Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu Ni alloys

Transcription

1 SUPPLEMENTARY INFORMATION DOI: /NMAT4477 Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu Ni alloys Tianru Wu 1, Xuefu Zhang 1, Qinghong Yuan 2, Jiachen Xue 3, Guangyuan Lu 1,4, Zhihong Liu 3, Huishan Wang 1, Haomin Wang 1, Feng Ding 5*, Qingkai Yu 3*, Xiaoming Xie 1,6*, Mianheng Jiang 1,6 1 State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences. 865 Changning Road, Shanghai , China. 2 Department of Physics, East China Normal University, 500 Dongchuan Road, Minhang, Shanghai , China. 3 Ingram School of Engineering, and MSEC, Texas State University, San Marcos, Texas, 78666, USA 4 University of Chinese Academy of Sciences, Beijing , China. 5 Institute of Textiles and Clothing, Hong Kong Polytechnic University, Kowloon, Hong Kong, China 6 School of Physical Science and Technology, ShanghaiTech University, 319 Yueyang Road, Shanghai , China * Correspondence to: xmxie@mail.sim.ac.cn, qy10@txstate.edu and feng.ding@polyu.edu.hk NATURE MATERIALS 1

2 Preparation and Characterization of Cu-Ni alloy As shown in Fig. S1, a 6 cm 6 cm Cu foil (25 μm, 99.8 %, Alfa-Aesar) was first electrochemically polished to reduce the surface roughness. Then the polished Cu foil was annealed at 1050 o C for 2 h in a mixture of Ar/H 2 (400/100 sccm) under atmospheric pressure (AP) to further increase its surface flatness and grain size. After annealing, the Cu foil was coated by a nickel layer with electroplating (see in Fig. S1b). The Ni-coated Cu foils were loaded into a 10 cm diameter CVD fused quartz tube. The annealing was carried out at 1050 ºC for 2h in a H 2 and Ar flow (H 2 /Ar=50 sccm/1000 sccm) at atmosphere pressure (the annealing process was often integrated into the growth process, as shown in Fig.S2). After that, the two layers of metals were completely inter-diffused to form Cu-Ni alloy foils in which the atomic proportions are determined by the amount of the deposited Ni film before annealing. It was found that grains in the Cu foil have a wide range of crystallographic orientations before annealing (Figs S1d-e). After high temperature annealing, strong (100) texture developed (Figs.S1d and S1f) and the average grain size increased to ~200 μm. Nickel plated on copper showed the same texture as that of copper and the texture was maintained after Cu-Ni alloying (Fig. S1d). Experimental parameters for a typical CVD graphene growth process For graphene growth process, the temperature was set up in the range of o C, and the total pressure was kept at 1 atm. At the same time, sccm methane (0.5 % CH 4 diluted in Ar), 15 sccm H 2 and 300 sccm Ar were fed into the CVD system. It is found that the growth rate of graphene domains is primarily determined by the growth temperature and methane partial pressure. Millimeter sized graphene domains could be obtained even after exposure to methane for only about ~5-10 min (as shown in Figs S2b-e). Fig.S2b gives typical optical picture of graphene domains on copper foil after 15min growth at 1050 º C, while Fig.S2c-e gives results on Cu85Ni15 at growth temperatures of 1050 ºC, 1070 ºC and 1100 ºC. Feedstock control for graphene growth on Cu-Ni alloy

