Transition Metal Dichalcogenide System Research Articles

Monolayer MoS2 and WS2 for Vertical Circular‐ Polarized‐Light‐Emitting Diode: from Fundamental Understanding to Device Architecture

AbstractLight‐emitting diodes (LEDs) have revolutionized lighting and displays due to their numerous advantages over conventional lighting mechanisms. Moreover, the directional nature of luminescent materials has spurred significant advancements in the development of circularly polarized LEDs, which hold transformative potential for applications in biomedical imaging, liquid crystal displays, spintronics, and valleytronics. The performance of circularly polarized LEDs mainly depends on the emitter material, which is this study's focus. In particular, semiconducting‐phase 2D monolayer MoS2 and WS2 are attractive emitter‐material candidates owing to their bandgap versatility, high carrier mobility, high exciton binding energy, polarized‐light‐emission properties, and unique spin–valley coupling. Several works have examined the fundamental light‐emission properties of monolayer MoS2 and WS2 from the perspectives of optoelectronic concepts, material fabrication, and device construction. This paper presents approaches to control, tune, and enhance these properties of monolayer MoS2 and WS2. Possible guidelines for monolayer‐material synthesis (top‐down and bottom‐up approaches) and device engineering of vertically stacked MoS2 and WS2 are presented. Finally, the review considers the material topological characteristics, outlines the challenges and potential of monolayer MoS2 and WS2 for developing high‐performance commercial circularly polarized LED devices, and proposes a technological roadmap for leveraging other monolayer transition metal dichalcogenide systems in optoelectronic devices.

Fluctuation-dominated quantum oscillations in excitonic insulators

The realization of excitonic insulators in transition metal dichalcogenide systems has opened the door to explorations of the exotic properties that such a state exhibits. We study theoretically the potential for excitonic insulators to show an anomalous form of quantum oscillations: the de Haas–van Alphen effect in an insulating system. We focus on the role of the interactions that generate the energy gap and show that it is crucial to consider quantum fluctuations that go beyond the mean-field treatment. Remarkably, quantum fluctuations can be dominant and lead to quantum oscillations that are significantly larger than those predicted using mean-field theory. Indeed, in experimentally accessible parameter regimes these fluctuation-generated quantum oscillations can even be larger than what would be found for the corresponding gapless system. Published by the American Physical Society 2024

Twisted graphene superlattices: resonating valence bond states and magnetic properties

Resonating valence bond (RVB) states are fundamental for understanding quantum spin liquids in two-dimensional (2D) systems. The RVB state is a collective phenomenon in which spins are uncoupled. 2D lattices such as triangular, honeycomb, and dice lattices were investigated using the Hubbard model and exact diagonalization method. We analyzed the total spin, spin–spin correlation functions, local magnetic moments, and spin and charge gaps as a function of on-site Coulomb repulsion, electron concentration, and electronic hopping parameters. Phase diagrams showed that RVB states can live in half-filled and hole-doped anisotropic triangular lattices. We found two types of RVB states: one in the honeycomb sublattice and the other in the centered hexagons in the triangular lattices. Owing to the novel discovery of exotic magnetic ordering in triangular moiré patterns in twisted bilayer graphene and transition metal dichalcogenide systems, our results provide physical insights into the onset of magnetism and possible spin liquid states in these layered materials.

Double Superconducting Dome of Quasi Two-Dimensional TaS2 in Non-Centrosymmetric van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (2D-vdWHs) based on transition metal dichalcogenides (TMDs) provide unparalleled control over electronic properties. However, the interlayer coupling is challenged by the interfacial misalignment and defects, which hinders a comprehensive understanding of the intertwined electronic orders, especially superconductivity and charge density wave (CDW). Here, by using pressure to regulate the interlayer coupling of non-centrosymmetric 6R-TaS2 vdWHs, we observe an unprecedented phase diagram in TMDs. This phase diagram encompasses successive suppression of the original CDW states from alternating H-layer and T-layer configurations, the emergence and disappearance of a new CDW-like state, and a double superconducting dome induced by different interlayer coupling effects. These results not only illuminate the crucial role of interlayer coupling in shaping the complex phase diagram of TMD systems but also pave a new avenue for the creation of a novel family of bulk heterostructures with customized 2D properties.

