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Highly Uniform Bilayer Graphene on Epitaxial Cu−Ni(111) Alloy Yuichiro Takesaki,† Kenji Kawahara,‡ Hiroki Hibino,⊥,¶ Susumu Okada,# Masaharu Tsuji,§ and Hiroki Ago*,†,‡,∥ †
Interdisciplinary Graduate School of Engineering Sciences, ‡Art, Science, and Technology Center for Cooperative Research (KASTEC), and §Research and Education Center of Carbon Resources, Kyushu University, Fukuoka 816-8580, Japan ⊥ Kwansei Gakuin University, Hyogo 669-1337, Japan ¶ NTT Basic Research Laboratories, NTT Corporation, Kanagawa 243-0198, Japan # Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan ∥ PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Band gap opening in bilayer graphene (BLG) under a vertical electric field is important for the realization of high performance graphene-based semiconductor devices, and thus, the synthesis of uniform and large-area BLG is required. Here we demonstrate the synthesis of a highly uniform BLG film by chemical vapor deposition (CVD) over epitaxial Cu−Ni (111) binary alloy catalysts. The relative concentration of Ni and Cu as well as the growth temperature and cooling profile was found to strongly influence the uniformity of the BLG. In particular, a slow cooling process after switching off the carbon feedstock is important for obtaining a uniform second layer, covering more than 90% of the total area. Moreover, low-energy electron microscopy (LEEM) study revealed the second layer grows underneath the first layer. We also investigated the stacking order by Raman spectroscopy and LEEM and found that 70−80% of bilayer graphene has Bernal stacking. The metastable 30°-rotated orientations were also observed both in the upper and lower layers. From our experimental observations, a new growth mode is proposed; the first layer grows during the CH4 supply on Cu−Ni alloy surface, while the second layer is segregated from the bulk alloy during the cooling process. Our work highlights the growth mechanism of BLG and offers a promising route to synthesize uniform and large-area BLG for future electronic devices.
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does not require control of edge structures like GNRs,17 making it a good candidate for practical applications. However, synthesis of wafer-scale BLG with high uniformity is still a challenging issue. Chemical vapor deposition (CVD) growth of uniform BLG has been reported on Cu foils by controlling the growth conditions,18−23 in spite that the low carbon solubility of Cu is known to selectively give MLG.24 However, this self-limiting growth mechanism on Cu catalyst makes it difficult to obtain a uniform BLG sheet, and in most of the previous works, smaller bilayer grains are observed mainly at the center of hexagonal grains of MLG,25−30 which indicates slower growth rate for the second layer as compared to the first layer. Once the first layer fully covers the Cu surface, the second layer growth ceases because the catalytically active Cu surface is blocked from CH4 gas. Thus, it is not straightforward to synthesize uniform BLG on Cu catalyst. Also, there is
INTRODUCTION
Extraordinary high carrier mobility as well as unique single atom-thick layered structure makes monolayer graphene (MLG) an attractive material for future electronic and photonic devices with high mechanical flexibility and optical transparency such as field-effect transistors, flexible devices, integrated circuits, chemical sensors, and optical communication.1,2 However, the absence of a band gap in the MLG hinders the development of graphene-based semiconductor devices. Thus, several methods to open a band gap in graphene have been proposed such as synthesis of one-dimensional graphene nanoribbons (GNRs),3 applying mechanical strain,4 chemical functionalization or molecular doping,5 and vertical biasing in bilayer graphene (BLG).6−13 It has been demonstrated that in the presence of a vertical electric field applied from both top and bottom dielectric gates, a band gap of BLG can be opened to be 0.2−0.3 eV.6−8 Compared to the other mentioned methods, vertical biasing in BLG is suitable for the fabrication of devices in large areas using top-down methods. Also, BLG is mechanically and chemically more robust than MLG14−16 and © XXXX American Chemical Society
Received: March 21, 2016 Revised: April 30, 2016
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DOI: 10.1021/acs.chemmater.6b01137 Chem. Mater. XXXX, XXX, XXX−XXX
