In this Letter, we used fluorescence microscopy to image the reversible transformation of individual CsPbCl3 nanocrystals to CsPbBr3, which enables us to quantify heterogeneity in reactivity among hundreds of nanocrystals prepared within the same batch. We observed a wide distribution of waiting times for individual nanocrystals to react as has been seen previously for cation exchange and ion intercalation. However, a significant difference for this reaction is that the switching times for changes in fluorescence intensity are dependent on the concentration of substitutional halide ions in solution (i.e., Br– or Cl–). On the basis of the high solid-state miscibility between CsPbCl3 and CsPbBr3, we develop a model in which the activation energy for anion exchange depends on the density of exchanged ions in the nanocrystal. The heterogeneity in reaction kinetics observed among individual nanocrystals limits the compositional uniformity that can be achieved in luminescent CsPbCl3–xBrx nanocrystals prepared by anion exchange.
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Fluorescence Microscopy of Single Lead Bromide Nanocrystals Reveals Sharp Transitions during Their Transformation to Methylammonium Lead Bromide
Control over the nucleation and growth of lead-halide perovskite crystals is critical to obtain semiconductor films with high quantum yields in optoelectronic devices. In this report, we use the change in fluorescence brightness to image the transformation of individual lead bromide (PbBr 2 ) nanocrystals to methylammonium lead bromide (CH 3 NH 3 PbBr 3 ) via intercalation of CH 3 NH 3 Br. Analyzing this reaction one nanocrystal at a time reveals information that is masked when the fluorescence intensity is averaged over many particles. Sharp rises in the intensity of single nanocrystals indicate they transform much faster than the time it takes for the ensemble average to transform. While the ensemble reaction rate increases with increasing CH 3 NH 3 Br concentration, the intensity rises for individual nanocrystals are insensitive to the CH 3 NH 3 Br concentration. To explain these observations, we propose a phase-transformation model in which the reconstructive transitions necessary to convert a PbBr 2 nanocrystal into CH 3 NH 3 PbBr 3 initially create a high energy barrier for ion intercalation. A critical point in the transformation occurs when the crystal adopts the perovskite phase, at which point the activation energy for further ion intercalation becomes progressively smaller. Monte Carlo simulations that incorporate this change in activation barrier into the likelihood of reaction events reproduce key experimental observations for the intensity trajectories of individual particles. The insights gained from this study may be used to further control the crystallization of CH 3 NH 3 PbBr 3 and other solution-processed semiconductors.
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- NSF-PAR ID:
- 10086875
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- ISSN:
- 2050-7526
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Anion exchange is a facile, post-synthetic method to tune the emission wavelength of colloidal cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals. While colloidal nanocrystals can exhibit size-dependent phase stability and chemical reactivity, the role of size in the mechanism of anion exchange in CsPbX3 nanocrystals has not been elucidated. We used single-particle fluorescence microscopy to monitor the transformation of individual CsPbBr3 nanocrystals to CsPbI3. By systematically varying the size of the nanocrystals and the concentration of substitutional iodide, we observed that smaller nanocrystals exhibit longer transition times in their fluorescence trajectories, while larger nanocrystals undergo a more abrupt transition during anion exchange. Monte Carlo simulations were used to rationalize the size-dependent reactivity, in which we varied how each exchange event affects the probability for further exchange. Greater cooperativity for simulated ion exchange leads to shorter transition times to complete the exchange. We propose that size-dependent miscibility between CsPbBr3 and CsPbI3 at the nanoscale controls the reaction kinetics. Smaller nanocrystals maintain a homogeneous composition during anion exchange. As the nanocrystal size increases, variations in the octahedral tilting patterns of the perovskite crystals lead to different structures for CsPbBr3 and CsPbI3. Thus, an iodide-rich region must first nucleate within larger CsPbBr3 nanocrystals, which is followed by rapid transformation to CsPbI3. While higher concentrations of substitutional anions can suppress this size-dependent reactivity, the inherent differences in reactivity between nanocrystals of different sizes are important to consider when scaling up this reaction for applications in solid-state lighting and biological imaging.more » « less
