Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals

Direct synthesis of CsPbX3 perovskite nanocrystal assemblies

Inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) possess many advantageous optoelectronic properties, making them an attractive candidate for light emitting diodes, lasers, or photodetector applications. Such perovskite NCs can form extended assemblies that further modify their bandgap and emission wavelength. In this article, a facile direct synthesis of CsPbX3 NC assemblies that are 1 μm in size and are composed of 10 nm-sized NC building blocks is reported. The direct synthesis of these assemblies with a conventional hot-injection method of the NCs is achieved through the judicious selection of the solvent, ligands, and reaction stoichiometry. Only under selective reaction conditions where the surface ligand environment is tuned to enhance the hydrophobic interactions between ligand chains of neighbouring NCs is self-assembly achieved. These assemblies possess narrow and red-shifted photoluminescence compared to their isolated NC counterparts, which further expands the colour gamut that can be rendered from inorganic perovskites. This is demonstrated through simple down-converting light emitters.

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.

CsPbX3 (X = Cl, Br, and I) Nanocrystals in Substrates toward Stable Photoluminescence: Nanoarchitectonics, Properties, and Applications

Cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) possess great potential in light-emitting diode applications because of their high brightness, low cost, tunable luminescence, and facile synthesis nature. However, these NCs are often disadvantaged by their instability in nonsolvent environment that hinders the practical applications of the material. In order to solve these issues, cesium lead halide NCs prepared using a solvent environment can be placed on substrates to retain the high stability and expand the applicability of the material. This Review focuses on the transfer of the all-inorganic cesium lead halide NCs (synthesized in solutions) onto matrix materials and their direct synthesis on these bases, including the inert shell growth (inorganic and organic shell), embedment in matrixes (e.g., metal organic frameworks, porous SiO2, glass, ZrO2, Al2O3, and AlOOH), and direct synthesis in substrates. In particular, the strategies for stability and PL property improvement of the materials are also summarized. The purpose of this Review is to provide inspiration for the encapsulation of cesium lead halide NCs with high brightness and stability in matrixes to expand the applicability of these materials in wide color gamut backlighting (e.g., white-light-emitting devices).

Stable and Bright CsPbX3 Nanocrystals in Metal-Organic Frameworks for White Light-Emitting Diodes

The stability of metal halide perovskite (CsPbX₃, X = Cl, Br, and I) nanocrystals (NCs) is crucial for their practical applications. In this paper, perovskite NCs were synthesized in situ in lead-based metal-organic frameworks (Pb-MOFs: [Pb₂(1,3,5-HBTC)₂(H₂O)₄]·H₂O), and we obtained stable and bright luminescence composites with different colors. Namely, CsPbBr₃@Pb-MOF composites were created by the in situ growth of CsPbBr₃ crystals (NCs) on Pb-MOF, which had high ion resistance, bright photoluminescence (PL), and excellent stability. The composites still had bright luminescence after 11 months of storage. The PL intensity of green-emitting CsPbBr₃@Pb-MOF composites was increased compared with as-prepared CsPbBr₃ NCs. Bright and stable blue- and red-emitting CsPbX₃@Pb-MOF composites were obtained by adjusting the amount of PbX₂ (X = Cl, Br, and I) in the synthesis process. These CsPbX₃ NCs were homogeneously distributed in Pb-MOF substrates. The growth of CsPbX₃ NCs in Pb-MOFs prevented NC aggregation and decreased surface defects against nonradiative recombination during emitting. Thus, the PL lifetime and stability were improved. Furthermore, white light-emission diodes were prepared using three color CsPbX₃@Pb-MOF composites with Commission Internationale de I'Eclairage color coordinates of (0.296, 0.316). This result provided an efficient way to overcome the limitation of chemical solution synthesis and improve the stability of CsPbX₃ NCs.

Macroporous perovskite nanocrystal composites for ultrasensitive copper ion detection

Accumulation of heavy metal ions, including copper ions (Cu²⁺), presents a serious threat to human health and to the environment. A substantial amount of research has focused on detecting such species in aqueous solutions. However, progress towards ultrasensitive and easy-to-use sensors for non-aqueous solutions is still limited. Here, we focus on the detection of copper species in hexane, realising ultra-sensitive detection through a fluorescence-based approach. To achieve this, a novel macroporous composite material has been developed featuring luminescent CsPbBr₃ nanocrystals (NCs) chemically adhered to a polymerized high internal phase emulsion (polyHIPE) substrate through surface thiol groups. Due to this thiol functionality, sub-monolayer NC formation is realised, which also renders outstanding stability of the composite in the ambient environment. Copper detection is achieved through a direct solution based immersion of the CsPbBr₃-(SH)polyHIPE composite, which results in concentration-dependent quenching of the NC photoluminescence. This newly developed sensor has a limit of detection (LOD) for copper as low as 1 × 10^-16 M, and a wide operating window spanning 10^-2 to 10^-16 M. Moreover, the composite exhibits excellent selectivity among different transition metals.

