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Controlled synthesis, characterization and applications of carbon nanotubes
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Content
Controlled Synthesis, Characterization and Applications
of Carbon Nanotubes
By
Jia Liu
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
Dec. 2014
Copyright 2014 Jia Liu
ii
Dedication
Dedicated to my family and friends
iii
Acknowledgements
Behind every great journey there is a support system of family and friends. It is with a great
deal of gratitude that I thank those who have guided, supported and inspirited me through the
most challenging and rewarding years of my life, I would not be the person I am today without
that love and support.
First and foremost, I would like to express my sincerest appreciation to my advisor,
Professor Chongwu Zhou for all his support, guidance and encouragement. Without his help
and supervision, this dissertation would not have been possible. His enthusiasm to science,
precision to detail and consistency to work was, and remains an inspiration for my future career.
I would like to express my deepest gratitude to Dr. Ming Zheng, our collaborator at NIST,
for all the invaluable advice and discussions, which helped me to become more mature and
effective as a researcher. Moreover, I would like to thank Dr. Lawrence Scott from Boston
College, Dr. Yang Hua from Suzhou University and Dr. Colin Nuckolls from Columbia
University for collaborating and providing valuable discussions, which have widened my
scientific views. I would also like to thank all my committee members Professor Steven
Cronin, Prof. Edward Goo, Prof. Susumu Takahashi and Dr. Ralf Haiges for their insightful
comments and suggestions.
In addition, I would like to thank all my former and current group members as well as my
USC friends: Dr. Bilu Liu, Dr. Gang Liu, Dr. Lewis Gomez, Dr. Fumiaki Ishikawa, Dr. Chuan
Wang, Dr. Po-Chiang Chen, Dr. Akshay Kumar, Dr. Alexandar Badmaev, Dr. Hsiao-Kang
Chang, Dr. Anuj Madaria and Dr. Yi Zhang, Dr. Jialu Zhange, Yue Fu, Haitian Chen, Yuchi
Che, Xue Lin, Zhen Li, Jing Qiu, Shelley Wang, Maoqing Yao, Nappadol Aroonyadet,
iv
Mingyuan Ge, Jiepeng Rong, Luyao Zhang, Xin Fang, Pyojae Kim, Younghyun Na, Rebecca
Lee, Kuan-The Li, Ning Yang, Liang Chen, Hui Gui, Pattaramon Vuttipittayamongkol, Ahmad
Abbas, Sen Cong,Yu Cao, Fanqi Wu, Anyi Zhang, Chenfei Shen, Yuqiang Ma, Yihang Liu and
Xuan Cao. It has been a great privilege to be a part of the nanolab at USC, thank you for sharing
those ups and downs with me, I could not have asked for a better team than that which I found
in every one of you.
It is impossible to list everyone who has graced me with their love and presence in my life.
Nevertheless, I would like to take a moment to thank my entire Westminster College and
RCNUWC family: President Barnie Forsythe, Dr. Caroline Perry, Pat Kirby, Dr. Kent Palmer,
Dr. Glen Frerichs, Dr. Bernie Hansert, Marta and Nhung for their caring, assistance, and
guidance.
Last but not least, a special thanks to my family and my American parents Bob and Regina
for their unconditional love, and support. Thank you so much for standing by me every step the
way, words cannot express my appreciation, you have inspired me to continue to strive to be the
best version of myself every day. You have truly been the best support system I could have
asked for and I thank you so much.
v
Table of Contents
Dedication .................................................................................................................................. ii
Acknowledgements ................................................................................................................... iii
List of tables............................................................................................................................... viii
List of Figures ..............................................................................................................................ix
Abstract ......................................................................................................................................xx
Chapter 1: Introduction to carbon nanotube ....................................................................................1
1.1 Background ..............................................................................................................................1
1.2 Structure of Carbon nanotubes .................................................................................................2
1.3 Electronic Properties of carbon nanotubes ...............................................................................4
1.4 Synthesis of carbon nanotubes .................................................................................................8
1.4.1 Arc Discharge ................................................................................................................8
1.4.2 Laser Ablation ................................................................................................................9
1.4.3 Chemical Vapor Deposition ............................................................................................... .11
1.5 Electronic applications of carbon nanotubes ..........................................................................13
Chapter 1 References .......................................................... ....................................................... 15
Chapter 2: Selective Synthesis of Semiconducting Carbon Nanotubes .........................................18
2.1 Introduction ............................................................................................................................18
2.2 Wafer-Scale Semiconducting Dominant Carbon Nanotube Synthesis ..................................20
2.3 Comparative Studies between IPA and Ethanol CVD ...........................................................22
2.3.1 Density and Diameter Comparison ..............................................................................22
2.3.2 Raman Comparison ......................................................................................................23
2.3.3 Aligned Nanotube Device Performance Comparison ..................................................25
2.3.4 Mass Spectrometry Measurement Comparison ...........................................................27
2.3.5 Control Experiment ......................................................................................................30
2.4 Electrical Measurement of Individual Nanotube Devices ......................................................32
2.5 Thin-film Transistor Performance of IPA nanotubes .............................................................34
vi
2.6 Summary ................................................................................................................................38
Chapter 2 Reference .......................................................................................................................39
Chapter 3: Chirality-controlled Synthesis of SWCNTs using VPE...............................................43
3.1 Introduction ............................................................................................................................43
3.2 Deposition and Activation of Chirality-pure Nanotube Seeds ...............................................45
3.3 VPE Cloning of SWCNTs ......................................................................................................48
3.4 Characterization of Cloned Nanotubes Using Microscopy ....................................................52
3.5 Chirality Identification by Raman Spectroscopy ...................................................................54
3.6 Electron Transport Measurement of Cloned Nanotubes ........................................................57
3.7 Discussion ..............................................................................................................................58
3.8 Summary ................................................................................................................................63
Chapter 3 Reference .....................................................................................................................65
Chapter 4: Chirality-dependent Vapour-phase Epitaxial Growth and Termination of Single-wall
Carbon Nanotubes ..........................................................................................................................68
4.1 Introduction ............................................................................................................................68
4.2 VPE-based Nanotube Cloning ................................................................................................70
4.3 Chirality-dependent Growth Rate and Length Distribution of SWCNTs ..............................72
4.4 Length Evolution Profiles of Semiconducting SWCNTs .......................................................73
4.5 Chirality-dependent Growth Rate and Lifetime of SWCNTs ................................................79
4.6 Atomic Illustration of Chirality-dependent SWCNT Growth via Diels-Alders Cycloaddition
Processes ......................................................................................................................................81
4.7 Chirality-dependent growth of armchair metallic SWCNTs ..................................................83
4.8 Summary ................................................................................................................................87
Chapter 4 Reference .....................................................................................................................88
Chapter 5: Nearly Exclusive Growth of Small Diameter Single-Wall Carbon Nanotube
Semiconductors from Organic Chemistry Synthetic End-Cap Molecules ....................................90
5.1 Introduction ............................................................................................................................90
5.2 Nanotube growth from C
50
H
10
...............................................................................................93
5.3 Multiple lasers Raman spectroscopic characterization ..........................................................99
5.4 Electrical Transport Property and Breakdown of SWCNT FETs ........................................106
5.5 Molecular seed size evolution and nanotube diameter-seed size relationship. ....................114
vii
5.6 DFT calculations ..................................................................................................................118
Chapter 5 Reference ...................................................................................................................127
Chapter 6: Conclusions and Future Directions ............................................................................131
6.1 Conclusion ............................................................................................................................131
6.2 Future Work on Mechanism Understanding ........................................................................132
6.2.1 In-situ SEM ................................................................................................................132
6.2.2 In-situ TEM ................................................................................................................134
6.3 Future work on Yield Improvement and Bulk Synthesis .....................................................137
Chapter 6 Reference ..................................................................................................................139
Bibliography ...............................................................................................................................140
viii
List of Tables
Table 1. Normalized value of relative intensity of possible species from mass spectrometry of
synthesis condition .........................................................................................................................30
Table 2.A summary of the six RBMs observed under the 405 nm laser and possible chirality
index assignments based on the Kataura plot ..............................................................................105
Table 3.A summary of the total number of SWCNTs from both individual and SWCNT array
FETs .............................................................................................................................................113
ix
List of Figures
Figure 1.1 Schematic illustrating a layer of graphene lattice rolled into an SWCNT .....................2
Figure 1.2 A sheet of graphene lattice (a) and classification of carbon nanotubes. (b1) armchair,
(b2) zigzag, and (b3) chiral nanotubes. ........................................................................4
Figure 1.3 (a) Three-dimensional view of the graphene π/π* bands and its 2D projection. (b)
Example of the allowed 1D subbands for a metallic tube. Schematic depicts (9,0). (c)
Example of the quantized 1D subbands for a semiconducting tube. Schematic shows
(10,0). The white hexagon defines the first Brillouin zone of graphene, and the black
dots in the corners are the graphene K points. .............................................................6
Figure 1.4 Electronic density of states for two (n,n) zigzag fibers: (a) (10,0) and (b)
(9,0).. ............................................................................................................................7
Figure 1.5 Schematics of the arc-discharge apparatus for nanotube synthesis. ...............................9
Figure 1.6 Schematic diagram of laser ablation method for production of carbon nanotubes..…10
Figure 1.7 Schematic diagram of the CVD system for production of carbon nanotubes..…....…11
Figure 1.8 Schematic of tip-growth mechanism for carbon nanotubes synthesized by the CVD
method. .......................................................................................................................12
Figure 1.9 Schematic of base-growth mechanism for carbon nanotubes synthesized by the CVD
method. .......................................................................................................................13
x
Figure 2.1 Wafer-scale carbon nanotube synthesis using isopropanol (a) Photograph of a four-
inch wafer with aligned nanotubes grown. (b) An SEM image of aligned nanotubes
grown from isopropanol carbon feedstock. ................................................................21
Figure 2.2 SEM (a) and AFM (b) images of isopropanol-synthesized carbon nanotubes. SEM (d)
and AFM (e) images of ethanol-synthesized carbon nanotubes. Diameter distribution
of nanotubes synthesized using isopropanol (c) and ethanol (f). ...............................23
Figure 2.3 Micro-Raman characterization. (a) RBMs of isopropanol-synthesized nanotubes and
(b) RBMs of ethanol-synthesized nanotubes using a 532 nm laser. (c) RBMs of
isopropanol-synthesized nanotubes and (d) RBMs of ethanol-synthesized nanotubes
using a 633 nm laser………………...………………………………………………25
Figure 2.4 (a) I
SD
-V
GS
curves of back-gate devices with isopropanol- and ethanol-synthesized
nanotubes. (b) On/off ratio distribution for devices with isopropanol- and ethanol-
synthesized carbon nanotubes. (c) Transfer and (d) Output characteristics of a
representative back-gated transistor with isopropanol-grown carbon nanotubes.......27
Figure 2.5 Mass spectra taken at the exhaust of the CVD system for ethanol (redtrace) and
isopropanol (blue trace). .............................................................................................29
Figure 2.6 AFM images of the semiconducting nanotubes before (a) and after (b) isopropanol
CVD process. Metallic nanotubes before (c) and after (d) isopropanol CVD
process… ....................................................................................................................32
Figure 2.7 (a) An SEM image of a representative individual nanotube device with a 4 μm channel
length. (b) Transfer characteristics of 38 individual nanotube transistors, with red
traces representing the metallic nanotubes and blue traces representing the
semiconducting ones. The percentage of devices with on/off ratio higher than 10 is
around 90%. The nanotubes were grown from isopropanol CVD. (c) Transfer
characteristics of a metallic single nanotube transistor. (d) Transfer characteristics of
a semiconducting single nanotube transistor. .............................................................34
xi
Figure 2.8 TFTs using networks of isopropanol-synthesized nanotubes. (a) An SEM image of a
nanotube network. (b) An SEM image of a back-gated TFT. (c) Transfer and (d)
output characteristics of the device in (b). .................................................................36
Figure 2.9 I-Vg and g
m
curve from a TFT carbon nanotube device with 50 μm channel
length…… ..................................................................................................................37
Figure 3.1 The chirality-pure nanotube seeds obtained from DNA-based separation are first
deposited onto various kinds of substrates including quartz and Si/SiO
2
, followed by
air annealing and water vapour annealing process to activate the ends for the ensuing
VPE growth. VPE with either methane or ethanol as the carbon feed stock is then
used to achieve chirality-controlled growth from these seeds. ..................................46
Figure 3.2 SEM images of the samples (a) without any annealing treatment, (b) with only air
annealing, (c) with only water vapour annealing, and (d) with both air and water
vapour annealing after the CVD cloning process. .......................................................48
Figure 3.3 SEM images of cloned nanotubes under different flow rate of H
2
and Ar. Here argon
flows through the ethanol bubbler and therefore determines the amount of ethanol
vapour entering the CVD system. The dots in the SEM images were etched markers on
substrates which were used to locate the nanotubes. ...................................................51
Figure 3.4 a, b, AFM images of (7,6) nanotubes before and after VPE using ethanol on quartz
substrates. The scale bars in a and b are 1 m. c, d, Length distribution of the (7,6)
nanotubes before and after VPE. e, f, Comparison of (7,6) nanotubes cloned on
quartz and Si/SiO
2
substrates. g, An SEM image showing (7,6) nanotubes cloned on
the quartz substrate using methane. h, i, SEM images of (6,5) nanotubes cloned on
quartz substrates using methane and ethanol, respectively.j, k, SEM images of (7,7)
nanotube cloned on quartz and Si/SiO
2
substrates. The scales bars are 50 m for e, f,
and j, 30 m for g, h, and i, and 10 m for k. .............................................................53
xii
Figure 3.5 AFM images of DNA (without nanotubes) after deposition on quartz (a), after VPE
(b), and an SEM image of the substrate after VPE (c), No nanotube growth was
observed on the substrates, excluding any possible effect from DNA on nanotube
cloning. .......................................................................................................................54
Figure 3.6 (a, b) Raman RBMs of (7,6) nanotubes before and after VPE. The spectra were taken
at different random locations on the quartz substrates with laser excitation energy of
1.96 eV. The peaks at 265 cm
-1
correspond to the RBM of (7,6) nanotubes. (c, d)
Raman RBMs of (7,7) nanotubes before and after VPE. The spectra were taken at
different random locations on the Si/SiO
2
substrates with laser excitation energy of
2.4 eV. The peaks at 249 cm
-1
correspond to the RBM of (7,7) nanotubes. (e) Raman
RBM spectra along a specific nanotube confirming that the chirality is preserved
after the VPE process. Inset, an SEM image (with artificial color) of the nanotube.
(f) Kataura plot shows the RBM frequencies and E
ii
of different nanotubes. The
values were taken from the reference. ........................................................................56
Figure 3.7 (a, b) Schematic a and an SEM image (with artificial color) b, of a back-gated (7,6)
nanotube transistor. The device consists of an individual (7, 6) nanotube, with L = 4
μm and 50-nm-thick SiO
2
gate dielectric. The scale bar is 5 m for (b. c). Transfer
characteristics (I
D
–V
G
) of the transistor measured at various V
D
biases. Inset, transfer
characteristics plotted in semi-logarithm scale with V
D
= 1 V. d, Output (I
D
–V
D
)
characteristics of the same device measured at various V
G
biases from -20 to 20
V…... ..........................................................................................................................58
Figure 3.8 (a) the effect of annealing. (b) Diels-Alder chemistry for the incorporation of C
2
H
2
and C
2
H
4
into an armchair site on the edge of an growing SWCNT seed. (c)Edge
structures of unzipped (6,5) and (9,1) tubes. The red letter a denotes an armchair site
on the edges. ...............................................................................................................62
Figure 3.9 SEM images of (6,5) and (9,1) CNTs after VPE for different growth times. a, (6,5) for
20 s with average length of 37.5µ m, b, (6,5) for 40s with average length of 35.7µ m, c,
(9,1) for 40s with average length of 11.9µ m, and d, (9,1) for 60s with average length
of 17.3µ m.FETs ..........................................................................................................63
xiii
Figure 4.1 Cloning yield improvement. (a) SEM images of cloned (6, 5) SWCNTs using
methane as carbon source. (b) SEM images of cloned (6, 5) SWCNTs using a
mixture of methane and ethylene as carbon source. ...................................................71
Figure 4.2 Length distributions and average length L
of three different (7, 7)/20s samples shows
the similar average length and thus small sample-to-sample variations. ...................73
Figure 4.3 (a, b, c) SEM images of VPE grown (6, 5), (8, 3), and (9, 1) SWCNTs with a growth
time of 20s, 20s, and 40s, respectively. Scale bars are 50 m for a, 20 m for both b
and c. (d, e, f,) Corresponding length distribution of SWCNTs from images (a, b, and
c). ...............................................................................................................................73
Figure 4.4 SEM images (left) and corresponding length distribution (right) of cloned (6,5)
SWCNTs with growth time of (a) 40s, (b) 60s, (c) 120s, and (d) 15min. ..................74
Figure 4.5 SEM images (left) and corresponding length distribution (right) of cloned (8,3)
SWCNTs with growth time of (a) 30s, (b) 40s, (c) 60s, (d) 120s, and (e) 15min. .....75
Figure 4.6 SEM images (left) and corresponding length distribution (right) of cloned (9,1)
SWCNTs with growth time of (a) 60s, (b) 120s, and (c) 15min…............................76
Figure 4.7 SEM images (left) and corresponding length distribution (right) of cloned (7,6)
SWCNTs with growth time of (a) 20s, (b) 40s, (c) 60s, (d) 120s, and (e) 15min...... 77
Figure 4.8 SEM images (left) and corresponding length distribution (right) of cloned (10,2)
SWCNTs with growth time of (a) 40s, (b) 60s, (c) 120s, and (d) 15min.FETs .........78
xiv
Figure 4.9 a, Length evolution profiles and fitted curves based on eq 3. for (6, 5), (8, 3), (9, 1),
(7, 6), and (10, 2) SWCNTs with growth times of 20s, 40s, 60s, 2 min, and 15 min.
Inset, chiral angle versus diameter for the above five semiconducting SWCNTs,
showing they belong to two subgroups with similar diameters in each one as
highlighted by different colours. b, Zoom-in plot of panel a shows the initial growth
period. c, d, Chiral-angle-dependent growth rate (R
0
) and lifetime (τ) of the above
five semiconducting SWCNTs. The vertical error bars in c and d correspond to the
errors of parameters extracted based on equation 3. .................................................81
Figure 4.10 (a, b, c) Cycloaddition of C
2
H
x
species to a (6, 5) SWCNT and the formation of six-
membered rings, leading to the continuous growth of this nanotube. (d) Addition of
CH
y
species leads to the formation of five-membered ring and consequently
nanotube growth stops. e, f, g, Addition of C
2
H
x
species to a (9, 1) SWCNT for its
continuous growth. h, Addition of CH
y
species leads to the growth stops. Multiple
arrows from (b to c) and (f to g) represent multiple addition reactions. ....................83
Figure 4.11 (a, b) Representative SEM images of cloned (6, 6) and (7, 7) SWCNTs with a
growth time of 20s. Scale bars are 20 m. (c, d) Length evolution of (6, 6) and (7, 7)
SWCNTs with growth time of 20s, 40s, and 60s. ......................................................85
Figure 4.12 SEM images (left) and corresponding length distribution (right) of cloned (6,6)
SWCNTs with growth time of (a) 40s, (b) 60s and cloned (7,7) SWCNTs with
growth time of (c) 40s, (d) 60s. ..................................................................................86
Figure 5.1 Scheme of proposed chirality-controlled nanotube growth from organic chemistry
synthesized molecular end-caps. (a) Structure of the bowl-shaped corannulene
molecule (C
20
H
10
). (b) Structure of the hemispherical C
50
H
10
molecule synthesized
from corannulene, which represents the end-cap plus a short sidewall segment of a
(5,5) SWCNT. (c) Proposed chirality controlled VPE growth of a (5, 5) SWCNT
from its molecular end-cap shown in b. The yellow, green and black atoms represent
carbon while the gray atoms represent hydrogen. ......................................................94
Figure 5.2 Energy dispersive X-ray (EDX) spectra (Figure S1a) of the C
50
H
10
molecules
deposited on quartz. For sample preparation, ~10 l of C
50
H
10
in toluene was
deposited on quartz and kept until dry to obtain thick deposits. The C signal comes
from C
50
H
10
molecules while the O and Si come from the quartz substrate. Zoom in
spectrum in Figure S1b shows that thereare no metal elements like Fe, Co, Ni
detected. ......................................................................................................................95
xv
Figure 5.3 An optical absorption spectrum of the C
50
H
10
molecules dissolved in toluene. A
solution with absorbance of 0.40 at 468 nm was used in this study. ..........................95
Figure 5.4 Representative SEM images of the quartz substrate coated with C
50
H
10
molecules
after the CVD process without seed pretreatment. .....................................................96
Figure 5.5 SEM and AFM characterization of nanotubes grown from C
50
H
10
molecular end-caps.
(a) Low magnification SEM image of as grown nanotubes. Inset is a digital camera
image of the quartz substrate after deposition of the C
50
H
10
molecules and drying,
where the red-orange areas correspond to a high density of C
50
H
10
molecules. (b), (c),
and (e) SEM images of as-grown SWCNTs at the locations indicated in image a. (d)
A high magnification SEM image of the area c. (f) An AFM image of a SWCNT
with a height of ~0.6 nm. Scale bar: (a) 1 mm, (b),(c),(e) 50 m, (d) 10 m, and (f)
200 nm. .......................................................................................................................97
Figure 5.6 Control experiments.(a) Low and (b) high magnification SEM images of the blank
quartz substrate after air and water vapour treatment followed by attempted CVD
growth, showing no growth of nanotubes in the absence of the C
50
H
10
molecules. ..99
Figure 5.7 Raman spectra of C
50
H
10
molecule-end-caps under 457 nm laser excitation. The black
curve was taken from bare quartz while the purple curves were taken from different
locations of quartz with C
50
H
10
deposited. The Raman signals under 633 nm and 514
nm lasers are much weaker than those of the 457 nm laser. In addition, the overall
Raman intensity is much weaker than Raman of as-grown nanotubes. ...................100
Figure 5.9 Raman spectra of the bare quartz substrate under different lasers. ............................101
Figure 5.8 (a), (b), (c) Raman RBM spectra of SWCNTs grown from C
50
H
10
molecular end-caps
excited by 633 nm (a), 514 nm (b), and 457 nm lasers (c). The peaks marked with
arrows are from SWCNTs and all the other peaks (marked with *) come from quartz
substrates. (d), (e), (f) RBM frequency distributions based on the above three lasers.
