In the past decade or so, there has been something of a revolution in electron microscopy, a technique central to much of materials science, and parts of solid state chemistry and condensed matter physics. That revolution has been based around developments both in hardware, improving electron optics, monochromation, camera sensitivity and spectrometer efficiency, and in software, with code now able to process vast data sets in a robust and speedy fashion to extract the key information. This proposal, for a 'multi-dimensional electron microscope', or MDEM, brings together many of these development into a single instrument that is dedicated to analysing materials at the atomic- and nano-scales in two and three dimensions. The flexibility and power of modern microscopy resides also in the ability to use multiple detectors, cameras and spectrometers simultaneously so that multiple signals can be acquired from a single electron beam position - this is known as 'multi-modal' microscopy and when combined with the MDEM approach leads to a remarkable detailed investigation of structure and composition, crystallography and physico-chemical behaviour.
The MDEM is based around a scanning electron microscope that can operate from low voltages (e.g. 60kV) to high voltages (e.g. 300kV), the former being used for the study of samples with low atomic number and/or of low dimension, such as graphene, where knock-on damage may be predominant, the latter for organic crystals where radiolysis can be hugely detrimental. The MDEM is designed to investigate samples that have previously been considered too beam-sensitive to examine with conventional methods. By using the latest generation of direct electron detectors, with remarkably sensitive and linear response, we are able to record diffraction patterns from organic crystals in just a few milliseconds, before the crystal degrades under the beam. We will apply this method to study the nanoscale defect structure in pharmaceutical crystals, the development of dislocations, stacking faults and twins and importantly the interfaces between dissimilar organic crystals. Remarkably little is known about the microstructure of processed 'semi-crystalline' polymers, especially aliphatic polymers such as polyethylene and related alkanes. By using scanning electron diffraction methods we will use the MDEM to reveal hitherto unseen polymer nano-structure.
Electron tomography, or 3D imaging, can now be extended to a huge range of nanoscale materials and can be combined with diffraction, x-ray and energy loss spectroscopy to provide a full 3D picture of the materials' structure, composition and crystallography. The method is almost universally applicable and the range of materials science enabled by this method is huge. The multi-modal multi-dimensional aspect of the MDEM means we are able to acquire vast amounts of information and new software algorithms will be developed to process the data in a robust, efficient and meaningful fashion. These algorithms use the latest ideas in machine learning and in compressed sensing, where prior information is built into any reconstruction or interpretation of the image, tomogram or spectrum.
There are numerous material systems and devices that will benefit from the MDEM approach and, in addition to those already mentioned, we present a few more examples: perovskite solar cells, nitride semiconductors, engineering alloys such as nano-structured steels and Ni-base superalloys, low-dimensional dichalcogenides , magnetic skyrmionic materials, heterogeneous catalysts, MOFs and metallic glasses.
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