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Electronic properties and relative stabilities of heterogeneous edge-decorated zigzag boron nitride nanoribbons
Journal of Alloys and Compounds 649 (2015) 1130e1135
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Electronic properties and relative stabilities of heterogeneous edge-decorated zigzag boron nitride nanoribbons L.L. Li*, X.F. Yu, X.J. Yang, X.H. Zhang, X.W. Xu, P. Jin, J.L. Zhao, X.X. Wang, C.C. Tang** Key Lab for Micro- and Nano-Scale Boron Nitride Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2015 Received in revised form 9 July 2015 Accepted 13 July 2015 Available online 15 July 2015
The wide band gap of boron nitride (BN) materials has been a major bottleneck for a wider application of BN in electronics. In this work, density functional theory computations were used to study the band structure of zigzag BN nanoribbons (BNNRs). Due to the ionic origin of the BN band gap, a heterogeneous edge decoration is an effective way to modulate the electronic band structure of BNNRs. This study demonstrates that a metallic behavior and magnetism can be realized by applying a NO2eNH2 pair edge decoration. Although the lone electron pair of the NH2 group is partly responsible for the metallic behavior, the effective potential difference induced by the donoreacceptor pair is also crucial for metallic behavior. Furthermore, these newly formed BNNRs were found to be more stable than H-passivated BNNRs. This simple chemical modification method offers great opportunities for the development of future BNNR-based electronic devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Boron nitride nanoribbons DFT Band structure Heterogeneous edge decoration
1. Introduction One-dimensional (1D) nanostructures, such as nanorods, nanotubes, nanowires, and nanobelts, are of great fundamental and technological significance due to their interesting electronic and physical properties associated with their low dimensionality and the occurrence of quantum confinement effects [1e4]. Recently, boron nitride nanoribbons (BNNRs) have been successfully synthesized by either cutting layered BN (hexagonal, h-BN) sheets or by unzipping BN nanotubes [5e7]. As a kind of novel 1D nanomaterial, BNNRs have also attracted much attention due to their excellent properties and high application potential. BNNRs are isoelectronic and have the same structure as graphene nanoribbons (GNRs). However, due to the large ionicity of the BeN chemical bond, BNNRs exhibit some unique properties that are different compared to the corresponding properties of GNRs. Zigzag BNNRs (zBNNRs) with bare edges are magnetic semiconductors and their band gap decreases with the ribbon width, which was confirmed both theoretically [8e11] and experimentally [12]. When the B edges were passivated with H atoms, a half-metallic behavior was
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.L. Li), [email protected] (C.C. Tang). http://dx.doi.org/10.1016/j.jallcom.2015.07.100 0925-8388/© 2015 Elsevier B.V. All rights reserved.
observed [13,14]. The dangling bonds of bare zBNNRs are rather active and can be easily passivated. When both edges of the ribbons were passivated with H atoms, the zBNNRs showed a semiconducting behavior, with a wide band gap close to the band gap of 2D BN monolayers [15]. However, this wide band gap has been a major bottleneck for a wider application of BNNRs in electronics. To promote the design of future BNNR-based devices, both the magnetic properties of BNNRs and possibilities to modify the band gap have been the focus of extensive studies. The formation of StoneeWales (SW) defects [16] as well as an edge modification [17] with F, Cl, OH, and NO2 groups can change the band gap of zBNNRs, although these decorated zBNNRs still retain their semiconducting behavior. In contrast, both an O and S edge-termination can result in a metallic behavior [18]. In addition to these homogeneous chemical edge decorations, the application of an external transverse electric field [19e21] or an axial strain [22] can significantly reduce the band gap of zBNNRs. The studies mentioned above indicated that the generation of an electrostatic potential difference between the two edges of a BNNR is a prerequisite for mediating the electronic band structure. Heterogeneous edge modification seems to be a promising approach to achieve such a potential difference. In this study, we used first-principle calculations based on density functional theory (DFT) to assess the electronic properties and relative stabilities of heterogeneous edge-decorated BNNRs.
