Structural, Electronic, and Magnetic Properties of the Semifluorinated

Jan 22, 2013 - Department of Physics, Hangzhou Normal University, Hangzhou, Zhejiang 310036, People,s Republic of China. ABSTRACT: Using density ...
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Structural, Electronic, and Magnetic Properties of the Semifluorinated Boron Nitride Bilayer: A First-Principles Study Yanli Wang*,† and Yi Ding*,‡ †

Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park Hangzhou, Zhejiang 310018, People’s Republic of China ‡ Department of Physics, Hangzhou Normal University, Hangzhou, Zhejiang 310036, People’s Republic of China ABSTRACT: Using density functional theory calculations with dispersion correction, we present a comprehensive study on the semifluorinated boron nitride (BN) bilayers. We find that the semifluorination leads to a nonbonded configuration in BN bilayers spontaneously, which would transform to the diamondized configuration via moderate pressure. Due to the combined effect of intrinsic intralayer strain and interlayer charge transfer, the ferromagnetic coupling is favored in both configurations, which exhibit intriguing half-metallic behavior for BN bilayers. While these nonbonded and diamondized configurations could undergo a 2 × 1 reconstruction, this requires overcoming a fair energy barrier owing to the sliding and warping of BN planes. With such reconstruction, the stability is greatly enhanced, and the system is converted into a nonmagnetic direct-band-gap semiconductor. Our studies demonstrate that the semifluorination can bring diverse electronic and magnetic properties into the BN bilayer, which could be half-metals or semiconductors depending on the structural types and would have potential applications in spintronics and nanodevices.

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INTRODUCTION Since the discovery in the year of 2004, graphene has attracted extensive attention in material science and condensed matter physics due to its unconventional mechanical, optical, and electronic properties.1−4 As the isoelectronic and isostructural analogues of graphene, the boron nitride nanostructures have also intrigued a lot of interest.5−8 In the experiments, the graphene-like boron nitride (BN) nanosheets have been successfully fabricated.9−14 Using the chemical-solution-derived method, the mono- and few-layer h-BN sheets have been obtained starting from single-crystalline hexagonal boron nitride (h-BN).9 The preparation of monolayer and bilayer hBN sheets has been reported by the chemical exfoliation in the solvent10 or the controlled energetic electron irradiation through selective sputtering.11,12 Recently, large-scale synthesis of h-BN nanofilms has been achieved by the chemical vapor deposition method.13,14 Although the two-dimensional BN sheet is an sp2-hybridized honeycomb structure, unlike the semimetallic behavior of graphene, the BN sheet opens a large gap due to the strong ionicity of B−N bonds.15 For various practical applications, it is desirable to control the band gaps and magnetic properties of BN systems. To this end, many strategies have been proposed to tailor the electronic structures of BN sheets, such as rolling the sheets into nanotubes,7,16,17 cutting them into nanoribbons,18−22 doping them with foreign atoms,23,24 chemical functionalization on the surface,25−27 and so on. Among these approaches, similar to graphene,28−31 the chemical functionalization is one of the most effective routes for BN systems. It has © 2013 American Chemical Society

been found that the full hydrogenation on both sides of the BN sheet reduces the band gap and induces an indirect-to-direct band gap transition.25,26 When the BN sheet is semihydrogenated on just one side, it becomes a ferromagnetic (FM) metal with hydrogen atoms adsorbed at the preferred boron sites.25,26 Besides hydrogenation, the fluorination is widely used to tailor the electronic properties of BN nanomaterials. In experiment, the BN nanotubes (BNNTs) have been fluorinated by introducing fluorine atoms at the stage of tube growth.32 Through density functional theory (DFT) calculations, it has been revealed that due to the fluorination the band gaps of BNNTs are reduced significantly,33,34 and magnetism could be induced into the systems.35,36 For the BN nanosheet, the full fluorination also decreases the band gap, and the gap value could be further modulated by external strains.37 When the sheet is semifluorinated, it exhibits an antiferromagnetic (AFM) semiconducting behavior.25,38,39 The AFM coupling could be altered to the FM one at a 6% compressive strain, which brings half-metallicity into the semifluorinated BN sheet.39 For the bilayer and multilayered BN nanosheets, the full fluorination on both sides transforms the layered structures into diamond-like nanofilms, which have reduced band gaps due to the strong inbuilt electric polarization.40 Since the chemical functionalization on both sides and a single side results in distinct electronic and magnetic properties, Received: November 12, 2012 Revised: January 18, 2013 Published: January 22, 2013 3114

