Close
About
FAQ
Home
Collections
Login
USC Login
Register
0
Selected
Invert selection
Deselect all
Deselect all
Click here to refresh results
Click here to refresh results
USC
/
Digital Library
/
University of Southern California Dissertations and Theses
/
One-dimensional nanostructures for chemical sensing, transparent electronics, and energy conversion and storage devices
(USC Thesis Other)
One-dimensional nanostructures for chemical sensing, transparent electronics, and energy conversion and storage devices
PDF
Download
Share
Open document
Flip pages
Contact Us
Contact Us
Copy asset link
Request this asset
Transcript (if available)
Content
ONE-DIMENSIONAL NANOSTRUCTURES FOR CHEMICAL SENSING,
TRANSPARENT ELECTRONICS, AND ENERGY CONVERSION
AND STORAGE DEVICES
by
Po-Chiang Chen
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(MATERIALS SCIENCE)
May 2010
Copyright 2010 Po-Chiang Chen
ii
Acknowledgements
First of all, I would like to thank my advisor, Professor Chongwu Zhou, offering me
a great opportunity to work under his guidance to explore the fantastic nanotechnology.
His excellent insight in science and engineering broadened my view and led me through
the challenges to the end of my Ph.D. work. I deeply appreciate the constant support
which I have received during the past four years.
I also want to thank my colleagues: Dr. Song Han, Dr. Daihua Zhang, Dr. Bo Lei,
Dr. Koungmin Ryu, Dr. Fumiaki Ishikawa, Dr. Guozhen Shen, Dr. Marco Crulli, Dr.
Paichun Chang, Alexander Badmaev, Lewis Gomez, Akshay Kumar, Hsiaokang Chang,
Saowalak Sucharoenchoke, Chuan Wang, Yi Zhang, Jialu Zhang, Anuj Madaria, Haitian
Chen, Yue Fu, Jing Xu, Jing Qiu, Yi Shi, Dongdong Li, Shih-jui Chen, Rui Zhang, Chao
Wu, Ting-wei Yeh, Zachary Lingley, and all other people for all of the support and
goodwill that you have shown me. It's truly humbling to know that so many of you are
pulling for me. I will cherish the memories of the past four year for the rest of my life.
In addition, the cooperators, Professor Mark Thompson, Professor Tobin Marks,
Professor David Janes, Professor Edward Goo, Professor Paul Daniel Dapkus, and
Professor Jiye Fang are true professionals and better yet, sages. Thank you so much for
offering me valuable suggestions and precious help.
Finally, I would have done nothing if not for the love, patience, support, and
encouragement from my family. Last but not least, I want to express my heartfelt thanks
to my wife Po-Yu Hsu for her support and encouragement. Thank you, my family, and I
love you forever.
iii
Tables of Contents
Acknowledgements ............................................................................................................. ii
List of Tables ..................................................................................................................... vi
List of Figures…………………………………………………………………………....vii
Abstract.................. …………………………………………….………………………...xiv
Chapter 1
Introduction ……………………………………………………………………………….1
1.1 Overview of One-Dimentional Nanostructures .........................................................1
1.2 Materials in Nano Scale .............................................................................................3
1.2.1 Electronic Transport Properties ...........................................................................4
1.2.2 Chemical Sensing Behaviors ...............................................................................7
1.2.3 Chemical Sensing Behaviors .............................................................................11
1.3 Applications of One-Dimentional Nanostructures...................................................13
1.4 Outline of This Work ...............................................................................................21
Chapter 1 References ................................................................................................22
Chapter 2
Nano Electronic Nose: A Hybrid Nanowire / Carbon Nanotube Sensor Array with
Integrated Micromachined Hotplates for Sensitive Gas Discrimination .......................... 30
2.1 Introduction ..............................................................................................................30
2.2 Experiments .............................................................................................................32
2.2.1 The concept of electronic nose ........................................................................ 32
2.2.2 Device Fabrications ......................................................................................... 33
2.2.3 Electrical measurements and chemical sensing experiments ........................... 34
2.3 Results and Discussions ...........................................................................................35
2.3.1 Electronic transport characterizations .............................................................. 36
2.3.2 Chemical sensing behavior .............................................................................. 38
2.3.3 Smell print library ............................................................................................ 42
2.3.4 Electronic Nose ................................................................................................ 44
2.4 Conclusion ...............................................................................................................46
Chapter 2 References ................................................................................................49
Chapter 3
2, 4, 6-Trinitrotoluene (TNT) Chemical Sensing Based on Aligned Single-Walled
Carbon Nanotubes and ZnO Nanowires ...................................................................... 54
3.1 Introduction ..............................................................................................................55
3.2 Experiments .............................................................................................................56
3.2.1 Device Fabrications ......................................................................................... 56
3.2.2 Chemical sensing experiments and set-up ....................................................... 58
iv
3.3 Results and Discussion ............................................................................................60
3.3.1 Flexible SWNT TNT chemical sensors ........................................................... 60
3.3.2 ZnO nanowire TNT chemical sensors ............................................................. 64
3.3.3 TNT sensing mechanism.................................................................................. 66
3.4 Conclusion ...............................................................................................................68
Chapter 3 References ................................................................................................70
Chapter 4
High-Performance Single-Crystalline Arsenic-Doped Indium Oxide Nanowires for
Transparent Thin Film Transistors and Active Matrix Organic Light-Emitting Diode
Displays……..................................................................................................................... 74
4.1 Introduction ..............................................................................................................75
4.2 Experiments .............................................................................................................77
4.2.1 The synthesis of As-doped In
2
O
3
nanowires ................................................... 77
4.2.2 Fabrication of As-doped In
2
O
3
nanowire TTFTs with an Al
2
O
3
dielectric ..... 78
4.2.3 Fabrication of As-doped In
2
O
3
nanowire transistors with a SAND dielectric . 78
4.2.4 OLED Deposition for the AMOLED display .................................................. 79
4.3 Results and Discussion ............................................................................................80
4.3.1 Characterizations of Arsenic-doped indium oxide nanowires ......................... 80
4.3.2 Electronic transport characteristics of As-doped In
2
O
3
nanowire TTFTs ....... 82
4.3.3 Fully transparent OLED driving circuitry........................................................ 90
4.3.4 AMOLED display and drive circuitry ............................................................. 91
4.4 Conclusion ...............................................................................................................94
Chapter 4 References ................................................................................................95
Chapter 5
Flexible and Transparent Supercapacitor based on In
2
O
3
Nanowire / Carbon
Nanotube Heterogeneous Films ........................................................................................ 99
5.1 Introduction ..............................................................................................................99
5.2 Experiments ...........................................................................................................100
5.3 Results and Discussion ..........................................................................................102
5.4 Conclusion .............................................................................................................108
Chapter 5 References ..............................................................................................109
Chapter 6
Inkjet Printing of Single Walled Carbon Nanotube / RuO
2
Nanowire
Supercapacitors on Cloth Fabrics and Flexible Substrates ............................................. 111
6.1 Introduction ............................................................................................................111
6.2 Experiments ...........................................................................................................114
6.2.1 Preparation of SWNT inks ............................................................................. 114
6.2.2 Synthesis of RuO
2
nanowires......................................................................... 114
6.2.3 Device Fabrication ......................................................................................... 115
6.3 Results and Discussion ..........................................................................................116
6.3.1 Characterizations of Inkjet Printed SWNT Films .......................................... 116
6.3.2 Electrochemical behaviors of inkjet printed SWNT supercapacitors ............ 119
v
6.3.3 Electrochemical behaviors of inkjet printed SWNT films ............................. 122
6.3.4 Electrochemical behaviors of RuO
2
/SWNT supercapacitors ........................ 126
6.4 Conclusion .............................................................................................................128
Chapter 6 References ..............................................................................................130
Chapter 7
Preparation and Characterization of Flexible Asymmetric Supercapacitors Based on
Transition-Metal-Oxide Nanowire / Single-Walled Carbon Nanotube Hybrid Thin
Film Electrodes………… ............................................................................................... 133
7.1 Introduction ............................................................................................................133
7.2 Experiments ...........................................................................................................137
7.2.1 Preparation of SWNT bulky paper ................................................................ 137
7.2.2 Material Synthesis and characterization ........................................................ 138
7.2.3 Preparation of hybrid-nanostructured films ................................................... 141
7.3 Results and Discussion ..........................................................................................144
7.3.1 Electrochemical behavior of hybrid-nanostructured films ............................ 144
7.3.2 Electrochemical behavior of hybrid-nanostructured asymmetric
supercapacitors ............................................................................................... 148
7.4 Conclusion .............................................................................................................154
Chapter 7 References ..............................................................................................155
Chapter 8
Conclusions and Future Work ........................................................................................ 158
8.1 Conclusions ............................................................................................................158
8.2 Future Work in chemical sensing applications ....................................................161
8.3 Future work in synthesis of 1-D nanostructures ...................................................163
Bibliography ....................................................................................................................166
vi
List of Tables
Table 1.1 Summary of the electronic properties of 1-D nanostructure
based FETs
14
Table 1.2 Chemical sensors based on 1-D nanostructures. 16
Table 2.1 Sensor Array Test Conditions
a
. 35
Table 2.2 Correlation matrix of hybrid sensor array operated at
different temperatures.
47
Table 3.1 Estimated TNT Concentrations. 59
Table 4.1 Summary of the electronic properties of different doped
and undoped metal-oxide material based TTFTs.
86
vii
List of Figures
Figure 1.1 Different 1-D nanostructures. 4
Figure 1.2 (a) SWNT is formed by wrapping a graphene sheet into a
cylinder by the chiral vector c
h
. (b) Illustrations of the
structure of metallic and semiconductor SWNTs .
7
Figure 1.3 The I-V
DS
family curves of single In
2
O
3
nanowire FET. 7
Figure 1.4 A summary of a few of the electronic, chemical, and optical
process occurring on metal oxides that can benefit from
reduction in size to the nanometer range.
9
Figure 1.5 Schematic of morphological changes occurring in Si during
electrochemical cycling.
12
Figure 1.6 (a) In
2
O
3
nanowire transparent transistors (b) I-V
DS
family
curves of a In
2
O
3
nanowire transparent transistor (c) I-V
G
curves
of a In
2
O
3
nanowire transparent transistor.
15
Figure 1.7 Ragone plot for different electrical energy storage and
conversion devices.
20
Figure 2.1 (a) Photograph of a chemical sensor chip with an integrated
micromachined hot plate. (b) Hybrid chemical sensor array chip
composed of four individual chemical sensors, including
individual In
2
O
3
nanowire, SnO
2
nanowire, ZnO nanowire, and
SWNT chemical sensor chips. The source-drain electrode
distance is about 3 μm.
36
Figure 2.2 Current-voltage curves of as-fabricated chemical sensors
measured at room temperature (a) and 200
o
C (b).
38
Figure 2.3 Sensing response to 500 ppm hydrogen (a) and 50 ppm ethanol
(b) diluted in air for SnO
2
nanowire, ZnO nanowire, In
2
O
3
nanowire, and SWNT sensors operated at 200
o
C. The Y-axis
represents ΔG/G
0
. Arrows indicate time when the tested
chemical was flown into the sensing chamber.
41
Figure 2.4 Representative sensor array response as bar graphs to NO
2
(a),
hydrogen (b), and ethanol (c). The solid bars and shaded bars
represent the sensing response at 25
o
C and 200
o
C, respectively.
44
viii
Figure 2.5 PCA scores and loading plots of the chemical sensor array
composed by 4 different nanostructure materials (a) and only 3
metal oxide nanowires (b) operated at different concentrations
and temperatures.
48
Figure 3.1 SEM image of vertical ZnO nanowires on a-sapphire. The scale
bar is 1 µm.
57
Figure 3.2 Set-up of Chemical Sensing Chamber. 59
Figure 3.3 Wearable transistors based on aligned nanotubes transferred to
fabric. (a) Schematic diagram showing a transistor structure that
uses polyethylene-coated fabric (b) The optical micrograph
showing an array of such transistors built on a flexible fabric. (c)
I-V
g
curves of a transistor (D = ~2 tubes / µm ) before and after
electrical breakdown. (d) I -V
g
curves at different V
ds
for the
device in (c). V
ds
is varied from -0.2 to -1 V in steps of -200 mV.
(e) I-V
ds
curves at different V
g
for the same device in (c). The
curves correspond to V
g
= 10 to -20 V in step of - 5 V.
60
Figure 3.4 (a) I-V curves taken in air, 5 ppm NO
2
, and three different TNT
concentrations. (b) Sensing response of a flexible SWNT sensor
to TNT. The normalized conductance change ( Δ G/ G
0
) is plotted
as a function of time with the sensor exposed to TNT of different
concentrations. Recovery was made by UV light (254 nm). The
inset shows TNT structure. (c) Plot of Δ G/ G
0
v.s. the TNT
concentration.
62
Figure 3.5 Six sensing cycles of the flexible CNT chemical sensor,
corresponding to NO
2
concentrations of 40 ppb, 80 ppb, 150
ppb, 1.25 ppm, 2.5 ppm, and 5 ppm.
64
Figure 3.6 (a) Schematic view of a ZnO nanowire transistor structure, with
Ti/Au deposited on ZnO nanowires as source and drain
electrodes. Inset: SEM image of the as-fabricated ZnO nanowire
chemical sensor. The scale bar is 2 μm. (b) I-V curves taken in
air and at different TNT concentrations. (c) Sensing response of
a ZnO nanowire chemical sensor to TNT. The normalized
conductance change ( Δ G /G
0
) is plotted as a function of time with
the sensor being exposed to TNT of different concentrations. (d)
Normalized conductance change ( Δ G/ G
0
) vs TNT concentrations
(C), which was fitted using S =1/ (A+B/C). Inset: 1/S vs 1/C
with linear line fit.
66
ix
Figure 3.7 Positive ion mass spectrum of (a) air, (b) 23 ppb TNT in air,
recorded at an electron energy of 70 eV.
69
Figure 4.1 Optical transmission spectra of a typical As-doped In
2
O
3
nanowire TTFT (Red curve), and a AMOLED display substrates
after transparent OLED deposition (Green curve).
80
Figure 4.2 (a) SEM image of As-doped indium oxide nanowires. Inset
figure shows an As-doped nanowire with a catalyst particle at the
very tip (inset). (b) EDS spectrum showing the chemical
composition of the As-doped indium oxide nanowires. (c)
Corresponding HR-TEM image of a single As-doped In
2
O
3
nanowire with a diameter of ~16 nm. The electron diffraction
pattern reveals a bcc crystal structure of As-doped In
2
O
3
nanowire (inset). (d) In another HR-TEM image, the (100)
planes are clearly visible, and oriented perpendicular to the
vertical axis.
82
Figure 4.3 Fabrication of fully transparent As-doped In
2
O
3
nanowire
transistors (a) Schematic diagram of As-doped In
2
O
3
nanowire
TTFT fabricated on an ITO glass substrate, with ALD-deposited
Al
2
O
3
or SAND as the dielectric layer and IAD-deposited ITO as
source and drain electrodes. (b) Optical photograph of fully
transparent As-doped In
2
O
3
nanowire transistors. The substrate
area is marked with a yellow frame for clarity. (c) Family of I
ds
-
V
DS
curves of a single As-doped In
2
O
3
nanowire TTFT with the
channel length of 1.8 μm. The gate voltage varied from -4.5 V to
4.5 V in a step of 1.5 V from bottom to top. Inset: SEM image of
an As-doped In
2
O
3
nanowire bridging ITO electrodes. (d)
Current versus gate voltage (I
ds
-V
g
) plot in the linear regime
(V
DS
= 200 mV). Red, green and blue curve correspond to linear-
scale I
ds
-V
g
, log scale I
ds
-V
g
, and μ, respectively. Inset shows an
SEM image of the nanowire transistor.
84
Figure 4.4 The device mobility, subthreshold voltage, and I
on
/I
off
ratio of 10
single nanowire TTFTs, with red line indicating average values
for the respective parameters.
86
Figure 4.5 (a) Family of I
ds
-V
DS
curves for a single As-doped In
2
O
3
nanowire transistor using SAND as the dielectric layer with a
channel length of 1.8 μm. The gate voltage is varied from -2.0 V
to 4.0 V in steps of 1.0 V from the bottom to top. (d) Current
versus gate voltage (I
ds
-V
g
) curves in the linear region (V
DS
= 200
mV). Red, green and blue curves correspond to a linear-scale I
ds
-
V
g
, log scale I
ds
-V
g
, and μ, respectively.
87
x
Figure 4.6 (a) The measurement setup showing As-doped In
2
O
3
nanowire
transistor configured as a common-source amplifier. (b)
Frequency response of AC gain of As-doped In
2
O
3
nanowire
TTFT. Solid blue line shows the measured frequency response
of As-doped In
2
O
3
nanowire TTFT, dashed red line represents
the simulated data of As-doped In
2
O
3
nanowire TTFT. The
green solid line shows simulation data for an optimized As-
doped In
2
O
3
nanowire TTFT. (c) Bilateral small-signal model
used in this study.
89
Figure 4.7 (a) Plot of the output current through the loaded OLED (I
OLED
)
versus V
in
with V
dd
at 5.0 V in linear scale (red line) and log
scale (blue line), respectively. Inset: Circuit diagram of an
OLED driven by a transparent As-doped In
2
O
3
nanowire
transistor. (b) OLED light intensity versus V
in
with V
dd
= 5.0 V.
Inset: OLED spectrum. (c) Optical images of the OLED under
V
in
= -3.0 V, 0.0 V, and 3.0 V.
90
Figure 4.8 An application of As-doped In
2
O
3
nanowire TTFT circuitry to
drive a seven segment AMOLED display. (a) Schematic of a
seven-segment digit of AMOLED pixel, which consist of one
switching transistor (T
1
), one driving transistor (T
2
), and one
storage capacitor (C
st
). The bias condition to operate the circuit
are -5 V to 5 V of scan line to full turn-off and turn-on, varying -
5 V to 5 V on the data line, 3 V on the V
dd
line, and 0 V on the
cathode line. (b) Optical photograph of fully transparent
AMOLED display before OLED layer deposition with the
substrate area marked with a yellow frame for clarity. The
feature on the background picture is clearly visible. (c) Optical
photograph of AMOLED animation. The seven segment digit
displays number 1, 3, and 6, respectively.
93
Figure 5.1 SEM images of CNT films, scale bar: 200 nm (a), PEDOT: CNT
films, scale bar: 200 nm (b), and In
2
O
3
nanowire / CNT
heterogeneous film, scale bar: 2 μm (c).
101
Figure 5.2 (a) Photograph of a flexible and transparent supercapacitor
fabricated using CNT films. (b) AFM image of entangled CNT
networks sitting on a transparent PET substrate. (c) Schematic
diagram of a flexible and transparent supercapacitor. The gray
color represents a Nafion film as separator between two In
2
O
3
nanowires/CNT heterogeneous film electrodes. (d)
Transmittance spectra of three different electrochemical
capacitors and a single CNT film.
103
xi
Figure 5.3 (a) Cyclic voltammograms (CVs) of a CNT film supercapacitor,
a PEDOT:CNT film supercapacitor, and a 30 wt% In
2
O
3
nanowires on CNT film supercapacitors. (b) Specific capacitance
vs. different weight of In
2
O
3
nanowires. The inserts are SEM
images with different weight of In
2
O
3
nanowires. The scale bars
in SEM pictures represent 2 μm.
106
Figure 5.4 (a) charge-discharge behavior of a In
2
O
3
nanowires/CNT
heterogeneous film electrochemical capacitor in 1M LiClO
4
electrolyte. (b) Cycle-life data of In
2
O
3
nanowires/CNT
heterogeneous film electrochemical capacitor. Specific
capacitance was calculated form GV measurements at constant
current of 0.5 A/g.
107
Figure 6.1 (a) SEM image and (b) XRD pattern of RuO
2
nanowires. 115
Figure 6.2 (a), (b), and (c) SEM images of a fabric with printed SWNT
films.
117
Figure 6.3 Photograph of cloth fabric before (a) and after (b) inkjet printing of
SWNT films. (c) Schematic diagram of an printed SWNT
supercapacitor using PVA/H
3
PO
4
as separator and electrolyte.
The inset shows a supercapacitor made of SWNT/ fabric rolled
around a pencil.
117
Figure 6.4 (a) Photograph of a PET substrate with SWNT films printed with
different size and number of repeated prints. The feature on the
background picture is clearly visible. (b) A SEM image of
printed SWNT films on a PET substrate and a photograph of a
supercapacitor built with SWNT/PET substrates.
118
Figure 6.5 (a) Conductance and transmittance of the printed SWNT films
on PET substrates as a function of different film thickness. (b)
Optical transmittance spectra of printed SWNT films on PET
substrates with different printed thickness from 20 nm to 200
nm.
119
Figure 6.6 Cyclic voltammetery of inkjet-printed SWNT supercapacitors on
PET substrates (a) and cloth fabrics (b) with PVA/H
3
PO
4
polymer electrolyte with different scan rates of 20, 50, and 100
mV/second. Galvanostatic charge/discharge curves measured
with a 1 mA/mg current density for inkjet-printed SWNT
supercapacitor on PET substrates (c) and cloth fabrics (d).
120
xii
Figure 6.7 Electrochemical impedance spectroscopy at 0.1 V bias voltage
on a supercapacitor built with SWNT/PET electrodes in 1 M
Na
2
SO
4
electrolyte.
123
Figure 6.8 Equivalent series resistance (ESR) and Power density of
SWNT/PET supercapacitors as a function of printed thickness.
124
Figure 6.9 Specific Capacitance of a SWNT/PET supercapacitor plotted v.s.
number of cycles.
125
Figure 6.10 (a) SEM image of RuO
2
nanowires dispersed on an inkjet-
printed SWNT film. The inset shows the sample at higher
magnification. (b) Cyclic voltammetery of RuO
2
nanowire /
inkjet-printed SWNT supercapacitors on PET substrates in
PVA/H
3
PO
4
polymer electrolyte with different scan rates of 50,
100, 200, 300, and 500 mV/second. (c) Galvanostatic
charge/discharge curves measured with a 1 mA/mg current
density for a RuO
2
nanowires / inkjet-printed SWNT
supercapacitor. (d) Electrochemical impedance spectroscopy for
a SWNT/PET supercapacitor (green curve) and a RuO
2
nanowire / inkjet-printed SWNT supercapacitor (red curve) in
0.3 M H
2
SO
4
electrolyte.
128
Figure 7.1 (a) Optical and (b) SEM image of SWNT films obtained by
using a vacuum filtration method.
138
Figure 7.2 (a) SEM image of β - MnO
2
nanowires with diameter of 20 nm
and length of 2-3 µm in average. (b) TEM image of β - MnO
2
nanowires. (c) High resolution TEM of a β - MnO
2
nanowire. (d)
A typical XRD spectrum of as-grown β - MnO
2
nanowires. (e)
SEM images of In
2
O
3
nanowires with diameter of 60 nm and
length of 10-100 µm in average. (f) TEM image of In
2
O
3
nanowires. (g) High resolution TEM of a In
2
O
3
nanowire. (h) A
typical XRD spectrum of as-grown In
2
O
3
nanowires.
141
Figure 7.3 Schematic diagram of an asymmetric supercapacitor composed
with MnO
2
nanowire / SWNT hybrid film as a cathode electrode,
and In
2
O
3
nanowire / SWNT hybrid film as an anode electrode.
142
Figure 7.4 A scratched MnO
2
nanowire / SWNT hybrid film. Inset (up):
SWNT films can be clearly observed underneath MnO
2
nanowire
networks. Inset (down): uniform MnO
2
nanowire network above
SWNT films
143
xiii
Figure 7.5 An as-fabricated In
2
O
3
nanowire / SWNT hybrid film. Inset: An
SWNT films underneath In
2
O
3
nanowires works as current
collecting electrodes.
144
Figure 7.6 Cyclic voltammetery in a three-electrode configuration with
different nanostructured thin film electrodes of (a) a bare SWNT
thin film electrode, (b) a MnO
2
nanowire / SWNT hybrid film
electrode, and (c) a In
2
O
3
nanowire / SWNT hybrid film
electrode in 1M Na
2
SO
4
electrolyte with different scan rates
ranging from 5 mV/sec to 100 mV/sec. (d) Comparative cyclic
voltammetery using MnO
2
nanowire / SWNT hybrid film and
In
2
O
3
nanowire / SWNT hybrid film as active electrode. The
scan rate of potential is 100 mV/sec.
146
Figure 7.7 Cyclic voltammograms of an optimized hybrid-nanostructured
asymmetric supercapacitor in 1M Na
2
SO
4
electrolyte with a scan
rate of 20 mV/sec (a), and with different scan rates of 5, 10, 20,
50, 75, and 100 mV/sec (b). (c) Galvanostatic charging /
discharging curves measured with a current density of 2 mA/cm
2
for an optimized hybrid-nanostructured asymmetric
supercapacitor in 1M Na
2
SO
4
electrolyte. (d) A comparison of
specific capacitance of a hybrid-nanostructured asymmetric
supercapacitor and a SWNT symmetric supercapacitor with
different discharging currents of 1, 2, 5, 10, 20 mA/cm
2
.
149
Figure 7.8 Galvanostatic charging/discharging curves measured with a
current density of 2 mA/cm
2
of a SWNT symmetric
supercapacitor in 1M Na
2
SO
4
electrolyte.
151
Figure 7.9 (a) Galvanostatic charging/discharging curves (I = 2 mA/cm
2
) of
an optimized asymmetric nanostructured supercapacitor in 1M
Na
2
SO
4
electrolyte. Cyclic voltammograms on an optimized
asymmetric nanostructured supercapacitor in 1M Na
2
SO
4
electrolyte with a scan rate of 20 mV/sec. (b) Coulombic
efficiency and specific capacitance of a hybrid-nanostructured
asymmetric supercapacitor in 1M Na
2
SO
4
electrolyte vs.
different cell voltage. (c) Photo image of a green LED connected
with the hybrid-nanostructured asymmetric supercapacitor
before/after discharging (inset). (d) A Rogone plot showing that
the hybrid-nanostructured asymmetric supercapacitors
outperform the SWNT symmetric supercapacitors and early
asymmetric supercapacitor data reported in literatures.
152
xiv
Figure 8.1 Schematic diagram of flexible ZnO pizeoelectronic chemical
sensors built on Kapton substrates without bending force (a) and
with bending force (b).
163
Figure 8.2 Horizontal fluidized bed systems (a) fix-bed type (b) photo
image of a fix-bed type horizontal fluidized bed system.
164
Figure 8.3 SEM images at (a) high magnification and (b) low magnification
of bulk synthesis of Si nanowires on Al
2
O
3
supported
nanoparticles using horizontal fluidized bed systems.
165
xv
Abstract
One-dimensional nanostructures have been intensively investigated and proven to
be of great potential as building blocks in different applications, including nano/micro
electronics, chemical/biological sensing, and more recently, energy conversion and
storage devices. Nanowires and carbon nanotubes made from organic/inorganic methods
are now available and give research opportunities for understanding of one-dimensional
nanostructures and their future applications. In this field, the research can be divided into
three categories: nanostructure synthesis, material characterizations, device fabrication
for applications. These three main thrusts govern the research route to understand the
one-dimensional nanostructures.
This dissertation follows above-mentioned elements and consists of eight chapters,
which will mainly concentrate on three most promising applications based on one-
dimensional nanostructures, including chemical sensing, transparent electronics, and
electrochemical capacitors. Following an overview and an introduction of fundamental
knowledge in one-dimensional nanostructures in Chapter 1, Chapter 2 and Chapter 3 will
take up the application of one-dimensional nanostructures into the chemical sensing field.
More specifically, Chapter 2 will carry out the important topic of “selectivity”, which
remains one of the challenging issues in the field, by using a nano electronic-nose sensor
array built on four different one-dimensional nanostructures. The sensor array performs a
great “discrimination power” and is capable to distinguish important industrial gases
distinctly.
xvi
The detection of explosives and nerve agents are still one of missing blocks in
current one-dimensional nanostructure based chemical sensors. Chapter 3 will introduce
2, 4, 6-trinitrotolune (TNT) sensors made of aligned single-walled carbon nanotubes
(SWNTs) and ZnO nanowires. The discussion will primarily focus on the TNT sensing
mechanism and the TNT sensing behavior of above two one-dimensional nanostructures.
Chapter 4 will concentrate on the synthesis of arsenic doped indium oxide (As-
In
2
O
3
) nanowire and its application in the transparent electronics. A comprehensive study
starting from nanowire synthesis, material characterizations, to electronic transport
properties will be presented. In the end, a transparent integrated circuit, which uses
nanowire transparent thin film transistors to control an active-matrix organic light
emitting diode (AMOLED) display, will be fabricated and demonstrated.
Chapter 5 will provide a versatile approach to produce hybrid-nanostructured thin
film electrodes by integrating transition-metal-oxide nanowires and SWNTs for the
application in the electrochemical capacitors. Here, the detailed device fabrication of the
first prototype of flexible and transparent supercapacitors will be presented, and the
device performance will be examined and discussed later on.
Chapter 6 will present another prototype of supercapacitor, which are allowed a
scalable fabrication and can be produced simply by utilizing a commercial inkjet printer.
The electrochemical characterizations of the inkjet-printed SWNT thin film electrodes
and the device performance of the first inkjet-printed supercapacitors will be discussed.
Besides, another hybrid-nanostructured thin film electrode composed of ruthenium oxide
xvii
(RuO
2
) nanowires and SWNTs will be introduced to improve the device performance
afterward.
In order to obtain high-performance supercapacitors with high power density and
energy density to drive electrical vehicles, Chapter 7 will be dedicated to discuss the
governing factors in aspects of the device structure and the choice of electrode materials.
A hybrid device structures built on two different hybrid-nanostructured thin film
electrodes will be adapted, and the detail of the electrode preparation, device fabrication
and optimization, and the electrochemical performance will be examined and presented.
Chapter 8, in the end, summarizes the above discussions and proposes future
research directions in one-dimensional nanostructures.
1
Chapter 1 Introduction
Nanotechnology and nanomaterials have become the flavor of the day, with recent
research achievements driven by academic curiosity and promising potential in kinds of
applications, ranging from chemical sensors, biological sensors, field effect transistors,
logic circuits to energy conversion and storage devices. Nanomaterials, regarding to
different dimensions, can be classified into three main categories, including zero-
dimension (e.g., quantum dots), one-dimension (e.g., nanowires, nanobelts, and
nanotubes), and two-dimension (e.g., graphene). One-dimensional nanostructures have
been shown to exhibit superior electrical, optical, mechanical, and thermal properties. In
this chapter, an overview and fundamental knowledge in one-dimensional nanostructures
are presented with an emphasis on transition-metal-oxide nanowires and single-walled
carbon nanotubes (SWNTs). In addition, a few potential applications based on one-
dimensional nanostructures are also addressed and discussed later on.
1.1 Overview of One-Dimensional Nanostructures
In 1959, Richard Feynman forecasted the development of nanotechnology in his
inspired talk entitled “There is plenty of room at the bottom”. Since then, following the
discovery of carbon nanotubes (CNTs) by S. Ijima in 1991 [1], one-dimensional (1-D)
nanostructures, including nanowires, nanorods, nanobelts, and nanotubes, have been a
subject of intensive research. Due to the significant quantum confinement effects raising
from tiny structures ranging from 1 nm to 100 nm, 1-D nanostructures exhibit quite
different physical properties in electronic transport, heat exchange, and mechanics,
compared to their bulk counterparts [2, 3]. Hence, 1-D nanostructures cover a wide range
2
of materials including insulators (e.g., SiO
2
and TiO
2
), metals (e.g., Au, Ag, and Pt), and
semiconductors (e.g., ZnO, In
2
O
3
, and SnO
2
). It is entirely not surprising that
semiconductor ones attract the most attentions due to the promising applications as
building blocks in electronic devices and circuits. In addition, 1-D inorganic nanowires
and SWNTs represent the smallest dimension structure, and can efficiently transport
electronic carriers, which result in the ballistic electronic transport in 1-D nanostructures.
And, as it is well-known, the ballistic electronic transport is the essential for the high
frequency operation in electronic devices [4]. Moreover, they can also exhibit device
functions as well. For instance, metallic ones (e.g., RuO
2
nanowires, Ni
4
Si
3
nanowires,
and metallic SWNTs) can work as “wiring elements” and semiconductor ones (e.g., Si
nanowires, ZnO nanowires, and semiconductor SWNTs) can serve as “device elements”
(i.e., active channel materials) in the architecture of integrated nanoscale circuits. Several
prototypes of nanoscale electronic and optoelectronic devices, including single nanowire
laser [5, 6], radio frequency devices [7-9], single electron transistors [10], memory units
[11, 12], logic circuits [13-15], and photo-detectors [16, 17], have been successfully
fabricated and demonstrated.
In addition to nano-electronics and micro-electronics, the applications of 1-D
nanostructures can fall into the three main categories (i.e., energy conversion and storage
cells, optical/biological/chemical sensing, and mechanics devices such as atomic force
microscopy (AFM) tips). These applications are resulted from their small diameter, large
surface to volume ratio (~ 1,000), superior optical and electronic sensitivities, and good
mechanical properties. Owing to the enhanced surface area (e.g., SWNTs ~ 2,630 m
2
/g in
theory), compared to thin film and bulk materials, nanowire and SWNT based chemical
3
sensors are expected to perform an unrivaled sensitivity, fast response, and low power
consumption [18-20]. For instance, indium oxide (In
2
O
3
) nanowire net sensors showed
the detection limit down to 5 ppb, while In
2
O
3
thin film chemical sensors merely
exhibited the detection limit of 5 ppm NO
2
[21]. Besides, nanowire based biological
sensors also showed single molecule detection [22, 23]. In addition to their excellent
device performance, low cost and remarkable simplicity in device fabrication have made
them more attractive than the conventional thin film based sensors.
Notwithstanding kinds of applications, the fundamental studies in 1-D nano-systems
are needed to explore the physics at nano-scales, such as electron transport, ionic
conductivity, lasing, phase transition, and electrochemical reactions. In this dissertation, a
number of 1-D nanostructures have been synthesized by using chemical vapor deposition
(CVD), laser ablation method, wet chemical synthesis, and so-called hydrothermal
method, shown in Figure 1.1. These 1-D nanostructures have also been fabricated as
nanoscale devices, typically field-effect transistors (FETs), chemical sensors, biological
sensors, and recently, energy conversion and storage devices. The device performance
and physical properties of these 1-D nanostructures have been systematically investigated.
As a brief introduction, the following parts will cover the fundamental knowledge and
potential applications of transition-metal-oxide nanowires and SWNTs.
1.2 Materials in Nano Scale
As discussed in the previous section, due to the small size, high surface-to-volume
ratio, and quantum-confinement effect, 1-D nanostructures have exhibited distinct
physical properties in electronics, optoelectronics, catalytic reactions, and
4
thermodynamics. Here, I would like to go into the detail of electronic transport, chemical
sensing, and electrochemical properties of transition-metal-oxide nanowires and SWNTs.
Figure 1.1 Different 1-D nanostructures
1.2.1 Electronic Transport Properties:
As it is well-known, SWNTs can be viewed as a sheet of graphene rolled into
seamless cylinders with nanometer-scale diameter (0.4 nm - 2 nm) and micrometer-scale
length. The electronic band structure of SWNTs can be expressed by considering the
covalent bonding of the carbon atoms arranged in a hexagonal lattice via sp
2
molecular
orbitals [24]. Depending on their chiralities and diameters, SWNTs can be metallic, semi-
metallic, or semiconductor, shown in Figure 1.2. For metallic nanotubes, the Fermi
In
2
O
3
nanowires ZnO nanowires SnO
2
nanowires
Si nanowires MnO
2
nanowires
AgV
4
O
11
nanowires MWNT
MgO nanowires
RuO
2
nanowires
5
energy intersects two bands of the one dimensional band structure and results in that
electron transport occurs ballistically over long nanotube lengths [25]; thus enable them
to carry high current density of 10
9
A/cm
2
, which can be an ideal interconnection element
in nano-circuits (e.g., the maximum current densities for normal metal (e.g. Ag) is about
10
6
A/cm
2
). Regarding to the semiconductor nanotubes, the electronic band structures
exhibit characteristic E
-1/2
van Hove type singularities, which is typical in 1-D system,
and lead to the varied energy band gap of (0.9/diameter) eV [26]. A more detailed
discussion about electronic band structure can be found in Ref. 27. In addition, with
extraordinary field-effect mobility of 79,000 cm
2
/Vs and intrinsic mobility of > 100,000
cm
2
/Vs [27], semiconductor nanotubes have been proposed for nanoelectronics
applications such as high frequency FETs (> 1 GHz), single electron memories, and bio-
chemical sensors.
Parallel to SWNTs, with the success of producing 1-D transition-metal-oxide
nanostructures, transition-metal-oxide nanowire based FETs can be readily made and the
electronic properties can be studied and expressed in analogy to fully depleted silicon-on-
insulator MOSFETs. The surface potential
f
φ in the FET channel is described by the 1-D
modified Poisson equation in the following equation: [28, 29]
2
22
() ()
()
f f gs bi
NW
x x
ex
x
φ φ φφ
ρ
λ ε
∂ − −
−= −
∂
(1)
where
gs
φ and
bi
φ are the gate and built-in potentials, ρ is the carrier density in the x-
direction,
NW
ε is the dielectric constant of nanowires, and λ is termed the nature length
( ( /)
NW ox ox NW
tt λ ε ε = ), which can be determined by the thickness of the gate oxide and
6
the nanowire. In order to obtain good electrostatic integrity and suppress the short-
channel effects (SCEs) of MOSFETs, λ must be scaled along with the gate length. For
nanowire MOSFETs, the diameter of nanowire could be reduced to several nanometers,
which can minimize the SCEs and represents a size scale that is challenging to current
photolithography technology. In addition, nanowire geometric structures enables gate
structures in semicylindrical and full-cylindrical, which can further decrease the SCEs
and improve the drive current for high-performance operations.
Here, FETs built on In
2
O
3
nanowires are used as examples to illustrate the
electronic properties of 1-D metal oxide nanostructures. The AFM image of Figure 1.3
[30] inset shows an In
2
O
3
nanowire contacted by two metallic electrodes (Ti/Au)
patterned atop the Si/SiO
2
substrate. These electrodes are used as the source / drain
electrodes while the silicon substrate is used as a back gate. Figure 1.3 shows the gate-
dependent drain current versus source-drain voltage (I-V
ds
) curves of the device measured
at room temperature. With the gate voltage varying from +15 V to -10 V, the
conductance of the nanowire was gradually suppressed. This behavior is in good
agreement with the well-known fact that In
2
O
3
is a n-type semiconductor due to the O
2
deficiency. Detailed analysis of the transistor data reveals good on /off ratio of 2.08 × 10
4
,
carrier concentration of 2.30 nm
-1
and effective mobility of 98.1 cm
2
/Vs.
Several approaches have been reported and suggested that the electronic
performance of transition-metal-oxide nanowire-based transistors can be improved by
using post-fabrication treatments, such as the thermal annealing under vacuum or the
exposure to UV/ozone. After the exposure to UV/ozone for 2 minutes, the device
performance parameters, including on/off ratio, mobility, and subthreshold slope of one
7
In
2
O
3
nanowire transistor, were improved to 10
6
, 514 cm
2
/Vs, and 160 mV/dec,
respectively [31].
Figure 1.2 (a) SWNT is formed by wrapping a graphene sheet into a cylinder by the
chiral vector c
h
. (b) Illustrations of the structure of metallic and semiconductor SWNTs
Figure 1.3 The I-V
ds
family curves of single In
2
O
3
nanowire FET
1.2.2 Chemical Sensing Behaviors:
With large surface-to-volume ratio and the Debye length comparable to their
small size, 1-D nanostructures have already displayed superior sensitivity to surface
1000
500
0
-500
-1000
I (nA)
-1.0 -0.5 0.0 0.5 1.0
V
ds
(V)
10 V
V
g
= 15 V
7 V
0 V
-7 V
-10 V
10
-2
10
-1
10
0
10
1
10
2
10
3
I (nA)
10 5 0 -5 -10
V
g
(V)
V
ds
= 0.32 V
Ti/Au
In
2
O
3
NW
8
chemical processes [18, 32-34]. For instance, Kong et al. [18] and Collins et al. [32] were
the first to demonstrate that certain molecular adsorptions can significantly change the
electrical conductance of SWNTs. Nitrogen dioxide (NO
2
), a well-known electron
acceptor, results in an increase in conductance of p-type semiconductor SWNTs while
NH
3
, an electron donor, causes a reduction in conductivity. Early experiments showed
that NO
2
binds on SWNT surface and withdraw approximately 0.1 e
-
per molecule, and
NH
3
adsorbate donates ~0.04 e
-
per molecule [35]. Besides, the detection limit of NO
2
can be improved down to 100 part per trillion (ppt) via coating polyethyleneimine (PEI)
molecules on SWNT surface [33].
Meanwhile, Wang et al. [36] reported the first SnO
2
nanobelt chemical sensor in
the detection of CO, ethanol, and NO
2
at elevated temperatures (300
o
C - 400
o
C). The
reported detection limit of NO
2
was 0.5 ppm in the synthetic air. The conductance of 1-D
nanostructures can be expressed as, [37]
2
0
( 2 )
4
Dw
G ne
l
π
µ
−
= (2)
where n
0
represents carrier concentration, μ represents mobility, l is the length of the
nanomaterials, D is the diameter of the nanomaterials, and w is the width of surface
charge region which is related to the Debye length of the nanomaterials.
The Debye length of sensing materials can be expressed as the following formula
obtained in the Schottky approximation [38]
1/2
()
s
D
eV
wL
kT
= (3)
1/2 0
2
0
()
D
kT
L
en
εε
=
(4)
9
where, ε
0
is the absolute dielectric constant, ε is the relative dielectric permittivity of the
structure, k is the Bolzmann’s constant, T is the temperature, and V
S
is the adsorbate-
induced band bending.
Figure 1.4 A summary of a few of the electronic, chemical, and optical process occurring
on metal oxides that can benefit from reduction in size to the nanometer range. [39]
Generally speaking, the response of the chemoresistors in ambient environment
can be defined as
1/2 1/2 1/2 10 0
01 0 1
00
4 4
( ) ( )( )
SS
GG
S ww V V
G D D en
εε −
= = −= ⋅ −
(5)
where, G
0
and G
1
are the conductance before and under exposure to chemicals. n
0
and n
1
represent the carrier concentration before and under exposure to chemicals. w
0
and w
1
represent the width of surface charge region before and under exposure of chemicals. V
S0
10
is the adsorbate-induced band bending due to oxygen molecule and moisture (V
S0
~ 0.1 eV
[47]) in ambient environment and V
S1
is the adsorbate-induced band bending from
exposure of chemicals (ex. NH
3
~ 0.25 eV [40]).
As one can see in the equation (5), the sensing response can be clearly attributed
to three different parts in the right term of the equation, including geometric factor (4/D),
electronic transport characterizations of nanomaterials (εε
0
/en
0
), and adsorbate-induced
band bending (
1/2 1/2
01 SS
VV − ) due to molecular adsorptions and reactions on transition-
metal-oxide material surface. The detail of reactions of chemisorbed molecules on
nanomaterial surface can be found in Ref. 38.
In the sense of practical applications, both sensitivity and selectivity are important
topics to the chemical sensing community. According to equation (5), there have been a
number of approaches developed to improve sensitivity, such as applying an external gate
voltage, doping metal impurities during material growth, modulating operation
temperature, and changing the geometric structures of nanomaterials. For instance, the
detection limit of a ZnO nanowire sensor to NO
2
is 10-fold better than the detection limit
of a ZnO nanorod sensor because nanowire has a higher surface-to-volume ratio than
nanorod [41]. And, also an In
2
O
3
porous nanotube film sensor exhibits a 100-fold higher
sensitivity than an In
2
O
3
nanowire mat sensor to ammonia at room temperature [42]. In
addition, by using chemical coating or nanoparticle decoration, both device sensitivity
and selectivity can be improved.
