Reducing Band Gap and Enhancing Carrier Mobility of Boron Nitride

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Reducing Band Gap and Enhancing Carrier Mobility of Boron Nitride Nanoribbons by Conjugated π Edge States Yu Wang,† Yafei Li,*,† and Zhongfang Chen*,‡ †

College of Chemistry and Materials Science, Jiangsu Key Laboratory of Biofunctional Materials, Nanjing Normal University, Nanjing, Jiangsu 210046, China ‡ Department of Chemistry, Institute for Functional Nanomaterials, University of Puerto Rico, Rio Piedras Campus, San Juan 00931, Puerto Rico ABSTRACT: The wide band gap of boron nitride (BN) materials has been a main bottleneck for their wide applications in electronics. In this work, by means of density functional theory (DFT) computations, we demonstrated that the band gaps of BN nanoribbons (BNNRs), with either zigzag or armchair edges, can be significantly reduced by C2H termination. The neighboring C2H terminal groups of BNNRs can covalently connect with each other, forming infinite carbon chains along the edges. The newly formed carbon chains introduce conjugated π edge states, which contribute to the band gap reduction of BNNRs. C2H termination can also result in higher carrier mobility for BNNRs. This simple strategy offers great opportunities for developing BNNR-based electronic devices.

1. INTRODUCTION Graphene,1,2 a single layer of graphite, has demonstrated intriguing physical properties and potential applications in nanoelectronics.3−5 The rise of graphene also stimulated the investigation of inorganic two-dimensional (2D) structures,6−8 among which boron nitride (BN) monolayer is one shining example. The BN monolayer, which is isoelectronic and structurally analogous to graphene, has been fabricated experimentally via different approaches.9−12 The BN monolayer possesses a number of properties rivaling or surpassing those of graphene. For example, the BN monolayer exhibits higher thermal stability and chemical inertness than graphene,13 which would facilitate its applications in some harsh environments. Especially, different from graphene which is semimetallic, the BN monolayer has a large energy band gap (>5 eV) due to the strong ionicity of B−N bonds originating from the considerable difference in electronegativity between B and N atoms. Experimentally, BN nanoribbons (BNNRs) can be obtained by cutting the BN monolayer or by unzipping the BN nanotubes (BNNTs) in the axial direction.14−16 The electronic properties of BNNRs have been extensively investigated. It was found that fully bare BNNRs with zigzag edges (zBNNRs) are magnetic metals, as confirmed by both theoretical17 and experimental18 studies. When B edges are passivated with H atoms, the half-bare zBNNRs can present the intriguing halfmetallicity property.19,20 However, the dangling bonds of bare BNNRs are rather active and can be easily passivated. When two edges are both passivated by H atoms, zBNNRs and armchair BNNRs (aBNNRs) are all semiconducting with a wide band gap close to that of 2D BN monolayer.21 © 2014 American Chemical Society

Due to the wide band gap, BNNRs are actually out of the scope of nanoelectronics applications. To conquer this bottleneck, significant theoretical efforts have been devoted to modulating the wide-band gap of BNNRs. It has been found that applying an external transverse electric field22−24 or an axial strain25 can significantly reduce the band gap for both zBNNRs and aBNNRs. However, these physical approaches are rather challenging for experimental realization since the required electric field or strain is rather high. Some theoretical studies revealed that the existence of defects,26−28 or graphene islands29−31 in BNNRs can reduce the band gap, however, the reduction range is inadequate and it is also difficult to flexibly construct defects or graphene islands in BNNRs. Chemical functionalization seems more promising toward tuning the electronic properties of BNNRs,32−38 especially for zBNNRs. For example, Wu et al.32 predicted that chemical decoration by monovalent groups (e.g., F, Cl) at edges can efficiently reduce the band gap of zBNNRs; Chen et al.33 theoretically demonstrated that the electronic and magnetic properties of zBNNRs can be significantly modified by changing the hydrogenation coverage; Lopez-Bezanilla et al.34 showed that oxygen (O) functionalization at edges can convert zBNNRs into metallic. However, the electronic properties of aBNNRs are rather robust toward such modifications.32,33,35 Theoretical studies39,40 demonstrated that the armchair edges of bare BNNRs are more stable in energy than zigzag Received: August 3, 2014 Revised: September 28, 2014 Published: October 5, 2014 25051

