Reconstruction of atomic resolution images from sparse datasets of radiation sensitive materials in scanning transmission electron microscopy

Scanning transmission electron microscopy is an imaging method that raster scans a focused high-energy electron beam across a thin sample and measures the electron intensity behind the specimen with either a small detector placed on axis (bright field) or a larger, ring-like detector placed off-axis (annular dark field). Both methods can yield atomically resolved images of crystals in projection, however, many materials such as organic crystals, minerals, hydrides or polymers are so beam sensitive that it is difficult to image them without destroying them. Even if they are not completely destroyed, they may undergo subtle changes in microstructure and / or chemical composition that may falsify apparent results.

In order to reduce the electron dose per area one can either

1. scan very fast and record a very noisy image of a hopefully unchanged material,
2. undersample the image by reducing the number of pixels used for the same region of interest and thus get a rather blurred image or
3. acquire a sparse dataset where the electron beam has skipped certain (random) positions. Some researchers claim the total electron dose required to reconstruct an image could thus be reduced by factors of 2, 10 or even 100.

Modern digital scan units can implement different scan procedures without flyback or drift issues. The question to be investigated is which scan strategy is best for a given imaging modality and material so that hopefully sub-nm images of beam-sensitive materials can be recorded in the future without them changing during observation.

This project will use extensive mathematical processing of large sparse datasets, both simulated and experimental, to reconstruct image details and study which scan strategies are best for different imaging modalities and different materials. The objective is ultimately to find out what the limits for atomic resolution in reconstructed images of beam sensitive are.