H2-free synthesis of monolayer graphene with controllable grain size by

plasma enhanced chemical vapor deposition

Yong Seung Kima, b, Jae Hong Leea, b, Sahng-Kyoon Jernga, b, Eunho Kima, c, Sunae Seoa,

b, Jongwan Junga, c, Seung-Hyun Chuna, b, *

a Graphene Research Institute, Sejong University, Seoul 143-747, Korea

b Department of Physics, Sejong University, Seoul 143-747, Korea

c Institute of Nano and Advanced Materials Engineering, Department of Nano Science

and Technology, Sejong University, Seoul 143-747 Korea

* Corresponding author: FAX: +82 2 3408 4316. E-mail address: schun@sejong.ac.kr (S.H.Chun)

Abstract

We report H2-free synthesis of high-quality graphene on Cu foil by plasma enhanced

chemical vapor deposition. The advantage of this method is the easily controllable grain

size of graphene by adjusting the plasma power. At a plasma power of 50~70 W, the

grain size of graphene can be increased by a factor of 4 compared to that of graphene

synthesized in H2-rich conditions. Raman spectra reveal that the quality of synthesized

graphene is as good as that of best quality graphene grown under H2-rich environments.

Moreover, the synthesis temperature of monolayer graphene can be reduced down to

650 °C. Our results indicate that plasma enhanced chemical vapor deposition is an

advantageous method not only for low-temperature synthesis but also for H2-free growth

of graphene, which provides the ultimate advantage of the controllable synthesis of high-

quality graphene with reduced flammability.

1. Introduction

Graphene is a two-dimensional carbon material that has been attracting great interest

because of its unique electrical properties and high potential for applications [1-4]. So far,

single-layer graphene has been made by a number of methods, including mechanical

exfoliation [5], epitaxial growth on silicon carbide (SiC) at high temperature [6-7], and

chemical vapor deposition on catalytic metal substrates [8-12]. Among them, the

chemical vapor deposition (CVD) method has been widely used to obtain large-scale

graphene with dimensions of up to 30 inches on transition metals [11]. However, due to

the high growth temperature (~1000 °C) required for thermal decomposition of

hydrocarbons, plasma enhanced chemical vapor deposition (PECVD) has been utilized

to achieve low-temperature growth of graphene by using reactive species generated in the

plasma. This method has already been used for the low-temperature growth of carbon

nanotubes (CNTs) with vertically aligned CNTs [13-17]. The advantages of PECVD are

also effective for the synthesis of graphene, but the high electric field aligned

perpendicular to the surface is not desirable because it prevents planar growth of

graphene and results in vertically standing graphene sheets [18-19]. Recently,

Nandamuri et al. firstly reported successful synthesis of single-layer graphene by

eliminating this electric field on the surface via remote-plasma enhanced CVD [20].

Since then, various methods of plasma excitation, such as direct current (DC) discharge

[21], radio-frequency (RF) discharge [22-23], and microwave (MW) plasma [24] are

employed in the PECVD process to achieve large-area graphene growth at reduced

substrate temperatures (~650 °C).

All of these processes include hydrogen gas in the mixture for the synthesis of

graphene, but the roles of plasma and hydrogen in the synthesis of PECVD graphene

have not been discussed in detail. Also, in contrast with thermal CVD, the size of

graphene grains and the growth parameters affecting their size are not studied in PECVD.

In this work, we report the H2-free synthesis of graphene on Cu foil with controllable

graphene grains by RF-PECVD. The growth parameters (such as plasma power, growth

time, temperature, and the composition of the inlet gas) are systemically controlled to

study their effects on the synthesis of graphene in an H2-free environment. Samples are

characterized by Raman spectroscopy, scanning electron microscopy (SEM), and optical

microscopy. When the graphene is synthesized in optimal conditions, high-quality

monolayer graphene with enlarged grains is obtained from H2-free mixtures. The size of

the graphene grains can be controlled systemically by varying the applied plasma power.

