Tunable Doping of Rhenium and Vanadium into Transition Metal Dichalcogenides for Two-Dimensional Electronics

Defect Engineering of 2D Semiconductors for Dual Control of Emission and Carrier Polarity

2D transition metal dichalcogenides (TMDCs) are considered as promising materials in post-Moore technology. However, the low photoluminescence quantum yields (PLQY) and single carrier polarity due to the inevitable defects during material preparation are great obstacles to their practical applications. Here, an extraordinary defect engineering strategy is reported based on first-principles calculations and realize it experimentally on WS2 monolayers by doping with IIIA atoms. The doped samples with large sizes possess both giant PLQY enhancement and effective carrier polarity modulation. Surprisingly, the high PL emission maintained even after one year under ambient environment. Moreover, the constructed p-n homojunctions shows high rectification ratio (≈2200), ultrafast response times and excellent stability. Meanwhile, the doping strategy is universally applicable to other TMDCs and dopants. This smart defect engineering strategy not only provides a general scheme to eliminate the negative influence of defects, but also utilize them to achieve desired optoelectronic properties for multifunctional applications.

Synthesis and Modulation of Low-Dimensional Transition Metal Chalcogenide Materials via Atomic Substitution

In recent years, low-dimensional transition metal chalcogenide (TMC) materials have garnered growing research attention due to their superior electronic, optical, and catalytic properties compared to their bulk counterparts. The controllable synthesis and manipulation of these materials are crucial for tailoring their properties and unlocking their full potential in various applications. In this context, the atomic substitution method has emerged as a favorable approach. It involves the replacement of specific atoms within TMC structures with other elements and possesses the capability to regulate the compositions finely, crystal structures, and inherent properties of the resulting materials. In this review, we present a comprehensive overview on various strategies of atomic substitution employed in the synthesis of zero-dimensional, one-dimensional and two-dimensional TMC materials. The effects of substituting elements, substitution ratios, and substitution positions on the structures and morphologies of resulting material are discussed. The enhanced electrocatalytic performance and photovoltaic properties of the obtained materials are also provided, emphasizing the role of atomic substitution in achieving these advancements. Finally, challenges and future prospects in the field of atomic substitution for fabricating low-dimensional TMC materials are summarized.

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Tuning Polarity in WSe2/AlScN FeFETs via Contact Engineering

Recent advancements in ferroelectric field-effect transistors (FeFETs) using two-dimensional (2D) semiconductor channels and ferroelectric Al0.68Sc0.32N (AlScN) allow high-performance nonvolatile devices with exceptional ON-state currents, large ON/OFF current ratios, and large memory windows (MW). However, previous studies have solely focused on n-type FeFETs, leaving a crucial gap in the development of p-type and ambipolar FeFETs, which are essential for expanding their applicability to a wide range of circuit-level applications. Here, we present a comprehensive demonstration of n-type, p-type, and ambipolar FeFETs on an array scale using AlScN and multilayer/monolayer WSe2. The dominant injected carrier type is modulated through contact engineering at the metal-semiconductor junction, resulting in the realization of all three types of FeFETs. The effect of contact engineering on the carrier injection is further investigated through technology-computer-aided design simulations. Moreover, our 2D WSe2/AlScN FeFETs achieve high electron and hole current densities of ∼20 and ∼10 μA/μm, respectively, with a high ON/OFF ratio surpassing ∼107 and a large MW of >6 V (0.14 V/nm).

Engineering the Electrical and Optical Properties of WS2 Monolayers via Defect Control

Two-dimensional (2D) materials as tungsten disulphide (WS2 ) are rising as the ideal platform for the next generation of nanoscale devices due to the excellent electric-transport and optical properties. However, the presence of defects in the as grown samples represents one of the main limiting factors for commercial applications. At the same time, WS2 properties are frequently tailored by introducing impurities at specific sites. Aim of this review paper is to present a complete description and discussion of the effects of both intentional and unintentional defects in WS2 , by an in depth analysis of the recent experimental and theoretical investigations reported in the literature. First, the most frequent intrinsic defects in WS2 are presented and their effects in the readily synthetized material are discussed. Possible solutions to remove and heal unintentional defects are also analyzed. Following, different doping schemes are reported, including the traditional substitution approach and innovative techniques based on the surface charge transfer with adsorbed atoms or molecules. The plethora of WS2 monolayer modifications presented in this review and the systematic analysis of the corresponding optical and electronic properties, represent strategic degrees of freedom the researchers may exploit to tailor WS2 optical and electronic properties for specific device applications.

