Extension of focal depth by electron quasi-Bessel beam in atomic-resolution scanning transmission electron microscopy

Extension of the focal depth of a sub-nanometer-sized electron probe in a scanning transmission electron microscope was demonstrated using an electron Bessel beam. We observed atomically-resolved annular dark-field scanning transmission electron microscope images of SrTiO 3 in the [001] direction with defocus using an electron quasi-Bessel beam generated by a ring-shaped aperture in an aberration-corrected probe-forming system. The experimental results show that the defocus range over which images with a 0.2 nm spatial resolution can be acquired using the electron quasi-Bessel beam is three times longer than the corresponding defocus range for a conventional beam.