Colloidal Metal-Halide Perovskite Nanoplatelets: Thickness-Controlled Synthesis, Properties, and Application in Light-Emitting Diodes

Efficient, Color-Stable, Pure-Blue Light-Emitting Diodes Based on Aromatic Ligand-Engineered Perovskite Nanoplatelets

Perovskite nanoplatelets (NPLs) show great potential for high-color-purity light-emitting diodes (LEDs) due to their narrow line width and high exciton binding energy. However, the performance of perovskite NPL LEDs lags far behind perovskite quantum dot-/film-based LEDs, owing to their material instability and poor carrier transport. Here, we achieved efficient and stable pure blue-emitting CsPbBr3 NPLs with outstanding optical and electrical properties by using an aromatic ligand, 4-bromothiophene-2-carboxaldehyde (BTC). The BTC ligands with thiophene groups can guide two-dimensional growth and inhibit out-of-plane ripening of CsPbBr3 NPLs, which, meanwhile, increases their structural stability via strongly interacting with PbBr64- octahedra. Moreover, aromatic structures with delocalized π-bonds facilitate charge transport, diminish band tail states, and suppress Auger processes in CsPbBr3 NPLs. Consequently, the LEDs demonstrate efficient and color-stable blue emissions at 465 nm with a narrow emission line width of 17 nm and a maximum external quantum efficiency (EQE) of 5.4%, representing the state-of-the-art CsPbBr3 NPL LEDs.

Realizing efficient broadband near-infrared emission and multimode photoluminescence switching via coordination structure modulation in Sb3+-doped 0D organic metal chlorides

Recently, organic Sb(III)-based metal halides have achieved significant results in the visible light region due to their efficient emission. However, realizing efficient broadband near-infrared (NIR) emission in such materials is a great challenge. Herein, we developed three different NIR emitters via a coordination structure modulation strategy in Sb3+-doped zero-dimensional organic metal chlorides of (C20H20P)2MnCl4, (C20H20P)2ZnCl4, and (C20H20P)2CdCl4 with tetrahedral structure. More specifically, after the dopant Sb3+ is inserted into the host lattice, the coordination structures of Sb3+ ions can change from [SbCl5]2- square-pyramidal configuration to [SbCl4]- clusters, which will bring a larger lattice distortion degree to the excited state compared to the ground state, resulting in a larger Stokes shift. Thus, efficient NIR emission with near-unity photoluminescence quantum yield (PLQY) can be obtained in Sb3+-doped compounds under 365 nm excitation. Moreover, Sb3+-doped NIR emitters also show remarkable stabilities, which prompts us to fabricate NIR phosphor conversion light-emitting diodes (pc-LEDs) and demonstrate their application in night vision. More interestingly, the Sb3+-doped (C20H20P)2MnCl4 shows tunable emission characteristics, which can be tuned from green to greenish-yellow, orange, red, and NIR emission under different external stimuli, and thus we can demonstrate the applications of this compound in quintuple-mode fluorescence anti-counterfeiting and information encryption.

Data-Driven Controlled Synthesis of Oriented Quasi-Spherical CsPbBr3 Perovskite Materials

Controlled synthesis of lead-halide perovskite crystals is challenging yet attractive because of the pivotal role played by the crystal structure and growth conditions in regulating their properties. This study introduces data-driven strategies for the controlled synthesis of oriented quasi-spherical CsPbBr3, alongside an investigation into the synthesis mechanism. High-throughput rapid characterization of absorption spectra and color under ultraviolet illumination was conducted using 23 possible ligands for the synthesis of CsPbBr3 crystals. The links between the absorption spectra slope (difference in the absorbance at 400 nm and 450 nm divided by a wavelength interval of 50 nm) and crystal size were determined through statistical analysis of more than 100 related publications. Big data analysis and machine learning were employed to investigate a total of 688 absorption spectra and 652 color values, revealing correlations between synthesis parameters and properties. Ex situ characterization confirmed successful synthesis of oriented quasi-spherical CsPbBr3 perovskites using polyvinylpyrrolidone and Acacia. Density functional theory calculations highlighted strong adsorption of Acacia on the (110) facet of CsPbBr3. Optical properties of the oriented quasi-spherical perovskites prepared with these data-driven strategies were significantly improved. This study demonstrates that data-driven controlled synthesis facilitates morphology-controlled perovskites with excellent optical properties.

Kinetically Controlled Synthesis of Quasi-Square CsPbI3 Nanoplatelets with Excellent Stability

Nanoplatelets (NPLs) share excellent luminescent properties with their symmetric quantum dots counterparts and entail special characters benefiting from the shape, like the thickness-dependent bandgap and anisotropic luminescence. However, perovskite NPLs, especially those based on iodide, suffer from poor spectral and phase stability. Here, stable CsPbI3 NPLs obtained by accelerating the crystallization process in ambient-condition synthesis are reported. By this kinetic control, the rectangular NPLs into quasi-square NPLs are tuned, where enlarged width endows the NPLs with a lower surface-area-to-volume ratio (S/V ratio), leading to lower surficial energy and thus improved endurance against NPL fusion (cause for spectral shift or phase transformation). The accelerated crystallization, denoting the fast nucleation and short period of growth in this report, is enabled by preparing a precursor with complete transformation of PbI2 into intermediates (PbI3 -), through an additional iodide supplier (e.g., zinc iodide). The excellent color stability of the materials remains in the light-emitting diodes under various bias stresses.

