Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation

Article abstract of DOI:10.1021/ja2105976

Submillimeter single-crystal monolayer and multilayer graphene domains were prepared by an atmospheric pressure chemical vapor deposition method with suppressing nucleation on copper foils through an annealing procedure. A facile oxidation visualization method was applied to study the nucleation density and morphology of graphene domains on copper foils. Scanning electron microscopy, transmission electron microscopy, atomic force microscopy, polarized optical microscopy, and Raman spectra showed that the submillimeter graphene domains were monolayer single crystals.

Full text of DOI:10.1021/ja2105976

Communication
pubs.acs.org/JACS
Controllable Synthesis of Submillimeter Single-Crystal Monolayer
Graphene Domains on Copper Foils by Suppressing Nucleation
Hong Wang,† Guanzhong Wang,*,† Pengfei Bao,† Shaolin Yang,† Wei Zhu,† Xing Xie,‡
and Wen-Jun Zhang‡
†Hefei National Laboratory for Physical Sciences at Microscale, and Department of Physics, University of Science and Technology of
China, Hefei, Anhui, 230026, P. R. China
‡Center of Super-Diamond and Advanced Films (COSDAF), and Department of Physics and Materials Science, City University of
Hong Kong, Hong Kong SAR, P. R. China
S
* Supporting Information
nucleation on copper foils by pregrowth longtime annealing.
ABSTRACT: Submillimeter single-crystal monolayer and
multilayer graphene domains were prepared by an
atmospheric pressure chemical vapor deposition method
with suppressing nucleation on copper foils through an
annealing procedure. A facile oxidation visualization
method was applied to study the nucleation density and
morphology of graphene domains on copper foils.
Scanning electron microscopy, transmission electron
microscopy, atomic force microscopy, polarized optical
microscopy, and Raman spectra showed that the
submillimeter graphene domains were monolayer single
crystals.
Graphene domains were synthesized on copper foils by
atmospheric pressure chemical vapor deposition (APCVD).
Copper foils were first annealed at 1045 °C for 3 h under 300
standard cubic centimeters per minute (sccm) Ar and 50 sccm
H2, and then a mixture of 0.5 sccm CH4 and 500 sccm H2 was
applied for graphene growth. After 15.5 min, CH4 and H2 were
shut off and 1000 sccm Ar was introduced for 30 s. Finally, the
substrates were cooled down from 1045 to 500 °C at a rate of
0.1 °C/s under 300 sccm Ar and 4 sccm H2 (Figure S1 in the
Supporting Information).
To make the graphene domains optically visible, the after-
growth copper foils were oxidized in air for about 1 min at 200
°C by a heating platform. After this procedure, the graphene
domain covered sites were well protected from oxidation21 and
appeared to be brighter dots over the darker oxidized copper by
bare-eye observation (Figure 1a). Under an optical microscope
he novel properties1,2 and the promising applications3−5
T
have motivated increasing attempts to synthesize high
quality graphene films since the monolayer graphene was
prepared.6−8 Large area uniform graphene films have been
prepared on Cu substrates by chemical vapor deposition
(CVD).9,10 Recently, various shapes of single-crystal graphene
domains such as hexagonal11−13 and rectangular14,15 shapes
have attracted a lot of research interest, but the domain size was
usually limited to tens of micrometers. Due to large numbers of
graphene nucleation at the early stages and a very fast growth
and mergence rate with the traditional CVD method, one can
only obtain micrometer-size individual single-crystal domains
or large area polycrystalline films.16 The domain boundaries
and rotational disorders17,18 in the two-dimensional crystals are
proven obstacles to electron transport.12 Thus, it is desirable to
prepare large-size high quality single-crystal graphene, which
brings attention to the precise growth mechanism of graphene
on a metal substrate. Although graphene grows from a nucleus,
across steps and grain boundaries on metal substrates, and
forms a graphene crystal domain, graphene crystals grown from
different nuclei without an orientation relation can only
coalesce to polycrystalline films. Therefore, reducing the
nucleation density is a feasible route to prepare large-size
single-crystal graphene domains. Li et al.19 employed a copper-
foil enclosure to synthesize large single-crystal graphene.
However, it is still a challenge to control the nucleation stage
for growth of supersized graphene single crystals.20
Figure 1. (a) Photo of the as-grown, separated graphene domains on
Cu foil. (b, c) Optical microscopy images of the graphene domains in
(a). The parallel dark stripes correspond to surface unevenness
induced by mechanical processing of Cu foil.
