Formation of a superatom monolayer using gas-phase-synthesized Ta@Si₁₆ nanocluster ions

The controlled assembly of superatomic nanocluster ions synthesized in the gas phase is a key technology for constructing a novel series of functional nanomaterials. However, it is generally difficult to immobilize them onto a conductive surface while maintaining their original properties owing to undesirable modifications of their geometry and charge state. In this study, it has been shown that this difficulty can be overcome by controlling the donor-acceptor interaction between nanoclusters and surfaces. Cations of Ta-atom-encapsulated Si₁₆ cage nanoclusters (Ta@Si₁₆) behaving as rare-gas-like superatoms are synthesized in the gas phase and deposited on conductive surfaces terminated with acceptor-like C₆₀ and donor-like α-sexithiophene (6T) molecules. Scanning tunneling microscopy and spectroscopy have demonstrated that Ta@Si₁₆ cations can be densely immobilized onto C₆₀-terminated surfaces while retaining their cage shape and positive charge, which is realized by creating binary charge transfer complexes (Ta@Si₁₆⁺-C₆₀^-) on the surfaces. The Ta@Si₁₆ nanoclusters exhibit excellent thermal stability on C₆₀-teminated surfaces similar to those in the gas phase, whereas the nanoclusters destabilize at room temperature on 6T-terminated surfaces owing to the loss of electronic closure via a change in the charge state.