⇦Chinese Physics Letters, 2020, Vol. 37, No. 10, Article code 108101 Scrolled Production of Large-Scale Continuous Graphene on Copper Foils Zhibin Zhang (张志斌)1†, Jiajie Qi (戚嘉杰)1†, Mengze Zhao (赵孟泽)1, Nianze Shang (尚念泽)1, Yang Cheng (程阳)1, Ruixi Qiao (乔瑞喜)2, Zhihong Zhang (张智宏)2, Mingchao Ding (丁铭超)1,3, Xingguang Li (李兴光)1, Kehai Liu (刘科海)4, Xiaozhi Xu (徐小志)5, Kaihui Liu (刘开辉)1,2, Can Liu (刘灿)1*, and Muhong Wu (吴慕鸿)1,2,4* Affiliations 1State Key Laboratory for Mesoscopic Physics, Frontiers Science Center for Nano-optoelectronics, School of Physics, Peking University, Beijing 100871, China 2International Centre for Quantum Materials, Collaborative Innovation Centre of Quantum Matter, Peking University, Beijing 100871, China 3Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 4Songshan Lake Materials Laboratory, Institute of Physics, Chinese Academy of Sciences, Guangdong 523808, China 5Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510631, China Received 12 July 2020; accepted 19 August 2020; published online 29 September 2020 Supported by the Beijing Natural Science Foundation (Grant No. JQ19004), the Key R&D Program of Guangdong Province (Grant Nos. 2019B010931001, 2020B010189001, 2018B010109009 and 2018B030327001), Bureau of Industry and Information Technology of Shenzhen (Graphene platform 201901161512), the National Natural Science Foundation of China (Grant Nos. 51991340, 51991342 and 51522201), the National Key R&D Program of China (Grant Nos. 2016YFA0300903 and 2016YFA0300804), the Beijing Excellent Talents Training Support (Grant No. 2017000026833ZK11), the Beijing Municipal Science & Technology Commission (Grant No. Z191100007219005), the Beijing Graphene Innovation Program (Z181100004818003), the Guangdong Innovative and Entrepreneurial Research Team Program (Grant No. 2016ZT06D348), the Science, Technology and Innovation Commission of Shenzhen Municipality (Grant No. KYTDPT20181011104202253), the National Postdoctoral Program for Innovative Talents (Grant No. BX20190016), and China Postdoctoral Science Foundation (Grant Nos. 2019M660280 and 2019M660281).
†These authors contributed equally to this work.
*Corresponding authors. Email: mhwu@pku.edu.cn; canliu@pku.edu.cn Citation Text: Zhang Z B, Qi J J, Zhao M Z, Shang N Z and Cheng Y et al. 2020 Chin. Phys. Lett. 37 108101    Abstract We report an efficient and economical way for mass production of large-scale graphene films with high quality and uniformity. By using the designed scrolled copper-graphite structure, a continuous graphene film with typical area of $200 \times 39$ cm$^{2}$ could be obtained in 15 min, and the production rate of the graphene film and space utilization rate of the CVD reactor can reach 520 cm$^{2}$$\cdot$min$^{-1}$ and 0.38 cm$^{-1}$$\cdot$min$^{-1}$, respectively. Our method provides a guidance for the industrial production of graphene films, and may also accelerate its large-scale applications. DOI:10.1088/0256-307X/37/10/108101 PACS:81.05.ue, 89.20.Bb, 81.15.Gh, 68.65.Pq © 2020 Chinese Physics Society Article Text Graphene has been widely investigated in the fields of materials, physics and chemistry due to its novel properties and various potential applications.[1–5] However, up to date, the industrial applications of graphene were limited to a small range, leaving most of its excellent mechanical, thermal and electrical properties underutilized. To fully utilize those properties, scalable production of high-quality graphene films in an economical and efficient way is in great demand.[6] At present, the most prevailing strategies for massive production of graphene flakes/films can be roughly divided into two groups: (i) chemical/electrochemical exfoliation of graphite in aqueous solution, which is cost-effective but of inferior quality;[7–11] (ii) chemical vapor deposition (CVD) growth of graphene on epitaxial substrates of metals or dielectrics,[8] which leads to growth of graphene with high quality and proves to be optimal in electronic and optoelectronic devices.[12–15] Since the realization of uniform growth of monolayer graphene on copper (Cu) foils,[13] the research interests of CVD-grown graphene have been focused on two points: large size and high quality.[16–20] Among all the techniques developed so far, the roll-to-roll growth method shows the best potential for industrialization.