Suppression of dynamic disorder in fullerenes at metal-organic interfaces

Organic semiconductors are prone to strong disorder effects that often exhibit significant dynamic characteristics. In this study, static disorder and dynamic disorder of fullerene molecules at the interface of Au(111) are directly distinguished using Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy. We consider an STM image consisting of hundreds of different molecules, as an ensemble which exhibits conductance fluctuations due to both dynamic disorder and static disorder. On the other hand, local conductance measurement of single molecules over time reflects only dynamic disorder. We demonstrate that dynamic disorder is always smaller than static disorder at the C₆₀/Au(111) interface due to structural constraints of molecules at the interface. Dynamic disorder in our experiment is due to small librations of individual molecules that are restricted by the metal surface, while static disorder is related to different bonding orientations of the C₆₀ cage that are frozen-in at room temperature. Our experimental results can be modeled with direct simulations of differential tunneling conductance.