Transition Metal Dichalcogenide Alloys Research Articles

Overcoming the thermodynamic barrier of alloying 2D transition metal dichalcogenides

Alloying of two-dimensional (2D) transition metal dichalcogenides (TMDs) enables modulation of their energy structure and bandgap, and thus, is crucially important for their integration into optical and electrooptical devices. However, achieving high yield and efficient alloying is often challenging due to the thermodynamic preference of one of the components over the other. Here, we develop a process for growing Mo1−xWxS2 alloys. While common chemical vapor deposition (CVD)-based growing techniques give limited alloying due to the thermodynamic dominance of MoS2 over WS2, we present here an efficient means to achieve high-yield alloying via a two-step CVD process. In this process, first, we grew monolayer WS2 flakes on a substrate, followed by the nucleation-diffusion second step. In this step, MoS2 nucleates mostly on the edges of the WS2, followed by diffusion of the Mo atoms into the flakes to achieve Mo1−xWxS2 alloys. Indeed, we obtained a large amount of alloy growth with different atomic compositions (i.e., various values of x). We employed several analysis methods, including Raman, photoluminescence, x-ray photoelectron spectroscopy, and electron dispersive spectroscopy, to characterize the properties of the grown alloys. These analyses demonstrate the transition from WS2- (x→1) to MoS2-like (x→0) behaviors, through alloy behavior (0<x<1), which combines the properties of both materials and presents a homogenous distribution of the diffused atoms. We also showed that controlling the diffusion time can efficiently determine the average atomic composition of the alloys. This work, therefore, overcomes the inherent thermodynamic barrier that limits the large-scale synthesis of TMD alloys, and thus, it paves the path toward their integration into advanced optoelectronic tunable bandgap applications.

Band Gap Engineering of MoxW1-xS2 Alloy Monolayers with Wafer-Scale Uniformity.

Modulating the band gap of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is critical for their application in a wider spectral range. Alloying has been demonstrated as an effective method for regulating the band gap of 2D TMDC semiconductors. The fabrication of large-area 2D TMDC alloy films with centimeter-scale uniformity is fundamental to the application of integrated devices. Herein, we report a liquid-phase precursor one-step chemical vapor deposition (CVD) method for fabricating a MoxW1-xS2 alloy monolayer with a large size and an adjustable band gap. Good crystalline quality and high uniformity on a wafer scale enable the continuous adjustment of its band gap in the range of 1.8-2.0 eV. Density functional theory calculations provided a deep understanding of the Raman-active vibration modes of the MoxW1-xS2 alloy monolayer and the change in the conductivity of the alloy with photon energy. The synthesis of large-area MoxW1-xS2 alloy monolayers is a critical step toward the application of 2D layered semiconductors in practical optoelectronic devices.

2D/2D Molybdenum Sulfo Selenides/Black Phosphorus Heterostructures for Supercapacitors and Light‐Driven Hydrogen Generation Applications

Abstract2D transition metal dichalcogenide alloys are considered as the promising duo for energy storage and catalyst for light‐driven energy conversion applications due to their novel physicochemical properties. However, heterostructuring with other 2D materials is an effective strategy to enhance the charge storage kinetics as well as comprehensive spectral light response and efficient charge separation. Herein, Molybdenum sulfo selenide (MoSSe)/black phosphorous (BP) heterostructure is synthesized by a one‐pot solvothermal technique, and energy storage and conversion efficiency are characterized. Interestingly, the fabricated symmetric supercapacitor based on the MoSSe/BP hybrid shows an exceptional capacitance of 230 mF cm−2 with an excellent energy density of 31.9 µWh cm−2 and a power density of 805.7 µW cm−2. To validate the experimental findings, Density Functional Theory (DFT) computational simulations are carried out concurrently. The lower diffusion energy barrier for electrolytic ions, in the case of hybrid MoSSe/BP compared to pristine MoSSe, supports the higher charge storage performance. Finally, MoSSe/BP nanocomposites' photocatalytic hydrogen evolution reaction (HER) performance is evaluated under 400 W of UV–vis light, with eosin Y dye acting as a sensitizer and triethanolamine acting as a sacrificial agent. The MoSSe/BP nanocomposite exhibits the maximum photocatalytic HER activity of 5718 µmol h−1 g−1, greater than the bare MoSSe nanostructure.

Growth of Highly Oriented (VNbMoTaW)S2 Layers.

