Nonepitaxial Wafer‐Scale Single‐Crystal 2D Materials on Insulators

Next‐generation nanodevices require 2D material synthesis on insulating substrates. However, growing high‐quality 2D‐layered materials, such as hexagonal boron nitride (hBN) and graphene, on insulators is challenging owing to the lack of suitable metal catalysts, imperfect lattice matching with substrates, and other factors. Therefore, developing a generally applicable approach for realizing high‐quality 2D layers on insulators remains crucial, despite numerous strategies being explored. Herein, a universal strategy is introduced for the nonepitaxial synthesis of wafer‐scale single‐crystal 2D materials on arbitrary insulating substrates. The metal foil in a nonadhered metal–insulator substrate system is almost melted by a brief high‐temperature treatment, thereby pressing the as‐grown 2D layers to well attach onto the insulators. High‐quality, large‐area, single‐crystal, monolayer hBN and graphene films are synthesized on various insulating substrates. This strategy provides new pathways for synthesizing various 2D materials on arbitrary insulators and offers a universal epitaxial platform for future single‐crystal film production.

Herein, we report a universal nonepitaxial synthesis (UNS) strategy for fabricating wafer-scale single-crystal 2D materials on diverse insulating substrates.High-quality 2D material layers were grown on both surfaces of our single-crystal metal foil, which was preplaced on an insulating substrate.A brief hightemperature treatment was applied, bringing the foil to an extremely soft near-molten state, which facilitated the adhesion of the metal foil-2D layer onto the insulator surface underneath.Eventually, the 2D material remained on the insulating substrate after cooling and removing the foil.This UNS strategy has been utilized to synthesize large-area, single-crystal, monolayer hBN and graphene on various insulators, as well as biand trilayer graphene after replacing the Cu(111) foil with the CuNi(111) alloy.This strategy is not limited to the type of insulating substrate or crystal lattice structure and can be widely applied to most single-crystal, polycrystalline, or amorphous insulating substrates.This approach is not only applicable to the growth of graphene and hBN but also expected to be extended to synthesizing other metal-substrate-based CVD-grown 2D materials.It also holds significance in achieving the growth of uniformly layered 2D van der Waals heterostructures, providing contributions for the future applications of mono-or multilayer 2D materials in electronic devices.

UNS Strategy
First, a nonadhered metal-insulator substrate system was designed and constructed by placing a prefabricated single-crystal Cu(111) foil on an insulating substrate (Figure 1a).The Cu(111) foil was loosely attached to the insulator surface, resulting in a considerable gap between them.This gap provided sufficient space for gases to enter and form 2D layers on the lower Cu(111) surface in a manner similar to the traditional CVD growth of 2D materials (Figure 1b).Monolayer hBN was then grown by subjecting the nonadhered Cu(111)-insulator substrate to the CVD system at 1050 °C under flowing hydrogen and argon.Subsequently, high-quality single-crystal monolayer hBN was formed uniformly on the top and bottom surfaces of our Cu(111) foil.After growth, the sample was moved to a high-temperature zone (preheated to 1097 °C), where the Cu(111) foil was heated and reached a near-molten state, became extremely soft, and adhered well to the surface of the insulator during this brief high-temperature treatment.Monolayer hBN grown on the lower surface was then pressed by the softened foil onto the insulator surface (Figure 1c).The high-temperature treatment time was fine-optimized to ensure that the Cu(111)-hBN was completely well attached onto the insulator surface while avoiding the excessive evaporation of Cu(111) (Figures S1 and S2, Supporting Information).The bottom surface of the Cu(111) foil was mostly flat and mirror-like following the heat treatment near the melting temperature, owing to rehardening on the underlying sapphire substrate (Figure S3, Supporting Information).The short duration of the hightemperature treatment and the continuously flowing gas environment prevented etching effects during this stage.Afterward, the sample was rapidly cooled by immediately moving from the high-temperature zone to a room-temperature zone in the quartz tube.After cooling, hydrogen plasma was used to remove the hBN film on the top Cu(111) foil surface, and the foil was then removed by overturned floating on an ammonium persulfate solution, leaving only monolayer hBN on the insulator (Figure 1d).Energy dispersive spectroscopy (EDS) was used to examine the sample surface to ensure that it was free of Cu residue (Figure S4, Supporting Information).Finally, a high-quality, large-area, single-crystal monolayer hBN film was grown on the insulating substrate.
A single-crystal metal substrate is essential to synthesizing high-quality, single-crystal 2D materials.In our study, singlecrystal Cu(111) foil was prepared from a polycrystalline foil using a contact-free-annealing method [32] and gently placed on the insulating substrate to ensure the required gap between the foil and insulator in the designed nonadhered metal-insulator substrate system.The large-area scanning electron microscopy (SEM) image reveals a uniform surface devoid of visible Cu grain boundaries of our single-crystal Cu(111) foil (Figure 1e).Electron backscatter diffraction (EBSD) inverse pole figure (IPF) maps of normal and transverse directions acquired in various regions show uniform contrast, confirming that the entire foil is a single crystal (Figure 1f).The 111 orientation of the foil is further confirmed by the X-ray diffraction (XRD) pattern, as shown in Figure 1g.In addition, a gap was observed between the Cu(111) foil and sapphire substrate from the cross-sectional SEM images (Figure 1h,i).