3 (1) Global feeding For graphene growth with global carbon feedstock feeding, the precursor flow was fixed as 120 sccm (0.5 % CH 4 diluted in Ar), as shown in Fig. S3a. (2) Local feeding For graphene growth from a controlled nucleus, a small feeding quartz pipette was set above Cu-Ni alloy foil in the 10 cm diameter quartz tube for the localized precursor feeding, as shown in Fig. S3b. At the center of the precursor flow, the concentration of precursor gas is highest and it decreases with the distance from the center owing to the dilution by Ar and H 2. During the growth after nucleation, the precursor flow was gradually increased to guarantee the enough carbon supply at the growth front of graphene on substrate. As shown in Fig. S3, the methane concentration (0.05 % CH 4 diluted in Ar) in the small quartz tube is increased from 8 to 24 sccm for a total of 2.5 h (8 sccm for 30 min, 12 sccm for 30 min, 16 sccm for 30 min, 20 sccm for 30 min, and 24 sccm for 30 min) during the growth procedure. A mixture of 1000 sccm Ar and 40 sccm H 2 in the large quartz tube were used as high diluted gas flow in order to ensure the single nucleus formation. It should be mentioned that the concentration of carbon feedstock at the local feeding region is critical to the formation of single graphene nucleus (Figs S3c-d). For example, under the experimental temperature of 1050 o C, only single nucleation was observed when the concentration of the methane flow on Cu 15 Ni 85 alloy was 8 sccm (Fig. S3c), while multiple nucleation could be observed when the concentration of methane flow was increased to 25 sccm (Fig. S3d). For pure copper and Cu 95 Ni 5, single nucleus control is not possible even with local precursor feeding conditions (Fig. S3e). The very low nucleation density and the moderate carbon solubility lead us to the special design of local precursor feeding. If we can introduce carbon precursor only to a local area, we can probably create controlled carbon super-saturation at designed location. As carbon super-saturation is one of the most important factors determining the nucleation, is may outperform other random contributing factors for nucleation, thus realizing single nucleus control. The experimental investigation confirmed that we found that right clue for the long-sought SCG synthesis through single nucleus control. It is important that the

4 concept is unique for appropriately formulated Cu-Ni alloy, which is not applicable for neither pure copper, nor for pure nickel substrate. Investigations of single graphene domain at nucleation stage by local feeding process We have carried out SEM investigations on graphene domains with lateral dimensions of about ~50 m, ~100 and ~500 m in a view area around 6 mm 4 mm. By carefully checking the graphene domain and the surroundings at different magnification, we confirm that we are really generating single nucleus on a wafer-scale substrate at nucleation stage by local feeding process (see Fig. S4). The capability of nucleation control at selected sites was also demonstrated in our experiments. At first, the nozzle was positioned to different sites on Cu-Ni substrates. After growth process by local feeding, single graphene domain forms at the selected sites (see Fig. S5). The experiments we demonstrated here indicate that the controllable technology can enable the growth of monolayer large-area SCG. Transmission electron microscope (TEM) and selected area electron diffraction (SAED) measurement Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies were carried out for crystalline structure characterizations of single graphene domain grown by local carbon feeding process (Fig. S6). A ~2 mm graphene domain was transferred onto an amorphous carbon-covered TEM grid. SEM image of the graphene on TEM grid shows clean and uniform structural integrity with the hexagonal shape well preserved (Fig. S6a). The high-resolution TEM image (Fig. S6b) indicates the graphene grain is monolayer. SAED measurement was used to confirm the crystallinity of the large graphene domain. Figs S6c h shows nine representative SAED patterns taken at the locations that are labelled correspondingly in Fig. S6a. The SAED patterns show the same set of hexagonal diffraction spots without rotation. Quantitative intensity analysis of one typical diffraction spots along the line (illustrated in Fig. S6d) was given in Fig. S6c. It can be seen that the intensity of the inner rings are always higher than that of the outer ones. These results unambiguously confirm that the graphene domain is a single crystalline monolayer. Tilted-angle-SAED measurements were also performed to verify the number of graphene layer. The

5 results and the analysis were given in Figs S7a-g. We can see that while increasing the tilting angle, the intensities of the diffraction peaks (0-110) and (1-210) decreased monotonously (see Fig. S7h), in agreement with its monolayer nature 1,2. The (0-110) diffraction peak broadens with increasing tilt angle (see Fig. S7i), also supporting that the graphene is monolayer 3. Low energy electron diffraction (LEED) investigations on a large single crystalline graphene domain Besides TEM/SAED investigations, we have performed LEED investigations on a ~1.5 cm graphene domain, the results are given in Fig. S8. LEED measurements were performed using an Elmitec GmbH LEEM III instrument. The largest field of view available in LEEM is about 75 m. The as-grown graphene samples were loaded into the LEEM instrument, and then annealed at ~200 C for overnight in ultra-high vacuum (base pressure ~ Torr). The measurements were taken at room temperature. Selected-area LEED was typically obtained from 30 m diameter regions of the surface. The consistent orientation of the LEED patterns acquired at multiple locations on the domain shows that the graphene domain is indeed a single crystal in centimeter size scale (Figs S8b-q). Raman mapping of inch-sized monolayer SCG by local feeding process A typical inch-sized graphene was transferred to a 2-inch silicon wafer with 300nm SiO 2 as shown by the optical picture in Fig. S9a. Raman microprobe spectroscopy (Thermo Fisher Scientific) using an Ar + laser (wavelength 532 nm) is utilized in the characterization of graphene. Fig. S9b shows Raman spectra collected at 50 locations selected randomly. Raman mapping were performed at typical locations on the large-area SCG as numbered in Fig. S9a and maps of intensity ratio of the 2D/G peak are shown in Figs S9c-k. The Raman results prove that the graphene domain is a single crystalline monolayer. Optical transmittance measurement for inch-sized monolayer SCG Optical transmittance measurements were also performed on an inch-sized graphene domain on quartz substrate. Optical micrograph of the sample is shown in Fig. S10. The transmittance of large-area SCG is ~ 97.4 % at 550 nm. The typical result indicates that the graphene sample is mono-layered.

