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

Data-Driven Mapping of the Cesium Cadmium Bromide Phase Space Utilizing a Soft-Chemistry Approach

Soft-chemistry techniques provide a versatile approach to synthesizing inorganic materials under mild conditions, enabling access to compositions and structures that are challenging to achieve through traditional thermodynamically driven solid-state methods. However, these solution-based routes often result in phase competition, requiring precise control over reaction conditions to achieve selective product formation. While one-variable-at-a-time (OVAT) approaches have traditionally been used for phase selection, data-driven strategies are emerging as more efficient methods for navigating complex synthetic spaces. Ternary metal halides, such as cesium cadmium bromides (Cs-Cd-Br), are of growing interest due to their potential in wide and ultrawide band gap applications. Unlike the well-studied cesium lead halide phases, the compositional diversity and solution-based synthesis of ternary Cs-Cd-Br phases remain largely unexplored. This study systematically investigates the synthetic phase space of the Cs-Cd-Br system by constructing a data-driven phase map. Using a common set of precursors and a standardized experimental procedure, we successfully synthesize all four known Cs-Cd-Br phases─CsCdBr3, Cs2CdBr4, Cs3CdBr5, and Cs7Cd3Br13─each exhibiting distinct structures, morphologies, and optical properties. Our findings highlight the potential of soft-chemistry methods for expanding the library of ternary metal halides and provide key insights into the thermodynamic and kinetic factors governing phase formation.

Templated self-organization of polymer-tethered gold nanoparticles into freestanding superlattices at the liquid-air interface

Programmable organization of uniform organic/inorganic functional building blocks into large-scale ordered superlattices has attracted considerable attention since the bottom-up self-organization strategy opens up a robust and universal route for designing novel and multifunctional materials with advanced applications in memory storage devices, catalysis, photonic crystals, and biotherapy. Despite making great efforts in the construction of superlattice materials, there still remains a challenge in the preparation of organic/inorganic hybrid superlattices with tunable dimensions and exotic configurations. Here, we report the spontaneous self-organization of polystyrene-tethered gold nanoparticles (AuNPs@PS) into freestanding organic/inorganic hybrid superlattices templated at the diethylene glycol-air interface. The resulting multilayer 3D superlattices exhibit hexagonally honeycomb and periodically tetrahedral lattices after the evaporation of AuNPs@PS building blocks at the liquid-air interface. Notably, a particular Moiré pattern originating from the twisted stacking of the adjacent layers is observed when the twist angle is 30°, leading to the exquisite quasi-crystalline packing with 12-fold rotational symmetry. In addition, the interparticle distance and gap within the 2D superlattice can be precisely regulated by adjusting the length of polymer segments, thereby generating distinctive 3D graphene-skeleton configurations in the freestanding superlattice. This finding presents a highly efficient and versatile way to artificially produce multifunctional organic/inorganic hybrid superlattice materials with adjustable dimensions and internal configurations.

Crystallization of 2D TiO2 Nanosheets via Oriented Attachment of 1D Coordination Polymer

Demystifying the molecular mechanism of growth is vital for the rational design, synthesis, and optimization of functional nanomaterials. Despite the promising perspectives and extensive efforts, the growth mechanism of atomically thin TiO2(B) nanosheets remains unclear, hence it is difficult to tune their band and surface structures. Herein, we report an oriented attachment-based crystallization mechanism of TiO2(B) nanosheets from a 1D titanium glycolate coordination polymer through hydrolysis and condensation. With time-tracking experiments, this 1D coordination polymer is found to be an intermediate in the synthesis of TiO2(B) nanosheets by using Ti alkoxides and chlorides as precursors, suggesting the universality of the 1D-to-2D growth mechanism. Such a side-to-side attachment pathway bridges the classical and nonclassical interpretations of crystallization, and meanwhile hints at the possibility of other 1D complexes as potential precursors for 2D materials.

Non-convergence of the blinking timescale of twelve-faceted perovskite nanocrystals observed through an advanced fluorescence correlation spectroscopy study

Single-particle photoluminescence measurements have been extensively utilized to investigate the charge carrier dynamics in quantum dots (QDs). Among these techniques, single dot blinking studies are effective for probing relatively slower processes with timescales >10 ms, whereas fluorescence correlation spectroscopy (FCS) studies are suited for recording faster processes with timescales typically <1 ms. In this study, we utilized scanning FCS (sFCS) to bridge the ms gap, thereby enabling the tracking of carrier dynamics across an extended temporal window ranging from μs to subsecond. We compared the sFCS data recorded on surface-immobilized twelve-faceted CsPbBr3 dodecahedron perovskite nanocrystals (d-PNCs) with the FCS data of the same nanocrystals in the solution phase. Although the two datasets exhibited similarities in a qualitative sense, they revealed notable quantitative differences. This is primarily attributed to the significantly varying immediate environments of PNCs in these two techniques, as well as the different temporal sizes of the observation windows available for the recording of carrier dynamics. The most intriguing finding of our study lies in the non-converging blinking timescale (τR) of d-PNCs in sFCS, despite this technique providing an extended temporal window size (≤328 ms) for studying carrier dynamics. We attribute this observation to PL blinking following power-law statistics, which causes the mean ON/OFF duration of blinking persuasive to the experimental integration time, making blinking occur across all timescales.