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diluted H2 flow, which improves crystallinity and surface flatness of metal catalyst.41 As we will see later, this process also induces the Cu− Ni alloy formation. Step 2 is the carbon supply with 0.1% CH4 feedstock, and effects of the growth temperature and time were studied. Step 3 is optimized by changing the cooling rate as well as by pausing the cooling at 850 and 750 °C for a certain period of time to enhance the segregation of the second graphene layer. The optimized growth condition is presented in the Supporting Information (Figure S1). After the CVD, as-grown graphene was transferred by a standard poly(methyl methacrylate) (PMMA)-assisted transfer method using an etching solution (aqueous solution of iron chloride (FeCl3) or ammonium persulfate ((NH4)2S2O8)) to dissolve the catalyst film.40 After etching the metal film, PMMA/graphene stack was transferred onto a SiO2(300 nm)/Si substrate. Then the PMMA was removed by dipping in hot acetone. Finally, the sample was thoroughly washed with a dilute HCl (10%) solution and ultrapure water. The catalyst metal films were characterized by X-ray photoelectron spectroscopy (XPS, Kratos AXIS-165), scanning electron microscope (SEM, HITACHI S-4800) equipped with an energy-dispersive X-ray spectroscope (EDS, Bruker QUANTAX FlatQUAD), and X-ray diffraction (XRD, RIGAKU RINT-III). Crystal orientations of asgrown graphene films were characterized by LEEM (Elmitec LEEM III). For the LEEM measurements, an as-grown graphene sample was vacuum annealed prior to the measurement to clean the graphene surface. Transferred graphene films were analyzed by optical microscopy (Nikon ECLIPSE ME600) and by Raman spectroscopy (Tokyo Instruments, Nanofinder30) using 532 nm excitation. The spatial distribution of the number of graphene layers was determined from the contrast of optical microscope images; the intensity of the green component was used to estimate the layer number.42 The optical transmittance was measured by UV−vis spectrometer (JASCO V-570) for the graphene sheet transferred on a quartz substrate. The relative stabilities of a graphene sheet on Cu(111) and Cu− Ni(111) films were studied by the DFT with the generalized gradient approximation (GGA)43 by combining with a van der Waals correction, a plane-wave basis set (25 Ry), and ultrasoft pseudo potentials.44 Graphene sheet, modeled by a planar π-conjugated molecule (C54H18), is adsorbed on metal surfaces simulated by four atom-thick metal slabs with various molecular orientations while fixing graphene−metal spacing to 0.35 nm under the open boundary condition normal to the surfaces.45
discussion on the growth mechanism of CVD-grown BLG in terms of the growth order of the upper and lower layers; some groups claim that the second layer grows on top of the first layer,19,20,22,23 while other groups claim the second layer grows below the first layer.26,27,31−33 Binary metal catalysts, such as Cu−Ni and Cu−Co foils, have also been investigated to obtain uniform BLG,34−37 which are expected to overcome the limit of self-limiting property of Cu catalyst, because Ni and Co have higher carbon solubility than Cu. However, the addition of Ni (or Co) to Cu catalyst can deteriorate the controllability of layer numbers because in principle the number of graphene layers tends to have some distribution ranging from monolayer to multilayers. Being different from monolayer growth on Cu, there is no reason to terminate the growth by the second layer so that the alloy catalysts sometimes result in the formation of nonuniform layer distribution with the main peak at the bilayer.35 Therefore, it is quite challenging to selectively synthesize BLG by CVD while suppressing the growth of monolayer and >3 layer graphene. In addition, commercially available metal alloy foils have prefixed metal concentrations, and thus, it is difficult to finely tune the BLG coverage.34 Furthermore, the investigation into the mechanism of selective growth of BLG is important for future wafer-scale production with high reproducibility. Stacking order in BLG is also important. Most of the previous CVD studies rely only on Raman spectroscopy, and the relative intensity of 2D band to G band (I2D/IG) and full line width of half-maximum of 2D band (FWHW2D) are used to distinguish the AB (Bernal) and twist stacking.20 However, the interlayer twist angle can be more complicated, which is sometimes difficult to judge only by the Raman spectroscopy.30,37,38 Here, we demonstrate the uniform synthesis of BLG on epitaxial Cu−Ni (111) alloy films deposited on sapphire by optimizing the Cu−Ni ratio as well as the growth conditions. Uniform BLG with 93% coverage was realized. The low-energy electron microscope (LEEM) measurement indicates that the second layer grows underneath the first layer. Density functional theory (DFT) calculations were also performed to understand the interaction between metal surface and graphene. On the basis of these results, we discuss the growth mechanism and stacking order of our BLG on Cu−Ni(111) alloy catalyst.