Unraveling the Luminescence Quenching Mechanism in Strong and Weak Quantum-Confined CsPbBr3 Triggered by Triarylamine-Based Hole Transport Layers

Luminescence quenching by hole transport layers (HTLs) is one of the major issues in developing efficient perovskite light-emitting diodes (PeLEDs), which is particularly prominent in blue-emitting devices. While a variety of material systems have been used as interfacial layers, the origin of such quenching and the type of interactions between perovskites and HTLs are still ambiguous. Here, we present a systematic investigation of the luminescence quenching of CsPbBr3 by a commonly employed hole transport polymer, poly[(9,9-dioctylfluorenyl-2,7diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine)] (TFB), in LEDs. Strong and weak quantum-confined CsPbBr3 (nanoplatelets (NPLs)/nanocrystals (NCs)) are rationally selected to study the quenching mechanism by considering the differences in their morphology, energy level alignments, and quantum confinement. The steady-state and time-resolved Stern-Volmer plots unravel the dominance of dynamic and static quenching at lower and higher concentrations of TFB, respectively, with a maximum quenching efficiency of 98%. The quenching rate in NCs is faster than that in NPLs owing to their longer PL lifetimes and weak quantum confinement. The ultrafast transient absorption results support these dynamics and rule out the involvement of Forster or Dexter energy transfer. Finally, the 1D 1H and 2D nuclear overhauser effect spectroscopy nuclear magnetic resonance (NOESY NMR) study confirms the exchange of native ligands at the NCs surface with TFB, leading to dark CsPbBr3-TFB ensemble formation accountable for luminescence quenching. This highlights the critical role of the triarylamine functional group on TFB (also the backbone of many HTLs) in the quenching process. These results shed light on the underlying reasons for the luminescence quenching in PeLEDs and will help to rationally choose the interfacial layers for developing efficient LEDs.

Ultrafast energy transfer dynamics in CsPbBr3 nanoplatelets-BODIPY heterostructure

Understanding and directing the energy transfer in nanocrystals-chromophore heterostructure is critical to improve the efficiency of their photocatalytic and optoelectronic applications. In this work, we studied the energy transfer process between inorganic-organic molecular complexes composed of cesium halide perovskite nanoplatelets (CsPbBr3 NPLs) and boron dipyrromethene (BODIPY) by photoluminescence spectroscopy (PL), time-correlated single photon-counting (TCSPC) and femtosecond transient absorption spectroscopy. The quenching of PL in CsPbBr3 NPLs occurred simultaneously with the PL enhancement of BODIPY implied the singlet energy transfer process. The rate of energy transfer has been determined by transient absorption spectrum as kET = 3.8 × 109 s-1. The efficiency of Förster energy transfer (FRET) has been quantitatively calculated up to 70%. Our work advances the understanding of the interaction between BODIPY and perovskite nanoplatelets, providing a new solution based on their optoelectronic and photocatalytic applications.

Highly sensitive quantitative detection of glycans on exosomes in renal disease serums using fluorescence signal amplification strategies

Exosomal glycoproteins play a significant role in many physiological and pathological processes. However, the detection of exosome surface glycans is currently challenged by the complexity of biological samples or the sensitivity of the methods. Herein, we prepared a novel fluorescent probe of biotin-functionalized nanocrystals (denoted as CdTe@cys-biotin) and applied it for the first time for the detection of the expression of exosomal surface glycans using a fluorescence amplification strategy. First, the dual affinity of TiO2 and CD63 aptamers of Fe3O4@TiO2-CD63 was utilized to rapidly and efficiently capture exosomes within 25 min. In this design, interference from other vesicles and soluble impurities can be avoided due to the dual recognition strategy. The chemical oxidation of NaIO4 oxidized the hydroxyl sites of exosomal surface glycans to aldehydes, which were then labeled with aniline-catalyzed biotin hydrazide. Using the high affinity between streptavidin and biotin, streptavidin-FITC and probes were successively anchored to the glycans on the exosomes. The fluorescent probe achieved the dual function of specific recognition and fluorescent labeling by modifying biotin on the surface of nanocrystals. This method showed excellent specificity and sensitivity for exosomes at concentrations ranging from 3.30 × 102 to 3.30 × 106 particles/mL, with a detection limit of 121.48 particles/mL. The fluorescent probe not only quantified exosomal surface glycans but also distinguished with high accuracy between serum exosomes from normal individuals and patients with kidney disease. In general, this method provides a powerful platform for sensitive detection of exosomes in cancer diagnosis.

Sterically Controlled Synthesis of Amine-Free CsPbBr3 Nanoplatelets for Stable, Pure-Blue Light Emission

Metal halide perovskite nanoplatelets (NPLs) have demonstrated excellent optical properties for light-emitting applications and achieved tunable blue luminescence through thickness control. However, their translation into electronic devices has lagged behind due to poor colloidal and film stability. The main reason for this is the deprotonation of their surface-capped ammonium passivating ligands, resulting in NPL aggregation. Here we report the first facile synthesis of amine-free pure-blue CsPbBr3 NPLs with outstanding thermal and light stability. This is achieved by utilizing an amine-free phosphine oxide route with a surface capping molecule exhibiting large steric hindrance to prevent NPL aggregation. Two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy suggests slower ligand exchange in amine-free NPLs compared to the conventional NPLs, which can be attributed to the strong binding strength of the designated ligand. Consequently, the amine-free NPLs exhibited superior stability against radiation, heat and moisture. We further demonstrate the importance of acid-base equilibrium in this amine-free synthesis route. Through solvent neutralization and passivation with various alkali carbonates, the resulting NPLs attained near-unity photoluminescence quantum yield (PLQY) and pure blue emission.