p-Type Two-Dimensional Semiconductors: From Materials Preparation to Electronic Applications

Two-dimensional (2D) materials are regarded as promising candidates in many applications, including electronics and optoelectronics, because of their superior properties, including atomic-level thickness, tunable bandgaps, large specific surface area, and high carrier mobility. In order to bring 2D materials from the laboratory to industrialized applications, materials preparation is the first prerequisite. Compared to the n-type analogs, the family of p-type 2D semiconductors is relatively small, which limits the broad integration of 2D semiconductors in practical applications such as complementary logic circuits. So far, many efforts have been made in the preparation of p-type 2D semiconductors. In this review, we overview recent progresses achieved in the preparation of p-type 2D semiconductors and highlight some promising methods to realize their controllable preparation by following both the top-down and bottom-up strategies. Then, we summarize some significant application of p-type 2D semiconductors in electronic and optoelectronic devices and their superiorities. In end, we conclude the challenges existed in this field and propose the potential opportunities in aspects from the discovery of novel p-type 2D semiconductors, their controlled mass preparation, compatible engineering with silicon production line, high-κ dielectric materials, to integration and applications of p-type 2D semiconductors and their heterostructures in electronic and optoelectronic devices. Overall, we believe that this review will guide the design of preparation systems to fulfill the controllable growth of p-type 2D semiconductors with high quality and thus lay the foundations for their potential application in electronics and optoelectronics.

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.

Controllable Synthesis of 2H-1T' Mox Re(1- x ) S2 Lateral Heterostructures and Their Tunable Optoelectronic Properties

Constructing heterostructures and doping are valid ways to improve the optoelectronic properties of transition metal dichalcogenides (TMDs) and optimize the performance of TMDs-based photodetectors. Compared with transfer techniques, chemical vapor deposition (CVD) has higher efficiency in preparing heterostructures. As for the one-step CVD growth of heterostructures, cross-contamination between the two materials may occur during the growth process, which may provide the possibility of one-step simultaneous realization of controllable doping and formation of alloy-based heterostructures by finely tuning the growth dynamics. Here, 2H-1T' Mox Re(1- x ) S2 alloy-to-alloy lateral heterostructures are synthesized through this one-step CVD growth method, utilizing the cross-contamination and different growth temperatures of the two alloys. Due to the doping of a small amount of Re atoms in 2H MoS2 , 2H Mox Re(1- x ) S2 has a high response rejection ratio in the solar-blind ultraviolet (SBUV) region and exhibits a positive photoconductive (PPC) effect. While the 1T' Mox Re(1- x ) S2 formed by heavily doping Mo atoms into 1T' ReS2 will produce a negative photoconductivity (NPC) effect under UV laser irradiation. The optoelectronic property of 2H-1T' Mox Re(1- x ) S2 -based heterostructures can be modulated by gate voltage. These findings are expected to expand the functionality of traditional optoelectronic devices and have potential applications in optoelectronic logic devices.

Dilute Rhenium Doping and its Impact on Defects in MoS2

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS2, indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS2. This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

Proximity-Induced Tunable Magnetic Order at the Interface of All-van der Waals-Layered Heterostructures

Spin-orbit coupling (SOC) plays a crucial role in controlling the spin-charge conversion efficiency, spin torque, and complex magnetic spin structures. In this study, we investigate the interplay between SOC and ferromagnetism in heterostructures of large-SOC and magnetic materials. We highlight the importance of the SOC-proximity effect on magnetic ordering in all-van der Waals-layered heterostructures, specifically Fe3GeTe2(FGT)/monolayer W1-xVxSe2 (x = 0 and 0.05). By increasing the SOC strength, we demonstrate various magnetic orderings induced at the interface of the heterostructure, including spin-flop, spin-flip, and inverted magnetization. Moreover, we show a sharp magnetic switching from antiferromagnetic state to ferromagnetic state in FGT/W0.95V0.05Se2, which is characteristic of the synthetic antiferromagnetic structure. This proof-of-concept result offers the possibility of interface-tailoring spintronics, including two-dimensional magnetoresistive random access memory toggle switching. Our findings provide insight into the design and development of next-generation spintronic devices by exploiting the interplay between SOC and magnetic ordering in all-van der Waals-layered heterostructures.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

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.