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RESULTS AND DISCUSSION CVD Growth of BLG on Cu−Ni Films. First, we examined the effect of the sputtering order of the Cu and Ni films on the sapphire c-plane (α-Al2O3(0001)) substrate. Here we used sapphire to prepare epitaxial metal films, as reported previously for Cu(111)/sapphire.39,40 Since sapphire is widely used as a substrate for GaN-based light emitting diodes,46 large sapphire wafers have become available with relatively low cost. Thus, sapphire can become an ideal substrate for high-quality graphene growth for advanced electronic applications. While fixing the nominal Ni ratio at 20%, we prepared two types of catalysts by changing the sputtering order, Cu400/Ni100/ sapphire and Ni100/Cu400/sapphire (the subscripts indicate the thickness in nm of the metal films; for example, Cu400 indicates 400 nm-thick Cu). We found that the deposition of Ni followed by Cu (Cu400/Ni100/sapphire) produces a much smoother surface after the graphene growth than the opposite order (Ni100/Cu 400 /sapphire). The result is shown in Supporting Information, Figure S2. As we will discuss later, at the high processing temperature during the CVD, these two metals diffuse into each other and form a Cu−Ni alloy film. In the case of Ni100/Cu400/sapphire, most of the surface Ni atoms diffuse into the bulk of the Cu film by replacing Cu atoms, thus producing a very rough surface. Roughness of the catalyst is
EXPERIMENTAL METHODS
For the growth of BLG, the stacked films of Cu and Ni deposited on cplane sapphire (α-Al2O3(0001)) were used as catalyst.39,40 Thin films of Cu and Ni were sequentially deposited by radio frequency (RF) magnetron sputtering machines (Shibaura 4ES and i-Miller) without being exposed to air to avoid oxidation at the interface between these two metal films. The substrate temperature was set at ∼420 and ∼200 °C for the deposition of Ni and Cu films, respectively, to obtain high crystallinity and smooth surface while avoiding surface oxidation. We varied the Cu/Ni nominal ratio by changing the thicknesses of the Cu and Ni films, while the total thickness was fixed to 500 nm. For example, to prepare 20% Ni, 100 nm Ni and 400 nm Cu were deposited. This nominal 20% Ni corresponds to 21.2 atomic % of Ni if we assume that both the sputtered Cu and Ni films are single crystals with different lattice constants. However, as the actual density of each sputtered film is not clear, we use the nominal thickness throughout this paper. For the growth of BLG, the substrate was set in a tubular reactor for the ambient pressure CVD using CH4, H2, and Ar gas. The growth process consists of three steps; (1) H2 annealing, (2) CH4 supply, and (3) cooling steps. Step 1 is annealing at both 500 and 1000 °C in a B
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Figure 1. (a) Schematics of the CVD growth of uniform bilayer graphene over a Cu−Ni alloy film deposited on a sapphire c-plane substrate. (b−d) Optical microscope images of graphene transferred on SiO2/Si substrates grown under different CVD conditions. (b) Temperature dependence for Cu-80%/Ni-20% catalyst. (c) Effect of slow cooling process instead of the rapid cooling used in panel b. (d) Optimization of the Ni concentration for the condition optimized in panels b and c. (e) Optical transmission spectra of BLG (red curve, Ni-22%/Cu-78%) and MLG (blue curve, pure Cu) transferred on quartz substrates. Inset shows the photograph of the samples on a SiO2/Si substrate.
temperature (1075 °C) is smaller than that grown at the lower temperature (1000 °C). We think that this can be ascribed to the improved crystallinity of the Cu−Ni alloy catalyst produced at high temperature. We have previously reported that the graphene segregation from a Co film is strongly affected by the crystallinity of the Co metal.41 A highly crystalline Co film gives much more uniform monolayer graphene than a polycrystalline Co film. Therefore, we speculate that the high CVD temperature enhances the crystallinity of the Cu−Ni thin film, giving a more uniform layer distribution.48 Since the graphene growth on high carbon solubility metals like Ni is driven by a segregation process,49 the cooling profile strongly influences the distribution of the number of graphene layers and film uniformity. In particular, we found that changing the cooling rate from the standard rapid quenching (i.e., immediately taking out the sample from the hot area) to a slow cooling (5 °C/min) dramatically increases the total BLG area up to 80% (see Figure 1c, left). The uniform segregation is consistent with previous report49 showing that relatively uniform graphene films can be obtained on Ni foil when the cooling rate is decreased. Previous in situ LEEM observations suggest that the segregation of the second layer takes place at around 850 °C.50 Thus, we examined the effect of keeping the
