Spatially Controlled Nucleation of Single-Crystal Graphene on Cu

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Spatially-Controlled Nucleation of SingleCrystal Graphene on Cu Assisted by Stacked Ni Ding Dong, Pablo Solís-Fernández, Hiroki Hibino, and Hiroki Ago ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b06265 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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Spatially-Controlled Nucleation of Single-Crystal Graphene on Cu Assisted by Stacked Ni

Ding Dong,† Pablo Solís-Fernández,‡ Hiroki Hibino,§ and Hiroki Ago,*,†,‡

†

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka 8168580, Japan ‡

§

Global Innovation Center (GIC), Kyushu University, Fukuoka 816-8580, Japan.

School of Science and Technology, Kwansei Gakuin University, Hyogo 669-1337, Japan

Keywords: chemical vapor deposition, single-crystal graphene, crystal growth, field-effect transistors

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Abstract In spite of recent progress of graphene growth using chemical vapor deposition, it is still a challenge to precisely control the nucleation site of graphene for the development of wafer-scale single-crystal graphene. In addition, the post-growth patterning used for device fabrication deteriorates the quality of graphene. Herein we demonstrate the site-selective nucleation of single-crystal graphene on Cu foil based on spatial control of the local CH4 concentration by a perforated Ni foil. The catalytically active Ni foil acts as CH4 modulator, resulting in millimeterscale single-crystal grains at desired positions. The perforated Ni foil also allows to synthesize patterned graphene without any post-growth processing.

Furthermore, the uniformity of

monolayer graphene is significantly improved when a plain Ni foil is placed below the Cu. Our findings offer a facile and effective way to control the nucleation of high-quality graphene, meeting the requirements of industrial processing.


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Among various methods to synthesize monolayer graphene, chemical vapor deposition (CVD) on Cu foils is the most promising to produce large-area graphene at relatively low cost.1–3 However, CVD-grown graphene usually consists of a number of grains separated by grain boundaries (GBs), which negatively impact the physical and electronic properties of graphene.1,4,5 Several studies have been devoted to decrease the nucleation density for obtaining large and isolated single-crystal graphene grains.6–13 This can be partly achieved by lowering the CH4 feedstock or pre-treating the Cu surface.11,14 More recently, it was shown that the presence of oxygen in the Cu decreases the graphene nucleation density, allowing to synthesize millimeter and even centimeter-sized single-grains.15–18

However, decreasing the nucleation density

generally results in unpractically long CVD times to obtain a complete graphene coverage.16,17 Compared with the attempt to increase a grain size, little attention has been paid on controlling the position of the graphene nucleation sites on the Cu foil. This lack of control induces the presence of untraceable GBs when the randomly-scattered large-grains are merged to form a continuous graphene layer. Controlling the position of the nucleation allows to obtain single grains either at a predetermined point for a complete coverage without GBs, or at pre-defined positions for the mass fabrication of device arrays and circuits at increased processing speeds. However, this can be a complex task, as graphene nucleation depends on a variety of factors, such as the CH4 concentration,14 the presence of impurities on the Cu,11,12,18–21 surface irregularities and defects,22–25 and the crystal structure of the Cu.26 Patterns of poly(methyl methacrylate) (PMMA) were used as controlled nucleation sites, but the graphene quality was degraded due to contamination from the PMMA.27

Recently, controlled growth of large

graphene grains has been attained by locally feeding the CH4 gas using a gas nozzle and Cu-Ni


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alloy to speed up the growth rate.28 However, this method requires a complicated setup and the use of an alloy as catalyst, which hinder its up-scaling. Here, we report that strategically located Ni foils enable us to produce spatially defined nucleation of millimeter-scale, pure monolayer, single-grain graphene, at a high growth rate. The local tuning of the CH4 feedstock is realized by perforated Ni foil masks, allowing for the localized growth of large graphene grains as well as for the growth of patterned graphene without the need of lithography processes. Moreover, when a plain Ni foil is placed below the Cu, bi-/few-layer graphene areas, which are usually observed in the graphene grown on Cu foils,1,3,13,15–17,29-31 are completely suppressed.

Our method offers a low-cost, simple, and

powerful way to control the nucleation of large single-crystals of monolayer graphene and to directly grow patterned graphene, which are expected to stimulate the development of practical applications of CVD graphene.

