Laura Piveteau - Characterizing inorganic materials with solid-state NMR spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is a powerful non-invasive analytical method to characterize not only small molecules in the solution-state but also large molecules, colloids, crystals and amorphous materials in the solid-state. Solid-state NMR spectroscopy accesses isotope-specifically the local chemical and electronic structure and does not pose any requirements to the crystallinity of samples. Hence, it can provide insight into the chemical composition, distribution and topology of materials by visualizing connectivity among nuclear spins as well as their structural geometry and molecular dynamics.

While solid-state NMR spectroscopy approaches are comparably well established for biological and organic solids, in the last years, rising efforts is being put into accessing also inorganic solids. The past reluctance to study inorganic materials with NMR spectroscopy - although almost every chemical element possesses at least one NMR-active isotope - is due to the low natural abundance and low sensitivity of most inorganic nuclei, which adds up to the already inherently low sensitivity of NMR spectroscopy.

Game changers are signal enhancing methods, such as the recently developed dynamic nuclear polarization (DNP) experiment. It requires sample formulations adapted to the type of studied material, which in case of success can lead to tens to hundreds fold signal enhancement, depending on the experimental setup. Such enhancement factors are often prerequisite to apply on inorganic nuclei the probably most powerful approach of NMR spectroscopy, namely two dimensional (2D) experiments. 2D NMR experiments correlate spectroscopic features and increase spectral resolution by separating spin interactions into different dimensions. In this presentation, the new avenues opened by DNP enhanced 2D NMR experiments will be presented on the example of colloidal semiconductor nanocrystals. In particular, anisotropic interactions, like the chemical shift or the quadrupole coupling, bear great potential as they contain geometrical information about the local structure of nuclei. They can help to distinguish and identify surface species or serve as sensitive probes of local and temporal structure fluctuations, as will be discussed on the illustrative case of lead-halide perovskite materials.