The tight-binding approximation and the recursion method are used to study the size dependence of the band gap for small CdS and ZnS crystallites (20–2500 atoms). Because of the lack of accurate experimental data, a simple model of the crystal is considered; one which has no dangling bonds and a symmetrical shape. It is then possible to have a good evaluation of the band gap, even for the largest crystallites. The optical-absorption spectra exhibit an excitonic peak; we determine the peak position from a simple evaluation of the binding energy. The results are compared with the results of other calculations based upon the effective-mass approximation and some experimental data.

• Received 7 February 1989

DOI:https://doi.org/10.1103/PhysRevB.39.10935