Electroluminescence in single layer MoS2

Mechanical and thermoelectric properties of ZrX2 and HfX2 (X = S and Se) from Van der Waals density-functional theory

The structural, mechanical, and thermoelectric characteristics of layered transition metal dichalcogenides MX2 (M = Zr, Hf; X = S, Se) have been studied using density functional theory along with van der Waals correction. The exchange-correlation functional, enhanced with corrections for van der Waals interactions, has been evaluated for the hexagonal bulk structures of these materials. The analysis of elastic properties reveals that these compounds exhibit brittleness at zero pressure and conform to Born's criteria for mechanical stability. Examination of elastic constants and moduli suggests that the compounds possess reasonable machinability, moderate hardness, and anisotropy in terms of sound velocity. Transport properties, including the Seebeck coefficient, electrical conductivity, thermal conductivity, and power factor, have been computed using the semi-classical Boltzmann theory implemented in the BoltzTraP code. All investigated compounds exhibit excellent thermoelectric performance at high temperatures. This result suggests that our compounds are highly promising candidate for practical utilization in the thermoelectric scope.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

Photo Luminescence and Radiative Carrier Losses in Monolayer Transition Metal Dichalcogenides

The carrier losses due to radiative recombination in monolayer transition metal dichalcogenides are studied using fully microscopic many-body models. The density- and temperature-dependent losses in various Mo- and W-based materials are shown to be dominated by Coulomb correlations beyond the Hartree-Fock level. Despite the much stronger Coulomb interaction in 2D materials, the radiative losses are comparable-if not weaker-than in conventional III-V materials. A strong dependence on the dielectric environment is found in agreement with experimental results.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Ultrafast Charge Transfer and Recombination Dynamics in Monolayer-Multilayer WSe2 Junctions Revealed by Time-Resolved Photoemission Electron Microscopy

The ultrafast carrier dynamics of junctions between two chemically identical, but electronically distinct, transition metal dichalcogenides (TMDs) remains largely unknown. Here, we employ time-resolved photoemission electron microscopy (TR-PEEM) to probe the ultrafast carrier dynamics of a monolayer-to-multilayer (1L-ML) WSe2 junction. The TR-PEEM signals recorded for the individual components of the junction reveal the sub-ps carrier cooling dynamics of 1L- and 7L-WSe2, as well as few-ps exciton-exciton annihilation occurring on 1L-WSe2. We observe ultrafast interfacial hole (h) transfer from 1L- to 7L-WSe2 on an ∼0.2 ps time scale. The resultant excess h density in 7L-WSe2 decays by carrier recombination across the junction interface on an ∼100 ps time scale. Reminiscent of the behavior at a depletion region, the TR-PEEM image reveals the h density accumulation on the 7L-WSe2 interface, with a decay length ∼0.60 ± 0.17 μm. These charge transfer and recombination dynamics are in agreement with ab initio quantum dynamics. The computed orbital densities reveal that charge transfer occurs from the basal plane, which extends over both 1L and ML regions, to the upper plane localized on the ML region. This mode of charge transfer is distinctive to chemically homogeneous junctions of layered materials and constitutes an additional carrier deactivation pathway that should be considered in studies of 1L-TMDs found alongside their ML, a common occurrence in exfoliated samples.

Split-Gate: Harnessing Gate Modulation Power in Thin-Film Electronics

With the increase in electronic devices across various applications, there is rising demand for selective carrier control. The split-gate consists of a gate electrode divided into multiple parts, allowing for the independent biasing of electric fields within the device. This configuration enables the potential formation of both p- and n-channels by injecting holes and electrons owing to the presence of the two gate electrodes. Applying voltage to the split-gate allows for the control of the Fermi level and, consequently, the barrier height in the device. This facilitates band bending in unipolar transistors and allows ambipolar transistors to operate as if unipolar. Moreover, the split-gate serves as a revolutionary tool to modulate the contact resistance by controlling the barrier height. This approach enables the precise control of the device by biasing the partial electric field without limitations on materials, making it adaptable for various applications, as reported in various types of research. However, the gap length between gates can affect the injection of the electric field for the precise control of carriers. Hence, the design of the gap length is a critical element for the split-gate structure. The primary investigation in this review is the introduction of split-gate technology applied in various applications by using diverse materials, the methods for forming the split-gate in each device, and the operational mechanisms under applied voltage conditions.

Polarized Tunneling Transistor for Ultralow-Energy-Consumption Artificial Synapse toward Neuromorphic Computing

Neural networks based on low-power artificial synapses can significantly reduce energy consumption, which is of great importance in today's era of artificial intelligence. Two-dimensional (2D) material-based floating-gate transistors (FGTs) have emerged as compelling candidates for simulating artificial synapses owing to their multilevel and nonvolatile data storage capabilities. However, the low erasing/programming speed of FGTs renders them unsuitable for low-energy-consumption artificial synapses, thereby limiting their potential in high-energy-efficient neuromorphic computing. Here, we introduce a FGT-inspired MoS2/Trap/PZT heterostructure-based polarized tunneling transistor (PTT) with a simple fabrication process and significantly enhanced erasing/programming speed. Distinct from the FGT, the PTT lacks a tunnel layer, leading to a marked improvement in its erasing/programming speed. The PTT's highest erasing/programming (operation) speed can reach ∼20 ns, which outperforms the performance of most FGTs based on 2D heterostructures. Furthermore, the PTT has been utilized as an artificial synapse, and its weight-update energy consumption can be as low as 0.0002 femtojoule (fJ), which benefits from the PTT's ultrahigh operation speed. Additionally, PTT-based artificial synapses have been employed in constructing artificial neural network simulations, achieving facial-recognition accuracy (95%). This groundbreaking work makes it possible for fabricating future high-energy-efficient neuromorphic transistors utilizing 2D materials.

