Ultra Low Dose Phase Contrast and Retrieval Imaging in Scanning Transmission Electron Microscopy via Electron Counting

Abstract

Scanning transmission electron microscopy (STEM) is an incredibly powerful tool for characterising materials to inform product development and solve mysteries in materials and biological sciences alike. In STEM, a focused electron beam is scanned across a region of interest on a sample and signals from the electron interactions with the sample material are collected to characterise it [1]. In recent decades, STEM has been used to probe both structural and functional properties of materials enabling important innovations in technologies for a more sustainable future. Traditionally, these insights have been gained by collecting the electrons that have been scattered to higher angles by the sample material, typically termed high-angle annular dark field (HAADF) STEM. However, the scattered intensity is proportional to Z^1.7, where Z is the atomic number, hampering the visibility of lighter elements. Additionally, the collection efficiency is poor making HAADF unsuitable for imaging beam sensitive materials where the electron dose impacting the sample must be reduced.

A candidate for high collection efficiency detection is four-dimensional scanning transmission electron microscopy (4D-STEM), where an image of the convergent beam electron diffraction (CBED) pattern is captured at every scan position [2]. This yields a four-dimensional dataset with a 2D image for every position in a 2D image, which contains an immense amount of information about the sample. However, most of the information about weakly scattering materials, like battery materials, metal organic framworks (MOFs), and biological specimens, is contained in the phase modulation imposed on the electron beam. Since physical detectors are unable to detect complex information, all phase information in the electron wavefunction is lost upon detection. There are several techniques for retrieving this phase information from the detected signal, the simplest of which utilises the fact that the center of mass (COM) of the CBED pattern is proportional to the gradient of the relative phase shift applied by the sample. Thus, the integrated center of mass (iCOM) retrieves the phase information and, due to the intensity being linear in Z and the high collection efficiency, enables characterisation of materials at significantly lower doses than traditional HAADF [3].

For ultra-low-dose imaging it is useful to scan the same area on the sample many times using a very fast scan speed so that the beam exposure is spaced out in shorter bursts to delay sample damage [4]. While 4D-STEM is very powerful, it significantly bottlenecks the practicable scan speed due to the high read-out overhead associated with reading out a full image at each scan position. To address this, this thesis focuses on the use of segmented detectors for estimation of the COM signal while scanning with extremely low dwell times below 500 ns. At these speeds, the finite detector response function to each electron detection event may cause artefacts in the recorded images. Therefore, a signal digitisation hardware is used to digitise the detector signal in real-time, such that the temporal position of each electron detection event -- and thus the integrity of the recorded image -- is maintained.

Finally, this thesis investigates the usefulness of segmented detectors and simple detector geometries for phase retrieval using iterative ptychography. The investigations presented paves the way for robust characterisation, with ultra-high resolution and dose efficiency, for beam sensitive materials.