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optimized structures of C2H-8-zBNNR and C2H-13-aBNNR are presented in Figure 2.

ones. Thus, the edges of experimentally synthesized BNNRs should prefer to crystallize in armchair shape. Note that BNNTs are known experimentally41−43 to have a preferential zigzag orientation, the BNNRs obtained by unzipping BNNTs are expected to be dominated by armchair ones. Therefore, for the practical application of BNNRs, it is urgent for us to figure out a feasible method which could also effectively reduce the band gap of aBNNRs. In this work, by means of density functional theory (DFT) computations, we revealed that the ethynyl radical (C2H) is the ideal terminal group for BNNRs with either zigzag or armchair edge in terms of reducing the band gap and enhancing carrier mobility. C2H groups can form zigzag or armchair carbon chains along the edge of BNNRs, which introduces conjugated π edge states to BNNRs and correspondingly lowers the band gap significantly.

Figure 2. Optimized structures of (a) 8-C2H-zBNNR and (b) 13-C2HaBNNR. The blue, pink, gray and white balls represent N, B, C and H atoms, respectively.

After full geometry optimization, the energetically most favorable edge configurations for B and N edges of C2H-8zBNNR both prefer to have neighboring C2H radicals connected to each other to form a zigzag carbon chain along the axial direction (Figure 2a). Specifically, C1 and C2 atoms of each C2H radical bind to the C2 atom of upper C2H radical and the C1 atom of bottom C2H radical, respectively. In this case, both the C1 and C2 atoms of the C2H radicals turn from sp to sp2 hybridization. As a result, the C1−C2 bond lengths for C2H8-zBNNR at the B and N edges (1.42 and 1.41 Å, respectively) are much longer than that of the isolated C2H radical (1.22 Å). The lengths of newly formed C1−B and C1−N bonds are 1.56 and 1.42 Å, respectively. In fact, the C2H termination makes zBNNRs have the same edge configuration as that of hydrogen terminated zigzag graphene nanoribbons (H-zGNRs). It is known that H-zGNRs with pure sp2 edge configuration have intriguing magnetic edge states.50−52 However, our computations revealed that C2H-8zBNNR has a nonmagnetic ground state. What makes the difference for zGNRs and zBNNRs? As shown in Figure 2a, for C2H-8-zBNNR, all the inner B and N atoms form three chemical bonds with neighboring atoms without leaving an unpaired electron. The edge C atoms can form alternative C C double bonds and C−C single bonds to ensure that all C atoms form four chemical bonds with neighboring atoms. Therefore, C2H-8-zBNNR has a nonmagnetic ground state since there is no unpaired electron in the whole ribbon. In contrast, according to the Clar’s rule,53,54 the unpaired electrons would appear at the edges for those zGNRs with pure sp2 edge configuration,55,56 which leads to a magnetic ground state. Similar to C2H-8-zBNNR, C2H groups at two edges of 13aBNNR also prefer to connect covalently to each other to form armchair carbon chains along the edges. Differently, for C2H13-aBNNR, C1 and C2 atoms of each C2H radical bind to the C1 atom of the bottom C2H radical and C2 atom of the upper C2H radical, respectively. The lengths of newly formed C1−C1 and C2−C2 bonds are 1.42 and 1.40 Å, respectively. The lengths of C1−B and C1−N bonds are 1.51 and 1.40 Å, respectively. The C atoms of C2H-13-aBNNR also can be fully saturated by forming alternative double and single bonds, leading to a nonmagnetic ground state. To get more information on the structural properties and chemical bonding of C2H-BNNRs, we also plotted the projected electronic density for C2H-8-zBNNR and C2H-13aBNNR. As shown in Figure 3, the electron distributions for both nanoribbons are quite localized in the inner parts, and N

