Asymmetric Growth of Tetragonal-Shaped Single-Crystalline

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C: Physical Processes in Nanomaterials and Nanostructures

Asymmetric Growth of Tetragonal-Shaped Single-Crystalline Graphene Flakes on Copper Foil by Annealing Treatment under Oxygen-Free Condition Biyun Shi, Qiao-Jun Cao, Qun Wang, Xu Han, Haifei Wu, Lei-Qiang Chu, Zebo Fang, Han Huang, Jian-Xin Tang, and Wei-Dong Dou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11897 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The Journal of Physical Chemistry

Asymmetric Growth of Tetragonal-shaped Single-Crystalline Graphene Flakes on Copper Foil by Annealing Treatment under Oxygen-Free Condition Bi-Yun Shi1, Qiao-Jun Cao1, Qun Wang1, Xu Han1, Hai-Fei Wu1, Lei-Qiang Chu1, Ze-Bo Fang1, Han Huang2, Jian-Xin Tang3, Wei-Dong Dou1,3, * 1

Laboratory of Low-dimensional Carbon Materials and Department of Physics, Shaoxing

University, Shaoxing 312000, China

2

Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of

Physics and Electronics, Central South University, Changsha 410083, China 3

Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of

Suzhou Nano Science and Technology, Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China *Corresponding authors. E-mail: [email protected] (Wei-Dong Dou) Abstract: Engineering graphene into particular shape is vital for potential industrial applications. To this end, better understanding of the growth mechanism is needed in order to control the growth behavior of graphene on substrate surface with specific shape. In this work, tetragonalshaped graphene single crystal (TS_GSC) with millimeter-scaled grain size were achieved on copper foil which was annealed at oxygen free condition (AOF) prior to graphene growth. The TS_GSC grains are featured by two dendric-frontiers at the shorter-edge (SE) sides and two

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sharp-frontiers at longer-edge (LE) sides of graphene grain. Combining scanning electron microscopy, optical microscopy and Raman mapping with carbon isotope labeling, we revealed for the first time an asymmetric growth behavior of TS_GSC grains on AOF treated copper substrate. It was supposed that the growth of graphene was determined by diffusion-limited aggregation mechanism at SE side while it was governed by edge-determined atom-attachment mechanism at LE side of graphene grain. In addition to single layer graphene, tetragonal shaped bilayer graphene with grain size over 200 m was also achieved on AOF treated copper substrate.


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1. Introduction Graphene, a single-layered carbon material with carbon atoms being bonded through sp2 hybridization and arranged into honeycombs, attracted tremendous scientific and technical attention since its first invention in 2004.1, 2 Graphene’s unique physical and chemical properties make it a star material which is frequently used in broad applications includes various sensors, hydrogen harvesting, Li-ion batteries, solar cells, etc. For the above-mentioned devices, graphene is usually used as electrode materials owning to its ultrahigh electric conductivity and optical transmittance. Besides electrode applications, graphene as semiconductor is equally attractive since it is superior to Si in aspects like electric and thermal conductivity, mechanical strength and flexibility, etc. However, difficulties in fabrication of graphene single crystal (GSC) must be overcome before the industrial application of graphene-related semiconductor. This attracted intense scientific and technological interest over fabrication of GSC with controlled sized, shape, thickness and stacking order.3-19 GSC usually was fabricated with chemical vapor deposition (CVD) method where copper foil was frequently chosen as catalytic substrate. Great efforts have been paid to improve the quality of GSC flakes, such as annealing treatment for better crystallinity of substrate,20-24 controlling the supplement of gases,25-29 employing enclosed or roll-up Cu foils,29,30 trapping gaseous carbon precursor inside a restricted region,31-35 and tuning the nucleation density by using oxygen.36-38 Millimeter-scale GSC was achieved with the approaches mentioned above. Recently, Vlassiouk et al even reported an approach toward industrial fabrication of continuous crystalline graphene on polycrystalline copper substrate.36 Single crystalline graphene of a foot long was achieved by the so-called advancing local control of the precursor concentrations CVD method. GSC flakes usually have hexagonal shape (HS_GSC), which resemble the symmetry of graphene lattice.28,40,41 However, in some cases, tetragonal-shaped GSC (TS_GSC) grains were


