Molecularly Designed Cluster-Surface Interaction for Halogen-like and Alkali-like Metal-Encapsulating Silicon Cage Superatoms on n- and p-Type Organic Substrates

Metal-encapsulating Si16 cage clusters (M@Si16) are promising superatoms (SAs) for designing tunable properties for their assembled materials by changing the central metal atom: halogen-like, rare-gas-like, and alkali-like characteristics appear for the central metal atom of groups 3, 4, and 5, respectively. To fabricate SA assemblies, metal-encapsulating M@Si16 SAs (M = Lu, Hf, and Ta) must be controllably immobilized on a substrate. Substrates decorated with organic molecules can facilitate optimization of a cluster-surface interaction because the molecular local interactions between SAs and predeposited organic molecules govern the electronic properties through molecular complexation. In this study, M@Si16 SAs are size-selectively soft-landed on organic substrates deposited with n-type fullerene (C60) and p-type hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC, C66H66), and the electronic states of M@Si16 on the organic substrates are characterized by X-ray and ultraviolet photoelectron spectroscopy. On the C60 substrate, all M@Si16 are fixed to be cationic, forming M@Si16+C60- via a charge transfer interaction, while on an HB-HBC substrate, M@Si16-HB-HBC+ (M = Lu and Hf) is formed with anionic M@Si16-. Together with density functional theory calculations, the charge preference of the M@Si16 SA is examined based on its chemical stability against O2 gas exposure; Lu@Si16 on HB-HBC is more robust toward O2 than that on C60, while Ta@Si16 on HB-HBC is less robust than that on C60. Depending on the SA properties, an appropriate selection of organic molecules for deposition provides a molecular designer concept for forming SA-assembled nanomaterials through the cluster-surface interaction.