Incoherence in atomic-resolution Z-contrast imaging

Abstract

Incoherent imaging of atomic clusters, monolayer rafts, or low index crystal projections at atomic resolution in a scanning transmission electron microscope (STEM) offers a number of distinct advantages over coherent imaging methods. These include an improved “interpretable” resolution and no experimental contrast reversals with specimen thickness or objective lens defocus. However, considerable controversy and confusion has arisen concerning the conditions under which the image of a given object can be interpreted using incoherent imaging theory. Here, we attempt to clarify the situation.Firstly, we consider the case of very thin specimens, such as heavy atoms, clusters of atoms, or extremely thin crystals. Experimentally, these specimens demand a relatively low annular detector inner radius to achieve an acceptable signal-to-noise ratio in the image. It is usual, therefore, to match the detector inner radius to the radius of the directly transmitted disc. In this conventional geometry, the annular dark-field signal will be dominated by coherent scattering, and the use of incoherent imaging theory may give rise to appreciable error close to the resolution of the STEM instrument.