Abstract
The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water for instance in catalysis, biophysics and nanotribology is extremely challenging due to its ultrafast dynamics, and the complex interplay of intermolecular and molecule-surface interactions. Here we present the first experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI, Bi2Te3). In addition to the technological relevance and scientific interest on the interfacial behaviour of water, understanding the interaction of TI surfaces with molecules is a key to design and manufacturing for future applications. However the surface chemistry of these materials has hitherto been hardly addressed and exploratory work on the motion of molecules on TI surfaces has been so far solely based on computational studies. By analysing the scattering lineshape from helium spinecho spectroscopy and comparing the results with van der Waals-corrected density functional theory calculations we are able to obtain a general insight into the diffusion and mobility of water on a topological insulator surface. Instead of the expected Brownian motion, we find strong evidence of a complex diffusion mechanism which follows an activated hopping motion on a corrugated potential energy surface and shows signatures of correlated motion with unusual repulsive interactions between the individual water molecules. From the experimental lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.