The photoluminescence mechanism of CsPb2Br5 microplates revealed by spatially resolved single particle spectroscopy

Unveiling the Structural Properties, Optical Behavior, and Thermoelectric Performance of 2D CsSn2Br5 Halide Obtained by Mechanochemistry

Metal halide perovskites with a two-dimensional structure are utilized in photovoltaics and optoelectronics. High-crystallinity CsSn2Br5 specimens have been synthesized via ball milling. Differential scanning calorimetry curves show melting at 553 K (endothermic) and recrystallization at 516 K (exothermic). Structural analysis using synchrotron X-ray diffraction data, collected from 100 to 373 K, allows for the determination of Debye model parameters. This analysis provides insights into the relative Cs-Br and Sn-Br chemical bonds within the tetragonal structure (space group: I4/mcm), which remains stable throughout the temperature range studied. Combined with neutron data, X-N techniques permit the identification of the Sn2+ lone electron pair (5s2) in the two-dimensional framework, occupying empty space opposite to the four Sn-Br bonds of the pyramidal [SnBr4] coordination polyhedra. Additionally, diffuse reflectance UV-vis spectroscopy unveils an indirect optical gap of approximately ∼3.3 eV, aligning with the calculated value from the B3LYP-DFT method (∼3.2 eV). The material exhibits a positive Seebeck coefficient as high as 6.5 × 104 μV K-1 at 350 K, which evolves down to negative values of -3.0 × 103 μV K-1 at 550 K, surpassing values reported for other halide perovskites. Notably, the thermal conductivity remains exceptionally low, between 0.32 and 0.25 W m-1 K-1.

Sizeable square CsPb2Br5 nanosheets for photodetection

All-inorganic perovskites have garnered significant attention in optoelectronics. Herein, square CsPb2Br5 nanosheets, with lateral dimensions of up to 200 μm and a thickness of less than 50 nm, were successfully synthesized via a straightforward aqueous method using HBr as a morphology-tailoring agent. A photodetector composed of a single nanosheet was subsequently fabricated and exhibited remarkable photodetection capabilities, demonstrating a detectivity of 5.98 × 109 Jones. These findings offer new perspectives on the synthesis and utilization of CsPb2Br5 and other perovskite nanostructures in optoelectronic devices.

Water-stable perovskite CsPb2Br5/CdSe quantum dot-based photoelectrochemical sensors for the sensitive determination of dopamine

A heterojunction of CdSe quantum dots in situ grown on the perovskite CsPb2Br5 (CsPb2Br5/CdSe) for water-stable photoelectrochemical (PEC) sensing was simply synthesized using the hot-injection method. Due to the inherent built-in electric field and the matching band structure between CsPb2Br5 and CdSe, the CsPb2Br5/CdSe p-n heterojunction demonstrates enhanced photoelectrochemical properties. Accelerated interfacial charge transfer and increased electron-hole pair separation enable hydrolysis-resistant CsPb2Br5/CdSe sensors to exhibit heightened sensitivity with an ultra-low detection limit (0.0124 μM) and a wide linear range (0.4-303.9 μM) in subsequent dopamine detection. Moreover, the CsPb2Br5/CdSe sensors show excellent anti-interference ability, as well as remarkable stability and reproducibility in water solvent. It is noteworthy that this work is conducted in an aqueous environment, which provides an inspiring and convenient way for photoelectric and photoelectrocatalysis applications based on water-resistant perovskites.

Parylene-Sealed Perovskite Nanocrystals Down-Shifting Layer for Luminescent Spectral Matching in Thin Film Photovoltaics

The present contribution aims to enhance solar cells' performance via the development of advanced luminescent down-shifting based on encapsulated nanostructured perovskite materials. Here, thin films of inorganic lead halide (CsPbBr₃) perovskite nanocrystal luminophores were synthetized, by hot-injection, deposited on glass substrates by spin-coating, and encapsulated with parylene type C, via chemical vapor deposition, to protect and stabilize the films. The optical properties of these thin films were characterized by absorption, emission and 2D contour spectra, their structure by X-ray diffraction and X-ray photoelectron spectroscopy, and the morphology by Scanning Transmission Electron microscopy. I-V curve and spectral response nanocrystalline silicon photovoltaic (nc-Si:H PV) cells were studied in the absence and presence of the perovskite and parylene luminescent down-shifting layers. The incorporation of the CsPbBr₃ nanocrystals and their encapsulation with the parylene type C polymeric coating led to an increase in the current generated and the spectral response of the PV cells in the regime of the nanocrystals' fluorescence emission. A 3.1% increase in the short circuit current density and a 5.6% increase in the power conversion efficiency were observed.

