Large-Scale Ab Initio Study of Size, Shape, and Doping Effects on Electronic Structure of Nanocrystals

Abstract

Semiconductor nanocrystals, such as quantum dots (QDs) and wires (QWs), often contain from a few thousands to more than 10⁶ atoms. It has been a great challenge to calculate the electronic structure of these large nanosystems using self-consistent first-principles method. In this chapter, recent development of calculations of nanocrystal physical properties using the large-scale ab initio pseudopotential or charge-patching methods is reviewed. The calculated size-dependent exciton energies and absorption spectra of QDs and QWs are in good agreement with experiments. The calculated ratios of bandgap increases between QWs and QDs are found to be material-dependent, and for most direct bandgap materials, this ratio is close to 0.586, as predicted by simple effective-mass approximation. We show that the electronic structure of a nanocrystal can be tuned not only by its size, but also by its shape. Therefore, the shape can be used as an efficient way to control the electronic structure of the nanocrystals. Changing the shape is expected to be more flexible and provides more variety of the electronic states than simply changing the size of the system. The special features of the electronic states obtained in different shapes of the nanocrystals can be used in various device applications. We also show that defect properties in QDs could be significantly different from those in bulk semiconductors. For example, although negatively charged DX^– center is unstable in bulk GaAs:Si with respect to the tetrahedral coordinated Si_Ga ^–, when the dot size is small enough, it becomes stable.