Zero-dimensional "excitons" in semiconductor clusters

Semiconductor crystallites of characteristic dimension\sim20-100Å can be made by precipitation in liquids and dielectrics. These crystallites have bulk-like internal lattices. The optical spectra show partially resolved discrete features that result from carrier spatial confinement in three dimensions. Spectral shifts of more than 1 eV are observed. The electronic wavefunctions of small crystallites are discussed within the effective mass approximation. The pattern of discrete hole states is qualitatively different in small crystallites than in slab superlattices. In small CdS crystallites, luminescence is observed from trapped carriers that may be in localized surface states. The recombination emission is strongly coupled to crystallite phonons.