Effect of oxygen on single-wall silicon carbide nanotubes studied by first-principles calculations

According to the predictions of first-principles calculations, single-wall silicon carbide nanotubes exhibit several unusual properties: they are semiconducting independently of their chirality, superior material for hydrogen storage, and have strong nonlinear optical coefficients. Nevertheless, only a single experiment indicates, in our knowledge, that a tubular form of silicon carbide (SiC) exists. It is known that the surface of bulk silicon carbide is oxidized in the presence of oxygen; therefore, oxygen may destabilize its tubular form. We applied ab initio density-functional theory calculations to investigate this important issue. We found that (i) the structure of silicon carbide nanotubes remain intact in ambient oxygen, (ii) but the oxygen molecule dissociates as interstitials on silicon carbide nanotubes even at room temperature, and (iii) the interstitial oxygen is a fast diffuser on SiC nanotubes.

Figure 1

(Color online) The optimized geometry of the metastable on-axis (a) and stable off-axis (b) interstitial oxygen in (8,0) nanotube. The cyan (small), yellow (big), and red (smallest) balls represent the carbon, silicon, and oxygen atoms, respectively.

Figure 2

(Color online) The optimized geometry of substitutional oxygen at $C$ site in (a) (6,6) and (b) (8,0) nanotubes. The cyan (small), yellow (big), and red (smallest) balls represent the carbon, silicon, and oxygen atoms, respectively. The transparent red (dark gray) and green (light gray) surfaces represent the isosurfaces of the wave function of the donor level in the $\Gamma$ point with positive and negative values, respectively. The calculated LDA donor level is at $\text{VBM}+0.89\text{eV}$ in (6,6) nanotube and $\text{VBM}+0.69\text{eV}$ in (8,0) nanotube, where VBM is the calculated valence band edge.

Figure 3

(Color online) Calculated density of states (DOS) of the perfect (black line) and defective (dotted gray line) (6,6) tubes. A smooth Gaussian broadening of 0.2 eV was applied in the construction of DOS. The occupied defect band clearly shows up in the band gap below the calculated Fermi level $\left(E_{\text{F}}\right)$.

Figure 4

(Color online) The optimized geometry of substitutional oxygen at Si site in (6,6) (a) and (8,0) (b) nanotubes. The cyan (small), yellow (big), and red (smallest) balls represent the carbon, silicon, and oxygen atoms, respectively. The transparent red (dark gray) and green (light gray) surfaces represent the isosurfaces of the wave function of the donor level in the $\Gamma$ point with positive and negative values, respectively. The calculated LDA donor and acceptor levels are at $\text{VBM}+0.55\text{eV}$ and $\text{VBM}+1.06\text{eV}$ in (6,6) nanotube, while at $\text{VBM}+0.88\text{eV}$ and $\text{VBM}+1.19\text{eV}$ in (8,0) nanotube, where VBM is the calculated valence-band edge.

Figure 5

(Color online) Calculated DOS of the perfect (black line) and defective (dotted gray line) (6,6) tubes. A smooth Gaussian broadening of 0.2 eV was applied in the construction of DOS. The occupied and unoccupied defect bands clearly show up in the band gap below and above the calculated Fermi level $\left(E_{\text{F}}\right)$.

Figure 6

(Color online) The optimized geometry of the pair of interstitial oxygen in a) (6,6) and b) (8,0) nanotubes. The cyan (small), yellow (big), and red (smallest) balls represent the carbon, silicon, and oxygen atoms, respectively.

Figure 7

(Color online) Metastable configurations of oxygen molecule approaching SiC nanotube: (a) Si-O-O-Si bridge and (b) Si-O-O-C bridge. The cyan (small), yellow (big), and red (smallest) balls represent the carbon, silicon, and oxygen atoms, respectively.

Figure 8

(Color online) The stages of the diffusion path of interstitial oxygen. Left: the most stable off-axis configuration. Right: metastable on-axis configuration. It is less stable by around 0.3 eV than the most stable configuration. Middle: the transition state during the diffusion that is around 0.29 eV higher in energy than the metastable configuration. The cyan (small), yellow (big), and red (smallest) balls represent the carbon, silicon, and oxygen atoms, respectively.