Interlayer friction and superlubricity in bilayer graphene and MoS2/MoSe2 van der Waals heterostructures
Introduction
Friction is a mechanical phenomenon that occurs on the contact interface with relative motion (or with a trend of relative motion) and hinders its relative motion. However, the process of friction is accompanied by energy dissipation, which is bound to cause losses. According to statistics, about one-fifth of the energy consumed in the earth each year is used to overcome the friction in various situations and processes [1]. Therefore, the control of tribological interactions (i.e., friction, adhesion, and wear) is necessary on all length scales [2]. In recent years, many researchers have devoted themselves to the study of friction on atomic scale, mainly for the miniaturization of devices, of which the ratio of surface area to volume has become quite large. For friction, wear and adhesion can significantly reduces the reliability and lifetime of devices, while the recently emerged two-dimensional materials may be a good solution to this problem. Take graphene as an example, the atoms in the same atomic layer combine through covalent bonding to form a monolayer structure with high modulus and high strength. Given that two-dimensional (2D) materials are free of dangling bonds and are bonded by weak van der Waals interaction, the low shear resistance enables low energy-cost sliding between layers. This characteristic makes these two-dimensional materials become promising solid lubricating materials in nano-electromechanical system (NEMS) [3,4].
In the 1990s, Sokoloff and Hirano predicted that if the sliding interface is rigid, structurally incompatible (i.e., non-matched), flat, molecular clean, and weakly bonded, there would be a physical state of almost zero friction [5,6]. This phenomenon is also known as superlubricity or ultra-low friction (friction coefficient μ ≤ 0.001), which has attracted great attention because 2D materials show excellent lubrication performance, even if they are only a few atomic layers [[7], [8], [9]]. Dienwiebel et al. [10,11] used a home-made friction microscope to examine the energy dissipation of the tungsten tip sliding on the graphite surface by measuring the relationship between the atomic friction force and the rotation angle. Their experiments confirmed that the ultra-low friction of graphite is due to the incommensurability between rotated graphite layers. In order to further investigating the effect of lattice commensurability on interlayer friction, Feng et al. [12] have utilized Friction force microscope (FFM) and scanning tunneling microscope (STM) and they found that under the van der Waals force between the probe tip and graphene nanoparticles, graphene shifted from commensurate state to incommensurate state at first, then quickly slipped to another commensurate state (temperature below 5 K). Leven et al. [13] found that when a relatively small graphene sheet slides on a larger graphene surface, the torque-induced reorientation of the graphene sheet may make the phenomenon of superlubricity at the homogeneous interface disappear. However, when a larger graphene sheet slides on the surface of h-BN, even when the mismatch angle between layers is zero, the inherent lattice mismatch between graphene and h-BN makes the interface sliding energy barrier very small, resulting in a very stable superlubricity behavior, free of the relative sliding direction of crystal planes. Moreover, comparing with the commensurate contact between graphene layers, there is a natural incommensurate contact configuration between graphene and other 2D materials due to the intrinsic lattice mismatch [14,15]. Therefore, in order to achieve superlubricity performance, the main focus is to achieve sustainable incommensurate sliding contact. Through a series of density functional theory (DFT) calculations, molecular dynamics (MD) simulations, strain engineering and registry index (RI) models, researchers proposed theoretically that the heterostructures composed of 2D layers with lattice mismatch and inherent incommensurate interface geometry can help to achieve robust superlubricity performance [14,[16], [17], [18], [19], [20], [21], [22]]. Song et al. [23] reported the experimental realization of robust superlubricity in microscale single-crystal heterostructures, while the interface between graphite and hexagonal boron nitride clearly showed that the superlubricity still exists even if the oriented contact is subjected to external force under environmental conditions. This shows evidently that van der Waals heterostructures have great potential in superlubricity.
Moiré patterns, is a kind of quasi-periodic patterns generated by the differences in periodicity or orientation of composing two-dimensional lattices. Especially in surface science, due to lattice mismatch and relative orientation, the heterostructures of 2D materials are mostly accompanied by Moiré patterns. These Moiré patterns provide periodic potentials that affect the electronic structure, which can lead to significant changes in physical properties. For example, in 2018, studies have shown that when the twist angle between two layers of graphene layers is adjusted to about 1.1° (magic angle), the system becomes a high-temperature superconductor [24,25]. It would be reasonable to speculate that Moiré patterns might also be able to lead to other new properties. The motivation for this study stems from the fact that the angle of torsion can affect the physical properties of two-layer or multi-layer 2D materials. In this work, we used molecular dynamics (MD) simulation to study the relationship between the Moiré patterns (of bilayer graphene, MoS2/MoSe2 heterostructures) and their superlubricity. Different Moiré periods corresponding to different twist angles, and the effects of other different factors (such as temperature, normal force, relative velocity, etc.) on interlayer friction were studied.
Section snippets
Simulation methods
In order to study the interlayer friction of graphene/graphene and MoS2/MoSe2 heterostructures, we constructed the bilayer structure of graphene/graphene and MoS2/MoSe2, as shown in Fig. 1 and Fig. 2, respectively. Specifically, the fixed layer, positioned at the bottom of the film, denotes the presence of the substrate. A flexible layer with arbitrary twist angles dragged atop the fixed graphene via a stage, which modeled as a rigid sheet duplicating of the flexible layer. Each atom in the
Results and discussion
To reduce potential energy corrugation during sliding, i.e. the energy required to surmount the sliding barrier, is essential to achieve low friction. Therefore, it is necessary to know the effective potential energy surface (PES) [32]. In order to exploit the mechanism of superlubricity, we performed molecular dynamics (MD) calculations on the bilayer graphene, as well as the MoS2/MoSe2 heterostructure. The interlayer interaction energy (in meV/atom) between the top flake and the substrate was
Conclusions
Based on molecular dynamics simulations, we have studied the relationship between twist angle and friction force of bilayer graphene and MoS2/MoSe2 heterostructures. The results show that the heterostructures with the intrinsic structural difference or the incommensurate contact structure obtained after a rotation at a certain angle have a lower friction coefficient and can lead to superlubricity between layers. The friction coefficient of bilayer twist angle graphene (incommensurate state) is
CRediT authorship contribution statement
Guoliang Ru: Methodology, Software, Writing - original draft. Weihong Qi: Conceptualization, Supervision, Writing - review & editing. Kewei Tang: Data curation, Software. Yaru Wei: Writing - review & editing. Taowen Xue: Visualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central Universities (Grant No.31020195C001), P. R. China.
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