Large-Scale Synthesis of Highly Luminescent Perovskite-Related CsPb2 Br5 Nanoplatelets and Their Fast Anion Exchange

Ligand-free template-assisted synthesis of stable perovskite nanocrystals with near-unity photoluminescence quantum yield within the pores of vaterite spheres

Ligand-free methods for the synthesis of halide perovskite nanocrystals are of great interest because of their excellent performance in optoelectronics and photonics. In addition, template-assisted synthesis methods have become a powerful tool for the fabrication of environmentally stable and bright nanocrystals. Here we develop a novel approach for the facile ligand-free template-assisted fabrication of perovskite nanocrystals with a near-unity absolute quantum yield, which involves CaCO₃ vaterite micro- and submicrospheres as templates. We show that the optical properties of the obtained nanocrystals are affected not mainly by the template morphology, but strongly depend on the concentration of precursor solutions, anion and cation ratio, as well as on adding defect-passivating rare-earth dopants. The optimized samples are further tested as infrared radiation visualizers exhibiting promising characteristics comparable to those that are commercially available.

Thermodynamic Stability, Structure, and Optical Properties of Perovskite-Related CsPb2Br5 Single Crystals under Pressure

CsPb₂Br₅ belongs to all inorganic perovskite-related quasi-two-dimensional materials that have attracted considerable attention due to their potential for optoelectronic applications. In this study, we solve numerous controversies on the physical properties of this material. We show that optical absorption in the visible spectrum and green photoluminescence are due to microcrystallites of the three-dimensional CsPbBr₃ perovskite settled on the CsPb₂Br₅ plates and that carefully cleaned crystal plates are devoid of these features. The high-pressure structural and spectroscopic experiments, performed on the single crystals free of CsPbBr₃ impurities, evidenced that the layered tetragonal structure of CsPb₂Br₅ is stable at least up to 6 GPa. The absorption edge is located in the ultraviolet at around 350 nm and continuously red shifts under pressure. Moderate band gap narrowing is well correlated to the pressure-induced changes in the crystal structure. Although the compressibility of CsPb₂Br₅ is much higher than for CsPbBr₃, the response in optical properties is weaker because the Pb–Br layers responsible for the optical absorption are much less affected by hydrostatic pressure than those built of Cs⁺ cations. Our study clarifies the confusing data in the literature on the optical properties and thermodynamic stability of CsPb₂Br₅.

The impact of genuinely hydrostatic pressure on the structure and optical properties of pure CsPb₂Br₅ was studied for the first time on single crystals. The results provide new information about the crystal stability, photoluminescence, and optical absorption.

Synthesis of Two-Dimensional CsPb2X5 (X = Br and I) with a Stable Structure and Tunable Bandgap by CsPbX3 Phase Separation

Perovskite-related materials with various dimensionalities have attracted sustained attention owing to their extraordinary electronic and optoelectronic properties, but it is still challenging in the synthesis of compounds with desired compositions and structures. Herein, a two-dimensional (2D) CsPb₂I₅ perovskite has been synthesized by the conversion of CsPbI₃ at high-pressure and high-temperature (high P-T) conditions, which is quenchable at ambient conditions. In situ synchrotron X-ray diffraction shows that high-pressure monoclinic CsPbI₃ converts into tetragonal CsPb₂I₅ and cubic CsI at 8.7 GPa upon heating from 644 to 666 K. Keeping the tetragonal structure stable, CsPb₂I₅ exhibits tunable optical properties with the bandgap changing from ∼2.4 eV at ambient pressure to ∼1.4 eV at 36.9 GPa. Further experiments demonstrate similar structural evolution in the typical three-dimensional CsPbBr₃ perovskite into 2D CsPb₂Br₅ at high P-T conditions, indicating that the conversion of CsPbX₃ (X = Br and I) into CsPb₂X₅ is ubiquitous.

Temperature-dependent dielectric properties of CsPb2Br5: a 2D inorganic halide perovskite

Two dimensional (2D) CsPb₂Br₅have been successfully synthesized via the chemical precipitation method. Detailed structural, morphological, optical, and dielectric studies of these materials have been performed. These 2D CsPb₂Br₅plates (of thickness around 200-300 nm) are ascribed to a tetragonal lattice system withI4/mcmspace group. The dielectric attributes such as dielectric constant, electrical modulus, loss factor, and the DC, and AC conductivities, are observed to be varying appreciably with temperature over an extensive frequency window of 10 Hz-50 MHz. The Nyquist plots are investigated using the Maxwell-Wagner equivalent circuit model, which shows the impact of grains and grain boundaries on the overall impedance. Both the free charge conductivity and space charge increase with an increment in temperature, as revealed from the modified Cole-Cole plot. The relaxation time and relaxation mechanism of 2D CsPb₂Br₅are estimated using the Kohlrausch-Williams-Watts equation. Variation in DC conductivity and relaxation time, as a function of temperature, closely resembles Arrhenius' behavior. Value of activation energy calculated from the DC conductivity corroborates with the same derived from relaxation time. The observation of high dielectric constant and nominal dielectric loss for CsPb₂Br₅perovskite offers enormous potential in energy harvesting and storage devices.

