Hyperpolarization of inorganic solids

Snaedis Björgvinsdóttir
EPFL, 2021
Thèse EPFL

Solid-state NMR can provide information about the atomic level structure and dynamics of materials. It directly probes symmetry and structure at nuclear sites, and is especially useful for investigation of disordered or amorphous solids that lack long range order. However, the application of solid-state NMR is sometimes limited by its relatively low sensitivity, caused by low concentrations and low gyromagnetic ratios of the magnetically active nuclei.

Dynamic nuclear polarization (DNP) can provide significant signal enhancements in magic-angle-spinning (MAS) NMR experiments. This is usually achieved by introducing stable organic radicals to the sample of interest, and transferring their large electron spin polarization to nearby nuclear spins via microwave irradiation near to the EPR frequency. Methods to hyperpolarize the bulk of solid materials containing protons are well established, whereas NMR of proton-free bulk materials remains challenging in many cases, especially when nuclear relaxation times are long, and if isotopic enrichment or paramagnetic doping to enhance relaxation rates are not feasible.

The overall objective of the work described in this thesis is to improve sensitivity in DNP enhanced solid-state NMR experiments, and to extend the application of DNP to systems that are currently difficult to access. In particular, this includes developing a strategy to hyperpolarize the bulk of proton-free inorganic materials.

In the first part, the classic flip-back method to recover bulk proton magnetization is combined with DNP of 1H containing solids with characteristic build-up times spanning two orders of magnitude. Gains in sensitivity in the 13C spectra of powdered crystalline theophylline, histidine and salicylic acid are reported, on top of the enhancements already provided by relayed DNP.

In the second part, a general strategy where the relayed DNP method is extended to proton-free inorganic materials is reported. The method uses a combination of impregnation DNP and slow spin diffusion between weakly magnetic nuclei such as 119Sn and 31P. Hyperpolarization is continuously generated at the surface either by direct DNP of the weakly magnetic nuclei, or by multiple bursts of cross polarization (CP) from protons in the wetting phase. Provided that bulk T1 values are long, even slow spin diffusion can then transfer the surface-generated hyperpolarization to the bulk, resulting in spectra which exceed the sensitivity of conventional solid-state NMR. As an example, multiple contact CP can provide a factor 50 gain in overall sensitivity of the 119Sn spectrum of SnO2, allowing access to materials that were previously unfeasible to study. Overall in this thesis, hyperpolarization transport by spin diffusion is confirmed between 31P nuclei in GaP and Sn2P2O7, 119Sn in SnO2, 113Cd in CdTe, 29Si in SiO2 (âº-quartz) and 6Li/7Li in lithium titanates, and shown to improve sensitivity in their bulk NMR spectra. Strategies to optimize bulk hyperpolarization are shown, as well as two-dimensional spin diffusion experiments which provide insight into the process of polarization transfer from surface to bulk.

In the third part, DNP is explored at the highest field and spinning frequencies available for DNP to date. NMR signal enhancements of 200 are reported at 21.1 T, enabled by 65 kHz MAS at 100 K. The fast spinning frequencies also yield high resolution DNP enhanced 1H-detected heteronuclear correlation spectra.

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Snaedis Björgvinsdóttir
EPFL, 2021
Thèse EPFL

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https://infoscience.epfl.ch/record/283340?ln=fr

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Solid-state magic-angle spinning NMR spectroscopy is a powerful technique to probe atomic-level microstructure and structural dynamics in metal halide perovskites. It can be used to measure dopant incorporation, phase segregation, halide mixing, decomposition pathways, passivation mechanisms, short-range and long-range dynamics, and other local properties. This Review describes practical aspects of recording solid-state NMR data on halide perovskites and how these afford unique insights into new compositions, dopants and passivation agents. We discuss the applicability, feasibility and limitations of H-1, C-13, N-15, N-14, Cs-133, Rb-87, K-39, Pb-207, Sn-119, Cd-113, Bi-209, In-115, F-19 and H-2 NMR in typical experimental scenarios. We highlight the pivotal complementary role of solid-state mechanosynthesis, which enables highly sensitive NMR studies by providing large quantities of high-purity materials of arbitrary complexity and of chemical shifts calculated using density functional theory. We examine the broader impact of solid-state NMR on materials research and how its evolution over seven decades has benefitted structural studies of contemporary materials such as halide perovskites. Finally, we summarize some of the open questions in perovskite optoelectronics that could be addressed using solid-state NMR. We, thereby, hope to stimulate wider use of this technique in materials and optoelectronics research.

NATURE PORTFOLIO2021

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Endogenous O-17 Dynamic Nuclear Polarization of Gd-Doped CeO2 from 100 to 370 K

Snaedis Björgvinsdóttir, David Lyndon Emsley, Michael Allan Hope, Georges Guévork Jean-Jacques Menzildjian, Bowen Zhang

O-17 NMR is an invaluable tool to study the structure and dynamics of oxide materials but remains challenging to apply in many systems. Even with isotopic enrichment, studies of samples with low masses and/or concentrations of the active species, such as thin films or interfaces, are limited by low sensitivity. Here, we show how endogenous dynamic nuclear polarization (DNP) can dramatically improve the sensitivity in the oxide-ion conductor Gd-doped CeO2, with a O-17 enhancement factor of 652 at 100 K. This is the highest enhancement observed so far by endogenous DNP or Gd3+ DNP, which is explained in terms of the electron paramagnetic resonance characteristics. The DNP properties are studied as a function of Gd concentration for both enriched and natural-abundance samples, and the buildup behavior shows that spin diffusion in O-17-enriched samples improves sensitivity by relaying hyperpolarization throughout the sample. Notably, efficient hyperpolarization could still be achieved at elevated temperatures, with enhancement factors of 320 at room temperature and 150 at 370 K, paving the way for the characterization of materials under operational conditions. Finally, the application of endogenous Gd3+ DNP is illustrated with the study of interfaces in vertically aligned nanocomposite thin films composed of Gd-CeO2 nanopillars embedded in a SrTiO3 matrix, where DNP affords selective enhancement of the different phases and enables a previously infeasible two-dimensional correlation experiment to be performed, showing spin diffusion between Gd-CeO2 and the solid-solid interface.

AMER CHEMICAL SOC2021

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Chemical Developments in DNP-NMR

Pascal Miéville

The Nuclear Magnetic Resonance (NMR) is an extraordinary tool to understand molecular structures. Its main limitation resides in its relatively low sensitivity when compared with other physico-chemical methods such as Mass Spectrometry (MS) or Optical Spectroscopy. Dynamic Nuclear Polarization (DNP), invented in the 50's but only widely applied in the last ten years, allows, by transferring the high polarization of paramagnetic centers to surrounding nuclei at low temperature, to enhance NMR signals by several orders of magnitude. This technique can be applied to solids at 100 K using Magic Angle Spinning (DNP-MAS) or in liquid phase through Dissolution-DNP as described by Ardenkjaer-Larsen in 2003. The efficiency of the polarization transfer is strongly dependent on the nature of the paramagnetic centers and on the way they are mixed with the target molecules. Part of this work will consist in the study of new ways to integrate paramagnetic centers into the matrix and also to limit their negative effects on NMR signals in liquid phase after dissolution. Because DNP-NMR has reached a stage where it should become a standard chemical method, a second objective of this thesis is to propose new chemical applications such as kinetic measurements that may be of interest in other domain of chemistry.

EPFL2011

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