Appl. Sci. Converg. Technol. 32 (1); 26-29 (2023)
https://doi.org/10.5757/ASCT.2023.32.1.26
eISSN: 2288-6559
Research Paper
Facile Synthesis of Wafer-Scale Single-Crystal
Graphene Film on Atomic Sawtooth Cu
Substrate
Received 7 November, 2022; revised 7 December, 2022; accepted 4 January, 2023
Seungjin Leea , Soo Ho Choia,b , Hayoung Koa , Soo Min Kimc∗ , and Ki Kang Kima,b∗
a Department
of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
c Department
of Chemistry, Sookmyoung Women’s University, Seoul 14072, Republic of Korea
b Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea
∗ Corresponding
author E-mail: soominkim@sookmyung.ac.kr, kikangkim@skku.edu
ABSTRACT
Wafer-scale single-crystal (SC) graphene film is highly required for industrial applications including electronic, photonic, and optoelectronic
devices. While SC graphene film has been successfully synthesized on SC Cu (111) or H-Ge (110) substrates, preparation methods of SC growth
substrate are still not practical. Here, we report the facile synthesis of wafer-scale SC graphene film on atomic sawtooth Cu substrate by means
of chemical vapor deposition. Atomic sawtooth Cu substrates are prepared by melting Cu foils on W foils and solidifying them. The substrates
are subsequently employed for synthesis of SC graphene without formation of grain boundaries. The electron diffraction patterns, visualized
by transmission electron microscopy, reveal that graphene film has the same lattice orientation over the whole region. Furthermore, Raman
mapping measurement demonstrates the homogeneity of the optical property of SC graphene films. Our method provides a new route for the
synthesis of SC graphene, as well as other two-dimensional materials.
Keywords: Two-dimensional materials, Graphene, Chemical vapor deposition, Atomic sawtooth Cu, Single-crystal
1. Introduction
Graphene is a single layer of graphite, consisting of carbon atoms
with hexagonal structure [1]. In recent decades, monolayer graphene
has gained attention as a candidate material in various fields including
electronic, optical, and optoelectronic applications due to its unique
physical properties such as high mechanical strength, thermal conductivity, and exceptional carrier mobility [1–3]. To apply graphene to the
practical applications, the large-area monolayer graphene film is required. In 2009, centimeter-scaled monolayer and few-layer polycrystalline (PC) graphene films were grown on catalytic Ni and Cu foils,
respectively, using chemical vapor deposition (CVD) [4,5]. However, the PC metal foils typically induced randomly-oriented graphene
grains, eventually forming PC graphene film when they merged together [6]. The grain boundaries in PC graphene film involved a lot of
atomic defects, degrading the intrinsic mechanical strength and electronic properties [7]. Therefore, the growth of large-area single-crystal
(SC) graphene film is highly demanded.
At earlier stages, improvements of graphene crystallinity were focused on by decreasing the number of nucleation seeds and simultaneously increasing the grain sizes via various methods such as thermal
annealing treatment, Cu surface cleaning treatment, Cu foil enclosure,
Cu oxidation treatment, and control of carbon feeding [8–12]. As a
result, centimeter-scaled graphene grains were achieved. But, using
such methods, it is still challenging to achieve SC graphene film at
large scale. In recent years, SC graphene film has been successfully
synthesized on SC Cu (111) foils, H-terminated Ge (110), and vicinal
Cu (110) and (111) surfaces via growing aligned graphene grains and
merging them without formation of grain boundaries [13–16]. This
strategy is very useful to achieve SC graphene film on wafer scale.
However, the preparation of such SC growth substrates requires complex processes such as solid phase epitaxy, crystallization, and longtime annealing treatment [11,17–21]. Therefore, a more productive
strategy for the preparation of SC growth substrate is still necessary
for practical applications.
Herein, we report a facile method for the preparation of an atomic
sawtooth Cu surface to grow SC graphene films. A large-scale atomic
sawtooth Cu substrate is obtained via a simple melting (at 1,100 ∘ C)solidification (at 1,050 ∘ C) process prior to the CVD synthesis of
graphene. The Miller index of atomic sawtooth Cu surface is random
but shows a specific high Miller index over the whole region. The lattice orientation of graphene grains on atomic sawtooth Cu surface is
confined by atomic step-edge, yielding coherently aligned graphene
grains. Such aligned graphene grains are eventually merged, forming
SC graphene film. The absence of a D-band in Raman spectra and high
oxidation resistivity at the merged region of aligned grains indicate
seamless stitching of grains. Furthermore, the single lattice orientation
of SC graphene film is confirmed by selected area electron diffraction
(SAED) pattern and transmission electron microscopy (TEM). Lastly,
homogeneous optical property of SC graphene film is verified using
Raman mapping analysis.
