Transmission Electron Microscopy of Organic Crystalline Material and Zeolites

Understanding and controlling the polymorphic form and microstructure of pharmaceutical and zeolitic materials is key to their ongoing application in the food, medical, and chemical industries. Bulk techniques traditionally used to characterise the properties of these materials are limited when it comes to the analysis of the fine atomic structure. In this work electron microscopy has been utilised for the characterisation of a model organic compound (theophylline form II) and a model zeolite (Sn-beta), both of which are highly sensitive to damage when irradiated by a high-energy electron beam (80 – 300 kV). Transmission electron microscope diffraction pattern analysis was used to determine the lifetime of theophylline form II under a number of controlled microscope and specimen conditions. Values of the characteristic electron fluence for damage varied from 11 ± 5 e-Å-2 to 42 ± 5 e-Å-2, with the longest lifetime observed at 300 kV accelerating voltage, at liquid nitrogen specimen temperature, and with a graphene specimen support substrate. Within this dose budget (i.e. the total dose required to locate, align, focus, and record an image of a specimen), atomic lattice information for theophylline form II was obtained by the use of scanning moiré fringes in bright-field scanning transmission electron microscopy. Resolution of the atomic lattice was less readily resolved by bright-field conventional transmission electron microscopy with a direct electron detector. Key to these results is the use of a workflow involving lowering the electron beam flux as much as possible, using 300 kV accelerating voltage (if the sample is >50 – 100 nm thick), and on-specimen focussing using the Ronchigram. Further improvements could be achieved by cooling the sample to liquid nitrogen temperatures, using a conductive specimen support substrate, and using a pixelated direct electron-counting detector where possible. A characteristic electron fluence for damage was determined for Sn-beta zeolites of 17000 ± 8000 e-Å-2 at 300 kV. Integrated differential phase contrast, scanning transmission electron microscopy revealed the projected atomic positions of the zeolite in the [100]/[010] directions. The use of high-angle annular dark-field scanning transmission electron microscopy in conjunction with integrated differential phase contrast imaging highlighted excess Sn in the form of SnO2 and identified potential, individual Sn atomic sites. The Sn could be in a number of different tetrahedral Si sites of the beta zeolite structure and image simulation was used to explore likely sites. Overall, recent improvements to transmission electron microscope characterisation of pharmaceuticals, zeolites, and similarly beam-sensitive inorganics have been discussed and this work demonstrates the potential for bright-field scanning transmission electron microscope techniques in this field

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