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Anisotropic Electrostatic Friction of Organic Molecules on ZnO Surfaces

We study the long-time self-diffusion of a single conjugated organic para-sexiphenyl (p-6P) molecule physisorbed on the inorganic ZnO $\left(10\overline{1}0\right)$ surface by means of all-atom molecular dynamics computer simulations. We find strongly anisotropic diffusion processes in which the diffusive motion along the polar [0001] direction of the surface can be many orders of magnitudes slower at relevant experimental temperatures than in the perpendicular direction. The observation can be rationalized by the underlying charge pattern of the electrostatically heterogeneous surface which imposes direction-dependent energy barriers to the motion of the molecule. Furthermore, the diffusive behavior is found to be normal and Arrhenius-like, governed by thermally activated energy barrier crossings. The detailed analysis of the underlying potential energy landscape shows, however, that in general the activation barriers cannot be estimated from idealized zero-temperature trajectories but must include the conformational and positional excursion of the molecule along its pathway. Furthermore, the corresponding (Helmholtz) free energy barriers are significantly smaller than the pure energetic barriers with implications on absolute rate prediction at experimentally relevant temperatures. Our findings suggest that adequately engineered substrate charge patterns could be possibly harvested to select desired growth modes of hybrid interfaces for optoelectronic device engineering.