Diffusion on a topological insulator surface: H2O on Bi2Te3(111)

Water is ubiquitous in everyday life, yet the simplest mechanisms controlling its molecular motion at a surface have to be revealed by experiment[1-3]. We have studied the diffusion of H2O on the Bi2Te3(111) surface using helium-3 spin-echo spectroscopy[4], a reciprocal-space technique that places tracer and collective diffusion on the same footing. While it is known that water does not react with Bi2Te3[5], information about the diffusion of adsorbates on topological insulator surfaces is scarce. Notably, surface diffusion measurements are capable of providing new bench-mark data for energy landscapes on topological insulator surfaces with their peculiar electronic surface effects.

The diffusion of H2O on in-situ cleaved single crystals of Bi2Te3(111) was studied by applying a water overpressure in the temperature range of 135 – 160 K. The molecular dynamics extracted from spin-echo measurements shows thermally activated diffusion with a jump mechanism and an activation energy of 40 meV. The dependence upon the momentum transfer is characteristic for jumps on a hexagonal lattice in accordance with the preferred adsorption site from density functional theory calculations. Furthermore, the measurements indicate that interactions between the individual water molecules play a significant role in the diffusion mechanism of water.

[1] A. Hodgson et al., Surf. Sci. Rep. 64, 381 (2009).
[2] S.-C. Heidorn et al. ACS Nano 9, 3572 (2015).
[3] J. Carrasco et al., Nat. Mater. Sci. 11, 667 (2012).
[4] A. Jardine et al., Prog. Surf. Sci. 84, 323 (2009).
[5] L. V. Yashina et al. ACS nano 7, 5181 (2013).