Compositional tunability, an indispensable parameter for modifying the properties of materials, can open up new applications for van der Waals (vdW) layered materials such as transition-metal dichalcogenides (TMDCs). To date, multielement alloy TMDC layers are obtained via exfoliation from bulk polycrystalline powders. Here, we demonstrate direct deposition of high-entropy alloy disulfide, (VNbMoTaW)S2, layers with controllable thicknesses on free-standing graphene membranes and on bare and hBN-covered Al2O3(0001) substrates via ultra-high-vacuum reactive dc magnetron sputtering of the VNbMoTaW target in Kr and H2S gas mixtures. Using a combination of density functional theory calculations, Raman spectroscopy, X-ray diffraction, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, we determine that the as-deposited layers are single-phase, 2H-structured, and 0001-oriented (V0.10Nb0.16Mo0.19Ta0.28W0.27)S2.44. Our synthesis route is general and applicable for heteroepitaxial growth of a wide variety of TMDC alloys and potentially other multielement alloy vdW compounds with the desired compositions.

Synthesis of component-controllable monolayer MoxW(1-x)S2ySe2(1-y) alloys with continuously tunable band gap and carrier type.

Alloying can effectively modify electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). However, efficient and simple methods to synthesize atomically thin TMD alloys need to be further developed. In this study, we synthesized 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys by using a new liquid phase edge epitaxy (LPEE) growth method with high controllability. This straightforward approach can be used to obtain monolayer materials and operates on a self-limiting growth mechanism. The process allows the liquid solution to come into contact with the two-dimensional grains only at their edges, resulting in epitaxy confined only along the in-plane direction, which produces exclusively monolayer epitaxy. By controlling the weight ratio of MoS2/WSe2 (MoSe2/WS2), 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys with different atomic ratios can be obtained on sapphire substrates, with band gap ranging from WS2 (1.55 eV) to MoSe2 (1.99 eV) and a continuously broad spectrum ranging from 623 nm to 800 nm. By adjusting the alloy composition, the carrier type and carrier mobility of alloy-based field-effect transistors can be modulated. In particular, the adjustable conductivity of MoxW(1-x)S2ySe2(1-y) alloys from n-type to bipolar type is achieved for the first time. This general synthetic strategy provides a foundation for the development of monolayer TMD alloys with multiple components and various 2D materials.

Exploring kinetic control for the growth of layered MoS2(1-x)Se2x alloy and its electrocatalytic activity in hydrogen evolution reaction

Although controlled growth of monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) alloys has recently been achieved, a comprehensive understanding of their growth mechanism remains a major challenge due to the complex nucleation kinetics. In this study, the synthesis of large-area ternary MoS2(1-x)Se2x alloys by atmospheric pressure chemical vapor deposition (CVD) is effectively and efficiently controlled. Both experimental and theoretical investigations are conducted to explore the influence of precursors on growth kinetics. The results suggest that the introduction of Se not only reduces nucleation sites but also lowers diffusion energy, thereby promoting lateral growth of the alloy, which is supported by density functional theory (DFT) calculations. Furthermore, the electrocatalytic performance of MoS2 film with discretely distributed triangle sheets is further enhanced by the incorporation of Se atoms during the hydrogen evolution reaction (HER). The Gibbs free energy of adsorbed hydrogen atom (ΔGH*) has been calculated by DFT, showing that the replacement of S with Se at the edge site compared with the in-plane site would significantly reduce the Gibbs free energy. This study is crucial for improving the comprehension of the growth dynamics of TMD alloys and promoting their utilization in electrocatalytic hydrogen generation.

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.

Excitons dynamic modulation by tailoring size of high-entropy Mo0.64W0.36S2 alloy

Two-dimensional transition metal disulfides with tailored optical response are vital to demonstrate versatile optoelectronic devices and nanophotonic elements. In this work, using transient absorption spectroscopy in conjunction with density functional theory calculations, we demonstrate ultrafast excitons absorption modulation via tailoring the size of a transition-metal dichalcogenide alloy Mo0.64W0.36S2 nanosheet. As the size of the nanosheet increases from 9 ± 5 to 190 ± 121 nm, peak positions of the A and B excitons vary from 666 to 675 nm and 626 to 638 nm, respectively. Furthermore, the decay lifetimes of A excitons slow down from 1.16 to 1.84 ps when the size changes from 9 ± 5 to 190 ± 121 nm. It is shown that the exciton diffusion and decay properties can be modulated by the components and structure modulation, which is beneficial for the optimal design and optimization of optoelectronic devices.

