Pure white-light emitting ultrasmall organic-inorganic hybrid perovskite nanoclusters

Structural Changes and Differences in Intrinsic Photoluminescence of Four Cationic Two-Dimensional Lead Halide Frameworks Modulated by Synthesis Temperature

Luminescent hybrid organolead halide materials with cationic inorganic frameworks and high chemical inertness have demonstrated broad application prospects in the visible light region. However, the corresponding relationship between structural changes and luminescence properties in such materials needs further clarification. Here, for the first time, we have successfully synthesized [Pb2X3](PDAH)(H2O) (X = Cl or Br, PDAH = C7H4NO4) single crystals via a facile hydrothermal method and then obtained [Pb11X14](PDA)4 (X = Cl or Br, PDA = C7H3NO4) at higher temperatures. The different synthesis temperatures resulted in significant differences in the luminescence properties of the two groups of structures. X-ray crystallography revealed the different degrees of distortion of the PbII centers' coordination environment between the two structures, which would significantly affect the electron-phonon coupling process under excited states and ultimately affect the emission properties originating from self-trapped excitons (STEs) of the two materials. In addition, density functional theory (DFT) calculations indicate that the two structures have different band gap characteristics due to the different proportions of inorganic and organic components, which also affect the optoelectronic properties of the two groups of materials. It is also worth mentioning that the broadband orange-light emission of [Pb11Br14](PDA)4 with a high photoluminescence quantum efficiency (PLQE) of 86% endows it with potential applications in WLEDs.

White light-emitting, biocompatible, water-soluble metallic magnesium nanoclusters for bioimaging applications

Ultra-small (1.6 nm), water-soluble, white light-emitting (WLE), highly stable (∼8 months) BSA templated metallic (Mg⁰) nanoclusters (fluorescent magnesium nanoclusters = FMNCs) is developed using the green and facile route. Synthesis was facilitated by the reduction of magnesium salt, where template bovine serum albumin is utilized as a reducing agent and ascorbic acid act as a capping agent to impart stability in water, thereby obtaining stabilized Mg⁰nanoclusters In solution, stabilized Mg⁰nanoclusters produce white light (450-620 nm with FWHM ∼120 nm) upon 366 nm light excitation. This white light emission was found to have a CIE coordinate of 0.30, 0.33 [pure white light CIE (0.33, 0.33)]. Taking advantage of WLE and ultrasmall size, FMNCs were used forin vitrofluorescence imaging of HaCaT cell lines, yielding blue (τ= 2.94 ns, with a relative of QY = 1.2 % w.r.t QS), green (τ= 3.07 ns; relative quantum yield of 4.6% w.r.t R6G) and red (τ= 0.3 ns) images. Further, incubation of FMNCs with HEK293 (Human embryonic kidney cell) and cancerous MDA-MB-231 (Breast cancer cell line) human cell lines yielded 100 % cell viability. Current work is envisioned to contribute significantly in the area of science, engineering, and nanomedicine.

Recent Advances in Blue Perovskite Quantum Dots for Light-Emitting Diodes

Metal halide perovskite nanostructures have sparked intense research interest due to their excellent optical properties. In recent years, although the green and red perovskite light-emitting diodes (PeLEDs) have achieved a significant breakthrough with the external quantum efficiency exceeding 20%, the blue PeLEDs still suffer from inferior performance. Previous reviews about blue PeLEDs focus more on 2D/quasi-2D or 3D perovskite materials. To develop more stable and efficient blue PeLEDs, a systematic review of blue perovskite quantum dots (PQDs) is urgently demanded to clarify how PQDs evolve. In this review, the recent advances in blue PQDs involving mixed-halide, quantum-confined all-bromide, metal-doped and lead-free PQDs as well as their applications in PeLEDs are highlighted. Although several excellent PeLEDs based on these PQDs have been demonstrated, there are still many problems to be solved. A deep insight into the advantages and disadvantages of these four types of blue-emitting PQDs is provided. Then, their respective potential and issues for blue PeLEDs have been discussed. Finally, the challenges and outlook for efficient and stable blue PeLEDs based on PQDs are addressed.

Enhancement of Photoluminescence Quantum Yield and Stability in CsPbBr3 Perovskite Quantum Dots by Trivalent Doping

We determine the influence of substitutional defects on perovskite quantum dots through experimental and theoretical investigations. Substitutional defects were introduced by trivalent dopants (In, Sb, and Bi) in CsPbBr3 by ligand-assisted reprecipitation. We show that the photoluminescence (PL) emission peak shifts toward shorter wavelengths when doping concentrations are increased. Trivalent metal-doped CsPbBr3 enhanced the PL quantum yield (~10%) and air stability (over 10 days). Our findings provide new insights into the influence of substitutional defects on substituted CsPbBr3 that underpin their physical properties.