Enhanced Efficiency, Broadened Excitation, and Tailored Er3+ Luminescence Triggered by Te4+ Codoping in Cs2NaYbCl6 Crystals for Multifunctional Applications

The quest for efficient and tunable luminescent materials has been at the forefront of research in the fields of chemistry and materials science. This work delves into the investigation of the luminescence properties of Er3+ ions triggered by 1% Te4+ in the environmentally benign perovskite Cs2NaYbCl6 (CNYC) crystals, aiming to enhance their efficiency and tune the luminescence color. The ratio of the green (2H11/2, 4S3/2-4I15/2) to red (4F9/2-4I15/2) emissions of Er3+ can be freely tunable by varying the concentration of Er3+ and producing the defects induced by codoping Te4+. The calculations reveal that the multiexcitonic excitations of Er3+ stem from f-f (4I15/2-4G11/2, 2H9/2) rather than d-f transitions. The broadened excitation, tuning of color, and enhancement of efficiency achieved in the luminescence perovskite crystals Cs2NaYbCl6:Te4+, Er3+ (CNYC:Te4+,Er3+) presents promising opportunities for the development of advanced optoelectronic devices with superior performance. Moreover, our investigation demonstrates the tunable luminescence response of CNYC:Er3+ to temperature variations, offering potential applications in temperature sensing.

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.

Energy transfer enhanced photoluminescence of 2D/3D CsPbBr3 hybrid assemblies

Energy transfer has been proven to be an effective method to optimize optoelectronic conversion efficiency by improving light absorption and mitigating nonradiative losses. We prepared 2D/3D CsPbBr3 hybrid assemblies at different reaction temperatures using the hot injection method and found that the photoluminescence quantum yields (PLQYs) of these hybrids were greatly enhanced from 53.4% to 72.57% compared with 3D nanocrystals (NCs). Femtosecond transient absorption measurements were used to study the PLQY enhancement mechanisms, and it was found that the hot carrier lifetime improved from 0.36 to 1.88 ps for 2D/3D CsPbBr3 hybrid assemblies owing to the energy transfer from 2D nanoplates to 3D NCs. The energy transfer benefits the excited carrier accumulation and prolonged hot carrier lifetime in 3D NCs in hybrid assemblies, as well as PLQY enhancement in materials.

Ligand-Enhanced Neodymium Doping of Perovskite Quantum Dots for Superior Exciton Confinement

In this study, all-inorganic perovskite quantum dots (QDs) for pure blue emission are explored for full-color displays. We prepared CsPbBr3 and Cs3NdCl6 QDs via hot injection methods and mixed in various ratios at room temperature for color blending. Nd-doped CsPb(Cl/Br)3 QDs showed a blueshift in emission, and the photoluminescence quantum yields (PLQY, ΦPL) were lower in the 460-470 nm range due to surface halogen and Cs vacancies. To address this, we introduced a silane molecule, APTMS, via a ligand exchange process, effectively repairing these vacancies and enhancing Nd doping into the lattice. This modification promotes the PLQY to 94% at 466 nm. Furthermore, combining these QDs with [1]Benzothieno[3,2-b][1]benzothiophene (BTBT), a conjugated small-molecule semiconductor, in a composite film reduced PLQY loss caused by FRET in solid-state QD films. This approach achieved a wide color gamut of 124% National Television System Committee (NTSC), using a UV LED backlight and RGB perovskite QDs in a BTBT-based organic matrix as the color conversion layer. Significantly, the photostability of this composite was enhanced when used as a color conversion layer (CCL) under blue-LED excitation.

Dual-heterodyne Kelvin probe force microscopy

We present a new open-loop implementation of Kelvin probe force microscopy (KPFM) that provides access to the Fourier spectrum of the time-periodic surface electrostatic potential generated under optical (or electrical) pumping with an atomic force microscope. The modulus and phase coefficients are probed by exploiting a double heterodyne frequency mixing effect between the mechanical oscillation of the cantilever, modulated components of the time-periodic electrostatic potential at harmonic frequencies of the pump, and an ac bias modulation signal. Each harmonic can be selectively transferred to the second cantilever eigenmode. We show how phase coherent sideband generation and signal demodulation at the second eigenmode can be achieved by using two numerical lock-in amplifiers configured in cascade. Dual-heterodyne KPFM (DHe-KPFM) can be used to map any harmonic (amplitude/phase) of the time-periodic surface potential at a standard scanning speed. The Fourier spectrum (series of harmonics) can also be recorded in spectroscopic mode (DHe-KPFM spectroscopy), and 2D dynamic images can be acquired in data cube mode. The capabilities of DHe-KPFM in terms of time-resolved measurements, surface photovoltage (SPV) imaging, and detection of weak SPV signals are demonstrated through a series of experiments on difference surfaces: a reference substrate, a bulk organic photovoltaic heterojunction thin film, and an optoelectronic interface obtained by depositing caesium lead bromide perovskite nanosheets on a graphite surface. The conclusion provides perspectives for future improvements and applications.

Toward Chiral Lasing from All-Solution-Processed Flexible Perovskite-Nanocrystal-Liquid-Crystal Membranes

Circularly polarized (CP) coherent light sources are of great potential for various advanced optical applications spanning displays/imaging to data processing/encryption and quantum communication. Here, the first demonstration of CP amplified spontaneous emission (ASE)/lasing from a free-standing and flexible membrane device is reported. The membrane device consists of perovskite nanocrystals (PNCs) and cholesteric liquid crystals (CLCs) layers sandwiched within a Fabry-Pérot (F-P) cavity architecture. The chiral liquid crystal cavity enables the generation of CP light from the device. The device is completely solution-processable and displays CP ASE with record dissymmetry factor (glum ) as high as 1.4, which is 3 orders of magnitude higher as compared with glum of CP luminescence of chiral ligand-capped colloidal PNCs. The device exhibits ultraflexibility as the ASE intensity remains unchanged after repeated 100 bending cycles and it is stable for more than 3 months with 80% of its original intensity. Furthermore, the ultraflexibility enables the generation of ASE from various objects of different geometric surfaces covered with the flexible perovskite membrane device. This work not only demonstrates the first CP ASE from a PNCs membrane with extremely high glum but also opens the door toward the fabrication of ultraflexible, extremely stable, and all solution-processable perovskite chiral laser devices.

