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The deformation induced atomic-scale stick-slip behaviors are unique and prominent friction characteristics of atomically thin two-dimensional (2D) materials. Here, the deformation induced atomic-scale frictional characteristics of atomically thin 2D materials under different load were explored at different velocity using atomic force microscopy. The evolution of contact quality and puckering forming play important role in atomic-scale frictional characteristics of 2D materials. The critical load was proposed for the strengthening effect at transient period of atomic-scale friction. The velocity-dependent evolution of contact quality affects the strengthening effect when the normal load is less than the critical load. Strengthening degree and distance increase with the increase of the normal load because of high puckering and strong interfacial bonding when the load exceeds the critical load. The suspended 2D materials on asperities of rough substrate possess stronger strengthening effect than smooth substrate because of large deformation space and flexibility. The stick time in stick-slip behavior elongated with the increase of load as the lattice of 2D materials on rough substrate is locally distorted by large deformation under high load. The correlation between the strengthening effect and friction of 2D materials could potentially provide deep insights into the tribological behavior of atomically thin 2D materials.

The atomic-scale steps are ubiquitous on graphene. The frictional behavior of graphene at atomic-scale steps under different relative humidity (RH) is very important for application of graphene as solid lubricant in complicated environment. Frictional properties at atomic-scale graphene steps were investigated using calibrated atomic force microscopy (AFM) in various RH. Lateral force at uncovered atomic step decreased with the increase of RH, while lateral force at covered step were independent of RH. Uncovered steps possessed higher work function than graphene plane from the scanning Kelvin probe microscopy (SKPM) measurement due to the existence of dangling bonds and oxygen-containing functional groups. Then the adsorption of water molecule on hydrophilic oxygen containing functional groups at uncovered step decreased the work function and reduced the lateral force under high RH. Also, lateral force at steps increased even fast than plane after plasma treatment, because dangling bonds is easier to be oxide functionalization. The studies provide a further understanding of frictional properties at atomic scale steps and could be helpful in the applications of graphene as solid lubricant in humid environment.

Graphene is one of the key building blocks for a broad range of flexible/stretchable electronics and has great potential as a solid lubricant in flexible micro/nanoelectromechanical systems, soft robot and microfluidic devices due to their excellent electrical, mechanical and tribological properties. The nanotribological properties of graphene on soft elastic substrate were studied using the calibrated atomic force microscopy (AFM). The nanotribological properties of graphene on soft elastic substrate were enhanced by the elastic deformation compared with the hard SiO2/Si substrate. Also the friction force of graphene on soft elastic substrate decreases with the increase of the thickness and shows a sub-linear dependence on the indentation depth. A novel model of the elastic deformation enhancing puckering effect was proposed to explain the nanotribological properties of graphene on soft elastic substrate. The atomic-scale frictional behaviors of graphene on soft elastic substrate confirmed the mechanism of the elastic deformation enhanced puckering effect. These studies can provide a fundamental understanding of graphene as a solid lubricant on soft elastic substrate for graphene-based flexible MEMS/NEMS devices.

Graphene as a well-known solid lubricant is widely used in micro- and nano-scale mechanical devices, decreasing the friction of graphene as far as possible is a perpetual task to improve the performance of these devices. A novel approach was proposed to decrease the friction of graphene against atomic force microscopy (AFM) tip by plasma treatment of the substrate. The plasma treatment of the substrate enhances the adhesive attraction between graphene and SiO 2 substrate by generating stronger van der Waals attraction. Enhancing the adhesive attraction can indeed decrease the friction of graphene, regardless of the thickness of graphene and the kinds of AFM tip. Longer time of plasma treatment results in stronger adhesive attraction, leads to smaller friction. The decreased friction is mainly due to the combined action of the suppressed puckering of graphene and the reduced ability of graphene to adjust its atomic configuration. This novel approach will promote the engineering application of graphene and other related 2D materials as lubricants in MEMS/NEMS.