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Download Hi-Res ImageDownload to MS-PowerPointCite This:Chem. Mater. 2016, 28, 13, 4583-4592
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Highly Uniform Bilayer Graphene on Epitaxial Cu–Ni(111) Alloy

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†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
*E-mail: [email protected]
Cite this: Chem. Mater. 2016, 28, 13, 4583–4592
Publication Date (Web):May 15, 2016
https://doi.org/10.1021/acs.chemmater.6b01137
Copyright © 2016 American Chemical Society
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Abstract

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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.

Introduction

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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 MLG(14-16) and 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 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.

Experimental Methods

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For the growth of BLG, the stacked films of Cu and Ni deposited on c-plane 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 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 as-grown 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)

Results and Discussion

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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/Cu400/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 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

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.

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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 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 report(49) 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 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 as-sputter 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 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.

Figure 2

Figure 2. Depth profiles of elements measured by XPS for the (a) as-sputtered 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 as-sputtered 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.

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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)(1)For the “Ni in Cu”, D0 and Q are 2.7 cm2/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 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 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 high-quality BLG with low defects.

Figure 3

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.

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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 AB-stacked 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, 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.

Figure 4

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.

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Figure 5

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.

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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. 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).
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 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 AB-stacked, large-area BLG.

Figure 6

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.

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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 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.

Figure 7

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.

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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.

Conclusions

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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 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 AB-stacked 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.

Supporting Information

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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

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  • Corresponding Author
    • Hiroki Ago - PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan;  Email: [email protected]
  • Authors
    • Yuichiro Takesaki - †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
    • Kenji Kawahara - †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
    • Hiroki Hibino - Kwansei Gakuin University, Hyogo 669-1337, Japan;  NTT Basic Research Laboratories, NTT Corporation, Kanagawa 243-0198, Japan
    • Susumu Okada - Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
    • Masaharu Tsuji - †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
  • Notes

    The authors declare no competing financial interest.

Acknowledgment

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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.

