J. Phys. Chem. C 2008, 112, 13571–13578
13571
Interaction of Iron Atoms with Pristine and Defective (8, 0) Boron Nitride Nanotubes G. Y. Gou, B. C. Pan,* and L. Shi* Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: April 1, 2008; ReVised Manuscript ReceiVed: June 15, 2008
Interaction of transition metal Fe atoms with pristine and defective (8, 0) boron nitride nanotubes have been investigated using density functional theory calculation. Our results indicate that a single Fe atom can adsorb on the outer wall of a perfect BN tube exothermically, and it can migrate along the tube surface easily, forming a stable Fe-Fe dimer for the double Fe atom adsorption. The presence of intrinsic defects, including single vacancy, divacancies, and Stone-Wales defects, can enhance the reactivity of BN tubes toward Fe. Special focus has been paid to the modulation on magnetic moment and spin configuration of atomic Fe adsorbed on the defect. Stable Fe-Fe dimer adsorption states exist on all intrinsic defects, characterized by distortion of local defect structures. Our findings propose a possible way for experiment to functionalize the BN tubes with transition metal iron. 1. Introduction 1991,1
Since their discovery in carbon nanotubes (CNTs) have attracted considerable interest because of their novel structural, electronic, and mechanical properties. Besides CNTs, another prevailing tubular nanomaterial, boron nitride nanotubes (BNNTs),2,3 have also been the subject of extensive study for years. In contrast to CNTs, BNNTs are semiconductors with a band gap width of about 5.8 eV,4 almost independent of tube chirality and morphology.5 In addition, BN tubes possess many intriguing properties comparable to CNTs, such as strong hardness, high thermal conductivity, and excellent mechanical properties.6-8 Especially, BNNTs are more chemically and thermally stable than their carbon analogues.9 Such advantages will make BNNTs ideal alternatives to CNTs for possible applications in nanoelectronic and optical devices. As far as optical and electronic applications of nanotubes are concerned, BNNTs can be superior to CNTs because of their uniform semiconducting properties. On the other hand, such uniform electronic properties may bring a hindrance to the extensive application of BNNTs. Thus, tuning their electronic structures will have potential impact on further application of BNNTs in both scientific and technological fields. Chemical functionalization of nanotubes with transition metal (TM) has proved to be an effective way for tuning their electronic properties. Experimentally, there has been some successful research performed on BNNTs/TM systems, such as Fe-Ni and pure Fe nanowire-filled BN multiwalled tubes,10,11 BNNT-supported gold nanoparticles,12 and YG-BNNTs filled with periodic magnetic Fe nanoparticles.13 Moreover, TM Fe is a commonly used catalyst in the synthesis of BNNTs.14 Therefore, a better understanding of the interaction between BNNTs and TM, especially Fe, is an extremely important subject. Because of the poor wetting properties of BN systems with metals, direct functionalization of BNNTs with TM is not easy, as far as perfect tubes are concerned. In fact, various defects * To whom correspondence should be addressed. E-mail: bcpan@ ustc.edu.cn,
[email protected].
