Layered Transition Metal Dichalcogenides Research Articles

Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature.

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX2 , where M= V, Fe, and Cr; T= W, Mo; X= S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, weshow that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS2 , V-WSe2 ). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe2 /WS2 , VSe2 /MoS2 ), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. Weattributed this to photon absorption at the TMD layer that generates electron-hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Vitality of Intralayer Vibration in hBN for Effective Long-Range Interlayer Hole Transfer across High Barriers in MoSe2/hBN/WSe2 Heterostructures.

Introducing the two-dimensional (2D) hexagonal boron nitride (hBN) between 2D transition metal dichalcogenide (TMD) layers promises convenient manipulation of the interlayer exciton (IX) and interlayer charge transfer in TMD/hBN/TMD heterostructures, while the role of inserted hBN layers during IX formation is controversial. Employing ab initio nonadiabatic molecular dynamics (NAMD) simulations and the electron-phonon coupling model, we systematically investigate interlayer hole transfer in MoSe2/WSe2 bilayers intercalated by hBN layers with various thicknesses. The conventional direct hole transfer from MoSe2 to WSe2 is decelerated by 2-3 orders of magnitude after the hBN insertion. Meanwhile, a novel channel intermediated by a deeper hole of WSe2 becomes dominant, where the intralayer shear mode of hBN plays a crucial role by reducing the energy barriers for this new channel. The unique role of hBN layers is revealed for the first time, enriching the knowledge of the underlying microscopic mechanisms and providing instructive guidance to practical van der Waals optoelectronic devices.

Interlayer engineering and electronic regulation of MoSe2 nanosheets rolled hollow nanospheres for high-performance sodium-ion half/full batteries

Layered transition metal dichalcogenides are promising candidates for sodium storage but suffering from low intrinsic electronic conductivity and limited interlayer spacing for fast electron/ion transport, which restricts their high-rate capability and cycling stability. In this work, rGO@MoSe2/NAC hierarchical architectures, consisting of conductive reduced graphene oxide (rGO) supported by hollow nanospheres that are rolled from superlattices of alternatively overlapped MoSe2 and N-doped amorphous carbon (NAC) monolayers, are synthesized as a high-performance sodium storage anode. Theoretical calculations reveal the intercalation of NAC monolayer between two adjacent MoSe2 monolayers improving electronic conductivity of MoSe2 in both surface and internal bulk to fully accelerate electron transport and enhance Na+ ​adsorption. The interoverlapped MoSe2/NAC superlattice featuring a wide interlayer expansion (72.3 ​%) of MoSe2 dramatically decreases Na+ ​diffusion barriers for fast insertion/extraction. Moreover, the hollow nanospheres and the rGO conductive network contribute to a robust hiberarchy that can well release internal stress and buffer the volume expansion, thereby enabling outstanding structural stability. Consequently, the rGO@MoSe2/NAC anode exhibits excellent high-rate capability of 194 mAh g−1 and ultralong cyclability of 12 ​000 cycles with a low capacity fading rate of 0.0038 ​% per cycle at an ultra-high current of 50 ​A ​g−1, delivering the best high-rate cycling performance to date. Remarkably, the Na3V2(PO4)3‖rGO@MoSe2/NAC full cells also present outstanding cycling stability (600 cycles) at 10C rate, which proves the great potential in fast-charging applications.

Synthesis of 1T-MoS2 sheets with large space distance between layers for high-rate aqueous Zn-ion batteries cathode

