Tunable Electronic and Magnetic Properties of Graphene Flake

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Tunable Electronic and Magnetic Properties of Graphene FlakeDoped Boron Nitride Nanotubes Zhaoyong Guan,† Weiyi Wang,† Jing Huang,*,†,‡ Xiaojun Wu,†,§,∥ Qunxiang Li,*,†,§ and Jinlong Yang†,§ †

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei, Anhui 230601, China § Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ∥ CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: Carbon-doped boron nitride nanostructures including nanosheets, nanoribbons, and nanotubes have drawn enormous research attention because of their tunable electronic properties and widespread applications. In this work, we explore the electronic and magnetic properties of graphene flake-doped singlewalled boron nitride nanotubes (BNNTs) on the basis of first-principles calculations. Theoretical results reveal that the band structures of these doped BNNTs can be effectively engineered by embedding graphene flakes with different sizes and shapes. Moreover, the Lieb theorem works for the triangle graphene flake-doped BNNTs, and the corresponding doped systems are ferromagnetic, originating from the spin-polarized interface states. All BNNTs embedded with the triangular graphene flakes with relatively small sizes are typical bipolar magnetic semiconductors, which can be easily tuned into half-metals by carrier doping, opening the door to their promising applications in spintronic devices.

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logical applications.18 The magnetism has been predicted in C/ BN nanotubes, in which the spin polarization of the interface states at the zigzag boundary connecting BNNT and CNT segments leads to the formation of one-dimensional itinerant ferromagnetic states.19−22 Electronic structures of ternary C/ BN nanotubes could be engineered by changing the composition and size of the Cx and (BN)y units, leading to either metallic or semiconducting behavior.23 Fan et al. found that the carrier confinement and charge separation at the interface in C/BN nanotubes are highly determined by the build-in electric field and band discontinuities.24,25 The transition property of the C/BN nanotubes depends on the connecting pattern, which significantly influences the static first hyperpolarizabilities.26 Negative different resistance and large rectifying behavior were observed in the C/BN nanotubes with certain compositions and joint configurations.27−29 These experimental and theoretical works mainly focused on hybrid heterojunctions based on CNTs and BNNTs. Based on C/BN hybrid nanotubes, to obtain redefined properties, the structural and electronic resemblances can be suitably modulated by altering compositions and arrangements of the

INTRODUCTION Recently, carbon-doped boron nitride nanostructures including nanosheets, nanoribbons, and nanotubes, have attracted increasing attention because of their tunable electronic properties and widespread applications. In contrast to carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) are semiconductors with a wide band gap of 5.9 eV, and this wide band gap is close to that of a hexagonal boron nitride (h-BN) sheet.1 Note that the band gaps of BNNTs are not sensitive to the tube chirality, radius, and wall−wall interaction.2 To reduce this wide band gap of BNNTs for potential applications, various methods, such as discharge and laser ablation,3,4 substitution reaction,5 chemical vapor deposition (CVD),6 hot-assisted hot filament CVD method,7,8 and in situ electron-beam irradiation inside,9,10 are used to prepare C-doped individual BNNTs9,10 and C/BN hybrid structures, i.e., C/BN nanotubes.11,12 The band gaps of these experimentally synthesized C/BN nanotubes can be effectively tuned by the percentage of substitution doping, which then provides a wide range of tunable electronic and magnetic properties.7,13 The rapid progress of the experimental investigations has brought many theoretical efforts.14−17 Previous investigations have revealed that quantum dots in CNTs or nanotube heterojunctions can be realized based on the spontaneous segregation processes, leading to the wide possible techno© XXXX American Chemical Society

Received: September 4, 2014 Revised: November 17, 2014

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BNNTs, respectively. In this work, we replace some of B and N atoms in BNNTs by other elements, which should have a very similar bond length with other hexagonal structures. As expected, graphene flakes are ideal candidates since they have both a hexagonal geometric structure with almost the same bond length. Then, it is very possible to alter the electronic properties and magnetism of BNNTs by embedding graphene nanoflakes within BNNTs. Here, we use the following notations: CN (CB) means that one B (N) atom is substituted with one C atom; C3B1N stands for a triangular graphene flake formed by substituting 3 B atoms and 1 N atom with 4 C atoms, etc. As illustrated in Figure 1, three types of graphene

C, B, and N atoms in these structures. For example, Du et al. have predicted the amazing electronic properties of C/BN single-walled nanotubes, and they have found that the armchair C/BN single-walled nanotube could be formed by connection of zigzag BN/C nanoribbons for bonding the original dangling bonds.30 However, to our knowledge, no comprehensive theoretical investigations have been carried out to explore the electronic structures and magnetism of BNNTs with embedded graphene flakes with different shapes and sizes so far. In recent years, much attention has been paid to heterostructures based on graphene and hexagonal boron nitride (h-BN) sheets, which could be successfully prepared by several methods (i.e., CVD and lithography).31−34 Kan et al. examineed the band structures and magnetism of BN sheets doped with different graphene nanoflakes.35 They revealed that a direct gap of these doped systems could be changed into an indirect gap by altering the interface bonding pattern, and the Lieb theorem holds for the triangle graphene flakes embedded in h-BN sheets.35 In this work, we explore the electronic structures and magnetic properties of BNNTs doped with graphene nanoflakes of various sizes and shapes by performing extensive firstprinciples calculations. Theoretical results clearly show that the number of midgap states and the size of the band gap are tunable by changing the embedded graphene flakes with different sizes and shapes, which offer effective ways to alter the electronic properties of BNNTs. Moreover, the Lieb theorem still works for these triangle graphene flake-doped BNNTs. The corresponding doped BNNTs are ferromagnetic due to the spin-polarized interface states. Interestingly, we find that all BNNTs embedded with the lar graphene flakes with relative small sizes are typical bipolar magnetic semiconductors (BMSs), which can be easily tuned into half-metals by carrier doping.

