Enhanced Exciton and Photon Confinement in Ruddlesden–Popper Perovskite Microplatelets for Highly Stable Low‐Threshold Polarized Lasing

At the heart of electrically driven semiconductors lasers lies their gain medium that typically comprises epitaxially grown double heterostuctures or multiple quantum wells. The simultaneous spatial confinement of charge carriers and photons afforded by the smaller bandgaps and higher refractive index of the active layers as compared to the cladding layers in these structures is essential for the optical‐gain enhancement favorable for device operation. Emulating these inorganic gain media, superb properties of highly stable low‐threshold (as low as ≈8 µJ cm^?2) linearly polarized lasing from solution‐processed Ruddlesden–Popper (RP) perovskite microplatelets are realized. Detailed investigations using microarea transient spectroscopies together with finite‐difference time‐domain simulations validate that the mixed lower‐dimensional RP perovskites (functioning as cladding layers) within the microplatelets provide both enhanced exciton and photon confinement for the higher‐dimensional RP perovskites (functioning as the active gain media). Furthermore, structure–lasing‐threshold relationship (i.e., correlating the content of lower‐dimensional RP perovskites in a single microplatelet) vital for design and performance optimization is established. Dual‐wavelength lasing from these quasi‐2D RP perovskite microplatelets can also be achieved. These unique properties distinguish RP perovskite microplatelets as a new family of self‐assembled multilayer planar waveguide gain media favorable for developing efficient lasers.     

Chemical vapor deposition growth of single-crystalline cesium lead halide microplatelets and heterostructures for optoelectronic applications

Orgaruc-inorganic hybrid halide perovskites,such as CH3NH3PbI3,have emerged as an exciting class of materials for solar photovoltaic applications;however,they are currently plagued by insufficient environmental stability.To solve this issue,all-inorganic halide perovskites have been developed and shown to exhibit significantly improved stability.Here,we report a single-step chemical vapor deposition growth of cesium lead halide (CsPbX3) microcrystals.Optical microscopy studies show that the resulting perovskite crystals predominantly adopt a square-platelet morphology.Powder X-ray diffraction (PXRD) studies of the resulting crystals demonstrate a highly crystalline nature,with CsPbC13,CsPbBr3,and CsPbI3 showing tetragonal,monoclinic,and orthorhombic phases,respectively.Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies show that the resulting platelets exhibit well-faceted structures with lateral dimensions of the order of 10-50 μm,thickness around 1 μm,and ultra-smooth surface,suggesting the absence of obvious grain boundaries and the single-crystalline nature of the individual microplatelets.Photoluminescence (PL) images and spectroscopic studies show a uniform and intense emission consistent with the expected band edge transition.Additionally,PL images show brighter emission around the edge of the platelets,demonstrating a wave-guiding effect in high-quality crystals.With a well-defined geometry and ultra-smooth surface,the square platelet structure can function as a whispering gallery mode cavity with a quality factor up to 2,863 to support laser emission at room temperature.Finally,we demonstrate that such microplatelets can be readily grown on a variety of substrates,including silicon,graphene,and other two-dimensional materials such as molybdenum disulfide,which can readily allow the construction of heterostructure optoelectronic devices,including a graphene/perovskite/ graphene vertically-stacked photodetector with photoresponsivity ＞ 105 A/W.The extraordinary optical properties of CsPbX3 platelets,combined with their ability to be grown on diverse materials to form functional heterostructures,can lead to exciting opportunities for broad optoelectronic applications.     

Two‐Photon Up‐Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi‐2D Perovskite Films

A new approach to generate a two‐photon up‐conversion photoluminescence (PL) by directly exciting the gap states with continuous‐wave (CW) infrared photoexcitation in solution‐processing quasi‐2D perovskite films [(PEA)₂(MA)₄Pb₅Br₁₆ with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two‐photon up‐conversion PL occurring in quasi‐2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two‐photon up‐conversion PL signal. This confirms that the gap states are indeed responsible for generating the two‐photon up‐conversion PL in quasi‐2D perovskites. Furthermore, mechanical scratching indicates that the different‐n‐value nanoplates are essentially uniformly formed in the quasi‐2D perovskite films toward generating multi‐photon up‐conversion light emission. More importantly, the two‐photon up‐conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi‐photon excitation. Polarization‐dependent up‐conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi‐photon up‐conversion light emission in quasi‐2D perovskites.     

