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

Author’s Accepted Manuscript Boron-nitride ad-unit and carbon ad-dimer defects in the boron nitride nanotubes Maryam Anafcheh, Reza Ghafouri, Fatemeh Ektefa, Mansour Zahedi www.elsevier.com/locate/jpcs

PII: DOI: Reference:

S0022-3697(14)00290-X http://dx.doi.org/10.1016/j.jpcs.2014.11.017 PCS7435

To appear in: Journal of Physical and Chemistry of Solids Received date: 24 July 2014 Revised date: 12 November 2014 Accepted date: 26 November 2014 Cite this article as: Maryam Anafcheh, Reza Ghafouri, Fatemeh Ektefa and Mansour Zahedi, Boron-nitride ad-unit and carbon ad-dimer defects in the boron nitride nanotubes, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2014.11.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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

Maryam Anafcheh1,c, Reza Ghafouria, Fatemeh Ektefaa, Mansour Zahedib a

Department of Chemistry, Shahr-e-Ray Branch, Islamic Azad University, Tehran, Iran

b

Department of Chemistry, Faculty of Sciences, Shahid Beheshti University, Evin,

19839-63113, Tehran, Iran c

Department of Chemistry, Faculty of Science, Alzahra University, Vanak, 19835-389,

Tehran, Iran Abstract We have applied density functional calculations to investigate carbon ad-dimer (CD) and boronnitride 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 (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. 1

Tel. : +98 9352362887 E-mail address: [email protected]

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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. Keywords: BN nanotubes, defect formation energy, DFT. 1. Introduction The structures of single-walled nanotubes (SWNTs) only in their ideal form could be described as a perfect graphene sheet wrapped up into a cylinder. In the process of preparations or modifications, various types of defects, such as vacancies, pentagons, heptagons, dopants, antisites or topological defects, on single-walled nanotubes can be formed [1-6]. These defects can be generated in either nonequilibrium microscopic growth [48-51] of SWNTs or via certain external perturbations, even though the energy cost is higher because of the formation of energetically unfavorable structural features [7-10]. A very important topological defect in carbon nanotubes (CNTs) is pentagon-heptagon pair, Stone-Wales (SW) [4] and carbon ad-dimer (CD) defect [5], leading to the formation of a 5-7-75 and 7-5-5-7 ring patterns, respectively, see Fig. 1. The SW defect is produced on a perfect nanotube by rotating a traditional CC bond by 90° about its center while CD defect formed by adsorption of a carbon dimer introduces two adjacent pentagons between two heptagons, i.e., the 7-5-5-7 arrangement, see Fig. 1. Various aspects of the Stone–Wales rearrangement in the CNTs such as identification by electron spin resonance and/or transmission electron microscopy TEM [11], defect formation energies [12], electronic properties [13], chemical reactivity [14, 15], and mechnical properties [16] have frequently been studied. In addition, different spectroscopic methods such as Raman vibrational spectroscopy have been used in order to qualify the type, and quantify the amount of defects present in CNTs [8]. A spectroscopic study using NMR signals revealed differences in the 13C NMR obtained for different orientations of SW defects on the

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sidewall of SWNTs [17]. The results indicated that 13C NMR might be able to detect the presence, and perhaps even quantify the concentration of SW defects. SW defects were also observed in boron nitride based materials mostly due to mechanical fractures [18]. These defects influence the properties of boron nitride nanotubes (BNNTs) in many aspects, including growth behavior and reactivity [18-20]. In addition to their structural significance, these SW defects have also been utilized as active sites for various processes. For example, to find new molecular storage materials, the adsorption of some diatomic and polyatomic molecules on the SW defects was studied and compared with the adsorption on perfect BNNTs [21]. The SW defects were also proposed to be employed to enhance the field emission of BNNTs. In spite of the relevance and large amount of theoretical works on SW defects so far, there have been some theoretical discussion on the carbon ad-dimer (CD) defects of SWNTs in the literature [21]. Horner et al. [23] studied the sidewall reactivity of armchair (5, 5) SWCNT in reacting with C2H4, O2 and O3 species, illustrating that the central C-C bond of CD defect in SWCNT is chemically more reactive than that of perfect sites. Then, Wang et al. [24] investigated the structural and electronic properties of hydrogenated armchair and zigzag SWCNTs with carbon ad-dimer (CD) defect and showed that the chemisorptions of two hydrogen atoms on the exterior sidewalls of CD defective armchair SWCNTs are thermodynamically more stable than on the perfect nanotubes, and slightly more stronger than on the CD defective zigzag ones. So it could be a good idea to investigate carbon ad-dimer (CD) defects in the BNNTs. Since boron and nitrogen are neighboring elements of carbon in the periodic table (CC units are isoelectronic with BN units) we will study the structural and electronic properties of armchair (n,