Figure 1. Side view of the optimized geometries of unit cells of hexagonal or triangle graphene flake-doped (5, 5) BNNTs: (a) HC12B12N, (b) T1-C10B6N, (c) T2-C6B10N.

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flake-doped BNNTs are examined: (1) H-CαBβN (α = β); here, α or β defines the number of the C atoms in the α or β sublattice. Figure 1a shows a hexagonal graphene flake embedded in a (5, 5) armchair BNNT, which contains 160 atoms in a unit cell. As for H-CαBβN, the number of C atoms in a hexagonal graphene flake-doped BNNT is equal to 6m2, where m stands for the hexatomic ring number on each edge. For example, the H-C12B12N structure is shown in Figure 1a; there are two hexatomic rings (m = 2) on each edge of the hexagonal graphene flake, which leads to 24 C atoms in this structure. (2) T1-CαBβN (α > β) and (3) T2-CαBβN (α < β); here, two triangular graphene flakes with the same size are introduced into BNNTs through different embedding patterns, as shown in Figure 1b,c, respectively. When a triangle graphene flake is embedded in a BNNT, there will be n2 C atoms in T1CαBβN and T2-CαBβN configurations. Here, the number of each edge C atom of the triangular graphene flake is labeled with n. For instance, the number of each edge C atoms is 4 in Figure 1b, so there are 16 C atoms in the triangle graphene flakedoped BNNT, denoted with T1-C10B6N. To explore the carrier doping effect on the electronic and magnetic properties, the triangle graphene flakes embedded BNNTs are charged by introducing the amount of carrier (x) to the supercell. The negative and positive values of x stand for electron and hole doping, respectively.

COMPUTATIONAL DETAILS The first-principles calculations are performed using the DMol3 package36,37 based on density functional theory (DFT). The exchange-correlation interaction is treated by the Perdew− Burke−Ernzerhof functional38 generalized gradient approximation. The basis set consists of the double numerical atomic orbitals augmented by polarization functions. Along the proposed nanotube axis, one-dimensional (1D) periodic boundary condition is applied. To neglect their neighboring interaction, the distance between the neighboring nanotubes is larger than 15.0 Å. The real-space global cutoff radii are chosen to be 4.3 Å, while the Monkhorst−Pack39 k-points are set to be 5, 10, and 26 for geometry optimization, total energy, and band structure calculations to sample the 1D Brillouin zone, respectively. In this work, all calculations are all-electron ones with scalar relativistic corrections. As for the self-consistent electronic structure calculations, the convergence criterions on the energy and electron density are set to be 10−6 hartree (1 hartree = 1 atomic unit = 27.21 eV). Geometry optimizations are performed with convergence criteria of 2 × 10−3 hartree/Å on the gradient, 5 × 10−3 Å on the displacement, and 1 × 10−5 hartree on the energy. The h-BN and BNNTs are used as testing systems to check the accuracy of our procedure. The B− N bond length in h-BN is predicted to be 1.45 Å, which is the experimental measured value. The calculated band gap of h-BN and (5, 5) BNNNT is 4.66 and 4.68 eV, respectively, which also agree well with previous results.1,40 There are two sublattices in BNNTs, which can be marked as α and β lattices, standing for B and N atoms’ locations in

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RESULTS AND DISCUSSION Geometric Structures. Before exploring electronic and magnetic properties of these graphene flake-doped BNNTs, we B

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optimize their geometric structures. We initially construct a BNNT by rolling up the optimized BN sheet (aBN = 1.45 Å), substitute B and N atoms with C atoms to form a graphene flake embedded in the BNNT, and fully relax all atomic positions in the proposed graphene flake-doped BNNT without any symmetry constraint. Then, we change the lattice constant (at least 5 different values for each computational system), and fully relax them again. Based on polynomial curve fitting, it is easy to find their equilibrium lattice constants according to the static self-consistently calculated total energies for these graphene flake-doped BNNTs with different constant lattices. At last, to obtain the most stable configuration, we again relax all atomic positions in the doped BNNT with the predicted equilibrium lattice constant. When a (5, 5) BNNT is embedded with a hexagonal graphene flake (m = 2), as shown in Figure 1a, the optimized structure indicates that the original cylinder nanotube still holds, meaning that graphene flakes can be embedded in BNNTs without compromising its mechanical integrity due to a small lattice mismatch. Moreover, as for embedding hexagonal graphene flakes with different sizes, we find that the C−B (C− N) distances just slightly change. For example, for H-C3B3N, the number of embedded C atoms is 6. The C−C bond lengths are about 1.41 Å, which is very close to the C−C bond length in perfect graphene (1.42 Å). The C−N and C−B bonds in HC3B3N vary from 1.41 to 1.42 Å, and from 1.50 to 1.53 Å, respectively. For H-C12B12N and H-C 27B27N, the C−N separations change in the range of 1.40−1.41 Å, and 1.41− 1.42 Å, and the C−B bond lengths change within the range of 1.51−1.54 Å, and 1.51−1.55 Å, respectively. These values are close to the previous theoretical results for graphene flakedoped h-BN sheets.35 For BNNTs doped with triangular graphene flakes using the T1-CαBβN configuration, as shown in Figure 1b, a triangle graphene flake is embedded into a BNNT via C−N bonds at edges and vertices, while the connections are realized via C−B bonds in the T2-CαBβN configuration. The C−N distances in the T1-C10B6N configuration at edges and vertices are predicted be about 1.41 and 1.42 Å, while the C−B bond lengths are about 1.51 and 1.55 Å at T2-C6B10N edges and vertices, respectively. These values just vary very slightly if the triangle graphene flake size changes. Note that the C−C and B−N bond lengths in all examined graphene flake-doped BNNTs range from 1.40 to 1.44 Å, and from 1.43 to 1.47 Å, which just slightly deviate from the C−C distance (ag = 1.42 Å) in graphene and the B−N separation aBN = 1.45 Å in BNNTs. This is easy to understand. The length difference between ag and aBN leads to small strains in these doped systems, and the C−C and B−N bonds are enlarged or compressed to release the strain energy. We also examine graphene flake-doped (6,6) and (7,7) armchair, (12,0) zigzag, and (4,8) helical BNNTs. We find that the ranges of C−N, C−B, C−C, and B−N bond lengths just change slightly, and they are insensitive to the diameter and chirality of BNNTs. Electronic Structures. On the basis of the optimized structures, we calculate the electronic structures of these proposed graphene flake-doped BNNTs. For systems with hexagonal graphene flakes, we find that they have nonmagnetic ground states. The band structures of H-CαBβN (m = 1 and 2) are plotted in Figure 2a,b, respectively. We find that all examined H-CαBβN systems are semiconductors with direct band gaps, in which the conduction band minimum (CBM)

