Controlling the nucleation and growth kinetics of lead halide perovskite quantum dots

A novel dual-mode biosensing platform based on Au@luminol and CdSe QDs for detection of trace heavy metal ions in PM2.5

Multi-component analysis of PM2.5, especially heavy metal ions, is very important for the study of air pollution. In this work, a novel dual-mode biosensing platform based on Au@luminol and CdSe QDs luminophores was constructed to achieve simultaneous detection of trace Mn2+ and Cd2+ in PM2.5. Interestingly, Au@luminol and CdSe QDs both had excellent fluorescence (FL) and electrochemiluminescence (ECL) performance, which provided feasibility for dual-mode detection. Based on this characteristic, this platform adopted a classical magnetic bead-assisted enzyme cleavage amplification strategy to convert trace Mn2+ and Cd2+ into a large number of Au@luminol and CdSe QDs probes, respectively, producing excellent positive and negative potential ECL signals in the presence of the only co-reactant H2O2. The above two probes were introduced into a and b regions of ITO electrode by DNA hybridization to realize the ECL-spatial-potential resolution and simultaneous detection of Mn2+ and Cd2+. In addition, the above two probes could also be directly used for FL detection of Mn2+ and Cd2+, further improving the detection accuracy. In general, this work focused on heavy metal pollution in atmospheric particulates by using a cleverly designed dual-mode biosensor, which provided a new idea for simultaneous detection of multi-component samples.

Facilitating Energy and Charge Transfer from CsPbBr3 Perovskite Nanocrystals via Ligand Shell Reconstruction

Efficiently extracting photon energy from colloidal lead halide perovskite nanocrystals (PNCs) as excitons and charge carriers is a crucial step in many applications of these materials. We herein report a functionalization strategy based on reconstructing the surface chemical environment of CsPbBr3 PNCs to strengthen the binding of acceptor motifs and, thereby, enhance energy and charge carrier transfer efficiency. A zwitterion ligand, 2-ammonium benzenesulfonate, was employed to protect the integrity of the PNC surface during a purification step for removing excess original synthetic ligands. Heterocyclic-carboxylate structures with strong chelating binding effects were utilized as the anchoring motifs to couple the acceptors to the PNC surface. Compared to directly applying the acceptors to as-synthesized PNCs, the new method achieved at least a 6-fold increase in transportation efficiency for both an oligothiophene triplet energy acceptor and a quinoline-derivative electron acceptor. NMR spectroscopy systematically analyzed the binding conditions of different surface ligands in each step of functionalization. The improved functionalization was attributed to the diminishment of competitive adsorption after the purification step. We identified the N-heterocyclic-carboxylate structure as the most effective anchoring group. Transient absorption spectroscopy was employed to monitor the triplet energy transfer and charge carrier migration processes in the PNC-acceptor complexes and evaluate their rate constants. Spectral and dynamic features for distinguishing the electron transfer process from triplet energy transfer were summarized. Our surface reconstruction strategy will benefit the development of PNC-based optoelectronics and promote the application of perovskite materials as photosensitizers in different photophysical and photochemical processes.

Atomic-Scale Insights into Flexoelectricity and the Enhanced Photovoltaic Effect at the Grain Boundary in Halide Perovskites

Grain boundaries (GBs) generally exist in halide perovskites and are often accompanied by structural distortions or composition segregation, significantly altering their optoelectronic properties. However, the atomic-scale mechanisms underpinning these effects remain elusive due to the inherent complexity of the GB structures. By employing aberration-corrected transmission electron microscopy, we directly visualize the atomic structures of GBs in halide perovskites, uncovering the emergence of flexoelectricity and associated polarization-induced shift-currents. We demonstrate that a large strain gradient at 52° GBs induces significant flexoelectric polarization. This flexoelectricity is observed across GBs with different compositions and misorientation angles. First-principles calculations confirm that such flexoelectric polarization can enhance the photovoltaic effect, resulting in a shift-current of ∼15 μA V-2. These findings uncover a previously unrecognized role of GBs in halide perovskites and provide new insights into leveraging GB engineering to achieve flexoelectricity and regulate optoelectrical properties in halide perovskites.

Aqueous Colloidal Perovskite Quantum Emitters

Aqueous solutions of nanoparticles are the cornerstones for applications in diagnostics, catalysis and more, where control over the nanoparticle's dispersion is pivotal to tailoring the final product properties. Of late, halide perovskite nanocrystals (HPNCs) with outstanding optoelectronic properties emerge as a class of semiconductor nanocrystals distinct from the incumbents. However, HPNCs are particularly susceptible to moisture induced degradation, limiting their utility and regulation in aqueous environments. Here, this hurdle is overcome to realize stable, mono-disperse, highly emissive HPNCs in aqueous environments even under ultra-dilute conditions. These colloidal HPNCs are synthesized via a facile room-temperature structural transformation-induced in situ core-shell self-assembly mechanism in contrast to the widely used pre-core-shell approach. The green HPNCs exhibit >80% photoluminescence quantum yield (PLQY) with excellent water dispersion stability (i.e., zeta potential >80 mV) even after >10,000 h in water. Unprecedented aqueous solution phase single-photon emission with g(2)(0) <0.2 at concentrations as low as ≈0.1 nM is measured. These aqueous HPNCs offer full color tunability that covers the entire Rec. 2020 standard. These findings will lay the foundation for innovative applications of HPNCs in aqueous environments, unlocking new opportunities for nanoscale sensing and optofluidics in photonics, environmental science, and materials engineering.

Nonradiative Energy Transfer between CsPbBr3 Nanoplatelets and AgInS2 Quantum Dots

Nonradiative energy transfer mechanisms, such as Förster resonance energy transfer (FRET), from a donor semiconductor to an acceptor may compete with the detrimental Auger recombination process within the semiconductor, thereby advancing their optoelectronic applications. While the FRET phenomenon has been intensively investigated in numerous perovskite and dye molecule pairs, FRET between thickness-controlled two-dimensional (2D) perovskite nanoplatelets (NPls) and zero-dimensional (0D) quantum dots (QDs) remains elusive. This work reports a thickness-driven FRET between CsPbBr3 NPls and AgInS2 QDs in a 2D/0D nanocomposite. The CsPbBr3/AgInS2 nanocomposite was fabricated using a simple postsynthetic approach and then characterized using X-ray diffraction and transmission electron microscopy. Using steady-state and time-resolved optical spectroscopic investigations, we show extremely efficient FRET between 2-monolayer (ML) CsPbBr3 NPls and AgInS2 QDs, having a FRET efficiency over 90%, while very poor FRET is observed between the 6 ML CsPbBr3/AgInS2 pair. We explain such thickness dependency by considering the spectral overlap between the donor (CsPbBr3) and the acceptor (AgInS2), which can be precisely controlled by altering the NPl thickness. The thickness-driven FRET efficiency between the CsPbBr3 NPls/AgInS2 QDs may lead to the design of a library of 2D perovskite-0D QD donor-acceptor pairs for optoelectronics.

Correlated Lattice Fluctuations in CsPbBr3 Quantum Dots Give Rise to Long-Lived Electronic Coherence

Electronic coherence is central to numerous areas of science, from quantum biology to quantum materials. In quantum materials, lead-halide perovskite (LHP) quantum dots (QDs) have been shown to support electronic coherence through observation of coherent single-photon emission and superfluorescence arising from spatial coherence at low temperatures. In contrast, direct measurement of temporal coherence between exciton states has been lacking. Here, we employ coherent multi-dimensional spectroscopy to observe an electronic coherence between exciton states in CsPbBr3 QDs that is long-lived at room temperature, surviving nearly three times longer than the electronic dephasing time. This observation of a long-lived electronic coherence at room temperature points to nearly perfectly correlated lattice fluctuations for each excitonic state in the superposition. These experiments reveal that the properties of LHP QDs extend to lattice dynamics that give rise to correlated fluctuations in the basis exciton states, a process that may next be optimized by design.

