Super Flat Graphene Growth on Candidate Substrate Selected by Molecular Dynamics—Theoretical and Experimental

Stability of functioning materials on a target surface is one of important factor for thin film growth. For thermodynamic stability, Schlom et. al. comprehensively investigate oxide materials face to silicon (Si) surface[1]. The paper is the reference on growth of oxide on Si surface, however, their study does not include any crystallographic consideration, such as lattice constants and orientation of crystal growth. By introducing an adsorption energy, the orientation of epitaxial film was predicted on candidate materials[2]. In this study, the absorption energy was again used to select candidate substrates for flat graphene growth, and graphene films were experimentally deposited on the candidate substrates by using a pulsed laser deposition (PLD), and showed flat surface of Ra ~ 63 pm close to super flat surface reported by molecular beam epitaxy [3].<br/><br/>A candidate substrate was selected by molecular dynamics (MD) simulation. A 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. Each carbon clusters and substrate surface were optimized before consisting supercells. On the sapphire surface, for an example, carbon cluster was placed on (1) aluminum or oxygen atoms, and the adsorption energy was estimated 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.<br/>The adsorption energy showed not much different on the candidate surfaces, however, carbon cluster vertically stood up on some candidate substrates, which prevents films to flatly grow on the surfaces. Carbon cluster of both six-ring and nanographene flatly covered the surface on only SrTiO substrate.<br/><br/>Carbon film was experimentally deposited on the candidate substrates by a PLD with 248 nm at the repetition rate of 2 Hz reduced by the slower Q-switched method [4]. On SrTiO substrate, an atomic force microscopy (AFM) showed a flat graphene grew with Ra ~ 63 nm. Interestingly, MD simulation showed 6-ring vertically stood up on Si substrate, and nano balls experimentally scattered on Si substrates.<br/><br/>Suitable substrate was selected by using a MD simulation on candidate substrates, and carbon film experimentally deposited on the candidate substrates. The simulation showed carbon cluster flatly cover on only SrTiO substrate, and flat graphene experimentally grew on SrTiO with Ra ~ 63 nm[5]. The same method was also applied onto oxide film, and crystal orientation of epitaxial film was predicted on Si (001) substrate. Magnesium oxide, for an example, was experimentally deposited on Si(001) substrate, and x-ray diffraction showed the simulation results agreed with the epitaxial film deposited on Si(001).<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] J. Zhang et. al., Angew. Chem. Int. Ed. 58, 14446 (2019).<br/>[4] S. Kaneko et. al., Jpn. J. Appl. Phys. 40, 4879 (2001).<br/>[5] S. Kaneko et. al., Sci. Rep. 12, 15809 (2022).