Molybdenum disulfide (MoS2), as a classical layered transition-metal dichalcogenide (TMD), shows great potential as an advanced Zn2+ storage cathode of aqueous Zn-ion batteries (ZIBs) due to high theoretical capacity and two-dimensional Zn2+ transport channels. However, the narrow interlayer distance and the poor electrical conduction of stable 2H-MoS2 limit its applications. In this work, the 1T-MoS2 sheets with good electrical conduction and large interlayer spacing of 9.5 Å (designated as MoS2-180) have been successfully prepared by a facile hydrothermal method. The interlaminar structural water induces stable existence of metastable 1T-MoS2 and large space distance of 6.08 Å between layers (that is larger than the size (5.5 Å) of hydrated Zn2+), and contributes to its excellent hydrophilicity, promoting transmission of hydrated Zn2+. The interlayer spacing-tailored MoS2-180 cathode exhibits high specific capacity (Cs) of 226.2 mAh g−1 at 0.10 A g−1 and impressive high-rate capability. The Cs at 5.00 A g−1 is extraordinarily up to 150.9 mAh g−1 and retains 66.7% of that at 0.10 A g−1. This is far superior to the most reported MoS2, MoS2 derivatives and MoS2/C composite cathodes. Moreover, the activated MoS2-180 cathode shows robust long-term cycling stability with Cs retention of 103.6% after 500 cycles at 1.00 A g−1. The mechanism behind these phenomena brings new insights into the structural water-induced 1T-MoS2 for high-performance aqueous ZIBs cathodes.

Polarization-tuneable excitonic spectral features in the optoelectronic response of atomically thin ReS2

The low crystal symmetry of rhenium disulphide (ReS2) leads to the emergence of dichroic optical and optoelectronic response, absent in other layered transition metal dichalcogenides, which could be exploited for device applications requiring polarization resolution. To date, spectroscopy studies on the optical response of ReS2 have relied almost exclusively in characterization techniques involving optical detection, such as photoluminescence, absorbance, or reflectance spectroscopy. However, to realize the full potential of this material, it is necessary to develop knowledge on its optoelectronic response with spectral resolution. In this work, we study the polarization-dependent photocurrent spectra of few-layer ReS2 photodetectors, both in room conditions and at cryogenic temperature. Our spectral measurements reveal two main exciton lines at energies matching those reported for optical spectroscopy measurements, as well as their excited states. Moreover, we also observe an additional exciton-like spectral feature with a photoresponse intensity comparable to the two main exciton lines. We attribute this feature, not observed in earlier photoluminescence measurements, to a non-radiative exciton transition. The intensities of the three main exciton features, as well as their excited states, modulate with linear polarization of light, each one acquiring maximal strength at a different polarization angle. We have performed first-principles exciton calculations employing the Bethe-Salpeter formalism, which corroborate our experimental findings. Our results bring new perspectives for the development of ReS2-based nanodevices.

The Role of Vacancy Dynamics in Two‐Dimensional Memristive Devices

AbstractTwo‐dimensional layered transition metal dichalcogenides (TMDCs) are promising memristive materials for neuromorphic computing systems. Despite extensive experimental work, the underlying switching mechanisms are still not understood, impeding progress in material and device functionality. This study reveals the dominant role of defect dynamics in the switching process of 2D TMDC materials. The switching process is governed by the formation and annihilation dynamics of a local vacancy depletion zone. It explains the distinct features of the device characteristics observed experimentally, including fundamentally different device behavior previously thought to originate from multiple mechanisms. Key influence factors are identified and discussed with a fully coupled and dynamic charge transport model for electrons, holes, and ionic point defects, including image‐charge‐induced Schottky barrier lowering (SBL). Thermal effects and local Joule heating are considered by coupling the transient heat transfer equation to the electronic properties. The model is validated with hysteresis and pulse measurements for various lateral 2D MoS2‐based devices, strongly corroborating the relevance of vacancy dynamics in TMDC devices and offering a new perspective on the switching mechanisms. The insights gained from this study can be used to extend the functional behavior of 2D TMDC memristive devices in future neuromorphic computing applications.

