Abstract:
In a recent study, a team from the Institute of Physics (IOP) of the Chinese Academy of Sciences (CAS) discovered a new superlubric system, i.e., large lattice mismatched van der Waals heterostructures. A friction coefficient below 10-6 was observed in the system, the lowest achieved against all other materials so far. The interface superlubricity in large lattice mismatched van der Waals heterostructures shows no twist-angle dependence. Most importantly, in such a system the interface friction could fully vanish if the interface were perfect, as the measured friction in experimentally explored samples comes from structural defects such as domain edges and surface steps.
Nowadays, 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.
Friction characterizations of vdW heterostructures [IMAGE: IOP]
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 a simple but good material platform to investigate superlubricity. In principle, an incommensurate van der Waals interface is an ideal system to study structural superlubricity.
Superlubricity of MoS2/graphite and MoS2/h-BN heterointerfaces [IMAGE: IOP]
In 2004, Joost W. M. Frenken’s group at Leiden University reported pioneer work on measurements of 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, Zheng Quanshui’s group at Tsinghua University discovered a self-retraction effect originating from such extremely low friction in a similar interface (PRL 2008, 100, 067205).
Origin of friction for three different heterointerfaces [IMAGE: IOP]
In such homogeneous vdW interfaces, commensurate contact arises when the twist angle is zero, leading to maximum friction, and small twist-angle contacts generate Moiré patterns; however, local commensurate contacts within each Moiré supercell still exist, leading to a rather large friction, whereas large twist-angle contacts are approximately incommensurate, which leads to a very small friction.
MD simulations of MoS2 flakes sliding on graphite [IMAGE: IOP]
A possible solution to eliminate the twist-angle dependence of friction in such vdW homointerfaces is to employ vdW heterointerfaces. In 2016, Zhang Guangyu’s group at the Institute of Physics (IOP) of the Chinese Academy of Sciences (CAS) studied the thermal stability of graphene-hBN heterointerfaces and found an interesting dynamic phenomenon: thermally induced rotation of graphene on hBN (PRL 2016, 116, 126101). Such rotations 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 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 of ~1.7 percent between graphene and hBN, these graphene-hBN heterointerfaces still suffer from pinning 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.
Effect of interface steps on the friction force [IMAGE: IOP]
Recently, Zhang’s group 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 percent and ~24.6 percent, 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 surprise, they found isotropic ultra-low friction coefficients all below 10-6 under any twist-angles in these vdW heterostructures. Such low friction coefficients of below 10-6 are lower than ever achieved in previous research.
In order to reveal the features 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 they totally vanish within the plane of the domain. Despite the domain edge, the group also investigated the surface steps on graphite or hBN and found a similar pinning effect. Collaborating with Tomas Polcar’ group at Czech Technical University, they also carried out molecular dynamic (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 will be helpful in 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 recently published in Nature Materials 2021.
For more information, please contact:
Zhang Guangyu
E-mail: gyzhang@iphy.ac.cn
Institute of Physics (IOP),
Chinese Academy of Sciences
Source: Institute of Physics (IOP),
Chinese Academy of Sciences