Using Perovskite Nanoparticles as Halide Reservoirs in Catalysis and as Spectrochemical Probes of Ions in Solution

Perovskite Nanocrystals: Superior Luminogens for Food Quality Detection Analysis

With the global limited food resources receiving grievous damage from frequent climate changes and ascending global food demand resulting from increasing population growth, perovskite nanocrystals with distinctive photoelectric properties have emerged as attractive and prospective luminogens for the exploitation of rapid, easy operation, low cost, highly accurate, excellently sensitive, and good selective biosensors to detect foodborne hazards in food practices. Perovskite nanocrystals have demonstrated supreme advantages in luminescent biosensing for food products due to their high photoluminescence (PL) quantum yield, narrow full width at half-maximum PL, tunable PL in the entire visible spectrum, easy preparation, and various modification strategies compared with conventional semiconductors. Herein, we have carried out a comprehensive discussion concerning perovskite nanocrystals as luminogens in the application of high-performance biosensing of foodborne hazards for food products, including a brief introduction of perovskite nanocrystals, perovskite nanocrystal-based biosensors, and their application in different categories of food products. Finally, the challenges and opportunities faced by perovskite nanocrystals as superior luminogens were proposed to promote their practicality in the future food supply.

Using the Photoluminescence Color Change in Cesium Lead Iodide Nanoparticles to Monitor the Kinetics of an External Organohalide Chemical Reaction by Halide Exchange

In this work, we demonstrate a photoluminescence-based method to monitor the kinetics of an organohalide reaction by way of detecting released bromide ions at cesium lead halide nanoparticles. Small aliquots of the reaction are added to an assay with known concentrations of CsPbI3, and the resulting Br-to-I halide exchange (HE) results in rapid and sensitive wavelength blueshifts (Δλ) due to CsPbBrxI3-x intermediate concentrations, the wavelengths of which are proportional to concentrations. An assay response factor, C, relates Δλ to Br- concentration as a function of CsPbI3 concentration. The observed kinetics, as well as calculated rate constants, equilibrium, and activation energy of the solvolysis reaction tested correspond closely to synthetic literature values, validating the assay. Factors that influence the sensitivity and performance of the assay, such as CsPbI3 size, morphology, and concentration, are discussed.

Tailoring CsPbBr3 Growth via Non-Polar Solvent Choice and Heating Methods

This study describes an investigation of the role of non-polar solvents on the growth of cesium lead halide (CsPbX₃ X = Br and I) nanoplatelets. We employed two solvents, benzyl ether (BE) and 1-octadecene (ODE), as well as two nucleation and growth mechanisms, one-pot, facilitated by microwave irradiation (MWI)-based heating, and hot-injection, using convection. Using BE and MWI, large mesoscale CsPbBr₃ nanoplatelets were produced, whereas use of ODE produced small crystallites. Differences between the products were observed by optical spectroscopies, which showed first band edge absorptions consistent with thicknesses of ∼9 nm [∼15 monolayer (ML)] for the BE-CsPbBr₃ and ∼5 nm (∼9 ML) for ODE-CsPbBr₃. Both products had orthorhombic crystal structures, with the BE-CsPbBr₃ revealing significant preferred orientation diffraction signals consistent with the asymmetric and two-dimensional platelet morphology. The differences in the final morphology were also observed for products formed via hot injection, with BE-CsPbBr₃ showing thinner square platelets with thicknesses of ∼2 ML and ODE-CsPbBr₃ showing similar morphologies and small crystallite sizes. To understand the role solvent plays in crystal growth, we studied lead plumbate precursor (PbBr_n^2-n) formation in both solvents, as well as solvent plus ligand solutions. The findings suggest that BE dissolves PbBr₂ salts to a higher degree than ODE, and that this BE to precursor affinity persists during growth.

Detection of Halomethanes Using Cesium Lead Halide Perovskite Nanocrystals

The extensive use of halomethanes (CH₃X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH₃I) as a model system, it is shown that cesium lead bromide (CsPbBr₃) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH₃I and release the iodide species. The extent of the halide exchange is directly dependent on the CH₃I concentration, with the photoluminescence emission of the CsPbBr₃ nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH₃I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.

