Enabling room temperature ferromagnetism in monolayer MoS2 via in situ iron-doping

Enhanced magnetic anisotropy of iridium dimers on antisite defects of two-dimensional transition-metal dichalcogenides

The combination of transition-metal (TM) elements with two-dimensional (2D) transition-metal dichalcogenides (TMDs) provides an effective route to realizing a 2D controllable magnetic order, leading to significant applications in multifunctional nanospintronics. However, in most TM atoms@TMDs nanostructures, it is challenging for the magnetic anisotropy energy (MAE) to exceed 30 meV when affected by the crystal field. Hence, the stronger magnetic anisotropy of TMDs has yet to be developed. Here, utilizing first-principle calculations based on density functional theory (DFT), a feasible method to enhance the MAEs of TMDs via configurating iridium dimers (Ir2) on 2D traditional and Janus TMDs with antisite defects is reported. Calculations revealed that 28 of the 54 configurations considered possessed structure-dependent MAEs of >60 meV per Ir2 in the out-of-plane direction, suggesting the potential for applications at room temperature. We also showed the ability to tune the MAE further massively by applying a biaxial strain as well as the surface asymmetric polarization reversal of Janus-type substrates. This approach led to changes to >80 meV per Ir2. This work provides a novel strategy to achieve tunable large magnetic anisotropy in 2D TMDs. It also extends the functionality of antisite-defective TMDs, thereby providing theoretical support for the development of magnetic nanodevices.

CO2 Stress-Driven Room Temperature Ferromagnetism of Ultrathin 2D Gallium Oxide

Spintronic devices work by manipulating the spin of electrons other than charge transfer, which is of revolutionary significance and can largely reduce energy consumption in the future. Herein, ultrathin two-dimensional (2D) non-van der Waals (non-vdW) γ-Ga2O3 with room temperature ferromagnetism is successfully obtained by using supercritical CO2 (SC CO2). The stress effect of SC CO2 under different pressures selectively modulates the orientation and strength of covalent bonds, leading to the change of atomic structure including lattice expansion, introduction of O vacancy, and transition of Ga-O coordination (GaO4 and GaO6). Magnetic measurements show that pristine γ-Ga2O3 is nonferromagnetic, whereas the SC CO2 treated γ-Ga2O3 exhibits obvious ferromagnetic behavior with an optimal magnetization of 0.025 emu g-1 and a Curie temperature of 300 K.

Magnetic doping in transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) are materials with unique electronic properties due to their two-dimensional nature. Recently, there is a large and growing interest in synthesizing ferromagnetic TMDCs for applications in electronic devices and spintronics. Apart from intrinsically magnetic examples, modification via either intrinsic defects or external dopants may induce ferromagnetism in non-magnetic TMDCs and, hence expand the application of these materials. Here, we review recent experimental work on intrinsically non-magnetic TMDCs that present ferromagnetism as a consequence of either intrinsic defects or doping via self-flux approach, ion implantation or e-beam evaporation. The experimental work discussed here is organized by modification/doping mechanism. We also review current work on density functional theory calculations that predict ferromagnetism in doped systems, which also serve as preliminary data for the choice of new doped TMDCs to be explored experimentally. Implementing a controlled process to induce magnetism in two-dimensional materials is key for technological development and this topical review discusses the fundamental procedures while presenting promising materials to be investigated in order to achieve this goal.

Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX2 , where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS2 , V-WSe2 ). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe2 /WS2 , VSe2 /MoS2 ), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron-hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Atomically Unraveling Highly Crystalline Self-Intercalated Tantalum Sulfide with Correlated Stacking Registry-Dependent Magnetism

In self-intercalated two-dimensional (ic-2D) materials, understanding the local chemical environment and the topology of the filling site remains elusive, and the subsequent correlation with the macroscopically manifested physical properties has rarely been investigated. Herein, highly crystalline gram-scale ic-2D Ta1.33S2 crystals were successfully grown by the high-pressure high-temperature method. Employing combined atomic-resolution scanning transmission electron microscopy annular dark field imaging and density functional theory calculations, we systematically unveiled the atomic structures of an atlas of stacking registries in a well-defined √3(a) × √3(a) Ta1.33S2 superlattice. Ferromagnetic order was observed in the AC' stacking registry, and it evolves into an antiferromagnetic state in AA/AB/AB' stacking registries; the AA' stacking registry shows ferrimagnetic ordering. Therefore, we present a novel approach for fabricating large-scale highly crystalline ic-2D crystals and shed light on a powerful means of modulating the magnetic order of ic-2D systems via stacking engineering, i.e., stackingtronics.

Above-Room-Temperature Ferromagnetism in Copper-Doped Two-Dimensional Chromium-Based Nanosheets

Two-dimensional ferromagnetic materials (2D-FMs) are expected to become ideal candidates for low-power, high-density information storage in next-generation spintronics devices due to their atomically ultrathin and intriguing magnetic properties. However, 2D-FMs with room-temperature Curie temperatures (Tc) are still rarely reported, which greatly hinders their research progress and practical applications. Herein, ultrathin Cu-doped Cr7Te8 FMs were successfully prepared and can achieve above-room-temperature ferromagnetism with perpendicular magnetic anisotropy via a facile chemical vapor deposition (CVD) method, which can be controlled down to an atomic thin layer of ∼3.4 nm. STEM-EDX quantitative analysis shows that the proportion of Cu to metal atoms is ∼5%. Moreover, based on the anomalous Hall effect (AHE) measurements in a six-terminal Hall bar device without any encapsulation as well as an out-of-plane magnetic field, the maximum Tc achieved ∼315 K when the thickness of the sample is ∼28.8 nm; even the ultrathin 7.6 nm sample possessed a near-room-temperature Tc of ∼275 K. Meanwhile, theoretical calculations elucidated the mechanism of the ferromagnetic enhancement of Cu-doped Cr7Te8 nanosheets. More importantly, the ferromagnetism of CVD-synthesized Cu-doped CrSe nanosheets can also be maintained above room temperature. Our work broadens the scope on room-temperature ferromagnets and their heterojunctions, promoting fundamental research and practical applications in next-generation spintronics.

Multiscale Synergetic Bandgap/Structure Engineering in Semiconductor Nanofibrous Aerogels for Enhanced Solar Evaporation

Solar-driven interface evaporation has been identified as a sustainable seawater desalination and water purification technology. Nonetheless, the evaporation performance is still restricted by salt deposition and heat loss owing to weak solar spectrum absorption, tortuous channels, and limited plane area of conventional photothermal material. Herein, the semiconductor nanofibrous aerogels with a narrow bandgap, vertically aligned channels, and a conical architecture are constructed by the multiscale synergetic engineering strategy, encompassing bandgap engineering at the atomic scale and structure engineering at the nano-micro scale. As a proof-of-concept demonstration, a Co-doped MoS2 nanofibrous aerogel is synthesized, which exhibits the entire solar absorption, superhydrophilic, and excellent thermal insulation, achieving a net evaporation rate of 1.62 kg m-2 h-1 under 1 sun irradiation, as well as a synergistically efficient dye ion adsorption function. This work opens up new possibilities for the development of solar evaporators for practical applications in clean water production.