2. Experimental details
2.1. Synthesis of SC graphene on atomic sawtooth Cu
substrate
High-purity Cu foils (0.127 mm thick, 99.99 %, Alfa Aesar) were
cleaned by ultrasonication in a nickel etchant (Nickel Etchant-TFB,
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Figure 1. (a) Schematics of CVD synthesis of graphene on PC Cu foil and SC atomic sawtooth Cu substrate. Simple melting-solidification produces SC atomic sawtooth
Cu substrate (bottom panel). On the other hand, thermal annealing offers PC Cu foil (upper panel), resulting in growth of SC and PC graphene films, respectively. (b
and c) Photographs of as-grown graphene films on (b) PC Cu foil and (c) SC atomic sawtooth Cu substrate.
Transene) for five minutes. The nickel etchant was rinsed with deionized (DI) water several times and removed by N2 blowing. As a support layer, W foils (0.1 mm thick, 99.95 %, Alfa Aesar) were cleaned
by ultrasonication in acetone and isopropyl alcohol for five minutes,
followed by N2 blowing. Two Cu foils were stacked on a W foil and
loaded into the center of the quartz tube. To melt Cu foils on the W
foil, the temperature of the quartz tube was elevated to 1,100 ∘ C for one
hour and maintained for five minutes. Molten Cu was slowly cooled
to 1,050 ∘ C over a period of 30 min. It should be noted that, due to
the surface tension of molten Cu, one Cu foil is not enough to cover
the entire surface of W foil. For the synthesis of SC graphene, CH4
(10 % diluted in H2 ) was supplied at 1,050 ∘ C with a flow rate of 0.02
sccm for 10–15 min. After the synthesis, the quartz tube was naturally
cooled to room temperature. The entire process was performed under
atmospheric pressure with flow rates of 200 and 3 sccm of Ar and H2
gases, respectively.
2.2. Bubble transfer of graphene
As a supporting layer, poly(methyl methacrylate) (PMMA) was
spun onto graphene on Cu substrates at 3,000 rpm for 60 s. The PMMA
layer was dried at 90 ∘ C for two mins. The sample and Pt foil were connected to cathode and anode of the power supply, respectively. The
PMMA/graphene layer was detached from Cu substrate by interfacial
H2 bubbles under applied voltages of 3–5 V, in 0.5 M NaOH solution.
Underlying NaOH residue was rinsed by DI water several times, and
the PMMA/graphene layer was transferred onto SiO2 /Si substrate (or
TEM grids). Prior to the characterization, the PMMA layer was removed by acetone vapor at 150 ∘ C.
2.3. Characterization
The orientations of the atomic sawtooth Cu substrates were analyzed by electron back scattered diffraction (EBSD) in a field emission scanning electron microscope (FE-SEM) (JSM-7000F, JEOL). The
morphologies of the atomic sawtooth Cu substrate and monolayer
SC graphene film were characterized by optical microscopy (Eclipse
LV150, Nikon), atomic force microscopy (AFM) (AFM 5000 II, Hitachi), and FE-SEM. The grain boundaries in PC graphene film were
selectively oxidized by ultraviolet (UV) light with a wavelength of
254 nm. The PC graphene film was loaded into a chamber and exposed to 20 mW UV light for 20 min. The crystal quality of graphene
was confirmed by micro-Raman spectroscopy (NTEGRA Spectra, NTMDT) and high-resolution transmission electron microscopy (HRTEM) (JEM ARM 200F, JEOL) with an acceleration voltage of 80 kV.
3. Results and discussion
Figure 1 shows two different methods of graphene synthesis on
PC Cu substrate and on atomic sawtooth Cu substrate via CVD. The
thermal annealing process at 1,050 ∘ C, below the Cu melting point, is
employed to smooth the Cu surface; then, the growth of graphene is
carried out. Such thermal annealing induces abnormal grain growth,
resulting in large Cu grain sizes. However, this process is unable to
achieve SC Cu foils. The photograph in Fig. 1(b) presents large Cu
grains and grain boundaries indicated by the white arrows. It should
be noted that some annealing conditions produced SC Cu (111) foils,
but specific care was required [20]. On the other hand, large-scale SC
atomic sawtooth Cu substrates, which consisted of periodic terraces
and step-edges, are simply obtained by replacing thermal annealing
with the melting-solidification process. To melt and maintain the flat
Cu surface, Cu foils stacked on W foils were melted at 1,100 ∘ C and
then solidified at 1,050 ∘ C. The photograph in Fig. 1(c) shows the flat
surface and absence of grain boundaries.
