Effect of microstructure on spontaneous polarization in amorphous solid water films

Amorphous solid water (ASW) films grown by vapor deposition below 110 K develop negative surface voltages V-s with respect to the substrate. This polarization is due to a partial alignment of the water molecules during condensation. Kelvin probe measurements show that the magnitude of the surface potential, vertical bar V-s vertical bar, increases linearly with film thickness at a rate that decreases with increasing deposition temperature. vertical bar V-s vertical bar decreases with increasing deposition temperature and increasing incidence angle of the vapor source. After film growth, vertical bar V-s vertical bar decreases irreversibly by 80% when the ice film is heated to similar to 30 K above the deposition temperature. The measurements of vertical bar V-s vertical bar as a function of film porosity indicate that polarization in ASW is governed by incompletely coordinated water molecules, dangling with unbalanced dipoles at the internal surface of the pores and weakly aligned by the anisotropic film-vacuum interface. This idea is supported by the strikingly similar behavior of vertical bar V-s vertical bar and the infrared absorption due to the most pliable, two-coordinated surface molecules with annealing temperature. (C) 2015 AIP Publishing LLC.