such as single vacancy, divacancies, and Stone-Wales defects are known to be present via electron beam irradiation or certain thermal perturbations of BNNT samples.15-17 The existence of such intrinsic defects can modify the properties of BNNTs and may also change the nature of the interaction between Fe and BN tubes. However, previous theoretical work on BNNTs/TM systems did not involve such an issue.18,19 In this paper, the adsorption of Fe atoms on perfect and defective BN tubes are systematically investigated using spinpolarized density functional theory (DFT) calculation. Experiments have shown that BNNTs prefer a zigzag orientation during the growth.20 Therefore, in our calculation, a single-wall (8, 0) BNNT has been employed as the prototype model for a pristine tube. Single atomic vacancy, divacancies, and Stone-Wales defects are considered as the representative defects on BNNTs. Without losing generality, our investigation covers the basic cases for adsorption of the double Fe atoms. Our calculations reveal that the Fe atoms can adsorb on perfect tubes exothermically. The presence of defects can enhance the reactivity between Fe and the tube. Especially, when the Fe atoms interact with spin-polarized single vacancy defects, their magnetic moments can be modulated. Our finding proposes a possible way for experiments to directly functionalize the BNNTs with transition metal iron. 2. Theoretical and Computational Details All calculations are carried out using SIESTA code,21-23 within the framework of the density functional theory (DFT). The norm-conserving pseudopotentials generated using the Troullier-Martins scheme,25 with atomic core and nonlocal components expressed in the fully separable form developed by Kleiman and Bylander,26,27 are used to represent the valence electrons. Sankey finite-range pseudo atomic orbitals (PAOs)29 are utilized as the split-valence double-ζ basis set (DZ) for the valence electrons of B and N, and double-ζ basis set with polarization (DZP) for Fe. The generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE)28 is adopted for the exchange-correlation potential. Spinunrestricted density functional theory calculation is used to obtain all the results presented in this work.
10.1021/jp802783p CCC: $40.75 2008 American Chemical Society Published on Web 08/08/2008
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Gou et al. TABLE 1: Calculated Binding Energies (Eb, in eV), the Bond Lengths (in Å) of Fe-B and Fe-N Bonds, the Charge Transferred (C, in e) from the Fe Atom to the Tube, and the Net Magnetic Moment (in µB) of the Fe Atom/Total System for Adsorption of a Single Fe Atom at Four Stable Sites on (8, 0) BNNT
Figure 1. Fully optimized configurations (top view) around an Fe atom adsorbed at the different sites on the outer surface of the (8, 0) tube: (a) hole, (b) bridge, (c) bridge-t, and (d) B-top. The B, N, and Fe atom are represented as white, black, and gold balls, respectively.
A periodical boundary condition is employed along the tube axis, and a vacuum region (at least 7 Å) is assumed between the tubes in their lateral direction. For the calculations of a perfect BN (8, 0) tube absorbed with Fe atoms, a supercell which is 3 times the primary cell along the tube axis is used. For the cases where Fe atoms interact with defective tubes, a supercell consisting of 1 × 1 × 3 primitive unit cells is employed for the single vacancy defects, and 1 × 1 × 4 for the divacancies and Stone-Wales defects. A set of 1 × 1 × 4 Monkhorst-Pack30 special k points is used to sample the Brillouin zone of the system. Our test calculations show that the results are not significantly altered upon increasing the k points. The conjugate gradient (CG) algorithm31 is adopted to fully relax the structures until the residual force acting on the each atom is no more than 0.04 eV/Å. 3. Results and Discussion A. Adsorption of the Fe Atoms on a Perfect Tube. 1. Adsorption of a Single Fe Atom. To examine the interaction between Fe and pristine BN tubes, adsorption of a single Fe atom on the outer surface of (8, 0) BNNT is initially investigated. Five nonequivalent adsorption sites are considered: (1) above the center of a BN hexagon (hole site), (2) over the center of an axial BN bond (bridge site), (3) over the center of a BN bond tilted to the tube axis (tilted bridge site, abbreviated as bridge-t site), (4) top of a B atom (B-top site), and (5) top of an N atom (N-top site). For each case, full structural relaxation has been performed, and the obtained configurations are displayed in Figure 1. It is found that except for the adsorption of the Fe atom at the N-top site, where Fe moves to the bridge-t site automatically, the other four adsorption configurations are stable. In order to evaluate the stability of the four configurations, the binding energy is calculated, which is given by the following expression:
