Bulk and Nanocrystalline Cesium Lead-Halide Perovskites as Seen by Halide Magnetic Resonance

Abstract

Lead-halide perovskites increasingly mesmerize researchers because they exhibit a high degree of structural defects and dynamics yet nonetheless offer an outstanding (opto)electronic performance on par with the best examples of structurally stable and defect-free semiconductors. This highly unusual feature necessitates the adoption of an experimental and theoretical mindset and the reexamination of techniques that may be uniquely suited to understand these materials. Surprisingly, the suite of methods for the structural characterization of these materials does not commonly include nuclear magnetic resonance (NMR) spectroscopy. The present study showcases both the utility and versatility of halide NMR and NQR (nuclear quadrupole resonance) for probing the structure and structural dynamics of CsPbX 3 (X = Cl, Br, I), in both bulk and nanocrystalline forms. The strong quadrupole couplings, which originate from the interaction between the large quadrupole moments of, e.g., the 35 Cl, 79 Br, and 127 I nuclei, and the local electric-field gradients, are highly sensitive to subtle structural variations, both static and dynamic. The quadrupole interaction can resolve structural changes with accuracies commensurate with synchrotron X-ray diffraction and scattering. It is shown that space-averaged site-disorder is greatly enhanced in the nanocrystals compared to the bulk, while the dynamics of nuclear spin relaxation indicates enhanced structural dynamics in the nanocrystals. The findings from NMR and NQR were corroborated by ab initio molecular dynamics, which point to the role of the surface in causing the radial strain distribution and disorder. These findings showcase a great synergy between solid-state NMR or NQR and molecular dynamics simulations in shedding light on the structure of soft lead-halide semiconductors.

Large-Area Nanocrystalline Caesium Lead Chloride Thin Films: A Focus on the Exciton Recombination Dynamics

“…We can envisage also two other possible origins of the different bands: a change in the stoichiometry of Cs, Pb, Cl [ 37 ] or different crystalline phase of CsPbCl 3 in the MCs and NC, in analogy with reports on hybrid perovskites [ 38 , 39 , 40 ]. Two recent papers [ 41 , 42 ] highlighted structural changes crossing between NC and MCs and therefore it is very likely to suppose that the different energies of the α and β bands reflect such changes. Presently, we cannot experimentally distinguish between the previous hypotheses about the origin of the β-band: however, it must be pointed out that our data clearly show the formation of extended electronic states as certified by the excitonic resonances of Figure 3 .…”

Exploiting the Lability of Metal Halide Perovskites for Doping Semiconductor Nanocomposites

“… 14 Another example is the lead-depleted Cs 4 PbX 6 structure where the [PbX 6 ] 4 – octahedra are isolated (0D). 13 , 15 − 17 A third closely related nonperovskite structure that has lower Cs + content and does not contain [PbX 6 ] 4 – octahedra, CsPb 2 X 5 , can be described as layers (2D) of [Pb 2 X 5 ] ® clusters separated by Cs + ions. 13 , 15 − 17 Because of the large difference in the involved atoms’ electronegativities, all these cesium lead halides exhibit mixed bonding nature.…”

Magnetic Resonance for Challenging Quadrupolar Halide Spins in Perovskites

“…Solid-state nuclear magnetic resonance (NMR) is playing an indispensable role in the research of perovskites due to its superb spectroscopic resolution of chemical structure for both organic and inorganic components. 3 …”

“…The work done by Piveteau et al 3 demonstrated that the challenges of halide signals in lead halide perovskites (CsPbX 3 ) can be tackled with a versatile choice of both NMR and nuclear quadrupole resonance (NQR) ( Figure 2 ). Here, NQR is a similar technique to NMR, instead using zero magnetic field to collect the signal of quadrupolar nuclei.…”

“…(c) Simulations of the 127 I NMR and NQR spectra for bulk γ-CsPbI 3 . Reproduced with permission from ref ( 3 ). Copyright 2020 American Chemical Society.…”