Boron-nitride ad-unit and carbon ad-dimer defects in the boron nitride nanotubes

• BNU defect formation of BNNTs with higher curvature is more favorable.
• The replacing of BN with carbon in defect sites leads to favorable configurations.
• Defect formation energies of CD defective BNNTs are all negative.
• C doping of defective sites in BNU (n, n) is exothermic, independent of tube diameter.
• Semiconducting properties of BNNTs are preserved after BNU and CD defect formation.

We have applied density functional calculations to investigate carbon ad-dimer (CD) and boron-nitride ad-unit (BNU) defect formation in armchair (n, n), n=4, 5, and 6, as well as zigzag (7, 0) BN nanotubes. According to our results, tube diameter, orientations of defects on the sidewall of BN nanotubes and the pyramidalization angles affect the BN ad-unit defect formation, such that the defect formation in the (4, 4) armchair BNNTs with the highest curvature is the most favorable while the defect in the (7, 0) BNNT which have similar diameter with the (4, 4) armchair BNNT cost larger formation energy because of smaller pyramidalization angles. Replacing of BN units with carbon dimers in defect sites leads to the removing frustrated B–B and N–N bonds, and an energetically favorable configuration for defective BNNTs. Then, the defect formation energies of CD defective BNNTs are all negative values, and also the carbon doping of defective sites of BNU defective BNNTs are exothermic and almost independent of the tube diameter. BNU and CD defects reduce the HOMO–LUMO gaps of BNNTs while electronic structures of host materials does not obviously change and only local energy levels appears in the DOS plots.

BN ad-unit defective BNNTs and carbon doped BN ad-unit defective BNNTs.Figure optionsDownload as PowerPoint slide