Figure 1

XPS survey spectra of freshly cleaved clean graphite using the XPS system described in Ref. 18 measured in HV conditions. No water is present within the detection limits of our XPS system, which is 0.1 atomic percent. Should water be present, a peak at a binding energy of 534 eV would be visible.Reuse & Permissions

Figure 2

Experimental results of step friction on HOPG. Lateral force line profiles acquired on (a) clean surfaces (HV conditions, normal $\text{force}=0.98$ nN) and (b) with water present (N${}_2$ conditions without pumping, normal $\text{force}=0.94$ nN). Red (black) data always indicate step-up (step-down) measurements. The load dependence of (c) the peak lateral force and (d) friction recorded at the step edge measured on both the clean surface and with water present using the same tip. Only unloading data is presented in (c) and (d). Error bars in (d) represent the $95\%$ confidence interval. (e) TEM image of the tip used to obtain data shown in (a)–(d). (f) A higher magnification image of the tip apex, also identifying the contacting asperity. The fit of the end asperity is shown by the dashed red line. Lateral force image (forward) showing lattice resolution of the graphite surface at a single step on (g) the clean surface and (h) with water present. Note the color scale in (g) and (h) indicates that lateral forces are more positive (negative) in brighter (darker) regions. Black lines in (h) and (g) indicate regions where lateral force line profiles are shown in (i) and (j), respectively. A TEM image of the contacting asperity of the tip used to acquire (h) is shown in Fig. 6b.Reuse & Permissions

Figure 3

(a) Friction force vs normal force data from Fig. 1d showing the overlap of loading (solid and bottom-half solid data points) and unloading (open and top-half solid data points) curves. Note that the friction measured on the terrace with water present (dry nitrogen: green stars) and on the clean surface (HV: green squares) have the same values. However, the friction observed at the step edge with water (nitrogen: blue stars) is slightly higher than on the clean surface (HV: blue squares). Fits of the unloading data on the clean surface (blue dotted line) and when water was present (blue dashed line) are $0.018\pm 0.002$ and $0.04\pm 0.01$, respectively. The fit determined on the surface with water was present was determined without the data point near 0 nN. (b) Force vs sample $z$-displacement curves taken with water present (nitrogen: red and black) and on the clean surface (HV: green and blue) show that the pull-off force has changed from $-0.8$ to $-3.0$ nN.Reuse & Permissions

Figure 4

Results from a repeated experiment with a different tip having a larger contacting asperity. The load dependence of (a) the peak lateral force and (b) friction force recorded at the step edge measured on both the clean (HV) surface and with water present (dry nitrogen conditions) with the same tip. Solid data points were taken during loading and open data points were taken during unloading. Red (black) friction data always indicate step-up (step-down) measurements. (d) TEM image of the tip apex showing that the tip has a $29\pm 1$ nm radius. The plane at which the surface contacts the tip asperity is marked by a dashed line.Reuse & Permissions

Figure 5

MD simulations of sliding on HOPG at steps. Lateral force line profiles acquired on (a) a clean surface (normal $\text{force}=0$ nN) and (b) with two layers of water molecules in the contact zone (normal $\text{force}=0$ nN). Red (black) data always indicate step-up (step-down) measurements. The load dependence of (c) peak lateral force and (d) friction measured on both the clean surface and with two layers of water molecules in the contact zone. Illustrations of the 2 nm radius tip sliding over the graphite substrate on (e) a clean surface and (f) with water molecules in the contact zone. (a), (b), (e), and (f) are from simulations conducted at 300 K. The load dependence results in (c) and (d) were performed at 10 K.Reuse & Permissions

Figure 6

Measurements of friction made with water present (in dry nitrogen conditions) for two different tips. The load dependence of the peak lateral forces and friction forces are shown in (a) for tip 1 and (c) for tip 2. Red (black) data always indicate step-up (step-down) measurements. Only unloading data is presented in (a) and (c). Blue (green) data always indicate friction measurements on steps (terraces). A TEM image of tip 1 is shown in (b), which has a radius of $5\pm 3$ nm. The plane the surface contacts the asperity of the tip is marked by a dashed line. Tip 1 was used to acquire the data shown in Fig. 2h.Reuse & Permissions

Figure 7

Measurements of friction made on the clean surface (under HV conditions) for two different tips. The load dependence of the peak lateral forces and friction forces are shown in (a) for tip 1 and (c) for tip 2. Red (black) data always indicate step-up (step-down) measurements. Only unloading data is presented (a) and (c). Blue (green) data always indicate friction measurements on steps (terraces). TEM images of tips 1 and 2 are given in (b) and (d), respectively. Tip 1 has a radius of $38\pm 5$ nm and tip 2 has a radius of $26.2\pm 0.5$ nm.Reuse & Permissions