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

Monolayers of transition metal dichalcogenides (TMD) exhibit excellent mechanical and electrical characteristics. Previous studies have shown that vacancies are frequently created during the synthesis, which can alter the physicochemical characteristics of TMDs. Even though the properties of pristine TMD structures are well studied, the effects of vacancies on the electrical and mechanical properties have received far less attention. In this paper, we applied first-principles density functional theory (DFT) to comparatively investigate the properties of defective TMD monolayers including molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2). The impacts of six types of anion or metal complex vacancies were studied. According to our findings, the electronic and mechanical properties are slightly impacted by anion vacancy defects. In contrast, vacancies in metal complexes considerably affect their electronic and mechanical properties. Additionally, the mechanical properties of TMDs are significantly influenced by both their structural phases and anions. Specifically, defective diselenides become more mechanically unstable due to the comparatively poor bonding strength between Se and metal based on the analysis of the crystal orbital Hamilton population (COHP). The outcomes of this study may provide the theoretical knowledge base to boost more applications of the TMD systems through defect engineering.

Theoretical understanding of electronic and mechanical properties of 1T' transition metal dichalcogenide crystals.

Transition metal dichalcogenides (TMDs) with a 1T' layer structure have recently received intense interest due to their outstanding physical and chemical properties. While the physicochemical behaviors of 1T' TMD monolayers have been widely investigated, the corresponding properties of layered 1T' TMD crystals have rarely been studied. As TMD monolayers do not have interlayer interactions, their physicochemical properties will differ from those of layered TMD materials. In this study, the electronic and mechanical characteristics of a range of 1T' TMDs are systematically examined by means of density functional theory (DFT) calculations. Our results reveal that the properties of 1T' TMDs are mainly affected by their anions. The disulfides are stiffer and more rigid, diselenides are more brittle. In addition, the 1T' polytype is softer than 2H TMDs. Comparison with the properties of the monolayers shows that the interlayer van der Waals forces can slightly weaken the TM-X covalent bonding strength, which can further influence the mechanical properties. These insights revealed by our theoretical studies may boost more applications of 1T' TMD materials.

Multiscale numerical simulation of in-plane mechanical properties of two-dimensional monolayers.

Many applications of two dimensional (2D) materials are often achieved through strain engineering, which is directly dependent on their in-plane mechanical characteristics. Therefore, understanding the in-plane mechanical characteristics of the 2D monolayers becomes imperative. Nevertheless, direct experimental measurements of in-plane mechanical properties of 2D monolayers face great difficulties due to the issues related to the availability of high-quality 2D materials and sophisticated facilities. As an alternative, numerical simulation has the potential to theoretically predict such properties. This review presents some recent progress in numerically exploring the in-plane mechanical properties of 2D materials, including first-principles density functional theory, force-field based classical molecular dynamics, and the finite-element method. The relevant case studies are provided to describe the applications of these methods along with their pros and cons. We hope that the multiscale simulation methods discussed in this review will inspire new ideas and boost further advances of the computational study on the in-plane mechanical properties of 2D materials.

Effect of Structural Phases on Mechanical Properties of Molybdenum Disulfide.

Molybdenum disulfide (MoS2) is a promising layer-structured material for use in many applications due to its tunable structural and electronic properties in terms of its structural phases. Its performance including efficiency and durability is often dependent on its mechanical properties. To understand the effects of the structural phase on its mechanical properties, a comparative study on the mechanical properties of bulk 2H, 3R, 1T, and 1T' MoS2 was conducted using the first-principles density functional theory. Since considerable applications of MoS2 are developed through strain engineering, the impact of the external pressure on its mechanical properties was also considered. Our results suggest a strong relationship between the mechanical properties of MoS2 and the structural symmetry of its crystal. Accordingly, the impacts of the external pressure on the mechanical properties of MoS2 also greatly vary with respect to the structural phases. Among all of the considered phases, the 2H and 3R MoS2 have a larger bulk modulus, Young's modulus, shear modulus, and microhardness due to their higher stability. Conversely, 1T and 1T' MoS2 are less strong. As such, 1T and 1T' MoS2 can be a better candidate for strain engineering.