Hydrogen evolution reaction catalyst with high catalytic activity by interplay between organic molecules and transition metal dichalcogenide monolayers

Introducing sulfur vacancies (SV) in monolayer molybdenum disulfide (MoS2) is a well-known approach to activate the inert basal planes for efficient hydrogen evolution reaction (HER). However, higher SV induction accelerates the disruption of the crystal structure. We report an efficient method for improving catalytic activity and durability via covalent melamine (MEL) functionalization at SV sites in the basal plane of transition metal dichalcogenide (TMD) monolayers. Functionalized 2H–MoS2 requires only an overpotential of 160 mV at 100 mAcm−2 in acidic medium with chronoamperometric durability of more than 14 h at 30 mAcm−2. Our theoretical results reveal that chemical functionalization creates novel HER active sites, where catalytic activity of the MEL−MoS2 catalyst arises from the interplay of the ring N sites of the MEL and the S atoms on the 2H–MoS2 surface. These findings of chemically coupled organic molecule−TMD system will pave new avenues for development of superior HER catalysts.

Pathways of Exciton Triggered Hot‐Carrier Injection at Plasmonic Metal−Transition Metal Dichalcogenide Interface

AbstractSurface plasmon induced hot‐carrier injection to semiconducting transition metal dichalcogenide (TMD) monolayers has been extensively studied. However, comprehensive understanding of the injection kinetics by fully considering the weak metal−TMD interaction and the TMDs’ exciton formation kinetics is missing. Here, a hot‐carrier injection pathway is elucidated by systematically investigating the interfacial interaction kinetics among different plasmonic metals and TMDs. The pathway highlights the exciton formation timescale as a threshold for interfacial carrier injection, before which plasmonic hot carriers and free electron−hole pairs are relaxed incoherently across the interface. The injected hot carriers will interact with excitons to form charged quasiparticles as trions, which have extended lifetime. The pathway reveals the fundamental mechanism of the plasmonic hot‐carrier−TMD interactions, opens the possibility of controllable manipulation of hot‐carrier injection process, and allows future research toward opto‐electrical guidance of trions in metal−TMD systems.

Hydrothermal synthesis and controlled growth of group-VIB W metal compound nanostructures from tungsten oxide to tungsten disulphide.

Two-dimensional lateral group-VIB transition metal dichalcogenides (TMDs) have attracted much attention in the fast evolving field of advanced photoelectric functional materials, but their controllable fabrication is challenging. Herein, an emerging synthetic route for sulfurization of tungsten oxide was developed. During the hydrothermal reaction, the optimization of the precursor selection and synthesis parameters led to the tunable properties of WO3-WSxOy-WS2 nanostructures. The vulcanization was thermodynamically favorably at low temperatures and in an environment with a sufficient S source, wherein WO3 was reduced by H atoms to WO3-x, and S atoms were preferentially adsorbed on O vacancies. The WSxOy nanostructures have a narrow band-gap attributed to the effect of S on the valence band top and electronic density of states by density functional theory. The photocurrent response and charge transfer properties of WSxOy were improved due to the charge transport between WS2 and WO3. Understanding the formation and transformation of WS2 nanostructures in solution contributes to the discovery of the important structure-efficiency relationship, which may be extended to other TMDs systems. Hence, extensive research efforts are still needed to develop safer and more efficient synthesis and modification methods to fully utilize the distinctive advantageous properties of TMDs in the photoelectric field.

At the Verge of Topology: Vacancy-Driven Quantum Spin Hall in Trivial Insulators.

Vacancies in materials structure─lowering its atomic density─take the system closer to the atomic limit, to which all systems are topologically trivial. Here we show a mechanism of mediated interaction between vacancies inducing a topologically nontrivial phase. Within an ab initio approach we explore topological transition dependence with the vacancy density in transition metal dichalcogenides. As a case of study, we focus on the PtSe2, for which the pristine form is a trivial semiconductor with an energy gap of 1.2 eV. The vacancies states lead to a large topological gap of 180 meV within the pristine system gap. We derive an effective model describing this topological phase in other transition metal dichalcogenide systems. The mechanism driving the topological phase allows the construction of backscattering protected metallic channels embedded in a semiconducting host.

