In now days, energy losses due to friction and wear account for approximately one-third of total human energy consumption. Achieving extremely low friction not only reduces energy consumption but also extends the life of machinery. Within this context, research on superlubricity has been a frontier in the area of tribology.  Superlubricity is a regime of motion in which friction vanishes or very nearly vanishes. Generally, a superlubricant interface should have a coefficient of friction less than 10^-3. Superlubricity may occur when two crystalline surfaces slide over each other in dry incommensurate contact. This effect, also called structural lubricity, was suggested in 1991 and verified with great accuracy between two graphite surfaces in 2004.

It is worth noting that natural van der Waals materials such as graphite, molybdenum disulfide, and hexagonal boron nitride (hBN) have been used as solid-state lubricants for more than a hundred years. Due to weak van der Waals (vdW) forces between adjacent layers in such materials, they are simple but a good material platform to investigate superlubricity. In principle, an incommensurate Van der Waals interface is an ideal system to study structural superlubricity.

In 2004, Joost W. M. Frenken's group at Leiden University reported the pioneer work on measurements of the extremely low friction between two graphite layers with a certain twisted angle (PRL 2004, 92, 126101), demonstrating a bench-mark progress on structural superlubricity. In 2008, Quanshui Zheng's group at Tsinghua University discovered a self-retraction effect originated from such extremely low friction in a similar interface (PRL 2008, 100, 067205).

In such homogeneous vdW interfaces, it is in commensurate contact when the twist angle is zero, leading to a maximum friction; small twist-angle contacts generate Moiré patterns, however, local commensurate contacts within each Moiré supercells still exist, leading to a rather large friction; large twist-angle contacts are approximately incommensurate, which leads to a very small friction.

A possible solution to eliminate the twist-angle dependence of friction in such vdW homointerfaces is to employ vdW heterointerfaces. In 2016, Guangyu Zhang's group at the Institute of Physics, Chinese Academy of Sciences (IoP-CAS) studied the thermal stability of graphene-hBN heterointerfaces and found an interesting dynamic phenomenon, i.e., thermally induced rotation of graphene on hBN (PRL 2016, 116, 126101). Such rotations would finally drive the heterointerface to a configuration with twist angle close to 0° or 30°, revealing the possible existence of structural superlubricity at this van der Waals heterogeneous interface at non-stable twist-angles.

Indeed, in 2018 Quanshui Zheng's group measured the friction of such heterointerfaces and verified the structural superlubricity phenomenon. They also found that the anisotropy of friction observed in the heterojunction was significantly lower than that measured in the homointerfaces (Nature Materials 2018, 17, 894). Due to the rather small lattice mismatch between graphene and hBN, i.e., ～1.7%, these graphene-hBN heterointerfaces still suffer from pining effects at small twist angles due to the presence of Moiré superlattices. It remains challenging to reveal a stable and isotropic structural superlubricity at such vdW interfaces.

Recently, Guangyu Zhang's groups @IoP-CAS has investigated systematically the superlubricity phenomenon in large lattice mismatched vdW heterointerfaces. Two typical interfaces were addressed, i.e. MoS₂/graphite and MoS₂/hBN with lattice mismatch of ～26.8% and ～24.6%, respectively. In order to measure the interface friction precisely, they set up an environment-controllable atomic force microscope  (AFM) system and developed relative lateral force measurement techniques for AFM.

To their surprises, they found isotropic ultra-low friction coefficients, i.e., all below 10^-6 under any twist-angles, in these vdW heterostructures. Such low friction coefficient of below 10^-6 is the lowest ever achieved in previous research.

In order to reveal the feature of such superlubricity in these large-lattice-mismatched vdW heterointerfaces, they also performed further size dependent measurements. Results indicate that the friction forces during sliding a domain of monolayer MoS₂ on graphite or hBN come dominantly from the MoS₂-domain-edge pinning effect; while it totally vanishes within the plane of the domain. Despite of the domain edge, they also investigated the surface steps on graphite or hBN and found similar pinning effect. Collaborated with Tomas Polcar' group at Czech Technical University, they also carried out molecular dynamics (MD) simulations on this domain-edge pinning effect. They found that the dynamics of the edge atoms present peculiar traits for distortions and potential energy fluctuations, leading to a significant contribution to the friction force which is consistent with the experimental results.

This study has explored in-depth the isotropic superlubricity in a class of new vdW heterointerfaces with large lattice mismatches. The message obtained from this research would be helpful on designing and applying advanced superlubric interfaces. The work, entitled "UItra-low friction and edge-pinning effect in large-lattice-mismatch van der Waals heterostructures", was published recently on Nature Materials 2021, https://doi.org/10.1038/s41563-021-01058-4. Dr. Liao Mengzhou in Guangyu Zhang's group @IoP-CAS is the first contributing author and Dr. Paolo Nicolini in Tomas Polcar’ group at Czech Technical University contributed for MD simulations.

This work was supported by the National Science Foundation, the Strategic Priority Research Program of CAS and grants from Songshan Lake Materials Laboratory.

Fig.1 | Friction characterizations of vdW heterostructures. (Images from IoP-CAS)

Fig.2 Superlubricity of MoS2/graphite and MoS2/h-BN heterointerfaces. (Images from IoP-CAS)

Fig.3 Origin of friction for three different heterointerfaces. (Images from IoP-CAS)

Fig.4 MD simulations of MoS2 flakes sliding on graphite. (Images from IoP-CAS)

Fig.5 Effect of interface steps on the friction force. (Images from IoP-CAS)

Contact:
Institute of Physics
Guangyu Zhang
Email: gyzhang@iphy.ac.cn

Key word:
Friction; Superlubricity; 2D van der Waals heterostructures; Interfaces

Abstract:
In a recent study, an IoP-CAS team discovered a new superlubric system, i.e. large lattice mismatched van der Waals heterostructures. Friction coefficient below 10^-6 was observed in such system and is the lowest achieved against all other materials so far. The interface superlubricity in large lattice mismatched van der Waals heterostructures show no twist-angle dependence. Most importantly, in such system the interface friction could fully vanish if the interface is perfect, and the measured friction in experimentally explored samples comes from those structural defects such as domain edges and surface steps.