Controllable phase modulation and electronic structures of monolayer MoSe2xTe2(1−x) alloys grown via molecular beam epitaxy

Controllable phase modulation and electronic structure are essential factors in the study of two-dimensional transition metal dichalcogenides due to their impact on intriguing physical properties and versatile optoelectronic applications. Here, we report the phase-controlled growth of ternary monolayer MoSe2xTe2(1−x) (0 ≤ x ≤ 1) alloys induced through in situ doping and composition tuning via molecular beam epitaxy. Our approach leverages the substitution of selenium for tellurium to lower the energy barrier of the semi-conducting 2H and semi-metallic 1T′ phase transition. The alloys’ lattice constants, Mo-3d binding energy and electronic bandgap were demonstrated to be tunable by varying the selenium composition (x), respectively. First-principles calculations agree well with our experimental results, revealing that the valence band bowing effect of the monolayer alloys is attributed to the difference in coupling between anions and cations. This work provides a new pathway for phase modulation growth and controllable electronic structure of ternary monolayer transition metal dichalcogenide alloys, which is of great significance for ohmic contact and band engineering in developing transistor device applications using two-dimensional semiconductors.

Alloy Engineering Allows On-Demand Design of Ultrasensitive Monolayer Semiconductor SERS Substrates.

The chemical mechanism (CM) of surface-enhanced Raman scattering (SERS) has been recognized as a decent approach to mildly amplify Raman scattering. However, the insufficient charge transfer (CT) between the SERS substrate and molecules always results in unsatisfying Raman enhancement, exerting a substantial restriction for CM-based SERS. In principle, CT is dominated by the coupling between the energy levels of a semiconductor-molecule system and the laser wavelength, whereas precise tuning of the energy levels is intrinsically difficult. Herein, two-dimensional transition-metal dichalcogenide alloys, whose energy levels can be precisely and continuously tuned over a wide range by simply adjusting their compositions, are investigated. The alloys enable on-demand construction of the CT resonance channels to cater to the requirements of a specific target molecule in SERS. The SERS signals are highly reproducible, and a clear view of the SERS dependences on the energy levels is revealed for different CT resonance terms.

Sodium and potassium ion storage in cation substituted 2D MoWSe2: insights into the effects of upper voltage cut-off

The superior properties, such as large interlayer spacing and the ability to host large alkali-metal ions, of two-dimensional (2D) materials based on transition metal di-chalcogenides (TMDs) enable next-generation battery development beyond lithium-ion rechargeable batteries. In addition, compelling but rarely inspected TMD alloys provide additional opportunities to tailor bandgap and enhance thermodynamic stability. This study explores the sodium-ion (Na-ion) and potassium-ion (K-ion) storage behavior of cation-substituted molybdenum tungsten diselenide (MoWSe2), a TMD alloy. This research also investigates upper potential suspension to overcome obstacles commonly associated with TMD materials, such as capacity fading at high current rates, prolonged cycling conditions, and voltage polarization during conversion reaction. The voltage cut-off was restricted to 1.5 V, 2.0 V, and 2.5 V to realize the material’s Na+ and K+ ion storage behavior. Three-dimensional (3D) surface plots of differential capacity analysis up to prolonged cycles revealed the convenience of voltage suspension as a viable method for structural preservation. Moreover, the cells with higher potential cut-off values conveyed improved cycling stability, higher and stable coulombic efficiency for Na+ and K+ ion half-cells, and increased capacity retention for Na+ ion half-cells, respectively, with half-cells cycled at higher voltage ranges.

Superconducting dome by tuning through a van Hove singularity in a two-dimensional metal

Chemical substitution is a promising route for the exploration of a rich variety of doping- and/or disorder-dependent collective phenomena in low-dimensional quantum materials. Here we show that transition metal dichalcogenide alloys are ideal platforms to this purpose. In particular, we demonstrate the emergence of superconductivity in the otherwise metallic single-layer TaSe2 by minute electron doping provided by substitutional W atoms. We investigate the temperature and magnetic field dependence of the superconducting state of Ta1-δWδSe2 with electron doping (δ) using variable temperature (0.34–4.2 K) scanning tunneling spectroscopy (STS). We unveil the emergence of a superconducting dome spanning 0.003 < δ < 0.03 with a maximized critical temperature of 0.9 K, a significant increase from that of bulk TaSe2 (TC = 0.14 K). Superconductivity emerges from an increase of the density of states (DOS) as the Fermi surface approaches a van Hove singularity due to doping. Once the singularity is reached, however, the DOS decreases with δ, which gradually weakens the superconducting state, thus shaping the superconducting dome. Lastly, our doping-dependent measurements suggest the development of a Coulomb glass phase triggered by disorder due to W dopants.

