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
/
Printed and flexible carbon nanotube macroelectronics
(USC Thesis Other)
Printed and flexible carbon nanotube macroelectronics
PDF
Download
Share
Open document
Flip pages
Contact Us
Contact Us
Copy asset link
Request this asset
Transcript (if available)
Content
PRINTED AND FLEXIBLE CARBON NANOTUBE
MACROELECTRONICS
By
Xuan Cao
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 2017
Copyright 2017 Xuan Cao
ii
Abstract
In this dissertation, I present my work on the development of printed carbon
nanotube (CNT) macroelectronics for large-area, low-cost and flexible
electronic applications such as sensors, active-matrix-based displays, and
electronic skin. Emerged as a solution-based and drop-on-demand patterning
technique, printing eliminates high-vacuum environment and multi-stage
photolithography needed in conventional micro-fabrication. Therefore,
printing is very suitable for manufacturing large-area flexible electronics with
low cost and rapid processing. The printing technologies we developed can be
divided into two methods.
One is top-contact self-aligned printing (SAP) for ultra-short-channel CNT
thin-film transistors (TFTs). Using top-contact self-aligned printing, we
addressed the following issues in fully printed TFTs. First of all, we have
successfully downscaled the channel length of fully printed CNT TFTs to sub-
micron, which is beyond the resolution of any existing printing technologies.
As a result, we have achieved unprecedented on-current density ~4.5 μA/μm
of fully printed CNT TFTs with high on-off ratio ~10
5
. This may open up the
door to fully printed high-performance TFTs for macroelectronic applications
iii
which need high current drive and high-speed TFTs, such as active-matrix
backplanes for display. Also, using top-contact SAP, we eliminated the contact
resistance caused by self-assembled monolayer (SAM) modification. Hence,
the device performance was further optimized. Furthermore, our printing
technique can be applied to other-materials-based systems like 2 dimensional
materials, metal oxides and so on. Overall, we believe this platform is
promising in producing high-performance fully printed TFTs.
The other method is screen printing as a scalable and low-cost approach for
fully printed CNT macroelectronics. Screen printing, with advantages of high
throughput, cost effectiveness, and simplicity, are universally used in industry
manufacturing. However, due to the lack of available metal, dielectric, and
semiconductor inks in the past years, screen printing was hardly used in
fabricating electronic devices. We made great efforts on developing new
functional inks, optimizing device configurations, and modifying printing
process. As a result, we have realized fully screen printed CNT TFTs with
good electrical performance and mechanical flexibility. Furthermore, we
developed fully printed CNT TFT backplanes and integrated them with
different types of sensors. Finally, we have achieved fully screen printed
active matrix electrochromic displays on flexible substrates. In this work, all
iv
the materials were formulated into screen printable ink and screen printing
served as the unique patterning technique. This tremendously simplified the
fabrication process and lowered the total cost of making such display.
Therefore, our screen printing work can be very important for future fully
printed large area and low-cost sensors, electronic skin, and displays.
In addition to printed CNT macroelectronics, we have also studied the
application of CNTs for ultraflexible electronics. Flexible thin-film transistors
based on semiconducting single-wall carbon nanotubes are promising for
flexible digital circuits, artificial skins, radio frequency devices, active-
matrix-based displays, and sensors due to the outstanding electrical properties
and intrinsic mechanical strength of carbon nanotubes. Nevertheless, previous
research effort only led to nanotube thin-film transistors with the smallest
bending radius down to 1 mm. We have realized the full potential of carbon
nanotubes by making ultraflexible and imperceptible p-type transistors and
circuits with a bending radius down to 40 μm. In addition, the resulted
transistors show mobility up to 12.04 cm
2
V
−1
S
−1
, high on−off ratio (∼10
6
),
ultralight weight (<3 g/m
2
), and good mechanical robustness (accommodating
severe crumpling and 67% compressive strain). Furthermore, the nanotube
circuits can operate properly with 33% compressive strain. On the basis of the
v
aforementioned features, our ultraflexible p-type nanotube transistors and
circuits have great potential to work as indispensable components for
ultraflexible complementary electronics.
This dissertation is presented with six chapters. Chapter 1 is an introduction
of carbon nanotubes regarding the structure, electrical properties, and
applications for micro/macroelectronics. For inkjet printed carbon nanotube
electronics, top-contact self-aligned printing is presented in chapter 2 to
downscale channel length of printed CNT TFTs to sub-micron. The resulted
devices show unprecedented current density compared with reported fully
printed nanotube TFTs. Chaper 3,4 are our screen printing work for fully
printed CNT TFTs, printed nanotube backplanes, and fully printed active-
matrix electrochromic display. Besides, we developed ultraflexible CNT
macroelectronics which will be presented in chapter 5. The last chapter,
chapter 6, is the summary and future direction of printed and flexible
macroelectronics.
vi
Acknowledgement
First of all, I would like to express my sincere gratitude to my advisor, Dr.
Chongwu Zhou, for offering me such a great opportunity to explore
nanotechnology and continuously supporting me over my Ph.D study. His
guidance, patience, and motivation greatly helped me obtain the capability of
problem solving, tremendously accumulate knowledge, and broaden my view
in nanotechnology. This laid a solid foundation for my future professional
career and more importantly, over the Ph.D research, working with Dr. Zhou,
I learned a truth to become a better scientist: maintain a seeking spirit, stay
out of your comfort zone, and always adapt to the change.
Also, I would like to thank my dissertation committee members, Prof. Wei Wu
and Prof. Andrea Armani for helping me with the dissertation defense process.
I am also grateful to Prof. Edward Goo and Prof. Steve Cronin serving as the
committee members of my qualifying exam.
In addition, I would like to thank my colleagues: Prof. Mark Thompson, Dr.
Donghai Zhu, Dr. Bilu Liu, Dr. Gang Liu, Dr. Haitian Chen, Dr. Yuchi Che,
vii
Dr. Xin Fang, Dr. Jiepeng Rong, Dr. Mingyuan Ge, Dr. Luyao Zhang, Dr. Hui
Gui, Liang Chen, Fanqi Wu, Yuqiang Ma, Yihang Liu, Qingzhou Liu, Anyi
Zhang, Christian Lau, Xiaofei Gu, Lang Shen, Haotian Shi, Bingya Hou, Dr.
Zhen Li, Sen Cong, Yu Cao, Chenfei Shen, and Jenny Lin, for all of your help
and goodwill you have shown me.
Finally, I would like to thank my family for the unconditional support, love,
and encouragement of my Ph.D study, emotionally helping me get out of the
dilemmas I met over these years.
viii
List of figures
Figure 1.1 …………………………………………………………………………………3
Structure of single-wall carbon nanotubes. (a) Chirality map of
SWNT, showing key parameters such as the chiral index, chiral angle, and diameter (b, c)
Atomic structure of (6,5) and (9,1) SWNTs, respectively.
Figure 1.2 …………………………………………………………………………………6
Examples of flexible macroelectronics including electronic skin, flexible display, flexible
AMOLED, epidermal electronics, X-ray imager, and humidity sensor.
Figure 1.3 …………………………………………………………………………………8
Forecast of flexible application based on printed electronics.
Figure 1.4 …………………………………………………………………………………9
Materials for thin-film transistors including carbon nanotubes, amorphous silicon, metal
oxide, organic semiconductors, and low-temperature poly silicon.
Figure 2.1 ……………………………………………………………………………….20
SEM image of the highly uniform printed carbon nanotube network.
ix
Figure 2.2 ………………………………………………………………………………22
Top-contact self-aligned printed ultrashort channel CNT thin film transistors. (a-d)
Schematic diagrams showing the fabrication process of a top-contact self-aligned printed
ultrashort channel nanotube transistor. (a) Schematic diagram showing the ink-jet printing
process of the first electrode on top of the pre-printed nanotube network. (b) Schematic
diagram showing the surface functionalization of the electrodes with a hydrophobic self-
assembled monolayer. (c) Schematic diagram showing the self-aligned ink-jet printing
process of the second electrode on the SAM-decorated surface of the first electrode before
dewetting. (d) Schematic diagram showing the ink-jet printing of ion gel dielectric. (e)
Optical image showing two printed electrodes defined by top-contact self-aligned printing
technique and the ultrashort channel formed between these two electrodes. (f) AFM and
SEM images showing an ultrashort channel of 400 nm with nanotubes between two printed
electrodes formed by self-aligned printing.
Figure 2.3 ………………………………………………………………………………23
(a) Optical microscope image showing the dewetting of the second electrode from the first
electrode surface about 5 minutes after printing. The color contrast on the first electrode
may come from solvent residue after the dewetting of the second electrode. (b) Optical
microscope image showing the trace of solvent was removed after sintering due to solvent
evaporation.
Figure 2.4 ………………………………………………………………………………25
Electrical characterization of fully-printed ultrashort channel CNT TFTs on willow glass.
(a) Transfer characteristics (ID-VG) of a representative ultrashort channel nanotube TFT
(L=400 nm, W=40 μm), measured at different VDS, from -0.1 V to -0.5 V with a step of -
0.1 V. (b) Transfer characteristics measured under VDS=-0.1V showing a current on/off
ratio of ~1x10
5
. The inset of this figure exhibits the gate leakage current as a function of
VG at VDS=-0.1V. (c) Output characteristic (ID-VD) of the same device measured at
x
different VG (from 0.5 V to -2 V with -0.5 V steps). (d) Comparison of channel length and
on-state current density of printed CNT TFTs between this work, and other work.
Figure 2.5 ………………………………………………………………………………26
Capacitance-voltage characteristic for the printed ultrashort channel CNT TFT, measured
at a frequency of 100 Hz.
Figure 2.6 ………………………………………………………………………………27
Output characteristic of the top-contact self-aligned printed ultrashort channel CNT TFT
in linear region.
Figure 2.7 ………………………………………………………………………………28
Statistical data of channel length measurements on printed ultrashort channel devices. (a)
SEM image of printed ultrashort channel with 15 evenly spaced channel length
measurements along the channel’s x axis. (b) Plot of channel length measurements from
(a). (c) Combined histogram of channel length measurements from 9 printed ultrashort
channel devices. (d) Combined plot of channel length measurements from 9 printed
ultrashort channel devices.
Figure 2.8 ………………………………………………………………………………32
Statistics analysis of 30 CNT TFTs with L=496 nm printed with top-contact self-aligned
printing technique. (a) On-state current density distributions of 30 TFTs measured at VG=-
1.5V and VDS=-0.1V. (b) Field-effect mobility distributions of 30 TFTs extracted from
transfer characteristics measured at VDS=-0.1V. (c) Threshold voltage distributions of 30
TFTs. (d) Current on/off ratio distributions of 30 TFTs measured at VDS=-0.1V.
xi
Figure 2.9 ………………………………………………………………………………33
Optical images showing the printed CNT TFTs with different channel lengths, 40 μm,
120 μm and 150 μm.
Figure 2.10 ………………………………………………………………………………37
Electrical characterization of printed CNT TFTs with different channel lengths. (a)
Transfer characteristics of representative printed CNT TFTs with channel lengths of 496
nm (violet), 40 μm (red), 120 μm (green), and 150 μm (blue). (b) Statistical study of 45
printed CNT TFTs showing on-state current density as a function of channel length. (c)
Statistical study of 45 printed CNT TFTs showing width-normalized total resistance as a
function of channel length.
Figure 3.1 ………………………………………………………………………………50
Fully screen-printed SWCNT TFTs on rigid and flexible substrates. (a) Schematic diagram
shows the fabrication process of fully printed top-gated SWCNT TFTs. (b), (c) Schematic
diagrams show the configuration of a fully printed TFT on PET substrate and screen
printing system, respectively. (d), (e) Optical images of fully printed TFT arrays on a 4
inch Si/SiO2 wafer (d) and a 12 × 12 cm PET sheet (e), respectively. (f) FE-SEM image
of deposited SWCNT film.
Figure 3.2 ………………………………………………………………………………53
Characterization of SWCNT TFTs printed with different ink dilution conditions. (a), (b)
Thicknesses of printed BTO and silver layers as a function of dilution ratios. (c) Transfer
(ID-VG) characteristics of TFTs printed with inks of different dilution ratios (Vsol/Vink),
measured at VDS = -1 V. (d) Transconductance exacted as a function of gate voltage.
xii
Figure 3.3 ………………………………………………………………………………55
Schematic diagrams and SEM images illustrate the capillary effect on BTO layers printed
with different dilution conditions. (a) Source and drain were printed with diluted silver ink
(Vsol/Vink = 1:4) and then diluted BTO ink (Vsol/Vink = 1:4) was printed as gate dielectric.
(b) Source and drain were printed with diluted silver ink (Vsol/Vink = 1:3) and then diluted
BTO ink (Vsol/Vink = 1:4) was printed as gate dielectric. The result shows a thinner BTO
layer (~5 μm) in (b) compared with the BTO layer (~ 6.5 μm) in (a).
Figure 3.4 ………………………………………………………………………………55
Profiles of printed electrode (a) using 1:3 silver ink, dielectric layer (b) using 1:4 BTO
ink, and gate (c) using undiluted silver ink, showing approximate thickness~3.3 μm, 5.1
μm, and 9.8 μm, respectively.
Figure 3.5 ………………………………………………………………………………56
Electrical characteristics of fully printed top-gated SWCNT TFTs on Si/SiO2 substrate,
with ink dilution of Vsol/Vink
= 1:3 for the silver source and drain, Vsol/Vink
= 1:4 for the
BTO dielectric, and undiluted silver for the gate. (a) Double-sweep of transfer
characteristics of a representative TFT measured at VDS = -1 V, showing very small
hysteresis. (b) Transfer characteristics under different drain voltages (from -1 to -3 V in
0.5 V steps). (c, d) Output characteristics of the same device in triode regime (c) and
saturation regime (d), respectively.
Figure 3.6 ………………………………………………………………………………58
Gate leakage current of a presentative printed SWCNT TFT as a function of gate voltage
at VDS = -1 V.
xiii
Figure 3.7 ………………………………………………………………………………59
Statistical analysis of 15 fully screen printed SWCNT TFTs showing (a) field-effect
mobility, (b) current on/off ratio, (c) on-current density, and (d) threshold voltage (Vth).
The calculated average values and standard deviations are included in each figure.
Figure 3.8 ………………………………………………………………………………61
Mechanical flexibility of fully printed flexible SWCNT TFTs on PET substrate, with ink
dilution of Vsol/Vink
= 1:3 for the silver source and drain, Vsol/Vink
= 1:4 for the BTO
dielectric, and undiluted silver ink for the gate. (a) Optical image of electrical
measurements on a printed TFT while bent. (b) Transfer (ID-VG) characteristics of a
representative device under different bending conditions of relaxed and bent at different
radii of curvature (R), measured at VDS = -1 V. (c) Field-effect mobility and (d) Ion/Ioff
plotted as a function of bending radius of curvature (R of relaxed state is infinite).
Figure 3.9 ………………………………………………………………………………63
Schematic diagrams showing the structure of the external OLED with aluminum (Al)
~100 nm, LiF~1 nm, tris (8-hydroxyquinoline) aluminum (Alq3) ~40 nm, 4,4’-bis[N- (1-
naphthyl) -N-phenylamino]biphenyl (NPD) ~40 nm and ITO.
Figure 3.10 ……………………………………………………………………………..64
Fully printed SWCNT TFTs for OLED control. (a) IOLED-VG family curves correspond to
values of VDD from 2.5 to 5 V in 0.5 V steps. (b) IOLED-VSS family curves correspond to
values of VG from -10 to 2 V in 2 V steps. (c) Optical images showing external OLED
intensity change versus VG with VDD = 3 V.
xiv
Figure 4.1 ……………………………………………………………………………80
Fully screen-printed active-matrix electrochromic display on flexible substrate. (a−e)
Schematic diagram showing the fabrication process and structure of a fully printed
AMECD. (f) Circuit diagram showing the configuration of as-printed AMECD. (g) A
photograph of 6 ×6 pixel flexible AMECD laminated on human skin displaying English
letter “U”. (h) SEM image showing the printed silver electrodes. The inset shows an SEM
image of the SWCNT network in the channel region. The use of the logo in (g) has been
authorized by the University of Southern California.
Figure 4.2 …………………………………………………………………………….…82
Electrical characterization of a fully screen-printed SWCNT backplane. (a) Transfer
characteristics of a representative TFT measured at VDS = -1 V. (b) Output characteristics
of the same device. VG is from -7.5 V to -2.5 V in 2.5 V steps. (c) Gate leakage current as
a function of gate voltage at VDS = -1 V. (d) Mobility map of an as-printed 6×6 active-
matrix backplane. (e), (f) Histograms of mobilities and current on-off ratios for 36 TFTs
in the backplane, showing mobility = 3.92 ± 1.08 cm
2
V
-1
s
-1
and Log (Ion/Ioff) = 3.71 ±
0.55.
Figure 4.3 ………………………………………………………………………………...87
Electrical characteristics of fully printed flexible electrochromic cells. (a)Top view
schematic diagram showing the lateral structure of a printed EC cell. (b) I-V characteristics
of a printed electrochromic cell under relaxed state and bent to various radiuses. (c)
Switching characteristics of a printed electrochromic cell, showing oxidation and reduction
processes over time with voltage at 5 V and – 5V. (d) Cyclability and (e) ambient stability
characterization of the fully screen-printed electrochromic display.
xv
Figure 4.4 ……………………………………………………………………………….88
Profiles of printed layers comprising the electrochromic cell
Figure 4.5 ……………………………………………………………………….………92
Fully screen-printed flexible AMECD. (a) Cross-section schematic diagram of a smart
pixel in AMECD. (b) Functionality test of a smart pixel configured by a TFT and an
electrochromic cell, suggesting the control capability of the TFT in turning the pixel on
and off. (c) Optical images of AMECD showing letters “U”, “S”, and “C”. The use of the
logo in (c) has been authorized by the University of Southern California.
Figure 5.1 ……………………………………………………………………….………105
Imperceptible carbon nanotube macroelectronics. (a) Schematic diagrams showing
fabrication procedure of carbon nanotube electronic foil. (b) Zoom-in schematic diagram
of fabricated electronic foil, showing TFTs and circuits. Inset is a SEM image of a typical
SWNT thin film. Scale bar is 500 nm. (c) Photograph of as-fabricated imperceptible
nanotube macroelectronics. Scale bar is 1 cm. (d,e) Ultra-thin carbon nanotube electronic
foil laminated onto human skin (d) and at rolled-up state (e). Scale bar: 2 cm in (d) and 1
cm in (e). (f) SEM image of the ultra-thin SWNT electronic foil laminated onto human hair
sitting on a substrate. Scale bar is 150 μm.
Figure 5.2 ……………………………………………………………………….………108
Electrical characterization of carbon nanotube thin-film transistors on 1.4 μm PET foil. (a)
Schematic diagram showing device configuration of a SWNT TFT with individual back-
gate on 1.4 μm PET substrate. (b) Transfer characteristics of a representative TFT with L
= 10 μm and W = 100 μm under VDS = −1 V. (c, d) Corresponding output characteristics
in triode regime (c) and saturation regime (d), respectively. VGS is from −0.2 to 1 V with
a step of 0.2 V, corresponding to curves from top to bottom. (e, f, g) Statistical study of 50
xvi
nanotube TFTs showing on−off ratio (e), field-effect mobility (f), and threshold voltage (g)
as functions of channel length.
Figure 5.3 ……………………………………………………………………….………110
Flexibility of nanotube TFTs on a 1.4 μm PET substrate. (a) Schematic diagram showing
the measurement of a TFT wrappingaround a cylinder. (b) Crumpled CNT electronic foil
(original size: 3 cm × 4 cm rectangle). Scale bar is 1 cm. (c) Transfer characteristics of a
representative TFT with L = 10 μm and W = 100 μm under relaxed (flat) state, after
crumpling, and bent with a radius of ∼220 μm. (d) Mobility, (e) logarithm on−off ratio,
and (f) threshold voltage of SWNT TFTs bent with a radius of ∼220 μm after different
bending cycles. (g) Mobility, (h) logarithm on−off ratio, and (i) threshold voltage of SWNT
TFTs after different crumpling cycles.
Figure 5.4 ………………………………………………………………………………112
SEM images of a crumpled electronic foil.
Figure 5.5 …………………………………………………………………….……...…113
Gate leakage current and electrical parameters of the representative device under three
conditions. (a) Gate leakage currents of the representative TFT with L = 10 μm and W =
100 μm under relaxed (flat) state, after crumpling, and bent with radius ~ 220 μm. (b)
Mobility, on-off ratio, and threshold voltage of the same SWNT TFT under relaxed (flat)
state, after crumpling, and bent with radius ~ 220 μm.
Figure 5.6 ……………………………………………………………………….………118
Stretch-induced effect of imperceptible carbon nanotube macroelectronics. (a) Illustration
of stretchable carbon nanotube macroelectronics. (b) SEM images of top view and cross-
section view of a wrinkled nanotube TFT under 67% compressive strain, showing the
bending radius of the active channel region, ∼40 μm. (c) Transfer characteristics of the
same SWNT TFT under different compressive strains, measured at VDS = −1 V. (d)
xvii
Corresponding normalized values of mobility, threshold voltage, and on−off ratio as
functions of compressive strain. (e) Schematic diagram of diode-load inverter. (f, g)
Electrical characteristics of an inverter in relaxed state (f) and undergoing 33%
compressive strain (g). Inset of (f): Photograph of an inverter in relaxed state. Scale bar is
100 μm. (h) Schematic diagram of diode-load NAND gates. (i, j) Output characteristics of
NAND gates in relaxed state (i) and undergoing 33% compressive strain (j). Inset of (i):
Photograph of a NAND gate in relaxed state. Scale bar is 100 μm. (k) Schematic diagram
of diode-load NOR gates. (l, m) Output characteristics of NOR gates in relaxed state (l)
and undergoing 33% compressive strain (m). Inset of (l): Photograph of a NOR gate in
relaxed state. The scale bar is 100 μm.
Figure 5.7 ……………………………………………………………………….………120
Gate leakage current of a SWNT TFT with L = 50 μm and W = 100 μm undergoing 0, 33%,
50%, 60% and 67% compressive strains.
Figure 5.8 ……………………………………………………………………….………122
SEM images of SWNT electronic foil on elastomer undergoing 33% compressive strain,
showing wrinkled mirco-structure. (a) top-view of the wrinkled plasitic electronics. Scale
bar is 200 μm. (b) Cross-section view of a tiny wrinkle, indicating radius of curvature ~ 2
μm. Scale bar is 5 μm.
Figure 6.1 ……………………………………………………………………….………136
Printed perovskite light-emitting diode (a) [15], electronic skin (b) [16], and photodetector
(c) [17]
xviii
Figure 6.2 ……………………………………………………………………….………137
High resolution printing technologies. (a) High-resolution flexographic printing using
CNT as a mask for silver and CdSe/ZnS quantum dots printing. [18] (b)
electrohydrodynamic printing for quantum dot light-emitting diode [19]
II
Table of contents
Abstract ............................................................................................................................................ ii
Acknowledgement ........................................................................................................................... vi
List of figures ................................................................................................................................ viii
1 Introduction to carbon nanotubes ......................................................................................... 1
1.1 Carbon nanotube structure ............................................................................................ 2
1.2 SWNTs for printed and flexible macroelectronics ......................................................... 4
1.3 References ..................................................................................................................... 10
2 Top-contact self-aligned printing for high-performance carbon nanotube thin-film
transistors with sub-micron channel length ................................................................................ 15
2.1 introduction ................................................................................................................... 15
2.2 Procedure of top-contact self-aligned printing for sub-micron channel length CNT
TFTs ………… ……… … ………… ……… … ………… ……… … ………… ……… … ………… ……… … ………… ……… … ….19
2.3 Electrical performance of fully printed CNT TFTs with submicron channel length 24
2.4 Summary ....................................................................................................................... 38
2.5 References ..................................................................................................................... 39
3 Screen printing as a scalable and low-cost approach for rigid and flexible thin-film
transistors using separated carbon nanotubes ............................................................................ 43
3.1 Introduction .................................................................................................................. 43
3.2 Development of fully screen printed SWCNT TFTs .................................................... 48
3.3 Electrical performance of fully screen printed SWCNT TFTs ................................... 56
3.4 Flexibility of fully screen printed CNT TFTs .............................................................. 60
3.5 Fully screen printed CNT TFTs for driving OLEDs ................................................... 62
3.6 Summary ....................................................................................................................... 65
3.7 References ..................................................................................................................... 67
4 Fully screen-printed, large-area, and flexible active-matrix electrochromic displays using
carbon nanotube thin-film transistors ......................................................................................... 72
4.1 Introduction .................................................................................................................. 72
4.2 Development of fully screen printed active matrix electrochromic display ................ 78
4.3 Electrical performance of fully screen printed active-matrix backplane based on
carbon nanotubes ..................................................................................................................... 81
4.4 Integration of the screen printed electrochromic cells and the printed CNT
backplane .................................................................................................................................. 86
III
4.5 Summary ...................................................................................................................... 93
4.6 References ..................................................................................................................... 94
5 Imperceptible and ultraflexible p‑type transistors and macroelectronics based on carbon
nanotubes ...................................................................................................................................... 99
5.1 Introduction .................................................................................................................. 99
5.2 Fabrication of ultraflexible SWNT macroelectronics ............................................... 103
5.3 Electrical performance of the ultraflexible SWNT TFTs ......................................... 106
5.4 Mechanical flexibility of the ultraflexible SWNT macroelectronics ........................ 109
5.5 Summary ..................................................................................................................... 124
5.6 References ................................................................................................................... 126
6 Conclusions and future work of printed and flexible CNT macroelectronics ................. 132
6.1 Conclusions ................................................................................................................. 132
6.2 Future direction of fully printed carbon nanotube macroelectronics ...................... 134
6.3 References ................................................................................................................... 138
Bibliography ............................................................................................................................... 141
1
1 Introduction to carbon nanotubes
Carbon nanotubes have attracted great research interest since they were first
discovered by Sumio Iijima in 1991.
1
As one of the most popular one
dimensional nanomaterials, carbon nanotubes can be considered as graphene
with honeycomb structure being rolled up into seamless cylinder. In term of
geometry, carbon nanotubes show large aspect ratio with the length in an order
of several micrometers and the diameter in an order of a nanometer.
2, 3
Regarding the electrical property, carbon nanotubes can be either metallic or
semiconducting depending on the chirality of the nanotubes.
4
Resulted from
the long mean free path (several hundreds of nanometers), the mobility of
semiconducting carbon nanotubes was experimentally proved to be >100000
cm
2
V
-1
s
-1
,
5, 6
which is much higher than the state-of-art monocrystalline
silicon with doping. On the other hand, metallic nanotubes can hold very large
current density (10
10
A/cm
2
), showing more promising current-carrying
capability than traditional metals and this can avoid the issue of
electromigration in copper used for interconnects in semiconductor industry.
7-9
2
Also, carbon nanotubes offer outstanding mechanical property with
Young’s modulus over 1 TPa and tensile strength over 200 GPa,
10
which is
very important for application in flexible and stretchable electronics. The
large aspect ratio of nanotubes can be utilized for biosensors and chemical
sensors.
11, 12
Overall, with the extraordinary geometric, electrical, and
mechanical properties, carbon nanotubes have become a promising
nanomaterial for many practical applications.
1.1 Carbon nanotube structure
A single-wall carbon nanotube (SWNT) can be viewed as a seamless
cylindrical tube rolled up by graphene. Depending on the ways of rolling up,
the resulted SWNTs can show different three-dimensional structure in terms
of atomic arrangement.
4
This can be described by chirality (n,m) or, in another
word, diameter (d) and chiral angle (θ) (Figure 1.1). The graphene lattice has
two basic vectors of a
1
= a(√3, 0) and a
2
= a( √3/2, 3/2), where a = 0.142 nm
is the carbon–carbon bond length. The chirality of nanotubes is defined by
chiral vector C
h,
which is given by
C
h
= na
1
ma
2
(n,m) (1)
Also, the diameter and chiral angle can be defined as the following:
3
𝑑 = √3𝑎 ( 𝑚 2
+ 𝑚𝑛 + 𝑛 2
)
1/2
/𝜋 (2)
𝜃 = 𝑡𝑎𝑛 −1
[√3𝑚 /( 𝑚 + 2𝑛 ) ] (3)
Based on the chiral angle, nanotubes can be categorized into 3 types, zigzag
nanotubes with θ = 0°; armchair nanotubes with θ = 30°; chiral nanotubes with
0° < θ < 30°.