Amplified Spontaneous Emission from Thermally Evaporated High-Quality Thin Films of CsPb(Br1-xYx)3 (Y = I, Cl) Perovskites

As a wavelength-tunable lasing material, perovskites are now generating a lot of scientific attention. Conventional solution-processed CsPbX₃ perovskite films sometimes suffer unavoidable pinhole defects and poor surface morphology, severely limiting their performance on amplified spontaneous emission (ASE) and lasing application. Herein, a thermal evaporation approach is explored in our work to achieve a uniform and high-coverage CsPb(Br_1-xY_x)₃ (Y = I, Cl) perovskites polycrystalline thin film. The ASE of these films was studied using a picosecond laser system. The ASE profile increases rapidly over the narrow peak in relation to the laser pump intensity, confirming the development of stimulated emission. ASE began when the energy density threshold was reached and ranged between 25 and 170 μJ/cm² per pulse for perovskite materials when replacing I with Br and then Cl. This work emphasizes the notable optical properties of high-quality perovskite thin films, leading to possible accessible uses in optoelectronic applications.

Insertion of metal cations into hybrid organometallic halide perovskite nanocrystals for enhanced stability: eco-friendly synthesis, lattice strain engineering, and defect chemistry studies

In this work, we developed a facile and environmentally friendly synthesis strategy for large-scale preparation of Cr-doped hybrid organometallic halide perovskite nanocrystals. In the experiment, methylammonium lead bromide, CH₃NH₃PbBr₃, was efficiently doped with Cr³⁺ cations by eco-friendly method at low temperatures to grow crystals via antisolvent-crystallization. The as-synthesized Cr³⁺ cation-doped perovskite nanocrystals displayed ∼45.45% decrease in the (100) phase intensity with an enhanced Bragg angle (2θ) of ∼15.01° compared to ∼14.92° of pristine perovskites while retaining their cubic (221/Pm-cm, ICSD no. 00-069-1350) crystalline phase of pristine perovskites. During synthesis, an eco-friendly solvent, ethanol, was utilized as an antisolvent to grow nanometer-sized rod-like crystals. However, Cr³⁺ cation-doped perovskite nanocrystals display a reduced crystallinity of ∼67% compared to pristine counterpart with ∼75% crystallinity with an improved contact angle of ∼72° against water in thin films. Besides, as-grown perovskite nanocrystals produced crystallite size of ∼48 nm and a full-width-at-half-maximum (FWHM) of ∼0.19° with an enhanced lattice-strain of ∼4.52 × 10^-4 with a dislocation-density of ∼4.24 × 10¹⁴ lines per m² compared to pristine perovskite nanocrystals, as extracted from the Williamson-Hall plots. The as-obtained stable perovskite materials might be promising light-harvesting candidates for optoelectronic applications in the future.

Synthesis of highly calibrated CsPbBr3 nanocrystal perovskites by soft chemistry

A new synthetic method for preparing highly calibrated CsPbBr₃ nanocrystal perovskites is described and analyzed using high-resolution scanning transmission electron microscopy. This new method based on soft chemistry leads to the large-scale production of nanocrystals. Such monodisperse nanocrystals allow for the deposition of homogeneous films, which provides new opportunities for the next generation of optoelectronic devices.

Spectrum Reconstruction with Filter-Free Photodetectors Based on Graded-Band-Gap Perovskite Quantum Dot Heterojunctions

