Electronic Structure of O-D Exciton Ground State in CdSe Nanocrystals
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ABSTRACT We present results on photoluminescence excitation spectra (PLE) of wurtzite CdSe nanocrystals (NCs) embedded in a glass with effective radii in the range 15-35 A. Information on the near band-gap absorption of an assembly of NCs is obtained by selecting a narrow energy range in the inhomogeneously broadened photoluminescence band. The size and shape dependence of the lowest exciton states are calculated for slightly non-spherical wurtzite NCs. The experimental results are in good agreement with the theoretical predictions when both shape and size dispersions are taken into account. INTRODUCTION The determination of semiconductor NC electronic structure has been the center of interest of a considerable number of theoretical works in the last ten years. In the strong confinement regime, the NCs electronic states are calculated separately for electrons and holes within the framework of the effective mass approximation1, 2 . Confined electron states are well described by one conduction band while the complexity of the valence states has to be considered for confined hole states. The NC eigenstates have an excitonic nature and are obtained by taking into account the electron-hole Coulomb interaction. The first effect of this interaction consists in a red shift of the eight-fold degenerate lowest electron-hole pair state. A second effect is the lifting of the degeneracy of the lowest exciton state due to the electron-hole short-range exchange interaction. The exciton lowest state is split into two states: the optically forbidden state and the optically active state, at higher energy. This exchange splitting is extremely small in bulk semiconductors, but is strongly enhanced for NCs in the strong confinement regime 3 .4 and should be treated in perturbation theory on the same level as the crystal field and the effect of shape anisotropy in nonspherical wurtzite NCs5 . The combination of all these effects gives a complicated fine structure of exciton states that has to be compared carefully to experimental spectral studies. Nowadays high quality CdSe semiconductor NCs with dimensions smaller than the bulk exciton Bohr radius are obtained by using different fabrication methods 67 . However, an inherent problem remains: the inhomogeneous broadening of the optical transitions which makes difficult the size dependence study. To reveal single size optical properties, spectroscopic techniques as hole burning, excitation of photoluminescence (PL) with a narrow spectral width in the lowenergy side of the first absorption band and PLE experiments with detection of PL in the blue side of the PL band have been used. Studies of the absorption spectra by hole burning 8 and PLE experiments 9 ,1° show that the multiple valence band model predicts with a good approximation the energy of the excited electronic levels. On the other hand, the nature of the long-lived and red shifted luminescent state observed in high quality CdSe NCs has remained for a long time a controversial question 11. This state was finally identified as the
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