Lead‐Free Sodium–Indium Double Perovskite Nanocrystals through Doping Silver Cations for Bright Yellow Emission

Lead‐free halide perovskite nanocrystals (NCs) have drawn wide attention for solving the problem of lead perovskites toxicity and instability. Herein, we synthesize the direct band gap double perovskites undoped and Ag‐doped Cs₂NaInCl₆ NCs by variable temperature hot injection. The Cs₂NaInCl₆ NCs have little photoluminescence because of dark self‐trapped excitons (STEs). The dark STEs can be converted into bright STEs by doping with Ag⁺ to produce a bright yellow emission, with the highest photoluminescence quantum efficiency of 31.1 %. The dark STEs has been directly detected experimentally by ultrafast transient absorption (TA) techniques. The dynamics mechanism is further studied. In addition, the Ag‐doped NCs show better stability than the undoped ones. This result provides a new way to enhance the optical properties of lead‐free perovskites NCs for high‐performance light emitters.     

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 %.     

Composition‐Dependent Hot Carrier Relaxation Dynamics in Cesium Lead Halide (CsPbX3, X=Br and I) Perovskite Nanocrystals

Cesium‐based perovskite nanocrystals (NCs) have outstanding photophysical properties improving the performances of lighting devices. Fundamental studies on excitonic properties and hot‐carrier dynamics in perovskite NCs further suggest that these materials show higher efficiencies compared to the bulk form of perovskites. However, the relaxation rates and pathways of hot‐carriers are still being elucidated. By using ultrafast transient spectroscopy and calculating electronic band structures, we investigated the dependence of halide in Cs‐based perovskite (CsPbX₃ with X=Br, I, or their mixtures) NCs on the hot‐carrier relaxation processes. All samples exhibit ultrafast (<0.6 ps) hot‐carrier relaxation dynamics with following order: CsPbBr₃ (310 fs)>CsPbBr_1.5I_1.5 (380 fs)>CsPbI₃ NC (580 fs). These result accounts for a reduced light emission efficiency of CsPbI₃ NC compared to CsPbBr₃ NC.     

Suppressing thermal quenching of lead halide perovskite nanocrystals by constructing a wide-bandgap surface layer for achieving thermally stable white light-emitting diodes

Lead halide perovskite nanocrystals as promising ultrapure emitters are outstanding candidates for next-generation light-emitting diodes (LEDs) and display applications, but the thermal quenching behavior of light emission has severely hampered their real-world applications. Here, we report an anion passivation strategy to suppress the emission thermal quenching behavior of CsPbBr₃ perovskite nanocrystals. By treating with specific anions (such as SO₄²⁻, OH⁻, and F⁻ ions), the corresponding wide-bandgap passivation layers, PbSO₄, Pb(OH)₂, and PbF₂, were obtained. They not only repair the surface defects of CsPbBr₃ nanocrystals but also stabilize the phase structure of the inner CsPbBr₃ core by constructing a core–shell like structure. The photoluminescence thermal resistance experiments show that the treated sample could preserve 79% of its original emission intensity up to 373 K, far superior to that (17%) of pristine CsPbBr₃. Based on the thermally stable CsPbBr₃ nanocrystals, we achieved temperature-stable white LED devices with a stable electroluminescence spectrum, color gamut and color coordinates in thermal stress tests (up to 373 K).

Highly thermotolerant CsPbBr₃ perovskite nanocrystals with anti-thermal quenching performance were obtained by constructing wide-bandgap passivation layers coated strongly on the perovskite surface.     

Impact of Zn-doping on the composition,stability, luminescence properties of silica coated all-inorganic cesium lead bromide nanocrystals and their biocompatibility

Cesium lead halide (CsPbX₃: X = I, Br, Cl) nanocrystals (NCs) are believed to be potential candidates for bioimaging applications. However, their low structural stability against polar solvents remains as a major limitation. To improve the NCs stability and maintain high emission intensity, we synthesized silica coated Zn-doped core@shell perovskite NCs via modified ligand assisted reprecipitation (LARP) synthetic method under relatively high humid condition. We systemically varied the composition inside the perovskite structure and then studied their photophysical properties and stability. Interestingly, the Zn-doping amount controls the ratio of CsPbBr₃ to Cs₄PbBr₆ perovskites inside the core and also facilitates the growth of (OA)₂PbBr₄ shell, enables overall increase in NCs emission intensity and stability. We observed green color emission from these NCs in the spectral range of 494-506 nm with a maximum photoluminescence quantum yield (PLQY) up to 88%. The optimized Zn-doped NCs exhibited nearly four times better water stability compared to the bare NCs and retain emission properties for several months even in highly polar solvents. Finally, we performed biocompatibility test of the NCs generated on biological samples and hydroponics test in a gardenia leaf for their potential bioimaging applications.     

