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Antiferromagnets exhibit high-speed spin responses in the terahertz frequency range and robustness against external magnetic fields, making them promising for nextgeneration high-speed, high-density spintronic devices. Recently, two-dimensional van der Waals magnetic systems, which possess rich antiferromagnetic ground states, have gained significant attention and serve as ideal platforms for studying low-dimensional antiferromagnetic physics. Detecting and controlling ultrafast spin dynamics in two-dimensional antiferromagnetic systems will lay the foundation for high-speed spintronic device applications. Antiferromagnets have no net macroscopic magnetization, making traditional optical methods, such as magneto-optical effects, challenging for detecting antiferromagnetic order in equilibrium states. However, in non-equilibrium states, the instantaneous magnetization generated by antiferromagnetic spin dynamics allows the use of time-resolved magneto-optical Kerr effect to detect coherent spin precession in antiferromagnets. Additionally, techniques such as linear dichroism spectroscopy, terahertz emission spectroscopy, and second harmonic generation have been employed in studying the dynamics of two-dimensional antiferromagnets. This paper introduces recent experimental progress on ultrafast spin dynamics in two-dimensional van der Waals antiferromagnetic systems, and briefly describes the coherent magnon excitations and corresponding mechanisms in two-dimensional antiferromagnets, including inverse magneto-optical effects/stimulated Raman scattering, orbital excitations, and exciton coupling. Furthermore, this paper discusses the critical slowing down of spin dynamics due to spin-lattice coupling effects and the amplification of coherent acoustic phonons. [ABSTRACT FROM AUTHOR]

Controlling light at the nanoscale by exploiting ultra‐confined polaritons—hybrid light and matter waves—in various van der Waals (vdW) materials empowers unique opportunities for many nanophotonic on‐chip technologies. So far, mainstream approaches have relied on interfacial techniques (e.g., refractive optics, meta‐optics, and moire engineering) to manipulate the polariton wavefront. Here, it is proposed that orbital angular momentum (OAM) of incident light can offer a new degree of freedom to structure vdW polaritons. With vortex excitations, a new class of accelerating polariton waves is observed—Airy‐like hyperbolic phonon polaritons (PhPs) in high‐symmetry orthorhombic vdW crystal α‐MoO3. In analogous to the well‐known Airy beams in free space, such Airy‐like PhPs also exhibit self‐accelerating, nonspreading, and self‐healing characteristics. Interestingly, the helical phase gradient of the vortex beam leads to asymmetry excitation of polaritons, as a result, the Airy‐like PhPs possess asymmetric propagation features even with a symmetric mode, analogous to the asymmetry hyperbolic shear polaritons in low‐symmetry crystals. The finding highlights the potential of OAM to manipulate polaritons in vdW materials, which can be further extended into a variety of applications such as active structured polaritonic devices. [ABSTRACT FROM AUTHOR]

Benefited from the lower dimensionality compared to their 3D counterpart, 2D flat‐band systems provide cleaner lattice models, easier experimental verification, and higher tunability, which make the 2D van der Waals (vdW) system an ideal playground for exploring flat‐band physics as well as their potential applications. Given the vast amount of research in the field of flat bands, a simple and efficient approach to search for realistic vdW materials with flat bands is still missing. Here, a two‐tier framework to filter and diagnose high‐quality flat‐band vdW materials by combining high‐throughput first‐principles calculations and the proposed 2D flat‐band score criterion is presented. Based on systematic geometrical analysis, 861 potential monolayer vdW materials are initially obtained amounting to 187,093 structures as stored in the Inorganic Crystal Structure Database. By applying the 2D flat‐band score criterion, 229 flat‐band candidates are efficiently identified, among which a sub‐catalog of 74 materials with flat bands right next to the Fermi level is further provided to facilitate experimental verification. All these efforts to screen experimentally available flat‐band candidates will certainly motivate continuing exploration toward the realization of this class of special materials and their applications in material science. [ABSTRACT FROM AUTHOR]

Amorphous solids are non‐equilibrium states of matter that lack long‐range structural order, and are recognized as key materials in electronics. Although 2D van der Waals materials have been intensively studied in the wide field of materials science, their amorphous states have been less studied experimentally. Here, a van der Waals ferromagnetic semiconductor CrGeTe3 transforms into an amorphous ferromagnetic metal upon irradiation with high‐energy Xe ions. Notably, the Curie temperature of the amorphous state reaches 200 K, which is three times higher than that of the crystalline phase. The anomalous Hall conductivity in the amorphous phase is governed by the extrinsic skew‐scattering mechanism, although conventional theory predicts that skew scattering is dominant only in ultra‐clean ferromagnetic metals. The present results call for a new theory of the anomalous Hall effect in highly disordered ferromagnetic conductors. Moreover, the unique change in magnetic and transport properties due to amorphization of the van der Waals material is expected to open up a new research field in materials science. [ABSTRACT FROM AUTHOR]

