Atomistic and Coarse-Grained Modeling Strategies for Thin Film Nucleation and Growth on Quasicrystalline Surfaces
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Atomistic and Coarse-Grained Modeling Strategies for Thin Film Nucleation and Growth on Quasicrystalline Surfaces James W. Evans1,3, Patricia A. Thiel2,3, and Bariş Ünal3,4 1 Department of Physics & Astronomy, Iowa State University, Ames, Iowa 50011, U.S.A. 2 Department of Chemistry, Iowa State University, Ames, Iowa 50011, U.S.A. 3 Ames Laboratory – USDOE, Iowa State University, Ames, Iowa 50011, U.S.A. 4 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge MA 02139, U.S.A. ABSTRACT Strategies are described for modeling the kinetics of non-equilibrium film growth during deposition of metals on quasicrystalline substrates. We review previous atomistic-level latticegas modeling and Kinetic Monte Carlo simulation for pseudomorphic (or commensurate) submonolayer growth based on a “disordered or irregular bond-network” (DBN) of neighboring adsorption sites. We describe extensions to treat strain effects and multilayer growth, and discuss a type of commensurate-incommensurate transition expected around 2-3 layers. We also describe a coarse-grained “step dynamics” modeling which tracks the dynamics of island edges in each layer rather than individual atoms. Step dynamics models can also include key aspects of the physics such as layer-dependent energetics, including quantum size effects, and strain effects. INTRODUCTION Interest in film growth by deposition of metals on quasicrystalline (QC) surfaces has been motivated in part by the possibility of transferring the quasiperiodic structure of the substrate template into the growing film [1,2]. Modeling is complicated by the complex non-periodic structure of the template which is reflected in the Potential Energy Surface (PES) describing the interaction of adsorbed atoms with the substrate. For film growth at lower temperatures [3], a key challenge is to describe the kinetics (not just the thermodynamics) of these non-equilibrium thin film growth processes on the relevant time- and length-scales. Molecular Dynamics cannot assess the relevant time scales, even assuming availability of reliable semi-empirical atomic interaction potentials. Thus, stochastic atomistic-level modeling, which implements relevant processes (thermally activated surface diffusion or hopping, and deposition) with probabilities proportional to their physical rates, is more appropriate and effective [3]. However, there are major, but not insurmountable, challenges since adsorbed atoms do not reside at a simple periodic lattice or grid of equivalent adsorption sites in each layer [4]. Thus, there is also some motivation to consider simpler coarse-grained (CG) approaches as described below [5]. Non-equilibrium features of growth which are very sensitive to hopping barriers or rates (kinetics), and not just to interactions (thermodynamics), include the following: (i) Nucleation and growth of 2D islands in the submonolayer regime; this process often involves heterogeneous nucleation at special trap sites, but has also been described using concepts of traditional surface nucle
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