Structural, Electronic, and Optical Properties of CdSe Nanocrystals: First-Principles Computations

We report on a series of complete ab initio calculations of CdSe nanocrystals with diameters up to 1.5~nm. For each nanocluster, the relaxed atomic structure, electronic states, and optical properties are calculated using the most thorough treatment to date. We predict a substantial structural relaxation of each cluster, especially at the surface, while still maintaing a wurtzite core. In contrast to Type IV and III/V nanoparticles, the predicted CdSe atomic structures are found to be relatively insensitive to the presence of surface passivation. We find that CdSe nanocrystals with ideal wurtzite structures are quasi-metallic and that surface relaxations, not passivation with organic ligands, are responsible for opening up a gap in the clusters. We predict the existence of a single midgap state, which is potentially responsible for sub-bandgap emission recently observed in CdSe nanoclusters. This work was performed under the auspices of the U.S. Department of Energy at the University of California/Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.