Tuning Crystal Plane Orientation in Multijunction and Hexagonal Single Crystalline CsPbBr3 Perovskite Disc Nanocrystals

Interfacial defect capture-induced abnormal fluorescence decay lifetime of CsPbBr3/CsPbBr3 nanodisks

We employed a two-step temperature-controlled strategy to obtain assembled CsPbBr3/CsPbBr3 nanodisks. Compared with conventional CsPbBr3 quantum dots, surface-state characterizations and exciton dynamics suggested that the interface defect prevented the rapid recombination of excitons, and extended the fluorescence lifetime by nearly 10-times.

7D High-Dynamic Spin-Multiplexing

Multiplexing technology creates several orthogonal data channels and dimensions for high-density information encoding and is irreplaceable in large-capacity information storage, and communication, etc. The multiplexing dimensions are constructed by light attributes and spatial dimensions. However, limited by the degree of freedom of interaction between light and material structure parameters, the multiplexing dimension exploitation method is still confused. Herein, a 7D Spin-multiplexing technique is proposed. Spin structures with four independent attributes (color center type, spin axis, spatial distribution, and dipole direction) are constructed as coding basic units. Based on the four independent spin physical effects, the corresponding photoluminescence wavelength, magnetic field, microwave, and polarization are created into four orthogonal multiplexing dimensions. Combined with the 3D of space, a 7D multiplexing method is established, which possesses the highest dimension number compared with 6 dimensions in the previous study. The basic spin unit is prepared by a self-developed laser-induced manufacturing process. The free state information of spin is read out by four physical quantities. Based on the multiple dimensions, the information is highly dynamically multiplexed to enhance information storage efficiency. Moreover, the high-dynamic in situ image encryption/marking is demonstrated. It implies a new paradigm for ultra-high-capacity storage and real-time encryption.

CsPbBr3 Perovskite Crack Platelet Nanocrystals and Their Biexciton Generation

Lead halide perovskite nanocrystals have been extensively studied in recent years as efficient optical materials for their bright and color-tunable emissions. However, these are mostly confined to their 3D nanocrystals and limited to the anisotropic nanostructures. By exploring the Cs-sublattice-induced metal(II) ion exchange with Pb(II), crack CsPbBr3 perovskite platelet nanocrystals having polar surfaces in all three directions are reported here, which remained different than reported standard square platelets. The crack platelets are also passivated with halides to enhance their brightness. Further, as these crack and passivated crack platelets have defects and polar surfaces, the exciton and biexciton generation in these platelets is investigated using femtosecond photoluminescence and transient absorption measurement at ambient as well as cryogenic temperatures, correlated with time-resolved single-particle photoluminescence spectroscopy, and compared with standard square platelets having nonpolar facets. These investigations revealed that the crack platelets and passivated crack platelets possess enhanced biexciton emission compared to square platelets due to the presence of polar surfaces in all three directions. These results provide insights into not only the design of the anisotropic nanostructures of ionic nanocrystals but also the possibility of tuning the single exciton to biexciton generation efficiency, which has potential applications in optoelectronic systems.

Doping Up the Light: A Review of A/B-Site Doping in Metal Halide Perovskite Nanocrystals for Next-Generation LEDs

All-inorganic metal halide perovskite nanocrystals (PeNCs) show great potential for the next generation of perovskite light-emitting diodes (PeLEDs). However, trap-assisted recombination negatively impacts the optoelectronic properties of PeNCs and prevents their widespread adoption for commercial exploitation. To mitigate trap-assisted recombination and further enhance the external quantum efficiency of PeLEDs, A/B-site doping has been widely investigated to tune the bandgap of PeNCs. The bandgap of PeNCs is adjustable within a small range (no more than 0.1 eV) by A-site cation doping, resulting in changes in the bond length of Pb-X and the angle of [PbX6]4. Nevertheless, B-site doping of PeNCs has a more significant impact on the bandgap level through modification of surface defect states. In this perspective, we delve into the synthesis of PeNCs with A/B-site doping and their impacts on the structural and optoelectronic properties, as well as their impacts on the performance of subsequent PeLEDs. Furthermore, we explore the A-site and B-site doping mechanisms and the impact of device architecture on doped PeNCs to maximize the performance and stability of PeLEDs. This work presents a comprehensive overview of the studies on A-site and B-site doping in PeNCs and approaches to unlock their full potential in the next generation of LEDs.