Monolayer hBN on Insulators
This designed substrate, i.e., the nonadhered Cu(111)-insulator, was used to prepare monolayer hBN.By modulating the parameters of the CVD growth and high-temperature treatment stages, we successfully obtained single-crystal monolayer hBN on various insulating substrates (Figure 2a).Optical images show differences in contrast between hBN-covered and bare regions on sapphire, SiO 2 /Si, and quartz substrates (Figure 2b-d).A typical Raman spectrum of synthesized hBN on SiO 2 /Si exhibits a sharp E 2g band at 1370.6 cm −1 with a full width at half maximum (FWHM) of ≈20.3 cm −1 (Figure 2e).The Raman map of the E 2g intensity further demonstrates the uniformity and high crystal quality of the synthesized monolayer hBN on SiO 2 /Si (Figure 2f).This hBN film has a smoother surface compared to monolayer hBN grown on the Cu(111) substrate, as evidenced by SEM (Figure 2g,h).The synthesized monolayer hBN film was also transferred onto a transmission electron microscopy (TEM) grid, and high-resolution TEM (HRTEM) images, and fast Fourier transform (FFT) patterns were then acquired at randomly selected locations over the entire TEM grid.The results revealed the honeycomb lattice structure and consistent crystal orientation, confirming the high crystal quality and single crystallinity of the synthesized monolayer hBN film (Figure 2i-l).These characterization results prove the high-quality, large-area, single-crystal monolayer nature of the synthesized hBN on insulating substrates using the developed UNS strategy.

