Effect of Halide Tuning on the Structural, Dielectric, and Optical Properties of Two-Dimensional 2-Chloroethylammonium Lead Halides

Layered hybrid organic-inorganic lead halides have gained a lot of attention for optoelectronic applications. A notable subset within this category is perovskites comprising halogenated amines since they may exhibit reduced band gap or polar order. We synthesized three compounds comprising 2-chloroethylammonium (CEA+) cations, with the chemical formula CEA2PbX4 (X = Cl, Br, I). X-ray diffraction studies show that at room temperature (RT), CEA2PbBr4 and CEA2PbI4 crystallize in Pbnm symmetry, with ordered CEA+ cations. CEA2PbBr4 and CEA2PbI4 undergo one structural phase transition (PT) into a disordered Pmnm phase near 315 and 360 K, respectively. CEA2PbCl4 shows a different packing of CEA+ with the organic chains oriented perpendicularly to the perovskite layers. It undergoes two PTs at 332 and 203 K from the high-temperature (HT) disordered I4/mmm phase to the partially ordered intermediate Pbnm phase and completely ordered low-temperature (LT) phase of the unknown space group. All compounds emit photoluminescence (PL): orange, yellow-green, and yellow for CEA2PbI4, CEA2PbCl4, and CEA2PbBr4, respectively, and bromide exhibits a very high quantum efficiency of 48%. Overall, our findings show that halide engineering strongly modulates hydrogen and halogen bonding strength, affecting the structural arrangement of building units, molecular dynamics, and thus optoelectronic properties.

Multi-Noncentrosymmetric Polar Order in 2D Hybrid Lead Chloride with Broadband Emission and High-Temperature Second-Harmonic Generation Switching

Two-dimensional lead halide perovskites represent a fascinating class of hybrid semiconductors for solar cell, light-emitting, nonlinear optical (NLO), and ferroelectric applications. A notable subset within this category is luminescent ferroelectrics, which have garnered considerable attention for their potential in integrated photoelectronic devices. In this study, we employed an organic amine halogenation strategy (also referred to as halogen engineering), which is renowned for its efficacy in inducing polar order through crystal engineering. Consequently, we synthesized a layered Ruddlesden-Popper (RP) lead chloride comprising 3-chloropropylammonium cations (CPA+), with the chemical formula CPA2PbCl4. This compound features as many as four temperature-dependent crystal phases, with phase transitions observed at T1 = 353.1 K (343.9 K), T2 = 211.7 K (208.6 K), and T3 = 182.0 K (178.2 K) in the heating (cooling) cycles. Employing a multitechnique approach─including thermal analysis, X-ray diffraction, dielectric and pyroelectric current measurements, Raman spectroscopy, and second-harmonic generation (SHG) studies─we determined the mechanisms of the structural phase transitions. Our findings demonstrate polar order of phase II (space group Cmc21), phase III (space group Pna21), and phase IV (space group Pca21), while also confirming the centrosymmetric nature of phase I (space group Cmce). X-ray diffraction data revealed that the I to II PT is of a ferroelectric nature, devoid of ferroelastic strain, a conclusion further supported by pyroelectric measurements. CPA2PbCl4 features negative linear thermal expansion and broadband emission, which transitions to white light above 180 K. Remarkably, CPA2PbCl4 also demonstrates high-temperature SHG on-off switching with a high contrast ratio of 300:1 along with good switching stability, as evidenced by SHG cycling studies at heating/cooling rates ranging from 5 to 50 K/min. This SHG study also sets new standards for the field of SHG switching by providing a method to quantify the thermal responsiveness of SHG-switchable materials using the treq (time requirement) parameter. Overall, our findings show that the halogenation strategy has led to the discovery of a rare example of an RP perovskite exhibiting coexistence of white-light emission, SHG on-off thermal bistability, ferroelectricity, and negative linear thermal expansion.

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.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Ultrafast Excitonic Response in Two-Dimensional Hybrid Perovskites Driven by Intense Midinfrared Pulses

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA)_{2}PbBr_{4} and (BA)_{2}(FA)Pb_{2}Br_{7} (BA^{+}=butylammonium; FA^{+}=formamidinium), reveal two distinct regimes of the temporal response. The first regime is dominated by a pump-induced transient expansion of the organic spacer layers that reduces the exciton oscillator strength, whereas the second regime arises from pump-induced lattice heating effects primarily associated with a spectral shift of the exciton energy. In addition, vibrational excitation enhances the biexciton emission, which we attribute to a stronger intralayer exciton confinement as well as vibrationally induced exciton detrapping from defect states. Our study provides fundamental insights regarding the impact of organic spacers on excitons in 2DHPs, as well as the excited-state dynamics and vibrational energy dissipation in these structurally diverse materials.

