Controllable Friction on Graphene via Adjustable Interfacial Contact Quality

Abstract Despite the numerous unique properties revealed through tribology research on graphene, the development of applications that utilize its rich tribological properties remains a long‐sought goal. In this article, a novel approach for reversible patterning of graphene's frictional properties using out‐of‐plane mechanical tapping is presented. The friction force between the atomic force microscopy (AFM) tip and the graphene film is increased by up to a factor of two, which can be attributed to variations in the interfacial binding strength between the graphene and substrate through the tapping process. The reversible and repeatable frictional properties of graphene make it a promising material for information storage applications with a high storage capacity of ≈1600 GB inch−2, allowing for direct writing and erasing of information, akin to a blackboard. These findings highlight the potential for friction tuning in lamellar materials and emphasize the significance of understanding nanoscale friction on graphene surfaces.

The influence of tapping times was investigated in this study to understand how repeated tapping affects friction.A 500 nm×500 nm region was tapped with a predefined tapping force of 87.3 nN, allowing for modulation of friction.Figure S1 demonstrates this process, utilizing a tapping array of 20×20 points.Initially, as shown in Figure S1(a), a high friction region was introduced; however, the boundary of this region was not well-defined, and the extent of friction enhancement was limited.Subsequent tapping improved these issues, as depicted in Figure S1(b).
To optimize the pattern and minimize the need for multiple tapping cycles, an alternating increase in tapping points was employed in our experiments.

Relaxation time effect on frictional patterning
The friction change induced by the mechanical tapping process has proven to be highly stable.
While we did not conduct a systematic investigation into the relaxation effects, we have observed that patterns prepared more than one month ago remain clearly visible in the friction channel.This is exemplified by the black rectangles as shown in Figure S2.We believe that in the absence of external applied fields, these patterns can be maintained for an extended period.where the high friction pattern can be clearly observed as indicated by black rectangles.

In-plane scan
To validate the importance of out-of-plane mechanical tapping, we performed a similar patterning process by replacing the out-of-plane tapping with an in-plane scan.Firstly, we continuously scanned a small area on a graphene plane using the same Si 3 N 4 tip with a normal force of 70 nN and a scan velocity of 2 µm/s.We then scanned a larger area to check for changes in friction.
Fig. S3a shows the friction map of graphene on a SiO 2 /Si substrate with a scan size of 5µm×5µm.
The white rectangles indicate the small in-plane scan area.

Erase with different normal forces
To investigate the effect of normal force on the erasing (scanning) process, we carried out erasing measurements with different normal forces.As shown in Fig. S4a, it took five scans for the pattern to completely disappear at a small normal load, while only two scans were needed for a larger normal force (Fig. S4b).We conclude that increasing the normal force during scanning can speed up the erasing process.

Tapping on a thick HOPG
We performed a similar patterning process a thick HOPG sample using the same Si 3 N 4 tip.To obtain a clean sample surface, the HOPG sample was freshly cleaved immediately before tapping.Fig. S5a and b show the topography and friction maps of the HOPG sample with a scan size of 2 µm × 2 µm and normal force of 1.1 nN.No obvious patterns were observed, except for some step edges introduced during the cleavage process.We then tapped on a small area of 650 nm × 650 nm with a tapping force of 98.5 nN, as indicated by the white rectangles.Fig. S5c and   d show the topography and friction maps of the same region after tapping.No obvious changes in the topography and friction maps were observed.Therefore, the out-of-plane tapping process can only be used for thin graphene sheets.

Tapping with different tips
After exfoliation, graphene sheets stay on the SiO 2 /Si substrate, and the interfacial contact quality between the graphene and the substrate degenerates due to the large jump-off force.To validate this assumption, we used a Si tip (PPP-LFMR from Nanosensors) to tap on the graphene.It is important to note that the silicon tip oxidizes into a silicon oxide tip in the atmosphere, so we indeed used a silicon oxide tip for the experiment.Both the silicon oxide and Si 3 N 4 tips had a similar radius of 10 nm.As expected, no obvious friction change was observed when using the silicon oxide tip (Fig. S6a and b).However, a friction difference was observed when using the Si 3 N 4 tip (Fig. S6).Since the binding energy for Si 3 N 4 /Graphene is much larger than that for SiO 2 /Graphene 1, 2 , the interfacial contact quality can be tuned during the retracting process.
Therefore, we conclude that using a tip with a larger binding energy is a key factor in writing on graphene.

Comparison with different storage media
In the study, we were able to achieve a minimum storage unit size of approximately 10 nm×10 nm using a tip with a radius of approximately 10 nm.The spacing between these storage units is

Figure S1 :Figure S2 :
Figure S1: Effects of tapping times on frictional patterning.Friction image after (a) One-time tapping.(b) Two-times tapping.Tapping array is 20×20 points and pre-tapping force is 87.3 nN.Tapping are is 500 nm×500 nm

Figure S3 :
Figure S3: In-plane scanning on a graphene sample has no effect on the writing.(a) Friction force map before in-plane scanning.(b) Friction force after in-plane scanning.

Figure S4 :
Figure S4: Erasing the pre-written pattern with reciprocal scanning using a normal force of (a) -2.2 nN and (b) 37.5 nN.

Figure S6 :
Figure S6: Friction maps before (a) and after (b) tapping by using a silicon oxide tip.Friction maps before (c) and after (d) tapping by using a Si 3 N 4 tip.