Stability of Carbon Cluster on Candidate Substrates Evaluated by Molecular Dynamics

Choice of substrate is one of important factors for synthesis of thin films. For oxide materials, Schlom et.al. comprehensively investigate the thermodynamic stability of more than 80 binary oxides for epitaxial growth on silicon substrates , however thermodynamics stability literary evaluates the thermal stability between oxide and silicon, and does not include any crystallographic factors such as lattice mismatch and crystal orientation. The thermal stability is literally potential use for epitaxial growth as Schlom mentioned in his report[1].<br/><br/>In previous report, we employed an absorption energy between oxide and silicon (Si) substrate to predict the direction of crystal growth of oxide films, and experimentally deposited oxide films on Si substrates[2]. In this study, molecular dynamics was applied on carbon clusters placed on variety of substrates with vacuum slab (supercell), and the absorption energy was estimated on the supercell. Carbon films were experimentally deposited on the target substrates by pulsed laser deposition (PLD) in carbon dioxide atmosphere[3].<br/><br/>For evaluation of absorption energy, supercell was consisted of carbon clusters placed on variety of substrates with vacuum slab. As carbon clusters, (1) carbon 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 SrTiO(001), for an example, carbon clusters were placed on either Sr, Ti or O atom. 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. The energy difference for self-consistent field was set at 1.0x10-6 Ha. Crystal structure of each substrates was optimized with periodic condition, and supercell consisting of slab and vacuum layer was used to optimize the top layer of substrate. After optimal surface was prepared on each substrates, supercell with graphitic cluster and slab was optimized, and total energy was estimated on supercell combined cluster and substrate (Etot_combined). Absorption energy was evaluated to be<br/><br/>Etot_sub + Etot_cluster - Etot_supercell,<br/><br/>where Etot_sub and Etot_cluster) were total energy of subs and graphitic cluster, respectively.<br/><br/>Among target substrates, the absorption energy were in range from 500 to 1000 J/mol. On the surface of Si(001), nanographene was stable with the absorption energy of ~940 J/mol, however 6-ring did not flatly cover the surface. The 6-ring vertically stood up on the Si(001), which impeded flat growth of graphene. Carbon clusters flatly covered only on SrTiO(001)[4].<br/><br/>For demonstration deposition, PLD method was employed with carbon dioxide atmosphere and the surface morphology was observed by atomic force microscopy (AFM). As expected, the surface morphology of SrTiO(001) shows flat surface with Ra ~ 63 pm, which is comparable with superflat CVD graphene annealed in carbon dioxide atmosphere (Ra∼76 pm)[5]. In general, the deposited clusters migrate on the target surface, and the clusters are captured by a kink. Interestingly, graphene showed layer by layer growth and grew not from kink but the terrace edge. We will show optimized structure of supercell with candidate substrates, and compared with experimental results.<br/><br/>Acknowledgments<br/>This study was supported in part by the Amada foundation under contract no. AF-2020227-B3, Tokyo Ohka Foundation for The Promotion of Science and technology no. 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/>References<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., ACS Omega 2, 1523 (2017).<br/>[4] S. Kaneko et. al., Sci. Rep. 12, 15809 (2022).<br/>[5] J. Zhang et. al, Angew. Chem. Int. Ed. 58, 14446 (2019).