Realization of Robust and Ambient-Stable Room-Temperature Ferromagnetism in Wide Bandgap Semiconductor 2D Carbon Nitride Sheets

Due to their weak intrinsic spin-orbit coupling and a distinct bandgap of 3.06 eV, 2D carbon nitride (CN) flakes are promising materials for next-generation spintronic devices. However, achieving strong room-temperature (RT) and ambient-stable ferromagnetism (FM) remains a huge challenge. Here, we demonstrate that the strong RT FM with a high Curie temperature (TC) up to ∼400 K and saturation magnetization (Ms) of 2.91 emu/g can be achieved. Besides, the RT FM exhibits excellent air stability, with Ms remaining stable for over 6 months. Through the magneto-optic Kerr effect, Hall device, X-ray magnetic circular dichroism, and magnetic force microscopy measurements, we acquired clear evidence of magnetic behavior and magnetic domain evolutions at room temperature. Electrical and optical measurements confirm that the Co-doped CN retains its semiconductor properties. Detailed structural characterizations confirm that the single-atom Co coordination and nitrogen defects as well as C-C covalent bonds are simultaneously introduced into CN. Density functional theory calculations reveal that introducing C-C bonds causes carrier spin polarization, and spin-polarized carrier-mediated magnetic exchange between adjacent Co atoms leads to long-range magnetic ordering in CN. We believe that our findings provide a strong experimental foundation for the enormous potential of 2D wide bandgap semiconductor spintronic devices.

Synthesis of component-controllable monolayer MoxW(1-x)S2ySe2(1-y) alloys with continuously tunable band gap and carrier type

Alloying can effectively modify electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). However, efficient and simple methods to synthesize atomically thin TMD alloys need to be further developed. In this study, we synthesized 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys by using a new liquid phase edge epitaxy (LPEE) growth method with high controllability. This straightforward approach can be used to obtain monolayer materials and operates on a self-limiting growth mechanism. The process allows the liquid solution to come into contact with the two-dimensional grains only at their edges, resulting in epitaxy confined only along the in-plane direction, which produces exclusively monolayer epitaxy. By controlling the weight ratio of MoS2/WSe2 (MoSe2/WS2), 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys with different atomic ratios can be obtained on sapphire substrates, with band gap ranging from WS2 (1.55 eV) to MoSe2 (1.99 eV) and a continuously broad spectrum ranging from 623 nm to 800 nm. By adjusting the alloy composition, the carrier type and carrier mobility of alloy-based field-effect transistors can be modulated. In particular, the adjustable conductivity of MoxW(1-x)S2ySe2(1-y) alloys from n-type to bipolar type is achieved for the first time. This general synthetic strategy provides a foundation for the development of monolayer TMD alloys with multiple components and various 2D materials.

On the electronic and spin-valley coupling of vanadium doped MoS2(1-x)Se2xmonolayers

Monolayers of MoS2with tunable bandgap and valley positions are highly demanding for their applications in opto-spintronics. Herein, selenium (Se) and vanadium (V) co-doped MoS2monolayers (vanadium doped MoS2(1-x)Se2x(V-MoSSe)) are developed and showed their variations in the electronic and optical properties with dopant content. Vanadium gets substitutionally (in place of Mo) doped within the MoS2lattice while selenium doped in place of sulfur, as shown by a detailed microstructure and spectroscopy analyses. The bandgap tunability with selenium doping can be achieved while valley shift is occurred due to the doping of vanadium. Chemical vapor deposition assisted grown MoS2(also selenium doped MoS2as shown here) is known for its n-type transport behavior while vanadium doping is found to be changing its nature to p-doping. Chirality dependent photoexcitation studies indicate a room temperature valley splitting in V-MoSSe (∼8 meV), where such a valley splitting is verified using density functional theory based calculations.

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Bridging Synthesis and Controllable Doping of Monolayer 4 in. Length Transition-Metal Dichalcogenides Single Crystals with High Electron Mobility

Epitaxial growth and controllable doping of wafer-scale atomically thin semiconductor single crystals are two central tasks to tackle the scaling challenge of transistors. Despite considerable efforts are devoted, addressing such crucial issues simultaneously under 2D confinement is yet to be realized. Here, an ingenious strategy to synthesize record-breaking 4 in. length Fe-doped transition-metal dichalcogenides (TMDCs) single crystals on industry-compatible c-plane sapphire without special miscut angle is designed. Atomically thin transistors with high electron mobility (≈146 cm² V^-1 s^-1 ) and remarkable on/off current ratio (≈10⁹ ) are fabricated based on 4 in. length Fe-MoS₂ single crystals, due to the ultralow contact resistance (≈489 Ω µm). In-depth characterizations and theoretical calculations reveal that the introduction of Fe significantly decreases the formation energy of parallel steps on sapphire surfaces and contributes to the edge-nucleation of unidirectional alignment TMDCs domains (>99%). This work represents a substantial leap in terms of bridging synthesis and doping of wafer-scale 2D semiconductor single crystals, which should promote the further device downscaling and extension of Moore's law.

Ferromagnetic single-atom spin catalyst for boosting water splitting

Heterogeneous single-atom spin catalysts combined with magnetic fields provide a powerful means for accelerating chemical reactions with enhanced metal utilization and reaction efficiency. However, designing these catalysts remains challenging due to the need for a high density of atomically dispersed active sites with a short-range quantum spin exchange interaction and long-range ferromagnetic ordering. Here, we devised a scalable hydrothermal approach involving an operando acidic environment for synthesizing various single-atom spin catalysts with widely tunable substitutional magnetic atoms (M₁) in a MoS₂ host. Among all the M₁/MoS₂ species, Ni₁/MoS₂ adopts a distorted tetragonal structure that prompts both ferromagnetic coupling to nearby S atoms as well as adjacent Ni₁ sites, resulting in global room-temperature ferromagnetism. Such coupling benefits spin-selective charge transfer in oxygen evolution reactions to produce triplet O₂. Furthermore, a mild magnetic field of ~0.5 T enhances the oxygen evolution reaction magnetocurrent by ~2,880% over Ni₁/MoS₂, leading to excellent activity and stability in both seawater and pure water splitting cells. As supported by operando characterizations and theoretical calculations, a great magnetic-field-enhanced oxygen evolution reaction performance over Ni₁/MoS₂ is attributed to a field-induced spin alignment and spin density optimization over S active sites arising from field-regulated S(p)-Ni(d) hybridization, which in turn optimizes the adsorption energies for radical intermediates to reduce overall reaction barriers.