Graphene on Arbitrary Insulators
In addition to the hBN film, our UNS strategy also applies to other 2D material systems.We have used this developed strategy to successfully synthesize nonepitaxial graphene film on various insulating substrates (Figure S5, Supporting Information).High-quality monolayer graphene films were obtained on commonly used quartz and SiO 2 /Si substrates by optimizing the conditions of CVD growth and high-temperature treatment (Figure 3a,b; Figure S6, Video S1, and Table S1, Supporting Information).Raman maps of the 2D-band frequency and 2D FWHM of graphene synthesized on insulating substrates (i.e., quartz and SiO 2 /Si) and of transferred graphene (grown on Cu(111) foil and then wet-transferred on these insulators) reveal that the synthesized monolayer graphene had smoother surfaces than the transferred samples and were devoid of visible folds (Figure 3c,d).In addition, the 2D bands were narrower (i.e., the FWHMs were smaller) in the typical Raman spectra of synthesized graphene compared with those of graphene transferred onto both insulating substrates (Figure 3e,f).Moreover, these 2D bands were blueshifted with respect to those of transferred graphene; we suggest that this blueshift is caused by the stronger interactions between graphene and the underlying insulating substrates.Additionally, monolayer graphene was successfully prepared on a bulk hBN crystal using our developed UNS strategy, as illustrated in Figure S7 (Supporting Information).In addition to the verified insulators (quartz, SiO 2 /Si, and hBN crystals), the developed strategy is expected to be universally applicable to most insulating substrates with smooth surfaces and good high-temperature stabilities.
We synthesized a wafer-scale single-crystal monolayer graphene film with weak light absorption on a 4 in.sapphire wafer (Figure 3g; Figures S8, Supporting Information).Uniform surfaces are observed on the large-area optical images acquired from fully covered monolayer graphene films synthesized on sapphire substrates (Figures S9 and S10, Supporting Information).Typical Raman maps of I D /I G and 2D FWHM confirm its high quality and uniform monolayer nature with the absence of apparent folds, in contrast with graphene grown on Cu(111) foil and then transferred on sapphire (Figure 3h; Figure S11, Supporting Information).Raman spectra collected from nine randomly selected positions show the negligible D band, further verifying that the synthesized graphene film was a highly crystalline, adlayer-free monolayer (Figure 3i).A noticeable blueshift and a small FWHM of the 2D band were observed for monolayer graphene synthesized on sapphire (Figure 3j).The Raman shift of 2D band indicated a possible biaxial strain in the graphene crystal structure and the charge-doping effect. [33] 2D-band frequency ( 2D ) versus G-band frequency ( G ) correlation diagram for various graphene samples is shown in Figure 3k, with reference values ( G = 1582 cm −1 ;  2D = 2692 cm −1 ) being chosen from a previous report on unstrained and undoped monolayer graphene; the relationships between the Gand 2D-band frequencies of charge-neutral graphene with biaxial strain () were assumed to be Δ G /Δ = −57.3cm −1 per % strain and Δ 2D /Δ = −160.3cm −1 per % strain, respectively. [34]he blue reference line with a slope of Δ 2D /Δ G = 0.75 represents the hole-doping effect. [35]Monolayer graphene grown on Cu(111) exhibited compressive strain, consistent with the previous report [9] .Graphene transferred onto the SiO 2 /Si substrate showed extremely low p-doping with negligible strain, whereas graphene transferred onto the sapphire substrate exhibited negligible p-doping and slight compressive strain.In contrast, monolayer graphene synthesized on the sapphire substrate exhibited considerable compressive strain, suggesting a strong interfacial coupling between the graphene film and underlying sapphire.A similar tendency of the strong interfacial coupling was observed in monolayer graphene synthesized on quartz and SiO 2 /Si substrates when analyzing the  2D - G diagram (Figure S12, Supporting Information).

Characterizations and Transport Properties
The surface morphology of synthesized monolayer graphene was also investigated in detail.SEM images of graphene synthesized on sapphire show a uniform surface devoid of noticeable adlayers (Figure 4a).Some small wrinkles (known as ripples) can   d,e) Large-area AFM images of the synthesized monolayer graphene and transferred monolayer graphene measured over an area of 5 × 5 μm 2 .f) AFM height profiles along lines marked in panels (d) and (e), and height distributions for synthesized (pink) and transferred (blue) graphene films extracted from AFM maps over a 500 × 500 nm 2 area in Figure S14 (Supporting Information).g) HRTEM image of synthesized graphene after being transferred to the TEM grid.h) FFT patterns obtained at ten randomly selected locations over the TEM grid.Scale bars, 5 nm −1 .The same orientation was observed in all patterns.
be observed in the high-resolution SEM image.In comparison, distinct folds (large wrinkles) were observed in the SEM images of graphene grown on the Cu(111) substrate (Figure 4b).These folds are perpendicular to the Cu(111) steps and are suggested to be formed during the cooling from a high growth temperature, ascribable to significant differences in thermal expansion coefficients of graphene and Cu(111). [9,12,36,37]The smooth surface of synthesized graphene is further evidenced by atomic force microscopy (AFM) topography maps (Figure 4c,d).The surface roughnesses of areas with dimensions of 1 × 1 and 5 × 5 μm 2 were determined to be 0.078 and 0.103 nm for monolayer graphene synthesized on sapphire.In contrast, the AFM topography maps of transferred graphene exhibit clear folds (Figure 4e; Figure S13, Supporting Information).The surface roughness of transferred graphene, calculated from an area of 5 × 5 μm 2 , was determined to be 0.790 nm.Upon comparison, the distanceheight profiles and surface-height distributions extracted from the AFM topography maps reveal that the surface of graphene synthesized on sapphire is significantly smoother than that of the transferred sample (Figure 4f; Figure S14, Supporting Information).Atom-resolved HRTEM images and a series of corresponding FFT patterns were acquired at randomly selected locations across the entire TEM grid, confirming the high crystal quality and single crystallinity of the synthesized graphene film (Figure 4g,h).
To further investigate the quality of the synthesized graphene film, graphene-based FETs (G-FETs) were fabricated on a SiO 2 /Si substrate, and the electronic performance was examined (Figure 5a-c; Figure S15, Supporting Information).The typical I SD versus V G -V D curves were measured at room temperature, and the electron and hole mobilities were calculated using the constant mobility model. [38]The average carrier mobility, calculated from eight different G-FETs, was 9.78 × 10 3 cm 2 V −1 s −1 for holes and 3.62 × 10 3 cm 2 V −1 s −1 for the electrons; these values are comparable to those reported for single-crystal graphene films grown on metal substrates (Figure 5d,e; Table S2, Supporting Information). [9,10]This good performance can be attributed to the smooth surface of the synthesized graphene film and its clean interface with the underlying insulator.In contrast, the graphene film, grown on Cu and transferred, exhibited hole mobilities of up to 5.68 × 10 3 cm 2 V −1 s −1 (with an average value of 3.94 × 10 3 cm 2 V −1 s −1 ) and electron mobilities of up to 2.45 × 10 3 cm 2 V −1 s −1 (with an average value of 1.32 × 10 3 cm 2 V −1 s −1 ).The comparatively poor performance could be attributed to contamination or cracks, as well as the graphene folds and adlayers in the channel regions (for an unbiased comparison, regions containing some folds and adlayers were randomly selected to pattern the G-FET channels).