Controllable Growth of Large-Scale Continuous ReS2 Atomic Layers

In recent years, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received significant attention due to their exceptional electrical and optical properties. Among these 2D materials, ReS2 distinguishes itself through its unique optical and conductance anisotropy. Despite concerted efforts to produce high-quality ReS2, the unique interlayer decoupling properties pose substantial challenges in growing large-area ReS2 thin films, with the preparation of single layers proving even more complex. In this work, large-scale continuous monolayer and bilayer ReS2 films were successfully grown on mica substrates using low-pressure chemical vapor deposition (LPCVD). Photodetectors were fabricated using the prepared high-quality ReS2 films, and the devices presented stable photoresponse and enhanced response sensitivity. The production of continuous ReS2 atomic layers heralds promising prospects for large-scale integrated circuits and advances the practical application of optoelectronics based on 2D layered materials.

Trends and prospects of 2-D tungsten disulphide (WS2) hybrid nanosystems for environmental and biomedical applications

Recently, 2D layered transition metal dichalcogenides (TMDCs) with their ultrathin sheet nanostructure and diversified electronic structure have drawn attention for various advanced applications to achieve high-performance parameters. Unique 2D TMDCs mainly comprise transition metal and chalcogen element where chalcogen element layers sandwich the transition metal element layer. In such a case, various properties can be enhanced and controlled depending on the targeted application. Among manipulative 2D TMDCs, tungsten disulphide (WS2) is one of the emerging nano-system due to its fascinating properties in terms of direct band gap, higher mobility, strong photoluminescence, good thermal stability, and strong magnetic field interaction. The advancement in characterization techniques, especially scattering techniques, can help in study of opto-electronic properties of 2D TMDCs along with determination of layer variations and investigation of defect. In this review, the fabrication and applications are well summarized to optimize an appropriate WS2-TMDCs assembly according to focused field of research. Here, the scientific investigations on 2D WS2 are studied in terms of its structure, role of scattering techniques to study its properties, and synthesis routes followed by its potential applications for environmental remediation (e.g., photocatalytic degradation of pollutants, gas sensing, and wastewater treatment) and biomedical domain (e.g., drug delivery, photothermal therapy, biomedical imaging, and biosensing). Further, a special emphasis is given to the significance of 2D WS2 as a substrate for surface-enhanced Raman scattering (SERS). The discussion is further extended to commercial and industrial aspects, keeping in view major research gaps in existing research studies.

2D Materials by Design: Intercalation of Cr or Mn between two VSe2 van der Waals Layers.

Insertion of metal layers between layered transition-metal dichalcogenides (TMDs) enables the design of new pseudo-2D nanomaterials. The general premise is that various metal atoms may adopt energetically favorable intercalation sites between two TMD sheets. These covalently bound metals arrange in metastable configurations and thus enable the controlled synthesis of nanomaterials in a bottom-up approach. Here, this method is demonstrated by the insertion of Cr or Mn between VSe2 layers. Vacuum-deposited transition metals diffuse between VSe2 layers with increasing concentration, arranging in ordered phases. The Cr3+ or Mn2+ ions are in octahedral coordination and thus in a high-spin state. Measured and computed magnetic moments are high for dilute Cr atoms, but with increasing Cr concentration the average magnetic moment decreases, suggesting antiferromagnetic ordering between Cr ions. The many possible combinations of transition metals with TMDs form a library for exploring quantum phenomena in these nanomaterials.

Impact of Carbon Impurities on Air Stability of MOCVD 2D-MoS2

Metal–organic chemical vapor deposition (MOCVD) is a key method for scalable synthesis of two-dimensional transition metal dichalcogenide (2D-TMDC) layers. However, it faces several challenges, such as the unintentional co-deposition of carbon impurities resulting from the pyrolysis of metal–organic precursors. This study investigates the chemical features of carbon and its impact on the photoluminescence property and air stability of 2D-MoS2. Using X-ray photoemission spectroscopy (XPS), it was found that the carbon impurities show characteristics similar to those of sp2-bonded graphitic carbon. Upon prolonged (20–40 weeks) exposure to the atmosphere, the incorporated carbon appears to react with 2D-MoS2, forming a MoS2−xCx solid solution. At the same time, a gradual decrease in the S/Mo ratio implies the formation of sulfur vacancies was also observed. These two processes lead to crystal degradation over time, as evidenced by the gradual quenching of the Raman and photoluminescence (PL) peaks. More detailed PL analyses suggest a charge transfer mechanism between sp2-carbon/2D-MoS2 and 2D-MoS2/air-adsorbates, which, in the short term, could alter PL emissions and appear to further intensify the degradation of 2D-MoS2 in the long-term. The findings highlight the strong impact of unintentionally co-deposited carbon on the optical properties and air stability of MOCVD 2D-MoS2 layers.

