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The physics of CdSe nanocrystals: structural and optical properties from ab initio

Authors: S. Botti

Ref.: Psi_k Confernce 2005, Schwäbisch Gmünd, Germany, 21/09/2005 (2005)

Abstract: In the past years, nanometer-sized CdSe clusters have attracted a large interest from the community. These nanocrystals can be prepared with a very narrow size distribution and have found important applications as chemical markers. In most experiments, the nanocrystals are formed by kinetically controlled precipitation, and are terminated with capping ligands (e.g. the TOPO molecule) which provide stabilization of the otherwise reactive dangling orbitals at the surface. Of course, when unsaturated bonds exist, they drive strong reconstructions which modify the electronic properties. However, the extent to which the cluster atomic structure resembles the corresponding bulk, as well as how the reconstruction of the surface affects the electronic properties are not well understood. We performed extensive density functional calculations of structural properties and absorption spectra of small CdSe nanocrystals, with a diameter ranging from 0.5 to 1.5 nm. We considered unsaturated clusters, obtained by structural relaxation of wurtzite bulk-like fragments, clusters whose surface was passivated with TOPO molecules, and fullerene-like cages. We found that the unsaturated systems exhibited very strong surface reconstructions. However, by passivating the Cd atoms at the surface with TOPO molecules, the clusters remained bulk-like, even for very small diameters. The optical spectra were obtained within time-dependent density functional theory. The optical absorption is strongly dependent on the size and the geometrical properties of the clusters. Furthermore, our results allowed us to interpret recent experiments of absorption in CdSe nanocrystals.