Molecular Beam Studies on Semiconductor Clusters: Polarizabilities and Chemical Bonding

This article discusses experiments that measure polarizabilities, permanent dipole moments, and absorption spectra of semiconductor clusters such as SiN, GaNAsM, and GeNTeM isolated in a molecular beam. The experiments show that the transition from the typical optoelectronic properties of macroscopic semiconductor crystals to the molecular properties of small polyhedral semiconductor molecules occurs in several steps. The bonding in nanometer‐sized semiconductor colloids with approximately 103–105 atoms is similar to that in the solid state. The bonding orbitals of the semiconductor crystals, which extend over a nanometer range, are, however, somewhat more strongly localized in these clusters and are therefore already slightly modified. Over the size range from 103 to 10 atoms, a gradual transition occurs from localized solid‐state‐like bonds to molecule‐like bonds. Interestingly, quantum size effects are found even in relatively small particles with about 102 atoms. These effects can be observed in the form of unusually small static polarizabilities of SiN clusters or in the absorption bands of GaNAsM clusters, which are strongly blue‐shifted relative to those of the solid state. For smaller clusters. These effects disappear over the size range 102−10 atoms, which is manifested by an increasing red‐shift of the absorption bands with decreasing cluster size. Small, semiconductor aggregates (clusters) which are free of ligands and typically consist of 10 to 103 atoms (and thus cannot yet be formed in a narrow size range by the techniques of colloid chemistry) can be generated and studied by molecular beam experiments. These experiments are described with reference to the measurement of polarizabilities of ligand‐free SiN and GaNAsM clusters, and simple physical chemistry models are discussed, which allow the interpretation of the polarizability in terms of the chemical bonds in the semiconductor clusters.