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Research Paper

Applied Science and Convergence Technology 2023; 32(1): 26-29

Published online January 30, 2023


Copyright © The Korean Vacuum Society.

Facile Synthesis of Wafer-Scale Single-Crystal Graphene Film on Atomic Sawtooth Cu Substrate

Seungjin Leea , Soo Ho Choia , b , Hayoung Koa , Soo Min Kimc∗ , and Ki Kang Kima , b∗

aDepartment of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
bCenter for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea
cDepartment of Chemistry, Sookmyoung Women’s University, Seoul 14072, Republic of Korea

Correspondence to:soominkim@sookmyung.ac.kr, kikangkim@skku.edu

Received: November 7, 2022; Revised: December 7, 2022; Accepted: January 4, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

Graphene isa 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 [13]. 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.

Atearlier 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 [812]. 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 [1316]. 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 [11,1721]. 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.

2.1. 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, 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.

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.

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.

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 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.

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.

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.

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.

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 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.

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.

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.

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.

S. Lee and S. H. Choi contributed equally to this work. This research was supported by the Institute for Basic Science (IBS-R011- D1) 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).

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