Aggregation-induced emission in lamellar solids of colloidal perovskite quantum wells

Achieving High Loading Capacity of Perovskite Nanocrystals in Pore-Reamed Metal-Organic Frameworks for Bright Scintillators

Lead halide perovskite nanocrystal (PNC) scintillators featuring a fast decay and a high radiation hardness have garnered significant attention. A high PNC loading is essential to ensure a strong X-ray absorption for scintillator applications, but concentrated PNCs tend to aggregate in the solid state, resulting in significant emission quenching. Employing a dispersion medium offers a promising strategy to produce high-loading PNC solids without agglomeration. Herein, we synthesize CsPbBr3 PNC/metal-organic framework (MOF) nanostructures to achieve high-loading PNCs within MOF hosts. The macroporous cavities of hierarchically porous (HP) MOFs can host more PNCs than the confined nanometer-scale spaces of microporous MOFs. Additionally, the surface-rich structure of MOFs aids in dispersing PNCs, effectively reducing aggregation-induced emission quenching. We find that HP-MOFs can achieve a high PNC loading ratio of 75%, as well as the less-aggregated PNCs. As a result, the PNC/HP-MOF scintillator exhibits a 2.3 times higher light yield than that of the PNC scintillator, primarily resulting from the enhanced luminance efficiency of well-dispersed PNCs. The bright and fast features of nanostructure scintillators enable static and dynamic X-ray imaging for industrial inspection applications. These findings highlight that constructing a high-loading nanostructure is crucial for advancing the X-ray imaging applications of PNC scintillators.

Strategies for Controlling Emission Anisotropy in Lead Halide Perovskite Emitters for LED Outcoupling Enhancement

In the last decade, momentous progress in lead halide perovskite (LHP) light-emitting diodes (LEDs) is witnessed as their external quantum efficiency (ηext) has increased from 0.1 to more than 30%. Indeed, perovskite LEDs (PeLEDs), which can in principle reach 100% internal quantum efficiency as they are not limited by the spin-statistics, are reaching their full potential and approaching the theoretical limit in terms of device efficiency. However, ≈70% to 85% of total generated photons are trapped within the devices through the dissipation pathways of the substrate, waveguide, and evanescent modes. To this end, numerous extrinsic and intrinsic light-outcoupling strategies are studied to enhance light-outcoupling efficiency (ηout). At the outset, various external and internal light outcoupling techniques are reviewed with specific emphasis on emission anisotropy and its role on ηout. In particular, the device ηext can be enhanced by up to 50%, taking advantage of the increased probability for photons outcoupled to air by effectively inducing horizontally oriented emission transition dipole moments (TDM) in the perovskite emitters. The role of the TDM orientation in PeLED performance and the factors allowing its rational manipulation are reviewed extensively. Furthermore, this account presents an in-depth discussion about the effects of the self-assembly of LHP colloidal nanocrystals (NCs) into superlattices on the NC emission anisotropy and optical properties.

Perovskite Light-Emitting Diodes with Quantum Wires and Nanorods

Perovskite materials, celebrated for their exceptional optoelectronic properties, have seen extensive application in the field of light-emitting diodes (LEDs), where research is as abundant as the proverbial "carloads of books." In this review, the research of perovskite materials is delved into from a dimensional perspective, with a focus on the exemplary performance of low-dimensional perovskite materials in LEDs. This discussion predominantly revolves around perovskite quantum wires and perovskite nanorods. Perovskite quantum wires are versatile in their growth, compatible with both solution-based and vapor-phase growth, and can be deposited over large areas-even on spherical substrates-to achieve commendable electroluminescence (EL). Perovskite nanorods, on the other hand, boast a suite of superior characteristics, such as polarization properties and tunability of the transition dipole moment, endowing them with the great potential to enhance light extraction efficiency. Furthermore, zero-dimensional (0D) perovskite materials like nanocrystals (NCs) are also the subject of widespread research and application. This review reflects on and synthesizes the unique qualities of the aforementioned materials while exploring their vital roles in the development of high-efficiency perovskite LEDs (PeLEDs).

