In the present paper, the cluster model and the charge self-consistent EHT method are used to study the chemisorptions of group VII elements on both Si (111) 1×1 and Ge(111)1×1 surfaces. The chemisorbed sites are determined by minimizing the total energy. For Cl, the absorption takes place on the atop sites for both Si (111) and Ge (111) surfaces. For I, the result shows that the threefold hollow sites of both surfaces are more preferable. For Br, both the atop and the threefold hollow sites seem to be possible on both surfaces. However, for Br/Si(lll)l×1 the atop site is more stable, whereas for Br/Ge (111)1×1 the threefold hollow site is more stable. Finally, the trend of the chemisorptions of the group VII atoms on the two surfaces are discussed and comparison with the recent SEXAFS results presented.
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