Journal of Solid State Chemistry 200 (2013) 294–298

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Journal of Solid State Chemistry

0022-45

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

Stability, electronic structures and transport properties of armchair (10, 10)BN/C nanotubes

H.P. Xiao, Chaoyu He, C.X. Zhang, L.Z. Sun n, Pan Zhou, Jianxin Zhong nn

Department of Physics, Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Xiangtan University, Xiangtan 411105, Hunan, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 28 November 2012

Received in revised form

4 January 2013

Accepted 19 January 2013Available online 6 February 2013

Keywords:

Boron nitride carbon hybrid nanotubes

Density functional theory

Non-equilibrium Green’s function

Electronic properties

Transport properties

96/$ - see front matter & 2013 Elsevier Inc. A

x.doi.org/10.1016/j.jssc.2013.01.026

esponding author. Fax: þ86 732 58292468.

responding author. Fax: þ86 732 58292468.

ail addresses: lzsun@xtu.edu.cn (L.Z. Sun), jxz

a b s t r a c t

Using the first-principle calculations, the stability and electronic properties of two novel types of four-

segment armchair (10, 10) BN/C hybrid nanotubes ((BN)5C5(BN)5C5NT and (BN)5C5(NB)5C5NT) as well

as two-segment armchair (10, 10) BN/C hybrid nanotubes (ðBN20�nCnNTs) are systematically investi-

gated. When n increases from 1 to 4, the band gap of ðBNÞ20�nCnNTs gradually decreases to a narrow

one. When 4rnr17, the electronic structure of carbon segment in ðBNÞ20�nCnNTs behaves as zigzag

graphene nanoribbons whose band gap is modulated by an inherent electric field of the BN segment.

ZGNR-like segments in (BN)5C5(BN)5C5NT and (BN)5C5(NB)5C5NT behave as narrow gap semiconductor

and metal, respectively, due to their different chemical environment. Moreover, the (BN)5C5(NB)5C5NT

can separate electron and hole carriers, indicating its potential application in solar cell materials.

Obvious transport enhancement around the Fermi level is found in the four-segment nanotubes,

especially a 6G0 transmission peak in the metallic (BN)5C5(NB)5C5NT.

& 2013 Elsevier Inc. All rights reserved.

1. Introduction

The discovery and large-scale synthesis of carbon nanotubes(CNTs) [1,2] and boron nitride nanotubes (BNNTs) [3,4] haveattracted enormous attention due to theirs potential applicationsin future nanotechnology. The quasi-one-dimensional (1D) CNTscan be metal or insulator depending on their chirality anddiameters [5], whereas BNNTs behave as wide band gap insula-tors [6] insensitive to their chirality and diameters. Inspired bythe comparability of atomic configuration between CNTs andBNNTs as well as the huge differences in their electronic proper-ties, many nanotubes with different stoichiometries of B, N and Celements (BNCNTs) have been theoretically investigated [7–11]and successfully synthesized [12–17]. One of the most importantmerits of such hybrid BNCNTs for application in nanodevice isthat their band structures can be modulated by changing theiratomic composition and configuration [8,11,16].

Recently, a novel type of BNC nanotubes (BNCNTs) is proposedby Du et al. [18] through hybridizing a zigzag BN nanoribbon(ZBNNR) segment and a zigzag graphene nanoribbon (ZGNR)segment. Their ab initio molecular dynamics (AIMD) simulationsreveal that (BN)0.5C0.5 armchair single wall nanotube (SWNT) canbe spontaneously formed via the interaction between a BNNR anda GNR at room temperature. Further AIMD and total energy

ll rights reserved.

hong@xtu.edu.cn (J. Zhong).

calculations [19] indicated that the stability of these two-segment BNCNTs is comparable to CNTs, and all BNCNTs areexpected to be well stable over room temperature as long as theirdiameters are larger than 0.4 nm. They also found that armchairBNCNTs are metallic except for those with diameters less than0.6 nm or 1–2 axis oriented zigzag carbon atomic chains aroundthe circumferences.