L.L. Li et al. / Journal of Alloys and Compounds 649 (2015) 1130e1135
Because different effective potentials at the two edges are highly desirable, a donor and an acceptor group should constitute a good chemical modification pair. We found that a heterogeneous edge decoration based on a NO2eNH2 pair can convert the behavior of the BNNR from semiconductor to a metallic behavior. To clarify the effect of the lone electron pair of the NH2 group, a systematic analysis of the electronic properties of BNNRs edge-decorated with a heterogeneous HeNH2 pair was then carried out. Furthermore, the stability of BNNRs with a heterogeneous NO2eNH2 pair edge decoration was studied by considering the Gibbs free energy of formation dG to promote the development of novel BNNR-based electronic devices. 2. Calculations The spin-polarized first-principles DFT computations were performed using a plane wave basis set with the projector augmented plane wave (PAW) [23] utilized to model the ioneelectron interaction as implemented in the Vienna ab initio simulation package (VASP) [24e26]. The generalized gradient approximation (GGA) method based on the PerdeweBurkeeEmzerh (PBE) [27] functional and a 450 eV cutoff for the plane wave basis set were adopted for all computations. Our supercells were large enough to ensure that the vacuum distance is at least 12 Å, so that an interaction between the individual nanoribbons and their periodic images can be neglected. For the calculation of the structural relaxation and the relaxation energy, the k-points were generated using the MonkhorstePack method with a grid size of 1 1 11, and the convergence threshold was set to 105 eV for the energy and 0.01 eV/Å for the force. The positions of all the atoms in the supercell were fully relaxed during the geometry optimization calculations. To avoid ambiguities regarding the free energy calculation results, the same energy cutoff and a similar k-grid sampling for convergence were adopted. For the calculation of the electronic band structures, a set of strings of 21 k-points connecting specific points of the Brillouin zone was used. 3. Results and discussion Following the convention established in literature [19e21], we define the width parameter Nz of the zBNNRs as the number of zigzag lines across the ribbon width, as illustrated in Fig. 1a for 10BNNR. Before addressing the donoreacceptor pair-modified nanoribbons, we revisit the primitive H edge-passivated BNNRs, establishing their geometrical structure and electronic band structure as a frame of reference. The optimized geometry of 10-BNNR with a width of 20.32 Å is shown in Fig. 1b. There are two types of BeN bonds in the zBNNRs. One is perpendicular to the axes (P-type BeN bond), and the other is slanted (S-type BeN bond). These two types of bonds are denoted by “P” and “S” in Fig. 1. The length of the Ptype BeN bond is in the range from 1.453 to 1.459 Å, which is a bit longer than the length of the S-type BeN bond (1.429e1.445 Å). Consequently, the BeN bond perpendicular to the axis is generally weaker than the slanted BeN bond. This is attributed to the interruption of the hexagonal network of BN sep bonds in the transverse direction. The energy band diagram of 10-BNNR is shown in Fig. 1c. The Hpassivated 10-BNNR exhibits an indirect band gap of 4.1 eV, and its band structure is characteristic for a non-spin-polarized material. Our results are in good agreement with the results of previous studies, indicating that the methodology adopted in this work yields a reliable description of the electronic and magnetic properties of zBNNRs. Similar to GNRs, BNNRs are built from an infinite number of 1D six-member rings, and all the atoms are sp2-
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hybridized and contribute to the s bond. Although BeN is isoelectronic to CeC, the covalence of the BeN bond essentially differs from the covalence of the CeC bond in the following two aspects. First, the p bond in BN formed from the p orbitals accepts electrons from the N lone-pair p orbital. Second, there is another remarkable difference between the BeN bond and the CeC bond concerning the scale of the p-electron conjugations. Since N is more electropositive than B (electronegativity: B ¼ 2.04, N ¼ 3.04), the p-electrons are highly localized around N, turning the BNNRs into a semiconducting material with a wide band gap, as shown in Fig. 1c and d. zBNNRs have two distinct edges terminated by N and B atoms, respectively, and consequentially called the N-edge and the B-edge. Because the BeN bond is highly polarized, the B-edge possesses distinct properties which are dramatically different compared to the properties of the N-edge. To obtain more information on the properties of 10-zBNNR, the partial density of states (PDOS) was calculated and is shown in Fig. 1d. For the pristine zBNNR, the top valence band is mainly dominated by the N atoms at the N-edge, while the conduction band is linked to the B atoms at the B-edge. The obvious difference in electronic properties between the Bedge and the N-edge of the zBNNR stimulated the subsequent investigation of heterogeneous edge decorations. Furthermore, considering the highly localized p electrons in the zBNNR, a chemical modification may provide a powerful pathway for enhancing the physical properties of pristine BNNRs, making them more attractive for applications. Therefore, a NH2 donor group was added to the N-edge and a NO2 acceptor group was added to the Bedge of the 10-zBNNR (labeled as NO2-[10-BNNR]-NH2). Both the NH2 and the NO2 group compensate the dangling sp2-s-orbital at the edge atoms. We expected that the potential difference induced by the donoreacceptor pair might weaken the polarization of the BeN covalent bond, thereby reducing the energy band gap of the BNNR. After completion of the geometrical optimization, the energetically most favorable edge configurations for the B and the N edges of NO2-[10-BNNR]-NH2 favor the connection of the N atoms to the ribbon and the OeNeO plane (HeNeH plane) being perpendicular to the axial direction of the ribbon (Fig. 2a). The planar BN structure of the ribbon does not change. However, the S-type BeN bond is obviously shortened and there is also a little protraction of the Ptype BeN bond in and close to the edges, reflecting that the local changes in the electronic structure are induced by edge modification. The electronic properties of NO2-[10-BNNR]-NH2 were studied by computing the electronic band structure, and the results are shown in Fig. 2d. The edge states close to the X point feature a spinup channel with a low resistance characteristic for metallic materials and a spin-down channel characteristic for semiconducting materials, indicating that NO2-[10-BNNR]-NH2 is a magnetic material. Furthermore, there is a spin-unpolarized band across the Fermi level close to the G point, which makes both spin channels conducting. To obtain further information, we computed the projected density of states (PDOS) for NO2-[10-BNNR]-NH2. First, as shown in Fig. 2e, the electronic states near the Fermi level can mainly be attributed to impurity states, and these states were evidently spin-polarized. There is little contribution from the pristine BNNR. Second, the electronic states corresponding to the pristine BNNR shift toward the Fermi level and the energy difference between the two edge states is reduced to 2.9 eV. Considering that the H-passivated BNNRs with edge states feature much larger band gaps, we can safely conclude that the edge states of BNNRs can be modified by the effective potential induced by the functional groups. Furthermore, the PDOS revealed that the conducting spinup channel close to the X point is linked to the N atoms of the NO2
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Fig. 1. Properties of a fully relaxed 10-zBNNR with H-passivated edges. (a), (b) Geometric structure; (c) band structure, and (d) PDOS.
