arXiv:1112.6319v1 [cond-mat.mes-hall] 23 Dec 2011 First-principles study of substitutional carbon pair and Stone-Wales defect complexes in boron nitride nanotubes Gunn Kim, 1 Jinwoo Park, 1 and Suklyun Hong 1 1 Department of Physics and Graphene Research Institute, Sejong University, Seoul 143-747, Korea (Dated: October 25, 2018) Using density functional theory, we study physical properties of boron nitride nanotubes (BNNTs) with the substitutional carbon pair defect. We also consider the Stone-Wales (SW) rearrangement of the C-C pair defect in the BNNT. The formation energy of an SW defect of the carbon dimer is approximately 3.1 eV lower than that of the SW-transformed B-N pair in the undoped BNNT. The activation energies show that the SW defect in the C-doped BNNT may be experimentally observed with a higher probability than in the undoped BNNT. Finally, we discuss the localized states originating from the carbon pair impurities. As a wide-band-gap nanomaterial[1–3], hexagonal boron nitride (h-BN) is a stable crystalline form con- sisting of equal numbers of boron and nitrogen atoms in a honeycomb arrangement[4–6]. As it has considerably high thermal and chemical stabilities[7], it can be used for high-temperature applications. The BN nanotube (BNNT)[8, 9] is a nano-sized seamless cylinder that can be considered a rolled-up h-BN sheet. It also has a wide band gap that is almost independent of the tube diame- ter, and chirality[10]. For sp 2 -bonded carbon nanostruc- tures such as fullerenes, carbon nanotubes and graphene, the Stone-Wales (SW) transformation[11] is believed to introduce topological defects or isomerization, which re- sults in a 90 ◦ rotation of two carbon atoms with respect to the midpoint of the C-C bond. When an SW defect is present in h-BN or BNNT, B- B and N-N bonds should be created, and the formation of these bonds increases the total energy of the system. Thus, the topological defect created by the SW rear- rangement is lacking. However, the formation of B-B and N-N bond pairs can be avoided if there is a C-C pair defect in h-BN (or BNNT). Using post-synthesis dop- ing, carbon impurities can be substituted in the h-BN layers[12–14]. During the formation of BNNTs and h- BN sheets using the chemical vapor deposition (CVD), carbon atoms may act as substitutional defects, because residual hydrocarbon may remain in the chamber, or car- bon impurities are solubilized in the metal catalyst. If carbon substitutional atoms congregate together in a lo- cal area in h-BN (or a bundle of BNNTs) by segrega- tion, they may form the interlayer (or intertube) con- duction channels. As mentioned above, the C-C pair de- fect in the h-BN network prohibits the formation of B-B and N-N bonds during the SW transformation. As the SW-transformed defects may affect the mechanical and electrical properties of the BN nanostrucutres, we should thus study these defects in the C-doped BN sheets or BN- NTs. This work may explain the formation of topological defects in the carbon-doped BNNTs or h-BN layers that can be produced in the CVD processes. In this paper, we present our first-principles study of the structural and electronic properties of the BNNTs containing two sub- stitutional carbon defects. The effect of carbon doping on the SW transformation is also examined. We carried out total energy calculations for our model systems based on the density functional theory[15]. The local density approximation (LDA)[16] with spin polar- ization was used for the exchange-correlation functional. Some calculations were repeated using the generalized gradient approximation. We found that the exchange- correlation functional did not change our main conclusion that the SW defect in the C-doped BNNT may be ob- FIG. 1: Optimized structures of (8,0) BNNTs with a C-C pair substitutional defect or the SW defects. (a) The BNNT with a C-C pair substitutional defect parallel to the tube axis, (b) the BNNT with a C-C pair substitutional defect with an oblique angle with respect to the tube axis. (c) and (d) show the SW-transformed C-C pair defect from (a) and (b), respectively. (e) and (f) are the BNNTs with SW defects which are parallel and tilted to the tube axis, respectively. Yellow, green and black balls represent nitrogen, boron and carbon atoms, respectively.