Coupling Localized Laser Writing and Nonlocal Recrystallization in Perovskite Crystals for Reversible Multidimensional Optical Encryption

The ability to generate and manipulate photoluminescence (PL) with high spatial resolution has been of primary importance for applications in micro-optoelectronics, while the emerging metal halide perovskites offer novel material platforms where diverse photonic functionalities and fine structuring are constantly explored. Herein, micro-PL patterns consisting of highly luminescent CsPbBr₃ nanocrystals (NCs) in nonluminescent perovskite crystals are directly fabricated by focused femtosecond laser irradiation. Further modulation with a moisture field leads to the selective dissolution of the laser-destabilized perovskite structures as revealed by density functional theory simulations, thus allowing for facile control of the reversible PL from the recrystallization of moisture-induced CsPbBr₃ NCs. By leveraging the coupled laser writing and moisture modulation, multimodal information encryption is realized by reversible encryption-reading and repeatable erasing-refreshing. This optical storage mechanism is also extended to 3D and 4D by realizing spatially and temporally resolved optical encryption. The coupled multifield modulation on perovskite crystals can enable potential applications in optical storage and encryption, and offer a novel solution for the creation and manipulation of localized PL structures with high temporal and spatial resolutions.

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.

Fabry-Perot Mode-Limited High-Purcell-Enhanced Spontaneous Emission from In Situ Laser-Induced CsPbBr3 Quantum Dots in CsPb2Br5 Microcavities

The patterned metal halide perovskites exhibit novel photophysical properties and high performance in photonic applications. Here, we show that a UV continuous wave laser can induce in situ crystallization of individual and patterned CsPbBr₃ quantum dots (QDs) inside the CsPb₂Br₅ microplatelets. The microplatelet acts as a natural Fabry-Perot cavity and causes the high-Purcell-effect-enhanced (by 287 times) cavity mode spontaneous emission of the embedded CsPbBr₃ QDs. The luminescence exhibits a superlinear emission intensity-excitation intensity relation I(p) ∝ p^2.83, and the exponent is much bigger than that of the free-space exciton spontaneous emission, suggesting arising of stimulated emission at higher photon concentrations. These laser-driven crystallized and patterned cavity mode luminescent perovskite QDs in a waterproof wider-bandgap perovskite microcavity act as an ideal platform for studying the cavity quantum electrodynamics phenomena and for applications in information storage and encryption, anticounterfeiting, and low-threshold lasers.

Assembling an Affinal 0D CsPbBr3/2D CsPb2Br5 Architecture by Synchronously In Situ Growing CsPbBr3 QDs and CsPb2Br5 Nanosheets: Enhanced Activity and Reusability for Photocatalytic CO2 Reduction

Conversion of CO₂ into valuable chemical feedstocks through artificial photosynthesis is an effective strategy to alleviate energy and environmental issues. Herein, we have developed a novel perovskite-based catalyst via in situ growing CsPbBr₃ quantum dots (QDs) on the affinal 2D CsPb₂Br₅ nanosheets for CO₂ photoconversion. CsPbBr₃ QDs were generated by peeling off layers from their cubic counterpart; meanwhile, CsPb₂Br₅ nanosheets were formed by heaping up the peeled layers. The resultant dual-phase composite exhibited outstanding activity and selectivity for photocatalytic conversion of gaseous CO₂ with a CO generation rate of 197.11 μmol g^-1 h^-1 under 300 W Xe lamp irradiation, which is 2.5 and 1.1 times higher than that of pure CsPb₂Br₅ or CsPbBr₃. Importantly, the fabricated dual-phase material presented extremely high stability and was able to maintain an unchangeable CO₂ conversion rate under wet air in the consecutive 10 h of recycling test. Furthermore, attributing to the in situ assembling strategy, the close contact allowed photo-generated electrons in CsPbBr₃ QDs to transfer rapidly to CsPb₂Br₅, and the affluent active sites in such an architecture enabled achieving enhanced CO₂ photoconversion activity. The present work provides an attractive approach for in situ constructing a consubstantial perovskite-based composite photocatalyst to ensure great stability and excellent activity for artificial photocatalytic CO₂ conversion.