Figure 2. Band structures of the hexagonal graphene flake-doped (5, 5) BNNTs for m = 1 and 2: (a) H-C3B3N (m = 1), (b) H-C12B12N (m = 2). Here, the spatial distributions of the VBM and CBM at the Γ point for the H-C12B12N system are plotted in the right panel of (b).

and the valence band maximum (VBM) locate at the Γ point (see the Supporting Information). The right panel of Figure 2b presents the spatial distribution of the CBM and VBM at the Γ point. Clearly, the edge C atoms in the hexagonal graphene flake give the dominative contribution to the wave functions of the VBM and CBM, and these B and N atoms directly connected to the edge C atoms also give somewhat contribution. These observations are totally different from that of the undoped (5, 5) BNNT, in which the CBM and VBM are mainly contributed by these B and N atoms in the BNNT, respectively. In general, the chemical activity is strongly related to the CBM and VBM. Hence, the chemically active sites in H-CαBβN are mainly limited within the graphene flake region. Moreover, we find that increasing the size of the graphene flake results in the band gap reduction of the doped BNNT. For example, the band gap of H-CαBβN with m = 1, 2, and 3 is predicted to be 3.48, 2.26, and 1.52 eV, respectively. Compared with the band gap (4.68 eV) of a pure (5, 5) BNNT, the significant band gap reduction in H-CαBβN systems originates from the emerging states within the band gap, which mainly come from the edge C atoms in the hexagonal graphene flake. Note that the number of these midgap states is tunable by changing the graphene flake size. The number of these midgap states increases with increasing the size of the graphene flake, as shown in Figure 2. To check the impact of diameter and chirality of BNNTs on their electronic structures, we examine the H-CαBβN systems for m = 1, 2, and 3 in (6, 6) armchair, (12, 0) zigzag, and (4, 8) helix BNNTs. Their corresponding band gaps are predicted to be 3.54, 2.30, and 1.58 eV; 3.56, 2.31, and 1.59 eV; and 3.51, 2.29, and 1.58 eV, respectively. These results ensure us to conclude that the band gap of the H-CαBβN system mainly depends on the size of the graphene flake, but it is very insensitive to the chirality and radius of the BNNT. Previous investigations have revealed that, under the different chemical potentials, graphene flakes with hexagonal and triangular shapes can exist in an h-BN sheet.41,42 This stimulates us to examine the triangular graphene flake-doped BNNTs. To determine their ground states, as examples, the energy difference (ΔE = EM − ENM) is predicted to be 218 and 201 meV for T1-C10B6N and T2-C6B10N; here, EM and ENM stand for the total energies by using spin-polarized and spin-restricted C

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To form a T1-CαBβN configuration, a triangle graphene flake bonds to a BNNT through C−N bonds. When these connections are realized by C−B bonds, this will result in a T2-CαBβN system. Previous reports have shown that the stability of the H, T1, and T2 configurations is different under the different conditions, and both T1 and T2 configurations can exist by changing the charged condition.42,43 Following the similar procedure for the T1-CαBβN case, now we turn to examine the triangle graphene flake-doped BNNTs with the T2-CαBβN configuration. The calculated band structures of T2CαBβN for n = 3−5 cases are shown in Figure 3b. Clearly, they are also semiconductors with direct or indirect band gaps. The separation between the CBM and VBM decreases with increasing the size of the triangle graphene flake due to the more doping states emerging. For example, the band gap of T2CαBβN for n = 2, 3, 4, and 5 cases is predicted to be about 0.82, 0.49, 0.25, and 0.11 eV, respectively. In contrast to the T1C αBβN case, n holes are brought into the T2-C αBβN configuration, and these holes will fill the occupied bands, which are mainly contributed by N 2p orbitals in the undoped BNNT. As a result, the doping states locate near the valence bands of the BNNT, as shown in Figure 3b. As seen in Figure 3, it is clear that hole doping in T2-CαBβN is more efficient than hole injecting T1-CαBβN in altering the band gap. For example, embedding the same size triangle graphene flake (n = 4), the band gap for T1-C10B6N and T2-C6B10N systems is about 0.65 and 0.25 eV, respectively. These above-discussed results ensure us to give a short conclusion: the electronic structures of BNNTs can be effectively tuned by embedding graphene flakes with different shapes and sizes. To check the impact of the diameter and chirality of BNNTs on their electronic structures, we also calculate the electronic structures of triangle graphene flakes with different sizes (n = 2−6) embedded in (7, 7) armchair, (12, 0) zigzag, and (4, 8) helix BNNTs with T1 and T2 configurations. Theoretical results clearly reveal that the larger size of the triangle graphene flake results in the smaller band gap. For example, the size of the triangle graphene flake embedded in a (7, 7) BNNT with a T1 configuration increases from n = 2 to 6, and the corresponding band gap decreases from 0.87 to 0.33 eV. For the same size of triangle graphene flake connecting to a BNNT through C−N or C−B bondings, the band gap for the T1 configuration is remarkably larger than that of with the T2 case. For instance, the band gap of the triangle graphene flake (n = 6) embedded in a (4, 8) helical BNNT is predicted to be about 0.22 and 0.03 eV for T1 and T2 configurations, respectively. These above-discussed results ensure us to give a short conclusion: the electronic structures of BNNTs can be effectively tuned by embedding graphene flakes with different shapes and sizes. Magnetic Properties. Both monolayer graphene and an hBN sheet are nonmagnetic; however, the triangular graphene flake is discovered to show a linear-scaling net spin because of topological frustration of the π-bonds.44 The triangular Nterminated nanodots and antidots are found to be stable above room temperature, and they are intrinsic spin-polarized at the Fermi level, which leads to the anisotropic spin transport and suggests the possibility of tunable spin filtering.45 Here, we will demonstrate that the tunable spin polarization and magnetic moment of these triangle graphene flake-doped BNNTs depend on the size as well as the shape of the embedded graphene flake. Moreover, the embedding bonds through either a C−N bond or a C−B bond can effectively alter the electronic

