Tunable Magnetism in Atomically Thin Itinerant Antiferromagnet with Room-Temperature Ferromagnetic Order.

Addressing the need for modulated spin configurations is crucial, as they serve as the foundational building blocks for next-generation spintronics, particularly in atomically thin structures and at room temperature. In this work, we realize intrinsic ferromagnetism in monolayer flakes and tunable ferro-/antiferromagnetism in (Fe0.56Co0.44)5GeTe2 antiferromagnets. Remarkably, the ferromagnetic ordering (≥1 L) and antiferromagnetic ordering (≥4 L) remain discernible up to room temperature. The TC (∼310 K) of the monolayer flakes sets a record high for known exfoliated monolayer van der Waals magnets. Within the framework of A-type antiferromagnetism, a notable odd-even layer-number effect at elevated temperatures (T = 150 K) is observed. Of particular interest is the strong ferromagnetic order in even-layer flakes at low temperatures. The intricate interplay among magnetic field strength, layer number, and temperature gives rise to a diverse array of phenomena, holding promise not only for new physics but also for practical applications.

Defective Tungsten Oxides with Stacking Faults for Proton Exchange Membrane Green-Hydrogen Generation.

Defects can introduce atomic structural modulation and tailor performance of materials. Herein, it demonstrates that semiconductor WO3 with inert electrocatalytic behavior can be activated through defect-induced tensile strains. Structural characterizations reveal that when simply treated in Ar/H2 atmosphere, oxygen vacancies will generate in WO3 and cause defective structures. Stacking faults are found in defects, thus modulating electronic structure and transforming electrocatalytic-inert WO3 into highly active electrocatalysts. Density functional theory (DFT) calculations are performed to calculate *H adsorption energies on various WOx surfaces, revealing the oxygen vacancy composition and strain predicted to optimize the catalytic activity of hydrogen evolution reaction (HER). Such defective tungsten oxides can be integrated into commercial proton exchange membrane (PEM) electrolyser with comparable performance toward Pt-based PEM. This work demonstrates defective metal oxides as promising non-noble metal catalysts for commercial PEM green-hydrogen generation.

Controlled synthesis of van der Waals CoS2for improved p-type transistor contact.

Two-dimensional (2D) van der Waals (vdW) p-type semiconductors have shown attractive application prospects as atomically thin channels in electronic devices. However, the high Schottky hole barrier of p-type semiconductor-metal contacts induced by Fermi-level pinning is hardly removed. Herein, we prepare a vdW 1T-CoS2nanosheet as the contact electrode of a WSe2field-effect transistor (FET), which shows a considerably high on/off ratio > 107and a hole mobility of ∼114.5 cm2V-1s-1. The CoS2nanosheets exhibit metallic conductivity with thickness dependence, which surpasses most 2D transition metal dichalcogenide metals or semimetals. The excellent FET performance of the CoS2-contacted WSe2FET device can be attributed to the high work function of CoS2, which lowers the Schottky hole barrier. Our work provides an effective method for growing vdW CoS2and opens up more possibilities for the application of 2D p-type semiconductors in electronic devices.

Stabilizing Cobalt Single Atoms via Flexible Carbon Membranes as Bifunctional Electrocatalysts for Binder-Free Zinc-Air Batteries.

Single-atom catalysts with high activity and efficient atom utilization have great potential in the electrocatalysis field, especially for rechargeable zinc-air batteries (ZABs). However, it is still a serious challenge to rationally construct a single-atom catalyst with satisfactory electrocatalytic activity and long-term stability. Here, we simultaneously realize the atomic-level dispersion of cobalt and the construction of carbon nanotube (CNT)-linked N-doped porous carbon nanofibers (NCFs) via an electrospinning strategy. In this hierarchical structure, the Co-N4 sites provide efficient oxygen reduction/evolution electrocatalytic activity, the porous architectures of NCFs guarantee the active site's accessibility, and the interior CNTs enhance the flexibility and mechanical strength of porous fibers. As a binder-free air cathode, the as-prepared catalysts deliver superdurability of 600 h at 10 mA cm-2 for aqueous ZABs and considerable flexibility and a small voltage gap for all-solid-state ZABs. This work provides an effective single-atom design/nanoengineering for superdurable zinc-air batteries.

Operando Direct Observation of Stable Water-Oxidation Intermediates on Ca2-xIrO4 Nanocrystals for Efficient Acidic Oxygen Evolution.

