Transition Metal Dichalcogenide Alloys Research Articles

Structural stability and intriguing electronic properties of two-dimensional transition metal dichalcogenide alloys.

Based on the first-principles calculations, we systematically studied the structures and electronic properties of two-dimensional (2D) transition metal dichalcogenide (TMD) alloys with half-to-half mixing of S and Se. Using the chemical potentials of S and Se, the energetic phase diagrams for both single phases and mixed phases of TMD were constructed. A new heterolayer structure (for Sc and Ti) and alternating structure (for Cr, Mn, Fe, Zr, Mo, and W) were proposed for the first time, which were thermodynamically stable for MSSe alloys under the S-poor (relatively low chemical potential of S) and Se-rich (relatively high chemical potential of Se) conditions, and further compared with the disordered structures. Moreover, band gaps, carrier effective mass, and work functions were calculated for these stable mixed phases. Compared to the single phases of MS2 and MSe2, MSSe alloys showed superior electronic properties including tunable band gaps and work functions. Importantly, the significantly reduced effective mass of the carriers in the MSSe alloys may induce higher carrier mobility, providing better performance of TMD materials in electronic devices.

Synthesis of a MoS2(1−x)Se2x ternary alloy on carbon nanofibers as the high efficient water splitting electrocatalyst

Binary transition metal dichalcogenides (TMDs) usually exhibit high hydrogen evolution reaction (HER) activities; however, the facile and efficient synthesis of ternary TMDs alloys remains a challenge. In this study, we reported an efficient method of synthesis for a MoS2(1−x)Se2x ternary alloy in a CVD system, and carbon nanofibers serve as the substrate. The MoS2(1−x)Se2x/CNFs hybrids were directly used as hydrogen evolution cathodes and exhibit lower onset potentials and excellent durability, suggesting they have significantly enhanced catalytic activity and could serve as effective and promising catalysts for the HER.

Thickness tunable transport in alloyed WSSe field effect transistors

We report the field effect transistor characteristics of exfoliated transition metal dichalcogenide alloy tungsten sulphoselenide. WSSe is a layered material of strongly bonded S-W-Se atoms having weak interlayer van der Waals forces with a significant potential for spintronic and valleytronic applications due to its polar nature. The X-ray photoelectron spectroscopy measurements on crystals grown by the chemical vapor transport method indicate a stoichiometry of the form WSSe. We report flake thickness tunable transport mechanism with n-type behavior in thin flakes (≤11 nm) and ambipolarity in thicker flakes. The devices with flake thicknesses of 2.4 nm–54.8 nm exhibit a maximum electron mobility of ∼50 cm2/V s along with an ION/IOFF ratio >106. The electron Schottky barrier height values of 35 meV and 52 meV extracted from low temperature I–V measurements for 3.9 nm and 25.5 nm thick flakes, respectively, indicate that an increase in hole current with thickness is likely due to lowering of the bandgap through an increase in energy of the valence band maximum.

Electronic band gaps and exciton binding energies in monolayerMoxW1−xS2transition metal dichalcogenide alloys probed by scanning tunneling and optical spectroscopy

Using optical absorption and scanning tunneling spectroscopy techniques, the authors investigate band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloyed crystals (Mo${}_{x}$W${}_{1-x}$S${}_{2}$). They observe a significant modulation of the excitonic transitions, quasiparticle band gaps, and exciton binding energies. These findings may hold promise for fundamental studies of intralayer phenomena and for potential applications of TMDC alloys in electronic and optoelectronic devices.

