Applied Surface Science 343 (2015) 128–132
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Growth characteristics of graphene synthesized via chemical vapor
deposition using carbon tetrabromide precursor
Taejin Choi a , Hanearl Jung a , Chang Wan Lee a , Ki-Yeung Mun b , Soo-Hyun Kim b ,
Jusang Park a , Hyungjun Kim a,∗
a
Nanodevice Laboratory, School of Electrical and Electronics Engineering, Yonsei University, Seodaemun-Gu, Seoul 120-749, Republic of Korea
Nano-Devices and Process Laboratory, School of Materials Science and Engineering, Yeungnam University, Dae-Dong, Gyeongsan-Si 712-749,
Republic of Korea
b
a r t i c l e
i n f o
Article history:
Received 24 November 2014
Received in revised form 13 March 2015
Accepted 17 March 2015
Available online 23 March 2015
Keywords:
Graphene
Carbon tetrabromide
Chemical vapor deposition
High yield
Bond dissociation energy
a b s t r a c t
A carbon tetrabromide (CBr4 ) precursor was employed for the chemical vapor deposition (CVD) of
graphene, and the graphene growth characteristics as functions of the following key factors were then
investigated: growth time, growth temperature, and the partial pressure of the precursor. The graphene
was transferred onto a SiO2 /Si substrate and characterized using transmission electron microscopy,
Raman spectroscopy, and X-ray photoelectron spectroscopy, and the electrical properties were measured
through the fabrication of field-effect transistors. Our results show that high yield and controllable growth
are possible via CVD used with a CBr4 precursor. Thus, CBr4 precursor is a new alternative candidate for
use in the mass production of graphene.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Graphene, a two dimensional sp2 hybridized carbon material,
has attracted considerable attention because of its remarkable
electrical, optical, chemical, and mechanical properties. These
properties include extremely high carrier mobility [1–3], high optical transparency [4], chemical stability, and high elasticity [5]. In
recent years, catalyst-assisted thermal chemical vapor deposition
(CVD) using a methane (CH4 ) precursor and a pure Cu foil has been
widely used for large-scale and high-quality graphene growth. This
is because of the self-limiting process at low pressure caused by the
low solubility of carbon in Cu [6–11]. However, graphene CVD using
CH4 on Cu foil (CH4 -graphene) requires a high growth temperature
(Ts ) of over 900 ◦ C and produces graphene with limited yield, which
is inappropriate for the mass production of this material. While
microwave plasma CVD using CH4 on Cu foils has been applied to
successfully stimulate the rapid growth of few-layer graphene over
a 100 s period, the experimental setup is complex and no significant
reduction in graphene film thickness has been shown [12].
The high Ts of CH4 -graphene is attributed to its high energy
barrier to dehydrogenation, which is related to the high bond
∗ Corresponding author Tel.: +82 2 2123 5773.
E-mail address: [email protected] (H. Kim).
http://dx.doi.org/10.1016/j.apsusc.2015.03.093
0169-4332/© 2015 Elsevier B.V. All rights reserved.
dissociation energy of C H (approximately 105 kcal·mol−1 at
room temperature) in CH4 [13]. Graphene growth using other
aliphatic hydrocarbon precursors also requires high Ts , again
owing to the high bond dissociation energy of C H (approximately 131 kcal·mol−1 for C2 H2 , 110 kcal·mol−1 for C2 H4 , and
98–100 kcal·mol−1 for C2 H6 and C3 H8 ) [11,14–16]. On the contrary, carbon tetrabromide (CBr4 ) has a bond dissociation energy
equal that of C Br (approximately 56 kcal·mol−1 ) [17,18], which is
lower than those of other halocarbon precursors (approximately
124 kcal·mol−1 for C— F in CF4 and 71 kcal·mol−1 for C Cl in CCl4 )
[19] and those of the aliphatic hydrocarbon precursors mentioned
above. Furthermore, CBr4 precursor forms reactive radicals at
approximately 300 ◦ C and reaches a sufficient vapor pressure of
1 Torr at 60 ◦ C [20].
However, low bond dissociation energy is neither a necessary
nor sufficient condition for low-temperature graphene growth.
