Journey of Making Cesium Lead Halide Perovskite Nanocrystals: What's Next

Probing the Structural Degradation of CsPbBr3 Perovskite Nanocrystals in the Presence of H2O and H2S: How Weak Interactions and HSAB Matter

Structural degradation of all inorganic CsPbBr3 in the presence of moisture is considered as one of its major limitations to use as an active component in various light-harvesting and light-emitting devices. Herein, we used two similar molecules, H2O and H2S, with similar structures, to follow the decomposition mechanism of CsPbBr3 perovskite nanocrystals. Interestingly, H2O acts as a catalyst for the decomposition of CsPbBr3, which is in contrast to H2S. Our experimental observations followed by density functional theory (DFT) calculations showed that the water molecule is intercalated in the CsPbBr3 perovskite whereas H2S is adsorbed in the (100) planes of CsPbBr3 by a weak electrostatic interaction. According to Pearson's hard-soft acid-base theory, both cations present in CsPbBr3 prefer soft/intermediate bases. In the case of the water molecule, it lacks a soft base and thus it is not directly involved in the reaction whereas H2S can provide a soft base and thus it gets involved in the reaction. Understanding the mechanistic aspects of decomposition can give different methodologies for preventing such unwanted reactions.

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.

Increasing the Performance and Stability of Red-Light-Emitting Diodes Using Guanidinium Mixed-Cation Perovskite Nanocrystals

Halide perovskite nanocrystals (PNCs) exhibit growing attention in optoelectronics due to their fascinating color purity and improved intrinsic properties. However, structural defects emerging in PNCs progressively hinder the radiative recombination and carrier transfer dynamics, limiting the performance of light-emitting devices. In this work, we explored the introduction of guanidinium (GA⁺) during the synthesis of high-quality Cs_1-xGA_xPbI₃ PNCs as a promising approach for the fabrication of efficient bright-red light-emitting diodes (R-LEDs). The substitution of Cs by 10 mol % GA allows the preparation of mixed-cation PNCs with PLQY up to 100% and long-term stability for 180 days, stored under air atmosphere and refrigerated condition (4 °C). Here, GA⁺ cations fill/replace Cs⁺ positions into the PNCs, compensating intrinsic defect sites and suppressing the nonradiative recombination pathway. LEDs fabricated with this optimum material show an external quantum efficiency (EQE) near to 19%, at an operational voltage of 5 V (50-100 cd/m²) and an operational half-time (t₅₀) increased 67% respect CsPbI₃ R-LEDs. Our findings show the possibility to compensate the deficiency through A-site cation addition during the material synthesis, obtaining less defective PNCs for efficient and stable optoelectronic devices.

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.

Impact of Bifunctional Ligands on Charge Transfer Kinetics in CsPbBr3-CdSe/CdS/ZnS Nanohybrids

Mixed dimensional nanohybrids (MDNHs) between zero-dimensional (0D) perovskites and two-dimensional (2D) II-VI semiconductors hold great potential for photonic device applications. An in-depth study to understand the shuttling of charge carriers is carried out utilizing bifunctional ligands such as 4-aminothiophenol (4-ATP), p-aminobenzoic acid, and 6-amino-2-naphthoic acid in the synthesis of MDNHs of CsPbBr₃ nanocrystals (NCs) and CdSe/CdS/ZnS core/shell/shell (CSS) nanoplatelets (NPLs). These MDNHs form donor-bridge-acceptor systems, where the electronic interaction is greatly influenced by the nature of ligands. The smaller size and stronger binding affinity of 4-ATP to CSS NPLs lead to a faster rate of charge transfer as compared to other linkers. Electronic structure calculations under the framework of density functional theory (DFT) confirms that in 4-ATP capped CSS NPLs, stronger electronic overlap occurs between CSS NPLs and 4-ATP at the valence band maxima (VBM). Furthermore, Poisson distribution modeling proposes that in 4-ATP linked MDNHs, the number of CSS NPLs around CsPbBr₃ NCs is highest.

Chemically Spiraling CsPbBr3 Perovskite Nanorods

Light emitting lead halide perovskite nanocrystals are currently emerging as the workhorse in quantum dot research. Most of these reported nanocrystals are isotropic cubes or polyhedral; but anisotropic nanostructures with controlled anisotropic directions still remain a major challenge. For orthorhombic CsPbBr₃, the 1D shaped nanostructures reported are linear and along either of the axial directions ⟨100⟩. In contrast, herein, spiral CsPbBr₃ perovskite nanorods in the orthorhombic phase are reported with unusual anisotropy having (101) planes remaining perpendicular to the major axis [201]. While these nanorods are synthesized using the prelattice of orthorhombic Cs₂CdBr₄ with Pb(II) diffusion, the spirality is controlled by manipulation of the compositions of alkylammonium ions in the reaction system which selectively dissolve some spiral facets of the nanorods. Further, as spirality varied with facet creation and elimination, these nanorods were explored as photocatalysts for CO₂ reduction, and the evolution of methane was also found to be dependent on the depth of the spiral nanorods. The entire study demonstrates facet manipulation of complex nanorods, and these results suggest that even if perovskites are ionic in nature, their shape could be constructed by design with proper reaction manipulation.