Regulating Phase Distribution of Dion-Jacobson Perovskite Colloidal Multiple Quantum Wells Toward Highly Stable Deep-Blue Emission

Metal halide perovskite colloidal quantum wells (CQWs) hold great promise for modern photonics and optoelectronics. However, current studies focus on Ruddlesden-Popper (R-P) phase perovskite CQWs that contain bilayers of monovalent long-chain alkylamomoniums between the separated perovskite octahedra layers. The bilayers are packed back-to-back via weak van der Waals interaction, resulting in inferior charge carrier transport and easier decomposition of perovskite. This report first creates a new type of perovskite colloidal multiple QWs (CMQWs) in the form of Dion-Jacobson (D-J) structure by introducing an asymmetric diammonium cation. Furthermore, the phase distribution is optimized by the synergistic effect of valeric acid and zwitterionic lecithin, finally achieving pure deep-blue emission at 435 nm with narrow full width at half maximum. The diammonium layer in D-J perovskite CMQWs features extremely short width of only ≈0.6 nm, thereby contributing to more effective charge carrier transport and higher stability. Through the continuous photoluminescence (PL) measurement and corresponding theoretical calculation, the higher stability of D-J perovskite CMQWs than that of R-P structural CMQWs is confirmed. This work reveals the inherent superior stability of D-J structural CMQWs, which opens a new direction for fabricating stable perovskite optoelectronics.

Self-assembly of perovskite nanoplates in colloidal suspensions

In recent years, perovskite nanocrystal superlattices have been reported with collective optical phenomena, offering a promising platform for both fundamental science studies and device engineering. In this same avenue, superlattices of perovskite nanoplates can be easily prepared on different substrates, and they too present an ensemble optical response. However, the self-assembly and optical properties of these aggregates in solvents have not been reported to date. Here, we report on the conditions for this self-assembly to occur and show a simple strategy to induce the formation of these nanoplate stacks in suspension in different organic solvents. We combined wide- and small-angle X-ray scattering and scanning transmission electron microscopy to evaluate CsPbBr3 and CsPbI3 perovskite nanoplates with different thickness distributions. We observed the formation of these stacks by changing the concentration of nanoplates and the viscosity of the colloidal suspensions, without the need for antisolvent addition. We found that, in hexane, the concentration for the formation of the stacks is rather high and approximately 80 mg mL-1. In contrast, in decane, dodecane, and hexadecane, we observe a much easier self-assembly of the nanoplates, presenting a clear correlation between the degree of aggregation and viscosity. We, then, discuss the impact of the self-assembly of perovskite nanoplates on Förster resonant energy transfer. Our predictions suggest an energy transfer efficiency higher than 50% for all the donor-acceptor systems evaluated. In particular, we demonstrate how the aggregation of these particles in hexadecane induces FRET for CsPbBr3 nanowires. For the n = 2 nanowires (donor) to the n = 3 nanowires (acceptor), the FRET rate was found to be 4.1 ns-1, with an efficiency of 56%, in agreement with our own predictions.

Self-assembled halide perovskite quantum dots in polymer thin films showing temperature-controlled exciton recombination

Rationally assembled supramolecular structures of organic chromophores or semiconductor nanomaterials show excitonic properties different from individual molecules or nanoparticles. We report polymer-assisted assembly formation and thermal modulation of excitonic recombination in self-assembled formamidinium lead bromide perovskite quantum dots.