Completely Amine-Free Open-Atmospheric Synthesis of High-Quality Cesium Lead Bromide (CsPbBr3) Perovskite Nanocrystals

Cesium lead halide perovskite nanocrystals (NCs) CsPbX₃ (X=Cl, Br, and I) have been prominent materials in the last few years due to their high photoluminescence quantum yield (PLQY) for light-emitting diodes and other significant applications in photovoltaics and optoelectronics. In colloidal CsPbX₃ synthesis, the most commonly used ligands are oleic acid and oleylamine. The latter plays an important role in surface passivation but may also be responsible for poor colloidal stability as a result of facile proton exchange leading to the formation of labile oleylammonium halide, which pulls halide ions out of the NC surface. Herein, a facile, efficient, completely amine-free synthesis of cesium lead bromide perovskite nanocrystals using hydrobromic acid as halide source and tri-n-octylphosphane as ligand under open-atmospheric conditions is demonstrated. Hydrobromic acid serves as labile source of bromide ion, and thus this three-precursor approach (separate precursors for Cs, Pb, Br) gives more control than a conventional single-source precursor for Pb and Br (PbBr₂). The use of HBr paved the way to eliminate oleylamine, and thus the formation of labile oleylammonium halide can be completely excluded. Various Cs:Pb:Br molar ratios were studied and optimum conditions for making very stable CsPbBr₃ NCs with high PLQY were found. These completely amine-free CsPbBr₃ perovskite NCs synthesized under bromine-rich conditions exhibit good stability and durability for more than three months in the form of colloidal solutions and films, respectively. Furthermore, stable tunable emission across a wide spectral range through anion exchange was demonstrated. More importantly, this work reports open-atmosphere-stable CsPbBr₃ NCs films exhibiting strong PL, which can be further used for optoelectronic device applications.     

Highly Luminescent Cesium Lead Halide Perovskite Nanocrystals with Tunable Composition and Thickness by Ultrasonication

We describe the simple, scalable, single‐step, and polar‐solvent‐free synthesis of high‐quality colloidal CsPbX₃ (X=Cl, Br, and I) perovskite nanocrystals (NCs) with tunable halide ion composition and thickness by direct ultrasonication of the corresponding precursor solutions in the presence of organic capping molecules. High angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) revealed the cubic crystal structure and surface termination of the NCs with atomic resolution. The NCs exhibit high photoluminescence quantum yields, narrow emission line widths, and considerable air stability. Furthermore, we investigated the quantum size effects in CsPbBr₃ and CsPbI₃ nanoplatelets by tuning their thickness down to only three to six monolayers. The high quality of the prepared NCs (CsPbBr₃) was confirmed by amplified spontaneous emission with low thresholds. The versatility of this synthesis approach was demonstrated by synthesizing different perovskite NCs.     

Revealing the Mechanism of Pressure-Induced Emission in Layered Silver-Bismuth Double Perovskites

Pressure-induced emission (PIE) associated with self-trapping excitons (STEs) in low-dimensional halide perovskites has attracted great attention for better materials-by-design. Here, using 2D layered double perovskite (C₆H₅CH₂CH₂NH₃⁺)₄AgBiBr₈ as a model system, we advance a fundamental physicochemical mechanism of the PIE from the perspective of carrier dynamics and excited-state behaviors of local lattice distortion. We observed a pressure-driven STE transformation from dark to bright states, corresponding a strong broadband Stokes-shifted emission. Further theoretical analysis demonstrated that the suppressed lattice distortion and enhanced electronic dimensionality in the excited-state play an important role in the formation of stabilized bright STEs, which could manipulate the self-trapping energy and lattice deformation energy to form an energy barrier between the potential energy curves of ground- and excited-state, and enhance the electron-hole orbital overlap, respectively.     