Effect of Dimensionality on Photoluminescence and Dielectric Properties of Imidazolium Lead Bromides

Hybrid organic–inorganic lead halide perovskites have emerged as promising materials for various applications, including solar cells, light-emitting devices, dielectrics, and optical switches. In this work, we report the synthesis, crystal structures, and linear and nonlinear optical as well as dielectric properties of three imidazolium lead bromides, IMPbBr₃, IM₂PbBr₄, and IM₃PbBr₅ (IM⁺ = imidazolium). We show that these compounds exhibit three distinct structure types. IMPbBr₃ crystallizes in the 4H-hexagonal perovskite structure with face- and corner-shared PbBr₆ octahedra (space group P6₃/mmc at 295 K), IM₂PbBr₄ adopts a one-dimensional (1D) double-chain structure with edge-shared octahedra (space group P1̅ at 295 K), while IM₃PbBr₅ crystallizes in the 1D single-chain structure with corner-shared PbBr₆ octahedra (space group P1̅ at 295 K). All compounds exhibit two structural phase transitions, and the lowest temperature phases of IMPbBr₃ and IM₃PbBr₅ are noncentrosymmetric (space groups Pna2₁ at 190 K and P1 at 100 K, respectively), as confirmed by measurements of second-harmonic generation (SHG) activity. X-ray diffraction and thermal and Raman studies demonstrate that the phase transitions feature an order–disorder mechanism. The only exception is the isostructural P1̅ to P1̅ phase transition at 141 K in IM₂PbBr₄, which is of a displacive type. Dielectric studies reveal that IMPbBr₃ is a switchable dielectric material, whereas IM₃PbBr₅ is an improper ferroelectric. All compounds exhibit broadband, highly shifted Stokes emissions. Features of these emissions, i.e., band gap and excitonic absorption, are discussed in relation to the different structures of each composition.

Three imidazolium lead bromides of various chemical compositions and crystal structures display broadband photoluminescence that can be tuned from bluish-green to orange. All compounds exhibit two structural phase transitions, which lead to interesting optical and electrical properties such SHG activity, ferroelectricity, or dielectric switching.

Thick-Layer Lead Iodide Perovskites with Bifunctional Organic Spacers Allylammonium and Iodopropylammonium Exhibiting Trap-State Emission

The nature of the organic cation in two-dimensional (2D) hybrid lead iodide perovskites tailors the structural and technological features of the resultant material. Herein, we present three new homologous series of (100) lead iodide perovskites with the organic cations allylammonium (AA) containing an unsaturated C═C group and iodopropylammonium (IdPA) containing iodine on the organic chain: (AA)₂MA_n-1Pb_nI_3n+1 (n = 3-4), [(AA)_x(IdPA)_1-x]₂MA_n-1Pb_nI_3n+1 (n = 1-4), and (IdPA)₂MA_n-1Pb_nI_3n+1 (n = 1-4), as well as their perovskite-related substructures. We report the in situ transformation of AA organic layers into IdPA and the incorporation of these cations simultaneously into the 2D perovskite structure. Single-crystal X-ray diffraction shows that (AA)₂MA₂Pb₃I₁₀ crystallizes in the space group P2₁/c with a unique inorganic layer offset (0, <1/2), comprising the first example of n = 3 halide perovskite with a monoammonium cation that deviates from the Ruddlesden-Popper (RP) halide structure type. (IdPA)₂MA₂Pb₃I₁₀ and the alloyed [(AA)_x(IdPA)_1-x]₂MA₂Pb₃I₁₀ crystallize in the RP structure, both in space group P2₁/c. The adjacent I···I interlayer distance in (AA)₂MA₂Pb₃I₁₀ is ∼5.6 Å, drawing the [Pb₃I₁₀]^4- layers closer together among all reported n = 3 RP lead iodides. (AA)₂MA₂Pb₃I₁₀ presents band-edge absorption and photoluminescence (PL) emission at around 2.0 eV that is slightly red-shifted in comparison to (IdPA)₂MA₂Pb₃I₁₀. The band structure calculations suggest that both (AA)₂MA₂Pb₃I₁₀ and (IdPA)₂MA₂Pb₃I₁₀ have in-plane effective masses around 0.04m₀ and 0.08m₀, respectively. IdPA cations have a greater dielectric contribution than AA. The excited-state dynamics investigated by transient absorption (TA) spectroscopy reveal a long-lived (∼100 ps) trap state ensemble with broad-band emission; our evidence suggests that these states appear due to lattice distortions induced by the incorporation of IdPA cations.