University of Southern California Dissertations and Theses

Chemical vapor deposition of graphene: synthesis, characterization, and applications

(USC Thesis Other)

Chemical vapor deposition of graphene: synthesis, characterization, and applications

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content

CHEMICAL V APOR DEPOSITION OF GRAPHENE: SYNTHESIS,
CHARACTERIZATION, AND APPLICATIONS

by
Yi Zhang

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

December 2012

Copyright 2012 Yi Zhang
ii
Dedication

This dissertation is dedicated to my family and my boyfriend Jian, for their constant
support, understanding, encouragement, and love.
iii
Acknowledgements
I want to thank my advisor Chongwu Zhou, for his guidance and support during my
five-year Ph. D study. Not only does he inspire me in academic research, but also teach
me the philosophy of life. I would not have this dissertation presented or achieved this
academic level without his help.
I would also like to thank Prof. Barry Thompson and Prof. Edward Goo for being
my dissertation committee members. The valuable suggestions they have made helped
me to improve my experiments and dissertation.
My thanks also go to my dear group members. I want to thank Dr. Lewis Gomez for
mentoring me during my junior years, thank Dr. Po-Chiang Chen, Dr. Fumiaki Ishikawa,
Dr. Alexander Badmaev, Dr. Koungmin Ryu, Dr. Akshay Kumar, Dr. Chuan Wang, Dr.
Hsiao-Kang Chang, Dr. Anuj Madaria, Haitian Chen, Jialu Zhang, Yue Fu, Yuchi Che,
Xue Lin, Jia Liu, Zhen Li, Jing Xu, Jing Qiu, Shelley Wang, Maoqing Yao, Nappadol
Aroonyadet, Luyao Zhang, Mingyuan Ge, Jiepeng Rong, Xin Fang, Pyojae Kim, Ahmad
Nabil Abbas, Pattaramon Vuttipittayamongkol, Ning Yang, Hui Gui, Younghyun Na,
Kuan-Teh Li, and Liang Chen, who always work shoulder by shoulder with me, enlighten
me when I face academic problems, help out when I encounter experimental difficulties.
iv
We encourage each other like brothers and sisters, I cherish the time we have spent
together in USC.
Lastly but the most importantly, I appreciate all the support, encouragement, and
constant love from my parents, my family, my boyfriend Jian Huang and all my friends.
Thanks for accompanying me whenever I needed, I would not have all those
achievements without you.
v
Table of Contents
Dedication ........................................................................................................................... ii
Acknowledgements ............................................................................................................ iii
List of Tables .................................................................................................................... viii
List of Figures .................................................................................................................... ix
Abstract ............................................................................................................................ xix
Chapter 1: Introduction ....................................................................................................... 1
1.1 Introduction of Graphene ............................................................................................. 1
1.2 Electrical Properties of Graphene ................................................................................ 2
1.3 Synthesis of Graphene by Chemical Vapor Deposition ............................................... 8
1.4 Different Graphene Growth Mechanism on Ni and Cu ..............................................11
1.4.1 Graphene Growth on Ni ....................................................................................11
1.4.2 Graphene Growth on Cu .................................................................................. 12
1.5 Outline of the Thesis .................................................................................................. 13
Chapter 1 References ....................................................................................................... 16
Chapter 2: Wafer-Scale Graphene by Chemical Vapor Deposition: Synthesis,
Transfer, and Devices ................................................................................................ 18
2.1 Introduction ................................................................................................................ 18
2.2 Single- Layer and Few-Layer Graphene Synthesis .................................................... 20
2.3 Micro-Raman Characterization of Graphene Films ................................................... 21
2.4 Transfer of Graphene films ........................................................................................ 23
2.5 Device Fabrication and Electrical Measurements ...................................................... 24
2.6 Optical Properties of CVD Graphene Films .............................................................. 26
2.7 Conclusion ................................................................................................................. 27
Chapter 2 References ....................................................................................................... 29
Chapter 3: Continuous, Highly Flexible, and Transparent Graphene Films by
Chemical Vapor Deposition for Organic Photovoltaics ........................................... 31
3.1 Introduction ................................................................................................................ 31
vi
3.2 Graphene Synthesis and Transfer .............................................................................. 34
3.2.1 CVD Graphene Synthesis and Characterization .............................................. 34
3.2.2 Transfer of CVD Graphene films and Characterization .................................. 35
3.3 Characterization of CVD Graphene Film as Transparent Electrode for Organic
Photovoltaics ............................................................................................................ 37
3.3.1 Film Thickness and Surface Roughness of Graphene ..................................... 37
3.3.2 Optical Transparency and Sheet Resistance of CVD Graphene films ............. 39
3.4 Flexible Transparent Electrodes: Graphene versus ITO ............................................ 42
3.5 Graphene OPV Cell Fabrication on Flexible Substrate and Device Performance
.................................................................................................................................. 45
3.5.1 Organic OPV Cell Fabrication with CVD Graphene and ITO ........................ 45
3.5.2 Cell Performance ............................................................................................. 48
3.6 Device Performance Model: The Lamber-W Function ............................................. 52
3.7 Performance of Highly Flexible Graphene OPV Cell under Bending ....................... 55
3.8 CVD Graphene Organic Photovoltaic Cells on Rigid Substrates .............................. 58
3.9 Conclusions ................................................................................................................ 61
Chapter 3 References ....................................................................................................... 62
Chapter 4: Comparison of Graphene Growth on Single-Crystalline and
Polycrystalline Ni by Chemical Vapor Deposition ................................................... 65
4.1 Introduction ................................................................................................................ 65
4.2 Graphene Synthesis on Both Ni film and Single Crystalline Ni (111) ...................... 68
4.3 Mechanism of Graphene Growth on Ni ..................................................................... 73
4.4 Raman Characterization for the Number of Graphene Layers ................................... 75
4.5 Raman Mapping for Large-Area Characterization .................................................... 78
4.6 Characterization on Transferred Graphene Films ...................................................... 81
4.7 Conclusion ................................................................................................................. 83
Chapter 4 References ....................................................................................................... 84
Chapter 5: Vapor Trapping Growth of Single-Crystalline Graphene Flowers:
Synthesis, Morphology, and Electronic Properties ................................................... 87
5.1 Introduction ................................................................................................................ 87
5.2 Large-Grain Graphene Synthesis Using a Vapor Trapping Method .......................... 89
5.3 Raman Characterization of Large-Grain Graphene ................................................... 92
5.4 TEM Characterization and Selected Area Electron Diffraction on Large-Grain
Graphene .................................................................................................................. 94
5.5 The Relation Between Graphene Morphology and Growth Parameters .................... 97
vii
5.6 The Relation Between Graphene Morphology and Cu Substrate: An Electron
Backscatter Spectroscopy Study ............................................................................ 102
5.7 Fabrication of Field Effect Transistors Using Large-Grain Grphene Flowers On
Si/SiO
2
.................................................................................................................... 105
5.8 Fabrication of Field Effect Transistors Using Large-Grain Grphene Flowers On
h-BN ....................................................................................................................... 107
5.9 Conclusion ............................................................................................................... 108
Chapter 5 References ......................................................................................................110
Chapter 6: Anisotropic Hydrogen Etching of Chemical Vapor Deposited Graphene ......113
6.1 Introduction ...............................................................................................................113
6.2 Synthesis of Graphene Using Chemical Vapor Deposition ......................................114
6.3 Anisotropic Etching of Graphene..............................................................................115
6.4 Raman Spectroscopy On Etched Graphene ..............................................................118
6.5 Temperature Dependence of the Anisotropic Etching ............................................. 122
6.6 Catalytic Effect of the Underlying Cu Substrate on Anisotropic Etching of
Graphene ................................................................................................................ 126
6.7 Conclusion ............................................................................................................... 129
Chapter 6 References ..................................................................................................... 131
Chapter 7: Conclusions and Future Directions ............................................................... 134
7.1 Conclusions .............................................................................................................. 134
7.2 Future Directions in CVD Graphene........................................................................ 136
7.2.1 Seeded Growth of Large Grain Graphene ..................................................... 136
7.2.2 Molecular Beam Assembly of Graphene Nanoribbons ................................. 139
7.2.3 Solution Based Graphene Nanoribbons ......................................................... 140
7.2.4 Helium Lithography for Graphene Nanoribbon Patterning and Electrical
Measurement ................................................................................................... 143
Chapter 7 References ..................................................................................................... 147
Bibliography ................................................................................................................... 148

viii
List of Tables

Table 3.1 Performance details of OPV cells built on PET. The structure of the
devices is given by [CVD-graphene/PEDOT/CuPc/C60/BCP/Al] and
[ITO/PEDOT/CuPc/C60/BCP/Al] for CVD graphene and ITO OPVs,
respectively. ...................................................................................................... 50
Table 3.2 Comparison of performance details of OPV cells built on glass
substrates. The structure of the devices is given by [CVD
GRAPHENE/PEDOT/CuPc/C60/BCP/Al] and
[ITO/CuPc/C60/BCP/Al] for CVD GRAPHENE and ITO OPVs,
respectively. ...................................................................................................... 61
Table 4.1 Parameters correlating graphene and Ni (111). ................................................. 67

ix
List of Figures

Figure 1.1 Mother of graphitic forms. Graphene is a 2D building material of all
other dimensionalities. It can be wrapped up into 0D buckyballs, rolled
into 1D nanotubes or stacked into 3D graphite (figure is taken from
Ref. 28). .............................................................................................................. 1
Figure 1.2 The unit cell (a) and Brillouin zone (b) of graphene are shown as
the dotted rhombus and the shaded hexagon, respectively.
i
a and
i
b ,
(i=1,2) are unit vectors and reciprocal lattice vectors, respectively.
Energy dispersion relations are obtained along the perimeter of the
dotted triangle connecting the high symmetry points Γ, Κ, and Μ
(figure is taken from Ref. 82). ............................................................................ 4
Figure 1.3 The energy dispersion relations for graphene are shown throughout
the whole region of the Brillouin zone. The inset shows the energy
dispersion along the high symmetry lines between the Γ, M, and K
points (figure is taken from Ref. 82). .................................................................. 6
Figure 1.4 (a) The energy dispersion relations for graphene. (b) Ambipolar
electric field effect in single-layer graphene. The insets show its
conical low-energy spectrum E(k), indicating changes in the position
of the Fermi energy E
F
with changing gate voltage V
g
. Positive
(negative) V
g
induce electrons (holes) in concentrations n = αV
g
where
the coefficient α ≈ 7.2 × 10
10
cm
–2
V
–1
for field-effect devices with a
300 nm SiO
2
layer used as a dielectric (figure b is taken from Ref. 28).
............................................................................................................................. 7
Figure 1.5 CVD system for graphene synthesis.................................................................. 9

x
Figure 1.6 Ni onSiO
2
/Si wafers and copper foils are loaded in the CVD graphene
system ............................................................................................................... 10
Figure 1.7 (a) schematic diagram of graphene formation on Ni. (b) Schematic
diagram of graphene atoms (yellow) on Ni (111) lattice (blue). ........................11
Figure 1.8 Schematic diagrams of graphene growth mechanism on Cu........................... 13
Figure 1.9 Outline of the dissertation ............................................................................... 14
Figure 2.1 (a) Schematic of full-wafer scale deposition of graphene layers on
polycrystalline nickel by CVD. (b) E-beam evaporated nickel film
(100 nm) on a 4” Si/SiO
2
wafer. (d) Atomic force microscopy (AFM)
image of nickel film after the chemical vapor deposition of graphene
layers. ................................................................................................................ 20
Figure 2.2 Raman spectrum obtained on as-synthesized graphene films on
Si/SiO
2
/Ni substrates. D, G and G’ Raman bands for graphene are
labeled on each spectrum. (a) Raman spectrum obtained on a single
layer graphene. A zoomed image of the G’ band on the right shows the
purely single Lorentzian fit of this peak (red trace), which is
characteristic of single-layer graphene. (b) Raman spectrum of bilayer
graphene. A nearly symmetric splitting and broadening of the G’ band
of this spectrum (shown on the right) is properly fit by a set of four
Lorentzian curves, characteristic of bilayer graphene. No G’ band was
found with a bulk graphite-like lineshape on the substrates sampled. .............. 22
Figure 2.3 Schematic diagram for the transfer process..................................................... 24

xi
Figure 2.4 (a) (upper left panel) shows a photograph of a 4-in wafer after
synthesis of deposited FLG on polycrystalline nickel; (upper right
panel) shows an AFM image of the few-layer graphene films on
Si/SiO
2
; (lower right panel) shows an optical micrograph of
transferred few-layer graphene on Si/SiO
2
; (lower left panel) shows a
typical micro-Raman spectrum of the transferred films. (b) 4-inch
wafer with back-gated few-layer graphene devices; insets show SEM
and AFM images of a typical device and device channel, respectively.
(c) I
DS
-V
DS
measurements for different gate voltages, V
G
= 2.5 V , 1.5 V
and -1.5 V for the black, red and blue curves, respectively and (d)
I
DS
-V
G
curve of one of the FET devices for V
DS
= 0.01 V . ............................... 26
Figure 2.5 (a) Photograph of a 1cm
2
FLG film transferred onto a glass substrate
(inside the box). (b) Transmittance of the FLG film shown in (a). ................... 27
Figure 3.1 (a) AFM image of a 300 nm Ni film deposited on Si/SiO
2
substrate
after high temperature annealing. (b) Typical X-ray diffraction
spectrum of annealed Ni film. ........................................................................... 35
Figure 3.2 (a) Low magnification TEM image of CVD graphene films. Inset
shows a selected area electron diffraction (SAED) pattern of the
few-layer graphene film. (b) Raman spectrum of CVD graphene film. ........... 37
Figure 3.3 (a) AFM image of a transferred CVD Graphene film onto glass
substrate. (b) Cross section measurement of the height of the CVD
Graphene. Typical thickness exhibited by the transferred films is found
within the range 1-3 nm. ................................................................................... 38
Figure 3.4 AFM images of the surface of CVD graphene, ITO and SWNT films
on glass. The scale bar in z-direction is 50 nm for all images. ......................... 39
Figure 3.5 Photographs showing highly transparent graphene films transferred
onto glass and PET are shown in (a) and (b), respectively. (c)
Transmission spectra for CVD graphene, ITO and SWNT films on
glass................................................................................................................... 40

xii
Figure 3.6 (a) Transmission spectra of CVD graphene with different sheet
resistance (R
Sheet
). (b) Comparison of R
Sheet
vs. light transmittance at
550 nm for CVD graphene and reduced GO films reported in the
literature. ........................................................................................................... 41
Figure 3.7 (a) Photograph illustrating high flexibility of CVD graphene
transferred on a PET flexible substrate. (b) and (c) AFM images of
the surface of CVD graphene and ITO films on PET, respectively. (d)
and (f) Conductance of the CVD graphene and ITO films on PET
substrates under bending conditions, respectively. The devices used
to monitor the conductance had channel width (W) = 1 mm, and
length (L) = 1 mm. (e) Optical images of CVD graphene (upper) and
ITO (lower) films on PET before and after being bent at the angles
specified in (b) and (c). Arrows show the direction of the bending. ................ 44
Figure 3.8 Schematic representation of the energy level alignment (top) and
construction of the heterojunction organic solar cell fabricated with
graphene as anodic electrode: CVD
graphene/PEDOT/CuPc/C60/BCP/Al............................................................... 47
Figure 3.9 Logarithmic (up) and linear (down) current density and power
density vs voltage characteristics of CVD graphene (a) and ITO (b)
OPV cells on PET under dark (red traces) and 100 mW/cm
2
AM1.5G
spectral illumination (blue traces). The output power density of the
cells is plotted on (a) and (b) as open circle traces. The structure of
the devices is given by [CVD graphene / PEDOT / CuPc / C
60
/ BCP /
Al] and [ITO / CuPc / C
60
/ BCP / Al] for CVD graphene and ITO
OPVs, respectively ............................................................................................ 49
Figure 3.10 Comparison of the modeled (solid lines) current density and power
density curves of the graphene and ITO devices obtained from the
Shockley equation against the experimentally (dots) obtained values. ............ 54
Figure 3.11 Current density vs. voltage characteristics of CVD graphene (a) or
ITO (b) photovoltaic cells under 100 mW/cm
2
AM1.5G spectral
illumination for different bending angles. Insets show photographs of
the experimental set up employed in the experiments. ..................................... 56
xiii
Figure 3.12 (a) Fill factor dependence of the bending angle for CVD graphene
and ITO devices shown in figure 3.11. (b) SEM images showing the
surface structure of CVD graphene (top) and ITO (bottom)
photovoltaic cells after being subjected to the bending angles
described in figure 3.11. .................................................................................... 58
Figure 3.13 Logarithmic (a) and linear (b) current density vs voltage
characteristics of CVD GRAPHENE and ITO photovoltaic cells on
glass under 100 mW/cm
2
AM1.5 spectral illumination. Structure of
the devices is given by [CVD GRAPHENE / PEDOT / CuPc / C
60
/
BCP / Al] and [ITO / CuPc / C
60
/ BCP / Al] for CVD GRAPHENE
and ITO OPVs, respectively.............................................................................. 60
Figure 4.1 XRD spectra and AFM images (inset) of Ni (111) substrate (a) and
polycrystalline Ni substrate (b), respectively. The color scale bar
corresponds to AFM images in a and b inset. ................................................... 68
Figure 4.2 (a). XRD spectra collected from polycrystalline Ni with fast (black),
medium (red), and slow (blue) annealing rates and XRD spectrum
collected from Ni (111) (green) after thermal annealing (XRD
spectrum is identical for Ni (111) using different annealing rates). (b).
Zoomed-in XRD spectra of peaks at 2θ = 52.16° (assigned as Ni (200)).
(c-j). Optical images taken after graphene CVD growth from
polycrystalline Ni with fast (c, d), medium (e, f) and slow (g, h)
annealing rates. (i, j). Optical images taken after graphene CVD
growth from Ni (111). ....................................................................................... 71
Figure 4.3 Schematic diagrams of graphene growth mechanism on Ni (111) (a)
and polycrystalline Ni surface (b). (c). Optical image of graphene/ Ni
(111) surface after the CVD process. The inset is a three dimensional
schematic diagram of a single graphene layer on Ni (111) surface. (d).
Optical image of graphene/ polycrystalline Ni surface after the CVD
process. The inset is a three dimensional schematic diagram of
graphene layers on polycrystalline Ni surface. Multiple layers formed
from the grain boundaries. ................................................................................ 74

xiv
Figure 4.4 (a-b). Ten typical Raman spectra of graphene grown on Ni (111) and
polycrystalline Ni, respectively. (c). The G’-to-G peak intensity ratio
(I
G’
/I
G
) v.s. the Full Width at Half Maximum (FWHM) of G’ bands of
graphene on both Ni (111) and polycrystalline Ni............................................ 77
Figure 4.5 (a). Maps of I
G’
/I
G
of 780 spectra collected on a 60*50 μm
2
area on the
Ni (111) surface and (b) 750 spectra collected on a 60*50 μm
2
area on
the polycrystalline Ni surface. Corresponding optical images to Ni
(111) Raman map and polycrystalline Ni Raman map (c and d). (e)
AFM image of graphene film transferred to SiO
2
/Si substrate from Ni
(111). (d) Height analysis of the thickness of graphene film. ........................... 79
Figure 4.6 Optical image of graphene transferred from Ni (111) (a) and
polycrystalline Ni film (b) to SiO
2
/Si substrate. Corresponding Raman
spectra taken from graphene transferred from Ni (111) (c) and
polycrystalline Ni (d). ....................................................................................... 81
Figure 4.7 Transmittance spectrum of graphene film transferred from Ni (111) .............. 83
Figure 5.1 (a) Schematic diagram of a vapor trapping CVD method for graphene
growth. (b) low and (c) high magnification SEM images of a six-lobe
graphene flower grown on Cu foil inside the vapor trapping tube. (d)
SEM image of a four-lobe graphene flower grown on Cu foil inside the
vapor trapping tube. (e) Graphene grown on Cu foil outside the
vapor-trapping tube. .......................................................................................... 90
Figure 5.2 SEM images of graphene flowers without using vapor trapping tube.
(a) low magnification. (b) high magnification. (scale bar: 50 μm) ................... 92
Figure 5.3 (a) A SEM image and (b) an optical microscope image of a six-lobe
graphene flower transferred on a Si/SiO
2
substrate. (c) Raman spectra
taken from location A, B, and C marked in Fig. 2b. (d-f) Raman map of
I
G
, I
2D
, and I
2D
/I
G
intensity ratio. Scale bar for (d)-(f) is 10 μm. The
color scale bar from bottom to top is 300, 500, 900, 1300, 1700, 2000
(d); 100, 600, 1200, 1800, 2400, 3200 (e); 1, 2, 3 (f). ...................................... 93

xv
Figure 5.4 (a) SEM image of graphene flowers transferred on a perforated SiN
TEM grid. (b) Zoomed-in SEM image of the graphene flower marked
using yellow dashed square in Fig. 3a. Each opening within the
graphene flower was marked by a letter (white: single-layer graphene;
yellow: A-B stacking bilayer graphene; blue: torn and folded graphene)
or a red cross (no graphene covered). (c) A bright field TEM image of
graphene suspended on SiN TEM grid. (d)-(f) Diffraction patterns
taken from opening H, Y, and O, respectively. (g) Diffraction pattern
taken from opening BB. (h) A bright field TEM image of torn and
folded graphene taken from opening AA. (i) Diffraction pattern taken
from opening AA. ............................................................................................. 95
Figure 5.5 SEM images of graphene grown using various recipe. The central
images with yellow frame in both left and right column are the same.
And the graphene was grown at 150 mTorr using 1:12.5 CH
4
/H
2
ratio.
The left set of SEM images with blue frame are graphene grown at
1:12.5 CH
4
/H
2
ratio with the total pressure varied from 80 mTorr to
400 mTorr. The right set of SEM images with red frame are graphene
grown at 150 mTorr with CH
4
/H
2
concentration ratio varied from 1:30
to 1:2. ................................................................................................................ 98
Figure 5.6 SEM images of graphene flowers using different growth time at 300
mTorr. (a) 5 min, (b) 10 min, (c) 20min, (d) 30 min. (scale bar: 50 μm)
......................................................................................................................... 101
Figure 5.7 (a) SEM image of a four-lobe graphene flower, (b) a six-lobe
graphene flower, and (c) another four-lobe graphene flower on the
same graphene sample. (d) SEM image of a sample with six-lobe, and
four-lobe graphene flowers, and copper grain boundaries. All SEM
images (a-d) were taken with samples tilted at 70° along Y axis. (e)-(h)
Corresponding EBSD orientation map image of the location
highlighted by the yellow dashed square in e, f, g, and h, respectively.
The color represents fcc crystalline orientation is shown on the right
side. ................................................................................................................. 103

xvi
Figure 5.8 (a) SEM image of a six-lobe graphene FET. Electrodes are marked by
different letters. The dashed blue square is the region of effective
graphene channel between electrode D and F. (b) Plot of drain current
(I
ds
) versus gate voltage (V
g
) minus Dirac point voltage (V
Dirac
) using D
and F as source and drain electrodes (black circles) and fitted FET
mobility curve (solid red line). The drain voltage (V
ds
) is 0.2 V. Inset is
a plot of drain current (I
ds
) versus drain voltage (V
ds
) at various gate
voltages. .......................................................................................................... 105
Figure 5.9 (a) SEM image of a hall-bar graphene/h-BN FET. (b) Plot of drain
current (I
ds
) versus gate voltage (V
g
). .............................................................. 108
Figure 6.1 (a) Schematic diagram of Cu etching mechanism. (b) SEM image of
graphene after etching on copper foil and (c) transferred onto Si/SiO
2

substrate . .........................................................................................................116
Figure 6.2 Energy-dispersive X-ray (EDX) spectra on different area of graphene
sample after etching. The red cross in each inset SEM image indicates
the location where each spectrum was taken. (a) An EDX spectrum
taken on a particle inside an etched hexagon. It shows strong silicon
and oxygen peak. (b) An EDX spectrum taken inside an etched
hexagon but not on a particle. No silicon peak is shown. (c) An EDX
spectrum taken on another particle inside an etched hexagon. It also
shows strong silicon and oxygen peak. (d) An EDX spectrum taken on
remaining graphene. No silicon peak is shown. ..............................................117
Figure 6.3 CVD graphene before and after etching. (a) as-grown CVD graphene
transferred onto Si/SiO
2
and a representative Raman spectrum as an
inset; (b) graphene etched by H
2
at 800 °C and transferred onto Si/SiO
2
.
Raman spectra (inset) show the intact graphene (pointed by red arrow)
and etched region (pointed by blue arrow); (c) Histogram of proportion
of angles of graphene etched edges. (d)-(f). Raman map of I
G
(d) (color
scale bar: 100,200,500,800,>800 a.u.), I
2D
(e) (color scale bar:
300,900,1500,2200,>2200 a.u.), and I
D
(f) (color scale bar:
40,80,120,>120 a.u.). The scale bar for d-f is 3 μm. ........................................119

xvii
Figure 6.4 (a) SEM image of etched graphene. A, B, C, and D are selected
locations across anisotropic edges of etched graphene. (b)
Corresponding Raman spectra taken from location A to D. (d) D band
intensity of point A to D. ................................................................................. 121
Figure 6.5 SEM images at different magnifications of graphene etched at
different temperatures and transferred onto Si/SiO
2
substrate. (a) and
(b). 700 °C; (c) and (d). 800 °C; (e) and (f). 900 °C; (g) and (h).
1000 °C. .......................................................................................................... 124
Figure 6.6 Percentage of graphene etched area at different temperatures. (a).
700 °C; (b). 800 °C; (c). 900 °C; (d). 1000 °C. Five regions were
randomly picked for the calculation of etched area, each region is 40
μm × 30 μm. (e) Etched area versus temperature plot. ................................... 126
Figure 6.7 (a) SEM image of CVD graphene transferred onto Si/SiO
2
substrate;
(b) A representative Raman spectrum of SLG; (c) SEM image of
transferred CVD graphene after H
2
annealing at 800 °C; (d) A
representative Raman spectrum of CVD graphene after H
2
annealing. .......... 127
Figure 7.1 SEM image of patterned PMMA seeds on Cu foil (a), graphene flower
arrays after CVD seeded growth (b), the edge between patterned
region and non-patterned region after graphene growth (c). .......................... 137
Figure 7.2 (a) Scheme for the synthesis of graphene ribbons using ring-opening
alkyne metathesis polymerization followed by a cascade cyclization.
(b) Scheme for the synthesis of armchair GNRs. ........................................... 140
Figure 7.3 Synthesis approach for GNRs in solution ..................................................... 141
Figure 7.4 (a) AFM image of spin-coated GNRs on Si/SiO
2
substrate. (b) Raman
spectra taken from aggregates and uniform thin film from spin-coated
GNR on Si/SiO
2
substrate. (c) UV-Vis spectra taken from GNR in
THF (black) and GNR in chlorobenzene (red) respectively. .......................... 142

xviii
Figure 7.5 SEM image of a GNR array patterned by helium lithography with 4
nm half-pitch. .................................................................................................. 144
Figure 7.6 SEM image of as-grown graphene hexagons on Cu foil (a), patterned
graphene hexagon devices (b), and a zoomed-in SEM image of
hexagon graphene devices (c). ........................................................................ 145

xix

Abstract

In this dissertation I discuss the synthesis of graphene using chemical vapor
deposition on Ni and Cu substrate, as well as various applications using CVD graphene.
Chapter 1 gives a brief introduction of graphene, the electrical properties of graphene,
and chemical vapor deposition method of graphene synthesis.
Chapter 2 discusses a simple, scalable and cost-efficient method to prepare graphene
using methane-based chemical vapor deposition on nickel films deposited over complete
Si/SiO
2
wafers. By using highly diluted methane, single- and few-layer graphene were
obtained, as confirmed by micro Raman spectroscopy. In addition, a transfer technique has
been applied to transfer the graphene film to target substrates via nickel etching.
Field-effect transistors based on the graphene films transferred to Si/SiO
2
substrates
revealed a weak p-type gate dependence, while transferring of the graphene films to glass
substrate allowed its characterization as transparent conductive films, exhibiting
transmittance of 80% in the visible wavelength range.
In chapter 3, continuous, highly flexible, and transparent few-layer graphene films
xx
synthesized from Ni film were implemented as transparent conductive electrodes (TCE) in
organic photovoltaic cells. Graphene films were synthesized by CVD, transferred to
transparent substrates, and evaluated in organic solar cell heterojunctions
(TCE/poly-3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper
phthalocyanine/fullerene/bathocuproine/aluminum). Key to our success is the continuous
nature of the CVD graphene films, which led to minimal surface roughness (~0.9 nm) and
offered sheet resistance down to 230 Ω/sq (at 72% transparency), much lower than stacked
graphene flakes at similar transparency. In addition, solar cells with CVD graphene and
indium tin oxide (ITO) electrodes were fabricated side-by-side on flexible polyethylene
terephthalate (PET) substrates and were confirmed to offer comparable performance, with
power conversion efficiencies (η) of 1.18 and 1.27%, respectively. Furthermore, CVD
graphene solar cells demonstrated outstanding capability to operate under bending
conditions up to 138°, whereas the ITO-based devices displayed cracks and irreversible
failure under bending of 60°. Our work indicates the great potential of CVD graphene films
for flexible photovoltaic applications.
In chapter 4, we discuss comparative study and Raman characterization on the
formation of graphene on single crystal Ni (111) and polycrystalline Ni substrates using
chemical vapor deposition. Preferential formation of monolayer/bilayer graphene on the
single crystal surface is attributed to its atomically smooth surface and the absence of grain
xxi
boundaries. In contrast, CVD graphene formed on polycrystalline Ni leads to higher
percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain
boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman
surface mapping reveals that the area percentages of monolayer/bilayer graphene are
91.4% for the Ni (111) substrate and 72.8% for the polycrystalline Ni substrate under
comparable CVD conditions. The use of single crystal substrates for graphene growth may
open ways for uniform high-quality graphene over large areas.
Chapter 5 discusses a vapor trapping method for the growth of large-grain,
single-crystalline graphene flowers with grain size up to 100 μm. Controlled growth of
graphene flowers with four lobes and six lobes has been achieved by varying the growth
pressure and the methane to hydrogen ratio. Surprisingly, electron backscatter diffraction
study revealed that the graphene morphology had little correlation with the crystalline
orientation of underlying copper substrate. Field effect transistors were fabricated based on
graphene flowers and the fitted device mobility could achieve ~ 4,200 cm
2
V
-1
s
-1
on
Si/SiO
2
and ~ 20,000 cm
2
V
-1
s
-1
on hexagonal boron nitride (h-BN). Our vapor trapping
method provides a viable way for large-grain single-crystalline graphene synthesis for
potential high-performance graphene-based electronics.
In chapter 6, a simple, clean, and highly anisotropic hydrogen etching method was
developed for chemical vapor deposited graphene catalyzed by the copper substrate. By
xxii
exposing CVD graphene on copper foil to hydrogen flow around 800 °C, we observed that
the initially continuous graphene can be etched to have many hexagonal openings. In
addition, we found that the etching is temperature dependent. Compared to other
temperatures (700, 900, and 1000 °C), etching of graphene at 800 °C is most efficient and
anisotropic. Of the angles of graphene edges after etching, 80% are 120°, indicating the
etching is highly anisotropic. No increase of the D band along the etched edges indicates
that the crystallographic orientation of etching is in the zigzag direction. Furthermore, we
observed that copper played an important role in catalyzing the etching reaction, as no
etching was observed for graphene transferred to Si/SiO
2
under similar conditions. This
highly anisotropic hydrogen etching technology may work as a simple and convenient way
to determine graphene crystal orientation and grain size and may enable the etching of
graphene into nanoribbons for electronic applications.
Brief conclusions are drawn in chapter 7. Future directions of graphene are also
discussed in chapter 7.