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it is natural to consider the BN bilayers with semifluorination. Will the semifluorination induce diamondization in the system as found in the case of full fluorination? Does the semifluorination lead to peculiar electronic and magnetic properties? In this paper, we pay close attention to the semifluorinated BN bilayer, which may possess more promising electronic properties than the monolayer counterpart. On the basis of firstprinciples calculations, we find that the semifluorination of the BN bilayer leads to a nonbonded configuration spontaneously, which could transform to the diamondized one via moderate pressure. In particular, both configurations favor the FM coupling and exhibit half-metallic behaviors. A 2 × 1 reconstruction could also occur in the nonbonded/diamondized configuration by overcoming a fair energy barrier, which converts the system into a nonmagnetic semiconductor.

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METHODS The first-principles calculations are carried out by the VASP code41 within a plane-wave basis and the projector-augmented wave pseudopotentials to describe the ion−electron interactions. For the exchange and correlation energy of electrons, the generalized gradient approximated functional of Perdew− Burke−Ernzerhof (PBE) form is adopted.42 Since the longrange interlayer interaction plays a crucial role in the geometries and binding energies of layered structures, the van der Waals (vdW) interactions are taken into account by using the semiempirical DFT-D2 method of Grimme with the inclusion of dispersion correction.43 The dipole corrections are also considered in our calculations due to the intrinsic polarization in the fluorinated BN sheets. The plane-wave cutoff energy is set to be 550 eV. The Monkhorst−Pack scheme is used to sample the Brillouin zone on a 25 × 25 × 1 k-mesh grid for the 1 × 1 unit cell and a 12 × 12 × 1 k-mesh grid for the 2 × 2 supercells. The optimizations of the lattice constants and the atom coordinates are performed by minimizing the total energies. All the sheets are fully relaxed until the force on each atom is less than 10−2 eV/Å, and the total energy changes are less than 10−4 eV. To verify the accuracy of our calculations, we first perform the calculations on the bulk h-BN. We obtain the intralayer lattice constant and interlayer spacing as 2.52 and 3.29 Å, respectively, in accordance with the experimental data of 2.5 and 3.3 Å.9

Figure 1. Optimized structures of the pristine BN bilayer with (a) AA and (b) AB stacking arrangements. The dashed lines in the top view denote the unit cell, and those in the side view show the interlayer spacing. The B and N atoms at the upper and lower layers are distinguished by adding the subscripts 1 and 2, respectively. (c)−(f) are the band structures and the total and partial DOSs for the BN bilayer with AA and AB stacking arrangements.

To determine the stabilities of bilayer BN, the binding energies are calculated as Eb = Ebilayer − Eupper − Elower, where the Ebilayer is the total energy of the bilayer sheet, and Eupper and Elower are the respective energies for the upper and lower layers with the same intralayer lattice constants as the bilayer sheet. With this definition, a more negative value of the binding energy corresponds to a more stable structure. Our calculations show that the Eb are −0.138 and −0.141 eV per formula unit (there are two B atoms and two N atoms) for the AA- and ABstacked BN bilayer, respectively. It indicates that unlike the AA stacking in the bulk h-BN, the BN bilayer prefers the AB stacking instead, which is in accordance with the previous studies.44,45 The AB stacking is also favored in the graphene bilayer46 and the graphene/BN heterobilayer.44,47 However, the energy difference between the AA- and AB-stacked BN bilayer is rather small (3 meV per formula unit), which suggests that the two stacking arrangements are almost energetically degenerate. Actually, both the AA- and AB-stacked BN bilayer sheets have been observed in the experiment.10 Figures 1(c) and (e) display the band structures of the AAand AB-stacked BN bilayer, respectively. It can be seen that the two stacking arrangements lead to similar semiconducting behaviors with large indirect band gaps. The gap values are identical for the AA and AB stacking sequences, which are 4.38 eV, in accordance with the previous LDA results of 4.3 eV.44 The large band gaps originate from the strong ionicity of B−N