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.

Light-Induced Wavelength Dependent Self Assembly Process for Targeted Synthesis of Phase Stable 1D Nanobelts and 2D Nanoplatelets of CsPbI3 Perovskites

Despite black cubic phase α-CsPbI₃ nanocrystals having an ideal bandgap of 1.73 eV for optoelectronic applications, the phase transition from α-CsPbI₃ to non-perovskite yellow δ-CsPbI₃ phase at room temperature remains a major obstacle for commercial applications. Since γ-CsPbI₃ is thermodynamically stable with a bandgap of 1.75 eV, which has great potential for photovoltaic applications, herein we report a conceptually new method for the targeted design of phase stable and near unity photoluminescence quantum yield (PLQY) two-dimensional (2D) γ-CsPbI₃ nanoplatelets (NPLs) and one-dimensional (1D) γ-CsPbI₃ nanobelts (NBs) by wavelength dependent light-induced assembly of CsPbI₃ cubic nanocrystals. This article demonstrates for the first time that by varying the excitation wavelengths, one can design air stable desired 2D nanoplatelets or 1D nanobelts selectively. Our experimental finding indicates that 532 nm green light-driven self-assembly produces phase stable and highly luminescent γ-CsPbI₃ NBs from CsPbI₃ nanocrystals. Moreover, we show that a 670 nm red light-driven self-assembly process produces stable and near unity PLQY γ-CsPbI₃ NPLs. Systematic time-dependent microscopy and spectroscopy studies on the morphological evolution indicates that the electromagnetic field of light triggered the desorption of surface ligands from the nanocrystal surface and transformation of crystallographic phase from α to γ. Detached ligands played an important role in determining the morphologies of final structures of NBs and NPLs from nanocrystals via oriented attachment along the [110] direction initially and then the [001] direction. In addition, XRD and fluorescence imaging data indicates that both NBs and NPLs exhibit phase stability for more than 60 days in ambient conditions, whereas the cubic phase α-CsPbI₃ nanocrystals are not stable for even 3 days. The reported light driven synthesis provides a simple and versatile approach to obtain phase pure CsPbI₃ for possible optoelectronic applications.

Chemically Sculpturing the Facets of CsPbBr3 Perovskite Platelet Nanocrystals

The facet chemistry of lead halide perovskite nanocrystals is critically important for determining their shape and interface ligand binding. In colloidal nanocrystals, these are mostly controlled by adopting specific synthetic strategies with a selection of the appropriate reactants. However, using selected ligands, the surface of preformed nanocrystals can be reconstructed without altering the crystal phase and lattice structure of their core. This has been shown here for hexagonal-shaped orthorhombic CsPbBr₃ platelet nanocrystals. When oleylammonium bromide was added to these postsynthesized platelets, all six edges and two planar facets are transformed from flat to wavy structures. With a variation in concentration, the crest-to-crest distance of these wavy platelets are also tuned. These became possible because of the oleylammonium ions, which changed the {200}, {012} and {020} facets of orthorhombic phase of CsPbBr₃ to the more compatible {110} and {002} facets simply by surface atom dissolution. This was also observed for multisegmented platelets having multiple junctions and even for platelets having a size of more than 200 nm. While shape modulations in ionic halide perovskite nanocrystals still face synthetic challenges, these results of surface reconstruction provide strong evidence of the possibility of sculpturing surface facets and shape changes in these nanostructures.