Negative refraction has a wide range of applications in diverse fields such as imaging, sensing, and waveguides and typically entails the fabrication of intricate metamaterials endowed with hyperbolic features. In contrast to artificially engineered hyperbolic materials, natural van der Waals (vdW) materials are more accessible owing to their inherent strong in-plane covalent bonding and weak interlayer interactions. However, most vdW materials manifest uniaxial crystal properties, which restrict their behavior solely to out-of-plane hyperbolicity. This characteristic poses a considerable challenge to their seamless integration via planar fabrication techniques, unless a suitable pattern is employed. Recent advances have identified natural biaxial α-MoO3 as a promising vdW material capable of exhibiting in-plane hyperbolicity. In this study, we performed numerical simulations demonstrating that negative refraction could be achieved by interfacing differently oriented α-MoO3 slabs coated with tunable graphene on a gold substrate. Our comprehensive analysis yielded three notable outcomes: negative refraction, simultaneous positive and negative refractions, and diffractionless propagation. These outcomes could be operated in a broad range of frequencies and achieved at all angles to offer a superior platform for the flexible manipulation of mid-infrared polaritons. Our findings provide valuable insights into the potential application of other two-dimensional vdW materials for advances in nanoscale super-resolution imaging, molecular sensing, and on-chip photonic integrated circuits.

Abstract Amorphous solids are non‐equilibrium states of matter that lack long‐range structural order, and are recognized as key materials in electronics. Although 2D van der Waals materials have been intensively studied in the wide field of materials science, their amorphous states have been less studied experimentally. Here, a van der Waals ferromagnetic semiconductor CrGeTe3 transforms into an amorphous ferromagnetic metal upon irradiation with high‐energy Xe ions. Notably, the Curie temperature of the amorphous state reaches 200 K, which is three times higher than that of the crystalline phase. The anomalous Hall conductivity in the amorphous phase is governed by the extrinsic skew‐scattering mechanism, although conventional theory predicts that skew scattering is dominant only in ultra‐clean ferromagnetic metals. The present results call for a new theory of the anomalous Hall effect in highly disordered ferromagnetic conductors. Moreover, the unique change in magnetic and transport properties due to amorphization of the van der Waals material is expected to open up a new research field in materials science.

One-dimensional (1D) van der Waals (vdW) materials have garnered significant attention for their unique structures and properties, making them promising candidates for various applications in optoelectronics, catalysis, energy storage, and sensors. However, further advancements are required to fully exploit their potential as energy storage devices. This study presents an anode design paradigm utilizing 1D vdW-based V2PS10 crystals characterized by large vdW gaps, a small bandgap, and distinctive chemical compositions that favor lithium storage. To enhance their performance, we employ surface modification techniques to create V2PS10 @C composites, where conductive carbon is integrated with V2PS10 crystals. This modification significantly improves electron transport and Li-ion diffusion within the composite anodes. The resulting composite anodes exhibit exceptional electrochemical properties, including a high discharge capacity of 1154 mAh g−1 after the 50th cycle at 0.1 A g−1, superior rate performance with a current density of 561 mAh g−1 at 10 A g−1, and prolonged cycling stability, maintaining a capacity of 605 mAh g−1 for up to 800 cycles at 5 A g−1. These remarkable electrochemical performances underscore the immense potential of 1D vdW-based materials as next-generation anodes for lithium-ion batteries.

MnSb2Te4 has a similar structure to an emerging material, MnBi2Te4. According to earlier theoretical studies, the formation energy of Mn antisite defects in MnSb2Te4 is negative, suggesting its inherent instability. This is clearly in contrast to the successful synthesis of experimental samples of MnSb2Te4. Here, the growth environment of MnSb2Te4 and the intrinsic defects are correspondingly investigated. We find that the Mn antisite defect is the most stable defect in the system, and a Mn-rich growth environment favors its formation. The thermodynamic equilibrium concentrations of the Mn antisite defects could be as high as 15% under Mn-poor conditions and 31% under Mn-rich conditions. It is also found that Mn antisite defects prefer a uniform distribution. In addition, the Mn antisite defects can modulate the interlayer magnetic coupling in MnSb2Te4, leading to a transition from the ideal antiferromagnetic ground state to a ferromagnetic state. The ferromagnetic coupling effect can be further enhanced by controlling the defect concentration. [ABSTRACT FROM AUTHOR]