Advances in Two-Dimensional Magnetic Semiconductors via Substitutional Doping of Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are two-dimensional (2D) materials with remarkable electrical, optical, and chemical properties. One promising strategy to tailor the properties of TMDs is to create alloys through a dopant-induced modification. Dopants can introduce additional states within the bandgap of TMDs, leading to changes in their optical, electronic, and magnetic properties. This paper overviews chemical vapor deposition (CVD) methods to introduce dopants into TMD monolayers, and discusses the advantages, limitations, and their impacts on the structural, electrical, optical, and magnetic properties of substitutionally doped TMDs. The dopants in TMDs modify the density and type of carriers in the material, thereby influencing the optical properties of the materials. The magnetic moment and circular dichroism in magnetic TMDs are also strongly affected by doping, which enhances the magnetic signal in the material. Finally, we highlight the different doping-induced magnetic properties of TMDs, including superexchange-induced ferromagnetism and valley Zeeman shift. Overall, this review paper provides a comprehensive summary of magnetic TMDs synthesized via CVD, which can guide future research on doped TMDs for various applications, such as spintronics, optoelectronics, and magnetic memory devices.

Edge-Based Two-Dimensional α-In2Se3-MoS2 Ferroelectric Field Effect Device

Heterostructures based on two-dimensional materials offer the possibility to achieve synergistic functionalities, which otherwise remain secluded by their individual counterparts. Herein, ferroelectric polarization switching in α-In₂Se₃ has been utilized to engineer multilevel nonvolatile conduction states in a partially overlapping α-In₂Se₃-MoS₂-based ferroelectric semiconducting field effect device. In particular, we demonstrate how the intercoupled ferroelectric nature of α-In₂Se₃ allows to nonvolatilely switch between n-i and n-i-n type junction configurations based on a novel edge state actuation mechanism, paving the way for subnanometric scale nonvolatile device miniaturization. Furthermore, the induced asymmetric polarization enables enhanced photogenerated carriers' separation, resulting in an extremely high photoresponse of ∼1275 A/W in the visible range and strong nonvolatile modulation of the bright A- and B- excitonic emission channels in the overlaying MoS₂ monolayer. Our results show significant potential to harness the switchable polarization in partially overlapping α-In₂Se₃-MoS₂ based FeFETs to engineer multimodal, nonvolatile nanoscale electronic and optoelectronic devices.

Gamma-Ray Irradiation Induced Ultrahigh Room-Temperature Ferromagnetism in MoS2 Sputtered Few-Layered Thin Films

Defect engineering is of great interest to the two-dimensional (2D) materials community. If nonmagnetic transition-metal dichalcogenides can possess room-temperature ferromagnetism (RTFM) induced by defects, then they will be ideal for application as spintronic materials and also for studying the relation between electronic and magnetic properties of quantum-confined structures. Thus, in this work, we aimed to study gamma-ray irradiation effects on MoS₂, which is diamagnetic in nature. We found that gamma-ray exposure up to 9 kGy on few-layered (3.5 nm) MoS₂ films induces an ultrahigh saturation magnetization of around 610 emu/cm³ at RT, whereas no significant changes were observed in the structure and magnetism of bulk MoS₂ (40 nm) films even after gamma-ray irradiation. The RTFM in a few-layered gamma-ray irradiated sample is most likely due to the bound magnetic polaron created by the spin interaction of Mo 4d ions with trapped electrons present at sulfur vacancies. In addition, density functional theory (DFT) calculations suggest that the defect containing one Mo and two S vacancies is the dominant defect inducing the RTFM in MoS₂. These DFT results are consistent with Raman, X-ray photoelectron spectroscopy, and ESR spectroscopy results, and they confirm the breakage of Mo and S bonds and the existence of vacancies after gamma-ray irradiation. Overall, this study suggests that the occurrence of magnetism in gamma-ray irradiated MoS₂ few-layered films could be attributed to the synergistic effects of magnetic moments arising from the existence of both Mo and S vacancies as well as lattice distortion of the MoS₂ structure.

A critical review of fabrication challenges and reliability issues in top/bottom gated MoS2field-effect transistors

The voyage of semiconductor industry to decrease the size of transistors to achieve superior device performance seems to near its physical dimensional limitations. The quest is on to explore emerging material systems that offer dimensional scaling to match the silicon- based technologies. The discovery of atomic flat two-dimensional materials has opened up a completely new avenue to fabricate transistors at sub-10 nanometer level which has the potential to compete with modern silicon-based semiconductor devices. Molybdenum disulfide (MoS₂) is a two-dimensional layered material with novel semiconducting properties at atomic level seems like a promising candidate that can possibly meet the expectation of Moore's law. This review discusses the various 'fabrication challenges' in making MoS₂based electronic devices from start to finish. The review outlines the intricate challenges of substrate selection and various synthesis methods of mono layer and few-layer MoS₂. The review focuses on the various techniques and methods to minimize interface defect density at substrate/MoS₂interface for optimum MoS₂-based device performance. The tunable band-gap of MoS₂with varying thickness presents a unique opportunity for contact engineering to mitigate the contact resistance issue using different elemental metals. In this work, we present a comprehensive overview of different types of contact materials with myriad geometries that show a profound impact on device performance. The choice of different insulating/dielectric gate oxides on MoS₂in co-planar and vertical geometry is critically reviewed and the physical feasibility of the same is discussed. The experimental constraints of different encapsulation techniques on MoS₂and its effect on structural and electronic properties are extensively discussed.

High Pressure-Driven Magnetic Disorder and Structural Transformation in Fe3 GeTe2 : Emergence of a Magnetic Quantum Critical Point

Among the recently discovered 2D intrinsic van der Waals (vdW) magnets, Fe₃ GeTe₂ (FGT) has emerged as a strong candidate for spintronics applications, due to its high Curie temperature (130 - 220 K) and magnetic tunability in response to external stimuli (electrical field, light, strain). Theory predicts that the magnetism of FGT can be significantly modulated by an external strain. However, experimental evidence is needed to validate this prediction and understand the underlying mechanism of strain-mediated vdW magnetism in this system. Here, the effects of pressure (0 - 20 GPa) are elucidated on the magnetic and structural properties of Fe₃ GeTe₂ by means of synchrotron Mössbauer source spectroscopy, X-ray powder diffraction and Raman spectroscopy over a wide temperature range of 10 - 290 K. A strong suppression of ferromagnetic ordering is observed with increasing pressure, and a paramagnetic ground state emerges when pressure exceeds a critical value, P_PM ≈ 15 GPa. The anomalous pressure dependence of structural parameters and vibrational modes is observed at P_C ≈ 7 GPa and attributed to an isostructural phase transformation. Density functional theory calculations complement these experimental findings. This study highlights pressure as a driving force for magnetic quantum criticality in layered vdW magnetic systems.

Enhanced Curie temperature and conductivity of van der Waals ferromagnet MgV2S4via electrostatic doping

A van der Waals intrinsic ferromagnet with double magnetic atom layers is of great interest for both revealing fundamental physics and exploring promising applications in low-dimensional spintronics. Here, the magnetic and electronic properties of the van der Waals ferromagnet MgV₂S₄ monolayer are studied under electrostatic doping using first-principles calculations. A MgV₂S₄ monolayer presents the desired physical properties such as that of being a half-semiconductor with a direct bandgap of 1.21 eV and a ferromagnetic ground state, and having a high Curie temperature of 462 K. Unlike the robust ferromagnetic ground state, magnetic anisotropy and Curie temperature are sensitive to electrostatic doping. Meanwhile, the transition from a semiconductor to a half-metal and the significant improvement in conductivity under electrostatic doping make the MgV₂S₄ monolayer a promising candidate for low-dimensional spintronic field-effect transistors.