References

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    Wang, Qing Hua; Shih, Chih-Jen; Paulus, Geraldine L. C.; Strano, Michael S.
    Journal of the American Chemical Society (2013), 135 (50), 18866-18875CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The covalent and noncovalent functionalization of bilayer graphene has been studied using an electrochem. process with aryl diazonium salts in the high conversion regime (D/G ratio >1), and Raman spectroscopic mapping and conductive at. force microscopy (cAFM) were used to study the resulting changes in the phys. and electronic structures. Covalent functionalization at high chem. conversion induced distinct changes in the Raman spectrum of bilayer graphene including the broadening and shift in position of the split G peak. Also, the D peak becomes active with four components. The broadening of the 2D22 and 2D21 components is a distinct indicator of covalent functionalization, whereas the decrease in intensity of the 2D11 and 2D12 peaks corresponds to doping. Conductive AFM imaging showed physisorbed species from noncovalent functionalization could be removed by mech. and elec. influence from the AFM tip, and that changes in cond. due to functionalization are inhomogeneous. These results allow one to distinguish covalent from noncovalent chem. as a guide for further studies of the chem. of bilayer graphene.
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  16. 16
    Bissett, M. A.; Takesaki, Y.; Tsuji, M.; Ago, H. Increased Chemical Reactivity Achieved by Asymmetrical ‘Janus’ Functionalisation of Graphene RSC Adv. 2014, 4, 52215– 52219 DOI: 10.1039/C4RA09724F
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    16
    Increased chemical reactivity achieved by asymmetrical 'Janus' functionalisation of graphene
    Bissett, Mark A.; Takesaki, Yuichiro; Tsuji, Masaharu; Ago, Hiroki
    RSC Advances (2014), 4 (94), 52215-52219CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Chem. functionalization is a promising method to tune the electronic structure of graphene, and the two-dimensional structure of graphene enables access to both of its faces for various types of functionalization. Here, we present the effect of covalent functionalization on the Raman spectrum in terms of monofacial (one-sided) and bifacial (two-sided) functionalization using both monolayer and bilayer graphene. Asym. or 'Janus' functionalization is found to provide significantly increased levels of doping compared to other schemes allowing for control over graphene's electronic structure as well as control over surface functionality.
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  17. 17
    Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Edge State in Graphene Ribbons: Nanometer Size Effect and Edge Shape Dependence Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 17954– 17961 DOI: 10.1103/PhysRevB.54.17954
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    17
    Edge state in graphene ribbons: nanometer size effect and edge shape dependence
    Nakada, Kyoko; Fujita, Mitsutaka; Dresselhaus, Gene; Dresselhaus, Mildred S.
    Physical Review B: Condensed Matter (1996), 54 (24), 17954-17961CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the d. of states. The charge d. in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for detg. the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixt. of zigzag and armchair sites. Examg. the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in detg. the d. of states near the Fermi level for graphite networks on a nanometer scale.
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  18. 18
    Lee, S.; Lee, K.; Zhong, Z. Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition Nano Lett. 2010, 10, 4702– 4707 DOI: 10.1021/nl1029978
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    18
    Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition
    Lee, Seunghyun; Lee, Kyunghoon; Zhong, Zhaohui
    Nano Letters (2010), 10 (11), 4702-4707CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The discovery of elec. field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples were limited to μm2 size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here the authors report homogeneous bilayer graphene films over at least a 2 in. × 2 in area, synthesized by CVD on Cu foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, at. force microscopy, and TEM. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by elec. transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is obsd. in 98% of the devices.
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  19. 19
    Yan, K.; Peng, H.; Zhou, Y.; Li, H.; Liu, Z. Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition Nano Lett. 2011, 11, 1106– 1110 DOI: 10.1021/nl104000b
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    19
    Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition
    Yan, Kai; Peng, Hailin; Zhou, Yu; Li, Hui; Liu, Zhongfan
    Nano Letters (2011), 11 (3), 1106-1110CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We report the epitaxial formation of bilayer Bernal graphene on copper foil via chem. vapor deposition. The self-limit effect of graphene growth on copper is broken through the introduction of a second growth process. The coverage of bilayer regions with Bernal stacking can be as high as 67% before further optimization. Facilitated with the transfer process to silicon/silicon oxide substrates, dual-gated graphene transistors of the as-grown bilayer Bernal graphene were fabricated, showing typical tunable transfer characteristics under varying gate voltages. The high-yield layer-by-layer epitaxy scheme will not only make this material easily accessible but reveal the fundamental mechanism of graphene growth on copper.
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  20. 20
    Liu, L.; Zhou, H.; Cheng, R.; Yu, W. J.; Liu, Y.; Chen, Y.; Shaw, J.; Zhong, X.; Huang, Y.; Duan, X. High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area AB-Stacked Bilayer Graphene ACS Nano 2012, 6, 8241– 8249 DOI: 10.1021/nn302918x
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    20
    High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area AB-Stacked Bilayer Graphene
    Liu, Lixin; Zhou, Hailong; Cheng, Rui; Yu, Woo Jong; Liu, Yuan; Chen, Yu; Shaw, Jonathan; Zhong, Xing; Huang, Yu; Duan, Xiangfeng
    ACS Nano (2012), 6 (9), 8241-8249CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical elec. field. Mech. exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few microns. Chem. vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixt. of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H2/CH4 ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temp. and low-pressure nucleation step is found to be crit. for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The elec. transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm2/V·s at room temp., comparable to that of the exfoliated bilayer graphene.
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  21. 21
    Sun, Z.; Raji, A.-R. O.; Zhu, Y.; Xiang, C.; Yan, Z.; Kittrell, C.; Samuel, E. L. G.; Tour, J. M. Large-Area Bernal-Stacked Bi-, Tri-, and Tetralayer Graphene ACS Nano 2012, 6, 9790– 9796 DOI: 10.1021/nn303328e
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    21
    Large-Area Bernal-Stacked Bi-, Tri-, and Tetralayer Graphene
    Sun, Zhengzong; Raji, Abdul-Rahman O.; Zhu, Yu; Xiang, Changsheng; Yan, Zheng; Kittrell, Carter; Samuel, E. L. G.; Tour, James M.
    ACS Nano (2012), 6 (11), 9790-9796CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Few-layer graphene, with Bernal stacking order, is of particular interest to the graphene community because of its unique tunable electronic structure. A synthetic method to produce such large area graphene films with precise thickness from 2 to 4 layers would be ideal for chemists and physicists to explore the promising electronic applications of these materials. Here, large-area uniform Bernal-stacked bi-, tri-, and tetralayer graphene films were successfully synthesized on a Cu surface in selective growth windows, with a finely tuned total pressure and CH4/H2 gas ratio. On the basis of the analyses obtained, the growth mechanism is not an independent homoexpitaxial layer-by-layer growth, but most likely a simultaneous-seeding and self-limiting process.
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  22. 22
    Wassei, J. K.; Mecklenburg, M.; Torres, J. A.; Fowler, J. D.; Regan, B. C.; Kaner, R. B.; Weiller, B. H. Chemical Vapor Deposition of Graphene on Copper from Methane, Ethane and Propane: Evidence for Bilayer Selectivity Small 2012, 8, 1415– 1422 DOI: 10.1002/smll.201102276
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    22
    Chemical Vapor Deposition of Graphene on Copper from Methane, Ethane and Propane: Evidence for Bilayer Selectivity
    Wassei, Jonathan K.; Mecklenburg, Matthew; Torres, Jaime A.; Fowler, Jesse D.; Regan, B. C.; Kaner, Richard B.; Weiller, Bruce H.
    Small (2012), 8 (9), 1415-1422CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    To study the effects of hydrocarbon precursor gases, graphene was grown by chem. vapor deposition from methane, ethane, and propane on copper foils. The larger mols. more readily produce bilayer and multilayer graphene, due to a higher carbon concn. and different decompn. processes. Single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane from 250 to 1000 mTorr. The bilayer graphene is AB-stacked as shown by selected area electron diffraction anal. Addnl. propane only produces a combination of single- to few-layer and turbostratic graphene. The percent coverage is investigated using Raman spectroscopy and optical, scanning electron, and transmission electron microscopies. The data are used to discuss a possible mechanism for the second-layer growth of graphene involving the different cracking pathways of the hydrocarbons.
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  23. 23
    Zhao, P.; Kim, S.; Chen, X.; Einarsson, E.; Wang, M.; Song, Y.; Wang, H.; Chiashi, S.; Xiang, R.; Maruyama, S. Equilibrium Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene ACS Nano 2014, 8, 11631– 11638 DOI: 10.1021/nn5049188
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    23
    Equilibrium Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene
    Zhao, Pei; Kim, Sungjin; Chen, Xiao; Einarsson, Erik; Wang, Miao; Song, Yenan; Wang, Hongtao; Chiashi, Shohei; Xiang, Rong; Maruyama, Shigeo
    ACS Nano (2014), 8 (11), 11631-11638CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Using ethanol as the carbon source, self-limiting growth of AB-stacked bilayer graphene (BLG) has been achieved on Cu via an equil. chem. vapor deposition (CVD) process. We found that during this alc. catalytic CVD (ACCVD) a source-gas pressure range exists to break the self-limitation of monolayer graphene on Cu, and at a certain equil. state it prefers to form uniform BLG with a high surface coverage of ∼94% and AB-stacking ratio of nearly 100%. More importantly, once the BLG is completed, this growth shows a self-limiting manner, and an extended ethanol flow time does not result in addnl. layers. We investigate the mechanism of this equil. BLG growth using isotopically labeled 13C-ethanol and selective surface aryl functionalization, and results reveal that during the equil. ACCVD process a continuous substitution of graphene flakes occurs to the as-formed graphene and the BLG growth follows a layer-by-layer epitaxy mechanism. These phenomena are significantly in contrast to those obsd. for previously reported BLG growth using methane as precursor.
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  24. 24
    Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils Science 2009, 324, 1312– 1314 DOI: 10.1126/science.1171245
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    24
    Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
    Li, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.
    Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
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  25. 25
    Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene ACS Nano 2011, 5, 6069– 6076 DOI: 10.1021/nn201978y
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    25
    Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene
    Vlassiouk, Ivan; Regmi, Murari; Fulvio, Pasquale; Dai, Sheng; Datskos, Panos; Eres, Gyula; Smirnov, Sergei
    ACS Nano (2011), 5 (7), 6069-6076CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Graphene CVD growth on Cu foil using methane as a C source is strongly affected by hydrogen, which appears to serve a dual role: an activator of the surface bound C that is necessary for monolayer growth and an etching reagent that controls the size and morphol. of the graphene domains. The resulting growth rate for a fixed methane partial pressure has a max. at hydrogen partial pressures 200-400 times that of methane. The morphol. and size of the graphene domains, as well as the no. of layers, change with hydrogen pressure from irregularly shaped incomplete bilayers to well-defined perfect single layer hexagons. Raman spectra suggest the zigzag termination in the hexagons as more stable than the armchair edges.
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  26. 26
    Wu, B.; Geng, D.; Guo, Y.; Huang, L.; Xue, Y.; Zheng, J.; Chen, J.; Yu, G.; Liu, Y.; Jiang, L.; Hu, W. Equiangular Hexagon-Shape-Controlled Synthesis of Graphene on Copper Surface Adv. Mater. 2011, 23, 3522– 3525 DOI: 10.1002/adma.201101746
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    26
    Equiangular Hexagon-Shape-Controlled Synthesis of Graphene on Copper Surface
    Wu, Bin; Geng, Dechao; Guo, Yunlong; Huang, Liping; Xue, Yunzhou; Zheng, Jian; Chen, Jianyi; Yu, Gui; Liu, Yunqi; Jiang, Lang; Hu, Wenping
