Formation, Morphology, and Effect of Complex Defects in Boron

Jun 14, 2011 - The other is the charge transfer between host-3 and C impurity because of the difference of their ...... Lee , C.; Yang , W.; Parr , R...
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Formation, Morphology, and Effect of Complex Defects in Boron Nitride Nanotubes: An ab initio Calculation Rui Liu, Jia Li, Gang Zhou,* Jian Wu, Bin-Ling Gu, and Wenhui Duan* Department of Physics, Tsinghua University, Beijing 100084, People0 s Republic of China ABSTRACT: The formation mechanism of the complex defect, consisting of a carbon (C) impurity and an atomic vacancy, and the corresponding effects on structure and properties of boron nitride nanotubes (BNNTs) are studied using density functional theory. It is found that the C impurity and the vacancy tend to bind together to form a pentagon-nonagon pair more stable than the isolated vacancy, because of geometrical and electronic effects. The combination of the cation vacancy and CB (C substitution for B) is preferable in N-rich conditions, while the combination of the anion vacancy and CN (C substitution for N) is favorable in B-rich conditions. The C-doping facilitates the formation of the vacancy in the growth of the BNNT. The interaction between the C impurity and the vacancy on the one hand quenches their respective local moments, making the system nonmagnetic; on the other hand substantially changes the density and distribution of localized gap states, reducing the activity. We suggest that the complex defect system is more appropriate for chemical applications, such as ion sieve and filter, than for physical applications.

I. INTRODUCTION Boron nitride nanotubes (BNNTs), the structural analogues of carbon nanotubes (CNTs), possess quite different physical and chemical properties from those of CNTs because of having two constituents instead of one constituent1 and have been attracting considerable attentions over the past decade. High thermal and chemical stabilities make BNNTs very attractive for specific applications, such as nanodevices in extreme conditions and protective shields of some nanomaterials.2 In electronic applications, however, the disadvantage of BNNTs relative to CNTs is the wide band gap (∼5.5 eV), independent of tube radius and helicity.1,3 To this end, numerous efforts have been made to reduce the band gap. Up to now, the most straightforward method is the doping of foreign atoms4,5 such as C and fluorine (F) in BNNTs. Honeycomb-structural BN and C materials have a good compatibility because of the isoelectronic and isostructural properties of BN pair and C2 dimer of the unit cell.6 C-doping is generally incorporated with the growth of BNNTs without causing any lattice damage, and the band gap of C-doped BNNTs can be reduced to ∼1 eV.4 In addition, previous calculations showed that C substitution for B or N of BNNTs induces a local moment at the doped site.7,8 Besides, vacancies811 and C-adatoms12 on the BNNTs generate new spin-polarized states in the gap and cause local moments. These predictions seem to be similar to those observed in magnetic fullerenes and CNTs.13 The discovery of the ferromagnetism in C nanostructures and nanomaterials has stimulated the enthusiasm of researchers from various fields.13,14 It is expected to open a new perspective in the fabrication of nano spintronic devices with low mass and stable properties, based on the sp-electron systems, for spin-based r 2011 American Chemical Society

quantum information processing.15 A more recent study, however, indicated that the spin-polarized states induced by vacancies in BNNTs are too localized to generate a collective magnetism,11 which is an essential requirement for any spintransport devices.16 In contrast, C-adsorption can activate the ferromagnetism of BNNTs under the help of negative charges,12 facilitating possible applications in spintronics and quantum information. These two results reflect the fact that, because of different physical origins, the defects made by different methods (e.g., electron irradiation, substitution, adsorption, and so on) in BNNTs might show different spin-polarized states, even having the same local moment, and further induce different coupling modes and properties. Therefore, the focus of this work is to understand the mechanism of the coupling of localized states of different defects in BNNTs, a representative of low-dimensional hybrid sp-electron systems unlike CNTs. To our best knowledge, comprehensive theoretical and experimental studies of the issue determining the applications of actual BNNTs, containing various defects, in devices were scarcely reported. Here we considered two kinds of defects, i.e., C impurity and vacancy, which are frequently encountered in experiments. By use of spin-polarized density functional theory, we studied the interaction between the C impurity and the vacancy and explored the effects on morphology and electronic structure of BNNTs. We discussed the formation mechanism of complex defect, i.e., C impurity-vacancy, from geometrical and electronic effects. Received: March 9, 2011 Revised: April 27, 2011 Published: June 14, 2011 12782

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Figure 1. Substitution sites of C impurity in the (10,0) BNNT with respect to the vacancy. Here we take the cases of (a) VB-CB and (b) VB-CN as examples. In part a, sites A, B, C, D, and E correspond to different distances and orientations between the CB and the VB impurities, respectively. Similar cases are described by sites A0 , B0 , C0 , D0 , E0 , and F0 for the CN impurity in part b. In the image, among the three NN atoms of the vacancy, two ones at the same circumference are labeled as host-1 and host-2, and the remaining one is labeled as host-3. Pink, blue, and gray balls represent boron, nitrogen, and carbon atoms, respectively.