Hence, the low-temperature growth of graphene was not the main
focus of this study. Rather, this study focused on the growth
characteristics of graphene with CBr4 and compared the findings
with the results of CH4 graphene. Paddison et al. observed that
CBr4 is decomposed into successive bromomethyl radicals under
hydrogen atmosphere [21]. These bromomethyl radicals are more
reactive than CBr4 , and they therefore evolve into more thermodynamically stable carbon species and form sp2 honeycombchained graphene on the metal surface [22]. Hence, interesting
T. Choi et al. / Applied Surface Science 343 (2015) 128–132
129
results about the CBr4 -based CVD growth of graphene can be
expected.
In this study, we examined graphene growth on Cu foil using
CVD and employing CBr4 precursor and systematically varied the
growth time (tg ), the partial pressure of the CBr4 precursor (Ps ), and
Ts . To the best of our knowledge, no studies have been published to
date pertaining to the synthesis of graphene using CBr4 precursor
as the carbon source. The CVD approach employing CBr4 precursor used in this study exhibits interesting experimental results,
specifically, high yield and controllable graphene growth.
2. Experimental
The experimental chamber used for graphene growth via CVD
consists of a quartz tube furnace pumped down to 10−3 Torr,
with the flow of hydrogen (99.9999%) and argon (99.999%) being
controlled using mass flow controllers. Here, a pure copper foil
(2 × 2 cm2 ) was cleaned in an ultrasonic bath using acetone, isopropyl alcohol (IPA), and deionized (DI) water. The Cu foil was
then heated to 1000 ◦ C at a rate of 30 ◦ C min−1 , and annealed in
an argon (600 sccm) and hydrogen (400 sccm) gas mixture flow for
30 min in order to reduce the native Cu oxide and to facilitate grain
growth [23]. CBr4 precursor (Sigma-Aldrich), vaporized in a canister at 50 ◦ C, was immediately introduced into the tube furnace
along with an Ar (1000 sccm) and H2 (200 sccm) gas mixture (total
chamber pressure of approximately 3.5 Torr). Ps , which was set to
1, 5, and 20 mTorr, was controlled by a needle valve and monitored using a digital vacuum gauge. The temperature dependence
was investigated by adjusting Ts to 400, 500, 600, 700, 800, and
1000 ◦ C, and the time dependence was also studied by changing tg
between 8, 15, 60, and 120 s. To facilitate a comparative study of the
growth yields of CVD-grown graphene using CBr4 precursor (CBr4 graphene) and CH4 -graphene, CH4 precursor with a Ps of 20 mTorr
was injected into the tube furnace during a growth step under the
fixed H2 /Ar gas flow and at Ts values of 800 and 1000 ◦ C. The low
temperature CVD below 800 ◦ C using CH4 was not tested because
this condition does not satisfy the minimum requirement for thermal CVD of graphene. Also, this process was carefully carried out
after the tube furnace was thoroughly cleaned.
A common transfer method was used to transfer the graphene
films to the target substrates, as described below. A 4% solution of
poly(methyl-methacrylate) (PMMA, Sigma-Aldrich) in anisole was
first spin coated on the graphene/Cu foil and dried for 30 min at
room temperature. Then, the graphene/PMMA film was released
from the Cu foil by chemically etching the foil with ammonium persulfate (0.2 M) solution for 12 h, followed by rinsing with DI water.
The floating PMMA-supported graphene was transferred onto a
SiO2 (285 nm)/Si substrate, or lacey grid. Next, the sample was
washed twice in a hot (50 ◦ C) acetone bath in order to remove the
PMMA. The same procedure was used to transfer the CVD graphene
to a heavily doped p-type silicon wafer with a 285-nm-thick SiO2
layer for device fabrication.
