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Single-wall carbon nanotubes separation and their device study

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Single-wall carbon nanotubes separation and their device study

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Single-Wall Carbon Nanotubes
Separation and Their Device Study

By
Hui Gui

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)
August, 2016

Copy right 2016 Hui Gui

i

Dedication
This dissertation is dedicated to my beloved family.

ii

Acknowledgement
I joined University of Southern California (USC) as a Ph.D student in 2011 and I have
enjoyed the time working and studying here over the past five years. Many people have
helped and encouraged me in my research and my personal life. Without them, I could
not make this dissertation possible.
First, I would like to give my sincere appreciation to my advisor Prof. Chongwu Zhou.
His enthusiasm to research, his rigorous scholarship and his persistence to work inspired
me to explore my research field and become a professional experimentalist. It is his
mentoring, support and patience that lead me to what I have achieved.
Meanwhile, I would like to thank Dr. Ming Zheng at National Institute of Standards
and Technology (NIST). My vision and knowledge was greatly boarded during working
together with Dr. Ming Zheng at NIST. I also thank Dr. Jeffery A. Fagan, Dr. Angela R.
Hight Walker, Dr. Jason K. Streit from NIST for their collaboration and valuable
discussion.
Moreover, I would also like to thank Dr. Qingwen Li from Suzhou Institute of
Nanotech and Nanobionics, Chinese Academy of Sciences. She introduced me to the
nanotube separation field and gave me suggestions and trainings. And thank Dr. Hongbo
Li and Dr. Hehua Jin for the guidance and friendship.
Furthermore, I would also like to thank my dissertation committee members Prof. Wei
Wu and Prof. Andrew Arman for their valuable instructions and suggestions, as well as
Prof. Stephen Cronin, Prof. Edward Goo for serving on my qualifying exam committee.

iii

In addition, I would like to thank all my former and current group members and friends:
Dr. Bilu Liu, Dr. Gang Liu, Dr. Jialu Zhang, Dr. Jia Liu, Dr. Hsiao-Kang Chang, Dr. Anuj
Madaria and Dr. Yi Zhang, Yue Fu, Dr. Haitian Chen, Dr. Yuchi Che, Zhen Li, Dr. Jing
Qiu, Dr. Shelley Wang, Dr. Maoqing Yao, Dr. Nappadol Aroonyadet, Dr. Mingyuan Ge,
Dr. Jiepeng Rong, Dr. Luyao Zhang, Dr. Xin Fang, Pyojae Kim, Younghyun Na, Rebecca
Lee, Liang Chen, Pattaramon Vuttipittayamongkol, Ahmad Abbas, Sen Cong, Yu Cao,
Xuan Cao, Fanqi Wu, Anyi Zhang, Chenfei Shen, Yuqiang Ma, Yihang Liu and Qingzhou
Liu. It has been a great time to be a part of the nanolab at USC, and thank you for sharing
those ups and downs with me. Your encouragement and friendship support me to stick to
my research and make my stay at USC a joyful journey.
Finally and most importantly, I would like to thank my husband Dongping Deng and
my parents for their unwavering love and support. Words cannot express my appreciation
to you. You are my forever motivation to go forward.

iv

Table of Contents
Dedication ..................................................................................................................................... i
Acknowledgement ....................................................................................................................... ii
List of figures .............................................................................................................................. vi
List of tables ................................................................................................................................ xi
Abstract ................................................................................................................................... xii
Chapter 1 Introduction ............................................................................................................... 1
1.1 Introduction of carbon nanotubes ................................................................................... 1
1.2 Physical structure of SWCNTs ........................................................................................ 2
1.3 Electronical structures of SWCNTs ................................................................................ 3
1.4 Electronic structure separation techniques .................................................................... 7
1.4.1 Ion-exchange chromatography (IEX) ........................................................................ 8
1.4.2 Density gradient ultracentrifugation (DGU) ........................................................... 10
1.4.3 Polymer selective extraction ..................................................................................... 12
1.4.4 Aqueous two-phase (ATP) separation ..................................................................... 13
1.5 Length sorting techniques .............................................................................................. 14
Chapter 1 References ............................................................................................................ 15
Chapter 2 Comparative study of gel chromatography separated Arc-discharge, HiPco and
CoMoCAT carbon nanotubes for thin-film transistor applications ................... 18
2.1 Introduction ..................................................................................................................... 18
2.2 Gel chromatography separation of SWCNTs ............................................................... 20
2.2.1 Gel chromatography nanotube separation method ................................................ 21
2.2.2 Gel chromatography separation results .................................................................. 22
2.3 Thin-film transistor performance and analysis ............................................................ 24
2.4. Application in display electronics ................................................................................. 37
2.5. Comparison between gel chromatography and DGU separated semiconducting
nanotubes ....................................................................................................................... 39
2.6 Conclusions ...................................................................................................................... 41
Chapter 2 References ............................................................................................................ 42
Chapter 3 Redox sorting of carbon nanotubes ....................................................................... 46
3.1. Introduction .................................................................................................................... 46
3.2. Redox sorting on aqueous-two phase separation ......................................................... 48
3.2.1 Redox modulation of nanotubes partitioning ......................................................... 48
3.2.2 Successive oxidation for bandgap separation ......................................................... 49
3.2.3 Summary of redox on ATP separation .................................................................... 55

v

3.3 Redox sorting on density gradient ultracentrifugation and gel chromatography ..... 56
3.4 Redox sorting on organic phase polyfluorene based extraction method .................... 61
3.5. Conclusions ..................................................................................................................... 63
Chapter 3 References ............................................................................................................ 63
Chapter 4 Facile and low-cost length sorting of single-wall carbon nanotubes by
precipitation and applications for thin-film transistors ...................................... 65
4.1 Introduction ..................................................................................................................... 65
4.2 Experimental methods .................................................................................................... 67
4.3 Results and discussion ..................................................................................................... 69
4.3.1 Polymer and salt precipitation of surfactant dispersed SWCNTs – a general
phenomenon ............................................................................................................... 69
4.3.2 Length sorting by polyelectrolytes ........................................................................... 70
4.3.3 ATP separation and thin-film transistors ............................................................... 77
4.4 Conclusions ...................................................................................................................... 82
Chapter 4 References ............................................................................................................ 83
Chapter 5 Conclusions and future directions ......................................................................... 86
5.1 conclusions ....................................................................................................................... 86
5.2 Future direction on separated semiconducting SWCNTs for flexible active-matrix
organic light-emitting diode display by screen printing .............................................. 87
Chapter 5 references ............................................................................................................. 89
Bibliography .............................................................................................................................. 91

vi

List of figures
Figure 1.1 (a) the illustration of SWCNTs structures. (b) The schematic representations
of SWCNT and MWCNT. (c) Armchair, zigzag and chiral SWCNTs. ...... 3
Figure 1.2 (a) The surface of the energy dispersion of graphene. (b) Dispersion of the
states of graphene. Inset is the Brillouin zone with high symmetry points.
.................................................................................................................... 5
Figure 1.3 (a) A first Brillouin zone of graphene with conic energy dispersions at six
K points. The allowed k ⊥ states in a SWNT are presented by dashed lines.
The band structure of a SWNTis obtained by cross-sections as indicated.
Zoom-ups of the energy dispersion near one of the K points are
schematically shown along with the cross-sections by allowed k ⊥ states and
resulting 1D energy dispersions for (b) a metallic SWNT and (c) a
semiconducting SWNT. ............................................................................. 6
Figure 1.4 The absorption spectra of the IEX separated 12-chirality semiconducting
fractions. ..................................................................................................... 9
Figure 1.5 DGU separation. (a) Schematic illustration of the DGU separated fractions
in a centrifuge tube with density gradients. (b) Absorption spectra of the
DGU separated metallic and separation fractions. ................................... 10
Figure 1.6 Illustration of the procedures of SWCNTs separation by multi-column gel
chromatography. ..................................................................................... 11
Figure 1.7 PFO chemical structure and photoluminescence excitation maps of PFO-
SWCNT sample in toluene solutions prepared using nanotubes grown by
the HiPco process as starting material showing the enrichment of
semiconducting nanotubes. ....................................................................... 13
Figure 1.8 Diagram of ATP system and the separation example of top phase and
bottom phase. ............................................................................................ 14
Figure 1.9 Length sorting techniques. (a) Illustration of SEC length sorting. (b)
Illustration of ultracentrifugation length sorting.

(c) Illustration of polymer
precipitation.

............................................................................................ 15
Figure 2.1 Raman shift of the arc discharge, HiPco and CoMoCAT nanotubes. ....... 21

vii

Figure 2.2 Comparison of gel-based column chromatographic separated nanotubes
synthesized by different methods. Optical absorption spectra of arc-
discharge nanotubes (a), HiPco nanotubes (b), and CoMoCAT nanotubes (c)
before (blue) and after (red) separation. Inset: Nanotube solutions after
separation. Length distribution of the separated semiconducting arc-
discharge nanotubes (d), HiPco nanotubes (e), and CoMoCAT nanotubes (f);
the average nanotube length is 540 nm, 617 nm, and 576 nm, respectively.
Inset: FE-SEM images of separated semiconducting nanotubes network
deposited on Si/SiO2 substrates with aminopropyltriethoxysilane (APTES)
functionalization, where the scale bar is 1 µm. (g) Schematic diagram of a
back-gated SN-TFT. (h) Optical microscope image of the SN-TFT array
fabricated on silicon substrate with 50 nm SiO2 acting as gate dielectric. ....
................................................................................................................... 23
Figure 2.3 Electrical properties of back-gated SN-TFTs using gel-based separated
semiconducting nanotubes synthesized with different methods. Normalized
transfer characteristics (ID/W–VG) of the SN-TFTs using semiconducting
arc-discharge nanotubes (a), HiPco nanotubes (b), and CoMoCAT
nanotubes (c) with various channel lengths (4, 10, 20, 50, and 100 μm) and
2000 μm channel width plotted in logarithm scale. Transfer characteristics
(red: linear scale, green: log scale) and gm–VG characteristics (blue) of a
typical SN-TFT (L = 10 μm, W = 2000 μm) using semiconducting arc-
discharge nanotubes (d), HiPco nanotubes (e), and CoMoCAT nanotubes (f).
(g)–(i) Output characteristics (ID–VD) of the same devices in (d)–(f). ...... 26
Figure 2.4 Statistical study and key device performance metrics comparison of SN-
TFTs using separated nanotubes with different synthetic methods. (a) Plot
of current density (Ion/W) versus inverse channel length for TFTs fabricated
on separated semiconducting nanotubes synthesized by arc-discharge (blue),
HiPco (red), and CoMoCAT (green) methods. Plot of (b) device resistance
and (c) average on/off ratio (Ion/Ioff) versus channel length for the same TFTs
characterized in (a). (d) Plots of on/off ratio (Ion/Ioff) versus drain voltage for
devices using three different kinds of semiconducting nanotubes with L =
50 μm and W = 1200 μm. (e) Trade-off between current density (Ion/W)
and on/off ratio (Ion/Ioff). (f) Relationship between device mobility and
channel length for three kinds of SN-TFTs. .............................................. 30
Figure 2.5 External OLED controlled by HiPco SN-TFTs. (a) Plot of the current through
the OLED (IOLED) versus VG with VDD= -8 V. The inset optical images show
the OLED intensity at certain gate voltages. And the inset schematic image

viii

is the diagram of the OLED control circuit. (b) IOLED-VDD Characteristics of
the OLED control circuit. Various curves correspond to various values of VG
from -10 to 0 V in 1 V steps. ...................................................................... 39
Figure 2.6 Comparison of key device performance metrics of SN-TFTs using
semiconducting arc-discharge nanotubes separated by DGU and gel-based
column chromatographic methods. (a) Optical absorption spectra of
semiconducting arc-discharge nanotubes separated by DGU (blue) and gel-
based column chromatographic (red) methods. (b) Current density (Ion/W)
versus inverse channel length for TFTs fabricated on semiconducting
nanotubes separated by DGU (blue) and gel-based (red) methods. Plots of
(b) average on/off ratio (Ion/Ioff) and (c) device mobility ( μdevice) versus
channel length for the same devices measured in (b). ............................... 41
Figure 3.1 Redox modulation of nanotube partition in 6% PEG + 6% DX two-phase
system. The same amount of arc-discharge SWCNTs (Hanwha) and an
identical surfactant composition 0.9% SC + 0.4% SDS are used in all cases.
Redox agent added in each case: 200 mM NaBH4 for the “reduced” regime;
70 mM NaBH4 for the “semi-reduced” regime, nothing for the “ambient”
regime; 0.5 mM NaClO for the “semi-oxidized” regime; and 2 mM NaClO
for the “oxidized” regime. .......................................................................... 49
Figure 3.2 Fractionation of HiPco nanotubes by successive oxidative extraction. (a)
Separation scheme and photographs of extracted fractions. (b) Optical
absorption spectra of the parent HiPco material and fractions successively
extracted from a PEG/DX ATP system with the aid of NaClO. “S1” and “S2”
are two successive fractions extracted into the top PEG phase from the
starting material, nanotubes remaining in the bottom DX phase are labelled
“M” fraction. ............................................................................................... 50
Figure 3.3 Spectroscopic characterization of fractions M3 to M6 successively extracted
from the M fraction shown in Figure 3.2. (a) Absorbance spectra of M3 to
M6. Vertical green lines indicate the wavelength positions of laser excitation
used for the resonance Raman measurement. (b) RBM profiles of M3 (black
traces) and M6 (magenta traces) measured at excitation wavelength of 457
nm, 514 nm, and 632 nm, respectively. The spectra are normalized by
integrated spectral area for easy comparison. ............................................. 52
Figure 3.4 Bandgap distributions of the extracted semiconducting (a) and metallic (b)
fractions. ..................................................................................................... 54

ix

Figure 3.5 Before (a) and after (b) photographs of a density gradient ultracentrifugation
separation with a range of added redox potential altering concentrations. The
separation of metallic (light blue) SWCNTs from the semiconducting
(bronze) SWCNTs is improved from the ambient condition by addition of
the small amount of oxidant NaClO (0.125 mM). Addition of 20 mM
reductant DTT reverses the nature of the SWCNTs isolated at the top of the
band. Spectra of aliquots collected from three of the tubes are shown in (c);
the location of the aliquots is given by the colored bar corresponding to the
trace color on the photograph, the pre-separation spectrum is given as the
black trace. .................................................................................................. 58
Figure 3.6 Gel chromatography elution pattern for (a) and (d): DTT–treated HiPco
tubes; (b) and (e): non-treated HiPco tubes, and (c) and (f): NaClO-treated
HiPco tubes. The three columns pictures in (a), (b) and (c) are taken at the
early, middle and later stage of the elution process. ................................... 60
Figure 3.7 Absorption spectra of HiPco SWCNT dispersions in toluene made with same
amount of PFO-bipy (1 mg / mL) and SWCNT (0.036 mg /mL) but under
different redox conditions. Blue trace: dispersion made with 10 mM vitamin
E added to the PFO-bipy /SWCNT mixture; black trace: control dispersion
with no redox agent added; red trace: dispersion made from the control by
adding 1/10 the sample volume of water, then bath sonication and
centrifugation to remove the newly formed aggregates. ............................ 62
Figure 4.1 (a) Schematic showing that adding polyelectrolyte (PMAA or PSS) to SDC-
dispersed SWCNTs leads to nanotube precipitation. (b) Molecular structures
of PMAA and PSS. (c) Length-sorting by forward sequential polymer
addition and SWCNT precipitation. ........................................................... 71
Figure 4.2 UV-vis-NIR absorption spectra of (a) 1% to 6% PMAA and (b) PSS
precipitated nanotubes. ............................................................................... 72
Figure 4.3 AFM images of nanotubes from the (a) 1%, (b) 2%, (c) 4%, and (d) 6%
PMAA fractions. The scale bar is 1 μm for all images. (e) Length
distributions of SWCNTs from the 1% and 4% PMAA fractions. (f) Average
nanotube lengths of the 1%, 2%, 4% and 6% PMAA fractions. ................ 74
Figure 4.4 AFM images of nanotubes from the (a) 1%, (b) 2%, (c) 4%, and (d) 6% PSS
fractions. The scale bar is 1 μm for all images. (e) Length distributions of the
1% and 4% PSS fractions. (f) Average nanotube lengths of the 1%, 2%, 4%
and 6% PSS fractions. ................................................................................ 75

