University of Southern California Dissertations and Theses

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

(USC Thesis Other)

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

PDF

Download a page range

Download transcript

Copy asset link

Request this asset

Transcript (if available)

Content

CHEMICAL VAPOR DEPOSITION OF GRAPHENE AND TWO-
DIMENSIONAL MATERIALS: SYNTHESIS, CHARACTERIZATION,
AND APPLICATIONS

by

Luyao 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
(MATERIAL SCIENCE)

May 2015

Copyright 2015 Luyao Zhang
ii

Abstract

Graphene, emerging in 2004 as a two-dimensional (2D) single layer material with
carbon atoms arranged in a honeycomb crystal lattice, exhibits physically fascinating and
technologically useful properties, triggers global boom in graphene research. Despite the
intense interest and remarkably rapid progress in this field during the past few years,
there are still serious challenges in achieving high quality, scalable, and low cost samples
needed for electronics and optoelectronics. Chemical vapor deposition (CVD) is explored
as a powerful technique to synthesize graphene and other two-dimensional materials in
large scale with low cost. In this dissertation, I present the opportunities, challenges and
solutions in producing graphene and other two-dimensional materials by chemical vapor
deposition.
The first chapter gives a brief introduction about properties, preparation and charac-
terization of graphene and hexagonal boron nitride (h-BN). Chapter 2 discusses the ap-
proaches to decrease graphene nucleation density on cooper foil and to synthesize large
grain, single-crystalline graphene by chemical vapor deposition. A vapor trapping method
is developed to grow graphene flowers with grain size up to 0.1mm at low pressure.
Hexagonal graphene with grain size over 2mm is also grown at ambient pressure. Select-
ed area electron diffraction (SAED) characterization confirms the single-crystalline na-
iii

ture of graphene grains, and electrical measurement demonstrates their excellent motilities.
Controlled synthesis and growth parameter study provide insight into the key factors that
affect the grain morphology. In the following chapter, bilayer and trilayer graphene with
various adlayer morphologies are produced by ambient pressure CVD, and characterized
by Raman and transmission electron microscopy (TEM). Growth mechanisms for both
concentric and nonconcentric adlayers are investigated. The proposed model for adlayer
growth, along with the statistical analysis, allows more controllable synthesis of bi-
layer/trilayer graphene. In chapter 4, we investigate the synthesis of h-BN by CVD. Vari-
ous growth conditions, including metal substrates, precursors, and growth parameters espe-
cially pressures, are studied to achieve better quality of BN films. In the last chapter, I pre-
sent the ongoing and future work of graphene applications and the 2D materials beyond gra-
phene. T-gate devices are fabricated with high quality CVD graphene on h-BN substrate for
RF applications, and the DC performance is compared with those fabricated on SiO
2
/Si sub-
strates, showing great potential for such type of system. Molybdenum disulfide (MoS2) in
the family of transition metal dichalcogenides (TMDs) is also grown by CVD on SiO
2
/Si for
the future study on 2D heterostructures.

iv

Dedication

This dissertation is dedicated to my son Daniel,
my husband Lizhong, and my dearest parents.

v

Acknowledgments

I am deeply thankful to my advisor, Professor Chongwu Zhou, whose inspiring guid-
ance, generous support, and consistent trust have facilitated tremendously the success of
this research. Professor Zhou has not only explored my potential in conducting impactful
research, but also offered me invaluable guidance on my pursuit of career and life goals.
Without his help, I would not have this dissertation presented.
Many thanks go to Prof. Edward Goo and Prof. Wei Wu for being me dissertation
committee members. I would also like to thank the other two members of my qualify
committees, Professor Steve Cronin and Professor Andrea Armani for their valuable time,
feedback, and suggestions.
In addition, I would like to thank Professor Huang Qiang and Porfessor Han Wang at
USC for collaboration, Dr. Matthew Mecklenburg for assistance with TEM experiment,
Dr. Donghai Zhu for the training of the cleanroom process, and Andy Chen in the materi-
al science department for all the helps he offered.
Special thanks also goes to my numerous friends, including my group members, Dr.
Yi Zhang, Dr. Chuan Wang, Dr. Haitian Chen, Dr. Bilu Liu, Dr. Yuchi Che, Dr. Jia Liu,
Dr. Gang Liu, Dr. Yun-cheng Lin, Dr. Mingyuan Ge, Dr. Jiepeng Rong, Dr. Shelley
Wang, Dr. Maoqing Yao, Dr. Noppadol Aroonyadet , Xin Fang, Ahmad Abbas, Xuan
vi

Cao, Hui Gui, Anyi Zhang, Yuqiang Ma, Liang Chen, Yu Cao, Fanqi Wu, Sen Cong,
Yihang Liu, Chengfei Shen, Yilin Huang, Han-wen Cheng, Yue Fu, Pattaramon
Vuttipittayamongko, Pyojae Kim, as well as all my other friends. They always encourage
me and help me when I feel upset or meet a problem. Their presence has made my five-
year graduate research life a pleasant and memorable journey.
Finally, I would like to express my sincerest appreciation to my dear husband and
parents for their unconditional love and support. Without their understanding, encour-
agement and sacrifice, I would not have all those achievements.

vii

Contents

Abstract ........................................................................................................................ ii
Dedication ................................................................................................................... iv
Acknowledgments ....................................................................................................... v
List of Figures .............................................................................................................. x
1 Introduction ................................................................................................................. 1
1.1 Introduction of Graphene ..................................................................................... 1
1.2 Electronic Property of Graphene .......................................................................... 2
1.3 Graphene Characterization ................................................................................... 3
1.3.1 Optical microscopy (OM) ...................................................................... 4
1.3.2 Atomic force microscopy (AFM) .......................................................... 4
1.3.3 Scanning electron microscopy (SEM) ................................................... 4
1.3.4 Transmission electron microscopy (TEM) ............................................ 5
1.3.5 Raman spectroscopy .............................................................................. 5
1.4 Synthesis of Graphene by Chemical Vapor Deposition ....................................... 7
1.4.1 Growth process ...................................................................................... 8
1.4.2 Metal substrate pretreatment ............................................................... 10
1.4.3 Graphene transfer ................................................................................ 11
1.5 Introduction of Hexagonal Boron Nitride .......................................................... 12
2 Growth of Large-grain Single-Crystalline Graphene ........................................... 15
2.1 Motivation .......................................................................................................... 15
2.2 Vapor-trapping Synthesis of Graphene Flowers ................................................ 17
2.3 Characterization of Graphene Flowers ............................................................... 19
2.3.1 Raman characterization ....................................................................... 19
2.3.2 TEM characterization .......................................................................... 20
viii

2.4 Growth Mechanism Study .................................................................................. 23
2.4.1 Growth parameters study ..................................................................... 23
2.4.2 Growth time study ............................................................................... 26
2.4.3 Seeded growth ..................................................................................... 27
2.4.4 Electron backscatter spectroscopy study ............................................. 29
2.5 Electric Property Measurement of Graphene Flowers ....................................... 33
2.6 Growth of Large Grain Hexagonal Graphene .................................................... 36
2.6.1 Cu substrate pretreatment .................................................................... 37
2.6.2 Annealing condition improvement ...................................................... 39
2.6.3 Growth condition improvement........................................................... 40
2.7 Conclusion .......................................................................................................... 42
3 Bilayer Graphene Synthesis by Chemical Vapor Deposition ............................... 44
3.1 Motivation and Related Work ............................................................................ 44
3.2 Ambient Pressure Synthesis of Bilayer/trilayer Graphene ................................. 47
3.3 Characterization of Bilayer/trilayer Graphene and Growth Mechanism Study . 49
3.3.1 AFM characterization .......................................................................... 49
3.3.2 Raman characterzation ........................................................................ 51
3.3.3 TEM characterzation ........................................................................... 52
3.3.4 Growth mechanism analysis ................................................................ 60
3.3.5 Growth parameters study ..................................................................... 63
3.4 Conclusion .......................................................................................................... 66
4 h-BN Synthesis by Chemical Vapor Deposition ..................................................... 67
4.1 Few-layer h-BN Film Growth on Ni Substrate .................................................. 67
4.2 Comparison of h-BN Grown on Ni and Cu Films ............................................. 69
4.3 Few-layer h-BN Grown on Cu Foil.................................................................... 71
4.4 Monolayer h-BN Grown on Cu Foil .................................................................. 72
4.5 Conclusion .......................................................................................................... 74
5 Ongoing and Future Works ..................................................................................... 75
5.1 High Quality Graphene for RF Applications ..................................................... 75
5.1.1 Motivation ........................................................................................... 75
5.1.2 Device fabrication and DC performance ............................................. 77
ix

5.2 Two-dimensional Materials beyond Graphene .................................................. 80
5.2.1 MoS
2
growth by CVD ......................................................................... 81
5.2.2 Two-dimensional material heterostructures growth ............................ 82
Bibliography .............................................................................................................. 84

x

List of Figures

Figure 1.1: Introduction of graphene .................................................................................. 2
Figure 1.2: Band structure of graphene ............................................................................... 3
Figure 1.3: Characterization of graphene ............................................................................ 6
Figure 1.4: Low pressure CVD system for graphene synthesis .......................................... 9
Figure 1.5: Flow chart of low pressure graphene CVD recipe on Cu foil ....................... 10
Figure 1.6: Schematic diagram of the transfer process .................................................... 12
Figure 1.7: Exfoliated and CVD few-layer h-BN ............................................................ 14
Figure 2.1: Graphene grain formation and large grain graphene ..................................... 16
Figure 2.2: Graphene flower by vapor-trapping growth .................................................. 18
Figure 2.3: Raman spectra and mapping of graphene flower .......................................... 20
Figure 2.4: TEM characterization of graphene flower ...................................................... 21
Figure 2.5: SEM images of graphene grown using various recipe ................................... 25
Figure 2.6: SEM images of graphene flowers using different growth time...........................
at 300 mTorr .................................................................................................................... 27
Figure 2.7: SEM images of graphene flower seeded growth ........................................... 29
Figure 2.8: EBSD mapping of graphene flowers ............................................................. 31
Figure 2.9: Back-gate graphene flower device measurement .......................................... 34
Figure 2.10: Graphene/h-BN device fabrication steps shown schematically .................. 35
xi

Figure 2.11: Graphene/h-BN flower device measurement ............................................... 36
Figure 2.12: ECP treatment of copper foil ........................................................................ 38
Figure 2.13: SEM image of as grown graphene on half-polished Cu foil ........................ 39
Figure 2.14: Effect of different annealing pressure .......................................................... 40
Figure 2.15: Large grain hexagonal graphene .................................................................. 42
Figure 3.1: Strucure of bilayer graphene .......................................................................... 45
Figure 3.2: SEM images showing bilayer/trilayer graphene regions………………………
in the center of graphene flowers by vapor trapping synthesis…………….......................
at 1000˚C with growth time of (a) 20min and (b) 40min. ................................................ 47
Figure 3.3: SEM images of as grown bilayer/trilayer graphene on copper………………
foil with four different kinds of morphologies ................................................................. 49
Figure 3.4: AFM characterization of the concentric trilayer graphene ............................. 50
Figure 3.5: AFM characterization of the edged nonconcentric trilayer graphene grain ... 51
Figure 3.6: Typical optical images and Raman spectra of bilayer/trilayer………...…….
graphene with different morphologies ............................................................................ 53
Figure 3.7: TEM characterization of bilayer/trilayer graphene ........................................ 55
Figure 3.8: Images of nonconcentric adlayers .................................................................. 56
Figure 3.9: TEM characterization of bilayer graphene ..................................................... 57
Figure 3.10: TEM characterization of trilayer graphene .................................................. 59
Figure 3.11: TEM characterization of bilayer graphene ................................................... 60
Figure 3.12: Adlayer growth from edges .......................................................................... 61
xii

Figure 3.13: Schematic diagrams propose two different mechanisms ………….…………
for graphene adlayer growth ............................................................................................. 62
Figure 3.14: Bilayer graphene morphology with different methane……………………….
concentration and growth temperatures ............................................................................ 65
Figure 4.1: h-BN grown on Ni films by CVD .................................................................. 69
Figure 4.2: Raman characterization of h-BN on Ni film .................................................. 69
Figure 4.3: Comparison of h-BN on Ni and Cu ................................................................ 70
Figure 4.4: Characterization of h-BN grown on Cu ......................................................... 72
Figure 4.5: Low pressure CVD of h-BN grown on Cu ..................................................... 73
Figure 5.1: Graphene T-gate device structure ................................................................... 76
Figure 5.2: Graphene T-gate device on h-BN ................................................................... 78
Figure 5.3: I-Vg of back gate graphene devices on h-BN and SiO
2
/Si substrate ............. 79
Figure 5.4: Structure and real samples of two-dimensional materials .............................. 80
Figure 5.5: Characterization of CVD MoS
2
...................................................................... 82

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 with sp2 bonds. It can be wrapped up into 0D fullerenes, rolled into
1D nanotubes, or stacked into 3D graphite (Figure 1.1(a)).
25
This 2D crystal was once
considered thermodynamically unstable and could not exist under room temperature. In
2004, an easy exfoliation method was developed by Novoselov, Geim and co-workers.
65

They used scotch tapes to isolate monolayer graphene from highly ordered pyrolytic
graphite (HOPG) and transfer it onto SiO
2
/Si substrates (Figure 1.1(b)). With a 300nm
oxide layer on Si, monolayer graphene is visible under optical microscope, and graphene
of different layers have contrast (Figure 1.1(c)), which is convenient for researchers to
identify them. Though this method only produces non-continuous graphene with size
limited to several microns, it is very accessible, and the quality of graphene is quite high.
Therefore, many research groups could prepare this material in their labs and use it for
study right away, triggering a “graphene storm”. Since the debut of graphene in 2004, its
unique physical, chemical, mechanical, and electrical properties have drawn a lot of in-
terests among scientists, and many applications have been developed.
2

1.2 Electronic Property of Graphene
Graphene has unique electronic properties that differ from other conventional three-
dimensional materials because of three reasons: two-dimensional structure, honey-comb
lattice, and all the lattice point are the same carbon atoms. Intrinsic or undoped graphene
is a zero-bandgap semimetal.
64
Figure 1.2(a) shows the 3D structure of graphene band.
1

The electrons in graphene are governed by the Dirac equation, and the electrons and
holes are called Dirac fermions. The interaction between electrons and the honeycomb
lattice causes the electrons to move in the material as if they have no mass. The energy-
momentum relation is linear for low energies near the six corners of the Brillouin zone
(Figure 1.2(b)), and these six corners are called the Dirac points. Monolayer graphene has
an ambipolar electric field effect, as shown in Figure 1.2(c). The charge carriers of gra-
Figure 1.1: Introduction of graphene
(a) Graphene as a building block for carbon structures
25
. (b) HOPG on scotch tape
to produce graphene by multiple exfoliations. (c) Image of graphene on SiO
2
/Si sub-
strate under optical microscope.
3

phene can be tuned continuously between electrons and holes. The carriers in graphene
can travel long distances without being scattered, and the carrier concentration can be as
high as 10
13
cm
-2
.
66
As a result, graphene has remarkable electron and hole mobility at
room temperature, with reported values in excess of 15,000 cm
2
·V
−1
·s
−1
,
65, 87
making it a
promising material for very fast electronic devices.

