| Abstract | Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s* tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells (s , p, . . .), while the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the biexciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and biexciton spectra. The quasidegenerate nature of the hole spectrum results in a correlated biexciton state, which makes it slowly convergent with basis size. We carry out a systematic study of the exciton and biexciton emission spectra as a function of the nanocrystal diameter and find that the interplay of repulsion between constituent excitons and correlation effects results in a change of the sign of biexciton binding energy from negative to positive at a critical nanocrystal size. |
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