Metal Doping of Strongly Confined Halide Perovskite Nanocrystals under Ambient Conditions

Halide perovskite nanocrystals are promising materials for optoelectronic applications. Metal doping provides an avenue to boost their performance further, e.g., by enhancing light emission, or to provide additional functionalities, such as nanoscale magnetism and polarization control. However, the synthesis of widely size-tunable nanocrystals with controlled doping levels has been inaccessible using traditional hot injection synthesis, preventing systematic studies on dopant effects toward device applications. Here, we report a versatile synthesis method for metal-doped perovskite nanocrystals with precise control over size and doping concentration under ambient conditions. Our room temperature approach results in fully size-tunable isovalent doping of CsPbX3 nanocrystals (X = Cl, Br, I) with various transition metals M2+ tested (M = Mn, Ni, Zn). This gives for the first time access to small, yet precisely doped quantum dots beyond the weak confinement regime reported so far. It also enables a comparative study of the photophysics across multiple size and dopant regimes, where we show dopant-induced localization to dominate over quantum confinement effects. This generalizable, facile synthesis method thus provides a toolbox for engineering perovskite nanocrystals toward light-emitting technologies under industrially relevant conditions.

Controlling the growth kinetics of CsPbX3 nanocrystals through the spatial confinement effect

Controlling CsPbX3 (X = Cl, Br, I) nanocrystal growth is challenging due to subsecond ionic metathesis. Here, spatial confinement at the hexane-acetonitrile interface enables precise control of kinetics, achieving ultra-wide PL tunability (434-520 nm for CsPbBr3, 541-662 nm for CsPbI3) from blue-violet to green and yellow-green to deep red.

Photon Management Through Energy Transfer in Halide Perovskite Nanocrystal-Dye Hybrids: Singlet vs Triplet Tuning

ConspectusPhotoinduced energy and electron transfer processes offer a convenient way to convert light energy into electrical or chemical energy. These processes remain the basis of operation of thin film solar cells, light emitting and optoelectronic devices, and solar fuel generation. In many of these applications, semiconductor nanocrystals that absorb in the visible and near-infrared region are the building blocks that harvest photons and initiate energy or electron transfer to surface-bound chromophores. Such multifunctional aspects make it challenging to steer the energy transfer pathway selectively. Proper selection of the semiconductor nanocrystal donor requires consideration of the nanocrystal bandgap, along with the alignment of valence and conduction band energies relative to that of the acceptor, in order to achieve desired output of energy or electron transfer.In this Account, we focus on key aspects of managing energy flow from excited semiconductor nanocrystals to surface-bound chromophores. The singlet and triplet characteristics of the semiconductor nanoparticle enable tuning of energy transfer pathways through bandgap engineering. In addition to the alignment of energy levels between the semiconductor donor and the singlet/triplet energy levels of the acceptor dye, other parameters such as spectral overlap, surface binding through functional groups, and rate of competing energy transfer pathways all play integral roles in directing energy transfer. For example, in a prototypical halide perovskite nanocrystal-rhodamine dye assembly, singlet energy transfer is observed when the donor is a high-bandgap semiconductor (e.g., CsPbBr3, Eg= 2.47 eV). However, when the donor is a low-bandgap semiconductor (e.g., CsPbI3, Eg = 1.87 eV), one observes only triplet energy transfer. Tuning of the donor bandgap with mixed halide perovskites (e.g., CsPb(BrxI1-x)3) allows for populations of both singlet and triplet excited states of the acceptor dye. Additionally, triplet characteristics of the donor semiconductor nanocrystal can be further enhanced through Mn doping which places low-energy triplet-active states within the nanocrystal donor.The ability to steer energy transfer pathways in a semiconductor nanocrystal-dye assembly finds its use in the design of semiconductor-multichromophoric films. Such hybrid films can down-shift or up-convert incident photons and deliver emission at desired wavelengths. By selecting high energy donor (e.g., CsPbBr3) one can down-shift the incident photons through energy transfer cascade, as in the case of the CsPbBr3-rubrene-tetraphenyldibenzoperiflanthene (DBP) system to populate singlet excited DBP (perylene derivative). On the other hand, when the donor energy is low as in the case of CsPbI3-rubrene-DBP, one can populate singlet DBP via triplet-triplet annihilation. Thus, by steering energy transfer pathways, it is possible to manage the photon flow and obtain desired emission output. Fundamental understanding of excited state processes responsible for energy transfer will assist in designing light harvesting assemblies that can manage photon delivery effectively in display devices and other optoelectronic devices.

Concentration Dependent Modulation in Optoelectronic Traits of Self-Collated CsPbBr3 Perovskites

Self-collation of perovskite nanocrystals into superstructures of larger length scales has been growing in research interest due to their dramatically enhanced performance in various nano-devices, modulating their optical and electrical traits. Herein, the unique concentration-dependent self-assembly of phenethylamine (PEA)-capped CsPbBr3 (PCPB) perovskites spanning a size range of nano to micron level without structural phase alteration is infered. By optimizing various synthetic parameters like PEA amount, and solvents, the self-coalescence in PCPB crystal growth is controlled. Furthermore, the highest-concentrated PCPB (C5) has improved the charge transfer (CT) efficiency to 1,4-Napthoquinone (NPQ), corroborated with stronger binding between C5 and NPQ, compared to the lowest-concentrated PCPB (C1). Incorporating NPQ into such concentration-dependent PCPB enhances their local conductance unveiling the CT-induced current rise, while the detrimental insulating property of PEA molecules reduces the conductance in C5 compared to C1. These outcomes offer a foundation for tailoring the properties of self-assembled perovskites for optoelectronic devices and energy conversion technologies.

Experimental Measurement of Particle-to-Particle Heat Transfer in Nanoparticle Solids

Thermal conductivity in nanoparticle solids has been previously reported in the range of 0.1-1 W m-1 K-1, which is a much smaller variation than the orders of magnitude differences achievable in electrical conductivity of similar systems. Both the low absolute magnitude of thermal conductivity and the relative insensitivity compared to electrical conductivity may be largely attributed to the poor interfacial thermal conductance of the many interfaces of the nanocrystal solid, but a direct experimental study of these interfaces is challenging. Here, we overcome this challenge via direct spectroscopic observation of heat flow within the components of a nanocrystal solid. These thermal transfers are studied by mixing two distinct types of particles: one that serves as a selectively excited antenna to inject heat and the other as the thermal acceptor to report the time-dependent change in temperature. Using transient spectroscopy, the equilibration between the heat donor and heat acceptor is observed to require ∼300 ps at room temperature, speeds up at reduced temperature, and has only weak sensitivity to the relative stoichiometry of the components or the intervening ligands. These results contrast strongly with the 10-20 ps time-scale of through-bond heat transfer at the ligand-particle surface and highlight the substantially lower interfacial thermal conductance of particle-to-particle transport without covalent bonding. It is also found, serendipitously, that the mixed composite films show an unexpected, substantially enhanced nonlinear absorption at the resonant wavelength of the plasmonic particles, which is tentatively attributed to a local field enhancement.

