Outstanding Challenges of Zero-Dimensional Perovskite Materials

Influence of Lewis basicity on the S2- induced synthesis of 0D Cs4PbBr6 hexagonal nanocrystals and its implications for optoelectronics

Perovskite nanocrystals (NCs) with their excellent optical and semiconductor properties have emerged as primary candidates for optoelectronic applications. While extensive research has been conducted on the 3D perovskite phase, the zero-dimensional (0D) form of this promising material in the NC format remains elusive. In this paper, a new synthesis strategy is proposed. According to the Hard-Soft Acid-Base (HSAB) principle, a novel class of hexagonal semiconductor nanocrystals (Cs4PbBr6 HNCs) derived from 0D perovskite Cs4PbBr6 is synthesized by doping an appropriate amount of PbS precursor solution into bromide. These Cs4PbBr6 HNCs are characterized and compared in detail to CsPbBr3 cubic nanocrystals (CsPbBr3 CNCs) as a reference. The Cs4PbBr6 HNCs exhibit significantly enhanced photoluminescence (PL) compared to CsPbBr3 CNCs, with an external quantum efficiency (EQE) reaching 24.19%. Furthermore, they demonstrate superior UV stability compared to CsPbBr3 CNCs. Comparative analysis of their physical properties and morphology, along with detailed investigations into band structures, density of states, and lifetime decay through DFT calculations, is provided. The practical application potential is validated by encapsulating them into backlight LEDs, covering 121.5% and 90.7% of the color gamut of NTSC and Rec. Our research provides comprehensive insights into the photophysical properties of inorganic halide perovskite nanomaterials and explores their potential in the field of optoelectronics.

Lattice doping of lanthanide ions in Cs2ZrCl6 nanocrystals enabling phase transition and tunable photoluminescence

Dopants can endow lead-free perovskite nanocrystals with novel photoelectric properties. However, understanding the effect of dopants on the structure and energy transfer of lead-free perovskite nanocrystals remains limited. In this work, we synthesize zero-dimensional Cs2ZrCl6 nanocrystals with a blue light quantum yield of up to 75.6% by an improved hot-injection method. And we introduce trace amounts of lanthanide ions (Ln3+) (<∼8%) in the lattice of nanocrystals and establish an effective energy transfer channel from self-trapped excitons (STEs) to various Ln3+ ions (Tb3+, Eu3+, Dy3+, Sm3+, and Pr3+), which can achieve tunable photoluminescence between red, green and blue. Interestingly, with increasing Ln3+ concentrations (>∼10%), the phase transition from the cubic phase Cs2ZrCl6:Ln3+ to the monoclinic phase Cs3LnCl6:Zr4+ occurred, while Zr4+ ions began to act as dopants. And a new energy transfer channel from dopant [ZrCl6]2- to host Ln3+ ions was established in the Cs3LnCl6 host accompanied by enhanced broadband photoluminescence excitation (PLE) and photoluminescence (PL). In particular, the photoluminescence quantum yield (PLQY) of Tb3+ ions increases from 0.77% to 54% upon the phase transition (under 276 nm excitation). Our study provides new insights into the effects of dopants on the structure of perovskite nanocrystals and is beneficial to the design of a variety of light-emitting materials for optoelectronic applications.

Ordered sulfonated polystyrene particle chains organized through AC electroosmosis as reinforcing phases in Polyacrylamide hydrogels

Polyacrylamide (PAM) hydrogels have garnered significant attention due to their unique swelling properties, biocompatibility, and stability, resulting in them being promising candidates for various applications, ranging from drug delivery to tissue engineering. However, traditional PAM hydrogels suffer from low strength and poor toughness, which limits their widespread use. In this study, based on the theory of filler-reinforced composites, we introduced ordered sulfonated polystyrene (SPS) particles into PAM hydrogels using electric field-assisted techniques. The effects of the geometric dimensions and filling concentration of SPS particles on thermal stability, swelling/deswelling behavior, and mechanical properties of composite hydrogels were investigated. When filled with ordered 100 nm SPS particles at a concentration of 2.0 g·L-1, the resulting SPS/PAM composite exhibited improved water retention capacity, as well as a fracture elongation of 316 % and a tensile strength of 23 kPa. These findings in the paper provide valuable insights into the understanding of PAM hydrogels and open up new avenues for the development of advanced hydrogel-based systems with enhanced performance and functionality.