[21–23] In most cases of this method, a winder roll is applied to deliver Cu stripes at a typical transport velocity of 10–50 cm$\cdot$min$^{-1}$ to achieve a full coverage of graphene on Cu surface.[24–26] In principle, the productivity is proportional to the exposed catalytic Cu surface area under the optimal growth condition.[27–31] Therefore, a larger equipment seems to be the final choice to further improve the yield, which in turn will also increase costs and space. Besides the roll-to-roll growth technique, a different synthesis method of graphene on rolled Cu has been recently reported, which has high efficiency in a much smaller reactor due to the increased exposed surface area of Cu.[32,33] However, the rolling construction of Cu foil as well as the growth temperature in these works should be precisely controlled to prevent the collapse and fusion of the adjacent Cu layers. Therefore, an efficient method that can balance the productivity and quality of large-scale graphene films is in great demand. In this work, we develop a scrolled production technique to realize the industrial-scale growth of graphene film with high quality in a commonly used laboratory CVD furnace. By adding a graphite paper as the separating layer, our method achieved high stacking density of Cu layers in a small CVD system. Meanwhile, with the excellent thermal stability, the graphite paper could also prevent the adjacent Cu from adhesion. In our experiments, the production rate of the graphene film and space utilization rate of the CVD reactor can reach 520 cm$^{2}$$\cdot$min$^{-1}$ and 0.38 cm$^{-1}$$\cdot$min$^{-1}$, respectively. Structural characterizations and transmittance tests of our large-scale graphene film revealed its high quality and uniformity. With such high productivity and quality, industrialization and commercialization of continuous graphene films will be highly accelerated. The scrolled production of a graphene film was performed in a commonly used CVD furnace as shown in Fig. 1(a), where a stacked Cu foil and a graphite paper were rolled coaxially onto a small inner quartz tube. Here, this inner quartz tube was used for three benefits: (i) to assist the rolling process of the Cu foil and the graphite paper and to facilitate the loading process of the scrolled sample; (ii) to improve the gas utilization rate by avoiding gas leakage from the inside cavity of the scrolled structure; (iii) to maintain this scrolled structure during the graphene growth at low temperatures. A pair of quartz holders on both ends of the inner quartz tube was used to avoid compaction of the bottom scrolled Cu [Fig. 1(b), upper panel]. With this design, the stacked structure is dense enough to fully utilize the small space, meanwhile loose enough for the flow of gas to form a uniform graphene film [Fig. 1(b), lower panel, the measurement of the gap size is shown in Fig. S1 in the Supplementary Materials]. Most importantly, with the assistance of the graphite paper, the shape of the Cu foil can remain intact after the high-temperature annealing (up to 1065 ℃ as shown in Fig. S2) and graphene growth in such a small CVD furnace (otherwise it will collapse and stick together as shown in Fig. S3). This simple design ensures the massive production of graphene without excessively relying on enlarging the CVD equipment. In our experiment, a $200 \times 39$ cm$^{2}$ Cu foil was scrolled with a graphite paper and loaded into the CVD system. After growth at the high temperature of 1050 ℃ for 15 min with the flow rate of CH$_{4}$/H$_{2}$ at 10 sccm/10 sccm, a full coverage of graphene on the Cu foil was realized [Fig. 1(d)]. The production rate of the graphene film is 520 cm$^{2}$$\cdot$min$^{-1}$, and the space utilization rate of the CVD reactor is as high as 0.38 cm$^{-1}$$\cdot$min$^{-1}$ (the volume of the reactor is $\sim $1373 cm$^{3}$). Compared with the previous works[27–33] using roll-to-roll growth, batch-to-batch growth and rolled Cu growth techniques, both the rates are superior [Fig. 1(c)]. This significantly increased productivity benefits from the designed scrolled Cu-graphite structure, which leads to the best utilization of space and feedstock simultaneously. In addition, to exclude the influence of the graphite paper on the synthesis of graphene, we conducted a controlled experiment without introducing CH$_{4}$, and no graphene film was found on Cu (Fig. S4), indicating that the graphite paper was the excellent separator and would not bring additional factors.
