Uniform hexagonal graphene flakes and
films grown on liquid copper surface
Dechao Geng1, Bin Wu1, Yunlong Guo, Liping Huang, Yunzhou Xue, Jianyi Chen, Gui Yu, Lang Jiang,
Wenping Hu, and Yunqi Liu2
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, People’s Republic of China
Edited by Hongjie Dai, Stanford University, Stanford, CA, and accepted by the Editorial Board March 5, 2012 (received for review January 7, 2012)
Unresolved problems associated with the production of graphene
materials include the need for greater control over layer number,
crystallinity, size, edge structure and spatial orientation, and a bet-
ter understanding of the underlying mechanisms. Here we report
a chemical vapor deposition approach that allows the direct synth-
esis of uniform single-layered, large-size (up to 10,000 μm2),
spatially self-aligned, and single-crystalline hexagonal graphene
flakes (HGFs) and their continuous films on liquid Cu surfaces.
Employing a liquid Cu surface completely eliminates the grain
boundaries in solid polycrystalline Cu, resulting in a uniform nu-
cleation distribution and low graphene nucleation density, but
also enables self-assembly of HGFs into compact and ordered struc-
tures. These HGFs show an average two-dimensional resistivity of
609 ? 200 Ω and saturation current density of 0.96 ? 0.15 mA∕μm,
demonstrating their good conductivity and capability for carrying
high current density.
atomic crystal ∣ electronic materials
and spintronic applications (1–3). It is critical to find ways of pre-
cisely controlling the graphene layer number (4–6), crystallinity,
size, edge structure, and even spatial orientation. The chemical
vapor deposition (CVD) approach is a powerful and cost-effec-
tive technique for the production of high-quality and large-scale
graphene films. In spite of the complexity of CVD procedures
involving different catalysts, carbon sources, and other variables,
the physical principles underlying this method are relatively sim-
ple. It is widely accepted that CVD mainly involves either surface
catalytic reaction (7, 8) or bulk carbon precipitation onto the
surface during cooling (9, 10) for catalysts with low-carbon
and high-carbon solubility, respectively. In both cases, graphene
nucleation on a catalyst surface is one of the critical steps in the
growth process. Various factors affect the initiation of the gra-
phene nucleation process, including the type (11, 12) or surface
microstructure of the catalyst, carbon source (13), carbon segre-
gation from metal-carbon melts (14), processing history, and
parameters in CVD growth (15–17). In general, nucleation den-
sities on substrates such as Cu or Ni are nonuniform. This non-
uniformity causes a large dispersion of both nucleus density and
size distribution of graphene, representing a general problem in
graphene CVD growth systems.
It has been found that low-pressure CVD synthesis of gra-
phene on Cu foil provides a good way of fabricating uniform
single-layer graphene films (7). Studies have shown that the con-
tinuous films were formed by connecting randomly oriented,
irregular-shaped, and micrometer-sized graphene flakes, result-
ing in the presence of a large amount of both low- and high-angle
grain boundaries composed of pentagons and heptagons, which
leads to a dramatic degradation in electronic properties com-
pared with those of pristine graphene (7, 18–20). Recently, we
(21) and others (22, 23) have shown that it is possible to grow
single-crystalline hexagonal graphene flakes (HGFs) with a pre-
dominance of zigzag edges at ambient pressure by controlling the
raphene has attracted considerable attention because of its
extraordinary physical properties and potential electronic
growth rate of graphene. The HGF is an ideal building block for
the construction of continuous graphene films and allows study
of their edge/orientation-dependent physics. The layer number
of HGFs was found to be strongly influenced by the gas flow ratio
of Ar to H2, an increase of which led to a change from mixed
single/multilayer to single-layer-dominated HGFs, consistent
with previous results (24). However, graphene nucleation prefer-
entially occurs on high-surface energy locations such as grain
boundaries or defects associated with solid polycrystalline Cu,
resulting in HGFs with inhomogeneous density and size distribu-
tion. In addition, the high graphene nucleation density and the
observed slow growth rate of HGFs result in an HGF size typi-
cally in the range of 1–10 μm in the diagonal direction (21–23).