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RESULTS AND DISCUSSION A. Pristine BN Bilayer. For the pristine BN bilayer (BN)2, two types of stacking arrangements are considered in our calculations as depicted in Figures 1 (a) and (b): (i) the AA stacking, in which the B (N) atoms at the upper layer locate on top of the N (B) ones at the lower layer; (ii) the AB stacking, where the upper N atoms sit on top of the lower B ones, while the upper B (lower N) atoms locate above (below) the hexagon centers of the lower (upper) BN layer. Our calculations show that the intralayer B−N bond length is 1.45 Å for both stacking sequences, which agrees well with the experiments (1.45 Å).10 The equilibrium interlayer distances are 3.13 and 3.09 Å for the AA and AB stacking, respectively, in good accordance with the previous LDA results (3.103 and 3.071 Å).44 The large interlayer spacing suggests no chemical bonds are formed between the two planes in the bilayer sheet. This is evidenced by the flatness of each BN plane, in which the B and N atoms bond to each other via sp2 hybridization. 3115

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and the B atoms at the lower layer, is reduced to 2.60 Å. The binding energy is calculated as −0.188 eV per formula unit (there are two B atoms, two N atoms, and one F atom in the unit cell of the F−(BN)2), comparable to that of the pristine bilayer sheet (−0.141 eV per formula unit). It means that the interlayer coupling is still weak in F−(BN)2, and the semifluorination of the BN bilayer results in a nonbonded configuration spontaneously. To measure the stability of fluorination, the formation energy is calculated as Ef = EF−(BN)2 − E(BN)2 − nEF2/2. Here, EF−(BN)2 and E(BN)2 are the total energies of the semifluorinated and pristine BN bilayer, respectively; EF2 is the energy of an isolated F2 molecule; and n is the number of fluorine atoms per formula unit cell. Our calculations show the nonbonded F−(BN)2 has a formation energy of −0.67 eV per F atom, which is slightly larger than that of the semifluorinated BN monolayer (−0.75 eV per F atom) by 0.08 eV. The small difference is reasonable considering that in the nonbonded F−(BN)2 the semifluorinated upper layer is only weakly coupled to the bottom BN sheet. The negative value of Ef indicates that the semifluorination of the BN bilayer is an exothermal process and could be possibly synthesized in the experiment. Similar to the semifluorinated BN monolayer (F−(BN)1), the F chemisorption on the BN bilayer induces one hole in each N atom at the upper layer, making the pz state half filled. As a result, local moments are created on those N atoms due to the unpaired pz electrons. For the magnetic ground states of F− (BN)1, both the AFM and FM states have been reported in previous studies.25,38,39,52 In view of this discrepancy, we carry out calculations on F−(BN)1. Consistent with the previous studies by Zhou et al.25 and Ma et al.,38,39 we find the ground state of F−(BN)1 corresponds to the AFM state, which is lower than the FM one by 11.8 meV per formula unit. To study the stable magnetic coupling in the nonbonded F−(BN)2, three magnetic configurations are considered: the nonmagnetic (NM) state, the ferromagnetic (FM) state, and the antiferromagnetic (AFM) state. Our calculations show that unlike the monolayer F−(BN)1 the bilayer counterpart of F− (BN)2 has the FM ground state, which is 0.009 and 0.26 eV per formula unit lower than the AFM and NM states, respectively. At the FM state, the total magnetic moment is 1 μB per formula unit, and the spin-density distributions in Figure 2(b) show most of the magnetism concentrates on the semifluorinated upper layer. From the Bader analysis,53,54 we obtain the atomic moments of N and F atoms at the upper layer as 0.70 and 0.22 μB, respectively, while the lower BN layer contributes only 0.078 μB to the total magnetic moment. Although the semifluorinated upper layer plays a dominant role in the magnetic properties of F−(BN)2, the bilayer structure and the monolayer counterpart of F−(BN)1 have different ground magnetic states since the latter prefers the AFM state instead.25,38,39 To address this discrepancy, we first analyze the structural difference between the two systems. For the F−(BN)2, the intralayer lattice constant is 2.56 Å, while it is 2.63 Å for the F−(BN)1. Compared to the F−(BN)1, the semifluorinated upper layer in the F−(BN)2 is compressed with a strain of 2.7%, which results from the lattice mismatch between the F−(BN)1 and the BN monolayer. It has been reported that for the F−(BN)1 the FM state becomes more stable than the AFM one at a 6% compression.39 Hence, only the 2.7% compression can not induce the magnetic transition. Considering that the charge doping could induce ferromagnet-

