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Controlled synthesis of large grain single crystal graphene by chemical vapor deposition
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Controlled synthesis of large grain single crystal graphene by chemical vapor deposition
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Content
CONTROLLED SYNTHESIS OF LARGE GRAIN SINGLE CRYSTAL
GRAPHENE BY CHEMICAL VAPOR DEPOSITION
by
Ning Yang
A Thesis Presented to the
FACULTY OF THE USC VITERBI SCHOOL OF ENGINEERING
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(ELECTRICAL ENGINEERING)
May 2012
Copyright 2012 Ning Yang
ii
Table of Contents
List of Figures .................................................................................................................... iii
Abstract ................................................................................................................................v
Chapter 1: Introduction ........................................................................................................1
Chapter 2: Graphene Basics .................................................................................................5
2.1 Carbon Allotropes ......................................................................................................5
2.2 Electronic Bandstructure of Graphene .......................................................................6
2.3 Optical Properties of Graphene ..................................................................................9
2.4 Phonon Dispersion Relationship of Graphene .........................................................10
2.5 Graphene Grain Boundary .......................................................................................11
Chapter 3: Polycrystalline Graphene Synthesis by Chemical Vapor Deposition (CVD) ..13
3.1 Growth Mechanism ..................................................................................................13
3.2 Experiment Procedure ..............................................................................................18
Chapter 4: Large Grain Single Crystal Graphene Synthesis by CVD ...............................26
4.1 Six-Lobe Large Area Single Crystal Graphene ........................................................27
4.2 Regular Square Single Layer Single Crystal Graphene ...........................................36
4.3 Hexagonal Single Crystal Graphene ........................................................................40
Chapter 5: Conclusion and Future Work ...........................................................................43
5.1 Conclusion ................................................................................................................43
5.2 Future Work .............................................................................................................43
References ..........................................................................................................................44
iii
List of Figures
Figure 1. The unit cell and Brillion zone of graphene .........................................................7
Figure 2. Graphene bandstructure ........................................................................................9
Figure 3. Neighbor atoms of a graphene plane up to fourth nearest neighbor ...................10
Figure 4. Phonon dispersion relationship...........................................................................11
Figure 5. Graphene grain boundary ...................................................................................12
Figure 6. Phase diagram for Ni, Co, Fe and Cu .................................................................15
Figure 7. Schematic of chemical vapor deposition of graphene on copper .......................16
Figure 8. As received copper and its surface condition .....................................................19
Figure 9. A typical CVD system set up .............................................................................20
Figure 10. Time dependence of experimental parameters: temperature, pressure, and gas
composition/flow rate ........................................................................................................22
Figure 11. Transfer process ................................................................................................24
Figure 12. SEM of six-lobe large grain single crystal graphene ........................................28
Figure 13. Optical image of six-lobe large grain single crystal graphene .........................29
Figure 14. Raman Mapping of G-band and D-band ..........................................................30
Figure 15. Raman Spectra of single layer six-lobe large grain single crystal graphene ....31
Figure 16. Copper enclosure and schematic process of CVD graphene on Cu .................32
iv
Figure 17. SEM of Cu enclosure grown six-lobe large grain single crystal graphene ......33
Figure 18. Schematic process of vapor trapping method ...................................................34
Figure 19. SEM of the vapor trapping grown six-lobe single crystal graphene ................35
Figure 20. SEM of the non-vapor trapping grown graphene .............................................36
Figure 21. SEM of regular square large grain single crystal graphene with annealing time
20 mins (left) and 3 hours (right) .......................................................................................37
Figure 22. SEM of regular square large grain single crystal graphene ..............................38
Figure 23. Optical image of regular square large grain single crystal graphene ...............39
Figure 24. Raman spectra of single layer square large grain single crystal graphene .......40
Figure 25. SEM regular square large grain single crystal graphene ..................................40
Figure 26. Raman spectra of mono layer (a), double layer (b) and triple layer (c) graphene
in hexagonal sample with the center occupied by multiple layers .....................................42
v
Abstract
This thesis presents a novel Chemical Vapor Deposition (CVD) method that can
produce six-lobe large grain single crystal graphene with grain size up to 80 µm. The new
CVD method introduces a reduced carbon source concentration and an intentionally
reduced gas flow rate in the initial stage. Compared with previously reported methods,
this method offers a promising pathway for investigation of the growth mechanism of
six-lobe large grain single crystal graphene CVD process.
Controlled synthesis of six-lobe, square and hexagonal shaped single crystal
graphene has been achieved by varying the pressure increasing method. The comparison
study gives a good indication of the dependent relationship between pressure increasing
methods and different single crystal graphene morphologies.
1
Chapter 1: Introduction
Graphene is a two-dimensional, honeycomb lattice arrangement with unique
physical properties. [1] The electron transport of graphene is described by Dirac-like
equation (zero effective mass), which give rise to extraordinary effects such as exhibit
giant intrinsic mobility, ballistic transport on sub-micrometer scale and half-integer quan-
tum Hall effect. [2] It is also a promising candidate material for high-frequency electronic
devices because of its high carrier mobility. The low absorbance of Graphene comple-
mented with its semi-metallic nature makes it potentially an ideal transparent conductor
where transparency and high conductivity is required. [3] Integrated devise will require
wafer scale deposition that can be possess using existing or post complementary metal
oxide semiconductor (CMOS) fabrication techniques. Implementation as a transparent
conductor will require uniform deposition over large areas with controllable number of
layers. These requirements have led to the development of a rapidly evolving research
thrust within the field of graphene based on deposition of high quality and uniform thin
films over large areas with controllable thickness.
