Photoemission spectroscopy and microscopy for Ta@Si₁₆ superatoms and their assembled layers

Superatoms (SAs) with specific compositions have the potential to significantly advance the field of nanomaterials science, leading to next-generation nanoscale functionalities. In this study, we fabricated assembled layers with tantalum metal-atom encapsulating silicon cage (Ta@Si₁₆) SAs on an organic C₆₀ substrate through deposition, and we characterized their electronic and optical properties by photoelectron spectroscopy and microscopy. The alkaline nature of Ta@Si₁₆ SAs reveals their electronic behaviors, such as charge transfer and electromagnetic near-field sensing, through two-photon photoemission (2PPE) spectroscopy and microscopy with a femtosecond laser. The evolution of the work function for Ta@Si₁₆ SAs on C₆₀, observed by 2PPE spectroscopy, demonstrates charge transfer complexation between the topmost C₆₀ layer and the first Ta@Si₁₆ layer, consistent with the electron-donating alkaline characteristics of Ta@Si₁₆ SAs. Specifically, a small amount of Ta@Si₁₆ SA deposition leads to a dramatic increase in 2PPE intensity, attributable to electromagnetic near-field enhancements, suggesting applications as sensitizers for nonlinear imaging in photoemission microscopy. For the assembled Ta@Si₁₆ SA layers, a plasmonic response of hν_p = 17.9 eV is spectroscopically identified, including their valence and conduction band structures, and the plasmonic energetics are discussed in the context of metal doping in bulk silicon.