Eb ) ET[BNNT + Fe] - ET[BNNT] - ET[Fe]
(1)
where ET[BNNT + Fe] is the total energy of BNNT with an adsorbed Fe atom, ET[BNNT] is the total energy of BNNT, and ET[Fe] is the energy of a single Fe atom. By definition, Eb < 0 corresponds to an exothermic procedure for the adsorption of Fe atoms. The calculated binding energies, structural, and other physical quantities are summarized in Table 1. The four configurations are almost equally stable and bear little deviation of the
site
Eb
bondFe-B
bondFe-N
C
µFe/µtotal
hole bridge bridge-t B-top
-0.61 -0.63 -0.61 -0.60
2.144 2.183 2.183 2.118
2.613, 2.622 × 2 2.382 2.271 -
0.352 0.312 0.310 0.379
4.030/4.00 4.052/4.00 4.058/4.00 4.093/4.00
corresponding Fe-B or Fe-N bond length. Here we choose the most exothermic adsorption configuration where Fe is adsorbed at the bridge site as an example for a detailed analysis. At the bridge site, Fe-B and Fe-N bond lengths are 2.18 and 2.38 Å, respectively. The B and N atoms (especially B) bonded with Fe shift slightly outward from the initial tube surface (Figure 1b), implying a strong interaction between Fe and the BN tube. From Mulliken population analysis, we find that atomic Fe interacts with the tube mainly through donating its 3d electrons to the tube, and its 4p (polarization basis set) electrons further enhance the spin polarization of the adsorbate. Since the pristine BN tubes are nonmagnetic, the net magnetic moment of the system (µ ) 4 µB) originates from magnetic Fe, and spin polarization mainly occurs at the adsorbed Fe atom. To gain deep insight into the chemical adsorption of an Fe atom on BNNT, the electronic properties are studied. Based on the spin-unrestricted calculation, total density of states (DOS) of a perfect and Fe adsorbed (at bridge site) (8, 0) BN tube are displayed in Figure 2, parts a and b, respectively. As seen in Figure 2b, additional states emerge within the gap region. These gap states mainly originate from the 3d and 4s electrons of the Fe atom, and the corresponding energy bands are little dispersed over the Brillouin zone in their band structures, implying their localized properties in the energy landscape. On the basis of the energetic calculation above, it is found that there is no obvious preference when a single Fe atom is adsorbed at one of four sites. One may wonder whether Fe could diffuse among these adsorption sites easily. To answer this question, we investigate the energy path when an Fe atom diffuses among four sites. Based on the structural features of BNNT, three pathways with high symmetry have been considered, which are along tube axial direction (path I), circularly and helically around the tube axis (paths II and III). A single Fe atom is set at several selected sites along these pathways. For each adsorbed case, we fully relax the entire system by fixing the relative position between Fe and the tube in the axial and azimuthal direction, while the related height of the Fe atom respective to the tube is optimized.32 The relative energy curves are obtained with respect to the different sites along three pathways. By evaluation of the energy barrier for an Fe atom diffusing along each pathway, the most economical path is estimated to be starting from the bridge site; the Fe atom moves across the B-top site to the hole site along an axial direction and then migrates circularly around the axis back to the bridge site, forming a continuous diffusion circuit (Figure 3). As described in Figure 3, there is a combined energy barrier of about 0.25 eV. It should be noted that the true minimum energy pathway (MEP), which can be identified with more accurate technique, may differ from our diffusion route. However, our calculated energy barrier can be seen as the upper bound of the true energy barrier. Moreover, considering the inaccuracy in the calculation of energy path (at the order of 0.1
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Figure 2. Density of states (DOS) of (a) a perfect (8, 0) BN tube and (b) the tube with a single Fe atom adsorbed at the bridge site. Red areas correspond to local density of states (LDOS) for the Fe atom. The density of states for majority-spin electrons are plotted upward and minority-spin downward. The Fermi level is represented as a dashed line.