The best quality graphene, in terms of structural integrity, is obtained by mechan-
ical exfoliation of highly orientated porolytic graphite (HOPG). [4] Although pristine
graphene has very low concentration of structural defects, the flake thickness, size and
location are largely uncontrollable. To overcome the disadvantage of small-scale produc-
tion of graphene using
2
mechanical exfoliation of HOPG, many chemical approaches have been developed, in-
cluding epitaxial growth on silicon carbide and ruthenium as well as two dimensional as-
sembly of reduced graphite oxides. Epitaxial growth provides high-quality multilayer
grapheme strongly interact with the substrate, but electrically isolated mono- or bi-layer
grapheme for device application has not been made.[5] On the other hand, the self-
assembly of soluble graphene sheets demonstrates the possibility of a low-cost produc-
tion method for large-scale transparent films. However, the assembled graphene films
show relatively low electrical conductivity due to the poor interlayer junction contact re-
sistance and the structural defects formed during the vigorous exfoliation and reduction
process. Compared with other graphene synthesis method, graphene growth on transition
metals such as Ni, Pd, Ru, Ir or Cu, has the distinct advantage of being able to provide
very large area of graphene films transferable to other substates. This advantage is espe-
cially true for the case of graphene growth on Cu substrate by Chemical Vapor Deposi-
tion (CVD) of methane as reported by Li et al. CVD of graphene on Cu yields a uniform
graphene film whose size is limited only by the size of the Cu substrate and the growth
system.
Despite the significant progress, CVD graphene is usually a polycrystalline film
made of small grains. [6] The grain boundaries degrade the electrical properties of the
resulting films and first principle quantum transport calculations predict that the periodic-
ity breaking disorder can adversely affect transport properties. Any of these defects can
give rise to higher surface chemical activity that would further disrupt the sp
2
bonding
nature of graphene, and thus impact graphene’s fundamental properties.
3
As the grain boundaries have been found to impede both transport and mechanical
properties, it is therefore very important to be able to synthesize large-grain, single-
crystalline graphene for various applications. [7],[8] To overcome the grain boundary is-
sue, large grain single crystal synthesis has been explored using several methods, such as
copper enclosure and vapor trapping. The pioneering work of Li et al10 demonstrated the
CVD growth of large graphene single crystals up to 0.5 mm in size, using a copper enclo-
sure. [9],[10] Such large graphene single crystals can be very important and may find ap-
plications for various electronic devices. However, the process depends on how the cop-
per enclosure is manually made and does not allow probing the inner gas environment.
On the other hand, vapor trapping method provides improved control and reproducibility;
and the open end of the vapor trapping tube enables the possibility of probing gas species
inside tube. However, the inner pressure distribution is still difficult to probe owing to the
complicated geographical structure of double tubing. To better understand the forming
mechanism of large grain single crystal graphene, here this paper report a smooth pres-
sure increase method using diluted methane to grow large-grain, single-crystalline gra-
phene with simple geographical setting.
In this work, a novel CVD method has been introduced with gradually and expo-
nentially slowly increased gas pressure, leading to the result of six-lobe single layer gra-
phene grain size up to 80 μm. According to literature, six-lobe flower shaped single layer
graphene are single crystal. Along with other single crystal graphene reported in litera-
ture, we performed a series of control experiments. By changing one constant while keep-
ing other condition the same, the graphene grain shape can be changed from six lobe, to
4
regular square to hexagonal shape. It gives a good indication for the forming mechanism
for different morphology. To achieve a better control of the partial pressure of graphene,
we used highly diluted (0.1%) methane instead of pure methane. The highly controllable
precursor effectively contributes to the smooth increase of the methane partial pressure.
The morphology study with variable pressure increase method indicates a strong relation-
ship between the large single crystal formation and smooth pressure increase method.
5
Chapter 2: Graphene Basics
In this Chapter, electronic band structure of Graphene has been calculated via
nearest neighbor Tight Binding method. In addition, phonon dispersion relationship of
Graphene has been discussed and calculated via fourth nearest neighbor Tight Binding
method. Both calculations were conducted via Matlab R2011b.
2.1. Carbon Allotropes
Carbon, one of the most common atoms on earth, occurs naturally in many forms
and as a component in countless substances. These are called allotropes of carbon. The
most common carbon allotropes are graphite and diamond. Carbon can occur as an unor-
dered mess of atoms, this is called amorphous carbon. A related form is glassy carbon
which has a semi-ordered structure with bonds resembling other forms. Finally, there are
three nanoscale forms of carbon that have attracted widespread attention over the last
decade because of their novel physical properties. These carbon nanostructures are called
buckyballs, carbon nanotubes, and graphene.
Diamond is the most stable form of pure carbon, formed under high temperatures
and pressures. It is a tetrahedral lattice with a carbon atom at each vertex. Each cabons
forms four covalent bonds with four neighboring atoms, completely filling its outer elec-
tron shell. Pure diamond has a wide bandgap and thus acts as transparent insulator.
st
to
th
is
yh
en
w
h
it
el
th
fo
ar
p
2
co
ce
All ot
tructure of g
o be experim
hick, 2D laye
s able to form
hedral of the
nt physical
wrapped up in
Carbo
exagonal str
t perfectly co
lectron each
he p
z
orbital
orm s-bonds
re largely re
ortant for de
2.2 Elec
The c
omb lattice
ell contains
ther allotrop
graphene. Alt
mentally stud
ers of sp2-bo
m the two-di
e fullerene f
properties.
nto 0D fuller
on has four e
ructure of gr
onjugated in
h, and the re
of a neighb
with other n
esponsible fo
etermining th
ctronic
crystalline st
structure, w
two carbon
pes of carbon
though grap
died, its basi
onded carbon
imensional g
family and th
Thus, keep
renes, rolled
electrons in
raphene pose
n sp
2
hybridi
emaining p
z
bor carbon a
neighboring
or its condu
he solid state
Bandst
tructure of g
which is show
atoms, atom
n can be co
hene is actu
ic structure i
n. It is inter
graphene, th
he cylinder-s
ping the sp
2
d into 1D nan
its valence l
es an alterna
ization. In th
has only on
atom to form
carbons. p-e
uction proper
e properties o
tructur
graphene ca
wn at Figure
m A and atom
onceptualized
ually the mos
is simple. G
resting that c
he planar loc
shaped carbo
2
hybridizat
notubes, or s
level with a
ate double b
his case its p
ne electron.
m a p-bond,
electrons in
rties, while
of graphene
re of Gr
an be describ
e 1 left. In m
m B. The rec
d as variatio
st recent carb
Graphene she
carbon with
al structure i
on nanotube
tion, the 2D
stacked into
configuratio
bond arrange
p
x
and p
y
orb
This p
z
orbi
while the re
graphene are
-orbitals
.
raphen
bed as a he
monolayer gr
ciprocal lattic
ons on the l
bon nanoma
eets are one-
sp
2
hybridiz
in the closed
es, all with d
D carbon ca
3D graphite
on of 2s
2
2p
2
ement that m
bitals contain
ital overlaps
emaining orb
e delocalized
are the mos
ne
exagon or ho
raphene, one
ce and its ve
6
attice
aterial
-atom
zation
d pol-
differ-
an be
e.
