Structural Phase Transformation in Strained Monolayer MoWSe₂ Alloy

Abstract

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) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe and WSe regions. Applying a bending strain blue-shifted the MoSe and WSe A Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe modes. In situ local Raman measurements suggested that the crack propagated primarily thorough MoSe-rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe monolayer containing nanoscopic WSe 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 matrix and WSe 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, indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe over the unalloyed counterparts.

Structural phase transitions in a MoWSe2 monolayer: Molecular dynamics simulations and variational autoencoder analysis

“…Results for the (10) orientation, where one of the Mo-Se bonds in the 2H lattice is perpendicular to the y axis, are presented in the Supplemental Material [19]. The precrack begins to propagate when the applied strain in the (11) orientation is increased to 2.5%, which is consistent with the experimental observation of the response of MoWSe 2 heterostructure under strain [1]. Figure 1(a) presents a snapshot of the primary crack at the critical strain corresponding to the onset of crack propagation.…”

“…A two-dimensional MoWSe 2 system was recently synthesized with chemical vapor deposition (CVD) and structural changes owing to mechanical deformation were examined with an in situ setup of a Raman spectrophotometer combined with a nanoindenter and scanning probe microscope [1]. The experiment reveals that the heterostructure contains randomly distributed nanoscale patches of WSe 2 in a matrix of MoSe 2 .…”

Strain Engineering of 2D Materials: Issues and Opportunities at the Interface

“…Interesting physical and chemical phenomena emerging at large strain level or specific strain state shall be further experimentally investigated. Examples include the structural phase transition of MX 2 (likely appearing at large biaxial strain), the uniform pseudomagnetic field in graphene (resulting from the uniform strain gradient), the unique flexoelectricity in 2D materials (favoring maximal strain gradient), and so on. In addition, although molecular simulations have demonstrated interesting wrinkling patterns associated with shear, realizing shear deformations in 2D materials is still experimentally elusive, which may present an alternative and useful deformation mode for the strain engineering of 2D materials …”

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures

“…2D TMD materials have aroused significant interest in the fields of electronic/optoelectronics owing to their layer‐dependent bandgap and excellent electronic mobility. Recently, 2D semiconducting alloy materials based on TMDs have exhibited outstanding characteristics in modulating the bandgap and carrier type owing to their tunable components . Among the various approaches, the CVD method for growing large‐scale monolayer ternary TMD alloys is a promising candidate.…”