Scanning electron microscopy (SEM; JSM-6700F) was used
to observe the morphology of the as-grown graphene and the
graphene transferred onto the SiO2 /Si substrate. Raman spectroscopic measurements (Jobin Yvon ARAMIS, Ar-ion laser excitation
wavelength: 532 nm, laser beam size: 1 ␮m2 ) were conducted to
investigate the optical properties of the graphene. High-resolution
transmission electron microscopy (HRTEM; Tecnai F20, FEI) was
used to measure the number of graphene layers, and X-ray photoelectron spectroscopy (XPS; Thermo VG, U.K., monochromated Al
X-ray sources) was utilized for the chemical analysis of the CBr4 graphene.
To fabricate back-gated graphene field effect transistors (BGFETs), which were used to evaluate the electrical properties of
Fig. 1. Raman spectra of the transferred CBr4 -graphene film grown at Ts = 1000 ◦ C
for tg = (a) 15 s, (b) 60 s, and (c) 120 s under Ps = 20 mtorr, and for tg = 120 s under
Ps = (d) 5 mtorr and (e) 1 mtorr.
the synthesized graphene, the CBr4 -graphene was transferred onto
285-nm-thick SiO2 on highly doped Si using the transfer technique
mentioned above. Photolithography was used to thermally evaporate Ti (10 nm)/Au (50 nm) for source and drain electrodes on
the transferred graphene films. Sequential O2 plasma etching was
then used to create a graphene channel with a length (L) of 6 ␮m
and width (W) of 4 ␮m. Four-probe electrical measurements were
performed at ambient conditions. The gate voltage was applied
through the back of the Si substrate, while the source-drain bias
was constant at 0.1 V.
3. Results and discussion
As stated above, graphene growth on Cu foil was conducted
through CVD employing CBr4 precursor, while tg , Ps , and Ts
were varied systematically. The graphene films grown on Cu foil
(Ts = 1000 ◦ C, Ps = 20 mTorr) for tg = 15, 60, and 120 s were transferred onto 285-nm-thick SiO2 substrates. The Raman spectra
(Fig. 1) of these CBr4 -graphene films are found to have three
primary peaks, D, G, and 2D, which are due to defects, the
doubly degenerate zone center E2g mode, and second-order zoneboundary phonons, respectively [24]. We measured the G to 2D
and D to G peak intensity ratios in order to characterize the CBr4 graphene. Note that these ratios are denoted in the literature as
I(G)/I(2D) and I(D)/I(G), respectively. We summarized the properties of CBr4 -graphene and the variable growth conditions as shown
in Table 1. Each of those graphene samples is allotted a letter of
the alphabet (from “sample a” to “sample e”), respectively. The
I(G)/I(2D) value and full-width half maximum (FWHM) of the 2D
peak (sample c; tg = 2 min) are approximately 1.25 and 74 cm−1 ,
respectively. These values indicate that multilayer graphene (MLG)
is grown on the Cu foils [25,26]. To obtain thinner graphene films, tg
was decreased to 1 min. The Raman spectrum of the resultant CBr4 graphene (sample b; tg = 1 min) shows that the I(G)/I(2D) value is
approximately 1.18 and the FWHM of the 2D peak is approximately
49 cm−1 , indicating the formation of thinner graphene films. For a
shorter growth period, the I(G)/I(2D) value of the CBr4 -graphene
(sample a; tg = 15 s) is approximately 0.85 and the FWHM of the
2D peak is approximately 45 cm−1 , indicating few-layer graphene
(FLG) formation. It is known that CVD FLG has an asymmetric 2D
peak with FWHM values of 45–54 cm−1 [11]. In this study, as Ps
drops to 5 mTorr (sample d), the 2D peak exhibits a FWHM of
approximately 54 cm−1 and an I(G)/I(2D) value of approximately
0.85, which is indicative of FLG. As Ps is further decreased to
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T. Choi et al. / Applied Surface Science 343 (2015) 128–132
Table 1
Summary of properties of graphene grown at various CVD conditions on Cu substrate.
