Self-pixelation through fracture in VO2 thin films

Vanadium dioxide (VO2) is an archetypal Mott material with a metal-insulator transition (MIT) near room temperature. In thin films, this transition is affected by substrate-induced strain but, as film thickness increases, the strain is gradually relaxed and the bulk properties are recovered. Epitaxial films of VO2 on (001)-oriented rutile titanium dioxide (TiO2) relax substrate strain by forming a network of fracture lines that crisscross the film along well-defined crystallographic directions. This work shows that the electronic properties associated with these lines result in a pattern that resembles a “street map” of fully strained metallic VO2 blocks separated by insulating VO2 stripes. Each block of VO2 is thus electronically self-insulated from its neighbors and its MIT can be locally induced optically with a laser, or electronically via the tip of a scanning probe microscope, so that the films behave functionally as self-patterned pixel arrays

Water Affinity and Surface Charging at the z‑Cut and y‑Cut LiNbO₃ Surfaces: An Ambient Pressure X‑ray Photoelectron Spectroscopy Study

Polarization dependence of water adsorption and desorption on LiNbO₃ surfaces was demonstrated using X-ray photoelectron spectroscopy (XPS) carried out in situ under near-ambient conditions. Positive and negative (0001) faces (z-cut) of the same crystal were compared for the same temperature and pressure conditions. Our results indicate a preferential adsorption on the positive face of the crystal with increasing water pressure and also higher desorption temperature of the adsorbed molecular water at the positive face. Adsorption measurements on the (1100) face (y-cut) showed also strong affinity to water, as observed for the z-cut positive surface. We found a direct relation between the capacity of the surface to discharge and/or to screen surface charges and the affinity for water of each face. XPS spectra indicate the presence of OH groups at the surface for all the conditions and surfaces measured