calculations, respectively. These results imply that these systems have spin-polarized ground states. The spin-polarized band structures of T1-CαBβN for n = 3−5 cases are presented in Figure 3a. Here, the red and blue lines stand for the spin-up and

Figure 3. (a) Spin-polarized band structures of the triangle graphene flake-doped (5, 5) BNNTs via the C−N bonds for n = 3−5, namely, T1-C6B3N (n = 3), T1-C10B6N (n = 4), and T1-C15B9N (n = 5). (b) Band structures of the triangle graphene flake-doped (5, 5) BNNTs via the C−B bonds for n = 3−5, namely, T1-C3B6N (n = 3), T1-C6B10N (n = 4), and T1-C9B15N (n = 5).

spin-down electrons, respectively. It is clear that they will be semiconductors with direct or indirect band gaps when the graphene flake size changes (see the Supporting Information). The valence electrons of B, C, and N atoms are different. When a C atom substitutes a B or N atom in a BNNT, one more electron or hole will be introduced into the doped system. As for T1-CαBβN, there are (n2 + n)/2 and (n2 − n)/2 replaced B and N atoms, which imply that the system is charged with (n2 + n)/2 electrons and (n2 − n)/2 holes, respectively. The net effect is n electrons being introduced in T1-CαBβN (n). These introduced electrons will fill the empty bands mainly contributed by B 2p orbitals in the BNNT. This leads to the dopant states, which lie at around the conduction bands of the BNNT; see Figure 3a. Then, the band gap of a (5, 5) BNNTs is significantly reduced due to these emerging doping states, which come from the edge C atoms in the doped triangle graphene flake. The calculated band gap of T1-CαBβN for n = 2, 3, 4, 5, and 6 cases is about 1.00, 0.79, 0.65, 0.54, and 0.41 eV, respectively. Since the band gap of T1-CαBβN depends on the size of triangle graphene flake, as shown in Figure 3a, to obtain a small band gap in the T1-CαBβN system, one needs to embed a large triangle graphene flake in a BNNT through C−N connections. A similar phenomenon has been observed in HCαBβN, as shown in Figure 2. D

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and magnetic properties of these triangle graphene flake-doped BNNTs. Because of the inversion symmetry, the free hexagonal graphene flakes are nonmagnetic, which leads to the hexagonal graphene flake-doped BNNTs remaining nonmagnetic, as shown in Figure 2. This situation will be different for the triangle graphene flakes. According to the Lieb theorem,46 because of the different numbers of the α and β sublattices, a triangular graphene flake should be spin-polarized. When a triangle graphene flake is embedded in a BNNT, the magnetism can still be kept, and the Lieb theorem still works. This is totally different from the transition metal (i.e., Ni) doped in an h-BN sheet; the atomic magnetism of the Ni atom disappears.47 To see how triangle graphene flake-doped BNNTs are polarized, we calculate their spin-polarized total and partial density of states (DOS). As an example, we plot the calculated spin-polarized DOS for the T2-C6B10N (n = 4) configuration in Figure 4a. Clearly, two DOS peaks for spin-up electrons

the Lieb theorem holds. Note that the B atom in edge C−B bonds also give a considerable contribution (about 18%) to the magnetic moment. The predicted magnetic moments of triangle graphene flakedoped BNNTs with T1 and T2 configurations are shown in Figure 5. According to the above definition, the T1-CαBβN (α >

Figure 5. Variation of magnetic moments of these examined systems with α − β. The solid and short dotted lines stand for the systems without and with carrier doping (i.e., introducing an electron or a hole), respectively.

β) systems have positive (α − β) values, whereas the T2-CαBβN (α < β) cases have negative (α − β) values. For two types of doping configurations, when a triangle graphene flake is embedded into a BNNT through the C−N or C−B bonds, then |α − β| electrons or holes would be introduced in the BNNT. As a result, the doped system has a magnetic moment of |α − β| μB due to the full spin polarization. This means that the magnetic moments of these doped BNNTs can be accurately modulated by changing the triangle graphene flake size. To examine the embedded pattern effect, we calculate the spin-polarized DOS values of the T1-C10B6N system and plotted them in Figure 6, in which the inset stands for its spin density. Figure 4. (a) Total and partial DOS of the doped (5, 5) BNNT in the T2-C6B10N configuration, and the red, black, cyan, blue, pink, and green lines stand for the total DOS, the partial DOS for 2p orbitals of the B, N, Cα, Cβ, and C atoms, respectively. (b) Spin density distribution of the doped system. (c) Spin density of the free curved graphene flake. Here, the isovalue is 0.02 e/Å3.