Tailoring Substitutional Sites for Efficient Lanthanide Doping in Lead-Free Perovskite Nanocrystals with Enhanced Near-Infrared Photoluminescence

The incorporation of rare earth lanthanide ions (Ln3+) into lead-free halide perovskite nanocrystals (NCs) is an effective and promising strategy to expand their optical, magnetic, and electrochemical properties. Herein, we designed and synthesized various Ln3+ (including Yb3+, Er3+, and Nd3+), doped Sb3+- or Bi3+-based and Sb3+/Bi3+ alloyed lead-free perovskite NCs, including vacancy-induced perovskite (A3B(III)2X9), double perovskite (A2B(I)B (III)X6), and layered-double perovskite (A4B(II)B(III)2X12) NCs with different energy transfer pathways to study the Ln3+ dopant photoluminescence (PL). While a small size mismatch between dopant ions and host substitutional sites are critical for efficient doping of many first-row transitional metal ion doped metal chalcogenides, surprisingly, the Ln3+ ions, including the large Nd3+ ions (112 pm), prefer smaller isovalent Sb(III) octahedral (Oh) sites (90 pm) instead of Bi(III) Oh sites (117 pm) in these lead-free perovskite NCs. Significantly, similar substitutional site-dependent Ln3+ doping efficiencies were obtained across all three different perovskite host lattices, despite differences in host-to-dopant energy transfer mechanisms, which can provide strong evidence of the preferred Sb3+ substitutional sites for lanthanide dopants in these lead-free perovskite lattices. The efficient Ln3+ doping in Sb3+-rich perovskite NCs leads to enhanced Ln3+ ion PL of the doped NCs. The preference of smaller Sb (III) over Bi(III) substitutional sites for Ln3+ dopants is attributed to the relatively high polarizabilities of lanthanide ions and the smaller cationic sites inside [SbX6]3- compared with [BiX6]3- octahedra. This study provides a fundamental understanding of Ln3+ doping behavior in lead-free perovskite NCs and opportunities for designing efficient Ln3+-doped functional materials by tuning the microenvironment of the host lattice for enhanced properties.

Coherent Multidimensional Spectroscopy Reveals Hot Exciton Cooling Landscapes in CsPbBr3 Quantum Dots

Hot exciton relaxation dynamics is one of the main processes in quantum dots (QD), conferring their functions in optoelectronic devices spanning photovoltaics and solar fuel generation to light emitting diodes, lasers, and quantum light sources. The challenge has been to monitor energy relaxation dynamics in parallel with resolution of excitation or excess energy. Here, we exploit the unique capacity of Coherent Multi-Dimensional Spectroscopy (CMDS) to provide the first observation of the hot exciton cooling landscape of a large size range of CsPbBr3 lead halide perovskite QD, notable for their impact on optoelectronic devices, as well as their strong and unique exciton-lattice coupling. The CMDS data reveal that the hot exciton relaxation landscape is a complex function of the energy. Ab initio quantum dynamics simulations rationalize the observed behavior through energy dependent nonadiabatic exciton-phonon coupling. This first observation of cooling landscapes in QD suggests that materials science that either accelerates or slows hot exciton cooling can better be understood as a landscape to optimize for applications.

Ligand-Engineered Hydrophilic Perovskite Enabling Surface Potential-Driven Anions Exchange for Multicolor Biosensing

The difficulty in designing zwitterionic ligands impedes the water-dispersed CsPbX3 perovskite nanocrystals (NCs) and their application as fast anion exchange (FAE) probes in biosensing. This study proposes a design paradigm for zwitterionic ligands predicated upon revealing the mechanism of the SN2 reaction between unsaturated alkylamines (Cn') and haloalkanoic acids (HAAs). Among them, the C=C bond can enhance the nucleophilicity of Cn' and promote the electrostatic adsorption of HAAs onto Cn', i.e., the geometric preorganization process, thereby initiating the SN2 reaction. Moreover, an appropriate "bridge" length enables HAAs to balance the geometric preorganization process and the Sigma hole intensity of the C-Br bond. Zwitterionic ligands derived from oleylamine (C18') and 5-bromovaleric acid (5-BVA) endow CsPbBr3 NCs with water dispersibility, an almost 100 % photoluminescence quantum yield, and enhanced surface potential, facilitating the capture of halide ions and driving the FAE reaction. Using AgI nanoparticles (NPs) as latent anion exchangers, a third FAE strategy is presented for multicolor biosensing. Such a robust biosensing strategy can generate wavelength shift and chromatic difference for biological target molecules, exemplified by H2S, and is ultimately applicable to multicolor assay in biological, environmental and food samples, demonstrating the immense potential of perovskite-based FAE probes in biosensing.

Emissive Traps Lead to Asymmetric Photoluminescence Line Shape in Spheroidal CsPbBr3 Quantum Dots

The morphology of quantum dots plays an important role in governing their photophysics. Here, we explore the photoluminescence of spheroidal CsPbBr3 quantum dots synthesized via the room-temperature trioctylphosphine oxide/PbBr2 method. Despite photoluminescence quantum yields nearing 100%, these spheroidal quantum dots exhibit an elongated red photoluminescence tail not observed in typical cubic quantum dots synthesized via hot injection. We explore the origins of this elongated red tail through structural and optical characterization including small-angle X-ray scattering, transmission electron microscopy and time-resolved, steady-state, and single quantum dot photoluminescence. From these measurements we conclude that the red tail originates from emissive traps. We show that treating spheroidal quantum dots with phenethylammonium bromide decreases the line shape asymmetry and increases passivation-consistent with emissive traps due to polar facets.

Eco-friendly synthesis of Cs3Bi2Br9 perovskite quantum dots using castor oil as solvent and ligand for leather anti-counterfeiting

All-inorganic cesium bismuth bromide perovskite quantum dots(Cs3Bi2Br9 PQDs) have emerged as promising alternatives to lead-based perovskite quantum dots with excellent performance due to their low toxicity, drawing extensive attention over recent decades. However, challenges remain in terms of their stability and the reliance on organic solvents during the preparation process. Herein, a novel synthesis method for Cs3Bi2Br9 PQDs is introduced, utilizing eco-friendly castor oil as both the solvent and ligand (CO-Cs3Bi2Br9). These PQDs display a vivid blue emission at 430 nm, with an impressive photoluminescence quantum yield (PLQY) of 21.2%. Furthermore, they maintain 97.3% of their fluorescence intensity after 72 h of environmental exposure. The effects of various components of castor oil, including ricinoleic, oleic and linoleic acid, on crystal growth and properties of the Cs3Bi2Br9 are investigated. Significantly, the presence of conjugated double bonds in linoleic acid, when used as a solvent, results in a PLQY of up to 53% for the synthesized Cs3Bi2Br9 PQDs. Moreover, CO-Cs3Bi2Br9 PQDs are introduced into leather by a layer-by-layer self-assembly method, the bright blue fluorescent pattern can be observed in the CO-Cs3Bi2Br9/leather under ultraviolet irradiation, indicating the leather anti-counterfeiting potential of CO-Cs3Bi2Br9 PQDs. This study opens a novel pathway for the sustainable synthesis of PQDs through the utilization of castor oil-derived natural green solvents.

CsPbBr3 Superstructures with Circularly Polarized Photolumines-Cence Obtained by the Self-Assembly and Annealing of Nanoclusters

We report a two-step approach to fabricate CsPbBr3 superstructures with strongly circularly polarized photoluminescence by self-assembly of nanoclusters on a substrate, followed by their annealing. In the first step, the nanoclusters self-assemble upon solvent evaporation, a process that forms mesoscopic superstructures whose geometrical arrangement at the μm-scale confers them optical chirality. In the second step, mild annealing of such superstructures induces the coalescence of the nanoclusters, accompanied by a continuous red shift of the photoluminescence up to 530 nm, with preservation of the μm-scale wires bundles and the chiral properties of the sample (glum=0.1). The successful chirality transfer from the initial nanoclusters assemblies to these final CsPbBr3 superstructures provides a convenient way to obtain circularly polarized emitters.