Spatial Control of Substitutional Dopants in Hexagonal Monolayer WS2 : The Effect of Edge Termination

The ability to control the density and spatial distribution of substitutional dopants in semiconductors is crucial for achieving desired physicochemical properties. Substitutional doping with adjustable doping levels has been previously demonstrated in 2D transition metal dichalcogenides (TMDs); however, the spatial control of dopant distribution remains an open field. In this work, edge termination is demonstrated as an important characteristic of 2D TMD monocrystals that affects the distribution of substitutional dopants. Particularly, in chemical vapor deposition (CVD)-grown monolayer WS₂ , it is found that a higher density of transition metal dopants is always incorporated in sulfur-terminated domains when compared to tungsten-terminated domains. Two representative examples demonstrate this spatial distribution control, including hexagonal iron- and vanadium-doped WS₂ monolayers. Density functional theory (DFT) calculations are further performed, indicating that the edge-dependent dopant distribution is due to a strong binding of tungsten atoms at tungsten-zigzag edges, resulting in the formation of open sites at sulfur-zigzag edges that enable preferential dopant incorporation. Based on these results, it is envisioned that edge termination in crystalline TMD monolayers can be utilized as a novel and effective knob for engineering the spatial distribution of substitutional dopants, leading to in-plane hetero-/multi-junctions that display fascinating electronic, optoelectronic, and magnetic properties.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

An Efficient, Short Stimulus PANC-1 Cancer Cell Ablation and Electrothermal Therapy Driven by Hydrophobic Interactions

Promising results in clinical studies have been demonstrated by the utilization of electrothermal agents (ETAs) in cancer therapy. However, a difficulty arises from the balance between facilitating the degradation of ETAs, and at the same time, increasing the electrothermal performance/stability required for highly efficient treatment. In this study, we controlled the thermal signature of the MoS₂ by harnessing MoS₂ nanostructures with M13 phage (MNM) via the structural assembling (hydrophobic interaction) phenomena and developed a combined PANC-1 cancer cell-MNM alternating current (AC)-stimulus framework for cancer cell ablation and electrothermal therapy. A percentage decrease in the cell viability of ~23% was achieved, as well as a degradation time of 2 weeks; a stimulus length of 100 μs was also achieved. Molecular dynamics (MD) simulations revealed the assembling kinetics in integrated M13 phage-cancer cell protein systems and the structural origin of the hydrophobic interaction-enabled increase in thermal conduction. This study not only introduced an 'ideal' agent that avoided the limitations of ETAs but also provided a proof-of-concept application of MoS₂-based materials in efficacious cancer therapy.

Determining optical material parameters with motion in structured illumination

A set of power measurements as a function of controlled nanopositioner movement of a planar film arrangement in a standing wave field is presented as a means to obtain the thicknesses and the dielectric constants to a precision dictated by noise in an exciting laser beam and the positioning and detector process, all of which can be refined with averaging. From a mutual information perspective, knowing the set of positions at which measurements are performed adds information. While applicable to any arrangement of planar films, the implementation considered involves thin transmissive membranes, as are employed in applications such as optomechanics. We show that measured power data as a function of object position provides sensitivity to the film refractive index and far-subwavelength thickness. Use of a cost function allows iterative retrieval of the film parameters, and a multi-resolution framework is described as a computationally efficient procedure. The approach is complementary to ellipsometry and could play an important role in routine film characterization studies for fields involving solid state material processing, as is common in the semiconductor device field.

Hybrid G/BN@2H-MoS2 Nanomaterial Composites: Structural, Electronic and Molecular Adsorption Properties

Hybrid structures often possess superior properties to those of their component materials. This arises from changes in the structural or physical properties of the new materials. Here, we investigate the structural, electronic, and gas-adsorption properties of hybrid structures made from graphene/hexagonal boron nitride and 2H-molybdenum disulfide (G/BN@MoS₂) monolayers. We consider hybrid systems in which the G/BN patch is at the Mo plane (model I) and the S plane (model II). We find that the implanted hexagon of G or BN in MoS₂ alters its electronic properties: G@MoS₂ (I,II) are metallic, while BN@MoS₂ (I) is an n-type conducting and BN@MoS₂ (II) is semiconducting. We study the molecular adsorption of some diatomic gases (H₂, OH, N₂, NO, CO), triatomic gases (CO₂, NO₂, H₂S, SO₂), and polyatomic gases (COOH, CH₄, and NH₃) on our hybrid structures while considering multiple initial adsorption sites. Our results suggest that the hybrid systems may be suitable materials for some applications: G@MOS₂ (I) for oxygen reduction reactions, BN@MoS₂ (I,II) for NH₃-based hydrogen production, and G@MoS₂ (I) and BN@MoS₂ (I,II) for filtration of No, Co, SO₂, H₂S, and NO₂.

A Simplified Method of the Assessment of Magnetic Anisotropy of Commonly Used Sapphire Substrates in SQUID Magnetometers

Solid-state wafers are indispensable components in material science as substrates for epitaxial homo- or heterostructures or carriers for two-dimensional materials. However, reliable determination of magnetic properties of nanomaterials in volume magnetometry is frequently affected by unexpectedly rich magnetism of these substrates, including significant magnetic anisotropy. Here, we describe a simplified experimental routine of magnetic anisotropy assessment, which we exemplify and validate for epi-ready sapphire wafers from various sources. Both the strength and the sign of magnetic anisotropy are obtained from carefully designed temperature-dependent measurements, which mitigate all known pitfalls of volume SQUID magnetometry and are substantially faster than traditional approaches. Our measurements indicate that in all the samples, two types of net paramagnetic contributions coexist with diamagnetism. The first one can be as strong as 10% of the base diamagnetism of sapphire [-3.7(1) × 10^-7 emu/gOe], and when exceeds 2%, it exhibits pronounced magnetic anisotropy, with the easy axis oriented perpendicularly to the face of c-plane wafers. The other is much weaker, but exhibits a ferromagnetic-like appearance. These findings form an important message that nonstandard magnetism of common substrates can significantly influence the results of precise magnetometry of nanoscale materials and that its existence must be taken for granted by both industry and academia.