Figure 1.1 Structure of single-wall carbon nanotubes. (a) Chirality map of
SWNT, showing key parameters such as the chiral index, chiral angle, and diameter(b, c)
Atomic structure of (6,5) and (9,1) SWNTs, respectively.[4]
4
1.2 SWNTs for printed and flexible
macroelectronics
With the outstanding electrical property, carbon nanotubes are promising for
solid-state devices.
13-15
The development of SWNT-based electronics can be
mainly classified into microelectronics and macroelectronics. On one hand,
the roadmap of microelectronics is to develop transistors with smaller and
smaller channel length and therefore improve the performance of transistors
in microprocessors and memories, following Moore’s Law.
16, 17
SWNTs, with
scatter-free ballistic transport behavior, show the prominent advantages for
field-effect transistors when the channel length is downscaled to an order of
ten nanometers.
18, 19
On the other hand, macroelectronics is moving towards
different direction and has become an irreplaceable element in modern
consumer electronics as shown in Figure 1.2, including electronic skin,
flexible display, flexible active –matrix organic light-emitting diode
(AMOLED), X-ray imager, epidermal electronics, and humidity sensor.
20-24
For macroelectronics, thin-film transistors (TFTs) have been developed as the
backbone, acting as “switches” for functional electronic devices like light-
emitting devices and sensors for consumer electronics.
25-27
Satisfying the need
to cover large area, TFTs are not required to have minimized feature size to
improve the integration density per area. Instead, they can be relatively “large”
5
in channel length (>1μm) as long as the performance is sufficient for the
macroelectronic applications such as driving an organic light emitting diode
(OLED) or a sensor. In this dissertation, we mainly discuss macroelectronics.
The blueprint of macroelectronics is to develop pixel-based, large-area, and
therefore flexible electronics to be used on curved surface for applications
such as flexible OLED TV , wearable electronics, electronic skin on robotics,
medical sensing systems, prosthesis and etc.
20-24
However, currently, modern
macroelectronics are mostly manufactured using conventional fabrication,
which needs high-vacuum environment for deposition, multistage lithography,
and subtractive etching process. As a consequence, this technology is
disadvantageous in many aspects.
28, 29
Firstly, limited by cost of high-vacuum
chamber, the technology is mainly applied in wafer-scale manufacturing,
which does not meet the requirement of scalable production of large-area (>1
m
2
) electronics. Secondly, the fabrication process includes multiple steps of
deposition and etching, which can significantly increase the total cost,
production time, and pollution to environment. Finally, due to the high
temperature used in the process (>200 ℃),
28, 29
flexible substrates such as
polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) have
been pushed out of the consideration.
6
Figure 1.2 Examples of flexible macroelectronics including electronic skin, flexible display,
flexible AMOLED, epidermal electronics, X-ray imager, and humidity sensor.
Printing technology emerged for fabricating macroelectronics has attracted
tremendous research interest due to its advantages in reducing cost, rapid
processing, and the scalability for large-area electronics. The basic concept
7
for printed electronics is to have functional materials such as metals,
insulators, and semiconductors formulated into printable inks, then print the
inks onto the desired substrates, and finally cure the printed ink to form
patterns. Compared with traditional fabrication, printing is an additive
patterning technology, eliminating multistage deposition and etching process
which are required in subtractive patterning. As a result, it dramatically
reduces the use of materials and total processing time for fabrication. Also,
printing can be carried out in ambient environment with good scalability and
high throughput. This is key for macroelectronics to be boosted for large-area
low-cost electronics in the future. Furthermore, with the solution-based
process, the temperature for curing printed inks is usually moderate (<140 ℃)
and therefore, this is compatible with flexible substrates.
28, 29
Hence, printed
and flexible electronics shows its bright future for different applications.
30
(Figure 1.3)
8
Figure 1.3 Forecast of flexible application based on printed electronics. [30]
CNT thin films show great potential for serving as the channel material of
TFTs due to its excellent electrical performance, outstanding mechanical
strength, operation stability, and low-temperature solution-based deposition.
29,
31-33
Nevertheless, the synthesis of carbon nanotubes offer a mixture of 1/3
metallic CNT and 2/3 semiconducting nanotubes.
34, 35
The existence of
metallic CNT in channel would remarkable degrade the on/off ratio and
therefore limit the application of CNT TFTs. Thanks to the great achievements
in CNT separation, ultra-high-purity semiconducting CNT solution (>99.99%)
is commercially available using polyfluorene (PFO)-assisted sorting.
36
This
paves a way for CNT thin film to become a competitive material for TFTs,
especially for printed and flexible TFTs. Figure 1.4 summarizes the materials
for current TFT technology, showing the comparison of CNTs, organic
9
mateials and traditional commercialized materials such as amorphous silicon,
poly-silicon, metal oxide.
37
However, for future macroelectronics which has
to be large-area, low-cost, and flexible, printing may be a realistic strategy. In
this regard, only CNTs and organic materials are taken into account with
printability in solution at room temperature. But CNTs show better stability
against moisture, oxygen, and moderate temperature, resulting in better
manufacturability and electrical performance in different environment. Hence,
I have combined printing as the patterning technique and CNTs as the
semiconductor to develop printed and flexible carbon nanotube
macroelectronics for future large-area and low-cost displays and sensing
systems.
Characteristic CNT TFT a-Si:H Oxide TFT Organic TFT Low Temp Poly-Si
Mobility
Good, 10 - 164
cm
2
/Vs Poor, 0.5 - 1 cm
2
/Vs Good, 1 - 40 cm
2
/Vs
Typical ≤ 1 cm
2
/Vs,
max. 46 cm2/Vs
Excellent, 10 -
500 cm
2
/Vs
Uniformity Good Excellent
Good with amorphous
type, poor with
crystalline type Good Poor
Stability Good Poor Poor Poor Excellent
Scalability Good, currently 4'' Excellent, >100'' Potential to 100'' Good, currently 4'' Limited to < 40''
Process
Temperature room temperature
Typical ~300 ˚C,
some low temp.
process can be
~150 ˚C
Typical ~200 ˚C, but
some need anneal at
350 ̊C Typical < 100 ˚C High, > 400 ˚C
Cost Low Low Medium Low High
Printability Yes No No Yes No
Flexiblity Yes No No Yes No
Availability Research stage Yes Yes Research stage Yes
Challenges Hysteresis
Poor mobility; poor
stability
Threshold voltage
unstable
Poor operational
stability
Uniformity, cost,
stability
Figure 1.4 Materials for thin-film transistors including carbon nanotubes, amorphous
silicon, metal oxide, organic semiconductors, and low-temperature poly silicon.
10
1.3 References
1. Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56-58.
2. Avouris, P. Carbon Nanotube Electronics. Chem. Phys. 2002, 281, 429-445.
3. Avouris, P. Molecular Electronics with Carbon Nanotubes. Accounts. Chem. Res.
2002, 35, 1026-1034.
4. Liu, B.; Wu, F.; Gui, H.; Zheng, M.; Zhou, C. Chirality-Controlled Synthesis and
Applications of Single-Wall Carbon Nanotubes. ACS Nano 2017, 11, 31-53.
5. Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Extraordinary Mobility in
Semiconducting Carbon Nanotubes. Nano Lett. 2004, 4, 35-39.
6. Zhou, X. J.; Park, J. Y.; Huang, S. M.; Liu, J.; McEuen, P. L. Band Structure,
Phonon Scattering, and the Performance Limit of Single-Walled Carbon Nanotube
Transistors. Phys. Rev. Lett. 2005, 95.
7. Close, G. F.; Yasuda, S.; Paul, B.; Fujita, S.; Wong, H. S. P. A 1 Ghz Integrated
Circuit with Carbon Nanotube Interconnects and Silicon Transistors. Nano Lett. 2008, 8,
706-709.
8. Li, J.; Ye, Q.; Cassell, A.; Ng, H. T.; Stevens, R.; Han, J.; Meyyappan, M. Bottom-
up Approach for Carbon Nanotube Interconnects. Appl. Phys. Lett. 2003, 82, 2491-2493.
9. Naeemi, A.; Meindl, J. D. Design and Performance Modeling for Single-Walled
Carbon Nanotubes as Local, Semiglobal, and Global Interconnects in Gigascale Integrated
Systems. Ieee. T. Electron Dev. 2007, 54, 26-37.
11
10. Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Exceptionally High Young's
Modulus Observed for Individual Carbon Nanotubes. Nature 1996, 381, 678-680.
11. Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Kam, N. W. S.; Shim, M.; Li, Y.
M.; Kim, W.; Utz, P. J.; Dai, H. J. Noncovalent Functionalization of Carbon Nanotubes for
Highly Specific Electronic Biosensors. Proceedings Of the National Academy Of Sciences
Of the United States Of America 2003, 100, 4984-4989.
12. Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai,
H. J. Nanotube Molecular Wires as Chemical Sensors. Science 2000, 287, 622-625.
13. Wang, C.; Takei, K.; Takahashi, T.; Javey, A. Carbon Nanotube Electronics -
Moving Forward. Chem. Soc. Rev. 2013, 42, 2592-2609.
14. 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, 495-U4.
15. Sun, D. M.; Timmermans, M. Y.; Tian, Y.; Nasibulin, A. G.; Kauppinen, E. I.;
Kishimoto, S.; Mizutani, T.; Ohno, Y. Flexible High-Performance Carbon Nanotube
Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156-161.
16. Kuhn, K. J. Moore's Law Past 32nm: Future Challenges in Device Scaling. Iwce-
13: 2009 13th International Workshop on Computational Electronics 2009, 37-40.
17. Schaller, R. R. Moore's Law: Past, Present, and Future. Ieee. Spectrum. 1997, 34,
52-+.
12
18. Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. J. Ballistic Carbon Nanotube
Field-Effect Transistors. Nature 2003, 424, 654-657.
19. Qiu, C. G.; Zhang, Z. Y.; Xiao, M. M.; Yang, Y. J.; Zhong, D. L.; Peng, L. M.
Scaling Carbon Nanotube Complementary Transistors to 5-Nm Gate Lengths. Science
2017, 355, 271-+.
20. Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.;
Drack, M.; Schwodiauer, R.; Graz, I.; Bauer-Gogonea, S.; Bauer, S.; Someya, T. An Ultra-
Lightweight Design for Imperceptible Plastic Electronics. Nature 2013, 499, 458-+.
21. Kim, D. H.; Lu, N. S.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S. D.; Wu, J.; Won,
S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L. Z.; Li,
M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.;
Huang, Y. G.; Coleman, T.; Rogers, J. A. Epidermal Electronics. Science 2011, 333, 838-
843.
22. Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A. User-
Interactive Electronic Skin for Instantaneous Pressure Visualization. Nat Mater 2013, 12,
899-904.
23. Takahashi, T.; Yu, Z.; Chen, K.; Kiriya, D.; Wang, C.; Takei, K.; Shiraki, H.; Chen,
T.; Ma, B.; Javey, A. Carbon Nanotube Active-Matrix Backplanes for Mechanically
Flexible Visible Light and X-Ray Imagers. Nano Lett 2013, 13, 5425-30.
24. Chou, H. H.; Nguyen, A.; Chortos, A.; To, J. W. F.; Lu, C.; Mei, J. G.; Kurosawa,
T.; Bae, W. G.; Tok, J. B. H.; Bao, Z. A. A Chameleon-Inspired Stretchable Electronic
13
Skin with Interactive Colour Changing Controlled by Tactile Sensing. Nat. Commun. 2015,
6.
25. Yeom, C.; Chen, K.; Kiriya, D.; Yu, Z.; Cho, G.; Javey, A. Large-Area Compliant
Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv. Mater.
2015.
26. Chen, P. C.; Fu, Y.; Aminirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.;
Zhou, C. W. Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and
Its Application in Organic Light Emitting Diode Control. Nano Lett. 2011, 11, 5301-5308.
27. Cao, X.; Chen, H. T.; Gu, X. F.; Liu, B. L.; Wang, W. L.; Cao, Y.; Wu, F. Q.; Zhu,
C. W. Screen Printing as a Scalable and Low-Cost Approach for Rigid and Flexible Thin-
Film Transistors Using Separated Carbon Nanotubes. Acs Nano 2014, 8, 12769-12776.
28. Aleeva, Y.; Pignataro, B. Recent Advances in Upscalable Wet Methods and Ink
Formulations for Printed Electronics. J. Mater. Chem. C 2014, 2, 6436-6453.
29. Sun, D. M.; Liu, C.; Ren, W. C.; Cheng, H. M. A Review of Carbon Nanotube- and
Graphene-Based Flexible Thin-Film Transistors. Small 2013, 9, 1188-205.
30.
http://www.iitc-conference.org/uploads/2/4/1/1/24114461/workshop_1_(morning).pdf
31. Cao, X.; Cao, Y.; Zhou, C. Imperceptible and Ultraflexible P-Type Transistors and
Macroelectronics Based on Carbon Nanotubes. ACS Nano 2016, 10, 199-206.
32. Wang, C.; Chien, J. C.; Takei, K.; Takahashi, T.; Nah, J.; Niknejad, A. M.; Javey,
A. Extremely Bendable, High-Performance Integrated Circuits Using Semiconducting
14
Carbon Nanotube Networks for Digital, Analog, and Radio-Frequency Applications. Nano
Lett. 2012, 12, 1527-1533.
33. Zhang, J. L.; Wang, C.; Zhou, C. W. Rigid/Flexible Transparent Electronics Based
on Separated Carbon Nanotube Thin-Film Transistors and Their Application in Display
Electronics. Acs Nano 2012, 6, 7412-7419.
34. Wang, C.; Zhang, J. L.; Ryu, K. M.; Badmaev, A.; De Arco, L. G.; Zhou, C. W.
Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display
Applications. Nano Lett. 2009, 9, 4285-4291.
35. Zhang, J. L.; Fu, Y.; Wang, C.; Chen, P. C.; Liu, Z. W.; Wei, W.; Wu, C.;
Thompson, M. E.; Zhou, C. W. Separated Carbon Nanotube Macroelectronics for Active
Matrix Organic Light-Emitting Diode Displays. Nano Lett. 2011, 11, 4852-4858.
36. Ding, J. F.; Li, Z.; Lefebvre, J.; Cheng, F. Y.; Dubey, G.; Zou, S.; Finnie, P.; Hrdina,
A.; Scoles, L.; Lopinski, G. P.; Kingston, C. T.; Simard, B.; Malenfant, P. R. L. Enrichment
of Large-Diameter Semiconducting Swcnts by Polyfluorene Extraction for High Network
Density Thin Film Transistors. Nanoscale 2014, 6, 2328-2339.
37.
http://semieurope.omnibooksonline.com/2012/semicon_europa/Plastic%20Electronics%2
0Conference/Plenary%20Session/03_Jennifer.Colegrove_DisplaySearch.pdf
<03_Jennifer.Colegrove_Displaysearch.Pdf>.
15
2 Top-contact self-aligned printing for
high-performance carbon nanotube
thin-film transistors with sub-micron
channel length
2.1 introduction
Semiconducting single-wall carbon nanotubes (SWCNTs) have been gaining
attention in developing fully printed thin-film transistors (TFTs) due to their
outstanding electric and mechanical properties, solution-based low-
temperature deposition process, and stability against oxygen and moisture.
1−6
Main-stream printing technologies such as inkjet printing,
7,8
aerosol-jet
printing,
9−11
screen printing,
12,13
gravure printing,
14,15
and flexographic
printing
16
have been used for fabricating CNT thin-film transistors, which
have great potential for applications in display electronics and sensing
systems.
13,15,17−19
These methods eliminate the need of using a high-vacuum
environment and multistage patterning process, thus paving the way for
scalable manufacturing of large-area, low-cost, and flexible electronics.
Although the reported fully printed CNT TFTs
8,12−15,19,20
show satisfactory
16
performance in terms of mobility and current on/off ratio, the channel lengths
of such devices are rather large (>20 μm) due to the limitation of printing
resolution and registration accuracy. Consequently, the on-state current
densities of fully printed CNT TFTs in those papers are very low (on the scale
between 10
−1
μA/μm and 10
−5
μA/μm). For example, our group published
screen-printed CNT TFTs with 105 μm channel length and a current density
of ∼10
−4
μA/μm.
12,13
Javey and Cho reported gravure-printed CNT TFTs with
85 μm channel length and a current density of ∼1.4 ×10
−4
μA/μm.
14,15
Inkjet
and aerosol-jet printed CNT TFTs with ion-gel gating have also been reported
with 25−100 μm channel length and an improved current density of ∼10
−1
-
10
−3
μA/μm.
7,8,19−21
For one of the most significant applications - current
driving for organic light emitting diodes, low on-state current density of
driving TFTs is disadvantageous since a large channel width is needed to
achieve desirable brightness of the integrated light-emitting devices, resulting
in a low aspect ratio. Also, such long-channel TFTs have low speed and high
operation voltage, which may limit their applications such as radio frequency
transistors.
22
Overall, downscaling of fully printed CNT TFTs is necessary to
further improve the performance and widen the potential applications for
scalable manufacturing of large-area and low-cost electronics. Previously
Sirringhaus’s group pioneered a back-contact self-aligned printing (SAP)
17
method to develop ultrashort-channel organic transistors.
22,23
However, due to
the vulnerability of organic materials to high temperature required by sintering
gold ink, the organic TFTs were configured by having the organic
semiconductor layer on top of the printed electrodes, thus forming back
contacts to the channel. This introduced a self-assembled monolayer (SAM)
between the semiconductor and metal contacts. As a result, the contact
resistance may increase remarkably. In addition, as the channel length is
downscaled to a sub-micron scale, a small effective thickness of dielectric
layer is required to have a strong gating on the short-channel devices.
Here we report a facile and highly reliable top-contact self-aligned printing
strategy to fabricate fully inkjet printed, ultrashort-channel (sub-micron), and
lateral-gate CNT TFTs on willow glass. In this study, we first printed the CNT
network and then printed the electrodes to form top contacts with a sub-micron
channel in between. Therefore, the gold electrodes were printed and sintered
on the semiconducting CNT network and directly formed good ohmic contacts,
eliminating the SAM which may cause a large contact resistance.
22
Furthermore, a high capacitance ion-gel material
24
has been employed as the
dielectric material with an effective thickness of sub-nanometer scale
provided by an electrical double layer.
25
Thus, we have further improved the
18
on-state current density and lowered the operation voltage of our devices with
the enhanced gating. The as-printed short-channel CNT TFTs show excellent
performance with a superior average on-state current density (4.5 μA/μm) at
low operation voltage of gate voltage (V
G
)= −1.5 V and source−drain voltage
(V
DS
) = −0.1 V , outstanding mobility (15.03 cm
2
V
−1
s
−1
), and high current
on/off ratio (∼10
5
). Compared with previous fully printed CNT TFTs, we
dramatically downscaled the printed TFTs from tens of micrometers to sub-
microns in channel length and realized great improvement in current density
by orders of magnitude. We believe the advantageous performance will be
highly desirable for future large-area high-definition printed displays,
electronic skins, radio frequency applications, and sensing systems.
19
2.2 Procedure of top-contact self-aligned printing
for sub-micron channel length CNT TFTs
Ultrashort channel CNT TFTs are printed on willow glass with lateral-gate
device architecture and ion gel as the gate dielectric. In principle, any
substrates which can go through the sintering temperature (∼250 °C) for gold
(to be discussed below) can be used, such as Si/SiO2 or glass substrate.
Specifically, here we chose willow glass as our substrate for demonstration.
As the first step, gold nanoparticle ink was printed as the lateral gate electrode,
followed by sintering at 250 °C for 1 h. As the second step, a semiconducting-
enriched SWCNT solution was printed and defined in the channel region. The
printed nanotube film was then annealed in air at 200°C to let the solvent
evaporate. As previously reported,
26
annealing at 400°C in vacuum can
remove the excess polymer. However, this temperature may be too high for
some potential substrates such as polyethylene terephthalate (PET) and,
therefore, is not used here. The quality of the printed nanotube film was then
inspected using field-emission scanning electron microscopy (FESEM), and
the SEM image (Figure 2.1) shows the highly uniform clean nanotube random
network.
20
Figure 2.1 SEM image of the highly uniform printed carbon nanotube network.
As the third step, the first gold electrode was printed to form a contact with
the nanotube network (Figure 2.2a). After sintering in air for 1 h, SAM
(1H,1H,2H,2H-perfluorodecanethiol (PFDT)) functionalization was
performed to modify the surface of the printed gold electrodes to be
hydrophobic (Figure 2.2b). Right after the treatment, the second gold
electrode was initially printed in a partially overlapping fashion with the first
electrode (Figure 2.2c). The gold ink landing on the surface of the SAM-
decorated first electrode was repelled by the hydrophobic surface, and a sub-
micron gap between the first electrode and the second electrode usually
formed about 5 min after the printing and when the solvent was drying (Figure
2.3).
23,27
We have observed that with 5 min of slow drying in air after printing,
1 μm
21
the contact line of the second electrode, which initially overlapped slightly
with the first electrode, retracted from the surface of the first electrode, leaving
a color contrast at the location where the second electrode dewetted from
(Figure 2.3a). The color contrast observed here may come from some residue
of the ink solvent after the ink dewetted. The sample was then sintered at
220 °C in air for 1 h, which worked to remove surfactants from the printed
gold particles and made the printed gold electrodes highly conductive.
400 nm 400 nm
Printed Au lateral gate
Printed Au electrode
Printed CNT network
Willow
glass
SAM
SAM functionalization
Ion gel printing
a b
c
d
e
Ion gel
Gate
Source & drain
Au ink
20 μm
f
Au Au
22
Figure 2.2 Top-contact self-aligned printed ultrashort channel CNT thin film transistors.
(a-d) Schematic diagrams showing the fabrication process of a top-contact self-aligned
printed ultrashort channel nanotube transistor. (a) Schematic diagram showing the ink-jet
printing process of the first electrode on top of the pre-printed nanotube network. (b)
Schematic diagram showing the surface functionalization of the electrodes with a
hydrophobic self-assembled monolayer. (c) Schematic diagram showing the self-aligned
ink-jet printing process of the second electrode on the SAM-decorated surface of the first
electrode before dewetting. (d) Schematic diagram showing the ink-jet printing of ion gel
dielectric. (e) Optical image showing two printed electrodes defined by top-contact self-
aligned printing technique and the ultrashort channel formed between these two electrodes.
(f) AFM and SEM images showing an ultrashort channel of 400 nm with nanotubes
between two printed electrodes formed by self-aligned printing.
After sintering, the color contrast observed in Figure 2.3a disappeared
completely (Figure 2.3b) due to solvent evaporation. The optical microscope
image (Figure 2.2e) shows a sub-micron channel defined by the top-contact
self-aligned printing, which was further characterized using atomic force
microscopy (AFM) and FESEM. Figure 2.2f shows a well-defined channel of
400 nm with a CNT random network within the channel. The fabrication
process of the device was completed with the printing of ion gel
24
as the gate
dielectric (Figure 2.2d). Once the ion-gel droplet landed on the device, it
spread out and formed a thin layer covering the channel region as well as the
lateral gate. Thanks to the electrolytic gating nature of the ion gel,
24
lateral-
gate device architecture can be used, which simplifies the printing process.
23
Figure 2.3. (a) Optical microscope image showing the dewetting of the second electrode
from the first electrode surface about 5 minutes after printing. The color contrast on the
first electrode may come from solvent residue after the dewetting of the second electrode.
(b) Optical microscope image showing the trace of solvent was removed after sintering
due to solvent evaporation.
Sintered 5 min
First Electrode First Electrode
Second
Electrode
Second
Electrode
(a) (b)
dewet
Trace of
solvent
20 μm 20 μm
24
2.3 Electrical performance of fully printed CNT
TFTs with submicron channel length
The electrical performance of the fully printed ultrashort channel CNT TFTs
was studied and is shown in Figure 2.4. The transfer characteristics of a
representative CNT TFT with channel length (L) = 400 nm and channel width
(W) = 40 μm, measured under different drain voltages (V
DS
) from −0.1 V to
−0.5 V in a step of −0.1 V , are presented in Figure 2.4a, showing a typical p-
type transistor behavior. Figure 2.4b exhibits the transfer curve of the
representative device in logarithm scale under V
DS
= −0.1 V , clearly showing
a high current on/off ratio of ∼1 × 10
5
. The threshold voltage of this TFT is
∼−0.2 V . Due to the high capacitance of the ion-gel gate dielectric, which
is measured to be ∼1.5 μF/cm
2
at 100 Hz (Figure 2.5), the transistor can be
operated with a gate voltage (V
G
) of <−1.5 V and achieve a high on-state
current density of 5.68 μA/μm at V
DS
= −0.1 V . The average current density is
4.5 μA/μm, measured over 30 devices at V
G
= −1.5 V and V
DS
= −0.1 V.
The field-effect mobility of this transistor is estimated to be 12.12 cm
2
V
−1
s
−1
,
calculated with the standard linear regime relation, 𝐼 𝐷 = 𝜇 𝐶 𝑖 𝑊 𝐿 (( 𝑉 𝐺𝑆
−
𝑉 𝑡 ℎ
) 𝑉 𝐷𝑆
−
𝑉 𝐷𝑆
2
2
) . Moreover, the gate leakage current as a function of gate
25
voltage at V
DS
= −0.1 V is shown in the inset of Figure 2.4b, indicating a small
gate leakage current (<10 nA) for the ion-gel gate dielectric. The output
characteristics of the same device (Figure 2.4c) show a typical current
saturation for the p-type (hole) transport due to the pinch-off effect.
Figure 2.4 Electrical characterization of fully-printed ultrashort channel CNT TFTs on
willow glass. (a) Transfer characteristics (ID-VG) of a representative ultrashort channel
nanotube TFT (L=400 nm, W=40 μm), measured at different VDS, from -0.1 V to -0.5 V
with a step of -0.1 V. (b) Transfer characteristics measured under VDS=-0.1V showing a
current on/off ratio of ~1x10
5
. The inset of this figure exhibits the gate leakage current as
a function of VG at VDS=-0.1V. (c) Output characteristic (ID-VD) of the same device
measured at different VG (from 0.5 V to -2 V with -0.5 V steps). (d) Comparison of channel
length and on-state current density of printed CNT TFTs between this work, and other
work.
-0.8 -0.4 0.0
-0.002
-0.001
0.000
V
DS
(V)
I
D
(A)
0.5 V
0 V
- 0.5 V
- 1.0 V
- 1.5 V
- 2 V
VG
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1E-3
0.01
0.1
1
10
100
1000
I
D
( A)
V
G
(V)
V
DS
= - 0.1 V
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-0.01
0.00
0.01
I
G
( A)
V
G
(V)
a
c
b
d
1 10 100 1000
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
I
on
/W ( m)
Channel Length ( m)
This work
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
-750
-600
-450
-300
-150
0
I
D
( A)
V
G
(V)
V
DS
= -0.1 V
V
DS
= -0.2 V
V
DS
= -0.3 V
V
DS
= -0.4 V
V
DS
= -0.5 V
Ref. 19
Ref. 28
Ref. 8
Ref. 30
Ref. 21
Ref. 14 Ref. 12&13
Ref. 20
Ref. 15
Ref. 31
Ref. 29
26
-2 -1 0 1 2
0.8
1.0
1.2
1.4
1.6
Capacitance ( F/cm
2
)
Voltage (V)
Frequency = 100 Hz
Figure 2.5 Capacitance-voltage characteristic for the printed ultrashort channel CNT TFT,
measured at a frequency of 100 Hz.
The linear region of the output curve (Figure 2.6) indicates good ohmic
contact between the printed gold electrodes and the printed nanotube network.
Our fully printed ultrashort channel CNT TFTs demonstrate a great success in
downscaling the channel length of printed nanotube transistors from tens of
micrometers to sub-microns.
27
-0.004 -0.002 0.000 0.002 0.004
-2.0x10
-5
0.0
2.0x10
-5
I
D
(A)
V
DS
(V)
0.5
0
-0.5
-1.0
-1.5
-2
VG
Figure 2.6 Output characteristic of the top-contact self-aligned printed ultrashort channel
CNT TFT in linear region.
In consideration of the solution-based printing technique, we note that there
is variation in the channel length within a single device and among different
devices. Figure 2.7a shows an SEM image of an as-printed ultra-short
channel. 15 locations along the channel (x direction) with 200 nm spacing
were measured for each device. The result for a representative device is
plotted in Figure 2.7b with location as the X axis and measured channel
length as the Y axis. The overall statistical data of channel length measured
from 9 devices with 135 locations are shown in Figure 2.7c, indicating a
normal distribution with average channel length of 496 nm and standard
deviation of 82 nm. The profile of channel length versus location (x)
28
measured from 9 devices is plotted in Figure 2.7d, showing the device-to-
device variation in channel length.
