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 Si₁₆ cage that encapsulates a metal atom. Specifically, group 5 metal-encapsulating Si₁₆ nanoclusters (M@Si₁₆: M = V, Nb, and Ta) exhibit alkali-like superatomic behavior on n-type C₆₀ substrates, where charge transfer between M@Si₁₆ and C₆₀ satisfies the 68-electron shell closure as M@Si₁₆ ⁺. The oxidation properties of M@Si₁₆ ⁺ 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@Si₁₆ are oxidized stepwise from the outer Si₁₆ cage to the central metal atom. While the nanoclusters share a common Si₁₆ cage, their chemical robustness depends on a superatomic “periodicity” (Ta@Si₁₆ > V@Si₁₆ > Nb@Si₁₆) which is explained by the electron density distributions of M@Si₁₆ investigated by DFT calculations.