Cold-Sintered All-Inorganic Perovskite Bulk Composite Scintillators for Efficient X-ray Imaging

Cost-effective bulk scintillators with high density, large-area, and long-term stability are desirable for high-energy radiation detections. Conventional bulk polycrystalline or single-crystal scintillators are generally synthesized by high-temperature approaches, and it is challenging to realize simultaneously high detectivity/responsivity, spatial resolution, and rapid time response. Here, we report the cold sintering of bulk scintillators (at 90 °C) based on an "emitter-in-matrix" principle, in which emissive CsPbBr3 nanocrystals are embedded in a durable and transparent Cs4PbBr6 matrix. These bulk scintillators exhibit high light yield (33,800 photons MeV-1), low detection limit (79 nGyair s-1), fast decay time (9.8 ns), and outstanding spatial resolution of 8.9 lp mm-1 to X-ray radiation and an energy resolution of 19.3% for γ-ray (59.6 keV) detection. The composite scintillator also shows exceptional stability against environmental degradation and cyclic X-ray radiation. Our results demonstrate a cost-effective strategy for developing perovskite-based bulk transparent scintillators with exceptional performance and high radioluminescence stability for high-energy radiation detection and imaging.

Analytical separation techniques: toward achieving atomic precision in nanomaterials science

The size- and shape-dependence of the properties are the most characteristic features of nanoscale matter. In many types of nanomaterials, there is a size regime wherein every atom counts. In order to fully realize the idea of 'maneuvering things atom by atom' envisioned by Richard Feynman, synthesis and separation of nanoscale matter with atomic precision are essential. It is therefore not surprising that analytical separation techniques have contributed tremendously toward understanding the size- as well as shape-dependent properties of nanomaterials. Fascinating properties of nanomaterials would not have been explored without the use of these techniques. Here we discuss the pivotal role of analytical separation techniques in the progress of nanomaterials science. We begin with a brief overview of some of the key analytical separation techniques that are of tremendous importance in nanomaterials research. Then we describe how each of these techniques has contributed to the advancements in nanomaterials science taking some of the nanosystems as examples. We discuss the limitations and challenges of these techniques and future perspectives.

Fabrication of Colloidal Cesium Metal Halide (CsMX: M = Fe, Co, and Ni) Nanoparticles and Assessment of Their Thermodynamic Stability by DFT Calculations

We synthesized colloidal cesium metal halide CsMX (M = Fe, Co, Ni; X = Cl, Br) nanoparticles (NPs) and assessed their crystal stability by density functional theory (DFT) calculations. We successfully synthesized Cs₃FeCl₅, Cs₃FeBr₅, Cs₃CoCl₅, Cs₃CoBr₅, CsNiCl₃, and CsNiBr₃ NPs. CsMX NPs with Fe and Co exhibited Cs₃M₁X₅ and Cs₂M₁X₄ structures depending on the reaction conditions; however, CsNiX NPs exhibited only the CsNiX₃ structure. The differences in structural stability by central metal ions were explained using spin-polarized DFT calculations. The analysis revealed tetragonal Cs₃M₁X₅ and orthorhombic Cs₂M₁X₄ structures to have similar thermodynamic stabilities in the case of Fe and Co, whereas the hexagonal CsMX₃ structure in the case of Ni was the most stable. Moreover, the calculation results were the same as the experimental results. In particular, cobalt-related Cs₃CoBr₅ NPs easily developed into Cs₂CoCl₄ nanorods with an increase in temperature.

Highly Stable and Efficient Mn2+ Doping Zero-Dimension Cs2ZnxPb1-xCl4 Alloyed Nanorods toward White Electroluminescent Light-Emitting Diodes

Zero-dimensional (0D) crystal structure perovskite NCs have reemerged as promising materials owing to their superior long-term stability; however, their poor conductivity leads to the inferior electrical performances and critically restricts the optoelectronic application of 0D perovskite materials. Herien, the alloyed 0D crystal structure Cs₂Zn_xPb_1-xCl₄ nanorods (NRs) have been synthesized by the modified hot-injection method, which emits bright blue-violet light at 408 nm, and the optimized photoluminescence quantum yield (PLQY) reaches 26%. The Cs₂Zn_0.88Pb_0.12Cl₄ NRs display more excellent air stability and an order of magnitude higher conductivity than CsPbCl₃ nanocube films. In addition, we dope Mn²⁺ ions into the Cs₂Zn_0.88Pb_0.12Cl₄ NRs, which accomplished the optimized PLQY of 40.3% and polarized emission with r = 0.19. The light-emitting diodes (LEDs) based on Mn²⁺ ion doped Cs₂Zn_0.88Pb_0.12Cl₄ NRs exhibit a chromaticity coordinate (CIE) of (0.36, 0.33), an EQE of 0.3%, and a maximum luminance of 98 cd m^-2. This work has enriched ideas for the production of white light perovskite LEDs.

