Synthesis of Large-Area Single-Crystal Graphene

Methods for growth of millimeter to centimeter-sized single-crystal, single-layer graphene (SLG) islands have been reviewed. Several strategies have been used to decrease the nucleation density of graphene so as to grow a large-area single-crystal island of SLG from a single nucleus; these are described.Methods for growth of centimeter to meter-scale single-crystal SLG films have been reviewed. The epitaxial growth of centimeter-scale single-crystal SLG, reliably achieved recently on Cu(111) foils and also with no adlayers, is explained.By ‘feeding’ carbon atoms by either diffusion at the interface between a graphene layer and the substrate, or diffusion through the substrate from its backside, sub-millimeter single-crystal BLG islands have been made on copper substrates. The growth of hexagonal single-crystal graphene isalnds with two to eight layers and sizes of hundreds of micrometers on Cu/Ni foils is reviewed.Large-area single-crystal AB-stacked bilayer/trilayer graphene films have been produced on single-crystal Cu/Ni(111) alloy foils by parametric study as a function of the concentration of Ni in the foils; this method is outlined.Single-crystal AB-stacked and ‘twisted’ BLG with a predetermined rotation angle has been prepared on a large scale by either stacking or folding large-area adlayer-free single-crystal SLG films; this is presented as a different pathway (if further developed and refined) to multilayer and single-crystal films, including with arbitrary twist angle, in contrast to the ‘as-grown’ multilayer films that are almost always AB-stacked (twist angle of zero). There have been breakthroughs in the mass production of graphene by chemical vapor deposition (CVD) and its practical applications have also been identified. Grain boundaries are typically present in ‘CVD graphene’ and adversely impact its properties. We summarize recent progress in growing large-area single-crystal graphene. Centimeter-scale single-crystal, truly single-layer graphene (SLG) films have been reportedly achieved on single-crystal Cu(111) foils by CVD growth, while meter-scale single-crystal SLG films have been reportedly produced with assistance of a roll-to-roll technique. The growth of uniform single crystals of bilayer or multilayer graphene over a large area remains an exciting challenge. Layer-by-layer transfer and the stacking of single-crystal SLG is considered a promising route to making new types of ‘single’ crystals or quasicrystals with specific numbers of layers and different stacking angles. There have been breakthroughs in the mass production of graphene by chemical vapor deposition (CVD) and its practical applications have also been identified. Grain boundaries are typically present in ‘CVD graphene’ and adversely impact its properties. We summarize recent progress in growing large-area single-crystal graphene. Centimeter-scale single-crystal, truly single-layer graphene (SLG) films have been reportedly achieved on single-crystal Cu(111) foils by CVD growth, while meter-scale single-crystal SLG films have been reportedly produced with assistance of a roll-to-roll technique. The growth of uniform single crystals of bilayer or multilayer graphene over a large area remains an exciting challenge. Layer-by-layer transfer and the stacking of single-crystal SLG is considered a promising route to making new types of ‘single’ crystals or quasicrystals with specific numbers of layers and different stacking angles. Graphene, a single layer of carbon atoms arranged in a honeycomb lattice (Figure 1A ), has attracted worldwide attention due to its unique 2D structure and excellent physical properties [1.Balandin A.A. et al.Superior thermal conductivity of single-layer graphene.Nano Lett. 2008; 8: 902-907Crossref PubMed Scopus (9126) Google Scholar, 2.Lee C. et al.Measurement of the elastic properties and intrinsic strength of monolayer graphene.Science. 2008; 321: 385Crossref PubMed Scopus (13407) Google Scholar, 3.Novoselov K.S. et al.Electric field effect in atomically thin carbon films.Science. 