Study on the Ultrafast Process of Perovskite Nanoparticles Modified by Different Alkyl Chains

Three kinds of perovskite nanoparticles encapsulated with different chain lengths of alkylammonium, (CH₃NH₃)_x(CH₃(CH₂)₃NH₃)_(1-x)PbBr₃ (NP-C₄), (CH₃NH₃)_x(CH₃(CH₂)₇NH₃)_(1-x)PbBr₃ (NP-C₈), and (CH₃NH₃)_x(CH₃(CH₂)₁₁NH₃)_(1-x)PbBr₃ (NP-C₁₂), are successfully prepared. X-ray powder diffraction experiments demonstrate that these three nanoparticles are all pure cubic phase. However, the compositions of these three nanoparticles are significantly different, as revealed by steady-state absorption spectra. NP-C₄ mainly consists of 2D perovskite with m (number of unit cell layers) = 1 and 3D perovskite. Instead, NP-C₈ and NP-C₁₂ are mainly composed of 2D perovskite with m = 3, 4, and 5. Time-resolved fluorescence spectra and femtosecond transient absorption spectra suggest the presence of energy transfer from 2D perovskite to 3D perovskite in these three nanoparticles. More importantly, the energy-transfer rate gradually decreases from NP-C₄ to NP-C₁₂. This result suggests that the composition of perovskite nanoparticles and their corresponding photophysical properties can be controlled by the chain length of alkylammonium. This provides a new insight for preparing novel perovskite nanoparticles for special applications.

Locally collective hydrogen bonding isolates lead octahedra for white emission improvement

As one of next-generation semiconductors, hybrid halide perovskites with tailorable optoelectronic properties are promising for photovoltaics, lighting, and displaying. This tunability lies on variable crystal structures, wherein the spatial arrangement of halide octahedra is essential to determine the assembly behavior and materials properties. Herein, we report to manipulate their assembling behavior and crystal dimensionality by locally collective hydrogen bonding effects. Specifically, a unique urea-amide cation is employed to form corrugated 1D crystals by interacting with bromide atoms in lead octahedra via multiple hydrogen bonds. Further tuning the stoichiometry, cations are bonded with water molecules to create a larger spacer that isolates individual lead bromide octahedra. It leads to zero-dimension (0D) single crystals, which exhibit broadband ‘warm’ white emission with photoluminescence quantum efficiency 5 times higher than 1D counterpart. This work suggests a feasible strategy to modulate the connectivity of octahedra and consequent crystal dimensionality for the enhancement of their optoelectronic properties.

Low dimensional lead halide perovskites possess intriguing optical properties that are still under debate. Here Cui et al. use hydrogen bonds containing spacers to synthesize highly luminescent perovskites with fully isolated lead-bromide octahedras and shed light on the origin of the emission.

Pressure-Induced Emission (PIE) of One-Dimensional Organic Tin Bromide Perovskites

Low-dimensional halide perovskites easily suffer from the structural distortion related to significant quantum confinement effects. Organic tin bromide perovskite C₄N₂H₁₄SnBr₄ is a unique one-dimensional (1D) structure in which the edge sharing octahedral tin bromide chains [SnBr₄^2-]_∞ are embraced by the organic cations C₄N₂H₁₄²⁺ to form the bulk assembly of core-shell quantum wires. Some unusual phenomena under high pressure are accordingly expected. Here, an intriguing pressure-induced emission (PIE) in C₄N₂H₁₄SnBr₄ was successfully achieved by means of a diamond anvil cell. The observed PIE is greatly associated with the large distortion of [SnBr₆]^4- octahedral motifs resulting from a structural phase transition, which can be corroborated by in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction spectra. The distorted [SnBr₆]^4- octahedra would accordingly facilitate the radiative recombination of self-trapped excitons (STEs) by lifting the activation energy of detrapping of self-trapped states. First-principles calculations indicate that the enhanced transition dipole moment and the increased binding energy of STEs are highly responsible for the remarkable PIE. This work will improve the potential applications in the fields of pressure sensors, trademark security, and information storage.