In the present work, the stability of two-segment armchair-(10, 10) ðBNÞ20�nCnNTs with diameter of about 12 A is system-atically investigated (here n represents the number of the zigzag Cchains in the hybrid nanotube, increasing from 0 to 20). Further-more, two four-segment BNCNTs ((BN)5C5(BN)5C5NT and(BN)5C5(NB)5C5NT) with symmetry dependent electronic propertyare proposed and investigated. We find that the two-segment andfour-segment hybrid BNCNTs not only produce versatile electro-nic properties, but also induce obvious transport enhancementaround the Fermi level for the four-segments nanotubes, espe-cially a 6G0 transmission peak in the metallic (BN)5C5(NB)5C5NT.

2. Computational methods and models

Armchair-(10, 10) tube with diameter of about 12 A containing20 zigzag chains of C or BN is chosen as the simulation model toinvestigate the effect of the variation of chemical composition on thetube’s stability and electronic properties. The two-segment systemsconsisting of ZGNR and ZBNNR are denoted as ðBNÞ20�nCnNTs,where n (n¼0–20) is the number of the zigzag C chains in the tube.The schematic diagram of the two-segment ðBNÞ20�nCnNTs and the

H.P. Xiao et al. / Journal of Solid State Chemistry 200 (2013) 294–298 295

two types of four-segment BNCNTs, (BN)5C5(BN)5C5NT and(BN)5C5(NB)5C5NT, are shown in Fig. 1. The two ZBNNR segmentsof (BN)5C5(BN)5C5NT have the same sequence along the circumfer-ence, namely –BN–BN–?C?–BN–BN–?C?–. It has a two-foldrotation axis, an inverse center and a mirror plane. Under suchsymmetry operations, the two ZGNR (ZBNNR) segments are equiva-lent. The (BN)5C5(NB)5C5NT system with a –BN–BN–?C?–NB–NB–?C?– order along the circumference has only a mirror plane. Inthis system, the two ZGNR segments are positioned at very differentchemical environments. The one connecting the adjacent ZBNNRsegments contains only C–B interface at its both side and the othercontains only C–N interface. Although the two segments of ZBNNRpositioned at the same local chemical environment are equivalent,the ZGNR segments in the system are inequivalent due to differentinterfaces. All two-probe systems adopted in the present workcontain a six unit cell scattering region (with length larger than10 A), two unit cell left- and right-electrode. A typical two-probesystem of (BN)5C5(NB)5C5NT is shown in Fig. 1.

We adopt the Vienna Ab initio Simulation Package (VASP) [20]to perform the density functional theory based (DFT) first-principles calculations to optimize the systems and investigatetheir stability and electronic properties. The projected augmentedwave (PAW) [21] potentials are chosen to describe the electron–ionic core interaction and the PBE version of the generalizedgradient approximation (GGA) [22] is adopted for the electronicexchange and correlation functional. A plane-wave basis set withthe kinetic energy cutoff of 420 eV is employed. To avoid theinteraction between the adjacent images in the periodic super-cells method, a vacuum region larger than 10 A perpendiculars tothe tube is adopted. All systems are fully optimized up to theresidual force on every atom to be less than 0.01 eV/A through theconjugate-gradient algorithm. The Brillouin zone (BZ) is sampledusing 1� 1� 11 Gamma-centered Monkhorst–Pack grids for the

Fig. 1. Structural diagram of two-segment ðBNÞ20�nCnNTs of n equal to 0, 1, 2, 19 and 20

probe system of (BN)5C5(NB)5C5NT is also shown.

Fig. 2. The calculated lattice constants (a), Gibbs free energies (b) and Egap1 and Eg

minimum of the first (second) conduction band and the maximum of the first (second

calculations of structure optimizing and total energy. To investi-gate the transmission conductance of the perfect (10, 10) CNT,two-segment (BN)10 C10NT, and four-segment (BN)5C5(BN)5C5NTand (BN)5C5(NB)5C5NT, we use the real-space non-equilibriumGreen’s function method implemented in the Atomistix ToolKit(ATK) package [23]. A double z plus polarization numerical orbitalbasis set is used and the mesh cutoff is set to 150 Ry (2040.75 eV).

