Abstract
Using numerical simulations, we study the atomic-scale frictional behaviors of monovacancy-defective graphene and single-layer molybdenum-disulfide (SLMoS2) based on the classical Prandtl–Tomlinson (PT) model with a modified interaction potential considering the Schwoebel–Ehrlich barrier. Due to the presence of a monovacancy defect on the surface, the frictional forces were significantly enhanced. The effects of the PT model parameters on the frictional properties of monovacancy-defective graphene and SLMoS2 were analyzed, and it showed that the spring constant of the pulling spring cx is the most influential parameter on the stick–slip motion in the vicinity of the vacancy defect. Besides, monovacancy-defective SLMoS2 is found to be more sensitive to the stick–slip motion at the vacancy defect site than monovacancy-defective graphene, which can be attributed to the complicated three-layer-sandwiched atomic structure of SLMoS2. The result suggests that the soft tip with a small spring constant can be an ideal candidate for the observation of stick–slip behaviors of the monovacancy-defective surface. This study can fill the gap in atomic-scale friction experiments and molecular dynamics simulations of 2D materials with vacancy-related defects.
Highlights
Atomic-scale friction has been at the forefront of scientific interest over the decades
According to previous lateral force microscopy (LFM) studies on the monovacancy-defective graphene [29,30] and the stepped surfaces [34–38], there is the Schwoebel–Ehrlich barrier existing in the vicinity of the vacancy defect, and the long-range interactions Vlong are varying, which means a second contribution to the tip–surface interaction potential should be considered
The enhanced frictional force in the vicinity of the vacancy defect is in agreement with that in the previous works about the monovacancy-defective graphene [29,30], and the sharp variations in tip–surface potentials and frictional forces can be attributed to the Schwoebel–Ehrlich barriers [31,32], which have been already observed in the friction at the atomic-scale surface steps of graphene [34–36,38] and MoS2 [37]
Summary
Atomic-scale friction has been at the forefront of scientific interest over the decades. As one of the most remarkable discoveries in nano-tribology, the atomic-scale stick–slip phenomenon appears in the time domain as a series of saw-tooth signals, and its period usually corresponds to the unit cell of the surface potential [11,12] This observation can be theoretically reproduced within classical mechanics by using the Prandtl–Tomlinson (PT) model, which describes the movement of a point-like tip connected to a support by a harmonic spring in constant-force mode of an idealized LFM. To study the mechanisms that control the frictional characteristics of the graphene and SLMoS2 with vacancy defects, we changed the effective masse of the system, the damping of the system, the sliding velocity of the tip, and the spring constant of the pulling spring to examine how the atomic-scale frictional behaviors depend on these variables. Ours is the first study revealing the influences of the classical PT model parameters on the atomic-scale frictional properties of a monovacancy-defective surface, and this work can make up for the deficiency of the LFM experiments and MD simulations when studying the atomic-scale frictional behaviors of 2D materials with vacancy-related defects
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