Solid State Communications 152 (2012) 802–805

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Structural and electronic properties of sulphur-doped boron nitride nanotubesSheetal Sharma, Pooja Rani, A.S. Verma ∗, V.K. JindalDepartment of Physics, Panjab University, Chandigarh, 160014, India

a r t i c l e i n f o

Article history:Received 29 November 2011Accepted 19 January 2012by E.L. IvchenkoAvailable online 25 January 2012

Keywords:A. Boron nitride nanotubeB. Density functional theoryD. Structural propertyD. Electronic property

a b s t r a c t

First-principles calculations based on density functional theory were performed to study the structuraland electronic properties of sulphur substitution-doped boron nitride (BN) nanotubes, using the theoryas implemented in SIESTA code, which uses non-conserving pseudo-potentials in fully non-local formand atomic orbitals as the basis set. The generalized gradient approximation (GGA) was used for theexchange–correlation (XC) potential. The tube selected was a (10, 0) BN nanotube that fell in the rangeof energy gap independent of the tube diameter. The electronic and structural properties for sulphursubstitution in the boron and the nitrogen sites were studied. The structural arrangement in equilibriumconditions for S shows an outward radial deformation around the sulphur atom in the tube. The bandgapof the pristine BN nanotubes was found to be significantly modified on doping.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Boron nitride nanotubes (BNNTs) were theoretically pre-dicted [1,2] and experimentally realized [3]; these are struc-turally similar to carbon nanotubes (CNTs). The wide range oftheir electronic properties from metallic to wide-bandgap semi-conductor, depending on their chemical composition, makes thempossible candidates for nanosize electronic devices [4,5]. More-over, electronic structure calculations indicate that, in contrast toCNTs, BNNTs are constant-bandgap materials, and thus they pro-vide an attractive opportunity for practical applications [6]. Thesetubes possess interesting properties, such as strong hardness, highthermal conductivity, and excellent mechanical properties, beingmore chemically and thermally stable than their carbon (C) ana-logues [7–10]. Therefore it is expected that BNNTs would be goodalternatives to CNTs for possible applications in nanoelectronics,since, allied with this stability advantage, they present uniformsemiconducting properties that can possibly be tuned according tospecific needs.

Blase et al. [2] theoretically calculated a bandgap of 5.5 eVfor BNNTs of diameter 1 nm and predicted that it is especiallyindependent of tube size or helicity. However, this claim has notbeen fully tested experimentally over a large range of nanotubesizes. Tailoring the electronic energy gap of these tubes is of greatinterest. A lot of research has been undertaken in this line to see theinfluence of doping and impurities on the electronic structure ofBNNTs [11–13]. The influence of sulphur doping on the electronic

∗ Corresponding author. Tel.: +91 565 2423417; +91 9412884655 (Mobile).E-mail addresses: ajay_phy@rediffmail.com, ssvmtr@rediffmail.com

(A.S. Verma).

0038-1098/$ – see front matter© 2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2012.01.038

structure of BNNTs has been rarely studied. In this study, wehave optimized S-doped BNNTs by a density functional theory(DFT) method and their structural stability has been investigated.The pristine and S-doped structures are allowed to relax by allgeometrical optimization.

2. Computational aspects

All DFT calculations were performed by using the SpanishInitiative for Electronic Simulation with thousands of atoms(SIESTA) computational code, which is based on the numericalatomic orbital density functional theory method; details can befound in [14]. The present ground-state calculations are basedon the first-principles pseudo-potential method within the den-sity functional formalism and the generalized gradient approxi-mation (GGA) using the Perdew–Burke–Ernzerhof functional [15]for the exchange–correlation energy. Norm-conserving pseudo-potentials for all atomswere generatedusing the Troullier–Martinsmethod [16] in non-relativistic form. A cut-off of 150 Ry for the gridintegration was utilized to represent the charge density. A simplecubic supercell with edge size 50 Å was used to ensure that theimages did not interact with each other. Eight Monkhorst–Packk-points were employed for the Brillouin zone integrations. Thestructures of the doped and undoped tubes were obtained byminimization of the total energy using the Hellmann–Feynmanforces [17]with Pulay-like correction. The structural optimizationswere performed using a conjugated gradient (CG) [18] procedureuntil the residual forces had values smaller than 0.001 eV/Å.

In the present study, we considered the representativemodel ofa (10, 0) zigzag single-walled BNNT of diameter 0.796 nm in whichthe ends of the nanotube were saturated by hydrogen atoms;see Fig. 1. Each zigzag model has three forms: pristine B60N60H20

S. Sharma et al. / Solid State Communications 152 (2012) 802–805 803

Fig. 1. Structure of optimized BNNTs: (a) pristine, (b) S doped at the B site, (c) S doped at the N site.

Table 1Values of structural parameters for S atom at the B site.

Distance nm Distance nm Distance nm Angle

1–2 = 0.16987 3–10 = 0.14555 6–7 = 0.14449 2–1–4 = 91.5484°1–3 = 0.17236 3–11 = 0.14680 8–9 = 0.14570 2–1–3 = 37.9557°1–4 = 0.22745 4–13 = 0.14127 9–10 = 0.14623 3–1–4 = 52.6422°2–7 = 0.14835 4–5 = 0.14130 11–12 = 0.144912–8 = 0.14676 5–6 = 0.14784 12–13 = 0.14837

Table 2Values of structural parameters for S atom at the N site.