A Bifunctional Optoelectronic Device for Photodetection and Photoluminescence Switching Based on Graphene/ZnTe/Graphene van der Waals Heterostructures

Controlling the dynamic processes, such as generation, separation, transport, and recombination, of photoexcited carriers in a semiconductor is foundational in the design of various devices for optoelectronic applications. One may imagine that if different processes can be manipulated in one single device and thus generate useful signals, a multifunctional device can be realized, and the toolbox for integrated optoelectronics will be expanded. Here, we revealed that in a graphene/ZnTe/graphene van der Waals (vdW) heterostructure, the carriers can be generated by illumination from visible to infrared frequencies, and thus, the detected spectrum range extends to the communication band, well beyond the band gap of ZnTe (2.26 eV). More importantly, we are able to control the competition between separation and recombination of the photoexcited carriers by an electric bias along the thickness-defined channel of the ZnTe flake: as the bias increases, the photodetecting performance, e.g. response speed and photocurrent, are improved due to the efficient separation of carriers; synchronously, the photoluminescence (PL) intensity decreases and even switches off due to the suppressed recombination process. The ZnTe-based vdW heterostructure device thus integrates both photodetection and PL switching functions by manipulating the generation, separation, transport, and recombination of carriers, which may inspire the design of the next generation of miniaturized optoelectronic devices based on the vdW heterostructures made by various thin flakes.

Janus MoAZ3H (A = Ge, Si; Z = N, P, As) monolayers: a new class of semiconductors exhibiting excellent photovoltaic and catalytic performances

Due to the asymmetrical structure in the vertical direction, Janus two-dimensional (2D) monolayer (ML) materials possess some unique physical properties, holding great promise for nanoscale devices. In this paper, based on the newly discovered MoA2Z4 (A = Si, Ge; Z = N, P, As) ML, we propose a class of 2D Janus MoAZ3H ML materials with good stability and excellent mechanical properties using first-principles calculations. We demonstrate that the novel Janus MoAZ3H ML materials are all semiconductors with bandgaps ranging from 0.69 to 2.44 eV, giving rise to good absorption in the visible light region. Especially, both MoSiN3H and MoGeN3H MLs can be used as catalysts for producing hydrogen through water splitting. This catalytic property is much more efficient than that of the MoA2Z4 ML, attributed to the intrinsic electric field induced by the vertical asymmetry effectively separating electrons and holes. More importantly, the carrier mobility of the MoAZ3H ML is up to 103-104 cm2 V-1 s-1 due to the large elastic modulus or small effective mass. Additionally, the electronic properties of the MoAZ3H ML can be easily tuned by strain. Our results suggest a new strategy for designing novel 2D Janus materials, which not only expands the members in the 2D MA2Z4-based Janus family, but also provide candidates with excellent performances in photovoltaic and catalytic fields.

2D Materials in the Display Industry: Status and Prospects

With advances in flexible electronics, innovative foldable, rollable, and stretchable displays have been developed to maintain their performance under various deformations. These flexible devices can develop more innovative designs than conventional devices due to their light weight, high space efficiency, and practical convenience. However, developing flexible devices requires material innovation because the devices must be flexible and exhibit desirable electrical insulating/semiconducting/metallic properties. Recently, emerging 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted considerable research attention because of their outstanding electrical, optical, and mechanical properties, which are ideal for flexible electronics. The recent progress and challenges of 2D material growth and display applications are reviewed and perspectives for exploring 2D materials for display applications are discussed.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Electroluminescence from Megasonically Solution-Processed MoS2 Nanosheet Films

Due to their superior optoelectronic properties, monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for electroluminescent devices. However, challenges in isolating optoelectronically active TMD monolayers using scalable liquid phase exfoliation have precluded electroluminescence in large-area, solution-processed TMD films. Here, we overcome these limitations and demonstrate electroluminescence from molybdenum disulfide (MoS2) nanosheet films by employing a monolayer-rich MoS2 ink produced by electrochemical intercalation and megasonic exfoliation. Characteristic monolayer MoS2 photoluminescence and electroluminescence spectral peaks at 1.88-1.90 eV are observed in megasonicated MoS2 films, with the emission intensity increasing with film thickness over the range 10-70 nm. Furthermore, employing a vertical light-emitting capacitor architecture enables uniform electroluminescence in large-area devices. These results indicate that megasonically exfoliated MoS2 monolayers retain their direct bandgap character in electrically percolating thin films even following multistep solution processing. Overall, this work establishes megasonicated MoS2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.

Molybdenum Disulfide as Tunable Electrochemical and Optical Biosensing Platforms for Cancer Biomarker Detection: A Review

Cancer is a common illness with a high mortality. Compared with traditional technologies, biomarker detection, with its low cost and simple operation, has a higher sensitivity and faster speed in the early screening and prognosis of cancer. Therefore, extensive research has focused on the development of biosensors and the construction of sensing interfaces. Molybdenum disulfide (MoS2) is a promising two-dimensional (2D) nanomaterial, whose unique adjustable bandgap shows excellent electronic and optical properties in the construction of biosensor interfaces. It not only has the advantages of a high catalytic activity and low manufacturing costs, but it can also further expand the application of hybrid structures through different functionalization, and it is widely used in various biosensors fields. Herein, we provide a detailed introduction to the structure and synthesis methods of MoS2, and explore the unique properties and advantages/disadvantages exhibited by different structures. Specifically, we focus on the excellent properties and application performance of MoS2 and its composite structures, and discuss the widespread application of MoS2 in cancer biomarkers detection from both electrochemical and optical dimensions. Additionally, with the cross development of emerging technologies, we have also expanded the application of other emerging sensors based on MoS2 for early cancer diagnosis. Finally, we summarized the challenges and prospects of MoS2 in the synthesis, functionalization of composite groups, and applications, and provided some insights into the potential applications of these emerging nanomaterials in a wider range of fields.

