In Situ Observation of Emission Behavior during Anion-Exchange Reaction of a Cesium Lead Halide Perovskite Nanocrystal at the Single-Nanocrystal Level

Single-Photon Emission from Organic Dye Molecules Adsorbed on a Quantum Dot via Energy Transfer

Single-photon emissions from individual emitters are crucial in fundamental science and quantum information technologies. Multichromophoric systems, comprising multiple dyes, can exhibit single-photon emissions through efficient annihilation between the excited states; however, exploring this phenomenon in complex systems remains a challenge. In this study, we investigated the photon statistics of emissions from multiple perylene bisimide (PBI) dyes adsorbed onto the surface of CdSe/ZnS quantum dots (QDs). When multiple PBIs were simultaneously excited by both direct excitation and energy transfer from the QD, multiphoton emissions from the PBIs were observed. Conversely, when the QDs were selectively excited, multiple PBIs exhibiting single-photon emission through energy transfer from the QDs to the PBIs were found. These results highlight the intriguing interplay between multichromophoric systems and QDs, offering valuable insights into the development of efficient single-photon sources in quantum information technologies.

Room-temperature epitaxial welding of 3D and 2D perovskites

Formation of epitaxial heterostructures via post-growth self-assembly is important in the design and preparation of functional hybrid systems combining unique properties of the constituents. This is particularly attractive for the construction of metal halide perovskite heterostructures, since their conventional solution synthesis usually leads to non-uniformity in composition, crystal phase and dimensionality. Herein, we demonstrate that a series of two-dimensional and three-dimensional perovskites of different composition and crystal phase can form epitaxial heterostructures through a ligand-assisted welding process at room temperature. Using the CsPbBr₃/PEA₂PbBr₄ heterostructure as a demonstration, in addition to the effective charge and energy transfer across the epitaxial interface, localized lattice strain was observed at the interface, which was extended to the top layer of the two-dimensional perovskite, leading to multiple new sub-bandgap emissions at low temperature. Given the versatility of our strategy, unlimited hybrid systems are anticipated, yielding composition-, interface- and/or orientation-dependent properties.

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.

Fluorescent dynamics of CsPbBr3 nanocrystals in polar solvents: a potential sensor for polarity

During synthesis, device processes, and applications of perovskite nanocrystals (NCs), there are usually inevitable interactions between perovskite NCs and polar solvents. To elaborately control the properties of perovskite NCs, investigating the effects of solvent polarity on perovskite NCs is thus highly important. Herein, fluorescent variations induced by different solvents into CsPbBr₃ NCs solution are systematically studied. In this report, it is found that when CsPbBr₃ NCs are treated with polar solvents, the fluorescence intensity decreases with a general redshift of fluorescence peak position. Moreover, the fluorescence quenching and peak position shift amplitude monotonously increase with the solvent polarity. Absorption spectra and fluorescent lifetime suggest that, with addition of polar solvents, the surface of NCs are destroyed and defect states are generated, leading to the fluorescent variations. Besides, dielectric constant of the solvent also increases with polarity, which may weaken the quantum confinement effect and decrease the exciton binding energy. We find the fluorescence may slightly blue shift if the emission of free carrier is strong enough with certain solvents, such as dimethylsulfoxide (DMSO). We also find the fluorescence intensity generally deceases to a stable state in 2 min, indicating quick interactions between CsPbBr₃ NCs and solvents. However, water continuously quenches the fluorescence of CsPbBr₃ NCs up to 72 h due to the poor miscibility between water and n-hexane. This work not only provides a comprehensive understanding on the fluorescent dynamics of CsPbBr₃ NCs in polar solvents but also affords a potential fluorescent indicator for solvent polarity.

A Highly Ordered Quantum Dot Supramolecular Assembly Exhibiting Photoinduced Emission Enhancement

Multicomponent supramolecular assembly systems enable the generation of materials with outstanding properties, not obtained from single-component systems, via a synergetic effect. Herein, we demonstrate a novel supramolecular coassembly system rendering highly ordered quantum dot (QD) arrangement structures formed via the self-assembly of azobenzene derivatives, where the photocontrollable photoluminescence (PL) properties of the QDs are realized based on photoisomerization. Upon mixing the assembled azobenzene derivatives and QDs in apolar media, a time-evolution coaggregation into hierarchical nanosheets with a highly ordered QD arrangement structure occurs. Upon photoirradiation, the nanosheets transform into ill-defined aggregates without arranged QDs together with enhancing the PL intensity. In days, the photoirradiated coaggregates undergo recovery of the PL properties corresponding to the arranged QDs through thermal isomerization.

Controlling the growth of a SiO2 coating on hydrophobic CsPbBr3 nanocrystals towards aqueous transfer and high luminescence

Silica coating can effectively solve the stability issue of lead halide perovskite nanomaterials. However, it is difficult to achieve aqueous SiO₂ coating on hydrophobic CsPbBr₃ nanocrystals (NCs). In this paper, the hydrolysis process of tetramethoxysilane was controlled to get a homogeneous SiO₂ coating or a NC/SiO₂ Janus structure. In step 1, the Cs₄PbBr₆ NCs were silanized using partially hydrolyzed tetramethoxysilane (PH-TMOS). During this process, the Si-OH groups which came from PH-TMOS were absorbed onto the surface of the Cs₄PbBr₆ NCs with the removal of hydrophobic oleic acid (OA) ligands. In step 2, phase transformation from Cs₄PbBr₆ to CsPbBr₃ occurred owing to the injection of water. Meanwhile, further hydrolysis of TMOS took place and generated cross-linked Si-O-Si. Because the silanization in step 1 created lots of growth sites, the condensation of SiO₂ was not limited to the interface between water and hexane. After growing for 12 h, the fully covered CsPbBr₃@SiO₂ capsules were prepared. The anion exchange reactions of the CsPbBr₃@SiO₂ capsules were studied. Only one even and symmetric PL peak was apparent during the anion exchange process, which was different from the bare CsPbBr₃ NCs. This result demonstrated that the SiO₂ shell can act as a buffer layer to block the direct contact of CsPbBr₃ with the excess PbBr₂ precursor in solution. Compared with the CsPbBr₃ NCs, CsPbBr₃@SiO₂ showed better stability in polar solvent and air. A bright green emission was also observed under UV light after 90 days. The successful preparation of CsPbBr₃@SiO₂ capsules with enhanced stability paves the way for the further development of lead halide perovskite nanomaterials.

Slow Anion-Exchange Reaction of Cesium Lead Halide Perovskite Nanocrystals in Supramolecular Gel Networks

Cesium lead halide perovskite nanocrystals are widely studied as among the most attractive emissive nanomaterials because of their high photoluminescence quantum yield and tunable emission wavelengths over the whole visible-light region by the halide ion-exchange reaction. However, the reactions were often observed in solution and generally very fast, which interferes with the fine-tuning capability of the emission properties. Here, we report a novel nanocrystal-organogel hybrid soft material in which the perovskite nanocrystals in a supramolecular gel exhibit extremely slow and inhomogeneous anion-exchange reactions that are different from those in solution. Furthermore, the inhomogeneous emission in the gel became homogeneous over several days due to a slow diffusion.

Dynamics of anion exchange in cesium lead halide (CsPbX3) perovskite nanocrystals

By investigating in situ PL spectra, the anion exchange of perovskite nanocrystals is divided into three stages, i.e. photon reabsorption process, initial anion exchange, and adequate anion exchange.