Bilayer and Trilayer Graphene
Beyond monolayer graphene, our developed UNS strategy is also suitable for fabricating few-layer graphene on insulating substrates (Figure 6a).Single-crystal CuNi(111) alloy foils with various Ni concentrations were manufactured by evaporating Ni layers on both sides of our Cu(111) foil and then re-annealing at a high temperature (Figure 6b).Large-area optical and SEM images without noticeable grain boundaries, as well as the uniform EBSD IPF maps verify the single crystallinity of our CuNi(111) (Figure 6c,d; Figure S16, Supporting Information).Following the graphene growth process, the CuNi(111)-graphene-sapphire sample was moved to a high-temperature zone for the nearmolten heat treatment, and few-layer graphene films were then obtained on the sapphire substrate using the UNS strategy (Figure S17, Supporting Information).No D band appeared in the typical Raman spectra of the synthesized bilayer graphene, confirming the high crystal quality (Figure 6e).The typical 2D FWHM is ≈50 cm −1 , which was fitted using four Lorentzian peaks, consistent with AB-stacked bilayer graphene. [39,40]The Raman map of the 2D FWHM is uniformly blue in color, with values of 45-55 cm −1 in most regions, exhibiting the uniform bilayer nature of the entire graphene film (Figure 6f).In contrast, the I G /I 2D ratio was higher in the typical Raman spectra of the synthesized trilayer graphene films (Figure 6g; Figure S18, Supporting Information).The FWHM of the 2D band (≈58 cm −1 ), fitted with six Lorentzian peaks, and the uniformity of the Raman map confirm the formation of ABA-stacked trilayer graphene on the sapphire substrate (Figure 6h), consistent with the previous report. [16]The 2D bands were narrower for the synthesized bilayer and trilayer graphene than those for the transferred samples (Figure S19, Supporting Information).The production of bilayer and trilayer graphene shows the feasibility of extending this UNS strategy to further synthesizing the multilayer (≥4 layers) 2D materials on insulating substrates.

Conclusion
In summary, we reported a universal strategy for nonepitaxially synthesizing the high-quality wafer-scale single-crystal 2D materials on insulating substrates.A nonadhered metal-insulator substrate system was designed to enable the growth of highquality 2D materials on both surfaces of a single-crystal metal foil.A brief high-temperature treatment first softened the singlecrystal foil, and the 2D material on the lower surface was pressed and adhered to the insulator surface.Consequently, large-area single-crystal 2D materials were synthesized nonepitaxially on various insulating substrates.The UNS strategy is not limited by the 2D materials (graphene, hBN, and other metal-based grown 2D layers), insulating substrates (kinds, phases, and crystallinity), and metal foils (single-crystal or polycrystalline Cu, Ni, or other alloys).This study is expected to provide ideal epitaxial templates for the further synthesis of more large-area single-crystal 2D materials on arbitrary insulating substrates, thereby facilitating the fabrication of 2D-material-based high-performance electronics in the future.

