Stability and Electronic Properties of a Novel C-BN Heteronanotube

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J. Phys. Chem. C 2009, 113, 13108–13114

Stability and Electronic Properties of a Novel C-BN Heteronanotube from First-Principles Calculations Zi-Yue Zhang, Zhuhua Zhang, and Wanlin Guo* Institute of Nano Science, Nanjing UniVersity of Aeronautics and Astronautics, Nanjing 210016, China ReceiVed: March 13, 2009; ReVised Manuscript ReceiVed: May 17, 2009

We report a family of heteronanotubes circumferentially consisting of a curled carbon ribbon and a curled boron nitride ribbon from first-principles calculations. By molecular dynamics simulations and total energy calculations, the stability of the C-BN heteronanotubes (C-BNNTs) is predicted to be comparable to that of carbon nanotubes, and all the C-BNNTs with diameters larger than 0.4 nm are expected to be stable well over room temperature. Zigzag C-BNNTs are found to be direct gap semiconductors with the band gaps varying depending on the tube diameters and the ratio of carbon dimer lines with respect to BN ones. In contrast, armchair C-BNNTs are metallic except for those with diameters less than 0.6 nm or 1-2 axisoriented zigzag carbon atomic chains around the circumferences, which become semiconducting. The versatile electronic properties in these heteronanotubes originate from an intricate interplay between the quantum confinement effect and the local tube curvature. I. Introduction 1

2

The discovery and large-scale synthesis of carbon nanotubes (CNTs) have triggered great scientific interests in the structural, mechanical, and electronic properties of tubular nanomaterials. It was found immediately that other hexagonal layer-like structures could also lead to microscopic tubular structures. Among these nanotubes, boron nitride nanotubes (BNNTs), which were first predicted in 19943 and synthesized shortly after,4 have attracted special interests. All BNNTs show large energy gaps (around 4.4 eV from local density approximation calculations) independent of tube chirality and diameters,5 in contrast with CNTs that can be metallic or semiconducting depending on their chirality and diameters.6 Despite having the same atomic lattice, the huge differences in electronic properties between the BNNT and the CNT inspired many scientific researchers to pay attention to the stoichiometric nanotubes consisting of B, C, and N elements, such as BC3, BC2N, and CNx nanotubes.7-15 By changing the chemical component, the electronic properties of this family of nanotubes can surprisingly exhibit a consistent modulation from metal to insulator.1,3,7,8 Additionally, some other BxNyCz nanotubes have been successfully synthesized, where the B, N, or C elements are wellproportioned in the tube walls.16 On the other hand, the nanotubes with local graphene and BN segments, which are called heteronanotubes, also start to be concerned. The lattice mismatch between graphene and BN leads to structural distortion around the junction between segments, which would make such nanotubes energetically unfavorable. For this reason, little study up to date is reported on this kind of nanotube.17-19 Some fundamental questions will be raised naturally: Are there new heteronanotubes stable above room temperature for possible synthesis? Is the structure or morphology of such nanotubes only a simple combination of individual carbon and BN sheets? How are the electronic structures of these nanotubes modulated by their chemical components, and how does electronic transport get through the nanotubes? However, previous theoretical studies * To whom all correspondence should be addressed: [email protected].

