Synthesis and transport properties of large-scale alloy Co(0.16)Mo(0.84)S2 bilayer nanosheets

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.

Recent advances in solution assisted synthesis of transition metal chalcogenides for photo-electrocatalytic hydrogen evolution

Hydrogen evolution from water splitting is considered to be an important renewable clean energy source and alternative to fossil fuels for future energy sustainability. Photocatalytic and electrocatalytic water splitting is considered to be an effective method for the sustainable production of clean energy, H₂. This perspective especially emphasizes research advances in the solution-assisted synthesis of transition metal chalcogenides for both photo and electrocatalytic hydrogen evolution applications. Transition metal chalcogenides (CdS, MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂, and WSe₂) have received intensified research interest over the past two decades on account of their unique properties and great potential across a wide range of applications. The photocatalytic activity of transition metal chalcogenides can further be improved by elemental doping, heterojunction formation with noble metals (Au, Pt, etc.), non-chalcogenides (MoS₂, In₂S₃, NiS_1-X), morphological tuning, through various solution-assisted synthesis processes, including liquid-phase exfoliation, heat-up, hot-injection methods, hydrothermal/solvothermal routes and template-mediated synthesis processes. In this review we will discuss recent developments in transition metal chalcogenides (TMCs), the role of TMCs for hydrogen production and various strategies for surface functionalization to increase their activity, different synthesis methods, and prospects of TMCs for hydrogen evolution. We have included a brief discussion on the effect of surface hydrogen binding energy and Gibbs free energy change for HER in electrocatalytic hydrogen evolution.

Emerging Synthesis Strategies of 2D MOFs for Electrical Devices and Integrated Circuits

The development of advanced electronic devices is boosting many aspects of modern technology and industry. The ever-increasing demand for advanced electrical devices and integrated circuits calls for the design of novel materials, with superior properties for the improvement of working performance. In this review, a detailed overview of the synthesis strategies of 2D metal organic frameworks (MOFs) acquiring growing attention is presented, as a basis for expansion of novel key materials in electrical devices and integrated circuits. A framework of controllable synthesis routes to be implanted in the synthesis strategies of 2D materials and MOFs is described. In short, the synthesis methods of 2D MOFs are summarized and discussed in depth followed by the illustrations of promising applications relating to various electrical devices and integrated circuits. It is concluded by outlining how 2D MOFs can be synthesized in a simpler, highly efficient, low-cost, and more environmentally friendly way which can open up their applicable opportunities as key materials in advanced electrical devices and integrated circuits, enabling their use in broad aspects of the society.

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano-electronics and nano-optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed-laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up-to-date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up-to-date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

Centimeter-Scale Few-Layer PdS2: Fabrication and Physical Properties

To develop next-generation electronic devices, novel semiconductive materials are urgently required. The transition metal dichalcogenides (TMDs) hold the promise of next generation of semiconductor materials for emerging electronic applications. As a member of the group-10 TMDs, PdS₂ has a notable layer-number-dependent band structure and tremendously high carrier mobility at room temperature. Here, we demonstrate the experimental realization of centimeter-scale synthesis of the few-layer PdS₂ by the combination of physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods. For the first time, the optical anisotropic properties of the few-layer PdS₂ were investigated through angle-resolved polarized Raman spectroscopy. Also, the evolution of Raman spectra was studied depending on the temperature in the range of 12-300 K. To further understand the electronic properties of the few-layer PdS₂, the field-effect transistor (FET) devices were fabricated and investigated. The electronic measurements of such FET devices reveal that the PdS₂ materials exhibit a tunable ambipolar transport mechanism with field-effect mobility of up to ∼388 cm² V^-1 s^-1 and the on/off ratio of ∼800, which were not reported before in the literature. To well understand the experimental results, the electronic structure of PdS₂ was determined using density functional theory (DFT) calculations. These excellent physical properties are very helpful in developing high-performance opto-electronic applications.