In summary, this dissertation starts from CVD graphene synthesis and fulfills with
various applications using as-synthesized graphene material, which proves the potential of
CVD graphene for device application, OPV cells, and other possible applications. With the
continuous improvement of graphene quality, as well as the promising application results
xxiii
shown in this dissertation, we should expect many more applications that exploit all kinds
of unique properties of graphene in near future.

1
Chapter 1: Introduction

1.1 Introduction of Graphene
Graphene is a flat monolayer of carbon atoms tightly packed into a two-dimensional
honeycomb lattice, and is a basic building block of graphitic materials of all other
dimensionalities (Figure 1.1). It can be wrapped up into 0D fullerenes, rolled into 1D
nanotubes or stacked into 3D graphite.
28

Figure 1.1 Mother of graphitic forms. Graphene is a 2D building material of all other
dimensionalities. It can be wrapped up into 0D buckyballs, rolled into 1D nanotubes or
stacked into 3D graphite (figure is taken from Ref. 28).
2

The 2D crystals were once considered thermodynamically unstable and could not
exist. Atomic monolayers have so far been known only as an integral part of larger 3D
structures, usually grown epitaxially on top of monocrystals with matching crystal lattices.
Without such a 3D base, 2D materials were presumed not to exist. Till 2004, an easy
isolation method of graphene was developed by Novoselov, Geim and co-workers, and
graphene crystals could be obtained on top of non-crystalline substrates.
This crystalline material was found not only to be continuous, but of high quality. The
charge carriers of graphene can travel thousands of interatomic distances without
scattering and the predicted charge carrier mobility is ~10
6
cm
2
V
-1
s
-1
. Due to its unique
structures and interesting properties, the debut of graphene attracted enormous interest
from researchers. The Nobel Prize in Physics 2010 was awarded jointly to Andre Geim and
Konstantin Novoselov for their groundbreaking experiments regarding the
two-dimensional material graphene.

1.2 Electrical Properties of Graphene
Figure 1.2a and b shows the unit cell and the Brillouin zone of graphene as a dotted
rhombus and shaded hexagon, respectively, where
1
a and
2
a are unit vectors in real
space, and
1
b and
2
b are reciprocal lattice vectors. In the x, y coordinates shown in the
figure 1.2, the real space unit vectors
1
a and
2
a of the hexagonal lattice are expressed
3
as:
)
2
,
2
3
( ),
2
,
2
3
(
2 1
a
a a
a
a a − = = , (1.1)
Where
°
= × = = = A a a a 46 . 2 3 42 . 1
2 1
is the lattice constant of graphene.
Correspondingly the unit vectors
1
b and
2
b of the reciprocal lattice are given by:
)
2
,
3
2
( ),
2
,
3
2
(
2 1
a a
b
a a
b
π π π π
− = = (1.2)
This corresponds to a lattice constant of
a 3
4π
in reciprocal space. The direction of
the unit vectors
2
b and
2
b of the reciprocal hexagonal lattice is rotated by 90° from
the unit vectors
1
a and
2
a of the hexagonal lattice in real space as shown in Figure 1.2.
The shaded area in Figure 1.2b shows the first Brillouin zone of graphene, where the
three high symmetry points Γ, Κ and Μ can be defined as the center, the corner, and the
center of the edge of the hexagonal Brillouin zone, respectively. Thus, the energy
dispersion relations of graphene can be calculated from the triangle Γ Κ Μ shown by the
dotted lines in Figure 1.2b.
82

4

Figure 1.2 The unit cell (a) and Brillouin zone (b) of graphene are shown as the dotted
rhombus and the shaded hexagon, respectively.
i
a and
i
b , (i=1,2) are unit vectors and
reciprocal lattice vectors, respectively. Energy dispersion relations are obtained along the
perimeter of the dotted triangle connecting the high symmetry points Γ, Κ, and Μ (figure
is taken from Ref. 82).

In graphene, three σ bonds hybridize in a sp
2
configuration, while the other 2pz orbital,
which is perpendicular to the graphene plane, makes π covalent bonds. It is known that the
π electrons are covalent and are the most important for determining the solid state
properties of graphene.
Two Bloch functions, constructed from atomic orbitals for the two inequivalent
carbon atoms at A and B, provide the basis functions for graphene. We solve the Hermitian
matrix H and the overlap integral matrix S to find the 2D energy dispersion relation of
graphene.








=
p
p
tf
tf
H
2
*
2
(k) -
(k) -
ε
ε
and








=
1 (k)
(k) 1
*
sf
sf
S (1.3)
5
where
p 2
ε is the site energy of the 2p atomic orbital and
)
2
k
( cos 2 (k)
y 3 2 / - 3 /
a
e e f
a ik a ik
x x
+ = (1.4)
where
C C
a a a a
- 2 1
3 = = =
Solving the secular equation det (H-ES) = 0 and using H and S as given in Eq. 1.3, the
eigenvalues ) (k E are obtained as a function ) k ( ω ,
x
k and
y
k :
(k) 1
(k)
(k)
2p
2
ω
ω ε
s
t
E
D g
m
±
=
±
(1.5)
where t is the transfer integral, s is the overlap of the electronic wave function on adjacent
sites, the + signs in the numerator and denominator go together giving the bonding π
energy band, and likewise for the – signs, which give the anti-bonding π
*
band, while the
function ) k ( ω is given by:
2
cos 4
2
cos
2
3
cos 4 1 ) k f( ) k (
2
2
a k a k
a k
y y
x
+ + = = ω
(1.6)

Figure 1.3 plots the energy dispersion relations of graphene throughout the Brillouin
zone and the inset shows the energy dispersion relations along the high symmetry axes
along the perimeter of the triangle Γ M K. Here t = 3.013 eV, s = 0.129 and ε
2p
= 0. The
upper half of the energy dispersion curves describes the π* anti-bonding energy band and
the lower half is the π bonding energy band. Interestingly, the π* anti-bonding and π
bonding bands are degenerate at the Κ points through which the Fermi energy passes.

6

Figure 1.3 The energy dispersion relations for graphene are shown throughout the whole
region of the Brillouin zone. The inset shows the energy dispersion along the high
symmetry lines between the Γ, M, and K points (figure is taken from Ref. 82).

Since there are two π electrons per unit cell, these electrons fully occupy the lower π
band. Since a detailed calculation of the density of states shows that the density of states
at the Fermi energy is zero, graphene is a zero-gap semiconductor. The existence of a zero
gap at the K points comes from the symmetry requirement that the two carbon atoms A
and B in the hexagonal lattice are equivalent to each other.
82

When the overlap integral s becomes zero, the π and π
*
bands become symmetrical
around E = ε
2p
. The energy dispersion relations in this case are commonly used as a simple
approximation for the electronic structure of a graphene layer:

7
2
cos 4
2
cos
2
3
cos 4 1 ) .k (k E
2
y x g2D
a k a k
a k
t
y y
x
+ + ± =
(1.7)

In the case where ε
2p
is taken as zero-point energy, the six corners of the Brillouin
zone are called Dirac points. Figure 1.4a depicts the energy dispersion of graphene around
Dirac points (one of the Dirac points is pointed out by a red arrow in Figure 1.4a). We can
see from Figure 1.4a that the energy band is exactly symmetric at Dirac points, where
undoped graphene is a perfect zero-bandgap semiconductor.
(a) (b)

Figure 1.4 (a) The energy dispersion relations for graphene. (b) Ambipolar electric field
effect in single-layer graphene. The insets show its conical low-energy spectrum E(k),
indicating changes in the position of the Fermi energy E
F
with changing gate voltage V
g
.
Positive (negative) V
g
induce electrons (holes) in concentrations n = αV
g
where the
coefficient α ≈ 7.2 × 10
10
cm
–2
V
–1
for field-effect devices with a 300 nm SiO
2
layer used
as a dielectric (figure b is taken from Ref. 28).

The charge carriers of graphene can be tuned continuously between electrons and
holes in concentrations n as high as 10
13
cm
-2
and their mobility μ can exceed 15,000
8
cm
2
V
-1
s
-1
even under ambient conditions.
1
In Figure 1.4b a graphene field effect transistor
exhibits a pronounced ambipolar electric field effect, with a 300 nm SiO
2
layer used as a
dielectric.

1.3 Synthesis of Graphene by Chemical Vapor Deposition
There are several main approaches for graphene synthesis: 1) micromechanical
cleavage of graphite
63, 67
, 2) chemical exfoliation of graphite oxide
89
, 3) epitaxial growth of
graphene on SiC surface
26,78,36
, 4) epitaxial growth of graphene on the surface of transition
metals, such as Ru (0001)
92
. However, there are still disadvantages of these methods, such
as some methods are not scalable for wafer-size synthesis and device fabrication
67,63
, the
graphene obtained is defective
89
or difficult to be transferred to target substrates
26,36, 78,92
.
Among all the strategies to produce graphene with large quantity, chemical vapor
deposition onto transition metal substrates becomes the most promising approach, which is
inexpensive and gives large area graphene. It opens a new route to large-area production of
high-quality graphene films for practical applications. Since 2008, graphene CVD has been
demonstrated on a variety of transition metals.
16-17, 58, 75, 91, 95, 99
Here in this thesis, we will
focus on Ni and Cu, which are the two major catalysts used for graphene CVD.
As shown in Figure 1.5, the CVD system for graphene synthesis is composed of
following parts: gas cylinders for gas feed, mass flow controllers (MFC) to control the flow
9
rates of gases, high temperature tube furnace, and the pressure control system to control the
pressure in the tube where graphene synthesis happens.
The substrates for graphene synthesis are loaded into the quartz tube of the high
temperature tube furnace first. The substrates can be evaporated Ni film on SiO
2
/Si wafers
or copper foils. The size of wafers/foils depends only on the dimension of the quartz tube.
Gas in
Gas out
Rack of wafers
9 feet-long growth furnace with three-zone
H
2
CH
4
Ar
Exhaust
MFC 1
MFC 2
MFC 3
High Temperature Tube Furnace
Quartz Tube
Viewport
Pressure Gauge
Pump
Pressure
control
system
Pressure
controller

Figure 1.5 CVD system for graphene synthesis

We place wafers on the wafer rack and load into the quartz tube. In the case of copper
foil, we roll the foil up and push it into the tube to reach the largest dimension of the copper
that can be accommodate into our CVD quartz tube. Therefore, scalable synthesis of
graphene is easy to achieve in our system (Figure 1.6).
10
Wafer rack
Rolled foil
CVD graphene on Ni film on SiO
2
/Si CVD graphene on Cu foil

Figure 1.6 Ni onSiO
2
/Si wafers and copper foils are loaded in the CVD graphene system

When the CVD synthesis of graphene starts, argon is introduced to the system to drive
away air. Then hydrogen is introduced to the system to form a reduction gas environment
while the temperature of the furnace increases from room temperature to high temperature
(900 °C or 1000 °C). When the temperature reaches the set point, methane is introduced to
the system as the carbon source for graphene synthesis. The synthesis time is from 10
minutes to 30 minutes and then methane is turned off, the temperature of furnace is brought
down to room temperature. The pressure control system controls the pressure from the
beginning to the end of the synthesis process. The mechanical pump pumps out all the
exhaust produced during the synthesis process.

11
1.4 Different Graphene Growth Mechanism on Ni and Cu
1.4.1 Graphene Growth on Ni
Compared with other metal catalysts, Ni has non-negligible carbon solubility at high
temperature.
40, 62, 70
Polycrystalline Ni substrates are firstly annealed in Ar/H
2
atmosphere
at 900~1000 °C to increase grain size, followed by exposing substrates to H
2
/CH
4
gas
mixture, when hydrocarbon decomposes and carbon atoms dissolve into Ni films to form a
solid solution. Finally, samples are cooled down in argon gas. The carbon solubility in Ni
decreases as temperature goes down, and carbon atoms diffuse out from bulk Ni and
precipitate on the surface to form graphene films.
Ni
CH
4
CH
4
CH
4
CH
4
Methane
decomposing and
carbon dissolving Cooling
Ni
Graphene
(a)
(b)

Figure 1.7 (a) schematic diagram of graphene formation on Ni. (b) Schematic diagram of
graphene atoms (yellow) on Ni (111) lattice (blue).

12
This process is illustrated in Figure 1.7a. Figure 1.7b shows an excellent lattice match
between graphene (yellow balls) and Ni (111) (blue balls) with densely packed hexagonal
lattice structure and similar lattice constant,
23
which makes Ni surface desirable for
graphene growth.

1.4.2 Graphene Growth on Cu
Since graphene formation on Ni is a segregation process, it is hard to suppress
multi-layers because there is always extra carbon dissolved in imperfect Ni films. On the
contrast, Cu has ultralow carbon solubility. Even if the hydrocarbon concentration is high
or the exposure time is long, there is only a small amount of carbon dissolved in Cu, and
most of the carbon source for graphene formation is from the CH
4
that is catalytically
decomposed on the Cu surface. After the first layer graphene is deposited, Cu surface is
fully covered and there is no catalyst exposed to hydrocarbon to promote decomposition
and growth. Thus, the graphene growth on Cu is a surface absorption process and
self-limiting, which makes it robust and insensitive to some growth parameters. Figure 1.8
illustrates the graphene growth mechanism on Cu.
13

Figure 1.8 Schematic diagrams of graphene growth mechanism on Cu.

1.5 Outline of the Thesis
The Outline of my Ph. D. thesis is summarized in Figure 1.9. This dissertation will
discuss the synthesis of graphene using chemical vapor deposition on both Ni and Cu
substrate and further extend to different applications using CVD graphene.
Chapter 1 gives a brief introduction of graphene. Chapter 2 to 4 focuses on graphene
on Ni. Chapter 2 discusses the development of the wafer-scale CVD synthesis of single and
few-layer graphene, along with the development of graphene transfer technique.
Full-wafer graphene field effect transistors have also been successfully fabricated on
transferred graphene on Si/SiO
2
. In chapter 3, this single and few-layer graphene films are
implanted into organic photovoltaic cells as anode materials, which show excellent device
performance in comparison with ITO OPV cells, as well as outstanding flexibility under
bending condition. To further improve the quality of graphene and obtain higher
14
percentage of single-layer graphene, chapter 4 discusses the use of Ni (111) for single-layer
graphene synthesis, which shows much higher single-layer graphene coverage than
polycrystalline Ni films.
Graphene film Graphene film
Metal Metal
Si/SiO Si/SiO
2 2
PMMA coating PMMA coating
Metal Metal
etching etching
PMMA / Graphene PMMA / Graphene
Substrate Substrate
Graphene Graphene
Transfer Acetone
2 μm
120°
120 °
120°
120 °
10 μm
0 10 20 30 40 50
50
40
30
20
10
0

Width (μm)
Length (μm)
0
0.25
0.50
0.75
1.0
91.4%
-20 -10 0 10 20 30 40
0.024
0.026
0.028
0.030
0.032
0.034
0.036
0.038

Gate Voltage (V)
Drain Current (mA)
Graphene on Ni Graphene on Cu
Transfer
Wafer-scale CVD graphene
CVD graphene for OPV
Graphene on Ni (111) Anisotropic etching
Large-grain graphene FET
Large-grain graphene synthesis

Figure 1.9 Outline of the dissertation

Chapter 5 and 6 discusses graphene on Cu. Chapter 5 presents the development of a
vapor trapping method for large-grain graphene synthesis, which achieves the synthesis of
graphene grain up to 100 micrometers. The large-grain graphene are then applied into field
effect transistors and showed device mobility ~4,200 cm
2
V
-1
s
-1
on Si/SiO
2
. We use h-BN
15
as substrate dielectric which further brought the mobility up to ~ 20,000 cm
2
V
-1
s
-1
. Along
with the process of graphene synthesis on Cu, an interesting anisotropic etching effect has
also been developed as a reverse reaction of graphene synthesis in chapter 6. This etching
method is simple, clean, and efficient, which may work as a simple and convenient way to
determine graphene crystal orientation and grain size, and may enable the etching of
graphene into nanoribbons for electronic applications.
Chapter 7 summarizes the work in chapter 2 to 6 and discusses future directions on
CVD graphene.

16
Chapter 1 References

16. Coraux, J.; N'Diaye, A. T.; Engler, M.; Busse, C.; Wall, D.; Buckanie, N.; Heringdorf,
F.-J. M. z.; van Gastel, R.; Poelsema, B.; Michely, T., Growth of Graphene on Ir(111) New
J. Phys. 2009, 11.
17. De Arco, L. G.; Zhang, Y .; Kumar, A.; Zhou, C., Synthesis, Transfer, and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Trans.
Nanotechnol. 2009, 8, 135-138.
23. Eizenberg, M.; Blakely, J. M., Carbon Monolayer Phase Condensation on Ni(111).
Surf. Sci. 1979, 82, 228-236.
26. Forbeaux, I.; Themlin, J. M.; Debever, J. M., Heteroepitaxial Graphite on
6h-Sic(0001): Interface Formation through Conduction-Band Electronic Structure. Phys
Rev B 1998, 58, 16396-16406.
28. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
36. Hass, J.; Feng, R.; Li, T.; Li, X.; Zong, Z.; de Heer, W. A.; First, P. N.; Conrad, E. H.;
Jeffrey, C. A.; Berger, C., Highly Ordered Graphene for Two Dimensional Electronics.
Appl Phys Lett 2006, 89, 143106.
40. International, A., Alloy Phase Diagrams. 2002; V ol. 3.
58. Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S., Evolution of Graphene Growth on Ni
and Cu by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268-4272.
62. Lopez, G. A.; Mittemeijer, E., The Solubility of C in Solid Cu. Scr. Mater. 2004, 51,
1-5.
63. Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S.,
The Structure of Suspended Graphene Sheets. Nature 2007, 446, 60-63.
67. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V . V .; Morozov, S.
V .; Geim, A. K., Two-Dimensional Atomic Crystals. P Natl Acad Sci USA 2005, 102,
10451-10453.
70. Oshima, C.; Nagashima, A., Ultra-Thin Epitaxial Films of Graphite and Hexagonal
Boron Nitride on Solid Surfaces. J. Phys.-Condes. Matter 1997, 9, 1-20.
75. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V .; Dresselhaus, M. S.;
Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical
Vapor Deposition. Nano Lett. 2009, 9, 30-35.
78. Rollings, E.; Gweon, G. H.; Zhou, S. Y .; Mun, B. S.; McChesney, J. L.; Hussain, B. S.;
Fedorov, A.; First, P. N.; de Heer, W. A.; Lanzara, A., Synthesis and Characterization of
Atomically Thin Graphite Films on a Silicon Carbide Substrate. J Phys Chem Solids 2006,
67, 2172-2177.
82. Salvetat, J. P.; Bonard, J. M.; Bacsa, R.; Stockli, T.; Forro, L., Physical Properties of
17
Carbon Nanotubes. Aip Conf Proc 1998, 442, 467-480.
89. Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y .;
Wu, Y .; Nguyen, S. T.; Ruoff, R. S., Synthesis of Graphene-Based Nanosheets Via
Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558-1565.
91. Sutter, P.; Sadowski, J. T.; Sutter, E., Graphene on Pt(111): Growth and Substrate
Interaction. Phys Rev B 2009, 80.
92. Sutter, P. W.; Flege, J. I.; Sutter, E. A., Epitaxial Graphene on Ruthenium. Nat. Mater.
2008, 7, 406-411.
95. Varykhalov, A.; Rader, O., Graphene Grown on Co(0001) Films and Islands:
Electronic Structure and Its Precise Magnetization Dependence. Phys Rev B 2009, 80.
99. Wang, S. M.; Pei, Y . H.; Wang, X.; Wang, H.; Meng, Q. N.; Tian, H. W.; Zheng, X. L.;
Zheng, W. T.; Liu, Y . C., Synthesis of Graphene on a Polycrystalline Co Film by
Radio-Frequency Plasma-Enhanced Chemical Vapour Deposition. J. Phys. D Appl. Phys.
2010, 43.

18
Chapter 2: Wafer-Scale Graphene by Chemical Vapor
Deposition: Synthesis, Transfer, and Devices

2.1 Introduction
Graphene is probably the best theoretically studied allotropic form of carbon. It
consists of a two-dimensional hexagonal arrangement of carbon atoms, with a quasi-linear
dispersion relation, for which the carrier effective mass is very low.
116
As a consequence, it
has a predicted mobility at room temperatures in the order of 10
6
cm
2
/Vs and an
experimentally measured mobility of 15,000 cm
2
/Vs. The high mobility of this material
opens the possibility of ballistic transport at submicron scales.
39, 66

Despite the advances in graphene research, and the numerous foreseen important
applications, implementation of graphene has been hampered due to the difficulty of
producing single or few-layers specimens over large areas. Three main methods have been
used to obtain single-layer or few-layer graphene (FLG): i) Epitaxial growth of graphene
obtained on 6H oriented SiC by vacuum annealing at 1400°C,
26
with the drawback of being
limited by the cost and size of SiC substrates; ii)

Micromechanical exfoliation of small
mesas of highly oriented pyrolytic graphite (HOPG),
66
which cannot be scaled to
19
wafer-size dimensions,

and iii) chemically-assisted exfoliation of intercalated graphite
compounds,
29, 96, 104
which typically leads to graphene with large amount of defects. An
alternative way is the chemical vapour deposition (CVD) of camphor on nickel,
88
which
led to growth of graphene of about twenty layers. Camphor, however, is a solid precursor
that needs to be sublimated, and its vapour pressure cannot be precisely controlled.
Recently, segregation of graphene was reported on Ni surfaces; however, several layers
were obtained instead of single-layer graphene, and the electronic properties of the
synthesized material were not evaluated.
111
Therefore, approaches that can provide
high-quality single- and few-layer graphene over large areas and the evaluation of their
electronic properties are still desired to meet realistic applications.
It is assumed that the carbon atoms dissolve into the Ni crystalline surface, and at
certain temperatures, they arrange epitaxially on the Ni (111) surface to form graphene.
Synthesized graphene films on Ni were recovered on Si/SiO
2
substrates for device
fabrication. In addition, we have achieved transferring the as-synthesized films to different
target substrates such as Si/SiO
2
and glass, which can enable wafer-scale
silicon-compatible fabrication of hybrid silicon/graphene electronics and transparent
conductive film applications.

20
2.2 Single- Layer and Few-Layer Graphene Synthesis
The schematic shown in Fig. 2.1a depicts the system setup employed in the CVD
deposition of graphene layers. Si/SiO
2
wafers of 4-in diameter were used as substrates to
deposit 100-nm-thick films of elemental Ni (Fig. 2.1b).

Gas in
Gas out
Substrate
1 μm 1 μm
(a)
(b) (c)

Figure 2.1 (a) Schematic of full-wafer scale deposition of graphene layers on
polycrystalline nickel by CVD. (b) E-beam evaporated nickel film (100 nm) on a 4”
Si/SiO
2
wafer. (d) Atomic force microscopy (AFM) image of nickel film after the
chemical vapor deposition of graphene layers.

Evaporated films were annealed at 300 or 800 °C in a 10:1 Ar:H
2
mixture. Heating
and cooling rates of 0.15 °C min
-1
allowed the formation of polycrystalline Ni domains
throughout the substrate. CVD synthesis of graphene was carried out at ambient pressure
by systematically varying parameters, such as temperature, gas composition, gas flow rate,
21
and deposition time. Next, we present results obtained by heating the substrates under a
flow of 600 sccm of H
2
up to 800 °C. After the target temperature was reached, methane
gas at a flow rate of 100 sccm was added to the hydrogen flow over the substrate, which
was lying horizontally inside the tube. The deposition process was conducted for 8 min.
We found that using diluted methane was key to the growth of single-layer graphene and
FLG (less than five layers), while using concentrated methane led to the growth of
multilayer graphene that resembled bulk graphite. Fig 2.1c shows an AFM image of
few-layer graphene after synthesis. Our graphene growth method could be extended to
other carbon feedstocks such as ethylene, acetylene, ethanol, and isopropanol, and other
catalytic films such as Fe and Co.

2.3 Micro-Raman Characterization of Graphene Films
We performed micro Raman analysis throughout the wafers after the chemical
vapour deposition process to confirm the formation of graphene layers on the Ni surface
and to obtain information about the quality and the number of layers deposited. Figure 2.2
shows Raman spectra taken at different locations on the synthesized films over Si/SiO
2
/Ni
substrates by using an excitation wavelength of 532 nm, with a power density of 2.0 mW
cm
-2
. Strong peaks near 1580 cm
-1
and 2690 cm
-1
were found. Analysis of the frequencies
and lineshapes of these peaks allows their assignment as the G and G’ bands of graphene
22
layers, respectively.
24
The peak located at 1345 cm
-1
corresponds to the D band of graphitic
carbon species, which is associated with the amount of defects in the crystalline structure
of the graphene layers. The low cross section of the D band confirms that synthesized films
are largely free of structural defects.

1200 1600 2000 2400 2800
Single layer
Raman Shift (cm
-1
)
2500 2600 2700 2800 2900
R am an shift (cm
-1
)
E xp
Fit
2500 2600 2700 2800 2900
R am an shift (cm
-1
)
E xp
Fit
1200 1600 2000 2400 2800
Bilayer
Raman Shift (cm
-1
)
G
G’
G
G’
D
D
(a)
(b)

Figure 2.2 Raman spectrum obtained on as-synthesized graphene films on Si/SiO
2
/Ni
substrates. D, G and G’ Raman bands for graphene are labeled on each spectrum. (a)
Raman spectrum obtained on a single layer graphene. A zoomed image of the G’ band on
the right shows the purely single Lorentzian fit of this peak (red trace), which is
characteristic of single-layer graphene. (b) Raman spectrum of bilayer graphene. A nearly
symmetric splitting and broadening of the G’ band of this spectrum (shown on the right)
is properly fit by a set of four Lorentzian curves, characteristic of bilayer graphene. No
G’ band was found with a bulk graphite-like lineshape on the substrates sampled.
23

Interlayer interactions affect the Raman fingerprints for single-layer, bilayer, and
few-layer graphene, allowing unambiguous identification of graphene layers.
24, 34
Figure
2.2a shows the Raman spectrum of single-layer graphene in the synthesized films. Single
Lorentzian fit of the G’ band is characteristic of monolayer graphene. On the other hand, a
subtle splitting, up-shift of nearly 15 wavenumbers and broadening observed in the G’
band that can be fit with four Lorentzian peaks, as shown in Figure 2.2b, which constitute
the spectroscopic signature of bilayer graphene.
24, 34

The domain size for the single-layer, bilayer, and few-layer graphene is typically
around 1-2 um, which is likely due to the grain size of the polycrystalline nickel film.
Extensive Raman characterization over as-synthesized samples consistently showed the
presence of graphene with less than five graphene layers.
12
No signature of multi-layer or
bulk graphite was found in the films deposited.

2.4 Transfer of Graphene films
Graphene on metal hampered the applications of graphene for nanoelectronics,
photovoltaic cells, and so on. The transfer of graphene onto different substrates is of great
importance for graphene applications. We developed a simple and effective graphene
transfer technique which ensured 100% graphene transfer onto a target substrate. A
schematic diagram of the transfer process is shown in Fig. 2.3. Graphene was first
24
spin-coated with a thin layer of polymethyl methacrylate (PMMA), and baked at 120 °C to
evaporate the solvent. The metal layer (Ni, Cu, or Cu foil) was then etched by floating the
substrate in Ni or Cu etchant. After 1 hour to 2 hours, the metal layer was completely
removed, leaving only PMMA/graphene film. The film was cleaned by DI water for
several times and then put onto a target substrate. PMMA film was then removed by
acetone, leaving graphene film on top of the targeting substrate.
Graphene film Graphene film
Metal Metal
Si/SiO Si/SiO
2 2
PMMA coating PMMA coating
Metal Metal
etching etching
PMMA / Graphene PMMA / Graphene
Substrate Substrate
Graphene Graphene
Transfer Acetone

Figure 2.3 Schematic diagram for the transfer process.

2.5 Device Fabrication and Electrical Measurements
Figure 2.4a shows a photograph of a Si/SiO
2
/Ni/Graphene wafer right after CVD
synthesis (upper left). Micro Raman spectrum taken on the films after transfer is shown in
lower left part of figure 2.4a, clearly showing very low D band intensity. This confirms that
graphene is largely defect-free after transfer. Upper and lower right panels of figure 2.4a
25
shows AFM and white-light microscopy images, of the graphene films on Si/SiO
2
substrate
after etching the Ni film, respectively. FLG were composed of micron-scale domains of
single-, bi- and few-layers of graphene with a maximum thickness of 5 layers, as confirmed
by micro Raman spectroscopy.
The transfer of graphene onto Si/SiO
2
allowed the fabrication of back-gated FETs at
large scale (Figure 2.4b). Micro-Raman measurements performed on the device channel
were consistent with a maximum of five graphene layers comprising the films (data not
shown). Four-probe measurements performed on the FLG films revealed a sheet resistance
of ~68 kΩ/sq. I
DS
-V
DS
characteristics depicted in Figure 2.4c shows that the drain current
increases with the increase of negative gate voltage, indicating a weak p-type behavior in
the films. Figure 2.4d shows the transfer characteristics for a device with channel width of
20 μm and channel length of 4 μm. Most devices were highly conductive and exhibited a
weak modulation of the drain current by the gate bias, which is consistent with a 2D
semimetal.
66
Compared to nanotubes,
45, 98
graphene FETs typically exhibit low on/off
ratios, which can be improved significantly by patterning graphene into nanoribbons.
98

Single graphene layer is a zero-gap semiconductor, but interlayer interactions bring in a
semimetal behavior in FLG. Therefore, the transfer characteristics observed in Figure 2.4d
can be attributed to a screened gating effect due to irregularities of the film and the
presence of more than two graphene layers in the films.
26
(c)
Drain
Source
2 μm 0.5 μm
(b)
(a)
0 -1 -2 -3 -4 -5
0
-1
-2
-3
I
DS
(mA)
V
DS
(V)
(d)
0 -1 -2 -3 -4 -5
0
-1
-2
-3
I
DS
(mA)
V
DS
(V)
(d)
(f)
(c)
-40 -20 0 20 40
3
3.2
3.4
3.6
3.8
I
DS
(μA)
V
G
(V)
(e)
-40 -20 0 20 40
3
3.2
3.4
3.6
3.8
I
DS
(μA)
V
G
(V)
(e)
100
Transmittance (%)
(g) 100
Transmittance (%)
(g)
(d)
5 μm 5 μm
1200 1800 2400 3000

Raman Shift (cm
-1
)

Figure 2.4 (a) (upper left panel) shows a photograph of a 4-in wafer after synthesis of
deposited FLG on polycrystalline nickel; (upper right panel) shows an AFM image of the
few-layer graphene films on Si/SiO
2
; (lower right panel) shows an optical micrograph of
transferred few-layer graphene on Si/SiO
2
; (lower left panel) shows a typical
micro-Raman spectrum of the transferred films. (b) 4-inch wafer with back-gated
few-layer graphene devices; insets show SEM and AFM images of a typical device and
device channel, respectively. (c) I
DS
-V
DS
measurements for different gate voltages, V
G
=
2.5 V , 1.5 V and -1.5 V for the black, red and blue curves, respectively and (d) I
DS
-V
G

curve of one of the FET devices for V
DS
= 0.01 V .