only focused on the possible ferromagnetism of a kind of axial heterojunction BxNyCz nanotubes,18 instead of a systematical study of essential stability and versatile electronic properties. In this work, we report a C-BN heteronanotube (C-BNNT) constructed circumferentially by a carbon ribbon and a BN ribbon from first-principles calculations. Both molecular dynamics (MD) and first-principles static simulations show that this kind of nanotube can remain an almost perfect atomic lattice far above room temperature, and, with increasing tube diameter, the tube stability is much enhanced, convincingly confirming the possibility of synthesizing such nanotubes. Experimentally, it is possible to use the developed methods that dope B and N into single-walled CNTs for synthesizing other BxNyCz nanotubes13,20 to synthesize the novel C-BNNT.With modulating the tube diameter and the chemical ratio of carbon and BN segments, such C-BNNTs show distinct local curvature change, which leads to versatile electronic properties. These findings highlight a new direction for band gap engineering, and also indicate potential applications in electronic and photonic devices. II. Computational Procedures All the first-principles calculations are carried out on the basis of the density functional theory (DFT),21,22 as implemented in the Vienna ab initio simulation package.23 In the calculations, the local density approximation is employed to describe the exchange-correlation potential. The energy cutoff is chosen to be 435 eV, and the Brillouin zone is sampled with 1 × 1 × 26 Monkhorst meshes. A unit cell is set up with a one-dimensional periodic boundary condition along the tube axis, and the vacuum space between two adjacent tubes is set up to 1 nm to eliminate the effect of direct tube-tube interaction. We adopt the same axial lattice constants for all nanotubes and uncurled nanoribbons of the same chirality, and all other optimization is performed within the nonrestricted scheme. The structures were fully relaxed using the conjugate gradient method until the force on each atom is less than 0.1 eV/nm. Our ab initio molecular dynamics (AIMD) simulations are carried out in the NVT ensemble by means of a No´se thermostat to control temperature.

10.1021/jp902246u CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

Novel C-BNNTs from First-Principles Calculations

Figure 1. Typical ball-and-stick models of (a) aC3-(BN)3NR, (b) zC3(BN)3NR, (c) zC3-(BN)3NT, and (d) aC3-(BN)3NT. The unit cell used for counting the number of the strips in carbon and BN segments is shown in rectangular boxes. The numbers of the unit cell and the atomic chain are presented on the right and left of C-BNNRs, respectively. Two types of C-C bonds (rC1 and rC2) and B-N bonds (rBN1 and rBN2) are particularly denoted. cB and cN present the cross sections of two cylinders formed by boron-carbon atoms and nitrogen-carbon atoms.

Newton’s equations of motion are integrated using the Verlet algorithm with a time step of 3 fs. Similar to the CNT,24 a single-walled C-BNNT can be described as a hexagonal C-BN layer rolled into a cylinder. We introduce the construction of C-BN nanoribbons (C-BNNRs) for rolling into the C-BNNTs. As shown in Figure 1, a C-BNNR is formed by a carbon ribbon in one part and a BN ribbon in the other part. The two parts are connected by C-B and C-N covalent bonds. The chosen unit cells to measure the ratio of BN and C components in zigzag and armchair C-BNNTs are denoted by a narrow rectangle, as shown in Figure 1a,b. We can find that a unit cell is composed of two dimer lines in zigzag C-BNNTs and two zigzag atomic chains in armchair C-BNNTs. In this work, to keep the symmetrical structure of the C-BNNT, tubes with even number of atomic chains have not been studied. The number of C unit in the nanotube is assumed to be m. We mark the (n, 0) C-BNNT as zCm-(BN)n-mNT and the (n, n) one as aCm-(BN)n-mNT. Take the C-BNNTs shown in Figure 1, for example, the (6, 0) C-BNNT is marked as zC3-(BN)3NT as there are three BN and C unit cells, and the (6, 6) C-BNNT with three BN and C units is marked as aC3-(BN)3NT. Since the zigzag and armchair C-BNNTs are rolled up from the C-BNNR with armchair and zigzag shaped edges, respectively, we mark the corresponding nanoribbon as a(z)Cm-(BN)n-mNT. III. Results and Discussion 1. Stability of C-BNNTs. In this part, we systematically study the stability of the C-BNNTs. We take a series of Cm(BN)n-mNTs for study, where m ) (n + 1)/2 for odd n, and m ) n/2 for even n. The thermodynamic stability of Cm(BN)n-mNTs can be determined by a competition between the increase in energy due to the bond-bending strain introduced by folding a single-layered sheet into a tube and the decrease in energy due to the dangling bonds on the edges of the sheet bonding together on the tube formation. Therefore, if the total energy of the tube is lower than that of the uncurled ribbon,