eV),32 the true energy barrier is still a small value. As a result of such a small energy barrier, the diffusion procedure can be achieved at a relatively low temperature, and a single Fe atom can migrate very easily on the outer wall of a perfect BNNT. 2. Adsorption and Diffusion of Two Fe Atoms on the Tube. Because of their high mobility, two separate Fe atoms diffusing along the tube may have a chance to encounter each other. In this case, how does the Fe atom already adsorbed at the stable site affect the diffusion of other Fe atoms nearby? In this subsection, we will systematically investigate this issue. In our calculation, several cases where the second Fe atom is placed at different positions around the first Fe atom adsorbed on a perfect (8, 0) BN tube are taken into consideration. After structural relaxations, the initial configurations automatically transform into two kinds of adsorption configuration, which are presented in Figure 4, parts a and c, respectively. In the first configuration (Figure 4 a), two Fe atoms bind separately on the tube. The average binding energy per Fe atom is almost the same as the case for adsorption of a single Fe atom; the second one (Figure 4c) with a lower energy is characterized with two Fe atoms bonding with each other, forming a stable Fe-Fe dimer. To better understand the interaction process between two Fe atoms, their minimum energy pathway (MEP) has been calculated using the nudged elastic band (NEB) method.33,34 In this method, a series of images are interpolated between the initial and final states of the reaction process. Meanwhile, a fictitious
spring force is introduced between all the intermediate images. The constrained optimization is achieved by minimizing the component of forces perpendicular to the band. In our calculation, we choose the separate Fe adsorption (state A in Figure 4) as the initial state and Fe-Fe dimer adsorption (state C) as the final state, with eight images inserted between them and the Fe-Fe distance taken as reaction coordinate. To precisely locate the highest energy image on the saddle point, the climbing image35 method is used after the regular NEB. The calculated MEP in the reaction process is displayed in the lower panel of Figure 4. The total energy of isolated Fe atom adsorption has been taken as energy zero. Relative to isolated Fe atoms, the structure of Fe dimer adsorption on the tube surface is about 1.85 eV lower in energy. Along this MEP, the system only has to overcome an energy barrier as small as 79 meV before reaching a stable state. The saddlepoint corresponds to the geometry where the Fe atom just crosses the top of a N atom (state B). Conversely, to separate two bonded Fe atoms, a large energy barrier of about 1.93 eV should be overcome. Thus, it is expected that the Fe-Fe dimer configuration will dominate in the double Fe atom adsorption on the outer wall of (8, 0) BNNT. Furthermore, it is found that the initial state differs from the final state not only in total energy but in the magnetic properties as well. For the two configurations, the magnetic moments of the system averaged on each adsorbed Fe (µ j ) are 4 and 3 µB respectively. This is understandable, since the electron configu-
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Figure 4. The minimum energy path (black circle) from adsorption of two separated Fe atoms (state A) to Fe dimer adsorption (state C) on the (8, 0) BN tube. Along the path, the system passes through a saddle point state (state B). The Fe-Fe distance is taken as the reaction coordinate, and the total energy of state A is set as energy zero. The empty circle represents the average magnetic moment per Fe atom (µ j) for every image. Solid lines are fitted to guide the eyes.
Figure 3. Diffusion circuit (upper panel) and relative energy curve (lower panel) with different adsorption sites where Fe is adsorbed on a (8, 0) BN tube. Here the diffusion circuit consists of two pathways: one is along the tube axis and the other is circularly around it. Three local minimum states (marked as A, B, and C) refer to adsorption at the bridge, B-top, and hole sites. The relative positions of these states on the diffusion circuit are measured by their axis coordinates z and radical angle θ, respectively. The total energy of state A has been taken as energy zero.
