C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5

.sc ienced i rec t .com

Avai lab le a t www

journal homepage: www.elsevier .com/ locate /carbon

Structural and charge transport characteristics of graphenelayers obtained from CVD thin film and bulk graphitematerials

Anastasia V. Tyurnina a,*, Kazuhito Tsukagoshi b, Hidefumi Hiura b,c

Alexander N. Obraztsov a,d

a Department of Physics, M.V. Lomonosov Moscow State University, Moscow 119991, Russiab International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba 305-0044, Japanc Green Innovation Research Laboratories, NEC Corporation, Tsukuba 305-8501, Japand Department of Physics and Mathematics, University of Eastern Finland, Joensuu 80101, Finland

A R T I C L E I N F O

Article history:

Received 16 May 2012

Accepted 2 September 2012

Available online 8 September 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.09.003

* Corresponding author: Fax: +33 438785117.E-mail address: turnina@polly.phys.msu.r

A B S T R A C T

We report an experimental comparative study of graphene layers produced by microme-

chanical cleavage of bulk graphite materials of different origins and graphite films obtained

by plasma enhanced chemical vapor deposition (PECVD). Structural characteristics of these

materials were evaluated using Raman spectroscopy and electron microscopy. Field effect

transistors (FETs) based on the PECVD graphene were produced using electron beam lithog-

raphy. Conductivity, carrier mobility and other characteristics of the PECVD graphene

obtained from Raman and FET tests were similar to the properties of graphene flakes

obtained from bulk graphite materials. Taking into account the scalability of the CVD fab-

rication, these results confirm the possible industrial use of graphene films obtained by this

method.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Since the demonstration of the potential possibility to obtain

micrometer scale graphene [1], a lot of efforts have been de-

voted to develop fabrication of single (SLG) and few layer graph-

ene (FLG) films with scale and dimensions appropriate for

industrial application [2]. Because of high optical transparency

in a wide spectral range [3–5] and prominent electronic proper-

ties [6–10] graphene applications are expected in optical ele-

ments, optoelectronic and electronic devices with the typical

lateral dimensions up to dozens of centimeters (see, e.g.

[4,5,11]). However, the experimental investigations were per-

formed mainly for graphene samples produced by the tradi-

tional scotch tape method from highly oriented pyrolitic

graphite (HOPG), Kish graphite or natural graphite [1,6–10,

er Ltd. All rights reservedu (A.V. Tyurnina).

12–15]. The method of micromechanical cleavage requires a

high purity condition and allows obtaining only a limited size

flakes randomly located on the substrate. Thus, it is obvious,

that the scotch tape method is not suitable for industry produc-

tion due to its low reproducibility.

Various kinds of methods for graphene production were

proposed with use of silicon carbide surface graphitization

in an ultra-high vacuum [8,16], dissociation of gaseous hydro-

carbons on Ni(111) [17] or copper foils [11], etc. Among the

different approaches chemical vapor deposition (CVD) has

proven to be a promising technique due to its low cost and

ability to provide a high quality FLG films on the large area

substrates [2]. In our previous works a plasma enhanced

chemical vapor deposition (PECVD) has been developed as

the method for production of large area (up to few square

.

50 C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5

centimeters with potentially large scalability) FLG, or graphite

of nanometer thickness [18]. Crystal ordering and specific

topology characteristics of the PECVD nanographite films

have been evaluated with use of Raman spectroscopy, scan-

ning (SEM), high resolution transmission electron microscopy

(HR TEM), and atomic force microscopy (AFM) [18,19]. In this

work, we study the structural properties of the PECVD graph-

ene in comparison with the SLG flakes produced from HOPG,

Kish graphite or natural graphite which are usually utilized in

the experimental investigations of graphene for field effect

transistor (FET), and we analyze electronic characteristics of

the FET produced using the PECVD graphene flakes.

Fig. 1 – Typical SEM image of the PECVD graphite film of few

nanometer thickness. The film consists of the atomically

flat domains separated from each other by the wrinkles

(bright lines).