Here we demonstrate that the use of liquid Cu is a particularly
effective means for controlling the nucleation process in gra-
phene CVD systems because it eliminates the grain boundaries
found in solid Cu and results in the production of uniform single-
layered, self-aligned, large-sized, single-domain HGFs and con-
tinuous monolayer films.
Results and Discussion
The approach involves the formation of liquid Cu phase on
quartz and W substrates at the growth temperature above Cu
melting point (Fig. S1). Fig. 1 A and B shows typical SEM images
of well-dispersed HGFs grown on liquid Cu spheres on a quartz
substrate. Raman measurements of HGFs (Fig. 1D) on the Cu
surface show the typical characteristics (25) of monolayer gra-
phene—namely a large I2D∕IGintensity ratio (~2.5–4) of the
two-dimensional (2D) and G bands, a symmetric 2D peak located
at 2;698 cm−1with FWHM of 35–40 cm−1—consistent with
uniform contrast observed in Fig. 1 A–C. The yield of monolayer
HGFs was very high, with the formation of only a few bilayer
or trilayer HGFs (Fig. S2 A–C). Importantly, these HGFs also
formed well-distributed assemblies on the surface of Cu spheres.
The dynamic changes in density and size of HGFs on Cu spheres
were monitored as shown in Fig. S2 D–F. The spatial arrangement
of HGFs on Cu spheres was uniform in all cases with the average
size of HGFs being about 5 μm, and the average distance between
HGFs decreasing with increasing growth time. These results
are consistent with surface nucleation and growth mechanism in
the case of growing graphene on solid Cu.
This approach of HGFs formed on flat liquid Cu/W surface is
illustrated in Fig. 2A, and these HGFs displayed similar features
Author contributions: D.G., B.W., and Y.L. designed research; D.G. and B.W. performed
research; Y.G., L.H., Y.X., J.C., G.Y., L.J., and W.H. contributed new reagents/analytic tools;
Y.G., L.H., Y.X., G.Y., L.J., and W.H. analyzed data; and B.W. and Y.L. wrote the paper.
The authors declare no conflict of interest.
This article is a PNASDirect Submission. H.D. is a guest editorinvited by the Editorial Board.
Freely available online through the PNAS open access option.
See Commentary on page 7951.
1D.G. and B.W. contributed equally to this work.
2To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
This article contains supporting information online at www.pnas.org/lookup/suppl/
7992–7996 ∣ PNAS ∣ May 22, 2012 ∣ vol. 109 ∣ no. 21www.pnas.org/cgi/doi/10.1073/pnas.1200339109
with the above Cu sphere system. Typically, HGFs were well-
dispersed on the surface, and there was no clear alignment rela-
tion between different HGFs when the HGFs were not fully cov-
ering the surface (Fig. 2B, Fig. S3A). Asthe density or coverage of
HGFs on the Cu surface increased, introducing spatial constraint
of the HGFs, the HGFs became self-aligned into an ordered
structure with the most compact packing arrangement (Fig. 2C),
mimicking the polycrystalline structure in metals. The edge-to-
edge alignment of HGFs led to the formation of low-angle grain
boundaries for adjacent HGFs. Remarkably, perfectly ordered
2D lattice structures of HGFs were obtained when the HGFs
possessed similar size (Fig. 2D). These observations indicate that
the translation or rotation of HGFs on a liquid Cu surface is in-
volved in the self-assembly of their ordered structures, and the
minimization of total HGF surface/edge energy on liquid Cu
surface may be responsible for the alignment.
The evolution from well-separated HGFs, to closely packed
structures, to continuous film is a direct result of the extended
nucleation and growth in the CVD system. We found that growth
for 40 min produced continuous monolayer graphene films, and
similar results were obtained with longer growth times (for exam-
ple, from 1–4 h). Fig. 2E shows a typical SEM image of a large
area of continuous graphene film grown for 2 h. The shape and
edges of the HGFs disappeared, and the appearance of the film
in images was similar to those grown at low pressure, as shown in
Fig. S3 B and C. Raman measurements were also performed on
many points of this film, and almost all of them exhibited a single-
layer nature, consistent with SEM and optical measurements.