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n), n = 4, 5, and 6, as well as zigzag (7, 0) BN nanotubes with a carbon ad-dimer (CD) and boron-nitride ad-unit (BNU) defect through density functional theory method. < Fig. 1>

2. Computational Method All density functional theory (DFT) quantum calculations are performed using Gaussian 98 program package [25]. Defect-free armchair (n, n) BNNTs with n= 4, 5, and 6, and also zigzag (7, 0) BNNT consist of six armchair and zigzag rings stacked along the tube axis, respectively. Hydrogen atoms are added at open ends to avoid dangling bonds. Geometries of all systems (perfect BNNTs and BNNTS with CD and BNU defect) are allowed to fully relax during the B3LYP/6-31G* optimization process [26]. Frequency calculations at the same level of theory are carried out for the systems and real frequencies are obtained, confirming that all of them are minimum energy structures. The standard 6-31G* basis set is employed due to being affordable and accurate enough for geometry optimization of even large molecules [20, 27]. The optimized structures of CD and BNU defective BNNTs as well as their parents are subjected to the calculations of the related properties such as total energies (Etot), defect formation energies (Er), and density of states (DOS) using the B3LYP/6-31G* level of theory. < Fig. 2>

3. Results and discussion 3.1. Stability and Defect Formation Energies At the first step of this study, each of the considered models of CD and BNU defective BNNTs and their parents are allowed to fully relax during the geometrical optimization at the

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level of the B3LYP/6-31G*. The obtained structural parameters of the perfect BNNTs are consistent with the literature, for example, B–N bond lengths in the middle of the tube are predicted to be 1.449-1.455 Å which can be compared to the previously reported values (1.401 and 1.458Å) [28, 29]. Since CD and BNU defects are produced by adsorption of a carbon dimer or a BN unit on hexagonal rings, defects with two different orientations for each BNNT are possible, one obtained by the adsorption of a cabon dimer or BN unit parallel to the tube axis, and the other obtained by adsorption of a slanted cabon dimer or BN unit. Consequently, in the present work, the considered configurations for CD and BNU defective (n, n) BNNTs are denoted by BNU(n, n)-I and CD(n, n)-I with parallel pentagon-pentagon junctions along tube axes and BNU(n, n)-II and CD(n, n)-II with slanted pentagon-pentagon junctions along tube axes. Figs. 2 and 3 show the optimized structures of the boron-nitride ad-unit and carbon addimer defective BNNTs, respectively, and the HOMO–LUMO energy gaps (Eg), and defect formation energies (Ef) of these models are given in Table 1. < Table. 1>

As mentioned above, the defect is formed on a perfect nanotube by adsorption of a carbon dimer or a BN unit, which introduces two adjacent pentagons between two heptagons. Then, it is assumed that the reaction presented in Eq. 1 is occurred: P-BNNT + XY→ D-BNNT

XY = BN and CC

(1)

where P-BNNT and D-BNNT represent a perfect and defective BNNT, respectively. Defect formation energies (the energies required to form ad-dimer defects) are calculated as follows: Ef = ED-BNNT - EP-BNNT -EXY

(2)