2. COMPUTATIONAL METHODOLOGY Our DFT computations were carried out by using an allelectron method within a generalized gradient approximation (GGA) for the exchange-correlation term, as implemented in the DMol3 code.44,45 The double numerical plus polarization (DNP) basis set and PBE functional46 were adopted in all computations. Self-consistent field (SCF) computations were performed with a convergence criterion of 10−6 a.u. on the total energy and electron density. To ensure high-quality numerical results, we chose the real-space global orbital cutoff radius (fine) in all computations. The Brillouin zones of nanoribbons were sampled by 1 × 1 × 10 k and 1 × 1 × 40 k points for geometric optimizations and electronic structure computations, respectively. 3. RESULTS AND DISCUSSION Ethynyl (C2H) RadicalThe Terminal Group. The C2H radical, first identified by Tucker et al. in 1974 in the Orion nebula,47 can be made via photolysis of acetylene (C2H2).48 Structurally it consists of two sp hybridized C atoms (labeled as C1 and C2, respectively) and one H atom (Figure 1). Our PBE

Figure 1. Scheme of the C2H radical. The black and white ball denote the C and H atom, respectively. The red point represents the unpaired electron.

computations showed that this radical has a doublet ground state (2Σ+) with the unpaired electron localized at the C1 atom; the computed C1C2 and C2−H bond lengths (1.22 and 1.07 Å, respectively) agree reasonably well those at the UHFCCSD(T)/cc-pVTZ level of theory (1.208 and 1.058 Å).49 Structural Properties of C2H-Terminated BNNRs (C2HBNNRs). To study the structural properties of C2H-terminated BNNRs (C2H-BNNRs), we chose the 8-zBNNR and 13aBNNR with a width of 22.1 and 22.5 Å, respectively, as two representatives. Following previous conventions,22−24 the ribbon width parameter Nz (Na) is defined as the number of zigzag chains for a zigzag ribbon and the number of dimer lines along the ribbon direction for an armchair ribbon. Initially, for both 8-zBNNR and 13-aBNNR, each edge atom was passivated with one C2H radical to terminate the dangling bond. The 25052

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contrast, H-8-zBNNR has a much larger indirect band gap 4.29 eV computed at the same theoretical levels. Thus, C2H termination is effective in reducing the band gap of zBNNRs. Does C2H termination also reduce the band gap of aBNNRs? To this end, we computed the band structure of C2H-13aBNNR. Encouragingly, C2H-13-aBNNR also has a rather low direct band gap of 0.69 eV (Figure 4b), compared with a 4.64 eV direct band gap of H-13-aBNNR. Besides C2H-8-zBNNR and C2H-13-aBNNR, we also computed the band gaps of a series of C2H terminated BNNRs. Figure 5 shows the band gap variation of C2H-

Figure 3. Projected electronic density of (a) C2H-8-zBNNR and (b) C2H-13-aBNNR.

atoms possess more electrons than B atoms, indicating the strong ionic feature of B−N bonds due to the remarkable difference in electronegativity between B and N atoms. In sharp contrast, the electron distributions for the edge C atoms of both nanoribbons are quite delocalized. There appears to be a “conduction” channel along the axial direction at each edge of BNNRs. These conduction channels were not found in HBNNRs.22−24 No doubt, the conduction channels of C2HBNNRs should correspond to the conjugated π states of sp2 hybridized carbon chains, which would make the electronic properties of of C2H-BNNRs distinguished from those of HBNNRs. Electronic Properties of C2H-BNNRs. We first computed the band structure of C2H-8-zBNNR. As shown in Figure 4a, C2H-8-zBNNR has a rather low direct band gap of 0.1 eV with the valence band maximum (VBM) and the conduction band minimum (CBM) both located at the Γ point. In sharp

Figure 5. Variation of band gaps of C2H-BNNRs as a function of the ribbon width.

aBNNRs and C2H-zBNNRs as a function of ribbon width. For C2H-zBNNRs, the band gap increases slightly with increasing ribbon width, and reaches to only 0.2 eV at width of 36 Å. For C2H-8-aBNNR, the band gap undergoes a tiny oscillation for

Figure 4. Band structures (left) and density of states (right) of 8-C2H-zBNNR (a) and 13-C2H-aBNNR (b). Partial charge density of the VBM (upper) and the CBM (lower) of 8-C2H-zBNNR (c) and 13-C2H-aBNNR (d) at the Γ point. The pink dashed line denotes the position of Fermi energy. 25053