rations of Fe can be altered by overlapping their 3d orbitals when two Fe atoms bond together. To investigate the change of magnetic moment with the relative distance between Fe atoms, µ j of each image is also plotted in Figure 4. It is found that µ j decreases from 4 to 3 µB abruptly when the Fe-Fe distance is less than 3.38 Å, accompanied by a rapid decrease of system energy. Thus, it is indicated that the energy gain from the initial to final state mainly originates from the interaction between two Fe atoms. B. Interaction between Fe Atoms and Intrinsic Defects. Previous theoretical work has indicated that the electronic properties of BNNTs can be greatly modified with existing intrinsic defects.15,36 It is also expected the reactivity between Fe and the tube can be enhanced because of the dangling bond (DB) and the localized states induced by these defects.37 In this section, four representative intrinsic defects are considered: single B and N atom vacancy (VB and VN), divacancies (Dv), and Stone-Wales (SW) defects in BNNT. The optimized structures of the intrinsic defects in a (8, 0) tube are displayed in the left panel of Figure 5. Each single vacancy defect exhibits one pentagon and one DB (5-1DB) geometry around the local defect region. The Dv defect is generated by removing a B-N pair, yielding a pentagon-octa-
gon-pentagon (5-8-5) structure from reconstruction of homoatomic B-B and N-N bonds. The SW defect is formed by rotating an axial B-N bond by π /2, resulting in a pentagonheptagon pair (5-7-7-5) configuration. 1. Adsorption of a Single Fe Atom on the DefectiWe Tube. Adsorption of a single Fe atom on a defective tube is considered initially. To ensure that the most stable configuration could be achieved,the Fe atom is placed at different positions around the defect. After structural relaxation, the configurations corresponding to the most stable states for an Fe atom adsorption on the VB, VN, Dv, and SW defects of (8, 0) BNNTs are obtained, which are displayed in the right panel of Figure 5. It is significant that for the adsorption of Fe at the single vacancy defects, the structures locally deformed by the 5-1DB defects are restored, as homoatomic B-B(N-N) bonds are broken and new Fe-B(N) bonds formed, while for the latter two defects, their topological structures are not significantly altered by the adsorbed Fe atom. The calculated binding energies and structural and other physical quantities are summarized in Table 2. Note that ET[BNNT] appearing in formula 1 is the total energy of the corresponding defective tube. Compared with adsorption on the perfect BNNT, interaction between Fe and the defective tube gets strengthened with the presence of intrinsic defect, yielding a smaller binding energy and shorter Fe-B and Fe-N bonds. Especially for the cases where Fe is adsorbed on VB and VN defects, their binding energies are -8.13 and -4.24 eV, respectively. Such a large energy gain means that newly formed Fe-B or Fe-N bonds are quite strong. Due to the ionizing nature of metallic Fe, it prefers to bond toward the atoms with high electron affinity. Therefore, compared with the adsorption of Fe at VN, the case of VB is characterized with a smaller binding energy, shorter chemical bonds, and larger amount of transferred charge between Fe and the tube. Moreover, because of the unpaired electrons induced by DB at the single vacancy defect,37 the interaction between the spin-
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Figure 6. Upper panel: DOS for the configurations where a single Fe atom is adsorbed at the different defect sites: (a) VB and (b) VN. Red areas correspond to LDOS of the Fe atom. The DOS for majority-spin electrons are plotted upward and minority-spin downward. The Fermi level (dotted line) is shifted to energy zero. Lower panel: The electron configurations of an isolated Fe atom (Oa), and the system where the Fe atom is adsorbed at VB (Ob) and VN site (Oc). The solid arrows (lines) correspond to the 3d electrons (orbitals) of Fe, dashed arrows represent the unpaired electrons from VB or VN defect, and dashed lines refer to their corresponding orbitals.
Figure 5. The optimal structures (left panel) around (a) VB, (b) VN, (c) Dv, and (d) SW defects and the most stable configurations (right panel) around a single Fe atom adsorbed (a) VB, (b) VN, (c) Dv, and (d) SW defects in the (8, 0) BN tube.