x

Figure 4.5 (a) Schematic of reverse sequential precipitation of SWCNTs, where
precipitations using 6%, 4%, 2%, and 1% PMAA were conducted
sequentially. (b) Length distribution of SWCNTs obtained by forward and
reverse precipitation using 1% PMAA. ...................................................... 77
Figure 4.6. (a) Absorption spectra of unsorted (black trace) plasma torch SWCNTs and
the semiconducting SWCNT enriched 1-R fraction (red trace). (b) An SEM
image of the 1-R nanotube thin-film network on a Si/SiO2 substrate. The
scale bar is 2 μm. (c) Transfer characteristics of TFTs fabricated with the 1-
R fraction with L=4 μm and W=400 μm. The blue arrows indicate the
forward and reverse scanning directions. (d) On-current density versus
inverted channel length of TFTs fabricated with the 1-R fraction. (e) on/off
ratio versus channel length of TFTs. (f) Relationship of mobility and channel
length of TFTs made with the 1-R fraction. ............................................... 79
Figure 4.7 (a) SEM image of the 6-R nanotube thin-film network on a Si/SiO2 substrate.
The scale bar is 2 μm (b) On-current density versus inverted channel length
of TFTs fabricated with the 6-R fraction. (c) on/off ratio versus channel
length of TFTs fabricated with 6-R fraction. (d) Relationship of mobility and
channel length of TFTs made with the 6-R fraction. .................................. 82
Figure 5.1 (a) The schematic diagram of the 1 transistor active-matrix OLED display.
(b) The schematic of the TFT on PET substrate with the materials used for
each layer. ................................................................................................... 88
Figure 5.2 The screen printed TFTs transfer curves with high uniformity. ............... 88

xi

List of tables
Table 2.1 Comparison of the properties of arc-discharge, HiPco and CoMoCAT
separated semiconducting nanotubes…………………………………….37
Scheme 3.1 Redox chemistry…………………………………………………………47

xii

Abstract
Single-wall carbon nanotube (SWCNT), a one-dimensional material, since its
discovery in 1993, has attracted tremendous interest from fundamental science to
advanced technological research. Its extraordinary electrical properties, such as intrinsic
high carrier mobility, current-carrying capacitance, as well as flexibility and transparency
have made SWCNTs a promising candidate for the future semiconductor material
replacing the traditional material Si for applications in thin-film transistors, circuits,
computers, displays especially flexible and transparent touch screens and so on. However,
the synthesized nanotubes are a mixture of metallic and semiconducting nanotubes with
broad length distribution. Despite the progress on the synthesis of semiconducting
nanotubes, its purity and yield still could not satisfy the requirements of various
applications. Therefore post-synthesis separation is necessary. This dissertation studies
the separation of SWCNTs including their metallicity, bandgap and length, and their thin-
film transistor (TFT) performance.
To demonstrate that the separated SWCNTs are desirable for macroelectronics
application and to better understand those separation mechanisms , I have (1) developed
a platform for comparing the advantages of the mainstream types of SWCNTs on TFT
applications separated by gel chromatography; (2) enabled bandgap-dependent separation
of SWCNTs and enhanced the reproducibility of the common separation methods by
redox chemistry; (3) proposed a mechanism that redox-induced surfactant reorganization
will affect the nanotube buoyancy in a density gradient field, affinity to polymer matrices,
and solubility in organic solvents; (4) developed a facile and low-cost length sorting

xiii

method and improved the TFTs performance with long SWCNTs by this length sorting
method.
This dissertation is structured as follows. Chapter one gives the introduction of carbon
nanotubes, including the structure, electrical property and current SWCNTs separation
methods. Chapter 2 shows a systematically comparative study of the thin-film transistor
performance of semiconducting SWCNTs separated by gel chromatography. It
demonstrates the advantage of each kinds of SWCNTs and compared the important
parameters like the on-current density, on/off ratio and mobility and their trade-offs.
Chapter 3 talks about the redox sorting of carbon nanotubes. The role of redox chemistry
is discovered on all of the mainstream nanotube separation methods, which results in
further understanding the mechanisms of all the separation techniques. This discovery
also enables the bandgap separation of nanotubes and more robust and reproducible
separation processes. Chapter 4 reports a facile and low-cost length sorting method for
SWCNTs based on a general phenomenon of length-dependent SWCNTs precipitation
by polymer and/or salt. The TFTs using the length sorted uniform long semiconducting
SWCNTs by precipitation have demonstrated the advantages in high performance than
that of short semiconducting nanotubes. Chapter 5 is the brief summary of my research
work and the future direction.

1

Chapter 1
Introduction
1.1 Introduction of carbon nanotubes
It is believed that carbon nanotubes (CNTs) were first observed in 1950s.
1
But a world-
wide attention was attracted only after 1991 when Sumio Ijima from Japan published the
paper in Nature demonstrating the multiwall structure of nanotubes.
2
Since the discovery
of carbon nanotubes, tremendous researches have been done both on the fundamental
scientific study to a wide range of technological applications. Carbon nanotubes with their
one-dimensional structure, due to the quantum confinement properties, exhibiting
extraordinary mechanical, thermal and electrical properties. It is reported that CNTs have
a Young’s Modulus of 1TPa.
3
Meanwhile, the thermal capacity of CNTs is 10 times
higher than copper which is well known for the high thermal conductivity in metal.
4
They
are also considered as a promising next generation semiconducting material for various
applications, such as transistors, circuits, sensors and so on, as they possess high intrinsic
carrier mobility
5
and current-carrying capacity
6-8
.
Carbon nanotubes are hollow cylinders of carbon atoms connected by carbon-carbon
bonds. Depending on the number of walls, they can be classified into single-wall carbon
nanotubes (SWCNTs), double-wall carbon nanotubes (DWCNTs) and multi-wall carbon
nanotubes (MWCNTs). Typically, SWCNTs have a diameter ranging from 0.5 to 3nm
while MWCNTs have a diameter from 2 to 200 nm. Their length can be from tens of

2

nanometers to several centimeters. As a result, typically nanotubes have large aspect
ratios. However, the carbon atom arrangement or how the graphene sheet is rolled up
leads to different nanotube structures. Here we will focus on the structure of SWCNTs.
1.2 Physical structure of SWCNTs
Carbon nanotubes are conceptually viewed as one or more sheets of graphene (a single-
layer of carbon atoms arranged in a honeycomb structure) rolling up by a certain direction.
SWCNT is a hollow cylinder considered as rolling up by one layer graphene seamlessly.
The properties of SWCNTs are strongly dependent on the rolling direction of the
graphene layer. As illustrated below in figure 1, the unit base vectors of the hexagonal
graphene lattice are a 1 and a 2. The vectors a 1 and a 2 combined with integers n and m
specify the roll-up vector C h= na 1+ma 2. The pair of integers (n,m) describes the structure
of SWCNTs, which is also known as the chirality of the SWCNT. The roll-up angle
between a 1 and C h is called the chiral angle θ. When the nanotube chirality (n,m) is
specified, the nanotube diameter d and the chiral angle θ can be calculated by the
following formulas:

d
a √

θ cos
1
2 2 √ 2

2

Where a=0.246 nm is the lattice constant of the graphene sheet. The chiral angel θ is
between 0 to 30°. When θ=0, that is n=0, they are zigzag nanotubes. And when θ=30 or
n=m, they are armchair nanotubes. If θ is between 0 and 30°, they are chiral nanotubes.

3

Figure 1.1 (a) the illustration of SWCNTs structures. Adapted from Jason. K. Streit’s
thesis. (b) The schematic representations of SWCNT and MWCNT.
9
(c) Armchair, zigzag
and chiral SWCNTs.

1.3 Electronical structures of SWCNTs
10-11

To understand the electronic structure of SWCNTs, we first need to know the electrical
band structure of graphene as SWCNTs are viewed as the rolling-up of graphene. Carbon
atom has 6 electrons and four of them are out shell valance electrons. They have four

4

atomic orbitals: 2s, 2px, 2py, and 2pz. As carbon atoms bond together forming graphene,
the three 2s, 2px, and 2py orbitals form three hybrid sp
2
orbitals that lie in the same plane
and a 2pz orbital that is vertical to the sp
2
orbitals plane. The sp
2
orbitals correspond to
the σ bonds formed between the adjacent carbon atoms. While the 2pz orbital takes on the
π covalent bonds out the plane of graphene, forming the delocalized states which are
responsible for the electric transport properties.
The band structure of graphene derived from π orbitals can be calculated by the tight-
binding approximations
12
and the energy of electrons in the π orbitals of graphene E is as
follow:
∓

1 4 cos √ 3 2
cos 2
4cos

2

Where γ0 is called as the tight-binding integral or transfer integral which measures the
strength of exchange interaction between nearest neighbor atoms. In the reciprocal k-
space E0=0. The negative sign and positive sign denote the valence bands of graphene
formed by bonding π and antibonding π* orbitals, respectively.
The surface plot of the energy dispersion E is shown in Figure 1.2a. The dispersion of
the states of graphene is along the high symmetry line of the reciprocal lattice of graphene.
As at the six K points shown in Figure 1.2a, the bandgap is zero, the graphene is
considered as semi-metallic.

5

Figure 1.2 (a) The surface of the energy dispersion of graphene. (b) Dispersion of the
states of graphene. Inset is the Brillouin zone with high symmetry points.
13

When graphene is rolled up, it forms a SWCNT. Therefore, the band structure of
SWCNTs could be derived from graphene by confining a proper boundary condition.
Consider a SWCNT is infinitely long cylinder, there will be two wave vectors to describe
it: the parallel one k || along the SWCNT axis and the vertical k ⊥along the circumference
of the SWCNT. Since the wave function repeats itself as it rotates 2 π around a nanotube,
the boundary condition would be:
k ⊥ .Ch= 2 π d k ⊥ =2 πq,
where q is an integer and d is the diameter of a SWCNT. As a consequence, the value of
k ⊥ is quantized for certain allowed equal-spaced values. Then the 1D band structure of
SWCNTs can be obtained by the cross-sectional cutting of the 2D band structure of
graphene shown in Figure 1.3a. So, when one value of k ⊥ passes through one of the six
K points of the first Brillouin zone, the bandgap will be 0 and it is a metallic SWCNT as

6

shown in Figure 1.3b. Otherwise, it will be a semiconducting SWCNT with a bandgap
shown in Figure 1.3c.

Figure 1.3 (a) A first Brillouin zone of graphene with conic energy dispersions at six
K points. The allowed k ⊥ states in a SWNT are presented by dashed lines. The band
structure of a SWCNT is obtained by cross-sections as indicated. Zoom-ups of the energy
dispersion near one of the K points are schematically shown along with the cross-sections
by allowed k ⊥ states and resulting 1D energy dispersions for (b) a metallic SWNT and
(c) a semiconducting SWNT.
11

Based on the above theory, after calculation, SWCNTs could be divided into metallic
or semiconducting based on the indices n and m by n-m=3q+p:
(1) when n-m=0, that is, p=0, k ⊥ passes through one of the K points and it will have a
continuous electronic state between valance band and conduction band and
therefore they have a zero bandgap. It is metallic. That means all armchair
nanotubes are true metallic. When p=0, but q ≠0, it is semi-metallic with bandgap

7

of a few kbT (meV) at room temperature. Semi-metallic is still considered as
metallic SWCNT.
(2) The rest SWCNTs with p=1 or 2, are semiconducting nanotubes. And their bandgap
is about a few eV, which inversely proportional to their diameters.
1.4 Electronic structure separation techniques
Large scale and high quality carbon nanotubes are commonly produced with
carbonaceous materials exposed metal catalyst under elevated temperature. The three
popular synthesis methods are (1) arc discharge, where a high temperature arc discharge
is generated between two short-distance graphite electrodes ; (2) pulsed laser vaporization,
where a laser beam is used to vaporize the graphite target; (3) chemical vapor deposition
(CVD), where a furnace is heated for the gas phase carbon feedstock chemical reaction.
The synthesized nanotubes always have various chiralities.
As we know, according to the tight binding theory, SWCNTs can be divided into
metallic and semiconducting nanotubes based on their electric properties. Many of the
applications in nanoelectronics and macroelectronics, demand high purity of
semiconducting or metallic SWCNTs. However, most of the synthesized nanotubes are a
mixture of about 1/3 of metallic and 2/3 semiconducting nanotubes. To date, some
research groups have reported controlled growth of highly enriched semiconducting
nanotubes or even single-chirality nanotubes,
14-15
however the yield and purity is still not
desirable for real application. Therefore, post-synthesis is necessary for high purity
semiconducting SWCNTs enrichment.
Since 2003, many powerful methods for chirality separation have been demonstrated
in the past decade or so. The separation techniques are including metallic/semiconducting,

8

diameter and chirality separation of SWCNTS. Ion exchange chromatorgraphy (IEX)
16-
17
, density gradient ultracentrifugation (DGU)
18
, gel chromatography
19-20
, polymer
selective wrapping
21
and aqueous two phase extraction (ATP)
22
can be used to achieve
high purity semiconducting or even single chirality nanotubes. All the techniques can
achieve high purity of metallic/semiconducting or diameter separation.
1.4.1 Ion-exchange chromatography (IEX)
In 2003, Zheng et al.
17
reported first successful separation of metallic and
semiconducting nanotubes via ion-exchange chromatography (IEX) separation of DNA-
wrapped SWCNTs. SWCNTs are wrapped by a single-strand DNA (ss-DNA) through
sonication to form stable individually dispersed suspension. Then the SWCNTs
suspension with DNA solution are injected into an ion-exchange column containing
anions that are thought to bind to the negatively charged phosphate groups of DNA. Later
hundreds ss-DNA sequences were tested to identify SWCNT recognition sequences,
which resulted in the purification of all 12 major semiconducting single-chirality
SWCNTs from a synthetic mixture which is shown in Figure 1.4.
16
Through single-point
scanning mutation and sequence motif variation of previously identified ss-DNA
sequences for semiconducting tubes, recognition sequences for the metallic armchair (6,6)
and (7,7) have also been identified, resulting in the purification of the corresponding
SWCNT species.
23
The mechanism of the IEX separation is becoming much clearer after
years of investigations. DNA–SWCNT interaction and the resulting hybrid structure are
dependent on both DNA sequence and the chirality of SWCNT. Some hybrids have well-
ordered DNA folding structure when the DNA-sequence is properly chosen. IEX is
basically a selection method that picks out ordered DNA-SWCNT hybrids that show

9

minimum electrostatic and electrodynamic interactions with the ion exchange resin. This
selection process and molecular recognition involved afford single chirality SWCNTs
with such high purity levels that few other methods can offer. However, the cost of DNA
should be taken into consideration concerning scale up of the process.

Figure 1.4 The absorption spectra of the IEX separated 12-chirality semiconducting
fractions.
16

10

1.4.2 Density gradient ultracentrifugation (DGU)
Another approach is to exploit differences in the buoyant densities among SWCNTs
of different structures. In a density gradient, the balance between buoyant force and
centrifugal force results in SWCNTs with different buoyant densities to locate in their
respective layers in the gradient, and therefore separation can be achieved. Hersam’s
group developed density gradient ultracentrifugation (DGU) approach to separate
surfactant coated SWCNTs according to their electronic structure and diameter.
18
Figure
1.5 shows the DGU separation results. When a chiral surfactant sodium cholate was used
for SWCNTs dispersion, enrichment of left- and right-handed (6, 5) SWCNTs were also
demonstrated.
24
A big step forward was the development by Weisman’s group of
nonlinear DGU, which resulted in the purification of 10 semiconducting single-chirality
SWCNTs and enantiomerically enriched fractions of seven pairs of (n,m) species.
25

Figure 1.5 DGU separation. (a) Schematic illustration of the DGU separated fractions
in a centrifuge tube with density gradients. (b) Absorption spectra of the DGU separated
metallic and separation fractions.
18

11

Gel chromatography has also been adopted for metallic and semiconducting as well as
single-chirality purification of surfactant dispersed SWCNTs.
19,

20
In this approach, as
illustrated in Figure 1.6, allyl dextran-based polymer beads are employed as stationary
phase. Adsorption of surfactant–dispersed SWCNTs on the stationary phase is highly
sensitive to the structure of SWCNTs, and can be tuned by the choice of surfactant used
for SWCNT coating, and for the mobile phase. Temperature also has strong effect on the
adsorption. By modulating the adsorption and desorption processes via surfactant and
temperature control, Kataura’s group showed that 13 chiralities of SWCNTs could be
separated with purity up to 93% in a large scale.
19
Subsequent work from the same group
26
,
27
demonstrated that simultaneous single-chirality and enantiomer purification of many
semiconducting SWCNTs could be achieved.

