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.
Bibliographical noteFunding Information:
This work was supported by National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (No. 2013R1A3A2042120, 2011-0030229, 2015R1A3A2066337, 2015R1A5A1009962, and 2015R1A1A1A05001560), the Nano·Material Technology Development Program through the NRF funded by the MSIP (No. 2016M3A7B4909668), and an Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government (18ZB1800, Development of Neuromorphic Hardware by using High Performance Memristor Device based on Ultra-thin Film Structure).
© 2018, The Author(s).
All Science Journal Classification (ASJC) codes
- Modelling and Simulation
- Materials Science(all)
- Condensed Matter Physics