Universality of strain-induced anisotropic friction domains on 2D materials

Van der Waals two-dimensional (2D) materials have shown various physical characteristics depending on their growth methods and conditions. Among those characteristics, the surface structural properties are crucial for the application of 2D materials, as the surface structures readily affect their atomic arrangements and/or interaction with substrates due to their atomic-scale thicknesses. Here, we report on the anisotropic friction domains of MoS2 grown not only by chemical vapor deposition (CVD) under various sulfur pressure conditions but also by a mechanical exfoliation process. The 180° periodicity of each domain and the 60° shift between adjacent domains indicate the presence of linearly aligned structures along the armchair direction of MoS2, which is determined by the optical second-harmonic generation method. The anisotropic friction domains of CVD-grown MoS2 flakes may be attributed to linearly aligned ripples caused by an inhomogeneous strain field distribution, which is due, in turn, to randomly formed nucleation sites on the substrate. The universality of the anisotropic frictional behaviors of 2D materials, including graphene, hBN, and WS2 with honeycomb lattice stacking, which differ from ReSe2 with a distorted triclinic 1T’ structure, supports our assumption based on the linearly aligned ripples along the crystallographic axes, which result from an inhomogeneous strain field. By measuring friction forces on graphene-like surfaces, South Korean researchers have uncovered evidence of ripple structures too tiny to see with typical electron microscopes. Devices made from two-dimensional (2D) materials often perform at less than their full theoretical potential. Bae Ho Park from Konkuk University in Seoul and colleagues suggest this discrepancy may arise from strain-induced patterns which appear when ultrathin crystals are deposited onto supports including silicon or glass. The team investigated 2D molybdenum disulfide surfaces with scanning probe microscopy to both record atomic-level topography and measure the transverse forces experienced by the moving probe tip. The resulting images revealed periodic frictional domains with geometries that suggest the presence of underlying surface ripples. These structures, which also appeared during scans of graphene and other films, can affect the electrical properties of 2D materials. We report on the anisotropic friction domains of MoS2 not only grown by chemical vapor deposition (CVD) under various sulfur pressure conditions, but also by mechanical exfoliation process. The 180° periodicity of each domain and the 60° shift between adjacent domains indicate the presence of linearly aligned structures along the armchair direction of MoS2. The universality of anisotropic frictional behaviors of 2D materials, including graphene, hBN, and WS2 with stacking honeycomb lattices supports our assumption based on linearly aligned ripples along the crystallographic axes, which result from an inhomogeneous strain field.