Synthesis, Photoluminescence and Vibrational Properties of Aziridinium Lead Halide Perovskites

Revisiting a (001)-oriented layered lead chloride templated by 1,2,4-triazolium: structural phase transitions, lattice dynamics and broadband photoluminescence

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I-II), T2 = 339 K (phase II-III), and T3 = 280 K (phase III-IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Colloidal Aziridinium Lead Bromide Quantum Dots

The compositional engineering of lead-halide perovskite nanocrystals (NCs) via the A-site cation represents a lever to fine-tune their structural and electronic properties. However, the presently available chemical space remains minimal since, thus far, only three A-site cations have been reported to favor the formation of stable lead-halide perovskite NCs, i.e., Cs+, formamidinium (FA), and methylammonium (MA). Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a facile colloidal synthesis of AZPbBr3 NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots (QDs) in the regime of strong quantum confinement. NMR and Raman spectroscopies confirm the stabilization of the AZ cations in the locally distorted cubic structure. AZPbBr3 QDs exhibit bright photoluminescence with quantum efficiencies of up to 80%. Stabilized with cationic and zwitterionic capping ligands, single AZPbBr3 QDs exhibit stable single-photon emission, which is another essential attribute of QDs. In particular, didodecyldimethylammonium bromide and 2-octyldodecyl-phosphoethanolamine ligands afford AZPbBr3 QDs with high spectral stability at both room and cryogenic temperatures, reduced blinking with a characteristic ON fraction larger than 85%, and high single-photon purity (g(2)(0) = 0.1), all comparable to the best-reported values for MAPbBr3 and FAPbBr3 QDs of the same size.

Phase Transitions, Dielectric Response, and Nonlinear Optical Properties of Aziridinium Lead Halide Perovskites

Hybrid organic-inorganic lead halide perovskites are promising candidates for next-generation solar cells, light-emitting diodes, photodetectors, and lasers. The structural, dynamic, and phase-transition properties play a key role in the performance of these materials. In this work, we use a multitechnique experimental (thermal, X-ray diffraction, Raman scattering, dielectric, nonlinear optical) and theoretical (machine-learning force field) approach to map the phase diagrams and obtain information on molecular dynamics and mechanism of the structural phase transitions in novel 3D AZRPbX3 perovskites (AZR = aziridinium; X = Cl, Br, I). Our work reveals that all perovskites undergo order-disorder phase transitions at low temperatures, which significantly affect the structural, dielectric, phonon, and nonlinear optical properties of these compounds. The desirable cubic phases of AZRPbX3 remain stable at lower temperatures (132, 145, and 162 K for I, Br, and Cl) compared to the methylammonium and formamidinium analogues. Similar to other 3D-connected hybrid perovskites, the dielectric response reveals a rather high dielectric permittivity, an important feature for defect tolerance. We further show that AZRPbBr3 and AZRPbI3 exhibit strong nonlinear optical absorption. The high two-photon brightness of AZRPbI3 emission stands out among lead perovskites emitting in the near-infrared region.

Raman study of pressure-induced phase transitions in imidazolium manganese- hypophosphite hybrid perovskite

By using Raman spectroscopy, we demonstrate that [IM]Mn(H₂POO)₃ is a highly compressible material that undergoes three pressure-induced phase transitions. Using a diamond anvil cell we performed high-pressure experiments up to 7.1 GPa, using paraffin oil as the compression medium. The first phase transition, which occurs near 2.9 GPa, leads to very pronounced changes in the Raman spectra. This behavior indicates that this transition is associated with very large reconstruction of the inorganic framework and collapse of the perovskite cages. The second phase transition, which occurs near 4.9 GPa, is associated with subtle structural changes. The last transition takes place near 5.9 GPa and it leads to further significant distortion of the anionic framework. In contrast to the anionic framework, the phase transitions have weak impact on the imidazolium cation. Pressure dependence of Raman modes proves that compressibility of the high-pressure phases is significantly lower compared to the ambient pressure phase. It also indicates that the contraction of the MnO₆ octahedra prevails over that of the imidazolium cations and hypophosphite linkers. However, compressibility of MnO₆ strongly decreases in the highest pressure phase. Pressure-induced phase transitions are reversible.

