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

Metal-encapsulating germanium cage nanoclusters, M@Ge 16 (M = lutetium (Lu) and tantalum (Ta)), consist of a single metal atom enclosed within a Ge 16 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@Ge 16 and Ta@Ge 16 were deposited onto hexa- tert -butyl-hexa- peri -hexabenzocoronene (HB-HBC)- and fullerene (C 60 )-modified highly oriented pyrolytic graphite (HOPG) substrates, respectively. Their chemical stability against O 2 was evaluated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Lu@Ge 16 – on HB-HBC remains intact even after 30 L of O 2 exposure, demonstrating better oxidation resistance than the Si 16 analogue Lu@Si 16 . Ta@Ge 16 + on C 60 undergoes oxidation after exposure to 1.0 × 10 4 L of O 2, indicating cage degradation and partial metal oxidation, although its initial oxidation resistance is comparable to that of Ta@Si 16 + on C 60 . Together with the better oxidative resistivity of Y@Ge 16 relative to Y@Si 16, 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 Ge 16 cage. This study highlights the critical role of geometric packing in achieving chemically resilient and electronically tunable designer BCSAs.