Slipping-Free Halide Perovskite Supercrystals from Supramolecularly-Assembled Nanocrystals

Supramolecularly assembled high-order supercrystals (SCs) help control the dielectric, electronic, and excitonic properties of semiconductor nanocrystals (NCs) and quantum dots (QDs). Ligand-engineered perovskite NCs (PNCs) assemble into SCs showing shorter excitonic lifetimes than strongly dielectric PNC films showing long photoluminescence (PL) lifetimes and long-range carrier diffusion. Monodentate to bidentate ligand exchange on ≈ 8 nm halide perovskite (APbX3 ; A:Cs/MA, X:Br/I) PNCs generates mechanically stable SCs with close-packed lattices, overlapping electronic wave functions, and higher dielectric constant, providing distinct excitonic properties from single PNCs or PNC films. From Fast Fourier Transform (FFT) images, time-resolved PL, and small-angle X-ray scattering, structurally and excitonically ordered large SCs are identified. An Sc shows a smaller spectral shift (<35 meV) than a PNC film (>100 meV), a microcrystal (>100 meV), or a bulk crystal (>100 meV). Also, the exciton lifetime (<10 ns) of an SC is excitation power-independent in the single exciton regime 〈N〉<1, comparable to an isolated PNC. Therefore, bidentate-ligand-assisted SCs help overcome delayed exciton or carrier recombination in halide perovskite nanocrystal assemblies or films.

Metal-Halide Perovskite Nanocrystal Superlattice: Self-Assembly and Optical Fingerprints

Self-assembly of nanocrystals into superlattices is a fascinating process that not only changes geometric morphology, but also creates unique properties that considerably enrich the material toolbox for new applications. Numerous studies have driven the blossoming of superlattices from various aspects. These include precise control of size and morphology, enhancement of properties, exploitation of functions, and integration of the material into miniature devices. The effective synthesis of metal-halide perovskite nanocrystals has advanced research on self-assembly of building blocks into micrometer-sized superlattices. More importantly, these materials exhibit abundant optical features, including highly coherent superfluorescence, amplified spontaneous laser emission, and adjustable spectral redshift, facilitating basic research and state-of-the-art applications. This review summarizes recent advances in the field of metal-halide perovskite superlattices. It begins with basic packing models and introduces various stacking configurations of superlattices. The potential of multiple capping ligands is also discussed and their crucial role in superlattice growth is highlighted, followed by detailed reviews of synthesis and characterization methods. How these optical features can be distinguished and present contemporary applications is then considered. This review concludes with a list of unanswered questions and an outlook on their potential use in quantum computing and quantum communications to stimulate further research in this area.

Heterostructural Nanosheets Consisting of Polycyclic Aromatic Hydrocarbon Shields and Layered Perovskite Cores for Optical Image Encryption

Optical image encryption technology, in which the emission on/off can be controlled by using specially appointed wavelengths, is useful in information storage and protection. Herein, we report a family of sandwiched heterostructural nanosheets, consisting of three-layered (n = 3) perovskite (PSK) frameworks in center with two different polycyclic aromatic hydrocarbons [triphenylene (Tp) and pyrene (Py)] in periphery. Both heterostructural nanosheets (Tp-PSK and Py-PSK) exhibit blue emissions under UVA-I irradiation; however, different photoluminescent properties are observed under UVA-II. A bright emission of Tp-PSK is attributed to the fluorescence resonance energy transfer (FRET) from Tp-shield to PSK-core, whereas the observed photoquenching phenomenon in Py-PSK is due to the competitive absorption between Py-shield and PSK-core. We exploited the unique photophysical features (on/off emission) of the two nanosheets in a narrow UV window (320-340 nm) for optical image encrypting.

An all-round AI-Chemist with a scientific mind

The realization of automated chemical experiments by robots unveiled the prelude to an artificial intelligence (AI) laboratory. Several AI-based systems or robots with specific chemical skills have been demonstrated, but conducting all-round scientific research remains challenging. Here, we present an all-round AI-Chemist equipped with scientific data intelligence that is capable of performing basic tasks generally required in chemical research. Based on a service platform, the AI-Chemist is able to automatically read the literatures from a cloud database and propose experimental plans accordingly. It can control a mobile robot in-house or online to automatically execute the complete experimental process on 14 workstations, including synthesis, characterization and performance tests. The experimental data can be simultaneously analysed by the computational brain of the AI-Chemist through machine learning and Bayesian optimization, allowing a new hypothesis for the next iteration to be proposed. The competence of the AI-Chemist has been scrutinized by three different chemical tasks. In the future, the more advanced all-round AI-Chemists equipped with scientific data intelligence may cause changes to the landscape of the chemical laboratory.