Sample
(a)
(b)
(c)
(d)
(e)
Partial pressure of CBr4 (mTorr)
20
20
20
5
1
Growth time (s)
15
60
120
120
120
Raman intensity ratio
I(G)/I(2D)
I(D)/I(G)
∼0.85
∼1.18
∼1.25
∼0.85
∼0.45
∼0.44
∼0.33
∼0.60
∼0.35
∼0.04
1 mTorr (sample e), single-layer graphene (SLG) film is obtained, as
shown by the symmetric 2D peak (FWHM: 39 cm−1 ) and I(G)/I(2D)
value of roughly 0.45 [24]. However, at a high Ps of 20 mTorr,
the I(D)/I(G) values are higher than the value of 0.04 obtained for
the low Ps of 1 mTorr. This means that low-quality FLG or MLG is
grown under high-Ps conditions, while high-quality CBr4 -graphene
(I(D)/I(G) < 0.1) is grown under low-Ps conditions.
Morphological data such as HRTEM and SEM images are important to support the above Raman data. The HRTEM images given
here show the transferred CBr4 -graphene films grown at tg = 15 s
(Fig. 2a) and tg = 1 min (Fig. 2b), which are both featured in the
Raman spectra (Fig. 1a and b). These images show that FLG is grown
for only 15 s while 4- to 5-layer MLG is grown for 1 min. The lattice fringes from these CBr4 -graphene films with an interplanar
distance of 0.34 nm can be seen in these figures, and these morphological data are consistent with the Raman spectra. As regards
observation of the CBr4 -graphene grown for a short tg of <60 s, the
SEM images of these CBr4 -graphene on Cu foil samples are shown
in supplementary Fig. S1. We can observe dense nucleation centers
(black spots) of CBr4 -graphene at tg = 8 s (supplementary Fig. S1a).
Further, at tg = 15 s, it can be seen that the continuous FLG film (supplementary Fig. S1b) almost covers the Cu surface. As transferred
onto a SiO2 /Si substrate, this graphene film has 90% FLG uniformity. Fully covered MLG film (supplementary Fig. S1c) is grown for
tg = 60 s. These results show that the use of CBr4 precursor enables
graphene to be rapidly and controllably grown (≤2 min).
FWHM of 2D peak (cm−1 )
#Graphene layers
∼45
∼49
∼74
∼54
∼39
Single or few layer
Few layer
Multilayer
Few layer
Single layer
The SEM image (Fig. 3a), Raman mapping image (Fig. 3b), and
its histogram (Fig. 3c) demonstrate the uniformity of the SLG transferred onto the SiO2 /Si substrate as mentioned above. This SLG film
contains only a 6% secondary graphene layer, thus, uniform SLG
film is grown under low Ps conditions. Also, in the XPS investigation of the SLG film (supplementary Fig. S2), no bromine content
is detected. This indicates that the CBr4 precursor is completely
decomposed into a C atom at Ts = 1000 ◦ C.
As previously stated, the electrical properties of the SLG film
were evaluated through the fabrication of BG-FETs. Fig. 3d shows
the representative ID –VG transfer curve (ID is the drain current
and VG is the gate voltage) of the BG-FET, which features a large
shift in the neutrality point to positive VG ; that is, p-type behavior is exhibited under ambient conditions [3,27]. The field-effect
mobility is extracted based on the ID /VG slope, which is fitted to the linear regime of the transfer curves using the equation
␮ = ((ID )/(VG ))(1/(VD COX )L/W). Here, L and W are the channel
length and width, respectively, and COX is the gate oxide capacitance of the SiO2 gate dielectric substrate. The histogram of the
extracted field-effect mobilities of 17 BG-FETs under ambient conditions is shown in supplementary Fig. S3. The average mobility for
the hole is 334 ± 225 cm2 /(V s) at room temperature, suggesting
that the examined single-layer CBr4 -graphene film is of reasonable
quality.
