Vacancy impacts on electronic and mechanical properties of MX2 (M = Mo, W and X = S, Se) monolayers

Equilibrium densities of intrinsic defects in transition metal diselenides of molybdenum and tungsten

Point defects are thermodynamically stabilized in all crystalline materials, with increased densities negatively impacting the properties and performance of transition metal dichalcogenides (TMDs). While recent point defect reduction methods have led to considerable improvements in the optoelectronic properties of TMDs, there is a clear need for theoretical work to establish the lower limit of defect densities, as represented by thermal equilibrium. To that end, an ab initio and thermodynamic analysis of the equilibrium densities of intrinsic point defects in MoSe2 and WSe2 is presented. The intrinsic defect formation energies at the limits of the selenium and metal-rich regimes are determined by density functional theory (DFT) and then augmented with elemental chemical potential functions to determine temperature- and pressure-dependent formation energies. Equilibrium defect densities are determined for MSe, SeM, vM, and vSe, where M and v, respectively, represent the metal and the vacancy, as a function of synthesis temperature and pressure. The effects of vibrational free energy contributions and treatment of the DFT exchange-correlation potential are found to be non-negligible. Calculated equilibrium densities are several orders of magnitude below reported defect densities in TMDs made by chemical vapor deposition, chemical vapor transport, and flux methods, thereby establishing that current synthesis methods are either kinetically limited or impurity dominated.

Temperature-dependent failure of atomically thin MoTe2

CONTEXT

In this study, we investigated the mechanical responses of molybdenum ditelluride (MoTe2) using molecular dynamics (MD) simulations. Our key focus was on the tensile behavior of MoTe2 with trigonal prismatic phase (2H-MoTe2) which was investigated under uniaxial tensile stress for both armchair and zigzag directions. Crack formation and propagation were examined to understand the fracture behavior of such material for varying temperatures. Additionally, the study also assesses the impact of temperature on Young's modulus and fracture stress-strain of a monolayer of 2H-MoTe2.

METHOD

The investigation was done using molecular dynamics (MD) simulations using Stillinger-Weber (SW) potentials. The tensile behavior was simulated for temperature for 10 K and then from 100 to 600 K with a 100-K interval. The crack propagation and formation of 10 K and 300 K 2H-MoTe2 for both directions at different strain rates was analyzed using Ovito visualizer. All the simulations were conducted using a strain rate of 10-4 ps-1. The results show that the fracture strength of 2H-MoTe2 in the armchair and zigzag direction at 10 K is 16.33 GPa (11.43 N/m) and 13.71429 GPa (9.46 N/m) under a 24% and 18% fracture strain, respectively. The fracture strength of 2H-MoTe2 in the armchair and zigzag direction at 600 K is 10.81 GPa (7.56 N/m) and 10.13 GPa (7.09 N/m) under a 12.5% and 12.47% fracture strain, respectively.

Defect engineering of 1T'MX2(M= Mo, W andX= S, Se) transition metal dichalcogenide-based electrocatalyst for alkaline hydrogen evolution reaction

The alkaline electrolyzer (AEL) is a promising device for green hydrogen production. However, their energy conversion efficiency is currently limited by the low performance of the electrocatalysts for the hydrogen evolution reaction (HER). As such, the electrocatalyst design for the high-performance HER becomes essential for the advancement of AELs. In this work, we used both hydrogen (H) and hydroxyl (OH) adsorption Gibbs free energy changes as the descriptors to investigate the catalytic HER performance of 1T' transition metal dichalcogenides (TMDs) in an alkaline solution. Our results reveal that the pristine sulfides showed better alkaline HER performance than their selenide counterparts. However, the activities of all pristine 1T' TMDs are too low to dissociate water. To improve the performance of these materials, defect engineering techniques were used to design TMD-based electrocatalysts for effective HER activity. Our density functional theory results demonstrate that introducing single S/Se vacancy defects can improve the reactivities of TMD materials. Yet, the desorption of OH becomes the rate-determining step. Doping defective MoS2with late 3d transition metal (TM) atoms, especially Cu, Ni, and Co, can regulate the reactivity of active sites for optimal OH desorption. As a result, the TM-doped defective 1T' MoS2can significantly enhance the alkaline HER performance. These findings highlight the potential of defect engineering technologies for the design of TMD-based alkaline HER electrocatalysts.