(g) Diameter distribution of SWCNTs derived from the RBM frequencies (h) Raman
D-band and G-band spectra of SWCNTs excited by a 457 nm laser. ......................103
xvi
Figure 5.10 (a) Raman G-band and D-band spectra of the SWCNTs grown from C
50
H
10
based on
three lasers. The high quality of the SWCNTs is confirmed by all three lasers. (b)
Alaser energy-dependent shift of the D-band frequency, i.e., the dispersive behavior,
is observed and is consistent with early reports on the Raman spectra of nanotubes
and graphene.
42,43
The dispersion relationship is( ∂ω
D
/ ∂E
laser
) = 45 cm
−1
/
eV.CNTs. ..................................................................................................................104
Figure 5.11 Raman spectra of as-grown SWCNTs taken from a 405 nm laser (CL-2000 Diode
pumped CrystaLaser LC, Renishaw, UK). Figure 11a is the RBM regime and Figure
11b is the D-band and G-band regime. In Figure 11a, the six green arrows indicate
six RBMs while all the other peaks (indicated by black asterisk *) originate from the
quartz substrate. No peaks were observed at ~340 cm
-1
for (5, 5) CNTs.................105
Figure 5.12 (a) Schematic of device structure of a back-gated individual SWCNT FET. (b)
Statistics of on/off current ratio distribution of 34 individual SWCNT FETs. (c), (d)
Representative transfer characteristics (I
DS
-V
G
) of an individual semiconducting. (e)
Transfer characteristic of an all-semiconducting nanotube-array-FET, with the inset
SEM image showing a total of four SWCNTs connected to both electrodes. (f)
Electrical breakdown experiments of the device in e. (g) Transfer characteristics of a
multiple-nanotube-FET before (blue) and after (red) electrical breakdown, with the
inset SEM image showing a total of two SWCNTs connected to both electrodes. (h)
Electrical breakdown experiments of the device in g. Scale bar of the SEM images: 5
m for b and d, 10 m for e, and 20 m for g. ........................................................108
Figure 5.13 A summary of the transfer characteristics (I
DS
-V
G
) of all individual SWCNT FETs.
The total devices number is 34 and 2 of them (red curves) show on/off ratio of <10
and are assigned to be metallic SWCNTs. Note that for true metallic SWCNTs, i.e.,
armchair (n, n) SWCNTs, the on/off ratio should be ~1. The red curves in this figure
show on/off ratio of 4.6 and 5.8, respectively. This behavior can be attributed to the
so-called semi-metallic SWCNTs with a small energy band gap.
49
We count these
red curves as metallic SWCNTs. ..............................................................................108
xvii
Figure 5.14 Output characteristics (I
DS
-V
D
) of typical semiconducting and metallic SWCNT
FETs, corresponding to the same devices in Figure 4c and Figure 4d in the main text,
respectively. ..............................................................................................................109
Figure 5.15 Electrical breakdown
50,51
of an all-semiconducting SWCNT array FETs. (a) Transfer
characteristic of the device before breakdown. (b) The breakdown experiments of
this device at a V
G
= -5 V. The gate dielectric (90 nm SiO
2
) was damaged at a V
D
=77
V (red arrow). This device was considered to have at least three semiconducting
SWCNTs in the channel and is counted as 3 for statistical analysis. (c) Gate current
(I
G
) during the breakdown process. Inset is the zoom in plot with V
D
<70 V. (d)
Transfer characteristic of the device after breakdown, showing that the device was
damaged. ...................................................................................................................111
Figure 5.16 Low magnification SEM images of the device shown in the inset of Figure 4e (a) and
Figure 4g (b). The whole channel area of the device is visible. Four SWCNTs and
two SWCNTs can be clearly discerned in image a and b, respectively FETs .........112
Figure 5.17 (a), (b) AFM images of as-deposited C
50
H
10
molecular aggregates on quartz (a) and
the C
50
H
10
molecules after pretreatment (b). (c) AFM examinations of the
relationship between as-grown nanotubes and the seed molecules. Images 1, 2, and 3
show an end of one nanotube, two ends of two different nanotubes, and both ends of
one nanotube, respectively. No big particles were found at the nanotube ends. Image
4 shows a big particle at the end of one SWCNT. The ends of the SWCNTs are
indicated by green arrows. The vertical bars are 15 nm for all AFM images. .........115
Figure 5.18 (a), (b) Size distributions of as deposited C
50
H
10
molecular seeds (a) and after air and
water vapour treatment (b). The average sizes of the clusters are 7.1 nm for a and 1.7
nm for b...... ..............................................................................................................115
Figure 5.19 AFM images of the C
50
H
10
molecules after (a) 300 C and (b) 400 C air treatment.
The bottom panels show corresponding height profiles of the white lines in images a
and b.……. ...............................................................................................................116
xviii
Figure 5.20
1
H NMR (400 MHz, C
6
D
6
) of C
50
H
10
after air treatment at 500 C. Inset shows the
zoom-in of 7-8 ppm area. The δ7.63 (s, 10H) peak
19
disappeared after the treatment
(blue arrow in the inset), indicating decomposition of the C
50
H
10
molecule. The
peaks indicated by black arrows originated from trace amount of diethyl ether in
C
6
D
6
solvent. ............................................................................................................118
Figure 5.21 Energy barriers and relative electronic energy (including ZPVE, zero point vibration
energy) profiles of the transformations from (5,m)into (5,m-1)SWCNT caps and the
structures of nanotube caps. (a) Comparisons of energy barriers from (5, 5) to (5, 4)
transformation between C
50
H
10
and C
50
H
9
, i.e., the pristine seed and the seed with
one hydrogen atom being extracted as the starting structures. (b), (c) Structures of
transition states (TS) of (5, 5) to (5, 4) transformation starting from C
50
H
10
(b) and
C
50
H
9
(c). (d) Transformation energy barriers of (5, m) to (5, m-1) SWCNTs. m=5,
4, 3, 2, or 1. (e), (f) Structure of formed (5, 4) (e) and (5, 3) (f) SWCNTs with the
formation of one and two 5-7 pairs. .........................................................................119
Figure 5.22 The relative energy profiles of the transformations from (5,4)into (5,3)SWCNT caps
at different bent regions (sites). The results show that the site 1, which locates nearby
the initial pentagon (red letter), has the lowest transformation barrier to form a new
pentagon-heptagon pairs (5-6 pairs, blue letter). ......................................................121
Figure 5.23 The relative energy profiles of the transformations from (5, 3)into (5, 2)SWCNT
caps at different bent regions (sites). The results show that the site 1, which locates
nearby the initial five-member-ring, has the lowest transformation barrier to form a
new 5-7 pairs. ...........................................................................................................121
Figure 5.24 The relative energy profiles of the transformations from (5, 2) into (5, 1) SWCNT
caps at different bent regions (sites). The results show that the site 1, which locates
nearby the initial five-member-ring, has the lowest transformation barrier to form a
new 5-7 pairs ............................................................................................................122
Figure 5.25 The relative Gibbs free energy profiles of the transformations from (5,m) into (5,m-
1) SWCNT caps of C
50
H
m+4
(m=5,4,3,2, respectively) at 298.15K (solid line) and
1173K(dash line).. ....................................................................................................122
xix
Figure 5.26 In situ measurements of oxygen concentration during SWCNT growth. The initial
oxygen concentration is higher than the detection limit (1000 ppm) of the sensor.
After H
2
flush (10 minutes) and furnace ramp-up (13 min) periods, the O
2
concentration drops to 23 ppm. The initial introduction of CH
4
and C
2
H
4
increases
the O
2
concentration to 77 ppm. Then, it decreases gradually and stabilizes at ~7
ppm. ..........................................................................................................................124
Figure 5.27 Comparative studies of Fe-grown SWCNTs under identical CVD conditions. (a), (b)
SEM images. (c) Representative Raman spectra of nanotubes grow from Fe catalyst.
The green arrows indicate RBMs while the asterisk * indicate peaks from the quartz
substrates. (d) Statistical analysis of the RBM distribution of SWCNTs grown from
Fe catalysts. The total number of RBM peaks is 151. The colored stripes of M
22
, S
33
,
M
11
, and S
22
correspond to the second electronic transition of metallic SWCNTs, the
third electronic transition of semiconducting SWCNTs, the first electronic transition
of metallic SWCNTs, and the second electronic transition of semiconducting
SWCNTs, respectively. ............................................................................................125
Figure 6.1 In-situ SEM observation of (6, 5) nanotube (a) SEM of the quartz substrate after
pretreatment. (b) After 5 minutes growth time. (c)(d) Same location as (c) after
second round of growth. respectively. ......................................................................134
Figure 6.2 (a) In-situ TEM observation of a SWCNT nucleation and growth from a Fe catalyst
under different time. (b) Indicating the active catalyst is Fe3C. Reprinted with the
permission from Ref
1
Copyright 2008 American Chemical Society respectively. .136
Figure 6.3 SEM image of (a) Nanotube growth under one atmosphere. (b) under low pressure
growth. respectively. ................................................................................................137
xx
Abstract
Single-walled carbon nanotubes (SWCNTs) possess superior electrical and optical
properties and hold great promise for electronic and biomedical applications. The properties of a
SWCNT strongly depend on its structure, therefore, a great effort of this dissertation has been
devoted to control the structure of single-walled carbon nanotubes (SWCNTs) during the
chemical vapor deposition (CVD) growth process. Despite the enormous investigation efforts
and the tremendous achievements made in the nanotube growth field over the past several
decades, controlled synthesis of nanotubes with predefined and precise structure still remain a
huge challenge. We have developed a new approach to the synthesis of SWCNTs with
predefined chirality by combining nanotube separation with growth. This novel approach can
also be used with organic molecules.
The thesis will start with a brief introduction of the carbon nanotube, then the synthesis and
characterization methods as well as the applications of SWCNTs is described in detail to
explain the importance of controlling the structure of the carbon nanotube.
In chapter 2, a novel carbon feedstock is introduced to selectively synthesize the
predominantly semiconducting nanotube by using the chemical vapor deposition (CVD)
method. The development of guided CVD growth of single-walled carbon nanotubes provides a
great platform for wafer-scale integration of aligned nanotubes into circuits and functional
electronic systems. However, the coexistence of metallic and semiconducting nanotubes is still
a major obstacle for the development of the carbon nanotube based on nanoelectronics. To
address this problem, we have developed a method to obtain predominantly semiconducting
xxi
nanotubes from direct CVD growth. By using isopropanol as the carbon feedstock, a
semiconducting nanotube purity of above 90% is achieved, which is unambiguously confirmed
by both electrical and micro-Raman measurements. A mass spectrometric study was performed
to elucidate the underlying chemical mechanism. Furthermore, high performance thin-film
transistors with an on/off ratio above 10
4
and mobility of up to 116 cm
2
/V∙s have been achieved
using the isopropanol-synthesized nanotube networks grown on silicon substrate. The method
reported in this contribution is easy to operate and the results are highly reproducible.
Therefore, such semiconducting predominantly single-walled carbon nanotubes could serve as
an important building block for future practical and scalable carbon nanotube electronics.
Chapter 3 presents our new approach to the selective synthesis of nanotube with predefined
chirality via the Vapor phase epitaxy (VPE) growth method. The lack of synthetic control in
chirality has long been recognized as a fundamental impediment in the science and application
of SWCNTs. Since the electronic property of a SWCNT strongly depends on its chirality,
1
previous efforts to address this issue have resulted in significant progress in the separation of
synthetic mixtures, which has yielded predominantly single-chirality nanotube species.
11-15
However, separation processes are limited by their small scale, their high cost, and the short
length (< 500 nm) of the resulting chirality-pure nanotubes. They are, therefore, not viable for
many, especially electronic device applications.
11
Here we demonstrate a general strategy for
producing both metallic and semiconducting SWCNTs with predefined chiralities by using
purified single-chirality nanotubes of (7, 6), (6, 5) and (7, 7) as seeds for subsequent metal-
catalyst-free growth, resembling the vapor phase epitaxy (VPE) commonly used for
semiconductor films.
xxii
To shed light on the growth mechanism of our VPE process, we carefully studied the
chirality-dependent growth kinetics and termination mechanism for seven single-chirality
nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7); covering near zigzag, medium
chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes. Details
of the experiment are presented in chapter 4. Our results reveal that the growth rates of
nanotubes increase with their chiral angles while the active lifetimes of the growth hold an
opposite trend. Consequently, the chirality distribution of a nanotube ensemble is jointly
determined by both growth rates and lifetimes. These results correlate nanotube structures and
properties with their growth behaviors and deepen our understanding of chirality-controlled
growth of nanotubes.
The VPE process is found to be highly robust and can also be used to synthesize SWCNTs
from various organic molecules, such as the (5, 5) end-cap. Details of the experiment results can
be found in chapter 5. Finally, a brief review and various future research directions are proposed
with some preliminary results.
1
Chapter 1: Introduction to the Carbon Nanotube
1.1 Background
It is believed that the first observation of tubular carbon nanostructure was made in
1952 by Radushkevich and Luckyanovich.
1
Since then many papers also reported the
bamboo like texture as well as the concentric texture which is determined from electron
diffraction.
2,3
However, due to the lack of communication and the limitation of the
instrument, it was not until Sumio Iijima’s Nature paper in 1991, nearly four decades
later, that the nanotube finally began to receive the recognition it deserves from the
research community. By using the high resolution transmission electron microscopy
(HRTEM), Sumio Iijima observed MWCNTs on the cathode of a carbon arc that was
used to produce fullerenes. This paper is also the first unambiguous evidence for the
possibility of growing carbon nanotubes without the need of any catalyst.
4
The SWCNT
was reported by both Iijima and Ichihashi from NEC
5
and Erthuane et al from IBM
6
in
1993. Since then SWCNTs attracted enormous research effort to study the properties
and achieve better control of the structure. Due to their long electron mean-free path,
SWCNTs are ballistic conductors of electrons and holes, which make them appealing as
building blocks in the next generation of electronics. From the first nanotube field effect
transistors demonstrated by both Dekker’s group at Delft University of Technology and
the Avouris’ group at IBM in 1998 to the first carbon nanotube computer in 2013,
7
the
carbon nanotube has shown vast potential for technology beyond silicon. However, to
fully utilize the superior properties of nanotubes, it is important to precisely control their
structures.
2
1.2 Structure of Carbon Nanotubes
8
Structurally, the nanotube can be pictured as a sheet of graphene lattice rolled up
into a seamless tube which is called single-walled carbon nanotube (SWCNT), as shown
in Fig.1.1. Additionally, depending on the number of graphene layers, a nanotube can
also be made up of more than two graphite sheet layers which is then called multi-
walled carbon nanotube (MWCNT). The diameter of SWCNT can range from sub-nm to
several nanometers while a MWCNT can have a diameter up to a few hundred of
nanometers. Due to the unique structure and remarkable properties, both kinds of
nanotubes have stimulated enormous interest for both fundamental research and future
applications.
Figure 1.1 Schematic illustrating a layer of graphene lattice rolled into an SWCNT.
9
The properties of the carbon nanotube are strongly dependent on its edge structure
which is specified by the chiral vector C
h
as shown in figure 1.2(a). The chiral vector C
h
can be expressed by the real space unit vectors a
1
and a
2
of the hexagonal lattice and is
defined as: C
h
= n a
1
+ m a
2,
where both n and m are integers. The nanotube structure is
often decided by n and m in a form of (n, m) which is called the chirality of the nanotube.
For instance, the chiral vector in Figure 1.2(a) is (m, n) = (4, 2), and the unit cell of this
3
nanotube is bounded by OABB’. We can picture this nanotube as contracted by cutting
the graphene along the lines of OB and BB’ and then rolling it up into a nanotube, so
that point O meets A, and point B meets B’.
Once (n, m) is specified, other structural properties, such as diameter (d) and chiral
angle (θ), can be determined by the following two equations:
𝑑 =
𝑎 𝑚 2
+𝑛 2
+𝑛 𝑚 𝜋
θ = cos
−1
(
2𝑛 +𝑚
2 𝑚 2
+𝑛 2
+𝑛 𝑚 )
Where a (0.246 nm) in the diameter calculation equation is the lattice constant of
the graphene sheet, and θ is defined as the angle between the chiral vector and the zigzag
direction. Because of the hexagonal symmetry of the graphene sheet and the chiral angle,
θ is ranged from 0
o
to 30
o
. When the chiral angle is equal to 0
o
or 30
o
, the nanotube is
called armchair (Fig.1.2 b1) and zigzag (Fig.1.2 b2), respectively. Both kinds of
nanotubes exhibit a spiral symmetry whose mirror image can be superposed on to the
original one, therefore armchair and zigzag nanotubes are also called achiral nanotubes.
Chiral nanotubes with a chiral angle between 0
o
to 30
o
are shown in Fig.1.2b3.
4
Figure 1.2 A sheet of graphene lattice (a) and classification of carbon nanotubes.(b1)
armchair, (b2) zigzag, and (b3) chiral nanotubes.
8
1.3 Electronic Properties of Carbon Nanotubes
10,11
The carbon nanotube can be pictured as a sheet of graphene rolled up as described in
section 1.2. The electronic structure of a SWCNT can be derived simply from that of
graphene. In graphene, three σ bonds hybridize in a sp2 configuration, while the other
2pz orbitals, which is perpendicular to the graphene plane and makes π covalent bonds. It
is known that the π electrons are valence electrons which are relevant for transport and
other solid state properties. Therefore, only π energy bands for graphene are considered
to determine the electrical properties of graphene. The two dimensional energy dispersion
relations for π bands of graphite E
2d
is given by:
𝐸 2𝑑 = ± 𝛾 0
[1 + 4 cos
3𝑘 𝑥 𝑎 2
cos
𝑘 𝑦 𝑎 2
+ 4 cos
2
𝑘 𝑦 𝑎 2
]
1/2
Where γ
0
is the C-C transfer energy, and the positive and negative E correspond to
the π* and π energy bands, respectively.
5
Figure 1.3 (a) shows the three-dimensional view of the graphene π/π* bands and its
2D projection. The conduction and valence bands touch at six K points in the Brillouin
zone, which causes graphene to be a semi-metallic material. In the single-walled carbon
nanotube, the wave vector K is quantized along the circumferential direction due to the
periodic boundary condition: K*C
h
= 2πq, where q is an integer. Therefore, only a
particular set of states are allowed with a spacing of 2/d. If one of these allowed states
passes through one of the K points, as shown in figure 1.3 (b), the nanotube will be
metallic and otherwise semiconducting (figure 1.3 (b)). Theoretical calculation indicates
that nanotubes will be metallic if n-m=3q and one third of all nanotubes are metallic at
room temperature. Further studies has shown that only armchair nanotubes, where n = m,
are truly metallic. Other metallic nanotubes open up a small gap due to the broken
symmetry caused by the curvature of nanotubes, which is inversely proportional to the
square of the diameter of the nanotubes.
12
Moreover, based on the energy dispersion relation, the density of states (DOS) of the
nanotube can be calculated. In figure 1.4 (a) and (b), the DOS for (10,0) and (9,0)
nanotubes are plotted in units of states per unit cell of 2D graphite, respectively.
The corresponding density of states of 2D graphite in both figures are plotted in
dotted lines for comparison . There is a noticeable difference between those two zigzag
nanotubes near the Fermi Level at E=0, where the DOS is zero for the semiconducting
nanotubes and non-zero for the metallic nanotubes. The singularities characteristic of
1D energy bands appear at the band edges of each energy band.
6
Figure 1.3(a) Three-dimensional view of the graphene π/π* bands and its 2D projection.
(b) Example of the allowed 1D subbands for a metallic tube. Schematic depicts (9,0). (c)
Example of the quantized 1D subbands for a semiconducting tube. Schematic shows
(10,0). The white hexagon defines the first Brillouin zone of graphene, and the black
dots in the corners are the graphene K points.
11
7
Figure 1.4 Electronic density of states for two (n,n) zigzag fibers: (a) (10,0) and
(b) (9,0).
10
8
1.4 Synthesis of Carbon Nanotubes
Being able to precisely control the structure, orientation, location and length of the
carbon nanotube with high yield and high quality during growth have been the central
focus in the nanotube field for the past several decades. Over the years, numerous
developments have been made in the controlled nanotube synthesis field. This chapter
describes the major synthesis methods of the nanotube with primary emphasis on the
single-walled carbon nanotube.
There are several techniques that have been developed for the large scale and high
quality production of carbon nanotubes. Among those techniques, the most widely used
methods are arc discharge, laser ablation and chemical vapor deposition (CVD).
1.4.1 Arc Discharge
The arc discharge method was used for the production of MWCNTs in Iijima’s
Nature paper in 1991, which led to a new era in the field of nanotube research. The
method reported in the paper employs two graphite electrodes inside a chamber filled
with an inert gas, and the two graphite electrodes are separated by a short distance which
is slightly different from the technique used for the production of C60 where graphite
electrodes were kept in contact (figure 1.5).
4,13
However, the yield was rather poor until
the significant improvement made by Ebbesen and Ajayan in 1992, where high
pressured helium was used in the arc-evaporation chamber.
14
Following these initial
studies, numerous studies have been carried out to further optimize the synthesis
condition to achieve high yield and better control.
15-17
In 2007, Marc Monthioux et al
9
reported that by using anodes made from either diamond powder or small grain graphite,
they could achieve much higher yield with their SWCNT production. Despite how much
progress has been made, the arc discharge process still remains costly, complex, and
difficult to control.
Figure 1.5 Schematics of the arc-discharge apparatus for nanotube synthesis.
18
1.4.2 Laser Ablation
Laser ablation is another efficient route to synthesize large quantities of SWCNTs
with narrow diameter distribution. Similarly to the arc discharge method, solid graphite is
used as the target. Moreover, metal particles such as Co, Fe are added to the graphite
10
targets for the synthesis of SWCNT. Carbon nanotubes generated from evaporation are
then condensed and collected in the downstream. A schematic diagram of the laser
ablation system is shown in figure 1.6.
19
During the process, an intense laser pulse is used
to ablate the graphite target which is placed in a heated furnace at 1200
o
C. A nanotube
synthesized from laser ablation is mostly in powdery morphology and in the form of
bundles consisting of tens of individually packed CNTs. Moreover, the diameter of the
nanotube can be modified by varying the growth conditions.
20
Nanotubes resulting from
the laser ablation method usually have higher quality than those produced by the arc
discharge method but at a much slower rate.
