Super Flat Graphene Grown on Substrate Selected by using Molecular Dynamics Calculation

Choice of substrate is one of most important factors for thin film growth. An extensive interdiffusion or chemical reaction might happen between substrate and target materials. For oxide materials, Schlom et. al. comprehensively investigate the thermodynamic stability of binary oxides for epitaxial growth on silicon (Si) substrates[1]. However, the thermodynamic stability does not include any crystal information such as lattice constants, orientation of crystal growth. In order to predict the orientation of crystal growth, molecular dynamics (MD) is empolyed to evaluate the stability of oxide cluster on Si substrates[2].<br/><br/>In this study, MD was used to select suitable substrates for graphene growth with flat surface. Supercell was consisted of carbon clusters placed on variety of substrates with vacuum slab. As carbon clusters, (1) C atom, (2) six-membered ring (6-ring) and (3) seven six-membered rings (nanographene) were placed on SrTiO, silicon and sapphire substrates. On the surface of sapphire substrate, carbon clusters were placed on either aluminum or oxygen atoms, and the absorption energy was estimated by using the density functional theory (DFT) with a semi-core pseudopotential. The generalized gradient approximation (GGA) method was used to obtain the electron density. Materials Studio and DMol3 were used for preparing and optimizing supercells, respectively[3].<br/><br/>The surface of substrate was optimized, and then the carbon clusters were placed on the candidate substrates. Although the absorption energies showed not large differences on various surfaces, 6-ring, for an example, stood vertically up on Si(001) substrate and covered flatly on SrTiO substrate. Only SrTiO surface was flatly covered by both 6-ring and nanographene after optimization by MD simulation. The SrTiO substrate was the first choice for flat graphene.<br/><br/>Super flat graphene is prepared by chemical vapor deposition (CVD) with post annexing in carbon dioxide atmosphere. Amorphous carbon is selectively etched in carbon dioxide atmosphere[4]. In this study, carbon films were experimentally deposited on target substrates in carbon dioxide atmosphere by pulsed laser deposition (PLD), and super flat surface (~60 pm) was observed on only SrTiO substrate, which agreed with the results of MD simulation. Since CVD graphene usually grows with intrinsic contamination on graphene surface during film growth, a post annealing is required to obtain a flat surface. However PLD prepared as-grown flat surface in carbon dioxide atmosphere.<br/><br/>This study was supported in part by Amada Foundation under contract AF-2020227-B3, Tokyo Ohka Foundation for Promotion of Science and Technology 22117 and the Collaborative Research Project of the Institute of Fluid Science, Tohoku University. Special acknowledgment to the National Cheng Kung University 90 and beyond (NCKU’90).<br/><br/>[1] D.G.Schlom et.al., J.Mater.Res. 11, 2757 (1996).<br/>[2] S.Kaneko et.al., Appl.Surf.Sci. 586, 152775 (2022).<br/>[3] S.Kaneko, et.al., Sci.Rep. 12, 15809 (2022).<br/>[4] J.Zhang et.al., Angew.Chem.Int.Ed. 58, 14446 (2019).