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 Vs 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, |Vs|, increases linearly with film thickness at a rate that decreases with increasing deposition temperature. |Vs| decreases with increasing deposition temperature and increasing incidence angle of the vapor source. After film growth, |Vs| decreases irreversibly by 80% when the ice film is heated to ∼30 K above the deposition temperature. The measurements of |Vs| 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 |Vs| and the infrared absorption due to the most pliable, two-coordinated surface molecules with annealing temperature.