3. Results and discussions

The pristine (10, 10) CNT (C20NT) and (10, 10) BNNT((BN)20NT) are narrow gap semiconductor and wide gap insulator,with lattice constant of 2.468 A and 2.515 A, respectively. There isa relative large mismatch in lattice constants between these twotypes of tubes and their hybrid will cost additional energies. Asshown in Fig. 2(a), the optimized lattice constants of the two-segment ðBNÞ20�nCnNTs lie between those of pure C20NT and(BN)20NT and linearly decrease with the increase in the numberof the zigzag C chains in the hybrid tube. Four-segment(BN)5C5(BN)5C5NT and (BN)5C5(NB)5C5NT hold the same latticeconstant of 2.486 A, which is close to that of the (BN)10C10NT(2.488 A). To evaluate the stability of these hybrid nanotubes,their Gibbs free energy ðdGÞ are calculated from the followingformula: dG¼ Ecoh�xBNmBN�xCmC , where Ecoh is the cohesiveenergy per atom of the tubes with different compositions, xi termis the molar fraction of atom i (i¼H, BN) in the tubes satisfyingxBNþxC¼1, and mi is the chemical potential of each constituentatom. mBN is chosen as the cohesive energy per pair BN of thesingle layered BN-sheet and mC is the cohesive energies per atomof the single layered graphene. The calculated dGs for all the two-segment ðBNÞ20�nCnNTs, the four-segment (BN)5C5(BN)5C5NT and(BN)5C5(NB)5C5NT are shown in Fig. 2(b). Due to the lattice

as well as four-segment (BN)5C5(BN)5C5NT and (BN)5C5(NB)5C5NT. A typical two-

ap2 energy band gaps (Egap1 (Egap2) means the energy difference between the

) valence band.) (c) for the two-segment ðBNÞ20�nCnNTs.

H.P. Xiao et al. / Journal of Solid State Chemistry 200 (2013) 294–298296

mismatch between these two types of segments, both the ZBNNRand ZGNR segments are less stable than the pristine ones. Thecalculated dGs increase with the increasing number of the zigzagC chains, whereas decrease slightly when the number of Zigzag Cchains is larger than 10. The four-segment hybrid tubes are lessstable than the two-segment ones due to the two B–C and twoN–C interfaces. The results of Gibbs free energy indicate that theBN and C constituents tend to separate from each other thusreducing the B–C or N–C interfaces, which is similar to the phaseseparation observed in the two dimensional BN/C domains [24].

Based on the above optimized structures, we investigate theelectronic structures of the two- and four-segment hybrid nano-tubes. The calculated band gaps of the two-segmentðBNÞ20�nCnNTs are summarized in Fig. 2(c). The band structuresof the (BN)20NT, (BN)19C1NT, (BN)18C2NT, (BN)17C3NT,(BN)16C4NT, (BN)15C5NT, (BN)10C10NT and C20NT are selected astypical examples and shown in Fig. 3. Our results indicate that thepristine (BN)20 is a wide band gap insulator with the gap of4.689 eV, whereas the pristine C20 is a narrow band gap semi-conductor with the gap of 0.036 eV. For the two-segment(BN)19C1NT, the single zigzag carbon chain introduces a pair ofextra bands which mainly derives from the carbon chain at eachside of the Fermi level with energy gap (denoted as Egap1 as inFig. 2(c), Egap1 is the difference between the minimum of the firstconduction band above the Fermi level and the maximum of thefirst valence band below the Fermi level) of 1.70 eV. As thenumber of the carbon chain increases from 1 to 4, the Egap1gradually decreases to 0.75 eV, as shown in Fig. 2(c). When thenumber is larger than 4 and smaller than 18 the hybrid nanotubesbehave as narrow band gap semiconductors. Interestingly, thedispersion of the two extra bands at each side of the Fermi levelfor (BN)19C1NT is very similar to that of ZGNR. As the carbon chainincreases, the dispersion characteristics of the two-segmentðBNÞ20�nCnNTs are much closer to that of ZGNR. Such behavior

Fig. 3. Band structures: (a) and (b) for the one-segment BN20NT and C20NT. (c), (d), (e

respectively.