    Advanced Materials (Weinheim, Germany) (2011), 23 (31), 3522-3525CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A large scale synthesis of equiangular hexagonal singular or multilayer graphene is reported by methane CVD on Cu surface at ambient pressure. The shape reflects the hexagonal graphene lattice, possessing either zigzag or armchair edges. The hexagon-shaped graphene shows no observable defects confirmed by Raman spectra, and is formed by nucleation and growth mechanism, thus allowing control of both d. and size. Moreover, the shape evolution follows an empirical rule that higher CH4 flow rate leads to shorter nucleation time, higher growth rates and larger deviations from equiangular hexagon shape. Based on these observations, we proposed a growth model that qual. establishes a connection between various exptl.. conditions and the final state of the grown graphene, and is in principle capable of predicting their results from different conditions in the Cu-Methane CVD system. Moreover, this system provides direct evidence of layer spatial arrangement in the case of multi-layer graphene flakes, leading to a growth model which describes the process of layer growth order and reasonably explains the important size correlation of different layers found in out expts. Finally, these graphene flakes show mobility higher than 1900 cm2 V-1s-1, demonstrating their good quality.
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    Nie, S.; Wu, W.; Xing, S.; Yu, Q.; Bao, J.; Pei, S.-S.; McCarty, K. F. Growth from Below: Bilayer Graphene on Copper by Chemical Vapor Deposition New J. Phys. 2012, 14, 093028 DOI: 10.1088/1367-2630/14/9/093028
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    27
    Growth from below: bilayer graphene on copper by chemical vapor deposition
    Nie, Shu; Wu, Wei; Xing, Shirui; Yu, Qingkai; Bao, Jiming; Pei, Shin-Shem; McCarty, Kevin F.
    New Journal of Physics (2012), 14 (Sept.), 093028/1-093028/9CODEN: NJOPFM; ISSN:1367-2630. (Institute of Physics Publishing)
    We evaluate how a second graphene layer forms and grows on Cu foils during chem. vapor deposition (CVD). LEED and microscopy is used to reveal that the second layer nucleates and grows next to the substrate, i.e., under a graphene layer. This underlayer mechanism can facilitate the synthesis of uniform single-layer films but presents challenges for growing uniform bilayer films by CVD. We also show that the buried and overlying layers have the same edge termination.
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  28. 28
    Kalbac, M.; Frank, O.; Kavan, L. The Control of Graphene Double-Layer Formation in Copper-Catalyzed Chemical Vapor Deposition Carbon 2012, 50, 3682– 3687 DOI: 10.1016/j.carbon.2012.03.041
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    28
    The control of graphene double-layer formation in copper-catalyzed chemical vapor deposition
    Kalbac, Martin; Frank, Otakar; Kavan, Ladislav
    Carbon (2012), 50 (10), 3682-3687CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
    The growth of graphene during Cu-catalyzed chem. vapor deposition was studied using 12CH4 and 13CH4 precursor gases. We suggest that the growth begins by the formation of a multilayer cluster. This seed increases its size but the growth speed of a particular layer depends on its proximity to the copper surface. The layer closest to the substrate grows fastest and thus further limits the growth rate of the upper layers. Nevertheless, the growth of the upper layers continues until the copper surface is completely blocked. It is shown that the upper layers can be removed by modification of the conditions of the growth by hydrogen etching.
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  29. 29
    Yan, Z.; Liu, Y.; Ju, L.; Peng, Z.; Lin, J.; Wang, G.; Zhou, H.; Xiang, C.; Samuel, E. L. G.; Kittrell, C.; Artyukhov, V. I.; Wang, F.; Yakobson, B. I.; Tour, J. M. Large Hexagonal Bi- and Trilayer Graphene Single Crystals with Varied Interlayer Rotations Angew. Chem. 2014, 126, 1591– 1595 DOI: 10.1002/ange.201306317
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    Yeh, C.-H.; Lin, Y.-C.; Nayak, P. K.; Lu, C.-C.; Liu, Z.; Suenaga, K.; Chiu, P.-W. Probing Interlayer Coupling in Twisted Single-Crystal Bilayer Graphene by Raman Spectroscopy J. Raman Spectrosc. 2014, 45, 912– 917 DOI: 10.1002/jrs.4571
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    Fang, W.; Hsu, A. L.; Caudillo, R.; Song, Y.; Birdwell, A. G.; Zakar, E.; Kalbac, M.; Dubey, M.; Palacios, T.; Dresselhaus, M. S.; Araujo, P. T.; Kong, J. Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy Nano Lett. 2013, 13, 1541– 1548 DOI: 10.1021/nl304706j
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    31
    Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy
    Fang, Wenjing; Hsu, Allen L.; Caudillo, Roman; Song, Yi; Birdwell, A. Glen; Zakar, Eugene; Kalbac, Martin; Dubey, Madan; Palacios, Tomas; Dresselhaus, Millie S.; Araujo, Paulo T.; Kong, Jing
    Nano Letters (2013), 13 (4), 1541-1548CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    The growth of large-area bilayer graphene was of technol. importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which dets. both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). The authors developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature assocd. with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, the authors were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chem. Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Also, by combining surface sensitive fluorination with such hybrid 12C/13C bilayer samples, the authors are able to identify that the 2nd layer grows underneath the 1st-grown layer, which is similar to a recently reported observation.
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  32. 32
    Hao, Y.; Wang, L.; Liu, Y.; Chen, H.; Wang, X.; Tan, C.; Nie, S.; Suk, J. W.; Jiang, T.; Liang, T.; Xiao, J.; Ye, W.; Dean, C. R.; Yakobson, B. I.; McCarty, K. F.; Kim, P.; Hone, J.; Colombo, L.; Ruoff, R. S. Oxygen-Activated Growth and Bandgap Tunability of Large Single-Crystal Bilayer Graphene Nat. Nanotechnol. 2016, 11, 426– 431 DOI: 10.1038/nnano.2015.322
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    32
    Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene
    Hao, Yufeng; Wang, Lei; Liu, Yuanyue; Chen, Hua; Wang, Xiaohan; Tan, Cheng; Nie, Shu; Suk, Ji Won; Jiang, Tengfei; Liang, Tengfei; Xiao, Junfeng; Ye, Wenjing; Dean, Cory R.; Yakobson, Boris I.; McCarty, Kevin F.; Kim, Philip; Hone, James; Colombo, Luigi; Ruoff, Rodney S.
    Nature Nanotechnology (2016), 11 (5), 426-431CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Bernal (AB)-stacked bilayer graphene (BLG) is a semiconductor whose bandgap can be tuned by a transverse elec. field, making it a unique material for a no. of electronic and photonic devices. A scalable approach to synthesize high-quality BLG is therefore crit., which requires minimal cryst. defects in both graphene layers and maximal area of Bernal stacking, which is necessary for bandgap tunability. Here we demonstrate that in an oxygen-activated chem. vapor deposition (CVD) process, half-millimeter size, Bernal-stacked BLG single crystals can be synthesized on Cu. Besides the traditional 'surface-limited' growth mechanism for SLG (1 st layer), we discovered new microscopic steps governing the growth of the 2 nd graphene layer below the 1 st layer as the diffusion of carbon atoms through the Cu bulk after complete dehydrogenation of hydrocarbon mols. on the Cu surface, which does not occur in the absence of oxygen. Moreover, we found that the efficient diffusion of the carbon atoms present at the interface between Cu and the 1 st graphene layer further facilitates growth of large domains of the 2 nd layer. The CVD BLG has superior elec. quality, with a device on/off ratio greater than 104, and a tunable bandgap up to ∼100 meV at a displacement field of 0.9 V nm-1.
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    Liu, X.; Fu, L.; Liu, N.; Gao, T.; Zhang, Y.; Liao, L.; Liu, Z. Segregation Growth of Graphene on Cu−Ni Alloy for Precise Layer Control J. Phys. Chem. C 2011, 115, 11976– 11982 DOI: 10.1021/jp202933u
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    Wu, Y.; Chou, H.; Ji, H.; Wu, Q.; Chen, S.; Jiang, W.; Hao, Y.; Kang, J.; Ren, Y.; Piner, R. D.; Ruoff, R. S. Growth Mechanism and Controlled Synthesis of AB-Stacked Bilayer Graphene on Cu-Ni Alloy Foils ACS Nano 2012, 6, 7731– 7738 DOI: 10.1021/nn301689m
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    34
    Growth Mechanism and Controlled Synthesis of AB-Stacked Bilayer Graphene on Cu-Ni Alloy Foils
    Wu, Yaping; Chou, Harry; Ji, Hengxing; Wu, Qingzhi; Chen, Shanshan; Jiang, Wei; Hao, Yufeng; Kang, Junyong; Ren, Yujie; Piner, Richard D.; Ruoff, Rodney S.
    ACS Nano (2012), 6 (9), 7731-7738CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Strongly coupled bilayer graphene (i.e., AB stacked) grows particularly well on com. 90-10" Cu-Ni alloy foil. However, the mechanism of growth of bilayer graphene on Cu-Ni alloy foils had not been discovered. Carbon isotope labeling (sequential dosing of 12CH4 and 13CH4) and Raman spectroscopic mapping were used to study the growth process. The mechanism of graphene growth on Cu-Ni alloy is by pptn. at the surface from carbon dissolved in the bulk of the alloy foil that diffuses to the surface. The growth parameters were varied to study their effect on graphene coverage and isotopic compn. Higher temp., longer exposure time, higher rate of bulk diffusion for 12C vs. 13C, and slower cooling rate all produced higher graphene coverage on this type of Cu-Ni alloy foil. The isotopic compn. of the graphene layer(s) could also be modified by adjusting the cooling rate. Large-area, AB-stacked bilayer graphene transferrable onto Si/SiO2 substrates was controllably synthesized.
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    Liu, W.; Kraemer, S.; Sarkar, D.; Li, H.; Ajayan, P. M.; Banerjee, K. Controllable and Rapid Synthesis of High-Quality and Large-Area Bernal Stacked Bilayer Graphene Using Chemical Vapor Deposition Chem. Mater. 2014, 26, 907– 915 DOI: 10.1021/cm4021854
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    35
    Controllable and Rapid Synthesis of High-Quality and Large-Area Bernal Stacked Bilayer Graphene Using Chemical Vapor Deposition
    Liu, Wei; Kraemer, Stephan; Sarkar, Deblina; Li, Hong; Ajayan, Pulickel M.; Banerjee, Kaustav
    Chemistry of Materials (2014), 26 (2), 907-915CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Bilayer graphene has attracted wide attention due to its unique band structure and bandgap tunability under specific (Bernal or AB) stacking order. However, it remains challenging to tailor the stacking order and to simultaneously produce large-scale and high-quality bilayer graphene. This work introduces a fast and reliable method of growing high-quality Bernal stacked large-area (>3 in. × 3 in.) bilayer graphene film or trilayer graphene domains (30 μm × 30 μm) using CVD on engineered Cu-Ni alloy catalyst films. The AB stacking order is evaluated by Raman spectra, electron diffraction pattern, and dual gate field-effect-transistor (FET) measurements, and a near-perfect AB stacked bilayer graphene coverage (>98%) was obtained. The synthesized bilayer and trilayer graphene with Bernal stacking exhibit electron mobility ≤3450 cm2/(V·s) and 1500 cm2/(V·s), resp., indicating comparable quality with respect to exfoliated bilayer and trilayer graphene. The record high (for CVD bilayer graphene) ON to OFF current ratios (up to 15) obtained for a large no. (>50) of dual-gated FETs fabricated at random across the large-area bilayer graphene film also corroborates the success of the authors' synthesis technique. Also, through catalyst engineering, growth optimization, and element anal. of catalyst, achieving surface catalytic graphene growth mode and precise control of surface carbon concn. are key factors detg. the growth of high quality and large area Bernal stacked bilayer graphene on Cu-Ni alloy. This discovery can not only open up new vistas for large-scale electronic and photonic device applications of graphene but also facilitate exploration of novel heterostructures formed with emerging beyond graphene two-dimensional at. crystals.
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  36. 36
    Lin, T.; Huang, F.; Wan, D.; Bi, H.; Xie, X.; Jiang, M. Self-Regulating Homogenous Growth of High-Quality Graphene on Co–Cu Composite Substrate for Layer Control Nanoscale 2013, 5, 5847– 5853 DOI: 10.1039/c3nr33124e
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    Lu, C.-C.; Lin, Y.-C.; Liu, Z.; Yeh, C.-H.; Suenaga, K.; Chiu, P.-W. Twisting Bilayer Graphene Superlattices ACS Nano 2013, 7, 2587– 2594 DOI: 10.1021/nn3059828
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    Twisting Bilayer Graphene Superlattices
    Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen
    ACS Nano (2013), 7 (3), 2587-2594CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decompn. of SiC; (ii) CVD on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycryst. structures. The present study studies bilayer graphene grown by ambient pressure CVD on polycryst. Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes result from the twist, with the angle detd. by TEM. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
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    Havener, R. W.; Zhuang, H.; Brown, L.; Hennig, R. G.; Park, J. Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene Nano Lett. 2012, 12, 3162– 3267 DOI: 10.1021/nl301137k
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    38
    Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene
    Havener, Robin W.; Zhuang, Houlong; Brown, Lola; Hennig, Richard G.; Park, Jiwoong
    Nano Letters (2012), 12 (6), 3162-3167CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Few-layer graphene is a prototypical layered material, whose properties are detd. by the relative orientations and interactions between layers. Exciting elec. and optical phenomena were obsd. for the special case of Bernal-stacked few-layer graphene, but structure-property correlations in graphene which deviates from this structure are not well understood. Here, the authors combine 2 direct imaging techniques, dark-field TEM (DF-TEM) and widefield Raman imaging, to establish a robust, 1-to-one correlation between twist angle and Raman intensity in twisted bilayer graphene (tBLG). The Raman G band intensity is strongly enhanced due to a previously unreported singularity in the joint d. of states of tBLG, whose energy is exclusively a function of twist angle and whose optical transition strength is governed by interlayer interactions, enabling direct optical imaging of these parameters. Also, findings suggest future potential for novel optical and optoelectronic tBLG devices with angle-dependent, tunable characteristics.