The average size of individual HGFs is determined by both
nucleation density and growth rate. Typical average size in
Fig. 2 B–D is about 20–30 μm. Increasing growth temperature
reproducibly leads to HGFs with average sizes of approximately
50 μm; and lowering CH4 flow rate leads to approximately
120 μm, as shown in Fig. 2 F and G, respectively. This large size
is a reflection of low nucleation density of HGFs in the liquid Cu
CVD system. The average growth rate of HGFs was estimated
to be 10–50 μm∕min on flat Cu/W, which is much higher than
the rate of 0.1–0.2 μm∕min observed for the case of HGFs grown
on a Cu solid surface (21). This result is highly important as
it shows that a liquid Cu surface favors the fast growth rate of
graphene without compromising its unique shape, highlighting
the possibility of realizing macroscopic-sized HGFs that are
otherwise difficult to achieve with slow growth.
Several differences were revealed between HGFs grown on
liquid and solid Cu surfaces. Although the latter produces a mix-
ture of single- and multilayer HGFs, small size, an inhomoge-
neous spatial dispersion, and random orientation, the former
results in HGFs with uniform single-layer characteristics, large
size, well-dispersed configurations, and a clear orientation rela-
SEM image showing well-dispersed, self-aligned HGFs on the surface of Cu
spheres grown using 10 sccm CH4∕300 sccm H2at 1,080 °C for 20 min. (B) The
corresponding magnified SEM image. (C) Optical image of HGFs on Cu
spheres showing the color contrast between separated HGFs and the Cu
surface, indicating the single-layer nature of the HGFs. (D) Typical Raman
spectrum of an HGF confirming its single-layer characteristics.
The growth of HGFs on liquid Cu spheres/quartz substrate. (A) Typical
image showing partially covered and well-dispersed HGFs using 6 sccm CH4∕300 sccm H2at 1,120 °C for 30 min. (C) SEM image of HGFs showing a compact
assembly of HGFs in which the dark and bright parts represent HGFs and the Cu surface, respectively. (D) SEM image of a near-perfect 2D lattice composed
of similar-sized HGFs. (E) SEM image of the sample for 2 h growth showing the continuous graphene film with uniform contrast. (F and G) SEM images of large-
sized HGFs showing that the average sizes are approximately 50 μm and approximately 120 μm using 1,140 °C and 1,160°C, respectively. Experimental con-
ditions from C and D are the same, using 6 sccm CH4∕300 sccm H2at 1,120 °C for 38 min.
The growth of HGFs on flat liquid Cu surfaces on W substrates. (A) Scheme showing CVD process for the synthesis of HGFs on liquid Cu surface. (B) SEM
Geng et al.PNAS
May 22, 2012
tion between different HGFs. These differences correlated well
with dramatic differences between the surface properties of liquid
and solid Cu. First, a liquid Cu surface completely eliminates the
grain boundaries, resulting in a low nucleation density (i.e., large
size) and more homogeneous nucleation on surface compared
to using solid Cu. The subsequent grain growth of HGF nuclei
is also uniform in all directions as indicated by the formation
of regular-shaped HGFs instead of equiangular-shaped HGFs,
possibly due to anisotropic solid Cu lattice. Second, liquid Cu
surface provides a higher C atom diffusion rate, favoring the fast
thus-transferred HGFs showing single-layer characteristics of HGFs and no detectable D-band. (C) Low-magnification TEM image showing an individual HGF.
(D–G) Selected area electron diffraction data for small regions indicated 1 to 4. These SAED data confirm the single-crystalline structure of the HGF as they show
the same set of sixfold symmetric diffraction points.
Raman and TEM characterizations. (A) Typical optical image of HGFs transferred onto 300 nm SiO2∕Si substrate. (B) Typical Raman spectroscopy of
gold electrodes. (B) The corresponding I–V curve of the device with a resistance and 2D resistivity values of approximately 87 Ω and 650 Ω, respectively.
(C) A plot of 2D resistivity of HGFs as a function of graphene width in many devices, in which transfer material, treatment, and graphene under tests
are indicated, with PSF-scratched HGFs for comparison. (D) A plot of saturation current density (Is) vs. HGF width measured for many devices. The dashed
line indicates the value of 0.44 mA∕μm for CVD-grown graphene from ref. 28. (Inset) The I–V curve of an HGF device with a width of 16.8 μm showing
the current saturation behavior. Note that the I–V curve becomes nonlinear at high current in this case. The arrow indicates the turning point of the current
and is used to calculate the saturation current density.