1.3 Graphene Characterization
The morphology and quality of graphene can be characterized by a number of tech-
niques. Optical microscopy and scanning electron microscopy usually show graphene
uniformity and morphology with a clear contrast between graphene and the substrate, and
atomic force microscopy can identify the layer number of graphene. Transmission elec-
Figure 1.2: Band structure of graphene
(a) Three-dimensional band structure shows energy dispersion relations for gra-
phene. (b) Zoom into low energy dispersion at one of the K points shows the elec-
tron hole symmetric Dirac cone structure. (c) Ambipolar electric field effect in
single-layer graphene.
25
The insets show its conical low-energy spectrum E(k),
indicating changes in the position of the Fermi energy EF with changing gate volt-
4

tron microscopy and Raman spectroscopy show more details of graphene structure, such
as stacking order and atomic defects.
1.3.1 Optical microscopy (OM)
Whether graphene is visible under optical microscope depends on the type of sub-
strate. Monolayer graphene absorbs approximately 2.3% of white light. It is almost trans-
parent on glass and PET films, therefore can be used as transparent electrodes. On Si wa-
fer with 300nm oxide layer, single layer and few layers of graphene can be easily identi-
fied from multiple layers of graphite, and the boundaries of graphene grain on the sub-
strate are quite clear (Figure 1.3 (a)).
72

1.3.2 Atomic force microscopy (AFM)
AFM in the tapping mode is a convenient tool to quickly determine the thickness of
few-layer graphene on a flat surface. Graphene sheets stack to form graphite with an in-
terplanar spacing of 0.335 nm. Due to the interaction of graphene with substrates and the
variations of equipment, the measured thickness of monolayer graphene on SiO
2
/Si sub-
strate ranges from 0.35 nm to 1 nm (Figure 1.3 (b)). AFM also demonstrates the surface
roughness, which shows wrinkles and folded areas for CVD graphene after transfer.
1.3.3 Scanning electron microscopy (SEM)
Low magnification SEM image of as grown graphene on metal substrate clearly
shows the metal grains with color contrast. More details of graphene morphology can be
revealed in the higher-resolution SEM image (Figure 1.3 (c)).
45
The Cu surface steps are
formed during thermal annealing, and the darker flakes indicate multiple-layer graphene.
5

Graphene “wrinkles” originate from the different thermal expansion coefficient of gra-
phene and the metal substrate which is copper in this case.
1.3.4 Transmission electron microscopy (TEM)
Transmission electron microscopy is one of the leading methods for imaging gra-
phene at the atomic level. Figure 1.3 (d) shows a low-magnification TEM image of trans-
ferred graphene with step-shaped edges.
106
The inset shows the selected area electron
diffraction (SAED) pattern of the single-layer graphene film, which confirms the gra-
phene lattice structure. With an aberration-corrected high resolution TEM, even atomic
structures of graphene, such as vacancy defects, grain boundaries, impurity dopants and
bond rotations can be explored.
1.3.5 Raman spectroscopy
The main features in Raman spectra of graphite-based materials are the G and D
bands and the second order of the D band, so-called 2D (or G’), all of which change in
shape, position, and relative intensity, and thus reflect the evolution of the structural and
electronic properties.
20
Figure 1.3 (e) is the colored optical image and corresponding Ra-
man spectra of a typical FLG from single- to four-layer, respectively. 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, and is ab-
sent in graphene samples with perfect atomic structure. The G band, standing at around
1580 cm
-1
is corresponding to in-plane carbon-atom stretching vibrations, and the intensi-
ty increases with graphene layer numbers due to more carbon atoms contributing to this
vibration mode. The 2D band induced by double-resonance processes is excitation-
6

energy dependent, and occurs at around 2700 cm
-1
under 2.33 eV green laser excitation.
For single-layer graphene, the 2D band is symmetric and can be fitted into only one Lo-
rentz peak.
Figure 1.3: Characterization of graphene
(a) Optical image of graphene transferred from the Ni surface to SiO
2
/Si substrate.
(b) AFM image of monolayer graphene on SiO
2
/Si. (c) High-resolution SEM image of
graphene grown on Cu. (d) Low magnification TEM image of graphene edges. Inset
shows monolayer graphene diffraction pattern. (e) Raman spectra of single to four
layers graphene.
7

1.4 Synthesis of Graphene by Chemical Vapor Deposi-
tion
Mechanical exfoliation of highly orientated polymeric graphite (HOPG) by using ad-
hesive tape produces graphene with almost the best quality, serves well for the study of
physical properties as well as single device demonstration. But the size of exfoliated gra-
phene is limited to several or tens of micrometers, and it takes efforts to identify single
layer graphene among other multilayer graphene and locate it on the substrate. To better
facilitate graphene into various applications such as transparent conductive electrodes,
device integration, energy storage and so on, a more reliable method with large quantity
and size is required. Since the discovery of graphene, it has been prepared using a num-
ber of techniques, such as chemical exfoliation,
13, 84
epitaxial growth on SiC,
73, 74
epitaxi-
al growth on surface of transition metals, such as Ru,
86
bottom-up assembly,
7
and chemi-
cal vapor deposition.
45, 106
Among all the strategies to produce graphene, chemical vapor
deposition (CVD) on transition metal substrates becomes the most promising approach,
since this method is inexpensive, highly productive, reproducible and easy to scale up.
Although chemical vapor deposition of graphene seems to be a new research field
that emerged after 2004, the formation of graphite films on hot transition metal surface
through carbon precipitation from carbon–metal solid solution has been known for over
50 years.
19, 32
The first work of controllable synthesis of monolayer and few-layer CVD
graphene on Ni substrate was reported in 2008.
106
Since then, CVD of large-scale gra-
phene with good quality has been explored from all kinds of aspects. To date, graphene
8

growth has been demonstrated on various transition metals: Ni, Cu,
45
Ru,
86
Pt,
23
Co,
88, 92

Ir,
14, 15
Rh,
27
Au,
67
Pd.
37
Among all of them, Ni and Cu are the two most important and
intensively studied materials. First of all, Ni and Cu have good catalytic power to form
high quality graphite films; Secondly, Ni and Cu are relatively cheaper and easier to ob-
tain than other metals, and easy for graphene transfer after growth. The carbon solubility
in Ni is much higher than that in Cu: for Ni, it is 0.6 weight % at 1326 ˚C, and for Cu, it
is 0.001~0.008 weight% at 1084 ˚C. The difference of carbon solubility in Ni and Cu
causes two distinctive growth mechanisms. For graphene formation on Ni, the growth
mechanism has been suggested to be a segregation process, leading to difficulty in sup-
pression of multi-layer formation. However, the graphene growth on Cu is suggested to
be a surface reaction process, which is self-limiting, robust, and insensitive to growth
parameters such as cooling rate and metal film grain boundary.
47

1.4.1 Growth process
A simple CVD setup requires a chamber or quartz tube for graphene growth, a gas
feedstock system and exhaust system connected to the chamber/tube to supply gas, and a
furnace to heat up the chamber/tube to the desired temperature. Graphene can be grown
on transition metal substrates under ambient pressure or low pressure. If low pressure is
used, a pressure control system consists of a pressure gauge, a valve and a pump is need-
ed. Figure 1.4 shows a schematic diagram of a general low pressure CVD setup in re-
search labs. Before growth, a metal substrate is loaded into the quartz tube. The size of
the metal substrate is only limited by the dimension of the quartz tube. Then, the quartz
9

tube is evacuated or purged with argon or hydrogen to remove air. During the CVD pro-
cess, the quartz tube is heated up, and gas species, usually hydrocarbon and hydrogen,
sometimes including argon for protection or dilution, are fed into the quartz tube, passing
through the hot zone, where hydrocarbon precursors decompose to carbon atoms at the
metal substrate surface. The synthesis time is from 10 minutes to 30 minutes, and then
the temperature of furnace is brought down to room temperature.
Based on different growth mechanisms for different metal catalysts, those active car-
bon atoms will either form single-layer and few-layers graphene on metal surface directly
at high temperature, or dissolve into metal films to form a carbon-metal solid solution,
and precipitate out upon cooling to form graphene films. Here, transition metals act as a
catalyst to crack hydrocarbon and provide a low energy pathways for reaction of forming
carbon rings. Figure 1.5 is a flow diagram of a general graphene growth recipe, where
Figure 1.4: Low pressure CVD system for graphene synthesis
10

methane is used as hydrocarbon source and copper foil is used as metal substrate. By
using plasma-enhanced chemical vapor deposition (PECVD), growth temperature could
be decreased from 900~1000°C to 500~600°C, because plasma can also provide enough
energy to generate active carbon species.

1.4.2 Metal substrate pretreatment
To improve graphene formation, metal substrates need to go through multiple pre-
treatment processes. Take copper substrate as an example. The commercial available
copper film mostly used by research groups is a 25 μm-thick polycrystalline foil. Prior to
loading it into chamber, this piece of copper foil is dipped into acetic acid to remove ox-
ide layer, and cleaned by DI water and organic solvent (acetone/IPA) to remove impuri-
Figure 1.5: Flow chart of low pressure graphene CVD recipe on Cu foil
0
200
400
600
800
1000
1200
0 20 40 60 80 100 120 140
Time (min)
Temperature ( °C)
Ramping Annealing Growth Coolingdown
Pressure = 40mTorr
H
2
= 7 sccm
Pressure = 500mTorr
H
2
= 50 sccm, CH
4
= 7sccm
11

ties, then blow dry. During temperature ramping, copper foil is usually protected in hy-
drogen atmosphere to avoid oxidation. At elevated temperature, before flow hydrocarbon,
copper foil is kept in hydrogen atmosphere for some time to further remove the oxidiza-
tion layer and increase copper grain size. This process is called pre-annealing. After an-
nealing, copper surface uniformity is better, and defect density is decreased, which lead
to improved graphene quality.

1.4.3 Graphene transfer
Unlike exfoliation method, for CVD graphene, a transfer process after growth is typi-
cally necessary for further characterizations and applications. Currently, there are two
methods for transfer: wet etching and bubbling transfer. Wet etching is mostly applied to
those relatively cheap metal films, which are also easily to be etched away by chemical
etchants. A schematic diagram of the wet etching transfer process is shown in Figure 1.6.
Graphene is first spin-coated by a thin layer of polymethyl methacrylate (PMMA), and
baked at 120 °C to evaporate the solvent. The metal layer (Ni, Cu, or Cu foil) is then re-
moved by Ni/Cu etchant, leaving only PMMA/graphene film floating on the solution.
Then the film is cleaned by DI water, picked up by a targeting substrate, left in room
temperature until the water between graphene and substrate evaporate away, and baked at
around 100 °C to further remove trapped water vapor, and baked at higher temperature
(up to 180 °C, depending on the targeting substrate and graphene morphology) to im-
prove adhesion of graphene to the substrate. Finally, PMMA film is removed by hot ace-
tone, leaving graphene film on top of the targeting substrate.
12

For those chemically inert and expensive metal films such as Pt, a bubbling transfer
process is more suitable. After CVD growth, same as wet etching, graphene on Pt sub-
strate is coated with a PMMA layer, followed by baking. Then the PMMA/graphene/Pt is
dipped into a NaOH aqueous solution and used as the cathode of an electrolysis cell.
23
At
cathode, a water reduction reaction takes place to produce H
2
bubbles at the interface
between the graphene and Pt substrate. The formation of a large number of H
2
bubbles
detaches PMMA/graphene layer from the Pt substrate in just few seconds. After that, the
floating PMMA/graphene layer is ready to be cleaned and picked up by targeting sub-
strates. The following steps are similar as wet etching.

1.5 Introduction of Hexagonal Boron Nitride
Hexagonal boron nitride (h-BN) is a layered material similar to graphite. It is com-
prised of alternating boron and nitrogen atoms in a honeycomb arrangement. Within each
layer of hexagonal BN, boron and nitrogen atoms have the same number and are bound
Figure 1.6: Schematic diagram of the transfer process
13

together by covalent bonds, whereas the layers are held together by weak van der Waals
forces. It is used as a good lubricant at both low and high temperatures and in vacuum.
Recently, h-BN receives lots of attention due to its attractive properties as a promis-
ing two-dimensional dielectric substrate for graphene electronics.
16, 22
First, it is an insu-
lator with a wide band gap ( ∼5.9 eV) and relative dielectric constant of 4. Secondly, h-
BN has an atomically smooth surface that is relatively free of dangling bonds and charge
traps. Thirdly, it has a similar lattice constant and hexagonal structure to graphene which
enable the tuning of its band gap. Last but not least, it has excellent thermal and chemical
stability. All of these figures make h-BN a perfect substrate for graphene electronics.
Boron nitride has amorphous forms and three crystalline forms: hexagonal, cubic and
wurtzite structure, and the hexagonal one is the most stable crystalline form. Currently
there are two methods to prepare h-BN films. One is mechanical exfoliation by scotch
tapes, and the other is chemical vapor deposition. Similar as those methods applied to
produce graphene, mechanical exfoliation produces h-BN with good quality, but sample
size is small and the thickness and location is uncontrollable (Figure 1.7 (a). On the other
hand, CVD approach produces materials with large scale and controllable thickness, but
crystalline structure is poorer, and there are defects in the lattice and impurity particles on
the film surface (Figure 1.7 (b)).
78

All the characterization methods mentioned previously for graphene can be applied to
h-BN. The typical thickness of single-layer h-BN is around 0.42 nm measured by AFM,
81

and the dominant Raman peak is at 1368 cm
-1
which can be assigned to the E
2g
vibration
mode of h-BN. Besides these methods, Fourier transform infrared spectroscopy (FTIR)
14

can analyze the h-BN lattice vibration modes, X-ray diffraction (XRD) can be used to
confirm the hexagonal crystal structure, and X-ray photoelectron spectroscopy (XPS) can
be applied to characterize the elemental stoichiometry of the synthesized h-BN samples
to see whether the B/N ratio is close to 1:1 and the sample is free of other impurities.
78
Figure 1.7: Exfoliated and CVD few-layer h-BN
(a) Optical image of exfoliated h-BN on SiO
2
/Si substrate. (b) Optical image of few-
layer CVD h-BN transferred on SiO
2
/Si.
15

Chapter 2
Growth of Large-grain Single-Crystalline Gra-
phene
2.1 Motivation
Graphene grows on metal substrates follows the sequence of nucleation, early-stage
growth, enlarging grain size, and merging together to form continuous graphene. In most
cases, graphene growth on metal substrates is not epitaxial and graphene nucleation is
randomly located on metal surface. The force between Cu substrate and graphene is very
weak
71, 99
. Therefore, the graphene grains are commonly not well aligned on the metal
substrates, and neighboring graphene grains may have different orientations.
In Figure 2-1(a), graphene grown on Cu foil is hexagonal in shape, and the SEM im-
age shows that the grain orientation is random in this region. When those grains merge
together, there will be grain boundaries formed between those grains. Even when the
whole grain is nucleated from the same site, it is possible that different lobes may show
different orientations with copper.
94

High resolution TEM study shows that the atomic structure of grain boundaries is a
series of disordered carbon rings, such as pentagons and heptagons instead of hexagons
(Figure 2.1(b))
31
. Studies found that grain boundaries negatively impact both transport
16

and mechanical properties
102, 105
. As a result, people put lots of effort to produce large-
grain single-crystalline graphene to facilitate its applications.
48, 49, 91, 98, 101

The early attempts to synthesize large grain graphene were done by Ruoff’s group in
2010
48, 49
. In one paper, they used a manually made copper bag (Figure 2.1(c)) and syn-
thesized large grain graphene with dendritic edges on the inner side of copper bag under
low pressure (Figure 2.1(d)). The graphene was characterized to be single crystals and the
size was up to 0.3 mm. They believed that the very low methane concentration and an
improved environment inside the Cu enclosure, where Cu vapor was in static equilibrium,
were the main reasons for low density of nuclei and thus large grain size. But this meth-
od has one drawback: the Cu enclosure is manually made and not quite reproducible.

Figure 2.1: Graphene grain formation and large grain graphene
(a) SEM image of hexagonal graphene grown on Cu foil. (b) High resolution TEM
shows atomic structure of graphene grain boundary. (c) Copper foil enclosure prior
to insertion in the furnace. (d) SEM image of graphene flowers grown by Cu enclo-
17

2.2 Vapor-trapping Synthesis of Graphene Flowers
In this work, we used a vapor trapping method to grow large-grain, single-crystalline
graphene flowers with controlled grain morphology as illustrated in Figure 2.2(a)
112
. Cu
foil was rolled up and put into a half inch quartz tube, which is open at one end and close
and the other end. We call it the “vapor trapping tube”. Then, the small tube was placed
inside a two-inch quartz tube of the CVD chamber with the open end facing to the gas
stream. Gases flown into the vapor trapping tube would be trapped inside, and therefore
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. A typical growth recipe was
as follows: 7 sccm H
2
was introduced to the 2 inch quartz tube CVD chamber at 40
mTorr, and the temperature was raised to 1000 °C in 40 minutes, and the Cu foil was
annealed at 1000 °C for 20 minutes. Then, 1 sccm CH
4
and 12.5 sccm H
2
were intro-
duced 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 tempera-
ture with the CH
4
and H
2
flow continuing. After growth, Cu foil was taken out of the va-
por trapping tube for characterization.
SEM images shows six-lobe graphene flowers grown on the bottom side of Cu foil
placed inside the vapor trapping tube (Figure 2.2(b-c)). The size of graphene flowers was
up to 100 μm. By varying the growth parameters, four-lobe graphene flowers were also
observed (Figure 2.2(d)). On the other side, the graphene grown on the Cu foil placed
outside the small vapor trapping tube was continuous film with slight etching (Figure
18

2.2(e)). This pronounced difference between graphene grown on Cu foil inside and out-
side 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 qua-
si-static reactant gas distribution which results in large flower-shaped graphene grains.