We report Ca2-xIrO4 nanocrystals exhibit record stability of 300 h continuous operation and high iridium mass activity (248 A gIr-1 at 1.5 VRHE) that is about 62 times that of benchmark IrO2. Lattice-resolution images and surface-sensitive spectroscopies demonstrate the Ir-rich surface layer (evolved from one-dimensional connected edge-sharing [IrO6] octahedrons) with high relative content of Ir5+ sites, which is responsible for the high activity and long-term stability. Combining operando infrared spectroscopy with X-ray absorption spectroscopy, we report the first direct observation of key intermediates absorbing at 946 cm-1 (Ir6+═O site) and absorbing at 870 cm-1 (Ir6+OO- site) on iridium-based oxides electrocatalysts, and further discover the Ir6+═O and Ir6+OO- intermediates are stable even just from 1.3 VRHE. Density functional theory calculations indicate the catalytic activity of Ca2IrO4 is enhanced remarkably after surface Ca leaching, and suggest IrOO- and Ir═O intermediates can be stabilized on positive charged active sites of Ir-rich surface layer.

Identification of the Active-Layer Structures for Acidic Oxygen Evolution from 9R-BaIrO3 Electrocatalyst with Enhanced Iridium Mass Activity.

Iridium-based perovskites show promising catalytic activity for oxygen evolution reaction (OER) in acid media, but the iridium mass activity remains low and the active-layer structures have not been identified. Here, we report highly active 1 nm IrOx particles anchored on 9R-BaIrO3 (IrOx/9R-BaIrO3) that are directly synthesized by solution calcination followed by strong acid treatment for the first time. The developed IrOx/9R-BaIrO3 catalyst delivers a high iridium mass activity (168 A gIr-1), about 16 times higher than that of the benchmark acidic OER electrocatalyst IrO2 (10 A gIr-1), and only requires a low overpotential of 230 mV to reach a catalytic current density of 10 mA cm-2geo. Careful scanning transmission electron microscopy, synchrotron radiation-based X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy analyses reveal that, during the electrocatalytic process, the initial 1 nm IrOx nanoparticles/9R-BaIrO3 evolve into amorphous Ir4+OxHy/IrO6 octahedrons and then to amorphous Ir5+Ox/IrO6 octahedrons on the surface. Such high relative content of amorphous Ir5+Ox species derived from trimers of face-sharing IrO6 octahedrons in 9R-BaIrO3 and the enhanced metallic conductivity of the Ir5+Ox/9R-BaIrO3 catalyst are responsible for the excellent acidic OER activity. Our results provide new insights into the surface active-layer structure evolution in perovskite electrocatalysts and demonstrate new approaches for engineering highly active acidic OER nanocatalysts.

Single-Atom-Layer Catalysis in a MoS2 Monolayer Activated by Long-Range Ferromagnetism for the Hydrogen Evolution Reaction: Beyond Single-Atom Catalysis.

Single-atom-layer catalysts with fully activated basal-atoms will provide a solution to the low loading-density bottleneck of single-atom catalysts. Herein, we activate the majority of the basal sites of monolayer MoS2 , by doping Co ions to induce long-range ferromagnetic order. This strategy, as revealed by in situ synchrotron radiation microscopic infrared spectroscopy and electrochemical measurements, could activate more than 50 % of the originally inert basal-plane S atoms in the ferromagnetic monolayer for the hydrogen evolution reaction (HER). Consequently, on a single monolayer of ferromagnetic MoS2 measured by on-chip micro-cell, a current density of 10 mA cm-2 could be achieved at the overpotential of 137 mV, corresponding to a mass activity of 28, 571 Ag-1 , which is two orders of magnitude higher than the multilayer counterpart. Its exchange current density of 75 µA cm-2 also surpasses most other MoS2 -based catalysts. Experimental results and theoretical calculations show the activation of basal plane S atoms arises from an increase of electronic density around the Fermi level, promoting the H adsorption ability of basal-plane S atoms.

Intrinsic Ferromagnetism in Mn-Substituted MoS2 Nanosheets Achieved by Supercritical Hydrothermal Reaction.

Doping atomically thick nanosheets is a great challenge due to the self-purification effect that drives the precipitation of dopants. Here, a breakthrough is made to dope Mn atoms substitutionally into MoS2 nanosheets in a sulfur-rich supercritical hydrothermal reaction environment, where the formation energy of Mn substituting for Mo sites in MoS2 is significantly reduced to overcome the self-purification effect. The substitutional Mn doping is convincingly evidenced by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine spectroscopy characterizations. The Mn-doped MoS2 nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.05 µB Mn-1 at room temperature. The intrinsic ferromagnetism is further confirmed by the reversibility of the magnetic behavior during the cycle of incorporating/removing Li codopants, showing the critical role of Mn 3d electronic states in mediating the magnetic interactions in MoS2 nanosheets.