TABLE 2: Calculated Binding Energies (Eb, eV), the Bond Lengths (Å) of Fe-B and Fe-N Bonds, the Charge Transferred (C, e) from Fe to the Defective Tube and the Net Magnetic Moment (µB) of the Fe Atom/Total System for Adsorption of a Single Fe Atom at Four Different Intrinsic Defect Sites in (8, 0) BNNT defect VB VN Dv SW
Eb
bondFe-B
bondFe-N
C
µFe/µtotal
-8.13 -4.24 -2.05 -1.06
1.870, 1.928 × 2 2.137 × 2 2.071
1.845, 1.893 × 2 2.369 × 2 2.293
0.690 0.209 0.414 0.324
3.876/5.00 1.541/1.00 3.666/4.00 4.006/4.00
polarized host defective tube and 3d electrons of Fe results in unusual magnetic properties: the magnetic moment of the adsorbed Fe is significantly altered from that of an isolated Fe atom, indicating the modulation on its electron configuration. For the nonmagnetic Dv and SW defects, the net magnetic moment of the adsorbate and charge transferred between Fe and the tubes are comparable with the results of adsorption on the perfect tubes; thus, there is no significant electron configuration modulation on the adsorbed Fe in cases of these two defects. Clearly, by interaction with different atomic vacancy defects, the adsorbed Fe atom can exhibit a completely distinct modulation effect on its magnetic moment. In what follows, these two kinds of systems, where Fe is adsorbed at VB and VN defects, will be investigated in detail and the physical origin of their different magnetic properties will be given.
The calculated density of states for the Fe adsorbed VB and VN defects are displayed in Figure 6, parts a and b, respectively. For the case where Fe is adsorbed at the VB defect, there exist occupied gap states in majority spin, and unoccupied gap states in minority spin, leading to completely spin-splitting HOMO and LUMO. However, the distribution of gap states in the energy landscape is altered in the Fe adsorbed VN defect: there are both occupied and unoccupied gap states in majority and minority spin, resulting in a lower occupation difference between spinup and spin-down electrons and therefore corresponding to a smaller magnetic moment of the system. The effect of adsorbed Fe on the electronic properties of the whole system mainly depends on the nature of the defect. Since the configuration of the Fe adsorbed single vacancy defect can be constructed by bonding an Fe atom directly to a “native” B or N atomic vacancy (the initial structure by removing a single B or N atom from the perfect tube, containing three DBs around the defect region38), these two “native” vacancy defects are under investigation. It is found that both defects produce unpaired electrons and introduce the spin-polarized gap states. However, with a different localized potential induced,39 two defects play completely distinct roles: in the B vacancy, there exist unoccupied gap states in minority spin. They correspond to three unpaired electrons from the three DBs (µ ) 3 µB), making the B vacancy as an acceptor-like defect. The N vacancy behaves as a donor-like defect, since its gap states appearing in majority spin are occupied, which originate from one spinpolarized electron from one of the DBs (µ ) 1 µB). When an Fe atom interacts with the two defects, different bonding characters arise. To better illustrate this procedure, the electron configurations for each defect and adsorbed Fe are depicted in the lower panel of Figure 6. According to Hund’s principle, six 3d electrons in an isolated Fe atom form the Oa configuration. When an Fe atom adsorbs at the B vacancy (three unpaired electrons represented as three upward dashed arrows), only one 3d electron in minority spin contributes to the acceptor-
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Figure 7. (a) The representative structures and the relative energy vs Fe-Fe distance for adsorption of two Fe atoms at (a) VB and (b) VN defects. The total energy minima of the respective systems are shifted to energy zero. Solid line curves are fitted to guide the eyes.
like defect to saturate one polarized electron, still leaving another two electrons unpaired. To form stable Fe-N bonds, one spinup 3d electron becomes spin-down, leaving an empty 3d orbital for electron pairs from two N atoms to occupy. This leads to the decrease of µFe, and the system is approximately in Ob electron configuration, characterized by a high-spin (HS) state. When an Fe atom binds at the N vacancy, the one polarized electron from donor-like defect (denoted as one downward dashed arrow) has to contribute to half-filled 3d orbitals in minority spin, since 3d orbitals in majority spin are full. Within a similar bonding mechanism, another two B atoms bond with Fe by occupying its empty 3d orbital, which is left by one of the 3d electrons in majority spin. As the valence electrons form the Oc configuration, the system is characterized by a low-spin (LS) state, with one unoccupied 3d orbital in majority spin and two in minority spin, which well matches its DOS in Figure 6b. Sun et al.40 have proposed a similar bonding mechanism to explain the bonding difference between adsorption of a single Fe atom and Fe chains on perfect CNTs. In their illustration, a 3d electron in the Fe atom can be transferred from spin-up configuration to spin-down when Fe adsorbs on a perfect CNT, and electron pairs from π bands of CNT occupy the empty 3d orbitals to form stable Fe-C bonds. For a perfect BNNT, where the π states are mainly bound around N atoms, there are no delocalized π bands available for bonding. However, for the defective BN tubes with single vacancy defects, their π bonds are broken, and through the interaction between unpaired electrons from the defects and 3d electrons of the Fe atom, nanotubes could supply electron pairs to occupy the empty 3d orbitals in Fe, which lead to the above-mentioned bonding model and the modulation effect on the magnetic moment of adsorbed Fe.
