We report on photoluminescence ͑PL͒ experiments in CdS nanocrystals fabricated by sequential ion implantation in Al 2 O 3 matrices. The PL spectrum and the spatial image of the PL intensity have been studied at 8 K using a scanning near-field optical microscope. The PL spectrum at each bright spot has been found to consist of narrow lines of various energies, although the spectrum measured by conventional optics shows a single and broad band locating below the free-exciton absorption energy. The origin of the sharp PL lines in CdS nanocrystals will be discussed.

We have fabricated cubic CdS films on GaAs substrates by means of a low-pressure metalorganic vapor deposition technique and studied their optical properties. The light-hole and heavy-hole exciton splitting due to the biaxial compressive strains in CdS films has been clearly observed in the photoreflectance spectrum. The temperature dependence of the free exciton energy has been determined for cubic CdS films on GaAs substrates.

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Many-body effects in strongly photoexcited single CdS nanocrystals embedded in Al2O3 matrices have been studied by a scanning near-field optical microscope at low temperatures. Under weak excitations, we find a sharp photoluminescence (PL) band originating from the exciton recombination. With an increase of the excitation laser energy, the PL intensity increases and the broad PL band is superimposed on the exciton-related sharp PL band. From a theoretical analysis, it was concluded that the broadening of the PL band is due to electron-hole pairs in CdS nanocrystals. The many-body effects of correlated exciton systems in CdS nanocrystals will be discussed.