Stabilizing far-from-equilibrium (Mo,Ti)S2 thin films by metal sulfurization at reduced temperature

We report the synthesis of large-area, high-Ti-content, Mo1−xTixS2 alloy thin films in the 2H phase at temperature as low as 500 °C using a scalable two-step method of metal film deposition, followed by sulfurization in H2S. Film processing at higher temperature accelerates Ti segregation, film coarsening, and the formation of TiS2 in the 1T phase. Crystal growth at higher temperature results in the formation of multiple binary sulfide phases, in agreement with the equilibrium phase diagram. Making highly metastable, smooth, and uniform single-phase alloy films, therefore, hinges on developing low-temperature processing. Our results are relevant to the development of technologies based on designer transition metal dichalcogenide alloys, including in photonic integrated circuits and gas sensing.

Alloyed transition-metal dichalcogenides (Mo1−xWxSe2) through a hydrothermal synthesis route: Probing layer-number-dependent band energies and band-gap bowing via scanning tunneling spectroscopy

In this work, we form alloyed transition-metal dichalcogenides (TMDs), ${\mathrm{Mo}}_{1--x}{\mathrm{W}}_{x}{\mathrm{Se}}_{2}$, through a hydrothermal synthesis procedure. Due to the identical effective ionic radius of the cations, the common-anionic alloys did not experience any lattice strain. A complete series of the common-anionic TMD alloys could thereby be synthesized; their nanosheets were formed by a liquid exfoliation method. The electronic properties of the alloyed nanosheets in their monolayer, bilayer, and trilayer forms were recorded separately through scanning tunneling spectroscopy (STS) in an extremely localized manner. From the STS studies, which have a correspondence to the density of states of the semiconductors, we have deliberated on their electronic properties and commented on the band edges of the nanosheets in the alloy series. The transport gap of the alloys in their monolayer, bilayer, and trilayer forms exhibited band-gap bowing, and also a layer-number-dependent bowing coefficient. That is, the bowing phenomenon interestingly occurred only in the few-layered TMD alloys and diminished to finally vanish in thicker nanosheets. The conduction band has been found to be responsible for such a nonlinear behavior of the transport gap; the results could be explained in terms of the molecular orbitals which form the band. The results have established a coherent description of the band-gap tuning in atomically thin 2D TMD alloys.

Anisotropic Rashba effect in two-dimensional non-Janus transition-metal dichalcogenide, MSSe (M=Mo,W) alloys

The breaking of the out of plane mirror symmetry in the Janus transition-metal dichalcogenides (TMDs) has made them appealing candidates for optoelectronic, spintronic, valleytronic, and photocatalytic applications. In this study, we compare the structural and electronic properties of the Janus structure with the more stable TMD alloys of the form $M\mathrm{SSe} (M=\mathrm{Mo},\mathrm{W})$. The charge distribution analysis and two-dimensional electron localization function analysis indicate that the geometry of the system plays a vital role in determining the magnitude of the metal to ligand charge transfer, which in turn dictates the stability of the alloys. The arrangement of the chalcogen atoms in the more stable alloy allows for a better charge distribution, resulting in superior stability. Band structure calculations reveal that both structures are semiconducting with identical direct band gaps and exhibit Rashba splitting near the \ensuremath{\Gamma} point in the presence of the spin-orbit coupling (SOC) effects. While in the Janus structures, the broken mirror symmetry gives rise to an out of plane uniform electric field, the reduced symmetry of the more stable alloy structure produces an anisotropic arrangement of the perpendicular dipole moments. The nonuniformity of the electric dipole moments is reflected in the Rashba effect, which has an anisotropic nature, unlike the isotropic Rashba splitting observed in the Janus structures.

Strain-engineering in two-dimensional transition metal dichalcogenide alloys

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive semiconductors for use in electronic, optoelectronic, and spintronic devices. This study examines how the electronic properties of 2D TMDs can be tuned for specific applications through a combination of alloying and applying strain. Group VIB TMDs (MoS2, MoSe2, WS2, and WSe2) are alloyed by mixing in the metal or chalcogen sublattices. Density functional theory is used to model the structures of the alloys at varying compositions and examine the electronic structure of the alloys under biaxial tensile and compressive strain. Alloying results in the continuous monotonic tuning of the direct bandgap between the limits of the pure components, with low bowing coefficients for all alloys. Applying strain results in a transition of the bandgap from direct to indirect at low values of tensile strain and higher values of compressive strain. Strain can also be used to increase or decrease the bandgap with low compressive strain or tensile strain, respectively. The shift rate, or the rate at which the bandgap changes with applied strain, changes monotonically with alloy composition. MoS2 is identified as the 2D TMD with the highest shift rate.