Experimental Section
Fabrication of Single-Crystal Cu(111) and CuNi(111) Foils: Polycrystalline Cu foil (Thermo Fisher Scientific) was precleaned using an ammonium persulfate solution and loaded into a CVD system for hightemperature contact-free annealing at 1070 °C under a flowing atmosphere of H 2 (99.999%,50 sccm) and Ar (99.999%, 50 sccm) at a pressure Bilayer and Trilayer Graphene Growth on CuNi(111) Substrates: The prefabricated CuNi(111) alloys with different Ni concentrations were used as the substrates for the bilayer and trilayer graphene growth.A CuNi(111) alloy substrate was cleaned and removed the surface oxidation using the ammonium persulfate solution and then loaded into the CVD system, followed by vacuum pumping to achieve an initial pressure of ≈0.1 Pa.Subsequently, the furnace was heated to 1070 °C and maintained for 30 min in H 2 (50 sccm) and Ar (50 sccm) environments.After that, the CH 4 (1% diluted in Ar, 10 sccm) was introduced into the CVD chamber to facilitate the growth of graphene, with a growth time of 30 min.After the graphene growth, the sample was rapidly removed from the high-temperature zone to the room-temperature zone, experiencing a quick cooldown in the same atmospheric conditions.The growth of graphene with different numbers of layers using the same conditions (temperature, pressure, gas composition, growth time, etc.), with the only variation being the use of different CuNi(111) alloys with varying Ni compositions.The Ni concentration plays a crucial role in determining the resulting layer number of the asgrown graphene on the CuNi(111) alloy.In this experiment, the formation of bilayer graphene on CuNi(111) foils was observed with a Ni concentration of 15%, achieving the highest coverage of bilayer graphene at ≈65%.Additionally, trilayer graphene was grown on CuNi(111) foils with a 20% Ni concentration, exhibiting a maximum coverage of ≈58% (Table S3, Supporting Information).
Synthesis of hBN and Graphene on Insulating Substrates: The CVD system was evacuated to ≈0.1 Pa before starting the hBN growth process.Then, a Cu(111)-sapphire substrate was loaded into the hBN CVD growth system.Ammonia borane (H 3 NBH 3 , 95%, Sigma-Aldrich) was used as the precursor, and the temperatures of the CVD system and precursor were set to 1050 and 80 °C, respectively.The hBN growth process was performed for 90 min under flowing H 2 (30 sccm) and Ar (15 sccm) to synthesize monolayer hBN films on the top and bottom surfaces of the Cu(111) foil.Following hBN growth, the sample was moved to a high-temperature zone in the CVD system to anneal at 1090-1150 °C.Finally, the sample was moved to a room-temperature zone and cooled without flowing gas.The hydrogen plasma was used to remove the upper-layered hBN film, and an ammonium persulfate solution was used to remove the Cu(111) foil.To synthesize graphene, a similar procedure was applied.Specifically, a Cu(111)-sapphire substrate was loaded into a graphene CVD growth system, and the tube furnace was heated to 1030 °C.The sample was moved into the growth zone (1030 °C) and annealed for 30 min under 10 sccm H 2 prior to growth.During growth, the furnace was maintained at 1030 °C for 30-60 min while CH 4 (5-20 sccm) and H 2 (5 sccm) were flowed into the chamber, with the pressure controlled to 10-30 Pa.Monolayer graphene films were thereby synthesized on the top and bottom surfaces of Cu(111).Following graphene growth, the sample was moved to a hightemperature zone, where the sample was heated from 1030 °C to the target temperature, which depended on the substrate material (for example, ≈1097 °C for sapphire and SiO 2 /Si and ≈1130 °C for quartz), for 2 min under flowing CH 4 (from 5 to 0 sccm) and H 2 (from 5 to 0 sccm) gas and then maintained for an additional 50-75 s without flowing gas.The sample was finally cooled by being moved into the room-temperature zone in the quartz tube without flowing gas.
Fabrication of G-FETs: G-FETs with channel lengths of 100 μm were fabricated from graphene on 300 nm SiO 2 /Si substrates.5 nm Cr and 20 nm Au were deposited using an e-beam evaporator as the source and drain electrodes after patterning using the traditional ultraviolet photolithography.All electrical measurements were conducted using the Keithley 4200-SCS at room temperature in the air.
Optical and Raman Characterizations: Optical images, Raman spectra, and Raman maps were obtained via confocal Raman spectroscopy (Alpha 300 R, WITec) at laser wavelengths of 532 and 488 nm using laser powers of 1.0 and 5.0 mW on the insulating and metal substrates, respectively.SEM, AFM, XRD, and TEM Characterizations: SEM (Merlin, Zeiss) was used to characterize the surface morphologies of graphene and Cu(111).EBSD and EDS maps were acquired using a Quattro SEM (Thermo Fisher Scientific).AFM (Dimension Icon, Bruker) was used to characterize the surface morphology and roughness of graphene.To prevent scratching or damaging the monolayer graphene surface, accurate surface morphology information was acquired using a solid Si tip in tapping mode (AFM probe type: NCHV-A, with a tip radius of 8 nm and a spring constant of 40 N m −1 ).XRD patterns of Cu(111) foil were obtained using an X-ray diffractometer (D8 ADVANCE Twin, Bruker) with Cu K radiation.HRTEM images of the graphene and hBN samples were obtained using a Titan Cs Image and Themis Z at 80 kV.

