Ab initio study of small zinc sulfide crystallites

In this work the size evolution of properties of small in vacuo zinc sulfide crystals is studied using the ab initio Hartree-Fock molecular orbital method. The calculations on finite crystallite clusters, ranging from Zn3S3 to Zn240S240, are made possible by using the transitional symmetry of two-electron integrals. The calculations show that the electronic structure of ZnS crystallites is local and is converging rapidly to those in bulk. The electron distribution on surfaces is non-uniform and a large charge fluctuation exists. Surface relaxation decreases this fluctuation somewhat. Surfaces have a strong influence on the energy spectrum. The energy bands are wide and gaps narrow if compared with the bulk system. The cohesive energy shows a linear dependence on the surface-to-volume ratio. The largest cluster has 80% of the extrapolated bulk cohesive energy. The extrapolated results are comparable with infinite systems calculated by standard methods.