Ultrathin CsPbX3 Nanowire Arrays with Strong Emission Anisotropy

1D nanowires of all‐inorganic lead halide perovskites represent a good architecture for the development of polarization‐sensitive optoelectronic devices due to their high absorption efficient, emission yield, and dielectric constants. However, among as‐fabricated perovskite nanowires with the lateral dimensions of hundreds nanometers so far, the optical anisotropy is hindered and rarely explored owing to the invalidating of electrostatic dielectric mismatch in the physical dimensions. Here, well‐aligned CsPbBr₃ and CsPbCl₃ nanowires with thickness T down to 15 and 7 nm, respectively, are synthesized using a vapor phase van der Waals epitaxial method. Strong emission anisotropy with polarization ratio up to ≈0.78 is demonstrated in the nanowires with T < 40 nm due to the electrostatic dielectric confinement. With the increasing of thickness, the polarization ratio remarkably reduces monotonously to ≈0.17 until T ≈140 nm; and further oscillates in a small amplitude owing to the wave characteristic of light. These findings not only represent a demonstration of perovskite‐based polarization‐sensitive light sources, but also advance fundamental understanding of their polarization properties of perovskite nanowires.     

Nanocube Superlattices of Cesium Lead Bromide Perovskites and Pressure‐Induced Phase Transformations at Atomic and Mesoscale Levels

Lead halide perovskites are promising materials for a range of applications owing to their unique crystal structure and optoelectronic properties. Understanding the relationship between the atomic/mesostructures and the associated properties of perovskite materials is crucial to their application performances. Herein, the detailed pressure processing of CsPbBr₃ perovskite nanocube superlattices (NC‐SLs) is reported for the first time. By using in situ synchrotron‐based small/wide angle X‐ray scattering and photoluminescence (PL) probes, the NC‐SL structural transformations are correlated at both atomic and mesoscale levels with the band‐gap evolution through a pressure cycle of 0 ? 17.5 GPa. After the pressurization, the individual CsPbBr₃ NCs fuse into 2D nanoplatelets (NPLs) with a uniform thickness. The pressure‐synthesized perovskite NPLs exhibit a single cubic crystal structure, a 1.6‐fold enhanced photoluminescence quantum yield, and a longer emission lifetime than the starting NCs. This study demonstrates that pressure processing can serve as a novel approach for the rapid conversion of lead halide perovskites into structures with enhanced properties.     

Oxalic Acid Enabled Emission Enhancement and Continuous Extraction of Chloride from Cesium Lead Chloride/Bromide Perovskite Nanocrystals

All‐inorganic cesium lead halide perovskite nanocrystals (NCs) have demonstrated excellent optical properties and an encouraging potential for optoelectronic applications; however, mixed‐halide perovskites, especially CsPb(Cl/Br)₃ NCs, still show lower photoluminescence quantum yields (PL QY) than the corresponding single‐halide materials. Herein, anhydrous oxalic acid is used to post‐treat CsPb(Cl/Br)₃ NCs in order to initially remove surface defects and halide vacancies, and thus, to improve their PL QY from 11% to 89% for the emission of 451 nm. Furthermore, due to the continuous chelating reaction with the oxalate ion, chloride anions from the mixed‐halide CsPb(Cl/Br)₃ perovskite NCs could be extracted, and green emitting CsPbBr₃ NCs with PL QY of 85% at 511 nm emission are obtained. Besides being useful to improve the emission of CsPb(Cl/Br)₃ NCs, the oxalic acid treatment strategy introduced here provides a further tool to adjust the distribution of halide anions in mixed‐halide perovskites without using any halide additives.     