Alkylammonium Halides for Facet Reconstruction and Shape Modulation in Lead Halide Perovskite Nanocrystals

ConspectusThe interactions of halides and ammonium ions with lead halide perovskite nanocrystals have been extensively studied for improving their phase stability, controlling size, and enhancing their photoluminescence quantum yields. However, all these nanocrystals, which showed intense and color tunable emissions, mostly retained the six faceted cube or platelet shapes. Shape tuning needs the creation of new facets, and instead of composition variations by foreign ions interactions/substitutions, these require facet stabilizations with suitable ligands. Among most of the reported cases of lead halide perovskites, alkyl ammonium ions are used as a capping agent, which substituted in the surface Cs(I) sites of these nanocrystals. Hence, new surface ligands having a specific binding ability with different facets other than those in cube/platelet shapes are required for bringing stability to new facets and, hence, for tuning their shapes.In this Account, interactions of alkyl ammonium ions on the surface of perovskite nanocrystals and their impact on surface reconstructions are reviewed. Emphasizing the most widely studied CsPbBr₃ nanocrystals, the usefulness and impact of alkyl ammonium ions on the phase stability, high-temperature annealing, enhancement of the brightness and doping in these nanocrystals are first discussed. Then, nanocrystals formed under limited primary alkyl ammonium ions and also with specific tertiary ammonium ions having new facets are elaborated. Further, the treatment of excess alkyl ammonium halides to these newly formed multifaceted polyhedron nanocrystals under different conditions, which led to armed and step-armed structures, are discussed. The change in optical properties during these shape transformations is also presented. Finally, the shape-change mechanism with alkyl ammonium halide-induced dissolutions of {200} and {112} facets and formation of {110} and {002} facets are discussed. Further, in summary, future prospects of new ligand designing for stabilizing new facets of perovskite nanocrystals and obtaining new shapes and properties are proposed.

Anchoring of Formamidinium Lead Bromide Quantum Dots on Ti3C2 Nanosheets for Efficient Photocatalytic Reduction of CO2

Metal halide perovskite with a suitable energy band structure and excellent visible-light response is a prospective photocatalyst for CO₂ reduction. However, the reported inorganic halide perovskites have undesirable catalytic performances due to phase-sensitive and severe charge carrier recombination. Herein, we anchor the FAPbBr₃ quantum dots (QDs) on Ti₃C₂ nanosheets to form a FAPbBr₃/Ti₃C₂ composite within a Schottky heterojunction for photocatalytic CO₂ reduction. Upon visible-light illumination, the FAPbBr₃/Ti₃C₂ composite photocatalyst exhibits an appealing photocatalytic performance in the presence of deionized water. The Ti₃C₂ nanosheet acts as an electron acceptor to promote the rapid separation of excitons and supply specific catalytic sites. An optimal electron consumption rate of 717.18 μmol/g·h is obtained by the FAPbBr₃/0.2-Ti₃C₂ composite, which has a 2.08-fold improvement over the pristine FAPbBr₃ QDs (343.90 μmol/g·h). Meanwhile, the FAPbBr₃/Ti₃C₂ photocatalyst also displays a superior stability during photocatalytic reaction. This work expands a new insight and platform for designing superb perovskite/MXene-based photocatalysts for CO₂ reduction.

A versatile approach for shape-controlled synthesis of ultrathin perovskite nanostructures

We demonstrate that ultrathin CsPbBr₃ nanostructures can be obtained by a simple mixing of precursor–ligand complexes under ambient conditions.

Impact of excitation energy on the excitonic luminescence of cesium lead bromide perovskite nanosheets

An excitation-energy-dependent luminescence phenomenon is reported in cesium lead bromide (CsPbBr₃) perovskite nanosheets. At 10 K, the relative integrated luminescence intensity between the trapped exciton (TX) emission and the free exciton (FX) emission shows an interesting tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such phenomenon, we develop a quantitative model on the basis of the biological population growth theory. A good agreement between experiment and theory is obtained. It is thus revealed that the lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level shall be the major cause of the observed phenomenon. These findings may help to deepen the current understanding of the complex luminescence mechanisms of these emerging light-emitting materials.