Figure 1 .
Figure 1.Universal nonepitaxial synthesis (UNS) strategy.a-d) Schematic of the synthesis process for 2D materials.a) Stage I: prefabricated singlecrystal metal foil is placed on an insulating substrate to form a nonadhered metal foil-insulator substrate.b) Stage II: 2D material islands are grown on both sides of the metal foil.c) Stage III: the metal foil begins to soften, reaching a nearly melted state; the 2D material layer is pressed onto the insulating substrate surface.d) Stage IV: the upper-layer 2D materials and Cu(111) foil are removed, with a 2D layer remaining on the insulating substrate.e) Largearea SEM image of our single-crystal Cu(111) foil.No grain boundaries appear.f) EBSD IPF maps collected from ten different regions across the entire Cu(111) foil; the distance between each region exceeds 3 mm.Scale bars, 50 μm.g) XRD pattern of our Cu(111) foil.Inset: photograph of the Cu(111) foil on sapphire.h) False-color SEM image of a typical cross-sectional structure of Cu(111)-sapphire substrate.i) Cross-sectional SEM image of the Cu(111) foil.

Figure 2 .
Figure 2. Synthesizing hBN on insulating substrates.a) Schematic of hBN synthesis on different insulating substrates using the developed UNS strategy.b-d) Optical images of monolayer hBN on sapphire, SiO 2 /Si, and quartz substrates.Weak light absorptions are observed in the hBN-covered areas compared to those of the bare substrates.e) Raman spectra acquired from synthesized monolayer hBN (red) and bare SiO 2 /Si substrate (blue) regions.The E 2g band position is ≈1370.6cm −1 , and the FWHM is ≈20.3 cm −1 .f) Typical Raman map of the E 2g intensity and correlated optical image of monolayer hBN on SiO 2 /Si.g,h) SEM images of monolayer hBN synthesized on g) SiO 2 /Si and h) Cu(111) substrates.i,j) HRTEM images of the synthesized monolayer hBN after transfer to the TEM grid.k) False-color HRTEM image from the area marked in panel (j).l) FFT patterns obtained from randomly selected locations on monolayer hBN.Scale bars, 5 nm −1 .

Figure 3 .
Figure 3. Nonepitaxial synthesis of graphene on insulators.a,b) Photographs and the optical images of synthesized monolayer graphene on a) quartz and b) SiO 2 /Si substrates.c,d) Raman maps of the 2D-band frequency and FWHM of synthesized and transferred graphene on c) quartz and d) SiO 2 /Si substrates.The same ranged color bars are used for the synthesized and transferred maps for comparison.e,f) Typical Raman spectra and 2D FWHM distributions of synthesized graphene (green) and transferred graphene (blue) on e) quartz and f) SiO 2 /Si substrates.g) Photograph of synthesized 3 in. 2single-crystal monolayer graphene on 4 in.sapphire wafer.h) Raman maps of the I D /I G ratio and 2D FWHM of monolayer graphene synthesized on sapphire.i) Raman spectra randomly collected from nine positions from synthesized graphene on sapphire.j) Plot of the 2D-band frequency versus 2D FWHM of synthesized graphene on sapphire and graphene transferred onto SiO 2 /Si.k) Correlation diagram of the 2D-band frequency versus G-band frequency.Data were collected from graphene films synthesized on sapphire (pink star), graphene transferred onto sapphire (pink circle) and SiO 2 /Si (blue circle), and graphene grown on Cu(111) without being transferred (green diamond).

