Stable fcc cage of III-IV mixed clusters with large energy gaps: Predictions based onab initiomolecular dynamics simulations

Abstract

Theoretical investigations based on ab initio molecular dynamics simulations predicts that group III-IV mixed clusters with a composition of $8:6$ form stable face centered cubic cagelike structures with large optical gaps. The ground-state geometries of these mixed clusters suggest that while group III elements ($\mathrm{Al}$, $\mathrm{Ga}$, and $\mathrm{In}$) are placed at the corners of the cube, group IV elements $(\mathrm{Si},\mathrm{Ge})$ occupy the centers of six faces, thus leading to form face centered cubic structure. The energy gap between the highest-occupied-lowest-unoccupied energy levels varies from $1.69\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for ${\mathrm{Al}}_{8}{\mathrm{Ge}}_{6}$ to $2.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for ${\mathrm{In}}_{8}{\mathrm{Si}}_{6}$. Contrary to the bulk behavior, the energy gap of these mixed clusters increases as one goes down for group III elements resulting in the largest gap of $2.66\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the ${\mathrm{In}}_{8}{\mathrm{Si}}_{6}$ cluster, which is much larger than the bare ${\mathrm{Si}}_{6}$ cluster $(2.12\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. It can be noted that this is the largest gap obtained for any $\mathrm{Si}$ cluster with metallic capping. Thus the most remarkable feature of these clusters is to have higher density of states along with larger optical gaps as compared to that of free ${\mathrm{Si}}_{6}$ or ${\mathrm{Ge}}_{6}$ clusters. Finally, the stability and physicochemical properties of these clusters have been analyzed based on LCAO-MO method taking all electrons into account.