Electro‐Optic Modulation in Hybrid Metal Halide Perovskites

Rapid and efficient conversion of electrical signals to optical signals is needed in telecommunications and data network interconnection. The linear electro‐optic (EO) effect in noncentrosymmetric materials offers a pathway to such conversion. Conventional inorganic EO materials make on‐chip integration challenging, while organic nonlinear molecules suffer from thermodynamic molecular disordering that decreases the EO coefficient of the material. It has been posited that hybrid metal halide perovskites could potentially combine the advantages of inorganic materials (stable crystal orientation) with those of organic materials (solution processing). Here, layered metal halide perovskites are reported and investigated for in‐plane birefringence and linear electro‐optic response. Phenylmethylammonium lead chloride (PMA₂PbCl₄) crystals are grown that exhibit a noncentrosymmetric space group. Birefringence measurements and Raman spectroscopy confirm optical and structural anisotropy in the material. By applying an electric field on the crystal surface, the linear EO effect in PMA₂PbCl₄ is reported and its EO coefficient is determined to be 1.40 pm V^?1. This is the first demonstration of this effect in hybrid metal halide perovskites, materials that feature both highly ordered crystalline structures and solution processability. The in‐plane birefringence and electro‐optic response reveal that layered perovskite crystals could be further explored for potential applications in polarizing optics and EO modulation.     

A Strategy toward New Low‐Dimensional Hybrid Halide Perovskites with Anionic Spacers

The low‐dimensional halide perovskites have received enormous attention due to their unique photovoltaic and optoelectronic performances. Periodic spacers are used to inhibit the growth of 3D perovskite and fabricate a 2D counterpart with layered structure, mostly based on organic/inorganic cations. Herein, by introducing organic anions (e.g., pentanedioic acid (PDA) and hexanedioic acid (HDA) simultaneously), leaf‐shaped (Cs₃Pb₂Br₅)₂(PDA–HDA) microplates with low‐dimensional structure are synthesized. They also exhibit significant photoluminescence (PL) centered at 540 nm with a narrow emission peak. The synthesis of single crystals of Pb(PDA) and Pb(HDA) allows to further clarify the crystal structure of (Cs₃Pb₂Br₅)₂(PDA–HDA) perovskite and its structural evolution mechanism. Moreover, the cooperative introduction of dicarboxylic acid pairs with appropriate lengths is thermodynamically favored for the low‐dimensional perovskite crystallization. The temperature‐dependent PL indicates a V‐shaped Stokes shift with elevated temperature that could be associated with the localization of excitons in the inorganic layers between organic dicarboxylic acid molecules. This work demonstrates low‐dimensional halide perovskite with anionic spacers, which also opens up a new approach to the growth of low‐dimensional organic–inorganic hybrid perovskite crystals.     

Efficient Red Perovskite Light‐Emitting Diodes Based on Solution‐Processed Multiple Quantum Wells

This paper reports a facile and scalable process to achieve high performance red perovskite light‐emitting diodes (LEDs) by introducing inorganic Cs into multiple quantum well (MQW) perovskites. The MQW structure facilitates the formation of cubic CsPbI₃ perovskites at low temperature, enabling the Cs‐based QWs to provide pure and stable red electroluminescence. The versatile synthesis of MQW perovskites provides freedom to control the crystallinity and morphology of the emission layer. It is demonstrated that the inclusion of chloride can further improve the crystallization and consequently the optical properties of the Cs‐based MQW perovskites, inducing a low turn‐on voltage of 2.0 V, a maximum external quantum efficiency of 3.7%, a luminance of ≈440 cd m^?2 at 4.0 V. These results suggest that the Cs‐based MQW LED is among the best performing red perovskite LEDs. Moreover, the LED device demonstrates a record lifetime of over 5 h under a constant current density of 10 mA cm^?2. This work suggests that the MQW perovskites is a promising platform for achieving high performance visible‐range electroluminescence emission through high‐throughput processing methods, which is attractive for low‐cost lighting and display applications.     