Figure 1.6 Schematic diagram of laser ablation method for production of carbon
nanotubes.
19
11
1.3.3 Chemical Vapor Deposition
Chemical vapor deposition (CVD) has been shown to be a powerful tool to
synthesize carbon nanotubes. Compared with other synthesis methods, CVD offers many
advantages including relative low growth temperature, easy operation, good
controllability, and low cost. A schematic diagram of the CVD system is shown in
figure1.7.
Figure 1.7 Schematic diagram of the CVD system for production of carbon nanotubes.
It is generally believed that the growth condition, catalyst, and supporting sub-
strate play an imperative role in determining the properties of the nanotubes. Therefore,
intensive efforts have been made to investigate the growth mechanism and the effect of
various process parameters involved in the CVD processes, including catalyst,
temperature, carbon sources, and carrier gas. Moreover, remarkable progress has been
achieved to control the orientation,
21-23
diameter,
24-27
and electronic property.
28-30
The synthesis of the carbon nanotube using CVD involving catalyst loaded in the
heated furnace with constant carbon source supply. At high temperature, solid metal
12
catalysts will deform and transform into liquid or semi-liquid droplets and those thermal
decomposed active carbon species will then dissolve in the liquid catalyst particles, when
the concentration of those active carbon species is high, ordered Sp2 carbon will start to
precipitate out, which leads to the formation of carbon nanotube growth and this is called
vapor-liquid-solid (VLS) mechanism.
31
There are two VLS growth modes: one is called
the tip growth mechanism and other is called the bottom growth mechanism.
In general, when the catalyst–substrate interaction is weak, hydrocarbon decom-
poses on the top surface of the metal, carbon diffuses down through the metal, and CNT
precipitates out across the metal bottom, pushing the whole metal particle off the
substrate (figure 1.8) and this is known as “tip-growth model”.
32
Figure 1.8 Schematic of tip-growth mechanism for carbon nanotubes synthesized by the
CVD method.
33
When the catalyst–substrate interaction is strong, initial hydrocarbon decomposi-
tion and carbon diffusion take place similar to that in the tip-growth case, but the CNT
precipitation fails to push the metal particle up; so the precipitation is compelled to
emerge out from the metal’s tip and subsequent hydrocarbon deposition takes place on
the lower peripheral surface of the metal, and dissolved carbon diffuses upward. Thus,
13
CNT grows up with the catalyst particle rooted on its base; hence, this is known as “base-
growth model”.
34
Figure 1.9 Schematic of base-growth mechanism for carbon nanotubes synthesized by
the CVD method.
33
1.5 Electronic Applications of Carbon Nanotubes
Inspired by all the achievements that have been made in the carbon nanotube
synthesis field, a lot of effort has also been invested in studying the properties of the
nanotube, especially the electrical transport properties due to its enormous potential in a
variety of novel applications such as transparent and flexible transistors, implantable
medical devices, and radio-frequency electronics.
35-38
The superior performance and
sensitivity of nanotube transistors over silicon transistors can also be used in applications
such as chemical, pressure and artificial skin sensors.
39-41
As an example, the chemical
stability of a SWNT field-effect transistor was exploited in a salt-water environment,
where the electrolyte potential was used to gate the carbon nanotube conduction. As the
nanotube transistors immersed, the gating efficiency was improved and the device
showed conceivable single molecule sensitivity even at low voltages.
42
Furthermore,
14
nanotube transistors can also act as an alternative to silicon technology beyond the
conventional scaling limits for the next generation of electronics.
43-45
The field-effect transistors made from individual single-walled nanotubes were
first reported more than a decade ago.
46,47
Since then, there have been enormous efforts
to explore the superior electrical properties of carbon nanotubes such as high current-
carrying capacity, ambipolar carrier transport, nearly hysteresis free and high carrier
mobility in the diffusive regime.
46-48
Additionally, researchers have also worked
extensively to improve the device performance by carefully studying the metal content in
relation to the nanotube diameter, improving the device structure and fabrication
technique, as well as scaling down the channel dimensions.
44,51-53
Up to date, individual
nanotube devices have demonstrated the measurement intrinsic mobility of 100 000
cm
2
/Vs at room temperature, conductance close to the ballistic transport limit (G=
4e
2
/h=155 µ S) with current-carrying capability up to 25 µ A pre tube, an on/off ratio that
can reach up to 10
7
and on-current density of 2.41 mA µ m
-1
which is more than four
times the diameter with normalized current density at an operating voltage of 0.5 V with
a channel length of sub 10 nm range.
49-52
15
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18
Chapter 2: Selective Synthesis of Semiconducting
Carbon Nanotubes
2.1 Introduction
The exceptional potential of using carbon nanotubes for beyond-silicon nanoelec-
tronics stems from the fact that they offer a combination of small size, high mobility,
ballistic transport, and small intrinsic capacitance.
1-9
However, one of the major
challenges remain to be solved is how to make the nanotube electronics practical and
scalable. To address this issue, the guided chemical vapor deposition (CVD) growth of
horizontally aligned single-walled carbon nanotubes on certain crystalline substrates like
quartz and sapphire developed by many groups including our own
10-12
is proven to be a
great platform for wafer-scale integration of aligned nanotubes into circuits and
functional electronic systems.
9,13,14
Due to significant effort devoted to the synthesis of single-walled carbon nano-
tubes, high density and good alignment have been achieved in previous studies,
15-17
which have great advantage on the device performance of nanotube electronics. However,
the co-existence of metallic and semiconducting nanotubes still remains to be a major
problem, which hampers further development of nanoelectronics based on carbon
nanotubes. To solve this problem, selective elimination using electrical breakdown is
widely used for device study.
18
However, this technique hurts the conductivity of
19
nanotubes and is hardly scalable to systems containing millions or billions of transistors.
Meanwhile, methods of separating metallic nanotubes from semiconducting ones have
been studied by using various approaches.
19-22
However, chemical separation treatment
induces defects in nanotubes and leads to contamination, which degrades the performance
of nanotubes in terms of mobility. Therefore, it is very desirable to achieve controlled
growth of semiconducting nanotubes through CVD directly.
Previously, semiconducting enriched nanotubes have been synthesized using
techniques such as gas phase
23
and supported catalyst CVD.
24-26
However, those
nanotubes typically come as bundles and consequently cannot be easily used for
nanoelectronic devices and circuits, which would prefer highly aligned nanotube
distributed over insulating substrates. For this goal, several groups have reported the
synthesis of predominant semiconducting nanotubes on flat substrates, including
approaches using ethanol/methanol mixture carbon feedstock,
27
UV irradiation during
CVD,
28
and plasma-enhanced CVD,
29-31
respectively. Nevertheless, some of those
techniques require complicated processes and the introduction of UV light or plasma may
decrease nanotube density. Moreover, the underlying mechanism for the selective growth
of semiconducting single-walled carbon nanotubes is not well understood and some
research groups have devoted efforts in this matter of hot debates.
32-35
Therefore, more
research is needed to develop a simple and reliable approach for the selective synthesis of
semiconducting nanotubes and to advance scientific understanding of the mechanism
behind.
Here, we report a facile, robust and high effective approach with the use of
isopropanol (isopropanol alcohol or IPA) to synthesize predominantly semiconducting
20
nanotubes from CVD directly. Using isopropanol as the feedstock, arrays of aligned
semiconducting carbon nanotubes have been grown on quartz substrates up to four inch
wafers. The percentage of semiconducting nanotubes is confirmed to be 97.6% for our
best sample and about 90% using both individual nanotube electrical measurements and
micro-Raman spectroscopy.
Furthermore, mass spectrometry was used to elucidate the underlying mechanism,
which suggests that the presence of right amount of water species from isopropanol
decomposition played a key role for the selective growth of semiconducting single-
walled nanotubes. It should be noted that we also performed nanotube synthesis on
silicon substrates using our isopropanol-based CVD process. As an example of
application, we have fabricated thin-film transistors (TFTs) from the as-grown
isopropanol-synthesized nanotube networks on silicon substrate and an on/off ratio over
10
4
and mobility up to 116 cm
2
/V ∙s have been achieved, which represents one of the best
reported performances for TFTs up to date. The method reported here is facile, robust,
and the results are highly reproducible. It could serve as a useful platform for practical
and scalable carbon nanotube electronics.
2.2 Wafer-Scale Semiconducting Dominant Carbon Nanotube
Synthesis
Aligned carbon nanotubes have been successfully synthesized on both small
substrates and four inch quartz wafers with isopropanol as the carbon feedstock, a picture
21
of the four inch quartz wafer after IPA synthesis is shown in Figure 2.1a. From scanning
electron microscopy(SEM) analysis, an average density of 4tubes/μm has been achieved
by using isopropanol as the carbon source and a representative SEM image is shown in
Figure 2.1b.
Figure 2.1 Wafer-scale carbon nanotube synthesis using isopropanol (a) Photograph of a
four-inch wafer with aligned nanotubes grown. (b) An SEM image of aligned nanotubes
grown from isopropanol carbon feedstock.
During our typical four inch wafer growth, quartz substrates were first annealed at
900 C for 1 hour in air to improve the alignment for nanotubes. To avoid wafer breakage,
extremely slow ramping rate (< 1 C/min) was used and constant gas flow rates were used
during both the ramping up and growth steps as reported in our previous publication.
9
After loading the quartz or silicon substrates with deposited Fe catalyst film into growth
furnace, 3600 standard cubic centimeter (sccm) of hydrogen (H
2
) was used during the
ramping up step. For nanotube growth, 1800 sccm of Ar flowing through an isopropanol
bubbler (kept at 0 C) and 1800 sccm of H
2
were used and the growth were conducted at
900 C. In order to achieve a uniform temperature throughout the entire wafer, a 9-foot-
long growth furnace with a three-zone temperature controller was used for the wafer-
scale synthesis.
a
b
22
2.3 Comparative Studies between IPA and Ethanol CVD
Ethanol has been widely used as the carbon source for nanotube synthesis.
However, nanotube result from ethanol growth generally shows the existence of one third
of metallic and two third of semiconducting nanotube. Here we demonstrate by using IPA
as the carbon feedstock, semiconducting nanotube can be synthesized with purity up to
98%. To further understand the mechanism and conform the selective synthesis of
semiconducting nanotube is exclusively result of the novel carbon source and not other
growth parameters, we carefully designed and performed a sequences of experiments to
compare the properties of carbon nanotubes synthesis from ethanol CVD with the
nanotubes synthesis from IPA CVD. Detailed the experimentation procedures and results
are described in the following text of this chapter.
2.3.1 Density and Diameter Comparison
The synthesis conditions of nanotubes growth on small substrates for both ethanol
and IPA CVD are very similar, in which a one inch quartz tube furnace was used with gas
flow rates of 300 sccm of H
2
and 160 sccm of Ar during growth process, where Ar passed
through a bubbler containing isopropanol or ethanol and kept at 0 C. From SEM images
we can see the density of nanotubes after ethanol and IPA synthesis are very similar.
Moreover, the AFM characterization suggests a diameter distribution of 1.38± 0.23 nm,
which is quite similar to the ethanol grown nanotubes with 1.49± 0.25 nm diameter
distribution as shown in Figure 2.2 c and f respectively.
23
Figure 2.2 SEM (a) and AFM (b) images of isopropanol-synthesized carbon
nanotubes.SEM (d) and AFM (e) images of ethanol-synthesized carbon nanotubes.
Diameter distribution of nanotubes synthesized using isopropanol(c) and ethanol (f).
2.3.2 Raman Comparison
Resonant Raman spectroscopy was further used to characterize the nanotubes
from different carbon feed stocks. Radial breathing modes (RBMs) were used to
distinguish the semiconducting nanotubes from metallic nanotubes. A number of RBM
spectra were acquired in both isopropanol and ethanol-synthesized nanotube samples by
applying lasers with wavelengths (energy) of 533 nm (2.33 eV), 632 nm (1.98 eV), and
785 nm (1.58 eV). Figure 2.3a and 2.3b show the Raman spectra recorded with 533 nm
excitation line for isopropanol- (Figure 3a) and ethanol-synthesized nanotubes (Figure
2.3b), respectively. Raman peaks from quartz substrate are labeled with “*”. The figures
0.5 1.0 1.5 2.0 2.5
0
1
2
3
4
5
6
7
d = 1.3765 0.233 nm
Number of nanotubes
Diameter (nm)
0.5 1.0 1.5 2.0 2.5 3.0
0
5
10
15
20
25
d = 1.4954+/-0.250nm
Number of nanotubes
Diameter (nm)
a
d
b
e
c
f
24
show that all RBM frequencies distribute from 140 to 180 cm
-1
, which belong to the
semiconducting nanotubes.
36
This is as expected because the nanotubes that are in
resonance with 533 nm laser excitation should be semiconducting according to the
diameter distributions of as-grown nanotubes.
In Figure 2.3c and 2.3d, the RBM spectra scanned using a 632 nm laser reveal
that all RBMs for isopropanol-synthesized nanotubes lie in the semiconducting region
(Figure2.3c), while RBMs for both semiconducting and metallic nanotubes were clearly
observed in the ethanol-synthesized nanotubes (Figure 2.3d), which is consistent with our
electrical measurements on both aligned nanotube array and individual nanotube devices.
As for the 785 nm laser, no obvious RBMs were observed in both samples (data not
shown) probably due to the low power of 785 nm laser we have. With the information
above, we further conclude that nanotubes produced from isopropanol as carbon
feedstock are predominant semiconducting ones as compared to ethanol.
In order to examine the quality of the as-grown nanotubes, the Raman D-band to
G-band ratio at multiple locations for both samples were measured. The results reveal
that the D/G ratios of nanotubes from isopropanol and ethanol are similar, ~0.04 on
average, indicating overall very high quality of nanotubes synthesized using both carbon
feed stocks. We note that only nanotubes in resonance with the laser energy would give
Raman signal.
25
Figure 2.3 Micro-Raman characterization. (a) RBMs of isopropanol-synthesized
nanotubes and (b) RBMs of ethanol-synthesized nanotubes using a 532 nm laser. (c)
RBMs of isopropanol-synthesized nanotubes and (d) RBMs of ethanol-synthesized
nanotubes using a 633 nm laser
2.3.3 Aligned Nanotube Device Performance Comparison
While Raman can be a very useful tool to characterize nanotubes, Raman spectra
alone does not provide definitive conclusion about what kind of nanotubes are present or
not present. We have therefore carried out extensive electrical measurements of the
aligned nanotube to compare the purity of nanotube synthesized from both ethanol and
IPA CVD.
100 120 140 160 180 200 220 240
Raman Shift (cm
-1
)
Intensity (a.u.)
*
*
Metallic Semiconducting
*
100 120 140 160 180 200 220 240
Raman Shift (cm
-1
)
Intensity (a.u.)
*
*
Metallic Semiconducting
*
100 120 140 160 180 200 220 240
Raman Shift (cm
-1
)
Intensity (a.u.)
*
Semiconducting Metallic
*
100 120 140 160 180 200 220 240
*
Intensity (a.u.)
Raman Shift (cm
-1
)
Semiconducting
Metallic
*
a b
c d
26
Aligned nanotubes using ethanol as the feedstock, which possess a similar nano-
tube density to isopropanol-synthesized samples (Figure 2.4 a, b) was synthesis on quartz
first, and then transferred onto Si/SiO2 substrate for the back-gated device fabrication.
Figure 2.4a compares the electric characteristics of aligned nanotube transistors with
isopropanol- (red trace) and ethanol-synthesized(blue trace) carbon nanotubes. Both
devices have channel widths of 50 μm and contain approximately 200 nanotubes in the
channel. The on/off ratio for the transistor using isopropanol-synthesized nanotubes is 42
as compared to 2.1 for the transistor using ethanol-synthesized nanotubes. If we follow
the method used in previous publication
27
and assume that the metallic nanotubes are
equally conductive as the semiconducting ones in their on-state, the percentage of
semiconducting nanotubes of the isopropanol-synthesized nanotube sample in Figure 2.4a
is estimated to be 97.6%. As a comparison, the ethanol-synthesized nanotube sample in
Figure 2.4a contains only roughly 52.4% of semiconducting nanotubes, which is not far
away from 67% semiconducting ratio as predicted by theory and validated by
experiments.
44
A number of such transistors using both isopropanol- and ethanol-synthesized
nanotubes were measured and the on/off ratio distributions were plotted in Figure 2.4b. It
is obvious that the average on/off ratio of the devices using isopropanol-synthesized
nanotubes is much higher than that using ethanol-synthesized nanotubes. The medium
on/off ratios are 20 and 3.5 for isopropanol- and ethanol-synthesized nanotubes,
respectively. Based on the analysis presented above, one can estimate the percentage of
semiconducting nanotubes to be 95% for isopropanol-synthesized nanotubes and 71.4%
for ethanol-synthesized nanotubes. The transfer characteristics and output characteristics
for representative transistors using isopropanol-synthesized nanotubes are shown in
Figure 2.4c and d, respectively. Both exhibit p-type behavior and linear I-V curves,
indicating ohmic contacts have been achieved.
27
Figure 2.4 (a) I
SD
-V
GS
curves of back-gate devices with isopropanol- and ethanol-
synthesized nanotubes. (b) On/off ratio distribution for devices with isopropanol- and
ethanol-synthesized carbon nanotubes. (c) Transfer and (d) Output characteristics of a
representative back-gated transistor with isopropanol-grown carbon nanotubes.
2.3.4 Mass Spectrometry Measurement Comparison
All the results above unambiguously confirm that predominately semiconducting
nanotubes were produced by using isopropanol as the carbon feed source. While this
finding is immensely useful for device applications using carbon nanotubes, it is
necessary to further understand the chemical mechanism behind such selective growth
behavior. We have performed a mass spectrometric
37
study to compare the CVD
processes for both isopropanol and ethanol, and the results are shown in Figure 2.5. We
-20 -10 0 10 20
0.01
0.1
1
10
52.4% semiconducting
I
on
/I
off
= 2.1
Isopropanol
Ethanol
I
SD
/W ( A/ m)
Gate Voltage (V)
I
on
/I
off
= 42
97.6% semiconducting
Isopropanol Ethanol
1
10
100
Samples
On/Off Ratio
Isopropanol
Ethanol
a b
-10 -5 0 5 10
0
2
4
6
8
10
12
14
0.2 V
0.4 V
0.6 V
0.8 V
Drain Current ( A)
Gate Voltage (V)
V
D
= 1 V
-1.0 -0.5 0.0 0.5 1.0
-15
-10
-5
0
5
10
15
Drain Current ( A)
Drain Voltage (V)
V
G
from -10 to 10 V
in 2 V step
c d
28
note that mass spectrometry is a very useful tool to decipher the secret of nanotube CVD.
It has been used before but the mechanism of the nanotube synthesis is still not very
clear.
38-41
In this experiment, mass spectrometry was performed with the Omnistar from
Pfeiffer (Model GSD301). In the measurement setup, the detector was connected to the
exhaust port of a one inch tube CVD system. Using this setup, the species at the CVD
exhaust port for both ethanol and isopropanol was quantitatively measured. Figure 4a
presents the intensity versus mass-to-charge ratio during synthesis. In order to get
accurate results during synthesis, both hydrogen and argon were first flowed for 5
minutes to purge the system of residual air. After that, the temperature was increased to
900° C with continued flow of hydrogen, and standard CVD was carried out at 900° C by
flowing argon through an ethanol or isopropanol bubbler. A set of mass spectra were
taken with a peak at M/Z = 20 for Ar
2+
in Figure 4 was used to normalize all the curves.
We took several mass spectra during the 15-minute growth period (blue trace for
isopropanol and red trace for ethanol), and the data was consistent throughout the entire
synthesis period. Here, the flow rates for both synthesis processes were 300 sccm of
hydrogen and 160 sccm of argon, the same with the growth procedure mentioned
previously.
On the basis of the mass spectrometric results, several main species and their
relative intensity are presented in Table 1.The main hydrocarbon species detected are
CH
3
+
, CH
4
and C
2
H
4
, while other species like C
2
H
2
+
and C
2
H
5
+
have trivial signals. By
calculating the ratios between the main hydrocarbon species and water for isopropanol
CVD, we obtained ratios of 3.9, 4.7 and 1.3 for CH
3
+
, CH
4
and C
2
H
4
, respectively.
Similarly, for ethanol CVD, we obtained ratios of 4.6, 5.4 and 3.1 for CH
3
+
, CH
4
and
C
2
H
4
, respectively
.
29
Interestingly, we noted that the hydrocarbons to water ratios are consistently
lower in isopropanol CVD than that in ethanol CVD, indicating a relatively high
concentration of water in isopropanol CVD process. Particularly, we note that the ratios
of C
2
H
4
to H
2
O are significantly different in two systems, i.e., 1.3 for isopropanol CVD
versus 3.1 for ethanol CVD. As water concentration has a profound effect on the selective
growth of SWCNTs with given property,
42
we believe this is the main difference between
the two systems.
Our results suggest that one can selective grow predominantly semiconducting
nanotubes when having the right amount of water vapor present in the CVD environment,
and isopropanol CVD happens to provide that environment. This is consistent with a
recent study where a suitable amount of water was directly introduced into CVD process
for the selective growth of semiconducting single-walled carbon nanotubes.
42
In contrast,
ethanol CVD produces relatively low water concentration than isopropanol CVD, and the
preferential growth of semiconducting nanotubes was not observed.
Figure 2.5 Mass spectra taken at the exhaust of the CVD system for ethanol (red trace)
and isopropanol (blue trace).
14 16 18 20 22 24 26 28 30 32 34
Ethanol
Isopropanol
M/Z
Relative intensity (a.u)
14 16 18 20 22 24 26 28 30 32 34
M/Z
Ethanol
Isopropanol
Relative intensity (a.u)
a b
30
Table 1. Normalized value of relative intensity of possible species from mass
spectrometry of synthesis condition.
M/Z Possible species Isopropanol Ethanol
15 CH
3
+
2.25 6.87
16 CH
4
2.7 8.15
18 H
2
O 0.58 1.5
26 C
2
H
2
+
0.049 0.32
28 C
2
H
4
0.75 4.7
29 C
2
H
5
+
0.152 1
2.3.5 Control Experiment
The mechanism of our observed preferential growth of semiconducting nanotubes
may involve multiple factors, such as suppression of metallic nanotube growth and/or
selective etching of metallic nanotubes in the isopropanol CVD environment after their
growth. In order to further investigate this point, we performed control experiments by
loading samples pre-deposited with separated high purity semiconducting nanotubes
(IsoNanoIntegris, 99% purity) and metallic nanotubes (IsoNanoIntegris, 98% purity) into
the same isopropanol CVD system. The use of such separated nanotubes clearly
distinguishes different behavior of semiconducting nanotubes with that of metallic ones
in the IPA CVD environment. Another reason of choosing these nanotubes is that they
31
have a very low content of metal residual, which excluding the re-growth of nanotubes
from metal catalysts.