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where ED-BNNT and EP-BNNT are the total energy of a defective BNNT and that of perfect BNNT, respectively. EXY is chemical potential for the CC and BN pairs, which are determined as the total energy per pair of atoms in the pure C and BN nanotubes. Among all the BNU defective BNNTs, only BNU(4, 4)-I have a negative Ef value, and the reaction energies for the other BNU(n, n) defective BNNTs are obtained to be positive values indicating the endothermic character of BN ad-unit defect formation in these cases. It seems that difference between formation energies of BN ad-unit defects may be due to the newly formed homonuclear B-B and N-N bonds, distortion of the tube wall from the cylindrical shape, and the orientation defects on the tube sidewall. In order to confirm this conjecture, the geometrical characteristics of optimized structures of the BNU defective BNNTs are discussed with the aim of giving better interpretations of these derivatives. In the BNU(n, n)-I defective BNNTs, the B1– N1 bond lengths, Fig. 2, on the pentagon-pentagon junctions are 1.469–1.471 Å, which are larger than those in the BNU(n, n)-II defective BNNTs (1.423–1.449 Å). B–N bonds on hexagonpentagon and heptagon-pentagon junctions in the two different pentagons have different lengths. For example, in defective BNNTs the hexagon-pentagon and heptagon-pentagon B–N bond lengths in the N3B2 pentagons are obtained to be 1.451-1.479 and 1.434-1.461 Å, respectively, while they are longer in the B3N2 pentagons, 1.474-1.503 and 1.461-1.497 Å, which can be due to the formation of frustrated B-B and N-N bonds. This structural bump is more blatant for longer homonuclear B–B bonds (1.655-1.671 and 1.591-1.598 Å in the BNU(n, n)-I and BNU(n, n)-II defective BNNTs, respectively) in the B3N2 pentagon in comparison to N-N ones (1.4921.507 Å and 1.430-1.444 Å in the BNU(n, n)-I and BNU(n, n)-II defective BNNTs, respectively) in the B2N3 pentagon rings. Moreover, the geometric parameters of the optimized structures of BN ad-unit defective BNNTs reveal that the tube wall at the defective sites of BNU(n, n)-I is

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distorted from the cylindrical shape, so that the BN unit relaxes outwardly from the surface of the tube wall, while in BNU(n, n)-II the BN unit moves inward to the tube center, such that two fused pentagons have a planar structure. In order to present an illustrative view, the pyramidalization angle (PA) is used to characterize the induced local pyramidalization at the sites of these atoms. The PA for an X atom is defined as 360 minus the sum of Y-X-Z bond angels of the considered atom; the larger the PA, the more the pyramidalized atom. Standard values in (4, 4) BNNTs are PA =2.56˚ and 8.21˚ for B and N, respectively, and by going to larger BNNT, (6, 6), decrease and reach 1.24˚ and 3.80˚ for B and N. The PA values of 20.32-23.01˚ and 30.32-34.45˚ at the B1 and N1 sites of BNU(n, n)-I defective BNNTs show an outward pyramidalization of BN units in the BN ad-unit defective BNNTs while larger PA values of N1 atoms show more outward pyramidalization of nitrogen atoms in comparison to boron ones. In comparison, in BNU(n, n)-II defective BNNTs, the B1 and N1 sites move inward to the tube center, and the PA of the B1 and N1 atoms are 6.38 and 2.54°, respectively which are closer to those in perfect BN tubes. Such structural features in the BNU(n, n)-I defective BNNTs lead to boat-like conformations for heptagonal rings. Therefore, the structural deformation due to the formation of BNU defects is more severe for BNU(n, n)-I defective BNNTs than for BNU(n, n)II defective BNNTs. As seen, the BNU defect formation energies depend on the orientation and on PA of the central atoms at the pentagon-pentagon ring fusion. Li et al. [18] and Yang et al. [19] have reported similar theoretical results on SW defective BNNTs and CNTs, respectively. In other words, the PA values, which is a measure of the degree of sp3 hybridization of an atom, at the pentagon-pentagon ring fusion of BNU(n, n)-I tubes are closer to the standard value of PA for sp3 hybridized atoms (31.5˚). As can be seen in Table 1, by going to the larger BN ad-unit defective BNNTs, decreasing of the curvature, pyramidalization angles of B and N atoms

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decrease. Therefore, our DFT results indicate that tube diameter and consequently the pyramidalization angles affect the defect formation, such that the defect formation of BNNTs with higher curvature (larger pyramidalization angles) might be more favorable because of both energetic and structural considerations. In order to further investigation, we employ a planar BN sheet (48 B atoms and 48 N atoms) to model a BNNT with an infinitely large diameter. Our large computed value for BN ad-unit defect formation energy (97.68 kcal/mol) underscores the difficulties in producing BN ad-unit defects in large-diameter BNNTs. Theoretical results for SW defect formation in single-walled BNNTs and CNTs are similar [18, 19]. Finally, in order to investigate BNU defects in zigzag BNNTs, we consider (7, 0) BNNT which have similar diameter (PA =2.56˚ and 8.21˚ for B and N, respectively) with (4, 4) armchair BNNT. Fig. 4 show the optimized structures of two different orientations of BN ad-unit defects in the (7, 0) BNNT. The PA values of 31.79˚ and 23.02˚ are obtained at N1 and B1 sites of BNU(7, 0)-II, respectively, which are larger than those calculated for N1 and B1 sites in the BNU (7, 0)-I, 8.99˚ and 7.81˚, respectively . Therefore, as expected, the BN ad-unit defect cost larger formation energy for the zigzag BNU(7, 0)-I (Ef = 59.83 kcal/mol) than for armchair BNU(4, 4)-I. The reverse trend is observed for BNU(7,0)-II and BNU(4, 4)-II defective BNNTs. < Fig. 3>