Efficient Room‐Temperature Phosphorescence from Organic–Inorganic Hybrid Perovskites by Molecular Engineering

Solution‐processed organic–inorganic hybrid perovskites are promising emitters for next‐generation optoelectronic devices. Multiple‐colored, bright light emission is achieved by tuning their composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room‐temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room‐temperature phosphorescence is activated by designing a mixed‐cation perovskite system to suppress nonradiative recombination. Multiple‐colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room‐temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.     

Room‐Temperature Stimulated Emission and Lasing in Recrystallized Cesium Lead Bromide Perovskite Thin Films

Cesium lead halide perovskites are of interest for light‐emitting diodes and lasers. So far, thin‐films of CsPbX₃ have typically afforded very low photoluminescence quantum yields (PL‐QY < 20%) and amplified spontaneous emission (ASE) only at cryogenic temperatures, as defect related nonradiative recombination dominated at room temperature (RT). There is a current belief that, for efficient light emission from lead halide perovskites at RT, the charge carriers/excitons need to be confined on the nanometer scale, like in CsPbX₃ nanoparticles (NPs). Here, thin films of cesium lead bromide, which show a high PL‐QY of 68% and low‐threshold ASE at RT, are presented. As‐deposited layers are recrystallized by thermal imprint, which results in continuous films (100% coverage of the substrate), composed of large crystals with micrometer lateral extension. Using these layers, the first cesium lead bromide thin‐film distributed feedback and vertical cavity surface emitting lasers with ultralow threshold at RT that do not rely on the use of NPs are demonstrated. It is foreseen that these results will have a broader impact beyond perovskite lasers and will advise a revision of the paradigm that efficient light emission from CsPbX₃ perovskites can only be achieved with NPs.     

Perovskite Materials for Light‐Emitting Diodes and Lasers

Organic–inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light‐emitting diodes, lasers, and light‐emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light‐emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A^?1 have been achieved. Although perovskite light‐emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light‐emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower‐dimensionality layered and three‐dimensional perovskites, nanostructures, charge‐transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light‐emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light‐emitting devices.     

Tailoring the Performances of Lead Halide Perovskite Devices with Electron‐Beam Irradiation

Lead halide perovskites are intensively studied in past few years due to their potential applications in optoelectronic devices such as solar cells, photodetectors, light‐emitting diodes (LED), and lasers. In addition to the rapid developments in material synthesis and device fabrication, it is also very interesting to postsynthetically control the optical properties with external irradiations. Here, the influences of very low energy (10–20 keV) electron beam of standard electron beam lithography are experimentally explored on the properties of lead halide perovskites. It is confirmed that the radiolysis process also happens and it can selectively change the photoluminescence, enabling the direct formation of nanolaser array, microsized light emitter array, and micropictures with an electron beam writer. Interestingly, it is found that discontinuous metallic lead layers are formed on the top and bottom surfaces of perovskite microplate during the radiolysis process, which can act as carrier conducting layers and significantly increase the photocurrent of perovskite photodetector by a factor of 217%. By using the electron beam with low energy to modify the perovskite, this method promises to shape the emission patterns for micro‐LED with well‐preserved optical properties and improves the photocurrent of photodetector.     

Aqueous Phase Exfoliating Quasi‐2D CsPbBr3 Nanosheets with Ultrahigh Intrinsic Water Stability

All‐inorganic cesium lead halide perovskite nanocrystals (NCs) have emerged as attractive optoelectronic materials due to the excellent optical and electronic properties. However, their environmental stability, especially in the presence of water, is still a significant challenge for their further commercialization. Here, ultrahigh intrinsically water‐stable all‐inorganic quasi‐2D CsPbBr₃ nanosheets (NSs) via aqueous phase exfoliation method are reported. Compared to conventional perovskite NCs, these unique quasi‐2D CsPbBr₃ nanosheets present an outstanding long‐term water stability with 87% photoluminescence (PL) intensity remaining after 168 h under water conditions. Moreover, the photoluminescence quantum yields (PLQY) of quasi‐2D CsPbBr₃ NSs is up to 82.3%, and these quasi‐2D CsPbBr₃ NSs also present good photostability of keeping 85% PL intensity after 2 h under 365 nm UV light. Evidently, such quasi‐2D perovskite NSs will open up a new way to investigate the intrinsic stability of all‐inorganic perovskites and further promote the commercial development of perovskite‐based optoelectronic and photovoltaic devices.     