2004; 306: 666Crossref PubMed Scopus (42802) Google Scholar, 4.Novoselov K.S. et al.Two-dimensional atomic crystals.Proc. Natl. Acad. Sci. U. S. A. 2005; 102: 10451-10453Crossref PubMed Scopus (8033) Google Scholar, 5.Bolotin K.I. et al.Ultrahigh electron mobility in suspended graphene.Solid State Commun. 2008; 146: 351-355Crossref Scopus (3770) Google Scholar]. 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Among various synthesis methods, chemical vapor deposition (CVD) has shown to be the most promising for the scalable production of large-area high-quality graphene films [17.Li X. et al.Synthesis of graphene films on copper foils by chemical vapor deposition.Adv. Mater. 2016; 28: 6247-6252Crossref PubMed Scopus (175) Google Scholar]. Ruoff and colleagues first reported the preparation of a centimeter-scale single-layer graphene (SLG) film on a commercial Cu foil by CVD in 2009 [18.Li X. et al.Large-area synthesis of high-quality and uniform graphene films on copper foils.Science. 2009; 324: 1312Crossref PubMed Scopus (8354) Google Scholar]. Unlike Ni with high carbon solubility (~0.9 at.% at 900°C [19.Lander J.J. et al.Solubility and diffusion coefficient of carbon in nickel: reaction rates of nickel-carbon alloys with barium oxide.J. Appl. 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However, grain boundaries (GBs) are often present in the CVD-grown graphene films and are reported to contain 5-, 7-, and/or distorted 6-membered rings (Figure 1D–F) [18.Li X. et al.Large-area synthesis of high-quality and uniform graphene films on copper foils.Science. 2009; 324: 1312Crossref PubMed Scopus (8354) Google Scholar,22.Huang P.Y. et al.Grains and grain boundaries in single-layer graphene atomic patchwork quilts.Nature. 2011; 469: 389-392Crossref PubMed Scopus (1406) Google Scholar,23.Wu Y. et al.Crystal structure evolution of individual graphene islands during CVD growth on copper foil.Adv. Mater. 2013; 25: 6744-6751Crossref PubMed Scopus (41) Google Scholar]. They are formed where two (or more) graphene islands (grown from different nuclei) that have different orientations join together [23.Wu Y. et al.Crystal structure evolution of individual graphene islands during CVD growth on copper foil.Adv. Mater. 2013; 25: 6744-6751Crossref PubMed Scopus (41) Google Scholar]. 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Mater. 2020; 32: 6078-6084Crossref Scopus (0) Google Scholar] and the existence of GBs reportedly influence the stacking order in bi- and multilayers [29.Hao Y. et al.Oxygen-activated growth and bandgap tunability of large single-crystal bilayer graphene.Nat. Nanotechnol. 2016; 11: 426-431Crossref PubMed Scopus (144) Google Scholar]. Consequently, the synthesis of large-area single-crystal graphene (thus, with no GBs) by reliable/reproducible and scalable methods is of great interest. In this review, we focus on developments in the CVD growth of large-area single-crystal graphene with different numbers of layers on Cu-based substrates, including Cu and Cu-based alloys. Large single-crystal SLG islands have been obtained by several methods on polycrystalline substrates. Recently, centimeter-scale adlayer-free single-crystal SLG films have been reproducibly produced on home-made single-crystal Cu(111) foils by removing all the carbon contaminants in the substrates. Mass production of single-crystal SLG films has been achieved by either a roll-to-roll technique or a home-designed pilot-scale CVD system. However, it is still challenging to grow large single-crystal bilayer or multilayer graphene by the CVD method, and layer-by-layer assembly of large-area single-crystal SLG films appears to be a promising way to produce them, especially with defined interlayer rotation angles. Based on the number of nuclei formed at the early stage of graphene growth, there are two strategies for the growth of large-area single-crystal SLG. The first is to ensure that graphene growth is from a single nucleus on a polycrystalline substrate, and the second is epitaxial growth from multiple nuclei on a single-crystal substrate (see Table 1).Table 1Representative Studies on the Growth of Large-Area Single-Crystal SLGStrategyMethodSummary of reported methodReported substrateaAbbreviation: Poly, polycrystalline.Reported sizeReported growth timeYearRefsSingle nucleusSurface smoothingAnnealing in Ar/H2 for 3 hPoly-Cu0.4 mm~15 min2012[42.Wang H. et al.Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation.J. Am. Chem. Soc. 2012; 134: 3627-3630Crossref PubMed Scopus (290) Google Scholar]Electro-polishing and suppressing Cu evaporation by stacking foilsPoly-Cu2 mm6 h2013[37.Chen S. et al.Millimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition.Adv. Mater. 2013; 25: 2062-2065Crossref PubMed Scopus (227) Google Scholar]Melting Cu followed with resolidificationPoly-Cu1 mm5 h2013[38.Mohsin A. et al.Synthesis of millimeter-size hexagon-shaped graphene single crystals on resolidified copper.ACS Nano. 2013; 7: 8924-8931Crossref PubMed Scopus (135) Google Scholar]Optimizing CVD parametersCu enclosure with low carbon supplyPoly-Cu0.5 mm~90 min2011[40.Li X. et al.Large-area graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper.J. Am. Chem. Soc. 2011; 133: 2816-2819Crossref PubMed Scopus (977) Google Scholar]Long-time annealing, a controlled pressure, relatively high growth temperature, and low methane pressurePoly-Cy2.3 mm125 min2012[36.Yan Z. et al.Toward the synthesis of wafer-scale single-crystal graphene on copper foils.ACS Nano. 2012; 6: 9110-9117Crossref PubMed Scopus (438) Google Scholar]Annealing in a non-reducing environment, low reactor pressure, and high H2/CH4 ratioPoly-Cu5 mm48 h2013[87.Zhou H. et al.Chemical vapour deposition growth of large single crystals of monolayer and bilayer graphene.Nat. Commun. 2013; 4: 2096Crossref PubMed Scopus (335) Google Scholar]Oxygen passivationO2 treatment at 1035°C for 5 minO-rich poly-Cu1 cm12 h2013[43.Hao Y. et al.The role of surface oxygen in the growth of large single-crystal graphene on copper.Science. 2013; 342: 720-723Crossref PubMed Scopus (736) Google Scholar]Annealing in Ar before growthPoly-Cu5.9 mm~10 h2013[45.Gan L. Luo Z. Turning off hydrogen to realize seeded growth of subcentimeter single-crystal graphene grains on copper.ACS Nano. 2013; 7: 9480-9488Crossref PubMed Scopus (176) Google Scholar]Heating in air at 200°C for 20 minPoly-Cu3 mm3 h2015[44.Ding D. et al.Behavior and role of superficial oxygen in Cu for the growth of large single-crystalline graphene.Appl. Surf. Sci. 2017; 408: 142-149Crossref Scopus (20) Google Scholar]Introducing O2 during growthPoly-Cu1 cm20 h2016[46.Guo W. et al.Oxidative-etching-assisted synthesis of centimeter-sized single-crystalline graphene.Adv. Mater. 2016; 28: 3152-3158Crossref PubMed Scopus (61) Google Scholar]SeedingUsing PMMA dots as seedsPoly-Cu18 μm20 min2011[48.Wu W. et al.Growth of single crystal graphene arrays by locally controlling nucleation on polycrystalline Cu using chemical vapor deposition.Adv. Mater. 2011; 23: 4898-4903Crossref PubMed Scopus (156) Google Scholar]Using graphene oxide flakes as seedsPoly-Cu150 μm20 min2014[47.Li Q. et al.Controllable seeding of single crystal graphene islands from graphene oxide flakes.Carbon. 2014; 79: 406-412Crossref Scopus (19) Google Scholar]Local feedingLocal feeding and multistep supply of methanePoly-Cu85Ni15 alloy3.8 cm2.5 h2016[51.Wu T. et al.Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu–Ni alloys.Nat. Mater. 2016; 15: 43-48Crossref PubMed Scopus (317) Google Scholar]Evolutionary selectionLocally feeding carbon source to a moving substratePoly-Cu90Ni10 alloy~30 cm~12 h2018[52.Vlassiouk I.V. et al.Evolutionary selection growth of two-dimensional materials on polycrystalline substrates.Nat. Mater. 2018; 17: 318-322Crossref PubMed Scopus (89) Google Scholar]Fast growthA molecular flow in a confined space with assistance of oxygenSingle-crystal Cu(100) foil3 mm10 min2016[55.Wang H. et al.Surface monocrystallization of copper foil for fast growth of large single-crystal graphene under free molecular flow.Adv. Mater. 2016; 28: 8968-8974Crossref PubMed Scopus (82) Google Scholar]Multistage carbon supply and second passivationSingle-crystal Cu(100) foil4 mm100 min2016[56.Lin L. et al.Rapid growth of large single-crystalline graphene via second passivation and multistage carbon supply.Adv. Mater. 