Chiral Two-Dimensional Hybrid Organic-Inorganic Perovskites for Piezoelectric Ultrasound Detection

Coupled Kite-to-Square Distortion Transition and Physical Properties in 2D Lead Halide Perovskite

2D lead halide perovskites showcase diverse electrical and optoelectrical properties due to their adaptable structural distortion, which dictates the symmetry characteristics of the material. To accommodate the geometric shape of the cation, the inorganic layer of the 2D perovskite often undergoes specific distortions such as lead-halide bond length elongation/compression and lead atom displacement. The resultant distortion manifests as a quadrilateral shape formed by Pb atoms from four adjacent four octahedrons. The degree of distortion increases as the quadrilateral deviates further from a square shape and vice versa. This quadrilateral shape not only visually represents the magnitude of distortion but also confirms its direction. During the transition from kite to square distortion under external stimuli, the positions of the Pb atoms vividly illustrate the symmetry-breaking process, corresponding to a shift from high to low symmetry states. The electrical and optoelectronic properties, including ferroelectricity, pyroelectricity, piezoelectricity, nonlinear optical properties, and characteristics related to bulky photovoltaic effects, some of them exhibit direction dependence nature. This perspective employed a visible structural distortion approach to elucidate symmetry breaking and coupling distortion transitions with eight optoelectronic physical properties in 2D layered perovskite. We review recent research advancements and outline current challenges that help us to understand the structure-property relationship of 2D perovskite.

Three-Dimensional Bimetallic Ammonium K-Eu Nitrate with a Rare (6,6)-Connected Ion Topology Exhibiting Structural Phase Transition and Photoluminescence Properties

A K-Eu bimetallic ammonium metal-nitrate three-dimensional (3D) framework incorporating R-N-methyl-3-hydroxyquinuclidine, (RM3HQ)2KEu(NO3)6 (RM3HQ = R-N-methyl-3-hydroxyquinuclidine, 1), was characterized and reported. Distinguishing from the former hybrid rare-earth double perovskites, 1 adopts a mixed corner- and face-sharing K+/Eu3+-centered polyhedral connectivity to form a 3D inorganic framework, showing a rare (6, 6)-connected ion topology with a 66 framework. Notably, 1 exhibits clear phase transition, and the switchable thermodynamic behavior is confirmed by variable-temperature dielectric measurements and second-harmonic generation response. Moreover, 1 also shows photoluminescence properties. The activator Eu3+ plays a crucial role in this process, leading to a significant narrow emission at 592 nm with a photoluminescence quantum yield (PLQY) of 20.76%. The fluorescence lifetime (FLT) of 1 is 4.32 ms. This finding enriches the bimetallic hybrid system for potential electronic and/or luminescence applications.

Multiple Functions of l-Thioproline in the Synthesis of Chiral Metal Bromides Showing Second-Harmonic-Generation Responses

Three new homochiral metal bromides, namely, (l-Htp)2Cu2Br4 (1), (l-Htp)(l-tp)CdBr3 (2), and (l-tp)2ZnBr2 (3), were prepared using l-thioproline as the chiral template. These compounds feature dimeric, chainlike, and monomeric structures. Their second-harmonic-generation (SHG) efficiencies are 0.1, 0.3, and 2.0 times that of KH2PO4, respectively. Density functional theory calculations were performed to reveal the origin of the SHG response of compound 3.

In Situ Chiral Template Approach to Synthesize Homochiral Lead Iodides for Second-Harmonic Generation

Homochiral halide perovskites have gained increasing attention because of their fascinating optoelectronic properties and prospective applications in laser technologies. However, the limited choice of chiral organic templates severely restricts their structural diversity and second-harmonic generation (SHG) effects. Here, we present an in situ chiral template approach for the synthesis of one-dimensional (1D) homochiral lead iodides. A chiral imine (L-ipp) template was generated in situ by reacting L-proline (L-pro) and acetone under ambient conditions. Notably, L-ipp can cooperate with L-pro to direct the formation of a homochiral lead iodide with dual chiral templates, which is unprecedented in crystalline metal halides. The homochiral lead iodide containing both L-ipp and L-pro shows a strong SHG response of 8.0 times that of KH2 PO4 (8.0×KDP). The SHG efficiency is one of the largest values reported to date for any homochiral lead halides under 1064 nm laser irradiation. A comparative study shows that homochiral 1D lead iodides containing either L-ipp or L-pro exhibit relatively weak SHG responses (≤1.0×KDP). This work demonstrates the advantage of using two different chiral templates over a single chiral template in enhancing the SHG responses of halide materials.