Microscopic studies of enhanced reactivity at structural faults in solids

Abstract

Conventional optical microscopy and, more particularly, a range of electron microscopic techniques have been adapted to identify the nature, and examine the chemical consequences, of emergent structural imperfections in single crystals of a number of anisotropic solids. Detailed studies have been carried out on graphite and MoS 2 , which are highly anisotropic both with respect to their reactivities and their dislocation behaviour; and which, in addition, react with oxygen to yield volatile products. Complications arising from the nucleation of new solid phases are therefore circumvented. Certain types of low-energy dislocations and stacking faults present in these solids are shown to be of little consequence as active centres in the oxidation process. High-energy dislocations are, however, extremely active. Indigeneous or created vacancies in the exterior surface also function as active centres. Their presence may be detected at concentrations greater than 1 in 10 10 of the exposed, basal-plane atoms, and their generation, particularly favoured by impingement of 1 D and 1 S oxygen atoms, may be monitored by an etch-decoration technique capable of detecting monatomic steps. In situ electron microscopic studies of photoinduced changes at, or immediately below, the surfaces of rather less anisotropic, organic molecular crystals, which do involve solid-state nucleation of product, again reveal the importance of certain types of dislocations in facilitating chemical transformations.