Structures and Stabilities of Clusters of Si12, Si18, and Si20 Containing Endohedral Charged and Neutral Atomic Species

Abstract

Electronic structure calculations based on density functional theory and Moller Plesset perturbation theory were performed on three isomers of Si 12 and on the endohedral clusters Si 12 containing neutral or charged atomic species. The existence of endohedral clusters depends on the Si 12 cage shape and the nature of the embedding species. Endohedral clusters of Li 0 ' 1 ' -1 , Na 0,1,-1 and He in Si 12 cages were found. In contrast, K + , Ne, F - , or Cl - do not form endohedral clusters with Si 12 due to their large size. All endohedral clusters that are minima on the potential energy surfaces are stable and have large HOMO-LUMO gaps (>1 eV). The stability order for the lithium and sodium clusters is: anionic clusters > neutral clusters > cationic clusters. The endohedral complex of two Li atoms with the Si 18 cage is lower in energy than the sum of the empty Si 18 cage and two Li atoms. In contrast, doping two Na atoms into the Si 18 cage forms an exohedral Na 2 Si 18 cluster. An endohedral cluster of Li 2 with Si 20 was also investigated and characterized. The stability of the endohedral complexes of two Li atoms in Si 18 and Si 20 suggest that silicon nanotubes, which are unstable, might be stabilized by an internal string of Li atoms.