17
It is known that separated nanotubes cannot completely imitate the
CVD process of aligned nanotube growth but the separated nanotubes used in this
experiment were extensively washed to remove the surfactant. After the samples went
through the standard isopropanol CVD process, no noticeable change in their density and
length was observed for both semiconducting and metallic nanotubes based on the
comparison of the AFM images (Figure 2.6). This indicates that the isopropanol CVD
does not provide selective etching of either metallic or semiconducting nanotubes. Our
results suggest that the selective growth of semiconducting nanotubes using isopropanol
carbon feedstock mostly likely occurs through suppression of the growth of metallic
nanotubes at the very beginning of nucleation.
This can be understood as short caps of metallic and semiconducting nanotubes
right after nucleation may have very different chemical reactivity.
28,43
Therefore, the
presence of the right amount of water species in the isopropanol CVD environment can
lead to preferential suppression of metallic nanotube growth.
32
Figure 2.6 AFM images of the Semiconducting nanotubes before (a) and after (b)
isopropanol CVD process. Metallic nanotubes before (c) and after (d) isopropanol CVD
process.
Our argument about the role of water for preferential growth of semiconducting
nanotubes is consistent with some of the previous work,
27,42
albeit with a lot difference in
experimental details. For instance, while ethanol alone does not lead to the preferential
growth of semiconducting SWCNTs, previously Ding et al.
27
have reported the use of
mixed ethanol and methanol for preferential growth, probably because decomposition of
methanol leads to more water species and OH radicals during the CVD process than
ethanol alone.
33
2.4 Electrical Measurement of Individual Nanotube Devices
To be more rigorous, the percentage of semiconducting nanotubes is further
evaluated by individual nanotube device measurements as illustrated in Figure 2.7. For
isopropanol-synthesized nanotubes, we patterned narrow channel devices by using a
stepper with a 0.5 μm resolution. A typical SEM image of the individual nanotube
transistor is shown in Figure 2.7a, whose channel is defined by projection photolithogra-
phy and oxygen plasma etching. A large number of such transistors were fabricated and
38 devices with one individual nanotube were selected, and their transfer characteristics
were measured as shown in Figure 2.7b.
For the measurements, the gate voltage (V
G
) was swept from -20 to 20 V with a
constant drain bias (V
D
) of 0.2 V. From the results, we found that the percentage of
devices with on/off ratios above 10 is ~90%, which is much higher than the theoretical 67%
semiconducting nanotubes ratio.
44
The typical transfer characteristics of metallic single
nanotube transistor and semiconducting single nanotube transistor are shown in Figure
2.7c and d respectively. Furthermore, the decent on-current of such individual nanotube
transistors confirms the preservation of good electrical quality in the isopropanol-
synthesized nanotubes.
34
Figure 2.7 (a) An SEM image of a representative individual nanotube device with a 4μm
channel length. (b) Transfer characteristics of 38 individual nanotube transistors, with red
traces representing the metallic nanotubes and blue traces representing the semiconduct-
ing ones. The percentage of devices with on/off ratio higher than 10 is around 90%. The
nanotubes were grown from isopropanol CVD. (c) Transfer characteristics of a metallic
single nanotube transistor. (d) Transfer characteristics of a semiconducting single
nanotube transistor.
a
b
-20 -10 0 10 20
10
-12
10
-11
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
Semiconducting
Metallic
Drain Current (A)
Gate Voltage (V)
-20 -10 0 10 20
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
VDS = 0.2V
Drain Current (mA)
Gate Voltage (V)
-20 -10 0 10 20
10
-10
10
-9
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
VDS = 0.1V
Drain Current (mA)
Gate Voltage (V)
c
d
35
2.5 Thin-film Transistor Performance of IPA nanotubes
The synthesis of predominantly semiconducting nanotubes using isopropanol may
find many applications. As an example, below we report the use of isopropanol nanotube
networks grown directly on silicon substrates for TFTs, which have great potential for
display electronics, and flexible and transparent electronics.
8,45-50
While the transistors
utilizing aligned array of semiconducting nanotubes reported above exhibit on/off ratios
in the range of 10 to 100, much higher on/off ratios are required for various applications
and can be achieved by producing random networks of isopropanol-synthesized carbon
nanotubes. By using such percolative carbon nanotube networks, the probability of
having a metallic nanotube conduction path between source and drain can be significantly
minimized, and thus high on/off ratios can be achieved as reported in our previous work
using separated nanotubes.
46-50
The fabrication of the isopropanol-synthesized nanotube
TFTs begins with the evaporation of 1 Å iron catalyst film over Si/SiO
2
substrates. The
similar isopropanol CVD process was used to grow nanotube networks on Si/SiO
2
substrates. Source and drain electrodes (Ti/Pd) were then patterned and back-gated
transistors were fabricated after etching away the unwanted nanotubes outside the active
channel region.
The SEM images of the as-grown carbon nanotube thin film on a Si/SiO
2
substrate is
shown in Figure 2.8a, and a representative nanotube TFT with a channel width of 20 μm
and a channel length of 50 μm is presented in Figure 2.8b. As shown in the image, the
nanotubes form a random network, which help to eliminate metallic nanotube conduction
path. The electrical characteristics of this device are plotted in Figure 2.8c and d. Figure
2.8c illustrates the transfer characteristics (I
D
-V
G
) at various drain biases in both linear
and logarithm scale and Figure 2.8d presents the output characteristics (I
D
-V
D
) at various
V
G
biases. The on/off ratio of this transistor is up to 10
4
(Figure 5c inset) and an effective
36
device mobility of 116 cm
2
/V ∙s is derived (detailed calculation is shown below), which
represents one of the best mobility for thin-film transistors published up to date. Our
result shows the great potential of using isopropanol-synthesized nanotube networks for
high-performance thin-film macroelectronics.
Figure 2.8 TFTs using networks of isopropanol-synthesized nanotubes. (a) An SEM
image of a nanotube network. (b) An SEM image of a back-gated TFT. (c) Transfer and
(d) output characteristics of the device in (b).
The effective mobility (μ) of transistor can be calculated using the following
equation:
μ=
L
V
d
C
w
W
∙
dI
d
dV
g
c
-20 -10 0 10 20
0.0
0.5
1.0
1.5
2.0
2.5
0.2 V
0.6 V
0.6 V
0.8 V
Drain Current ( A)
Gate Voltage (V)
V
D
= 1 V
-5 -4 -3 -2 -1 0
-10
-8
-6
-4
-2
0
V
G
from -10 to 10 V
in 2 V step
Drain Current ( A)
Drain Voltage (V)
a
b
d
-20 -10 0 10 20
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Drain Current ( A)
Gate Voltage (V)
V
DS
= 0.2 V
37
Here, L is the channel length and W is the channel width. C
W
is the specific
capacitance per unit area of nanotube channel, which can be calculated in following
equation:
Here, D is the density of nanotube, C
Q
is the quantum capacitance of nanotubes,
ε
s
is the dielectric constant, t is the thickness of the dielectric layer, and R is the radius of
nanotube. In our case, the channel length of the transistor is 50 μm and width is 20 μm.
Based on the SEM image, the density D is around 1 tube/μm, t = 500 nm, R = 1 nm, ε
s
is
approximately 4, and the quantum capacitance C
Q
of 4.0× 10
−10
F/m was used according
to previous publication. The transconductance g
m
extracted from I-V
GS
is shown in Figure
2.9. A peak effective mobility of 116cm
2
V
-1
S
-1
is achieved for this TFT nanotube device.
-20 -10 0 10 20
0.0
0.5
1.0
1.5
2.0
2.5
g
m
( s)
I
ds
Drain Current ( A)
Gate Voltage (V)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
g
m
Figure 2.9 I-Vg and g
m
curve from a TFT carbon nanotube device with 50 μm channel
length.
38
2.6 Summary
To summarize, we have developed a facile, robust, and reproducible CVD ap-
proach to grow predominantlysemiconducting carbon nanotubes by using isopropanol as
the carbon feedstock. Both electrical and Raman characterization unambiguously
confirms the significantly improved percentage of semiconducting nanotubes. We have
also performed mass spectrometric study for different carbon feed stocks to understand
the underlying mechanism behind this finding and found that water concentration may
plays a critical role in the observed selective growth behavior.Finally, TFTs with an
on/off ratio up to 10
4
and mobility up to 116 cm
2
/V∙s were demonstrated using the
isopropanol-synthesized nanotubes, which showthe great potential for future applications
in nanotube-based macroelectronics.
39
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43
Chapter 3: Chirality-controlled Synthesis of
SWCNTs using VPE
3.1 Introduction
Single-wall carbon nanotubes (SWCNTs) possess superior electrical and optical
properties and hold great promise for electronic and biomedical applications.
1-10
Since the
electronic property of a SWCNT strongly depends on its chirality, the lack of synthetic
control in chirality has long been recognized as a fundamental impediment in the science
and application of SWCNTs. Previous efforts to address this issue have resulted in
significant progress in separation of synthetic mixtures, yielding predominantly single-
chirality nanotube species.
11-15
However, separation processes are limited by their small
scale, high cost, and short length (< 500 nm) of the resulting chirality-pure nanotubes.
They are therefore not suitable for many, especially electronic device applications.
Realistic application of SWCNTs in electronic devices and integrated circuits
requires that the electronic type of the nanotubes be well controlled. Selective synthesis
of semiconducting or metallic predominated SWCNTs has already been achieved through
controlled chemical vapour deposition (CVD) growth,
16-18
enabling the fabrication of
nanotube field-effect transistors using predominantly semiconducting SWCNTs.
19,20
As a
next step, synthesis of SWCNTs with predefined chirality is highly desired. However,
this has been an elusive goal for a long time in the nanotube field.
44
For metal-catalyzed nanotube synthesis by CVD, it is generally believed that the
diameters of the nanotubes are determined by the size of the catalytic metal particles.
Unfortunately, attempts to control the size of the catalysts in hope of achieving chirality-
controlled nanotube growth have not been successful.
21
Nanotube “cloning” has been
studied with
22,23
and without
24
metal catalyst particles showing various degree of success;
however, controlled synthesis of nanotubes with predefined chiralities has not been
achieved. In contrast, vapour phase epitaxy (VPE) such as molecular beam epitaxy and
metal organic CVD has long been used by the semiconductor industry to grow two-
dimensional epitaxial films which can preserve the crystalline structure of the starting
substrate. It is therefore very intriguing to explore the potential of using VPE for one-
dimensional nanotube synthesis.
Here, we demonstrate a VPE-analogous general strategy for producing nanotubes
of predefined chirality. Our strategy is to combine nanotube separation with synthesis to
achieve controlled growth of nanotubes with preselected chirality. Tremendous progress
has been made in recent years in SWCNT separation.
11-15,25,26
In particular, DNA-based
chromatographic separation allows purification of both semiconducting and metallic
single-chirality SWCNTs through the use of different DNA sequences.
11, 26
In this work,
three exemplary chirality-pure (7, 6), (6, 5), and (7, 7) SWCNT seeds with purity up to 90%
are used. These serve as the starting templates for the ensuing catalyst-free chirality-
controlled nanotube cloning process.
We show that purified single-chirality nanotube seeds of both semiconducting and
metallic SWCNTs are significantly elongated through a catalyst-free VPE process,
45
producing horizontally aligned SWCNT arrays on quartz substrates and randomly
oriented SWCNTs on Si/SiO
2
substrates, with lengths more than tens of micrometers .
Raman characterization confirms that the original chiralities of the nanotube seeds are
preserved in the extended portion of the nanotubes, and the semiconducting nature of the
grown (7, 6) and (6, 5) SWCNTs are further confirmed using electrical measurements on
individual nanotubes.
3.2 Deposition and Activation of Chirality-pure Nanotube
Seeds
The schematic of the chirality-controlled SWCNTs cloning process is illustrated
in Figure 1shown below. First, the chirality-pure (7, 6), (6, 5), and (7, 7) nanotube seeds
were obtained using the DNA-based chromatographic purification.
12,26
Seed solutions
with a concentration of ≈ 0.5 μg/mL were drop-deposited onto quartz or Si/SiO
2
substrates and incubated for 30 minutes to 1 week followed by gentle rinse with D-I
water and blow-dry with N
2
.
46
Figure 3.1 The chirality-pure nanotube seeds obtained from DNA-based separation are
first deposited onto various kinds of substrates including quartz and Si/SiO
2
, followed by
air annealing and water vapour annealing process to activate the ends for the ensuing
VPE growth. VPE with either methane or ethanol as the carbon feed stock is then used to
achieve chirality-controlled growth from these seeds.
The as-deposited seeds then went through an air and water vapour annealing
process, which we call the cleaning process. This step removes the DNA wrapped on the
nanotube seeds, and activates the nanotube ends so that carbon reaction intermediates
from decomposed feed stock can be efficiently added. The cleaning process involves first
annealing in a 2.54 cm (1-inch) tube furnace at 200 ° C in air for 30 minutes. The
temperature was then increased to 400 ° C with the flow of 300 standard cubic centimeter
per minute (sccm) hydrogen. Once the elevated temperature was reached, argon through a
water bubbler kept at room temperature with a flow rate of 100sccm was set to flow for 3
(6, 5)
Chirality-pure seeds from
DNA-based separation
Nanotube cloning with
controlled chirality
Air annealing and
water vapor annealing
Vapor Phase
Epitaxy
Deposition of
the seeds
(7,6)
Metallic
SWCNT
Semiconducting
SWCNT
(7, 7)
47
minutes. After the air and water vapour annealing and cooling down, VPE at 900 ° C with
either ethanol or methane as carbon feed source was used for nanotube growth.
From a serial of controlled experiments, we found that the cleaning process is
essential to achieve VPE growth of nanotube as show in the SEM images in figure 3.2.As
we can see, without any cleaning treatment, only a small number of curvy nanotube
bundles were observed shown in Figure 3.2 (a). With only air annealing, some individual
cloned nanotubes present, while most of the nanotubes were still bundled together (figure
3.2b). With only water vapour annealing, as shown in figure 3.2c, most of the cloned
nanotubes were individually dispersed with no aggregation. And when both air and water
vapour annealing was used before VPE, the results were similar to the sample with only
water vapour annealing, while the sample looks slightly cleaner and the nanotube length
were slightly longer, SEM images after VPE growth is shown in figure 3.2d. These results
indicate that the water vapour annealing process plays an important role in the cloning as it
improves the density of the cloned nanotubes significantly, yielding clean and straight
nanotubes.
48
Figure 3.2 SEM images of the samples (a) without any annealing treatment, (b) with only
air annealing, (c) with only water vapour annealing, and (d) with both air and water vapour
annealing after the CVD cloning process.
3.3 VPE Cloning of SWCNTs
Both methane and ethanol were found to be suitable as the carbon feed source to
achieve chirality-controlled growth from these seeds. For both cases, 300 sccm of
hydrogen was introduced during the temperature ramping up process, which may
terminate nanotube ends with hydrogen atoms. For the ethanol-based VPE growth, 300
sccm H
2
and 160 sccm Ar flowing through an ethanol bubbler kept at 0 ° C was used; for
the methane-based VPE growth, 2000 sccm CH
4
and 300 sccm H
2
were used. For both
cases, the growth was carried out for15 minutes. We have conducted systematic
explorations to define the stoichiometry for each species during the VPE cloning and to
49
find the best recipe for cloning growth. In these experiments, 10 samples with similar
density of nanotube seeds were prepared and went through air and water vapour cleaning
process together and followed by cloning growth using ethanol as carbon source. We first
investigated the effect of hydrogen concentration during cloning by varying the flow rate
of hydrogen from 200 sccm to 400 sccm, while keeping the flow rate of argon constant at
160 sccm. After VPE cloning, we used SEM to carefully examine the length, density and
uniformity of cloned nanotubes. The results are shown in figure 3.3 a-d. Our results
indicate that cloned nanotubes have the highest density when 300 sccm of hydrogen and
160 sccm of argon are used, while higher or lower flow rates of hydrogen reduce the
yield of cloned nanotubes. Too high concentration of hydrogen is unfavourable probably
because it leads to hydrogenation and etching of nanotubes. On the other hand, too low
concentration of hydrogen will likely produce less ordered carbon structures such as
amorphous carbon, which may hamper the nanotube growth, as suggested by literature
reports.
27-29
We have also studied the effect of carbon concentration by keeping the hydrogen
flow rate constant at 250 sccm and changing the argon flow rate from 200 sccm to 108
sccm. The result is shown in figure 3.3 e-h. The highest density was achieved when
hydrogen flow rate is 250 sccm and argon flow rate is 128 sccm. We note a very
interesting observation from these studies: the highest density occurred when the ratio
between hydrogen and argon is around 2 (250/128 = 1.95 for the figure 3.3 g;
300/160=1.88 for the figure 3.3 c).
50
d
c
b
a
51
Figure 3.3 SEM images of cloned nanotubes under different flow rate of H
2
and Ar. Here
argon flows through the ethanol bubbler and therefore determines the amount of ethanol
vapour entering the CVD system. The dots in the SEM images were etched markers on
substrates which were used to locate the nanotubes.
h
g
f
e
52
3.4 Characterization of Cloned Nanotubes Using Microscopy
We used atomic force microscopy (AFM) and scanning electron microscopy
(SEM) to characterize SWCNTs before and after VPE cloning. Representative AFM
images of (7,6) SWCNT before and after cloning using ethanol on quartz substrates are
presented in figure 3.4a and b, respectively. The average length of the nanotubes after
cloning was measured to be (34.5± 17.7) μm figure 3.4d, significantly longer than the
average length of (0.34±0.15) μm for the as-purified (7,6) nanotube seeds Figure 3.4c.
The measured diameter of the cloned nanotubes were approximately 0.9 nm, consistent
with the (7,6) nanotube diameter (d = 0.89 nm). As control experiments, blank quartz
substrates and quartz substrates deposited with DNA solution but without any nanotube
seeds were subjected to the air and water vapour annealing, and then the VPE cloning. No
nanotube growth was observed after the VPE process figure 3.5. This demonstrates that
the long SWCNTs were indeed grown from the nanotube seeds.
We also carefully studied the effect of substrates on the VPE cloning process. On
Si/SiO
2
substrates figure 3.4e, random orientation was observed for the cloned nanotubes
irrespective of the gas flow direction. For nanotube cloning performed on the ST-cut
quartz substrates, the cloned SWCNTs were found to be horizontally aligned along the
crystal orientation.
30
This should be very useful for the fabrication of nanotube transistors
and integrated circuits.
Overall, the nanotube VPE process was found to be very robust and highly
reproducible. Various carbon feed stocks including methane and ethanol, and different
nanotube seeds including (7, 6), (6, 5), and (7, 7) were used successfully. The SEM
53
images of (7,6) nanotubes cloned with methane figure3.4g, (6,5) nanotubes cloned with
methane figure3.4h and ethanol figure3.4i, respectively, indicate that similar growth were
achieved for all the cases. VPE cloning process is also demonstrated for the (7,7)
armchair metallic nanotube using both Si/SiO
2
figure3.4j and quartz figure3.4k as
substrate.
Figure 3.4 a, b, AFM images of (7,6) nanotubes before and after VPE using ethanol on
quartz substrates. The scale bars in a and b are 1 m. c, d, Length distribution of the (7,6)
nanotubes before and after VPE. e, f, Comparison of (7,6) nanotubes cloned on quartz
and Si/SiO
2
substrates. g, An SEM image showing (7,6) nanotubes cloned on the quartz
substrate using methane. h, i, SEM images of (6,5) nanotubes cloned on quartz substrates
using methane and ethanol, respectively. j, k, SEM images of (7,7) nanotube cloned on
quartz and Si/SiO
2
substrates. The scales bars are 50 m for e, f, and j, 30 m for g, h, and
i, and 10 m for k.
0.0 0.2 0.4 0.6 0.8
0
5
10
15
20
25
30
35
Number of nanotubes
Length ( m)
Length before cloning
d = 0.34 ± 0.15 m
(7,6) nanotubes
methane VPE
g e f
h i
SiO
2
Quartz
(6,5) nanotubes
methane VPE
(6,5) nanotubes
ethanol VPE
j
(7,7) nanotubes
methane VPE
(7,7) nanotubes
methane VPE k
10 20 30 40 50 60 70 80
0
1
2
3
4
5 Length after cloning
d = 34.5 ± 17.7 m
Number of nanotubes
Length ( m)
a
b
c
d
Gas flow direction
54
Figure 3.5 AFM images of DNA (without nanotubes) after deposition on quartz (a), after
VPE (b), and an SEM image of the substrate after VPE (c), No nanotube growth was
observed on the substrates, excluding any possible effect from DNA on nanotube cloning.
3.5 Chirality Identification by Raman Spectroscopy
To determine whether the original chiralities were preserved during the cloning
process, we used micro-Raman to characterize the pristine and cloned SWCNTs. For the
(7,6) case, we used 1.96 eV laser excitation, which is close to the second optical
transition E
22
= 1.92 eV of the (7,6) nanotubes.
31
To further enhance the signal-to-noise
ratio, surface-enhanced Raman spectroscopy was used after e-beam evaporation of 5 nm
silver on the substrate.
2.5µm
VPE with DNA as seeds
b
2.5µm
DNA after deposition
a
c
100 µm
d
55
The radial breathing mode (RBM) Raman spectra taken at different random
locations on the substrates before and after cloning are presented in figure 3.6a and b,
respectively. For small diameter nanotubes such as (7, 6), (6, 5), and (7,7), adjacent
nanotubes have very distinct RBM frequencies (figure 3.6f)
32
, allowing unambiguous
chirality assignment by determining the RBM frequency and the laser excitation energy.
Figure 3.6a and b show predominant peaks at 265 cm
-1
, which correspond to the RBM of
(7, 6) nanotubes, indicating that the chirality is indeed preserved in the cloned nanotubes.
The minority peaks at 300 cm
-1
and 307 cm
-1
can be attributed to the (8, 3) and (9, 1)
impurities.
