Heterodimerization via the covalent bonding of Ta@Si₁₆ nanoclusters and C₆₀ molecules

Initial products prepared via the surface immobilization of Ta-atom-encapsulated Si₁₆ cage (Ta@Si₁₆) nanoclusters on solid surfaces terminated with monolayer films of C₆₀ molecules were investigated using scanning tunneling microscopy (STM). The STM results indicated that marked aggregation and desorption of surface-immobilized Ta@Si₁₆ nanoclusters were not induced, even after thermal annealing at -500 K, whereas the local vertical and lateral positions of the Ta@Si₁₆ nanoclusters with respect to neighboring adsorption sites in the C₆₀ film were modified. This local positional transition occurred on C₆₀ monolayer films weakly bonded (via van der Waals forces) to substrates such as highly oriented pyrolytic graphite (HOPG) but did not occur on C₆₀ monolayer films covalently bonded to substrates such as Si(111)7 × 7. These results indicated that the heterodimer consisting of a Ta@Si₁₆ nanocluster and a C₆₀ molecule, Ta@Si₁₆-C₆₀, was formed as an initial product via covalent bonding, which inhibited wide-range surface migration of the Ta@Si₁₆ nanoclusters but allowed them to locally change their positions via thermally activated precessional motion. In addition, the transition temperature of the local positional shift was found to decrease as the area density of the surface-immobilized Ta@Si₁₆ nanoclusters increased, indicating that the barrier height of the precessional motion of the Ta@Si₁₆-C₆₀ heterodimer was decreased due to accumulation of the elastic strain energy generated in the C₆₀ films.