Ab initio theoretical study of hydrogen and its interaction with boron acceptors and nitrogen donors in single-wall silicon carbide nanotubes

Silicon carbide nanotubes have a great potential for biological applications. It is of interest to explore the electronic properties of these nanotubes, and how those are modified in the presence of impurities. Hydrogen is a common impurity that can appear during the growth of silicon carbide nanotubes or in the environment. In this paper we studied the properties of one and two hydrogen atoms in armchair and zigzag silicon carbide nanotubes by ab initio supercell calculations beyond the standard density functional theory. We found that a single hydrogen atom is an amphoteric defect: it can act as a donor as well as an acceptor, depending on the adsorption site. However, both sites are nearly equally stable; therefore these defects compensate each other in SiC nanotube semiconductors. We found that hydrogen can absorb onto SiC nanotubes when atomic hydrogen is present in the environment. In addition, we investigated the incorporation of hydrogen by applying BH and NH molecules in the environment of SiC nanotubes. The simulations predict that boron dissolves into SiC nanotubes together with hydrogen; thus boron can be used to raise the concentration of hydrogen in SiC nanotubes. Nitrogen is also incorporated with hydrogen. We predict that the shallow boron acceptor and the nitrogen donor may be activated in these processes.

Figure 1

(Color online) The optimized geometry of ${\mathrm{H}}_i$ bound to a carbon atom in (a) the outer and (b) the inner configuration and to a silicon atom in (c) the outer and (d) the inner configuration in an (8,0) single-wall SiC nanotube. The large yellow (smaller cyan) balls are the silicon (carbon) atoms, while the smallest ball represents the hydrogen atom. The localization of the highest occupied level is also shown. The red (blue) transparent lobes depict the positive (negative) value of the wave function.

Figure 2

Calculated hybrid-Hamiltonian band structure of (a) perfect (8,0) SiC nanotube supercell, (b) hydrogen bound to carbon atom (C-H defect), and (c) hydrogen bound to silicon atom (Si-H defect) in the outer configurations in (8,0) SiC nanotube 128-atom supercell close to the band gap. The solid and dotted lines represent the spin-up and spin-down bands in the defective supercells, respectively. The spin-up donor level in the band gap is occupied by one electron for the C-H defect. The unoccupied spin-down acceptor level falls in the gap for the Si-H defect.

Figure 3

(Color online) Optimized geometry of the closest pair of hydrogen atoms in (8,0) SiC nanotubes: (a) H-C-Si-H, (b) $\mathrm{H}\left(u\right)\text{−}\mathrm{C}\text{−}\mathrm{Si}\text{−}\mathrm{H}\left(d\right)$, (c) $\mathrm{H}\left(d\right)\text{−}\mathrm{C}\text{−}\mathrm{Si}\text{−}\mathrm{H}\left(u\right)$, (d) H-C-Si-C-H, and (e) H-Si-C-Si-H defect. The large yellow (smaller cyan) balls are the silicon (carbon) atoms, while the smallest ball represents the hydrogen atom. The localization of the highest occupied level is also shown. The red (blue) transparent lobes depict the positive (negative) value of the wave function.

Figure 4

(Color online) The optimized geometry of ${\mathrm{H}}_i$ bound to carbon atom in (a) the outer and (b) the inner configuration and silicon atom in (c) the outer and (d) the inner configuration in a (6,6) single-wall SiC nanotube. The large yellow (smaller cyan) balls are the silicon (carbon) atoms, while the smallest ball represents the hydrogen atom. The localization of the highest occupied level is also shown. The red (blue) transparent lobes depict the positive (negative) value of the wave function.

