The p(4 × 2) surface reconstruction on epitaxial WO3 thin films

Surface reconstructions induced by reducing epitaxial WO3(1 0 0) thin films grown on LaAlO3(1 0 0) were studied using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). When the WO3(1 0 0) surface was annealed in either O2 or NO2 at 620 K, the surface was predominantly covered by a p(2 × 2) reconstruction with a low density of elongated (1 × 1) islands. Raising the annealing temperature to 800 K reduced the film and the surface was dominated by p(4 × 2) and p(2 × 4) reconstructions, although a few small patches of the p(2 × 2) surface could still be detected. Annealing the p(4 × 2) surface in ultra-high-vacuum (UHV) at 800 K resulted in a (1 × 1) LEED pattern. The STM images of the p(4 × 2) surface were dominated by 0.1 nm deep troughs separating bright rows. The appearance of the rows was sensitive to the imaging bias with a 2× periodicity along the rows seen at high biases and a 1× periodicity at low biases. These results could be explained by a structural model based on incomplete (1 × 1) terraces that form as reduced W5+ ions migrate from the surface into the bulk.