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13109 the nanotube could be stable in free space. We thus compared the total energies per atom of Cm-(BN)n-mNTs and their corresponding ribbons as shown in Figure 2a. It is found that the energy difference between a Cm-(BN)n-mNT and the corresponding ribbon increases with increasing tube diameter and becomes steady at 0.08 eV for the zigzag tubes and 0.23 eV for the armchair ones when the diameter is more than 0.6 nm. The smaller energy difference for zCm-(BN)n-mNTs is related to the evidently lower total energies of their ribbons than that of zigzag ribbons with similar widths. It is mainly due to the difference of structural relaxations between two types of Cm(BN)n-mNRs. As shown in Figure 1a,b, the structural optimization for zigzag ribbons is rather small, while the unsaturated B-N dimer lines at two sides of armchair ones are buckled apparently, which lowers the total energy of the armchair nanoribbon system. On the other hand, we also note that the total energies of Cm-(BN)n-mNTs are lower than those of corresponding ribbons except for the (5, 0) and (2, 2) nanotubes. Therefore, the smallest Cm-(BN)n-mNTs stable in free space are the (6, 0) and (3, 3) for the zigzag and armchair forms, which have diameter of about 0.48 and 0.42 nm, respectively. As the total energies of the zC3-(BN)2NT and aC1-(BN)1NT are higher than that of their uncurled state, calculating the approximate energy paths of uncurling the tubes into their planar ribbons is thus necessary to exactly examine their stability. We axially uncurl the two tubes from the junctions formed by the B-C and N-C bonds because they have the lowest binding energy among all the covalent bonds referred in these systems. This method has been used to successfully predict the smallest single-wall CNTs in free space by Sawada et al.25 and the validity of this method has been confirmed comparable to that of expensive nudged elastic band method.26 The total energies of optimized structures as a function of uncurling distance L is shown in Figure 2b. It is found that the energy increases with L, and then an energy barrier of 0.204 and 0.277 eV/atom must be overcome to uncurl the zC3-(BN)2NT and aC1-(BN)1NT into planar ribbons, respectively. The energy barriers for uncurling the two tubes corresponds to more than 1500 K,27 mainly due to the creation of dangling bonds at the uncurled edges. Therefore, even such ultrasmall Cm-(BN)n-mNTs have high stability. The relative stability of the proposed Cm-(BN)n-mNTs compared to the BNNTs and CNTs is very meaningful. To address this issue, we adopt the approach customary used in tertiary phase thermodynamics to account for chemical compositions. This method has been utilized to analyze the relative stability of endohedral silicon nanowires,28 graphene nanoribbons (GNRs)29,30 as well as small BNNTs.31 In this approach, the formation energy δG for the Cm-(BN)n-mNT, BNNT, and CNT are respectively defined as δG ) E(χi) - ∑iχi µi, where E(χi) is the total energy per atom of a nanotube with given composition and dimensions, χi (i ) C, BN) is the molar fraction of the C atom and BN pair in the nanotube, satisfying the relation ∑iχi ) 1, and µi is the chemical potential of the constituent i at a given state. We choose µC and µBN as the total energy per atom (BN pair) of a single infinite graphene and BN sheets, respectively. In this way, we can directly compare the formation energies among Cm-(BN)n-mNTs and CNTs with different compositions as shown in Figure 2c where a smaller value of δG indicates relatively higher stability. The formation energies for Cm-(BN)n-mNTs are approximately equal to that of CNTs,32 suggesting the comparable stability between them. On the other hand, we note that the formation energies of Cm-(BN)n-mNTs with different chirality will not come into a uniform trend with

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Figure 2. Stability of Cm-(BN)n-mNTs, where m ) (n + 1)/2 for odd n, and m ) n/2 for even n. (a) Comparison of the total energies per atom for Cm-(BN)n-mNTs with that of the corresponding ribbons as a function of tube diameter. The green and blue symbols corresponding to the right y-axis represent the total energy difference between z(a)C-BNNRs and a(z)C-BNNTs. (b) Energy paths for unrolling the zC3-(BN)2NT and the aC1-(BN)1NT into corresponding ribbons. The configuration of an opening state of the zC3-(BN)2NT is also shown in the inset. (c) Formation energies per atom for Cm-(BN)n-mNTs, BNNTs, and CNTs as a function of tube diameter. The inset presents that the strain energy per atom is proportional to radius-2 for the zigzag and armchair Cm-(BN)n-mNTs. (d) Relaxed structures of the zC3-(BN)3NT and the aC2-(BN)1NT at 1600K after 3 ps AIMD simulations.