Figure 2.2: Graphene flower by vapor-trapping growth
(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 out-
side the vapor-trapping tube.
19

2.3 Characterization of Graphene Flowers
2.3.1 Raman characterization
The graphene flowers were also transferred onto Si/SiO
2
substrates for further inves-
tigation. SEM image and OM image of a six-lobe graphene flower are shown in Figure
2.3 (a) and (b). The color contrast of the graphene is very uniform in both SEM and the
optical image, except the central part which is darker than the lobes in a hexagonal shape.
Raman spectra were taken from different locations on the graphene flower, indicating by
letters in Figure 2.3(b). The Raman spectrum in black was taken from the area outside the
graphene flower and did not show any G or 2D peak of graphene as expected. The Ra-
man spectrum in red was taken from the area of graphene lobes and presents typical fea-
tures of single-layer graphene: the I
2D
/I
G
ratio is ~0.5, and the full width at half-maximum
(FWHM) of 2D band is ~33 cm
-1
. The Raman spectrum in blue collected from the center
of the graphene flower represents bilayer graphene
9
with I
2D
/I
G
ratio ~1 and the FWHM
of 2D band ~53 cm
-1
. This is a typical spectrum for AB stacking bilayer graphene.
To further investigate large surface area of the graphene flower, Raman mapping was
performed on the whole flower and maps of I
G,
I
2D
, I
2D
/I
G
ratio and I
D
were collected and
shown in Figure 2.3(d), (e), (f) and (g), 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 (less than 5µm) in the center, which is believed to
be the nucleation site.
20

2.3.2 TEM characterization
To confirm that each graphene flower is a single-crystalline grain, we transferred gra-
phene to perforated SiN TEM grid and used selected area electron diffraction (SAED) to
study the crystalline structure and grain size of graphene. SEM image of graphene on
TEM grid in Figure 2.4(a) shows that graphene retains its flower shape after transfer.
Figure 2.4(b) is a zoomed-in SEM image of the graphene flower highlighted by yellow
dashed line in Figure 2.4(a). In order to measure the grain size of graphene, we did SAED
Figure 2.3: Raman spectra and mapping of graphene flower
(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 (b). (d-f) Raman map of I
G
, I
2D
, I
2D
/I
G
ratio and I
D
. Scale bar for (d)-(g)
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); 120, 180, 240 (g)
21

on graphene at every opening within the graphene flower, and compared the orientation
of the diffraction patterns. As shown in Figure 2.4(b), three different kinds of diffraction
patterns were marked by white letters, yellow letters, and blue letters, respectively. The
openings which were not covered by graphene membrane were marked by red crosses.
Figure 2.4: TEM characterization of graphene flower
(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
(a). 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 gra-
phene) or a red cross (no graphene covered). (c) TEM image of graphene covered one
TEM grid opening. (d)-(f) Diffraction patterns taken from opening H, Y, and O, re-
spectively. (g) A bright field TEM image of graphene suspended on SiN TEM grid. (h)
Diffraction pattern taken from opening BB. (i) Diffraction pattern taken from opening
AA. (j) A bright field TEM image of folded graphene taken from opening AA.
22

Figure 2.4(c) and (g) show 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. Figure 2.4 (d), (e), and (f) are images of three
representative diffraction patterns from opening H, Y, and O. We observe that the open-
ings marked with white letters all show one set of symmetric six-fold electron diffraction
pattern that orientates to the same direction. These white letters cover all six lobes of the
graphene flower, indicating that the graphene flower is a single-crystalline grain.
Figure 2.4 (h) 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 AB stacking bilayer graphene
100
. This observation is also in accordance with
the Raman spectra showing that the center of graphene flower consists of bilayer gra-
phene.
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, mainly during the removal of PMMA by hot acetone. Figure 2.4 (i) shows dif-
fraction pattern taken from opening AA, which is covered by torn and folded graphene
film as the bright field TEM image (Figure 2.4(j)) shows. The folded graphene membrane
becomes multiple-layered, and hence the diffraction patterns show multiple sets of dif-
fraction spots. 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 Figure 2.4 (i).
23

Once we exclude torn and folded graphene due to the transfer, we can conclude that
SAED confirms that the graphene flower is a single-crystalline graphene grain, and the
center of the graphene flower is AB stacking bilayer graphene, which is consistent with
the Raman characterization.

2.4 Growth Mechanism Study
2.4.1 Growth parameters study
For graphene chemical vapor deposition on copper, various crystal grain morpholo-
gies have been reported, including six-lobe flowers,
53, 112
four-lobe stars,
48
dendritic
snowflakes,
49
squares,
91
and hexagons.
96, 98
The morphology dependence on growth pa-
rameters is a relatively complicated issue due to the polycrystalline nature and high sur-
face roughness of the Cu substrate, which causes local variations in growth conditions
across the substrate. Among all the parameters (growth temperature, Cu crystal orienta-
tion, growth time, flow rate, etc.), two factors, total growth pressure and methane-to-
hydrogen-ratio (CH
4
:H
2
), are found to the dominant factors that affect the graphene mor-
phology significantly. To investigate the correlation between the grain morphology and
those two factors, 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 mTorr)
to conduct a series of growth process.
Figure 2.5 shows the growth results using various conditions when a reaction time of
30 minutes and growth temperature of 1000°C was used. With methane-to-hydrogen ratio
24

fixed at 1:12.5, we carried out graphene vapor trapping CVD at total pressure of 80, 100,
125, 150, 200, 300, and 400 mTorr. Corresponding SEM images are shown in the right
column in Figure 2.5. 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. 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 left column of SEM images in Figure 2.5, graphene grains were small and close to
hexagonal shape at 1:30 ratio, and then changed to mostly four-lobe structures when the
methane-to-hydrogen ratio increased to 1:20. The grains exhibited shapes 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 CH
4
:H
2
to 1:10, the graphene grains became
irregular. For CH
4
:H
2
=1:5, the graphene islands tended to connect with each other, leav-
ing only small gaps in between. When CH
4
:H
2
was brought up to 1:2, graphene grew into
continuous film with multi-layer patches in some locations.

25

Figure 2.5: SEM images of graphene grown using various recipe
26

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 diffu-
sion/deposition and hydrogen etching
110
. 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 direc-
tions to grow into graphene lobes, and when the carbon supply increases further, the gra-
phene 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 pres-
sure as the underlying mechanism includes both carbon diffusion/deposition and hydro-
gen etching.

2.4.2 Growth time study
To study the evolution of graphene grain morphology with growth time, we carried
out graphene CVD using different growth time under the pressure of 300 mTorr and
CH
4
:H
2
= 1:12.5, and results are shown in Figure 2.6. We observed mostly six-lobe gra-
phene 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
27

morphology became irregular due to disturbed diffusion path of carbon by adjacent flakes.
The adjacent graphene nucleuses seem to squeeze each other, and compete for more car-
bon source to grow and enlarge the size, which disturb the active carbon gradient and
affect the grain morphology. This effect only occurs when the neighboring grains are
close enough. Also, we found that the nucleation density of different growth time was
similar, indicating that nucleation only happens during the initial stage of growth.

2.4.3 Seeded growth
In order to study the effect of nearby grains on the graphene flower morphology, we
need to intentionally control the nucleation site. We used e-beam lithography to pattern
Figure 2.6: 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)
28

PMMA seeds on Cu foil. PMMA as an e-beam resist was over exposed at the seed area
and cross-linked
96
. After develop, PMMA on unexposed area was removed, leaving the
seed array on the copper surface. Since PMMA is a carbon based material, we believe
that the seeds will act like as carbon-rich points on Cu foil, which serve as nucleation
centers as well as provide carbon source initiating graphene growth. Figure 2.7(a) shows
patterned PMMA seeds on Cu foil. The distance between each seed is 20 μm and the size
of seeds is 0.8 μm.
The patterned Cu foil was then loaded into a vapor-trapping tube in CVD system for
growth. The growth recipe was similar as the typical one for graphene flowers, with sys-
tem pressure of 190 mTorr during growth, CH
4
:H
2
=1:12.5, and growth time of 20min.
Figure 2.7(b) is a SEM image taken at the edge of patterned area and non-patterned area
after CVD growth. On the patterned area, graphene flowers nucleated from seeds and
form uniform array. A few spots without seed patterning had graphene nucleated, which
may due to defects on Cu foil. On the other hand, the graphene flowers grown on the
non-patterned area were random without any order. This result indicates that the pre-
patterned seeds are effective for the control of graphene nucleation. In Figure 2.7(c) and
(d), the zoom-in SEM images clearly show the difference in graphene flower shape. The
graphene flowers on the non-patterned region have uniform four-lobe structure, while
shapes of most graphene flowers on the patterned area are irregular. Since these regions
are close, the local environment like temperature and gas flow is almost identical during
the growth. The distinction in the morphology should be mainly caused by competition of
neighboring grains for carbon source.
29

2.4.4 Electron backscatter spectroscopy study
Although the thermodynamics of the growth system remains the same regardless
whether the absolute pressure is atmospheric pressure (AP), low pressure (LP) or under
ultrahigh vacuum conditions (UHV), the kinetics of the growth phenomenon are different,
leading to a variation in the graphene morphology.
5
After annealing, the surface of the Cu
Figure 2.7: SEM images of graphene flower seeded growth
(a) SEM image of PMMA seeds pattered on Cu foil by E-beam lithography. (b)
SEM image of graphene flower arrays grown by vapor trapping method on Cu foil
with patterned PMMA seeds. Yellow dashed line indicates the boundary with and
without PMMA seeds. (c) Zoom-in SEM image of graphene flowers grown from
PMMA seeds. (d) Zoom-in SEM image of graphene flowers grown on the area with-
out PMMA seeds.
30

foil is mainly in (100) orientation which is 4-fold symmetric. But the graphene honey-
comb lattice has a 6-fold symmetry. At very low pressure, graphene morphology is large-
ly related to the Cu crystal orientation. Under low pressure condition (eg: below 10 Torr),
graphene islands typically have shapes with high perimeter-to-area ratio, such as lobes,
flowers, and dendrites, and the grain shape is remarkably affected by the CH
4
:H
2
ratio.
Increasing hydrogen partial pressure can suppress the dendritic growth and lead to islands
with smoother and more planar edges. At atmospheric pressure, there is not a clear rela-
tionship between the graphene island orientation and the Cu surface crystallography due
to the weak interaction between graphene and Cu substrates, and graphene grains are
more likely to develop into 6-fold symmetry, which is a hexagon.
To further study the correlation between the morphology of graphene grains and the
underneath copper surface, electron backscatter diffraction (EBSD) was used to investi-
gate the copper surface after graphene growth. EBSD is conducted using a SEM equipped
with an EBSD detector and can be used to index the crystallographic orientation of cop-
per films underneath the graphene islands directly. Figure 2.8 shows EBSD images of
copper surface covered by graphene flowers with different shapes. Figure 2.8 (a), (b), and
(c) are SEM images of graphene flowers grown at different locations on the same copper
substrate with images taken with the sample tilted at 70° for EBSD. The graphene grains
on this sample were mostly six-lobe flowers, with a few four-lobe flowers in some loca-
tions. Corresponding EBSD quality map and orientation map images Figure 2.8 (a), (b),
and (c) are shown in Figure 2.8 (e), (f), (g), and (i), (j), (k), respectively. EBSD orienta-
tion map image in Fig. 2-8 i and j show similar but slightly different green colors which
31

are close to Cu (110), as indicated in the color scale bar. EBSD orientation map in Figure
2.8 (k) shows orange-magenta color, indicating a crystal orientation close to Cu (100).

Moreover, we investigated a sample with copper grain boundaries and both four-
and six-lobe graphene flowers grown on the surface. Figure 2.8(d) shows a SEM image
Figure 2.8: EBSD mapping of graphene flowers
(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) Cor-
responding EBSD quality image of the location in a, b, c, and d, respectively. (i)-(l)
Corresponding EBSD orientation map image of the location in e, f, g, and h, respec-
tively. The color represents fcc crystalline orientation is shown on the right side.
32

of a graphene sample grown using a different recipe. Copper grain boundaries and both
four- and six-lobe graphene flowers are shown in the SEM image. The corresponding
EBSD orientation map in Figure 2.8 (l) shows crystalline index close to Cu (100), with
slight 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. All the above information indicates that graphene
grains can grow into different morphology on the same copper substrate, and the under-
neath copper crystalline orientation is not closely related to the morphology of the gra-
phene grains grown on top.
Although previous study show that for the CVD growth on polycrystalline copper foil,
substrate crystallography affects graphene growth by different growth rate, defects num-
ber and nucleation density,
95
the results from EBSD illustrate that the morphology of
graphene grains do not have much correlation with the crystalline structure of the under-
neath copper substrate. In fact, both four-lobe and six-lobe graphene morphology have
been observed and reported by many research groups on polycrystalline copper foil or
single crystal copper
48, 49, 71, 89, 94
. The major reason might due to the weak interaction
between graphene and underneath copper substrate. Therefore, we believe that the mor-
phology 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 correla-
tion with the beneath copper substrate.

33

2.5 Electric Property Measurement of Graphene Flow-
ers
To further evaluate the quality of the large-grain single-crystalline graphene flowers,
we transferred the as-grown graphene flowers onto a highly doped p-type silicon sub-
strate with 300 nm thermal oxide as the gate dielectric and fabricated back-gated gra-
phene FETs. 5 nm Ti and 50 nm Pd were deposited as source and drain electrodes by e-
beam evaporation. Figure 2.9 (a) shows a SEM image of the graphene FET. Five elec-
trodes were marked from A to E, and a central electrode was marked as F. Figure 2.9(b)
shows a representative plot (shown in black circles) of drain current (I
ds
) versus gate volt-
age (V
g
) 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 μFE using the equation

(2.1)
where R
total
= V
ds
/I
ds
is the total resistance of the device including the channel resistance
and contact resistance Rc, 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
113
. The capacitive carrier density n is
related to the gate voltage via the equation

(2.2)
where the term on the right-hand side of Equation 2.2 describe the carrier density induced
by the back gate via the back-gate electrostatic capacitance C
ox
. We did not include the
34

quantum capacitance in graphene on the right-hand side of Equation 2.2 since it was neg-
ligible 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
Figure 2.9(b) was the curve for fitted field effect mobility. Without a well-defined chan-
nel 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 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, indicating the high
quality of the synthesized graphene flowers. Figure 2.9(c) is a plot of drain current (I
ds
)
versus drain voltage (V
ds
) at various gate voltages. The drain current increases linearly
Figure 2.9: Back-gate graphene flower device measurement
(a) SEM image of a six-lobe graphene FET. Electrodes are marked by different let-
ters. The dashed blue square is the region of effective graphene channel between elec-
trodes 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. (c) Plot of
drain current (I
ds
) versus drain voltage (V
ds
) at various gate voltages.
35

with the increase of drain voltage at different gate voltages, indicating the Ohmic contact
between graphene and Pd electrodes.
The graphene devices on Si/SiO
2
substrates are highly disordered because of the dan-
gling bonds of SiO
2
and charge traps between graphene and SiO
2
. To further increase the
device mobility, a better dielectric substrate is highly desirable. Hexagonal boron nitride
(h-BN) is an appealing substrate for this purpose. We exfoliated h-BN onto Si/SiO
2
sub-
strate with pre-patterned align marks and transferred large-grain graphene flowers on it.
The graphene grains will randomly attach to the BN and the regions are defined under
SEM. Then, Hall-bar structure was fabricated on exfoliated graphene on h-BN (Figure
2.11 (a)) by e-beam pattern and oxygen-plasma etching. 5 nm Ti and 50 nm Au were
deposited as source and drain electrodes. The whole process is illustrated in Figure 2.10.

Figure 2.11(b) 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 13.5 μm and
width of 4.5 μm. We also fitted the curve with the same equation used in the back-gated
Figure 2.10: Graphene/h-BN device fabrication steps shown schematically
Firstly, h-BN is exfoliated onto a SiO
2
substrate with existing alignment marks. After
that, CVD grown graphene flower is transferred onto the entire chip. Then, gra-
phene is patterned with e-beam lithography and reactive ion etching. In the end, S/D
electrodes are fabricated with e-beam lithography and e-beam evaporation of Ti/Au.
36

device on Si/SiO
2
(shown in blue curve). 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
34
and from 8,000 to 13,000 cm
2
V
-1
s
-1
from Gannett et al
22
. Our large-grain graphene
flower shows great potential for the application of high mobility graphene-based nanoe-
lectronics.