A co-doped 2D transition metal dichalcogenide system with the potential of having enhanced photocatalytic properties

Pristine and co-doped tungsten sulfide nanoparticles were successfully synthesized using hydrothermal method. These nanoparticles were characterized using microscopic and spectroscopic techniques. To determine morphological properties, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed. UV–Vis DRS was used to evaluate optical properties for light-harvesting measurements of nanoparticles. Assessing the rate of recombination of the photogenerated electrons and holes, electrochemical impedance spectroscopy (EIS) and photoluminescence spectroscopy (PL) were used. Band edge positions and respective shifts were determined from XPS including the chemical states of the dopants. The co-doping of rare earth metals on the pristine material proved to be light harvesting reducing the band energy gap energy by 0.05 eV of the pristine material resulting in a red shift. Amongst the prepared photocatalyst at different content, 3%Ce/Gd-WS2 showed to have high photocurrent response, lower rate of recombination and lower charge transfer resistance indicating its potential as in photocatalytic applications such as water-splitting and water treatment.

Local field effects in ultrafast light–matter interaction measured by pump-probe spectroscopy of monolayer MoSe2

Abstract Using ultrafast resonant pump-probe spectroscopy in an unconventional experimental setup we investigate the spectral shape and dynamics of absorption features related to the A exciton in an hexagonal boron nitride (hBN)/MoSe2/hBN van der Waals heterostructure. While in a pure two-level system a pump-probe experiment measures the occupation or the polarization dynamics, depending on the time ordering of the pulse pair, in the transition metal dichalcogenide (TMD) system both quantities get thoroughly mixed by strong exciton–exciton interaction. We find that for short positive delays the spectral lines experience pronounced changes in their shape and energy and they relax to the original situation on a picosecond time scale. For negative delays, distinctive spectral oscillations appear indicating the first-time observation of perturbed free induction decay for a TMD system. The comparison between co-circular and cross-circular excitation schemes further allows us to investigate the rapid inter-valley scattering. By considering a three-level system as a minimal model including the local field effect, excitation-induced dephasing (EID), and scattering between the excited states we explain all phenomena observed in the experiment with excellent consistency. Our handy model can be even further reduced to two levels in the case of a co-circular excitation, for which we derive analytic expressions to describe the detected signals. This allows us to trace back the spectral shapes and shifts to the impact of local field effect and EID thus fully reproducing the complex behavior of the observed effects.

Efficient Spin-Orbit Torque Generation in Semiconducting WTe2 with Hopping Transport.

Spin-orbit torques (SOTs) from transition metal dichalcogenide systems (TMDs) in conjunction with ferromagnetic materials are recently found to be attractive in spintronics for their versatile features. However, most of the previously studied crystalline TMDs are prepared by mechanical exfoliation, which limits their potentials for industrial applications. Here, we show that amorphous WTe2 heterostructures deposited by magnetron sputtering possess a sizable damping-like SOT efficiency of ξDLWTe2 ≈ 0.20 and low damping constant of α = 0.009 ± 0.001. Only an extremely low critical switching current density of Jc≈ 7.05 × 109 A/m2 is required to achieve SOT-driven magnetization switching. The SOT efficiency is further proved to depend on the W and Te relative compositions in the co-sputtered W100-xTex samples, from which a sign change of ξDLWTe2 is observed. In addition, the electronic transport in amorphous WTe2 is found to be semiconducting and is governed by a hopping mechanism. With the above advantages and rich tunability, amorphous and semiconducting WTe2 serves as a unique SOT source for future spintronics applications.

1D chain structure in 1T′-phase 2D transition metal dichalcogenides and their anisotropic electronic structures

The quasi-metallic 1T'-phase is one of the notable polymorphic two-dimensional transition metal dichalcogenide phases and possesses a unique structure, comprising a one-dimensional zigzag transition metal chain along a single axis. This structure gives rise to unprecedented anisotropic electronic properties, including new anisotropic mid-infrared plasmon excitations. The emergence of highly correlated electronic modes in such anisotropic systems is relevant to the intrinsic electronic and optical properties of two-dimensional transition metal dichalcogenides. In this review, we highlight the unique features of these materials in their anisotropic quasi-metallic 1T'-phase, not found in the other structural phases. We discuss how this anisotropic structure brings about the onset of unique physical phenomena such as the electronic band inversion, anomalously large magnetoresistance, and intrinsic or induced superconductivity in multiple transition metal dichalcogenide systems. The unique structural properties of these systems have led to spatially resolved transport and optical properties that can be further manipulated for use in a wide variety of applications ranging from nano-electronics to ultra-sensitive photonic systems but also their incorporation in high performance field-electric transistors, photodetectors, hydrogen evolution reactions, and piezoelectric devices cater to technologies of the future.