2.6 Optical Properties of CVD Graphene Films
Figure 2.5a shows a photograph of a ~1 cm
2
FLG film transferred on glass exhibiting
high transparency to naked eyes. Figure 2.5b shows that the transmittance spectrum of the
transferred FLG film in the visible wavelength range is ~80%, which is about less than 10
27
graphene layer films. Due to the simultaneous good electrical conductivity and high
transparency of the synthesized graphene films, they are likely to find application as
transparent conductors.
(f)
V
G
(V) V
G
(V)
400 500 600 700 800
0
20
40
60
80
100
Transmittance (%)
Wavelegth (nm)
(g)
400 500 600 700 800
0
20
40
60
80
100
Transmittance (%)
Wavelegth (nm)
(g) (a) (b)

Figure 2.5 (a) Photograph of a 1cm
2
FLG film transferred onto a glass substrate (inside
the box). (b) Transmittance of the FLG film shown in (a).

2.7 Conclusion
In summary, this work demonstrates a simple, scalable and effective method to
synthesize monolayer and few-layer graphene films by using methane-based CVD on
nickel films, transfer of the synthesized films to different target substrates and their
evaluation as transparent conducting films. Graphene produced over Si/SiO
2
wafers can be
very useful for device fabrication, and our approach may serve as the foundation for the
growth of single-domain graphene over macro-scale areas such as complete wafers.
Growth of graphene on single-crystalline nickel is currently under way with the goal of
28
significantly increasing graphene grain size. This approach constitutes a significant
advance towards the production of thin films of graphene at industrial scales and has
important implications for future graphene-related electronic devices.

29
Chapter 2 References

12. Cancado, L. G.; Reina, A.; Kong, J.; Dresselhaus, M. S., Geometrical Approach for the
Study of G ' Band in the Raman Spectrum of Monolayer Graphene, Bilayer Graphene, and
Bulk Graphite. Physical Review B 2008, 77.
24. Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.;
Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K., Raman Spectrum of
Graphene and Graphene Layers. Physical Review Letters 2006, 97, 187401.
26. Forbeaux, I.; Themlin, J. M.; Debever, J. M., Heteroepitaxial Graphite on
6h-Sic(0001): Interface Formation through Conduction-Band Electronic Structure. Phys
Rev B 1998, 58, 16396-16406.
29. Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B., A Chemical Route to
Graphene for Device Applications. Nano Letters 2007, 7, 3394-3398.
34. Gupta, A.; Chen, G.; Joshi, P.; Tadigadapa, S.; Eklund, P. C., Raman Scattering from
High-Frequency Phonons in Supported N-Graphene Layer Films. Nano Letters 2006, 6,
2667-2673.
39. Hwang, E. H.; Adam, S.; Das Sarma, S., Transport in Chemically Doped Graphene in
the Presence of Adsorbed Molecules. Physical Review B 2007, 76, 195421.
45. Kang, S. J.; Kocabas, C.; Ozel, T.; Shim, M.; Pimparkar, N.; Alam, M. A.; Rotkin, S.
V.; Rogers, J. A., High-Performance Electronics Using Dense, Perfectly Aligned Arrays of
Single-Walled Carbon Nanotubes. Nat Nanotechnol 2007, 2, 230-236.
66. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;
Grigorieva, I. V.; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films.
Science 2004, 306, 666-669.
88. Somani, P. R.; Somani, S. P.; Umeno, M., Planer Nano-Graphenes from Camphor by
Cvd. Chemical Physics Letters 2006, 430, 56-59
96. Viculis, L. M.; Mack, J. J.; Kaner, R. B., A Chemical Route to Carbon Nanoscrolls.
Science 2003, 299, 1361.
98. Wang, C.; Ryu, K. M.; Badmaev, A.; Patil, N.; Lin, A.; Mitra, S.; Wong, H. S. P.;
Zhou, C., Device Study, Chemical Doping, and Logic Circuits Based on Transferred
Aligned Single-Walled Carbon Nanotubes. Applied Physics Letters 2008, 93.
104. Wu, J.; Becerril, H. A.; Bao, Z.; Liu, Z.; Chen, Y.; Peumans, P., Organic Solar
Cells with Solution-Processed Graphene Transparent Electrodes. Applied Physics Letters
2008, 92, 263302.1-263302.3.
111. Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., Graphene
Segregated on Ni Surfaces and Transferred to Insulators. Appl. Phys. Lett. 2008, 93,
113103.
30
116. Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P., Experimental Observation of
the Quantum Hall Effect and Berry's Phase in Graphene. Nature 2005, 438, 201-204.

31
Chapter 3: Continuous, Highly Flexible, and Transparent
Graphene Films by Chemical Vapor Deposition for Organic
Photovoltaics

3.1 Introduction
Solar energy harvesting using organic photovoltaic (OPV) cells has been
proposed as a means to achieve low-cost energy due to their ease of manufacture, light
weight and compatibility with flexible substrates.
72
A critical aspect of this type of
optoelectronic device is the transparent conductive electrode through which light couples
into the device. Conventional OPVs typically use transparent indium tin oxide (ITO) or
fluorine doped tin oxide (FTO) as such electrodes.
1, 72
However, the scarcity of indium
reserves, intensive processing requirements, and highly brittle nature of metal oxides
8, 42,
84
impose serious limitations on the use of these materials for applications where cost,
physical conformation, and mechanical flexibility are important.
Carbon nanotubes
5, 79, 105
and nanowires
44, 52
have been used as alternative
materials for electrodes in OPVs, but the roughness of such films were comparable to
or larger than a typical device thickness, which may lead to significant shunt losses. In
32
contrast, graphene is a one-atom thick, two-dimensional crystalline arrangement of
carbon atoms with a quasi-linear dispersion relation, and predicted mobility on the order
of 10
6
cm
2
/V·s for a charge carrier concentration n
i
~ 10
12
cm
-2
.
116

Graphene monolayer has a transparency of 97-98%
64
and the sheet resistance of
undoped graphene is of the order of ~6 kΩ.
7, 20, 28, 93
Graphene films are suitable for
applications as transparent conductive electrodes where low sheet resistance and high
optical transparency are essential. Conventional methods to obtain graphene thin films
such as epitaxial growth,
26
micromechanical exfoliation of graphite
66
and exfoliation of
chemically oxidized graphite
29, 96
are either expensive, unscalable or yield graphene with
limited conductivity due to a high defect density.
Recently, graphene films obtained from reduced graphene oxide (GO) have been
explored for applications as transparent electrodes in solar cells.
22, 100, 104
However, the
devices obtained exhibited rather moderate performance, leakage current under dark
conditions, and moderate power conversion efficiency of < 0.4 %. The moderate
performance of these devices may be attributed to several factors, including: i) reduction
of oxygen functionalities on the graphene oxide flakes does not completely restore the π
conjugation in the films, and ii) the vacuum filtration or spin coating methods used to
prepare reduced graphene oxide films lead to stacked graphene flakes and thus significant
flake-to-flake resistance.
33
Eda et. al.
22
reported doped reduced graphene oxide films with a sheet resistance
of 40 kΩ/sq, a transparency of 64%, and a solar cell conversion efficiency of 0.1%, while
Wu et. al.
104
reported reduced graphene oxide films of 5–10
3
kΩ/sq, > 80% transparency,
and a conversion efficiency of 0.4%. Efforts to improve percolation on the graphene
electrode include the use of reduced GO combined with carbon nanotube films, but this
approach requires extra processing steps.
94
As a result, continuous, highly flexible, and
transparent graphene films are still highly desirable for photovoltaic applications.
Chemical vapor deposition (CVD) has surged as an important method to obtain
high quality graphene films.
32, 50, 74, 111
In particular, films with sheet resistance of 280
Ω/sq (80% transparent) and 770 Ω/sq (90% transparent) have been reported for graphene
synthesized on Ni films, while sheet resistance of 350 Ω/sq (90% transparent) has been
reported for CVD graphene on Cu films, which represents a good advance in the use of
graphene as transparent conductive films. Another advantage of CVD is its scalability, we
have reported wafer-scale synthesis and transfer of single- and few-layer graphene for
device fabrication.
32

In this chapter we explore the implementation of large-area, smooth few-layer
CVD graphene films as the anode material in flexible and rigid OPV cells with multilayer
structure. Such films exhibit sheet resistance and transparency controlled in the range of
230 Ω/sq at 72% transparency, and 8.3 kΩ/sq at 91% transparency. The use of CVD
34
graphene is attractive because other graphene films, which are formed by stacked
micron-size flakes, suffer from flake-to-flake contact resistance and high roughness. In
contrast, grain boundaries of CVD graphene films have the advantage of being formed in
situ during synthesis; such a process is expected to minimize contact resistance between
neighboring graphene domains and may result in smoother films with better conducting
properties. Solar cells made with CVD graphene exhibited performance that compares to
ITO devices and surpasses that of ITO devices under bending conditions, exhibiting
power conversion efficiencies of 1.18% and being operational under bending conditions
up to 138°.

3.2 Graphene Synthesis and Transfer
3.2.1 CVD Graphene Synthesis and Characterization
Elemental Ni was thermally evaporated onto pre-cleaned Si/SiO
2
substrates up to a
thickness of ~1000 Å. Subsequently, Ni/Si/SiO
2
substrates were taken into a sealed
high-temperature furnace and heated to 900 °C under a hydrogen flow rate of 600 sccm.
The Ni surface was annealed at 900 °C for 20 min and then graphene synthesis was
obtained at 900 °C by flowing methane at a flow rate of 100 sccm for 8 minutes.
35

40 50 60 70 80

2θ (degrees)
Intensity (a.u.)
Ni (111)
Ni(200)
Si(100)
(a)
(b)

Figure 3.1 (a) AFM image of a 300 nm Ni film deposited on Si/SiO
2
substrate after high
temperature annealing. (b) Typical X-ray diffraction spectrum of annealed Ni film.

A Rigaku x-ray diffractometer equipped with a 12 kW rotating anode x-ray generator
was employed to investigate the distribution of crystalline planes on the annealed
polycrystalline Ni. Figure 3.1a shows an AFM image of a typical annealed Ni surface.
Irregular and faceted-shape surface are consistent with polycrystalline formation. X-ray
diffraction spectrum shown in Figure 3.1b reveals the presence of (111) and (200) planes.
Furthermore, it is clear that the surface of annealed Ni film is comprised predominantly by
the (111) plane.

3.2.2 Transfer of CVD Graphene films and Characterization
Graphene films were synthesized by chemical vapor deposition on a thermally
annealed polycrystalline nickel surface comprised mostly of the (111) plane. This
36
synthesis yields a continuous film comprised of monolayer and few-layer graphene with
low defect density, as indicated by TEM imaging and diffraction and micro Raman
spectroscopy measurements performed on the transferred films.
The transfer of graphene onto a TEM grid is the same as described in Chapter 2.4.
Free-standing PMMA/CVD Graphene was obtained by etching the underlying
polycrystalline Ni surface with Ni etchant. Consecutively, acetone is used to dissolve the
PMMA layer and allow the deposition of clean CVD Graphene films on standard grids for
TEM analysis. Figure 3.2a shows a low magnification TEM image of the deposited CVD
graphene films, while the selected area diffraction pattern along the z- direction shows
well ordered typical graphite lattice structure.
Further spectroscopic evidence is obtained by micro-Raman. Figure 3.2b shows the
Raman spectrum corresponding to the sample analyzed in Figure 3.2a. The observed
G-band centered at 1581 cm
-1
is characteristic of the C-C stretching in the sp
2
structure of
graphene. Another band of interest, the G’-band that appears at 2697 cm
-1
, presents a
fairly symmetric lineshape that suggests the films are comprised of few layers of
graphene (1-5 layers). The very low D-band intensity at 1354 cm
-1
with respect to the
intensity of the G-band, indicates a low defect density in the transferred CVD graphene.
37

1500 2000 2500 3000

Raman shift (cm
-1
)
(a) (b)

Figure 3.2 (a) Low magnification TEM image of CVD graphene films. Inset shows a
selected area electron diffraction (SAED) pattern of the few-layer graphene film. (b)
Raman spectrum of CVD graphene film.

3.3 Characterization of CVD Graphene Film as Transparent Electrode
for Organic Photovoltaics
3.3.1 Film Thickness and Surface Roughness of Graphene
The thin film nature of OPV devices requires control of layer thickness and
morphology to reduce the possibility of leakage current and shorts.
100, 104
Therefore,
thickness and surface smoothness of the transparent electrode in OPVs are important for
good device performance. As a point of reference, we compared the thickness and
roughness given by CVD graphene films against single-walled carbon nanotubes (SWNT)
and ITO films, which are materials that have been amply reported in the literature as
transparent electrodes.
83

38
The thickness of CVD graphene obtained at the above-mentioned synthesis
conditions is on the order of 1-3 nm. We perform height profile measurements on the
transferred films on glass to estimate the thickness of the graphene film. Figure 3.3a shows
an AFM image of an opening in the graphene film with clear edges. Figure 3.3b shows the
height profile along the straight line depicted in Figure 3.3a.

(a)
1.0 nm
(b)

Figure 3.3 (a) AFM image of a transferred CVD Graphene film onto glass substrate. (b)
Cross section measurement of the height of the CVD Graphene. Typical thickness
exhibited by the transferred films is found within the range 1-3 nm.

A height step of ~1 nm can be clearly observed between the substrate surface and the
graphene film. Larger vertical distances can be found between the substrate surface and the
CVD graphene edge step as well. However, due to irregularities on Ni surface, and as
transferred graphene films may suffer from bending, folding and mechanical stress, they
may not be lying fully extended and flattened on the receiving substrate. Thus, the lowest
vertical distance within the profile edge steps can be regarded as a good estimate of the film
39
thickness. In this case, a thickness of ~1 nm indicates the CVD graphene can be as thin as
bilayer graphene.
Figure 3.4 compares AFM images of the CVD graphene, a commercial
150-nm-thick ITO film (R
Sheet
= 20 Ω/sq) and a 30-nm-thick SWNT film obtained by the
filtration/PDMS transfer method (R
Sheet
= 1.1 kΩ/sq). The r.m.s. surface roughness
measured for CVD graphene, ITO and SWNT films was 0.9, 0.7 and 8.4 nm for graphene,
ITO and SWNT, respectively; such values were consistent with roughness previously
reported for graphene,
100
ITO
48
and SWNT
112
films. Roughness values obtained indicate
that CVD graphene was nearly 10 times smoother and thinner than SWNT films, with
surface roughness comparable to that of ITO. In addition, CVD graphene and SWNT
films passivated with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS) showed r.m.s surface roughness of 0.8 and 5.1, respectively.
Graphene roughness: 0.9 nm ITO roughness: 0.7 nm SWNT roughness: 8.4 nm

Figure 3.4 AFM images of the surface of CVD graphene, ITO and SWNT films on glass.
The scale bar in z-direction is 50 nm for all images.

3.3.2 Optical Transparency and Sheet Resistance of CVD Graphene films
High transparency is also necessary for the use of CVD graphene as a substitute for
40
transparent metal oxide electrodes in OPVs. Figure 3.5a, b shows photographs displaying
see-through areas (dotted lines) of 2 and 1.3 cm
2
of graphene films after being transferred
to glass and polyethylene terephthalate (PET), respectively. Thorough inspection of the
graphene films using scanning electron microscopy also confirmed the formation of
continuous films without any visible cracks.
(a)
400 600 800 1000
0
20
40
60
80
100
Transmittance (%)
W avelength (nm)
CVD Graphene
ITO
SWNT
(c) (b)

Figure 3.5 Photographs showing highly transparent graphene films transferred onto glass
and PET are shown in (a) and (b), respectively. (c) Transmission spectra for CVD
graphene, ITO and SWNT films on glass.

Optical transmittance of the transferred graphene films in the visible and near
infrared range was measured using a Varian50 spectrophotometer in the wavelength range
of 400-1100 nm. Figure 3.5c depicts the wavelength dependence of the optical
transparency of the CVD graphene, ITO and SWNT films displayed in Figure 3.4. For
ITO films, the transmittance peaks at 535 nm, while the transmittance increases
monotonically with the increase in wavelength of the incident light from T = 86% (at 400
nm) to T = 95% (at 1100 nm) and from T = 70% (at 400 nm) to T = 82% (at 1100) nm, in
graphene and SWNT films respectively.
41
Light transmission in graphene is dictated by absorption, due to the π-conjugated
system. As a consequence of this, as CVD graphene films become thinner, transparency
is expected to increase. In principle, the sheet resistance of a graphene film comprised of
several graphene layers should decrease for each layer added;
56
therefore it is expected
that the thicker the film, the larger the number of layers, the smaller the sheet resistance,
but simultaneously, the lower the transparency.
We were able to tune the transparency and sheet resistance of graphene films by
varying the synthesis conditions. Figure 3.6a shows that highly transparent CVD
graphene films can be obtained at the expense of higher resistance. Sheet resistance as
low as 230 Ω/sq (with T=72%) and optical transparency as high as 91% (with R
Sheet
= 8.3
kΩ/sq) can be achieved, and therefore a compromise between these parameters must be
met for specific applications.
70 75 80 85 90 95 100
10
-1
10
1
10
3
10
5
10
7
R
sheet
(kΩ/sq)
Transmittance (%)
CVD Graphene
Eda et al.
Blake et al.
Li et al.
Wu et al.
400 600 800 1000
0
20
40
60
80
100
Transmittance (%)
Wavelength (nm)
8.30 kΩ/sq
3.40 kΩ/sq
0.70 kΩ/sq
0.23 kΩ/sq
a
b

Figure 3.6 (a) Transmission spectra of CVD graphene with different sheet resistance
(R
Sheet
). (b) Comparison of R
Sheet
vs. light transmittance at 550 nm for CVD graphene and
reduced GO films reported in the literature.
42

Further characterization of the CVD graphene films is shown in Figure 3.6b, where
we compare sheet resistance and transparency of CVD graphene against reduced GO
films reported in the literature. Analysis of Figure 3.6b shows that graphene synthesized
by CVD exhibits better transparency/R
Sheet
ratio than the reduced graphene oxide films
reported so far.
7, 21, 55, 104
The transparency/R
Sheet
ratio of CVD graphene can be further
improved to yield films with 700 Ω/sq and ~ 90% transparency.
74

3.4 Flexible Transparent Electrodes: Graphene versus ITO
To investigate the flexibility of the CVD graphene electrodes and its influence on the
performance of flexible OPV cells, we transferred CVD graphene films onto PET
substrates (Figure 3.7a) and compared the electrical conductivity of graphene and ITO
films under bending conditions. Figure 3.7b and 3.7c show AFM images of CVD
graphene (R
Sheet
= 500 Ω/sq and T = 75 %) and ITO (R
Sheet
= 25 Ω/sq, T = 86 %), on PET.
The r.m.s. surface roughness of ITO on PET was 1.1 nm, nearly 60% higher than on
glass.
100 nm thick aluminum metal contacts were thermally deposited through a shadow
mask onto the above mentioned films. Two-probe electrical measurements were
performed on both films by direct contact of tungsten microprobes to the aluminum
43
electrodes, soldering the probe tips to the aluminum pads to assure good electrical contact
on each measurement. Performing this process for each bending angle allowed us to
monitor the change in conductance of the film with the bending angle (inset Figure 3.7d).
The conductance of the graphene/PET film remained virtually unperturbed by bending
(Figure 3.7d) even after several complete bending cycles and decreased by only 7.9%
after 100 bending cycles.
In contrast, Figure 3.7f shows three clearly defined regions that describe the typical
behavior of ITO conductance under bending conditions. For bending angles from 0° to
~130° a steady decrease in the conductance of the ITO film by three orders of magnitude
with increased bending angle was observed. Interestingly, immediately after a critical
angle (128°) conductance suddenly fell by six orders of magnitude. Finally, after the
critical angle was reached, the conductance of the film continued to decrease even when
bending angle decreases; an open circuit (σ ≤ 10
-12
S) was obtained after only one
bending cycle.
44

f

d

e

20 μm

20

20 μm

20 μm

b
c

ITO

Graphene

Graphene

0.5 μm

ITO

0.5 μm

0 40 80 120 160
10
-8
10
-6
10
-4
10
-2
10
0
10
2
Conductance (mS)
Bending angle (2θ)

Bending Graphene/PET
Recovery Graphene/PET
θ θ
0 40 80 120 160
10
-8
10
-6
10
-4
10
-2
10
0
10
2
Conductance (mS)

Bending angle (2θ)
Bending ΙΤΟ/ΠΕΤ
Recovery ΙΤΟ/ΠΕΤ
b
20 μm 20 μm
20 μm 20 μm
c
a

Figure 3.7 (a) Photograph illustrating high flexibility of CVD graphene transferred on a
PET flexible substrate. (b) and (c) AFM images of the surface of CVD graphene and ITO
films on PET, respectively. (d) and (f) Conductance of the CVD graphene and ITO films
on PET substrates under bending conditions, respectively. The devices used to monitor
the conductance had channel width (W) = 1 mm, and length (L) = 1 mm. (e) Optical
images of CVD graphene (upper) and ITO (lower) films on PET before and after being
bent at the angles specified in (b) and (c). Arrows show the direction of the bending.

The fact that the conductivity of the ITO film did not recover after bending the
ITO film back to lower radius of curvature can be associated with the development of
multiple discontinuity scattering sites on the brittle ITO film that were generated by
45
tensile strain under bending and may further develop under compressive stress while
decreasing the bending angle. Optical microscopy images were collected on the ITO/PET
and Graphene/PET films. Figure 3.7e shows optical micrographs of graphene and ITO
films before and after the first bending cycle (0°150°0°). As can be seen, under the
microscope resolution, very pronounced cracks were developed on the ITO film, while
the graphene film remained intact. These results demonstrate the advantage of CVD
graphene in terms of mechanical flexibility over ITO films, which opens new avenues for
robust, flexible, and lightweight transparent CVD graphene electrodes in OPVs.

3.5 Graphene OPV Cell Fabrication on Flexible Substrate and Device
Performance
3.5.1 Organic OPV Cell Fabrication with CVD Graphene and ITO
Although CVD graphene films clearly outperform ITO as transparent conductive
electrodes on flexible PET substrates under bending conditions, it is important to
implement this material into working OPVs in order to evaluate its performance. Thus,
we fabricated OPV cells on PET substrates using graphene and ITO as transparent
electrodes, under identical experimental conditions.
Graphene electrodes were fabricated by transferring as-grown CVD graphene films
46
onto pre-cleaned, 100 µm thick, PET substrates. PET substrates coated with ITO were
obtained from Southwall Technologies Inc. Both substrates were solvent cleaned and
passivated by spin coating a thin layer (10 nm) of
poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) with R
Sheet
= 1 kΩ/sq
(PEDOT:PSS).
Use of the PEDOT:PSS coating as the electron blocking layer decreased the
conductivity of the PEDOT:PSS/CVD Graphene film to 2.1 kΩ/sq, while for the
PEDOT/ITO film it remained ~1 kΩ/sq. PEDOT:PSS was expected to help mitigate the
brittle nature of the ITO electrode to enhance its performance under bending conditions,
and interestingly, PEDOT:PSS passivation of ITO was also found to improve the
rectification behavior of the devices. Finally, the planarizing effect afforded by the
PEDOT:PSS treatment is desirable to compensate for possible folding or wrinkles that
may accompany the CVD graphene film transfer process or irregular wetting between the
electrode and the cell active layers, which would yield device shorting or shunt losses.
The substrates were taken into high vacuum conditions where the organic thin films
and the aluminum cathode were consecutively deposited by thermal evaporation. The
multilayered configuration employed (Figure 3.8) is given as: CVD graphene [<5 nm] or
ITO / PEDOT:PSS / Copper phthalocyanine (CuPc) [40 nm] / Fullerene (C
60
) [40 nm] /
Bathocuproine (BCP) [10 nm] / Aluminum (Al). Aluminum cathodes were deposited
47
through a shadow mask with circular openings of 0.75 mm
2
. Optical excitation of the
CuPc (C
60
) leads to the donation of an electron (hole) to C
60
(CuPc) and the
photogenerated charge carriers are swept to the external contacts producing a measurable
light-generated current.

Substrate
Graphene
CuPc
PEDOT
BCP
C
60
Al
Graphene
CuPc
C
60
Al 4.5eV
BCP
4.3eV
PEDOT
5.2eV
2.2eV
5.2eV
3.3eV
4.0eV
6.2eV
6.4eV
3.0eV
Graphene
CuPc
C
60
Al 4.5eV
BCP
4.3eV
PEDOT
5.2eV
2.2eV
5.2eV
3.3eV
4.0eV
6.2eV
6.4eV
3.0eV

Figure 3.8 Schematic representation of the energy level alignment (top) and construction
of the heterojunction organic solar cell fabricated with graphene as anodic electrode:
CVD graphene/PEDOT/CuPc/C60/BCP/Al.
48

3.5.2 Cell Performance
Current density vs. voltage or J(V) characteristics were measured in air at room
temperature in the dark and under spectral mismatch corrected 100 mW/cm
2
white light
illumination from an AM 1.5G filtered 300 W Xenon arc lamp (Newport Co.). Routine
spectral mismatch correction for ASTM G173-03 was performed using a filtered silicon
photodiode calibrated by the National Renewable Energy Laboratory (NREL) to reduce
measurement errors. Chopped monochromatic light (250 Hz, 10 nm FWHM) and lock-in
detection was used to perform all spectral responsivity and spectral mismatch correction
measurements.
We compared the J(V) characteristics of a typical photovoltaic cell obtained with
CVD graphene (R
sheet
: 3.5 kΩ/sq, T: 89%) against a typical cell obtained with an ITO
anode (R
Sheet
: 25 Ω/sq, T: 96%), that were fabricated under identical experimental
conditions. Figure 3.9a and 3.9b show semi-log (up) and linear (down) J(V) plots
obtained from CVD graphene and ITO OPV cells, respectively. Red and blue traces
correspond to the current density measured in the dark and under illumination,
respectively. The output power density of the cells (P), which is given by P = J·V, is
shown in Figures 3.9a and 3.9b as open circle traces for which the maximum point on the
curve corresponds to the maximum output power density (P
max
) of the device.
49
-0.8 -0.4 0.0 0.4 0.8
-8
-4
0
4
8
10
-8
10
-6
10
-4
10
-2
-1
0
1
(A/cm
2
) (mA/cm
2
)
Voltage (V)

Current density
Dark (mA/cm
2
)
Light (mA/cm
2
)
ITO
Power (mW/cm
2
)
Power Density (mW/cm
2
)
-0.8 -0.4 0.0 0.4 0.8
-8
-4
0
4
8
10
-8
10
-6
10
-4
10
-2
-1
0
1
(A/cm
2
) (mA/cm
2
)
Voltage (V)

Current density
Dark (mA/cm
2
)
Light (mA/cm
2
)
Graphene
Power (mW/cm
2
)
Power Density (mW/cm
2
)
a
b

Figure 3.9 Logarithmic (up) and linear (down) current density and power density vs
voltage characteristics of CVD graphene (a) and ITO (b) OPV cells on PET under dark
(red traces) and 100 mW/cm
2
AM1.5G spectral illumination (blue traces). The output
power density of the cells is plotted on (a) and (b) as open circle traces. The structure of
the devices is given by [CVD graphene / PEDOT / CuPc / C
60
/ BCP / Al] and [ITO /
CuPc / C
60
/ BCP / Al] for CVD graphene and ITO OPVs, respectively
50

For an incident power density, P
inc
= 100 mW/cm
2
, the power conversion efficiency
(η = P
max
/P
inc
) and other performance parameters are summarized in Table 3.1. It is
clearly observed from the semi-log plots in Figures 3.9a and 3.9b that both devices have
nearly identical open circuit voltage (V
oc
) (for J=0) of 0.48 V under illumination
conditions, which suggests similar recombination behavior in both cells. Furthermore,
from Figures 3.9a, it can be seen that unlike OPVs reported for reduced GO anodes,
94, 104

we did not observe large leakage current densities from any of the CVD graphene OPV
cells.

Anode
J
sc
(mA/cm
2
)
V
oc
(V) FF η η η η            
CVD graphene 4.73 0.48 0.52 1.18
ITO 4.69 0.48 0.57 1.27

Table 3.1 Performance details of OPV cells built on PET. The structure of the devices is
given by [CVD-graphene/PEDOT/CuPc/C60/BCP/Al] and
[ITO/PEDOT/CuPc/C60/BCP/Al] for CVD graphene and ITO OPVs, respectively.

The J(V) characteristics of the CVD graphene cell under illumination showed a
short-circuit photocurrent density (J
sc
) (for V=0) of 4.73 mA/cm
2
, an open-circuit voltage
(V
oc
) of 0.48 V and a maximum power (P
max
) of 1.18 mW/cm
2
, to yield a fill factor (FF)
51
of 0.52 and overall power conversion efficiency (η) of 1.18%. The control device, using
an ITO anode on PET, gave J
sc
of 4.69 mA/cm
2
, V
oc
of 0.48 V and P
max
of 1.27 mW/cm
2
,
for a FF of 0.57 and an efficiency of 1.27%. Performance details for both cells are
summarized in Table 3.1.
Analysis of Figures 3.9a and 3.9b reveals that despite the lower transparency and
higher R
Sheet
of the CVD graphene electrode, CVD graphene solar cell exhibits an output
power density nearly 93% of that shown by the ITO device. We also observed that CVD
graphene OPV cells were more sensitive to the anode conductivity, and hence, to its
capacity to pull holes from the active layers than to its transparency.
The fact that the two cells gave very similar device performance is encouraging,
especially considering that the ITO substrate gave ~100-fold lower R
Sheet
and higher
transparency than the CVD graphene film, which would favor the performance of the
ITO device.
101
This may be rationalized by considering that, as demonstrated above, the
sheet resistance increases to similar values on both electrodes after being coated with
PEDOT:PSS. In this case, charge injection from the active layers of the OPV cells may be
limited by the PEDOT:PSS layer, thus yielding similar performance on both cells. We
fabricated OPV cells on PET/PEDOT:PSS substrates without graphene or ITO and all of
them produced open circuit characteristics. Although PEDOT:PSS was used on both,
graphene and ITO OPV cells, the performance of the cells was measured by puncturing
52
the PEDOT:PSS layer to contact the underlying electrode material, which confirms that
CVD graphene and ITO anodes, instead of PEDOT:PSS are the ultimate electrodes in the
hole extraction process of the devices.