increasing tube diameter as observed for BNNTs. It indicates that the formation energies of Cm-(BN)n-mNTs are somewhat sensitive to tube chirality: Zigzag forms are found to be more stable than armchair ones, which is opposite to CNTs.33 This chirality-dependent trend of the formation energy with tube diameter of Cm-(BN)n-mNTs is well quantified in the fitting formula of δG ) R/R2, as shown in the inset of Figure 2c, where R is the radius of tubes and R is calculated to be 2.1 for the zigzag forms and 2.4 for the armchair ones. In addition to the static calculation of stability, we further confirm the mechanical stability of Cm-(BN)n-mNTs by AIMD simulations. It is shown that even the smallest stable zC3(BN)3NT and aC2-(BN)1NT can be stable at temperatures up to 1600 K throughout a 3 ps simulation duration. The relaxed structures of both tubes at 1600 K are shown in Figure 2d, where the atomic configuration remains nearly undistorted. This is very consistent with the predictions by the energy change path method. Although from statistical viewpoint a few atoms may reach the global minimum energy without completely overcoming the energy barriers, leading to lower realistic stability, such high energy barriers predicted from multiple computing techniques should prove that the free-standing Cm-(BN)n-mNTs are stable well above room temperature. 2. Structure Properties of C-BNNTs. There are two kinds of C-C bonds (rC1 and rC2) and B-N bonds (rBN1 and rBN2) in the C-BNNTs, as shown in Figure 1c,d. rC1 and rBN1 represent the covalent bonds along the tube axis, while rC2 and rBN2 represent those around the circumference. All bonds along the tube axis become larger, and those around the circumference become shorter with increasing tube diameter. As seen in Figure 3, in both zigzag and armchair C-BNNTs, rC1 is shorter than rC2 with the tube diameter smaller than 0.65 nm and larger in contrast. However, rBN1 is shorter than rBN2 in zigzag tubes, which is contrary to the situation in armchair tubes. The

Figure 3. Bond length difference of (rC1 - rC2) and (rBN1 - rBN2) for the optimized Cm-(BN)n-mNTs, where m ) (n + 1)/2 for odd n, and m ) n/2 for even n.

difference between rBN1 and rBN2 becomes more significant with decreasing tube diameter. These changes in bond length are attributed to the bond strain induced by the curvature K of the tube wall, which is inversely proportional to tube radius, as will be specially examined later. In our optimized Cm-(BN)n-mNTs, the boron and nitrogen atoms move inward and outward, respectively, forming a buckling structure that is common in polar atomic surface.34 At the same time, the carbon semicircles also alternatively move slightly inward and outward, connecting with boron planar and nitride planar, respectively, resulting in two cylinders denoted by cB and cN in Figure 1c. The cross sections of cB and cN are shown in Figure 4, where the buckling structure is visible. We have calculated the mean radiuses for the semicircles formed by C, B, and N atoms as indicated by RC, RB, and RN, and found RC < RB < RN. Because of the difference between RN and RB decreasing monotonically with increasing the tube size, we adopt the average values of them as the radius of BN segment RBN.

Novel C-BNNTs from First-Principles Calculations

Figure 4. Curvature separations between C and BN semicylinders of the optimized Cm-(BN)n-mNTs, where m ) (n + 1)/2 for odd n, and m ) n/2 for even n. The inset presents the cross sections cB and cN of two cylinders formed by boron-carbon atoms and nitrogen-carbon atoms in the zC3-(BN)3NT. The mean radiuses of the semicircles formed by C, B, and N atoms are marked as RC, RB, and RN, respectively.

Figure 5. (a) Band structure of the zC6-(BN)5NT. (b) Total and projected density of states for the zC6-(BN)5NT (black dash line). The partial densities for the C, B, and N atoms are presented by the green, blue, and red solid lines, respectively. The Fermi level is located at 0 eV indicated by red dash line. (c) Charge-density contours [38 e/(Å)3] for the LUCO and HOCO of the zC6-(BN)5NT. Green, red and blue balls denote C atoms, N atoms, and B atoms, respectively.