Spectrum reconstruction with filter-free microspectrometers has attracted much attention owing to their promising potential in in situ analysis systems, on-chip spectroscopy characterizations, hyperspectral imaging, etc. Further efforts in this field can be devoted to improving the performance of microspectrometers by employing high-performance photosensitive materials and optimizing the reconstruction algorithms. In this work, we demonstrate spectrum reconstruction with a set of photodetectors based on graded-band-gap perovskite quantum dot (PQD) heterojunctions using both calculation and machine learning algorithms. The photodetectors exhibit good photosensitivities with nonlinear current-voltage curves, and the devices with different PQD band gaps show various spectral responsivities with different cutoff wavelength edges covering the entire visible range. Reconstruction performances of monochromatic spectra with the set of PQD photodetectors using two different algorithms are compared, and the machine learning method achieves relatively better accuracy. Moreover, the nonlinear current-voltage variation of the photodetectors can provide increased data diversity without redundancy, thus further improving the accuracy of the reconstructed spectra for the machine learning algorithm. A spectral resolution of 10 nm and reconstruction of multipeak spectra are also demonstrated with the filter-free photodetectors. Therefore, this study provides PQD photodetectors with the corresponding optimized algorithms for emerging flexible microspectrometer systems.

Atomic-Scale Imaging and Nano-Scale Mapping of Cubic α-CsPbI3 Perovskite Nanocrystals for Inverted Perovskite Solar Cells

Colloidal synthesized cubic α-CsPbI₃ perovskite nanocrystals having a smaller lattice constant (a = 6.2315 Å) compared to the standard structure, and nanoscale mapping of their surfaces are reported to achieve superior photovoltaic performance under 45-55% humidity conditions. Atomic scale transmission electron microscopic images have been utilized to probe the precise arrangement of Cs, Pb, and I atoms in a unit cell of α-CsPbI₃ NCs, which is well supported by the VESTA structure. Theoretical calculation using density functional theory of our experimental structure reveals the realization of direct band to band transition with a lower band gap, a higher absorption coefficient, and stronger covalent bonding between the Pb and I atoms in the [PbI₆]^4- octahedral, as compared to reported standard structure. Nanoscale surface mapping using Kelvin probe force microscopy yielding contact potential difference (CPD) and conductive atomic force microscopy for current mapping have been employed on α-CsPbI₃ NCs films deposited on different DMSO doped PEDOT:PSS layers. The difference of CPD value under dark and light illumination suggests that the hole injection strongly depends on the interfaces with PEDOT:PSS layer. The carrier transport through grain interiors and grain boundaries in α-CsPbI₃ probed by the single-point c-AFM measurements reveal the excellent photosensitivity under the light conditions. Finally, inverted perovskite solar cells, employing α-CsPbI₃ NCs film as an absorber layer and PEDOT:PSS layer as a hole transport layer, have been optimized to achieve the highest power conversion efficiency of 10.6%, showing their potential for future earth abundant, low cost, and air stable inverted perovskite photovoltaic devices.

High-yield synthesis of CsPbBr3nanoparticles: diphenylphosphine as a reducing agent and its effect in Pb-seeding nucleation and growth

Based on the reported nucleation mechanisms for CsPbX₃and II-VI/IV-VI quantum dots, CsPbBr₃nanoparticles with a higher reaction-yield (up to 393% mass-increment) were synthetized by the hot-injection method. The introduction of diphenylphosphine (DPP) as a reducing agent improved nanoparticle nucleation and growth, giving out evidence for Pb-seeding in CsPbBr₃nanoparticles formation. Additionally, a clear influence of the DPP in a CsPbBr₃-Cs₄PbBr₆incomplete phase transformation was observed, marked by the appearance of several PbBr₂nanoparticles. This indicated the need for an improved ratio between the stabilizing agents and the precursors, due to the increased number of nucleation sites produced by DPP. The resulting CsPbBr₃nanoparticles showed high quality, as they displayed 70%-90% photoluminescence quantum yield; narrow size distribution with an average nanoparticle size of∼10 nm; and the characteristic cubic morphology reported in previous works. This increment in CsPbBr₃nanoparticles' reaction yield will contribute to making them a more attractive option for different optoelectronic applications.

Rapid separation and purification of lead halide perovskite quantum dots through differential centrifugation in nonpolar solvent

We report the rapid separation and purification of lead halide perovskite quantum dots (QDs) in a nonpolar solvent by using a convenient and efficient differential separation method. Size-selective precipitation effectively separates the perovskite QDs from larger aggregates and provides direct evidence for strong quantum confinement in the photoluminescence (PL). Significantly, the size-selected perovskite QDs are readily well-dispersed in a nonpolar solvent and remain stable in ambient air (humidity > 60%) for >20 days. These enable measurement of the electronic band structure of versatile perovskite QDs as a function of size for the first time. Despite a clear blue-shift of the optical bandgap, the lowest unoccupied molecular orbital (LUMO) readily moves towards the vacuum level while the highest occupied molecular orbital (HOMO) changes slightly, in good agreement with that observed in the quantum size effect tuning of quasi-2D perovskites and colloidal semiconductor QDs. The results demonstrate the possibility of utilizing differential centrifugation as a novel method to attain size-dependent tunability for property-specific perovskite-QD based optoelectronic applications.