2016; 28: 4671-4677Crossref PubMed Scopus (41) Google Scholar]Continuously supply oxygen from oxide substratePoly-Cu0.3 mm5 s2016[58.Xu X. et al.Ultrafast growth of single-crystal graphene assisted by a continuous oxygen supply.Nat. Nanotechnol. 2016; 11: 930-935Crossref PubMed Scopus (187) Google Scholar]Continuously supply fluorine from metal fluoride substratePoly-Cu1 mm5 s2019[59.Liu C. et al.Kinetic modulation of graphene growth by fluorine through spatially confined decomposition of metal fluorides.Nat. Chem. 2019; 11: 730-736Crossref PubMed Scopus (20) Google Scholar]Multiple nuclei‘Seamless stitching’ of aligned graphene islands on single-crystal substratesMagnetron sputtering on single-crystal MgO(111)Cu(111) film–10 min2012[62.Ogawa Y. et al.Domain structure and boundary in single-layer graphene grown on Cu(111) and Cu(100) films.J. Phys. Chem. Lett. 2012; 3: 219-226Crossref Scopus (174) Google Scholar]Repeated chemomechanical polishing and annealingCu(111) foil3 × 6 cm21 h2015[68.Nguyen V.L. et al.Seamless stitching of graphene domains on polished copper (111) foil.Adv. Mater. 2015; 27: 1376-1382Crossref PubMed Scopus (180) Google Scholar]Temperature-gradient-driven annealingCu(111) foil5 × 50 cm220 min2017[70.Xu X. et al.Ultrafast epitaxial growth of metre-sized single-crystal graphene on industrial Cu foil.Sci. Bull. 2017; 62: 1074-1080Crossref Scopus (198) Google Scholar]Magnetron sputteringCu(111) film4 inch2 h2017[69.Deng B. et al.Wrinkle-free single-crystal graphene wafer grown on strain-engineered substrates.ACS Nano. 2017; 11: 12337-12345Crossref PubMed Scopus (61) Google Scholar]Electroplating Ni onto Cu(111) foils followed with annealing at 1050°C for 4–6 hoursCu/Ni(111) foils (1.3 to 8.6 at.% Ni)2 × 8 cm25 min2018[54.Huang M. et al.Highly oriented monolayer graphene grown on a Cu/Ni(111) alloy foil.ACS Nano. 2018; 12: 6117-6127Crossref PubMed Scopus (0) Google Scholar]Contact-free annealingCu(111) foil32 cm21 h2019[72.Luo D. et al.Adlayer-free large-area single crystal graphene grown on a Cu(111) foil.Adv. Mater. 2019; 31: 1903615Crossref Scopus (18) Google Scholar]Two-step magnetron sputteringCu90Ni10(111) film4 inch10 min2019[71.Deng B. et al.Scalable and ultrafast epitaxial growth of single-crystal graphene wafers for electrically tunable liquid-crystal microlens arrays.Sci. Bull. 2019; 64: 659-668Crossref Scopus (15) Google Scholar]a Abbreviation: Poly, polycrystalline. Open table in a new tab At an early stage, researchers tried to grow a large-area single-crystal graphene island from a single nucleus on a polycrystalline substrate, based on the fact that, in some situations, the single graphene island grows epitaxially once it has been nucleated and does not change its lattice orientation when it crosses the GBs of the polycrystalline metal substrate (Figure 2A ) [30.Wang H. et al.Lateral homoepitaxial growth of graphene.CrystEngComm. 2014; 16: 2593-2597Crossref Scopus (7) Google Scholar, 31.Dong J. et al.How graphene crosses a grain boundary on the catalyst surface during chemical vapour deposition growth.Nanoscale. 2018; 10: 6878-6883Crossref PubMed Google Scholar, 32.Zhang X. et al.How the orientation of graphene is determined during chemical vapor deposition growth.J. Phys. Chem. Lett. 2012; 3: 2822-2827Crossref Scopus (67) Google Scholar]. Thus, it is critical to control and suppress the nucleation density of graphene, which is reported to be determined by the number of active sites and the concentration of active carbon species [formed by the decomposition of CH4, e.g., CHx (x = 3, 2, 1, 0)] on the substrate surface [33.Kim H. et al.Activation energy paths for graphene nucleation and growth on Cu.ACS Nano. 2012; 6: 3614-3623Crossref PubMed Scopus (277) Google Scholar,34.Braeuninger-Weimer P. et al.Understanding and controlling Cu-catalyzed graphene nucleation: the role of impurities, roughness, and oxygen scavenging.Chem. Mater. 