groups and the metallic band at the vicinity of the G point mainly stems from the pz orbitals of the NH2 groups. Thus, the lone electron pair in the NH2 group is responsible for the metallic behavior
of the edge-modified nanoribbons. To get more information on the structural properties and chemical bonding, we calculated the charge density behavior and plotted the projected charge density
Fig. 2. Properties of a 10-BNNR with a NO2eNH2 pair edge decoration. (a) Geometric structure; (b) Projected electronic density of NO2-[4-BNNR]-NH2; (c) Spin densities of the magnetic ground state for an isosurface value of 103 e/Å3; (d) Electronic band structure, with the dashed line denoting the positions of the Fermi level; (e) PDOS; (f) PDOS for the nitrogen atoms in the NH2 groups.
L.L. Li et al. / Journal of Alloys and Compounds 649 (2015) 1130e1135
for NO2-[10-BNNR]-NH2 in Fig. 2b. The electron distribution is quite localized in the inner parts, and N atoms possess more electrons than B atoms, indicating the ionic feature of BeN bonds. However, the electron distribution for the N atoms of NH2 group is quite delocalized, which is in good agreement with the PDOS results. The spin charge density distribution was also plotted in Fig. 2c. The spin density is localized around the N atoms of the NO2 groups. A systematic study of the electronic properties of the NO2eNH2decorated x-zBNNRs (x ¼ 4e9) has been conducted and the results are shown in Fig. 3. Similar to the previous results, a metallic band close to the G point was observed for all these ribbons. To further investigate the effect of the lone electron pair in the NH2 group on the Fermi level, we substituted the NO2 groups with H atoms and the results obtained for the HeNH2-decorated 10BNNR are shown in Fig. 4aec. However, BNNRs with a HeNH2 termination are semiconducting and the energy gap is 0.64 eV. According to Fig. 4c, the electronic states of the pristine BNNR shift toward the Fermi level due to the effective potential induced by the functional groups, and the energy difference between these two
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edge states is reduced to 3.4 eV. This is because the H, NO2 and NH2 groups compensate the dangling sp2 s orbitals at each edge site and have little contribution to the p orbitals of the edge states. But, the higher electron density will lift the energy of occupied orbitals. The energy difference between two edge states is thus reduced. Srivastava [10] have reported a band gap of 0.45 eV and 0.40 eV for CleH and HeCl terminated zBNNRs (Nz ¼ 8) repectively. They also observed downward shifting of electronic states in the valence band caused by the effective potential induced by the functional groups. Our result reveals that the metallic behavior of the BNNRs edge-decorated with NO2eNH2 pairs is not caused by the impuritystates introduced by the functional groups alone. The band shift due to the effective potential induced by the functional groups also plays an important role. The relative stability of edge-modified BNNRs is very important in practice since it determines whether this nanostructure can be obtained experimentally. Because the structures investigated in this study have different chemical compositions, the binding energy per atom is not a suitable measure to compare their relative
Fig. 3. Comparison of the electronic band structure of BNNRs edge-decorated with a NO2eNH2 pair obtained for different ribbon widths. (a) NO2-[4-BNNR]-NH2, (b) NO2-[5-BNNR]NH2, (c) NO2-[6-BNNR]-NH2, (d) NO2-[7-BNNR]-NH2, (e) NO2-[8-BNNR]-NH2, (f) NO2-[9-BNNR]-NH2. The dashed lines denote the position of the Fermi level.