Enhanced Photodetection Performance of Photodetectors Based on Indium-Doped Tin Disulfide Few Layers

Two dimensional (2D) tin disulfide (SnS₂) has attracted growing interest as a promising high performance photodetector with superior performance such as fast response time, high responsivity, and good stability. However, SnS₂-based photodetectors still face great challenges, and the photodetection performance needs to be improved for practical applications. Herein, indium-doped SnS₂ (In-SnS₂) few layers were exfoliated from CVT-grown single crystals, which were synthesized by chemical vapor transport. Photodetectors based on In-SnS₂ few layers were fabricated and detected. Compared with photodetectors based on pristine SnS₂, photodetectors based on In-SnS₂ few layers exhibited better photodetection performances, including higher responsivities, higher external quantum efficiencies, and greater normalized detectivities. The responsivity (R), external quantum efficiency (EQE), and normalized detectivity (D*) were increased by up to 2 orders of magnitude after In doping. Considering responsivity and response time, the photodetector based on 1.4 at. % In-SnS₂ few layers exhibited an optimal photodetection performance with a high R of 153.8 A/W, a high EQE of 4.72 × 10⁴ %, a great D* of 5.81 × 10¹² Jones, and a short response time of 13 ms. Our work provides an efficient path to enhance photodetection performances of photodetectors based on SnS₂ for future high-performance optoelectronic applications.

A Linear Relationship between the Charge Transfer Amount and Level Alignment in Molecule/Two-Dimensional Adsorption Systems

We systematically study the adsorption of tetrathiafulvalene (TTF), tetracyanoquinodimethane (TCNQ), and tetracyanoethylene (TCNE) on a variety of two-dimensional (2D) monolayers with weak van der Waals (vdW) interactions based on density functional theory. We confirm that TTF can act as an effective donor when its highest occupied molecular orbital (HOMO) level is higher than the conduction band minimum (CBM) state of 2D materials, while TCNQ and TCNE can act as effective acceptors when their lowest unoccupied molecular orbital (LUMO) levels are lower than the valence band maximum (VBM) state of 2D materials. Moreover, our calculations reveal a linear relationship between the charge transfer amount and level alignment between the molecule and 2D monolayer. In other words, the charge transfer is linearly dependent on the energy difference between the HOMO level and 2D CBM state for the donor molecule or the energy difference between the LUMO level and 2D VBM state for the acceptor molecule. The linear relationship indicates that the charge transfer is insensitive to the local binding environments due to the weak vdW interaction. However, the linear relationship cannot be applied to atoms or molecules that are chemisorbed on 2D materials.

Transition-Metal Substitution-Induced Lattice Strain and Electrical Polarity Reversal in Monolayer WS2

The physical and chemical properties of transition metal dichalcogenides can be effectively tuned by doping or alloying, which is essential for their practical applications. However, the microstructure evolutions and their effects on the physical properties induced by alloying from hetero-atoms with different outermost electronic structures are still unclear. Here, we synthesized Nb-substituted WS₂ with various Nb concentrations showing unusual changes of optical behaviors and continuous electrical polarity reversal. The fully softened Raman mode, rapidly quenched photoluminescence, and severe electron scattering can be attributed to the combined effects of charge doping and lattice strain caused by atomic Nb doping. Three types of substitution modes of Nb atoms in the WS₂ lattice were observed directly from atomic-resolution scanning transmission electron microscopy. Density functional theory calculations further confirm the role of lattice strain in the evolutions of optical and electrical characteristics. With increasing Nb concentration, n-type, ambipolar, and p-type field-effect transistors can be achieved, indicating the capacity of this doping method to engineer the properties of two-dimensional materials for future electronic applications.

