Constructing Stable and Potentially High-Performance Hybrid Organic-Inorganic Perovskites with "Unstable" Cations

Single Molecular Semi-Sliding Ferroelectricity/Multiferroicity

In recent years, the unique mechanism of sliding ferroelectricity with ultralow switching barriers has been experimentally verified in a series of 2-dimensional (2D) materials. However, its practical applications are hindered by the low polarizations, the challenges in synthesis of ferroelectric phases limited in specific stacking configurations, and the low density for data storage since the switching process involves large-area simultaneous sliding of a whole layer. Herein, through first-principles calculations, we propose a type of semi-sliding ferroelectricity in the single metal porphyrin molecule intercalated in 2D bilayers. An enhanced vertical polarization can be formed independent on stacking configurations and switched via sliding of the molecule accompanied by the vertical displacements of its metal ion anchored from the upper layer to the lower layer. Such semi-sliding ferroelectricity enables each molecule to store 1 bit data independently, and the density for data storage can be greatly enhanced. When the bilayer exhibits intralayer ferromagnetism and interlayer antiferromagnetic coupling, a considerable difference in Curie temperature between 2 layers and a switchable net magnetization can be formed due to the vertical polarization. At a certain range of temperature, the exchange of paramagnetic-ferromagnetic phases between 2 layers is accompanied by ferroelectric switching, leading to a hitherto unreported type of multiferroic coupling that is long-sought for efficient "magnetic reading + electric writing".

Rational Design of a Super-Alkali Compound with Reversible Photoluminescence

The zero-dimensional (0D) (H₅O₂)(C₄H₁₄N₂S₂)₂BiCl₈: Sb³⁺ single crystal is obtained by the cooling crystallization method. Surprisingly, this compound shows reversible photoluminescence (PL) upon H₅O₂⁺Cl^- removal and insertion. To be specific, the release of H₅O₂⁺Cl^- resulted in red-orange emission with a very low photoluminescence quantum yield (PLQY). While on the reuptake of it, a bright yellow emission with a nearly 10-fold increase of PLQY was observed. Density functional theory (DFT) calculations and temperature-dependent PL experiments reveal that significant [SbCl₆]^3- octahedron distortion induced by guest (H₅O₂⁺Cl^-) removal at the ground state, especially at the excited state, is responsible for the disparate PL performance. Encouragingly, we also found that (C₄H₁₄N₂S₂)₂BiCl₇: Sb³⁺ exhibits a fast response (<3 s) to dilute hydrochloric acid with naked-eye perceivable PL color changes, rendering it a potential sensing material for hydrochloric acid.

Bimetallic superalkali substitution in the CsPbBr3 perovskite: Pseudocubic phases and tunable bandgap

Perovskites attract attention as efficient light absorbers for solar cells due to their high-power conversion efficiency (up to 24%). The high photoelectric conversion efficiency is greatly affected by a suitable band structure. Cation substitution can be an effective approach to tune the electronic band structure of lead halide perovskites. In this work, superalkali cations were introduced to replace the Cs⁺ cation in the CsPbBr₃ material. The bimetallic superalkalis (LiMg, NaMg, LiCa, and NaCa) were inserted since they are structurally simple systems and have a strong tendency to lose one electron to achieve a closed-shell cation. The cation substitution in the lead halide perovskite leads to changes in the shape of both valence and conduction bands compared to CsPbBr₃. Introducing superalkali cations produces extra electronic states close to the Fermi level, which arise from the formation of alkali earth metal states at the top of the valence band. Our first-principles computations reveal that bimetallic superalkali substitution decreases the bandgap of the perovskite. The bandgaps of MgLi-PbBr₃ (1.35 eV) and MgNa-PbBr₃ (1.06 eV) are lower than the bandgap of CsPbBr₃ (2.48 eV) and within the optimal bandgap (i.e., 1.1-1.4 eV) for single-junction solar cells. Thus, the MgLi-PbBr₃ and MgNa-PbBr₃ inorganic perovskites are promising candidates for high-efficiency solar cells.

Two-Dimensional van der Waals Ferroelectrics: Scientific and Technological Opportunities

Recent breakthroughs in two-dimensional (2D) van der Waals ferroelectrics have been impressive, with a series of 2D ferroelectrics having been realized experimentally. The discovery of ferroelectric order in atom-thick layers not only is important for exploring the interplay between dimensionality and ferroelectric order but may also enable ultra-high-density memory, which has attracted significant interest. However, understanding of 2D ferroelectrics goes beyond simply their atomic-scale thickness. In this Perspective, I suggest possible innovations that may resolve a number of conventional issues and greatly transform the roles of ferroelectrics in nanoelectronics. The major obstacles in the commercialization of nanoelectronic devices based on current ferroelectrics involve their insulating and interfacial issues, which hinder their combination with semiconductors in nanocircuits and reduce their efficiency in data reading/writing. In comparison, the excellent semiconductor performance of many 2D ferroelectrics may enable computing-in-memory architectures or efficient ferroelectric photovoltaics. In addition, their clean van der Waals interfaces can greatly facilitate their integration into silicon chips, as well as the popularization of nondestructive data reading and indefatigable data writing. Two-dimensional ferroelectrics also give rise to new physics such as interlayer sliding ferroelectricity, Moiré ferroelectricity, switchable metallic ferroelectricity, and unconventional robust multiferroic couplings, which may provide high-speed energy-saving data writing and efficient data-reading strategies. The emerging 2D ferroelectric candidates for optimization will help resolve some current issues (e.g., weak vertical polarizations), and further exploitation of the aforementioned advantages may open a new era of nanoferroelectricity.