locating at about −0.08 and −0.19 eV below the Fermi level are occupied, while two DOS peaks for spin-down electrons lying above the Fermi level at 0.17 and 0.27 eV are empty, which are mainly contributed by the edge C atoms in the triangle graphene flake. This observation results in the magnetism in the T2-C6B10N system. That is to say, the magnetic moment of the T2-C6B10N system is 4.0 μB, in contrast to both nonmagnetic pristine BNNTs and hexagonal graphene flake-doped BNNTs. To clearly illustrate the origination of the predicted magnetic moments of doped BNNTs with triangle graphene flakes, we plot the spin density distribution in Figure 4b for the T2-C6B10N (n = 4) system. It is clear that the C atoms in the edge C−B bonds give the dominative contribution to the predicted magnetic moment. As shown in Figure 4b,c, the spin density mainly localizes around the edge C atoms of the triangle graphene flake, which is similar to the spin distribution of an isolated triangle graphene flake with the same curved structure. This observation indicates that BNNTs are promising candidates to assemble magnetic triangle graphene flakes, and

Figure 6. Total and partial DOS of the doped (5, 5) BNNT in the T1C10B6N configuration, and the inset stands for the corresponding spin density.

Although their magnetic moments have the same value (i.e., 4 μB for T1-C10B6N and T2-C6B10N cases), and all graphene flake induced midgap states are fully polarized, as shown in Figure 6, we observe the following different features for T1 and T2 configurations with embedding the same size of a triangle graphene flake: (1) The positions of the occupied and the empty midgap states for the spin-up and spin-down electrons, respectively, are different. (2) For the T1 configuration, the N atoms in the edge C−N bonds give more contribution to the magnetic moment, than that of the B atoms in the edge C−B connections. For example, they give about 21% and 18% E

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contribution to the predicted 4 μB for T1-C10B6N and T2-C6B10N systems, respectively. The reason is that, at N atoms, there are more accumulated electrons, which induces the more spinpolarized for the edge N sites, compared with the edge B atoms. On the basis of the Mulliken population analysis, in T1-C10B6N, each edge N atom gains about 0.38 e, while each edge B atom in T2-C6B10N loses about 0.30 e. It should be pointed out that electronic structures of triangle graphene flakes embedded in (5, 5) BNNTs near the Fermi level are mainly determined by the C−B and C−N interface states (see the Supporting Information), which are similar to that of in-planar graphene flakes embedded in a BN sheet.35 The above predicted metal-free ferromagnetism originates from the pure 2p electron. It is well-established that the magnetic system without any transition metals holds the great advantage for spintronics since these 2p electron systems have the relatively long spin relaxation time due to the weak spin− orbit coupling.48 Thus, the triangular graphene flakes-doped BNNTs with tunable metal-free magnetism are promising for applications in spintronic devices. Carrier Doping Effect. In general, these synthesized C/BN hybrid nanostructures are usually positively or negatively charged depending on the experimental conditions.49 For example, Natalia et al. have pointed out that an atomically thin target tends to be positively charged for the emission of secondary electrons when it is irradiated by the high energy electrons.42 Moreover, carrier doping is a commonly used method to effectively modulate the materials properties.50,51 Now, we turn to explore the carrier doping effect on the electronic and magnetic properties of the triangle graphenedoped BNNTs. First, we examine the carrier doping effect on their magnetic moments by introducing an additional electron or a hole in the unit cell. The calculated results are also presented in Figure 5. It is interesting to see that doping either an electron or a hole will reduce the magnetic moment with 1 μB, which is not related to the embedded pattern in these doped BNNTs. For example, both T1-C21B15N and T2-C15B21N systems without carrier doping have 6 μB, while their magnetic moments are all predicted to be 5 μB for doping either an electron or a hole. This is easy to understand this observation, since the systems with T1 and T2 configurations are fully spin-polarized. The doped electron will fill the first empty state locating above the Fermi level for the spin-down electrons, while the introduced hole will occupy the first filled state lying below the Fermi level for the spin-up electrons. Then, the reduction of the number difference between the spin-up and spin-down electrons is 1 exactly. As a result, the reduction of the magnetic moment is 1 μB. As shown in Figures 3, 4a, and 6, these triangle graphene flake-doped BNNTs (i.e., n < 5) are all typical bipolar magnetic semiconductors (BMSs). Now, we turn to explore the carrier doping effect on the main features of BMS. In our previous work,52 we defined three important energy parameters (Δ1, Δ2, and Δ3) to characterize BMSs, where Δ1 represents the spin-flip gap between the VB edges and CB edges channels, while Δ1 + Δ2 and Δ2 + Δ3 reflect the spin-conserved gaps for the two spin channels, respectively. These energy parameters (Δ1, Δ2, and Δ3) are summarized in Table 1. Clearly, the values of Δ1, Δ2, and Δ3 can be effectively tuned by changing the graphene flake size, the shape, and the embedded pattern. With changing the size and embedded pattern of the triangle graphene flake, the values of the three parameters change. For example, the values of Δ1, Δ2, and Δ3 are predicted to be 0.65,

Table 1. Calculated Δ1, Δ2, and Δ3 (in eV) for the Triangle Graphene Flake-Doped BNNTs (n = 3−5) with T1 and T2 Configurations T1

C6B3N

C10B6N

C15B10N

Δ1 Δ2 Δ3 T2 Δ1 Δ2 Δ3

0.80 2.60 0.79 C3B6N 0.48 0.83 2.84

0.65 2.32 0.90 C6B10N 0.25 1.06 2.53

0.53 2.05 0.94 C10B15N 0.12 1.18 2.39

2.32, and 0.90 eV, for the T1-C10B6N system, and 0.25, 1.06, and 2.53 eV for the T2-C6B10N configuration, whereas, for the T1C21B15N system, they are 0.42, 1.85, and 0.93 eV, as shown in Figure 7.

Figure 7. Spin-polarized DOS of the doped (5, 5) BNNT with T1C10B6N, T2-C6B10N, and T1-C21B15N configurations. Here, the values of Δ1, Δ2, and Δ3 are labeled with their line lengths for clarity.

