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

Direct imaging, three-dimensional interaction spectroscopy, and friction anisotropy of atomic-scale ripples on MoS2

“…1a, recorded on a multilayer MoS2 flake, clearly demonstrates the layer-dependence of friction that is a ubiquitous characteristic of 2D materials 15 , no linearly aligned structures, i.e. ripples, are observed on the flake surface, along the lines of previous FFM work performed on this material 15,17,20 .…”

“…Considering that the direction dependence, i.e., anisotropy, of friction can be potentially an important design parameter for 2Dmaterial-based solid lubrication in small-scale mechanical systems, we performed FFM measurements to probe friction anisotropy on MoS 2 flakes exfoliated onto SiO 2 . Our work was additionally motivated by previous reports of friction anisotropy on 2D materials including graphene and MoS 2 , where twofold and sixfold symmetries have been reported [16][17][18][19] , as well as irregular anisotropic behavior 20 . While twofold anisotropic behavior was tentatively explained by the presence of ripples 16,17 or stripes formed by molecular adsorbates 18 , sixfold anisotropy was ascribed to the hexagonal symmetry of the atomic structure of the involved materials 19 .…”

“…Motivated by the absence of AFM data in the literature demonstrating the presence of atomic-scale ripples on 2D materials, we performed experiments to answer the question of whether the imaging of atomic-scale ripples on MoS 2 can be accomplished with an AFM-based approach. Based on the defining role that ripples are thought to play in the friction anisotropy of such materials 16,17 , we initially conducted FFM experiments on few-layer MoS 2 samples exfoliated onto SiO 2 , whereby topographical and friction force maps are recorded simultaneously as the AFM tip slides on the sample surface in contact mode 23 . While the friction force map of Fig.…”

Friction Anisotropy of MoS2: Effect of Tip–Sample Contact Quality

“…In previous studies 21, [25][26][27][28]30 in which the AFM tip itself was used to probe friction anisotropy of 2D materials, both six-fold 21,30 and two-fold symmetric friction anisotropy [25][26][27][28] have been observed. In studies where the sample was rotated with a fixed scanning direction, two-fold symmetry was observed 24,25,27,28 . The two-fold symmetry was attributed to the existence of inplane strain induced by topographic wrinkles formed due to the difference in thermal contraction of the substrate compared to the 2D material after deposition [35][36][37] .…”

“…21 (see Figure S1b). In cases where the sample was rotated relative to the scanning direction, some studies reported 60 o periodicity (i.e., six-fold symmetry), including measurements on monolayer graphene [22][23][24] , while others [25][26][27][28][29] observed 180 o periodicity (i.e., twofold symmetry). The latter was attributed to out-of-plane elastic deformation of the substrate resulting from oriented linear wrinkles (sometimes referred to as ripples) in the 2D material where scanning perpendicular to the wrinkles led to higher friction.…”