The long-term goal of the electron microscopy research group is to develop and extend the understanding and techniques of charged particle microscopy, to allow the most complete and accessible set of atomic scale information to be gained on the widest possible variety of materials. This encompasses improving fundamental understanding of charged particle beam – sample interactions; creating improved charged particle optics arrangements for microscopes; enhancing and making the most use of charged particle detector technologies; and developing means and methods of processing the vast amounts of data now produced in charged particle microscopes. These approaches aim to simplify and ease the gaining of atomic resolution insights from material observations and help identify effects not readily visible or expected in the inspected material. Applications span the full range of natural and synthetic condensed matter samples, though currently are particularly relevant to materials development for new energy generation and storage materials, novel materials for medical therapeutics and diagnostics, and advanced computing elements, for example.

Diffractive Imaging Reconstruction Techniques

One of the greatest optical imaging breakthroughs in recent years has been the development of robustly convergent iterative algorithms to solve the so-called ‘phase-problem’ [1, 2], whereby the quantitative phase shift imparted upon a wave passing through a sample can be determined from a series of diffraction-type patterns, obtained from illuminating selected regions of the sample. This technique, called ptychography[3], though now relatively mature in the x-ray synchrotron and light optical communities, is still emerging in electron microscopy (EM), with first practical demonstrations of robust iterative ptychography in 2011[4].

Practical use in EM following this has been hindered by the availability of detectors that are fast enough, pixelated, radiation hard (being able to sustain intense high-energy electron bombardment) and have a high-enough quantum efficiency. This has recently been addressed, with so-called hybrid direct-electron detectors being developed, which we now have at UVic (see facilities). Combined with ptychography, these detectors enable new electron microscope spatial resolution levels[5], as has also recently been demonstrated at UVic.

Furthermore, the general technique of collecting diffraction data from many illumination positions, referred to as either scanning electron diffraction (SED)[6] or 4D-Scanning Transmission Electron Microscopy (4D-STEM)[7], offers many new capabilities for characterising materials. The practical application and development of these techniques and algorithms is being researched within the group.

Phased and Shaped Electron Beams

Research to better ‘shape’ the electron beam, particularly to overcome lens aberrations, has been active since the beginnings of electron microscopy [8]. Modern electron microscope aberration correctors are the culmination of extensive work, relying upon high tolerance multipole magnetic and/or electrostatic lenses and electronics, coupled with computational controls [9]. Alternatives to using these multipole lenses include mirror correctors and placing partially transmissive objects in the beam path[12-14]. The latter approaches, where the beam no longer traverses purely through vacuum before reaching the sample, or before reaching the detectors has seen particular appeal in recent years, owing to the relative ease with which nm level accuracy can now be achieved on microfabricated components.

This is exemplified in experiments that placed holographic gratings into electron microscopes to produce vortex beams [16], where the resulting orbital angular momentum offers a new means to explore magnetic properties, for example [17, 18]. Since this work, a wide variety of beam profiles have been produced, see e.g. [15, 19]. It was also found that shaped or ‘phase sculpted’ beams, produced by phase-plates or gratings placed in the beam, offer advantages for ptychographic methods and 4D-STEM by either improved robustness in the reconstruction [20] or promoting stronger or more linear contrast [21]. Phase plates are also used in conventional TEM to improve phase contrast[22]. Indeed, recent developments of hole-free Zernike type plates [23, 24] have dramatically increased their use, and they are now becoming more commonplace in cryo-EM biological molecule structure determination[25], through offering superior phase contrast transfer characteristics.

Such elements are being microfabricated at UVic to shape the electron beam, which combined with our dual aberration corrected TEM equipped with multiple bi-prisms, allows the creation of unique electron beam profiles. These are being researched for enhanced information transfer from ‘phase-objects’ such as biological samples, and for investigation of magnetic phases and plasmonic behaviour of advanced materials at the ultimate resolution.

Sample Modification in the Electron Microscope

Electron beam induced changes in the sample, either temporary or permanent, often become the ultimate limit of the usefulness of the information provided by the electron microscope[26]. Changes induced include atomic displacement, e-beam sputtering, structural damage, mass loss, specimen heating, and material deposition, and electrostatic charging. These processes typically occur on timescales < 1 sec, and in proteins, for example, changes significant enough to result in incorrect structure determination can occur with electron doses above 10 e2, at typical beam energies[27]. Techniques to investigate and mitigate this generally involve some degree of electron dose fractionation, so that in effect time resolved imaging is employed, which can for example be drift corrected[28]. Alternatively or in combination with this it appears that carefully controlling the timing of electrons arriving on the sample can be used to mitigate damage mechanisms [29]. Using our pixelated direct electron detector, with its unique high-speed triggering and exposure capabilities, these effects are being investigated.

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