Realizing Room-Temperature Ferromagnetism in Molecular-Intercalated Antiferromagnet VOCl.

Two-dimensional (2D) van der Waals (vdW) magnets are gaining attention in fundamental physics and advanced spintronics, due to their unique dimension-dependent magnetism and potential for ultra-compact integration. However, achieving intrinsic ferromagnetism with high Curie temperature (TC) remains a technical challenge, including preparation and stability issues. Herein, we develop an applicable electrochemical intercalation strategy to decouple interlayer interaction and guide charge doping in antiferromagnet VOCl, thereby inducing robust room-temperature ferromagnetism. The expanded vdW gap isolates the neighboring layers and shrinks the distance between the V-V bond, favoring the generation of ferromagnetic (FM) coupling with perpendicular magnetic anisotropy (PMA). Element-specific X-ray magnetic circular dichroism (XMCD) directly proves the source of the ferromagnetism. Detailed experimental results and density functional theory (DFT) calculations indicate that the charge doping enhances the FM interaction by promoting the orbital hybridization between t2 g and eg. Our work sheds new light on a promising way to achieve room-temperature ferromagnetism in antiferromagnets, thus addressing the critical materials demand for designing spintronic devices. This article is protected by copyright. All rights reserved.

Probing the Néel-Type Antiferromagnetic Order and Coherent Magnon-Exciton Coupling in Van Der Waals VPS3.

2D van der Waals (vdW) antiferromagnets have received intensive attention due to their terahertz resonance, multilevel magnetic-order states, and ultrafast spin dynamics. However, accurately identifying their magnetic configuration still remains a challenge owing to the lack of net magnetization and insensitivity to external fields. In this work, the Néel-type antiferromagnetic (AFM) order in 2D antiferromagnet VPS3 with the out-of-plane anisotropy, which is demonstrated by the temperature-dependent spin-phonon coupling and second-harmonic generation (SHG), is experimentally probed. This long-range AFM order even persists at the ultrathin limit. Furthermore, strong interlayer exciton-magnon coupling (EMC) upon the Néel-type AFM order is detected based on the monolayer WSe2 /VPS3 heterostructure, which induces an enhanced excitonic state and further certifies the Néel-type AFM order of VPS3 . The discovery provides optical routes as the novel platform to study 2D antiferromagnets and promotes their potential applications in magneto-optics and opto-spintronic devices.

Intrinsic room-temperature ferromagnetism in a two-dimensional semiconducting metal-organic framework.

The development of two-dimensional (2D) magnetic semiconductors with room-temperature ferromagnetism is a significant challenge in materials science and is important for the development of next-generation spintronic devices. Herein, we demonstrate that a 2D semiconducting antiferromagnetic Cu-MOF can be endowed with intrinsic room-temperature ferromagnetic coupling using a ligand cleavage strategy to regulate the inner magnetic interaction within the Cu dimers. Using the element-selective X-ray magnetic circular dichroism (XMCD) technique, we provide unambiguous evidence for intrinsic ferromagnetism. Exhaustive structural characterizations confirm that the change of magnetic coupling is caused by the increased distance between Cu atoms within a Cu dimer. Theoretical calculations reveal that the ferromagnetic coupling is enhanced with the increased Cu-Cu distance, which depresses the hybridization between 3d orbitals of nearest Cu atoms. Our work provides an effective avenue to design and fabricate MOF-based semiconducting room-temperature ferromagnetic materials and promotes their practical applications in next-generation spintronic devices.

Engineering a local acid-like environment in alkaline medium for efficient hydrogen evolution reaction.

Tuning the local reaction environment is an important and challenging issue for determining electrochemical performances. Herein, we propose a strategy of intentionally engineering the local reaction environment to yield highly active catalysts. Taking Ptδ- nanoparticles supported on oxygen vacancy enriched MgO nanosheets as a prototypical example, we have successfully created a local acid-like environment in the alkaline medium and achieve excellent hydrogen evolution reaction performances. The local acid-like environment is evidenced by operando Raman, synchrotron radiation infrared and X-ray absorption spectroscopy that observes a key H3O+ intermediate emergence on the surface of MgO and accumulation around Ptδ- sites during electrocatalysis. Further analysis confirms that the critical factors of the forming the local acid-like environment include: the oxygen vacancy enriched MgO facilitates H2O dissociation to generate H3O+ species; the F centers of MgO transfers its unpaired electrons to Pt, leading to the formation of electron-enriched Ptδ- species; positively charged H3O+ migrates to negatively charged Ptδ- and accumulates around Ptδ- nanoparticles due to the electrostatic attraction, thus creating a local acidic environment in the alkaline medium.