Amine-Terminated Carbon Dots Linking Hole Transport Layer and Vertically Oriented Quasi-2D Perovskites through Hydrogen Bonds Enable Efficient LEDs

Close attention to the interfaces of solution-processed metal halide perovskite-based light-emitting devices (LEDs) is crucial for their optimal performance. Solution processing of these devices typically leads to the formation of van der Waals interfaces with a weak connection between different functional layers, leaving great room for improvement in charge transport through strengthening of the interlayer interaction. Here, we have realized a hydrogen-bond-assisted interface that makes use of ultrasmall amine-terminated carbon dots to enhance the interaction between the hole transport layer made of PEDOT:PSS and the hybrid lead bromide perovskite emitting layer, which not only promotes the hole injection efficiency but also orients the quasi-2D perovskite crystals penetrating the vertical direction of the device without any, or very few, horizontal grain boundaries, which has a profound effect on the photophysical and transport properties of the emitting layer. As a result, LEDs based on quasi-2D perovskites show up to 24.5% external quantum efficiency, 80 000 cd m^-2 brightness, and over 5-fold extended longevity.

Tough, stable and self-healing luminescent perovskite-polymer matrix applicable to all harsh aquatic environments

Gelatinous underwater invertebrates such as jellyfish have organs that are transparent, luminescent and self-healing, which allow the creatures to navigate, camouflage themselves and, indeed, survive in aquatic environments. Artificial luminescent materials that can mimic such functionality can be used to develop aquatic wearable/stretchable displays and water-resistant devices. Here, a luminescent composite that is simultaneously transparent, tough and can autonomously self-heal in both dry and wet conditions is reported. A tough, self-healable fluorine elastomer with dipole–dipole interactions is synthesized as the polymer matrix. It exhibits excellent compatibility with metal halide perovskite quantum dots. The composite possesses a toughness of 19 MJ m−3, maximum strain of 1300% and capability to autonomously self-heal underwater. Notably, the material can withstand extremely harsh aqueous conditions, such as highly salty, acidic (pH = 1) and basic (pH = 13) environment for more than several months with almost no decay in mechanical performance or optical properties.

Mechano-optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots

Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr₃) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20-300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors.

Structure and Surface Passivation of Ultrathin Cesium Lead Halide Nanoplatelets Revealed by Multilayer Diffraction

The research on two-dimensional colloidal semiconductors has received a boost from the emergence of ultrathin lead halide perovskite nanoplatelets. While the optical properties of these materials have been widely investigated, their accurate structural and compositional characterization is still challenging. Here, we exploited the natural tendency of the platelets to stack into highly ordered films, which can be treated as single crystals made of alternating layers of organic ligands and inorganic nanoplatelets, to investigate their structure by multilayer diffraction. Using X-ray diffraction alone, this method allowed us to reveal the structure of ∼12 Å thick Cs-Pb-Br perovskite and ∼25 Å thick Cs-Pb-I-Cl Ruddlesden-Popper nanoplatelets by precisely measuring their thickness, stoichiometry, surface passivation type and coverage, as well as deviations from the crystal structures of the corresponding bulk materials. It is noteworthy that a single, readily available experimental technique, coupled with proper modeling, provides access to such detailed structural and compositional information.

