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

Innovations of metallic contacts on semiconducting 2D transition metal dichalcogenides toward advanced 3D-structured field-effect transistors

2D semiconductors, represented by transition metal dichalcogenides (TMDs), have the potential to be alternative channel materials for advanced 3D field-effect transistors, such as gate-all-around field-effect-transistors (GAAFETs) and complementary field-effect-transistors (C-FETs), due to their inherent atomic thinness, moderate mobility, and short scaling lengths. However, 2D semiconductors encounter several technological challenges, especially the high contact resistance issue between 2D semiconductors and metals. This review provides a comprehensive overview of the high contact resistance issue in 2D semiconductors, including its physical background and the efforts to address it, with respect to their applicability to GAAFET structures.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Evolution of Metal Tellurides for Energy Storage/Conversion: From Synthesis to Applications

Metal telluride (MTe)-based nanomaterials have emerged as a potential alternative for efficient, highly conductive, robust, and durable electrodes in energy storage/conversion applications. Significant progress in the material development of MTe-based electrodes is well-sought, from the synthesis of its nanostructures, integration of MTes with supporting materials, synthesis of their hybrid morphologies, and their implications in energy storage/conversion systems. Herein, an extensive exploration of the recent advancements and progress in MTes-based nanomaterials is reviewed. This review emphasizes elucidating the fundamental properties of MTes and providing a systematic compilation of its wet and dry synthesis methods. The applications of MTes are extensively summarized and discussed, particularly, in energy storage and conversion systems including batteries (Li-ion, Zn-ion, Li-S, Na-ion, K-ion), supercapacitor, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and CO2 reduction. The review also emphasizes the future prospects and urgent challenges to be addressed in the development of MTes, providing knowledge for researchers in utilizing MTes in energy storage and conversion technologies.

Vanadium Metal Doping of Monolayer MoS2 for p-Type Transistors and Fast-Speed Phototransistors

Modulating the electrical properties of two-dimensional (2D) materials is a fundamental prerequisite for their development to advanced electronic and optoelectronic devices. Substitutional doping has been demonstrated as an effective method for tuning the band structure in monolayer 2D materials. Here, we demonstrate a facile selective-area growth of vanadium-doped molybdenum disulfide (V-doped MoS2) flakes via pre-patterned vanadium-metal-assisted chemical vapor deposition (CVD). Optical microscopy characterization revealed the presence of flake arrays. Transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were employed to identify the chemical composition and crystalline structure of as-grown flakes. Electrical measurements indicated a light p-type conduction behavior in monolayer V-doped MoS2. Furthermore, the response time of phototransistors based on V-doped MoS2 monolayers exhibited a remarkable capability of 3 ms, representing approximately 3 orders of magnitude faster response than that observed in pure MoS2 phototransistors. This work hereby provides a feasible approach to doping of 2D materials, promising a scalable pathway for the integration of these materials into emerging electronic and optoelectronic devices.

A Molecular Coordination Strategy for Regulating the Interface of MoS2 Field Effect Transistors

Chemically modifying monolayer two-dimensional transition metal dichalcogenides (TMDs) with organic molecules provides a wide range of possibilities to regulate the electronic and optoelectronic performance of both materials and devices. However, it remains challenging to chemically attach organic molecules to monolayer TMDs without damaging their crystal structures. Herein, we show that the Mo atoms of monolayer MoS2 (1L-MoS2) in defect states can coordinate with both catechol and 1,10-phenanthroline (Phen) groups, affording a facile route to chemically modifying 1L-MoS2. Through the design of two isomeric molecules (LA2 and LA5) comprising catechol and Phen groups, we show that attaching organic molecules to Mo atoms via coordinative bonds has no negative effect on the crystal structure of 1L-MoS2. Both theoretical calculation and experiment results indicate that the coordinative strategy is beneficial for (i) repairing sulfur vacancies and passivating defects; (ii) achieving a long-term and stable n-doping effect; and (iii) facilitating the electron transfer. Field effect transistors (FETs) based on the coordinatively modified 1L-MoS2 show high electron mobilities up to 120.3 cm2 V-1 s-1 with impressive current on/off ratios over 109. Our results indicate that coordinatively attaching catechol- or Phen-bearing molecules may be a general method for the nondestructive modification of TMDs.