Furthermore, the synthesis temperature of monolayer graphene is reduced to 650 °C. The

results are compared with those of graphene grown in H2-rich conditions in order to

provide insights into the roles of plasma and hydrogen in the PECVD synthesis of

graphene.

2. Experimental methods

2.1 Synthesis of graphene film

The graphene films studied in this work were grown on 25 μm-thick Cu foils (Alfa

Aesar, 99.8 %, #13382) by RF-PECVD. The chamber is equipped with an inductively

coupled plasma reactor and pumped with a turbomolecular pump (Osaka Vacuum LTD,

TG1003), keeping the base pressure as low as ~10-7 Torr. Polycrystalline Cu foil was cut

into 7x7 cm2 pieces and mounted in the chamber without any pre-cleaning treatment.

Five different stages were employed to synthesize graphene films on Cu foil using

methane (CH4) as a carbon source. The Cu substrate was heated to the growth

temperature (650 ~ 830 °C) at the heating rate of 3 °C/s without any gas flow into the

chamber. When it reached the target temperature, H2 gas was introduced into the

chamber at the flow rate of 40 standard cubic centimeters per minute (sccm). Hydrogen

gas was discharged by an RF power of 50 W for 2 min to eliminate surface oxides on the

copper foil. Then, the chamber was purged with Ar at the flow rate of 100 sccm for 2

min to remove residual hydrogen gas. During the graphene growth stage, RF plasma was

generated for a specified growth time under a continuous flow of argon (or hydrogen, 40

sccm) and methane (1 sccm), while the pressure was kept at 10 mTorr. The plasma

power was varied from 0 to 120 W, the substrate temperature from 650 to 830 °C, and the

growth time from 1 to 4 min. Subsequently, the sample was cooled down rapidly to room

temperature at a cooling rate of 3 °C/s by turning off the heating power, and then it was

taken out for characterization.

2.2 Transfer of graphene film

All the synthesized graphene films were transferred onto SiO2/Si substrates by

etching the Cu foil in an aqueous solution of iron chloride (FeCl3) [8]. Prior to wet-

etching, the surface of graphene/Cu was spin-coated with poly-methyl methacrylate

(PMMA, 950 A2), followed by baking at 150 °C for 3 min. The purpose of the PMMA

coating is to protect graphene films from cracks and tears during the transfer process [9,

12]. When Cu foil was dissolved completely, the PMMA/graphene membrane was

washed with deionized water and placed on the SiO2/Si substrate. Finally the PMMA

film was dissolved and removed by acetone.

2.3 Characterization

The surface morphologies of graphene films on Cu substrates are analyzed by SEM

(Tescan, VEGA-3) and optical microscopy (Leica, DM2500M). The crystallinity and

the structural information of synthesized graphene are obtained by micro-Raman

spectroscopy (Renishaw, inVia system) using a 514.5 nm laser. In order to estimate the

uniformity of synthesized graphene, Raman spectra are measured at three different

points in each graphene film. The averaged data of these points are presented in the

plots of intensity ratios and the full width at half maximum (FWHM). The point-to-

point variation of the Raman spectra is displayed as an error-bar. A minimal laser

power of 2mW is used carefully during the measurements to avoid any damage or

heating of the graphene films.

3. Results and discussion

3.1 Effect of synthesis time

We firstly investigated the quality of graphene synthesized in an H2-free

environment (Ar:H2:CH4 = 40:0:1 sccm) for different growth times from 1 to 4 min,

while the plasma power and the growth temperature are fixed at 50 W and 830 °C,

respectively. Fig. 1(a) shows the Raman spectra of synthesized graphene, in which the

three most prominent peaks are the D peak at ~1350 cm-1, the G peak at ~1580 cm-1, and

the 2D peak at ~2680 cm-1. The D peak is a defect-induced Raman feature observed in

the disordered sample or at the edge of the graphene [25-28]. The G peak is known to be

associated with the doubly degenerate phonon mode at the Brillouin zone center,

indicating sp2 carbon networks in the sample [25-28]. The intensity ratio of the D peak to

the G peak (ID/IG) is less than 0.2, indicating a low defect density [the lower curve in Fig.

