Strain energy and electronic structures of silicon carbide nanotubes: Density functional calculations

We perform density functional calculations for the geometrics, strain energy, and electronic structures of silicon carbide nanotubes ($\mathrm{SiCNT}$’s). We find that the strain energy in $\mathrm{SiCNT}$’s is as higher as $0.686\mathrm{eV}∕\mathrm{atom}$ relative to $3\mathrm{C}\text{−}\mathrm{SiC}$ for (5,5) $\mathrm{SiCNT}$ and decreases with increasing tube diameter. All the $\mathrm{SiCNT}$’s are semiconductors, the band gap of which increases with increasing tube diameter. In contrast to $3\mathrm{C}\text{−}\mathrm{SiC}$, zigzag $\mathrm{SiCNT}$ has a direct band gap at the $\Gamma$ point, whereas armchair and chiral tubes have an indirect band gap. The highest occupied valance band and the lowest unoccupied conduction band highly localize to $\mathrm{C}$ and $\mathrm{Si}$ atoms, respectively. Hydrogen-decorated $\mathrm{SiCNT}$’s display the characters of $p$- or $n$-type semiconductors depending on the adsorbing site.

Figure 1

Optimized supercells of (a) (8,0), (b) (5,5), and (9,3) $\mathrm{SiCNT}$’s. Gray and black balls represent silicon and carbon atoms, respectively. The diameters of these tubes are $8.01\mathrm{\AA }$ for (8,0), $8.64\mathrm{\AA }$ for (5,5), and $10.77\mathrm{\AA }$ for (9,3) $\mathrm{SiCNT}$’s.

Figure 2

Variation of strain energy $\left(E_s\right)$ in $\mathrm{SiCNT}$’s relative to $3\mathrm{C}\text{−}\mathrm{SiC}$ with tube diameter (D). The solid and dotted lines are the fit to $E_s=E_0+\alpha ∕D^2$ (see text).

Figure 3

Band structures along the tube axis of (a) (8,0), (b) (5,5), (c) (16,0), and (d) (9,3) $\mathrm{SiCNT}$’s. The Fermi levels $\left(E_F\right)$ of these tubes are denoted by the dashed lines in the figure.

Figure 4

Total valance charge density contours in the tangent planes of (a) (5,5) and (b) (8,0) $\mathrm{SiCNT}$’s. The solid circles indicate the positions of $\mathrm{Si}$ atoms.

Figure 5

Dependence of band gap of armchair (open uptriangles), zigzag (solid uptriangles), and chiral (open square and star) $\mathrm{SiCNT}$’s as a function of the tube diameter.

Figure 6

PDOS of (5,5) $\mathrm{SiCNT}$ projected to $\mathrm{Si}$ (solid line) and $\mathrm{C}$ (dotted line), respectively. The energy at Fermi level is denoted by the dashed arrow in this figure.

Figure 7

Optimized supercells and DOS of $\mathrm{H}$-decorated (5,5) $\mathrm{SiCNT}$. The $\mathrm{H}$ atom chemically adsorbs above a $\mathrm{C}$ [(a) and (b)] and $\mathrm{Si}$ [(c) and (d)] on the tube wall. The Fermi levels $\left(E_F\right)$ are indicated by the dashed lines. The local maps near the adsorbing sites are also presented as the insets of this figure.