Typically, the detection limits of chemical sensors are also related to the signal-to-
noise ratio, which has been discussed in carbon nanotube based chemical sensors.
11
However, until now works addressing the noise level of metal oxide based chemical
sensors has been lacking. This is still a task which needs to be considered in this field.
1.2.3 Electrochemical Properties:
With the advantage of short diffusion ion length, 1-D nanostructures can be one of the
most promising materials to overcome the kinetic problems (i.e. slow cation and electron
diffusion) existing in conventional electrode materials for the applications in
electrochemical energy conversion and storage cells. In considerations of the solid-state
ionic diffusion of in electrode materials, the mean diffusion time, τ
eq
, can be determined
by the diffusion coefficient (D), and the diffusion length (L), according the following
equation [43],
2
/ 2
eq
LD τ = (6)
And, the diffusion coefficient can be expressed by using the Stokes-Einstein relation
of ( )/ ( )
B
D k T Ze µ = ⋅⋅ ⋅ , which is proportional to the material mobility ( μ). According to
the above equations, there are two approaches to overcome the kinetic problems of
electrode materials. One is to enhance the diffusion coefficient by enhancing the mobility
of electrode materials (e.g., via a doping process), which also leads to improve material
conductivity. The other method is to further decrease the cation and electron diffusion
length by using nanostructured materials. For instance, a reduction of diffusion length
from 10 μm (the typical value of conventional electrode materials) to 50 nm (average
diameter of 1-D nanostructures), for a LiCoO
2
electrode with diffusion coefficient of 10
-
11.6
cm
2
/s from electrochemical impedance spectroscopy [44], the τ
eq
can dramatically
reduce from ~ 50,000 seconds to ~1 second, which leads to fast charging/discharging
12
behavior in the electrochemical cells. Thus, the “going nano” effect is so significant that
most of research efforts have been devoted in developing new nanostructures.
In addition to short ion diffusion length, large surface area (A) can be another
advantage from 1-D nanostructures. According to the Cottrell equation,
1/2 *
1/2
()
()
nFAD C
it
t π
= (6)
where n is the number of electron exchange between oxidation and reduction, F is
Faraday constant (9.65 ×10
4
C/mol), C is the solution concentration, and t is diffusion
time. The charging/discharging current is proportional to the surface area of electrode
materials; therefore results in a fast charging/discharging rate and high power density in
1-D nanostructure based electrochemical cells.
Figure 1.5 Schematic of morphological changes occurring in Si during electrochemical
cycling [45].
13
Last but not least, 1-D nanostructures offer the other advantage of providing better
accommodation of strain from cation insertion/removal process, which significantly
improve the cycle life of electrochemical batteries. For example, in lithium ion batteries,
Si thin film electrodes usually exhibit a 4-fold of volume expansion after lithium ion
inserting into the crystal structures, which results in a permanent damage of electrodes
and a loss of active materials; therefore decrease the specific capacity within short
cycling numbers. The problem cannot be solved until recently Chan et al. adapted Si
nanowires as the anode material and successfully showed good capacity retention even
after 50 cycles of charging/discharging measurements [45].
However, 1-D nanostructures also can lead to some disadvantages, including an
increase of undesirable electrode/electrolyte reactions due to large surface area, lower
volumetric energy density because of inferior packing of materials, and potentially more
complex synthesis. Among them, undesirable electrode/electrolyte reactions can cause
self-discharging, poor cycling and calendar life, which are still one of unsolved issues to
1-D nanostructures applied in electrochemical conversion and storage devices.
1.3 Applications of One-Dimensional Nanostructures
As we discussed in the above section, compared to 2-D and 3-D materials, 1-D
nanostructures have larger surface area and superior physical properties, which have
attracted steadily growing interest. In brief, current applications of 1-D nanomaterials can
fall into three primary areas: electronics, chemical/biological sensors, and energy
conversion and storage devices.
14
In Table 1.1, we summarize the 1-D nanostructure based FETs described in
literature and their corresponding electronic transport properties. Among them, transistors
made of SWNTs, In
2
O
3
nanowires, ZnO nanowires, and SnO
2
nanowires have been
widely reported. As one can see, FETs fabricated with these four materials exhibit high
electron mobility, high transconductance, and low threshold voltage (not shown here),
which are critical for applications in nano-electronics and micro-electronics. Besides, it is
still a challenge to develop p-type transition-metal-oxide nanowires and n-type SWNTs
with good electronic performance. Doping is a good approach to achieve both p-type
nanowires and n-type SWNTs (Table 1.1). For instance, Xiang et al. reported p-type ZnO
nanowires with phosphorus as dopants during nanowire synthesis and showed effective
mobility of 1.7 cm
2
/Vs and carrier concentration of 2.2 ×10
7
cm
-1
. We note that the
device performance can be improved via the optimization of device structures and design.
Materials
Carrier
Concentration
Mobility
(cm
2
/V•S)
Transconductance
(μS)
Synthesis
Method
Ref
SWNT (p-type) N/A 20 1.7×10
-3
Thermal CVD [47]
SWNT: Sc
contact (n-type)
N/A 4650 25 Thermal CVD [48]
ZnO (n-type) 5×10
15
cm
-3
1175 0.06 Thermal CVD [49]
ZnO:P (p-type) 2.2×10
7
cm
-1
1.7 0.1 Thermal CVD [46]
ZnO:Ga (n-type) 3.9×10
18
cm
-3
10.2 N/A Thermal CVD [50]
In
2
O
3
(n-type) 8.56×10
7
cm
-1
1450 5.87 Laser Ablation [51]
In
2
O
3
(n-type) 1.02×10
7
cm
-1
71 1×10
-3
Laser Ablation [52]
SnO
2
(n-type) 2.98×10
7
cm
-1
172 0.18 Laser Ablation [53]
SnO
2
(n-type) 1.5×10
8
cm
-1
40 3.1×10
-3
Laser Ablation [54]
SnO
2
:Ta (n-type) N/A 156 2.94 Thermal CVD [55]
T
i
O
2
(n-type) 4.5×10
17
cm
-3
0.2 3×10
-3
Thermal CVD [56]
V
2
O
5
(n-type) N/A
9.6×10
-3
(192K)
N/A
Solution
Synthesis
[57]
Ga
2
O
3
(p-type) 5.3×10
8
cm
-1
9.6×10
-2
9.88×10
-8
Thermal CVD [58]
α-Fe
2
O
3
(n-type) 1.59×10
8
cm
-1
9.6×10
-3
8.2×10
-6
Thermal CVD [59]
α-Fe
2
O
3
(p-type)
4.9×10
6
cm
-1
3.09×10
-2
2.2×10
-3
Thermal CVD [60]
Table 1.1, Summary of the electronic properties of 1-D nanostructure based FETs.
15
In contrast to traditional opaque FETs, In
2
O
3
is known to be one of the best
candidates for transparent electronics applications due to its large band gap (~3.6 eV at
room temperature) and superior electron transport properties. Figure 1.6 (a) shows the
optical image of transparent In
2
O
3
nanowire transistors with ITO electrodes patterned
atop a high-k dielectric layer (50 nm HfO
2
) on an ITO /glass substrate, showing very
good transparency of > 80% in visible light regime. I-V
ds
curves of one device with the
gate voltage varied from -5 V to 5 V are shown in Figure 1.6 (b), which displays a high
on-current of ~3.8 μA at 2 V with zero gate voltage. Figure 1.6 (c) is the linear-scale and
log-scale drain current versus gate voltage (I
DS
-V
G
) characteristics for the same In
2
O
3
nanowire transistor at V
ds
= 100 mV. Detailed analysis revealed good on/off ratio ~ 10
5
,
effective mobility of 168.7 cm
2
/Vs, and threshold voltage of -1.8 V. The electronic
performance of these transparent In
2
O
3
nanowire transistors is comparable with or even
better than traditional In
2
O
3
nanowire transistors, made using low-k dielectric and non-
transparent electrodes. In parallel, semiconductor SWNTs have also been reported in the
applications of transparent electronics.
Figure 1.6 (a) In
2
O
3
nanowire transparent transistors (b) I-V
DS
family curves of an In
2
O
3
nanowire transparent transistor (c) I-V
G
curves of a In
2
O
3
nanowire transparent transistor.
12
10
8
6
4
2
0
I
DS
(µΑ)
2.0 1.5 1.0 0.5 0.0
V
DS
(V)
2.5
2.0
1.5
1.0
0.5
0.0
I
DS
(µΑ)
-4 -2 0 2 4
V
G
(V)
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
(b) (c)
(a)
16
Material Material Type Sensor Type Target Gas Media Detection Limit Response Time Ref.
ZnO
Nanorods Chemoresistor
NO 2
NO
N 2 O
Air
10 ppm (125
o
C) 30 sec [65]
Et-OH 1 ppm(350
o
C)
N/A [66]
H 2 S
0.1 ppm (RT)
0.05 ppm (130
o
C)
Nanowires FET
NO 2
NH 3
Ar
1 ppm
0.5%
N/A [41]
Nanowires
Chemoresistor
Ozone Humid air 150 ppb (350
o
C ) N/A [67]
Multi-nanowires Et-OH
Air
1 ppm (300
o
C) N/A [68]
Nanowire Paste
HCHO
LPG
90 gasoline
CO
NH 3
50 ppm
500 ppm
50 ppm (330
o
C)
500 ppm
50 ppm
N/A [64]
Pt-ZnO Multi-nanorods H 2 N 2 500 ppm N/A [61]
In 2 O 3
Single nanowire
FET
Chemoresistor
NH 3
NO 2
Air
0.02%
20 ppb
2 min
20 min
[69]
[21]
Single nanowire
Chemoresistor
CO
H 2
Et-OH
Air
10 ppm
500 ppm (275
o
C)
1 ppm
<1 min [70]
CH 4
H 2 S
HCHO
50 ppm (250
o
C) N/A [63]
NO
N 2 O
0.1 ppm (150
o
C) 20 sec [65]
Ozone Humid air 75 ppb (400
o
C) N/A [68]
Multi-nanowires
NH 3
NO 2
Air
500 ppm
5 ppb
6 min
16 min
[21]
Acetone Humid air 25 ppm (400
o
C)
6 min
16 min
[71]
WO 3 Nanowire Chemoresistor
NO 2
NO
N 2 O
Air 0.1 ppm (250
o
C) 10 sec [65]
TiO 2
Nanowire
Chemoresistor
Et-OH Air 2% (550
o
C) N/A [72]
Nanobelt O 2 N 2 200 ppb N/A [73]
Nanotube H 2 N 2 1000 ppm (290
o
C) 150 sec [74]
SnO 2
Single nanowire Chemoresistor
CO
NH 3
Air
25 ppm (280
o
C)
100 ppb (300
o
C)
N/A [75]
CO
NO 2 Humid air
0.1 ppm (400
o
C) N/A [36]
Ozone 75 ppb (400
o
C) N/A [68]
Nanoribbon
Chemoresistor
NO 2 Air 3 ppm (photoinduced) < 1min [76]
Nanobelt DMMP
Air
78 ppb (500
o
C) N/A [77]
Nanotube
NH 3
Et-OH
10 ppm (200
o
C)
10 ppm (200
o
C)
5 sec [78]
Nanorods Et-OH 10 ppm (300
o
C)) N/A [79]
Nanofiber film Et-OH 100 ppb (300
o
C) N/A [80]
SnO 2 :In Multi-nanowire Et-OH 10 ppm (400
o
C) 2 sec [81]
Au-SnO 2 Nanobelt CO 50 ppm (400
o
C) ~ 1 min [82]
V 2 O 5
Nanobelts
Chemoresistor
Et-OH Air 5 ppm (400
o
C) 30~50 sec [83]
Nanofibers
1-butylamine
a-propanol
Tolune
NH 3
N 2
30 ppb
1000 ppm
1000 ppm
10 ppm
N/A [84]
CuO Nanoribbons Chemoresistor HCHO Air 5 ppm N/A [62]
SWNT
Nanotubes FET
NH 3
NO 2
Air
0.1%
2 ppm
1200 sec
1000 sec
[18]
Multi-nanotubes Chemoresistor
Nitrotoluene
NO 2
N 2
262 ppb
44 ppb
N/A [35]
PEI-SWNT Nanotube FET
NH 3
NO 2
Air
100 ppm
100 ppt
500 sec
400 sec
[36]
Pd-SWNT Multi-nanotubes FET H 2 S Air 50 ppm 300 sec [85]
Table 1.2, chemical sensors based on 1-D nanostructures.
17
The application of 1-D nanostructures is not restricted to nano- and micro- electronics.
They have also demonstrated promising potential and unique advantage in chemical and
biological sensors. Table 1.2 summarizes some important characterizations of 1-D
nanostructures with various sensor types, different geometric structures, and measuring
environments. In Table 1.2, compared to sensors made of Pt-ZnO thin films (~20 nm), Pt-
ZnO nanorods (diameter: 50-150 nm) based sensors achieved a 3-fold increase of
sensitivity due to higher surface-to-volume ratio of nanorods, which again indicated the
superior sensitivity of 1-D nanostructures [61]. However, the detection limits of these
metal-oxide nanostructured sensors to some chemicals are still far behind the detection
limits required by the U.S. Department of Health and Human Services. For example, the
ATSDR minimal risk levels (MRLs) of HCHO (Formaldehyde) is 0.04 ppm, which is far
beyond the detection limits reported from CuO nanoribbon sensors (5 ppm, [62]), In
2
O
3
nanowire sensors (50 ppm, [63]), and ZnO nanowire paste sensors (~50 ppm, [64]).
Much work still needs to be done to improve the sensing detection limits. We note that
both 1-D nanostructure and conventional film based chemical sensors can offer good
chemical sensing performance; however, 1-D nanostructures can offer certain advantages
such as precise diameter control, easy integration into transistor configuration, and single
crystalline material quality.
Besides of sensitivity, the selectivity is still one of the most concerned topics in the
chemical sensing society. Electronic nose technique is one of the most promising
approaches to carry out the selectivity problem and has attracted a lot of efforts in
developing new e-nose systems, such as KAMINA technology and chemical sensor
18
arrays. Sensor arrays made of thin film metal-oxides have been widely discussed, but
only limited work on 1-D nanostructure based sensor arrays has been performed.
In addition to the foregoing discussions, recently, due to depleting fossil fuels and
environmental issues, the interest of developing alternative energy conversion and
storage devices, with characteristics of high energy density, high power density, and low
cost, has increased to greater extent. Because 1-D nanostructures can provide short
diffusion length to ions, thus resulting in high charging/discharging rates and high power
density, they have been viewed as building components for next-generation
electrochemical energy conversion and storage devices. More recently, there have been a
lot of exciting breakthrough in 1-D nanostructure based lithium ion batteries and
electrochemical capacitors (supercapacitors), which can provide enough high energy
density and power density to drive electrical vehicles (EVs) or hybrid EVs (HEVs). For
instance, Futaba et al. reported highly-packed and aligned SWNT thin films as
supercapacitor electrodes, which provided an enhanced energy density of 69.4 Wh/kg and
power density of 43.3 kW/kg [86]. To further improve the device performance, an
approach which combined SWNT and transition-metal-oxide nanostructures, has been
proposed. This method not only took fully advantages of the electrical double layer
capacitance from SWNTs along with the Faradaic process from transition-metal-oxide
nanostructures, but also used lithium salt electrolyte to build hybrid supercapacitors.
Ramaprabhu et al. have demonstrated that ruthenium oxide (RuO
2
)/multi-walled carbon
nanotube (MWNT) nanocrystalline composites exhibited higher specific capacitance (138
F/g) and energy density (36.8 Wh/kg) than pure MWNT electrodes of 67 F/g and 21.4
Wh/kg [87]. Besides, compared to 0-D In
2
O
3
nanostructures, 1-D In
2
O
3
nanorods
19
exhibited a higher specific capacitance than In
2
O
3
nanospheres. This can be attributed to
that 1-D nanostructures provide more redox reaction active sites, and inner/outer charges
than those of 0-D nanostructure [88]. In addition, 1-D nanostructures usually can provide
better electrode contact than 0-D nanostructures; thus reduce the contact resistance, and
enhance the power density of devices.
Regarding to the applications in lithium ion battery applications, Si nanowires have
been adapted as anode material, which showed a specific capacity of 4,200 mAh/g and
superior cycling life than its bulk counterpart. Besides, MWNTs also exhibited an
excellent specific capacity of 460 mAh/g, which is much better than the theoretical
capacity of 372 mAh/g of graphite [89]. This can be attributed to the nature of high
surface area, and new reactions merely happened in nanoscale materials. In addition, due
to the “going nano” effect, different working mechanisms in lithium ion batteries can be
practical. For instance, lithium alloy materials including Si nanowires and Sn nanowires
have performed excellent specific capacity without enormous volume swings compared
to their bulk counterparts. Also for conversion-reaction nanomaterials, such as Fe
3
O
4
,
CuO, NiO, and In
2
O
3
etc., the phenomena of large voltage separation between charging
and discharging have also overcome by using nanostructured materials.
20
Figure 1.7 Ragone plot for different electrical energy storage and conversion devices [90].
21
1.4 Outline of This Work
Academic and applied interests provided the motivation for the studies of 1-D
nanostructures; hence my research has primarily focused on the synthesis of 1-D
nanostructures and their applications in different fields. In particular, I would like to
center on three important applications based 1-D nanostructures in this dissertation,
which are chemical sensing, transparent electronics, and electrochemical capacitors. The
dissertation is accordingly organized into eight chapters: Following Chapter 1, Chapter 2
will introduce a powerful electronic nose technique by integrating micro-machined
hotplates with four different 1-D nanostructures, including SWNTs, In
2
O
3
nanowires,
SnO
2
nanowires, and ZnO nanowires; Chapter 3 discusses the fabrication of 2, 4, 6-
trinitrotoluene (TNT) chemical sensors built on aligned SWNTs and ZnO nanowires,
followed by a comprehensive study of TNT sensing mechanism by using a mass-
spectrometer; Chapter 4 focuses on the synthesis of doped transition-metal-oxide
nanowires and its applications in the transparent electronics and the controlling of active
matrix organic light-emitting diode (AMOLED) displays; Chapter 5 is reserved to give
the first flexible and transparent energy storage devices, established on hybrid-
nanostructured thin film electrodes by using In
2
O
3
nanowires and SWNTs; Chapter 6
presents a generic approach of producing supercapacitors by simply using a commercial
inkjet printer with SWNT inks, and the integration of RuO
2
nanowires and printed SWNT
films certainly improves the device performance; Chapter 7 studies the governing factors
of high-performance supercapacitors by using different 1-D nanostructures and an
asymmetric device structure, while Chapter 8, in the end, summarizes the above
discussions and proposes future research directions in 1-D nanostructures.
22
Chapter 1 References
1. Iijima S. Helical microtubules of graphitic carbon. Nature 1991; 354 (6348): 56-
58.
2. Wang Z L. Nanowires and nanobelts: materials, properties and devices. Metal
and semiconductor nanowires volume I, Springer, New York (2006).
3. Rao C N R, and Govindaraj A. Nanotubes and nanowires, RSC publishing,
Cambridge, UK (2005).
4. Martel R, Schmidt T, Shea H R, Hertel T, and Avouris P. Single- and multi-wall
carbon nanotube field-effect transistors. Applied Physics Letters 1998; 73 (17):
2447-2449. Javey A, Guo J, Wang Q, Lundstrom M, and Dai H J. Ballistic
carbon nanotube field-effect transistors. Nature 2003; 424 (6949): 654-657.
5. Duan X F, Huang Y , Agarwal R, and Lieber C M. Single-nanowire electrically
driven lasers. Nature 2003; 421 (6920): 241-245.
6. Huang M H, Mao S, Feick H, Yan H Q, Wu Y Y , Kind H, Weber E, Russo R,
Yang P D. Room-temperature ultraviolet nanowire nanolasers. Science 2001;
292 (5523): 1897-1899.
7. Kocabas C, Kim H S, Banks T, Rogers J A, Pesetski A A, Baumgardner J E,
Krishnaswamy S V , Zhang H. Radio frequency analog electronics based on
carbon nanotube transistors. Proceedings of the National Academy of Sciences
of the United States of America 2008; 105 (5): 1405-1509.
8. Wang D, Yu Z, McKernan S, and Burke P J. Ultrahigh frequency carbon
nanotube transistor based on a single nanotube. IEEE Transactions on
Nanotechnology 2007; 6 (4): 400-403.
9. Sun Y G, Rogers J A. Inorganic semiconductors for flexible electronics. 2007; 19
(15): 1897-1916.
10. Nilsson H A, Duty T, Abay S, Wilson C, Wagner J B, Thelander C, Delsing P,
Samuelson L. A radio frequency single-electron transistor based on an InAs/InP
heterostructure nanowire. Nano Letters 2008; 8 (3): 872-875.
11. Lu W, Lieber C M. Nanoelectronics from the bottom up. Nature Materials 2007;
6 (11): 841-850.
12. Li C, Fan W D, Lei B, Zhang D H, Han S, Tang T, Liu X L, Liu Z Q, Asano S,
Meyyappan M, Han J, and Zhou C. Multilevel memory based on molecular
devices. Applied Physics Letters 2004; 84 (11); 1949-1951.
23
13. Javey A, Nam S, Friedman R S, Yan H, Lieber C M. Layer-by-layer assembly of
nanowires for three-dimensional, multifunctional electronics. Nano Letters 2007;
7 (3): 773-777.
14. Ryu K, Badmaev A, Wang C, Lin A, Patil N, Gomez L, Kumar A, Mitra S, Wong
H S P, Zhou C. CMOS-Analogous wafer-scale nanotube-on-insulator approach
for submicrometer devices and integrated circuits using aligned nanotubes. Nano
Letters 2009; 9 (1): 189-197.
15. Cao Q, Kim H S, Pimparkar N, Kulkarni J P, Wang C J, Shim M, Roy K, Alam
M A, Rogers J A. Medium-scale carbon nanotube thin-film integrated circuits on
flexible plastic substrates. Nature 2008; 454 (7203): 495-500.
16. Fan Z Y , Ho J C, Jacobson Z A, Razavi H, Javey A. Large-scale, heterogeneous
integration of nanowire arrays for image sensor circuitry. Proceedings of the
National Academy of Sciences of the United States of America 2008; 105 (32):
11066-11070.
17. Jiang Y , Zhang W J, Jie J S, Meng X M, Fan X, Lee S T. Photoresponse
properties of CdSe single-nanoribbon photodetectors. Advanced Functional
Materials. 2007; 17 (11): 1795-1800.
18. Kong J. Franklin N R, Zhou C, Chapline M G, Peng S, Cho K J, Dai H J.
Nanotube molecular wires as chemical sensors. Science 2000; 287 (5453): 622-
625.
19. Li C, Zhang D H, Liu X L, Han S, Tang T, Han J, Zhou C. In
2
O
3
nanowires as
chemical sensors. Applied Physics Letters 2003; 82 (10): 1613-1615.
20. Lieber C M. Nanoscale science and technology: Building a big future from small
things. MRS Bulletin 2003; 28 (7): 486-491.
21. Zhang D H, Liu Z Q, Li C, Tang T, Liu X L, Han S, Lei B, Zhou C. Detection of
NO
2
down to ppb levels using individual and multiple In
2
O
3
nanowire devices.
Nano Letters 2004; 4 (10): 1919-1924.
22. Cui Y , Wei Q Q, Park H K, Lieber C M. Nanowire nanosensors for highly
sensitive and selective detection of biological and chemical species. Science
2001; 293 (5533): 1289-1292.
23. Patolsky F, Zheng G, Lieber C M. Nanowire sensors for medicine and the life
sciences. Nanomedicine 2006; 1 (1): 51-65.
24. Raty J Y , Gygi F, Galli G. Growth of carbon nanotubes on metal nanoparticles: A
microscopic mechanism from ab initio molecular dynamics simulations.
Physical Review Letters 2005; 95 (9): 96103-96106.
24
25. Tans S J, Devoret M H, Dai H J, Thess A, Smalley R E, Geerligs L J, Dekker C.
Individual single-wall carbon nanotubes as quantum wires. Nature 1997; 386
(6624): 474-477.
26. Bandaru P R. Electrical properties and applications of carbon nanotube
structures. Journal of Nanoscience and Nanotechnology 2007; 7 (4/5):1239-1267.
27. DUrkop T, Getty S A, Cobas E, Fuhrer, M S. Extraordinary mobility in
semiconducting carbon nanotubes. Nano Letters 2004; 4 (1): 35-39.
28. Lu W, Xie P, and Lieber C M. Nanowire transistor performance limits and
applications. IEEE Transactions on Electron Devices 2008; 55 (11): 2859-2876.
29. Young K K. Short-channel effect in fully depleted SOI MOSFETs. IEEE
Transactions on Electron Devices 1989; 36 (2): 399-402.
30. Li C, Zhang D H, Han S, Liu X L, Tang T, Zhou C. Diameter-controlled growth
of single-crystalline In
2
O
3
nanowires and their electronic properties. Advanced
Materials 2003; 15 (2): 143-146.
31. Ju S, Facchetti A, Xuan Y , Liu J, Ishikawa F, Ye P, Zhou C, Marks T J, Janes D B.
Fabrication of fully transparent nanowire transistors for transparent and flexible
electronics. Nature Nanotechnology 2007; 2 (6): 378-384.
32. Collins P G, Bradley K, Ishigami M, Zettl A. Extreme oxygen sensitivity of
electronic properties of carbon nanotubes. Science 2000; 287 (5459): 1801-1804.
33. Peng Q, Vermesh O, Grecu M, Javey A, Wang O, Dai H J, Peng S, Cho K.
Toward large arrays of multiplex functionalized carbon nanotube sensors for
highly sensitive and selective molecular detection. Nano Letters 2003; 3(3): 347-
351.
34. McAlpine M C, Ahmad H, Wang D, Heath J R. Highly ordered nanowire arrays
on plastic substrates for ultrasensitive flexible chemical sensors. Nature
Materials 2007; 6 (5): 379-384.
35. Zhang Y, Suc C, Liu Z, Li J. Carbon nanotubes functionalized by NO
2
:
Coexistence of charge transfer and radical transfer. Journal of Physical
Chemistry B 2006; 110 (45): 22462-22470.
36. E. Comini, G. Faglia, G. Sberveglieri, Z. W. Pan, and Z. L. Wang, Stable and
highly sensitive gas sensors based on semiconducting oxide nanobelts. Applied
Physics Letters 2002; 81 (10): 1869-1871.
25
37. V. V. Sysoev, B. K. Button, K. Wepsoec, S. Dmitriev, and A. Kolmakov, Toward
the nanoscopic "electronic nose": hydrogen vs carbon monoxide discrimination
with an array of individual metal oxide nano- and mesowire sensors. Nano
Letters 2006; 6 (8): 1584-1588.
38. N. Barsan, and U. Weimar, Conduction model of metal oxide gas sensors.
Journal of Electroceramics 2001; 7 (7):143-167.
39. S. Semancik, and D. F. Cox, Fundamental characterization of clean and gas-
dosed tin oxide. Sensors and Actuators1987; 12 (2):101-106.
40. M. Batzill, and U. Diebold, The surface and materials science of tin oxide.
Journal of Surface Science 2005; 79 (2-4):47-154.
41. Z. Fan, and J. G. Lu, Gate-refreshable nanowire chemical sensors. Applied
Physics Letters 2006; 86 (12): 123510-123512.
42. N. Du, H. Zhang, B. Chen, X. Ma, Z. Liu, J. Wu, and D. Yang, Porous indium
oxide nanotubes: layer-by-layer assembly on carbon-nanotube templates and
application for room-temperature NH
3
gas sensors. Advanced Materials 2007. 19
(12): 1641-1645.
43. Guo Y, Hu J, Wan L. Nanostructured materials for electrochemical energy
conversion and storage devices. Advanced Materials 2008; 20 (23): 2878-2887.
44. Okubo M, Hosono E, Kim J, Enomoto M, Kojima N, Kudo T, Zhou H, Honma I.
Nanosize effect on high-rate Li-ion intercalation in LiCoO
2
electrode. Journal of
the American Chemical Society 2007; 129 (23): 7444-7452.
45. Chan C K, Peng H, Liu G, McIlwrath K, Zhang X F, Huggins R A, Cui Y . High-
performance lithium battery anodes using silicon nanowires. Nature
Nanotechnology 2008; 3(1): 31-35.
46. B. Xiang, P. Wang, X. Zhang, S. A. Dayeh, D. P. R. Aplin, C. Soci, D. Yu, and D.
Wang, Rational synthesis of p-type zinc oxide nanowire arrays using simple
chemical vapor deposition. Nano Letters 2007; 7 (2): 323-326.
47. Martel R, Schmidt T, Shea H R, Hertel T, Avouris P. Single- and multi-wall
carbon nanotube field-effect transistors. Applied Physics Letters 1998; 73 (17):
2447-2449.
48. Zhang Z, Wang S, Ding, L, Liang X, Pei T, Shen J, Xu H, Chen Q, Cui R, Li Y,
Peng L- M. Self-aligned ballistic n-type single-walled carbon nanotube field-
effect transistors with adjustable threshold voltage. Nano Letters 2008; 8 (11):
3696-3710.
26
49. S. Ju, K. Lee, M. Yoon, A. Facchetti, T. J. Marks, and D. B. Janes, High
performance ZnO nanowire field effect transistors with organic gate
nanodielectrics: effects of metal contacts and ozone treatment. Nanotechnology
2007; 18 (15): 155201-155205.
50. G.-D. Yuan, W.-J. Zhang, J.-S. Jie, X. Fan, J.-X. Tang, I. Shafiq, Z.-Z. Ye, C.-S.
Lee, and S.-T. Lee, Tunable n-type conductivity and transport properties of Ga-
doped ZnO nanowire array. Advanced Materials 2008; 20 (1): 168-173.
51. S. Ju, F. N. Ishikawa, P. Chen, H. Chang, and C. Zhou, High performance In
2
O
3
nanowire transistors using organic gate nanodielectrics. Applied Physics Letters
2008; 92 (24): 222105-222107.
52. C. Li, B. Lei, W. Fan, D. Zhang, M.Meyyappan, and C. Zhou, Molecular
memory based on nanowire - molecular wire heterostructures. Journal of
Nanoscience and Nanotechnology 2007; 7 (1): 138-150.
53. S. Ju, P. Chen, C. Zhou, Y. Ha, A. Facchetti, T. M. Marks, S. K. Kim, S.
Mohammadi, and D. B. Janes, 1/f noise of SnO
2
nanowire transistors. Applied
Physics Letters 2008; 92 ( ): 243120-243122.
54. Z. Liu, D. Zhang, S. Han, C. Li, T. Tang, W. Jin, X. Liu, B. Lei, and C. Zhou,
Laser ablation synthesis and electron transport studies of tin oxide nanowires.
Advanced Materials 2003; 20 ( ): 1754-1757.
55. E. N. Dattoli, Q. Wan, W. Guo, Y. Chen, X. Pan, and W. Lu, Fully transparent
thin-film transistor devices based on SnO
2
nanowires " Nano Letters 2007; 7 (8):
2463-2469.
56. J. M. Baik, M. H. Kim, C. Larson, X. Chen, S. Guo, A. M. Wodtke, and M.
Moskovits, High-yield TiO
2
nanowire synthesis and single nanowire field-effect
transistor fabrication. Applied Physics Letters 2008; 92 (24): 242111-242113.
57. G. T. Kim, J. Muster, V . Krstic, J. G. Park, Y . W. Park, S. Roth, and M. Burghard,
Field-effect transistor made of individual V
2
O
5
nanofibers. Applied Physics
Letters 2000; 76 (11): 1875-1877.
58. P. Chang, Z. Fan, W. Tseng, A. Rajagopal, and J. G. Lu, Beta-Ga
2
O
3
nanowires:
synthesis, characterization, and p-channel field-effect transistor. Applied Physics
Letters 2005; 87 (22): 222102-222104.
59. Z. Fan, X. Wen, S. Yang, and J. G. Lu, Controlled p- and n-type doping of Fe
2
O
3
nanobelt field effect transistors. Applied Physics Letters 2005; 87 (1):13113-
13115.
27
60. Y . Lee, Y . Chueh, C. Hsieh, M. Chang, L. Chou, Z. L. Wang, Y . Lan, C. Chen, H.
Kurata, S. Isoda, p-Type alpha-Fe
2
O
3
Nanowires and their n-type transition in a
reductive ambient. Small 2007; 3 (8):1356-1361.
61. L. C. Tien, P. W. Sadik, D. P. Norton, L. F. Voss, S. J. Pearton, H. T. Wang, B. S.
Kang, F. Ren, J. Jun, and J. Lin, Hydrogen sensing at room temperature with Pt-
coated ZnO thin films and nanorods. Applied Physics Letters 2005; 87 (22):
222106-222108.
62. L. C. Short, and T. Benter, Selective measurement of HCHO in urine using direct
liquid-phase fluorimetric analysis. Clinical Chemistry and Laboratory Medicine
2005; 43 (2):178-182.
63. P. Xu, Z. Cheng, Q. Pan, J. Xu, Q. Xiang, W. Yu, and Y. Chu, High aspect ratio
In
2
O
3
nanowires: synthesis, mechanism and NO
2
gas-sensing properties. Sensors
and Actuators B 2008; 130 (2): 802-808.
64. Y. C. Xu, Y. Li, and J. Shen, Gas sensing properties of ZnO nanorods prepared
by hydrothermal method. Journal of Material Science 2005; 40 (11): 2919-2921.
65. Y. S. Kim, S. C. Ha, and K. Kim, Room-temperature semiconductor gas sensor
based on nonstoichiometric tungsten oxide nanorod film. Applied Physics
Letters 2005. 86 (21): 213105-213107.
66. C. Wang, X. Chu, and M. Wu, Detection of H
2
S down to ppb levels at room
temperature using sensors based on ZnO nanorods. Sensors and Actuators B
2006; 113 (1): 320-323.
67. G. Sberveglieri, C. Baratto, E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, and A.
V omiero, Synthesis and characterization of semiconducting nanowires for gas
sensing. Sensors and Actuators B 2007; 121 (1): 208-213.
68. Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, and C. L. Lin,
Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors.
Applied Physics Letters 2004; 84 (18): 3654-3656.
69. C. Li, D. Zhang, X. Liu, S. Han, T. Tang, J. Han, and C. Zhou, In
2
O
3
nanowires
as chemical sensors. Applied Physics Letters 2003; 82 (10):1613-1616.
70. K. Ryu, D. Zhang, and C. Zhou, High-performance metal oxide nanowire
chemical sensors with integrated micromachined hotplates. Applied Physics
Letters 2008; 92 (9): 093111-093113.
71. A. Vomiero, S. Bianchi, E. Comini, G. Faglia, M. Ferroni, and G.Sberveglieri,
Controlled growth and sensing properties of In
2
O
3
Nanowires. Crystal Growth &
Design 2007; 7 (12):2500-2504.
28
72. L. Francioso, A.M. Taurino, A. Forleo, and P. Siciliano, TiO
2
nanowires array
fabrication and gas sensing properties. Sensors and Actuators B 2008; 130 (1):
70-76.
73. Y . Wang, G. Du, H. Liu, D. Liu, S. Qin, N. Wang, C. Hu, X. Tao, J. Jiao, J. Wang,
and Z. L. Wang, Nanostructured sheets of Ti-O nanobelts for gas sensing and
antibacterial applications. Advanced Functional Materials 2008; 18 (7): 1131-
1137.
74. O. K. Varghese, D. Gong, M. Paulose, K. G. Ong, and C. A. Grimes, Hydrogen
sensing using titania nanotubes. Sensors and Actuators B 2003; 93 (1-3): 338-
344.
75. D. C. Meier, S. Semancik, B. Button, E. Strelcov, and A. Kolmakov, Coupling
nanowire chemiresistors with MEMS microhotplate gas sensing platforms.
Applied Physics Letters 2007; 91 (6): 63118-63120.
76. M. Law, H. Kind, B. Messer, F. Kim, and P. Yang, Photochemical sensing of
NO
2
with SnO
2
nanoribbon nanosensors at room temperature. Angewandte
Chemie International Edition 2002; 41 (13): 2405-2408.
77. C. Yu, Q. Hao, S. Saha, L. Shi, X. Kong, and Z. L. Wang, Integration of metal
oxide nanobelts with microsystems for nerve agent detection. Applied Physics
Letters 2005; 86 (6): 063101-063103.
78. G. X. Wang, J. S. Park, M. S. Park, and X. L. Gou, Synthesis and high gas
sensitivity of tin oxide nanotubes. Sensors and Actuators B 2008; 131 (1): 313-
317.
79. Y. J. Chen, L. Nie, X. Y. Xue, Y. G. Wang, and T. H. Wang, Linear ethanol
sensing of SnO
2
nanorods with extremely high sensitivity. Applied Physics
Letters 2006; 88 (8): 83105-83107.
80. Z. Ying, Q. Wan, Z. T. Song and S. L. Feng, SnO
2
nanowhiskers and their
ethanol sensing characteristics. Nanotechnology 2004; 15 (11):1682-1684.
81. X. Y. Xue, Y. J. Chen, Y. G. Liu, S. L. Shi, Y. G. Wang, and T. H. Wang,
Synthesis and ethanol sensing properties of indium-doped thin oxide nanowires.
Applied Physics Letters 2006; 88 (20): 201907-201909.
82. L. H. Qian, K. Wang, Y. Li, H. T. Fang, Q. H. Lu, and X. L. Ma, CO sensor
based on Au-decorated SnO
2
nanobelt. Materials Chemistry and Physics 2006;
100 (1): 82-84.
83. J. Liu, X. Wang, Q. Peng, and Y. Li, Vanadium Pentoxide nanobelts: highly
selective and stable ethanol sensor materials. Advanced Materials 2005; 17 (8-9):
764-767.
29
84. I. Raible, M. Burghard, U. Schlecht, A. Yasuda, and T. Vossmeyer, V
2
O
5
nanofibers: novel gas sensors with extremely high sensitivity and selectivity to
amines. Sensors and Actuators B 2005; 106 (2): 730-735.
85. A. Star, V. Joshi, S. Skarupo, D. Thomas, and J. -C. P. Gabriei, Gas sensor array
based on metal-decorated carbon nanotubes. Journal of Physical Chemistry B,
110 (42): 21014-21020, 2006.
86. Futaba D N, Hata K, Yamada T, Hiraoka T, Hayamizu Y , Kakudate Y , Tanaike O,
Hatori H, Yumura M, Iijima S. Shape-engineerable and highly densely packed
single-walled carbon nanotubes and their application as super-capacitor
electrodes. Nature Materials 2006; 5 (12): 987-994.
87. Reddy A L M, Ramaprabhu S. Nanocrystalline metal oxides dispersed
multiwalled carbon nanotubes as supercapacitor electrodes. Journal of Physical
Chemistry C 2007; 111 (21): 7727-7734.
88. Chang J, Lee W, Mane R S, Cho B W, Han S. Morphology-dependent
electrochemical supercapacitor properties of indium oxide. Electrochemical and
solid state letters 2008; 11 (1): A9-A11.
89. Claye A S, Fisher J E, Huffman C B, Rinzler A G, Smalley R E. Solid-state
electrochemistry of the Li single wall carbon nanotube system. Journal of the
Electrochemical Society 2000; 147 (8): 2845-2852.
90. Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials
2008; 7 (11): 845-854.
30
Chapter 2 Nano Electronic Nose: A Hybrid Nanowire / Carbon Nanotube Sensor
Array with Integrated Micromachined Hotplates for Sensitive Gas Discrimination
A novel hybrid chemical sensor array composed of individual In
2
O
3
nanowires,
SnO
2
nanowires, ZnO nanowires, and single-walled carbon nanotubes (SWNTs) with
integrated micromachined hotplates for sensitive gas discrimination was demonstrated.
Key features of our approach include the integration of nanowire and carbon nanotube
sensors, precise control of the sensor temperature using the micromachined hotplates, and
the use of Principal Component Analysis (PCA) for pattern recognition. This sensor
array was exposed to important industrial gases such as hydrogen, ethanol and nitrogen
dioxide at different concentrations and sensing temperatures, and an excellent selectivity
was obtained to build up an interesting “smell-print” library of these gases. Principal
component analysis of the sensing results showed great discrimination of those three
tested chemicals, and in-depth analysis revealed clear improvement of selectivity by the
integration of carbon nanotube sensors. This nano electronic nose approach has great
potential to detect and discriminate a wide variety of gases, including explosives and
nerve agents.
2.1 Introduction
Chemical sensing based on a wide variety of nanostructures has attracted great
interest in the past few years because of their remarkable properties such as high
sensitivity, low cost, and easy fabrication when used as chemical sensors [1-17]. For
31
instance, semiconducting single-walled carbon nanotube (SWNT) field effect transistors
(FETs) can be used to detect critical industrial gases, NO
2
and NH
3
[3]. Using SnO
2
nanobelt FETs, detection of dimethyl methylphosphonate (DMMP), which is a nerve
agent simulant, can be down to 78 ppb [4]. In our previous work, we also presented
highly sensitive In
2
O
3
nanowire sensors for which the detection limit of NO
2
was in the
ppb level [5]. To improve the device sensitivity, chemical coating treatments [7-9] or
nanoparticle decoration [6, 10-13] are widely used. For example, the sensitivity was
improved by using Pd-coated SnO
2
nanowire chemoresistors [14], and surface-
functionalized flexible silicon nanowire FETs [15] have been used to detect organic
vapors at room temperature.
Recently, several research groups have worked on establishing selective chemical
sensing for thin film/nanostructure sensors. For instance, the principal component
analysis (PCA) was recently applied to nanotube-based chemical sensors and then
extended also to nanowire sensors [9]. The KAMINA technology [18], which is based on
using a temperature gradient in a sensor array, has also been used for both thin films and
nanostructures [19, 20]. This method, however, requires rather high power consumption
(~ 1 W) [21] and lacks precise control over the temperature for individual sensors.
In this letter, we report a brand-new platform, which can serve as an “electronic
nose” with good “discrimination factors”, built with four different semiconducting
nanostructured materials including In
2
O
3
nanowires, SnO
2
nanowires, ZnO nanowires,
and SWNTs. This integration of both semiconducting metal oxide nanowire and
semiconducting SWNT sensors provides an important discrimination factor for improved
selectivity due to the significant sensing mechanisms between the carbon nanotubes and
32
the metal oxide nanowires. Furthermore, the integrated micromachined hot plate enables
us to control temperature individually and accurately for each sensor, with additional
advantages such as ultra low power consumption (~ 60 mW) and short response time (20
o
C to 300
o
C within 1 minute) [22]. This temperature control works as the second
discrimination factor. With these two discrimination factors, we achieved a “smell-print”
library by detecting important industrial gases such as hydrogen, ethanol and nitrogen
dioxide. Furthermore, the analyzed sensing results, with aid from the PCA method, [23-
25] demonstrated a clear success in the discrimination in these gases. Our approach
shows great potential to become a realistic “electronic nose” with the advantage of
compact size, ultra low power consumption, and easy fabrication.
2.2 Experiments
2.2.1 The Concept of Electronic Nose
As illustrated in the section IV, metal oxide nanostructured chemical sensors have
shown good sensitivity toward a broad range of chemicals. However, a critical issue in
the development of chemical sensor systems for practical applications is the “selectivity”.
One promising way to achieve selectivity among different chemicals is to build
“electronic noses”. The idea of electronic noses was inspired by the mechanisms of
human olfaction. In general, basic elements of an electronic nose system include an
“odour” sensor array, a data pre-processor, and a pattern recognition (PARC) engine. [26]
Among them, a sensor array which mimics the olfactory receptor cells situated in the roof
of the nasal cavity of human beings is like signal receptors.
33
The concept of “chemical sensor array” for odour reorganization was demonstrated
by Persaud and Dodd in 1982. [27] After that, considerable effort has been directed to
develop different types of sensor arrays such as chemoresistors, acoustic/SAW sensors,
optical sensors, and potentiometric transducers. On the other hand, sensor arrays made by
different active materials were also studied. For example, Li et al. [28] and Heath et al.