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Table 1. Carrier Effective Masses (m*), Stretching Modules (C), Deformation Potential Constant (E1), and Carrier Mobility of Investigated C2H- and H-BNNRs C2H-8-zBNNR C2H-13-aBNNR H-8-zBNNR H-13-aBNNR

electron hole electron hole electron hole electron hole

m* (in m0)

C(eV/Å)

E1 (eV)

μ (cm2/(V s))

0.09 0.08 0.32 0.33 1.74 0.85 0.70 0.58

398.58

7.20 4.84 4.26 2.96 9.8 3.44 3.96 2.46

2292.95 6031.09 896.45 1773.04 11.30 268.66 234.35 805.18

367.17 310.63 268.35

and VBM (for hole). The DP theory has been successfully employed to predict the carrier mobility of many 1D systems.58−60 The computed results for C2H- and H-BNNRs are summarized in Table 1. According to our computations, H-8-zBNNR has a me and mh of 1.74 and 0.85 m0 (m0 is the free electron mass), respectively, while H-13-aBNNR has a me and mh of 0.70 and 0.58 m0, respectively. Note that our results achieve good agreement with previous studies.22,25 Remarkably, C2H-8zBNNR has a much lower me and mh of 0.09 and 0.08 m0, respectively, while C2H-13-aBNNR has a me and mh of 0.32 and 0.33 m0, respectively. The lower carrier effective masses of C2HBNNRs also results in higher carrier mobility than those of HBNNRs, as shown in Table 1. Especially, the carrier mobility of C2H-8-zGNR (2292.95 and 6031.09 cm2/(V s) for electron and hole, respectively) are not only higher in an order of magnitude than those of H-8-zBNNR, but also much higher than those of silicon bulk (∼1500 and ∼450 cm2/(V s) for electron and hole, respectively), indicating that C2H-BNNRs could be a promising candidate for future microelectronics. Stability of C2H-BNNRs. To assess the stability of C2HBNNRs, we calculated their respective edge formation energy (Eedge), which is defined as

those narrow ribbons, and then converges to a constant value of 0.7 eV as the width increases. Therefore, C2H termination can result in a rather low direct band gap for both zBNNRs and aBNNRs, which is superior to previously explored terminal groups, such as H,21 F,32 and OH.35 Especially, these C2HBNNRs all have a direct band gap, which would facilitate their applications in optoelectronics. Our above computations showed that the band gaps of both zBNNRs and aBNNRs can be notably lowered by C2H termination. What is the underlying mechanism? First, we computed the total and partial density of states (DOS) for both C2H-8-zBNNR and C2H-13-aBNNR. As shown in Figure 4, for both BNNRs, the electronic states near the Fermi level mainly are mainly contributed to by C-2p states and slightly from 2pstates of B and N atom. To get some deeper insights, we also plotted the partial charge densities associated with the VBM and CBM. For C2H-8-zBNNR, the VBM mainly localizes at the C atoms of the B edge, while the CBM is mainly contributed by C atoms of N edge, and the B and N atoms at edges also contribute to the VBM and CBM (Figure 4c). For C2H-13aBNNR, both VBM and CBM are mainly from carbon atoms at two edges, and are also slightly contributed by the edge B/N atoms (Figure 4d). Therefore, the electronic properties of C2Hterminated BNNR are dominated by the edge conjugated π states and to a less extent by the intrinsic edge states. Considering that H-BNNRs with edge states present much larger band gaps, we can safely conclude that the remarkable band gap reduction in C2H-BNNRs can be ascribed to the presence of edge-conjugated π states. Carrier Mobility of C2H-BNNRs. For device applications, not only the band gap but also the carrier mobility is important. Therefore, we then computed the carrier mobility of C2HBNNRs and compared it with those of H-BNNRs on the basis of the deformation potential (DP) theory proposed by Bardeen and Shockley.57 According to the DP theory, the carrier mobility (μ) of one-dimensional (1D) systems can be given by the expression: μ=