Figure 2.7 Statistical data of channel length measurements on printed ultrashort channel
devices. (a) SEM image of printed ultrashort channel with 15 evenly spaced channel length
measurements along the channel’s x axis. (b) Plot of channel length measurements from
(a). (c) Combined histogram of channel length measurements from 9 printed ultrashort
channel devices. (d) Combined plot of channel length measurements from 9 printed
ultrashort channel devices.
Our sub-micron nanotube TFTs show excellent electrical performance in
terms of high on-state current density, low-voltage drive, outstanding on/off
ratio, and good mobility. Especially, our printed ultrashort channel TFTs show
a dramatically improved on-state current density of 4.5 μA/μm at V
DS
= −0.1
0 500 1000 1500 2000 2500 3000
0
100
200
300
400
500
600
700
800
900
1000
1
2
3
4
5
6
7
8
9
Channel Length (nm)
x (nm)
Device
Number
200 400 600 800
0
10
20
30
Number of Measurements
Channel Length (nm)
L = 496 82 nm
a
0 500 1000 1500 2000 2500 3000
0
100
200
300
400
500
600
700
Channel Length (nm)
x (nm)
b
c d
29
V and V
G
= −1.5 V , which is several orders of magnitude higher than other
printed CNT TFTs. Figure 2.4d clearly shows the comparison of channel
length and the on-state current density for printed CNT TFTs between this
work and other previous reported works.
8,12−15,19−21,28−32
From Figure 2.4d, we
can observe that our printed CNT TFTs have the shortest channel length and
the highest on-state current density at a low drain voltage of −0.1 V and a
small gate voltage of −1.5 V . Detailed comparison of all the representative
printed CNT TFTs reported so far has been presented in Table 2.1, including
the printing method, gate dielectric material, metal contact, on-state current
density, etc. Based on Table 2.1, we can also conclude that our fully printed
CNT TFTs have the smallest channel length and the best on-state current
density. The ultrashort channel length may enable scaling up the number of
transistors per unit area, which may be promising for applications such as
printed CNT TFT active backplanes for high-resolution sensing, display, etc.
Moreover, the high current density at low-voltage operations makes these
printed CNT TFTs good candidates for display applications and low-power
consumption portable electronics.
30
Table 2.1 Comparison of on-state current density for printed CNT TFTs.
Reference L
(μm)
Method Dielectric Electrode V
G
(V)
V
DS
(V)
I
on
/W
(μA/μm)
Year
This
work
0.496 Ink-jet Ion Gel Au -1.5 -0.1 4.5 2016
Ref 28 100 Ink-jet Ion Gel Ag -1 0.1 0.075 2008
Ref 8 75 Ink-jet Ion Gel
(PEI/LiClO
4
)
Ag 1 0.5 0.002 2011
Ref 29 150 Gravure
+Ink-jet
BTO Ag -1.5 -0.1 0.000013 2011
Ref 21 200 Ink-jet Ion Gel Metallic
CNTs
-1.5 -0.1 0.003 2013
Ref 14 85 Gravure BTO+PMMA Ag -1.5 -0.1 0.00014 2013
Ref 12 105 Screen BTO Ag -1.5 -0.1 0.0001 2014
Ref 15 100 Gravure BTO Ag -1.5 -0.1 0.000016 2015
Ref 19 25 Aerosol Ion Gel Au/PEDOT -1.5 -0.1 0.18 2016
Ref 30 100 Aerosol SiO
2
Ag (double
layer)
-1.5 -0.1 0.00094 2016
Ref 30 100 Aerosol SiO
2
Metallic
CNT
-1.5 -0.1 0.0014 2016
Ref 30 100 Aerosol SiO
2
Au -1.5 -0.1 0.0016 2016
Ref 31 350 Ink-jet Polymer
(PV3D3)
Ag -1.5 -0.1 0.000015 2016
Ref 13 105 Screen BTO Ag -1.5 -0.1 0.0001 2016
Ref 20 150 Ink-jet BTO/PDMS Unsorted
CNTs
-1.5 -0.1 0.000064 2016
ξ
If values were not provided in the manuscripts explicitly, they were estimated from the I-V
curves presented in the manuscripts. On-state current density of this work are the average value
of 30 printed ultrashort channel CNT TFTs.
In order to assess the uniformity of the fully printed ultrashort channel CNT
TFTs, 30 transistors with a channel length of ∼496 nm have been printed, and
the electrical performance has been tested. Statistics studies of the key device
parameters, including the on-state current density, field-effect mobility,
threshold voltage, and the current on/off ratio, of the 30 printed transistors
have been conducted and presented in Figure 2.8. Judging from the on-state
current density distributions of the 30 printed CNT TFTs (Figure 2.8a), these
31
devices show good uniformity with an on-state current density of 4.5 ± 2.5
μA/μm at V
DS
= −0.1 V and V
G
= −1.5 V. We note that while the variation in
channel length affects the variation in the on-current, it does not fully account
for the on-current variation, as the variation in the CNT density can be another
important factor. Field-effect charge carrier mobilities of the 30 printed TFTs
are extracted from the transfer curves measured at V
DS
= −0.1 V . The statistic
distributions of mobilities are shown in Figure 2.8b, illustrating uniform
mobility with an average value of 15.03 cm
2
V
−1
s
−1
. Moreover, the transistors
have threshold voltages of −0.45 ± 0.22 V (Figure 2.8c). Furthermore, Figure
2.8d shows the distribution of current on/off ratios, with log
10
(I
on
/I
off
) of 4.81
± 0.56, which is comparable with previously published ion-gel-gated CNT
TFT work.
10
The semiconducting nanotube ink was purchased from
Nanointegris Inc. with ultrahigh purity of >99.99%, which is believed to be
the reason for the observed high on/off ratio >10
4
. As a result, even if 1 out of
10000 nanotubes is metallic, we should be able to obtain high on/off ratio.
Overall, printing of ultrashort channel CNT TFTs with self-aligned printing
approach is highly reliable and reproducible, and all of the printed sub-micron
nanotube TFTs show highly uniform electrical performance.
32
Figure 2.8 Statistics analysis of 30 CNT TFTs with L=496 nm printed with top-contact
self-aligned printing technique. (a) On-state current density distributions of 30 TFTs
measured at VG=-1.5V and VDS=-0.1V. (b) Field-effect mobility distributions of 30 TFTs
extracted from transfer characteristics measured at VDS=-0.1V. (c) Threshold voltage
distributions of 30 TFTs. (d) Current on/off ratio distributions of 30 TFTs measured at
VDS=-0.1V.
In order to fully reveal the benefits of channel length downscaling, systematic
comparison was carried out between the printed CNT TFTs with 496 nm
channel length and TFTs with channel lengths of 40, 120, and 150 μm. A
control group of 15 CNT TFTs was printed with channel lengths of 40, 120,
and 150 μm (Figure 2.9), following the same printing procedure as for short
channel devices.
2 3 4 5 6 7
0
2
4
6
8
10
12
14
16
18
20
Counts
log
10
(I
on
/I
off
)
0 2 4 6 8 10 12 14
0
2
4
6
8
10
12
14
16
18
I
on
/W ( A/ m)
Counts
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
0
2
4
6
8
10
12
14
16
18
Counts
V
th
(V)
0 3 6 9 12 15 18 21 24 27
0
1
2
3
4
5
6
7
8
Mobility (cm
2
V
-1
s
-1
)
Counts
a b
c d
I
on
/W
4.5±2.5 μA/μm
mobility
15.03 ±9.16 cm
2
V
-1
s
-1
V
th
-0.45±0.22 V
Log
10
(I
on
/I
off
)
4.81±0.56
33
Figure 2.9 Optical images showing the printed CNT TFTs with different channel lengths,
40 μm, 120 μm and 150 μm.
All of the printed TFTs with long channel lengths were electrically tested with
exactly the same measurement parameters of the short channel devices, and
the comparison was made and is presented in Figure 2.10a,b. Transfer
characteristics of four representative CNT TFTs, with channel-lengths of 496
nm, 40 μm, 120 μm, and 150 μm, are shown in Figure 2.10a, indicating a
much higher on-state current density for the 496 nm CNT transistor compared
to transistors with longer channel lengths. Figure 2.10b further shows the on-
state current density of these printed transistors as a function of the channel
length, exhibiting a decreasing trend of the on-state current density as the
channel length increases. Moreover, the width-normalized total resistance
(R·W) of CNT TFTs as a function of channel length is exhibited in Figure
2.10c. As channel length scales from 496 nm to 40, 120, and 150 μm, the R·W
50 μm
40 μm 120 μm
150 μm
34
increases by a factor of 10, 34 and 50, respectively. Overall, the printed
ultrashort channel CNT TFTs show a remarkably high on-state current density
at a low-voltage operation, which makes it promising for low power
consumption displays and portable electronics. By now, we have talked about
the printing of CNT TFTs with sub-micron channel length using a top-contact
self-aligned printing strategy. For future work, tunable channel length and
channel width may be highly desirable for wide applications. The channel
length of TFTs defined by the self-aligned printing technique is mainly
determined by the repulsion force between the metal nanoparticle ink and the
SAM-modified first electrode. Many factors can be adjusted to tune the
channel length. First of all, by choosing different thiol-based SAMs, the
surface hydrophobicity of the first electrode can be altered, and different
channel lengths can be achieved. Second, the concentration of the SAM and
the functionalization time may affect the hydrophobicity of the electrode
surface and yield different channel lengths. Moreover, the viscosity of the
metal nanoparticle ink can also be tuned to achieve the desirable channel
length. The channel width of the printed TFTs is basically controlled by the
printing process, which can be varied in a range from 20 to 1000 μm. In
addition, many different kinds of ion gels
25
have been reported so far and can
be used as the gate dielectric in replacement of the ion gel we used in this
35
paper for the self-aligned printed transistors. Furthermore, the dielectric
materials for the top-contact self-aligned printed TFTs are not limited to ion
gel. Indeed, the top-contact self-aligned printing technique is compatible with
various state-of-the-art dielectric deposition technologies. Many dielectric
materials can be deposited as the gate dielectric layer. For example, a thin
layer (∼20 nm) of atomic layer deposition (ALD)-deposited aluminum oxide
can be a great alterative dielectric material for the self-aligned printed TFTs.
Last but not least, the top-contact self-aligned printing technique is not limited
to CNT TFTs. Instead, a wide range of channel materials, such as two-
dimensional transition-metal dichalcogenide monolayers, organic materials
(which can be stable up to 200−280 °C), etc., are compatible with this printing
strategy and are worth exploration.
36
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
I
on
/W( A/ m)
V
G
(V)
0.496 m
40 m
120 m
150 m
0 50 100 150
0.1
1
10
I
on
/W( A/ m)
Chanel Length ( m)
a
b
c
0 50 100 150
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
R·W (MΩ·μm)
Channel Length ( m)
37
Figure 2.10 Electrical characterization of printed CNT TFTs with different channel
lengths. (a) Transfer characteristics of representative printed CNT TFTs with channel
lengths of 496 nm (violet), 40 μm (red), 120 μm (green), and 150 μm (blue). (b)
Statistical study of 45 printed CNT TFTs showing on-state current density as a function
of channel length. (c) Statistical study of 45 printed CNT TFTs showing width-
normalized total resistance as a function of channel length.
38
2.4 Summary
In conclusion, we have successfully downscaled the channel length of fully
printed CNT TFTs to a sub-micron level with a top-contact self-aligned
printing technique, which bypassed the resolution limit of inkjet printing
technology. In addition, high-capacitance ion gel has been printed as the gate
dielectric to further promote the on-state current density at low-voltage
operation. Notably, the fully printed sub-micron CNT TFTs exhibit superior
electrical performance, with an outstanding average on-state current density
of 4.5 μA/μm at V
G
= −1.5 V and V
DS
= −0.1 V , large average carrier mobility
of 15.03 cm
2
V
−1
s
−1
, and high current on/off ratio of ∼1 × 10
5
. Especially, our
printed ultrashort channel CNT TFTs demonstrate a huge improvement in
terms of on-state current density, which is several orders of magnitude higher
than previously reported printed CNT TFTs.
8,12−15,19−21,28−32
The successful
channel-length downscaling and on-state current improvement make printed
ultrashort-channel CNT TFTs a capable candidate as a low-cost, high-
definition solution for display electronics, sensing systems, electronic skins,
etc.
39
2.5 References
1. 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, 495-500.
2. Park, J. U.; Hardy, M.; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. K.;
Lee, C. Y .; Strano, M. S.; Alleyne, A. G.; Georgiadis, J. G.; Ferreira, P. M.; Rogers, J. A.
High-Resolution Electrohydrodynamic Jet Printing. Nat. Mater. 2007, 6, 782-789.
3. Sun, D. M.; Timmermans, M. Y .; Tian, Y .; Nasibulin, A. G.; Kauppinen, E. I.;
Kishimoto, S.; Mizutani, T.; Ohno, Y . Flexible High-Performance Carbon Nanotube
Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156-161.
4. Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A. User-
Interactive Electronic Skin for Instantaneous Pressure Visualization. Nat. Mater. 2013, 12,
899-904.
5. Wang, C.; Takei, K.; Takahashi, T.; Javey, A. Carbon Nanotube Electronics -
Moving Forward. Chem. Soc. Rev. 2013, 42, 2592-2609.
6. Wang, C.; Zhang, J. L.; Ryu, K. M.; Badmaev, A.; De Arco, L. G.; Zhou, C. W.
Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display
Applications. Nano Lett. 2009, 9, 4285-4291.
7. Cai, L.; Zhang, S.; Miao, J.; Yu, Z.; Wang, C. Fully Printed Foldable Integrated
Logic Gates with Tunable Performance Using Semiconducting Carbon Nanotubes. Adv.
Funct. Mater. 2015, 25, 5698-5705.
8. Chen, P. C.; Fu, Y .; Aminirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.;
Zhou, C. W. Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and
Its Application in Organic Light Emitting Diode Control. Nano Lett. 2011, 11, 5301-5308.
9. Ha, M.; Xia, Y .; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M. C.;
Frisbie, C. D. Printed, Sub-3V Digital Circuits on Plastic from Aqueous Carbon Nanotube
Inks. ACS Nano 2010, 4, 4388-4395.
40
10. Ha, M. J.; Seo, J. W. T.; Prabhumirashi, P. L.; Zhang, W.; Geier, M. L.; Renn, M. J.;
Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Aerosol Jet Printed, Low V oltage, Electrolyte
Gated Carbon Nanotube Ring Oscillators with Sub-5 μs Stage Delays. Nano Lett. 2013, 13,
954-960.
11. Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur,
A. High-Speed, Inkjet-Printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary
Ring Oscillators. Nano Lett. 2014, 14, 3683-3687.
12. Cao, X.; Chen, H. T.; Gu, X. F.; Liu, B. L.; Wang, W. L.; Cao, Y .; Wu, F. Q.; Zhou,
C. W. Screen Printing as a Scalable and Low-Cost Approach for Rigid and Flexible Thin-
Film Transistors Using Separated Carbon Nanotubes. ACS Nano 2014, 8, 12769-12776.
13. Cao, X.; Lau, C.; Liu, Y .; Wu, F.; Gui, H.; Liu, Q.; Ma, Y .; Wan, H.; Amer, M. R.;
Zhou, C. W. Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochromic
Displays Using Carbon Nanotube Thin-Film Transistors. ACS Nano 2016, 10, 9816-9822.
14. Lau, P. H.; Takei, K.; Wang, C.; Ju, Y .; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.; Javey,
A. Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on Flexible
Substrates. Nano Lett. 2013, 13, 3864-3869.
15. Yeom, C.; Chen, K.; Kiriya, D.; Yu, Z.; Cho, G.; Javey, A. Large-Area Compliant
Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv. Mater.
2015, 27, 1561-1566.
16. Higuchi, K.; Kishimoto, S.; Nakajima, Y .; Tomura, T.; Takesue, M.; Hata, K.;
Kauppinen, E. I.; Ohno, Y . High-Mobility, Flexible Carbon Nanotube Thin-Film
Transistors Fabricated by Transfer and High-Speed Flexographic Printing Techniques.
Appl. Phys. Express 2013, 6, 085101.
17. Zhang, J. L.; Fu, Y .; Wang, C.; Chen, P. C.; Liu, Z. W.; Wei, W.; Wu, C.; Thompson,
M. E.; Zhou, C. W. Separated Carbon Nanotube Macroelectronics for Active Matrix
Organic Light-Emitting Diode Displays. Nano Lett. 2011, 11, 4852-4858.
18. Xu, W.; Zhao, J.; Qian, L.; Han, X.; Wu, L.; Wu, W.; Song, M.; Zhou, L.; Su, W.;
Wang, C.; Nie, S.; Cui, Z. Sorting of Large-Diameter Semiconducting Carbon Nanotube
41
and Printed Flexible Driving Circuit for Organic Light Emitting Diode (OLED). Nanoscale
2014, 6, 1589-1595.
19. Li, H.; Tang, Y .; Guo, W.; Liu, H.; Zhou, L.; Smolinski, N. Polyfluorinated
Electrolyte for Fully Printed Carbon Nanotube Electronics. Adv. Funct. Mater. 2016, 26,
6914-6920.
20. Cai, L.; Zhang, S.; Miao, J.; Yu, Z.; Wang, C. Fully Printed Stretchable Thin-Film
Transistors and Integrated Logic Circuits. ACS Nano 2016,
DOI:10.1021/acsnano.6b07190
21. Sajed, F.; Rutherglen, C. All-Printed and Transparent Single Walled Carbon
Nanotube Thin Film Transistor Devices. Appl. Phys. Lett. 2013, 103, 143303.
22. Noh, Y . Y .; Zhao, N.; Caironi, M.; Sirringhaus, H. Downscaling of Self-Aligned,
All-Printed Polymer Thin-Film Transistors. Nat. Nanotechnol. 2007, 2, 784-789.
23. Caironi, M.; Gili, E.; Sakanoue, T.; Cheng, X. Y .; Sirringhaus, H. High Yield, Single
Droplet Electrode Arrays for Nanoscale Printed Electronics. ACS Nano 2010, 4, 1451-1456.
24. Hyun, W. J.; Secor, E. B.; Rojas, G. A.; Hersam, M. C.; Francis, L. F.; Frisbie, C.
D. All-Printed, Foldable Organic Thin-Film Transistors on Glassine Paper. Adv. Mater.
2015, 27, 7058-7064.
25. Kim, S. H.; Hong, K.; Xie, W.; Lee, K. H.; Zhang, S. P.; Lodge, T. P.; Frisbie, C. D.
Electrolyte-Gated Transistors for Organic and Printed Electronics. Adv. Mater. 2013, 25,
1822-1846.
26. Brady, G. J.; Way, A. J.; Safron, N. S.; Evensen, H. T.; Gopalan, P.; Arnold, M. S. Quasi-
ballistic carbon nanotube array transistors with current density exceeding Si and GaAs. Sci.
Adv. 2016, 2, e1601240.
27. Zhao, N.; Chiesa, M.; Sirringhausa, H.; Li, Y . N.; Wu, Y . L. Self-Aligned Inkjet
Printing of Highly Conducting Gold Electrodes with Submicron Resolution. J. Appl. Phys.
2007, 101, 064513.
42
28. Vaillancourt, J.; Zhang, H. Y .; Vasinajindakaw, P.; Xia, H. T.; Lu, X. J.; Han, X. L.;
Janzen, D. C.; Shih, W. S.; Jones, C. S.; Stroder, M.; Chen, M. Y . H.; Subbaraman, H.;
Chen, R. T.; Berger, U.; Renn, M. All Ink-Jet-Printed Carbon Nanotube Thin-Film
Transistor on a Polyimide Substrate with an Ultrahigh Operating Frequency of over 5 GHz.
Appl. Phys. Lett. 2008, 93, 243301.
29. Noh, J.; Jung, M.; Jung, K.; Lee, G.; Lim, S.; Kim, D.; Kim, S.; Tour, J. M.; Cho,
G. Integrable Single Walled Carbon Nanotube (SWNT) Network Based Thin Film
Transistors Using Roll-to-Roll Gravure and Inkjet. Org. Electron. 2011, 12, 2185-2191.
30. Cao, C. Y .; Andrews, J. B.; Kumar, A.; Franklin, A. D. Improving Contact Interfaces
in Fully Printed Carbon Nanotube Thin-Film Transistors. ACS Nano 2016, 10, 5221-5229.
31. Lee, D.; Yoon, J.; Lee, J.; Lee, B. H.; Seol, M. L.; Bae, H.; Jeon, S. B.; Seong, H.;
Im, S. G.; Choi, S. J.; Choi, Y . K. Logic Circuits Composed of Flexible Carbon Nanotube
Thin-Film Transistor and Ultra-Thin Polymer Gate Dielectric. Sci. Rep. 2016, 6, 26121.
32. Vuttipittayamongkol, P.; Wu, F. Q.; Chen, H. T.; Cao, X.; Liu, B. L.; Zhou, C. W.
Threshold V oltage Tuning and Printed Complementary Transistors and Inverters Based on
Thin Films of Carbon Nanotubes and Indium Zinc Oxide. Nano Res. 2015, 8, 1159-1168.
43
3 Screen printing as a scalable and low-
cost approach for rigid and flexible
thin-film transistors using separated
carbon nanotubes
3.1 Introduction
Printing technology in manufacturing electronics has drawn tremendous
interest during the past few decades.
1-12
Compared with traditional fabrication
approaches in which multistaged photolithography and vacuum deposition are
required, printing is a cost-effective and scalable technology with high
throughput and is highly compatible with low-temperature processing, which
provides an important way in mass production of large-area flexible
electronics at extremely low cost.
13,14
Among various kinds of printed
electronics, separated single-wall carbon nanotube (SWCNT) thin-film
transistors (TFTs) have attracted growing attention due to their high mobility,
high on/off ratio, low operation voltage, and potential application in flexible
electronics.
15-19
Recently, intensive research effort has been devoted to
develop low-cost printed SWCNT TFTs and integrated circuits with high
performance. The printing techniques used in the past research can be mainly
44
divided into two groups. The first one is of high registration accuracy
represented by aerosol-jet printing and inkjet printing. The second one is of
high scalability and throughput represented by gravure printing and
flexographic printing. Up until now, inkjet and aerosol-jet printing have been
used for fabricating SWCNT TFTs,
20-22
2T1C pixel control circuits for organic
light-emitting diode (OLED) control
23
and digital circuit applications.
20,24,25
The advantages of those two printing techniques are the high printing
resolution and uniformity,
9
which endow printed devices with small
dimension and decent electrical performance. Nevertheless, those techniques,
limited by low scalability and throughput, may not be suitable for mass
production to further reduce the cost of printed electronics. For research of
highly scalable printing technology of SWCNT TFTs, pioneer work has been
done by Cho's group which demonstrated gravure-printing-based R2R process
for manufacturing carbon nanotube (CNT) radio frequency identifications,
26
full adders
27
and D flip-flops.
28
However, the performance is moderate due to
the nature of printed layers and the quality of the active channel material.
29
Recently, as a milestone, Javey's and Cho's group reported fully gravure-
printed large-area flexible top-gated CNT TFTs with excellent electrical
performance.
29
In their work, semiconductor-concentrated nanotube solution
was used as channel material and high-k barium titanate (BTO)/poly(methyl
45
methacrylate) hybrid ink was printed as gate dielectric, which significantly
improved the mobility (μ) and on/off ratio of the printed SWCNT devices.
Moreover, the printed devices showed low operation voltage (<10 V) and
outstanding mechanical bendability. Later in 2013, flexographic printing and
transfer were reported by Ohno's group for SWCNT TFT fabrication, starting
from SWCNTs grown by chemical vapor deposition.
30
Mobility of 157 cm
2
V
-1
s
-1
was achieved by using polyimide as the gate dielectric for the back-
gated devices.
Screen printing, in which screen masks are used to deposit materials onto
large-area substrates with high throughput, is considered as one of the scalable
printing techniques and has been widely used in printed electronics.
1,7,31-33
Benefiting from its simplicity, scalability and environment-friendly
process,
1,31
this technique shows tremendous potential for mass production of
large-area electronics at very low cost. There are two advantages of screen
printing when compared with other scalable printing techniques such as
gravure or flexographic printing. First, the masks for screen printing are
usually made of fabric or stainless steel mesh, which are much more cost-
effective than engraved metal masks for gravure printing.
13
The second
advantage is that alignment between the screen mask and the substrate can be
46
easily performed right before printing, as the screen mask is semitransparent,
and both the screen mask and the substrate are planar. These advantages
enables the alignment to be performed in a parallel-plate fashion with good
accuracy.
In contrast, the gravure printing and flexographic printing usually have either
the mask or the substrate on a roller, making alignment in both X and Y
direction difficult. Hence, the alignment capability of screen printing makes
this technique particularly suitable for fabrication of multilayered structures.
A lot of research has been done in screen-printed TFTs, especially for organic
TFTs.
32
However, the performance is usually limited by the inherent
properties of the organic channel materials.
In this paper, we report the first fully screen-printed top-gated TFTs on rigid
and flexible substrates using semiconductor-enriched SWCNT solutions.
Silver (Ag) source (S) and drain (D), high-k barium titanate dielectric and Ag
gate (G) were printed sequentially in the fabrication process with low-
temperature baking (∼140 ℃). The printed devices showed mobility up to
7.67 cm
2
V
-1
s
-1
, low operation voltage (<10 V), current on/off ratio of 10
4
∼
47
10
5
, and excellent mechanical flexibility. In addition, OLED control capability
of printed SWCNT TFTs was demonstrated by connecting an external OLED
to a representative TFT. Overall, the good electrical characteristics, low-
temperature and cost-effective fabrication process, and outstanding
mechanical flexibility of fully printed SWCNT TFTs suggest screen printing
is very promising to become a practical technique for device and display
applications.
48
3.2 Development of fully screen printed SWCNT
TFTs
Figure 3.1a illustrates the fabrication process of printed TFTs on rigid Si/SiO2
wafer and flexible poly(ethylene terephthalate) (PET) substrate. The surface
of substrate was first functionalized with poly-L-lysine and then a uniform
random SWCNT network was formed by immersing the substrate into
semiconductor-enriched SWCNT solution for 35 min. Second, Ag source,
drain, and barium titanate dielectric were printed, followed by etching of
unwanted SWCNTs that were not covered by Ag electrodes and BTO
dielectric. Finally, Ag gate was printed. More details of the experiments are
shown in the Method Section. Figure 3.1b shows the configuration of a fully
printed TFT on PET substrate and Figure 3.1c is a schematic diagram
exemplifying the screen printer and screen printing process. Ink was first
applied on the screen mask by a stainless spatula and then spread by a
squeegee on the whole patterned area. Then the squeegee was moved across
the mask with certain pressure so that the ink was squeezed through the mesh
in the desired pattern area and then printed on the substrate. Eventually, the
printed layer was cured in an oven at 140 ℃. In the printing process, mask
specifications, clearance, squeegee angle, printing speed, ink properties, and
49
pressure are key factors which determine the quality of printed layers.
31
In
addition, there are X-Y-θ adjustment micrometers on the printer to ensure
good alignment between different layers. Figure 3.1d, e shows optical images
of printed 12 × 10 TFT arrays on a Si/SiO2 and a PET substrate, respectively.
The SWCNT network was inspected by field-emission scanning electron
microscopy (FE-SEM) as shown in Figure 3.1f.
50
Figure 3.1 Fully screen-printed SWCNT TFTs on rigid and flexible substrates. (a)
Schematic diagram shows the fabrication process of fully printed top-gated SWCNT
TFTs. (b), (c) Schematic diagrams show the configuration of a fully printed TFT on PET
substrate and screen printing system, respectively. (d), (e) Optical images of fully printed
TFT arrays on a 4-inch Si/SiO2 wafer (d) and a 12 × 12 cm PET sheet (e), respectively.
(f) FE-SEM image of deposited SWCNT film.
