Ferroelasticity in Two-Dimensional Hybrid Ruddlesden-Popper Perovskites Mediated by Cross-Plane Intermolecular Coupling and Metastable Funnel-like Phases.

Ferroelasticity is a phenomenon in which a material exhibits two or more equally stable orientation variants and can be switched from one form to another under an applied stress. Recent works have demonstrated that two-dimensional layered organic-inorganic hybrid Ruddlesden-Popper perovskites can serve as ideal platforms for realizing ferroelasticity, however, the ferroelastic (FE) behavior of structures with a single octahedra layer such as (BA)2PbI4 [BA = CH3(CH2)3NH3+] has remained elusive. Herein, by using a combined first-principles and metadynamics approach, the FE behavior of (BA)2PbI4 under mechanical and thermal stresses is uncovered. FE switching is mediated by cross-plane intermolecular coupling, which could occur through multiple rotational modes, rendering the formation of FE domains and several metastable paraelastic (PE) phases. Such metastable phases are akin to wrinkled structures in other layered materials and can act as a "funnel" of hole carriers. Thermal excitation tends to flatten the kinetic barriers of the transition pathways between orientation variants, suggesting an enhanced concentration of metastable PE states at high temperatures, while halogen mixing with Br raises these barriers and conversely lowers the concentration of PE states. These findings reveal the rich structural diversity of (BA)2PbI4 domains, which can play a vital role in enhancing the optoelectronic properties of the perovskite and raise exciting prospects for mechanical switching, shape memory, and information processing.

Molecular Principles of the Excited-State Intramolecular Thiol Proton Transfers in 3-thiolflavone Derivatives.

The excited-state intramolecular proton transfer (ESIPT) effect has attracted considerable attention due to its potential applications in photoluminescent materials. However, only a few theoretical reports have investigated the ESIPT process involving sulfur-hydrogen bonds. Herein, we systematically investigated the ESIPT effect of three 3-thiolflavone derivatives containing sulfur-hydrogen bonds with M06-2X functional combined Def2-TZVP basis set. The intramolecular sulfur-hydrogen bonds were confirmed in the ground and excited states via analyzing the bond lengths, interaction energies, and infrared vibrational spectra. Besides, we demonstrated that the electron-withdrawing group led to a more stable tautomer compared to the electron-donating group. Conversely, the electron-donating group played a crucial role in reducing the energy barrier of the ESIPT reaction due to the strengthening of the hydrogen bond in the excited state. Interestingly, the substituent group can determine the excited-state electronic properties of keto tautomers. Specifically, the electron-withdrawing group caused significant improvement in the nπ* transition configuration, significantly reducing the radiation rate. The electron-donating group increased the proportion of the ππ* transition configuration, thus, the keto tautomer had bright emission. We expect these findings to open new avenues for designing potential luminescent materials.

Additively Manufactured Dual-Faced Structured Fabric for Shape-Adaptive Protection.

Fabric-based materials have demonstrated promise for high-performance wearable applications but are currently restricted by their deficient mechanical properties. Here, this work leverages the design freedom offered by additive manufacturing and a novel interlocking pattern to for the first time fabricate a dual-faced chain mail structure consisting of 3D re-entrant unit cells. The flexible structured fabric demonstrates high specific energy absorption and specific strength of up to 1530 J kg-1 and 5900 Nm kg-1 , respectively, together with an excellent recovery ratio of ≈80%, thereby overcoming the strength-recoverability trade-off. The designed dual-faced structured fabric compares favorably against a wide range of materials proposed for wearable applications, attributed to the synergetic strengthening of the energy-absorbing re-entrant unit cells and their unique topological interlocking. This work advocates the combined design of energy-absorbing unit cells and their interlocking to extend the application prospects of fabric-based materials to shape-adaptive protection.

Abnormal behavior of preferred formation of the cationic vacancies from the interior in a γ-GeSe monolayer with the stereo-chemical antibonding lone-pair state.

Two-dimensional (2D) materials tend to have the preferable formation of vacancies at the outer surface. Here, contrary to the normal notion, we reveal a type of vacancy that thermodynamically initiates from the interior part of the 2D backbone of germanium selenide (γ-GeSe). Interestingly, the Ge-vacancy (VGe) in the interior part of γ-GeSe possesses the lowest formation energy amongst the various types of defects considered. We also find a low diffusion barrier (1.04 eV) of VGe, which is half of those of sulfur vacancies in MoS2. The facile formation of mobile VGe is rooted in the antibonding coupling of the lone-pair Ge 4s and Se 4p states near the valence band maximum, which also exists in other gamma-phase MX (M = Sn, Ge; X = S, Te). The VGe is accompanied by a shallow acceptor level in the band gap and induces strong infrared light absorption and p-type conductivity. The VGe located in the middle cationic Ge sublattice is well protected by the surface Se layers - a feature that is absent in other atomically thin materials. Our work suggests that the unique well-buried inner VGe, with the potential of forming structurally protected ultrathin conducting filaments, may render the GeSe layer an ideal platform for quantum emitting, memristive, and neuromorphic applications.

Strong Edge Stress in Molecularly Thin Organic-Inorganic Hybrid Ruddlesden-Popper Perovskites and Modulations of Their Edge Electronic Properties.

