Exciton states and optical properties of CdSe nanocrystals - art. no. 245318

The optical spectra of CdSe nanocrystals up to 55 Angstrom in diameter are analyzed in a wide range of energies from the fine structure of the low-ene rgy excitations to the so-called high-energy transitions. We apply a symmet ry-based method in two steps. First we take the tight-binding (TB) paramete rs from the bulk sp(3)s* TB model, extended to include the spin-orbit inter action. The full single-particle spectra are obtained from an exact diagona lization by using a group-theoretical treatment. The electron-hole interact ion is next introduced: Both the Coulomb (direct) and exchange terms are co nsidered. The high-energy excitonic transitions are studied by computing th e electric dipole transition probabilities between single-particle states, while the transition energies are obtained by taking into account the Coulo mb interaction. The fine structure of the lowest excitonic states is analyz ed by including the electron-hole exchange interaction and the wurtzite cry stal-field terms in the exciton Hamiltonian. The latter is diagonalized in the single electron-hole pair excitation subspace of progressively increasi ng size until convergence. The peaks in the theoretical transition spectra are then used to deduce the resonant and nonresonant Stokes shifts, which a re compared with their measured values in photoluminescence experiments. We find that the final results depend on the crystal-field term, the relative size of the surface, and the degree of saturation of the dangling bonds. T he results show satisfactory agreement with available experimental data.