In the next section, we investigate the carbon ad-dimer defect formed on BNNTs to address three questions: 1) Does removing of frustrated B-B and N-N bonds (frustration effect) lead to more stable compounds? 2) Are BNNTs favorable for carbon ad-dimer defect? 3) Are BNU defect sites in the BNNTs chemically more reactive than perfect sites? Our results may be

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useful for further studies in doping or functionalization of BNU defective BNNTs and construction of nanodevices. Replacing of boron-nitride units with carbon dimers at defect sites leads to the uniform B-N bond lengths in the pentagonal and heptagonal rings (Fig. 3), and energetically favorable configurations (Table 1) for CD(n, n)-I models. The calculated C=C bond lengths (1.379-1.383 Å, as compared to the 1.35 Å of H2C=CH2) are slightly shorter than normal C-C bond lengths in CNTs (1.42 Å) [29]. As seen in Table 1, the defect formation energies of carbon ad-dimer defective BNNTs are all negative values. Defect formation energies (Ef) for the carbon ad-dimer defective BNNTs are obtained (at the B3LYP/6-31G* level of theory) to be -90.28, -79.61 and 69.94 kcal/mol for the CD (4, 4)-I, CD (5, 5)-I and CD (6, 6)-I, respectively, which are more negative than those obtained for the respective CD (n, n)-II models. Generally, the calculated defect formation energies for the carbon ad-dimer defective BNNTs are found to be more negative than those obtained for BNU (4, 4), which show carbon ad-dimer defects are more favorable in comparison to BN ad-unit defects because of removing frustrated B-B and N-N bonds. Therefore, in order to investigate doping of the BN ad-unit defective BNNTs with carbon dimer, it is assumed that BN unit at the pentagon-pentagon junction is replaced with the C=C one and the following reaction takes place: BNU (n, n) + CC→ CD (n, n) + BN

(3)

We define the energy of reaction, Er, in the usual way as follows: Er = E[CD (n, n)] + E[BN] - E[BNU (n, n)] – E[CC]

(4)

where E[CD (n, n)] and E[BNU (n, n)] are the total energies of the CD and BNU defective BNNTs; ECC and EBN are the chemical potentials for the CC and BN pairs, which are determined as the total energy per pair of atoms in the pure C and BN nanotubes. As can be seen in Table 1, the reaction

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energies of C-doping of the BN ad-unit defective BNNTs are all negative values, indicating that the doping reactions are exothermic. The reaction energies obtained at the B3LYP/6-31G* level of theory for these isomers range from -86.07 to -67.41 kcal/mol, meaning that the carbon doping of defective sites on the exterior sidewalls of BN ad- unit defective BNNTs is almost independent of the tube diameter. < Fig. 4>

3.2. Electronic structures Previous theoretical studies pointed out that the electronic structures of nanotubes can be modified in the presence of some defects or dopants. To this aim, Fig. 5 illustrates a comparison between densities of states (DOS) of: (a) perfect (5, 5) BNNTs, (b) BN ad-unit defective (5, 5) BNNT, BNU (5, 5)-I, and (c) carbon-doped BNU defective (5, 5) BNNT, CD (5, 5)-I, and the HOMO–LUMO energy gaps (Eg) of these models are given in Table 1. At this point it is necessary to mention that the HOMO-LUMO gaps, Eg, computed with hybrid functional is large because of the high energy of the virtual orbitals. Then, we stress that the focus here is not to find precise HOMO-LUMO gaps for a given composition; instead, the primary purpose is to study the evolution and the trend of HOMO-LUMO gaps in the considered models, and this is just an approximate comparison. The HOMO-LUMO gaps of the perfect BNNTs are obtained to be 5.72-5.75-eV, reasonably consistent with previous theoretical results [6, 11]. BN ad-unit defects only reduce the HOMO-LUMO gaps by 0.68-1.07 eV, 11.92-18.60 , in line with some previously reported results [31]. For example, using cluster model and PBC computations, Li et al. [18] found that semiconducting properties of (8,0) BNNTs are preserved after the introduction of SW defects, and only a small reduction in the HOMO-LUMO gaps occurred. In the condensed

