Study of the Structural and Electronic Properties of Neutral and Charged Niobium-Doped Silicon Clusters: Niobium Encapsulated in Silicon Cages

We performed systematic structure searches for low energy structures of neutral and singly charged niobium-doped silicon clusters NbSinQ (n = 2-20; Q = 0, +/- 1) by means of the CALYPSO structure searching method. A large population of low energy clusters is collected from the unbiased structure search. Subsequent geometry optimizations using density-functional theory with the B3LYP exchange-correlation functional are carried out to determine structural patterns and relative stabilities of various low energy candidates for Nb-doped silicon clusters. Based on the calculated binding energies along with measured photoelectron spectroscopy data, we are able to confirm that our lowest energy structures are the true minima. It is shown that the localized position of the Nb impurity atom in NbSin0/+/-(1) clusters gradually moves from the convex capping position, to surface-substituted, to the concave, and in the end to the encapsulated state as the number of Si atoms increases from 2 to 20. The lowest energy isomer of both neutral and anionic NbSi12 cluster is very stable in a high-symmetry endohedral D-6h structure in which the Nb atom is placed at the center of a regular hexagonal prism of Si atoms. This makes it an attractive building block for cluster-assembled materials.