Signatures of nontrivial Rashba metal states in a transition metal dichalcogenides Josephson junction

Nontrivial Rashba metal states in conventional semiconductor materials generated by both Rashba spin–orbit coupling and ferromagnetic exchange coupling coexisting were recently predicted and exploited. Single layered transition metal dichalcogenides (TMDC) featuring those states and their potential applications have been less focused. We find that, in the materials with Rashba spin–orbit coupling only, nontrivial Rashba metallic states can be manipulated by an external gate voltage. Based on extensive numerical simulations, the relationships between the supercurrent and nontrivial Rashba metallic states in the TMDC Josephson junction have been investigated. It is shown that, in the absence of the Rashba spin–orbit coupling, the critical supercurrent exhibits a stark difference between normal Rashba metal state and anomalous Rashba metal state in the finite junction as compared to the case of the short junction. While in the case of the finite Rashba spin–orbit coupling, the critical supercurrent demonstrates a reentrant behavior when Fermi level sweeps from anomalous Rashba metal state to Rashba ring metal state. Intriguingly, not only at the conversion of the nontrivial Rashba metallic states but also in the Rashba ring metal state the reentrant behavior exhibits again, which could be well explained by the mixing of spin-triplet Cooper pairs with spin-singlet Cooper pairs in Ising superconductor. Such a reentrant effect offers a new way to detect Ising superconductivity based on the TMDC systems. Meanwhile our study also clarified that the nontrivial Rashba metallic state plays an important role in controlling the supercurrent in the TMDC Josephson junction, which is useful for designing future superconducting devices.

Unconventional Magnetism in Layered Transition Metal Dichalcogenides

In this contribution to the MDPI Condensed Matter issue in Honor of Nobel Laureate Professor K.A. Müller I review recent experimental progress on magnetism of semiconducting transition metal dichalcogenides (TMDs) from the local-magnetic probe point of view such as muon-spin rotation and discuss prospects for the creation of unique new device concepts with these materials. TMDs are the prominent class of layered materials, that exhibit a vast range of interesting properties including unconventional semiconducting, optical, and transport behavior originating from valley splitting. Until recently, this family has been missing one crucial member: magnetic semiconductor. The situation has changed over the past few years with the discovery of layered semiconducting magnetic crystals, for example CrI 3 and VI 2 . We have also very recently discovered unconventional magnetism in semiconducting Mo-based TMD systems 2H-MoTe 2 and 2H-MoSe 2 [Guguchia et. al., Science Advances 2018, 4(12)]. Moreover, we also show the evidence for the involvement of magnetism in semiconducting tungsten diselenide 2H-WSe 2 . These results open a path to studying the interplay of 2D physics, semiconducting properties and magnetism in TMDs. It also opens up a host of new opportunities to obtain tunable magnetic semiconductors, forming the basis for spintronics.

Anisotropic Collective Charge Excitations in Quasimetallic 2D Transition-Metal Dichalcogenides.

The quasimetallic 1T′ phase 2D transition‐metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in‐plane electronic correlations are explored, and new mid‐infrared plasmon excitations in 1T′ phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first‐principles study which analyzes the charge dynamics across multiple axes of the atomic‐layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long‐range coupling is responsible for the coherent 1D charge dynamics and the spin–orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D‐chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.

Transition Metal Dichalcogenides as Strategy for High Temperature Electron-Hole Superfluidity

Condensation of spatially indirect excitons, with the electrons and holes confined in two separate layers, has recently been observed in two different double layer heterostructures. High transition temperatures were reported in a double Transition Metal Dichalcogenide (TMD) monolayer system. We briefly review electron-hole double layer systems that have been proposed as candidates for this interesting phenomenon. We investigate the double TMD system WSe 2 /hBN/MoSe 2 , using a mean-field approach that includes multiband effects due to the spin-orbit coupling and self-consistent screening of the electron-hole Coulomb interaction. We demonstrate that the transition temperature observed in the double TMD monolayers, which is remarkably high relative to the other systems, is the result of (i) the large electron and hole effective masses in TMDs, (ii) the large TMD band gaps, and (iii) the presence of multiple superfluid condensates in the TMD system. The net effect is that the superfluidity is strong across a wide range of densities, which leads to high transition temperatures that extend as high as T B K T = 150 K.