Figure 1.6 Illustration of the procedures of SWCNTs separation by multi-column gel
chromatography.
19

12

1.4.3 Polymer selective extraction
Polyfluorene based selective extraction (Figure 1.7) is developed by Nicholas’s group
in 2007.
21
Polymers such as poly(9,9-dioctylfl uorene) (PFO) and its compounds,
poly(9,9-dioctylfl ourene- alt -(1,4-benzo-2,10,3-thiadiazole) (F8BT), regioregular
poly(3-alkylthiophene) (rr-P3AT), regioregular poly(3-dodecylthiophene) (rr-P3DDT),
are critical for dispersion of the s-SWCNTs in organic solvents (typically toluene and
xylene). As a small fraction of semiconducting nanotubes are selectively wrapped by the
polymer and therefore dispersed in the organic solvent during sonication, while the rest
of semiconducting and metallic nanotubes that are insoluble can be removed by
ultracentrifugation. This method is quick as the separation process takes only short time
of sonication and centrifugation. It can achieve high purity semiconducting SWCNTs
with high average nanotube length (1-2 µm). However, the yield is very low and typically
around 1%. Meanwhile, the polymers used to selective wrap semiconducting nanotubes
are still expensive with comparable price to DNA.

13

Figure 1.7 PFO chemical structure and photoluminescence excitation maps of PFO-
SWCNT sample in toluene solutions prepared using nanotubes grown by the HiPco
process as starting material showing the enrichment of semiconducting nanotubes.
21

1.4.4 Aqueous two-phase (ATP) separation
Recently, Zheng et al. introduced the aqueous two-phase (ATP) separation technique,
a technique widely used for biochemical separations, for SWCNT separation.
22
A
polymer aqueous two-phase (ATP) system exploits polymer phase separation
phenomenon to create two separate but permeable water phases of slightly different
structures.
28-29
As a result, hydration energy of a solute is rescaled differently in the two
phases, leading to uneven solute distribution. This can be exploited for separation. Both
surfactant-dispersed and DNA-dispersed SWCNTs can be separated by ATP to achieve

14

high purity metallic/semiconducting and single-chirality SWCNT purification.
30-34
Figure
1.8 shows the ATP system and the separation results. Overall, compared with other
existing CNT separation methods, the ATP separation method is quick, low cost, and
feasible to scale up.

Figure 1.8 Diagram of ATP system and the separation example of top phase and
bottom phase.
22

1.5 Length sorting techniques
The electronic structure separation techniques are very important and are widely
studied for all kinds of applications. However, many fundamental studies and applications
also need length sorted nanotubes. For example, in electronic application such as thin-
film transistors, long nanotubes is preferred for less sheet resistance and therefore high
performance. Several length sorting methods for SWCNTs, such as the size exclusion
chromatography (SEC)
35-36
, ultracentrifugation
37-38
and polymer precipitation
39
have
been reported as effective length sorting methods for SWCNTs (Figure 1.9). As the flow
speed of nanotubes in a gel with certain pore sizes depends on the nanotube lengths, SEC
can separate nanotubes by length with high resolution, but it needs expensive set of liquid

15

chromatography system to achieve small amount of length sorted nanotubes,
Ultracentrifugation takes advantage of the minor difference of the buoyancy of SWCNTs
with different length to keep them into layers with different density. This method has a
preparation scale yet the separation needs long time ultracentrifugation as well as low
resolution. The polymer precipitation method uses the depletion force between nanotubes
and polymers to induce nanotubes precipitation according to their length. This is a simple,
quick and low-cost method for nanotube length sorting with reasonable resolution.

Figure 1.9 length sorting techniques. (a) Illustration of SEC length sorting.
36
(b)
Illustration of ultracentrifugation length sorting.
38
(c) Illustration of polymer
precipitation.
39

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18

Chapter 2
Comparative study of gel
chromatography separated Arc-discharge,
HiPco and CoMoCAT carbon nanotubes for
thin-film transistor applications
2.1 Introduction
Since first discovered in 1991
1
, carbon nanotubes (CNT) have attracted huge attention
due to their extraordinary electrical properties such as high intrinsic carrier mobility and
current-carrying capacity
2-4
. Significant progress has been made toward making
nanoscale transistors based on the individual or aligned CNTs for nanoelectronics
5-12
, we
reported in 2009 that thin-films of separated carbon nanotubes can work as the channel
material for thin-film transistors (TFT) used in display back-panel electronics
13
. Other
popular TFT channel materials, such as amorphous silicon
14
and organic materials
15-17

suffer from their low carrier mobility, while polycrystalline silicon,
18-19
and metal
oxide
20-21
typically require high-cost and high-temperature processing. Compared with all
the materials above, CNT thin-films have the advantages of low-cost room-temperature
processing, superb transparency, excellent flexibility, high device performance, and
compatibility with printing technologies. During the past years, inspired by the density-

19

gradient ultracentrifuge (DGU) carbon nanotube separation method developed by Hersam
and his coworkers
22-23
, high-performance TFTs using pre-separated semiconducting
nanotubes have been fabricated by us and several other groups
13, 24-25
. In those previous
reports, transistors exhibiting high on/off ratio (>10
5
) as well as excellent current drive
capability (~ 1 µA/µm), and their applications such as digital logic circuits
26-27
,
transparent electronics
28
and active matrix organic light-emitting diode (AMOLED)
displays
29
have been demonstrated.
Recently, several groups
30-31
have reported a gel-based column chromatographic
nanotube separation method, which is very simple and inexpensive. By this method, high-
purity semiconducting and even single chirality nanotubes
32-33
can be separated, and
devices fabricated using gel-based separated nanotubes show excellent electrical
performance
34
. Due to these merits, gel-based separated semiconducting nanotubes look
very promising for TFT applications such as display electronics. In spite of the significant
progress reported so far, many interesting issues remain to be studied. For example,
among all the mainstream nanotubes, which kind of nanotubes is most suitable for TFT
applications? Is the nanotube diameter a key factor affecting the gel-based separated
nanotube thin-film transistor (SN-TFT) performance? Do gel-based separated nanotubes
have electrical properties as good as DGU-based separated nanotubes?
To answer the above-mentioned questions, we report a comparative and systematic
study of three kinds of mainstream carbon nanotubes separated using low-cost gel-based
column chromatography for thin-film transistor applications. Our work includes the
following essential components. (1) We carried out gel-based column chromatography
for arc-discharge nanotubes (Carbon Solutions, Inc), HiPco nanotubes (Unidym, Inc), and
CoMoCAT nanotubes (Sigma Aldrich, Inc). High-purity semiconducting nanotubes were

20

achieved; (2) SN-TFTs were fabricated using the three kinds of gel-based separated
semiconducting nanotubes, and key device performance metrics such as on-current
density, on/off ratio, sheet resistance and device mobility are directly compared. Based
on the detailed analysis, we have revealed a trade-off between transistor mobility and
on/off ratio depending on the nanotube diameter, and find that large-diameter nanotubes
offers high mobility, while small-diameter nanotubes can provide high on/off ratio; (3) In
addition, we have also compared the electrical properties of gel-based and DGU-based
semiconducting nanotubes, and similar electrical performance was observed for both
kinds of semiconducting nanotubes. Our gel-based SN-TFT platform shows significant
advantages over conventional platforms with respect to cost, scalability, reproducibility,
and device performance, and suggests a practical and realistic approach for nanotube-
based AMOLED display applications.
2.2 Gel chromatography separation of SWCNTs
Three kinds of carbon nanotubes, namely arc-discharge nanotubes, HiPco nanotubes,
and CoMoCAT nanotubes were selected and studied in this work. Figure 2.1 shows the
Raman spectra of the three kinds of nanotubes. These three kinds of nanotubes have very
high quality, as their Raman G/D ratio is similar (G/D ratio is 35, 50, and 28 for arc-
discharge, HiPco and CoMoCAT semiconducting nanotubes), but very distinct diameter
distribution. Arc-discharge nanotubes have diameters about 1.3 nm to 1.7 nm, which is
larger than HiPco nanotubes (0.7 nm to 1.2 nm) and CoMoCAT nanotubes (~ 0.7 nm).
Due to this diameter difference, different optical and electrical properties were observed
for these three kinds of separated nanotubes, which will be discussed later in this chapter.

21

Figure 2.1 Raman shift of the arc discharge, HiPco and CoMoCAT nanotubes.

2.2.1 Gel chromatography nanotube separation method
To carry out the gel chromatography separation of the three types of nanotubes,
individually dispersed nanotube solution is prepared successful. Carbon nanotube
dispersions of the three type are prepared as follows: Arc-discharge nanotubes were
dispersed in water with the assistant of sodium cholate (SC) with a concentration of 1
mg/mL, while HiPco and CoMoCAT nanotubes were dispersed in aqueous solution
assisted by 1% sodium dodecyl sulfate (SDS, Sigma-Aldrich (99%)) at 0.3 mg/ml and 1
mg/mL, respectively. All three kinds of nanotubes were sonicated using a tip-type
ultrasonic homogenizer (Sonicator 3000, Misonix) for 2 hours at 9 Watts in a water/ice
bath. After sonication, the solution was centrifuged to remove any possible bundles or
impurities (20,000 rpm for 3 hour at 14 °C). The resulting supernatant was collected as
Arc-discharge, HiPco and CoMoCAT SWNT solutions.
To do gel-based column chromatography for SWCNTs dispersion, first, Sephacryl
medium (GE Healthcare, Inc.) was filled into a typical column (30 cm in length and 2 cm
in width). Second, the column was equilibrated by flushing with 1% SDS solution. The

200 300 1400 1600 1800
Intensity (a.u.)
Raman shift (cm
-1
)
Arc discharge
HiPCO
CoMoCAT

22

nanotube solution was then added to the column. After that 1% SDS solution was used to
elute the column, and metallic nanotubes were sorted out during this elution step.
Following that, for arc-discharge and HiPco nanotubes, 1% SC solution was added into
the column to wash out the remaining semiconducting nanotubes, while 1% SDS+0.04%
SC solution was used to get semiconducting CoMoCAT nanotubes.
2.2.2 Gel chromatography separation results
After gel chromatography separation, we characterized the separated three types of
nanotubes. Figure 2.2 shows the comparison of gel-based column chromatographic
separated arc-discharge, HiPco and CoMoCAT nanotubes. The optical absorption spectra
before (blue) and after (red) gel-based separation are plotted in Figs. 2.2a–2.2c (a: arc-
discharge nanotubes; b: HiPco nanotubes; c: CoMoCAT nanotubes). Based on these
curves, one can estimate the purity of the separated semiconducting nanotubes, which is
98%, 92% and 95%, respectively. The purity of separated semiconducting nanotubes is
calculated using the optical absorption spectroscopy evaluation
35
, which is well accepted
by researchers in the carbon nanotube field. This estimation method typically has an error
of around a few percent. In this regard, the differences in the purities we obtained with
one round of gel chromatography separation are within the error margin of the absorption
spectroscopy evaluation. The diameter information can also be extracted, where arc-
discharge semiconducting nanotubes exhibit a diameter range of 1.3 nm to 1.7 nm, while
HiPco and CoMoCAT nanotubes show diameters in the range 0.8 nm to 1 nm and ~0.7
nm, respectively. Because of their different diameter distribution, these three kinds of
separated nanotubes exhibit different optical properties, which can be seen from the peak
positions in the optical absorption spectra, as well as the color of the separated

23

semiconducting nanotube solutions shown in the insets of Figs. 2.2a–2.2c (light brown
for arc-discharge semiconducting nanotubes; dark green for HiPco semiconducting
nanotubes; purple for CoMoCAT semiconducting nanotubes).

Figure 2.2 Comparison of gel-based column chromatographic separated nanotubes
synthesized by different methods. Optical absorption spectra of arc-discharge nanotubes
(a), HiPco nanotubes (b), and CoMoCAT nanotubes (c) before (blue) and after (red)
separation. Inset: Nanotube solutions after separation. Length distribution of the separated
semiconducting arc-discharge nanotubes (d), HiPco nanotubes (e), and CoMoCAT
nanotubes (f); the average nanotube length is 540 nm, 617 nm, and 576 nm, respectively.
Inset: FE-SEM images of separated semiconducting nanotubes network deposited on
Si/SiO2 substrates with aminopropyltriethoxysilane (APTES) functionalization, where
the scale bar is 1 µm. (g) Schematic diagram of a back-gated SN-TFT. (h) Optical
microscope image of the SN-TFT array fabricated on silicon substrate with 50 nm SiO2
acting as gate dielectric.

24

Other than purity and diameter, nanotube length also plays a crucial role in nanotube
thin-film transistor performance. To characterize the length distribution of the three kinds
of gel-based separated semiconducting nanotubes, more than one hundred tubes from
each kind were imaged and measured by field-emission scanning electron microscopy
(FE-SEM), and the histograms are shown in Figure 2.2d–2.2f. From these plots, one can
find that arc-discharge nanotubes, HiPco nanotubes, and CoMoCAT nanotubes show
similar average nanotube lengths, which are 540 nm, 617 nm and 576 nm, respectively.
This similarity of nanotube length distribution is due to the fact that similar nanotube
dispersion recipe was employed for all three kinds of nanotubes. As all these three kinds
of separated nanotubes are similar in terms of semiconducting purity and nanotube length,
they can be the ideal materials to study the effect of diameter on the electrical performance
of SN-TFTs.
2.3 Thin-film transistor performance and analysis
To fabricate SN-TFT devices, high density, uniform separated nanotube thin films
were deposited on Si/SiO2 substrates using the solution-based aminosilane-assisted
separated nanotube deposition technique.
13
FE-SEM was used to inspect the samples after
nanotube assembly and the SEM images of the arc-discharge, HiPco, and CoMoCAT
semiconducting nanotubes deposited on Si/SiO2 substrates are shown in the inset of Figs.
2.2d–2.2f, respectively. The nanotube deposition recipes were carefully adjusted so as all
these three kinds of nanotubes had similar area nanotube density, which was measured to
be 32–41 tubes/ μm
2
for arc-discharge nanotubes, 27–38 tubes/ μm
2
for HiPco nanotubes,
and 26–36 tubes/ μm
2
for CoMoCAT nanotubes. After nanotube assembly, the deposited
separated nanotube thin films were used for back-gated SN-TFTs fabrication. 50 nm SiO2

25

was used to act as the back-gate dielectric. The source and drain electrodes were patterned
by photo-lithography, and 1 nm Ti and 50 nm Pd were deposited followed by a lift-off
process to form the source and drain metal contacts. Finally, since the separated nanotube
thin film cover the entire wafer, in order to achieve accurate channel length and width,
and to remove any possibility of leakage in the devices, one more step of photo-
lithography plus O2 plasma treatment was used to remove the unwanted nanotubes outside
the device channel regions. A schematic diagram of the back-gated SN-TFTs and the
optical microscope image of a fabricated SN-TFT array are shown in Figs. 2.2g and 2.2h,
respectively.
Electrical performance of gel-separated arc-discharge, HiPco and CoMoCAT TFTs is
compared in Figure 2.3. Such SN-TFTs were made with channel width (W) of 200, 400,
800, 1200, 1600, and 2000 μm, and channel length (L) of 4, 10, 20, 50, and 100 μm. Based
on these devices, we have carried out systematic measurement and analysis of the
electrical performance of the SN-TFTs. Figs. 2.3a–2.3c show the normalized transfer
characteristics (ID/W-VG) of the SN-TFTs using arc-discharge (Fig. 2.3a), HiPco (Fig.
2.3b) and CoMoCAT (Fig. 2.3c) semiconducting nanotubes with various channel lengths
(4, 10, 20, 50, and 100 μm) and fixed channel width (2000 μm) plotted in logarithm scale.
All the curves were measured at VD = 1V. From the figures, the following behaviors can
be observed: (1) Devices from all nanotube samples show p-type field-effect behavior
and very high on/off ratios; (2) As the device channel length increases, the on/off ratio
increases while the on-current decreases. In addition, all three kinds of devices exhibit
on/off ratio higher than 10
6
when the channel length is longer than 50 μm; (3) The devices
using larger-diameter (arc-discharge > HiPco > CoMoCAT) semiconducting nanotubes

26

exhibit better on-current but lower on/off ratio than the devices using smaller-diameter
semiconducting nanotubes.