known to negatively affect the quality of the synthesized graphene.41,47 Therefore, we have intensively studied Cu/Ni/ sapphire substrates for the catalytic synthesis of BLG. The growth process of BLG on the Cu/Ni/sapphire substrate is schematically illustrated in Figure 1, panel a. Figure 1, panels b−d summarize the results of our systematic investigation of the growth conditions. The distribution of the number of graphene layers determined from the optical images is shown as a pie graph for each condition. We first studied the effect of the growth temperature (Figure 1b). CVD temperatures of 1000 °C resulted in BLG coverage reaching 70%. However, the BLG grains are relatively small, and most of them contain areas of tri- or multilayer graphene, which make it difficult to fully cover the catalyst surface only with BLG. We found that increasing the growth temperature to 1075 °C strongly suppresses the formation of tri- or multilayer graphene, although the obtained BLG coverage is lower (25%). Generally, a high CVD temperature increases the carbon solubility in metal films and also enhances the catalytic dissociation of the CH4 feedstock, both of which are expected to increase the total amount of segregated carbon atoms, leading to multilayer graphene. However, as can be seen in Figure 1, panel b, the total area of bi- and multilayer graphene grown at the high C
DOI: 10.1021/acs.chemmater.6b01137 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials sample at a constant temperature of 850 °C for 1 h during cooling down the sample. Introducing this additional step in the cooling process further improved the BLG uniformity, which resulted in a BLG coverage of 90% (Figure 1c, right). Finally, the Ni concentration was tuned to obtain the highest BLG coverage, as shown in Figure 1, panel d. While increasing the nominal Ni concentration to 25% gave a larger portion of tri- and multilayer graphene flakes, a Ni concentration of 22% resulted in the growth of uniform BLG with a 93% coverage. This indicates that a precise control of the Ni concentration is quite important for producing high BLG coverage. In the case of commercially available Cu−Ni alloy foils, only predefined Ni concentrations are available. However, our sputtering method for producing Cu/Ni films offers a higher controllability, as the film thicknesses of the Cu and Ni films can be changed independently. This, together with the higher crystallinity arising from the use of the sapphire substrate, allows us to obtain improved results to those obtained using conventional Cu−Ni alloy foils. From our experimental results, we found that the cooling profile and the Ni concentration are the most influential parameters for increasing the BLG coverage. It should be also noted that for our growth conditions, the catalyst surface is always fully covered by a continuous monolayer graphene layer (see Figure 1b,d), suggesting different growth modes for the first and the second layers. The uniform growth of BLG was further confirmed by optical transmittance measurements after transfer on a quartz substrate, as shown in Figure 1, panel e. Our BLG sheet showed a transmittance of 95.9 ± 0.6% for 550 nm wavelength, while for a MLG sheet grown on Cu(111)/sapphire, the transmittance increases to 97.9 ± 0.6%. These values are consistent with the theoretical absorption values of 2.3% and 4.6% for MLG and BLG, respectively.51 Inset of Figure 1, panel e shows a photograph of a MLG and a BLG sheet transferred on a same SiO2(300 nm)/Si substrate. The difference in optical contrast can be clearly appreciated from this photograph. The reproducibility of our method was confirmed by different CVD runs (see Figure S3 of the Supporting Information) and different sputter batches. Therefore, our method offers a versatile and reliable way to synthesize uniform BLG in large scale. Characterization of Cu−Ni Alloy Catalysts. The inset of Figure 2, panel a shows the photograph of an as-sputtered Cu/ Ni/sapphire (22%-Ni) and after the graphene growth. The assputter film shows the characteristic color of metallic Cu, as the Cu film is located over the Ni film. After the CVD, the film presents gray-colored surface, indicating the Ni diffusion into the upper Cu film. This observation was supported by the XPS depth-profile measurements (see Figure 2a,b). Surfaces of the films were etched by an Ar-ion beam, and XPS spectra were collected at different depths to obtain the profiles. The etching rate was calibrated by another experiment using a thin bare Cu film. In the as-sputtered film (Figure 2a), the substrate was mostly composed of Cu down to the studied depths (∼180 nm), which were below the Cu film thickness (∼390 nm). Some trace amounts of C atoms are present on the surface, which may come from impurities (such as adsorbed CO2 and organic molecules) due to the exposure after the sputtering. The Ni concentration significantly increased after the graphene growth with the relative concentration of Cu and Ni becoming almost constant in the measured depth (see Figure 2b). A similar behavior was observed in the elemental mappings for the cross-sections of the substrate measured by EDS (Figure
Figure 2. Depth profiles of elements measured by XPS for the (a) assputtered Cu/Ni film and (b) that after the CVD for BLG growth. Blue, red, and black plots show Cu, Ni, and C, respectively. Inset of panel a is a photograph of the samples. The surface color of the assputtered film (left) changed to silver after the CVD (right), reflecting the formation of Cu−Ni alloy. (c) Cross-sectional EDS mapping images for Cu and Ni. (d) XRD profiles of the as-sputtered film and that after the BLG-CVD.
