Rotational transitions in a C₆₀ monolayer on the WO ₂/W(110) surface

Variable-temperature scanning tunneling microscopy (STM) is shown to be an effective technique to study two-dimensional phase transitions. Observations show that a monolayer of C₆₀ deposited on an ultrathin WO₂ layer grown on the W(110) surface undergoes a structural phase transition at 259 K, similar in temperature to that of bulk C₆₀. In turn, a kinetic transition has been observed at 220 K, which is significantly higher than that of the bulk C₆₀ crystal (90 K). This difference is attributed to interactions between the molecular overlayer and the substrate, as well as correlation effects within the C₆₀ film. Different types of molecular nanomotion, such as rotation, spinning, and switching between different orientations, have been observed. STM measurements are supported by density functional theory calculations, which provide confirmation of different orientations of C₆₀ on the WO₂ thin film.