Synthesis of Co-Doped MoS2 Monolayers with Enhanced Valley Splitting

Internal magnetic moments induced by magnetic dopants in MoS₂ monolayers are shown to serve as a new means to engineer valley Zeeman splitting (VZS). Specifically, successful synthesis of monolayer MoS₂ doped with the magnetic element Co is reported, and the magnitude of the valley splitting is engineered by manipulating the dopant concentration. Valley splittings of 3.9, 5.2, and 6.15 meV at 7 T in Co-doped MoS₂ with Co concentrations of 0.8%, 1.7%, and 2.5%, respectively, are achieved as revealed by polarization-resolved photoluminescence (PL) spectroscopy. Atomic-resolution electron microscopy studies clearly identify the magnetic sites of Co substitution in the MoS₂ lattice, forming two distinct types of configurations, namely isolated single dopants and tridopant clusters. Density functional theory (DFT) and model calculations reveal that the observed enhanced VZS arises from an internal magnetic field induced by the tridopant clusters, which couples to the spin, atomic orbital, and valley magnetic moment of carriers from the conduction and valence bands. The present study demonstrates a new method to control the valley pseudospin via magnetic dopants in layered semiconducting materials, paving the way toward magneto-optical and spintronic devices.

Recent advances in two-dimensional ferromagnetism: materials synthesis, physical properties and device applications

Two-dimensional (2D) ferromagnetism is critical for both scientific investigation and technological development owing to its low-dimensionality that brings in quantization of electronic states as well as free axes for device modulation. However, the scarcity of high-temperature 2D ferromagnets has been the obstacle of many research studies, such as the quantum anomalous Hall effect (QAHE) and thin-film spintronics. Indeed, in the case of the isotropic Heisenberg model with finite-range exchange interactions as an example, low-dimensionality is shown to be contraindicated with ferromagnetism. However, the advantages of low-dimensionality for micro-scale patterning could enhance the Curie temperature (T_C) of 2D ferromagnets beyond the T_C of bulk materials, opening the door for designing high-temperature ferromagnets in the 2D limit. In this paper, we review the recent advances in the field of 2D ferromagnets, including their material systems, physical properties, and potential device applications.

Phase Transition and Superconductivity Enhancement in Se-Substituted MoTe2 Thin Films

Consecutively tailoring few-layer transition metal dichalcogenides MX₂ from 2H to T_d phase may realize the long-sought topological superconductivity in a single material system by incorporating superconductivity and the quantum spin Hall effect together. Here, this study demonstrates that a consecutive structural phase transition from T_d to 1T' to 2H polytype can be realized by increasing the Se concentration in Se-substituted MoTe₂ thin films. More importantly, the Se-substitution is found to dramatically enhance the superconductivity of the MoTe₂ thin film, which is interpreted as the introduction of two-band superconductivity. The chemical-constituent-induced phase transition offers a new strategy to study the s^+- superconductivity and the possible topological superconductivity, as well as to develop phase-sensitive devices based on MX₂ materials.

Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe₂ monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe₂. Transmission electron microscopy characterization exhibits that Pb_0.036Sn_0.964Se₂ nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe₂ field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb_0.036Sn_0.964Se₂ monolayer show n-type behavior with a high on/off ratio of 10⁶ which is higher than any values of SnSe₂ FETs reported at the moment. The estimated carrier concentration of Pb_0.036Sn_0.964Se₂ is approximately six times lower than that of SnSe₂. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe₂ monolayer has significant potential in future nanoelectronic and optoelectronic applications.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

FeS2 /CoS2 Interface Nanosheets as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Electrochemical water splitting to produce hydrogen and oxygen, as an important reaction for renewable energy storage, needs highly efficient and stable catalysts. Herein, FeS₂ /CoS₂ interface nanosheets (NSs) as efficient bifunctional electrocatalysts for overall water splitting are reported. The thickness and interface disordered structure with rich defects of FeS₂ /CoS₂ NSs are confirmed by atomic force microscopy and high-resolution transmission electron microscopy. Furthermore, extended X-ray absorption fine structure spectroscopy clarifies that FeS₂ /CoS₂ NSs with sulfur vacancies, which can further increase electrocatalytic performance. Benefiting from the interface nanosheets' structure with abundant defects, the FeS₂ /CoS₂ NSs show remarkable hydrogen evolution reaction (HER) performance with a low overpotential of 78.2 mV at 10 mA cm^-2 and a superior stability for 80 h in 1.0 m KOH, and an overpotential of 302 mV at 100 mA cm^-2 for the oxygen evolution reaction (OER). More importantly, the FeS₂ /CoS₂ NSs display excellent performance for overall water splitting with a voltage of 1.47 V to achieve current density of 10 mA cm^-2 and maintain the activity for at least 21 h. The present work highlights the importance of engineering interface nanosheets with rich defects based on transition metal dichalcogenides for boosting the HER and OER performance.