(Figure 1b, c), the domains usually have a regular square shape
with some jagged edges. Large domains were measured to be
about 0.4 × 0.4 mm2. Hexagonal shaped domains can also be
prepared under the same growth conditions, but only these two
Here we report the growth of large monolayer single-crystal
graphene domains with a notable suppression of graphene
Received: November 11, 2011
Published: February 10, 2012
© 2012 American Chemical Society
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shape domains were found in our samples. Recently, Wu et al.
demonstrated that rectangular domains can only grow on
Cu(111) while hexagonal domains can form on all other non-
(111) Cu surfaces.15
We employed a large flow of H2 to dilute CH4 in the growth
process. According to Li et al.,16 low CH4 concentration lead to
a reduction of graphene nucleation. Our attempts also show
that a high CH4 concentration leads to the rapid growth of
graphene over the Cu substrates, while highly diluted CH4
reduces the growth rate and makes it more controllable in
growing separated graphene domains (Figure S2). If only the
CH4 supply was shut off and the sample was cooled in a high
concentration H2 atmosphere (300 sccm Ar and 40 sccm H2)
after the growth stage, the graphene domains would appear to
be etched into rectangular openings (Figure S3). Otherwise, no
visible etching damage was observed on the sample cooled in a
highly diluted H2 atmosphere (300 sccm Ar and 4 sccm H2)
(Figure S4). In accordance with the copper-catalyzed etching
mechanism proposed by Zhang et al. recently,22 we believe that
low concentration H2 reduces the etch rate considerably and
results in no significant etching damage.
The presence of impurities and defects on the surface of a
substrate affects nucleation behavior very considerably.23 They
may act as active heteronuclei in the early stage of the graphene
growth.24 Gao et al.25 demonstrated that the graphene
nucleation rate near a step edge may be 104−107 times greater
than that on a terrace due to a significantly lower nucleation
barrier. At the center of some graphene domains, a spikelike
structure was observed in SEM images (Figure 2e, S5) and was
identified as impurities in the Cu substrate. It is suggested that
several impurities which have a much lower nucleation barrier
may act as heteronuclei at the early stage of the growth. High
temperature substrate annealing is known to be helpful for the
reduction of volatile impurities, contaminants, and defects on a
copper surface, thus leading to the suppression of graphene
nucleation. The influence of annealing and growth times on
graphene nucleation density and domain size was studied.
Figure 2a−c demonstrated that the graphene domain density
decreases with prolonged annealing time. This indicates that a
longer annealing time notably induces a reduction of nucleation
density. Analogously, Figure 2d−f demonstrated that the
domain size increases with prolonged growth time. It was
about tens of micrometers with a growth time of 8 min (Figure
2d) and rapidly increased to about 100 and 350 μm with a
growth time of 11 and 15.5 min (Figure 2e), respectively. The
graphene domains started to merge if the growth time was
extended further (Figure S6). On the other hand, graphene
domains can hardly be observed for growth times less than 1
min under our growth conditions.
Figure 2. (a, b) SEM images of graphene domains on copper foils with
different pregrowth annealing times (20 min and 3 h respectively). (c)
The average graphene nucleation density with respect to annealing
time. The growth time was 11 min in (a−c). (d, e) SEM images of
graphene domains with different growth times (8 and 15.5 min
respectively). The color contrast within a graphene domain represents
different Cu grains after annealing, indicating that the graphene
domain can grow continuously across Cu crystal grain boundaries. (f)
The lateral length of the square domains as a function of the growth
time. The annealing time in (d−f) was 3 h.
Figure 3a shows an optical micrograph of a square graphene
domain transferred to a 300 nm SiO2/Si substrate with Sun’s
method.26 The high-magnification image of a corner of the
domain is shown in Figure 3b. The uniform color contrast of
the optical micrograph indicates that the film has excellent
thickness uniformity. Atomic force microscopy (AFM) was
employed to identify the number of layers of the sample. The
height profile across a crack shows that the thickness of the
graphene domain is about 0.6 nm, corresponding to a CVD
derived monolayer graphene.8
Figure 3. (a) Low, (b) high magnification optical microscopy and (c)
AFM image of a square graphene domain transferred to SiO2/Si
substrate. (d) A height profile was taken from the position indicated in
(c).
Raman spectroscopy was used to evaluate the quality,
thickness, and uniformity of the single-crystal graphene
domains. Figure 4 shows Raman mapping of the D-band
(Figure 4a), G-band (Figure 4b), and 2D-band (Figure 4c)
recorded at the corner area of a graphene domain. A typical
Raman spectrum is displayed in Figure 4d, in which the G-peak
(∼1591 cm−1) and 2D-peak (∼2689 cm−1) were shown. The
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Communication
Figure 4. (a−c) D-band, G-band, and 2D-band Raman maps of a
corner of a square-shaped graphene domain. (d) Raman spectra taken
in the circled area shown in (c).
I2D/IG intensity ratio is about 3, and the full width at half-
maximum (fwhm) of the 2D-peak is about 33 cm−1. These
results confirm that the graphene is monolayer. Furthermore,
the disorder-induced D-peak (∼1345 cm−1) is very weak,
indicating the high quality of the graphene film.
