Mechanical\nProperties of Atomically Thin\nTungsten Dichalcogenides: WS₂, WSe₂, and WTe₂

Abstract

Two-dimensional\n(2D) tungsten disulfide (WS₂), tungsten\ndiselenide (WSe₂), and tungsten ditelluride (WTe₂) draw increasing attention due to their attractive properties deriving\nfrom the heavy tungsten and chalcogenide atoms, but their mechanical\nproperties are still mostly unknown. Here, we determine the intrinsic\nand air-aged mechanical properties of mono-, bi-, and trilayer (1–3L)\nWS₂, WSe₂, and WTe₂ using a complementary\nsuite of experiments and theoretical calculations. High-quality 1L\nWS₂ has the highest Young’s modulus (302.4 ±\n24.1 GPa) and strength (47.0 ± 8.6 GPa) of the entire family,\noverpassing those of 1L WSe₂ (258.6 ± 38.3 and 38.0\n± 6.0 GPa, respectively) and WTe₂ (149.1 ± 9.4\nand 6.4 ± 3.3 GPa, respectively). However, the elasticity and\nstrength of WS₂ decrease most dramatically with increased\nthickness among the three materials. We interpret the phenomenon by\nthe different tendencies for interlayer sliding in an equilibrium\nstate and under in-plane strain and out-of-plane compression conditions\nin the indentation process, revealed by the finite element method\nand density functional theory calculations including van der Waals\ninteractions. We also demonstrate that the mechanical properties of\nthe high-quality 1–3L WS₂ and WSe₂ are\nlargely stable in air for up to 20 weeks. Intriguingly, the 1–3L\nWSe₂ shows increased modulus and strength values with aging\nin the air. This is ascribed to oxygen doping, which reinforces the\nstructure. The present study will facilitate the design and use of\n2D tungsten dichalcogenides in applications such as strain engineering\nand flexible field-effect transistors.