Sm and Gd Contacts in 2D Semiconductors for High-Performance Electronics and Spintronics

Two-dimensional (2D) semiconductors have attracted great attention due to their rich electronic properties and even been considered to have the potential to extend Moore's Law. However, the Schottky barrier between the metal and 2D semiconductor is formed due to the metal-induced gap states (MIGS), which greatly hinder the development of 2D semiconductor transistors in large-scale integrated circuits. Meanwhile, most air-stable 2D semiconductors are nonmagnetic, limiting the possibility of spintronic application. Here, we report a new strategy to suppress the MIGS and reduce the Schottky barrier height on 2D semiconductors (MoS2, WS2, and WSe2) by using lanthanide metal (Sm and Gd) contacts. It was found the lanthanide contacts exhibit a good Ohmic property with a near-zero Schottky barrier. As a result, the carrier mobility of MoS2 transistors reaches 118 cm2/(V s). Furthermore, Gd-contact MoS2 transistors show the typical magnetic property where the magnetoresistance reaches 2.7% at 5 K. By studying its spin valve effect, it was demonstrated that the nonlocal magnetoresistance is 4.1% and spin polarization is 3.25%. This study provides a promising pathway for high-performance 2D electronic and spintronics, which may open a new strategy for future computing-in-memory architecture.

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.

A One-Step Self-Flowering Method toward Programmable Ultrathin Porous Carbon-Based Materials for Microwave Absorption and Hydrogen Evolution

Ultrathin 2D porous carbon-based materials offer numerous fascinating electrical, catalytic, and mechanical properties, which hold great promise in various applications. However, it remains a formidable challenge to fabricate these materials with tunable morphology and composition by a simple synthesis strategy. Here, a facile one-step self-flowering method without purification and harsh conditions is reported for large-scale fabrication of high-quality ultrathin (≈1.5 nm) N-doped porous carbon nanosheets (NPC) and their composites. It is demonstrated that the layered tannic/oxamide (TA/oxamide) hybrid is spontaneously blown, exfoliated, bloomed, in situ pore-formed, and aromatized during pyrolysis to form flower-like aggregated NPC. This universal one-step self-flowering system is compatible with various precursors to construct multiscale NPC-based composites (Ru@NPC, ZnO@NPC, MoS2 @NPC, Co@NPC, rGO@NPC, etc.). Notably, the programmable architecture enables NPC-based materials with excellent multifunctional performances, such as microwave absorption and hydrogen evolution. This work provides a facile, universal, scalable, and eco-friendly avenue to fabricate functional ultrathin porous carbon-based materials with programmability.

Concentration Phase Separation of Substitution-Doped Atoms in TMDCs Monolayer

The density and spatial distribution of substituted dopants affect the transition metal dichalcogenides (TMDCs) materials properties. Previous studies have demonstrated that the density of dopants in TMDCs increases with the amount of doping, and the phenomenon of doping concentration difference between the nucleation center and the edge is observed, but the spatial distribution law of doping atoms has not been carefully studied. Here, it is demonstrated that the spatial distribution of dopants changes at high doping concentrations. The spontaneous formation of an interface with a steep doping concentration change is named concentration phase separation (CPS). The difference in the spatial distribution of dopants on both sides of the interface can be identified by an optical microscope. This is consistent with the results of spectral analysis and microstructure characterization of scanning transmission electron microscope. According to the calculation results of density functional theory, the chemical potential has two relatively stable energies as the doping concentration increases, which leads to the spontaneous formation of CPS. Understanding the abnormal phenomena is important for the design of TMDCs devices. This work has great significance in the establishment and improvement of the doping theory and the design of the doping process for 2D materials.

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.