Organic-inorganic hybrid Ruddlesden-Popper perovskites (HRPPs) have gained much attention for optoelectronic applications due to their high moisture resistance, good processability under ambient conditions, and long functional lifetimes. Recent success in isolating molecularly thin hybrid perovskite nanosheets and their intriguing edge phenomena have raised the need for understanding the role of edges and the properties that dictate their fundamental behaviors. In this work, we perform a prototypical study on the edge effects in ultrathin hybrid perovskites by considering monolayer (BA)2PbI4 as a representative system. On the basis of first-principles simulations of nanoribbon models, we show that in addition to significant distortions of the octahedra network at the edges, strong edge stresses are also present in the material. Structural instabilities that arise from the edge stress could drive the relaxation process and dominate the morphological response of edges in practice. A clear downward shift of the bands at the narrower ribbons, as indicative of the edge effect, facilitates the separation of photoexcited carriers (electrons move toward the edge and holes move toward the interior part of the nanosheet). Moreover, the desorption energy of the organic molecule can also be much lower at the free edges, making it easier for functionalization and/or substitution events to take place. The findings reported in this work elucidate the underlying mechanisms responsible for edge states in HRPPs and will be important in guiding the rational design and development of high-performance layer-edge devices.

The photocatalytic mechanism of organic dithienophosphole derivatives as highly efficient photo-redox catalysts.

The use of organic photo-redox catalysts to initiate well-controlled photochemical reactions has aroused great interest. The development of visible light-driven photocatalytic reactions, which enable rapid and efficient synthesis of fine products, is highly desired from the perspective of being able to achieve low cost, good reversibility, and environmental friendliness. Herein, the organic photocatalytic cycle, with organic dithienophosphole (DTP) derivatives Ph-DTP and TPA-DTP as the photo-redox catalysts, and iodonium salt (Ar2I+) and ethyl 4-(dimethylamino)benzoate (EDB) as the respective acceptor and donor substrates, is fully analyzed by using density functional theory and dissociative electron transfer theory. We show that the strong redox potentials in the excited state as well as the sufficiently long-lived excited state of both DTP derivatives are a robust driving force for activating the electron acceptor Ar2I+ in the activation process. Moreover, the activation barriers of electron transfer are only 0.43-11.9 kcal mol-1 for the different activation pathways. During the deactivation process, the reaction energy profiles indicate that EDB plays a vital role in reducing DTPs˙+ to their initial states. Importantly, the activation barriers and rate constants in both activation and deactivation processes obtained in this study are better than those of classic Cu-based and metal-free Ph-PTZ-based photo-redox catalysts. The excellent performance of both DTP derivatives thus enables them to be highly efficient organic photo-redox catalysts.

Emissive Nature and Molecular Behavior of Zero-Dimensional Organic-Inorganic Metal Halides Bmpip2MX4.

Zero-dimensional (0D) organic-inorganic metal halides, with their high stability and broadband emission features, have aroused great interest in optoelectronic applications. Metal halides of the type Bmpip2MX4 (M = Pb, Sn, or Ge; X = I or Br) have 0D disphenoidal coordinated structures that offer an excellent opportunity to investigate their emissive nature and molecular behavior. Herein, the photophysical properties and carrier transport behavior of 0D Bmpip2MX4 metal halides are studied by using density functional theory. Our results indicate that Bmpip2MX4 metal halides present broadband emission widths and significant Stokes shifts. In particular, Bmpip2SnBr4 possesses the largest Stokes shift (1.981 eV) and the shortest exciton self-trapping time, demonstrating the best photoluminescence emission ability. Bmpip2GeI4 exhibits the lowest electron-hole creation energy and the best photoresponse capacity. Moreover, Bmpip2PbI4 demonstrates superior transport capabilities with high carrier mobilities of 4.56 × 10-3 and 2.51 × 10-7 cm2 V-1 s-1 for hole and electron carriers, respectively, which makes it comparable even with typical hole transport materials (e.g., RR P3HT, ∼10-4 cm2 V-1 s-1). These findings highlight exciting opportunities for the future development and application of such kinds of 0D metal halides in optoelectronics.

Water dissociation and hydrogen evolution on the surface of Fe-based bulk metallic glasses.

Fe-Based bulk metallic glasses (BMGs) with a composition of Fe48Cr15Mo14C15B6Y2 have recently been reported with good application performance due to their excellent mechanical and chemical properties, showing excellent corrosion resistance, remarkable forming and processing ability, ultrahigh yield strength and large elasticity. Here, we report on a new functional application for such Fe-based BMGs, which can exhibit better catalytic performance than the pristine Fe surface. The hydrogen evolution and dissociation processes of one and two H2O molecules on both BMG and pristine Fe surfaces are investigated using first-principles calculations. The energy barriers of the dissociation processes on the BMG surface are lower than those on the pristine Fe surface. Moreover, the structural configurations along the dissociation path during hydrogen evolution show that it is easier for H2O molecules to dissociate into H2 on the surface of the BMG, rendering it a more active catalyst than the pristine Fe surface. Analyses on the electronic structures show further evidence that the BMG surface has a stronger ability to facilitate charge transfer at the interface and is more inclined to accept transferred charges, thereby promoting its catalytic activity. These findings shed light on understanding the functional applications of BMGs and are anticipated to be highly meaningful for further experimental investigations.