Structure and bonding of small semiconductor clusters

We calculate the geometrical and electronic structure of small ${\mathrm{Si}}_{\mathrm{n}}$, ${\mathrm{Si}}_{\mathrm{n}}$${\mathrm{}}^{+}$, and ${\mathrm{Si}}_{\mathrm{n}}$${\mathrm{}}^{\mathrm{-}}$ clusters up to sites n=14 within a combined tight-binding–density-functional-theory scheme. Especially stable structures for n=6 and 10 coincide with observed abundancies in the experimental mass spectra. All equilibrium structures are found to be close packed, with a different bonding than found in the bulk fragments. A transition to bulklike open structures is estimated to occur at cluster sizes n≊${10}^2$–${10}^3$. .AE