Strain-Engineered Rippling and Manipulation of Single-Layer WS₂by Atomic Force Microscopy

Surface ripple, as an important factor of corrugations in two-dimensional (2D) atomic crystals, plays important roles in determining their mechanical and physical properties. Here, we systematically investigated the strain-engineered rippling structure and manipulation of the rippling domain in monolayer WS2 flakes via atomic force microscopy (AFM). The rippling structure was introduced by the in-plane compression applied through the underlying SiO2/Si substrate during the rapid cooling process of post-growth. The zigzag-orientated rippling domains with three-equivalent directions were visualized by transverse shear microscopy (TSM) and friction force microscopy and further determined via angle-dependent TSM. Furthermore, these rippling domains can be precisely manipulated by controlling the AFM scanning, and various rippling patterns were formed by the AFM lithography. The manipulation mechanisms were phenomenally discussed based on their strain-induced anisotropic mechanical properties, the film-substrate mechanical model, and the dynamic strain-induced anisotropic puckering effects. Our study will be beneficial in understanding and controlling not only the rippling structures but also the rippling-related electronic and optical properties of 2D materials.