According to the curvature formula K ∼ 1/R, we find that KC > KBN, and both of them decrease with increasing tube size. Figure 4 shows the separation between KC and KBN as a function of n, which exhibits an oscillation dependent on whether n is odd or even. Since there is one more carbon unit in the tube with odd n, the distortion of carbon segment wall is stronger, leading to the enhancement of KC. For example, the value of KC - KBN for the zC6-(BN)5NT is 0.02 nm-1 more than that for the zC5(BN)5NT. 3. Electronic Properties of C-BNNTs. The BNNTs are usually regarded as insulators independent of their chirality, while the CNTs can be semiconducting or metallic depending on their chiralities and diameters. Hence, it should be interesting to know what the electronic properties of the C-BNNTs would be like and how they change with tube chirality, diameters, as well as the percentage of C and BN components. a. Zigzag C-BNNTs. Distinct from zigzag CNTs,35 all zCm(BN)n-mNTs are shown to be semiconductors with direct band gaps. Figure 5 shows the band structure and density of states for the zC6-(BN)5NT, as well as the isosurface plots of the charge density of two bands around the Fermi level (EF). The calculated gap of zC6-(BN)5NT is 0.829 eV. Both the highest occupied crystalline orbital (HOCO) and the lowest unoccupied crystalline orbital (LUCO) are located at the Gamma point, with the corresponding electron states mainly distributed on the carbon segment. The comparison of the density of states in Figure 5b also indicates that the electronic properties of the composite structure, at least in the region around the Fermi energy (which

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Figure 6. Energy gaps of the zCm-(BN)6-mNTs and zCm-(BN)15-mNTs as a function of the percentage of carbon. The inset is the variation of band gaps of A-GNRs as a function of the carbon unit number m, where a unit consists of two carbon dimer lines.

is the most important of such fields as molecular electronics), are dominantly determined by the carbon segments, which can be regarded as a bent armchair GNR (A-GNR). Now we investigate the dependence of the band gaps of zCm(BN)n-mNTs on their components and diameters. First, we change the ratio of m and n - m, namely, the numbers of the C and BN units, within a given tube diameter. The band gaps as a function of carbon ratio for the (6, 0) and (15, 0) Cm-(BN)n-mNTs are shown in Figure 6. It is found that the gap of a (15, 0) Cm-(BN)n-mNT (∆l) is inversely proportional to the number of carbon unit, separated into three groups with a hierarchy of gap size given by ∆3x > ∆3x-2 > ∆3x-1 (x ) 1-5). For example, the first three tubes with m ) 1, 2, 3 have energy gaps of 1.31, 1.29, and 1.36 eV, respectively. To understand such unique fluctuation of gap value, we remove the BN segment from the zCm-(BN)n-mNTs and only calculate the energy gap of the remaining A-GNR, as shown in the inset of Figure 6. It is shown that the energy gaps of the A-GNR as a function of unit number m are also well separated into three different categories, which is in excellent agreement with the previous study of A-GNRs.36 It confirms that the semiconducting properties in zCm-(BN)n-mNTs are mainly due to the quantum confinement effect in the carbon segment. However, we note that the gap size hierarchy of A-GNRs is now changed to ∆3x-1 > ∆3x > ∆3x-2 (x ) 1-5), which is different from that of corresponding zCm-(BN)n-mNTs. This difference results from the curvature effect in tubes, which becomes more significant in small tubes, as will be discussed next. For the (6, 0) Cm-(BN)n-mNTs, the band gap increases monotonically with decreasing number of carbon units m, from 0.05 eV of the zC5-BN1NT to 0.70 eV of the zC1-BN5NT. The band structures of the (6, 0) Cm-(BN)n-mNTs as well as the (6, 0) CNT and BNNT are systematically shown in Figure 7a-g. It is sure that the quantum confinement effect is mainly responsible for the gap increase as in the (15, 0) Cm(BN)n-mNTs. Also, the curvature effect can not be neglected for small-diameter Cm-(BN)n-mNTs either, and even carries greater weight for the change of energy gap. As shown in Figure 7h, the curvature of C (BN) segment decreases (increases) with decreasing value of m, which results in the weakened (enhanced) π*-σ* hybridization and consequently leads to the raising (lowering) of the lowest π* state of the C (BN) section [rose (blue) color in Figure 7a-g]. As a result of the BN’s ionicity (site potential asymmetry), the lowest π* state of the BN segment is rarely lower than that of the C segment. In the following, we fix the percent of carbon unit m/n and investigate the variations in electronic properties as a function of tube diameters. First, we calculated the band gaps of zCm-