It should be noted that our bonding model is based on the isolated orbital picture, ignoring the hybrid effect and charge redistribution among 3d orbitals of Fe. Actually, when Fe interacts with single vacancy defects, the hybrid effect between d orbitals and s, p orbitals from the defective tube cannot change the total number of polarized electrons, but the charge redistribution among the 3d orbitals can lead to an alteration in the magnetic moment of adsorbed Fe. Therefore, our model works well to illustrate the total magnetic moment of the system (µtotal), but inconsistency occurs in µFe. 2. Interaction between Two Fe atoms and Intrinsic Defects. As demonstrated, spin configuration of the Fe atom adsorbed on a single vacancy defect can be modulated. When another incoming Fe atom bonds with this Fe, what will their structural and spin configurations be? In this subsection, the systems of defective tubes adsorbed with two Fe atoms are under investigation. Starting from the structure of one Fe atom adsorbed single vacancy defect, another Fe is initially placed at different positions around the first Fe atom. Stable configurations with different Fe-Fe distances are obtained after structural optimization, and the total energy of system only changes with the relative Fe-Fe distance. Figure 7 plots the total energy of the respective system as a function of the Fe-Fe distance. The obtained energy curves clearly exhibit the interaction between two metallic atoms, where the energy minimum corresponds to the equilibrium Fe-Fe bond length. For the adsorption of two Fe atoms at the VB defect, the optimal bond length the Fe-Fe bond is found to be 2.24 Å, and the binding energy is -1.24 eV, whereas in the case of the VN defect, there is a shorter equilibrium bond length (2.19 Å), accompanied by a smaller binding energy (-1.96 eV).
Interaction of Fe Atoms with Boron Nitride Nanotubes Figure 7 shows that the two systems share similar energy curves but are distinct in their magnetic properties. In the first system, its magnetic moment decreases from 9 to 7 µB when RFe-Fe is less than the equilibrium distance. Moreover, for the structure with the shortest Fe-Fe bond (RFe-Fe ) 2.195 Å), there is a net antiparallel magnetization between two bonded Fe atoms, resulting in the smallest magnetic moment of the system (µ ) 3 µB). On the contrary, the system where two Fe atoms are adsorbed on VN remains constant in its magnetic moment, independent of the Fe-Fe bond length. This discrepancy can be understood from the spin configuration of the initial Fe atom adsorbed at two kinds of single vacancy defects. For the Fe atom adsorbed at VB, its 3d electrons are characterized with an HS configuration. According to Hund’s principle, d electrons with an HS configuration can easily change their spin configuration. This may explain the observed magnetic moment distribution in the case of the two Fe atom adsorption on VB. When the Fe-Fe distance is closer to the equilibrium bond length, the spin configurations of the 3d electrons are altered with the effective overlap between the 3d orbitals. A similar effect is also observed for the double Fe atom adsorption on the perfect BNNT (Figure 4). Especially for the ultrashort Fe-Fe bond, a large overlap between the d orbitals results in significant alteration of the electron occupation in majority and minority spin of two Fe atoms. Thus, for the different Fe-Fe distance, corresponding to different amounts of overlapping 3d orbitals, the system undergoes the different spin configuration. On the other hand, the 3d electrons of the Fe atom adsorbed at VN are characterized with an LS configuration. When this Fe atom bonds with another Fe, there is no effective overlap, since its 3d orbitals in both majority and minority spin are not completely filled. Thus, the Fe adsorbed at VN cannot form other electron configuration when bonding with the incoming Fe atom. On the basis of the results above, it is concluded that the single vacancy defect based BNNTs/Fe systems can be precisely detected by measuring their magnetic moments with electron spin resonance (ESR) technique. In the first part of this section, we have revealed that the topological