[29] have reported using functionalized carbon nanotubes [28] and Si nanowires [29] as
active materials for chemical sensor arrays of electronic nose systems. Sensor arrays
made of thin film metal oxides have been widely discussed [30-33]. However, only
limited work on semiconducting oxide based sensor arrays has been performed.
2.2.2 Device Fabrications
The hybrid nanostructure sensor array with integrated hot plates began with the
fabrication of SiN membranes. The detail of fabrication of SiN membranes can be found
in literatures [22]. In brief, we deposited 100 nm SiN on both sides of Si wafers using a
low temperature CVD. After that, a photolithography process was carried out to pattern
SiN membranes, followed by CF
4
plasma to etch unwanted SiN membranes and 10 wt%
KOH to remove Si substrate underneath SiN membranes. We can obtained uniform SiN
membranes with a thickness of 100 nm. To achieve diversity in the gas response,
chemical sensors were fabricated with four different single-crystalline nanomaterials
including In
2
O
3
nanowires, SnO
2
nanowires, ZnO nanowires, and SWNTs, which were
obtained using different growth methods described below. The In
2
O
3
and SnO
2
nanowires
were synthesized using a laser ablation method as described in our previous reports [34,
35]. These nanowires, with diameters of about 20 nm and lengths of 5 μm, exhibited
defect-free crystalline structure and good electronic characteristics. Vertical ZnO
34
nanowires grown on a-sapphire with diameters of 50 nm and lengths of 10 μm were
obtained via a vapor-solid (VS) growth method following previous reports [36, 37].
SWNTs were synthesized directly on SiN membranes via chemical vapor deposition
(CVD) at 900
o
C with ferritin serving as catalysts, and a methane/ethylene/hydrogen gas
mixture as the carbon feedstock [38]. The metal oxide nanowires were sonicated into
isopropanol alcohol (IPA) to form different suspensions and then dispersed on the SiN
membrane side of the substrates until a reasonable nanowire density was achieved.
Photolithography was applied to pattern both source and drain electrodes for chemical
sensors and the heating electrodes of hot plates, followed by 100 nm platinum (Pt)
deposition using e-beam evaporation. After device fabrication, high voltage bias was
applied to burn the unwanted SWNTs between heating electrodes and sensor electrodes.
2.2.3 Electrical Measurements and Chemical Sensing Experiments
The electron transport measurements of nanowire and SWNT chemical sensors and
the micromachined hot plates were carried out using an Agilent 4156B semiconductor
parameter analyzer. The chemical sensing experiments were performed using a home-
made chemical sensing setup described in our previous work [5, 39-42]. Briefly, the
fabricated sensor chips were mounted on a chip carrier inside a sealed chamber with an
electrical feedthrough and gas inlet/outlet. The assembled devices from three different
metal oxide nanowires and SWNTs were exposed to nitrogen dioxide, ethanol and
hydrogen under different concentrations and at varied temperatures (Table 2.1)
accomplished by the micro-machined hot plates.
35
Test gas Concentration (ppm)
500
Hydrogen 1000
2000
50
Ethanol 100
200
NO
2
0.1
1
a
The sensing experiments were carried out at room temperature and 200
o
C.
Table 2.1: Sensor Array Test Conditions
a
.
2.3 Results and Discussions
Figure 2.1 (a) shows the photograph of a finished chemical sensor chip, where two
arrays of metal electrodes for contacting dispersed nanowires/nanotubes and a wider
zigzag Pt heating electrode can be clearly seen residing on a suspended SiN membrane
(the bright green area). A chemical sensor array, composed of four different sensor chips,
each with individual In
2
O
3
nanowire, SnO
2
nanowire, ZnO nanowire, and a SWNT mat,
was examined using a field emission scanning electron microscopy (FESEM) and optical
microscopy, and the corresponding images are depicted in Figure 2.1 (b). The distance
between figure-shaped source and drain electrodes is 3 μm and each integrated chip is 1
cm × 1 cm in size.
36
Figure 2.1 (a) Photograph of a chemical sensor chip with an integrated micromachined
hot plate. (b) Hybrid chemical sensor array chip composed of four individual chemical
sensors, including individual In
2
O
3
nanowire, SnO
2
nanowire, ZnO nanowire, and SWNT
chemical sensor chips. The source-drain electrode distance is about 3 μm.
2.3.1 Electronic Transport Characterizations
The conductance of the nanowire and SWNT sensors at different temperatures (25
o
C
and 200
o
C) were examined and the corresponding current-voltage (I-V) curves are
shown in Figure 2.2. The curves indicate that all these nanomaterials exhibited low-
SnO
2
In
2
O
3
Heater
Sensors
ZnO
(a)
(b)
SWNTs
37
resistance contacts with the Pt source and drain electrodes. Separate nanowire/nanotube
devices were made with gate electrodes to examine the n-type semiconducting
characteristics of nanowire sensors and p-type semiconducting characteristics of SWNT
sensors. Evident from Figure 2.2 (b), the nanosensors became more conductive at high
temperature than at room temperature. For example, the electrical conductance of the
In
2
O
3
nanowire, SnO
2
nanowire, ZnO nanowire, and a SWNT mat sensor increased from
20 μS, 15 μS, 2 μS, and 5 μS at 25
o
C to 23.5 μS, 18.5 μS, 5.5 μS, and 18 μS at 200
o
C,
respectively. The increase in temperature caused the thermal excitation of carriers and/or
the removal of moisture from the nanostructure surface, thus leading to increased carrier
concentrations and higher conductance for the semiconducting metal oxide nanowires
and SWNTs. In addition, the enhanced reactivity toward surrounding gas molecules at
elevated temperatures, and operating the sensors at temperature above 100
o
C can
eliminate the complicated effect of moisture. In addition, separate nanowire/nanotube
devices were made with gate electrodes to confirm the n-type semiconducting
characteristics of nanowire sensors and p-type semiconducting characteristics of SWNT
sensors.
Regarding the performance of the micromachined hot plates, features like easy
temperature tuning and ultra low power consumption were examined. The temperature of
the nanosensor hot plate could be easily tuned to any temperature between 25
o
C to 350
o
C by applying a voltage bias in the range of 0 V to 10 V. For example, a voltage of 8 V
was used to raise the hotplate temperature to 300
o
C within 1 minute, and then maintained
at this temperature with a power consumption of 60 mW [22]. The detail can be found in
38
Ref. 22. This power consumption compares favorably with the ~ 1 W consumption of the
KAMINA technique [21].
Figure 2.2. Current-voltage curves of as-fabricated chemical sensors measured at room
temperature (a) and 200
o
C (b).
2.3.2 Chemical Sensing Behavior
The molecular sensing mechanism for semiconducting nanomaterials mainly relies
on the change of electrical conductivity introduced by the surface redox process between
nanomaterials and surrounding environment, which can be expressed as [43],
1/2 1/2 1/2 10 0
01
00
4
( ) ( )( )
SS
GG
RS relative sensor response V V
G D en
εε −
= ≈ ⋅−
(1)
where G
1
and G
0
denote the nanowire/nanotube conductance under and before the
exposure, respectively. n
0
represent the carrier concentration before exposure to
chemicals. D is the diameter of the nanomaterials. ε
0
is the absolute dielectric constant, ε
is the relative dielectric permittivity of the nanostructured materials. V
S0
is the adsorbate-
induced band bending due to molecules in dry air environment and V
S1
is the adsorbate-
induced band bending from the exposure of chemicals. The detail can be found in the Ref.
44. To fully understand the different sensing behaviors in different nanomaterials, several
-10
-5
0
5
10
I
DS
(µΑ)
-0.4 -0.2 0.0 0.2 0.4
V
DS
(V)
25
o
C
SnO 2
In 2O 3
SWNT
ZnO
200
o
C
SnO 2
ZnO
SWNT
-12
-6
0
6
12
I
DS
(µΑ)
-0.4 -0.2 0.0 0.2 0.4
V
DS
(V)
In 2O 3
39
theoretical works have been reported to estimate the adsorbate-induced band bending
energy and charge transfer to different nanomaterials. For instance, the band bending
energy for NH
3
to SnO
2
and (10, 0) carbon nanotube is ~ 0.25 eV [45] and ~ 0.15 eV [46],
respectively, which reveals the different chemical reactions between SnO
2
and carbon
nanotubes. Thus, an integration of different nanostructured materials could provide a
discrimination factor in our e-nose system.
As is well known, hydrogen and ethanol are not easy to be detected at room
temperature using solid-state sensors. Armed with the integrated hot plates, we
successfully detected hydrogen and ethanol in ambient environment with only localized
heating to the nanowire/nanotube sensor. Figure 2.3 (a) plots the isothermal response of
the current flowing in metal oxide nanowire and SWNT sensors as a function of time
under 500 ppm hydrogen diluted in air at 200
o
C. As one can see, the electrical current
increased by 12.4%, 17.7%, and 10.7% for the SnO
2
nanowire, ZnO nanowire, and In
2
O
3
nanowire chemical sensors, respectively. This can be understood, as hydrogen exhibits
reducing gas behavior for n-type semiconducting metal oxide nanowires by desorbing
absorbed oxygen ions and producing oxygen vacancies [47]. As a result, the nanowire
conductance increased with the metal oxide nanowire sensors operated at 200
o
C. In
contrast, a decrease of 18% in conductance was observed in SWNT chemical sensor
operated at 200
o
C with the introduction of 500 ppm hydrogen, as shown in Figure 2.3 (a).
Furthermore, with the SWNT sensor operated at room temperature, we did not observe
any significant conductance change even with the introduction of 2000 ppm hydrogen.
Some early reported literatures suggested that the sensing response can happen in the
interface of nanomaterials and contact electrodes due to the chemisorption instead of
40
physisorption of hydrogen onto the Pt electrodes [48, 49]. However, the interaction
between the interface of nanomaterials and contact electrodes are complex and might not
clear till now. Thus further studies are required to investigate the sensing mechanism.
Similar to hydrogen, ethanol also works as a reducing gas to n-type
semiconducting metal-oxide nanowires, which can be easily observed from Figure 2.3 (b).
The electrical conductance was initially stable; however, upon exposure to 50 ppm
ethanol diluted in dry air, the conductance was observed to increase by 14.8%, 1.277%,
and 16.1% for SnO
2,
ZnO
,
and In
2
O
3
nanowire chemical sensors, respectively with the
nanosensors kept at 200
o
C using the micromachined hot plates. In the meantime, no
response was observed with the SWNT chemical sensor even with the introduction of
200 ppm ethanol at 200
o
C. As we know, oxygen molecules are usually adsorbed on the
surface of various metal oxide nanowires and form negative oxygen ions. These ions lead
to a reduction in electron concentration in n-type semiconductors. Once exposed to
ethanol vapor at elevated temperatures, these oxygen ions can react with ethanol
absorbed on the surface and release electrons back to nanowires. The mechanism is as
below [50].
) ( ) ( ) ( ) (
4 2 5 2
ads O H ads M H O H C s M s O OH H C − + − + → + + (2)
where (s) and (ads) indicate a surface site or an adsorbed species, respectively.
41
0.012
0.008
0.004
0.000
0.18
0.12
0.06
0.00
0.09
0.06
0.03
0.00
0.18
0.12
0.06
0.00
-0.18
-0.12
-0.06
0.00
400 300 200 100 0
Time (sec)
SWNTs
In
2
O
3
NWs
ZnO NWs
SnO
2
NWs
ΔG/G
0
0.15
0.10
0.05
0.00
SWNTs
In
2
O
3
NWs
ZnO NWs
SnO
2
NWs
ΔG/G
0
0.15
0.10
0.05
0.00
0.03
0.02
0.00
-0.02
200 150 100 50 0
Time (sec)
0.012
0.008
0.004
0.000
0.18
0.12
0.06
0.00
0.09
0.06
0.03
0.00
0.18
0.12
0.06
0.00
-0.18
-0.12
-0.06
0.00
400 300 200 100 0
Time (sec)
SWNTs
In
2
O
3
NWs
ZnO NWs
SnO
2
NWs
ΔG/G
0
0.15
0.10
0.05
0.00
SWNTs
In
2
O
3
NWs
ZnO NWs
SnO
2
NWs
ΔG/G
0
0.15
0.10
0.05
0.00
0.03
0.02
0.00
-0.02
200 150 100 50 0
Time (sec)
Figure 2.3 Sensing response to 500 ppm hydrogen (a) and 50 ppm ethanol (b) diluted in
air for SnO
2
nanowire, ZnO nanowire, In
2
O
3
nanowire, and SWNT sensors operated at
200
o
C. The Y-axis represents ΔG/G
0
. Arrows indicate time when the tested chemical
was flown into the sensing chamber.
Previously, several researchers reported the detection of ethanol vapor using multi-
walled carbon nanotube (MWNT) thin film sensors at high temperatures and SWNT
devices with high gate bias at room temperature in dry nitrogen environment as well [51,
52]. The possible sensing mechanisms in carbon nanotube devices can be attributed to (i)
the change of bulk doping on nanotube body, (ii) the Schottky barrier induced by gas
adsorption at nanotube/metal contact interface, or (iii) the electrochemical reaction of
b a Ethanol Hydrogen
42
chemical and substrate surface [51, 52]. However, in our case, it is interesting that we did
not observe significant change in SWNT nanosensor under exposure of ethanol. In
comparison of Ref. 52, the observation can be attributed to the different device design,
which means that no gate voltage was applied to our SWNT sensors, and the SiN surface
we used can be very different from the Si/SiO
2
surface used in Ref. 52. The SWNT
sensors, therefore, provide us great discrimination between two different reducing gases,
hydrogen and ethanol, with reduced conductance observed for hydrogen exposure, and an
absence of conductance change for ethanol exposure.
2.3.3 Smell Print Library
Further measurements using our hybrid nanostructure sensor array were
accomplished with different concentrations of hydrogen, ethanol and NO
2
at different
temperatures (table 1). Through these sensing measurements, we can go toward a sensor
response library of “smell-print” with the use of this nano-sensor array, thus creating an
“electronic nose”. The histogram in Figure 2.4 summarizes the sensing response of these
tested gases. The solid bars and shaded bars represent the sensing response at 25
o
C and
200
o
C, respectively, and the height of each bar indicates the relative sensor response (RS)
in conductance which is described as equation (1). As shown in Figure 2.4, SWNT
chemical sensors showed significantly different response to each chemical, as compared
to In
2
O
3,
SnO
2,
and ZnO nanowire sensors. For example, the RS of the SWNT sensor was
about 0.23 at 200
o
C and 0.02 at room temperature upon exposure to 0.1 ppm NO
2.
A
negative response (RS ~ -0.2) at 200
o
C and no response (RS ~ 0) at room temperature
were observed in the case of H
2
sensing at 500 ppm. Furthermore, there was no observed
response (RS ~ 0) to ethanol at room temperature and even at 200
o
C for various
43
concentrations. As we discussed in the early section, the sensing mechanisms between
metal oxide nanowire and SWNT sensors are quite different [43-46]. Thus, metal oxide
nanowire sensors usually exhibit opposite sensing response in comparisons of SWNT
sensors upon exposure to the same chemical as enriched carrier concentration. Therefore,
the SWNT chemical sensor worked as a good “discrimination factor” for our electronic
nose study.
Figure 2.4 Representative sensor array response as bar graphs to NO
2
(a), hydrogen (b),
and ethanol (c). The solid bars and shaded bars represent the sensing response at 25
o
C
and 200
o
C, respectively.
SWNTs
500
2000
1000 500
1000
2000
500
1000
500
1000
2000
In
2
O
3
SnO
2
ZnO
Hydrogen
SWNTs In
2
O
SnO
2
ZnO
Ethanol
200
50
10
200
50
100
200
50
100
200 50 100
50
100
(a)
(b)
2000
44
Figure 2.4: Continued
Figure 2.4 Representative sensor array response as bar graphs to NO
2
(a), hydrogen (b),
and ethanol (c). The solid bars and shaded bars represent the sensing response at 25
o
C
and 200
o
C, respectively.
2.3.4 Electronic Nose
For data analysis, there are several sutable tools, such as Independent Component
Analysis (ICA), Linear Discrimination Analysis (LDA), and Principal Component
Analysis (PCA) [53]. Among them, both LDA and PCA provide optimal discrimination
power [54], but have not been fully explored for nanostructure-based chemical sensor
arrays. Generally speaking, LDA is a supervised method (i.e. need calibration of both
sensor signals and respective concentrations), which usually requires a relatively large
number of samples from each analyte class. PCA, on the other hand, is a powerful
unsupervised method (i.e., only require independent variable information such as sensor
responses) and is good for various qualitative applications in understanding relationship
in sensing data [54]. It provides an efficient method for reducing the dimensionality of a
data set by performing a covariance analysis between various factors [23].
For a data
matrix M, with sensing responses from m different samples as row and n sensors as
NO
2
0.1 1 0.1 1 0.1 1
0.1 1
SWNTs In
2
O
SnO
2
ZnO
(c)
45
column (in our case, m = 8 for eight sensing conditions in Table 1, and n = 8 for four
different sensors operated at two temperatures), the PCA approach carries out singular
value decomposition (SVD) [54],
which allows projection of the data set in 2-D or 3-D
principal component space. As a result, a correlation between the relative positions of
factors and contribution of each sensor in the loading plot (principal component diagram)
can be visualized. Thus, PCA not only reveals the redundant information in the data set,
but also groups the data with similar characteristics.
To study the discrimination ability of the hybrid sensor array, the PCA analysis was
carried out with the aid of commercial statistical software (STATISTIXL 1.7 Beta
version). The processed data set contains the sensor responses from 8 sensing
components (4 sensors with 2 different operation temperatures) in a sensor array to
different chemicals with different concentrations. Figure 5a shows the score and loading
plots of the first two principal components (PC1 and PC2) for the data of the metal oxide
nanowire and SWNT sensor array responses to the tested chemicals, hydrogen, ethanol,
and NO
2
, respectively. The cumulative variance of PC1 and PC2 is 98.1%, which is
considered to be good in retaining the originality of the data [23].
These chemicals were
clearly separated to 3 clusters in principal component space, which indicates that this
sensor array provides a high discrimination ability in respect to these chemicals.
Comparing to conventional chemical sensing results and sensors, the hybrid sensor array
developed in this work can distinguish chemicals by their nature rather than their
concentrations. In addition, we have found that the integration of carbon nanotube
sensors played an important role in improving the selectivity of the electronic nose
system.
46
Figure 2.5 (b) displays the PCA plot based on sensing results from only the In
2
O
3
,
SnO
2
, and ZnO nanowires. Although NO
2
, hydrogen and ethanol can still be
distinguished to certain degree, there is a slight overlap between ethanol and hydrogen
clusters. By comparing Figure 2.5 (a) and 2.5 (b), one can clearly see the importance of
integrating both metal oxide nanowires and carbon nanotubes for the maximized
discrimination power of electronic noses. Table 2.2 shows the correlation matrix of
hybrid sensor array operated at different temperatures. As one can see, the strong
correlation is found among most of metal oxide nanowire sensors operated at both high
and room temperatures. Only ZnO nanowire sensor operated at 200
o
C appears to be
moderately independent from other metal oxide nanowire sensors. Besides, SWNT
sensors also show a strong correlation at both high and room operation temperatures to
each other. This probably can be attributed to that individual sensors in a sensor array
have poor specificity in chemicals [23]. Further experiments/modeling are needed to
investigate this observation. Besides, in order to identify and quantify the test chemicals,
other classification techniques (ex. LDA and artificial neural networks) need to be
applied in future works.
2.4 Conclusion
In summary, we demonstrated a brand-new hybrid chemical sensor array composed
of In
2
O
3
nanowire, SnO
2
nanowire, ZnO nanowire, and SWNT sensors, which were
integrated with micromachined hot plates. Key to our success includes the integration of
nanowire and carbon nanotube sensors, precise control of the sensor temperature, and the
use of Principal Component Analysis for data processing. The response of these sensors
47
to hydrogen, ethanol, and NO
2
were measured at different concentrations and at both
room temperature and 200
o
C. Principal component analysis of the sensing results
showed great discrimination of those three tested chemicals, and in-depth analysis
revealed clear improvement of selectivity by the integration of carbon nanotube sensors.
This nano electronic nose approach has great potential to detect and discriminate a wide
variety of gases, including explosives and nerve agents.
SnO 2 (RT) SnO 2 (HT) In 2O 3 (RT) In 2O 3 (HT)
SWNT
(RT)
SWNT
(HT)
ZnO (RT) ZnO (HT)
SnO 2 (RT) 1.000 0.976 0.997 0.972 -0.959 -0.933 0.998 0.374
SnO 2 (HT) - 1.000 0.986 0.996 -0.985 -0.959 0.970 0.421
In 2O 3 (RT) - - 1.000 0.982 -0.977 -0.954 0.995 0.383
In 2O 3 (HT) - - - 1.000 -0.981 -0.943 0.966 0.350
SWNT
(RT)
- - - - 1.000 0.984 -0.950 -0.400
SWNT
(HT)
- - - - - 1.000 -0.923 -0.517
ZnO (RT) - - - - - - 1.000 0.369
ZnO (HT) - - - - - - - 1.000
Table 2.2 Correlation matrix of hybrid sensor array operated at different temperatures.
48
Figure 2.5 PCA scores and loading plots of the chemical sensor array composed by 4
different nanostructure materials (a) and only 3 metal oxide nanowires (b) operated at
different concentrations and temperatures.
49
Chapter 2 References
1. Cui Y , Wei Q, Park H K, Lieber C M. Nanowire nanosensors for highly sensitive and
selective detection of biological and chemical species. Science 2001; 293 (5533):
1289-1292.
2. Maiti A, Rodriguez J A, Law M, Kung P, McKinney J R, Yang P. SnO
2
nanoribbons as
NO
2
sensors: insights from first principles calculations. Nano Letters 2003; 3 (8):
1025-1028.
3. Kong J, Franklin N R, Zhou C, Chapine M G, Peng S, Cho K, Dai H. Nanotube
molecular wires as chemical sensors Science. 2000; 287 (5543): 622-625.
4. Yu C, Han Q, Saha S, Shi L, Kong X, Wang Z L. Integration of metal oxide
nanobelts with microsystems for nerve agent detection. Applied Physics Letters
2005; 86 (6): 063101-063103.
5. Zhang D, Liu Z, Li C, Tang T, Liu X, Han S, Lei, Zhou C. DNA-decorated carbon
nanotubes for chemical sensing. Nano Letters 2004; 4 (9): 1919-1924.
6. Staii C, Johnson A T Jr, Chen M, Gelperin A. Free energy landscape of a
DNA−carbon Nanotube hybrid using replica exchange molecular dynamics. Nano
Letters 2005; 5 (2): 1774-1778.
7. Lao C, Li Y, Wong C P, Wang Z L. Enhancing the electrical and optoelectronic
performance of nanobelt devices by molecular surface functionalization. Nano
Letters 2007; 7 (5): 1323-1328.
8. Qi P, Vermesh O, Grecu M, Javey A, Wang Q, Dai H. Toward large arrays of
multiplex functionalized carbon nanotube sensors for highly sensitive and
selective molecular detection. Nano Letters 2003; 3 (3): 347-351.
9. Lu Y, Partridge C, Meyyappan M, Li J. A carbon nanotube sensor array for
sensitive gas discrimination using principal component analysis. Journal of
Electroanalytical Chemistry 2006; 593 (1-2): 105110.
10. Star A, Joshi V, Skarupo S, Thomas D, Gabriei J C P. Gas sensor array based on
metal-decorated carbon nanotubes. Journal of Physical Chemistry B 2006; 110:
21014-21020.
11. Tien L C, Sadik P W, Norton D P, Voss L F, Pearton S J, Wang H T, Kang B S,
Ren F, Jun J, Lin J. Hydrogen sensing at room temperature with Pt-coated ZnO
thin films and nanorods. Applied Physics Letters 2005; 87 (22): 222106-22108.
12. Sysoev V V, Button B K, Wepsiec K, Dmitriev S, Kolmakov A. Gradient
microarray electronic nose based on percolating SnO
2
nanowire sensing elements.
Nano Letters 2006; 6 (8): 1584-1588.
50
13. Chen X H, Moskovits M. Observing catalysis through the agency of the
participating electrons: surface-chemistry-induced current changes in a tin oxide
nanowire decorated with silver. Nano Letters 2007; 7 (3): 807-812.
14. Kolmakov A, Klenov D O, Lilach Y, Stemmer S, Moskovits M. Enhanced gas
sensing by individual SnO
2
nanowires and nanobelts functionalized with Pd
catalyst particles. Nano Letters 2005; 5 (4): 667-673.
15. McApline M C, Ahmad H, Wang D, Heath J R. Highly ordered nanowire arrays
on plastic substrates for ultrasensitive flexible chemical sensors. Nature Materials
2007; 6 (5): 379-384.
16. Snow E S, Perkins F K, Houser E J, Badescu S C, Reinecke T L. Chemical
detection with a single-walled carbon nanotube capacitor. Science 2005; 307
(5717): 1942-1945.
17. Fan Z Y, Lu J G. Gate-refreshable nanowire chemical sensors. Applied Physics
Letters 2005; 86 (12): 123510-123512.
18. “Patents,” DE PS 4423289, EP 0769141B, US 5783154, JP 2980688C
19. Goschnick J, Haeringer D, Kiselev I. Multicomponent quantification with a novel
method applied to gradient gas sensor microarray signal patterns. Sensors and
Actuators B 2007; 127 (1): 237-241.
20. Sysoev V V, Goschnick J, Schneider T, Strelcov E, Kolmakov A. A gradient
microarray electronic nose based on percolating SnO
2
nanowire sensing elements.
Nano Letters. 2007; 7 (10): 3182-3188.
21. Arnold C, Harms M, Goschnick, J. Air quality monitoring and fire detection with
the Karlsruhe electronic micronose KAMINA. IEEE Sensors Journal 2002; 2 (3):
179-188.
22. Ryu K, Zhang D, Zhou C. High-performance metal oxide nanowire chemical
sensors with integrated micromachined hotplates. Applied Physics Letters 2008; 9
(9): 093111-03113.
23. Srivastave A K, Dravid V P. On the performance evaluation of hybrid and mono-
class sensor arrays in selective detection of VOCs: A comparative study .Sensors
and Actuators B 2006; 117 (1): 244-252.
24. Scorsone E, Pisanelli A M, Persaud K C. Development of an electronic nose for
fire detection. Sensors and Actuators B 2006; 116 (1-2): 55-61.
25. Scott S M, James D, Ali Z. Data analysis for electronic nose systems.
Microchimica Acta 2007; 156 (3-4): 183-207.
51
26. Craven M A, Gardner J W, Bartlett P N. Electronic Noses-development and future
prospects. Trends in Analytical Chemistry 1996; 15: 486-493.
27. Persaud K, Dodd G H. Analysis of discrimination mechanisms of the mammalian
olfactory system using a model nose. Nature 1982; 299: 352-355.
28. Lu Y, Partridge C, Meyyappan M, Li J. A carbon nanotube sensor array for
sensitive gas discrimination using principal component analysis. Journal of
Electroanalytical Chemistry 2006; 593: 105-110.
29. McApline M C, Ahmad H, Wang D, Heath J. Highly ordered nanowire arrays on
plastic substrates for ultrasensitive flexible chemical sensors. Nature Materials
2007; 6: 1-6.
30. Althainz P, Dahlke A, Frietsch-Klarhof M, Goschnick J, Ache H J. Reception
tuning of gas-sensor microsystem by selective coating. Sensors and Actuators B
1995; 24-25: 366-369.
31. Folch J, Capdevila X G, Segarra M, Morante J R. Solid electrolyte multisensor
system for detecting O2, CO, and NO2. Journal of the Electrochemical Society
2007; 154: J201-J208.
32. Moseley P T. New trends and future prospects of thick-and thin-film gas sensors.
Sensors and Actuators B 1991; 3: 167-174.
33. Then D, Vidic A, Ziegler C H. A highly sensitive self-oscillating cantilever array
for the quantitative and qualitative analysis of organic vapor mixtures. Sensors
and Actuators B 2006; 117: 1-9.
34. Li C, Zhang D, Han S, Liu X, Tang T, Zhou C. Diameter-controlled growth of
single-crystalline In
2
O
3
nanowires and their electronic properties. Advanced
Materials 2003; 15 (2): 143-146.
35. Liu Z, Zhang D, Han S, Li C, Tang T, Jin W, Liu X, Lei B, Zhou C. Laser ablation
synthesis and electron transport studies of tin oxide nanowires. Advanced
Materials 2003; 15 (20): 1754-1757.
36. Xiang B, Wang P, Zhang X, Dayeh S A, Aplin D P R, Soci C, Yu D, Wang D.
Rational synthesis of p-Type zinc oxide nanowire arrays using simple chemical
vapor deposition. Nano Letters 2007; 7 (2): 323-326.
37. Han S, Zhang D, Zhou C. Synthesis and electronic properties of ZnO/CoZnO
core-shell nanowires. Applied Physics Letters. 2006; 88 (13): 133109-133111.
38. Liu X, Lee C, Han S, Li C, Zhou C. Molecular Nanoelectronics American
Scientific Publishers 2003; 2-38.
52
39. Li C, Lei B, Zhang D, Liu X, Han S, Tang T, Rouhanizadeh M, Hsiai T, Zhou C.
Chemical sensor using semiconducting metal oxide nanowires. Applied Physics
Letters 2003; 83 (19): 4014-4016.
40. Li C, Zhang D, Liu X, Han S, Tang T, Han J, Zhou C. In
2
O
3
nanowires as
chemical sensors. Applied Physics Letters 2003; 82 (10): 1613-1615.
41. Li C, Zhang D, Han S, Liu X, Zhou C. Surface treatment and doping dependence
of In
2
O
3
nanowires as ammonia sensors. Journal of Physical Chemistry B 2003;
107 (45): 12451-12455.
42. Zhang D, Li C, Liu X, Han S, Tang T, Zhou C. Doping dependent NH
3
sensing for
indium oxide nanowires. Applied Physics Letters 2003; 83 (19): 1845-1847.
43. Chen P C, Shen G, Zhou C. Chemical sensors and electronic noses based on one-
dimensional metal oxide nanostructures. IEEE Transactions on Nanotechnology
2008; 7: 668-682.
44. Barsan N, Weimar U. Conduction model of metal oxide gas sensors. Jornal of
Electroceramics 2001; 7 (3): 143-167.
45. Batzill M, Diebold U. The surface and materials science of tin oxide. Progress in
Surface Science 2005; 79 (2-4): 47-154.
46. Zhao J J, Buldum A, Han J, Lu J. Gas molecule adsorption in carbon nanotubes
and nanotube bundles. Nanotechnology 2002; 13 (2): 195-200.
47. Fryberger T B, Semancik S. Conductance response of Pd/SnO (110) model gas
sensors to H
2
and O
2
. Sensors and Actuators B 1990; 2 (4): 305-309.
48. Dag S, Ozturik Y, Ciraci S, Yildirim T. Adsorption and dissociation of hydrogen
molecules on bare and functionalized carbon nanotubes. Physics Review B 2005;
72 (15): 155404-155411.
49. Kumar M K, Ramaprabhu S. Nanostructured Pt functionlized multiwalled carbon
nanotube based hydrogen sensor. Journal of Physical Chemistry B 2006; 110 (23):
11291-11298.
50. Comini E, Faglia G, Sberveglieri G, Calestani D, Zanotti L, Zha M. Tin oxide
nanobelts electrical and sensing properties. Sensors and Actuators B 2005; 111-
112 (1): 2-6.
51. Sin M L Y, Chow G C T, Wong G M K, Li W J, Leong P H W. Ultralow-power
alcohol vapor sensors using chemically functionalized multiwalled carbon
nanotubes. IEEE Transactions on Nanotechnology 2007; 6 (6): 571-577.
53
52. Someya T, Small J, Kim P, Nuckolls C, Yardley J T. Alcohol vapor sensors based
on single-walled carbon nanotube field effect transistors. Nano Letters 2003; 3 (7):
877-881.
53. Delac K, Grgic M, Grgic S. Independent comparative study of PCA, ICA, and
LDA on the FERET data set. International Journal of Imaging System
Technolology 2005; 15 (5): 252-260.
54. Jurs P C, Bakken G A, McClelland H E. Computational methods for the analysis
of chemical sensor array data from volatile analytes. Chemistry Review. 2000;
100 (7): 2649-2678.
54
Chapter 3 2, 4, 6-Trinitrotoluene (TNT) Chemical Sensing Based on Aligned Single-
Walled Carbon Nanotubes and ZnO Nanowires
To face the increased threat of terrorism and the need of homeland security, the
detection of explosives and nerve agents has been attracted a great deal of attention. In
this paper, we demonstrate for the first time the TNT sensors based on aligned SWNTs
and ZnO nanowires. We first transferred aligned SWNTs onto a piece of fabric as the
active material and successfully fabricated flexible SWNT chemical sensors, which have
great potential for wearable electronics. After electrical breakdown, these flexible TNT
sensing devices were exposed to a trace of TNT molecules and exhibited an excellent
sensitivity down to 8 ppb at room temperature. Besides, to realize the concept of
electronic nose (e-nose) system for explosives, we also fabricated chemical sensors based
on ZnO nanowires with a TNT detection limit of 40 ppb at room temperature. To the best
of our knowledge, this is the lowest detection limit reported so far using metal oxide
nanowire based chemical sensors to detect explosive chemicals. Furthermore, the
detection limit of our chemical sensors is close to the requirement of 1.5 ppb TNT set by
U.S. Occupational Safety and Health Administration.
As is well known, the TNT molecule decomposition is a complex process. To fully
understand the underlying detection behaviors of our devices, we used a positive ion
mass spectroscopy to study the sensing behaviors. The TNT mass spectra suggest that the
existence of the nitric oxide (NO) molecules might be one of the major groups which
results in the reduced conductance of both aligned SWNTs and ZnO nanowire based
chemical sensors.
55
3.1 Introduction
Chemical sensors based on one-dimensional (1-D) nanostructures have attracted a
great deal of attention, due to exquisite sensitivity and fast response to surrounding
environment [1-5]. In addition, both carbon nanotubes and metal oxide nanowires are
promising candidates for building an electronic nose (e-nose) system [6, 7]. Among these
materials, semiconductor single-walled carbon nanotubes (SWNTs) are molecular-scale
wires composed entirely of surface atoms, which should be ideal for the direct electrical
detection and are expected to exhibit excellent sensitivity to surrounding chemical and
biological species [8-10]. Kong et al. initially utilized SWNT field effect transistors
(FETs) to detect nitrogen dioxide (NO
2
) and ammonia (NH
3
), and demonstrated the
detection limits of 2 ppm for NO
2
and 0.1 % for NH
3
[11]. Subsequently, such SWNT-
based chemical sensors have been applied to detect a wide variety of chemicals and the
detection limits have been significantly improved. Qi et al. fabricated large arrays of
functionalized SWNT sensors, and the minimum detecting limit of NO
2
went down to
100 ppt [12]. In addition, metal oxide nanowires have been widely studied and
demonstrated with great potential in chemical sensing applications [13-16].
Recently, due to the threat of terrorism and the need of homeland security,
significant effort has been made in the detection of both explosives and nerve agents,
such as 2, 4, 6- trinitrotoluene (TNT), 2, 4- dinitrotoluene (DNT), hexogen (DRX), and
dimethyl methylphosphonate (DMMP) [17-20]. One of the leading candidates is 1-D
nanostructure-based chemoresistors or FETs. Snow et al. and Wang et al. have reported
the detection of DMMP in ppb level by using SWNT and SnO
2
nanowire-based chemical
sensors, respectively [21, 22].
However, to our knowledge, there were only a few reports
56
on the use of 1-D nanostructure-based chemoresistor and FETs for detecting explosives
and the detection mechanism is still unclear [23, 24].
In addition, electronic devices fabricated on mechanically flexible substrates have
recently attracted enormous attention, due to proliferation of handheld and wide
applications in portable electronics, aerospace science and civil engineering. Currently,
conventional mircofabrication techniques or printing methods can be applied to SWNTs
on plastic substrates to form devices, which provide a solution to inexpensive mass-
production and conformable electronics [25-27].
Herein, we report the transfer of aligned semiconductor SWNTs onto cloth fabric
and successful fabrication of flexible SWNT chemical sensors, which have great potential
for wearable electronics. These SWNT chemical sensors exhibited good sensitivity of
trace chemical vapors, including 8 ppb TNT and 40 ppb NO
2
, at room temperature.
Besides, to realize the concept of electronic nose (e-nose) system for explosives, we also
fabricated ZnO nanowire-based chemical sensors, which exhibited the detection limit of
60 ppb for TNT molecules at room temperature. To our knowledge, this is the first TNT
sensor built on metal oxide nanowires. In addition, the detection limit of our chemical
sensors is close to the requirement of 1.5 ppb TNT set by U.S. Occupational Safety and
Health Administration. The flexible TNT sensors could find immediate applications in
systems with the demand of mechanical flexibility, light weight, and high sensitivity.
3.2 Experiments:
3.2.1 Device Fabrications
The fabrication of flexible SWNT chemical sensors started with the synthesis of
SWNTs on quartz substrates using a chemical vapor deposition (CVD) method, which
57
have been reported by us and other groups [28-30]. After growth, we adapted a facile
method [31] to transfer the aligned nanotubes from the original substrate to fabric. In
brief, a 100 nm thick gold film was first deposited on the original substrate with aligned
SWNTs, followed by applying a thermal tape to peel off the gold film and nanotubes
from the growth substrate. The gold film with SWNTs on the thermal tape were pressed
against a piece of textile fabric, which was pre-coated with polyethylene at elevated
temperature and then transferred from thermal tape onto textile fabric, which had a 50-nm
Ti as a back gate electrode and a 2-µm thick SU-8 as gate dielectric layer. The thermal
tape was released, and then KI/I
2
gold etchant was then applied to remove gold films.
Finally, Ti (0.5 nm) and Pd (40 nm) were deposited on the transferred SWNTs as
source/drain electrodes. A schematic diagram of a flexible SWNT chemical sensor is
shown in Figure 3.3(a).
Figure 3.1 SEM image of vertical ZnO nanowires on a-sapphire. The scale bar is 1 µm.
58
Vertical ZnO nanowires grown on an a-sapphire with diameters of 50 nm and
lengths of 10 μm (shown in Figure 3.1) were obtained via a vapor-solid (VS) growth
method, which exhibited defect-free crystalline structure and good electronic
characteristics. As-grown ZnO nanowires were sonicated into isopropanol alcohol (IPA)
to form a nanowire suspension and then dispersed on a Si/SiO
2
substrate until a suitable
nanowire density was achieved. Photolithography was applied to pattern both source and
drain electrodes for chemical sensors, followed by 5 nm Ti / 45 nm Au deposition using
an e-beam evaporator. The schematic diagram of ZnO chemical senor is shown in Figure
3.4 (a).
3.2.2 Chemical Sensing Experiments and Set-up
The chemical sensing experiments were performed using a home-made chemical
sensing setup described in our previous work [33-38]. Briefly, the as-made flexible
SWNT sensors and ZnO nanowire sensors were mounted on a chip carrier inside a sealed
chamber with an electrical feedthrough and gas inlet/outlet, shown in Figure 3.2. The
generation of diluted TNT vapor was carried out following reported literature [46, 47].
The TNT vapor was generated from TNT powder (99.0%, Chem Service) with/without
heating, and air was used as carrier gas in the experiments. 1.5 gram TNT powder (99.0%
Chem Service) was placed in a 10 ml glass vial, which served as a reservoir of saturated
vapor. A stream of dry air was used to carry the TNT vapor out, and then was further
diluted by another stream of dry air to obtain various concentrations. The air flow was
directed toward the sensor surface, and the flow rates were controlled by mass flow
controllers. The TNT concentration was calculated based on the saturated vapor pressure
and the ratio of dilution, following Ref. 23. The details for the estimated TNT
59
concentrations can be found in the Supporting Information. In order to simulate the
practical environment, some parts of gas line were exposed to indoor light.
Nitrogen dioxide (NO
2
), a well-known electron acceptor, was also adapted in this
work. Different concentrations of NO
2
were introduced into the sensing chamber to
investigate the sensitivity of our devices and TNT sensing mechanism.
Figure 3.2 Set-up of Chemical Sensing Chamber
Temperature
(
o
C)
Vapor Pressure
(10
-3
mm-Hg)[a]
Estimated
Concentration
(ppm)
TNT / Air Flow
Rate (sccm)
Estimated TNT
Concentration
25 0.23 0.3 50 / 2000 8 ppb
180 / 2500 23 ppb
40 0.34 0.5 130 / 1000 60 ppb
160 / 500 0.15 ppm
210 / 500 0.2 ppm
60 0.54 0.8 215 / 400 0.4 ppm
70 0.66 0.9 260 / 400 0.6 ppm
90 0.97 1.4 270 / 320 1.1 ppm
Table 3-1 Estimated TNT Concentrations.
UV light viewport
Gas In Gas Out
Sensor Chip
60
3.3 Results and Discussion
3.3.1 Flexible SWNT TNT Chemical Sensors
Figure 3.3 Wearable transistors based on aligned nanotubes transferred to fabric. (a)
Schematic diagram showing a transistor structure that uses polyethylene-coated fabric (b)
The optical micrograph showing an array of such transistors built on a flexible fabric. (c)
I-V
g
curves of a transistor (D = ~2 tubes / µm ) before and after electrical breakdown. (d)
I -V
g
curves at different V
ds
for the device in (c). V
ds
is varied from -0.2 to -1 V in steps of
-200 mV. (e) I-V
ds
curves at different V
g
for the same device in (c). The curves
correspond to V
g
= 10 to -20 V in step of - 5 V.
Figure 3.3 (b) shows an optical photograph of flexible aligned SWNT FETs on a
textile fabric, where the nanotubes can be clearly seen bridging two electrodes in the
SEM image in Figure 3.3 (b) Figure 3.3 (c) displays the current-gate voltage (I-V
g
)
characteristics of a typical flexible transistor on fabric before and after electrical
breakdown. The device showed significant improvement for the on/off ratio from 2.3 to
3×10
3
, accompanied by decrease of the on-state current due to metallic carbon nanotube
removal. After electrical breakdown, further I-V
g
and current vs. drain-source voltage (I-
V
ds
) measurements were also carried out. Figure 3.3 (d) shows the current magnitude
61
(│I│) as a function of gate voltage (V
g
) at different V
ds
from -0.2 to -1 V with a step of -
200 mV, showing a pronounced p-type semiconductor behavior. We extracted the
subthreshold swing of this device to be 2 V/decade and the on/off ratio as high as 10
5
. In
addition, Figure 3.3 (e) shows a set of I-V
ds
curves at different gate voltages. The highest
current obtained at V
g
= -20 V and V
ds
= -5 is 11.8 µA, and the device also showed
moderate current saturation behavior. Our results are comparable with early reported
work on both rigid and flexible substrates [31, 32].
Figure 3.4 (a) displays the I-V curves (at V
g
= 0V) obtained under different TNT
concentrations of 0.15, 0.4, and 0.6, and 5 ppm NO
2
as a comparison. These curves are
found to be rather linear and indicate the Ohmic contact nature for the sensing devices in
diluted TNT and NO
2
environment. With the increasing TNT concentration, the device
conductance was monotonically suppressed from 3 μS (in air) to 2 μS (in 0.6 ppm TNT).
On the other hand, the device conductance was increased to 4.2 μS while the device was
exposed to 5 ppm NO
2
. The NO
2
, known as a strong electron-withdrawing compound, is
expected to increase the hole concentration in p-type nanotubes, thus leading to the
observed higher conductance. Besides, a sensing experiment of six sensing cycles with
six different NO
2
concentrations ranging from 40 ppb to 5 ppm has been carried out and
can be found in the Supporting Information. Figure 3.4 (b) plots the change in SWNT
conductance normalized by the initial conductance at gate bias V
g
= 0V for TNT sensing.
