Characterization of Frictional Properties of Single-Layer Molybdenum-Disulfide Film Based on a Coupling of Tip Radius and Tip⁻Sample Distance by Molecular-Dynamics Simulations.

Lateral-force microscopy is a powerful tool to study the frictional properties of two-dimensional materials. However, few works distinctly reveal the correlation between the tip radius with the tip⁻sample distance and the frictional properties of the two-dimensional (2D) materials. We performed molecular-dynamics simulations to study the atomic-scale friction of a typical two-dimensional single-layer molybdenum disulfide (SLMoS₂). The effects of tip radius and tip⁻sample distance on the frictional properties were analyzed and discussed. The frictional force⁻sliding-distance curves show typical stick⁻slip behaviors, and the periodicity can be used to characterize the lattice constants of SLMoS₂. Sub-nanoscale stick-slip movements occur in one-lattice sliding periods along with only the armchair (AC) direction and only when the tip radius is smaller than 3 Å with 1.47 Å tip-sample distance. At the same tip⁻sample distance, a smaller tip can provide a more detailed characterization and higher-precision frictional properties of SLMoS₂. A larger tip is capable of providing comparative frictional properties of SLMoS₂ at a proper vertical tip⁻sample distance, compared with the small tip.