Eedge =

1 (Eribbon − NBNμBN − NC2HμC H ) 2 2L

where Eribbon is the total energy of a C2H-BNNR, μBN and μC2H are the chemical potential of one BN pair and C2H radical, respectively. NBN and NC2H are the number of BN pairs and C2H radical in a given BNNR. In our study, μBN is the total energy per BN pair of infinite 2D BN monolayer, and μC2Hk is equal to half of the total energy of the C4H2 molecule, which can be seen as the dimer of C2H radicals. According to this definition, the more stable edge has a lower Eedge. We examined the variation of Eedge as a function of ribbon width for C2H-BNNRs (Figure 5). For comparison, the Eedge of a series of H-BNNRs were also computed. The definition of Eedge for H-BNNRs is similar to that of C2H-BNNRs, and the half of the total energy of the H2 molecule is taken as the chemical potential of H (μH)61 As shown in Figure 6, the Eedge of C2H-zBNNRs and C2HaBNNRs are rather robust in response to the increase of ribbon width. The average Eedge of C2H-zBNNRs is around −1.33 eV/ Å, which is higher than that of C2H-aBNNRs (−1.55 eV/Å). The negative Eedge of C2H-BNNRs manifest the higher stability relative to the 2D BN monolayer and C4H2 molecule. Especially, the Eedge of C2H-BNNRs are much lower than those of H-BNNRs, indicating that C2H termination can produce much more stable edges than hydrogen termination.

eℏ2C (2πkBT )1/2 |m*|3/2 E12

where ℏ is the reduced Planck constant, kB is Boltzmann constant, T is the temperature (300 K). C is the stretching modulus, which is defined as C= [∂2E/∂ε2]/a0, where E is the total energy of the ribbon, a0 is the lattice constant, and ε = (a − a0)/a0 is the applied uniaxal strain along the axis direction of the ribbon. m* is the carrier effective mass (electron, me; hole, mh) and is defined as m* = ℏ2[∂2E(k)/∂k2]−1, where k is the coordinate vector in reciprocal, E(k) is the dispersion relation. E1 is the deformation potential (DP) constant and is define as E1 = ∂Eedge/∂ε, where Eedge is the value of CBM (for electron) 25054

