Surface structure and spectroscopy of WO2(012)

The strong metal-metal bonds inherent to the crystal structure of WO2 have a pronounced effect on the electronic characteristics of this transition-metal oxide. The results of the bonding are clearly identifiable in the photoemission spectra of polycrystalline WO2 samples. For the first time, single crystal surface studies have also been carried out. Single crystals of WO2 with well-developed (012) faces were grown by a vapour transport method, and surface cleaning procedures involving ion bombardment and low-pressure oxygen annealing were developed. Photoemission spectroscopy and low energy electron diffraction were used to characterise the surface composition and long range order. LEED patterns reveal the expected c2mm symmetry, based on a model (012) surface unit cell of dimensions 5.6 Å × 21 Å. Details of the surface structure were examined using high-resolution STM. The periodicity in filled-state STM images is determined by the separation between the centres of the metal-metal bonds rather than individual atoms. Rows of greyscale maxima are observed corresponding to the chains of WO6 octahedra which run in the [100] direction. The terminating (012) ionic plane most probably has stoichiometry W2O4. STM images demonstrate that the chains of paired WO6 octahedra in this outer layer undergo pronounced lateral relaxations, bringing them into positions between the rows of chains in the underlying W2 ionic plane. Atomic-scale steps and other defects have been identified. © 1997 Elsevier Science B.V. All rights reserved