Deriving Chiroptical Properties from Intrinsically Achiral Building Blocks of One-Dimensional CsPbBr3 Perovskite Nanowires

Chirality is a ubiquitous feature in biological systems and occurs even in certain inorganic crystals. Interestingly, some inorganic nanocrystals have been shown to possess chirality, despite their achiral bulk forms. However, the mechanism of chirality formation and chiroptical responses in such nanocrystals is still ambiguous due to the presence of chiral organic ligands used to passivate such nanocrystals. Here, we recognize intrinsic chiroptical responses from lead halide perovskite nanowires with different length scales. Cube-connected nanowires with minimum interfacial contacts make their arrangement chiral for chiroptical responses even in the absence of chiral ligands. The chiral nanowires with varying lengths serve as a systematic platform for improving dissymmetric factors significantly with increasing lengths. The dissymmetric factor of the longest nanowires reaches 1.4 × 10-2, which is the highest among the intrinsic chiral perovskite nanocrystals at present. The nanowires generate circularly polarized luminescence, which has been seldom reported in halide perovskite nanocrystals in the absence of any chiral ligands. Furthermore, we find that chirality exists in the basic unit consisting of two corner-connected cubes in the form of a dimer. The intrinsic chirality of the nanowires is determined by the lattice rotation of connected cubes along the interfacial boundaries, which is different from the commonly observed chirality induced by chiral ligands. Such chiral lead halide perovskite nanocrystals with robust chiroptical properties provide an ideal platform for understanding the origin of intrinsic chirality and the rational design of anisotropic chiral nanostructures.

Ultra-Long Carrier Lifetime of Spiral Perovskite Nanowires Realized through Cooperative Strategy of Selective Etching and Passivation

Spiral inorganic perovskite nanowires (NWs) possess unique morphologies and properties that allow them highly attractive for applications in optoelectronic and catalytic fields. In popular solution-based synthesis methodology, however, challenges persist in simultaneously achieving precise and facile control over morphological twisting and fantastic carrier lifetimes. Here, a cooperative strategy of concurrently employing selective etching and ligand engineering is applied to facilitate the formation of spiral CsPbBr3 perovskite NWs with an ultralong carrier lifetime of ≈2 µs. Specifically, a novel amine of 1-(p-tolyl)ethanamine is introduced to functionalize as both a selective etchant and the source of forming an effective ligand to passivate the exposed facets, favoring the structural twisting and the enhancement of carrier lifetimes. The twisting behaviors are dependent on the etch ratios, which are essentially associated with the densities of grain boundaries and dislocations in the NWs. The ultralong carrier lifetime and long-term stability of the spiral NWs open up new possibilities for all-inorganic perovskites in optoelectronic and photocatalytic fields, while the cooperative synthesis strategy paves the way for exploring complex spiral structures with tunable morphology and functionality.

Unveiling the Influence of Brightness Heterogeneity in Fluorescence Correlation Spectroscopy of Perovskite Nanocrystals

Nanoparticles (NPs), including perovskite nanocrystals (PNCs) with single photon purity, present challenges in fluorescence correlation spectroscopy (FCS) studies due to their distinct photoluminescence (PL) behaviors. In particular, the zero-time correlation amplitude [g2(0)] and the associated diffusion timescale (τD) of their FCS curves show substantial dependency on pump intensity (IP). Optical saturation inadequately explains the origin of this FCS phenomenon in NPs, thus setting them apart from conventional dye molecules, which do not manifest such behavior. This observation is apparently attributed to either photo-brightening or optical trapping, both lead to increased NP occupancy (N) in the excitation volume, consequently reducing the g2(0) amplitude [since g2(0) α 1/N] at high IP. However, an advanced FCS study utilizing alternating laser excitation at two different intensities dismisses such possibilities. Further investigation into single-particle blinking behaviors as a function of IP reveals that the intensity dependence of g2(0) primarily arises from the brightness heterogeneity prevalent in almost all types of NPs. This report delves into the complexities of the photophysical properties of NPs and their adverse impacts on FCS studies.

