Two-dimensional wetting: the role of atomic steps on the nucleation of thin water films on BaF2(111) at ambient conditions.

The interaction of water with freshly cleaved BaF(2)(111) surfaces at ambient conditions (room temperature and under controlled humidity) has been studied using scanning force microscopy in different operation modes. The images strongly suggest a high surface diffusion of water molecules on the surface indicated by the accumulation of water at step edges forming two-dimensional bilayered structures. Steps running along the 110 crystallographic directions show a high degree of hydrophilicity, as evidenced by small step-film contact angles, while steps running along other directions exhibiting a higher degree of kinks surprisingly behave in a quite opposite way. Our results prove that morphological defects such as steps can be crucial in improving two-dimensional monolayer wetting and stabilization of multilayer grown on surfaces that show good lattice mismatch with hexagonal ice.

Influence of the macroscopic shape of the tip on the contrast in scanning polarization force microscopy images.

We demonstrate that a quantitative analysis of the contrast obtained in electrostatic force microscopy images that probe the dielectric response of the sample (scanning polarization force microscopy (SPFM)) requires numerical simulations that take into account both the macroscopic shape of the tip and the nanoscopic tip apex. To simulate the SPFM contrast, we have used the generalized image charge method (GICM), which is able to accurately deal with distances between a few nanometers and several microns, thus involving more than three orders of magnitude. Our numerical simulations show that the macroscopic shape of the tip accounts for most of the SPFM contrast. Moreover, we find a quasi-linear relation between the working tip-sample distance and the contrast for tip radii between 50 and 200 nm. Our calculations are compared with experimental measurements of the contrast between a thermally grown silicon oxide sample and a few-layer graphene film transferred onto it.

Thin water films grown at ambient conditions on BaF(2)(111) studied by scanning polarization force microscopy.

The interaction of water with freshly cleaved BaF(2)(111) surfaces has been studied using scanning force microscopy operated in different modes at room temperature and under controlled humidity. The Kelvin probe microscopy (KPM) mode has been used to study the evolution of the surface potential differences (SPDs). In the 20%-50% relative humidity (RH) range, adsorbed water forms two-dimensional solidlike bilayers (islands). The SPD between water islands and the bare substrate surface exhibits a sign crossover from negative ( approximately -30 mV) at low RHs to positive ( approximately +50 mV) at higher RHs, evidencing a cooperative and irreversible flipping of the preferential orientation of water dipoles, from pointing toward the surface evolving into the opposite direction. The KPM results suggest that the classical hexagonal (I(h)) bilayer configuration is not the most favorable structure.