Perovskite Quantum Dots with Ultrahigh Solid-State Photoluminescence Quantum Efficiency, Superior Stability, and Uncompromised Electrical Conductivity

All-inorganic perovskite quantum dots (PQDs), potentially applicable to high-performance display technologies, are facing challenges when the superior luminescence properties with high stability and uncompromised electrical conductivity are combined. Here, by introducing hexylamine sulfate and reducing the reaction rate, we managed to optimize the surface sacrificial coating of CsPbBr₃ QDs. As a result, the colloidal PQDs show a photoluminescence quantum efficiency (PLQE) of 95.8% in solution, and an internal PLQE as high as 97.8% in solid-state films. As far as stability is concerned, the PQDs not only show excellent resistance to polar solvent but also can retain over 84% of the initial PL intensity after continuous heating at 100 °C for 60 min. More importantly, the superior stability is achieved without compromising electrical conductivity. The light-emitting diodes made from these PQDs show a current efficiency of 8.9 cd A^-1 with excellent thermal stability.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Organic Lead Halide Nanocrystals Providing an Ultra-Wide Color Gamut with Almost-Unity Photoluminescence Quantum Yield

The most attractive aspect of perovskite nanocrystals (NCs) for optoelectronic applications is their widely tunable emission wavelength, but it has been quite challenging to tune it without sacrificing the photoluminescence quantum yield (PLQY). In this work, we report a facile ligand-optimized ion-exchange (LOIE) method to convert room-temperature spray-synthesized, perovskite parent NCs that emit a saturated green color to NCs capable of emitting colors across the entire visible spectrum. These NCs exhibited exceptionally stable and high PLQYs, particularly for the pure blue (96%) and red (93%) primary colors that are indispensable for display applications. Surprisingly, the blue- and red-emissive NCs obtained using the LOIE method preserved the cubic shape and cubic phase structure that they inherited from their parent NCs, while exhibiting high crystallinity and high color-purity. Together with the parent green-emissive NCs, the obtained blue- and red-emissive NCs provided a very wide color gamut, corresponding to a Digital Cinema Initiatives-P3 of 140% or an International Telecommunication Union Recommendation BT.2020 of 102%. With the superior optical merits of these LOIE-manipulated NCs, a corresponding color conversion luminescence device provided a high external quantum efficiency (10.5%) and extremely high brightness (970 000 cd/m²). This study provides a valid route toward highly stable, extremely emissive, and panchromatic perovskite NCs with potential use in a variety of future optoelectronic applications.

Materials, photophysics and device engineering of perovskite light-emitting diodes

Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

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.

Symmetry Breaking Induced Anisotropic Carrier Transport and Remarkable Thermoelectric Performance in Mixed Halide Perovskites CsPb(I1-xBrx)3

We present a combination of first-principles calculations and the Boltzmann transport theory to understand the carrier transport and thermoelectric performance of mixed halide perovskite alloys CsPb(I_1-xBr_x)₃ with different Br compositions. Our computational results correlate the conduction band splitting in CsPb(I_1-xBr_x)₃ to the significant anisotropy in their carrier transport properties, such as effective masses and deformation potential constants. Such band splitting originates from the symmetry-broken crystal structures of CsPb(I_1-xBr_x)₃ polymorphs: with residue stresses/strains in asymmetric CsPb(I_1-xBr_x)₃, nondegenerate orbitals reconstruct the conduction band and reduce the Pb-halide antibonding character along certain directions. While the Seebeck coefficient (S) and the relaxation time-normalized electrical conductivity (σ/τ) show weak directional anisotropy, the carrier relaxation time (τ) is highly direction-dependent. The reconstruction of the conduction band finally leads to significantly anisotropic and enhanced thermoelectric power factors (PF = S²σ) in CsPb(I_1-xBr_x)₃ compared to those in pure CsPbI₃ and CsPbBr₃, showing anomalous nonlinear alloy behavior. A delicate balance between S²σ and combined measurement of the carrier effective mass and deformation potential constant, m*E_DP, is confirmed. The lattice thermal conductivities of CsPb(I_1-xBr_x)₃ are significantly suppressed compared to those of their pure counterparts due to strong mass disordering and strain fields upon halogen substitution. As a result, symmetry breaking in CsPb(I_1-xBr_x)₃ leads to anisotropy in carrier transport, high PF, and scattered phonon transport (ultralow thermal conductivity), concurrently contributing to their promising thermoelectric figures of merit (ZT) up to 1.7 at room temperature. The principles behind the asymmetry-induced factors would serve as new design concepts to tailor the thermoelectric properties of alloys, mixtures, superlattices, and low-dimensional materials.