Squeegee
Clearance
Squeegee angle
Substrate
Mask Ink
5 cm
a
b c
d e f
5 cm
Ag gate
BTO dielectric
Ag S&D
PET
1 μm
Substrate
SWCNT
network
SWCNT
incubation
S&D
patterning
Ag
electrode
BTO
Dielectric
patterning
Excessive
SWCNT
etching
Gate
patterning
Ag gate
51
We carried out significant amount of work to optimize the printed TFT
performance by using BTO ink and Ag ink of different concentrations
prepared vis dilution. The characteristics of TFTs printed with different ink
dilution ratios are shown in Figure 3.2. We selected TFTs with channel length
(L) ∼ 105 μm and channel width (W) ∼ 1000 μm as examples. BTO and Ag
inks were purchased from commercial sources and then diluted using
diethylene glycol ethyl ether acetate as the solvent with different volume ratio
(V
sol
/V
ink
). Figure 3.2a, b shows the thickness variation of printed BTO and
Ag layers as a function of V
sol
/V
ink
. Due to the nature of screen printing, ink
used for this technique requires high viscosity, which results in inherently
thick printed layers. In Figure 3.2a, b, the thicknesses of undiluted (UD) BTO
and Ag layers after printing are 8.1 and 10 μm, respectively, measured using
a profilometer. The thickness variation from sample to sample was observed
to be around 0.5 μm. We note the thickness can be reduced by diluting the
inks. For example, diluted BTO ink with V
sol
/V
ink
= 1:4 led to printed BTO
layer of ∼5 μm in thickness, while diluted Ag ink with V
sol
/V
ink
= 1:4 and
V
sol
/V
ink
= 1:3 led to Ag electrode of ∼6 μm and ∼3.5 μm, respectively. To
facilitate printing of multiple layers for the SWCNT TFTs, we needed Ag
source and drain electrodes with height that would not negatively affect the
printing of subsequent BTO, so we focused on Ag inks diluted with volume
52
ratios of 1:4 and 1:3. The transfer characteristics of SWCNT TFTs printed
using inks of different dilution ratios are shown in Figure 3.2c, d. To study the
effect of BTO ink dilution, we can first compare the curves in Figure 3.2c, d
corresponding to diluted Ag ink with V
sol
/V
ink
= 1:3. When undiluted BTO ink
was used, the printed SWCNT TFTs show low on-current (∼0.4 μA) and low
transconductance (∼0.059 μS/mm) at V
G
= -10 V and V
DS
= -1 V. Remarkably,
on-current and the peak of transconductance (Figure 3.2d) increased up to ∼3
μA and ∼0.27 μS/mm when we used the 1:4 diluted BTO for dielectric layer.
We attribute the improved on-current to both improvement in gate capacitance
and mobility when thinned dielectric was used. First of all, at a given gate
voltage (V
G
), thinner gate dielectric can lead to higher gate capacitance and
stronger gate-channel coupling, and thus the number of carriers in the channel
increases. Moreover, based on the multiple trap and release (MTR) model,
33
the increased charge in the channel can lead to greater filling of interface traps,
causing the ratio of free to trapped carriers to increase. As a result, devices
with thinner dielectrics would show higher field-effect mobility. Both the
enhanced mobility and increased number of carriers contributed to larger
drain current (ID) and transconductance when diluted BTO was used.
53
Figure 3.2 Characterization of SWCNT TFTs printed with different ink dilution conditions.
(a), (b) Thicknesses of printed BTO and silver layers as a function of dilution ratios. (c)
Transfer (ID-VG) characteristics of TFTs printed with inks of different dilution ratios
(Vsol/Vink), measured at VDS = -1 V. (d) Transconductance exacted as a function of gate
voltage.
In addition, we observed that Ag inks of different dilution ratios also affect
the performance of printed devices even when the same kind of BTO was used
for gate dielectric printing. As shown in Figure 3.2c, d, with the same dilution
ratio of BTO, more diluted Ag ink for source and drain would improve the on-
current and peak transconductance. Interestingly, we found that dilution of Ag
b
d
0.0 0.1 0.2 0.3
2
4
6
8
10
Dilution ratio (V
sol
/V
ink
)
Ag
Thickness ( m)
0.0 0.1 0.2 0.3
2
4
6
8
10
Dilution ratio (V
sol
/V
ink
)
BTO
Thickness ( m)
a
c
-10 -5 0 5 10
0
1
2
3
V
G
(V)
-I
D
( A)
1:4 BTO & 1:3 Ag
1:4 BTO & 1:4 Ag
UD BTO & 1:3 Ag
UD BTO & 1:4 Ag
-10 -5 0 5 10
0.0
0.1
0.2
0.3
V
G
(V)
Transconductance ( S/mm)
54
ink for source and drain had an important effect on the thickness of the
subsequent printed BTO dielectric, which had a direct impact on the device
performance. Here we propose that capillary effect plays a critical role in the
solution-based printing process. After source and drain printing, a “trench”
with width = 105 μm in the channel region was formed between the rather
high source and drain electrodes. A deeper trench formed by less-diluted Ag
source and drain would have stronger capillary effect and thus more BTO
material would be trapped in the trench when BTO was printed. Hence, the
resulted printed BTO was thicker than the BTO layer in more-diluted S/D case.
As shown in Figure 3.3, with the same BTO ink with V
sol
/V
ink
= 1:4 for
dielectric, the resulted BTO layer when 1:3 diluted Ag ink was used for source
and drain is ∼5 μm in thickness and thinner than the ∼6.5 μm BTO layer when
1:4 diluted Ag ink was used for source and drain patterning. Nonetheless,
overdiluted BTO (V
sol
/V
ink
> 1:4) caused gate leakage through pinholes in the
gate dielectric. Hence we optimized the ink dilution as V
sol
/V
ink
= 1:3 for Ag
source and drain, Vsol/Vink = 1:4 for BTO gate dielectric, and undiluted Ag
ink for gate for the following study, and the profiles of the printed electrode,
dielectric layer, and gate are shown in Figure 3.4 as examples.
55
Figure 3.3 Schematic diagrams and SEM images illustrate the capillary effect on BTO
layers printed with different dilution conditions. (a) Source and drain were printed with
diluted silver ink (Vsol/Vink = 1:4) and then diluted BTO ink (Vsol/Vink = 1:4) was printed
as gate dielectric. (b) Source and drain were printed with diluted silver ink (Vsol/Vink =
1:3) and then diluted BTO ink (Vsol/Vink = 1:4) was printed as gate dielectric. The result
shows a thinner BTO layer (~5 μm) in (b) compared with the BTO layer (~ 6.5 μm) in
(a).
Figure 3.4 Profiles of printed electrode (a) using 1:3 silver ink, dielectric layer (b) using
1:4 BTO ink, and gate (c) using undiluted silver ink, showing approximate thickness~3.3
μm, 5.1 μm, and 9.8 μm, respectively.
10 μm
a b
BTO
AG AG
AG AG
BTO
10 μm
BTO BTO
0.0 0.5 1.0 1.5 2.0
0
2
4
6
8
Height ( m)
Position ( m)
0.0 0.5 1.0
0
2
4
6
8
10
Height ( m)
Position ( m)
a b c
0.0 0.5 1.0 1.5 2.0
-1
0
1
2
3
4
Height ( m)
Position ( m)
56
3.3 Electrical performance of fully screen printed
SWCNT TFTs
On the basis of the optimized dilution conditions, the electrical performance
of fully printed SWCNT TFTs on rigid Si/SiO2 substrate was further studied,
as shown in Figure 3.5.
Figure 3.5 Electrical characteristics of fully printed top-gated SWCNT TFTs on Si/SiO2
substrate, with ink dilution of Vsol/Vink
= 1:3 for the silver source and drain, Vsol/Vink
= 1:4
for the BTO dielectric, and undiluted silver for the gate. (a) Double-sweep of transfer
characteristics of a representative TFT measured at VDS = -1 V, showing very small
hysteresis. (b) Transfer characteristics under different drain voltages (from -1 to -3 V in
0.5 V steps). (c, d) Output characteristics of the same device in triode regime (c) and
saturation regime (d), respectively.
-5.0 -2.5 0.0
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
I
D
( A)
V
DS
(V)
V
G
from -5 V
to 0 V in 1 V
steps
V
DS
from -1 V to -3 V
in 0.5 V steps
L = 105 μm
W = 1000 μm
μ = 7.67 cm
2
V
-1
S
-1
-10 -5 0 5 10
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
-I
D
(A)
V
G
(V)
-10 -5 0 5 10
0
2
4
6
8
10
-I
D
( A)
V
G
(V)
a b
c d
-0.10 -0.05 0.00 0.05 0.10
-40
-20
0
20
40
I
D
(nA)
V
DS
(V)
V
G
from -5 V
to 0 V in 1 V
steps
57
Transfer characteristics of a representative TFT with L = 105 μm and W =
1000 μm are shown in Figure 3.5a. The device exhibits current on/off ratio of
∼3 ×10
4
, extremely small hysteresis, and normalized peak transconductance
of∼0.43 μS/mm at V
DS
= -1 V. The mobility was calculated with the following
equation:
23
μ =
𝐿 𝑊 1
𝐶 𝑜𝑥
𝑉 𝑆𝐷
𝑑 𝐼 𝑆𝐷
𝑑 𝑉 𝐺
where L and W are the channel length and width of the device. V
SD
is the
source and drain voltage and is 1 V. I
SD
is the current flowing from source to
drain and V
G
is the gate voltage. C
ox
is the gate capacitance per unit area and
can be calculate using the following equation if we consider the SWCNT
network as a uniform thin-film:
23
C
𝑜𝑥
=
𝜀 0
𝜀 𝑟 𝑡 𝑜𝑥
= 6.195 × 10
−9
F/cm
2
where ε
r
is the relative dielectric constant of the BTO (∼35 for the BTO ink
we used), ε
0
is the vacuum dielectric constant, and t
ox
is the thickness of the
BTO layer (∼5 μm). Then the calculated field effect mobility is 7.67 cm
2
V
-1
s
-1
. The gate leakage current as a function of gate voltage at V
DS
= -1 V is
shown in Figure 3.6, and the small leakage current (<0.5 nA) indicates
excellent insulating property of the printed BTO dielectric layer. The transfer
curves with V
DS
ranging from -0.1 to 0.1 V are shown in Figure 3.5b. The
58
output curves in Figure 3.5c exhibits a clear linear regime, illustrating good
ohmic contacts between the printed Ag electrode and the SWCNT network.
Additionally, current saturation due to pinch-off effect was clearly observed
as shown in Figure 3.5d. Statistical study of 15 fully screen printed SWCNT
TFTs with L ∼ 105 μm and W ∼ 1000 μm is shown in Figure 3.7, exhibiting
good uniformity in terms of mobility, current on/off ratio, on-current density,
and threshold voltage.
Figure 3.6. Gate leakage current of a presentative printed SWCNT TFT as a function of
gate voltage at VDS = -1 V.
-10 -5 0 5 10
-400.0p
0.0
400.0p
V
G
(V)
I
G
(A)
59
Figure 3.7 Statistical analysis of 15 fully screen printed SWCNT TFTs showing (a) field-
effect mobility, (b) current on/off ratio, (c) on-current density, and (d) threshold voltage
(Vth). The calculated average values and standard deviations are included in each figure.
0 2 4 6 8 10 12 14 16
0
1
2
3
4
5
6
7
8
9
Mobility (cm
2
V
-1
s
-1
)
Index of devices
0 2 4 6 8 10 12 14 16
0
1
2
3
4
5
6
Log (I
on
/I
off
)
Index of devices
0 2 4 6 8 10 12 14 16
0
1
2
3
4
5
Index of devices
I
on
/W ( A/mm)
0 2 4 6 8 10 12 14 16
0
1
2
3
4
5
6
Threshold voltage (V)
Index of devices
a b
c d
μ = 4.41 1.11 cm
2
V
-1
s
-1
Log(I
on
/I
off
)= 4.45 0.38
I
on
/W= 2.81 0.57 μS/mm V
th
= 4.04 1.08 V
60
3.4 Flexibility of fully screen printed CNT TFTs
After the demonstration of fully printed high-performance SWCNT TFTs on
rigid substrate, screen printing was also used in fabricating SWCNT TFTs on
flexible substrate. Figure 3.8 shows the mechanical flexibility of fully screen-
printed TFTs on PET substrate. Flexible TFTs were wrapped onto glass vials
of different radii (R) during the measurement. The optical image of the
measurement setup is shown in Figure 3.8a and the transfer characteristics of
different bending conditions are shown in Figure 3.8b. The extracted mobility
and on/off ratio as a function of bending radius in Figure 3.8c,d indicate that
there is little measurable degradation of the flexible SWCNT TFT while bent
with radius of curvature down to 3 mm. Our results indicate the great
mechanical stability and flexibility of the SWCNT network and the TFT
structure.
61
Figure 3.8 Mechanical flexibility of fully printed flexible SWCNT TFTs on PET substrate,
with ink dilution of Vsol/Vink
= 1:3 for the silver source and drain, Vsol/Vink
= 1:4 for the
BTO dielectric, and undiluted silver ink for the gate. (a) Optical image of electrical
measurements on a printed TFT while bent. (b) Transfer (ID-VG) characteristics of a
representative device under different bending conditions of relaxed and bent at different
radii of curvature (R), measured at VDS = -1 V. (c) Field-effect mobility and (d) Ion/Ioff
plotted as a function of bending radius of curvature (R of relaxed state is infinite).
-10 -5 0 5 10
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
Relaxed
R = 10.8 mm
R = 5.8 mm
R = 3 mm
-I
D
( )
V
G
(V)
2 cm
∞
a b
d
0 4 8 12
1
2
3
4
5
Mobility (cm
2
V
-1
s
-1
)
Radius of Curvature (mm)
0 4 8 12
10
1
10
2
10
3
10
4
10
5
10
6
On/Off ratio
Radius of Curvature (mm)
∞
c
62
3.5 Fully screen printed CNT TFTs for driving
OLEDs
On the basis of the measured high on/off ratio, high mobility, small hysteresis,
and low operation voltage, we further studied the application of these fully
printed low-cost SWCNT TFTs in display electronics. For proof of concept,
a typical printed top-gated TFT was connected to an external OLED, whose
structure is shown in Figure 3.9. The schematic diagrams of the measured
circuits are in the insets of Figure 3.10a-c. The current which flows through
the OLED (I
OLED
) is plotted as a function of gate voltage at different V
DD
in
Figure 3.10a. A driving current of 22.5 μA was measured at V
G
= -20 V and
V
DD
= 5 V, which is more than sufficient for driving the OLED that requires
1 μA to have observable light emission. The family curves of I
OLED
- V
ss
measured at different gate voltages are shown in Figure 3.10b, and Figure
3.10c indicates that the intensity of light emission changed with increasing V
G
.
The OLED was very bright at V
G
= -10 V, V
DD
= 3 V and then the light
intensity was reduced by increasing V
G
. Eventually, the OLED was turned off
at V
G
= 6 V. On the basis of the data above, we demonstrated that the fully
screen-printed SWCNT TFTs exhibited good OLED control capability and
63
might have potential for large-area, flexible and low-cost display electronics
applications.
Figure 3.9 Schematic diagrams showing the structure of the external OLED with
aluminum (Al) ~100 nm, LiF~1 nm, tris (8-hydroxyquinoline) aluminum (Alq3) ~40 nm,
4,4’-bis[N- (1-naphthyl) -N-phenylamino]biphenyl (NPD) ~40 nm and ITO.
Al
LiF
Alq
3
NPD
ITO
64
Figure 3.10 Fully printed SWCNT TFTs for OLED control. (a) IOLED-VG family curves
correspond to values of VDD from 2.5 to 5 V in 0.5 V steps. (b) IOLED-VSS family curves
correspond to values of VG from -10 to 2 V in 2 V steps. (c) Optical images showing
external OLED intensity change versus VG with VDD = 3 V.
-20 -10 0 10 20
0
5
10
15
20
25
I
OLED
( )
V
G
(V)
V
DD
from 2.5 to 5 V
in 0.5 V steps
V
G
from -10 to 2 V
in 2 V steps
-10 -5 0 5 10
0
5
10
15
20
I
OLED
()
V
ss
(V)
V
G
V
ss
V
G
V
DD
= 3V
V
G
V
DD
a b
V
G
= - 10 V
V
G
= - 6 V V
G
= - 2 V V
G
= 2 V
c
V
G
= 6 V
65
3.6 Summary
In summary, we have fabricated fully screen-printed top-gated SWCNT TFTs
on both rigid and flexible substrates. In this study, semiconductor-enriched
SWCNT solution was used as channel materials and high-k gate dielectric was
printed. In addition, the optimization of ink dilution was studied and the
printed devices exhibited mobility up to 7.67 cm
2
V
-1
s
-1
, on/off ratio of 10
4
∼
10
5
, minimal hysteresis, low operation voltage, and outstanding mechanical
flexibility. On the basis of the good performance, the OLED driving capability
of fully printed TFTs was demonstrated. Our work shows that screen printing
has great potential for mass production of large-area, cost-effective and high-
performance CNT TFTs for applications in macroelectronic applications. One
concern about screen printing may be the printing resolution, which is related
to several factors. First of all, specifications of the screen mask is an important
factor including the mechanical strength of the metal mesh, emulsion
thickness, wire diameter, and opening ratio. Recently, 6 μm resolution screen
printing has been reported by improving the quality of the screen masks.
34
Second, ink and substrate also affect the printing resolution. Most of the inks
used for screen printing are composed of nanoparticles, binders, and solvents.
The diameter of the nanoparticles and affinity of the ink to the substrate play
66
significant roles influencing the printing resolution. Overall, screen printing
holds great promise for high-resolution printing in the future.
67
3.7 References
1. Bao, Z. N.; Feng, Y.; Dodabalapur, A.; Raju, V. R.; Lovinger, A. J. High-
Performance Plastic Transistors Fabricated by Printing Techniques. Chem. Mater. 1997, 9,
1299-1301.
2. Berggren, M.; Nilsson, D.; Robinson, N. D. Organic Materials for Printed
Electronics. Nat. Mater. 2007, 6, 3-5.
3. Cho, J. H.; Lee, J.; Xia, Y.; Kim, B.; He, Y. Y.; Renn, M. J.; Lodge, T. P.; Frisbie,
C. D. Printable Ion-Gel Gate Dielectrics for Low-Voltage Polymer Thin-Film Transistors
on Plastic. Nat. Mater. 2008, 7, 900-906.
4. Li, Y. N.; Wu, Y. L.; Ong, B. S. Facile Synthesis of Silver Nanoparticles Useful
for Fabrication of High-Conductivity Elements for Printed Electronics. J. Am. Chem. Soc.
2005, 127, 3266-3267.
5. Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai,
R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364-367.
6. Park, J. U.; Hardy, M.; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. K.;
Lee, C. Y.; Strano, M. S.; Alleyne, A. G.; Georgiadis, J. G., et al. High-Resolution
Electrohydrodynamic Jet Printing. Nat. Mater. 2007, 6, 782-789.
7. Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya,
T. Stretchable Active-Matrix Organic Light-Emitting Diode Display Using Printable
Elastic Conductors. Nat. Mater. 2009, 8, 494-499.
68
8. Sekitani, T.; Takamiya, M.; Noguchi, Y.; Nakano, S.; Kato, Y.; Sakurai, T.;
Someya, T. A Large-Area Wireless Power-Transmission Sheet Using Printed Organic
Transistors and Plastic Mems Switches. Nat. Mater. 2007, 6, 413-417.
9. Singh, M.; Haverinen, H. M.; Dhagat, P.; Jabbour, G. E. Inkjet Printing-Process
and Its Applications. Adv. Mater. 2010, 22, 673-685.
10. Willmann, J.; Stocker, D.; Dorsam, E. Characteristics and Evaluation Criteria of
Substrate-Based Manufacturing. Is Roll-to-Roll the Best Solution for Printed Electronics?
Org. Electron. 2014, 15, 1631-1640.
11. Yan, H.; Chen, Z. H.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.;
Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors.
Nature 2009, 457, 679-686.
12. Fukuda, K.; Takeda, Y.; Yoshimura, Y.; Shiwaku, R.; Tran, L. T.; Sekine, T.;
Mizukami, M.; Kumaki, D.; Tokito, S. Fully-Printed High-Performance Organic Thin-
Film Transistors and Circuitry on One-Micron-Thick Polymer Films. Nat. Commun. 2014,
10.1038/ncomms5147.
13. Aleeva, Y.; Pignataro, B. Recent Advances in Upscalable Wet Methods and Ink
Formulations for Printed Electronics. J. Mater. Chem. C 2014, 2, 6436-6453.
14. Sun, D. M.; Liu, C.; Ren, W. C.; Cheng, H. M. A Review of Carbon Nanotube- and
Graphene-Based Flexible Thin-Film Transistors. Small 2013, 9, 1188-1205.
69
15. Sun, D. M.; Timmermans, M. Y.; Tian, Y.; Nasibulin, A. G.; Kauppinen, E. I.;
Kishimoto, S.; Mizutani, T.; Ohno, Y. Flexible High-Performance Carbon Nanotube
Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156-161.
16. Wang, C.; Takei, K.; Takahashi, T.; Javey, A. Carbon Nanotube Electronics -
Moving Forward. Chem. Soc. Rev. 2013, 42, 2592-2609.
17. Wang, C.; Zhang, J. L.; Ryu, K. M.; Badmaev, A.; De Arco, L. G.; Zhou, C. W.
Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display
Applications. Nano Lett. 2009, 9, 4285-4291.
18. 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, 495-500.
19. Zhang, J. L.; Wang, C.; Zhou, C. W. Rigid/Flexible Transparent Electronics Based
on Separated Carbon Nanotube Thin-Film Transistors and Their Application in Display
Electronics. Acs Nano 2012, 6, 7412-7419.
20. Ha, M. J.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M.
C.; Frisbie, C. D. Printed, Sub-3v Digital Circuits on Plastic from Aqueous Carbon
Nanotube Inks. Acs Nano 2010, 4, 4388-4395.
21. Okimoto, H.; Takenobu, T.; Yanagi, K.; Miyata, Y.; Shimotani, H.; Kataura, H.;
Iwasa, Y. Tunable Carbon Nanotube Thin-Film Transistors Produced Exclusively via
Inkjet Printing. Adv. Mater. 2010, 22, 3981-3986.
70
22. Xu, W.; Zhao, J.; Qian, L.; Han, X.; Wu, L.; Wu, W.; Song, M.; Zhou, L.; Su, W.;
Wang, C.; Nie, S.; Cui, Z. Sorting of Large-Diameter Semiconducting Carbon Nanotube
and Printed Flexible Driving Circuit for Organic Light Emitting Diode (Oled). Nanoscale
2014, 6, 1589-1595.
23. Chen, P. C.; Fu, Y.; Aminirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.;
Zhou, C. W. Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and
Its Application in Organic Light Emitting Diode Control. Nano Lett. 2011, 11, 5301-5308.
24. Ha, M. J.; Seo, J. W. T.; Prabhumirashi, P. L.; Zhang, W.; Geier, M. L.; Renn, M.
J.; Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Aerosol Jet Printed, Low Voltage, Electrolyte
Gated Carbon Nanotube Ring Oscillators with Sub-5 μs Stage Delays. Nano Lett. 2013, 13,
954-960.
25. Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur,
A. High-Speed, Inkjet-Printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary
Ring Oscillators. Nano Lett. 2014, 14, 3683-3687.
26. Jung, M.; Kim, J.; Noh, J.; Lim, N.; Lim, C.; Lee, G.; Kim, J.; Kang, H.; Jung, K.;
Leonard, A. D., et al. All-Printed and Roll-to-Roll-Printable 13.56-Mhz-Operated 1-Bit Rf
Tag on Plastic Foils. Ieee. T. Electron Devices 2010, 57, 571-580.
27. Noh, J.; Jung, K.; Kim, J.; Kim, S.; Cho, S.; Cho, G. Fully Gravure-Printed Flexible
Full Adder Using Swnt-Based Tfts. Ieee. Electron Device Lett. 2012, 33, 1574-1576.
28. Noh, J.; Jung, M.; Jung, K.; Lee, G.; Kim, J.; Lim, S.; Kim, D.; Choi, Y.; Kim, Y.;
Subramanian, V., et al. Fully Gravure-Printed D Flip-Flop on Plastic Foils Using Single-
Walled Carbon-Nanotube-Based Tfts. Ieee. Electron Device Lett. 2011, 32, 638-640.
71
29. Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.;
Javey, A. Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on
Flexible Substrates. Nano Lett. 2013, 13, 3864-3869.
30. Higuchi, K.; Kishimoto, S.; Nakajima, Y.; Tomura, T.; Takesue, M.; Hata, K.;
Kauppinen, E. I.; Ohno, Y. High-Mobility, Flexible Carbon Nanotube Thin-Film
Transistors Fabricated by Transfer and High-Speed Flexographic Printing Techniques.
Appl. Phys. Express 2013, 6, 085101.
31. Menard, E.; Meitl, M. A.; Sun, Y. G.; Park, J. U.; Shir, D. J. L.; Nam, Y. S.; Jeon,
S.; Rogers, J. A. Micro- and Nanopatterning Techniques for Organic Electronic and
Optoelectronic Systems. Chem. Rev. 2007, 107, 1117-1160.
32. Ryu, G. S.; Kim, J. S.; Jeong, S. H.; Song, C. K. A Printed Otft-Backplane for
Amoled Display. Org. Electron. 2013, 14, 1218-1224.
33. Singh, V. K.; Mazhari, B. Impact of Scaling of Dielectric Thickness on Mobility in
Top-Contact Pentacene Organic Thin Film Transistors. J. Appl. Phys. 2012, 111, 034905.
34. Http://www.kuroda-electric.eu/Ultra-Fine-Pattern-Screen-Printing
72
4 Fully screen-printed, large-area, and
flexible active-matrix electrochromic
displays using carbon nanotube thin-
film transistors
4.1 Introduction
Printed electronics enable additive patterning, solution-based processing at
low-temperatures, outstanding scalability, and the elimination of high-
vacuum conditions.
1,2
Compared with conventional microfabrication,
printing simplifies the manufacturing process from multistage
photolithography and deposition to one-step additive patterning,
1
which
dramatically accelerates the manufacturing process. Furthermore, the
processing temperature for printing technology is typically below 130 ° C,
1
which is compatible with commonly used flexible plastic substrates such as
polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).
Printing is also a solution-based patterning technique and does not require
high-vacuum conditions needed in conventional material deposition
73
processes such as metal evaporation, atomic layer deposition, and so on. As
a result, printed electronics can remarkably reduce the cost of manufacturing
and thus enable large-area, low-cost, and flexible displays for various
applications such as disposable tags, single-use medical electronics, and
smart home appliances.
3−7
Due to the aforementioned advantages, printed displays have attracted
growing interest among the research community.
8−10
Although considerable
efforts have been spent on developing printed organic light-emitting diodes
(OLEDs),
11,12
progress has been hampered by the poor stability of organic
molecules against moisture. Instead, researchers have recently focused their
attention on developing printed polymer light-emitting diodes (PLEDs) and
quantum-dot light-emitting diodes (QLEDs).
9,13−15
These devices require the
ability to print thin layers to achieve a relatively low operation voltage.
Meanwhile, the surface roughness should also be minimized to avoid leakage
current that leads to non-radiative recombination of electrons and holes. In
this regard, inkjet printing satisfies these requirements and has been used to
74
fabricate PLEDs and QLEDs. However, the low throughput of inkjet printing
is problematic for the mass production of large-area displays.
3
Electroluminescent displays (EL) are another group of light-emitting devices
and can be printed using high-throughput and scalable printing techniques
such as screen printing and gravure printing.
16
Nevertheless, most printed EL
displays usually operate at relatively high voltage to achieve a desirable
brightness (>50 V),
17,18
limiting their application due to their high-power
consumption and safety concerns. Electrochromic displays are a group of
reflective displays that can reversibly change their reflectivity upon the
application of a bias voltage.
8
Because electrochromic materials like
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)
fundamentally operate by reflecting light rather than emitting,
electrochromic displays are more comfortable for reading and can serve as a
nonvolatile display with extremely low-power consumption. Due to their
device configuration, low operation voltage, and solution-based fabrication
process, electrochromic displays are ideal candidates for printed
displays.
19−22
Nevertheless, electrochromic displays are especially
susceptible to cross-talk, in which adjacent pixels are partially activated upon
addressing a single pixel. This cross-talk issue can be solved by integrating
75
electrochromic pixels into an active-matrix structure. Scientists have thus
made great strides in fabricating active-matrix electrochromic displays
(AMECD).
8,22
However, in those studies, the active-matrix backplane was
configured by printing organic thin-film transistors, which unfortunately
showed rather low mobility (<0.01 cm
2
V
-1
s
-1
).
22
Also, those studies
employed multiple patterning techniques such as bar coating, inkjet printing,
screen printing, and lamination to fabricate a single electrochromic device,
12
which remarkably increased the complexity of manufacturing.
Our group has focused on screen printing in consideration of its inherent
scalability and the simplicity of layer alignment performed in a parallel-plate
fashion.
3
Here, we report fully screen-printed active-matrix backplanes on
PET substrates using silver (Ag) nanoparticles as the conductor,
semiconducting single-wall carbon nanotubes (SWCNTs) as the conduction
channel, and barium titanate (BTO) as the insulator. The as-printed thin-film
transistors (TFTs) in the active-matrix backplane show an outstanding carrier
mobility of 3.92 ± 1.08 cm
2
V
−1
s
−1
, current on−off ratio I
on
/I
off
∼ 10
4
, and
good uniformity, resulting in an excellent platform for active-matrix-based
display and sensing systems. Additionally, we have developed fully screen-
printed electrochromic cells configured using a
76
silver/electrolyte/PEDOT:PSS lateral structure with great ambient stability
and cyclability. Combining these two components, we successfully
demonstrated a fully screen printed active-matrix electrochromic display.