Metallic CuCo2S4 nanosheets of atomic thickness as efficient bifunctional electrocatalysts for portable, flexible Zn-air batteries

Optimized catalysts show great potential for renewable energy storage and conversion. Herein, we report metallic CuCo2S4 nanosheets (NSs) of atomic thickness as efficient bifunctional electrocatalysts for use in portable, flexible Zn-air batteries. The metallic CuCo2S4 NSs of atomic thickness with 4-atom-thick to 6-atom-thick layers are confirmed by temperature-dependent electrical resistance measurements and atomic force microscopy. Furthermore, extended X-ray absorption fine structure spectroscopy confirms that CuCo2S4 NSs with sulfur vacancies can further increase the OER activity. Due to high electrical conductivity and ultrathin nanosheet structure with abundant defects, CuCo2S4 NSs exhibit excellent reversible oxygen catalytic performance with an overpotential of 287 mV (at j = 10 mA cm-2) for the oxygen evolution reaction (OER) and an onset potential of 0.90 V for the oxygen reduction reaction (ORR). Additionally, the portable, flexible Zn-air battery using CuCo2S4 NSs as the air-cathode displays a high open circuit voltage and strong rechargeable capacity for 18 h. The present study highlights the importance of designing metallic catalysts having atomic thickness with surface defects for highly efficient and stable renewable energy storage and conversion.

Structural stability and magnetic-exchange coupling in Mn-doped monolayer/bilayer MoS2

Ferromagnetic (FM) two-dimensional (2D) transition metal dichalcogenides (TMDs) have potential applications in modern electronics and spintronics and doping of TMDs with transition metals can enhance the magnetic characteristics.

Damage-free and rapid transfer of CVD-grown two-dimensional transition metal dichalcogenides by dissolving sacrificial water-soluble layers

As one of the most important family members of two-dimensional (2D) materials, the growth and damage-free transfer of transition metal dichalcogenides (TMDs) play crucial roles in their future applications. Here, we report a damage-free and highly efficient approach to transfer single and few-layer 2D TMDs to arbitrary substrates by dissolving a sacrificial water-soluble layer, which is formed underneath 2D TMD flakes simultaneously during the growth process. It is demonstrated, for monolayer MoS₂, that no quality degradation is found after the transfer by performing transmission electron microscopy, Raman spectroscopy, photoluminescence and electrical transport studies. The field effect mobility of the post-transfer MoS₂ flakes was found to be improved by 2-3 orders compared with that of the as-grown ones. This approach was also demonstrated to be applicable to other TMDs, other halide salts as precursors, or other growth substrates, indicating its universality for other 2D materials. Our work may pave the way for material synthesis of future integrated electronic and optoelectronic devices based on 2D TMD materials.

A two-dimensional Fe-doped SnS2 magnetic semiconductor

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. In this study, we present a high-quality Fe-doped SnS₂ monolayer exfoliated using a micromechanical cleavage method. Fe atoms were doped at the Sn atom sites, and the Fe contents are ∼2.1%, 1.5%, and 1.1%. The field-effect transistors based on the Fe_0.021Sn_0.979S₂ monolayer show n-type behavior and exhibit high optoelectronic performance. Magnetic measurements show that pure SnS₂ is diamagnetic, whereas Fe_0.021Sn_0.979S₂ exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ~31 K. Density functional theory calculations show that long-range ferromagnetic ordering in the Fe-doped SnS₂ monolayer is energetically stable, and the estimated Curie temperature agrees well with the results of our experiment. The results suggest that Fe-doped SnS₂ has significant potential in future nanoelectronic, magnetic, and optoelectronic applications.

