Structural Phase Transformation in Strained Monolayer MoWSe2 Alloy

Two-dimensional (2D) materials exhibit different mechanical properties from their bulk counterparts owing to their monolayer atomic thickness. Here, we have examined the mechanical behavior of 2D molybdenum tungsten diselenide (MoWSe<inf>2</inf>) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe<inf>2</inf> and WSe<inf>2</inf> regions. Applying a bending strain blue-shifted the MoSe<inf>2</inf> and WSe<inf>2</inf> A<inf>1g</inf> Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe<inf>2</inf> modes. In situ local Raman measurements suggested that the crack propagated primarily thorough MoSe<inf>2</inf>-rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe<inf>2</inf> monolayer containing nanoscopic WSe<inf>2</inf> regions akin to the experiment. Raman spectra calculated from MD trajectories of crack propagation confirmed the emergence of intermediate peaks in the strained monolayer alloy, mirroring experimental results. The simulations revealed that the stress buildup around the crack tip caused an irreversible structural transformation from the 2H to 1T phase both in the MoSe<inf>2</inf> matrix and WSe<inf>2</inf> patches. This was corroborated by high-angle annular dark-field images. Crack branching and subsequent healing of a crack branch were also observed in WSe<inf>2</inf>, indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe<inf>2</inf> over the unalloyed counterparts.