cpl-37-10-108101-fig1.png
Fig. 1. CVD growth of large-scale graphene with the scrolled growth technique. (a) Schematic of the scrolled Cu-graphite structure, the graphite paper is used as the separator and a small inner quartz tube is used as the core of the structure. (b) Upper panel: photograph of the apparatus, lower panel: schematic of the gas flow in the confined space between Cu and the graphite paper, the model shown here is for CH$_{4}$ molecules. (c) Comparison of the production rate (orange column) and productivity (pink rhombus) in the previous works[27–33] with this work. (d) Photograph of the continuous graphene film on Cu with an area of $200 \times 39$ cm$^{2}$.
In addition to the coverage of graphene, the quality of graphene is another general concern about this close-packed configuration. To find the most appropriate condition for the high-quality growth of graphene, the flow ratio of CH$_{4}$/H$_{2}$ was fixed at 1,[23] while the flow rate of CH$_{4}$/H$_{2}$ varied from 80 sccm/80 sccm, 40 sccm/40 sccm to 10 sccm/10 sccm. With the CH$_{4}$ concentration decreasing, the graphene films tend to be more uniform as shown in optical images [Fig. 2(a) and Fig. S5]. Raman spectra (with excitation laser wavelength of 633 nm) were conducted in randomly chosen regions on three typical samples correspondingly. As the concentration of CH$_{4}$ decreases, the intensity ratio of the 2D peak and G peak ($I_{\rm 2D}$/$I_{\rm G}$) increases, and the full width at half maximum (FWHM) of the 2D peak decreases, indicating that the multi-layer area of the sample is greatly reduced. The graphene film with best quality is acquired under the CH$_{4}$ flow rate of 10 sccm, with Raman spectra showing the intensity ratio of D peak and G peak ($I_{D}$/$I_{\rm G}$) less than 0.02 [Fig. 2(b)]. This phenomenon can be understood qualitatively as follows: with the flow rate of CH$_{4}$ and H$_{2}$ increasing (40 sccm and 80 sccm) in the atmospheric pressure CVD (APCVD) system, the thermal dissociation and collision become much more intense, and the nucleation of graphene will take place more easily on high energy sites, thus multilayer graphene with high density of defects will emerge;[34,35] while in the lower CH$_{4}$ concentration case, the nucleation and adlayer growth are effectively suppressed, leading to the formation of the high-quality graphene film [Fig. 2(c)].[36]
cpl-37-10-108101-fig2.png
Fig. 2. The dependence of the graphene quality on CH$_{4}$ and H$_{2}$ concentration. (a) Optical images of the as-grown graphene films with different CH$_{4}$ concentrations. These images are of the same size. (b) Raman spectra of the corresponding samples in (a). (c) Schematic of the graphene growth under different CH$_{4}$ concentrations.
To further evaluate the quality and uniformity of the as-grown graphene in an extended region of the 2-m-long Cu, we selected 5 positions on the Cu foil with intervals of $\sim $0.5 m, and transferred them onto the 300 nm SiO$_{2}$/Si substrates. Raman spectra (with excitation laser wavelength of 532 nm) of these 5 different samples all show typical monolayer characteristics without D peaks [Fig. 3(a)]. In addition, Raman mappings were also carried out on the 5 samples correspondingly: the FWHM of the 2D peak, $I_{\rm 2D}$/$I_{\rm G}$ and $I_{D}$/$I_{\rm G}$ are all of high uniformity at a 100-µm-scale area [Figs. 3(b)–3(d) and Figs. S6–S8]. These results demonstrate that even the Cu foil is stacked in such a scrolled regime, the obtained graphene film is still of high quality and uniformity at large scale.[33]
cpl-37-10-108101-fig3.png
Fig. 3. Raman characterization of the transferred graphene. (a) Raman spectra of the 5 transferred samples on 300 nm SiO$_{2}$/Si. (b)–(d) Raman mappings of 2D peaks' FWHMs (b), $I_{\rm 2D}$/$I_{\rm G}$ (c) and $I_{D}$/$I_{\rm G}$ (d), respectively. The subscripts 1–5 represent the samples shown in (a). These maps are of the same size.