Studying the effect of dimensions and spacer ligands on the optical properties of 2D lead iodide perovskites

In recent years, metal-halide perovskites (MHPs) have emerged as highly promising optoelectronic materials based on their exceptional properties and versatility in applications such as solar cells, light-emitting devices, and radiation detectors. This study investigates the optical properties of two-dimensional (2D) MHPs, with the Ruddlesden-Popper structure, comparing three morphologies-bulk poly-crystals, colloidal nanoplatelets (NPs), and thin films, aiming to bridge between the bulk and nano dimensionalities. By synthesizing bulk 2D MHPs using long alkyl ammonium spacers, typically found in colloidal systems, and NPs using shorter ligands suitable for bulk growth, we elucidate the relationship between these materials' structural modifications and optical characteristics. We propose the existence of two regions in these 2D MHPs, which differ in their optoelectronic properties and are associated with "bulk" and "surface" regions. Specifically, for poly-crystals, we observe the appearance of a lower energy "bulk" phase associated with the stacking of many 2D sheets, apparent both in absorption and photoluminescence. For NPs, this stacking is hindered, and hence, only the "surface" phase exists. With the elongation of the spacer chain, the poly-crystal becomes more similar to the NPs. For thin films, an interesting phenomenon is observed - the rapid film formation mechanism forces a more colloid-like structure for the shorter ligands and a more poly-crystal-like structure for the longer ones. Overall, this study bridging the different dimensions of 2D MHPs may support new possibilities for future research and development in this innovative field.

Fluoride Ion Passivation of CsPbBr3 Nanocrystals at Room Temperature for Highly Efficient and Stable White Light-Emitting Diodes

Inorganic halide perovskite nanocrystals (NCs) are regarded as promising emitters for light-emitting diodes due to their bright and narrow emission. However, surface defects often result in trap states and ion migration, which remains a huge challenge for high-quality perovskite NCs. Herein, fluoride ions are introduced into CsPbBr3 perovskite NCs at room temperature through the chelation of ligands. Experimental results demonstrate that these fluoride ions from inorganic salts can improve the average lifetime and crystallinity of CsPbBr3 NCs. Meanwhile, the resulting photoluminescence quantum yield is optimized up to 99.02%, and it has high stability to water, heat, and ultraviolet light. Density functional theory calculations show that fluoride ions have a higher binding energy compared to other ligands, which not only removes the electron trapping center but also increases the halogen ion migration energy. By mixing green-emission CsPbBr3 NCs and red-emission K2SiF6:Mn4+ phosphors on a blue chip, the fabricated white light emitting diode shows a high luminous efficiency of 147.8 lm/W, a wide color gamut (129% for NTSC), and CIE coordinates of (0.3160, 0.3051). Furthermore, the photoluminescence intensity decreased by only 2.9% after 48 h of continuous operation.

Strategies for Stabilizing Metal Halide Perovskite Light-Emitting Diodes: Bulk and Surface Reconstruction of Perovskite Nanocrystals

Light-emitting colloidal lead halide perovskite nanocrystals (PeNCs) are considered promising candidates for next-generation vivid displays. However, the operational stability of light-emitting diodes (LEDs) based on PeNCs is still lower than those based on polycrystalline perovskite films, which requires an understanding of defect formation in PeNCs, both inside the crystal lattice ("bulk") and at the surface. Meanwhile, uncontrollable ion redistribution and electrochemical reactions under LED operation can be severe, which is also related to the bulk and surface quality of PeNCs, and a well-designed device architecture can boost carrier injection and balance radiative recombination. In this review, we consider bulk and surface reconstruction of PeNCs by enhancing the crystal lattice rigidity and rationally selecting the surface ligands. Degradation pathways of PeNCs under applied voltage are discussed, and strategies are considered to avoid both undesirable ion migration and electrochemical reactions in the PeNC films. Subsequently, other critical issues hindering the commercial application of PeNC LEDs are discussed, including the toxicity of Pb in lead halide perovskites, scale-up deposition of PeNC films, and design of active-matrix prototypes for high-resolution LED modules.

Surface chemistry-engineered perovskite quantum dot photovoltaics

The discovery and synthesis of colloidal quantum dots (QDs) was awarded the Nobel Prize in Chemistry in 2023. Recently, the development of bulk metal halide perovskite semiconductors has generated intense interest in their corresponding perovskite QDs. QDs, more broadly known as nanocrystals, constitute a new class of materials that differ from both molecular and bulk materials. They have rapidly advanced to the forefront of optoelectronic applications owing to their unique size-, composition-, surface- and process-dependent optoelectronic properties. More importantly, their ultrahigh surface-area-to-volume ratio enables various surface chemistry engineering strategies to tune and optimize their optoelectronic properties. Finally, three-dimensional confined QDs, offering nearly perfect photoluminescent quantum yield, slow hot-carrier cooling time, especially their colloidal synthesis and processing using industrially friendly solvents, have revolutionized the fields of electronics, photonics, and optoelectronics. Particularly, in emerging perovskite QD-based PVs, the advancement of surface chemistry has boosted the record power conversion efficiency (PCE) to 19.1% within a five-year period, surpassing all other colloidal QD photovoltaics (PVs). Given the rapid enhancement of device performances, perovskite QD PVs have attracted significant attention. Further study of semiconducting perovskite QDs will lead to advanced surface structures, a deeper understanding of halide perovskites, and enhanced PCE. In this review article, we comprehensively summarize and discuss the emerging perovskite QD PVs, providing insights into the impact of surface chemical design on their electronic coupling, dispersibility, stability and defect passivation. The limitations of current perovskite QDs mainly arise from their "soft" ionic nature and dynamic surface equilibrium, which lead to difficulties in the large-scale synthesis of monodispersed perovskite QDs and conductive inks for high-throughput printing techniques. We present that the development of surface chemistry is becoming a platform for further improving PCE, aiming to reach the 20% milestone. Additionally, we discuss integrating artificial intelligence to facilitate the mass-production of perovskite QDs for large-area, low-cost PV technology, which could help address significant energy challenges.

Transition from Light-Induced Phase Reconstruction to Halide Segregation in CsPbBr3-xIx Nanocrystal Thin Films

Inorganic metal-halide perovskite materials pave the way for many applications ranging from optoelectronics to quantum information due to their low cost, high photoluminescence and energy conversion efficiencies. However, light-induced bandgap instability due to ion migration in mixed-halide perovskites remains a significant challenge to the efficiency of optoelectronic devices. Thus, we combined hyperspectral fluorescence microspectroscopy and computational methods to understand the underlying transition mechanism between phase reconstruction and segregation in CsPbBr3-xIx (0 < x < 3) nanocrystal thin films. Our outcomes have shown that samples with x = 1.0 and x = 1.5 exhibit halide migration, favoring Br enrichment locally. In this case, an interplay between photo and thermal activation promotes the expulsion of I- from the perovskite lattice and generates a reconstruction of Br-rich domains, forming the CsPbBr3 phase. Thus, thermodynamic parameters such as the halide activation energy and phase reconstruction diffusibility were obtained by combining the kinetic parameters from linear unmixing data and Fick's second law. Moreover, we observed that the Br-I interdiffusion followed an Arrhenius-like behavior over laser-induced temperature increase. On the other hand, for samples with x = 2.0, phase segregation occurred due to the larger CsPbBrI2 nanocrystal size, iodine content and the high laser intensity employed. These three combined effects modify transport and recombination due to the reduction of charge carrier diffusion length (LD = 10.2 nm) and bandgap. Thus, iodide ions diffuse from the nanocrystal surface to the core forming a "type-II heterostructure", promoting a red shift in the fluorescence spectrum, which is characteristic of phase segregation. Furthermore, real-time dark recovery of light-induced halide segregation is reported for CsPbBrI2 nanocrystal thin films. Finally, the possible halide migration mechanism and physical origins of the transition between these phenomena are pointed out.

Room Temperature Superfluorescence from an Electron-Hole Liquid

Superfluorescence, a coherent burst of light from an excited ensemble of emitters, is a crucial quantum optical phenomenon with far-reaching implications in nanophotonics and many-body optical processes. Despite its observation in various systems, realizing superfluorescence in an electron-hole plasma (EHP) at room temperature has remained a formidable challenge, hindering the development of continuous-wave and electrically excited superfluorescence devices. Herein, we address this challenge by condensing the high-density EHP into an electron-hole liquid (EHL) at room temperature, thereby preserving quantum coherence. Using a model system of nanocrystal thin films, we demonstrate the first experimental observation of room temperature superfluorescence from an EHL. Key attributes heralding superfluorescence include a redshift of ∼94  meV from uncorrelated exciton emission, a fluence-dependent delayed growth of macroscopic coherence with abrupt radiative decay ∼1250 times faster than spontaneous emission, a distinct quadratic fluence dependence with a clear threshold, and Burnham-Chiao ringing. These findings open up exciting possibilities for developing electrically pumped colloidal nanocrystals lasers and quantum technologies operating at room temperature.

