Chemical Characterization of an Alkali-Like Superatom Consisting of a Ta-Encapsulating Si₁₆ Cage

Chemical characterization was performed for an alkali-like superatom consisting of a Ta-encapsulating Si₁₆ cage, Ta@Si₁₆, deposited on a graphite substrate using X-ray photoelectron spectroscopy (XPS) to element-specifically clarify the local electronic structure of the cage atoms. The XPS spectra derived from Ta 4f and Si 2p core levels have been well modeled with a single chemical component, revealing the formation of a symmetric Si cage around the Ta atom in the deposited nanoclusters. On chemical treatments by heating or oxygen exposure, it is found that the deposited Ta@Si₁₆ is thermally stable up to 700 K and is also exceptionally less reactive toward oxygen compared to other Ta–Si nanoclusters, although some heat degradation and oxidation accompany the treatments. These results show the promising possibility of applying Ta@Si₁₆ as a building block to fabricate cluster-assembled materials consisting of naked nanoclusters.