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Figure 7. Band structures of the zCm-(BN)6-mNTs where the value of m:(n - m) is (a) 6:0, (b) 5:1, (c) 4:2, (d) 3:3, (e) 2:4, (f) 1:5, and (g) 0:6. The blue and rose lines are the π* states of BN and C, respectively. The Fermi level is located at 0 eV indicated by red dash line. (h) The curvature of the zCm-(BN)6-mNTs as a function of the percentage of carbon.

Figure 8. Energy gaps of various types of the zigzag C-BNNTs as a function of n (see text).

(BN)n-mNTs, where m ) (n + 1)/2 for odd n, and m ) n/2 for even n. It is found that the gap value of the zCm-(BN)n-mNTs increases rapidly in the region of small diameter because of the stronger quantum confinement effect in the narrow carbon segment. When n is more than 11, the band gaps oscillate with tube diameter, similar to that in zigzag CNTs.37 It should be noted that two adjacent tubes with the same number of carbon units have similar gaps. For example, zC5-(BN)4NT and zC5(BN)5NT show gaps of 0.617 and 0.639 eV, respectively, further confirming the important role of quantum confinement effect in the carbon segment on the gap of the tube. Of course, the curvature effect deriving from different tube diameters is responsible for the small difference between the two gap values. To more clearly reflect the curvature effect, we perform a series of calculations of the diameter dependence of the gap of zC3(BN)n-3NTs, with fixed width of carbon segment. It is found that the gap value of the zC3-(BN)n-3NT dramatically increases with increasing n (Figure 8), just because the curvature effect in the carbon segment is weakened in the larger nanotube. Obviously, the gap would be converged to that of a flat passivated A-GNR with six carbon dimer lines width with further increasing n. These consequences adequately prove the importance of the local curvature effect on the energy gap of the whole tube. We thus conclude that the interesting change of electronic properties with the chemical component result from the coupled effect of quantum confinement and local tube curvature.

