Ab initio calculations of structural and electronic properties of CdTe clusters

We present results of a study on small stoichiometric CdnTen (1 <= n <= 6) clusters and a few nonstoichiometric CdmTen [(m,n=1,4,13,16,19); (m not equal n)] clusters using the density functional formalism and projector augmented wave method within the generalized gradient approximation. Structural properties viz. geometry, bond length, symmetry, and electronic properties such as the highest occupied molecular orbital (HUMO) and lowest unoccupied molecular orbital (LUMO) gap, binding energy, ionization potential, nature of bonding, etc., have been analyzed. The initial geometries of nonstoichiometric clusters were considered as fragments of the bulk with T-d symmetry. It was observed that upon relaxation, the symmetry changes for the Cd-rich clusters whereas the Te-rich clusters retain their symmetry. It may be mentioned that the Te p lone pair repulsion drives the Te atoms to the surface and renders stability to the clusters. The Cd-rich clusters develop a HOMO-LUMO gap due to relaxation whereas there is no considerable change in the HOMO-LUMO gap of the Te-rich clusters. Thus, the symmetry of a cluster seems to be an important factor in determining the HOMO-LUMO gap. To render the surface of a quantum dot inert, passivation is essential. In the present work, we have passivated the nonstoichiometric clusters using fictitious "hydrogen like" pseudoatoms. It was observed that passivation removed the states in the HOMO-LUMO gap region resulting in widening of the gap. The symmetry of the clusters, however, remains unchanged upon passivation.