Initial Stages of the Contact between a Metallic Tip and Carbon Nanotubes

We present an experimental and theoretical study of the interaction between a tip and single-walled carbon nanotubes (SWCNT) lying on a ${\mathrm{SiO}}_2$ surface. Adhesion and jump-to-contact forces (JC) are measured in a high vacuum system for SWCNT with diameters ($D$) ranging from 1.4 to 4.5 nm; we find adhesion forces $\sim 3.2\mathrm{nN}$ and JC forces $\sim 2.4\mathrm{nN}$. Simulations yield adhesion forces in agreement with experiment showing that for $D\sim 1.4\mathrm{nm}$ half of this force is due to tip-SWCNT and $\mathrm{tip}\mathrm{\text{−}}{\mathrm{SiO}}_2$ van der Waals interactions.

Figure 1

(a) Force vs piezoelectric displacement curve taken in high vacuum. The origin of the horizontal axis is arbitrarily set at the tip-sample contact position. The thin red and thick black lines represent the approaching and withdrawing branches, respectively, as indicated by the arrows. Inset: scheme of the experimental setup. (b) Jump to contact (filled square dots) and adhesion force (open triangles) as a function of the nanotube diameter where the indentation experiment is carried out. The straight lines have been drawn just as a guide to the eyes. Inset: AFM topography image showing a SWCNT contacted to a gold electrode.

Figure 2

Force versus tip-nanotube distance ($Z$) for (8,8)-SWCNT. Black line (open squares): pushing the tip; red line (filled circles): retrieving the tip at point E. The tip-nanotube geometry for the points B, C, D, E is also shown. $Z$ is defined as the distance between the upper Au-tip layer and lower C layer of the nanotubes; see geometry for point B.

Figure 3

Contact forces (a), and electrical conductance (b), as a function of $Z$ for different $(n,n)$ nanotubes [$n=4$, 6, 8, 10]. In Figs. 3a we have added a second curve for the (10,10) case (open triangles) in which the vdW interaction between the tip and nanotube (but not the substrate) is also included. Inset: channel contributions to the conductance for a (10,10) CNT.

Figure 4

(a) Forces and (b) frequency shifts as a function of $Z$ (see Fig. 2) for a (10,10) nanotube. Blue (filled triangles): short-range tip-nanotube interaction; black (filled squares): total tip-nanotube interaction; red (open circles): total tip-nanotube plus tip-substrate interaction. The inset in (b) shows a larger $Z$ interval where the frequency shift is represented for the tip-nanotube system (black filled squares) and for the tip-nanotube-substrate system (red open circles). For the sake of comparison the experimental frequency shift (green line) has been also plotted.