Tuning 2D Magnetism in Cobalt Monoxide Nanosheets Via In Situ Nickel-Doping

Element doping has become an effective strategy to engineer the magnetic properties of two-dimensional (2D) materials and is widely explored in van der Waals layered transition metal dichalcogenides. However, the high-concentration substitution doping of 2D nonlayered metal oxides, which can preserve the original crystal texture and guarantee the homogeneity of doping distribution, is still a critical challenge due to the isotropic bonding of closed-packed structures. In this work, the synthesis of high-quality 2D nonlayered nickel-doped cobalt monoxide nanosheets via in situ atmospheric pressure chemical vapor deposition method is reported. High-resolution transmission electron microscopy confirmed that nickel atoms are doped at the intrinsic cobalt atom sites. The nickel doping concentration is stable at ≈15%, superior to most magnetic dopants doping in 2D materials and metal oxides. Magnetic measurements showed that pristine cobalt monoxide is nonferromagnetic, whereas nickel-doped cobalt monoxide exhibits robust ferromagnetic behavior with a Curie temperature of ≈180 K. Density functional theory calculations reveal that nickel atoms can improve the internal ferromagnetic correlation, giving rise to significant ferromagnetic performance of cobalt monoxide nanosheets. These results provide a valuable case for tuning the competing correlated states and magnetic ordering by substitution doping in 2D nonlayered oxide semiconductors.

All-Transfer Electrode Interface Engineering Toward Harsh-Environment-Resistant MoS2 Field-Effect Transistors

Nanoscale electronic devices that can work in harsh environments are in high demand for wearable, automotive, and aerospace electronics. Clean and defect-free interfaces are of vital importance for building nanoscale harsh-environment-resistant devices. However, current nanoscale devices are subject to failure in these environments, especially at defective electrode-channel interfaces. Here, harsh-environment-resistant MoS₂ transistors are developed by engineering electrode-channel interfaces with an all-transfer of van der Waals electrodes. The delivered defect-free, graphene-buffered electrodes keep the electrode-channel interfaces intact and robust. As a result, the as-fabricated MoS₂ devices have reduced Schottky barrier heights, leading to a very large on-state current and high carrier mobility. More importantly, the defect-free, hydrophobic graphene buffer layer prevents metal diffusion from the electrodes to MoS₂ and the intercalation of water molecules at the electrode-MoS₂ interfaces. This enables high resistances of MoS₂ devices with all-transfer electrodes to various harsh environments, including humid, oxidizing, and high-temperature environments, surpassing the devices with other kinds of electrodes. The work deepens the understanding of the roles of electrode-channel interfaces in nanoscale devices and provides a promising interface engineering strategy to build nanoscale harsh-environment-resistant devices.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

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.

Large-Scale Ultra-Robust MoS2 Patterns Directly Synthesized on Polymer Substrate for Flexible Sensing Electronics

Synthesis of large-area patterned MoS₂ is considered the principle base for realizing high-performance MoS₂ -based flexible electronic devices. Patterning and transferring MoS₂ films to target flexible substrates, however, require conventional multi-step photolithography patterning and transferring process, despite tremendous progress in the facilitation of practical applications. Herein, an approach to directly synthesize large-scale MoS₂ patterns that combines inkjet printing and thermal annealing is reported. An optimal precursor ink is prepared that can deposit arbitrary patterns on polyimide films. By introducing a gas atmosphere of argon/hydrogen (Ar/H₂ ), thermal treatment at 350 °C enables an in situ decomposition and crystallization in the patterned precursors and, consequently, results in the formation of MoS₂ . Without complicated processes, patterned MoS₂ is obtained directly on polymer substrate, exhibiting superior mechanical flexibility and durability (≈2% variation in resistance over 10,000 bending cycles), as well as excellent chemical stability, which is attributed to the generated continuous and thin microstructures, as well as their strong adhesion with the substrate. As a step further, this approach is employed to manufacture various flexible sensing devices that are insensitive to body motions and moisture, including temperature sensors and biopotential sensing systems for real-time, continuously monitoring skin temperature, electrocardiography, and electromyography signals.