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  39. 39
    Hu, B.; Ago, H.; Ito, Y.; Kawahara, K.; Tsuji, M.; Magome, E.; Sumitani, K.; Mizuta, N.; Ikeda, K.; Mizuno, S. Epitaxial Growth of Large-Area Single-Layer Graphene over Cu(111)/Sapphire by Atmospheric Pressure CVD Carbon 2012, 50, 57– 65 DOI: 10.1016/j.carbon.2011.08.002
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    Epitaxial growth of large-area single-layer graphene over Cu(1 1 1)/sapphire by atmospheric pressure CVD
    Hu, Baoshan; Ago, Hiroki; Ito, Yoshito; Kawahara, Kenji; Tsuji, Masaharu; Magome, Eisuke; Sumitani, Kazushi; Mizuta, Noriaki; Ikeda, Ken-ichi; Mizuno, Seigi
    Carbon (2012), 50 (1), 57-65CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
    We report the atm. pressure CVD growth of single-layer graphene over a cryst. Cu(1 1 1) film heteroepitaxially deposited on c-plane Al2O3. Orientation-controlled, epitaxial single-layer graphene is achieved over the Cu(1 1 1) film on Al2O3, while a polycryst. Cu film deposited on a Si wafer gives non-uniform graphene with multilayer flakes. Moreover, the CVD temp. is found to affect the quality and orientation of graphene grown on the Cu/Al2O3 substrates. The CVD growth at 1000° gives high-quality epitaxial single-layer graphene whose orientation of hexagonal lattice matches with the Cu(1 1 1) lattice which is detd. by the Al2O3's crystallog. direction. At lower CVD temp. of 900°, low-quality graphene with enhanced Raman D band is obtained, and it showed 2 different orientations of the hexagonal lattice; one matches with the Cu lattice and another rotated by 30°. C isotope-labeling expt. indicates rapid exchange of the surface-adsorbed and gas-supplied C atoms at the higher temp., resulting in the highly crystd. graphene with energetically most stable orientation consistent with the underlying Cu(1 1 1) lattice.
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  40. 40
    Ago, H.; Kawahara, K.; Ogawa, Y.; Tanoue, S.; Bissett, M. A.; Tsuji, M.; Sakaguchi, H.; Koch, R. J.; Fromm, F.; Seyller, T.; Komatsu, K.; Tsukagoshi, K. Epitaxial Growth and Electronic Properties of Large Hexagonal Graphene Domains on Cu(111) Thin Film Appl. Phys. Express 2013, 6, 075101 DOI: 10.7567/APEX.6.075101
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    40
    Epitaxial growth and electronic properties of large hexagonal graphene domains on Cu(111) thin film
    Ago, Hiroki; Kawahara, Kenji; Ogawa, Yui; Tanoue, Shota; Bissett, Mark A.; Tsuji, Masaharu; Sakaguchi, Hidetsugu; Koch, Roland J.; Fromm, Felix; Seyller, Thomas; Komatsu, Katsuyoshi; Tsukagoshi, Kazuhito
    Applied Physics Express (2013), 6 (7), 075101/1-075101/4CODEN: APEPC4; ISSN:1882-0778. (Japan Society of Applied Physics)
    Large hexagonal single-cryst. domains of single-layer graphene are epitaxially grown by ambient-pressure chem. vapor deposition over a thin Cu(111) film deposited on c-plane sapphire. The hexagonal graphene domains with a max. size of 100 μm are oriented in the same direction due to the epitaxial growth. Reflecting high crystallinity, a clear band structure with the Dirac cone is obsd. by angle-resolved photoelectron spectroscopy (ARPES), and a high carrier mobility exceeding 4000 cm2 V-1 s-1 was obtained on SiO2/Si at room temp. This epitaxial approach combined with large domain growth is expected to contribute to future electronic applications.
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    Ago, H.; Ito, Y.; Mizuta, N.; Yoshida, K.; Hu, B.; Orofeo, C. M.; Tsuji, M.; Ikeda, K.; Mizuno, S. Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire ACS Nano 2010, 4, 7407– 7414 DOI: 10.1021/nn102519b
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    41
    Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire
    Ago, Hiroki; Ito, Yoshito; Mizuta, Noriaki; Yoshida, Kazuma; Hu, Baoshan; Orofeo, Carlo M.; Tsuji, Masaharu; Ikeda, Ken-ichi; Mizuno, Seigi
    ACS Nano (2010), 4 (12), 7407-7414CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Epitaxial CVD growth of uniform single-layer graphene is demonstrated over Co film crystd. on c-plane sapphire. The single cryst. Co film is realized on the sapphire substrate by optimized high-temp. sputtering and successive H2 annealing. This cryst. Co film enables the formation of uniform single-layer graphene, while a polycryst. Co film deposited on a SiO2/Si substrate gives a no. of graphene flakes with various thicknesses. Also, an epitaxial relation between the as-grown graphene and Co lattice is obsd. when synthesis occurs at 1000°; the direction of the hexagonal lattice of the single-layer graphene completely matches with that of the underneath Co/sapphire substrate. The orientation of graphene depends on the growth temp. and, at 900°, the graphene lattice is rotated at 22 ± 8° with respect to the Co lattice direction. The authors' work expands a possibility of synthesizing single-layer graphene over various metal catalysts. Also, the authors' CVD growth gives a graphene film with predefined orientation, and thus can be applied to graphene engineering, such as cutting along a specific crystallog. direction, for future electronics applications.
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    Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making Graphene Visible Appl. Phys. Lett. 2007, 91, 063124 DOI: 10.1063/1.2768624
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    Making graphene visible
    Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K.
    Applied Physics Letters (2007), 91 (6), 063124/1-063124/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Microfabrication of graphene devices used in many exptl. studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepd. on top of Si wafers with a certain thickness of SiO2. The authors study graphene's visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. By using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current std.) and, esp., ≈100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quant. describe the exptl. data.
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    Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865– 3868 DOI: 10.1103/PhysRevLett.77.3865
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    Generalized gradient approximation made simple
    Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Hamada, I.; Otani, M. Comparative van der Waals Density-Functional Study of Graphene on Metal Surfaces Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 153412 DOI: 10.1103/PhysRevB.82.153412
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    Comparative van der Waals density-functional study of graphene on metal surfaces
    Hamada, Ikutaro; Otani, Minoru
    Physical Review B: Condensed Matter and Materials Physics (2010), 82 (15), 153412/1-153412/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    We present a comparative van der Waals d.-functional (vdW-DF) study of graphene adsorbed on (111) surfaces of Ni, Cu, Pd, Ag, Au, and Pt, using the second version of vdW-DF (vdW-DF2) of Lee et al. [Phys. Rev. B 82, 081101(R) (2010)] and the exchange functional (C09) developed by Cooper [Phys. Rev. B 81, 161104(R) (2010)]. We show that the use of the vdW-DF2 correlation together with the C09 exchange yields the most satisfactory results. Adsorption geometries of graphene are in good agreement with available expt. data, and the electronic structure of graphene varies depending on the nature of the substrate. Band-gap opening at the K point obsd. on the Ni(111) surface is reproduced reasonably well.
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    Otani, M.; Sugino, O. First-Principles Calculations of Charged Surfaces and Interfaces: A Plane-Wave Nonrepeated Slab Approach Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 115407 DOI: 10.1103/PhysRevB.73.115407
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    First-principles calculations of charged surfaces and interfaces: A plane-wave nonrepeated slab approach
    Otani, M.; Sugino, O.
    Physical Review B: Condensed Matter and Materials Physics (2006), 73 (11), 115407/1-115407/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    A new first-principles computational approach to a charged surface/interface is presented. The surface is modeled as a slab imposed with boundary conditions to screen the excess surface charge. To treat this model, which is nonperiodic in the surface normal direction, a std. pseudopotential plane-wave scheme is modified at the Poisson solver part with the help of the Green's function technique. Benchmark calcns. are done for Al/Si(111) with the bias voltage applied between the surface and the model scanning tunneling microscopy (STM) tip, the model back gate, or the model soln. The calcns. are found to be efficient and stable, and their implementation is found to be easy. Because of the flexibility, the scheme is considered to be applicable to more general exptl. situations.
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    Amano, H. Growth of GaN Layers on Sapphire by Low-Temperature-Deposited Buffer Layers and Realization of p-type GaN by Magnesium Doping and Electron Beam Irradiation Angew. Chem. 2015, 54, 7764– 7769 DOI: 10.1002/anie.201501651
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    Luo, Z.; Lu, Y.; Singer, D. W.; Berck, M. E.; Somers, L. A.; Goldsmith, B. R.; Johnson, A. T. C. Effect of Substrate Roughness and Feedstock Concentration on Growth of Wafer-Scale Graphene at Atmospheric Pressure Chem. Mater. 2011, 23, 1441– 1447 DOI: 10.1021/cm1028854
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    Effect of Substrate Roughness and Feedstock Concentration on Growth of Wafer-Scale Graphene at Atmospheric Pressure
    Luo, Zhengtang; Lu, Ye; Singer, Daniel W.; Berck, Matthew E.; Somers, Luke A.; Goldsmith, Brett R.; Johnson, A. T. Charlie
    Chemistry of Materials (2011), 23 (6), 1441-1447CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The growth of large-area graphene on catalytic metal substrates is a topic of both fundamental and technol. interest. We have developed an atm. pressure chem. vapor deposition (CVD) method that is potentially more cost-effective and compatible with industrial prodn. than approaches based on synthesis under high vacuum. Surface morphol. of the catalytic Cu substrate and the concn. of carbon feedstock gas were found to be crucial factors in detg. the homogeneity and electronic transport properties of the final graphene film. The use of an electropolished metal surface and low methane concn. enabled the growth of graphene samples with single layer content exceeding 95%. Field effect transistors fabricated from CVD graphene made with the optimized process had room temp. hole mobilities that are a factor of 2-5 larger than those measured for samples grown on as-purchased Cu foil with larger methane concn. A kinetic model is proposed to explain the obsd. dependence of graphene growth on catalyst surface roughness and carbon source concn.
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    Zhang, Y.; Gomez, L.; Ishikawa, F. N.; Madaria, A.; Ryu, K.; Wang, C.; Badmaev, A.; Zhou, C. Comparison of Graphene Growth on Single-Crystalline and Polycrystalline Ni by Chemical Vapor Deposition J. Phys. Chem. Lett. 2010, 1, 3101– 3107 DOI: 10.1021/jz1011466
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    48
    Comparison of Graphene Growth on Single-Crystalline and Polycrystalline Ni by Chemical Vapor Deposition
    Zhang, Yi; Gomez, Lewis; Ishikawa, Fumiaki N.; Madaria, Anuj; Ryu, Koungmin; Wang, Chuan; Badmaev, Alexander; Zhou, Chongwu
    Journal of Physical Chemistry Letters (2010), 1 (20), 3101-3107CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The authors report a comparative study and Raman characterization of the formation of graphene on single crystal Ni (111) and polycryst. Ni substrates using CVD. Preferential formation of monolayer/bilayer graphene on the single crystal surface is attributed to its atomically smooth surface and the absence of grain boundaries. In contrast, CVD graphene formed on polycryst. Ni leads to a higher percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman surface mapping reveals that the area percentages of monolayer/bilayer graphene are 91.4% for the Ni (111) substrate and 72.8% for the polycryst. Ni substrate under comparable CVD conditions. The use of single crystal substrates for graphene growth may open ways for uniform high-quality graphene over large areas.
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    Reina, A.; Thiele, S.; Jia, X.; Bhaviripudi, S.; Dresselhaus, M. S.; Schaefer, J. A.; Kong, J. Growth of Large-Area Single- and Bi-Layer Graphene by Controlled Carbon Precipitation on Polycrystalline Ni Surfaces Nano Res. 2009, 2, 509– 516 DOI: 10.1007/s12274-009-9059-y
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    49
    Growth of large-area single- and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces
    Reina, Alfonso; Thiele, Stefan; Jia, Xiaoting; Bhaviripudi, Sreekar; Dresselhaus, Mildred S.; Schaefer, Juergen A.; Kong, Jing
    Nano Research (2009), 2 (6), 509-516CODEN: NRAEB5; ISSN:1998-0124. (Springer)
    We report graphene films composed mostly of one or two layers of graphene grown by controlled carbon pptn. on the surface of polycryst. Ni thin films during atm. chem. vapor deposition (CVD). Controlling both the methane concn. during CVD and the substrate cooling rate during graphene growth can significantly improve the thickness uniformity. As a result, one- or two- layer graphene regions occupy up to 87% of the film area. Single layer coverage accounts for 5%-11% of the overall film. These regions expand across multiple grain boundaries of the underlying polycryst. Ni film. The no. d. of sites with multilayer graphene/graphite (>2 layers) is reduced as the cooling rate decreases. These films can also be transferred to other substrates and their sizes are only limited by the sizes of the Ni film and the CVD chamber. Here, we demonstrate the formation of films as large as 1 in2. These findings represent an important step towards the fabrication of large-scale high-quality graphene samples.
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  • Abstract

