Surface Chemistry on Microclusters: Recent Results from Silicon and Germanium Cluster Beams
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SURFACE CHEMISTRY ON MICROCLUSTERS: RECENT RESULTS FROM SILICON AND GERMANIUM CLUSTER BEAMS J. M. ALFORD AND R. E. SMALLEY Rice Quantum Institute and Department of Chemistry Rice University, Houston, Texas 77251
ABSTRACT Supersonic cluster beam techniques have recently produced some fascinating new information as to the surface chemistry and physics of small (2-100 atom) clusters of various semiconductors. For example, it appears that silicon clusters exhibit a remarkably pronounced alternation in reactivity as a function of cluster size. For ammonia chemisorption certain clusters such as Si 3 3 +, Si 3 9 +, and Si 4 5 + have been found to be almost completely inert, while neighboring clusters such as Si 3 6 and Si45+ chemisorb ammonia readily. Such sharply patterned reactivity results may provide significant clues as to the detailed nature of semiconductor surface restructuring and the consequent effects of this restructuring upon the surface chemistry.
INTRODUCTION
Over the past few years a new avenue for fundamental research into the properties of metal and semiconductor surfaces has begun to emerge. It involves the study of such small pieces of the bulk surface that the individual atoms may be counted, and explanations for the detailed surface chemistry and physics may be sought as though these species were actually just large molecules. With current techniques it is fairly straight forward to generate cold, fairly intense supersonic beams containing clusters from two to several hundi=d atoms in size. Although these constitute rather large molecules, still most of the atoms are on the surface, and the properties of this surface have a dominate influence on all aspects of the cluster chemistry and physics. In a very literal sense these small atomic clusters are then "molecular models" of real surfaces. As is the case with all simple models, these cluster models should not be expected to be perfect replicas of the real thing. They will be useful as models only to the extent that some of the essential new physics and chemistry that we need to learn about real surfaces will also hold sway in the small cluster. If so, it may be that many of the new insights will best be gained first on these small clusters. The hope of course is that many of the powerful techniques we normally associate with small molecule science can be extended to these cluster models. Particularly intriguing is the notion that it may soon be possible to calculate the physical and chemical properties of these small clusters by high level quantum chemical techniques, and to test the effectiveness of approximations used in these techniques by direct comparison with detailed experimental measures on exactly the same cluster in the laboratory. The cluster models of surfaces are most likely to be of direct use when one concentrates on the microscopic details of surface chemistry, especially when this chemistry occurs on the surface of what is primarily a covalently bonded material. So a natural early topic of concentrated cluster research has been c
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