2016; 28: 8905-8915Crossref PubMed Scopus (79) Google Scholar]. The surface conditions of the substrate, such as roughness and cleanliness [35.Han G.H. et al.Influence of copper morphology in forming nucleation seeds for graphene growth.Nano Lett. 2011; 11: 4144-4148Crossref PubMed Scopus (292) Google Scholar, 36.Yan Z. et al.Toward the synthesis of wafer-scale single-crystal graphene on copper foils.ACS Nano. 2012; 6: 9110-9117Crossref PubMed Scopus (438) Google Scholar, 37.Chen S. et al.Millimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition.Adv. Mater. 2013; 25: 2062-2065Crossref PubMed Scopus (227) Google Scholar, 38.Mohsin A. et al.Synthesis of millimeter-size hexagon-shaped graphene single crystals on resolidified copper.ACS Nano. 2013; 7: 8924-8931Crossref PubMed Scopus (135) Google Scholar], and the CVD parameters, including growth temperature [39.Li X. et al.Graphene films with large domain size by a two-step chemical vapor deposition process.Nano Lett. 2010; 10: 4328-4334Crossref PubMed Scopus (760) Google Scholar] and partial pressure of CH4 (or ratio of H2/CH4) [40.Li X. et al.Large-area graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper.J. Am. Chem. Soc. 2011; 133: 2816-2819Crossref PubMed Scopus (977) Google Scholar], reportedly influence the nucleation density of graphene. Pretreatment of the metal substrate, including surface polishing, acid etching, and high temperature annealing in a reductive atmosphere, is still widely used to clean and flatten the substrate surface and decrease the number of active sites. A growth temperature (above 1000°C) with a low partial pressure of the carbon precursor that leads to a relatively low concentration of active carbon species was typically used to grow single-crystal graphene islands [37.Chen S. et al.Millimeter-size single-crystal graphene by suppressing evaporative loss of Cu during low pressure chemical vapor deposition.Adv. Mater. 2013; 25: 2062-2065Crossref PubMed Scopus (227) Google Scholar,41.Chen X. et al.Chemical vapor deposition growth of 5 mm hexagonal single-crystal graphene from ethanol.Carbon. 2015; 94: 810-815Crossref Scopus (51) Google Scholar,42.Wang H. et al.Controllable synthesis of submillimeter single-crystal monolayer graphene domains on copper foils by suppressing nucleation.J. Am. Chem. Soc. 2012; 134: 3627-3630Crossref PubMed Scopus (290) Google Scholar]. In addition to decreasing the flow rate of the carbon precursor, Li and colleagues showed that by folding a Cu foil to form a ‘pocket’, an extremely low methane partial pressure is achieved inside the pocket and a 0.5-mm single-crystal graphene island with an average growth rate of 6 μm/min could be grown on the interior side of the Cu pocket [40.Li X. et al.Large-area graphene single crystals grown by low-pressure chemical vapor deposition of methane on copper.J. Am. Chem. Soc. 2011; 133: 2816-2819Crossref PubMed Scopus (977) Google Scholar]. In 2013, Hao and colleagues first reported the role of oxygen in the graphene growth process. They showed that oxygen on the Cu surface not only decreases the graphene nucleation density by passivating Cu surface active sites (Figure 2C) but also accelerates the growth of the graphene island by shifting the growth kinetics from edge-attachment-limited to diffusion-limited (Figure 2D) [43.Hao Y. et al.The role of surface oxygen in the growth of large single-crystal graphene on copper.Science. 2013; 342: 720-723Crossref PubMed Scopus (736) Google Scholar]. Individual islands with diameters larger than 1 cm were grown on oxygen-rich Cu foils after a 12-h growth (Figure 2B). Following this, various methods to introduce surface oxygen on Cu foils have been reported, including heating the Cu foil in air [44.Ding D. et al.Behavior and role of superficial oxygen in Cu for the growth of large single-crystalline graphene.Appl. Surf. Sci. 2017; 408: 142-149Crossref Scopus (20) Google Scholar], annealing it in an inert atmosphere [45.Gan L. Luo Z. Turning off hydrogen to realize seeded growth of subcentimeter single-crystal graphene grains on copper.ACS Nano. 2013; 7: 9480-9488Crossref PubMed Scopus (176) Google Scholar], and introducing O2 during the graphene growth [46.Guo W. et al.Oxidative-etching-assisted synthesis of centimeter-sized single-crystalline graphene.Adv. Mater. 