bonds. However, owing to the difference in stacking sequence, the electronic structures of the BN bilayer are influenced by the sublattice exchange symmetry effects, as found in the graphene bilayer sheets.48,49 In the AA-stacked BN bilayer, the B1 (N1) atom at the upper layer is equivalent to the B2 (N2) one at the lower layer, thus they make equal contributions to the total density of states (DOSs) in the whole energy range, as shown in Figure 1(d). The top valence bands originate from the p orbitals of N1 and N2 atoms, while the bottom conduction bands are from the p orbitals of B1 and B2 ones. On the other hand, the two BN planes are no longer equivalent in the ABstacked bilayer sheet. Figure 1(f) shows that the top valence bands and the bottom conduction bands are mainly from the p orbitals of lower N2 and upper B1 atoms, respectively. For the AB-stacked BN bilayer, as seen from Figure 1(e), the lowest conduction band is quite flat from the K to M point, and the K point is only slightly higher in energy than the M point, thus leading to an indirect band gap. Our results are consistent with the previous studies by Yang et al.50 and Balu et al.51 However, Zhong et al.44 and Ribeiro et al.45 have reported a direct band gap. The difference in the band gap type could be attributed to the different computational codes employed, which use different basis sets. It seems that a plane-wave basis set predicts an indirect band gap, while the atomic-like basis set leads to a direct one for the BN bilayer. B. Nonbonded Configuration for Semifluorination. When the F atoms are chemisorbed on the upper surface of BN bilayer, the semifluorinated bilayer sheet is formed. For ease of discussion, the semifluorinated BN bilayer is denoted as F− (BN)2. In this work, we pay main attention to the F−(BN)2 with AB stacking arrangement, since the AA-stacked F−(BN)2 behaves quite similarly as the AB one (see Section E). Considering that the F atoms prefer to adsorb on top of B atoms in the BN nanostructures,25,34 we study the F−(BN)2 with F atoms binding with B atoms at the upper layer. The F atoms are initially placed on top of the B ones at a height of 3 Å from the upper BN plane. After full structural optimization, the upper layer is buckled, while the lower layer remains essentially flat, as shown in Figure 2(a). The buckling height of the upper layer, defined as the perpendicular distance between the B and N atoms, is 0.39 Å. Due to the buckling, the interlayer spacing, which is the distance between the N atoms at the upper layer

Figure 2. (a) Optimized structure of the F−(BN)2 with the nonbonded configuration. (b) The spin-up (left panel) and spindown (right panel) band structures and the spin density distributions at the FM state. (c) The total and partial DOSs for the nonbonded F− (BN)2. 3116