Recent Advances in All-Inorganic Zero-Dimensional Metal Halides

All-inorganic zero-dimensional (0D) metal halides are composed of isolated metal halide polyhedrons bridged by monovalent alkali metal ions. The unique structure gives rise to molecule-like electronic configuration and consequently highly attractive optical properties. In comparison with their three-dimensional (3D) counterparts, the 0D metal halides exhibit characteristic features such as broadband emission and long-term stability. In addition, 0D metal halides can be constructed from a diverse range of metal ions and permit high-level impurity doping, thereby offering great structural designability and spectral tunability. This Review surveys recent advances in 0D metal halides, including crystal preparation, luminescence modulation, and emerging applications.

State of the Art and Prospects for Halide Perovskite Nanocrystals

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

Highly efficient radiative recombination in intrinsically zero-dimensional perovskite micro-crystals prepared by thermally-assisted solution-phase synthesis

Zero-dimensional (0D) quantum confinement can be achieved in perovskite materials by the confinement of electron and hole states to single PbX6 4- perovskite octahedra. In this work, 0D perovskite (Cs4PbBr6) micro-crystals were prepared by a simple thermally-assisted solution method and thoroughly characterized. The micro-crystals show a high level of crystallinity and a high photoluminescence quantum yield of 45%. The radiative recombination coefficient of the 0D perovskite micro-crystals, 1.5 × 10-8 s-1 cm3, is two orders of magnitude higher than that of typical three-dimensional perovskite and is likely a strong contributing factor to the high emission efficiency of 0D perovskite materials. Temperature dependent luminescence measurements provide insight into the role of thermally-activated trap states. Spatially resolved measurements on single 0D perovskite micro-crystals reveal uniform photoluminescence intensity and emission decay behaviour suggesting the solution-based fabrication method yields a high-quality and homogenous single-crystal material. Such uniform emission reflects the intrinsic 0D nature of the material, which may be beneficial to device applications.

Shining Emitter in a Stable Host: Design of Halide Perovskite Scintillators for X-ray Imaging from Commercial Concept

Halide perovskite (HP) nanocrystals (NCs) have recently shown great potential for X-ray detection and imaging. However, the practical application still has a long way to go with many technical requirements waiting to be fulfilled, including structure optimization, stability enhancement, and cost reduction. A design principle in this beginning stage is urgently needed but still lacking. Herein, with an "emitter-in-matrix" principle refined from commercial scintillators, CsPbBr₃@Cs₄PbBr₆ with emissive CsPbBr₃ NCs embedded inside a solid-state Cs₄PbBr₆ host is subjected to X-ray sensing and imaging. The Cs₄PbBr₆ matrix not only enhances the attenuation of X-rays but also dramatically improves the stability of CsPbBr₃ NCs. A favorable optical design with the Cs₄PbBr₆ matrix being transparent to the emission from CsPbBr₃ NCs enables efficient light output. As a result, stable and sensitive scintillation response to X-ray signals is demonstrated with superior linearity and ultrahigh time resolution. In order to show the huge potential for practical applications, X-ray imaging using a large-area film (360 mm × 240 mm) by the blade-coating technique is carried out to obtain a high-quality image of interior structures invisible to the human eye. In addition to the above advantages in optics, CsPbBr₃@Cs₄PbBr₆ also enjoys facile solution synthesis with large scalability, excellent repeatability, and low cost.

Synthesis and dimensional control of CsPbBr3 perovskite nanocrystals using phosphorous based ligands

Nanocrystals of the inorganic perovskite, CsPbBr₃, display outstanding photo-physical properties, making them ideal for next generation optical devices. However, the typical combination of oleic acid and oleylamine ligands employed in their synthesis is easily displaced, leading to poor stability that can hinder their applicability. In this work, we look toward the replacement of the oleic acid and amine with phosphorous-based ligands. We synthesize CsPbBr₃ nanocrystals using an oleylamine/alkylphosphonic acid combination with near perfect monodispersity with the ability to tune the bandgap by varying the alkyl chain length. We further investigate the replacement of the oleylamine giving a ligand combination of alkylphosphonic acid/trioctylphosphine oxide for perovskite nanocrystal nucleation and growth. This combination is typical for the widely studied metal chalcogenide synthesis and in our study with CsPbBr₃ yields a pure phase perovskite.