2
. The
makes
n one
s with
bitals
d and
st im-
oney-
e unit
ectors
7
are shown at Figure 1 right as well. And the nearest-neighbor distance ac-c is about 1.42
Å.
Figure 1. The unit cell and Brillion zone of graphene
a
i
and b
i
are unit vectors and reciprocal unit vectors, and high symmetry points are
shown. From Fig. 1, the unit vectors are:
2
11
,, ,
22
1
33
a= a a a = a a
22
a
is the lattice constant of graphene, is about 2.46Å. Correspondingly, the recip-
rocal unit vectors are:
2
22 2 2
,, ,
1
b= b =
aa 3a 3a
To get tight binding calculation result for mono-layer graphene, we will assume
that each carbon atom is sp
2
hybridized. Each carbon atom contains four valence elec-
trons, and we will assume that three of them, for example, 2s, 2p
x
and 2p
y
will form three
sp
2
hybrid orbitals. Three atomic orbitals from one atom, and there are two atoms in one
8
unit cell, so in total there will be six σ bands, three of them are bonding bands, and the
other three are anti-bonding bands. The left atom orbitals, two 2p
z
atomic orbitals for the
two atoms, will for two π bands, bonding and anti-bonding.
We should notice that, here we consider the π bands and σ bands separately. Alt-
hough there are two atoms is a graphene unit cell, and each atom contains four atomic
orbitals, so in total there should eight atomic orbital and we need an eight by eight matrix.
If we considerate the π bands and σ bands are independent, then the problem is simplified
to a two by two matrix for π bands and another six by six matrix for σ bands. Actually,
this is a result if we believe that graphene has the planar hexagonal lattice structure. We
assume carbon atoms are at the xy plane. While assuming the s parameter s=
A B
=0, we can calculate the Hamiltonian as follows,
zz
pA p B
tH r R H r R
22
323 23 3 23
2cos( )
2
xx y x y x x
aa a a a a a
ik ik ik ik ik ik ik
y
AB
ka
Hte e e te e
2
2
()
,
()
1()
,
() 1
z
z
zz
p
AA AB
BA BB p
AA AB
pAp B
BA BB
tf k
HH
H
HH tf k
ss sf k
sssrRrR
ss sf k
2
2
222
22
2
2
()()
det 0
()()
( ) ()() ()()
()
1() ()
1()
z
z
zz
z
z
p
ij ij
p
pp
p
p
EtsEfk
HEs
tsEfk E
E t sE f k E t sE f k
tf k
sf k E t f k E
sf k
9
Figure 2. Graphene bandstructure
2.3 Optical Properties of Graphene
Graphene's unique optical properties produce an unexpectedly high opacity for an
atomic monolayer, with a startlingly simple value: it absorbs 2.3% of white light, where a
is the fine-structure constant. This is a consequence of the unusual low-energy electronic
structure of monolayer graphene that features electron and hole conical bands meeting
each other at the Dirac point, which is qualitatively different from more common quadrat-
ic massive bands. Based on the Slonczewski-Weiss-McClure (SWMcC) band model of
graphite, the interatomic distance, hopping value and frequency cancel when the optical
conductance is calculated using the Fresnel equations in the thin-film limit.
10
This has been confirmed experimentally, but the measurement is not precise
enough to improve on other techniques for determining the fine-structure constant. The
band gap of graphene can be tuned from 0 to 0.25 eV (about 5 micrometer wavelength)
by applying voltage to a dual-gate bilayer graphene field-effect transistor (FET) at room
temperature.
2.4 Phonon Dispersion Relationship of Graphene
We start with an approach for calculating the phonon dispersion relationship with-
in a force constant model, in which inter-atomic forces are represented by spring con-
stants. Although the model is simple, it is able to approximate the model with experi-
mental results as closely as possible by increasing the number of force constants. In this
calculation a fourth nearest neighbor model, as shown in Figure 3, is used. The result of
the phonon dispersion calculation is shown in Figure 4.
Figure 3. Neighbor atoms of a graphene plane up to fourth nearest neighbor
11
Figure 4. Phonon dispersion relationship
2.5 Graphene Grain Boundary
This investigation focuses on the growth of graphene via chemical vapor deposi-
tion, as described in detail below. In this process, carbon atoms adhere to the surface of a
metal substrate under high temperatures. Once a carbon atom occupies a position on the
surface of the substrate it pushes other carbons to the side, creating a one atom thick layer
of carbon. As the temperature is lowered the carbon crystallizes into a layer of graphene.
Unavoidably, the graphene crystallization will start at various places on the sur-
face of the substrate before the entire area has formed a lattice. Each initial crystallization
is referred to as a nucleation site, and establishes an orientation for the lattice that grows
from it.[11], [12] As various crystal regions grow out from nucleation sites, their borders
12
will meet and a discrepancy will probably occur between the lattice orientations of each
region. This will create a definite boundary between regions. Growth stops when every
region is surrounded by such boundaries (or the edge of the substrate). At this point the
regions are called domains.
Figure 5. Graphene grain boundary
In a sense, domain boundaries represent defects in the crystal structure of the gra-
phene, since along those lines the bonding of the carbon atoms does not follow the simple
Bravais lattice from a repetition of the unit cell. This acts as a barrier for charge transport
phenomena and an exception to graphene’s optical properties (both discussed below).
Therefore, it is desirable to maximize the size of domains to limit the frequency of do-
main boundaries.
13
Chapter 3: Polycrystalline Graphene Synthe-
sis by Chemical Vapor Deposition (CVD)
Recently development on uniform single layer graphene deposition on top of cop-
per allowed the access of high quality material production over large areas. Chemical va-
por deposition (CVD) is an attractive approach to graphene production due to its capabil-
ity of producing large area deposition and the lack of intense mechanical and/or chemical
treatments.