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matter physics the band gap originates from the delocalized states of the host material and the defects introduce additional localized states within the gap. Since defects are formed inevitably during the growth of nanotubes, it is important to note that the introduction of BNU defects into the BN nanotubes does not obviously change electronic structure of BNNTs, implying that the band gap of BN nanotubes almost may be retained at the presence of defects and there are just one or two additional defect states. These electronic properties will make BN nanotubes suitable material for many potential applications, particularly in nanoelectronics. In the CD (n, n) models, the Eg decreases to 4.15-4.27 eV (by about 25.66-27.51% changes in comparison to those of perfect BNNTs and 9.78-17.70% in comparison to those of BN ad-unit defective BNNTs) and a local energy level appears in the DOS plots of BN ad-unit defective BNNTs after the C-doping of BN units at the pentagon-pentagon ring fusion of defective sites, see Fig. 5. < Fig. 5>

4. Conclusion: We investigate carbon ad-dimer (CD) and boron-nitride BN ad-unit (BNU) defect formation in armchair (n, n), n = 4, 5, and 6, as well as zigzag (7, 0) BN nanotubes at the B3LYP/6-31G(d) level. Some interesting results are found as follows. (i) Our DFT results indicate that defect orientation, tube diameter, and consequently the pyramidalization angles affect the BN ad-unit defect formation, such that the defect formation of BNNTs with higher curvature and larger pyramidalization angles are more favorable. (ii) The BN ad-unit defect in the (7, 0) BNNT which have similar diameter with (4, 4) armchair BNNT cost larger formation energy than armchair BNNTs because of the smaller pyramidalization angles. (iii) The replacing of boron and nitrogen units with carbon dimers in defect sites leads to the energetically favorable

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configurations for defective BNNTs. (iv) The defect formation energies of carbon ad-dimer defective BNNTs are all negative values which can be due to the removing frustrated B-B and NN bonds. (v) The carbon doping of defective sites on the exterior sidewalls of BN ad-unit defective BNNTs are exothermic and also almost independent of the tube diameter. (vi) BN adunit defects in the BNNTs and C-doping of BN ad-unit defective BNNTs reduce the HOMOLUMO gaps while electronic structures of host materials (BNNTs and BN ad-unit defects ones) do not obviously change and only local energy levels appear in the DOS plots.

Acknowledgements We are grateful to Professor Seik Weng Ng for making us available his software (G98W) and hardware (machine time) facilities. The financial support of Research Council of Shahid Beheshti University is gratefully acknowledged. References: [1] M. Bockrath, W. Liang, D. Bozovic, J.H. Hafner, C.M. Lieber, M. Tinkham, H. Park, Science 291 (2001) 283-285. [2] T. Maltezopoulos, A. Kubetzka, M. Morgenstern, R. Wiesendanger, S.G. Lemay, C. Dekker, Appl. Phys. Lett. 83 (2003) 1011-1013. [3] A. Hashimoto, K. Suenaga, A. Gloter, K. Urita, S. Iijima, Nature 430 (2004) 870-873. [4] A.J. Stone, D.J. Wales, Chem. Phys. Lett. 128 (1986) 501-503. [5] D. Orlikowski, M.B. Nardelli, J. Bernholc, C. Roland, Phys. Rev. Lett. 83 (1999) 4132-4135. [6] H.J. Choi, J. Ihm, S.G. Louie, M.L. Cohen, Phys. Rev. Lett. 84 (2000) 2917. [7] H.F. Bettinger, T. Dumitrica, G.E. Scuseria, B.I. Yakobson, Phys. Rev. B 65 (2002) 041406. [8] Y. Miyamoto, A. Rubio, S. Berber, M. Yoon, D. Tomanek, Phys. Rev. B 69 (2004) 121413.