Biexciton Emission in CsPbBr3 Nanocrystals: Polar Facet Matters

The metal halide perovskite nanocrystals exhibit a remarkable tolerance to midgap defect states, resulting in high photoluminescence quantum yields. However, the potential of these nanocrystals for applications in display devices is hindered by the suppression of biexcitonic emission due to various Auger recombination processes. By adopting single-particle photoluminescence spectroscopy, herein, we establish that the biexcitonic quantum efficiency increases with the increase in the number of facets on cesium lead bromide perovskite nanocrystals, progressing from cube to rhombic dodecahedron to rhombicuboctahedron nanostructures. The observed enhancement is attributed mainly to an increase in their surface polarity as the number of facets increases, which reduces the Coulomb interaction of charge carriers, thereby suppressing Auger recombination. Moreover, Auger recombination rate constants obtained from the time-gated photon correlation studies exhibited a discernible decrease as the number of facets increased. These findings underscore the significance of facet engineering in fine-tuning biexciton emission in metal halide perovskite nanocrystals.

Competition among recombination pathways in single FAPbBr3 nanocrystals

Single particle level microscopy of immobilized FAPbBr3 nanocrystals (NCs) has elucidated the involvement of different processes in their photoluminescence (PL) intermittency. Four different blinking patterns are observed in the data from more than 100 NCs. The dependence of PL decays on PL intensities brought out in fluorescence lifetime intensity distribution (FLID) plots is rationalized by the interplay of exciton- and trion-mediated recombinations along with hot carrier (HC) trapping. The high intensity-long lifetime component is attributed to neutral exciton recombination, the low intensity-short lifetime component is attributed to trion assisted recombination, and the low intensity-long lifetime component is attributed to hot carrier recombination. Change-point analysis (CPA) of the PL blinking data reveals the involvement of multiple intermediate states. Truncated power law distribution is found to be more appropriate than power law and lognormal distribution for on and off events. Probability distributions of PL trajectories of single NCs are obtained for two different excitation fluences and wavelengths (λex = 400, 440 nm). Trapping rate (kT) prevails at higher power densities for both excitation wavelengths. From a careful analysis of the FLID and probability distributions, it is concluded that there is competition between the HC and trion assisted blinking pathways and that the contribution of these mechanisms varies with excitation wavelength as well as fluence.

Facet Dependent Photoluminescence Blinking from Perovskite Nanocrystals

Photoluminescence (PL) blinking of nanoparticles, while detrimental to their imaging applications, may benefit next-generation displays if the blinking is precisely controlled by reversible electron/hole injections from an external source. Considerable efforts are made to create well-characterized charged excitons within nanoparticles through electrochemical charging, which has led to enhanced control over PL-blinking in numerous instances. Manipulating the photocharging/discharging rates in nanoparticles by surface engineering can represent a straightforward method for regulating their blinking behaviors, an area largely unexplored for perovskite nanocrystals (PNCs). This work shows facet engineering leading to different morphologies of PNCs characterized by distinct blinking patterns. For instance, examining the PL intensity trajectories of single PNCs, representing the instantaneous photon count rate over time, reveals that the OFF-state population significantly increases as the number of facets increases from six to twenty-six. This study suggests that extra-faceted PNCs, owing to their polar facets and expanded surface area, render them more susceptible to photocharging, which results in larger OFF-state populations. Furthermore, the fluorescence correlation spectroscopy (FCS) study unveils that the augmented propensity for photocharging in extra-faceted PNCs can also originate from their greater tendency to form complexes with neighboring molecules.

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.

Probing the Structural Degradation of CsPbBr3 Perovskite Nanocrystals in the Presence of H2O and H2S: How Weak Interactions and HSAB Matter

Structural degradation of all inorganic CsPbBr3 in the presence of moisture is considered as one of its major limitations to use as an active component in various light-harvesting and light-emitting devices. Herein, we used two similar molecules, H2O and H2S, with similar structures, to follow the decomposition mechanism of CsPbBr3 perovskite nanocrystals. Interestingly, H2O acts as a catalyst for the decomposition of CsPbBr3, which is in contrast to H2S. Our experimental observations followed by density functional theory (DFT) calculations showed that the water molecule is intercalated in the CsPbBr3 perovskite whereas H2S is adsorbed in the (100) planes of CsPbBr3 by a weak electrostatic interaction. According to Pearson's hard-soft acid-base theory, both cations present in CsPbBr3 prefer soft/intermediate bases. In the case of the water molecule, it lacks a soft base and thus it is not directly involved in the reaction whereas H2S can provide a soft base and thus it gets involved in the reaction. Understanding the mechanistic aspects of decomposition can give different methodologies for preventing such unwanted reactions.