CsPbBr3–CdS heterostructure: stabilizing perovskite nanocrystals for photocatalysis

The instability of cesium lead bromide (CsPbBr₃) nanocrystals (NCs) in polar solvents has hampered their use in photocatalysis. We have now succeeded in synthesizing CsPbBr₃–CdS heterostructures with improved stability and photocatalytic performance. While the CdS deposition provides solvent stability, the parent CsPbBr₃ in the heterostructure harvests photons to generate charge carriers. This heterostructure exhibits longer emission lifetime (τ_ave = 47 ns) than pristine CsPbBr₃ (τ_ave = 7 ns), indicating passivation of surface defects. We employed ethyl viologen (EV²⁺) as a probe molecule to elucidate excited state interactions and interfacial electron transfer of CsPbBr₃–CdS NCs in toluene/ethanol mixed solvent. The electron transfer rate constant as obtained from transient absorption spectroscopy was 9.5 × 10¹⁰ s⁻¹ and the quantum efficiency of ethyl viologen reduction (Φ_EV⁺˙) was found to be 8.4% under visible light excitation. The Fermi level equilibration between CsPbBr₃–CdS and EV²⁺/EV⁺˙ redox couple has allowed us to estimate the apparent conduction band energy of the heterostructure as −0.365 V vs. NHE. The insights into effective utilization of perovskite nanocrystals built around a quasi-type II heterostructures pave the way towards effective utilization in photocatalytic reduction and oxidation processes.

The insights into effective utilization of perovskite nanocrystals built around a CsPbBr₃–CdS heterostructure pave the way towards their utilization in photocatalytic reduction and oxidation processes.     

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.     

Synthesis and Localized Photoluminescence Blinking of Lead-Free 2D Nanostructures of Cs3Bi2I6Cl3 Perovskite

Two-dimensional (2D) lead-free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in-depth understanding on their shape-controlled charge-carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single-particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs₃Bi₂I₆Cl₃, by solution-based method. We applied fluorescence microscopy and super-resolution optical imaging at single-particle level to investigate their morphology-dependent PL properties. Narrow emission line widths and passivation of non-radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super-resolution optical image of the NS from localization-based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.     

Novel hydrostable Z-scheme Ag/CsPbBr3/Bi2WO6 photocatalyst for effective degradation of Rhodamine B in aqueous solution

In this study, a hydrostable Z-scheme Ag/CsPbBr₃/Bi₂WO₆ photocatalyst was designed and fabricated for the degradation of Rhodamine B (RhB). The structural instability of CsPbX₃ perovskites in water is one of the main obstacles that restrict their practical application in photocatalytic wastewater treatment. The photocatalyst was prepared in three steps: passivation of CsPbBr₃ nanocrystals (NCs) with 3-mercaptopropionic acid (MPA), construction of a heterojunction between MPA-passivated CsPbBr₃ NCs and Bi₂WO₆ ultrathin nanosheets, and doping Ag nanoparticles as charge mediators in the heterojunction. The as-obtained 5%Ag/20%CsPbBr₃/Bi₂WO₆ exhibits good stability and excellent photocatalytic activity. The degradation rate is 93.9% in 120 min, which is 4.41 times than that of Bi₂WO₆.     

Synthesis of double perovskite and quadruple perovskite nanocrystals through post-synthetic transformation reactions

Lead-free halide perovskite nanocrystals (NCs) represent a group of emerging materials which hold promise for various optical and optoelectronic applications. Exploring facile synthetic methods for such materials has been of great interest to not only fundamental research but also technological implementations. Herein, we report a fundamentally new method to access lead-free Bi-based double perovskite (DP) and quadruple perovskite (or layered double perovskite, LDP) NCs based on a post-synthetic transformation reaction of Cs₃BiX₆ (X = Cl, Br) zero-dimensional (0D) perovskite NCs under mild conditions. The produced NCs show good particle uniformity, high crystallinity, and comparable optical properties to the directly synthesized NCs. The relatively slow kinetics and stop-on-demand feature of the transformation reaction allow real-time composition–structure–property investigations of the reaction, thus elucidating a cation-alloyed intermediate-assisted transformation mechanism. Our study presented here demonstrates for the first time that post-synthetic transformation of 0D perovskite NCs can serve as a new route towards the synthesis of high-quality lead-free perovskite NCs, and provides valuable insights into the crystal structures, excitonic properties and their relationships of perovskite NCs.

Lead-free perovskite nanocrystals are synthesized by post-synthetic transformation reactions. The post-synthetic transformations show the structural flexibility of zero-dimensional perovskite nanocrystal materials.     