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    Figure 1

    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.

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    Figure 2

    Figure 2. Depth profiles of elements measured by XPS for the (a) as-sputtered 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 as-sputtered 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.

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    Figure 3

    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.

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    Figure 4

    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.

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    Figure 5

    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.

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    Figure 6

    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.

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    Figure 7

    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.

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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvVCmurrF&md5=ad161af2eba592988bc7f374fec47f7f
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      Bissett, M. A.; Takesaki, Y.; Tsuji, M.; Ago, H. Increased Chemical Reactivity Achieved by Asymmetrical ‘Janus’ Functionalisation of Graphene RSC Adv. 2014, 4, 52215– 52219 DOI: 10.1039/C4RA09724F
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      16
      Increased chemical reactivity achieved by asymmetrical 'Janus' functionalisation of graphene
      Bissett, Mark A.; Takesaki, Yuichiro; Tsuji, Masaharu; Ago, Hiroki
      RSC Advances (2014), 4 (94), 52215-52219CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Chem. functionalization is a promising method to tune the electronic structure of graphene, and the two-dimensional structure of graphene enables access to both of its faces for various types of functionalization. Here, we present the effect of covalent functionalization on the Raman spectrum in terms of monofacial (one-sided) and bifacial (two-sided) functionalization using both monolayer and bilayer graphene. Asym. or 'Janus' functionalization is found to provide significantly increased levels of doping compared to other schemes allowing for control over graphene's electronic structure as well as control over surface functionality.
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    17. 17
      Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Edge State in Graphene Ribbons: Nanometer Size Effect and Edge Shape Dependence Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 17954– 17961 DOI: 10.1103/PhysRevB.54.17954
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      17
      Edge state in graphene ribbons: nanometer size effect and edge shape dependence
      Nakada, Kyoko; Fujita, Mitsutaka; Dresselhaus, Gene; Dresselhaus, Mildred S.
      Physical Review B: Condensed Matter (1996), 54 (24), 17954-17961CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Finite graphite systems having a zigzag edge exhibit a special edge state. The corresponding energy bands are almost flat at the Fermi level and thereby give a sharp peak in the d. of states. The charge d. in the edge state is strongly localized on the zigzag edge sites. No such localized state appears in graphite systems having an armchair edge. By utilizing the graphene ribbon model, we discuss the effect of the system size and edge shape on the special edge state. By varying the width of the graphene ribbons, we find that the nanometer size effect is crucial for detg. the relative importance of the edge state. We also have extended the graphene ribbon to have edges of a general shape, which is defined as a mixt. of zigzag and armchair sites. Examg. the relative importance of the edge state for graphene ribbons with general edges, we find that a non-negligible edge state survives even in graphene ribbons with less developed zigzag edges. We demonstrate that such an edge shape with three or four zigzag sites per sequence is sufficient to show an edge state, when the system size is on a nanometer scale. The special characteristics of the edge state play a large role in detg. the d. of states near the Fermi level for graphite networks on a nanometer scale.
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    18. 18
      Lee, S.; Lee, K.; Zhong, Z. Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition Nano Lett. 2010, 10, 4702– 4707 DOI: 10.1021/nl1029978
      [ACS Full Text ACS Full Text], [CAS], Google Scholar
      18
      Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition
      Lee, Seunghyun; Lee, Kyunghoon; Zhong, Zhaohui
      Nano Letters (2010), 10 (11), 4702-4707CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The discovery of elec. field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples were limited to μm2 size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here the authors report homogeneous bilayer graphene films over at least a 2 in. × 2 in area, synthesized by CVD on Cu foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, at. force microscopy, and TEM. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by elec. transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is obsd. in 98% of the devices.
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    19. 19
      Yan, K.; Peng, H.; Zhou, Y.; Li, H.; Liu, Z. Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition Nano Lett. 2011, 11, 1106– 1110 DOI: 10.1021/nl104000b
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      19
      Formation of Bilayer Bernal Graphene: Layer-by-Layer Epitaxy via Chemical Vapor Deposition
      Yan, Kai; Peng, Hailin; Zhou, Yu; Li, Hui; Liu, Zhongfan
      Nano Letters (2011), 11 (3), 1106-1110CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We report the epitaxial formation of bilayer Bernal graphene on copper foil via chem. vapor deposition. The self-limit effect of graphene growth on copper is broken through the introduction of a second growth process. The coverage of bilayer regions with Bernal stacking can be as high as 67% before further optimization. Facilitated with the transfer process to silicon/silicon oxide substrates, dual-gated graphene transistors of the as-grown bilayer Bernal graphene were fabricated, showing typical tunable transfer characteristics under varying gate voltages. The high-yield layer-by-layer epitaxy scheme will not only make this material easily accessible but reveal the fundamental mechanism of graphene growth on copper.
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    20. 20
      Liu, L.; Zhou, H.; Cheng, R.; Yu, W. J.; Liu, Y.; Chen, Y.; Shaw, J.; Zhong, X.; Huang, Y.; Duan, X. High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area AB-Stacked Bilayer Graphene ACS Nano 2012, 6, 8241– 8249 DOI: 10.1021/nn302918x
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      20
      High-Yield Chemical Vapor Deposition Growth of High-Quality Large-Area AB-Stacked Bilayer Graphene
      Liu, Lixin; Zhou, Hailong; Cheng, Rui; Yu, Woo Jong; Liu, Yuan; Chen, Yu; Shaw, Jonathan; Zhong, Xing; Huang, Yu; Duan, Xiangfeng
      ACS Nano (2012), 6 (9), 8241-8249CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Bernal-stacked (AB-stacked) bilayer graphene is of significant interest for functional electronic and photonic devices due to the feasibility to continuously tune its band gap with a vertical elec. field. Mech. exfoliation can be used to produce AB-stacked bilayer graphene flakes but typically with the sizes limited to a few microns. Chem. vapor deposition (CVD) has been recently explored for the synthesis of bilayer graphene but usually with limited coverage and a mixt. of AB- and randomly stacked structures. Herein we report a rational approach to produce large-area high-quality AB-stacked bilayer graphene. We show that the self-limiting effect of graphene growth on Cu foil can be broken by using a high H2/CH4 ratio in a low-pressure CVD process to enable the continued growth of bilayer graphene. A high-temp. and low-pressure nucleation step is found to be crit. for the formation of bilayer graphene nuclei with high AB stacking ratio. A rational design of a two-step CVD process is developed for the growth of bilayer graphene with high AB stacking ratio (up to 90%) and high coverage (up to 99%). The elec. transport studies demonstrate that devices made of the as-grown bilayer graphene exhibit typical characteristics of AB-stacked bilayer graphene with the highest carrier mobility exceeding 4000 cm2/V·s at room temp., comparable to that of the exfoliated bilayer graphene.
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    21. 21
      Sun, Z.; Raji, A.-R. O.; Zhu, Y.; Xiang, C.; Yan, Z.; Kittrell, C.; Samuel, E. L. G.; Tour, J. M. Large-Area Bernal-Stacked Bi-, Tri-, and Tetralayer Graphene ACS Nano 2012, 6, 9790– 9796 DOI: 10.1021/nn303328e
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      21
      Large-Area Bernal-Stacked Bi-, Tri-, and Tetralayer Graphene
      Sun, Zhengzong; Raji, Abdul-Rahman O.; Zhu, Yu; Xiang, Changsheng; Yan, Zheng; Kittrell, Carter; Samuel, E. L. G.; Tour, James M.
      ACS Nano (2012), 6 (11), 9790-9796CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Few-layer graphene, with Bernal stacking order, is of particular interest to the graphene community because of its unique tunable electronic structure. A synthetic method to produce such large area graphene films with precise thickness from 2 to 4 layers would be ideal for chemists and physicists to explore the promising electronic applications of these materials. Here, large-area uniform Bernal-stacked bi-, tri-, and tetralayer graphene films were successfully synthesized on a Cu surface in selective growth windows, with a finely tuned total pressure and CH4/H2 gas ratio. On the basis of the analyses obtained, the growth mechanism is not an independent homoexpitaxial layer-by-layer growth, but most likely a simultaneous-seeding and self-limiting process.
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    22. 22
      Wassei, J. K.; Mecklenburg, M.; Torres, J. A.; Fowler, J. D.; Regan, B. C.; Kaner, R. B.; Weiller, B. H. Chemical Vapor Deposition of Graphene on Copper from Methane, Ethane and Propane: Evidence for Bilayer Selectivity Small 2012, 8, 1415– 1422 DOI: 10.1002/smll.201102276
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      22
      Chemical Vapor Deposition of Graphene on Copper from Methane, Ethane and Propane: Evidence for Bilayer Selectivity
      Wassei, Jonathan K.; Mecklenburg, Matthew; Torres, Jaime A.; Fowler, Jesse D.; Regan, B. C.; Kaner, Richard B.; Weiller, Bruce H.
      Small (2012), 8 (9), 1415-1422CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      To study the effects of hydrocarbon precursor gases, graphene was grown by chem. vapor deposition from methane, ethane, and propane on copper foils. The larger mols. more readily produce bilayer and multilayer graphene, due to a higher carbon concn. and different decompn. processes. Single- and bilayer graphene can be grown with good selectivity in a simple, single-precursor process by varying the pressure of ethane from 250 to 1000 mTorr. The bilayer graphene is AB-stacked as shown by selected area electron diffraction anal. Addnl. propane only produces a combination of single- to few-layer and turbostratic graphene. The percent coverage is investigated using Raman spectroscopy and optical, scanning electron, and transmission electron microscopies. The data are used to discuss a possible mechanism for the second-layer growth of graphene involving the different cracking pathways of the hydrocarbons.
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    23. 23
      Zhao, P.; Kim, S.; Chen, X.; Einarsson, E.; Wang, M.; Song, Y.; Wang, H.; Chiashi, S.; Xiang, R.; Maruyama, S. Equilibrium Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene ACS Nano 2014, 8, 11631– 11638 DOI: 10.1021/nn5049188
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      23
      Equilibrium Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene
      Zhao, Pei; Kim, Sungjin; Chen, Xiao; Einarsson, Erik; Wang, Miao; Song, Yenan; Wang, Hongtao; Chiashi, Shohei; Xiang, Rong; Maruyama, Shigeo
      ACS Nano (2014), 8 (11), 11631-11638CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Using ethanol as the carbon source, self-limiting growth of AB-stacked bilayer graphene (BLG) has been achieved on Cu via an equil. chem. vapor deposition (CVD) process. We found that during this alc. catalytic CVD (ACCVD) a source-gas pressure range exists to break the self-limitation of monolayer graphene on Cu, and at a certain equil. state it prefers to form uniform BLG with a high surface coverage of ∼94% and AB-stacking ratio of nearly 100%. More importantly, once the BLG is completed, this growth shows a self-limiting manner, and an extended ethanol flow time does not result in addnl. layers. We investigate the mechanism of this equil. BLG growth using isotopically labeled 13C-ethanol and selective surface aryl functionalization, and results reveal that during the equil. ACCVD process a continuous substitution of graphene flakes occurs to the as-formed graphene and the BLG growth follows a layer-by-layer epitaxy mechanism. These phenomena are significantly in contrast to those obsd. for previously reported BLG growth using methane as precursor.
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    24. 24
      Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils Science 2009, 324, 1312– 1314 DOI: 10.1126/science.1171245
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      24
      Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils
      Li, Xuesong; Cai, Weiwei; An, Jinho; Kim, Seyoung; Nah, Junghyo; Yang, Dongxing; Piner, Richard; Velamakanni, Aruna; Jung, Inhwa; Tutuc, Emanuel; Banerjee, Sanjay K.; Colombo, Luigi; Ruoff, Rodney S.
      Science (Washington, DC, United States) (2009), 324 (5932), 1312-1314CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Graphene was attracting great interest because of its distinctive band structure and phys. properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. The authors grew large-area graphene films of the order of centimeters on Cu substrates by CVD using methane. The films are predominantly single-layer graphene, with a small percentage (<5%) of the area having few layers, and are continuous across Cu surface steps and grain boundaries. The low soly. of C in Cu appears to help make this growth process self-limiting. The authors also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities ≤4050 cm2/V-s at room temp.
