Bromopropane as a novel bromine precursor for the completely amine free colloidal synthesis of ultrastable and highly luminescent green-emitting cesium lead bromide (CsPbBr3) perovskite nanocrystals

Highly Bright and Stable CsPbX3@Cs4PbX6 Hexagonal Nanoarchitectonics Created by Controlling Dissolution-Recrystallization of CsPbX3 Nanomaterials

CsPbBr3@Cs4PbBr6 hexagonal NCs with a bright photoluminescence (PL) peak of 456 nm are created through the dissolution-recrystallization of CsPbBr3 nanoplatelets. Small CsPbBr3 nanocrystals are encapsulated in hexagonal Cs4PbBr6 during recrystallization to form a core-shell structure and keep high brightness and stability. The recrystallization kinetics is systematically investigated to explore the roles of methyl acetate, oleylamine, and n-hexane. Result further indicates that core/shell NCs remained high PL under a variety of harsh conditions (e.g., light irradiation and heat treatment) because of Cs4PbX6 shell and the controlling of recrystallization. Their initial PL intensity is remained after 4 months of storage under ambient conditions and continuous exposure to UV lamp for 180 min. The bright PL is also maintained even treatment at 120 °C. To indicate the universality of this synthesis method, CsPbX3@Cs4PbX6 hexagonal NCs with different emission colors are fabricated by changing temperature, solvent viscosity, and precursors (e,g, oleylamine and halogens). These core-shell samples reveal bright and stable green, orange, and red PL. Because of its high stability, the core/shell NCs are dispersed in flexible films to create diverse patterns. The films also exhibit high brightness and excellent stability. This strategy opens a novel avenue for the application of perovskite nanomaterials in the display field.

Surface and optical properties of phase-pure silver iodobismuthate nanocrystals

The study of surface defects is one of the forefronts of halide perovskite research. In the nanoscale regime, where the surface-to-volume ratio is high, the surface plays a key role in determining the electronic properties of perovskites. Perovskite-inspired silver iodobismuthates are promising photovoltaic absorbers. Herein, we demonstrate the colloidal synthesis of phase pure and highly crystalline AgBiI4 nanocrystals (NCs). Surface-sensitive spectroscopic techniques reveal the rich surface features of the NCs that enable their impressive long-term environmental and thermal stabilities. Notably, the surface termination and its passivation effects on the electronic properties of AgBiI4 are investigated. Our atomistic simulations suggest that a bismuth iodide-rich surface, as in the case of AgBiI4 NCs, does not introduce surface trap states within the band gap region of AgBiI4, unlike a silver iodide-rich surface. These findings may encourage the investigation of surfaces of other lead-free perovskite-inspired materials.

Highly stable, substrate-free, and flexible broadband halide perovskite paper photodetectors

In this work, we aim to fabricate a highly stable and flexible perovskite paper photodetector based on a Zn-doped MA_0.6FA_0.4PbI₃ perovskite and CNC. The paper photodetector has been successfully synthesized by the vacuum filtration method and deposited with interdigitated electrodes. The paper photodetector exhibits a significant photoresponse with a responsivity of 0.23 A W^-1 under 650 nm light irradiation when operated at 5 V. The stability of the paper photodetector has also been tested and it shows high photoresponse after 30 days under ambient conditions. Therefore, this paper photodetector holds promise for developing efficient, stable, and flexible optoelectronic devices in the future.

Exploring the Role of Short Chain Acids as Surface Ligands in Photoinduced Charge Transfer Dynamics from CsPbBr3 Perovskite Nanocrystals

The most commonly used surface capping ligands, like oleic acid and oleylamine, passivate the surface of perovskite nanocrystals (PNCs) to enhance their stability and optical properties. However, due to their inherent insulating nature, charge transport across the surface of the PNCs is hindered, limiting their application in devices. In this study, we have post-treatment CsPbBr₃ PNCs with short chain ligands benzoic acid (BA) and ascorbic acid (AA) and observed that both acid-treated PNCs show enhanced stability and optical properties. Still, BA-treated PNCs show the highest charge transport rate due to their conjugating nature. The photoelectrochemical measurements also show the most efficient electron flow across the surface of the PNC with BA-treated PNCs. A longer carrier lifetime and fast charge transfer make BA-treated PNCs ideal candidates for application in real-life devices.

Green synthesis of highly stable CsPbBr3 perovskite nanocrystals using natural deep eutectic solvents as solvents and surface ligands

Perovskite nanocrystals (PNCs) have attracted widespread attention as promising materials for the optoelectronic field due to their remarkable photophysical properties and structural tunability. However, their poor stability and the use of toxic organic solvents in the preparation process have severely restricted their practical applications. Herein, a facile, rapid and toxic organic solvent-free synthesis strategy of CsPbBr₃ PNCs was developed for the first time via the ligand-assisted reprecipitation (LARP) method using natural deep eutectic solvents (NADESs) as solvents and surface ligands. In this method, the NADESs not only functioned as solvents for green synthesis, but also served simultaneously as surface ligands of CsPbBr₃ PNCs to significantly improve their optical properties and stability. The as-synthesized CsPbBr₃ PNCs exhibited high photoluminescence quantum yield (PLQY, ∼96.8%), narrow full width at half-maximum (FWHM, ∼18.8 nm) and a high stability that retained 82.9% of PL intensity after 70 days. This work provides a new strategy for the green synthesis of PNCs, which promises feasibility for the industrial large-scale synthesis of high-quality PNCs.

