Extended Depth of Field for High Resolution Scanning Transmission Electron Microscopy

TL;DR

This paper demonstrates the application of optical microscopy-based extended depth-of-field techniques to high-resolution STEM imaging, enabling all-in-focus images and topography maps of nanoparticles despite the limited depth of field.

Contribution

It introduces the novel use of established optical microscopy algorithms to extend the depth of field in aberration-corrected STEM images.

Findings

Achieved extended depth of focus from ~6 nm to in-focus images of nanoparticles.

Automatically generated all-in-focus images and topography maps.

Validated techniques on Pt-Co nanoparticles on carbon support.

Abstract

Aberration-corrected scanning transmission electron microscopes (STEM) provide sub-angstrom lateral resolution; however, the large convergence angle greatly reduces the depth of field. For microscopes with a small depth of field, information outside of the focal plane quickly becomes blurred and less defined. It may not be possible to image some samples entirely in focus. Extended depth-of-field techniques, however, allow a single image, with all areas in-focus, to be extracted from a series of images focused at a range of depths. In recent years, a variety of algorithmic approaches have been employed for bright field optical microscopy. Here, we demonstrate that some established optical microscopy methods can also be applied to extend the ~6 nm depth of focus of a 100 kV 5th-order aberration-corrected STEM (alpha_max = 33 mrad) to image Pt-Co nanoparticles on a thick vulcanized carbon support. These techniques allow us to automatically obtain a single image with all the particles in focus as well as a complimentary topography map.