Metal thio(seleno) phosphates MPX3 attract considerable attention with potential application in spintronics and other types of devices. Here, ferromagnetism in an Ising type diluted magnetic semiconductor Fe1‐xZnxPS3 is reported. Bulk single crystals of Fe1‐xZnxPS3 have been synthesized via chemical vapor transport method and characterized by X‐ray diffraction. The weak ferromagnetism associated with spin/cluster spin glass emerges at 30% Zn doping. A nearly static ferromagnetic order is observed in the highly diluted magnets Fe0.1Zn0.9PS3. X‐ray photoemission measurements indicate introduction of holes and associated Fe3+ in samples with ferromagnetic order. Density functional theory calculations confirm the Ising‐type ferromagnetic state in highly diluted compounds with hole doping. The interplay between bound magnetic polarons induces impurity‐band‐exchange and superexchange interactions can explain the evolution of magnetism with Zn doping. The findings suggest that the magnetic diluted FePS3 system is a candidate for potential applications in magnetic devices, and its quantum mechanism should be explored by using microscopic techniques in the near future. [ABSTRACT FROM AUTHOR]

Quantum emitters in solid-state materials are highly promising building blocks for quantum information processing and communication science. Recently, single-photon emission from van der Waals materials has been reported in transition metal dichalcogenides and hexagonal boron nitride, exhibiting the potential to realize photonic quantum technologies in two-dimensional materials. Here, we report the generation of room temperature single-photon emission from exfoliated and thermally annealed single crystals of van der Waals α-MoO 3 . The second-order correlation function measurement displays clear photon antibunching, while the luminescence intensity exceeds 0.4 Mcts/s and remains stable under laser excitation. The theoretical calculation suggests that an oxygen vacancy defect is a possible candidate for the observed emitters. Together with photostability and brightness, quantum emitters in α-MoO 3 provide a new avenue to realize photon-based quantum information science in van der Waals materials.

The electric dipole in materials is closely associated with their electronic transport, optical properties, and mechanical behavior. Here, we have employed the differential phase contrast (DPC) technique of the scanning transmission electron microscopy technique (STEM) to directly analyze the local electric dipole at the sub-Angstrom scale. By utilizing DPC-STEM technology, we successfully visualized the ferroelectric polarization of van der Waals material 3R α-In 2 Se 3 and directly confirmed the dipole interlocking effect (DIE) between in-plane (IP) and out-of-plane (OOP) polarizations. Through density functional theory (DFT) calculations and structural analysis, we discovered that this DIE is caused by the central asymmetry of the middle Se atoms of each monolayer and that the reversal of polarization is accompanied by the emergence of an intermediate phase, β-In 2 Se 3 . Leveraging the DIE, we developed a multidirectional ferroelectric memristor that can effectively modulate the IP polarization by applying an OOP pulse voltage.

The bulk van der Waals (vdW) single‐crystalline indium selenide (β‐InSe) displays exceptional plasticity in a layered crystalline form at both micro‐ and macroscale. However, the nanoscale origin of plasticity remains unclear. Herein, an atomic‐level study on the deformation mechanisms of InSe by using first‐principles calculations is reported. Remarkable anisotropic elasticity is observed in the vdW InSe layered crystal, and the stiffness is dramatically softened because of the vdW gap in the layered structure. The simulations capture the distinct fracture modes in the uniaxial tensile deformation, depending on the loading directions—brittle fracture in the [100] and [110] directions while ductile failure in the [001] direction. The InSe layered crystal structure exhibits superplastic deformability under uniaxial compression. Different transition pathways, including interlayer tangling, amorphization, and cross‐linking, are tracked along respective deformation directions. The unprecedented plasticity of InSe layered crystals can be attributed to the phase transition coupled with interlayer gliding and cross‐layer dislocation slipping. This study deepens our understanding of the deformation mechanisms of layered materials at the atomistic level and provides insights into tailoring material properties for low‐dimensional material design based on its deformation mechanisms. [ABSTRACT FROM AUTHOR]

Van der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low‐dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high‐mobility charge carriers, which is an essential requirement for high‐speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, TaCo2Te2 is introduced as an air‐stable, high‐mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. It is found that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after 4 months of monitoring using an optical microscope. Signatures of canted antiferromagntic behavior are also observed at low‐temperature. TaCo2Te2 shows a metallic character and a large, nonsaturating, anisotropic magnetoresistance. Furthermore, the Hall data and quantum oscillation measurements reveal the presence of both electron‐ and hole‐type carriers and their high mobility. [ABSTRACT FROM AUTHOR]

