Origin of Compact Triangular Islands in Metal-on-Metal Growth

Abstract

The microscopic origin of compact triangular islands on close-packed surfaces is identified using kinetic Monte Carlo simulations with energy barriers obtained from density-functional calculations. In contrast to earlier accounts, corner diffusion anisotropy is found to control the shape of compac$ĩ slands at intermediate temperatures.-CEIVEE We rationalize the correlation between the orientation 6P L dendrites grown at low temperatures and triangular islands grown at higher temperatures, and explain why in some systems dendrites grow fat before turning compact. 66~30.Fq, 68.35.Fx, Epitaxial growth of metals is governed by a handful of elementary atomic diffusion processes, where motion of atoms along and across clusters of adsorbed atoms is inherently different from diffusion on plain terraces [1,2]. In particular, sites of low coordination and symmetry do not only always influence growth, but very often directly control it [3]. For example, diffusion processes at kinks and corners define the shape of atomic aggregates [3], the mechanism by which these migrate across the surface [3,4], and the very growth mode itself [5]. Here low-symmetry diffusion processes are shown to have additional important roles, and three outstanding morphology issues in epit axial growth are resolved. JUL 2 \ t$lgIn a seminal scanning-tunneling-microscopy (STM) study of homoepitaxial growth of Pt (1 11), Michely and coworkers [6] observed several beautiful transitions in surface morphology upon increasing the substrate temperature T. Focusing on compact island shapes, triangular islands bounded by A-steps at 400 K were observed to become inverted at 640 K, bounded by B-steps (see Fig. 1 for clarification of step types). A simple explanation fon triangular islands was proposed by Michely et at. [6]: growing islands should advance faster perpendicular to steps with lower adatom mobility. Accordingly, islands would tend to become triangular, with the faster growing steps disappearing. The shape transition from A-to Bstep-bounded islands was attributed to a cross-over in the rate for diffusion of atoms along the two types of steps. Specifically, an anisotropy in both activation barriers and prefactors for edge diffusion was proposed to induce a cross-over in the relative growth speed of these steps, inverting the triangular islands. Density-functional calculations for A1/Al(ll 1) by Stumpf and Scheffler [71indeed do show anisotropic edge diffusion, with barriers for diffusion along A-and B-steps of 0.32 and 0.39-0.42 eV, respectively. With the additional (and vaguely motivated) assumption of a 100 times larger prefactor for edge diffusion along B-steps, the shape transition, experiment ally observed for Pt/Pt (111) [6], could be qualitatively reproduced in kinetic Monte Carlo (KMC) simulations of Al(lll) growth [8].By resealing semi-empirically calculated energy barriers for Pt/Pt(111), Jacobsen and coworkers [9] have been able to find a parameter set that reproduces the experimentally observed transitions, but with the governing aniso...

A molecular view of heterogeneous catalysis

“…23 Here, again, the statistical dynamics of the surface is included in detail and a large number of possible reaction pathways are included in a kinetic Monte Carlo description of the system. 24 The resulting rates are in excellent agreement with the detailed surface science experiments on single crystals at low pressures 25 ͓see Fig. 4͑b͔͒.…”

First-principles kinetic modeling in heterogeneous catalysis: an industrial perspective on best-practice, gaps and needs

“…105 For similar reasons, it is also much more involved to integrate numerical kMC-based microkinetic formulations into fluid dynamical simulations at the reactor level (vide infra). In consequence, application of kMC simulations in chemical engineering has to date been restricted to a few seminal works 1,48,[129][130][131][132][133][134][135][136][137][138] and significant efforts to provide these functionalities in easy-to-use program packages are required to popularize this technique.…”

Diffusion rates of Cu adatoms on Cu(111) in the presence of an adisland nucleated at fcc or hcp sites

“…A large number of theoretical and experimental works has been devoted to homoepitaxial growth, e.g., on Pt [3,4,5,6,7], Cu [8,9,10], Ag [11,12], Rh [13,14,15], Al [16,17,18]... In this respect, the understanding of adisland shapes is of fundamental importance since they can indirectly influence the growth mode.…”