A wafer-scale Bernal-stacked bilayer graphene film obtained on a dilute Cu (0.61 at Ni) foil using atmospheric pressure chemical vapour deposition

A bilayer graphene film was synthesized on a dilute Cu (0.61 at% Ni) foil using atmospheric pressure chemical vapour deposition (AP-CVD). Atomic force microscopy average step height analysis, scanning electron microscopy micrographs and the Raman optical microscopy images and spectroscopy data supported by selected area electron diffraction data showed that the bilayer graphene film obtained on a dilute Cu (0.61 at% Ni) foil is of high-quality, continuous over a wafer-scale (scale of an entire foil) and mainly Bernal stacked. These data clearly showed the capability of a dilute Cu (0.61 at% Ni) foil for growing a wafer-scale bilayer graphene film. This capability of a dilute Cu (0.61 at% Ni) foil was ascribed primarily to the metal surface catalytic activity of Cu and Ni catalyst. A wafer-scale bilayer graphene film obtained on a dilute Cu (0.61 at% Ni) foil has a sheet resistance of 284 U sq 1 (measured using a fourpoint probe station). Time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy showed a high surface concentration of Ni in the dilute Cu (0.61 at% Ni) foil which altered the surface catalytic activity of the Cu to grow a wafer-scale bilayer graphene film.


