Size Quantization in Layered Semiconductor Colloids with Tetrahedral Bonding: HgI2

Small-particle-size colloids of red HgI₂ show relatively sharp blue-shifted optical absorption peaks in the UV that are attributed to size quantization effects. Within the framework of a simple model that only considers transitions between the lowest quantum states of electrons and holes in HgI₂, the optical data are consistent with crystallite structures containing up to four HgI₂ layers with dimensions in the layer plane ranging from 10 to 26 Å. The lowest two energy peaks could not be accommodated by particles containing only a single layer of HgI₂. Published data on layered PbI₂ colloids are also believed to be incompatible with a single PbI₂ layer if more acceptable values of the effective masses for electrons and holes in PbI₂ are used in the analysis. Preliminary calculations indicate that the optical data are also consistent with a single particle size showing multiple transitions between higher energy quantum levels of electrons and holes. The possible existence of magic number in HgI₂ colloids is inconclusive based on the optical data and electron microscopy.