A comparison of calculated images generated by six modes of transmission electron microscopy

Abstract

Six modes of transmission electron microscopy (TEM) are compared by a numerical simulation of the image formation. The comparison includes five modes of the conventional electron microscope (CEM) (axial bright field, Unwin's phase plate, central stop dark field, tilted beam dark field, conical illumination dark field) and the annular detector mode of the scanning transmission electron microscope (STEM). It is assumed that the illumination is perfectly coherent and that the interaction between electron beam and specimen may be described as an entirely elastic event. Furthermore, the influence of radiation damage and noise is neglected. The restrictions due to these approximations are discussed and shown to be unessential to the conclusions. Recent results from single-atom scattering theory are used to describe the specimen, a one-dimensional model of a thin carbon film carrying six osmium atoms. The calculated through-focus series demonstrates the nonlinearity of most modes of the CEM, which generates interference artifacts and introduces focusing difficulties and the risk of misinterpretation of the micrographs. STEM images, however, are shown not to be subject to these artifacts, which thereby explains the excellent quality of micrographs generated by this new instrument.