EXCITONS IN CDSE QUANTUM DOTS

Recent advances in material synthesis have provided samples with CdSe quantum dots with a degree of monodispersity high enough to allow for observation of excited exciton states and their size dependence [Norri s et at, Phys. Rev. Lett. 72, 2612 (1994)]. Here we report theoretical results for these exciton states using the effective bond-orbital mod el (EBOM) for the hole and single-band effective-mass theory (EMT) for the electron in an iterative Hartree scheme including the Coulomb int eraction and finite offsets. We present results for hole energies, exc iton energies, and exciton oscillator strengths and compare with exper iments and other theoretical results. Our results are found to account for most of the important features of the experimental absorption spe ctra by Norris er al. In particular, experimental states corresponding to the exciton ground state (1 Gamma(8)-1S(e)), as well as the 2 Gamm a(8)-1S(e) and 3 Gamma(8)-1S(e) excited states, have been identified. Also, a set of experimental exciton states observed lifted with an ene rgy close to the spin-orbit splitting lambda approximate to 420 meV ab ove the exciton ground state have been identified as Gamma(7)-1S(e) sp in-orbit split-off states with large oscillator strengths. A nonpertur bative study of the effects of the crystal-field splitting, which is i nherent in hexagonal CdSe quantum dots, revealed patterns of avoided c rossings, accompanied with redistribution of oscillator strengths, bet ween different exciton states for increasing values of the crystal-fie ld splittings. In CdSe where the crystal-held splitting is approximate to 25 meV, the splitting is not expected to have a significant effect on the present quantum dot absorption spectra.