Results and Discussion Site-selective synthesis of single-crystal, bi-/few-layer-free monolayer graphene Figure 1a shows a schematic of the process employed for the CVD growth of single-crystalline graphene at predefined positions of the Cu foil. The Cu foil was oxidized just before the CVD, to decrease the graphene nucleation.15 This pre-oxidized Cu foil was then sandwiched between two Ni foils during the CVD, in contrast to conventional CVD growth that uses only a preoxidized (or pristine) Cu foil. The upper Ni foil has a circular hole at the center (diameter varies between 2 – 6 mm, see Supporting Information, Figure S1a,b), and is employed to modify the spatial concentration of CH4 in the surroundings of the Cu. Ni has a higher catalytic activity toward CH4 decomposition and a much higher C solubility than Cu.30 As we shall discuss later,


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this induces a decrease of the CH4 concentration in the vicinity of the Ni. Thus, the local CH4 concentration becomes higher at the center of the Ni hole, triggering the graphene nucleation on the Cu just below the hole, while suppressing it on the rest of the Cu surface. This allows us to control the nucleation site of single-crystal graphene, obtaining a large graphene grain at a specific area of the Cu foil. As shown in Figure 1b, a single graphene grain was obtained in the center of a ~ 17 × 17 mm2 Cu piece by using a Ni foil with a 2 mm-diameter hole. This corresponds to a nucleation density as low as 0.34 nuclei/cm2, which is the lowest value reported for Cu foils as far as we know (see Supporting Information, Table S1).10–18,31,32 It is worth to mention that this density can be further reduced by using larger Cu and Ni foils. By increasing the size of the hole, the growth rate of the graphene can be increased, although the accuracy on the position slightly decreases. However, even when a Ni foil with a relatively large hole (6 mm diameter) was used, a single graphene grain was still obtained, located only ~ 1 mm off the center of the hole (Figure 1c, 60 ppm CH4, 90 min CVD reaction). We checked the reproducibility of this site-selective nucleation.

Figure S2 and Table S2 of Supporting

Information summarize the result of 12 CVD runs with different CH4 concentrations and growth times using a Ni foil with a 6 mm circular hole. We found a reasonably wide range of the growth conditions gives single graphene grain (around 67 % of the performed CVD runs with a 6 mm hole, see Figure S2). There is a tendency that the number of grains increases with CVD time or CH4 concentration (Table S2). However, densities attained were always much lower than those when the top Ni foil was absent (see Supporting Information, Figure S3). When using a smaller hole with 2 mm diameter, a single grain was always obtained under the tested conditions (Supporting Information, Table S3).


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The magnified image of the grain (Figure 1d) clearly shows that the shape is hexagonal, suggesting the formation of single-crystalline graphene. The grain also shows a uniform optical contrast after being transferred onto a SiO2 substrate (Figure 1e), except for the presence of some polymer residues due to the transfer process. This implies that the grain is a uniform monolayer of graphene. As we shall see later, the absence of bi-/few-layer areas is a direct consequence of the presence of the bottom Ni foil. The result of the CVD dramatically changed when no Ni foils were used (Figure 1k), with a large number of graphene grains nucleated at random positions across the Cu surface (Figure 1l; note that for this figure, the CVD time was shortened to avoid a complete merging of the grains). Supporting Information Figure S3 shows that an increase of CH4 concentration increases the total area of graphene. Thus, the almost full coverage of the Cu surface is an indication of a higher local CH4 concentration when no Ni is used (Figure 1l). Another difference is the presence of many bi- to few-layer graphene areas when no Ni foils are employed, as indicated by the optical contrast seen after transfer onto SiO2 (Figure 1m). Thus, our method can offer a facile way to control the nucleation of graphene without the formation of multi-layer grains, which are widely seen in the literature for the graphene grown of on Cu foils.1,11,16,32–34 It should be noted that our approach here is completely different from those reported for graphene growth using Ni-Cu alloys instead of pure Cu foils.28,35–37 In our case, there is only a slight physical contact between the Cu and Ni foils without any external pressure, and thus no substantial alloying occurs between the Ni and Cu foils. Accordingly, they are easily detached from each other after the CVD. This was confirmed by X-ray diffraction (XRD) measurements of the Ni foils after the CVD, as will be discussed later. We also confirmed that the Ni foils can