1(b)]. The 2D peak, which originates from a second-order Raman process, is widely used

to determine the thickness of graphene [25-28]. For single-layer graphene, it is well-

known that the intensity ratio of the 2D peak to the G peak (I2D/IG) is higher than 2 [9, 25,

27-30], and the FWHM of the 2D band is close to 30 cm-1 (e.g., 24 ~ 30 cm-1 in

mechanically exfoliated graphene and 31 ~ 34 cm-1 in thermal CVD graphene) [9, 25, 27-

30]. In our graphene films, the ratio of I2D/IG is much higher than two (3.5 ~ 4.0) [the

upper curve in Fig. 1(b)] and the FWHM of the single Lorentzian 2D band is less than 30

cm-1 (27.5 ~ 29.5 cm-1) [Fig. 1(c)] regardless of the synthesis time. These results indicate

that high-quality monolayer graphene can be synthesized in an H2-free environment by

PECVD.

Fig. 1 - (a) Raman spectra of graphene synthesized at 830 °C with Ar/CH4 (40:1) mixtures

for various synthesis times with a plasma power of 50 W. (b) Intensity ratios of the D and

2D peaks to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit.

SEM images of graphene synthesized with different growth times are shown in Fig.

2(a-c). Nucleation of graphene domains with an average domain size of ~2 µm is

observed when synthesis lasts for 1 min [Fig. 2(a)]. The optical image and the Raman

maps, obtained from graphene transferred onto a SiO2 substrate, clearly show graphene

grains with dimensions of a few microns [Fig. 2(d-f)]. As the growth time increases,

these domains continue to grow laterally [Fig. 2(b)] and coalesce into larger ones [Fig.

2(c)], resulting in a fully covering monolayer of graphene on the Cu surface when the

growth time is longer than 3 min. The SEM image in Fig. 2(c) shows a continuous

graphene film with wrinkles and non-uniform dark flakes similar to those in thermal

CVD-grown graphene [9-10]. Our observations of the nucleation and coalescence

process of graphene, which have not been reported by others in the PECVD of graphene,

suggest that the synthesis mechanism of plasma-assisted graphene on Cu foil is similar to

that of thermal CVD, a surface adsorption process of carbon.

Fig. 2 - SEM image of graphene on Cu foil synthesized at 830 °C for (a) 1, (b) 2, and (c) 3

min in Ar/CH4 (40:1) environments with a plasma power of 50 W. (d) Optical image and

the Raman map of (e) G and (f) 2D peak intensity obtained from the graphene synthesized

for 1 min after transferring it onto a SiO2 substrate.

3.2 Effect of plasma power

Fig. 3(a) shows the plasma power dependence of Raman spectra from synthesized

graphene when the growth temperature and time are fixed at 830 °C and 3 min,

respectively. For the graphene film synthesized without RF plasma, the intensity of the D

peak is as strong as that of the G peak and a notable disorder-induced D´ peak (~1620

cm-1) [25] is observed to the right of the G peak [Fig. 3(a)]. These peaks are reduced

remarkably when the RF plasma assists during the graphene growth [Fig. 3(a)]. The low

ID/IG (< 0.2) observed in plasma-assisted graphene suggests that the defect density is

reduced by the use of plasma [bottom of Fig. 3(b)]. This enhancement of Raman spectra

can be attributed to the easier dissociation of methane by RF plasma into active species

(CHx<4) which decompose more easily to solid carbon than methane does [16]. Due to

the advantages of plasma, the graphene synthesized under RF plasma exhibits better

crystallinity and a higher growth rate at relatively low temperatures. Fig. 3(d) shows a

partially covering graphene film when it is synthesized for 3 min without RF plasma. On

the other hand, a full covering of graphene is always observed when it is synthesized for