2.6 Growth of Large Grain Hexagonal Graphene
Several pieces of studies have found that graphene growth is not limited by the Cu
substrate underneath, which means graphene can grow over Cu grain boundaries
70
. In
principle, as long as graphene nucleation density is low enough, for example, one nuclea-
tion per cm
2
, graphene can grow into millimeter grain size. However, as simple as we
expected, it is quite tricky to decrease nucleation density in the experiment. Impurities,
Figure 2.11: Graphene/h-BN flower device measurement
(a) SEM image of a hall-bar graphene/h-BN FET. (b) Plot of drain current (I
ds
) versus
gate voltage (V
g
).
37

surface roughness, grain boundaries can act as carbon source sink and initiate nucleation.
One straightforward method to suppress nucleation is to decrease carbon source by reduc-
ing methane partial pressure, and increase hydrogen partial pressure to facilitate the etch-
ing effect which is the reverse of the graphene growth reaction. This can be done by
flowing highly diluted methane with large flow rate of hydrogen, or by tuning the system
pressure and grow under low pressure. One issue related to this strategy is that under such
conditions, the graphene growth rate would be extremely slow, and it may take even days
to grow a large grain graphene with reasonable size, which is not energy efficient. There-
fore, besides the vapor-trapping growth under low pressure, we applied diluted methane
in argon and synthesized hexagonal graphene under ambient pressure.

2.6.1 Cu substrate pretreatment
During our experiment, we found that Cu foil quality is a key factor in graphene
growth. Cu substrates from various vendors may present completely different properties
for graphene growth under the same condition, probably due to their different purity,
thickness, processing procedure, etc. Therefore, it is critical to develop a routine to im-
prove the Cu foil quality in a reproducible and convenient way.
Previously, people used acetic acid and organic solvents (IPA, acetone) to remove the
native oxide and other contaminations on the Cu surface before loading it into the furnace
for growth. But some of the impurity particles may still be trapped inside the scratches on
the surface. After growth, we observe graphene islands nucleate and grow along those
defective lines. For this reason, we applied electro-chemical polishing (ECP), which is a
38

technique widely used to improve micro smoothness, micro topology, and material
brightness by anodic dissolving of the substrate in an electrolyte with an external source
of electricity. We made a simple polishing setup by clipping two pieces of Cu foils as
anode and cathode, dipping them into the electrolyte solution, and connected to voltage
supply (Figure 2.12(a)). We found that the optical condition for polishing is using 2V
with 75% phosphoric acid, 15% EG (ethylene glycol) and 10% DI water as the electro-
lyte. After 15 min polishing, the Cu foil surface becomes more smooth and free of parti-
cles compared to the one without polishing, as the SEM images shown in Figuer 2.12 (b)
and (c).

To confirm the Cu foil after ECP treatment have better quality for graphene synthesis,
we loaded the half polished Cu foil into the furnace and did CVD by flowing 25sccm 0.1%
CH
4
in Argon, 30sccm H
2
, and 455 sccm Argon for growth under ambient pressure at
1025 °C for 30 min. After growth, the backside of the Cu foil was examined under SEM.
In Figure 2.13, the SEM image shows the boundary of the polished and unpolished area.
Figure 2.12: ECP treatment of copper foil
(a) Photo of experiment set up for electro-chemical polishing. (b) SEM image of 25
μm Cu foil. (c) SEM image of 25 μm Cu foil after 10 min polishing.
39

It is quite obvious that the nucleation density in the top region is much higher than that in
the bottom area. This verifies that ECP does remove the surface defects and decrease the
nucleation density effectively.

2.6.2 Annealing condition improvement
Besides the quality of Cu foil, the annealing condition also affects the graphene
growth. For graphene synthesized at low pressure, the pre-annealing process is to flow
hydrogen at low pressure for 20 to 30 min. Under high temperature and low pressure, Cu
atoms move fast on the surface, evaporate and diffuse to the low temperature zone, which
takes away some impurities, but also generates surface defects. To suppress Cu evapora-
tion at high temperature, high pressure annealing can be applied. We tried annealing at
low pressure (1 Torr) and high pressure (1500 Torr) and did growth at the same condition
Figure 2.13: SEM image of as grown graphene on half-polished Cu foil
The red dash line is the boundary of polished and unpolished area.
40

with different growth time, and the results are shown in Fig. 2-14 (a) and (b). Under the
same SEM magnification, the low pressure annealing sample has more than ten nuclea-
tion sites in this area, while the high pressure annealing sample only has one nucleation
site, indicating the success of the high pressure annealing. But after long time annealing
at high pressure, there are many white particles deposited on the sample surface, which
might come from the impurities imbedded deep inside the bulk Cu. These undesirable
particles were also deposited on the graphene surface and hard to remove. By combining
low pressure and high pressure annealing, we may be able to decrease the nucleation
density and get rid of these particles.

2.6.3 Growth condition improvement
We also found that when growth temperature rises, desorption rate of active carbon
from the surface will also increase, which will lower down the possibility for carbon ab-
Figure 2.14: Effect of different annealing pressure
(a) SEM image of as grown graphene on Cu foil for 10 min with pre-annealing in 1
Torr Ar/H
2
at 1070 °C for 20min. (b) SEM image of as grown graphene on Cu foil for
30 min with pre-annealing in 1500 Torr Ar/H
2
at 1070 °C for 4 h.
41

sorption and graphene nucleation. So we applied temperature of 1075 °C which is close
to the Cu melting point for large grain graphene growth. The typical graphene growth
recipe is as follows: ECP polished Cu foil is loaded into 1 inch furnace and the quartz
tube is evacuated by mechanical pump for 5 min. Then, 30 sccm H
2
was introduced to the
chamber at 500 mTorr, and the temperature rose to 1075 °C in 40 minutes. Then, 100
sccm Argon and 100 sccm H
2
were introduced and mechanical pump was shut off to
bring the system pressure to ~1400 Torr. Then Cu foil was annealed at 1075 °C for 120
minutes. After that, the pressure of the system was brought up to ambient pressure, and
20 sccm 0.1% diluted CH
4
in Ar, 30 sccm H
2
and 450 sccm Ar were introduced into the
chamber for graphene growth. After 2 h, the CVD chamber was cooled down to room
temperature. The typical growth results are shown in Fig. 2-15 (a) and (b). By baking the
Cu foil on the hotplate for 10 min at 200 °C, the Cu uncovered by graphene was oxidized
while the area covered by graphene was protected
91
. Therefore, it shows contrast under
the optical microscope and we can spot the graphene grains directly, as Fig. 2-15 (a)
shows.
From the SEM image, we can see that the graphene grain is hexagonal, and the diag-
onal length is up to 400 μm. From these primary results, we found the chief factors for
large grain graphene synthesis: low methane concentration, high hydrogen partial pres-
sure, high growth temperature and long-time annealing at high pressure. By combining
all those conditions and fine tuning the recipe, it is anticipated to be able to synthesize
large grain graphene and push the grain size to its limit.

42

2.7 Conclusion
In summary, we developed a vapor trapping method to grow large-grain, single-
crystalline graphene flowers with controlled grain morphology and grain size up to 100
μm. Raman spectra indicate that the graphene flowers have high quality single-layer gra-
phene 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 of ~4,200 cm
2
V
-1
s
-1

Figure 2.15: Large grain hexagonal graphene
(a) Optical image of as grown graphene on Cu foils with 10 min air oxidation at
200 °C. (b) SEM image of large grain graphene grown on polished Cu foil.
43

on Si/SiO
2
and ~20,000 cm
2
V
-1
s
-1
on h-BN was extracted, indicating that the large-grain
single-crystalline graphene is of great potential for graphene-based nanoelectronics.
For the large-grain hexagonal graphene synthesis by ambient pressure CVD, copper
foils is found to a key factor. Electro-chemical polishing, high temperature and long-time
annealing of the Cu foil helped to reduce the volatile impurities, contaminants, and de-
fects on Cu surface, thus leading to the suppression of graphene nucleation. The synthe-
sized graphene had a hexagonal shape and the grain size was over 400 µm.

44

Chapter 3
Bilayer Graphene Synthesis by Chemical Vapor
Deposition
3.1 Motivation and Related Work
Single layer graphene is a promising material to replace silicon for the next genera-
tion electronics. However, single layer graphene is intrinsically semimetal and has no
energy band gap. To open up a bandgap, people tried chemical doping or patterning na-
nometer-width graphene ribbons.
29, 79, 80, 82
Recent studies have shown that bilayer gra-
phene (BLG) and trilayer graphene (TLG) with two or three monolayer graphene stacked
together demonstrated rich physical properties lacking in monolayer graphene. Both ex-
perimental and theoretical investigations reveal that AB-stacked BLG
111
and ABC-
stacked TLG
57
exhibit a tunable band gap when a perpendicular electric field was applied.
On the other hand, for twisted BLG, the electronic properties are quite sensitive to the
rotation angle.
55
This enables band structure engineering by controlling stacking order
and interlayer coupling in BLG and TLG, as Figure 3.1 shows. This unusual characteris-
tic of bilayer graphene has attracted significant interest for fundamental studies, and the
unique ability of controlling the band gap and the exciton energy can lead to new possi-
bilities of bilayer graphene based nanoelectronic and nanophotonic devices operating at
room temperature.
45

Earlier studies on bilayer graphene were based on the exfoliated sample, which are all
Bernal stacking. Later on, bilayer graphene with various stacking order was synthesized
on Cu foil by chemical vapor deposition, and the characteristic properties for different
stacking orders were characterized by Raman and TEM. The earliest work of bilayer gra-
phene CVD was done by Zhong’s group
39
. They successfully synthesized wafer scale
homogeneous bilayer graphene by CVD using pure methane flow with depletion of hy-
drogen (70sccm CH
4
), low growth pressure (0.5Torr), and slower cooling rate
(18°C/min). Since the graphene grown on copper is predominantly monolayer, to break
the self-limit effect of the growth process, Zhongfan Liu’s group introduced a second
growth process.
100
They first synthesized uniform monolayer graphene on Cu foil, and
then placed another piece of fresh Cu foil at the upstream as an efficient catalyst to con-
tinuously decompose CH
4
and did a second step growth at the downstream copper film.
Figure 3.1: Strucure of bilayer graphene
(a) Schematic diagram shows that AB stacking bilayer graphene has a tunable band
gap with a vertical electric field. (b) ) Schematic diagram shows two overlaid gra-
phene lattices generate Moiré patterns.
46

At high temperature, carbon radicals or small carbon fragments generated from the up-
stream Cu foil were transported downstream and deposited epitaxially onto the existing
monolayer graphene film, arranging into thermodynamically stable Bernal stacking struc-
ture and forming hexagonal bilayer regions.
Most bilayer or few-layer graphene synthesized on Cu foil by CVD are not homoge-
neous, which means that the two layers do not have the same size. After the monolayer
regions merge together to become continuous, the bilayer regions show as isolated is-
lands in the monolayer region. These smaller regions, referred to as adlayers, are be-
lieved to form underneath the first layer by low-energy electron diffraction (LEED) and
low-energy electron microscopy (LEEM).
62
Because of the weak binding force between
graphene and copper films, active hydrocarbon CH
x
formed by methane decomposition
on bare copper can either move from sides of graphene to the gap between graphene and
copper surface
44, 62
, or diffuse through the defect hole on the graphene into the gap
114
, and
nucleate at the same nucleation site of the top layer graphene and grow into smaller
adlayers next to the copper. In our previous work of vapor-trapping growth, we found
AB stacking bilayer regions in the graphene flower center as Figure 3.2 demonstrated.
Though bilayer/trilayer graphene could be synthsized, understanding the mechanism
for adlayers growing into varies orientations and the effect of the growth parameters on
stacking orders still remains a challenge. Therefore, there is a strong motivation to study
the growth mechanism of few-layer graphene and control their stacking orders.

47

3.2 Ambient Pressure Synthesis of Bilayer/trilayer
Graphene
In this study, ambient pressure was used for graphene growth. Unlike low pressure
growth where pure methane is used as carbon source, ambient pressure growth requires
ultralow methane concentration to suppress the nucleation density and growth rate and
therefore achieve large single-crystalline hexagonal graphene with few layers. In our
study, diluted methane in argon with concentration of 0.1% was applied. Firstly, 127µm
copper foil (Alfa Aesar purity 99.9%) was electrochemically polished under 2 volts for
10min, followed by rinse with DI water and blow-dry. Then the copper foil was loaded
into a 1 inch quartz tube furnace and annealed at 1050 ˚C for 1 hour with protection of
Ar/H
2
(100/50sccm) flow. After that, the temperature was changed to the targeting tem-
Figure 3.2: SEM images showing bilayer/trilayer graphene regions in the center of
graphene flowers by vapor trapping synthesis at 1000˚C with growth time of (a)
20min and (b) 40min.
48

perature. Once the growth temperature was reached (ranging from 1000 ˚C to 1075 ˚C),
30 sccm diluted methane (0.1% in Argon), 30 sccm hydrogen and 440 sccm argon were
fed into the chamber, and the growth was carried out at ambient pressure for 30min. The
gas flow was kept the same during the cooling down.
By tuning the flow rate of diluted methane and argon, the methane concentration was
varied from 40 ppm to 90 ppm. All the graphene grains show hexagonal shape with
straight edges under our growth condition. Most of the grain nucleation only occurs at
the initial stage of the growth, and the grains will merge together to form a continuous
graphene film when the growth time is increased.
Figure 3.3 is a typical SEM image of as-produced graphene on copper foil (70 ppm
methane, grown at 1025 ˚C for 30min). Four types of graphene adlayer morphology are
presented and marked by red letter ABCD. The contrast within the individual graphene
islands in regions A, B and C indicates that there are adlayers in the grain. In regions
marked as A, the adlayers are not concentric and share a few edges with the larger hex-
agonal grain, showing an irregular shape. In region B, two grains have concentric hexag-
onal adlayers aligned parallel to the large grain, while in region C, three concentric hexa-
gons twist 30˚ with the larger layer. In region D, the contrast of the graphene flake is con-
stant through the whole grain, but darker than the nearby islands, indicating a uniform
few-layer grain. These four kinds of morphologies coexist at the same region on copper
foil, where the local micro-environment was essentially close during the growth, reveal-
ing the complexity of graphene growth by CVD. Here, the nonconcentric adlayer mor-
49

phology is particularly interesting since it takes a remarkable percentage in all kinds of
morphologies and consistently appears.

3.3 Characterization of Bilayer/trilayer Graphene and
Growth Mechanism Study
3.3.1 AFM characterization
In order to illustrate the structure details of the bilayer and trilayer graphene sample,
the as-produced graphene was transferred onto Si substrate with 300nm oxide layer and
characterized by atomic force microscope (AFM) (Digital Instrument Dimensional 3100,
tapping mode). Figure 3.4 demonstrated a concentric trilayer graphene grain with three
layers aligned without any twist. The height profiles extracted from the grain edges
shown steps around 0.7-1.1nm for each layer.