Exploring room-temperature ferromagnetism in WXBC (X = W, Mn, Fe) monolayers

Two-dimensional (2D) transition metal boron-carbide is a novel material that has unique properties suitable for advanced spintronics and storage applications. Through first-principles calculations based on density functional theory (DFT) calculations, we report a new class of stable 2D ceramic WXBC (X = W, Mn, Fe) monolayers. We find that all WXBC monolayers prefer a ferromagnetic ground state with metallic electronic property. DFT calculations proved that WXBC monolayers exhibit good energetic, mechanical, and dynamic stabilities. More importantly, these monolayers exhibit large magnetic anisotropy energy (MAE) of 1213 μeV, 247 μeV and 20 μeV per magnetic atom for W₂BC, WMnBC, and WFeBC, respectively. An out-of-plane easy axis (EA) magnetization direction is found for W₂BC whereas the EA for WMnBC and WFeBC are in-plane. By performing Monte Carlo (MC) simulations based on the 2D Heisenberg model, we predict Curie temperatures (T_C) of 155 K for the W₂BC monolayer. The Berezinskii–Kosterlitz–Thouless transition (BKT) temperature values of WMnBC and WFeBC are as high as 374.69 K and 417.39 K. For further investigations, the adsorption properties of Li, Na, and K atoms on WXBC (atm-WXBC) systems are examined. It is revealed that the initial ferromagnetic metallic properties of bare WXBC monolayers are maintained for all atm-WXBC systems. The obtained strong chemisorption energies are high enough to make adsorbed Li, Na, and K immobile on WXBC surfaces. All these findings demonstrate the unique potential of WXBC monolayers as multifunctional candidates for advanced magnetic device and storage applications.

Two-dimensional (2D) transition metal boron-carbide is a novel material that has unique properties suitable for advanced spintronics and storage applications.

Fe-Doped MoS2 Nanozyme for Antibacterial Activity and Detoxification of Mustard Gas Simulant

The peroxidase-like catalytic activity of various nanozymes was extensively applied in various fields. In this study, we have demonstrated the preparation of Fe-doped MoS₂ (Fe@MoS₂) nanomaterials with enhanced peroxidase-like activity of MoS₂ in a co-catalytic pathway. In view of Fenton reaction, the peroxidase-like Fe@MoS₂ nanozyme prompted the decomposition of hydrogen peroxide (H₂O₂) to a reactive hydroxyl radical (·OH). The efficient decomposition of H₂O₂ in the presence of Fe@MoS₂ has been employed toward the antibacterial activity and detoxification of mustard gas simulant. The combined effect of Fe@MoS₂ and H₂O₂ showed remarkable antibacterial activity against the drug-resistant bacterial strain methicillin-resistant Staphylococcus aureus and Escherichia coli with the use of minimal concentration of H₂O₂. Fe@MoS₂ was further applied for the detoxification of the chemical warfare agent sulfur mustard simulant, 2-chloroethyl ethyl sulfide, by selective conversion to the nontoxic sulfoxide. This work demonstrates the development of a hybrid nanozyme and its environmental remediation from harmful chemicals to microbes.

Synthesis of Group VIII Magnetic Transition-Metal-Doped Monolayer MoSe2

The limitation on the spintronic applications of van der Waals layered transition-metal dichalcogenide semiconductors is ascribed to the intrinsic nonmagnetic feature. Recent studies have proved that substitutional doping is an effective route to alter the magnetic properties of two-dimensional transition-metal dichalcogenides (TMDs). However, highly valid and repeatable substitutional doping of TMDs remains to be developed. Herein, we report group VIII magnetic transition metal-doped molybdenum diselenide (MoSe₂) single crystals via a one-pot mixed-salt-intermediated chemical vapor deposition method with high controllability and reproducibility. The high-angle annular dark-field scanning transmission electron microscopy studies further confirm that the sites of Fe are indeed substitutionally incorporated into the MoSe₂ monolayer. The Fe-doped MoSe₂ monolayer with a concentration from 0.93% to 6.10% could be obtained by controlling the ratios of FeCl₃/Na₂MoO₄. Moreover, this strategy can be extended to create Co(Ni)-doped MoSe₂ monolayers. The magnetic hysteresis (M-H) measurements demonstrate that group VIII magnetic transition-metal-doped MoSe₂ samples exhibit room-temperature ferromagnetism. Additionally, the Fe-doped MoSe₂ field effect transistor shows n-type semiconductor characteristics, indicating the obtainment of a room-temperature dilute magnetic semiconductor. Our approach is universal in magnetic transition-metal substitutional doping of TMDs, and it inspires further research interest in the study of related spintronic and magnetoelectric applications.

Strategies for Controlled Growth of Transition Metal Dichalcogenides by Chemical Vapor Deposition for Integrated Electronics

In recent years, transition metal dichalcogenide (TMD)-based electronics have experienced a prosperous stage of development, and some considerable applications include field-effect transistors, photodetectors, and light-emitting diodes. Chemical vapor deposition (CVD), a typical bottom-up approach for preparing 2D materials, is widely used to synthesize large-area 2D TMD films and is a promising method for mass production to implement them for practical applications. In this review, we investigate recent progress in controlled CVD growth of 2D TMDs, aiming for controlled nucleation and orientation, using various CVD strategies such as choice of precursors or substrates, process optimization, and system engineering. We then survey different patterning methods, such as surface patterning, metal precursor patterning, and postgrowth sulfurization/selenization/tellurization, to mass produce heterostructures for device applications. With these strategies, various well-designed architectures, such as wafer-scale single crystals, vertical and lateral heterostructures, patterned structures, and arrays, are achieved. In addition, we further discuss various electronics made from CVD-grown TMDs to demonstrate the diverse application scenarios. Finally, perspectives regarding the current challenges of controlled CVD growth of 2D TMDs are also suggested.

Substantially Enhanced Properties of 2D WS2 by High Concentration of Erbium Doping against Tungsten Vacancy Formation

Doping in 2D materials is an important method for tuning of band structures. For this purpose, it is important to develop controllable doping techniques. Here, we demonstrate a substitutional doping strategy by erbium (Er) ions in the synthesis of monolayer WS₂ by chemical vapor deposition. Substantial enhancements in photoluminescent and photoresponsive properties are achieved, which indicate a tungsten vacancy suppression mechanism by Er filling. Er ion doping in the monolayer WS₂ is proved by X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS), fluorescence, absorption, excitation, and Raman spectra. 11.5 at% of the maximum Er concentration is examined by energy dispersive X-ray spectroscopy (EDX). Over 6 times enhancement of intensities with 7.9 nm redshift in peaks are observed from the fluorescent spectra of Er-doped WS₂ monolayers compared with their counterparts of the pristine WS₂ monolayers, which agrees well with the density functional theory calculations. In addition, over 11 times of dark current, 469 times of photocurrents, photoresponsivity, and external quantum efficiency, and two orders of photoresponse speed are demonstrated from the Er-doped WS₂ photodetector compared with those of the pristine WS₂ device. Our findings prove rare-earth doping in 2D materials, the exciting and ideal technique for substantially enhanced photoluminescent and photoresponsive properties.