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ism in the hexagonal group III−V nanosheets,55 we examine the charge transfer in the F−(BN)2. Due to the intrinsic polarization induced by F chemisorption, a small amount of electrons is transferred from the lower BN sheet to the semifluorinated upper layer, which is 0.034 e per formula unit from the Bader analysis. Hence we calculate the F−(BN)1 with a lattice constant of 2.56 Å and charged with 0.034 e per formula unit and find that the FM state is energetically more favorable than the AFM state by 0.016 eV per formula unit. Therefore, through a combined effect of the intrinsic intralayer strain and a small amount of interlayer charge transfer, the FM coupling is favored in the bilayer structure of F−(BN)2. Figure 2(b) depicts the spin-polarized band structures of the nonbonded F−(BN)2 at the FM state. Interestingly, the system exhibits a half-metallic behavior. The spin-up bands are insulating with a large indirect band gap of 4.73 eV, while the spin-down ones are metallic with three bands crossing the Fermi level. The half-metal gap, defined as the distance between the Fermi level and the valence band maximum of the spin-up bands, is 0.37 eV. The partial DOS (PDOS) diagram Figure 2(c) indicates that the partially filled bands for the spindown electrons come from the p orbitals of N and F atoms at the upper layer, which is in accordance with the spin density distributions in Figure 2(b). Using the energy difference between the AFM and FM states and the mean field theory, the Curie temperature is estimated to be about 100 K by using the formala of γkBTC/2 = EAFM − EFM,56 where γ is the dimension of the system; kB is the Boltzmann constant; and EAFM and EFM are the energies of the system at the AFM and FM states, respectively. It shows that the half-metallicity in the nonbonded F−(BN)2 could only be sustained at low temperature. C. Diamondized Configuration for Semifluorination. Although the nonbonded F−(BN)2 has a promising halfmetallic behavior, there are still some limitations on its practical applications. First, it is difficult to manipulate the F−(BN)2 as a whole because the two layers in it are only weakly coupled. Second, the half-metallicity only exists at low temperature. A possible resolution to the first problem is enhancing the interlayer interaction via chemical bonding. As a matter of fact, it is found that when the initial interlayer distance in F−(BN)2 is reduced to 2.2 Å the chemical bonds are formed between the two layers after full structural relaxation, which gives rise to a diamondized configuration as depicted in Figure 3(a). Owing to the interlayer bond formation, both the upper and lower layers are buckled with the buckling heights of 0.54 and 0.33 Å, respectively. The interlayer B2−N1 bond length is 1.65 Å, only slightly larger than the intralayer ones of B1−N1 (1.59 Å) and B2−N2 (1.53 Å). For the diamondized configuration, the binding energy Eb is −0.356 eV per formula unit, and the formation energy Ef is −0.821 eV per formula unit. Both values are lower than those of the nonbonded F−(BN)2 by about 0.15 eV per formula unit, which demonstrates the stronger stability of the diamondized configuration. The increased structural stability originates from the substantial energy gain from the interlayer bond formation, which is larger than the energy cost to deform the two layers in the bilayer sheet. Although the diamondized configuration is energetically more favorable than the nonbonded one, the diamondization can not occur spontaneously. It implies there is an energy barrier for the transition from the nonbonded to the diamondized configuration. To search for the transition state and determine the energy barrier, we employ the climbing image nudged elastic band (CI-NEB) method 57,58 as

Figure 3. (a) Optimized structure of the diamondized F−(BN)2. The buckling height for each layer and the interlayer bond length is denoted. (b) The minimum-energy path for the structural transformation from the nonbonded to the diamondized configuration. Insets plot the configurations of the initial (I), the transition (T), and the final (F) states along the reaction pathway.

implemented in VASP. Along the reaction pathway from the nonbonded to the diamondized configuration, we obtain an energy barrier of 0.031 eV per formula unit, as shown in Figure 3(b). It is an order of magnitude smaller than that of the semifluorinated graphene bilayer (0.22 eV per formula unit).59 The difference mainly stems from the attractive Coulomb interaction between B and N atoms in the F−(BN)2. For the transition state, the buckling heights of the upper and lower layers are 0.43 and 0.12 Å, respectively. The interlayer distance is 2.16 Å, which is 0.44 Å smaller than that of the initial state. To overcome the energy barrier, the required pressure is estimated to be 1.93 GPa by the formula of P = (ET − EI)/ (A·(dI − dT)), where A is the area of the unit cell; ET and EI are the total energies; and dI and dT are the interlayer distances for the initial and transition states, respectively. Since the required pressure is moderate, we expect that the preparation of diamondized F−(BN)2 could be realized in the experiments with a similar approach for synthesizing diamondized bilayer graphene.60 Moreover, the diamondized configuration of F− (BN)2 could be retained when the external pressure is released. Figure 4(a) displays the spin-polarized band structures of the diamondized F−(BN)2 at the FM state, which resembles those of the nonbonded counterpart. Apparently, the half-metallicity is maintained in the diamondized configuration with the spinup bands insulating and the spin-down ones metallic. Although the half-metal gap of 0.13 eV is smaller than that of the nonbonded configuration (0.37 eV), it is still large enough to be operated at room temperature. The PDOSs analysis in Figure 4(b) shows that the metallic spin-down bands are mainly from the p orbitals of N2 atoms at the lower layer. There are also small contributions from the p orbitals of N1 and F atoms at the upper layer. The calculated total magnetic moment is 1 μB per formula unit, in which 0.65 μB comes from the unpassivated N2 atoms. The rest is contributed by the N1 and F atoms with 0.26 and 0.09 μB, respectively. These obtained atomic moments are in line with the spin density distributions in Figure 4(a). In the diamondized configuration, the energies of different magnetic states follow the order of EFM < EAFM < ENM. The energy difference between the FM and AFM states is 3117

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Figure 5. Side view (a) and top view (b) of the reconstructed F− (BN)2. The dashed lines in (b) denote the unit cell of the reconstructed configuration. The two intralayer lattice constants are also marked.