Mo-Based Heterogeneous Interface and Sulfur Vacancy Synergistic Effect Enhances the Fenton-like Catalytic Performance for Organic Pollutant Degradation

Heterogeneous Fenton-like reactions (HFLRs) based on the in situ electrochemical generation of hydrogen peroxide (H₂O₂) are one of the green methods to remediate organic pollutants in wastewater. However, the design of Fenton-like catalysts with specific active sites and high pollutant degradation rate is still challenging. Here, MoS₂-MoC and MoS₂-Mo₂N catalytic cathodes with heterojunctions were successfully prepared, and the mechanism by which hydroxyl radicals and singlet oxygen (¹O₂) were generated cleanly without adding chemical additives other than oxygen was clarified. The composite catalysts contained more sulfur vacancies, and the catalytic cathode achieved a high paracetamol pollutant degradation efficiency with 0.17 kWh g^-1 TOC specific energy consumption. And almost 5 times higher activity was achieved compared to a pure MoS₂ catalytic cathode. Experimental studies confirmed that the production of ¹O₂ was based on the transformation of superoxide radicals by Mo⁶⁺, and ¹O₂ accounted for approximately 66% of the total degradation and enhanced the nonradical behavior in the reaction. This work provides a sustainable strategy for pollutant utilization, which is valuable for solving the difficult problems of HFLRs and developing new environmental remediation technologies.

Recent Progress in Contact Engineering of Field-Effect Transistor Based on Two-Dimensional Materials

Two-dimensional (2D) semiconductors have been considered as promising candidates to fabricate ultimately scaled field-effect transistors (FETs), due to the atomically thin thickness and high carrier mobility. However, the performance of FETs based on 2D semiconductors has been limited by extrinsic factors, including high contact resistance, strong interfacial scattering, and unintentional doping. Among these challenges, contact resistance is a dominant issue, and important progress has been made in recent years. In this review, the Schottky-Mott model is introduced to show the ideal Schottky barrier, and we further discuss the contribution of the Fermi-level pinning effect to the high contact resistance in 2D semiconductor devices. In 2D FETs, Fermi-level pinning is attributed to the high-energy metal deposition process, which would damage the lattice of atomically thin 2D semiconductors and induce the pinning of the metal Fermi level. Then, two contact structures and the strategies to fabricate low-contact-resistance short-channel 2D FETs are introduced. Finally, our review provides practical guidelines for the realization of high-performance 2D-semiconductors-based FETs with low contact resistance and discusses the outlook of this field.

Evolution Application of Two-Dimensional MoS2-Based Field-Effect Transistors

High-performance and low-power field-effect transistors (FETs) are the basis of integrated circuit fields, which undoubtedly require researchers to find better film channel layer materials and improve device structure technology. MoS₂ has recently shown a special two-dimensional (2D) structure and superior photoelectric performance, and it has shown new potential for next-generation electronics. However, the natural atomic layer thickness and large specific surface area of MoS₂ make the contact interface and dielectric interface have a great influence on the performance of MoS₂ FET. Thus, we focus on its main performance improvement strategies, including optimizing the contact behavior, regulating the conductive channel, and rationalizing the dielectric layer. On this basis, we summarize the applications of 2D MoS₂ FETs in key and emerging fields, specifically involving logic, RF circuits, optoelectronic devices, biosensors, piezoelectric devices, and synaptic transistors. As a whole, we discuss the state-of-the-art, key merits, and limitations of each of these 2D MoS₂-based FET systems, and prospects in the future.

Lowering Contact Resistances of Two-Dimensional Semiconductors by Memristive Forming

Two-dimensional semiconductors have great potential for beyond-silicon electronics. However, because of the lack of controllable doping methods, Fermi level pinning, and van der Waals (vdW) gaps at the metal-semiconductor interfaces, these devices exhibit high electrical contact resistances, restricting their practical applications. Here, we report a general contact-resistance-lowering strategy by constructing vertical metal-semiconductor-metal memristor structures at the contact regions and setting them into a nonvolatile low-resistance state through a memristive forming process. Through this, we reduce the contact resistances of MoS₂ field-effect transistors (FETs) by at least one order of magnitude and improve the on-state current densities of MoTe₂ FETs by about two orders of magnitude. We also demonstrate that this strategy is applicable to other two-dimensional semiconductors, including MoSe₂, WS₂, and WSe₂, and a variety of contact metals, including Au, Cu, Ni, and Pd. The good stability and universality indicate the great potential for technological applications.