Magnetic Anisotropy Control with Curie Temperature above 400 K in a van der Waals Ferromagnet for Spintronic Device

The technological appeal of van der Waals ferromagnetic materials is the ability to control magnetism under external fields with desired thickness toward novel spintronic applications. For practically useful devices, ferromagnetism above room temperature or tunable magnetic anisotropy is highly demanded but remains challenging. To date, only a few layered materials exhibit unambiguous ferromagnetic ordering at room temperature via gating techniques or interface engineering. Here, it is demonstrated that the magnetic anisotropy control and dramatic modulation of Curie temperature (T_c ) up to 400 K are realized in layered Fe₅ GeTe₂ via the high-pressure diamond-anvil-cell technique. Magnetic phases manifesting with in-plane anisotropic, out-of-plane anisotropic and nearly isotropic magnetic states can be tuned in a controllable way, depicted by the phase diagram with a maximum T_c up to 360 K. Remarkably, the T_c can be gradually enhanced to above 400 K owing to the Fermi surface evolution during a pressure loading-deloading process. Such an observation sheds light on the understanding and control of emergent magnetic states in practical spintronic applications.

Vanadium-Doped Molybdenum Diselenide Atomic Layers with Room-Temperature Ferromagnetism

Two-dimensional diluted magnetic semiconductors with high Curie temperature are highly sought after because of their potential applications in spintronics. Development of new techniques for preparation of high quanlity diluted magnetic semiconductors is critical for their applications. In this study, vanadium-doped molybdenum selenide, a new diluted magnetic semiconductor, was synthesized by a single-step chemical vapor deposition method. The merit of this method is that the molybdenum and vanadium precursors can be supplied to the growth substrate uniformly. Photoluminescence measurements reveal that the band gap of MoSe 2 decreases after doping, which can be attributed to the formation of impurity energy band caused by p-type doping at the valence band maximum. Thus, the V-doped MoSe 2 still maintains the semiconducting characteristics. Vibrating sample magnetometer studies clearly show the ferromagnetism of V-doped MoSe 2 at room temperature. DFT calculations illustrates the joint contribution of V dopants and nearby atoms to the magnetic moments. This study provides future prospects for the multifunctional application of two-dimensional materials.

Reducing Contact Resistance and Boosting Device Performance of Monolayer MoS2 by In Situ Fe Doping

2D semiconductors are emerging as plausible candidates for next-generation "More-than-Moore" nanoelectronics to tackle the scaling challenge of transistors. Wafer-scale 2D semiconductors, such as MoS₂ and WS₂ , have been successfully synthesized recently; nevertheless, the absence of effective doping technology fundamentally results in energy barriers and high contact resistances at the metal-semiconductor interfaces, and thus restrict their practical applications. Herein, a controllable doping strategy in centimeter-sized monolayer MoS₂ films is developed to address this critical issue and boost the device performance. The ultralow contact resistance and perfect Ohmic contact with metal electrodes are uncovered in monolayer Fe-doped MoS₂ , which deliver excellent device performance featured with ultrahigh electron mobility and outstanding on/off current ratio. Impurity scattering is suppressed significantly thanks to the ultralow electron effective mass and appropriate doping site. Particularly, unidirectionally aligned monolayer Fe-doped MoS₂ domains are prepared on 2 in. commercial c-plane sapphire, suggesting the feasibility of synthesizing wafer-scale 2D single-crystal semiconductors with outstanding device performance. This work presents the potential of high-performance monolayer transistors and enables further device downscaling and extension of Moore's law.

Chemical Vapor Deposition of Ferrimagnetic Fe3O4 Thin Films

Ultrathin magnetic materials with room-temperature ferromagnetism/ferrimagnetism hold great potential in spintronic applications. In this work, we report the successful controllable growth of Fe3O4 thin films using a facile chemical vapor deposition method. Room-temperature ferrimagnetism was maintained in the as-grown Fe3O4 thin films down to 4 nm. Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy analysis were conducted to reveal the structure and quality of the Fe3O4 film. Magnetization measurement showed ferrimagnetic hysteresis loops in all Fe3O4 thin films. A saturation magnetization of 752 emu/cm3 was observed for the 4 nm Fe3O4 film, which was higher than that of bulk Fe3O4 materials (480 emu/cm3). Additionally, the Verwey transition at ~120 K was visible for the Fe3O4 thin films. This work provides an alternative method of synthesizing ferrimagnetic ultrathin films for electronic, spintronic, and memory device applications.

Growth of highly conducting MoS2-xNx thin films with enhanced 1T' phase by pulsed laser deposition and exploration of their nanogenerator application

High-quality growth of MoS_2-xN_x films is realized on single-crystal c-Al₂O₃ substrates by the pulsed laser deposition (PLD) in ammonia rendering highly stable and tunable 1Tʹ/2H biphasic constitution. Raman spectroscopy reveals systematic enhancement of 1Tʹ phase component due to the incorporation of covalently bonded N-doping in MoS₂ lattice, inducing compressive strain. Interestingly, the film deposited at 300 mTorr NH₃ shows ∼80% 1Tʹ phase. The transport measurements performed on MoS_2-xN_x films deposited at 300 mTorr NH₃ display very low room temperature resistivity of 0.03 mΩ-cm which is 100 times enhanced over the undoped MoS₂ grown under comparable conditions. A triboelectric nanogenerator (TENG) device containing biphasic MoS_2-xN_x film as an electron acceptor exhibits a clear enhancement in the output voltage as compared to the pristine MoS₂. Device architecture, p-type N doping in MoS₂ lattice, favorably increased work-function, multiphasic component of MoS₂, and increased surface roughness synergistically contribute to superior TENG performance.

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MoS_2-xN_x films grown on c-Al₂O₃ and ITO/PET by pulsed laser deposition in NH₃

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p-type doping with high conductivity and 1T’+2H dual polymorph state is realized

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Increased work-function of MoS_2-xN_x films over pristine MoS₂ is realized

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Impressive Triboelectric Nanogenerator application is demonstrated with MoS_2-xN_x

Escalating Ferromagnetic Order via Se-Vacancies Near Vanadium in WSe2 Monolayers

Magnetic order has been proposed to arise from a variety of defects, including vacancies, antisites, and grain boundaries, which are relevant in numerous electronics and spintronics applications. Nevertheless, its magnetism remains controversial due to the lack of structural analysis. The escalation of ferromagnetism in vanadium-doped WSe₂ monolayer is herein demonstrated by tailoring complex configurations of Se vacancies (Se_Vac ) via post heat-treatment. Structural analysis of atomic defects is systematically performed using transmission electron microscopy (TEM), enabled by the monolayer nature. Temperature-dependent magnetoresistance hysteresis ensures enhanced magnetic order after high-temperature heat-treatment, consistent with magnetic domain analysis from magnetic force microscopy (MFM). The vanadium-Se vacancy pairing is a key to promoting ferromagnetism via spin-flip by electron transfer, predicted from density-functional-theory (DFT) calculations. The approach toward nanodefect engineering paves a way to overcome weak magnetic order in diluted magnetic semiconductors (DMSs) for renovating semiconductor spintronics.

Tunable tunneling magnetoresistance in in-plane double barrier magnetic tunnel junctions based on B vacancy h-NB nanoribbons

Magnetic tunnel junctions (MTJs) have attained new opportunities due to the emergence of two-dimensional (2D) magnetic materials after they were proposed more than forty years ago. Here, an in-plane double barrier magnetic tunnel junction (IDB-MTJ) based on B vacancy h-NB nanoribbons has been proposed firstly, and the transport properties have been studied using density functional theory combined with the nonequilibrium Green's function method. Due to its unique structural characteristics, the tunneling magnetoresistance (TMR) ratio can be tuned and the maximum TMR can reach 1.86 × 10⁵. The potential applications of the IDB-MTJ in magnetic random-access memories and logical computation have also been discussed. We find that the IDB-MTJs have great potential in magnetic random-access memories and logical computation applications.