6 Conceptual proposal for the synthesis of larger SCG In our study, we achieved the synthesis of ~1.5 inch high-quality SCG with mobility of ~12,000 cm 2 /V s from a controlled single nucleus on Cu-Ni alloy in 2.5 hours. Our work demonstrated the feasibility of large-area SCG from a controlled single nucleus and is a reliable starting point for large SCG, say > 4. The current limited size of SCG is attributed to limited space and apparatus design in our CVD system. Here, we propose to design an apparatus with a rotating substrate and movable pipette for feeding precursor gases (Fig. S11), which is expected to carry out the synthesis of SCG larger than 4. Inspired by the structure of a gramophone, the metal substrate in CVD rotates like the disc on gramophone and the pipette nozzle can moves outwards from the center of rotation. The pipette nozzle always locates at the edge of as-grown single crystalline graphene domain and only provides appropriate growth atmosphere at the growth front. The effects of different Ni content and cooling rates on the growth of graphene To make sure that the mechanism results in only monolayer graphene, we have performed extensive investigations with cooling rates ranging from 10 o C/min to 10 o C/sec on Cu 85 Ni 15. Surface morphology of graphene on Si substrate (with a 300 nm SiO 2 layer) was taken by optical microscopy (Leica material Microscope DM6000M). Typical optical images and Raman spectra displayed in Figs S12a-b clearly indicate that the graphene films are always in monolayer. If we use an alloy with much higher nickel content, say Cu 50 Ni 50, we will get multilayer graphene with a high cooling rate (10 C/sec) as shown in Fig. S12c. Here, we also explored the influences of several parameters on the graphene nucleation density, domain size and layer number. Twenty five sets of growth parameters (P1-P25) were listed in Table. S1. Isotopic experiment for graphene on Cu90Ni10, Cu85Ni15 and Cu80Ni20 At the beginning of feeding carbon precursor, 13 CH 4 was used for forming the graphene nuclei and followed sizable graphene grains. Thereafter, carbon precursor was switched to 12 CH 4. The carbon isotope compositions of the graphene grains were characterized by Raman mapping, using wavenumber of 2D band as the indicator, where wavenumber 2580 cm -1 (blue) represents pure 13 C and wavenumber 2675

7 cm -1 (red) represents pure 12 C, shown as the color bar in Fig. 3 in the main text. A 532 nm excitation laser with a 50 objective lens in Thermo Scientific DXR micro-raman microscope was used for acquisition of Raman spectra and maps of the graphene domains. As shown in Fig. 3d, 13 C contents immediately decreased to ~ 0 %, ~ 60 %, and ~ 70 % after switching isotopes for Cu 90 Ni 10, Cu 85 Ni 15 and Cu 80 Ni 20, respectively, and afterwards, the 13 C contents gradually decreased for Cu 85 Ni 15 and Cu 80 Ni 20, respectively. It is also noticed that, around the edge of graphene domains on Cu 85 Ni 15 and Cu 80 Ni 20, the 13 C contents tend to stop dropping, but seem like a constant. We attribute the stop of the dropping of 13 C contents to the non-equilibrium precipitates of carbon from substrates after turning off precursor gases during cooling. During cooling process, the precipitate of carbon mainly contributes the carbon supply, where the 13 C still take a part in substrates and won t be diluted anymore. The attached video shows the extension of growth front of graphene during growth by highlighting the 13 C contents. The color in the map indicates the intensity of Raman spectra at the wavenumber, where 2D band locates at the growth front, represented by the star sign. The colors from red to blue represent the intensity from maximum to minimum. It is observed that the red zone becomes wider and wider after switching isotopes, because the 13 C content decreases slower and slower with time, in the other word, the difference of the 13 C contents at adjacent locations along the growth orientation become smaller during the growth. Secondary ion mass spectrometry (SIMS) measurements SIMS can provide detailed chemical information of the surface and sub-surface of a material with high-accuracy depth profiling. In this study, we performed TOF-SIMS (ION-TOF V) depth profile analysis to further test the component of alloys and the carbon dissolution in Cu-Ni alloys. TOF SIMS mapping area is about μm 2, and the image lateral resolution is less than 1 μm. A Cs 1+ ion beam sputtering was used for layer removal, and total analysis depth is >100 nm. It could be observed that Cu and Ni were completely inter-diffused to each other to form Cu-Ni alloy foils with the atomic proportions determined by the amount of the deposited Ni film at high temperature annealing (shown in Fig. S13).