Compared to other work on AMECD,
8,22
we have reduced the complexity of
manufacturing by only using screen printing for all the patterning processes.
Therefore, the cost of printed large-area flexible AMECD is further reduced
by the simplified printing process, which is important for the mass
production of large-area reflective displays. Also, SWCNT TFTs are
promising for flexible and printed displays due to their outstanding
electronic performance, printability, and mechanical flexibility.
23−28
In this
study, SWCNT TFTs configured by silver electrodes and BTO dielectric
layers show more advantageous performance and stability than organic TFTs
and hence play a better role in active-matrix backplanes. Finally, in previous
work on AMECD,
22
PEDOT:PSS was employed as the conducting lines
connecting pixels, which caused considerable voltage loss due to its poor
conductivity. Instead, we have replaced PEDOT:PSS with silver lines to
avoid any considerable voltage loss. However, we only used PEDOT:PSS in
the electrochromic pixels as the active material showing color change by
oxidation and reduction. The Ag/electrolyte/PEDOT:PSS lateral structure
77
we developed for electrochromic cells shows good functionality,
manufacturability, and stability.
Overall, we believe the fully screen-printed flexible active-matrix
electrochromic display using SWCNT TFTs, with outstanding electrical
performance and cost-effective fabrication process, establishes a promising
platform for low-cost, large-area, and flexible printed displays.
78
4.2 Development of fully screen printed active-
matrix electrochromic display
The fabrication process of the active-matrix electrochromic display is
outlined in Figure 4.1a−e. First, high-purity semiconducting SWCNTs were
incubated on a 5 × 5 cm
2
PET substrate. Second, silver source and drain
electrodes and data lines were printed, followed by the printing of a BTO
layer on the channel region of each TFT, as shown in Figure 4.1a. The
printed BTO layer was used as a hard mask for oxygen plasma etching to
remove the unwanted SWCNTs outside the TFT region and avoid crosstalk
between adjacent pixels (Figure 4.1b). Then another BTO layer was printed
as a passivation layer to protect the data lines and the ground lines. Finally,
scan lines, ground lines, PEDOT:PSS layer, and electrolyte were screen
printed sequentially, as shown in Figure 4.1c−e. Figure 1f is a circuit diagram
of the as-printed AMECD, where each pixel is composed of a SWCNT TFT
and an electrochromic cell. Figure 4.1g is a photograph of a fully screen-
printed AMECD laminated on human skin, displaying the letter “U” with
excellent color contrast. The detailed procedure for operating the as-printed
AMECD will be described below. Figure 4.1h and inset are scanning
electron microscopy images showing printed Ag source/drain and SWCNT
network in the channel.
79
80
Figure 4.1 Fully screen-printed active-matrix electrochromic display on flexible substrate.
(a−e) Schematic diagram showing the fabrication process and structure of a fully printed
AMECD. (f) Circuit diagram showing the configuration of as-printed AMECD. (g) A
photograph of 6 ×6 pixel flexible AMECD laminated on human skin displaying English
letter “U”. (h) SEM image showing the printed silver electrodes. The inset shows an SEM
image of the SWCNT network in the channel region. The use of the logo in (g) has been
authorized by the University of Southern California.
81
4.3 Electrical performance of fully screen printed
active-matrix backplane based on carbon nanotubes
A key step toward a fully screen-printed AMECD is to optimize the device
performance and uniformity of the printed SWCNT TFT array. The electrical
characteristics are shown in Figure 4.2.
-4 -2 0
0.0
-2.5
-5.0
V
G
= -2.5 V
V
G
= -5.0 V
I
D
( A)
V
D
(V)
V
G
= -7.5 V
-10 0 10
-1.0n
0.0
1.0n
I
G
(A)
V
G
(V)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0 2 4 6 8
0
2
4
6
8
Number of devices
Mobility (cm
2
V
-1
s
-1
)
2 3 4 5
0
5
10
15
Number of devices
Log (I
on
/I
off
)
a b
c
d
e f
Mobility (cm
2
V
-1
s
-1
)
-15 -10 -5 0 5 10 15
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
-I
D
(A)
V
G
(V)
82
Figure 4.2 Electrical characterization of a fully screen-printed SWCNT backplane. (a)
Transfer characteristics of a representative TFT measured at VDS = -1 V. (b) Output
characteristics of the same device. VG is from -7.5 V to -2.5 V in 2.5 V steps. (c) Gate
leakage current as a function of gate voltage at VDS = -1 V. (d) Mobility map of an as-
printed 6×6 active-matrix backplane. (e), (f) Histograms of mobilities and current on-off
ratios for 36 TFTs in the backplane, showing mobility = 3.92 ± 1.08 cm
2
V
-1
s
-1
and Log
(Ion/Ioff) = 3.71 ± 0.55.
In this work, the printed backplane contains 6 × 6 SWCNT TFTs with
channel length (L) ∼ 105 μm and channel width (W) ∼ 1000 μm. Based on
the recipe we reported in previous work,
3
diluted inks were used for source,
drain, and dielectric layer patterning, whereas the gate was printed with
undiluted silver ink, resulting in thicknesses of 3 μm for both the source and
drain, 5 μm for the BTO dielectric layer, and 10 μm for the gate. Figure 4.2a
shows the transfer characteristics of a representative SWCNT TFT at source
drain voltage (V
DS
) = −1 V, exhibiting on-state current density of 3.62
μA/mm at gate voltage (V
G
) = −15 V with an average on−off ratio of 10
4
.
The output characteristics of the same device in the saturation region and
linear region are shown in Figure 4.2b, suggesting the pinch-off effect and
good ohmic contact between the SWCNT network and silver electrodes.
Moreover, the dependence of gate leakage current on gate voltage is
83
illustrated in Figure 4.2c. Overall, with applied V
DS
= −1 V, the gate leakage
current is smaller than 1 nA in the range of V
G
from −15 to 15 V, indicating
the high quality of as-printed BTO for the gate dielectric. In this work, we
used the parallel plate model to extract the mobility of printed TFTs, as done
in our previous work
3,29
. Accordingly, the statistical study of the mobility of
our 6 × 6 TFT array is shown by the mobility mapping in Figure 2d as well
as the histogram in Figure 4.2e. As shown, the average mobility obtained is
∼3.92 cm
2
V
−1
s
−1
with a standard deviation ∼1.08 cm
2
V
−1
s
−1
. Similarly,
I
on
/I
off
ratio of this TFT array is shown in Figure 4.2f, with a log(Ion/Ioff)
having an average of 3.71 and a standard deviation of 0.55. Based on these
statistical data, the outstanding electrical performance of the fully screen-
printed TFT array is comparable with Javey and Cho’s work on gravure
printed CNT TFTs
24
and backplanes.
30,31
This enhanced electrical
performance can enable new applications such as monolithically integrated
active-matrix organic light-emitting diode (AMOLED), which requires
relatively high mobility. On the other hand, we believe the excellent
uniformity of the as made 6 × 6 pixel backplane is desirable for practical
applications such as large-area active-matrix-based flexible displays and
sensing systems.
84
It should be noted that considerable effort was spent on developing fully
screen-printed electrochromic cells. When designing printed electrochromic
cells, there are two possible configurations: a lateral configuration and a
vertical configuration. The lateral structure consists of two electrodes and a
layer of electrolyte bridging them.
32
In the vertical structure, the electrolyte
is sandwiched between two electrodes. Compared with the lateral structure,
the vertical structure effectively reduces the carrier path length to the
thickness of electrolyte, resulting in a more rapid color switching response.
However, for printing processes, the vertical structure may cause current
leakage between the two electrodes for several reasons. First, the printed
electrolyte may not be very dense or flat, and pinholes can be induced after
printing the top electrode when the solvent is evaporated during baking.
Because all the materials are formulated into solution-based inks to facilitate
the printing process, the solvent employed to carry electrode materials such
as PEDOT may also dissolve a portion of printed electrolyte, consequently
creating more pinholes. Previously, a lamination technique, using electrodes
patterned on two separate substrates and a layer of electrolyte in between as
an adhesion layer, was employed to avoid the leakage issue.
22
85
In an effort to avoid the lamination step and further simplify the
manufacturing process, we have chosen the lateral structure to facilitate the
screen printing process. Based on our observations, the switching time of our
electrochromic cells is about 2−5 s, which is consistent with previous reports
of lateral electrochromic cells.
8
The screen-printed electrochromic cell
consists of three components: ground lines, electrolyte, and PEDOT:PSS as
the active material for coloration. For every smart pixel, there exists one TFT
integrated with an EC cell. Notably, compared with the other published work
on printed electrochromic displays,
8,20−22,32
all the layers of our smart pixels
were deposited using screen printing. This eliminates the needs for
combining multiple patterning techniques like bar coating, inkjet printing,
lamination, and so on. Furthermore, with the high throughput, scalability,
and relatively accurate alignment, the screen printing method is highly
desirable for mass production of large-area printed electronics. Therefore,
the simplicity of our fabrication scheme with screen printing would be
important for practical applications of printed backplanes due to the low cost,
simple, and fast processing.
86
4.4 Integration of the screen printed
electrochromic cells and the printed CNT
backplane
Figure 4.3a is a schematic diagram showing the printed lateral-structure EC
cell.
87
Figure 4.3 Electrical characteristics of fully printed flexible electrochromic cells. (a)Top
view schematic diagram showing the lateral structure of a printed EC cell. (b) I-V
characteristics of a printed electrochromic cell under relaxed state and bent to various
radiuses. (c) Switching characteristics of a printed electrochromic cell, showing oxidation
and reduction processes over time with voltage at 5 V and – 5V. (d) Cyclability and (e)
ambient stability characterization of the fully screen-printed electrochromic display.
0 20 40 60 80
-60
-40
-20
0
20
40
Current ( A)
Time (s)
a
b c
-4 -2 0 2 4
-20
-10
0
10
20
5 Cycles
10 Cycles
25 Cycles
50 Cycles
75 Cycles
100 Cycles
Current ( A)
Voltage (V)
-4 -2 0 2 4
-20
-10
0
10
20
Current ( A)
5 Cycles
Day 1
Day 7
Voltage (V)
d e
-4 -2 0 2 4
-30
-20
-10
0
10
20
30
Relaxed
R = 20 mm
R = 10 mm
R = 5 mm
Current ( A)
Voltage (V)
PEDOT:PSS
Source
Ag Line
Electrolyte
Drain
88
The thickness profiles of the printed silver layer, electrolyte, and PEDOT:PSS
are reported in Figure 4.4, showing thicknesses of 9, 8, and 2.5 μm,
respectively.
Figure 4.4 Profiles of printed layers comprising the electrochromic cell
The current−voltage characteristics of a printed electrochromic cell under a
relaxed state and bent to radiuses of 20, 10, and 5 mm are shown in Figure
4.3b, which demonstrates that the printed EC cell operates reliably under
bending. When the applied bias changed from −3 to 3 V, the maximum current
flowing through the pixel was around 25 μA. The switching characteristics of
the EC cell at 5 V and −5 V are shown in Figure 4.3c. The PEDOT:PSS was
oxidized upon the application of ∼5 V, showing a exponentially decreasing
current from 25 μA to 13 μA. On the other hand, when the voltage changed to
−5 V after oxidation, a high initial current of −58 μA was observed, which
was consistent with the high initial reduction current due to the
89
electrochemical reactions occurring in the electrochromic cell.
22
However, it
showed lower saturation current of −7 μA when PEDOT:PSS was reduced to
a semiconducting state with lower conductivity. Figure 4.3d shows the similar
switching characteristics of the screen-printed EC cell after sweeping bias
from −3 to 3 V and then back to −3 V, for 5, 10, 25, 50, 75, and 100 cycles,
suggesting excellent cyclability of the as-printed EC cell. We also carried out
stability test of printed EC cells in ambient environment, as shown in Figure
4.3e. Negligible degradation of the electrical performance was observed after
7 days in air. The I−V characteristic measurements in Figure 4.3b, d, e used a
sweeping rate of 20 mV between data points at 60 ms intervals. The oxidation
and reduction curves in Figure 4.3c were taken at 100 ms intervals.
Overall, we have realized fully screen-printed, lateral-structure
electrochromic cells on flexible substrates with outstanding electrical
performance, remarkable cyclability, and stability in ambient environment.
We have additionally realized the integration of fully printed EC cells with
the printed backplane on PET substrates. Figure 4.5a shows the structure of a
fully screen-printed smart pixel consisting of a TFT and an EC cell. Based on
the measurements in Figure 4.3, we have carried out a functionality test of a
smart pixel, as shown in Figure 4.5b, to demonstrate the printed TFT’s control
90
capability over the EC cell. We first built a baseline with scan line voltage
(V
Scan
) = 0 V and data line voltage (V
Data
) = 0 V, and drain current (I
D
) is ∼0
A. When V
Scan
changed to −10 V, the TFT was switched to the on-state. With
V
Data
= 4 V, we observed a typical exponentially decreasing drain current,
indicating the oxidation of PEDOT:PSS and resulting in a gray/transparent
state. Then we turned the TFT off by changing V
Scan
to 10 V, which caused
the drain current to drop to 10 nA, suggesting the smart pixel remains turned
off after switching off the TFT. After that, we changed the biases to V
Scan
=
−10 V and V
Data
= −4 V. As expected, we observed the typical switching
characteristics of reduction of the EC cell, showing a dark-blue PEDOT:PSS
pattern. Finally, we turned the pixel off by applying V
Scan
= 10 V. Up until
this point, we have demonstrated the control capability of the screen-printed
SWCNT TFT on screen printed EC cell, showing excellent coloration and
retention behavior by changing V
Scan
and V
Data
. Based on this result, we further
addressed the pixels individually and have successfully enabled the printed
AMECD to display letters “U”, “S”, and “C” in Figure 4.5c. Notably, the
decoloration of the as-printed reflective EC cells is negligible after addressing,
suggesting an intrinsic “memory effect” in which the pixel color is retained
after the applied voltage is removed. This leads to extremely low power
consumption of the reflective AMECD, as static images can be displayed for
91
hours without having to maintain an applied voltage. This feature of
electrochromic displays will be essential for practical applications in large-
area, cost-effective, and flexible display electronics. Encouraged by the data
above, we believe this demonstration of fully screen printed AMECD using a
SWCNT backplane establishes the foundation for future research and
practical application of printed large-area, low-cost, and flexible displays.
a
b
c
0 50 100 150 200 250
-8.0x10
-6
-4.0x10
-6
0.0
4.0x10
-6
8.0x10
-6
I
D
(A)
Time (s)
V
Scan
= 0 V
V
Data
= 0 V
V
Scan
=-10 V
V
Data
= 4 V
V
Scan
= 10 V
V
Data
= 4 V
V
Scan
=-10 V
V
Data
=- 4 V
V
Scan
= 10 V
V
Data
=- 4 V
BTO
Electrolyte
Ground
Source Drain
Gate
PET
PEDOT
V
Data
V
Scan
92
Figure 4.5 Fully screen-printed flexible AMECD. (a) Cross-section schematic diagram of
a smart pixel in AMECD. (b) Functionality test of a smart pixel configured by a TFT and
an electrochromic cell, suggesting the control capability of the TFT in turning the pixel on
and off. (c) Optical images of AMECD showing letters “U”, “S”, and “C”. The use of the
logo in (c) has been authorized by the University of Southern California.
93
4.5 Summary
We have fabricated fully screen-printed active-matrix electrochromic displays
based on carbon nanotube thin-film transistors on flexible substrates. In this
study, 6 × 6 pixel active-matrix backplanes were fully screen printed using
semiconducting-enriched SWCNTs as the channel material, leading to
excellent electrical performance with mobility of 3.92 ± 1.08 cm
2
V
−1
s
−1
and
on−off current ratios ∼10
4
. Then we developed fully screen-printed lateral-
structured electrochromic cells, which were monolithically integrated with the
printed nanotube backplane. Switching characteristics, stability, and
flexibility of as-printed EC cells were investigated, suggesting excellent
robustness and electrical performance of screen printed AMECD. Our work
has demonstrated that the fully screen-printed SWCNT active-matrix
electrochromic display can be a desirable platform for large-area, low-cost,
and flexible displays.
94
4.6 References
1. Aleeva, Y.; Pignataro, B. Recent Advances in Upscalable Wet Methods and Ink
Formulations for Printed Electronics. J. Mater. Chem. C 2014, 2, 6436-6453.
2. Park, S.; Vosguerichian, M.; Bao, Z. A Review of Fabrication and Applications of
Carbon Nanotube Film-Based Flexible Electronics. Nanoscale 2013, 5, 1727-52.
3. Cao, X.; Chen, H. T.; Gu, X. F.; Liu, B. L.; Wang, W. L.; Cao, Y.; Wu, F. Q.; Zhu,
C. W. Screen Printing as a Scalable and Low-Cost Approach for Rigid and Flexible Thin-
Film Transistors Using Separated Carbon Nanotubes. ACS Nano 2014, 8, 12769-12776.
4. Chen, P. C.; Fu, Y.; Aminirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.;
Zhou, C. W. Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and
Its Application in Organic Light Emitting Diode Control. Nano Lett. 2011, 11, 5301-5308.
5. Lilja, K. E.; Backlund, T. G.; Lupo, D.; Virtanen, J.; Hamalainen, E.; Joutsenoja,
T. Printed Organic Diode Backplane for Matrix Addressing an Electrophoretic Display.
Thin Solid Films 2010, 518, 4385-4389.
6. Ryu, G. S.; Kim, J. S.; Jeong, S. H.; Song, C. K. A Printed Otft-Backplane for
Amoled Display. Org. Electron. 2013, 14, 1218-1224.
7. Xu, W.; Zhao, J.; Qian, L.; Han, X.; Wu, L.; Wu, W.; Song, M.; Zhou, L.; Su, W.;
Wang, C.; Nie, S.; Cui, Z. Sorting of Large-Diameter Semiconducting Carbon Nanotube
and Printed Flexible Driving Circuit for Organic Light Emitting Diode (Oled). Nanoscale
2014, 6, 1589-95.
95
8. Andersson, P.; Forchheimer, R.; Tehrani, P.; Berggren, M. Printable All-Organic
Electrochromic Active-Matrix Displays. Adv. Funct. Mater. 2007, 17, 3074-3082.
9. Kim, B. H.; Onses, M. S.; Lim, J. B.; Nam, S.; Oh, N.; Kim, H.; Yu, K. J.; Lee, J.
W.; Kim, J. H.; Kang, S. K.; Lee, C. H.; Lee, J.; Shin, J. H.; Kim, N. H.; Leal, C.; Shim,
M.; Rogers, J. A. High-Resolution Patterns of Quantum Dots Formed by
Electrohydrodynamic Jet Printing for Light-Emitting Diodes. Nano Lett. 2015, 15, 969-73.
10. Ren, M. S.; Gorter, H.; Michels, J.; Andriessen, R. Ink Jet Technology for Large
Area Organic Light-Emitting Diode and Organic Photovoltaic Applications. J. Imaging.
Sci. Techn. 2011, 55.
11. Kopola, P.; Tuomikoski, M.; Suhonen, R.; Maaninen, A. Gravure Printed Organic
Light Emitting Diodes for Lighting Applications. Thin Solid Films 2009, 517, 5757-5762.
12. Tekoglu, S.; Hernandez-Sosa, G.; Kluge, E.; Lemmer, U.; Mechau, N. Gravure
Printed Flexible Small-Molecule Organic Light Emitting Diodes. Org. Electron. 2013, 14,
3493-3499.
13. Kong, Y. L.; Tamargo, I. A.; Kim, H.; Johnson, B. N.; Gupta, M. K.; Koh, T. W.;
Chin, H. A.; Steingart, D. A.; Rand, B. P.; McAlpine, M. C. 3d Printed Quantum Dot Light-
Emitting Diodes. Nano Lett. 2014, 14, 7017-7023.
14. Tait, J. G.; Witkowska, E.; Hirade, M.; Ke, T. H.; Malinowski, P. E.; Steudel, S.;
Adachi, C.; Heremans, P. Uniform Aerosol Jet Printed Polymer Lines with 30 Mu M Width
for 140 Ppi Resolution Rgb Organic Light Emitting Diodes. Org. Electron. 2015, 22, 40-
43.
96
15. Youn, H.; Park, H. J.; Guo, L. J. Printed Nanostructures for Organic Photovoltaic
Cells and Solution-Processed Polymer Light-Emitting Diodes. Energy Technol. 2015, 3,
340-350.
16. Lee, D. H.; Choi, J. S.; Chae, H.; Chung, C. H.; Cho, S. M. Screen-Printed White
Oled Based on Polystyrene as a Host Polymer. Curr. Appl. Phys. 2009, 9, 161-164.
17. Johnston, D.; Barnardo, C.; Fryer, C. Passive Multiplexing of Printed
Electroluminescent Displays. J. Soc. Inf. Disp. 2005, 13, 487-491.
18. Kim, J.-Y.; Bae, M. J.; Park, S. H.; Jeong, T.; Song, S.; Lee, J.; Han, I.; Yoo, J. B.;
Jung, D.; Yu, S. Electroluminescence Enhancement of the Phosphor Dispersed in a
Polymer Matrix Using the Tandem Structure. Org. Electron. 2011, 12, 529-533.
19. Said, E.; Andersson, P.; Engquist, I.; Crispin, X.; Berggren, M. Electrochromic
Display Cells Driven by an Electrolyte-Gated Organic Field-Effect Transistor. Org.
Electron. 2009, 10, 1195-1199.
20. Kawahara, J.; Ersman, P. A.; Engquist, I.; Berggren, M. Improving the Color
Switch Contrast in Pedot:Pss-Based Electrochromic Displays. Org. Electron. 2012, 13,
469-474.
21. Andersson Ersman, P.; Kawahara, J.; Berggren, M. Printed Passive Matrix
Addressed Electrochromic Displays. Org. Electron. 2013, 14, 3371-3378.
22. Kawahara, J.; Andersson Ersman, P.; Nilsson, D.; Katoh, K.; Nakata, Y.; Sandberg,
M.; Nilsson, M.; Gustafsson, G.; Berggren, M. Flexible Active Matrix Addressed Displays
97
Manufactured by Printing and Coating Techniques. J. Polym. Sci., Part B: Polym. Phys.
2013, 51, 265-271.
23. Ha, M. J.; Seo, J. W. T.; Prabhumirashi, P. L.; Zhang, W.; Geier, M. L.; Renn, M.
J.; Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Aerosol Jet Printed, Low Voltage, Electrolyte
Gated Carbon Nanotube Ring Oscillators with Sub-5 Mu S Stage Delays. Nano Lett. 2013,
13, 954-960.
24. Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.;
Javey, A. Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on
Flexible Substrates. Nano Lett. 2013, 13, 3864-9.
25. Xu, W.; Liu, Z.; Zhao, J.; Xu, W.; Gu, W.; Zhang, X.; Qian, L.; Cui, Z. Flexible
Logic Circuits Based on Top-Gate Thin Film Transistors with Printed Semiconductor
Carbon Nanotubes and Top Electrodes. Nanoscale 2014, 6, 14891-7.
26. Cai, L.; Wang, C. Carbon Nanotube Flexible and Stretchable Electronics.
Nanoscale Res. Lett. 2015, 10, 1013.
27. Cai, L.; Zhang, S.; Miao, J.; Yu, Z.; Wang, C. Fully Printed Foldable Integrated
Logic Gates with Tunable Performance Using Semiconducting Carbon Nanotubes. Adv.
Funct. Mater. 2015, 25, 5698-5705.
28. 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, 495-U4.
98
29. Cao, X.; Cao, Y.; Zhou, C. W. Imperceptible and Ultraflexible P-Type Transistors
and Macroelectronics Based on Carbon Nanotubes. ACS Nano 2016, 10, 199-206.
30. Lee, W.; Koo, H.; Sun, J.; Noh, J.; Kwon, K. S.; Yeom, C.; Choi, Y.; Chen, K.;
Javey, A.; Cho, G. A Fully Roll-to-Roll Gravure-Printed Carbon Nanotube-Based Active
Matrix for Multi-Touch Sensors. Scientific Reports 2015, 5.
31. Yeom, C.; Chen, K.; Kiriya, D.; Yu, Z.; Cho, G.; Javey, A. Large-Area Compliant
Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv. Mater.
2015.
32. Andersson, P.; Nilsson, D.; Svensson, P. O.; Chen, M. X.; Malmstrom, A.;
Remonen, T.; Kugler, T.; Berggren, M. Active Matrix Displays Based on All-Organic
Electrochemical Smart Pixels Printed on Paper. Adv. Mater. 2002, 14, 1460-+.
99
5 Imperceptible and ultraflexible p‑type
transistors and macroelectronics
based on carbon nanotubes
5.1 Introduction
Single-wall carbon nanotubes (SWNTs) are an ideal channel material of thin-
flexible macroelectronics,
1−5
film transistors (TFTs) for displays, and
sensors.
6−8
Compared with traditional channel materials such as amorphous
silicon and polysilicon, carbon nanotubes show excellent mechanical
flexibility
9−15
and can be solutiondeposited
10,11
or printed
16,17
onto flexible
substrates at room temperature, enabling low-cost fabrication of large-area
flexible electronics.
5,9
Besides, the outstanding intrinsic electrical properties
of semiconducting SWNTs exhibit considerable advantages over organic
materials for TFTs in terms of mobility, on−off ratio, and stability against
moisture and oxygen.
5,9,10,18−20
Hence, intensive research efforts have been
devoted to flexible SWNT TFTs for applications in digital circuits, active-
matrix-based displays, and sensors.
1,21−25
TFTs, with SWNT networks as
channel and thin films of metals as source, drain, and gate electrodes, have
100
been experimentally demonstrated with relatively high mechanical flexibility
and stability.
1,10
In order to improve the bendability of SWNT TFTs,
researchers previously focused on optimizing dielectrics of TFTs, trying to
find those that can sustain large tensile strains. Elastomeric dielectrics were
proposed by Q. Cao et al., and the flexible SWNT TFTs can be bent to a radius
of curvature as small as 3.3 mm.
26
Importantly, C. Wang used atomic layer
deposition and e-beam evaporation to deposit nAl2O3 and SiO2 as the
dielectric layer and fabricated high-performance flexible SWNT integrated
circuits with a minimal bending radius of ∼1.27 mm.10 Employing poly(vinyl
alcohol) as the dielectric layer, S. Aikawa et al. pushed the bending radius
down to 1 mm.
27
Overall, failure of the fragile dielectric layer remains as the
obstacle limiting the flexibility of SWNT TFTs and circuits.
Recently, a new class of “imperceptible” or “epidermal” electronics has
emerged.
28−32
Using ultrathin flexible substrates, the imperceptible electronics
would be at low strain level at extremely small bending radius according to
the calculations in the literature.
33,34
Based on this concept, ultraflexible
organic electronics have been demonstrated for various applications.
29,30,33,35,36
However, the ultraflexible organic transistors show low mobility (∼1 to 3 cm
2
V
−1
s
−1
)
29,33
and might suffer from poor long-term stability in air. Although the
101
reported indium−gallium−zinc oxide (IGZO) transistors exhibited high
mobility (∼26 cm
2
V
−1
s
−1
),
34
they are n-type and cannot be converted to p-
type. However, modern electronics usually require complementary circuits,
which would combine both n-type and p-type transistors to realize ultralow
static power dissipation and reliable operation with large noise margin. As a
result, the development of high-performance ultraflexible p-type transistors is
highly desirable to realize ultraflexible complementary circuits.
Here we report imperceptible and ultraflexible p-type SWNT TFTs and
circuits fabricated directly on 1.4 μm poly (ethylene terephthalate) (PET) film.
The resulting SWNT TFTs exhibit excellent electrical performance with field-
effect mobility up to 12.04 cm
2
V
−1
s
−1
and an on−off ratio of ∼10
6
. Moreover,
the ultra-lightweight SWNT electronics (3 g/m
2
) show unprecedented
mechanical flexibility compared with previously reported flexible SWNT
electronics. Our fabricated TFTs showed little performance degradation when
bent at a radius of curvature of ∼40 μm, crumpled like a piece of paper, or
even under 67% compressive strain. The as-fabricated inverters, NAND gates,
and NOR gates maintained their electrical performance and functionalities
even when 33% compressive strain was introduced to form tiny micro-
wrinkles on the electronic foil. The aforementioned good electrical
102
performance, excellent flexibility, and unique lightweight property endow the
SWNT-based ultraflexible electronics with a promising future for next-
generation wearable electronics, flexible displays, sensing systems, and
digital electronics.