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be repeatedly used without any decrease on their efficiency nor requiring any further treatment between the successive CVD runs (see Figure S4). By simply arranging multiple holes in the Ni mask at selected positions, it is also possible to obtain several graphene grains on the Cu surface at preselected positions. Figure 1f-j shows one example of three isolated graphene grains grown using a Ni mask having three circular holes. Figures 1f and 1g-j show as-grown graphene grains and those after the transfer, respectively. In spite of the relatively large size of the employed Cu foil (~ 150 mm2), graphene nucleation occurred only at the uncovered areas. This result is essential for speeding up the growth process, allowing to grow single-grain graphene arrays even in a wafer scale, which is promising for automating the processing of graphene for practical applications at industrial scales. Especially interesting is the combination of the multi-holed Ni masks with graphene growth techniques on epitaxial Cu(111) films that allow to control the orientation of the graphene grains, potentially leading to a seamless stitching of the different grains.38 The quality of the graphene grown on the Cu foil sandwiched between two Ni foils was investigated by Raman spectroscopy after transferring to a SiO2 substrate (Figure 2a). The spectra were collected at different positions of the sample surface, indicated by the colored circles in the optical image (Figure 2 inset). No significant D band (~1350 cm-1) was observed in the spectra, except for some specific positions (green spectrum) that correspond to wrinkles formed during the transfer to the SiO2 substrate (Figure 2b). This D-band is usually related to the presence of defects, grain boundaries, and/or wrinkles, and thus its absence clearly indicates the high quality of the graphene.39 The relative intensity of the 2D band (~2680 cm–1) with respect to the G band (~1585 cm–1) was ~2, supporting that the graphene is monolayer (see Figure 2c).30


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To confirm that the isolated graphene grain obtained below the Ni hole is single crystal, lowenergy electron microscopy (LEEM) was performed for an as-grown grain on the Cu foil. An optical image of the grain is shown in the center of Figure 2d. Electron diffraction patterns were collected across the grain surface, at the areas marked by the blue circles. All the diffraction patterns related to the graphene (six diffraction spots marked by dotted yellow circles) are oriented along the same direction, implying the growth of a single-crystal graphene grain. Together with the hexagonal diffraction pattern originated in the graphene, some other additional diffractions spots were also observed (Figure 2d).

These additional diffractions change

depending on the measured position, and are probably originated from the faceting of the underlying polycrystalline Cu foil surface.

Interestingly, the CVD graphene extended

continuously over the grain boundaries of the Cu (see e.g. the dark horizontal line intersecting the graphene hexagonal domain), as reported elsewhere.15

Role of the upper Ni foil In the previous section, we have shown that the graphene growth on a Cu foil can be tuned by the presence of nearby Ni foils. In this section, we will discuss the role of the Ni foil placed above the Cu, and how it enables the site-selective nucleation of graphene. It should be noted here that there is only a slight physical contact between the upper Ni foil and the Cu foil, and due to the roughness of both foils there exists a small gap (< 1 mm) between them. Thus, the effect of this Ni foil can be due to a modification of its environment, in particular a regulation of the local CH4 concentration. To better understand the effect of the upper Ni foil, a CVD was conducted with the perforated Ni foil on top of the Cu but without the Ni foil underneath (see the inset on Figure 3a). Also, it is be noted that the CH4 concentrations used in this section are lower