We have also performed Raman mapping along a specific nanotube as shown in
figure 3.6e, confirming that the chirality is preserved along the whole nanotube. Similar
studies were carried out for (7, 7) nanotubes using 2.4 eV laser excitation, Raman spectra
of (7, 7) before (Fig. 3c) and after VPE growth (figure 3.6d) showed predominant peaks
at 249 cm
-1
, which correspond to the RBM of (7, 7) nanotubes. The weak peak at 303 cm
-
1
is attributed to the Si/SiO
2
substrate background. The RBM intensity of cloned
nanotubes is weaker than that of the nanotube seeds. This is because many nanotube
seeds got etched away during the annealing and subsequent VPE step, thus leading to
reduced nanotube number density and reduced Raman intensity.
56
Figure 3.6 (a, b) Raman RBMs of (7,6) nanotubes before and after VPE. The spectra
were taken at different random locations on the quartz substrates with laser excitation
energy of 1.96 eV. The peaks at 265 cm
-1
correspond to the RBM of (7,6) nanotubes.(c,
d)Raman RBMs of (7,7) nanotubes before and after VPE. The spectra were taken at
different random locations on the Si/SiO
2
substrates with laser excitation energy of 2.4 eV.
The peaks at 249 cm
-1
correspond to the RBM of (7,7) nanotubes. (e) Raman RBM
spectra along a specific nanotube confirming that the chirality is preserved after the VPE
process. Inset, an SEM image (with artificial color) of the nanotube. (f)Kataura plot
shows the RBM frequencies and E
ii
of different nanotubes. The values were taken from
the reference.
29
230 240 250 260 270 280 290 300 310
1.9
2.0
2.1
2.2
2.3
2.4
2.5
Laser Energy
Laser Energy
(6,5)
(7,7)
E
ii
(eV)
Raman shift (cm
-1
)
(7,6)
Laser Energy
200 220 240 260 280 300 320
Intensity (a.u.)
Raman shift (cm
-1
)
RBM
= 265 cm
-1
(7,6) nanotubes
(7,6) after cloning
200 220 240 260 280 300 320
Intensity (a.u.)
Raman shift (cm
-1
)
RBM
= 265 cm
-1
(7,6) nanotubes
(7,6) before cloning
150 200 250 300 350 400 450 500
Intensity (a.u.)
Raman shift (cm
-1
)
RBM
= 249 cm
-1
(7,7) nanotubes
(7,7) before cloning
150 200 250 300 350 400 450 500
Intensity (a.u.)
Raman shift (cm
-1
)
RBM
= 249 cm
-1
(7,7) nanotubes
(7,7) after cloning
c
d
f
200 220 240 260 280 300 320
Intensity (a.u.)
Raman shift (cm
-1
)
RBM
= 265 cm
-1
(7,6) nanotubes
e
a
b
57
3.6 Electron Transport Measurement of Cloned Nanotubes
Back-gated nanotube field effect transistors were fabricated to further characterize
the electrical properties of the cloned (7, 6) and (6, 5) nanotubes. The device schematic
and an SEM image of a representative device consisting of an individual (7, 6) nanotube
are shown in figure 3.7a and b, respectively. In brief, the cloned (7,6) nanotubes grown
on quartz substrates were transferred to Si/SiO
2
substrates with 50 nm SiO
2
using
methods described in our previous publication
9
. This was followed by formation of Ti/Pd
(0.5/50 nm) source/drain metal contacts using lithography and lift-off techniques. After
production, the device had a channel length of L = 4 μm and a 50-nm-thick SiO
2
gate
dielectric.
The transfer characteristics (I
D
–V
G
) measured at various V
D
biases, shown in
figure 3.7c, clearly indicate that the device is made of a semiconducting nanotube. This is
consistent with the (7,6) chirality. Moreover, this nanotube delivered a respectable on-
current of 5.4 μA at V
D
= 1 V and V
G
= -15 V, which corresponds to ~ 6 mA/μm when
normalized by the nanotube diameter 0.89 nm. The maximum transconductance
measured at V
D
= 1 V was further calculated to be 0.74 mS/μm. The linear output (I
D
–V
D
)
characteristics of the same device shown in figure 3.7d also indicated formation of ohmic
contacts between the nanotube and the metal contacts.
We measured a total of 17 such devices produced with individual cloned (7, 6)
nanotubes and found that all of them exhibit similar semiconducting behavior. Similar
electrical characteristics were also observed for transistors made from cloned (6, 5)
nanotubes. These electrical measurements further confirm that chirality-controlled
58
cloning was achieved, and moreover demonstrate the preservation of superior electrical
properties in the cloned nanotubes.
Figure 3.7(a, b) Schematic a and an SEM image (with artificial color) b, of a back-gated
(7,6) nanotube transistor. The device consists of an individual (7, 6) nanotube, with L = 4
μm and 50-nm-thick SiO
2
gate dielectric. The scale bar is 5 m for (b. c).Transfer
characteristics (I
D
–V
G
) of the transistor measured at various V
D
biases. Inset, transfer
characteristics plotted in semi-logarithm scale with V
D
= 1 V. d, Output (I
D
–V
D
)
characteristics of the same device measured at various V
G
biases from -20 to 20 V.
3.7 Discussion
What is the mechanism of the VPE-based SWCNT cloning? In what follows, we
attempt to provide a molecular model to explain the effect of annealing, SWCNT growth,
and chirality-dependent growth rate. We emphasize the tentative nature of this model,
-15 -10 -5 0 5 10 15
0
1
2
3
4
5
6
0.2 V
0.4 V
0.6 V
Drain Current ( A)
Gate Voltage (V)
V
D
= 1 V
0.8 V
-1.0 -0.5 0.0 0.5 1.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
Drain Current( A)
Drain Voltage (V)
V
G
from -20 to 20 V
in 5 V step
a
b
c
d
-15 -10 -5 0 5 10 15
0.01
0.1
1
10
Drain current ( A)
Gate voltage (V)
V
D
= 1 V
59
and present it in the spirit of stimulating further exploration of the VPE-based SWCNT
cloning.
We find that annealing is a critical step for successful SWCNT cloning. The edges
of purified SWCNT seeds are created by the sonication process for dispersion. Cavitation
ruptures C-C bonds, most likely leading to oxidation of carbons at the cutting edges to
form –COOH, or reconstruction of carbons at the edges to form 5 or 7 membered ring
structures, making the edges not suitable for continued growth. Air oxidation and
subsequent hydrogen and water treatment most likely remove these defective carbons and
expose more reactive hydrogen-terminated sp
2
carbon edges for growth (figure 3.8a).
What are the chemical reactions responsible for the observed nanotube growth in
the absence of metal catalyst? VPE growth takes place at high enough temperature to
thermally decompose carbon feedstock (methane or ethanol). There is a large body of
literatures covering extensive studies on the pyrolysis process. To simplify our discussion
and without loss of generality, we focus on the methane case
33-35
. Overall pyrolysis of
methane can be described by the reaction CH
4
-> C (solid) + 2H
2
. It starts with the rate-
limiting radical generation reaction: CH
4
-> CH
3
.
+ H
.
. Following this is the formation of
a variety of hydrocarbon species, including acetylene C
2
H
2
and ethylene C
2
H
4
.
Indeed, we have detected C
2
H
2
and C
2
H
4
under our VPE growth condition using
mass spectrometry, consistent with literature reports. Interestingly, also well-documented
is the generation of high molecular weight species by the pyrolysis process, including
polycyclic aromatic hydrocarbons, tar, and coke. As a matter of fact, at temperatures
above 850 º C, solid carbon and hydrogen are thermodynamically more favored than any
60
other hydrocarbon products
36
. Collectively, these results suggest that the chemistry
between graphitic carbon structures and pyrolysis products is rich and may be exploited
for SWCNT growth.
In the context of these previous works, the essence of SWCNT cloning by VPE is
to incorporate certain pyrolysis products into a graphitic carbon structure defined by the
purified SWCNT seed. We propose that this is achieved by covalent addition of C
2
H
2
and
C
2
H
4
at the SWCNT edge. More specifically, we propose that the chemistry between
C
2
H
2
/C
2
H
4
and SWCNT edge follows the classic organic chemistry reaction for the
generation of six-membered rings: Diels-Alder cycloaddition. As shown in figure 3.8b,
acetylene C
2
H
2
can be incorporated into a site with armchair configuration through Diels-
Alder reaction (reaction 1), followed by a rearomatization step (reaction 2). Similar
reactions between C
2
H
2
and polycyclic aromatic hydrocarbons in organic solvents have
been recently shown and proposed as the basis for metal-catalyst free synthesis of
SWCNTs by Port et al.
37,38
. Likewise, C
2
H
4
can also be incorporated into an armchair
site through Diels-Alder reaction (reaction 3). The C-C single bond created by reaction 3
can be converted into a double bond via a radical (CH
3
.
or H
.
) catalyzed step (reaction 4).
In both cases, the final product is a new six-membered sp
2
carbon ring built on the
SWCNT edge. The whole process can be repeated at other existing or newly built
armchair sites resulting in elongation of the SWCNT with conservation of chirality
defined by the seed. Growth termination may occur as a result of edge reactions with
other reactive pyrolysis products.
The Diels-Alder chemistry in the cloning mechanism requires the presence of
armchair sites on the SWCNT edge. A naturally consequence of this requirement is
61
chirality-dependent growth rate. In general, for a (n,m) tube, there are m armchair sites at
the edge, and the rate of carbon incorporation should be proportional to m. The rate of
tube length elongation R can be obtained after normalizing the rate of carbon
incorporation by the tube circumference:
𝑅 ∝
𝑚
𝑛 2
+𝑛 𝑚 +𝑚 2
(1)
Fig. 5c illustrates the end edges of two semiconducting tubes of the same diameter
(6,5) and (9,1), unzipped along tube axis. According to the above analysis, (6,5) tubes
should grow 5 times faster than (9,1). Precise rate measurement is challenging due to
short growth duration and non-synchronized initiation of growth
39
. Nevertheless, our
preliminary data does show that (6,5) tubes grow noticeable faster than (9,1) tubes (figure
3.9). Interestingly, our conclusion of chirality dependent SWCNT growth rate for VPE is
similar to that of Ding et al. for metal-catalyzed SWCNT growth.
40
This is because the armchair growth site in our model is equivalent to the “kink”
or “cozy corner” in the model of Ding et al. It should be noted that there are alternative
mechanisms proposed for graphene edge growth without metal catalyst, such as hydrogen
atom catalyzed addition of C
2
H
2.
41
Further investigations are needed to develop
quantitative growth rate measurement and to evaluate the role of different mechanisms in
the VPE cloning process.
62
Figure 3.8 (a) the effect of annealing.(b) Diels-Alder chemistry for the incorporation of
C
2
H
2
and C
2
H
4
into an armchair site on the edge of an growing SWCNT seed. (c)Edge
structures of unzipped (6,5) and (9,1) tubes. The red letter a denotes an armchair site on
the edges.
63
Figure 3.9 SEM images of (6,5) and (9,1) CNTs after VPE for different growth times. a,
(6,5) for 20 s with average length of 37.5µ m, b, (6,5) for 40s with average length of
35.7µ m, c, (9,1) for 40s with average length of 11.9µ m, and d, (9,1) for 60s with average
length of 17.3µ m.
3.8 Summary
In conclusion, we have demonstrated that chirality-controlled SWCNT growth
can be achieved using catalyst-free VPE growth, which produces single-chirality
horizontally aligned nanotubes with significantly longer lengths than the as-purified seeds.
Although this work only focused on the semiconducting (7, 6), (6, 5), and metallic (7,7)
nanotube species, we believe that the cloning process described here should be applicable
to other semiconducting and metallic chiralities.
26
Our cloning method therefore opens
up possibility to study catalyst-free nanotube VPE growth mechanism as a function of
64
chirality. With its versatility and reproducibility, the cloning technology platform should
greatly benefit the device and circuit application of nanotubes. Moreover, future research
on improving the cloning yield, density, and precise positioning of the nanotube seeds
could potentially lead to the long-dreamed integrated functional “carbon-only” electronic
systems.
65
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68
Chapter 4: Chirality-dependent Vapour-phase
Epitaxial Growth and Termination of Single-wall
Carbon Nanotubes
4.1 Introduction
Single-wall carbon nanotubes (SWCNTs) can be conceptually considered as roll
up cylinders from graphene sheets. The structure of a SWCNT is uniquely determined by
a pair of integers, (n, m), named chiral indices, or equivalently, diameter (d) and chiral
angle ( θ).
1
The electronic and optical characteristics of SWCNTs relay on their
chiralities
2,3
, and chirality-controlled preparation is crucial for both basic research and
many practical applications of SWCNTs.
4,5
Direct synthesis
6-11
has demonstrated certain
degree of success to control the structure and properties of SWCNTs, but still far away
from the goal of single chirality growth. From a synthetic point of view, the final product
distribution, i.e., the population of each (n, m) SWCNT, can be expressed as:
P n, m ∝ N n, m, t,… ×
t
0
R n, m, t,… dt (1)
Here, P is the population, N is the nucleation density, R is the growth rate, and t is
the growth time for specific (n, m) SWCNT. Therefore, the population of each SWCNT
is jointly determined by N and R, corresponding to the nanotube nucleation and growth
steps, respectively. It is speculated that both steps may be dependent on the chirality of
nanotubes, and are influenced by a vast amount of experimental parameters, e.g.,
catalysts, catalyst supports, and their pretreatments,
6,7,9-11
growth time,
12
temperature,
69
6,13
carbon precursors and their feeding rates,
8
making chirality-controlled SWCNT
synthesis an intractable task.
Previous theoretical simulations have investigated the nanotube nucleation
processes and revealed chirality-dependent nucleation possibilities (or nucleation density
N) based on thermodynamic considerations.
14,15
However, experimental observation of
chirality-dependent nanotube nucleation is extremely challenging owning to their very
fast nucleation dynamics
16
and very small scale,
17
which require characterization
methods to have ultrafast response time and ultrahigh spatial resolution. Following
nucleation is the nanotube growth kinetics (elongation), which is characterized by the
chirality-dependent nanotube growth rate (R) and lifetime ( τ). Chirality-dependent growth
rate was only very recently been discussed
18,19
while for the chirality-dependent lifetime
( τ) of nanotubes, as far as we known, have rarely been studied. We note that in nearly all
nanotube synthetic methods including chemical vapour deposition (CVD), arc discharge,
and laser ablating, neither nucleation nor growth step can be fixed or isolated, making
chirality-controlled growth an elusive goal.
Recently, we have developed a vapour phase epitaxial (VPE) based cloning
strategy that uses DNA-separated chirality pure nanotube seeds for chirality-controlled
synthesis of SWCNTs.
20
The uniqueness of such process is that the single chirality
nanotube seeds, which initiate succeeding nanotube growth, are pre-existing already.
Thanks to this advantage, the nanotube nucleation step, which has been demonstrated to
be an intractable process to control,
14,15
is circumvented. Therefore, the VPE process
serves as a valuable platform to unambiguously distinguish nucleation from growth
processes and to focus solely on chirality-dependent growth kinetics of SWCNTs.
70
Inspired by this deliberation, in this Letter, we have systematically studied a total of
seven single chirality SWCNTs for VPE cloning growth to investigate the different
growth behaviors of these SWCNTs and correlate to their chiralities. Noticeably, the
SWCNTs we chose contains both metallic and semiconducting ones, which span a large
chiral angle range but with similar diameters, thus singling out the impact of chiral angles
on the growth rates and lifetimes of SWCNTs.
4.2 VPE-based Nanotube Cloning
The VPE based cloning process follows our previous report
20
with some
improvements. Single-chirality SWCNTs used for the current study were purified
following the previously published procedures.
21,22
Briefly, for a target (n, m) chirality
SWCNT, a specific ssDNA sequence identified previously was used to disperse either
HiPCO or CoMoCAT synthetic mixture. The dispersion was then fractionated by ion-
exchange chromatography (IEX). The fraction containing the desired (n, m) SWCNTs
was identified by UV-vis-near IR absorption spectroscopy.
The first step for VPE-based cloning is to deposit nanotube seeds on quartz
growth substrates. We found that the deposition recipes for each chirality SWCNTs
varied largely and related to various properties of seeds solution including their
concentration (which is characterized based on the optical absorption intensity at E
22
for
semiconducting SWCNTs and E
11
for metallic SWCNTs), lengths, and possibly the
structure of DNA motif which was used to separate and disperse the nanotubes in
solution. In addition, the surface cleanness of the substrates was also found to influence
the seed adsorption significantly. The deposition time, which varied from 1 hr to up to 10
71
days, was carefully optimized to obtain nanotube seeds with reasonable density on
substrates for each seeds.
Three step pre-treatments were preformed before VPE cloning. First, the nanotube
seeds on substrates underwent an air treatment at 200 C for 30 min in a one inch CVD
furnace. Second, a H
2
O vapour treatment was conducted at 400 C for 3 min. During
H
2
O treatment, Ar (100 sccm) flowed through a vial containing H
2
O kept at 25 C, and
H
2
(300 sccm) was directly introduced into the furnace. The above two treatments may
remove defective edge structures and eliminate some functional groups. Third, a high
temperature H
2
(300 sccm) treatment was conducted to further clean the nanotube seeds
and to expose the fresh edges of seeds. After the above pre-treatments, mixture carbon
sources (CH
4
/C
2
H
4
=1800/10 sccm) together with H
2
(300 sccm) were introduced to
initiate nanotube growth at 900 C. We found that the addition of 10 sccm C
2
H
4
increase
the yield of our VPE cloning process noticeably (figure 4.1).
For experiments with short growth times, we (i) open the furnace to quickly cool
down the system and (ii) flow large amount of H
2
(1000 sccm) right after the set time
reached to control the growth time
23
.
Figure 4.1: Cloning yield improvement. (a) SEM images of cloned (6, 5) SWCNTs using
methane as carbon source. (b) SEM images of cloned (6, 5) SWCNTs using a mixture of
methane and ethylene as carbon source.
72
Multiple samples grew under the same condition show similar average lengths as
shown in figure 4.2, indicating the reliability and good reproducibility of short time
growth experiments.
Figure 4.2 Length distributions and average length L
of three different (7, 7)/20s samples
shows the similar average length and thus small sample-to-sample variations.
4.3 Chirality-dependent Growth Rate and Length Distribution
of SWCNTs
To obtain information on the growth kinetics of SWCNTs, we first performed
experiments at very short growth time of tens of seconds. Figure 3.4a, b, and c presents
SEM images of (6, 5), (8, 3), and (9, 1) SWCNTs with a growth duration of 20s, 20s, and
40s, respectively. These three nanotubes are semiconducting type with nearly identical
diameters, but distinct chiral angles. Specifically, (6, 5) is a near armchair SWCNT with a
chiral angle of 27.0
o
, (8, 3) has a chiral angle of 15.3
o
, while (9, 1) represents a near
zigzag SWCNT with a chiral angle of only 5.2
o
. We further analyzed the length
information of these three samples and deduced their average lengths. As can be clearly
seen from figure 3.4d, 1e, and 1f, these three kinds of nanotubes have very different
length distribution. For example, (6, 5) SWCNT has an average length(𝐿
)of 37.5 mafter
5 10 15 20 25 30 35 40 45 50
0
10
20
30
40
Counts
Length ( m)
5 10 15 20 25
0
5
10
15
20
25
Counts
Length ( m)
0 5 10 15 20 25
0
5
10
15
20
25
Counts
Length ( m)
6.4 um is 1003d,
6.6 um is unknown, from a previous talk
8.0 um is 186#
a b c
3# sample
(7, 7), 20 sec
L=8.0 m
1# sample
(7, 7), 20 sec
L=6.4 m
2# sample
(7, 7), 20 sec
L=6.6 m
73
20s growth, which is much longer than (8, 3) SWCNT under the same growth duration,
i.e., 8.4 m. For (9, 1) SWCNT, only after a minimum time of 40s could we observe
sufficient amount of SWCNTs, which have an average length of 11.9 m.
Figure 4.3(a, b, c) SEM images of VPE grown (6,5), (8,3), and (9,1)SWCNTs with a
growth time of 20s, 20s, and 40s, respectively. Scale bars are 50 m for a, 20 m for both
b and c.(d, e, f,) Corresponding length distribution of SWCNTs from images (a, b, and c).
4.4 Length Evolution Profiles of Semiconducting SWCNTs
While the results from very short growth time do suggest significant differences
among these three SWCNTs, they are insufficient to extract the growth kinetics of each
SWCNT. To get comprehensive information on the growth kinetics, we systematically
studied the growth of five types of semiconducting SWCNTs under various growth times,
i.e., 20s, 30s, 40s, 60s, 120s, and 900s (figure 4.54.6, 4.7, and 4.8).
0 10 20 30 40 50 60
0
10
20
30
40
50
Counts
Length ( m)
0 20 40 60 80 100
0
5
10
15
20
25
30
Counts
Length ( m)
0 3 6 9 12 15 18 21
0
5
10
15
20
25
30
Counts
Length ( m)
d e f
a b c
(6, 5), 20 sec
L=37.5 m
(8, 3), 20 sec
L=8.4 m
(9, 1), 40 sec
L=11.9 m
74
Figure 4.4 SEM images (left) and corresponding length distribution (right) of cloned (6,5)
SWCNTs with growth time of (a) 40s, (b) 60s, (c) 120s, and (d) 15min.
75
Figure 4.5 SEM images (left) and corresponding length distribution (right) of cloned (8,3)
SWCNTs with growth time of (a) 30s, (b) 40s, (c) 60s, (d) 120s, and (e) 15min.
76
Figure 4.6 SEM images (left) and corresponding length distribution (right) of cloned (9,1)
SWCNTs with growth time of (a) 60s, (b) 120s, and (c) 15min.
77
Figure 4.7 SEM images (left) and corresponding length distribution (right) of cloned (7,6)
SWCNTs with growth time of (a) 20s, (b) 40s,(c) 60s, (d) 120s, and (e) 15min.
78
Figure 4.8 SEM images (left) and corresponding length distribution (right) of cloned
(10,2) SWCNTs with growth time of (a) 40s, (b) 60s,(c) 120s, and (d) 15min.
79
4.5 Chirality-dependent Growth Rate and Lifetime of
SWCNTs
By extra all the length information from these five the semiconducting nanotubes
grown under different time, is shown in the colored dots in figure 4.9a. It is evident that
these nanotubes possess distinct growth kinetics, especially during the initial growth
period, as shown in figure 4.9b.To quantitatively analysis the nanotube growth kinetics
and to model the growth process that has finite period, i.e., growth plus termination, we
assume the average growth rate (R
t
) of a (n, m) SWCNT at time t follows exponential
kinetics:
(2)
Here, R
0
and τ is the average initial growth rate and lifetime for a SWCNT.
