Faceting Induced by Ultrathin Metal Films: A First Principles Study

J. G. Che, C. T. Chan, C. H. Kuo, T. C. Leung
1997 Physical Review Letters  
Using first principles calculations, we studied the overlayer growth mode and the substrate stability when ultrathin layers of various metals are grown on a Mo(111) substrate. We found that the growth mode is Stranski-Krastanov, and the overlayer can induce the substrate to facet, in accordance with recent experimental observations. The growth-induced instability of the substrate towards faceting is driven by the enhancement of the surface energy anisotropy. However, faceting can be forbidden
more » ... can be forbidden in some cases if the overlayer adsorption does not lower the surface formation energy significantly. [S0031-9007(97)04532-8] PACS numbers: 68.35.Bs, 68.35.Md, 71.15.Nc Ultrathin metal films supported on metal substrates can have novel physical and chemical properties that render them useful in applications such as magnetic technology, catalysis, and material science. These novel properties depend on factors such as the overlayer-substrate interaction, the overlayer growth mode, and the substrate morphological change. Recently, some metal overlayers are observed to induce the substrate to facet [1] . Such a flat to hilland-valley surface reconstruction involves morphological changes in a macroscopic scale, and is quite different from the more familiar atomic scale adatom-induced surface reconstruction [2]. The main objective of this paper is to use first principles calculations to study the overlayer growth and the induced substrate faceting. We focus on various metallic overlayers on bcc (111) substrates, where there exist comprehensive experimental data [1, 3, 4] . These systems are particularly worth studying since they involve simultaneously the physics of the surfaces, thin films, bimetallic interfaces, and reconstructions in macroscopic length scales. Each of these aspects is interesting in its own right and when combined together, they present a complex and challenging problem that mandates the use of ab initio calculations for an accurate description at the atomic level. This is to our knowledge the first attempt using first principles calculations to address directly the problem of overlayer-induced faceting. When thin metal layers are grown on the (111) surface of bcc metals like Mo and W, the following generic features are observed [1,3,4]: (1) The clean surfaces are stable. (2) Annealing is needed to observe the overlayerinduced faceting. (3) The (111) surfaces facet to triangular pyramids with [112] orientations. The pyramids are made up of the substrate atoms (Mo or W) coated with a thin wetting layer of overlayer atoms. The observed faceting is thus a macroscopic morphological change of the substrate induced by the overlayer. (4) Excess overlayer atoms form 3D islands after the completion of one wetting "physical monolayer" (PML), which is defined as the number of geometrical monolayers needed to shadow all the substrate atoms. One PML is two geometrical monolayers for bcc(112) and three geometrical monolayers for bcc(111). (5) Only some metals like Au, Rh, Pt, Ir, and Pd (with Pauling electronegativity .2) cause faceting. Others do not. (6) A critical coverage of approximately one PML is needed to induce the facet formation. The general phenomenon of faceting has attracted much attention for almost a century [5, 6] , and the thermodynamic driving force is attributed to the surface energy anisotropy [5] [6] [7] [8] . Low index clean metal surfaces seldom facet since the anisotropy is usually small, but stable metal surfaces can facet upon adsorption of O and Cl [9] . Embeddedatom [10] and earlier local-density approximation (LDA) results [11] indicate that metal overlayers can also enhance surface energy anisotropy significantly, although these phenomena are actually rather subtle since isoelectronic metals like Cu, Ag, and Au can behave differently [1] . We seek to understand these observations by first principles calculations. We choose Mo as the substrate, and we consider the energetics of the adsorption and growth of pseudomorphic layers of different fcc metals (Cu, Ag, Au, Pd, Pt) on different orientations of the Mo substrate. The calculations are done using the local density formalism [12] and norm conserving scalar-relativistic pseudopotentials [13] . We employ a mixed basis set [14] , which consists of both numerical orbitals centered on the atomic sites, and plane waves with a kinetic energy up to 11.5 Ry. The k points are sampled on a uniform grid of not less than 64 points in the surface Brillouin zones. The substrate is represented by a slab of 11 layers of Mo, and the overlayers are added as additional pseudomorphic layers on either side of the slab. The slabs are separated by a distance of 9.5 Å. All atomic positions are fully relaxed. A schematic top view of bcc (111) and (112) is given in Fig. 1 . Since the [112] direction makes the smallest angle with [111], it is natural for the (111) surface to facet to pyramids exposing three equivalent facets of ͕112͖ surfaces, provided that ͕112͖ has a lower surface energy and that the surface energy anisotropy is big enough to 4230 0031-9007͞97͞79(21)͞4230(4)$10.00
doi:10.1103/physrevlett.79.4230 fatcat:fbgaqsngircfnkuc7snzazdkce