Hydrochromic CsPbBr3 Nanocrystals for Anti-Counterfeiting

Hydrochromic materials that can reversibly change color upon water treatment have attracted much attention owing to their potential applications in diverse fields. Herein, for the first time, we report that space-confined CsPbBr₃ nanocrystals (NCs) are hydrochromic. When CsPbBr₃ NCs are loaded into a porous matrix, reversible transition between luminescent CsPbBr₃ and non-luminescent CsPb₂Br₅ can be achieved upon the exposure/removal of water. The potential applications of hydrochromic CsPbBr₃ NCs in anti-counterfeiting are demonstrated by using CsPbBr₃ NCs@mesoporous silica nanospheres (around 100 nm) as the starting material. Owing to the small particle size and negatively charged surface, the as-prepared particles can be laser-jet printed with high precision and high speed. We demonstrate the excellent stability over repeated transformation cycles without color fade. This new discovery may not only deepen the understanding of CsPbX₃, but also open a new way to design CsPbX₃ materials for new applications.     

Water‐Assisted Size and Shape Control of CsPbBr3 Perovskite Nanocrystals

Lead‐halide perovskites are well known to decompose rapidly when exposed to polar solvents, such as water. Contrary to this common‐place observation, we have found that through introducing a suitable minor amount of water into the reaction mixture, we can synthesize stable CsPbBr₃ nanocrystals. The size and the crystallinity, and as a result the band gap tunability of the strongly emitting CsPbBr₃ nanocrystals correlate with the water content. Suitable amounts of water change the crystallization environment, inducing the formation of differently shaped perovskites, namely spherical NCs, rectangular nanoplatelets, or nanowires. Bright CsPbBr₃ nanocrystals with the photoluminescence quantum yield reaching 90 % were employed for fabrication of inverted hybrid inorganic/organic light‐emitting devices, with the peak luminance of 4428 cd m^?2 and external quantum yield of 1.7 %.     

Revealing the Formation Mechanism of CsPbBr3 Perovskite Nanocrystals Produced via a Slowed‐Down Microwave‐Assisted Synthesis

We developed a microwave‐assisted slowed‐down synthesis of CsPbBr₃ perovskite nanocrystals, which retards the reaction and allows us to gather useful insights into the formation mechanism of these nanoparticles, by examining the intermediate stages of their growth. The trends in the decay of the emission intensity of CsPbBr₃ nanocrystals under light exposure are well correlated with their stability against decomposition in TEM under electron beam. The results show the change of the crystal structure of CsPbBr₃ nanocrystals from a deficient and easier to be destroyed lattice to a well crystallized one. Conversely the shift in the ease of degradation sheds light on the formation mechanism, indicating first the formation of a bromoplumbate ionic scaffold, with Cs‐ion infilling lagging a little behind. Increasing the cation to halide ratio towards the stoichiometric level may account for the improved radiative recombination rates observed in the longer reaction time materials.     

Colloidal CsPbBr3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots

Traditional CdSe‐based colloidal quantum dots (cQDs) have interesting photoluminescence (PL) properties. Herein we highlight the advantages in both ensemble and single‐nanocrystal PL of colloidal CsPbBr₃ nanocrystals (NCs) over the traditional cQDs. An ensemble of colloidal CsPbBr₃ NCs (11 nm) exhibits ca. 90 % PL quantum yield with narrow (FWHM=86 meV) spectral width. Interestingly, the spectral width of a single‐NC and an ensemble are almost identical, ruling out the problem of size‐distribution in PL broadening. Eliminating this problem leads to a negligible influence of self‐absorption and Förster resonance energy transfer, along with batch‐to‐batch reproducibility of NCs exhibiting PL peaks within ±1 nm. Also, PL peak positions do not alter with measurement temperature in the range of 25 to 100 °C. Importantly, CsPbBr₃ NCs exhibit suppressed PL blinking with ca. 90 % of the individual NCs remain mostly emissive (on‐time >85 %), without much influence of excitation power.     

Alloying n‐Butylamine into CsPbBr3 To Give a Two‐Dimensional Bilayered Perovskite Ferroelectric Material

Cesium‐lead halide perovskites (e.g. CsPbBr₃) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr₃, we design the first cesium‐based two‐dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C₄H₉NH₃)₂CsPb₂Br₇ ( 1 ). Strikingly, 1 shows a high Curie temperature (T_c=412 K) above that of BaTiO₃ (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm^?2), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs⁺ ions. To our knowledge, such a 2D bilayered Cs⁺‐based metal–halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large “on/off” ratios of photoconductivity (>10³).     