Consequently, the average nanotube length (L
t
) at time t would be:
(3)
We used equation 3 to fit the length evolution profiles of the preceding mentioned
five SWCNTs (solid curves in figure 4.9a) and extracted the two parameters, i.e.,R
0
and
τfor each nanotube. Here we emphasize that the nanotubes we studied fall into two
subgroups with very similar diameters in each subgroup (inset of figure 4.9a), thus
excluding any effect of nanotube diameter on their growth kinetics.
12,24
80
Since all these five nanotubes are semiconducting ones, the only noticeable
structural parameter difference within each subgroup is the chiral angle. Therefore, the
above results suggest a clear chiral angle dependent growth behavior of SWCNTs in
VPE-based cloning process.
We further plotted chiral angle versus growth rate and lifetime for these five
SWCNTs in figure 4.9c and d. As can be clearly discerned from figure 4.9c, nanotube
growth rate increases with increasing its chiral angle, a result similar to that of a very
recent report on metal catalyst driven CVD grown nanotubes
19
. More interestingly, we
observed an opposite trend for chiral angle dependent lifetimes, i.e., the lifetime of a
nanotube decreases with increasing its chiral angle (figure 4.9d). Specifically, both (9, 1)
and (10, 2) nanotubes possess lifetime of ~80s under our growth condition, which are
much longer than (8, 3) nanotube with a lifetime of ~40s and (6, 5) and (7, 6) with
lifetime of less than 20s. Therefore, we emphasize that the final product distribution of a
SWCNT ensemble neither solely relies on their growth rates nor their active lifetime as
understood previously, but on their product R
0
× τ.
81
Figure 4.9 a, Length evolution profiles and fitted curves based on eq 3. for (6,5), (8,3),
(9,1), (7,6), and (10, 2) SWCNTs with growth times of 20s, 40s, 60s, 2min, and 15 min.
Inset, chiral angle versus diameter for the above five semiconducting SWCNTs, showing
they belong to two subgroups with similar diameters in each one as highlighted by
different colours. b, Zoom-in plot of panel a shows the initial growth period. c, d, Chiral-
angle-dependent growth rate (R
0
) and lifetime ( τ) of the above five semiconducting
SWCNTs. The vertical error bars in c and d correspond to the errors of parameters
extracted based on equation 3.
4.6 Atomic Illustration of Chirality-dependent SWCNT
Growth via Diels-Alders Cycloaddition Processes
We propose that the above chirality dependent growth rate and lifetime
phenomena can be interpreted in the framework of Diels-Alder cycloaddition
mechanism
16,20,25
. According to the Diels-Alder chemistry, active carbon species will
only be added to the armchair sites (serve as dienophile) on the open edges of SWCNTs
0 200 400 600 800
0
10
20
30
40
(10,2)
(7,6)
(8,3)
(9,1)
Length ( m)
Growth time (s)
(6,5)
a b
c d
0 20 40 60 80 100 120 140
0
10
20
30
40
(10,2)
(7,6)
(8,3)
(9,1)
Length ( m)
Growth time (s)
(6,5)
0.68 0.72 0.76 0.80 0.84 0.88 0.92
0
5
10
15
20
25
30
(10,2)
(7,6)
(9,1)
(8,3)
Chiral angle (
o
)
SWCNT diameter (nm)
(6,5)
0 5 10 15 20 25 30
0
1
2
3
4
(7,6)
(6,5)
(8,3)
(10,2)
Growth rate (R
0
, m/s)
Chiral angle (
o
)
(9,1)
0 5 10 15 20 25 30
0
20
40
60
80
(7,6)
(6,5)
(8,3)
(10,2)
Lifetime ( , s)
Chiral angle (
o
)
(9,1)
Size: 28 and 32
82
during VPE based cloning process.
20
Note that a particular (n, m) SWCNT has a total of
m armchair sites (figure 4.10a and e) which can simultaneously accept coming carbon
species for nanotube growth (figure 4.10a-c and figure 4.10e-g).
Therefore, it is a natural consequence that nanotubes with large m possess high
growth rate. In real nanotube growth process, however, the situation may not be that ideal
and we speculate that each adding step has a certain failure possibility. For example,
addition of CH
x
or C
3
H
x
species instead of C
2
H
x
will generate five- or seven-membered
ring on nanotubes (figure 4.10d, h), which in turn will prevent further nanotube growth
via DA chemistry. Based on recent theoretical studies,
16,26
the formation of six-
membered ring is more energetically favourable than other kinds of structures especially
when the nanotube segment is long.
One should note that even a small difference in energy would lead to a huge
difference in event possibility owning to their exponential relationship, suggesting it
would have a rather low possibility for five or seven-membered ring being formed during
the process. Nevertheless, as discussed above, it still has a very low but certain possibility
of five or seven-membered rings formation, which in turn will terminate nanotube growth.
We emphasize that for nanotubes with large m, the adding events happen more frequently
than those nanotubes with small m. Consequently, it will take much shorter time for these
larger m nanotubes to form the five or seven-membered rings than the smaller m ones
from statistical point of view, suggesting a shorter lifetime for larger m nanotubes. Note
that this does not necessarily mean lifetime is directly proportional to 1/m, as the
nanotube diameter may also play a role on this.
24
Nevertheless, it is safe to conclude,
based on the Diels-Alder process illustrated in figure 4.10, that SWCNTs with larger
83
mwould have higher growth rates and shorter active lifetimes. This conclusion agrees
very well with our experimental observations as for nanotubes with similar diameters,
large chiral angle corresponds to large m.
Figure 4.10 (a, b, c)Cycloaddition of C
2
H
x
species to a (6, 5) SWCNT and the formation
of six-membered rings, leading to the continuous growth of this nanotube.(d) Addition of
CH
y
species leads to the formation of five-membered ring and consequently nanotube
growth stops. e, f, g, Addition of C
2
H
x
species to a (9, 1) SWCNT for its continuous
growth. h, Addition of CH
y
species leads to the growth stops. Multiple arrows from (b to
c) and (f to g) represent multiple addition reactions.
4.7 Chirality-dependent growth of armchair metallic SWCNTs
Among all kinds of SWCNTs, armchair (n, n) nanotubes are of particular interest
since they are the only true metallic nanotubes with zero bandgaps and linear energy
C
2
H
x
C
2
H
x
CH
y
C
2
H
x
CH
y
a b c d
e f g h
C
2
H
x
(9, 1)
(6, 5)
84
dispersion relationships at Dirac point.
1
Our DNA-based separation method is capable of
purifying such armchair metallic nanotubes, for example, (6, 6) and (7, 7), with high
purity.
22
Here, we chose these two kinds of armchair nanotubes and studied their growth
kinetics. Showing in figure 4.11a and b are the SEM images of the (6, 6) and (7, 7)
SWCNTs with a growth time of 20s. Surprisingly, we found that both nanotubes have
relatively short average length (11.5 m for (6, 6) and 6.4 m for (7, 7)). We further
studied the length evolution of these two armchair SWCNTs with growth time of 40s and
60s (figure 4.12). As shown in figure 4.11c and d, both nanotubes stop growth within 20s,
indicating a very short lifetime which is consistent with our experimental observations
and Diels-Alder cycloaddition mechanism proposed on large chiral angle SWCNTs.
While it is difficult to accurately calculate the growth rate for these two nanotubes
since they saturated within the shortest experimental time, we would like to comment on
their saturated lengths and compare with their semiconducting counterparts, e.g., (6, 6)
versus (6, 5). The most noticeable difference is that (6, 6) nanotubes possess much
shorter, ~1/3, saturated length as compared with (6, 5) ones, which has never been
reported before. We note one interesting phenomenon is that in real SWCNT samples,
armchair ones abnormally do not occupy a large population based on the electron
diffraction analysis.
27
This result is consistent with our observation here shown that
armchair nanotubes possess shorter saturated lengths than other nanotubes, i.e., they are
less abundant in a nanotube ensemble. We noticed that armchair nanotubes are the only
true metallic ones with zero bandgap, thus it is intriguing to study whether the electronic
properties of nanotubes have a strong effect on the growth. Further experiments which
can well control the growth period into very short time, e.g., few seconds,
12
combined
85
with theoretical simulations, may help to understand the abnormal behaviour of armchair
SWCNTs, which would open up a novel window to exploring metallic/semiconducting
controlled synthesis.
Figure 4.11(a,b)Representative SEM images of cloned (6,6) and (7,7) SWCNTs with a
growth time of 20s. Scale bars are 20 m.(c,d) Length evolution of (6,6) and (7,7)
SWCNTs with growth time of 20s, 40s, and 60s.
20 30 40 50 60
10
12
14
Length ( m)
Growth time (s)
20 30 40 50 60
6
8
10
Length ( m)
Growth time (s)
c d
a b
0221 version
Most recent
(6, 6) (7, 7)
86
Figure 4.12 SEM images (left) and corresponding length distribution (right) of cloned
(6,6) SWCNTs with growth time of (a) 40s, (b) 60s and cloned (7,7) SWCNTs with
growth time of (c) 40s, (d) 60s.
87
4.8 Summary
In summary, we have isolated nanotube elongation and termination from their
nucleation and studied chirality-dependent growth rate and active lifetime of both
semiconducting and metallic SWCNTs in a novel VPE-based cloning platform. Our
research reveals distinct growth behaviours among SWCNTs and correlate with their
structures within the framework of Diels-Alder chemistry. The abnormal growth
behaviour of armchair nanotubes suggests a possible correlation between nanotube
growth and their electronic characteristics. This work provides direct experimental
evidence on the chirality-dependent growth kinetics of single chirality nanotubes which
can guide further synthetic processes design toward chirality pure SWCNT synthesis.
88
Chapter 4 Reference
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19 Rao, R., Liptak, D., Cherukuri, T., Yakobson, B. I. & Maruyama, B. In situ evidence for
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90
Chapter 5: Nearly Exclusive Growth of Small Diameter
Single-Wall Carbon Nanotube Semiconductors from
Organic Chemistry Synthetic End-Cap Molecules
5.1 Introduction
The inability of scientists to synthesize single-wall carbon nanotubes (SWCNTs)
possessing uniform electronic properties and chirality represents the major impediment to
their widespread applications. Recently, there is growing interest to explore and
synthesize well-defined carbon nanostructures, including fullerenes, short nanotubes, and
sidewalls of nanotubes, aiming for controlled synthesis of SWCNTs. One noticeable
advantage of such processes is that no metal catalysts are involved, and the produced
nanotubes will be free of metal contamination. These methods, however, suffer
shortcomings of either low yield or poor controllability of nanotube uniformity.
Here, we report a brand-new approach to achieve high efficiency metal-free
growth of nearly pure SWCNT semiconductors, as supported by extensive spectroscopic
characterization, electrical transport measurements, and density functional theory
calculations. Our strategy combines bottom-up organic chemistry synthesis with vapour
phase epitaxy elongation. We identify a strong correlation between the electronic
properties of SWCNTs and their diameters in nanotube growth. This study not only
provides material platforms for electronic applications of SWCNTs, but also contributes
91
to fundamental understanding of the growth mechanism and controlled synthesis of
SWCNTs.
SWCNTs represent attractive materials for the next generation nanoelectronics,
macroelectonics, and optoelectronics, owing to their intrinsic small dimensions, excellent
electronic and optical properties, chemical inertness, mechanical robustness, and other
outstanding properties.
1-3
To transform the electronics applications of SWCNTs from a
sought-after dream goal to a high-impact reality, the electronic properties of SWCNTs
must be precisely controlled.
4-6
It has been well documented that SWCNTs can be either
metals or semiconductors, depending critically on their geometrical structures, or
specifically, their chirality. Currently, researchers believe that the chirality, and therefore
the electronic properties, of a SWCNT become fixed during the initial nucleation step and
that the follow-up steady growth stage will not change that chirality, but will just extend
the nanotube length. This is supported by the fact that ultra-long SWCNTs typically
possess the same chirality along their entire length, unless the growth conditions change.
7
Therefore, nanotube nucleation control at the initial stage is the key to solving the
structure and property heterogeneity problem.
Metal nanoparticles with sizes of only a few nanometers are traditional catalysts
for the synthesis of SWCNTs, and nanotubes with narrow heterogeneity have been
successfully grown with varying degree of success in the past decade.
8-16
On the other
hand, there has been increasing interest in recent years in using structurally well-defined
carbon nanomaterials
17-26
to initiate nanotube growth, with an aim of producing uniform
SWCNTs. Moreover, the potential influence of metal contamination on the properties and
applications of nanotubes will be eliminated in such metal-free growth systems.
27
For
92
example, nanotube cloning has been demonstrated by Zhang, Liu and co-workers
22
as
well as by our own group
23,24
recently, with the newly grown SWCNT segments having
the same chirality as the seeds employed, based on Raman spectroscopic analysis.
Nevertheless, the yield of the cloning process is still rather low at the current stage,
largely due to very low areal number densities of nanotube seeds. On the other hand,
recent studies have shown fullerenes
28-30
and sidewall rings of nanotubes
31
can also serve
as seed for SWCNTs growth, but the products are mixtures of metallic and
semiconducting SWCNTs.
Here, we report a new nanotube growth routine that combines bottom-up organic
chemistry synthesis with an elongation method resembling vapour phase epitaxy (VPE)
to achieve metal-catalyst-free growth of nearly pure semiconducting SWCNTs and their
aligned arrays. We used pure, structurally well-defined molecular end-caps of nanotubes,
which are completely metal-free, to initiate further nanotube growth; scanning electron
microscopy (SEM) and atomic force microscopy (AFM) characterization confirm the
growth of horizontally aligned SWCNTs with high yield, indicating the high efficiency of
the molecular end-caps. Furthermore, Raman spectroscopic analysis with multiple lasers
indicates the very small diameters of the as-grown SWCNTs, and single nanotube and
nanotube array field-effect transistor (FET) measurements unambiguously confirm the
nearly exclusive growth of semiconducting SWCNTs, with purity higher than 97%, the
highest purity so far from a direct growth strategy. The mechanism of SWCNTs grown
from carbon nanostructures, including the relationship between seed size and nanotube
diameter, chirality-evolution process of SWCNTs, and the origin of the selective growth
93
of semiconducting nanotubes are studied via experimental and density functional theory
(DFT) calculations.
5.2 Nanotube growth from C
50
H
10
We start with the corannulene molecule (C
20
H
10
) for the synthesis of the end-caps
of nanotubes, aiming at chirality-controlled growth of SWCNTs via molecular cap
engineering, as illustrated in Figure 1. Corannulene itself is the smallest curved subunit of
C
60
fullerene and is a bowl-shaped non-planar molecule, with a bowl-depth of 0.87 Å
(Figure 1a).
32,33
It is actually the oldest known bowl-shaped polycyclic aromatic
hydrocarbon (PAH) and was first synthesized in 1966,
34,35
long before the discovery of
fullerenes. Significantly, the synthesis of corannulene has recently been scaled up to
produce kilogram quantities,
36
making it by far the most attractive molecular precursor
for bottom-up synthesis of nanotubes. Corannulene was subjected to direct chlorination
with iodine monochloride, a 5-fold Negishi coupling, and flash vacuum pyrolysis (FVP),
successively, to synthesize a hemispherical molecule, C
50
H
10
,
19
which represents the end-
cap plus a short sidewall segment of a (5, 5) chirality SWCNT, as shown in Figure 1b.
94
Figure 5.1 Scheme of proposed chirality-controlled nanotube growth from organic
chemistry synthesized molecular end-caps. (a) Structure of the bowl-shaped corannulene
molecule (C
20
H
10
). (b) Structure of the hemispherical C
50
H
10
molecule synthesized from
corannulene, which represents the end-cap plus a short sidewall segment of a (5,5)
SWCNT. (c) Proposed chirality controlled VPE growth of a (5, 5) SWCNT from its
molecular end-cap shown in b. The yellow, green and black atoms represent carbon while
the gray atoms represent hydrogen.
The as-synthesized C
50
H
10
molecules have a nominal purity of 100% without
other isomers, and they are quite soluble in common solvents like toluene (C
6
H
5
CH
3
),
dichloromethane (DCM, CH
2
Cl
2
), and acetonitrile (ACN, CH
3
CN).We point out metal
elements such as Fe, Co, Ni, Cu, etc., which can act as catalysts for CVD growth of
nanotubes, were not used in the above organic chemistry synthesis process. Therefore, it
is safe to conclude that the as-formed C
50
H
10
molecules are free of such metals, and result
is further confirmed by the energy dispersive X-ray characterization (Figure 2). Later, we
propose to use this nanotube-end-cap for the chirality-controlled synthesis of SWCNTs
via a strategy resembling VPE (Figure 1c). The feasibility of VPE growth of SWCNTs
Organic chemistry
synthesis
Vapour phase
epitaxy
Corannulene (C
20
H
10
)
(5, 5) end-cap (C
50
H
10
)
(5, 5) nanotube
a
b
c
95
has recently been demonstrated in our work using DNA-separated nanotube seeds.
23,24
Figure 5.2Energy dispersive X-ray (EDX) spectra (Figure S1a) of the C
50
H
10
molecules
deposited on quartz. For sample preparation, ~10 l of C
50
H
10
in toluene was deposited
on quartz and kept until dry to obtain thick deposits. The C signal comes from C
50
H
10
molecules while the O and Si come from the quartz substrate. Zoom in spectrum in
Figure S1b shows that there are no metal elements like Fe, Co, Ni detected.
Our original objective was to grow (5, 5) SWCNTs using the C
50
H
10
end-caps, as
shown in Figure 1b 1c. The as-synthesized red-orange powder of C
50
H
10
was first
dissolved in toluene to make a stable solution, and the absorption spectrum of C
50
H
10
molecular in toluene is shown in figure 3.
Figure 5.3 An optical absorption spectrum of the C
50
H
10
molecules dissolved in toluene.
A solution with absorbance of 0.40 at 468 nm was used in this study.
0 10 20 30 40
Intensity (a.u.)
Energy (keV)
0 1 2 3 4
Si
Intensity (a.u.)
Energy (keV)
C O
Fe/Co/Ni
a b
400 500 600 700 800 900
0.0
0.4
0.8
1.2
1.6
Absorbance (a.u.)
Wavelength (nm)
Abs=0.40 at 468 nm
96
Then, we dispersed the molecules onto quartz substrates via spin coating or drop
casting, and the substrates were subjected to a horizontal CVD furnace for subsequent
nanotube growth. We initially conducted nanotube growth experiments without any
pretreatment of C
50
H
10
. In these experiments, we systematically varied and studied
multiple parameters, e.g., the growth temperatures (from 700 C to 975 C), flow rates
and proportions of CH
4
/H
2
/C
2
H
4
and consequently their partial pressures, and growth
times (from tens of seconds to 1 hour). However, no nanotubes grew under any of these
conditions. Instead, we observed two typical features on the quartz surface after the CVD
process, i.e., a clean surface without anything grown on it or a dirty surface with dense
amorphous carbon deposits (Figure 4).
Figure 5.4 Representative SEM images of the quartz substrate coated with C
50
H
10
molecules after the CVD process without seed pretreatment.
97
Such amorphous carbon deposition was typically observed for growth conditions
with either high CH
4
and C
2
H
4
partial pressures or high growth temperatures, which
result in considerable thermal pyrolysis of carbon sources, as evidenced by the
blackening of the quartz reaction tube.
Next, we reexamined the whole process and considered the possibility that rims of
the C
50
H
10
molecules might be covered by other C
50
H
10
molecules or solvents,
embedding the active edges inside larger aggregates, as supported by the AFM
characterization. Faced with this conjecture, we speculated that pretreatment might be
necessary to initiate nanotube growth from this molecular end-cap. After extensive
exploration, ultimately, we found that high temperature air treatment, followed by water
vapour treatment, is very effective in activating C
50
H
10
for nanotube growth. In particular,
we learned that air oxidation at 500 C followed by water vapour treatment at 900 C
gives the highest nanotube yield as shown in Figure 5.
Figure 5.5 SEM and AFM characterization of nanotubes grown from C
50
H
10
molecular end-caps.
(a) Low magnification SEM image of as grown nanotubes. Inset is a digital camera image of the
quartz substrate after deposition of the C
50
H
10
molecules and drying, where the red-orange areas
correspond to a high density of C
50
H
10
molecules. (b), (c), and (e) SEM images of as-grown
SWCNTs at the locations indicated in image a. (d) A high magnification SEM image of the area c.
(f) An AFM image of a SWCNT with a height of ~0.6 nm. Scale bar: (a) 1 mm, (b),(c),(e) 50 m,
(d) 10 m, and (f) 200 nm.
a b c
b
e
0.5
0 0.6 nm
c
d e f
98
We used SEM to examine the overall growth efficiency of nanotubes from
pretreated C
50
H
10
. The phenomenon of the coffee ring effect can effectively and clearly
demonstrate that nanotubes were indeed grown from pretreated C
50
H
10
molecules. To
demonstrate this, we deposited ~5 l of C
50
H
10
solution in toluene onto a quartz substrate
and allowed it to nearly dry under ambient conditions. During the drying process, most of
the C
50
H
10
molecules were left on the boundary of the solvent, due to capillary force.
37
The inset of Figure 5a shows a photo image of such a substrate, in which the red-orange
deposit of C
50
H
10
molecules localized mostly along a circle can be clearly discerned.
Figure 5a shows a low magnification SEM image of this substrate after the nanotube
growth process, using C
50
H
10
pretreated in air at 500 C and water vapour at 900 C prior
to nanotube growth; a bright circle-shaped area is visible. Zoom-in SEM images of the
square areas b and c (Figure 5b and Figure 5c) clearly show the highly efficient growth of
dense SWCNTs. High magnification SEM characterization shows that the nanotubes are
aligned along the quartz surface (Figure 5d). We also found nanotubes at relatively low
density inside the coffee ring boundary (area e in Figure 5a), derived from the small
amount of C
50
H
10
molecules deposited inside the coffee ring (Figure 5e). Some nanotubes
grown at high density areas are less aligned and bended (Figures 5b-5d), because the
existing molecular clusters or other nanotubes on substrate can result in changes of the
growth directions of nanotubes. In contrast, the nanotubes grown at low density areas are
typically straight (Figure 5e).
