My research focuses on the development and application of advanced techniques in scanning transmission electron microscopy (STEM).
STEM is a powerful tool for investigating the structure and composition of materials at atomic resolution, and the combination of Z-contrast annular dark field (ADF) imaging, electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX) have propelled STEM to great popularity in materials science. These signals are available simultaneously and are relatively straightforward to interpret, providing quantitative information on what types of atoms are where. In addition the fine structure of EELS is determined by the local densities of states (DOS) providing insight into local electronic structure.
We aim to push the capabilities of STEM yet further to bring even greater benefits for understanding materials. A major avenue of our research involves 4D STEM and electron ptychography.
In 4D STEM a 2D image of the convergent beam electron diffraction (CBED) is recorded at each probe position with a pixelated detector in the 2D raster of a STEM scan, thus building up a 4D dataset. Compared to conventional STEM imaging modes this 4-cube of data provides far greater flexibility for producing signals. Signals that would conventionally not be possible simultaneously can all be obtained from a single 4D dataset. For instance, ADF images with different arbitrary and overlapping angular integration ranges can be obtained from a single scan.
However the advantages of 4D STEM go well beyond flexibility. Because of the abundance of information on the details of the probe position dependent scattering contained in the 4D data, more sophisticated methods can be used to yield greater information about the sample. For instance one can determine the center of mass of the scattering, which can be linked to local electromagnetic fields inside the sample. Electron ptychography makes use of the interference of diffracted CBED disks with the the undiffracted beam to solve the so called phase problem. We have recently developed the method into an extremely dose efficient imaging mode, and are working to combine this with the latest developments in high speed cameras for both greater clarity imaging of beam sensitive materials at low doses and even greater sensitivity imaging for materials that can withstand higher electron doses.
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