Abstract
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.
Highlights
Two-dimensional (2D) materials are not stable in freestanding forms[1]
transverse force microscopy (TFM) images were obtained by using the longitudinal scan mode of Friction force microscopy (FFM), where the scanning direction of a probe tip is parallel to the cantilever axis, because it produces much clearer domain contrasts than the conventional lateral scan mode of FFM and can display the crystallographic orientation in situations where LFM cannot[13,18,19,20]
In summary, we have investigated the universal characteristics of nanoscale friction domains in 2D materials, such as graphene, hexagonal boron nitride (hBN), WS2, and MoS2 with a hexagonal honeycomb structure
Summary
Stable flakes of 2D materials can be obtained through interaction with the substrates. Such interaction gives rise to an inhomogeneous strain in the flakes. 2D materials exhibit surface corrugations, such as ripples, wrinkles, and bubbles[2,3,4,5,6,7]. Such surface corrugations are crucial for the electrical and mechanical properties of 2D materials, the characteristics of the ripples have rarely been revealed, mainly due to their small size and aspect ratio[8]. Friction force microscopy (FFM) is a useful tool to study the characteristics of ripples[9] and periodic deformation[10,11,12]
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