2c). After the CVD, EDS mapping showed a uniform distribution for both Cu and Ni throughout the whole film depth. These results clearly indicate the interdiffusion of Ni atom to the Cu layer (and vice versa), which resulted in the formation of Cu−Ni metal alloy. Because the diffusion is a thermally activated process, the diffusion coefficient, D, of Ni (or Cu) atoms in a Cu (Ni) film can be expressed by the following equation:52 ⎛ Q ⎞ ⎟ D = D0 exp⎜ − ⎝ RT ⎠
(1)
For the “Ni in Cu”, D0 and Q are 2.7 cm /s and 236 kJ/mol, respectively.52 For “Cu in Ni”, D0 and Q are 5.7 cm2/s and 258 kJ/mol, respectively. From these values, the diffusion coefficients can be determined for the CVD temperature (1075 °C); the diffusion of Ni in the Cu matrix is 1.9 × 10−9 cm2/s, while Cu diffusion in Ni is 5.7 × 10−10 cm2/s. Therefore, the diffusion coefficient of Ni in Cu matrix is about three-times higher than that of Cu in Ni matrix. This can be another reason why the Ni/Cu/sapphire showed a rougher surface after the CVD (see Figure S2). One interesting point of Figure 2, panel b is that a small amount of C atoms with a concentration of 1−2% was observed in the bulk of the Cu−Ni alloy film, clearly contrasting with the behavior seen before the CVD (Figure 2a). This shows that a significant amount of C is being dissolved into the Cu−Ni metal alloy during the CVD process. As discussed later, we speculate that such dissolved C atoms in the bulk of the alloy contribute to the growth of large-area BLG. We also measured the XRD patterns for the metal films, as shown in Figure 2, panel d. The as-sputtered Cu/Ni/sapphire substrate showed diffraction peaks at 43.4° and 44.6°, which are assigned to the face-centered cubic (fcc) (111) diffractions from Cu(111) and Ni(111) planes, respectively. After the CVD, only one sharp peak lying between these two diffraction peaks was observed at 43.7°. According to the previous XPS 2
D
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Figure 3. (a) Optical micrograph of transferred BLG and (b) the corresponding Raman mapping images of the relative intensity of 2D band to G band (I2D/IG). (c) Raman spectra marked in panel b. Inset shows the 2D band shape of the spectrum 2, which is fitted with four Lorentzian curves (the gray broken lines). The fitted curve is shown by the red broken line. Distribution of (d) I2D/IG ratio and (e) fwhm2D measured for 300 different spots of a transferred graphene sheet. Red and gray data show AB stacked and twist BLG grains, respectively.
Figure 4. (a) BF-LEEM image and (b−d) corresponding diffraction patterns. The diffraction was measured at the points 1, 2, and 3, marked in panel a. In , panels b−d, “1L G” denotes the diffraction from monolayer graphene, while “upper G” and “lower G” are those from upper and lower graphene sheets of bilayer graphene, respectively.
intense 2D band compared to the G band (point 3 in the mapping). As MLG also shows a more intense 2D band than G band, its results are difficult to differentiate between MLG and twisted BLG areas only from Raman mapping. However, comparison with the optical micrograph shows no difference in the contrast between these areas. This indicates that the thickness of the graphene is the same and they both correspond to BLG, thus suggesting the presence of islands of twisted BLG in addition to the AB-stacked BLG that covers most of the Cu− Ni surface. Raman spectra measured at the points marked in Figure 3, panel b are shown in Figure 3, panel c. The intensity of defect-related D band was very low in our BLG sheet regardless of the stacking type, signifying the growth of highquality BLG with low defects. The stacking type of BLG also influences the shape and width of Raman 2D band (fwhm2D).20 We observed that the AB-stacked BLG showed a broad 2D band with a fwhm of 44− 63 cm−1, while for twisted BLG, the fwhm was 38−46 cm−1. As shown in the inset of Figure 3, panel c, the 2D band of the ABstacked area can be deconvoluted into four Lorentzian components, as in the case of exfoliated BLG flakes.53 This also proves the formation of AB-stacked BLG. The statistical distributions of I2D/IG and fwhm2D are plotted in Figure 3,
and EDS observations, this peak can be assigned to the diffraction from the Cu−Ni(111) alloy. Therefore, a Cu−Ni alloy having fcc(111) plane is formed during the CVD process, allowing for the presence of a certain amount of C atoms in the alloy bulk for the effective BLG growth. The relative concentration of Ni is calculated to be 25% from Vegard’s law, which is a similar value to that obtained from the XPS depth profile. We speculate that the slight discrepancy from the initial value is due to evaporation of some surface Cu atoms during the CVD, as the Cu melting temperature (1084 °C) is close to the CVD temperature (1075 °C), while that of Ni (1455 °C) is much higher. Stacking Order in CVD-Grown BLG. Figure 3, panels a and b show the optical micrograph and the corresponding Raman mapping image of the intensity ratio of the 2D and G bands (I2D/IG) measured for a transferred BLG sheet. It is acknowledged that Raman spectroscopy can be used to determine the stacking type in BLG, mainly from the I2D/IG ratio.20,53 In brief, this ratio is below 1 for AB stacking, while for twisted BLG, the ratio is usually over 1 (in most cases the ratio is ∼2).20 From Figure 3, panel b, it is seen that most of the area corresponds to AB stacking, represented by blue color (points 1 and 2 in the mapping image). Some isolated areas show a more E