In situ synthesis of high-quantum-efficiency and stable bromide-based blue-emitting perovskite nanoplatelets

We present a facile synthetic approach for the growth of two-dimensional CsPbBr₃ nanoplatelets (NPLs) in the temperature range of 50-80 °C via the vacuum-assisted low-temperature (VALT) method. In this method, we utilized the solubility of the PbBr₂ precursor at temperatures high than the reaction temperature, thus making Br available during the reaction to form NPLs with fewer defects. The high chemical availability of Br during the reaction changes the growth dynamics and formation of highly crystalline nanoplatelets. Using this method, we have synthesized NPLs with an emission wavelength range of 450 to 485 nm that have high photoluminescence quantum yields (PLQY) from 80 to 100%. The synthesized NPLs retain their initial PLQY of about 80% after one month at ambient conditions. The formation of NPLs with fewer defects and enhanced radiative recombination was further confirmed by X-ray diffraction (XRD), reduced Urbach energy, time-resolved photocurrent measurements, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FTIR) spectroscopy. Additionally, we utilized the synthesized NPLs for the fabrication of down-conversion light emitting diodes (LEDs), and the electroluminescence peak was barely shifted compared to the photoluminescence peak.

Amine-Free Synthetic Route: An Emerging Approach to Making High-Quality Perovskite Nanocrystals for Futuristic Applications

In recent years, colloidal cesium lead halide (CsPbX₃) perovskite nanocrystals (PNCs) have attracted significant attention from researchers due to their unique optical properties and potential use in optoelectronic applications. In colloidal synthesis, oleic acid and oleylamine are commonly used as surface-capping ligands. Although oleylamine plays a crucial role in maintaining the colloidal stability and surface passivation of PNCs, its dynamic equilibrium with oleic acid leads to the formation of labile oleylammonium, which pulls halides from the surface of PNCs and thus degrades the crystals. In this Perspective, we summarize the various approaches for eliminating the amines to make high-quality, photostable, and amine-free CsPbX₃ PNCs. In addition, we look over the prospects of these PNCs regarding stability in different environmental conditions, photoluminescence properties, and optoelectronic device performance. This perspective will give a broad overview of amine-free PNCs starting from their synthesis, challenges, and optoelectronic properties to their future prospects.

Spray-driven halide exchange in solid-state CsPbX3 nanocrystal films

CsPbI₃ perovskite nanocrystals (NCs) are promising building blocks for photovoltaics and optoelectronics. However, they exhibit an essential drawback in the form of phase stability: α-phase, with a ∼1.80 eV bandgap, can easily experience a phase transition to a non-radiative orthorhombic δ-phase in an ambient environment. This leads to the need to carry out the CsPbI₃-based device fabrication in an inert atmosphere, which is technologically inconvenient and expensive. One of the most successful approaches proposed to overcome this problem is synthesizing mixed halide CsPbBr_3-xI_x NCs to improve the stability of the α-phase perovskite structure. However, the formation of high-quality thin films of CsPbBr_3-xI_x NCs with high PLQY is challenging owing to the degradation of their optical properties after deposition on a substrate. This work presents spray coating to carry out a solid-state anion exchange in CsPbBr₃ NCs thin films at ambient conditions with low-demanding reaction conditions. This constitutes a novel open-air and annealing-free technology to manufacture CsPbBr_3-xI_x NC thin films with high optical quality and record high photoluminescence quantum yields (PLQY) based on spray-driven halide (Br^- to I^-) anion exchange in a solid-state phase. Besides, tunable emission wavelengths between 520 and 670 nm can be obtained from CsPbBr_3-xI_x NC films using accurate tuning volumes of HI solution sprayed over the initial surface of CsPbBr₃ film to provide the halide exchange. The optical quality of the halide-exchanged PNCs films remains practically identical to that of initial Br-containing layers, with a remarkable PLQY enhancement after anion exchange, from ∼61% for CsPbBr₃ thin films emitting at 520 nm to ∼84% for mixed halide CsPbBr_3-xI_x film emitting at 640 nm. The huge potential of the system is confirmed by demonstrating a low-threshold amplified spontaneous emission.