Introduction
2][3] However, many of these applications are restricted by the zero band gap of graphene. 4,56][7] Hence, graphene synthesis has been focused on growing high-quality and large-area AB-stacked bilayer graphene.Chemical vapour deposition (CVD) is a favourable synthesis technique for graphene since it can grow high-quality and large-area or wafer-scale graphene, which is important for electronic devices. 8,9In addition, atmosphericpressure CVD is technologically more accessible for graphene growth.
Generally, CVD synthesis of graphene starts with the decomposition of hydrocarbon into active carbon atoms on catalytic metal substrates (e.g.1][12][13][14] In CVD graphene growth, Cu is a favourable catalytic metal substrate due to its very low solubility of carbon (i.e.<0.001 at% at 1000 C), 15 low cost, high etchability and capability of growing a homogeneous monolayer graphene lm.7][18] Such a challenge for Cu is typically ascribed primarily to the low decomposition rate of hydrocarbon gas on the substrate surface. 17,19,20CVD synthesis of graphene on a Cu substrate typically favours monolayer graphene growth due to the very low solubility of carbon in Cu. 19 According to Harpale et al., 21 a surface-to-bulk diffusion of carbon atoms in Cu is restricted by preferential carbon-carbon bonds formation (i.e.carbon-carbon dimer pairs) over Cu-carbon bonds.Therefore, isothermal CVD synthesis of graphene on Cu occurs predominantly during the hydrocarbon exposure for several minutes. 19n contrast to Cu, Ni is known to have higher decomposition rate of hydrocarbon and higher solubility of carbon (i.e.$1.3 at% at 1000 C (ref.22)) which leads to a sufficient supply of active carbon atoms for CVD synthesis of graphene multilayers. 17,20However, a CVD multilayer graphene lm on Ni typically has non-uniform and randomly rotated layers of graphene due to non-uniform precipitation or segregation of carbon atoms from different grains surfaces and grain boundaries. 12,23nterestingly, since CVD synthesis of graphene on Cu substrates is limited to the surface of the catalyst (favours monolayer graphene growth), a Cu surface engineered with Ni has a capability of growing large-area multilayers of graphene due to Ni since it has higher decomposition rate of hydrocarbon compared to Cu.In previous studies, Cu/Ni thin lms and commercial Cu-Ni alloys have demonstrated such capability, including the growth of large-area AB-stacked bilayer graphene. 20,24,25In these studies, the Cu/Ni thin lms have Ni concentrations >5 at% (ref.17, 24 and 26) and commercial Cu (88.0 wt%)-Ni (9.9 wt%) 27 and Cu (67.8 wt%)-Ni (31.0 wt%) 20 foils have Ni bulk concentrations of $11 at% (ref.27) and $33 at% (ref.20) respectively, which are much higher than the Ni bulk concentration of 0.61 at% in the dilute Cu (0.61 at% Ni) foil used in this study.In non-dilute Cu-Ni foils (i.e.Cu foils with high Ni bulk concentrations), CVD graphene growth is known to dominate from segregation or precipitation processes which leads to variation in the thickness uniformity and stacking order in multilayer graphene lms. 20,23,25,26Therefore, the idea of a dilute Cu (0.61 at% Ni) foil is aimed at obtaining high surface concentration of Ni (1 to 3 at%) in Cu (0.61 at% Ni) foil through bulk-to-surface diffusion of Ni while maintaining a low bulk concentration of Ni (<1 at%) in Cu(Ni) foil during hydrocarbon exposure for graphene growth.Mainly, the aim of using a dilute Cu (0.61 at% Ni) foil is to obtain a large-area AB-stacked bilayer graphene predominantly during the hydrocarbon exposure for several minutes.Liu et al. 17 synthesized a high-quality and large-area AB-stacked bilayer graphene lm using Cu (1200 nm)/Ni (400 nm) thin lms which had a Ni surface concentration of about 3 at% during low pressure CVD graphene growth.Though their study shows a Ni surface concentration of about 3 at% in Cu (1200 nm)/Ni (400 nm) thin lms, these lms have a Ni bulk concentration of about 25 at% which could lead to CVD graphene growth by precipitation processes and that would lead to variation in the thickness uniformity and stacking order in multilayer graphene lms.In addition, annealed Cu-Ni thin lms have a preferential (111) surface which favourably grows monolayer graphene, in contrast, annealed Alfa Aesar Cu foil for graphene growth has a preferential (001) surface which causes compact graphene island formation.It is worth noting that the study of Liu et al. 17 prepared graphene lms at a temperature of 920 C and background pressure of 0.2 mbar using CVD, but we are aiming at using atmospheric background pressure (AP-CVD) and temperatures higher than 920 C (i.e.970 C).In a simplied view of the kinetics of the CVD process which are different for both low pressure and atmospheric pressure CVD, 19 to get high quality/purity graphene layers in CVD the background pressure of the CVD substrate should be minimized to the high vacuum limit, particularly, at CVD temperatures around 900 C (especially in the case when methane is a source of active carbon species).Therefore, the lower the background pressure of the CVD substrate (Low Pressure (LP-CVD)), the lower the density of impurities and residual gas in the system the higher the quality of graphene layers. 19In contrast to LP-CVD, AP-CVD grows defective/low-quality graphene layers at CVD temperatures around 900 C.However, at temperatures higher than 900 C (i.e.$1000 C), AP-CVD grows high-quality (acceptable quality) graphene layers.
This study focused on the AP-CVD synthesis and characterization of a high-quality and wafer-scale (scale of an entire foil) AB-stacked bilayer graphene lm obtained on a dilute Cu (0.61 at% Ni) foil and compared the growth to the results of AP-CVD growth under identical conditions on pure Cu foil (for monolayer and bilayer graphene lms obtained on pure Cu foils see Fig. S1-S4 in the ESI †).Atomic force microscopy (AFM) average step height analysis showed the thickness of bilayer graphene, scanning electron microscopy (SEM) micrographs showed uniform and continuous graphene layers and the Raman optical microscopy images and spectroscopy data supported by selected area electron diffraction (SAED) data showed highquality and continuous (wafer-scale) AB-stacked bilayer graphene for the graphene lm obtained on the dilute Cu (0.61 at% Ni) foil, while bilayer graphene growth on the Cu foil showed bilayer domains on a monolayer graphene background (Fig. S3 and S4 in the ESI †).The wafer-scale bilayer graphene lm obtained on a dilute Cu (0.61 at% Ni) foil has a sheet resistance of 284 U sq À1 .Aer growth, a high surface concentration of Ni compared to the Ni bulk concentration in dilute Cu (0.61 at% Ni) foil was conrmed and quantied with time-of-ight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) respectively.