structures of the nonmagnetic Dv and SW defects are not significantly altered by adsorption of a singe Fe atom and there is no modulation on spin configuration of adsorbed Fe. As illustrated by Zobelli et al.,15 such defects cannot provide half-filled shallow states for doping, and thus interaction of spin degenerate defective states with the Fe atom cannot make any difference in majority and minority spin. Experimentally, there must be chances for more than one Fe atom to interact with BNNT samples containing intrinsic defects. In this situation, how will the Dv and SW defects react with them? To answer this question, the adsorption of two Fe atoms on Dv and SW defects are taken as the representative examples. After placing two Fe atoms at different positions around the Dv or SW defect, full relaxation has been performed to obtain a stable structure. As displayed in Figure 8, parts a and b, two of the most stable configurations of the Fe adsorbed Dv and SW defects are characterized with distortion of the topological structures for Dv and SW. For adsorption of the double Fe atoms at Dv, one Fe atom incorporates into the BN network by breaking the B-B and N-N bonds, with another Fe moving outward from the tube outer wall, forming an Fe-Fe dimer. The direction of Fe-Fe dimer is either parallel or perpendicular to the tube axis, making a 0.20 eV difference in the binding energies of the two structures. In the case of a SW defect, Fe can react with defective BNNT easily to form N-Fe-N and B-Fe-N bonds, by breaking homoatomic N-N bonds, whereas
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Figure 8. Examples for local structures of (a) Dv and (b) SW defects distorted with the adsorption of double Fe atoms.
the other Fe atom binds either with the first Fe atom or the B-B bond of the SW defect in the respective configuration, with the former one 0.21 eV more stable than the latter. Mulliken population analysis indicates that there are no magnetic moment modulations on the two Fe atoms adsorbed at Dv and SW defects. The nonmagnetic Dv and SW defects can also effectively “capture” incoming Fe atoms, just the same as single vacancy defects. Therefore, intrinsic defects such as single vacancy, divacancies, and SW defects can enhance the reactivity of BNNTs toward Fe. It may be expected that, at high concentration of Fe substitutional impurities, these intrinsic defects can make it possible to synthesize BNNT-supported Fe nanoparticles. Our finding reveals that intrinsic defects are “active” sites for BNNTs to react with Fe atoms, which provide the possibility for functionalization of the BN tubes with transition metal iron via electron beam irradiation or certain thermal perturbations of BNNT samples. 4. Conclusions In summary, we have systematically investigated the structural, electronic, and magnetic properties for the adsorption of the single and double Fe atoms on the perfect and defective (8, 0) BN nanotubes, where the intrinsic defects, such as single vacancy, divacancies, and Stone-Wales defects are taken into consideration. We find that a single Fe atom can adsorb on the tube surface exothermically. This single Fe atom can migrate on the sidewall of BNNT easily and tends to form a stable Fe-Fe dimer in the case of the double Fe atom adsorption. The interaction between Fe atoms and the tube are enhanced with the existence of intrinsic defects in BNNT. Especially for the adsorption of a single Fe atom at single vacancy defects, the local deformation induced by defects are restored, and two vacancy defects make the different modulation effect on spin configurations of adsorbed Fe. We have proposed a bonding mechanism to explain the interaction between a single Fe atom and B or N vacancy defect. The local structures of divacancies and Stone-Wales defects are not significantly altered with adsorption of a single Fe atom but are completely distorted through interaction with the two. It is suggested that by introduction of intrinsic defects, it is possible to functionalize the BN tube with transition metal Fe, which is an important issue for fabricating BN nanotube-supported nanosize iron particles.
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