As control experiments, we performed nanotube synthesis experiments using
blank quartz substrates, following the identical air and water pretreatment, and no
SWCNTs growth was observed (Figure 6). Overall, the above experiments
99
unambiguously demonstrate that nanotube growth is indeed initiated by the deposited
C
50
H
10
molecules. We used AFM to study the diameters of the as-grown SWCNTs, and
found that most nanotubes have heights below 1 nm. For example, Figure 5f shows a
representative AFM image of a SWCNT with a diameter of ~0.6 nm. We have found,
however, that the as-grown SWCNTs contain some bundles, which introduces
uncertainty with the use of AFM for nanotube diameter measurements.
Figure 5.6 Control experiments.(a) Low and (b) high magnification SEM images of the
blank quartz substrate after air and water vapour treatment followed by attempted CVD
growth, showing no growth of nanotubes in the absence of the C
50
H
10
molecules.
5.3 Multiple lasers Raman spectroscopic characterization
To analyse further the diameter, chirality, and quality of the SWCNTs grown
from the pretreated C
50
H
10
molecular, we performed systematic Raman spectroscopic
analysis with multiple lasers. We found that the C
50
H
10
molecules themselves show very
weak Raman signals under short laser wavelength (Figures 7).
100
Figure 5.7 Raman spectra of C
50
H
10
molecule-end-caps under 457 nm laser excitation.
The black curve was taken from bare quartz while the purple curves were taken from
different locations of quartz with C
50
H
10
deposited. The Raman signals under 633 nm and
514 nm lasers are much weaker than those of the 457 nm laser. In addition, the overall
Raman intensity is much weaker than Raman of as-grown nanotubes.
Figures 8a, 8b, and 8c show representative Raman spectra of as-grown SWCNTs
with laser excitation wavelengths of 633 nm (Figure 8a), 514 nm (Figure 8b), and 457 nm
(Figure 8c), respectively. The top x-axis in each plot shows the diameters of the
SWCNTs, deduced from the relationship between the frequencies of radial breathing
modes (RBMs) in Raman spectra and nanotube diameters,
38
using the equation d
t
=
223.5
ω
RBM
−12.5
. Here, d
t
is the diameter of nanotube in nanometer, and ω
RBM
is the frequency
of the RBM in cm
-1
. The peaks marked with arrows are RBMs, while all the other peaks
(indicated by asterisks) are from the quartz substrates (Figure 9). In these Raman spectra,
most of the RBM peaks are located at wavelengths above 240 cm
-1
, indicating that small
1000 1200 1400 1600 1800 2000
★
★
★
457 nm laser
C
50
H
10
Blank quartz
Intensity (a.u.)
Raman shift (cm
-1
)
101
diameter nanotubes, with d
t
<1 nm, have been grown. Statistical analyses of the RBM
frequencies of SWCNTs based on the three laser excitations are shown in Figures 8d, 8e,
and 8f, and the diameter distribution of SWCNTs derived from the Raman
characterizations are plotted in Figure 3g, which gives an average nanotube diameter of
0.79 nm, based on more than 100 RBMs.
Figure 5.9 Raman spectra of the bare quartz substrate under different lasers.
We analysed the chirality information on the SWCNTs that was obtained by
excitation with the three lasers used. Here, we emphasize that for such small diameter
SWCNTs (<1 nm), Raman spectra are unambiguous with respect to chirality assignments,
since adjacent SWCNTs have very distinct E
ii
values and RBM frequencies. Surprisingly,
we discovered that most of the nanotubes are actually semiconductors, e.g., (8, 3), (6, 1),
102
and (5, 1) in Figure 8a, (10, 2), (7, 3), and (5, 4) in Figure 3b, and (8, 1) in Figure 8c.
More interestingly, we observed some RBMs at very high frequencies that cannot be
attributed to any (n, m) chiralities, since the E
ii
values of such small diameter SWCNTs
are currently unavailable from either theory or experiments. For instance, the RBMs at
~517 cm
-1
are the peaks most frequently observed when using the 633 nm laser (Figures
8a and 8d), which corresponds to nanotube diameters of ~0.44 nm. We note that only (5,
1) SWCNT should show a RBM peak at this frequency. Therefore, we assign these
RBMs to (5, 1) nanotubes.
To the best of our knowledge, this is one of the smallest diameter SWCNTs ever
reported to have been grown without the necessity of a template confinement.
39,40
We
point out that our growth method not only provides a practical way to synthesize such
ultra-small diameter SWCNTs and produces valuable material platforms to allow studies
of their exotic properties, but also provides critical information on the build-up of the
electronic transition energy database of these small nanotubes, which is of fundamental
importance to the study of structure-property relationships of SWCNTs and curvature-
induced electronic property changes.
41
103
Figure 5.8 (a), (b), (c) Raman RBM spectra of SWCNTs grown from C
50
H
10
molecular end-caps
excited by 633 nm (a), 514 nm (b), and 457 nm lasers (c). The peaks marked with arrows are
from SWCNTs and all the other peaks (marked with *) come from quartz substrates. (d), (e), (f)
RBM frequency distributions based on the above three lasers. (g) Diameter distribution of
SWCNTs derived from the RBM frequencies (h) Raman D-band and G-band spectra of SWCNTs
excited by a 457 nm laser.
100 200 300 400 500
(5, 1)
(6, 1)
633 nm
Intensity (a.u.)
Raman shift (cm
-1
)
Diameter (nm)
2.0 1.0 0.7 0.5
(8, 3)
a d
b e
100 200 300 400 500 600
0
5
10
15
20
633 nm
Counts
RBM frequency (cm
-1
)
100 200 300 400 500 600
0
2
4
6
8
10
514 nm
Counts
RBM frequency (cm
-1
)
100 200 300 400 500 600
0
4
8
12
16
Counts
RBM frequency (cm
-1
)
457 nm
0.5 1.0 1.5 2.0 2.5 3.0
0
10
20
30
40
50
Counts
Diameter (nm)
Average: 0.79 nm
Sd: 0.49 nm
N=114
1300 1400 1500 1600
G-band
Intensity (a.u.)
Raman shift (cm
-1
)
457 nm
D-band
c f
g h
100 200 300 400 500
(5,4)
(7,3)
Intensity (a.u.)
Raman shift (cm
-1
)
(10,2)
514 nm
Diameter (nm)
2.0 1.0 0.7 0.5
100 200 300 400 500
(8,1)
Intensity (a.u.)
Raman shift (cm
-1
)
(6,3)
457 nm
Diameter (nm)
2.0 1.0 0.7 0.5
★
★
★
★
★
★
★
★
★
★
★
★
★
★
★
★
★
104
In addition, the very low defect-induced D-band to tangential G-band intensity
ratio (<0.01) suggests a high quality of SWCNTs grown from the pretreatedC
50
H
10
molecules (Figure 8h and Figures 10).
42
Figure 5.10 (a) Raman G-band and D-band spectra of the SWCNTs grown from C
50
H
10
based on three lasers. The high quality of the SWCNTs is confirmed by all three lasers. (b)
Alaser energy-dependent shift of the D-band frequency, i.e., the dispersive behavior, is
observed and is consistent with early reports on the Raman spectra of nanotubes and
graphene.
42,43
The dispersion relationship is(∂ ω
D
/∂E
laser
) = 45 cm
−1
/eV.
We note that the above three lasers are not in resonance with the (5, 5) SWCNTs.
We also used a 405 nm laser, which could excite (5, 5) SWCNTs,
44
to characterize our
sample. However, we did not observe any RBMs at ~340 cm
-1
related to (5, 5) nanotubes,
indicating no or very small population of (5, 5) chirality in the sample. In fact, when
using the 405 nm laser, we observed many fewer RBMs than the other three lasers
(Figure 11 and Table 1). This is understandable since the 405 nm laser has high energy
photons, and only very few nanotubes are in resonance with this laser based on the
Kataura plot.
105
Figure 5.11 Raman spectra of as-grown SWCNTs taken from a 405 nm laser (CL-2000
Diode pumped CrystaLaser LC, Renishaw, UK). Figure 11a is the RBM regime and
Figure 11b is the D-band and G-band regime. In Figure 11a, the six green arrows indicate
six RBMs while all the other peaks (indicated by black asterisk *) originate from the
quartz substrate. No peaks were observed at ~340 cm
-1
for (5, 5) CNTs.
45
Table 2. A summary of the six RBMs observed under the 405 nm laser and possible
chirality index assignments based on the Kataura plot.
44
No. 1 2 3 4 5 6
RBM
(cm
-1
)
382 517 382 381 296 380
(n, m) (6,2)/(7,1) N.A. (6,2)/(7,1) (6,2)/(7,1) N.A. (6,2)/(7,1)
106
5.4 Electrical Transport Property and Breakdown of SWCNT
FETs
Raman spectroscopic characterization points to a trend that nanotubes grown from
pretreated C
50
H
10
may be enriched with semiconducting SWCNTs (Figure 8). It is
extremely difficult, however, to determine the precise metallic/semiconducting ratio of
SWCNTs by Raman spectroscopy, owing to their resonant nature.
42
To obtain a
quantitative value for the proportion of semiconducting nanotubes reliably and rigorously,
we performed systematic electrical transport measurements based on individual SWCNT
FETs, as well as on aligned array SWCNT FETs combined with the electrical breakdown
technique. We first transferred as-grown SWCNTs from quartz to Si/SiO
2
(90 nm oxide),
using a polymer-mediated transfer process,
46,47
and fabricated bottom gate FET devices
(Figure 12a).
We have fabricated a total of 13 chips and have tested more than 1000 devices,
among which 147 working devices with nanotubes in the channel areas were identified.
Among these devices, about 1/4 of them show individual SWCNTs in the channel, while
the other 3/4 show parallel SWCNTs forming an array in the channel. Occasionally, a
few devices were observed in which no SWCNTs were directly connected to the source
and drain electrodes; instead, they formed a random network inside the channel. Such
network devices are not included in the following discussion.
107
108
Figure 5.12 (a) Schematic of device structure of a back-gated individual SWCNT FET. (b)
Statistics of on/off current ratio distribution of 34 individual SWCNT FETs. (c), (d)
Representative transfer characteristics (I
DS
-V
G
) of an individual semiconducting.(e) Transfer
characteristic of an all-semiconducting nanotube-array-FET, with the inset SEM image showing a
total of four SWCNTs connected to both electrodes. (f) Electrical breakdown experiments of the
device in e. (g) Transfer characteristics of a multiple-nanotube-FET before (blue) and after (red)
electrical breakdown, with the inset SEM image showing a total of two SWCNTs connected to
both electrodes. (h) Electrical breakdown experiments of the device in g. Scale bar of the SEM
images: 5 m for b and d, 10 m for e, and 20 m for g.
We plot the distribution of on/off ratios of individual SWCNT FETs (Figure 12b
and Figure 13) and set an on/off ratio criterion of 10 to distinguish metallic SWCNTs
from semiconducting ones.
12,13,48
The total number of individual SWCNT FETs is 34, and
among them, 32 have on/off ratios larger than 10, giving a semiconducting SWCNT ratio
of 32/34=94.1%.
Figure 5.13 A summary of the transfer characteristics (I
DS
-V
G
) of all individual SWCNT
FETs. The total devices number is 34 and 2 of them (red curves) show on/off ratio of <10
109
and are assigned to be metallic SWCNTs. Note that for true metallic SWCNTs, i.e.,
armchair (n, n) SWCNTs, the on/off ratio should be ~1. The red curves in this figure
show on/off ratio of 4.6 and 5.8, respectively. This behavior can be attributed to the so-
called semi-metallic SWCNTs with a small energy band gap.
49
We count these red curves
as metallic SWCNTs.
Figure 12c shows the typical transfer characteristics (I
DS
-V
G
) of a semiconducting
SWCNT FET (SEM image of the device shown in the inset), which shows p-type
behavior with an on/off current ratio of ~3.7 x 10
4
. As a comparison, Figure 4d shows the
transfer characteristics of another individual SWCNT FET (SEM image shown in the
inset) with an on/off ratio of ~7 (based on the red curve in Figure 3d with V
D
=0.4 V). The
output curves (I
DS
-V
D
) of the nanotube FETs shows linear behavior at small V
D
regimes
(Figure 14), indicating the ohmic contact between the SWCNTs and the electrodes.
Taking into account the evidence from Raman spectroscopic analysis with multiple lasers
in Figure 8 that most of the nanotubes grown from pretreated C
50
H
10
possess rather small
diameters, we attribute the curves in Figures 12dto so called semi-metallic SWCNTs,
such as (6, 3) SWCNTs, as detected by Raman characterization in Figure 12c.
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
-80
-60
-40
-20
0
I
DS
(nA)
V
D
(V)
V
G
= -5V to 0V
in 1V step
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
-60
-40
-20
0
V
G
= -5V to 0V
in 1V step
I
DS
(nA)
V
D
(V)
a b
110
Figure 5.14 Output characteristics (I
DS
-V
D
) of typical semiconducting and metallic
SWCNT FETs, corresponding to the same devices in Figure 4c and Figure 4d in the main
text, respectively.
Such small-diameter semi-metallic SWCNTs have small but finite band gaps
between their conduction band and their valance band in the electronic density of states
and thus show gate dependence behavior,
49
as evidenced in Figures 12d. However, the
band gaps of semi-metallic SWCNTs are typically very small, e.g., ~ 10 meV, which is
reflected by the obvious ambipolar transport behavior observed in Figures 12d.Here we
point out that the semi-metallic nanotubes with on/off ratio less than 10 (Figure 12d) are
counted as metallic SWCNTs. Actually, we did not observe even a single device with
on/off ratio of 1, which would correspond to a true metallic armchair SWCNT.
From SEM observations, we found that a large number of devices contain more
than one SWCNT in the channel. For example, the SEM image in the inset of Figure 12e
shows a four-SWCNT-array connected to the two electrodes. The transfer characteristic
of this device, shown in Figure 12e, clearly demonstrates the semiconducting behavior,
with an on/off ratio of ~520 for this nanotube-array-FET. The transfer characteristics of
all such nanotube-array-FETs are summarized in Figure 13.
We used the electrical breakdown technique
50,51
to count the actual number of
SWCNTs for the device in Figure 12e. As shown in Figure 12f, we observed a total of
three sudden decreases in I
DS
and, therefore, three SWCNTs being broken during the
process. V
D
cannot be increased further since the gate oxide will be damaged at V
D
of
~80 V (Figure 15).
111
Figure 5.15 Electrical breakdown
50,51
of an all-semiconducting SWCNT array FETs. (a)
Transfer characteristic of the device before breakdown. (b) The breakdown experiments
of this device at a V
G
= -5 V. The gate dielectric (90 nm SiO
2
) was damaged at a V
D
=77 V
(red arrow). This device was considered to have at least three semiconducting SWCNTs
in the channel and is counted as 3 for statistical analysis. (c) Gate current (I
G
) during the
breakdown process. Inset is the zoom in plot with V
D
<70 V. (d) Transfer characteristic of
the device after breakdown, showing that the device was damaged.
We noted that after the breakdown of the 3
rd
SWCNT at V
D
of ~70 V, there is
still current flow in the channel area, suggesting that at least one more SWCNTis still
connected to the electrodes. Taking the SEM image (inset of Figure 12e and Figure 16a)
and the breakdown experiments (Figure 12f) together, we conclude that there were
originally a total of four semiconducting SWCNTs in this device, i.e., all the visible
112
SWCNTs in the SEM image are indeed connected to both electrodes. This is reasonable,
since we first transferred nanotubes and then conducted the electrodes deposition, putting
the electrodes on top of the SWCNTs with good contact.
Figure 5.16Low magnification SEM images of the device shown in the inset of Figure 4e
(a) and Figure 4g (b). The whole channel area of the device is visible. Four SWCNTs and
two SWCNTs can be clearly discerned in image a and b, respectively.
Similarly, we have performed systematically electrical breakdown experiments on
nanotube-array FETs with on/off ratios <10. Figure 12g shows the transfer curves of such
a device before and after electrical breakdown. The blue curve in Figure 12g shows the
initial measurement of the device, which exhibits an on/off ratio of ~5. SEM inspection
reveals two SWCNTs connected to both electrodes (Inset of Figure 12g and Figure 16b).
After the electrical breakdown of the first metallic SWCNT at V
G
= +5V (Figure
12h), the on/off ratio of the device increased to >4 x 10
3
, indicating only one metallic
SWCNT in this device. Therefore, we conclude that this device contains one metallic
a b
113
SWCNT and one semiconducting SWCNT. Combining this electrical breakdown and
counting technique with SEM imaging, as well as the individual SWCNT FET results, we
have identified a total of 264 semiconducting SWCNTs and 8 metallic ones (Table 2),
giving a semiconducting SWCNT ratio of 264/272=97.1%. The error for these statistics
is given by equation𝛿 = 1.96 ×
σ
2
N
= 1.96 ×
p(1−p)
N
= 2%. Here, 𝛿 is the statistic
error, σ is the standard deviation, N is the number of SWCNTs, p is the semiconducting
SWCNT purity, and the confidence coefficient is set as 0.95. We note that this study is
not only the very first example of the selective growth of semiconducting SWCNTs by a
metal-free process, but also stands among the highest purity of semiconducting SWCNTs
reported so far from a direct growth approach. Such small diameter semiconducting
SWCNTs are preferred for short channel transistors since small diameter nanotubes
exhibit much smaller OFF state current than large diameter ones.
52,53
Table 3. A summary of the total number of SWCNTs from both individual and SWCNT
array FETs
Total
devices
Total
SWCNTs
Metallic
SWCNTs
Semiconducting
SWCNTs
Individual
SWCNT FETs
34 34 2 32
SWCNT array
FETs
93 238 6 232
114
5.5 Molecular seed size evolution and nanotube diameter-seed
size relationship.
So far, little is known about the growth mechanism of nanotubes from metal-free
carbonaceous molecular seeds such as fullerenes,
28,30,54
short nanotubes,
23,24,28
and carbon
nanorings.
31
It is therefore important to investigate the mechanism of nanotube growth
from the pretreated C
50
H
10
molecular end caps, which will benefit further development of
molecular seeds for structure-controlled nanotube growth. In this study, we focus on the
following two major aspects: i) The relationship between the size of nanotubes and the
sizes of seeds from which nanotubes are grown, ii) the underlying mechanism for the
chirality changed growth of SWCNTs and selective growth of semiconducting-
predominated SWCNTs.
To shed some light on these issues, we first used AFM to study in detail the size
evolution of deposited C
50
H
10
clusters. AFM examination of the as-deposited C
50
H
10
molecules shows an average particle size of 7.1 nm (Figure 17a and Supplementary
Figure S15a). Because the diameter of a single C
50
H
10
molecule is approximately 1 nm,
19
this suggests aggregation of tens of C
50
H
10
molecules into large clusters. It is obvious that
such large molecular aggregates are not suitable for SWCNT growth, as observed in our
experiments without seed pretreatment. After air and water vapour treatment at 500 C
and 900 C, respectively, the sizes of the clusters decreased dramatically, leading to an
average size of 1.7 nm (Figure 17b and Figure 18b), which is found to be much smaller
than the clusters resulting from treatment of the C
50
H
10
molecules in air at 300 C and
400 C(Figure 19)
115
Figure 5.17(a), (b) AFM images of as-deposited C
50
H
10
molecular aggregates on quartz
(a) and the C
50
H
10
molecules after pretreatment (b). (c) AFM examinations of the
relationship between as-grown nanotubes and the seed molecules. Images 1, 2, and 3
show an end of one nanotube, two ends of two different nanotubes, and both ends of one
nanotube, respectively. No big particles were found at the nanotube ends. Image 4 shows
a big particle at the end of one SWCNT. The ends of the SWCNTs are indicated by green
arrows. The vertical bars are 15 nm for all AFM images.
Figure 5.18 (a), (b) Size distributions of as deposited C
50
H
10
molecular seeds (a) and
after air and water vapour treatment (b). The average sizes of the clusters are 7.1 nm for a
and 1.7 nm for b.
200 nm
1 2
3 4
1 m
500 nm
a
b
c
0 2 4 6 8 10 12 14 16 18 20
0
5
10
15
20
25
30
Counts
Size (nm)
As deposited
1 2 3 4 5
0
5
10
15
20
Counts
Size (nm)
After pretreatment
a b
116
Figure 5.19 AFM images of the C
50
H
10
molecules after (a) 300 C and (b) 400 C air
treatment. The bottom panels show corresponding height profiles of the white lines in
images a and b.
There are several mechanisms that may lead to a reduction in the cluster sizes. For
instance, high-temperature-induced sublimation of C
50
H
10
molecules out of the clusters,
burning and degradation reactions of C
50
H
10
molecules when exposed to air and water at
high temperatures, and fragmentation and coalescence of the C
50
H
10
molecules. These
processes can lead to changes not only of the cluster sizes but also of the actual structures
of the individual molecules, which is evidenced by nuclear magnetic resonance
spectroscopic analysis (NMR, Figure20).After SWCNT growth, we used AFM to
carefully examine tips for many nanotubes. We found that most SWCNTs do not have
larger particles at their tips (Images 1, 2, and 3 in Figure 17c) and that only a small
portion of SWCNTs (< 10%) have particles much larger than the diameters of the
nanotubes (Image 4 in Figure 17d). This phenomenon shows a sharp contrast with a
117
recent study on nanotubes grown from fullerenes where much larger particles were
frequently observed at the tips of the nanotubes.
29
We speculate that this is an important
difference between C
50
H
10
and fullerene, since the latter needs to be opened first to form
a cap, which may bring randomization in terms of cluster sizes and structures. AFM
characterization shows that the sizes of the seed clusters are pretty small right before
nanotube growth (Figure 18b) and that most of the nanotubes possess diameters
comparable to those of the seed sizes (Figure 17c). This suggests that most SWCNTs
grow from individual (structure changed) molecules or very small aggregates and
explains the selective growth of small diameter SWCNTs presented above. Noticeably, in
this study we observed the growth of ultra-small SWCNTs, e.g., (5, 4) and (5, 1), which
are rarely reported under any other nanotube growth process. Typically, a template is
needed for the nucleation and growth of such ultra-small SWCNTs, and the grown
nanotubes are confined inside the template.
39,40
118
Figure 5.20
1
H NMR (400 MHz, C
6
D
6
) of C
50
H
10
after air treatment at 500 C. Inset
shows the zoom-in of 7-8 ppm area. The δ7.63 (s, 10H) peak
19
disappeared after the
treatment (blue arrow in the inset), indicating decomposition of the C
50
H
10
molecule. The
peaks indicated by black arrows originated from trace amount of diethyl ether in C
6
D
6
solvent.