3.6 Device Performance Model: The Lamber-W Function
To estimate the impact of resistive losses on device performance the J(V)
dependence under illumination was modeled according to a modified form of the
Shockley equation, which is commonly applied to describe the current density (J) vs.
voltage (V) characteristics of organic solar cells, given by:

ph
p
s
t
s
s
J
R
JR V
nV
JR V
J J −
−
+






−







 −
= 1 exp (3.1)

where R
s
, R
p
, J
s
, J
ph
, n, and V
t
are the lumped series resistance, lumped parallel
resistance, reverse-bias saturation current-density, photocurrent-density, diode ideality
factor, and thermal voltage respectively for a single diode circuit model. As a practical
matter, equation 3.1 was resolved by expressing it in terms of the Lambert-W function
37

to give

53
) (
) (
) (
) (
exp
) (
0
p s
s ph p
p s
s ph s
t
p
p s t
p s s
s
t
R R
V J J R
R R
J J R V
nV
R
R R nV
R R J
W
R
nV
J
+
− +
−


















+
+ +
+
= (3.2)

Where W
0
represents Lambert’s function of the form W(x)e
W(x)
=x(V)
4, 37, 41, 69
.
In Figure 3.10 the modeled J(V) and output power density obtained according to
equation 3.2 are plotted as solid lines for the CVD graphene and ITO cells, depicted in
Figures 3.9a and b, respectively. The modeled data are compared against the
experimentally measured values and plotted as open symbols in Figure 3.10. The strong
similarity between the graphene and ITO device demonstrates that the CVD graphene
based devices may be described by the generalized Shockley equation in the same way
that their ITO based counterparts are commonly discussed.
Modeling the data in this way allows us to estimate to what extent series resistive
losses, parallel conductance, and recombination processes may impact device
performance. The model ideality factors, parallel resistances and saturation
current-densities were all comparable for the ITO and CVD graphene devices under
illumination, having values of n = 2.4 and 2.6, R
p
=1.47 kΩcm
2
and 1.62 kΩcm
2
, and J
s

=2.0 μA/cm
2
and 3.1 μA/cm
2
,
respectively, suggesting that the recombination and leakage
processes are similar for both devices.

54

0.0 0.1 0.2 0.3 0.4 0.5
-5
-4
-3
-2
-1
0
0.0
0.5
1.0
1.5
ITO
Graphene
Voltage (V)

Current density (mA/cm
2
)
Power density (mW/cm
2
)

Figure 3.10 Comparison of the modeled (solid lines) current density and power density
curves of the graphene and ITO devices obtained from the Shockley equation against the
experimentally (dots) obtained values.

The model series resistance calculated from equation 3.2 for the CVD graphene
device is 12.6 Ωcm
2
, which is less than 5 times that of the ITO device with R
s
= 2.6 Ω
cm
2
, while the model photocurrent density (J
ph
) for the CVD graphene device (4.75
mA/cm
2
) is higher than J
ph
for the ITO device (4.66 mA/cm
2
). This indicates that the
power output of the graphene based device is primarily limited by charge transport losses
rather than optical transmittance losses. This constitutes a very promising result for CVD
graphene transparent electrodes, which perform comparably to ITO, despite carrying a
55
relatively higher sheet resistance.

3.7 Performance of Highly Flexible Graphene OPV Cell under Bending
Given the good performance of OPVs with graphene electrodes, the question
remains if such devices will perform well under strain-stress conditions. Current-voltage
characteristics under bending of CVD graphene and ITO solar cells are shown in Figures
3.11a and b, respectively.
We observed that the performance of both devices was slightly degraded upon
bending. For instance, solar cells using CVD graphene electrodes withstood bending
angles (curvature radii, surface strain) up to 138° (4.1 mm, 2.4%) while exhibiting good
solar cell performance. In sharp contrast, ITO cells only withstood bending to 36° (15.9
mm, 0.8%) while showing poor performance, and failed completely to become an open
circuit after being bent to 60° (9.5 mm, 1%). It is important to note that, with increased
bending angle, the current density dropped for CVD graphene and ITO devices, while
their open circuit voltage remained virtually unchanged. In some cases this effect can be
associated with decreased illumination of the devices during bending. However, as both
cells are subjected to similar bending conditions, the marked difference exhibited in the
conversion efficiency between them cannot be attributed to irregular illumination induced
by bending, but may be related to the presence of micro cracks on the ITO device.
56

a
-0.4 0.0 0.4 0.8
-4
-2
0
2
4

Current density (mA/cm
2
)
Voltage (V)

Dark
2θ = θ = θ = θ = 0
2θ = θ = θ = θ = 83
2θ = θ = θ = θ = 138
CVD-G
Bending angle(2 Θ Θ Θ Θ)=138° ° ° °
Θ
Light
Bending angle(2 Θ Θ Θ Θ)=138° ° ° °
Θ
Bending angle(2 Θ Θ Θ Θ)=138° ° ° °
Θ
Light
-0.4 0.0 0.4 0.8
-4
-2
0
2
4

Current density (mA/cm
2
)
Voltage (V)

Dark
2θ = θ = θ = θ = 0
2θ = θ = θ = θ = 83
2θ = θ = θ = θ = 138
CVD-G
Bending angle(2 Θ Θ Θ Θ)=138° ° ° °
Θ
Light
Bending angle(2 Θ Θ Θ Θ)=138° ° ° °
Θ
Bending angle(2 Θ Θ Θ Θ)=138° ° ° °
Θ
Light

-0.4 0.0 0.4 0.8
-4
0
4

Dark
2θ = θ = θ = θ = 0
2θ = θ = θ = θ = 36
2θ = θ = θ = θ = 60
Voltage (V)
Current density (mA/cm
2
)

ITO
Light
Bending angle(2 Θ Θ Θ Θ)=36° ° ° °
Θ
Light
Bending angle(2 Θ Θ Θ Θ)=36° ° ° °
Θ
Bending angle(2 Θ Θ Θ Θ)=36° ° ° °
Θ
-0.4 0.0 0.4 0.8
-4
0
4

Dark
2θ = θ = θ = θ = 0
2θ = θ = θ = θ = 36
2θ = θ = θ = θ = 60
Voltage (V)
Current density (mA/cm
2
)

ITO
Light
Bending angle(2 Θ Θ Θ Θ)=36° ° ° °
Θ
Light
Bending angle(2 Θ Θ Θ Θ)=36° ° ° °
Θ
Bending angle(2 Θ Θ Θ Θ)=36° ° ° °
Θ
b

Figure 3.11 Current density vs. voltage characteristics of CVD graphene (a) or ITO (b)
photovoltaic cells under 100 mW/cm
2
AM1.5G spectral illumination for different bending
angles. Insets show photographs of the experimental set up employed in the experiments.

57
To further investigate this, we plotted the fill factor vs. the bending angle of the
OPV cells with CVD graphene and ITO electrodes (Figure 3.12a). The fill factor
(FF=P
max
/J
sc
V
oc
) depends strongly on the output power of the cell, and is directly related
to the cell conversion efficiency (η) by
η = FF
J
sc
V
oc
P
inc
×100 (3.6)

Gradual degradation of the initial fill factor, and hence, the conversion efficiency was
observed on the CVD graphene cell as the bending angle increased; in contrast, the fill
factor of the ITO device rapidly decayed to zero when bent at around 60°.
Furthermore, we performed SEM measurements to investigate changes in film
morphology that may have been introduced by bending of the devices. Figure 3.12b
shows the appearance of micro-cracks throughout the ITO device, while no signs of
micro-cracks or fissures were observed on the graphene device. Development of
micro-cracks generated by mechanical stress in ITO, even at small bending angles, can
substantially increase the film resistance, which has a key impact in reducing the fill
factor.
58
0 20 40 60 80 100 120 140
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Fill Factor
Bending Angle (2θ)

Graphene
ITO
a b
Graphene/PET
ITO/PET
100 μm 50 μm
50 μm 100 μm
Figure 3.12 (a) Fill factor dependence of the bending angle for CVD graphene and ITO
devices shown in figure 3.11. (b) SEM images showing the surface structure of CVD
graphene (top) and ITO (bottom) photovoltaic cells after being subjected to the bending
angles described in figure 3.11.

This agrees well with the observed decrease in output current density and power
conversion efficiency of the solar cells without observing appreciable change in the V
oc
.
CVD graphene, being of organic nature and more flexible, surpasses the performance of
ITO, which may easily crack under slight bending albeit PEDOT:PSS passivation.
Therefore, the brittle nature of ITO plays a major role in the resulting poor performance
of ITO-flexible organic solar cells, while the CVD graphene thin films exhibited good
performance as flexible transparent electrodes.

3.8 CVD Graphene Organic Photovoltaic Cells on Rigid Substrates
In order to explore the performance of graphene OPVs on rigid substrates, we
59
fabricated solar cells on CVD graphene films transferred on glass (R
Sheet
: 1.2 kΩ/sq, T:
82% at 550 nm) and glass substrates coated with ITO (Thickness: 150 nm, R
Sheet
: 20 Ω/sq,
T: 84% at 550 nm). J(V) characteristics of the fabricated devices are plotted in semi-log
(Figure 3.13a) and linear (Figure 3.13b) scale for both devices.
Again, CVD graphene solar cells on rigid transparent substrate showed distinct
diode behavior with little leakage current at reverse bias in the dark, while exhibiting
high dark current under forward bias. Under illumination, the ITO device gives J
sc
of 5.41
mA/cm
2
, V
oc
of 0.47 V , FF of 0.54 and power conversion efficiency (η) of 1.39%. On the
other hand, the CVD graphene device exhibited J
sc
of 3.45 mA/cm
2
, V
oc
of 0.47 V , FF of
0.47 and (η) of 0.75%.
Comparison of these devices shows that even though conversion efficiency of the
CVD graphene device is lower, the overall performance of the CVD graphene
photovoltaic cell is competitive, with FF comparable to that of the control ITO device.
Higher transparency of the ITO film may lead to a higher exciton generation rate, which
in turn is reflected in higher J
sc
values. However, smoothness and thickness of the
graphene film may favor charge injection and transport. The disparate power conversion
efficiency observed between the two cells can be attributed to the higher sheet resistance
and lower transparency of the graphene electrode in the G-OPV .

60
-0.4 -0.2 0.0 0.2 0.4 0.6
-6
-4
-2
0
2
4
6
Voltage (V)

Current density (mA/cm
2
)
ITO Dark
ITO Light
Graphene Dark
Graphene Light

b
a
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
10
-8
10
-6
10
-4
10
-2
10
0
Current density (A/cm
2
)
Voltage (V)
Graphene Dark
Graphene Light
ITO Dark
ITO Light

Figure 3.13 Logarithmic (a) and linear (b) current density vs voltage characteristics of
CVD GRAPHENE and ITO photovoltaic cells on glass under 100 mW/cm
2
AM1.5
spectral illumination. Structure of the devices is given by [CVD GRAPHENE / PEDOT /
CuPc / C
60
/ BCP / Al] and [ITO / CuPc / C
60
/ BCP / Al] for CVD GRAPHENE and ITO
OPVs, respectively.
61
Table 3.2 summarizes representative solar cell performance parameters measured
for the CVD graphene and ITO cells as well as reduced GO devices reported in the
literature. Clearly, CVD graphene in all cases, on rigid or flexible substrates, compares
favorably against reduced GO as transparent anode in OPV cells.

Anode J
sc
(mA/cm
2
) V
oc
(V) FF η η η η (%)
CVD graphene 3.45 0.47 0.47 0.75
ITO 5.41 0.47 0.54 1.39
Red. GO (Wu et al.) 2.10 0.48 0.34 0.40
Red. GO (Wang et al.) 1.00 0.70 0.36 0.26

Table 3.2 Comparison of performance details of OPV cells built on glass substrates. The
structure of the devices is given by [CVD GRAPHENE/PEDOT/CuPc/C60/BCP/Al] and
[ITO/CuPc/C60/BCP/Al] for CVD GRAPHENE and ITO OPVs, respectively.

3.9 Conclusions
In this chapter, we demonstrate a feasible, scalable and effective method to
employ CVD graphene as highly transparent, continuous and flexible electrodes for
OPVs. This approach constitutes a significant advance towards the production of
transparent conductive electrodes in solar cells. CVD graphene meets the most important
criteria of abundance, low cost, conductivity, stability, electrode/organic film
compatibility and flexibility that are necessary to replace ITO in organic photovoltaics,
which may have important implications for future organic optoelectronic devices.
62
Chapter 3 References

1. Andersson, A.; Johansson, N.; Bröms, P.; Yu, N.; Lupo, D.; Salaneck, W. R., Fluorine
Tin Oxide as an Alternative to Indium Tin Oxide in Polymer Leds. Advanced Materials
1998, 10, 859-863.
4. Banwell, T. C.; Jayakumar, A., Exact Analytical Solution for Current Flow through
Diode with Series Resistance. Electronics Letters 2000, 36, 291-292.
5. Barnes, T. M.; X. Wu, J. Z.; Duda, A.; Lagemaat, J. v. d.; Coutts, T. J.; Weeks, C. L.;
Britz, D. A.; Glatkowski, P., Single-Wall Carbon Nanotube Networks as a Transparent
Back Contact in Cdte Solar Cells. Applied Physics Letters 2007, 90, 243503.1-243503.3.
7. Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.;
Ponomarenko, L. A.; Morozov, S. V .; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov,
K. S., Graphene-Based Liquid Crystal Device. Nano Letters 2008, 8, 1704-1708.
8. Boehme, M.; Charton, C., Properties of Ito on Pet Film in Dependence on the Coating
Conditions and Thermal Processing. Surface & Coatings Technology 2005, 200 932- 935.
20. Echtermeyer, T. J.; Lemme, M. C.; Bolten, J.; Baus, M.; Ramsteiner, M.; Kurz, H.,
Graphene Field-Effect Devices. The European Physical Journal - Special Topics 2007, 148,
19-26.
21. Eda, G.; Fanchini, G.; Chhowalla, M., Large-Area Ultrathin Films of Reduced
Graphene Oxide as a Transparent and Flexible Electronic Material. Nat Nanotechnol 2008,
3, 270-274.
22. Eda, G.; Lin, Y .-Y .; Miller, S.; Chen, C.-W.; Su, W.-F.; Chhowalla, M., Transparent and
Conducting Electrodes for Organic Electronics from Reduced Graphene Oxide. Applied
Physics Letters 2008, 92, 233305.1-233305.3.
26. Forbeaux, I.; Themlin, J. M.; Debever, J. M., Heteroepitaxial Graphite on
6h-Sic(0001): Interface Formation through Conduction-Band Electronic Structure. Phys
Rev B 1998, 58, 16396-16406.
28. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
29. Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B., A Chemical Route to
Graphene for Device Applications. Nano Letters 2007, 7, 3394-3398.
32. Gomez, L.; Zhang, Y .; Kumar, A.; Zhou, C., Synthesis, Transfer and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Transactions on
Nanotechnology 2009, 8, 135-138.
37. Hayes, B., Why W? American Scientist 2005, 93, 104-108.
41. Jain, A.; Kapoor, A., Exact Analytical Solutions of the Parameters of Real Solar Cells
Using Lambert W-Function. Sol. Energy Mater. Sol. Cells 2004, 81, 269-277.
63
42. Jansseune, T., Indium Price Soars as Demand for Displays Continues to Grow
Compound Semiconductor 2005, 11, 34-35.
44. Kang, M.-G.; Guo, L. J., Nanoimprinted Semitransparent Metal Electrodes and Their
Application in Organic Light-Emitting Diodes. Advanced Materials 2007, 19, 1391-1396.
48. Kim, K.-B.; Tak, Y .-H.; Han, Y .-S.; Baik, K.-H.; Yoon, M.-H.; Lee, M.-H.,
Relationship between Surface Roughness of Indium Tin Oxide and Leakage Current of
Organic Light-Emitting Diode. Japanese Journal of Applied Physics 2003, 42, L438-L440.
50. Kim, K. S.; Zhao, Y .; Jang, H.; Lee, S. Y .; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.;
Choi, J. Y .; Hong, B. H., Large-Scale Pattern Growth of Graphene Films for Stretchable
Transparent Electrodes. Nature 2009, 457, 706-710.
52. Lee, J.-Y .; Connor, S. T.; Cui, Y .; Peumans, P., Solution-Processed Metal Nanowire
Mesh Transparent Electrodes. Nano Letters 2008, 8, 689-692.
55. Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H., Highly Conducting
Graphene Sheets and Langmuir-Blodgett Films. Nat Nano 2008, 3, 538-542.
56. Li, X.; Zhu, Y .; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.;
Ruoff, R. S., Transfer of Large-Area Graphene Films for High-Performance Transparent
Conductive Electrodes. Nano Letters 2009, 9, 4359-4363.
64. Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.;
Peres, N. M. R.; Geim, A. K., Fine Structure Constant Defines Visual Transparency of
Graphene. Science 2008, 320, 1308.
66. Novoselov, K. S.; Geim, A. K.; Morozov, S. V .; Jiang, D.; Zhang, Y .; Dubonos, S. V .;
Grigorieva, I. V .; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films.
Science 2004, 306, 666-669.
69. Ortiz-Conde, A.; García Sánchez, F. J.; Muci, J., Exact Analytical Solutions of the
Forward Non-Ideal Diode Equation with Series and Shunt Parasitic Resistances.
Solid-State Electronics 2000, 44, 1861-1864.
72. Peumans, P.; Yakimov, A.; Forrest, S. R., Small Molecular Weight Organic Thin-Film
Photodetectors and Solar Cells. Journal of Applied Physics 2003, 93, 3693-3723.
74. Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V .; Dresselhaus, M. S.; Kong,
J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor
Deposition. Nano Letters 2008, 9 30-35.
79. Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.; Dennler, G.; Sariciftci,
N. S.; Hu, L.; Gruner, G., Organic Solar Cells with Carbon Nanotube Network Electrodes.
Applied Physics Letters 2006, 88, 233506.1-233506.3
83. Sangeeth, C. S. S.; Manu, J.; Reghu, M., Charge Transport in Transparent Conductors:
A Comparison. Journal of Applied Physics 2009, 105, 063713.1-063713.6.
84. Scott, J. C.; Kaufman, J. H.; Brock, P. J.; DiPietro, R.; Salem, J.; Goitia, J. A.,
Degradation and Failure of Meh-Ppv Light-Emitting Diodes. Journal of Applied Physics
1996, 79, 2745-2751.
64
93. Tan, Y . W.; Zhang, Y .; Bolotin, K.; Zhao, Y .; Adam, S.; Hwang, E. H.; Das Sarma, S.;
Stormer, H. L.; Kim, P., Measurement of Scattering Rate and Minimum Conductivity in
Graphene. Physical Review Letters 2007, 99, 246803.1-246803.4.
94. Tung, V . C.; Chen, L.-M.; Allen, M. J.; Wassei, J. K.; Nelson, K.; Kaner, R. B.; Yang,
Y ., Low-Temperature Solution Processing of Graphene#Carbon Nanotube Hybrid
Materials for High-Performance Transparent Conductors. Nano Letters 2009, 9,
1949–1955.
96. Viculis, L. M.; Mack, J. J.; Kaner, R. B., A Chemical Route to Carbon Nanoscrolls.
Science 2003, 299, 1361.
100. Wang, X.; Zhi, L.; Mllen, a. K., Transparent, Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells. Nano Letters 2008, 8, 323-327.
101. Wang, X.; Zhi, L.; Tsao, N.; Tomovic, Z.; Li, J.; Müllen, K., Transparent Carbon
Films as Electrodes in Organic Solar Cells. Angewandte Chemie International Edition
2008, 47, 2990-2992.
104. Wu, J.; Becerril, H. A.; Bao, Z.; Liu, Z.; Chen, Y .; Peumans, P., Organic Solar
Cells with Solution-Processed Graphene Transparent Electrodes. Applied Physics Letters
2008, 92, 263302.1-263302.3.
105. Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.;
Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G., Transparent, Conductive
Carbon Nanotube Films. Science 2004, 305, 1273-1276.
111. Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., Graphene Segregated
on Ni Surfaces and Transferred to Insulators. Appl. Phys. Lett. 2008, 93, 113103.
112. Zhang, D.; Ryu, K.; Liu, X.; EvgueniPolikarpov; Ly, J.; Tompson, M. E.; Zhou, C.,
Transparent, Conductive, and Flexible Carbon Nanotube Films and Their Application in
Organic Light-Emitting Diodes. Nano Letters 2006, 6, 1880-1886.
116. Zhang, Y . B.; Tan, Y . W.; Stormer, H. L.; Kim, P., Experimental Observation of the
Quantum Hall Effect and Berry's Phase in Graphene. Nature 2005, 438, 201-204.

65
Chapter 4: Comparison of Graphene Growth on
Single-Crystalline and Polycrystalline Ni by Chemical Vapor
Deposition

4.1 Introduction
Graphene is a monolayer, two-dimensional material which exhibits exceptionally
high crystal and electronic quality.
28
Graphene has been attracting great interests because
of its outstanding thermodynamic stability, extremely high charge-carrier mobilities, and
excellent mechanical stiffness.
68
Tremendous efforts have been made to explore both
physical properties
9, 65-66
and applications of graphene, such as the opening of bandgap of
graphene,
2, 51, 102, 106
graphene as transparent conductive electrodes,
54, 90
graphene
nanomechanical resonators,
13
graphene supercapacitors
109
and so on. Micromechanical
cleavage of graphite has allowed the study of fundamental properties of graphene due to
the high quality, scarce presence of structural defects, and low levels of unintentional
doping of the exfoliated graphene.
63, 67
However, this approach is not scalable and the
development of new methods to obtain graphene by scalable methods has surged as an
active research area.
33, 36, 78, 89, 92

66
Recently, chemical vapor deposition (CVD) has raised its popularity in the synthesis
of graphene as a scalable and cost effective approach.
17, 50, 53, 58, 111
Polycrystalline Ni has
been shown to be a good substrate for graphene synthesis by CVD, but the percentage of
monolayer or bilayer graphene is limited by the grain size of crystalline Ni obtained after
thermal annealing of Ni thin film
17, 75
. It was pointed out that CVD graphene on Ni can be
divided into two categories: multilayer graphene (≥ 3 layers) and monolayer/bilayer
graphene, as micro-Raman can distinguish multilayer from monolayer/bilayer, but cannot
distinguish monolayer from bilayer.
76
We, among other groups, have reported the synthesis
of wafer-scale few-layer graphene by CVD on the surface of polycrystalline Ni.
17
Our
results suggest that during the synthesis carbon atoms tend to segregate on nucleation sites
on the Ni surface to form multiple-layer graphene grains. The formation of such multilayer
domains is believed to be correlated to different factors including the abundance of defects
and grain boundaries on the polycrystalline Ni substrate. It is therefore particularly
interesting to investigate the formation of graphene on single crystal Ni due to the absence
of interface boundaries, and Ni (111) is especially interesting due to the excellent lattice
match between graphene/graphite and Ni (111) face, where the hexagonal lattice constant
is 2.497 Å for Ni (111) and 2.46 Å for graphite
23
as shown in Table 4.1.

67
Ni (111) interstitial distance (Å) 1.412
C-C bond length in graphene (Å) 1.420
Ni hexagonal lattice constant at 750 °C (Å) 2.497
C lattice constant in graphene (Å) 2.461
Lattice constant mismatch (%) 1.5
Table 4.1 Parameters correlating graphene and Ni (111).

In this chapter, we will discuss the influence of the concentration of Ni interface
boundaries on the formation of such multilayer graphene domains. Synthesis of graphene
by CVD on the (111) face of single crystal Ni favors the formation of highly uniform
monolayer/bilayer graphene on the Ni surface, and simultaneously hinders the formation of
multilayer graphene domains. Our results are understood on the basis of the
diffusion-segregation model for carbon precipitation on Ni surface,
86
where the uniform
and grain-boundary-free surface of Ni (111) single crystal provides a smooth surface for
uniform graphene formation. In contrast, the rough surface of polycrystalline Ni with
abundant grain boundaries facilitates the formation of multilayer graphene. Micro-Raman
surface mapping reveals that the area percentages of monolayer/bilayer graphene are
91.4% for the Ni (111) substrate and 72.8% for the polycrystalline Ni substrate under
comparable CVD conditions.
68

4.2 Graphene Synthesis on Both Ni film and Single Crystalline Ni (111)
To prepare samples for CVD growth, single crystal Ni was obtained from Crystal
Base Co., Ltd, while polycrystalline Ni was prepared by depositing 500 nm of Ni metal
(99.999 % purity) onto a SiO
2
/Si wafer by e-beam evaporation. Both the Ni (111) substrate
and polycrystalline Ni film substrate were loaded into a CVD chamber. The chamber was
heated up to 900 °C under 600 sccm H
2
, and the samples were annealed for 15 min under
900 °C. Shown in Figure 4.1a and b are the X-Ray Diffraction (XRD) spectra taken after
the thermal annealing of both substrates.
40 50 60
0
200000
400000
600000
800000
1000000

2θ θ θ θ ( ( ( (degrees)
Ni (111)

Intensity (a.u.)
40 50 60
0
1000
2000
3000
4000
5000

2θ θ θ θ ( ( ( (degrees)
Intensity (a.u.)
Ni (111)
Ni (200)
(a) (b)
200nm

Figure 4.1 XRD spectra and AFM images (inset) of Ni (111) substrate (a) and
polycrystalline Ni substrate (b), respectively. The color scale bar corresponds to AFM
images in a and b inset.

There is only a single peak corresponding to Ni (111) in the XRD spectrum taken from
the Ni (111) substrate (Figure 4.1a) as expected. The XRD spectrum of polycrystalline Ni,
69
however, shows a strong Ni (111) peak and a very weak Ni (200) after the annealing
process, which indicates the presence of predominantly Ni (111) grains with a smaller
population of Ni (100) grains. The Atomic Force Microscopy (AFM) images shown in
Figure 4.1a and b insets further indicate the difference between those two substrates in
terms of the surface roughness: The surface of Ni (111) is considerably smoother (average
roughness = 5.3 nm) than the polycrystalline Ni (average roughness = 36.3 nm) after
thermal annealing. In addition, the polycrystalline Ni surface acquired a multigrain-like
appearance after thermal annealing.
The process of graphene growth on Ni can be divided into two parts: the first is carbon
segregation from bulk Ni to Ni surface in an intermediate temperature range (~ 1065-1180
K),
86
and the second is carbon precipitation which happens when the system temperature
decreases (<1065 K).
86
It is reported that both carbon segregation and precipitation tend to
happen at the grain boundaries.
86
The impurities in transition metals tend to segregate at
grain boundaries which can be good nucleation sites for carbon segregation.
76
Also, the
grain boundaries can act as active sites for the interaction of carbon atoms and lattice
vacancies during cooling.
86
Therefore, the grain boundary plays an important role in both
the carbon segregation and precipitation.
To fully elucidate the effect of grain boundaries, we have performed a series of
experiments to prepare polycrystalline and single crystal Ni samples. Three different
70
annealing rates (175 °C/min, 58 °C/min, 27 °C/min) were chosen from fast to slow for the
annealing process of polycrystalline Ni, and the medium annealing rate (58 °C/min) was
applied to anneal the Ni (111). XRD spectra were collected after the thermal annealing
process as shown in Figure 4.2a. The first three spectra from top to bottom correspond to
fast, medium and slow rate annealing on polycrystalline Ni, and the last one is collected
from Ni (111) after annealing. All the spectra were normalized for further analysis. While
all four samples show a strong peak corresponding to Ni (111), the polycrystalline Ni
samples display an additional Ni (200) peak at 2θ = 52.16°, with intensity being highest for
fast annealing and lowest for slow annealing. In contrast, the single crystalline Ni XRD
spectrum shows no peak for Ni (200) (Figure 4.2b). Therefore, we can conclude that slower
thermal annealing favors the formation of crystalline Ni (111) grains with less grain
boundaries for polycrystalline Ni samples.
The synthesis of graphene film on Ni (111) and polycrystalline Ni film was done in
the same CVD furnace with a 1 inch quartz tube. Different flow rates of CH
4
were used for
Ni (111) and polycrystalline Ni film respectively. The Ni (111) (or polycrystalline Ni film)
substrate was first loaded into the furnace, then 600 sccm H
2
flow was added into the
furnace for 15 minutes while the temperature increased from the room temperature to
900 °C. 80 sccm CH
4
(25 sccm for Ni film) was introduced to the furnace at 900 °C for 10
minutes. The furnace was then cooled done to room temperature under a cooling rate of
71
16 °C/min.

35 40 45 50 55 60
2θ θ θ θ (degrees)
Intensity (a.u.)

fast
medium
slow
single crystal
50 51 52 53 54 55
2θ θ θ θ (degrees)
Intensity (a.u.)

(a)
(b)
(c)
(d)
(e)
(f)
(h)
(j)
(g)
(i)

Figure 4.2 (a). XRD spectra collected from polycrystalline Ni with fast (black), medium
(red), and slow (blue) annealing rates and XRD spectrum collected from Ni (111) (green)
after thermal annealing (XRD spectrum is identical for Ni (111) using different annealing
rates). (b). Zoomed-in XRD spectra of peaks at 2θ = 52.16° (assigned as Ni (200)). (c-j).
Optical images taken after graphene CVD growth from polycrystalline Ni with fast (c, d),
medium (e, f) and slow (g, h) annealing rates. (i, j). Optical images taken after graphene
CVD growth from Ni (111).