Thermal and Chemical Phase Engineering of Two-Dimensional Ruthenate

Monolayer ruthenate nanosheets obtained by exfoliating layered ruthenium oxide exhibit excellent electrical conductivity, redox activity, and catalytic activity, which render them suitable for advanced electronic and energy devices. However, to fully exploit the benefits, we require further structural insights into a complex polymorphic nature and diversity in relevant electronic states of two-dimensional (2D) ruthenate systems. In this study, the 2D structures, stability, and electronic states of 2D ruthenate are investigated on the basis of thermal and chemical phase engineering approaches. We reveal that contrary to a previous report, exfoliation of an oblique 1T phase precursor leads to nanosheets having an identical phase without exfoliation-induced phase transition to a 1H phase. The oblique 1T phase in the nanosheets is found to be metastable and, thus, transforms successively to a rectangular 1T phase upon heating. A phase-controllable synthesis via Co doping affords nanosheets with metastable rectangular and thermally stable hexagonal 1T phases at a Co content of 5-10 and 20 at%, respectively. The 1T phases show metallic electronic states, where the d-d optical transitions between the Ru 4d (t_2g) orbital depend on the symmetry of the Ru framework. The Co doping in ruthenate nanosheets unexpectedly suppresses the redox and catalytic activities under acidic conditions. In contrast, the Co^2+/3+ redox pair is activated and produces conductive nanosheets with high electrochemical capacitance in an alkaline condition.

Engineering the Local Atomic Configuration in 2H TMDs for Efficient Electrocatalytic Hydrogen Evolution

The introduction of heteroatoms is a widely employed strategy for electrocatalysis of transition metal dichalcogenides (TMDs). This approach activates the inactive basal plane, effectively boosting the intrinsic catalytic activity. However, the effect of atomic configurations incorporated within the TMDs' lattice on catalytic activity is not thoroughly understood owing to the lack of controllable synthetic approaches for highly doped TMDs. In this study, we demonstrate a facile approach to realizing heavily doped MoS₂ with a high doping concentration above 16% via intermediate-reaction-mediated chemical vapor deposition. As the V doping concentration increased, the incorporated V atoms coalesced in a manner that enabled both the basal plane activation and electrical conductivity enhancement of MoS₂. This accelerated the kinetics of the hydrogen evolution reaction (HER) through the reduced Gibbs free energy of hydrogen adsorption, as evidenced by experimental and theoretical analyses. Consequently, the coalesced V-doped MoS₂ exhibited superior HER performance, with an overpotential of 100 mV at 10 mA cm^-2, surpassing the pristine and single-atom-doped counterparts. This study provides an intriguing pathway for engineering the atomic doping configuration of TMDs to develop efficient 2D nanomaterial-based electrocatalysts.

Liquid Phase Edge Epitaxy of Transition-Metal Dichalcogenide Monolayers

Precise monolayer epitaxy is important for two-dimensional (2D) semiconductors toward future electronics. Here, we report a new self-limited epitaxy approach, liquid phase edge epitaxy (LPEE), for precise-monolayer epitaxy of transition-metal dichalcogenides. In this method, the liquid solution contacts 2D grains only at the edges, which confines the epitaxy only at the grain edges and then precise monolayer epitaxy can be achieved. High-temperature in situ imaging of the epitaxy progress directly supports this edge-contact epitaxy mechanism. Typical transition-metal dichalcogenide monolayers (MX₂, M = Mo, W, and Re; X = S or Se) have been obtained by LPEE with a proper choice of molten alkali halide solvents (AL, A = Li, Na, K, and Cs; L = Cl, Br, or I). Furthermore, alloying and magnetic-element doping have also been realized by taking advantage of the liquid phase epitaxy approach. This LPEE method provides a precise and highly versatile approach for 2D monolayer epitaxy and can revolutionize the growth of 2D materials toward electronic applications.