Figure 3.3: SEM images of as grown bilayer/trilayer graphene on copper foil with
four different kinds of morphologies
50

Figure 3.5 demonstrated a nonconcentric few-layer graphene with adlayer covered the
top right part of grain. The height profile extracted from the grain edges together with the
optical image shown that the left part was monolayer and right part was trilayer. The pe-
riodic lines on the graphene surface were wrinkles of graphene.
Figure 3.4: AFM characterization of the concentric trilayer graphene
(a) Optical image of the graphene grain. Scale bar is 10 µm. (b) Zoomed in AFM
height image of the graphene grain marked by yellow dashed square in panel a
shows graphene edges and islands. (c-e) Corresponding cross sectional height pro-
files of graphene edges indicated by blue, green and red dashed lines in panel b. Step
height was calculated by the vertical distance between the average heights of each
stage.
51

3.3.2 Raman characterzation
Micro-Raman is a powerful tool to determine the stacking order and coupling of lay-
ers in few-layer graphene. Here, we applied Raman (Renishaw, 532nm laser) to charac-
terize the adlayers of different morphologies. In Figure 3.6, four types of BLG/TLG were
demonstrated in panels (a)-(d) and the corresponding Raman spectra in (e)-(h) were taken
from the points marked by red, blue and green circles. The quite low intensity of D band
(~ 1350cm
-1
) in all spectra suggests the high quality of BLG/TLG graphene after transfer.
The Raman spectra in (e) together with the optical image in (a) indicate that the left part
of this graphene grain is monolayer while the right part is ABA stacked trilayer, with
Figure 3.5: AFM characterization of the edged nonconcentric trilayer graphene
grain (a) Optical image of the graphene. Scale bar is 10 µm. (b) Zoomed in AFM
height image of the graphene grain marked by yellow dashed square in panel a
shows graphene edges and wrinkles. (c-e) Corresponding cross sectional height pro-
files of graphene edges indicated by blue, green and red dashed lines in panel b.
52

I
2D
/I
G
~0.7 and full width at half maximum (FWHM) of 2D peak ~65cm
-1
.
114
The Raman
data in (f) shows the characteristic spectra of ABA stacked TLG with I
2D
/I
G
~0.9, FWHM
of 2D peak ~56cm
-1
in the blue curve, and I
2D
/I
G
~0.6, FWHM of 2D peak ~62cm
-1
in the
green curve, which are consistent with the optical image shown in (b). The blue curve in
Figure 3.6(g) has I
2D
/I
G
~3.6, FWHM of 2D peak ~28cm
-1
, showing a typical spectrum of
bilayer graphene with 30˚ twist angle. The last set of images in (d) and (h) present a uni-
form ABA trilayer graphene grain with I
2D
/I
G
~0.6 and fwhm of 2D peak ~68cm
-1
. The
uniform trilayer may come from the simultaneous seeding mechanism proposed by Sun,
Z et al.
85

3.3.3 TEM characterzation
Although AFM and Raman are successful to characterize the few-layer graphene
grain, the complexity in the few-layer graphene growth requires a more accurate and de-
tailed technique to determine the layer number, twist angles and grain boundaries of the
adlayers in other types of few-layer graphene. Therefore, we used selected area electron
diffraction (SAED) assisted with dark-field transmission electron microscopy (DF-TEM)
to provide the precise structural information of the BL/TL graphene. Graphene was firstly
transferred onto gold TEM grid supported by C-flat holey carbon films and examined
under SEM to locate the interested areas. Then the corresponding areas were inspected
under TEM and the selected area diffraction patterns were acquired from graphene grain
suspended over holes in the support membrane. In this way, we establish correlations
between the macro-scale morphology and atomic level structure of the adlayer graphene.
53

Figure 3.6: Typical optical images and Raman spectra of bilayer/trilayer graphene
with different morphologies (a-d) Optical images of hexagonal graphene domains
transferred onto SiO2 (300nm)/Si substrate, showing (a) partially covered trilayer
from the edge, (b) concentric trilayers aligned in the center, (c) concentric bilayer
island twisted 30 degree in the center, and (d) uniform trilayer graphene. Colored
circles indicate the positions where the Raman spectra were taken. Scale bars are 10
µm. (e-h) Corresponding Raman spectra taken at the indicated points at panels (a)
to (d).
54

Here, we focused on nonconcentric adlayer, which is challenging to determine its
stacking orders by Raman or optical images. Three samples were prepared and more than
twenty locations were examined. The representative results were demonstrated below.
The transmission electron microscopy (TEM) was performed by JEOL JEM-2100F at
200 kV for Figure 3.7 and 3.9, and FEI Titan 80-300 at 80 kV for Figure 3.10 and 3.11.
Figure 3.7 displays a representative grain where the adlayers share the same boundary
with the larger layer. Panel (a) is a SEM image, showing several hexagonal graphene
grains merging together and suspending on TEM grid. The edges of grain were highlight-
ed by yellow dash lines and the regions with adlayers were marked by blue dash lines.
SAED patterns were acquired from the four regions labeled as CDEF and the correspond-
ing images were shown in panels (c)-(f).
Region C had two sets of six-fold diffraction patterns with 30˚ rotation. The set with
higher intensity was measured along the red arrow direction and the intensity ratio of the
outer {1-210} and inter {0-110} peak was ~2, suggesting AB stacking bilayers. The other
set had the same orientation with SAED pattern in region D and similar intensity ratio of
I{1-210}/I{0-110} ~0.5 along the blue arrows, indicating a continuous monolayer. Re-
gion E had only one set of diffraction pattern with characteristic AB stacked order. Re-
gion F also had one set of spots and displayed monolayer feature. To conclude, the corre-
sponding structure diagram in panel (b) shows that in the top-right grain, the adlayer is
AB stacking bilayer and twisted 30˚ with the larger monolayer, and in the bottom-left
grain, the triangle-shaped adlayer goes from one edge to the grain center and AB stacked
with the larger monolayer.
55

For nonconcentric adlayers, besides those which share one or few edges with the
larger layer, there are other nonconcentric few layers look like adlayers patches scattered
randomly under the large hexagonal layer, as those presented in Figure 3.8. Since the
Figure 3.7: TEM characterization of bilayer/trilayer graphene
(a) SEM image of a TEM grid covered by hexagonal graphene grains with edges
indicated by yellow dash lines. The scale bar is 50µm. Areas with darker color are
marked as C and E, showing more than one graphene layer. (b) Schematic dia-
gram of the graphene grains in (a). (c) SAED patterns taken at area C show iden-
tical images. The spot intensity taken along the red arrow direction indicates AB
stacking bilayer graphene, and the other set of diffraction pattern with 30˚ twist
shows monolayer graphene. (d) Identical SAED patterns taken at area D showing
monolayer graphene. (e-f) SAED patterns taken at area D and E show AB stack-
ing bilayer graphene and monolayer graphene respectively.
56

shapes of these adlayers are not uniform hexagons mostly, and their sizes are small, we
also examined such adlayers under TEM and the results are shown in following figures.

Figure 3.9 presents a region where two hexagonal grains connected together. The out-
er shape of the hexagonal grains in panel (a) shows a misalignment of 30˚ between the
two grains. The two connected grains were colored by light blue and red with an estimat-
ed boundary, and the small adlayer region was marked by dark blue. The center area was
enlarged in panel (b) and the following data was acquired from hole C, G and H. Panel (c)
was the SAED pattern taken in hole C, showing two sets of diffraction patterns with a 30˚
rotation. Since graphene grain boundary and twisted few layers both show more than one
set of peaks in SAED, we put an objective aperture in the diffraction plane as indicated
by the blue and red circle in (c), and obtained two dark field TEM images with false color
for each region. By overlapping these two images, the graphene structure was clearly
Figure 3.8: Images of nonconcentric adlayers
(a) SEM image of the graphene grains on copper. (b) Optical image of part of a
graphene grain transferred on SiO
2
/Si substrate. Both images show that adlayer
patches also exist at locations other than the grain center.
57

illustrated in panel (d). The color difference shown a clear borderline marked by the
white dashed line, indicating a grain boundary.

For further confirmation, the electron beam was focused in a smaller area and diffrac-
tion pattern was taken separately at the circled region in panel (d). The corresponding
Figure 3.9: TEM characterization of bilayer graphene
(a) Low magnification TEM image of a bilayer graphene region. Inset is a sche-
matic diagram of the graphene grains. The region outlined with red dot in (a) is
magnified in (b). (c) SAED patterns taken from hole C. The grains were selected
using an objective aperture in the diffraction plane separately, and individual
grains are shown in (d) with false colors. Within the hole C, two diffraction pat-
terns were acquired. (e) was taken from the area marked as “e” and (f) was from
the area marked as “f”. (g) and (h) are the SAED pattern and corresponding
spots intensities taken in hole G and H, respectively.
58

SAED images were presented in panel (e) and (f). The spots intensity profiles confirmed
that region “e” was monolayer graphene and region “f” was AB stacking bilayer gra-
phene. Panel (g) was acquired at hole G, also shown a AB stacking bilayer structure of
the same orientation as in panel (f), indicating the whole adlayer marked by the dark blue
color had Bernal stacking with the light blue island. Panel (h) was acquired at hole H,
showing two sets of monolayer diffraction spots, indicating a monolayer structure with a
grain boundary of 30˚, which was coincident with the 30˚ rotation between the two grains.
Based on the characterization results, we got the schematic diagram of the grain structure
drawn in the inset of panel (a).
Figure 3.10 shows two groups of TEM characterization data of the small trilayers.
Panel (a) and (d) were bright field TEM images, showing the continuous graphene film
suspended on the holey carbon film support. The SAED pattern were taken within the
hole and shown in panel (b) and (e), both having two sets of diffraction patterns with a
30˚ rotation. Peak intensity profiles were extracted along the directions marked by the
arrows to determine the layer numbers in each set. The red set implies AB stacking bi-
layers and the blue set indicates monolayer. Putting an objective aperture in the diffrac-
tion plane as indicated by blue and red circles, two spatial images with false color were
obtained and stacked together, as displayed in panel (c) and (f). The results indicated that
the small adlayers were AB stacking bilayer, and twisted 30˚ with the upper larger mono-
layer.
Among all the adlayers we examined, most of them stack either in Bernal stacking or
twisted 30˚ with the upper layer. We only found one region out of twenties, where there
59

was an adlayer twisted 7˚ with the larger monolayer and formed a grain boundary in the
smaller adlayers as shown in Figure 3.11. The schematic diagram of the structure is
shown in panel c. This indicates that AB stacking and 30˚ twisting have the smallest for-
mation energy barrier, which is consistent with the theoretical calculation.
76

Figure 3.10: TEM characterization of trilayer graphene
(a-c) are from the one location and (d-f) are from the other location. (a) and (d)
are bright field TEM images. Insets are the schematic diagram of the correspond-
ing graphene structure. (b) and (e) are SAED patterns taken from hole (a) and
(d), respectively. Placing an objective aperture in the diffraction plane as indicat-
ed by blue and red circles on the diffraction pattern in (b) and (e) separately and
acquired two dark-field images for each one. Overlap them and generate a false
color image in (c) and (f).
60

3.3.4 Growth mechanism analysis
The sharp distinction between those concentric and nonconcentric adlayers indi-
cates different growth mechanisms. Nevertheless, it is not intuitive to figure out how do
Figure 3.11: TEM characterization of bilayer graphene
(a) SAED pattern showing three sets of diffraction patterns. Spots intensities taken
along the arrow directions indicate all of them are monolayer. The rotation be-
tween the yellow dot and blue dot is 7˚; the rotation between the yellow dot and red
dot is 30˚. Putting aperture at the spot marked by circles separately and acquired
three dark-field images. Overlap the red and blue ones and generate a false color
image in (b). The red and blue regions were in the same layer with a grain bounda-
ry. The white dash line indicates the grain boundary. The dark-field image taken
by putting aperture at the yellow dot is shown in (c). Inset is a schematic diagram
of the graphene grain.
61

those nonconcentric adlayers nucleate and grow. One possible explanation is that the
concentric adlayers and non-concentric adlayers nucleate and grow at different stages.
For the nonconcentric adlayers, suppose they nucleate underneath the top layer at some
points other than the grain center after the first layer is formed, they would still grow into
smaller hexagonal patches as Figure 3.8 and 3.10 show.
However, as Figures 3.3(a) and 3.12(a) demonstrated, these nonconcentric adlayers
seem to nucleated from parts of the grain edges after the top layers were formed and grew
forward to the grain center, leaving an irregular front end in the middle. The AFM image
in Figure 3.12 (b) further shows that the right part of the grain which is trilayer has a
sharp boundary at edges, but stepped border in the middle, indicating that the adlayer
growth started from the grain edge and stopped somewhere in the middle. It is interesting
that within a small region, those nonconcentric adlayers usually locate at the same side of
the grains, and the directions are not the same throughout the whole sample, excluding
Figure 3.12: Adlayer growth from edges
(a) Optical image of hexagonal graphene grains with nonconcentric adlayers. Scale
bar is 10 µm. (b) AFM height image of hexagonal graphene grain with nonconcentric
adlayers on the top right part. The green arrows indicate the growth front. Scale bar
is 5 µm. (c) Schematic diagram of the formation of the adlayers from the edges.

62

the effect of gas flow. One possible explanation is that the copper steps formed at high
temperature during the annealing process leave larger gaps on one side than the other as
Figure 3.12 (c) shows, promoting the adlayer growth at the favorable side. On the other
hand, for the concentric adlayers, no matter how they stack with the larger top layer, they
display a regular hexagonal shape. These adlayers are proposed to nucleate either simul-
taneously with the top monolayer or at the early stage of top lay growth and expand in a
slower growth rate due to the limited carbon supply, forming an inverted wedding-cake
structure. The comparison of these two growth mechanisms is illustrated in Figure 3.13.
During our experiments, we found that adlayers mainly stack with the top layer in
AB stacking order or twisted 30˚. This is different from the case of monolayer graphene
growth on copper, in which the orientations of the grain are randomly distributed due to
the weak interaction between graphene and copper. It is also interesting to notice that
adlayer growth stops at top layer graphene grain boundaries, as shown in Figure 3.9 and
Figure 3.13: Schematic diagrams propose two different mechanisms for graphene
adlayer growth
(a) Nucleate at graphene edge and grow towards the center; (b) Nucleate at grain
center and grow together with larger layer
63

Figure 3.12. It seems that grain boundary has higher energy barrier for graphene adlayers
to grow across it, while in the case of monolayer graphene growth on copper, the growth
front of graphene can go over the grain boundary of underneath copper films. Both of
these phenomena suggest that atomic structure of the upper layer graphene has strong
influence on the formation of the underneath adlayers. The weak interaction between the
copper and upper layer graphene leaves gaps for active carbon source diffusion and cre-
ates a nanochamber for adlayers to grow epitaxially with the top layer graphene. There-
fore, the adlayer graphene formation could be controlled by regulating the growth of the
top monolayer graphene.

3.3.5 Growth parameters study
To investigate the effect of growth parameters on adlayer morphologies, methane
concentration and growth temperature were selected as the two most important factors
and studied systematically. Figure 3.14 shows the representative SEM images of the
morphology evolution with different methane concentration and temperature during
growth. Figure 3.14 (a-d) is the morphology changes with methane concentration varying
from 40ppm to 90ppm, and Figure 3.14 (e-h) is the results with growth temperature
changing from 1000˚C to 1075˚C. Interestingly, we found that methane concentration
and growth temperature had similar influence on the BLG/TLG morphology. At low me-
thane concentration (40ppm) and relatively low growth temperature (1000˚C), there are
few multilayers and most domains are monolayer graphene. When the methane concen-
tration or the growth temperature increases, concentric adlayers appear in some of the
64

grains. With further rise of these two parameters, the nonconcentric adlayers emerge and
gradually dominates as the largest percentage of adlayers.

65

Statistical analysis was done by examining over 100 grains for each growth tempera-
ture and the results of each kind of adlayer morphology were presented in Figure 3.14(i),
showing the same trend. There was negligible concentric bilayer with twist angle other
than 30˚, and the ratio of aligned AB stacking bilayer and 30˚ twisted bilayer was close to
1:1. Meanwhile, hydrogen partial pressure also plays a critical role in graphene nuclea-
tion and growth. We found that when the hydrogen flow rate was increased from 30sccm
to 40sccm (partial pressure from 46 Torr to 61 Torr), there was no graphene growth in
30min, suggesting that the methane concentration we used was already at the bottom line
to initiate graphene nucleation. Since the adlayer percentage rises with increasing me-
thane concentration and growth temperature, the bilayer/trilayer growth could be promot-
ed or prohibited by tuning these two factors.
Figure 3.14: Bilayer graphene morphology with different methane concentration
and growth temperatures
SEM images were taken after graphene transfer. Scale bars are 20µm. (a-d) Typical
SEM images of graphene grown at methane concentration of 40ppm, 50ppm,
70ppm, and 90ppm, respectively. Other growth conditions were kept the same: hy-
drogen flow rate was 30sccm, temperature was 1050˚C, growth time was 60min for
(a) and 30min for (b-d). (e-h) Typical SEM images of graphene grown at tempera-
ture of 1000˚C, 1025˚C, 1050˚C, and 1070˚C, respectively. Other growth conditions
were maintained the same: hydrogen flow rate was 30sccm, methane concentration
was 60ppm, growth time was 60min for (e) and 30min for (f-h). (i) Histograms of the
percentage of four kinds of graphene grain morphologies at different growth tem-
peratures. Statistical data was based on counting more than 100 randomly chosen
grains for each growth condition.
66

3.4 Conclusion
In conclusion, we have used APCVD to synthesize bilayer and trilayer graphene with
various adlayer morphologies. Raman, SEM and SAED combined with DF-TEM were
applied to resolve the key structural factors of the adlayers, including layer numbers,
stacking orders and grain boundaries. Unlike the concentric adlayers which grow simul-
taneously on the same nucleation center with the upper layer, the nonconcentric adlayers
do not share the same nucleation site with the upper hexagonal grain and may nucleate
after the upper layer is formed. The percentage of these nonconcentric adlayers increas-
es with rising methane concentration and growth temperatures. Our model based on the
epitaxial adlayer formation between the copper surface and the upper layer graphene pro-
vides guidelines for controlling the coverage and stacking order distribution of BLG and
TLG by modifying the growth parameters.