Electrical characterization of HGFs. (A) SEM image of one typical two-terminal device based on an individual HGF contacted by 30 nm top and bottom
www.pnas.org/cgi/doi/10.1073/pnas.1200339109Geng et al.
growth of HGFs that is one of the critical factors responsible for
large size. Third, the floating HGFs on liquid Cu surface self-
assemble into a compact, ordered structure. This alignment of
HGFs is difficult to realize in solid Cu surfaces, as the epitaxial
alignment of HGFs brought about by a solid Cu lattice is weak
(21, 23). Finally, the production of dominated single-layer HGFs
is also exceptional, as using similar experimental conditions to
grow HGFs on solid Cu mainly resulted in significant amounts
of multilayer HGFs characterized by a central dark area in
SEM or optical images (21, 22). We speculate that the high
mobility of Cu atoms in the liquid state may erase the nucleation
vacancies, preventing growth of a second layer on the same nu-
cleus. Control experiments were further performed to illustrate
the role of the liquid Cu phase (Fig. S4, Fig. S5).
HGFs were transferred onto 300 nm SiO2∕Si substrate
(Fig. 3A) and transmission electron microscopy (TEM) grid for
Raman spectroscopy and crystalline structure characterizations
using poly (methyl methacrylate) (PMMA) or polysulfone (PSF)
supporting layers (see Methods). The shape and position of peaks,
and the intensity ratio between 2D and G peaks, confirmed the
single-layer nature (Fig. 3B) (25). Twelve individual HGFs with
different sizes were tested by selected area electron diffraction
(SAED) on different locations of each HGF. The single-crystal-
line nature of all 12 HGFs was confirmed by the observations of
the same set of sixfold symmetric diffraction spots at different
locations (the maximum distance between two locations was
about 45 μm, Fig. S6), as shown in Fig. 3 C–G.
We fabricated field-effect transistor (FET) devices using indi-
vidual HGFs transferred onto 300 nm SiO2∕Si substrates. More
than 90% of the devices showed linear and reproducible I–V
curves, demonstrating the ohmic contact obtained between HGFs
and Au electrodes using our device fabrication method (21, 26).
Fig. 4 A and B show a typical SEM image of a single-layer HGF
device together with its current-voltage (I–V) curve measured un-
der ambient conditions (see more cases in Fig. S7). The resistance
of the device is approximately 87 Ω. Fig. 4C shows a plot of 2D
HGF resistivity (defined as R × W∕L, where W is the width of
the HGF, L is the channel length of the device, and R is the re-
sistance) as a function of HGF width. The average value of 2D
resistivity of the HGFs is 609 ? 200 Ω approaching approxi-
mately 230 Ω for pristine peel-off graphene (27). Importantly,
measurements were also taken on several small PSF-scratched
HGFs that were obtained by cutting large-width PSF-HGFs.
The 2D resistivity showed no dependence on the width of HGFs,
showing that the electrical properties of large HGFs are micro-
We also observed clear current saturation in all measured
devices, as shown in Fig. 4D and its Inset, as has also been recently
observed for CVD-grown graphene with long channel lengths
(28). Large saturation current densities (defined as saturation
current divided by graphene device width) were found from
I–V curves of these two-terminal devices. The value of the satura-
tion current for the HGFs was 0.96 ? 0.15 mA∕μm, approxi-
mately twice that (0.44 mA∕μm) for CVD-grown graphene (28),
indicating its capability of carrying high current density. It should
be mentioned that HGFs grown on solid Cu have similar values of
both 2D resistivity and saturation current density with those
grown on liquid Cu. In addition, FET measurements were also
performed on these devices, and the average hole mobility values
in HGF devices fell into a range (1;000–2;500 cm2V−1s−1),
consistent with that of HGFs grown on a solid Cu surface (21,
29) and those of the typical results (7, 13, 15, 30–32) for graphene
produced on Cu (Fig. S7, Fig. S8, Table S1).
In summary, we have demonstrated that the use of liquid Cu is a
particularly effective means for controlling the nucleation process
single-layered, self-aligned, large-sized, single-domain HGFs
and continuous monolayer films. The combined data of Raman
spectra,TEM, and electrical tests reveal a single-crystallinenature,
reasonable carrier mobility, high conductivity, and the capability
for carrying a large current of HGFs grown on liquid Cu surface.