Liquid-precursor-intermediated synthesis of atomically thin transition metal dichalcogenides

With the rapid development of integrated electronics and optoelectronics, methods for the scalable industrial-scale growth of two-dimensional (2D) transition metal dichalcogenide (TMD) materials have become a hot research topic. However, the control of gas distribution of solid precursors in common chemical vapor deposition (CVD) is still a challenge, resulting in the growth of 2D TMDs strongly influenced by the location of the substrate from the precursor powder. In contrast, liquid-precursor-intermediated growth not only avoids the use of solid powders but also enables the uniform distribution of precursors on the substrate through spin-coating, which is much more favorable for the synthesis of wafer-scale TMDs. Moreover, the spin-coating process based on liquid precursors can control the thickness of the spin-coated films by regulating the solution concentration and spin-coating speed. Herein, this review focuses on the recent progress in the synthesis of 2D TMDs based on liquid-precursor-intermediated CVD (LPI-CVD) growth. Firstly, the different assisted treatments based on LPI-CVD strategies for monolayer 2D TMDs are introduced. Then, the progress in the regulation of the different physical properties of monolayer 2D TMDs by substitution of the transition metal and their corresponding heterostructures based on LPI-CVD growth are summarized. Finally, the challenges and perspectives of 2D TMDs based on the LPI-CVD method are discussed.

Graphene and Beyond: Recent Advances in Two-Dimensional Materials Synthesis, Properties, and Devices

Since the isolation of graphene in 2004, two-dimensional (2D) materials research has rapidly evolved into an entire subdiscipline in the physical sciences with a wide range of emergent applications. The unique 2D structure offers an open canvas to tailor and functionalize 2D materials through layer number, defects, morphology, moiré pattern, strain, and other control knobs. Through this review, we aim to highlight the most recent discoveries in the following topics: theory-guided synthesis for enhanced control of 2D morphologies, quality, yield, as well as insights toward novel 2D materials; defect engineering to control and understand the role of various defects, including in situ and ex situ methods; and properties and applications that are related to moiré engineering, strain engineering, and artificial intelligence. Finally, we also provide our perspective on the challenges and opportunities in this fascinating field.

Bifunctional Monolayer WSe2/Graphene Self-Stitching Heterojunction Microreactors for Efficient Overall Water Splitting in Neutral Medium

Developing efficient bifunctional electrocatalysts in neutral media to avoid the deterioration of electrodes or catalysts under harsh environments has become the ultimate goal in electrochemical water splitting. This work demonstrates the fabrication of an on-chip bifunctional two-dimensional (2D) monolayer (ML) WSe₂/graphene heterojunction microreactor for efficient overall water splitting in a neutral medium (pH = 7). Through the synergistic atomic growth of the metallic Cr dopant and graphene stitching contact on the 2D ML WSe₂, the bifunctional WSe₂/graphene heterojunction microreactor consisting of a full-cell configuration demonstrates excellent performance for overall water splitting in a neutral medium. Atomic doping of metallic Cr atoms onto the 2D ML WSe₂ effectively facilitates the charge transfer at the solid-liquid interface. In addition, the direct growth of the self-stitching graphene contact with the 2D WSe₂ catalyst largely reduces the contact resistance of the microreactor and further improves the overall water splitting efficiency. A significant reduction of the overpotential of nearly 1000 mV at 10 mA cm^-2 at the Cr-doped WSe₂/graphene heterojunction microreactor compared to the ML pristine WSe₂ counterpart is achieved. The bifunctional WSe₂/graphene self-stitching heterojunction microreactor is an ideal platform to investigate the fundamental mechanism of emerging bifunctional 2D catalysts for overall water splitting in a neutral medium.