Defect engineering for thermal transport properties of nanocrystalline molybdenum diselenide

Molybdenum diselenide (MoSe2) is attracting great attention as a transition metal dichalcogenide (TMDC) due to its unique applications in micro-electronics and beyond. In this study, the role of defects in the thermal transport properties of single-layer MoSe2 is investigated using non-equilibrium molecular dynamics (NEMD) simulations. Specifically, this work quantifies how different microstructural defects such as vacancies and grain boundaries (GBs) and their concentration (N) alter the thermal conductivity (TC) of single crystal and nanocrystalline MoSe2. These results show a significant drop in thermal conductivity as the concentration of defects increases. Specifically, point defects lower the TC of MoSe2 in the form of N-β where β is 0.5, 0.48 and 0.36 for VMo, VMo-Se and VSe vacancies, respectively. This study also examines the impact of grain boundaries on the thermal conductivity of nanocrystalline MoSe2. These results suggest that GB migration and stress-assisted twinning along with localized phase transformation (2H to 1T) are the primary factors affecting the thermal conductivity of nanocrystalline MoSe2. Based on MD simulations, TC of polycrystalline MoSe2 increases with the average grain size (d̄) in the form of d̄4.5. For example, the TC of nanocrystalline MoSe2 with d̄ = 11 nm is around 40% lower than the TC of the pristine monocrystalline sample with the same dimensions. Finally, the influence of sample size and temperature is studied to determine the sensitivity of quantitative thermal properties to the length scale and phonon scattering, respectively. The results of this work could provide valuable insights into the role of defects in engineering the thermal properties of next generation semiconductor-based devices.

Morphology Transition with Temperature and its Effect on Optical Properties of Colloidal MoS2 Nanostructures

Morphology plays a crucial role in determining the chemical and optical properties of nanomaterials due to confinement effects. We report the morphology transition of colloidal molybdenum disulfide (MoS2) nanostructures, synthesized by a one-pot heat-up method, from a mix of quantum dots (QDs) and nanosheets to predominantly nanorods by varying the synthesis reaction temperature from 90 to 160 °C. The stoichiometry and composition of the synthesized QDs, nanosheets, and nanorods were quantified to be MoS2 using energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses. A nanostructure morphology transition due to variation in the reaction temperature resulted in a photoluminescence quantum yield enhancement from 0 to 4.4% on increasing the temperature from 90 to 120 °C. On further increase in the temperature to 160 °C, a decrease in the quantum yield to 3.06% is observed. Red-shifts of ≈18 and ≈140 nm in the emission maxima and absorption edge, respectively, are observed for the synthesized nanostructures with an increase in the reaction temperature from 90 to 160 °C. The change in the quantum yield is attributed to the change in shape and hence confinement of charge carriers. To the best of our knowledge, microscopic analysis of variation in shape and optical properties of colloidal MoS2 nanostructures with temperature, explained by a nonclassical growth mechanism is presented here for the first time.

Edge contacts accelerate the response of MoS2 photodetectors

We use a facile plasma etching process to define contacts with an embedded edge geometry for multilayer MoS₂ photodetectors. Compared to the conventional top contact geometry, the detector response time is accelerated by more than an order of magnitude by this action. We attribute this improvement to the higher in-plane mobility and direct contacting of the individual MoS₂ layers in the edge geometry. With this method, we demonstrate electrical 3 dB bandwidths of up to 18 MHz which is one of the highest values reported for pure MoS₂ photodetectors. We anticipate that this approach should also be applicable to other layered materials, guiding a way to faster next-generation photodetectors.

Fluorescence off-on nanosensor based on MoS2 nanosheets and oligonucleotides for the alternative detection of mercury(II) ions or silver(I) ions

As traditional methods for detection of heavy metal pollution in water involve complex procedures and require expensive equipment, there is a great deal of interest in the development of rapid and simple methods for determining heavy metal ions in water. Here, a nanobiosensor based on molybdenum disulphide (MoS₂) nanosheets and fluorophore (FAM) labeled oligonucleotides was proposed, and fluorescence spectroscopy was adopted for detection of Hg²⁺ or Ag⁺ ions in aqueous solution. The principle underlying detection by the sensor involves the formation of T-Hg²⁺-T or C-Ag⁺-C mismatches by single-stranded DNA (ssDNA) rich in thymine (T) or cytosine (C), thereby forming stable double-stranded DNA (dsDNA) structures. By exploiting the different adsorption capacity of MoS₂ nanosheets for ssDNA and dsDNA, when oligonucleotides were in a single chain state, MoS₂ nanosheets possessed a strong adsorption capacity for ssDNA, resulting in fluorescence quenching of FAM. After the addition of Hg²⁺ or Ag⁺, ssDNA formed double chains structure, the fluorescence recovered due to the weak adsorption capacity of MoS₂ nanosheets for dsDNA. Along this line, an "off-on" mode fluorescence nanobiosensor was designed to alternatively detect these two heavy metal ions in water. The sensor showed high sensitivity and excellent selectivity for both Hg²⁺ and Ag⁺ ions, with minimum detection limits of 6.8 nM and 8.9 nM, respectively.