Excitons in semiconductors and molecules are widely utilized in photovoltaics and optoelectronics, and high-temperature coherent quantum states of excitons can be realized in artificial electron-hole bilayers and an exotic material of an excitonic insulator (EI). Here, we investigate the band gap evolution of a putative high-temperature EI Ta 2 NiSe 5 upon surface doing by alkali adsorbates with angle-resolved photoemission and density functional theory (DFT) calculations. The conduction band of Ta 2 NiSe 5 is filled by the charge transfer from alkali adsorbates, and the band gap decreases drastically upon the increase of metallic electron density. Our DFT calculation, however, reveals that there exist both structural and excitonic contributions to the band gap tuned. While electron doping reduces the band gap substantially, it alone is not enough to close the band gap. In contrast, the structural distortion induced by the alkali adsorbate plays a critical role in the gap closure. This work indicates a combined electronic and structural nature for the EI phase of the present system and the complexity of surface doping beyond charge transfer.

Adsorption of alkali atoms onto material surfaces is widely utilized for controlling electronic properties and is particularly effective for two-dimensional materials. While tuning the chemical potential and band gap and creating quantum-confined states are well established for alkali adsorption on semiconductors, the effects on semimetallic systems remain largely elusive. Here, utilizing angle-resolved photoemission spectroscopy measurements and density functional theory calculations, we disclose the creation of two-dimensional electron gas and the quantum-confined Lifshitz transition at the surface of a Weyl semimetal T d -MoTe 2 by potassium adsorption. Electrons from potassium adatoms are shown to be transferred mainly to the lowest unoccupied band within the gapped part of the Brillouin zone, which, in turn, induces strong surface band bending and quantum confinement in the topmost layer. The quantum-confined topmost layer evolves from a semimetal to a strong metal with a Lifshitz transition departing substantially from the bulk band. The present finding and its underlying mechanism can be exploited for the creation of electronic heterojunctions in van der Waals semimetals.

The dependence of electromechanical behavior on strain in ferroelectric materials can be leveraged as parameter to tune ferroelectric properties such as the Curie temperature. For van der Waals materials, a unique opportunity arises because of wrinkling, bubbling, and Moiré phenomena accessible due to structural properties inherent to the van der Waals gap. Here, we use piezoresponse force microscopy and unsupervised machine learning methods to gain insight into the ferroelectric properties of layered CuInP2S6 where local areas are strained in-plane due to a partial delamination, resulting in a topographic bubble feature. We observe significant differences between strained and unstrained areas in piezoresponse images as well as voltage spectroscopy, during which strained areas show a sigmoid-shaped response usually associated with the response measured around the Curie temperature, indicating a lowering of the Curie temperature under tensile strain. These results suggest that strain engineering might be used to further increase the functionality of CuInP2S6 through locally modifying ferroelectric properties on the micro- and nanoscale. [ABSTRACT FROM AUTHOR]

The van der Waals semiconductor Bi 4 O 4 SeCl 2 has recently attracted great interest due to its extremely small lattice thermal conductivity, which may find possible application in the field of energy conversion. Herein, we accurately predict the thermoelectric transport properties of Bi 4 O 4 SeCl 2 using first-principles calculations and Boltzmann transport theory, where the carrier relaxation time is obtained by fully considering the electron-phonon coupling. It is found that a maximum p -type ZT value of 3.1 can be reached at 1100 K along the in-plane direction, which originates from increased Seebeck coefficient induced by multivalley band structure, as well as enhanced electrical conductivity caused by relatively stronger intralayer bonding. Besides, it is interesting to note that comparable p - and n -type ZT values can be realized in certain temperature regions, which is very desirable in the fabrication of thermoelectric modules.

Magnetism in two dimensions is traditionally considered an exotic phase mediated by spin fluctuations, but far from collinearly ordered in the ground state. Recently, 2D magnetic states have been discovered in layered van der Waals compounds. Their robust and tunable magnetic state by material composition, combined with reduced dimensionality, foresee a strong potential as a key element in magnetic devices. Here, a class of 2D magnets based on metallic chlorides is presented. The magnetic order survives on top of a metallic substrate, even down to the monolayer limit, and can be switched from perpendicular to in-plane by substituting the metal ion from iron to nickel. Using functionalized STM tips as magnetic sensors, local exchange fields are identified, even in the absence of an external magnetic field. Since the compounds are processable by molecular beam epitaxy techniques, they provide a platform with large potential for incorporation into current device technologies., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)

