Structural and electronic properties of $4\mathrm{\AA }$ carbon nanotubes on $\mathrm{Si}\left(001\right)$ surfaces

The structural and electronic properties of a single-walled $4\mathrm{\AA }$ (3,3) carbon nanotube (CNT) adsorbed on the $\mathrm{Si}\left(001\right)$ surface are studied using a first-principles method. It is found that for CNTs parallel to the dimer rows, the surface trench is the most stable adsorption site, while CNTs perpendicular to the dimer rows prefer to be adsorbed between Si surface dimers. The stable structures are characterized by four $\mathrm{C}\text{−}\mathrm{Si}$ covalent bonds and large binding energies, $0.38–0.39\mathrm{eV}$ per unit length of the CNT. The electronic properties of the adsorbed structures are sensitive to the CNT adsorption sites. The structure is semiconducting when the adsorbed CNT is at the trench site, while it is metallic when the CNT is adsorbed between Si dimers. The metal-semiconductor transition of the CNT at the surface trench site is due to the formation of $\mathrm{C}\text{−}\mathrm{Si}$ covalent bonds. The adsorption of a $3\mathrm{\AA }$ (2,2) CNT inside a (6,6) CNT on $\mathrm{Si}\left(001\right)$ surfaces is also discussed.

Figure 1

(Color online) (a) The side and top views of the optimized atomic structure, (b) the band structure, and (c) the density of states of the (3,3) CNT adsorbed on the $\mathrm{Si}\left(001\right)$ surface at site $A$ with $T_{\Vert }$ geometry. $J^{\prime }$-$\Gamma$ $\left(\Gamma \text{−}J\right)$ is parallel (perpendicular) to the tube axis. The Fermi level is indicated by the dashed lines. Inset: The top view of the atomic structure of the isolated (3,3) CNT (not drawn to the same scale as the adsorbed structure).

Figure 2

The band structures for (a) the isolated (3,3) CNT, (b) the deformed (3,3) CNT, calculated with the deformed geometry as it is adsorbed at site $A$ with $T_{\Vert }$ geometry on the Si surface but with the Si and H atoms removed, and (c) the deformed (3,3) CNT as in (b) but with four H atoms attached to the four C atoms participating in the formation of $\mathrm{C}\text{−}\mathrm{Si}$ bonds. The Fermi levels are indicated by the dashed lines.

Figure 3

(Color online) Contour plots of the ELF on a plane passing through two $\mathrm{C}\text{−}\mathrm{Si}$ bonds for the (3,3) CNT adsorbed on the $\mathrm{Si}\left(001\right)$ surface (a) at site $A$ with $T_{\Vert }$ geometry, and (b) at site $C$ with $T_{\perp }$ geometry.

Figure 4

(Color online) (a) The side and top views of the optimized atomic structure, (b) the band structure, and (c) the density of states of the (3,3) CNT adsorbed on the $\mathrm{Si}\left(001\right)$ at site $C$ with $T_{\perp }$ geometry. $J$-$\Gamma$ $\left(\Gamma \text{−}{J}^{\prime }\right)$ is parallel (perpendicular) to the tube axis. The Fermi level is indicated by the dashed lines. Inset: The top view of the atomic structure of the isolated (3,3) CNT (not drawn to the same scale as the adsorbed structure).

Figure 5

The band structures for (a) the deformed (3,3) CNT, calculated with the deformed geometry as it is adsorbed at site $C$ with $T_{\perp }$ geometry on the Si surface but with the Si and H atoms removed, (b) the deformed (3,3) CNT as in (a), but with four H atoms attached to the four C atoms participating in the formation of $\mathrm{C}\text{−}\mathrm{Si}$ bonds, and (c) the $\mathrm{Si}\left(001\right)$ surface by removing the adsorbed CNT at site $C$ with $T_{\perp }$ geometry. The Fermi levels are indicated by the dashed lines.

Figure 6

(Color online) (a) The side view of the optimized atomic structure, (b) the band structure, and (c) the density of states of the double-walled (2,2)@(6,6) CNT adsorbed on the $\mathrm{Si}\left(001\right)$ surface at the most stable adsorption site $C$ with $T_{\perp }$ geometry. $J$-$\Gamma$ $\left(\Gamma \text{−}{J}^{\prime }\right)$ is parallel (perpendicular) to the tube axis. The Fermi level is indicated by the dashed lines.