Figure 2.3. Electrical properties of back-gated SN-TFTs using gel-based separated
semiconducting nanotubes synthesized with different methods. Normalized transfer
characteristics (ID/W–VG) of the SN-TFTs using semiconducting arc-discharge nanotubes
(a), HiPco nanotubes (b), and CoMoCAT nanotubes (c) with various channel lengths (4,
10, 20, 50, and 100 μm) and 2000 μm channel width plotted in logarithm scale. Transfer
characteristics (red: linear scale, green: log scale) and gm–VG characteristics (blue) of a
typical SN-TFT (L = 10 μm, W = 2000 μm) using semiconducting arc-discharge
nanotubes (d), HiPco nanotubes (e), and CoMoCAT nanotubes (f). (g)–(i) Output
characteristics (ID–VD) of the same devices in (d)–(f).

Figs. 2.3d–2.3f exhibit the transfer characteristics (red: linear scale, green: log scale)
and gm-VG characteristics (blue) of typical SN-TFTs using three kinds semiconducting
nanotubes measured at VD = 1V. All the devices have a channel length of 10 μm and width
h
-10 -5 0 5 10
10
-16
10
-14
10
-12
10
-10
10
-8
10
-6
V
D
= 1 V
HiPco
L = 4 m
L = 10 m
L = 20 m
L = 50 m
L = 100 m

I
D
/ W ( A/ m)
Gate Voltage (V)
-10 -5 0 5 10
10
-14
10
-12
10
-10
10
-8
10
-6
V
D
= 1 V
Arc-discharge
L = 4 m
L = 10 m
L = 20 m
L = 50 m
L = 100 m

I
D
/ W ( A/ m)
Gate Voltage (V)
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
-10 -5 0 5 10
0
100
200
300
400
500
600
700
800
V
D
= 1 V
Arc-discharge

Gate Voltage (V)
Drain Current ( A)
0
20
40
60
80
100
120
140
Transconductance ( S)
Drain Current (A)
-10 -5 0 5 10
0
20
40
60
80
100
120
140

Gate Voltage (V)
Drain Current ( A)
0
10
20
30
40
50
60
70
80
Transconductance ( S)
Drain Current (A)
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
V
D
= 1 V
HiPco

d
-10 -5 0 5 10
0
5
10
15
20
25
30
35
40

Gate Voltage (V)
Drain Current ( A)
0
4
8
12
16
20
Transconductance ( S)
Drain Current (A)
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
V
D
= 1 V
CoMoCAT

-5 -4 -3 -2 -1 0
-3500
-3000
-2500
-2000
-1500
-1000
-500
0
Arc-discharge
V
G
from -10 V to 0 V
with a step of 1 V

Drain Current ( A)
Drain Voltage (V)
-5 -4 -3 -2 -1 0
-250
-200
-150
-100
-50
0
HiPco
V
G
from -10 V to 0 V
with a step of 1 V

Drain Current ( A)
Drain Voltage (V)
-5 -4 -3 -2 -1 0
-60
-50
-40
-30
-20
-10
0
CoMoCAT
V
G
from -10 V to 0 V
with a step of 1 V

Drain Current ( A)
Drain Voltage (V)
-10 -5 0 5 10
10
-16
10
-14
10
-12
10
-10
10
-8
V
D
= 1 V
CoMoCAT
L = 4 m
L = 10 m
L = 20 m
L = 50 m
L = 100 m

I
D
/ W ( A/ m)
Gate Voltage (V)

27

of 2000 μm. Based on these plots, one can find the key device performance metrics of
these three devices. For the arc-discharge SN-TFT (Fig. 2.3d), the on-current density
(Ion/W) at VD = 1V and VG = -10V is measured to be 0.34 μA/ μm, and on/off ratio is 2
×10
4
. The transconductance (gm) can also be extracted from the maximum slope of the
transfer characteristics, which is 113 μS. For the HiPco SN-TFT (Fig. 2.3e), on-current
density is 0.066 μA/ μm, on/off ratio is 3.6×10
6
, and transconductance is 33 μS. For
CoMoCAT SN-TFT (Fig. 2.3f), the on-current density, on/off ratio, and transconductance
are calculated to be 0.0175 μA/ μm, 1.6×10
7
, and 10.6 μS, respectively. The corresponding
output characteristics (ID-VD) of these three SN-TFTs are also plotted in Figs. 2.3g–2.3i,
respectively. Under small VD biases, the devices exhibit linear behavior, indicating that
ohmic contacts are formed between the metal electrodes and the nanotubes. Saturation
behaviour was observed when more negative VD was applied, indicating nice field-effect
operation.
As mentioned previously, the separated arc-discharge, HiPco, and CoMoCAT
nanotubes have distinctively different diameters, but are similar in length and network
density. To get a more comprehensive understanding of the diameter dependent electrical
performance behaviour, we have compared the key device performance metrics such as
on-current density, channel sheet resistance, on/off ratio, and device mobility for SN-
TFTs based on those three kinds of gel-based separated. Figure 2.4 summarizes the results
after the measurement of 180 SN-TFTs with different semiconducting nanotube
diameters, and various channel lengths and channel widths. Figure 2.4a shows the
normalized on-current densities (Ion/W) of the transistors with various channel lengths
measured at VD = 1 V and VG = -10 V, showing that the on-current density is
approximately inversely proportional to the channel length for all three kinds of

28

semiconducting nanotubes. The highest on-current density is measured to be 1 μA/ μm,
which comes from SN-TFTs using separated arc-discharge semiconducting nanotubes
with a channel length of 4 μm. Overall, with the same device dimension, SN-TFTs using
arc-discharge nanotubes provides about 5 times higher on-current density than the ones
using HiPco nanotubes, and about 17 times higher on-current density than the ones with
CoMoCAT nanotubes. This conclusion is also consistent with the data shown in Fig. 2.3.
To understand the reason for this on-current density difference, we have analyzed the
contact resistivity and channel sheet resistance of these three different kinds of devices
using the transfer length method (TLM). For each transistor, we know that the total device
resistance (Rtot) is equal to the sum of the contact resistance (Rc) and channel resistance
(Rch). As Rch = R □L/W, where R □ is the sheet resistance of the separated nanotube film,
the total resistance can be described as: Rtot = Rc + R □L/W or RtotW = RcW+ R □L. This
means that at fixed channel width, the scaled device resistance (RtotW) follows a linear
relationship with the channel length, while the slope stands for the sheet resistance (R □)
and the intercept corresponds to the scaled contact resistance (RcW). Therefore, using the
scaled device resistance data obtained at gate bias of -10 V with different channel length,
we can derive the scaled contact resistance and channel sheet resistance of the three
different kinds of SN-TFTs, and the results are plotted in Figure 2.4b. From this figure,
one can find that the contact resistances for SN-TFTs are negligible compared with
channel sheet resistance: The calculated channel sheet resistances for arc-discharge SN-
TFTs, HiPco SN-TFTs, and CoMoCAT SN-TFTs are 0.28 M Ω/ □, 1.10 M Ω/ □ and 6.52
M Ω/ □, respectively. As the sheet resistance is dominated by the tube-to-tube junction
resistance
36
, we can conclude that large-diameter nanotubes provide smaller junction
resistance than small-diameter nanotubes. One contributing factor can be that large-

29

diameter nanotubes have larger tube-to-tube contact area, and therefore smaller junction
resistance. Other factors may include how holes transport from one nanotube to another,
which needs further study for a thorough understanding. Overall, we find that SN-TFTs
using large-diameter nanotubes are superior to the ones using small-diameter nanotubes
in terms of channel sheet resistance, which is also the reason why higher on-current
density is observed for devices fabricated with larger-diameter nanotubes. As noted
before, the differences in the purities of our three kinds of nanotubes are within the error
margin of the absorption spectroscopy evaluation. In addition, the metallic nanotube
density is way below the percolation threshold to form a conductive path from source to
drain, and we therefore believe the above mentioned difference in on-current density
mainly comes from the semiconducting nanotube networks.
Besides on-current density, the other important device parameter is current on/off ratio,
which is calculated as the current at VG = –10 V divided by the minimum current measured
for VD = 1 V; the results are plotted in Fig. 2.4c. From this plot, one can find that as the
channel length increases, the average on/off ratio of all three kinds of SN-TFTs increases,
which is due to the decrease in the probability of percolative transport through metallic
nanotubes as the device channel length increases. It is worth noting that for HiPco and
CoMoCAT SN-TFTs, we observe that the average on/off ratio decreases slightly when
the channel length is longer than 50 μm. This is because although both the on-current and
off-current should decrease when the channel length increases, when the channel length
is sufficiently long, the off-current will reach the noise level of the measurement
equipment (Agilent 4156 B Semiconducting Parameter Analyzer with an accuracy of 1
pA), and then would not decrease further. Therefore, we observed a slightly decrease of
the on/off ratio for long channel devices.

30

Figure 2.4 Statistical study and key device performance metrics comparison of SN-
TFTs using separated nanotubes with different synthetic methods. (a) Plot of current
density (Ion/W) versus inverse channel length for TFTs fabricated on separated
semiconducting nanotubes synthesized by arc-discharge (blue), HiPco (red), and
CoMoCAT (green) methods. Plot of (b) device resistance and (c) average on/off ratio
(Ion/Ioff) versus channel length for the same TFTs characterized in (a). (d) Plots of on/off
ratio (Ion/Ioff) versus drain voltage for devices using three different kinds of
semiconducting nanotubes with L = 50 μm and W = 1200 μm. (e) Trade-off between
current density (Ion/W) and on/off ratio (Ion/Ioff). (f) Relationship between device mobility
and channel length for three kinds of SN-TFTs.
10
3
10
4
10
5
10
6
10
7
0.0
0.2
0.4
0.6
0.8

I
on
/W ( A/ m)
Arc discharge
HiPco
CoMoCAT

On/Off ratio
0 20406080 100
1
10
Arc discharge
HiPCO
CoMoCAT

Mobility (cm
2
V
-1
S
-1
)
Channel Length ( m)
0 20406080 100
0
200
400
600
800
Arc discharge
HiPco
CoMoCAT

Resistance (M  m)
Channel Length ( m)
0.00 0.05 0.10 0.15 0.20 0.25
0.0
0.2
0.4
0.6
0.8
1.0 Arc discharge
HiPCO
CoMoCAT

I
on
/W ( A/ m)
1/L ( m
-1
)
0 20406080 100
10
2
10
3
10
4
10
5
10
6
10
7
10
8
Arc discharge
HiPco
CoMoCAT

On/Off ratio
Chanel Length ( m)
-1 -2 -3 -4 -5
10
2
10
3
10
4
10
5
10
6

Arc discharge
HiPco
CoMoCAT
On/Off ratio
Drain Voltage (V)
a b
ef
cd

31

In addition, from Fig. 2.4c, we also observe that for the same channel length, the on/off
ratios of arc-discharge SN-TFTs is lower than the on/off ratios of HiPco and CoMoCAT
SN-TFTs, which means large-diameter SN-TFTs have higher off-current than small-
diameter SN-TFTs. There are two possible sources for the off-current, which are the
percolative transport through metallic nanotubes and the thermal excitation of carriers
through semiconducting nanotubes.
37
For short channel devices, the former source is
believed to be the main reason of the off-current because the channel length is comparable
to the length of nanotubes. On the other hand, when the channel length is much longer
than nanotube length, based on the 2D stick model
38
, the percolation threshold density
(N) can be expressed as the following equation:
2
)
236 . 4
(
1
l
N

 ,
where l is the average length of the nanotubes. If we take arc-discharge nanotubes as
an example (l = 540 nm), N can be calculated to be 20 tubes/ μm
2
. As only about 2% of
the separated arc-discharge nanotubes are metallic, in order to form a metallic pathway
for the long channel devices, the total nanotube density needs to reach 1000 tubes/ μm
2
,
which is much higher than the actual nanotube density we measured (32–41 tubes/ μm
2
).
Therefore, percolative transport through metallic nanotubes is negligible for long channel
devices, which suggests that the off-current mainly comes from the thermal excitation of
carriers. As we know, the bandgap (Eg) of a nanotube is inversely proportional to the
diameter (d) of the nanotube, which can be written as Eg = 2 γoac-c/d, where γo is the C–C
tight-binding overlap energy, and ac-c is the nearest-neighbor C–C distance (0.142 nm).
Based on literature, γo is around 2.7 eV,
3-4
so that the bandgap ranges for arc-discharge
nanotubes, HiPco nanotubes and CoMoCAT nanotubes are 0.45–0.59 eV, 0.77–0.95 eV,

32

and 1.09 eV, respectively. Thermal excitation can be strongly suppressed for small-
diameter separated nanotube TFTs because of their large bandgap. However, for large-
diameter nanotubes (arc-discharge nanotubes), due to their small bandgap, non-negligible
amounts of thermally excited carriers can be present in the semiconducting nanotubes and
flow through the channel to form a non-negligible off-current, which leads to lower on/off
ratio than small-diameter nanotube devices (HiPco and CoMoCAT SN-TFTs). It is worth
noting that due to the above-mentioned noise limit of the equipment and the high device
resistance for CoMoCAT nanotube thin films, CoMoCAT SN-TFTs exhibit lower on/off
ratio than HiPco SN-TFTs when the channel lengths are long. However, the off-current
of CoMoCAT SN-TFTs is actually lower than HiPco SN-TFTs before it reaches the noise
level at shorter channel lengths.
The conclusion above is further supported by the results shown in Fig. 2.4d, where
typical devices using three kinds of nanotubes with the same channel dimension (L = 50
μm, W = 1200 μm) were characterized. This plot shows that the on/off ratio of the arc-
discharge SN-TFTs decreases when the source-to-drain voltage increases, while the
on/off ratios for HiPco and CoMoCAT SN-TFTs remain the same under different drain
biases. The decrease in the on/off ratio for arc-discharge SN-TFTs can be attributed to
the fact that carriers will gain more energy under a high source-to-drain bias, and therefore,
more carriers will be able to transport through the channel due to thermal excitation,
which will result in a higher off-current and a lower on/off ratio. In contrast, the wide
bandgap of HiPco and CoMoCAT semiconducting nanotubes can effectively suppress the
thermal excitation even under a high source-to-drain voltage, and thus a nearly constant
on/off ratio is observed. This phenomenon further proves that instead of percolative
transport through metallic nanotubes, thermal excitation of carriers is the main source of

33

the off-current for long channel SN-TFTs. This diameter-dependent on/off ratio
behaviour also suggests that small-diameter nanotubes are preferred for applications
which need high biases and high on/off ratios.
Interestingly, the data shown in Figs. 2.4a and 2.4c also reveal a trade-off between
drive-current and on/off ratio, both of which are key parameters for display applications.
On the one hand, larger-diameter nanotubes and shorter channel length can help to
achieve higher on-current density due to the small sheet resistance. On the other hand, the
narrow bandgap associated with large-diameter nanotubes will give rise to more thermal
excitation, and shorter channel length can also increase the possibility of percolative
transport through metallic nanotubes, thus leading to higher off-current and lower on/off
ratio. The plot in Fig. 2.4e clearly illustrates this trade-off. One of the most promising
applications of carbon nanotube thin-film transistors is AMOLED display electronics,
where current drive capability and on/off ratio are the most crucial parameters. Unlike
liquid crystal displays (LCD), where a voltage-controlled circuit is applied, a current-
controlled circuit is required for AMOLED displays, which means the current flow
through the driving transistors will directly go through the OLED pixels, and therefore,
determine the output light intensity of the OLED. For this reason, high current drive
capability is required for TFTs used in AMOLED displays to create sufficient light
intensity within a certain area. For a 40-inch high-definition television (HDTV), in order
to reach a brightness of 600 Cd/m
2
, a current of about 12 μA needs to be delivered to a
pixel with an area of 153 × 460 μm
2
,
39
which means that, if a two-transistor control circuit
is applied in each pixel, a minimum unit areal current drive of 0.00034 μA/ μm
2
needs to
be satisfied for the driving transistors in the circuitry. Besides driving current, on/off ratio
is another crucial parameter for display electronics. As progressive scanning is used in