Despite its relatively short history, several groups has already reported excellent
device characteristics such as high motilities up to 7250 cm
2
V
-1
s
-1
at low temperature and
large area growth up to 30 inches. However most of the early works are focused on the
production of polycrystalline graphene, whose domain size was limited to tens of mi-
crons.
3.1 Growth Mechanism
It has been known for over 40 years that CVD of hydrocarbons on reactive nickel
or transition-metal-carbide surface can produce thin grapheitic layers. It was surmised
that the formation of graphite was the consequence of diffusion and segregation of carbon
impurities from the bulk to the surface during the annealing and cooling stages. The car-
14
bon solubility in the metal and the growth conditions determine the deposition mecha-
nism which ultimately also defines the morphology and thickness of the graphene films.
The precipitation of graphite via formation of transition metal and carbon solid or
liquid solution has been widely studied and the mechanism has been verified for all
known catalysts for graphite. Many experimental conditions have been found to be im-
portant. Among them, the cooling rate and exposure time to carbon have shown to be the
most important condition for graphite properties.
Recently, graphene growth by chemical vapor deposition on polycrystalline Ni
and Cu substrates has received a lot of attention because of cost, grain size, etchability
and their wide use and acceptance by the semiconductor industry.
Ni has been studied intensively as a suitable catalyst for high quality graphite
formation. However, it is noticeable that the formation mechanism of highly crystalline
sp2 carbon on Ni is different than on Copper. The phase diagram of Ni and C reveals that
the solubility of carbon in nickel at temperature above 800 °C forms a solid solution and
lowering the temperature decreases the solubility, allowing carbon to diffuse out of Ni.
The formation of metastable Ni
3
C phase promotes the precipitation of carbon out of Ni.
Carbon prefers to precipitate from the grain boundaries; therefore the thickness of graph-
ite is larger at the grain boundary than within the grains. Thus, the uniformity and layer
control of graphene deposited on Ni is hard to achieve. Co and Fe show similar catalytic
behavior as can be surmised from the phase diagrams. The ability to form sp
2
crystalline
carbon from solid solutions of various transition metals is dependent on their carbon af-
finity. The most suitable catalyst for graphitic carbon formation are those transition met-
al
fa
m
b
is
w
o
ls that have
aces by form
Graph
methane gas o
on as shown
s very low c
weight % for
f copper wi
low affinity
ming weak bo
Figur
hene on copp
over a coppe
n by the fact
compared w
Ni at ~1326
ith carbon c
towards car
onds.
re 6. Phase d
per is in prin
er substrate t
that there is
ith Co and N
6 °C, and ~0
an be attrib
rbon but are
diagram for N
nciple straigh
typically hel
no carbide p
Ni (0.001-0
0.9% weight
bute to the it
still able to
Ni, Co, Fe an
htforward, in
ld at 1000 °C
phase. The s
.008 weight
for Co at ~1
ts stable 3d
stabilize car
nd Cu [5]
nvolves the d
C. Cu has lo
solubility of
t % at 1084
1320 °C). T
d-electron sh
rbon on thei
decompositi
ow affinity to
f carbon in co
°C for Cu,
The low reac
hell configur
15
r sur-
ion of
o car-
opper
~0.6
ctivity
ration
{
pu
th
p
it
m
st
ev
en
fo
p
lo
la
qu
[Ar]3d
10
4s
1
}
ulsions. As
he electron
eculiar comb
ty to form in
mation.
Graph
trates exhibi
ver graphene
nce of multi
In con
oils, in term
ercentage (l
ow-on studie
arge as 30 i
uality graph
Figur
} of copper.
a result, Cu
ns in the sp
2
bination of v
ntermediate s
hene deposit
it mobility u
e grown on
layers at the
ntrast to Ni,
s of uniform
ess than 5%
es have dem
nches. The
ene over larg
re 7. Schema
The symme
can form o
2
hybridized
very low affi
soft bonds m
ted on poly
up to 3700 cm
Ni seems is
e grain bound
exceptional
m deposition
%) of the are
monstrated th
growth on
ge area read
atic of chem
etrical electr
nly soft bon
carbon to th
finity betwee
makes coppe
ycraystalline
m
2
V
-1
s
-1
and
s fundamenta
daries, and th
results has
n of high qua
a having few
he growth o
copper is si
dily accessibl
mical vapor d
ron distribut
nds with carb
he empty 4s
en copper an
er a true cata
Ni and tra
d half-intege
ally limited
he high solu
been achiev
ality single-
w layers. Th
of single lay
imple and s
le.
deposition of
tion minimiz
rbon via cha
s states of co
nd carbon alo
alyst for grap
ansfered into
er quantum H
by its small
ubility of carb
ved on polyc
-layer graphe
he initial and
yered graphe
straightforwa
f graphene o
zes reciproc
arge transfer
opper. Hence
ong with the
phitic carbon
o insulating
Hall effect. H
l grain size,
rbon.
crystalline co
ene, with a
d subsequen
ene over are
ard, making
n copper
16
al re-
from
e this
e abil-
n for-
sub-
How-
pres-
opper
small
nt fol-
eas as
high
17
The growth mechanism of graphene on copper is surface related and rather due to
carbon segregation or precipitation from the bulk. Substantial evidence for this has been
reported by Ruoff group using isotopic labeling of the methane precursor gas. By taking
advantage of the fact that the Raman modes of 12C and 13C differ slightly in energy,
they were able to monitor the progressive enlargement of graphene domains on copper.
From the isotopically labeled Raman analysis result, it is shown that the growth time and
cooling rate does not affect the graphene thickness.
The specific growth parameters that have been utilized for achieveing the best
graphene has been summarized in Table 1. Most of the depositions have been performed
on copper foils with thickness ranging from 25-50 um. The most commonly used temper-
ature is 1000 °C, while other temperatures ranging from 800-950 °C have also been re-
ported. The pressure used can be either low pressure (0.5 - 50 Torr) or atmospheric pres-
sure of methane and hydrogen gas mixture at various ratios as indicated in Table 1. Thus
far, experiments have shown little influence evidence of deposition parameter on the
physical and electrical properties of as-grown graphene on copper.
There are many ways to affect the outcome of a CVD graphene growth run. Since
the growth dynamics of carbon deposition and domain growth have not yet been fully
understood, finding the proper balance of these controls are mainly based on experiments.
