Two-dimensional Transition-metal Dichalcogenide Alloys Research Articles

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.

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.

Modeling of two-dimensional MoxW<sub>1−x</sub>S<sub>2y</sub>Se<sub>2(1−y)</sub> alloy band structure

Objectives. Two-dimensional transition metal dichalcogenides (TMDs) are utilized for various optical applications due to the presence in these materials of a direct band gap corresponding to the visible and near-infrared spectral regions. However, a limited set of existing TMDs makes the region of the used spectral range discrete. The most effective way to solve this problem is to use two-dimensional TMD films based on multicomponent alloys, including three or more different chemical elements (while TMDs consist of two). By varying their morphological composition, one can control the value of the band gap and thus their optical absorption spectrum. However, since the band gap in such structures is highly nonlinear as far as their chemical composition is concerned, it can be challenging to select the required concentration in order to achieve uniform absorption. In this regard, the purpose of this work is to theoretically determine the dependence of the band gap of four-component two-dimensional MoxW1–xS2ySe2(1–y) alloys on their morphological composition.Methods. The calculations were performed within the framework of the density functional theory using the Quantum Espresso software package. Flakes of two-dimensional TMDs alloys were prepared from bulk TMDs crystals by mechanical exfoliation on a Si/SiO2 substrate. An experimental study of the photoluminescence characteristics was carried out using photoluminescence microscopy-spectroscopy. Results. In this work, the dependence of the band gap on the morphological composition of two-dimensional MoxW1–xS2ySe2(1–y) alloys was determined. Upon varying the composition of TMDs alloys, it was found that the band gap changes from 1.43 to 1.83 eV. The obtained theoretical results are in qualitative agreement with the experimental data.Conclusions. The minimum band gap is observed in alloys close to MoSe2, while alloys close to WS2 have the maximum band gap value.

Controllable synthesis of WS2(1-x)Se2x monolayers with fast photoresponse by a facile chemical vapor deposition strategy

Two-dimensional transition metal dichalcogenides (TMDs) alloys such as WS2(1-x)Se2x have attracted much interest in the fundamental science and device fabrication for their composition-tunable chemical and physical properties. However, the traditional chemical vapor deposition (CVD) methods often rely on precursors with high melting point such as WO3, WS2 and WSe2. Here, we report a facile CVD strategy for the formation of high quality WS2(1-x)Se2x monolayers with composition-tunable optical and electronic properties by using highly evaporated sodium tungstate dihydrate (Na2WO4·2H2O) precursor as W source. The as-grown samples exhibit uniform allocation of W, S, and Se elements across the domain and the Se (or S) composition can be continually modulated from 0 to 2. The band gap evolves continuously from 1.97 eV for WS2 to intermediate value for WS2(1-x)Se2x and then to 1.63 eV for WSe2. Electrical transport studies further reveal the transition from n-type for WS2 to bipolar (or slightly n-type) for WS2(1-x)Se2x and then to p-type for WSe2. Furthermore, the as-grown WS2(1-x)Se2x-photodetectors exhibit good photo response to visible light (532 nm) with a fast response time of about 20 ms. Our facile and precise synthesis strategy can be used as a general route to fabricating such composition modulated 2D TMD alloys for optoelectronic applications.

Composition- and layer-dependent bandgap of two-dimensional transition metal dichalcogenides alloys

Abstract Based on a bandgap thermodynamic approach, the composition- and layer-dependent bandgap energy of two-dimensional transition metal dichalcogenides (TMDCs) alloys is modeled without any adjustable parameters. The bandgap energy of TMDCs alloys increases with decreasing layer number at the same composition. Moreover, the bowing effect gets weaker as the layer number decreases. The model predictions agree well with available experiment and simulation results.

Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys

Composition modulation of two-dimensional transition-metal dichalcogenides (TMDs) has introduced an enticing prospect for the synthesis of Van der Waals alloys and lateral heterostructures with tunable optoelectronic properties. Phenomenologically, the optoelectronic properties of alloys are entangled to a strain that is intrinsic to synthesis processes. Here, we report an unprecedented biaxial strain that stems from the composition modulation of monolayer TMD alloys (e.g., MoS2xSe2(1 - x)) and inflicts fracture on the crystals. We find that the starting crystal (MoSe2) fails to adjust its lattice constant as the atoms of the host crystal (selenium) are replaced by foreign atoms (sulfur) during the alloying process. Thus, the resulting alloy forms a stretched lattice and experiences a large biaxial tensile strain. Our experiments show that the biaxial strain relaxes via formation of cracks in interior crystal domains or through less constraint bounds at the edge of the monolayer alloys. Griffith’s criterion suggests that defects combined with a sulfur-rich environment have the potential to significantly reduce the critical strain at which cracking occurs. Our calculations demonstrate a substantial reduction in fracture-inducing critical strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.

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.

Dendritic growth of monolayer ternary WS2(1-x)Se2x flakes for enhanced hydrogen evolution reaction.

Two-dimensional transition-metal dichalcogenides (TMDs) have attracted much research interest in the hydrogen evolution reaction (HER) due to their superior electrocatalytic properties. Beyond binary TMDs, ternary TMD alloys, as electrocatalysts, were also gradually acknowledged for their remarkable efficiency in HER. Herein, we successfully synthesized monolayer dendritic ternary WS2(1-x)Se2x flakes possessing abundant active edge sites on a single crystalline SrTiO3 (STO(100)). And the obtained dendritic WS2(1-x)Se2x flakes could be transferred intact to arbitrary substrates, for example, SiO2/Si and Au foils. Intriguingly, the transferred dendritic WS2(1-x)Se2x flakes on Au foil demonstrate a significant HER performance, reflected by a rather lower Tafel slope of ∼69 mV dec-1 and a much higher exchange current density of ∼50.1 μA cm-2 outshining other CVD-grown two-dimensional TMD flakes. Furthermore, our new material shows excellent stability in electro-catalyzing the HER, suggestive of its robustness for being an excellent electrocatalyst. We believe that this work broadens the outlook for the synthesis of two-dimensional TMDs toward satisfying the applications in electrocatalysis.

NANOELECTRONICS. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface.

Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.

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.

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.

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.