Si clusters are more metallic than bulk Si

Dipole polarizabilities were computed using density functional theory for silicon clusters over a broad range of sizes up to N = 147 atoms. The calculated total effective polarizabilities, which include contributions from permanent dipole moments of the clusters, are in very good agreement with recently measured values. We show that the permanent dipole contributions are most important for clusters in the intermediate size range and that the measured polarizabilities can be used to distinguish between energetically nearly degenerate cluster isomers at these sizes. We decompose the computed total polarizabilities α into the so-called dipole and charge transfer contributions, α^p and α^q, using a site-specific analysis. When the per-atom values of these quantities are plotted against N^-1/3, clear linear trends emerge that can be extrapolated to the large size limit (N^-1/3→0), resulting in a value for αN of 30.5 bohrs³/atom that is significantly larger than the per-atom polarizability of semiconducting bulk Si, 25.04 bohrs³/atom. This indicates that Si clusters possess a higher degree of metallicity than bulk Si, a conclusion that is consistent with the strong electrostatic screening of the cluster interiors made evident by the analysis of the calculated atomic polarizabilities.