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than the standard 60 ppm used in the previous, in order to prevent a complete graphene coverage due to the lack of the bottom Ni. In the case of Fig. 3a, oxygen-free Cu foil (denoted as “O-free Cu”) was used to increase the nucleation density of graphene, which enables us to count the spatial distribution of graphene grains within the 6 mm hole. A composed optical image of the Cu after the CVD (26 ppm CH4) can be seen in Figure 3a, where the numbers indicate the average density in grains/cm2 determined for each of the colored sections (the uncolored original image is displayed in Figure S5). This image shows that graphene nucleation is completely suppressed at the Cu areas below the Ni foil. In the uncovered Cu areas, both the nucleation density and the grain size increased with the distance from the edge of the Ni to the center of the hole. Similar behavior can be observed for pre-oxidized Cu foils (denoted as “O-rich Cu”). In that case, to compensate for the low nucleation density observed in O-rich Cu, the Ni foil was placed next to the Cu foil during the CVD (see a schematic of the configuration in Figure 3b). This allows us to study the Cu at longer distances from the Ni (in our case up to 4 cm on the Cu edge on the opposite side of the Ni), compared with the 3 mm in the case of using perforated Ni foils (i.e., 6 mm diameter hole). Figure 3b shows a plot of the nucleation density of graphene on the Cu foil as a function of the distance from the Ni foil, measured for three different CH4 concentrations. The data was collected from the images of the Cu foils after the CVD (Figures 3c and S6), and it shows that graphene nucleation is significantly suppressed near the Ni foil, where no graphene grains were observed. From there, the grain density increases with the distance from the Ni foil until it reaches a saturation value (see the graph in Figure 3b). The distance of saturation decreases with the increase of the CH4 concentration (note that for the lowest CH4 concentration of 15 ppm this saturation point is presumably reached at a distance


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longer than the size of the Cu foil). Additionally, the graphene density increases with the CH4 concentration for any given distance to the Ni. Thus, the maximum densities at a distance of ~ 4 cm from the Ni range from 12 cm-2 to 40 cm-2 for 15 and 25 ppm CH4 concentrations, respectively. This agrees well with our observations in conventional CVD growth (i.e. without using Ni foils), which shows how increasing the CH4 concentration during the CVD increases the density of graphene nuclei (see Supporting Information, Figure S3). To rule out any possible influence of the gas flow direction or temperature gradients along the reaction tube on the observed results, we placed two Ni foils at both ends of the O-rich Cu foil, as presented in Figure S7. The center of the Cu foil showed the maximum graphene density, confirming the essential role of Ni foils on the nucleation of graphene. Combining all the above data, it is apparent that the presence of Ni decreases the local CH4 concentration on its surroundings, thus decreasing the graphene nucleation density without requiring any physical contact with the Cu. As we will show later, this tuning of the local CH4 concentration can be developed to directly grow graphene patterns with any arbitrary shape. It is highly likely that the Ni foil absorbs C atoms from its environment via enhanced CH4 decomposition,40 locally decreasing the amount of CH4 available to the Cu. To verify this role of Ni foil, we have replaced the mask and bottom foils by Cu, which has a much lower C solubility (see Supporting Information, Figure S8). Under the same CVD condition used to obtain a single grain with the Ni foils (Fig. 1c-e), graphene now covered most of the Cu surface, growing even outside of the mask hole. This highlights the importance of using a mask with a high C solubility metal (in the present case, Ni) to achieve the growth of isolated graphene grains at controlled positions. Although other transition metals, such as W, have been used to reduce the graphene grain density on Cu by acting as a sink for C atoms,41 it has not yet been studied the


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possibility to control the nucleation sites. The optical micrographs of the Ni foil after the CVD present a dark color, which support the hypothesis of the C absorption by the Ni (see Figure S9). Here, a relatively high CH4 concentration was used to clarify the changes occurring to the Ni foil during the CVD process (see Experimental Section). The presence of C on the Ni foil after the CVD was further confirmed by a combination of XRD and Raman measurements performed after exposure to different doses of CH4 feedstock. The XRD pattern of a pristine Ni foil showed three diffraction peaks at 2θ = 44º, 51º, and 76º, corresponding to the diffraction from Ni(111), Ni(200) and Ni(220) planes, respectively (Figure 3d). The relative intensity of the Ni diffraction peaks significantly changed after exposure to CH4 during the CVD, suggesting a structural change of the Ni foil with enlargement of Ni (100) and (111) grains normal to the Ni foil during the CVD process. Exposure to low CH4 concentrations did not show evidence of the presence of C on the Ni foils by XRD and Raman, as shown below. The C concentration in Ni after low CH4 exposure was also check by X-ray photoelectron spectroscopy (XPS) depth profiling, but C concentration was below the detection limit after surface etching of ∼5 nm (see Supporting Information, Figure S10). This is not surprising, given that the CH4 concentration used (23 ppm), is well below that commonly used in the literature for growing graphene on Ni foils (CH4 ~ 0.65 %).42 For the medium CH4 exposure doses (by increasing both the CVD time and CH4 concentration), a weak diffraction peak centered at 2θ = 26º started to appear (purple in Figure 3d). This additional peak can be assigned to the diffraction from the graphite (002) plane, which corresponds to the stacking direction of graphite. The intensity of the graphite (002) peak became stronger and an extra graphite (004) peak also became clear when the CH4 dose was further increased (high CH4 exposure, blue data). The presence of these diffraction peaks