Perovskite nanocrystals fluorescence nanosensor for ultrasensitive detection of trace melamine in dairy products by the manipulation of inner filter effect of gold nanoparticles

Barium sulfate-coated CsPbBr₃ perovskite nanocrystals (CsPbBr₃ NCs@BaSO₄) was successfully synthesized that exhibited stable and intense fluorescence property in aqueous buffer. With the CsPbBr₃ NCs@BaSO₄ as signal readout, an ultrasensitive fluorescence nanosensor was developed for turn-on determination of melamine by the manipulation of inner filter effect of citrate-protected gold nanoparticles (AuNPs). The fluorescence of the CsPbBr₃ NCs@BaSO₄ was remarkably quenched by the AuNPs due to inner filter effect. This inner filter effect could be weakened by the addition of melamine as a result of melamine-triggering aggregation of the AuNPs and subsequently led to a recovery in the fluorescence of the CsPbBr₃ NCs@BaSO₄. The recovery ratio was proportional to the concentration of melamine in the range of 5.0-500.0 nmol/L. The limit of detection was 0.42 nmol/L and the relative standard deviation was 4.0% for the repetitive determination of 500.0 nmol/L melamine solution (n = 11). The nanosensor was successfully applied to analysis of melamine in dairy product samples.

Doping and ion substitution in colloidal metal halide perovskite nanocrystals

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals

Metal halide perovskite semiconductor nanocrystals have emerged as a lucrative class of materials for many optoelectronic applications. By leveraging the synthetic toolboxes developed from decades of research into more traditional semiconductor nanocrystals, remarkable progress has been made across these materials in terms of their structural, compositional, and optoelectronic control. Here, we review this progress in terms of their underlying formation stages, synthetic approaches, and postsynthetic treatment steps. This assessment highlights the rapidly maturing nature of the perovskite nanocrystal field, particularly with regard to their lead-based derivatives. It further demonstrates that significant challenges remain around precisely controlling their nucleation and growth processes. In going forward, a deeper understanding of the role of precursors and ligands will significantly bolster the versatility in the size, shape, composition, and functional properties of these exciting materials.

Bulk and surface DFT investigations of inorganic halide perovskites screened using machine learning and materials property databases

In the recent past, there has been proliferation in high-throughput density functional theory and data-driven explorations of materials motivated by a need to reduce physical testing and costly computations for materials discovery. This has, in conjunction with the development of open-access materials property databases, encouraged accelerated and more streamlined discovery and screening of technologically relevant materials. In this work, we report our results on the screening and DFT studies of one such class of materials, i.e. ABX₃ inorganic halide perovskites (A, B and X representing the monovalent, divalent and halide ions respectively) using a coupled machine-learning (ML) and density functional theory (DFT) approach. Utilizing the support vector machine algorithm, we predict the formability of 454 inorganic halide compounds in the perovskite phase. Compounds with a formation probability P≥ 0.8 are further checked for thermodynamic stability in at least one of these three open materials databases - Materials Project (MP), Automatic FLOW for Materials Discovery (AFLOW) and Open Quantum Materials Database (OQMD). The shortlisted candidate perovskites are then considered for DFT computations. Taking input geometries from MP's structure predictor, the optimized lattice parameters and computed band gaps (BG) for all screened compounds are compared with predictions across all databases. Subsequently, detailed studies on low index surfaces are presented for two halide perovksites - RbSnCl₃ and RbSnBr₃- having band-gaps in the favourable range for photovoltaics (PV). Different possible (100), (110) and (111) surface terminations are investigated for each of these compositions and the atomic relaxations, surface energies and electronic band structures are reported for each termination. To the best of our knowledge, no surface studies have been reported in the literature for any of the halide perovskites present in our database. These studies, therefore, are an important step towards gaining a fundamental understanding of the interfacial properties of perovskites, which can help facilitate further breakthroughs in the PV technology.