On direct synthesis of high quality APbX3 (A = Cs+, MA+ and FA+; X = Cl-, Br- and I-) nanocrystals following a generic approach

Direct synthesis of APbX₃ [A = Cs⁺, methylammonium (MA⁺) or formamidinium (FA⁺) and X = Cl^-, Br^- or I^-] perovskite nanocrystals (NCs) following a generic approach is a challenging task even today. Motivated by our recent success in obtaining directly high-quality red/NIR-emitting APbI₃ NCs employing 1,3-diiodo-5,5-dimethylhydantoin (DIDMH) as an iodide precursor, we explore here whether violet/green-emitting APbCl₃ and APbBr₃ NCs can also be obtained using the chloro- and bromo-analog of DIDMH keeping in mind that a positive outcome will provide the generic protocol for direct synthesis of all APbX₃ NCs using similar halide precursors. It is shown that green-emitting APbBr₃ NCs with near-unity PLQY and violet-emitting CsPbCl₃ NCs with an impressive PLQY of ∼70%, mixed-halide NCs, CsPb(Cl/Br)₃ and CsPb(Br/I)₃, emitting in the blue and yellow-orange region with PLQYs of 87-95% and 68-98%, respectively can indeed be obtained employing the bromo- and chloro-analog of DIDMH. These NCs exhibit remarkable stability under different conditions including the polar environment. Femtosecond pump-probe studies show no ultrafast carrier trapping in these systems. The key elements of the halide precursors that facilitated the synthesis and the factors contributing to the excellent characteristics of the NCs are determined by careful analysis of the data. The results are of great significance because a direct method of obtaining highly luminescent and stable APbX₃ NCs (except violet-emitting hybrid NCs) is eventually identified and the work provides valuable insight into the selection of appropriate halide precursors for the development of superior systems.

Inner Filter Effect as a Boon in Perovskite Sensing Systems to Achieve Higher Sensitivity Levels

Perovskite quantum dots (PQDs) exhibit exceptional fluorescence property and are potential candidates for fluorescent metal-ion sensors. The present work shows the presence of inner filter effect (IFE) in perovskite sensing systems and its significance in enhancing the detection limits. Two different sensing systems (with a different extent of IFE), one with simple long-chain monodentate ligand-capped PQDs and the other with short-chain bidentate ligand capped PQDs, were developed toward sensing Co²⁺. The fluorescence quenching mechanism is elucidated and is observed to be a combination of Forster resonance energy transfer (FRET) and IFE. The electrostatic interaction of donor (D) with acceptor (A) and its distance for energy transfer was appropriate and was well within the requirement for a good energy transfer from PQDs (donor) to Co²⁺ ions (acceptor) facilitating partial FRET. Also, the spectral overlap of absorption of excited and emitted radiation (of PQDs) with that of Co²⁺ allows a significant amount of IFE. PQDs were successfully modified for lesser spectral overlap with reduced IFE. The reduction in IFE adversely drops the detection levels from 0.733 × 10^-7 to 0.7970 × 10^-6 on modification. This work provides insights into the design and development of high sensing perovskite probes with manipulation of IFE and also shows the importance of IFE to be considered during the study of such sensing systems, which has been neglected so far in perovskite systems.

Enhancement of photoluminescence and the stability of CsPbX3 (X = Cl, Br, and I) perovskite nanocrystals with phthalimide passivation

Cesium lead halide perovskite nanocrystals (CsPbX₃ NCs) have been the flourishing area of research in the field of photovoltaic and optoelectronic applications because of their excellent optical and electronic properties. However, they suffer from low stability and deterioration of photoluminescence (PL) properties post-synthesis. In this work, we demonstrate that incorporating an additional ligand can further enhance the optical properties and stability of NCs. Here, we introduced phthalimide as a new surface passivation ligand into the oleic acid/oleylamine system in situ to get near-unity photoluminescence quantum yield (PLQY) of CsPbBr₃ and CsPbI₃ perovskite NCs. Phthalimide passivation dramatically improves the stability of CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs under ambient light and UV light. The PL intensity was recorded for one year, which showed a dramatic improvement for CsPbBr₃ NCs. Nearly 11% of PL can be retained even after one year with phthalimide passivation. CsPbCl₃ NCs exhibit 3 times higher PL with phthalimide and retain 12% PL intensity even after two months, while PL of as-synthesized NCs completely diminishes. Under continuous UV light illumination, the PL intensity of phthalimide passivated NCs is well preserved, while the as-synthesized NCs exhibit negligible PL emission in 2 days. About 40% and 25% of initial PL is preserved for CsPbBr₃ and CsPbCl₃ NCs in the presence of phthalimide. CsPbI₃ NCs with phthalimide exhibit PL even after 2 days, while PL for as-synthesized NCs rapidly declined in the first 10 h. The presence of phthalimide in CsPbI₃ NCs could maintain stability even after a week, while the as-synthesized NCs underwent a transition to the non-luminescent phase within 4 days. Furthermore, blue, green, yellow, and red-emitting diodes using CsPbCl_1.5Br_1.5, CsPbBr₃, CsPbBr_1.5I_1.5, CsPbI₃ NCs, respectively, are fabricated by drop-casting NCs onto blue LED lights, which show great potential in the field of display and lighting technologies.