Synthesis and Localized Photoluminescence Blinking of Lead‐Free 2D Nanostructures of Cs3Bi2I6Cl3 Perovskite

Two‐dimensional (2D) lead‐free halide perovskites have generated enormous perception in the field of optoelectronics due to their fascinating optical properties. However, an in‐depth understanding on their shape‐controlled charge‐carrier recombination dynamics is still lacking, which could be resolved by exploring the photoluminescence (PL) blinking behaviour at the single‐particle level. Herein, we demonstrate, for the first time, the synthesis of nanocrystals (NCs) and 2D nanosheets (NSs) of layered mixed halide, Cs₃Bi₂I₆Cl₃, by solution‐based method. We applied fluorescence microscopy and super‐resolution optical imaging at single‐particle level to investigate their morphology‐dependent PL properties. Narrow emission line widths and passivation of non‐radiative defects were evidenced for 2D layered nanostructures, whereas the activation of shallow trap states was recognized at 77 K. Interestingly, individual NCs were found to display temporal intermittency (blinking) in PL emission. On the other hand, NS showed temporal PL intensity fluctuations within localized domains of the crystal. In addition, super‐resolution optical image of the NS from localization‐based method showed spatial inhomogeneity of the PL intensity within perovskite crystal.     

Colloidal Synthesis and Optical Properties of All‐Inorganic Low‐Dimensional Cesium Copper Halide Nanocrystals

Low‐dimensional metal halides have recently attracted extensive attention owing to their unique structure and photoelectric properties. Herein, we report the colloidal synthesis of all‐inorganic low‐dimensional cesium copper halide nanocrystals (NCs) by adopting a hot‐injection approach. Using the same reactants and ligands, but different reaction temperatures, both 1D CsCu₂I₃ nanorods and 0D Cs₃Cu₂I₅ NCs can be prepared. Density functional theory indicates that the reduced dimensionality in 1D CsCu₂I₃ compared to 0D Cs₃Cu₂I₅ makes the excitons more localized, which accounts for the strong emission of 0D Cs₃Cu₂I₅ NCs. Subsequent optical characterization reveals that the highly luminescent, strongly Stokes‐shifted broadband emission of 0D Cs₃Cu₂I₅ NCs arises from the self‐trapped excitons. Our findings not only present a method to control the synthesis of low‐dimensional cesium copper halide nanocrystals but also highlight the potential of 0D Cs₃Cu₂I₅ NCs in optoelectronics.     

Exploring Highly Efficient Broadband Self-Trapped-Exciton Luminophors: from 0D to 3D Materials

The review summarizes our recent reports on brightly-emitting materials with varied dimensionality (3D, 2D, 0D) synthesized using “green” chemistry and exhibiting highly efficient photoluminescence (PL) originating from self-trapped exciton (STE) states. The discussion starts with 0D emitters, in particular, ternary indium-based colloidal quantum dots, continues with 2D materials, focusing on single-layer polyheptazine carbon nitride, and further evolves to 3D luminophores, the latter exemplified by lead-free double halide perovskites. The review shows the broadband STE PL to be an inherent feature of many materials produced in mild conditions by “green” chemistry, outlining PL features general for these STE emitters and differences in their photophysical properties. The review is concluded with an outlook on the challenges in the field of STE PL emission and the most promising venues for future research.     

Glycine-Functionalized CsPbBr3 Nanocrystals for Efficient Visible-Light Photocatalysis of CO2 Reduction

Capping ligands are indispensable for the preparation of metal-halide-perovskite (MHP) nanocrystals (NCs) with good stability; however, the long alkyl-chain capping ligands in conventional MHP NCs will be unfavorable for CO₂ adsorption and hinder the efficient carrier separation on the surface of MHP NCs, leading to inferior catalytic activity in artificial photosynthesis. Herein, CsPbBr₃ nanocrystals with short-chain glycine as ligand are constructed through a facile ligand-exchange strategy. Owing to the reduced hindrance of glycine and the presence of the amine group in glycine, the photogenerated carrier separation and CO₂ uptake capacity are noticeably improved without compromising the stability of the MHP NCs. The CsPbBr₃ nanocrystals with glycine ligands exhibit a significantly increased yield of 27.7 μmol g⁻¹ h⁻¹ for photocatalytic CO₂-to-CO conversion without any organic sacrificial reagents, which is over five times higher than that of control CsPbBr₃ NCs with conventional long alkyl-chain capping ligands.     