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Figure 5. LEEM analysis of the CVD-grown BLG sheet on a Cu−Ni thin film. (a−c) BF LEEM images. (a) Uniform BLG. (b) Different area with MLG, which appears dark. (c) Magnified image of white circle in panel b. (d) Reflection spectra of spots 1, 2, 3 shown in panel c. (e−h) Top rows: LEEM images measured with different diffraction angle conditions. The colored areas indicate the upper/lower layers with rotation angles with respect to the original Cu(111) diffraction. Areas marked with white lines indicate the grains, which have Bernal (AB) stacking. Middle rows: Selected area diffraction taken from the points marked in the top rows. Bottom images of panels e−h show orientations of upper and lower graphene layers determined for the positions A−D from the corresponding diffraction patterns. All scale bars are 5 μm.
panels d and e. For fair evaluation, we collected Raman spectra at 300 different spots from a 1.5 mm × 1.5 mm area of a transferred graphene sheet. From these distributions, we conclude that 70−80% of the BLG has AB stacking, while the remaining has twisted stacking. To further determine the orientation of our BLG, we measured LEEM for the as-grown sample on the Cu−Ni(111) alloy film. This technique allows us to precisely determine rotation angles between the graphene layers by measuring electron diffraction patterns, which cannot be obtained by Raman spectroscopy. In addition, LEEM measurement provides information about the graphene orientation with respect to the underlying Cu−Ni(111) lattice. Figure 4 shows examples of electron diffraction patterns from mono- and bilayer graphene grown on the Cu−Ni(111) alloy. It should be noted that this is not a representative area of the sample. Instead, an area including different structures (monolayer, AB and twisted bilayer graphene) has been selected to demonstrate the effectiveness of the LEEM measurement. At the brightest area (position 1) in Figure 4, panel a, where monolayer graphene is present (the number of layer was confirmed by independent measurement of the electron reflection spectrum (see Figure 5d)), we observed six strong diffraction spots
originating in monolayer graphene (Figure 4b). In addition, weak diffraction spots are also seen at the position 1, which consist of three brighter and three darker spots. These spots with different intensities originate in Cu−Ni(111) surface, reflecting the three-fold symmetry of the fcc(111) plane. The relatively weaker diffraction from the Cu−Ni compared with graphene can be explained by a screening effect of incident electrons by the graphene layer before reaching the Cu−Ni surface as well as the screening of scattered electrons from the Cu−Ni surface by the graphene layer. The above results indicate that the as-grown MLG is rotated 30° from the Cu−Ni lattice. At the position 2 (Figure 4c), only one set of six diffraction spots was observed, with identical orientation to that of MLG (Figure 4b). Here, no diffraction spots from the Cu−Ni lattice are present due to an enhanced electron screening effect by BLG. Therefore, at the position 2, BLG has AB stacking with both graphene layers rotated from the Cu−Ni lattice by 30°. At the position 3, a new set of diffraction spots rotated by 30° is observed, presumably originating in the bottom graphene layer. The set of six diffraction spots from the lower layer has an identical intensity proving the six-fold symmetry of graphene. These spots are weaker due to the presence of the upper layer. F
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To further understand plausible orientations of graphene on a Cu−Ni alloy catalyst, we performed DFT calculations using a planar π-conjugated molecule, C54H18, adsorbed on Cu(111) and Cu−Ni(111) surfaces as a model system of as-grown graphene. Here, we take 7 × 7 lateral periodicity, and Cu−Ni alloy was simulated by randomly replacing 20% of the Cu atoms by Ni. Energy profiles are plotted in Figure 6, panels a
Thus, in this area, the upper layer is rotated from the Cu−Ni lattice by 30°, while the lower layer matches the orientation of the Cu−Ni lattice (0°). This indicates that the position 3 corresponds to a 30°-twisted BLG region. Interestingly, the orientation of the upper layer (areas 2 and 3) matches that of the MLG region (area 1). Thus, it is highly likely that the upper graphene layer uniformly covers the whole view area of Figure 4, panel a. This strongly suggests that the second layer (i.e., the segregated layer) is grown below the first layer. Figure 5, panel a shows bright-field (BF) LEEM image of more representative area of our BLG sample. Uniform contrast was observed in the whole view area, indicating the presence of large and uniform BLG. For the detailed investigation of stacking order, we have chosen a specific area (Figure 5b) that contains both bilayer and monolayer graphene. The dark area seen in Figure 