8 The SIMS measurement provided more decisive information to understand the local nucleation feature and growth conditions on Cu-Ni alloy. Surface enrichment of carbon was detected on the Cu-Ni alloy after the substrate was exposed to methane for few minutes. The surface concentration of carbon reaches atoms cm -3 and raises with the increasing of the proportion of Ni content (see Fig. S13). However, the carbon concentration is far below the saturation state in the bulk. The effect of growth parameters for graphene growth on Ni-Cu alloy substrate Here, we investigated some reaction conditions of graphene grown on Cu-Ni alloy substrate (Fig. S14 and Table. S1). From the data that have been presented, modulating the content of Ni in Cu-Ni alloy is the key factor in reducing graphene nuclei density and enabling the fast growth of larger-size graphene domains during a short growth time. We have carefully investigated the nucleation rates on a number of factors, the results are summarized in Table S1. It can be found that the nucleation density drops dramatically with increased nickel content in the alloy (Fig. S14a). The comparison of nucleation densities on Cu 85 Ni 15 and that on pure Cu are shown in Fig. S14b and S14c. It was found that the nucleation density on Cu 85 Ni 15 still keeps at a very low profile even under the condition of increasing methane flow rate. However, the increase of methane flow significantly affect the density of the nucleus on copper and Cu 95 Ni 5 substrates. It means that low nucleation density on Cu-Ni alloy could be easily realized because of the more extensive growth conditions. Density Functional Theory calculations For CH 4 decomposition, a Cu (100) or Ni doped Cu (100) surface was modeled by a slab of four atomic layers in which atoms in the bottom layer were fixed in their respective bulk positions. For carbon diffusion in the bulk metal substrate, the Cu (100) or Ni doped Cu (100) surface was modeled by a slab of seven atomic layers with the bottom two layers fixed to mimic the half-infinite catalyst structure. A vacuum region of more than 10 Å was used to ensure decoupling between neighboring slabs. All calculations were performed within the framework of density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP) 4,5. The electronic exchange and correlation are included

9 through the generalized gradient approximation (GGA) in the Perdew Burke Ernzerhof (PBE) 6 form. The spin polarized projector-augmented wave (PAW) 7 method was used to describe the electronic interaction, and the plane-wave kinetic-energy cutoff was set as 400 ev. During the structural relaxation, all atoms except those fixed ones were fully relaxed until the force on any given atom was smaller than 0.02 ev/å. A k-point mesh was used for a 4 4 Cu surface unit cell. The climbing-image nudged elastic band (cneb) 8 method was exploited to locate the transition states of CH 4 decomposition on the substrate surface, as well as C diffusion in the bulk of substrate. The formation energy of an adsorbed/dissolved C atom is defined as: E f = E C+Sub E Sub - E C@G Where E C+Sub is the energy of substrate with C atom dissolved in the surface or bulk, E Sub is the energy of the substrate, and E C@G is the energy of carbon in graphene. Device fabrication and measurement As shown in Fig. S15a, single crystalline graphene domains were transferred onto hexagonal boron nitride (h-bn) flakes pre-deposited on SiO 2 /Si substrate, followed by fabrication of graphene devices in Hall bar configuration. Transport measurements were carried out on h-bn substrate. As shown in Fig. S15a, resistance as a function of the back gate voltage of graphene on h-bn exhibits relatively narrow and symmetric Dirac peak with its charge neutrality point located at V g =25 V. Hall mobility ( Hall ) in our CVD graphene was extracted from magneto-resistance measurements by using Hall R R xy xx L 1. B is the magnetic-field intensity. L and W are channel length (4 µm) and width (1 µm), W B respectively. The carrier mobility extracted for several different samples ranges from 10,000-20,000 cm 2 /V s at room temperature. Graphene has two surfaces exposed to the environment. Because of the transfer process involved and defects induced by device fabrication, the transport measurement of CVD graphene exhibits some randomness in the Dirac point. Actually, the doping in our results was mainly caused by the impurities or residues at the interface in between graphene and h-bn. In some samples