2. Samples preparation and characterization

The samples of graphite films of nanometer thickness were

produced by the PECVD technique from a hydrogen and

methane gas mixture activated by a DC discharge. The de-

tailed description of this method is presented elsewhere

[18]. The graphite films were grown on square (10 · 10 mm)

substrates made of nickel sheets of about 0.5 mm thickness.

Those nickel sheets were produced by rolling of polycrystal-

line nickel. A gas mixture of hydrogen and methane (in pro-

portion of H2:CH4 = 100:5) was used in the PECVD procedure

with total pressure of about 10 Pa and with total gas flow of

about 500 sccm. To remove oxide layer and possible impuri-

ties from the nickel surface the substrates were exposed

initially in pure hydrogen plasma at the same pressure with

substrate temperature of about 900 �C during 15 min. After

that methane with proportion mentioned above was

introduced into the gas. The carbon deposition starts with

introduction of methane at the same substrate temperature

(of about 900 �C determined by an optical pyrometer) and con-

tinues for 5–10 min depending on desirable thickness of the

graphite film of nanometer thickness.

The micromechanical cleavage method similar to that de-

scribed in Ref. [1] was used to prepare graphene flakes from

different bulk graphite materials including HOPG (Furuuchi

Kagaku, Japan), Kish graphite (SPI West Chester, PA 19381,

USA), natural graphite (Madagascar graphite) and the PECVD

graphite films of nanometer thickness. The PECVD graphite

films were used as-grown without any special treatments

and without separation from the nickel substrate. The micro-

mechanically pilled out graphene flakes were transferred

onto the clean silicon substrate surface to perform a compar-

ative study of their structural and electrical properties. A low-

resistivity silicon wafer was used to prepare the substrates of

5 · 5 · 0.5 mm covered by 300 nm of silicon oxide layer. Usage

of this type of substrate allows determination the number of

graphene layer in transferred flakes through the analysis of

optical microscopy images [20,21].

Raman spectroscopy is a powerful tool to evaluate struc-

tural characteristics, defect presence and thickness of the

graphene samples. In order to compare the structural quality

of as-prepared graphene flakes, Raman investigation was per-

formed using T64000 Jobin Yvon instrument with the laser

excitation at wavelength of 514.5 nm. The optical microscopy

(OLYMPUS BX51) attached to the Raman spectrometer allows

the laser beam focusing to a spot size of 1–2 lm.

Electronic properties of the graphene layers prepared from

the PECVD graphene were evaluated by measuring the FET

characteristics. The FETs with the top and bottom gates elec-

trodes were produced using electron beam lithography (EBL)

facility ELIONIX ELS-7500RS similar to that described else-

where [22]. The samples were mounted on chip carriers and

sealed in a vacuum sample tube. During the measurements,

the sample tube was immersed in liquid nitrogen, and thus

the sample temperature was about 77 K.

The routine characterizations of as-grown PECVD graphite

film of nanometer thickness were performed using SEM (LEO

Supra 50 PV). The overall observation shows that the PECVD

film is more or less homogeneous on whole nickel substrate

surface with dimensions up to 20 · 20 mm. At micrometer

resolution (Fig. 1) the main topology features of the film are

the atomically flat domains with characteristic size of about

1 lm and the wrinkles between the domains. The typical

height of the wrinkles is about 30 nm. Previously we have

reported that the most probable reason for the origin of the

wrinkles is post-grown deformation of the PECVD films be-

cause of difference in the thermal expansion coefficients of

graphite and nickel [17,19]. At the same time in according to

Raman and AFM measurements the flat area domains have

atomically smooth surfaces approximately parallel to the

nickel substrate. These surfaces consist of highly ordered

FLG films with thickness varying between one to tens graph-

ene layers [17,19].

3. Results and discussion

Four graphene fragments for each type of graphite sources

were selected using an optical microscopy [20,21] to carry

out subsequent comparative study of their properties. The

typical optical images of the handpicked objects are shown

in Fig. 2. The microphotos were analyzed by the image pro-

cessing program ‘‘ImageJ 1.37v’’ to determine the number

and the size of graphene layers in these samples. We were

able to prepare uniform graphene monolayers peeled off from

the PECVD graphite film of nanometer thickness and from

Kish graphite (see examples in Fig. 2a and c, correspondingly).