A New, Thorough Look on Unusual and Neglected Group III-VI Compounds Toward Novel Perusals

The attention on group III-VI compounds in the last decades has been centered on the optoelectronic properties of indium and gallium chalcogenides. These outstanding properties are leading to novel advancements in terms of fundamental and applied science. One of the advantages of these compounds is to present laminated structures, which can be exfoliated down to monolayers. Despite the large knowledge gathered toward indium and gallium chalcogenides, the family of the group III-VI compounds embraces several other noncommon compounds formed by the other group III elements. These compounds present various crystal lattices, among which a great deal is offered from layered structures. Studies on aluminium chalcogenides show interesting potential as anodes in batteries and as semiconductors. Thallium (Tl), which is commonly present in the +1 oxidation state, is one of the key components in ternary chalcogenides. However, binary Tl-Q (Q = S, Se, Te) systems and derived films are still studied for their semiconducting and thermoelectric properties. This review aims to summarize the biggest features of these unusual materials and to shed some new light on them with the perspective that in the future, novel studies can revive these compounds in order to give rise to a new generation of technology.

Optical force exerted on the two dimensional transition-metal dichalcogenide coated dielectric particle by Gaussian beam

Two-dimensional transition-metal dichalcogenide (TMDC) exhibits a series of distinctive optical and electrical characteristics, which make it has a good application prospect in the field of optical manipulation. Based on the Mie theory, we investigate the radiation force exerted on the TMDC wrapped dielectric particle by Gaussian wave. Theoretical calculations show that the optical force spectra exhibit two resonant peaks in the visible region, which are generated by the interband exciton transitions in TMDC. Magnitude and morphology of the excitonic peaks could be modulated effectively by tuning the number of coated TMDC layers. Furthermore, the excitonic peaks transform significantly with particle size due to the variation of coupling strength between the dielectric particle and TMDC coating. The investigation provides potential applications in optical manipulations and light-matter interactions.

Two Dimensional Heterostructures for Optoelectronics: Current Status and Future Perspective

Researchers have found various families of two-dimensional (2D) materials and associated heterostructures through detailed theoretical work and experimental efforts. Such primitive studies provide a framework to investigate novel physical/chemical characteristics and technological aspects from micro to nano and pico scale. Two-dimensional van der Waals (vdW) materials and their heterostructures can be obtained to enable high-frequency broadband through a sophisticated combination of stacking order, orientation, and interlayer interactions. These heterostructures have been the focus of much recent research due to their potential applications in optoelectronics. Growing the layers of one kind of 2D material over the other, controlling absorption spectra via external bias, and external doping proposes an additional degree of freedom to modulate the properties of such materials. This mini review focuses on current state-of-the-art material design, manufacturing techniques, and strategies to design novel heterostructures. In addition to a discussion of fabrication techniques, it includes a comprehensive analysis of the electrical and optical properties of vdW heterostructures (vdWHs), particularly emphasizing the energy-band alignment. In the following sections, we discuss specific optoelectronic devices, such as light-emitting diodes (LEDs), photovoltaics, acoustic cavities, and biomedical photodetectors. Furthermore, this also includes a discussion of four different 2D-based photodetector configurations according to their stacking order. Moreover, we discuss the challenges that remain to be addressed in order to realize the full potential of these materials for optoelectronics applications. Finally, as future perspectives, we present some key directions and express our subjective assessment of upcoming trends in the field.

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

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

The effects of spin-orbit coupling on optical properties of monolayer [Formula: see text] due to mechanical strains

We have studied the optical conductivity of a quasi two-dimensional [Formula: see text] in the presence of external magnetic field and spin-orbit coupling. Specially, we address the frequency dependence of optical conductivity due to spin-orbit interaction. Using linear response theory the behavior of optical conductivity has been obtained within Green's function method. We have also considered the effects of uniaxial and biaxial in-plane strain on the optical absorption of [Formula: see text] layer. In the absence of external magnetic field with negative uniaxial strain parameter, optical conductivity includes Drude weight at zero frequency limit while Drude weight vanishes for [Formula: see text] layer under positive uniaxial strain. Our results show that the increase of uniaxial positive strain parameter causes to move the position peak to the higher frequencies. In contrast to uniaxial strain case, the Drude weight in optical conductivity appears at positive biaxial strain value 0.15. Also we have studied the effects of magnetic field, electron doping, hole doping in the presence of spin-orbit coupling on frequency dependence of optical conductivity of [Formula: see text] in details. The magnetic field dependence of optical absorption shows a monotonic decreasing behavior for each value of temperature in the absence of strain parameter.

Sensitivity-enhanced optical pressure sensor based on MoS2

A sensitivity-enhanced optical pressure sensor based on molybdenum disulfide (MoS₂) is proposed. The sensing principle is that the pressure causes the deformation of the polydimethylsiloxane (PDMS) pressure structure above the MoS₂ film, leading to the change of the ambient refractive index, so that a measurable light propagation difference in the waveguide under the film is created to reflect the micro changes of the pressure. The pressure is finally numerically converted to the wavelength shift of the interference peak of the obtained spectrum. The process is simulated and analyzed using MoS₂ dielectric film, in contrast with that using graphene dielectric film. It turns out that under same conditions, the MoS₂ film has a more distinct modulation effect on light than that of the graphene film. Experiments using the real sensor prototype are carried out and the results show that the pressure measuring sensitivity is improved to 96.02 nm/kPa in the pressure range of 0-0.6 kPa, which is much higher than the typical optical pressure sensors. The proposed optical pressure sensor based on MoS₂ is of high potential to support ultra-sensitive pressure detection in many applications.

Hybrid electroluminescent devices composed of (In,Ga)N micro-LEDs and monolayers of transition metal dichalcogenides

We demonstrate a novel electroluminescence device in which GaN-based μ-LEDs are used to trigger the emission spectra of monolayers of transition metal dichalcogenides, which are deposited directly on the μ-LED surface. A special μ-LED design enables the operation of our structures even within the limit of low temperatures. A device equipped with a selected WSe₂ monolayer flake is shown to act as a stand-alone, electrically driven single-photon source.