Experimental
Graphene synthesis and transfer onto 300 nm SiO 2 /Si substrates Cu foil samples ($20 Â 20 mm 2 ) were obtained from a high purity (99.8%) 25 mm thick annealed Cu foil for graphene growth ordered from Alfa Aesar.The surface of obtained Cu foil samples was cleaned by immersing samples in aqueous nitric acid for 15 s to dissolve impurities, then in distilled water followed by a ultra-sonic bath with acetone and isopropanol and dry-blowing with N 2 to remove water residues. 28A dilute Cu (0.61 at% Ni) foil was obtained by doping a Cu foil (mass ¼ 268 mg) with Ni (mass ¼ 1.5 mg).A 116 nm thin layer of high purity (99.99%)Ni was thermally evaporated onto a Cu foil in a vacuum chamber with a pressure of 3 Â 10 À3 Pa.Aer evaporation of Ni onto Cu, the Cu/Ni sample was annealed at 950 C for 8 h under an argon atmosphere to obtain a homogeneous distribution of Ni concentration (0.61 at%) in Cu foil.Inductively coupled plasma optical emission spectrometry conrmed 0.61 at% Ni concentration in dilute Cu (0.61 at% Ni) foil.Pure Cu and Cu (0.61 at% Ni) foils were simultaneously loaded in AP-CVD at a centre of a quartz tube for bilayer graphene growth.
Cu and Cu (0.61 at% Ni) foils were kept under Ar (300 sccm) and H 2 (100 sccm) while the temperature was ramped from room temperature to 1050 C at a heating rate of 0.5 C s À1 and was maintained at this temperature for 20 min to obtain large Cu grains.Aer 20 min, the temperature was cooled at a cooling rate of À0.2 C s À1 to 980 C. At 980 C, the bilayer graphene lms on Cu and Cu (0.61 at% Ni) foils were obtained from a mixture of Ar (300 sccm), H 2 (9 sccm) and CH 4 (10 sccm) for 5 min.Immediately aer growth, the CH 4 ow was closed and the quartz tube was pushed to the cooler region of the furnace where samples rapidly cooled down to 600 C within 90 s and then to a temperature of less than 80 C before the samples were taken out.
The graphene thin lms obtained on foils were transferred onto 300 nm SiO 2 /Si substrates and TEM grids for TEM/SAED measurements by spin coating (at 3000 rpm for 30 s) a thin layer of polymethyl methacrylate (PMMA) (average M w $ 996 000 by GPC dissolved in chlorobenzene with a concentration of 46 mg mL À1 ) on the as-grown graphene on foils.The PMMA/graphene/foils were placed in 1 M iron nitrate to etch off Cu and Cu(Ni).PMMA/graphene lms oated in the etchant aer the foils were etched.These lms were then transferred using a polyethylene terephthalate (PET) to the 5% hydrochloride (HCl), then, to deionized (DI) water to dissolve the iron nitrate.Subsequently, the PMMA/graphene lms were transferred onto 300 nm thick SiO 2 /Si substrates.Finally, PMMA was removed by placing samples in the acetone bath for 6 h. 29