Previous studies on graphene growth using CH4 precursor and
Cu catalyst have shown that high-temperature CVD above 900 ◦ C is
Fig. 2. HRTEM images of the transferred CBr4 -graphene films grown at tg = (a) 15 s and (b) 1 min (Ts = 1000 ◦ C, Ps = 20 mTorr).
T. Choi et al. / Applied Surface Science 343 (2015) 128–132
131
Fig. 3. (a) SEM image and (b) Raman mapping (50 × 50 ␮m2 ) of 2D FWHM value (cm−1 ) of the single layer CBr4 -graphene transferred onto the SiO2 /Si substrate, (c) histogram
resulting from the Raman mapping showing relative counts versus 2D FWHM, (d) ID –VG transfer curve of a BG-FET using the single-layer graphene; inset photo is optical
microscopy image of the BG-FET.
critical for high-quality graphene to be obtained [28–30]. As shown
in Fig. 4, the Raman spectrum of CH4 -graphene grown at Ts = 800 ◦ C
has broad D and G peaks, indicating low-quality graphitic carbon.
Interestingly, it is apparent here that high-quality graphene growth
is possible at Ts = 800 ◦ C using CBr4 precursor. Note that thermal
decomposition of the hydrocarbon precursor at high temperatures
causes unstable carbon adatoms to assemble into graphene on the
surface of the Cu substrate. It has recently been suggested that the
growth process involves multiple steps and intermediate stages
such as partial and complete dehydrogenation of the CH4 , surface
diffusion of carbon adatoms, merging, the formation of nucleation
centers, and subsequent stitching of grains [31]. The complete
dehydrogenation of CH4 to a final C atom is an endothermic process,
and the corresponding activation energy is approximately 4 eV,
which can be converted into 92 kcal/mol [32]. This value is smaller
than the bond dissociation energy of CH4 , because the Cu catalyst
reduces the activation energy barrier to the thermal decomposition
Fig. 4. Raman spectra of CBr4 -graphene and CH4 -graphene samples grown at
Ts = 1000 and 800 ◦ C with the same partial pressure (20 mTorr) of CBr4 and CH4
precursors.
of the CH4 absorbed on the Cu surface at high temperature. Thus,
CBr4 precursor, which has a significantly smaller bond dissociation energy, is well decomposed into a C atom on a Cu substrate
at Ts = 800 ◦ C, compared to the above case of CH4 , as shown in our
results. Further, the XPS Br 3d spectra (supplementary Fig. S4b)
indicate that there is no Br content in the CBr4 -graphene grown at
Ts values of over 700 ◦ C, supporting the above demonstration of the
complete decomposition of CBr4 . That is, CBr4 is a useful precursor
for the controlled CVD growth of good-quality graphene conducted
at lower temperatures than those necessary when CH4 is used.
On the other hand, below Ts = 600 ◦ C, the CBr4 precursor does
not become fully dissociated; thus, graphitic carbon film with Br
content is deposited on the Cu substrate. The XPS spectrum of
the CBr4 -graphene (Ts = 600 ◦ C) in supplementary Fig. S4b shows
that the Br 3d peak consists of prominent peaks at binding energy
(BE) values of 70.7 eV (Br 3d5/2 ) and 72.1 eV (Br 3d3/2 ), accounting
for the presence of C Br bonds, and 67.7 and 69.5 eV, which can
be assigned to Br− species. The peak area ratio of C Br in the Br
3d spectrum is 55%, which invariably intimates the formation of
covalently bonded bromine on the graphene [33]. Also, the Raman
spectrum (supplementary Fig. S4a) exhibits broad D and G peaks,
large I(D)/I(G) ratios, and small 2D peaks, which are attributed to
a higher defect density in the covalently functionalized graphene
network [34,35].