Eight cycles have been successively recorded, corresponding to eight different TNT
concentrations ranging from 8 ppb to 1.1 ppm, respectively. The relative sensor response
(RS) in conductance is defined as
62
0
0
0
G
G G
G
G
RS
−
=
∆
= (1)
where G
0
and G denote the nanotube conductance before and after the exposure,
respectively.
Figure 3.4 (a) I-V curves taken in air, 5 ppm NO
2
, and three different TNT concentrations.
(b) Sensing response of a flexible SWNT sensor to TNT. The normalized conductance
change ( Δ G/ G
0
) is plotted as a function of time with the sensor exposed to TNT of
different concentrations. Recovery was made by UV light (254 nm). The inset shows
TNT structure. (c) Plot of Δ G/ G
0
v.s. the TNT concentration.
Each cycle of sensing was started by desorbing the attached molecules with
ultraviolet (UV) light ( λ~254 nm) irradiation (at the point A, taking the first cycle as an
example), as previously demonstrated [39]. The SWNT conductance kept increasing until
63
the UV light was turned off at point B. The conductance decreased gradually, due to the
electron-hole recombination and re-adsorption of oxygen molecules and moisture in the
air. After a relatively stabilized state was reached, diluted TNT vapor (TNT in air, 8 ppb)
was introduced to the airflow at point C. The lowest detectable TNT concentration was
found to be ~ 8 ppb in air. With higher concentration TNT used, more pronounced
conductance modulation was observed, as shown in Figure 2b.
Figure 3.4 (c) shows a linear dependency between the normalized sensor response
and the TNT concentration, which can be fitted as ΔG/G
0
=-0.004 - 0.059C. The reduced
conductance we observed for the SWNT sensors under TNT exposure is interesting, and
will be further discussed below based on the results of positive ion mass spectra. We note
that reduced conductance was previously reported for DNT sensing using SWNT
networks and reduced graphene oxides [22-40], but no previous report can be found on
the effect of TNT exposure on SWNT conductance. In addition, TNT molecules are easy
to decompose into small molecular fragments under light illumination or heat treatments,
which might influence the sensing behaviors of SWNTs [40].
As a result, our wearable
SWNT-based sensing devices showed good sensitivity to TNT at room temperature, and
the detection limit is comparable with the conductive polymer TNT sensors [41].
Figure 3.5 shows six sensing cycles with a flexible aligned semiconductor SWNT
chemical sensor, corresponding to NO
2
/ air concentrations of 40 ppb, 80 ppb, 150 ppb,
1.25 ppm, 2.5 ppm, and 5 ppm, respectively. The NO
2
molecules, known as strong
electron-withdrawing groups, increased hole concentration of the p-type nanotubes and
therefore led to increased conductance. The flexible aligned semiconductor SWNT
chemical sensor showed a detection limit of 40 ppb for NO
2
.
64
Figure 3.5 Six sensing cycles of the flexible CNT chemical sensor, corresponding to NO
2
concentrations of 40 ppb, 80 ppb, 150 ppb, 1.25 ppm, 2.5 ppm, and 5 ppm.
3.3.2 ZnO Nanowire TNT Chemical Sensors
To further investigate the TNT sensing mechanism, we have also studied the effect
of TNT on ZnO nanowire sensors, which would be essential components to be combined
with carbon nanotube sensors for electronic nose systems [6]. Figure 3a shows a
schematic diagram of a ZnO nanowire sensor. The synthesis of ZnO nanowire and device
fabrication can be found in the Supporting Information. The SEM image (inset of Figure
3.6 (a)) reveals the dimension of a single ZnO nanowire sensor with the channel length of
2 μm. The I-V curves (at V
g
= 0V) obtained under the different TNT concentrations from
60 ppb to 1.36 ppm are shown in Figure 3.6 (b). With increasing TNT concentration, the
device conductance was monotonically suppressed from 3 μS (in air) to 0.5 μS (in 1.36
ppm TNT). Figure 3.6 (c) plots the changes in ZnO nanowire conductance normalized
against the initial conductance. Seven cycles have been successively recorded,
corresponding to seven different TNT/air concentrations ranging from 60 ppb to 1.36
ppm, respectively. Similar to semiconductor SWNTs, with increasing TNT concentration,
more pronounced conductance modulation was observed. The lowest detectable
65
concentration is ~ 60 ppb for a ZnO nanowire sensor. To our knowledge, this is the
lowest detection limit reported so far using metal oxide nanowire based chemical sensors
to detect TNT.
Figure 3.6 (d) plots the derived normalized sensor response as a function of TNT
concentrations (C). The curve is rather linear at low TNT concentration (< 200 ppb), but
tends to saturate when the TNT partial pressure went up. This curve can be well fitted
with the following equation:
0
1
GA
B
G
C
∆
=
+
(2)
with A = 0.086 and B = 16.74, which has also been further confirmed by the linear fitting
of RS
-1
and C
-1
shown in the inset of Figure 3d. The results can be understood as the
surface coverage of adsorbed molecules follows Langmuir isotherm [33]. At lower
concentrations, the chemical sensor exhibited a linear dependence between the
normalized sensor response and the TNT concentration. At higher concentrations, the
surface coverage tends to saturate and hence leads to the saturation response observed in
Figure 3.6 (d). In comparisons of ZnO nanowire based TNT sensors, SWNT based
sensors did not exhibit a saturation response at the higher TNT concentrations (shown in
Figure 3.4 (c)). We tentatively attributed this difference to the varied SWNT and
nanowire density in our sensing devices. For SWNT chemical sensors, the transferred
carbon nanotube density was about 4 tubes/μm in average (channel width is 50 μm),
which provided a lot of reaction sites for TNT molecules and resulted in the unsaturated
behavior at high TNT concentrations. On the other hand, the ZnO nanowire chemical
sensors, with nanowire density less than 1 wire/μm, provided less reaction sites for TNT
66
molecules than SWNT chemical sensors and could saturate easily at high TNT
concentrations.
Figure 3.6 (a) Schematic view of a ZnO nanowire transistor structure, with Ti/Au
deposited on ZnO nanowires as source and drain electrodes. Inset: SEM image of the as-
fabricated ZnO nanowire chemical sensor. The scale bar is 2 μm. (b) I-V curves taken in
air and at different TNT concentrations. (c) Sensing response of a ZnO nanowire
chemical sensor to TNT. The normalized conductance change ( Δ G/ G
0
) is plotted as a
function of time with the sensor being exposed to TNT of different concentrations. (d)
Normalized conductance change ( Δ G/ G
0
) vs TNT concentrations (C), which was fitted
using S =1/ (A+B/C). Inset: 1/S vs 1/C with linear line fit.
3.3.3 TNT Sensing Mechanism
Since the TNT molecule decomposition process is complex, more detailed
experiments are required to understand the surface adsorption behavior of TNT and its
decomposition products. For this purpose, a positive ion mass spectrometer (Omnistar,
67
GSD 310O, Pfeiffer) with the ion beam intensity of 70 eV, was connected to the sensing
chamber. A standard mass spectrum of air at 25
o
C is shown in Figure 3.7 (a). We can see
a typical electron ionization (EI) mass spectrum of air with a base peak at m/z 40 due to
the presence of argon ions (Ar
+
). Major ions in the mass spectrum of air are due to N
2
at
m/z 28, O
2
at m/z 32, hydroxyl group at m/z 34, and carbon dioxide (CO
2
) at m/z 44. For
the peaks located between m/z 14 and m/z 22, they can be attributed to the second
ionization energy of N
2
, O
2
, hydroxyl group, and CO
2
in air. These characteristic peaks
from air can be used as a reference for the identification of TNT molecules. Figure 3.7 (b)
shows the mass spectrum of 23 ppb TNT in air. As one can observe in the inset of Figure
3.7 (b), in comparison to the mass spectrum of air, there are two additional peaks in the
spectrum, including NO molecules (m/z at 30) and NO
2
molecules (m/z at 46). Among
them, NO molecule is a well-known electron donor to SWNTs, and is a signature
decomposition product of TNT molecules because of the photolysis under the
experimental conditions [42, 43]. For NO
2
molecule, it is known as an electron-
withdrawing group. However, the ratio between NO and NO
2
can be estimated to be 20,
which is consistent with previous reports that NO is the most abundant product from TNT
decomposition. Our results suggest that the reduction in nanotube conductance is induced
by the abundance of NO relative to NO
2
in the decomposition products of TNT. In
addition to NO and NO
2
, there are other decomposition products in the sensing chamber
which might influence the electron or hole concentrations in sensing materials, such as
nitrobenzene, RDX, nitromethane, and DNT [40, 44], which are beyond the detection
limit of our mass spectrometer.
68
On the other hand, for ZnO nanowire sensors, the dominant mechanism [45] is that
NO can react with oxygen ions on the metal oxide surface and convert to NO
2
. (shown in
equation (3)) [45]. The adsorbed NO
2
molecules are strong electron-withdrawing groups,
and thus cause a reduced conductance for ZnO nanowire sensing devices, consistent with
our observation reported in Figure 3.
22 surface gas
NO O e NO O
−− − −
+ +→ + (3)
3.4 Conclusion
In summary, we have successfully demonstrated TNT chemical sensing based on
both SWNTs and ZnO nanowires. We have transferred aligned SWNTs onto cloth fabric
and successfully fabricated flexible SWNT TNT sensors, which can be used for wearable
electronics. These flexible SWNT chemical sensors exhibited good sensitivity towards
trace TNT molecules down to 8 ppb diluted with air as carrying gas, and the detection
limit is comparable to conductive-polymer based TNT sensors. In addition, ZnO
nanowire sensors were fabricated and the TNT detection limit was 60 ppb. A positive ion
mass spectrometer was employed to understand the sensing mechanism, and revealed the
presence of NO and NO
2
as decomposition products of TNT under room light irradiation.
While the relative abundance of NO is believed to induce the conductance decrease for
SWNT sensors, the reaction of NO with oxygen ions on ZnO nanowire surface can
convert to NO
2
and lead to reduced conductance for ZnO nanowire sensor. The flexible
TNT sensors could find immediate applications in systems with the demand of
mechanical flexibility, light weight, and high sensitivity.
69
Figure 3.7 Positive ion mass spectrum of (a) air, (b) 23 ppb TNT in air, recorded at an
electron energy of 70 eV.
2.0x10
-9
1.5
1.0
0.5
0.0
Intensity (a.u.)
50 40 30 20 10
Mass (m/z)
2.0x10
-9
1.5
1.0
0.5
0.0
Intensity (a.u.)
50 40 30 20 10
Mass (m/z)
2.0x10
-12
1.5
1.0
0.5
0.0
Intensity (a.u.)
34 32 30 28
Mass (m/z)
2.0x10
-12
1.5
1.0
0.5
0.0
Intensity (a.u.)
46 44 42 40
Mass (m/z)
2.0x10
-12
1.5
1.0
0.5
0.0
Intensity (a.u.)
34 32 30 28
Mass (m/z)
2x10
-13
1
Intensity (a.u.)
46 44 42 40
Mass (m/z)
NO
NO
2
(a)
Air
(b)
TNT @ 25
o
C
70
Chapter 3 References
1. Wang Z L. Functional Oxides Nanobelts – Materials, properties and potential
applications in Nanosystems and Biotechnology. Annual Review of Physical
Chemistry 2004; 55: 159-196.
2. Kolmakov A, Moskovits M. Chemical sensing and catalysis by one-dimensional
metal-oxide nanostructures. Annual Review of Materials Research 2004; 34: 151-
180.
3. Lu J G, Chang P, Fan Z. Quasi-one-dimensional metal oxide materials-synthesis,
properties and applications. Material Science Engineering Report 2006; 52: 49-91.
4. Snow E S, Perkins F K, Robinson J A. Chemical vapor detection using single-
walled carbon nanotubes. Chemical Society Reviews 2006; 35: 790-798.
5. Chen P C, Shen G, Zhou C. Chemical sensors and electronic noses based on one-
dimensional metal oxide nanostructures. IEEE Transactions on Nanotechnology
2008; 7: 668-682.
6. Chen P C, Ishikawa F N, Chang H, Ryu K, Zhou C. A nanoelectronic nose: a
hybrid nanowire/carbon nanotube sensor array with integrated micromachined
hotplates for sensitive gas discrimination. Nanotechnology 2009; 20 (12):
125503-125510.
7. Sysoev V V, Goschnick J, Schneider T, Strelcov E, Kolmakov A. A gradient
microarray electronic nose based on percolating SnO
2
nanowire sensing elements.
Nano Letters. 2007; 7 (10): 3182-3188.
8. Richard C, Balavoine F, Schultz P, Ebbesen T W, Mioskowski C. Supramolecular
self-assembly of lipid derivatives on carbon nanotubes. Science 2003; 330(5620):
775-778.
9. Cao Q, Rogers J A. Ultrathin films of single-walled carbon nanotubes for
electronics and sensors: a review of fundamental and applied aspects. Advanced
Materials 2009; 21 (1): 29-53.
10. Snow E S, Perkins F K, Houser E J, Badescu S D, Reinecke T L. Chemical
detection with a single-walled carbon nanotube capacitor. Science 2005; 307
(5717): 1942-1945.
11. Kong J, Franklin N R, Zhou C, Chapline M G, Peng S, Cho K, Dai H. Nanotube
molecular wires as chemical sensors. Science 2000; 287 (5543): 622-625.
71
12. Qi P, Vermesh O, Grecu M, Javey A, Wang Q, Dai H. Toward large arrays of
multiplex functionalized carbon nanotube sensors for highly sensitive and
selective molecular detection. Nano Letters 2003; 3 (3): 347-351.
13. Comini E, Faglia G, Sberveglieri G, Pan Z W, Wang Z L. Stable and highly
sensitive gas sensors based on semiconducting oxide nanobelts. Applied Physics
Letters 2002; 81 (10): 1869-1871.
14. Kolmakov A, Klenov D O, Lilach Y, Stemmer S, Moskovits M. Enhanced gas
sensing by individual SnO
2
nanowires and nanobelts functionalized with Pd
catalyst particles. Nano Letters 2005; 5 (4): 667-673.
15. McApline M C, Ahmad H, Wang D, Heath J R. Highly ordered nanowire arrays
on plastic substrates for ultrasensitive flexible chemical sensors. Nature Materials
2007; 6 (5): 379-384.
16. Li Q H, Liang Y X, Wan Q, Wang T H. Oxygen sensing characteristics of
individual ZnO nanowire transistors. Applied Physics Letters 2005; 85 (26): 6389-
6391.
17. Sohn H, Sailor M J, Magde D, Trogler W C. Detection of Nitroaromatic
explosives based on photoluminescent polymers containing metalloles. Journal of
the American Chemistry Society 2003; 125 (13): 3821-3830.
18. Goldman E R, Medintz I L, Whitley J L, Hayhurst A, Clapp A R, Uyeda H T,
Deschamps J R, Lessman M E, Mattoussi H. Trace explosives signatures from
World War II unexploded undersea ordnance. Journal of the American Chemistry
Society 2005; 127 (18): 6744-6751.
19. Novak J P, Snow E S, Houser E J, Park D, Stepnowski J L, McGill R A. Nerve
agent detection using networks of single-walled carbon nanotubes. Applied
Physics Letters 2003; 83 (19): 4026-4028.
20. Wang F, Gu H, Swager T M. Carbon nanotube/polythiophene chemiresistive
sensors for chemical warfare agents. Journal of the American Chemistry Society
2008; 130 (16): 5392-5393.
21. Yu C, Hao Q, Saha S, Shi L, Kong X, Wang Z L. Integration of metal oxide
nanobelts with microsystems for nerve agent detection. Applied Physics Letters
2005; 86 (24): 063101-063103.
22. Snow E S, Perkins F K. Capacitance and conductance of single-walled carbon
nanotubes in the presence of chemical vapors. Nano Letters 2005; 5 (12): 2414-
2417.
23. Staii C, Johnson Jr A T, Chen M, Gelperin A. DNA-decorated carbon nanotubes
for chemical sensing. Nano Letters 2005; 5 (9): 1774-1778.
72
24. Li J, Lu Y, Ye Q, Cinke M, Han J, Meyyappan M. Carbon nanotube sensors for
gas and organic vapor detection. Nano Letters 2003; 3 (7): 929-933.
25. Ju S, Xuan Y, Ye P, Janes D B, Ishikawa F, Zhou C, Lu G, Facchetti A, Marks T J.
Fabrication of fully transparent nanowire transistors for transparent and flexible
electronics. Nature Nanotechnology 2007; 2 (11): 378-384.
26. Bradley K, Gabriel J C P, Gruner G. Flexible nanotube electronics. Nano Letters
2003; 3 (10): 1353-1355.
27. Kang S J, Kocabas C, Ozel T, Shim M, Pimparkar N, Alam M A, Rotkin S V,
Rogers J A. High-performance electronics using dense, perfectly aligned arrays of
single-walled carbon nanotubes. Nature Nanotechnology 2007; 2 (11): 230-236.
28. Lim H J, Lee D Y, Oh Y J. A comparison between several vibration-powered
piezoelectric generators for standalone systems. Sensors and Actuators A 2006;
125 (2): 405-410.
29. Han S, Liu X, Zhou C. Template-free directional growth of single-walled carbon
nanotubes on a- and r- plane. Journal of the American Chemistry Society 2005;
127 (15): 5294-5295.
30. Kocabas C, Hur S, Gaur A, Meitl A M, Shim M, Rogers J A. Guided growth of
large-scale, horizontally aligned arrays of single-walled carbon nanotubes and
their use in thin-film transistors. Small 2005; 1 (11): 1110-1116.
31. Ishikawa F N, Chang H K, Ryu K, Chen P, Badmaev A, De Arco Gomez L, Shen
G, Zhou C. Transparent electronics based on transfer printed aligned carbon
nanotubes on rigid and flexible substrates. ACS Nano, 2008; 3: 73-79.
32. Cao Q, Kim H S, Pimparkar N, Kulkarni J P, Wang C J, Shim M, Roy K, Alam M
A, Rogers J A. Medium-scale carbon nanotube thin-film integrated circuits on
flexible plastic substrates. Nature 2008; 454 (7203): 495-500.
33. Zhang D, Liu Z, Li C, Tang T, Liu X, Han S, Lei B, Zhou C. Detection of NO
2
down to ppb levels using individual and multiple In
2
O
3
nanowire devices. Nano
Letters 2004; 4 (10): 1919-1924.
34. Li C, Lei B, Zhang D, Liu X, Han S, Tang T, Rouhanizadeh M, Hsiai T, Zhou C.
Chemical sensor using semiconducting metal oxide nanowires. Applied Physics
Letters 2003; 83 (19): 4014-4016.
35. Li C, Zhang D, Liu X, Han S, Tang T, Han S, Zhou C. In
2
O
3
nanowires as
chemical sensors. Applied Physics Letters 2003; 82 (10): 1613-1615.
73
36. Li C, Zhang D, Han S, Liu X, Zhou C. Surface treatment and doping dependence
of In
2
O
3
nanowires as ammonia sensors. Journal of Physical Chemistry B 2003;
107 (45): 12451-12455.
37. Zhang D, Li C, Liu X, Han S, Tang T, Zhou C. Doping dependent NH
3
sensing for
indium oxide nanowires. Applied Physics Letters 2003; 83 (19): 1845-1847.
38. Ryu K, Zhang D, Zhou C. High-performance metal oxide nanowire chemical
sensors with integrated micromachined hotplates. Applied Physics Letters 2008; 9
(9): 093111-03113.
39. Zhang D, Li C, Han S, Liu X, Tang T, Jin W, Zhou C. Ultraviolet photodetection
properties of indium oxide nanowires. Applied Physics A 2003; 77 (1): 163-166.
40. Jeremy T R, Perkins F K, Snow E S, Wei Z, Sheehan P E. Reduced graphene
oxide molecular sensors. Nano Letters 2008; 8 (10): 3137-3140.
41. Lemire G W, Simeonsson J B, Sausa R C. Monitoring of vapor-phase nitro
compounds using 226-nm radiation: fragmentation with subsequent NO
resonance-enhanced multiphoton ionization detection. Analytical Chemistry 1993;
65 (5): 529-533.
42. Pinnaduwage L A, Gehl A, Hedden D L, Muralidharan G, Thundat T, Lareau R T,
Sulchek T, Manning L, Rogers B, Jones M, Adams J D. Explosives: a microsensor
for trinitrotoluene vapour. Nature 2003; 425 (7127): 474-478.
43. Bartberger M D, Liu W, Ford E, Miranda K M, Switzer C, Fukuto J M, Farmer P
J, Wink D A, Houk K N. The reduction potential of nitric oxide (NO) and its
importance to NO biochemistry. Proceeding of the National Academy of Science
on the United States 2002; 99 (17): 10958-10963.
44. Schmelling C D, Gray K A. Photocatalytic transformation and mineralization of
2,4,6-trinitrotoluene (TNT) in TiO
2
slurries. Water Research 1995; 29 (12): 2651-
2662.
45. Oxley J C, Smith J L, Zhou Z L, Mckenney R L J. Thermal Decomposition
Studies on NTO and NTO/TNT1. Phys. Chem. 1995; 99 (25): 10383-10391.
46. Lim H J, Lee D Y, Oh Y J. A comparison between several vibration-powered
piezoelectric generators for standalone systems. Sensors and Actuators A 2006;
125 (2): 405-411.
47. Tao S, Li G. Porphyrin-doped mesoporous silica films for rapid TNT detection.
Colloid Polym. Sci. 2007; 285 (7): 721-728.
74
Chapter 4 High-Performance Single-Crystalline Arsenic-Doped Indium Oxide
Nanowires for Transparent Thin Film Transistors and Active Matrix Organic
Light-Emitting Diode Displays
In this chapter, we report high-performance arsenic (As) -doped indium oxide
(In
2
O
3
) nanowires for transparent electronics, including their implementation in
transparent thin-film transistors (TTFTs) and transparent active-matrix organic light-
emitting diodes (AMOLED) displays. The As-doped In
2
O
3
nanowires were synthesized
using a laser ablation process, and then fabricated into TTFTs with indium-tin-oxide
(ITO) as the source, drain and gate electrodes. The nanowire TTFTs on glass substrates
exhibit very high device mobilities (~1,490 cm
2
V
-1
s
-1
), current on/off ratios (5.7 × 10
6
),
steep subthreshold slopes (88 mV/dec), and a saturation current of 60 μA for a single
nanowire. By using a self-assembled nanodielectric (SAND) as gate dielectric, the device
mobilities and saturation current can be further improved up to 2,560 cm
2
V
-1
s
-1
and 160
μA, respectively. All devices exhibit good optical transparency (~81 % on average) in the
visible spectral range. In addition, the nanowire TTFTs were utilized to control green
OLEDs with varied intensities. Furthermore, a fully integrated seven-segment AMOLED
display was fabricated with a good transparency of 40 % and with each pixel controlled
by two nanowire transistors. This work demonstrates that the performance enhancement
possible by combining nanowire doping and self-assembled nanodielectrics enables
silicon-free electronic circuitry for low power consumption, optically transparent, high-
frequency devices assembled near room temperature.
75
4.1 Introduction
The concept of transparent electronics (also called invisible circuits) was first
proposed in 1997 [1], and offers the attraction of optical transparency and, in principle,
low temperature processing. There are currently numerous research efforts on transparent
electronics, due to its great potential to make significant commercial impact, including in
displays [2-5], solar cells [6, 7], charge-coupled devices (CCDs) [8], and UV detectors [9,
10]. The core technology to realize the transparent electronics requires the development
of high-performance transparent thin film transistors (TTFTs), with high device
mobilities, moderate carrier concentrations, low threshold voltages, and steep
subthreshold slopes [11]. Currently, TTFTs fabricated with amorphous or polycrystalline
transparent conducting oxide (TCO) thin films have been widely studied, including ZnO,
SnO
2
, CuAlO
2
and many other semiconductor oxides [1, 12-16]. However, TTFTs made
from these materials usually exhibit rather low mobilities (0.2-120 cm
2
V
-1
sec
-1
) and high
threshold voltages (V
th
: 10-20 V) [14-17]. For instance, TTFTs with amorphous indium
gallium oxide (a-IGO) films display device mobility of 7 cm
2
V
-1
sec
-1
, a current on-off
ratio of 10
4
, and an inverter gain of 1.5 on glass substrates [18]. These results clearly
indicate that the performance of TCO thin film-based TTFTs may limit their operation in
high frequency applications and has significant room for further improvement.
Recently, one-dimensional (1-D) nanostructured materials, including single-walled
carbon nanotubes (SWNTs) and semiconductor metal oxide nanowires, have been
considered as another material choice for TTFT fabrications. [19-22]. In comparison to
conventional TCO-based TTFTs, nanomaterials synthesized through a simple chemical
76
vapor deposition (CVD) method can easily provide high-quality single-crystalline
nanostructures, which are highly desirable in most electronic and optoelectronic devices
[23-25]. In addition, nanostructured material-based TTFTs have added advantages, such
as versatile compatibility with a variety of device substrates, which thereby extends their
applicability to flexible electronics, due to the compatibility of nanostructured TTFTs
with low temperature processing. Among these nanomaterials, In
2
O
3
nanowires having a
wide energy gap (~3.75 eV), a single crystalline nanostructure, and high device mobility,
is one of the best candidates for high performance TTFTs [20, 26]. Our research group
and Ju et al. has also demonstrated In
2
O
3
nanowire TTFTs with a device mobility as high
as 514 cm
2
V
-1
sec
-1
. However, this performance is still low compared to non-transparent
devices [27], raising the question of whether further improvement might be possible
through doping of the nanowires. While doping methods have been employed for
controlling carrier concentration and transport properties in nanowires, their effects are
not always predictable, and remain a challenging issue in nano-electronics science and
technology, which constrains nanowire based TTFT applications [21, 28].
In considering the enablers of next-generation displays, having good optical
transparency and/or mechanical flexibility, high performance transparent and/or flexible
TFTs will be essential. For example, the circuitry of an AMOLED pixel usually contains
one driving transistor and one switching transistor. The driving transistors must carry
sufficient current to the OLED pixels, and the switching transistors must be able to
operate at 60 - 120 Hz for the human eye to see an integrated image of successively
presented patterns in a video display (so-called retinal persistence). Currently,
polycrystalline silicon (poly-Si) and amorphous silicon (a-Si) are widely used as the
77
“back-panel” electronics for AMOLED displays. However, these back-panels are usually
optically opaque, not compatible with flexible substrates, and have drawbacks such as
low mobilities, high threshold voltages, and in the case of a-Si, poor current-carrying
capacity [11].
In this chapter, we first report the synthesis of arsenic (As)-doped In
2
O
3
nanowires
on Si/SiO
2
substrates and then fabricate TTFTs by transferring the doped nanowires to
glass substrates with pre-patterned indium-tin oxide (ITO) gate electrodes and an Al
2
O
3
or a self-assembled nanodielectric (SAND) gate insulator, followed by patterning
transparent ITO source and drain electrodes. It will be seen that the as-fabricated As-
doped In
2
O
3
nanowire TTFTs perform as typical n-type FETs with a high device mobility
(1,490 cm
2
V
-1
sec
-1
) and optical transparency of near 81% in the visible wavelength
regime. We further examine the AC gain of these nanowire TTFTs, to the best of our
knowledge, the first AC gain study of nanowire TTFTs. The results indicate good
frequency response, ~1.5 kHz with a unity-gain frequency of ~18.8 GHz. Moreover, we
then show that As-doped In
2
O
3
nanowire TTFTs can be used to drive organic light-
emitting diodes (OLEDs) with tunable emitting intensities, including a seven-segment
AMOLED display enabled by a nanowire TTFT “back-panel”.
4.2 Experiments
4.2.1 The Synthesis of As-doped In
2
O
3
Nanowires
10 nm gold nanoparticles were dispersed on a Si/SiO
2
substrate and utilized as
catalysts in the nanowire synthesis. The substrate was then placed into a quartz tube at the
down stream end of a furnace, while an InAs target was placed at the upper stream of the
78
furnace. During the laser ablation process, the chamber was maintained at 760 Torr,
700°C with a constant flow of 150 standard cubic centimetres (sccm) of Ar mixed with
10 % H
2
. The typical reaction time was about 50 minutes. After cooling down, the
samples were characterized using FESEM, TEM, XRD, and EDS.
4.2.2 Fabrication of As-doped In
2
O
3
Nanowire TTFTs with an Al
2
O
3
Dielectric
For transparent nanowire transistors, fabrication started with the deposition of a 50
nm Al
2
O
3
dielectric layer by atomic layer deposition (ALD) on a commercial ITO glass
(Sheet Resistance = 20 Ω/ □) at 250
o
C. The ITO glass served as the back gate of the
transparent transistors. The As-doped In
2
O
3
nanowires were then dispersed on the
substrates as described early. After pattering source and drain electrodes by
photolithography, ITO was deposited by ion-assisted deposition (IAD) at room
temperature (R
sheet
= 60 Ω/□) with an ITO (In
2
O
3
:SnO
2
= 9:1) target purchased from
William Advanced Materials. The schematic diagram is shown in the Figure 4-3 (a), inset.
Next, the electronic parameters of as-fabricated nanowire TTFTs were examined by using
Agilent 4156B instrumentation.
4.2.3 Fabrication of As-doped In
2
O
3
Nanowire Transistors with a SAND Dielectric
After the nanowire synthesis, the as-grown nanowires were removed from the
Si/SiO
2
substrates by ultra-sonication to yield a suspension in isopropyl alcohol (IPA),
and were then dispersed onto a silicon substrate coated with SAND dielectric layer. The
process was performed repeatedly until the desired nanowire density was achieved. The
nanowire orientation is random, but with desired density. We can achieve a yield of 80%
for the transistors. The SAND dielectric films used in this study consisted of layer-by-
layer self-assembled multilayers deposited using procedures described elsewhere. The
79
SAND dielectric is composed of self-assembled multilayers that include the following
layer building blocks: (i) α,ω-difunctionalized hydrocarbon chains that block charge
transport due to their saturated structures, (ii) highly polarizable stilbazolium π-electron
layers that stabilize charge carriers in the channel with oriented dipoles, and (iii) glassy
siloxane polymer layers that planarize the surface and enhance structural robustness by
cross-linking and filling pinholes. Next, photolithography was applied to pattern both
source and drain electrodes to bridge the nanowires, followed by metal deposition of 5
nm Ti/ 45 nm Au using an e-beam evaporation. The degenerately doped silicon substrate
serves as the back gate.
4.2.4 OLED Deposition for the AMOLED Display
After the fabrication of the As-doped In
2
O
3
nanowire TTFT back-panel, the chip
was cleaned with UV/ozone for 10 min. The cleaned back-panel was then placed in the
vacuum chamber of an evaporator. All compounds were purified using temperature
gradient vacuum sublimation prior to deposition. A shadow mask was placed on the
nanowire TTFT back-panel, after which organic layers (20 nm of NPD, 25 nm of 8%
Irppy / CBP, and 400 Ǻ of BCP) were deposited by thermal evaporation from resistively
heated tantalum boats at a rate of about 2 Ǻ/s, followed by the cathode consisting 10 Ǻ of
LiF and 150 Ǻ of aluminum. The AMOLED devices were then connected with a home-
made electrical measurement jig to drive the AMOLED displays. The transmittance
spectrum reveals ~81% optical transmission of As-doped In
2
O
3
nanowire TTFT,
essentially identical to that of bare glass or glass/ITO substrates. After OLED layer
deposition, the transmittance spectrum of AMOLED display is about 35% optical
transmission, shown in Figure 4.1.
80
100
80
60
40
20
0
Transmittance (%)
800 700 600 500 400
Wavelength (nm)
Figure 4.1 Optical transmission spectra of a typical As-doped In
2
O
3
nanowire TTFT (Red
curve), and a AMOLED display substrates after transparent OLED deposition (Green
curve).
4.3 Results and Discussion
4.3.1 Characterizations of Arsenic-doped Indium Oxide Nanowires
As-doped In
2
O
3
nanowires used in this study were grown on a Si/SiO
2
substrate by
a laser ablation process. In contrast to our previous work [29] using argon mixed with
oxygen as the carrier gas, we intentionally added hydrogen to the carrying gas, to
suppress the oxidation processes and incorporate a small amount of As into the In
2
O
3
nanowires. The surface morphology and crystal structure of the As-doped In
2
O
3
nanowires was further characterized by field-emission scanning electron microscopy
(FESEM), high-resolution transmission electron microscopy (HRTEM), selected area
electron diffraction pattern (SAED), and energy dispersion X-ray spectrometry (EDS).
Figure 4.2 (a) shows a typical SEM image of as-grown As-doped In
2
O
3
nanowires which
are ~5 - 10 μm long with a diameter of 15 - 30 nm. An Au/In alloy particle, with a
diameter of ~16 nm, can be clearly observed at the very tip of the nanowire, supporting
81
growth via a so-called vapor-liquid-solid (VLS) growth mechanism (Figure 4.2 (a), inset).
In addition, the As-doped In
2
O
3
nanowires have perfectly smooth surfaces without any
amorphous coating and have extremely uniform diameters. To further evaluate the
composition of the As-doped In
2
O
3
nanowires, EDS was performed during the TEM
investigation. Figure 4.2 (b) shows a typical EDS spectrum of an individual As-doped
In
2
O
3
nanowire, which indicates the presence of As elements in In
2
O
3
nanowires. The
atomic ratio of In : As is estimated to 100 : 4. The atomic density of indium atoms in
In
2
O
3
is about 3.12×10
22
atom/cm
3
[30], and the dopant density of 4% As dopants can be
estimated to be 1.25×10
21
atom/cm
3
. However, the actual amount of As atoms
incorporated into the crystal sites of In
2
O
3
nanowires are not clear and required more
detail experiments.
The crystallography of these As-doped nanowires was also studied using HRTEM
and SAED. HRTEM confirms that each As-doped In
2
O
3
nanowire has a perfect single
crystalline structure without any noticeable dislocations or defects, and the corresponding
SAED reveals that the phase of As-doped nanowires is body-centered cubic (bcc), shown
in Figure 4.2 (c). The interspacing between each plane is 0.506 nm, corresponding to the
(200) plane in the bcc As-doped In
2
O
3
nanowire crystal structure (shown in Figure 4.2
(d)), with the lattice constant (a) of 1.012 nm slightly expanded versus that of undoped
In
2
O
3
nanowires (a = 1.01 nm) [29]. The expanded lattice constant can be attributed to
the incorporation of As into the In
2
O
3
nanowires. Taken together, these results indicate
that these nanowires grown using the present laser ablation process exhibit high
crystalline quality, with the In
2
O
3
nanowire structural integrity preserved after As dopant
incorporation.
82
Figure 4.2 (a) SEM image of As-doped indium oxide nanowires. Inset figure shows an
As-doped nanowire with a catalyst particle at the very tip (inset). (b) EDS spectrum
showing the chemical composition of the As-doped indium oxide nanowires. (c)
Corresponding HR-TEM image of a single As-doped In
2
O
3
nanowire with a diameter of
~16 nm. The electron diffraction pattern reveals a bcc crystal structure of As-doped In
2
O
3
nanowire (inset). (d) In another HR-TEM image, the (100) planes are clearly visible, and
oriented perpendicular to the vertical axis.
4.3.2 Electronic Transport Characteristics of As-doped In
2
O
3
Nanowire TTFTs
It is well-known that doping processes can substantially enhance the electrical
properties of nanowire materials in a controlled fashion, and that zinc, antimony and
arsenic are all effective n-type dopants which increase the carrier concentration in In
2
O
3
thin films [31, 32]. Here we focus on As-doped In
2
O
3
nanowires as the semiconductor in
TTFT devices. After the nanowire synthesis, the as-grown nanowires were removed from
83
the Si/SiO
2
substrates by ultra-sonication to yield a suspension in isopropyl alcohol
(IPA), and were then dispersed onto an ITO substrate with 50 nm Al
2
O
3
dielectric. The
process was performed repeatedly until the desired nanowire density was achieved and
then ITO was deposited as source and drain electrodes. The nanowire orientation is
random, but with desired density, we can achieve a yield of 80% for the transistors.
Figure 4.3 (a) shows a schematic of an As-doped In
2
O
3
nanowire TTFT on an ITO glass
substrate, with an atomic layer deposition (ALD)-derived high-k Al
2
O
3
(thickness of 50
nm, k
eff
~ 9.0) dielectric layer, and ion-assisted deposition-derived (IAD) ITO (thickness
of 100 nm, sheet resistance ~ 60 Ω/□) source and drain electrodes, with the
semiconductor-doped metal oxide nanowires as the active channel. Figure 4.3 (b)
displays the optical image of an As-doped In
2
O
3
nanowire TTFT substrate, with the
background photograph visible through the transistor regions. The transmittance
spectrum in the Figure 4.1 reveals ~81% optical transmission, essentially identical to that
of bare glass or glass/ITO substrates.
Detailed electronic transport measurements were carried out to characterize the
electronic properties of single As-doped In
2
O
3
nanowire TTFTs. The measured drain
current (I
ds
) versus source-drain voltage (V
DS
) characteristics of a representative single
nanowire TTFT is shown in Figure 4.3 (c). The channel length of the as-fabricated
TTFTs is ~1.8 μm, shown in the inset SEM image of Figure 4.3 (d) (scale bar = 1μm).
The device exhibits typical enhancement-mode n-type semiconductor transistor
behaviour with an on-current (I
on
) of ~60 μA while V
g
= 4.5 V and V
DS
= 3 V. Note that
doping helps to reduce the contact resistance in In
2
O
3
nanowire FETs, shown by the
absence of a Schottky barrier in Figure 4.3 (c).
84
Figure 4.3 Fabrication of fully transparent As-doped In
2
O
3
nanowire transistors (a)
Schematic diagram of As-doped In
2
O
3
nanowire TTFT fabricated on an ITO glass
substrate, with ALD-deposited Al
2
O
3
or SAND as the dielectric layer and IAD-deposited
ITO as source and drain electrodes. (b) Optical photograph of fully transparent As-doped
In
2
O
3
nanowire transistors. The substrate area is marked with a yellow frame for clarity.
(c) Family of I
ds
-V
DS
curves of a single As-doped In
2
O
3
nanowire TTFT with the channel
length of 1.8 μm. The gate voltage varied from -4.5 V to 4.5 V in a step of 1.5 V from
bottom to top. Inset: SEM image of an As-doped In
2
O
3
nanowire bridging ITO
electrodes. (d) Current versus gate voltage (I
ds
-V
g
) plot in the linear regime (V
DS
= 200
mV). Red, green and blue curve correspond to linear-scale I
ds
-V
g
, log scale I
ds
-V
g
, and μ,
respectively. Inset shows an SEM image of the nanowire transistor.
Figure 4.3 (d) shows the drain current (I
ds
) versus gate voltage (V
g
) characteristics
for a single transparent As-doped In
2
O
3
nanowire transistor. The doped In
2
O
3
nanowire
devices display I
on
/I
off
= 5.7×10
6
, a peak subthreshold slope (S) = 88 mV/dec, and
threshold voltage (V
T
) = 0.5 V, while V
DS
= 200 mV. The mobility ( μ) was calculated
from the maximum transconductance by applying the equations 1 and 2 [20].
2
()
DS
DS i g
dI L
device mobility
V C dV
µ= ⋅ (1)
85
where L is the channel length and C
i
is the specific capacitance of single doped nanowire
calculated as follows,
26
0
2 / ln(2 / )
is
C L hr πε ε = (2)
where, r is the radius of doped In
2
O
3
nanowires, h is the thickness of the dielectric layer ,
and ε
s
is the dielectric constant of ALD deposited Al
2
O
3
. It is found that the device
mobility varies from ~ 1,080 - 1,490 cm
2
V
-1
sec
-1
as the gate bias is increased from 1.0 V
to 3.0 V. In comparisons to other nanowire TFTs, the present mobilities are substantially
greater, with ZnO = 20 cm
2
V
-1
sec
-1
, In
2
O
3
= 35 cm
2
V
-1
sec
-1
, and SnO
2
= 15 cm
2
V
-1
sec
-1
[11]. This can be understood from the single-crystal nature of the As-doped In
2
O
3
nanowires and the formation of relatively high-quality interfaces. Note that the present
device performance rivals or exceeds that of previously reported doped and un-doped
metal oxide nanowire TTFTs [20, 21, 33, 34]. Table 4.1 summarizes the relevant data
which suggest that the As doping should significantly enhance overall nanowire device
performance. Detailed statistical study of ten single nanowire TTFTs (Figure 4.4) reveals
that the device mobility is 1,200 cm
2
V
-1
sec
-1
in average with a standard deviation of 210
cm
2
V
-1
sec
-1
, the subthreshold voltage is 170 mV/dec with a standard deviation of 90
mV/dec, and the I
on
/I
off
ratio is from 10
5
to 10
7
.
86
Figure 4.4 The device mobility, subthreshold voltage, and I
on
/I
off
ratio of 10 single
nanowire TTFTs, with red line indicating average values for the respective parameters.
Material
On-Current
( μA)
Mobility
(cm
2
/VS)
On/off ratio
Subthreshold
Slope (mV/dec)
Ref.
In 2O 3 NW < 1 98.1 2×10
4
N/A 25
In 2O 3 NW 10 514 10
6
160 19
In 2O 3 Thin film 800 120 10
5
80 12
ZnO 2 96 10
6
300 19
Ta-SnO 2 NW 20 120 10
5
270 20
Sb- SnO 2 NW 22 550 10
5
170 33
Zn- In 2O 3 NW 8.5 80 10
6
700 34
As- In 2O 3 NW 60 1,490 5.7×10
6
88 This work
Table 4.1 Summary of the electronic properties of different doped and undoped metal-
oxide material based TTFTs.
87
160
120
80
40
0
I
DS
(µΑ)
1.5 1.2 0.9 0.6 0.3 0.0
V
DS
(V)
(a)
20
15
10
5
0
I
DS
(µΑ)
5 4 3 2 1 0 -1 -2 -3 -4
V
g
(V)
10
-9
10
-8
10
-7
10
-6
10
-5
I
DS
(A)
2500
2000
1500
1000
500
0
µ
eff
(cm
2
/Vs)
(b)
Figure 4.5 (a) Family of I
ds
-V
DS
curves for a single As-doped In
2
O
3
nanowire transistor
using SAND as the dielectric layer with a channel length of 1.8 μm. The gate voltage is
varied from -2.0 V to 4.0 V in steps of 1.0 V from the bottom to top. (d) Current versus
gate voltage (I
ds
-V
g
) curves in the linear region (V
DS
= 200 mV). Red, green and blue
curves correspond to a linear-scale I
ds
-V
g
, log scale I
ds
-V
g
, and μ, respectively.
To further modify device performance, the 50 nm Al
2
O
3
gate dielectric was replaced
with an organic self-assembled nanodielectric (SAND), layer (thickness ~ 16 nm, k
eff
~5)
[35]. The mobility of a single As-doped In
2
O
3
nanowire TFT is enhanced to ~ 2,560
cm
2
V
-1
sec
-1
, for V
g
and V
DS
= 4.4 V and 200 mV, respectively. In addition, I
on
is increased
to 160 μA at V
g
= 3.0 V and V
DS
= 1.5 V, and S increased to 149 mV/dec (Figures 4.5 (a)
and Figure 4.5 (b)). These SAND effects can be attributed to: the advantageous
characteristics of the high-capacitance dipolar organic nanoscopic dielectrics [36, 37].
Similar I
on
/I
off
ratio (~10
4
) was previously reported for ZnO/SAND nanowire TFTs [38],
and may result from the gate leakage current. The gate leakage current was measured
between the source/drain pads and the back gate electrode (with overlapping area of ~
2×10
5
μm
2
) to be about 1 nA for the 16 nm SAND dielectric and 20 pA for the 50 nm
Al
2
O
3
dielectric at V
g
= 3 V. This gate leakage current for SAND dielectric sets a lower
limit for the off current that can be reliably measured, and is likely a reason for the
88
observed I
on
/I
off
ratio of 10
4
for the SAND-gated devices. We note that much of the
measured leakage current may be conduction between the source/drain pads and the back
gate electrode underneath, and thus the leakage current could be greatly reduced by using
a patterned gate electrode with minimized overlap with the source and drain electrodes.