Self-Assembly of Perovskite CsPbBr3 Quantum Dots Driven by a Photo-Induced Alkynyl Homocoupling Reaction

Herein, we report the facile growth of three-dimensional CsPbBr₃ perovskite supercrystals (PSCs) self-assembled from individual CsPbBr₃ perovskite quantum dots (PQDs). By varying the carbon chain length of a surface-bound ligand molecule, 1-alkynyl acid, different morphologies of PSCs were obtained accompanied by an over 1000-fold photoluminescence improvement compared with that of PQDs. Systematic analyses have shown, for the first time, that under UV irradiation, CsBr, the byproduct formed during PQDs synthesis, could effectively catalyze the homocoupling reaction between two alkynyl groups, which further worked as a driving force to push forward the self-assembly of PQDs.

Synthetic factors affecting the stability of methylammonium lead halide perovskite nanocrystals

Lead halide perovskite nanocrystals (PNCs) have emerged as promising candidates for use in optoelectronic devices. Significant focus has been directed towards optimising synthetic conditions to obtain PNCs with tunable emission properties. However, the reproducible production of stable PNC dispersions is also crucial for fabrication and scale-up of these devices using liquid deposition methods. Here, the stability of methylammonium lead halide (MAPbX3 where X = Br, I) PNCs produced via the ligand-assisted reprecipitation process is explored. We have focussed on understanding how different combinations of specific synthetic factors - dilution, halide source and ratio as well as capping-ligand concentration - affect the stability of the resultant PNC dispersion. Photoluminescence spectroscopy, transmission electron microscopy and dynamic light scattering studies revealed that subtle changes in the reaction conditions lead to significant changes in the particle morphology and associated optical properties, often with catastrophic consequences on stability. This study highlights the importance of designing PNC dispersions in order to make more efficient and reliable optoelectronic devices.

Scalable photonic sources using two-dimensional lead halide perovskite superlattices

Miniaturized photonic sources based on semiconducting two-dimensional (2D) materials offer new technological opportunities beyond the modern III-V platforms. For example, the quantum-confined 2D electronic structure aligns the exciton transition dipole moment parallel to the surface plane, thereby outcoupling more light to air which gives rise to high-efficiency quantum optics and electroluminescent devices. It requires scalable materials and processes to create the decoupled multi-quantum-well superlattices, in which individual 2D material layers are isolated by atomically thin quantum barriers. Here, we report decoupled multi-quantum-well superlattices comprised of the colloidal quantum wells of lead halide perovskites, with unprecedentedly ultrathin quantum barriers that screen interlayer interactions within the range of 6.5 Å. Crystallographic and 2D k-space spectroscopic analysis reveals that the transition dipole moment orientation of bright excitons in the superlattices is predominantly in-plane and independent of stacking layer and quantum barrier thickness, confirming interlayer decoupling.

Decoupled high-order MQW superlattices are desired to design miniaturized photonic sources but they are yet to be realized in scalable ways. Here Jagielski et al. achieve this goal using multiple-stacked colloidal lead halide perovskite quantum wells separated by atomically thin quantum barriers.

Understanding the Ligand Effects on Photophysical, Optical, and Electroluminescent Characteristics of Hybrid Lead Halide Perovskite Nanocrystal Solids

There has been a tremendous amount of interest in developing high-efficiency light-emitting diodes (LEDs) based on colloidal nanocrystals (NCs) of hybrid lead halide perovskites. Here, we systematically investigate the ligand effects on EL characteristics by tuning the hydrophobicity of primary alkylamine ligands used in NC synthesis. By increasing the ligand hydrophobicity, we find (i) a reduced NC size that induces a higher degree of quantum confinement, (ii) a shortened exciton lifetime that increases the photoluminescence quantum yield, (iii) a lowering of refractive index that increases the light outcoupling efficiency, and (iv) an increased thin-film resistivity. Accordingly, ligand engineering allows us to demonstrate high-performance green LEDs exhibiting a maximum external quantum efficiency up to 16.2%. The device operational lifetime, defined by the time lasted when the device luminance reduces to 85% of its initial value, LT85, reaches 243 min at an initial luminance of 516 cd m^-2.

