Superstructures and defect structures revealed by atomic-scale STM imaging ofWO3(001)

Abstract

Atomic-scale images of the (001) surface of monoclinic WO3 have been obtained by scanning tunneling microscopy. Termination of the surface W02 plane with half a monolayer of oxygen ions gives a surface periodicity Q2 times that of the underlying ReO&-like framework. An additional periodicity is introduced by tilting of WO6 octahedra to produce alternately long and short O-O separations along the [110]direction. This produces a (2 X 2) superstructure approximating the space group c2mm. Prominent defect lines at the surface are attributed to missing oxygen ions.Tungsten trioxide (WO3) is an important 5d transitionrnetal oxide that finds application in a number of technologically important devices including thin-film electrochromic displays and H2S gas sensors. The material undergoes at least five phase changes at temperatures between 100 and 1000 K. These are associated with tilting and distortion of a framework of corner-sharing WO6 octahedra to give structures with lower symmetry than the parent cubic structure of

Oxygen-induced p(3×1) reconstruction of the W(100) surface

“…In both cases, the average periodicity measured along the [010] direction is 3.0 ± 0.1Å, while the average periodicity measured along the orthogonal [001] direction is 9.0 ± 0.3Å. These values correspond well to the interatomic periodicity of the W(100) surface (3.165Å) and do not match the periodicities expected for either the bulk WO 2 or WO 3 (100) surfaces [12,17].…”

“…3. It is assumed that W atoms are imaged at the surface, though STM studies of the WO 3 (001) surface suggest that oxygen atoms may alternatively be resolved [12,13]. In both cases, the average periodicity measured along the [010] direction is 3.0 ± 0.1Å, while the average periodicity measured along the orthogonal [001] direction is 9.0 ± 0.3Å.…”

“…At elevated temperatures (T > 600 K), it undergoes a transition to an orthorhombic phase. While pure stoichiometric WO 3 has a bandgap of 2.6 eV, it is possible by reducing the crystal to perform STM and LEED on single crystal and thin film surfaces [11][12][13][14][15][16]. Tungsten dioxide WO 2 is another important oxide of tungsten, which displays room temperature metallic conductivity.…”

Mechanisms, Kinetics, and Dynamics of Oxidation and Reactions on Oxide Surfaces Investigated by Scanning Probe Microscopy

“…In tungsten oxide, the WO 6 octahedra can tilt and buckle in a variety of ways, leading to a series of temperature‐dependent phase transitions between different structures that deviate slightly from the ideal cubic structure 47–49. The {100} planes form the lowest energy surfaces 50, 51. Non‐polar stoichiometric {100} surfaces can be created by capping every other five‐fold coordinated W cation exposed on a WO 2 plane with a terminal oxygen, atom resulting in the c(2 × 2) structure pictured in Figure 50–52.…”

Local Structure and Properties of Oxide Surfaces: Scanning Probe Analyses of Ceramics

“…Some compounds in this class have been imaged and several reconstructions observed. [48][49][50][51][52] Atomic-resolution STM images have been used in analyses of phenomena other than reconstructions in oxides. For example, the effect of electron correlation has been observed directly in an image of a defect in (001) NiO, 28 magnetic ordering has been identified from corrugation amplitudes in iron oxides, 53 and temperature-dependent conductance in uranium oxide (UO 2 ) has been manipulated to produce images.…”