2D Transition Metal Dichalcogenides Alloys Research Articles

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.

In situ strain electrical atomic force microscopy study on two‐dimensional ternary transition metal dichalcogenides

AbstractAtomically thin two‐dimensional (2D) alloys have attracted wide interests of study recently due to their potential in flexible electronic and optoelectronic applications. In particular, monolayer transition metal dichalcogenide (TMD) alloys have emerged as unique 2D semiconductors with tunable bandgaps, by means of alloying. However, response of surface electrical potential and barrier height to strain for 2D TMD alloys–electrode interface is rarely explored. Apparently, revealing such strain‐dependent evolution of electrical properties is crucial for developing advanced 2D TMD based flexible electronics and optoelectronics. Here we performed in situ strain Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C‐AFM) investigations of monolayer Mo0.4W0.6Se2 on Au coated flexible substrate, where controlled uniaxial tensile strain is applied. Both contact potential difference (CPD) and Schottky barrier heights (SBH) of monolayer Mo0.4W0.6Se2 show obvious decreases with the increase of strain, which is mainly due to the strain‐induced increment of TMD electron affinity. Our in situ strain photoluminescence (PL) measurements also indicate the changes of electronic band structures under strain. We further exploit the substrate effects on CPD by study the monolayer alloy on the mostly used substrates of SiO2/Si and indium tin oxide (ITO)/glass. Our findings could strengthen the foundation for the potential applications of 2D TMD and their alloys in the fields of strain sensors, flexible photodetectors, and other wearable electronic devices.image

Large-area synthesis of monolayer MoTexSe2-x alloys by chemical vapor deposition

Great achievements have been made in the alloying of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), which can allow tunable bandgaps for practical applications in optoelectronic devices. However, telluride-based TMDs alloys were less studied due to the difficulties of sample synthesis. Here, in this work, we report the large-area synthesis of monolayer MoTexSe2-x alloys with a controllable Te composition by a modified alkali metal halide assisted chemical vapor deposition method. Photoluminescence (PL) spectra demonstrate that the bandgap changes from 1.55 to 1.38 eV with increasing Te concentrations. The behaviors of Raman modes of MoSe2 are well explained by a phonon confinement model. The density functional theory calculation provides insights into the atomic vibration images and better understanding of the Raman properties of MoTexSe2-x alloys. Our work provides a simple method to synthesize large-scale 2H phase Te-based 2D TMDs alloys for their further applications in near-infrared devices.

Mechanical elasticity and piezoelectricity in monolayer transition-metal dichalcogenide alloys

Two-dimensional (2D) materials have attracted considerable attention due to their application in nanoscale devices. The intrinsic physical properties of 2D materials are crucial for their practical applications. In this work, the structural, electronic, mechanical and piezoelectric properties of transition metal dichalcogenide (TMD) monolayer alloys were investigated using first-principles calculations. The results show that for MX2(1-x)Y2x (where M = Cr, Mo and W; and X, Y = S, Se and Te) monolayer alloys, the electronic, mechanical and piezoelectric properties greatly depend on the composition. The band gap decreases with increasing x in MX2(1-x)Y2x alloys if the Y atom has a larger atomic number than the X atom. The in-plane stiffness and piezoelectric coefficients of 2D MX2(1-x)Y2x alloys lie in the range between that of MX2 to MY2 monolayers. Thus, these properties can be finely tuned by modifying the chemical composition. These results provide a useful guideline for the design of nanoscale devices based on 2D TMD alloys.

Atomic-Level Customization of 4 in. Transition Metal Dichalcogenide Multilayer Alloys for Industrial Applications.

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large-scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS2(1- x ) Se2 x ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic-level substitution combined with a solution-based large-area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS1.62 Se0.38 , MoS1.37 Se0.63 , MoS1.15 Se0.85 , and MoS0.46 Se1.54 alloys, as well as MoS2 and MoSe2 . Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large-scale 2D TMD alloys are implemented. Se-stimulated strain effects and a detailed mechanism for the Se substitution in the MoS2 crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure-phase MoS2 and MoSe2 , are unambiguously achieved.

Chemical Selectivity at Grain Boundary Dislocations in Monolayer Mo1-xWxS2 Transition Metal Dichalcogenides.

Grain boundaries (GBs) are unavoidable crystal defects in polycrystalline materials and significantly influence their properties. However, the structure and chemistry of GBs in 2D transition metal dichalcogenide alloys have not been well established. Here we report significant chemical selectivity of transition metal atoms at GB dislocation cores in Mo1-xWxS2 monolayers. Different from classical elastic field-driven dislocation segregation in bulk crystals, the chemical selectivity in the 2D crystals originates prominently from variation of atomic coordination numbers at dislocation cores. This observation provides atomic insights into the topological effect on the chemistry of crystal defects in 2D materials.

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.