3 min with RF plasma [Fig. 2(c)]. Since graphene grains are not observable in the fully

covered case, the graphene films synthesized for 1 min are used for the SEM image in

Fig. 3(e-h). As the plasma power increases from 0 to 70 W, the FWHM of the 2D band

decreases from 41 to 29 cm-1 [Fig. 3(c)] and the ratio of I2D/IG increases from 2 to 4 [top

of Fig. 3(b)] with an enlargement of the average size of graphene grains from 0.5 to 2 µm

[Fig. 3(e, f, i), Fig. 2(a) for 50 W]. When the plasma power is higher than 70 W, the

opposite results are observed for both grain size and Raman spectra. The average size of

graphene grains reduces from 2 to 0.5 µm [Fig. 3(g-i)], the I2D/IG decreases from 4 to 3

[top of Fig. 3(b)] and the FWHM of the 2D band increases from 30 to 35 cm-1 [top of Fig.

3(c)]. These observations indicate that the optimal plasma power for enhanced grain size

and Raman spectra is 50 ~ 70 W. To the best of our knowledge, this plasma power

dependent variation of grain size has not been reported previously in PECVD graphene.

In thermal CVD, it has been well-known that atomic hydrogen plays a very important

role in determining the dimensions of graphene grains because it acts not only as an

activator of surface-bound carbon but also as an etching reagent that controls the size of

the resulting graphene domains [31]. Considering this role of hydrogen in graphene

growth, the enlarged grain size under optimum plasma power could be attributed to the

atomic hydrogen generated by RF plasma during the process of the dissociation of

methane into active species. A theoretical model of inductively coupled plasma shows

that more than 80 % of incoming CH4 dissociates to other species, such as H, H2, CH3,

C2H4, C2H6, etc., when the applied plasma power is higher than 50 W [16-17, 32-33].

The resulting mole fraction of atomic hydrogen is higher than 0.1 % in this condition.

Although the actual intensity of H would be lower than the calculated one in our system

because of the H2-free environment, it will increase with the plasma power and become

high enough to affect the resulting size of graphene grains. Further studies are required

to investigate the actual partial pressure of atomic hydrogen in an H2-free environment

under various plasma powers.

Fig. 3 - (a) Raman spectra of graphene grown for 3 min at 830 °C under an H2-free

environment with various plasma powers. (b) Plasma power dependence of intensity ratio

of the D and 2D peak to the G peak. (c) FWHM of 2D band obtained from single

Lorentzian fit. The lines are guides for the eye. SEM images of graphene on Cu foil grown

(d) for 3 min without plasma and (e-g) for 1 min with different plasma powers from 30 to

120 W. To observe the graphene grains, 1 min-grown graphene films are used for the SEM

images. (i) Average size of graphene grains as a function of applied plasma power.

3.3 Effect of inlet gas composition

We synthesized graphene films in an H2-rich environment (Ar:H2:CH4 = 0:40:1

sccm) in order to compare their grain size and quality with that of graphene grown in H2-

free conditions (Ar:H2:CH4 = 40:0:1 sccm). The plasma power and temperature are fixed

at 50 W and 830 °C, respectively. In the H2-rich environment, a similar growth process

comprising nucleation and coalescence of graphene is observed in the SEM images [Fig.

4(a-c)]. However, the average size of graphene grains and the synthesis time required to

produce a full covering of graphene are reduced to ~0.5 µm and 1 min, respectively. Fig.

4(d) shows the time dependent evolution of graphene grains in both H2-free and H2-rich

environments. A 3 times higher growth rate and a 4 times reduced grain size are

observed in H2-rich mixtures, which can be attributed to the higher partial pressure of

atomic hydrogen generated by RF plasma from hydrogen molecules. The higher

intensity of hydrogen leads to greater dissociation of methane by the hydrogen

abstraction reaction (CH4 + H CH3 + H2) and an increase in the growth rate of

graphene. The Raman spectra in Fig. 4(e) show a high I2D/IG (~3.4) and a low FWHM of

the 2D band (~27.9 cm-1), similar to that of H2-free graphene, indicating that high

crystallinity graphene can be obtained in both H2-free and H2-rich environments. Our

observation suggests that hydrogen is not necessarily required in the PECVD of graphene.

