Improving 4DSTEM measurements of atomic charge and electrostatic potential via energy filtration

Abstract

In the past few years, data acquired using four dimensional scanning electron microscopy (4DSTEM) has been used to measure and map atomic potentials with unprecedented resolution and sensitivity to light elements in 2D, or quasi-2D materials [1-3]. These results either rely on integrated center of mass measurements (iCOM) [4] or on ptychographic reconstruction algorithms. iCOM is a conceptually simple technique in which the center of mass of the forward scattered beam is measured as the beam is rastered across the sample, and then converted into electric field, charge density or atomic potential, while the more complex and computationally intensive ptychographic algorithms optimize the atomic potential, among other parameters, to best fit a forward scattering model to the acquired data [5-7]. These other parameters can include finite thickness with the introduction of multislice algorithms, partial coherence, uncertainty with respect to electron probe position on the sample and aberrations in the probe itself, at the cost of increased computation times and more hyperparameters to modify. While ptychography can model many of these non-ideal experimental settings, the reconstruction algorithms cannot yet appropriately model inelastic electron scattering events, which is an increasing percentage of all electrons as the sample thickness increases and therefore a limit on the reconstruction quality. In order to effectively reconstruct atomic potentials in thicker samples then, these electrons must either be accounted for in the forward scattering model, or removed altogether during the experiment.