Figure 4.2c-j depict the optical images of Ni substrates after graphene synthesis.
Figure 4.2c and d, e and f, and g and h correspond to polycrystalline Ni samples obtained
with fast, medium and slow thermal annealing rates, respectively. The dark regions are
72
confirmed to consist of multilayer graphene (≥3 layers), while the light regions are
confirmed to be monolayer/bilayer graphene using micro-Raman spectroscopy. According
to Figure 4.2c-h, the percentage of multilayer graphene formation increases as the
polycrystallinity of the Ni substrate increases. These results suggest that the formation of
multilayer graphene can be attributed to the increase of carbon segregation localized at
polycrystalline grain boundaries, while the formation of monolayer/bilayer graphene is
mainly obtained on the flat central areas of large crystalline grains.
Furthermore, images obtained on Ni (111) substrate after graphene synthesis are
shown in Figure 4.2i and j. Analysis of the graphene growth on single crystal Ni (111)
(Figures 4.2i and j) reveals the scarce formation of multilayer graphene grains. The
observation can be understood by considering the absence of inter-plane grain boundaries
on the surface of Ni (111) and therefore a shortage of nucleation sites for multilayer
graphene formation. Thus, mostly monolayer/bilayer graphene is uniformly formed on the
surface of Ni (111) single crystal. Synthesis of graphene utilizing different methane
concentrations was also conducted. When a methane concentration of 0.65% was used, the
graphene growth was not continuous on either Ni (111) or polycrystalline Ni. And when we
further decreased the methane concentration to 0.50%, no graphene growth was observed
on either substrate. On the other hand, when we increased the methane concentration
higher than 12% (other growth conditions remained the same), multilayer graphene also
73
formed on Ni (111) substrate.

4.3 Mechanism of Graphene Growth on Ni
Based on the discussion above, a graphene growth mechanism is proposed in Figure
4.3. Figure 4.3a and b give schematic diagrams of possible mechanism of the formation of
graphene during the carbon segregation and precipitation on Ni (111) and polycrystalline
Ni surfaces, respectively.
Due to the high solubility of carbon in Ni, carbon first diffuses into bulk Ni, and then
segregates and precipitates onto Ni surface. In the carbon/Ni (111) system, the surface of
Ni (111) is very smooth with almost no grain boundaries, allows uniform segregation of
carbon onto the Ni (111) surface, and thus tends to form single layer graphene. In contrast,
in the carbon/ polycrystalline Ni system, the Ni surface is heavily populated by the grain
boundaries, especially inter-plane grain boundaries, which allow the accumulation of
carbon at these sites during the segregation phase and lead to the formation of multilayer
graphene.

74
(b)
(c)
(d)
(a)
Monolayer
graphene
CH
4
Carbon solution in
polycrystalline Ni
C
C
C
C
Grain boundary of
polycrystalline Ni
C
C
Multilayer
graphene
Monolayer
graphene
Monolayer
graphene
Monolayer
graphene
Monolayer
graphene

Figure 4.3 Schematic diagrams of graphene growth mechanism on Ni (111) (a) and
polycrystalline Ni surface (b). (c). Optical image of graphene/ Ni (111) surface after the
CVD process. The inset is a three dimensional schematic diagram of a single graphene
layer on Ni (111) surface. (d). Optical image of graphene/ polycrystalline Ni surface after
the CVD process. The inset is a three dimensional schematic diagram of graphene layers
on polycrystalline Ni surface. Multiple layers formed from the grain boundaries.

Optical images were taken from both Ni (111) and polycrystalline Ni substrates.
Figure 4.3c shows that the surface of Ni (111) has a relatively uniform color with only a
few dark dots. In contrast, the surface of polycrystalline Ni has many dark grains, as shown
in Figure 3d. Both the dark dots and dark grains have been confirmed to be multilayer
graphene by micro-Raman, while the rest of the surface is confirmed to be
75
monolayer/bilayer graphene. These experimental results well match our proposed “grain
boundary mechanism” for graphene formation. The inset schematic diagram in Figure 4.3c
shows the formation of graphene in an ideal case: a well-ordered graphene sheet on Ni (111)
surface without any grain boundaries. The formation of multilayer graphene on
polycrystalline Ni is illustrated in the inset of Figure 4.3d, red and green represents two
graphene layers which segregate and precipitate from grain boundaries. More layers will
continue segregating and precipitating from the boundaries depends on the concentration
of carbon in bulk Ni. The misalignment between two polycrystalline grains may provide
abundant nucleation sites for carbon atoms to segregate from the boundaries. Therefore,
multilayer graphene tends to form at the boundaries, while monolayer graphene tends to
form on Ni (111) surface.

4.4 Raman Characterization for the Number of Graphene Layers
The formation of graphene layers on Ni surface was confirmed by micro-Raman
spectroscopy after the CVD process. The information of the defects of graphene, as well as
the number of graphene layers can also be derived from Raman spectra.
Figure 4.4a and b show ten typical spectra collected from different locations on the
synthesized graphene films on Ni (111) and polycrystalline Ni, respectively. The low
intensity of D band (~ 1350 cm
-1
) confirms that the graphene formed on both Ni (111) and
76
polycrystalline Ni surfaces are of low defects. Peaks located at ~ 1590 cm
-1
and ~ 2700
cm
-1
are assigned as G and G’ bands of the graphene layers, respectively.
All ten spectra collected from the graphene on Ni (111) in Figure 4a show
single-Lorentzian lineshape and narrow linewidth (25 – 55 cm
-1
). Furthermore, all ten
spectra exhibit G’ to G peak intensity ratios (I
G’
/I
G
) larger than one, which are considered
fingerprints for the formation of monolayer/bilayer graphene, as previously reported.
75

In contrast, the ten typical Raman spectra collected from the graphene on
polycrystalline Ni in Figure 4.4b can be divided into two groups. The first group of spectra
are similar to spectra in Figure 4a, which have single Lorentzian profile for G’ band,
narrow linewiths for G’ peaks, and I
G’
/I
G
> 1. On the other hand, the second group of
spectra have noticeable up-shift of ~ 15 cm
-1
in the G’ band, as well as broadening of G’
band linewidth (~50 cm
-1
) that can be fit with four or more Lorentzian peaks.
It is known that the G’ peak position exhibits up-shift with the increase of number of
layer of graphene, and the fitting of G’ peak with two or more Lorentzian peaks is a
signature of multilayer graphene.
24, 34
Moreover, I
G’
/I
G
for the second group of Raman
spectra is smaller than one, which is characteristics of multilayer graphene.
12, 24, 34

Altogether, all the Raman spectra collected from the graphene film on single crystal Ni
show the formation of monolayer/bilayer graphene, while only about 50% of the spectra
collected from the graphene film grown on polycrystalline Ni correspond to
77
monolayer/bilayer graphene.
50 75 100
0
1
2
3
Single crystal
Polycrystalline
I
G'
/ I
G
G' FWHM
1200 1600 2000 2400 2800

Raman Shift (cm
-1
)
(a)
(b)
(c)
1200 1600 2000 2400 2800
Raman Shift (cm
-1
)

Figure 4.4 (a-b). Ten typical Raman spectra of graphene grown on Ni (111) and
polycrystalline Ni, respectively. (c). The G’-to-G peak intensity ratio (I
G’
/I
G
) v.s. the Full
Width at Half Maximum (FWHM) of G’ bands of graphene on both Ni (111) and
polycrystalline Ni.
78

Figure 4.4c shows a plot of I
G’
/I
G
values versus Full Width at Half Maximum (FWHM)
of G’ bands of micro-Raman spectra taken at random locations on the graphene films
grown on single crystal and polycrystalline Ni. It is clearly shown in Figure 4.4c that all
spectra of graphene on single crystal Ni surface have higher I
G’
/I
G
values (all higher than
one) and narrower FWHM, while only half of the spectra of graphene on polycrystalline Ni
have high I
G’
/I
G
values and narrow FWHM. Therefore, the result from Figure 4.4c confirms
that the graphene on single crystal Ni is dominantly monolayer/bilayer, while the
percentage of multilayer graphene is much higher on polycrystalline Ni.

4.5 Raman Mapping for Large-Area Characterization
In order to have a better idea of how the graphene grows in large area on both Ni (111)
and polycrystalline Ni, about 800 Raman spectra were collected over 3000 μm
2
area with 2
μm spacing between each data point on both surfaces.
79
0 10 20 30 40 50
50
40
30
20
10
0

Width (μm)
Length (μm)
0
0.25
0.50
0.75
1.0
91.4%
(a)
0 10 20 30 40 50
40
30
20
10
0

Width (μm)
Length (μm)
0
0.25
0.50
0.75
1.0
72.8%
(b)
(c) (d)
10 μm 10 μm
30 nm
500 nm
(e) (f)
0.2 0.4 0.6 0.8 1.0
-2
0
2
4
6
8

Height (nm)
Distance (μ μ μ μm)
0.93 nm

Figure 4.5 (a). Maps of I
G’
/I
G
of 780 spectra collected on a 60*50 μm
2
area on the Ni (111)
surface and (b) 750 spectra collected on a 60*50 μm
2
area on the polycrystalline Ni
surface. Corresponding optical images to Ni (111) Raman map and polycrystalline Ni
Raman map (c and d). (e) AFM image of graphene film transferred to SiO
2
/Si substrate
from Ni (111). (d) Height analysis of the thickness of graphene film.

The I
G’
/I
G
values were then extracted from the spectra. Figure 4.5a and b show the
80
I
G’
/I
G
contour maps of graphene on Ni (111) and polycrystalline Ni, respectively. 91.4% of
Raman spectra collected from the graphene on Ni (111) surface has I
G’
/I
G
higher than one,
which is a hallmark of monolayer/bilayer graphene. In contrast, the percentage of
monolayer/bilayer graphene is only 72.8% from the graphene grown on polycrystalline Ni.
Moreover, the narrow spacing of 2 μm between data points for the Raman measurements
enables the confirmation of continuous graphene deposition on both substrates. Figure 4.5c
and d are the corresponding optical images of the two Raman maps, which are in
accordance with the Raman maps: dark regions in the optical images correspond to
multilayer graphene in Raman maps, while the rest is confirmed to be monolayer/bilayer
graphene.
Furthermore, we transferred graphene to SiO
2
/Si substrate using the same method
described in Chapter 2.4, and measured the thickness of our graphene film by AFM. Figure
4.5e shows an AFM image of transferred graphene from Ni (111) substrate. The thickness
was measured from the edge of graphene film to the opening (shown in the white line).
Figure 4.5f is the height analysis of the graphene film from the white line drawn on 4.5e.
The thickness of the graphene film is 0.93 nm, which is considered to be one or two layers
due to the surface roughness of SiO
2
/Si substrate.

81
4.6 Characterization on Transferred Graphene Films
Optical images and Raman spectra were obtained on the transferred graphene film
from Ni (111) and polycrystalline Ni film respectively for comparison study. Transfer of
graphene from Ni (111) and polycrystalline Ni was done using the same method described
in Chapter 2.4. Comparing Figure 4.6a and b, the graphene transferred from Ni (111)
(shown in Figure 4.6a) is more uniform than graphene transferred from polycrystalline Ni
(shown in Figure 4.6b).
1200 1600 2000 2400 2800
0
2000
4000
6000
8000
10000
Intensity (a.u.)

Raman Shift (cm
-1
)
1200 1600 2000 2400 2800
0
2000
4000
6000
8000
10000

Raman Shift (cm
-1
)
Intensity (a.u.)
(a) (b)
(c)
(d)

Figure 4.6 Optical image of graphene transferred from Ni (111) (a) and polycrystalline Ni
film (b) to SiO
2
/Si substrate. Corresponding Raman spectra taken from graphene
transferred from Ni (111) (c) and polycrystalline Ni (d).

82
It is confirmed by Raman spectroscopy that the graphene transferred from Ni (111) is
monlayer/bilayer as shown in Figure 4.6c. The G’ band can be fitted by a single Lorentzian
peak and has a linewidth of ~30 cm
-1
. I
G’
/I
G
is also higher than one. Raman spectra
collected from different locations on the substrate are shown in Figure 4.6d. Raman spectra
colored in red, black, and blue were taken from regions pointed by arrows in red, black, and
blue respectively. The shape of G’ peak in red contains two components G’
1
and G’
2
, which
indicates the formation of graphite. The G’ peak in black has a symmetric shape with a
linewidth of ~60 cm
-1
, indicating the formation of few-layer graphene (less than five). The
G’ peak in blue has a linewidth of ~30 cm
-1
and can be fitted by a single Lorentzian peak
indicating monolayer/bilayer graphene.
Graphene from Ni (111) was also transferred to glass for the transmittance
measurement to investigate the number of layers of graphene. The transmittance was
measured using a Varian50 spectrophotometer in the wavelength range of 400-1000 nm.
The transmittance is 95.1% at 550 nm, which indicate that the average thickness of the
transferred graphene film is approximately two layers (Figure 4.7).
83
400 600 800 1000
0
20
40
60
80
100

Transmittance (%)
Wavelength (nm)

Figure 4.7 Transmittance spectrum of graphene film transferred from Ni (111)

4.7 Conclusion
In summary, we found that preferential formation of monolayer/bilayer graphene on
the single crystal surface is attributed to its atomically smooth surface and the absence of
grain boundaries. In contrast, CVD graphene formed on polycrystalline Ni leads to higher
percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain
boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman
surface mapping reveals that the area percentages of monolayer/bilayer graphene are
91.4% for the Ni (111) substrate and 72.8% for the polycrystalline Ni substrate under
comparable CVD conditions.
84
Chapter 4 References

2. Bai, J. W.; Zhong, X.; Jiang, S.; Huang, Y .; Duan, X. F., Graphene Nanomesh. Nat
Nanotechnol 2010, 5, 190-194.
9. Bolotin, K. I.; Ghahari, F.; Shulman, M. D.; Stormer, H. L.; Kim, P., Observation of the
Fractional Quantum Hall Effect in Graphene. Nature 2009, 462, 196-199.
12. Cancado, L. G.; Reina, A.; Kong, J.; Dresselhaus, M. S., Geometrical Approach for the
Study of G ' Band in the Raman Spectrum of Monolayer Graphene, Bilayer Graphene, and
Bulk Graphite. Physical Review B 2008, 77.
13. Chen, C. Y .; Rosenblatt, S.; Bolotin, K. I.; Kalb, W.; Kim, P.; Kymissis, I.; Stormer, H.
L.; Heinz, T. F.; Hone, J., Performance of Monolayer Graphene Nanomechanical
Resonators with Electrical Readout. Nat Nanotechnol 2009, 4, 861-867.
17. De Arco, L. G.; Zhang, Y .; Kumar, A.; Zhou, C., Synthesis, Transfer, and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Trans.
Nanotechnol. 2009, 8, 135-138.
23. Eizenberg, M.; Blakely, J. M., Carbon Monolayer Phase Condensation on Ni(111).
Surf. Sci. 1979, 82, 228-236.
24. Ferrari, A. C.; Meyer, J. C.; Scardaci, V .; Casiraghi, C.; Lazzeri, M.; Mauri, F.;
Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K., Raman Spectrum of
Graphene and Graphene Layers. Physical Review Letters 2006, 97, 187401.
28. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
33. Green, A. A. H., M. C., Emerging Methods for Producing Monodisperse Graphene
Dispersions. Journal of Physical Chemistry Letters 2010, 1, 544-549.
34. Gupta, A.; Chen, G.; Joshi, P.; Tadigadapa, S.; Eklund, P. C., Raman Scattering from
High-Frequency Phonons in Supported N-Graphene Layer Films. Nano Letters 2006, 6,
2667-2673.
36. Hass, J.; Feng, R.; Li, T.; Li, X.; Zong, Z.; de Heer, W. A.; First, P. N.; Conrad, E. H.;
Jeffrey, C. A.; Berger, C., Highly Ordered Graphene for Two Dimensional Electronics.
Appl Phys Lett 2006, 89, 143106.
50. Kim, K. S.; Zhao, Y .; Jang, H.; Lee, S. Y .; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.;
Choi, J. Y .; Hong, B. H., Large-Scale Pattern Growth of Graphene Films for Stretchable
Transparent Electrodes. Nature 2009, 457, 706-710.
51. Kim, M.; Safron, N. S.; Han, E.; Arnold, M. S.; Gopalan, P., Fabrication and
Characterization of Large-Area, Semiconducting Nanoperforated Graphene Materials.
Nano Letters 2010, 10, 1125-1131.
53. Levendorf, M. P.; Ruiz-Vargas, C. S.; Garg, S.; Park, J., Transfer-Free Batch
85
Fabrication of Single Layer Graphene Transistors. Nano Lett. 2009, 9, 4479-4483.
54. Lewis Gomez, Y . Z., Cody Schlenker, Koungmin Ryu, Mark Thompson, Chongwu
Zhou, Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor
Deposition for Organic Photovoltaics. ACS Nano 2010.
58. Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S., Evolution of Graphene Growth on Ni
and Cu by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268-4272.
63. Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S.,
The Structure of Suspended Graphene Sheets. Nature 2007, 446, 60-63.
65. Novoselov, K. S.; Geim, A. K.; Morozov, S. V .; Jiang, D.; Katsnelson, M. I.;
Grigorieva, I. V .; Dubonos, S. V .; Firsov, A. A., Two-Dimensional Gas of Massless Dirac
Fermions in Graphene. Nature 2005, 438, 197-200.
66. Novoselov, K. S.; Geim, A. K.; Morozov, S. V .; Jiang, D.; Zhang, Y .; Dubonos, S. V .;
Grigorieva, I. V .; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films.
Science 2004, 306, 666-669.
67. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V . V .; Morozov, S.
V .; Geim, A. K., Two-Dimensional Atomic Crystals. P Natl Acad Sci USA 2005, 102,
10451-10453.
68. Obraztsov, A. N., Chemical Vapour Deposition Making Graphene on a Large Scale.
Nat Nanotechnol 2009, 4, 212-213.
75. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V .; Dresselhaus, M. S.;
Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical
Vapor Deposition. Nano Lett. 2009, 9, 30-35.
76. Reina, A. T., S.; Jia, X.; Bhaviripudi, S.;Dresselhaus, M. S.; Schaefer, J. A.; Kong, J. ,
Growth of Large-Area Single- and Bi-Layer Graphene by Controlled Carbon Precipitation
on Polycrystalline Ni Surfaces. Journal of nano research 2009, 2, 509-516.
78. Rollings, E.; Gweon, G. H.; Zhou, S. Y .; Mun, B. S.; McChesney, J. L.; Hussain, B. S.;
Fedorov, A.; First, P. N.; de Heer, W. A.; Lanzara, A., Synthesis and Characterization of
Atomically Thin Graphite Films on a Silicon Carbide Substrate. J Phys Chem Solids 2006,
67, 2172-2177.
86. Shelton, J. C.; Patil, H. R.; Blakely, J. M., Equilibrium Segregation of Carbon to a
Nickel (111) Surface - Surface Phase-Transition. Surf Sci 1974, 43, 493-520.
89. Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y .;
Wu, Y .; Nguyen, S. T.; Ruoff, R. S., Synthesis of Graphene-Based Nanosheets Via
Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558-1565.
90. Sukang Bae, H. K., Youngbin Lee, Xiangfan Xu, Jae-Sung Park, Yi Zheng, Jayakumar
Balakrishnan, Tian Lei, Hye Ri Kim, Young Il Song, Young-Jin Kim, Kwang S. Kim,
Barbaros Ozyilmaz, Jong-Hyun Ahn,Byung Hee Hong, and Sumio Iijima, Roll-to-Roll
Production of 30-Inch Graphene Films for Transparent Electrodes. Nat Nanotechnol 2010.
92. Sutter, P. W.; Flege, J. I.; Sutter, E. A., Epitaxial Graphene on Ruthenium. Nat. Mater.
86
2008, 7, 406-411.
102. Wang, X. R.; Dai, H. J., Etching and Narrowing of Graphene from the Edges. Nat.
Chem. 2010, 2, 661-665.
106. Wu. M.; Tse, J. S. J., J. S., Unzipping of Graphene by Fluorination. Journal of
Physical Chemistry Letters 2010, 1, 1934-1937.
109. Yu, D. S. D., L. M., Self-Assembled Graphene/Carbon Nanotube Hybrid Films for
Supercapacitors. Journal of Physical Chemistry Letters 2010, 1, 467-470.
111. Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., Graphene Segregated
on Ni Surfaces and Transferred to Insulators. Appl. Phys. Lett. 2008, 93, 113103.

87
Chapter 5: Vapor Trapping Growth of Single-Crystalline
Graphene Flowers: Synthesis, Morphology, and Electronic
Properties

5.1 Introduction
Graphene is a two-dimensional, honeycomb lattice arrangement with unique physical
properties.
28, 65-66, 116
To overcome the disadvantage of small-scale production of graphene
using mechanical exfoliation of highly orientated polymeric graphite (HOPG), chemical
vapor deposition (CVD) of large-area single-layer graphene on metal films has been
explored from various aspects.
17, 50, 57, 60, 75, 110-111
Despite the significant progress, CVD
graphene is usually a polycrystalline film made of small grains.
38
As the grain boundaries
have been found to impede both transport
59, 108, 110
and mechanical properties
87
, it is
therefore very important to be able to synthesize large-grain, single-crystalline graphene
for various applications. The pioneering work of Li et al
60
demonstrated the CVD growth
of large graphene single crystals up to 0.5 mm in size, using a copper enclosure. Such large
graphene single crystals can be very important and may find applications for various
88
electronic devices, however, the process depends on how the copper enclosure is manually
made, and the copper enclosure does not allow probing the gas environment inside. Due to
the utmost importance, alternative means to produce large-grain single-crystalline
graphene can be very beneficial for applications and the growth mechanism study.
In this chapter, we will discuss a vapor trapping method to grow large-grain,
single-crystalline graphene with controlled grain morphology. The grain size of
six-lobe-flower-shape grains can achieve 100 μm, with high quality single-layer graphene
as lobes and bilayer graphene as centers. Selected area electron diffraction (SAED) has
been applied to confirm the single-crystalline nature of the graphene flowers, and
systematic study of the graphene morphology versus growth parameters (total pressure and
methane-to-hydrogen ratio) has been performed. We found that the graphene morphology
can be well controlled by tuning the total pressure of the CVD system, and the
methane-to-hydrogen ratio. In addition, electron backscatter diffraction (EBSD) study
indicates that the graphene morphology has little correlation with the crystal orientation of
the underlying copper substrate. Field effect transistors (FETs) have been fabricated based
on the large-grain graphene flowers, and the fitted device mobility could achieve ~ 4,200
cm
2
V
-1
s
-1
on Si/SiO
2
and ~ 20,000 cm
2
V
-1
s
-1
on hexagonal boron nitride (h-BN).

89
5.2 Large-Grain Graphene Synthesis Using a Vapor Trapping Method
Graphene synthesis was done using a vapor trapping chemical vapor deposition
method as illustrated in Fig. 5.1a. Cu foil was rolled up and put into a half inch small quartz
tube, which is open only at one end. The half inch quartz tube was then placed inside a
two-inch quartz tube of the CVD chamber. Gases flown into the small quartz tube would be
trapped inside, and therefore we believe that the small quartz tube would result in gas
composition and gas flow rate different from outside the tube, thus leading to interesting
growth results of graphene. Another piece of Cu foil was sometimes placed outside the
small vapor trapping tube for comparison.
7 sccm H
2
was introduced to the CVD chamber at 40 mTorr, and the temperature was
brought up to 1000 °C in 40 minutes. The Cu foils were annealed at 1000 °C for 20 minutes.
1 sccm CH
4
and 12.5 sccm H
2
were then introduced into the CVD chamber for graphene
growth. The pressure was kept at 200 mTorr for 30 minutes during the growth. The CVD
chamber was cooled down to room temperature with the flow of 1 sccm CH
4
and 12.5 sccm
H
2
continuing.

90
(a)
(d)
(c)
20 μm
5 μm
Copper foil
Vapor-trapping
tube
(b)
(e)
50 μm
100 μm 10 μm

Figure 5.1 (a) Schematic diagram of a vapor trapping CVD method for graphene growth.
(b) low and (c) high magnification SEM images of a six-lobe graphene flower grown on
Cu foil inside the vapor trapping tube. (d) SEM image of a four-lobe graphene flower
grown on Cu foil inside the vapor trapping tube. (e) Graphene grown on Cu foil outside
the vapor-trapping tube.

Fig. 5.1b and c show SEM images of six-lobe graphene flowers grown on the bottom
side of Cu foil placed inside the vapor trapping tube. The size of graphene flowers is up to
91
100 μm (Fig. 5.1b). By varying the growth parameters, we also observed four-lobe
graphene flowers grown on Cu foil as shown in Fig. 5.1d. Interestingly, the graphene
grown on the Cu foil placed outside the small vapor trapping tube did not show any
“flower” shape, but continuous graphene film with slight etching
115
was found instead (Fig.
5.1e). The pronounced difference between graphene grown on Cu foil inside and outside
the vapor-trapping tube indicates that the vapor trapping tube does change the local
environment inside the tube, especially in reducing the carbon supply and creating a
quasi-static reactant gas distribution which results in large flower-shaped graphene grains.
We also observed graphene flowers grown on copper foil without the vapor trapping
tube by using reduced methane flow rate (0.5 sccm CH
4
and 25 sccm H
2
at a total pressure
of 150 mTorr, and other growth parameters are the same as the recipe using vapor trapping
tube). Fig. 5.2a and b are SEM images of as-grown graphene flowers without vapor
trapping tube at different magnifications. However, the shape of flowers was not as
uniform as the ones using vapor trapping tube. Our vapor trapping approach has little
variation from run to run, and the open end of the vapor trapping tube may enable probing
of gas species inside using techniques such as mass spectrometer.

92
b
a

Figure 5.2 SEM images of graphene flowers without using vapor trapping tube. (a) low
magnification. (b) high magnification. (scale bar: 50 μm)

5.3 Raman Characterization of Large-Grain Graphene
The graphene flowers were successfully transferred onto Si/SiO
2
substrates for further
investigation using the transfer technique described in Chapter 2.4. SEM image and optical
microscope image of a six-lobe graphene flower are shown in Fig. 5.3a and b. The color
contrast of the graphene flower is very uniform in both SEM and the optical image, except
that the central part takes a hexagonal shape and is darker than the lobes.
Raman spectra were taken from different locations on the transferred graphene
sample. The Raman spectrum in black in Fig. 5.3c was taken from the area outside the
graphene flower (marked by letter A in Fig. 5.3b), which did not show any G or 2D peak of
graphene as expected. In contrast, the Raman spectrum in red in Fig. 2c was taken from the
93
area of graphene lobes (marked by letter B in Fig. 2b). It presents typical features of
single-layer graphene: the I
2D
/I
G
intensity ratio is ~0.5, and the full width at half-maximum
(FWHM) of 2D band is ~33 cm
-1
. The Raman spectrum in blue in Fig. 5.3c was collected
from the center of the graphene flower (marked by letter C in Fig. 5.3b). The I
2D
/I
G

intensity ratio is ~1 and the FWHM of 2D band ~53 cm
-1
, which represents bilayer
graphene.
14

1200 1600 2000 2400 2800

Intensity (a.u.)
Raman Shift (cm
-1
)
A
B
C
(a) (b)
(d)
(c)
20 μm
(e) (f)
20 μm
A
B
C

Figure 5.3 (a) A SEM image and (b) an optical microscope image of a six-lobe graphene
flower transferred on a Si/SiO
2
substrate. (c) Raman spectra taken from location A, B,
and C marked in Fig. 2b. (d-f) Raman map of I
G
, I
2D
, and I
2D
/I
G
intensity ratio. Scale bar
for (d)-(f) is 10 μm. The color scale bar from bottom to top is 300, 500, 900, 1300, 1700,
2000 (d); 100, 600, 1200, 1800, 2400, 3200 (e); 1, 2, 3 (f).

To further investigate large surface area of the graphene flower, Raman maps of I
G
, I
2D
,
94
and I
2D
/I
G
intensity ratio

were collected and shown in Fig. 5.3d, e, and f, respectively. The
maps show very uniform G and 2D band for the graphene flower, with only a little PMMA
residue on the lower lobe. Therefore, Raman spectroscopy shows that the graphene flower
is mainly single-layer graphene, with a small bilayer region in the center, which is believed
to be the nucleation site.

5.4 TEM Characterization and Selected Area Electron Diffraction on
Large-Grain Graphene
The grain size of graphene is of great importance in device application since grain
boundaries may affect the transport properties of graphene FETs, and decrease the device
mobility. Previously low energy electron microscopy (LEEM) was used to investigate
graphene grain size,
60
but access to LEEM is usually not widely available. Here we report
the use of selected area electron diffraction (SAED) as a reliable method to study the
crystalline structure and grain size of graphene, which can be performed with readily
available transmission electron microscopy (TEM).
As-grown graphene flowers were transferred onto a perforated SiN TEM grid. SEM
image of graphene on TEM grid in Fig. 5.4a shows that graphene retains its flower shape
after transfer. Fig. 5.4b is a zoomed-in SEM image of the graphene flower highlighted by
yellow dashed line in Fig. 5.4a.
95

Figure 5.4 (a) SEM image of graphene flowers transferred on a perforated SiN TEM grid.
(b) Zoomed-in SEM image of the graphene flower marked using yellow dashed square in
Fig. 3a. Each opening within the graphene flower was marked by a letter (white:
single-layer graphene; yellow: A-B stacking bilayer graphene; blue: torn and folded
graphene) or a red cross (no graphene covered). (c) A bright field TEM image of
graphene suspended on SiN TEM grid. (d)-(f) Diffraction patterns taken from opening H,
Y , and O, respectively. (g) Diffraction pattern taken from opening BB. (h) A bright field
TEM image of torn and folded graphene taken from opening AA. (i) Diffraction pattern
taken from opening AA.

In order to measure the grain size of graphene, we did SAED on graphene at every
opening within the graphene flower, and compared the orientation of the diffraction
patterns. As shown in Fig. 5.4b, three different kinds of diffraction patterns were marked
96
by white letters, yellow letters, and blue letters, respectively. The openings which were not
covered by graphene membrane were marked by red crosses.
Fig. 5.4c shows a bright field TEM image of graphene membrane at a TEM grid
opening covered by graphene membrane. The graphene membrane is clean, uniform, and
smooth within the opening. We observe that the openings marked with white letters all
show one set of symmetric six-fold electron diffraction pattern that orientates to the same
direction. Fig. 5.4d, e, and f are images of three representative diffraction patterns from
opening H, Y , and O. One can see that the openings with white letters cover all six lobes of
the graphene flower, indicating that the graphene flower is a single-crystalline grain.
Fig. 5.4g shows the diffraction pattern taken from the opening BB located at the center
of the graphene flower, also displaying one set of symmetric six-fold diffraction spots. The
outer set of diffraction spots are from equivalent planes {1-210}, showing higher
(approximately twice) intensity than the inner set from {1-100}. This is a key feature for
A-B stacking bilayer graphene.
61
This observation is also in accordance with the Raman
spectra (Fig. 5.3c) showing that the center of graphene flower consists of bilayer graphene.
We also observed that the graphene membrane was torn or folded at some of the
openings of the TEM grid. This might be due to the surface tension caused by the transfer
process. Fig. 5.4h shows a bright field TEM image taken from opening AA, which is
covered by torn and folded graphene film. The folded graphene membrane becomes
97
multiple-layered, and hence the diffraction patterns taken from folded graphene membrane
show multiple sets of diffraction spots in Fig. 5.4i. We marked all the openings with torn
and folded graphene membrane with blue letters, and their diffraction patterns are all
similar to the one shown in Fig. 5.4i. Once we exclude torn and folded graphene due to the
transfer, we can conclude that SAED on each opening covered by the graphene flower
confirms that the graphene flower is a single-crystalline graphene grain, and the center of
the graphene flower is A-B stacking bilayer graphene.