Previous investigations have shown that the spin filtering effect can be simply realized in a BMS by carrier doping.52,53 To clearly illustrate the similar results observed in these triangle graphene flake-doped BNNTs, we take the T1-C15B10N system as an example. The calculated DOS values without carrier doping, doping with an electron, or doping with a hole, are plotted in Figure 8a−c, respectively. As for the system without carrier doping, it is a BMS with Δ1 = 0.53 eV, Δ2 = 2.05 eV, and Δ3 = 0.94 eV. There is an empty DOS peak for the spin-

Figure 8. Spin-polarized DOS of the doped (5, 5) BNNT with the T1C10B6N case: (a) system without carrier doping, (b) doping with an electron, (c) doping with a hole. F

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down electrons and filled DOS peaks for the spin-up electrons locating at about 0.56 and −0.15 eV, labeled with A and B, respectively. When an electron is introduced to the system, it will fill the lowest empty midgap state for the spin-down electrons, and then the peak A crosses the Fermi level. This implies that the channel for the spin-down electrons is metallic conducting. After an electron doping, the position of the peak B for the spin-up electrons shifts downward and locates at −0.50 eV. The band gap for the spin-up electrons is predicted to be 1.53 eV. This observation will be different for doping with a hole, which will occupy the upmost filled state for the spin-up electrons. The peak B crosses the Fermi level, indicating the half-metallicity for the spin-up channel, while the band gap for the spin-down electrons is about 2.53 eV, and hence the spinup channel is semiconducting. That is to say, the spin filtering phenomenon, conducting with either the spin-up or the spindown channel, can be successfully realized by carrier doping. Note that the separation between the peaks A and B has a relatively small value (Δ1 = 0.12 eV) in the T2 configuration compared with the T1 situation (Δ1 = 0.53 eV), which suggests that a BMS in the T2 configuration can be more easily tuned into a half-metal by carrier doping. Moreover, the values of Δ1 + Δ2 and Δ2 + Δ3 for the T1 configuration are 2.59 and 2.99 eV, whereas they are 1.30 and 3.57 eV for the T2 case, respectively. These values are enough to stabilize the predicted half-metallicity. It should be pointed out that, with increasing the size of the graphene flake, the doped BNNTs (i.e., n > 6) will be spin-gapless semiconductors (SGSs).54 Compared with BMSs, because the Δ1 is zero for these SGSs, they can be more easily tuned into half-metallic by carrier doping. To check effect of the chirality of BNNTs, test calculations for triangle graphene flakes with different sizes (n ranges from 3 to 5) embedded in zigzag (12, 0) and helical (4, 8) BNNTs are also carried out. We find that they are still BMSs, which can also be modulated into half-metals via carrier doping, regardless of the chirality of BNNTs. These findings imply that these triangle graphene flake-doped BNNTs hold great potential for applications in molecular spintronic devices. In general, the size of the band gap will be underestimated using the general gradient approximation (i.e., PBE functional used in our calculations). To predict the accurate band gap, various hybrid functionals including Heyd−Scuseria−Ernzerhof (HSE06)55 are commonly adopted. Unfortunately, the HSE06 calculations for these proposed systems are not bearable for our current resources. Then, we turn to perform test calculations using the DFT+U method. The band gap of a perfect (5, 5) BNNT is predicted to be 4.68 eV at the GGA-PBE level, while the experimental measured band gap (about 5.5 eV) can be obtained by the DFT+U (i.e., U = 6.0 eV) method. Using the DFT+U method, we obtain the similar dispersion curves for the valence and conduction bands, while the position of conduction bands is up-shifted, and the separations among these subbands change. Fortunately, the Lieb theorem still works. The triangle graphene flakes embedded in BNNTs are FM due to the spinpolarized interface states, and the BMS feature and metal-free ferromagnetism in these triangle graphene flake-doped BNNTs survive to the choice of computational method.

shape and size or altering the embedded pattern can effectively modulate the band structures and magnetic properties of these doped BNNTs. The band gaps of these doped systems decrease with increasing the size of graphene flakes. Moreover, the Lieb theorem holds for the embedded triangle graphene flakes in BNNTs, which are ferromagnetic due to the spin-polarized interface states. All BNNTs embedded with triangular graphene flakes with relatively small sizes are typical bipolar magnetic semiconductors, and they can be tuned into half-metals by carrier doping. These theoretical findings open the door for the graphene flake-doped BNNTs to the applications in spintronics.

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ASSOCIATED CONTENT

S Supporting Information *

Calculated band structures of H-CαBβN, T1-CαBβN, and T2CαBβN systems within narrow energy windows, and the partial DOS of the edge C, B, and N atoms in the T1-C10B6N and T2C6B10N systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-551-63607125. Fax: +86-551-63603748 (J.H.). *E-mail: [email protected] (Q.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Shuanglin Hu for useful discussions. This work was partially supported by the National Natural Science Foundation of China (Nos. 21273208, 21473168, 21121003), by the National Key Basic Research Program (Nos. 2014CB921101 and 2011CB921400), by the Strategic Priority Research Program (B) of the CAS (No. XDB01020000), by the Fundamental Research Funds for the Central Universities, by the Anhui Provincial Natural Science Foundation (1408085QB26), by the China Postdoctoral Science Foundation (No. 2012M511409), and by the SCCAS, Shanghai and USTC Supercomputer Centers.