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observed.36,37,42,43 And the formation tetragonal grains were attributed to either the substrate symmetry, e.g. four-fold symmetry of Cu (100) surface, or the reconstruction of Cu surface. Previous works reported that TS_GSC grains have nearly squared-shaped and dendric-edges. These characters represent the symmetric nature of Cu (100) substrate and the diffusion-limited aggregation (DAL) mechanism for growth of graphene. However, one question is still remained unclear, i.e. what is role of edge nature for the growth of GSC flakes? Compared to single layer graphene (SLG), the growth of few layer graphene (FLG) was even harder because the catalytic activity of the Cu surface will be suppressed when Cu substrate was fully covered with SLG.

44, 45

To modulate the carbon solubility of substrate, Cu-Ni foil was

utilized but the uniformity of FLG is limited due to nonuniform alloy composition.46-48 Recently, Fang and Hao et al reported O-assistant approach to fabricate half-millimeter single crystalline bilayer graphene. It was revealed that the existence of oxygen on copper surface enhanced the carbon dissolution in bulk Cu and its succedent segregation at copper surface as graphene grow, leading to the preferentially growth of FLG.26,49 However, only partial coverage of bilayer graphene has been obtained by using this method. In addition to the grain size, improvement in thickness uniformity of FLG is also a challenge. Therefore, more studies are still needed for a better understanding about the CVD growth of graphene. In this study, we focus on the growth of tetragonal GSC grains. Millimeter-scaled TS_GSC flakes were achieved. We observed for the first time the asymmetric growth behavior of the TS_GSC grain on copper foil, which is attributed to an edge-determined effect. The sharp frontiers of TS_GSC grains are supposed to be zigzag edge. Additionally, the basal plane of the TS_GSC flakes is characterized with periodically corrugated morphology. These characters enable potential applications as edge- or strain-based sensors or as template for growth of nanostructure.50-56 In addition to single-layered TS_GSC, a bi-layered TS_GSC grain with domain size over 200μm were also reported.


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Figure 1 Schematic illustration of graphene fabrication processes. 2. Experimental Section Graphene synthesis: The fabrication of graphene on copper foil was conducted on a homemade low-pressure CVD system. The basement pressure of the CVD chamber is as low as 0.01Pa. Commercial Cu foil (99.8% purity, 25m thick, Alfa Aesar) was used as the catalytic substrate for graphene growth. Copper foils were folded into enclosed pockets with size of 2.5 × 6.0mm2 after being cleaned by ethanol and dried with pure gaseous N2. Then the enclosed copper pocket was loaded into the CVD chamber for graphene growth. The schematic procedure of fabrication process is illustrated in Fig. 1. The copper pocket was heated up to 1035℃ in 1 hour (Furnace warming) under Ar flow rate of 300sccm, and maintained for 2 hours (Cu annealing). The copper substrate was annealed under either oxygen rich (AOR, O2 flow rate of 0.15 sccm) or oxygen free (AOF, O2 flow rate of zero)