34

most display circuits nowadays, each pixel will only be programmed for a very short time
during one frame time. In order to have a smooth picture, the switching transistors of each
display pixel need to have high enough on/off ratio to keep the light intensity constant.
The larger the display, the higher the required on/off ratio, and based on the literature
40
,
on/off ratios need to reach 10
6
for 256 grayscale 1080P displays. From Figure 2.4e, we
can see that for the three kinds of SN-TFTs, if an on/off ratio of 10
6
is required, the highest
measured on-current density for arc-discharge SN-TFTs is 0.17 μA/ μm, which comes
from a device with a channel length of 20 μm, while the highest on-current densities for
HiPco and CoMoCAT SN-TFTs are 0.11 μA/ μm (L = 10 μm) and 0.014 μA/ μm (L = 10
μm), respectively. In addition, the values of the maximum on-current drive per unit area,
are 0.0085 μA/ μm
2
for arc-discharge SN-TFTs, 0.011 μA/ μm
2
for HiPco SN-TFTs, and
0.0014 μA/ μm
2
for CoMoCAT SN-TFTs. One interesting finding is that although having
the same channel geometry, arc-discharge SN-TFTs exhibit about 5 times higher on-
current density than HiPco SN-TFTs, and these two kinds of SN-TFTs provide similar
maximum on-current drive per unit area for devices with an on/off ratio higher than 10
6
.
This is due to the trade-off between on-current density and device on/off ratio. Overall,
based on our analysis, all three kinds of SN-TFTs meet the basic requirements for
transistors used in AMOLED displays. As a proof of concept, we have connected a typical
HiPco SN-TFTs to an external OLED to control the output light intensity, and the results
are shown in Supporting Information.
Besides the on-current density and on/off ratio, we have also characterized device
mobility ( μdevice) for all three kinds of SN-TFTs. Device mobility of the SN-TFTs is
extracted following the equation:

35

dm
device
Dox g Dox
dI g LL
VC W dV V C W
   

where L and W are the device channel length and width, VD = 1 V, and Cox is the gate
capacitance per unit area. Here, we use the following equation
41, 42
:
1
11 00
0
0
sinh(2 / ) 1
ln ( )
2
ox
ox Q
ox
t
CC
R




  
 






to calculate the gate capacitance as it considers the electrostatic coupling of nanotubes.
1
0

 stands for the density of nanotubes and is measured to be around 10 tubes/ μm,
CQ=4.0 × 10
-10
F/m is the quantum capacitance of nanotubes.
43
And
0 ox
  =3.9 × 8.85×
10
-14
is the dielectric constant at the interface. The device mobilities of three kinds of SN-
TFTs are plotted in Fig. 2.4f. The arc-discharge SN-TFTs give the highest mobility, which
is around 17 cm
2
/Vs, whereas HiPco and CoMoCAT SN-TFTs show lower mobilities
around 5 cm
2
/Vs and 1 cm
2
/Vs, respectively. These data illustrate that large-diameter SN-
TFTs provide higher mobility than small-diameter SN-TFTs, which is consistent with the
device sheet resistance analysis discussed above. We note that the mobility of around 17
cm2/Vs we report here for separated arc discharge tubes is lower than the mobility of
around 30 cm2/Vs we reported previously.
26
One reason is that we used relatively low
nanotube density (32–41 tubes/ μm
2
) to achieve a high on/off ratio (e.g. >106 for L ≥50
μm), while the previous study
26
used 41 tubes/ μm
2
to achieve higher mobility at the cost
of lower on/off ratio (~10
5
). Other factors affecting the mobility include batch-to-batch
variation of nanotube quality, variation in nanotube surfactants and length as well as
different gate dielectric structures.
25

One important issue we want to point out is that although mobility is the key metric for
other thin-film transistor channel materials, it is not the best parameter to evaluate the

36

performance of carbon nanotube TFTs. The reason is that mobility can only directly
reflect the current drive capability, but cannot reveal on/off ratio information for the given
transistors. This may not be a problem for other TFT channel materials, such as
amorphous silicon, polycrystalline silicon, or metal oxides, because all these materials
have constant on/off ratio regardless of the channel geometry once their doping level or
elemental composition is fixed. However, due to the existence of metallic nanotubes,
carbon nanotube TFTs exhibit a trade-off between on-current density and on/off ratio
depending on the transistor geometry. Therefore, it is not fair to evaluate the performances
of different nanotube TFTs by just comparing the mobility, since this cannot account for
the on/off ratio difference between different transistors. One good example is the
comparison between arc-discharge and HiPco SN-TFTs we discussed above. Although
arc-discharge SN-TFTs show about three times higher mobility than HiPco SN-TFTs,
these two devices exhibit similar on-current drive when the device on/off ratio is required
to be higher than 10
6
. For this reason, we think for carbon nanotube TFTs, it is better to
compare the maximum current drive capability for a given on/off ratio requirement rather
than compare the device mobility alone.
Based on the analysis above, we have found that different separated semiconducting
nanotubes exhibit different electrical properties. Table 1 summarizes all the differences
we have discussed so far for arc-discharge, HiPco and CoMoCAT separated nanotubes,
including semiconducting nanotube purity, nanotube diameter, electrical bandgap,
maximum on-current density for devices with on/off ratio higher than 10
6
, device mobility,
and channel sheet resistance. Overall, we find that large-diameter nanotubes provide
smaller sheet resistance, higher transconductance, and higher device mobility. Hence,
large-diameter nanotubes have advantage in applications which require high carrier

37

mobility, such as radio frequency circuits. In contrast, small-diameter nanotubes show
higher on/off ratio and smaller off-current, which may be preferred for digital circuit
applications, where on/off ratio and power consumption are big concerns. Also, we can
conclude that all three kinds of separated nanotubes satisfy the general requirement of
AMOLED display applications, which demand a certain current drive and high on/off
ratio.
Table 2.1 Comparison of the properties of arc-discharge, HiPco and CoMoCAT
separated semiconducting nanotubes

Arc-
discharge
HiPco CoMoC
AT
Purity 98% 92% 95%
Diameter (nm) 1.3-1.7 0.8-1 ~0.7
Energy band(eV) 0.45-0.59 0.77-0.95 ~1.09
On-current density
when on/off
ratio >10
6
( μA/ μm)
0.17 0.11 0.014
Device Mobility
(cm
2
V
-1
S
-1
)
17 5 1
Sheet resistance
(MΩ/□)
0.28 1.10 6.52

2.4. Application in display electronics
Our ability to fabricate high performance gel-based SN-TFTs enabled us to further
explore their application in display electronics. For the proof of concept purpose, an
organic light-emitting diode (OLED) was connected to and controlled by a typical HiPco
SN-TFT device. In order to control the OLED, device on-current and on/off ratio are
crucial. Here the device channel length and channel width were selected to be 20 μm and

38

1200 μm so that the transistor can provide enough current while the on/off reaches 10
6

and therefore can meet the requirement for controlling the OLED to switch on and off. A
standard NPD/Alq3 OLED (2 × 2 mm
2
) was employed in this study with a multi-layered
configuration given as ITO/4-4’-bis[N-(1-naphthyl)-N-phenyl-amino]bi-phenyl (NPD)
[40 nm]/tris(8-hydroxyquinoline) aluminium (Alq3) [40 nm]/LiF [1 nm]/aluminium (Al)
[100 nm]. The schematic of the OLED control circuit is shown in the inset of Fig. 2.5a,
where the drain of the driving transistor was connected to an external OLED and a
negative voltage (VDD) was applied to the cathode of the OLED. Current flow through
OLED (IOLED) can be modified by varying the voltage applied to VG, as directly revealed
in Fig. 2.5a where current versus VG characteristics are plotted with a fixed VDD of –8 V.
From this figure and the inset optical photographs taken at certain gate voltages, one can
find that the light intensity of the OLED is modulated by the gate voltage and it can be
fully turned on and turned off when VG are biased at –10 V and 10 V, respectively.
Furthermore, current flow through the OLED was also measured by sweeping the VDD
while also changing the input voltage VG as plotted in Fig. 2.5b. The figure illustrates that
the tested OLED has a threshold voltage of about 3 V and it will be turned on when the
controlling transistor is in the “ON” state and the supply voltage is higher than the OLED
threshold voltage.

39

-9 -8 -7 -6 -5 -4 -3 -2 -1 0
0
10
20
30
40
50
V
G
from -10 V to 0 V
with a step of 1 V

I
OLED
( A)
Drain Voltage (V)

Figure 2.5 External OLED controlled by HiPco SN-TFTs. (a) Plot of the current
through the OLED (IOLED) versus VG with VDD= -8 V. The inset optical images show the
OLED intensity at certain gate voltages. And the inset schematic image is the diagram of
the OLED control circuit. (b) IOLED-VDD Characteristics of the OLED control circuit.
Various curves correspond to various values of VG from -10 to 0 V in 1 V steps.

2.5. Comparison between gel chromatography and DGU
separated semiconducting nanotubes
In addition to the comparison of electrical performance between different kinds of gel-
based separated semiconducting nanotubes, we are also very interested in the device
performance comparison between SN-TFTs using nanotubes separated by gel-based
column chromatography and nanotubes separated by other main stream nanotube
separation methods, especially the density gradient ultracentrifugation (DGU) method.
To carry out this comparison, we first compared the optical absorption spectra of
semiconducting arc-discharge nanotubes separated by DGU (blue curve) and gel-based
column chromatographic (red curve) methods, which are shown in Fig.2.6a. Here we
chose 99%-separated semiconducting nanotubes purchased from Nanointegris Inc. as the
reference DGU-based separated nanotube sample. From this plot, one can find that very

40

similar optical absorption spectra were obtained for DGU-based and gel-based separated
nanotubes, which suggests that these two kinds of nanotubes share similar purity as well
as diameter distribution. Starting with these two kinds of semiconducting nanotubes, we
have fabricated 120 SN-TFTs (60 SN-TFTs having each kind of nanotube) and compared
their performance. Figures 2.6b–2.6d summarize the key statistical parameters of the
DGU-based and gel-based arc-discharge SN-TFTs. The on-current density and on/off
ratio information can be found in Figs. 2.6b and 2.6c, which reveal that DGU-based
separated SN-TFTs provide slightly higher on-current density but lower on/off ratio than
gel-based SN-TFTs. This behaviour may due to the fact that DGU-separated nanotubes
have longer average length (~1 μm) than gel-based arc-discharge nanotubes (540 nm)
leading to fewer nanotube-to-nanotube junctions and therefore lower sheet resistance, but
a higher probability of metallic nanotube pass for DGU-separated nanotubes. Besides on-
current and on/off ratio, device mobility is also studied in Fig. 2.6d. This figure indicates
that gel-based arc-discharge SN-TFTs show slightly higher mobility than DGU-based
SN-TFTs when the channel length is longer than 20 μm. Overall, although there are some
small differences between gel-based and DGU-based arc-discharge SN-TFTs, these two
kinds of devices give similar electrical performance, which suggests that in terms of
electrical properties, gel-based separated semiconducting nanotubes are comparable to
DGU separated semiconducting nanotubes.

41

Figure 2.6 Comparison of key device performance metrics of SN-TFTs using
semiconducting arc-discharge nanotubes separated by DGU and gel-based column
chromatographic methods. (a) Optical absorption spectra of semiconducting arc-
discharge nanotubes separated by DGU (blue) and gel-based column chromatographic
(red) methods. (b) Current density (Ion/W) versus inverse channel length for TFTs
fabricated on semiconducting nanotubes separated by DGU (blue) and gel-based (red)
methods. Plots of (b) average on/off ratio (Ion/Ioff) and (c) device mobility ( μdevice) versus
channel length for the same devices measured in (b).

2.6 Conclusions
We report gel-based column chromatographic nanotube separation of different kinds
of nanotubes and their application in macroelectronics, including progress on the detailed
analysis of key performance metrics of devices using gel-based arc-discharge, HiPco and
CoMoCAT semiconducting nanotubes, and direct comparison of the electrical properties
d
b a
c
0.00 0.05 0.10 0.15 0.20 0.25
0.0
0.3
0.6
0.9
1.2
1.5
DGU seperation
Gel seperation

I
on
/W ( A/ m)
1/L ( m
-1
)
0 20406080 100
10
2
10
3
10
4
10
5
10
6
10
7
DGU seperation
Gel seperation

On/Off ratio
Channel Length ( m)
1 2 345
4
6
8
10
DGU seperation
Gel seperation

Mobility (cm
2
V
-1
S
-1
)
Channel Length ( m)
400 600 800 1000 1200

DGU separation
Gel separation
Absorbance(a.u.)
Wavelength (nm)
S
33
M
11
S
22

42

of gel-based and DGU-based separated semiconducting nanotubes. We have revealed a
trade-off between transistor mobility and on/off ratio, depending on the nanotube
diameter. While large-diameter nanotubes (arc-discharge) lead to high device mobility,
small-diameter nanotubes (HiPco and CoMoCAT) can provide high on/off ratios (>10
6
)
for transistors with comparable dimensions. In addition, based on our analysis, gel-based
SN-TFTs have satisfied the requirements of large scale AMOLED high definition
displays and can be a promising candidate for the transistors used in next generation
displays. Moreover, we have pointed out that due to the trade-off between on-current
density and on/off ratio for SN-TFTs, instead of mobility, maximum on-current density
for devices with on/off ratio above a certain threshold should be the main parameter to
evaluate the electrical performance of carbon nanotube thin-film transistors. Furthermore,
we have carried out a comparison between gel-based and DGU-based separated
nanotubes, and found that both methods can provide separated nanotubes with similar
electrical performance. Our work represents significant advance in gel chromatography
separated nanotube thin-film electronics, and may provide a guide to future research on
SN-TFT based macroelectronics.

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46

Chapter 3
Redox sorting of carbon nanotubes
3.1. Introduction
The discovery of single-wall carbon nanotubes (SWCNTs) has unveiled the existence
of not just one but a family of several hundred stable macromolecules.
1
They are simple
in composition and atomic structure  all made of carbon atoms locally bonded in the
hexagon geometry of graphene, but variations in the helical twist angle (  ) of the
hexagons and in tube diameter (d) result in a diverse set of nanotube electronic structures.
On the basis of theoretical analysis
2-5
and experimental observation,
6,

7
all SWCNTs can
be ranked in an order according to the width of their electronic bandgap: armchair metallic
tubes (  = 30°) with zero bandgap; non-armchair semimetallic tubes with small (< 100
meV) but nonvanishing bandgaps that scale as cos(3 )/d
2
; and semiconducting tubes with
bandgaps that scale as 1/d.
The bandgap-based nanotube ranking manifests itself in solution phase SWCNT redox
chemistry, a research subject started over a decade ago with the emergence of effective
nanotube dispersion and separation methods. Strano et al. first observed that dissolved
oxygen at low pH suppresses SWCNT optical absorption and resonance Raman cross-
sections in a bandgap-dependent fashion: metallic tubes are more sensitive than
semiconducting tubes, and among the latter, the smaller bandgap/larger diameter tubes
are more sensitive than larger bandgap/smaller diameter tubes.
8
Shortly after, Zheng and

47

Diner showed that outer sphere electron transfer between SWCNTs and small-molecule
oxidants also exhibits the same bandgap dependence, and interpreted the oxygen- and pH-
dependent optical response as the result of an outer-sphere electron transfer redox reaction
between SWCNTs and oxygen.
9
Many ensuing studies have aimed at determining the
redox potential of SWCNTs and its correlation with bandgap.
10,

11
To further explore
SWCNT redox chemistry, we find it useful to take a note from the classical coordination
chemistry of transition metal ions, where ligand modulation of redox potential and ligand
reorganization upon electron transfer are abundantly documented. If we view a dispersed
SWCNT as a coordination complex, coupling is expected between the nanotube and the
coordinating surfactant layer in electron transfer reactions.