The FLG fragments with different number of graphene mono-

layers of 1–5 were produced from HOPG and from natural

Fig. 2 – Typical optical micrograph images of graphene flakes obtained from PECVD and different graphite sources by the

micromechanical cleavage with the use of the scotch tape and transferred onto the oxidized silicon wafer: (a) PECVD graphite

film of nanometer thickness, (b) HOPG, (c) Kish graphite and (d) natural graphite.

C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5 51

graphite (see examples in Fig. 2b and d, correspondingly). Pre-

sented in Fig. 2 FLG flakes contain the monolayer thickness

area with dimensions large enough to focus inside it the laser

beam during Raman spectroscopy measurements (i.e. more

than 2 lm).

Raman spectra of graphene samples produced from differ-

ent graphite materials are presented in Fig. 3. The spectra re-

corded in a range of 1000–3000 cm�1 are typical for a SLG with

the G peak position at around 1580 cm�1 and the 2D band of

second-order scattering located at around 2700 cm�1 [18,23].

1350 1425 1500 1575 2600 2650 2700 2750

43

21

Inte

nsity

, a. u

.

D G

2D

Raman Shift, cm-1

1 Kish - graphite2 PECVD3 HOPG4 Natural graphite

Fig. 3 – Raman spectra obtained for the graphene samples

produced from different graphite materials and deposited

into on Si:SiO2 (300 nm) substrates. The spectra were

normalized to the intensity of the G peak and shifted for

visibility along the Y axis (intensity).

There is no signal detected above the noise level at the Raman

shift frequency corresponding to, so called, ‘disordered graph-

ite line D’ at about 1350 cm�1. These peculiarities indicate

rather perfect graphene material crystallinity at least in the

area size comparable with the dimensions of excitation laser

beam spot (about 1–2 lm). All spectra presented in Fig. 3 were

normalized to the intensity of the G peak. For clear visibility

baselines of the Raman spectra 2, 3, and 4 are shifted (down)

along the Y axis (intensity). The profiles of the Raman bands

were fitted by the Lorentzian shapes and obtained fitting

parameters are shown in Table 1: position – m, full width at

half maximum (FWHM) – Dm and ratio of intensities for the

lines G and 2D – IG/I2D. The values of these fitting parameters

were determined with accuracy better than 0.1%. The shape

analysis of the 2D line confirms that all selected graphene

fragments (or investigated area of those) consist of a mono-

layer of graphene. Their crystallography perfection is con-

firmed additionally by the value of the 2D peaks intensity

which is much higher than that of the G-lines (see data of

IG/I2D in Table 1). Raman spectra analysis shows that the prop-

erties of the graphene samples produced from the PECVD

graphite films possesses crystallographic perfection compa-

rable with that of graphene flakes peeled off from graphite

materials of other types.

At the same time the presence of some minor difference in

Raman spectra should be noted. These differences are the

most evident in the form and position of the G line of Raman

spectrum for the graphene flake produced from natural

graphite (spectrum 4 in Fig. 3). FWHM value of the G line for

this type of graphene samples is about 6.2 cm�1 which is

much smaller than FWHM values of the G lines of Raman

spectra of other types of the graphene sample. In addition,

the G line for the natural graphite samples is shifted towards

higher frequencies and the intensity ratio IG/I2D is relatively

higher (see Table 1) in comparison with those of the other

Table 1 – Raman spectra parameters obtained from fitting of experimental data presented in Fig. 3 for SLG on Si:SiO2 (300 nm)substrates.

Source of graphene Raman spectrum parameters

mG, cm�1 DmG, cm�1 m2D, cm�1 Dm2D, cm�1 IG/I2D

Kish graphite 1579 14.8 2671 29.5 0.143PECVD 1581 9.9 2669 25.1 0.14HOPG 1581 10.0 2672 26.4 0.144Natural graphite 1588 6.2 2674 24.9 0.18