Figure 5

The calculated hybrid-Hamiltonian band structure of (a) perfect (6,6) SiC nanotube supercell, (b) hydrogen bound to carbon atom (C-H defect) in the outer configuration, (c) C-H defect in the inner configuration, and (d) hydrogen bound to silicon atom (Si-H defect) in the outer configuration in (6,6) SiC nanotube 96-atom supercell close to the band gap. The solid and dotted lines represent the spin-up and spin-down bands in defective supercells, respectively. The spin-up donor level in the band gap is occupied by one electron for C-H defects. The unoccupied spin-down acceptor level falls in the gap for Si-H defects.

Figure 6

(Color online) Optimized geometry of the closest pair of hydrogen atoms in (6,6) SiC nanotubes: (a) H-C-Si-H, (b) $\mathrm{H}\left(u\right)\text{−}\mathrm{C}\text{−}\mathrm{Si}\text{−}\mathrm{H}\left(d\right)$, (c) $\mathrm{H}\left(d\right)\text{−}\mathrm{C}\text{−}\mathrm{Si}\text{−}\mathrm{H}\left(u\right)$, (d) H-C-Si-C-H, and (e) H-Si-C-Si-H defect. The large yellow (smaller cyan) balls are the silicon (carbon) atoms, while the smallest ball represents the hydrogen atom. The localization of the highest occupied level is also shown. The red (blue) transparent lobes depict the positive (negative) value of the wave function.

Figure 7

(Color online) The calculated formation enthalpy of (a) C-H@(8,0), (b) Si-H@(8,0), (c) C-H@(6,6), and (d) Si-H@(6,6) defects in the outer configurations. The five lines from bottom to top are associated with a partial pressure of the appropriate species of 1, 0.1, 0.01, 0.001, and $0.0001\mathrm{atm}$ in red, magenta, violet, blue, and green colors, respectively. In (e), the temperature and pressure dependency of the chemical potential of single hydrogen atom are shown. The top five straight lines correspond to the ${\mathrm{H}}_2$ source while the bottom five dotted lines correspond to the atomic H source. The five lines show the chemical potential at different values of the partial pressure depicted on the lines in atm and also colored as explained above. The zero value of the chemical potential is set to the total energy of the species in vacuum without the zero-point energy.

Figure 8

(Color online) The calculated formation enthalpy of (a) H-C-Si-H@(8,0) and (b) H-C-Si-H@(6,6) defects assuming ${\mathrm{H}}_2$ gas in the environment and thermal equilibrium. The five lines from bottom to top are associated with a partial pressure of ${\mathrm{H}}_2$ gas of 1, 0.1, 0.01, 0.001, and $0.0001\mathrm{atm}$ in red, magenta, violet, blue, and green colors, respectively.

Figure 9

(Color online) The optimized geometry of boron and hydrogen complexes: large yellow (smaller cyan) balls are the silicon (carbon) atoms. Boron (smaller green ball) and hydrogen (small white ball) atoms are labeled. (a) ${\mathrm{B}}_{\mathrm{C}}\mathrm{H}$ complex and the calculated total charge density (ice-blue color). (b) ${\mathrm{B}}_{\mathrm{Si}}\mathrm{H}$ complex and the calculated total charge density (ice-blue color). (c) ${\mathrm{B}}_{\mathrm{Si}}\mathrm{H}$ complex and the calculated localization of the highest occupied level. The red (blue) transparent lobes depict the positive (negative) value of the wave function.

Figure 10

(Color online) The optimized geometry of nitrogen and hydrogen complexes: large yellow (smaller cyan) balls are the silicon (carbon) atoms. Nitrogen (small deep blue ball) and hydrogen (smallest white ball) atoms are labeled. (a) Metastable ${\mathrm{N}}_{\mathrm{C}}\mathrm{H}$ complex and the calculated localization of the highest occupied defect level. (b) ${\mathrm{N}}_{\mathrm{C}}\mathrm{H}$ complex and the calculated total charge density (ice-blue color). (c) ${\mathrm{N}}_{\mathrm{C}}\mathrm{H}$ complex and the calculated localization of the highest occupied level. The red (blue) transparent lobes depict the positive (negative) value of the wave function.