The observed intensity under phase-contrast imaging conditions is a complicated function of the microscope and the thin solid. The important parameters of the microscope are the beam divergence, the energy spread of the fast electrons, and the objective lens excitation. From these parameters, a transfer function is defined that operates on the scattered wavefunction. The important parameters of the thin solid are the thickness, the atomic coordinates, and the scattering strength of the atoms. These parameters define the phase and amplitude of the Bloch waves excited in the crystal. As a result of these complexities, image interpretation is quite often problematic. Moreover, for those models that do accurately predict the experimental intensity, uniqueness is still in question. Since the indicated parameters limit the resolution capability of the microscope and this limit can not be overcome, the task at hand is to exploit the consequences of these parameters.There are two imaging conditions that produce interpretable data for periodic crystals: those where the intensity maxima correspond to atomic columns (white-atom images), and those where the intensity maxima correspond to atomic tunnels (black-atom images.)
Efficient phase contrast imaging in STEM using a pixelated detector. Part 1: Experimental demonstration at atomic resolution