Observation of reduced (1 × 1) terraces on WO 3(001) surfaces using scanning tunnelling microscopy

Abstract

Ordered (1 × 1) terraces on a WO 3(001) surface have been resolved using scanning tunnelling microscopy. (1 × 1) surfaces were observed after sample treatments involving both simple vacuum annealing and argon ion bombardment/anneal cycles. The (1 × 1) surface was observed to coexist with (2 × 2) and (√2 × √2)R45° reconstructed areas of the surface. The (√2 × √2)R45° reconstruction was only amenable to imaging at positive sample biases (i.e. under tunnelling into empty sample states), as is commonly expected for n-type semiconducting transition metal oxides. In contrast, both the (1 × 1) and (2 × 2) reconstructions have been imaged at negative sample biases (tunnelling from filled sample states) as well as at positive bias. These observations suggest that the (1 × 1) reconstruction arises from termination of the bulk structure in a bare WO 2 plane in which all the W ions are reduced from W(VI) to W(V).