Means to suppress them are under further investigation.
The present devices exhibit high mobilities and steep subthreshold slopes, which are
desired properties for fast device operation, such as switching transistors in AMOLED
displays. The unity-gain frequency of the nanowire TTFTs was estimated using equation
3, where (V
g
-V
T
) is the gate overvoltage (the applied gate voltage in excess of the
threshold voltage) [39].
2
()
2
gT
T
VV
f
L
µ
π
−
= (3)
In this approximation, the unity-gain frequency of our TTFTs can reach 18.8 GHz. In
addition, we also investigated the AC gain and radio-frequency response (RFR) from the
As-doped In
2
O
3
nanowire TTFTs. Figure 4.6 (a) shows the experimental set-up for AC
measurements. The nanowire TTFT is configured as common-source amplifier, and DC
offsets are applied to the gate and drain electrodes to bias it in saturation, near the
maximum g
m
(~1.8 μS). Figure 4.6 (b) shows the RFR of the device measured by
applying digital signal processing (DSP) to the time domain signals (Blue line), simulated
data for the as-fabricated TTFT (Red line), and simulated data for an optimized device
structure (Green line). A gain of ~6 dB can be observed at low frequency with a roll-off
89
around 500 Hz and a cut-off frequency (f
-3 dB
) of ~1.5 kHz, which is comparable to earlier
work.
Figure 4.6 (a) The measurement setup showing As-doped In
2
O
3
nanowire transistor
configured as a common-source amplifier. (b) Frequency response of AC gain of As-
doped In
2
O
3
nanowire TTFT. Solid blue line shows the measured frequency response of
As-doped In
2
O
3
nanowire TTFT, dashed red line represents the simulated data of As-
doped In
2
O
3
nanowire TTFT. The green solid line shows simulation data for an optimized
As-doped In
2
O
3
nanowire TTFT. (c) Bilateral small-signal model used in this study.
To understand the frequency response of our As-doped In
2
O
3
nanowire TTFTs, a
bilateral small-signal model
39
(shown in Figure 4.6 (c)) is applied to model the AC
characteristics of the present TTFTs. The simulated result (red line) agrees well with the
measured data, suggesting that the operation frequency is strongly influenced by the
parasitic capacitance associated with the source and drain contact electrodes (~ 157 pF)
and the probes (~ 100 pF). By optimizing the device dimensions and structure (i.e., use
short channel widths and reduce overlaps between source and drain contact electrodes),
and adopting an active probe (~ 1 MΩ) for the measurements, the cut-off frequency (f
-3
90
dB
) can be increased to 400 MHz, shown in the Figure 4.6 (b) (green line). The simulated
results therefore suggest that As-doped In
2
O
3
nanowire TTFTs offer great potential in
high frequency device applications. Further optimization of the design toward functional,
transparent integrated circuits is in progress.
4.3.3 Fully Transparent OLED Driving Circuitry
Figure 4.7 (a) Plot of the output current through the loaded OLED (I
OLED
) versus V
in
with
V
dd
at 5.0 V in linear scale (red line) and log scale (blue line), respectively. Inset: Circuit
diagram of an OLED driven by a transparent As-doped In
2
O
3
nanowire transistor. (b)
OLED light intensity versus V
in
with V
dd
= 5.0 V. Inset: OLED spectrum. (c) Optical
images of the OLED under V
in
= -3.0 V, 0.0 V, and 3.0 V.
The ability to fabricate high performance As-doped In
2
O
3
nanowire TTFTs enabled
further exploration of transparent circuit applications. Controlling a variable-intensity
OLED was selected as the next target. Here a monochrome OLED is wire-bonded on a
91
breadboard together with an As-doped In
2
O
3
nanowire TTFT chip (Figure 4.7). The inset
of Figure 4.7 (a) shows the circuit diagram of the experimental setup, where one TTFT is
connected to an external OLED, and V
dd
is applied to drain of the transistor. By
controlling V
in
that provides a gate voltage for the transistor with fixed V
dd
, the voltage
drop across the OLED can be controlled. Figure 3a shows the current flowing through the
OLED, which is successfully modulated via V
in
by a factor of ~300, thus controlling the
OLED light intensity, as shown in Figure 4.7 (b). OLED light intensity below 1 Cd/m
2
is
usually defined as the off-state and above 1 Cd/m
2
as the on-state [40].
Although the
present off-state: on-state light intensity ratio is ~5, the effect is clearly visible in Figure
4.7 (c) where the OLED is operated with V
in
= -3 (left), 0 (middle), and 3 (right) V,
respectively.
4.3.4 AMOLED Display and Drive Circuitry
The discussion above demonstrates the high-performance characteristics of As-
doped In
2
O
3
nanowire/SAND TTFTs, including high device mobility and fast device
operation, can be employed as switching and driving transistors in an AMOLED display
circuitry. Here, we go one step further to fabricate an AMOLED display using high-
performance As-doped In
2
O
3
TTFTs based on our earlier collaborative results on
AMOLED display fabrication together with our co-operators [41]. Figure 4.8 (a) shows
the equivalent circuit diagram of seven segment AMOLED display circuitry. Note that
the driving circuit of each OLED pixel consists of one switching transistor (T1), one
driving transistor (T2), and one storage capacitor (C
st
). T1 is employed to select a
specified pixel and transfer data through the data line to the OLED. T2 is employed to
control the current supplied to the OLED pixel, which is adjusted by controlling the V
g
of
92
T2, equal to the voltage difference between the ends of C
st
. The storage capacitor is
employed to store data during one period for time-varying operations. The EL opening
area of each segment is ~0.18 mm
2
.
AMOLED display fabrication begins by patterning the OLED ITO anode and
individually addressed bottom gate electrodes by photolithography and wet etching. Next,
50 nm ALD Al
2
O
3
is deposited on the patterned ITO gate and OLED anodes. Following
Al
2
O
3
deposition, contact holes above the OLED anodes, as bottom gate electrode
contacts for each pixel, are fabricated through a wet etching process. After that, a
suspension of As-doped In
2
O
3
nanowires in 2-propanol alcohol is dispersed on the device
substrate. A thin layer of source and drain electrodes (20 nm Al) are then deposited by e-
beam evaporation and patterned by lift-off. Following source and drain electrode
patterning, a 200 nm thick SiO
2
layer is deposited by e-beam evaporation to passivate the
device and planarize the nanowire transistors for OLED fabrication.
Figure 4.8 (b) shows an optical image of a 1” × 1” inch AMOLED display substrate
before OLED deposition, with a background image visible through the display region.
The optical transmittance spectrum (Figure 4-1) reveals that the optical transmittance
values of the AMOLED display before and after OLED layer deposition are ~ 81% and ~
35% in the visible region, respectively. The optical transmittance should be readily
increased using a more transparent OLED design. The display consists of 12 seven-
segment OLED pixels, 84 OLED unit pixels, and 156 nanowire TTFTs. For each seven-
segment OLED pixel, the scan lines for all unit pixels are individually controlled, as are
the data lines. Figure 4.8 (c) shows optical images of the seven segment pixels with
93
different numerical digits at different data line voltages (V
data
= -5 to 5 V) with different
scan line voltages (V
scan
= -5 to 5 V) and fixed V
dd
(= 3 V). To the best of our knowledge,
this is the first demonstration of a seven-segment AMOLED display driven entirely by
TTFT circuits.
Figure 4.8 An application of As-doped In
2
O
3
nanowire TTFT circuitry to drive a seven
segment AMOLED display. (a) Schematic of a seven-segment digit of AMOLED pixel,
which consist of one switching transistor (T
1
), one driving transistor (T
2
), and one storage
capacitor (C
st
). The bias condition to operate the circuit are -5 V to 5 V of scan line to full
turn-off and turn-on, varying -5 V to 5 V on the data line, 3 V on the V
dd
line, and 0 V on
the cathode line. (b) Optical photograph of fully transparent AMOLED display before
OLED layer deposition with the substrate area marked with a yellow frame for clarity.
The feature on the background picture is clearly visible. (c) Optical photograph of
AMOLED animation. The seven segment digit displays number 1, 3, and 6, respectively.
94
4.4 Conclusion
In summary, we have demonstrated the great potential of As-doped In
2
O
3
nanowires
for high-performance transparent electronics. With the aid of arsenic dopants and a self-
assembled gate nanodielectric, As-doped In
2
O
3
nanowire TTFTs with transparent ITO
contacts show good transparency and excellent transistor performance such as high
mobility, high on/off ratio, low operation voltage, and steep subthreshold slope. A
saturation device mobility of 1,490 cm
2
V
-1
s
-1
is achieved on glass substrates, which is the
highest TTFT device mobility reported so far. We further examined the AC gain from a
single As-doped In
2
O
3
nanowire TTFT, and the results indicate good frequency response
and a unity-gain frequency of 18.8 GHz. In addition, the TTFTs were further utilized to
construct a transparent circuit and used to control a variable-intensity OLED. Moreover,
an AMOLED display with good transparency was also fabricated which generates
numerical displays. Our results suggest that As-doped In
2
O
3
nanowires have great
potential to serve as building blocks for future transparent electronics.
95
Chapter 4 References
1. Kawazoe H, Yasukawa M, Hyodo H, Kurita M, Yanagi H, Hosono H, Kawaoe H.
P-type electrical conduction in transparent thin films of CuAlO
2
. Nature 1997;
389 (6654): 939-942.
2. Hirao T, Furuta M, Furuta H, Matsuda T, Hiramatsu T, Hokari H, Yoshida M.
High mobility top-gate zinc oxide thin-film transistors (ZnO-TFTs) for active-
matrix liquid crystal displays. SID 06 Digit Digest 2006; 18-20.
3. Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosono H. Thin-film
transistor fabricated in single-crystalline transparent oxide semiconductor. Science
2003; 300 (5623): 1269-1272.
4. Chae J, Appasamy S, Jian K. Patterning of indium tin oxide by projection
photoablation and lift-off process for fabrication of flat-panel displays. Applied
Physics Letters 2007; 90 (26): 261102-261104.
5. Rogers J A, Bao Z, Baldwin K, Dodabalapur A, Crone B, Raju V R, Kuck V , Katz
H, Amundson K, Ewing J, Drzaic P. Paper-like electronic displays: large-area
rubber-stamped plastic sheets of electronics and microencapsulated
electrophoretic inks. Proceedings of the National Academy of Sciences of the
United States of America 2001; 98 (9): 4835-4840.
6. Oregan G, Gratzel M. A Low-cost, high-efficiency solar-cell based on dye-
sensitized colloidal TiO
2
films. Nature 1991; 353 (6346): 737-740.
7. Minami T. New N-type transparent conducting oxides. MRS Bulletin 2000; 25:
38-44.
8. Samant S, Gopal A. Study of a prototype high quantum efficiency thick
scintillation crystal video-electronic portal imaging device. Medical Physics 2006;
33 (8): 2783-2791.
9. Fortunato E, Barquinha P, Goncalves A, Marques A, Pereria L, Martins R. Recent
advances in zno transparent thin film transistors. Thin Solid Films 2005; 487 (1-2):
205-211.
10. Soci C, Zhang A, Xiang B, Dayeh S, Aplin D P R, Park J, Bao X, Lo Y, Wang D.
ZnO nanowire UV photodetectors with high internal gain. Nano Letters 2007; 7
(4): 1003-1009.
11. Carcia P F, McLean R S, Reilly M H, Nunes G Jr. Transparent ZnO thin-film
transistor fabricated by rf magnetron sputtering. Applied Physics Letters 2003; 82
(7): 1117-1119.
96
12. Wang L, Yoon M H, Lu G, Yang Y, Facchetti A, Marks T J. High-performance
transparent inorganic-organic hybrid thin-film N-type transistors. Nature
Materials 2006; 5 (11): 893-900.
13. Philip J, Punnoose A, Kim B I, Reddy K M, Layne S, Holmes J O, Satpati B,
LeClair P R, Santos T S, Moodera J S. Carrier-controlled ferromagnetismin
transparent oxide semiconductors. Nature Materials 2006; 5 (4): 298-304.
14. Hoffman R L, Norris B J, Wager J F. ZnO-based transparent thin-film transistors.
Applied Physics Letters 2003; 82 (5): 733-735.
15. Ueda K, Hase T, Yanagi H, Kawazoe H, Hosono H, Ohta H, Orita M, Hirano M.
Epitaxial growth of transparent P-type conducting CuGaO2 thin films on sapphire
(001) substrates by pulsed laser deposition. Journal of Applied Physics 2001; 89
(3): 1790-1793.
16. Inoue S, Ueda K, Hosono H, Hamada N. electronic structure of the transparent P-
type semiconductor (LaO)CuS. Physical Review B 2001; 64 (24): 245211-245215.
17. Chiang H Q, Hong D, Hung C M, Presley R E, Wager J F, Park C H, Keszler D A,
Herman G S. Thin-film transistors with amorphous indium gallium oxide channel
layers. Journal of Vacuum Science B 2006; 24 (6): 2702-2705.
18. Gruner G. Carbon nanotube films for transparent and plastic electronics. Journal
of Materials Chemistry 2006; 16 (35): 3533-3539.
19. Ju S Y, Facchetti A, Xuan Y, Liu J, Ishikawa F, Ye P D, Zhou C W, Marks T J,
Janes D B. Fabrication of fully transparent nanowire transistors for transparent
and flexible electronics. Nature Nanotechnology 2007; 2 (6): 378-384.
20. Dattoli E N, Wan Q, Guo W, Chen Y B, Pan X Q, Lu W. Fully transparent thin-
film transistor devices based on SnO
2
nanowires. Nano Letters 2007; 7 (8): 2463-
2469.
21. Ishikawa F N, Chang H K, Ryu K, Chen P C, Badmaev A, De Arco L G, Shen G Z,
Zhou C W. Transparent electronics based on transfer printed aligned carbon
nanotubes on rigid and flexible substrates. ACS Nano 2009; 3 (1): 73-79.
22. Zhang W P, Zu R D, Wang Z L. Nanobelts of semiconducting oxides. Science
2001; 291 (5510): 1947-1949.
23. Duan X, Huang Y, Cui, Wang J, Lieber C M. Indium phosphide nanowires as
building blocks for nanoscale electronic and optoelectronic devices. Nature 1998;
409: 66-69.
97
24. Huang M H, Mao S, Feick H, Yan H Q, Wu Y Y , Kind H, Weber E, Russo R, Yang,
P D. Room-temperature ultraviolet nanowire nanolasers. Science 2001; 292
(5523): 1897-1899.
25. Zhang D H, Li C, Han S, Liu X L, Tang T, Jin W, Zhou C W. Electronic transport
studies of single-crystalline In
2
O
3
nanowires. Applied Physics Letters 2003; 82 (1):
112-114.
26. Ju S, Ishikawa F N, Chen P C, Chang H K, Zhou C W, Ha Y G, Liu J, Faccheti A,
Marks T J, Janes D B. High performance In
2
O
3
nanowire transistors using organic
gate nanodielectrics. Applied Physics Letters 2008; 92 (9): 222105-222107.
27. Xiang B, Wang P, Zhang X, Dayeh S A, Aplin D P R, Soci C, Yu D, Wang D.
Rational synthesis of P-type zinc oxide nanowire arrays using simple chemical
vapor deposition. Nano Letters 2007; 7 (2): 323-328.
28. Wager J F, Keszler D A, Presley R E. Transparent electronics. Springer Science +
Business Media, LLC 2008, New York., U.S.A.
29. Li C, Zhang D, Han S, Liu X, Tang T, Zhou C. Diameter-controlled growth of
single-crystalline In
2
O
3
nanowires and their electronic properties. Advanced
Materials 2003; 15 (2): 143-146.
30. Asikanen T, Ritala M, Li W M, Lappalainen R, Leskela M. Modifying ALE
grown In
2
O
3
films by Benzoly Fluoride Pulses. Applied Surface Science 1977;
112: 231-235.
31. Lee C H, Kuo C V, Lee C L. Effects of heat treatment and ion doping of indium
oxide. Thin Solid Film 1989; 173 (1): 59-66.
32. Tahar R B H, Ban T, Ohya Y, Takahashi Y. Tin doped indium oxide thin films:
electrical properties. Journal of Applied Physics 1998; 83 (5): 2631-2645.
33. Wan Q, Dattoli E N, Lu W. Doping dependent electrical characteristics of SnO
2
nanowires. Small 2008; 4 (4): 451-454.
34. Zhang W, Jie J, He Z, Tao S, Fan X, Zhou Y, Yuan G, Luo L, Zhang W, Lee C S,
Lee S T. Single zinc-doped indium oxide nanowire as driving transistor for
organic light-emitting diode. Applied Physics Letters 2008; 92 (15): 153312-
153314.
35. DiBenedetto S, Facchetti A, Ratner M A, Marks T J. Molecular self-assembled
monolayers and multilayers as gate dielectrics for organic thin film transistor
applications. Advanced Materials, 2009; 21 (14-15): 1407-1433.
98
36. Ju S, Lee K, Janes D B, Yoon M Y, Facchetti A, Marks T J. Nanowire field-effect
transistors enabled by self-assembled organic gate nanodielectrics. Nano Letters
2005; 5 (11): 2281-2286.
37. Yoon M Y, Facchetti A, Marks T J. σ - π Molecular dielectric multilayers for low-
voltage organic thin-film transistors. Proceedings of the National Academy of
Sciences 2005; 102 (13): 4678-4682.
38. Ju S, Lee K, Yoon M H, Facchetti A, Marks T J, Janes D B. High-performance
zno nanowire field-effect transistors with organic nanodielectrics: effects of metal
contacts and ozone treatment. Nanotechnology 2007; 18 (15): 155201-155211.
39. Akinwande D, Close G F, Wong H S P. Analysis of the frequency response of
carbon nanotube transistors. IEEE Transactions on Nanotechnology 2006; 5 (5):
599-605.
40. Kafafi Z H. Organic electroluminescence. Taylor & Francis Group 2005. Boca
Raton, U.S.A.
41. Ju S, Li J, Liu J, Chen P C, Ha Y , Ishikawa F, Chang H, Zhou C, Faccheti A, Janes
D B, Marks T J. Transparent active matrix organic light-emitting diode displays
driven by nanowire transistor circuitry. Nano Letters 2008; 8 (4): 997-1004.
99
Chapter 5 Flexible and Transparent Supercapacitor based on In
2
O
3
Nanowire/
Carbon Nanotube Heterogeneous Films
In this chapter, a supercapacitor with the features of optical transparency and
mechanical flexibility has been fabricated using metal oxide nanowire / carbon nanotube
heterogeneous film, and studies found that the power density can reach 7.48 kW/kg after
galvanostatic measurements. In addition, to study the stability of flexible and transparent
supercapacitor, the device was examined for a large number of cycles and showed a good
retention of capacity (~ 88%). This approach could work as the platform for future
transparent and flexible nanoelectronics.
5.1 Introduction
There has been great interest recently in both flexible and transparent electronics
such as transparent and flexible active matrix organic light-emitting diode (AMOLED)
display, which may find applications in heads-up display, automobile wind-shield display,
and conformable products [1].
However, to realize fully transparent and/or flexible
devices, one may also consider making transparent and/or flexible energy conversion and
storage units with high energy storage and power density. Electrochemical capacitor
(supercapacitor), with properties of high energy storage, small size, and lightweight, has
become one of the best candidates of energy storage devices [2-5]. Though some
supercapacitors built on carbon nanotubes (CNTs) have been reported [6-9], the
performance of these supercapacitors (so-called electrical double-layer capacitors
(EDLCs)) is usually not as good as redox supercapacitors made of metal oxide materials
(ex. RuO
2
, MnO
2
, IrO
2
) [10, 11]. On the other hand, these supercapacitors are usually
100
neither transparent nor flexible, which greatly limit their real applications in flexible or
transparent electronics. It is still of great interest to develop transparent and flexible
supercapacitors.
In this paper, we report a prototype of high performance flexible and transparent
electrochemical supercapacitors based on metal oxide nanowire / carbon nanotube
heterogeneous films. The typical supercapacitor structure is made of a polymer
electrolyte layer sandwiched between two transparent and flexible nanowire / nanotube
film electrodes. The device structure includes the following features. First of all, we
chose metal oxide nanowires dispersed on CNT films as active materials. Owing to their
unique properties of high aspect ratio and short diffusion path length to ions, metal oxide
nanowires can provide high surface area, fast charge/discharge, and facial redox reactions
and thus can be one of good candidates for electrochemical capacitors. Here, In
2
O
3
nanowires are used as an example, and the general concept can be applied to many other
nanowires. Secondly, we used transparent flexible polymer membrane with nonaqueous
electrolyte to work as a separator and electrolyte. The third feature is that we optimized
the nanowire / nanotube film thickness for mechanical flexibility and optical transparency,
which allowed us to achieve flexible and transparent supercapacitors based on In
2
O
3
nanowire / CNT heterogeneous films.
5.2 Experiments
Hybrid In
2
O
3
nanowires / CNT films were prepared as follows. First, CNT films
were fabricated by vacuum filtration method following previous report [12-14].
In brief,
to make an uniform CNT film, a CNT suspension (0.0025 mg/ml carbon nanotubes (arc-
101
discharge P3 nanotubes, Carbon Solutions Inc.) with 1 wt% aqueous sodium dodecyl
sulfate (SDS)) was filtered through a porous alumina filtration membrane (pore size: 200
nm, Whatman). As the solvent went through the membrane, the CNTs were trapped on
the membrane surface thus forming a homogenous entangled network. An adhesive and
flat poly (dimethysiloxane) (PDMS) stamp was adapted to peel the CNT film off of the
filtration membrane and then released it onto a polyethylene terephtalate (PET) substrate
on hotplate at 100 ℃. The SEM image of a transferred CNT film on PET substrate is
shown in Figure 5.1(a). The thickness of the film is roughly 60 nm and the conductivity is
~ 570 S/cm.
Figure 5.1 SEM images of CNT films, scale bar: 200 nm (a), PEDOT: CNT films, scale
bar: 200 nm (b), and In
2
O
3
nanowire / CNT heterogeneous film, scale bar: 2 μm (c).
In
2
O
3
nanowires with a diameter of ~ 20 nm and a length of ~ 5 μm were
synthesized by a pulsed laser deposition (PLD) method [15]. The detail can be found in
early reports. The as-grown nanowires were sonicated into IPA solutions and then
dispersed upon transferred CNT films to form In
2
O
3
nanowire /CNT heterogeneous film
(Figure 5.1 (c)) for transparent and flexible supercapacitor study. For comparison, we
also fabricated supercapacitors using transferred CNT films and poly (3,4-
ethylenedioxythiophene) (PEDOT) / CNT hybrid films. The PEDOT: CNT films were
made by spin-coating of a PEDOT film with a thickness of ~ 10 nm. The SEM image
102
shown in Fig 1(b) exhibited an uniform PEDOT :CNT film with a the scale bar of 200
nm.
5.3 Results and Discussion
Figure 5.2 (a) shows a photograph of a supercapacitor made of transferred CNT
films, which gives mechanical flexibility and optical transparency. A typical AFM image
of a transferred CNT film on a PET substrate is shown in Figure 5.2 (b). It can be seen
that dense and homogenous networks of entangled CNTs are formed. The schematic
diagram of the supercapacitor in this work is depicted in Figure 5.2 (c). The device was
composed of two transparent electrodes (In
2
O
3
nanowire / CNT heterogeneous films on
PET substrates) sandwiched with a Nafion 117 membrane. Here, the Nafion 117
membrane served as a transparent spacer and nonaqueous 1 M LiClO
4
(LiClO
4
in a
mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethylene carbonate
(DMC), in a volume ratio of EC/DEC/DMC=1:1:1) was adapted as electrolyte in this
study. The transparency of the supercapacitors built on In
2
O
3
nanowires/ CNT
heterogeneous films, transferred CNT films, and PEDOT: CNTs films was measured and
the results are shown in Figure 5.2 (d). As is well-known, In
2
O
3
is a wide band gap
material (3.57 eV) which has been widely applied in the transparent electronics. With an
existence of 30 wt% In
2
O
3
nanowires above CNT films, the transmittance of a In
2
O
3
nanowires/CNT heterogeneous film supercapacitor exhibits merely 3-5% degradation in
whole spectral region in comparisons of a transferred CNT film supercapacitor and a
PEDOT: CNT film supercapacitor. The transparency of the PEDOT: CNT film
103
supercapacitor (green line) is almost the same as a transferred CNT film supercapacitor
(brown line) over a spectral range from 300 nm to 800 nm.
Figure 5.2 (a) Photograph of a flexible and transparent supercapacitor fabricated using
CNT films. (b) AFM image of entangled CNT networks sitting on a transparent PET
substrate. (c) Schematic diagram of a flexible and transparent supercapacitor. The gray
color represents a Nafion film as separator between two In
2
O
3
nanowires/CNT
heterogeneous film electrodes. (d) Transmittance spectra of three different
electrochemical capacitors and a single CNT film.
The electrochemical properties and capacitance measurements of supercapacitors
were studied in a two-electrode system by cyclic voltammetry (CV) and galvanostatic
(GV) charge-discharge measurements using a Potentiostat/Galvanostat (Princeton
Applied Research 273A). Figure 5.3 (a) shows the results of typical CV measurements of
three different electrochemical capacitors. The scan range is between 0.2 V and -0.6 V
104
with a scan rate of 20 mV/sec. The capacitive behaviors of three different devices are
clearly observed in this figure with near rectangular-shaped voltammograms and large
CV currents. Specific capacitance of a supercapacitor can be obtained through the
following equation [6]:
1
( / ) ( )
i
CF g
vm
= , (1)
where ν is the scan rate, i is the corresponding current of the applied voltage, and m is
the weight of the active electrodes. The calculated specific capacitance of transferred
CNT film supercapacitor is about 25.4 F/g and PEDOT: CNT film supercapacitor is 33
F/g. These results are in a good agreement with the previously reported result from highly
dense CNT network [6] and 85% CNT / 15% PEDOT supercapacitors [16] in aqueous
electrolyte. In addition, the specific capacitance of PEDOT: CNT film supercapacitor is
much better than a supercapacitor made of PEDOT-PSS carbon nanotube fibers (6.25 F/g)
in nonaqueous electrolyte [17]. This can be attributed to larger surface to volume ratio of
CNTs than CNT fibers (diameter: 50-500 nm), which improves the influence of different
morphology. The effect of PEDOT coated over CNT films might result in a higher
electric conductivity and provide more accessible area of CNT networks [17, 18]. In
addition, as one can see in Figure 5.3 (a), the In
2
O
3
nanowire /CNT heterogeneous film
supercapacitor exhibits an even higher specific capacitance (64 F/g) than both transferred
CNT film and PEDOT: CNT film supercapacitors. We have further prepared and
measured supercapacitors with various In
2
O
3
nanowire quantity. With the increasing
amount of In
2
O
3
nanowires dispersed upon CNT films, the specific capacitance of the
heterogeneous supercapacitor can be dramatically improved up to 64 F/g (0.007 mg of
In
2
O
3
nanowire), which can be clearly observed in Figure 5.3 (b). This can be understood
105
since the pseudocapacitance from the reversible redox transitions of adhesive In
2
O
3
nanowires contributes to the overall capacitance [19, 20].
The result of GV measurements of In
2
O
3
nanowire / CNT heterogeneous film
supercapacitor is presented in Fig 4 (a). The charge/discharge experiment was carried out
at a constant current of 0.5 A/g and a high operation voltage of 2.0 V. The slope of the
discharge curve can be used to determine the specific capacitance by using the equation
below [2],
12
11
()
/
sp
I
C
dV dt m m
= + , (2)
where I represents the discharge current, dV/dt is the slope of the discharge curve, and m
is the weight per electrode of the active material. The specific capacitance, for this device
shown, is about 64 F/g confirming qualitatively the results from the CV measurements. In
addition, the specific energy density (W=CV
2
/2, where V is the potential) is 1.29 Wh/kg
and power density ( P = W/ Δ t, Δt is the discharge time) is 7.48 kW/kg. These values reveal
that the performance of In
2
O
3
nanowire / CNT film supercapacitor is competitive to early
reported work [3] in terms of high power density and energy density. Besides, similar
observation with other early literatures, IR (Internal Resistance) drop was observed in
each branch of the entire curves in Figure 5.3 (a) because of the equivalent series
resistance of the In
2
O
3
nanowire / CNT film electrodes, the nonaqueous electrolyte and
the contact resistance between the electrodes. Through using different nanostructured
materials (ex. Au/CNT core-shell structures [21]) or right combination of active
materials/electrodes and electrolyte, one can minimize the internal resistance and thus
106
improve the device performance in terms of higher specific capacitance and power
density.
Figure 5.3 (a) Cyclic voltammograms (CVs) of a CNT film supercapacitor, a
PEDOT:CNT film supercapacitor, and a 30 wt% In
2
O
3
nanowires on CNT film
supercapacitors. (b) Specific capacitance vs. different weight of In
2
O
3
nanowires. The
inserts are SEM images with different weight of In
2
O
3
nanowires. The scale bars in SEM
pictures represent 2 μm.
In order to study the stability of In
2
O
3
nanowire / CNT heterogeneous film
supercapacitor, the device was examined by GV measurements for a large number of
cycles. The cycling process was performed at the same as above-mentioned conditions
for 500 cycles. Figure 5.4 (b) reveals the variation of specific capacitance as a function of
cycle number. After first 100 cycles, there is a little specific capacitance decrease from 64
F/g to 53 F/g. Then, the specific capacitance retains the same values and minimal fade up
to 500 cycles. The specific capacitance fading of In
2
O
3
nanowire / CNT heterogeneous
film supercapacitors probably can be attributed to the dissociation of In
2
O
3
nanowire
during the redox process in GV measurements, which is also reported in the literatures
for other metal oxide nanostructured materials [20, 22].
More works need to be done in
107
clarifying the mechanism. In comparisons to supercapacitors made by other transition
metal oxide nanostructured materials [22, 23], this observation indicates a good stability
of In
2
O
3
nanowire / CNT heterogeneous films for long-term capacitor applications.
Figure 5.4 (a) charge-discharge behavior of a In
2
O
3
nanowires/CNT heterogeneous film
electrochemical capacitor in 1M LiClO
4
electrolyte. (b) Cycle-life data of In
2
O
3
nanowires/CNT heterogeneous film electrochemical capacitor. Specific capacitance was
calculated form GV measurements at constant current of 0.5 A/g.
108
5.4 Conclusion
In summary, we made a prototype of flexible and transparent supercapacitors built
on In
2
O
3
nanowires / CNT heterogeneous films. Studies found that the performance of
CNT heterogeneous film supercapacitors can be significantly improved by dispersion of
In
2
O
3
nanowires on CNT films due to the redox transitions of In
2
O
3
nanowires without
degradation in transparency. Enhanced specific capacitance, power density, energy
density, and long operation cycles have been realized with the incorporation of In
2
O
3
nanowires. Extensive studies are underway to further improve the device performance
and fill the gap of practical applications.
109
Chapter 5 References
1. Ju S, Li J, Liu J, Chen P, Ha Y, Ishikawa I N, Chang H, Zhou C, Facchetti A,
Janes D B, Marks T. J. Transparent active matrix organic light-emitting diode
displays driven by nanowire transistor circuitry. Nano Letters 2008; 8 (4): 997-
1004.
2. Pushparaj V L, Shaijumon M M, Kumar A, Murugesan S, Ci L, Vajtai R,
Linhardt J R, Nalamasu O, P M. AjayanP M. Flexible energy storage devices
based on nanocomposite paper. Proceedings of the National Academy of
Sciences of the United States of America 2007; 104 (34): 13574-13577.
3. Arico A S, Bruce P, Scrosati B, Tarascon J M, Schalkwijk W V. Nanostructured
materials for advanced energy conversion and storage devices. Nature Materials
2005; 4 (5): 366-377.
4. Burke A. Ultracapacitors: why, how, and where is the technology 2000. Journal
of Power Sources 2000; 91 (1): 37-51.
5. Conway B E. Electrochemical Capacitors. Kluwer Academic Publishers. New
York (1999).
6. Kaempgen M, Ma J., Gruner G, Wee G, Mhaisalkar S G. Bifunctional carbon
nanotube networks for supercapacitors. Applied Physics Letters 2007; 90 (26):
264104-264106.
7. Futaba D N, Hata K, Yamada T, Hiraoka T, Hayamizu Y , Kakudate Y , Tanaike O,
Hatori H, Yumura M, Iijima S. Nature Materials 2006; 5 (12): 987-994.
8. Che G L, Lakshmi B B, Fisher E R, Martin C R. Carbon nanotubule membranes
for electrochemical energy storage and production. Nature 1998; 393 (6683):
346-349.
9. Niu C, Sichel E K, Hoch R, Moy D, Tennent H. High power electrochemical
capacitors based on carbon nanotube electrodes. Applied Physics Letters 1997;
70 (11): 1480-1482.
10. Hu C, Chang K, Lin M, Wu Y . Design and tailoring of the nanotubular arrayed
architecture of hydrous RuO2 for next generation supercapacitors. Nano Letters
2006; 6 (12): 2690-2695.
11. Subramanian V, Zhu H, Vajtai R, Ajayan P M, Wei B. Hydrothermal synthesis
and pseudocapacitance properties of MnO2 nanostructures. Journal of Physical
Chemistry B 2005; 109 (43): 20207-20214.
110
12. Wu Z, Chen Z, Du X, Logan J M, Sippel J, Nikolou M, Kamaras K, Reynolds J
R, Tanner D B, Hebard A F, Rinzler A G. Transparent, conductive carbon
nanotube films. Science 2004; 305 (5688): 1273-1276.
13. Zhang D, Ryu K, Liu X, Polikarpov E, Ly J, Tompson M E, Zhou C. Transparent,
conductive, and flexible carbon nanotube films and their application in organic
light-emitting diodes. Nano Letters 2006; 6 (9): 1880-1886.
14. Zhou Y, Hu L, Gruner G. A method of printing carbon nanotube thin films.
Applied Physics Letters 2006; 88 (12): 123109-123111.
15. Li C, Zhang D, Han S, Liu X, Tang T, Zhou C. Diameter-controlled growth of
single-crystalline In2O3 nanowires and their electronic properties. Advanced
Materials 2003; 15 (2): 143-146.
16. Lota K, Khomenko V, Frackowiak E. Capacitance properties of poly(3,4-
ethylenedioxythiophene)/carbon nanotubes composites. Journal of Physics and
Chemistry of Solids 2004; 65 (2-3): 295-301.
17. Gallegos A K C, Rincon M E. Carbon nanofiber and PEDOT-PSS bilayer
systems as electrodes for symmetric and asymmetric electrochemical capacitor
cells. Journal of Power Sources 2006; 162 (1): 743-747.
18. Liu R, Lee S B. MnO2/Poly(3,4-ethylenedioxythiophene) coaxial nanowires by
one-step coelectrodeposition for electrochemical energy storage. Journal of the
American Chemical Society 2008; 130 (10): 2942-2493.
19. Chang J, Lee W, Mane R S, Cho B W, Han S. Morphology-dependent
electrochemical supercapacitor properties of indium oxide. Electrochemical and
Solid-State Letters 2008; 11 (1): A9-A11.
20. Prasad K R, Koga K, Miura N. Electrochemical deposition of nanostructured
indium oxide: High-performance electrode material for redox supercapacitors.
Chemistry of Materials 2004; 16 (10): 1845-1847.
21. Shaijumon M M, Ou F S, Ci L, Ajayan P M. Synthesis of hybrid nanowire arrays
and their application as high power supercapacitor electrodes. Chemical
Communications 2008; 20: 2373-2375.
22. Ragupathy P, Vasan H N, Munichandraiah N. Cobalt hydroxide as a capacitor
material: Tuning its potential window. Journal of Electrochemistry Society 2008;
155 (11): A855-A861.
23. Chang J, Hsu S, Tsai W, Sun I. A novel electrochemical process to prepare a
high-porosity manganese oxide electrode with promising pseudocapacitive
performance . Journal of Power Sources 2008; 177(2): 676-680.
111
Chapter 6 Inkjet Printing of Single Walled Carbon Nanotube / RuO
2
Nanowire
Supercapacitors on Cloth Fabrics and Flexible Substrates
In this chapter, single walled carbon nanotube (SWNT) thin film electrodes were
printed on flexible substrates and cloth fabrics by using SWNT inks and an off-the-shelf
inkjet printer, with features of controlled pattern geometry (0.4 cm
2
~ 6 cm
2
), location,
controllable thickness (20 nm ~ 200 nm), and tunable electrical condcutivity. The as-
printed SWNT films were then sandwiched together with a piece of printable polymer
electrolyte to form flexible and wearable supercapacitors, which displayed good
capacitive behavior even after 1,000 charging/discharging cycles. Furthermore, a simple
and efficient route to produce ruthenium oxide (RuO
2
) nanowire / SWNT hybrid films
has been developed, and studies found that the knee frequency of hybrid thin film
electrodes can reach 1,500 Hz, which is much higher than the knee frequency of our bare
SWNT electrodes (~ 158 Hz). In addition, with the integration of RuO
2
nanowires, the
printed SWNT supercapacitor performance were significantly improved in terms of the
specific capacitance of 138 F/g, power density of 96 kW/kg, and energy density of 18.8
Wh/kg. The results indicate the potential of printable energy storage devices and the
significant promise in applications of wearable energy storage devices.
6.1 Introduction
Recently, due to the limited availability of fossil fuels and the development of
hybrid electrical vehicles, there has been an increasing demand for next generation high-
power energy sources [1-4]. However, to provide the peak power, conventional charge
devices, such as batteries, need to be bulky and heavy, which are not suitable for the next
112
generation of portable electronic devices, with the requirements of light weight, small
thickness, and good flexibility [5-8]. Consequently, electrochemical capacitors (so-called
supercapacitors), with the advantages of high power density (> 1-10 kW/kg), high energy
density (0.5-10 Wh/kg), high cycling ability (>10,000), and light weight, have attracted
enormous interest and are considered to be one of the most promising energy conversion
and storage devices to fulfill future energy storage needs [8-10]. Usually, according to the
charge storage mechanism, supercapacitors can be divided into two different categories,
including electrical double layer capacitors (EDLCs) and redox supercapacitors
(pseudocapacitors) [11]. For EDLCs, carbonaceous materials, such as activated carbons,
carbon fibers, aerogels, and nanostructured carbon materials, have been widely reported
and applied in commercial products [12-14].
The high accessible surface area (~ 430 m
2
/g), low electrical resistivity (0.016
Ohm-cm), uniform pore size distribution (average < 2 nm), and high knee frequency (~
100 Hz) of single-walled carbon nanotubes (SWNTs) render them as one of the most
attractive materials in carbonaceous materials [14, 15]. For instance, Gruner et al.
adapted SWNT bulky paper as EDLC electrodes with a specific capacitance of 39 F/g
and a power density of 5.8 kW/kg in 1M H
2
SO
4
electrolyte [16]. An et al. have reported a
measured power density of 20 kW/kg and an energy density of 7 Wh/kg in a solution of
7.5 N KOH, using a heat-treated arc-discharge SWNT network [15]. In addition,
“printable energy storage devices”, which can be eventually achieved simply by using
large-scale, solution-based, and roll-to-roll printing, can be one of the most important
solutions [17].
Recent success examples include using a spray method to produce SWNT
thin film electrodes and supercapacitors [18], and using the Meyer rod coating method to
113
spread carbon nanotubes on paper for both supercapacitors and lithium ion batteries [19].
While the above-mentioned methods typically produces continuous films of carbon
nanotubes with no control over geometry and position, inkjet printing methods would
provide the capability of printing carbon nanotube films in controlled geometries and at
specific locations [20-22]. However, there has been little report on the electrochemical
characteristics of inkjet-printed SWNT films and their applications in electrochemical
supercapacitors.
Here, we report supercapacitors based on inkjet printing of SWNT films obtained
by using an off-the-shelf inkjet printer to print SWNT inks onto different substrates,
including flexible substrates and cloth fabrics. In the present report, the inkjet printing
method not only provides a noncontact deposition method for obtaining SWNT films, but
also allows us to readily control the pattern geometry, location, electrical conductivity,
film thickness, and uniformity. In order to further improve the device performance, we
combined ruthenium oxide (RuO
2
) nanowires with printed SWNT films, where the RuO
2
nanowires were synthesized using a chemical vapor deposition (CVD) method. Owing to
its unique properties, such as metallic conductivity, intrinsic reversibility of surface redox
reactions, and ultrahigh pseudocapacitance, RuO
2
has become one of the most promising
electrode materials for electrochemical capacitors [23-25]. As we proposed in the early
work [26], instead of using only SWNT films, we integrated RuO
2
nanowires together
with SWNT films and fabricated RuO
2
nanowires / printed SWNT hybrid films.
Importantly, the printed supercapacitors can be fully integrated with the fabrication
process of current printed electronics.
114
6.2 Experiments
6.2.1 Preparation of SWNT Inks
The fabrication of inkjet-printed SWNT supercapacitors starts with the
functionalization of carbon nanotubes [27]. Arc-discharge nanotubes (P3 nanotubes from
Carbon Solutions Inc.) were mixed with 1 wt% aqueous sodium dodecyl sulfate (SDS) in
D. I. water to make a highly dense SWNT suspension with a typical concentration of 0.2
mg/mL. The addition of SDS surfactant further improves the solubility of SWNTs by
sidewall functionalization. The as-prepared SWNT solution was then ultrasonically
agitated using a probe sonicator for ~ 20 min with the intensity of 200 Watts, followed by
centrifugation to separate out undissolved SWNT bundles and impurities. It was
important to prevent nozzle clogging during the printing due the flocculation of long
SWNTs in a solution. Thus a moderate SWNT length (500 nm ~ 1.5 μm) was adapted in
this study.
6.2.2 Synthesis of RuO
2
Nanowires
Ruthenium oxide RuO
2
nanowires were synthesized using a thermal chemical vapor
deposition (CVD) system [33]. A 5 nm gold film was deposited on Si/SiO
2
substrates as
catalysts using an e-beam evaporator, followed by annealing at 700
o
C for 30 minutes.
The substrate was then placed into a quartz tube at the down stream end of a furnace,
while stoichiometric RuO
2
powders (Sigma-Aldrich 99.999%, metal basis) were utilized
as precursor and placed at the center of the furnace. During the growth, the quartz tube
was maintained at a pressure of 10 Torr and a temperature of 960 °C, with a constant
flow of 100 standard cubic centimetres (sccm). The typical reaction time was about 3-4
115
hours. After cooling down, the samples were characterized using field emission scanning
electron microscope (FESEM) and x-ray diffractometer (XRD).
Figure 6.1 (a) shows a SEM image of RuO
2
nanowires with a typical diameter of ~
100-200 nm and length of 5-10 μm in average. The XRD pattern of as-synthesized RuO
2
nanowires, shown in Figure 6.1 (b), is confirmed by the characteristic diffraction peaks of
the RuO2 (110), (101), (200), (211), and (002) planes at 2θ values of about 28.2° , 34.6° ,
40.2° , 53.7° , and 60.2°, respectively (JCPDS 40-1290). The diffraction pattern also
reveals the phase of nanowires is rutile-structured RuO
2
, with lattice constant values of a
= 0.45 nm and c = 0.31 nm.
Figure 6.1 (a) SEM image and (b) XRD pattern of RuO
2
nanowires.