bond angle is 122.9°, which are quite close to those of the pristine BN monolayer (1.45 Å and 120°). It indicates that the B2b−N2b zigzag chain is sp2-bonded. On the other hand, all the other B and N atoms are sp3 hybridized, thus forming longer B−N bond lengths larger than 1.50 Å. In the reconstructed F− (BN)2, owing to the reduced symmetry, the two intraplane lattice constants are slightly different, which are 5.07 and 5.14 Å as shown in Figure 5(b). The buckling heights are 0.69 and 0.86 Å for the upper and lower layers, respectively, which are larger than the corresponding values in the diamondized configuration. For the reconstructed F−(BN)2, we obtain a formation energy of −1.09 eV per F atom, which is lower than the nonbonded and diamondized configurations by 0.42 and 0.27 eV per F atom, respectively. It means there is a remarkable energy gain from the 2 × 1 reconstruction. Since in the nonbonded and diamondized configurations there exist dangling bonds localized at the N atoms, these dangling bonds could be saturated via atomic rearrangement, which lowers the energy of the system effectively. However, due to the parallel movement and large buckling of the lower BN layer, there exists an energy barrier for the structural transition from the nonbonded/diamondized to the reconstructed configuration. Our CI-NEB calculations show that when the nonbonded configuration is used as the initial state, the obtained energy barrier is 0.068 eV per formula unit. Likewise, the transition from the diamondized to the reconstructed configuration needs to overcome a barrier of 0.16 eV per formula unit. The higher energy cost in the latter case is attributed to the bond breaking evolved during the structural transformation. Therefore, it is easier for the experimenters to synthesize the reconstructed F−(BN)2 from the nonbonded configuration. The band structure of the reconstructed F−(BN)2 is depicted in Figure 6(a). It can be seen that unlike the halfmetallic nonbonded and diamondized configurations, the reconstructed configuration turns out to be a nonmagnetic semiconductor. Such a semiconducting feature discriminates the metallicity found in the semihydrogenated graphene bilayer with similar reconstruction,64 which is mainly due to the localization of the electronic states in the BN systems. For the

Figure 4. (a) Spin-up (left panel) and spin-down (right panel) band structures and spin density distributions, and (b) the total and partial DOSs for the diamondized F−(BN)2 at the FM state.

0.025 eV per formula unit, which is much larger than that of the nonbonded configuration (0.009 eV per formula unit). Using such energy difference and the mean field theory, the Curie temperature is predicted to be 290 K for the diamondized configuration, which is much higher than that of the nonbonded counterpart. The enhanced FM coupling in the diamondized configuration stems from the remarkable charge transfer from the lower layer to the upper layer. The Bader analysis gives that it is 0.30 e per formula unit, which is an order of magnitude larger than that in the nonbonded configuration (0.034 e per formula unit). D. Reconstructed Configuration for Semifluorination. Note that the lower layer of the diamondized F−(BN)2 resembles the N-terminated (111) surface of the cubic BN crystal (c-BN). Actually, the lattice constant of the diamondized F−(BN)2 is 2.59 Å, quite analogous to the experimental data of 2.6 Å for the surface unit cell of c-BN (111).61 Previous studies have revealed that the c-BN (111) surface gains substantial energy by transforming to a 2 × 1 reconstructed configuration.62 Similar behaviors have also been reported in the C (111) surface.63 Motivated by these studies, we investigate the reconstruction of F−(BN)2 with a 2 × 1 pattern, as illustrated in Figure 5. The reconstructed configuration can be obtained from the nonbonded or diamondized F−(BN)2 via parallel movement and warping of the lower BN layer. The movement is achieved by shifting the lower layer along the armchair direction by a half B−N bond length. The lower layer also undergoes warping to form alternate zigzag chains upward and downward, which facilitates the bond formation with the upper layer. The upward and downward zigzag chains are referred to as B2a−N2a and B2b−N2b, respectively, as denoted in Figure 5(a). It can be seen that the B2a−N2a zigzag chain bonds to the upper layer, which gives rise to the five- and seven-membered rings in the reconstructed configuration. The B2a−N2a zigzag chain is essentially flat, while the B2b−N2b one is slightly tilted. In the B2b−N2b zigzag chain, the B−N bond length is 1.46 Å, and the 3118