Fig. 4 - SEM images of graphene films synthesized for (a) 20, (b) 30, and (c) 60 sec in

H2/CH4 (40:1) mixtures with a plasma power of 50 W. (d) Time dependence of average

grain size of graphene synthesized under different inlet gas compositions. (e) Raman

spectra of graphene films grown in H2-rich conditions.

3.4 Effect of growth temperature

The effect of substrate temperature on graphene synthesis in an H2-free environment

is investigated in a temperature range from 650 to 830 °C. In this series of growth

experiments, the plasma power and the synthesis time were maintained at 50 W and 3

min, respectively. Fig. 5(a-c) shows Raman spectra of graphene film grown at different

substrate temperatures. As the temperature decreases, ID/IG increases from 0.2 to 1.3 and

I2D/IG decreases from 4.0 to 2.2 [Fig. 5(b)]. The higher intensity of the D peak [Fig. 5(a)]

and the broader FWHM of the 2D band [Fig. 5(c)] at lower growth temperatures suggest

that there are gradual increments of disorder for graphene synthesized at lower

temperatures. Interestingly, even at the substrate temperature of 650 °C, the ratio of

I2D/IG is higher than 2 and a single-Lorentzian 2D peak is observed, indicating that

monolayer graphene is synthesized at this temperature.

Fig. 5 - (a) Raman spectra of graphene films on SiO2/Si substrate grown for 3 min at various

temperatures with a plasma power of 50 W and H2-free mixtures. (b) Intensity ratios of the

D and 2D peaks to the G peak. (c) FWHM of 2D band of synthesized graphene.

4. Conclusion

We presented the H2-free synthesis of high-quality monolayer graphene on Cu foil

with a controllable grain size by PECVD. The Raman spectra of the synthesized

graphene were as good as those of graphene films grown under H2-rich environments,

showing a high ratio of I2D/IG (3.5 ~ 4.0) and a low FWHM of the 2D band (< 30 cm-1).

The control over the grain size of synthesized graphene by adjusting the plasma power,

achieved in this work for the first time, is the prominent advantage of the H2-free

synthesis of graphene, which provides useful insights for understanding the growth

mechanism of PECVD graphene and for optimization of the growth process to further

improve the quality of graphene. The obtained graphene grains were 4 times larger than

those grown in an H2-rich environment. We suggest that the variation in the size of

graphene grains according to the plasma power and inlet gas composition could be

attributed to the different amount of atomic hydrogen generated by the RF plasma. Since

the actual pressure of atomic hydrogen in the H2-free environment is not known, further

studies are required to fully understand the growth mechanism of plasma-assisted

graphene. Our results suggest that PECVD is an advantageous method for the H2-free

growth of graphene, providing the ultimate advantage of the controlled synthesis of

graphene and reduced flammability.

Acknowledgments

This research was supported by the Priority Research Centers Program (2011-

0018395), the Basic Science Research Program (2011-0026292), and the Center for

Topological Matter in POSTECH (2011-0030786) through the National Research

Foundation of Korea (NRF) funded by the Ministry of Education, Science and

Technology (MEST).

References

[1] Kim P, Zhang YB, Tan YW, Stormer HL. Experimental observation of the

quantum Hall effect and Berry's phase in graphene. Nature. 2005;438(7065):201-4.

[2] Geim AK, Novoselov KS. The rise of graphene. Nat Mater. 2007;6(3):183-91.

[3] Geim AK, MacDonald AH. Graphene: Exploring carbon flatland. Phys Today.

2007;60(8):35-41.

[4] Castro Neto AH, Guinea F, Peres NMR, Novoselov KS, Geim AK. The electronic

properties of graphene. Rev Mod Phys. 2009;81(1):109-62.

[5] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al.

