Clusters in solution: Growth and optical properties of layered semiconductors with hexagonal and honeycombed structures

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Abstract
Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed‐packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 Å thick, whose lateral dimensions vary from 12 to 90 Å depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six‐membered rings. The optical absorption spectra of PbI2 and BiI3 can be quantitatively understood in terms of the measured cluster size distributions.