Highly efficient radiative recombination in intrinsically zero-dimensional perovskite micro-crystals prepared by thermally-assisted solution-phase synthesis

Zero-dimensional (0D) quantum confinement can be achieved in perovskite materials by the confinement of electron and hole states to single PbX₆⁴⁻ perovskite octahedra. In this work, 0D perovskite (Cs₄PbBr₆) micro-crystals were prepared by a simple thermally-assisted solution method and thoroughly characterized. The micro-crystals show a high level of crystallinity and a high photoluminescence quantum yield of 45%. The radiative recombination coefficient of the 0D perovskite micro-crystals, 1.5 × 10⁻⁸ s⁻¹ cm³, is two orders of magnitude higher than that of typical three-dimensional perovskite and is likely a strong contributing factor to the high emission efficiency of 0D perovskite materials. Temperature dependent luminescence measurements provide insight into the role of thermally-activated trap states. Spatially resolved measurements on single 0D perovskite micro-crystals reveal uniform photoluminescence intensity and emission decay behaviour suggesting the solution-based fabrication method yields a high-quality and homogenous single-crystal material. Such uniform emission reflects the intrinsic 0D nature of the material, which may be beneficial to device applications.

0D Cs₄PbBr₆ perovskite microcrystals exhibit a radiative recombination coefficient two orders of magnitude higher than typical 3D perovskite.

All-Inorganic Perovskite CsPb2Br5 Nanosheets for Photodetector Application Based on Rapid Growth in Aqueous Phase

All-inorganic cesium lead-halide perovskites exhibit a great development prospect in optoelectronic devices owing to their stability and remarkable optoelectronic properties. Herein, we investigate the solution-processed synthesis of perovskite CsPb₂Br₅ nanosheets by using aqueous and ethanol as solvents. The results show that the aqueous environment ensures the phase formation of CsPb₂Br₅ and that the supersaturated solution in ethanol boosts nucleation of the nanosheets. The substrate temperature is the key factor for the evolution of morphology and the variation of the thickness of CsPb₂Br₅ nanosheets. Lower substrate temperature (<35 °C) is conducive to the formation of evenly distributed nanosheets with less stacking. The spatial and time-resolved fluorescence spectra indicate the heterogeneity of the defect density and the recombination process in different nanosheet regions. The photodetector based on the prepared CsPb₂Br₅ nanosheet displays an excellent switching current ratio (9 × 10²), a short rise and decay time (43 and 83 ms, respectively), and good stability (75% of the initial current after 90 days in air). In addition, the mechanical stability and flexibility of the photodetector on the flexible substrate are investigated for 500 bending cycles.

In Situ Growth of 3D/2D (CsPbBr3/CsPb2Br5) Perovskite Heterojunctions toward Optoelectronic Devices

Two-dimensional (2D) CsPb₂Br₅ exhibits intriguing functions in enhancing the performance of optoelectronic devices in terms of environmental stability and luminescence properties when composited with other perovskites in different dimensionalities. We built a type I three-dimensional (3D) CsPbBr₃/2D CsPb₂Br₅ heterojunction through phase transition where CsPbBr₃ quantum dots in situ grew into 2D CsPb₂Br₅. A thorough growth mechanism study in combination with excited state dynamic investigations via femtosecond spectroscopy and first-principles calculations revealed that the type I hierarchy enhanced the stability of the heterojunction and spurred its luminous quantum yield by prolonging the lifetime of photogenerated carriers. Mixing the heterojunction with other phosphors yielded white-light-emitting diodes with a color rendering index of 94%. The work thus not only offered one new avenue for building heterojunctions by using the "soft crystal" nature of perovskites but also disentangled the enhanced luminescence mechanism of the heterojunction that can be harnessed for promising applications in the luminescence and display fields.

Curing the fundamental issue of impurity phases in two-step solution-processed CsPbBr3 perovskite films

Inorganic lead halide perovskite CsPbBr₃ offers attractive photophysical properties and phase stability for high-performance optoelectronic devices. However, CsPbBr₃ films produced by the classic solution-based two-step method are always accompanied with impurity phases of CsPb₂Br₅ and Cs₄PbBr₆, which represents a major efficiency-limiting factor for future advances of CsPbBr₃-based devices. The challenge lies in the complexity of the Cs-Pb-Br phase system, requiring both spatially and temporally precise control of the precursor stoichiometry during solution-phase growth of CsPbBr₃ films. By adopting 2-methoxyethanol as the solution conversion medium instead of commonly applied methanol, the reaction between CsBr and PbBr₂ can be finely controlled to yield single phase CsPbBr₃ films within a few minutes; extending the solution-conversion step to 24 h does not alter the phase purity of resulting CsPbBr₃ films. The present work paves the way to regulate the crystal growth behaviors of two-step solution-processed CsPbBr₃ films by simple solvent engineering.

