Structure, Stability and Electron Counting Rules in Transition Metal Encapsulated Silicon and Germanium Clusters

Abstract

Transition metal (TM) doped silicon and germanium clusters have been studied widely over the last two-three of decades. The initial motivation was to understand metal-semiconductor interfaces, and to couple magnetism in the TM atoms with the semiconducting properties of silicon. However, later on, the focus shifted to production of stable, TM encapsulated silicon or germanium cage clusters, possibly having magnetic moment. The most fundamental question the experiments threw up is, what are the most stable clusters in these series? And, how to understand their stability? Like other branches of physics and chemistry, simple electron counting rules, such as the 18-electron rule of inorganic and organometallic chemistry, and the idea of shell filling in a spherical electron gas, were invoked to explain stability of some of the clusters. However, different clusters were found to be most stable under different experimental conditions, making it somewhat unclear whose stability one has to explain. Faced with such challenges, a large number of experimental and theoretical studies were devoted to elucidating these issues. Quite early on, some authors (Sen and Mitas, e.g.) argued that electron counting rules may not explain all the observations in TM–Silicon clusters, though some of the observations can ostensibly be explained by such rules. The most recent works from Bandyopadhyaya and Sen, and Khanna and co-workers show that electron counting rules only have limited applicability to these clusters. A detail review of the literature discussing these issues is presented, and the still open questions are pointed out.