Surface Alloy Phases of Immiscible Metals: A Semiempirical Study of Au Growth on Ni(110)
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GUILLERMO BOZZOLO*, RODRIGO IBANEZ-MEIER** AND JOHN FERRANTE`** * Analex Corporation, 3001 Aerospace Parkway, Brook Park, OH, 44142-1003 WSA,Inc. , Palo Alto, CA 94301. ***National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH 44135.
ABSTRACT Recent experiments using scanning tunneling microscopy (STM) show evidence for the formation of surface alloys of metals with a broad miscibility gap. Such is the case for Au deposited on Ni(110), where STM images indicate that at low coverage Au atoms squeeze Ni atoms out of the surface layer forming a surface alloy while the Ni atoms form islands on the surface. We present results of a theoretical modelling of this phenomenon using the BFS method for alloys. We find evidence which strongly support the conclusions drawn from experiment. Ni island formation and alternative short range order patterns are discussed, as well as the transition from a surface alloy phase at low coverages to phase separation at higher coverages.
Atomic resolution scanning tunneling microscopy experiments carried out by Pleth Nielsen et al [1] show that in spite of the broad miscibility gap in the phase diagram of Ni-Au, Au atoms substitute Ni atoms on the Ni(110) surface thus forming a surface alloy. The experiments indicate the formation of a pattern of Au dimers in the surface plane for low Au coverage, with the substituted Ni atoms located in chains along the close-packed (cp) direction following the fcc stacking of the substrate. Low energy ion scattering experiments by Boerma et al [2] corroborate these results. Building on the ideas suggested by Pleth Nielsen et al [1], we extend this analysis by presenting the essential results concerning the growth process using BFS, a recently developed semiempirical method for alloys [3]. The goal of this work is to find theoretical arguments to support the experimental findings or, in other words, to answer the question if the configuration seen experimentally can be reproduced by theoretical calculations. As we will show below, this analysis basically agrees with the experimental results and provides some insight to the different ingredients and processes that lead to the observed patterns. Moreover, our study suggests that the surface alloying process is favored only at low coverages, leading to phase separation as the Au coverage reaches a certain critical range. The BFS method is based on the idea that the energy of formation of an alloy is the superposition of individual contributions ei of non-equivalent atoms in the alloy [3]:
.r=e +g,
_ c0f).
(1)
ej has two components: a strain energy ec, computed with equivalent crystal theory (ECT) [4], that accounts for the actual geometrical distribution of the atoms surrounding atom i, computed as if all its neighbors were of the same atomic species, and a chemical energy cc - ecO, which takes into account the fact that some of the neighbors of atom i may be of a different chemical species. For eq we interpret the chemical composition as a defect of an otherwise pure crystal
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