Emergence of a Thermal Hysteresis of Electrical Resistance by Thinning in 1T-TiSe2

AbstractWe measured the temperature dependence of the resistance in thinned samples of a layered transition metal dichalcogenide 1T-TiSe2. As a result, a thermal hysteresis of resistance was discovered in most samples with a thickness of 10 μm or less, but not in samples thicker than 100 μm. The onset temperature of this hysteresis was almost the same as the onset temperature of the charge density wave (CDW) transition, suggesting that this hysteresis is related to the CDW. All the samples with hysteresis exhibited one or a few jumps of resistance in the 110–190 K range in cooling. Moreover, the hysteresis was related not only to the history of temperature below the CDW transition temperature, but also to that above the transition temperature. To interpret these anomalous results, we discuss the effect of impurities on the domain structure of the CDW.

Optical signatures of Förster-induced energy transfer in organic/TMD heterostructures

Hybrid van der Waals heterostructures of organic semiconductors and transition metal dichalcogenides (TMDs) are promising candidates for various optoelectronic devices, such as solar cells and biosensors. Energy-transfer processes in these materials are crucial for the efficiency of such devices, yet they are poorly understood. In this work, we develop a fully microscopic theory describing the effect of the Förster interaction on exciton dynamics and optics in a WSe2/tetracene heterostack. We demonstrate that the differential absorption and time-resolved photoluminescence can be used to track the real-time evolution of excitons. We predict a strongly unidirectional energy transfer from the organic to the TMD layer. Furthermore, we explore the role temperature has in activating the Förster transfer and find a good agreement to previous experiments. Our results provide a blueprint to tune the light-harvesting efficiency through temperature, molecular orientation and interlayer separation in TMD/organic heterostructures.

Investigating the Fundamental Conditions for Quantitative Growth to Obtain High-Quality WS2 Using a Process of Physical Vapor Deposition

Two-dimensional layered transition-metal dichalcogenides (2D-TMDs) have garnered significant attention due to their layer number-dependent electronic properties, making them promising candidates for atomically thin electronics and optoelectronics. However, current research has primarily focused on exfoliated TMD materials, which have limitations in size, layer number control, and yield. Therefore, a crucial challenge remains in producing large single TMD crystals with precise control over the layer number. A comprehensive understanding and precise control of the growth conditions are imperative to address this challenge. This study systematically investigated key growth conditions, including temperature, precursor flow, growth duration, material quantity, gas flow, and slide position. By optimizing these parameters, we successfully synthesized TMD materials with an impressive size of 850 µm. Notably, we achieved the preparation of monolayer WS2 single crystals on a large scale within a remarkably short duration of 10 min, exhibiting a lateral growth rate of up to 1.4 μm/s, which is comparable to the best-exfoliated monolayers. The findings from our study provide a robust pathway for the rapid growth of high-quality TMD single crystals, facilitating further advancements in this field.

Charge density wave and superconductivity in 6R-TaS[formula omitted

The layered transition metal dichalcogenide compounds 1T-TaS2 and 4H-TaS2 are well known for their exotic properties, which include charge density wave, superconductivity, Mott transition, etc., and lately quantum spin liquid. Here, we report the magnetic, transport and transmission electron microscopy study of the charge density wave and superconductivity in 6R-TaS2 which is a relatively less studied polymorph of this dichalcogenide TaS2. Our high temperature electron microscopy reveals multiple charge density wave transitions between room temperature and 660K. Magnetization, and the electrical resistivity measurements in the temperature range of 2-400 K reveal that 6R-TaS2 undergoes a charge density wave transition around 305 K and is followed by a transition to a superconducting state around 3.5 K. The low temperature specific heat measurement exhibits anomaly associated with the superconducting transition around 2.4 K. The estimated Ginzburg Landau parameter suggests that this compound lies at the extreme limit of type-II superconductivity.