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Figure 7. Spin-polarized band structures and spin density distributions at the FM state for the AA-stacked F−(BN)2 with (a) nonbonded and (b) diamondized configurations. The optimized structure of the reconstructed AA-stacked F−(BN)2 from (c) the side view and (d) the top view. The dashed lines in (d) denote the unit cell. (e) The band structure of the reconstructed configuration.

configuration the most stable structure. Moreover, the reconstruction eliminates the magnetism and converts the system into a semiconductor with a direct band gap of 2.86 eV, as shown in Figure 7(e). From the energetic point of view, the energy differences between the AA- and AB-stacked F−(BN)2 (ΔE = EAA − EAB) are 0.008, 0.098, and −0.052 eV per formula unit for the nonbonded, diamondized, and reconstructed configurations, respectively. All these values are larger than that of the pristine counterpart (0.003 eV per formula unit), indicating that the semifluorination increases the energy difference between the two stacking patterns.

Figure 6. (a) Band structure and partial charge densities for the VBM and CBM of the reconstructed F−(BN)2. (b) The total (upper panel) and partial (middle and bottom panels) DOSs for the reconstructed F−(BN)2. The middle and bottom panels are the partial DOSs for the atoms at the upper and lower layers, respectively.

reconstructed F−(BN)2, it has a direct band gap of 2.79 eV at the Γ point. The partial charge densities for the VBM and the CBM are plotted in Figure 6(a). It shows that the VBM is doubly degenerate and mainly contributed by the p orbitals of N and F atoms at the upper layer, while the CBM is mainly composed of the p orbitals of B2b atoms at the lower layer. These results agree well with the PDOS diagram in Figure 6(b). The separated distributions of the VBM and CBM make the reconstructed F−(BN)2 a candidate for photovoltaic applications. E. Semifluorination with AA Stacking Arrangement. Finally, we discuss the F−(BN)2 with AA stacking arrangement briefly. It is found that the AA-stacked F−(BN)2 behaves quite similarly to the AB-stacked counterpart. The semifluorination of the AA-stacked BN bilayer also leads spontaneously to a nonbonded configuration. It could transform to the diamondized F−(BN)2 by overcoming an energy barrier of 0.077 eV per formula unit. Accompanied with the structural transition, there is an energy gain of 0.064 eV per formula unit owing to the interlayer bond formation in the diamondized configuration. The band structures in Figures 7(a) and (b) show that both the nonbonded and diamondized configurations are half metals at the ground FM state. The spin density distributions show the magnetism mainly localizes at the unpassivated N atoms. When the AA-stacked F−(BN)2 undergoes a 2 × 1 reconstruction as displayed in Figures 7(c) and (d), the energy of the system is lowered by 0.48 eV per formula unit compared to the nonbonded configuration, making the reconstructed

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CONCLUSION In summary, the structural, electronic, and magnetic properties of the F−(BN)2 are investigated by first-principles calculations. For the F−(BN)2, there are three possible configurations: the nonbonded, diamondized, and reconstructed configurations. The nonbonded F−(BN)2 is formed spontaneously after semifluorination of the BN bilayer. It could transform to the other two configurations by surmounting an energy barrier. For the transition to the diamondized configuration, a small energy barrier could be conquered by a moderate pressure, while the barrier is larger for the transformation to the reconstructed configuration due to the parallel movement and warping of the lower BN layer. Accompanied with the transformation, the system gains substantial energy from the diamondization/ reconstruction. Owing to a combined effect of the intrinsic intralayer strain and the interlayer charge transfer, both the nonbonded and diamondized configurations favor the FM coupling and exhibit an interesting half-metallic behavior, while the reconstructed configuration is nonmagnetic and presents a semiconducting characteristic with a direct band gap. Our studies demonstrate that the semifluorinated BN bilayer could be half-metals or semiconductors depending on the structural types, which endows it a promising material for potential applications in spintronics and nanodevices. 3119

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y. Wang); [email protected] (Y. Ding). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge the support from NSFC (11104249, 11104052), ZJNSF (Z6110033), and Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics (20120906).

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(64) Zhou, J.; Wang, Q.; Sun, Q.; Jena, P. Appl. Phys. Lett. 2011, 98, 063108-1−3.

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dx.doi.org/10.1021/jp311189a | J. Phys. Chem. C 2013, 117, 3114−3121