Revealing water films structure from force reconstruction in dynamic AFM

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 (I_h), showed characteristic oscillations in the attractive regime with a periodicity of 3.7 Å. This distance matches the thickness of a bilayer of I_h ice and is absent in force profiles on (111) CaF₂ surfaces, which show a different lattice parameter. A thickness of 2.6 Å 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 I_h growth of the first water layer on BaF₂ crystals is not observed, the matching of the lattice parameter between I_h and BaF₂ does induce a strong ordering of the water films and the formation of ice-like structures, even at room temperature.