52 C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5

samples. Taking into account that the Raman measurements

were carried out under the same conditions for all samples,

one can be sure that relative variations in the Raman peaks

positions of the samples of graphene produced from natural

graphite could not originate from their heating by the laser

radiation. Simultaneously detected frequency shift, decrease

of the line width and increase of the IG/I2D ratio may be ex-

plained as a result of the presence of impurities in the graph-

ene samples. Such impurities are well known to provide

changes of electrical conductivity via doping [24]. Also, such

kind of impurities in SLG or FLG might lead to the significant

reduction of charge carrier mobility [24]. We further empha-

size that all these relatively minor deviations in the Raman

spectrum were only detected for the graphene sample ob-

tained from natural graphite. All other investigated graph-

enes were produced from the artificially synthesized

graphite samples. The artificial production suggests forma-

tion of graphite under controlled (clean) conditions. Thus,

the observed differences in Raman spectrum of graphene

flakes peeled off from natural graphite can be assigned to

the impurities presented in the natural material formed

without any special control.

Another distinctive feature is observed for Raman spec-

trum of graphene prepared from Kish graphite (curve 1 in

Fig. 3). The G band has FWHM is about 15 cm�1, that is wider

in comparison with those of other types of the graphene sam-

ples, where this value did not exceed 10 cm�1 (see Table 1).

This Raman line width increase corresponds to relatively

Fig. 4 – (a) Optical micrograph images of (a) few PECVD graphene

wafer and (b) FET device prepared from the SLG flake. The locati

the flake shown in image (a) is marked with black dotted line. So

by thermal deposition of Ti/Au layers of 5 and 50 nm thickness, r

e-beam deposition of Al layer of 30 nm thickness; 5 nm Al2O3 top

Schematic view of the dual gate configuration of fabricated FET

and channel length of 9 lm. (For interpretation of the references

version of this article.)

poorer structural ordering that can also influence the carrier

mobility. Despite of the noted differences, obtained Raman

spectra indicate that all the samples have quite similar struc-

tural characteristics which are specific for rather high ordered

single graphene layer.

In order to examine the electronic properties of the PECVD

graphene layers, we manufactured and characterized SLG

based field effect transistor device. The parameters of the

fabricated FET device were compared with those reported in

previous publications for similar devices produced from other

types of graphite materials [25,26]. Fig. 4a shows the optical

image of initial PECVD SLG flake. SLG based FET devices were

fabricated with EBL technique and one of the optical micro-

photos is presented in Fig. 4b The double gate electrode con-

figuration, schematically shown in Fig. 4c, was used for the

FET fabrication similar to that described elsewhere [22]. The

electrode configuration of the FET, presented in Fig. 4b, was

realized using EBL. Four contacts (numbered 8, 19, 4 and 3

in Fig. 4b) made of titanium and gold layers of 5 and 50 nm

thicknesses correspondingly, were evaporated onto the sur-

face of the PECVD graphene fragment from Fig. 4a Four con-

tacts configuration of FET allows elimination of the contact

resistance influence on the charge transport characteristic

measurements. The top electrode and gate (numbered 2 in

Fig. 4b) were formed by exposing on air of 30 nm alumina

layer. The top gate Al layer produced by e-beam sputtering

is oxidized for several hours under air exposure to form a thin

(�5 nm) top gate dielectric layer of Al2O3 [22].

flakes on top of an oxidized (300 nm of SiO2) highly doped Si

on of area from (a) is denoted by dashed line; the contour of

urce and drain contacts, marked as 3, 4, 8 and 19 are formed

espectively. The top gate electrode, marked as 2, is formed by

gate dielectric was formed by exposing the sample to air. (c)

device. As-prepared FET device has channel width of 2 lm

to color in this figure legend, the reader is referred to the web

-0.5 0.0 0.5 1.0 1.5

0.2

0.4

0.6

I sd.µ

A

Vtg. V-0.5 0.0 0.5 1.0 1.5

1x105

2x105

3x105

4x105

5x105

Vtg. V

R. Ohm(a) (b)

Fig. 5 – (a) Dual-gate transfer curve of the PECVD graphene FET formed on the Si substrate covered by 300 nm SiO2. The

current was measured for both positive and negative top-gate voltages. (b) Typical ambipolar dependence of SLG resistance R

on the top gate voltage Vtg. The measurements were performed at constant bottom gate voltage Vbg value of 30 V.