A solution-processed zero-dimensional all-inorganic perovskite scintillator for high resolution gamma-ray spectroscopy detection

Low dimensional scintillators have been successfully and widely applied in the radiation-detection community for home security, scientific research, and imaging. We have grown zero-dimensional Cs₄PbBr₆ materials with CsPbBr₃ nanocrystals embedded by using a solution-growth method at low temperature. We have demonstrated the scintillation properties of these 0D nanoscintillators with high luminescence quantum efficiency for green light. In addition, we have successfully achieved pulse height spectra with this 0D perovskite material, which has been demonstrated with fast decay time (<10 ns), high energy resolution (3.0 ± 0.1%, 59.5 keV from ²⁴¹Am), high light yield (64 000 photons per MeV) and long term stability under various atmospheres (moisture, radiation). These results are much better than those of the most widely used commercial products such as NaI(Tl). It also demonstrates that this 0D all-inorganic material can be employed as a scintillator for ionization radiation applications such as X-ray imaging and spectroscopy applications.

Efficiency Improvement of MAPbI3 Perovskite Solar Cells Based on a CsPbBr3 Quantum Dot/Au Nanoparticle Composite Plasmonic Light-Harvesting Layer

We demonstrate a method to enhance the power conversion efficiency (PCE) of MAPbI3 perovskite solar cells through localized surface plasmon (LSP) coupling with gold nanoparticles:CsPbBr3 hybrid perovskite quantum dots (AuNPs:QD-CsPbBr3). The plasmonic AuNPs:QD-CsPbBr3 possess the features of high light-harvesting capacity and fast charge transfer through the LSP resonance effect, thus improving the short-circuit current density and the fill factor. Compared to the original device without Au NPs, a 27.8% enhancement in PCE of plasmonic AuNPs:QD-CsPbBr3/MAPbI3 perovskite solar cells was achieved upon 120 μL Au NP solution doping. This improvement can be attributed to the formation of surface plasmon resonance and light scattering effects in Au NPs embedded in QD-CsPbBr3, resulting in improved light absorption due to plasmonic nanoparticles.

Green-Emitting Lead-Free Cs4SnBr6 Zero-Dimensional Perovskite Nanocrystals with Improved Air Stability

We report the synthesis and characterization of nanocrystals of a novel fully inorganic lead-free zero-dimensional perovskite, Cs₄SnBr₆. Samples are made of crystals with an average size of ∼20 nm with green emission centered around 530 nm. Interestingly, both colloidal suspensions and thin films show an enhanced air stability with respect to that of any other previous tin-based nanocrystalline system, with emission persisting for tens of hours under laboratory air.

CsPbBr3-Cs4PbBr6 composite nanocrystals for highly efficient pure green light emission

All-inorganic perovskite CsPbBr₃-Cs₄PbBr₆ composite nanocrystals (NCs) were synthesized via a convenient solution process without inert gas protection and systematically studied as green phosphors for light emitting diode (LED) applications. While colloidal composite NCs emit green color with an emission peak at around 515 nm, their thin films on top of blue GaN chips exhibit a redshift of ∼15-20 nm due to subsequential aggregation in solid state. The Commission Internationale De I'eclairage (CIE) chromaticity coordinates of the green LEDs assembled by composite NCs reached (0.201, 0.746) with nearly 100% green ratio. Moreover, the pure green LED displayed a luminous efficiency of 45 lm W^-1 under 10 mA driving current. The colloidal CsPbBr₃-Cs₄PbBr₆ composite NCs have the photoluminescence quantum yield (PLQY) as high as 74%. The high PLQY originates from cubic CsPbBr₃ NCs well passivated by the zero-dimensional Cs₄PbBr₆ matrix, substantially suppressing the nonradiative recombination. The comparison between the green LEDs fabricated with pure and composite perovskites imply that the CsPbBr₃-Cs₄PbBr₆ composites with higher quantum yield could be an effective way to get the brightness and the stability of pure green LEDs.

Delayed Photoluminescence and Modified Blinking Statistics in Alumina-Encapsulated Zero-Dimensional Inorganic Perovskite Nanocrystals

We demonstrate enhancement of the photoluminescence (PL) properties of individual zero-dimensional (0D) Cs₄PbBr₆ perovskite nanocrystals (PNCs) upon encapsulation by alumina using an appropriately modified atomic layer deposition method. In addition to the increased PL intensity and improved long-term stability of encapsulated PNCs, our single-particle studies reveal substantial changes in the PL blinking statistics and the persistent appearance of the long-lived, "delayed" PL components. The blinking patterns exhibit a modification from the fast switching between fluorescent ON and OFF states found in bare PNCs to a behavior with longer ON states and more isolated OFF states in alumina-encapsulated PNCs. Controlled exposure of 0D nanocrystals to moisture suggests that the observed PL lifetime changes may be related to water-induced "reservoir" states that allow for longer-lived charge storage with subsequent back-feeding into the emissive states. Viable encapsulation of PNCs with metal oxides that can preserve and even enhance their PL properties can be utilized in the fabrication of extended structures on their basis for optoelectronic and photonic applications.