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indicates the formation of multi-layer graphene or thin graphitic films on the surface of the Ni foil at the higher CH4 exposures. It is noteworthy that the XRD patterns do not show the presence of Cu in the Ni foil after the CVD nor any shift of the Ni peaks, providing that a substantial alloying has not occurred between the two metals.28 This was confirmed by XPS inspection of the top surface of the Ni mask, which did not show the presence of any Cu (Supporting Information, Figure S10). However, given the high employed temperatures, it can be expected that some of the Cu atoms diffuse to the Ni foil in the areas of contact between the two foils. Examination of these surfaces by XPS showed that some Cu exists on the surface of the Ni foil in contact to the Cu foil on which no Ni was observed (Supporting Information, Figure S11). This is probably due to lower melting temperature of Cu than Ni so that Cu evaporates more easily than Ni. However, we note that in practice this is not an issue, because the same Ni foil can be reused without any decrease on its effectivity, as was already shown in Figure S4. Raman spectra of the Ni foils exposed to the medium and high CH4 doses showed characteristic G and 2D bands (Figure 3e). The appearance of these bands also indicates the formation of few-layer graphene (medium doses) or thin graphite (high doses) on the Ni foil surface. This is related to the well-known catalytic graphitization on Ni, owing to the high catalytic activity and high carbon solubility of Ni metal.40,43 We believe that such high catalytic activity of the Ni metal suppresses the nucleation of monolayer graphene on a Cu surface when Ni is placed close to the Cu foil by modulating the local CH4 concentration. The observed superficial graphene/graphite can be completely removed from the Ni foil by annealing in H2 at 1075ºC, as evidenced by the disappearance of the associated peaks both in the XRD and Raman data (green data in Figure 3d,e).40 Due to a much higher carbon solubility of Ni than Cu, the Ni


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foil can be recycled for the site-selective graphene growth without any additional annealing process after the CVD itself. As shown in Figure S4, we confirmed that a Ni foil can be continuously used for more than 30 times without losing its ability to control the nucleation density on Cu. This is important for industrial-scale processing of graphene, for which the same large-area Ni foils can be repeatedly used without supposing any increase of the manufacturing costs.

Role of the bottom Ni foil Figure 4a compares the nucleation densities of graphene grown on a Cu foil and that with a Ni foil placed below, for different CH4 concentrations. The Ni foil significantly decreased the graphene nucleation density on the Cu surface by a factor of ~ 2 for the same CH4 concentration. In the case of the isolated Cu foil, the Cu surface was mostly covered with graphene for CH4 concentrations higher than 15 ppm, due to the high nucleation density (see Figure S3). On the other hand, the Cu foil with Ni gave lower nucleation densities with isolated graphene grains even at 23 ppm CH4, while coalescence similar to that on bare Cu occurred above 26 ppm (Figure S3). Such low density of graphene nuclei enables us to synthesize isolated singlecrystalline graphene grains with lateral sizes up to ~ 5.7 mm (see Figure 4b), at a relatively high growth rate (~12 µm/min). Further increase of the CH4 concentration from 15 ppm to 20 ppm produced 4 mm hexagonal graphene in only 4 hours (growth rate ~17 µm/min). Note that these growth rates are among the highest reported so far on the growth of isolated, millimeter-sized graphene grains on Cu foils.13,15–17,29,31


