Adhesion and friction of a multiwalled carbon nanotube sliding against single-walled carbon nanotube

The adhesion and friction at crossed nanotube junctions were investigated in ambient by using an atomic force microscope (AFM) in tapping mode. A multiwalled carbon nanotube (MWNT) tip attached to a conventional AFM probe was scanned across a single-walled carbon nanotube (SWNT) suspended over a $2\text{−}\mu \mathrm{m}$-wide trench. The interaction between nanotubes was found to critically depend on the morphology of the MWNT tip, which was determined from force-distance curves and scans performed against hard trench surface. The interaction between nanotubes caused the attenuation of vibrational amplitude of AFM cantilever, from which the frictional force between nanotubes was obtained by analyzing the dissipated vibrational power of cantilever. The adhesive force between nanotubes was measured from the vertical cantilever deflection when the MWNT tip end detached from the shell of the SWNT. From the friction and adhesion data, an experimental value of coefficient of friction of $0.006\pm 0.003$ is obtained for the sliding between nanotubes, which is comparable to the reported value of graphite on nanoscale. The shear strength between nanotubes is derived to be $4\pm 1\mathrm{MPa}$ by using a continuum model, which is in accordance with the value of $2\mathrm{MPa}$ reported for the sliding of MWNT on graphite in ambient. It is nearly 2 orders of magnitude larger than the interlayer shear strength of $0.05\mathrm{MPa}$ reported for MWNT in vacuum. We attribute the difference between the intertube and the interlayer frictions to the presence of water at the nanotube-nanotube interface in ambient.

Figure 1

SEM image of single-walled carbon nanotubes suspended on the top of a microtrench made of polycrystalline silicon.

Figure 2

Schematic of the experimental setup for tribological measurements between individual nanotubes. The double arrow indicates the fast scanning direction ($x$ direction).

Figure 3

(Color online) (a) Cantilever tapping amplitude and (b) vertical deflection signals (representative of at least $10\text{cycles}$ reproducible within 5%) as a function of distance, and (c) SEM/schematic images of the multiwalled carbon nanotube tip during the first cycle, during the test, and after $\sim 500\text{cycles}$. The SEM image for the tip during the test was not taken; instead, a tip schematic is drawn. At the tip end, some shells of the MWNT were stripped off (refer to the text for details), leaving a thin tube with smaller diameter extended from a thick base.

Figure 4

(Color online) From left to right, the images correspond to (a) cantilever tapping amplitude signals and (b) cantilever vertical deflection recorded during the trace and retrace scans and their difference. The deflection was captured on a separate scan immediately after the capture of amplitude. Profiles are depicted below each image for the scan lines marked by the arrows. The scan lines marked by the upper arrows are on the tube support layer. The scan lines marked by the lower arrows are through the suspended SWNT. The scan size is $4\mu \mathrm{m}$ and the tip end is about $100\mathrm{nm}$ lower than the tube support layer. The position of the scan area relative to the trench is depicted in the top schematic.

Figure 5

(Color online) Cantilever tapping amplitude and vertical deflection signals extracted from the scan lines through the suspended SWNT (a) and on the tube support layer (b) from Fig. 4. The MWNT tip contact geometries and forces corresponding to three points for SWNT ($A_N$, $B_N$, and $C_N$) and tube support layer ($A_T$, $B_T$, and $C_T$) are illustrated to the right. The tapping amplitude signal measured at point $A_N$ for each scan lines is used to calculate the frictional force between nanotubes while the vertical deflection signal measured at point $C_N$ is used for the calculation of the adhesion force between nanotubes.

Figure 6

Cantilever tapping amplitude (trace-retrace) calculated from the captured images with the tip end at the same height as the tube support layer. The profile is given for the scan line marked by the arrow.