Samples characterization
The step height analysis of graphene thickness was obtained using a Dimension Icon AFM (Bruker) with nanoscope analysis soware in ScanAsyst contact mode.SEM micrographs of the prepared graphene lms were observed with a Zeiss Ultra Plus 55 eld emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 1.0 kV.Prepared graphene lms were characterized with a WITec Alpha 300 micro-Raman imaging system with 532 nm excitation laser.Raman spectra were measured at room temperature with the laser power set below 2 mW in order to minimize heating effects.Electron diffraction patterns of graphene samples were obtained with high-resolution transmission electron microscopy (HRTEM) (Jeol JEM-2100F Field Emission Electron Microscope, with a maximum analytical resolution of 200 kV and a probe size under 0.5 nm).The graphene lm sheet resistance measurements were carried out at room temperature using a Signatone four-point probe station, and a DC current in the range of 0-2.0 mA was used.The surface elemental map images of Cu and dilute Cu (0.61 at% Ni) foils were obtained with timeof-ight secondary ion mass spectrometry (TOF-SIMS) using a Ga + primary ion beam and the analyses were carried out over an area of 500 Â 500 mm 2 and ion sputter gun area of 1000 Â 1000 mm 2 .The mass spectra were calibrated to the following mass peaks in positive mode: Al, Na, Ni, Fe, Si, C, C 2 H 5 , K and Cu.The Ni surface concentration in dilute Cu (0.61 at% Ni) foil was quantied with X-ray photoelectron spectroscopy (XPS).A Physical Electronics VersaProbe 5000 instrument was used employing a 100 mm monochromatic Al-Ka to irradiate the Cu (0.61 at% Ni) foil surface.Photoelectrons were collected using a 180 hemispherical electron energy analyzer.The Cu (0.61 at% Ni) foil was analyzed at a 45 angle between the foil surface and the path to the analyzer.Survey spectra were obtained at the pass energy of 117.5 eV, with a step size of 0.1 eV.The highresolution spectra of elements, C 1s, Cu 2p, Ni 2p, and O 1s were measured to obtain the chemical composition of the foil surface.High-resolution spectra were obtained at the pass energy of 23.5 eV, with a step size of 0.05 eV.The spectra were obtained before and aer the foil were sputtered at a rate of 0.3 nm min À1 with an Ar beam operating at 500 V and 150 mA for several cycles while measuring the spectra aer each sputter duration.All binding energies were referenced to that of the binding energy of the Fermi level (E f ¼ 0 eV).The SEM micrographs in Fig. 2(a) and (b) show uniform and continuous bilayer graphene lm (at low and high magnications respectively) obtained on a dilute Cu (0.61 at% Ni) foil and transferred onto a 300 nm SiO 2 /Si substrate.The area of the graphene lm shown in the SEM micrographs in Fig. 2 is the same as those of other parts of the lm (see Fig. S5 in the ESI †), suggesting a uniform and continuous graphene lm over entire graphene lm.In contrast, SEM micrographs of the bilayer graphene lm obtained on pure Cu foil (Fig. S3 in the ESI †) shows non-uniform layers of graphene (lighter areas corresponding to monolayer and darker areas to multilayer (bilayer) graphene).Nonetheless, CVD synthesis of graphene on Cu favours monolayer graphene, hence its bilayer graphene shows bilayer domains on a monolayer graphene background. 19In the high magnication image (Fig. 2(b)), it can be seen that wrinkles due to graphene transfer are fewer in the bilayer graphene compared to monolayer graphene lm transferred under  In the Raman spectrum of high-quality graphene, the main features that are observable are the G-band mode ($1590 cm À1 ) and the 2D-band mode ($2690 cm À1 ).1][32] Fig. 3 shows the Raman data of a bilayer graphene lm obtained on a dilute Cu (0.61 at% Ni) foil and transferred onto a 300 nm SiO 2 /Si substrate.In Fig. 3(a) and (b), the Raman optical microscope images (at low and higher magnications respectively) also show a uniform and continuous graphene lm over a large-area (analysed area) of graphene lm obtained on a dilute Cu (0.61 at% Ni) foil in agreement with the photographic image (Fig. 1(a) for the sample transferred onto SiO 2 ) and SEM images (Fig. 2).Fig. 3(b) shows a slightly higher contrast than that of a monolayer graphene (Fig. S1(b) in the ESI †) since the optical microscope images of graphene lms display a colour contrast between monolayer and bi or multilayer graphene lms.Fig. 3(c) shows the average Raman spectrum of spectra acquired from a 30 mm 2 area (indicated with a square box in Fig. 3(b)) of a bilayer graphene lm.2][33] S1 in the ESI †), these features demonstrate the characteristics of bilayer graphene.Similar results are obtained from other parts of the graphene lm which suggest a continuous bilayer graphene lm (Fig. S6 in the ESI †).Fang et al. 33 have identied the AB-stacked bilayer graphene with 2D peaks FWHMs in the range of $40-70 cm À1 (with a cut-off FWHM of 70 cm À1 ) using a CVD graphene prepared on Cu foil.