5, panel b corresponds to monolayer, while the blight area is assigned to bilayer graphene. The electron reflectivity measured at positions 1, 2, and 3 are plotted in Figure 5, panel d. The profiles confirm that positions 2 and 3 correspond to bilayer, while spot 1 is monolayer graphene.54 Figure 5, panel c presents the BF-LEEM image of the circled area in Figure 5, panel b measured at 40 eV, where the contrast reflects the information on the stacking order of the BLG. Figure 5, panels e−h and Figure S4 present detailed analyses of graphene grain orientation for the area shown in Figure 5, panel c. From the diffraction patterns, we could categorize four different regions, AB (0°/0°, Figure 5e), twist (30°/0°, Figure 5f), AB (30°/30°, Figure 5g), and twist (0°/30°, Figure 5h). Each of these pairs of angles shows the relative orientation with respect to the underlying Cu−Ni(111) lattice of the upper (first angle) and the lower (second angle) layers, where the stacking order was determined from the relative intensities of the electron diffraction spots, as was explained in Figure 4. The atomic structures of these four pairs of graphene orientations are indicated in the bottom panels of Figure 5, panels e−h. The monolayer area (position 1 in Figure 5c) has 30°-rotation with respect to the Cu−Ni lattice, as determined from Figure 5, panel g. This monolayer is expected to extend to the region B of Figure 5, panel f. The diffraction spot marked with the dark green circle in Figure 5, panel f is coming from the lower layer considering the weak diffraction spot. Therefore, we confirmed that layer which is a part of position 1 corresponds to the upper layer. The present LEEM results indicate that there is certain selectivity for 0°-orientation in addition to 30°-orientation for both the upper and lower graphene layers. We speculate that the addition of Ni atoms into the Cu(111) lattice lowers the original three-fold symmetry, as most likely the Ni atoms randomly distribute on the surface and the bulk of the Cu. Therefore, the lowering surface symmetry by the Ni atoms may be one of possible reasons. In addition, our recent work suggests that high CVD temperatures above 1040 °C induce thermal fluctuations that may result in the formation of rotated grains.55 It is worthwhile to note that hexagonal BLG grains observed on Cu foil were reported to frequently have a second layer whose angle is rotated by 30° from the first layer.25 Trilayer graphene grown on Cu foil at 1050 °C was also reported to have major stacking with a combination of 0° and 30° orientations at each layer.56 As shown in the Supporting Information (Figure S5), we observed rotated hexagonal BLG grains on an epitaxial Cu(111) film. Therefore, the 30°-rotation in BLG is a commonly observed phenomenon even on Cu catalyst (without Ni).
Figure 6. Binding energies of planar π-conjugated molecules adsorbed on (a) Cu and (b) Cu−Ni(111) alloy surfaces. The binding energies were determined by DFT calculations for relative orientations. Right panels show the atomic images of 0° and 30° orientation. Yellow and green atoms indicate Cu and Ni atoms, respectively.
and b for the molecule adsorbed on the Cu(111) and Cu− Ni(111), respectively. In both cases, the absolute biding energy is the highest at 0°, signifying that the orientation registered by the fcc(111) is the most stable configuration. The absolute binding energy was found to be higher for the Cu−Ni alloy than Cu. This suggests a stronger interaction between C−Ni than that of C−Cu, being consistent with the previous calculation showing a higher binding energy for graphene−Ni interface than graphene−Cu.57 Calculations also show that there is a metastable structure corresponding to 30°-rotated orientation for both Cu and Cu−Ni catalysts (Figure 6a,b). Interestingly, the energy difference between the most stable and metastable configurations of C54H18/Cu−Ni system (17 meV) is much smaller than that of C54H18/Cu system (34 meV). This result suggests that the Cu−Ni alloy gives metastable 30°rotated orientation more frequently than that of pure Cu catalyst. Further fine-tuning of the growth condition may be necessary for the experimental realization of perfectly ABstacked, large-area BLG. Growth Mechanism of Uniform BLG on Cu−Ni Alloy Film. On the basis of the experimental results presented, we propose a growth mechanism of BLG over the Cu−Ni(111) alloy catalyst in Figure 7. As confirmed by the XPS and the EDS (Figure 2), the Cu/Ni stacked film forms a Cu−Ni alloy during the high-temperature CVD process (Figure 7a,b). At G
DOI: 10.1021/acs.chemmater.6b01137 Chem. Mater. XXXX, XXX, XXX−XXX
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Figure 7. Schematic of the growth mechanism of BLG on Cu−Ni metal alloy catalyst. (a) As-sputtered, Cu/Ni stacked film. (b) Cu−Ni alloy is formed during heating the substrate for graphene growth. (c) MLG starts to grow on the alloy surface, while some carbon atoms diffuse into the bulk of the alloy. (d) MLG covers the whole alloy surface with a sufficient amount of dissolved C atoms. (e) During cooling process, the second layer is segregated to form BLG. Yellow, green, black, and red show Cu, Ni, C, and H atoms, respectively.