Highly Emissive Colloidal Nanocrystals of a "2.5-Dimensional" Monomethylhydrazinium Lead Bromide

The ability to control materials at the nanoscale has advanced optoelectronic devices, such as LEDs, displays, and quantum light sources. A new frontier is controlling exciton properties beyond quantum size confinement, achieved through single monolayer heterostructures. In the prototypical example of transition metal dichalcogenide heterostructures and moiré superlattices, excitons with long lifetimes, strong binding energies, and tunable dipole moments have been demonstrated and are ideal for optoelectronics and quantum applications. Expanding this material platform is crucial for further progress. This study introduces colloidal nanocrystals (NCs) of monomethylhydrazinium lead bromide (MMHPbBr3), a novel lead halide perovskite (LHP) with a unique "2.5-dimensional" electronic structure. While the spatial dimensionality of the NC extends in all three dimensions, these NCs exhibit excitonic properties intermediate between 2D and 3D LHPs. Density functional theory (DFT) calculations show that MMHPbBr3 features spatially separated electron and hole wave functions, with electrons delocalized in 3D and holes confined in 2D monolayers. Synthesized via a rapid colloidal method, these NCs were characterized by using techniques such as 4D-STEM and nuclear magnetic resonance, confirming their monoclinic structure. Optical analysis revealed size-dependent properties and 3D quantum confinement effects, with three distinct photoluminescence (PL) bands at cryogenic temperatures corresponding to excitons with varying interlayer coupling. PL spectroscopy of single MMHPbBr3 NCs reveals their photon emission statistics, expanding their potential for unconventional quantum material designs.

Perovskite Nanocrystal Self-Assemblies in 3D Hollow Templates

Highly ordered nanocrystal (NC) assemblies, namely, superlattices (SLs), have been investigated as materials for optical and optoelectronic devices due to their unique properties based on interactions among neighboring NCs. In particular, lead halide perovskite NC SLs have attracted significant attention owing to their extraordinary optical characteristics of individual NCs and collective emission processes like superfluorescence (SF). So far, the primary method for preparing perovskite NC SLs has been the drying-mediated self-assembly method, in which the colloidal NCs spontaneously assemble into SLs during solvent evaporation. However, this method lacks controllability because NCs form random-sized SLs at random positions on the substrate, rendering NC assemblies in conjunction with device structures, such as photonic waveguides or microcavities, challenging. Here, we demonstrate template-assisted self-assembly to deterministically place perovskite NC SLs and control their geometrical properties. A solution of CsPbBr3 NCs is drop-casted on a substrate with lithographically defined hollow structures. After solvent evaporation and removal of excess NCs from the substrate surface, NCs remain only in the templates, thereby defining the position and size of these NC assemblies. We performed photoluminescence (PL) measurements on these NC assemblies and observed signatures of SF, similar to those in spontaneously assembled SLs. Our findings are crucial for optical devices that harness embedded perovskite NC assemblies and enable fundamental studies on how these collective effects can be tailored through the SL geometry.

Efficient Energy Transfer from Quantum Dots to Closely-Bound Dye Molecules without Spectral Overlap

Quantum dots (QDs) are semiconductor nanocrystals whose optical properties can be tuned by altering their size. By combining QDs with dyes we can make hybrid QD-dye systems exhibiting energy transfer (ET) between QDs and dyes, which is important in sensing and lighting applications. In conventional QDs that need a shell to passivate surface defects, ET usually proceeds through Förster resonance energy transfer (FRET) that requires significant spectral overlap between QD emission and dye absorbance, as well as large oscillator strengths of those transitions. This considerably limits the choice of dyes. In contrast, perovskite QDs do not require passivating shells for bright emission, which makes ET mechanisms beyond FRET accessible. This work explores the design of a CsPbBr3 QD-dye system to achieve efficient ET from CsPbBr3 QDs to dyes with dimethyl iminium binding groups where the close binding of dyes to CsPbBr3 surface facilitates spatial wavefunction overlap. Using steady-state and time-resolved photoluminescence experiments, we demonstrate that efficient ET from CsPbBr3 to dyes with minimal spectral overlap proceeds via the Dexter exchange-type mechanism, which overcomes the conventional restriction of spectral overlap that severely limits the tunability of these systems. This approach opens new avenues for QD-molecule hybrids for a wide range of applications, such as lighting.

Enhancing the Performance of Perovskite Environmental Sensors through the Synergistic Effect of the Natural Antioxidant

All-inorganic perovskite CsPbX3 is considered to be the next-generation optical material due to its excellent optical properties and potential applications in optoelectronics. However, the inherent ionic crystal property makes it susceptible to interaction with moisture and oxygen in the ambient atmosphere and hinders the development of stable perovskite devices. Herein, a natural and nontoxic molecule, ascorbic acid (AA), is introduced to improve the performance of perovskite nanocrystals. Experimental results reveal that the strong coordination between carbonyl groups and undercoordinated Pb2+, together with the hydrogen bonding interaction between hydroxyl groups and defects of halide ions in AA, suppress nonradiative recombination. In addition, with the synergistic effect of the C═O and -OH groups in AA, the perovskite crystal structure exhibits excellent stability due to hydrogen bonding. Finally, the demonstration of humidity sensing based on perovskites has been presented. It is shown that the humidity sensor exhibits good sensitivity in the range of 30%-80% relative humidity, with a detection limit of 3.9% and a response time of 46 s. This work provides a low-cost, nontoxic, and efficient modification approach and demonstrates an easy way to improve the performance and humidity stability of perovskite.

Photon Reabsorption and Surface Plasmon Modulating Exciton-to-Dopant Energy Transfer Dynamics in Mn:CsPb(BrCl)3 Quantum Dots

Exciton-to-dopant energy transfer (ET) dynamics of Mn:CsPbX3 quantum dots (QDs), which is dominated by diverse physical factors, requires more comprehensive understanding. Here, the concentration-dependent photon reabsorption effect on ET dynamics has been meticulously analyzed in colloidal Mn:CsPb(BrCl)3 QDs. The results indicate that the photons emitted by the smaller QDs are absorbed by the larger QDs, effectively providing additional excitation light to the latter. The reabsorbed photons play a crucial role in significantly enhancing the ET process in the larger QDs. Additionally, the Mn:CsPb(BrCl)3 QDs/Poly(methyl methacrylate)/Ag/SiO2 multilayer films were fabricated to study the influence of the surface plasmon (SP) on ET dynamics. The results reveal that resonant energy transfer between excitons and SP via dipole interactions can regulate the ET process and Mn2+ emission intensity by controlling the distance between the SP and excitons. These findings provide insights into Mn:CsPbX3 QD ET dynamics and potential methods for controlling their luminescence performance in practical applications.

A Brief Review of Perovskite Quantum Dot Solar Cells: Synthesis, Property and Defect Passivation

Perovskite quantum dot solar cells (PQDSCs), as the promising candidate for the next generation of solar cell, have garnered the significant attention over the past decades. However, the performance and stability of PQDSCs are highly dependent on the properties of interfaces between the perovskite quantum dots (PQDs) and the other layers in the device. This work provides a brief overview of PQDSCs, including the synthesis of PQDs, the characteristics and preparation methods of PQDs, the photoelectric properties as the light absorption layer and optimization methods for PQDSCs with high efficiency. Future directions and potential applications are also highlighted.

