Abstract
Mass transport at surfaces determines the kinetics of processes such as heterogeneous catalysis and thin-film growth, with the diffusivity being controlled by excitation across a translational barrier. Here, we use neutron time-of-flight and spin-echo spec-troscopy to follow the nanoscopic motion of triphenylphosphine (P(C6H5)3 or PPh3) adsorbed on exfoliated graphite. Together with force-field molecular dynamics simulations, we show that the motion is similar to that of a molecular motor, i.e. PPh3 " rolls " over the surface with an almost negligible activation energy for rotations and motion of the phenyl groups and a comparably small activation energy for translation. The unique behaviour of PPh3 is due to its three-point binding with the surface: Together with van der Waals corrected density functional theory calculations, we illustrate that the adsorption energy of PPh3 increases considerably compared to molecules with flat adsorption geometry, yet the effective diffusion barrier for translational motion increases only slightly. We rationalise these results in terms of molecular symmetry, structure and contact angle, illustrating that the molecular degrees of freedom in larger molecules, i.e. rotations and conformational changes, are intimately connected with the diffusivity.