One of the most important variables to affect CVD outcome is the amount of reaction
gases. Increase methane provides more carbon atoms to deposit (and more nucleation
sites leading to more domains), while increase hydrogen promotes the reaction also in-
creases chemical processes on the copper and surrounding environment. The temperature
18
also affects the rate of reaction. Impurities in the copper substrate detract from the growth
process by encouraging nucleation sites and thus hindering the formation of contiguous
carbon domains. Annealing time of the copper can affect the level of impurity for the
same reason. The geometry of the growth chamber affects the deposition rate of carbon
due to its effect on gas flow patterns. Finally, leakage in the vacuum system further de-
tract from the growth, as oxygen from the air oxidizes the copper, making the carbon at-
oms unable to adhere to the copper surface and inhibit the deposition.
While not used in this investigation, it should be notices that a fairly common var-
iant on CVD is that of plasma-enhanced CVD (PECVD). PECVD works in a similar way,
but in addition to using a furnace to provide heat energy, an radio frequency AC current
is passed through the substrate. This spark ionizes the gases in the chamber, enhancing
the deposition onto the substrate.
3.2 Experiment Procedure
3.2.1 Substrate Preparation and Pre-treatment
The growth recipe used in this investigation follows that of Ruoff group. Gra-
phene was growth on 0.025 mm copper foil from Alfa Aesar. A piece of foil was cut ap-
proximately 2cm by 3cm. In order to flatten the sample, the copper foil was sandwiched
between two glasses. Before the insertion into the furnace, a wet chemical pre-treatment
process by dipping in acetic acid was introduced in order to partially remove Cu
2
O. The
sample was dipped in acetone for 10 seconds, cleaned by water, then in acetic acid for 10
m
re
fu
th
o
4
an
ob
tr
ph
n
st
ra
st
minutes, clea
emaining IP
urnace tube.
Once
he gas tanks
f 2 sccm, ma
0 mins.
Then
nneal the cop
btaining lar
reatment sev
hene. First,
ative oxide (
tage prior to
anged the s
tructural def
ned by wate
A was remo
the Cu foils
and pumped
aintaining a
Figure
the furnace
pper. The pr
rge graphen
vers several
by annealin
(CuO, Cu
2
O
o deposition
urface morp
fects).
er, and then c
oved using c
s were loade
d down to ba
pressure of
8. As receiv
was held at
re-treatment
ne domains
important fu
ng Cu substr
O) is removed
increases th
phology (int
cleaned by a
compressed
ed, the furna
ase pressure
27 mTorr. T
ed copper an
t the same t
of the coppe
in polycry
functions tha
rate in hydro
d from the a
he grain size
troduction o
acetone and
air, and the
ace vacuum
e. Hydrogen
Then the furn
nd its surfac
temperature
er foils has b
ystalline gra
at ensure hig
ogen at 100
as received C
e of the poly
of atomic s
isopropyl al
e copper then
system was
was then flo
nace heated
e condition
for 20 addit
been found t
aphene synt
gh quality d
00 °C befor
Cu. In additi
ycrystalline
steps, elimin
lcohol (IPA)
n loaded int
s closed off
owed at flow
up to 1000
tional minut
to be importa
thesis. The
deposition of
re deposition
on, the anne
copper and
nation of su
19
). The
to the
from
w rate
°C in
tes to
ant in
pre-
f gra-
n, the
ealing
rear-
urface
20
3.2.2 Chemical Vapor Deposition
The synthesis process, as detailed in Fig 3 with specific flow rate, started with
heating the substrate to desired temperature and anneal in hydrogen for pre-treatment. At
the end of this period, growth was started by introducing methane, which acts as carbon
source.
Figure 9. A typical CVD system set up
The specific growth parameters that have been utilized for achieving the best gra-
phene has been summarized in Table 1. Most of the depositions have been performed on
copper foils with thickness ranging from 25-50 μm.
The most commonly used temperature is 1000 °C, while other temperatures rang-
ing from 800-950 °C have also been reported. The pressure used can be either low pres-
sure (0.5 - 50 Torr) or atmospheric pressure of methane and hydrogen gas mixture at var-
ious ratios as indicated in Table 1.
21
Duration of this deposition phase was 30 mins, and the end of this process was
signaled by reducing the heat source at the appropriate cooling rate and allowing the fur-
nace to return to room temperature.
Ref Description annealing Growth and Cooling Morphology Mobility
cm
2
V
-1
s
-1
[9] LPCVD on
Cu foil
H2 2sccm,
40 mTorr,
1000C,
20 min
H2:CH4=2:35 sccm, 500
mTorr, 1000C, 10min;
Cooling rate= 40~300
C/min
Continuous (10min),
95% monolayer; 4-
lobe flower (1 min):
grain size <5um
4050
[14] Roll-to-roll
production
on Cu foil
H2 2sccm,
90mTorr,
1000C, 30
min
H2:CH4=8:24 sccm, 460
mTorr, 1000 C,
30min; Cooling rate=
600 C/min, flow H2 90
mTorr
Continuous 7350
[13] Two-step
LPCVD on
Cu foil
H2 2sccm,
40mTorr,
1000C, 20
min
H2:CH4=2:7 sccm, 160
mTorr, 1035 C,
25min;
Four lobe flower,
grain size~20 um
Continuous with
two-step process
800-15000
[15] LPCVD on
Cu foil, Cu
enclosure
H2=2sccm,
27mTorr,
1000C
H2:CH4:Ar= 50:1 (100
ppm), 450 sccm,
1000 C, 20~30 min
Continuous
[22] LPCVD 1000 C, 1h Pressure<500 mTorr, P
(CH4)=1 mTorr,
P(H2)=200 or 350
mTorr, 1000 C, 0~250
min; fast cool to room
temperature
Flower-irregular-
hexagon
Table 1. growth parameters summary from literature
Hydrogen, coming either from the carbon precursor, typically CH
4
, or from the H
2
used as dilute gas can also have a role in the graphene CVD growth. Furthermore, hydro-
gen is also used in the pretreatment. Its interaction with the substrate can affect the sub-
sequent CH
4
chemisorptions kinetics. Although hydrogen has been considered to be inac-
ti
fo
in
si
ti
sp
ab
ad
hy
ex
ch
ive in regard
orm a numb
nto the catal
ites for hydr
ions, removi
pecies in CV
bstraction by
ddition, hyd
ydrogen play
xist with hyd
hange surfac
Figu
ds to recomb
ber of impor
lyst and com
ocarbon and
ing hydrogen
VD is the CH
y H atoms.