Ultrathin Two-Dimensional Multinary Layered Metal Chalcogenide Nanomaterials

Ultrathin two-dimensional (2D) layered transition metal dichalcogenides (TMDs), such as MoS₂ , WS₂ , TiS₂ , TaS₂ , ReS₂ , MoSe₂ and WSe₂ , have attracted considerable attention over the past six years owing to their unique properties and great potential in a wide range of applications. Aiming to achieve tunable properties and optimal application performances, great effort is devoted to the exploration of 2D multinary layered metal chalcogenide nanomaterials, which include ternary metal chalcogenides with well-defined crystal structures, alloyed TMDs, heteroatom-doped TMDs and 2D metal chalcogenide heteronanostructures. These novel 2D multinary layered metal chalcogenide nanomaterials exhibit some unique properties compared to 2D binary TMD counterparts, thus holding great promise in various potential applications including electronics/optoelectronics, catalysis, sensors, biomedicine, and energy storage and conversion with enhanced performances. This article focuses on the state-of-art progress on the preparation, characterization and applications of ultrathin 2D multinary layered metal chalcogenide nanomaterials.

Progress on Electronic and Optoelectronic Devices of 2D Layered Semiconducting Materials

2D layered semiconducting materials (2DLSMs) represent the thinnest semiconductors, holding many novel properties, such as the absence of surface dangling bonds, sizable band gaps, high flexibility, and ability of artificial assembly. With the prospect of bringing revolutionary opportunities for electronic and optoelectronic applications, 2DLSMs have prospered over the past twelve years. From materials preparation and property exploration to device applications, 2DLSMs have been extensively investigated and have achieved great progress. However, there are still great challenges for high-performance devices. In this review, we provide a brief overview on the recent breakthroughs in device optimization based on 2DLSMs, particularly focussing on three aspects: device configurations, basic properties of channel materials, and heterostructures. The effects from device configurations, i.e., electrical contacts, dielectric layers, channel length, and substrates, are discussed. After that, the affect of the basic properties of 2DLSMs on device performance is summarized, including crystal defects, crystal symmetry, doping, and thickness. Finally, we focus on heterostructures based on 2DLSMs. Through this review, we try to provide a guide to improve electronic and optoelectronic devices of 2DLSMs for achieving practical device applications in the future.

Chemical Vapor Deposition Growth of Degenerate p-Type Mo-Doped ReS2 Films and Their Homojunction

Substitutional doping of transition metal dichalcogenide two-dimensional materials has proven to be effective in tuning their intrinsic properties, such as band gap, transport characteristics, and magnetism. In this study, we realized substitutional doping of monolayer rhenium disulfide (ReS₂) with Mo via chemical vapor deposition. Scanning transmission electron microscopy demonstrated that Mo atoms are successfully doped into ReS₂ by substitutionally replacing Re atoms in the lattice. Electrical measurements revealed the degenerate p-type semiconductor behavior of Mo-doped ReS₂ field effect transistors, in agreement with density functional theory calculations. The p-n diode device based on a doped ReS₂ and ReS₂ homojunction exhibited gate-tunable current rectification behaviors, and the maximum rectification ratio could reach up to 150 at V_d = -2/+2 V. The successful synthesis of p-type ReS₂ in this study could largely promote its application in novel electronic and optoelectronic devices.