Stable Monolayer Transition Metal Dichalcogenide Ordered Alloys with Tunable Electronic Properties

From first-principles calculations, we discover highly stable monolayer transition metal dichalcogenides (TMDs) ternary alloys consisting of group 5 and 7 transition metal elements. We show for Nb1–xRexS2, Ta1–xRexS2 and their selenide counterparts that the 1H ordered alloy structures for x ≤ 0.5 are thermodynamically stable, with formation energies an order of magnitude lower than currently known TMD alloys such as MoxW1–xS2, so that they could potentially be synthesizable using chemical vapor deposition or exfoliation techniques. This class of TMD alloys offer a wide tunable bandgap range of ∼1 eV, displaying metallic to semiconducting behavior versus alloy composition. Importantly, at x = 0.5, the alloys are valence isoelectronic with MoS2. These stoichiometric compounds, Nb0.5Re0.5S2, Ta0.5Re0.5S2, and their selenide counterparts, exhibit band features similar to MoS2, but possess significantly smaller bandgaps (∼1 to 1.2 eV). As a result, compared to MoS2 and WS2, this class of alloy TMDs display enhanced absorbance in the visible range of the solar spectrum where the solar spectral intensity is the strongest. These ordered monolayer TMD alloys could open doors for designing ultrathin solar absorbers with improved performance.

On the Stability and Electronic Structure of Transition-Metal Dichalcogenide Monolayer Alloys Mo1−xXxS2−ySey with X = W, Nb

Layered transition-metal dichalcogenides have extraordinary electronic properties, which can be easily modified by various means. Here, we have investigated how the stability and electronic structure of MoS 2 monolayers is influenced by alloying, i.e., by substitution of the transition metal Mo by W and Nb and of the chalcogen S by Se. While W and Se incorporate into the MoS 2 matrix homogeneously, forming solid solutions, the incorporation of Nb is energetically unstable and results in phase separation. However, all three alloying atoms change the electronic band structure significantly. For example, a very small concentration of Nb atoms introduces localized metallic states, while Mo 1 - x W x S 2 and MoS 2 - y Se y alloys exhibit spin-splitting of the valence band of strength that is in between that of the pure materials. Moreover, small, but evident spin-splitting is introduced in the conduction band due to the symmetry breaking. Therefore, transition-metal dichalcogenide alloys are interesting candidates for optoelectronic and spintronic applications.

Photocarrier dynamics in transition metal dichalcogenide alloy Mo0.5W0.5S2.

We report a transient absorption study of photocarrier dynamics in transition metal dichalcogenide alloy, Mo0.5W0.5S2. Photocarriers were injected by a 400-nm pump pulse and detected by a 660-nm probe pulse. We observed a fast energy relaxation process of about 0.7 ps. The photocarrier lifetime is in the range of 50 - 100 ps, which weakly depends on the injected photocarrier density and is a few times shorter than MoS2 and WS2, reflecting the relatively lower crystalline quality of the alloy. Saturable absorption was also observed in Mo0.5W0.5S2, with a saturation energy fluence of 32 μJ cm(-2). These results provide important parameters on photocarrier properties of transition metal dichalcogenide alloys.

Superlinear composition-dependent photocurrent in CVD-grown monolayer MoS2(1-x)Se2x alloy devices.

Transition metal dichalcogenides (TMDs) have emerged as a new class of two-dimensional materials that are promising for electronics and photonics. To date, optoelectronic measurements in these materials have shown the conventional behavior expected from photoconductors such as a linear or sublinear dependence of the photocurrent on light intensity. Here, we report the observation of a new regime of operation where the photocurrent depends superlinearly on light intensity. We use spatially resolved photocurrent measurements on devices consisting of CVD-grown monolayers of TMD alloys spanning MoS2 to MoSe2 to show the photoconductive nature of the photoresponse, with the photocurrent dominated by recombination and field-induced carrier separation in the channel. Time-dependent photoconductivity measurements show the presence of persistent photoconductivity for the S-rich alloys, while photocurrent measurements at fixed wavelength for devices of different alloy compositions show a systematic decrease of the responsivity with increasing Se content associated with increased linearity of the current-voltage characteristics. A model based on the presence of different types of recombination centers is presented to explain the origin of the superlinear dependence on light intensity, which emerges when the nonequilibrium occupancy of initially empty fast recombination centers becomes comparable to that of slow recombination centers.

Two-dimensional transition metal dichalcogenide alloys: preparation, characterization and applications.