Ligand & band gap engineering: tailoring the protocol synthesis for achieving high-quality CsPbI3 quantum dots

Hot-injection has become the most widespread method used for the synthesis of perovskite quantum dots (QDs) with enormous interest for application in optoelectronic devices. However, there are some aspects of the chemistry involved in this synthesis that have not been completely investigated. In this work, we synthesized ultra-high stable CsPbI₃ QDs for more than 15 months by controlling two main parameters: synthesis temperature and the concentration of capping ligands. By increasing the capping ligand concentration during the QD synthesis, we were able to grow CsPbI₃ in a broad range of temperatures, improving the photophysical properties of QDs by increasing the synthesis temperature. We achieved the maximum photoluminescence quantum yield (PLQY) of 93% for a synthesis conducted at 185 °C, establishing an efficient surface passivation to decrease the density of non-radiative recombination sites. Under these optimized synthesis conditions, deep red LEDs with an External Quantum Efficiency (EQE) higher than 6% were achieved. The performance of these LEDs is higher than that of the reported CsPbI₃ QD-LEDs containing standard capping agents, without additional elements or further element exchange. We show that it is possible to produce stable CsPbI₃ QDs with high PLQY and red emission beyond the requirement of the Rec. 2020 standards for red color.

Lead-Free Cs4CuSb2Cl12 Layered Double Perovskite Nanocrystals

Concerns about the toxicity of lead-based perovskites have aroused great interest for the development of alternative lead-free perovskite-type materials. Recently, theoretical calculations predict that Pb²⁺ cations can be substituted by a combination of Cu²⁺ and Sb³⁺ cations to form a vacancy-ordered layered double perovskite structure with superior optoelectronic properties. However, accessibilities to this class of perovskite-type materials remain inadequate, hindering their practical implementations in various applications. Here, we report the first colloidal synthesis of Cs₄CuSb₂Cl₁₂ perovskite-type nanocrystals (NCs). The resulting NCs exhibit a layered double perovskite structure with ordered vacancies and a direct band gap of 1.79 eV. A composition-structure-property relationship has been established by investigating a series of Cs₄Cu_xAg_2-2xSb₂Cl₁₂ perovskite-type NCs (0 ≤ x ≤ 1). The composition induced crystal structure transformation, and thus, the electronic band gap evolution has been explored by experimental observations and further confirmed by theoretical calculations. Taking advantage of both the unique electronic structure and solution processability, we demonstrate that the Cs₄CuSb₂Cl₁₂ NCs can be solution-processed as high-speed photodetectors with ultrafast photoresponse and narrow bandwidth. We anticipate that our study will prompt future research to design and fabricate novel and high-performance lead-free perovskite-type NCs for a range of applications.

Trapping and Detrapping in Colloidal Perovskite Nanoplatelets: Elucidation and Prevention of Nonradiative Processes through Chemical Treatment

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr₃ nanoplatelets, before and after chemical treatment with PbBr₂ that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr₂. More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.

Facets Directed Connecting Perovskite Nanocrystals

Connecting nanocrystals with removal of interface ligand barriers is one of the key steps for efficient carrier transportation in optoelectronic device fabrication. Typically, ion migration for crystal deformation or connection with other nanocrystals needs a solvent as medium. However, on the contrary, this has been observed for CsPbBr₃ perovskite nanocrystals in film where nanocrystals were swollen to get wider and fused with adjacent nanocrystals in self-assembly on film during solvent evaporation. Depending on precursor composition and exposed facets, again these connections could be programmed for tuning their connecting directions leading to different shapes. Aging further on solid substrate, these were also turned to continuous film of nanostructures eliminating all interparticle gaps on the film. This transformation could be ceased at any point of time, simply by heating or adding sufficient ligands. Analysis suggested that these unique and controlled connections were only observed with polyhedron shaped nanostructures with certain compositions and not with traditionally cubes. Details of this solid-surface transformation during solvent evaporation were analyzed, and an interparticle material transfer type mechanism was proposed. As these observations were not seen in chalcogenide and oxide nanocrystals and exclusively observed in perovskite nanocrystals, this would add new fundamentals to the insights of crystal growths of nanocrystals and would also help in obtaining films of connecting nanocrystals.

Doping the Smallest Shannon Radii Transition Metal Ion Ni(II) for Stabilizing α-CsPbI3 Perovskite Nanocrystals

Red emitting α-CsPbI₃ nanocrystals are highly phase sensitive to ambient exposure, and B-site doping with suitable cations is adopted as one of the most feasible approaches for their phase stability. There are several reports herein: Ni(II) ions having the smallest transition metal Shannon radii were explored for doping in these nanocrystals. This successfully stabilized the cubic phase and retained the intense emission of nanocrystals for nearly 2 months. Being the smallest ion, the halide octahedra in the perovskite lattice were expected to provide high restraint ability toward δ-CsPbI₃. Comparing with postsynthesis iodide treatments, the importance of doping in high temperature reaction was discussed. Finally, these doped nanocrystals were explored for photovoltaic devices and showed comparable efficiency (9.1%) to different other similar doped nanocrystals. Hence, the finding reported here is a step forward for understanding the insights of phase stability of α-CsPbI₃ perovskite nanocrystals.