AVS 50th International Symposium
    Surface Science Friday Sessions
       Session SS2-FrM

Paper SS2-FrM5
Surface Phase Transitions Upon Reduction of WO@sub 3@(100) Thin Films

Friday, November 7, 2003, 9:40 am, Room 328

Session: Oxide Surfaces and Interfaces
Presenter: M. Li, Yale University
Authors: M. Li, Yale University
E.I. Altman, Yale University
A. Posadas, Yale University
C.H. Ahn, Yale University
Correspondent: Click to Email
The evolution of surface morphology and reconstructions induced by reducing epitaxial WO@sub 3@(100) thin films grown on LaAlO@sub 3@(100) was studied using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). When the WO@sub 3@(100) surface was annealed in either O@sub 2@ or NO@sub 2@ at 620 K, the surface was predominantly covered by a p(2x2) reconstruction with a low density of elongated (1x1) islands. Raising the annealing temperature to 800 K led to a net reducing environment even in the presence of O@sub 2@ or NO@sub 2@. Under these conditions, the surface was dominated by a two domain p(4x2)/p(2x4) reconstruction, although a few small patches of the p(2x2) surface could still be detected. Further annealing at 800 K in oxygen led to the appearance of p(3x2) domains on the p(4x2) dominated surface and a more diffuse p(4x2) LEED pattern. The surface was exclusively populated by a p(3x2) reconstruction upon annealing in UHV above 800 K while a p(3x2) LEED pattern could also be observed. After further annealing in UHV above 800 K, (1x1) islands appeared on the surface and the p(3x2) LEED pattern became more diffuse. The (1x1) area increased with annealing time in UHV and eventually exclusively (1x1) terminated terraces with straight steps due to crystallographic shear planes were observed. The STM images of the p(4x2) and p(3x2) surfaces were dominated by 0.1 nm deep troughs separating bright rows with 4x and 3x spacings respectively between rows. The appearance of the rows was sensitive to the imaging bias with a 2x periodicity along the rows seen at high biases and a 1x periodicity at low biases. These results could be explained by a structural model based on incomplete (1x1) terraces that form as reduced W@super 3@ ions migrate from the surface into the bulk.