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L.L. Li et al. / Journal of Alloys and Compounds 649 (2015) 1130e1135
Fig. 4. Properties of a 10-BNNR with a HeNH2 pair edge decoration. (a) Structure; (c) Electronic band structure, with the dashed line denoting the position of the Fermi level; (d) Projected density of states (PDOS).
stability. Therefore, we adopt the approach customary used in tertiary phase thermodynamics to account for chemical composition and utilized previously to qualitatively analyze the relative stability of endohedral silicon nanowires [28]. This approach has been frequently employed to investigate the relative stability of nanostructures based on GNR [29,30]. The Gibbs free energy of formation dG for edge-modified BNNRs with different ribbon widths was defined as
dG ¼ Eribbon ecB $mB ecN $mN ecO $mO ecH $mH
(1)
and used to determine the relative stabilities of these edgemodified nanoribbons. In Eq. (1), Eribbon is the cohesive energy per atom of the chemically functionalized BNNR, ci (i ¼ B, N, O and H, respectively) represents the mole fraction of the i atoms in the nanoribbon, satisfying the relation cH þ cO þ cB þ cN ¼ 1. The binding energy per atom of the H2 and O2 molecules are denoted by mH and mO, respectively, and mB and mN denote the cohesive energies per B atom and N atom in an infinite pristine BN single layer, respectively. According to these definitions, the more stable the edge, the lower dG. We examined the variation of dG as a function of the ribbon width for NO2-[zBNNR]-NH2 (Fig. 5). For comparison, the dG values for several H-passivated BNNRs were also computed. The definition of dG for H-passivated BNNRs is similar to the definition given above for NO2-[zBNNR]-NH2. A negative Gibbs free energy of formation suggests that the formation of NO2-[zBNNR]-NH2 is energetically favorable. The values of dG obtained for NO2-[zBNNR]-NH2 are much lower than those obtained for H-passivated BNNRs, indicating that the NO2eNH2 termination results in more stable edges than the hydrogen termination and suggesting a potential experimental route for transforming BNNRs to NO2-[zBNNR]-NH2. Apparently, the S-type BeN bond is strengthened due to the effective potential induced by the NO2eNH2 pair, which explains the theoretical results. With increasing ribbon width, the dG value of both the pristine
Fig. 5. Gibbs free energy of formation dG calculated for zBNNRs and NO2-[zBNNR]-NH2 (Nz ¼ 4e10) as a function of the ribbon width.
BNNRs and the edge-modified BNNRs increases monotonically. In addition, it should be mentioned that both NO2 and NH2 can also react with the inner atoms of the BNNRs. However, under real experimental conditions, these functional groups would react with the more reactive atoms at the ribbon edges first, and the addition of functional groups to the basal plane of the BNNRs can be neglected provided that the concentration of functional groups is adequately low.
4. Conclusions In summary, we have conducted a detailed study on the electronic properties and relative stabilities of BNNRs with a heterogenous NO2eNH2 pair edge decoration. The electronic states at the edges were found to shift due to the effective potential induced by the functional groups. However, with the introduction of impurity states, the functionalized ribbons became magnetic and showed a
L.L. Li et al. / Journal of Alloys and Compounds 649 (2015) 1130e1135
metallic behavior. Moreover, these zBNNRs with a heterogeneous edge-decoration are more stable than H-saturated BNNRs. Our theoretical results are expected to promote the design of future BNNR-based electronic devices. Acknowledgments This work was supported by the Science and Technology Innovation Fund for Outstanding Youth at the Hebei University of Technology (Grant No. 2012007), the Program for Changjiang Scholars and the Innovative Research Team at the Hebei University of Technology (PCSIRT, Grant No. IRT13060), and the National Natural Science Foundation of China (Grant No. 51372066, 51172060 and 51401074). References [1] [2] [3] [4] [5]
Q. Tang, Z. Zhou, Prog. Mater. Sci. 58 (2013) 1244. M.S. Xu, T. Liang, M. Shi, H. Chen, Chem. Rev. 113 (2013) 3766. K.J. Koski, ACS Nano 7 (2013) 3739. B. Peng, L.L. Li, W.Q. Ji, F.Y. Cheng, J. Chen, J. Alloy Compd. 484 (2009) 308. H.B. Zeng, C.Y. Zhi, Z.H. Zhang, X.L. Wei, X.B. Wang, W.L. Guo, Y. Bando, D. Golberg, Nano Lett. 10 (2010) 5049. [6] K.J. Erickson, A.L. Gibb, A. Sinitskii, M. Rousseas, N. Alem, J.M. Tour, A.K. Zettl, Nano Lett. 11 (2011) 3221. [7] Y.L. Liao, Z. Chen, J.W. Connell, C.C. Fay, C. Park, J.W. Kim, Y. Lin, Adv. Funct.