Materials. Cu foils (99.8% purity) that were 25-μm thick and 50-m thick W foils
(99.95%) were obtained from Alfa Aesar. One to three pieces of Cu foils were
directly placed onto quartz substrates, and various-sized liquid Cu spheres
were formed on the quartz surface during the high-temperature annealing
process due to the nonwetting nature between Cu and quartz. Similarly,
two to four pieces of Cu foil were directly put on W foil for growing HGFs
on a flat liquid Cu surface. Electroplated Cu films on W substrates from CuSO4
aqueous solution (256 g/L) were also used.
CVD GrapheneSynthesis andTransfer.Prior tographenegrowth, theCVD 2.54-
cm quartz tube was pumped to approximately 5 Pa to clean the system, and
then filled with 200 standard cubic cm per min (sccm) H2followed by heating
the furnace (Lindberg/Blue M, TF55035A) to the desired temperature above
the melting point of Cu over 30–40 min. Subsequently, annealing for 30 min
was employed. In the case of Cu spheres on quartz substrates, different
temperatures and annealing times were employed to study the growth me-
chanism and the relationship between experimental conditions and the
properties of the resulting HGF (Fig. S4). In each case, changing temperature
was realized by simply switching off the furnace, and the temperature
dropped from 1,080 °C to desired one in about 2–4 min. Then the furnace
was turned on until the desired temperature was obtained. At the beginning
of growth, the H2flow rate was changed to the desired value, and CH4was
then introduced to the chamber with the required value for a certain time.
Finally, CH4was turned off, and the system was cooled down to room tem-
perature at the cooling rate of about 25°C∕min. In the case of Cu on W foil,
after the annealing process, typical growth conditions were 6 sccm CH4and
300 sccm H2at 1,120 °C for 28 min to 4 h. In this case of 28 min growth, no
HGFs were grown. This fact was used to evaluate HGF growth rate. The ex-
perimental parameters are described inthe corresponding figure captions for
each case. Note that there is no observable Cu deposition on the quartz tube
after many runs of graphene growth, consistent with low vapor pressure of
liquid Cu (∼0.05 Pa at 1,120 °C). The HGFs grown on flat Cu/W surfaces were
also transferred to 300 nm SiO2∕Si substrates and TEM grids by PMMA-
assisted or PSF (average molecular weight 22,000), assisted methods similar
to those reported previously. PMMA and PSF supporting films were removed
by acetone and chloroform rinsing, respectively.
Characterization of HGFs. The samples were characterized by SEM (Hitachi
S-4800, 1 kV and 15 kV), optical microscopy, Raman spectroscopy (Renishaw
Invia plus, with laser excitation of 514 nm and spot size of 1–2 μm), and TEM
(Tecnai G2 F20 U-TWIN, operated at 200 kV).
Device Fabrication and Electrical Properties of HGFs and Films. The electrical
properties of HGFs were measured after they were transferred onto
300 nm SiO2∕Si substrates. FET devices based on HGFs were fabricated using
our previous method (21, 26). Briefly, 2–5-μm wide nanowires (a rigid H type
anthracene derivative) (26) were deposited on individual HGFs, and then a
30 nm gold film was evaporated on the sample. Finally, the nanowires were
removed by a micromanipulator, and the desired electrodes were fabricated
by mechanically scratching the gold film to make isolated FET devices. The
tests, including measuring I–V curves and back-gated FET properties of HGFs,
were conducted with a Keithley 4200 analyzer at room temperature in air,
and 2D resistivity and saturation current density for HGFs were calculated
from the data. The mobility of charge carriers is extracted from the equation
VDis the voltage between source and drain electrodes, and Coxis the gate
capacitance per unit area.
dVg, where L and W are the device channel length and width,
ACKNOWLEDGMENTS. We thank Prof. Z.Y. Zhang, Prof. L.M. Peng, and Prof.
X.L. Liang for their valuable discussions and help about device characteriza-
tion. This work was supported by the National Basic Research Program of
China (2011CB932700, 2011CB808403, 2011CB932303, and 2009CB623603),
the National Natural Science Foundation of China (61171054, 60736004,
20973184, 20825208, and 60911130231), and the Chinese Academy of
Geng et al. PNAS
May 22, 2012
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