Controllable growth of wafer-scale monolayer transition metal dichalcogenides ternary alloys with tunable band gap

The tuning of band gap is very important for the application of two-dimensional (2D) materials in optoelectronic devices. Alloying of 2D transition metal dichalcogenides (TMDCs) is an important way to tune the wide band gap. In this study, we report a multi-step vapor deposition method to grow monolayer TMDC ternary alloy films with wafer scale, including Mo1−x W x S2, Mo1−x W x Se2 and MoS2x Se2(1−x), which are accurately controllable in the elemental proportion (x is from 0 to 1). The band gap of the three 2D ternary alloy materials are continuously tuned for the whole range of metal and chalcogen compositions. The metal compositions are controlled by the as-deposited thickness. Raman, photoluminescence, elemental maps and TEM show the high spatial homogeneity in the compositions and optical properties across the whole wafer. The band gap can be continuously tuned from 1.86 to 1.99 eV for Mo1−x W x S2, 1.56 to 1.65 eV for Mo1−x W x Se2, 1.56 to 1.86 eV for MoS2x Se2(1−x). Electrical transport measurements indicate that Mo1−x W x S2 and MoS2x Se2(1−x) monolayers show n-type semiconductor behaviors, and the carrier types of Mo1−x W x Se2 can be tuned as n-type, bipolar and p-type. Moreover, this control process can be easily generalized to other 2D alloy films, even to quaternary or multi-element alloy materials. Our study presents a promising route for the preparation of large-scale homogeneous monolayer TMDC alloys and the application for future functional devices.

A KMnO4-Generated Colloidal Electrolyte for Redox Mediation and Anode Protection in a Li-Air Battery.

The rechargeable lithium-oxygen (Li-O2) battery has the highest theoretical specific energy density of any rechargeable batteries and could transform energy storage systems if a practical device could be attained. However, among numerous challenges, which are all interconnected, are polarization due to sluggish kinetics, low cycle life, small capacity, and slow rates. In this study, we report on use of KMnO4 to generate a colloidal electrolyte made up of MnO2 nanoparticles. The resulting electrolyte provides a redox mediator for reducing the charge potential and lithium anode protection to increase cycle life. This electrolyte in combination with a stable binary transition metal dichalcogenide alloy, Nb0.5Ta0.5S2, as the cathode enables the operation of a Li-O2 battery at a current density of 1 mA·cm-2 and specific capacity ranging from 1000 to 10 000 mA·h·g-1 (corresponding to 0.1-1 mA·h·cm-2) in a dry air environment with a cycle life of up to 150. This colloidal electrolyte provides a robust approach for advancing Li-air batteries.

Continuous Color‐Tunable Light‐Emitting Devices Based on Compositionally Graded Monolayer Transition Metal Dichalcogenide Alloys (Adv. Mater. 44/2022)

Light-Emitting Devices A color-tunable light-emitting diode is realized by Jiang Pu, Yasumitsu Miyata, Taishi Takenobu, and co-workers in article number 2203250 using compositionally graded monolayer transition metal dichalcogenide alloys. By controlling the light-emitting positions in the alloys, the composition gradient of the bandgap enables continuous and reversible light emission with energies ranging from 2.1 to 1.7 eV. The results provide a new approach for exploring monolayer semiconductor alloy based broadband optoelectronic device applications.

Continuous Color-Tunable Light-Emitting Devices Based on Compositionally Graded Monolayer Transition Metal Dichalcogenide Alloys.

The diverse series of transition metal dichalcogenide (TMDC) materials has been employed in various optoelectronic applications, such as photodetectors, light-emitting diodes, and lasers. Typically, the detection or emission range of optoelectronic devices is unique to the bandgap of the active material. Therefore, to improve the capability of these devices, extensive efforts have been devoted to tune the bandgap, such as gating, strain, and dielectric engineering. However, the controllability of these methods is severely limited (typically ≈0.1eV). In contrast, alloying TMDCs is an effective approach that yields a composition-dependent bandgap and enables light emissions over a wide range. In this study, a color-tunable light-emitting device using compositionally graded TMDC alloys is fabricated. The monolayer WS2 /WSe2 alloy grown by chemical vapor deposition shows a spatial gradient in the light-emission energy, which varies from 2.1 to 1.7eV. This alloy is incorporated in an electrolyte-based light-emitting device structure that can tune the recombination zone laterally. Thus, a continuous and reversible color-tunable light-emitting device is successfully fabricated by controlling the light-emitting positions. The results provide a new approach for exploring monolayer semiconductor-based broadband optical applications.