Research ArticleAPPLIED PHYSICS

Chemical and physical origins of friction on surfaces with atomic steps

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Science Advances  09 Aug 2019:
Vol. 5, no. 8, eaaw0513
DOI: 10.1126/sciadv.aaw0513
  • Fig. 1 Schematic illustration and atomic-scale rendering of a silica AFM tip sliding up and down a single-layer graphene step edge on an atomically flat graphite surface.

    The silica tip model represents the native oxide at the apex of the Si AFM tip used in the experimental study. This model system enables both experimental and computational studies that isolate the chemical and physical origins of friction.

  • Fig. 2 Lateral force (solid lines) and height profile (dashed lines) measured at the graphene step edge with a silica AFM tip.

    The normal force applied to the tip was 36.7 nN, and the sliding speed was 500 nm/s. In the step-up direction, the positive lateral force means that the graphene step edge is resisting tip sliding. In the step-down direction, the negative lateral force is resistive to the tip sliding and the positive (or upward deviation from the negative trend) force is assistive to the tip sliding. The inset is the AFM topographic image of the graphene step edge obtained after repeated friction measurements at applied normal forces varying from 7.3 to 36.7 nN (fig. S3A); the postscan image shows no damage of the friction-tested region (white line). The height of the step edge is 0.34 nm, corresponding to the sum of the thickness of one graphene layer and the interlayer spacing between adjacent graphene layers.

  • Fig. 3 Load dependence of friction force and corresponding COF.

    (A) Friction force measured with the silica AFM tip under various applied normal loads. The step-up resistive, step-down resistive, and step-down assistive forces are determined as marked in Fig. 2. The mean and SD were calculated from values of multiple measurements, where each measurement involved averaging over 128 scans. The SDs of the experimental values are similar to or smaller than the size of symbols. (B) Friction force calculated from reactive MD simulations. Note that, for the step-down case, a positive assistive lateral force corresponds to a negative friction force. (C) COF calculated from the load dependence of friction force, which is the slope of the least squares fitting lines in (A) and (B). The error bar in (C) indicates the uncertainty in the calculated slope. Because friction force for the cases of step-down resistive and step-down assistive decreases as the applied load increases, negative COF is obtained.

  • Fig. 4 Reactive MD simulation showing the origins of chemical and physical effects on friction.

    (A and B) Lateral force, (C and D) shear strain of atoms in the silica where the sign indicates direction relative to sliding, and (E and F) number of hydrogen bonds formed between the graphene step edge and the silica, calculated from simulations as a function of center-of-mass position of the tip with respect to the graphene step edge for (A, C, and E) step-up and (B, D, and F) step-down. The normal load applied to the silica tip is 10 nN, and the sliding speed is 10 m/s. The topographic height change measured with the center of mass of the counter surface is shown with dashed lines in (A) and (B) on the secondary y axis. The white and gray background areas are the lower and upper terraces, respectively. The snapshots of the shear strain of atoms in the silica and the hydrogen bonds bridging two surfaces at three locations for both step-up and step-down are also shown.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/8/eaaw0513/DC1

    Supplementary Text

    Fig. S1. PM-RAIRS spectra of graphite surface and Au surface.

    Fig. S2. Front and side views of the MD simulation box.

    Fig. S3. Lateral force of an AFM tip sliding across the graphene step edge.

    Fig. S4. MD simulations of a tip sliding across a graphene step edge.

    Fig. S5. Out-of-plane vibration of the carbon atoms on the upper and lower terraces.

    Fig. S6. Out-of-plane deformation of C─OH groups terminating the graphene step edge.

    Fig. S7. Force analysis diagram of the AFM tip at a graphene step edge.

    Fig. S8. Lateral force of the AFM tip at multilayer graphene step edges.

    Fig. S9. Water contact angle measurement on Si wafers.

    Fig. S10. Applied normal force error in AFM experiments.

    References (4447)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. PM-RAIRS spectra of graphite surface and Au surface.
    • Fig. S2. Front and side views of the MD simulation box.
    • Fig. S3. Lateral force of an AFM tip sliding across the graphene step edge.
    • Fig. S4. MD simulations of a tip sliding across a graphene step edge.
    • Fig. S5. Out-of-plane vibration of the carbon atoms on the upper and lower terraces.
    • Fig. S6. Out-of-plane deformation of C─OH groups terminating the graphene step edge.
    • Fig. S7. Force analysis diagram of the AFM tip at a graphene step edge.
    • Fig. S8. Lateral force of the AFM tip at multilayer graphene step edges.
    • Fig. S9. Water contact angle measurement on Si wafers.
    • Fig. S10. Applied normal force error in AFM experiments.
    • References (4447)

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