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    25. 25
      Vlassiouk, I.; Regmi, M.; Fulvio, P.; Dai, S.; Datskos, P.; Eres, G.; Smirnov, S. Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene ACS Nano 2011, 5, 6069– 6076 DOI: 10.1021/nn201978y
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      25
      Role of Hydrogen in Chemical Vapor Deposition Growth of Large Single-Crystal Graphene
      Vlassiouk, Ivan; Regmi, Murari; Fulvio, Pasquale; Dai, Sheng; Datskos, Panos; Eres, Gyula; Smirnov, Sergei
      ACS Nano (2011), 5 (7), 6069-6076CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Graphene CVD growth on Cu foil using methane as a C source is strongly affected by hydrogen, which appears to serve a dual role: an activator of the surface bound C that is necessary for monolayer growth and an etching reagent that controls the size and morphol. of the graphene domains. The resulting growth rate for a fixed methane partial pressure has a max. at hydrogen partial pressures 200-400 times that of methane. The morphol. and size of the graphene domains, as well as the no. of layers, change with hydrogen pressure from irregularly shaped incomplete bilayers to well-defined perfect single layer hexagons. Raman spectra suggest the zigzag termination in the hexagons as more stable than the armchair edges.
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    26. 26
      Wu, B.; Geng, D.; Guo, Y.; Huang, L.; Xue, Y.; Zheng, J.; Chen, J.; Yu, G.; Liu, Y.; Jiang, L.; Hu, W. Equiangular Hexagon-Shape-Controlled Synthesis of Graphene on Copper Surface Adv. Mater. 2011, 23, 3522– 3525 DOI: 10.1002/adma.201101746
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      26
      Equiangular Hexagon-Shape-Controlled Synthesis of Graphene on Copper Surface
      Wu, Bin; Geng, Dechao; Guo, Yunlong; Huang, Liping; Xue, Yunzhou; Zheng, Jian; Chen, Jianyi; Yu, Gui; Liu, Yunqi; Jiang, Lang; Hu, Wenping
      Advanced Materials (Weinheim, Germany) (2011), 23 (31), 3522-3525CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A large scale synthesis of equiangular hexagonal singular or multilayer graphene is reported by methane CVD on Cu surface at ambient pressure. The shape reflects the hexagonal graphene lattice, possessing either zigzag or armchair edges. The hexagon-shaped graphene shows no observable defects confirmed by Raman spectra, and is formed by nucleation and growth mechanism, thus allowing control of both d. and size. Moreover, the shape evolution follows an empirical rule that higher CH4 flow rate leads to shorter nucleation time, higher growth rates and larger deviations from equiangular hexagon shape. Based on these observations, we proposed a growth model that qual. establishes a connection between various exptl.. conditions and the final state of the grown graphene, and is in principle capable of predicting their results from different conditions in the Cu-Methane CVD system. Moreover, this system provides direct evidence of layer spatial arrangement in the case of multi-layer graphene flakes, leading to a growth model which describes the process of layer growth order and reasonably explains the important size correlation of different layers found in out expts. Finally, these graphene flakes show mobility higher than 1900 cm2 V-1s-1, demonstrating their good quality.
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    27. 27
      Nie, S.; Wu, W.; Xing, S.; Yu, Q.; Bao, J.; Pei, S.-S.; McCarty, K. F. Growth from Below: Bilayer Graphene on Copper by Chemical Vapor Deposition New J. Phys. 2012, 14, 093028 DOI: 10.1088/1367-2630/14/9/093028
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      27
      Growth from below: bilayer graphene on copper by chemical vapor deposition
      Nie, Shu; Wu, Wei; Xing, Shirui; Yu, Qingkai; Bao, Jiming; Pei, Shin-Shem; McCarty, Kevin F.
      New Journal of Physics (2012), 14 (Sept.), 093028/1-093028/9CODEN: NJOPFM; ISSN:1367-2630. (Institute of Physics Publishing)
      We evaluate how a second graphene layer forms and grows on Cu foils during chem. vapor deposition (CVD). LEED and microscopy is used to reveal that the second layer nucleates and grows next to the substrate, i.e., under a graphene layer. This underlayer mechanism can facilitate the synthesis of uniform single-layer films but presents challenges for growing uniform bilayer films by CVD. We also show that the buried and overlying layers have the same edge termination.
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    28. 28
      Kalbac, M.; Frank, O.; Kavan, L. The Control of Graphene Double-Layer Formation in Copper-Catalyzed Chemical Vapor Deposition Carbon 2012, 50, 3682– 3687 DOI: 10.1016/j.carbon.2012.03.041
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      28
      The control of graphene double-layer formation in copper-catalyzed chemical vapor deposition
      Kalbac, Martin; Frank, Otakar; Kavan, Ladislav
      Carbon (2012), 50 (10), 3682-3687CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      The growth of graphene during Cu-catalyzed chem. vapor deposition was studied using 12CH4 and 13CH4 precursor gases. We suggest that the growth begins by the formation of a multilayer cluster. This seed increases its size but the growth speed of a particular layer depends on its proximity to the copper surface. The layer closest to the substrate grows fastest and thus further limits the growth rate of the upper layers. Nevertheless, the growth of the upper layers continues until the copper surface is completely blocked. It is shown that the upper layers can be removed by modification of the conditions of the growth by hydrogen etching.
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    29. 29
      Yan, Z.; Liu, Y.; Ju, L.; Peng, Z.; Lin, J.; Wang, G.; Zhou, H.; Xiang, C.; Samuel, E. L. G.; Kittrell, C.; Artyukhov, V. I.; Wang, F.; Yakobson, B. I.; Tour, J. M. Large Hexagonal Bi- and Trilayer Graphene Single Crystals with Varied Interlayer Rotations Angew. Chem. 2014, 126, 1591– 1595 DOI: 10.1002/ange.201306317
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      There is no corresponding record for this reference.
    30. 30
      Yeh, C.-H.; Lin, Y.-C.; Nayak, P. K.; Lu, C.-C.; Liu, Z.; Suenaga, K.; Chiu, P.-W. Probing Interlayer Coupling in Twisted Single-Crystal Bilayer Graphene by Raman Spectroscopy J. Raman Spectrosc. 2014, 45, 912– 917 DOI: 10.1002/jrs.4571
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      There is no corresponding record for this reference.
    31. 31
      Fang, W.; Hsu, A. L.; Caudillo, R.; Song, Y.; Birdwell, A. G.; Zakar, E.; Kalbac, M.; Dubey, M.; Palacios, T.; Dresselhaus, M. S.; Araujo, P. T.; Kong, J. Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy Nano Lett. 2013, 13, 1541– 1548 DOI: 10.1021/nl304706j
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      31
      Rapid Identification of Stacking Orientation in Isotopically Labeled Chemical-Vapor Grown Bilayer Graphene by Raman Spectroscopy
      Fang, Wenjing; Hsu, Allen L.; Caudillo, Roman; Song, Yi; Birdwell, A. Glen; Zakar, Eugene; Kalbac, Martin; Dubey, Madan; Palacios, Tomas; Dresselhaus, Millie S.; Araujo, Paulo T.; Kong, Jing
      Nano Letters (2013), 13 (4), 1541-1548CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      The growth of large-area bilayer graphene was of technol. importance for graphene electronics. The successful application of graphene bilayers critically relies on the precise control of the stacking orientation, which dets. both electronic and vibrational properties of the bilayer system. Toward this goal, an effective characterization method is critically needed to allow researchers to easily distinguish the bilayer stacking orientation (i.e., AB stacked or turbostratic). The authors developed such a method to provide facile identification of the stacking orientation by isotope labeling. Raman spectroscopy of these isotopically labeled bilayer samples shows a clear signature assocd. with AB stacking between layers, enabling rapid differentiation between turbostratic and AB-stacked bilayer regions. Using this method, the authors were able to characterize the stacking orientation in bilayer graphene grown through Low Pressure Chem. Vapor Deposition (LPCVD) with enclosed Cu foils, achieving almost 70% AB-stacked bilayer graphene. Also, by combining surface sensitive fluorination with such hybrid 12C/13C bilayer samples, the authors are able to identify that the 2nd layer grows underneath the 1st-grown layer, which is similar to a recently reported observation.
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    32. 32
      Hao, Y.; Wang, L.; Liu, Y.; Chen, H.; Wang, X.; Tan, C.; Nie, S.; Suk, J. W.; Jiang, T.; Liang, T.; Xiao, J.; Ye, W.; Dean, C. R.; Yakobson, B. I.; McCarty, K. F.; Kim, P.; Hone, J.; Colombo, L.; Ruoff, R. S. Oxygen-Activated Growth and Bandgap Tunability of Large Single-Crystal Bilayer Graphene Nat. Nanotechnol. 2016, 11, 426– 431 DOI: 10.1038/nnano.2015.322
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      32
      Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene
      Hao, Yufeng; Wang, Lei; Liu, Yuanyue; Chen, Hua; Wang, Xiaohan; Tan, Cheng; Nie, Shu; Suk, Ji Won; Jiang, Tengfei; Liang, Tengfei; Xiao, Junfeng; Ye, Wenjing; Dean, Cory R.; Yakobson, Boris I.; McCarty, Kevin F.; Kim, Philip; Hone, James; Colombo, Luigi; Ruoff, Rodney S.
      Nature Nanotechnology (2016), 11 (5), 426-431CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Bernal (AB)-stacked bilayer graphene (BLG) is a semiconductor whose bandgap can be tuned by a transverse elec. field, making it a unique material for a no. of electronic and photonic devices. A scalable approach to synthesize high-quality BLG is therefore crit., which requires minimal cryst. defects in both graphene layers and maximal area of Bernal stacking, which is necessary for bandgap tunability. Here we demonstrate that in an oxygen-activated chem. vapor deposition (CVD) process, half-millimeter size, Bernal-stacked BLG single crystals can be synthesized on Cu. Besides the traditional 'surface-limited' growth mechanism for SLG (1 st layer), we discovered new microscopic steps governing the growth of the 2 nd graphene layer below the 1 st layer as the diffusion of carbon atoms through the Cu bulk after complete dehydrogenation of hydrocarbon mols. on the Cu surface, which does not occur in the absence of oxygen. Moreover, we found that the efficient diffusion of the carbon atoms present at the interface between Cu and the 1 st graphene layer further facilitates growth of large domains of the 2 nd layer. The CVD BLG has superior elec. quality, with a device on/off ratio greater than 104, and a tunable bandgap up to ∼100 meV at a displacement field of 0.9 V nm-1.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhslyjtLc%253D&md5=a19c8c23d36d962ef3bc01103e47ee9b
    33. 33
      Liu, X.; Fu, L.; Liu, N.; Gao, T.; Zhang, Y.; Liao, L.; Liu, Z. Segregation Growth of Graphene on Cu−Ni Alloy for Precise Layer Control J. Phys. Chem. C 2011, 115, 11976– 11982 DOI: 10.1021/jp202933u
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      Wu, Y.; Chou, H.; Ji, H.; Wu, Q.; Chen, S.; Jiang, W.; Hao, Y.; Kang, J.; Ren, Y.; Piner, R. D.; Ruoff, R. S. Growth Mechanism and Controlled Synthesis of AB-Stacked Bilayer Graphene on Cu-Ni Alloy Foils ACS Nano 2012, 6, 7731– 7738 DOI: 10.1021/nn301689m
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      34
      Growth Mechanism and Controlled Synthesis of AB-Stacked Bilayer Graphene on Cu-Ni Alloy Foils
      Wu, Yaping; Chou, Harry; Ji, Hengxing; Wu, Qingzhi; Chen, Shanshan; Jiang, Wei; Hao, Yufeng; Kang, Junyong; Ren, Yujie; Piner, Richard D.; Ruoff, Rodney S.
      ACS Nano (2012), 6 (9), 7731-7738CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Strongly coupled bilayer graphene (i.e., AB stacked) grows particularly well on com. 90-10" Cu-Ni alloy foil. However, the mechanism of growth of bilayer graphene on Cu-Ni alloy foils had not been discovered. Carbon isotope labeling (sequential dosing of 12CH4 and 13CH4) and Raman spectroscopic mapping were used to study the growth process. The mechanism of graphene growth on Cu-Ni alloy is by pptn. at the surface from carbon dissolved in the bulk of the alloy foil that diffuses to the surface. The growth parameters were varied to study their effect on graphene coverage and isotopic compn. Higher temp., longer exposure time, higher rate of bulk diffusion for 12C vs. 13C, and slower cooling rate all produced higher graphene coverage on this type of Cu-Ni alloy foil. The isotopic compn. of the graphene layer(s) could also be modified by adjusting the cooling rate. Large-area, AB-stacked bilayer graphene transferrable onto Si/SiO2 substrates was controllably synthesized.
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      Liu, W.; Kraemer, S.; Sarkar, D.; Li, H.; Ajayan, P. M.; Banerjee, K. Controllable and Rapid Synthesis of High-Quality and Large-Area Bernal Stacked Bilayer Graphene Using Chemical Vapor Deposition Chem. Mater. 2014, 26, 907– 915 DOI: 10.1021/cm4021854
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      35
      Controllable and Rapid Synthesis of High-Quality and Large-Area Bernal Stacked Bilayer Graphene Using Chemical Vapor Deposition
      Liu, Wei; Kraemer, Stephan; Sarkar, Deblina; Li, Hong; Ajayan, Pulickel M.; Banerjee, Kaustav