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Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, J.L. Andres, C. Gonzalez, M. Head- Gordon, E.S. Replogle, J.A. Pople, Gaussian 98, Gaussian Inc, Pittsburgh PA, 1998. [26] A.D. Becke, J. Chem. Phys. 98 (1993) 5648-5652. [27] R. Ghafouri, M. Anafcheh, M. Zahedi, Physica E 58 (2014) 94-100. [28] M. Mirzaei, Physica E 41(2009) 883-885. [29] S-P. Ju, Y-C. Wang, T-W. Lien, Nanoscale Res Lett 6 (2011) 160. [30] Z. Zhou, M. Steigerwald, M. Hybertsen, L. Brus, R.A. Friesner, J Am Chem Soc 126 (2004) 3597-3607. [31] H.J. Xiang, J.L. Yang, J.G. Hou, Q.S. Zhu, Phys Rev B 68 (2003) 035427.

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Table 1. HOMO and LUMO energy levels(eV), HOMO-LUMO energy gaps (Eg in eV), defect formation energies (Ef in kcal/mol), reaction energies (Er in Kcal/mol), and pyramidalization angles of the defect sites (PA (B1) and PA(N1) in degree) for BN ad-unit defective BNNTs (BNU (n, n)) along with those for the C-doped BN ad-unit defective BNNTs (CD (n, n)).

EHOMO

ELUMO

Eg

Ef

Er

PA(B1)

PA(N1)

BNU(4, 4)-I

-6.10

-1.07

5.04

-5.11

23.02

34.45

BNU(4, 4)-II

-5.95

-1.23

4.72

29.08

0.44

2.19

BNU(5, 5)-I

-6.03

-1.01

5.02

7.46

21.06

31.06

BNU(5, 5)-II

-5.94

-1.24

4.70

23.89

0.55

2.68

BNU(6, 6)-I

-5.98

-0.95

5.03

15.68

20.32

30.28

BNU(6, 6)-II

-5.90

-1.22

4.68

21.05

2.73

0.65

BNU(7, 0)-I

-6.10

-1.42

4.68

59.83

7.81

8.99

BNU(7, 0)-II

-6.02

-1.44

4.58

19.30

16.20

31.79

CD(4, 4)-I

-5.54

-1.39

4.15

-90.28

-85.17

21.85

21.92

CD(4, 4)-II

-5.46

-1.20

4.26

-52.90

-73.94

5.08

11.51

CD(5, 5)-I

-5.48

-1.32

4.16

-79.61

-86.07

18.98

19.11

CD(5, 5)-II

-5.48

-1.22

4.26

-43.56

-67.45

5.09

10.03

CD(6, 6)-I

-5.56

-1.28

4.27

-69.94

-85.62

16.85

16.83

CD(6, 6)-II

-5.43

-1.23

4.20

-32.16

-63.27

5.12

9.83

CD(7, 0)-I

-5.67

-1.39

4.28

-35.98

-95.80

5.47

5.48

CD(7, 0)-II

-5.61

-1.43

4.18

-77.61

-86.90

20.85

18.76

15

FIGURE CAPTIONS: Fig. 1- Schematic structures of a) Stone-Wales (SW) and b) carbon ad-dimer (CD) defect, leading to the formation of a 5-7-7-5 and 7-5-5-7 ring patterns, respectively, Fig. 2- The optimized structures of two different orientations of boron-nitride ad-unit defects on the tube sidewall of armchair BN nanotubes, BNU (n, n)-I and BNU(n, n)-II n=4, 5, and 6. Fig. 3- The optimized structures of two different orientations of carbon ad-dimer defects on the tube sidewall of armchair BN nanotubes, CD(n, n)-I and CD(n, n)-II n=4, 5, and 6. Fig. 4- The optimized structures of two different orientations of boron-nitride ad-unit and carbon ad-dimer defects on the tube sidewall of zigzag (7, 0) BN nanotube. Fig. 5- Calculated density of states for (a) perfect (5, 5) BNNT, (b) BN ad-unit defective BNNT, BNU(5, 5)-I, and (c) carbon-doped BNU defective (5, 5) BNNT, CD(5, 5)-I.

HIGHLIGHTS: 9 BNU defect formation of BNNTs with higher curvature is more favorable. 9 The replacing of BN with carbon in defect sites leads to favorable configurations. 9 Defect formation energies of CD defective BNNTs are all negative. 9 C doping of defective sites in BNU (n, n) is exothermic, independent of tube diameter. 9 Semiconducting properties of BNNTs are preserved after BNU and CD defect formation.

16

Graphical Abstract

BN ad-unit defective BNNTs and carbon doped BN ad-unit defective BNNTs

17

Figure 1

Fig. 1

Figure 2

Fig. 2

Figure 3

Fig. 3

Figure 4

Fig. 4

Figure 5

Fig. 5