Effect of the dipole moment on the formation of trap states in nanosized CdS clusters
Dr. Anuar Aldongarov (L.N. Gumilyov Eurasian National University).
In this presentation 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
We aslo have tested effect of variation of structures of the large CdS clusters with the same stoichiometry formula on the electronic transitions and dipole moments using DFTB method. The results show that the variations of structures with the same stoichiomestry may provide significant change of dipole moment and electronic transitions at the same time. Usually increase of dipole moment correlates with the decrease of energy of first electronic transition.
DFTB calculations for very large clusters (300–1000 atoms) have also demonstrated that the location of surface defects play important role in formation of trap states. We have created surface defects by removing SH groups in directions along and against the dipole moment. The results show that probability of formation of trap states increases if the termination of SH group provided increase of dipole moment and vice versa.
Based on the obtained results we may conclude that symmetric distributions of charges over the volume of CdS nanosized crystals may provide the best passivation strategy.