The structural and electronic properties of NbSin−/0 (n = 3–12) clusters: anion photoelectron spectroscopy and ab initio calculations

Abstract

Niobium-doped silicon clusters, NbSi_n⁻ (n = 3–12), were generated by laser vaporization and investigated by anion photoelectron spectroscopy. The structures and electronic properties of NbSi_n⁻ anions and their neutral counterparts were investigated with ab initio calculations and compared with the experimental results. It is found that the Nb atom in NbSi_n^−/0 prefers to occupy the high coordination sites to form more Nb–Si bonds. The most stable structures of NbSi_3–7^−/0 are all exohedral structures with the Nb atom face-capping the Si_n frameworks. At n = 8, both the anion and neutral adopt a boat-shaped structure and the openings of the boat-shaped structures remain unclosed in NbSi_9–10^−/0 clusters. The most stable structure of the NbSi₁₁⁻ anion is endohedral, while that of neutral NbSi₁₁ is exohedral. The global minima of both the NbSi₁₂⁻ anion and neutral NbSi₁₂ are D_6h symmetric hexagonal prisms with the Nb atom at the center. The perfect D_6h symmetric hexagonal prism of NbSi₁₂⁻ is electronically stable as it obeys the 18-electron rule and has a shell-closed electronic structure with a large HOMO–LUMO gap of 2.70 eV. The molecular orbital analysis of NbSi₁₂⁻ suggests that the delocalized Nb–Si₁₂ ligand interactions may contribute to the stability of the D_6h symmetric hexagonal prism. The AdNDP analysis shows that the delocalized 2c–2e Si–Si bonds and multicenter-2e NbSi_n bonds are important for the structural stability of the NbSi₁₂⁻ anion.