drogen can b
ys an impor
drogen atom
ce reaction p
ure 10. Time
ination and
tant process
mpete with
d carbon radi
n from the s
H
3
radical, w
Third, hydro
e active in th
tant role in t
ms on the sur
properties of
dependence
and gas co
desorbing o
ses. First, the
CH
4
for che
ical on the su
surface. It is
which on the
ogen can pa
he competiti
the C sp
3
-sp
rface of Cu c
f carbon atom
e of experime
omposition/f
f the catalys
e H
2
and/or
emisorption
urface by su
generally b
e surface und
assivate defe
ion of CH
x
d
p
2
transition.
catalyst, and
ms.
ental parame
flow rate [10
st surface, it
the atomic
s. Second, a
ubsequent H-
elieved that
dergoes succ
ects and grai
deposition/C
Therefore, c
d their mutua
eters: tempe
0]
can actually
H can in-di
atomic H cr
-abstraction
the main gr
cessive hydr
in boundarie
C-etching. At
carbon atom
al interaction
rature, press
22
y per-
iffuse
reates
reac-
rowth
rogen
es. In
t last,
ms co-
n may
sure,
23
Characterization of these as-grown samples began with optical microscopy to
identify the amount and relative thickness of deposition. Further imaging and identifica-
tion was done with scanning electron microscopy (SEM). Raman spectroscopy (Ren-
ishaw inVia Raman Microscope, 532 nm laser, laser power output ~15 mW, spot size 1
um2) was also performed to identify surface coverage and to characterize the deposited
layers. From the analysis of Raman peaks, it is possible to obtain useful structural infor-
mation of graphene. The Raman spectrum provides unique fingerprints for single layer
graphene. Varying graphene layer thickness can also be discerned optically be the shade
intensity of the area, which thin layer appearing lighter than thicker ones. Atomic Force
Microscopy (AFM) is usually not preferred in prior to transfer, since thickness measure-
ments of the graphene on Cu are difficult to quantify.
3.2.3 Transfer Process
Another key advantage to CVD graphene growth is the ability to be transferred to
arbitrary substrate. Graphene growth is observed on both sides of the copper foil. During
our transfer process, graphene on one side of the copper can be transferred on to a 300
nm SiO
2
substrate. The schematic diagram is shown as Figure 11. First, a thin layer of
poly(methyl methacrylate) (PMMA) (MicroChem 950 A6) diluted with anisole (1:1) was
spin coated on the as grown graphene on one side of the foil at 3500 rpm for a minute.
Second, the PMMA coated samples were baked at 180 C for 10 min. Subsequently, cop-
24
per was wet etched using a commercially available etchant FeCl
3
for 30-60 mins, result-
ing graphene/PMMA films floating in the etchant.
The transfer is typically done by depositing a protective polymeric [ Polydime-
thylsiloxane (PDMS) or poly(methylmethacrylate) (PMMA)] coating on top of the gra-
phene thin film and etching the underlying copper in iron chloride [Fecl3 in HCl/H
2
O].
Other Cu substrate etching recipes including HCl and HNO
3
in H
2
O can also be used. In
our experiment, both HCl and Fecl3 have been used etchant. Since HCl releases corrosive
vapor and the etching rate of copper is very slow, FeCl
3
is preferred because it slowly and
effectively etches the copper without forming gaseous products or precipitates.
Figure 11. Transfer process
The removal process of Cu substrate is performed by immersing the polymer spin
coated graphene into etchant until a free-standing graphene/polymer membrane floating
in the solution can be observed. The strength of the polymer is strong enough to allow
25
membrane to be handled onto another substrate. Once transferred, the polymer is re-
moved by dissolving with acetone for the case of PMMA.
26
Chapter 4: Large Grain Single Crystal Gra-
phene Synthesis by CVD
Graphene grown by chemical vapor deposition (CVD) has been receiving signifi-
cant attention recently because of the case with which large-area films can be grown, but
so far most CVD graphene films are polycrystalline, consisting of numerous grain
boundaries. A typical CVD process of graphene synthesis on copper starts with the nu-
cleation of individual graphene grains randomly distributed across the copper surface.
These grains continue to grow with time and eventually merge together to form a contin-
uous polycrystalline film.
The presence of grain boundaries has been found to be detrimental to the transport
properties. It’s known that the surface defects will help increase surface reactions with
the ambient such as adsorbents or with deposited dielectrics. The presence of heptagons
and pentagons in the network of hexagons has been observed experimentally, and first
principle quantum transport calculations predict that the periodicity breaking disorder can
adversely affect transport properties. [21] Any of these defects can give rise to higher sur-
face chemical activity that would further disrupt the sp
2
bonding nature of graphene, and
thus impact graphene’s fundamental properties. Therefore it’s very important to be able
to synthesis large-grain, single-crystalline graphene for various applications.
Recent results have shown that the individual graphene grains before the for-
mation of the continuous film can be a four-lobed polycrystalline single layer, a six-lobed
27
single crystal single layer, a hexagonal single crystal single layer, a square single crystal
single layer, or a hexagonal single crystal few layer, depending on CVD parameters.
Since grain boundary is detrimental and it’s hard avoid grain boundaries in polycrystal-
line graphene, especially in the case of device arrays and circuits. It is therefore necessary
to synthesize large scale, high-quality single crystal graphene films.
4.1 Six-Lobe Large Area Single Crystal Graphene
Six-lobe large area single crystal graphene synthesis was first reported from
Ruoff’s group using Cu enclosure method with low pressure CVD. The method provides
good demonstration for CVD growthof large area single crystal graphene up to 0.5 mm in
size. However, the Cu enclosure geometry is manually made, thus the inner environment
is poorly controlled. In addition, the enclosed geometry of Cu enclosure does not allow to
be probed, therefore makes it even harder to understand the inner environment. In order
to overcome these problem, a vapor trapping method was reported which provides higher
yield, better reproductively and the possibility to be probed. However, this method still
relies on a complicated geometry, and the inner environment and working mechanism is
unknown yet. Therefore it requires a simple geometry based synthesis recipe that can
provide better envision into the six-lobe large grain single crystal forming mechanism
problem. In this report, a novel recipe was introduced with exponentially increased and
diluted methane as precursor. In the meantime, previously reported method has been re-
produced as a comparison.