P-Type 2D Semiconductors for Future Electronics

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Performance improvement in monolayered SnS2 double-gate field-effect transistors via point defect engineering

Owing to the relatively high carrier mobility and on/off current ratio, monolayered SnS₂ has the advantage of suppressing drain-to-source tunneling for short channels, rendering it a promising candidate in field-effect transistor (FET) applications. To extend the scaling limit of the channel length, we propose to rationally modulate the electronic properties of monolayered SnS₂ through the customized design of point defects and simulate its performance limit in sub-5 nm double-gate FETs (DGFETs), using density functional theory combined with nonequilibrium Green's function formalism. Among all types of point defects, the Se atom as a substitutional dopant (Se_S) can nondegenerately inject electrons into each monolayered (ML) SnS₂ 2 × 4 × 1 supercell, whereas the Sn vacancy (V_Sn) defect exhibits an opposite doping effect. By adjusting the lateral Schottky barrier height between electrodes and the channel region, the on-state current (I_on), on/off ratio, delay time, and power-delay product in the formed n-type Se_S-doped SnS₂ and p-type V_Sn-doped SnS₂ DGFETs with a channel length of 4.5 nm have been remarkably improved, fulfilling the requirements of the International Technology Roadmap for Semiconductors (ITRS) for high-performance applications in the 2028 horizon. Our work unveils the great significance of point defect engineering for applications in ultimately scaled electronics.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode

In this study, selective Nb doping (P-type) at the WS₂ layer in a WS₂-MoS₂ lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO₃ > WO₃ > Nb₂O₅) enable the separation of the growth temperature of intrinsic MoS₂ to 700 °C (first grown inner layer) and Nb-doped WS₂ to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS₂ layer is selectively controlled from approximately 1.87 × 10⁷/cm² at 1.5 at.% Nb to approximately 1.16 × 10¹³/cm² at 8.1 at.% Nb, while the electron carrier density in n-type MoS₂ shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS₂-MoS₂ heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (10⁵) and a minimum channel resistance (10⁸ Ω). With this optimized photodiode (8.1 at.% Nb at V_g = -30 V), an I_photo/I_dark ratio of 6000 and a detectivity of 1.1 × 10¹⁴ Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

WSe2-VSe2 Alloyed Nanosheets to Enhance the Catalytic Performance of Hydrogen Evolution Reaction

Tuning the electronic structures of transition metal dichalcogenides (TMD) is essential for their implementation in next-generation energy technologies. In this study, we synthesized composition-tuned WSe₂-VSe₂ (W_1-xV_xSe₂, x = 0-1) alloyed nanosheets using a colloidal reaction. Alloying the semiconducting WSe₂ with VSe₂ converts the material into a metallic one, followed by a 2H-to-1T phase transition at x = 0.7. Over a wide composition range, WSe₂ and VSe₂ are atomically immiscible and form separate ordered domains. The miscible alloy at x = 0.1 displayed enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in an acidic electrolyte. This trend was correlated with the d-band center via a volcano-type relationship. Spin-polarized density functional theory calculations consistently predicted the atomic immiscibility, which became more significant at the 2H-1T phase transition composition. The Gibbs free energy of H adsorption on the basal planes (Se or hole sites) and the activation barriers along the Volmer-Heyrovsky reaction pathway supported the enhanced HER performance of the alloy phase, suggesting that the dispersed V-doped structures were responsible for the best HER catalytic activity. Our study demonstrates how the atomic structure of TMD alloy nanosheets plays a crucial role in enhancing catalytic activity.

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.

Nontrivial Doping Evolution of Electronic Properties in Ising-Superconducting Alloys

Transition metal dichalcogenides offer unprecedented versatility to engineer 2D materials with tailored properties to explore novel structural and electronic phase transitions. In this work, the atomic-scale evolution of the electronic ground state of a monolayer of Nb_{1- δ} Mo_δ Se₂ across the entire alloy composition range (0 < δ < 1) is investigated using low-temperature (300 mK) scanning tunneling microscopy and spectroscopy (STM/STS). In particular, the atomic and electronic structure of this 2D alloy throughout the metal to semiconductor transition (monolayer NbSe₂ to MoSe₂ ) is studied. The measurements enable extraction of the effective doping of Mo atoms, the bandgap evolution and the band shifts, which are monotonic with δ. Furthermore, it is demonstrated that collective electronic phases (charge density wave and superconductivity) are remarkably robust against disorder and further shown that the superconducting T_C changes non-monotonically with doping. This contrasting behavior in the normal and superconducting state is explained using first-principles calculations. Mo doping is shown to decrease the density of states at the Fermi level and the magnitude of pair-breaking spin fluctuations as a function of Mo content. These results paint a detailed picture of the electronic structure evolution in 2D TMD alloys, which is of utmost relevance for future 2D materials design.

