The excitonic fine structure of colloidal nanocrystals

Abstract

In this thesis, the newly developed atomic effective pseudopotential (AEP) method is presented. The use of AEPs allows us to bypass a self-consistent procedure and to address eigenstates around a certain region of the spectrum (e.g., around the band-gap). Therefore, AEP enables the study of the optical properties of many-atoms systems, where mainly the energy states around band-gap are involved. The nanocrystals NCs in all calculations are terminated by pseudo hydrogen atoms and relaxed artificially by the way they are generated. To improve the underestimated band-gap and effective masses that inherited from the local density approximation (LDA) results, an empirical correction is applied to the non-local part of norm-conserving pseudopotential. To obtain the fine structure (FS) and optical properties of NCs, the screened configuration interaction (CI) theory is used in combination with AEP methods. We focus on three materials: CdSe, InP, and HgTe for their wide applications and fabrications. The results on CdSe NCs show good agreement with experiments. The qualitative disagreement between AEP and effective mass approximation (EMA) results on the FS of CdSe NCs suggests an improvement of EMA. The comparison of the wurtzite (WZ) and zinc-blende (ZB) FS as a function of size and ellipticity show that the absorption of significantly oblate ZB nanocrystals can be very similar to "spherical" WZ nanocrystals. On the other hand, if the nanocrystals have no ellipticity but different crystal structures, the fine structure differs significantly. Conversely, structures with high ellipticity, either oblate or prolate, with different crystal structures, show similar fine structures. The results on InP NCs show good agreement between AEP and the semi-empirical pseudopotential method (SEPM). Both theories reproduce well the experimental measurements on the dark-bright (DB) splitting and the optical gap of NCs. The results on HgTe NCs are in good comparison with measured optical gaps. The calculated DB splitting, size-dependent absorption spectra and the exciton-trion splitting can be useful for further studies on this promising material. For the outlook, the study of the exciton lifetime still needs more development. From the single-particle (SP) results obtained by AEP method, the research on the biexciton and trion is totally possible and that will contribute more understanding about the optical properties of NCs.