Atomic and electronic structure of quantum dots on the basis of CdSe

Within the framework of the density functional theory, comparative calculations of the total energy and electronic states of Cd$_{n}$Se$_{n}$ nanoparticles with a structure of three types: wurtzite, sphalerite and NaCl were performed. It has been shown that for $n \le 72$, the formation of a NaCl type structure is energetically favorable. However, extrapolation of the energy values per Cd—Se atom pair shows that for $n > 130$ (corresponding to a size of about $2$ nm), wurtzite-type particles can be more advantageous than particles with the NaCl structure. The electronic structure of Cd$_{n}$Se$_{n}$, Cd$_{n}$S$_{n}$, and Zn$_{n}$S$_{n}$ nanoparticles, as well as CdSe/CdS and CdSe/CdS/ZnS quantum dots, has been studied. It is shown that the ZnS shell not only increases the band gap of a quantum dot, but also significantly increases the intensity of its emission due to the appearance of electronic states near the band gap.