Flow synthesis and multidimensional parameter screening enables exploration and optimization of copper oxide nanoparticle synthesis

Copper-based nanoparticles (NPs) are highly valued for their wide-ranging applications, with particular significance in CO2 reduction. However current synthesis methods encounter challenges in scalability, batch-to-batch variation, and high energy costs. In this work, we describe a novel continuous flow synthesis approach performed at room temperature to help address these issues, producing spherical, colloidally stable copper(ii) oxide (CuO) NPs. This approach leverages stabilizing ligands like oleic acid, oleylamine, and soy-lecithin, a novel choice for CuO NPs. The automated flow platform facilitates facile, real-time parameter screening of Cu-based nanomaterials using optical spectroscopy, achieving rapid optimization of NP properties including size, size dispersity, and colloidal stability through tuning of reaction parameters. This study highlights the potential of continuous flow synthesis for efficient parameter exploration to accelerate understanding, optimization, and eventually enable scale-up of copper-based NPs. This promises significant benefits for various sectors, including energy, healthcare, and environmental conservation, by enabling reliable production with reduced energy and cost requirements.

Increased Brightness and Reduced Efficiency Droop in Perovskite Quantum Dot Light-Emitting Diodes Using Carbazole-Based Phosphonic Acid Interface Modifiers

We demonstrate the use of [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz) and [2-(3,6-di-tert-butyl-9H-carbazol-9-yl)ethyl]phosphonic acid (t-Bu-2PACz) as anode modification layers in metal-halide perovskite quantum dot light-emitting diodes (QLEDs). Compared to conventional QLED structures with PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrenesulfonate)/PVK (poly(9-vinylcarbazole)) hole-transport layers, the QLEDs made with phosphonic acid (PA)-modified indium tin oxide (ITO) anodes show an over seven-fold increase in brightness, achieving a brightness of 373,000 cd m-2, one of the highest brightnesses reported to date for colloidal perovskite QLEDs. Importantly, the onset of efficiency roll-off, or efficiency droop, occurs at ∼1000-fold higher current density for QLEDs made with PA-modified anodes compared to control QLEDs made with conventional PEDOT:PSS/PVK hole transport layers, allowing the devices to sustain significantly higher levels of external quantum efficiency at a brightness of >105 cd m-2. Steady-state and time-resolved photoluminescence measurements indicate that these improvements are due to a combination of multiple factors, including reducing quenching of photoluminescence at the PEDOT:PSS interface and reducing photoluminescence efficiency loss at high levels of current density.

Breaking the Boundaries of the Goldschmidt Tolerance Factor with Ethylammonium Lead Iodide Perovskite Nanocrystals

We report the synthesis of ethylammonium lead iodide (EAPbI3) colloidal nanocrystals as another member of the lead halide perovskites family. The insertion of an unusually large A-cation (274 pm in diameter) in the perovskite structure, hitherto considered unlikely due to the unfavorable Goldschmidt tolerance factor, results in a significantly larger lattice parameter compared to the Cs-, methylammonium- and formamidinium-based lead halide perovskite homologues. As a consequence, EAPbI3 nanocrystals are highly unstable, evolving to a nonperovskite δ-EAPbI3 polymorph within 1 day. Also, EAPbI3 nanocrystals are very sensitive to electron irradiation and quickly degrade to PbI2 upon exposure to the electron beam, following a mechanism similar to that of other hybrid lead iodide perovskites (although degradation can be reduced by partially replacing the EA+ ions with Cs+ ions). Interestingly, in some cases during this degradation the formation of an epitaxial interface between (EAxCs1-x)PbI3 and PbI2 is observed. The photoluminescence emission of the EAPbI3 perovskite nanocrystals, albeit being characterized by a low quantum yield (∼1%), can be tuned in the 664-690 nm range by regulating their size during the synthesis. The emission efficiency can be improved upon partial alloying at the A site with Cs+ or formamidinium cations. Furthermore, the morphology of the EAPbI3 nanocrystals can be chosen to be either nanocube or nanoplatelet, depending on the synthesis conditions.

Bidentate Lewis Base Ligand-Mediated Surface Stabilization and Modulation of the Electronic Structure of CsPbBr3 Perovskite Nanocrystals

The desorption of conventional ligands from the surface of halide perovskite nanocrystals (NCs) often causes their structural instability and deterioration of the optoelectronic properties. To address this challenge, we present an approach of using a bidentate Lewis base ligand, namely, 1,4-bis(diphenylphosphino)butane (DBPP), for the synthesis of CsPbBr3 NCs. The phosphine group of DBPP has a strong interaction with the PbBr2 precursor, forming a highly crystalline intermediate complex during the reaction. In the presence of oleic acid, the uncoordinated phosphine group of DBPP is converted into the phosphonium cation, which strongly binds to the surface bromide of the formed CsPbBr3 NCs through hydrogen bonding. Density functional theory calculations suggest that DBPP can strongly bind to the undercoordinated lead and surface bromide ions of CsPbBr3 NCs through its unprotonated and protonated phosphine groups, respectively. The robust binding of DBPP to the surface of perovskite NCs helps to preserve their structural integrity under various environmental stresses. Moreover, the electron density and energy levels are regulated in DBPP-capped CsPbBr3 NCs by the donation of electrons from the ligands to the NCs, resulting in their improved photocatalytic CO2 reduction performance. Our study highlights the potential of using bidentate ligands to stabilize the surface of perovskite NCs and modulate their optical and electronic properties.

Enabling Multicolor Information Encryption: Oleylammonium-Halide-Assisted Reversible Phase Conversion between Cs4PbX6 and CsPbX3 Nanocrystals

Recently, halide perovskites have been recognized for their thermochromic characteristics, showing significant potential in information encryption applications. However, the limited luminescence color gamut hinders the encryption of complex multicolor information. Herein, for the first time, multicolor thermochromic perovskites with luminescence covering the entire visible spectrum have been designed. Oleylammonium halide salts facilitate a reversible phase transformation between nonluminescent Cs4PbX6 nanocrystals (NCs) and luminescent CsPbX3 NCs upon heating or cooling. This process occurs without the need for external addition or removal of ligands or metal salts, enabling efficient and intelligent information encryption. A proof-of-concept demonstration successfully encrypts and decrypts multicolor digital information. This work not only advances the understanding of phase transformations in perovskites but also highlights their significant potential for information encryption applications.

Towards non-blinking and photostable perovskite quantum dots

Surface defect-induced photoluminescence blinking and photodarkening are ubiquitous in lead halide perovskite quantum dots. Despite efforts to stabilize the surface by chemically engineering ligand binding moieties, blinking accompanied by photodegradation still poses barriers to implementing perovskite quantum dots in quantum emitters. To date, ligand tail engineering in the solid state has rarely been explored for perovskite quantum dots. We posit that attractive intermolecular interactions between low-steric ligand tails, such as π-π stacking, can promote the formation of a nearly epitaxial ligand layer that significantly reduces the quantum dot surface energy. Here, we show that single CsPbBr3 quantum dots covered by stacked phenethylammonium ligands exhibit nearly non-blinking single photon emission with high purity (~ 98%) and extraordinary photostability (12 hours continuous operation and saturated excitations), allowing the determination of size-dependent exciton radiative rates and emission line widths of CsPbBr3 quantum dots at the single particle level.

A CsPb0.95Ni0.05Br3 NCs-based fluorescence sensor for rapidly and accurately evaluating trace water in edible oils along with the structure destruction and dissolution

Metal ions with smaller radii than Pb2+ can stabilize CsPbBr3 NCs' cubic structure by lattice shrinkage, but lacking sensing research. Herein, Ni-substituting CsPbBr3 NCs were prepared to rapidly and accurately detect water content (WC) in edible oils. CsPb0.95Ni0.05Br3 NCs had the highest fluorescence intensity, approximately 125 % of CsPbBr3 NCs. The results displayed that CsPb0.95Ni0.05Br3 NCs were uniformly quadrilateral crystalline packing (8.78 ± 0.28 nm particle size) with inter-planar distances of 0.41, 0.33, and 0.29 nm. Given the fluorescence quenching behavior, a superior linear curve between fluorescence-decreased ratio and WC was established within 0-3 ‰ (v/v) and a detection limitation of 0.042 ‰. Furthermore, excellent precision and accuracy were verified in various oils with a relative error of 2.06 %. It was suggested that water destroyed and dissolved CsPb0.95Ni0.05Br3 NCs' crystal structure to induce fluorescence quenching. Thus, Pb-site substitutions of CsPbBr3 NCs enhanced the sensing performance, enlightening other elements-substituted CsPbBr3 NCs for sensing.

