Mobile Surface Traps in CdSe Nanocrystals with Carboxylic Acid Ligands

We have performed ab initio calculations of electronic properties of the realistic Cd-rich CdSe nanocrystals with covalently bound carboxylic acid (X-type) ligands. Configurations both with and without surface traps can be prepared depending on the amount and geometry of the adsorbed ligands. We find that Cd and Se dangling bonds do not necessarily create surface traps, whereas traps originating from ligands can form near the top of the valence band. Some of the ligands are found to be mobile on the surface and this mobility is accompanied by a spectral diffusion of the associated trap energy levels. This provides the first atomistic example of the processes required to explain the emission wavelength and lifetime variations, and blinking of the nanocrystals.