Herein, a high-quality gate stack (native HfO 2 formed on 2D HfSe 2 ) fabricated via plasma oxidation is reported, realizing an atomically sharp interface with a suppressed interface trap density (D it ≈ 5 × 10 10 cm -2 eV -1 ). The chemically converted HfO 2 exhibits dielectric constant, κ ≈ 23, resulting in low gate leakage current (≈10 -3 A cm -2 ) at equivalent oxide thickness ≈0.5 nm. Density functional calculations indicate that the atomistic mechanism for achieving a high-quality interface is the possibility of O atoms replacing the Se atoms of the interfacial HfSe 2 layer without a substitution energy barrier, allowing layer-by-layer oxidation to proceed. The field-effect-transistor-fabricated HfO 2 /HfSe 2 gate stack demonstrates an almost ideal subthreshold slope (SS) of ≈61 mV dec -1 (over four orders of I DS ) at room temperature (300 K), along with a high I on /I off ratio of ≈10 8 and a small hysteresis of ≈10 mV. Furthermore, by utilizing a device architecture with separately controlled HfO 2 /HfSe 2 gate stack and channel structures, an impact ionization field-effect transistor is fabricated that exhibits n-type steep-switching characteristics with a SS value of 3.43 mV dec -1 at room temperature, overcoming the Boltzmann limit. These results provide a significant step toward the realization of post-Si semiconducting devices for future energy-efficient data-centric computing electronics., (© 2024 The Authors. Advanced Materials published by Wiley‐VCH GmbH.)

Strain solitons are quasi-dislocations that form in van der Waals materials to relieve the energy associated with lattice or rotational mismatch. Novel electronic properties of strain solitons were predicted and observed. To date, strain solitons have been observed only in exfoliated crystals or mechanically strained crystals. The lack of a scalable approach toward the generation of strain solitons poses a significant challenge in the study of and use of their properties. Here, we report the formation of strain solitons with epitaxial growth of bismuth on InSb(111)B by molecular beam epitaxy. The morphology of the strain solitons for films of varying thickness is characterized with scanning tunneling microscopy, and the local strain state is determined from atomic resolution images. Bending in the solitons is attributed to interactions with the interface, and large angle bending is associated with edge dislocations. Our results enable the scalable generation of strain solitons.

The edges of van der Waals materials exhibit unique physical and chemical properties, and they are promising for applications in many fields, such as optoelectronics, energy storage, and catalysis. Van der Waals material nanostructures with a controllable high density of edges are difficult to produce by current fabrication methods. In the present study, a simple nanomachining process to fabricate van der Waals nanowires with a high density of edges is proposed. This method used a linear-edge diamond tool to cut the basal plane of van der Waals film materials into a one-dimensional nanowire at the nanoscale. Experimental tests were performed to investigate the influences of the cutting thickness, film thickness, cutting direction, and material properties on the machining outcomes. The results showed that the van der Waals materials possessed low Young's moduli ranging from 24 to 238 GPa by cutting with a cutting thickness of larger than 30 nm, and the out-of-plane cutting direction led to the best machining quality and controllable preparation of van der Waals nanowires. To support the interpretation of the process outcomes, molecular dynamics simulation and transmission electron microscopy were performed to reveal the material-removal mechanism during nanocutting of van der Waals materials. From analysis of the chip-deformation process, interlayer slipping was found to dominate the plastic processing of the van der Waals materials, accompanied by intralayer bending and intralayer fracture in the out-of-plane cutting direction. By contrast, the brittle removal state occurred when cutting in the in-plane direction. This study provides important insights into the material-removal mechanism of van der Waals materials prepared by nanoscale mechanical cutting. [Display omitted] • A new method to fabricate van der Waals nanowires with exposed edges is proposed. • The observation of slip steps reveals the chip formation mechanism dominated by interlayer slip for the first time. • The brittle fragments induced by the crack propagation are the main deformation forms in the in-plane cutting direction. • High-quality nanowires can be formed with moderate Young's modulus, small slipping and large cleavage energy of materials. [ABSTRACT FROM AUTHOR]

We report on the Tomonaga-Luttinger liquid (TLL) behavior in fully degenerate 1D Dirac Fermions. A ternary van der Waals material Nb 9 Si 4 Te 18 incorporates in-plane NbTe 2 chains, which produce a 1D Dirac band crossing Fermi energy. Tunneling conductance of electrons confined within NbTe 2 chains is found to be substantially suppressed at Fermi energy, which follows a power law with a universal temperature scaling, hallmarking a TLL state. The obtained Luttinger parameter of ∼0.15 indicates a strong electron-electron interaction. The TLL behavior is found to be robust against atomic-scale defects, which might be related to the Dirac electron nature. These findings, combined with the tunability of the compound and the merit of a van der Waals material, offer a robust, tunable, and integrable platform to exploit non-Fermi liquid physics.