Zhang et al. b. Armchair C-BNNTs. We then examine the electronic properties of armchair Cm-(BN)n-mNTs. Most of aCm-(BN)n-mNTs are metallic. The band structure of an aC3-(BN)3NT is shown in Figure 8a, where two bands near EF cross each other at k ) 2π/3. However, aCl-(BN)mNTs can also become semiconductor in the following two cases: (1) When the diameter is less than 0.6 nm, for example, aC2-(BN)2NT (Figure 9b), is an indirect gap semiconductor because of the strong curvature effect, which leads to a symmetry breaking of the carbon segment. (2) When there is only one carbon unit in the tube, such as the aC1-(BN)5NT in Figure 9c, a direct energy gap is obtained, which will be particularly discussed later. As noted in Figure 9a,b, there are two flat bands at the region 2π/3 e |k| e π near EF mainly from the two junctions. These states decay into the center of carbon segment, similar to the edge states in zigzag GNRs (Z-GNRs),38 which give rise to a very large density of states around EF but can not result in magnetism due to large dispersion of these states. Despite metallic or semiconducting aCm-(BN)n-mNTs, the two states around EF are not solely confined in the carbon section. The occupied states are contributed by the C-B bonding configuration, and the unoccupied states are derived from the C-N boundary, as shown by the partial charge density of states in Figure 9d-f. Since the aCm-(BN)n-mNT is obtained by rolling up a corresponding ribbon with zigzag shape edges, the electronic properties root in the characters of the ribbon consisting of a Z-GNR and a zigzag BN nanoribbon. The energy bands near EF of pristine Z-GNRs are mainly contributed by the two edge states.37,38 In contrast, for the zigzag BN ribbon, the highest valence band is originated from the orbitals of the N edge atoms, while the lowest conductance band is mainly localized at the B edge atoms.39 Therefore the occupied π orbitals of N have much lower energy than the unoccupied π orbitals of B, while the π orbitals of C locate in the middle. When C and BN nanoribbons form a junction, the interaction between the π orbitals of C and N (B) results in a higher (lower) orbital originate from C-N(B) junction.40 We have also studied how the component ratio affects the electronic properties of aCm-(BN)n-mNTs. Taking the (6, 6) aCm-(BN)n-mNTs, for example, we have examined the change of band structure with decreasing m. It is shown that the tube transfers from metal (Figure 9a) to semiconductor when the value of m/(n - m) is decreased to 1/5 (Figure 9c), suggesting that the electronic properties of aCm-(BN)n-mNTs can hardly be modulated by varying the component ratio. This unique character is related to the junction states near EF. Further examining the charge density distributions shows that the two junction states decay significantly only in the carbon segment (see Figure 9d-f). It thus can be imagined that, with decreasing number of carbon units, the Coulomb interaction between the two junction states is enhanced, which impels the occupied (unoccupied) orbital to move downward (upward). Moreover, when we fixed the unit number of the C segment to a single one and only increase the number of BN units, the direct band gaps of these aC1-(BN)n-1NTs (n ) 3-8) remain stable around 0.77 eV, independent of tube diameters. This steady direct gap in aC1-BNn-1NTs suggests potential photoelectric applications, such as for red light emitting devices. IV. Conclusions In conclusion, we have systematically investigated the stability and electronic properties of single-walled C-BNNTs

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Figure 9. Band structures of (a) the aC3-(BN)3NT, (b) the aC2-(BN)2NT, and (c) the aC1-(BN)5NT. The Fermi level is located at 0 eV indicated by a red dashed line. The charge-density contours [38 e/(Å)3] for the two bands around the Fermi level for the aC3-(BN)3NT, aC2-(BN)2NT, and aC1-(BN)5NT are shown in d-f, respectively. Green, red, and blue balls denote C, N, and B atoms, respectively.

through first-principles calculations. Our calculations indicate that the C-BNNTs possess high thermodynamic stability comparable to that of CNTs, and all C-BNNTs can be stable above room temperature when the diameter is larger than 0.4 nm. Concerning the electronic properties, all zigzag C-BNNTs are direct gap semiconductors with band gaps tunable between 0.09 and 0.14 eV within our calculated diameter range. For an armchair tube, only the C-BNNTs with a rather low ratio of carbon component have steady direct band gaps of about 0.77 eV or those with diameters less than 0.6 nm are indirect semiconductors, and all the rest are metallic. In zigzag C-BNNTs, both the LUCO and HOCO states derive from the carbon segment, while in armchair C-BNNTs, the LUCO and HOCO near EF are from the hybrid states distributed on the two segment junctions. A peculiar interplay between the quantum confinement effect and the local tube curvature leads to the interesting change of electronic properties with the chemical ratio, which should have potential applications in nanotube gap engineering and nanoscale optoelectronic devices. Acknowledgment. This work is supported by the 973 Program (2007CB936204), National NSF (10732040), Jiangsu Province NSF (BK2008042), and the MOE (705021, IRT0534) of China. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (3) Rubio, A.; Corkill, J. L.; Cohen, M. L. Phys. ReV. B 1994, 49, R5081. (4) Chopra, N. G.; Luyken, R. L.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966. (5) Blase´, X.; Rubio, A.; Louie, S. G.; Cohen, M. L. Phys. ReV. B 1995, 51, 6868. (6) Odom, T. W.; Huang, J. L.; Kim, P.; Lieber, C. M. J. Phys. Chem. B 2000, 104, 2794. (7) Miyamoto, Y.; Rubio, A.; Louie, S. G.; Cohen, M. L. Phys. ReV. B 1994, 50, 18360. (8) Miyamoto, Y.; Rubio, A.; Cohen, M. L.; Louie, S. G. Phys. ReV. B 1994, 50, 4976.

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