Results and discussion
1][32] In monolayer graphene, the 2D peak has a single Lorentzian feature. 30In AB-stacked bilayer graphene the electronic band splits into two conduction and two valence bands and the split causes splitting of the phonon bands into two components which give rise to four peaks in the Raman 2D peak with peak frequencies at approximately 2655, 2680, 2700, and 2725 cm À1 and FWHMs equal to that of monolayer graphene. 301][32] The amplitudes of these four Lorentzians are relative, meaning, two Lorentzians at $2680 and $2700 cm À1 (inner peaks in 2D peak) have almost the same intensity and are higher than the other two at $2655 and $2725 cm À1 (outer peaks in 2D peak). 32For non-AB stacked bilayer graphene, the 2D peak is a single Lorentzian as in monolayer graphene, but with a larger FWHM and upshied frequency from that of monolayer graphene. 31ig. 4(c) shows the Raman spectra from data mapped in Fig. 4(a) and the 2D peaks were tted with four Lorentzians which demonstrate features of AB-stacked bilayer graphene.
Table 1 shows a summary of the analysis results of the Raman spectra of monolayer and bilayer graphene lms obtained on Cu (shown in the ESI †) and dilute Cu (0.61 at% Ni) foils.In this table, it can be seen that graphene features of bilayer graphene obtained on Cu foil overlap with those of monolayer graphene, suggesting the presence of a signicant fraction of monolayer graphene in the prepared bilayer graphene lm.In contrast to the bilayer graphene lm obtained on Cu foil, the bilayer graphene lm obtained on Cu (0.61 at% Ni) foil shows different features compared to monolayer graphene features as would be expected in Raman analysis of monolayer and multilayer (bilayer) graphene.The Raman spectral analysis showed that the bilayer graphene lm obtained on the dilute Cu (0.61 at% Ni) foil is predominantly AB-stacked bilayer graphene and that was further supported by electron diffraction analysis.
Fig. 5(a) shows a typical TEM image of the bilayer graphene lm obtained on Cu (0.61 at% Ni) foil and transferred on a lacey carbon TEM grid (see Fig. S7(a) in the ESI † for the low magni-cation TEM image).In Fig. 5(a), regions A and B shown in a hole of a lacey carbon TEM grid show an area without graphene and with graphene respectively.Fig. 5(b) shows a typical high magnication TEM image of graphene in region B of Fig. 5(a), and (c) shows a SAED pattern from the corresponding area which shows two sets of hexagonal diffraction spots.TEM diffraction patterns were analysed using a diffraction ring proler, which was developed for phase identication in complex microstructures. 35   hexagon and (1À110) for inner hexagon respectively. 36The electron diffraction patterns obtained at different positions of the graphene lm show similar results (see Fig. S7 in the ESI †).
It is known that the relative intensities of the spots in the outer hexagon are twice the intensities of the spots in the inner hexagon for AB-stacked bilayer graphene (shown with a schematic view in the gure inset). 32,33,36Therefore, the diffraction data (similar to that obtained from other spots of the same lm) show that the graphene lm obtained on dilute Cu (0.61 at% Ni) foil is predominantly AB-stacked bilayer graphene as evidenced by relative intensities shown in Fig. 5(d) in agreement with the Raman data above.A four-point probe/sheet resistance measuring system for thin lms was used to measure the sheet resistance of the bilayer graphene lm transferred onto the 300 nm SiO 2 /Si substrate and was obtained as 284 U sq À1 (see Fig. S8 in the ESI †).A sheet resistance of 284 U sq À1 measured for the bilayer graphene obtained on Cu (0.61 at% Ni) foil in this study is in the same order of magnitude with that measured from AB-stacked bilayer (287 U sq À1 ) graphene lm in ref. 20.