Soft crystal lattice and large anharmonicity facilitate the self-trapped excitonic emission in ultrathin 2D nanoplates of RbPb2Br5

Self-trapping of excitons (STE) and concomitant useful broadband emission in low-dimensional metal halides occur due to strong electron–phonon coupling, which exhibit potential applications in optoelectronics and solid-state lighting. Lattice softness and high anharmonicity in the low-dimensional structure can lead to transient structural distortion upon photoexcitation that should promote the spatial localization or trapping of charge carriers, which is essential for STE. Herein, we report the ligand-assisted reprecipitation synthesis of ultrathin (∼3.5 nm) two-dimensional (2D) metal halide, RbPb₂Br₅ nanoplates (NPLs), which demonstrate highly Stokes shifted and broadband emission covering most parts of the visible to near IR range (500–850 nm) with a long-lived photoluminescence (PL) lifetime. The excitation wavelength independent emission and emission wavelength independent excitation spectra along with the analogous PL decay kinetics of bulk and NPLs suggest the intrinsic nature of broadband emission. The experimental low sound velocity (∼1090 m s⁻¹) and associated low bulk and shear moduli (10.10 and 5.51 GPa, respectively) indicate the large anharmonicity and significantly soft lattice structure, which trigger the broadband STE emission in 2D NPLs of RbPb₂Br₅. Strong electron-longitudinal optical (LO) phonon coupling results in broadband STE emission in 2D RbPb₂Br₅ NPLs.

The presence of large anharmonicity and lattice softness in low dimensional metal halide, RbPb₂Br₅ nanoplates, trigger strong electron–phonon coupling and consequently highly Stokes shifted broadband emission originates from self-trapped excitons.     

Ultrafast Charge Delocalization Dynamics of Ambient Stable CsPbBr3 Nanocrystals Encapsulated in Polystyrene Fiber

CsPbBr₃ nanocrystals (NCs) encapsulated in a transparent polystyrene (PS) fiber matrix (CsPbBr₃@PS) have been synthesized to protect the NCs. The ultrafast charge delocalization dynamics of the embedded NCs have been demonstrated, and the results are compared with the pristine CsPbBr₃ in toluene. The electrospinning method was employed for the preparation of CsPbBr₃@PS fibers by using a polystyrene solution doped with pre-synthesized CsPbBr₃ and characterized by XRD, HRTEM, and X-ray photoelectron spectroscopy (XPS). Energy level diagrams of CsPbBr₃ and PS suggest that CsPbBr₃@PS fibers make a type I core–shell structure. The carrier cooling for CsPbBr₃@PS fibers is found to be much slower than pure CsPbBr₃ NCs. This observation suggests that photoexcited electrons from CsPbBr₃ NCs get delocalized from the conduction band of the perovskite to lowest unoccupied molecular orbital (LUMO) of the PS fiber matrix. The CsPbBr₃@PS fibers possess remarkable stability under ambient conditions as well as in water over months. The clear understanding of charge carrier relaxation dynamics of CsPbBr₃ confined in PS fibers could help to design robust optoelectronic devices.     

How Chemical Bonding Impacts Halide Perovskite Nanocrystals Growth to Bulk Films: Implication of Pb−X Bond on Growth Kinetics

Lead halide perovskites nanocrystals have emerged as a leading candidate in perovskite solar cells and light-emitting diodes. Given their favorable, tunable optoelectronic properties through modifying the size of nanocrystals, it is imperative to understand and control the growth of lead halide perovskite nanocrystals. However, during the nanocrystal growth into bulk films, the effect of halide bonding on growth kinetics remains elusive. To understand how a chemical bonding of Pb−X (covalency and ionicity) impact on growth of nanocrystals, we have examined two different halide perovskite nanocrystals of CsPbCl₃ (more ionic) and CsPbI₃ (more covalent) derived from the same parent CsPbBr₃ nanocrystals. Tracking the growth of nanocrystals by monitoring the spectral features of bulk peaks (at 445 nm for Cl and at 650 nm for I) enables us to determine the growth activation energy to be 92 kJ/mol (for CsPbCl₃) versus 71 kJ/mol (for CsPbI₃). The electronegativity of halides in Pb−X bonds governs the bond strength (150–240 kJ/mol), characteristics of bonding (ionic versus covalent), and growth kinetics and resulting activation energies. A fundamental understanding of Pb−X bonding provides a significant insight into controlling the size of the perovskite nanocrystals with more desired optoelectronic properties.