All Inorganic Halide Perovskites Nanosystem: Synthesis,Structural Features,Optical Properties and Optoelectronic Applications

The recent success of organometallic halide perovskites (OHPs) in photovoltaic devices has triggered lots of corresponding research and many perovskite analogues have been developed to look for devices with comparable performance but better stability. Upon the preparation of all inorganic halide perovskite nanocrystals (IHP NCs), research activities have soared due to their better stability, ultrahigh photoluminescence quantum yield (PL QY), and composition dependent luminescence covering the whole visible region with narrow line‐width. They are expected to be promising materials for next generation lighting and display, and many other applications. Within two years, a lot of interesting results have been observed. Here, the synthesis of IHPs is reviewed, and their progresses in optoelectronic devices and optical applications, such as light‐emitting diodes (LEDs), photodetectors (PDs), solar cells (SCs), and lasing, is presented. Information and recent understanding of their crystal structures and morphology modulations are addressed. Finally, a brief outlook is given, highlighting the presently main problems and their possible solutions and future development directions.     

Tunable Dual‐Emission in Monodispersed Sb3+/Mn2+ Codoped Cs2NaInCl6 Perovskite Nanocrystals through an Energy Transfer Process

Double halide perovskites are a class of promising semiconductors applied in photocatalysis, photovoltaic devices, and emitters to replace lead halide perovskites, owing to their nontoxicity and chemical stability. However, most double perovskites always exhibit low photoluminescence quantum efficiency (PLQE) due to the indirect bandgap structure or parity‐forbidden transition problem, limiting their further applications. Herein, the self‐trapped excitons emission of Cs₂NaInCl₆ by Sb‐doping, showing a blue emission with high PLQE of 84%, is improved. Further, Sb/Mn codoped Cs₂NaInCl₆ nanocrystals are successfully synthesized by the hot‐injection method, showing a tunable dual‐emission covering the white‐light spectrum. The studies of PL properties and dynamics reveal that an energy transfer process can occur between the self‐trapped excitons and dopants (Mn²⁺). The work provides a new perspective to design novel lead‐free double perovskites for realizing a unique white‐light emission.     

Hybrid Lead Halide Perovskites for Ultrasensitive Photoactive Switching in Terahertz Metamaterial Devices

The recent meteoric rise in the field of photovoltaics with the discovery of highly efficient solar‐cell devices is inspired by solution‐processed organic–inorganic lead halide perovskites that exhibit unprecedented light‐to‐electricity conversion efficiencies. The stunning performance of perovskites is attributed to their strong photoresponsive properties that are thoroughly utilized in designing excellent perovskite solar cells, light‐emitting diodes, infrared lasers, and ultrafast photodetectors. However, optoelectronic application of halide perovskites in realizing highly efficient subwavelength photonic devices has remained a challenge. Here, the remarkable photoconductivity of organic–inorganic lead halide perovskites is exploited to demonstrate a hybrid perovskite–metamaterial device that shows extremely low power photoswitching of the metamaterial resonances in the terahertz part of the electromagnetic spectrum. Furthermore, a signature of a coupled phonon–metamaterial resonance is observed at higher pump powers, where the Fano resonance amplitude is extremely weak. In addition, a low threshold, dynamic control of the highly confined electric field intensity is also observed in the system, which could tremendously benefit the new generation of subwavelength photonic devices as active sensors, low threshold optically controlled lasers, and active nonlinear devices with enhanced functionalities in the infrared, optical, and the terahertz parts of the electromagnetic spectrum.     