Exciton-Polariton Properties in Planar Microcavity of Millimeter-Sized Two-Dimensional Perovskite Sheet

In this work, the self-organized two-dimensional (2D) perovskite crystal sheet was synthesized for exciton-polariton studies. The layered 2D perovskite sheets naturally form Fabry-Pérot (F-P) optical cavities and exhibit evident cavity polariton modes. In the region of F-P cavity polariton modes, an exciton-polariton is formed by the superposition of strongly coupled exciton and photon states and the decreased spacing between two adjacent oscillation modes can be well explained by the formation of exciton polarities. Rabi splitting energy (Ω) evaluated from the energy-wavevector (E-k) dispersion relation of exciton-polariton coupling was about 259 ± 10 meV. For edge emissions of the laminated structure, the thickness-dependent quantum well exerts great influence on the band-gap absorption and emission. This work studies exciton-polariton properties in layered 2D perovskite sheets, making it possible application for polariton devices in the strong light-matter interaction region.

Facile synthesis of a dual-phase CsPbBr3–CsPb2Br5 single crystal and its photoelectric performance

The emerging metal-halide perovskites are promising for next generation optoelectronic devices. Recently, all-inorganic halide perovskites have been developed and show significantly improved stability compared with organic–inorganic hybrid halide perovskites. Here, we report a facile method based on the coffee ring effect of solvents to synthesize dual-phase CsPbBr₃–CsPb₂Br₅ single crystal microsheets for the first time. The prepared dual-phase CsPbBr₃–CsPb₂Br₅ single crystal is composed of a tetragonal crystalline phase of CsPb₂Br₅ and a monoclinic phase of CsPbBr₃ according to X-ray diffraction (XRD) patterns. The sharp XRD peaks indicate the high crystallinity of the as-synthesized dual-phase CsPbBr₃–CsPb₂Br₅ microsheets. CsPbBr₃ is mainly distributed on the edge of the microsheets based on photoluminescence (PL) mapping images. Besides, a photodetector based on the dual-phase CsPbBr₃–CsPb₂Br₅ microsheets exhibits good performance with a high on/off photocurrent ratio of 300 and a photoresponsivity of 2.68 mA W⁻¹. The rise and decay times of the CsPbBr₃–CsPb₂Br₅ microsheet photodetector are around 25.3 ms and 29.6 ms, respectively. The experimental results indicate that the dual-phase CsPbBr₃–CsPb₂Br₅ microsheet could be a good candidate for the fabrication of high-performance micro photodetectors compatible with practical applications.

Dual-phase CsPbBr₃–CsPb₂Br₅ single crystal microsheets were synthesized by a simple method based on the coffee ring effect.

Green Emission Induced by Intrinsic Defects in All-Inorganic Perovskite CsPb2Br5

All-inorganic perovskites with improved stability are expected to be better candidates for optoelectronics, compared to organic-inorganic hybrid perovskites. A new member of all-inorganic perovskites, CsPb₂Br₅, has attracted great attention for its promising applications in optoelectronic devices. However, the origins of the green emission in CsPb₂Br₅ have been actively debated. By using first-principles calculations, we find that Cs_Pb and V_Br are dominant intrinsic defects independent of the growth conditions within the stable region of CsPb₂Br₅. Interestingly, we suggest that individual intrinsic defects do not lead to the green emission of CsPb₂Br₅, while the donor-acceptor pair recombination of Cs_Pb and V_Br possibly does. Our findings provide new insights into the experimental controversy about the green emission and its origins in CsPb₂Br₅ from the perspective of intrinsic defects, which help to extend the application of CsPb₂Br₅ in optoelectronic devices.

Extrinsic Green Photoluminescence from the Edges of 2D Cesium Lead Halides

Since the first report of the green emission of 2D all-inorganic CsPb₂ Br₅ , its bandgap and photoluminescence (PL) origin have generated intense debate and remained controversial. After the discovery that PL centers occupy only specific morphological structures in CsPb₂ Br₅ , a two-step highly sensitive and noninvasive optical technique is employed to resolve the controversy. Same-spot Raman-PL as a static property-structure probe reveals that CsPbBr₃ nanocrystals are contributing to the green emission of CsPb₂ Br₅ ; pressure-dependent Raman-PL with a diamond anvil cell as a dynamic probe further rules out point defects such as Br vacancies as an alternative mechanism. Optical absorption under hydrostatic pressure shows that the bandgap of CsPb₂ Br₅ is 0.3-0.4 eV higher than previously reported values and remains nearly constant with pressure up to 2 GPa in good agreement with full-fledged density functional theory (DFT) calculations. Using ion exchange of Br with Cl and I, it is further proved that CsPbBr_{3- x} X_x (X = Cl or I) is responsible for the strong visible PL in CsPb₂ Br_{5- x} X_x . This experimental approach is applicable to all PL-active materials to distinguish intrinsic defects from extrinsic nanocrystals, and the findings pave the way for new design and development of highly efficient optoelectronic devices based on all-inorganic lead halides.