Electronic properties of the WO2 (001) polar surface of tungsten trioxide

Tungsten trioxide WO3 shows promising perspectives in many fields of research. Surface states appear to play a fundamental role on the overall functionality of the material. Experimental evidences showed the capability to consistently obtain meta-stable polar surfaces, characterized by large amount of excess charge on the surface. In this study, we investigate the WO2 terminated polar (001) surface, by density functional theory calculations. Our data suggest a tri-dimensional nature of the excess electrons, delocalized across the entire slab, compared to the usually observed two-dimensional electron gas behavior, in the case of common transitional metal oxide. The material is expected to be able to host excess charge also in localized states (namely polarons). Furthermore, this work also suggests a deep coupling between the antiferroelectric distorsions, the oxygen vacancies at the topmost layer, and the dispersion of the excess charge. Our results, constitute overall solid bases to further investigate these connections and a valid starting point towards the opportunity to tune the electronic properties of the WO3 surface.