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condition depends on the target samples. After that, the growth stage begins with introducing of methane (flow rate of 0.5 sccm) into CVD chamber while furnace temperature was kept at 1035℃. Mixed gas of Ar, H2 and O2 was also backfilled into CVD chamber as graphene grew.36 The flow rate of Ar, H2 and O2 was set to be 300sccm, 100sccm and 0.06sccm, respectively. The growth time (g_t) is changed from 0.5 to 4 hours depends on the required samples. The growth stage is terminated with the shutoff of methane dosage, then the graphene/copper sample was cooled down to room temperature (RT) within 1 hour (cooling-down stage) under Ar flow of 300sccm. Graphene transfer: Graphene grown on copper foil was transferred onto SiO2/Si substrate with the assistance of polymethyl methacrylate (PMMA). The sample was floated in ammonium persulfate for 24h to etch off the Cu foil. Then PMMA was removed by hot acetone (50℃) after the graphene/PMMA film was transferred onto SiO2/Si substrate. Characterization: A Zeiss Axio optical microscope (OM) and a Zeiss Supra 55 scanning electron microscopy (SEM) were used to investigate the morphology of graphene samples. The Raman spectra and Raman maps were used to reveal to quality and stacking properties of graphene grains. And the Raman measurements were taken on a Horiba HR Evolution Raman spectrometer using laser excitation of 532nm. And the X-ray diffraction (XRD) measurements were conducted on a Rigaku Smart-Lab XRD facility using Cu Ka emission.


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The edge nature as well as the morphology of graphene grain on copper foil was revealed by using a Bruke Multimode 8 atomic force microscopy (AFM).

Figure 2 SEM and XRD results of graphene on AOR and AOF treated copper foil show the correlation between the shape of graphene grain and crystalline structure of copper substrate. (a, b) SEM images of HS_GSC flakes on AOR-treated (a) and AOF-treated (b) Cu foil. (c, d) statistical results of HS_GSC and TS_GSC flakes on AOR-treated (c) and AOFtreated (d) Cu foil, (e) XRD results measured from as-received (top panel), AOF- (middle panel) and AOR-treated (bottom panel) copper foils. 3. Results and Discussions The morphology of GSC flakes on the annealing-treated copper substrates were measured by using SEM. Fig.2 shows the typical SEM images of GSC grains on copper foils treated with AOR and AOF procedures, respectively. HS_GSC grains were clearly observed for the


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samples which were treated with AOR procedure prior to graphene growth stage. This observation is in good agreement with our previous study.36 In contrast, TS_GSC grains become dominated for the copper substrate annealed according to AOF procedure prior to graphene growth stage. Although HS_GSC grains can also be observed for AOF treated samples, their percentage are much smaller than that of TS_GSC grains. Fig. 2c and 2d shows the statistical results of TS_GSC and HS_GSC grains on AOR and AOF treated samples. These results clearly indicate that annealing-treatment of Cu under sufficient oxygen condition leads to preferable formation of HS_GSC grains while annealing Cu under oxygen-free condition results in the domination of TS_GSC grains. To discover the correlation between shapes of GSC flakes and the crystallinity of Cu substrate, XRD measurements were conducted on the as-received and annealing-treated copper foils (see Fig. 2e). Two primary peaks were observed for the as-received Cu foil, which are peaked at 2θ = 43.2° and 50.3°, respectively. According to literature, the former peak corresponds to the reflex from Cu(111) facet while the latter is a reflex from Cu(100) orientation.43 The details of the primary diffraction peaks (α and β) were shown in the inset of Fig.2e. Beside the reflex form (111) and (100) facets, several shoulder peaks were also observed in the vicinity of reflexes from Cu(111) and (100) facets. This indicates the polycrystalline nature of the as-received copper foil. Usually, the crystallinity of metal will be improved after annealing-treatment at temperatures close to melting point. After AOF treatment, Cu(100) component was strongly enhanced while the (111) phase was substantially suppressed (see


