Profile Imaging of Surfaces

Abstract

Profile imaging is a method whereby the atomic resolution capabilities of the modern electron microscope can be harnessed to provide direct information about the topography of particular types of surfaces. However, several prerequisites must be satisfied before the surface features visible in the profile image can be considered as genuinely representative of the specific material. The microscope must be properly adjusted (i.e. focus, alignment and astigmatism), the surface in profile must be thin (typically 100Å or less), and it should be free of contaminants or surface reaction products. In the absence of contamination or other amorphous specimen regions, however, correct operating conditions are difficult to establish. Moreover, it is not obvious that any surface reactions or reconstructions that occur along the thin profile edge of the sample are related to bulk surface morphology. Finally, facilities forin situsurface treatment must be considered as essential for materials with reactive surfaces. In this paper, recent progress in surface profile imaging is briefly reviewed, keeping in mind these diverse requirements. The possibility for dynamic image viewing and recording without loss of performance levels should also be appreciated.OxidesClean fracture surfaces of many maximally valent oxides can be obtained by the simple expedient of crushing under a clean solvent with a mortar and pestle, and the ubiquitous holey carbon support film solves the problem of a means for microscope adjustment. Alternative techniques of specimen preparation such as ion-milling or chemical thinning may be required in order to obtain extended areas of a specific surface and orientation, but care is then required to avoid surface, or near-surface, modifications. Typical projected cation spacings in the major low-index zones are such that it is possible to resolve individual cation columns, and profile images such as the [001] SnO2 crystal shown in Fig. 1 clearly reveal the atomic (re-)arrangements at the surface. Several electron-beam-induced processes during extended observations of oxide surfaces have been documented. These include surface diffusion, as well as surface, and near-surface, reduction due to electron-stimulated desorption (ESD) of oxygen. In some maximally-valent transition metal oxides, the epitaxial development of a lower (metallic) oxide takes place, but a variety of end-products can in general be anticipated depending on such factors as the initial surface cleanliness, the residual microscope vacuum and the degree of covalency/ionicity of the original oxide.