67

Chapter 4
h-BN Synthesis by Chemical Vapor Deposition
The hexagonal boron nitride (h-BN) with strong in-plane bonds, large band gap, and
planar structure provide an ideal flat, insulating, and inert surface for graphene device,
which has been shown to improve the device mobility dramatically. Scalable synthesis of
monolayer h-BN films using CVD, which can produce h-BN in large quantities at low
costs, has the potential to galvanize further research in graphene electronics and the pro-
duction of optoelectronic devices in industry
81
. Compared to the exfoliation method, the
common drawback of CVD approach is the low quality. Since BN exist in both cubic
phase and hexagonal phase, the synthesized BN may have cubic phase or even amor-
phous structure in hexagonal lattice, which degrades the crystalline structure of the h-BN.
Generally speaking, substrate and precursor are two key factors in CVD growth. By care-
fully choosing the precursor and preparing the substrate, atomically smooth surface mor-
phology and uniform crystalline structure can be achieved. In this chapter, we will dis-
cuss the methods we tried to synthesize large area h-BN by CVD.

4.1 Few-layer h-BN Film Growth on Ni Substrate
Ni films have strong catalytic effect for both graphene and h-BN synthesis. In this
experiment, we evaporated Ni films onto SiO
2
/Si substrate and did high temperature an-
nealing to obtain a polycrystalline substrate. Then, we grew few-layer h-BN films on Ni,
68

using borazine as a precursor in the ambient pressure chemical vapor deposition (APCVD)
system. Borazine (B
3
N
3
H
6
) is a relative small molecule compared to polymers and is liq-
uid under room temperature. It has 1:1 B/N ratio, does not exhibit the high toxicity and its
dehydrogenation reaction on metal surfaces occurs over a very broad temperature range.
During the experiment, borazine precursor was sealed in a glass bubbler and kept at
4 °C in a refrigerator to maintain a constant vapor pressure. A 1-10 sccm flow of N
2
was
supplied into the bubbler and carried the precursor into the furnace, passing through the
surface of Ni at temperature of 400 °C for about 10min. Then the temperature was slowly
(~ 6°C/min) raised to 1000 °C for a postannealing process for another 30 min. After the
sample was cooled down to room temperature, a colorful film can be observed on the Ni
surface (Figure 4.1 (a-b)). The color may due to the thick and different h-BN grain thick-
ness. From the optical images, the h-BN films were multilayer, and the grain size was
similar to the Ni film grain size.
After the CVD synthesis was finished, the h-BN film was transferred by coating the
h-BN/Ni substrates with a thin layer of PMMA. After etching the underlying polycrystal-
line Ni with a commercial Ni etchant, the PMMA/h-BN film was detached from the
SiO
2
/Si substrate and transferred to DI water and was suspended on the surface of water
to remove the Ni etchant residue. Subsequently, the h-BN film was transferred to SiO
2
/Si
substrate as Figure 4.1(c) shows. Raman was used to characterize the h-BN film quality.
The Raman spectrum in Figure 4.2(b) shows one dominant peak at 1369 cm
-1
, which can
be assigned to the E
2g
vibration mode of h-BN.
69

4.2 Comparison of h-BN Grown on Ni and Cu Films
During our experiment, we found that the Ni film quality regarding the surface rough-
ness and grain size effect the h-BN growth a lot. A smooth and less defective Ni surface
will reduce the amount of allotropes (c-BN) and impurity particles that are crucial for
Figure 4.2: Raman characterization of h-BN on Ni film
(a) Optical image of few-layer h-BN film on Ni films under Raman Spectroscopy.
(b) Raman spectrum of h-BN on Ni film excited by green laser.
Figure 4.1: h-BN grown on Ni films by CVD
(a) Photo of as grown h-BN on Ni/SiO
2
/Si substrates. (b) Optical image of h-BN on
Ni films. (c) Optical image of h-BN transferred onto SiO
2
/Si substrate.

70

high-quality h-BN growth. Thus, we tried several methods to improve the Ni film quality,
such as using c-plane sapphire as a substrate for Ni evaporation and applying different
annealing conditions for Ni film, to achieve a single crystalline Ni film for uniform h-BN
growth. For comparison, we also evaporated Cu films onto SiO
2
/Si substrates and did
similar annealing process to obtain polycrystalline Cu films. It is noticed that Cu films
have larger grain size (~ 10 μm) and smoother surface than Ni (~ 1-3 μm grain size) as
shown in Figure 4.3(a)-(d).

We loaded Ni films and Cu films into the CVD system separately and did h-BN
growth under the similar condition. The growth condition for Ni and Cu was the same.
Figure 4.3: Comparison of h-BN on Ni and Cu
Optical image of 500 nm Cu (a) and 300nm Ni (b) films on SiO
2
/Si substrates after
H
2
/N
2
annealing at 900 °C for 15 min. AFM image of annealed (c) Cu and (d) Ni
films. Image scale is 5 μm and the height scale is 200 nm for Cu and 300 nm for Ni.
Raman spectra of h-BN grown on (e) Cu and (f) Ni films.
71

Due to the lower melting point of Cu, the postannealing condition was a little bit different
to decrease the evaporation of copper during the annealing process. The postannealing
condition for Cu films was ramping from 400 °C to 1000 °C in 1 h and keeping for 20
min, while the condition for Ni films was ramping to 1000 ramping in 1.5 h and keeping
for 40 min. After growth, both films were examined by Raman. The h-BN on Ni films
shows a strong Raman signal around 1367.7 cm
-1
, while the film on Cu shows a very
broad and weak band around that range. This indicates that the catalytic effect of Ni is
stronger than Cu for h-BN growth.

4.3 Few-layer h-BN Grown on Cu Foil
Since borazine is sensitive to light, heat and moisture, and also more expensive, am-
monia borane (NH
3
−BH
3
) is used as a BN precursor, which is a white powder at room
temperature and easily accessible and more stable under ambient conditions than bora-
zine. It melts around 106 °C, and the thermal decomposition reaction to hydrogen, mon-
omeric aminoborane (BH
2
NH
2
) and borazine takes place in the temperature range of
77−137 °C accompanied by heat evolution. The synthesis was carried out on Cu foil in an
ambient pressure CVD system. Effective surface diffusion of the adsorbed BN rings for
the formation of atomically flat BN surface is restricted on a Cu foil with a rough surface
morphology. Therefore, Cu foil was firstly polished and annealed in a mixed N
2
/H
2
gas
flow of 100 sccm for 30 min at 1000 °C. Then, ammonia borane stored in a glass flask
was heated and sublimated at around 110 °C using a heating belt, and the vapor was car-
ried out with an N
2
gas flow of 25 sccm into the CVD chamber for about 30 min. After
72

that, the precursor supply was shut off and the temperature was cooled down. During the
synthesis, ammonia borane was boiling vigorously and generated a large amount of vapor
which caused thick layer of h-BN deposited on Cu foil. After synthesis, the sample was
characterized by SEM and Raman and the results were shown in Figure 4.4.
From the SEM image, the h-BN film is continuous and the thickness is larger than
1 μm. The Raman peak is around 1370 cm
-1
and is a little bit broader than the sample
grown on Ni films, which can be ascribed to the smaller grain size of h-BN films
61
.

4.4 Monolayer h-BN Grown on Cu Foil
Previous APCVD synthesis of h-BN was only able to obtain few layer h-BN without a
good control on the number of layers
38, 78
. Since h-BN growth on Cu foil is not self-
limited as graphene, to reduce the layer number further, low pressure CVD is applied to
control the vapor pressure of the precursor. Under the low pressure, the heating tempera-
Figure 4.4: Characterization of h-BN grown on Cu
(a) Schematic diagram of CVD system for h-BN synthesis under low pressure. (b)
SEM image of as grown monolayer h-BN on Cu foils. (c) Raman spectrum of h-BN
on Ni film excited by green laser.
73

ture for NH
3
−BH
3
precursor is reduced from 110 °C to 80 °C by a hot water base. Figure
4.5(a) shows a schematic diagram of LPCVD for monolayer h-BN synthesis. With short
time growth, the h-BN was not continuous and the initial flakes were triangular instead of
hexagonal as in the case of graphene
35
, as shown in Figure 4.5(c). After 50 min growth,
the h-BN film became almost continuous as Figure 4.5 (b) shows. After growth, h-BN
was also transferred to SiO
2
/Si substrate for Raman characterization. Due to the mono-
layer natural, the Raman signal was very weak, and was only observed with long time
accumulation.

Figure 4.5: Low pressure CVD of h-BN grown on Cu
(a) Schematic diagram of CVD system for h-BN synthesis under low pressure. (b)
SEM image of as grown monolayer h-BN by NH
3
−BH
3
precursor on Cu foils. (c)
SEM image of isolated monolayer h-BN grains on Cu foil.
74

4.5 Conclusion
In summary, by changing the metal substrates, precursors and growth pressure, we
have demonstrated several conditions to synthesize few-layer and monolayer h-BN films.
We compared the Ni and Cu substrates, and found that Ni films have stronger catalytic
effect for h-BN growth than Cu, while Cu substrates have a smoother surface and larger
grain size, which produced h-BN with uniform layer number.

75

Chapter 5
Ongoing and Future Works
5.1 High Quality Graphene for RF Applications
5.1.1 Motivation
Graphene has attracted tremendous attention as a channel material for future elec-
tronic devices due to its outstanding electronic properties.
2, 8, 75
The high charge carrier
mobility and saturation velocity, as well as strong carrier density modulation by electric
field, result in high transconductance for graphene field-effect transistors (FETs).
17, 50, 60,
109
Moreover, single atomic layer thickness of graphene provides ultimate electrostatic
geometry for scaling down device dimension. These properties make graphene FETs
especially promising for future radio frequency (RF) analog applications in millimeter
wave range and beyond, as they may not require large on/off current ratios. Moreover,
experimental and theoretical estimates predict possible graphene transistors operation at
terahertz (THz) frequencies.
10, 46
Additionally, ambipolar transport in graphene transistors
offers additional functionality in analog circuits, such as frequency multipliers,
4, 36
mix-
ers
21
and multi-mode amplifiers. Recently, graphene FETs with cut-off frequencies (fT)
up to 100-300 GHz range have been achieved,
16, 80
which approach the performance of
the state-of-the-art conventional semiconductor devices. Graphene transistors have been
fabricated on exfoliated graphene flacks, and large-are graphene film prepared by epitax-
ial growth and chemical vapor deposition (CVD).
3, 51, 97
Compared with exfoliation and
76

epitaxial growth, CVD method is scalable and low cost. Therefore, CVD-grown large-
area graphene film of high quality provides a good platform for scalable device and
circuit fabrication.
Previously, we developed an optimized self-aligned device design of T-shaped top
gate structure. In this design, the Al/Al
2
O
3
T-shaped gate stack with thin high quality
dielectric of equivalent oxide thickness down to 2-3 nm is obtained and allows self-
aligned source and drain formation by depositing metal with the T-gate mask, which
scales down the gate lengths to 100 nm, as Figure 5.1 shows. Scalable high yield fabrica-
tion of graphene FETs has been achieved through combining T-gate fabrication proce-
dure with large-area graphene films synthesized by CVD method.
As reported in literatures, the mobility of exfoliated graphene from the HOPG is
much higher than mobility of the graphene grown by the chemical vapor deposition. In
order to improve the material properties of graphene transistors, impurities and defects
Figure 5.1: Graphene T-gate device structure
(a) Schematic of self-aligned T-shaped gate device. (b) Cross section SEM image
of T-shaped gate device
77

concentration should be minimized by thorough optimization of synthesis and transfer
methods. Additionally, the performance of graphene FET is not only limited by the
intrinsic property of graphene, but also the substrate. The trapped charge, phonon
scattering from the substrate material, especially the non uniformity in graphene cause by
the surface roughness, will affect the electrical transport in graphene. Therefore, we
propose to utilize atomically smooth surfaces of such materials as hexagonal boron
nitride (h-BN) as substrates for fabrication of graphene transistors. It is expected to
significantly improve the mobility of graphene, and, hence, improve transconductance
and current saturation of graphene transistors.

5.1.2 Device fabrication and DC performance
Before fabrication of top gate devices, we first fabricated the back gate devices of
graphene on h-BN to test the electrical perforamnce of graphene sample we grown by
CVD. The fabrication process was similar to the flow shown in Figure 2.10. The
continuous graphene was synthesized by low pressure CVD and clearfully transferred
onto exfoliated h-BN on Si substrate with align markers. Then the area of graphene/hBN
was located by optical microscope and the graphene was patterned by E-beam
lithography. Then the electrodes was designed and patterned by E-beam lithography
followered by E-beam evaporation and lift-off of Ti/Au. After every step of fabrication,
devices were annealed in Ar/H
2
gas at ~300˚C to remove the reside of E-beam resist.
The quality of the samples was characterzied during the fabrication process to ensure
the intrinsic performance of graphene, as shown in Figure 5.2. Exfoliated h-BN flakes
78

with size larger than 20µm and thickness smaller than 50nm were selected as Figure 5.2(a)
shown. The SEM image was taken after graphene was patterned, and the AFM image in
panel (c) and Raman spectrum in panel (d) confimed that graphene channel was
continous, smooth, clean and maintained good quality.

In comparison, the same type of graphene was transferred onto SiO
2
/Si substrate with
the same transfer method and back gate devices were fabricated by photo lithography.
The I-Vg curve was measured and demonstrated in Figure 5.3. For the graphene on
SiO
2
/Si device, the hysteresis was large, and the graphene was highly p-doped with Dirac
Figure 5.2: Graphene T-gate device on h-BN
(a) Optical image of continuous graphene on exfoliated BN. (b) SEM image of pat-
terned graphene on BN. (c) AFM image of the channel of patterned back-gate de-
vice. (d) Raman spectrum of the graphene in the channel area.
79

point around 40V. The fitted mobility was ~3000 cm
2
/V/s, and the contact resistance was
~ 1500 Ω. On the other hand, for the graphene on h-BN device, the two curves coincide
without hysteresis, and the Dirac point was around 0V, indicating quite low doping. The
fitted mobility was ~10,000 cm
2
/Vs, and the contact resistance was ~ 400 Ω.
After demonstrating the superior quality of graphene on h-BN devices, T-gate is fab-
ricated with the similar recipe and this work is going on. In our previous recipe to obtain
the Al
2
O
3
dielectric, we baked the device in air to oxidize Al near the interface of gra-
phene and Al gate. However, in the graphene/h-BN, the graphene surface is so clean and
smooth that the interface between graphene and Al gate is very difficult for oxygen diffu-
sion and penetration. It may require other methods such as atomic layer deposition (ALD)
to grow the Al
2
O
3
dielectric layer. Since this fabrication process requires E-beam lithog-
raphy to pattern devices on exfoliated h-BN and the yield is low, further improvement
Figure 5.3: I-Vg of back gate graphene devices on h-BN and SiO
2
/Si substrate
(a) Graphene device on exfoliated BN. (b) Device with same graphene on SiO
2
/Si.
-20 -10 0 10 20
-25
-50
-75
-100
-125

I
d
( A)
V
bg
(V)
VdS = -0.1V
W = 10 m, L = 8 m
-60 -40 -20 0 20 40 60 80 100 120
-20
-30
-40
W = 10 m, L = 8 m

I
d
( A)
V
bg
(V)
VdS = -0.1V
(a) (b)
80

can be done by using CVD synthesized continuous BN to facilitate the wafer scale fabri-
cation with photolithography.

