From Si 16 to Ge 16 : Geometric Origins of Robustness in Binary Cage Superatoms on Organic Substrates

Metal-encapsulating germanium cage nanoclusters, M@Ge16 (M = lutetium (Lu) and tantalum (Ta)), consist of a single metal atom enclosed within a Ge16 cage and can be regarded as binary cage superatoms (BCSAs) exhibiting enhanced geometric and electronic stability due to dense atomic packing and closed-shell electron configurations. In this study, Lu@Ge16 and Ta@Ge16 were deposited onto hexa-tert-butyl-hexa-peri-hexabenzocoronene (HB-HBC)- and fullerene (C60)-modified highly oriented pyrolytic graphite (HOPG) substrates, respectively. Their chemical stability against O2 was evaluated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Lu@Ge16– on HB-HBC remains intact even after 30 L of O2 exposure, demonstrating better oxidation resistance than the Si16 analogue Lu@Si16. Ta@Ge16+ on C60 undergoes oxidation after exposure to 1.0 × 104 L of O2, indicating cage degradation and partial metal oxidation, although its initial oxidation resistance is comparable to that of Ta@Si16+ on C60. Together with the better oxidative resistivity of Y@Ge16 relative to Y@Si16, these results suggest that the enhanced robustness of Ge-based BCSAs arises from a better spatial match between the larger Y and Lu atoms and the Ge16 cage. This study highlights the critical role of geometric packing in achieving chemically resilient and electronically tunable designer BCSAs.