Optical measurements on ZnS nanoclusters have been carried out to investigate surface effects along with quantum size effects. ZnS nanocrystals have been synthesized in the range of 1.5–2.5 nm, using different chemical methods as well as electronic passivating procedures. The size of nanoparticles has been estimated from empirical pseudopotential calculations. We have obtained a significantly narrower size distribution of ZnS nanocrystals than reported in earlier published results. We observed band gap luminescence in mercaptoethanol capped ZnS nanocrystals. Effects of various defect levels on the luminescent behavior of ZnS nanoparticles have been examined

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ZnS quantum dots having a narrow size distribution are chemically synthesized and characterized as regards their linear and non-linear optical properties. Quantum-size effects are revealed by the size-dependent blue-shift observed in the optical absorption spectra. The third-order optical non-linearity of a colloidal solution of ZnS quantum dots is studied by a third-harmonicgeneration technique at the fundamental wavelength of 1.06 µm. The third-harmonic signal is size dependent and increases with particle size over the range 7 to 21 Å. The third-harmonic signal of ZnS dots is also dependent on the choice of embedding medium (for example, DMF and acetonitrile). The results are qualitatively explained on the basis of the local field effect.

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the empirical pseudo-potential to calculate the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a function of shape and size of the quantum dots. Our studies explain the building up of the bulk band structure when the size of the dot is much larger than the bulk Bohr exciton radius. We present our investigations of HOMO-LUMO gap variation with size, for CdSe, ZnSe and GaAs quantum dots. The calculated excitonic energies are sensitive to the shape and size of quantum dots and are in good agreement with experimental HOMO-LUMO gaps for CdSe quantum dots. The agreement improves as experimentally observed lattice contraction is incorporated in pseudo-potential calculations for ZnSe quantum dots. Electronic structure evolution, as the size of quantum dot increases, is presented for CdSe, ZnSe and GaAs quantum dots.