Scheme 3.1 Redox chemistry

In this work, we expand the redox chemistry of SWCNTs by investigating the effect of
electron transfer on surfactant coating structures in the context of a number of SWCNT
separation processes. Scheme 3.1 summarizes the basic chemistry at the focal point of
this study. In the scheme, a black circle represents an individual nanotube, while the red
and blue lines denote surfactant coating layer. Our findings suggest that electron transfer
between SWCNTs and exogenous redox molecules induces surfactant coating layer
reorganization, which in turn changes the interaction between dispersed SWCNTs and

48

their solvent media, therefore altering the outcome of a number of separation processes.
In keeping with solid state physics nomenclature, we denote the oxidized, resting, and
reduced state by p, i, and n, respectively.
3.2. Redox sorting on aqueous-two phase separation
3.2.1 Redox modulation of nanotubes partitioning
We first observe a strong redox effect in the separation of SWCNTs by the recently
developed polymer aqueous two-phase (ATP) method.
12-14
Arc-discharge SWCNTs are
used. The dispersion and separation procedure is referred to Khripin et al..
14
Figure 3.1
shows the redox modulation of SWCNT partition in a polyethylene glycol (PEG)/dextran
(DX) system. Arc-discharge SWCNTs are used in this particular experiment, but similar
results are also observed for nanotubes with other diameter ranges. As shown in Figure
3.1, five distinct partition regimes can be created under different redox conditions:
1.“reduced” regime, where both metallic and semiconducting tubes are found in the DX
phase; 2. “semi-reduced” regime, where metallic and semiconducting tubes are found in
PEG and DX phase, respectively; 3. “ambient” regime, where both metallic and
semiconducting tubes are found in the top PEG phase; 4. “semi-oxidized” regime, where
metallic and semiconducting tubes are found in DX and PEG phase respectively,
reversing the partition found in the “semi-reduced” regime; 5. “oxidized” regime, where
both semiconducting and metallic tubes are pushed to the bottom DX phase.

49

Figure 3.1 Redox modulation of nanotube partition in 6% PEG + 6% DX two-phase
system. The same amount of arc-discharge SWCNTs (Hanwha) and an identical
surfactant composition 0.9% SC + 0.4% SDS are used in all cases. Redox agent added in
each case: 200 mM NaBH4 for the “reduced” regime; 70 mM NaBH4 for the “semi-
reduced” regime, nothing for the “ambient” regime; 0.5 mM NaClO for the “semi-
oxidized” regime; and 2 mM NaClO for the “oxidized” regime.

3.2.2 Successive oxidation for bandgap separation
While both oxidants and reductants can generate partition conditions for SWCNT
sorting, we find that the oxidative process gives a broader tuning range for partition
control. This is due in part to a built-in oxidant gradient across the two polymer phases.
It has been reported that PEG has mild reducing capability.
15
Consistent with this, by
monitoring changes in SWCNT optical absorbance induced by oxidation, we find that ~
mM NaClO are consumed by 6% PEG within one minute. In contrast, NaClO is quite
stable in DX and capable of oxidizing nanotubes. When NaClO is added to a PEG/DX
ATP system, PEG cannot effectively consume NaClO residing in the DX phase, unless
the two phases are mixed to enhance cross-phase NaClO diffusion. This phenomenon
allows us to design an oxidative extraction procedure as follows (Figure 3.2a): 1. start
from the “oxidized” regime by adding sufficient amount of NaClO; 2. shift the system to
“semi-oxidized” regime by repeatedly mixing the ATP system to gradually reduce the

50

oxidative effect; 3. remove the top PEG phase, and add fresh top phase to reconstitute the
ATP system; 4. obtain more fractions by repeating steps 2 and 3, until all the nanotubes
are extracted from the bottom to the top phase.

Figure 3.2 Fractionation of HiPco nanotubes by successive oxidative extraction. (a)
Separation scheme and photographs of extracted fractions. (b) Optical absorption
spectra of the parent HiPco material and fractions successively extracted from a PEG/DX
ATP system with the aid of NaClO. “S1” and “S2” are two successive fractions extracted
into the top PEG phase from the starting material, nanotubes remaining in the bottom DX
phase are labelled “M” fraction.

51

Figure 3.2 shows the result of a HiPco nanotube fractionation by such a successive
extraction process using an ATP system composed of 6% PEG + 6% DX and 0.9% SC +
1 % SDS. In the first step, 1 mM NaClO is added so that the system is in the “oxidized”
regime with all the nanotubes residing in the bottom DX phase. Upon mixing and phase
settling, smaller diameter/larger bandgap semiconducting tubes are extracted into the top
PEG phase (fraction S1, panels a and b of Figure 3.2). The top phase is then removed and
replaced with a fresh PEG phase. Upon further mixing and settling, a 2
nd
fraction
containing larger diameter/small bandgap semiconducting tubes is extracted (fraction S2,
panels a and b of Figure 3.2). The change in bandgap from S1 to S2 is clearly indicated
by the shift in E11 and E22 absorption peak towards longer wavelengths (Figure 3.2b).
Remaining in the bottom DX phase are largely metallic tubes (fraction M, panels a and b
of Figure 3.2), which are further fractionated by repeating the PEG phase extraction
procedure.
Figure 3.3a shows the absorption spectra of four successively extracted metallic
fractions M3 to M6 from the M fraction. We notice that the spectrum of M6 closely
resembles that of the armchair-enriched fractions reported by Haroz et al..
16
To determine
the difference in chirality distribution in the metallic fractions, we have measured the
Raman radial breathing mode (RBM) profiles of M3 and M6 using three different
excitation wavelengths. As shown in Figure 3.3b, metallic RBM peaks from M6 have
higher peak intensities at lower Raman shifts. Chirality assignment further reveals that
M6 has more RBM peak intensities from armchair or near-armchair tubes. This implies
that M3 is enriched in smaller diameter and non-armchair metallic tubes with higher
cos(3 )/d
2
values, whereas M6 is enriched in armchair or near-armchair and larger
diameter tubes with lower cos(3 )/d
2
values.

52

Figure 3.3 Spectroscopic characterization of fractions M3 to M6 successively extracted
from the M fraction shown in Figure 3.2. (a) Absorbance spectra of M3 to M6. Vertical
green lines indicate the wavelength positions of laser excitation used for the resonance
Raman measurement. (b) RBM profiles of M3 (black traces) and M6 (magenta traces)
measured at excitation wavelength of 457 nm, 514 nm, and 632 nm, respectively. The
spectra are normalized by integrated spectral area for easy comparison.
400 600 800 1000 1200
0.0
0.2
0.4
0.6
0.8
1.0

Absorbance
Wavelength (nm)
M3
M4
M5
M6
150 200 250 300 350

Intensity (arb. units)
Raman shift (cm
-1
)
M3
M6
semi
(6,6)
(7,4)
457 nm
(7,7)
(8,5)
(9,3)
(11,8)
(12,6)
(13,4)
(12,3)
(10,3)
(7,6)
514 nm
632 nm
(a)
(b)

53

Figure 3.4 compares bandgap distributions calculated for fractions S1, S2, M3 and M6
shown in Figure 3.2b and Figure 3.3a. Figure 3.4a shows that smaller diameter (larger
bandgap) SWCNTs are first extracted in S1 followed by larger diameter (smaller bandgap)
SWCNTs in S2. Figure 3.4b demonstrates that this oxidative extraction method is
additionally able to fractionate metallic nanotubes by their vanishingly small bandgap,
with fraction M3 enriched in larger non- zero bandgap semi-metals and fraction M6
enriched in zero-bandgap armchair metals.
For each semiconducting fraction, relative abundances were estimated by analyzing
the corresponding absorption spectra (Figure 3.2b). After subtracting a linear background,
Voigt profiles were applied to spectrally simulate the resonant E11 absorption peaks of
each identified (n,m) species. Assuming a similar absorption cross section for each
nanotube structure, the relative concentrations were found from the spectrally integrated
values obtained from the fitting, each normalized by the sum of all integrated absorbance
peaks. The optical bandgap was then determined from the positions of the simulated
absorption peaks and the results were binned into a histogram.
Due to the many spectrally overlapping peaks found in the metallic region of the
absorption spectrum, Raman measurements (shown in Figure 3.3b) were instead analyzed
to estimate the relative abundances of metallic fractions M3 and M6. For each fraction,
Raman data obtained from each of the three excitation wavelengths were spectrally
integrated using Lorentzian profiles to approximate the RBM peaks of the different (n,m)
structures. To correct for differences in concentration, the integrated RBM values were
scaled by the absorbance values found at the excitation wavelength used to acquire the
Raman data. Finally, to calculate relative abundances, each scaled RBM peak was
normalized by the sum of all scaled RBM peaks in each of the three Raman measurements.

54

For non-armchair metals, small curvature-induced bandgaps were estimated using the
analytical expression derived by Kane and Mele
5
. Armchair species have a finite density
of electronic states at the Fermi level and thus possess no electronic bandgap. The results
were combined and binned into a histogram.
Taken together, our results demonstrate that the oxidative extraction process purifies
SWCNTs in the order of large bandgap semiconducting tubes, small bandgap
semiconducting tubes, semi-metals of non-zero bandgap, and armchair tubes of zero-
bandgap.

Figure 3.4 Bandgap distributions of the extracted semiconducting (a) and metallic (b)
fractions.

Metallic bandgap (meV)
0 10 203040
Relative abundance (%)
0.0
0.1
0.2
0.3
0.4
0.5
M6
0.0
0.1
0.2
0.3
0.4
0.5
0.6
M3
Semiconducting optical bandgap (eV)
0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
0.0
0.1
0.2
0.3
0.4
S2
Relative abundance (%)
0.0
0.1
0.2
0.3
0.4
0.5
S1
ab

55

3.2.3 Summary of redox on ATP separation
The oxidative extraction method has a number of distinct features. First, it is capable
of fractionating metallic tubes by their vanishingly small bandgap, a feat that no other
nanotube separation method has ever demonstrated. Second, we note that the oxidative
extraction order for semiconducting tubes is opposite to that of the recently developed
surfactant SDS/sodium deoxycholate (SDC)-based extraction, where a gradual increase
(decrease) of the SDS (SDC) concentration leads to PEG phase extraction of larger
diameter tubes first, followed by smaller diameter ones.
12-13
These two different
mechanisms may be employed orthogonally to achieve better separation. Finally, in
comparison with our previously reported SDS/SC based extraction process for
metal/semiconductor separation,
14
the oxidative extraction works with a greatly expanded
surfactant concentration range (0.4% to 1% for SC, 0.4 to 1% for SDS) and temperature
range (15°C – 30 °C), making the process much more reliable and robust. In light of the
newly identified role played by redox, we suggest that our previously reported SDS/SC
based extraction of metal/semiconductor SWCNTs is enabled by dissolved oxygen, the
oxidation potential of which is sensitively dependent on factors such as temperature and
pH, consequently making the process vulnerable to uncontrollable external changes.
Mechanistically, redox induced change in nanotube partition in the ATP system is most
likely the result of a surfactant coating layer reorganization triggered by electron transfer.
Oxidation/reduction necessarily alters the electronic configuration or electron wave
function of a nanotube, which in turn should alter the bonding between the nanotube and
surfactant molecules. To accommodate changes in binding affinity, the surfactant layer

56

must reorganize in composition and/or spatial arrangement, leading to a rescaling of
solvation energies in the two phases.
3.3 Redox sorting on density gradient ultracentrifugation
and gel chromatography
The above analysis suggests that the redox effect should be a general phenomenon
observable in other SWCNT separation processes. Indeed, a number of reported pH- and
oxygen-dependent separation phenomena in density gradient ultracentrifugation
(DGU)
17-18
and gel chromatography
19-20
are likely due to redox-triggered surfactant
reorganization. To find more concrete evidence, we have made direct observation of the
redox effect on DGU and gel chromatography separations.
The experimental details for redox sorting on DGU are as follows: A density gradient
containing water-filled electric arc nanotubes
21
was prepared and run in a preparative
ultracentrifuge (Beckman L80-XP) to demonstrate the change in SWCNT density with
the solution redox condition. Eight, three-layer gradients consisting of a 3.7 mL (26 %
(volume/volume) iodixanol, 1.125 % SC, 1.125 % SDS) top layer,  0.7 mL middle layer
(30 % iodixanol, 1.125% SC, 1.125 % SDS) containing the dispersed SWCNTs, and a
0.5 mL bottom layer (34 % iodixanol, 1.125 % SC, 1.125 % SDS) were constructed in
Beckman optiseal ½” centrifuge tubes (#362185). At ambient condition, nothing is added.
Use NaClO and Dithiothreitol (DTT) to tune the oxidized and reduced condition.
Prior to constructing the gradients in the centrifuge tube, an amount of either oxidant,
100 X diluted 5 M stock NaClO, or reducing agent, 1 M DTT, was added to both the top
and middle layers for each specific centrifuge tube to generate a range of solution redox

57

conditions, including a no additive (ambient) condition. A photograph of one tube
showing the constructed density gradients prior to centrifugation and noting the amount
of modifier added is shown in Figure 3.5a. The nanotubes were then ultracentrifuged (VTi
65.2 rotor) at 6810 rad/s (65 kRPM) for 1 h at 20 C. Figure 3.5b is a photograph of the
results; the addition of oxidant dramatically changes the average density of the SWCNTs,
whereas the change with addition of the reducing agent is less dramatic. However, close
inspection notes that the color at the top of the main SWCNT band changes from blue-
gray to bronze with the addition of the reducing agent. The best separation of metallic
and semi-conducting tubes was achieved for addition of 2.5 µL/mL (or 0.125 mM) of the
oxidant, followed by the ambient conditions. Absorbance spectra of the top and bottom
bands for these two samples show clear metal-semiconducting separation (Figure 3.5c);
the spectra are normalized to one at 810 nm to aid comparison. The approximate positions
of the extracted bands are shown for each fraction corresponding to the color of the
absorbance trace; the spectra of the parent dispersion are shown by the black trace.

58

Figure 3.5 Before (a) and after (b) photographs of a density gradient ultracentrifugation
separation with a range of added redox potential altering concentrations. The separation
of metallic (light blue) SWCNTs from the semiconducting (bronze) SWCNTs is
improved from the ambient condition by addition of the small amount of oxidant NaClO
(0.125 mM). Addition of 20 mM reductant DTT reverses the nature of the SWCNTs
isolated at the top of the band. Spectra of aliquots collected from three of the tubes are
shown in (c); the location of the aliquots is given by the colored bar corresponding to the
trace color on the photograph, the pre-separation spectrum is given as the black trace.

59

For gel chromatographic separation, 0.2 mL HiPco (Unidym) SWCNTs dispersion was
loaded onto the prepared gel column. Nanotubes are found to stay in the top of the gel
column. A solution of 1% SDS is used to elute nanotubes. The elution is collected at 0.15
mL per fraction.
To examine the redox effect, we treated aliquots of the same HiPco dispersion under
three different conditions before loading it onto the gel column: 50 mM DTT added to
the HiPco dispersion, nothing added to the HiPco dispersion, and 50 mM NaClO added
the HiPco dispersion. The separation outcome of DTT treated HiPco dispersion is shown
in Figure 3.6a. When washed by 1% SDS, a red band comes down first, and then the blue
band mixed with the red band. The remaining back materials stuck on the column are
most likely amorphous carbon impurities, as this dispersion was not extensively
prepurified. Absorption spectra of selected fractions are shown in Figure 3.6d. The
measurement shows that the early red fraction is metallic enriched (black trace M),
whereas the later blue to brown fraction are enriched in semiconducting SWCNT species.
We measured one early and one late semiconducting SWCNT fractions (S1 and S2) and
found that the elution order is small-diameter semiconducting SWCNTs first and large-
diameter semiconducting SWCNTs second.
The non-treated HiPco dispersion was found to have a different elution pattern as
shown in Figures 3.6b and 3.6e. In this case the early metallic fractions contained a
substantial amount of semiconducting SWCNTs as contaminants. For the later
semiconductor enriched fractions, larger-diameter tubes came out first followed by small-
diameter tubes (opposite to the reducing condition).

60

Figures 3.6c and 3.6f show the elution pattern for the 50mM NaClO treated HiPco
dispersion. In this case, two well separated bands are developed. The earlier band is
enriched in metallic tubes but has substantial semiconducting tube contamination. The
later bands contain mostly semiconducting tubes but displays little diameter separation.

Figure 3.6 Gel chromatography elution pattern for (a) and (d): DTT–treated HiPco
tubes; (b) and (e): non-treated HiPco tubes, and (c) and (f): NaClO-treated HiPco tubes.
The three columns pictures in (a), (b) and (c) are taken at the early, middle and later stage
of the elution process.