Carbon Dots with Dual-Emissive, Robust, and Aggregation-Induced Room-Temperature Phosphorescence Characteristics

Carbon dots (CDs) with dual-emissive, robust, and aggregation-induced RTP characteristics are reported for the first time. The TA-CDs are prepared via hydrothermal treatment of trimellitic acid and exhibit unique white prompt and yellow RTP emissions in solid state under UV excitation (365 nm) on and off, respectively. The yellow RTP emission of TA-CDs powder should be resulted from the formation of a new excited triplet state due to their aggregation, and the white prompt emission is due to their blue fluorescence and yellow RTP dual-emissive nature. The RTP emission of TA-CDs powder was highly stable under grinding, which is very rare amongst traditional pure organic RTP materials. To employ the unique characteristics of TA-CDs, advanced anti-counterfeiting and information encryption methodologies (water-stimuli-response producing RTP) were preliminarily investigated.

Light Generation in Lead Halide Perovskite Nanocrystals: LEDs, Color Converters, Lasers, and Other Applications

Facile solution processing lead halide perovskite nanocrystals (LHP-NCs) exhibit superior properties in light generation, including a wide color gamut, a high flexibility for tuning emissive wavelengths, a great defect tolerance and resulting high quantum yield; and intriguing electric feature of ambipolar transport with moderate and comparable mobility. As a result, LHP-NCs have accomplished great achievements in various light generation applications, including color converters for lighting and display, light-emitting diodes, low threshold lasing, X-ray scintillators, and single photon emitters. Herein, the considerable progress that has been made thus far is reviewed along with the current challenges and future prospects in the light generation applications of LHP-NCs.

Inorganic and Layered Perovskites for Optoelectronic Devices

Organic-inorganic halide perovskites are making breakthroughs in a range of optoelectronic devices. Reports of >23% certified power conversion efficiency in photovoltaic devices, external quantum efficiency >21% in light-emitting diodes (LEDs), continuous-wave lasing and ultralow lasing thresholds in optically pumped lasers, and detectivity in photodetectors on a par with commercial GaAs rivals are being witnessed, making them the fastest ever emerging material technology. Still, questions on their toxicity and long-term stability raise concerns toward their market entry. The intrinsic instability in these materials arises due to the organic cation, typically the volatile methylamine (MA), which contributes to hysteresis in the current-voltage characteristics and ion migration. Alternative inorganic substitutes to MA, such as cesium, and large organic cations that lead to a layered structure, enhance structural as well as device operational stability. These perovskites also provide a high exciton binding energy that is a prerequisite to enhance radiative emission yield in LEDs. The incorporation of inorganic and layered perovskites, in the form of polycrystalline films or as single-crystalline nanostructure morphologies, is now leading to the demonstration of stable devices with excellent performance parameters. Herein, key developments made in various optoelectronic devices using these perovskites are summarized and an outlook toward stable yet efficient devices is presented.

Perovskite-Based Artificial Multiple Quantum Wells

Semiconductor quantum well structures have been critical to the development of modern photonics and solid-state optoelectronics. Quantum level tunable structures have introduced new transformative device applications and afforded a myriad of groundbreaking studies of fundamental quantum phenomena. However, noncolloidal, III-V compound quantum well structures are limited to traditional semiconductor materials fabricated by stringent epitaxial growth processes. This report introduces artificial multiple quantum wells (MQWs) built from CsPbBr₃ perovskite materials using commonly available thermal evaporator systems. These perovskite-based MQWs are spatially aligned on a large-area substrate with multiple stacking and systematic control over well/barrier thicknesses, resulting in tunable optical properties and a carrier confinement effect. The fabricated CsPbBr₃ artificial MQWs can be designed to display a variety of photoluminescence (PL) characteristics, such as a PL peak shift commensurate with the well/barrier thickness, multiwavelength emissions from asymmetric quantum wells, the quantum tunneling effect, and long-lived hot-carrier states. These new artificial MQWs pave the way toward widely available semiconductor heterostructures for light-conversion applications that are not restricted by periodicity or a narrow set of dimensions.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Metal Halide Perovskite Nanosheet for X-ray High-Resolution Scintillation Imaging Screens