Activating the Electrocatalysis of MoS2 Basal Plane for Hydrogen Evolution via Atomic Defect Configurations

Point defects of heteroatoms and vacancies can activate the inert basal plane of molybdenum sulfide (MoS₂ ) to improve its performance on catalyzing the hydrogen evolution reaction (HER). However, the synergy between heteroatoms and vacancies is still unclear. Here, a chemical vapor deposition-assisted in situ vanadium (V) doping method is used to synthesize monolayer MoS₂ with abundant and tunable vacancies and V-dopants in the lattice. Ten delicate defect configurations are prepared to provide a complex system for the relationship investigation between microstructure and catalytic performance. The combination of on-chip electrochemical tests and theoretical calculations indicates that the HER performance greatly depends on the type and amount of defect configurations. The optimal configuration is that three V atoms are aggregated and accompanied by abundant sulfur vacancies, in which, H atoms directly interact with Mo and V atoms to form the most stable metal-bridge structure. The on-chip measurements also confirm that the sample with high concentrations of this type of defect configuration exhibits the best catalytic performance, indicating the efficient synergy in the optimal configuration. The revealed effects of defect configurations are expected to inspire the design and regulation of high-efficiency 2D catalysts.

Efficient double-layer sintering of titanomagnetite concentrate

The traditional titanomagnetite sintering process consumes high fuel and produces weak-strength sinter. In this study, double-layer sintering was used to solve above problems. The theoretical analysis and sintering pot test results showed that sintering of feed bed constituted by two different-basicity layers could improve mineralization and permeability. By using the double layer structure of sintering bed and controlling the basicities of lower and upper layer (2.5 and 1.5, respectively), the yield, productivity, and reduction disintegration index (RDI_+3.15) were 67.32%, 1.65 t m² h⁻¹, 49.68% respectively, which were improved 33.12%, 1.74%, and 9.27%, respectively than those obtained by the traditional sintering process. Meanwhile, 1.69 kg t⁻¹ of solid fuel consumption and nearly 10% of electricity and gas consumption for sintering were saved. It was demonstrated that using different basicities for upper and lower layer of sintering bed would promote formation of silica-ferrite of calcium and aluminum (SFCA) with simultaneous reduction of perovskite, improving the sinter quality.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Review of strategies toward the development of alloy two-dimensional (2D) transition metal dichalcogenides

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention owing to their prosperity in material research. The inimitable features of TMDCs triggered the emerging applications in diverse areas. In this review, we focus on the tailored and engineering of the crystal lattice of TMDCs that finally enhance the efficiency of the material properties. We highlight several preparation techniques and recent advancements in compositional engineering of TMDCs structure. We summarize different approaches for TMDCs such as doping and alloying with different materials, alloying with other 2D metals, and scrutinize the technological potential of these methods. Beyond that, we also highlight the recent significant advancement in preparing 2D quasicrystals and alloying the 2D TMDCs with MAX phases. Finally, we highlight the future perspectives for crystal engineering in TMDC materials for structure stability, machine learning concept marge with materials, and their emerging applications.

Salt-assisted chemical vapor deposition of two-dimensional transition metal dichalcogenides

Salt-assisted chemical vapor deposition (SA-CVD), which uses halide salts (e.g., NaCl, KBr, etc.) and molten salts (e.g., Na2MoO4, Na2WO4, etc.) as precursors, is one of the most popular methods favored for the fabrication of two-dimensional (2D) materials such as atomically thin metal chalcogenides, graphene, and h-BN. In this review, the distinct functions of halogens (F, Cl, Br, I) and alkali metals (Li, Na, K) in SA-CVD are first clarified. Based on the current development in SA-CVD growth and its related reaction modes, the existing methods are categorized into the Salt 1.0 (halide salts-based) and Salt 2.0 (molten salts-based) techniques. The achievements, advantages, and limitations of each technique are discussed in detail. Finally, new perspectives are proposed for the application of SA-CVD in the synthesis of 2D transition metal dichalcogenides for advanced electronics.

Impurity-Induced Emission in Re-Doped WS2 Monolayers

Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS₂ substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe-Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.