Photoelectrochemical performance of ligand-free CsPb2Br5 perovskites

A pure aqueous synthesis strategy for ligand-free CsPb2Br5 nanoplatelets and pure-phase single crystal is reported. The CsPb2Br5 perovskite displays excellent photoelectrochemical activity in the absence of other electron acceptors.

A handy imaging sensor array based on the phase transformation from CsPbBr3 to CsPb2Br5: highly sensitive and rapid detection of water content in ethanol

This work proposes a handy imaging sensor array based on the phase transformation from CsPbBr3 to CsPb2Br5 for highly sensitive and rapid detection of the water content in ethanol.

Ligand Assisted Control of Photoluminescence in Organometal Perovskite Nanocrystals

Organometal perovskite nanocrystals have shown remarkable properties not only in photovoltaics, but also in light-emitting devices. In this work colloidal nanocrystals of organometal perovskite CH3NH3PbBr3 (MAPBr) with effective visible photoluminescence were synthesized by the ligand assisted reprecipitation method. The studies were carried out by photoluminescence spectroscopy and optical transmission spectroscopy. Analysis of the photoluminescence and transmission spectra showed that by changing the concentration of the ligands oleylamine and octylamine, it is possible to control the size of nanocrystals and the photoluminescence wavelength due to the quantum confinement effect. It was shown that the increase in ligands concentration in MAPBr perovskite nanocrystals (NCs) solutions decreases the width of the peak which indicates a better quality of the obtained nanocrystals. An increase in the band gap indicates a decrease in the size of the nanocrystals. Replacing the ligands in the colloidal perovskite NCs solutions leads to shift of the photoluminescence peak from 456 to 535 nm.

Metastability and Seeding Effects in the Mechanochemical Hybrid Lead(II) Iodide Formation

The mechanism for the mechanochemical synthesis of (C(NH₂ )₃ )₃ PbI₅ 3 and (C(NH₂ )₃ )₄ PbI₆ 4 and their conversion into each other is presented. We investigated the synthesis of 3 at different frequencies and energies using in situ powder X-ray diffraction. By splitting the reaction into single parts we could prove that the formation of 3 is simply dependent on the energy and mixing speed. The nucleation of 4 instead is slightly negative dependent on the energy but dependent on the mixing speed, while its growth is mostly independent of any influence. We were able to influence the reaction pathways by seeding the mixture with a small amount of powdery 4. The formation of 4 is very likely an auto-catalytic process. 3 instead is metastable. It can be stabilized by energy, which beside mechanochemistry can also be achieved by temperature. The results showcases the complex nature of mechanochemical reactions.

Lead halide perovskites with aggregation-induced emission feature coupled with gold nanoparticles for fluorescence detection of heparin

Herein, a new kind of lead halide perovskite (LHP, (C₁₂H₂₅NH₃)₂PbI₄) with aggregation-induced emission (AIE) feature is developed as a fluorescent probe for heparin (Hep). The LHPs exhibit high emission when they aggregate in water. Interestingly, a few picomoles of dispersed gold nanoparticles (AuNPs) can quench the emission of LHPs, but the aggregated AuNPs are invalid. When protamine (Pro) is mixed with AuNPs at first, the negatively charged AuNPs aggregate through electrostatic interaction, producing the AIE recovery. Nevertheless, Hep disturbs the interaction between AuNPs and Pro due to its strong electrostatic interaction with Pro. Therefore, the dispersed AuNPs quench the fluorescence of LHPs again. A response linear range of Hep of 0.8-4.2 ng ml^-1is obtained, and the detection limit is 0.29 ng ml^-1. Compared with other probes for determination of Hep with AuNPs, this strategy exhibits better sensitivity due to the small quantity of AuNPs used. Finally, it is also successfully applied to detect Hep in human serum samples with satisfactory recoveries.