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(2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451−10453. (3) Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, P. A. A Roadmap for Graphene. Nature 2012, 490, 192−200. (4) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156−6214. (5) Tang, Q.; Zhou, Z.; Chen, Z. Graphene Related Materials: Tuning Properties by Functionalization. Nanoscale 2013, 5, 4541− 4583. (6) Tang, Q.; Zhou, Z. Graphene-Analogues Low-Dimensional Materials. Prog. Mater. Sci. 2013, 58, 1244−1315. (7) Xu, M. S.; Liang, T.; Shi, M.; Chen, H. Graphene-like TwoDimensional Materials. Chem. Rev. 2013, 113, 3766−3798. (8) Koski, K. J. The New Skinny in Two-Dimensional Nanomaterials. ACS Nano 2013, 7, 3739−3743. (9) Meyer, J. C.; Chuvilin, A.; Algara-Siller, G.; Biskupek, J.; Kaiser, U. Selective Sputtering and Atomic Resolution Imaging of Atomically Thin Boron Nitride Membranes. Nano Lett. 2009, 9, 2683−2689. (10) Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. LargeScale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties. Adv. Mater. 2009, 21, 2889−2893. (11) Pacilé, D.; Meyer, J. C.; Girit, C. O.; Zettl, A. The TwoDimensional Phase of Boron Nitride: Few-Atomic-Layer Sheets and Suspended Membranes. Appl. Phys. Lett. 2008, 92, 133107. (12) Golberg, D.; Bando, Y.; Huang, Y.; Terao, T.; Mitome, M.; Tang, C. Boron Nitride Nanotubes and Nanosheets. ACS Nano 2010, 4, 2979−2993. (13) Li, L. H.; Cevenka, J.; Watanabe, K.; Taniguchi, T.; Chen, Y. Strong Oxidation Resistance of Atomically Thin Boron Nitride Nanosheets. ACS Nano 2014, 8, 1457−1462. (14) Zeng, H.; Zhi, C.; Zhang, Z.; Wei, X.; Wang, X.; Guo, W.; Bando, Y.; Golberg, D. “White Graphenes”: Boron Nitride Nanoribbons via Boron Nitride Nanotube Unwrapping. Nano Lett. 2010, 10, 5049−5055. (15) Erickson, K. J.; Gibb, A. L.; Sinitskii, A.; Rousseas, M.; Alem, N.; Tour, J. M.; Zettl, A. K. Longitudinal Splitting of Boron Nitride Nanotubes for the Facile Synthesis of High Quality Boron Nitride Nanoribbons. Nano Lett. 2011, 11, 3221−3226. (16) Liao, Y. L.; Chen, Z.; Connell, J. W.; Fay, C. C.; Park, C.; Kim, J. W.; Lin, Y. Chemical Sharpening, Shortening, and Unzipping of Boron Nitride Nanotubes. Adv. Funct. Mater. 2014, 24, 4497−4506. (17) Barone, V.; Peralta, J. E. Magnetic Boron Nitride Nanoribbons with Tunable Electronic Properties. Nano Lett. 2008, 8, 2210−2214. (18) Terrones, M.; Charlier, J. C.; Gloter, A.; Cruz-Silva, E.; Terrés, E.; Li, Y. B.; Zanolli, Z.; Dominguez, J. M.; Terrones, H.; Bando, Y.; Golberg, D. Experimental and Theoretical Studies Suggesting the Possibility of Metallic Boron Nitride Edges in Porous Nanourchins. Nano Lett. 2008, 8, 1026−1032. (19) Zheng, F.; Zhou, G.; Liu, Z.; Wu, J.; Duan, W.; Gu, B.; Zhang, S. B. Half Metallicity along the Edge of Zigzag Boron Nitride Nanoribbons. Phys. Rev. B 2008, 78, 205415. (20) Lai, L.; Lu, J.; Wang, L.; Luo, G. F.; Zhou, J.; Qin, R.; Gao, Z. X.; Mei, W. N. Magnetic Properties of Fully Bare and Half-Bare Boron Nitride Nanoribbons. J. Phys. Chem. C 2009, 113, 2273−2276. (21) Du, A.; Smith, S. C.; Lu, G. First-Principles Studies of Electronic Structure and C-Doping Effect in Boron Nitride Nanoribbon. Chem. Phys. Lett. 2007, 447, 181−186. (22) Park, C.-H.; Louie, S. G. Energy Gaps and Stark Effect in Boron Nitride Nanoribbons. Nano Lett. 2008, 8, 2200−2203. (23) Zheng, F. W.; Liu, Z. R.; Wu, J.; Duan, W. H.; Gu, B.-L. Scaling Law of the Giant Stark Effect in Boron Nitride Nanoribbons and Nanotubes. Phys. Rev. B 2008, 78, 085423.

Figure 6. Variation of Eedge of C2H-zBNNRs and C2H-aBNNRs as a function of ribbon width. The inset figure is the variation of the Eedge of H-zBNNRs and H-aBNNRs as a function of ribbon width.

Moreover, it should be specified that the C2H radical also can react with the inner atoms of BNNRs. However, under real experimental conditions, C2H radicals would react with the more reactive edge atoms first, and the addition of C2H to the basal plane of BNNRs would be overwhelmed providing that the concentration of C2H is adequately low.

4. CONCLUSIONS To summarize, we demonstrated that the C2H radical is an ideal terminal group for BNNRs in terms of reducing the wide band gap and enhancing carrier mobility. C2H radicals can form interesting sp2 hybridized carbon chains at the edges of BNNRs and hence introduce the conjugated π states to BNNRs. As a result, the band gaps of both aBNNRs and zBNNRs can be significantly reduced. Moreover, C2H-BNNRs have lower carrier effective masses than H-BNNRs, which would be an advantage for their applications in electronic devices. C2HBNNRs also have higher stability than H-BNNRs. Our theoretical results would shed some light on the possible ways to promote the design of BNNRs-based devices.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support in China by NSFC (21403115), startup funds of Nanjing Normal University (184080H20145), and Jiangsu Specially Appointed Professor Plan, and in USA by the Department of Defense (Grant W911NF-12-1-0083) is gratefully acknowledged. The computational resources utilized in this research were provided by the Shanghai Supercomputer Center.