28
4.1.1 Diluted Methane Method
Graphene synthesis was done using a novel chemical vapor deposition method
with pressure increased exponentially as illustrated in Fig 1. The precursor used in the
experiment is 0.1% diluted methane in argon and hydrogen.
Figure 12. SEM of six-lobe large grain single crystal graphene
29
Since the precursor gas is 1000 time diluted, there is a lot of space to manipulate
the carbon source concentration under desirable precision. And by introducing the gas
flow gradually and exponentially, which implies that on the initial stage, the increase
speed of gas concentration is exponentially slow. However, it’s not always the slower the
better, after certain limitation, a slower increase rate does not lead to a better result.
First, after the sample is loaded, the furnace is heated up to 1000 °C in 40 mins,
with a constant hydrogen flow rate 2 sccm, and permanent pressure 2 Torr. The pressure
reader used in this system has the range from 0 to 1500 Torr with a minimum readout of
1 Torr. Then, Cu foil is annealed in the same hydrogen flow for 30 mins. After annealing,
a gas combination of 25 sccm of diluted CH
4
and 4 sccm of H
2
was introduced into the
chamber for graphene growth.
Figure 13. Optical image of six-lobe large grain single crystal graphene
30
(a)
(b)
Figure 14. Raman Mapping of G-band and D-band
The pressure was increased gradually and exponentially. The whole growth last
30 mins. In the first ten mins, the pressure was increased with a constant increase rate of
31
1 T/min. Then the pressure increase rate enhanced to 2 T/min for 5 mins, followed by a
10 mins period of time with pressure increase rate of 3 T/min. In the end, the pressure
increase rate was changed into 6 T/min for 5 mins. After the procedure, the total pressure
inside the chamber is 80Torr. Cool down the system with a fully open pump valve, the
flow rate of CH4 and H2 was kept constant. Figure 2 and Figure 3 show SEM images of
six-lobe graphene grown on the bottom side of Cu foil. The size of six-lobe graphene
grain is up 80 um.
Figure 15. Raman Spectra of single layer six-lobe large grain single crystal gra-
phene
After growth, the grapheme films were transferred to SiO
2
/Si substrates as de-
scribed by Li et al. in order to analyze the film by Raman spectroscopy and perform elec-
trical measurements. Fig 4 is the optical image of graphene on top of 300 um SiO
2
sub-
strate. Figure 6 show Raman spectra recorded at the graphene region (Figure 15), the D
band (Figure 14b) and the G band (Figure 14a). The spectra show that the growing mate-
ri
o
4
fo
ce
ti
ial was indee
f graphene o
4.1.2 Co
The co
oil and then
edure mentio
ial. Graphene
Fig
ed graphene,
only.
opper E
opper enclos
crimping the
oned in Chap
e grew on bo
gure 16. Cop
, with a low
nclosur
sure method
e three rema
pter 3, but em
oth the insid
pper enclosur
D band inte
re Meth
d was perform
aining sides.
mployed slig
de and outsid
re and schem
nsity across
hod
med by bend
The basic gr
ghtly lower m
de of the Cu
matic proces
the domain
ding a 25 um
rowth was s
methane flow
enclosure.
ss of CVD gr
and the pres
m thick coppe
imilar to the
w rates and
raphene on C
32
sence
er
e pro-
par-
Cu
33
The graphene grow on the outside showed behavior similar to graphene growth
reported by Li et al. In contrast, the growth on the inside showed a much lower density of
nuclei followed by very large domain growth after extended period of time. Because of
the enclosed copper geometry, the inner environment is unable to be probed and thus the
precise growth conditions inside the enclosure are not well understood. Figure is copper
foil enclosure prior to insertion in the furnace and a schematic plot of the CVD system for
graphene on copper.
Figure 17. SEM of Copper enclosure grown six-lobe large grain single crystal graphene
Figure 17 is a SEM image of the copper enclosure grown graphene. In the exper-
iment, 2sccm of high purity (99.8%) CH
4
and 2 sccm of H
2
was introduced, the pressure
is 40 mTorr. Growth temperature is 1000 °C and growth time is 30 min.
34
4.1.3 Vapor Trapping Method
In the vapor trapping method, Cu foil was rolled up and put into a half inch small
quartz tube, which is open only at one end. The half inch quartz tube was then placed in-
side a two-inch quartz tube of the CVD chamber.
Figure 18. Schematic process of vapor trapping method
Gases flown into the small tube would be trapped inside, and therefore possibly
the small quartz tube would result in gas flow rate or gas composition different from out-
side the tube, thus leading to the six-lobe graphene growth. Another piece of Cu foil was
sometimes placed outside the small vapor trapping tube for comparison. 7 sccm H
2
was
introduced to the CVD chamber at 40 mTorr, and the temperature was brought up to 1000
°C in 40 minutes. The Cu foils were annealed at 1000 °C for 20 minutes. 1 sccm CH
4
and
Copper foil
Vapor-trapping
tube
35
12.5 sccm H
2
were then introduced into the CVD chamber for graphene growth. The
pressure was kept at 200 mTorr for 30 minutes during the growth. The CVD chamber
was cooled down to room temperature with the flow of 1 sccm CH
4
and 12.5 sccm H
2
continuing.
Figure 19 and 20 show SEM images of six-lobe graphene flowers grown on the
bottom side of Cu foil placed inside the vapor trapping tube. The size of graphene flowers
is up to 100 μm (Fig. 1b). Interestingly, the graphene grown on the Cu foil placed outside
the small vapor trapping tube did not show any “flower” shape, but continuous graphene
film with slight etching was found instead (Fig. 18). The pronounced difference between
graphene grown on Cu foil inside and outside the vapor-trapping tube indicates that the
vapor trapping tube does change the local environment inside the tube, which results in
large flower-shape graphene grains.