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Mater 24 (2014) 4497. [8] V. Barone, J.E. Peralta, Nano Lett. 8 (2008) 2210. [9] C.-H. Park, S.G. Louie, Nano Lett. 8 (2008) 2200. [10] P. Srivastava, N.K. Jaiswal, G.K. Tripathi, Solid State Commun. 185 (2014) 41e46. [11] H.M. Rai, N.K. Jaiswal, P. Srivastava, R. Kurchania, J. Comput. Theor. Nanosci. 10 (2013) 368e375. s, Y.B. Li, Z. Zanolli, [12] M. Terrones, J.C. Charlier, A. Gloter, E. Cruz-Silva, E. Terre J.M. Dominguez, H. Terrones, Y. Bando, D. Golberg, Nano Lett. 8 (2008) 1026. [13] F. Zheng, G. Zhou, Z. Liu, J. Wu, W. Duan, B. Gu, S.B. Zhang, Phys. Rev. B 78 (2008) 205415. [14] L. Lai, J. Lu, L. Wang, G.F. Luo, J. Zhou, R. Qin, Z.X. Gao, W.N. Mei, J. Phys. Chem. C 113 (2009) 2273. [15] Du, S.C. Smith, G. Lu, Chem. Phys. Lett. 447 (2007) 181. [16] W. Chen, Y. Li, G.T. Yu, Z. Zhou, Z.J. Chen, Chem. Theory Comput. 5 (2009) 3088. [17] X. Wu, M. Wu, X.C. Zeng, Front. Phys. China 4 (2009) 367. [18] J. Lopez-Bezanilla, S. Huang, H. Terrones, B.G. Sumpter, Nano Lett. 11 (2011) 3267. [19] C.-H. Park, S.G. Louie, Nano Lett. 8 (2008) 2200. [20] F.W. Zheng, Z.R. Liu, J. Wu, W.H. Duan, B.-L. Gu, Phys. Rev. B 78 (2008) 085423. [21] Z.H. Zhang, W.L. Guo, Phys. Rev. B 77 (2008) 075403. [22] J. Qi, X. Qian, L. Qi, Q. Feng, D. Shi, J. Li, Nano Lett. 12 (2012) 1224. [23] P.E. Bloechl, Phys. Rev. B 50 (1994) 17953. [24] G. Kresse, J. Heffner, J. Phys. Rev. B 54 (1996) 11169. [25] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558. [26] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. [27] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [28] T. Dumitrica, M. Hua, B.I. Yakobson, Phys. Rev. B 70 (2004) 241303. [29] V. Barone, O. Hod, G.E. Scuseria, Nano Lett. 6 (2006) 2748. [30] E.J. Kan, Z.Y. Li, J.L. Yang, J.G. Hou, J. Am. Chem. Soc. 130 (2008) 4224.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Electronic properties and relative stabilities of heterogeneous edge-decorated zigzag boron nitride nanoribbons L.L. Li*, X.F. Yu, X.J. Yang, X.H. Zhang, X.W. Xu, P. Jin, J.L. Zhao, X.X. Wang, C.C. Tang** Key Lab for Micro- and Nano-Scale Boron Nitride Materials in Hebei Province, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 June 2015 Received in revised form 9 July 2015 Accepted 13 July 2015 Available online 15 July 2015
The wide band gap of boron nitride (BN) materials has been a major bottleneck for a wider application of BN in electronics. In this work, density functional theory computations were used to study the band structure of zigzag BN nanoribbons (BNNRs). Due to the ionic origin of the BN band gap, a heterogeneous edge decoration is an effective way to modulate the electronic band structure of BNNRs. This study demonstrates that a metallic behavior and magnetism can be realized by applying a NO2eNH2 pair edge decoration. Although the lone electron pair of the NH2 group is partly responsible for the metallic behavior, the effective potential difference induced by the donoreacceptor pair is also crucial for metallic behavior. Furthermore, these newly formed BNNRs were found to be more stable than H-passivated BNNRs. This simple chemical modification method offers great opportunities for the development of future BNNR-based electronic devices. © 2015 Elsevier B.V. All rights reserved.
Keywords: Boron nitride nanoribbons DFT Band structure Heterogeneous edge decoration
1. Introduction One-dimensional (1D) nanostructures, such as nanorods, nanotubes, nanowires, and nanobelts, are of great fundamental and technological significance due to their interesting electronic and physical properties associated with their low dimensionality and the occurrence of quantum confinement effects [1e4]. Recently, boron nitride nanoribbons (BNNRs) have been successfully synthesized by either cutting layered BN (hexagonal, h-BN) sheets or by unzipping BN nanotubes [5e7]. As a kind of novel 1D nanomaterial, BNNRs have also attracted much attention due to their excellent properties and high application potential. BNNRs are isoelectronic and have the same structure as graphene nanoribbons (GNRs). However, due to the large ionicity of the BeN chemical bond, BNNRs exhibit some unique properties that are different compared to the corresponding properties of GNRs. Zigzag BNNRs (zBNNRs) with bare edges are magnetic semiconductors and their band gap decreases with the ribbon width, which was confirmed both theoretically [8e11] and experimentally [12]. When the B edges were passivated with H atoms, a half-metallic behavior was
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.L. Li), [email protected] (C.C. Tang). http://dx.doi.org/10.1016/j.jallcom.2015.07.100 0925-8388/© 2015 Elsevier B.V. All rights reserved.
