Pushing the dose limit of atomic-resolution imaging: A 4D-STEM case study of NaCl.

Beam-induced damage fundamentally limits the characterization of beam-sensitive materials by scanning transmission electron microscopy (STEM), since structural information must be recorded before the electron beam irreversibly modifies the specimen. Here, sodium chloride (NaCl) is employed as a model beam-sensitive ionic crystal to investigate beam-induced structural evolution and, more importantly, the low-dose regime in which useful structural information can be recovered prior to significant damage. Four-dimensional STEM (4D-STEM) datasets were acquired at 200 kV using a Timepix3 direct electron detector and reconstructed using real-time integrated centre of mass (riCoM) imaging. We first establish the characteristic damage behaviour of NaCl by intentionally using high electron doses and by following its evolution under continued irradiation. Across different dwell times and raster scan orientations, damage develops reproducibly into square-faceted voids whose boundaries remain aligned with the 〈100〉 crystallographic directions of the rock-salt lattice, demonstrating that the resulting morphology is governed predominantly by the intrinsic crystallography rather than by the scan geometry. Having established this characteristic damage response, we subsequently investigate the low-dose imaging limit of NaCl. A multiframe dose-fractionated acquisition series is used to follow the beam-induced structural degradation using a dose of 130 e-Å-2. A normalized cross-correlation with respect to the first frame provides a quantitative measure of the progressive damage evolution. This furthermore enables evaluating the onset of measurable damage as a function of accumulated dose. Once this onset is found, we acquire another frame series with an electron dose of 32 e-Å-2. These results demonstrate how dose-efficient 4D-STEM acquisition, combined with the riCoM technique, can extend the accessible imaging regime of highly beam-sensitive ionic materials while maintaining atomic resolution.