Toward Edge Engineering of Two-Dimensional Layered Transition-Metal Dichalcogenides by Chemical Vapor Deposition.

The manipulation of edge configurations and structures in atomically-thin transition metal dichalcogenides (TMDs) for versatile functionalization has attracted intensive interest in recent years. The chemical vapor deposition (CVD) approach has shown promise for TMD edge engineering of atomic edge configurations (1H, 1T or 1T'-zigzag or armchair edges) as well as diverse edge morphologies (1D nanoribbons, 2D dendrites, 3D spirals, etc.). These edge-rich TMD layers offer versatile candidates for probing the physical and chemical properties and exploring potential applications in electronics, optoelectronics, catalysis, sensing, and quantum technologies. In this Review, we present an overview of the current state-of-the-art in the manipulation of TMD atomic edges and edge-rich structures using CVD. We highlight the vast range of distinct properties associated with these edge configurations and structures and provide insights into the opportunities afforded by such edge-functionalized crystals. The objective of this Review is to motivate further research and development efforts to use CVD as a scalable approach to harness the benefits of such crystal-edge engineering.

Synthesis and Formulation of Ternary Transition Metal Dichalcogenide Alloys for Additive Electronic Manufacturing

Compared with the most investigated graphene, layered transition metal dichalcogenides (TMDs) have received significant attention due to their various chemical compositions and great potential electronic application. In order to tailor and engineer properties of TMDs and therefore enhance their efficiency toward commercial applications, several methods have been employed including reducing dimensionality, creating inter and intra-heterostructures, introducing inter stain, and alloying. Among these, alloying via chemical vapor transport (CVT) is scalable, cost- efficient, and controllable method of tuning TMD bulk crystal properties. Alloying of TMDs can be categorized into several types including metal replacement, dichalcogenide replacement, and both metal and dichalcogenide replacement. In a typical TMD CVT reaction, high-purity precursors are mixed in a desired stochiometric ratio, vacuumed, and sealed in an ampoule to form a closed system. The resulting TMD crystal can then be used for additional processes, e.g., exfoliation to yield 2D TMD nanosheets. Additive electronics manufacturing is a promising technique for the scalable fabrication of electronic devices, including sensors and energy storage devices. For an ink to be compatible for different printing modalities the ink rheology including viscosity, surface tension, and solid requires to be tuned to obtain suitable fluid dynamic parameters for jetting the ink. This work summarizes the development, synthesis, characterization, and formulation of two-dimensional ternary TMDCs ink for aerosol jet printing (AJP) technology. Ball milling assisted liquid exfoliation is utilized for synthesizing ternary TMDs nanomaterials under controlled condition. The resulting nanomaterial is developed into nanomaterial ink compatible with AJP. Detailed analysis of the material characterization and ink properties are required to optimize the fluid dynamics and properties of the ink. After printing the formulated ink using AJP on various substrates, post-printing process techniques is required to be investigated for printed nanomaterials inks to achieve bulk-like performance for the printed structures. Our results highlight the innovations in synthesis and formulation of ternary transition metal dichalcogenide nanomaterial inks for additive manufacturing of electronic devices such as sensors, solar cells, and energy storage devices.

