Revealing Water Films Structure from Force Reconstruction in Dynamic AFM

Abstract

The structure of water films in contact with surfaces has direct implications in many important interfacial processes, from biology to climatology, as well as in ice nucleation. Here we report on the detection of individual ice-like water layers adsorbed on surfaces in ambient conditions. Reconstructed force profiles obtained in amplitude modulation atomic force microscopy (AM-AFM) on top of (111) BaF2 surfaces, with a lattice constant close to the distance of facing water molecules in hexagonal ice (Ih), showed characteristic oscillations in the attractive regime with a periodicity of 3.7 A. This distance matches the thickness of a bilayer of Ih ice and is absent in force profiles on (111) CaF2 surfaces, which show a different lattice parameter. A thickness of 2.6 A is measured for the first water layer in contact with the surface, corresponding to a high-density liquid film structure predicted from calculations in the literature. Our results indicate that, although epitaxial Ih growth of the first water...