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REFERENCES

(1) Golberg, D.; Bando, Y.; Huang, Y.; Terao, T.; Mitome, M.; Tang, C. Boron Nitride Nanotubes and Nanosheets. ACS Nano 2010, 4, 2979−2993. (2) Rubio, A.; Corkill, J. L.; Cohen, M. L. Theory of Graphitic Boron Nitride Nanotubes. Phys. Rev. B 1994, 49, 5081−5084. (3) Zhang, Y.; Gu, H.; Suenaga, K.; Iijima, S. Heterogeneous Growth of B-C-N Nanotubes by Laser Ablation. Chem. Phys. Lett. 1997, 279, 264−269. (4) Sen, R.; Satishkumar, B. C.; Govindaraj, A.; Harikumar, K. R.; Raina, G.; Zhang, J. P. B−C−N, C−N and B−N Nanotubes Produced by the Pyrolysis of Precursor Molecules over Co Catalysts. Chem. Phys. Lett. 1998, 287, 671−676. (5) Han, W. Q.; Cumings, J.; Huang, X.; Bradley, K.; Zettl, A. Synthesis of Aligned BxNyCz Nanotubes by a Substitution-reaction Route. Chem. Phys. Lett. 2001, 346, 368−372. (6) Wu, J.; Han, W. Q.; Walukiewicz, W.; Ager, J. W.; Shan, W.; Haller, E. E. Raman Spectroscopy and Time-Resolved Photoluminescence of BN and BxNyCz Nanotubes. Nano Lett. 2004, 4, 647−650. (7) Wang, W. L.; Bai, X. D.; Liu, K. H.; Xu, Z.; Golberg, D.; Bando, Y. Direct Synthesis of B−C−N Single-Walled Nanotubes by Bias-

■

SUMMARY We perform extensive first-principles calculations to explore the electronic structures and magnetism of BNNTs doped with graphene flakes with different sizes and shapes. Theoretical results clearly reveal that either changing the graphene flake G

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Article

Assisted Hot Filament Chemical Vapor Deposition. J. Am. Chem. Soc. 2006, 128, 6530−6531. (8) Xu, Z.; Lu, W.; Wang, W.; Gu, C.; Liu, K.; Bai, X. Converting Metallic Single-Walled Carbon Nanotubes into Semiconductors by Boron/Nitrogen Co-Doping. Adv. Mater. 2008, 20, 3615−3619. (9) Wei, X.; Wang, M. S.; Bando, Y.; Golberg, D. Electron-BeamInduced Substitutional Carbon Doping of Boron Nitride Nanosheets, Nanoribbons, and Nanotubes. ACS Nano 2011, 5, 2916−2922. (10) Wei, X.; Wang, M. S.; Bando, Y.; Golberg, D. Post-Synthesis Carbon Doping of Individual Multi-Walled Boron Nitride Nanotubes via Electron-Beam Irradiation. J. Am. Chem. Soc. 2010, 132, 13592− 13593. (11) Miyamoto, Y.; Rubio, A.; Cohen, M. L.; Louie, S. G. Chiral Tubules of Hexagonal BC2N. Phys. Rev. B 1994, 50, 4976−4979. (12) Weng, S. Z.; Cherrey, K.; Chopra, N. G.; Blase, X.; Miyamoto, Y.; Rubio, A. Synthesis of BxNyCz Nanotubules. Phys. Rev. B 1995, 51, 11229−11232. (13) Lam, K. T.; Lu, Y.; Feng, Y. P.; Liang, G. Stability and Electronic Structure of Two Dimensional Cx(BN)y Compound. Appl. Phys. Lett. 2010, 98, 022101. (14) Blase, X.; Charlier, J. C.; De Vita, A.; Car, R. Theory of Composite BxNyCz Nanotube Heterojunctions. Appl. Phys. Lett. 1997, 70, 197−199. (15) An, W.; Turner, C. H. Linking Carbon and Boron-Nitride Nanotubes: Heterojunction Energetics and Band Gap Tuning. J. Phys. Chem. Lett. 2010, 1, 2269−2273. (16) Huang, B.; Si, C.; Lee, H.; Zhao, L.; Wu, J.; Gu, B. L. Intrinsic Half-Metallic BN-C nanotubes. Appl. Phys. Lett. 2010, 97, 043115. (17) da Rocha Martins, J.; Chacham, H. Disorder and Segregation in B−C−N Graphene-type Layers and Nanotubes: Tuning the Band Gap. ACS Nano 2011, 5, 385−393. (18) Blase, X. Properties of Composite BxCyNz Nanotubes and Related Heterojunctions. Comput. Mater. Sci. 2000, 17, 107−114. (19) Choi, J.; Kim, Y. H.; Chang, K. J.; Tomanek, D. Itinerant Ferromagnetism in Heterostructured C/BN Nanotubes. Phys. Rev. B 2003, 67, 125421. (20) Wu, R. Q.; Liu, L.; Peng, G. W.; Feng, Y. P. Magnetism in BN Nanotubes Induced by Carbon Doping. Appl. Phys. Lett. 2005, 86, 122510. (21) Wu, M. M.; Zhong, L.; Wang, Q.; Sun, Q.; Pandey, R.; Jena, P. Anisotropy and Transport Properties of Tubular C-BN Janus Nanostructures. J. Phys. Chem. C 2011, 115, 23978−23983. (22) Guo, C. S.; Fan, W. J.; Zhang, R. Q. Spin Polarization of the Injected Carriers in C-doped BN Nanotubes. Solid State Commun. 2006, 137, 246−248. (23) Fan, Y.; Zhao, M.; He, T.; Wang, Z.; Zhang, X.; Xi, Z. Electronic Properties of BN/C Nanotube Heterostructures. J. Appl. Phys. 2010, 107, 094304. (24) Fan, Y. C.; Hou, K. Y.; Wang, Z. H.; He, T.; Zhang, X. J.; Zhang, H. Y.; Dong, J. M.; Liu, X. D.; Zhao, M. W. Theoretical Insights into the Built-In Electric Field and Band Offsets of BN/C Heterostructured Zigzag Nanotubes. J. Phys. D: Appl. Phys. 2011, 44, 095405. (25) Liu, H.; Zhang, H.; Song, J.; Zhang, Z. Electronic Structures of an (8, 0) Boron Nitride/Carbon Nanotube Heterojunction. J. Semicond. 2010, 31, 013001. (26) Zhong, R. L.; Sun, S. L.; Xu, H. L.; Qiu, Y. Q.; Su, Z. M. BN Segment Doped Effect on the First Hyperpolarizibility of Heteronanotubes: Focused on an Effective Connecting Pattern. J. Phys. Chem. C 2013, 117, 10039−10044. (27) Zhao, P.; Liu, D. S.; Zhang, Y.; Su, Y.; Liu, H. Y.; Li, S. J. Electronic Transport Properties of Zigzag Carbon and Boron-NitrideNanotube Heterostructures. Solid State Commun. 2012, 152, 1061− 1066. (28) Xiao, H. P.; He, C.; Zhang, C. X.; Sun, L. Z.; Zhou, P.; Zhong, J. Stability, Electronic Structures and Transport Properties of Armchair (10, 10) BN/C Nanotubes. J. Solid State Chem. 2013, 200, 294−298. (29) Zhang, Z. Y.; Miao, C.; Guo, W. Nano-solenoid: Helicoid Carbon-Boron Nitride Heteronanotube. Nanoscale 2013, 5, 11902− 11909.