Further discussion of the growth properties of CBr4 -graphene,
including rapid nucleation and additive layer growth, is given
here. The bond dissociation energy of CBr4 is not as high as
that of hydrocarbon sources, as mentioned above. When the substrate temperature increases, CBr4 can decompose into successive
halocarbon intermediates such as CBr3 (tribromomethyl), CBr2
(dibromomethyl), CBr (bromomethylene), and finally into a C atom
[21]. These halocarbon intermediates are more reactive than CBr4
[22], so they evolve into thermodynamically more stable carbon atoms and form sp2 honeycomb-chained graphene on the Cu
surface. We assume that this Br content, which is chemically conjugated with the Cu substrate, comes from the dissociation of Br
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T. Choi et al. / Applied Surface Science 343 (2015) 128–132
species from the CBr4 precursor. This means that the Cu substrate
helps the dissociation of the C Br bonds of the CBr4 precursor
through the formation of surface copper bromide. Surprisingly, Br
content at the Br 3d core level and Cu Br bonds at the Cu 2p3/2
core level of the as-grown CBr4 graphene are observed before the
transfer process, as shown in supplementary Fig. S5. These Cu Br
components shift the XPS Cu 2p3/2 peak to approximately 0.6 eV
compared to the pure Cu peak of CH4 -graphene [36]. In other words,
the Cu substrate plays the critical role of catalyzing the CBr4 decomposition, with the result that the generated C adatoms nucleate sp2
hybridized C nuclei under the H2 /Ar mixing gas atmosphere.
In previous reports, higher CH4 concentration was found to
lead to gas phase reactions in the bulk gas flow, depositing MLG
with higher defect density [37]. Meanwhile, at low CH4 concentrations, SLG is synthesized by the rate limiting step in the surface
reaction regime. Considering our results, we have demonstrated
that our CVD using CBr4 is similar to the case involving CH4 . At
higher CBr4 concentrations (Ps = 5−20 mTorr), thick graphene film
with higher defect density is rapidly grown on the Cu substrate,
as shown in Table 1 and supplementary Fig. S1. Obviously, both
graphene growth processes using a Cu catalyst under a H2 /Ar
atmosphere with higher CBr4 and CH4 concentration are not selflimiting. Finally, at low CBr4 concentrations, SLG with little defect
density is grown.
The results discussed above show that CBr4 precursor can
be employed in the high yield and controllable CVD growth
of graphene. Also, the electrical and optical properties of CBr4 graphene can be improved through further optimization, by
controlling the surface properties of the Cu substrate and the experimental parameters. Nevertheless, this study on CVD graphene
using CBr4 precursor allows this substance to be presented as a good
alternative precursor for use in the mass production of graphene.
4. Conclusions
We synthesized graphene film on Cu foil using CVD with CBr4
precursor for the first time, under various CVD conditions. Our
results show that CBr4 precursor can be employed in the high
yield and controllable growth of CVD graphene. The low bond
dissociation energy of CBr4 allows lower temperature growth
(800 ◦ C) of high-quality graphene film, compared to the growth
temperature of CH4 -graphene (1000 ◦ C). Also, the thermal decomposition of CBr4 generates reactive C adatoms, which are engaged
in graphene nucleation, resulting in the rapid growth of graphene.
This growth is dependent on the CBr4 concentration in a H2 /Ar
atmosphere. Thus, CBr4 can be adopted as a CVD precursor for
higher yield graphene production, compared to that achieved using
CH4 precursor.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF), funded by the Korea Ministry of Science,
ICT, & Future Planning (MSIP) (No. NRF-2014R1A2A1A11052588),
the Fundamental R&D Program for Core Technology of Materials,
funded by the Ministry of Trade, Industry and Energy, Republic of
Korea (No. 10050296), and the Center for Integrated Smart Sensors,
funded by the Ministry of Science, ICT & Future Planning, as a Global
Frontier Project (CISS-2011-0031848).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.
2015.03.093.
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