Probing defects in molecular materials: From polymers to metal–organic frameworks

Summary

Molecular materials span glasses, semi-crystalline polymers, and highly crystalline framework materials and molecular crystals. They are bound to a common cause in materials science by their characteristic molecular sub-components: their strong covalent bonds strongly direct interatomic structure but much weaker and often anisotropic interactions drive intermolecular interactions. Complex defects and disorder can appear in the domain of molecular packing. Yet compared to inorganic materials our understanding of the details of such defect and disorder modes remains in its infancy. This project will seek to advance state-of-the-art microscopy, diffraction and total scattering, and spectroscopy techniques to unravel atomic and molecular pictures in what are characteristically non-periodic features of these materials.

Defect-engineering in metal–organic frameworks (MOFs) has arisen as a key route to modulating their functional properties such as by controlling their pore size distributions. Likewise, controlling the crystallisation of many polymers is critical to performance in energy materials (organic light emitting diodes, photovoltaics, and emerging electronics) as well as in determining their mechanical properties. The precise, microscopic mechanisms of these disorder types—known to determine functional properties at the macroscopic length scale—remains obscured. A few instances show significant promise: Our group has developed nanobeam electron diffraction in the scanning transmission electron microscopes (STEM) to reveal coherent defect domains in MOFs [1], rotational disorder in crystal-amorphous MOF composites [2], as well as dislocations in molecular crystals [3] and molecular packing in polymer semiconductors [4].

This project will take these tools as a launch point to develop the next generation of tools for the characterisation of defects and disorder in molecular materials. Key routes to develop over the project may include cryogenic cooling of samples to examine hydration or dehydration, precession electron diffraction for electron beam pair distribution function analysis, and diffuse scattering analysis for molecular vibrational mode analysis (phonons). The project includes opportunities to tailor directions to the interests of the researcher and adapt to discoveries of previously unknown disorder types along the way. We are keen to support and develop an inclusive community of researchers, with the aim for equity for marginalised researchers. If you are interested, please email Sean Collins (s.m.collins@leeds.ac.uk) for informal inquiries.

[1] D.N. Johnstone et al. J. Am. Chem. Soc. 142 (2020) 13081–13089.
[2] A.F. Sapnik et al. Commun Chem 6 (2023) 1–12.
[3] S.T. Pham et al. ArXiv preprint (2023) https://doi.org/10.48550/arXiv.2308.16589.
[4] A.J. Sneyd et al. Science Advances 7 (2021) eabh4232.

Additional information

The project will have a primary focus on structural and chemical characterisation. The PhD researcher will receive training to drive forward their research using approaches including electron diffraction, pair distribution function analysis, and vibrational, optical, and core ionisation spectroscopy. The project will draw on materials science, chemistry, physics, and engineering principles. Applications are welcome from undergraduate degrees in any related field.

The advanced electron microscopy techniques used in the project will involve the use of equipment at the University of Leeds and the Leeds Electron Microscopy and Spectroscopy (LEMAS) Centre at the Bragg Centre for Materials Research. Instrumentation at LEMAS includes a new Tescan Tensor dedicated scanning transmission electron microscope, one of the first two installed in the world, as well as cryogenic plunge-freezing and cryo-transfer, focused ion beam facilities, and capabilities for electron beam spectroscopy, diffraction, and tomography (three-dimensional imaging).

The PhD researcher working on this project will join an enthusiastic and creative team of researchers working across a range of topics from electron microscopy of organic semiconductors and small molecule organic crystals to MOF synthesis. The research group is joint between the School of Chemical and Process Engineering and the School of Chemistry at the University of Leeds.

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