Excellent Thermoelectric Properties in monolayer WSe2 Nanoribbons due to Ultralow Phonon Thermal Conductivity

By using first-principles calculations combined with the nonequilibrium Green's function method and phonon Boltzmann transport equation, we systematically investigate the influence of chirality, temperature and size on the thermoelectric properties of monolayer WSe2 nanoribbons. The results show that the armchair WSe2 nanoribbons have much higher ZT values than zigzag WSe2 nanoribbons. The ZT values of armchair WSe2 nanoribbons can reach 1.4 at room temperature, which is about seven times greater than that of zigzag WSe2 nanoribbons. We also find that the ZT values of WSe2 nanoribbons increase first and then decrease with the increase of temperature, and reach a maximum value of 2.14 at temperature of 500 K. It is because the total thermal conductance reaches the minimum value at 500 K. Moreover, the impact of width on the thermoelectric properties in WSe2 nanoribbons is not obvious, the overall trend of ZT value decreases lightly with the increasing temperature. This trend of ZT value originates from the almost constant power factor and growing phonon thermal conductance.

3d transition metal doping-induced electronic structures and magnetism in 1T-HfSe2 monolayers

By performing first-principles calculations, we explore the structural, electronic and magnetic properties of 3d transition metal (TM) atom-doped 1T-HfSe₂ monolayers.

Booming Development of Group IV-VI Semiconductors: Fresh Blood of 2D Family

As an important component of 2D layered materials (2DLMs), the 2D group IV metal chalcogenides (GIVMCs) have drawn much attention recently due to their earth-abundant, low-cost, and environmentally friendly characteristics, thus catering well to the sustainable electronics and optoelectronics applications. In this instructive review, the booming research advancements of 2D GIVMCs in the last few years have been presented. First, the unique crystal and electronic structures are introduced, suggesting novel physical properties. Then the various methods adopted for synthesis of 2D GIVMCs are summarized such as mechanical exfoliation, solvothermal method, and vapor deposition. Furthermore, the review focuses on the applications in field effect transistors and photodetectors based on 2D GIVMCs, and extends to flexible devices. Additionally, the 2D GIVMCs based ternary alloys and heterostructures have also been presented, as well as the applications in electronics and optoelectronics. Finally, the conclusion and outlook have also been presented in the end of the review.

Large-Size 2D β-Cu2 S Nanosheets with Giant Phase Transition Temperature Lowering (120 K) Synthesized by a Novel Method of Super-Cooling Chemical-Vapor-Deposition

2D triangular β-Cu₂ S nanosheets with large size and high quality are synthesized by a novel method of super-cooling chemical-vapor-deposition. The phase transition of this 2D material from β-Cu₂ S to γ-Cu₂ S occurs at 258 K (-15 °C), and such transition temperature is 120 K lower than that of its bulk counterpart (about 378 K).

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.

Chemical Vapor Deposition of Monolayer Mo(1-x)W(x)S2 Crystals with Tunable Band Gaps

Band gap engineering of monolayer transition metal dichalcogenides, such as MoS₂ and WS₂, is essential for the applications of the two-dimensional (2D) crystals in electronic and optoelectronic devices. Although it is known that chemical mixture can evidently change the band gaps of alloyed Mo_1−xW_xS₂ crystals, the successful growth of Mo_1−xW_xS₂ monolayers with tunable Mo/W ratios has not been realized by conventional chemical vapor deposition. Herein, we developed a low-pressure chemical vapor deposition (LP-CVD) method to grow monolayer Mo_1−xW_xS₂ (x = 0–1) 2D crystals with a wide range of Mo/W ratios. Raman spectroscopy and high-resolution transmission electron microscopy demonstrate the homogeneous mixture of Mo and W in the 2D alloys. Photoluminescence measurements show that the optical band gaps of the monolayer Mo_1−xW_xS₂ crystals strongly depend on the Mo/W ratios and continuously tunable band gap can be achieved by controlling the W or Mo portion by the LP-CVD.

Recent Advances in Two-Dimensional Materials beyond Graphene

The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here, we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.

Electrical and magnetic properties of FeS2 and CuFeS2 nanoplates

The as-made pyrite (FeS₂) and chalcopyrite (CuFeS₂) nanoplates exhibit abnormal strong ferromagnetic behavior due to the dangling bonds on surface.