Irreversibility in Anion Exchange Between Cesium Lead Bromide and Iodide Nanocrystals Imaged by Single-Particle Fluorescence

Abstract

Anion exchange is a powerful method to tune the emission wavelength of perovskite CsPbX 3 (X = Cl, Br, or I) nanocrystals across the visible spectrum. CsPbX 3 nanocrystals can possess a number of crystal structures that depend on both their composition and size. Understanding the role these structural variations play during anion exchange is important for their future application in optoelectronic devices. In this work, we used fluorescence microscopy to monitor reaction trajectories of individual nanocrystals as they undergo anion exchange between CsPbBr 3 and CsPbI 3 . By varying the concentration of substitutional ion used to induce anion exchange, we quantify differences in reaction times for exchange in opposite directions. CsPbI 3 nanocrystals undergo more abrupt shifts in their emission characteristics as they transform to CsPbBr 3 , while CsPbBr 3 nanocrystals exhibit a smoother transition during their transformation to CsPbI 3 . Simulations of anion exchange using Monte Carlo trajectories are consistent with a larger degree of structural reorganization when CsPbI 3 nanocrystals undergo anion exchange. Together, these results reveal that there are structural differences between CsPbI 3 nanocrystals synthesized by the hot-injection method and those produced by anion exchange. The narrower distribution of reaction times when CsPbI 3 nanocrystals transform to CsPbBr 3 may produce better compositional homogeneity when this reaction is scaled-up to incorporate these materials into optoelectronic devices.

In Situ Observation of Emission Sites during the Halide Exchange Reaction in Single Cesium Lead Halide Perovskite Nanocrystals

“…In accordance with a previous report on PL polarization in a single formamidinium lead bromide (FAPbBr 3 ) PNC, the PL anisotropy of the single PNCs used in this work is attributed to lattice distortion. − After drop-casting of the DDA-HI solution, the PL anisotropy value decreased as the reaction progressed. We propose that the decrease in PL anisotropy is attributed to lattice rearrangement due to the difference in the ionic radii of Br – and I – (1.96 and 2.20 Å, respectively). − Because the Br – ions in PNCs were not completely exchanged, the halide composition within a single CsPbBr x I (3– x ) PNC became inhomogeneous. The inhomogeneous halide distribution caused a nonuniform lattice parameter within a single PNC.…”

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond

“… (D) Illustration of structural differences in when as-synthesized CsPbI 3 NCs undergo anion exchange to CsPbBr 3 . Reprinted from ( Wang et al., 2020a ). Copyright © 2020, American Chemical Society.…”

“…found that the formation of mixed-halide perovskite NCs was dominated by both lattice reconstruction and anion-exchange processes rather than the general anion exchange model ( Bi et al., 2018 ). Later, fluorescence microscopy was used to monitor the reaction trajectories of anion exchange between CsPbBr 3 and CsPbI 3 NCs ( Wang et al., 2020a ). It was found that CsPbI 3 NCs underwent more abrupt shifts in their emission characteristics when they transformed to CsPbBr 3 NCs, while CsPbBr 3 NCs exhibited a smoother transition during their transformation to CsPbI 3 NCs ( Figure 13 D), highlighting the structural differences between CsPbX 3 NCs directly synthesized by the HI method and those prepared by anion exchange.…”

Stochastic Noise in Single-Nanoparticle Catalysis

“…Over the past decade, tremendous progress has been made in this direction. In-situ microscopic, nanoscopic, and single-molecule-level spectroscopic tools − have elucidated the elementary catalytic events occurring on the surface of the catalyst in real time. ,− Owing to their improved spatial and temporal resolution, as compared to conventional bulk-level interrogations, these studies have revealed that catalysts are heterogeneous. Catalytic activity can vary spatiallyon nanometer and micrometer scalesand also sometimes in time. − Such spatial variation of catalytic behavior is often attributed to static differences in morphologies, structural features (facets, edges, or steps), or chemical environments of individual grains of the catalyst. − Temporal changes in activity are assigned to dynamics in the surface structure or composition of the catalyst resulting from processes such as adsorbate-induced surface restructuring, poisoning of the surface, or sint...…”