Figure 4 .
Figure 4. Characterization of the synthesized monolayer graphene.a,b) SEM images of monolayer graphene a) synthesized on the sapphire substrate and b) grown on the Cu(111) foil.Apparent graphene fold and Cu steps are indicated by arrows.c) AFM topography image of graphene synthesized on sapphire over the 1 × 1 μm 2 measurement area.d,e)Large-area AFM images of the synthesized monolayer graphene and transferred monolayer graphene measured over an area of 5 × 5 μm 2 .f) AFM height profiles along lines marked in panels (d) and (e), and height distributions for synthesized (pink) and transferred (blue) graphene films extracted from AFM maps over a 500 × 500 nm 2 area in FigureS14(Supporting Information).g) HRTEM image of synthesized graphene after being transferred to the TEM grid.h) FFT patterns obtained at ten randomly selected locations over the TEM grid.Scale bars, 5 nm −1 .The same orientation was observed in all patterns.

Figure 5 .
Figure 5. Transport property measurements.a,b) Schematic and photograph of fabricated G-FETs on a 4 in.SiO 2 /Si wafer.c) Optical image and the corresponding true component Raman map of the graphene channel.Au, graphene, and SiO 2 /Si are shown in yellow, green, and blue, respectively.d) Electronic transport characteristic curves measured at room temperature.The blue line indicates the hole region (V G −V D < 0 V), and the red line indicates the electron region (V G −V D > 0 V).Inset: typical I SD versus V G −V D curve.e) I SD versus V G −V D curves from G-FETs of the synthesized graphene (blue) and transferred graphene (pink).

Figure 6 .
Figure 6.Few-layer graphene on insulating substrates.a) Schematic of the structure of few-layer graphene between the CuNi(111) foil and the sapphire substrate.b) Photograph of the fabricated single-crystal CuNi(111) alloy foil.c) Large-area optical image of the single-crystal CuNi(111) foil.d) SEM image and EDS map of the CuNi(111) foil.Ni and Cu elements are shown in red and green, respectively.e) Typical Raman spectra of the synthesized bilayer graphene and corresponding Lorentzian fits of the 2D band (an FWHM of ≈50 cm −1 ).f) Raman maps of the 2D FWHM of bilayer graphene.g) Typical Raman spectra of trilayer graphene with a 2D FWHM of ≈58 cm −1 .The six Lorentzian fits of the 2D band are shown here.h) 2D FWHM Raman map of trilayer graphene synthesized on sapphire.
of ≈760 Torr, which formed a single-crystal Cu(111) foil.The single-crystal CuNi(111) alloy was fabricated by depositing Ni layers on both sides of the prepared Cu(111) foil using an e-beam evaporator.A double-sided deposition approach was employed to achieve a more uniform distribution of Ni atoms in Cu(111) and the fabrication of the desired CuNi(111) alloy was expedited.Various Ni layers were deposited to create CuNi alloys with different Ni compositions.The specific Ni layer thickness was calculated based on the target Ni proportion in CuNi(111) alloys and the Cu(111) foil thickness.In these experiments, the Cu(111) foil thickness was measured, the desired Ni/Cu ratio was determined, and the required Ni layer thickness was calculated based on the equation as C (Ni in CuNi alloy) = 100% × [thickness of deposited Ni layers]/[total thickness of CuNi foil].Slight changes of concentration may occur after the postannealing and growth process due to different evaporation rates of Cu and Ni at high temperatures.After depositing the Ni layer, Ni-plated Cu(111) foils were introduced into the CVD system.The furnace was heated to 1070 °C, and the sample was annealed at this temperature under H 2 (50 sccm) and Ar (50 sccm) gas flows at a pressure of 760 Torr.Following a 4 h annealing process, the sample was cooled to room temperature, and the single-crystal CuNi(111) alloy foil was obtained.