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

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

Phase control of quasi-2D perovskites and improved light-emitting performance by excess organic cations and nanoparticle intercalation

The optoelectronic properties of quasi-two-dimensional organic-inorganic hybrid perovskites can be tuned by controlling the formation of Ruddlesden-Popper type phases, which enables diverse device applications such as photovoltaics and light-emitting diodes (LEDs). Herein, the influence of excess organic cations on the phase formation of (PEA)2MAn-1PbnBr3n+1 is systematically investigated with various mixing ratios to discover the phase distribution beneficial for light-emitting diodes. It is found that PEA cations exceeding Pb ions in molar ratio are required to produce small-n phases in the films with a strong photoluminescence, while excess MA cations enable the formation of more large-n phases. Low electrical conductivity inherent to the properties of quasi-2D perovskites is further lowered by the introduction of excess organic cations. This is overcome by the intercalation of zinc oxide (ZnO) nanoparticles (NPs) into the blocking layers composed of PEA cations. Importantly, these metal oxide NPs also modulate the phase distribution, enabling the realization of bright green quasi-2D perovskites with a better stability and a maximum luminance of nearly 60 000 cd m-2, which is the highest brightness compared to the so far reported quasi-2D perovskite LEDs incorporating organic cations.

Flexible Polymer-Assisted Mesoscale Self-Assembly of Colloidal CsPbBr3 Perovskite Nanocrystals into Higher Order Superstructures with Strong Inter-Nanocrystal Electronic Coupling

Surface-passivating ligands, although ubiquitous to colloidal nanocrystal (NC) syntheses, play a role in assembling NCs into higher order structures and hierarchical superstructures, which has not been demonstrated yet for colloidal CsPbX₃ (X = Cl, Br, and I) NCs. In this work, we report that functional poly(ethylene glycols) (PEG₆-Y, Y = -COOH and -NH₂) represent unique surface-passivating ligands enabling the synthesis of near-uniform CsPbBr₃ NCs with diameters of 3.0 nm. The synthesized NCs are assembled into individual pearl necklaces, bundled pearl necklaces, lamellar, and nanorice superstructures, in situ. It is believed a variety of forces, including van der Waals attractions between hydrophilic PEG tails in a nonpolar solvent and dipole-dipole attraction between NCs, drive mesoscale assembly to form superstructures. Furthermore, postsynthetic ligand treatment strengthens the argument for polymer-assisted mesoscale assembly as pearl necklace assemblies can be successfully converted into either lamellar or nanorice structures. We observe an ∼240 meV bathochromic shift in the lowest energy absorption peak of CsPbBr₃ NCs when they are present in the lamellar and nanorice assemblies, representing strong inter-NC electronic coupling. Moreover, pearl necklace structures are spontaneously assembled into micrometer length scale twisted ribbon hierarchical superstructures during storage of colloidal CsPbBr₃ NCs. The results show that the self-assembled superstructures of CsPbBr₃ NCs are now feasible to prepare via template-free synthesis, as self-assembled structures emerge in the bulk solvent, a process that mimics biological systems except for the use of nonbiological surface ligands (PEG₆-Y). Taken together, emergent optoelectronic properties and higher order superstructures of CsPbBr₃ NCs should aid their potential use in solid-state devices and simplify scalable manufacturing.

MA2CoBr4: lead-free cobalt-based perovskite for electrochemical conversion of water to oxygen

We have synthesized a lead-free stable organic–inorganic perovskite (MA₂CoBr₄) by using non-hazardous solvents such as methanol and ethanol, which are eco-friendly and safe to handle in comparison to DMF, toluene, etc.

Revealing Photoluminescence Modulation from Layered Halide Perovskite Microcrystals upon Cyclic Compression

Halide perovskites show promise for high-efficiency solar energy conversion and light-emitting diode devices owing to their bandgap, which falls within the visible optical range. However, due to their rigidity, GPa pressures are necessary to control the complex interplay between their electronic and crystallographic structure. Layered perovskites are likely to be controlled using much lower pressures by exploiting the optical anisotropy of the embedded organic molecules in the structure. This work introduces layered perovskite microplatelets and demonstrates the extreme sensitivity of their emission to cyclic mechanical loading in the range of tens of MPa. A drastic change in their emission is observed in situ, from near-white to an enhanced blue color. This process is reversible, as is evident from a hysteresis loop in the photoluminescence (PL) intensity of the microplatelets. A combination of experimental analysis and computational modelling shows that such behavior cannot be attributed to changes in the crystallographic structure of the flakes. Instead, it suggests that, thanks to their structural anisotropy, microplate alignment and reorientation are responsible for the observed PL modulation. The possibility to tune the optical emission of layered perovskite crystals via low pressures makes them highly interesting as active materials in applications where stress sensing or light modulation is desired.