      Chemistry of Materials (2014), 26 (2), 907-915CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Bilayer graphene has attracted wide attention due to its unique band structure and bandgap tunability under specific (Bernal or AB) stacking order. However, it remains challenging to tailor the stacking order and to simultaneously produce large-scale and high-quality bilayer graphene. This work introduces a fast and reliable method of growing high-quality Bernal stacked large-area (>3 in. × 3 in.) bilayer graphene film or trilayer graphene domains (30 μm × 30 μm) using CVD on engineered Cu-Ni alloy catalyst films. The AB stacking order is evaluated by Raman spectra, electron diffraction pattern, and dual gate field-effect-transistor (FET) measurements, and a near-perfect AB stacked bilayer graphene coverage (>98%) was obtained. The synthesized bilayer and trilayer graphene with Bernal stacking exhibit electron mobility ≤3450 cm2/(V·s) and 1500 cm2/(V·s), resp., indicating comparable quality with respect to exfoliated bilayer and trilayer graphene. The record high (for CVD bilayer graphene) ON to OFF current ratios (up to 15) obtained for a large no. (>50) of dual-gated FETs fabricated at random across the large-area bilayer graphene film also corroborates the success of the authors' synthesis technique. Also, through catalyst engineering, growth optimization, and element anal. of catalyst, achieving surface catalytic graphene growth mode and precise control of surface carbon concn. are key factors detg. the growth of high quality and large area Bernal stacked bilayer graphene on Cu-Ni alloy. This discovery can not only open up new vistas for large-scale electronic and photonic device applications of graphene but also facilitate exploration of novel heterostructures formed with emerging beyond graphene two-dimensional at. crystals.
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    36. 36
      Lin, T.; Huang, F.; Wan, D.; Bi, H.; Xie, X.; Jiang, M. Self-Regulating Homogenous Growth of High-Quality Graphene on Co–Cu Composite Substrate for Layer Control Nanoscale 2013, 5, 5847– 5853 DOI: 10.1039/c3nr33124e
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      Lu, C.-C.; Lin, Y.-C.; Liu, Z.; Yeh, C.-H.; Suenaga, K.; Chiu, P.-W. Twisting Bilayer Graphene Superlattices ACS Nano 2013, 7, 2587– 2594 DOI: 10.1021/nn3059828
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      Twisting Bilayer Graphene Superlattices
      Lu, Chun-Chieh; Lin, Yung-Chang; Liu, Zheng; Yeh, Chao-Hui; Suenaga, Kazu; Chiu, Po-Wen
      ACS Nano (2013), 7 (3), 2587-2594CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decompn. of SiC; (ii) CVD on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycryst. structures. The present study studies bilayer graphene grown by ambient pressure CVD on polycryst. Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes result from the twist, with the angle detd. by TEM. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
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      Havener, R. W.; Zhuang, H.; Brown, L.; Hennig, R. G.; Park, J. Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene Nano Lett. 2012, 12, 3162– 3267 DOI: 10.1021/nl301137k
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      38
      Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene
      Havener, Robin W.; Zhuang, Houlong; Brown, Lola; Hennig, Richard G.; Park, Jiwoong
      Nano Letters (2012), 12 (6), 3162-3167CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Few-layer graphene is a prototypical layered material, whose properties are detd. by the relative orientations and interactions between layers. Exciting elec. and optical phenomena were obsd. for the special case of Bernal-stacked few-layer graphene, but structure-property correlations in graphene which deviates from this structure are not well understood. Here, the authors combine 2 direct imaging techniques, dark-field TEM (DF-TEM) and widefield Raman imaging, to establish a robust, 1-to-one correlation between twist angle and Raman intensity in twisted bilayer graphene (tBLG). The Raman G band intensity is strongly enhanced due to a previously unreported singularity in the joint d. of states of tBLG, whose energy is exclusively a function of twist angle and whose optical transition strength is governed by interlayer interactions, enabling direct optical imaging of these parameters. Also, findings suggest future potential for novel optical and optoelectronic tBLG devices with angle-dependent, tunable characteristics.
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    39. 39
      Hu, B.; Ago, H.; Ito, Y.; Kawahara, K.; Tsuji, M.; Magome, E.; Sumitani, K.; Mizuta, N.; Ikeda, K.; Mizuno, S. Epitaxial Growth of Large-Area Single-Layer Graphene over Cu(111)/Sapphire by Atmospheric Pressure CVD Carbon 2012, 50, 57– 65 DOI: 10.1016/j.carbon.2011.08.002
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      Epitaxial growth of large-area single-layer graphene over Cu(1 1 1)/sapphire by atmospheric pressure CVD
      Hu, Baoshan; Ago, Hiroki; Ito, Yoshito; Kawahara, Kenji; Tsuji, Masaharu; Magome, Eisuke; Sumitani, Kazushi; Mizuta, Noriaki; Ikeda, Ken-ichi; Mizuno, Seigi
      Carbon (2012), 50 (1), 57-65CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
      We report the atm. pressure CVD growth of single-layer graphene over a cryst. Cu(1 1 1) film heteroepitaxially deposited on c-plane Al2O3. Orientation-controlled, epitaxial single-layer graphene is achieved over the Cu(1 1 1) film on Al2O3, while a polycryst. Cu film deposited on a Si wafer gives non-uniform graphene with multilayer flakes. Moreover, the CVD temp. is found to affect the quality and orientation of graphene grown on the Cu/Al2O3 substrates. The CVD growth at 1000° gives high-quality epitaxial single-layer graphene whose orientation of hexagonal lattice matches with the Cu(1 1 1) lattice which is detd. by the Al2O3's crystallog. direction. At lower CVD temp. of 900°, low-quality graphene with enhanced Raman D band is obtained, and it showed 2 different orientations of the hexagonal lattice; one matches with the Cu lattice and another rotated by 30°. C isotope-labeling expt. indicates rapid exchange of the surface-adsorbed and gas-supplied C atoms at the higher temp., resulting in the highly crystd. graphene with energetically most stable orientation consistent with the underlying Cu(1 1 1) lattice.
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    40. 40
      Ago, H.; Kawahara, K.; Ogawa, Y.; Tanoue, S.; Bissett, M. A.; Tsuji, M.; Sakaguchi, H.; Koch, R. J.; Fromm, F.; Seyller, T.; Komatsu, K.; Tsukagoshi, K. Epitaxial Growth and Electronic Properties of Large Hexagonal Graphene Domains on Cu(111) Thin Film Appl. Phys. Express 2013, 6, 075101 DOI: 10.7567/APEX.6.075101
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      40
      Epitaxial growth and electronic properties of large hexagonal graphene domains on Cu(111) thin film
      Ago, Hiroki; Kawahara, Kenji; Ogawa, Yui; Tanoue, Shota; Bissett, Mark A.; Tsuji, Masaharu; Sakaguchi, Hidetsugu; Koch, Roland J.; Fromm, Felix; Seyller, Thomas; Komatsu, Katsuyoshi; Tsukagoshi, Kazuhito
      Applied Physics Express (2013), 6 (7), 075101/1-075101/4CODEN: APEPC4; ISSN:1882-0778. (Japan Society of Applied Physics)
      Large hexagonal single-cryst. domains of single-layer graphene are epitaxially grown by ambient-pressure chem. vapor deposition over a thin Cu(111) film deposited on c-plane sapphire. The hexagonal graphene domains with a max. size of 100 μm are oriented in the same direction due to the epitaxial growth. Reflecting high crystallinity, a clear band structure with the Dirac cone is obsd. by angle-resolved photoelectron spectroscopy (ARPES), and a high carrier mobility exceeding 4000 cm2 V-1 s-1 was obtained on SiO2/Si at room temp. This epitaxial approach combined with large domain growth is expected to contribute to future electronic applications.
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      Ago, H.; Ito, Y.; Mizuta, N.; Yoshida, K.; Hu, B.; Orofeo, C. M.; Tsuji, M.; Ikeda, K.; Mizuno, S. Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire ACS Nano 2010, 4, 7407– 7414 DOI: 10.1021/nn102519b
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      41
      Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire
      Ago, Hiroki; Ito, Yoshito; Mizuta, Noriaki; Yoshida, Kazuma; Hu, Baoshan; Orofeo, Carlo M.; Tsuji, Masaharu; Ikeda, Ken-ichi; Mizuno, Seigi
      ACS Nano (2010), 4 (12), 7407-7414CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Epitaxial CVD growth of uniform single-layer graphene is demonstrated over Co film crystd. on c-plane sapphire. The single cryst. Co film is realized on the sapphire substrate by optimized high-temp. sputtering and successive H2 annealing. This cryst. Co film enables the formation of uniform single-layer graphene, while a polycryst. Co film deposited on a SiO2/Si substrate gives a no. of graphene flakes with various thicknesses. Also, an epitaxial relation between the as-grown graphene and Co lattice is obsd. when synthesis occurs at 1000°; the direction of the hexagonal lattice of the single-layer graphene completely matches with that of the underneath Co/sapphire substrate. The orientation of graphene depends on the growth temp. and, at 900°, the graphene lattice is rotated at 22 ± 8° with respect to the Co lattice direction. The authors' work expands a possibility of synthesizing single-layer graphene over various metal catalysts. Also, the authors' CVD growth gives a graphene film with predefined orientation, and thus can be applied to graphene engineering, such as cutting along a specific crystallog. direction, for future electronics applications.
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      Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making Graphene Visible Appl. Phys. Lett. 2007, 91, 063124 DOI: 10.1063/1.2768624
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      Making graphene visible
      Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K.
      Applied Physics Letters (2007), 91 (6), 063124/1-063124/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Microfabrication of graphene devices used in many exptl. studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepd. on top of Si wafers with a certain thickness of SiO2. The authors study graphene's visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. By using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300 nm (the current std.) and, esp., ≈100 nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quant. describe the exptl. data.
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      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple Phys. Rev. Lett. 1996, 77, 3865– 3868 DOI: 10.1103/PhysRevLett.77.3865
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      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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      Hamada, I.; Otani, M. Comparative van der Waals Density-Functional Study of Graphene on Metal Surfaces Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 153412 DOI: 10.1103/PhysRevB.82.153412
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      Comparative van der Waals density-functional study of graphene on metal surfaces
      Hamada, Ikutaro; Otani, Minoru
      Physical Review B: Condensed Matter and Materials Physics (2010), 82 (15), 153412/1-153412/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We present a comparative van der Waals d.-functional (vdW-DF) study of graphene adsorbed on (111) surfaces of Ni, Cu, Pd, Ag, Au, and Pt, using the second version of vdW-DF (vdW-DF2) of Lee et al. [Phys. Rev. B 82, 081101(R) (2010)] and the exchange functional (C09) developed by Cooper [Phys. Rev. B 81, 161104(R) (2010)]. We show that the use of the vdW-DF2 correlation together with the C09 exchange yields the most satisfactory results. Adsorption geometries of graphene are in good agreement with available expt. data, and the electronic structure of graphene varies depending on the nature of the substrate. Band-gap opening at the K point obsd. on the Ni(111) surface is reproduced reasonably well.
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      Otani, M.; Sugino, O. First-Principles Calculations of Charged Surfaces and Interfaces: A Plane-Wave Nonrepeated Slab Approach Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 115407 DOI: 10.1103/PhysRevB.73.115407
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      First-principles calculations of charged surfaces and interfaces: A plane-wave nonrepeated slab approach
      Otani, M.; Sugino, O.
      Physical Review B: Condensed Matter and Materials Physics (2006), 73 (11), 115407/1-115407/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      A new first-principles computational approach to a charged surface/interface is presented. The surface is modeled as a slab imposed with boundary conditions to screen the excess surface charge. To treat this model, which is nonperiodic in the surface normal direction, a std. pseudopotential plane-wave scheme is modified at the Poisson solver part with the help of the Green's function technique. Benchmark calcns. are done for Al/Si(111) with the bias voltage applied between the surface and the model scanning tunneling microscopy (STM) tip, the model back gate, or the model soln. The calcns. are found to be efficient and stable, and their implementation is found to be easy. Because of the flexibility, the scheme is considered to be applicable to more general exptl. situations.
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      Amano, H. Growth of GaN Layers on Sapphire by Low-Temperature-Deposited Buffer Layers and Realization of p-type GaN by Magnesium Doping and Electron Beam Irradiation Angew. Chem. 2015, 54, 7764– 7769 DOI: 10.1002/anie.201501651
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      Luo, Z.; Lu, Y.; Singer, D. W.; Berck, M. E.; Somers, L. A.; Goldsmith, B. R.; Johnson, A. T. C. Effect of Substrate Roughness and Feedstock Concentration on Growth of Wafer-Scale Graphene at Atmospheric Pressure Chem. Mater. 2011, 23, 1441– 1447 DOI: 10.1021/cm1028854
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      Effect of Substrate Roughness and Feedstock Concentration on Growth of Wafer-Scale Graphene at Atmospheric Pressure
      Luo, Zhengtang; Lu, Ye; Singer, Daniel W.; Berck, Matthew E.; Somers, Luke A.; Goldsmith, Brett R.; Johnson, A. T. Charlie
      Chemistry of Materials (2011), 23 (6), 1441-1447CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The growth of large-area graphene on catalytic metal substrates is a topic of both fundamental and technol. interest. We have developed an atm. pressure chem. vapor deposition (CVD) method that is potentially more cost-effective and compatible with industrial prodn. than approaches based on synthesis under high vacuum. Surface morphol. of the catalytic Cu substrate and the concn. of carbon feedstock gas were found to be crucial factors in detg. the homogeneity and electronic transport properties of the final graphene film. The use of an electropolished metal surface and low methane concn. enabled the growth of graphene samples with single layer content exceeding 95%. Field effect transistors fabricated from CVD graphene made with the optimized process had room temp. hole mobilities that are a factor of 2-5 larger than those measured for samples grown on as-purchased Cu foil with larger methane concn. A kinetic model is proposed to explain the obsd. dependence of graphene growth on catalyst surface roughness and carbon source concn.