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Another interesting effect of the underneath Ni foil is presented in Figure 4c-f. In the samples grown without a Ni foil there are always some bi- and few-layer areas in the monolayer graphene (Figures 4c,d and S12). However, when a Ni foil was placed below the Cu foil we found that the monolayer graphene is completely uniform and free from any bi-/few-layer grains (Figure 4e,f). The presence of bi-/few-layer areas is an issue commonly observed in graphene growth on Cu foils,1,11,16,32–34 and it becomes particularly significant when oxygen is used to decrease the nucleation density33 or the CH4 concentration is increased16. Albeit this can be exploited for the growth of bilayer graphene,16,33,34 the inhomogeneity in the layer number can decrease the quality of monolayer graphene.44 In this work, we demonstrate that by simply using a bottom Ni foil a complete monolayer graphene with sizes as large as 5×5 cm2 can be readily obtained (Figure 4f inset and Figure S13). Finally, it is worthwhile to note that the Ni foil placed below the Cu also prevents the growth of graphene on the backside of the Cu foil (see Figure S14). In conventional CVD using Cu foils, graphene is formed on both sides of the Cu, requiring to remove the graphene from one of the sides by plasma etching or any other method before the transfer to another substrate.45 This additional processing is inconvenient and can potentially damage/contaminate the graphene on the other side. By simply placing a Ni foil below the Cu foil, the graphene formation can be completely suppressed at the backside, which makes the graphene transfer process easier, faster, and more efficient.

Patterned growth of monolayer graphene Our Ni mask method is not limited to the site-selective nucleation of single-crystal graphene. The method can be further applied for the direct synthesis of patterned graphene, as shown in


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Figure 5. This is done by either prolonging the reaction time or increasing the CH4 concentration so that the graphene extends its growth front until the edges of the Ni mask. Thus, we can synthesize graphene patterns with arbitrary shapes by simply placing the appropriate Ni mask on the Cu foil. In addition, as we discussed before, the Ni masks can be used many cycles (> 30 times). Some examples are shown in Figure 5a-e; the patterns of monolayer graphene, such as arrays of circles and squares as well as a word “Graphene”, were easily obtained (Figure 5a-e). Although the graphene is not necessary single-crystalline in this case, it is still important to show the possibility of the direct synthesis of patterned graphene without the aid of any lithography processes, avoiding the post-growth etching of graphene, that increase the complexity of the processing and can deteriorate the quality of the graphene. To demonstrate the potential of this method, an array of graphene field-effect transistors (FETs) was produced without using any lithography processes, including the patterning and deposition of electrodes. The graphene channels for the FET were directly grown on a 4-inch sapphire wafer with a thin Cu film on top, and the metal electrodes were deposited using a shadow mask after transferring the graphene to a SiO2/Si substrate. These devices show the ambipolar transfer curve expected for graphene (Figure 5f).46 The carrier mobility was 2,000 – 3,000 cm2/Vs at 300 K probably due to the difficulty of transferring such large graphene (the FET channel length and width are millimeter scale) without wrinkles and impurities.

Mechanism of site-selective nucleation of monolayer graphene From the experimental observations, the mechanism of site-selective nucleation of monolayer graphene on the stacked Cu catalyst is presented in Supporting Information (Figure S15). The key of our strategy is to utilize a perforated Ni foil mask on top of the Cu to locally control the


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carbon source supply. The high catalytic activity of Ni metal towards CH4 decomposition results in a selective nucleation of graphene at center of the Ni hole (arrows 1 and 2 in Figure S15). On the other hand, the underlying Ni foil can trap the excess carbon species diffused from the top surface of Cu, possibly through GBs of the Cu foil. In addition, the presence of the bottom Ni foil prevents the bottom surface of the Cu to act as carbon supply to the top surface, as evidenced by the absence of graphene on the backside.

These effects are expected to suppress the

formation of bi-/few-layer graphene and reduce the nucleation density of monolayer graphene (arrows 3 and 4).

Conclusions We demonstrate the possibility to control the location of single-crystal graphene grains. This was achieved by stacking a perforated Ni foil mask on top of the Cu foil during the CVD. The presence of the Ni foil tunes the local concentration of CH4, decreasing the nucleation density as well as controlling the nucleation sites on the Cu foil. This allowed us to increase the CH4 concentration, speeding up the growth rate up to values comparable to the best ones obtained so far. The presence of a second Ni foil below the Cu completely suppressed the formation of bi/few-layer graphene in monolayer graphene. This also inhibited the growth of graphene on the back side of the Cu foil. Together, the use of the Ni foils realized one of the lowest reported nucleation densities for graphene on Cu foils (< 0.34 nuclei/cm2), while adding the advantage of the control in the position. The top Ni foil can also be used for the direct growth of patterned graphene without relying on any lithographic processes. These advantages of our methods are expected to promote the development of practical applications of CVD graphene.