10 measured, the Dirac point locates near zero of gate voltage. The mobility in our measurement was practically independent of temperature, indicating that it was still limited by charge impurity scattering. As shown in Fig. S15c, magneto-transport measurement shows quantum Hall states at all integer filling factors from 2-6 at a magnetic field of 9T, indicating the four-folded degeneracy of the Landau levels is lifted. Percolation analysis (1) The largest Ni cluster The largest Ni cluster size measured for 5, 10, 15, 20, 25 % are 9, 34, 91, 5660, 24022, 31693, respectively. The number of Ni atoms in the largest Ni cluster are %, 0.32 %, %, %, 89.4 % of the total Ni atoms for 5 %, 10 %, 15 %, 20 % and 25 % Ni contents respectively. A sudden jump from less than 100 (< 0.5 %) to more than 5000 (>25 %) at the percolation threshold (19.9 %) can be clearly seen, which implies that continuous Ni chains/channels will be built if the Ni contents is greater than the threshold content (19.9 %). And after the threshold (e.g., for 25 % Ni content), the largest Ni cluster contains distributed uniformly inside the box and thus C solubility inside the alloy can be greatly increased and their diffusion can be very fast. (2) The distribution of the Ni clusters that have at least one atom on the topmost layer From the above results, we can conclude that, for Ni content less than 10 %, continuous Ni chains (clusters) that has one atom on the top-most surface are distributed on the surface and may reach only 2-3 Ni layers depth from the surface; For Ni content greater than the percolation threshold, 19.9 %, the continuous Ni chains go everywhere inside the alloy; For Ni content between 10 % and 20 %, the chains may reach the depth of a few nm or ~ 5-10 Ni layer depth. (3) Mechanism of graphene CVD growth at different Ni content Based on the C diffusion model, that C atoms can quickly dissolve inside or diffuse outside the Cu-Ni alloy along a Ni chain only, we can conclude that:

11 i. There are no sufficient C dissolved into bulk for Ni content less than 10 % and most C precursors are on the surface of the alloy. So the mechanism of graphene growth is self-limited, same as that of Cu. ii. The dissolution and precipitation of the C atoms into/out of the Cu-Ni alloy can be very quick for Ni content > 20 %; There is sufficient C solubility inside the Ni and the exchange between the surface atom and bulk atom is very frequent. So, the mechanism of graphene growth is precipitation driven. iii. In the medium range of %, even though there are enough C atoms dissolved into the Cu-Ni alloy, their precipitation to the surface will be very difficult because C atom going from one Ni cluster to another one needs to overcome a large barrier, ~ 3 ev. For the case of 15 %, some C atoms can reach the depth of ~1-2 nm through continuous Ni chain and their diffusion to the surface can be fast. Therefore, during graphene growth, the C supply comes from the surface diffusion and bulk precipitation from limited depth. And thus 15 % Ni content allows the growth near the boundary between the two mechanisms surface-mediated and the bulk-precipitation. References 1 Jannik C. M., et al. The structure of suspended graphene sheets. Nature 446, (2007). 2 Lee S., et al. Wafer Scale Homogeneous Bilayer Graphene Films by Chemical Vapor Deposition. Nano Lett. 10, (2010). 3 Meyer, J. C., et al. On the roughness of single- and bi-layer graphene membranes. Solid State Commun.143, (2007). 4. Kresse, G. & Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, (1996). 5. Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B. 54, (1996).