55
5.6 DFT calculations
We further used DFT calculation to understand how the chirality of nanotubes
evolves during their growth process. We note that by converting one adjacent hexagon-
hexagon (6-6) pair into one pentagon-heptagon (5-7) pair in C
50
H
10
molecules, the
chirality of thus grown nanotubes will change from (5, m) to (5, m-1). Interestingly, our
calculations show that the extraction of few hydrogen atoms from C
50
H
10
will
significantly reduce the energy barriers for chirality transformations. For example, Figure
21a show a comparison of energy barriers between C
50
H
10
and C
50
H
9
for transforming (5,
5) SWCNT into a (5, 4) chirality. Figure 21b and 21c are the structures of the transition
119
states (TS) from (5, 5) to (5, 4) using C
50
H
10
(Figure 21b) and C
50
H
9
(Figure 21c) as the
starting seeds. It is evidenced from Figure 21a that if we start from C
50
H
9
, the
transformation barrier is three times lower than that of starting from pristine C
50
H
10
(62.8
versus 171.0 kcal/mol). Since oxygen and water were used during seed pretreatment, it is
reasonable to assume that some hydrogen atoms will be extracted during the pretreatment.
This suggests that it is easier for the treated seeds (with some hydrogen atoms being
extracted) to grow nanotubes in a chirality-changed fashion. Then, we performed
systematical calculations on the transformation of other chiralities from (5, m) to (5, m-1),
with m equals to 5, 4, 3, 2, and 1.
Figure 5.21 Energy barriers and relative electronic energy (including ZPVE, zero point
vibration energy) profiles of the transformations from (5,m) into (5,m-1)SWCNT caps
and the structures of nanotube caps. (a) Comparisons of energy barriers from (5, 5) to (5,
4) transformation between C
50
H
10
and C
50
H
9
, i.e., the pristine seed and the seed with one
hydrogen atom being extracted as the starting structures. (b), (c) Structures of transition
states (TS) of (5, 5) to (5, 4) transformation starting from C
50
H
10
(b) and C
50
H
9
(c). (d)
Transformation energy barriers of (5, m) to (5, m-1) SWCNTs. m=5, 4, 3, 2, or 1. (e), (f)
Relative energy (kcal / mol)
C
50
H
10
C
50
H
9
0.0
171.0
62.8
30.7
34.9
Relative energy (kcal / mol)
(5, 5) to (5, 4)
(5, 4) to (5, 3)
(5, 3) to (5, 2)
(5, 2) to (5, 1)
(5, 1) to (5, 0)
0.0
30.7
62.8
38.3
37.6
30.7
28.1
13.5
11.4
4.3
-0.7
(n, m)
SWCNT
TS of (n, m)
to (n, m-1)
(n, m-1)
SWCNT
(5, 5)
SWCNT
TS of (5, 5)
to (5, 4)
(5, 4)
SWCNT
TS for C
50
H
10 TS for C
50
H
9
(5, 4) SWCNT
(C
50
H
9
)
(5, 3) SWCNT
(C
50
H
8
)
(a)
(b) (c)
(d)
(e) (f)
5
5 5
7
7
7
120
Structure of formed (5, 4) (e) and (5, 3) (f) SWCNTs with the formation of one and two
5-7 pairs.
Here, we find that the succeeding 5-7 pairs prefer to form by converting 6-6 pairs
which are initially nearby the preformed pentagon since this configuration has the lowest
barrier (Figures 22, 23, and 24). Figure 21d shows the energy barriers of such
transformations at zero Kelvin and Figure 21e and 6f show the structures of caps that can
form (5, 4) and (5, 3) nanotubes. The highest barrier is the transformation of (5, 5) to (5,
4) chirality. After this step, the transformations of (5, 4) to (5, 3) and (5, 2) to (5, 1) are
found to have slightly lower barriers than that of (5, 3) to (5, 2) and (5, 1) to (5, 0)
transformations. The trend is the same for high temperatures of 1173 K and 273 K
(Figures 25). The possibility of such transformation to occur can be described as 𝑝 ∝
𝑒 (−
𝐸 𝑏 𝑘 𝑇 )
. Here p is the possibility of the chirality transformation, E
b
is the energy barrier, k
is the Boltzmann constant, and T is the temperature. We note that these transformations
are not difficult to occur at temperature of ~1173 K (nanotube growth temperature in this
study), and this can explain the high-yield, but chirality-changed growth of SWCNTs
from the treated molecules. Moreover, based on Raman analysis in Figure 3, we also
observed that some nanotubes have diameters larger than the C
50
H
10
seeds, for example,
(10, 2) and (8, 3). This should be originated from the aggregation of a few seeds into
relatively large structures (Figure 17) and consequently, nanotubes with diameters larger
than (5, 5) were grown.
121
Figure 5.22 The relative energy profiles of the transformations from (5,4)into
(5,3)SWCNT caps at different bent regions (sites). The results show that the site 1, which
locates nearby the initial pentagon (red letter), has the lowest transformation barrier to
form a new pentagon-heptagon pairs (5-6 pairs, blue letter).
Figure 5.23 The relative energy profiles of the transformations from (5, 3) into (5, 2)
SWCNT caps at different bent regions (sites). The results show that the site 1, which
locates nearby the initial five-member-ring, has the lowest transformation barrier to form
a new 5-7 pairs.
(5,4)SWCNT(C
50
H
9
)
(5,3)SWCNT(C
50
H
8
)
site1
site2 site3
site4
5
7
5
7
5
7
(5,3)SWCNT(C
50
H
8
)
site1
site2
site3
7
5
122
Figure 5.24 The relative energy profiles of the transformations from (5, 2) into (5, 1)
SWCNT caps at different bent regions (sites). The results show that the site 1, which
locates nearby the initial five-member-ring, has the lowest transformation barrier to form
a new 5-7 pairs.
Figure 5.25 The relative Gibbs free energy profiles of the transformations from (5,m)
into (5,m-1) SWCNT caps of C
50
H
m+4
(m=5,4,3,2, respectively) at 298.15K (solid line)
and 1173K(dash line).
nearby 55R defects
nearby 7R defects
(5,4)SWCNT(C
50
H
9
)
(5,3)SWCNT(C
50
H
8
)
123
Previous theoretical
14,56,57
and experimental
58
studies revealed that nanotubes with
different chiralities and electronic properties have different stabilities, and metallic
nanotubes are generally less stable than semiconducting ones. Researchers have explored
such stability and reactivity differences between metallic and semiconducting SWCNTs
to realize selective growth of, for example, semiconducting SWCNTs, by introducing
external chemical or physical interaction into the CVD environment. Oxidative species
like OH radicals,
41
O
2
gas,
59
and H
2
O vapour
15,60
were found to be able to preferentially
suppress the growth of metallic SWCNTs. Similar phenomena have also been reported by
using ultraviolent-assisted CVD.
13
To examine whether a similar mechanism governs our
process, we conducted the following experiments. First, we used a trace oxygen detector
to monitor the concentration of oxygen in situ during the CVD growth of SWCNTs from
pretreated C
50
H
10
molecular seeds. The results show that the oxygen concentration ranged
from a few ppm to ~77 ppm during the SWCNT growth process (Figure 26). We note
that this concentration is two to three orders of magnitude lower than those in the early
reports where a few hundred ppm to a few thousand ppm of H
2
O and O
2
were
intentionally added to enable the selective growth of semiconducting SWCNTs.
15,59,60
Second, in a separate experiment, we then grew SWCNTs under identical CVD
conditions in the absence of C
50
H
10
molecular seeds, employing a commonly-used Fe
catalyst. Raman analysis shows that SWCNTs grown from Fe have a rather broad
diameter distribution and do not show any noticeable enrichment of either metallic or
semiconducting SWCNTs (Supplementary Figure S23). Collectively, the above two
experiments reveal that trace oxygen residue in our CVD system does not play an
124
important role for the selective growth of semiconducting SWCNTs and that the
selectivity appear to originate from the molecular seeds used.
Figure 5.26 In situ measurements of oxygen concentration during SWCNT growth. The
initial oxygen concentration is higher than the detection limit (1000 ppm) of the sensor.
After H
2
flush (10 minutes) and furnace ramp-up (13 min) periods, the O
2
concentration
drops to 23 ppm. The initial introduction of CH
4
and C
2
H
4
increases the O
2
concentration
to 77 ppm. Then, it decreases gradually and stabilizes at ~7 ppm.
59
125
Figure 5.27 Comparative studies of Fe-grown SWCNTs under identical CVD conditions.
(a), (b) SEM images. (c) Representative Raman spectra of nanotubes grow from Fe
catalyst. The green arrows indicate RBMs while the asterisk * indicate peaks from the
quartz substrates. (d) Statistical analysis of the RBM distribution of SWCNTs grown
from Fe catalysts. The total number of RBM peaks is 151. The colored stripes of M
22
, S
33
,
M
11
, and S
22
correspond to the second electronic transition of metallic SWCNTs, the third
electronic transition of semiconducting SWCNTs, the first electronic transition of
metallic SWCNTs, and the second electronic transition of semiconducting SWCNTs,
respectively.
Previous DFT calculations suggest that structure-dependent stability differences
of metallic versus semiconducting SWCNTs are much more significant in the small
diameter regime than in medium or large diameter regime.
14
As nanotubes grown from
pretreated C
50
H
10
end-caps possess exceptionally small diameters, we speculate that
nanotube diameter-induced stability differences between metallic and semiconducting
SWCNTs, which may play a central role at the very small diameter regime, might be the
100 200 300 400 500
*
*
*
*
*
Intensity (a.u.)
Raman shift (cm
-1
)
Fe grown SWNT
514 nm laser
*
50 100 150 200 250 300 350 400 450
0
5
10
15
Counts
RBM frequency (cm
-1
)
M
22
S
33
M
11
S
22
a b
c d
126
key reason for the preferential growth of semiconducting SWCNTs in this study. The
nucleation and growth of such small diameter SWCNTs on flat substrates may relate to
the special molecular seeds used here, which serve as nanotube end-caps and stabilize
nuclei of very small diameter nanotubes. Further study is clearly warranted.
In summary, a nanotube-end-cap molecule, C
50
H
10
, prepared by bottom-up
organic chemistry synthesis, was used for the first time to grow SWCNTs having nearly
pure semiconducting properties by a metal-free process. Various growth conditions were
tested, and their effects on nanotube growth efficiency were studied. The diameter,
chirality, and electronic properties of the nanotubes grown from this molecular end-cap
were studied in detail via microscopy, spectroscopy, and electrical transport
characterization. DFT calculations show that the treated molecules facilitate chirality-
changed growth of SWCNTs. The exceptional small diameter feature of the SWCNTs
grown from pretreated C
50
H
10
molecules, combined with the diameter-dependent stability
differences between semiconducting and metallic SWCNTs, are proposed to be the key
origin for the nearly exclusive growth of semiconducting SWCNTs. This study not only
establishes an efficient approach to grow nearly pure SWCNT semiconductors, but also
provides valuable new insight into the selective growth mechanism of SWCNTs.
127
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131
Chapter 6: Conclusions and Future Directions
6.1 Conclusion
In summary, this thesis focuses on controlled synthesis of carbon nanotube. First
of all, type-controlled synthesis is realized by using isopropanol as the carbon feedstock.
This approach of controlling the type is found to be highly reproducible and resulted
semiconducting nanotubes have very high quality and high purity which could be used as
an important building block for next generation of electronics. However, to fully realize
the potential of nanotube in many applications especially nanoelectronics, it is important
to precisely control the structure of produced nanotubes. Here we reported our strategy to
achieve controlled synthesis with predefined chirality by using separated nanotubes as the
seed instead of metal catalyst. We found that our VPE approach is an effective method to
elongate the carbon nanotube seed without changing their chirality. Moreover, their
growth behavior is future investigate in hope to shine light on the growth mechanism of
the VPE process, in hope to improve the yield to enable their use in industrial
applications. Furthermore, the VPE process is found to be highly robust, and can be used
with varies carbon based seed. By combine VPE process with organic chemistry
synthetic end-cap molecules, we were able to achieve nearly exclusive growth of small
diameter semiconducting SWCNT. All those development could fundamentally change
the situation and greatly accelerate SWNT research and development.
132
6.2 Future Work on Mechanism Understanding
Metal catalyst-free growth has become increasingly important in recent years,
largely due to the difficulty in synthesize metal particle with exact same structure, as well
as the inability of metal particle maintain its morphology and structure at high
temperature. As a result, the produced nanotube would result with varies chiralities, and
this inhomogeneity has been the bottleneck in the field and limited the use of carbon
nanotube in many applications.
The idea of using carbon structure as seed can surpass the problem raised by
metal catalyst as mentioned above. In this thesis, we have demonstrate the potential of
metal catalyst system in achieving single chirality controlled carbon nanotube growth,
however, the mechanism understanding of such system is still remain unclear. The lack
of complete understanding of nanotube growth mechanism is the major reason that
hinders chirality-controlled synthesis of SWNTs. We have proposed the growth DA
chemistry, and result from growth rate study seems to agree with the proposed
mechanism, nevertheless under such a complex system, it is important to explore the
detail of the growth by employing in techniques that can result directly observation of the
growth process in real time.
6.2.1 In-situ SEM
SEM is a powerful imaging technique to characterize the structure of various
materials, especially nanomaterials. The ability to observe structure transformation
133
overtime, make in-situ SEM an attractive method to study the mechanism of any novel
growth techniques. Moreover, SEM can inspect a large area at once, so multiple
nanotubes can be studied simultaneous. In collaboration with Dr. Homma, the growth
condition in SEM chamber was established as show in figure 6.1. There are two modes
available in this SEM system, namely low vacuum mode and high vacuum mode. The
CVD process can be conducted at both modes and the maximum allowed pressure was
1× 10
-3
Pa and 30Pa for high and low vacuum mode respectively. The growth would
perform under the low vacuum mode, and after synthesis the system would then switch to
the high vacuum mode to perform in-situ SEM imaging.
Pretreatment is conducted before load substrate in the SEM chamber to activate
these seeds. The typical pretreatment condition included air oxidation and water
treatment. Air oxidation was performed at 500° C for 5min, and then following water
treatment was conducted at 500° C or 700° C for 4min with 0.1sccm water and
450sccmAr/H
2
at pressure around 3.0× 10
3
Pa. Our experimental results indicated that the
pretreatment was very essential. Without proper pretreatment, almost no grown was
observed from the seed. Figure 6.1 a shows the quartz surface after the pretreatment, the
average length of nanotube seed is 300 nm which is too short to be visible under SEM,
after 10 minutes growth in the SEM chamber, much longer tubes can be found on the
quartz surface (figure 6.1 b).
Furthermore, we performed second time growth using the same growth condition.
Figure 6.1 c shows the SEM image after the initial 10 minutes growth. After SEM
inspection and after 10 minutes growth time, several newly grown CNTs were found as
indicated by black dot box in Figure1d.Preliminary results from the in-situ SEM
134
measurement showed a strong evidence of the cloning process, which also help to
provide a deeper understanding of the growth mechanism. Moreover, in-situ monitoring
the growth process might also help to comprehensively evaluate the influence of
experimental parameters on its growth.
Figure 6.1 In-situ SEM observation of (6, 5) nanotube (a) SEM of the quartz substrate
after pretreatment. (b) After 5 minutes growth time. (c)(d) Same location as (c) after
second round of growth.
6.2.2 In-situ TEM
TEM is another powerful imaging technique that has been widely used to study
the property and structure of various nanomaterials. One of the most important
5µm
5µm
8µm 8µm
c
d
a
b
135
advantages of TEM is the high resolution, the ability to provide structure information of
the materials with atomic resolution. With such high resolution, we can clearly see the
structure fluctuations of seeds after the elongation process.
Recently, in-situ TEM become increasingly important, especially in
understanding the growth mechanism studies. From in-situ TEM measurement,
transitional states can be easily reviewed as well as the correlation between growth
conditions with final product can be investigated. Therefore, in-situ TEM has been
widely used to study the mechanism of nanotubes grow from metal catalyst. Figure 6.2
shows a measurement from in-situ TEM observation of a SWCNT nucleation and growth
from a Fe catalyst under different time. Result from the in-situ measurement revealed that
the catalysts for SWCNTs are single crystalline Fe
3
C with clear crystalline lattices, and
CNTs are formed by surface precipitation and cap lifting-up.
1
136
Figure 6.2 (a) In-situ TEM observation of a SWCNT nucleation and growth from a Fe
catalyst under different time. (b) Indicating the active catalyst is Fe3C. Reprinted with the
permission from Ref
1
Copyright 2008 American Chemical Society
Despite the enormous progress has been made in mechanism understanding of
nanotubes growth from metal catalyst, in-situ evidence of the cloning process is still
limited. Nevertheless, in-situ measurement can provide valuable guidance for the
controllable synthesis of CNTs, and provide useful information on the active sites for
nanotube growth, as well as determine how active carbon species is added to the open
ends of the nanotubes. We anticipate that in situ TEM observations will further provide
helpful guidance on our CVD cloning process and help to optimize the CVD process to
realize scalable cloning growth of single-chirality SWNTs.
137
6.3 Future work on Yield Improvement and Bulk Synthesis
Chirality-controlled synthesis has been demonstrated in this thesis, however the
yield is still too low for industrial applications. Current condition uses 900
o
C as the
growth temperature, however under such high temperature many nanotube seeds would
be etched away during the initial growth stage due to their short length and high defect
density which would result high reactivity. One way to lower the temperature is to
employ carbon precursors that can thermally decomposed at lower temperature. Qian et
al. found by adding C
2
H
2
or C
2
H
4
, the activation and decomposition of methane can be
significantly enhanced.
2
In addition to vary the gas precursors, lower the pressure would
also lead to lower the growth temperature and improve the efficiency. Our preliminary
result did show a vast improvement in the yield as shown in the SEM image in figure 6.3,
comparing one atmosphere (a) with low pressure growth (b).
Figure 6.3 SEM image of (a) Nanotube growth underone atmosphere. (b)under low
pressure growth.
1µm
25µm
a
b
138
To further increase nanotube yield under VPE growth process, and increase the
production capacity towards bulk application, we propose to replace 2D flat quartz
substrate with 3D spherical substrate.
3
It is a common strategy in the catalyst industry to use 3D catalyst support
materials to improve the yield of the products in catalytic reactions. Specifically, we
propose to deposit our DNA-separated chirality-pure SWNTs onto zeolite and other
porous materials to achieve bulk growth of SWNTs with identical chirality. Zeolite has
large specific surface areas (SSA) and porous structures, which can accommodate SWNT
seeds at high density to enable high yield growth. Another advantage of using 3D
substrate is that the end of nanotube seeds may be suspended in such case, which may
increase the chance of nanotube seeds to contact with reactive carbon sources and
therefore, increase the VPE yield. It has been demonstrated that 3D substrates can be
used to grow nanotubes at tons scale under conventional metal-catalyzed CVD process,
31
showing the great potential of using 3D substrate to dramatically increase the yield of our
VPE process toward large amount of SWNTs with the same chirality.
As the nanotube basic studies and practical applications have been severally
hindered by the lack of bulk amount of SWNTs with controlled chirality, it is reasonable
to believe that the outcome of this task will fundamentally change the situation and
greatly accelerate SWNT research and development.
139
Chapter 6 Reference
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Abstract (if available)
Abstract
Single-walled carbon nanotubes (SWCNTs) possess superior electrical and optical properties and hold great promise for electronic and biomedical applications. The properties of a SWCNT strongly depend on its structure, therefore, a great effort of this dissertation has been devoted to control the structure of single-walled carbon nanotubes (SWCNTs) during the chemical vapor deposition (CVD) growth process. Despite the enormous investigation efforts and the tremendous achievements made in the nanotube growth field over the past several decades, controlled synthesis of nanotubes with predefined and precise structure still remain a huge challenge. We have developed a new approach to the synthesis of SWCNTs with predefined chirality by combining nanotube separation with growth. This novel approach can also be used with organic molecules. ❧ The thesis will start with a brief introduction of the carbon nanotube, then the synthesis and characterization methods as well as the applications of SWCNTs is described in detail to explain the importance of controlling the structure of the carbon nanotube. ❧ In chapter 2, a novel carbon feedstock is introduced to selectively synthesize the predominantly semiconducting nanotube by using the chemical vapor deposition (CVD) method. The development of guided CVD growth of single-walled carbon nanotubes provides a great platform for wafer-scale integration of aligned nanotubes into circuits and functional electronic systems. However, the coexistence of metallic and semiconducting nanotubes is still a major obstacle for the development of the carbon nanotube based on nanoelectronics. To address this problem, we have developed a method to obtain predominantly semiconducting nanotubes from direct CVD growth. By using isopropanol as the carbon feedstock, a semiconducting nanotube purity of above 90% is achieved, which is unambiguously confirmed by both electrical and micro-Raman measurements. A mass spectrometric study was performed to elucidate the underlying chemical mechanism. Furthermore, high performance thin-film transistors with an on/off ratio above 10⁴ and mobility of up to 116 cm²/V∙s have been achieved using the isopropanol-synthesized nanotube networks grown on silicon substrate. The method reported in this contribution is easy to operate and the results are highly reproducible. Therefore, such semiconducting predominantly single-walled carbon nanotubes could serve as an important building block for future practical and scalable carbon nanotube electronics. ❧ Chapter 3 presents our new approach to the selective synthesis of nanotube with predefined chirality via the Vapor phase epitaxy (VPE) growth method. The lack of synthetic control in chirality has long been recognized as a fundamental impediment in the science and application of SWCNTs. Since the electronic property of a SWCNT strongly depends on its chirality,¹ previous efforts to address this issue have resulted in significant progress in the separation of synthetic mixtures, which has yielded predominantly single-chirality nanotube species.¹¹⁻¹⁵ However, separation processes are limited by their small scale, their high cost, and the short length (< 500 nm) of the resulting chirality-pure nanotubes. They are, therefore, not viable for many, especially electronic device applications.¹¹ Here we demonstrate a general strategy for producing both metallic and semiconducting SWCNTs with predefined chiralities by using purified single-chirality nanotubes of (7, 6), (6, 5) and (7, 7) as seeds for subsequent metal-catalyst-free growth, resembling the vapor phase epitaxy (VPE) commonly used for semiconductor films. ❧ To shed light on the growth mechanism of our VPE process, we carefully studied the chirality-dependent growth kinetics and termination mechanism for seven single-chirality nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7)
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Liu, Jia
(author)
Core Title
Controlled synthesis, characterization and applications of carbon nanotubes
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Publication Date
09/22/2014
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09/22/2014
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Zhou, Chongwu (
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ariajia@gmail.com
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