Conduction band-edge valley splitting in two-dimensional ferroelectric AgBiP2S6 by magnetic doping: towards electron valley-polarized transport

Two-dimensional valleytronic systems, using the valley index of carriers to perform logic operations, serves as the basis of the next-generation information technologies. For efficient use of the valley degree of freedom, the major challenge currently is to lift the valley degeneracy to achieve valley splitting. In this work, using first-principles calculations, we propose that valley splitting can be readily achieved in a ferroelectric AgBiP₂S₆ monolayer by TM doping (TM = V, Cr, Mn, Fe, Co, and Ni), which is highly feasible in experiments. In sharp contrast to most previous reports of valley-related features in the valence band-edge, the pristine AgBiP₂S₆ monolayer has a direct band-gap located at K/K′ points of the Brillouin zone and harbors strong coupled spin and valley physics around the conduction band-edge, due to inversion symmetry breaking combined with strong spin–orbit coupling. By TM-doping, the local magnetic moment can be introduced into the system, which can destroy the valley degeneration of the conduction band-edge and induce valley splitting. Especially in a V-doped system, accompanied with a large valley splitting (26.8 meV), there is a serious n-type doping in AgBiP₂S₆. The efficient electron-doping moves the Fermi level just located between the conduction band minimum of the K/K′ valleys, which is suitable for valley-polarized transport. Moreover, the valley-polarized index can be flipped by applying a small magnetic field to rotate the magnetocrystalline direction. The magnitude of valley splitting relies on the strength of orbital hybridization between the TM-d and Bi-p states and can be tuned continually by applying biaxial strain. Under an in-plane electric field, such valley degeneracy breaking would give rise to the long-sought anomalous valley Hall effect, which is crucial to design a valleytronic device.

The anomalous valley Hall effect in two-dimensional ferroelectric AgBiP₂S₆ by magnetic doping offering a possible avenue to realize data storage using valley splitting.

Spin-Selective Hole-Exciton Coupling in a V-Doped WSe2 Ferromagnetic Semiconductor at Room Temperature

While valley polarization with strong Zeeman splitting is the most prominent characteristic of two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors under magnetic fields, enhancement of the Zeeman splitting has been demonstrated by incorporating magnetic dopants into the host materials. Unlike Fe, Mn, and Co, V is a distinctive dopant for ferromagnetic semiconducting properties at room temperature with large Zeeman shifting of band edges. Nevertheless, little known is the excitons interacting with spin-polarized carriers in V-doped TMDs. Here, we report anomalous circularly polarized photoluminescence (CPL) in a V-doped WSe₂ monolayer at room temperature. Excitons couple to V-induced spin-polarized holes to generate spin-selective positive trions, leading to differences in the populations of neutral excitons and trions between left and right CPL. Using transient absorption spectroscopy, we elucidate the origin of excitons and trions that are inherently distinct for defect-mediated and impurity-mediated trions. Ferromagnetic characteristics are further confirmed by the significant Zeeman splitting of nanodiamonds deposited on the V-doped WSe₂ monolayer.

Quantum Internal Structure of Plasmons

Plasmons are usually described in terms of macroscopic quantities such as electric fields and currents. However, as fundamental excitations of metals, they are also quantum objects with internal structure. We demonstrate that this can induce an intrinsic dipole moment which is tied to the quantum geometry of the Hilbert space of plasmon states. This quantum geometric dipole offers a unique handle for manipulation of plasmon dynamics via density modulations and electric fields. As a concrete example, we demonstrate that scattering of plasmons with a nonvanishing quantum geometric dipole from impurities is nonreciprocal, skewing in different directions in a valley-dependent fashion. This internal structure can be used to control plasmon trajectories in two dimensional materials.

Large-Area Epitaxial Film Growth of van der Waals Ferromagnetic Ternary Chalcogenides

Following the first experimental realization of intrinsic ferromagnetism in 2D van der Waals (vdW) crystals, several ternary metal chalcogenides with unprecedented long-range ferromagnetic order have been explored. However, the synthesis of large-area 2D ternary metal chalcogenide thin films is a great challenge, and a generalized synthesis has not been demonstrated yet. Here, a quick and scalable synthesis of epitaxially aligned ferromagnetic ternary metal chalcogenide thin films (Cr₂ Ge₂ Te₆ , Cr₂ Si₂ Te₆ , Mn₃ Si₂ Te₆ ) is reported. The synthesis is based on the flux-controlled surface diffusion of Te on metal (Cr, Mn)-deposited wafer (Ge, Si) substrates. Magnetic anisotropy study of the epitaxial ternary thin films reveals the intrinsic magnetic easy axis; out-of-plane direction for Cr₂ Ge₂ Te₆ and Cr₂ Si₂ Te₆ , and in-plane direction for Mn₃ Si₂ Te₆ . In addition to the synthesis, this work creates an opportunity for transfer-free device fabrication for realizing magnetoelectronics based on the electrical control of both charge and spin degrees of freedom in 2D ferromagnetic semiconductors.

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.

First-principles computational exploration of ferromagnetism in monolayer GaS via substitutional doping

Using first-principles calculations, functionalization of the monolayer-GaS crystal structure through N or Cr-doping at all possible lattice sites has been investigated. Our results show that pristine monolayer-GaS is an indirect-bandgap, non-magnetic semiconductor. The bandgap can be tuned and a magnetic moment (MM) can be induced by the introduction of N or Cr atomic anion/cation doping in monolayer GaS. For instance, the intrinsic character of monolayer GaS can be changed by substitution of N for the S-site to p-type, while substitution of Cr at the S-site or Ga-site induces half-metallicity at sufficiently high concentrations. The defect states are located in the electronic bandgap region of the GaS monolayer. These findings help to extend the application of monolayer-GaS structures in nano-electronics and spintronics. Since the S-sites at the surface are more easily accessible to doping in experiment, we chose the S-site for further investigations. Finally, we perform calculations with ferromagnetic (FM) and antiferromagnetic (AFM) alignment of the MMs at the dopants. For pairs of impurities of the same species at low concentrations we find Cr atoms to prefer the FM state, while N atoms prefer the AFM state, both for impurities on opposite surfaces of the GaS monolayer and for impurities sharing a common Ga neighbor sitting at the same surface. Extending our study to higher concentrations of Cr atoms, we find that clusters of four Cr atoms prefer AFM coupling, whereas the FM coupling is retained for Cr atoms at larger distance arranged on a honeycomb lattice. For the latter arrangement, we estimate the FM Curie temperatureT_Cto be 241 K. We conclude that the Cr-doped monolayer-GaS crystal structure offers enhanced electronic and magnetic properties and is an appealing candidate for spintronic devices operating close to room temperature.

Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe₂O₃ nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe₂O₃ nanoflakes epitaxially grown on α-Al₂O₃ substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe₂O₃ thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe₂O₃ nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

Doping Concentration Modulation in Vanadium-Doped Monolayer Molybdenum Disulfide for Synaptic Transistors

Doping is an effective way to modify the electronic property of two-dimensional (2D) materials and endow them with additional functionalities. However, wide-range control of the doping concentrations in monolayer 2D materials with large-scale uniformity remains challenging. Here, we report in situ chemical vapor deposition growth of vanadium-doped monolayer molybdenum disulfide (MoS₂) with widely tunable doping concentrations ranging from 0.3 to 13.1 atom %. The key to regulate the doping concentration lies in the use of appropriate vanadium precursors with different doping abilities, which also generate large-scale uniform doping to MoS₂. Artificial synaptic transistors were fabricated using the heavily doped MoS₂ as the channel material. Synaptic potentiation, depression, and repetitive learning processes were mimicked by the gate-tunable changes of channel conductance in such transistors with abundant vanadium atoms to trap/detrap electrons. This work develops a feasible method to dope monolayer 2D semiconductors and demonstrates their applications in artificial synaptic transistors.

Two-dimensional magneto-photoconductivity in non-van der Waals manganese selenide

Deficient intrinsic species and suppressed Curie temperatures (T_c) in two-dimensional (2D) magnets are major barriers for future spintronic applications. As an alternative, delaminating non-van der Waals (vdW) magnets can offset these shortcomings and involve robust bandgaps to explore 2D magneto-photoconductivity at ambient temperature. Herein, non-vdW α-MnSe₂ is first delaminated as quasi-2D nanosheets for the study of emerging semiconductor, ferromagnetism and magneto-photoconductivity behaviors. Abundant nonstoichiometric surfaces induce the renormalization of the band structure and open a bandgap of 1.2 eV. The structural optimization strengthens ferromagnetic super-exchange interactions between the nearest-neighbor Mn²⁺, which enables us to achieve a high T_c of 320 K well above room temperature. The critical fitting of magnetization and transport measurements both verify that it is of quasi-2D nature. The above observations are evidenced by multiple microscopic and macroscopic characterization tools, in line with the prediction of first-principles calculations. Profiting from the negative magnetoresistance effect, the self-powered infrared magneto-photoconductivity performance including a responsivity of 330.4 mA W^-1 and a millisecond-level response speed are further demonstrated. Such merits stem from the synergistic modulation of magnetic and light fields on photogenerated carriers. This provides a new strategy to manipulate both charge and spin in 2D non-vdW systems and displays their alluring prospects in magneto-photodetection.

Enhanced Electrical Performance of Monolayer MoS2 with Rare Earth Element Sm Doping

Rare earth (RE) element-doped two-dimensional (2D) transition metal dichalcogenides (TMDCs) with applications in luminescence and magnetics have received considerable attention in recent years. To date, the effect of RE element doping on the electronic properties of monolayer 2D-TMDCs remains unanswered due to challenges including the difficulty of achieving valid monolayer doping and introducing RE elements with distinct valence and atomic configurations. Herein, we report a unique strategy to grow the Sm-doped monolayer MoS₂ film by using an atmospheric pressure chemical vapor deposition method with the substrate face down on top of the growth source. A stable monolayer triangular Sm-doped MoS₂ was achieved. The threshold voltage of an Sm-doped MoS₂-based field effect transistor (FET) moved from -12 to 0 V due to the p-type character impurity state introduced by Sm ions in monolayer MoS₂. Additionally, the electrical performance of the monolayer MoS₂-based FET was improved by RE element Sm doping, including a 500% increase of the on/off current ratio and a 40% increase of the FET's mobility. The electronic property enhancement resulted from Sm doping MoS₂, which led internal lattice strain and changes in Fermi energy levels. These findings provide a general approach to synthesize RE element-doped monolayer 2D-TMDCs and to enrich their applications in electrical devices.

2D Polarized Materials: Ferromagnetic, Ferrovalley, Ferroelectric Materials, and Related Heterostructures

The emergence of 2D polarized materials, including ferromagnetic, ferrovalley, and ferroelectric materials, has demonstrated unique quantum behaviors at atomic scales. These polarization behaviors are tightly bonded to the new degrees of freedom (DOFs) for next generation information storage and processing, which have been dramatically developed in the past few years. Here, the basic 2D polarized materials system and related devices' application in spintronics, valleytronics, and electronics are reviewed. Specifically, the underlying physical mechanism accompanied with symmetry broken theory and the modulation process through heterostructure engineering are highlighted. These summarized works focusing on the 2D polarization would continue to enrich the cognition of 2D quantum system and promising practical applications.

Antiferromagnetic proximity coupling between semiconductor quantum emitters in WSe2 and van der Waals ferromagnets

van der Waals ferromagnets have gained significant interest due to their unique ability to provide magnetic response even at the level of a few monolayers. Particularly in combination with 2D semiconductors, such as the transition metal dichalcogenide WSe2, one can create heterostructures that feature unique magneto-optical response in the exciton emission through the magnetic proximity effect. Here we use 0D quantum emitters in WSe2 to probe for the ferromagnetic response in heterostructures with Fe3GT and Fe5GT ferromagnets through an all-optical read-out technique that does not require electrodes. The spectrally narrow spin-doublet of the WSe2 quantum emitters allowed to fully resolve the hysteretic magneto-response in the exciton emission, revealing the characteristic signature of both ferro- and antiferromagnetic proximity coupling that originates from the interplay among Fe3GT or Fe5GT, a thin surface oxide, and the spin doublets of the quantum emitters. Our work highlights the utility of 0D quantum emitters for probing interface magnetic dipoles in vdW heterostructures with high precision. The observed hysteretic magneto response in the exciton emission of quantum emitters adds further new degrees of freedom for spin and g-factor manipulation of quantum states.

The effects of substitutional Fe-doping on magnetism in MoS2and WS2monolayers

Doping of two-dimensional (2D) semiconductors has been intensively studied toward modulating their electrical, optical, and magnetic properties. While ferromagnetic 2D semiconductors hold promise for future spintronics and valleytronics, the origin of ferromagnetism in 2D materials remains unclear. Here, we show that substitutional Fe-doping of MoS₂and WS₂monolayers induce different magnetic properties. The Fe-doped monolayers are directly synthesized via chemical vapor deposition. In both cases, Fe substitutional doping is successfully achieved, as confirmed using scanning transmission electron microscopy. While both Fe:MoS₂and Fe:WS₂show PL quenching and n-type doping, Fe dopants in WS₂monolayers are found to assume deep-level trap states, in contrast to the case of Fe:MoS₂, where the states are found to be shallow. Usingμm- and mm-precision local NV^-magnetometry and superconducting quantum interference device, we discover that, unlike MoS₂monolayers, WS₂monolayers do not show a magnetic phase transition to ferromagnetism upon Fe-doping. The absence of ferromagnetism in Fe:WS₂is corroborated using density functional theory calculations.