White perovskite based lighting devices

Hybrid organic–inorganic and all-inorganic metal halide perovskites have been one of the most intensively studied materials during the last few years.

Synthesis and characterization of Mn-doped CsPb(Cl/Br)3 perovskite nanocrystals with controllable dual-color emission

Colloidal Mn-doped CsPb(Cl/Br)₃ NCs were synthesized at different MnCl₂-to-PbBr₂ molar feed ratios or reaction temperatures to tune their color emission.

Bright Tail States in Blue-Emitting Ultrasmall Perovskite Quantum Dots

All-inorganic lead halide perovskite quantum dots (CsPbBr₃ QDs) are attracting significant research interests because of their highly efficient light-emitting performance combined with tunable emission wavelength facilely realized by ion exchange. However, blue emission from perovskite QDs with strong quantum confinement is rarely reported and suffers from lower luminescence efficiency. Here we report blue-emitting ultrasmall (∼3 nm) CsPbBr₃ QDs with photoluminescence (PL) quantum yield as high as 68%. Using time-resolved and steady-state PL spectroscopy, we elucidate the mechanism of the highly efficient PL as recombination of excitons localized in radiative band tail states. Through analyzing the spectral-dependent PL lifetime and the PL line shape, we obtain a large band tail width of ∼80 meV and a high density of state of ∼10²⁰ cm^-3. The relaxation of photocarriers into the radiative tail states suppresses the capture by nonradiative centers. Our results provide solid evidence for the positive role of band tail states in the optical properties of lead halide perovskites, which can be further tailored for high-performance optoelectronic devices.

Ultrastable, cationic three-dimensional lead bromide frameworks that intrinsically emit broadband white-light

We have discovered the first two lead halide materials adopting a purely cationic inorganic 3D topology. The highly distorted Pb^II centers afford strong electron–phonon coupling in a deformable lattice and unusual broadband white-light emission as an intrinsic property.

Herein, we report the unusual broadband white-light emission as an intrinsic property from two cationic lead bromide frameworks. This is the first time that the metal halide materials adopting a purely inorganic positively-charged three-dimensional (3D) topology have been synthesized, thus affording highly distorted Pb^II centers. The single-component white-light emitters achieve an external quantum efficiency of up to 5.6% and a correlated color temperature of 5727 K, producing typical white-light close to that of fluorescent light sources. Unlike the air/moisture-sensitive 3D organolead halide perovskites, our cationic materials are chemically “inert” over a wide range of pH as well as aqueous boiling condition. Importantly, these long-sought ultrastable lead halide materials exhibit undiminished photoluminescence upon continuous UV-irradiation for 30 days under atmospheric condition (∼60% relative humidity, 1 bar). Our mechanistic studies indicate the broadband emission have contributions from the self-trapped excited states through electron-vibrational coupling in the highly deformable and anharmonic lattice, as demonstrated by variable-temperature photoluminescence/absorption spectra as well as X-ray crystallography studies. The chemical robustness and structural tunability of the 3D cationic bromoplumbates open new paths for the rational design of hybrid bulk emitters with high photostability.

One-Step Preparation of Cesium Lead Halide CsPbX3 (X = Cl, Br, and I) Perovskite Nanocrystals by Microwave Irradiation

CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) are competitive emitting materials for illumination and display because of their outstanding photophysical properties. However, the conventional synthetic approaches suffer from low yields, complex procedures, and toxic chemicals. In this work, we demonstrate a one-step microwave-assisted approach to prepare CsPbX₃ NCs. The homogeneous heating and rapid temperature increment of microwave preparation facilitate the growth of CsPbX₃ NCs, producing the NCs with high photoluminescence quantum yields up to 90%, narrow emission full-width at half-maximum, and emission color tunable from blue to red. By optimizing the preparation conditions of the microwave-assisted approach, CsPbX₃ NCs with cation- and halide anion-controlled emission properties, tunable reaction rate, and enhanced stability are prepared. Light-emitting diode (LED) prototypes are further fabricated by employing the as-prepared CsPbX₃ NCs as the color conversion materials on commercially available 365 nm GaN LED chips.