5.2 Two-dimensional Materials beyond Graphene
Besides graphene which could exist in a free-standing form as an atomic single layer,
it was proved that other layered materials could be exfoliated down to a monolayer as
well. During the past three to four years, other two-dimensional layered materials, such as
hexagonal boron nitride (hBN), molybdenum disulphide (MoS
2
), tungsten diselenide
(WSe
2
), and black phosphors (BP), are emerging as an exciting material system for a new
generation of atomically thin electronics
43, 68, 104
and optoelectronics
56, 107
due to their
unique electronic and optical properties.
58, 69, 83, 115
(Figure 5.4) These materials have a
natural bandgap which is lack in pristine graphene. In this regards, the monolayer transi-
tion metal dichalcogenides (TMDCs) in the form of MX
2
(M = Mo, W, Ti, Zr, Hf, V, Nb,
Ta, Re; X = S, Se, T) is particularly interesting due to their intriguing physical properties
and exciting prospects for a variety of applications.
Figure 5.4: Structure and real samples of two-dimensional materials
81

5.2.1 MoS
2
growth by CVD
Among the large family of 2D TMDCs, one of the most well studied materials is
MoS
2
. It has an intrinsic direct band gap of 1.8 eV which is lack in graphene, and facili-
tates the transistors and logic circuits based on single layers of this material.
68
Moreover,
the absence of dangling bonds together with thermal stability up to 1,100 °C make MoS
2

interesting for nanoelectronic applications. Similar as graphene, the most common ap-
proaches for obtaining single- and few-layer-thick MoS
2
include mechanical exfoliation
of large crystals using scotch tape, chemical exfoliation by dispersing in a solvent with
the appropriate surface tension, molecule/atom intercalation in order to exfoliate these
layers and enable their dispersion in polar solvents, and chemical vapor deposition.
12, 33

93,
108
The reliable synthesis of single- and few-layer MoS
2
is an essential first step for char-
acterizing the layer-dependent changes in its properties, as well as providing pathways
for the integration into a multitude of applications. Here, we use chemical vapor deposi-
tion method to grow monolayer MoS
2
crystals on SiO
2
/Si.
We followed recent work for the CVD growth of monolayer MoS
2
and developed
our own recipes using a 1-inch furnace.
41
In a typical experiment, sulfur (99.95%, 30 mg,
Sigma-Aldrich) was put in a boat in the upstream with a temperature of ∼300˚C during
growth. MoO
3
powder (99.99%, 15 mg, Sigma-Aldrich) used as the Mo source was put in
the middle zone with a temperature of 650˚C. The growth substrate was SiO
2
/Si, which
was put on the top of the MoO
3
boat and facing down. The distance between sulfur and
MoO
3
was 17-19 cm. The furnace was purged by high purity Ar for 10 min. Then, 50
sccm of Ar was introduced to the system, and the furnace temperature was increased to
82

650˚C rapidly in 10 min. The growth took 5min and was naturally cooled under 50 sccm
of Ar after growth. Figure 5.5(a) and (b) show a typical optical microscopic image of the
as-grown triangle-shaped MoS
2
. The lateral dimensions of these triangles are found to be
5-30µm. Afterwards, Raman spectroscopic was conducted to evaluate the number of lay-
ers, as shown in Figure 5.5(c). The distance between the in-plane E
2g
mode at ~383cm
-1

and out-of-plane A
1g
mode at ~403cm
-1
was ~19cm
-1
, indicating the formation of pre-
dominately monolayer MoS
2
, consistent with early reports.
40, 41, 52

5.2.2 Two-dimensional material heterostructures growth
While efforts to develop devices based on two-dimensional materials are ongoing,
another exciting strategy that is rapidly emerging is to combine different two-dimensional
crystals in artificially produced layered materials. These 2D layers can be integrated into
a monolayer (lateral 2D heterostructure) or a multilayer stack (vertical 2D heterostruc-
ture). The atomically thin geometry of these 2D materials can allow for band structure
Figure 5.5: Characterization of CVD MoS
2

(ab) Optical image of MoS
2
. on SiO
2
/Si. (c) Raman spectrum of the sample in (b)
using 532nm laser.
83

modulation in both vertical and horizontal direction as well as forming atomically sharp
heterojunctions, providing access to new properties and applications beyond component
2D atomic crystals.
6, 11, 18, 24, 26, 30, 59, 90, 103, 107
In principle, a whole new range of devices
could be created by properly combining highly conductive graphene, insulating boron
nitride and semiconducting TMDCs such as MoS
2
or WS
2
.
63, 90

To create such heterojunctions, multiple steps of aligned transfer need to be applied to
stack the isolated crystals together. But no matter how carefully it is done, the transfer
process always involves interfacial contamination, which is not desired for reliable fabri-
cation. Therefore, people tried to produce heterojunctions directly by chemical vapor
deposition or vapor transport deposition through the successive growth of a second mate-
rial (for example, MoSe
2
or WSe
2
) at the edge or on top of an existing domain of a first
material (graphene, etc.).
18, 26, 28, 30, 42, 54, 77
Currently, we are working on this direction
with the matured CVD recipe of graphene, h-BN, MoS
2
, WeS
2
, and new recipes of NbS
2

in developing.

84

Bibliography
1. Andrei, E. Y.; Li, G. H.; Du, X., Electronic Properties of Graphene: A Perspective
From Scanning Tunneling Microscopy and Magnetotransport. Rep. Prog. Phys. 2012, 75.
2. Avouris, P., Graphene: Electronic and Photonic Properties and Devices. Nano Lett.
2010, 10, 4285-4294.
3. Badmaev, A.; Che, Y.; Li, Z.; Wang, C.; Zhou, C., Self-Aligned Fabrication of
Graphene RF Transistors with T-Shaped Gate. ACS Nano 2012, 6, 3371-3376.
4. Bai, J.; Duan, X.; Huang, Y., Rational Fabrication of Graphene Nanoribbons Using a
Nanowire Etch Mask. Nano Lett. 2009, 9, 2083-2087.
5. 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.
6. Britnell, L.; Gorbachev, R. V.; Jalil, R.; Belle, B. D.; Schedin, F.; Mishchenko, A.;
Georgiou, T.; Katsnelson, M. I.; Eaves, L.; Morozov, S. V.; Peres, N. M. R.; Leist, J.;
Geim, A. K.; Novoselov, K. S.; Ponomarenko, L. A., Field-Effect Tunneling Transistor
Based on Vertical Graphene Heterostructures. Science 2012, 335, 947-950.
7. Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.;
Seitsonen, A. P.; Saleh, M.; Feng, X.; Mullen, K.; Fasel, R., Atomically precise bottom-
up fabrication of graphene nanoribbons. Nature 2010, 466, 470-473.
8. Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K., The
Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109-162.
9. 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. 2011,11, 3519–3525.
10. Chen, Z.; Lin, Y.-M.; Rooks, M. J.; Avouris, P., Graphene nano-ribbon electronics.
Physica E Low Dimens Syst Nanostruct. 2007, 40, 228-232.
11. Cheng, R.; Li, D.; Zhou, H.; Wang, C.; Yin, A.; Jiang, S.; Liu, Y.; Chen, Y.; Huang,
Y.; Duan, X., Electroluminescence and Photocurrent Generation from Atomically Sharp
WSe
2
/MoS
2
Heterojunction p-n Diodes. Nano Lett. 2014,14, 5590–5597.
12. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H., The chemistry
of two-dimensional layered transition metal dichalcogenide nanosheets. Nature Chem.
2013, 5, 263-75.
85

13. Coleman, J. N., Liquid Exfoliation of Defect-Free Graphene. Acc. Chem. Res. 2012,
46, 14-22.
14. Coraux, J.; N'Diaye, A. T.; Busse, C.; Michely, T., Structural Coherency of Graphene
on Ir(111). Nano Lett. 2008, 8, 565-570.
15.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.
16.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.
17. Ding, L.; Wang, Z.; Pei, T.; Zhang, Z.; Wang, S.; Xu, H.; Peng, F.; Li, Y.; Peng, L.-
M., Self-Aligned U-Gate Carbon Nanotube Field-Effect Transistor with Extremely Small
Parasitic Capacitance and Drain-Induced Barrier Lowering. ACS Nano 2011, 5, 2512-
2519.
18. Duan, X.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li, H.; Wu, X.; Tang, Y.;
Zhang, Q.; Pan, A.; Jiang, J.; Yu, R.; Huang, Y.; Duan, X., Lateral epitaxial growth of
two-dimensional layered semiconductor heterojunctions. Nat Nanotechnol. 2014, 9,
1024–1030.
19. Eizenberg, M.; Blakely, J. M., Carbon Monolayer Phase Condensation on Ni(111).
Surf. Sci. 1979, 82, 228-236.
20. Ferrari, A. C., Raman Spectroscopy of Graphene and Graphite: Disorder, Electron-
phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47-
57.
21. Fiori, G.; Iannaccone, G., Simulation of Graphene Nanoribbon Field-Effect
Transistors. Electron Device Lett. IEEE 2007, 28, 760-762.
22. 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.
23. Gao, L. B.; Ren, W. C.; Xu, H. L.; Jin, L.; Wang, Z. X.; Ma, T.; Ma, L. P.; Zhang, Z.
Y.; Fu, Q.; Peng, L. M.; Bao, X. H.; Cheng, H. M., Repeated Growth and Bubbling
Transfer of Graphene With Millimetre-size Single-crystal Grains Using Platinum.
Nat.Commun. 2012, 3, 699.
24. Geim, A. K.; Grigorieva, I. V., Van der Waals heterostructures. Nature 2013, 499,
419-25.
86

25. Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-191.
26. Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.;
Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.;
Zhou, W.; Ajayan, P. M., Vertical and in-plane heterostructures from WS
2
/MoS
2

monolayers. Nat. Mater. 2014,13, 1135–1142.
27. Gotterbarm, K.; Zhao, W.; Hofert, O.; Gleichweit, C.; Papp, C.; Steinruck, H.-P.,
Growth and oxidation of graphene on Rh(111). Phys. Chem. Chem. Phys. 2013, 15,
19625-19631.
28. Han, G. H.; Rodríguez-Manzo, J. A.; Lee, C.-W.; Kybert, N. J.; Lerner, M. B.; Qi, Z.
J.; Dattoli, E. N.; Rappe, A. M.; Drndic, M.; Johnson, A. T. C., Continuous Growth of
Hexagonal Graphene and Boron Nitride In-Plane Heterostructures by Atmospheric
Pressure Chemical Vapor Deposition. ACS Nano 2013, 7, 10129-10138.
29. Han, M. Y.; Özyilmaz, B.; Zhang, Y.; Kim, P., Energy Band-Gap Engineering of
Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98, 206805.
30. Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J.; Beanland, R.; Ross, J. S.; Rivera, P.;
Yao, W.; Cobden, D. H.; Xu, X., Lateral heterojunctions within monolayer MoSe-WSe
semiconductors. Nat Mater 2014,13, 1096–1101.
31. 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.
32. Karu, A. E.; Beer, M., Pyrolytic Formation of Highly Crystalline Graphite Films. J.
Appl. Phys. 1966, 37, 2179.
33. Kathleen M. McCreary , A. T. H., Jeremy T. Robinson , Enrique Cobas ,James C.
Culbertson , Adam L. Friedman , Glenn G. Jernigan , and Berend T. Jonker, Large-Area
Synthesis of Continuous and Uniform MoS
2
Monolayer Films on Graphene. Adv. Funct.
Mater. 2014,41, 6449–6454.
34. 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.
35. Kim, K. K.; Hsu, A.; Jia, X. T.; Kim, S. M.; Shi, Y. S.; Hofmann, M.; Nezich, D.;
Rodriguez-Nieva, J. F.; Dresselhaus, M.; Palacios, T.; Kong, J., Synthesis of Monolayer
Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition. Nano Lett. 2012,
12, 161-166.
87

36.Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price,
B. K.; Tour, J. M., Longitudinal unzipping of carbon nanotubes to form graphene
nanoribbons. Nature 2009, 458, 872-876.
37. Kwon, S. Y.; Ciobanu, C. V.; Petrova, V.; Shenoy, V. B.; Bareno, J.; Gambin, V.;
Petrov, I.; Kodambaka, S., Growth of Semiconducting Graphene on Palladium. Nano Lett.
2009, 9, 3985-3990.
38. Lee, K. H.; Shin, H. J.; Lee, J.; Lee, I. Y.; Kim, G. H.; Choi, J. Y.; Kim, S. W., Large-
Scale Synthesis of High-Quality Hexagonal Boron Nitride Nanosheets for Large-Area
Graphene Electronics. Nano Lett. 2012, 12, 714-718.
39. Lee, S.; Lee, K.; Zhong, Z. H., Wafer Scale Homogeneous Bilayer Graphene Films
by Chemical Vapor Deposition. Nano Lett. 2010, 10, 4702-4707.
40. Lee, Y.-H.; Yu, L.; Wang, H.; Fang, W.; Ling, X.; Shi, Y.; Lin, C.-T.; Huang, J.-K.;
Chang, M.-T.; Chang, C.-S.; Dresselhaus, M.; Palacios, T.; Li, L.-J.; Kong, J., Synthesis
and Transfer of Single-Layer Transition Metal Disulfides on Diverse Surfaces. Nano Lett.
2013, 13, 1852-1857.
41. Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu,
Y.-C.; Wang, J. T.-W.; Chang, C.-S.; Li, L.-J.; Lin, T.-W., Synthesis of Large-Area
MoS2 Atomic Layers with Chemical Vapor Deposition. Adv. Mater. 2012, 24, 2320-2325.
42. Levendorf, M. P.; Kim, C. J.; Brown, L.; Huang, P. Y.; Havener, R. W.; Muller, D.
A.; Park, J., Graphene and Boron Nitride Lateral Heterostructures for Atomically Thin
Circuitry. Nature 2012, 488, 627-632.
43. Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y.,
Black phosphorus field-effect transistors. Nat Nano 2014, 9, 372-377.
44. Li, Q. Y.; Chou, H.; Zhong, J. H.; Liu, J. Y.; Dolocan, A.; Zhang, J. Y.; Zhou, Y. H.;
Ruoff, R. S.; Chen, S. S.; Cai, W. W., Growth of Adlayer Graphene on Cu Studied by
Carbon Isotope Labeling. Nano Lett. 2013, 13, 486-490.
45. Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung,
I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S., Large-Area Synthesis of High-
Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312-1314.
46. Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H., Chemically Derived, Ultrasmooth
Graphene Nanoribbon Semiconductors. Science 2008, 319, 1229-1232.
47. 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.
88

48. 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.
49. 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.
50. Liao, L.; Bai, J.; Cheng, R.; Lin, Y.-C.; Jiang, S.; Qu, Y.; Huang, Y.; Duan, X., Sub-
100 nm Channel Length Graphene Transistors. Nano Lett. 2010, 10, 3952-3956.
51. Liao, L.; Lin, Y.-C.; Bao, M.; Cheng, R.; Bai, J.; Liu, Y.; Qu, Y.; Wang, K. L.;
Huang, Y.; Duan, X., High-speed Graphene Transistors With A Self-aligned Nanowire
Gate. Nature 2010, 467, 305-308.
52.Liu, K.-K.; Zhang, W.; Lee, Y.-H.; Lin, Y.-C.; Chang, M.-T.; Su, C.-Y.; Chang, C.-S.;
Li, H.; Shi, Y.; Zhang, H.; Lai, C.-S.; Li, L.-J., Growth of Large-Area and Highly
Crystalline MoS2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12, 1538-1544.
53. Liu, L. X.; Zhou, H. L.; Cheng, R.; Yu, W. J.; Liu, Y.; Chen, Y.; Shaw, J.; Zhong, X.;
Huang, Y.; Duan, X. F., High-Yield Chemical Vapor Deposition Growth of High-Quality
Large-Area AB-Stacked Bilayer Graphene. ACS Nano 2012, 6, 8241-8249.
54. Liu, Z.; Ma, L. L.; Shi, G.; Zhou, W.; Gong, Y. J.; Lei, S. D.; Yang, X. B.; Zhang, J.
N.; Yu, J. J.; Hackenberg, K. P.; Babakhani, A.; Idrobo, J. C.; Vajtai, R.; Lou, J.; Ajayan,
P. M., In-plane Heterostructures of Graphene and Hexagonal Boron Nitride with
Controlled Domain Sizes. Nat Nanotecnol. 2013, 8, 119-124.
55. Lopes dos Santos, J. M. B.; Peres, N. M. R.; Castro Neto, A. H., Graphene Bilayer
with a Twist: Electronic Structure. Phys. Rev. Lett. 2007, 99, 256802.
56. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A., Ultrasensitive
photodetectors based on monolayer MoS2. Nat Nano 2013, 8, 497-501.
57. Lui, C. H.; Li, Z.; Mak, K. F.; Cappelluti, E.; Heinz, T. F., Observation of an
electrically tunable band gap in trilayer graphene. Nat. Phys. 2011, 7, 944-947.
58. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS2: A New
Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.
59. Meng-Yu Lin, C.-E. C., Cheng-Hung Wang, Chen-Fung Su, Chi Chen, Si-Chen Lee,
and Shih-Yen Lin, Toward epitaxially grown two-dimensional crystal hetero-structures:
89