Engineering electronic structure of atomically thin two-dimensional (2D) materials is of great importance to their potential applications. In comparison to numerous other approaches, such as strain and chemical functionization, alloying can continuously tune the band gaps in a wide energy range. Atomically thin 2D alloys have been prepared and studied recently due to their potential use in electronic and optoelectronic applications. In this review, we first summarize the preparation methods of 2D alloys (mainly on transition metal dichalcogenide (TMD) monolayer alloys), including mechanical exfoliation, physical vapor deposition (PVD), chemical vapor deposition (CVD) and chalcogen exchange. Then, atomic-resolution imaging, Raman and photoluminescence (PL) spectroscopy characterization of 2D alloys are reviewed, in which band gap tuning is discussed in detail based on the PL experiments and theoretical calculations. Finally, applications of 2D alloys in field-effect transistors (FETs), photocurrent generation and hydrogen evolution catalysis are reviewed.

Order-disorder phase transitions in the two-dimensional semiconducting transition metal dichalcogenide alloys Mo(1-x)W(x)X₂ (X = S, Se, and Te).

A combination of density functional theory, an empirical model, and Monte Carlo simulations is used to shed light on the evolution of the atomic distribution in the two-dimensional semiconducting transition metal dichalcogenide alloys Mo1−xWxX2 (X = S, Se, and Te) as a function of the W concentration and temperature. Both random and ordered phases are discovered and the origin of the phase transitions is clarified. While the empirical model predicts at x = 1/3 and 2/3 ordered alloys, Monte Carlo simulations suggest that they only exist at low temperature due to a small energetic preference of Mo-X-W over Mo-X-Mo and W-X-W interactions, explaining the experimental observation of random alloy Mo1−xWxS2. Negative formation energies point to a high miscibility. Tunability of the band edges and band gaps by alteration of the W concentration gives rise to a broad range of applications.

Controllable Synthesis of Band-Gap-Tunable and Monolayer Transition-Metal Dichalcogenide Alloys

The electronic and optical properties of transition metal dichalcogenide (TMD) materials are directly governed by their energy gap; thus, the band gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayer TMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here we systematically compared the two approaches of forming MoS2xSe2(1-x) monolayer alloys: selenization of MoS2 and sulfurization of MoSe2. The optical energy gap of as-grown CVD MoS2 can be continuously modulated from 1.86 eV (667 nm) to 1.57 eV (790 nm) controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe2 and thus the MoSe2/MoS2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.

Two-dimensional molybdenum tungsten diselenide alloys: photoluminescence, Raman scattering, and electrical transport.

Two-dimensional transition-metal dichalcogenide alloys have attracted intense attention due to their tunable band gaps. In the present work, photoluminescence, Raman scattering, and electrical transport properties of monolayer and few-layer molybdenum tungsten diselenide alloys (Mo1-xWxSe2, 0 ≤ x ≤ 1) are systematically investigated. The strong photoluminescence emissions from Mo1-xWxSe2 monolayers indicate composition-tunable direct band gaps (from 1.56 to 1.65 eV), while weak and broad emissions from the bilayers indicate indirect band gaps. The first-order Raman modes are assigned by polarized Raman spectroscopy. Second-order Raman modes are assigned according to its frequencies. As composition changes in Mo1-xWxSe2 monolayers and few layers, the out-of-plane A1g mode showed one-mode behavior, while B2g(1) (only observed in few layers), in-plane E2g(1), and all observed second-order Raman modes showed two-mode behaviors. Electrical transport measurement revealed n-type semiconducting transport behavior with a high on/off ratio (>10(5)) for Mo1-xWxSe2 monolayers.

Active guests in the MoS2/MoSe2 host lattice: efficient hydrogen evolution using few-layer alloys of MoS(2(1-x))Se(2x).

Few-layer transition metal dichalcogenide alloys based on molybdenum sulphoselenides [MoS2(1-x)Se2x] possess higher hydrogen evolution (HER) activity compared to pristine few-layer MoS2 and MoSe2. Variation of the sulphur or selenium content in the parent dichalcogenides reveals a systematic structure-activity relationship for different compositions of alloys, and it is found that the composition MoS1.0Se1.0 shows the highest HER activity amongst the catalysts studied. The tunable electronic structure of MoS2/MoSe2 upon Se/S incorporation probably assists in the realization of high HER activity.

