Electro-Optic Modulation Using Metal-Free Perovskites

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Molecularly thin, two-dimensional all-organic perovskites

Recently, the emergence of all-organic perovskites with three-dimensional (3D) structures has expanded the potential applications of perovskite materials. However, the synthesis and utilization of all-organic perovskites in 2D form remain largely unexplored because the design principle has not been developed. We present the successful synthesis of a metal-free 2D layered perovskite, denoted as the Choi-Loh van der Waals phase (CL-v phase), with the chemical formula A2B2X4, where A represents a larger-sized cation compared to B and X denotes an anion. The CL-v phase exhibits a van der Waals gap enabled by interlayer hydrogen bonding and can be exfoliated or grown as molecularly thin 2D organic crystals. The dielectric constants of the CL-v phase range from 4.8 to 5.5 and we demonstrate their potential as gate dielectrics for thin-film transistors.

Terahertz Emission via Optical Rectification in a Metal-Free Perovskite Crystal

We report on the emission of high-intensity pulsed terahertz radiation from the metal-free halide perovskite single crystal methyl-DABCO ammonium iodide (MDNI) under femtosecond illumination. The power and angular dependence of the THz output implicate optical rectification of the 800 nm pump as the mechanism of THz generation. Further characterization finds that, for certain crystal orientations, the angular dependence of THz emission is modulated by phonon resonances attributable to the motion of the methyl-DABCO moiety. At maximum, the THz emission spectrum of MDNI is free from significant phonon resonances, resulting in THz pulses with a temporal width of <900 fs and a peak-to-peak electric field strength of approximately 0.8 kV cm-1-2 orders of magnitude higher than any other reported halide perovskite emitters. Our results point toward metal-free perovskites as a promising new class of THz emitters that brings to bear many of the advantages enjoyed by other halide perovskite materials. In particular, the broad tunability of optoelectronic properties and ease of fabrication of perovskite materials opens up the possibility of further optimizing the THz emission properties within this material class.

Metal-Free Perovskites for X-Ray Detection

Metal-free perovskites are a promising class of materials for X-ray detection due to their unique structural, optical, and electrical properties. Here, we first delve into the stoichiometry and geometric argument of metal-free perovskites. Followed, the alternative A/B/X ions and hydrogen-bonding are clearly introduced to further optimize the materials' stability and properties. Finally, we provide a comprehensive overview of their potential applications for flexible X-ray images and prospects for metal-free perovskite development. In conclusion, metal-free perovskite is a promising material for X-ray detection. Its stoichiometric and geometric parameters, ion, and hydrogen bond selection, and application prospects are worthy of further study.

Metal-free enantiomorphic perovskite [dabcoH2]2+[H3O]+Br- 3 and its one-dimensional polar polymorph

The structure and stoichiometry of a new metal-free and ammonium-free compound [dabcoH₂]²⁺H₃O⁺Br^- ₃ (where [dabcoH₂]²⁺ = 1,4-di-aza-bicyclo-[2.2.2]octane dication) correspond to the general formula ABX ₃ characteristic of perovskites. In enantiomorphic trigonal polymorph α of [dabcoH₂]²⁺H₃O⁺Br^- ₃, the corner-sharing [H₃O]Br₆ octahedra combine into a 3D framework embedding [dabcoH₂]²⁺ dications in pseudo-cubic cages. In the more dense polymorph β, the face-sharing [H₃O]Br₆ octahedra form 1D polyanionic columns separated by [dabcoH₂]²⁺ dications. These different topologies correlate with different crystal fields around the cations and their different disorder types: orientational disorders of [dabcoH₂]²⁺ dications and H₃O⁺ cations in polymorph α and positional disorder of [H₃O]⁺ cations in polymorph β. The orientational disorder increases the lengths of OH⋯Br hydrogen bonds in polymorph α, but NH⋯Br distances of ordered dabcoH₂ dications are longer in polymorph β. The presence of polar [H₃O]⁺ cations in [dabcoH₂]²⁺H₃O⁺Br^- ₃ polymorphs offers additional polarizability of the centres compared with analogous metal-free [dabcoH₂]²⁺[NH₄]⁺Br^- ₃ perovskite.

Omni-functional crystal: Advanced methods to characterize the composition and homogeneity of lithium niobate melts and crystals

Lithium niobate (LN) is a type of multifunctional dielectric and ferroelectric crystal that is widely used in acoustic, optical, and optoelectronic devices. The performance of pure and doped LN strongly depends on various factors, including its composition, microstructure, defects, domain, and homogeneity. The structure and composition homogeneity can affect both the chemical and physical properties of LN crystals, including their density, Curie temperature, refractive index, and piezoelectric and mechanical properties. In terms of practical demands, both the composition and microstructure characterizations these crystals must range from the nanometer scale up to the millimeter and wafer scales. Therefore, LN crystals require different characterization technologies when verifying their quality for various device applications. Optical, electrical, and acoustic technologies have been developed, including x-ray diffraction, Raman spectroscopy, electron microscopy, and interferometry. To obtain detailed structural information, advanced sub-nanometer technologies are required. For general industrial demands, fast and non-destructive technologies are preferable. This review outlines the advanced methods used to characterize both the composition and homogeneity of LN melts and crystals from the micro- to wafer scale.

Structural and Functional Insights into Metal-Free Perovskites

In the past three years, metal-free perovskites have garnered significant interest as promising candidates for utilization in next-generation wearable electronics. A variety of different molecular structures for these perovskites have been designed for different applications. However, there is still no systematic understanding that can elucidate the relationship between the structural details and properties of perovskites. This would provide a helpful guide for designing a metal-free perovskite with the desired packing structure and properties. Herein, we summarize recently reported structural and functional insights into metal-free perovskites. The underlying design of the molecular structure and its role in the packing structure and resulting properties are explained. In addition, important factors and challenges in the design of a molecular structure that will be useful for future applications are discussed. This information will help enrich the library of potential structures and future applications of metal-free perovskites, which is believed to be much larger than is currently known.