5.5 The Relation Between Graphene Morphology and Growth
Parameters
During our synthesis process, it was observed that the morphology of large graphene
grains changed when the parameters changed in the CVD system. Among various growth
parameters, the total pressure of the CVD system and methane-to-hydrogen ratio were
found to be two important parameters that were closely related to the morphology of
graphene grains. To investigate the correlation between the grain morphology and the total
pressure and methane-to-hydrogen ratio, we varied the total pressure at a fixed methane to
hydrogen ratio (1:12.5), and also varied methane-to-hydrogen ratio at a fixed total pressure
(150 mT) to conduct a series of growth process.
98
Total
Pressure
CH
4
/ H
2
Ratio
50 μm
50 μm
50 μm
80
100
125
150
200
300
400
mT
1:30
1:20
1:15
1:12.5
1:10
1:5
1:2
50 μm

Figure 5.5 SEM images of graphene grown using various recipe. The central images with
yellow frame in both left and right column are the same. And the graphene was grown at
150 mTorr using 1:12.5 CH
4
/H
2
ratio. The left set of SEM images with blue frame are
graphene grown at 1:12.5 CH
4
/H
2
ratio with the total pressure varied from 80 mTorr to
400 mTorr. The right set of SEM images with red frame are graphene grown at 150 mTorr
with CH
4
/H
2
concentration ratio varied from 1:30 to 1:2.
99
Fig. 5.5 shows the growth results using various conditions when a reaction time of 30
minutes was used. With methane-to-hydrogen ratio of 1:12.5, we carried out graphene
CVD using the vapor trapping method at total pressure of 80, 100, 125, 150, 200, 300, and
400 mTorr. Corresponding SEM images are shown in the left column in Fig. 5.5.
We observed that the graphene grains changed from irregular small flakes (80 mTorr)
to mostly four-lobe grains (100 mTorr), and then changed to irregular patterns in between
four-lobe and six-lobe flowers (125 mTorr), then to mostly six-lobe flowers (150 and 200
mTorr). When we further increased the total pressure to 300 mTorr, the six-lobe graphene
flowers turned to irregular shape when a reaction time of 30 minutes was used. The
individual graphene grains tended to coalesce with each other when the total pressure was
increased to 400 mTorr, leaving small gaps between irregular graphene grains.
Interestingly, similar results were obtained when we kept the total pressure at 150
mTorr and gradually increased the methane to hydrogen ratio from 1:30 to 1:2. As shown
in the right column of SEM images in Fig. 5.5, graphene grains were small and close to
hexagonal shape at 1:30 ratio, and then the graphene grains changed to mostly four-lobe
structures when the methane-to-hydrogen ratio increased to 1:20. The grains exhibited
shape between four-lobe and six-lobe flowers for CH
4
:H
2
=1:15, and exhibited mostly
six-lobe flowers for CH
4
:H
2
=1:12.5. When we further increased the methane to hydrogen
ratio to 1:10, the graphene grains became irregular. For CH
4
:H
2
=1:5, the graphene islands
100
tended to connect with each other, leaving only small gaps in between. When
methane-to-hydrogen ratio was brought up to 1:2, graphene grew into continuous film with
multi-layer patches in some locations.
All the observations indicate that increasing the total pressure of the CVD system has
a similar effect on the morphology of graphene grains as increasing the
methane-to-hydrogen ratio. The morphology of graphene grains changed from small
irregular flakes to graphene flowers with lobe structures, and eventually coalesced into a
quasi continuous graphene film, with the increase of total pressure or methane-to-hydrogen
ratio. We believe that the graphene growth is a balance between carbon
diffusion/deposition and hydrogen etching.
115
When the carbon supply is low (at low
CH
4
/H
2
ratio or low total pressure), the graphene nucleates and forms some initial
structures, but the grains of graphene are small because of limited carbon supply and the
etching effect of hydrogen. When the carbon supply increases, carbon diffuse along
particular directions to grow into graphene lobes, and when the carbon supply increases
further, the graphene grains grow close to each other and the original along-the-lobe carbon
diffusion is perturbed. The morphology of graphene depends on both CH
4
/H
2
ratio and the
total pressure as the underlying mechanism includes both carbon diffusion/deposition and
hydrogen etching.
101
a b
c d

Figure 5.6 SEM images of graphene flowers using different growth time at 300 mTorr. (a)
5 min, (b) 10 min, (c) 20min, (d) 30 min. (scale bar: 50 μm)

In addition, we carried out graphene CVD under the pressure of 300 mTorr using
different growth time, with results shown in Fig. 5.6. We observed lobed graphene flowers
with growth time of 5 minutes and 10 minutes. When growth time of 20 minutes and 30
minutes were used, the graphene grains grew close to each other, and the morphology
became irregular due to disturbed diffusion path of carbon by adjacent flakes.

102
5.6 The Relation Between Graphene Morphology and Cu Substrate: An
Electron Backscatter Spectroscopy Study
To further study the correlation between the morphology of graphene grains and the
underneath copper surface, electron backscatter diffraction (EBSD) was used to investigate
the copper surface after graphene growth.
Fig. 5.7 shows EBSD images of copper surface covered by graphene flowers with
different shapes. Fig. 5.7a, b, and c are SEM images of graphene flowers grown at different
locations on the same copper substrate (CH
4
:H
2
=1:12.5, 200 mTorr) with images taken
with the sample tilted at 70° for EBSD. The graphene grains on this sample were mostly
six-lobe flowers (e.g. Fig. 5.7b), with a few four-lobe flowers in some locations (e.g. Fig.
5.7a and c). Corresponding EBSD orientation map images for the locations highlighted by
yellow dashed squares in Fig. 5.7a, b, and c are shown in Fig. 5.7f, g, and h, respectively.
EBSD orientation map image in Fig. 5.7e and f show similar but slightly different green
colors which are close to Cu (110), as indicated in the color scale bar. EBSD orientation
map in Fig. 5.7g shows orange-magenta color, indicating a crystal orientation close to Cu
(100).
103

Figure 5.7 (a) SEM image of a four-lobe graphene flower, (b) a six-lobe graphene flower,
and (c) another four-lobe graphene flower on the same graphene sample. (d) SEM image
of a sample with six-lobe, and four-lobe graphene flowers, and copper grain boundaries.
All SEM images (a-d) were taken with samples tilted at 70° along Y axis. (e)-(h)
Corresponding EBSD orientation map image of the location highlighted by the yellow
dashed square in e, f, g, and h, respectively. The color represents fcc crystalline
orientation is shown on the right side.

All the above information indicates that graphene grains can grow into different
morphology on the same copper substrate, and the underneath copper crystalline
orientation is not closely related to the morphology of the graphene grains grown on top.
Moreover, we investigated a sample with copper grain boundaries and both four- and
six-lobe graphene flowers grown on the surface. Fig. 5.7d shows a SEM image of a
graphene sample grown using a different recipe (CH
4
:H
2
=1:12.5, 125 mTorr). Copper
grain boundaries and both four- and six-lobe graphene flowers are shown in the SEM
image. The corresponding EBSD orientation map of the highlighted location using a
yellow dashed square in Fig. 5.7h shows crystalline index close to Cu (100), with slight
104
orientation difference between adjacent copper grains. The black points in the EBSD
images correspond to locations where the instrument cannot determine the crystal
orientation with high confidence.
The results from EBSD indicate that for the CVD growth on polycrystalline copper
foil, the morphology of graphene grains do not have much correlation with the crystalline
structure of the underneath copper substrate. In fact, both four-lobe
59, 73, 103
and six-lobe
60,
97
graphene morphology have been observed and reported by many research groups on
polycrystalline copper foil
59-60, 97, 103
or single crystal copper.
73

Recently, Rasool et al
73
reported that the interaction between graphene and
underneath copper substrate was weak, and since the copper atoms are almost freely
mobile, they may act as carbon carriers to extend the graphene grains. Therefore, we
believe that the morphology of graphene grains is mostly related to the local environment
close to the copper substrate, and can be tuned by varying growth parameters (pressure,
methane-to-hydrogen ratio, flow rate etc), but the graphene morphology does not have
much correlation with the beneath copper substrate.

105
5.7 Fabrication of Field Effect Transistors Using Large-Grain Grphene
Flowers On Si/SiO
2

To evaluate the quality of the large-grain single-crystalline graphene flowers, we
transferred the as-grown graphene grains onto a highly doped p-type silicon substrate with
300 nm thermal oxide as the gate dielectric and fabricated back-gated graphene FETs.
A
B
C
F
E
D
20 μm
-20 -15 -10 -5 0 5
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12

V
G
- V
Dirac
(V)
Drain Current (mA)
DF
fitted curve
-0.4 -0.2 0.0 0.2 0.4
-0.8
-0.4
0.0
0.4
0.8
-80V
-40V
0V
40V
80V
Drain Voltage (V)
Drain Current (mA)

(a) (b)

Figure 5.8 (a) SEM image of a six-lobe graphene FET. Electrodes are marked by different
letters. The dashed blue square is the region of effective graphene channel between
electrode D and F. (b) Plot of drain current (I
ds
) versus gate voltage (V
g
) minus Dirac
point voltage (V
Dirac
) using D and F as source and drain electrodes (black circles) and
fitted FET mobility curve (solid red line). The drain voltage (V
ds
) is 0.2 V. Inset is a plot
of drain current (I
ds
) versus drain voltage (V
ds
) at various gate voltages.

5 nm Ti and 50 nm Pd were deposited as source and drain electrodes by e-beam
evaporation. Fig. 5.8a shows a SEM image of the graphene FET. Five electrodes were
marked from A to E, and a central electrode was marked as F. Fig. 5.8b shows a
representative plot (shown in black circles) of drain current (I
ds
) versus gate voltage (V
g
)
106
minus Dirac point voltage (V
Dirac
) using D and F as source and drain electrodes. The drain
voltage (V
ds
) is 0.2 V.
We fitted the curve to retrieve the field effect mobility μ
FE
using the equation
C
FE
total
R
W
L
n n e
R +
+
=
2 2
0
1
μ
(5.1)
where R
total
= V
ds
/I
ds
is the total resistance of the device including the channel resistance and
contact resistance R
c
, e is the electron charge, L and W are the length and width of the
graphene channel, respectively, and n
0
and n are the carrier density due to residual
impurities and back-gate modulation, respectively.
117
The capacitive carrier density n is
related to the gate voltage via the equation
ox
Dirac BG
C
ne
V V = − (5.2)
where the term on the right-hand side of Equation 5.2 describe the carrier density induced
by the back gate via the back-gate electrostatic capacitance C
ox
. We did not include the
quantum capacitance in graphene on the right-hand side of Equation 5.2 since it was
negligible compared to the back-gate induced carrier density in our back-gated devices.
We fitted the back-gated device DF using this method, and the red curve shown in Fig. 5.8b
was the curve for fitted field effect mobility. Without a well-defined channel of the device,
we estimated the device dimension by using the region of graphene grain marked by the
dashed blue rectangular. The estimated L and W were 12 μm and 7.5 μm, and the fitted field
107
effect mobility was ~ 4,200 cm
2
V
-1
s
-1
. This value is comparable with the mobility of large
single crystal graphene reported recently,
60
indicating the high quality of the synthesized
graphene flowers. The inset of Fig. 5.8b is a plot of drain current (I
ds
) versus drain voltage
(V
ds
) at various gate voltages. The drain current increases linearly with the increase of drain
voltage at different gate voltages, indicating the Ohmic contact between graphene and Pd
electrodes.

5.8 Fabrication of Field Effect Transistors Using Large-Grain Grphene
Flowers On h-BN
The graphene devices on Si/SiO
2
substrates are highly disordered because of the
dangling bonds of SiO
2
and charge traps between graphene and SiO
2
. To further increase
device mobility, a better dielectric substrate is highly desirable. Hexagonal boron nitride
(h-BN) is an appealing substrate, because it has an atomically smooth surface that is
relatively free of dangling bonds and charge traps. It also has a lattice constant similar to
that of graphite, and has large optical phonon modes and a large electrical bandgap.
18

Large-grain graphene based hall-bar devices have been fabricated on exfoliated h-BN
(Fig. 5.9a). Fig. 5.9b shows a plot (shown in black circles) of drain current (I
ds
) versus gate
voltage (V
g
) of one of the graphene/h-BN devices, with a channel length of 13.5μm and
108
width of 4.5μm. We also fitted the curve with the same equation used in the back-gated
device on Si/SiO
2
(shown in blue curve in Fig. 5.9d). The extracted hole and electron
mobility is ~ 10,000 cm
2
V
-1
s
-1
and ~ 20,000 cm
2
V
-1
s
-1
, respectively. The previously
reported mobility at room temperature of small-grain CVD graphene/h-BN was ~ 1,200
cm
2
V
-1
s
-1
from Kim et al
47
and from 8,000 to 13,000 cm
2
V
-1
s
-1
from Gannett et al.
27
Our
large-grain graphene flower shows great potential for the application of high mobility
graphene-based nanoelectronics.
-20 -10 0 10 20 30 40
0.024
0.026
0.028
0.030
0.032
0.034
0.036
0.038

Gate Voltage (V)
Drain Current (mA)
5 μm
(a) (b)

Figure 5.9 (a) SEM image of a hall-bar graphene/h-BN FET. (b) Plot of drain current (I
ds
)
versus gate voltage (V
g
).

5.9 Conclusion
In summary, we developed a vapor trapping method to grow large-grain,
single-crystalline graphene with controlled grain morphology and grain size up to 100 μm.
109
Raman spectra indicate that the graphene flowers have high quality single-layer graphene
as lobes and bilayer graphene as centers. SAED confirms the single-crystalline nature of
graphene flowers. Systematic study of the graphene morphology versus growth parameters
and EBSD study indicate that the graphene morphology mostly relates to the local
environment around the growth area, and does not have much correlation with the
crystalline orientation of the underneath copper substrate. FETs have been fabricated based
on the large-grain graphene flowers, and high device mobility ~ 4,200 cm
2
V
-1
s
-1
on Si/SiO
2

and ~ 20,000 cm
2
V
-1
s
-1
on h-BN have been achieved, indicating that the large-grain
single-crystalline graphene is of great potential for graphene-based nanoelectronics.
Further effort toward location controlled growth of single-crystalline graphene flowers of
even larger grain size is currently underway.

110

Chapter 5 References

14. Chen, S. S.; Cai, W. W.; Piner, R.; Suk, J. W.; Wu, Y . P.; Ren, Y . J.; Kang, J. Y .; Ruoff,
R. S., Synthesis and Characterization of Large-Area Graphene and Graphite Films on
Commercial Cu-Ni Alloy Films. Nano Lett. 2011, ASAP.
17. De Arco, L. G.; Zhang, Y .; Kumar, A.; Zhou, C., Synthesis, Transfer, and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Trans.
Nanotechnol. 2009, 8, 135-138.
18. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.;
Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J., Boron Nitride Substrates for High-Quality
Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726.
27. Gannett, W.; Regan, W.; Watanabe, K.; Taniguchi, T.; Crommie, M. F.; Zettl, A.,
Boron Nitride Substrates for High Mobility Chemical Vapor Deposited Graphene. Appl
Phys Lett 2011, 98.
28. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
38. Huang, P. Y .; Ruiz-Vargas, C. S.; Van der Zande, A. M.; Whitney, W. S.; Levendorf, M.
P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y .; Park, J.; McEuen, P. L.;
Muller, D. A., Grains and Grain Boundaries in Single-Layer Graphene Atomic Patchwork
Quilts. Nature 2011, 469, 389.
47. Kim, E.; Yu, T. H.; Song, E. S.; Yu, B., Chemical Vapor Deposition-Assembled
Graphene Field-Effect Transistor on Hexagonal Boron Nitride. Appl Phys Lett 2011, 98.
50. Kim, K. S.; Zhao, Y .; Jang, H.; Lee, S. Y .; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.;
Choi, J. Y .; Hong, B. H., Large-Scale Pattern Growth of Graphene Films for Stretchable
Transparent Electrodes. Nature 2009, 457, 706-710.
57. Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni,
A.; Jung, I.; Tutuc, E., Large-Area Synthesis of High-Quality and Uniform Graphene Films
on Copper Foils. Science 2009, 324, 1312-1314.
59. Li, X. S.; Magnuson, C. W.; Venugopal, A.; An, J. H.; Suk, J. W.; Han, B. Y .; Borysiak,
M.; Cai, W. W.; Velamakanni, A.; Zhu, Y . W.; Fu, L. F.; V ogel, E. M.; V oelkl, E.; Colombo,
L.; Ruoff, R. S., Graphene Films with Large Domain Size by a Two-Step Chemical Vapor
Deposition Process. Nano Lett. 2010, 10, 4328-4334.
60. Li, X. S.; Magnuson, C. W.; Venugopal, A.; Tromp, R. M.; Hannon, J. B.; V ogel, E. M.;
Colombo, L.; Ruoff, R. S., Large-Area Graphene Single Crystals Grown by Low-Pressure
Chemical Vapor Deposition of Methane on Copper. J. Am. Chem. Soc 2011, 133,
111
2816-2819.
61. Liu, Z. F.; Yan, K.; Peng, H. L.; Zhou, Y .; Li, H., Formation of Bilayer Bernal
Graphene: Layer-by-Layer Epitaxy Via Chemical Vapor Deposition. Nano Lett. 2011, 11,
1106-1110.
65. Novoselov, K. S.; Geim, A. K.; Morozov, S. V .; Jiang, D.; Katsnelson, M. I.;
Grigorieva, I. V .; Dubonos, S. V .; Firsov, A. A., Two-Dimensional Gas of Massless Dirac
Fermions in Graphene. Nature 2005, 438, 197-200.
66. Novoselov, K. S.; Geim, A. K.; Morozov, S. V .; Jiang, D.; Zhang, Y .; Dubonos, S. V .;
Grigorieva, I. V .; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films.
Science 2004, 306, 666-669.
73. Rasool, H. I.; Song, E. B.; M., M.; Regan, B. C.; Wang, K. L.; Weiller, B. H.;
Gimzewski, J. K., Atomic-Scale Characterization of Graphene Grown on Copper (100)
Single Crystals. J. Am. Chem. Soc 2011, 133, 12536-12543.
75. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V .; Dresselhaus, M. S.;
Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical
Vapor Deposition. Nano Lett. 2009, 9, 30-35.
87. Shenoy, V . B.; Grantab, R.; Ruoff, R. S., Anomalous Strength Characteristics of Tilt
Grain Boundaries in Graphene. Science 2010, 330, 946-948.
97. Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S., Role of
Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene. ACS
Nano 2011, 5, 6069-6076.
103. Wofford, J. M.; Nie, S.; McCarty, K. F.; Bartelt, N. C.; Dubon, O. D., Graphene
Islands on Cu Foils: The Interplay between Shape, Orientation, and Defects. Nano Lett.
2010, 10, 4890-4896.
108. Yazyev, O. V .; Louie, S. G., Electronic Transport in Polycrystalline Graphene. Nat.
Mater. 2010, 9, 806-809.
110. Yu, Q. K.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J. F.; Su, Z. H.; Cao, H. L.; Liu, Z.
H.; Pandey, D.; Wei, D. G., Control and Characterization of Individual Grains and Grain
Boundaries in Graphene Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10,
443-449.
111. Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., Graphene Segregated
on Ni Surfaces and Transferred to Insulators. Appl. Phys. Lett. 2008, 93, 113103.
115. Zhang, Y .; Li, Z.; Kim, P.; Zhang, L.; Zhou, C. W., Anisotropic Hydrogen Etching of
Chemical Vapor Deposited Graphene. ACS Nano 2011, 6, 126-132.
116. Zhang, Y . B.; Tan, Y . W.; Stormer, H. L.; Kim, P., Experimental Observation of the
Quantum Hall Effect and Berry's Phase in Graphene. Nature 2005, 438, 201-204.
117. Zhang, Z. Y .; Wang, Z. X.; Xu, H. L.; Wang, S.; Ding, L.; Zeng, Q. S.; Yang, L. J.; Pei,
112
T. A.; Liang, X. L.; Gao, M.; Peng, L. M., Growth and Performance of Yttrium Oxide as an
Ideal High-K Gate Dielectric for Carbon-Based Electronics. Nano Lett. 2010, 10,
2024-2030.

113
Chapter 6: Anisotropic Hydrogen Etching of Chemical Vapor
Deposited Graphene

6.1 Introduction
Graphene, a two dimensional, honey comb arrangement of carbon atoms has drawn
significant attention with its interesting physical properties.
9, 28, 66
In terms of the
preparation of graphene, chemical vapor deposition (CVD) has raised its popularity as a
scalable and cost effective approach for graphene synthesis.
17, 46, 50, 53, 57, 75, 111, 114
Among
various CVD approaches, since the debut of CVD graphene synthesis using copper as a
substrate reported by Li et al,
57
the copper-catalyzed decomposition of methane to form
graphene has been studied from a variety of aspects.
6, 58, 60, 110
It is well known that
single-layer graphene (SLG) is formed on copper foil substrate by decomposition of
methane, resulting in the formation of graphene and hydrogen. The roles of various
parameters such as pressure
6
and hydrogen
97
in chemical vapor deposition of graphene
have been studied in some recent papers. One can imagine that the reverse reaction could
also occur by exposing graphene to hydrogen and would effectively result in etching.
However, the reverse reaction has not been systematically studied, such as the role of
114
temperature and copper substrate, even though etching using other processes such as gas
phase chemical etching
102
and hydrogen plasma etching
107, 113
have led to important
applications including formation of graphene nanoribbons.
In this chapter, we have studied anisotropic hydrogen etching of chemical vapor
deposited graphene as the reverse reaction of graphene growth. We observed that
continuous graphene could be etched into hexagonal openings by exposing CVD graphene
on copper foil to hydrogen flow at high temperatures. The etching is found to be
temperature-dependent, and 800 °C offers the most efficient and anisotropic etching. 80%
of the angles of graphene edges after etching at 800 °C are 120°.
We assigned the etched edges to < 0 2 11 > edges, which are the zigzag edges of
graphene. Moreover, we observed that copper played an important role in catalyzing the
etching reaction, as no etching was observed for graphene transferred to Si/SiO
2
substrate
under similar conditions. This highly anisotropic hydrogen etching technology may work
as a simple and convenient way to determine graphene grain size and crystal orientation,
and may enable the etching of graphene into nanoribbons with smooth and ordered zigzag
edges without compromising the quality of graphene.

6.2 Synthesis of Graphene Using Chemical Vapor Deposition
Our work started with the preparation of CVD graphene. SLG was grown on copper
115
foil using CVD similar to the process reported by Li et al
57
.
Graphene growth was done using copper foil (99.8%, Alpha Aesar) as a substrate.
Copper foil was loaded into a 2’’ fused quartz tube and heated to 1000 °C with the flow of
7 sccm H
2
at 40 mTorr. The Cu foil was then annealed at 1000 °C for 20 minutes. 7 sccm
CH
4
and 30 sccm H
2
were introduced to the CVD system at 500 mTorr for 30 minutes for
graphene growth. The temperature was decreased to 600 °C without changing flow rates
and pressure. CH
4
was then turned off at 600 °C, and the system was cooled down to room
temperature with the flow of 30 sccm H
2
at 500 mTorr. Anisotropic etching of graphene by
hydrogen was done in the same CVD chamber as graphene growth, with the flow of 30
sccm H
2
at 500 mTorr, and the etching was carried out for 30 minutes at 800 °C.

6.3 Anisotropic Etching of Graphene
After the synthesis of CVD graphene, anisotropic etching of graphene was done in the
same CVD chamber at 800 °C with the flow of hydrogen. As illustrated in Figure 6.1a, the
etching reaction is the reverse reaction of graphene growth, as hydrogen can react with
carbon in graphene and produce methane.
We observed that almost every etched hexagon had a particle in the center, as shown
in the SEM image in Figure 6.1b. The particles are probably come from the silicon dioxide
impurity in Cu foil, and initiated the etching reaction by breaking the carbon-carbon bond
116
of graphene in locations they were deposited.

Anisotropic etching
Cu
H
2
CH
4
Cu
2
(a)
(b) (c)
2 μm
120°
120°
120°
120°
1 μm

Figure 6.1 (a) Schematic diagram of Cu etching mechanism. (b) SEM image of graphene
after etching on copper foil and (c) transferred onto Si/SiO
2
substrate .

Energy-dispersive X-ray spectroscopy has been performed to test the composition of
the particles (Figure 6.2). Strong silicon and oxygen peaks from the spectrum taken on one
of the particles indicate that the particles are silicon dioxide. After small openings were
etched on graphene, copper substrate underneath would serve as catalyst for hydrogen to
react with carbon in graphene, and hence accelerate the etching reaction of graphene.

117
+
1 μm
Counts (a.u.)
Energy (keV)
+
1 μm
Counts (a.u.)
Energy (keV)
+
1 μm
Counts (a.u.)
Energy (keV)
+
1 μm
Counts (a.u.)
Energy (keV)
(a) (b)
(d) (c)

Figure 6.2 Energy-dispersive X-ray (EDX) spectra on different area of graphene sample
after etching. The red cross in each inset SEM image indicates the location where each
spectrum was taken. (a) An EDX spectrum taken on a particle inside an etched hexagon.
It shows strong silicon and oxygen peak. (b) An EDX spectrum taken inside an etched
hexagon but not on a particle. No silicon peak is shown. (c) An EDX spectrum taken on
another particle inside an etched hexagon. It also shows strong silicon and oxygen peak.
(d) An EDX spectrum taken on remaining graphene. No silicon peak is shown.

Interestingly, the catalytic etching reaction is highly anisotropic in our experimental
condition. As shown in the SEM image in Figure 6.1c taken on the graphene transferred to
a Si/SiO
2
substrate, the openings in graphene after etching turn out to be very regular
118
hexagonal shape, and edges with 120° angles can be found in many etched corners. While
we report hydrogen etching of graphene on copper substrate in this paper, similar reaction
may occur for graphene on other substrates such as nickel. Even though the SiO
2

nanoparticles in our current work are randomly located, controlled positioning of SiO
2

nanoparticles can be achieved by combining patterning and deposition of SiO
2
particles.
For instance, by using assembled nanospheres as shadow mask for SiO
2
deposition, one
should achieve SiO
2
particle deposition at desired locations.
80
Alternatively, our observed
hydrogen etching of graphene may also work after initiating openings in graphene via
patterning and oxygen plasma etching.
113

6.4 Raman Spectroscopy On Etched Graphene
We have conducted micro-Raman spectroscopy on both as-grown CVD graphene and
graphene after etching reaction. As-grown graphene was transferred onto 300 nm Si/SiO
2

substrate using the reported transfer technique.
17

As shown in the SEM image after transfer in Figure 6.3a, the graphene is continuous,
clean, and uniform over large area. Raman spectra were collected over the entire substrate
to determine the number of layers, as well as the quality of graphene. The inset of Figure
6.3a shows a typical Raman spectrum of the as-transferred graphene. The I
2D
/I
G
intensity
ratio is 2.2, the 2D peak has a full width at half maximum (FWHM) of ~ 33 cm
-1
, which
119
confirms the formation of SLG.
57
The absence of D peak indicates that the graphene after
transfer maintains high quality.

(a) (b)
(d) (e) (f)
1 μm
1200 1600 2000 2400 2800
Intensity (a. u.)

Raman Shift (cm
-1
)
1 μm
1200 1600 2000 2400 2800

Intensity (a. u.)
Raman Shift (cm
-1
)
0
20
40
60
80
100
150 120 90

Propotion of angles (%)
Angle (degrees)
60
800 °C
(c)

Figure 6.3 CVD graphene before and after etching. (a) as-grown CVD graphene
transferred onto Si/SiO
2
and a representative Raman spectrum as an inset; (b) graphene
etched by H
2
at 800 °C and transferred onto Si/SiO
2
. Raman spectra (inset) show the
intact graphene (pointed by red arrow) and etched region (pointed by blue arrow); (c)
Histogram of proportion of angles of graphene etched edges. (d)-(f). Raman map of I
G
(d)
(color scale bar: 100,200,500,800,>800 a.u.), I
2D
(e) (color scale bar:
300,900,1500,2200,>2200 a.u.), and I
D
(f) (color scale bar: 40,80,120,>120 a.u.). The
scale bar for d-f is 3 μm.

Figure 6.3b shows the SEM image of graphene etched at 800 °C after transfer. Raman
spectra were taken from the remaining graphene and one of the etched hexagons,
respectively. The red curve in the inset of Figure 6.3b represents the Raman spectrum of the
remaining graphene (pointed by the red arrow), which does not show the presence of D
120
peak, indicating the remaining graphene after etching has little defects.
35, 43
The blue curve
represents the Raman spectrum inside the etched hexagons (pointed by the blue arrow),
which almost does not show any G and 2D band intensity, indicating the removal of
graphene by anisotropic etching of hydrogen.
We also studied the orientation of the etching by measuring the angles of graphene
edges after etching. As shown in Figure 6.3c, 80% of the 139 measured angles were 120°,
indicating that the etching is along specific crystal orientation of graphene. It was reported
that the pitting and channeling by metal
3, 11, 15, 30-31
and SiO
2
77
nanoparticles tended to
happen along < 0 2 11 > direction, which corresponded to the zigzag edge of graphene.
Theoretical study also reveals that in the graphene gasification reaction, since the C-C
bond on the armchair face is the weakest one, it is more favorable to cleavage compared to
the C-C bond on the zigzag face.
15, 71
Therefore, we can assign the etching orientation to
zigzag face of graphene. From the result shown in Figure 6.3c, we can conclude that the
copper-assisted hydrogen etching is highly anisotropic and efficient, which etches
graphene along the zigzag direction.
We further investigated the surface of etched graphene by Raman surface mapping.
The step of Raman surface mapping was 1.5 μm, in both X and Y direction. Figure 6.3d-f
are Raman maps of I
G
, I
2D
, and I
D
, respectively. I
G
, I
2D
maps show the anisotropic behavior
of the etched patterns, which mostly have edges of 120°. The I
D
map in Figure 6.3f shows
121
that D peak intensity is uniform over the remaining graphene area.
1200 1600 2000 2400 2800

Intensity (a.u.)
Raman Shift (cm
-1
)
A
B
C
D
(a)
(b)
(c)
2 μm
A
C
B
D
120
160
200
240
280
320
360
400
D band intensity

Intensity (a.u.)
Location
A B C D

Figure 6.4 (a) SEM image of etched graphene. A, B, C, and D are selected locations
across anisotropic edges of etched graphene. (b) Corresponding Raman spectra taken
from location A to D. (d) D band intensity of point A to D.
122

Raman spectra taken from selected regions across etched edges are shown in Figure
6.4. Figure 6.4a shows a SEM image of etched graphene, with letter A to D indicating
locations inside graphene (A and D) and at the etched edges (B and C). Figure 6.4b shows
corresponding Raman spectra taken from location A to D. D band in each spectrum is low
no matter whether the spectrum is taken from the inside region of graphene (A and D), or
from the edge of remaining graphene (B and C). Figure 6.4c shows a plot of D band
intensity of spectra taken from A to D. We can tell that the D band intensity at edges of
remaining graphene does not increase compared to the inner region, indicating that the
edges are smooth and along the zigzag direction.
19, 97

6.5 Temperature Dependence of the Anisotropic Etching
Temperature always plays a significant role in chemical reactions. For a better
understanding of the etching process, we studied the influence of temperature in the
catalytically etching reaction. Four different temperatures were investigated in our
experiment. The SLG samples used in the etching experiments were prepared in the same
round of growth reaction, ensuring the uniformity and consistency of the starting SLG in
each etching reaction.
After etching at 700 °C for 30 minutes, the etching was observed to be mild and is
123
probably along the grain boundaries of graphene, therefore leading to graphene islands
shown in Figure 6.5a and b using different magnifications. Only a small amount of etched
hexagons are observed inside the graphene islands after etching at 700 °C for 30 minutes.
When 800 °C was used for etching, more area of graphene was etched away, and the etched
patterns were mostly hexagonal as seen in Figure 6.5c and d. We could also observe that
the edges of etched graphene were mostly 120° in Figure 6.5d.
When 900 °C was used for etching, we observed a decrease of the etched area of
graphene, as well as less anisotropic etched patterns. When the temperature was further
brought up to 1000 °C, the percentage of etched graphene became even lower. The edge of
the etched patterns turned to be round, indicating the decrease of anisotropic property of
the etching. We did not observe any etching effect when the temperature decreased to
600 °C, and it can be understood as the etching reaction will only happen when the
temperature reaches certain degrees to overcome the activation energy of the breaking of
carbon-carbon bonds. On the other hand, since the etching reaction is exothermic, the
reaction is unfavorable if the reaction temperature is very high. That is why we observed
the weakening of etching effect when the temperature increased to 900 °C and 1000 °C.

