Incorporation of topological defects and atomic impurities on the carbon nanotube surface: A DFT study of AD-dimer defects

Abstract

We applied density functional calculations to investigate the 7-5-5-7 defects obtained by the addition of an X₂ dimer (X = B, Al, Ga, C, Si, Ge, N, P, As, O, S, and Se) to the zigzag (6, 0), (7, 0), and (8, 0) and armchair (4, 4), (5, 5), (6, 6) single-walled carbon nanotubes (SWCNTs). Two different orientations for X₂ ad-dimer defects were considered. According to our results, defect formation energies depend strongly on the nanotube diameter and the orientation of the defect. Moreover, it was found that defect formation energies of the X₂ ad-dimer-defective SWCNTs depend upon the nature of X–X, being in the series of B/C/N/O (first row atoms) < Ga/Ge/As/Se (third-row atoms) < Al/Si/P/S (second-row atoms). X₂ ad-dimer defects reduce the HOMO-LUMO gaps (E_g) of the zigzag (n, 0) SWCNTs by only about 1.06%-8.53%, while decrease the E_g of the armchair (n, n) SWCNTs by about 10.03%-42.29%, in comparison with those of perfect SWCNTs. Our results indicated that tube diameter has a very slight effect on the adsorption energy of two hydrogen atoms on the exterior sidewalls of XAD-defective SWCNTs. Moreover, the adsorption of hydrogen atoms on the XAD-defective armchair SWCNTs is stronger than on the XAD-defective zigzag ones, depending strongly on the type of ad-dimer dopants. A decreasing trend is observed for E_r/H values as the absolute values of the natural charge of the X atoms increases.