Therefore, in the case of DGU, the buoyancy of SWCNTs, or the banding of SWCNT
fractions are strongly modulated by redox potential. At oxidized condition, the separation
purity is better than ambient and reduced condition. Whereas in the case of gel
chromatography, the elution pattern for semiconducting tubes is also dependent on redox
condition. At ambient condition, the semiconducting SWCNTs shows that small diameter
tubes elute out first and larger diameter tubes later, however, at reduced condition, the
elution process is reversed and at oxidized condition, the diameter trend disappears. This

61

result imply that the binding affinity to the stationary phase polymer matrix is changed
by surfactant reorganization.
3.4 Redox sorting on organic phase polyfluorene based
extraction method
We find that redox also affects SWCNT dispersion in organic solvents, further
extending the role of redox in SWCNT separation. Polyfluorene and related polymer
structures have been used for efficient extraction of semiconducting tubes in non-polar
solvents.
22
The mechanism of the selective extraction remains elusive despite many
investigations. We have examined a few commercially available polyfluorene derivatives
for the dispersion of various sources of SWCNTs. In all cases, we find that oxidizing
condition enhances selective dispersion with a concomitant lowering of dispersion yield,
whereas reducing condition does just the opposite. Figure 3.7 shows an example of the
redox effect on the dispersion of HiPco nanotubes by (PFO-bipy). A PFO-bipy /SWCNT
mass ratio = 28 is used for the experiment. Consistent with a literature report,
23
selective
dispersion of semiconducting tubes is compromised by the excess amount of PFO-bipy,
as evident by the appearance of metallic SWCNT features in the (400 to 600) nm region
(grey area in Figure 3.7) of the absorption spectrum (Figure 3.7, black trace). A mild
oxidation treatment via the addition of 1/10 the sample volume of water followed by bath
sonication induces preferential metallic tube aggregation, such that the remaining
dispersion (Figure 3.7, red trace) is highly enriched in semiconducting tubes. In contrast,
in the presence of 10 mM reducing agent vitamin E, PFO-bipy disperses nearly all chiral
species non-selectively (Figure 3.6, blue trace). This finding shows that solubility of
polymer-wrapped SWCNTs in organic solvents is strongly dependent on the redox status

62

of the solvent environment. Ambient redox condition fortuitously enables certain
polymers to selectively extract semiconducting tubes under a narrow polymer/SWCNT
mass ratio. Our finding suggests that controlled oxidation may be used to enhance the
semiconducting tube selectivity for those polymers
24
that lack the capability under
ambient conditions.

Figure 3.7 Absorption spectra of HiPco SWCNT dispersions in toluene made with
same amount of PFO-bipy (1 mg / mL) and SWCNT (0.036 mg /mL) but under different
redox conditions. Blue trace: dispersion made with 10 mM vitamin E added to the PFO-
bipy /SWCNT mixture; black trace: control dispersion with no redox agent added; red
trace: dispersion made from the control by adding 1/10 the sample volume of water, then
bath sonication and centrifugation to remove the newly formed aggregates.

600 900 1200 1500
0.0
0.3
0.6
0.9
1.2

Absorbance
Wavelength (nm)
vitamin E added
control
water treated

63

3.5. Conclusions
In summary, we have shown that redox strongly affects the structure of the surfactant
coating layer, and consequently nanotube separation outcome. Ambient redox condition
set by the dissolved oxygen and uncontrolled pH (in aqueous systems) is conducive to a
number of SWCNT separation processes, but may not necessarily represent the optimum
conditions. Revelation of the hidden role played by redox highlights the importance of
regulating this parameter for more reproducible separation outcome, and points to the
possibility of redox tuning of coating layer structures to enhance separation resolution.
Going beyond nanotube separation, it is foreseeable that redox triggered surfactant
reorganization may also affect other colloidal behavior of SWCNTs.
Chapter 3 References
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9. Zheng, M.; Diner, B. A., Solution Redox Chemistry of Carbon Nanotubes. Journal of the
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11. Schäfer, S.; Cogan, N. M. B.; Krauss, T. D., Spectroscopic Investigation of Electrochemically
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12. Zhang, M.; Khripin, C. Y.; Fagan, J. A.; McPhie, P.; Ito, Y.; Zheng, M., Single‐Step Total
Fractionation of Single‐Wall Carbon Nanotubes by Countercurrent Chromatography. Analytical
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13. Fagan, J. A.; Khripin, C. Y.; Silvera Batista, C. A.; Simpson, J. R.; Haroz, E. H.; Hight Walker,
A. R.; Zheng, M., Isolation of specific small‐diameter single‐wall carbon nanotube species via
aqueous two‐phase extraction. Adv Mater 2014, 26 (18), 2800‐4.
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15. Luo, C.; Zhang, Y.; Zeng, X.; Zeng, Y.; Wang, Y., The role of poly(ethylene glycol) in the
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Simard, B., High‐Yield, Single‐Step Separation of Metallic and Semiconducting SWCNTs Using
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18. Antaris, A. L.; Seo, J.‐W. T.; Brock, R. E.; Herriman, J. E.; Born, M. J.; Green, A. A.; Hersam,
M. C., Probing and Tailoring pH‐Dependent Interactions between Block Copolymers and Single‐
Walled Carbon Nanotubes for Density Gradient Sorting. The Journal of Physical Chemistry C
2012, 116 (37), 20103‐20108.
19. Hirano, A.; Tanaka, T.; Urabe, Y.; Kataura, H., pH‐ and Solute‐Dependent Adsorption of
Single‐Wall Carbon Nanotubes onto Hydrogels: Mechanistic Insights into the
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20. Flavel, B. S.; Kappes, M. M.; Krupke, R.; Hennrich, F., Separation of single‐walled carbon
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21. Fagan, J. A.; Huh, J. Y.; Simpson, J. R.; Blackburn, J. L.; Holt, J. M.; Larsen, B. A.; Walker, A.
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Chapter 4
Facile and low-cost length sorting of
single-wall carbon nanotubes by
precipitation and applications for thin-film
transistors
4.1 Introduction
Single-wall carbon nanotubes (SWCNTs) have unique electrical properties which
make them promising candidates for future applications in nanoelectronics and
macroelectronics.
1-4
Structural polydispersity of SWCNTs, both in their atomic structures
and in their length, is, however, a major issue that needs to be solved to enable their
applications in electronics and many other areas. Significant progress in nanotube
separation according to metallic/semiconducting character, chirality, and diameter has
been made utilizing effective methods such as ion exchange chromatography (IEX),
5, 6

density gradient ultracentrifugation(DGU),
7
gel chromatography,
8-10
polymer selective
wrapping,
11
and aqueous two-phase (ATP) extraction.
12-19

Addressing SWCNT length polydispersity to improve application performances is
motivated by numerous publications reporting length-dependent results.
20-26
For example,

66

short nanotubes are more effective for drug delivery,
23-24
while long tubes are preferred
in SWCNT thin-film transistors (TFTs).
21,

25, 26-27
In nanotube network TFTs, removal
of shorter tubes is important because along a conducting path longer nanotubes would
form fewer tube-tube junctions than shorter tubes. As a result, networks formed by longer
nanotubes are expected to exhibit lower sheet resistance, higher on-current, and higher
carrier mobility.
3, 26
Because as-prepared nanotube dispersions usually have broad length
distributions, the examples above indicate that in addition to metal/semiconductor and
chirality sorting, it is also highly desirable to sort SWCNTs based on their lengths.
Several methods have been reported for SWCNT length sorting. Size exclusion
chromatography (SEC)
28, 29
has been used for nanotube length sorting to achieve narrow
length distributions. Other methods such as ultracentrifugation
30, 31
, cross flow filtration
32
,
and flow-field flow fractionation
33
, have also been employed for length sorting of
nanotubes. However, the above-mentioned methods require expensive instrumentation
and are not readily scalable. Recently, some of us (C. Y. K. and M. Z.) reported that a
polymer precipitation
34
method can be used to effectively sort SWCNTs by their lengths.
In this method, a charge-neutral polymer, polyethylene glycol (PEG), is introduced to a
dispersion of DNA-wrapped SWCNTs, inducing a molecular-crowding effect that leads
to length-dependent nanotube precipitation. In comparison with other methods mentioned
above, this polymer precipitation method does not require expensive instruments, and is
a facile and scalable length sorting method. Demonstration of the method was limited
however to ss-DNA and RNA dispersed SWCNTs, and was not reported for SWCNTs
dispersed via any of the much more broadly utilized surfactants.
Given the desirability to fractionate SWCNT dispersions by length, the utility of
implementing a similar precipitation method to length sort SWCNT dispersions made

67

with low-cost surfactants is clear. Unfortunately, adding PEG alone to SWCNTs
dispersed with the commonly used surfactant sodium deoxycholate (SDC) does not
readily result in SWCNT precipitation. We found however that the addition of extra salt,
such as NaSCN, to the mixture of PEG and SDC-SWCNT dispersion, re-enables selective
precipitation. This prompted us to conduct a systematic investigation on the general
phenomenon of polymer and salt induced precipitation of surfactant dispersed carbon
nanotubes.
In this chapter, we report an important extension of the polymer precipitation method
to the length sorting of surfactant-dispersed SWCNTs. In particular, we show that two
polyelectrolytes - polymethacrylate (PMAA) and polystyrene sulfonate (PSS), can be
applied to SDC-dispersed SWCNTs to achieve precipitation based nanotube length
fractionation. Significantly, we show that applying this new length-sorting methodology
to low-cost raw nanotubes from the plasma torch method results in fractions of relatively
long and uniform length SWCNTs suitable for subsequent multistep ATP extraction of
the metallic and semiconducting SWCNT subpopulations. TFTs fabricated with the
resulting semiconducting SWCNTs exhibit excellent performance with the mobility as
high as 18 cm
2
/(Vs) and Ion/Ioff ratio up to 10
7
.
4.2 Experimental methods
SWCNT dispersion: Plasma torch SWCNTs were dispersed in 20 g/L SDC at 4mg/ mL
by sonication (tip sonicator, 0.64 cm, Thomas Scientific). The sonication condition was
1 h at about 0.9 W/mL. The sonication vial was placed in an ice bath to maintain low
temperature. After sonication, the suspension was centrifuged for 2 h at 1885 rad s
-1
(18
krpm) at 10 °C in a JA-20 rotor (Beckman-Coulter). After centrifugation the supernatant
was decanted and used as the parent dispersion for the length sorting experiments.

68

Polyelectrolyte precipitation: Two schemes for sequential precipitation were used in
this work following previous report.34 In forward sequential precipitation, properly
diluted PMAA or PSS solution was mixed with SWCNT dispersion to the final polymer
concentration of 1 wt. %. The mixture was vortexed and then incubated for 4 h at room
temperature (23 °C). Mild 5 min centrifugation at 943 rad/s (9 krpm, JA-20 rotor,
Beckman-Coulter), is then sufficient to pellet a fraction of the SWCNT mass, referred to
hereafter as the “precipitate” or “pellet”. After removing the supernatant, the pellet was
resuspended in 2 wt. % sodium cholate (SC), and the fraction was labelled “1 % PMAA
(or PSS)”. Additional PMAA or PSS was then added to the supernatant to increase the
polymer concentration (to 2 wt. %, 4 wt. %, and finally 6 wt. %), and the above
precipitation steps were repeated to yield additional pellet fractions. In reverse sequential
precipitation, the highest polymer concentration is used in initial step. In this method the
supernatant is retained as the sorted fraction, and a lesser polymer concentration solution
is added to dilute and resuspend the pellet; the resuspended pellet is then incubated /
pelleted to produce the next sequential fraction.
Multistep ATP extraction: We followed the general procedure given by Khripin et al.
12
and Gui et al..
14
For the plasma torch SWCNTs, 3-5 steps were needed to obtain a top
phase that is highly enriched in semiconducting nanotubes. To remove the polymers, we
added 0.1M NaSCN to the top phase and precipitated out all SWCNTs to form a small
volume pellet with light centrifugation (5 min, 943 rad/s (9 krpm)). The pellet was
resuspended in 0.4% SC+0.1% SDS, and was used directly for thin-film deposition.
Optical absorption characterization: A Varian Cary 5000 spectrometer was used to
characterize the UV-vis-NIR absorption of the nanotubes. For liquid samples spectra were
measured through a quartz microcuvette with a 10 mm path length.

69

Thin-film transistor fabrication: To form a SWCNT network on a Si/SiO2 substrate, we
first prepared poly-L-lysine functionalized substrates according to a previously reported
procedure.35 Functionalized substrates were then immersed in the SWCNT solution for
5.5 h. Subsequently the substrate was rinsed with deionized (DI) water and blown dry
with N2. A 50 nm SiO2 layer was used as the back-gate dielectric. Standard
photolithography for TFT fabrication was used as described previously.
35

Atomic Force Microscopy (AFM): Clean Si/SiO2 substrates were used for SWCNT
deposition. The SC-dispersed SWCNT sample was diluted by 5 times with around 0.1 M
NaSCN and was deposited on to the substrate by a 5-10 min incubation. After incubation,
samples were rinsed by DI water and blown dry using nitrogen. AFM imaging was done
on a Bruker Dimension Icon AFM in the peak-force tapping mode (ScanAsyst) using the
respective ScanAsyst-Air probes. Typically, more than one hundred SWCNTs from each
fraction were measured for statistical analysis.
4.3 Results and discussion
4.3.1 Polymer and salt precipitation of surfactant dispersed
SWCNTs – a general phenomenon
Polymer precipitation via molecular-crowding-induced cluster formation has been used
to fractionate DNA-wrapped nanotubes according to their lengths.
34
In this work, we
found that precipitation of SWCNT dispersions is a general phenomenon: it can be
induced by addition of polymers, salts, and their combinations, and is applicable to a
variety of aqueous SWCNT dispersions. For SWCNTs dispersed by anionic surfactants
(e.g. SDC, SC, Sodium dodecylbenzenesulfonate, DNA), length-dependent precipitation
can be induced by addition of many different chemicals. These include (1) neutral

70

polymers such as PEG, poly (2-ethyl-2-oxazoline), and PVP when in combination with
added salt such as NaSCN; (2) anionic polyelectrolytes such as PMAA, PSS and PAA;
and (3) salt alone at high enough concentrations. For SWCNTs dispersed by neutral
surfactants such as Triton X-405, (NH4)2SO4 was identified as a very effective agent to
induce length-dependent precipitation.
Our observations strongly suggest that nanotube precipitation cannot be fully
accounted for by the simple depletion force based mechanism
34
proposed for charge-
neutral crowding agents. Further investigations are needed to understand the physical
mechanism behind the general precipitation phenomenon. We suggest that the physics
behind the phenomenon is that of phase separation of two substances (SWCNTs, and
polymers and/or salt) at high enough concentrations: the precipitated is the SWCNT-rich
phase, and the supernatant is the polymer and/or salt-rich phase. Since longer tubes
contribute less entropy to the free energy of the SWCNT-rich phase than shorter tubes
with same mass fraction, it is expected that longer tubes undergo phase separation more
readily than shorter tubes. This is analogous to the well-studied polymer-polymer phase
separation, where longer-chain polymers are more readily phase-separated.
36

4.3.2 Length sorting by polyelectrolytes
We found that polyelectrolytes, such as PMAA or PSS are the most convenient to use
for the precipitation of surfactant-dispersed nanotubes. A schematic of the forward
precipitation scheme using these molecules is illustrated in Figure 4.1. The structures of
PMAA and PSS are shown in Figure 4.1b. As the polymer concentration increases
gradually, progressively shorter and shorter tubes are precipitated out. Length sorting can

71

thus be achieved by repeated cycles of polymer addition and SWCNT pellet removal
(Figure 4.1c).

Figure 4.1. (a) Schematic showing that adding polyelectrolyte (PMAA or PSS) to
SDC-dispersed SWCNTs leads to nanotube precipitation. (b) Molecular structures of
PMAA and PSS. (c) Length-sorting by forward sequential polymer addition and SWCNT
precipitation.

In this work, we used inexpensive raw SWCNTs mass-produced by the plasma torch
method. The price of the plasma torch SWCNTs is about 10 times lower than that of arc
discharge SWCNTs; however the as-synthesized plasma torch SWCNTs contain a large
amount of impurities such as C60, amorphous carbon and catalysts. SDC was used to
disperse the raw SWCNTs. A detailed dispersion preparation method is given in the
Methods and Materials section. To initiate the forward sequential precipitation process,
we added PMAA at a final concentration of 1% to the SWCNT dispersion. After thorough
mixing and shaking, mild centrifugation was used to collect the precipitated pellet. The
supernatant was collected and extra PMAA was added to the final concentration of 2% to

72

precipitate out more SWCNTs. The procedure was then repeated at PMAA concentrations
of 4% and 6% respectively, as shown in Figure 1c. Each precipitated pellet was collected
and resuspended in 2% SC. UV-vis-NIR absorption spectra of the re-dispersed pellets are
shown in Figure 4.2. The spectra indicate that the precipitation sorting process cause some
minor changes in diameter distribution.