Table 2 – Charge carrier mobilities, obtained from charac-teristics of FET made of graphenes deposited onto Sisubstrates covered by 300 nm SiO2 (except the FET fabri-cated from suspended graphene).

References Characteristics of FET device

Size of FET devices,(channel width ·

channel length) lm2

Charge carriermobility,

cm2/V s; (on/offresistance ratio)

Current work 9 · 2 5 · 103; (�50)[12] (Suspendedgraphene)

N/A 106; (N/A)

[25] (APCVD) 1 · 3 3.8 · 103; (N/A)[29] (APCVD) 4 · 6 4 · 103; (�1,5)[11] (LPCVD) 3 · 2 7 · 103; (�14)[30] (LPCVD) 5 · 2 2.7 · 103; (�6)

C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5 53

The charge carriers transport characteristics of as-pre-

pared FET fabricated using the PECVD graphene (Fig. 4) were

investigated using a 6-probe station under liquid nitrogen

temperature of 77 K. Fig.5 shows the resulting (a) dual-gate

transfer curve and (b) RV dependence. The current was mea-

sured for both positive and negative top-gate voltages. The

gate leak current was typically 10 nA which was much smaller

than the current between the source and drain (�1 lA). A

charge carrier inversion peak at the top gate voltage of about

0.5 V has been observed. This voltage value corresponds to

the Dirac point position which can be shifted from 0 V due

to several factors explained elsewhere [27]. In Fig. 5b, depen-

dence of the PECVD graphene resistance, R, on the voltage ap-

plied to the top gate, Vtg is represented. The characteristic of

PECVD graphene based FET is typical for a strong ambipolar

electric field effect, which is as well commonly observed for

the FET prepared using graphene flakes peeled off from HOPG

or natural graphite [1,28]. By analyzing presented transport

characteristics we can conclude that the electrical properties

of charge carriers in the PECVD graphene are similar to that

observed in graphene layers produced from other types of

graphite (HOPG, Kish graphite, natural graphite) [1,28,30].

Based on the data of the IV dependence, Fig. 5a, the electron

and hole carrier mobility estimations were performed using

the formula: (dIsd/dVgate)|Vsd = W * l * C0 * Vsd/L [31]. Here

(dIsd/dVgate) is taken from linear parts of IV curve, W and L

are width and length of the channel, l – mobility, C0 – capac-

itance. The mobility values in our experiments were 2700 and

5000 cm2/V s for electrons and holes, respectively. These val-

ues are lower than the record one shown for suspended

graphene flake exfoliated from natural graphite [12] (see

Table 2), yet still quite high in comparison to state of the art

Si based FETs. Thus, one can conclude that PECVD single-

and multi-graphene flakes are very attractive for practical

application in FET based circuits. Also it should be mentioned

that investigated FET device demonstrated on/off resistance

ratio around �50, which is typical for recent graphene based

FET devices (Table 2). It is well known that the poor on/off ra-

tio of graphene resulting from its zero band gap and the per-

spective to improve that value is another challenge topic,

described in [32,33].

Similar results for on/off ratio and mobility values have

been demonstrated recently for FET devices manufactured

using graphene synthesized by low pressure thermal CVD

(LPCVD) and atmospheric pressure thermal CVD (APCVD) on

copper foils (Table 2). These types of CVD techniques are con-

sidered nowadays to be a possible solution for the large-scale

single graphene or FLG production [11,25,29,30]. However, as

shown in Table 2, transport measurements for the graphene

grown by that way were performed for FET of micrometer

size. This is, in fact, because the large-scale CVD graphene

or FLG films consists of small single crystal domains with a

number of defects on their boundaries [28,30]. These defects

impede the charge carrier transport in the film. Other disad-

vantage of the thermal CVD in comparison with the PECVD

consists in higher substrate temperature (1000–1050 �C)

which may be critical for industrial applications [34]. It is also

worth to note, that among different state of the art CVD set-

ups for large-scale graphene production PECVD has proven

to be a possible way for even further lowering of the growth

process temperature [34]. However, reported mobility values

for FET made from such low temperature PECVD graphene

were either much lower (than typical values presented in

Table 2) or were not reported at all.