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middle panel of Fig.2e). In contrast, AOR treatment leaded to the predomination of Cu(111) phase. It was supposed that the Cu substrate was purified after long time of AOR treatment along with since oxygen can consume infinitesimal impurities such as carbon and sulfur.36 To support this suspicious, we measured the elemental distribution of sulfur on copper surface with energy dispersive spectrometer (EDS). The corresponding EDS results were shown in supplementary information (Fig. S1). The copper foil used for EDS measurements was pretreated by annealing under oxygen-rich condition for 1 hour. Sulfur segregated due to high temperature annealing and aggregated into lined grains on the surface of Cu substrate. This observation demonstrated the purifying effect of annealing under oxygen sufficient condition. The purified Cu foil tends to undergo (111) recrystallization according to the law of minimized formation energy. Now, it is clear that AOF and AOR treatments lead to Cu(100) and Cu(111) facets, respectively. Considering the symmetry of (100) and (111) surface, we conclude that the shapes of graphene flakes represent the symmetric nature of corresponding substrates. We noted that the carbon flux may also result in the shape variation of graphene grains. In a recent work, Xu et al revealed that carbon flux during growth stage could also lead to the shape variation of graphene grains.57 However, this effect can be ruled out in this study since the flux of carbon precursor was set to be the same value for either tetragonal and hexagonal shaped graphene samples.


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Figure 3 Typical OM image and Raman results of the typical TS_GSC grains. (a) OM image, (b) Raman spectrum, (c-f) Raman maps of D peak intensity (c), G peak intensity (d), 2D peak intensity (e) and width of 2D peak (f). Raman mapping was taken over an area of 1.3 × 1.7 mm2 which was marked with solid rectangle in (a). In our recent work, we demonstrated that millimeter-scaled hexagonal GSC flakes can be fabricated on Cu(111) surface with CVD processes under moderate oxygen partial pressure.36 High quality millimeter-sized hexagonal shaped graphene can also be found in supplementary material (Fig. S2). In this study, we report the millimeter-scaled tetragonalshaped graphene grains which were fabricated by employing AOF annealing procedure. Fig.3 shows the OM image and Raman results of two TS_GSC flakes on SiO2/Si wafer. The


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lateral dimension of the typical TS_GSC grains is about 0.5 × 1.5 mm2. Typical Raman spectrum shown in Fig. 3b reveals the predominant features of graphene, i.e. G peak (~1588 cm ―1) which corresponds to in-plane vibration of sp2 carbon atoms and 2D peak (~2684 cm ―1) which is originated from a two-phonon double resonance Raman process. The I2D IG ratio is about 2:1, and the 2D band can be well fitted with single Lorentz line. These features indicate the single layer nature of the graphene flake. In addition, the defect peak (D, ~1342cm ―1) is almost undatable, which demonstrates that the TS_GSC flakes are nearly defect-free. To further verify the quality of the graphene flakes, Raman mapping measurements were carried out on selected region of TS_GSC sample (see Fig. 3a). Fig. 3c3e show the Raman maps using the peak intensity of D, G and 2D band, respectively. The D band is almost undetectable and intensity of G and 2D band is nearly constant over the measured region. In addition, the width of 2D feature which was measured as the full-width at half maximum (FWHM) of 2D peak is even unchanged across the whole measured region (see Fig. 3f). These results undoubtably revealed the defect-free and single layer natures of the tetragonal shaped graphene samples.


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Figure 4 OM images and the Raman map show the asymmetric growth behavior and edge natures of TS_GSC grain. (a) OM image reveals the rough and its neighbored smooth grain edges of a TS_GSC grain, (b) OM image shows the region where Raman map was taken (the region marked with rectangle), (e) Raman map using carbon isotope labelling reveals the asymmetric growth behavior of TS_GSC grain along orthogonal directions. Now, we pay attention to the growth dynamics of the TS_GSC domains on AOF treated Cu substrate. It has been revealed that AOF treatment leads to Cu(100) recrystallization which is four-fold symmetric. This implies that the diffusion barriers of carbon atom along two orthogonal directions are identical to each other. Thus, squared graphene domains should be expected if the growth of graphene is merely governed by DLA mechanism.