Fig. 6(a) and (b) show the map images of TOF-SIMS secondary ion intensities measured from a dilute Cu (0.61 at% Ni) foil surface of the as-received sample (i.e.without surface sputtering with an ion gun) and aer surface cleaning for 3 min with ion sputtering respectively.The foil was annealed under graphene growth conditions without methane source.Alfa Aesar Cu foil doped with Ni to obtain a dilute Cu (0.61 at% Ni) foil for graphene growth has a purity of 99.8% and about 0.2% unknown-impurities.The TOF-SIMS data (Fig. 6) shows the presence of Na, Al, Si, C 2 H 5 , K, Fe and Ni impurities in the Cu (0.61 at% Ni) surface and subsurface layers (bulk layers).These impurities have a potential to inuence the CVD graphene growth and the effect of each impurity will be determined by its metal-carbon interaction energy, metal-methane It is desirable to quantify the TOF-SIMS secondary-ion intensities measured; however, the quantication in TOF-SIMS is complicated because of the strong dependence of the secondary-ion yield on the matrix effects (target chemical and electronic character). 37,38urthermore, the surface fractional concentration of Ni in the dilute Cu (0.61 at% Ni) foil with an as-grown bilayer graphene lm was quantied with X-ray photoelectron spectroscopy.In the analysis, a resolved angle between the foil surface and the path to the analyzer focuses analysis within the topmost ($5) atomic layers.In this instance, the topmost ($5) atomic layers consist of two atomic layers of bilayer graphene and 2 atomic layers of Cu foil.The foil surface was sputter cleaned with ions for several cycles while measuring the spectra of elements, C 1s, Cu 2p, Ni 2p (shown in Fig. 7(a)-(c)) and O 1s aer each sputter cycle, to obtain the chemical composition of the foil surface (Table 2).In Fig. 7 and Table 2, it can be seen that before surface sputter cleaning, C 1s have high concentrations compared to Cu 2p substrate, O 1s (adsorbed from air) and Ni 2p and that conrms a lm of graphene on the foil surface.Aer a 2 min sputter cycle, Ni 2p shows a surface fractional concentration of 1.2 at% and the presence of C 1s and O 1s (restricted to the surface) suggest that the analysis is within the rst atomic layer of a Cu foil (see Fig. 7 and Table 2).
Interestingly, aer a 5 min sputter cycle, Ni 2p, C 1s and O 1s are not detected and Cu shows a fractional concentration of 99.9 at% which correspond to a relatively pure Cu.In this instance, a 5 min sputter cleaning at a rate of 0.3 nm min À1 is equivalent to a removal of 1.5 nm thick material which in this instance consist of a bilayer graphene ($1 nm thick including surface adsorbed carbon and oxygen from air) and approximately the rst two atomic layers of Cu ($0.5 nm).Accordingly, the analysis shown here aer 5 min sputter cleaning are from the topmost subsurface atomic layers of Cu as conrmed by the absence (zero concentrations) of C 1s and O 1s which are restricted to the surface of Cu.In brief, this analysis conrms a surface alloying of Cu with Ni (similar to the TOF-SIMS data above) while maintaining relatively pure Cu in the topmost subsurface atomic layers of the Cu.However, a Ni surface fractional concentration of 1.2 at% should be larger than 1.2 at%, at least 2.1 at% as calculated in Fig. S9 in the ESI, † because 1.2 at% is the fractional/average value of Ni concentration measured in the presence of other species rather than Cu alone by XPS.
Fig. 8 shows the C 1s core level spectra of the as-grown bilayer graphene lm on dilute Cu (0.61 at% Ni) foil.0][41] The tted sp 2 C]C peak has a dominating intensity which conrms the sp 2 hybridization property of graphene in the as-grown bilayer graphene lm, 30,41  the low-intensity oxide peaks could be due to adsorbed oxygen or carbon bonded oxygen during synthesis of the graphene lm.The p-p* electrons transition enhances the carbon to carbon bonds in graphene and conrms the high quality of the graphene (suggested by the Raman data) since the p-p* bonds determine the fundamental electronic properties of graphene. 30,40