Prospects of Z-Scheme Photocatalytic Systems Based on Metal Halide Perovskites

Considering the attractive optoelectronic properties of metal halide perovskites (MHPs), their introduction to the field of photocatalysis was only a matter of time. Thus far, MHPs have been explored for the photocatalytic generation of hydrogen, carbon dioxide reduction, organic synthesis, and pollutant degradation applications. Of growing research interest and possible applied significance are the currently emerging developments of MHP-based Z-scheme heterostructures, which can potentially enable efficient photocatalysis of highly energy-demanding redox processes. In this Perspective, we discuss the advantages and limitations of MHPs compared to traditional semiconductor materials for applications as photocatalysts and describe emerging examples in the construction of MHP-based Z-scheme systems. We discuss the principles and material properties that are required for the development of such Z-scheme heterostructure photocatalysts and consider the ongoing challenges and opportunities in this emerging field.

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.

Scalable synthesis of colloidal CsPbBr3 perovskite nanocrystals with high reaction yields through solvent and ligand engineering

The ligand assisted reprecipitation (LARP) technique is an accessible and facile method that can synthesize metal halide perovskite nanocrystals (PNCs) under ambient conditions. However, low product yields of less than 30% for LARP and its contemporary methods are indicative of highly inefficient reactions. In this work we apply the principles of green chemistry to the LARP technique for synthesizing CsPbBr3 PNCs and help address this issue. Through these efforts, high product yields of ∼70% are achieved using stochiometric Cs : Pb precursor ratios. This is realized by (i) substituting the conventional toluene (TOL) anti-solvent with ethyl acetate (EA) and (ii) replacing the conventionally used unsaturated oleylamine ligand with the shorter saturated octylamine ligand. These changes also result in a 60% molar reduction in total ligand concentration and a 62.5% reduction in solvent waste during purification. The synthesized PNCs are comparable to the TOL-LARP reference in crystal quality, morphology and phase, with their photoluminescence quantum yields being readily enhanced to over 80% through additions of RNH3Br ligands. The spectral versatility of these materials is demonstrated through post-synthetic chloride and iodide halide anion exchange, which readily yields tunable CsPbX3 derivatives across the visible spectrum. Our EA-LARP protocol is further shown to be readily upscaled to ∼0.5 L, while maintaining good nanocrystal properties and a product yield of 60%.

Potassium doping-induced variations in the structures and photoelectric properties of a MAPbI3 perovskite and a MAPbI3/TiO2 junction

The intrinsic and K⁺-doping-induced properties of a MAPbI₃ material and a MAPbI₃/TiO₂ junction have been elucidated.

Advances in the Stability of Halide Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals are promising candidates for next-generation optoelectronics because of their facile synthesis and their outstanding and size-tunable properties. However, these materials suffer from rapid degradation, similarly to their bulk perovskite counterparts. Here, we survey the most recent strategies to boost perovskite nanocrystals stability, with a special focus on the intrinsic chemical- and compositional-factors at synthetic and post-synthetic stage. Finally, we review the most promising approaches to address the environmental extrinsic stability of perovskite nanocrystals (PNCs). Our final goal is to outline the most promising research directions to enhance PNCs' lifetime, bringing them a step closer to their commercialization.

Facile purification of CsPbX3 (X = Cl-, Br-, I-) perovskite nanocrystals

CsPbI₃ perovskite nanocrystals are a promising optoelectronic material when stabilized in their cubic phase. While ongoing efforts have addressed this structural challenge through a variety of meta-stabilization approaches, the postsynthesis purification of these nanocrystal dispersions has remained a challenge. In this article, we undertake a detailed investigation into the chemical, optical, and structural changes that arise during purification of CsPbI₃ nanocrystals. It is found that nanocrystal degradation can only be avoided through the judicious control of additives within each purification cycle. Under optimized additive-to-nanocrystal ratios, multiple purification cycles can be readily achieved, while retaining the quality and phase stability of the CsPbI₃. This facile purification protocol ensures the preparation of high purity and high quality CsPbI₃ nanocrystal inks that are suitable for better characterization or integration in optoelectronic devices. The approach has been generalized for CsPbX₃ (X = Cl^-, Br^-, and I^-).