A long-standing pursuit in materials science is to identify suitable magnetic semiconductors for integrated information storage, processing, and transfer. Van der Waals magnets have brought forth new material candidates for this purpose. Recently, sharp exciton resonances in antiferromagnet NiPS 3 have been reported to correlate with magnetic order, that is, the exciton photoluminescence intensity diminishes above the Néel temperature. Here, it is found that the polarization of maximal exciton emission rotates locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnet order hidden in previous neutron scattering and optical experiments. Furthermore, defect-bound states are suggested as an alternative exciton formation mechanism that has yet to be explored in NiPS 3 . The supporting evidence includes chemical analysis, excitation power, and thickness dependent photoluminescence and first-principles calculations. This mechanism for exciton formation is also consistent with the presence of strong phonon side bands. This study shows that anisotropic exciton photoluminescence can be used to read out local spin chain directions in antiferromagnets and realize multi-functional devices via spin-photon transduction., (© 2023 Wiley-VCH GmbH.)

Micromechanical exfoliation of two-dimensional (2D) van der Waals materials has triggered an explosive interest in 2D material research. The extension of this idea to 1D van der Waals materials, possibly opening a new arena for 1D material research, has not yet been realized. In this paper, we demonstrate that 1D nanowire with sizes as small as six molecular ribbons, can be readily achieved in the Ta2(Pd or Pt)3Se8 system by simple micromechanical exfoliation. Exfoliated Ta2Pd3Se8 nanowires are n-type semiconductors, whereas isostructural Ta2Pt3Se8 nanowires are p-type semiconductors. Both types of nanowires show excellent electrical switching performance as the channel material for a field-effect transistor. Low-temperature transport measurement reveals a defect level inherent to Ta2Pd3Se8 nanowires, which enables the observed electrical switching behavior at high temperature (above 140 K). A functional logic gate consisting of both n-type Ta2Pd3Se8 and p-type Ta2Pt3Se8 field-effect transistors has also been successfully achieved. By taking advantage of the high crystal quality derived from the parent van der Waals bulk compound, our findings about the exfoliated Ta2(Pd or Pt)3Se8 nanowires demonstrate a new pathway to access single-crystal 1D nanostructures for the study of their fundamental properties and the exploration of their applications in electronics, optoelectronics, and energy harvesting. [ABSTRACT FROM AUTHOR]

Transition metal trichalcogenides (TMTs), a family of van der Waals materials, have gained increasing interests from the discovery of magnetism in few-layer forms. Although TMTs with 3 d transition metal elements have been studied extensively, much less is explored for the 4 d and 5 d cases, where the interesting interplay between electron correlations and the relativistic spin-orbit coupling is expected. Using ab initio calculations, we here investigate the electronic property of TMTs with 4 d and 5 d transition metal elements. We show that the band structures exhibit multiple node-like features near the Fermi level. These are the remnant of multiple Dirac cones that were recently discovered in the monolayer cases. Our results indicate that the peculiar two-dimensional multiple Dirac cones are concealed even in the layered bulk systems. [ABSTRACT FROM AUTHOR]

The ill effects of prolonged use of rifamycin antibiotics such as rifampicin accentuates its need for detection in the environment as well as in biological fluids. Antibiotics in water and soil are long-lasting, bio-accumulative, and hazardous to aquatic species as well as human health. To address this issue, a sensing platform has been developed using Molybdenum diselenide (MoSe 2) embedded on reduced graphene oxide (rGO) functionalized with β-cyclodextrin (β-cd) polymer. The formation of hybrid composite was validated with X-ray diffraction analysis (XRD), Raman spectroscopy, fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM) with EDX analysis. The formation of microspheres were observed with hexagonal crystal system and P63/mmc space group. Furthermore, the composite was employed to fabricate an efficient electrochemical sensor for detecting the widely used antibiotic, rifampicin (RIF). The results reveal excellent activity of the sensor with a limit of detection (LOD) of 28 nM in a linear working range from 0.019 to 374.5 μM. The sensor also exhibited a high sensitivity of 11.64 μA μM−1 cm−2. Additionally, the sensor showed appreciable recovery range when monitored in real-samples such as human serum and urine, and industrial water, and fish samples. [Display omitted] • MoSe 2 microspheres were formed by aggregation of primary nano-sized particles. • The MoSe 2 was anchored with β-cd functionalized rGO to form an efficient catalyst (MoSe 2 /rGO/β-cd). • The large surface area, and guest-host interaction, facilitated rich interfacial electroactive sites for detection of RIF. • Sensitive detection of RIF was possible with lower LOD at 28 nM. • The real-time applicability was successfully implied in environmental and biological samples. [ABSTRACT FROM AUTHOR]