Electron-phonon interaction and ultrafast photoemission from doped monolayer MoS2

We have examined the effect of electron-phonon coupling on photoluminescence and ultrafast response of electron doped monolayer MoS₂, using a combination of density functional theory, time dependent density functional theory, and many-body theory. For small doping (∼1-3%) of interest here, the electron-phonon coupling parameter is modest (∼0.1-0.2) but its effect on the emissive properties and response of the system to femtosecond (fs) laser pulses is striking. We find an ultrafast (fs) relaxation of the electronic subsystem as well as a high fluence of visible light emission induced by electron phonon interaction. Together with high carrier mobility, these features of monolayer MoS₂ may be relevant for optoelectronic technologies.

Molecular Van Der Waals Heterojunction Photodiodes Enabling Dipole-Induced Polarity Switching

Solid-state devices capable of controlling light-responsive charge transport at the molecular scale are essential for developing molecular optoelectronic technology. Here, a solid-state molecular photodiode device constructed by forming van der Waals (vdW) heterojunctions between standard molecular self-assembled monolayers and two-dimensional semiconductors such as WSe₂ is reported. In particular, two non-functionalized molecular species used herein (i.e., tridecafluoro-1-octanethiol and 1-octanethiol) enable bidirectional modulation of the interface band alignment with WSe₂ , depending on their dipole orientations. This dipole-induced band modulation at the vdW heterointerface leads to the opposite change of both photoswitching polarity and rectifying characteristics. Furthermore, compared with other molecular or 2D photodiodes at a similar scale, these heterojunction devices exhibit significantly enhanced photo-responsive performances in terms of photocurrent magnitude, open-circuit potential, and switching speed. This study proposes a novel concept of the solid-state molecular optoelectronic device with controlled functions and enhanced performances.

Advances of Various Heterogeneous Structure Types in Molecular Junction Systems and Their Charge Transport Properties

Molecular electronics that can produce functional electronic circuits using a single molecule or molecular ensemble remains an attractive research field because it not only represents an essential step toward realizing ultimate electronic device scaling but may also expand our understanding of the intrinsic quantum transports at the molecular level. Recently, in order to overcome the difficulties inherent in the conventional approach to studying molecular electronics and developing functional device applications, this field has attempted to diversify the electrical characteristics and device architectures using various types of heterogeneous structures in molecular junctions. This review summarizes recent efforts devoted to functional devices with molecular heterostructures. Diverse molecules and materials can be combined and incorporated in such two- and three-terminal heterojunction structures, to achieve desirable electronic functionalities. The heterojunction structures, charge transport mechanisms, and possible strategies for implementing electronic functions using various hetero unit materials are presented sequentially. In addition, the applicability and merits of molecular heterojunction structures, as well as the anticipated challenges associated with their implementation in device applications are discussed and summarized. This review will contribute to a deeper understanding of charge transport through molecular heterojunction, and it may pave the way toward desirable electronic functionalities in molecular electronics applications.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS2

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS₂) are investigated using first-principles calculations combined with the Boltzmann transport equation. Due to a large acoustic-optical phonon gap of 1.85 THz, the four-phonon (4ph) scattering significantly reduces the room-temperature phononic thermal conductivity (κ_ph). With the further inclusion of phonon-electron scattering, κ_ph reduces to 1.78 W mK^-1. Nevertheless, the total thermal conductivity (κ_total) of 7.82 W mK^-1 is dominated by the electronic thermal conductivity (κ_e) of 6.04 W mK^-1. Due to the electron-phonon coupling, κ_e differs from the typical estimation based on the Wiedemann-Franz law with a constant Sommerfeld value. This work provides new insights into the physical mechanisms for thermal transport in metallic 2D systems with strong anharmonic and electron-phonon coupling effects. The phonon scattering beyond three-phonon (3ph) scattering and even κ_e are typically overlooked in computations, and the constant Sommerfeld value is widely used for separating κ_e and κ_ph from the experimental thermal conductivity. These conclusions have implications for both the computational and experimental measurements of the thermal properties of transition metal dichalcogenides.

Facilitating Uniform Large-Scale MoS2, WS2 Monolayers, and Their Heterostructures through van der Waals Epitaxy

The fabrication process for the uniform large-scale MoS₂, WS₂ transition-metal dichalcogenides (TMDCs) monolayers, and their heterostructures has been developed by van der Waals epitaxy (VdWE) through the reaction of MoCl₅ or WCl₆ precursors and the reactive gas H₂S to form MoS₂ or WS₂ monolayers, respectively. The heterostructures of MoS₂/WS₂ or WS₂/MoS₂ can be easily achieved by changing the precursor from WCl₆ to MoCl₅ once the WS₂ monolayer has been fabricated or switching the precursor from MoCl₅ to WCl₆ after the MoS₂ monolayer has been deposited on the substrate. These VdWE-grown MoS₂, WS₂ monolayers, and their heterostructures have been successfully deposited on Si wafers with 300 nm SiO₂ coating (300 nm SiO₂/Si), quartz glass, fused silica, and sapphire substrates using the protocol that we have developed. We have characterized these TMDCs materials with a range of tools/techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), micro-Raman analysis, photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and selected-area electron diffraction (SAED). The band alignment and large-scale uniformity of MoS₂/WS₂ heterostructures have also been evaluated with PL spectroscopy. This process and resulting large-scale MoS₂, WS₂ monolayers, and their heterostructures have demonstrated promising solutions for the applications in next-generation nanoelectronics, nanophotonics, and quantum technology.

