Nanoscopic diffusion of water on a topological insulator

Abstract

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of intermolecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi 2 Te 3. Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi 2 Te 3. Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.

Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene

“…We note that those calculations were performed on free-standing graphene while our measurements are on Ni(111) supported graphene and in particular, the ripples giving rise to the ultra-fast droplet diffusion 26 are suppressed by the substrate 28 . We also note that our measurements on graphene indicate a higher diffusion and hopping rate than experimental values for other substrates 24,43 or in nano-confinement 47 and that the interfacial motion of water is many orders of magnitude faster than bulk diffusion in amorphous solid water 48 (see Supplementary Table 1 and Supplementary Discussion).…”

“…The single-particle dephasing rate (blue points) is periodic in ΔK rising from the origin and returning to α = 0 at about ΔK = 2.9 Å −1 in the Γ M direction. The periodicity indicates that motion takes place by a jump mechanism and the data are well described by a simple model for jump diffusion 27,43 ,…”

Alkali metal adsorption on metal surfaces: new insights from new tools

“…[18][19][20] The technique has been used to study adsorbate motion in a wide range of systems and materials, including alkali metals, organic molecules, and water, and for understanding inelastic scattering processes from surface phonons. 4,5,7,[20][21][22][23][24][25] A direct comparison between candidate energy landscapes from DFT and the experiment provides strong support for a diffusion mechanism involving hops on a Bravais lattice. It follows that degenerate sites within the unit cell, such as degenerate HCP and FCC sites, can be excluded.…”

Inter-adsorbate forces and coherent scattering in helium spin-echo experiments

“…However, the analysis would be identical for a simulation using Monte-Carlo methods where the trajectories for each adsorbate are generated by random hops on a specified lattice. 39 The resulting lineshape (eqn (6)) is simpler to analyse, since intra-cell motion is absent, but the results should be the same, as long as the Monte-Carlo algorithm generates the correct statistical occupancy of sites. 10 The form-factor in helium scattering is relatively little studied and it is a significant challenge, if eqn (8) is to be applied more widely in quasi-elastic scattering experiments with helium atoms.…”