Abstract text (incl. figure legends and references)

The state of vanishing friction known as superlubricity has important applications for energy saving and increasing the lifetime of micro- or nanomechanical systems with sliding components (1). Superlubric behavior has been found in two-dimensional graphene or graphite because of the weak interlayer van der Waals force and shows a strong dependence on twist angle. Experimentally, M. Dienwiebel et al. recognized two narrow angular regions with a distance of about 61 deg. ± 2 deg. showing high friction, whereas nearly zero friction was observed between these two angular regions. This indicates that the ultralow friction is associated with incommensurability between the rotated graphite layers (2). Theoretically, H. Z. Bai et al. revealed the bilayer graphene (BLG) superlubricity with a coefficient of friction of 0.0003 at a twist angle of 3°, and this coefficient of friction is about 26 times lower than a non-superlubric state (3). Despite the insightful theoretical works, the frictional properties within a small twist angle regime and the dynamic twist process during sliding between atomic layers of graphene have not been directly observed in experiments due to the great challenges of such experiments.

Here, we report the first time direct observation of dislocation activity and twisting in BLG by in situ straining inside a transmission electron microscopy (Fig. 1A). Combining with the 4D STEM technique, strains and twist angles were evaluated at each strain step (Fig. 1A) and their relationship was established to understand the superlubric properties of BLG. In our experiment, a monolayer-bilayer-monolayer graphene configuration was verified (Fig. 2B). As depicted in Fig. 2B-D, the width of the bilayer region decreased as well as the dislocation network changed during straining. With a BLG change of 22 nm (Fig. 2C), dislocation in BLG converted to the Moiré superlattice configuration, indicating the twist between the two monolayers. When it comes to a BLG width change of 121 nm (Fig. 2D), a higher dislocation density is reached, indicating a further increase in twist angle. The analysis on dislocation types revealed that the predominate screw type dislocations with Burgers vectors and remain, while dislocations with Burgers vectors changed from mixed type to mainly screw type (Fig. 2E-G). Meanwhile, the evaluation of twist angle distribution in BLG shows that the twist angle increases along with the continuous sliding, and these results agree well with the twist angle calculated from the Moiré superlattice periodicity (Fig. 2A). Eventually, the relationship between strain and twist angle reveals a clear decreasing tendency in strain with the increasing of twist angle (Fig. 2B), indicating a gradual change towards superlubric behavior in BLG.

In conclusion, we developed an in situ staining method combined with 4D-STEM to realize the direct observation of dislocation activities and investigation of the twist-dependent superlubric behavior in BLG. This method can be taken as a versatile way to study the strain-related physical properties of other 2D materials.

(1) S. Kawai et al. Science, , 351, 957 (2016).

(2) M. Dienwiebel, et al. Phys. Rev. Lett., 92, 12 (2004).

(3) H. Z. Bai, et al. Carbon, 191, 28 (2002).

(4) X. Zhou, et al. to be published.

Fig. 1 (A) Schematic representation of the straining process and 4D STEM set-up for strain and twist angle evaluation. (B to D) The sequence DF-TEM images taken from the reflection describe the dislocation network and twist in BLG at various strain steps. (E to G) Drawing of the dislocations in the BLG region corresponding to B to D.

Fig. 2 (A) Twist angle distribution at various strain steps. (B) Strain-twist angle correlation, showing a decreasing trend of strain with the increasing of twist angle.