Thermodynamically and Kinetically Controlled Nucleation and Growth of Halide Perovskite Single Crystals

Halide perovskites are ideal for next-generation optical devices and photovoltaics. Although perovskite single-crystals show reproducible optoelectronic properties, significant variations in the crystal size, anisotropy, density, defects, photoluminescence (PL), and carrier lifetime affect the sample properties and device performances. Homogenous size and shape FA/MAPbBr3 single microcrystals (MCs) with controlled edge lengths, crystal densities, PL lifetimes, and PL intensities are prepared by thermodynamically controlling and kinetically separating the crystal nucleation-growth processes using optimum N-cyclohexyl-2-pyrrolidone (CHP) concentration. The crystal growth kinetics at different CHP concentrations and temperatures are estimated spectroscopically by measuring the concentration of Pb (II). High-density cubic MCs with a homogenous size distribution, high PL intensities, and long PL lifetimes are obtained within minutes at high temperatures by the controlled addition of the pyrrolidone derivative. Conversely, the crystal size nonlinearly increases with time at low temperatures. The isotropically grown high-density single crystals at controlled nucleation-growth rates at 190 °C with 20% CHP show the highest PL intensity and the longest PL lifetimes. This method offers thermodynamic and kinetic control of perovskite single-crystal growth with shape control.

Ultrasmall water-stable CsPbBr3 quantum dots with high intensity blue emission enabled by zeolite confinement engineering

Ultrasmall CsPbBr3 perovskite quantum dots (PQDs) as promising blue-emitting materials are highly desired for full-color display and lighting applications, but their inferior efficiency and poor ambient stability hinder extensive applications. Herein, a "break-and-repair" strategy has been developed to tightly confine monodispersed ultrasmall CsPbBr3 PQDs in a zeolite. In this strategy, the CsPbBr3 PQDs are introduced into the zeolite via a high temperature evaporation method, wherein the perovskite precursors break the zeolite framework, and amino acids and silane are then used to fix the damaged framework and lock the perovskite QDs within the matrix. By modulating the synthetic conditions to control the growth of CsPbBr3, PQDs with ultrasmall size of 2 nm have been obtained in the zeolite, giving emission centered at 460 nm with a high quantum yield of 76.93%. Strikingly, the PQDs@zeolite composite exhibits water-induced reversible photoluminescence promoted by the coordination between the amino acids and PQDs in a dynamic manner, achieving enhanced water stability (14 days in aqueous solution). This work provides a new perspective for the synthesis of water-stable blue-emitting perovskite composites for potential applications in lighting fields.

Room-Temperature, Homogeneous, Single-Step, and Large-Scale Synthesis of Perovskite Nanoplatelets for Blue Light-Emitting Diodes

Inorganic perovskite nanocrystals (IPNCs) have attracted considerable attention due to their excellent optoelectronic properties. However, problems arise from anion migration during the preparation of a blue light-emitting diode (LED), and only small-scale syntheses have been conducted on a laboratory scale. By using only Br as the anion here, CsPbBr3 was synthesized in the form of nanoplatelets to eliminate the effects of anion migration and to prepare an inorganic perovskite nanoplatelet (IPNPL) emitting blue light. In addition, the synthesis was performed under ambient conditions at room temperature, and the synthetic process was shortened to enable large-scale synthesis. We used a 1 L bottle for large-scale synthesis, and a photoluminescence quantum yield (PLQY) of 78% was observed at 460 nm. We fabricated LEDs by using IPNPLs, and we observed an electroluminescence peak at 461 nm. The developed synthetic method is expected to pave the way for commercialization of IPNCs and the next-generation display market.

Tuning the Driving Force for Charge Transfer in Perovskite-Chromophore Systems

Understanding the interplay between the kinetics and energetics of photophysical processes in perovskite-chromophore hybrid systems is crucial for realizing their potential in optoelectronics, photocatalysis, and light-harvesting applications. By combining steady-state optical characterizations and transient absorption spectroscopy, we have investigated the mechanism of interfacial charge transfer (CT) between colloidal CsPbBr3 nanoplatelets (NPLs) and surface-anchored perylene derivatives and have explored the possibility of controlling the CT rate by tuning the driving force. The CT driving force was tuned systematically by attaching acceptors with different electron affinities and by varying the bandgap of NPLs via thickness-controlled quantum confinement. Our data show that the charge-separated state is formed by selectively exciting either the electron donors or acceptors in the same system. Upon exciting attached acceptors, hole transfer from perylene derivatives to CsPbBr3 NPLs takes place on a picosecond time scale, showing an energetic behavior in line with the Marcus normal regime. Interestingly, such energetic behavior is absent upon exciting the electron donor, suggesting that the dominant CT mechanism is energy transfer followed by ultrafast hole transfer. Our findings not only elucidate the photophysics of perovskite-molecule systems but also provide guidelines for tailoring such hybrid systems for specific applications.

The Anisotropic Complex Dielectric Function of CsPbBr3 Perovskite Nanorods Obtained via an Iterative Matrix Inversion Method

Colloidal lead halide perovskite nanorods have recently emerged as promising optoelectronic materials. However, more information about how shape anisotropy impacts their complex dielectric function is required to aid the development of applications that take advantage of the strongly polarized absorption and emission. Here, we have determined the anisotropy of the complex dielectric function of CsPbBr3 nanorods by analyzing the ensemble absorption spectra in conjunction with the ensemble spectral fluorescence anisotropy. This strategy allows us to distinguish the absorption of light parallel and perpendicular to the main axis so that the real and imaginary components of the dielectric function along each direction can be determined by the use of an iterative matrix inversion (IMI) methodology. We find that quantum confinement gives rise to unique axis-dependent electronic features in the dielectric function that increase the overall fluorescence anisotropy in addition to the optical anisotropy that results from particle shape, even in the absence of quantum confinement. Further, the procedure outlined here provides a strategy for obtaining anisotropic complex dielectric functions of colloidal materials of varying composition and aspect ratios using ensemble solution-phase spectroscopy.

