Abstract
Aberration-correction reduces the depth of field in scanning transmission electron microscopy and allows three-dimensional imaging by depth-sectioning. This imaging mode offers the potential for sub-Ångstrom lateral resolution and nanometer-scale depth sensitivity. For biological samples, which may be many microns across and where high lateral resolution may not always be needed, optimizing the depth resolution even at the expense of lateral resolution may be desired, aiming to image through thick specimens. Although there has been extensive work examining and optimizing the probe formation in two-dimensions, there is less known about the probe shape along the optical axis. Here we examine the probe shape in three-dimensions in an attempt to better understand the depth-resolution in this mode. We present examples of how aberrations change the probe shape in three-dimensions and demonstrate that off-axial aberrations may need to be considered for focal series of large areas. We show that oversized or annular apertures theoretically improve the vertical resolution for 3D imaging of nanoparticles. When imaging nanoparticles of several nanometer size, the regular STEM can thereby be optimized such that the vertical full width at half maximum approaches that of the aberration corrected STEM with a standard aperture.
