Prospects of annular differential phase contrast applied for optical sectioning in STEM.

The annular differential phase contrast (ADPC) mode in a third-order spherical aberration-corrected scanning transmission electron microscope (STEM) has recently been realized at an operating voltage of 300 kV by inserting a physical Fresnel phase plate in front of the objective lens and by using a detector geometry which matches that of the Fresnel phase plate [1]. By image calculation we explore the feasibility of this mode for the voltage range of 20-80kV. Alternatively, we mimic the Fresnel phase plate material-free with the help of the adjustable aberrations of the corrector. The additional correction of chromatic aberration, fifth-order spherical aberration and image spread improves significantly the resolution and contrast. Under these advanced conditions it is possible to achieve optical sectioning in the ADPC mode with atomic resolution and a depth of field shorter than 3 Å for an accelerating voltage of 30 kV. Moreover, we show that the contrast obtained in the ADPC mode is clearly superior over the contrast in incoherent bright-field (IBF) and high-angle annular dark-field (HAADF), the two other common methods in STEM. We propose that with the advanced ADPC method applied in STEM, the investigation of the inner structure of thick samples will be possible without slicing.