54 C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5

4. Conclusions

We performed comparative study of structural and electronic

characteristics of graphene samples produced by microme-

chanical cleavage from PECVD graphite films, HOPG, Kish

graphite, and natural graphite. The analysis of Raman spectra

shows high crystalline order and similarity of structural prop-

erties of PECVD samples and the reference samples produced

from other types of graphene material. The FET devices man-

ufactured from the PECVD graphene demonstrate strong

ambipolar field effect, and the charge carrier transport prop-

erties similar to that for FETs produced with the use of graph-

ene sheets peeled off from HOPG, Kish graphite, and natural

graphite. The charge transport characteristics obtained for

the manufactured FET device have the highest values in com-

parison with other reported for PECVD graphenes. PECVD

technique used in this work essentially enables highly con-

trollable and reproducible large scale graphene fabrication

for mass production of FET based integrated circuits.

Acknowledgements

This work was performed in frame of collaboration agree-

ment between NIMS and MSU and was partially supported

by the Grants of the Russian Foundation for Basic Research

(10-02-01194-a), Ministry of Education and Science of the Rus-

sian Federation (Contract Nos. 16.740.11.0071 and

6.740.11.0763), by the Grant of the President of the Russian

Federation (contract MK-16.120.11.3035), by the Grants-in-

Aid for Scientific Research (No. 21241038) from the Ministry

of Education, Culture, Sports, Science and Technology of Ja-

pan (MEXT) and by the Funding Program for World-Leading

Innovative R&D on Science and Technology (FIRST Program)

from the Japan Society for the Promotion of Science (JSPS).

R E F E R E N C E S

[1] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y,Dubonos SV, et al. Electric field effect in atomically thincarbon films. Science 2004;306:666–9.

[2] Obraztsov AN. Chemical vapour deposition : makinggraphene on a large scale. Nat Nanotechnol 2009;4:212–3.

[3] Nair RR, Blake P, Grigorenko AN, Novoselov KS, Booth TJ,Stauber T, et al. Fine structure constant defines visualtransparency of graphene. Science 2008;320:1308.

[4] Wang X, Zhi L, Mullen K. Transparent, conductive grapheneelectrodes for dye-sensitized solar cells. Nano Lett2008;8(1):323–7.

[5] Weber CM, Eisele DM, Rabe JP, Liang Y, Feng X, Zhi L, et al.Graphene-based optically transparent electrodes forspectroelectrochemistry in the UV–vis region. Small2009;6(2):184–9.

[6] Castro Neto AH, Guinea F, Peres NMR, Novoselov KS, GeimAK. The electronic properties of graphene. Rev Mod Phys2009;81(1):109–62.

[7] Novoselov KS, McCann E, Morozov SV, Falko VI, KatsnelsonMI, Zeitler U, et al. Unconventional quantum Hall effect andBerry’s phase of 2p in bilayer graphene. Nat Phys2006;2:177–80.

[8] Ohta T, Bostwick A, Seyller T, Horn K, Rotenberg E.Controlling the electronic structure of bilayer graphene.Science 2006;313:951–4.

[9] Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI,Grigorieva IV, et al. Two-dimensional gas of massless Diracfermions in graphene. Nature 2005;438:197–200.

[10] Zhang Y, Tan YW, Stormer HL, Kim P. Experimentalobservation of the quantum Hall effect and Berry’s phase ingraphene. Nature 2005;438:201–4.

[11] Bae S, Kim H, Lee Y, Xu X. Roll-to-roll production of 30-inchgraphene films for transparent electrodes. Nat Nanotechnol2010;5:574–8.

[12] Neugebauer P, Orlita M, Faugeras C, Barra AL, Potemski M.How perfect can graphene be? Phys Rev Lett 2009;103.136403–1-4.

[13] Lemme MC, Echtermeyer TJ, Baus M, Kurz H. A graphenefield-effect device. IEEE Electron Device Lett 2007;28(4):282–4.

[14] Geim, A. Graphene update. Portland, Oregon, Bull APS55(2),2010.

