Michael Allan Hope

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Dynamic Nuclear Polarization of Inorganic Halide Perovskites

David Lyndon Emsley, Michael Allan Hope, Dominik Józef Kubicki, Aditya Mishra, Gabriele Stevanato

NMR, EPR, XRD datasets and SEM image for the research article titled "Dynamic Nuclear Polarization of Inorganic Halide Perovskites". For further details see the readme.txt file.

EPFL Infoscience2023

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Dynamic Nuclear Polarization of Inorganic Halide Perovskites

David Lyndon Emsley, Michael Allan Hope, Dominik Józef Kubicki, Aditya Mishra, Gabriele Stevanato

The intrinsic low sensitivity of nuclear magnetic resonance (NMR) experiments limits their utility for structure determination of materials. Dynamic nuclear polarization (DNP) under magic angle spinning (MAS) has shown tremendous potential to overcome this key limitation, enabling the acquisition of highly selective and sensitive NMR spectra. However, so far, DNP methods have not been explored in the context of inorganic lead halide perovskites, which are a leading class of semiconductor materials for optoelectronic applications. In this work, we study cesium lead chloride and quantitatively compare DNP methods based on impregnation with a solution of organic biradicals with doping of high-spin metal ions (Mn2+) into the perovskite structure. We find that metal-ion DNP provides the highest bulk sensitivity in this case, while highly surface-selective NMR spectra can be acquired using impregnation DNP. The performance of both methods is explained in terms of the relaxation times, particle size, dopant concentration, and surface wettability. We envisage the future use of DNP NMR approaches in establishing structure–activity relationships in inorganic perovskites, especially for mass-limited samples such as thin films.

2023

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Light-Induced Halide Segregation in 2D and Quasi-2D Mixed-Halide Perovskites

Manuel Cordova, David Lyndon Emsley, Michael Allan Hope, Aditya Mishra

Photoinduced halide segregation hinders widespread application of three-dimensional (3D) mixed-halide perovskites. Much less is known about this phenomenon in lower-dimensional systems. Here, we study photoinduced halide segregation in lower-dimensional mixed iodide-bromide perovskites (PEA2MAn-1Pbn(BrxI1-x)3n+1, with PEA+: phenethylammonium and MA+: methylammonium) through time-dependent photoluminescence (PL) spectroscopy. We show that layered two-dimensional (2D) structures render additional stability against the demixing of halide phases under illumination. We ascribe this behavior to reduced halide mobility due to the intrinsic heterogeneity of 2D mixed-halide perovskites, which we demonstrate via 207Pb solid-state NMR. However, the dimensionality of the 2D phase is critical in regulating photostability. By tracking the PL of multidimensional perovskite films under illumination, we find that while halide segregation is largely inhibited in 2D perovskites (n = 1), it is not suppressed in quasi-2D phases (n = 2), which display a behavior intermediate between 2D and 3D and a peculiar absence of halide redistribution in the dark that is only induced at higher temperature for the quasi-2D phase.

AMER CHEMICAL SOC2023

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