12 6. Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, (1996). 7. Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B. 50, (1994). 8. Mills, G., Jónsson, H. & Schenter, G. K. Reversible work transition state theory: application to dissociative adsorption of hydrogen. Surf. Sci. 324, (1995). Figure S1. Preparation and characterization of Cu-Ni alloy. a) Electroplating setup for Ni layer deposition on Cu foils. b) Optical image of Cu-Ni bilayer by electroplating process. c) Surface morphology of the electroplated Ni layer on Cu foils after the electroplating process. d) XRD spectra of Cu and Cu-Ni bilayer before and after the annealing process.(e) EBSD of Cu grains before high temperature annealing.(f) EBSD of Cu-Ni alloy after high temperature annealing.

13 Figure S2. Experimental parameters for a typical graphene CVD growth process. a) Time dependence of experimental parameters for growing graphene grains. b-e) Optical images of graphene domains obtained on Cu foils at 1050 o C for 90 min and on Cu 85 Ni 15 foils at 1050 o C, 1070 o C and 1100 o C for 15 min.

14 Figure S3. Feedstock control for graphene growth on Cu-Ni alloy. a) Experimental parameters for growing graphene by global and local feeding of methane. b) Schematic diagram of experimental setup for local methane feeding in CVD process. c) Optical image of single graphene domain grown on Cu 85 Ni 15 alloy at 1050 o C for 60 min with diluent methane of 8 sccm. d) Optical image of multi graphene domains grown on Cu 85 Ni 15 alloy at 1050 o C for 60 min with diluent methane of 25 sccm. e) Optical image of graphene grains grown on Cu foils at 1050 o C for 30 min with diluted methane at 5 sccm.

15 Figure S4. SEM investigations showing single nucleus graphene domain with different sizes by using local feeding process. a) Illustration of viewing locations of single graphene domain grown at 1050 o C. b) An optical image showing a typical graphene nucleus on an inch-scale alloy foil. c-e) SEM images showing single nucleus at a 6mm 4mm area for graphene nucleus with different-sizes. Please note that images marked with 1-4 were taken above, below, to the right and to the left side of the nucleus. The nucleus was always displayed in the image to guide the reader for the exact viewing position.

16 Figure S5 Nucleation control of graphene at a selected position on Ni-Cu foil. a) Schematic diagram of the graphene nucleation by moving the feeding nozzle to the selected position of substrates. b-f) Optical images of graphene domains grown at the selected locations of Cu-Ni substrate.

17 Figure S6. TEM/SAED investigations showing that the graphene prepared by local feeding method is mono-layered and single crystalline. a) SEM image of a ~2 mm sized graphene domain transferred to a TEM grid. b) High-resolution TEM image of monolayer graphene. c) Intensities of the diffraction spots along the line illustrated in d. d-l) SAED patterns taken from nine representative regions indicated in a.

18 Figure S7. Tilt-angle SAED investigations justifying that the graphene is mono-layered. a-g) SAED patterns of graphene at different tilt angles of 0, 5, 10, 15, 20, 25 and 30. h) Evolution of intensities of the (0-110) and (1-210) SAED peaks and FWHM of the (0-110) SAED peaks with tilt angles. i) Intensities of the (0-110) peaks showing a significant broadening with increasing tilting angles.

19 Figure S8. LEED investigations proving the single crystallinity of graphene at centimeter scale. a) Optical image of a graphene domain grown on the alloy substrate. b-q) LEED patterns taken from representative regions marked in a. The extra weak LEED spots came from the faceted alloy substrate.

20 Figure S9. Raman investigations justifying that the large-area SCG is mono-layered. a) Optical image of an inch-sized graphene transferred to a silicon wafer (with a 300 nm SiO 2 layer). b) Raman spectrum randomly taken from ~50 locations on the inch-sized graphene. c-k) Raman mapping of 2D (~ 2680 cm 1 ) to G (~ 1580 cm 1 ) peak intensity ratio (I 2D /I G ) at typical locations marked in a.

21 Figure S10. Optical transmittance measurement of an inch-sized graphene domain transferred to quartz substrate. The transmittance at 550 nm is ~ 97.4 %, indicating that the graphene is mono-layered. The inset shows the optical image of the graphene sit on the quartz substrate. Figure S11. Schematic diagram of the CVD setup for larger SCG synthesis. A rotated substrate and careful control of local precursor gas supply are designed.

