Manipulation of Nanotubes on Surfaces

The interaction between nanotubes and their underlying substrate is sensitive to the details of the particular surface. In order for an AFM tip to controllably manipulate nanotubes across a surface, it is crucial that the tip/nanotube interaction be greater than the adhesion/friction force between the nanotube and the substrate. Depending on the experimental goals, one might, for example, require surfaces with high friction, in order to pin and manipulate the nanotubes into desired configurations. However, if one wants to measure certain physical properties such as stress and strain, a low frictional surface is necessary to decouple the surface contributions from the intrinsic mechanical properties of the nanotube.

Hertel et al. [59] investigated the interaction between a H-passivated Si (100) surface and deposited MWNTs using non-contact mode AFM. For example, the AFM image of nanotubes in Fig. 13 illustrates one type of elastic deformation that may occur when nanotubes are deposited onto a surface. At the crossing point of the two overlapping MWNTs, the measured height is six nanometers less than the sum of the individual diameters. In addition, the width of the upper nanotube shortly before and after crossing the lower

Fig. 13. AFM non-contact mode image of several overlapping MWNTs. The upper tubes are seen to wrap around the lower ones which are slightly compressed [59]

tube appears broadened. This observed axial and radial deformation of the tubes may be explained by a strong surface/nanotube interaction. As the upper nanotube is forced to bend over the lower nanotube, the strain energy increases; however, this is compensated for by a gain in binding energy between the surface and the lower nanotube, which attempts to maximize its contact area with the substrate. The strength of the attractive force between the nanotubes and the surface may be estimated using a 1D model where the profile along the tube axis is determined by the balance of strain and adhesion energy. In this framework, Hertel and co-workers [59] found that the binding energy is determined primarily by van der Waals interactions and approaches up to 0.8 ± 0.3eV/Afor multi-wall nanotubes 10 nm in diameter on hydrogen-terminated silicon.

An important consequence of this large binding energy is that nanotubes will tend to distort and conform to the substrate topography. In addition, nanotubes can be pinned in highly strained configurations on different substrates after manipulation with an AFM tip [5,59,60]. Wong et al. [5] and Falvo et al. [60] exploited high substrate/nanotube friction in order to apply lateral stresses at specified locations along a MWNT to produce translations and bends (Fig. 14).

Fig. 14. AFM tapping-mode images (0.9|i.m x 0.9|i.m) of a 4.4 nm-diameter MWNT before and after bending on an oxidized Si substrate. After bending, buckling occurs along the nanotube axis [5]

When bent to large angles, the nanotubes exhibited raised features, which correspond to locations along the nanotube where the tube has buckled. This buckling behavior was shown to be reversible in both experimental studies. The motion of nanotubes in contact with a surface can occur via rolling, sliding or a combination of these processes. The details of this behavior are expected to depend sensitively on the substrate as well as where the AFM tip contacts the nanotube to induce motion. Falvo et al. [61] found that if the AFM tip pushes the MWNTs from the end on a mica or graphite surface, a single stick-slip peak in the lateral force trace is observed. These peaks are attributed to the pinning force between the nanotube and the substrate that must be overcome before motion can proceed. However, when the nanotube is manipulated from its side on mica, the resulting motion is an in-plane rotation of the nanotube about a pivot point dependent upon the position of the AFM tip. This sliding behavior is illustrated in Fig. 15. However, if the

Fig. 15. Sliding a multi-walled carbon nanotube. (a) Tube in its original position. Grid lines are overlaid so that one of the grid axes corresponds to the original orientation of the tube axis. (b) Tube's orientation after AFM manipulation. The pivot point and push point are indicated by the bottom and top arrows, respectively. Inset shows the lateral force trace during a sliding manipulation [61]

Fig. 15. Sliding a multi-walled carbon nanotube. (a) Tube in its original position. Grid lines are overlaid so that one of the grid axes corresponds to the original orientation of the tube axis. (b) Tube's orientation after AFM manipulation. The pivot point and push point are indicated by the bottom and top arrows, respectively. Inset shows the lateral force trace during a sliding manipulation [61]

nanotube was pushed from the side on a graphite surface, new behavior was observed: a lateral stick-slip motion with the absence of in-plane rotation. Hence the nanotube appeared to undergo a stick-slip rolling motion, which was topographically verified due to the asymmetrically shaped nanotube cap. Upon comparison of the lateral force measurements for rolling and sliding, Falvo et al. [61] discovered that the slip-stick peaks in rolling are higher than the lateral force needed to sustain sliding, although quantitative force values were not reported. Thus unlike macroscopic systems where rolling is preferred over sliding, the energy cost for rolling in nanoscale systems is larger than that of the sliding cases.

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