Phase Behavior and Role of Organic Additives for Self-Doped CsPbI3 Perovskite Semiconductor Thin Films

The phase change of all-inorganic cesium lead halide (CsPbI3) thin film from yellow δ-phase to black γ-/α-phase has been a topic of interest in the perovskite optoelectronics field. Here, the main focus is how to secure a black perovskite phase by avoiding a yellow one. In this work, we fabricated a self-doped CsPbI3 thin film by incorporating an excess cesium iodide (CsI) into the perovskite precursor solution. Then, we studied the effect of organic additive such as 1,8-diiodooctane (DIO), 1-chloronaphthalene (CN), and 1,8-octanedithiol (ODT) on the optical, structural, and morphological properties. Specifically, for elucidating the binary additive-solvent solution thermodynamics, we employed the Flory-Huggins theory based on the oligomer level of additives' molar mass. Resultantly, we found that the miscibility of additive-solvent displaying an upper critical solution temperature (UCST) behavior is in the sequence CN:DMF > ODT:DMF > DIO:DMF, the trends of which could be similarly applied to DMSO. Finally, the self-doping strategy with additive engineering should help fabricate a black γ-phase perovskite although the mixed phases of δ-CsPbI3, γ-CsPbI3, and Cs4PbI6 were observed under ambient conditions. However, the results may provide insight for the stability of metastable γ-phase CsPbI3 at room temperature.

All-Inorganic CsPbBr3 Perovskite Nanocrystals Synthesized with Olive Oil and Oleylamine at Room Temperature

Room temperature (RT) synthesis of the ternary cesium lead bromide CsPbBr₃ quantum dots with oleic acid and oleylamine ligands was developed by Zeng and coworkers in 2016. In their works, the supersaturated recrystallization (SR) was adopted as a processing method without requiring inert gas and high-temperature injection. However, the oleic acid ligand for haloplumbate is known to be relatively unstable. Hence, in this work, we employed the eco-friendly olive oil to replace the oleic acid portion for the SR process at RT. Resultantly, we found that the cube-shaped nanocrystal has a size of ~40-42 nm and an optical bandgap of ~2.3 eV independent of the surface ligands, but the photoluminescence lifetime (τ_av) and crystal packing are dependent on the ligand species, e.g., τ_av = 3.228 ns (olive oil and oleylamine; here less ordered) vs. 1.167 ns (oleic acid and oleylamine). Importantly, we explain the SR mechanism from the viewpoint of the classical LaMer model combined with the solvent engineering technique in details.

Pressure effect on the physical, mechanical, and thermal properties of ternary halide perovskite AgCaCl3: a first-principles study

CONTEXT AND RESULTS

As an inorganic halide perovskite material, AgCaCl₃, characterized by its high stability and environmental friendliness, is considered a potential candidate for major applications in optoelectronics and lens manufacturing. This work aimed to determine the electronic properties such as density of state (DOS) and band structure (BS) of AgCaCl3. The results showed that the material has an indirect band gap almost invariably at 1.5 eV in the pressure range studied. The dielectric function [Formula: see text], absorption coefficient [Formula: see text], optical conductivity [Formula: see text], reflectivity [Formula: see text], and the refractive index [Formula: see text] showed clearly that the perovskite AgCaCl₃ preserved its optical characteristics within the chosen pressure range investigated. The calculated elastic constants C₁₁, C₁₂, and C₁₄ as dynamic stability criteria for the elastic moduli such as bulk modulus (B), shear modulus (G), Young's modulus (Y), Poisson's ratio ([Formula: see text]), and anisotropy factor (A) showed that the material is a ductile plastic. Debye temperature ([Formula: see text]), isobaric and isochoric heat capacities (C_P, C_V), coefficient of the thermal expansion (α), Gibbs free energy (G), and entropy (S) were also studied. The results obtained provide a theoretical basis for experimental work and offer the possibility of future industrial applications of AgCaCl₃.

COMPUTATIONAL AND THEORETICAL TECHNIQUES

Density functional theory (DFT) calculations as implemented in the Wien2K code were used to study the mechanical and thermal properties of AgCaCl₃ perovskite over a pressure range. Lattice parameters, electronic, and optical properties are optimized with the approximation of the generalized gradient of the Perdew-Burke-Ernzerhof function (PBE-GGA) function. The mechanical and thermodynamic properties were calculated using ElaStic and Gibbs2 codes, and the properties of AgCaCl₃ over the pressure range investigated were predicted.