Monodispersed CsPb2Br5@SiO2 Core-Shell Nanoparticles as Luminescent Labels for Biosensing

Despite the rising advances in the field of metal halide perovskite nanocrystals (NCs), the exploitation of such nanoparticles as luminescent labels for ex vivo imaging and biosensing is still unclear and in the early stages of investigation. One of the major challenges toward the implementation of metal halide perovskite NCs in biosensing applications is to produce monodispersed nanoparticles with desired surface characteristics and compatible with aqueous environments. Here, we report the synthesis of monodispersed spherical CsPb₂Br₅@SiO₂ core-shell nanoparticles by post-synthetic chemical transformation of 3D CsPbBr₃ NCs in the presence of tetraethyl orthosilicate and a critical water/ammonia ratio. This method involves an ammonia-mediated and ammonia-induced "top-down" transformation of as-synthesized 3D CsPbBr₃ NCs to smaller CsPb2Br5 nanoclusters (ca. 2-3 nm), which trigger a seed-mediated silica growth, yielding monodispersed spherical blue luminescent (λ_emission = 432 nm) CsPb₂Br₅@SiO₂ perovskite nanoparticles. By adjusting the reaction conditions, core-shell nanoparticles of a 36.1 ± 4.5 nm diameter, which preserve their optical properties in water, were obtained. Besides that, the viability of the developed nanoparticles as a luminescent label for biosensing has been proven by specific biorecognition of the IgG protein in a direct immunoassay. Our work sheds light on the chemical processes and transformations involved in the silica nucleation mechanism in the presence of perovskite nanoparticles and opens the way for the future rational design of the next generation of semiconductor NC luminescent biological labels.

Ligand-mediated synthesis of compositionally related cesium lead halide CsPb2X5 nanowires with improved stability

Compositionally related cesium lead halide materials, such as CsPb2X5, have attracted great interest due to their considerable optoelectronic/optical properties as well as improved stability. Currently, CsPb2Br5 nanocrystals can be well-designed by tuning the ligands or precursor ratio, whereas, CsPb2X5 (with Cl- or I-) nanocrystals can only be obtained by the anion exchange method. Herein, we report a method to directly synthesize CsPb2X5 facilitated by thiol ligands. The morphology of CsPb2X5 can be designed as a nanowire. Importantly, the stability of directly synthesized CsPb2X5 nanowires is much improved when compared with the stabilities of the materials obtained by the anion-exchange method. We believe that this method will promote the application of 1D tetragonal CsPb2X5 in optoelectronics, optics and other fields.

Single component Mn-doped perovskite-related CsPb2ClxBr5-x nanoplatelets with a record white light quantum yield of 49%: a new single layer color conversion material for light-emitting diodes

Single component nanocrystals (NCs) with white fluorescence are promising single layer color conversion media for white light-emitting diodes (LED) because the undesirable changes of chromaticity coordinates for the mixture of blue, green and red emitting NCs can be avoided. However, their practical applications have been hindered by the relative low photoluminescence (PL) quantum yield (QY) for traditional semiconductor NCs. Though Mn-doped perovskite nanocube is a potential candidate, it has been unable to realize a white-light emission to date. In this work, the synthesis of Mn-doped 2D perovskite-related CsPb₂Cl_xBr_5-x nanoplatelets with a pure white emission from a single component is reported. Unlike Mn-doped perovskite nanocubes with insufficient energy transfer efficiency, the current reported Mn-doped 2D perovskite-related CsPb₂Cl_xBr_5-x nanoplatelets show a 10 times higher energy transfer efficiency from perovskite to Mn impurities at the required emission wavelengths (about 450 nm for perovskite emission and 580 nm for Mn emission). As a result, the Mn/perovskite dual emission intensity ratio surprisingly elevates from less than 0.25 in case of Mn-doped nanocubes to 0.99 in the current Mn-doped CsPb₂Cl_xBr_5-x nanoplatelets, giving rise to a pure white light emission with Commission Internationale de l'Eclairage (CIE) color coordinates of (0.35, 0.32). More importantly, the highest PL QY for Mn-doped perovskite-related CsPb₂Cl_xBr_5-x nanoplatelets is up to 49%, which is a new record for white-emitting nanocrystals with single component. These highly luminescent nanoplatelets can be blended with polystyrene (PS) without changing the white light emission but dramatically improving perovskite stability. The perovskite-PS composites are available not only as a good solution processable coating material for assembling LED, but also as a superior conversion material for achieving white light LED with a single conversion layer.

CsPb2 Br5 Single Crystals: Synthesis and Characterization

CsPb₂ Br₅ is a ternary halogen-plumbate material with close characteristics to the well-reported halide perovskites. Owing to its unconventional two-dimensional structure, CsPb₂ Br₅ is being looked at broadly for potential applications in optoelectronics. CsPb₂ Br₅ investigations are currently limited to nanostructures and powder forms of the material, which present unclear and conflicting optical properties. In this study, we present the synthesis and characterization of CsPb₂ Br₅ bulk single crystals, which enabled us to finally clarify the material's optical features. Our CsPb₂ Br₅ crystal has a two-dimensional structure with Pb₂ Br₅^- layers spaced by Cs⁺ cations, and exhibits approximately 3.1 eV indirect band gap with no emission in the visible spectrum.