6.2.3 Device Fabrication
A commercial Epson piezoelectric printer (Artisan 50) with a resolution of 1,440 ×
1,440 dots per inch (dpi) was used in this study. For printing, the obtained SDS-
functionalized SWNT inks were loaded into cleaned Epson T078120 (black) ink
cartridges through a syringe and allowed to equilibrate for several minutes before printing
was performed. Pattern designed in Epson Print CD were printed onto transparent
(a)
(b)
116
poly(ethylene terephthalate) (PET) sheets, cloth fabrics, and SiO
2
/Si substrates. The
printed film thickness was determined from topographical analysis of the films by using
atomic force microscopy (AFM) (Digital Instruments, Dimension 3100). The mass of the
SWNTs deposited on each substrate was determined by weighing the substrates before
and after printing. The gel electrolyte was prepared by mixing poly(vinyl alcohol) (PVA)
powder with water (1g of PVA/ 10 mL of D.I. water) and 2 mL phosphoric acid (H
3
PO
4
)
[18]. Upon evaporation of excess water in a vacuum oven at 60
o
C, the gel electrolyte
solidified. The solid PVA/H
3
PO
4
electrolyte functioned as both the separator between
two SWNT electrodes and the electrolyte for ion transportation.
6.3 Results and Discussion
6.3.1 Characterizations of Inkjet Printed SWNT Films
Figure 6.2 (a) shows the SEM image of a SWNT film with multiple prints (× 190)
onto a piece of cloth fabric. The photograph of a printed pattern of 1 inch
2
is shown in
Figure 6.3 (a) and (b). As the dispensed ink dried, the nanotubes formed tangled, rather
dense, and homogeneous networks on the surface of each fiber with typical bundle length
of ~ 0.2-1.8 μm and diameter of 9-20 nm, which can be easily observed in Figure 6.2 (b)
and Figure 6.2 (c). Afterward, two as-printed SWNT films on cloth fabric were used as
thin film electrodes without any further treatment, and sandwiched together with a
polymer electrolyte to form an electrochemical capacitor, as shown in Figure 6.3 (c). The
inset photograph shows a real inkjet-printed supercapacitor wrapped on a pencil.
117
Figure 6.2 (a), (b), and (c) SEM images of a fabric with printed SWNT films.
Figure 6.3 Photograph of cloth fabric before (a) and after (b) inkjet printing of SWNT films. (c)
Schematic diagram of an printed SWNT supercapacitor using PVA/H
3
PO
4
as separator and
electrolyte. The inset shows a supercapacitor made of SWNT/ fabric rolled around a pencil.
In addition, we also printed SWNTs on a 4 inch PET sheet with different pattern
geometry (0.4 cm
2
~ 6 cm
2
), location, and print number (× 40, × 80, × 120, and × 200),
shown in Figure 6.4 (a). Electrically conductive SWNT patterns could be achieved
merely through multiple prints over the same pattern, and the optical transmittance is
about 80% in the visible light region ( λ~ 400 nm to 700 nm, minimum of 20 repetitions
on PET substrates). As one can see in Figure 6.4 (b), just like the printed SWNT films on
cloth fabric, the inkjet-printed SWNT films on a PET substrate also exhibits tangled and
randomly oriented networks on surface, and can be fabricated as an electrochemical
capacitor without any further treatment. Our inkjet-printed method exhibits many
advantages such as ability of controlling pattern geometry, pattern location, film
(a)
(b) (c)
118
thickness, electrical conductivity, and optical transparency. In addition, our method is
compatible with other substrates, which could open up pathways towards realizing
wearable electronics.
Figure 6.4 (a) Photograph of a PET substrate with SWNT films printed with different size
and number of repeated prints. The feature on the background picture is clearly visible. (b)
A SEM image of printed SWNT films on a PET substrate and a photograph of a
supercapacitor built with SWNT/PET substrates.
To assess the electrical conductivity and the optical transparency of printed
SWNT films, we have performed both four-probe dc measurements and transmittance
measurements on inkjet-printed SWNT films with different film thickness, shown in
Figure 6.5 (a). With each successive inkjet printing, the nanotube film thickness (t)
increased, thereby increasing the conductivity from 0.54 S/cm (t = 20 nm) to 1,562 S/cm
(t = 200 nm). The improved conductivity can be attributed to the better percolation of the
deposited SWNTs which improves the number of electrical pathways. However, the
increased printed thickness resulted in more light being absorbed, thereby reducing the
optical transparency from 80% (t = 20 nm) down to 12% (t = 200 nm) in the visible light
region, as shown in Figure 6.5 (b).
(a) (b)
119
The sheet resistance (R
s
) of our inkjet-printed SWNT films (78 Ohm/ with a
thickness of 0.2 μm) compares favorably with previous reported work (R
s
= 40k Ohm/
with × 90 prints for Ref. 21, and 100k Ohm/ for Ref. 20). We note that the high
conductivity of printed nanotube films is important for studying the electrochemical
behavior of printed SWNT films and supercapacitors.
Figure 6.5 (a) Conductance and transmittance of the printed SWNT films on PET
substrates as a function of different film thickness. (b) Optical transmittance spectra of
printed SWNT films on PET substrates with different printed thickness from 20 nm to
200 nm.
6.3.2 Electrochemical Behaviors of Inkjet Printed SWNT Supercapacitors
The printed SWNT films on PET substrate (SWNT/PET) used for the fabrication
of supercapacitors were typically printed for a number of 200 times, and had a sheet
resistance of 78 Ohm/ with a thickness of 0.2 μm and an optical transparency of ~10%
in average. For SWNT films printed on cloth fabrics (SWNT/fabric) with similar print
numbers (× 200 prints), the sheet resistance is usually about 815 Ohm/ . As mentioned
earlier, Two inkjet-printed SWNT film electrodes and a gel polymer electrolyte were
sandwiched together to form an electrochemical energy storage device (shown in Figure
6.3 (c) and Figure 6.4 (b)). Cyclic Voltammetery (CV) measurements were carried out to
(b) (a)
120
evaluate the stability of the electrochemical cells under the voltage range from 0 V to 1 V.
Galvanostatic (GV) charge/discharge measurements (0-1 V) were used to evaluate the
specific capacitance (C
sp
), power density, and the internal resistance (IR) of the devices in
a two-electrode configuration.
Figure 6.6 Cyclic voltammetery of inkjet-printed SWNT supercapacitors on PET
substrates (a) and cloth fabrics (b) with PVA/H
3
PO
4
polymer electrolyte with different
scan rates of 20, 50, and 100 mV/second. Galvanostatic charge/discharge curves
measured with a 1 mA/mg current density for inkjet-printed SWNT supercapacitor on
PET substrates (c) and cloth fabrics (d).
Figure 6.6 (a) shows the CV curves of a SWNT/PET supercapacitor, with
different scan rates of 20, 50, and 100 mV/sec, which showed good electrochemical
stability and capacitive behavior in printed SWNT thin film electrodes with a gel polymer
electrolyte. The quasi-rectangular shape of these curves can be attributed to the presence
of 3-6% carboxylic acid group (-COOH) attached on the sidewall of nanotubes, which
showed a signature of pseudocapacitance in the region of 0.2 V [28]. The
1.0
0.8
0.6
0.4
0.2
0.0
Potential (V)
200 150 100 50 0
Time (sec)
1.0
0.8
0.6
0.4
0.2
0.0
Potential (V)
220 165 110 55 0
Time (sec)
-2
-1
0
1
2
Current (A/g)
1.0 0.8 0.6 0.4 0.2 0.0
Potential (V)
PET
-1.0
0.0
1.0
Current (A/g)
1.0 0.8 0.6 0.4 0.2 0.0
Potential (V)
Fabric
PET Fabric
(a)
(b)
(c) (d)
IR drop
121
pseudocapacitive behavior was further confirmed by the impedance measurements to be
discussed below. For SWNTs/fabric supercapacitors, the CV curves (shown in Figure 6.6
(b)) obtained were also quite regular and of rectangular shape, similar to SWNT/PET
supercapacitors, but with a smaller current density due to the higher sheet resistance of
the SWNT films printed on cloth fabrics. The increased sheet resistance of SWNTs/fabric
might be caused by the reduced percolation of SWNT coating due to the fiber nature of
the fabrics, which can be observed in Figure 6.2 (b).
A typical GV charging/discharging behavior of our SWNT/PET supercapacitor,
with a charging/discharging current density of 1 mA/mg, is presented in Figure 6.6 (c).
As one can easily observe, the charging/discharging curves show good capacitive
behavior and the voltage drop of our device is rather small (~ 0.05 V), which is
comparable with early reported work [16, 18]. For SWNT/fabric supercapacitors, the GV
charging/discharging measurement also exhibited good capacitive behavior, shown in
Figure 6.6 (d). However, the voltage drop (~ 0.22 V) is much higher than the voltage
drop of SWNT/PET supercapacitors. As we discussed early, it can be attributed to the
high sheet resistance of SWNT films printed on cloth fabric. The specific capacitance
was calculated from the charge/discharge curves, according to the following equation
below [8, 26, 29],
12
11
( )( )
/
sp
I
C
dV dt m m
= +
−
(1)
where I is the applied discharging current, m
1
and m
2
are the mass of each electrode, and
dV/dt is the slope of the of discharge curve after voltage drop. The specific capacitance of
122
SWNT/PET and SWNTs/fabric supercapacitor is about 65 F/g and 60 F/g, respectively.
Besides, the power density (P) can be obtained using the following equation [14],
2
4
V
P
RM
= (2)
where V is the applied voltage, R is the equivalent series resistance (ESR), and M is the
total mass of the printed SWNT film electrode. The measured power density of
SWNT/PET and SWNT/fabric supercapacitor is about 4.5 kW/kg and 3.0 kW/kg,
respectively. The specific energy density (E
sp
) of our devices were calculated following
using E
sp
= 0.5 C
sp
V
2
. The calculated specific energy density is about 8.2 Wh/kg and 6.1
Wh/kg for SWNT/PET and SWNT/fabric supercapacitors, respectively.
6.3.3 Electrochemical Behaviors of Inkjet Printed SWNT Films
To determine the frequency response and the ESR of inkjet-printed SWNT thin film
electrodes, electrochemical impedance spectroscopy (EIS) measurements were performed.
The measurements were carried at a dc bias voltage of 0 V, with a 10 mV amplitude
sinusoidal signal, using a Gamry Reference 600 potentiostat/galvanostat in 1 M Na
2
SO
4
electrolyte. The Nyquist plot of the multiple printed SWNT film electrodes (× 200) is
shown in Figure 6.7, which shows that the imaginary part of impedance sharply increases
at lower frequency, confirming the capacitive behavior of printed SWNT films. The
presence of a semicircle arises from the double-layer capacitance coupled with a Faradaic
reaction resistance and a series resistance of the solution in contact with printed SWNT
films, which confirms the redox reaction happens as below [28].
C OH C O H e
CO e CO
+−
−−
> − ⇔> = + +
>= + ⇔>=
(3)
123
Besides, the impedance curve intersects the real axis (Re (Z)) at a 45
o
angle, which is
consistent with the porous nature of the electrode when saturated with electrolyte.
30, 31
The knee frequency of the printed SWNT films is about 158 Hz, which suggests that
most of its stored energy is accessible at frequency below 158 Hz.
Figure 6.7 Electrochemical impedance spectroscopy at 0.1 V bias voltage on a
supercapacitor built with SWNT/PET electrodes in 1 M Na
2
SO
4
electrolyte.
In addition, to investigate the relationship of the ERS and power density of our
printed SWNT thin film electrodes, we performed EIS measurements on samples with
different thickness (40 nm, 80 nm, 0.1 μm, 0.17 μm, and 0.2 μm) of printed SWNT films
in 1 M Na
2
SO
4
electrolyte. The ESR of printed SWNT thin film electrodes usually can be
extracted from the high frequency part of EIS curves [32]. For instance, the ESR of a 0.2
μm SWNT films is about 90.7 Ohm, which can be observed in Figure 6.7. Figure 6.8
shows the ESR and the power density as a function of printed SWNT film thickness. With
increasing film thickness, the ESR gradually decreases, whereas the power density
1600
1200
800
400
2000 1500 1000 500 0
Re (Z) / Ohm
0.8 Hz
158 Hz
-Im (Z) / Ohm
124
increases and saturates at the thickness of 0.2 μm. A 0.2 μm SWNT film shows a power
density of 22.3 kW/kg, which is comparable with that of CNT bulky paper [16].
Figure 6.8 Equivalent series resistance (ESR) and Power density of SWNT/PET
supercapacitors as a function of printed thickness.
To evaluate the stability of printed SWNT thin film supercapacitors, we carried
out long charging/discharging measurements using printed SWNT/PET supercapacitors
in polymer electrolyte as an example. The values of specific capacitance with respect to
charging/discharging cycle number were measured (up to 1,000 cycles), and the results
indicate that the specific capacitance of our supercapacitor maintains good stability
without any noticeable decreasing of capacitance after 1,000 cycles, shown in Figure 6.9.
(b)
10
1
10
2
10
3
10
4
10
5
ESR (Ohm)
200 150 100 50 0
Thickness (nm)
25
20
15
10
5
0
Power Density (kW/kg)
125
Figure 6.9 Specific Capacitance of a SWNT/PET supercapacitor plotted v.s. number of
cycles.
The above results and analyses have summarized the reliable capacitive ability of
our inkjet-printed SWNT supercapacitors, in terms of good capacitive behavior and
stability after a number of charging/discharging cycles. However, there is still a lot of
room for further improvement, such as high ESR and low energy density. As we
illustrated in the early work [26], we provided a simple and efficient solution of
fabricating hybrid nanostructured electrodes with the integration of metal oxide
nanowires and carbon nanotube films, which significantly improved the device
performance due to the pseudocapacitance contributed from metal oxide nanowires.
Herein, instead of using In
2
O
3
nanowires, we synthesized RuO
2
nanowires via a thermal
CVD method [33] and integrated nanowires with printed SWNT films. Figure 6.1 (a)
shows the SEM image of RuO
2
nanowires with a typical diameter of ~ 100-200 nm and
length of 5-10 μm. The as-synthesized RuO
2
nanowires were sonicated into isopropanol
70
60
50
40
30
20
10
0
Specific Capacitance (F/g)
1000 800 600 400 200 0
Number of Cycle
126
alcohol (IPA) to form a nanowire suspension and then dispersed on the printed SWNT
films on a PET substrate until a reasonable nanowire density was achieved. A RuO
2
nanowire network can be clearly observed in Figure 6.10 (a). Some RuO
2
flakes from the
nanowire synthesis can also be found on the sample surface. The inset figurer reveals the
density of dispersed RuO
2
nanowires is about ~ 6 nanowires/μm, and the SWNT film can
be clearly observed underneath RuO
2
nanowires.
6.3.4 Electrochemical Behaviors of RuO
2
/SWNT Supercapacitors
Two RuO
2
nanowire/SWNT hybrid films on PET substrates were then
sandwiched together with a PVA/H
3
PO
4
polymer electrolyte to form an electrochemical
cell. A typical CV behavior of the hybrid supercapacitor are shown in Figure 6.10 (b),
with different scan rates of 50 mV/sec, 100 mV/sec, 200 mV/sec, 300 mV/sec, and 500
mV/sec. The curves displayed a quasi-rectangular shape with a higher current density
than printed SWNT film supercapacitors (Figure 6.6 (a) and Figure 6.6(b)), which can be
attributed to the low ESR of the hybrid RuO
2
nanowires/printed SWNT films. The shape
of these CV curves is also different compared to the printed SWNT supercapacitors,
which can be due to the pseudocapacitance contributed from RuO
2
nanowires through the
following electrochemical protonation [34].
22
() RuO H e RuO OH
δδ
δδ
+−
−
+ +→ (1 0) δ ≥≥ (4)
To evaluate the performance of supercapacitors built on RuO
2
nanowires/SWNT
heterogeneous films, GV charging/discharging experiments were performed with a
charging/discharging current of 8 mA/mg. The results (shown in Figure 6.10 (c))
exhibited a negligible IR drop of ~0.02 V, a specific capacitance of 135 F/g, a power
density of 96 kW/kg, and an energy density of 18.8 Wh/kg. The performance of our
127
supercapacitors is close to the early reported work using 15 wt.% RuO
2
nanoparticles
decorated CNT films
35
and RuO
2
.xH
2
O/CNT composites as supercapacitor electrodes
[36].
Figure 6.10 (d) illustrates the results of impedance spectroscopy on the bare SWNT
films and the RuO
2
nanowires/inkjet-printed SWNT films in 0.3 M H
2
SO
4
solution, at a
dc bias voltage of 0 V, with a 10 mV amplitude sinusoidal signal. In comparison to the
bare SWNT film electrodes, the Nyquist plot of the RuO
2
nanowire/inkjet-printed SWNT
film electrodes show that the imaginary part of impedance sharply increases at lower
frequency, which indicates that SWNT films retain their electron-transfer capability with
the integration of RuO
2
nanowires [37]. It can be seen that the diameters of semicircle in
the Nyquist plot of the RuO
2
nanowire/SWNT hybrid films is smaller than that of the
bare SWNT films, which means the electrochemical reaction on the electrode/electrolyte
interface of RuO
2
nanowire/SWNT hybrid films is more facile than bare printed SWNT
thin film electrodes [38]. In addition, from the point intersecting with the real axis in the
range of high frequency (10 kHz), the ESR of the RuO
2
nanowires/printed SWNT film
electrodes (21.86 Ohm) is lower than that of the bare SWNT film electrodes (43 Ohm),
showing that the integration of RuO
2
nanowires with SWNT film electrodes increases the
conductivity of printed SWNT film electrodes. According to Equation 2, the RuO
2
nanowire/SWNT hybrid films are expected to possess higher power density and better
rate behavior than SWNT thin film electrodes in H
2
SO
4
electrolyte. The knee frequency
of RuO
2
nanowire/inkjet-printed SWNT is about 1.5 kHz, which is much higher than the
knee free frequency of printed SWNT film electrodes (~158 Hz).
128
Figure 6.10 (a) SEM image of RuO
2
nanowires dispersed on an inkjet-printed SWNT
film. The inset shows the sample at higher magnification. (b) Cyclic voltammetery of
RuO
2
nanowire / inkjet-printed SWNT supercapacitors on PET substrates in PVA/H
3
PO
4
polymer electrolyte with different scan rates of 50, 100, 200, 300, and 500 mV/second.
(c) Galvanostatic charge/discharge curves measured with a 1 mA/mg current density for a
RuO
2
nanowires / inkjet-printed SWNT supercapacitor. (d) Electrochemical impedance
spectroscopy for a SWNT/PET supercapacitor (green curve) and a RuO
2
nanowire /
inkjet-printed SWNT supercapacitor (red curve) in 0.3 M H
2
SO
4
electrolyte.
6.4 Conclusion
In summary, we have produced SWNT thin film electrodes on different
substrates by simply using SWNT inks and a commercial inkjet printer, with good control
over pattern geometry, pattern location, film thickness, electrical conductivity, and
optical transparency. The as-fabricated printed SWNT supercapacitors exhibited good
capacitive behavior and stability after long charging/discharging cycles, which revealed
the potential in the applications of wearable energy storage devices. In addition, we have
10 µm
1 µm
(a) (b)
(c) (d)
1.0
0.8
0.6
0.4
0.2
Potential (V)
200 150 100 50 0
Time (sec)
15.0
7.5
0.0
-7.5
-15.0
Current (A/g)
1.0 0.8 0.6 0.4 0.2 0.0
Potential (V)
800
600
400
200
-Im (Z) / Ohm
1000 800 600 400 200 0
Re (Z) /Ohm
0.6 Hz
315 Hz
158 Hz
1.5 kHz
SWNTs
RuO /SWNT
2
129
developed a simple and efficient method to produce hybrid nanostructured electrodes via
the integration of RuO
2
nanowires and printed SWNT films. The supercapacitors built on
RuO
2
nanowire/printed SWNT hybrid thin film electrodes displayed an enhanced device
performance, in aspects of specific capacitance of 138 F/g, power density of 96 kW/kg,
and energy density of 18.8 Wh/kg. Our results suggest that printable electrochemical
capacitors hold significant promise in applications of wearable energy storage devices,
and can be fully integrated with the fabrication process of current printed electronics.
130
Chapter 6 References
1. Winter M, Brodd R J. What are batteries, fuel cells, and supercapacitors.
Chemical Reviews 2004; 104 (10): 4245-4269.
2. Long J W, Dunn B, Rolison D R, White H S. Three-dimensional battery
architecture. Chemical Reviews 2004; 104 (10): 4463-4492.
3. Kang K, Meng Y S, Bréger J, Grey C P, Ceder G. Electrodes with high power and
high capacity for rechargeable lithium batteries. Science 2006; 311 (5763): 977-
980.
4. Yoon J, Baca A J, Park S I, Elvikis P, Geddes III J B, Li L, Kim R H, Xiao J,
Wang S, Kim T H, Motala M J, Ahn B Y, Duoss E B, Lewis J A, Nuzzo R G,
Ferreira P M, Huang Y, Rockett A, Rogers J A. Ultrathin silicon solar microcells
for semitransparent, mechanically flexible and microconcentrator module designs.
Nature Materials 2008; 7 (11): 907-915.
5. Liu J, Cao G, Yang Z, Wang D, Dubois D, Zhou X, Graff G L, Pederson L R,
Zhang J G. Oriented nanostructures for energy conversion and storage.
ChemSusChem 2008; 1 (8-9): 676-697.
6. Pasquier A D, Plitz I, Menocal S, Amatucci G. A comprehensive study of Li-ion
battery, supercapacitor, and nonaqueous asymmetric hybrid devices for
automotive applications. Journal of Power Sources 2003; 115 (1): 171-178.
7. Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials
2008; 7 (11): 845-854.
8. Pushparaj V L, Shaijumon M M, Kumar A, Murugesan S, Ci L, Vajtai R,
Linhardt R J, Nalamasu O, Ajayan P M. Flexible energy storage devices based on
nanocomposite paper. Proceeding of the National Academy Sciences of the
United States 2007; 104 (34): 13574-13577.
9. Arico A S, Bruce P, Scrosati B, Tarascon T M, Schalkwijk W V. Nanostructured
materials for advanced energy conversion and storage devices. Nature Materials
2005; 4 (5): 366-377.
10. Futaba D N, Hata K, Yamada T, Hiraoka T, Hayamizu Y, Kakudate Y, Tanaike O,
Hatori H, Yumura M, Iijima S. Shape-engineerable and highly densely packed
single-walled carbon nanotubes and their application as super-capacitor electrode.
Nature Materials 2006; 5 (12): 987-994.
11. Conway B E. Electrochemical supercapacitors – Scientific fundamentals and
technological applications. Kluwer Academic / Plenum Publishers 1999, New
York, U.S.A.
131
12. Frackowiak E. Carbon materials for supercapacitor application. Physical
Chemistry Chemical Physics 2007; 9 (15): 1774–1785.
13. Frackowiak E, Beguin F. Carbon materials for the electrochemical storage of
energy in capacitors. Carbon 2001; 39 (6): 937-950.
14. Zhang L L, Zhao X S. Carbon-based materials as supercapacitor electrodes.
Chemical Society Reviews 2009; 38 (9): 2520-2531.
15. An K H, Kim W S, Park Y S, Choi Y C, Lee S M, Chung D C, Bae D J, Lim S C,
Lee Y H. Supercapacitors using single-walled carbon nanotube electrodes.
Advanced Materials 2001; 13 (7): 497-500.
16. Kaempgen M, Ma J, Gruner G, Wee G, Mhaisalker S G. Bifuncational carbon
nanotube networks for supercapacitors. Applied Physics Letters 2007; 90 (26):
264104-264106.
17. Kiebele A, Gruner G. Carbon nanotube based battery architecture. Applied
Physics Letters 2007; 91 (14): 144104-144106.
18. Kaempgen M, Chan C K, Ma J, Cui Y, Gruner G. Printable thin film
supercapacitors using single-walled carbon nanotubes. Nano Letters 2009; 9 (8):
1919-1923.
19. Hu L, Choi J W, Yang Y, Jeong S, Mantia F L, Cui L F, Cui Y. Highly conductive
paper for energy-storage devices. Proceeding of the National Academy of
Sciences of the United Sates 2009; 106 (51): 21490-21494.
20. Small W R, Panhuis M I H. Inkjet printing of transparent, electrically conducting
single-walled carbon-nanotube composites. Small 2007; 3 (9): 1500-1509.
21. Kords K, Mustonen T, Tth G, Jantunen H, Lajunen M, Soldano C, Talapatra S,
Kar S, Vajtai R, Ajayan P M. Inkjet printing of electrically conductive patterns of
carbon nanotubes. Small 2006; 2 (8-9): 1021-1025.
22. Song J W, Kim J, Yoon Y H, Choi B S, Kim J H, Han C S. Inkjet printing of
single-walled carbon nanotubes and electrical characterization of the line pattern.
Nanotechnology 2008; 19 (9): 95702-95707.
23. Hu C C, Chang K H, Lin M C, Wu Y T. Design and tailoring of the nanotubular
arrayed architecture of hydrous RuO
2
for next generation supercapacitors. Nano
Letters 2006; 6 (12): 2690-2695.
24. Ardizzone A, Fregonara G, Trasatti S. “Inner” and “outer” active surface of RuO
2
electrodes. Electrochema Acta 1990; 35 (1): 263-267.
132
25. Trasatti S. Physical electrochemistry of ceramic oxides. Electrochema Acta 1991;
36 (2): 225-241.
26. Chen P C, Shen G, Sukcharoenchoke S, Zhou C. Applied Physics Letters 2009; 94
(4): 43113-43115.
27. Zhang D, Ryu K, Liu X, Polikarpov E, Ly J, Tompson M E, Zhou C. Transparent,
conductive, and flexible carbon nanotube films and their application in organic
light-emitting diodes. Nano Letters 2006; 6 (9): 1880-1886.
28. Frackowiak E, Metenier K, Bertagna V, Beguin F. Supercapacitor electrodes from
multiwalled carbon nanotubes. Applied Physics Letters 2000; 77 (15): 2421-2423.
29. Shaijumon M M, Ou F S, Ci L, Ajayan P M. Synthesis of hybrid nanowire arrays
and their application as high power supercapacitor electrodes. Chemical
Communications 2008; 20: 2373-2375.
30. Barsoukov E, Macdonald J R. Impedance spectroscopy theory, experiment, and
applications. John Wiley & Sons, Inc. 2005, Hoboken, New Jersey, U.S.A.
31. Niu C, Sichel E K, Hoch R, Moy D, Tennent H. High power electrochemical
capacitors based on carbon nanotube electrodes. Applied Physics Letters 1997; 70
(11): 1480-1482.
32. Khomenko V, Raymundo-Pieñro E, Béguin F. Optimization of an asymmetric
manganese oxide /activated carbon capacitor working at 2V in aqueous medium. J.
of Power Sources 2006; 153 (1): 183-190.
33. Liu Y L, Wu Z Y, Lin K J, Huang J J, Lin Y H, Jian W B, Lin J J. Growth of
single-crystalline RuO
2
nanowires with one- and two- nanocontact electrical
characterizations. Applied Physics Letters 2007; 90 (1): 13105-13107.
34. Ramani M, Haran B S, White R E, Popov B N. Synthesis and characterization of
hydrous ruthenium oxide-carbon supercapacitors. Journal of Electrochemistry
Society 2001; 148 (4): A374-380.
35. Sun Z, Liu Z, Han B, Miao S, Du J, Miao Z. Microstructural and electrochemical
characterization of RuO2/CNT composite synthesized in supercritical diethyl
amine. Carbon 2006; 44 (5): 888-893.
36. Qin X, Durbach S, Wu G T. Electrochemical characterization on
RuO
2
xH
2
O/carbon nanotubes composite electrodes for high energy density
supercapacitors. Carbon 2004; 42 (2): 451-453.
37. Ye J S, Cui H F, Liu Z, Lim T M, Zhang W D, Sheu F S. Preparation and
characterization of aligned carbon nanotube-ruthenium oxide nanocomposites for
supercapacitors. Small 2005; 1 (5): 560-565.
133
Chapter 7 Preparation and Characterization of Flexible Asymmetric
Supercapacitors Based on Transition-Metal-Oxide Nanowire / Single-Walled
Carbon Nanotube Hybrid Thin Film Electrodes
In this chapter, we have successfully fabricated flexible asymmetric supercapacitors
(ASCs) based on transition-metal-oxide nanowire / single-walled carbon nanotube
(SWNT) hybrid thin film electrodes. These hybrid nanostructured films, with advantages
of mechanical flexibility, uniform layered structures, and mesoporous surface
morphology, were produced by using a filtration method. Here, manganese dioxide
nanowire / SWNT hybrid films worked as the positive electrode and indium oxide
nanowire / SWNT hybrid films served as the negative electrode in a designed ASC. In
our design, charges can be stored not only via electrochemical double layer capacitance
from SWNT films, but also through reversible faradic process from transition-metal-
oxide nanowires. In addition, to obtain stable electrochemical behavior during
charging/discharging cycles in a 2 V potential window, the mass balance between two
electrodes has been optimized. Our optimized hybrid-nanostructured ASC exhibited a
superior device performance with specific capacitance of 184 F/g, energy density of 25.5
Wh/kg, and columbic efficiency of ~ 90%. In addition, our ASCs exhibited a power
density of 50.3 kW/kg, which is 10-fold higher than early reported ASC work. The high-
performance hybrid-nanostructured ASCs can find applications in conformal electrics,
portable electronics, and electrical vehicles.
7.1 Introduction
Electrochemical capacitors (so-called supercapacitors), with desirable properties of
134
high power density (10 times more power than batteries), fast charging (with seconds),
excellent cycling stability, small size, and light weight, have become one of the most
promising candidates for next-generation power devices [1-4]. Besides, with the
complementary characteristics to rechargeable batteries and fuel cells, supercapacitors
have been used in many applications such as power back-up, pacemakers, air bags, and
electrical vehicles [5-7]. Currently, most of commercial supercapacitors are made of
high-surface-area carbonaceous materials typically with a specific capacitance of ~ 4 F/g,
a power density of 3-4 kW/kg, and an energy density of 3-4 Wh/kg in both aqueous
electrolyte and organic electrolyte [8]. However, these supercapacitors might not provide
sufficient energy / power densities or efficiencies to drive low-emission hybrid cars and
trucks.
Therefore, the challenge for current supercapacitor technology is to improve the
energy density without sacrificing the power density and the cycle life. According to the
equation of
2
1
2
E CV = , obviously, the attempt to improve the energy density can be
realized by maximizing the device capacitance (C) and/or the cell voltage (V) [9]. An
efficient way to increase the cell voltage is to use organic electrolytes, since organic
electrolytes usually can provide a wider voltage window with better electrochemical
stability than aqueous electrolytes. For instance, tetraethylammonium tetrafluoroborate
(TEABF
4
) in acetonitrile (AN) has been used as an electrolyte in supercapacitors, which
allows the supercapacitors to be charged/discharged up to 2.0 V or 2.3 V [10]. However,
organic electrolytes are usually more expensive and less conductive than aqueous
electrolytes (σ
LiPF6:EC:DEC
~ 10 mS/cm; σ
1M Na2SO4
> 100 mS/cm), which result in low
135
specific capacitance and high equivalent series resistance in supercapacitors, and
preclude supercapacitors to reach high power density. In addition, AN is not
environmentally-friendly and can cause a toxic effect to human organs, which is against
the requirement of “green electrolyte” for next-generation power devices. In this regard,
aqueous electrolyte-based supercapacitors are more attractive because of highly-safety,
low-cost, and environmental-friendly.
An alternative approach to improve energy density is to develop hybrid
electrochemical capacitors (so-called asymmetric supercapacitors) [11-13], which can
also provide a wider operating potential window compared to symmetric supercapacitors.
For an asymmetric supercapacitor, the active material used in one electrode is usually
different from the other electrode in a cell system. For instance, ruthenium oxide (RuO
2
)
can be used as the positive electrode [14], while activated carbon (AC) can work as the
negative electrode in an asymmetric supercapacitor [15]. Moreover, asymmetric
supercapacitors can make use of the different potential windows of the two electrodes to
increase the maximum operation voltage of the aqueous electrolyte in the cell system,
which results in an improved specific capacitance and energy density. In comparisons to
conventional electrical double layer capacitors (EDLCs), Qu et al. reported to use δ -
manganese dioxide as the positive electrode and AC as the negative electrode in an
asymmetric supercapacitor, with operation voltage of 1.8 V, energy density of 28.4
Wh/kg, and power density of 150 W/kg in an aqueous electrolyte [16]. Due to the
improved energy density, attention has been paid to such transition-metal-oxide / AC
asymmetric supercapacitors. Asides from that, Yuan et.al presented an asymmetric
supercapacitor built on two different transition-metal-oxide materials, and the devices
136
exhibited energy density of 23 Wh/kg, and improved power density of 1.4 kW/kg at a
high discharging current of 25 mA/cm
2
[17]. However, none of them could supply a
power density which could be comparable to that of SWNT EDLCs (23 kW/kg) in an
aqueous gel electrolyte [18].
This can be attributed to the poor conductance of the
transition-metal-oxide based electrodes, which leads to the decrease of the power density
of the asymmetric supercapacitors.
To address this issue, we have developed an easy and efficient method to improve
the conductivity of transition-metal-oxide based electrodes by integrating transition-
metal-oxide nanowires together with single-walled carbon nanotubes (SWNTs) to form
hybrid nanostructured films, which have been applied as electrodes of flexible and
transparent supercapacitors [19]. In our early work, the incorporation of transition-metal-
oxide nanowires contributed pseudocapacitance to SWNT thin film EDLCs; thus
improved the device performance in the aspects of specific capacitance and power density.
In this letter, using a similar concept, we prepared hybrid-nanostructured thin film
electrodes by using two different transition-metal-oxide nanowires including manganese
dioxide (MnO
2
) nanowires and indium oxide (In
2
O
3
) nanowires together with SWNTs,
and fabricated hybrid-nanostructured asymmetric supercapacitors. In this asymmetric cell
system, MnO
2
nanowire / SWNT hybrid films served as the positive electrode, while
In
2
O
3
nanowire / SWNT hybrid films functioned as the negative electrode with a neutral
electrolyte. In order to obtain a stable 2 V operation potential of hybrid-nanostructured
asymmetric supercapacitors, the mass balance between the two electrodes of the cell
system has been optimized. The optimized hybrid-nanostructured asymmetric
supercapacitors was stably operated up to 2 V with the specific capacitance of 184 F/g,
137
power density of 50.3 kW/kg, and energy density of 25.5 Wh/kg. In comparison to the
early reported asymmetric supercapacitors, our hybrid-nanostructured asymmetric
supercapacitors exhibit better power density, which can be attributed to the integration of
SWNTs. In addition, our design not only takes full advantage of the electrical double-
layer capacitance from SWNTs and the pseudocapacitance from transition-metal-oxide
nanowires, but also improves the conductivity of transition-metal-oxide nanowire films,
which leads to high energy density and high power density of our asymmetric
supercapacitors. Furthermore, SWNT films also worked as current collecting electrodes,
which further reduced the total device weight by excluding metal current collecting
electrodes used in conventional supercapacitors.
7.2 Experiment
7.2.1 Preparation of SWNT Bulky Paper
The fabrication of hybrid-nanostructured asymmetric supercapacitors began with
the preparation of functionalized carbon nanotube (CNT) solutions. The detail can be
found in our early work [20]. In brief, arc-discharge carbon nanotubes (P3 nanotubes
from Carbon Solutions Inc.) were mixed with 1 wt% aqueous sodium dodecyl sulfate
(SDS) in distilled water to make a highly dense SWNT suspension with a typical
concentration of 0.1 mg/mL. The addition of SDS surfactants further improves the
solubility of SWNTs by sidewall functionalization. This SWNT solution was then
ultrasonically agitated using a probe sonicator for ~ 20 min, followed by centrifugation to
separate out undissolved SWNT bundles and impurities. To make an uniform SWNT thin
film electrode, the as-prepared SWNT suspension was filtered through a porous alumina
138
filtration membrane (pore size: 200 nm, Whatman). As the solvent went through the
membrane, SWNTs were trapped on the membrane surface and formed a homogenous
entangled network. After the filtration, significant amount of distilled water was applied
to remove the remaining SDS surfactants.
Figure 7.1 (a) Optical and (b) SEM image of SWNT films obtained by using a vacuum
filtration method
During the trapped SWNT film became dry, the “bulky paper” thus formed was
gently peeled off from the filtration membrane, which can be observed in Figure 7.1. The
mass of as-fabricated SWNT bulky papers and hybrid nanostructured films were
determined by a micro-balance after filtrations. Typical mass loading of a 2-inch-
diameter SWNT bulky paper was about ~ 8 mg, with a film thickness of 2.2 µm and sheet
resistance of 13-16 Ohm/square, which is comparable to that of early reported SWNT
networks [18, 21, 22]. The electrode size used in this work was about 0.5 cm
2
.
7.2.2 Material Synthesis and Characterization
MnO
2
nanowires were synthesized by using a so-called hydrothermal method
reported elsewhere [23]. In brief, Mn(CH
3
COO)
2
• 4H
2
O and Na
2
S
2
O
8
(99.999%, Sigma
Aldrich) were dissolved in 100 ml distilled water with a molar ratio of 1 : 1 at room
temperature, and stirred by a magnetic stirrer to form a clear and homogeneous solution.
(a)
(b)
139
The mixed solution was then transferred to a 130 ml Teflon-lined stainless steel autoclave
and heated at 120
o
C for 12 hours in an electrical oven for hydrothermal reactions. After
the reaction, the products were washed with deionized water and ethanol to remove the
sulfate ions and other remains by filtration. Then the products were dried in a vacuum
oven at 100
o
C for 12 hours.
In
2
O
3
nanowires were synthesized by using a thermal chemical vapor deposition
(CVD) method. A 5 nm gold film was deposited on Si/SiO
2
substrates as catalysts using
an e-beam evaporator, followed by annealing at 700
o
C for 30 minutes. The substrates
were then placed into a quartz tube at the downstream position of a furnace, while
stoichiometric In
2
O
3
powders (99.99%, Alfa-Aesar) mixed with graphite powders were
utilized as precursor and also placed at the center of a furnace. During the growth, the
quartz tube was maintained at a pressure of 1 atm and a temperature of 900 °C, with a
constant flow of 120 standard cubic centimetres (sccm). The typical reaction time was
about 50 minutes. The as-grown nanowires were characterized by using field-emission
scanning electron microscopy (FESEM, Philips S-2000), high resolution transmission
electron microscopy (HR-TEM, JEOL 100-CX), and x-ray diffractometer (XRD).
Figure 7.2 (a) shows a typical SEM image of as-grown MnO
2
nanowires with
typical length of ~ 2-3 μm and diameter of ~ 20 nm in average. These nanowires have
smooth surface without any amorphous coating and have extremely uniform diameters,
which can be easily observed in the low magnification TEM image (Figure 7.2 (b)). In
addition, there is no noticeable dislocation or defect in MnO
2
nanowires, and the
corresponding HR-TEM image, showed in Figure 7.2 (c), exhibits a perfect single
crystalline structure with a very well lattice fringe, corresponded to the d-spacing of 0.31
140
nm of β - phase MnO
2
(β - MnO
2
) crystal structure [24]. To further evaluate the
crystalline structure, Figure 7.2 (d) shows the result of XRD measurement. The XRD
pattern also confirmed the crystalline structure of as-grown nanowires to be β - MnO
2
nanowires, which are tetragonal symmetry with P42/mnm space group and lattice
constants of a = 4.388 nm and c = 2.865 nm (JCPDS data (PDF-01-072-1984)).
Furthermore, there is no extra peak observed in the XRD spectrum, which confirms the
high crystalline nature of the MnO
2
nanowires and is in an agreement with the HR-TEM
observations. Figure 7.2 (e) displays the SEM image of CVD synthesized In
2
O
3
nanowires which are 10-100 μm long with diameter of 50-100 nm in average. Similar to
MnO
2
nanowires, both low-magnification TEM (Fig. 1 (f)) and HR-TEM (Figure 7.2 (e))
suggest that each In
2
O
3
nanowire has a perfect single crystalline structure without any
noticeable dislocations or defects. The interspacing between each plane is 0.505 nm,
corresponding to the (200) plane in the body-centered cubic (bcc) In
2
O
3
nanowire crystal
structure, with a lattice constant of a =1.01 nm [25, 26]. Although the XRD pattern
shown in Figure 7.2 (h) exhibits two extra Au peaks due to the existence of Au catalysts,
the XRD diffraction patterns also indicate that these nanowires exhibit high crystalline
quality [24].
141
Figure 7.2 (a) SEM image of β - MnO
2
nanowires with diameter of 20 nm and length of
2-3 µm in average. (b) TEM image of β - MnO
2
nanowires. (c) High resolution TEM of a
β - MnO
2
nanowire. (d) A typical XRD spectrum of as-grown β - MnO
2
nanowires. (e)
SEM images of In
2
O
3
nanowires with diameter of 60 nm and length of 10-100 µm in
average. (f) TEM image of In
2
O
3
nanowires. (g) High resolution TEM of a In
2
O
3
nanowire. (h) A typical XRD spectrum of as-grown In
2
O
3
nanowires.
7.2.3 Preparation of Hybrid-Nanostructured Films
To produce hybrid nanostructured films, the as-grown transition-metal-oxide
nanowires were sonicated into isopropyl alcohol (IPA) solutions and then dispersed upon
a SWNT film / AAO membrane to form transition-metal-oxide nanowire / SWNT hybrid
films by a filtration method. As the suspension went through the SWNT film / filtration
membrane, the nanowires were trapped upon the SWNT films and formed intertwined
mesh. The “hybrid-nanostructured thin films” were gently peeled off, while the trapped
nanowire/SWNT films got dry. These hybrid nanostructured films exhibited
600
400
200
Intensity (a.u.)
60 50 40 30 20
2 Theta (degrees)
(222
(440
(400
10 nm
100 nm
(001)
1 µm
(e)
(f)
(h)
(g)
1200
800
400
Intensity (a.u.)
70 60 50 40 30 20
2 Thetha (degree)
100 nm
2 µm
(a) (b)
(c)
(d)
(100)
(101)
(111)
(211)
(220)
142
characterizations of mechanical flexibility, uniform layered structures, and mesoporous
surface morphology. Figure 7.3 shows the schematic diagram of an asymmetric
supercapacitor built on two different hybrid-nanostructured films. As one can easily
observe, tthe asymmetric cell system is composed of a nitrocellulose film as the separator,
a MnO
2
nanowire / SWNT film as the positive electrode, an In
2
O
3
nanowire / SWNT film
as the negative electrode, and aqueous solution as the electrolyte.
Figure 7.3 Schematic diagram of an asymmetric supercapacitor composed with MnO
2
nanowire / SWNT hybrid film as a cathode electrode, and In
2
O
3
nanowire / SWNT hybrid
film as an anode electrode.
Figure 7.4 shows a typical MnO
2
nanowire / SWNT hybrid film electrode. MnO
2
nanowires are uniformly coated upon SWNT films. To check the interface between the
SWNT networks and MnO
2
nanowire mesh, we intentionally stretched a MnO
2
nanowire
/ SWNT hybrid film electrode. Uniform SWNT networks underneath the MnO
2
nanowire
films can be clearly observed in the inset of Figure 7.4, and the MnO
2
nanowires are
well-distributed and form a homogenous film on SWNT films. Similar to the MnO
2
nanowire /SWNT hybrid films, Figure 7.5 shows well-dispersed In
2
O
3
nanowire mesh on
(a)
MnO
2
NWs / SWNTs
In
2
O
3
NWs / SWNTs
Nitrocellulose Separator
143
SWNT networks forming a layer-by-layer structure. The inset displays the SWNT
networks below the In
2
O
3
nanowire films. By using the filtration method, we produced
conformal, binder-free, all-nanostructured-materials hybrid-nanostructured films with
highly tunable surfaces, which allows aqueous electrolytes to fully wet the nanowire
mesh and the SWNTs. In addition, we note that uniform-dispersed transition-metal-oxide
nanowire films and a layer-by-layer structure are critical in this study, since charges can
uniformly distribute on each electrode and the cell voltage can split equally on both
electrodes.
Figure 7.4 A scratched MnO
2
nanowire / SWNT hybrid film. Inset (up): SWNT films can
be clearly observed underneath MnO
2
nanowire networks. Inset (down): uniform MnO
2
nanowire network above SWNT films
5 µm
2 µm
500 nm
(b)
144
Figure 7.5 An as-fabricated In
2
O
3
nanowire / SWNT hybrid film. Inset: An SWNT films
underneath In
2
O
3
nanowires works as current collecting electrodes.