Scintillators, which are capable of converting ionizing radiation into visible photons, are an integral part of medical, security, and commercial diagnostic technologies such as X-ray imaging, nuclear cameras, and computed tomography. Conventional scintillator fabrication typically involves high-temperature sintering, generating agglomerated powders or large bulk crystals, which pose major challenges for device integration and processability. On the other hand, colloidal quantum dot scintillators cannot be cast into compact solid films with the necessary thickness required for most X-ray applications. Here, we report the room-temperature synthesis of a colloidal scintillator comprising CsPbBr₃ nanosheets of large concentration (up to 150 mg/mL). The CsPbBr₃ colloid exhibits a light yield (∼21000 photons/MeV) higher than that of the commercially available Ce:LuAG single-crystal scintillator (∼18000 photons/MeV). Scintillators based on these nanosheets display both strong radioluminescence (RL) and long-term stability under X-ray illumination. Importantly, the colloidal scintillator can be readily cast into a uniform crack-free large-area film (8.5 × 8.5 cm² in area) with the requisite thickness for high-resolution X-ray imaging applications. We showcase prototype applications of these high-quality scintillating films as X-ray imaging screens for a cellphone panel and a standard central processing unit chip. Our radiography prototype combines large-area processability with high resolution and a strong penetration ability to sheath materials, such as resin and silicon. We reveal an energy transfer process inside those stacked nanosheet solids that is responsible for their superb scintillation performance. Our findings demonstrate a large-area solution-processed scintillator of stable and efficient RL as a promising approach for low-cost radiography and X-ray imaging applications.

Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair

The easily tunable emission of halide perovskite nanocrystals throughout the visible spectrum makes them an extremely promising material for light-emitting applications. Whereas high quantum yields and long-term colloidal stability have already been achieved for nanocrystals emitting in the red and green spectral range, the blue region currently lags behind with low quantum yields, broad emission profiles, and insufficient colloidal stability. In this work, we present a facile synthetic approach for obtaining two-dimensional CsPbBr₃ nanoplatelets with monolayer-precise control over their thickness, resulting in sharp photoluminescence and electroluminescence peaks with a tunable emission wavelength between 432 and 497 nm due to quantum confinement. Subsequent addition of a PbBr₂-ligand solution repairs surface defects likely stemming from bromide and lead vacancies in a subensemble of weakly emissive nanoplatelets. The overall photoluminescence quantum yield of the blue-emissive colloidal dispersions is consequently enhanced up to a value of 73 ± 2%. Transient optical spectroscopy measurements focusing on the excitonic resonances further confirm the proposed repair process. Additionally, the high stability of these nanoplatelets in films and to prolonged ultraviolet light exposure is shown.

The Role of Metal Halide Perovskites in Next-Generation Lighting Devices

The development of smart illumination sources represents a central challenge for current technology. In this context, the quest for novel materials that enable efficient light generation is essential. Metal halide compounds with perovskite crystalline structure (ABX₃) have gained tremendous interest in the last five years since they come as easy-to-prepare high performance semiconductors. Perovskite absorbers are driving the power-conversion-efficiencies of thin film photovoltaics to unprecedented values. Nowadays, mixed-cation, mixed-halide lead perovskite solar cells reach efficiencies consistently over 20% and promise to get close to 30% in multijunction devices when combined with silicon cells at no surcharge. Nonetheless, perovskites' fame extends further since extensive research on these novel semiconductors has also revealed their brightest side. Soon after their irruption in the photovoltaic scenario, demonstration of efficient color tunable-with high color purity-perovskite emitters has opened new avenues for light generation applications that are timely to discuss herein.