Publication date: 14th January 2021
Lead-halide perovskite (LHP) nanocrystals (NCs) have emerged over the last years as attractive light emitters and are being increasingly investigated for their application in LEDs, LCD displays, and many more optoelectronic devices.1 Their bright photoluminescence (PL) spans the entire visible spectral range and exhibits narrow emission line widths (<100 meV) as well as a very high quantum yields of up to 100%. Unlike in other semiconductor NCs, the luminescence in LHP NCs is not produced by quantum confinement. Instead, it is ascribed to the high defect-tolerance of the perovskite structure.2 However, these emissive properties are absent in bulk LHPs, which raises the question of their origins in LHP NCs. Detailed characterization of the bulk and nanocrystalline forms of LHP materials remains challenging, inter alia because of the softness of the LHP structure, but it is essential for a better understanding of the structure-property relationship of these materials.
We used halide NMR and halide nuclear quadrupole resonance (NQR) spectroscopy to investigate the structure and dynamics of both bulk and nanocrystalline CsPbX3 (X = Cl, Br, I) perovskites. We exploited the strong quadrupole couplings of the halide nuclei, which originate from the interaction between the large quadrupole moments of the halide nuclei and the local electric-field gradients. The latter are especially strong in LHPs, thus making the quadrupole interaction highly sensitive to subtle structural variations, both in time and space. The comparison of the NMR and NQR spectra of nanocrystalline and bulk materials revealed larger structural disorder and accelerated dynamics in NCs compared to bulk. This was corroborated by ab-initio molecular dynamics, pointing out the role of the surface in causing radial distribution of strain and disorder. Thus, halide NMR and NQR are some of the few characterization methods that can probe both the bulk and nancrystalline forms of LHPs and they may this way provide the structural and dynamical insights fundamental to a better understanding of the exceptional properties of these materials.
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