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Experimental Section: Sample preparation. Cu foils were received from Nilaco Co. (99.9 % purity, 80 µm thick) and Tanaka Kikinzoku Kogyo Co. (electro-polished, 99.9% purity, 80 µm thick). Nilaco Cu foils were chemically polished before its use by immersion in an ammonium persulfate (APS) etchant solution in water (0.2 M) for 3 min, and then rinsed in deionized water. The surface of the Cu foils was oxidized by heating at 250 °C for 10 min in air prior to the graphene CVD. Energy Dispersive X-Ray Spectroscopy (EDX) measurements were performed to estimate the oxygen concentration for pristine and the pre-oxidized Cu foils.

The surface oxygen

concentration of the pristine Cu foil (~10 at.% due to surface oxidation and impurity) increased to ~30 at.% after the pre-oxidation. Thin Cu films (2 µm) were obtained by radio-frequency magnetron sputtering (Shibaura Mechatronics Corp. CFS-4ES) on c-plane sapphire substrates. Ni foils were purchased from Nilaco Corporation (99+ % purity). The Ni foils were used without any pretreatment. The Ni-Cu-Ni sandwich structure was made by placing the Cu foil on the top of a piece of Ni foil (200 µm thick) with a similar size. Then, a Ni foil (50 µm, 80 µm, or 200 µm thick) with pre-designed holes was placed on top of the Cu foil, with gaps below 1 mm. Graphene synthesis. The Ni-Cu-Ni sandwich structure was loaded into the CVD chamber, and graphene was grown by ambient pressure CVD with CH4, H2, and Ar gases at 1075 °C. During the heating up to the reaction temperature (1075 °C), only Ar was flowed. After reaching 1075 °C, H2 (2%) and CH4 (concentration varied from 10 to 200 ppm) were added to the Ar. The growth time was set from 30 to 480 min, depending on the required size of the graphene grain. Finally, the reaction was quenched by rapidly taking out the substrate from the heating zone, followed by pumping of the chamber to vacuum.


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For the experiments shown in Figs. 3d,e and S9, we used the following conditions. Low CH4 dose: CH4 23 ppm, reaction time 120 min. Middle dose: 95 ppm, 120 min. High dose: 95 ppm, 240 min. Graphene transfer. For the transfer from the Cu foil, the graphene surface was covered with a PMMA film by spin coating. After removing the Cu film by immersing in an aqueous solution of APS, the PMMA/ graphene stack was transferred onto a SiO2/Si wafer. Finally, the PMMA film was removed by dipping the substrate into acetone. Characterizations. Raman spectra were measured with a Nanofinder30 (Tokyo Instruments) using a 532 nm excitation laser. The crystallinity and crystallographic orientation of Ni foils were measured by an X-ray diffractometer (XRD) (RIGAKU RINT-III) using Cu-Kα radiation (1.5418 Å). Crystal orientations of the as-grown graphene films were characterized by LEED in an ultrahigh vacuum chamber (8 × 10−9 Pa) using a BDL600IR (OCI, Canada). The chemical composition of the Ni and Cu foils were measured by X-ray photoelectron spectroscopy (XPS, Kratos AXIS-165). The depth profile was measured by etching the foils with Ar ions. Backgated graphene FETs were made with a transferred graphene pattern using Au metal electrodes deposited thorough a shadow mask.

Transport properties were measured using a B1500A

semiconductor analyzer (Keysight Technologies) at room temperature and under vacuum conditions (∼1.5 × 10−4 Pa).


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Supporting Information available: Complementary optical images, Raman, and other related data. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author：： *E-mail: [email protected]

Acknowledgements: This work was supported by JSPS KAKENHI Grant Number JP15H03530, JP15K13304, JP16H0091, New Energy and Industry Technology Development Organization (NEDO), and PRESTO-JST. D.D. acknowledges the receipt of the MEXT scholarship.


