Influence of Magnetic Sublattice Ordering on Skyrmion Bubble Stability in 2D Magnet Fe5GeTe2.

The realization of above room-temperature ferromagnetism in the two-dimensional (2D) magnet Fe5GeTe2 represents a major advance for the use of van der Waals (vdW) materials in practical spintronic applications. In particular, observations of magnetic skyrmions and related states within exfoliated flakes of this material provide a pathway to the fine-tuning of topological spin textures via 2D material heterostructure engineering. However, there are conflicting reports as to the nature of the magnetic structures in Fe5GeTe2. The matter is further complicated by the study of two types of Fe5GeTe2 crystals with markedly different structural and magnetic properties, distinguished by their specific fabrication procedure: whether they are slowly cooled or rapidly quenched from the growth temperature. In this work, we combine X-ray and electron microscopy to observe the formation of magnetic stripe domains, skyrmion-like type-I, and topologically trivial type-II bubbles, within exfoliated flakes of Fe5GeTe2. The results reveal the influence of the magnetic ordering of the Fe1 sublattice below 150 K, which dramatically alters the magnetocrystalline anisotropy and leads to a complex magnetic phase diagram and a sudden change of the stability of the magnetic textures. In addition, we highlight the significant differences in the magnetic structures intrinsic to slow-cooled and quenched Fe5GeTe2 flakes.

Identifying the Origin of Thermal Modulation of Exchange Bias in MnPS3/Fe3GeTe2 van der Waals Heterostructures.

The exchange bias phenomenon, inherent in exchange-coupled ferromagnetic and antiferromagnetic systems, has intrigued researchers for decades. Van der Waals materials, with their layered structures, offer an ideal platform for exploring exchange bias. However, effectively manipulating exchange bias in van der Waals heterostructures remains challenging. This study investigates the origin of exchange bias in MnPS3/Fe3GeTe2 van der Waals heterostructures, demonstrating a method to modulate nearly 1000% variation in magnitude through simple thermal cycling. Despite the compensated interfacial spin configuration of MnPS3, a substantial 170 mT exchange bias is observed at 5 K, one of the largest observed in van der Waals heterostructures. This significant exchange bias is linked to anomalous weak ferromagnetic ordering in MnPS3 below 40 K. The tunability of exchange bias during thermal cycling is attributed to the amorphization and changes in the van der Waals gap during field cooling. The findings highlight a robust and adjustable exchange bias in van der Waals heterostructures, presenting a straightforward method to enhance other interface-related spintronic phenomena for practical applications. Detailed interface analysis reveals atom migration between layers, forming amorphous regions on either side of the van der Waals gap, emphasizing the importance of precise interface characterization in these heterostructures.

Strong angular and spectral narrowing of electroluminescence in an integrated Tamm-plasmon-driven halide perovskite LED.

Next-generation light-emitting applications such as displays and optical communications require judicious control over emitted light, including intensity and angular dispersion. To date, this remains a challenge as conventional methods require cumbersome optics. Here, we report highly directional and enhanced electroluminescence from a solution-processed quasi-2-dimensional halide perovskite light-emitting diode by building a device architecture to exploit hybrid plasmonic-photonic Tamm plasmon modes. By exploiting the processing and bandgap tunability of the halide perovskite device layers, we construct the device stack to optimise both optical and charge-injection properties, leading to narrow forward electroluminescence with an angular full-width half-maximum of 36.6° compared with the conventional isotropic control device of 143.9°, and narrow electroluminescence spectral full-width half-maximum of 12.1 nm. The device design is versatile and tunable to work with emission lines covering the visible spectrum with desired directionality, thus providing a promising route to modular, inexpensive, and directional operating light-emitting devices.

Probing Self-Diffusion of Guest Molecules in a Covalent Organic Framework: Simulation and Experiment.