124
10 μm
3 μm
10 μm
3 μm
10 μm
3 μm
(a)
(c)
(e)
(b)
(d)
(f)
10 μm 3 μm
(g) (h)

Figure 6.5 SEM images at different magnifications of graphene etched at different
temperatures and transferred onto Si/SiO
2
substrate. (a) and (b). 700 °C; (c) and (d).
800 °C; (e) and (f). 900 °C; (g) and (h). 1000 °C.

125
The decrease of anisotropic property of etching can also be attributed to the increase
of temperature. Based on the theoretical calculation of hydrogen addition to zigzag and
armchair edges,
71
the cleavage of C-C bond from armchair edge is much easier than from
zigzag edge. Therefore, when temperature is relatively low, the selectivity between etching
from zigzag edge and etching from armchair edge is high since the activation energy of
breaking C-C bond from armchair edge is much lower. When temperature is high enough
to overcome the energy barrier of breaking C-C bond from zigzag edge, the anisotropic
behavior of the etching process is much weakened. This explains why we observed that the
anisotropic behavior became less pronounced when temperature was enhanced from
800 °C to 900 °C and then 1000 °C.
We further calculated the percentage of etched area of graphene at different
temperatures. Five different regions from the samples were randomly picked for each
temperature for the calculation of the etched area, and the size of each region was 40 μm ×
30 μm. According to the histograms in Figure 6.6a-d, we could tell that the percentage of
etched area of graphene increased from 700 °C to 800 °C, but then dropped from 800 °C to
1000 °C. In Figure 6.6e, we calculated the mean values of the etched area of graphene at
different temperatures, and it could be clearly seen that the percentage of etched area
increased from 25% at 700 °C to 58% at 800 °C, then dropped to 49% at 900 °C, and
further decreased to 30% at 1000 °C.
126

1 2 3 4 5
0
20
40
60
80
100

Etched area (%)
Region
1 2 3 4 5
0
20
40
60
80
100

Etched area (%)
Region
1 2 3 4 5
0
20
40
60
80
100

Etched area (%)
Region
700 °C 800 °C 900 °C
(a) (c) (b)
(d)
1 2 3 4 5
0
20
40
60
80
100

Etched area (%)
Region
1000 °C
(e)
700 800 900 1000
0
20
40
60
80
Temperature ( °C )

Etched area (%)

Figure 6.6 Percentage of graphene etched area at different temperatures. (a). 700 °C; (b).
800 °C; (c). 900 °C; (d). 1000 °C. Five regions were randomly picked for the calculation
of etched area, each region is 40 μm × 30 μm. (e) Etched area versus temperature plot.

6.6 Catalytic Effect of the Underlying Cu Substrate on Anisotropic
Etching of Graphene
In order to confirm the catalytic function of copper substrate, further experiment was
carried out for comparison. We loaded our transferred graphene sample into the same CVD
system, and the sample was kept at 800 °C for 30 minutes with the flow of 30 sccm H
2
. The
system pressure was maintained at 500 mTorr during the process.
127
Figure 6.7a is the SEM image of as-grown CVD graphene after transfer, which is
continuous and uniform over large area. A typical Raman spectrum of the transferred
graphene is shown in Figure 6.7b, which is almost identical to the Raman spectrum in the
inset of Figure 6.3a, indicating SLG with very low defect density. After annealing in
hydrogen, we did not observe any obvious anisotropic etched patterns on graphene. The
white lines in Figure 6.7c should be due to the formation of wrinkles in graphene due to the
thermal cycling.
1200 1600 2000 2400 2800

Intensity (a.u.)
Raman shift (cm
-1
)
1200 1600 2000 2400 2800

Intensity (a.u.)
Raman shift (cm
-1
)
2 μm
(a) (b)
(c) (d)
2 μm

Figure 6.7 (a) SEM image of CVD graphene transferred onto Si/SiO
2
substrate; (b) A
representative Raman spectrum of SLG; (c) SEM image of transferred CVD graphene
after H
2
annealing at 800 °C; (d) A representative Raman spectrum of CVD graphene
after H
2
annealing.
128

Raman spectrum in Figure 6.7d was taken from the sample after hydrogen annealing,
which showed a strong D peak ~ 1350 cm
-1
, which can be attributed to disorder in
graphene.
19
Also, the Raman spectra showed broadening of G and 2D peaks, as well as a
decrease of intensity of the 2D peak, indicating the graphene has become more defective
after annealing in hydrogen. Altogether, we can conclude that the etching reaction is
catalyzed by copper and the anisotropic etching would not happen if there is no copper
participating in the reaction.
The anisotropic property of the catalytic etching can be applied to determine the
crystalline orientation of CVD graphene and to estimate the grain size of graphene. If the
hexagonal etched patterns are parallel to each other within certain area, one can conclude
that the area should be within a single graphene grain.
We can use the SEM image in Figure 6.1c as an example. The hexagonal patterns are
well aligned with each other over the inspected area, and hence over ~ 6 μm × 2 μm area
should be one single graphene grain. In comparison, in Figure 6.3b the bottom left etched
pattern is not parallel to the middle three hexagonal etched patterns, which implies that the
bottom left part of the graphene may have a different crystalline orientation from the
graphene grain containing the middle three etched hexagons. This simple etching method
provides a useful tool to estimate the grain size of graphene, as compared to more
129
sophisticated technologies such as scanning tunneling microscopy (STM),
110
transmission
electron microscopy (TEM),
38, 49
and low energy electron microscopy (LEEM).
60, 103

Studying the etching reaction also helps to better understand and control the graphene
growth condition. We can also observe the hexagonal etched patterns on the as-grown
graphene if we turn methane off after 30 minute of graphene growth, with only hydrogen
flowing in the CVD system during cooling down process. This phenomenon indicates that
the graphene growth and hydrogen etching are competitive with each other. The graphene
growth reaction is endothermic and will only happen and reach to equilibrium when the
temperature increases to certain degrees. During the cooling down process, the former
equilibrium breaks and the reaction shifts from graphene growth to hydrogen etching at a
certain temperature, since the etching reaction is exothermic and will be more favorable
when temperature decreases. Our observation suggests that one should keep methane
flowing during the cooling process to suppress hydrogen etching in order to grow
continuous CVD graphene films.

6.7 Conclusion
In summary, we have developed an anisotropic hydrogen etching method of graphene
catalyzed by copper substrate. The anisotropic etching is the reverse reaction of CVD
graphene growth, and it is simple, clean, and highly efficient at 800 °C and 500 mTorr with
130
hydrogen gas flow. 80% of the 139 angles measured at graphene edges are confirmed to be
along < 0 2 11 > zigzag direction.
Moreover, we observed that copper played an important role in catalyzing the etching
reaction, as no etching was observed for graphene transferred to Si/SiO
2
under similar
conditions. This highly anisotropic hydrogen etching technology may work as a simple and
convenient way to determine graphene grain size and crystal orientation, and may enable
the etching of graphene into nanoribbons for electronic applications.

131
Chapter 6 References

3. Baker, R. T. K.; Chludzinski, J. J., Catalytic Gasification of Graphite by Chromium
and Copper in Oxygen, Steam and Hydrogen. Carbon 1981, 19, 75-82.
6. Bhaviripudi, S.; Jia, X. T.; Dresselhaus, M. S.; Kong, J., Role of Kinetic Factors in
Chemical Vapor Deposition Synthesis of Uniform Large Area Graphene Using Copper
Catalyst. Nano Lett. 2010, 10, 4128-4133.
9. Bolotin, K. I.; Ghahari, F.; Shulman, M. D.; Stormer, H. L.; Kim, P., Observation of the
Fractional Quantum Hall Effect in Graphene. Nature 2009, 462, 196-199.
11. Campos, L. C.; Manfrinato, V . R.; Sanchez-Yamagishi, J. D.; Kong, J.; Jarillo-Herrero,
P., Anisotropic Etching and Nanoribbon Formation in Single-Layer Graphene. Nano Lett.
2009, 9, 2600-2604.
15. Ci, L.; Xu, Z. P.; Wang, L. L.; Gao, W.; Ding, F.; Kelly, K. F.; Yakobson, B. I.; Ajayan,
P. M., Controlled Nanocutting of Graphene. Nano Res. 2008, 1, 116-122.
17. De Arco, L. G.; Zhang, Y .; Kumar, A.; Zhou, C., Synthesis, Transfer, and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Trans.
Nanotechnol. 2009, 8, 135-138.
19. Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R., Perspectives
on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751-758.
28. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
30. Goethel, P. J.; Yang, R. T., Mechanism of Graphite Hydrogenation Catalyzed by
Nickel. J. Catal. 1987, 108, 356-363.
31. Goethel, P. J.; Yang, R. T., The Tunneling Action of Group-Viii Metal Particles in
Catalyzed Graphite Hydrogenation. J. Catal. 1988, 114, 46-52.
35. Gupta, A. K.; Russin, T. J.; Gutierrez, H. R.; Eklund, P. C., Probing Graphene Edges
Via Raman Scattering. ACS Nano 2009, 3, 45-52.
38. Huang, P. Y .; Ruiz-Vargas, C. S.; Van der Zande, A. M.; Whitney, W. S.; Levendorf, M.
P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y .; Park, J.; McEuen, P. L.;
Muller, D. A., Grains and Grain Boundaries in Single-Layer Graphene Atomic Patchwork
Quilts. Nature 2011, 469, 389.
43. Jiao, L. Y .; Wang, X. R.; Diankov, G.; Wang, H. L.; Dai, H. J., Facile Synthesis of
High-Quality Graphene Nanoribbons. Nat. Nanotechnol. 2010, 5, 321-325.
46. Karu, A. E.; Beer, M., Pyrolytic Formation of Highly Crystalline Graphite Films. J.
Appl. Phys. 1966, 37, 2179.
49. Kim, K.; Lee, Z.; Regan, W.; Kisielowski, C.; Crommie, M. F.; Zettl, A., Grain
Boundary Mapping in Polycrystalline Graphene. ACS Nano 2011, 5, 2142-2146.
132
50. Kim, K. S.; Zhao, Y .; Jang, H.; Lee, S. Y .; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.;
Choi, J. Y .; Hong, B. H., Large-Scale Pattern Growth of Graphene Films for Stretchable
Transparent Electrodes. Nature 2009, 457, 706-710.
53. Levendorf, M. P.; Ruiz-Vargas, C. S.; Garg, S.; Park, J., Transfer-Free Batch
Fabrication of Single Layer Graphene Transistors. Nano Lett. 2009, 9, 4479-4483.
57. Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni,
A.; Jung, I.; Tutuc, E., Large-Area Synthesis of High-Quality and Uniform Graphene Films
on Copper Foils. Science 2009, 324, 1312-1314.
58. Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S., Evolution of Graphene Growth on Ni
and Cu by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268-4272.
60. Li, X. S.; Magnuson, C. W.; Venugopal, A.; Tromp, R. M.; Hannon, J. B.; V ogel, E. M.;
Colombo, L.; Ruoff, R. S., Large-Area Graphene Single Crystals Grown by Low-Pressure
Chemical Vapor Deposition of Methane on Copper. J. Am. Chem. Soc 2011, 133,
2816-2819.
66. Novoselov, K. S.; Geim, A. K.; Morozov, S. V .; Jiang, D.; Zhang, Y .; Dubonos, S. V .;
Grigorieva, I. V .; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films.
Science 2004, 306, 666-669.
71. Pan, Z. J.; Yang, R. T., The Mechanism of Methane Formation from the Reaction
between Graphite and Hydrogen. J. Catal. 1990, 123, 206-214.
75. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V .; Dresselhaus, M. S.;
Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical
Vapor Deposition. Nano Lett. 2009, 9, 30-35.
77. Ren, W. C.; Gao, L. B.; Liu, B. L.; Wu, Z. S.; Jiang, C. B.; Cheng, H. M.,
Crystallographic Tailoring of Graphene by Nonmetal Sio(X) Nanoparticles. J. Am. Chem.
Soc 2009, 131, 13934.
80. Ryu, K.; Badmaev, A.; Gomez, L.; Ishikawa, F.; Lei, B.; Zhou, C. W., Synthesis of
Aligned Single-Walled Nanotubes Using Catalysts Defined by Nanosphere Lithography. J.
Am. Chem. Soc 2007, 129, 10104.
97. Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S., Role of
Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene. ACS
Nano 2011, 5, 6069-6076.
102. Wang, X. R.; Dai, H. J., Etching and Narrowing of Graphene from the Edges. Nat.
Chem. 2010, 2, 661-665.
103. Wofford, J. M.; Nie, S.; McCarty, K. F.; Bartelt, N. C.; Dubon, O. D., Graphene
Islands on Cu Foils: The Interplay between Shape, Orientation, and Defects. Nano Lett.
2010, 10, 4890-4896.
107. Yang, R.; Zhang, L. C.; Wang, Y .; Shi, Z. W.; Shi, D. X.; Gao, H. J.; Wang, E. G.;
Zhang, G. Y ., An Anisotropic Etching Effect in the Graphene Basal Plane. Adv. Mater. 2010,
22, 4014-4019.
133
110. Yu, Q. K.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J. F.; Su, Z. H.; Cao, H. L.; Liu, Z.
H.; Pandey, D.; Wei, D. G., Control and Characterization of Individual Grains and Grain
Boundaries in Graphene Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10,
443-449.
111. Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., Graphene Segregated
on Ni Surfaces and Transferred to Insulators. Appl. Phys. Lett. 2008, 93, 113103.
113. Zhang, G. Y .; Shi, Z. W.; Yang, R.; Zhang, L. C.; Wang, Y .; Liu, D. H.; Shi, D. X.;
Wang, E. G., Patterning Graphene with Zigzag Edges by Self-Aligned Anisotropic Etching.
Adv. Mater. 2011, 23, 3061.
114. Zhang, Y .; Gomez, L.; Ishikawa, F. N.; Madaria, A.; Ryu, K.; Wang, C. A.; Badmaev,
A.; Zhou, C. W., Comparison of Graphene Growth on Single-Crystalline and
Polycrystalline Ni by Chemical Vapor Deposition. J. Phys. Chem. Lett. 2010, 1,
3101-3107.

134
Chapter 7: Conclusions and Future Directions

7.1 Conclusions
As a conclusion, this dissertation discusses the synthesis of graphene using chemical
vapor deposition and its various applications. To overcome the challenge of graphene
synthesis in large scale, I have developed the chemical vapor deposition method for
wafer-scale graphene synthesis. An efficient transfer technique has also been developed to
transfer graphene from metal films to Si/SiO
2
, glass, PET, and other target substrates for
potential applications. This as-synthesized graphene films have also been applied as anode
materials for organic photovoltaic cells, which demonstrate excellent device performance
as ITO OPV cells. Furthermore, the graphene OPV cells overbeat ITO OPV cells under
bending condition due to its outstanding flexibility. To increase the percentage of
single-layer graphene, I have explored single crystalline Ni (111) for CVD graphene
synthesis. Owe to its surface smoothness and lack of grain boundaries, Ni (111) serves as a
better substrate for single-layer graphene synthesis than polycrystalline Ni. To overcome
the grain size limit of graphene, I have developed a vapor trapping method for large-grain
graphene synthesis using CVD. Field effect transistors have been fabricated using the
135
large-grain graphene flowers on both Si/SiO
2
and h-BN substrate and high device mobility
has been achieved. An anisotropic etching method of graphene using hydrogen has also
been developed, which is simple, clean, and left with smooth edges of graphene after
etching.
In this dissertation, chapter 2, 3, and 4 focus on graphene on Ni, which discuss the
wafer-scale synthesis of graphene, the transfer technique, the application of graphene for
OPV cells, as well as the comparison between Ni (111) and polycrystalline Ni. Chapter 5
and 6 focuses on graphene on Cu, which discuss large-grain graphene synthesis and
electrical measurements, and the anisotropic etching of graphene using hydrogen.
This dissertation starts from CVD graphene synthesis and fulfills with various
applications using as-synthesized graphene material, which proves the potential of CVD
graphene for device application, OPV cells, and other possible applications. With the
continuous improvement of graphene quality, as well as the promising application results
shown in this dissertation, we should expect many more applications that exploit all kinds
of unique properties of graphene in near future.

136
7.2 Future Directions in CVD Graphene
7.2.1 Seeded Growth of Large Grain Graphene
In chapter 5 we discussed the synthesis of large-grain single-crystalline graphene
flowers. The grain size of graphene increased from several micrometers to 100
micrometers which largely improved the device performance. Nevertheless, it is still not
very convenient for large-area device fabrication since the locations of large-grain
graphene flowers are random, which means we cannot control the nucleation of graphene
flowers.
Here we propose a method to use e-beam lithography to pattern PMMA seeds on Cu
foil, which serve as nucleation centers as well as provide carbon source for graphene
growth. We believe that the seeds will act like defect points on Cu foil, which may
facilitate the nucleation of graphene. Since PMMA is a carbon source, graphene may grow
easier from the PMMA seeds than any other locations, hence realize the control of
nucleation of graphene. Figure 7.1a shows patterned PMMA seeds on Cu foil. The distance
between each seed is 20 μm and the size of seeds is 0.8 μm.
137
20 μm
(a)
40 μm
(b)
100 μm
(c)

Figure 7.1 SEM image of patterned PMMA seeds on Cu foil (a), graphene flower arrays
after CVD seeded growth (b), the edge between patterned region and non-patterned
region after graphene growth (c).
138

The patterned Cu foil was then loaded into a vapor-trapping tube in CVD system for
growth. 25 sccm H
2
was introduced to the system for 40 minutes while the temperature was
brought up to 1000 °C. The sample was then annealed for 20 minutes at 1000 °C and 2
sccm of CH
4
was introduced to the system for graphene growth. The growth time was 20
minutes and the system pressure was 190 mTorr. The temperature was ramped down to
room temperature while keeping both CH
4
and H
2
flowing in the system.
Figure 7.1b is a SEM image of graphene flower arrays after CVD growth on the
patterned seeds. We can see that graphene flowers nucleated from seeds and form uniform
array. Only several locations between seed have graphene nucleated, which may due to
defects on Cu foil. The SEM image (Figure 7.1c) taken from the edge between patterned
area and non-patterned area clearly shows the difference. The region with patterned seeds
shows controlled graphene growth while the non-patterned region shows graphene flowers
without any order.
The preliminary results of seeded growth indicate that pre-patterned seeds are
effective for the controlling of graphene nucleation hence control the growth location of
graphene. The distance between seeds, the e-beam lithography recipe, and the growth
recipe still need to be fine-tuned to grow graphene flowers with large grain size and more
uniform morphology.

139
7.2.2 Molecular Beam Assembly of Graphene Nanoribbons
Graphene is a truly remarkable two-dimensional material with fascinating properties;
however, the lack of a bandgap has hindered graphene from many electronic applications.
To render a bandgap for room temperature operation, reliable production of graphene
nanoribbons (GNRs) with width ~ 1-2 nm and atomically smooth edges would be of
paramount importance, and may lead to opportunities for both fundamental research and
important applications such as flexible electronics.
In collaboration with Dr. Colin Nuckolls in Columbia University and Dr. Mark
Thompson in USC, we are following general scheme shown in Figure 7.2 to create
graphene nanoribbons. It leverages Dr. Nuckolls’ recent advances at developing
ring-opening alkyne metathesis polymerization
25, 85
in scheme a. This allows us to create
living polymers that have controlled endgroups using solution-based chemistry. The
cascade cyclization would be highly possible happen on Au (111) surface and the
atomically precise ribbons would form after dehydrogenation.
Scheme b shows 10,10’-dibromo-9,9’-bianthryl as precursor monomers for GNR with
armchair edges as previously reported by Cai et al.
10
The precursor monomers were
synthesized by Dr. Thompson’s group.
We sublimed the precursor monomers from a high vacuum evaporator onto receiving
substrate. Single crystal Au (111) on mica was used as receiving substrate, and was heated
constantly for precursor dehalogenation. After depositing precursor molecules, the
140
substrate was raised to 400 °C for self-assembled dehydrogenation and form GNRs with
zigzag edges.
We will design more precursor monomers for zigzag edges and sawtooth-like edges
which can be used for the study of electrical properties of differentGNRs.

[Mask]
1. ROMP
2. Unmask
Cascade
cyclization
(b)
(a)

Figure 7.2 (a) Scheme for the synthesis of graphene ribbons using ring-opening alkyne
metathesis polymerization followed by a cascade cyclization. (b) Scheme for the
synthesis of armchair GNRs.

7.2.3 Solution Based Graphene Nanoribbons
In collaboration with Dr. Klaus Müllen in Max Planck Institute for Polymer Research,
we are working on solution-based GNRs synthesized using Diels-Alder reaction shown in
Figure 7.3 by Dr. Müllen’s group.
141

Figure 7.3 Synthesis approach for GNRs in solution

These organic molecules assembled ribbons have estimated average length of 260 nm
to 500 nm. The nanoribbons are dispersed in THF and Chlorobenzene for further
characterizations. The estimated height and width of the nanoribbons is ~ 1 nm and 0.7 nm
respectively.
AFM image in Figure 7.4a shows GNRs dispersed on Si/SiO
2
substrate. Ribbons are
pointed out using red arrows. The measured height of ribbons is ~0.8 nm and the length is ~
1 μm. Raman spectra taken on both aggregates (black curve) and uniform thin film (red
curve) on spin-coated GNR sample are shown in Figure 7.4b. We can see G band (1593
cm
-1
) and 2D band (2741 cm
-1
) which are two key features of graphene structures. The D
band (1319 cm
-1
) which represents point defects has a relatively large intensity because the
edges of GNRs synthesized using this method are occupied by methyl groups.
142
1 μm
1000 1500 2000 2500 3000 3500

Raman Shift (cm
-1
)
Intensity (a.u.)
400 500 600 700 800 900 1000 1100
0.0
0.5
1.0
1.5
2.0
2.5

Absorption (a.u.)
Wavelength (nm)
GNR in THF
GNR in Chlorobenzene
(a)
(b)
(c)

Figure 7.4 (a) AFM image of spin-coated GNRs on Si/SiO
2
substrate. (b) Raman spectra
taken from aggregates and uniform thin film from spin-coated GNR on Si/SiO
2
substrate.
(c) UV-Vis spectra taken from GNR in THF (black) and GNR in chlorobenzene (red)
respectively.
143

The relatively narrow absorption band at 570 nm in the UV-Vis absorption spectra in
Figure 7.4c means the good conjugation and homogeneity of the sample. If the final
reaction is not complete and there are partially fused species, the spectrum becomes much
broader and unstructured.
The TEM characterization of GNRs is still ongoing and we are also fabricating GNR
based devices using dielectrophoresis in order to study the electrical properties of GNRs.

7.2.4 Helium Lithography for Graphene Nanoribbon Patterning and Electrical
Measurement
In comparison with the bottom-up methods for GNR synthesis mentioned in chapter
7.2.2 and 7.2.3, we are also working on a top-down fabrication method for atomically
précised GNRs in collaboration with Dr. Wei Wu in USC.
We have recently used helium lithography to pattern CVD graphene into 4 nm GNRs
(shown in Figure 7.5) by Joul heating. The ribbons are continuous and uniform through the
patterned area. This method may lead to graphene nanoribbons with atomically smooth
edges.
81
This approach is also promising to be scaled to complete wafers by using
nanoimprint lithography, which may realize large-scale graphene device integration.
What we also take into consideration is the orientation of GNRs. In order to compare
144
the GNR devices with different type of ribbons (zigzag and armchair), we need to find a
method to determine the direction of patterned GNRs.

Figure 7.5 SEM image of a GNR array patterned by helium lithography with 4 nm
half-pitch.

A new recipe for the growth of graphene hexagons has been developed since
hexagonal graphene have their edges along to zigzag direction.
107
A 25 μm thick Cu foil
was loaded into a 2 inch quartz tube in CVD system for growth. 25 sccm H
2
was introduced
to the system for 40 minutes while the temperature was brought up to 1000 °C. The sample
was then annealed for 20 minutes at 1000 °C and 0.1 sccm of CH
4
was introduced to the
system for graphene growth. The growth time was 15 minutes and the system pressure was
145
35 Torr. The temperature was ramped down to room temperature while keeping both CH
4

and H
2
flowing in the system. Figure 7.6 a shows a SEM image of as-grown graphene
hexagons.
50 μm
20 μm
10 μm
(a)
(b)
(c)

Figure 7.6 SEM image of as-grown graphene hexagons on Cu foil (a), patterned graphene
hexagon devices (b), and a zoomed-in SEM image of hexagon graphene devices (c).
146

We selected graphene grains in good hexagonal shape for device fabrication, with
electrodes along the hexagon edges and perpendicular to the edges to fabricate ribbons
with armchair edges and zigzag edges respectively. Figure 7.6b shows patterned graphene
hexagon devices and Figure 7.6c shows a zoomed-in SEM image of four electrodes
perpendicular to hexagon edges.
We will then perform the helium lithography to pattern 4 nm GNRs between
electrodes. Electrical measurements of GNR devices will be performed for the study of
electrical properties for both zigzag and armchair ribbons.

147

Chapter 7 References

10. Cai, J. M.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.;
Seitsonen, A. P.; Saleh, M.; Feng, X. L.; Mullen, K.; Fasel, R., Atomically Precise
Bottom-up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470-473.
25. Fischer, F. R.; Nuckolls, C., Design of Living Ring-Opening Alkyne Metathesis. Ang.
Chem. Int. Ed. 2010, 49, 7257-7260.
81. Safron, N., Barrier-Guided Growth of Micro- and Nano-Structured Graphene. APS
2012 March Meeting Abstract 2012.
85. Sedbook, D. F., Bidentate Phenoxides as Ideal Activating Ligands for Living Ring
Opening Alkyne Metathesis Polymerization. Submitted to JACS 2012.
107. Yang, R.; Zhang, L. C.; Wang, Y.; Shi, Z. W.; Shi, D. X.; Gao, H. J.; Wang, E. G.;
Zhang, G. Y., An Anisotropic Etching Effect in the Graphene Basal Plane. Adv. Mater.
2010, 22, 4014-4019.

148

Bibliography

1. Andersson, A.; Johansson, N.; Bröms, P.; Yu, N.; Lupo, D.; Salaneck, W. R., Fluorine
Tin Oxide as an Alternative to Indium Tin Oxide in Polymer Leds. Advanced Materials
1998, 10, 859-863.
2. Bai, J. W.; Zhong, X.; Jiang, S.; Huang, Y.; Duan, X. F., Graphene Nanomesh. Nat
Nanotechnol 2010, 5, 190-194.
3. Baker, R. T. K.; Chludzinski, J. J., Catalytic Gasification of Graphite by Chromium
and Copper in Oxygen, Steam and Hydrogen. Carbon 1981, 19, 75-82.
4. Banwell, T. C.; Jayakumar, A., Exact Analytical Solution for Current Flow through
Diode with Series Resistance. Electronics Letters 2000, 36, 291-292.
5. Barnes, T. M.; X. Wu, J. Z.; Duda, A.; Lagemaat, J. v. d.; Coutts, T. J.; Weeks, C. L.;
Britz, D. A.; Glatkowski, P., Single-Wall Carbon Nanotube Networks as a Transparent
Back Contact in Cdte Solar Cells. Applied Physics Letters 2007, 90, 243503.1-243503.3.
6. Bhaviripudi, S.; Jia, X. T.; Dresselhaus, M. S.; Kong, J., Role of Kinetic Factors in
Chemical Vapor Deposition Synthesis of Uniform Large Area Graphene Using Copper
Catalyst. Nano Lett. 2010, 10, 4128-4133.
7. Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.;
Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W.; Geim, A. K.; Novoselov,
K. S., Graphene-Based Liquid Crystal Device. Nano Letters 2008, 8, 1704-1708.
8. Boehme, M.; Charton, C., Properties of Ito on Pet Film in Dependence on the Coating
Conditions and Thermal Processing. Surface & Coatings Technology 2005, 200 932- 935.
9. Bolotin, K. I.; Ghahari, F.; Shulman, M. D.; Stormer, H. L.; Kim, P., Observation of
the Fractional Quantum Hall Effect in Graphene. Nature 2009, 462, 196-199.
10. Cai, J. M.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.;
Seitsonen, A. P.; Saleh, M.; Feng, X. L.; Mullen, K.; Fasel, R., Atomically Precise
Bottom-up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470-473.
11. Campos, L. C.; Manfrinato, V. R.; Sanchez-Yamagishi, J. D.; Kong, J.; Jarillo-Herrero,
P., Anisotropic Etching and Nanoribbon Formation in Single-Layer Graphene. Nano Lett.
2009, 9, 2600-2604.

