Tight-Binding Method and Multiband Effective Mass Theory Applied to CdS Nanocrystals: Single-Particle Effects and Optical Spectra Fine Structure

Optical spectra of CdS nanocrystals are interpreted by using both the atomistic tight-binding method and multiband effective mass theory. Both methods correctly describe the energy splitting between the two lowest optically active transitions and their relative strengths, providing the same labeling of the two main absorption peaks of the spectrum. Our calculations unambiguously show that these peaks correspond to the 1S3/2 f 1s and 1P3/2 f 1p transitions. Both zinc blende and wurtzite-type structures for CdS nanocrystals are considered. Similar optical spectra are predicted for the two lattice structures. We also study how the spectrum, and in particular, the 1S3/2- 1S1/2 splitting, is changed by modifying parameters, within the experimental uncertainties, including size and shape fluctuations, surface passivation and spin-orbit coupling. Our results are robust to small variations in all of these parameters. 1