Room-temperature electrical control of polarization and emission angle in a cavity-integrated 2D pulsed LED

Devices based on two-dimensional (2D) semiconductors hold promise for the realization of compact and versatile on-chip interconnects between electrical and optical signals. Although light emitting diodes (LEDs) are fundamental building blocks for integrated photonics, the fabrication of light sources made of bulk materials on complementary metal-oxide-semiconductor (CMOS) circuits is challenging. While LEDs based on van der Waals heterostructures have been realized, the control of the emission properties necessary for information processing remains limited. Here, we show room-temperature electrical control of the location, directionality and polarization of light emitted from a 2D LED operating at MHz frequencies. We integrate the LED in a planar cavity to couple the polariton emission angle and polarization to the in-plane exciton momentum, controlled by a lateral voltage. These findings demonstrate the potential of TMDCs as fast, compact and tunable light sources, promising for the realization of electrically driven polariton lasers.

2D semiconductors offer a promising platform for the realization of compact and CMOS-compatible optoelectronic components. Here, the authors report the realization of light-emitting diodes based on 2D WSe2 integrated with a planar cavity, showing the electrical control of the emission angle and polarization at room temperature.

Advances in Two-Dimensional Materials for Optoelectronics Applications

The past one and a half decades have witnessed the tremendous progress of two-dimensional (2D) crystals, including graphene, transition-metal dichalcogenides, black phosphorus, MXenes, hexagonal boron nitride, etc., in a variety of fields. The key to their success is their unique structural, electrical, mechanical and optical properties. Herein, this paper gives a comprehensive summary on the recent advances in 2D materials for optoelectronic approaches with the emphasis on the morphology and structure, optical properties, synthesis methods, as well as detailed optoelectronic applications. Additionally, the challenges and perspectives in the current development of 2D materials are also summarized and indicated. Therefore, this review can provide a reference for further explorations and innovations of 2D material-based optoelectronics devices.

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe₂, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe₂ can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., − 2 to − 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe₂ was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p–n diode using a 0.8 nm-thick MoTe₂ flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10⁻⁴. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Efficiency Roll-Off Free Electroluminescence from Monolayer WSe2

Exciton-exciton annihilation (EEA) is a nonradiative process commonly observed in excitonic materials at high exciton densities. Like Auger recombination, EEA degrades luminescence efficiency at high exciton densities and causes efficiency roll-off in light-emitting devices. Near-unity photoluminescence quantum yield has been demonstrated in transition metal dichalcogenides (TMDCs) at all exciton densities with optimal band structure modification mediated by strain. Although the recombination pathways in TMDCs are well understood, the practical application of light-emitting devices has been challenging. Here, we demonstrate a roll-off free electroluminescence (EL) device composed of TMDC monolayers tunable by strain. We show a 2 orders of magnitude EL enhancement from the WSe₂ monolayer by applying a small strain of 0.5%. We attain an internal quantum efficiency of 8% at all injection rates. Finally, we demonstrate transient EL turn-on voltages as small as the band gap. Our approach will contribute to practical applications of roll-off free optoelectronic devices based on excitonic materials.

Functionalization of 2D MoS2 Nanosheets with Various Metal and Metal Oxide Nanostructures: Their Properties and Application in Electrochemical Sensors

Two-dimensional transition metal dichalcogenides (2D TMDs) have gained considerable attention due to their distinctive properties and broad range of possible applications. One of the most widely studied transition metal dichalcogenides is molybdenum disulfide (MoS₂). The 2D MoS₂ nanosheets have unique and complementary properties to those of graphene, rendering them ideal electrode materials that could potentially lead to significant benefits in many electrochemical applications. These properties include tunable bandgaps, large surface areas, relatively high electron mobilities, and good optical and catalytic characteristics. Although the use of 2D MoS₂ nanosheets offers several advantages and excellent properties, surface functionalization of 2D MoS₂ is a potential route for further enhancing their properties and adding extra functionalities to the surface of the fabricated sensor. The functionalization of the material with various metal and metal oxide nanostructures has a significant impact on its overall electrochemical performance, improving various sensing parameters, such as selectivity, sensitivity, and stability. In this review, different methods of preparing 2D-layered MoS₂ nanomaterials, followed by different surface functionalization methods of these nanomaterials, are explored and discussed. Finally, the structure-properties relationship and electrochemical sensor applications over the last ten years are discussed. Emphasis is placed on the performance of 2D MoS₂ with respect to the performance of electrochemical sensors, thereby giving new insights into this unique material and providing a foundation for researchers of different disciplines who are interested in advancing the development of MoS₂-based sensors.

Nano-bio interactions of 2D molybdenum disulfide

Two-dimensional (2D) molybdenum disulfide (MoS₂) is an ultrathin nanomaterial with a high degree of anisotropy, surface-to-volume ratio, chemical functionality and mechanical strength. These properties together enable MoS₂ to emerge as a potent nanomaterial for diverse biomedical applications including drug delivery, regenerative medicine, biosensing and bioelectronics. Thus, understanding the interactions of MoS₂ with its biological interface becomes indispensable. These interactions, referred to as "nano-bio" interactions, play a key role in determining the biocompatibility and the pathways through which the nanomaterial influences molecular, cellular and biological function. Herein, we provide a critical overview of the nano-bio interactions of MoS₂ and emphasize on how these interactions dictate its biomedical applications including intracellular trafficking, biodistribution and biodegradation. Also, a critical evaluation of the interactions of MoS₂ with proteins and specific cell types such as immune cells and progenitor/stem cells is illustrated which governs the short-term and long-term compatibility of MoS₂-based biomedical devices.