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Surprisingly, squared grain was rarely observed. In present study, the TS_GSC grains are characterized with unequal length and width. This phenomenon implies that the growth speed of graphene is asymmetric along the two orthogonal directions on Cu(100) surface, leading to the belt-like shape of TS-GSC grains. Additionally, the graphene frontier at longer-edge (LE) side is distinct from that of shorter-edge (SE) side. Graphene frontier at LE side is characterized with smooth and sharp edges while the SE side is featured by dendric branches. We also investigated the growth kinetic by visualizing the time evolution of graphene growth with C isotope labeling and Raman mapping. The belt-shape as well as the edge nature of graphene at SE and LE side remained as TS_GSC grain grew. In addition, the length of SE edges is in proportion with that of LE edges as TS_GSC grain grew. Considering the four-fold symmetry of the Cu(100) surface, it is reasonable to speculate that the diffusion mobility should be the same value for carbon atoms at LE and SE side of graphene grains. Nevertheless, we found that graphene grew in different speed along the two orthogonal directions. This implies that, apart from DLA mechanism, other factors do influence the growth of TS_GSC grains. In the inset of Fig. 4a, we show a schematic illustration of the TS_GSC grain frontiers. According to the symmetry graphene lattice, the frontiers of TS grain should be featured by either arm-chaired (AC) edge or zig-zag (ZZ) edge, respectively. Theoretical calculations have shown that the energy required to form armchair- and zigzag-edges is approximately the same. However, compared to ZZ edge, the AC edge has a little bit of higher degree of activity. So, compared to ZZ edge, carbon attachment to


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AC edge is energetically more favorable.58 Therefore, carbon atoms are preferred to be attached to AC edge of graphene grain, leading to the preferential growth along corresponding directions. Based on this speculation, we propose that the dendric frontiers (SE side) of TS_GSC grain are featured by AC edge while the smooth frontiers (LE side) are characterized with ZZ edge. It is well known that the dendric branches were resulted from random diffusion of carbon atom on copper substrate. So, it can be concluded that the growth of graphene at the SE side is governed by DLA mechanism while growth of graphene at LE side is dominated by edge-determined atom-attachment (EDA) behavior. This observation is in discrepancy with the case of HS_GSC grains on Cu(111) substrate where the growth of graphene grains are governed by DLA mechanism irrespective of edge nature.36


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Figure 5 OM image AFM images showing the morphology of TS_GSC grain. (a) OM image showing the region where AFM measurements were taken (the white square), (b) heightmode AFM image, (c) amplitude-mode AFM image, (d-e) height-mode AFM images region 1(d), 2(e) and 3(f), respectively. The scale bar is 5m for (b) and (c), and is 1m for (d-e). To further support this argument, we measured the edge morphology of a TS_GSC grain on copper foil by using AFM. AFM images shown in Fig. 5 were taken from the corner of a TS_GSC grain which is marked by the solid square in Fig. 5a. An edge-sensitive mode of AFM measurement, named amplitude-AFM was also used to identify the edge nature of TS_GSC grain more clearly (see Fig. 5c). These results clearly reveal the sharp (smooth) edge at LE side and the dendric (or rough) nature at SE side of graphene grains. This observation is in good agreement with the OM images and Raman maps shown in Fig. 3 and Fig. 4, respectively. Since the AOF-treated copper substrate is dominated by four-fold symmetric (100) component, it is reasonable to speculate that the diffusion barrier of carbon atoms along the orthogonal directions is identical to each other. However, distinct frontier shapes were revealed. This indicates that the edge natures play an important role for the asymmetric growth of TS_GSC grains. Now, we investigate the morphology of the TS_GSC gains. Fig. 5c- 5e shows AFM images of copper surface taken from the selected regions in Fig. 5b. It was shown that graphenecovered Cu regions are corrugated. In contrast, pure Cu regions without graphene are