Distance nm Angle

1–2 = 0.18874 2–1–3 = 42.6723°1–3 = 0.19160 3–1–4 = 44.8439°1–4 = 0.19155 2–1–4 = 41.9048°

(Fig. 1(a)), B atom doped by S atom B59SN60H20(Fig. 1(b)), and Natom doped by S atom B60N59SH20 (Fig. 1(c)).

3. Results and discussion

First, the pristine BN nanotube was structurally optimized, andthe calculated average B–N bond length was found to be 1.453 Å,which is in accordance with previously reported values [19,20];then two possible substitution of the sulphur atom, replacingeither the boron or the nitrogen atom in a BN nanotube, werecarried out. Fig. 1 shows the optimized BNNTs for the pure andthe two differently doped configurations. The relevant atoms arenumbered in Fig. 2 to describe the structural changes after doping.All the parameters (bond lengths and bond angles) are listed inTables 1 and 2. The distortion is more pronounced near the dopingsite as compared to the rest of the tube. A close look at theparameters reveals that in both cases there is an outward localstructural distortion along the radial direction at the substitutedatom, which is expected because of the larger atomic radius of thesulphur atom, but it is more pronounced in the case of doping atthe B site [21]. Small values of angles at the doping site are anindication of larger distortion in the radial direction.

3.1. Energy band structure and density of states

We now that, for hexa-boron nitride (h-BN), nitrogen is muchmore electronegative than boron; therefore, the electrons aremainly localized on pi-orbitals of N in the valence band and canhardly be promoted over a wide gap to the conduction band(pi-orbitals of B), and thus h-BN shows poor conductivity, havingthe character of an insulator.

Fig. 2. Localized structures of the fully relaxed doped system (labelled).

For the pristine tube, the highest occupied molecular orbitalto lowest unoccupied molecular orbital (HOMO–LUMO) gap hasbeen found to be 4.10385 eV, indicating that it is a wide-bandgap semiconductor, which agrees well with results in theliterature [2,22]. Energy band structures near the Fermi level arepresented for pristine and doped BNNTs in Fig. 3. The Fermi energyfor pristine, S-doped at the N site, and S-doped at the B sitenanotubes are−2.98 eV,−2.19 eV, and−3.70 eV, respectively (thelevels are not shown in Fig. 3). The bandgap diminishes to 3.97 eVfor doping at theN site (as shown in Fig. 3(b)). Similarly for S dopingat the B site (as is clear from the Fig. 3(c)) besides diminishingbandgap 3.72 eV significantly and also provides a new energy levelinside the gap. The band density increases in both the valence bandand the conduction band after doping.

The density of states (DOS) has been plotted for all BNNTs forcomparison purposes. The Fermi level is localized at zero energy.The DOS changes in all aspects, as shown in Fig. 4, and showsbroadening of the band due to the increased DOS induced by thedoping atom. For B site doping, as discussed above, a localizedstate can be seen at about ∼1.0eV above the top of the valenceband in Fig. 4 (b). This empty level makes it easier for electronsto get excited from the filled valence band to the acceptor levelthan to the conduction band, and due to this a hole will form inthe filled band, making the system a p-type semiconductor. Hencewe can say that doping of a sulphur atom at the boron site resultsin reducing valence electron and therefore adding an electron isnecessary, which gives a negative charge to the impurity which isnow binded by the hole formed.

The DOS for the BNNT that is S doped at the nitrogen site isshown in Fig. 4(c). In this case, we are very surprised that theoccupancy at the Fermi level (here 0.0 eV) is not zero, but has afinite value, causing the lowest conduction band to be filled at thepoint, localized in the conduction band, and thereby changing thebehavior of the pristine BNNT from insulator behavior to metallicbehavior. This can be explained, as the sulphur atom acts as anelectron donor and transfers 0.469 e− to the tube.

804 S. Sharma et al. / Solid State Communications 152 (2012) 802–805

a b c

Fig. 3. Energy levels of BNNTs: (a) pristine, (b) S doped at the B site, (c) S doped at the N site.

Fig. 4. Calculated ab initio DOSs for the BNNTs: (a) pristine, (b) S doped at the B site, (c) S doped at the N site. The Fermi energy is shifted to 0.0 eV.

4. Conclusions

We performed a DFT study to investigate the physicalproperties of the electronic structure of S-doped (10, 0) BNnanotubes. For the two configurations of doping (i.e., S at the B siteand S at the N site), the electronic structures differ appreciably:although we find an outward radial deformation for both cases, itis more significant in the case of S atom doping at the B site. Bandstructure results indicate that the BNNTs behave as an impurity-doped semiconductor if the S atom is substituted at the B site andthat they have metallic behavior if the S atom is substituted at theN site. Hence, it is the impurity that determines the conductivity ofthese materials.

Acknowledgments

The authors would like to thank the SIESTA team, HPCC PanjabUniversity, Chandigarh, for the computational facilities, and UGC,CSIR, New Delhi, for financial support.

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