Surface-Enhanced NMR Spectroscopy

Dynamic Nuclear Polarization (DNP) is currently one of the most efficient ways of enhancing sensitivity in solid-state NMR experiments. The DNP protocol consists in doping a sample with a small amount of para-magnetic species, typically nitroxide biradicals, and carrying out a magic-angle spinning (MAS) solid-state NMR experiment under microwave irradiation at around 100 K. Maximum theoretical enhancements that can be achieved this way reach 660, translating into the possibility of studying atomic-level composition of dilute surface species at picomolar concentrations that would be otherwise out of reach. Recent years wit-nessed very fast development of DNP instrumentation (high-power and high-frequency microwave sources, higher magnetic fields, faster sample spinning), polarizing agents (rigid dinitroxide biradicals) and new sam-ple preparation techniques (universally applicable and straightforward protocols, solutions for DNP of highly reactive organometallic species) which brought about sensitivity enhancements close to the theoret-ical maximum. The experimental part of this thesis reviews recent advances in the field of MAS DNP. We explores ways of improving currently existing polarizing agents by fine-tuning their performance (function-alization, deuteration). In particular, we have developed a new polarizing agent, TEKPol2, which performs better than current state-of-the-art TEKPol. We have subsequently explored factors that limit DNP en-hancements under typical experimental conditions. We describe how by simply mixing the DNP sample with dielectric particles (such as NaCl, KBr, sapphire) enhancements can be boosted by a factor of 2.5. We have rationalized this phenomenon in terms of improved microwave propagation through the sample using finite-element simulations. Further, we have applied conventional solid-state NMR to probe atomic level microstructure to determine key properties of hybrid lead halide perovskites materials. Methylammonium (MA)- and formamidinium (FA)-based lead halide perovskites attract much attention owing to their impressive performance as photo-voltaic materials, currently providing power conversion efficiencies over 22%. We have used 14N, 2H, 13C, 133Cs, 87Rb, 39K and 1H solid-state MAS NMR to explain in detail cation reorientation dynamics, phase com-position, and phase separation phenomena in Cs-, Rb-, K- guanidinium (GUA)-, MA-, and FA-containing pho-tovoltaic perovskites, both in bulk and on thin films prepared according to the usual device fabrication procedure. We have presented the first quantitative, cation-specific picture of cation reorientation in MA/FA and GUA/MA materials, which explains the exceptionally long charge carrier lifetimes observed in these materi-als. Our findings have provided a link between charge carrier lifetimes of ABX3-type lead halide perovskites and the structure of A-cation site. We also found that rubidium and potassium are not incorporated into the 3D lattice of materials previously described in the literature. Cesium and GUA, on the other hand, are easily incorporated. These advances were possible through the use of mechanochemistry as a straightfor-ward way of preparing large quantities of high quality materials for NMR studies.

Emsley, David Lyndon
Lausanne, EPFL

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 Record created 2018-01-25, last modified 2019-05-09

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