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Abstract Organic–inorganic hybrid perovskites have emerged as promising optoelectronic materials for applications in photovoltaic and optoelectronic devices. Particularly, 2D layer‐structured hybrid perovskites are of great interest due to their remarkable optical and electrical properties, which can be easily tuned by selecting suitable organic and inorganic moieties during the material synthesis. Here, the solution‐phase growth of a large square‐shaped single‐crystalline 2D hybrid perovskite, phenethylammonium lead bromide (C6H5C2H4NH3)2PbBr4(PEPB), with thickness as few as 3 unit cell layers is demonstrated. Compared to bulk crystals, the 2D PEPB nanocrystals show a major blueshifted photoluminescence (PL) peak at 409 nm indicating an increase in bandgap of 40 meV. Besides the major peak, two new PL peaks located at 480 and 525 nm are observed from the hybrid perovskite nanocrystals. PEPB nanocrystals with different thicknesses show different colors, which can be used to estimate the thickness of the nanocrystals. Time‐resolved reflectance spectroscopy is used to investigate the exciton dynamics, which exhibits a biexponential decay with an amplitude‐weighted lifetime of 16.7 ps. The high‐quality 2D (C6H5C2H4NH3)2PbBr4nanocrystals are expected to have high PL quantum efficiency and potential applications for light‐emitting devices.
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Recent progress has been made on the synthesis and characterization of metal halide perovskite magic-sized clusters (PMSCs) with ABX 3 composition ( A = C H 3 N H 3 + or Cs + , B = P b 2 + , and X = C l − , Br - , or I - ). However, their mechanism of growth and structure is still not well understood. In our effort to understand their structure and growth, we discovered that a new species can be formed without the CH 3 NH 3 + component, which we name as molecular clusters (MCs). Specifically, CH 3 NH 3 PbBr 3 PMSCs, with a characteristic absorption peak at 424 nm, are synthesized using PbBr 2 and CH 3 NH 3 Br as precursors and butylamine (BTYA) and valeric acid (VA) as ligands, while MCs, with an absorption peak at 402 nm, are synthesized using solely PbBr 2 and BTYA, without CH 3 NH 3 Br. Interestingly, PMSCs are converted spontaneously overtime into MCs. An isosbestic point in their electronic absorption spectra indicates a direct interplay between the PMSCs and MCs. Therefore, we suggest that the MCs are precursors to the PMSCs. From spectroscopic and extended X-ray absorption fine structure (EXAFS) results, we propose some tentative structural models for the MCs. The discovery of the MCs is critical to understanding the growth of PMSCs as well as larger perovskite quantum dots (PQDs) or nanocrystals (PNCs).more » « less
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Abstract Cesium methylammonium lead iodide (Cs
x MA1−x PbI3) nanocrystals were obtained with a wide range of A‐site Cs‐MA compositions by post‐synthetic, room temperature cation exchange between CsPbI3nanocrystals and MAPbI3nanocrystals. The alloyed Csx MA1−x PbI3nanocrystals retain their photoactive perovskite phase with incorporated Cs content,x , as high as 0.74 and the expected composition‐tunable photoluminescence (PL). Excess methylammonium oleate from the reaction mixture in the MAPbI3nanocrystal dispersions was necessary to obtain fast Cs‐MA cation exchange. The phase transformation and degradation kinetics of films of Csx MA1−x PbI3nanocrystals were measured and modeled using an Avrami expression. The transformation kinetics were significantly slower than those of the parent CsPbI3and MAPbI3nanocrystals, with Avrami rate constants,k , at least an order of magnitude smaller. These results affirm that A‐site cation alloying is a promising strategy for stabilizing iodide‐based perovskites.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8TC06470A