of the carbon segment in two-segment ðBNÞ20�nCnNTs is verysimilar to the isolated graphene strip called ‘‘nanoroad’’ formedby selective hydrogenation on graphene predicated by Singh et al.[25]. However, we find that although the carbon segments of two-segment ðBNÞ20�nCnNTs behave as semiconductors and their bandstructures are very similar to those of the pristine ZGNRs, theirband gaps are very different from those of the correspondingpristine ZGNRs with same width. Such a phenomenon can beattributed to two reasons: (i) inherent electric field (IEF) of theZBNNR segment which tends to reduce the energy gaps of thecarbon segments. Such effect is similar to the report of Kan et al.[26]; (ii) Peierls distortion [27,28] enlarging the band gaps of thecarbon segments due to strong quantum confinement. The ZBNNRsegment of the two-segment ðBNÞ20�nCnNTs will induce an inher-ent electric field (IEF) exerting on the carbon segment due to thedifferent work function at N and B side of ZBNNR [29]. When thenumber of the carbon chain in ðBNÞ20�nCnNTs is smaller than 4,both the effects of IEF and the Peierls distortion of narrow GNRsmodulate the band gap of the carbon segment. Such Peierlsdistortion is similar to the report of Tozzini et al. [28] in graphenenanoribbons sculpted in graphene. When the number of thecarbon chain in ðBNÞ20�nCnNTs is larger than 3, Peierls distortionbecomes very weak and the IEF is the main reason dominating theenergy gap of the carbon segments, which lead to the relativelysmall band gaps of the carbon segments in comparison with thoseof the pristine ZGNRs with same width. The above results indicatethat it will be an effective band structure engineering approach tosandwich the ZGNRs into the ZBNNRs.

Although the four-segment (BN)5C5(BN)5C5NT and(BN)5C5(NB)5C5NT have the same chemical constituent to that ofthe two-segment (BN)10C10NT (a narrow band gap semiconductorwith the gap of 0.017 eV), they exhibit obviously differentelectronic properties. Band structures and local density of statesfor the two four-segment hybrid nanotube are shown in

), (f) and (g) for the two-segment BN20�nCnNTs of n equals to 1, 2, 3, 4, 5 and 10,

Fig. 4. Band structures and local density of states of the four-segment (BN)5C5(BN)5C5NT (a) and (BN)5C5NB5C5NT (b).

Fig. 5. Transport spectra of the one-segment C20NT (a), two-segment (BN)10 C10NT

(b), four-segment (BN)5C5(BN)5C5NT (c) and (BN)5C5(NB)5C5NT (d).

H.P. Xiao et al. / Journal of Solid State Chemistry 200 (2013) 294–298 297

Fig. 4(a) and (b). (BN)5C5(BN)5C5NT is a narrow band gapsemiconductor with the gap of 0.10 eV and (BN)5C5(NB)5C5NT atypical metal. From the structural point of view, the two ZGNRsegments in (BN)5C5(BN)5C5NT locate at the same chemicalenvironment within similar two types of C–N and C–B interfaces.According to the local density of states shown in Fig. 4 (a), we cansee that these two equivalent ZGNR segments make the samecontribution to the total density of state. The two equivalentZBNNR segments also contribute the same density of state locatedat relative low energy. Furthermore, the two segments of ZGNR in(BN)5C5(BN)5C5NT are modulated by the equal IEFs of the ZBNNRsegments and behave as narrow band gap semiconductor, result-ing in the whole hybrid tube behave as a narrow band gapsemiconductor. The characteristics of the electronic propertiesof this novel (BN)5C5(BN)5C5NT are very similar to those of the4-ZGNR/ZBNNR hybrid nanoribbon reported in our previous work[30].

As for (BN)5C5(NB)5C5NT, although the two ZBNNR segmentsare inequivalent, they possess very similar local chemical envir-onments. Each ZBNNR segment holds a B–C interface and a N–Cinterface. They possess almost the same contributions to the totalenergy states as confirmed from the analysis on their local densityof states as shown in Fig. 4(b). However, the two ZGNR segmentsin (BN)5C5(NB)5C5NT are located at very different chemicalenvironments. Moreover, unlike the case of (BN)5C5(BN)5C5NT,there is no IEF exert on the two ZGNR segments because one ofthem has only C–B interfaces and the other has only C–Ninterfaces. Interestingly, the C–B interfaces and C–N interfacesshow electron and hole doping effect on the two ZGNR segments,respectively. From Fig. 4(b) we can see that the density of states ofthe ZGNR segment with only C–B interfaces (denoted as 1 in thefigure) shift to the low energy because the additional electronsinject into the ZGNR segment from the interfacial B atoms,indicating electron doping behavior. As for the ZGNR segmentwith only C–N interfaces, its density of states (denoted as 3 in theFig. 4(b)) shift to higher energy and show more empty statesbecause the C–N interface inject electrons from the interfacial Catoms to the adjacent ZBNNR segments, behaving as hole dopingeffects on the carbon segment. Such charge transfer results in acoexist state at the Fermi-level exhibiting typical metallic prop-erty. The above results indicate that the four-segment(BN)5C5(NB)5C5NT hybrid nanotube can separate the electronand hole into two ZGNR segments. Such behavior is very essential

for the solar cell material. Our results show that the four-segment(BN)5C5(NB)5C5NT hybrid nanotube might be an interestingcandidate for the solar cell materials.