To understand the surface structure of Cu foils after forming
atomic sawtooth Cu substrate, EBSD measurement was conducted.
An EBSD inverse pole figure (IPF) map of the thermally annealed Cu
foils, in Fig. 2(a), shows five different colors and Miller indices, indicating the formation of PC Cu foils. In contrast, the atomic sawtooth
Cu surface shows only a single color in the EBSD IPF maps in Fig.
2(b). The Miller indices are random upon the ‘batch-by-batch’, which
include (951), (4 -1 -9), and (-11 -14 21). These Miller indices have
different terrace facets, with low indices of (100), (110), or (111), and
step edges. AFM measurement was further conducted to observe the
Figure 2. (a and b) EBSD IPF mapping images of (a) PC Cu foil and (b) SC atomic sawtooth Cu substrate. (c) AFM topography image of the atomic sawtooth Cu substrate.
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Figure 3. (a and b) SEM images of as-grown graphene grains on (a) PC Cu foil and (b) atomic sawtooth substrate. (c and d) SEM images of (c) misaligned and (d)
aligned graphene grains after UV treatment for 20 min. The misaligned grains are oxidized along the grain boundary, whereas oxidation does not occur in the aligned
graphene grains. (e and f) Schematics and Raman I2D /IG mapping images and (g) Raman spectra extracted from boundary regions in (e) misaligned and (f) aligned
graphene grains. (h) Schematic for growth of SC graphene on atomic sawtooth substrate: step 1. nucleation at step edges for coherently aligned graphene grains, step
2. seamless stitching of aligned graphene grains, and step 3. growth of SC graphene film.
surface morphologies. A representative AFM image of the atomic sawtooth Cu surface in Fig. 2(c) shows a very flat surface with straight Cu
step lines, which cross over by an angle of ~76.8 ∘ . This angle is clearly
distinct from the angle of 60 ∘ in Cu (111) surface [19]. This again
demonstrates the formation of the atomic sawtooth Cu surface.
To observe the growth behavior on different substrates, graphene
grains were synthesized on PC Cu and SC atomic sawtooth Cu surfaces. While randomly-oriented hexagonal graphene grains on PC
surface are visible in scanning electron microscope (SEM) images
[Fig. 3(a)], well-aligned hexagonal graphene grains on SC atomic sawtooth surface are apparently observed [Fig. 3(b)]. This implies that
the atomic sawtooth surface provokes the growth of aligned graphene
grains.
To verify the seamless stitching between aligned grains in the
merged region, UV ozone treatment was executed [22]. UV treatment
was conducted under ~50 % humidity condition for 20 min to selectively oxidize the defective grain boundaries [22]. While there is a
white-colored oxidation line indicated by the yellow circle in the SEM
image of misaligned graphene domains after UV treatment [Fig. 3(c)],
no noticeable lines are seen in the aligned graphene grains [Fig. 3(d)].
This indicates that coherently aligned graphene grains merge without
forming grain boundaries.
The absence of grain boundaries was further characterized by Raman mapping measurements after being transferred onto SiO2 /Si substrates. Angles between misaligned hexagonal graphene grains have
typically random degrees [i.e., 98 ∘ in Fig. 3(e)] and darker regions
can be seen to have distinctly emerged at boundary regions between
misaligned grains in the Raman mapping image of the 2D band and
G band intensity ratio (I2D /IG ). In contrast, only an angle of 120 ∘
is observed for the aligned grains and uniform color contrast is observed in the aligned graphene grains [Fig. 3(f)]. The representative Raman spectra extracted from the boundary regions in aligned
graphene grains clearly show the absence of a D band (at ~1,340 cm−1 ),
whereas a D band is found in misaligned graphene grains [Fig. 3(g)].