(30) Du, A. J.; Ying, C.; Zhu, Z. H.; Lu, G. P.; Smith, S. C. C-BN Single-Walled Nanotubes from Hybrid Connection of BN/C Nanoribbons: Prediction by ab initio Density Functional Calculations. J. Am. Chem. Soc. 2009, 131, 1682−1683. (31) Jin, C.; Lin, F.; Suenaga, K.; Iijima, S. Fabrication of a Freestanding Boron Nitride Single Layer and Its Defect Assignments. Phys. Rev. Lett. 2009, 102, 195505. (32) Kotakoski, J.; Jin, C. H.; Lehtinen, O.; Suenaga, K.; Krasheninnikov, A. V. Electron Knock-On Damage in Hexagonal Boron Nitride Monolayers. Phys. Rev. B 2010, 82, 113404. (33) Ci, L. J.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430−35. (34) Liu, Z.; Ma, L.; Shi, G.; Zhou, W.; Gong, Y.; Lei, S. In-Plane Heterostructures of Graphene and Hexagonal Boron Nitride with Controlled Domain Sizes. Nat. Nanotechnol. 2013, 8, 119−124. (35) Kan, M.; Zhou, J.; Wang, Q.; Sun, Q.; Jena, P. Tuning the Band Gap and Magnetic Properties of BN Sheets Impregnated with Graphene Flakes. Phys. Rev. B 2011, 11, 205412. (36) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (37) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756−7764. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (39) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (40) Topsakal, M.; Akturk, E.; Ciraci, S. First-Principles Study of Two- and One-Dimensional Honeycomb Structures of Boron Nitride. Phys. Rev. B 2009, 79, 115442. (41) Liu, Y.; Bhowmick, S.; Yakobson, B. I. BN White Graphene with “Colorful” Edges: The Energies and Morphology. Nano Lett. 2011, 11, 3113−3116. (42) Berseneva, N.; Krasheninnikov, A. V.; Nieminen, R. M. Mechanisms of Postsynthesis Doping of Boron Nitride Nanostructures with Carbon from First-Principles Simulations. Phys. Rev. Lett. 2011, 107, 035501. (43) Zhou, Y.; Wang, Z.; Yang, P.; Gao, F. Novel Electronic and Magnetic Properties of Graphene Nanoflakes in a Boron Nitride Layer. J. Phys. Chem. C 2012, 116, 758−7586. (44) Wang, W. L.; Meng, S.; Kaxiras, E. Graphene NanoFlakes with Large Spin. Nano Lett. 2007, 8, 241−245. (45) Du, A.; Chen, Y.; Zhu, Z.; Amal, R.; Lu, G. Q.; Smith, S. C. Dots versus Antidots: Computational Exploration of Structure, Magnetism, and Half-Metallicity in Boron-Nitride Nanostructures. J. Am. Chem. Soc. 2009, 131, 17354−17359. (46) Lieb, E. H. Two Theorems on the Hubbard Model. Phys. Rev. Lett. 1989, 62, 1201−1204. (47) Huang, B.; Xiang, H.; Yu, J.; Wei, S. H. Effective Control of the Charge and Magnetic States of Transition-Metal Atoms on SingleLayer Boron Nitride. Phys. Rev. Lett. 2012, 108, 206802. (48) Sanvito, S. Molecular Spintronics. Chem. Soc. Rev. 2011, 40, 3336−3355. (49) Wei, X.; Tang, D. M.; Chen, Q.; Bando, Y.; Golberg, D. Local Coulomb Explosion of Boron Nitride Nanotubes under Electron Beam Irradiation. ACS Nano 2013, 7, 3491−3497. (50) Yin, L. C.; Cheng, H. M.; Saito, R. Triangle Defect States of Hexagonal Boron Nitride Atomic Layer: Density Functional Theory Calculations. Phys. Rev. B 2010, 81, 153407. (51) Huang, B.; Lee, H. Defect and Impurity Properties of Hexagonal Boron Nitride: A First-Principles Calculation. Phys. Rev. B 2012, 86, 245406. (52) Li, X. X.; Wu, X. J.; Li, Z. Y.; Yang, J. L.; Hou, J. G. Bipolar Magnetic Semiconductors: A New Class of Spintronics Materials. Nanoscale 2012, 4, 5680−5685. (53) Menezes, M. G.; Capaz, R. B. Half-Metallicity Induced by Charge Injection in Hexagonal Boron Nitride Clusters Embedded in Graphene. Phys. Rev. B 2012, 86, 195413. H

dx.doi.org/10.1021/jp5089349 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article

(54) Wang, X. L. Proposal for a New Class of Materials: Spin Gapless Semiconductors. Phys. Rev. Lett. 2008, 100, 156404. (55) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2006, 124, 219906.

I

dx.doi.org/10.1021/jp5089349 | J. Phys. Chem. C XXXX, XXX, XXX−XXX