Stable fcc cage of III-IV mixed clusters with large energy gaps: Predictions based on ab initio molecular dynamics simulations

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 (Al, Ga, and In) are placed at the corners of the cube, group IV elements (Si,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 eV for Al₈Ge₆ to 2.66 eV for In₈Si₆. 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 eV for the In₈Si₆ cluster, which is much larger than the bare Si₆ cluster (2.12 eV). It can be noted that this is the largest gap obtained for any 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 Si₆ or Ge₆ clusters. Finally, the stability and physicochemical properties of these clusters have been analyzed based on LCAO-MO method taking all electrons into account.