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atomically flat. The corrugated character in graphene covered regions is confirmed by the red-shift of graphene G and 2D band in Raman spectrum which are sensitive to the bending of graphene basal plane (see Fig. S3 in supplementary material).59 It was reported that oxygen-induced p-doping of graphene can result in shifting of G and 2D band.60-62 In that cases, G and 2D band blue-shifted which is in contrast to the present case. In addition, the hole-doping usually leads to the intensity decrease and line broadening in 2D mode of graphene. These spectroscopic variations were absent from this study. So, the red-shifting of graphene G and 2D band is less-likely attributed to the doping effect. Actually, the strain in graphene can also lead to variation of graphene G and 2D band. Ferrai et al observed an obvious red-shifting of graphene 2D band when uniaxial strain was exerted on graphene.63 Considering the similarity between our observation with the results in literature, we conclude that the red-shifting of Raman G and 2D band of bunched graphene is attributed to the strain owning to the corrugated morphology of bunched graphene. Previously, the corrugated morphology was attributed to step bunching (SB) of copper surface. And SB was supposed to be driven by the relaxation of compressive strain exerted onto graphene overlayer during the cooling stage of CVD process.64-66 In a recent work, Yi et al found that SB can even be formed in a case where the compressive strain is absence.67 By combining theoretical studies with in-situ experimental observations, the authors revealed that the formation of surface SB is enabled by rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer


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in the vicinity of steps. It was reported that the step-height and adjacent distance of the step bunching are determined by the flux of diffused Cu atom which is sensitive to the substrate temperature.67 Specifically, the flux will be reduced exponentially as substrate temperature decreases. As shown in Fig. 5d, the step-height and adjacent distance of the step bunching are ~30nm and ~400nm, respectively. These values reduced greatly to ~2nm and ~100nm when the graphene growth temperature was decreased to 950℃ (see supplementary information, Fig. S4). These observations seem to be agreed with Yi’s conclusion.

Figure 6 SEM images of a HS_GSC grain (a) on copper foil treated with AOR procedure and a TS_GSC grain (b) on copper foil treated with AOF procedure. The round-shaped FLG grains are marked with short arrows in panel (a). Now, we turn our attention to the growth of FLG. Fig. 6a and 6b show the typical SEM image of graphene flakes on AOR- and AOF-treated cu substrate, respectively. The growth time (g_t) was set to be 1h for both cases. Round-shaped FLG grains are easily observed


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from HS_GSC flake on AOR-treated Cu foil, while similar grains are rarely observed in the case of TS_GSC flakes on AOF-treated copper foil. It was reported that the round-shaped FLG grains were resulted from carbon segregation.68 Hao et al reported that carbon atoms dissolved into bulk copper from interior side of copper pocket and dissolved out from the exterior side of the copper pocket.26 And bilayer graphene as large as 1.2 mm was achieved by using oxygen activated growth on polycrystal copper foil. In this study, we find that FLG grains are only observable on the interior side of copper pocket. So, we speculate that carbon diffused from exterior to interior side of copper pocket under the graphene growth condition reported in this study. Compared with AOF-treated Cu pocket, growth of FLG on AOR-Cu substrate seems easier, but with poor thickness uniformity. This one of the main differences for FLG growth on AOR- and AOF-treated substrates. We speculate that this discrepancy may be attributed to the difference of carbon dissolution in AOR- and AOFtreated Cu substrate. To support this speculation, we check the graphene coverage on exterior side of Cu pockets which were treated with AOR and AOF procedures, respectively (see Fig. S5 in supplementary information). For AOF case, the exterior side of Cu pocket is nearly completely covered by continuous graphene after half-hour growth. The quick growth of graphene on the exterior side of Cu pocket reduced the carbon dissolution. As a result, growth of FLG on the interior side of AOF treated copper pocket was depressed. In contrast, for AOR case, graphene on the exterior side of Cu pocket is still discontinuous even after growing for three hours, leaving plenty of bare copper regions which may serve


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as channels for carbon dissolution. As a consequence, growth FLG boomed on the interior side of AOR-treated Cu pocket.