Conclusions
This study demonstrated the synthesis of a wafer-scale (on the scale of an entire foil) and high-quality AB-stacked bilayer graphene lm on a dilute Cu (0.61 at% Ni) foil using AP-CVD.AFM, SEM, Raman, TEM/SAED and four-point probe/sheet resistance analysis showed that a bilayer graphene lm obtained on a dilute Cu (0.61 at% Ni) foil is of high-quality, continuous (wafer-scale) and mainly Bernal stacked.This study clearly showed the capability of a dilute Cu (0.61 at% Ni) foil for growing a wafer-scale bilayer graphene lm compared to a pure Cu foil which is known to grow bilayer domains on a monolayer graphene background in AP-CVD (see Fig. S3 and S4 in the ESI †).The capability of a dilute Cu (0.61 at% Ni) foil for growing a wafer-scale bilayer graphene lm was ascribed to the carbon solubility and the metal surface catalytic activity of Cu and Ni in

Fig. 1 (
Fig.1(a) shows photographic images of the Cu (0.61 at% Ni) foil ($20 Â 20 mm 2 ) used in AP-CVD growth of a wafer-scale (on the scale of an entire foil) bilayer graphene and transferred bilayer graphene lm on 300 nm SiO 2 /Si substrate with a continuous lm.In Fig.1(b), an AFM average step height prole across the graphene edge shown in the AFM micrograph shows that the thickness of the graphene lm obtained on a dilute Cu (0.61 at% Ni) foil is about 1.4 nm, suggesting bilayer graphene.The SEM micrographs in Fig.2(a) and (b) show uniform and continuous bilayer graphene lm (at low and high magnications respectively) obtained on a dilute Cu (0.61 at% Ni) foil and transferred onto a 300 nm SiO 2 /Si substrate.The area of the graphene lm shown in the SEM micrographs in Fig.2is the same as those of other parts of the lm (see Fig.S5in the ESI †), suggesting a uniform and continuous graphene lm over entire graphene lm.In contrast, SEM micrographs of the bilayer graphene lm obtained on pure Cu foil (Fig.S3in the ESI †) shows non-uniform layers of graphene (lighter areas corresponding to monolayer and darker areas to multilayer (bilayer) graphene).Nonetheless, CVD synthesis of graphene on Cu favours monolayer graphene, hence its bilayer graphene shows bilayer domains on a monolayer graphene background.19In the high magnication image (Fig.2(b)), it can be seen that wrinkles due to graphene transfer are fewer in the bilayer graphene compared to monolayer graphene lm transferred under

Fig. 1 A
Fig. 1 A continuous wafer-scale bilayer graphene film obtained using AP-CVD.(a) Photographs of the Cu (0.61 at% Ni) foil ($20 Â 20 mm 2 ) with an as-grown bilayer graphene film and transferred bilayer graphene film on a 300 nm SiO 2 /Si substrate.(b) AFM image (showing the edge) of bilayer graphene transferred onto a SiO 2 /Si substrate and height profile measured along the dotted line.
Fig. 4(a) and (b) show the mapping of the 2D peaks FWHMs and of the corresponding 2D to G peaks intensities ratio (I 2D /I G ) respectively of Raman spectra acquired from 30 mm 2 areas of a bilayer graphene lm obtained on a dilute Cu (0.61 at% Ni) foil.The 2D peaks FWHMs

Fig. 2 (
Fig.2 (a and b) SEM micrographs of a bilayer graphene film (at low and high magnifications respectively) obtained on a dilute Cu (0.61 at% Ni) foil and transferred onto a 300 nm SiO 2 /Si substrate.

Fig. 3 (
Fig.3 (a and b) Raman optical microscope images of a bilayer graphene film (at low and higher magnifications respectively) obtained on a dilute Cu (0.61 at% Ni) foil and transferred onto a 300 nm SiO 2 /Si substrate.(c) Average Raman spectrum of spectra acquired from a 30 mm 2 area (indicated with a square box in (b)) of a bilayer graphene film.

Fig. 5 (
d) shows the diffraction rings intensity prole which was indexed using the Miller-Bravais indices (hkil) for graphite where peaks at d ¼ 1.23 Å and peak d ¼ 2.13 Å in Fig. 5(d) correspond to indices (1À210) for outer

Fig. 4
Fig. 4 (a) The mapping of 2D peaks FWHMs and (b) of the corresponding 2D to G peaks intensities ratio (I 2D /I G ) for bilayer graphene film obtained on a dilute Cu (0.61 at% Ni) foil and transferred onto 300 nm SiO 2 /Si substrate.(c) Raman spectra from data mapped in (a) and the 2D peaks solid-lines are Lorentzians fits.

Fig. 5
Fig. 5 (a) TEM image of bilayer graphene film obtained on Cu (0.61 at% Ni) foil and transferred onto a lacey carbon TEM grid (regions A and B shown in a hole of a lacey carbon TEM grid show an area without graphene and with graphene respectively).(b) A high magnification TEM image of graphene in region B of (a).(c) A selected area electron diffraction (SAED) pattern from an area shown in (b) and showing two sets of hexagonal diffraction spots.(d) The diffraction rings intensity profile of two sets of hexagonal diffraction spots in (c) and the inset to the figure shows a schematic view of the AB-stacked bilayer graphene and diffraction rings.

Fig. 6
Fig. 6 (a) The map images of TOF-SIMS secondary ion intensities measured from a dilute Cu (0.61 at% Ni) foil surface of the as-received sample (i.e.without surface sputtering with ion gun) and (b) after surface cleaning for 3 min with ion sputtering.The foil was annealed under graphene growth conditions without methane source.

Table 1
Summary of the analysis results of the Raman spectra of monolayer and bilayer graphene films obtained on Cu (shown in the ESI) and dilute Cu (0.61 at% Ni) foils and transferred onto 300 nm SiO 2 /Si substrates for characterization