Hole‐Boosted Cu(Cr,M)O2 Nanocrystals for All‐Inorganic CsPbBr3 Perovskite Solar Cells

The all‐inorganic CsPbBr₃ perovskite solar cell (PSC) is a promising solution to balance the high efficiency and poor stability of state‐of‐the‐art organic–inorganic PSCs. Setting inorganic hole‐transporting layers at the perovskite/electrode interface decreases charge carrier recombination without sacrificing superiority in air. Now, M‐substituted, p‐type inorganic Cu(Cr,M)O₂ (M=Ba²⁺, Ca²⁺, or Ni²⁺) nanocrystals with enhanced hole‐transporting characteristics by increasing interstitial oxygen effectively extract holes from perovskite. The all‐inorganic CsPbBr₃ PSC with a device structure of FTO/c‐TiO₂/m‐TiO₂/CsPbBr₃/Cu(Cr,M)O₂/carbon achieves an efficiency up to 10.18 % and it increases to 10.79 % by doping Sm³⁺ ions into perovskite halide, which is much higher than 7.39 % for the hole‐free device. The unencapsulated Cu(Cr,Ba)O₂‐based PSC presents a remarkable stability in air in either 80 % humidity over 60 days or 80 °C conditions over 40 days or light illumination for 7 days.     

Triple‐Mode Emission of Carbon Dots: Applications for Advanced Anti‐Counterfeiting

Photoluminescence (PL), up‐conversion PL (UCPL), and phosphorescence are three kinds of phenomena common to light‐emitting materials, but it is very difficult to observe all of them simultaneously when they are derived from a single material at room temperature. For the first time, triple‐mode emission (that is, PL, UCPL, and room temperature phosphorescence (RTP)) is reported, which relies on a composite of the luminescent carbon dots (CDs) prepared from m‐phenylenediamine and poly(vinyl alcohol) (PVA). Moreover, the CDs‐PVA aqueous dispersion is nearly colorless and demonstrates promise as a triple‐mode emission ink in the field of advanced anti‐counterfeiting.     

Amino‐Assisted Anchoring of CsPbBr3 Perovskite Quantum Dots on Porous g‐C3N4 for Enhanced Photocatalytic CO2 Reduction

Halide perovskite quantum dots (QDs) have great potential in photocatalytic applications if their low charge transportation efficiency and chemical instability can be overcome. To circumvent these obstacles, we anchored CsPbBr₃ QDs (CPB) on NH_x‐rich porous g‐C₃N₄ nanosheets (PCN) to construct the composite photocatalysts via N?Br chemical bonding. The 20 CPB‐PCN (20 wt % of QDs) photocatalyst exhibits good stability and an outstanding yield of 149 μmol h^?1 g^?1 in acetonitrile/water for photocatalytic reduction of CO₂ to CO under visible light irradiation, which is around 15 times higher than that of CsPbBr₃ QDs. This study opens up new possibilities of using halide perovskite QDs for photocatalytic application.     

5‐(2‐Mercaptoethyl)‐1H‐tetrazole: Facile Synthesis and Application for the Preparation of Water Soluble Nanocrystals and Their Gels

A facile method for the preparation of the novel capping ligand 5‐(2‐mercaptoethyl)‐1H‐tetrazole for the stabilization of water‐soluble nanocrystals was developed. This effective synthetic procedure is based on the cycloaddition of sodium azide to 3,3′‐dithiobis(propionitrile) followed by the reductive cleavage of a S?S bond with triphenylphosphine. The structure of the synthesized compound was confirmed by single‐crystal X‐ray analysis. A target tetrazole was successfully applied for the direct aqueous synthesis of CdTe and Au nanocrystals. CdTe nanocrystals capped with 5‐(2‐mercaptoethyl)‐1H‐tetrazole were found to reveal high photoluminescence efficiencies (up to 77 %). Nanocrystals capped with this tetrazole ligand are able to build 3D structures in a metal‐ion‐assisted gelation process in aqueous solution. Critical point drying of the as‐formed hydrogels allowed the preparation of the corresponding aerogels, while preserving the mesoporous structure.     