All-Inorganic Lead-Free Doped-Metal Halides for Bright Solid-State Emission from Primary Colors to White Light

Metal halides have been explored with the aid of strong photoluminescence for optical and optoelectronic applications. However, the preparation of lead (Pb)-free solid-state emitters with high photoluminescence quantum yields (PLQYs) and tunable emission remains exceptionally challenging. Herein, we report metal ion (Cu(I), Mn(II), and Sn(II))-doped Cs₃ZnI₅ single crystals that are primary color (violet, green, and orange/red) emitters with extremely high PLQYs. Whereas the Mn-doping leads to bright green emissions with 100% PLQY, the Cu- and Sn-doping give rise to blue and red emissions with PLQYs of 57 and 64%, respectively. Interestingly, higher Mn doping results in white light emissive crystals as a side product, which are found to be Mn-doped CsI single crystals. The bright white light emissive crystals can be synthesized in a pure form in large quantities and exhibit a high color rendering index (CRI) of 78 and CIE coordinates of (0.30, 0.38), which are close to daylight conditions. To the best of our knowledge, this is the first demonstration of white light emission from a complete inorganic system. Importantly, the single crystals of all colors exhibit high long-term stability as their PLQY remains unchanged even after 2 months of preparation, and are thermally stable up to 600 °C.

Real-Time In Situ Observation of CsPbBr3 Perovskite Nanoplatelets Transforming into Nanosheets

The manipulation of nano-objects through heating is an effective strategy for inducing structural modifications and therefore changing the optoelectronic properties of semiconducting materials. Despite its potential, the underlying mechanism of the structural transformations remains elusive, largely due to the challenges associated with their in situ observations. To address these issues, we synthesize temperature-sensitive CsPbBr₃ perovskite nanoplatelets and investigate their structural evolution at the nanoscale using in situ heating transmission electron microscopy. We observe the morphological changes that start from the self-assembly of the nanoplatelets into ribbons on a substrate. We identify several paths of merging nanoplates within ribbons that ultimately lead to the formation of nanosheets dispersed randomly on the substrate. These observations are supported by molecular dynamics simulations. We correlate the various paths for merging to the random orientation of the initial ribbons along with the ligand mobility (especially from the edges of the nanoplatelets). This leads to the preferential growth of individual nanosheets and the merging of neighboring ones. These processes enable the creation of structures with tunable emission, ranging from blue to green, all from a single material. Our real-time observations of the transformation of perovskite 2D nanocrystals reveal a route to achieve large-area nanosheets by controlling the initial orientation of the self-assembled objects with potential for large-scale applications.

Vertex-Oriented Cube-Connected Pattern in CsPbBr3 Perovskite Nanorods and Their Optical Properties: An Ensemble to Single-Particle Study

The design of cube-connected nanorods is accomplished by connecting seed nanocrystals of a defined shape in a particular orientation or by etching selective facets of preformed nanorods. In lead halide perovskite nanostructures, which retain mostly a hexahedron cube shape, such patterned nanorods can be designed with the anisotropic direction along the edge, vertex, or facet of seed cubes. Combining the Cs-sublattice platform for transforming metal halides to halide perovskites with facet-specific ligand binding chemistry, herein, vertex-oriented patterning of nanocubes in one-dimensional (1D) rod structures is reported. By tuning the length of host metal halides, their lengths could also be tuned from 100 nm to nearly 1000 nm. The symmetry of the hexagonal phase of host halide CsCdBr₃ and product orthorhombic CsPbBr₃ helped in maintaining the vertex [201] as the anisotropic direction. Neutral exciton recombination rates, extracted from photoluminescence blinking traces, showed a systematic increase from isolated cubes to cube-connected nanorods of various lengths. Efficient coupling of wave functions in vertex-oriented cube assemblies permits exciton delocalization. Our findings on carrier delocalization in cube-connected nanorods along their vertex direction having minimum interfacial contacts provide valuable insights into the fundamental chemistry of assembling anisotropic halide perovskite nanostructures as conducting wires.

Eliminating Nanocrystal Surface Light Loss and Ion Migration to Achieve Bright Mixed-Halide Blue Perovskite LEDs

Blue light-emittin g diodes (LEDs) are important components for perovskite electroluminescence applications, which still suffer from insufficient luminescence efficiency and poor stability. In Cl/Br mixed perovskite NCs, surficial defects cause severe light failure and ion migration, the in-depth mechanism of which is also not clear. To gain insights into these issues, we employ the ligand post-addition approach for mixed Cl/Br NCs by using octylammonium hydrobromide (OctBr) ligands, which effectively decrease surficial light loss and block ion migration pathways. The passivated CsPbCl_1.5Br_1.5 NCs exhibit exceptional blue emission with 95% PLQY, and the electroluminescence spectra of LEDs are located at the initial positions at the initial states. The treated NC blue devices show a negligible color shift as the voltage increases, which proves that electric-field-driven ion migration is drastically suppressed. In addition, OctBr-treated CsPbCl_1.5Br_1.5 and CsPbClBr₂ NC LEDs show high external quantum efficiencies of 2.42 and 3.05% for emission peaks at 456 and 480 nm, respectively. Our work identified the nature of NC surface defects and provided a surficial modification approach to develop high-performance and color-stable blue mixed-halide perovskite LEDs.