Manganese-Doped One-Dimensional Organic Lead Bromide Perovskites with Bright White Emissions

Single-component white-emitting phosphors are highly promising to simplify the fabrication of optically pumped white light-emitting diodes. To achieve white emission, precise control of the excited state dynamics is required for a single-component system to generate emissions with different energies in the steady state. Here, we report a new class of white phosphors based on manganese (Mn)-doped one-dimensional (1D) organic lead bromide perovskites. The bright white emission is the combination of broadband blue emission from the self-trapped excited states of the 1D perovskites and red emission from the doped Mn²⁺ ions. Because of the indirect nature of the self-trapped excited states in 1D perovskites, there is no energy transfer from these states to the Mn²⁺ ions, resulting in an efficient dual emission. As compared to the pristine 1D perovskites with bluish-white emission, these Mn-doped 1D perovskites exhibit much higher color rendering index of up to 87 and photoluminescence quantum efficiency of up to 28%.

Stable and conductive lead halide perovskites facilitated by X-type ligands

Lead halide perovskites exhibit outstanding optoelectronic and optical properties. However, some applications of perovskites are hindered by their instability in polar environments; thus, how to balance stability with conductivity is a great challenge. Here, we report a new approach of using X-type ligands to address this issue. Surface treatments containing multi-step ligand exchanges and ion filling were necessary to obtain X-type ligand-protected perovskites. Performances of this material show that: (1) the crystal structure of perovskites is stable in ethanol; (2) surface defects can be fixed by a photoactivation process and photoluminescence intensity can be enhanced to 136%; and (3) electronic devices fabricated from such materials show stabilility even after washing with ethanol. X-type ligand-protected perovskites with high stability and good conductivity are promising new materials for wide applications in electronic and optoelectronics devices.

Giant five-photon absorption from multidimensional core-shell halide perovskite colloidal nanocrystals

Multiphoton absorption processes enable many technologically important applications, such as in vivo imaging, photodynamic therapy and optical limiting, and so on. Specifically, higher-order nonlinear absorption such as five-photon absorption offers significant advantages of greater spatial confinement, increased penetration depth, reduced autofluorescence, enhanced sensitivity and improved resolution over lower orders in bioimaging. Organic chromophores and conventional semiconductor nanocrystals are leaders in two-/three-photon absorption applications, but face considerable challenges from their small five-photon action cross-sections. Herein, we reveal that the family of halide perovskite colloidal nanocrystals transcend these constraints with highly efficient five-photon-excited upconversion fluorescence-unprecedented for semiconductor nanocrystals. Amazingly, their multidimensional type I (both conduction and valence band edges of core lie within bandgap of shell) core-shell (three-dimensional methylammonium lead bromide/two-dimensional octylammonium lead bromide) perovskite nanocrystals exhibit five-photon action cross-sections that are at least 9 orders larger than state-of-the-art specially designed organic molecules. Importantly, this family of halide perovskite nanocrystals may enable fresh approaches for next-generation multiphoton imaging applications.

CsPbxMn1-xCl3 Perovskite Quantum Dots with High Mn Substitution Ratio

CsPbX₃ (X = Cl, Br, I) perovskite quantum dots (QDs) are potential emitting materials for illumination and display applications, but toxic Pb is not environment- and user-friendly. In this work, we demonstrate the partial replacement of Pb with Mn through phosphine-free hot-injection preparation of CsPb_xMn_1-xCl₃ QDs in colloidal solution. The Mn substitution ratio is up to 46%, and the as-prepared QDs maintain the tetragonal crystalline structure of the CsPbCl₃ host. Meaningfully, Mn substitution greatly enhances the photoluminescence quantum yields of CsPbCl₃ from 5 to 54%. The enhanced emission is attributed to the energy transfer of photoinduced excitons from the CsPbCl₃ host to the doped Mn, which facilitates exciton recombination via a radiative pathway. The intensity and position of this Mn-related emission are also tunable by altering the experimental parameters, such as reaction temperature and the Pb-to-Mn feed ratio. A light-emitting diode (LED) prototype is further fabricated by employing the as-prepared CsPb_xMn_1-xCl₃ QDs as color conversion materials on a commercially available 365 nm GaN LED chip.

Solvent-free, mechanochemical syntheses of bulk trihalide perovskites and their nanoparticles

For the first time, we have synthesized APbBr₃ (A = Cs⁺/MA⁺/FA⁺, where MA⁺ = CH₃NH₃⁺ and FA⁺ = CH(NH₂)₂⁺) bulk as well as nanoparticles (NPs) by solid-state reactions at room temperature.