Tunable Electronic Properties of Two-Dimensional Transition Metal Dichalcogenide Alloys: A First-Principles Prediction.

We investigated the composition-dependent electronic properties of two-dimensional transition-metal dichalcogenide alloys (WxMo1-xS2) based on first-principles calculations by applying the supercell method and effective band structure approximation. It was found that hole effective mass decreases linearly with increasing W composition, and electron effective mass of alloys is always larger than that of their binary constituents. The different behaviors of electrons and holes in alloys are attributed to the fact that metal d-orbitals have different contributions to conduction bands of MoS2 and WS2 but almost identical contributions to valence bands. We examined the conduction polarity of WxMo1-xS2 monolayer alloys with four metal electrode materials (Au, Ag, Cu, and Pd). It suggests the main carrier type for transport in transistors could change from electrons to holes as W composition increases if high work function metal contacts were used. The tunable electronic properties of two-dimensional transition-metal dichalcogenide alloys make them attractive for electronic and optoelectronic applications.

Monolayer semiconducting transition metal dichalcogenide alloys: Stability and band bowing

The stability and band bowing effects of two-dimensional transition metal dichalcogenide alloys MX2(1−x)X′2x (M = Mo, W, and X, X′ = S, Se, Te) are investigated by employing the cluster expansion method and the special quasi-random structure approach. It is shown that for (S, Se) alloys, there exist stable ordered alloy structures with concentration x equal to 1/3, 1/2, and 2/3, which can be explained by the small lattice mismatch between the constituents and a large additional charge exchange, while no ordered configuration exists for (Se, Te) and (S, Te) alloys at 0 K. The calculated phase diagrams indicate that complete miscibility in the alloys can be achieved at moderate temperatures. The bowing in lattice constant for the alloys is quite small, while the bowing in band gap, and more so in band edge positions, is much more significant. By decomposing the formation of alloy into multiple steps, it is found that the band bowing is the joint effect of volume deformation, chemical difference, and a low-dimensionality enhanced structure relaxation. The direct band gaps in these alloys continuously tunable from 1.8 eV to 1.0 eV, along with the moderate miscibility temperatures, make them good candidates for two-dimensional optoelectronics.

Two-Dimensional Transition Metal Dichalcogenide Alloys: Stability and Electronic Properties.

Using density-functional theory calculations, we study the stability and electronic properties of single layers of mixed transition metal dichalcogenides (TMDs), such as MoS2xSe2(1-x), which can be referred to as two-dimensional (2D) random alloys. We demonstrate that mixed MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured using chemical-vapor deposition technique or exfoliated from the bulk mixed materials. By applying the effective band structure approach, we further study the electronic structure of the mixed 2D compounds and show that general features of the band structures are similar to those of their binary constituents. The direct gap in these materials can continuously be tuned, pointing toward possible applications of 2D TMD alloys in photonics.

High pressure transport studies in transition metal dichalcogenide alloys

We have studied the electrical resistivity and Hall coefficient in the layered transition metal dichalcogenide alloy systems Hf x Ti 1- x Se 2 and PtSe 1.4S 0.6 down to 4 K at hydrostatic pressures in the range 0 to 12 kbar. A novel rhodium plus 0.5 at.% iron versus chromel thermocouple was used for high pressure thermometry. The Hf x Ti 1- x Se 2 system spans the range from semimetallic TiSe 2 to semiconducting HfSe 2. Band uncrossing can be unambiguously determined from the pressure dependence of the Hall coefficient, and occurs between x = 0.3 and 0.5. In Hf x Ti 1- x Se 2 we have followed the behaviour of the phase transition present in TiSe 2. PtSe 1.4S 0.6 is weakly semimetallic at 1 bar, and shows a transition to a more insulating state below 150 K. Under pressure metallic character at room temperature is improved, though there is still evidence for a transition to a higher resistivity state below 150 K.