Polar ice freezing on ferroelectric LiNbO3 surfaces as measured by low T KPFM and PFM

Abstract

Under ambient conditions, all surfaces are covered by a thin film of water, ranging in thickness from a fraction of a monolayer to a macroscopic film. Even the thinnest water layer can affect the physical and chemical properties of the substrate. Ferroelectric materials show strong electric fields at the surface. These electric fields can be screened by different mechanisms, namely intrinsic (charge carriers or defects) or extrinsic (chemical environment or adsorbates), that play a crucial role in the stabilization of bulk polarization. In this sense, when considering a ferroelectric material, the electrostatic interactions between the surface and the water molecules electric dipole emerge as new critical aspect for both, the behavior of adsorbed water layers and the polarization dynamics. LiNbO3 (LN) is a ferroelectric oxide perovskite widely used in nonlinear optics and acoustic applications, mainly due to the bulk properties of the material. We applied Near-ambient Xray photoelectron spectroscopy (XPS) to study water adsorption and desorption on ferroelectric oxide crystals such as LiNbO3 at the line CIRCE in ALBA and demonstrated polarization dependent water adsorption and desorption processes and surface discharge dynamics, and probed the existence of a water monolayer even after high vacuum conditions, which stayed on polarized surfaces for temperatures above 250 ºC. But the strong interaction between water molecules electric dipole and ferroelectric surfaces also plays an important role on the formation of ice structures on polarized surfaces. When water layers on ferroelectric polarized surfaces go through a face transition into solid state, the ice layers formed on theses surfaces are also strongly affected by the polarization state. At cryogenic temperatures almost fully polarized ice layers can be formed, with polarization direction depending on the substrate. In this work, we have studied polarization structure of ice layers at 3.5 K on periodically poled LN (PPLN) surfaces by KPFM in a LowT Attocube AFM system, and probed the parallel alignment of the water molecules in agreement with the polarization direction of PPLN as probed by low T PFM characterization.