Local Structure in Crystalline, Glass and Melt States of a Hybrid Metal Halide Perovskite.

The pursuit of structure-property relationships in crystalline metal halide perovskites (MHPs) has yielded an unprecedented combination of advantageous characteristics for wide-ranging optoelectronic applications. While crystalline MHP structures are readily accessible through diffraction-based structure refinements, providing a clear view of associated long-range ordering, the local structures in more recently discovered glassy MHP states remain unexplored. Herein, we utilize a combination of Raman spectroscopy, solid-state nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy, in situ X-ray diffraction (XRD) and pair distribution function (PDF) analysis to investigate the coordination environment in crystalline, glass and melt states of the 2D MHP [(S)-(-)-1-(1-naphthyl)ethylammonium]2PbBr4. While crystalline SNPB shows polarization-dependent Raman spectra, the glassy and melt states exhibit broad features and lack polarization dependence. Solid-state NMR reveals disordering at the organic-inorganic interface of the glass due to significant spatial disruption in the tethering ammonium groups and the corresponding dihedral bond angles connecting the naphthyl and ammonium groups, while still preserving substantial naphthyl group registry and remnants of the layering from the crystalline state (deduced from XRD analysis). Moreover, PDF analysis demonstrates the persistence of corner-sharing PbBr6 octahedra in the inorganic framework of the melt/glass phases, but with a loss of structural coherence over length scales exceeding approximately one octahedron due to disorder in the inter- and intraoctahedra bond angles/lengths. These findings deepen our understanding of diverse MHP structural motifs and how structural alterations within the MHP glass affect properties, offering potential for advancing next-generation phase change materials and devices.

Surface-binding molecular multipods strengthen the halide perovskite lattice and boost luminescence.

Reducing the size of perovskite crystals to confine excitons and passivating surface defects has fueled a significant advance in the luminescence efficiency of perovskite light-emitting diodes (LEDs). However, the persistent gap between the optical limit of electroluminescence efficiency and the photoluminescence efficiency of colloidal perovskite nanocrystals (PeNCs) suggests that defect passivation alone is not sufficient to achieve highly efficient colloidal PeNC-LEDs. Here, we present a materials approach to controlling the dynamic nature of the perovskite surface. Our experimental and theoretical studies reveal that conjugated molecular multipods (CMMs) adsorb onto the perovskite surface by multipodal hydrogen bonding and van der Waals interactions, strengthening the near-surface perovskite lattice and reducing ionic fluctuations which are related to nonradiative recombination. The CMM treatment strengthens the perovskite lattice and suppresses its dynamic disorder, resulting in a near-unity photoluminescence quantum yield of PeNC films and a high external quantum efficiency (26.1%) of PeNC-LED with pure green emission that matches the Rec.2020 color standard for next-generation vivid displays.

Static and Dynamic Disorder in Formamidinium Lead Bromide Single Crystals.

We show that formamidinium-based crystals are distinct from methylammonium-based halide perovskite crystals because their inorganic sublattice exhibits intrinsic local static disorder that coexists with a well-defined average crystal structure. Our study combines terahertz-range Raman scattering with single-crystal X-ray diffraction and first-principles calculations to probe the evolution of inorganic sublattice dynamics with temperature in the range of 10-300 K. The temperature evolution of the Raman spectra shows that low-temperature, local static disorder strongly affects the crystal structural dynamics and phase transitions at higher temperatures.

Enabling Natural Abundance 17O Solid-State NMR by Direct Polarization from Paramagnetic Metal Ions.

Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing its applications and the quality of NMR spectroscopy as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample, enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low-sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal-ion-based DNP, the polarization agents are distributed in the bulk sample and act as a source of both relaxation and polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted, and the entire sample can be directly polarized. We exploit this finding to measure high-quality NMR spectra of 17O in the electrode material Li4Ti5O12 doped with Fe(III) despite its low abundance and long relaxation time.

Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials.

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li4Ti5O12. In addition for being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.