Intrinsic Room-Temperature Ferromagnetism in V2C MXene Nanosheets.

Two-dimensional (2D) materials with intrinsic magnetic properties are intensively explored due to their potential applications in low-power-consumption electronics and spintronics. To date, only a handful of intrinsic magnetic 2D materials have been reported. Here, we report a realization of intrinsic ferromagnetic behavior in 2D V2C MXene nanosheets through layer mismatch engineering. The V2C MXene nanosheets with a small-angle twisting show a robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.013 emu/g at room temperature. An in-depth study has been performed by X-ray absorption spectroscopy as well as electron paramagnetic resonance (EPR) and photoelectron spectroscopy analyses. It has been revealed that the symmetry-broken interlayer twisting reduced the degeneracy of V 3d states and the van Hove singularity. This led to a redistribution of the density of electronic states near the Fermi level and consequently activated the Stoner ferromagnetism with improved density of itinerant d electrons. This work highlights V2C MXene as a promising intrinsic room-temperature ferromagnetic material with potential applications in spintronics or spin-based electronics.

Embedding atomic cobalt into graphene lattices to activate room-temperature ferromagnetism.

Graphene is extremely promising for next-generation spintronics applications; however, realizing graphene-based room-temperature magnets remains a great challenge. Here, we demonstrate that robust room-temperature ferromagnetism with TC up to ∼400 K and saturation magnetization of 0.11 emu g-1 (300 K) can be achieved in graphene by embedding isolated Co atoms with the aid of coordinated N atoms. Extensive structural characterizations show that square-planar Co-N4 moieties were formed in the graphene lattices, where atomically dispersed Co atoms provide local magnetic moments. Detailed electronic structure calculations reveal that the hybridization between the d electrons of Co atoms and delocalized pz electrons of N/C atoms enhances the conduction-electron mediated long-range magnetic coupling. This work provides an effective means to induce room-temperature ferromagnetism in graphene and may open possibilities for developing graphene-based spintronics devices.

Interface electron transfer and thickness dependent transport characteristics of La0.7Sr0.3VO3 thin films.

La0.7Sr0.3VO3 (LSVO) thin films, 5-30 unit cells (u.c.) in thickness, have been epitaxially deposited on (0 0 1) SrTiO3 (STO) single crystal substrates. Although LSVO is metallic in bulk, insulating behavior is observed, from 2 to 390 K, in LSVO films less than 9 u.c. in thickness, while thicker films show a metal-insulator transition with the critical temperature increasing with the decrease of film thickness. X-ray absorption spectra reveal a charge transfer across the LSVO/STO interface for a continuous increase of V valence in LSVO, as well as a decrease of Ti valence in interfacial STO, with the LSVO film thickness increases. The transport characteristics are discussed in terms of enhanced electron localization due to the reduction of film thickness and V 3d band filling induced by the charge transfer.

Interlayer Photoelectron Transfer Boosted by Bridged RuIV Atoms in GaS Nanosheets for Efficient Water Splitting.

Photocatalytic water splitting over layered nanosheet (NS) catalysts has caught a lot of attention for renewable hydrogen fuel production. However, the weak van der Waals interlayer interactions make it a great challenge to realize an effective dissociation of photogenerated excitons and efficient charge transfer across the interior of layered catalysts during the photocatalysis process. Here, we propose an intercalation strategy of high-valence RuIV atoms to render two-dimensional GaS NS photocatalysts with rapid electron-hole dissociation and long photocarrier lifetime in visible-light-driven water splitting. Experimental and theoretical results unravel that the intercalated single-site Ru, confined in interlayer of GaS NSs, with a hexagonal structural configuration of "Ru1-S6", can serve as an electron-trapped high-speed channel toward simultaneously accelerating electron-hole pairs dissociation and promoting photoelectron transportation through the van der Waals interlayer. Consequently, the as-developed Ru-intercalated GaS NSs can give a notable H2 production rate of 340 µmol g-1 h-1 under visible-light irradiation and an apparent yield of 7% at 420 nm, 38 times that of pure GaS NSs. This study opens up a feasible way for a new design of highly active layered photocatalysts toward high-efficiency solar energy conversion.