high temperatures (1000−1075 °C), the CH4 feedstock is decomposed into carbon intermediate species, CHx (x = 0−3), due to reaction catalyzed by the Cu−Ni alloy. Some of the C atoms diffuse into the bulk of the alloy (Figure 7c), as was observed by the depth-profile of XPS (see Figure 2b). It should be noted that regardless of the growth conditions employed, after the CVD, the surface of the catalyst is always fully covered with a monolayer graphene layer, as seen in Figure 1, panels b− d. Therefore, it is highly likely that the first layer grows via surface reaction, as reported when using a pure Cu metal as a catalyst.24 Since more than 70% of the surface atoms of the Cu−Ni alloy are Cu atoms, it is reasonable that the monolayer is formed on the alloy surface (Figure 7c,d). Thus, we consider that the growth of the first layer occurs during the supplying of the CH4 gas. Most probably, the dissolution of carbon atoms into the Cu− Ni bulk takes place simultaneously with the surface growth of the monolayer graphene. Once the alloy surface is covered with MLG, the catalytic decomposition of the CH4 ceases. Accordingly, we observed that extending the CH4 reaction time does not significantly influence the BLG coverage. During the cooling process, the second layer is formed via carbon segregation (Figure 7e). Thus, this second layer should be formed below the first layer, which was confirmed by our LEEM measurement, as discussed in Figures 4 and 5. Here, the Ni concentration determines the total amount of dissolved C atoms, and the cooling profile strongly affects the amount of segregated C atoms from the alloy bulk. This is in accordance with our observations that both Ni concentration and cooling profile strongly influence the BLG coverage (see Figure 1). Previous reports explain the BLG growth in Cu−Ni catalyst by a pure segregation process.34,36 As far as we know, this is the first report that proposes different growth modes for the first and second layers, stressing the importance of both segregation and surface processes for the synthesis of uniform BLG.
different growth modes for the upper and the lower layers. In this, the upper graphene layer grows during the exposure to CH4 at high temperature (surface growth), while the lower graphene layer is formed during the cooling process (segregation growth). Our Raman spectroscopy measurements indicated that about 70−80% of the BLG is AB-stacked. Furthermore, the LEEM measurements revealed that the ABstacked BLG has two main orientations, 0°/0° and 30°/30°, with respect to the underlying Cu−Ni(111) lattice. The DFT calculations showed that the 0°-oriented is the most stable configuration, while 30°-oriented configuration is a metastable state. Also, the energy difference between these two configurations is suggested to be smaller for the Cu−Ni alloy than pure Cu catalyst. Our work provides a reliable way to synthesize BLG in large-scale, which can be applied to graphene-based high-performance semiconductor devices.
CONCLUSIONS The uniform growth of BLG with a coverage of 93% of the catalyst surface is demonstrated by using epitaxial Cu−Ni(111) alloy film deposited on c-plane sapphire. Our systematic study indicated that the Ni concentration and the cooling profile during CVD strongly influence the BLG coverage. We proposed a new growth mechanism, which explains the
ACKNOWLEDGMENTS This work is supported by PRESTO-JST and KAKENHI (Grant Nos. 15H03530, 15K13304, and 16H00917 from JSPS). We thank Dr. Miura of the Center of Advanced Instrumental Analysis of Kyushu University for the XPS measurements. We acknowledge Mr. Tanoue and Mr. Kinoshita for the experimental help of bilayer graphene syntheses.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01137. Experimental procedure, effect of the Cu−Ni stacking order, additional optical microscope and LEEM images, and AB-stacked and twist bilayer graphene grains grown on Cu(111) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.chemmater.6b01137 Chem. Mater. XXXX, XXX, XXX−XXX
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