Electric field effect in atomically thin carbon films. Science. 2004;306(5696):666-9.

[6] Berger C, Song ZM, Li XB, Wu XS, Brown N, Naud C, et al. Electronic

confinement and coherence in patterned epitaxial graphene. Science.

2006;312(5777):1191-6.

[7] Emtsev KV, Bostwick A, Horn K, Jobst J, Kellogg GL, Ley L, et al. Towards

wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nat

Mater. 2009;8(3):203-7.

[8] Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, et al. Large-scale pattern

growth of graphene films for stretchable transparent electrodes. Nature.

2009;457(7230):706-10.

[9] Li XS, Cai WW, An JH, Kim S, Nah J, Yang DX, et al. Large-Area Synthesis of

High-Quality and Uniform Graphene Films on Copper Foils. Science.

2009;324(5932):1312-4.

[10] Li XS, Zhu YW, Cai WW, Borysiak M, Han BY, Chen D, et al. Transfer of

Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes.

Nano Lett. 2009;9(12):4359-63.

[11] Bae S, Kim H, Lee Y, Xu XF, Park JS, Zheng Y, et al. Roll-to-roll production of

30-inch graphene films for transparent electrodes. Nat Nanotechnol. 2010;5(8):574-8.

[12] Reina A, Jia XT, Ho J, Nezich D, Son HB, Bulovic V, et al. Large Area, Few-

Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett.

2009;9(1):30-5.

[13] Bower C, Zhou O, Zhu W, Werder DJ, Jin SH. Nucleation and growth of carbon

nanotubes by microwave plasma chemical vapor deposition. Appl Phys Lett.

2000;77(17):2767-9.

[14] Bower C, Zhu W, Jin SH, Zhou O. Plasma-induced alignment of carbon

nanotubes. Appl Phys Lett. 2000;77(6):830-2.

[15] Hiramatsu M, Shiji K, Amano H, Hori M. Fabrication of vertically aligned carbon

nanowalls using capacitively coupled plasma-enhanced chemical vapor deposition

assisted by hydrogen radical injection. Appl Phys Lett. 2004;84(23):4708-10.

[16] Delzeit L, McAninch I, Cruden BA, Hash D, Chen B, Han J, et al. Growth of

multiwall carbon nanotubes in an inductively coupled plasma reactor. J Appl Phys.

2002;91(9):6027-33.

[17] Denysenko IB, Xu S, Long JD, Rutkevych PP, Azarenkov NA, Ostrikov K.

Inductively coupled Ar/CH4/H-2 plasmas for low-temperature deposition of ordered

carbon nanostructures. J Appl Phys. 2004;95(5):2713-24.

[18] Malesevic A, Vitchev R, Schouteden K, Volodin A, Zhang L, Van Tendeloo G, et

al. Synthesis of few-layer graphene via microwave plasma-enhanced chemical vapour

deposition. Nanotechnology. 2008;19(30):305604.

[19] Malesevic A, Kemps R, Vanhulsel A, Chowdhury MP, Volodin A, Van

Haesendonck C. Field emission from vertically aligned few-layer graphene. J Appl Phys.

2008;104(8):084301.

[20] Nandamuri G, Roumimov S, Solanki R. Remote plasma assisted growth of

graphene films. Appl Phys Lett. 2010;96(15):154101.

[21] Kim JH, Castro EJD, Hwang YG, Lee CH. Synthesis of Few-layer (Graphene on

a Ni Substrate by Using DC Plasma Enhanced Chemical Vapor Deposition (PE-CVD). J

Korean Phys Soc. 2011;58(1):53-7.

[22] Terasawa T-o, Saiki K. Growth of graphene on Cu by plasma enhanced chemical

vapor deposition. Carbon. 2012;50(3):869-74.

[23] Zheng WT, Qi JL, Zheng XH, Wang X, Tian HW. Relatively low temperature

synthesis of graphene by radio frequency plasma enhanced chemical vapor deposition.

Appl Surf Sci. 2011;257(15):6531-4.