Synergetic Effect of Substitutional Dopants and Sulfur Vacancy in Modulating the Ferromagnetism of MoS2 Nanosheets.

The activation and modulation of the magnetism of MoS2 nanosheets are critical to the development of their application in next-generation spintronics. Here, we report a synergetic strategy to induce and modulate the ferromagnetism of the originally nonmagnetic MoS2 nanosheets. A two-step experimental method was used to simultaneously introduce substitutional V dopants and sulfur vacancy (Vs) in the MoS2 nanosheet host, showing an air-stable and adjustable ferromagnetic response at room temperature. The ferromagnetism could be modulated by varying the content of Vs through Ar plasma irradiation of different periods, with a maximum saturation magnetization of 0.011 emu g-1 reached at the irradiation time of 6 s (s). Experimental characterizations and first-principles calculations suggest that the adjustable magnetization is attributed to the synergetic effect of the substitutional V dopants and Vs in modulating the band structure of MoS2 nanosheets, resulting from the strong hybridization between the V 3d state and the Vs-induced impurity bands. This work suggests that the synergetic effect of substitutional V atoms and Vs is a promising route for tuning the magnetic interactions in two-dimensional nanostructures.

Beating the exclusion rule against the coexistence of robust luminescence and ferromagnetism in chalcogenide monolayers.

Monolayer chalcogenide semiconductors with both luminescent and ferromagnetic properties are dreamed for simultaneous polarization and detection of the valley degree of freedom in valleytronics. However, a conventional chalcogenide monolayer lacks these coexisting properties due to their mutually exclusive origins. Herein we demonstrate that robust ferromagnetism and photoluminescence (PL) could be achieved in a (Co, Cr)-incorporated single monolayer MoS2, where the ferromagnetic interaction is activated by Co ions, and the nonradiative recombination channels of excitons is cut off by Cr ions. This strategy brings a 90-fold enhancement of saturation magnetization and 35-fold enhancement of PL intensity than the pristine MoS2 monolayer. The main reasons for the coexisting ferromagnetism and PL are the electronic interactions between the impurity bands of atop Cr adatoms and substitutional Co atoms, as well as the increased content of neutral exciton. Our findings could extend the applications of two-dimensional chalcogenides into spintronics, valleytronic and photoelectric devices.

Efficient oxygen evolution electrocatalysis in acid by a perovskite with face-sharing IrO6 octahedral dimers.

The widespread use of proton exchange membrane water electrolysis requires the development of more efficient electrocatalysts containing reduced amounts of expensive iridium for the oxygen evolution reaction (OER). Here we present the identification of 6H-phase SrIrO3 perovskite (6H-SrIrO3) as a highly active electrocatalyst with good structural and catalytic stability for OER in acid. 6H-SrIrO3 contains 27.1 wt% less iridium than IrO2, but its iridium mass activity is about 7 times higher than IrO2, a benchmark electrocatalyst for the acidic OER. 6H-SrIrO3 is the most active catalytic material for OER among the iridium-based oxides reported recently, based on its highest iridium mass activity. Theoretical calculations indicate that the existence of face-sharing octahedral dimers is mainly responsible for the superior activity of 6H-SrIrO3 thanks to the weakened surface Ir-O binding that facilitates the potential-determining step involved in the OER (i.e., O* + H2O → HOO* + H+ + e¯).

Reversible Tuning of the Ferromagnetic Behavior in Mn-Doped MoS2 Nanosheets via Interface Charge Transfer.

Reversible manipulation of the magnetic behavior of two-dimensional van der Waals crystals is crucial for expanding their applications in spin-based information-processing technologies. However, to date, most experimental approaches to tune the magnetic properties are single way and have very limited practical applications. Here, we report an interface charge-transfer method for obtaining a reversible and air-stable magnetic response at room temperature in Mn-doped MoS2 nanosheets. By adsorption of benzyl viologen (BV) molecules as the charge donor, the saturation magnetization of Mn-doped MoS2 nanosheets is enhanced by a magnitude of 60%, and the magnetization can be restored to the original value when the adsorbed BV molecules are removed. This cycle can be repeated many times on the same sample without detectable degradation. Experimental characterizations and first-principles calculations suggest that the enhanced magnetization can be attributed to the increase of Mn magnetic moment because of the enriched electrons transferred from BV molecules. This work shows that interface charge transfer may open up a new pathway for reversibly tuning the exchange interactions in two-dimensional nanostructures.