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.

Optimizing Perovskite Surfaces to Enhance Post-Treatment for Efficient Blue Mixed-Halide Perovskite Light-emitting Diodes

The halide postdeposition treatment technique is a widely used strategy for mitigating defects in perovskite. However, when applied to mixed-halide perovskites, it often leads to surface and internal halide heterogeneity, which compromises luminescence performance and spectral stability. In this work, blue mixed-halide 3D perovskites are engineered with acetate (Ac⁻)-rich surfaces to optimize the post-treatment process and achieve halide homogeneity. The findings demonstrate that the strong interaction between surface Ac⁻ ions and Pb2+ ions significantly reduces the formation of halide vacancy defects caused by the washing effect of isopropanol during post-treatment. This defect reduction slows the infiltration of halide ions into the perovskite lattice, providing more time for surface reconstruction and minimizing the accumulation of introduced halide ions at the surface. As a result, a mild halide redistribution occurs, promoting the formation of a uniform mixed-halide perovskite phase. This approach enabled the development of blue mixed-halide 3D PeLEDs with a record external quantum efficiency of 19.28% (emission peak at 482 nm), comparable to state-of-the-art blue reduced-dimensional perovskite-based PeLEDs. Additionally, the device demonstrated a narrowband and stable electroluminescence spectrum with a full width at half maximum (FWHM) of less than 16 nm.

25.71 %-Efficiency FACsPbI3 Perovskite Solar Cells Enabled by A Thiourea-based Isomer

Various isomers have been developed to regulate the morphology and reduce defects in state-of-the-art perovskite solar cells (PSCs). To insight the structure-function-effect correlations for the isomerization of thiourea derivatives on the performance of the PSCs, we developed two thiourea derivatives [(3,5-dichlorophenyl)amino]thiourea (AT) and N-(3,5-dichlorophenyl)hydrazinecarbothioamide (HB). Supported by experimental and calculated results, it was found that AT can bind with undercoordinated Pb2+ defect through synergistic interaction between N1 and C=S group with a defect formation energy of 1.818 eV, which is much higher than that from the synergistic interaction between two -NH- groups in HB and perovskite (1.015 eV). Moreover, the stronger interaction between AT and Pb2+ regulates the crystallization process of perovskite film to obtain a high-quality perovskite film with high crystallinity, large grain size, and low defect density. Consequently, the AT-treated FACsPbI3 device engenders an efficiency of 25.71 % (certified as 24.66 %), which is greatly higher than control (23.74 %) and HB-treated FACsPbI3 devices (25.05 %). The resultant device exhibits a remarkable stability for maintaining 91.0 % and 95.2 % of its initial efficiency after aging 2000 h in air condition or tracking at maximum power point for 1000 h, respectively.

Dynamic Lock-And-Release Mechanism Enables Reduced ΔG at Low Temperatures for High-Performance Polyanionic Cathode in Sodium-Ion Batteries

Low-temperature synthesis of polyanionic cathodes for sodium-ion batteries is highly desirable but often plagued by prolonged reaction times and suboptimal crystallinity. To address these challenges, a novel self-adaptive coordination field regulation (SACFR) strategy based on a dynamic lock-and-release (DLR) mechanism is introduced. Specifically, urea is used as a DLR carrier during synthesis, which dynamically "locks" and "releases" vanadium ions for controlled release, simultaneously "locking" H+ ions to enhance phosphate group release, thereby creating a self-adaptive coordination field that can intelligently respond to real-time demands of the reaction system. This dynamic coordination behavior contributes to both an improvement in reaction kinetics and a significant reduction in Gibbs free energy change (ΔG). As a result, the kinetic efficiency and thermodynamic spontaneity of the reaction are greatly enhanced, enabling the efficient synthesis of high-crystalline Na3V2O2(PO4)2F (NVOPF) at 90 °C within just 3 hours. The as-prepared NVOPF cathode exhibits exceptional rate performance and ultra-stable cycling stability across a broad temperature range. Furthermore, the successful kilogram-scale synthesis underscores the practical potential of the innovative strategy. This work pioneers the regulation of coordination field chemistry for polyanionic cathode synthesis, providing transformative insights into material design.

Ultrafast Spin Relaxation of Charge Carriers in Strongly Quantum Confined Methylammonium Lead Bromide Perovskite Magic-Sized Clusters

Spin relaxation of charge carriers in strongly quantum confined perovskite magic-sized clusters has been probed, for the first time, by using polarization-controlled femtosecond transient absorption (fs-TA) spectroscopy. Fs-TA measurements with a circularly polarized pump and probe allowed for the determination of the exciton spin relaxation lifetime (∼1.5 ps) at room temperature based on the dynamics of a photoinduced absorption (PIA) feature peaked at 458 nm. This spin lifetime is shorter than that of perovskite quantum dots (PQDs) with a larger size, and the results suggest that exciton confinement and defects likely play a more important role in these strongly quantum confined magic-sized clusters with a larger surface-to-volume ratio.

The Photophysics of Perovskite Emitters: from Ensemble to Single Particle

Halide perovskite emitters are a groundbreaking class of optoelectronic materials possessing remarkable photophysical properties for diverse applications. In perovskite light emitting devices, they have achieved external quantum efficiencies exceeding 28%, showcasing their potential for next-generation solid-state lighting and ultra high definition displays. Furthermore, the demonstration of room temperature continuous-wave perovskite lasing underscores their potential for integrated optoelectronics. Of late, perovskite emitters are also found to exhibit desirable single-photon emission characteristics as well as superfluorescence or superradiance phenomena for quantum optics. With progressive advances in synthesis, surface engineering, and encapsulation, halide perovskite emitters are poised to become key components in quantum optical technologies. Understanding the underpinning photophysical mechanisms is crucial for engineering these novel emergent quantum materials. This review aims to provide a condensed overview of the current state of halide perovskite emitter research covering both established and fledging applications, distill the underlying mechanisms, and offer insights into future directions for this rapidly evolving field.

Revealing and Manipulating Hidden Fine-Structure Coherence of Bright Excitons in CsPbI3 Perovskite Quantum Dots

Observation and understanding of fine-structure splitting of bright excitons in lead halide perovskite quantum dots (QDs) are crucial to their emerging applications in quantum light sources and exciton coherence manipulation. Recent studies demonstrate that ensemble-level polarization-resolved transient absorption spectroscopy can reveal the quantum beats arising from the coherence between two fine-structure levels. Here we report the observation of an extra fine-structure quantum coherence hidden in previous studies by using cryo-magnetic quantum beat spectroscopy. In ∼6 nm CsPbI3 QDs, two splitting energies of 0.25 and 1.20 meV were observed at 1.7 K, which gradually increased to 0.74 and 1.55 meV, respectively, when a longitudinal magnetic field up to 7 T was applied. The field dependence allowed us to extract two distinct nominal Landé g-factors corresponding to QDs with different orientations with respect to the external field.