7.3 Results and Discussion
7.3.1 Electrochemical Behavior of Hybrid-Nanostructured Films
Electrochemical measurements were carried out with a potentiostat/galvanostat
(263, Princeton Applied Research) in 1 M Na
2
SO
4
electrolyte. Galvanostatic (GV)
charging/discharging measurements were used to determine the specific capacitance (C
sp
),
power density, and the internal resistance (IR) of the devices in a two-electrode
configuration. Cyclic Voltammetery (CV) measurements were performed to evaluate the
stability and the electrochemical behavior of our hybrid-nanostructured films under
different potential window from -0.6 V to 0.8 V in a three-electrode configuration. A
hybrid-nanostructured film, an Ag/AgCl (saturated NaCl) assembly, and a platinum wire
were used as the working electrode, the reference electrode, and the counter electrodes,
respectively. The potential range of MnO
2
and In
2
O
3
hybrid nanostructured films extends
from 0.0 to 0.8 V and -0.6 to 0.2 V vs. Ag/AgCl, respectively, while In
2
O
3
nanowires are
more stable at more negative potentials. The CV results of SWNT bulky papers, MnO
2
500 nm
(c)
20 µm
145
nanowire / SWNT hybrid films, and In
2
O
3
nanowire / SWNT hybrid films in the aqueous
electrolyte are presented in Figure 7.6. Figure 7.6 (a) shows the results of CV
measurements of SWNT bulky papers with different scan rates of 5 and 20 mV/sec in the
potential range of 0.0V and 0.8V. The rectangular shape of these curves reveals a good
electrical double layer capacitance behavior of our SWNT bulky papers. The cyclic
voltammograms of MnO
2
nanowire / SWNT hybrid films with scan rates of 5 mV/sec, 20
mV/sec, 50 mV/sec, and 100 mV/sec, can be found in Figure 7.6 (b), which shows a
quasi-rectangular shape of these curves. As it is well-known, MnO
2
has been considered
as a promising electrode material exhibiting ideal capacitive behavior in a mild aqueous
electrolyte with a stable potential range up to 1.2 V [13]. The redox transition is based on
the injection and ejection of cations and electrons, in which cations (Na
+
) intercalate into
MnO
2
lattice and correspondingly Mn(IV) become Mn(III) to balance the charges [27]
and can be expressed as following [12]
() ()
a b an bn
MnO OH nH ne MnO OH
+−
−+
+ +↔ (1)
Where MnO
a
(OH)
b
and MnO
a-n
(OH)
b+n
represent interfacial MnO
2
‧H
2
O in higher and
lower oxidation states, respectively. The quasi-rectangular shapes are close to the
behavior of EDLCs, even though Faradaic process dominates the electrochemical
behavior of MnO
2
nanowire networks in an aqueous electrolyte [16]. In addition, the
SWNT films underneath MnO
2
nanowire networks also contributed electrical double
layer capacitance which might influence the CV shapes of MnO
2
nanowire / SWNT
hybrid films.
146
Figure 7.6 Cyclic voltammetery in a three-electrode configuration with different
nanostructured thin film electrodes of (a) a bare SWNT thin film electrode, (b) a MnO
2
nanowire / SWNT hybrid film electrode, and (c) a In
2
O
3
nanowire / SWNT hybrid film
electrode in 1M Na
2
SO
4
electrolyte with different scan rates ranging from 5 mV/sec to
100 mV/sec. (d) Comparative cyclic voltammetery using MnO
2
nanowire / SWNT hybrid
film and In
2
O
3
nanowire / SWNT hybrid film as active electrode. The scan rate of
potential is 100 mV/sec.
Figure 7.6 (c) displays the cyclic voltammograms of In
2
O
3
nanowire / SWNT
hybrid films with scan rates of 5 mV/sec, 20 mV/sec, 50 mV/sec, and 100 mV/sec.
Similar to the MnO
2
nanowire / SWNT hybrid films, the In
2
O
3
nanowire / SWNT films
exhibit a quasi-rectangular shape with Faradaic process contributed by the In
2
O
3
nanowires [28]. In comparison to Figure 7.6 (a), as one can easily see, the cyclic
voltammograms of both hybrid-nanostructured films are quite different due to the
-4.0
-2.0
0.0
2.0
4.0
Current (A/g)
-0.6 -0.4 -0.2 0.0 0.2
Potential (V v.s. Ag/Ag/Cl)
1.5
1.0
0.5
0.0
-0.5
-1.0
Current (A/g)
0.8 0.6 0.4 0.2 0.0
Potential (V v.s. Ag/Ag/Cl)
-3.0
-1.5
0.0
1.5
3.0
Current (A/g)
0.8 0.6 0.4 0.2 0.0
Potential (V v.s. Ag/Ag/Cl)
-4.0
-2.0
0.0
2.0
4.0
Current (A/g)
0.8 0.4 0.0 -0.4
Potential (V v.s. Ag/AgCl)
In 2O 3 NW
/SWNT
MnO 2 NW
/SWNT
5 mV/sec
10 mV/sec
50 mV/sec
100 mV/sec
5 mV/sec
10 mV/sec
50 mV/sec
100mV/sec
5 mV/sec
15 mV/sec
(a) SWNT (b) MnO
2
NW/SWNT
(c) In
2
O
3
NW/SWNT (d)
147
Faradaic process contributed from transition-metal-oxide nanowires. The specific
capacitance of those active materials can be obtained via the following equation [29, 30]:
1
( / ) ( )
i
CF g
vm
= , (2)
where ν is the scan rate, i is the corresponding current of the applied voltage, and m is the
weight of the active materials. With this equation, the specific capacitance of SWNT
bulky paper is calculated to be about 80 F/g, MnO
2
nanowire / SWNT hybrid film is 253
F/g, and In
2
O
3
nanowire / SWNT hybrid film is 201 F/g.
By expressing the total cell voltage as the sum of the potential range of MnO
2
nanowire / SWNT hybrid film and In
2
O
3
nanowire / SWNT hybrid film, we are able to
estimate that the hybrid-nanostructured asymmetric supercapacitors can be operated up to
1.4 V. Figure 7.6 (d) shows the cyclic voltammograms obtained in a three-electrode cell
from MnO
2
nanowire / SWNT hybrid film electrode and In
2
O
3
nanowire / SWNT hybrid
film electrode in 1 M Na
2
SO
4
electrolyte. It is easy to observe that MnO
2
nanowire /
SWNT hybrid film has a stable electrochemical behavior in positive polarization and
In
2
O
3
nanowire / SWNT hybrid film has a stable electrochemical behavior in negative
polarization. Hence, in order to obtain a capacitor operating in a 1.4 V voltage window, it
is necessary to control the experimental conditions for MnO
2
nanowire / SWNT hybrid
film to work in the potential window range from 0.2 V to 0.8V and In
2
O
3
nanowire /
SWNT hybrid film work in the potential window range from -0.6 V to 0.2 V, which will
ensure a safe performance of both electrodes during long cycling. In this way, we can
avoid the decomposition of aqueous electrolyte at 1 V in a symmetric cell system. In
addition, more negative potential (for reduction) and positive potential (for oxidation) can
be achieved, since both the hydrogen and oxygen evolution reactions are presumably
148
kinetically limited on these transition-metal-oxide nanowires and SWNTs. As a result,
the operation window of MnO
2
nanowires can extend from -0.1 V to 1.2 V and In
2
O
3
nanowires can enlarge from -1.0 V to 0.2 V vs. Ag/AgCl in 1M Na
2
SO
4
electrolyte.
Moreover, unlike a symmetric supercapacitor, which the applied voltage can split
equally between the two electrodes due to using the same material and having the same
mass in each electrode, in asymmetric supercapacitors, the voltage-split depends on the
capacitance of the active materials in each electrode. The capacitance is usually related to
the mass and the specific capacitance of the active material [13]. Thus, in order to split
voltage equally, we have to optimize the mass balance between the two electrodes in our
cell system following the relationship of qq
+ −
= , where q
+
means the charges stored at
positive electrode and q
-
means the charges stored at negative electrode [11].
The stored
charge can be expressed by the following the equation [11]:
SP
qC m E = ∗∗∆ (3)
where E ∆ is the potential range of charging/discharging process, m is the mass of each
electrode. Since the mass loading of SWNTs in each electrode is the same, the optimal
mass ratio between the electrodes should be
2 23
/
MnO In O
mm = 0.74 in the hybrid-
nanostructured asymmetric cell system.
7.3.2 Electrochemical Behavior of Hybrid-Nanostructured Asymmetric
Supercapacitors
Figure 7.7 (a) shows the cyclic voltammograms at different cell voltages for a
hybrid-nanostructured asymmetric supercapacitor with optimal mass ratio between two
149
electrodes. With a scan rate of 20 mV/sec, the hybrid-nanostructured supercapacitor
shows an ideal capacitive behavior with quasi-rectangular CV curves even at potential
window up to 2.0 V in 1M Na
2
SO
4
electrolyte. The cell could maintain an ideal
capacitive behavior even at the high scan rate of 100 mV/sec, which exhibits the
desirable fast-charging/discharging property for power devices, shown in Figure 7.7 (b).
Figure 7.7 Cyclic voltammograms of an optimized hybrid-nanostructured asymmetric
supercapacitor in 1M Na
2
SO
4
electrolyte with a scan rate of 20 mV/sec (a), and with
different scan rates of 5, 10, 20, 50, 75, and 100 mV/sec (b). (c) Galvanostatic
charging/discharging curves measured with a current density of 2 mA/cm
2
for an
optimized hybrid-nanostructured asymmetric supercapacitor in 1M Na
2
SO
4
electrolyte.
(d) A comparison of specific capacitance of a hybrid-nanostructured asymmetric
supercapacitor and a SWNT symmetric supercapacitor with different discharging currents
of 1, 2, 5, 10, 20 mA/cm
2
.
Figure 7.7 (c) displays the 10 cycles of galvanostatic charging-discharging curves
of an asymmetric supercapacitor with a constant current of 2 mA/cm
2
in the potential
2.0
1.5
1.0
0.5
0.0
Potential (V)
1800 1500 1200 900 600 300 0
Time (sec)
200
160
120
80
40
Specific Capacitance (F/g)
20 15 10 5 0
Current Density (mA/cm
2
)
Hybrid-nanostructured
Asymmetric Supercapacitor
SWNT Symmetric
Supercapacitor
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Current (mA)
2.0 1.5 1.0 0.5 0.0
Potential (V)
-4.0
-2.0
0.0
2.0
4.0
Current (mA)
2.0 1.5 1.0 0.5 0.0
Potential (V)
20 mV/sec
5, 10, 20,
50, 100 mV/sec
(a)
(b)
(c) (d)
150
range between 0.01 V and 2.01 V. The symmetry of the charging and discharging
characteristics shows good capacitive behavior and the specific capacitance has been
evaluated from the charging-discharging curves, according to the following equation [19]:
1 1
( )( )
/
sp
I
C
dV dt m m
+−
= +
−
(4)
where I is the applied discharging current, m
+
and m
-
are the mass of positive and
negative electrode, and the dV/dt is the slope of the of discharge curve after IR drop. The
power density and the energy density can be calculated using the following equation [11]:
2
4
V
P
RM
= (5)
22
11
28
sp
E CV MC V = = (6)
where V is the applied voltage, R is the equivalent series resistance (ESR), and M is the
total mass of the hybrid-nanostructured film electrodes, C is the total capacitance of the
hybrid-nanostructured asymmetric supercapacitor (
4
SP
C
C
M
= ) [31]. The calculated
specific capacitance of a hybrid-nanostructured asymmetric supercapacitor is about 184
F/g, while the power density and energy density can be improved up to 50.3 kW/kg and
25.5 Wh/kg, respectively. We also carried out the GV measurements to symmetric
SWNT symmetric supercapacitors, shown in Figure 7.8. The specific capacitance is
merely 80 F/g, with the power density of 11.4 kW/kg and the energy density of 4 Wh/kg.
We further investigated the device performance of hybrid-nanostructured asymmetric
supercapacitors and symmetric SWNT symmetric supercapacitor by using different
charging/discharging currents. Figure 7.7 (d) shows the specific capacitance of a SWNT
151
symmetric supercapacitor and a hybrid-nanostructured asymmetric supercapacitor as a
function of different discharging current density. The decrease of specific capacitance of
both supercapacitors can be attributed to the decrease of the utilization efficiency of
active materials with increasing discharging current. The hybrid-nanostructured
asymmetric supercapacitors performed the specific capacitance of 90 F/g even at a
discharging current of 20 mA/cm
2
.
Figure 7.8 Galvanostatic charging/discharging curves measured with a current density of
2 mA/cm
2
of a SWNT symmetric supercapacitor in 1M Na
2
SO
4
electrolyte.
Figure 7.9 (a) shows the galvanostatic charging/discharging curves of one hybrid-
nanostructured asymmetric supercapacitor with different maximum cell voltage. The
specific discharge capacitance was improved with increasing cell voltage, and the
charging/discharging behavior is capacitive with symmetric charge-discharge curves up
to 1.5 V. However, with increasing cell voltage, non-capacitive behavior with non-
symmetric charge-discharge curve can be found. Therefore, to determine the optimal cell
voltage, the coulombic efficieny should be evaluated, according to the following equation
[17]
152
Figure 7.9 (a) Galvanostatic charging/discharging curves (I = 2 mA/cm
2
) of an optimized
asymmetric nanostructured supercapacitor in 1M Na
2
SO
4
electrolyte. Cyclic
voltammograms on an optimized asymmetric nanostructured supercapacitor in 1M
Na
2
SO
4
electrolyte with a scan rate of 20 mV/sec. (b) Coulombic efficiency and specific
capacitance of a hybrid-nanostructured asymmetric supercapacitor in 1M Na
2
SO
4
electrolyte vs. different cell voltage. (c) Photo image of a green LED connected with the
hybrid-nanostructured asymmetric supercapacitor before/after discharging (inset). (d) A
Rogone plot showing that the hybrid-nanostructured asymmetric supercapacitors
outperform the SWNT symmetric supercapacitors and early asymmetric supercapacitor
data reported in literatures.
100%
d
c
q
q
η= × (7)
where q
d
and q
c
are the total amount of discharge and charge of the capacitor obtained
form the galvanostatic experiments exhibited in Figure 7.9 (a). Figure 7.9 (b) represents
the coulombic efficiency and the average specific discharge capacitance of both
electrodes as the function of the cell voltage in five hybrid-nanostructured asymmetric
2.0
1.5
1.0
0.5
0.0 Cell Voltage (V)
200 150 100 50 0
Time (sec)
200
180
160
140
120
Specific Capacitance (F/g)
2.1 1.8 1.5 1.2 0.9 0.6
Cell Voltage (V)
100
95
90
85
80
η (%)
1
2
4
10
2
4
100
Energy Density (Wh/kg)
10
0
10
2
10
4
10
6
Power Density (W/kg)
SWNT Symmetric
Supercapacitor
(a)
(b)
(c)
(d)
153
supercapacitors. Whereas the capacitance increases with the cell voltage, the coulombic
efficiency decreases significantly when the voltage is above 1.8 V. This might be
attributed to di-hydrogen that hydrogen produced during the negative polarization of the
In
2
O
3
nanowires in the negative electrode. More detailed experiments are required to
investigate the phenomena. Therefore, the cell optimal voltage should not higher than 1.8
V.
To show the practical applications of hybrid-nanostructured asymmetric
supercapacitors, we connected one hybrid-nanostructured asymmetric supercapacitor
with a green light-emitted diode (LED) and successfully lighten it, which can be observed
in the inset of Figure 7.9 (c). To highlight the superior device performance of hybrid-
nanostructured asymmetric supercapacitors, Figure 7.9 (d) shows the Ragone plots of
SWNT symmetric supercapacitors and hybrid-nanostructured asymmetric supercapacitors.
All the data were calculated based on the total mass of active materials of two electrodes.
It can be seen that the hybrid-nanostructured asymmetric supercapacitors exhibit much
higher energy density and power density than the SWNT symmetric supercapacitors. In
addition, in comparisons of previous reported asymmetric configurations, our hybrid-
nanostructured asymmetric supercapacitors display a comparable energy density and
higher power density beyond early reported work [14, 16, 17, 32], which can be
attributed to the integration of transition-metal-oxide nanowires and SWNTs to form a
binder-free, hybrid-nanostructured thin film electrodes, and thus improved the
condcutivity of electrodes and power density of asymmetric cells.
154
7.4 Conclusion
In summary, we have developed a simple and efficient way to obtain flexible,
mesoporous, and uniform hybrid-nanostructured thin film electrodes for supercapacitor
study. Our hybrid-nanostructured asymmetric supercapacitors exhibited superior device
performance than SWNT symmetric supercapacitors, in the aspects of operation voltage,
specific capacitance, energy density, and power density. The superior performance can be
attributed to (1) the enhanced charge storage contributed from electrical double-layer
capacitance from SWNT films and pseudocapacitance from transition-metal-oxide
nanowires, (2) good conductivity by using SWNTs as conductivity agent in our design,
and (3) the optimized mass balance, which allowed us to operate the devices in a 2 V
potential window with stable electrochemical behavior. In addition, the total weight of
our devices can be further reduced because of excluding metal current collecting
electrodes and binders. The optimized asymmetric supercapacitors in aqueous electrolyte
have performed an improved device performance in specific capacitance of 184 F/g,
power density of 50.3 kW/kg, and energy density of 25.5 Wh/kg. The high performance
asymmetric supercapacitors can be applied in the need of conformal electronics, portable
electronics, and electrical vehicles.
155
Chapter 7 References
1. Winter M, Brodd R J. What are batteries, fuel cells, and supercapacitors.
Chemical Reviews 2004; 104 (10): 4245-4269.
2. Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials
2008; 7 (11): 845-854.
3. Pushparaj V L, Shaijumon M M, Kumar A, Murugesan S, Ci L, Vajtai R, Linhardt
R J, Nalamasu O, Ajayan, P M. Flexible energy storage devices based on
nanocomposite paper. Proceeding of the National Academy of Sciences of the
United States 2007; 104 (34): 13574-13577.
4. Arico A S, Bruce P, Scrosati B, Tarascon T M, Schalkwijk W V. Nanostructured
materials for advanced energy conversion and storage devices. Nature Materials
2005; 4 (5): 366-377.
5. Futaba D N, Hata K, Yamada T, Hiraoka T, Hayamizu Y, Kakudate Y, Tanaike O,
Hatori H, Yumura M, Iijima S. Shape-engineerable and highly densely packed
single-walled carbon nanotubes and their application as super-capacitor electrode
Nature Materials 2006; 5 (12): 987-994.
6. Liu J, Cao G, Yang Z, Wang D, Dubois D, Zhou X, Graff G L, Pederson L R,
Zhang J G. Oriented nanostructures for energy conversion and storage.
ChemSusChem 2008; 1 (8-9): 676-697.
7. Pasquier A D, Plitz I, Menocal S, Amatucci G. A comprehensive study of Li-ion
battery, supercapacitor, and nonaqueous asymmetric hybrid devices for
automotive applications. Journal of Power Sources 2003; 115 (1): 171-178.
8. Conway B E. Electrochemical supercapacitors – Scientific fundamentals and
technological applications. Kluwer Academic / Plenum Publishers 1999, New
York, U.S.A.
9. Zhang L L, Zhao X S. Carbon-based materials as supercapacitor electrodes.
Chemical Society Reviews 2009; 38 (9): 2520-2531.
10. Frackowiak E. Carbon materials for supercapacitor application. Physical
Chemistry Chemical Physic 2007; 9 (15): 1774–1785.
11. Khomenko V, Raymundo-Piñero E, Béguin F. Optimisation of an asymmetric
manganese oxide/actived carbon capacitor working at 2V in aqueous medium.
Journal of Power Sources 2006; 153 (1): 183-190.
156
12. Khomenko V, Raymundo-Piñero E, Frackowiak E, Béguin F. High-voltage
asymmetric supercapacitors operating in aqueous electrolyte. Applied Physics A.
2006; 82 (4): 567-573.
13. Cottineau T, Toupin M, Delahaye T, Brousse T, Bélanger D. Nanostructured
transition metal oxides for aqueous hybrid electrochemical supercapacitors.
Applied Physics A 2006; 82 (4): 599-106.
14. Algharaibeh Z, Liu X, Pickup P G. An asymmetric anthraquinone-modified
carbon/ruthenium oxide supercapacitor. Journal of Power Sources 2009; 187 (2):
1640-1643.
15. Duffy N W, Baldsing W, Pandolfo A G. The nickel-carbon asymmetric
supercapacitor-Performance, energy density and electrode mass rations.
Electrochimica Acta 2008; 54 (2): 535-539.
16. Qu Q, Zhang P, Wang B, Chen Y, Tian S, Wu Y, Holze R. Electrochemical
performance of MnO
2
nanorods in neutral aqueous electrolyte as a cathode for
asymmetric supercapacitors. Journal of Physical Chemistry C. 2009; 113 (31):
14020-14027.
17. Yuan C Z, Gao B, Zhang X G. Electrochemical capacitance of NiO/Ru
0.35
V
0.65
O
2
asymmetric electrochemical capacitor. Journal of Power Sources 2007; 173 (1):
606-612.
18. Kaempgen M, Chan C K, Ma J, Cui Y, Gruner G. Printable thin film
supercapacitors using single-walled carbon nanotubes. Nano Letters 2009; 9 (5):
1919-1923.
19. Chen P C, Shen G, Sukcharoenchoke S, Zhou C. Flexible and Transparent
Supercapacitor based on In
2
O
3
Nanowire / Carbon Nanotube Heterogeneous
Films Applied Physics. Letters 2009; 94 (4): 43113-43115.
20. Zhang D, Ryu K, Liu X, Polikarpov E, Ly J, Tompson M E, Zhou C. Transparent,
conductive, and flexible carbon nanotube films and their application in organic
light-emitting diodes. Nano Letters 2006; 6 (9): 1880-1886.
21. Hu L, Choi J W, Yang Y, Jeong S, Mantia F L, Cui L F, Cui Y. Highly conductive
paper for energy-storage devices. Proceeding of the National Academy of Science
of the United States 2009; 106 (51): 21490-21494.
22. Lee S W, Kim B S, Chen S, Yang S H, Hammond P T. Layer-by-layer assembly of
all carbon nanotube ultrathin films for electrochemical applications. Journal of the
American Chemistry Society 2009; 131 (2): 671-679.
157
23. Kim D K, Muralidharan P, Lee H W, Ruffo R, Yang Y, Chan C K, Peng H,
Huggins R A, Cui Y. Spinel LiMnO nanorods as lithium ion battery cathodes.
Nano Letters 2008; 8 (11): 3948-3952.
24. Wei M, Konishi Y, Zhou H, Sugihara H, Arakawa H. Synthesis of single-crystal
manganese dioxide nanowires by a soft chemical process. Nanotechnology 2006;
16 (2): 245-249.
25. Wyckoff R W G. Crystal Structures, Interscience, 1968, New York.
26. Li C, Zhang D, Han S, Liu X, Tang T, Zhou C. Diameter-controlled growth of
single-crystalline In
2
O
3
nanowires and their electronic properties. Advanced
Materials 2003; 15 (2): 143-146.
27. Xu C, Du H, Li B, Kang F, Zeng Y. Asymmetric activated carbon-manganese
dioxide capacitors in mild aqueous electrolyte containing alkaline-earth cations.
Journal of Electrochemical Society 2009; 156 (6): A435-441.
28. Prasad K R, Koga K, Miura N. Electrochemical deposition of nanostructured
indium oxide: high-performance electrode material for redox supercapacitors.
Chemistry of Matterials 2004; 16 (10): 1845-1847.
29. Shaijumon M M, Ou F S, Ci L, Ajayan P M. Synthesis of hybrid nanowire arrays
and their application as high power supercapacitor electrodes. Chemical
Communications 2008; 20: 2373-2375.
30. Frackowiak E, Beguin F. Carbon materials for the electrochemical storage of
energy in capacitors. Carbon 2001; 39 (6): 937-950.
31. Jurewicz K, Frackowiak E, Beguin F. Towards the mechanism of electrochemical
hydrogen storage in nanostructured carbon materials. Applied Physics A 2004; 78
(7): 981-987.
32. Wang G Z, Zhang B L, Yu Z L, Qu M Z. Manganese oxide/MWNTs composite
electrodes for supercapacitors. Solid State Ionics 2005; 176 (11-12): 1169-1174.
158
Chapter 8 Conclusions and Future Work
With the better understanding of the phenomena at nanoscale as well as the
utilization of one-dimensional (1-D) nanostructures for the good of human beings,
substantial progress have been made in the synthesis of different 1-D nanostructures and
bridging the technique gap from laboratory to real life. The vigorous developments in 1-
D nanostructures and nanotechnology cannot be only attributed to the small size of 1-D
nanostructures, but also their unique physical properties, such as quantized conductance,
short ion diffusion length, and size-dependent Debye length, which have rendered them
with important applications in electronics, chemical/biological sensing, photovoltaic cells,
and electrochemical energy conversion and storage devices.
The previous chapters have systematically discussed my research work on three
important applications based on one-dimensional (1-D) nanostructures including
chemical sensing, transparent electronics, and electrochemical capacitors. This chapter
will summarize the above discussions through Chapter 2 to Chapter 7, and propose future
research directions of potential impacts.
8.1 Conclusions
The ongoing research work in the chemical sensing study have become not only
obtaining 1-D nanostructures for higher “sensitivity”, but also integrating them into
micro-scale devices for better “selectivity”, which remains one of challenging tasks in the
chemical sensing field. Chapter 2 introduced a nano electronic-nose sensor array built on
four different 1-D nanostructures, which were integrated with micro-machined hotplates
159
in a sensing chip. The discussion has primarily focused on the incorporation of In
2
O
3
nanowires, ZnO nanowires, SnO
2
nanowires, and single-walled carbon nanotubes
(SWNTs) in order to obtain a sensor array, which enabled us to obtain the “discrimination
factor” to distinguish different chemicals. Other than that, different sensing temperatures
controlled by micro-machined hotplates worked as the other “discrimination factor” to
our electronic-nose sensor array. The chapter has given the detail of the fabrication
process of the nano electronic-nose sensor array and the sensing experiments, followed
by obtaining a “smell library” of important industrial gases, including NO
2
, H
2
, and
ethanol. In addition, after analyzing sensing data via the principal component analysis,
our nano electronic-nose sensor arrays showed good pattern reorganization in above
gases; hence confirmed the success of coping with the challenging task of selectivity in
different chemicals.
Asides from sensitivity and selectivity of industrial gases, due to the increasing
demand of homeland security and threat of terrorism, the detection of explosives and
nerve agents is receiving more and more attention. To address this issue, in Chapter 3, we
have fabricated 2, 4, 6- trinitrotoluene (TNT) sensors using aligned SWNTs and ZnO
nanowires. The detection limit of our SWNT TNT sensor can be down to 8 ppb at room
temperature. Detailed sensing experiments were carried out to understand the sensing
behaviors of the 1-D nanostructures. In addition, as it is well-known, the decomposition
of TNT molecules is a complicated process; thus a comprehensive study of the TNT
sensing mechanism has been explored by using a mass spectroscopy in the end of this
work.
Chapter 4 focused on the synthesis and the application of arsenic-doped In
2
O
3
(As-
160
In
2
O
3
) nanowires in transparent electronics and explored the feasibility of using As-In
2
O
3
nanowire transparent thin film transistors (TTFTs) in controlling active-matrix organic
light-emitting diode (AMOLED) displays. The chapter presented our extensive studies of
As-In
2
O
3
nanowires starting from nanowire synthesis, material characterizations,
electronic transport properties, to circuit integration of OLED and nanowire TTFTs.
Furthermore, detailed electrical measurements in As-In
2
O
3
nanowire field effect
transistors on self-assembled nanodielectric (SAND) organic and inorganic (Al
2
O
3
)
dielectric layers have been performed, and confirmed the excellent electronic transport
performance of our As-In
2
O
3
nanowires.
Besides the applications in chemical sensing and transparent electronics using
semiconductor 1-D nanostructures, we also proposed a versatile approach to produce so-
called hybrid-nanostructured thin films for the applications in electrochemical capacitors.
In Chapter 5, the first application was based on In
2
O
3
nanowire / SWNT hybrid
nanostructured thin film electrodes. Based on that, we successfully conceived and
fabricated highly transparent and flexible supercapacitors, which not only performed an
ideal electrochemical behavior but also exhibited good capacitance retention after a long
cycling measurement. We note that the approach developed in this work is universally
applicable to any 1-D nanostructures; therefore may be crucial in the development of
next-generation supercapacitors and lithium ion batteries.
To carter for the industry manufacturing, a mass-production approach, which utilizes
the inkjet-printing method as an example, has been presented in Chapter 6. The inkjet-
printing method provided a good controlling in pattern geometry, pattern location, film
thickness, and electrical conductivity. In addition, the electrochemical properties as well
161
as the possible charge storage mechanism were proposed. Finally, an improved device
performance was achieved through the integration of ruthenium oxide (RuO
2
) nanowires
together with printed SWNT films, and leaded to higher knee frequency of 1,500 Hz and
power density of 96 kW/kg, compared to bare SWNT-based supercapacitors.
Following the previous chapters, Chapter 7 tackled the factors which govern the
performance of supercapacitors in terms of operation voltage, energy density, and power
density. An asymmetric device structure is found to be a key factor for any high-
performance supercapacitors, which was realized by using two different hybrid-
nanostructured thin film electrodes as active materials in the cathode and the anode in the
cell system. Besides, the operating potential window, which is related to the output
energy density of supercapacitors, has been fine-tuned through the optimized
combination of positive and negative electrodes. Finally, a mass balance between two
electrodes has been optimized to ensure a stable electrochemical behavior in a wide
potential window. Our optimized hybrid-nanostructured asymmetric supercapacitors
exhibited a superior device performance with energy density of 25.4 Wh/kg, power
density of 50.3 kW/kg, and columbic efficiency of ~ 90%. We believe that the high-
performance hybrid-nanostructured asymmetric supercapacitors can find applications in
conformal electronics, portable electronics, and electrical vehicles.
8.2 Future Work in chemical sensing applications
We have carried out systematic studies on 1-D nanostructure based chemical sensors
and discussed two of the most important topics in chemical sensing society, including
electronic-nose technique and explosive chemical sensor. The wearable and flexible
162
SWNT chemical sensors have demonstrated good sensitivity to a trace of TNT vapor, and
nano electronic-nose sensor arrays have performed good discrimination power in
important industrial gases. These studies assert a foundation firmly for future research to
address the challenge remains. For instance, although 1-D nanostructures have showed
high sensitivity to various gases, the sensitivity is still not far beyond than that of 2-D
thin film sensing materials. The following is a list of possible ideas; some, however, are
mutually exclusive. One of future work on 1-D nanostructure based chemical sensors will
be focused on improving sensitivity by using “side-effects”, such as photo-induced effect
(e.g. TiO
2
), piezo-induced effect (e.g. ZnO), and phase-change effect (e.g. VO
2
and
Fe
3
O
4
). Kolmakov et al. have just proposed using a phase-change material, single crystal
VO
2
micro-ribbons as active material to detect Ar, He, and H
2
. Although the sensitivity is
far behind of current chemoresistors and FETs (S ~ 141 torr to He), the results have
suggested a new working mechanism and applications of phase change materials.
Besides of sensitivity, there are still a lot of room which can be improved in the
selectivity for practical applications in our life, in terms of easy fabrication, low cost, low
power consumption, and good integration with conformal electronics. Thus, instead of
phase-change materials, we propose to use ZnO nanowires as the sensing material for our
next generation chemical sensor study. ZnO, one of well-known piezo-electronic
materials, has been widely studied due to its unique opto-electrical and electronic
properties and a promising potential in nano electrical generators. However, only few
works have been done by utilizing its piezo-electronic properties in the application of
chemical sensor.
163
Herein, we present a new generation of chemical sensors built on ZnO nanowires on
flexible substrates, with characteristics of simple fabrication, mechanical flexibility, low
power consumption, and good selectivity. A schematic diagram of the flexible device is
shown below. As one can see, rather than single nanowire chemical sensor, we adapt
nanowire mat across several different electrodes, which provide minimal fabrication,
high-quality device, and high device yielding of ~ 100%. With different bending angles,
each device would be under different pressure, which generates different piezo-induced
effect to each device. Thus even only using one sensing material, each device can still
work as a different sensing component, which provide the desired “discrimination power”
for electronic nose system. In addition, due to “going-nano” effect, the chemical sensors
would only consume very little power for sensing applications.
Figure 8-1 Schematic diagram of flexible ZnO pizeoelectronic chemical sensors built on
Kapton substrates without bending force (a) and with bending force (b).
8.3 Future work in synthesis of 1-D nanostructures
To overcome the technique gap from laboratory to real life applications in
electronics and energy storage devices, the production of 1-D nanostructures with
164
characteristics of large quantity, high quality, doping capability, and low cost will be one
of most critical issue to the nanotechnology society. Regarding to the mass production of
carbon nanotubes, many approaches have been reported and demonstrated to produce
SWNTs and MWNTs in a large quantity, such as the arc-discharge process, the high-
pressure CO conversion (Hipco), and the so-called super-growth method on metal
substrates, which the synthesis efficiency is 100 times higher than the laser ablation
method. On the other hand, regarding to 1-D nanowires, there has been no report so far.
To address this issue, Si nanowires are used as an example, and we present easy but
efficient method for continuous production of silicon nanowires in both fixed bed and
fluidized bed catalytic chemical vapor deposition systems, which comprises the following
steps: preparing supported metal catalysts, adding supported nanosized metal catalysts
into reacting chamber, and growing silicon nanowires on the catalyst support by using
chemical vapor deposition of silicon atoms. The silicon nanowires are usually with a
diameter of 10-50 nm and a length of 5-20 μm in average. The apparatus is easy to
operate, has a high reaction rate, and can be used to produce silicon nanowires with high
degree of crystallization, high purity, and high yield.
Figure 8-2 Horizontal fluidized bed systems (a) Fix-bed type (b) Photo image of a fix-bed
type horizontal fluidized bed systems.
MFCs
Computer
Controlled
System Furnace
165
Figure 8-3 SEM images at (a) high magnification and (b) low magnification of bulk
synthesis of Si nanowires on Al
2
O
3
supported nanoparticles using horizontal fluidized
bed systems.
166
Bibliography
Akinwande D, Close G F, Wong H S P. Analysis of the frequency response of carbon
nanotube transistors. IEEE Transactions on Nanotechnology 2006; 5 (5): 599-605.
Algharaibeh Z, Liu X, Pickup P G. An asymmetric anthraquinone-modified
carbon/ruthenium oxide supercapacitor. Journal of Power Sources 2009; 187 (2):
1640-1643.
Althainz P, Dahlke A, Frietsch-Klarhof M, Goschnick J, Ache H J. Reception tuning
of gas-sensor microsystem by selective coating. Sensors and Actuators B 1995; 24-25
(1-3): 366-369.
An K H, Kim W S, Park Y S, Choi Y C, Lee S M, Chung D C, Bae D J, Lim S C, Lee
Y H. Supercapacitors using single-walled carbon nanotube electrodes. Advanced
Materials 2001; 13 (7): 497-500.
Ardizzone A, Fregonara G, Trasatti S. “Inner” and “outer” active surface of RuO
2
electrodes. Electrochema Acta 1990; 35 (1): 263-267.
Arico A S, Bruce P, Scrosati B, Tarascon J M, Schalkwijk W V. Nanostructured
materials for advanced energy conversion and storage devices. Nature Materials
2005; 4 (5): 366-377.
Arnold C, Harms M, Goschnick, J. Air quality monitoring and fire detection with the
Karlsruhe electronic micronose KAMINA. IEEE Sensors Journal 2002; 2 (3): 179-
188.
Asikanen T, Ritala M, Li W M, Lappalainen R, Leskela M. Modifying ALE grown
In
2
O
3
films by Benzoly Fluoride Pulses. Applied Surface Science 1977; 112: 231-235.
Baik J M, Kim M H, Larson C, Chen X, Guo S, Wodtke A M, Moskovits M. High-
yield TiO
2
nanowire synthesis and single nanowire field-effect transistor fabrication.
Applied Physics Letters 2008; 92 (24): 242111-242113.
Bandaru P R. Electrical properties and applications of carbon nanotube structures.
Journal of Nanoscience and Nanotechnology 2007; 7 (4/5):1239-1267.
Barsan N, Weimar U. Conduction model of metal oxide gas sensors. Journal of
Electroceramics 2001; 7 (3):143-167.
Barsoukov E, Macdonald J R. Impedance spectroscopy theory, experiment, and
applications. John Wiley & Sons, Inc. 2005, Hoboken, New Jersey, U.S.A.
167
Bartberger M D, Liu W, Ford E, Miranda K M, Switzer C, Fukuto J M, Farmer P J,
Wink D A, Houk K N. The reduction potential of nitric oxide (NO) and its importance
to NO biochemistry. Proceeding of the National Academy of Science on the United
States 2002; 99 (17): 10958-10963.
Batzill M, Diebold U. The surface and materials science of tin oxide. Journal of
Surface Science 2005; 79 (2-4): 47-154.
Bradley K, Gabriel J C P, Gruner G. Flexible nanotube electronics. Nano Letters 2003;
3 (10): 1353-1355.
Burke A. Ultracapacitors: why, how, and where is the technology 2000. Journal of
Power Sources 2000; 91 (1): 37-51.
Cao Q, Kim H S, Pimparkar N, Kulkarni J P, Wang C J, Shim M, Roy K, Alam M A,
Rogers J A. Medium-scale carbon nanotube thin-film integrated circuits on flexible
plastic substrates. Nature 2008; 454 (7203): 495-500.
Cao Q, Rogers J A. Ultrathin films of single-walled carbon nanotubes for electronics
and sensors: a review of fundamental and applied aspects. Advanced Materials 2009;
21 (1): 29-53.
Carcia P F, McLean R S, Reilly M H, Nunes G Jr. Transparent ZnO thin-film
transistor fabricated by rf magnetron sputtering. Applied Physics Letters 2003; 82 (7):
1117-1119.
Chae J, Appasamy S, Jian K. Patterning of indium tin oxide by projection
photoablation and lift-off process for fabrication of flat-panel displays. Applied
Physics Letters 2007; 90 (26): 261102-261104.
Chan C K, Peng H, Liu G, McIlwrath K, Zhang X F, Huggins R A, Cui Y. High-
performance lithium battery anodes using silicon nanowires. Nature Nanotechnology
2008; 3(1): 31-35.
Chang J, Lee W, Mane R S, Cho B W, Han S. Morphology-dependent electrochemical
supercapacitor properties of indium oxide. Electrochemical and solid state letters
2008; 11 (1): A9-A11.
Chang J, Hsu S, Tsai W, Sun I. A novel electrochemical process to prepare a high-
porosity manganese oxide electrode with promising pseudocapacitive performance .
Journal of Power Sources 2008; 177(2): 676-680.
Chang P, Fan Z, Tseng W, Rajagopal A, Lu J. G. Beta-Ga
2
O
3
nanowires: synthesis,
characterization, and p-channel field-effect transistor. Applied Physics Letters 2005;
87 (22): 222102-222104.
168
Che G L, Lakshmi B B, Fisher E R, Martin C R. Carbon nanotubule membranes for
electrochemical energy storage and production. Nature 1998; 393 (6683): 346-349.
Chen P C, Shen G, Zhou C. Chemical sensors and electronic noses based on one-
dimensional metal oxide nanostructures. IEEE Transactions on Nanotechnology 2008;
7 (6): 668-682.
Chen P C, Ishikawa F N, Chang H, Ryu K, Zhou C. A nanoelectronic nose: a hybrid
nanowire/carbon nanotube sensor array with integrated micromachined hotplates for
sensitive gas discrimination. Nanotechnology 2009; 20 (12): 125503-125510.
Chen P C, Shen G, Sukcharoenchoke S, Zhou C. Applied Physics Letters 2009; 94 (4):
43113-43115.
Chen Y J, Nie L, Xue X Y, Wang Y G, Wang T H. Linear ethanol sensing of SnO
2
nanorods with extremely high sensitivity. Applied Physics Letters 2006; 88 (8):
83105-83107.
Chen X H, Moskovits M. Observing catalysis through the agency of the participating
electrons: surface-chemistry-induced current changes in a tin oxide nanowire
decorated with silver. Nano Letters 2007; 7 (3): 807-812.
Chiang H Q, Hong D, Hung C M, Presley R E, Wager J F, Park C H, Keszler D A,
Herman G S. Thin-film transistors with amorphous indium gallium oxide channel
layers. Journal of Vacuum Science B 2006; 24 (6): 2702-2705.
Claye A S, Fisher J E, Huffman C B, Rinzler A G, Smalley R E. Solid-state
electrochemistry of the Li single wall carbon nanotube system. Journal of the
Electrochemical Society 2000; 147 (8): 2845-2852.
Collins P G, Bradley K, Ishigami M, Zettl A. Extreme oxygen sensitivity of electronic
properties of carbon nanotubes. Science 2000; 287 (5459): 1801-1804.
Comini E, Faglia G, Sberveglieri A, Pan Z W, Wang Z L. Stable and highly sensitive
gas sensors based on semiconducting oxide nanobelts. Applied Physics Letters 2002;
81 (10): 1869-1871.
Comini E, Faglia G, Sberveglieri G, Calestani D, Zanotti L, Zha M. Tin oxide
nanobelts electrical and sensing properties. Sensors and Actuators B 2005; 111-112
(1): 2-6.
Conway B E. Electrochemical Capacitors. Kluwer Academic Publishers. New York
(1999).
Cottineau T, Toupin M, Delahaye T, Brousse T, Bélanger D. Nanostructured transition
metal oxides for aqueous hybrid electrochemical supercapacitors. Applied Physics A
2006; 82 (4): 599-106.
169
Craven M A, Gardner J W, Bartlett P N. Electronic Noses-development and future
prospects. Trends in Analytical Chemistry 1996; 15 (9): 486-493.
Cui Y , Wei Q Q, Park H K, Lieber C M. Nanowire nanosensors for highly sensitive
and selective detection of biological and chemical species. Science 2001; 293 (5533):
1289-1292.
Dag S, Ozturik Y, Ciraci S, Yildirim T. Adsorption and dissociation of hydrogen
molecules on bare and functionalized carbon nanotubes. Physics Review B 2005; 72
(15): 155404-155411.
Dattoli E N, Wan Q, Guo W, Chen Y B, Pan X Q, Lu W. Fully transparent thin-film
transistor devices based on SnO
2
nanowires. Nano Letters 2007; 7 (8): 2463-2469.
Delac K, Grgic M, Grgic S. Independent comparative study of PCA, ICA, and LDA
on the FERET data set. International Journal of Imaging System Technolology 2005;
15 (5): 252-260.
DiBenedetto S, Facchetti A, Ratner M A, Marks T J. Molecular self-assembled
monolayers and multilayers as gate dielectrics for organic thin film transistor
applications. Advanced Materials, 2009; 21 (14-15): 1407-1433.
Du N, Zhang H, Chen B, Ma X, Liu Z, Wu J, Yang D. Porous indium oxide nanotubes:
layer-by-layer assembly on carbon-nanotube templates and application for room-
temperature NH
3
gas sensors. Advanced Materials 2007. 19 (12): 1641-1645.
Duan X F, Huang Y , Agarwal R, and Lieber C M. Single-nanowire electrically driven
lasers. Nature 2003; 421 (6920): 241-245.
Duan X, Huang Y , Cui, Wang J, Lieber C M. Indium phosphide nanowires as building
blocks for nanoscale electronic and optoelectronic devices. Nature 1998; 409 (66-69):
66-69.
Duffy N W, Baldsing W, Pandolfo A G. The nickel-carbon asymmetric supercapacitor-
Performance, energy density and electrode mass rations. Electrochimica Acta 2008;
54 (2): 535-539.