These results strongly support the seamless stitching between aligned
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graphene grains synthesized on atomic sawtooth Cu substrate. The
schematic in Fig. 3(h) provides a model for the growth of SC graphene
film on the atomic sawtooth surface: step 1. nucleation at step edges
for coherently aligned graphene grains, step 2. seamless stitching of
aligned graphene grains, and step 3. growth of SC graphene film.
Centimeter-scaled SC graphene film was successfully synthesized
on atomic sawtooth Cu substrate with prolonged growth time. To
characterize the SC graphene film, the sample was transferred onto
SiO2 /Si substrate using a bubble transfer method [Inset of Fig. 4(a)].
The color contrast in the optical image is almost identical, implying
similar thickness of the SC graphene film over the entire region [Fig.
4(a)]. Some dark lines can be observed in the optical image due to
the presences of wrinkled and folded graphene; these lines were induced during the synthesis and transfer processes [23]. The AFM topography image also shows homogeneous color contrast, with wrinkles [Fig. 4(b)] that typically form by thermal expansion difference
between Cu and graphene film during cooling process after synthesis. Furthermore, Raman intensity mapping image for I2D /IG clearly
presents the homogeneity of the optical characteristic in the SC monolayer graphene film [Fig. 4(c)]. The uniform color contrast in the mapping image also indicates the absence of bi- and tri-layer graphene islands on the film.
After transfer onto a TEM grid, the SC graphene film crystal orientation was further characterized by TEM. Coherently aligned single
hexagonal dots can be observed in the SAED patterns, which were
measured at nine different spots (indicated by Roman numerals) in an
area of 300 × 300 𝜇m2 [Figs. 4(d) and 4(e)]. This indicates the single
orientation of SC graphene film over the whole region. The HR-TEM
image clearly shows the hexagonal atomic structure of graphene, with
d-spacing of 0.21 nm [Fig. 4(f)]; this value agrees well with that in
a previous report [24]. Consequently, large-scale SC graphene film
is synthesized on atomic sawtooth Cu substrate via simple meltingsolidification process.
ASCT Vol. 32 No. 1 (2023); https://doi.org/10.5757/ASCT.2023.32.1.26
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Figure 4. (a) Optical (inset: photograph), (b) AFM topography, and (c) Raman I2D /IG mapping images of SC graphene film after transfer onto SiO2 /Si substrate. (d) SEM
image and (e) corresponding nine SAED patterns of SC graphene film transferred onto TEM grid. The SAED patterns are obtained from nine different regions indicated
by Roman numerals in (d). (f) HR-TEM image of graphene film.
4. Conclusions
In summary, we have successfully synthesized centimeter-scaled
SC graphene film on atomic sawtooth Cu substrate. A simple meltingsolidification process produced atomic sawtooth Cu substrates, which
have random high Miller indices. Due to the atomic step-edges in
the substrate, coherently aligned graphene grains are synthesized and
merged to form SC graphene film. UV treatment and Raman D band
analysis show the absence of crystal imperfections at the merged region in the aligned grains. The single crystallinity of the graphene
film is further confirmed by the aligned SAED patterns in HR-TEM.
In addition, the homogenous optical property of the SC graphene film
is verified by Raman mapping measurements. We believe that our approach not only provides a facile route for synthesizing SC graphene
film, but also opens a new avenue for practical applications based on
large-scale 2D films.
Acknowledgments
S. Lee and S. H. Choi contributed equally to this work. This research was supported by the Institute for Basic Science (IBS-R011D1) and the Next-generation Intelligence Semiconductor Program
(2022M3F3A2A01072215) through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT. K.
K. K. would like to acknowledge the Basic Science Research Program
through the NRF, funded by the Ministry of Science, ICT and Future
Planning (2020R1A4A3079710 and 2022R1A2C2091475). S. M. K.
would like to acknowledge support from the Basic Science Research
Program through the NRF, funded by the Ministry of Science, ICT
and Future Planning (2020R1A2B5B03002054, 2022R1A2C2009292,
and 2022R1A4A3030766).
Conflicts of Interest
The authors declare no conflicts of interest.
Appl. Sci. Converg. Technol. | Vol. 32, No. 1 | January 2023
ORCID
Seungjin Lee
Soo Ho Choi
Hayoung Ko
Soo Min Kim
Ki Kang Kim
https://orcid.org/0000-0003-2222-0429
https://orcid.org/0000-0002-9927-0101
https://orcid.org/0000-0002-1566-9925
https://orcid.org/0000-0002-1404-9572
https://orcid.org/0000-0003-1008-6744
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