[24] Kim J, Ishihara M, Koga Y, Tsugawa K, Hasegawa M, Iijima S. Low-temperature

synthesis of large-area graphene-based transparent conductive films using surface wave

plasma chemical vapor deposition. Appl Phys Lett. 2011;98(9):091502.

[25] Dresselhaus MS, Malard LM, Pimenta MA, Dresselhaus G. Raman spectroscopy

in graphene. Phys Rep. 2009;473(5-6):51-87.

[26] Ferrari AC. Raman spectroscopy of graphene and graphite: Disorder, electron-

phonon coupling, doping and nonadiabatic effects. Solid State Commun. 2007;143(1-

2):47-57.

[27] Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F, et al. Raman

spectrum of graphene and graphene layers. Phys Rev Lett. 2006;97(18):187401.

[28] Pimenta MA, Dresselhaus G, Dresselhaus MS, Cancado LG, Jorio A, Saito R.

Studying disorder in graphite-based systems by Raman spectroscopy. Phys Chem Chem

Phys. 2007;9(11):1276-91.

[29] Shen ZX, Ni ZH, Wang HM, Kasim J, Fan HM, Yu T, et al. Graphene thickness

determination using reflection and contrast spectroscopy. Nano Lett. 2007;7(9):2758-63.

[30] Cao HL, Yu QK, Jauregui LA, Tian J, Wu W, Liu Z, et al. Electronic transport in

chemical vapor deposited graphene synthesized on Cu: Quantum Hall effect and weak

localization (vol 96, 122106, 2010). Appl Phys Lett. 2010;96(25):259901.

[31] Vlassiouk I, Regmi M, Fulvio P, Dai S, Datskos P, Eres G, et al. Role of

Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene. Acs

Nano. 2011;5(7):6069-76.

[32] Hash DB, Meyyappan M. Model based comparison of thermal and plasma

chemical vapor deposition of carbon nanotubes. J Appl Phys. 2003;93(1):750-2.

[33] Meyyappan M, Delzeit L, Cassell A, Hash D. Carbon nanotube growth by

PECVD: a review. Plasma Sources Sci T. 2003;12(2):205-16.

Figure Captions

Fig. 1. (a) Raman spectra of graphene synthesized at 830 °C with Ar/CH4 (40:1) mixtures for

various synthesis times with a plasma power of 50 W. (b) Intensity ratios of the D and

2D peaks to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit.

Fig. 2. SEM image of graphene on Cu foil synthesized at 830 °C for (a) 1, (b) 2, and (c) 3 min

in Ar/CH4 (40:1) environments with a plasma power of 50 W. (d) Optical image and the

Raman map of (e) G and (f) 2D peak intensity obtained from the graphene synthesized

for 1 min after transferring it onto an SiO2 substrate.

Fig. 3. (a) Raman spectra of graphene grown for 3 min at 830 °C under an H2-free environment

with various plasma powers. (b) Plasma power dependence of intensity ratio of the D

and 2D peak to the G peak. (c) FWHM of 2D band obtained from single Lorentzian fit.

The lines are guides for the eye. SEM images of graphene on Cu foil grown (d) for 3

min without plasma and (e-g) for 1 min with different plasma powers from 30 to 120 W.

To observe the graphene grains, 1 min-grown graphene films are used for the SEM

images. (i) Average size of graphene grains as a function of applied plasma power.

Fig. 4. SEM images of graphene films synthesized for (a) 20, (b) 30, and (c) 60 sec in H2/CH4

(40:1) mixtures with a plasma power of 50 W. (d) Time dependence of average grain

size of graphene synthesized under different inlet gas compositions. (e) Raman spectra

of graphene films grown in H2-rich conditions.

Fig. 5. (a) Raman spectra of graphene films on SiO2/Si substrate grown for 3 min at various

temperatures with a plasma power of 50 W and H2-free mixtures. (b) Intensity ratios of

the D and 2D peaks to the G peak. (c) FWHM of 2D band of synthesized graphene.