Novelty All-Inorganic Titanium-Based Halide Perovskite for Highly Efficient Photocatalytic CO2 Conversion

Lead halide perovskite materials have great potential for photocatalytic reaction due to their low fabrication cost, unique optical absorption coefficient, and suitable band structures. However, the main problems are the toxicity and instability of the lead halide perovskite materials. Therefore, a facile synthetic method is used to prepare lead-free environmentally friendly Cs₂ TiX₆ (X = Cl, Cl_0.5 Br_0.5 , Br) perovskite materials. Their structural and optical characteristics are systematically investigated. The band gaps of the produced samples are illustrated to be from 1.87 to 2.73 eV. Moreover, these materials can keep high stability in harsh environments such as illumination and heating, and the Cs₂ Ti(Cl_0.5 Br_0.5 )₆ microcrystals demonstrate the yields of 176 µmol g^-1 for CO and 78.9 µmol g^-1 for CH₄ after light irradiation for 3 h, which is of the first report of Ti-based perovskite photocatalysts. This finding demonstrates that the Ti-based perovskites will create opportunities for photocatalytic applications, which may offer a new idea to construct low-cost, eco-friendly, and bio-friendly photocatalysts.

Water-Driven Synthesis of Deep-Blue Perovskite Colloidal Quantum Wells for Electroluminescent Devices

Perovskite colloidal quantum wells (QWs) are promising to realize narrow deep-blue emission, but the poor optical performance and stability suppress their practical application. Here, we creatively propose a water-driven synthesis strategy to obtain size-homogenized and strongly confined deep-blue CsPbBr₃ QWs, corresponding to three monolayers, which emit at the deep-blue wavelength of 456 nm. The water controls the orientation and distribution of the ligands on the surface of the nanocrystals, thus inducing orientated growth through the Ostwald ripening process by phagocytizing unstable nanocrystals to form well-crystallized QWs. These QWs present remarkable stability and high photoluminescence quantum yield of 94 %. Furthermore, we have prepared light-emitting diodes based on the QWs via the all-solution fabrication strategy, achieving an external quantum efficiency of 1 % and luminance of 2946 cd m^-2 , demonstrating state-of-the-art brightness for perovskite QW-based LEDs.

Electron Trapping Prolongs the Lifetime of Charge-Separated States in 2D Perovskite Nanoplatelet-Hole Acceptor Complexes

Two-dimensional (2D) lead halide perovskite nanoplatelets (NPLs) are promising materials for blue light emission because of the strong quantum confinement in the 2D morphology. However, the identity of carrier traps and the trap influence on charge transfer in these NPLs remain unclear. Herein, transient absorption studies revealed two types of electron traps in 3 monolayer lead bromide perovskite NPLs with trapping lifetime of 9.0 ± 0.6 and 516 ± 59 ps, respectively, while no hole traps were observed. Systematic charge transfer experiments show that electron traps have negligible influence on ultrafast electron transfer or hole transfer but extend the half-lifetime of the charge-separated state from 2.1 ± 0.1 to 68 ± 3 ns after hole transfer, which is explained by the reduced electron-hole overlap. This work contributes to the understanding of the fundamental carrier dynamics in 2D perovskite NPLs and offers guidelines for boosting their performance in optoelectronics and photocatalysis.

Ligands in Lead Halide Perovskite Nanocrystals: From Synthesis to Optoelectronic Applications

Ligands are indispensable for perovskite nanocrystals (NCs) throughout the whole lifetime, as they not only play key roles in the controllable synthesis of NCs with different sizes and shapes, but also act as capping shell that affects optical properties and electrical coupling of NCs. Establishing a systematic understanding of the relationship between ligands and perovskite NCs is significant to enable many potential applications of NCs. This review mainly focuses on the influence of ligands on perovskite NCs. First of all, the ligands-dominated size and shape control of NCs is discussed. Whereafter, the surface defects of NCs and the bonding between ligands and perovskite NCs are classified, and corresponding post-treatment of surface defects via ligands is also summarized. Furthermore, advances in engineering the ligands towards the high performance of optoelectronic devices based on perovskite NCs, including photodetector, solar cell, light emitting diode (LED), and laser, and finally to potential challenges are also discussed.

Modulation of the Excitation States in All-Inorganic Halide Perovskites via Sb3+ and Bi3+ Codoping

Sb³⁺-doped halide perovskites are promising candidates for solid-state lighting due to their diverse fluorescent colors and high efficiency. However, the mismatched high excitation energy with commercial UV chips is one of the critical issues to be addressed. Herein, a Bi³⁺ codoping strategy was established as a general and efficient approach to modulate the excitation spectrum from the Sb³⁺-doping center in all-inorganic perovskites of Cs₂InCl₅·H₂O, Cs₂NaInCl₆, and Rb₃InCl₆. The incorporated Bi³⁺ greatly enhanced the splitting of the A band (¹S₀-³P₁ transition) and boosts the enormous redshift of the low-energy branch in all these systems. The interactions persist strongly even at extremely low doping concentrations, suggesting a dipole-based long-range interaction. The results provide an in-depth insight into the contribution mechanism of Bi³⁺ to Sb³⁺ in all-inorganic perovskites, which throws light upon tuning the excitation spectrum of broadband emission from the extrinsic self-trapped exciton (STE).

