Defect-Controlled Transport Properties of Metallic Atoms along Carbon Nanotube Surfaces

The diffusion mechanism of indium atoms along multiwalled carbon nanotubes is studied by means of photoemission spectromicroscopy and density functional theory calculations. The unusually high activation temperature for diffusion ($\approx 700\mathrm{K}$), the complex C $1s$ and In $3d_{5/2}$ spectra, and the calculated adsorption energies and diffusion barriers suggest that the indium transport is controlled by the concentration of defects in the C network and proceeds via hopping of indium adatoms between C vacancies.

Figure 1

C $1s$ (a) and In $3d$ images (b) from the cross section of multiwalled CNT array taken at room temperature. The vacuum side (on the left) and the substrate Si (on the right) are distinctly darker in the C $1s$ map. Labels 1–3 indicate the points used for spectroscopic analysis (see Fig. 2).

Figure 2

In $3d_{5/2}$ spectra acquired at the set of points indicated in Fig. 1 as a function of temperature (markers). The components, $A$, $B$, and $C$, providing the best fit to the spectra (thick black line) are displayed by thin lines. Their energy positions relative to the binding energy the metallic line ($M$) of the metal islands, denoted as 0, are indicated by arrows. The energy shifts are interpreted in terms of the bonding configurations shown in Fig. 3.

Figure 3

Calculated In-$3d$ core-level shifts ($\Delta$, with respect to metallic BE) and adsorption energies ($E_{\mathrm{ads}}$) for the most stable configurations of an In adatom, displayed in yellow (gray), on pristine and defective graphene (in black). The labels $A$, $B$, and $C$ refer to the components shown in Fig. 2.

Figure 4

C $1s$ peak acquired from CNTs, ion bombarded HOPG, and HOPG.