Carbon-substituting in (4,4) boron nitride nanotube: Density functional study of boron-11 and nitrogen-14 electric field gradient tensors

Density functional theory (DFT) study is performed to investigate the influence of carbon substituting in a representative model of armchair boron nitride nanotubes (BNNTs). To this aim, the electric field gradient (EFG) tensors at the sites of B-11 and N-11 nuclei are calculated in two models of (4,4) single-walled BNNT. Model one (raw) consists of 36 B and 36 N atoms with 12 saturating H atoms of two mouths while 7 B and 7 N atoms are substituted by 14 C atoms like a wire in model two (C-substituted). The converted EFG tensors to measurable nuclear quadrupole resonance (NQR) parameters, quadrupole coupling constant (C-Q) and asymmetry parameter (eta(Q)), reveal that the C-Q values in the length of raw BNNT are divided into some layers with equal magnitude and among them the mouth layers have the largest C-Q magnitudes. In the C-substituted model, in addition to the mouth layers, the C-Q of those B and N nuclei directly bonded to C atoms are increased to the magnitudes as large as those mouth nuclei meaning that the active sites are increased in the C-substituted BNNT model. It is worth noting that the NQR parameters of other nuclei rather than those directly bonded to C and also those in the first neighborhood of C atoms are almost in equal values in the two models. Comparing the results with a recent study on zigzag BNNT (Mirzaei M et al., Z. Naturforsch A 62: 56, 2007) reveals that armchair and zigzag BNNTs show almost similar electronic properties. However, there is a significant difference in the electronic properties of those B and N atoms located at the mouth of the two BNNTs whose mouths are similar in armchair, whereas there are two different mouths (B-mouth and N-mouth) in zigzag BNNT.