observed [13,14]. The dangling bonds of bare zBNNRs are rather active and can be easily passivated. When both edges of the ribbons were passivated with H atoms, the zBNNRs showed a semiconducting behavior, with a wide band gap close to the band gap of 2D BN monolayers [15]. However, this wide band gap has been a major bottleneck for a wider application of BNNRs in electronics. To promote the design of future BNNR-based devices, both the magnetic properties of BNNRs and possibilities to modify the band gap have been the focus of extensive studies. The formation of StoneeWales (SW) defects [16] as well as an edge modification [17] with F, Cl, OH, and NO2 groups can change the band gap of zBNNRs, although these decorated zBNNRs still retain their semiconducting behavior. In contrast, both an O and S edge-termination can result in a metallic behavior [18]. In addition to these homogeneous chemical edge decorations, the application of an external transverse electric field [19e21] or an axial strain [22] can significantly reduce the band gap of zBNNRs. The studies mentioned above indicated that the generation of an electrostatic potential difference between the two edges of a BNNR is a prerequisite for mediating the electronic band structure. Heterogeneous edge modification seems to be a promising approach to achieve such a potential difference. In this study, we used first-principle calculations based on density functional theory (DFT) to assess the electronic properties and relative stabilities of heterogeneous edge-decorated BNNRs.
L.L. Li et al. / Journal of Alloys and Compounds 649 (2015) 1130e1135
Because different effective potentials at the two edges are highly desirable, a donor and an acceptor group should constitute a good chemical modification pair. We found that a heterogeneous edge decoration based on a NO2eNH2 pair can convert the behavior of the BNNR from semiconductor to a metallic behavior. To clarify the effect of the lone electron pair of the NH2 group, a systematic analysis of the electronic properties of BNNRs edge-decorated with a heterogeneous HeNH2 pair was then carried out. Furthermore, the stability of BNNRs with a heterogeneous NO2eNH2 pair edge decoration was studied by considering the Gibbs free energy of formation dG to promote the development of novel BNNR-based electronic devices. 2. Calculations The spin-polarized first-principles DFT computations were performed using a plane wave basis set with the projector augmented plane wave (PAW) [23] utilized to model the ioneelectron interaction as implemented in the Vienna ab initio simulation package (VASP) [24e26]. The generalized gradient approximation (GGA) method based on the PerdeweBurkeeEmzerh (PBE) [27] functional and a 450 eV cutoff for the plane wave basis set were adopted for all computations. Our supercells were large enough to ensure that the vacuum distance is at least 12 Å, so that an interaction between the individual nanoribbons and their periodic images can be neglected. For the calculation of the structural relaxation and the relaxation energy, the k-points were generated using the MonkhorstePack method with a grid size of 1 1 11, and the convergence threshold was set to 105 eV for the energy and 0.01 eV/Å for the force. The positions of all the atoms in the supercell were fully relaxed during the geometry optimization calculations. To avoid ambiguities regarding the free energy calculation results, the same energy cutoff and a similar k-grid sampling for convergence were adopted. For the calculation of the electronic band structures, a set of strings of 21 k-points connecting specific points of the Brillouin zone was used. 3. Results and discussion Following the convention established in literature [19e21], we define the width parameter Nz of the zBNNRs as the number of zigzag lines across the ribbon width, as illustrated in Fig. 1a for 10BNNR. Before addressing the donoreacceptor pair-modified nanoribbons, we revisit the primitive H edge-passivated BNNRs, establishing their geometrical structure and electronic band structure as a frame of reference. The optimized geometry of 10-BNNR with a width of 20.32 Å is shown in Fig. 1b. There are two types of BeN bonds in the zBNNRs. One is perpendicular to the axes (P-type BeN bond), and the other is slanted (S-type BeN bond). These two types of bonds are denoted by “P” and “S” in Fig. 1. The length of the Ptype BeN bond is in the range from 1.453 to 1.459 Å, which is a bit longer than the length of the S-type BeN bond (1.429e1.445 Å). Consequently, the BeN bond perpendicular to the axis is generally weaker than the slanted BeN bond. This is attributed to the interruption of the hexagonal network of BN sep bonds in the transverse direction. The energy band diagram of 10-BNNR is shown in Fig. 1c. The Hpassivated 10-BNNR exhibits an indirect band gap of 4.1 eV, and its band structure is characteristic for a non-spin-polarized material. Our results are in good agreement with the results of previous studies, indicating that the methodology adopted in this work yields a reliable description of the electronic and magnetic properties of zBNNRs. Similar to GNRs, BNNRs are built from an infinite number of 1D six-member rings, and all the atoms are sp2-
