Computational investigations into the structural and electronic properties of CdnTen (n = 1–17) quantum dots

Abstract

Size-tunability of the electronic and optical properties of semiconductor quantum dots and nanoclusters is due to the quantum size effect, which causes variations in the electronic excitations as the particle boundaries are changed. Recently, CdSe and CdTe quantum dots have been used in energy harvesting devices. Despite these promising practical applications, a complete understanding of the electronic transitions associated with the surfaces of the nanoparticles is currently lacking and is difficult to achieve experimentally. Computational methods could provide valuable insights and allow us to understand the electronic and optical properties of quantum dots and nanoclusters. Hollow cage and endohedral or core–shell cage structures for Cd_nTe_n clusters have been reported before. We have performed systematic density functional theory (DFT) studies on the structure and electronic properties of the Cd_nTe_n (n = 1–17) clusters. As the number of atoms increases in the Cd_nTe_n clusters, the predicted geometries change from simple planar structures to more complicated 3D-structures. Two classes of the most stable structures were elucidated for clusters with n = 10–17: (i) hollow cage structures with an empty center; and (ii) endohedral or core–shell cage structures with one or more atoms inside the cage. Noticeably higher highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps were observed for the hollow cage isomers as compared to the core–shell structures. The highest occupied molecular orbitals of all of the clusters studied were shown to be localized on the surface of the cage for the hollow cage structures, while in the case of the core–shell structures, the HOMO electron densities were found to be distributed both on surface and the interior of the structures. Most of the small size clusters Cd_nTe_n (n = 2–9) showed minimal values for the dipole moments (close to zero) owing to the highly ordered and symmetric configurations of these structures. For isomers of the larger clusters (n = 10–17), it was observed that the core–shell structures have higher values for the dipole moments than the hollow cage species because of the highly symmetric structures of the hollow cages. Core–shell cage structures exhibited lower polarizability than the respective hollow cage structures.