An ab initio MO study of the interaction between Li and Na with a dangling bond on a Si(111) surface model cluster

Abstract

The interaction between alkali metals (Li and Na) and Si(111) surface cluster models has been investigated by means of an ab-initio MO method including electron correlation for both ground state and lowlying excited states. Three cluster models composed of Si 4H 9, Si 7H 15 and Si 10H 15 were chosen as the models of a dangling bond in a Si(111) surface. Potential energy curves, calculated as a function of SiM distance for two alkali metal systems ( M = Li and Na), showed that the alkali metals can form a strong chemical bond with the surface dangling bond. The bonding nature between the metal and the surface dangling bond at the ground state is composed of charge interaction in which an electron is transferred from the metal to the surface. At the first exited state, the bonding is varied to a strong repulsive interaction between the metal and the dangling bond due to a σ * interaction. According to the calculation of excitation energies and charge populations, the interaction generates different levels from those of the cluster alone. The first electronic excitation for the interaction systems is attributed to a charge transfer band from the Si dangling bond to the alkali metal. The absorption band appearing at a higher energy region was attributed to a local excitation within the Na atom (3 s → 3 p). The bonding natures at the ground and low-lying excited states are discussed on the basis of the theoretical results.