On the importance of electron-electron and electron-phonon scatterings and energy renormalizations during carrier relaxation in monolayer transition-metal dichalcogenides

Anab initiobased fully microscopic many-body approach is used to study the carrier relaxation dynamics in monolayer transition-metal dichalcogenides. Bandstructures and wavefunctions as well as phonon energies and coupling matrix elements are calculated using density functional theory. The resulting dipole and Coulomb matrix elements are implemented in the Dirac-Bloch equations to calculate carrier-carrier and carrier-phonon scatterings throughout the whole Brillouin zone (BZ). It is shown that carrier scatterings lead to a relaxation into hot quasi-Fermi distributions on a single femtosecond timescale. Carrier cool down and inter-valley transitions are mediated by phonon scatterings on a picosecond timescale. Strong, density-dependent energy renormalizations are shown to be valley-dependent. For MoTe₂, MoSe₂and MoS₂the change of energies with occupation is found to be about 50% stronger in the Σ and Λ side valleys than in theKandK' valleys. However, for realistic carrier densities, the materials always maintain their direct bandgap at theKpoints of the BZ.

Broadband, Ultra-High-Responsive Monolayer MoS2/SnS2 Quantum-Dot-Based Mixed-Dimensional Photodetector

Atomically thin two-dimensional (2D) materials have gained significant attention from the research community in the fabrication of high-performance optoelectronic devices. Even though there are various techniques to improve the responsivity of the photodetector, the key factor limiting the performance of the photodetectors is constrained photodetection spectral range in the electromagnetic spectrum. In this work, a mixed-dimensional 0D/2D SnS₂-QDs/monolayer MoS₂ hybrid is fabricated for high-performance and broadband (UV-visible-near-infrared (NIR)) photodetector. Monolayer MoS₂ is deposited on SiO₂/Si using chemical vapor deposition (CVD), and SnS₂-QDs are prepared using a low-cost solution-processing method. The high performance of the fabricated 0D/2D photodetector is ascribed to the band bending and built-in potential created at the junction of SnS₂-QDs and MoS₂, which enhances the injection and separation efficiency of the photoexcited charge carriers. The mixed-dimensional structure also suppresses the dark current of the photodetector. The decorated SnS₂-QDs on monolayer MoS₂ not only improve the performance of the device but also extends the spectral range to the UV region. Photoresponsivity of the device for UV, visible, and NIR region is found to be ∼278, ∼ 435, and ∼189 A/W, respectively. Fabricated devices showed maximum responsivity under the visible region attributed to the high absorbance of monolayer MoS₂. The response time of the fabricated device is measured as ∼100 ms. These results reveal that the development of a mixed-dimensional (0D/2D) SnS₂-QDs/MoS₂-based high-performance and broadband photodetector is technologically promising for next-generation optoelectronic applications.

A Waveguide-Integrated Two-Dimensional Light-Emitting Diode Based on p-Type WSe2/n-Type CdS Nanoribbon Heterojunction

Transition metal dichalcogenides (TMDs) have emerged as two-dimensional (2D) building blocks to construct nanoscale light sources. To date, a wide array of TMD-based light-emitting devices (LEDs) have been successfully demonstrated. Yet, their atomically thin and planar nature entails an additional waveguide/microcavity for effective optical routing/confinement. In this sense, integration of TMDs with electronically active photonic nanostructures to form a functional heterojunction is of crucial importance for 2D optoelectronic chips with reduced footprint and higher integration capacity. Here, we report a room-temperature waveguide-integrated light-emitting device based on a p-type monolayer (ML) tungsten diselenide (WSe₂) and n-type cadmium sulfide (CdS) nanoribbon (NR) heterojunction diode. The hybrid LED exhibited clear rectification under forward biasing, giving pronounced electroluminescence (EL) at 1.65 eV from exciton resonances in ML WSe₂. The integrated EL intensity against the driving current shows a superlinear profile at a high current level, implying a facilitated carrier injection via intervalley scattering. By leveraging CdS NR waveguides, the WSe₂ EL can be efficiently coupled and further routed for potential optical interconnect functionalities. Our results manifest the waveguided LEDs as a dual-role module for TMD-based optoelectronic circuitries.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Injection-free multiwavelength electroluminescence devices based on monolayer semiconductors driven by an alternating field

Two-dimensional (2D) semiconductors have emerged as promising candidates for various optoelectronic devices especially electroluminescent (EL) devices. However, progress has been hampered by many challenges including metal contacts and injection, transport, and confinement of carriers due to small sizes of materials and the lack of proper double heterostructures. Here, we propose and demonstrate an alternative approach to conventional current injection devices. We take advantage of large exciton binding energies in 2D materials using impact generation of excitons through an alternating electric field, without requiring metal contacts to 2D materials. The conversion efficiency, defined as the ratio of the emitted photons to the preexisting carriers, can reach 16% at room temperature. In addition, we demonstrate the first multiwavelength 2D EL device, simultaneously operating at three wavelengths from red to near-infrared. Our approach provides an alternative to conventional current-based devices and could unleash the great potential of 2D materials for EL devices.

