Sub-ångström resolution ptychography in a scanning electron microscope at 20 keV.

Achieving sub-ångström (<1 Å) resolution in electron microscopy typically requires a high-energy (>30 keV) beam and a transmission electron microscope (TEM) fitted with an aberration corrector and a high efficiency, high pixel-count camera. The cost, space- and personnel-requirements of such TEMs is prohibitive for many laboratories. Low-energy (≤30 keV) scanning electron microscopes (SEMs), in comparison, offer a simpler, more compact, and cost-effective electron microscopy platform. Lower beam energies also have the potential to provide greater information from thin, light-element samples. Here, we demonstrate a sub-ångström resolution of 0.67 Å using a 20 keV SEM operated in transmission mode, enabled by ptychographic reconstruction. This resolution corresponds to a resolution-to-wavelength ratio of 7.8, surpassing previous results from electron ptychography and conventional imaging approaches. The gain in resolution is achieved through a multi-slice ptychographic algorithm incorporating diffraction distortion correction, coupled with a SEM fitted with a cold field emission source, immersion lens, simple projector lens, and a hybrid direct detector optimized for low-energy electrons. This approach holds immediate promise for high-resolution imaging of 2D materials and, with further development, may enable structural studies of small (<100 kDa) proteins.