Figure 4.2 UV-vis-NIR absorption spectra of (a) 1% to 6% PMAA and (b) PSS
precipitated nanotubes.

We then characterized the length distributions of the precipitated fractions using AFM.
Figures 4.3a-4.3d show the AFM images of SWCNTs collected from 1%, 2%, 4% and 6%
PMAA precipitation. It is evident that as the PMAA concentration increases from 1% to
6%, the average length of the sorted SWCNTs decreases gradually. To be more
quantitative, we have measured the lengths of more than 100 nanotubes from each of the
four precipitated fractions. Figure 4.3e shows length distribution histograms for the 1%
and 4% PMAA precipitated fractions. SWCNTs from the 1% PMAA precipitation are
distributed mainly in the 0.4 to 0.7 µm length range, while SWCNTs from the 4% PMAA
precipitation pellet are more generally in the 0.1 to 0.5µm length range. Statistical
analysis (Figure 4.3f) shows that the average tube length is 0.65 µm, 0.45 µm, 0.35 µm,

73

and 0.25 µm for the 1%, 2%, 4% and 6% PMAA precipitated fractions, respectively.
These results demonstrate that simple PMAA precipitation can yield clear length sorting
of surfactant-dispersed nanotubes: longer nanotubes are precipitated out at lower PMAA
concentration and then shorter and shorter nanotubes are precipitated out as PMAA
concentration increases.
Besides PMAA, we have also used another polyelectrolyte PSS to achieve nanotube
length sorting by the forward sequential precipitation method (Figure 1c). The AFM
images in Figures 3a-3d are from 1%, 2%, 4% and 6% PSS fractions. Figures 3e and 3f
provide length distribution statistics for the PSS precipitated tubes.

74

Figure 4.3 AFM images of nanotubes from the (a) 1%, (b) 2%, (c) 4%, and (d) 6%
PMAA fractions. The scale bar is 1 μm for all images. (e) Length distributions of
SWCNTs from the 1% and 4% PMAA fractions. (f) Average nanotube lengths of the 1%,
2%, 4% and 6% PMAA fractions.

75

Figure 4.4 AFM images of nanotubes from the (a) 1%, (b) 2%, (c) 4%, and (d) 6%
PSS fractions. The scale bar is 1 μm for all images. (e) Length distributions of the 1% and
4% PSS fractions. (f) Average nanotube lengths of the 1%, 2%, 4% and 6% PSS fractions.

76

Previous studies have shown that the optimal PEG molecular weight range is 6~12 kDa
for length sorting of DNA-wrapped nanotubes.
34
To investigate the polymer chain length
effect on length sorting of surfactant dispersed SWCNTs, we tested PSS polymers with
molecular weight of 70 kDa, 200 kDa and 1000 kDa, respectively, and found similar
length sorting effect.
What we have described so far are results from forward sequential precipitation, i.e.
increasing the polymer concentration gradually to precipitate out longer tubes first.
Alternatively, reverse sequential precipitation as shown in Figure 4.5a can also be used
for length sorting. In this case, one starts the precipitation with the highest polymer
concentration to obtain shorter tubes in the supernatant first, then gradually reduces the
concentration of added polymer to the re-dispersed pellet to obtain progressively longer
tubes. By comparing length distributions of the longest tube fractions obtained by the
forward and reverse precipitations, we found that they yield similar length distributions,
except that the fraction from the forward precipitation contained in general more short
tubes from 0.1 to 0.4µm. As a result, nanotubes from forward precipitation have an
average length of 0.65 µm, slightly shorter than reverse precipitation which has an
average length of 0.7 µm. This is understandable, as revers precipitation removes shorter
nanotubes at each precipitation step (4 steps in total) and at last only the longest nanotubes
will survive, while forward precipitation only has one step to remove shorter nanotubes
from the longest fraction.

77

Figure 4.5. (a) Schematic of reverse sequential precipitation of SWCNTs, where
precipitations using 6%, 4%, 2%, and 1% PMAA were conducted sequentially. (b) Length
distribution of SWCNTs obtained by forward and reverse precipitation using 1% PMAA.

4.3.3 ATP separation and thin-film transistors
Length sorted nanotubes can be further processed to achieve enrichment of
semiconducting nanotubes for electronic applications. Here we used the ATP extraction
method for metallic/semiconducting SWCNT separation. The 1% PMAA precipitated
SWCNTs with an average length of 650 nm, were resuspended in 2% SC and directly
used for ATP separation. Detailed information on the separation is given in the Materials
and Methods section. Figure 4.6a shows the absorption spectra of the starting SWCNT
dispersion (black trace), and semiconducting tube fraction 1-R extracted from the long

78

nanotubes of the 1% PMAA fraction (red trace). The latter spectrum shows no metallic
absorption features in the metallic transition region (M11, wavelengths  600 nm - 800
nm). The short nanotubes of the 6% PMAA fraction have also been separated by ATP to
yield a semiconducting SWCNT enriched fraction (6-R). We found that it is easier to
enrich semiconducting nanotubes from longer tube fractions , while more steps are
needed to achieve the same semiconducting purity using shorter tube fractions. Polymers
in the 1-R and 6-R fractions were removed by introducing 0.1 M NaSCN to precipitate
out the nanotubes, and then re-suspending the pelleted nanotubes in a mixture of 0.4%
SC+0.1% SDS for easier deposition on Si/SiO2 substrate to form thin-film networks. It is
worth noting that in addition to length fractionation, precipitation is also a convenient
way to remove non-nanotube chemicals such as excess amount of surfactants, graphitic
impurities, and polymers in nanotube fractions obtained from ATP separations (these
components are assumed to partition volumetrically and thus are vastly reduced in a
precipitation/dilution sequence).

79

Figure 4.6 (a) Absorption spectra of unsorted (black trace) plasma torch SWCNTs and
the semiconducting SWCNT enriched 1-R fraction (red trace). (b) An SEM image of the
1-R nanotube thin-film network on a Si/SiO2 substrate. The scale bar is 2 μm. (c) Transfer
characteristics of TFTs fabricated with the 1-R fraction with L=4 μm and W=400 μm.
The blue arrows indicate the forward and reverse scanning directions. (d) On-current
density versus inverted channel length of TFTs fabricated with the 1-R fraction. (e) on/off
ratio versus channel length of TFTs. (f) Relationship of mobility and channel length of
TFTs made with the 1-R fraction.

To demonstrate the utility of the length-sorted semiconducting nanotubes, we
fabricated TFTs using the longest semiconducting fraction 1-R, since long nanotubes are

80

expected to have few nanotube-nanotube junction along a conducting path, and therefore
higher on-current and mobility. The SEM image of a typical thin-film network made of
the 1R fraction is shown in Figure 4.6b. Using the thin film network made of the 1-R
tubes, we fabricated back-gated transistors. The channel widths (W) of the devices were
chosen to be (200, 400, 800, 1,200, 1,600, and 2,000) μm, while channel lengths (L) were
(4, 10, 20, 50 and 100) μm. Figure 4.6c shows a typical forward and reverse drain current-
versus-gate voltage (ID –VG) of a device with L=4 μm and W=400 μm, indicating p-type
transfer characteristics. The output characteristics, i.e. drain current versus drain voltage
(ID–VD) plotted at different gate voltages in both the linear regime (-0.1V–0.1V) and
saturation regime (-3V–0V). In the linear regime at small VD, the contacts formed
between metal electrodes and the nanotubes are Ohmic contacts. When VD is more
negative, drain current saturation is observed, indicating a well-behaved field-effect
operation. Figure 4.6d is the on-current density versus channel length (ID/W–L). The on-
current density is inversely proportional to the channel length, demonstrating high
uniformity of the devices. The highest on-current density is 0.75µA/µm, which is
comparable to the literature reports
37, 38
.
Figure 4.6e shows the Ion/Ioff ratio versus channel length for TFTs made with the 1-R
nanotubes. We can see that the on/off ratio is higher than 10
5
even at the shortest channel
length (4 µm). When the channel length is increased to 20 µm, the on/off ratio is improved
to 10
7
. There is a slight decrease of the on/off ratio to 10
6
when the channel length is
further increased to 50 µm and 100 µm. This is because the off-current decreases due to
the lower probability of forming percolation path at longer channel lengths, and when it
becomes comparable with the noise level of the measurement equipment
35
there will be
a slight decrease of the on/off ratio. Except for those that have channel lengths of 4 µm,

81

all the TFT devices have on/off ratio higher than 10
6
. The on/off ratio of the TFTs we
fabricated is comparable to the highest values reported in the literature.
39
Figure 4.6f is a
plot of the mobility versus channel length for the devices. The mobility, which is
calculated based on the parallel plate model, ranges from 14 to 18 cm
2
/(Vs), indicating
good uniformity in device performance. At 20 μm channel length, on/off ratio is highest
and reaches 10
7
while the mobility remains at 17 cm2/(Vs). At 50 μm channel length, the
mobility reaches the maximum of 18 cm
2
/(Vs) and the on/off ratio remains higher than
10
6
. The mobility we obtain is higher than other random nanotube network TFTs.
35
We
also fabricated TFTs using the shortest fraction 6-R. While the on/off ratio of TFTs made
with 6-R is similar to those made of 1-R, the on-current density (maximum 0.26 µA/µm)
and mobility (1-4 cm
2
/(Vs) )are much lower which is shown in figure 4.7. These data
demonstrate that high performance TFT devices with high on-current density, on/off ratio
and mobility can be fabricated using the inexpensive raw plasma torch SWCNTs after
length sorting and metal/semiconductor separation.

82

Figure 4.7 (a) SEM image of the 6-R nanotube thin-film network on a Si/SiO2 substrate.
The scale bar is 2 μm (b) On-current density versus inverted channel length of TFTs
fabricated with the 6-R fraction. (c) on/off ratio versus channel length of TFTs fabricated
with 6-R fraction. (d) Relationship of mobility and channel length of TFTs made with the
6-R fraction.

4.4 Conclusions
In this work, we have expanded the precipitation based length sorting method to
surfactant-dispersed SWCNTs. Utilizing the polyelectrolytes PMAA and PSS, we
realized the separation of surfactant-dispersed nanotubes by their lengths. The length-
sorted SWCNTs fractions were found to be readily further separable by ATP for
semiconducting tube enrichment. We applied the facile length sorting and
metallic/semiconducting separation methods to the low-cost plasma torch SWCNTs.
TFTs fabricated with long semiconducting nanotubes showed on/off ratios of 10
5
- 10
7

83

and mobilities of 14-18 cm
2
/ (Vs). Our findings shed new lights on the phase behaviour
of colloidal SWCNTs at high polymer and salt concentrations, and demonstrate that
precipitation-based length sorting has great potential as a facile, low-cost and scalable
method to produce length sorted semiconducting nanotubes for macroelectronics
applications.
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Chapter 5
Conclusions and future directions
5.1 conclusions
As a conclusion, this dissertation discussed about the single-wall carbon nanotubes
separation and their thin-film transistor (TFT) applications. To demonstrate that the
separated SWCNTs are desirable for macroelectronics application and to better
understand those separation mechanisms, I have developed a platform for comparing the
advantages of the mainstream SWCNTs on TFT applications separated by gel
chromatography. And I also discovered the key parameter-redox chemistry that affects
various separation methods, which enhanced the reproducibility of separation results and
enabled bandgap-dependent nanotube separation. A general mechanism that redox-
induced surfactant reorganization will affect the nanotube buoyancy in a density gradient
field, affinity to polymer matrices, and solubility in organic solvents is also proposed to
better understand the various separation performances. I reported for the first time a
general phenomenon of a length-dependent precipitation of surfactant-dispersed carbon
nanotubes by polymers, salts, and their combinations and developed a facile and low-cost
method for SWCNTs length sorting which improved the nanotube TFTs performance
greatly.

87

5.2 Future direction on separated semiconducting SWCNTs
for flexible active-matrix organic light-emitting diode display
by screen printing
Active-matrix organic light-emitting diode display (AMOLED) has been widely
studied as a next generation display technology because of the color purity and brightness,
wide view angle, low power consumption and good flexibility.
1
SWCNTs have the
advantage of high intrinsic carrier mobility and current-carrying capacity
2-4
, and intrinsic
flexibility, which make them a promising channel material for the active-matrix TFTs.
The advantages of SWCNT TFTs have been demonstrated by photolithography based
fabrication
5-6
and ink-jet printing
7
. However, no one reported screen printing for the
flexible AMOLED. Screen printing is a low-cost and scalable method as
photolithography based fabrication process requires repeated photolithography steps and
high vacuum conditions and ink-jet printing suffers from low throughput.
Screen printing is widely used in printing electronics which uses screen masks to
deposit materials. To print the active-matrix TFTs, poly(ethylene terephthalate) (PET) is
used for the flexible substrate. The schematic of the active-matrix design is shown in
Figure 5.1a. As shown in Figure 5.1b, silver nanoparticle ink is used for source/drain,
scan line and data line, and gate line printing. While barium titanate (BTO) paste is used
for dielectric material. The separated semiconducting SWCNTs are used for the channel
material of TFTs.

88

Figure 5.1 (a) The schematic diagram of the 1 transistor active-matrix OLED display.
(b) The schematic of the TFT on PET substrate with the materials used for each layer.
8

The screen printed TFTs show uniform performance. Figure 5.2a shows the transfer
curve of the TFTs in the same row. The TFT performance can be further improved by
tuning the nanotube thin-film density and quality as well as the printing recipe. After
finishing the active-matrix printing, the OLEDs are integrated into it. The printed TFTs
can drive the OLEDs and show the on and off states.
-15 -10 -5 0 5 10 15
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4

Gate Voltage (V)
-I
D
 A 

Figure 5.2 The screen printed TFTs transfer curves with high uniformity.

89

The challenges are (1) how to get better printed TFTs performance with higher on current
and on/off ratio; (2) how to improve the yet of the OLEDs as we know OLEDs deposition
condition is critical and easy to be dead; (3) encapsulation is needed to increase the
OLEDs lifespan.
The screen printed flexible active-matrix OLED display using separated
semiconducting SWCNTs as the channel materials for TFTs will be a low-cost and
scalable method. Some other components such as mechanical and chemical sensors can
also be integrated into the active-matrix backplane for other applications, for example
electronic skins. This will demonstrate the high potency of SWCNTs used in active-
matrix display and the advantage of low-cost and high throughput screen printing
technology.

Chapter 5 references
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structure of atomically resolved carbon nanotubes. Nature 1998, 391 (6662), 59‐62.
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90

8. Cao, X.; Chen, H.; Gu, X.; Liu, B.; Wang, W.; Cao, Y.; Wu, F.; Zhou, C., Screen printing as a
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Abstract (if available)

Abstract Single-wall carbon nanotube (SWCNT), a one-dimensional material, since its discovery in 1993, has attracted tremendous interest from fundamental science to advanced technological research. Its extraordinary electrical properties, such as intrinsic high carrier mobility, current-carrying capacitance, as well as flexibility and transparency have made SWCNTs a promising candidate for the future semiconductor material replacing the traditional material Si for applications in thin-film transistors, circuits, computers, displays especially flexible and transparent touch screens and so on. However, the synthesized nanotubes are a mixture of metallic and semiconducting nanotubes with broad length distribution. Despite the progress on the synthesis of semiconducting nanotubes, its purity and yield still could not satisfy the requirements of various applications. Therefore post-synthesis separation is necessary. This dissertation studies the separation of SWCNTs including their metallicity, bandgap and length, and their thin-film transistor (TFT) performance. ❧ To demonstrate that the separated SWCNTs are desirable for macroelectronics application and to better understand those separation mechanisms, I have (1) developed a platform for comparing the advantages of the mainstream types of SWCNTs on TFT applications separated by gel chromatography

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Asset Metadata

Creator Gui, Hui (author)

Core Title Single-wall carbon nanotubes separation and their device study

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Publication Date 07/29/2016

Defense Date 05/26/2016

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag length sorting,metallic/semiconducting separation,OAI-PMH Harvest,single wall carbon nanotube,thin-film transistor

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Armani, Andrea M. (committee member), Wu, Wei (committee member)

Creator Email guihui09@gmail.com,hgui@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-286752

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Legacy Identifier etd-GuiHui-4670.pdf

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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