To investigate the origin of the excellent performance of this scrolled growth technique, we examined the crystal structure of the Cu foil since it can significantly affect the epitaxial quality of graphene. In our experiments, the as-received commercial Cu foil was polycrystalline and the XRD 2$\theta$ scan results indicated that they were (200) oriented (Fig. S9). After the annealing process, it showed a single pronounced Cu(111) peak [Fig. 4(a)]. Furthermore, the EBSD mapping proved that the single crystal was of large-scale uniformity [Fig. 4(b)]. These results demonstrated that the Cu foil was transformed into Cu(111) single crystals during the high-temperature annealing process,[37] and each single crystal region was at least of centimeter scale. This phenomenon can be understood as follows: during annealing at a reduced atmosphere, the main driving force for the abnormal grain growth is surface energy minimization.[38] Thus the Cu(111) facet which has the lowest surface energy is obtained (Table S1). For graphene growth on such the Cu(111) substrates, typical high-resolved TEM (HRTEM) images revealed the single-crystal nature of graphene without detectable defects [Fig. 4(c)]. With further decreasing the growth time, aligned graphene domains on Cu foil can be found, and the alignment level reaches $> 98$% [Figs. 4(d)–4(f)]. As is widely accepted, Cu(111) is the most ideal substrate for the growth of single-crystal graphene due to their same rotation symmetry and the small lattice mismatch (4%).[31] Thus, we can attribute the high-quality growth of the graphene film to the seamless stitching of separated graphene domains on Cu(111) single crystals (details are shown in Fig. S10).
cpl-37-10-108101-fig4.png
Fig. 4. Characterizations of single-crystal Cu foils. [(a),(b)] XRD 2$\theta$-scan spectrum (a) and EBSD IPF map (b) of the annealed Cu, proving a large-scale single-crystal Cu(111) formed in the scrolled structure. (c) Representative HRTEM image of monolayer graphene showing a uniform crystal lattice structure. Inset: corresponding fast Fourier transform (FFT) pattern. (d)–(f) SEM images of unidirectionally aligned graphene domains obtained at different areas on one single-crystal Cu(111) grain. These images are of the same size.
cpl-37-10-108101-fig5.png
Fig. 5. Transmittance characterization of graphene transferred onto fused silica. (a) Photograph of the graphene featuring a good transparency to the underneath BoYa Tower in Peking University. The size of the fused silica is 4 inch. (b) Visible spectrum of the representative transferred graphene film. (c) The transmittance at 550 nm of the transferred graphene films at different areas.
Using this scrolled production technique, we are able to obtain uniform large-scale graphene films efficiently, and various applications of this film are already expected. Here, we transferred a transparent graphene film onto a 4-inch fused silica for examination [Fig. 5(a)], and the BoYa Tower in Peking University is clearly seen through this transparent film. The optical transmittance at 550 nm is reduced by $\sim $2.3% at different positions [Figs. 5(b) and 5(c)], which implies a single layer property of the transferred sample.[39] Sheet resistance of the graphene film was also measured, and an average value of $\sim$$450\,\Omega$/sq was obtained (Fig.  S11). The measured field-effect transistor mobility of the graphene film was $\sim $2600 cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$ at 1.5 K (Fig. S12). The excellent optical transparency and good electrical property of the as-grown graphene demonstrate its capability as transparent electrodes in electronic and optoelectronic devices. In summary, a scrolled Cu-graphite structure has been designed to successfully synthesize a $200 \times 39$ cm$^{2}$ uniform graphene film. The use of graphite separator is a crucial strategy to increase the stacking density of the Cu substrate, which leads to the fast growth of graphene in a limited space. The production rate of the graphene film reaches 520 cm$^{2}$$\cdot$min$^{-1}$, and the space utilization rate of the CVD reactor is as high as 0.38 cm$^{-1}$$\cdot$min$^{-1}$, which are several times or even magnitude higher than those in the literature available. Large single crystals of the Cu substrate formed in our scrolled structure through high temperature annealing treatment are proved to ensure the quality of the epitaxial-grown films. Our method thus implies a suitable balance between productivity and quality of the massive production of graphene films. We believe that this scrolled growth technique will be a significant promotion to the large-scale industrialization and commercialization of graphene films. 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