2016; 28: 3152-3158Crossref PubMed Scopus (61) Google Scholar], all of which have indicated that the presence of surface oxygen is effective in decreasing the nucleation density of graphene. Seeding growth is a common method for the synthesis of various single-crystal materials. Graphene oxide flakes [47.Li Q. et al.Controllable seeding of single crystal graphene islands from graphene oxide flakes.Carbon. 2014; 79: 406-412Crossref Scopus (19) Google Scholar], exfoliated thin graphite flakes [30.Wang H. et al.Lateral homoepitaxial growth of graphene.CrystEngComm. 2014; 16: 2593-2597Crossref Scopus (7) Google Scholar], and prepatterned polymethyl methacrylate (PMMA) dots [48.Wu W. et al.Growth of single crystal graphene arrays by locally controlling nucleation on polycrystalline Cu using chemical vapor deposition.Adv. Mater. 2011; 23: 4898-4903Crossref PubMed Scopus (156) Google Scholar] have been used as the seeds for graphene growth on Cu. Wu and colleagues reported the controllable growth of arrays of single-crystal graphene islands with sizes of tens of microns using prepatterned PMMA dots as the nucleation seeds, which might be a promising method for quantity production of single-crystal graphene islands of a certain size (Figure 2E) [48.Wu W. et al.Growth of single crystal graphene arrays by locally controlling nucleation on polycrystalline Cu using chemical vapor deposition.Adv. Mater. 2011; 23: 4898-4903Crossref PubMed Scopus (156) Google Scholar]. A single-crystal graphene film was reported on a polycrystalline Pt foil if graphene islands with aligned orientation were first transferred to the substrate, followed by growth from their edges [49.Jang H.-S. et al.Toward scalable growth for single-crystal graphene on polycrystalline metal foil.ACS Nano. 2020; 14: 3141-3149Crossref PubMed Scopus (2) Google Scholar]. The key point of using seeds for graphene growth is to ensure that graphene grows only from the seeds rather than nucleating elsewhere. It should always be remembered that undesired nucleation may occur in the empty space between seeds as a result of the supersaturation of active carbon species formed by decomposition of the carbon source, if the seed density is too low [50.Ding D. et al.Spatially controlled nucleation of single-crystal graphene on Cu assisted by stacked Ni.ACS Nano. 2016; 10: 11196-11204Crossref PubMed Scopus (28) Google Scholar]. A 1.5-inch single-crystal graphene island was reportedly obtained on a Cu85Ni15 alloy foil after 2.5-h growth from one single nucleus by locally feeding the carbon precursor through a quartz nozzle (Figure 2F) [51.Wu T. et al.Fast growth of inch-sized single-crystalline graphene from a controlled single nucleus on Cu–Ni alloys.Nat. Mater. 2016; 15: 43-48Crossref PubMed Scopus (317) Google Scholar]. A new approach was reported by Vlassiouk and colleagues for the synthesis of a foot-long single-crystal graphene film on a moving polycrystalline Cu90Ni10 alloy foil at rates up to 2.5 cm/h (Figure 2G). With a moving substrate and localized feeding of the carbon precursor, the fastest-growing island orientation reportedly overwhelms any slower-growing islands, resulting in a single-crystal graphene film [52.Vlassiouk I.V. et al.Evolutionary selection growth of two-dimensional materials on polycrystalline substrates.Nat. Mater. 2018; 17: 318-322Crossref PubMed Scopus (89) Google Scholar]. It should be noted that localized feeding of the carbon precursor can be used only for a Cu/Ni alloy with proper composition and is not suitable for the graphene growth on pure Cu or pure Ni. This might be due to the fact that the nucleation density of graphene on the Cu/Ni alloy is much lower than that on Cu under the same conditions [53.Liu Y. et al.How low nucleation density of graphene on CuNi alloy is achieved.Adv. Sci. 2018; 5: 1700961Crossref Scopus (8) Google Scholar] and that the carbon solubility of Cu/Ni alloys (such as Cu85Ni15 or Cu90Ni10) is not high enough to dissolve all th