Covalent organic frameworks (COFs) are a class of porous materials whose sorption properties have so far been studied primarily by physisorption. Quantifying the self-diffusion of guest molecules inside their nanometer-sized pores allows for a better understanding of confinement effects or transport limitations and is thus essential for various applications ranging from molecular separation to catalysis. Using a combination of pulsed field gradient nuclear magnetic resonance measurements and molecular dynamics simulations, we have studied the self-diffusion of acetonitrile and chloroform in the 1D pore channels of two imine-linked COFs (PI-3-COF) with different levels of crystallinity and porosity. The higher crystallinity and porosity sample exhibited anisotropic diffusion for MeCN parallel to the pore direction, with a diffusion coefficient of Dpar = 6.1(3) × 10-10 m2 s-1 at 300 K, indicating 1D transport and a 7.4-fold reduction in self-diffusion compared to the bulk liquid. This finding aligns with molecular dynamics simulations predicting 5.4-fold reduction, assuming an offset-stacked COF layer arrangement. In the low-porosity sample, more frequent diffusion barriers result in isotropic, yet significantly reduced diffusivities (DB = 1.4(1) × 10-11 m2 s-1). Diffusion coefficients for chloroform at 300 K in the pores of the high- (Dpar = 1.1(2) × 10-10 m2 s-1) and low-porosity (DB = 4.5(1) × 10-12 m2 s-1) samples reproduce these trends. Our multimodal study thus highlights the significant influence of real structure effects such as stacking faults and grain boundaries on the long-range diffusivity of molecular guest species while suggesting efficient intracrystalline transport at short diffusion times.

Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite.

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Exploring Layered Disorder in Lithium-Ion-Conducting Li3Y1-xInxCl6.

Li3Y1-xInxCl6 undergoes a phase transition from trigonal to monoclinic via an intermediate orthorhombic phase. Although the trigonal yttrium containing the end member phase, Li3YCl6, synthesized by a mechanochemical route, is known to exhibit stacking fault disorder, not much is known about the monoclinic phases of the serial composition Li3Y1-xInxCl6. This work aims to shed light on the influence of the indium substitution on the phase evolution, along with the evolution of stacking fault disorder using X-ray and neutron powder diffraction together with solid-state nuclear magnetic resonance spectroscopy, studying the lithium-ion diffusion. Although Li3Y1-xInxCl6 with x ≤ 0.1 exhibits an ordered trigonal structure like Li3YCl6, a large degree of stacking fault disorder is observed in the monoclinic phases for the x ≥ 0.3 compositions. The stacking fault disorder materializes as a crystallographic intergrowth of faultless domains with staggered layers stacked in a uniform layer stacking, along with faulted domains with randomized staggered layer stacking. This work shows how structurally complex even the "simple" series of solid solutions can be in this class of halide-based lithium-ion conductors, as apparent from difficulties in finding a consistent structural descriptor for the ionic transport.

Covalent Organic Frameworks as Single-Site Photocatalysts for Solar-to-Fuel Conversion.

Single-site photocatalysts (SSPCs) are well-established as potent platforms for designing innovative materials to accomplish direct solar-to-fuel conversion. Compared to classical inorganic porous materials, such as zeolites and silica, covalent organic frameworks (COFs)─an emerging class of porous polymers that combine high surface areas, structural diversity, and chemical stability─are attractive candidates for SSPCs due to their molecular-level precision and intrinsic light harvesting ability, both amenable to structural engineering. In this Perspective, we summarize the design concepts and state-of-the-art strategies for the construction of COF SSPCs, and we review the development of COF SSPCs and their applications in solar-to-fuel conversion from their inception. Underlying pitfalls concerning photocatalytic characterization are discussed, and perspectives for the future development of this burgeoning field are given.

Harnessing Van der Waals CrPS4 and Surface Oxides for Nonmonotonic Preset Field Induced Exchange Bias in Fe3GeTe2.