Ionic liquid gating induced self-intercalation of transition metal chalcogenides

Ionic liquids provide versatile pathways for controlling the structures and properties of quantum materials. Previous studies have reported electrostatic gating of nanometer-thick flakes leading to emergent superconductivity, insertion or extraction of protons and oxygen ions in perovskite oxide films enabling the control of different phases and material properties, and intercalation of large-sized organic cations into layered crystals giving access to tailored superconductivity. Here, we report an ionic-liquid gating method to form three-dimensional transition metal monochalcogenides (TMMCs) by driving the metals dissolved from layered transition metal dichalcogenides (TMDCs) into the van der Waals gap. We demonstrate the successful self-intercalation of PdTe2 and NiTe2, turning them into high-quality PdTe and NiTe single crystals, respectively. Moreover, the monochalcogenides exhibit distinctive properties from dichalcogenides. For instance, the self-intercalation of PdTe2 leads to the emergence of superconductivity in PdTe. Our work provides a synthesis pathway for TMMCs by means of ionic liquid gating driven self-intercalation.

Electrochemical imaging correlated to hydrogen evolution reaction on transition metal dichalcogenide, WS2

The ability of two-dimensional, layered transition-metal dichalcogenides (TMDs) to promote hydrogen evolution reaction (HER) has attracted much attention for hydrogen-generation applications. Their electrocatalytic active sites originally come from unique electronic properties at the edges. Here, we use scanning electrochemical cell microscopy (SECCM) to visualize and quantify the electrochemical HER activities of tungsten disulfide (WS2) islands. We show that the HER correlated current distribution is affected by the number of WS2 layers. SECCM imaging provides a direction for future designs in TMD-based electrocatalysts.

Centimeter-level MoS2 films with controllable number of layers by face-to-face chemical vapor deposition strategy

Two-dimensional layered transition metal dichalcogenides (TMDCs) are highly promising in field-effect-transistors (EFTs), photodetectors, flexible devices, but the limitation of uncontrollable size and number of layers is a huge challenge for the development of these devices. Although some breakthroughs like wafer-level MoS2 have been achieved on specific substrates such as copper foil or sapphire using advanced chemical vapor deposition, electrical and optoelectronic testing requires the transfer of material from these substrates to SiO2/Si substrates. In this paper, we developed a face-to-face confined space CVD scheme for the preparation of centimeter-level MoS2 thin films on amorphous SiO2/Si substrate. In contrast to traditional confined-space CVD scheme where the MoO3 powder is often piled like a small hill in the small quartz boat, The MoO3 powder is evenly spread in the small quartz boat, facing directly up to the growing substrate with a “face-to-face” growth mode. Therefore, the nucleation centers are uniformly distributed on the growing substrate, finally resulting in a uniform and clean continuous film originating from the coalescence of numerous MoS2 domains grown up from these nucleation centers. The layer number can be well controlled by adjusting growing parameters like growing temperature, gas flow rate and growth time. Our facile but robust face-to-face growth strategy may be a great supplementary to the wafer-level growth of 2D TMDCs.

Intensity-Dependent Optical Response of 2D LTMDs Suspensions: From Thermal to Electronic Nonlinearities.

The nonlinear optical (NLO) response of photonic materials plays an important role in the understanding of light-matter interaction as well as pointing out a diversity of photonic and optoelectronic applications. Among the recently studied materials, 2D-LTMDs (bi-dimensional layered transition metal dichalcogenides) have appeared as a beyond-graphene nanomaterial with semiconducting and metallic optical properties. In this article, we review most of our work in studies of the NLO response of a series of 2D-LTMDs nanomaterials in suspension, using six different NLO techniques, namely hyper Rayleigh scattering, Z-scan, photoacoustic Z-scan, optical Kerr gate, and spatial self-phase modulation, besides the Fourier transform nonlinear optics technique, to infer the nonlinear optical response of semiconducting MoS2, MoSe2, MoTe2, WS2, semimetallic WTe2, ZrTe2, and metallic NbS2 and NbSe2. The nonlinear optical response from a thermal to non-thermal origin was studied, and the nonlinear refraction index and nonlinear absorption coefficient, where present, were measured. Theoretical support was given to explain the origin of the nonlinear responses, which is very dependent on the spectro-temporal regime of the optical source employed in the studies.