Bromobenzene aliphatic nucleophilic substitution guided controllable and reproducible synthesis of high quality cesium lead bromide perovskite nanocrystals

We develop a new chemical design for the controllable and reproducible synthesis of high quality CsPbBr₃ perovskite nanocrystals in one step based on bromobenzene and alkane amine aliphatic nucleophilic substitution.

Understanding the Surface Properties of Halide Exchanged Cesium Lead Halide Nanoparticles

This report describes a characterization study of the surfaces of CsPbBr₃ and CsPbBr_{3- x}I _x perovskite nanoparticles (NPs) obtained via a simultaneous purification and halide exchange (HE) postsynthetic processing technique. We studied composition-dependent NP-ligand interactions via diffusion ordered NMR (DOSY) and quantified resulting photoluminescence quantum yield (QY) as a function of halide exchange as well as ligand exchange. Importantly, ligand binding strength and QY were found to decrease when successive purification and/or halide/ligand exchange steps were taken without careful concurrent additions of acid and base ligands. This suggests that ligands added during postsynthetic processing steps are localized at the surface of the NP, passivating open surface sites. Further, we show that CsPbBr_{3- x}I _x with increasing CsPbI₃ character, obtained via the same method, have decreasing ligand density, from 6.4 to 1.4 to 0.2 nm^-2, indicating the composition-dependence of surface ligand binding, which also has consequences on the QY of the resulting mixed-halide NPs. These results shed further light on the importance of ion-ligand moiety additions during purification and halide exchange of highly emissive CsPbBr₃ NPs to maintain their as-synthesized properties, as well as the intrinsic differences in surfaces binding and photostability between near-unity QY CsPbBr₃ and mixed-halide CsPbBr_{3- x}I _x NPs.

Tuning the Optical Properties of CsPbBr3 Nanocrystals by Anion Exchange Reactions with CsX Aqueous Solution

Topotactic anion exchange has been developed to tune the composition and band gap energies of cesium lead halide (CsPbX₃) perovskite nanocrystals (NCs). However, current anion exchange methods either require harsh conditions or take a long time to realize substantial substitution. Here, we present a method to modulate the composition of colloidal CsPbBr₃ NCs through ultrasonication-assisted anion exchange with CsX (X = Cl, I) solution. Efficient anion exchange of CsPbBr₃ NCs with Cl^- or I^- is realized with substitution ratio up to 93% and preservation of the pristine shape and structure of CsPbBr₃ NCs. This anion exchange results in tunable emission, covering the whole visible spectral range, with relatively high photoluminescence quantum yield, narrow emission bandwidths, and good stability. This work provides a facile and efficient way to engineer the properties of halide perovskite NCs and has great potential for large-scale production of compositionally diverse perovskite NCs.

Ligand-mediated synthesis of compositionally related cesium lead halide CsPb2X5 nanowires with improved stability

Compositionally related cesium lead halide materials, such as CsPb2X5, have attracted great interest due to their considerable optoelectronic/optical properties as well as improved stability. Currently, CsPb2Br5 nanocrystals can be well-designed by tuning the ligands or precursor ratio, whereas, CsPb2X5 (with Cl- or I-) nanocrystals can only be obtained by the anion exchange method. Herein, we report a method to directly synthesize CsPb2X5 facilitated by thiol ligands. The morphology of CsPb2X5 can be designed as a nanowire. Importantly, the stability of directly synthesized CsPb2X5 nanowires is much improved when compared with the stabilities of the materials obtained by the anion-exchange method. We believe that this method will promote the application of 1D tetragonal CsPb2X5 in optoelectronics, optics and other fields.

Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX3 (X = Cl, Br, I) Colloidal Nanocrystals

In the past few years, hybrid organic-inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15-25 nm formamidinium CH(NH₂)₂PbX₃ (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415-740 nm, full width at half-maximum (fwhm) of 20-44 nm, and radiative lifetimes of 5-166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI₃ nanocrystals paves the way for perovskite quantum dot photovoltaics.