Two-dimensional van der Waals (vdW) heterostructures are an attractive platform for studying exchange bias due to their defect-free and atomically flat interfaces. Chromium thiophosphate (CrPS4), an antiferromagnetic material, possesses uncompensated magnetic spins in a single layer, rendering it a promising candidate for exploring exchange bias phenomena. Recent findings have highlighted that naturally oxidized vdW ferromagnetic Fe3GeTe2 exhibits exchange bias, attributed to the antiferromagnetic coupling of its ultrathin surface oxide layer (O-FGT) with the underlying unoxidized Fe3GeTe2. Anomalous Hall measurements are employed to scrutinize the exchange bias within the CrPS4/(O-FGT)/Fe3GeTe2 heterostructure. This analysis takes into account the contributions from both the perfectly uncompensated interfacial CrPS4 layer and the interfacial oxide layer. Intriguingly, a distinct and nonmonotonic exchange bias trend is observed as a function of temperature below 140 K. The occurrence of exchange bias induced by a "preset field" implies that the prevailing phase in the polycrystalline surface oxide is ferrimagnetic Fe3O4. Moreover, the exchange bias induced by the ferrimagnetic Fe3O4 is significantly modulated by the presence of the van der Waals antiferromagnetic CrPS4 layer, forming a heterostructure, along with additional iron oxide phases within the oxide layer. These findings underscore the intricate and complex nature of exchange bias in van der Waals heterostructures, highlighting their potential for tailored manipulation and control.

Downsizing Porphyrin Covalent Organic Framework Particles Using Protected Precursors for Electrocatalytic CO2 Reduction.

Covalent organic frameworks (COFs) are promising electrocatalyst platforms owing to their designability, porosity, and stability. Recently, COFs with various chemical structures are developed as efficient electrochemical CO2 reduction catalysts. However, controlling the morphology of COF catalysts remains a challenge, which can limit their electrocatalytic performance. Especially, while porphyrin COFs show promising catalytic properties, their particle size is mostly large and uncontrolled because of the severe aggregation of crystallites. In this work, a new synthetic methodology for rationally downsized COF catalyst particles is reported, where a tritylated amine is employed as a novel protected precursor for COF synthesis. Trityl protection provides high solubility to a porphyrin precursor, while its deprotection proceeds in situ under typical COF synthesis conditions. Subsequent homogeneous nucleation and colloidal growth yield smaller COF particles than a conventional synthesis, owing to suppressed crystallite aggregation. The downsized COF particles exhibit superior catalytic performance in electrochemical CO2 reduction, with higher CO production rate and faradaic efficiency compared to conventional COF particles. The improved performance is attributed to the higher contact area with a conductive agent. This study reveals particle size as an important factor for the evaluation of COF electrocatalysts and provides a strategy to control it.

Rational Design of 2D Supramolecular Networks Switchable by External Electric Fields.

The reversible formation of hydrogen bonds is a ubiquitous mechanism for controlling molecular assembly in biological systems. However, achieving predictable reversibility in artificial two-dimensional (2D) materials remains a significant challenge. Here, we use an external electric field (EEF) at the solid/liquid interface to trigger the switching of H-bond-linked 2D networks using a scanning tunneling microscope. Assisted by density functional theory and molecular dynamics simulations, we systematically vary the molecule-to-molecule interactions, i.e., the hydrogen-bonding strength, as well as the molecule-to-substrate interactions to analyze the EEF switching effect. By tuning the building block's hydrogen-bonding ability (carboxylic acids vs aldehydes) and substrate nature and charge (graphite, graphene/Cu, graphene/SiO2), we induce or freeze the switching properties and control the final polymorphic output in the 2D network. Our results indicate that the switching ability is not inherent to any particular building block but instead relies on a synergistic combination of the relative adsorbate/adsorbate and absorbate/substrate energetic contributions under surface polarization. Furthermore, we describe the dynamics of the switching mechanism based on the rotation of carboxylic groups and proton exchange, which generate the polarizable species that are influenced by the EEF. This work provides insights into the design and control of reversible molecular assembly in 2D materials, with potential applications in a wide range of fields, including sensors and electronics.