Ultra-small α-CsPbI3 perovskite quantum dots with stable, bright and pure red emission for Rec. 2020 display backlights

The synthesis of α-CsPbI₃ perovskite quantum dots (QDs) with pure red emission around 630 nm is in high demand for display backlight application. However, the phase transition of α-CsPbI₃ to yellow non-emitting δ-CsPbI₃ has been proven to be a great challenge for the classic colloidal synthesis route for perovskite QDs in octadecene (ODE). Herein, we report a novel colloidal synthesis route by replacing ODE with lauryl methacrylate (LMA) as the reaction solvent to improve the solubility of precursors, resulting in small sized α-CsPbI₃ QDs with a diameter of only 4.2 nm, which are the smallest red PQDs reported so far. The corresponding CsPbI₃ QD films exhibit a tunable photoluminescence (PL) emission peak in the bright pure red region of 627 to 638 nm. The CsPbI₃ QD polymer composite films with PL emission at 630 nm exhibit a superior photoluminescence quantum yield (PLQY) and photostability to mixed halide CsPbBrI₂ films under intense illumination. Perovskite light emitting diodes (LED) with the color gamut reaching 96% of the Rec. 2020 standard are achieved using these films. This study provides a high-performance pure red fluorescent material with a robust, low-cost, and reproducible colloidal chemistry that will pave the way for the adoption of perovskite QDs in display backlight application.

Atomically flat semiconductor nanoplatelets for light-emitting applications

The last decade has witnessed extensive breakthroughs and significant progress in atomically flat two-dimensional (2D) semiconductor nanoplatelets (NPLs) in terms of synthesis, growth mechanisms, optical and electronic properties and practical applications. Such NPLs have electronic structures similar to those of quantum wells in which excitons are predominantly confined along the vertical direction, while electrons are free to move in the lateral directions, resulting in unique optical properties, such as extremely narrow emission line width, short photoluminescence (PL) lifetime, high gain coefficient, and giant oscillator strength transition (GOST). These unique optical properties make NPLs favorable for high color purity light-emitting applications, in particular in light-emitting diodes (LEDs), backlights for liquid crystal displays (LCDs) and lasers. This review article first introduces the intrinsic characteristics of 2D semiconductor NPLs with atomic flatness. Subsequently, the approaches and mechanisms for the controlled synthesis of atomically flat NPLs are summarized followed by an insight on recent progress in the mediation of core/shell, core/crown and core/crown@shell structures by selective epitaxial growth of passivation layers on different planes of NPLs. Moreover, an overview of the unique optical properties and the associated light-emitting applications is elaborated. Despite great progress in this research field, there are some issues relating to heavy metal elements such as Cd²⁺ in NPLs, and the ambiguous gain mechanisms of NPLs and others are the main obstacles that prevent NPLs from widespread applications. Therefore, a perspective is included at the end of this review article, in which the current challenges in this stimulating research field are discussed and possible solutions to tackle these challenges are proposed.

Suppression of Thermal Quenching for CsPbX3 (X = Cl, Br, and I) Quantum Dots via the Hollow Structure of SrTiO3 and Light-Emitting Diode Applications

All-inorganic perovskite quantum dots (PQDs, CsPbX₃, X = Cl, Br, and I) show outstanding application prospects in the field of photoelectric devices. In recent years, the development of PQDs has greatly improved their stability to water, oxygen, and light. However, thermal quenching of PQDs greatly limits their practical application. Herein, we embed PQDs into ATiO₃ (A = Ca, Ba, and Sr) of three different mesoporous spherical structures to explore the effect on thermal quenching of PQDs. Because of the unique mesoporous hollow microsphere structure and low thermal conductivity of SrTiO₃, it can effectively block the heat transfer and improve the thermal quenching of PQDs. The photoluminescence (PL) intensity of CsPbBr₃@SrTiO₃ composites is 72.6% of the initial intensity after heating to 120 °C. Moreover, the PL intensity of CsPbBr₃@SrTiO₃ composites remains about 80% of the initial value even when stored in air for 20 days or irradiated by 365 nm UV light for 48 h. A neutral white light-emitting diode is assembled by a blue chip, CsPbBr₃@SrTiO₃ composites, and red phosphor of K₂SiF₆:Mn⁴⁺, which has a color temperature of 5389 K and a color gamut covered 133% of National Television Standards Committee (NTSC).

Programmable precise kinetic control over crystal phase, size, and equilibrium in spontaneous metathesis reaction for Cs-Pb-Br nanostructure patterns at room temperature

Growth kinetics involved in spontaneous random clustering of perovskite precursors to a particular cesium-lead-bromide (Cs-Pb-Br) nanocrystal (NC) is a poorly understood phenomenon and its spectroscopic investigation is highly challenging. There is scarcely any method that has been optimized yet in which perovskites and their related NCs of a particular size can be grown, viewed, or tuned to another by reaction handling. Here, for the first time, we shed light on the largely overlooked process of growth kinetics of these transformations throughout the reaction trajectory of anionic [PbBr_x]^n- crystallization dictated by Cs⁺ cation and report a simple and direct approach to control the metathesis reaction between two precursors (specifically Cs⁺- and PbBr₂-associated oligomeric complexes) in one solvent at room temperature to monitor the NC growth characteristics in a stepwise manner even in the early stages of nucleation. Altering the molar ratio of the two precursors up to a factor of 10 leads to the formation of three prominent phases (CsPbBr₃, Cs₄PbBr₆, CsBr) as dictated by Cs⁺ to trigger distinct morphological forms (nanobelts, nanoplatelets, rhombohedral NCs, pseudo-rhombic NCs, spherical CsBr NCs, cubic CsBr NCs) including a transient phase that is formed out of linearly self-assembled CsPbBr₃ clusters. Our results pave the way towards understanding spontaneous crystallization to develop well-defined, hassle-free routes for diverse perovskite NCs in a simple yet controlled manner.