Quantum dots assembled from an aziridinium based hybrid perovskite displaying tunable luminescence

3D hybrid perovskites based upon small organic cations gave start to a new intensively growing class of semiconducting materials. Here we report on the elaboration of quantum dots of a recently emerged new perovskite (AzrH)PbBr₃ (AzrH = aziridinium cation). By employing the antisolvent precipitation technique and stabilization with a cationic surfactant we succeeded in obtaining quantum dots that display tunable luminescence. This piece of work shows the perspective of aziridinium-based materials for the elaboration of advanced photonic nanostructures.

Effect of Pd(II) uptake on high-temperature phase transitions in a hybrid organic-inorganic perovskite semiconductor

Hybrid organic-inorganic perovskites (HOIPs) have been widely studied for their interesting functions and potential applications. Here, we report a novel sulfur-containing hybrid organic-inorganic perovskite based on a one-dimensional ABX₃-type compound: [C₃H₇N₂S]PbI₃ ([C₃H₇N₂S]⁺ is 2-amino-2-thiazolinium) (1). Compound 1 undergoes two high-temperature phase transitions at 363 K and 401 K, respectively, showing a band gap of 2.33 eV, and has a narrower band gap compared to other one-dimensional materials. Moreover, by introducing thioether groups into the organic component, 1 has the ability to uptake Pd(II) ions. Compared with previously reported low-temperature isostructural phase transition sulfur-containing hybrids, the molecular motion of 1 becomes more intense under the stimulation of high temperature, leading to changes in the space group during the two phase transitions (Pbca → Pmcn → Cmcm), which are no longer the previous isostructural phase transitions. Significant changes in the phase transition behavior and semiconductor properties before and after metal absorption make it possible to monitor the absorption process of metal ions. The study of the effect of Pd(II) uptake on phase transitions may be helpful to reveal the mechanism of phase transitions more deeply. This work will broaden the hybrid organic-inorganic ABX₃-type semiconductor family and pave the way for the development of organic-inorganic hybrid-based multifunctional phase transition materials.

Solvent-mediated crystallization of (TMS)2BiBr5·DMSO: a new 0D hybrid halide perovskite

Lead-free hybrid halide perovskites have recently gained enormous research attention because of their excellent optical properties. Herein, we report a novel zero-dimensional (0D) hybrid halide perovskite, (TMS)₂BiBr₅·DMSO, stabilized by the dimethyl sulfoxide (DMSO) solvent. The structure contains isolated [BiBr₅OS(CH₃)₂]^2- anionic polyhedra and the cations [(CH₃)₃S]⁺ (trimethyl sulfonium ion, TMS) balance the charge, making it a 0D halide perovskite. Non-interaction of the trans Br atoms (with respect to the Bi-DMSO bond) with the cationic TMS resulted in shorter trans Bi-Br bonds as compared to other Bi-Br bonds in the isolated BiBr₅·DMSO polyhedra. The optical properties study reveals that the compound is an indirect band gap type with a band gap of 2.79 eV. From the PL study it is observed that the compound shows emission in the blue region.

Two metal-free perovskite molecules with different 3D frameworks show reversible phase transition, dielectric anomaly and SHG effect

Three-dimensional (3D) hybrid organic-inorganic perovskites (HOIPs) have attracted tremendous research interest due to their unique structure and promising applications. However, research on the design, synthesis and properties of this kind of metal-free crystalline material is still in the exploratory stage. Herein, two 3D perovskite molecules [1,4-3.2.2-dabcn]NH₄Br₃ (1) and [1,4-3.2.2-dabcn]NH₄I₃·0.5H₂O (2) were obtained by reacting 1,4-diazabicyclo[3.2.2]nonane (1,4-3.2.2-dabcn) with NH₄X (X = Br and I) in the corresponding concentrated halogen acids. The single X-ray diffraction results demonstrated that the inorganic framework structures in compounds 1 and 2 constructed with NH₄Br and NH₄I are completely different, caused by the radius of the bromide ion being smaller than that of the iodide ion. The 3D framework of compound 1 is constructed with a coplanar dimer [(NH₄)₂Br₆]^2- as the basic building unit, leading to the expanded 3D perovskite framework structure with a larger cavity to accommodate the 1,4-3.2.2-dabcn molecule. Nevertheless, compound 2 adopts a familiar 3D crystal framework structure with corner-sharing [(NH₄)I₆] octahedra, where the [1,4-3.2.2-dabcn] cations and water solvent molecule are confined in the cavities enclosed by the octahedra. Notably, both compounds exhibit reversible phase transition, dielectric anomaly and the second harmonic generation (SHG) effect. From the perspective of molecular design, this work is of great significance to guide the construction of new 3D metal-free perovskite molecular materials with reversible properties.