Despite belonging to a well-studied family of transition metal trihalides, VI 3 has received significant attention only recently. As a hard ferromagnetic van der Waals compound with a large coercivity, it attracted much attention because of its potential use in atomically thin spintronic and optoelectronic devices. However, practical exploration of VI 3 is challenging due to its instability under ambient conditions. Here, we present a comprehensive set of optical, x-ray diffraction, magnetization, and ellipsometric measurements of VI 3 and demonstrate that, similarly to the related van der Waals ferromagnet CrI 3 , the degradation process is accelerated by the presence of moisture. The VI 3 surface was covered by selected media commonly used in physical measurements to test its stability and lower the degradation rate three times or higher, providing practical information for experimentalists interested. The decomposition study at ambient conditions shows that the VI 3 single crystal can be used for most of the bulk, magnetization, and optical measurements without any noticeable change of physical properties, as the significant degradation appears first after ∼2 h of exposition as illustrated, e.g., by the evolution of the ferromagnetic T 1 and T 2 transitions. The ellipsometric measurement demonstrates that even the surface remains optically stable for at least 5 min. • The degradation of VI 3 is accelerated by the presence of moisture, similarly to CrI 3. • VI 3 crystals are stable for most of the bulk experiments after a 2 h-exposition. • Ellipsometry shows that the surface remains optically stable for at least 5 min. [ABSTRACT FROM AUTHOR]

Activating metamagnetic transitions between ordered states in van der Waals magnets and devices bring great opportunities in spintronics. We show that external pressure, which enhances the interlayer hopping without introducing chemical disorders, triggers multiple metamagnetic transitions upon cooling in the topological van der Waals magnets Mn(Bi 1- x Sb x ) 4 Te 7 , where the antiferromagnetic interlayer superexchange coupling competes with the ferromagnetic interlayer coupling mediated by the antisite Mn spins. The temperature-pressure phase diagrams reveal that while the ordering temperature from the paramagnetic to ordered states is almost pressure-independent, the metamagnetic transitions show nontrivial pressure and temperature dependence, even re-entrance. For these highly anisotropic magnets, we attribute the former to the ordering temperature being only weakly dependent on the intralayer parameters and the latter to the parametrically different pressure and temperature dependence of the two interlayer couplings. Our independent probing of these disparate magnetic interactions paves an avenue for efficient magnetic manipulations in van der Waals magnets.

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl 3 , CrBr 3 , CrI 3 , is characterized by the low-temperature (LT) phase adopting the trigonal BiI 3 -type while the structure of the high-temperature (HT) phase is monoclinic of AlCl 3 type ( C 2/ m ). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl 3 and CrI 3 . We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl 3 , VBr 3 , and VI 3 by measuring specific heat, magnetization, and x-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the HT phase has a higher symmetry while the LT structure reveals a lower symmetry. The structural phase transition between them shows no measurable hysteresis in contrast to CrX 3 . Possible relations of the evolution of the ratio c / a of the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed., (© 2022 IOP Publishing Ltd.)

Stacking two-dimensional van der Waals (vdW) materials rotated with respect to each other show versatility for studying exotic quantum phenomena. In particular, anisotropic layered materials have great potential for such twistronics applications, providing high tunability. Here, we report anisotropic superconducting order parameters in twisted Bi 2 Sr 2 CaCu 2 O 8+ x (Bi-2212) vdW junctions with an atomically clean vdW interface, achieved using the microcleave-and-stack technique. The vdW junctions with twist angles of 0° and 90° showed the maximum Josephson coupling, comparable to that of intrinsic Josephson junctions. As the twist angle approaches 45°, Josephson coupling is suppressed, and eventually disappears at 45°. The observed twist angle dependence of the Josephson coupling can be explained quantitatively by theoretical calculation with the d -wave superconducting order parameter of Bi-2212 and finite tunneling incoherence of the junction. Our results revealed the anisotropic nature of Bi-2212 and provided a novel fabrication technique for vdW-based twistronics platforms compatible with air-sensitive vdW materials.

Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI 3 , readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air-stable and layer-dependent ferromagnetic (FM) van der Waals crystal, CrPS 4 , using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics.

Metal halides are a class of layered materials with promising electronic and magnetic properties persisting down to the two-dimensional limit. While most recent studies focused on the trihalide components of this family, the rather unexplored metal dihalides are also van der Waals layered systems with distinctive magnetic properties. Here we show that the dihalide NiBr 2 grows epitaxially on a Au(111) substrate and exhibits semiconducting and magnetic behavior starting from a single layer. Through a combination of a low-temperature scanning-tunneling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy, and photoemission electron microscopy, we identify two competing layer structures of NiBr 2 coexisting at the interface and a stoichiometrically pure layer-by-layer growth beyond. Interestingly, X-ray absorption spectroscopy measurements revealed a magnetically ordered state below 27 K with in-plane magnetic anisotropy and zero-remanence in the single layer of NiBr 2 /Au(111), which we attribute to a noncollinear magnetic structure. The combination of such two-dimensional magnetic order with the semiconducting behavior down to the 2D limit offers the attractive perspective of using these films as ultrathin crystalline barriers in tunneling junctions and low-dimensional devices.

Realizing a state of matter in two dimensions has repeatedly proven a novel route of discovering new physical phenomena. Van der Waals (vdW) materials have been at the center of these now extensive research activities. They offer a natural way of producing a monolayer of matter simply by mechanical exfoliation. This work demonstrates that the possible multiferroic state with coexisting antiferromagnetic and ferroelectric orders persists down to the bilayer flake of NiI 2 . By exploiting the optical second-harmonic generation technique, both magnitude and direction of the ferroelectric order, arising from the cycloidal spin order, are successfully traced. The possible multiferroic state's transition temperature decreases from 58 K for the bulk to about 20 K for the bilayer. Our observation will spur extensive efforts to demonstrate multifunctionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.

We predict that layer antiferromagnetic bilayers formed from van der Waals (vdW) materials with weak interlayer versus intralayer exchange coupling have strong magnetoelectric response that can be detected in dual-gated devices where internal displacement fields and carrier densities can be varied independently. We illustrate this strong temperature-dependent magnetoelectric response in bilayer CrI 3 at charge neutrality by calculating the gate voltage-dependent total magnetization through Monte Carlo simulations and mean-field solutions of the anisotropic Heisenberg model informed from density functional theory and experimental data and present a simple model for electrical control of magnetism by electrostatic doping.

Multi‐valued logic (MVL) computing, which uses more than three logical states, is a promising future technology for handling huge amounts of data in the forthcoming "big data" era. The feasibility of MVL computing depends on the development of new concept devices/circuits beyond the complementary metal oxide semiconductor (CMOS) technology. This is because many CMOS devices are required to implement basic MVL functions, such as multilevel NOT, AND, and OR. In this study, a novel MVL device is reported with a complementarily controllable potential well, featuring the negative differential transconductance (NDT) phenomenon. This NDT device implemented on the WS2–graphene–WSe2 van der Waals heterostructure is evolved to a double‐NDT device operating on the basis of two consecutive NDT phenomena via structural engineering and parallel device configuration. This double‐NDT device is intensively analyzed via atomic force microscopy, kelvin probe force microscopy, Raman spectroscopy, and temperature‐dependent electrical measurement to gain a detailed understanding of its operating mechanism. Finally, the operation of a quaternary inverter configured with the double‐peak NDT device and a p‐channel transistor through Cadence circuit simulation is theoretically demonstrated. [ABSTRACT FROM AUTHOR]

Multivalued logic (MVL) devices/circuits have received considerable attention because the binary logic used in current Si complementary metal-oxide-semiconductor (CMOS) technology cannot handle the predicted information throughputs and energy demands of the future. To realize MVL, the conventional transistor platform needs to be redesigned to have two or more distinctive threshold voltages (V TH s). Here, we report a finding: the photoinduced drain current in graphene/WSe 2 heterojunction transistors unusually decreases with increasing gate voltage under illumination, which we refer to as the light-induced negative differential transconductance (L-NDT) phenomenon. We also prove that such L-NDT phenomenon in specific bias ranges originates from a variable potential barrier at a graphene/WSe 2 junction due to a gate-controllable graphene electrode. This finding allows us to conceive graphene/WSe 2 -based MVL logic circuits by using the I D -V G characteristics with two distinctive V TH s. Based on this finding, we further demonstrate a light-triggered ternary inverter circuit with three stable logical states (ΔV out of each state <0.05 V). Our study offers the pathway to substantialize MVL systems.