149
12. Cancado, L. G.; Reina, A.; Kong, J.; Dresselhaus, M. S., Geometrical Approach for the
Study of G ' Band in the Raman Spectrum of Monolayer Graphene, Bilayer Graphene, and
Bulk Graphite. Physical Review B 2008, 77.
13. Chen, C. Y.; Rosenblatt, S.; Bolotin, K. I.; Kalb, W.; Kim, P.; Kymissis, I.; Stormer, H.
L.; Heinz, T. F.; Hone, J., Performance of Monolayer Graphene Nanomechanical
Resonators with Electrical Readout. Nat Nanotechnol 2009, 4, 861-867.
14. Chen, S. S.; Cai, W. W.; Piner, R.; Suk, J. W.; Wu, Y. P.; Ren, Y. J.; Kang, J. Y.; Ruoff,
R. S., Synthesis and Characterization of Large-Area Graphene and Graphite Films on
Commercial Cu-Ni Alloy Films. Nano Lett. 2011, ASAP.
15. Ci, L.; Xu, Z. P.; Wang, L. L.; Gao, W.; Ding, F.; Kelly, K. F.; Yakobson, B. I.; Ajayan,
P. M., Controlled Nanocutting of Graphene. Nano Res. 2008, 1, 116-122.
16. Coraux, J.; N'Diaye, A. T.; Engler, M.; Busse, C.; Wall, D.; Buckanie, N.; Heringdorf,
F.-J. M. z.; van Gastel, R.; Poelsema, B.; Michely, T., Growth of Graphene on Ir(111) New
J. Phys. 2009, 11.
17. De Arco, L. G.; Zhang, Y.; Kumar, A.; Zhou, C., Synthesis, Transfer, and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Trans.
Nanotechnol. 2009, 8, 135-138.
18. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.;
Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J., Boron Nitride Substrates for
High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726.
19. Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R., Perspectives
on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751-758.
20. Echtermeyer, T. J.; Lemme, M. C.; Bolten, J.; Baus, M.; Ramsteiner, M.; Kurz, H.,
Graphene Field-Effect Devices. The European Physical Journal - Special Topics 2007,
148, 19-26.
21. Eda, G.; Fanchini, G.; Chhowalla, M., Large-Area Ultrathin Films of Reduced
Graphene Oxide as a Transparent and Flexible Electronic Material. Nat Nanotechnol 2008,
3, 270-274.
22. Eda, G.; Lin, Y.-Y.; Miller, S.; Chen, C.-W.; Su, W.-F.; Chhowalla, M., Transparent
and Conducting Electrodes for Organic Electronics from Reduced Graphene Oxide.
Applied Physics Letters 2008, 92, 233305.1-233305.3.
23. Eizenberg, M.; Blakely, J. M., Carbon Monolayer Phase Condensation on Ni(111).
Surf. Sci. 1979, 82, 228-236.
24. Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.;
Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K., Raman Spectrum of
Graphene and Graphene Layers. Physical Review Letters 2006, 97, 187401.
25. Fischer, F. R.; Nuckolls, C., Design of Living Ring-Opening Alkyne Metathesis. Ang.
Chem. Int. Ed. 2010, 49, 7257-7260.

150
26. Forbeaux, I.; Themlin, J. M.; Debever, J. M., Heteroepitaxial Graphite on
6h-Sic(0001): Interface Formation through Conduction-Band Electronic Structure. Phys
Rev B 1998, 58, 16396-16406.
27. Gannett, W.; Regan, W.; Watanabe, K.; Taniguchi, T.; Crommie, M. F.; Zettl, A.,
Boron Nitride Substrates for High Mobility Chemical Vapor Deposited Graphene. Appl
Phys Lett 2011, 98.
28. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
29. Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B., A Chemical Route to
Graphene for Device Applications. Nano Letters 2007, 7, 3394-3398.
30. Goethel, P. J.; Yang, R. T., Mechanism of Graphite Hydrogenation Catalyzed by
Nickel. J. Catal. 1987, 108, 356-363.
31. Goethel, P. J.; Yang, R. T., The Tunneling Action of Group-Viii Metal Particles in
Catalyzed Graphite Hydrogenation. J. Catal. 1988, 114, 46-52.
32. Gomez, L.; Zhang, Y.; Kumar, A.; Zhou, C., Synthesis, Transfer and Devices of
Single- and Few-Layer Graphene by Chemical Vapor Deposition. IEEE Transactions on
Nanotechnology 2009, 8, 135-138.
33. Green, A. A. H., M. C., Emerging Methods for Producing Monodisperse Graphene
Dispersions. Journal of Physical Chemistry Letters 2010, 1, 544-549.
34. Gupta, A.; Chen, G.; Joshi, P.; Tadigadapa, S.; Eklund, P. C., Raman Scattering from
High-Frequency Phonons in Supported N-Graphene Layer Films. Nano Letters 2006, 6,
2667-2673.
35. Gupta, A. K.; Russin, T. J.; Gutierrez, H. R.; Eklund, P. C., Probing Graphene Edges
Via Raman Scattering. ACS Nano 2009, 3, 45-52.
36. Hass, J.; Feng, R.; Li, T.; Li, X.; Zong, Z.; de Heer, W. A.; First, P. N.; Conrad, E. H.;
Jeffrey, C. A.; Berger, C., Highly Ordered Graphene for Two Dimensional Electronics.
Appl Phys Lett 2006, 89, 143106.
37. Hayes, B., Why W? American Scientist 2005, 93, 104-108.
38. Huang, P. Y.; Ruiz-Vargas, C. S.; Van der Zande, A. M.; Whitney, W. S.; Levendorf,
M. P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y.; Park, J.; McEuen, P. L.;
Muller, D. A., Grains and Grain Boundaries in Single-Layer Graphene Atomic Patchwork
Quilts. Nature 2011, 469, 389.
39. Hwang, E. H.; Adam, S.; Das Sarma, S., Transport in Chemically Doped Graphene in
the Presence of Adsorbed Molecules. Physical Review B 2007, 76, 195421.
40. International, A., Alloy Phase Diagrams. 2002; Vol. 3.
41. Jain, A.; Kapoor, A., Exact Analytical Solutions of the Parameters of Real Solar Cells
Using Lambert W-Function. Sol. Energy Mater. Sol. Cells 2004, 81, 269-277.
42. Jansseune, T., Indium Price Soars as Demand for Displays Continues to Grow
Compound Semiconductor 2005, 11, 34-35.

151
43. Jiao, L. Y.; Wang, X. R.; Diankov, G.; Wang, H. L.; Dai, H. J., Facile Synthesis of
High-Quality Graphene Nanoribbons. Nat. Nanotechnol. 2010, 5, 321-325.
44. Kang, M.-G.; Guo, L. J., Nanoimprinted Semitransparent Metal Electrodes and Their
Application in Organic Light-Emitting Diodes. Advanced Materials 2007, 19, 1391-1396.
45. Kang, S. J.; Kocabas, C.; Ozel, T.; Shim, M.; Pimparkar, N.; Alam, M. A.; Rotkin, S.
V.; Rogers, J. A., High-Performance Electronics Using Dense, Perfectly Aligned Arrays of
Single-Walled Carbon Nanotubes. Nat Nanotechnol 2007, 2, 230-236.
46. Karu, A. E.; Beer, M., Pyrolytic Formation of Highly Crystalline Graphite Films. J.
Appl. Phys. 1966, 37, 2179.
47. Kim, E.; Yu, T. H.; Song, E. S.; Yu, B., Chemical Vapor Deposition-Assembled
Graphene Field-Effect Transistor on Hexagonal Boron Nitride. Appl Phys Lett 2011, 98.
48. Kim, K.-B.; Tak, Y.-H.; Han, Y.-S.; Baik, K.-H.; Yoon, M.-H.; Lee, M.-H.,
Relationship between Surface Roughness of Indium Tin Oxide and Leakage Current of
Organic Light-Emitting Diode. Japanese Journal of Applied Physics 2003, 42, L438-L440.
49. Kim, K.; Lee, Z.; Regan, W.; Kisielowski, C.; Crommie, M. F.; Zettl, A., Grain
Boundary Mapping in Polycrystalline Graphene. ACS Nano 2011, 5, 2142-2146.
50. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.;
Choi, J. Y.; Hong, B. H., Large-Scale Pattern Growth of Graphene Films for Stretchable
Transparent Electrodes. Nature 2009, 457, 706-710.
51. Kim, M.; Safron, N. S.; Han, E.; Arnold, M. S.; Gopalan, P., Fabrication and
Characterization of Large-Area, Semiconducting Nanoperforated Graphene Materials.
Nano Letters 2010, 10, 1125-1131.
52. Lee, J.-Y.; Connor, S. T.; Cui, Y.; Peumans, P., Solution-Processed Metal Nanowire
Mesh Transparent Electrodes. Nano Letters 2008, 8, 689-692.
53. Levendorf, M. P.; Ruiz-Vargas, C. S.; Garg, S.; Park, J., Transfer-Free Batch
Fabrication of Single Layer Graphene Transistors. Nano Lett. 2009, 9, 4479-4483.
54. Lewis Gomez, Y. Z., Cody Schlenker, Koungmin Ryu, Mark Thompson, Chongwu
Zhou, Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor
Deposition for Organic Photovoltaics. ACS Nano 2010.
55. Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H., Highly Conducting
Graphene Sheets and Langmuir-Blodgett Films. Nat Nano 2008, 3, 538-542.
56. Li, X.; Zhu, Y.; Cai, W.; Borysiak, M.; Han, B.; Chen, D.; Piner, R. D.; Colombo, L.;
Ruoff, R. S., Transfer of Large-Area Graphene Films for High-Performance Transparent
Conductive Electrodes. Nano Letters 2009, 9, 4359-4363.
57. Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni,
A.; Jung, I.; Tutuc, E., Large-Area Synthesis of High-Quality and Uniform Graphene
Films on Copper Foils. Science 2009, 324, 1312-1314.
58. Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S., Evolution of Graphene Growth on Ni
and Cu by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268-4272.
152
59. Li, X. S.; Magnuson, C. W.; Venugopal, A.; An, J. H.; Suk, J. W.; Han, B. Y.;
Borysiak, M.; Cai, W. W.; Velamakanni, A.; Zhu, Y. W.; Fu, L. F.; Vogel, E. M.; Voelkl,
E.; Colombo, L.; Ruoff, R. S., Graphene Films with Large Domain Size by a Two-Step
Chemical Vapor Deposition Process. Nano Lett. 2010, 10, 4328-4334.
60. Li, X. S.; Magnuson, C. W.; Venugopal, A.; Tromp, R. M.; Hannon, J. B.; Vogel, E.
M.; Colombo, L.; Ruoff, R. S., Large-Area Graphene Single Crystals Grown by
Low-Pressure Chemical Vapor Deposition of Methane on Copper. J. Am. Chem. Soc 2011,
133, 2816-2819.
61. Liu, Z. F.; Yan, K.; Peng, H. L.; Zhou, Y.; Li, H., Formation of Bilayer Bernal
Graphene: Layer-by-Layer Epitaxy Via Chemical Vapor Deposition. Nano Lett. 2011, 11,
1106-1110.
62. Lopez, G. A.; Mittemeijer, E., The Solubility of C in Solid Cu. Scr. Mater. 2004, 51,
1-5.
63. Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S.,
The Structure of Suspended Graphene Sheets. Nature 2007, 446, 60-63.
64. Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.;
Peres, N. M. R.; Geim, A. K., Fine Structure Constant Defines Visual Transparency of
Graphene. Science 2008, 320, 1308.
65. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.;
Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A., Two-Dimensional Gas of Massless Dirac
Fermions in Graphene. Nature 2005, 438, 197-200.
66. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;
Grigorieva, I. V.; Firsov, A. A., Electric Field Effect in Atomically Thin Carbon Films.
Science 2004, 306, 666-669.
67. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S.
V.; Geim, A. K., Two-Dimensional Atomic Crystals. P Natl Acad Sci USA 2005, 102,
10451-10453.
68. Obraztsov, A. N., Chemical Vapour Deposition Making Graphene on a Large Scale.
Nat Nanotechnol 2009, 4, 212-213.
69. Ortiz-Conde, A.; García Sánchez, F. J.; Muci, J., Exact Analytical Solutions of the
Forward Non-Ideal Diode Equation with Series and Shunt Parasitic Resistances.
Solid-State Electronics 2000, 44, 1861-1864.
70. Oshima, C.; Nagashima, A., Ultra-Thin Epitaxial Films of Graphite and Hexagonal
Boron Nitride on Solid Surfaces. J. Phys.-Condes. Matter 1997, 9, 1-20.
71. Pan, Z. J.; Yang, R. T., The Mechanism of Methane Formation from the Reaction
between Graphite and Hydrogen. J. Catal. 1990, 123, 206-214.
72. Peumans, P.; Yakimov, A.; Forrest, S. R., Small Molecular Weight Organic Thin-Film
Photodetectors and Solar Cells. Journal of Applied Physics 2003, 93, 3693-3723.

153
73. Rasool, H. I.; Song, E. B.; M., M.; Regan, B. C.; Wang, K. L.; Weiller, B. H.;
Gimzewski, J. K., Atomic-Scale Characterization of Graphene Grown on Copper (100)
Single Crystals. J. Am. Chem. Soc 2011, 133, 12536-12543.
74. Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong,
J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor
Deposition. Nano Letters 2008, 9 30-35.
75. Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V.; Dresselhaus, M. S.;
Kong, J., Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical
Vapor Deposition. Nano Lett. 2009, 9, 30-35.
76. Reina, A. T., S.; Jia, X.; Bhaviripudi, S.;Dresselhaus, M. S.; Schaefer, J. A.; Kong, J. ,
Growth of Large-Area Single- and Bi-Layer Graphene by Controlled Carbon Precipitation
on Polycrystalline Ni Surfaces. Journal of nano research 2009, 2, 509-516.
77. Ren, W. C.; Gao, L. B.; Liu, B. L.; Wu, Z. S.; Jiang, C. B.; Cheng, H. M.,
Crystallographic Tailoring of Graphene by Nonmetal Sio(X) Nanoparticles. J. Am. Chem.
Soc 2009, 131, 13934.
78. Rollings, E.; Gweon, G. H.; Zhou, S. Y.; Mun, B. S.; McChesney, J. L.; Hussain, B. S.;
Fedorov, A.; First, P. N.; de Heer, W. A.; Lanzara, A., Synthesis and Characterization of
Atomically Thin Graphite Films on a Silicon Carbide Substrate. J Phys Chem Solids 2006,
67, 2172-2177.
79. Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.; Dennler, G.; Sariciftci,
N. S.; Hu, L.; Gruner, G., Organic Solar Cells with Carbon Nanotube Network Electrodes.
Applied Physics Letters 2006, 88, 233506.1-233506.3
80. Ryu, K.; Badmaev, A.; Gomez, L.; Ishikawa, F.; Lei, B.; Zhou, C. W., Synthesis of
Aligned Single-Walled Nanotubes Using Catalysts Defined by Nanosphere Lithography. J.
Am. Chem. Soc 2007, 129, 10104.
81. Safron, N., Barrier-Guided Growth of Micro- and Nano-Structured Graphene. APS
2012 March Meeting Abstract 2012.
82. Salvetat, J. P.; Bonard, J. M.; Bacsa, R.; Stockli, T.; Forro, L., Physical Properties of
Carbon Nanotubes. Aip Conf Proc 1998, 442, 467-480.
83. Sangeeth, C. S. S.; Manu, J.; Reghu, M., Charge Transport in Transparent Conductors:
A Comparison. Journal of Applied Physics 2009, 105, 063713.1-063713.6.
84. Scott, J. C.; Kaufman, J. H.; Brock, P. J.; DiPietro, R.; Salem, J.; Goitia, J. A.,
Degradation and Failure of Meh-Ppv Light-Emitting Diodes. Journal of Applied Physics
1996, 79, 2745-2751.
85. Sedbook, D. F., Bidentate Phenoxides as Ideal Activating Ligands for Living Ring
Opening Alkyne Metathesis Polymerization. Submitted to JACS 2012.
86. Shelton, J. C.; Patil, H. R.; Blakely, J. M., Equilibrium Segregation of Carbon to a
Nickel (111) Surface - Surface Phase-Transition. Surf Sci 1974, 43, 493-520.

154
87. Shenoy, V. B.; Grantab, R.; Ruoff, R. S., Anomalous Strength Characteristics of Tilt
Grain Boundaries in Graphene. Science 2010, 330, 946-948.
88. Somani, P. R.; Somani, S. P.; Umeno, M., Planer Nano-Graphenes from Camphor by
Cvd. Chemical Physics Letters 2006, 430, 56-59
89. Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.;
Wu, Y.; Nguyen, S. T.; Ruoff, R. S., Synthesis of Graphene-Based Nanosheets Via
Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558-1565.
90. Sukang Bae, H. K., Youngbin Lee, Xiangfan Xu, Jae-Sung Park, Yi Zheng, Jayakumar
Balakrishnan, Tian Lei, Hye Ri Kim, Young Il Song, Young-Jin Kim, Kwang S. Kim,
Barbaros Ozyilmaz, Jong-Hyun Ahn,Byung Hee Hong, and Sumio Iijima, Roll-to-Roll
Production of 30-Inch Graphene Films for Transparent Electrodes. Nat Nanotechnol 2010.
91. Sutter, P.; Sadowski, J. T.; Sutter, E., Graphene on Pt(111): Growth and Substrate
Interaction. Phys Rev B 2009, 80.
92. Sutter, P. W.; Flege, J. I.; Sutter, E. A., Epitaxial Graphene on Ruthenium. Nat. Mater.
2008, 7, 406-411.
93. Tan, Y. W.; Zhang, Y.; Bolotin, K.; Zhao, Y.; Adam, S.; Hwang, E. H.; Das Sarma, S.;
Stormer, H. L.; Kim, P., Measurement of Scattering Rate and Minimum Conductivity in
Graphene. Physical Review Letters 2007, 99, 246803.1-246803.4.
94. Tung, V. C.; Chen, L.-M.; Allen, M. J.; Wassei, J. K.; Nelson, K.; Kaner, R. B.; Yang,
Y., Low-Temperature Solution Processing of Graphene#Carbon Nanotube Hybrid
Materials for High-Performance Transparent Conductors. Nano Letters 2009, 9,
1949–1955.
95. Varykhalov, A.; Rader, O., Graphene Grown on Co(0001) Films and Islands:
Electronic Structure and Its Precise Magnetization Dependence. Phys Rev B 2009, 80.
96. Viculis, L. M.; Mack, J. J.; Kaner, R. B., A Chemical Route to Carbon Nanoscrolls.
Science 2003, 299, 1361.
97. Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S., Role
of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene.
ACS Nano 2011, 5, 6069-6076.
98. Wang, C.; Ryu, K. M.; Badmaev, A.; Patil, N.; Lin, A.; Mitra, S.; Wong, H. S. P.;
Zhou, C., Device Study, Chemical Doping, and Logic Circuits Based on Transferred
Aligned Single-Walled Carbon Nanotubes. Applied Physics Letters 2008, 93.
99. Wang, S. M.; Pei, Y. H.; Wang, X.; Wang, H.; Meng, Q. N.; Tian, H. W.; Zheng, X. L.;
Zheng, W. T.; Liu, Y. C., Synthesis of Graphene on a Polycrystalline Co Film by
Radio-Frequency Plasma-Enhanced Chemical Vapour Deposition. J. Phys. D Appl. Phys.
2010, 43.
100. Wang, X.; Zhi, L.; Mllen, a. K., Transparent, Conductive Graphene Electrodes for
Dye-Sensitized Solar Cells. Nano Letters 2008, 8, 323-327.

155
101. Wang, X.; Zhi, L.; Tsao, N.; Tomovic, Z.; Li, J.; Müllen, K., Transparent Carbon
Films as Electrodes in Organic Solar Cells. Angewandte Chemie International Edition
2008, 47, 2990-2992.
102. Wang, X. R.; Dai, H. J., Etching and Narrowing of Graphene from the Edges. Nat.
Chem. 2010, 2, 661-665.
103. Wofford, J. M.; Nie, S.; McCarty, K. F.; Bartelt, N. C.; Dubon, O. D., Graphene
Islands on Cu Foils: The Interplay between Shape, Orientation, and Defects. Nano Lett.
2010, 10, 4890-4896.
104. Wu, J.; Becerril, H. A.; Bao, Z.; Liu, Z.; Chen, Y.; Peumans, P., Organic Solar
Cells with Solution-Processed Graphene Transparent Electrodes. Applied Physics Letters
2008, 92, 263302.1-263302.3.
105. Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.;
Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G., Transparent, Conductive
Carbon Nanotube Films. Science 2004, 305, 1273-1276.
106. Wu. M.; Tse, J. S. J., J. S., Unzipping of Graphene by Fluorination. Journal of
Physical Chemistry Letters 2010, 1, 1934-1937.
107. Yang, R.; Zhang, L. C.; Wang, Y.; Shi, Z. W.; Shi, D. X.; Gao, H. J.; Wang, E. G.;
Zhang, G. Y., An Anisotropic Etching Effect in the Graphene Basal Plane. Adv. Mater.
2010, 22, 4014-4019.
108. Yazyev, O. V.; Louie, S. G., Electronic Transport in Polycrystalline Graphene.
Nat. Mater. 2010, 9, 806-809.
109. Yu, D. S. D., L. M., Self-Assembled Graphene/Carbon Nanotube Hybrid Films
for Supercapacitors. Journal of Physical Chemistry Letters 2010, 1, 467-470.
110. Yu, Q. K.; Jauregui, L. A.; Wu, W.; Colby, R.; Tian, J. F.; Su, Z. H.; Cao, H. L.;
Liu, Z. H.; Pandey, D.; Wei, D. G., Control and Characterization of Individual Grains and
Grain Boundaries in Graphene Grown by Chemical Vapour Deposition. Nat. Mater. 2011,
10, 443-449.
111. Yu, Q. K.; Lian, J.; Siriponglert, S.; Li, H.; Chen, Y. P.; Pei, S. S., Graphene
Segregated on Ni Surfaces and Transferred to Insulators. Appl. Phys. Lett. 2008, 93,
113103.
112. Zhang, D.; Ryu, K.; Liu, X.; EvgueniPolikarpov; Ly, J.; Tompson, M. E.; Zhou,
C., Transparent, Conductive, and Flexible Carbon Nanotube Films and Their Application
in Organic Light-Emitting Diodes. Nano Letters 2006, 6, 1880-1886.
113. Zhang, G. Y.; Shi, Z. W.; Yang, R.; Zhang, L. C.; Wang, Y.; Liu, D. H.; Shi, D. X.;
Wang, E. G., Patterning Graphene with Zigzag Edges by Self-Aligned Anisotropic Etching.
Adv. Mater. 2011, 23, 3061.

156
114. Zhang, Y.; Gomez, L.; Ishikawa, F. N.; Madaria, A.; Ryu, K.; Wang, C. A.;
Badmaev, A.; Zhou, C. W., Comparison of Graphene Growth on Single-Crystalline and
Polycrystalline Ni by Chemical Vapor Deposition. J. Phys. Chem. Lett. 2010, 1,
3101-3107.
115. Zhang, Y.; Li, Z.; Kim, P.; Zhang, L.; Zhou, C. W., Anisotropic Hydrogen
Etching of Chemical Vapor Deposited Graphene. ACS Nano 2011, 6, 126-132.
116. Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P., Experimental Observation of
the Quantum Hall Effect and Berry's Phase in Graphene. Nature 2005, 438, 201-204.
117. Zhang, Z. Y.; Wang, Z. X.; Xu, H. L.; Wang, S.; Ding, L.; Zeng, Q. S.; Yang, L. J.;
Pei, T. A.; Liang, X. L.; Gao, M.; Peng, L. M., Growth and Performance of Yttrium Oxide
as an Ideal High-K Gate Dielectric for Carbon-Based Electronics. Nano Lett. 2010, 10,
2024-2030.

Abstract (if available)

Abstract In this dissertation I discuss the synthesis of graphene using chemical vapor deposition on Ni and Cu substrate, as well as various applications using CVD graphene. ❧ Chapter 1 gives a brief introduction of graphene, the electrical properties of graphene, and chemical vapor deposition method of graphene synthesis. ❧ Chapter 2 discusses a simple, scalable and cost-efficient method to prepare graphene using methane-based chemical vapor deposition on nickel films deposited over complete Si/SiO2 wafers. By using highly diluted methane, single- and few-layer graphene were obtained, as confirmed by micro Raman spectroscopy. In addition, a transfer technique has been applied to transfer the graphene film to target substrates via nickel etching. Field-effect transistors based on the graphene films transferred to Si/SiO₂ substrates revealed a weak p-type gate dependence, while transferring of the graphene films to glass substrate allowed its characterization as transparent conductive films, exhibiting transmittance of 80% in the visible wavelength range. ❧ In chapter 3, continuous, highly flexible, and transparent few-layer graphene films synthesized from Ni film were implemented as transparent conductive electrodes (TCE) in organic photovoltaic cells. Graphene films were synthesized by CVD, transferred to transparent substrates, and evaluated in organic solar cell heterojunctions (TCE/poly-3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper phthalocyanine/fullerene/bathocuproine/aluminum). Key to our success is the continuous nature of the CVD graphene films, which led to minimal surface roughness (~0.9 nm) and offered sheet resistance down to 230 Ω/sq (at 72% transparency), much lower than stacked graphene flakes at similar transparency. In addition, solar cells with CVD graphene and indium tin oxide (ITO) electrodes were fabricated side-by-side on flexible polyethylene terephthalate (PET) substrates and were confirmed to offer comparable performance, with power conversion efficiencies (η) of 1.18 and 1.27%, respectively. Furthermore, CVD graphene solar cells demonstrated outstanding capability to operate under bending conditions up to 138°, whereas the ITO-based devices displayed cracks and irreversible failure under bending of 60°. Our work indicates the great potential of CVD graphene films for flexible photovoltaic applications. ❧ In chapter 4, we discuss comparative study and Raman characterization on the formation of graphene on single crystal Ni (111) and polycrystalline Ni substrates using chemical vapor deposition. Preferential formation of monolayer/bilayer graphene on the single crystal surface is attributed to its atomically smooth surface and the absence of grain boundaries. In contrast, CVD graphene formed on polycrystalline Ni leads to higher percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman surface mapping reveals that the area percentages of monolayer/bilayer graphene are 91.4% for the Ni (111) substrate and 72.8% for the polycrystalline Ni substrate under comparable CVD conditions. The use of single crystal substrates for graphene growth may open ways for uniform high-quality graphene over large areas. ❧ Chapter 5 discusses a vapor trapping method for the growth of large-grain, single-crystalline graphene flowers with grain size up to 100 μm. Controlled growth of graphene flowers with four lobes and six lobes has been achieved by varying the growth pressure and the methane to hydrogen ratio. Surprisingly, electron backscatter diffraction study revealed that the graphene morphology had little correlation with the crystalline orientation of underlying copper substrate. Field effect transistors were fabricated based on graphene flowers and the fitted device mobility could achieve ~ 4,200 cm²V⁻¹s⁻¹ on Si/SiO₂ and ~ 20,000 cm²V⁻¹s⁻¹ on hexagonal boron nitride (h-BN). Our vapor trapping method provides a viable way for large-grain single-crystalline graphene synthesis for potential high-performance graphene-based electronics. ❧ In chapter 6, a simple, clean, and highly anisotropic hydrogen etching method was developed for chemical vapor deposited graphene catalyzed by the copper substrate. By exposing CVD graphene on copper foil to hydrogen flow around 800 °C, we observed that the initially continuous graphene can be etched to have many hexagonal openings. In addition, we found that the etching is temperature dependent. Compared to other temperatures (700, 900, and 1000 °C), etching of graphene at 800 °C is most efficient and anisotropic. Of the angles of graphene edges after etching, 80% are 120°, indicating the etching is highly anisotropic. No increase of the D band along the etched edges indicates that the crystallographic orientation of etching is in the zigzag direction. Furthermore, we observed that copper played an important role in catalyzing the etching reaction, as no etching was observed for graphene transferred to Si/SiO₂ under similar conditions. This highly anisotropic hydrogen etching technology may work as a simple and convenient way to determine graphene crystal orientation and grain size and may enable the etching of graphene into nanoribbons for electronic applications. ❧ Brief conclusions are drawn in chapter 7. Future directions of graphene are also discussed in chapter 7. ❧ In summary, this dissertation starts from CVD graphene synthesis and fulfills with various applications using as-synthesized graphene material, which proves the potential of CVD graphene for device application, OPV cells, and other possible applications. With the continuous improvement of graphene quality, as well as the promising application results shown in this dissertation, we should expect many more applications that exploit all kinds of unique properties of graphene in near future.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

Chemical vapor deposition of graphene and two-dimensional materials: synthesis, characterization, and applications

PDF

Controlled synthesis of large grain single crystal graphene by chemical vapor deposition

PDF

Fabrication of polymer films on liquid substrates via initiated chemical vapor deposition: controlling morphology and composition

PDF

Controlled synthesis, characterization and applications of carbon nanotubes

PDF

Synthesis, characterization, and device application of two-dimensional materials beyond graphene

PDF

Controlling polymer film patterning, morphologies, and chemistry using vapor deposition

PDF

Vapor phase deposition of dense and porous polymer coatings and membranes for increased sustainability and practical applications

PDF

Novel processing of liquid substrates via initiated chemical vapor depostion

PDF

Fabrication of silicon-based membranes via vapor-phase deposition and pyrolysis of organosilicon polymers

PDF

In2O3 COVID-19 biosensors and two-dimensional materials: synthesis and applications

PDF

Formation of polymer gels, films, and particles via initiated chemical vapor deposition onto liquid substrates

PDF

Scale-up of vapor-phase deposition of polymers: towards large-scale processing

PDF

Fabrication and study of organic and inorganic optoelectronics using a vapor phase deposition (VPD)

PDF

Fabrication, deposition, and characterization of size-selected metal nanoclusters with a magnetron sputtering gas aggregation source

PDF

Growth, characterization of gallium arsenide based nanowires and application in photovoltaic cells

PDF

Carbon material-based nanoelectronics

PDF

Modification of electrode materials for lithium ion batteries

PDF

Carbon nanotube nanoelectronics and macroelectronics

PDF

One-dimensional nanomaterials for electronic and sensing applications

PDF

Surface functionalization of nanomaterials and development of field effect transistor nanobiosensors

Asset Metadata

Creator Zhang, Yi (author)

Core Title Chemical vapor deposition of graphene: synthesis, characterization, and applications

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 08/22/2012

Defense Date 07/30/2012

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

Tag anisotropic etching,applications,chemical vapor deposition,graphene,graphene OPV,large-grain,OAI-PMH Harvest,single-crystalline graphene,transfer,vapor-trapping,wafer-scale

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Goo, Edward K. (committee member), Thompson, Barry C. (committee member)

Creator Email remainyi@gmail.com,zhang17@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-91487

Unique identifier UC11288331

Identifier usctheses-c3-91487 (legacy record id)

Legacy Identifier etd-ZhangYi-1161.pdf

Dmrecord 91487

Document Type Dissertation

Rights Zhang, Yi

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA