Superatomic Nanoclusters Comprising Silicon or Aluminum Cages

This chapter describes two superatoms, each comprising a central atom and a silicon or aluminum cage. Binary nanoclusters (NCs) at optimized mixing ratios are key components in designing the functionalities relevant to their electronic properties. To form chemically robust functional NCs, it is important to design the cooperatively synergistic effects between the electronic and geometric structures because these stabilize the individual NCs not only against charge transfer into the corresponding cations or anions but also against structural perturbations in their assemblies. Among binary NCs, synergistic effects are particularly expected when one central atom encapsulating cage structure completes a specific electron shell because electronic and geometric factors can operate simultaneously. Although the term “superatom” is widely used when the valence electrons in NCs complete an electron shell, more synergistic effects appear when the superatom adopts a closepacked structure, such as a highly symmetric cage as a binary cage superatom. Representative examples are given for one central atom encapsulated by silicon and aluminum cages, M@Si₁₆ and X@Al₁₂, their formation and characterization are described, and a large-scale synthetic approach is established for M@Si₁₆. The perspectives for binary cage superatom assembly are discussed in terms of theoretical calculations.