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    48. 48
      Zhang, Y.; Gomez, L.; Ishikawa, F. N.; Madaria, A.; Ryu, K.; Wang, C.; Badmaev, A.; Zhou, C. Comparison of Graphene Growth on Single-Crystalline and Polycrystalline Ni by Chemical Vapor Deposition J. Phys. Chem. Lett. 2010, 1, 3101– 3107 DOI: 10.1021/jz1011466
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      48
      Comparison of Graphene Growth on Single-Crystalline and Polycrystalline Ni by Chemical Vapor Deposition
      Zhang, Yi; Gomez, Lewis; Ishikawa, Fumiaki N.; Madaria, Anuj; Ryu, Koungmin; Wang, Chuan; Badmaev, Alexander; Zhou, Chongwu
      Journal of Physical Chemistry Letters (2010), 1 (20), 3101-3107CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The authors report a comparative study and Raman characterization of the formation of graphene on single crystal Ni (111) and polycryst. Ni substrates using CVD. Preferential formation of monolayer/bilayer graphene on the single crystal surface is attributed to its atomically smooth surface and the absence of grain boundaries. In contrast, CVD graphene formed on polycryst. Ni leads to a higher percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman surface mapping reveals that the area percentages of monolayer/bilayer graphene are 91.4% for the Ni (111) substrate and 72.8% for the polycryst. Ni substrate under comparable CVD conditions. The use of single crystal substrates for graphene growth may open ways for uniform high-quality graphene over large areas.
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    49. 49
      Reina, A.; Thiele, S.; Jia, X.; Bhaviripudi, S.; Dresselhaus, M. S.; Schaefer, J. A.; Kong, J. Growth of Large-Area Single- and Bi-Layer Graphene by Controlled Carbon Precipitation on Polycrystalline Ni Surfaces Nano Res. 2009, 2, 509– 516 DOI: 10.1007/s12274-009-9059-y
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      49
      Growth of large-area single- and bi-layer graphene by controlled carbon precipitation on polycrystalline Ni surfaces
      Reina, Alfonso; Thiele, Stefan; Jia, Xiaoting; Bhaviripudi, Sreekar; Dresselhaus, Mildred S.; Schaefer, Juergen A.; Kong, Jing
      Nano Research (2009), 2 (6), 509-516CODEN: NRAEB5; ISSN:1998-0124. (Springer)
      We report graphene films composed mostly of one or two layers of graphene grown by controlled carbon pptn. on the surface of polycryst. Ni thin films during atm. chem. vapor deposition (CVD). Controlling both the methane concn. during CVD and the substrate cooling rate during graphene growth can significantly improve the thickness uniformity. As a result, one- or two- layer graphene regions occupy up to 87% of the film area. Single layer coverage accounts for 5%-11% of the overall film. These regions expand across multiple grain boundaries of the underlying polycryst. Ni film. The no. d. of sites with multilayer graphene/graphite (>2 layers) is reduced as the cooling rate decreases. These films can also be transferred to other substrates and their sizes are only limited by the sizes of the Ni film and the CVD chamber. Here, we demonstrate the formation of films as large as 1 in2. These findings represent an important step towards the fabrication of large-scale high-quality graphene samples.
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      Odahara, G.; Otani, S.; Oshima, C.; Suzuki, M.; Yasue, T.; Koshikawa, T. In-situ Observation of Graphene Growth on Ni(111) Surf. Sci. 2011, 605, 1095– 1098 DOI: 10.1016/j.susc.2011.03.011
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      50
      In-situ observation of graphene growth on Ni(111)
      Odahara, Genki; Otani, Shigeki; Oshima, Chuhei; Suzuki, Masahiko; Yasue, Tsuneo; Koshikawa, Takanori
      Surface Science (2011), 605 (11-12), 1095-1098CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      Graphene growth of mono-, bi-, and tri-layers on Ni(111) through surface segregation was obsd. in situ by low energy electron microscopy. The C segregation was controlled by adjusting substrate temp. from 1200 K to 1050 K. After the completion of the first layer at 1125 K, the second layer grew at the interface between the first-layer and the substrate at 1050 K. The third layer also started to grow at the same temp., 1050 K. All the layers exhibited a 1 × 1 at. structure. The edges of the first-layer islands were straight lines, reflecting the hexagonal at. structure. On the other hand, the shapes of the second-layer islands were dendritic. The edges of the third-layer islands were again straight lines similar to those of the first-layer islands. The phenomena presumably originate from the changes of interfacial-bond strength of the graphene to Ni substrate depending on the graphene thickness. No nucleation site of graphene layers was directly obsd. All the layers expanded out of the field of view and covered the surface. The no. of nucleation sites is extremely small on Ni(111) surface. This finding might open the way to grow the high quality, single-domain graphene crystals.
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      Fine Structure Constant Defines Visual Transparency of Graphene
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      Science (Washington, DC, United States) (2008), 320 (5881), 1308CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      There is a small group of phenomena in condensed matter physics that is defined only by the fundamental consts. and does not depend on material parameters. Examples are the resistivity quantum, h/e2 (h is Planck's const. and e the electron charge), that appears in a variety of transport expts. and the magnetic flux quantum, h/e, playing an important role in the physics of supercond. By and large, sophisticated facilities and special measurement conditions are required to observe any of these phenomena. We show that the opacity of suspended graphene is defined solely by the fine structure const., α = e2/ℏc ≈ 1/137 (where c is the speed of light), the parameter that describes coupling between light and relativistic electrons and that is traditionally assocd. with quantum electrodynamics rather than materials science. Despite being only one atom thick, graphene is found to absorb a significant (πα = 2.3%) fraction of incident white light, a consequence of graphene's unique electronic structure.
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      Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers Phys. Rev. Lett. 2006, 97, 187401 DOI: 10.1103/PhysRevLett.97.187401
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      Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K.
      Physical Review Letters (2006), 97 (18), 187401/1-187401/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Graphene is the 2-dimensional building block for C allotropes of every other dimensionality. Its electronic structure is captured in its Raman spectrum that clearly evolves with the no. of layers. The D peak 2nd order changes in shape, width, and position for an increasing no. of layers, reflecting the change in the electron bands via a double resonant Raman process. The G peak slightly down-shifts. This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
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      Hibino, H.; Kageshima, H.; Maeda, F.; Nagase, M.; Kobayashi, Y.; Yamaguchi, H. Microscopic Thickness Determination of Thin Graphite Films Formed on SiC from Quantized Oscillation in Reflectivity of Low-Energy Electrons Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 075413 DOI: 10.1103/PhysRevB.77.075413
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      Microscopic thickness determination of thin graphite films formed on SiC from quantized oscillation in reflectivity of low-energy electrons
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      Physical Review B: Condensed Matter and Materials Physics (2008), 77 (7), 075413/1-075413/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Low-energy electron microscopy (LEEM) was used to measure the reflectivity of low-energy electrons from graphitized SiC(0001). The reflectivity shows distinct quantized oscillations as a function of the electron energy and graphite thickness. Conduction bands in thin graphite films form discrete energy levels whose wave vectors are normal to the surface. Resonance of the incident electrons with these quantized conduction band states enhances electrons to transmit through the film into the SiC substrate, resulting in dips in the reflectivity. The dip positions are well explained using tight-binding and 1st-principles calcns. The graphite thickness distribution can be detd. microscopically from LEEM reflectivity measurements.
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      Ago, H.; Ohta, Y.; Hibino, H.; Yoshimura, D.; Takizawa, R.; Uchida, Y.; Tsuji, M.; Okajima, T.; Mitani, H.; Mizuno, S. Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces Chem. Mater. 2015, 27, 5377– 5385 DOI: 10.1021/acs.chemmater.5b01871
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      Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces
      Ago, Hiroki; Ohta, Yujiro; Hibino, Hiroki; Yoshimura, Daisuke; Takizawa, Rina; Uchida, Yuki; Tsuji, Masaharu; Okajima, Toshihiro; Mitani, Hisashi; Mizuno, Seigi
      Chemistry of Materials (2015), 27 (15), 5377-5385CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The growth of single-layer graphene on Cu metal by chem. vapor deposition (CVD) is a versatile method for synthesizing high-quality, large-area graphene. It is known that high CVD temps., close to the Cu melting temp. (1083 °C), are effective for the growth of large graphene domains, but the growth dynamics of graphene over the high-temp. Cu surface is not clearly understood. We investigated the surface dynamics of the single-layer graphene growth by using heteroepitaxial Cu(111) and Cu(100) films. At relatively lower temps., 900-1030 °C, the as-grown graphene showed the identical orientation with the underlying Cu(111) lattice. However, when the graphene was grown above 1040 °C, a new stable configuration of graphene with 3.4° rotation became dominant. This slight rotation is interpreted by the enhanced graphene-Cu interaction due to the formation of long-range ordered structure. Further increase of the CVD temp. resulted in graphene which is rotated with wide angle distributions, suggesting the enhanced thermal fluctuation of the Cu lattice. The band structures of CVD graphene grown at different temps. are well correlated with the obsd. structural change of the graphene. The strong impact of high CVD temp. on a Cu catalyst was further confirmed by the structural conversion of a Cu(100) film to Cu(111) which occurred during the high-temp. CVD process. Our work presents important insight into the growth dynamics of CVD graphene, which can be developed to high-quality graphene for future high-performance electronic and photonic devices.
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      Zhao, H.; Lin, Y.-C.; Yeh, C.-H.; Tian, H.; Chen, Y.-C.; Xie, D.; Yang, Y.; Suenaga, K.; Ren, T.-L.; Chiu, P.-W. Growth and Raman Spectra of Single-Crystal Trilayer Graphene with Different Stacking Orientations ACS Nano 2014, 8, 10766– 10773 DOI: 10.1021/nn5044959
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      Growth and Raman Spectra of Single-Crystal Trilayer Graphene with Different Stacking Orientations
      Zhao, Haiming; Lin, Yung-Chang; Yeh, Chao-Hui; Tian, He; Chen, Yu-Chen; Xie, Dan; Yang, Yi; Suenaga, Kazu; Ren, Tian-Ling; Chiu, Po-Wen
      ACS Nano (2014), 8 (10), 10766-10773CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Understanding the growth mechanism of graphene layers in chem. vapor deposition (CVD) and their corresponding Raman properties is technol. relevant and of importance for the application of graphene in electronic and optoelectronic devices. Here, we report CVD growth of single-crystal trilayer graphene (TLG) grains on Cu and show that lattice defects at the center of each grain persist throughout the growth, indicating that the adlayers share the same nucleation site with the upper layers and these central defects could also act as a carbon pathway for the growth of a new layer. Statistics shows that ABA, 30-30, 30-AB, and AB-30 make up the major stacking orientations in the CVD-grown TLG, with distinctive Raman 2D characteristics. Surprisingly, a high level of lattice defects results whenever a layer with a twist angle of θ = 30° is found in the multiple stacks of graphene layers.
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      Vanin, M.; Mortensen, J. J.; Kelkkanen, A. K.; Garcia-Lastra, J. M.; Thygesen, K. S.; Jacobsen, K. W. Graphene on Metals: A van der Waals Density Functional Study Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 081408 DOI: 10.1103/PhysRevB.81.081408
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      Graphene on metals: A van der Waals density functional study
      Vanin, M.; Mortensen, J. J.; Kelkkanen, A. K.; Garcia-Lastra, J. M.; Thygesen, K. S.; Jacobsen, K. W.
      Physical Review B: Condensed Matter and Materials Physics (2010), 81 (8), 081408/1-081408/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      We use d. functional theory (DFT) with a recently developed van der Waals d. functional (vdW-DF) to study the adsorption of graphene on Co, Ni, Pd, Ag, Au, Cu, Pt, and Al(111) surfaces. In contrast to the local-d. approxn. (LDA) which predicts relatively strong binding for Ni,Co, and Pd, the vdW-DF predicts weak binding for all metals and metal-graphene distances in the range 3.40-3.72 Å. At these distances the graphene band structure as calcd. with DFT and the many-body G0W0 method is basically unaffected by the substrate, in particular there is no opening of a band gap at the K point.
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    • 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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