103
5.2 Fabrication of ultraflexible SWNT
macroelectronics
Figure 4.1a illustrates our proposed scheme for fabricating flexible SWNT
macroelectronics on ultrathin PET substrates. First, a Si/SiO2 substrate was
spin-coated with polydimethylsiloxane (PDMS) as a reusable supporting
substrate for easy handling. Second, an ultrathin PET film with a thickness of
1.4 μm, as the substrate for SWNT TFTs and circuits, was laminated onto the
supporting substrate for the following fabrication. After that, microfabrication
was carried out to produce SWNT TFTs and circuits with successive gate
electrode patterning, gate dielectric deposition, SWNT deposition, and
formation of source and drain electrodes and interconnects. A detailed
fabrication process for SWNT TFTs and circuits is described in the Methods
section. Finally, the as-made electronic foil was peeled off from the
supporting substrate for further electrical characterization. Figure 4.1b shows
a zoom-in schematic diagram of SWNT TFTs and circuits on the fabricated
electronic foil. The inset of Figure 4.1b shows a scanning electron microscope
(SEM) image of a typical carbon nanotube thin film that we use to make
SWNT TFTs. Figure 4.1c shows a photograph of an as-fabricated electronic
foil with a size of 3 cm × 4 cm. Significantly, the proposed scheme is
104
compatible with a standard semiconductor manufacturing process, which
enables accurate alignment to realize the complicated configuration of circuits
and scalable manufacturing at relatively low cost.
a
d
e
f
Peel
off
b
TFT Circuit
Human
hair
PDMS
PET
Si/SiO
2
Laminate
Micro-
Fabrication
c
105
Figure 5.1 Imperceptible carbon nanotube macroelectronics. (a) Schematic diagrams
showing fabrication procedure of carbon nanotube electronic foil. (b) Zoom-in schematic
diagram of fabricated electronic foil, showing TFTs and circuits. Inset is a SEM image of
a typical SWNT thin film. Scale bar is 500 nm. (c) Photograph of as-fabricated
imperceptible nanotube macroelectronics. Scale bar is 1 cm. (d,e) Ultra-thin carbon
nanotube electronic foil laminated onto human skin (d) and at rolled-up state (e). Scale bar:
2 cm in (d) and 1 cm in (e). (f) SEM image of the ultra-thin SWNT electronic foil laminated
onto human hair sitting on a substrate. Scale bar is 150 μm.
Besides compatibility with the standard semiconductor manufacturing
process, our fabricated SWNT TFTs and circuits are ultraflexible, extremely
bendable, imperceptible, ultrathin (∼1.4 μm), and ultralightweight (3 g/m2).
Figure 1d shows the fabricated SWNT electronic foil laminated onto human
skin, and the human subject reported that the foil was essentially
imperceptible because of the ultralight weight and conformal lamination due
to the ultraflexibility. The photograph in Figure 1e shows the SWNT
electronic foil rolled up with a radius of curvature of ∼1 mm, and Figure 1f
shows the SEM image of the SWNT electronic foil laminated onto human hair
sitting on a substrate, indicating the extreme bendability and minimal
thickness of the nanotube electronics. We believe that ultralight-weight,
flexible, and imperceptible SWNT electronics are highly advantageous in
large-area wearable electronics for sensor and display applications.
106
5.3 Electrical performance of the ultraflexible
SWNT TFTs
We first characterized the direct current (DC) performance of the as-fabricated
SWNT TFTs on ultrathin substrates. Figure 2a shows the schematic diagram
of an SWNT TFT on a 1.4 μm PET substrate. The SWNT TFT was controlled
by an individual back-gate with Al2O3/SiO2 (20 nm/5 nm) stacked in series
as the gate dielectric. The purpose of the SiO2 dielectric layer was to facilitate
the deposition of ultrahigh semiconducting purity SWNT networks onto the
gate dielectric. A 50 nm palladium (Pd) layer, together with 1 nm titanium (Ti)
as an adhesion-promotion layer, was used as the metal for the source, drain,
and gate electrodes. Figure 5.2b−d delineates the DC performance of a
representative SWNT TFT with a channel length (L) of 10 μm and a channel
width (W) of 100 μm. The SWNT TFT exhibited p-type transistor behavior
with hysteresis that is typical for SWNT transistors (Figure 5.2b). With a
drain-to-source bias (V
DS
) of −1 V, the on-current was −30 μA at a gate-to-
source bias (V
GS
) of −2 V, while the off-current of the TFT was −9 pA at VGS
= 0.35 V, indicating a high on−off ratio of 106. The field-effect mobility of
the TFT is extracted to be 12.04 cm2 V−1 s−1 using the formula μ =
(L/W)[1/(CV
DS
)](dI
D
/dV
GS
), where I
D
is the drain-to-source current, and C is
107
the gate capacitance, estimated with the parallel plate model. The I
D
−V
DS
curves at different gate bias (Figure 5.2c) show excellent linear behavior,
indicating good ohmic contacts between source/drain electrodes and SWNT
networks. Output characteristics of the same SWNT TFT are shown in Figure
5.2d, and excellent current saturation behavior can be clearly observed. The
mobility of our p-type ultraflexible SWNT TFTs is much higher than the
mobility of other reported p-type ultraflexible organic TFTs (∼1 to 3 cm
2
V
−1
s
−1
for the best devices)
29,33
and is comparable with the values reported for
ultraflexible n-type IGZO TFTs (∼26 cm
2
V
−1
s
−1
for the best device).
34
In
order to evaluate the uniformity of the device performance, we also carried
out a statistical study of 50 fabricated SWNT TFTs with various channel
lengths, and the results are shown in Figure 5.2e−g. Our fabricated SWNT
TFTs have an on−off ratio centered between 10
5
and 10
6
, average mobility
centered between 5 and 8 cm
2
V
−1
s
−1
, and threshold voltage centered between
−0.5 and −1 V. Overall, the uniformity of our fabricated SWNT TFTs is very
good and ready for practical applications
108
Figure 5.2 Electrical characterization of carbon nanotube thin-film transistors on 1.4 μm
PET foil. (a) Schematic diagram showing device configuration of a SWNT TFT with
individual back-gate on 1.4 μm PET substrate. (b) Transfer characteristics of a
representative TFT with L = 10 μm and W = 100 μm under VDS = −1 V. (c, d)
Corresponding output characteristics in triode regime (c) and saturation regime (d),
respectively. VGS is from −0.2 to 1 V with a step of 0.2 V, corresponding to curves from
top to bottom. (e, f, g) Statistical study of 50 nanotube TFTs showing on−off ratio (e), field-
effect mobility (f), and threshold voltage (g) as functions of channel length.
-1.5 -1.0 -0.5 0.0
0
-5
-10
-15
I
D
( A)
V
DS
(V)
-0.05 0.00 0.05
-1.0
-0.5
0.0
0.5
1.0
I
D
( A)
V
DS
(V)
V
GS
from -0.2 V to 1 V
in 0.2 V steps
0 20 40 60 80 100
0.0
-0.5
-1.0
-1.5
Threshold voltage (V)
Channel Length ( m)
0 20 40 60 80 100
0
2
4
6
8
10
Mobility (cm
2
/V/s)
Channel Length ( m)
b c d
e f g
a
0 20 40 60 80 100
0
1
2
3
4
5
6
7
Log( I
on
/I
off
)
Channel Length ( m)
1.4 μm PET
Al
3
O
2
/SiO
2
20/5 nm
Ti/Pd 1/50 nm
SWNT network
Ti/Pd
Ti/Pd SWNT
Ti/Pd
Al
3
O
2
/SiO
2
PET
-2 -1 0 1 2
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
-I
D
( )
V
GS
(V)
109
5.4 Mechanical flexibility of the ultraflexible SWNT
macroelectronics
In order to characterize the flexibility of the ultrathin SWNT electronic foil,
two experiments that consist of extreme bending and crumpling were carried
out. In the first test, as shown in the schematic diagram in Figure 5.3a, we
tightly wrapped our fabricated SWNT electronic foil around a cylinder with a
radius of curvature of ∼220 μm.
0
2
4
6
8
10
100 10
Mobility (cm
2
V
-1
s
-1
)
Bending Cycles
0 1
-2 -1 0 1 2
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
R = 220 um
After crumpling
Flat
- I
D
(A)
V
G
(V)
a b c
0
2
4
6
8
100 10
Log (I
on
/I
off
)
Bending Cycles
0 1
0.0
-0.5
-1.0
-1.5
-2.0
100 10
Threshold voltage (V)
Bending Cycles
0 1
0
2
4
6
8
100 10
Mobility (cm
2
V
-1
s
-1
)
Crumpling Cycles
0 1
0
2
4
6
8
100 10
Log (I
on
/I
off
)
Crumpling Cycles
0 1
0.0
-0.5
-1.0
-1.5
-2.0
100 10
Threshold voltage (V)
Crumpling Cycles
0 1
d e f
g h i
110
Figure 5.3 Flexibility of nanotube TFTs on a 1.4 μm PET substrate. (a) Schematic
diagram showing the measurement of a TFT wrapping around a cylinder. (b) Crumpled
CNT electronic foil (original size: 3 cm × 4 cm rectangle). Scale bar is 1 cm. (c) Transfer
characteristics of a representative TFT with L = 10 μm and W = 100 μm under relaxed (flat)
state, after crumpling, and bent with a radius of ∼220 μm. (d) Mobility, (e) logarithm
on−off ratio, and (f) threshold voltage of SWNT TFTs bent with a radius of ∼220 μm after
different bending cycles. (g) Mobility, (h) logarithm on−off ratio, and (i) threshold voltage
of SWNT TFTs after different crumpling cycles.
To calculate the tensile strain when the single wall carbon nanotube (SWNT)
electronic foil was wrapped tightly around a cylinder with radius ~ 220 μm,
we used the following formula:
ε =
1
𝑅 ×
𝑑 𝑠 + 𝑑 𝑓 2
×
𝜒 ∙ 𝛾 2
+ 2 ∙ 𝜒 ∙ 𝛾 + 1
𝜒 ∙ 𝛾 2
+ 𝜒 ∙ 𝛾 + 𝛾 + 1
In the above formula, R is the bending radius, 𝑑 𝑠 is the thickness of the
substrate, and 𝑑 𝑓 is the thickness of SWNT thin-film transistor (TFT). 𝛾 =
𝑑 𝑓 /𝑑 𝑠 and 𝜒 = 𝑌 𝑓 /𝑌 𝑠 where 𝑌 𝑓 and 𝑌 𝑠 are the Young’s modulus of SWNT
TFT and the substrate, respectively. We assume 𝑌 𝑓 = 𝑌 𝑠 and the above
formula can be further simplified:
111
ε =
1
𝑅 ×
𝑑 𝑠 + 𝑑 𝑓 2
In this case, a tensile strain of ∼0.3%, parallel to the drain-to-source current
direction, was applied to the SWNT TFTs. The electrical performance of the
SWNT TFTs under tensile strain was measured in ambient air. In the second
test, the same fabricated SWNT electronic foil was severely crumpled and
then flattened. Figure 5.3b shows the SWNT electronic foil severely crumpled
from its original size as a 3 cm × 4 cm rectangle, and SEM images of the
crumpled electronic foil can be found in the Figure 5.4.
112
Figure 5.4 SEM images of a crumpled electronic foil.
The electrical performance of the crumpled SWNT TFTs was also
characterized in ambient air. Figure 5.3c compares the transfer characteristics
of a representative SWNT TFT with L = 10 μm and W = 100 μm in three
conditions: relaxed status, bent with a radius of curvature of ∼220 um, and
after crumpling. The SWNT TFT exhibited p-type behavior in all three
conditions without any discernible change of its on-current. A measurable
increase of off-current was observed in the case of the crumpled TFT, while
113
the off-current in the other two cases showed no measurable change. This can
be attributed to the increased leakage current through the gate dielectric after
the TFT was crumpled (shown in Figure 5.5a).
Figure 5.5 Gate leakage current and electrical parameters of the representative device
under three conditions. (a) Gate leakage currents of the representative TFT with L = 10
μm and W = 100 μm under relaxed (flat) state, after crumpling, and bent with radius ~
220 μm. (b) Mobility, on-off ratio, and threshold voltage of the same SWNT TFT under
relaxed (flat) state, after crumpling, and bent with radius ~ 220 μm.
Figure 5.5a shows that the gate leakage currents of the TFT at bent and relaxed
status exhibited no identifiable difference, while the gate leakage current of
-2 -1 0 1 2
10
-15
10
-14
10
-13
10
-12
10
-11
10
-10
10
-9
I
G
(A)
V
G
(V)
0
2
4
6
8
10
12
14
Flat Crumpled
Mobility (cm
2
V
-1
S
-1
)
R = 220 μm
0.0
-0.5
-1.0
-1.5
-2.0
V
th
0
3
6
9
12
Log (I
on
/I
off
)
a
b
114
the crumpled TFT showed an increase by a factor of 10, which confirms our
assumption. However, since the off-current for all three conditions was less
than 100 pA and significantly smaller than the on-current, the electrical
performance of the SWNT TFTs showed no big change (Figure 5.5b). From
Figure 5.5b, the mobility remained between 10 and 12 cm
2
V
−1
s
−1
, the on−off
ratio remained more than 10
6
, and the shift of the threshold voltage was less
than 0.2 V. As expected, there was no significant change of the electrical
performance of the SWNT TFTs when the devices were in different bending
conditions. In order to test the reliability of our platform, we further performed
bending and crumpling tests with nine SWNT TFTs with 100 cycles of
bending and 100 cycles of crumpling for each device. Figure 5.3d−f plot the
mobility, the logarithm on−off ratio, and the threshold voltage of the devices
under testing with 0 (before bending), 1, 10, and 100 bending cycles,
respectively, and overall the changes in device performance were very small.
The mobility changed from 5.12 ± 1.81 cm
2
V
−1
s
−1
to 7.35 ± 2.19 cm
2
V
−1
s
−1
,
the logarithm on−off ratio changed from 4.76 ± 0.57 to 5.33 ± 0.47, and the
threshold voltage changed from −0.95 ± 0.18 V to −1.26 ± 0.18 V after 100
bending cycles. Figure 5.3g−i plot the mobility, the logarithm on−off ratio,
and the threshold voltage of the devices under testing with 0 (before
crumpling), 1, 10, and 100 crumpling cycles, respectively, and the changes in
115
device performance were small as well. The mobility changed from 5.96 ±
1.62 cm
2
V
−1
s
−1
to 5.29 ± 1.55 cm
2
V
−1
s
−1
, the logarithm on−off ratio changed
from 5.30 ± 0.44 to 5.12 ± 0.55, and the threshold voltage changed from −1.09
± 0.16 V to −0.95 ± 0.06 V after 100 crumpling cycles. On the basis of the
test results, all nine SWNT TFTs under testing after a total of 200 bending and
crumpling cycles still maintained good performance, confirming that our
platform is reliable. Two reasons contribute to the ultraflexibility of our
fabricated SWNT TFTs. First of all, metal thin films and carbon nanotubes
have good intrinsic mechanical flexibility.
37−39
Several groups including our group have reported flexible carbon nanotube
electronics.
2,8,10,21
In addition, by decreasing the thickness of the substrate to
1.4 μm, we significantly reduced the tensile strain under similar bending
conditions. Therefore, the dielectric layer would survive even at sharp bending
or crumpling. In terms of the bending experiments, thanks to the ultrathin PET
substrate, the tensile strain applied to the SWNT TFTs was calculated to be
∼0.3% when the SWNT electronic foil was wrapped tightly around the
cylinder with a radius of ∼220 μm. The small tensile strain (∼0.3%) would
not cause significant failure of the dielectric layer, which explained the
unchanged electrical performance. In terms of the crumpling experiments, the
116
high yield that we observed (nine devices each surviving 100 cycles of
crumpling) suggests that the crumpling did not lead to strain sufficiently high
to cause Al2O3 dielectric failure due to the ultrathin PET substrate. Our
platform of combining carbonnanotubes with ultrathin substrates greatly
improves the flexibility and mechanical stability of carbon nanotube
electronics when compared with previously reported flexible SWNT
TFTs.
10,26,27
With the bending test described above, it is usually experimentally
challenging to achieve a bending radius below 100 μm. As a result, we
adopted a stretch compatibility test to further reduce the radius of curvature
for the electrical measurement and characterize the robustness of our platform.
117
-2 -1 0 1 2
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
0
33%
50%
60%
67%
-I
D
(A)
V
GS
(V)
0.0 0.5 1.0 1.5 2.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
V
OUT
(V)
Vdd = 0.6V
V
IN
(V)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Gain
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Output (V)
NAND V
DD
= 0.6 V
Logic "0" = 0 V
Logic "1" = 2 V
Gate A 0 0 1 1
Gate B 0 1 0 1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Gate A 0 0 1 1
Gate B 0 1 0 1
Output (V)
NAND V
DD
= 0.6 V
Logic "0" = 0 V
Logic "1" = 2 V
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Gate A 0 0 1 1
Gate B 0 1 0 1
Output (V)
NOR V
DD
= 0.6 V
Logic "0" = 0 V
Logic "1" = 2 V
b
c
e
i
R ~ 40 μm
1.4 μm PET
0 20 40 60
0.0
0.2
0.4
0.6
0.8
1.0
Mobility
V
th
Log (I
on
/I
off
)
Normalized Value
Compressive strain (%)
Release strain d
V
IN
V
OUT
V
DD
V
IN
V
DD
V
OUT GND
f
g
V
OUT
V
B
V
A
V
DD
h
V
DD
V
A
V
B
V
OUT
GND
j
V
B
V
OUT
V
DD
V
A
k
V
DD
V
A
V
B
V
OUT GND
l
m
Laminate
Pre-stretched
elastomer
a
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Gate A 0 0 1 1
Gate B 0 1 0 1
Output (V)
NOR V
DD
= 0.6 V
Logic "0" = 0 V
Logic "1" = 2 V
0.0 0.5 1.0 1.5 2.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
V
OUT
(V)
V
IN
(V)
0.0
0.2
0.4
0.6
0.8
1.0
Gain
118
Figure 5.6 Stretch-induced effect of imperceptible carbon nanotube macroelectronics. (a)
Illustration of stretchable carbon nanotube macroelectronics. (b) SEM images of top view
and cross-section view of a wrinkled nanotube TFT under 67% compressive strain,
showing the bending radius of the active channel region, ∼40 μm. (c) Transfer
characteristics of the same SWNT TFT under different compressive strains, measured at
VDS = −1 V. (d) Corresponding normalized values of mobility, threshold voltage, and
on−off ratio as functions of compressive strain. (e) Schematic diagram of diode-load
inverter. (f, g) Electrical characteristics of an inverter in relaxed state (f) and undergoing
33% compressive strain (g). Inset of (f): Photograph of an inverter in relaxed state. Scale
bar is 100 μm. (h) Schematic diagram of diode-load NAND gates. (i, j) Output
characteristics of NAND gates in relaxed state (i) and undergoing 33% compressive strain
(j). Inset of (i): Photograph of a NAND gate in relaxed state. Scale bar is 100 μm. (k)
Schematic diagram of diode-load NOR gates. (l, m) Output characteristics of NOR gates
in relaxed state (l) and undergoing 33% compressive strain (m). Inset of (l): Photograph of
a NOR gate in relaxed state. The scale bar is 100 μm.
Figure 5.6a illustrates the process for the stretch compatibility and mechanical
robustness test. The fabricated SWNT electronic foil with TFTs and circuits
was laminated onto a Pre-stretched elastomer (3 M VHB) with desired total
strain. Then the pre-stretched elastomer was slowly released. Over the process,
wrinkled microstructures of the electronic foil were formed due to the
compressive strain applied on the ultrathin PET film. Figure 5.6b shows the
SEM images of SWNT TFTs under 67% compressive strain. The left top-view
SEM image clearly shows that out-of-plane wrinkles formed after the strain
was transferred from the pre-stretched elastomer to the SWNT electronic foil.
119
The right cross-section SEM image shows the active channel region of one
SWNT TFT with a bending radius of ∼40 μm, which would lead to ∼1.7%
local tensile strain. We note that the local tensile strain depends on the local
bending radius and can be different from the number 67% due to the wrinkles
formed on the electronic foil. However, the use of 67% compressive strain is
better to clearly describe our experiment. We measured the electrical
performance of the SWNT TFT under different compressive strains. Figure
5.6c shows the transfer characteristics of a representative SWNT TFT with L
= 50 μm and W = 100 μm undergoing various compressive strains. There is
no apparent degradation of the SWNT TFT even under 67% compressive
strain. The small change in the off-current can be attributed to the variations
of the gate leakage current under different compressive strains (Figure 5.7).
120
Figure 5.7 Gate leakage current of a SWNT TFT with L = 50 μm and W = 100 μm
undergoing 0, 33%, 50%, 60% and 67% compressive strains.
121
Figure 5.6d depicts normalized values of mobility, threshold voltage, and
current on−off ratio under various compressive strains for the same device.
For various compressive strains up to 67%, the change in mobility is less than
5%, the change in threshold voltage is less than 7%, and the change in
logarithmic on/off ratio is less than 4%. While devices with local bending
radius down to 40 μm showed good stretch compatibility and mechanical
robustness (e.g., Figure 5.6b−d), we note that finer wrinkles with even smaller
bending radius did lead to failure of the devices, and the overall yield of the
devices on the stretch-released sample was ∼10%. Besides SWNT TFTs, we
also evaluated the stretch compatibility and mechanical robustness of the
fabricated SWNT circuits to demonstrate that our platform is also suitable for
complex circuits and systems. For proof of this concept, the fabricated basic
logic blocks such as inverters, NAND gates, and NOR gates were configured
with diode-load since SWNT transistors usually show p-type transistor
behavior. To evaluate the flexibility of the circuits, we applied 33%
compressive strain to the SWNT electronic foil to form tiny wrinkles (Figure
5.8).
122
Figure 5.8 SEM images of SWNT electronic foil on elastomer undergoing 33%
compressive strain, showing wrinkled mirco-structure. (a) top-view of the wrinkled plasitic
electronics. Scale bar is 200 μm. (b) Cross-section view of a tiny wrinkle, indicating radius
of curvature ~ 2 μm. Scale bar is 5 μm.
The wrinkles typically had a bending radius of ∼50−60 μm. The fabricated
SWNT circuits were tested in ambient air with and without 33% compressive
strain, and the results are shown in Figure 5.6e−m. Figure 5.6e shows the
schematic diagram of diode-load inverters, and Figure 5.6f,g shows the
voltage transfer curves (VTCs) of diode-load inverters with and without
compressive strain. The inset of Figure 5.6f shows a photograph of an SWNT
diode-load inverter. Inverters still functioned properly after 33% compressive
strain with a slight decrease in the inverter gain. Figure 5.6h shows the
schematic diagram of diode-load NAND gates, and Figure 5.6i,j shows the
output characteristics of diode-load NAND gates before and after 33%
compressive strain. The inset of Figure 5.6i shows a photograph of an SWNT
diode-load NAND gate. NAND gates also functioned correctly after a 33%
R ~ 2 μm
a b
123
compressive strain with a slight decrease in both output high voltage level
(V
OH
) and output low voltage level (V
OL
). The schematic diagram of SWNT
diode-load NOR gates is shown in Figure 5.6k, and the inset of Figure 5.6l
shows a photograph of a diode-load NOR gate. SWNT diode-load NOR gates
showed similar behaviors to the SWNT diode-load NAND gates (Figure
5.6l,m). We note that the 67% compressive strain we used for single transistor
measurement and 33% compressive strain we used for SWNT circuits are by
no means limits of our transistor operation. With further optimization of the
transistor/circuit design, we believe that even better performance can be
achieved. The above results confirm that, besides fabricated SWNT TFTs, the
fabricated SWNT circuits on ultrathin substrates also have outstanding stretch
compatibility and mechanical robustness.
124
5.5 Summary
In conclusion, we have developed a reliable approach for realizing high-
performance and ultraflexible SWNT TFTs and circuits on a 1.4 μm PET
substrate. The fabricated SWNT electronic foil is ultrathin, is ultra-
lightweight, and exhibits excellent electrical performance and mechanical
flexibility due to the properties of carbon nanotubes and the ultrathin
substrates, which help to reduce tensile strains. The fabricated SWNT TFTs
can be bent with a bending radius down to ∼40 μm and even severely
crumpled without discernible degradation of their electrical performance.
Moreover, the SWNT TFTs can also sustain up to 67% compressive strain
without apparent change compared with their performance without any strain.
Besides the SWNT TFTs, the SWNT circuits also show good performance
with up to 33% compressive strain. Another important advantage of our
platform is that it is compatible with the standard semiconductor
manufacturing process, which enables large-scale low-cost fabrication of our
SWNT electronic foil. Our platform can be further improved by incorporating
n-type materials (e.g., IGZO) to achieve complementary circuits that can
reduce the circuit power consumption and improve the reliability of circuit
operation. On the basis of all the aforementioned advantages, our platform
125
shows great potential for next-generation electronics such as ultraflexible
digital electronics, artificial skins, active-matrix-based displays, and sensors.
126
5.6 References
1. Cao, Q.; Kim, H. S.; Pimparkar, N.; Kulkarni, J. P.; Wang, C.; 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, 495-500.
2. Chen, H.; Cao, Y.; Zhang, J.; Zhou, C. Large-Scale Complementary
Macroelectronics Using Hybrid Integration of Carbon Nanotubes and IGZO Thin-Film
Transistors. Nat. Commun. 2014, 5, 4097.
3. Sun, D. M.; Timmermans, M. Y.; Tian, Y.; Nasibulin, A. G.; Kauppinen, E. I.;
Kishimoto, S.; Mizutani, T.; Ohno, Y. Flexible High-Performance Carbon Nanotube
Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156-161.
4. Ha, M.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M. C.;
Frisbie, C. D. Printed, Sub-3v Digital Circuits on Plastic from Aqueous Carbon Nanotube
Inks. Acs Nano 2010, 4, 4388-4395.
5. Park, S.; Vosguerichian, M.; Bao, Z. A Review of Fabrication and Applications of
Carbon Nanotube Film-Based Flexible Electronics. Nanoscale 2013, 5, 1727-1752.
6. Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A. User-
Interactive Electronic Skin for Instantaneous Pressure Visualization. Nat. Mater. 2013, 12,
899-904.
7. Yeom, C.; Chen, K.; Kiriya, D.; Yu, Z.; Cho, G.; Javey, A. Large-Area Compliant
Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv. Mater.
2015, 27, 1561-1566.
127
8. Zhang, J.; Wang, C.; Zhou, C. Rigid/Flexible Transparent Electronics Based on
Separated Carbon Nanotube Thin-Film Transistors and Their Application in Display
Electronics. Acs Nano 2012, 6, 7412-7419.
9. Wang, C.; Takei, K.; Takahashi, T.; Javey, A. Carbon Nanotube Electronics--
Moving Forward. Chem. Soc. Rev. 2013, 42, 2592-2609.
10. Wang, C.; Chien, J. C.; Takei, K.; Takahashi, T.; Nah, J.; Niknejad, A. M.; Javey,
A. Extremely Bendable, High-Performance Integrated Circuits Using Semiconducting
Carbon Nanotube Networks for Digital, Analog, and Radio-Frequency Applications. Nano
Lett. 2012, 12, 1527-1533.
11. Wang, C.; Zhang, J.; Ryu, K.; Badmaev, A.; De Arco, L. G.; Zhou, C. Wafer-Scale
Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display Applications.
Nano Lett. 2009, 9, 4285-4291.
12. Snell, A. J.; Spear, W. E.; Lecomber, P. G.; Mackenzie, K. Application of
Amorphous-Silicon Field-Effect Transistors in Integrated-Circuits. Appl. Phys. A: Mater.
Sci. Process. 1981, 26, 83-86.
13. Uchikoga, S. Low-Temperature Polycrystalline Silicon Thin-Film Transistor
Technologies for System-on-Glass Displays. MRS Bull. 2002, 27, 881-886.
14. Xu, F.; Wu, M. Y.; Safron, N. S.; Roy, S. S.; Jacobberger, R. M.; Bindl, D. J.; Seo,
J. H.; Chang, T. H.; Ma, Z. Q.; Arnold, M. S. Highly Stretchable Carbon Nanotube
Transistors with Ion Gel Gate Dielectrics. Nano Lett. 2014, 14, 682-686.
128
15. Chae, S. H.; Yu, W. J.; Bae, J. J.; Duong, D. L.; Perello, D.; Jeong, H. Y.; Ta, Q.
H.; Ly, T. H.; Vu, Q. A.; Yun, M., et al. Transferred Wrinkled Al2O3 for Highly
Stretchable and Transparent Graphene-Carbon Nanotube Transistors. Nat. Mater. 2013, 12,
403-409.