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Figure 1. (a) Schematic of the production of a spatially-controlled, single-crystal graphene grain on a Cu foil. The Cu foil is sandwiched between two Ni foils during the CVD, which control of the local CH4 concentration. (b) As-grown, mm-sized single graphene grain obtained by using the method shown in (a). The position of the hole (2 mm diameter) in the upper Ni foil is indicated by the white circle. Graphene was visualized by slightly oxidizing the Cu foil after the CVD growth. (c) A graphene grain obtained by using a Ni mask with a 6 mm hole. Magnified images before (d) and after the transfer (e). (f-j) Simultaneous site-selective nucleation of three graphene grains on a 15 × 10 mm2 Cu foil by a Ni mask with three holes. Images show the asgrown grains (f) and those after the transfer (g). (h-j) shows magnified images of the individual grains from (g). (k) Schematic of a conventional CVD growth of graphene on Cu foil without the assistance of Ni foils. (l) As-grown graphene using the same CH4 concentration as in (c-e) but without placing Ni foils on either side of the Cu. (m) Optical micrograph of the sample shown in (l) after the transfer. The graphene is monolayer but contains many bi-/few-layer graphene areas.


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Figure 2. (a) Raman spectra of a single graphene grain grown on Cu sandwiched by Ni foils, after transfer on SiO2. Each spectrum was collected from the spots marked in the optical image with the same color. Upper-right side of the image (dark) is the graphene, while the lower-left side (light) is the bare SiO2. (b,c) Raman mapping images of the same area shown in (a). Intensity ratios of ID/IG (b) and I2D/IG (c). (d) Optical image of an as-grown graphene grain (center) and LEED patterns collected at the marked positions.


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Figure 3. (a) Composite optical microscope image of an O-free Cu foil after CVD with a top Ni foil having a 6 mm hole. The edge of the Ni hole is located over the external colored circle. The numbers indicate the densities of graphene grains in cm-2 measured at the colored areas. Inset shows a schematic of the sample configuration. (b) Schematic of the setup to determine the effect of Ni on the nucleation density, with a Ni foil located on one side of a 4 cm-long O-rich Cu foil. The graph shows the nucleation density of graphene on the Cu foil as a function of distance from the Ni foil for three different CH4 concentrations. (c) Optical image of the O-rich Cu foil after the CVD with 20 ppm CH4. (d) XRD profiles and (e) Raman spectra collected on a 50 µm-thick pristine Ni foil (black curves), after being exposed to increasing CH4 doses for different CVD procedures while located on top of a Cu foil (red, purple and blue, in the order of increasing CH4 dose), and after being annealed in H2 at high temperatures (green).


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Figure 4. (a) Plot of the graphene nucleation density against the CH4 concentration for Cu foil with (blue) and without (red) a bottom Ni foil (growth time 2 h). (b) Optical micrograph of an isolated 4 mm graphene grain, obtained by placing a Ni foil below the Cu and extending the growth time to 6 h with a CH4 concentration of 15 ppm. Insets in (b) show as-grown isolated grains on Cu, with sizes of (top) 5.7 mm by extending the growth time to 8 h with 15 ppm CH4, and (bottom) 4 mm obtained in 4 h by increasing the CH4 concentration to 20 ppm. (c-f) Optical images of graphene grown without (c,d) and with (e,f) a bottom Ni foil, while the rest of the growth conditions are essentially the same (CH4 concentration 20 ppm, reaction time 1 h, 1060 ºC). (d) and (f) show the magnified images marked in squares of (c) and (e), respectively. Inset of (f) shows a 4-inch SiO2 wafer with a transferred 5×5 cm2 pure single-layer graphene grown with a bottom Ni foil.


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Figure 5. (a,b) Directly grown arrays of graphene circles (a) and squares (b) by using Ni masks. The graphene patterns are visualized by post-growth oxidation of the Cu foil. (c-e) Patterned growth of the word “Graphene” by using a corresponding Ni mask. (c) is the corresponding Ni mask. (f) Transfer curve of a graphene FET from a directly patterned array of FET devices on a 4-inch Cu wafer. Top inset shows the wafer after the CVD. Bottom insets show an enlargement of the graphene squared on the top inset, and an image of a device fabricated using no lithography processes.


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