Towards the evaluation of defects in MoS2 using cryogenic photoluminescence spectroscopy.

Characterization of the type and density of defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) is important as the nature of these defects strongly influences the electronic and optical properties of the material, especially its photoluminescence (PL). Defect characterization is not as straightforward as it is for graphene films, where the D and D' Raman scattering modes easily indicate the density and type of defects in the graphene layer. Thus, in addition to the Raman scattering analysis, other spectroscopic techniques are necessary to perform detailed characterization of atomically thin TMD layers. We demonstrate that PL spectroscopy performed at liquid helium temperatures reveals the key fingerprints of defects in TMDs and hence provides valuable information about their origin and concentration. In our study, we address defects in chemical vapor deposition (CVD)-grown MoS2 monolayers. A significant difference is observed between the as-grown monolayers compared with the CVD-grown monolayers transferred onto a Si/SiO2 substrate, which contain extra defects due to the transfer process. We demonstrate that the temperature-dependent Raman and PL micro-spectroscopy techniques enable disentangling the contributions and locations of various defect types in TMD systems.

SnS2 Hollow Spheres: Template‐Free Synthesis and Growth Mechanism

Recent advances in 2D transition metal dichalcogenides (TMDs) have led to a variety of promising technologies for nanoelectronics, photonics, sensing, energy storage, and optoelectronics. Among these dichalcogenides, tin sulfide (SnS2) has received great attention due to its high optical absorption coefficient, high theoretical capacitance, high natural abundance of precursor chemicals, and minimal impact of these on the environment (green chemistry). It is crucial to obtain materials with varied morphologies because the chemical and physical properties are dependent on the morphology. The controlled synthesis of SnS2 with a specific morphology, a hollow sphere, is of significant interest. Herein, a facile, template‐free, one‐step solvothermal method to prepare SnS2 spheres with a hollow structure is demonstrated. The size of the SnS2 hollow spheres is uniform, at 2 μm in diameter, and the walls of the sphere are only 300 nm thick. The hollow structure forms due to the different specific surface energies of the SnS2 sheets and the fluorine‐doped tin oxide (FTO) substrate. Furthermore, the possible growth mechanism of these SnS2 hollow spheres is proposed, which may be applied to other 2D TMD systems.

Spatial Mapping of Hot-Spots at Lateral Heterogeneities in Monolayer Transition Metal Dichalcogenides.

Lateral heterogeneities in atomically thin 2D materials such as in-plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano- and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot-spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS2 and WS2 do not largely impact the distribution of heat dissipation, while GBs of MoS2 appreciably localize heating in the device. High-resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure-property-processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all-2D electronic circuitry with enhanced functionalities, lifetime, and performance.

Phase Identification and Strong Second Harmonic Generation in Pure ε-InSe and Its Alloys.

Two-dimensional material indium selenide (InSe) has offered a new platform for fundamental research in virtue of its emerging fascinating properties. Unlike 2H-phase transition-metal dichalcogenides (TMDs), ε phase InSe with a hexagonal unit cell possesses broken inversion symmetry in all the layer numbers, and predicted to have a strong second harmonic generation (SHG) effect. In this work, we find that the as-prepared pure InSe, alloyed InSe1- xTe x and InSe1- xS x ( x = 0.1 and 0.2) are ε phase structures and exhibit excellent SHG performance from few-layer to bulk-like dimension. This high SHG efficiency is attributed to the noncentrosymmetric crystal structure of the ε-InSe system, which has been clearly verified by aberration-corrected scanning transmission electron microscopy (STEM) images. The experimental results show that the SHG intensities from multilayer pure ε-InSe and alloyed InSe0.9Te0.1 and InSe1- xS x ( x = 0.1 and 0.2) are around 1-2 orders of magnitude higher than that of the monolayer TMD systems and even superior to that of GaSe with the same thickness. The estimated nonlinear susceptibility χ(2) of ε-InSe is larger than that of ε-GaSe and monolayer TMDs. Our study provides first-hand information about the phase identification of ε-InSe and indicates an excellent candidate for nonlinear optical (NLO) applications as well as the possibility of engineering SHG response by alloying.