First-Principles Calculations of Carbon Nanotubes Adsorbed on Si(001)

We report first-principles calculations on the adsorption of a metallic (6,6) single-walled carbon nanotube (SWCN) on the Si(001) surface. We find stable geometries for the nanotube between two consecutive dimer rows where C-Si chemical bonds are formed. The binding energy in the most stable geometry is found to be $0.2\mathrm{e}\mathrm{V}/\AA$. Concerning the electronic properties, the most stable structure shows an increase in the density of states near the Fermi level due to the formation of C-Si bonds enhancing the metallic character of the nanotube by the contact with the surface. These properties may lead one to consider metallic SWCNs adsorbed on Si substrates for interconnections and contacts on future nanoscale devices. Finally, the nature of the nanotube-surface interaction for nanotubes of larger diameters is also discussed.

Figure 1

Equilibrium geometry for the adsorbed CNT on the Si(001) surface for the most stable structure (T0 geometry). The distances are in $\AA$ and the charge transfers from the CNT (positive) to the surface (negative) are indicated in units of electrons per unit cell. Small spheres are C atoms and the larger spheres are Si atoms.

Figure 2

Electronic band structure for the CNT and the Si(001) surface along directions parallel ($\Gamma –J^{\prime }$) and perpendicular ($\Gamma –J$) to the dimer trench or the tube axis. (a) The isolated CNT. (b) The Si surface plus the CNT at $5\AA$ from the dimers. (c) The adsorbed CNT on the most stable structure (T0 geometry). The DOS along $\Gamma –J^{\prime }$ is also shown. The zero of energy is taken as the Fermi level, indicated by dashed lines.

Figure 3

Charge density contour plots for the adsorbed nanotube on Si(001) in the T0 geometry on a plane passes through a C-Si bond. (a) Total charge density. (b) Charge density of the subbands that cross the Fermi level. The contour spacings are ${10}^{-2}$ and $4\times {10}^{-5}e/{\AA }^3$, respectively.

Figure 4

Schematic representation of the contact region for the CNT in the T0 geometry. The arc represents the contact region for a $100\AA$ diameter CNT at the same configuration. The shaded area is our estimation for the region where C-Si bonds are formed for the larger CNT, using the same distance $h$ calculated for the small CNT.