Perovskite Random Lasers, Process and Prospects

Random lasers (RLs) are a kind of coherent light source with optical feedback based on disorder-induced multiple scattering effects instead of a specific cavity. The unique feedback mechanism makes RLs different from conventional lasers. They have the advantages of small volume, flexible shape, omnidirectional emission, etc., and have broad application prospects in the fields of laser illumination, speckle-free imaging, display, and sensing. Colloidal metal-halide perovskite nanomaterials are a hot research field in light sources. They have been considered as desired gain media owing to their superior properties, such as high photoluminescence, tunable emission wavelengths, and easy fabrication processes. In this review, we summarize the research progress of RLs based on perovskite nanomaterials. We first present the evolution of the RLs based on the perovskite quantum dots (QDs) and perovskite films. The fabrication process of perovskite nano-/microstructures and lasers is discussed in detail. After that, the frontier applications of perovskite RLs are discussed. Finally, the challenges are discussed, and the prospects for further development are proposed.

Engineering the Optical Properties of CsPbBr3 Nanoplatelets through Cd2+ Doping

Lead halide perovskite nanoplatelets (NPls) attract significant attention due to their exceptional and tunable optical properties. Doping is a versatile strategy for modifying and improving the optical properties of colloidal nanostructures. However, the protocols for B-site doping have been rarely reported for 2D perovskite NPls. In this work, we investigated the post-synthetic treatment of CsPbBr₃ NPls with different Cd²⁺ sources. We show that the interplay between Cd²⁺ precursor, NPl concentrations, and ligands determines the kinetics of the doping process. Optimization of the treatment allows for the boosting of linear and nonlinear optical properties of CsPbBr₃ NPls via doping or/and surface passivation. At a moderate doping level, both the photoluminescence quantum yield and two-photon absorption cross section increase dramatically. The developed protocols of post-synthetic treatment with Cd²⁺ facilitate further utilization of perovskite NPls in nonlinear optics, photonics, and lightning.

Recent progress of single-halide perovskite nanocrystals for advanced displays

Light-emitting diodes based on lead halide perovskite nanocrystals (LHP NCs) have shown an astonishing increase in efficiency in just several years of academic research, reaching high external quantum efficiencies exceeding 20%. The extensive color-tunability and narrow emission bandwidth of LHP NCs, in particular, are of great importance in the creation of the next generation of ultra-high-definition displays, as defined by the Rec. 2020 standard recommendation. In fact, whereas the colour of LHP NCs can be easily tuned by the compositions of halogens, the ion migration in mixed-halide perovskites under the electric field will seriously affect the spectral stability and operational lifetimes of perovskite light-emitting diodes (PeLEDs). Therefore, it is essential to realize efficient colour-saturated PeLEDs based on single-halide perovskite NCs. In this review, we focus on the recent progress in LHP NC-based PeLEDs and highlight the strategy of tuning the spectral emission based on quantum confinement or cation alloying/doping in single-halide perovskite NCs. Finally, we will give an outlook on future research avenues for preparing high-efficiency pure green, red and blue PeLEDs based on single-halide perovskite NCs.

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles

A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Enhancing the Intrinsic and Extrinsic Stability of Halide Perovskite Nanocrystals for Efficient and Durable Optoelectronics

Over the past few years, metal halide perovskite nanocrystals have been at the forefront of colloidal semiconductor nanomaterial research because of their fascinating properties and potential applications. However, their intrinsic phase instability and chemical degradation under external exposures (high temperature, water, oxygen, and light) are currently limiting the real-world applications of perovskite optoelectronics. To overcome these stability issues, researchers have reported various strategies such as doping and encapsulation. The doping improves the optical and photoactive phase stability, whereas the encapsulation protects the perovskite NCs from external exposures. This perspective discusses the rationale of various strategies to enhance the stability of perovskite NCs and suggests possible future directions for the fabrication of optoelectronic devices with long-term stability while maintaining high efficiency.

Elucidating the Role of Antisolvents on the Surface Chemistry and Optoelectronic Properties of CsPbBrxI3-x Perovskite Nanocrystals

Colloidal lead-halide perovskite nanocrystals (LHP NCs) have emerged over the past decade as leading candidates for efficient next-generation optoelectronic devices, but their properties and performance critically depend on how they are purified. While antisolvents are widely used for purification, a detailed understanding of how the polarity of the antisolvent influences the surface chemistry and composition of the NCs is missing in the field. Here, we fill this knowledge gap by studying the surface chemistry of purified CsPbBr_xI_3-x NCs as the model system, which in itself is considered a promising candidate for pure-red light-emitting diodes and top-cells for tandem photovoltaics. Interestingly, we find that as the polarity of the antisolvent increases (from methyl acetate to acetone to butanol), there is a blueshift in the photoluminescence (PL) peak of the NCs along with a decrease in PL quantum yield (PLQY). Through transmission electron microscopy and X-ray photoemission spectroscopy measurements, we find that these changes in PL properties arise from antisolvent-induced iodide removal, which leads to a change in halide composition and, thus, the bandgap. Using detailed nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR) measurements along with density functional theory calculations, we propose that more polar antisolvents favor the detachment of the oleic acid and oleylamine ligands, which undergo amide condensation reactions, leading to the removal of iodide anions from the NC surface bound to these ligands. This work shows that careful selection of low-polarity antisolvents is a critical part of designing the synthesis of NCs to achieve high PLQYs with minimal defect-mediated phase segregation.

Revisiting the nontemplate approach for the synthesis of highly green emissive hybrid perovskite nanocrystals: platelets or spheres?

The long-standing debate about the morphology of colloidal methylammonium lead bromide perovskites nanocrystals, manufactured by our nontemplate synthetic strategy reported in 2014, is now resolved; specifically, the highest green emissive single nanoplatelets (of up to 93%) with long-term chemical and photochemical stability have been obtained after suitable purification steps.