DUrkop T, Getty S A, Cobas E, Fuhrer, M S. Extraordinary mobility in
semiconducting carbon nanotubes. Nano Letters 2004; 4 (1): 35-39.
Fan Z Y , Ho J C, Jacobson Z A, Razavi H, Javey A. Large-scale, heterogeneous
integration of nanowire arrays for image sensor circuitry. Proceedings of the National
Academy of Sciences of the United States of America 2008; 105 (32): 11066-11070.
Fan Z, Lu J G. Gate-refreshable nanowire chemical sensors. Applied Physics Letters
2006; 86 (12): 123510-123512.
170
Fan Z, Wen X, Yang S, Lu J G. Controlled p- and n-type doping of Fe
2
O
3
nanobelt
field effect transistors. Applied Physics Letters 2005; 87 (1):13113-13115.
Folch J, Capdevila X G, Segarra M, Morante J R. Solid electrolyte multisensor system
for detecting O
2
, CO, and NO
2
. Journal of the Electrochemical Society 2007; 154 (7):
J201-J208.
Fortunato E, Barquinha P, Goncalves A, Marques A, Pereria L, Martins R. Recent
advances in zno transparent thin film transistors. Thin Solid Films 2005; 487 (1-2):
205-211.
Frackowiak E, Beguin F. Carbon materials for the electrochemical storage of energy
in capacitors. Carbon 2001; 39 (6): 937-950.
Frackowiak E, Metenier K, Bertagna V, Beguin F. Supercapacitor electrodes from
multiwalled carbon nanotubes. Applied Physics Letters 2000; 77 (15): 2421-2423.
Francioso L, Taurino A M, Forleo A, Siciliano P. TiO
2
nanowires array fabrication and
gas sensing properties. Sensors and Actuators B 2008; 130 (1): 70-76.
Fryberger T B, Semancik S. Conductance response of Pd/SnO (110) model gas
sensors to H
2
and O
2
. Sensors and Actuators B 1990; 2 (4): 305-309.
Futaba D N, Hata K, Yamada T, Hiraoka T, Hayamizu Y, Kakudate Y, Tanaike O,
Hatori H, Yumura M, Iijima S. Shape-engineerable and highly densely packed single-
walled carbon nanotubes and their application as super-capacitor electrode. Nature
Materials 2006; 5 (12): 987-994.
Gallegos A K C, Rincon M E. Carbon nanofiber and PEDOT-PSS bilayer systems as
electrodes for symmetric and asymmetric electrochemical capacitor cells. Journal of
Power Sources 2006; 162 (1): 743-747.
Goldman E R, Medintz I L, Whitley J L, Hayhurst A, Clapp A R, Uyeda H T,
Deschamps J R, Lessman M E, Mattoussi H. Trace explosives signatures from World
War II unexploded undersea ordnance. Journal of the American Chemistry Society
2005; 127 (18): 6744-6751.
Goschnick J, Haeringer D, Kiselev I. Multicomponent quantification with a novel
method applied to gradient gas sensor microarray signal patterns. Sensors and
Actuators B 2007; 127 (1): 237-241.
Gruner G. Carbon nanotube films for transparent and plastic electronics. Journal of
Materials Chemistry 2006; 16 (35): 3533-3539.
Guo Y, Hu J, Wan L. Nanostructured materials for electrochemical energy conversion
and storage devices. Advanced Materials 2008; 20 (23): 2878-2887.
171
Han S, Zhang D, Zhou C. Synthesis and electronic properties of ZnO/CoZnO core-
shell nanowires. Applied Physics Letters. 2006; 88 (13): 133109-133111.
Han S, Liu X, Zhou C. Template-free directional growth of single-walled carbon
nanotubes on a- and r- plane. Journal of the American Chemistry Society 2005; 127
(15): 5294-5295.
Hirao T, Furuta M, Furuta H, Matsuda T, Hiramatsu T, Hokari H, Yoshida M. High
mobility top-gate zinc oxide thin-film transistors (ZnO-TFTs) for active-matrix liquid
crystal displays. SID 06 Digit Digest 2006; 18-20.
Hoffman R L, Norris B J, Wager J F. ZnO-based transparent thin-film transistors.
Applied Physics Letters 2003; 82 (5): 733-735.
Hu C, Chang K, Lin M, Wu Y . Design and tailoring of the nanotubular arrayed
architecture of hydrous RuO2 for next generation supercapacitors. Nano Letters 2006;
6 (12): 2690-2695.
Hu L, Choi J W, Yang Y, Jeong S, Mantia F L, Cui L F, Cui Y. Highly conductive
paper for energy-storage devices. Proceeding of the National Academy of Sciences of
the United Sates 2009; 106 (51): 21490-21494.
Huang M H, Mao S, Feick H, Yan H Q, Wu Y Y , Kind H, Weber E, Russo R, Yang P
D. Room-temperature ultraviolet nanowire nanolasers. Science 2001; 292 (5523):
1897-1899.
Iijima S. Helical microtubules of graphitic carbon. Nature 1991; 354 (6348): 56-58.
Inoue S, Ueda K, Hosono H, Hamada N. electronic structure of the transparent p-type
semiconductor (LaO)CuS. Physical Review B 2001; 64 (24): 245211-245215.
Ishikawa F N, Chang H K, Ryu K, Chen P, Badmaev A, De Arco Gomez L, Shen G,
Zhou C. Transparent electronics based on transfer printed aligned carbon nanotubes
on rigid and flexible substrates. ACS Nano, 2008; 3 (1): 73-79.
Jiang Y , Zhang W J, Jie J S, Meng X M, Fan X, Lee S T. Photoresponse properties of
CdSe single-nanoribbon photodetectors. Advanced Functional Materials. 2007; 17
(11): 1795-1800.
Javey A, Nam S, Friedman R S, Yan H, Lieber C M. Layer-by-layer assembly of
nanowires for three-dimensional, multifunctional electronics. Nano Letters 2007; 7
(3): 773-777.
Javey A, Guo J, Wang Q, Lundstrom M, and Dai H J. Ballistic carbon nanotube field-
effect transistors. Nature 2003; 424 (6949): 654-657.
172
Jeremy T R, Perkins F K, Snow E S, Wei Z, Sheehan P E. Reduced graphene oxide
molecular sensors. Nano Letters 2008; 8 (10): 3137-3140.
Ju S, Li J, Liu J, Chen P C, Ha Y, Ishikawa F, Chang H, Zhou C, Faccheti A, Janes D
B, Marks T J. Transparent active matrix organic light-emitting diode displays driven
by nanowire transistor circuitry. Nano Letters 2008; 8 (4): 997-1004.
Ju S, Lee K, Janes D B, Yoon M Y, Facchetti A, Marks T J. Nanowire field-effect
transistors enabled by self-assembled organic gate nanodielectrics. Nano Letters 2005;
5 (11): 2281-2286.
Ju S, Chen P, Zhou C, Ha Y, Facchetti A, Marks T J, Kim S K, Mohammadi S, Janes
D B. 1/f noise of SnO
2
nanowire transistors. Applied Physics Letters 2008; 92 (24):
243120-243122.
Ju S, Facchetti A, Xuan Y , Liu J, Ishikawa F, Ye P, Zhou C, Marks T J, Janes D B.
Fabrication of fully transparent nanowire transistors for transparent and flexible
electronics. Nature Nanotechnology 2007; 2 (6): 378-384.
Ju S, Lee K, Yoon M, Facchetti A, Marks T J, Janes D B. High performance ZnO
nanowire field effect transistors with organic gate nanodielectrics: effects of metal
contacts and ozone treatment. Nanotechnology 2007; 18 (15): 155201-155205.
Ju S, Ishikawa F N, Chen P, Chang H, and Zhou C. High performance In
2
O
3
nanowire
transistors using organic gate nanodielectrics. Applied Physics Letters 2008; 92 (22):
222105-222107.
Jurewicz K, Frackowiak E, Beguin F. Towards the mechanism of electrochemical
hydrogen storage in nanostructured carbon materials. Applied Physics A 2004; 78 (7):
981-987.
Jurs P C, Bakken G A, McClelland H E. Computational methods for the analysis of
chemical sensor array data from volatile analytes. Chemistry Review. 2000; 100 (7):
2649-2678.
Kaempgen M, Ma J., Gruner G, Wee G, Mhaisalkar S G. Bifunctional carbon nanotube
networks for supercapacitors. Applied Physics Letters 2007; 90 (26): 264104-264106.
Kaempgen M, Chan C K, Ma J, Cui Y, Gruner G. Printable thin film supercapacitors
using single-walled carbon nanotubes. Nano Letters 2009; 9 (8): 1919-1923.
Kafafi Z H. Organic electroluminescence. Taylor & Francis Group 2005. Boca Raton,
U.S.A.
Kang K, Meng Y S, Bréger J, Grey C P, Ceder G. Electrodes with high power and
high capacity for rechargeable lithium batteries. Science 2006; 311 (5763): 977-980.
173
Kang S J, Kocabas C, Ozel T, Shim M, Pimparkar N, Alam M A, Rotkin S V, Rogers
J A. High-performance electronics using dense, perfectly aligned arrays of single-
walled carbon nanotubes. Nature Nanotechnology 2007; 2 (11): 230-236.
Kawazoe H, Yasukawa M, Hyodo H, Kurita M, Yanagi H, Hosono H, Kawaoe H. P-
type electrical conduction in transparent thin films of CuAlO
2
. Nature 1997; 389
(6654): 939-942.
Khomenko V, Raymundo-Pieñro E, Béguin F. Optimization of an asymmetric
manganese oxide /activated carbon capacitor working at 2V in aqueous medium. J. of
Power Sources 2006; 153 (1): 183-190.
Khomenko V, Raymundo-Piñero E, Frackowiak E, Béguin F. High-voltage
asymmetric supercapacitors operating in aqueous electrolyte. Applied Physics A.
2006; 82 (4): 567-573.
Kiebele A, Gruner G. Carbon nanotube based battery architecture. Applied Physics
Letters 2007; 91 (14): 144104-144106.
Kim D K, Muralidharan P, Lee H W, Ruffo R, Yang Y , Chan C K, Peng H, Huggins R
A, Cui Y . Spinel LiMnO nanorods as lithium ion battery cathodes. Nano Letters 2008;
8 (11): 3948-3952.
Kim G T, Muster J, Krstic V, Park J G, Park Y W, Roth S, Burghard M. Field-effect
transistor made of individual V
2
O
5
nanofibers. Applied Physics Letters 2000; 76 (14):
1875-1877.
Kim Y S, Ha S C, Kim K. Room-temperature semiconductor gas sensor based on
nonstoichiometric tungsten oxide nanorod film. Applied Physics Letters 2005. 86 (21):
213105-213107.
Kocabas C, Kim H S, Banks T, Rogers J A, Pesetski A A, Baumgardner J E,
Krishnaswamy S V , Zhang H. Radio frequency analog electronics based on carbon
nanotube transistors. Proceedings of the National Academy of Sciences of the United
States of America 2008; 105 (5): 1405-1509.
Kocabas C, Hur S, Gaur A, Meitl A M, Shim M, Rogers J A. Guided growth of large-
scale, horizontally aligned arrays of single-walled carbon nanotubes and their use in
thin-film transistors. Small 2005; 1 (11): 1110-1116.
Kolmakov A, Klenov D O, Lilach Y , Stemmer S, Moskovits M. Enhanced gas sensing
by individual SnO
2
nanowires and nanobelts functionalized with Pd catalyst particles.
Nano Letters 2005; 5 (4): 667-673.
Kong J. Franklin N R, Zhou C, Chapline M G, Peng S, Cho K J, Dai H J. Nanotube
molecular wires as chemical sensors. Science 2000; 287 (5453): 622-625.
174
Kolmakov A, Moskovits M. Chemical sensing and catalysis by one-dimensional
metal-oxide nanostructures. Annual Review of Materials Research 2004; 34: 151-180.
Kords K, Mustonen T, Tth G, Jantunen H, Lajunen M, Soldano C, Talapatra S, Kar S,
Vajtai R, Ajayan P M. Inkjet printing of electrically conductive patterns of carbon
nanotubes. Small 2006; 2 (8-9): 1021-1025.
Kumar M K, Ramaprabhu S. Nanostructured Pt functionlized multiwalled carbon
nanotube based hydrogen sensor. Journal of Physical Chemistry B 2006; 110 (23):
11291-11298.
Law M, Kind H, Messer B, Kim F, Yang P. Photochemical sensing of NO
2
with SnO
2
nanoribbon nanosensors at room temperature. Angewandte Chemie International
Edition 2002; 41 (13): 2405-2408.
Lao C, Li Y, Wong C P, Wang Z L. Enhancing the electrical and optoelectronic
performance of nanobelt devices by molecular surface functionalization. Nano Letters
2007; 7 (5): 1323-1328.
Lee C H, Kuo C V , Lee C L. Effects of heat treatment and ion doping of indium oxide.
Thin Solid Film 1989; 173 (1): 59-66.
Lee S W, Kim B S, Chen S, Yang S H, Hammond P T. Layer-by-layer assembly of all
carbon nanotube ultrathin films for electrochemical applications. Journal of the
American Chemistry Society 2009; 131 (2): 671-679.
Lee Y, Chueh Y, Hsieh C, Chang M, Chou L, Wang Z L, Lan Y, Chen C, Kurata H,
Isoda S. P-type alpha-Fe
2
O
3
Nanowires and their n-type transition in a reductive
ambient. Small; 3 (8):1356-1361.
Lemire G W, Simeonsson J B, Sausa R C. Monitoring of vapor-phase nitro
compounds using 226-nm radiation: fragmentation with subsequent NO resonance-
enhanced multiphoton ionization detection. Analytical Chemistry 1993; 65 (5): 529-
533.
Li C, Fan W D, Lei B, Zhang D H, Han S, Tang T, Liu X L, Liu Z Q, Asano S,
Meyyappan M, Han J, and Zhou C. Multilevel memory based on molecular devices.
Applied Physics Letters 2004; 84 (11); 1949-1951.
Li C, Lei B, Fan W, Zhang D, Meyyappan M, Zhou C. Molecular memory based on
nanowire - molecular wire heterostructures. Journal of Nanoscience and
Nanotechnology 2007; 7 (1): 138-150.
Li C, Zhang D H, Liu X L, Han S, Tang T, Han J, Zhou C. In
2
O
3
nanowires as
chemical sensors. Applied Physics Letters 2003; 82 (10): 1613-1615.
175
Li C, Zhang D H, Han S, Liu X L, Tang T, Zhou C. Diameter-controlled growth of
single-crystalline In
2
O
3
nanowires and their electronic properties. Advanced Materials
2003; 15 (2): 143-146.
Li Q H, Liang Y X, Wan Q, Wang T H. Oxygen sensing characteristics of individual
ZnO nanowire transistors. Applied Physics Letters 2005; 85 (26): 6389-6391.
Lim H J, Lee D Y, Oh Y J. A comparison between several vibration-powered
piezoelectric generators for standalone systems. Sensors and Actuators A 2006; 125
(2): 405-410.
Liu J, Cao G, Yang Z, Wang D, Dubois D, Zhou X, Graff G L, Pederson L R, Zhang J
G. Oriented nanostructures for energy conversion and storage. ChemSusChem 2008;
1 (8-9): 676-697.
Liu R, Lee S B. MnO
2
/Poly(3,4-ethylenedioxythiophene) coaxial nanowires by one-
step coelectrodeposition for electrochemical energy storage. Journal of the American
Chemical Society 2008; 130 (10): 2942-2493.
Liu X, Lee C, Han S, Li C, Zhou C. Molecular Nanoelectronics American Scientific
Publishers 2003; 2-38.
Liu Y L, Wu Z Y, Lin K J, Huang J J, Lin Y H, Jian W B, Lin J J. Growth of single-
crystalline RuO
2
nanowires with one- and two- nanocontact electrical
characterizations. Applied Physics Letters 2007; 90 (1): 13105-13107.
Liu Z, Zhang D, Han S, Li C, Tang T, Jin W, Liu Z, Lei B, Zhou C. Laser ablation
synthesis and electron transport studies of tin oxide nanowires. Advanced Materials
2003; 15 (20): 1754-1757.
Lieber C M. Nanoscale science and technology: Building a big future from small
things. MRS Bulletin 2003; 28 (7): 486-491.
Liu J, Wang X, Peng Q, Li Y. Vanadium Pentoxide nanobelts: highly selective and
stable ethanol sensor materials. Advanced Materials 2005; 17 (6): 764-767.
Long J W, Dunn B, Rolison D R, White H S. Three-dimensional battery architecture.
Chemical Reviews 2004; 104 (10): 4463-4492.
Lota K, Khomenko V, Frackowiak E. Capacitance properties of poly(3,4-
ethylenedioxythiophene)/carbon nanotubes composites. Journal of Physics and
Chemistry of Solids 2004; 65 (2-3): 295-301.
Lu J G, Chang P, Fan Z. Quasi-one-dimensional metal oxide materials-synthesis,
properties and applications. Material Science Engineering Report 2006; 52 (1-3): 49-
91.
176
Lu W, Lieber C M. Nanoelectronics from the bottom up. Nature Materials 2007; 6
(11): 841-850.
Lu W, Xie P, and Lieber C M. Nanowire transistor performance limits and
applications. IEEE Transactions on Electron Devices 2008; 55 (11): 2859-2876.
Lu Y, Partridge C, Meyyappan M, Li J. A carbon nanotube sensor array for sensitive
gas discrimination using principal component analysis. Journal of Electroanalytical
Chemistry 2006; 593 (1-2): 105110.
Maiti A, Rodriguez J A, Law M, Kung P, McKinney J R, Yang P. SnO
2
nanoribbons
as NO
2
sensors: insights from first principles calculations. Nano Letters 2003; 3 (8):
1025-1028.
Martel R, Schmidt T, Shea H R, Hertel T, Avouris P. Single- and multi-wall carbon
nanotube field-effect transistors. Applied Physics Letters 1998; 73 (17): 2447-2449.
McAlpine M C, Ahmad H, Wang D, Heath J R. Highly ordered nanowire arrays on
plastic substrates for ultrasensitive flexible chemical sensors. Nature Materials 2007;
6 (5): 379-384.
Meier D C, Semancik S, Button B, Strelcov E, Kolmakov A. Coupling nanowire
chemiresistors with MEMS microhotplate gas sensing platforms. Applied Physics
Letters 2007; 91 (6): 63118-63120.
Minami T. New N-type transparent conducting oxides. MRS Bulletin 2000; 25: 38-44.
Moseley P T. New trends and future prospects of thick-and thin-film gas sensors.
Sensors and Actuators B 1991; 3 (3): 167-174.
Nilsson H A, Duty T, Abay S, Wilson C, Wagner J B, Thelander C, Delsing P,
Samuelson L. A radio frequency single-electron transistor based on an InAs/InP
heterostructure nanowire. Nano Letters 2008; 8 (3): 872-875.
Niu C, Sichel E K, Hoch R, Moy D, Tennent H. High power electrochemical
capacitors based on carbon nanotube electrodes. Applied Physics Letters 1997; 70
(11): 1480-1482.
Nomura K, Ohta H, Ueda K, Kamiya T, Hirano M, Hosono H. Thin-film transistor
fabricated in single-crystalline transparent oxide semiconductor. Science 2003; 300
(5623): 1269-1272.
Novak J P, Snow E S, Houser E J, Park D, Stepnowski J L, McGill R A. Nerve agent
detection using networks of single-walled carbon nanotubes. Applied Physics Letters
2003; 83 (19): 4026-4028.
177
Okubo M, Hosono E, Kim J, Enomoto M, Kojima N, Kudo T, Zhou H, Honma I.
Nanosize effect on high-rate Li-ion intercalation in LiCoO
2
electrode. Journal of the
American Chemical Society 2007; 129 (23): 7444-7452.
Oregan G, Gratzel M. A Low-cost, high-efficiency solar-cell based on dye-sensitized
colloidal TiO
2
films. Nature 1991; 353 (6346): 737-740.
Oxley J C, Smith J L, Zhou Z L, Mckenney R L J. Thermal Decomposition Studies on
NTO and NTO/TNT1. Phys. Chem. 1995; 99 (25): 10383-10391.
Pasquier A D, Plitz I, Menocal S, Amatucci G. A comprehensive study of Li-ion
battery, supercapacitor, and nonaqueous asymmetric hybrid devices for automotive
applications. Journal of Power Sources 2003; 115 (1): 171-178.
Patolsky F, Zheng G, Lieber C M. Nanowire sensors for medicine and the life
sciences. Nanomedicine 2006; 1 (1): 51-65.
Persaud K, Dodd G H. Analysis of discrimination mechanisms of the mammalian
olfactory system using a model nose. Nature 1982; 299: 352-355.
Philip J, Punnoose A, Kim B I, Reddy K M, Layne S, Holmes J O, Satpati B, LeClair
P R, Santos T S, Moodera J S. Carrier-controlled ferromagnetismin transparent oxide
semiconductors. Nature Materials 2006; 5 (4): 298-304.
Pinnaduwage L A, Gehl A, Hedden D L, Muralidharan G, Thundat T, Lareau R T,
Sulchek T, Manning L, Rogers B, Jones M, Adams J D. Explosives: a microsensor for
trinitrotoluene vapour. Nature 2003; 425 (7127): 474-478.
Prasad K R, Koga K, Miura N. Electrochemical deposition of nanostructured indium
oxide: High-performance electrode material for redox supercapacitors. Chemistry of
Materials 2004; 16 (10): 1845-1847.
Pushparaj V L, Shaijumon M M, Kumar A, Murugesan S, Ci L, Vajtai R, Linhardt J R,
Nalamasu O, P M. AjayanP M. Flexible energy storage devices based on
nanocomposite paper. Proceedings of the National Academy of Sciences of the United
States of America 2007; 104 (34): 13574-13577.
Qi P, Vermesh O, Grecu M, Javey A, Wang Q, Dai H. Toward large arrays of
multiplex functionalized carbon nanotube sensors for highly sensitive and selective
molecular detection. Nano Letters 2003; 3 (3): 347-351.
Qian L H, Wang K, Li Y, Fang H T, Lu Q H, Ma X L. CO sensor based on Au-
decorated SnO
2
nanobelt. Materials Chemistry and Physics 2006; 100 (1): 82-84.
Qin X, Durbach S, Wu G T. Electrochemical characterization on RuO
2
xH
2
O/carbon
nanotubes composite electrodes for high energy density supercapacitors. Carbon 2004;
42 (2): 451-453.
178
Qu Q, Zhang P, Wang B, Chen Y, Tian S, Wu Y, Holze R. Electrochemical
performance of MnO
2
nanorods in neutral aqueous electrolyte as a cathode for
asymmetric supercapacitors. Journal of Physical Chemistry C. 2009; 113 (31): 14020-
14027.
Ragupathy P, Vasan H N, Munichandraiah N. Cobalt hydroxide as a capacitor
material: Tuning its potential window. Journal of Electrochemistry Society 2008; 155
(11): A855-A861.
Raible I, Burghard M, Schlecht U, Yasuda A, V ossmeyer T. V
2
O
5
nanofibers: novel
gas sensors with extremely high sensitivity and selectivity to amines. Sensors and
Actuators B 2005; 106 (2): 730-735.
Ramani M, Haran B S, White R E, Popov B N. Synthesis and characterization of
hydrous ruthenium oxide-carbon supercapacitors. Journal of Electrochemistry Society
2001; 148 (4): A374-380.
Rao C N R, and Govindaraj A. Nanotubes and nanowires, RSC publishing,
Cambridge, UK (2005).
Raty J Y , Gygi F, Galli G. Growth of carbon nanotubes on metal nanoparticles: A
microscopic mechanism from ab initio molecular dynamics simulations. Physical
Review Letters 2005; 95 (9): 96103-96106.
Richard C, Balavoine F, Schultz P, Ebbesen T W, Mioskowski C. Supramolecular
self-assembly of lipid derivatives on carbon nanotubes. Science 2003; 330(5620):
775-778.
Reddy A L M, Ramaprabhu S. Nanocrystalline metal oxides dispersed multiwalled
carbon nanotubes as supercapacitor electrodes. Journal of Physical Chemistry C 2007;
111 (21): 7727-7734.
Rogers J A, Bao Z, Baldwin K, Dodabalapur A, Crone B, Raju V R, Kuck V, Katz H,
Amundson K, Ewing J, Drzaic P. Paper-like electronic displays: large-area rubber-
stamped plastic sheets of electronics and microencapsulated electrophoretic inks.
Proceedings of the National Academy of Sciences of the United States of America
2001; 98 (9): 4835-4840.
Ryu K, Badmaev A, Wang C, Lin A, Patil N, Gomez L, Kumar A, Mitra S, Wong H S
P, Zhou C. CMOS-Analogous wafer-scale nanotube-on-insulator approach for
submicrometer devices and integrated circuits using aligned nanotubes. Nano Letters
2009; 9 (1): 189-197.
Ryu K, Zhang D, Zhou C. High-performance metal oxide nanowire chemical sensors
with integrated micromachined hotplates. Applied Physics Letters 2008; 92 (9):
093111-093113.
179
Samant S, Gopal A. Study of a prototype high quantum efficiency thick scintillation
crystal video-electronic portal imaging device. Medical Physics 2006; 33 (8): 2783-
2791.
Sberveglieri G, Baratto C, Comini E, Faglia G, Ferroni M, Ponzoni A, V omiero A.
Synthesis and characterization of semiconducting nanowires for gas sensing. Sensors
and Actuators B 2007; 121 (1): 208-213.
Schmelling C D, Gray K A. Photocatalytic transformation and mineralization of
2,4,6-trinitrotoluene (TNT) in TiO
2
slurries. Water Research 1995; 29 (12): 2651-
2662.
Scorsone E, Pisanelli A M, Persaud K C. Development of an electronic nose for fire
detection. Sensors and Actuators B 2006; 116 (1-2): 55-61.
Scott S M, James D, Ali Z. Data analysis for electronic nose systems. Microchimica
Acta 2007; 156 (3-4): 183-207.
Semancik S, Cox D F. Fundamental characterization of clean and gas-dosed tin oxide.
Sensors and Actuators1987; 12 (2):101-106.
Shaijumon M M, Ou F S, Ci L, Ajayan P M. Synthesis of hybrid nanowire arrays and
their application as high power supercapacitor electrodes. Chemical Communications
2008; 20 (20): 2373-2375.
Short L C, Benter T. Selective measurement of HCHO in urine using direct liquid-
phase fluorimetric analysis. Clinical Chemistry and Laboratory Medicine 2005; 43
(2):178-182.
Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nature Materials 2008;
7 (11): 845-854.
Sin M L Y, Chow G C T, Wong G M K, Li W J, Leong P H W. Ultralow-power
alcohol vapor sensors using chemically functionalized multiwalled carbon nanotubes.
IEEE Transactions on Nanotechnology 2007; 6 (6): 571-577.
Small W R, Panhuis M I H. Inkjet printing of transparent, electrically conducting
single-walled carbon-nanotube composites. Small 2007; 3 (9): 1500-1509.
Snow E S, Perkins F K, Houser E J, Badescu S C, Reinecke T L. Chemical detection
with a single-walled carbon nanotube capacitor. Science 2005; 307 (5717): 1942-
1945.
Snow E S, Perkins F K, Robinson J A. Chemical vapor detection using single-walled
carbon nanotubes. Chemical Society Reviews 2006; 35 (9): 790-798.
180
Snow E S, Perkins F K. Capacitance and conductance of single-walled carbon
nanotubes in the presence of chemical vapors. Nano Letters 2005; 5 (12): 2414-2417.
Soci C, Zhang A, Xiang B, Dayeh S, Aplin D P R, Park J, Bao X, Lo Y , Wang D. ZnO
nanowire UV photodetectors with high internal gain. Nano Letters 2007; 7 (4): 1003-
1009.
Sohn H, Sailor M J, Magde D, Trogler W C. Detection of Nitroaromatic explosives
based on photoluminescent polymers containing metalloles. Journal of the American
Chemistry Society 2003; 125 (13): 3821-3830.
Someya T, Small J, Kim P, Nuckolls C, Yardley J T. Alcohol vapor sensors based on
single-walled carbon nanotube field effect transistors. Nano Letters 2003; 3 (7): 877-
881.
Song J W, Kim J, Yoon Y H, Choi B S, Kim J H, Han C S. Inkjet printing of single-
walled carbon nanotubes and electrical characterization of the line pattern.
Nanotechnology 2008; 19 (9): 95702-95707.
Srivastave A K, Dravid V P. On the performance evaluation of hybrid and mono-class
sensor arrays in selective detection of VOCs: A comparative study .Sensors and
Actuators B 2006; 117 (1): 244-252.
Star A, Joshi V, Skarupo S, Thomas D, Gabriei J. -C. P. Gas sensor array based on
metal-decorated carbon nanotubes. Journal of Physical Chemistry B 2006; 110 (42):
21014-21020.
Staii C, Johnson A T Jr, Chen M, Gelperin A. Free energy landscape of a
DNA−carbon Nanotube hybrid using replica exchange molecular dynamics. Nano
Letters 2005; 5 (2): 1774-1778.
Subramanian V, Zhu H, Vajtai R, Ajayan P M, Wei B. Hydrothermal synthesis and
pseudocapacitance properties of MnO
2
nanostructures. Journal of Physical Chemistry
B 2005; 109 (43): 20207-20214.
Sun Y G, Rogers J A. Inorganic semiconductors for flexible electronics. 2007; 19 (15):
1897-1916.
Sun Z, Liu Z, Han B, Miao S, Du J, Miao Z. Microstructural and electrochemical
characterization of RuO2/CNT composite synthesized in supercritical diethyl amine.
Carbon 2006; 44 (5): 888-893.
Sysoev V V , Button B K, Wepsoec K, Dmitriev S, Kolmakov A. Toward the
nanoscopic "electronic nose": hydrogen vs carbon monoxide discrimination with an
array of individual metal oxide nano- and mesowire sensors. Nano Letters 2006; 6 (8):
1584-1588.
181
Sysoev V V, Button B K, Wepsiec K, Dmitriev S, Kolmakov A. Gradient microarray
electronic nose based on percolating SnO
2
nanowire sensing elements. Nano Letters
2006; 6 (8): 1584-1588.
Tahar R B H, Ban T, Ohya Y, Takahashi Y. Tin doped indium oxide thin films:
electrical properties. Journal of Applied Physics 1998; 83 (5): 2631-2645.
Tans S J, Devoret M H, Dai H J, Thess A, Smalley R E, Geerligs L J, Dekker C.
Individual single-wall carbon nanotubes as quantum wires. Nature 1997; 386 (6624):
474-477.
Tao S, Li G. Porphyrin-doped mesoporous silica films for rapid TNT detection.
Colloid Polym. Sci. 2007; 285 (7): 721-728.
Then D, Vidic A, Ziegler C H. A highly sensitive self-oscillating cantilever array for
the quantitative and qualitative analysis of organic vapor mixtures. Sensors and
Actuators B 2006; 117 (1): 1-9.
Tien L C, Sadik P W, Norton D P, Voss L F, Pearton S J, Wang H T, Kang B S, Ren F,
Jun J, Lin J. Hydrogen sensing at room temperature with Pt-coated ZnO thin films
and nanorods. Applied Physics Letters 2005; 87 (22): 222106-222108.
Trasatti S. Physical electrochemistry of ceramic oxides. Electrochema Acta 1991; 36
(2): 225-241.
Ueda K, Hase T, Yanagi H, Kawazoe H, Hosono H, Ohta H, Orita M, Hirano M.
Epitaxial growth of transparent P-type conducting CuGaO2 thin films on sapphire
(001) substrates by pulsed laser deposition. Journal of Applied Physics 2001; 89 (3):
1790-1793.
Varghese O K, Gong D, Paulose M, Ong K G, Grimes C A. Hydrogen sensing using
titania nanotubes. Sensors and Actuators B 2003; 93 (1-3): 338-344.
V omiero A, Bianchi S, Comini E, Faglia G, Ferroni M, Sberveglieri G. Controlled
growth and sensing properties of In
2
O
3
nanowires. Crystal Growth & Design 2007; 7
(12): 2500-2504.
Wager J F, Keszler D A, Presley R E. Transparent electronics. Springer Science +
Business Media, LLC 2008, New York., U.S.A.
Wan Q, Li Q H, Chen Y J, Wang T H, He X L, Li J P, Lin C L. Fabrication and
ethanol sensing characteristics of ZnO nanowire gas sensors. Applied Physics Letters
2004; 84 (18): 3654-3656.
Wan Q, Dattoli E N, Lu W. Doping dependent electrical characteristics of SnO
2
nanowires. Small 2008; 4 (4): 451-454.
182
Wang C, Chu X, Wu M. Detection of H
2
S down to ppb levels at room temperature
using sensors based on ZnO nanorods. Sensors and Actuators B 2006; 113 (1): 320-
323.
Wang D, Yu Z, McKernan S, and Burke P J. Ultrahigh frequency carbon nanotube
transistor based on a single nanotube. IEEE Transactions on Nanotechnology 2007; 6
(4): 400-403.
Wang F, Gu H, Swager T M. Carbon nanotube/polythiophene chemiresistive sensors
for chemical warfare agents. Journal of the American Chemistry Society 2008; 130
(16): 5392-5393.
Wang G X, Park J S, Park M S, Gou X L. Synthesis and high gas sensitivity of tin
oxide nanotubes. Sensors and Actuators B 2008; 131 (1): 313-317.
Wang G Z, Zhang B L, Yu Z L, Qu M Z. Manganese oxide/MWNTs composite
electrodes for supercapacitors. Solid State Ionics 2005; 176 (11-12): 1169-1174.
Wang L, Yoon M H, Lu G, Yang Y, Facchetti A, Marks T J. High-performance
transparent inorganic-organic hybrid thin-film N-type transistors. Nature Materials
2006; 5 (11): 893-900.
Wang Y, Du G, Liu H, Liu D, Qin S, Wang N, Hu C, Tao X, Jiao J, Wang J, Wang Z L.
Nanostructured sheets of Ti-O nanobelts for gas sensing and antibacterial applications.
Advanced Functional Materials 2008; 18 (7): 1131-1137.
Wang Z L. Nanowires and nanobelts: materials, properties and devices. Metal and
semiconductor nanowires volume I, Springer, New York (2006).
Wang Z L. Functional oxides nanobelts – materials, properties and potential
applications in nanosystems and biotechnology. Annual Review of Physical
Chemistry 2004; 55: 159-196.
Wei M, Konishi Y, Zhou H, Sugihara H, Arakawa H. Synthesis of single-crystal
manganese dioxide nanowires by a soft chemical process. Nanotechnology 2006; 16
(2): 245-249.
Winter M, Brodd R J. What are batteries, fuel cells, and supercapacitors. Chemical
Reviews 2004; 104 (10): 4245-4269.
Wu Z, Chen Z, Du X, Logan J M, Sippel J, Nikolou M, Kamaras K, Reynolds J R,
Tanner D B, Hebard A F, Rinzler A G. Transparent, conductive carbon nanotube films.
Science 2004; 305 (5688): 1273-1276.
Wyckoff R W G. Crystal Structures, Interscience, 1968, New York.
183
Xiang B, Wang P, Zhang X, Dayeh S A, Aplin D P R, Soci C, Yu D, Wang D. Rational
synthesis of p-type zinc oxide nanowire arrays using simple chemical vapor
deposition. Nano Letters 2007; 7 (2): 323-326.
Xu C, Du H, Li B, Kang F, Zeng Y. Asymmetric activated carbon-manganese dioxide
capacitors in mild aqueous electrolyte containing alkaline-earth cations. Journal of
Electrochemical Society 2009; 156 (6): A435-441.
Xu P, Cheng Z, Pan Q, Xu J, Xiang Q, Yu W, Chu Y. High aspect ratio In
2
O
3
nanowires: synthesis, mechanism and NO
2
gas-sensing properties. Sensors and
Actuators B 2008; 130 (2): 802-808.
Xu Y C, Li Y, Shen J. Gas sensing properties of ZnO nanorods prepared by
hydrothermal method. Journal of Material Science 2005; 40 (11): 2919-2921.
Xue X Y, Chen Y J, Liu Y G, Shi S L, Wang Y G, Wang T H. Synthesis and ethanol
sensing properties of indium-doped thin oxide nanowires. Applied Physics Letters
2006; 88 (20): 201907-201909.
Ye J S, Cui H F, Liu Z, Lim T M, Zhang W D, Sheu F S. Preparation and
characterization of aligned carbon nanotube-ruthenium oxide nanocomposites for
supercapacitors. Small 2005; 1 (5): 560-565.
Ying Z, Wan Q, Song Z T and Feng S L. SnO
2
nanowhiskers and their ethanol sensing
characteristics. Nanotechnology 2004; 15 (11):1682-1684.
Yoon M Y, Facchetti A, Marks T J. σ - π Molecular dielectric multilayers for low-
voltage organic thin-film transistors. Proceedings of the National Academy of
Sciences 2005; 102 (13): 4678-4682.
Yoon J, Baca A J, Park S I, Elvikis P, Geddes III J B, Li L, Kim R H, Xiao J, Wang S,
Kim T H, Motala M J, Ahn B Y, Duoss E B, Lewis J A, Nuzzo R G, Ferreira P M,
Huang Y, Rockett A, Rogers J A. Ultrathin silicon solar microcells for
semitransparent, mechanically flexible and microconcentrator module designs. Nature
Materials 2008; 7 (11): 907-915.
Young K K. Short-channel effect in fully depleted SOI MOSFETs. IEEE Transactions
on Electron Devices 1989; 36 (2): 399-402.
Yu C, Hao Q, Saha S, Shi L, Kong X, Wang Z L. Integration of metal oxide nanobelts
with microsystems for nerve agent detection. Applied Physics Letters 2005; 86 (6):
063101-063103.
Yuan C Z, Gao B, Zhang X G. Electrochemical capacitance of NiO/Ru
0.35
V
0.65
O
2
asymmetric electrochemical capacitor. Journal of Power Sources 2007; 173 (1): 606-
612.
184
Zhang D, Ryu K, Liu X, Polikarpov E, Ly J, Tompson M E, Zhou C. Transparent,
conductive, and flexible carbon nanotube films and their application in organic light-
emitting diodes. Nano Letters 2006; 6 (9): 1880-1886.
Zhang D H, Li C, Han S, Liu X L, Tang T, Jin W, Zhou C W. Electronic transport
studies of single-crystalline In
2
O
3
nanowires. Applied Physics Letters 2003; 82 (1):
112-114.
Zhang D H, Liu Z Q, Li C, Tang T, Liu X L, Han S, Lei B, Zhou C. Detection of NO
2
down to ppb levels using individual and multiple In
2
O
3
nanowire devices. Nano
Letters 2004; 4 (10): 1919-1924.
Zhang D, Liu Z, Li C, Tang T, Liu X, Han S, Lei, Zhou C. DNA-decorated carbon
nanotubes for chemical sensing. Nano Letters 2004; 4 (9): 1919-1924.
Zhang D, Li C, Han S, Liu X, Tang T, Jin W, Zhou C. Ultraviolet photodetection
properties of indium oxide nanowires. Applied Physics A 2003; 77 (1): 163-166.
Zhang L L, Zhao X S. Carbon-based materials as supercapacitor electrodes. Chemical
Society Reviews 2009; 38 (9): 2520-2531.
Zhang W, Jie J, He Z, Tao S, Fan X, Zhou Y, Yuan G, Luo L, Zhang W, Lee C S, Lee
S T. Single zinc-doped indium oxide nanowire as driving transistor for organic light-
emitting diode. Applied Physics Letters 2008; 92 (15): 153312-153314.
Zhang W P, Zu R D, Wang Z L. Nanobelts of semiconducting oxides. Science 2001;
291 (5510): 1947-1949.
Zhang Y, Suc C, Liu Z, Li J. Carbon nanotubes functionalized by NO
2
: Coexistence
of charge transfer and radical transfer. Journal of Physical Chemistry B 2006; 110
(45): 22462-22470.
Zhang Z, Wang S, Ding, L, Liang X, Pei T, Shen J, Xu H, Chen Q, Cui R, Li Y, Peng
L- M. Self-aligned ballistic n-type single-walled carbon nanotube field-effect
transistors with adjustable threshold voltage. Nano Letters 2008; 8 (11): 3696-3710.
Zhao J J, Buldum A, Han J, Lu J. Gas molecule adsorption in carbon nanotubes and
nanotube bundles. Nanotechnology 2002; 13 (2): 195-200.
Zhou Y, Hu L, Gruner G. A method of printing carbon nanotube thin films. Applied
Physics Letters 2006; 88 (12): 123109-123111.
Abstract (if available)
Abstract
One-dimensional nanostructures have been intensively investigated and proven to be of great potential as building blocks in different applications, including nano/micro electronics, chemical/biological sensing, and more recently, energy conversion and storage devices. Nanowires and carbon nanotubes made from organic/inorganic methods are now available and give research opportunities for understanding of one-dimensional nanostructures and their future applications. In this field, the research can be divided into three categories: nanostructure synthesis, material characterizations, device fabrication for applications. These three main thrusts govern the research route to understand the one-dimensional nanostructures.
Linked assets
University of Southern California Dissertations and Theses
Conceptually similar
PDF
Zero-dimensional and one-dimensional nanostructured materials for application in photovoltaic cells
PDF
Applications of one-dimensional structured nanomaterials as biosensors and transparent electronics
PDF
Nanomaterials for macroelectronics and energy storage device
PDF
One-dimensional nanomaterials for electronic and sensing applications
PDF
Nanomaterials for energy storage devices and electronic/optoelectronic devices
PDF
Semiconducting metal oxide nanostructures for scalable sensing applications
PDF
One-dimensional nanomaterials: synthesis and applications
PDF
GaAs nanowire optoelectronic and carbon nanotube electronic device applications
PDF
Synthesis and properties study of Q1D semiconductor nanostructures
PDF
Synthesis, characterization, and device application of two-dimensional materials beyond graphene
PDF
Synthesis, assembly, and applications of single-walled carbon nanotube
PDF
New materials and device structure for organic light-emitting diodes
PDF
Novel nanomaterials for electronics, optoelectronics and sensing applications
PDF
Synthesis and application of one-dimensional nanomaterials
PDF
Optoelectronic, thermoelectric, and photocatalytic properties of low dimensional materials
PDF
Printed electronics based on carbon nanotubes and two-dimensional transition metal dichalcogenides
PDF
Nanostructure interaction modeling and estimation for scalable nanomanufacturing
PDF
Plasmonic enhancement of catalysis and solar energy conversion
PDF
Carbon nanotube macroelectronics
PDF
Single-wall carbon nanotubes separation and their device study
Asset Metadata
Creator
Chen, Po-Chiang
(author)
Core Title
One-dimensional nanostructures for chemical sensing, transparent electronics, and energy conversion and storage devices
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Materials Science
Publication Date
05/07/2011
Defense Date
03/08/2010
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
active-matrix organic light emitting diode,AMOLED display,carbon nanotubes,chemical sensing,metal oxide nanowires,OAI-PMH Harvest,supercapacitors,transparent electronics
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Zhou, Chongwu (
committee chair
), Dapukus, Daniel (
committee member
), Goo, Edward K. (
committee member
)
Creator Email
pcchen1977@gmail.com,pochianc@usc.edu
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-m3049
Unique identifier
UC1133742
Identifier
etd-Chen-3588 (filename),usctheses-m40 (legacy collection record id),usctheses-c127-340880 (legacy record id),usctheses-m3049 (legacy record id)
Legacy Identifier
etd-Chen-3588.pdf
Dmrecord
340880
Document Type
Dissertation
Rights
Chen, Po-Chiang
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Repository Name
Libraries, University of Southern California
Repository Location
Los Angeles, California
Repository Email
cisadmin@lib.usc.edu
Tags
active-matrix organic light emitting diode
AMOLED display
carbon nanotubes
chemical sensing
metal oxide nanowires
supercapacitors
transparent electronics