Speaker
Description
Heterostructures made up of two-dimensional materials are excellent platforms to achieve twist-angle-independent ultra-low friction due to their weak interlayer van der Waals interactions and natural lattice mismatches. However, for finite-size interfaces, the domain edges can still play a role on the friction process, and their overall effect remains still unclear. In a recent publication[1], we reported on the superlubricity phenomenon and on the edge-pinning effect for the MoS$_2$/graphite and the MoS$_2$/h-BN heterostructures (two prototypical 2D materials). By means of atomic force microscopy experiments, we found that the coefficients of friction of such interfaces can be below 10$^{-6}$. The relevance of our findings is at least twofold: on the one hand we have proved that the contribution to friction coming from the contact area is basically vanishing for van der Waals heterostructures (the value of coefficient of friction that we estimate is the lowest reported in the literature so far), but on the other hand we have shown that a non-vanishing component of friction (which scales with the flake's perimeter) is still present. The latter aspect (and in particular the supporting computational study) is the main focus of this contribution. In fact, molecular dynamics and static simulations corroborate the experimental findings, and help to clarify the role of domain edges and interface steps in the friction process. Thanks to a detailed structural and energetic analysis, it was possible to highlight the contribution of sulfur atoms placed at the edge of the flakes. They turned out to present enhanced distortions (with respect to the atoms in the center of the flake) and they are more prone to get locked into locally commensurated structures. This then leads to the above-mentioned edge-pinning effect, and ultimately brings a greater contribution to the final friction force. The results provide more information on the sliding mechanism of finite low-dimensional structures, which is vital to understand the friction process of laminar solid lubricants.
References
[1] M. Liao, P. Nicolini et al., Nat. Mater., 21, 47-53 (2022).
