Surface phase transitions and related surface defect structures upon reduction of epitaxial $WO3(100)$ thin films: A scanning tunneling microscopy study Available to Purchase

The evolution of surface morphology and reconstructions induced by reducing epitaxial $WO3(100)$ thin films on $LaAlO3(100)$ were studied using scanning tunneling microscopy (STM). As the films were reduced the surface transformed from $p(2×2)$ to $p(5×2)$ to $p(4×2)$ to $p(3×2)$ and finally to $(1×1);$ at intermediate stages mixtures of the phases were observed. As the surface structure changed the films became progressively darker indicating a connection between surface structure and bulk reduction. The formation of the $p(5×2)$ and $p(4×2)$ structures from the $p(2×2)$ structure could be explained by the ordering of vacancies created by $W5+$ migration into the bulk into troughs. The ordered troughs gave the surface a stranded appearance in STM images. As the surface was reduced the troughs narrowed to create the $p(3×2)$ surface; reduction of this surface gave way to a $(1×1)$ surface with all the W ions reduced to $5+.$ Half-height steps due to crystallographic shear planes intersecting the surface characterized the $(1×1)$ surface. Several defect structures were characteristic of the $p(n×2)$ surfaces including domain boundaries oriented parallel to the strands and along [011], in-plane line defects across strands, wide strands, and crosslinks above strands. Structural models for these defects are proposed based on STM results. The [011] domain boundaries are attributed to the formation of stress domains on a reconstructed surface with uniaxial stress induced by the strands and troughs.