We also found other evidence that impurities served as
nucleation centers in the growth of graphene domains. As
shown in Figure 5a, a graphene domain with an impurity center
Figure 6. (a) Low magnification TEM image of a corner in a square
graphene domain transferred to TEM grids. (b) TEM image of a
cracked area on the graphene domain. (c) High-resolution TEM image
taken from region marked with arrow in (b). (d) SAED of the six areas
numbered in (a).
patterns were taken from six different sites of the graphene
domain. As shown in Figure 6d, only one set of hexagonal
diffraction spots without rotation was observed, revealing the
detected area is a single crystalline film. SAED patterns
recorded from other areas of the domain also displayed
identical diffraction spots, indicating the whole graphene
domain is a single crystalline film. In addition, we employed
a direct domain visualization method27 proposed by Kim et al.
to confirm the crystallinity of the submillimeter graphene
domains. Polarized optical microscopy (POM) images show
similar colors within each graphene domain (Figure S7),
indicating no domain boundaries in the submillimeter graphene
domains.
It was also found that the cooling rate can significantly affect
the layer numbers of graphene domains. High-quality
monolayer graphene domains can only be obtained with low
cooling rates, while high cooling rates result in multilayer
graphene domains with more defects, as confirmed by Raman
spectra. Bhaviripudi et al. also revealed that multilayer graphene
can be obtained on copper foils by APCVD and the growth was
not always self-limiting under APCVD conditions.28 Thus,
more research should be done to understand the precise
mechanism of single-crystal graphene domain formation on
copper foils by APCVD.
In conclusion, we have developed an annealing method to
control the nucleation density of graphene on copper foils by
APCVD and synthesized submillimeter-size graphene domains.
A facile oxidation visualization method was applied to study the
nucleation density and morphology of graphene domains on
copper substrates. SEM, TEM (HRTEM and SAED), AFM,
POM, and Raman spectra showed that a submillimeter
graphene domain was a monolayer single crystal. Suppressing
nucleation through an annealing procedure offers an effective
way to grow large graphene single crystals.
Figure 5. (a) SEM image of multinucleation in the center of a large
graphene domain on SiO2/Si substrate. The white areas are some
cracks caused during the transfer process. (b) Raman spectra taken in
the circled area shown in (a).
which appeared as a multinucleation site was transferred onto a
SiO2/Si substrate. This terraced structure of a few layers of
graphene is similar to Robertson’s result,11 but with a much
larger domain size. Raman spectra of different positions on the
terraced structure are shown in Figure 5b. The green-circled
area in the big graphene domain shows I2D/IG = 3 and fwhm
(2D) = 33 cm−1, which corresponds to a monolayer graphene,
while the blue-circled (pink-circled) spot shows I2D/IG = 0.95
and fwhm (2D) = 60 cm−1 (I2D/IG = 0.58 and fwhm (2D) = 72
cm−1), respectively, indicating increased graphene layers toward
the center of the terraced structure.
Figure 6a shows a low magnification transmission electron
microscopy (TEM) image of a corner in a square graphene
domain (as marked by a white dashed line). Figure 6b shows a
TEM image of a cracked area of the graphene domain. The
high-resolution TEM image (Figure 6c) taken from the
arrowed region marked in Figure 6b indicates the graphene
domain is monolayer. To identify the crystallinity of the
graphene domains, selected area electron diffraction (SAED)
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(23) Mullin, J. W. Crystallization, 3rd ed.; Butterworth Heinemann:
London, 1993.
(24) Nie, S.; Wofford, J. M.; Bartelt, N. C.; Dubon, O. D.; McCarty,
K. F. Phys. Rev. B 2011, 84, 155425.
(25) Gao, J.; Yip, J.; Zhao, J.; Yakobson, B. I.; Ding, F. J. Am. Chem.
Soc. 2011, 133, 5009.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
(26) Sun, Z.; Yan, Z.; Yao, J.; Beitler, E.; Zhu, Y.; Tour, J. M. Nature
2010, 468, 549.
(27) Kim, D. W.; Kim, Y. H.; Jeong, H. S.; Jung, H.-T. Nat.
Nanotechnol. 2012, 7, 29.
(28) Bhaviripudi, S.; Jia, X.; Dresselhaus, M. S.; Kong, J. Nano Lett.
2010, 10, 4128.
AUTHOR INFORMATION
■
Corresponding Author
gzwang@ustc.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Lei Zhou and Dr. Xiaoshuang Shen for their help
in the preparation of this paper and Mr Gongpu Li for his
assistance in TEM experiments. This work was supported by
the National Basic Research Program of China
(2011CB921400, 2009CB939900) and the Natural Science
Foundation of China (Grant No. 50772100).
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