All‐in‐One Cellulose Nanocrystals for 3D Printing of Nanocomposite Hydrogels

Cellulose nanocrystals (CNCs) with >2000 photoactive groups on each can act as highly efficient initiators for radical polymerizations, cross‐linkers, as well as covalently embedded nanofillers for nanocomposite hydrogels. This is achieved by a simple and reliable method for surface modification of CNCs with a photoactive bis(acyl)phosphane oxide derivative. Shape‐persistent and free‐standing 3D structured objects were printed with a mono‐functional methacrylate, showing a superior swelling capacity and improved mechanical properties.     

Templated‐Assembly of CsPbBr3 Perovskite Nanocrystals into 2D Photonic Supercrystals with Amplified Spontaneous Emission

Perovskite nanocrystals (NCs) have revolutionized optoelectronic devices because of their versatile optical properties. However, controlling and extending these functionalities often requires a light‐management strategy involving additional processing steps. Herein, we introduce a simple approach to shape perovskite nanocrystals (NC) into photonic architectures that provide light management by directly shaping the active material. Pre‐patterned polydimethylsiloxane (PDMS) templates are used for the template‐induced self‐assembly of 10 nm CsPbBr₃ perovskite NC colloids into large area (1 cm²) 2D photonic crystals with tunable lattice spacing, ranging from 400 nm up to several microns. The photonic crystal arrangement facilitates efficient light coupling to the nanocrystal layer, thereby increasing the electric field intensity within the perovskite film. As a result, CsPbBr₃ 2D photonic crystals show amplified spontaneous emission (ASE) under lower optical excitation fluences in the near‐IR, in contrast to equivalent flat NC films prepared using the same colloidal ink. This improvement is attributed to the enhanced multi‐photon absorption caused by light trapping in the photonic crystal.     

Stable Blue Luminescent CsPbBr3 Perovskite Nanocrystals Confined in Mesoporous Thin Films

Creating CsPbBr₃ perovskite nanocrystals with bright blue emission is challenging because their optical properties depend sensitively on structure. Growing perovskites in mesoporous templates bypasses some of these purification issues because the size of the nanocrystal is governed by the dimensions of the pores. Mesoporous silica consisting of aligned channels with tunable diameter can be easily synthesized and used as a template. When the perovskite solution evaporates and retreats, some of the liquid remains trapped in the interconnecting pores by discontinuous dewetting. The precursor crystallizes, generating stable ca. 3.1 nm blue‐emitting perovskite nanocrystals. The mesoporous template also serves as a protective barrier to preserve the optical properties of the CsPbBr₃ from atmospheric conditions. Compared to the bulk crystals and the powder composite, the strong blue‐shift of the emission peak in the film is accompanied by a decrease in the longer lifetime component and an 8‐fold increase in the external quantum efficiency.     

Aqueous Precursor Induced Morphological Change and Improved Water Stability of CsPbBr3 Nanocrystals

In the literature, lead halide perovskites are very notable for their degradation in the presence of polar solvents, such as water. In contrast, in this research, it is observed that adding a minor amount of water into the precursor solution can improve the stability and photoluminescence quantum yield of CsPbBr₃ nanocrystals through a ligand-assisted reprecipitation (LARP) method. In this way, the shape and phase transformation from CsPbBr₃ nanoplates to CsPbBr₃/Cs₄PbBr₆ nanorods and Cs₄PbBr₆ nanowires can be controlled with increasing water content in the precursor solution. Upon adding water up to an ideal amount, CsPbBr₃ maintains its phase and nanoplate morphology. The key role of water amount for tuning the crystallinity, stability, morphology, optical properties, and phase transformation of cesium lead halide perovskite nanocrystals will be beneficial in the future commercialization of optoelectronics.     

