Structure and reactivity of oxide surfaces: new perspectives from scanning tunnelling microscopy

Abstract

Scanning tunnelling microscopy (STM) has emerged in the past few years as a uniquely powerful tool for the investigation of oxide surfaces. STM is capable of real-space imaging of periodic structures with atomic resolution and of characterising local atomic arrangements associated with defect sites. The power of the technique is further enhanced by the ability to probe filled and empty density of states profiles at specific atomic sites by monitoring variations in tunnelling currents with applied voltage. The interpretation of STM images is not always unambiguous, a question of central importance being whether oxygen or metal ions appear as maxima in the images. The necessity for adequate sample conductivity imposes some constraints on the applicability of the technique, although developments in instrumentation and in techniques for sample preparation are helping to overcome these limitations. The range and limitations of the technique are illustrated by reference to work on tungsten oxides, titanium dioxide and iron oxides. The review concludes with a discussion of recent developments in the study of molecular adsorbates.