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hybridized and contribute to the s bond. Although BeN is isoelectronic to CeC, the covalence of the BeN bond essentially differs from the covalence of the CeC bond in the following two aspects. First, the p bond in BN formed from the p orbitals accepts electrons from the N lone-pair p orbital. Second, there is another remarkable difference between the BeN bond and the CeC bond concerning the scale of the p-electron conjugations. Since N is more electropositive than B (electronegativity: B ¼ 2.04, N ¼ 3.04), the p-electrons are highly localized around N, turning the BNNRs into a semiconducting material with a wide band gap, as shown in Fig. 1c and d. zBNNRs have two distinct edges terminated by N and B atoms, respectively, and consequentially called the N-edge and the B-edge. Because the BeN bond is highly polarized, the B-edge possesses distinct properties which are dramatically different compared to the properties of the N-edge. To obtain more information on the properties of 10-zBNNR, the partial density of states (PDOS) was calculated and is shown in Fig. 1d. For the pristine zBNNR, the top valence band is mainly dominated by the N atoms at the N-edge, while the conduction band is linked to the B atoms at the B-edge. The obvious difference in electronic properties between the Bedge and the N-edge of the zBNNR stimulated the subsequent investigation of heterogeneous edge decorations. Furthermore, considering the highly localized p electrons in the zBNNR, a chemical modification may provide a powerful pathway for enhancing the physical properties of pristine BNNRs, making them more attractive for applications. Therefore, a NH2 donor group was added to the N-edge and a NO2 acceptor group was added to the Bedge of the 10-zBNNR (labeled as NO2-[10-BNNR]-NH2). Both the NH2 and the NO2 group compensate the dangling sp2-s-orbital at the edge atoms. We expected that the potential difference induced by the donoreacceptor pair might weaken the polarization of the BeN covalent bond, thereby reducing the energy band gap of the BNNR. After completion of the geometrical optimization, the energetically most favorable edge configurations for the B and the N edges of NO2-[10-BNNR]-NH2 favor the connection of the N atoms to the ribbon and the OeNeO plane (HeNeH plane) being perpendicular to the axial direction of the ribbon (Fig. 2a). The planar BN structure of the ribbon does not change. However, the S-type BeN bond is obviously shortened and there is also a little protraction of the Ptype BeN bond in and close to the edges, reflecting that the local changes in the electronic structure are induced by edge modification. The electronic properties of NO2-[10-BNNR]-NH2 were studied by computing the electronic band structure, and the results are shown in Fig. 2d. The edge states close to the X point feature a spinup channel with a low resistance characteristic for metallic materials and a spin-down channel characteristic for semiconducting materials, indicating that NO2-[10-BNNR]-NH2 is a magnetic material. Furthermore, there is a spin-unpolarized band across the Fermi level close to the G point, which makes both spin channels conducting. To obtain further information, we computed the projected density of states (PDOS) for NO2-[10-BNNR]-NH2. First, as shown in Fig. 2e, the electronic states near the Fermi level can mainly be attributed to impurity states, and these states were evidently spin-polarized. There is little contribution from the pristine BNNR. Second, the electronic states corresponding to the pristine BNNR shift toward the Fermi level and the energy difference between the two edge states is reduced to 2.9 eV. Considering that the H-passivated BNNRs with edge states feature much larger band gaps, we can safely conclude that the edge states of BNNRs can be modified by the effective potential induced by the functional groups. Furthermore, the PDOS revealed that the conducting spinup channel close to the X point is linked to the N atoms of the NO2
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Fig. 1. Properties of a fully relaxed 10-zBNNR with H-passivated edges. (a), (b) Geometric structure; (c) band structure, and (d) PDOS.
groups and the metallic band at the vicinity of the G point mainly stems from the pz orbitals of the NH2 groups. Thus, the lone electron pair in the NH2 group is responsible for the metallic behavior
of the edge-modified nanoribbons. To get more information on the structural properties and chemical bonding, we calculated the charge density behavior and plotted the projected charge density
Fig. 2. Properties of a 10-BNNR with a NO2eNH2 pair edge decoration. (a) Geometric structure; (b) Projected electronic density of NO2-[4-BNNR]-NH2; (c) Spin densities of the magnetic ground state for an isosurface value of 103 e/Å3; (d) Electronic band structure, with the dashed line denoting the positions of the Fermi level; (e) PDOS; (f) PDOS for the nitrogen atoms in the NH2 groups.