Elucidation of Photoluminescence Blinking Mechanism and Multiexciton Dynamics in Hybrid Organic–Inorganic Perovskite Quantum Dots

Halide perovskites (ABX₃) have emerged as promising materials in the past decade owing to their superior photophysical properties, rendering them potential candidates as solar cells, light‐emitting diode displays, and lasing materials. To optimize their utilization into optoelectronic devices, fundamental understanding of the optical behaviors is necessary. To reveal the comprehensive structure–property relationship, CH₃NH₃PbBr₃ (MAPbBr₃) perovskite quantum dots (PQDs) of three different sizes are prepared by controlling the precipitation temperature. Photoluminescence (PL) blinking, a key process that governs the emission efficiency of the PQD materials, is investigated in detail by the time‐resolved spectroscopic measurements of individual dots. The nature of the generated species in the course of blinking events is identified, and the mechanism governing the PL blinking is studied as a function of PQD sizes. Further, the practical applicability of MAPbBr₃ PQDs is assessed by studying the multiexciton dynamics under high photoexcitation intensity under which most of the display devices work. Ultrafast transient absorption spectroscopy helped in uncovering the volume‐dependent Auger recombination rates, which are further explored by comparing the early‐time transitions related to surface trap states and higher band states.     

Spontaneous Self‐Assembly of Perovskite Nanocrystals into Electronically Coupled Supercrystals: Toward Filling the Green Gap

Self‐assembly of nanoscale building blocks into ordered nanoarchitectures has emerged as a simple and powerful approach for tailoring the nanoscale properties and the opportunities of using these properties for the development of novel optoelectronic nanodevices. Here, the one‐pot synthesis of CsPbBr₃ perovskite supercrystals (SCs) in a colloidal dispersion by ultrasonication is reported. The growth of the SCs occurs through the spontaneous self‐assembly of individual nanocrystals (NCs), which form in highly concentrated solutions of precursor powders. The SCs retain the high photoluminescence (PL) efficiency of their NC subunits, however also exhibit a redshifted emission wavelength compared to that of the individual nanocubes due to interparticle electronic coupling. This redshift makes the SCs pure green emitters with PL maxima at ≈530–535 nm, while the individual nanocubes emit a cyan‐green color (≈512 nm). The SCs can be used as an emissive layer in the fabrication of pure green light‐emitting devices on rigid or flexible substrates. Moreover, the PL emission color is tunable across the visible range by employing a well‐established halide ion exchange reaction on the obtained CsPbBr₃ SCs. These results highlight the promise of perovskite SCs for light emitting applications, while providing insight into their collective optical properties.     

Fluorinated 2D Lead Iodide Perovskite Ferroelectrics

Hybrid perovskite materials are famous for their great application potential in photovoltaics and optoelectronics. Among them, lead‐iodide‐based perovskites receive great attention because of their good optical absorption ability and excellent electrical transport properties. Although many believe the ferroelectric photovoltaic effect (FEPV) plays a crucial role for the high conversion efficiency, the ferroelectricity in CH₃NH₃PbI₃ is still under debate, and obtaining ferroelectric lead iodide perovskites is still challenging. In order to avoid the randomness and blindness in the conventional method of searching for perovskite ferroelectrics, a design strategy of fluorine modification is developed. As a demonstration, a nonpolar lead iodide perovskite is modified and a new 2D fluorinated layered hybrid perovskite material of (4,4‐difluorocyclohexylammonium)₂PbI₄, 1 , is obtained, which possesses clear ferroelectricity with controllable spontaneous polarization. The direct bandgap of 2.38 eV with strong photoluminescence also guarantees the direct observation of polarization‐induced FEPV. More importantly, the 2D structure and fluorination are also expected to achieve both good stability and charge transport properties. 1 is not only a 2D fluorinated lead iodide perovskite with confirmed ferroelectricity, but also a great platform for studying the effect of ferroelectricity and FEPV in the context of lead halide perovskite solar cells and other optoelectronic applications.