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REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Gregorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. 25055

dx.doi.org/10.1021/jp5078328 | J. Phys. Chem. C 2014, 118, 25051−25056

The Journal of Physical Chemistry C

Article

(46) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (47) Tucker, K. D.; Kutner, M. L.; Thaddeus, P. The Ethynyl Radical C2HA New Interstellar Molecule. Astrophys. J. 1974, 193, L115. (48) Fahr, A. Ultraviolet Absorption Spectrum and Cross-Sections of Ethynyl (C2H) Radicals. J. Mol. Spectrosc. 2003, 217, 249−254. (49) Fortenberry, R. C.; King, R. A.; Stanton, J. F.; Crawford, T. D. A Benchmark Study of the Vertical Electronic Spectra of the Linear Chain, Radicals C2H and C4H. J. Chem. Phys. 2010, 132, 144303. (50) Fujita, M.; Wakabayashi, K.; Nakada, K.; Kusakabe, K. Peculiar Localized State at Zigzag Graphite Edge. J. Phys. Soc. Jpn. 1996, 65, 1920−1923. (51) Nakada, K.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Edge State in Graphene Ribbons: Nanometer Size Effect and Edge Shape Dependence. Phys. Rev. B 1996, 54, 17954−17961. (52) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Energy Gaps in Graphene Nanoribbons. Phys. Rev. Lett. 2006, 97, 216803. (53) Clar, E. Polycyclic Hydrocarbons; Academic Press: New York, 1964. (54) Clar, E. The Aromatic Sextet; Wiley: London, 1972. (55) Wassmann, T.; Seitsonen, A. P.; Saitta, A. M.; Lazzeri, M.; Mauri, F. Clar’s Theory, π-Electron Distribution, and Geometry of Graphene Nanoribbons. J. Am. Chem. Soc. 2010, 132, 3440−3451. (56) Li, Y. F.; Zhou, Z.; Carlos, C. R.; Chen, Z. F. Preserving the Edge Magnetism of Zigzag Graphene Nanoribbons by Ethylene Termination: Insights by Clar’s Rule. Sci. Rep. 2013, 3, 2030. (57) Bardeen, J.; Shockley, W. Deformation Potential and Mobilities in Non-polar Crystals. Phys. Rev. 1950, 80, 72−80. (58) Long, M.-Q.; Tang, L.; Wang, D.; Wang, L.; Shuai, Z. Theoretical Predictions of Size-Dependent Carrier Mobility and Polarity in Graphene. J. Am. Chem. Soc. 2009, 131, 17728−17729. (59) Long, M.; Tang, L.; Wang, D.; Li, Y.; Shuai, Z. Electronic Structure and Carrier Mobility in Graphdiyne Sheet and Nanoribbons: Theoretical Predictions. ACS Nano 2011, 5, 2593−2600. (60) Cai, Y.; Zhang, G.; Zhang, Y.-W. Polarity-Reversed Robust Carrier Mobility in Monolayer MoS2 Nanoribbons. J. Am. Chem. Soc. 2014, 136, 6269−6275. (61) Huang, B.; Lee, H.; Gu, B.-L.; Liu, F.; Duan, W. H. Edge Stability of Boron Nitride Nanoribbons and Its Application in Designing Hybrid BNC Structures. Nano Res. 2012, 5, 62−72.