22 Figure S12. Typical optical images and Raman spectra of graphene samples prepared on alloys at 1050 o C with different Ni content and different cooling rates. a) Graphene grown on Cu 85 Ni 15 with a cooling rate of 10 o C/min. b) Graphene grown on Cu 85 Ni 15 with a cooling rate of 10 o C/sec. c) Graphene grown on Cu 50 Ni 50 with a cooling rate of 10 o C/sec. The homogeneous optical contrast and the intensity Ratio of the 2D/G Raman peaks clearly indicates that in a wide temperature range, the graphene grown on Cu 85 Ni 15 is monolayer. Multilayer graphene is evident on the Cu 50 Ni 50 by the inhomogeneous optical contrast and the Raman spectrum at typical location.

23 Figure S13. SIMS measurement of C surface segregation on Cu-Ni alloys with different Ni content. a) The concentration of C atom in Cu-Ni alloy with 10 at%, 15 at% and 20 at% Ni content, respectively, after graphene growth by APCVD at 1050 o C for 15 min. The inset is SIMS measurement of the component of cooled Cu-Ni alloy after high temperature annealing. b-c) SIMS image depth profile of C in the different depth of Cu foil (0-5 nm and nm).d-i) SIMS image depth profile of C in the different depth of Ni 15 Cu 85 foil (0-5 nm, 5-10 nm, nm, 20-40, and nm). The color scale represents secondary ion intensity.

24 Figure S14. Effects of alloy composition and carbon precursor flow rate on the nucleation density prepared at 1050 o C. a) Graphene nucleation density as a function of Ni content. b) Graphene nucleation density as a function of precursor flow rate on Cu 85 Ni 15. c) Graphene nucleation density as a function of precursor flow ratee on pure Cu.

25 Table S1. Growth conditions of P1-P25 in the AP-CVD system. Parameters for Components global feeding of Alloy Growth Temperature ( o C) 0.5 % CH 4 Growth (sccm) time (min) Nucleation Domain density (/cm 2 ) size (μm) P1 Cu 85 Ni <20 ~1100 P2 Cu 85 Ni <10 ~1700 P3 Cu 85 Ni <10 ~2550 P4 Cu <20 ~1050 P5 Cu 90 Ni < P6 Cu 80 Ni <20 ~850 P7 Cu 85 Ni <20 ~1100 P8 Cu 85 Ni <20 ~1100 P9 Cu 85 Ni <100 ~1100 P10 Cu < P11 Cu > P12 Cu > Parameters for Components local feeding of Alloy Growth 0.05 % CH 4 Temperature ( o C) (sccm) Growth time (min) Nucleation Domain density (/cm 2 ) size (μm) P13 Cu 85 Ni Stepwise 120 Single nucleus ~10500 from 8 to 20 P14 Cu 85 Ni Stepwise from 8 to Single nucleus ~35000 P15 Cu 85 Ni Single nucleus P16 Cu 85 Ni Multiple nuclei / P17 Cu 95 Ni Multiple nuclei / P18 Cu Multiple nuclei / Parameters for Components graphene films of Alloy Growth Temperature ( o C) 0.5 % CH 4 Growth Cooling rate (sccm) time (min) Layer number P19 Cu 85 Ni o C/min monolayer P20 Cu 85 Ni o C/sec monolayer P21 Cu 75 Ni o C/min monolayer P22 Cu 75 Ni o C/min 1-2 layers P23 Cu 75 Ni o C/sec 1-2 layers P24 Cu 50 Ni o C/min \ P25 Cu 50 Ni o C/sec multilayer

26 Figure S15. Electrical transport measured in a graphene device made from a large graphene domain. a) Transfer characteristics of the device at 300 K, the inset shows an optical microscopy image of a device on h-bn flake. Plots of R xx and R xy as a function of gate voltage at b) T = 300K and B = 3T, c) T = 2K and B = 9T. Quantum Hall effect fan diagram of d) longitudinal resistance (R xx ) and e) Hall resistance (R xy ) as a function of V g and magnetic field B at 2K.

27 Figure S16. The percolation analysis of Cu-Ni alloy. a) The maximum Ni cluster size in the simulated system (10 3 nm 3 ) vs. the Ni contents. b-f) The distribution of the Ni clusters that has at least one atom on the top-most layer in the simulated box and their depth analysis.