An Asymmetry Field-Effect Phototransistor for Solving Large Exciton Binding Energy of 2D TMDCs

The probing of fundamental photophysics is a key prerequisite for the construction of diverse optoelectronic devices and circuits. To date, though, photocarrier dynamics in 2D materials remains unclear, plagued primarily by two issues: a large exciton binding energy, and the lack of a suitable system that enables the manipulation of excitons. Here, a WSe₂ -based phototransistor with an asymmetric split-gate configuration is demonstrated, which is named the "asymmetry field-effect phototransistor" (AFEPT). This structure allows for the effective modulation of the electric-field profile across the channel, thereby providing a standard device platform for exploring the photocarrier dynamics of the intrinsic WSe₂ layer. By controlling the electric field, this work the spatial evolution of the photocurrent is observed, notably with a strong signal over the entire WSe₂ channel. Using photocurrent and optical spectroscopy measurements, the physical origin of the novel photocurrent behavior is clarified and a room-temperature exciton binding energy of 210 meV is determined with the device. In the phototransistor geometry, lateral p-n junctions serve as a simultaneous pathway for both photogenerated electrons and holes, reducing their recombination rate and thus enhancing photodetection. The study establishes a new device platform for both fundamental studies and technological applications.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

Optical Characterization of Few-Layer PtSe2 Nanosheet Films

Thin films of transition-metal dichalcogenides are potential materials for optoelectronic applications. However, the application of these materials in practice requires knowledge of their fundamental optical properties. Many existing methods determine optical constants using predefined models. Here, a different approach was used. We determine the sheet conductance and absorption coefficient of few-layer PtSe2 in the infrared and UV-vis ranges without recourse to any particular model for the optical constants. PtSe2 samples with a thickness of about 3-4 layers were prepared by selenization of 0.5 nm thick platinum films on sapphire substrates at different temperatures. Differential reflectance was extracted from transmittance and reflectance measurements from the front and back of the sample. The film thickness, limited to a few atomic layers, allowed a thin-film approximation to calculate the optical conductance and absorption coefficient. The former has a very different energy dependence in the infrared, near-infrared, and visible ranges. The absorption coefficient exhibits a strong power-law dependence on energy with an exponent larger than three in the mid-infrared and near-infrared regions. We have not observed any evidence for a band gap in PtSe2 thin layers down to an energy of 0.4 eV from our optical measurements.

MXene monolayer Mn2ZnN2: a promising robust intrinsic half-metallic nanosheet

Two-dimensional half-metallic ferromagnets are promising in spintronics. In recent years, the half-metallicity and the magnetic properties of the MXene materials have been the research hotspots of new materials due to their unique crystal characteristics and wide applications. In this paper, the MXene nanosheet Mn₂ZnN₂was predicted as a kind of robust intrinsic half-metallic nanosheet whose magnetic moment per unit is the integer 6.00μ_Bbased on the first principles calculations. The half-metallic character and the magnetic moment of this nanosheet mainly result from the spin-polarized Mn-ions induced by the crystal field. If the absolute biaxial compression strain is lower than 3.0%, the half-metallicity remains well and the magnetic moment per unit is always 6.00μ_B, indicating that its half-metallicity and magnetic properties are stable within a certain pressure range. More importantly, the magnetic moment per unit is elevated from 6.00μ_Bto 9.00μ_Band the half-metallic energy gap increases evidently after an electron is removed from this nanosheet, suggesting that the half-metallicity and magnetic properties of this nanosheet may be improved via tuning its charge state.

Spontaneous n-Doping in Growing Monolayer MoS2 by Alkali Metal Compound-Promoted CVD

Monolayer MoS₂ has emerged as one of the most promising candidate materials for future semiconductor devices because of its fascinating physical properties and optoelectronic performance. Recently, the utilization of alkali metal compounds as promoters in CVD growth has been demonstrated to be a facile strategy for growing monolayer MoS₂ and other 2D TMDs with large domain sizes. In this work, we systematically investigated the residues derived from alkali metal compounds and the spontaneous n-doping effect on monolayer MoS₂ in alkali metal compound-promoted CVD growth. When using NaOH and other alkali metal compounds as promoters, it is found that the Raman peak of the A_1g mode red shifted with a broadening width and the PL intensity of the A peak decreased with a red shift, which was attributed to the spontaneous n-doping effect during growth. Moreover, the growth using varying amounts of NaOH promoter suggests that the n-doping level could be controlled by the amount of promoter. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) showed the existence of cation-derived residues in the form of a Na-O cluster physiosorbed on top of monolayer MoS₂, which was also confirmed by the transfer experiment. The NaOH treatment experiment and density functional theory (DFT) calculations demonstrate that sodium hydroxide clusters, which could be converted from a combination of Na-O clusters and water vapor, could produce an n-doping effect on monolayer MoS₂. This study provides a facile route to controllably grow monolayer 2D materials with a desired doping level without further treatment.

Boosting quantum yields in two-dimensional semiconductors via proximal metal plates

Monolayer transition metal dichalcogenides (1L-TMDs) have tremendous potential as atomically thin, direct bandgap semiconductors that can be used as convenient building blocks for quantum photonic devices. However, the short exciton lifetime due to the defect traps and the strong exciton-exciton interaction in TMDs has significantly limited the efficiency of exciton emission from this class of materials. Here, we show that exciton-exciton interaction in 1L-WS₂ can be effectively screened using an ultra-flat Au film substrate separated by multilayers of hexagonal boron nitride. Under this geometry, induced dipolar exciton-exciton interaction becomes quadrupole-quadrupole interaction because of effective image dipoles formed within the metal. The suppressed exciton-exciton interaction leads to a significantly improved quantum yield by an order of magnitude, which is also accompanied by a reduction in the exciton-exciton annihilation (EEA) rate, as confirmed by time-resolved optical measurements. A theoretical model accounting for the screening of the dipole-dipole interaction is in a good agreement with the dependence of EEA on exciton densities. Our results suggest that fundamental EEA processes in the TMD can be engineered through proximal metallic screening, which represents a practical approach towards high-efficiency 2D light emitters.

The short exciton lifetime and strong exciton-exciton interaction in transition metal dichalcogenides limit the efficiency of exciton emission. Here, the authors show that exciton-exciton interaction in monolayer WS₂ can be screened using proximal metal plates, leading to an improved quantum yield.