Quantifying the Thermodynamics of Ligand Binding to CsPbBr3 Quantum Dots

Cesium lead halide perovskites are an emerging class of quantum dots (QDs) that have shown promise in a variety of applications; however, their properties are highly dependent on their surface chemistry. To this point, the thermodynamics of ligand binding remain unstudied. Herein, ¹H NMR methods were used to quantify the thermodynamics of ligand exchange on CsPbBr₃ QDs. Both oleic acid and oleylamine native ligands dynamically interact with the CsPbBr₃ QD surface, having individual surface densities of 1.2–1.7 nm^?2. 10‐Undecenoic acid undergoes an exergonic exchange equilibrium with bound oleate (K_eq=1.97) at 25 °C while 10‐undecenylphosphonic acid undergoes irreversible ligand exchange. Undec‐10‐en‐1‐amine exergonically exchanges with oleylamine (K_eq=2.52) at 25 °C. Exchange occurs with carboxylic acids, phosphonic acids, and amines on CsPbBr₃ QDs without etching of the nanocrystal surface; increases in the steady‐state PL intensities correlate with more strongly bound conjugate base ligands.     

Encapsulating Palladium Nanoparticles Inside Mesoporous MFI Zeolite Nanocrystals for Shape‐Selective Catalysis

Pd nanoparticles were successfully encapsulated inside mesoporous silicalite‐1 nanocrystals (Pd@mnc‐S1) by a one‐pot method. The as‐synthesized Pd@mnc‐S1 with excellent stability functioned as an active and reusable heterogeneous catalyst. The unique porosity and nanostructure of silicalite‐1 crystals endowed the Pd@mnc‐S1 material general shape‐selectivity for various catalytic reactions, including selective hydrogenation, oxidation, and carbon–carbon coupling reactions.     

Phase‐Stable CsPbI3 Nanocrystals: The Reaction Temperature Matters

High temperature colloidal synthesis for obtaining thermal, colloidal and phase‐stable CsPbI₃ nanocrystals with near‐unity quantum yield is reported. While standard perovskite synthesis reactions were carried out at 160 °C (below 200 °C), increase of another ≈100 °C enabled the alkylammonium ions to passivate the surface firmly and prevented the nanocrystals from phase transformation. This did not require any inert atmosphere storage, use of heteroatoms, specially designed ligands, or the ice cooling protocol. Either at high temperature in reaction flask or in the crude mixture or purified dispersed solution; these nanocrystals were observed stable and retained the original emission. Different spectroscopic analyses were carried out and details of the surface binding of alkyl ammonium ligands in place of surface Cs in the crystal lattice were investigated. As CsPbI₃ is one of the most demanding optical materials, bringing stability by proper surface functionalization without use of secondary additives would indeed help in wide spreading of their applications.     

Direct Conversion of Bulk Metals to Size‐Tailored,Monodisperse Spherical Non‐Coinage‐Metal Nanocrystals

Monodisperse non‐noble metal nanocrystals (NCs) that are highly uniform in shapes and particle size are much desired in various advanced applications, and are commonly prepared by either thermal decomposition or reduction, where reactive organometallic precursors or/and strong reducing agents are mandatory; however, these are usually toxic, costly, or suffer a lack of availability. Bulk Group 12 metals can now be converted into ligand‐protected, highly crystalline, monodisperse spherical metal NCs with precisely controlled sizes without using any precursors and reducers. The method is based on low‐power NIR‐laser‐induced size‐selective layer‐by‐layer surface vaporization. The monodisperse Cd NCs show pronounced deep‐UV (DUV) localized surface plasmon resonance making them highly competitive DUV‐plasmonic materials. This approach will promote appreciably the emergence of a wide range of monodisperse technically important non‐coinage metal NCs with compelling functionalities.     

Manganese‐Doping‐Induced Quantum Confinement within Host Perovskite Nanocrystals through Ruddlesden–Popper Defects

The concept of doping Mn²⁺ ions into II–VI semiconductor nanocrystals (NCs) was recently extended to perovskite NCs. To date, most studies on Mn²⁺ doped NCs focus on enhancing the emission related to the Mn²⁺ dopant via an energy transfer mechanism. Herein, we found that the doping of Mn²⁺ ions into CsPbCl₃ NCs not only results in a Mn²⁺‐related orange emission, but also strongly influences the excitonic properties of the host NCs. We observe for the first time that Mn²⁺ doping leads to the formation of Ruddlesden–Popper (R.P.) defects and thus induces quantum confinement within the host NCs. We find that a slight doping with Mn²⁺ ions improves the size distribution of the NCs, which results in a prominent excitonic peak. However, with increasing the Mn²⁺ concentration, the number of R.P. planes increases leading to smaller single‐crystal domains. The thus enhanced confinement and crystal inhomogeneity cause a gradual blue shift and broadening of the excitonic transition, respectively.