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for NO2-[10-BNNR]-NH2 in Fig. 2b. The electron distribution is quite localized in the inner parts, and N atoms possess more electrons than B atoms, indicating the ionic feature of BeN bonds. However, the electron distribution for the N atoms of NH2 group is quite delocalized, which is in good agreement with the PDOS results. The spin charge density distribution was also plotted in Fig. 2c. The spin density is localized around the N atoms of the NO2 groups. A systematic study of the electronic properties of the NO2eNH2decorated x-zBNNRs (x ¼ 4e9) has been conducted and the results are shown in Fig. 3. Similar to the previous results, a metallic band close to the G point was observed for all these ribbons. To further investigate the effect of the lone electron pair in the NH2 group on the Fermi level, we substituted the NO2 groups with H atoms and the results obtained for the HeNH2-decorated 10BNNR are shown in Fig. 4aec. However, BNNRs with a HeNH2 termination are semiconducting and the energy gap is 0.64 eV. According to Fig. 4c, the electronic states of the pristine BNNR shift toward the Fermi level due to the effective potential induced by the functional groups, and the energy difference between these two
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edge states is reduced to 3.4 eV. This is because the H, NO2 and NH2 groups compensate the dangling sp2 s orbitals at each edge site and have little contribution to the p orbitals of the edge states. But, the higher electron density will lift the energy of occupied orbitals. The energy difference between two edge states is thus reduced. Srivastava [10] have reported a band gap of 0.45 eV and 0.40 eV for CleH and HeCl terminated zBNNRs (Nz ¼ 8) repectively. They also observed downward shifting of electronic states in the valence band caused by the effective potential induced by the functional groups. Our result reveals that the metallic behavior of the BNNRs edge-decorated with NO2eNH2 pairs is not caused by the impuritystates introduced by the functional groups alone. The band shift due to the effective potential induced by the functional groups also plays an important role. The relative stability of edge-modified BNNRs is very important in practice since it determines whether this nanostructure can be obtained experimentally. Because the structures investigated in this study have different chemical compositions, the binding energy per atom is not a suitable measure to compare their relative
Fig. 3. Comparison of the electronic band structure of BNNRs edge-decorated with a NO2eNH2 pair obtained for different ribbon widths. (a) NO2-[4-BNNR]-NH2, (b) NO2-[5-BNNR]NH2, (c) NO2-[6-BNNR]-NH2, (d) NO2-[7-BNNR]-NH2, (e) NO2-[8-BNNR]-NH2, (f) NO2-[9-BNNR]-NH2. The dashed lines denote the position of the Fermi level.
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Fig. 4. Properties of a 10-BNNR with a HeNH2 pair edge decoration. (a) Structure; (c) Electronic band structure, with the dashed line denoting the position of the Fermi level; (d) Projected density of states (PDOS).
stability. Therefore, we adopt the approach customary used in tertiary phase thermodynamics to account for chemical composition and utilized previously to qualitatively analyze the relative stability of endohedral silicon nanowires [28]. This approach has been frequently employed to investigate the relative stability of nanostructures based on GNR [29,30]. The Gibbs free energy of formation dG for edge-modified BNNRs with different ribbon widths was defined as
dG ¼ Eribbon ecB $mB ecN $mN ecO $mO ecH $mH
(1)
and used to determine the relative stabilities of these edgemodified nanoribbons. In Eq. (1), Eribbon is the cohesive energy per atom of the chemically functionalized BNNR, ci (i ¼ B, N, O and H, respectively) represents the mole fraction of the i atoms in the nanoribbon, satisfying the relation cH þ cO þ cB þ cN ¼ 1. The binding energy per atom of the H2 and O2 molecules are denoted by mH and mO, respectively, and mB and mN denote the cohesive energies per B atom and N atom in an infinite pristine BN single layer, respectively. According to these definitions, the more stable the edge, the lower dG. We examined the variation of dG as a function of the ribbon width for NO2-[zBNNR]-NH2 (Fig. 5). For comparison, the dG values for several H-passivated BNNRs were also computed. The definition of dG for H-passivated BNNRs is similar to the definition given above for NO2-[zBNNR]-NH2. A negative Gibbs free energy of formation suggests that the formation of NO2-[zBNNR]-NH2 is energetically favorable. The values of dG obtained for NO2-[zBNNR]-NH2 are much lower than those obtained for H-passivated BNNRs, indicating that the NO2eNH2 termination results in more stable edges than the hydrogen termination and suggesting a potential experimental route for transforming BNNRs to NO2-[zBNNR]-NH2. Apparently, the S-type BeN bond is strengthened due to the effective potential induced by the NO2eNH2 pair, which explains the theoretical results. With increasing ribbon width, the dG value of both the pristine
Fig. 5. Gibbs free energy of formation dG calculated for zBNNRs and NO2-[zBNNR]-NH2 (Nz ¼ 4e10) as a function of the ribbon width.
BNNRs and the edge-modified BNNRs increases monotonically. In addition, it should be mentioned that both NO2 and NH2 can also react with the inner atoms of the BNNRs. However, under real experimental conditions, these functional groups would react with the more reactive atoms at the ribbon edges first, and the addition of functional groups to the basal plane of the BNNRs can be neglected provided that the concentration of functional groups is adequately low.
4. Conclusions In summary, we have conducted a detailed study on the electronic properties and relative stabilities of BNNRs with a heterogenous NO2eNH2 pair edge decoration. The electronic states at the edges were found to shift due to the effective potential induced by the functional groups. However, with the introduction of impurity states, the functionalized ribbons became magnetic and showed a
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metallic behavior. Moreover, these zBNNRs with a heterogeneous edge-decoration are more stable than H-saturated BNNRs. Our theoretical results are expected to promote the design of future BNNR-based electronic devices. Acknowledgments This work was supported by the Science and Technology Innovation Fund for Outstanding Youth at the Hebei University of Technology (Grant No. 2012007), the Program for Changjiang Scholars and the Innovative Research Team at the Hebei University of Technology (PCSIRT, Grant No. IRT13060), and the National Natural Science Foundation of China (Grant No. 51372066, 51172060 and 51401074). References [1] [2] [3] [4] [5]
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