[15] Forbeaux I, Themlon JM, Debever JM. Heteroepitaxialgraphite on 6H–SiC(0001): interface formation throughconduction-band electronic structure. Phys Rev B1998;58(24):16396–406.

[16] Gamo Y, Nagoshima A, Wakabayashi M, Terai M, Oshima C.Atomic structure of monolayer graphite formed on Ni(111).Surf Sci 1997;374(1):61–4.

[17] Obraztsov AN, Obraztsova EA, Tyurnina AV, Zolotukhin AA.Chemical vapor deposition of thin graphite films ofnanometer thickness. Carbon 2007;45(10):2017–21.

[18] Obraztsov AN, Tyurnina AV, Obraztsova EA, Zolotukhin AA,Liu B, Chin KS, et al. Raman scattering characterization ofCVD graphite films. Carbon 2007;46(6):963–8.

[19] Tyurnina AV, Ismagilov RR, Chuvilin AV, Obraztsov AN.Topology peculiarities of graphite films of nanometerthickness. Phys Status Solidi B 2010;247:3010–3.

[20] Blake P, Hill EW, Castro Neto AH, Novoselov KS, Jiang D, YangR, et al. Making graphene visible. Appl Phys Lett 2007;91.063124–1-2.

[21] Casiraghi C, Hartschuh A, Lidorikis E, Qian H, HarutyunyanH, Gokus T, et al. Rayleigh imaging of graphene andgraphene layers. Nano Lett 2007;7(9):2711–7.

[22] Miyazaki H, Odaka S, Sato T, Tanaka S, Goto H, Kanda A, et al.Inter-layer screening length to electric field in thin graphitefilm. Appl Phys Express 2008;1. 034007–1-3.

[23] Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, MauriF, et al. Raman spectrum of graphene and graphene layers.Phys Rev Lett 2006;97. 187401–1-4.

[24] Casiraghi C, Pissana S, Novoselov KS, Ferrari AC, Geim A.Raman fingerprint of charged impurities in graphene. ApplPhys Lett 2007;91. 233108–1-3.

[25] Wei L, Hong L, Chuan X, Khatami Y, Banerjee K. Synthesis ofhigh-quality monolayer and bilayer graphene on copperusing CVD. Carbon 2011;49:4122–30.

[26] Mattevi C, Kim H, Chhowalla M. A review of chemical vapourdeposition of graphene on copper. J Mater Chem2011;21:3324–34.

[27] Schwierz F. Graphene transistors. Nat Nanotechnol2010;5:587–96.

[28] Novoselov KS, Morozov SV, Mohinddin TMG, PonomarenkoLA, Elias DC, Yang R, et al. Electronic properties of graphene.Phys Status Solidi (B) 2007;244(11):4106–11.

[29] Li XS, Cai WW, An JH, Kim S, Nah J, Yang DX, et al. Large-areasynthesis of high-quality and uniform graphene films on Cufoils. Science 2009;324:1312–4.

[30] Song HS, Li SL, Miyazaki H, Sato S, Hayashi K, Yamada A,et al. Origin of the relatively low transport mobility ofgraphene grown through chemical vapor deposition. Sci Rep2012;2(337):1–6.

C A R B O N 5 2 ( 2 0 1 3 ) 4 9 – 5 5 55

[31] Stassen AF, De Boer RWI, Iosad NN, Morpurgo AF. Influenceof the gate dielectric on the mobility of rubrene single-crystal field-effect transistors. Appl Phys Lett2004;85:3899–901.

[32] Fengnian X, Damon BF, Yu ML, Phaedon A. Graphene field-effect transistors with high on/off current ratio and largetransport band gap at room temperature. Nano Lett2010;10(2):715–8.

[33] Lin MW, Ling C, Zhang Y, Yoon HJ, Cheng MMC, Agapito LA,et al. Room-temperature high on/off ratio in suspendedgraphene nanoribbon field-effect transistors.Nanotechnology 2011;22. 265201–1-7.

[34] Kim J, Ishihara M, Koga Y, Tsugawa K, Hasegawa M, Iijima S.Low-temperature synthesis of large-area graphene-basedtransparent conductive films using surface wave plasmachemical vapor deposition. Appl Phys Lett 2011;98. 091502–1-3.