In Situ Formation of Iodide Precursor for Perovskite Quantum Dots with Application in Efficient Solar Cells

Perovskite quantum dots (PQDs) become a kind of competitive material for fabricating high-performance solar cells due to their solution processability and outstanding optoelectronic properties. However, the current synthesis method of PQDs is mostly based on the binary-precursor method, which results in a large deviation of the I/Pb input ratio in the reaction system from the stoichiometric ratio of PQDs. Herein, a ternary-precursor method with an iodide source self-filling ability is reported for the synthesis of the CsPbI3 PQDs with high optoelectronic properties. Systematically experimental characterizations and theoretical calculations are conducted to fundamentally understand the effects of the I/Pb input molar ratio on the crystallographic and optoelectronic properties of PQDs. The results reveal that increasing the I/Pb input molar ratio can obtain ideal cubic structure PQDs with iodine-rich surfaces, which can significantly reduce the surface defects of PQDs and realize high orientation of PQD solids, facilitating charge carrier transport in the PQD solids with diminished nonradiative recombination. Consequently, the PQD solar cells exhibit an impressive efficiency of 15.16%, which is largely improved compared with that of 12.83% for the control solar cell. This work provides a feasible strategy for synthesizing high-quality PQDs for high-performance optoelectronic devices.

Quantum-Confined Perovskite Nanocrystals Enabled by Negative Catalyst Strategy for Efficient Light-Emitting Diodes

The perovskite nanocrystals (PeNCs) are emerging as a promising emitter for light-emitting diodes (LEDs) due to their excellent optical and electrical properties. However, the ultrafast growth of PeNCs often results in large sizes exceeding the Bohr diameter, leading to low exciton binding energy and susceptibility to nonradiative recombination, while small-sized PeNCs exhibit a large specific surface area, contributing to an increased defect density. Herein, Zn2+ ions as a negative catalyst to realize quantum-confined FAPbBr3 PeNCs with high photoluminescence quantum yields (PL QY) over 90%. Zn2+ ions exhibit robust coordination with Br- ions is introduced, effectively retarding the participation of Br- ions in the perovskite crystallization process and thus facilitating PeNCs size control. Notably, Zn2+ ions neither incorporate into the perovskite lattice nor are absorbed on the surface of PeNCs. And the reduced growth rate also promotes sufficient octahedral coordination of PeNC that reduces defect density. The LEDs based on these optimized PeNCs exhibits an external quantum efficiency (EQE) of 21.7%, significantly surpassing that of the pristine PeNCs (15.2%). Furthermore, the device lifetime is also extended by twofold. This research presents a novel approach to achieving high-performance optoelectronic devices.

Determination of the particle size distribution of cube-shaped colloidal perovskite quantum dots from photoluminescence spectra: A combined theoretical-experimental approach

A theoretical-experimental approach is proposed to convert the photoluminescence spectra of colloidal perovskite quantum dot ensembles into accurate estimates for their intrinsic particle size distribution functions. Two main problems were addressed and properly correlated: the size dependence of the first excitonic transition in a single cube-shaped quantum dot and the inhomogeneous broadening of the fluorescence line shape due to the size nonuniformity of the chemically prepared quantum dot suspension in addition to the single-dot homogeneous broadening. By applying the reported methodology to CsPbBr3 quantum dot samples belonging to the strong and intermediate confinement regimes, the calculated size distributions exhibited close agreement with those obtained from transmission electron microscopy, with precise estimates for the average particle size and standard deviation. Specifically for strongly confined ultrasmall CsPbBr3 quantum dots, the presented spectroscopic model for size distribution computation is based on a new analytical expression for the size-dependent bandgap, which was developed within the framework of the finite-depth square-well effective mass approximation accounting for band nonparabolicity effects. Such a quantum mechanical approach correctly predicts the expected transition to the intermediate confinement regime in sufficiently large quantum dots, which are traditionally described by the well-known bandgap equation in the infinite potential barrier limit with a spatially correlated electron-hole wavefunction and nonparabolic carrier effective masses. The proposed calculation scheme originates from general theoretical considerations so that it can be readily adapted to semiconductor quantum dots of many other systems, from all inorganic metal halides to hybrid perovskite materials, regardless of the adopted chemical synthesis route.

Reductive pathways in molten inorganic salts enable colloidal synthesis of III-V semiconductor nanocrystals

Colloidal quantum dots, with their size-tunable optoelectronic properties and scalable synthesis, enable applications in which inexpensive high-performance semiconductors are needed. Synthesis science breakthroughs have been key to the realization of quantum dot technologies, but important group III-group V semiconductors, including colloidal gallium arsenide (GaAs), still cannot be synthesized with existing approaches. The high-temperature molten salt colloidal synthesis introduced in this work enables the preparation of previously intractable colloidal materials. We directly nucleated and grew colloidal quantum dots in molten inorganic salts by harnessing molten salt redox chemistry and using surfactant additives for nanocrystal shape control. Synthesis temperatures above 425°C are critical for realizing photoluminescent GaAs quantum dots, which emphasizes the importance of high temperatures enabled by molten salt solvents. We generalize the methodology and demonstrate nearly a dozen III-V solid-solution nanocrystal compositions that have not been previously reported.

Antisolvent controls the shape and size of anisotropic lead halide perovskite nanocrystals

Colloidal lead halide perovskite nanocrystals have potential for lighting applications due to their optical properties. Precise control of the nanocrystal dimensions and composition is a prerequisite for establishing practical applications. However, the rapid nature of their synthesis precludes a detailed understanding of the synthetic pathways, thereby limiting the optimisation. Here, we deduce the formation mechanisms of anisotropic lead halide perovskite nanocrystals, 1D nanorods and 2D nanoplatelets, by combining in situ X-ray scattering and photoluminescence spectroscopy. In both cases, emissive prolate nanoclusters form when the two precursor solutions are mixed. The ensuing antisolvent addition induces the divergent anisotropy: The intermediate nanoclusters are driven into a dense hexagonal mesophase, fusing to form nanorods. Contrastingly, nanoplatelets grow freely dispersed from dissolving nanoclusters, stacking subsequently in lamellar superstructures. Shape and size control of the nanocrystals are determined primarily by the antisolvent's dipole moment and Hansen hydrogen bonding parameter. Exploiting the interplay of antisolvent and organic ligands could enable more complex nanocrystal geometries in the future.

Exploring Nanoscale Perovskite Materials for Next-Generation Photodetectors: A Comprehensive Review and Future Directions

The rapid advancement of nanotechnology has sparked much interest in applying nanoscale perovskite materials for photodetection applications. These materials are promising candidates for next-generation photodetectors (PDs) due to their unique optoelectronic properties and flexible synthesis routes. This review explores the approaches used in the development and use of optoelectronic devices made of different nanoscale perovskite architectures, including quantum dots, nanosheets, nanorods, nanowires, and nanocrystals. Through a thorough analysis of recent literature, the review also addresses common issues like the mechanisms underlying the degradation of perovskite PDs and offers perspectives on potential solutions to improve stability and scalability that impede widespread implementation. In addition, it highlights that photodetection encompasses the detection of light fields in dimensions other than light intensity and suggests potential avenues for future research to overcome these obstacles and fully realize the potential of nanoscale perovskite materials in state-of-the-art photodetection systems. This review provides a comprehensive overview of nanoscale perovskite PDs and guides future research efforts towards improved performance and wider applicability, making it a valuable resource for researchers.

Pure-Phase Perovskite Quantum Well for Green Light-Emitting Diodes

Perovskite multiple quantum wells (MQWs) have shown great potential in the field of light-emitting diodes (LEDs). However, the random formation of QWs with varying well widths (n numbers) often leads to suboptimal interface defects and charge transport issues. Here, we reveal that the crystallization sequence of bromide-based perovskite MQWs is large-n QWs preceding small-n QWs. With this insight, we prevent the crystallization of subsequent small-n QWs by reducing the crystallization rate, ultimately resulting in the crystallization of only n = 5 QWs. This reduction in the crystallization rate is achieved through the chemical interaction of dual additives with perovskite constituents. Additionally, the chemical interaction effectively passivates the uncoordinated lead ions defects. Consequently, pure-phase perovskite QWs with a high photoluminescence quantum efficiency of 75% are achieved. The resulting green LEDs achieve a peak external quantum efficiency of 17.1% and a maximum luminance of 29,480 cd m-2, which is attractive for full-color display applications of perovskites.