Defect complexes in carbon and boron nitride nanotubes.

The effect of defect complexes on the stability, structural and electronic properties of single-walled carbon nanotubes and boron nitride nanotubes is investigated using the ab initio pseudopotential density functional method implemented in the Castep code. We found more substantial atomic relaxations in the zig-zag carbon nanotube than the armchair one. We find that the B(C)B(c) defect introduced in both zig-zag and armchair carbon nanotubes results in a semimetallic system. Similarly to the carbon nanotubes, the relaxation energies in the zig-zag boron nitride nanotubes are lower than in the armchair system. We find that creating a C(B)B(N) in the boron nitride nanotube, changes the system to metallic. The zig-zag configuration is energetically more stable than the armchair one in both the boron-rich and nitrogen-rich environments. The interaction between the carbon impurity and the antisite was investigated: we find that C(B)B(N) is preferable in the B-rich environment, and C(N)N(B) is preferable in the N-rich environment. We determine that in both zig-zag and armchair systems, B(N)N(B) is stable with the heats of formation of -5.77 eV and -8.69 eV, respectively.

Ab initio studies of vacancies in (8,0) and (8,8) Single-walled carbon and boron nitride nanotubes.

A systematic study of vacancies in single-walled carbon nanotubes and boron nitride nanotubes was carried out. First principles calculations within the framework of density functional theory using the CASTEP code are used to optimize fully the geometries of the systems. The generalized gradient approximation is used for the exchange-correlation functional. We find that the pristine single-walled carbon nanotubes have lower heats of formation compared with the boron nitride nanotubes, consistent with other findings. The zig-zag (8,0) carbon nnaotube has a slightly lower (-3.32 eV) heat of formation compared to the armchair (8,8) configuration (-3.25 eV). Comparison of the heats of formation of the vacancy systems is made and we draw conclusions about the relative stability of these defects. The heats of formation and atomic relaxations of the vacancies are explained as resulting from the tendency of the affected ions to recover the lost electronic coordination. For the boron nitride nanotube, we find that the vacancies on the nitrogen and boron site, namely V(N), and V(B), are respectively the more stable vacancies in the B- and N-rich environments. The electronic structure of the single vacancies also depends on the nanotube chirality.

Vacancy complexes in carbon and boron nitride nanotubes.

The effect of divacancies on the stability, structural and electronic properties of carbon and boron nitride nanotubes is studied using the ab initio density functional method. V(B)B(N) is more stable in the boron-rich and less stable in the nitrogen-rich growth conditions, and V(N)N(B) is more stable in the nitrogen-rich than in the boron-rich conditions. We find that stoichiometric defects V(B)V(N), V(B)C(N) and V(N)C(B) are stable in both the boron and nitrogen rich environments. The relaxation energy in the V(C)V(C) is lower in the armchair than in the zig-zag and the opposite trend is seen for V(C)B(C) and V(C)N(C). The divacancy is found to be particularly effective in changing the band gap of the semiconducting nanotubes due to the appearance of additional energy levels within the band gap region. For the zig-zag systems, we observe a drastic reduction of the band gap in V(B)B(N), V(N)N(B) and V(N)C(B) and a complete removal of the band gap in V(B)V(N) and V(B)C(N), negating the semiconducting behaviour of the nanotube.

First principles studies of extrinsic and intrinsic defects in boron nitride nanotubes.

Spin polarized density functional theory has been used to investigate the structural stability and electronic properties of extrinsic and intrinsic defects in boron nitride nanotubes. Carbon substitutional defects under nitrogen rich and boron-rich growth conditions have the lowest heats of formation compared to boron and nitrogen antisites. Creating a defect reduces the band gap of the nanotube in both armchair and zig-zag geometries. We show that the substitutional carbon atom affects the electronic properties of the nanotube in such a way that it transforms from insulator to a semiconductor or metal. Antisites are stable in the reverse atmosphere and have the main characteristic that among all defects they have the highest heats of formations in both the zig-zag and armchair nanotubes.