Figure 7 OM images and the corresponding Raman spectra and map of the typical roundshaped FLG (a) and blade-shaped bilayer graphene grain (b). The scale bar is 50m for (a) and 100 m for (b). Although growth of FLG is depressed in AOF-treated case, FLG grains can also be formed on the AOF-treated Cu substrate provided that the growth time is long enough. Interestingly, compared with AOR-treated case, FLG on AOF-treated Cu substrate bares merit of better thickness uniformity. Fig. 7 shows the OM images and the corresponding Raman maps of a FLG grain on AOR- and AOF-treated Cu foil, respectively. Fig. 7a shows a round-shaped FLG grain on AOR-treated copper substrate. The size of the grain is shrunk


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with the increment of layer number. The layer number of round-shaped FLG is hard to be controlled. As revealed by the Raman spectra, the thickness of graphene is even over 10 layers at centered position. This indicates that the segregation of carbon atoms is poorly controlled for the case of AOR treated Cu foil. For comparison, we also show a FLG grain grown on Cu foil which was treated by AOF procedure prior to a 4h of growth time (Fig. 7b,). The Raman 2D peak of this grain can be fitted with four components, corresponding to the four allowed excitation modes of from conductive band to valance band of graphene.69 In addition, the map of I2D IG perfectly matches the corresponding OM image of the same graphene grain. These results reveal the double layer nature of graphene which has AB stacking ordering. The size of bilayer graphene is over 200m. Interestingly, the shape of bilayer graphene is roughly tetragonal (blade shape) which resembles the symmetry of the substrate. This observation seems to indicate that AOF treatment is preferred for growth of FLG grains with larger grain sized and better thickness uniformity. 4. Conclusions The growth behavior of graphene was studied as a function of symmetry of the copper substrate. Annealing Cu foil under oxygen-rich and oxygen-free condition leads to the domination of Cu (111) and Cu (100) surface orientation, respectively. AOR treatment usually leads to preferential growth of HS_GSC grains, while AOF treatment results in the domination of TS_GSC grains on copper substrate. For TS_GSC grains, an asymmetric


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growth behavior was observed for the first time. The TS-GSC grains were featured by two dendric frontiers and two sharp frontiers. This indicates that the growth of graphene at dendric-border sides is determined by DLA mechanism while is governed by EDA mechanism at sharp-border side. In addition to the single layered graphene grains, the formation of few layer graphene grains was also investigated. AOR procedure usually results in round-shaped FLG grains which are poor in thickness uniformity. Whereas AOF treatment can lead to formation of large-sized bilayer graphene with better thickness uniformity. The low carbon dissolution is supposed to be accounted for the improvement of FLG quality in the case of AOF treatment. Supporting Information Additional information on EDS results of annealed Cu, Raman map of HS_GSC, AFM image and Raman spectroscopy of bunched graphene, graphene coverage on exterior side of graphene/Cu pocket. Acknowledgement The work described in this paper was supported by grants from the National Natural Science Foundation of China (No. 61474077, 11874427 and 51872186) and Natural Science Foundation of Zhejiang Province (No. LY19F040005). H. H acknowledges support


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from the Innovation-Driven project of Central South University (Grants No. 2017CX018) and from the Natural Science Foundation of Hunan province (Grants No. 2016JJ1021). Author Contributions W.-D Dou conceived the experiment, supervised the project. B. -Y Shi, Q. Wang, X. Han, and W. -D Dou conducted the experiments. All authors analysis the data. References 1. Novoselov, K. S.; Geim,A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. 2. Geim,A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. 3. Echtermeyer, T. J.; Lemme, M. C.; Baus, M.; Szafranek, B. N.; Geim, A. K.; Kurz, H. Nonvolatile Switching in Graphene Field-Effect Devices, IEEE Electron Device Lett. 2008, 29, 952–954. 4. Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material., Nat. Nanotechnol. 2008, 3, 270– 274.


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TOC Graphic


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Figure 1 264x117mm (96 x 96 DPI)





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graphic abstract 170x121mm (96 x 96 DPI)


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Asymmetric Growth of Tetragonal-Shaped Single-Crystalline

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