Single and double MoS2/graphene hetero-structures by chemical vapor depositions. Appl.
Phys. Lett. 2014, 105.
60. Moon, J.; Curtis, D.; Bui, S.; Hu, M.; Gaskill, D.; Tedesco, J.; Asbeck, P.; Jernigan,
G.; VanMil, B.; Myers-Ward, R., Top-gated Epitaxial Graphene FETs on Si-face SiC
Wafers with a Peak Transconductance of 600 mS/mm. IEEE Electron Device Lett. 2010,
31, 260-262.
61. Nemanich, R. J.; Solin, S. A.; Martin, R. M., Light Scattering Study of Boron-Nitride
Micro-crystals. Phys. Rev. B 1981, 23, 6348-6356.
62. Nie, S.; Wu, W.; Xing, S. R.; Yu, Q. K.; Bao, J. M.; Pei, S. S.; McCarty, K. F.,
Growth From Below: Bilayer Graphene on Copper by Chemical Vapor Deposition. New
J. Phys 2012, 14.
63. Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K.,
A roadmap for graphene. Nature 2012, 490, 192-200.
64. 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.
65.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.
66. Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.;
Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K., Room-temperature Quantum Hall
Effect in Graphene. Science 2007, 315, 1379-1379.
67. Oznuluer, T.; Pince, E.; Polat, E. O.; Balci, O.; Salihoglu, O.; Kocabas, C., Synthesis
of graphene on gold. Appl. Phys. Lett. 2011, 98, -.
68. RadisavljevicB; RadenovicA; BrivioJ; GiacomettiV; KisA, Single-layer MoS2
transistors. Nat Nano 2011, 6, 147-150.
69. Ramasubramaniam, A., Large excitonic effects in monolayers of molybdenum and
tungsten dichalcogenides. Phys. Rev. B 2012, 86, 115409.
70. Rasool, H. I.; Song, E. B.; Allen, M. J.; Wassei, J. K.; Kaner, R. B.; Wang, K. L.;
Weiller, B. H.; Gimzewski, J. K., Continuity of Graphene on Polycrystalline Copper.
Nano Lett. 2011, 11, 251-256.
90

71. 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.
72. 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 Lett. 2009, 9, 30-35.
73. Riedl, C.; Coletti, C.; Starke, U., Structural and electronic properties of epitaxial
graphene on SiC(0001): a review of growth, characterization, transfer doping and
hydrogen intercalation. J. Phys. D: Appl. Phys. 2010, 43, 374009.
74. 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.
75. Schwierz, F., Graphene transistors. Nat Nano 2010, 5, 487-496.
76. Shallcross, S.; Sharma, S.; Pankratov, O. A., Twist boundary in graphene: energetics
and electric field effect. J. Phys. Condens. Matter. 2008, 20, 454224.
77. Shi, Y.; Zhou, W.; Lu, A. Y.; Fang, W.; Lee, Y. H.; Hsu, A. L.; Kim, S. M.; Kim, K.
K.; Yang, H. Y.; Li, L. J.; Idrobo, J. C.; Kong, J., van der Waals epitaxy of MoS2 layers
using graphene as growth templates. Nano Lett. 2012, 12, 2784-91.
78. Shi, Y. M.; Hamsen, C.; Jia, X. T.; Kim, K. K.; Reina, A.; Hofmann, M.; Hsu, A. L.;
Zhang, K.; Li, H. N.; Juang, Z. Y.; Dresselhaus, M. S.; Li, L. J.; Kong, J., Synthesis of
Few-Layer Hexagonal Boron Nitride Thin Film by Chemical Vapor Deposition. Nano
Lett. 2010, 10, 4134-4139.
79. Shinde, P. P.; Kumar, V., Direct band gap opening in graphene by BN doping: Ab
initio calculations. Phys. Rev. B 2011, 84, 125401.
80. Son, Y.-W.; Cohen, M. L.; Louie, S. G., Energy Gaps in Graphene Nanoribbons.
Phys. Rev. Lett. 2006, 97, 216803.
81. Song, L.; Ci, L. J.; Lu, H.; Sorokin, P. B.; Jin, C. H.; Ni, J.; Kvashnin, A. G.;
Kvashnin, D. G.; Lou, J.; Yakobson, B. I.; Ajayan, P. M., Large Scale Growth and
Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Lett. 2010, 10, 3209-
3215.
82. Soo Park, C.; Zhao, Y.; Lee, J.-H.; Whang, D.; Shon, Y.; Song, Y.-H.; Jin Lee, C.,
Tunable bandgap of a single layer graphene doped by the manganese oxide using the
electrochemical doping. Appl. Phys. Lett. 2013, 102, 032106.
91

83. Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F.,
Emerging Photoluminescence in Monolayer MoS
2
. Nano Lett. 2010, 10, 1271-1275.
84.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.
85.Sun, Z. Z.; Raji, A. R. O.; Zhu, Y.; Xiang, C. S.; Yan, Z.; Kittrel, C.; Samuel, E. L. G.;
Tour, J. M., Large-Area Bernal-Stacked Bi-, Tr-, and Tetralayer Graphene. ACS Nano
2012, 6, 9790-9796.
86. Sutter, P. W.; Flege, J.-I.; Sutter, E. A., Epitaxial Graphene on Ruthenium. Nat.
Mater. 2008, 7, 406-411.
87. Tao, L.; Lee, J.; Li, H.; Piner, R. D.; Ruoff, R. S.; Akinwande, D., Inductively heated
synthesized graphene with record transistor mobility on oxidized silicon substrates at
room temperature. Appl. Phys. Lett. 2013, 103, 183115.
88. Varykhalov, A.; Rader, O., Graphene Grown on Co(0001) Films and Islands:
Electronic Structure and Its Precise Magnetization Dependence. Phys. Rev. B 2009, 80,
035437.
89. 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.
90. Wang, H.; Liu, F.; Fu, W.; Fang, Z.; Zhou, W.; Liu, Z., Two-dimensional
heterostructures: fabrication, characterization, and application. Nanoscale 2014, 6,
12250-12272.
91. Wang, H.; Wang, G.; Bao, P.; Yang, S.; Zhu, W.; Xie, X.; Zhang, W.-J., Controllable
Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper
Foils by Suppressing Nucleation. J. Am. Chem. Soc. 2012, 134, 3627-3630.
92. 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, 455402.
93. Wang, X.; Feng, H.; Wu, Y.; Jiao, L., Controlled Synthesis of Highly Crystalline
MoS2 Flakes by Chemical Vapor Deposition. J. Am. Chem. Soc. 2013, 135, 5304-5307.
94. 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.
92

95. Wood, J. D.; Schmucker, S. W.; Lyons, A. S.; Pop, E.; Lyding, J. W., Effects of
Polycrystalline Cu Substrate on Graphene Growth by Chemical Vapor Deposition. Nano
Lett. 2011, 11, 4547-4554.
96. Wu, W.; Jauregui, L. A.; Su, Z.; Liu, Z.; Bao, J.; Chen, Y. P.; Yu, Q., Growth of
Single Crystal Graphene Arrays by Locally Controlling Nucleation on Polycrystalline Cu
Using Chemical Vapor Deposition. Adv. Mater. 2011, 23, 4898-4903.
97. Wu, Y.; Jenkins, K. A.; Valdes-Garcia, A.; Farmer, D. B.; Zhu, Y.; Bol, A. A.;
Dimitrakopoulos, C.; Zhu, W.; Xia, F.; Avouris, P., State-of-the-art Graphene High-
frequency Electronics. Nano Lett. 2012, 12, 3062-3067.
98. Wu, Y. M. A.; Fan, Y.; Speller, S.; Creeth, G. L.; Sadowski, J. T.; He, K.; Robertson,
A. W.; Allen, C. S.; Warner, J. H., Large Single Crystals of Graphene on Melted Copper
Using Chemical Vapor Deposition. ACS Nano 2012, 6, 5010-5017.
99. Xu, Z. P.; Buehler, M. J., Interface Structure and Mechanics Between Graphene and
Metal Substrates: A First-principles Study. J. Phys. Condens. 2010, 22.
100. Yan, K.; Peng, H. L.; Zhou, Y.; Li, H.; Liu, Z. F., Formation of Bilayer Bernal
Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition. Nano Lett. 2011, 11,
1106-1110.
101. Yan, Z.; Lin, J.; Peng, Z.; Sun, Z.; Zhu, Y.; Li, L.; Xiang, C.; Samuel, E. L.; Kittrell,
C.; Tour, J. M., Toward the Synthesis of Wafer-Scale Single-Crystal Graphene on
Copper Foils. ACS Nano 2012, 6, 9110-9117.
102. Yazyev, O. V.; Louie, S. G., Electronic Transport in Polycrystalline Graphene.
Nat.Mater. 2010, 9, 806-809.
103. Yexin Deng, Z. L., Nathan J. Conrad, Han Liu, Yongji Gong, Sina Najmaei,
Pulickel M. Ajayan, Jun Lou, Xianfan Xu, and Peide D. Ye, Black Phosphorus–
Monolayer MoS2 van der Waals Heterojunction p–n Diode. ACS Nano 2014, 8, 8292–
8299.
104. Yu, L.; Lee, Y. H.; Ling, X.; Santos, E. J.; Shin, Y. C.; Lin, Y.; Dubey, M.; Kaxiras,
E.; Kong, J.; Wang, H.; Palacios, T., Graphene/MoS2 hybrid technology for large-scale
two-dimensional electronics. Nano Lett. 2014, 14, 3055-63.
105. 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.
93

106. 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.
107. Yu, W. J.; Liu, Y.; Zhou, H.; Yin, A.; Li, Z.; Huang, Y.; Duan, X., Highly efficient
gate-tunable photocurrent generation in vertical heterostructures of layered materials. Nat
Nano 2013, 8, 952-958.
108. Yu, Y.; Li, C.; Liu, Y.; Su, L.; Zhang, Y.; Cao, L., Controlled Scalable Synthesis of
Uniform, High-Quality Monolayer and Few-layer MoS2 Films. Sci. Rep. 2013, 3.
109. Zeng, Q.; Wang, S.; Yang, L.; Wang, Z.; Zhang, Z.; Peng, L.; Zhou, W.; Xie, S.,
Doping-free Fabrication of Carbon Nanotube Thin-film Diodes and Their Photovoltaic
Characteristics. Nano Res. 2012, 5, 33-42.
110. 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.
111. Zhang, Y.; Tang, T. T.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.;
Shen, Y. R.; Wang, F., Direct observation of a widely tunable bandgap in bilayer
graphene. Nature 2009, 459, 820-3.
112. Zhang, Y.; Zhang, L. Y.; Kim, P.; Ge, M. Y.; Li, Z.; Zhou, C., Vapor Trapping
Growth of Single-Crystalline Graphene Flowers: Synthesis, Morphology, and Electronic
Properties. Nano Lett. 2012, 12, 2810-2816.
113. 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.
114. Zhao, H.; Lin, Y.-C.; Yeh, C.-H.; Tian, H.; Chen, Y.-C.; Xie, D.; Yang, Y.; Suenaga,
K.; Ren, T.-L.; Chiu, P.-W., Growth and Raman Spectra of Single-Crystal Trilayer
Graphene with Different Stacking Orientations. ACS Nano 2014, 8, 10766-10773.
115. Zhao, W.; Ribeiro, R. M.; Toh, M.; Carvalho, A.; Kloc, C.; Castro Neto, A. H.; Eda,
G., Origin of Indirect Optical Transitions in Few-Layer MoS
2
, WS
2
, and WSe
2
. Nano Lett.
2013, 13, 5627-5634.

Abstract (if available)

Abstract Graphene, emerging in 2004 as a two-dimensional (2D) single layer material with carbon atoms arranged in a honeycomb crystal lattice, exhibits physically fascinating and technologically useful properties, triggers global boom in graphene research. Despite the intense interest and remarkably rapid progress in this field during the past few years, there are still serious challenges in achieving high quality, scalable, and low cost samples needed for electronics and optoelectronics. Chemical vapor deposition (CVD) is explored as a powerful technique to synthesize graphene and other two-dimensional materials in large scale with low cost. In this dissertation, I present the opportunities, challenges and solutions in producing graphene and other two-dimensional materials by chemical vapor deposition. ❧ The first chapter gives a brief introduction about properties, preparation and characterization of graphene and hexagonal boron nitride (h-BN). Chapter 2 discusses the approaches to decrease graphene nucleation density on cooper foil and to synthesize large grain, single-crystalline graphene by chemical vapor deposition. A vapor trapping method is developed to grow graphene flowers with grain size up to 0.1mm at low pressure. Hexagonal graphene with grain size over 2mm is also grown at ambient pressure. Selected area electron diffraction (SAED) characterization confirms the single-crystalline nature of graphene grains, and electrical measurement demonstrates their excellent motilities. Controlled synthesis and growth parameter study provide insight into the key factors that affect the grain morphology. In the following chapter, bilayer and trilayer graphene with various adlayer morphologies are produced by ambient pressure CVD, and characterized by Raman and transmission electron microscopy (TEM). Growth mechanisms for both concentric and nonconcentric adlayers are investigated. The proposed model for adlayer growth, along with the statistical analysis, allows more controllable synthesis of bi-layer/trilayer graphene. In chapter 4, we investigate the synthesis of h-BN by CVD. Various growth conditions, including metal substrates, precursors, and growth parameters especially pressures, are studied to achieve better quality of BN films. In the last chapter, I present the ongoing and future work of graphene applications and the 2D materials beyond graphene. T-gate devices are fabricated with high quality CVD graphene on h-BN substrate for RF applications, and the DC performance is compared with those fabricated on SiO₂/Si substrates, showing great potential for such type of system. Molybdenum disulfide (MoS₂) in the family of transition metal dichalcogenides (TMDs) is also grown by CVD on SiO₂/Si for the future study on 2D heterostructures.

Linked assets

University of Southern California Dissertations and Theses

Conceptually similar

PDF

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

PDF

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

PDF

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

PDF

Substrate-induced effects in thermally cycled graphene & electron and thermoelectric transports across 2D material heterostructures

PDF

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

PDF

Carbon material-based nanoelectronics

PDF

Controlled synthesis, characterization and applications of carbon nanotubes

PDF

Plasmonic enhancement of catalysis and solar energy conversion

PDF

Inorganic/organic hybrid materials and grafted coatings via vapor phase deposition processes

PDF

Nanostructured silicon anode and sulfur cathode for lithium rechargeable batteries

PDF

One-dimensional nanomaterials for electronic and sensing applications

PDF

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

PDF

Modification of electrode materials for lithium ion batteries

PDF

Zero-dimensional and one-dimensional nanostructured materials for application in photovoltaic cells

PDF

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

PDF

One-dimensional nanostructures for chemical sensing, transparent electronics, and energy conversion and storage devices

PDF

The patterning of polymer thin films on porous substrates via initiated chemical vapor deposition

PDF

Biophysical studies of nanosecond pulsed electric field induced cell membrane permeabilization

PDF

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

PDF

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

Asset Metadata

Creator Zhang, Luyao (author)

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

School Viterbi School of Engineering

Degree Doctor of Philosophy

Degree Program Materials Science

Publication Date 03/13/2015

Defense Date 01/16/2015

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

Tag bilayer,CVD,graphene,large-grain,OAI-PMH Harvest

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Zhou, Chongwu (committee chair), Goo, Edward K. (committee member), Wu, Wei (committee member)

Creator Email luyaozha@usc.edu,zlyzju@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-539918

Unique identifier UC11297890

Identifier etd-ZhangLuyao-3232.pdf (filename),usctheses-c3-539918 (legacy record id)

Legacy Identifier etd-ZhangLuyao-3232.pdf

Dmrecord 539918

Document Type Dissertation

Format application/pdf (imt)

Rights Zhang, Luyao

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