16. Chen, P.; Fu, Y.; Aminirad, R.; Wang, C.; Zhang, J.; Wang, K.; Galatsis, K.; Zhou,
C. Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits and Its
Application in Organic Light Emitting Diode Control. Nano Lett. 2011, 11, 5301-5308.
17. Xu, W.; Zhao, J.; Qian, L.; Han, X.; Wu, L.; Wu, W.; Song, M.; Zhou, L.; Su, W.;
Wang, C., et al. Sorting of Large-Diameter Semiconducting Carbon Nanotube and Printed
Flexible Driving Circuit for Organic Light Emitting Diode (Oled). Nanoscale 2014, 6,
1589-1595.
18. Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors
and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015-1022.
19. Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances
on Plastic. Nature 2004, 428, 911-918.
20. Gelinck, G. H.; Huitema, H. E.; van Veenendaal, E.; Cantatore, E.; Schrijnemakers,
L.; van der Putten, J. B.; Geuns, T. C.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B. H.,
et al. Flexible Active-Matrix Displays and Shift Registers Based on Solution-Processed
Organic Transistors. Nat. Mater. 2004, 3, 106-110.
21. Cao, X.; Chen, H.; Gu, X.; Liu, B.; Wang, W.; Cao, Y.; Wu, F.; Zhou, C. Screen
Printing as a Scalable and Low-Cost Approach for Rigid and Flexible Thin-Film
Transistors Using Separated Carbon Nanotubes. Acs Nano 2014, 8, 12769-12776.
129
22. Sun, D. M.; Timmermans, M. Y.; Kaskela, A.; Nasibulin, A. G.; Kishimoto, S.;
Mizutani, T.; Kauppinen, E. I.; Ohno, Y. Mouldable All-Carbon Integrated Circuits. Nat.
Commun. 2013, 4, 2302.
23. Jung, M.; Kim, J.; Noh, J.; Lim, N.; Lim, C.; Lee, G.; Kim, J.; Kang, H.; Jung, K.;
Leonard, A. D., et al. All-Printed and Roll-to-Roll-Printable 13.56-MHz-Operated 1-Bit
RF Tag on Plastic Foils. IEEE Trans. Electron Devices 2010, 57, 571-580.
24. Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.;
Javey, A. Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on
Flexible Substrates. Nano Lett. 2013, 13, 3864-3869.
25. Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur,
A. High-Speed, Inkjet-Printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary
Ring Oscillators. Nano Lett. 2014, 14, 3683-3687.
26. Cao, Q.; Hur, S. H.; Zhu, Z. T.; Sun, Y. G.; Wang, C. J.; Meitl, M. A.; Shim, M.;
Rogers, J. A. Highly Bendable, Transparent Thin-Film Transistors That Use Carbon-
Nanotube-Based Conductors and Semiconductors with Elastomeric Dielectrics. Adv.
Mater. 2006, 18, 304-+.
27. Aikawa, S.; Einarsson, E.; Thurakitseree, T.; Chiashi, S.; Nishikawa, E.; Maruyama,
S. Deformable Transparent All-Carbon-Nanotube Transistors. Appl. Phys. Lett. 2012, 100.
28. Kim, D. H.; Lu, N.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S.; Wu, J.; Won, S. M.;
Tao, H.; Islam, A., et al. Epidermal Electronics. Science 2011, 333, 838-843.
130
29. Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.;
Drack, M.; Schwodiauer, R.; Graz, I.; Bauer-Gogonea, S., et al. An Ultra-Lightweight
Design for Imperceptible Plastic Electronics. Nature 2013, 499, 458-463.
30. White, M. S.; Kaltenbrunner, M.; Glowacki, E. D.; Gutnichenko, K.; Kettlgruber,
G.; Graz, I.; Aazou, S.; Ulbricht, C.; Egbe, D. A. M.; Miron, M. C., et al. Ultrathin, Highly
Flexible and Stretchable Pleds. Nat. Photonics 2013, 7, 811-816.
31. Yeo, W. H.; Kim, Y. S.; Lee, J.; Ameen, A.; Shi, L.; Li, M.; Wang, S.; Ma, R.; Jin,
S. H.; Kang, Z., et al. Multifunctional Epidermal Electronics Printed Directly onto the Skin.
Adv. Mater. 2013, 25, 2773-2778.
32. Jeong, J. W.; Yeo, W. H.; Akhtar, A.; Norton, J. J.; Kwack, Y. J.; Li, S.; Jung, S.
Y.; Su, Y.; Lee, W.; Xia, J., et al. Materials and Optimized Designs for Human-Machine
Interfaces via Epidermal Electronics. Adv. Mater. 2013, 25, 6839-6846.
33. Fukuda, K.; Takeda, Y.; Yoshimura, Y.; Shiwaku, R.; Tran, L. T.; Sekine, T.;
Mizukami, M.; Kumaki, D.; Tokito, S. Fully-Printed High-Performance Organic Thin-Film
Transistors and Circuitry on One-Micron-Thick Polymer Films. Nat. Commun. 2014, 5,
4147.
34. Salvatore, G. A.; Munzenrieder, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Strebel, I.;
Buthe, L.; Troster, G. Wafer-Scale Design of Lightweight and Transparent Electronics
That Wraps around Hairs. Nat. Commun. 2014, 5, 2982.
35. Drack, M.; Graz, I.; Sekitani, T.; Someya, T.; Kaltenbrunner, M.; Bauer, S. An
Imperceptible Plastic Electronic Wrap. Adv. Mater. 2015, 27, 34-40.
131
36. Kaltenbrunner, M.; White, M. S.; Glowacki, E. D.; Sekitani, T.; Someya, T.;
Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with High
Flexibility. Nat. Commun. 2012, 3, 770.
37. Thostenson, E. T.; Ren, Z. F.; Chou, T. W. Advances in the Science and
Technology of Carbon Nanotubes and Their Composites: A Review. Compos. Sci. Technol.
2001, 61, 1899-1912.
38. Li, T.; Suo, Z. G.; Lacour, S. P.; Wagner, S. Compliant Thin Film Patterns of Stiff
Materials as Platforms for Stretchable Electronics. J. Mater. Res. 2005, 20, 3274-3277.
39. Li, T.; Huang, Z. Y.; Suo, Z.; Lacour, S. P.; Wagner, S. Stretchability of Thin Metal
Films on Elastomer Substrates. Appl. Phys. Lett. 2004, 85, 3435-3437.
132
6 Conclusions and future work of
printed and flexible CNT
macroelectronics
6.1 Conclusions
In this dissertation, I have presented the development of our innovative
platforms for printed and flexible CNT macroelectronics. Regarding printed
CNT macroelectronics, our roadmap was branched into two strategies: top-
contact self-aligned printing for high-performance sub-micron CNT TFTs and
screen printing for large-area high-throughput manufacturing of CNT TFT
backplanes. Due to the development of top-contact self-aligned printing, we
have successfully bypassed the limitation in resolution of inkjet printing and
therefore achieved sub-micron channel length of fully printed CNT TFTs.
Combined with ion-gel gating, we have improved the on-current current
density to be the highest among fully printed CNT TFTs, which is crucial for
current driving display application and high speed transistors for radio-
frequency application. Also, screen printing, as a scalable and low-cost
method, has been adopted to make fully printed CNT TFTs. We have achieved
133
fully screen printed CNT TFTs with high mobility, on-off ratio, and excellent
mechanical flexibility. Based on this, fully screen printed active-matrix
electrochromic display has been developed on flexible substrate, showing the
great potential of screen printing for large-area, low-cost, and flexible active-
matrix-based displays and sensing systems.
In addition, we have done in-depth investigation of ultraflexible CNT
macroelectronics on 1.4 μm PET substrate. The resulted CNT TFTs showed
unprecedented mechanical flexibility with bending radius down to 40 μm,
while maintaining excellent electrical performance of mobility centered
between~5 - 8 cm
2
V
-1
s
-1
and on-off ratio 10
5
-10
6
. Moreover, the CNT TFTs
can also sustain up to 67% compressive strain without apparent change
compared with their performance without any strain. Besides the CNT TFTs,
Also, we have made flexible and stretchable logic circuits including inverters,
NAND gates, and NOR gates, showing good performance with up to 33%
compressive strain. Our platform can be further improved by incorporating n-
type materials (e.g., IGZO) to achieve complementary circuits that can reduce
the circuit power consumption and improve the reliability of circuit operation.
134
6.2 Future direction of fully printed carbon
nanotube macroelectronics
Overall, although printing shows the advantages in scalability and cost
effectiveness, there still remains several challenges in further boosting printed
CNT macroelectronics for practical applications.
Firstly, ink development is one of the most important topics in printed
electronics. Although the current PFO-based CNT sorting can produce
semiconducting CNT solution with very high purity (>99.99%),
1
the sorted
nanotubes are still wrapped by PFO polymer, which may increase the
resistance of tube-to-tube junctions. In addition, PFO polymer is an expensive
material which may potentially increase the spending for mass production.
Hence, more cost-effective and cleaner methods need to be developed.
Regarding the dielectric materials for TFT technology, organic materials with
good intrinsic printability in solution suffer from low dielectric constants and
degradation at high temperature (e.g 140 ℃ required for curing metal inks),
2,
3
which leads to the weak gating and the performance degradation of the
printed TFTs. Metal oxide such as BTO, with the high dielectric constant,
seems to be a good candidate of printable dielectric materials.
4-7
Nevertheless,
135
it is very challenging to control the diameter distribution of the nanoparticle
inks. Moreover, without the advantage of crosslink happened in polymer
formation, it is difficult to call a balance between a thin film with good gating,
and a dense film to minimize the gate leakage current. Therefore,
organic/inorganic mixture is currently considered to be the answer and needs
more research effort.
8-10
For metal inks, instead of old-fashion metal
nanoparticles, 1D metal nanowires and carbon nanotubes can provide more
desirable features such as flexibility and transparency.
11-14
Finally and more
importantly, functional inks related to sensing and display applications have
been attracting a lot of research interest: printable pressure sensitive inks for
electronic skin, light-emitting perovskites for LEDs, electrochromic material
for non-volatile displays, and so on.
5,15,16
Therefore, the ink development will
tremendously improve the performance of printed CNT TFTs and broaden the
applications in large-area printed sensors and displays.
136
Figure 6.1 Printed perovskite light-emitting diode (a) [15], electronic skin (b) [16], and
photodetector (c) [17]
Secondly, compared with conventional deposition and lithography, printing,
as a solution-based patterning, shows the limitation in resolution and the
controllability upon the thickness of printed layers. The most commonly used
printing techniques like inkjet printing, gravure printing, screen printing, and
flexographic printing are with relatively low resolution (>20 μm in feature
size) and result in a thick film (usually, >100 nm) with considerable surface
roughness. These issues should be addressed in the future to further improve
the integration density, electrical performance, uniformity, and
manufacturability of printed electronics. For example, high-resolution
flexographic printing has been developed by A. John Hart’s group using
carbon nanotube forest as the media to carry nanoparticle inks.
18
The resulted
films show very small features and surface roughness. Also,
electrohydrodynamic printing reported by John Rogers’ group has shown the
capability of printing tiny features.
19
137
Figure 6.2 High resolution printing technologies. (a) High-resolution flexographic printing
using CNT forest as masks for silver and CdSe/ZnS quantum dots printing. [18] (b)
electrohydrodynamic printing for quantum dot light-emitting diode [19]
138
6.3 References
1. Ding, J. F.; Li, Z.; Lefebvre, J.; Cheng, F. Y.; Dubey, G.; Zou, S.; Finnie, P.; Hrdina,
A.; Scoles, L.; Lopinski, G. P.; Kingston, C. T.; Simard, B.; Malenfant, P. R. L. Enrichment
of Large-Diameter Semiconducting Swcnts by Polyfluorene Extraction for High Network
Density Thin Film Transistors. Nanoscale 2014, 6, 2328-2339.
2. Chen, J. H. Organic Low-Dielectric Constant Materials for Microelectronics.
Introduction To Organic Electronic And Optoelectronic Materials And Devices 2008, 133,
845-866.
3. Aleeva, Y.; Pignataro, B. Recent Advances in Upscalable Wet Methods and Ink
Formulations for Printed Electronics. J. Mater. Chem. C 2014, 2, 6436-6453.
4. Cao, X.; Chen, H. T.; Gu, X. F.; Liu, B. L.; Wang, W. L.; Cao, Y.; Wu, F. Q.; Zhu,
C. W. Screen Printing as a Scalable and Low-Cost Approach for Rigid and Flexible Thin-
Film Transistors Using Separated Carbon Nanotubes. Acs Nano 2014, 8, 12769-12776.
5. Cao, X.; Lau, C.; Liu, Y. H.; Wu, F. Q.; Gui, H.; Liu, Q. Z.; Ma, Y. Q.; Wan, H.
C.; Amer, M. R.; Zhou, C. W. Fully Screen-Printed, Large-Area, and Flexible Active-
Matrix Electrochromic Displays Using Carbon Nanotube Thin-Film Transistors. Acs Nano
2016, 10, 9816-9822.
6. Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.;
Javey, A. Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on
Flexible Substrates. Nano Lett 2013, 13, 3864-9.
139
7. Yeom, C.; Chen, K.; Kiriya, D.; Yu, Z.; Cho, G.; Javey, A. Large-Area Compliant
Tactile Sensors Using Printed Carbon Nanotube Active-Matrix Backplanes. Adv. Mater.
2015.
8. Pramanik, N. C.; Il Seok, S. High-Dielectric Constant Inorganic-Organic Hybrid
Materials Prepared with Sol-Gel-Derived Crystalline Batio3. Jpn. J. Appl. Phys. 2008, 47,
531-537.
9. Sarasqueta, G.; Choudhury, K. R.; Kim, D. Y.; So, F. Organic/Inorganic
Nanocomposites for High-Dielectric-Constant Materials. Appl. Phys. Lett. 2008, 93.
10. Guo, M.; Hayakawa, T.; Kakimoto, M.; Goodson, T. Organic Macromolecular
High Dielectric Constant Materials: Synthesis, Characterization, and Applications. J. Phys.
Chem. B 2011, 115, 13419-13432.
11. Jakubowska, M.; Sloma, M.; Mlozniak, A. Printed Transparent Electrodes
Containing Carbon Nanotubes for Elastic Circuits Applications with Enhanced Electrical
Durability under Severe Conditions. Materials Science And Engineering B-Advanced
Functional Solid-State Materials 2011, 176, 358-362.
12. Cao, Q.; Zhu, Z. T.; Lemaitre, M. G.; Xia, M. G.; Shim, M.; Rogers, J. A.
Transparent Flexible Organic Thin-Film Transistors That Use Printed Single-Walled
Carbon Nanotube Electrodes. Appl. Phys. Lett. 2006, 88.
13. Zhao, Y. F.; Zou, W. J.; Li, H.; Lu, K.; Yan, W.; Wei, Z. X. Large-Area, Flexible
Polymer Solar Cell Based on Silver Nanowires as Transparent Electrode by Roll-to-Roll
Printing. Chinese. J. Polym. Sci. 2017, 35, 261-268.
140
14. Araki, T.; Mandamparambil, R.; van Bragt, D. M. P.; Jiu, J.; Koga, H.; van den
Brand, J.; Sekitani, T.; den Toonder, J. M. J.; Suganuma, K. Stretchable and Transparent
Electrodes Based on Patterned Silver Nanowires by Laser-Induced Forward Transfer for
Non-Contacted Printing Techniques. Nanotechnology 2016, 27.
15. Bade, S. G.; Li, J.; Shan, X.; Ling, Y.; Tian, Y.; Dilbeck, T.; Besara, T.; Geske, T.;
Gao, H.; Ma, B.; Hanson, K.; Siegrist, T.; Xu, C.; Yu, Z. Fully Printed Halide Perovskite
Light-Emitting Diodes with Silver Nanowire Electrodes. ACS Nano 2016, 10, 1795-801.
16. Khan, S.; Lorenzelli, L.; Dahiya, R. S. Screen Printed Flexible Pressure Sensors
Skin. 2014 25th Annual Semi Advanced Semiconductor Manufacturing Conference (Asmc)
2014, 219-224.
17. Mattana, G.; Briand, D. Recent Advances in Printed Sensors on Foil. Materials
Today 2016, 19, 88-99.
18. Kim, S.; Sojoudi, H.; Zhao, H.; Mariappan, D.; McKinley, G. H.; Gleason, K. K.;
Hart, A. J. Ultrathin High-Resolution Flexographic Printing Using Nanoporous Stamps.
Science Advances 2016, 2.
19. Kim, B. H.; Onses, M. S.; Lim, J. B.; Nam, S.; Oh, N.; Kim, H.; Yu, K. J.; Lee, J.
W.; Kim, J. H.; Kang, S. K.; Lee, C. H.; Lee, J.; Shin, J. H.; Kim, N. H.; Leal, C.; Shim,
M.; Rogers, J. A. High-Resolution Patterns of Quantum Dots Formed by
Electrohydrodynamic Jet Printing for Light-Emitting Diodes. Nano Lett 2015, 15, 969-73.
141
Bibliography
Aikawa, S.; Einarsson, E.; Thurakitseree, T.; Chiashi, S.; Nishikawa, E.;
Maruyama, S. Deformable Transparent All-Carbon-Nanotube Transistors.
Appl. Phys. Lett. 2012, 100.
Aleeva, Y.; Pignataro, B. Recent Advances in Upscalable Wet Methods and Ink
Andersson Ersman, P.; Kawahara, J.; Berggren, M. Printed Passive Matrix Addressed
Electrochromic Displays. Org. Electron. 2013, 14, 3371-3378.
Andersson, P.; Forchheimer, R.; Tehrani, P.; Berggren, M. Printable All-Organic
Electrochromic Active-Matrix Displays. Adv. Funct. Mater. 2007, 17, 3074-3082.
Araki, T.; Mandamparambil, R.; van Bragt, D. M. P.; Jiu, J.; Koga, H.; van den Brand,
J.; Sekitani, T.; den Toonder, J. M. J.; Suganuma, K. Stretchable and Transparent
Electrodes Based on Patterned Silver Nanowires by Laser-Induced Forward Transfer
for Non-Contacted Printing Techniques. Nanotechnology 2016, 27.
Avouris, P. Carbon Nanotube Electronics. Chem. Phys. 2002, 281, 429-445.
Avouris, P. Molecular Electronics with Carbon Nanotubes. Accounts. Chem. Res. 2002,
35, 1026-1034.
Bade, S. G.; Li, J.; Shan, X.; Ling, Y.; Tian, Y.; Dilbeck, T.; Besara, T.; Geske, T.; Gao,
H.; Ma, B.; Hanson, K.; Siegrist, T.; Xu, C.; Yu, Z. Fully Printed Halide Perovskite
Light-Emitting Diodes with Silver Nanowire Electrodes. ACS Nano 2016, 10, 1795-
801.
142
Bao, Z. N.; Feng, Y.; Dodabalapur, A.; Raju, V. R.; Lovinger, A. J. High-Performance
Plastic Transistors Fabricated by Printing Techniques. Chem. Mater. 1997, 9, 1299-
1301.
Berggren, M.; Nilsson, D.; Robinson, N. D. Organic Materials for Printed Electronics.
Nat. Mater. 2007, 6, 3-5.
Brady, G. J.; Way, A. J.; Safron, N. S.; Evensen, H. T.; Gopalan, P.; Arnold, M. S. Quasi-
ballistic carbon nanotube array transistors with current density exceeding Si and GaAs.
Sci. Adv. 2016, 2, e1601240.
Cai, L.; Wang, C. Carbon Nanotube Flexible and Stretchable Electronics. Nanoscale
Res. Lett. 2015, 10, 1013.
Cai, L.; Zhang, S.; Miao, J.; Yu, Z.; Wang, C. Fully Printed Foldable Integrated Logic
Gates with Tunable Performance Using Semiconducting Carbon Nanotubes. Adv.
Funct. Mater. 2015, 25, 5698-5705.
Cai, L.; Zhang, S.; Miao, J.; Yu, Z.; Wang, C. Fully Printed Stretchable Thin-Film
Transistors and Integrated Logic Circuits. ACS Nano 2016,
DOI:10.1021/acsnano.6b07190
Caironi, M.; Gili, E.; Sakanoue, T.; Cheng, X. Y .; Sirringhaus, H. High Yield, Single
Droplet Electrode Arrays for Nanoscale Printed Electronics. ACS Nano 2010, 4, 1451-
1456.
Cao, C. Y .; Andrews, J. B.; Kumar, A.; Franklin, A. D. Improving Contact Interfaces in
Fully Printed Carbon Nanotube Thin-Film Transistors. ACS Nano 2016, 10, 5221-5229.
Cao, Q.; Hur, S. H.; Zhu, Z. T.; Sun, Y. G.; Wang, C. J.; Meitl, M. A.; Shim,
M.; Rogers, J. A. Highly Bendable, Transparent Thin-Film Transistors That
143
Use Carbon-Nanotube-Based Conductors and Semiconductors with
Elastomeric Dielectrics. Adv. Mater. 2006, 18, 304-+.
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, 495-U4.
Cao, Q.; Zhu, Z. T.; Lemaitre, M. G.; Xia, M. G.; Shim, M.; Rogers, J. A. Transparent
Flexible Organic Thin-Film Transistors That Use Printed Single-Walled Carbon
Nanotube Electrodes. Appl. Phys. Lett. 2006, 88.
Cao, X.; Cao, Y.; Zhou, C. Imperceptible and Ultraflexible P-Type Transistors and
Macroelectronics Based on Carbon Nanotubes. ACS Nano 2016, 10, 199-206.
Cao, X.; Chen, H. T.; Gu, X. F.; Liu, B. L.; Wang, W. L.; Cao, Y.; Wu, F. Q.; Zhu, C.
W. Screen Printing as a Scalable and Low-Cost Approach for Rigid and Flexible Thin-
Film Transistors Using Separated Carbon Nanotubes. Acs Nano 2014, 8, 12769-12776.
Cao, X.; Lau, C.; Liu, Y. H.; Wu, F. Q.; Gui, H.; Liu, Q. Z.; Ma, Y. Q.; Wan, H. C.;
Amer, M. R.; Zhou, C. W. Fully Screen-Printed, Large-Area, and Flexible Active-
Matrix Electrochromic Displays Using Carbon Nanotube Thin-Film Transistors. Acs
Nano 2016, 10, 9816-9822.
Cao, X.; Lau, C.; Liu, Y .; Wu, F.; Gui, H.; Liu, Q.; Ma, Y .; Wan, H.; Amer, M. R.; Zhou,
C. W. Fully Screen-Printed, Large-Area, and Flexible Active-Matrix Electrochromic
Displays Using Carbon Nanotube Thin-Film Transistors. ACS Nano 2016, 10, 9816-
9822.
144
Chae, S. H.; Yu, W. J.; Bae, J. J.; Duong, D. L.; Perello, D.; Jeong, H. Y.; Ta,
Q. H.; Ly, T. H.; Vu, Q. A.; Yun, M., et al. Transferred Wrinkled Al2O3 for
Highly Stretchable and Transparent Graphene-Carbon Nanotube Transistors.
Nat. Mater. 2013, 12, 403-409.
Chen, H.; Cao, Y.; Zhang, J.; Zhou, C. Large-Scale Complementary
Macroelectronics Using Hybrid Integration of Carbon Nanotubes and IGZO
Thin-Film Transistors. Nat. Commun. 2014, 5, 4097.
Chen, J. H. Organic Low-Dielectric Constant Materials for Microelectronics.
Introduction To Organic Electronic And Optoelectronic Materials And Devices 2008,
133, 845-866.
Chen, P. C.; Fu, Y.; Aminirad, R.; Wang, C.; Zhang, J. L.; Wang, K.; Galatsis, K.;
Zhou, C. W. Fully Printed Separated Carbon Nanotube Thin Film Transistor Circuits
and Its Application in Organic Light Emitting Diode Control. Nano Lett. 2011, 11,
5301-5308.
Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Kam, N. W. S.; Shim, M.; Li, Y. M.;
Kim, W.; Utz, P. J.; Dai, H. J. Noncovalent Functionalization of Carbon Nanotubes for
Highly Specific Electronic Biosensors. Proceedings Of the National Academy Of
Sciences Of the United States Of America 2003, 100, 4984-4989.
Cho, J. H.; Lee, J.; Xia, Y.; Kim, B.; He, Y. Y.; Renn, M. J.; Lodge, T. P.; Frisbie, C.
D. Printable Ion-Gel Gate Dielectrics for Low-Voltage Polymer Thin-Film Transistors
on Plastic. Nat. Mater. 2008, 7, 900-906.
145
Chou, H. H.; Nguyen, A.; Chortos, A.; To, J. W. F.; Lu, C.; Mei, J. G.; Kurosawa, T.;
Bae, W. G.; Tok, J. B. H.; Bao, Z. A. A Chameleon-Inspired Stretchable Electronic
Skin with Interactive Colour Changing Controlled by Tactile Sensing. Nat. Commun.
2015, 6.
Close, G. F.; Yasuda, S.; Paul, B.; Fujita, S.; Wong, H. S. P. A 1 Ghz Integrated Circuit
with Carbon Nanotube Interconnects and Silicon Transistors. Nano Lett. 2008, 8, 706-
709.
Ding, J. F.; Li, Z.; Lefebvre, J.; Cheng, F. Y.; Dubey, G.; Zou, S.; Finnie, P.; Hrdina,
A.; Scoles, L.; Lopinski, G. P.; Kingston, C. T.; Simard, B.; Malenfant, P. R. L.
Enrichment of Large-Diameter Semiconducting Swcnts by Polyfluorene Extraction
Drack, M.; Graz, I.; Sekitani, T.; Someya, T.; Kaltenbrunner, M.; Bauer, S.
An Imperceptible Plastic Electronic Wrap. Adv. Mater. 2015, 27, 34-40.
Durkop, T.; Getty, S. A.; Cobas, E.; Fuhrer, M. S. Extraordinary Mobility in
Semiconducting Carbon Nanotubes. Nano Lett. 2004, 4, 35-39.
Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic
Appliances on Plastic. Nature 2004, 428, 911-918.
Fukuda, K.; Takeda, Y.; Yoshimura, Y.; Shiwaku, R.; Tran, L. T.; Sekine, T.;
Mizukami, M.; Kumaki, D.; Tokito, S. Fully-Printed High-Performance
Organic Thin-Film Transistors and Circuitry on One-Micron-Thick Polymer
Films. Nat. Commun. 2014, 5, 4147.
146
Gelinck, G. H.; Huitema, H. E.; van Veenendaal, E.; Cantatore, E.;
Schrijnemakers, L.; van der Putten, J. B.; Geuns, T. C.; Beenhakkers, M.;
Giesbers, J. B.; Huisman, B. H., et al. Flexible Active-Matrix Displays and
Shift Registers Based on Solution-Processed Organic Transistors. Nat. Mater.
2004, 3, 106-110.
Guo, M.; Hayakawa, T.; Kakimoto, M.; Goodson, T. Organic Macromolecular High
Dielectric Constant Materials: Synthesis, Characterization, and Applications. J. Phys.
Chem. B 2011, 115, 13419-13432.
Ha, M. J.; Seo, J. W. T.; Prabhumirashi, P. L.; Zhang, W.; Geier, M. L.; Renn, M. J.;
Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Aerosol Jet Printed, Low Voltage, Electrolyte
Gated Carbon Nanotube Ring Oscillators with Sub-5 Mu S Stage Delays. Nano Lett.
2013, 13, 954-960.
Ha, M. J.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M. C.;
Frisbie, C. D. Printed, Sub-3v Digital Circuits on Plastic from Aqueous Carbon
Nanotube Inks. Acs Nano 2010, 4, 4388-4395.
Ha, M.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam,
M. C.; Frisbie, C. D. Printed, Sub-3v Digital Circuits on Plastic from Aqueous
Carbon Nanotube Inks. Acs Nano 2010, 4, 4388-4395.
Higuchi, K.; Kishimoto, S.; Nakajima, Y.; Tomura, T.; Takesue, M.; Hata, K.;
Kauppinen, E. I.; Ohno, Y. High-Mobility, Flexible Carbon Nanotube Thin-Film
Transistors Fabricated by Transfer and High-Speed Flexographic Printing Techniques.
Appl. Phys. Express 2013, 6, 085101.
147
http://semieurope.omnibooksonline.com/2012/semicon_europa/Plastic%20Electronic
s%20Conference/Plenary%20Session/03_Jennifer.Colegrove_DisplaySearch.pdf
<03_Jennifer.Colegrove_Displaysearch.Pdf>.
Http://www.kuroda-electric.eu/Ultra-Fine-Pattern-Screen-Printing
Hyun, W. J.; Secor, E. B.; Rojas, G. A.; Hersam, M. C.; Francis, L. F.; Frisbie, C. D.
