Oxidative reactivity of alkali-like superatoms of group 5 metal-encapsulating Si16 cage nanoclusters

It is crucial to control the reactivity of surface silicon atoms for applications in miniaturized silicon-based nanodevices. Here we demonstrate that reactive silicon atoms are made unreactive by forming a Si-16 cage that encapsulates a metal atom. Specifically, group 5 metalencapsulating Si-16 nanoclusters (M@ Si-16: M=V, Nb, and Ta) exhibit alkali-like superatomic behavior on n-type C-60 substrates, where charge transfer between M@ Si-16 and C60 satisfies the 68-electron shell closure as M@ Si-16(+). The oxidation properties of M@ Si-16(+) are investigated by X-ray photoelectron spectroscopy, revealing that the chemical stability of the caged silicon surface towards oxygen is enhanced by a factor of 104 compared to a crystalline silicon surface, and that M@ Si-16 are oxidized stepwise from the outer Si-16 cage to the central metal atom. While the nanoclusters share a common Si-16 cage, their chemical robustness depends on a superatomic periodicity (Ta@ Si-16 > V@ Si-16 > Nb@ Si-16) which is explained by the electron density distributions of M@ Si-16 investigated by DFT calculations.