Figure 19. SEM of the vapor trapping grown six-lobe large grain single crystal graphene
10 μm
4
G
su
th
gr
fo
th
sc
Fi
4.2 Reg
Graphe
In a
uppressed nu
he article, an
raphene dom
or 3 h under
hen a mixtur
ccm H2 was
igure 20. SE
ular Sq
ene
recently rep
ucleation th
n intentional
mains were s
r 300 standar
re of 0.5 sccm
s applied for
EM of the no
quare S
ported artic
hat provides
lly increased
synthesized
rd cubic cen
m and 50 sc
graphene gr
on-vapor trap
Single L
le, Wang e
submillimet
d annealing
on copper f
ntimeters per
cm H2, and
rowth. After
pping grown
Layer S
et al. introd
ter single cr
time of 3 h
foils by atmo
r minute (sc
then a mixtu
r 15.5 min, C
n graphene
Single C
duced a CV
rystal mono
hours was us
ospheric pre
cm) Ar and
ure of 0.5 sc
CH
4
and H
2
w
Crystal
VD method
olyer grapien
sed. In the p
essure at 104
50 sccm H
2
ccm CH
4
and
were shut of
36
with
ne. In
paper,
45 °C
2
, and
d 500
ff and
37
1000 sccm Ar was introduced for 30s. Finally, the substrates were cooled down from
1045 to 500 C at a rate of 0.1 C/s under 300 Ar and 4 sccm of H2. The result they have
got is regular square single crystal graphene.
Figure 21. SEM of regular square large grain single crystal graphene with annealing time
20 mins (left) and 3 hours (right)
However, under another estimation of theirs, they did a control experiment with
annealing time of 20 mins and 3 hours. Although the nucleation sites have been signifi-
cantly decreased, the morphology of square shaped graphene has not been changed.
Therefore elongated annealing time and suppressed nucleation does not play an role in
the morphology determination of regular square graphene growth.
In our study, we reproduced the regular square shaped graphene in many condi-
tions. A typical result is shown in Fig 11. In the experiment, a same precursor as 4.1.1 is
used, 0.1% CH4 diluted in Ar. In order to form a control experiment as 4.1.1, experiment
parameters were kept as close as possible. In this case, the CH
4
/Ar flow rate is 25 sccm
and the H2 flow rate is 4 sccm. Total pressure is 35 Torr. Figure 23 is an optical image of
the square graphene. It’s interesting to be noticed that occasionally, we can find small six
38
lobed graphene grain, even when most of the sample is covered by a same shape of
square. Figure 13 is the Raman spectra of square graphene, confirmed the material is sin-
gle layer graphene.
(a)
(b)
Figure 22. SEM of regular square large grain single crystal graphene
39
(a)
(b) (c)
Figure 23. Optical image of regular square large grain single crystal graphene
40
Figure 24. Raman spectra of single layer regular square large grain single crystal gra-
phene
4.3 Hexagonal Single Crystal Graphene
Hexagonal Graphene can be achieved under various conditions. In order to com-
pare with previous experiments, here we used same 0.1% CH
4
diluted in Ar. While keep-
ing the total pressure constant as 35 Torr, changing the gas composition from 25:4 sccm
CH4: H2 to 25:20 sccm CH4:H2. The result change from square shaped graphene to hex-
agonal graphene. Fig.15 is the SEM of hexagonal graphene.
41
(a)
(b)
Figure 25. SEM regular square large grain single crystal graphene
42
(a)
(b)
(c)
Figure 26. Raman spectra of single layer (a), double layer (b) and triple layer (c) gra-
phene in hexagonal sample with the center occupied by multiple layers
43
Chapter 5: Conclusion and Future Work
5.1 Conclusion
In this report, we introduced a novel CVD method that can produce six-lobe large
grain single crystal graphene. Considering previously reported method, both Cu enclosure
and vapor trapping, relies on complicated geometry. By exponentially reduce the flow
rate of the carbon source in the initial stage of the growth; the new method produced
large area six-lobe graphene with simple traditional CVD geometry. This method gives a
clear indication and provides a good pathway for further understanding the exact mecha-
nism of large-grain six-lobe single crystal graphene.
As a comparison, we have extensively discussed about different morphologies of
large area single crystal graphene and their forming methods. The comparison study
gives a good indication of the key parameters for different shapes.
5.2 Future Work
Transport study is needed for the large-grain single crystal graphene grown by
novel CVD method, in order to ensure this CVD method can provide as high quality of
graphene as other CVD methods.
Selected area electron diffraction (SAED) is required to further confirm that the
same graphene domain is single crystal.
44
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Abstract (if available)
Abstract
This thesis presents a novel Chemical Vapor Deposition (CVD) method that can produce six-lobe large grain single crystal graphene with grain size up to 80 um. The new CVD method introduces a reduced carbon source concentration and an intentionally reduced gas flow rate in the initial stage. Compared with previously reported methods, this method offers a promising pathway for investigation of the growth mechanism of six-lobe large grain single crystal graphene CVD process. Controlled synthesis of six-lobe, square and hexagonal shaped single crystal graphene has been achieved by varying the pressure increasing method. The comparison study gives a good indication of the dependent relationship between pressure increasing methods and different single crystal graphene morphologies.
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University of Southern California Dissertations and Theses
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Asset Metadata
Creator
Yang, Ning
(author)
Core Title
Controlled synthesis of large grain single crystal graphene by chemical vapor deposition
School
Viterbi School of Engineering
Degree
Master of Science
Degree Program
Electrical Engineering
Publication Date
05/08/2012
Defense Date
03/29/2012
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
chemical vapor deposition,large grain graphene,OAI-PMH Harvest,single crystal graphene
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Zhou, Chongwu (
committee chair
), Cronin, Stephen B. (
committee member
), Lu, Jia Grace (
committee member
)
Creator Email
ningy@usc.edu,ynnning@gmail.com
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c3-34323
Unique identifier
UC11289932
Identifier
usctheses-c3-34323 (legacy record id)
Legacy Identifier
etd-YangNing-807.pdf
Dmrecord
34323
Document Type
Thesis
Rights
Yang, Ning
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
Access Conditions
The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
Repository Name
University of Southern California Digital Library
Repository Location
USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA
Tags
chemical vapor deposition
large grain graphene
single crystal graphene