Modeling electronic excitations/formation of trap states in semiconducting nanocrystals

In this chapter we review recent results of modelling of formation of trap states and ideas on the optimal way of passivation based on these results. Passivation of quantum dots is a crucial condition determining their optical properties. Dangling bonds on the surface of nanocrystals were commonly considered as the main source of trap states. Recent studies clearly indicate that presence of dangling bonds not always lead to formation of trap states. We also present a new idea on formation of trap states, which considers the effect of the ground state dipole moment. Results obtained via density functional theory calculations indicate the correlation between the dipole moment value and formation of deep trap states. A correlation between the dipole moment value and the deepness of the trap states locations was demonstrated using the Cd4S cluster as a model basis and different number of SH groups as passivating agents. Namely, the high values of the dipole moment provide a higher number of trap states. Rearrangement of the same number of SH groups also indicates the dipole moment's effect on the electronic spectra. The application of an electrostatic field oriented against the dipole moment vector also confirms the importance of the dipole moment in formation of optical properties of nanocrystals.