All-Printed, Foldable Organic Thin-Film Transistors on Glassine Paper. Adv. Mater.
2015, 27, 7058-7064.
Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56-58.
Jakubowska, M.; Sloma, M.; Mlozniak, A. Printed Transparent Electrodes Containing
Carbon Nanotubes for Elastic Circuits Applications with Enhanced Electrical
Durability under Severe Conditions. Materials Science And Engineering B-Advanced
Functional Solid-State Materials 2011, 176, 358-362.
Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. J. Ballistic Carbon Nanotube
Field-Effect Transistors. Nature 2003, 424, 654-657.
Jeong, J. W.; Yeo, W. H.; Akhtar, A.; Norton, J. J.; Kwack, Y. J.; Li, S.; Jung,
S. Y.; Su, Y.; Lee, W.; Xia, J., et al. Materials and Optimized Designs for
Human-Machine Interfaces via Epidermal Electronics. Adv. Mater. 2013, 25,
6839-6846.
Johnston, D.; Barnardo, C.; Fryer, C. Passive Multiplexing of Printed
Electroluminescent Displays. J. Soc. Inf. Disp. 2005, 13, 487-491.
148
Jung, M.; Kim, J.; Noh, J.; Lim, N.; Lim, C.; Lee, G.; Kim, J.; Kang, H.; Jung, K.;
Leonard, A. D., et al. All-Printed and Roll-to-Roll-Printable 13.56-Mhz-Operated 1-
Bit Rf Tag on Plastic Foils. Ieee. T. Electron Devices 2010, 57, 571-580.
Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.;
Tokuhara, T.; Drack, M.; Schwodiauer, R.; Graz, I.; Bauer-Gogonea, S., et al.
An Ultra-Lightweight Design for Imperceptible Plastic Electronics. Nature
2013, 499, 458-463.
Kaltenbrunner, M.; White, M. S.; Glowacki, E. D.; Sekitani, T.; Someya, T.;
Sariciftci, N. S.; Bauer, S. Ultrathin and Lightweight Organic Solar Cells with
High Flexibility. Nat. Commun. 2012, 3, 770.
Kawahara, J.; Andersson Ersman, P.; Nilsson, D.; Katoh, K.; Nakata, Y.; Sandberg, M.;
Nilsson, M.; Gustafsson, G.; Berggren, M. Flexible Active Matrix Addressed Displays
Manufactured by Printing and Coating Techniques. J. Polym. Sci., Part B: Polym. Phys.
2013, 51, 265-271.
Kawahara, J.; Ersman, P. A.; Engquist, I.; Berggren, M. Improving the Color Switch
Contrast in Pedot:Pss-Based Electrochromic Displays. Org. Electron. 2012, 13, 469-
474.
Khan, S.; Lorenzelli, L.; Dahiya, R. S. Screen Printed Flexible Pressure Sensors Skin.
2014 25th Annual Semi Advanced Semiconductor Manufacturing Conference (Asmc)
2014, 219-224.
Kim, B. H.; Onses, M. S.; Lim, J. B.; Nam, S.; Oh, N.; Kim, H.; Yu, K. J.; Lee, J. W.;
Kim, J. H.; Kang, S. K.; Lee, C. H.; Lee, J.; Shin, J. H.; Kim, N. H.; Leal, C.; Shim,
149
M.; Rogers, J. A. High-Resolution Patterns of Quantum Dots Formed by
Electrohydrodynamic Jet Printing for Light-Emitting Diodes. Nano Lett 2015, 15, 969-
73.
Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur, A.
High-Speed, Inkjet-Printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary
Ring Oscillators. Nano Lett. 2014, 14, 3683-3687.
Kim, D. H.; Lu, N. S.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S. D.; Wu, J.; Won, S.
M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L. Z.; Li,
M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.;
Huang, Y. G.; Coleman, T.; Rogers, J. A. Epidermal Electronics. Science 2011, 333,
838-843.
Kim, J.-Y.; Bae, M. J.; Park, S. H.; Jeong, T.; Song, S.; Lee, J.; Han, I.; Yoo, J. B.;
Jung, D.; Yu, S. Electroluminescence Enhancement of the Phosphor Dispersed in a
Polymer Matrix Using the Tandem Structure. Org. Electron. 2011, 12, 529-533.
Kim, S. H.; Hong, K.; Xie, W.; Lee, K. H.; Zhang, S. P.; Lodge, T. P.; Frisbie, C. D.
Electrolyte-Gated Transistors for Organic and Printed Electronics. Adv. Mater. 2013,
25, 1822-1846.
Kim, S.; Sojoudi, H.; Zhao, H.; Mariappan, D.; McKinley, G. H.; Gleason, K. K.; Hart,
A. J. Ultrathin High-Resolution Flexographic Printing Using Nanoporous Stamps.
Science Advances 2016, 2.
Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H.
J. Nanotube Molecular Wires as Chemical Sensors. Science 2000, 287, 622-625.
150
Kong, Y. L.; Tamargo, I. A.; Kim, H.; Johnson, B. N.; Gupta, M. K.; Koh, T. W.; Chin,
H. A.; Steingart, D. A.; Rand, B. P.; McAlpine, M. C. 3d Printed Quantum Dot Light-
Emitting Diodes. Nano Lett. 2014, 14, 7017-7023.
Kopola, P.; Tuomikoski, M.; Suhonen, R.; Maaninen, A. Gravure Printed Organic
Light Emitting Diodes for Lighting Applications. Thin Solid Films 2009, 517, 5757-
5762.
Kuhn, K. J. Moore's Law Past 32nm: Future Challenges in Device Scaling. Iwce-13:
2009 13th International Workshop on Computational Electronics 2009, 37-40.
Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho,
G.; Javey, A. Fully Printed, High Performance Carbon Nanotube Thin-Film
Transistors on Flexible Substrates. Nano Lett. 2013, 13, 3864-3869.
Lee, D. H.; Choi, J. S.; Chae, H.; Chung, C. H.; Cho, S. M. Screen-Printed White Oled
Based on Polystyrene as a Host Polymer. Curr. Appl. Phys. 2009, 9, 161-164.
Lee, D.; Yoon, J.; Lee, J.; Lee, B. H.; Seol, M. L.; Bae, H.; Jeon, S. B.; Seong, H.; Im,
S. G.; Choi, S. J.; Choi, Y . K. Logic Circuits Composed of Flexible Carbon Nanotube
Thin-Film Transistor and Ultra-Thin Polymer Gate Dielectric. Sci. Rep. 2016, 6, 26121.
Li, H.; Tang, Y .; Guo, W.; Liu, H.; Zhou, L.; Smolinski, N. Polyfluorinated Electrolyte
for Fully Printed Carbon Nanotube Electronics. Adv. Funct. Mater. 2016, 26, 6914-
6920.
Li, J.; Ye, Q.; Cassell, A.; Ng, H. T.; Stevens, R.; Han, J.; Meyyappan, M. Bottom-up
Approach for Carbon Nanotube Interconnects. Appl. Phys. Lett. 2003, 82, 2491-2493.
Li, T.; Huang, Z. Y.; Suo, Z.; Lacour, S. P.; Wagner, S. Stretchability of Thin
Metal Films on Elastomer Substrates. Appl. Phys. Lett. 2004, 85, 3435-3437.
151
Li, T.; Suo, Z. G.; Lacour, S. P.; Wagner, S. Compliant Thin Film Patterns of
Stiff Materials as Platforms for Stretchable Electronics. J. Mater. Res. 2005,
20, 3274-3277.
Li, Y. N.; Wu, Y. L.; Ong, B. S. Facile Synthesis of Silver Nanoparticles Useful for
Fabrication of High-Conductivity Elements for Printed Electronics. J. Am. Chem. Soc.
2005, 127, 3266-3267.
Lilja, K. E.; Backlund, T. G.; Lupo, D.; Virtanen, J.; Hamalainen, E.; Joutsenoja, T.
Printed Organic Diode Backplane for Matrix Addressing an Electrophoretic Display.
Thin Solid Films 2010, 518, 4385-4389.
Liu, B.; Wu, F.; Gui, H.; Zheng, M.; Zhou, C. Chirality-Controlled Synthesis and
Mattana, G.; Briand, D. Recent Advances in Printed Sensors on Foil. Materials Today
2016, 19, 88-99.
Menard, E.; Meitl, M. A.; Sun, Y. G.; Park, J. U.; Shir, D. J. L.; Nam, Y. S.; Jeon, S.;
Rogers, J. A. Micro- and Nanopatterning Techniques for Organic Electronic and
Optoelectronic Systems. Chem. Rev. 2007, 107, 1117-1160.
Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.;
Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364-367.
Naeemi, A.; Meindl, J. D. Design and Performance Modeling for Single-Walled
Carbon Nanotubes as Local, Semiglobal, and Global Interconnects in Gigascale
Integrated Systems. Ieee. T. Electron Dev. 2007, 54, 26-37.
152
Noh, J.; Jung, K.; Kim, J.; Kim, S.; Cho, S.; Cho, G. Fully Gravure-Printed Flexible
Full Adder Using Swnt-Based Tfts. Ieee. Electron Device Lett. 2012, 33, 1574-1576.
Noh, J.; Jung, M.; Jung, K.; Lee, G.; Lim, S.; Kim, D.; Kim, S.; Tour, J. M.; Cho, G.
Integrable Single Walled Carbon Nanotube (SWNT) Network Based Thin Film
Transistors Using Roll-to-Roll Gravure and Inkjet. Org. Electron. 2011, 12, 2185-2191.
Noh, Y . Y .; Zhao, N.; Caironi, M.; Sirringhaus, H. Downscaling of Self-Aligned, All-
Printed Polymer Thin-Film Transistors. Nat. Nanotechnol. 2007, 2, 784-789.
Okimoto, H.; Takenobu, T.; Yanagi, K.; Miyata, Y.; Shimotani, H.; Kataura, H.; Iwasa,
Y. Tunable Carbon Nanotube Thin-Film Transistors Produced Exclusively via Inkjet
Printing. Adv. Mater. 2010, 22, 3981-3986.
Park, J. U.; Hardy, M.; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. K.; Lee,
C. Y.; Strano, M. S.; Alleyne, A. G.; Georgiadis, J. G., et al. High-Resolution
Electrohydrodynamic Jet Printing. Nat. Mater. 2007, 6, 782-789.
Park, S.; Vosguerichian, M.; Bao, Z. A Review of Fabrication and
Applications of Carbon Nanotube Film-Based Flexible Electronics. Nanoscale
2013, 5, 1727-1752.
Pramanik, N. C.; Il Seok, S. High-Dielectric Constant Inorganic-Organic Hybrid
Materials Prepared with Sol-Gel-Derived Crystalline Batio3. Jpn. J. Appl. Phys. 2008,
47, 531-537.
Qiu, C. G.; Zhang, Z. Y.; Xiao, M. M.; Yang, Y. J.; Zhong, D. L.; Peng, L. M. Scaling
Carbon Nanotube Complementary Transistors to 5-Nm Gate Lengths. Science 2017,
355, 271-+.
153
Ren, M. S.; Gorter, H.; Michels, J.; Andriessen, R. Ink Jet Technology for Large Area
Organic Light-Emitting Diode and Organic Photovoltaic Applications. J. Imaging. Sci.
Techn. 2011, 55.
Ryu, G. S.; Kim, J. S.; Jeong, S. H.; Song, C. K. A Printed Otft-Backplane for Amoled
Display. Org. Electron. 2013, 14, 1218-1224.
Said, E.; Andersson, P.; Engquist, I.; Crispin, X.; Berggren, M. Electrochromic Display
Cells Driven by an Electrolyte-Gated Organic Field-Effect Transistor. Org. Electron.
2009, 10, 1195-1199.
Sajed, F.; Rutherglen, C. All-Printed and Transparent Single Walled Carbon Nanotube
Thin Film Transistor Devices. Appl. Phys. Lett. 2013, 103, 143303.
Salvatore, G. A.; Munzenrieder, N.; Kinkeldei, T.; Petti, L.; Zysset, C.; Strebel,
I.; Buthe, L.; Troster, G. Wafer-Scale Design of Lightweight and Transparent
Electronics That Wraps around Hairs. Nat. Commun. 2014, 5, 2982.
Sarasqueta, G.; Choudhury, K. R.; Kim, D. Y.; So, F. Organic/Inorganic
Nanocomposites for High-Dielectric-Constant Materials. Appl. Phys. Lett. 2008, 93.
Schaller, R. R. Moore's Law: Past, Present, and Future. Ieee. Spectrum. 1997, 34, 52-
+.
Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T.
Stretchable Active-Matrix Organic Light-Emitting Diode Display Using Printable
Elastic Conductors. Nat. Mater. 2009, 8, 494-499.
154
Sekitani, T.; Takamiya, M.; Noguchi, Y.; Nakano, S.; Kato, Y.; Sakurai, T.; Someya,
T. A Large-Area Wireless Power-Transmission Sheet Using Printed Organic
Transistors and Plastic Mems Switches. Nat. Mater. 2007, 6, 413-417.
Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic
Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9,
1015-1022.
Singh, M.; Haverinen, H. M.; Dhagat, P.; Jabbour, G. E. Inkjet Printing-Process and Its
Applications. Adv. Mater. 2010, 22, 673-685.
Singh, V. K.; Mazhari, B. Impact of Scaling of Dielectric Thickness on Mobility in
Top-Contact Pentacene Organic Thin Film Transistors. J. Appl. Phys. 2012, 111,
034905.
Snell, A. J.; Spear, W. E.; Lecomber, P. G.; Mackenzie, K. Application of
Amorphous-Silicon Field-Effect Transistors in Integrated-Circuits. Appl.
Phys. A: Mater. Sci. Process. 1981, 26, 83-86.
Sun, D. M.; Liu, C.; Ren, W. C.; Cheng, H. M. A Review of Carbon Nanotube- and
Graphene-Based Flexible Thin-Film Transistors. Small 2013, 9, 1188-1205.
Sun, D. M.; Timmermans, M. Y.; Kaskela, A.; Nasibulin, A. G.; Kishimoto,
S.; Mizutani, T.; Kauppinen, E. I.; Ohno, Y. Mouldable All-Carbon Integrated
Circuits. Nat. Commun. 2013, 4, 2302.
155
Sun, D. M.; Timmermans, M. Y.; Tian, Y.; Nasibulin, A. G.; Kauppinen, E.
I.; Kishimoto, S.; Mizutani, T.; Ohno, Y. Flexible High-Performance Carbon
Nanotube Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156-161.
Sun, D. M.; Timmermans, M. Y.; Tian, Y.; Nasibulin, A. G.; Kauppinen, E. I.;
Kishimoto, S.; Mizutani, T.; Ohno, Y. Flexible High-Performance Carbon Nanotube
Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156-161.
Tait, J. G.; Witkowska, E.; Hirade, M.; Ke, T. H.; Malinowski, P. E.; Steudel, S.;
Adachi, C.; Heremans, P. Uniform Aerosol Jet Printed Polymer Lines with 30 Mu M
Width for 140 Ppi Resolution Rgb Organic Light Emitting Diodes. Org. Electron. 2015,
22, 40-43.
Takahashi, T.; Yu, Z.; Chen, K.; Kiriya, D.; Wang, C.; Takei, K.; Shiraki, H.; Chen, T.;
Ma, B.; Javey, A. Carbon Nanotube Active-Matrix Backplanes for Mechanically
Flexible Visible Light and X-Ray Imagers. Nano Lett 2013, 13, 5425-30.
Tekoglu, S.; Hernandez-Sosa, G.; Kluge, E.; Lemmer, U.; Mechau, N. Gravure Printed
Flexible Small-Molecule Organic Light Emitting Diodes. Org. Electron. 2013, 14,
3493-3499.
Thostenson, E. T.; Ren, Z. F.; Chou, T. W. Advances in the Science and
Technology of Carbon Nanotubes and Their Composites: A Review. Compos.
Sci. Technol. 2001, 61, 1899-1912.
Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Exceptionally High Young's Modulus
Observed for Individual Carbon Nanotubes. Nature 1996, 381, 678-680.
156
Uchikoga, S. Low-Temperature Polycrystalline Silicon Thin-Film Transistor
Technologies for System-on-Glass Displays. MRS Bull. 2002, 27, 881-886.
Vaillancourt, J.; Zhang, H. Y .; Vasinajindakaw, P.; Xia, H. T.; Lu, X. J.; Han, X. L.;
Janzen, D. C.; Shih, W. S.; Jones, C. S.; Stroder, M.; Chen, M. Y . H.; Subbaraman, H.;
Chen, R. T.; Berger, U.; Renn, M. All Ink-Jet-Printed Carbon Nanotube Thin-Film
Transistor on a Polyimide Substrate with an Ultrahigh Operating Frequency of over 5
GHz. Appl. Phys. Lett. 2008, 93, 243301.
Vuttipittayamongkol, P.; Wu, F. Q.; Chen, H. T.; Cao, X.; Liu, B. L.; Zhou, C. W.
Threshold V oltage Tuning and Printed Complementary Transistors and Inverters Based
on Thin Films of Carbon Nanotubes and Indium Zinc Oxide. Nano Res. 2015, 8, 1159-
1168.
Wang, C.; Chien, J. C.; Takei, K.; Takahashi, T.; Nah, J.; Niknejad, A. M.;
Javey, A. Extremely Bendable, High-Performance Integrated Circuits Using
Semiconducting Carbon Nanotube Networks for Digital, Analog, and Radio-
Frequency Applications. Nano Lett. 2012, 12, 1527-1533.
Wang, C.; Hwang, D.; Yu, Z.; Takei, K.; Park, J.; Chen, T.; Ma, B.; Javey, A.
User-Interactive Electronic Skin for Instantaneous Pressure Visualization. Nat.
Mater. 2013, 12, 899-904.
Wang, C.; Takei, K.; Takahashi, T.; Javey, A. Carbon Nanotube Electronics--
Moving Forward. Chem. Soc. Rev. 2013, 42, 2592-2609.
Wang, C.; Zhang, J. L.; Ryu, K. M.; Badmaev, A.; De Arco, L. G.; Zhou, C. W. Wafer-
Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display
Applications. Nano Lett. 2009, 9, 4285-4291.
157
Wang, C.; Zhang, J. L.; Ryu, K. M.; Badmaev, A.; De Arco, L. G.; Zhou, C. W. Wafer-
Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display
Applications. Nano Lett. 2009, 9, 4285-4291.
White, M. S.; Kaltenbrunner, M.; Glowacki, E. D.; Gutnichenko, K.;
Kettlgruber, G.; Graz, I.; Aazou, S.; Ulbricht, C.; Egbe, D. A. M.; Miron, M.
C., et al. Ultrathin, Highly Flexible and Stretchable Pleds. Nat. Photonics 2013,
7, 811-816.
Willmann, J.; Stocker, D.; Dorsam, E. Characteristics and Evaluation Criteria of
Substrate-Based Manufacturing. Is Roll-to-Roll the Best Solution for Printed
Electronics? Org. Electron. 2014, 15, 1631-1640.
Xu, F.; Wu, M. Y.; Safron, N. S.; Roy, S. S.; Jacobberger, R. M.; Bindl, D. J.;
Seo, J. H.; Chang, T. H.; Ma, Z. Q.; Arnold, M. S. Highly Stretchable Carbon
Nanotube Transistors with Ion Gel Gate Dielectrics. Nano Lett. 2014, 14, 682-
686.
Xu, W.; Liu, Z.; Zhao, J.; Xu, W.; Gu, W.; Zhang, X.; Qian, L.; Cui, Z. Flexible Logic
Circuits Based on Top-Gate Thin Film Transistors with Printed Semiconductor Carbon
Nanotubes and Top Electrodes. Nanoscale 2014, 6, 14891-7.
Yan, H.; Chen, Z. H.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.;
Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors.
Nature 2009, 457, 679-686.
158
Yeo, W. H.; Kim, Y. S.; Lee, J.; Ameen, A.; Shi, L.; Li, M.; Wang, S.; Ma, R.;
Jin, S. H.; Kang, Z., et al. Multifunctional Epidermal Electronics Printed
Directly onto the Skin. Adv. Mater. 2013, 25, 2773-2778.
Youn, H.; Park, H. J.; Guo, L. J. Printed Nanostructures for Organic Photovoltaic Cells
and Solution-Processed Polymer Light-Emitting Diodes. Energy Technol. 2015, 3,
340-350.
Zhang, J. L.; Fu, Y .; Wang, C.; Chen, P. C.; Liu, Z. W.; Wei, W.; Wu, C.; Thompson, M.
E.; Zhou, C. W. Separated Carbon Nanotube Macroelectronics for Active Matrix
Organic Light-Emitting Diode Displays. Nano Lett. 2011, 11, 4852-4858.
Zhang, J. L.; Wang, C.; Zhou, C. W. Rigid/Flexible Transparent Electronics Based on
Separated Carbon Nanotube Thin-Film Transistors and Their Application in Display
Electronics. Acs Nano 2012, 6, 7412-7419
Zhao, N.; Chiesa, M.; Sirringhausa, H.; Li, Y . N.; Wu, Y . L. Self-Aligned Inkjet Printing
of Highly Conducting Gold Electrodes with Submicron Resolution. J. Appl. Phys. 2007,
101, 064513.
Zhao, Y. F.; Zou, W. J.; Li, H.; Lu, K.; Yan, W.; Wei, Z. X. Large-Area, Flexible
Polymer Solar Cell Based on Silver Nanowires as Transparent Electrode by Roll-to-
Roll Printing. Chinese. J. Polym. Sci. 2017, 35, 261-268.
Zhou, X. J.; Park, J. Y.; Huang, S. M.; Liu, J.; McEuen, P. L. Band Structure, Phonon
Scattering, and the Performance Limit of Single-Walled Carbon Nanotube Transistors.
Phys. Rev. Lett. 2005, 95.
Abstract (if available)
Abstract
In this dissertation, I present my work on the development of printed carbon nanotube (CNT) macroelectronics for large‐area, low‐cost and flexible electronic applications such as sensors, active‐matrix‐based displays, and electronic skin. Emerged as a solution‐based and drop‐on‐demand patterning technique, printing eliminates high‐vacuum environment and multi‐stage photolithography needed in conventional micro‐fabrication. Therefore, printing is very suitable for manufacturing large‐area flexible electronics with low cost and rapid processing. The printing technologies we developed can be divided into two methods. ❧ One is top‐contact self‐aligned printing (SAP) for ultra‐short‐channel CNT thin‐film transistors (TFTs). Using top‐contact self‐aligned printing, we addressed the following issues in fully printed TFTs. First of all, we have successfully downscaled the channel length of fully printed CNT TFTs to sub‐micron, which is beyond the resolution of any existing printing technologies. As a result, we have achieved unprecedented on‐current density ~4.5 μA/μm of fully printed CNT TFTs with high on‐off ratio ~10⁵. This may open up the door to fully printed high‐performance TFTs for macroelectronic applications which need high current drive and high‐speed TFTs, such as active‐matrix backplanes for display. Also, using top‐contact SAP, we eliminated the contact resistance caused by self‐assembled monolayer (SAM) modification. Hence, the device performance was further optimized. Furthermore, our printing technique can be applied to other‐materials‐based systems like 2‐dimensional materials, metal oxides and so on. Overall, we believe this platform is promising in producing high‐performance fully printed TFTs. ❧ The other method is screen printing as a scalable and low‐cost approach for fully printed CNT macroelectronics. Screen printing, with advantages of high throughput, cost effectiveness, and simplicity, are universally used in industry manufacturing. However, due to the lack of available metal, dielectric, and semiconductor inks in the past years, screen printing was hardly used in fabricating electronic devices. We made great efforts on developing new functional inks, optimizing device configurations, and modifying printing process. As a result, we have realized fully screen printed CNT TFTs with good electrical performance and mechanical flexibility. Furthermore, we developed fully printed CNT TFT backplanes and integrated them with different types of sensors. Finally, we have achieved fully screen printed active matrix electrochromic displays on flexible substrates. In this work, all the materials were formulated into screen printable ink and screen printing served as the unique patterning technique. This tremendously simplified the fabrication process and lowered the total cost of making such display. Therefore, our screen printing work can be very important for future fully printed large area and low‐cost sensors, electronic skin, and displays. ❧ In addition to printed CNT macroelectronics, we have also studied the application of CNTs for ultraflexible electronics. Flexible thin‐film transistors based on semiconducting single‐wall carbon nanotubes are promising for flexible digital circuits, artificial skins, radio frequency devices, active‐matrix‐based displays, and sensors due to the outstanding electrical properties and intrinsic mechanical strength of carbon nanotubes. Nevertheless, previous research effort only led to nanotube thin‐film transistors with the smallest bending radius down to 1 mm. We have realized the full potential of carbon nanotubes by making ultraflexible and imperceptible p‐type transistors and circuits with a bending radius down to 40 μm. In addition, the resulted transistors show mobility up to 12.04 cm² V⁻¹ S⁻¹, high on−off ratio (∼10⁶), ultralight weight (<3 g/m²), and good mechanical robustness (accommodating severe crumpling and 67% compressive strain). Furthermore, the nanotube circuits can operate properly with 33% compressive strain. On the basis of the aforementioned features, our ultraflexible p‐type nanotube transistors and circuits have great potential to work as indispensable components for ultraflexible complementary electronics. ❧ This dissertation is presented with six chapters. Chapter 1 is an introduction of carbon nanotubes regarding the structure, electrical properties, and applications for micro/macroelectronics. For inkjet printed carbon nanotube electronics, top‐contact self‐aligned printing is presented in chapter 2 to downscale channel length of printed CNT TFTs to sub‐micron. The resulted devices show unprecedented current density compared with reported fully printed nanotube TFTs. Chapter 3,4 are our screen printing work for fully printed CNT TFTs, printed nanotube backplanes, and fully printed active‐matrix electrochromic display. Besides, we developed ultraflexible CNT macroelectronics which will be presented in chapter 5. The last chapter, chapter 6, is the summary and future direction of printed and flexible macroelectronics.
Linked assets
University of Southern California Dissertations and Theses
Conceptually similar
PDF
Carbon nanotube macroelectronics
PDF
Printed electronics based on carbon nanotubes and two-dimensional transition metal dichalcogenides
PDF
Single-wall carbon nanotubes separation and their device study
PDF
Nanomaterials for macroelectronics and energy storage device
PDF
Carbon nanotube nanoelectronics and macroelectronics
PDF
GaAs nanowire optoelectronic and carbon nanotube electronic device applications
PDF
Carbon material-based nanoelectronics
PDF
Electronic and optoelectronic devices based on quasi-metallic carbon nanotubes
PDF
Synthesis, assembly, and applications of single-walled carbon nanotube
PDF
Multiwall carbon nanotubes reinforced epoxy nanocomposites
PDF
Graphene and carbon nanotubes: synthesis, characterization and applications for beyond silicon electronics
PDF
Nanoelectronics based on gallium oxide and carbon nanotubes
PDF
Raman spectroscopy and electrical transport in suspended carbon nanotube field effect transistors under applied bias and gate voltages
PDF
Controlled synthesis, characterization and applications of carbon nanotubes
PDF
Optoelectronic properties and device physics of individual suspended carbon nanotubes
PDF
Light Emission from Carbon Nanotubes and Two-Dimensional Materials
PDF
In2O3 COVID-19 biosensors and two-dimensional materials: synthesis and applications
PDF
In-situ characterization of nanoscale opto-electronic devices through optical spectroscopy and electron microscopy
PDF
Analysis of carbon nanotubes using nanoelectromechanical oscillators
PDF
Raman spectroscopy of carbon nanotubes under axial strain and surface-enhanced Raman spectroscopy of individual carbon nanotubes
Asset Metadata
Creator
Cao, Xuan
(author)
Core Title
Printed and flexible carbon nanotube macroelectronics
School
Viterbi School of Engineering
Degree
Doctor of Philosophy
Degree Program
Materials Science
Publication Date
03/28/2017
Defense Date
03/07/2017
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
carbon nanotubes,flexible,macroelectronics,OAI-PMH Harvest,printing,thin-film transistor
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Zhou, Chongwu (
committee chair
), Armani, Andrea (
committee member
), Wu, Wei (
committee member
)
Creator Email
caox@usc.edu,caoxuan1234@gmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c40-350710
Unique identifier
UC11258394
Identifier
etd-CaoXuan-5147.pdf (filename),usctheses-c40-350710 (legacy record id)
Legacy Identifier
etd-CaoXuan-5147.pdf
Dmrecord
350710
Document Type
Dissertation
Rights
Cao, Xuan
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Access Conditions
The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA
Tags
carbon nanotubes
flexible
macroelectronics
thin-film transistor