Finally, we investigate the transport properties of pristine one-segment C20NT, two-segment (BN)10C10NT, and four-segment(BN)5C5(BN)5C5NT and (BN)5C5(NB)5C5NT. The calculated trans-mission conductances for these four systems are shown in Fig. 5.As shown in Fig. 5(a), there is a plateau of 2G0 within the energywindow of [�0.78 eV, 0.78 eV] for C20NT. The transport propertyof the two-segment (BN)10C10NT (as shown in Fig. 5(b)) is similarto that of the 9-ZGNR [30,31]; there is a plateau of 1G0 within theenergy window of [�0.95 eV, 0.88 eV] and a narrow plateau of2G0 within the energy area [�0.6 eV, 0.12 eV] derived from theappearance of the two edge states. The band gap of the two-segment (BN)10C10NT is only 0.017 eV, which is submerged in theenvironment thermal fluctuations (300 K electrode temperatureused in our present work) of about 0.027 eV, so that there is no

H.P. Xiao et al. / Journal of Solid State Chemistry 200 (2013) 294–298298

transmission dip at the Fermi level in its transmission spectrum.Moreover, the asymmetry distribution of the transmission con-ductance is derived from the modulation effect of the IEF of theZBNNR segment on the ZGNR segment. As shown in Fig. 5(c) and(d), the two four-segment hybrid nanotubes hold remarkabletransport enhancement in comparison with that of the C20 andtwo-segment tubes. A 4G0 transport plateau appears in thetransport spectra of (BN)5C5(BN)5C5NT and (BN)5C5(NB)5C5NT inthe energy areas of [�0.25 eV, 0.1 eV] and [�0.2 eV, 0.1 eV],respectively. These transport enhancements are mainly contrib-uted by the four energy bands near the Fermi level as shown inFig. 4. Differently, there is a transmission dip around the Fermilevel for (BN)5C5(BN)5C5NT, whereas a transmission peak up to6G0 in (BN)5C5(NB)5C5NT. The difference can be understood fromtheir band structures: (BN)5C5(NB)5C5NT is metal and(BN)5C5(BN)5C5NT is a narrow band gap semiconductor. The bandgap in (BN)5C5(BN)5C5NT is larger enough (0.1 eV) to conquer theenvironment thermal fluctuations, which results in thetransmission dip.

4. Conclusion

Two types of novel four-segment (BN)5C5(BN)5C5NT and(BN)5C5(NB)5C5NT as well as a series of two-segment armchair-(10, 10) ðBNÞ20�nCnNTs are systematically investigated on theirelectronic and transport properties. For the two-segmentðBNÞ20�nCnNTs, its band gap is effectively modulated by thechemical constituent, and most of them behave as narrow bandgap semiconductors. The four-segment (BN)5C5(BN)5C5NT and(BN)5C5(NB)5C5NT behave as narrow band gap semiconductorand metal, respectively. Moreover, we find that (BN)5C5(NB)5C5NTcan separate the electron and the hole, which indicates itspotential application in solar cell materials. Obvious transportenhancements are realized in the four-segment tubes, especiallyin the metallic (BN)5C5(NB)5C5NT where a transmission peak upto 6G0 appears in its transmission spectrum around theFermi level.

Acknowledgments

This work is supported by the National Natural ScienceFoundation of China (Grant Nos. 10874143 and 10774127), the

Program for New Century Excellent Talents in University (GrantNo. NCET-10-0169), the Scientific Research Fund of Hunan Pro-vincial Education Department (Grant No. 10K065), the SpecializedResearch Fund for the Doctoral Program of Higher Education(20094301120004), and the Hunan Provincial Innovation Founda-tion for Postgraduate (Grant No. CX2012B273).

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