(24) Zhang, Z. H.; Guo, W. L. Energy-Gap Modulation of BN Ribbons by Transverse Electric Fields: First-Principles Calculations. Phys. Rev. B 2008, 77, 075403. (25) Qi, J.; Qian, X.; Qi, L.; Feng, Q.; Shi, D.; Li, J. StrainEngineering of Band Gaps in Piezoelectric Boron Nitride Nanoribbons. Nano Lett. 2012, 12, 1224−1208. (26) Chen, W.; Li, Y.; Yu, G. T.; Zhou, Z.; Chen, Z. Electronic Structure and Reactivity of Boron Nitride Nanoribbons with StoneWales Defects. J. Chem. Theory Comput. 2009, 5, 3088−3095. (27) Li, X.; Wu, X.; Zeng, X. C.; Yang, J. Band-Gap Engineering via Tailored Line Defects in Boron Nitride Nanoribbons, Sheet, and Nanotubes. ACS Nano 2012, 6, 4104−4112. (28) Tang, S.; Zhang, S. Electronic and Magnetic Properties of Hybrid Boron Nitride Nanoribbons and Sheets with 5−7 Line Defects. J. Phys. Chem. C 2013, 117, 17309−17318. (29) Gao, Y.; Zhang, Y.; Chen, P.; Li, Y.; Liu, M.; Gao, T.; Ma, D.; Chen, Y.; Cheng, Z.; Qiu, X.; et al.et al. Toward Single-Layer Uniform Hexagonal Boron Nitride−Graphene Patchworks with Zigzag Linking Edges. Nano Lett. 2013, 13, 3439−3443. (30) Bhowmick, S.; Singh, A. K.; Yakobson, B. I. Quantum Dots and Nanoroads of Graphene Embedded in Hexagonal Boron Nitride. J. Phys. Chem. C 2011, 115, 9885−9893. (31) Jung, J.; Qiao, Z.; Niu, Q.; MacDonald, A. H. Transport Properties of Graphene Nanoroads in Boron Nitride Sheets. Nano Lett. 2013, 13, 3439−3443. (32) Wu, X.; Wu, M.; Zeng, X. C. Chemically Decorated Boron− Nitride Nanoribbons. Front. Phys. China 2009, 4, 367−372. (33) Chen, W.; Li, Y.; Yu, G. T.; Li, C.; Zhang, S. B.; Zhou, Z.; Chen, Z. Hydrogenation: A Simple Approach To Realize Semiconductor− Half-Metal−Metal Transition in Boron Nitride Nanoribbons. J. Am. Chem. Soc. 2010, 132, 1699−1705. (34) Lopez-Bezanilla, A.; Huang, J.; Terrones, H.; Sumpter, B. G. Boron Nitride Nanoribbons Become Metallic. Nano Lett. 2011, 11, 3267−3273. (35) Lopez-Bezanilla, A.; Huang, J.; Terrones, H.; Sumpter, B. G. Structure and Electronic Properties of Edge-Functionalized Armchair Boron Nitride Nanoribbons. J. Phys. Chem. C 2012, 116, 15675− 15681. (36) Krepel, D.; Hod, O. Effects of Edge Oxidation on the Structural, Electronic, and Magnetic Properties of Zigzag Boron Nitride Nanoribbons. J. Chem. Theory Comput. 2014, 10, 373−380. (37) Liu, Y.; Wu, X.; Zhao, Y.; Zeng, X. C.; Yang, J. Half-Metallicity in Hybrid Graphene/Boron Nitride Nanoribbons with Dihydrogenated Edges. J. Phys. Chem. C 2011, 115, 9442−9450. (38) Tang, Q.; Bao, J.; Li, Y.; Zhou, Z.; Chen, Z. Tuning Band Gaps of BN Nanosheets and Nanoribbons via Interfacial Dihalogen Bonding and External Electric Field. Nanoscale 2014, 6, 8624−8634. (39) Mukherjee, R.; Bhowmick, S. Edge Stabilities of Hexogonal Boron Nitride Nanoribbons: A First-Principles Study. J. Chem. Theory Comput. 2011, 7, 720−724. (40) Huang, B.; Lee, H.; Gu, B. L.; Liu, F.; Duan, W. H. Edge Stability of Boron Nitride Nanoribbons and Its Applications in Designing Hybrid BNC Structures. Nano. Res. 2012, 5, 62−72. (41) Loiseau, A.; Willaime, F.; Demoncy, N.; Hug, G.; Pascard, H. Boron Nitride Nanotubes with Reduced Numbers of Layers Synthesized by Arc Discharge. Phys. Rev. Lett. 1996, 76, 4737. (42) Menon, M.; Srivastava, D. Structure of Boron Nitride Nanotubes: Tube Closing versus Chirality. Chem. Phys. Lett. 1999, 307, 407. (43) Arenal, R.; Kociak, M.; Loiseau, A.; Miller, D.-J. Determination of Chiral Indices of Individual Single- and Double-Walled Boron Nitride Nanotubes by Electron Diffraction. Appl. Phys. Lett. 2006, 89, 073104. (44) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (45) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756−7764. 25056

dx.doi.org/10.1021/jp5078328 | J. Phys. Chem. C 2014, 118, 25051−25056