In particular, we confirmed that the coupling between the two defect states quenches the respective local moments of the constituent defects, but induces more localized gap states. This opens a new perspective in the chemistry applications of BN nanomaterials.

II. CALCULATION METHOD AND MODEL We chose the (10,0) BNNT as a representative of zigzag ones frequently encountered in experiments17 to start our study. Structural optimizations and electronic structure calculations were carried out using the Vienna ab initio simulation package (VASP)18 within the framework of spin-polarized density functional theory. The generalized gradient approximation of Perdew BurkeErnzerhof (PBE) exchange correlation functional19 and the projector augmented wave potentials describing the electronion interaction20 were employed, respectively. A tetragonal 1  1  3 supercell was adopted, which is sufficient to make the interaction between defects negligible, with a vacuum region of 12 Å between tubes for eliminating the tubetube interaction. Integration over the Brillouin zone was done using the MonkhorstPack scheme21 with a 1  1  3 k-point mesh for structural optimizations and a 1  1  7 k-point mesh for electronic structure calculations, respectively. All atoms were fully relaxed until the Hellmann Feynman force on each atom was less than 10 meV/Å. In the study on the formation of complex defect, the relative position of C impurity and vacancy is an important parameter, which is to a certain extent compatible with the interaction between the impurity and the vacancy. We took the vacancy as the reference center to identify the substitution site of C impurity within the tube wall. The nearest-neighbor (NN) atoms of the vacancy are defined as host 1, 2 and 3, shown by image in Figure 1. III. RESULTS AND DISCUSSION The defects considered here are C substitution for B (CB), C substitution for N (CN), cation vacancy (VB), and anion vacancy (VN), also corresponding to four combinations of complex defects, that is, VB-CB, VB-CN, VN-CB, and VN-CN, respectively. For a vacancy in BNNTs, there are two different sets of lattice (substitution) sites, according to the substitution of C for B or N. Taking the cases of the VB as examples, we merely show the substitution sites of C impurity in the (10,0) BNNT in Figure 1. Sites A (A0 ), B (B0 ), C (C0 ), D (D0 ), E (E0 ), and (F0 ), respectively, correspond to different near neighbors of the VB for CB (CN)

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Figure 2. (a) Optimized structure of VB-CB complex (on the left), relative to the VB (on the right). (b) Final (optimized) structure of VBCN complex (on the right), evolving from initial structure (on the left). Note that optimized structures of VN-CN and VN-CB complexes are the same as those of parts a and b, respectively. Pink, blue, and gray balls represent boron, nitrogen, and carbon atoms, respectively. In part b, the two NN nitrogen atoms of the vacancy at host-1 and 3 sites are especially highlighted in red and green colors.

impurity. Total energy calculations show that for the VB-CB and VN-CN combinations, the impurity prefers to stay at the C site, while in the VB-CN and VN-CB combinations the B0 site is favorable for the impurity. For the vacancy (either VB or VN) of zigzag BNNTs, in general, the two NN atoms along the tube circumference (i.e., host-1 and host-2 in the image of Figure 1) are directly bonded because of the curvature effect and electronic effect,11 and a 51DB (one pentagon and one dangling bond located at the nonagon, as shown in the right panel of Figure 2a) configuration is formed accordingly.8,9 The left panel of Figure 2a shows the optimized structure of VB-CB (or VN-CN) complex, where only one type of constituent, B (or N), is removed from the BNNT and substituted by the C atom. We can see that the C impurity has less effect on the structure and stability of the vacancy, that is, the 51DB configuration of the vacancy, as well as the bonding of host-1 and host-2, is completely preserved. Only the stress, induced by the size difference between the substituted B or N atom and the impurity, drives host-3 near the impurity (i.e., the CN and CB bond lengths are, respectively, 1.29 and 1.41 Å for VB-CB and VN-CN complexes) and inward along the radial direction relative to the isolated vacancy with the 51DB configuration11 (i.e., the distance outward from the tube surface for host-3 is 0.20 Å in the VB-CB complex, far smaller than that of 0.65 Å in the VB; while in the VN-CN complex the value remains basically unchanged like that of 0.22 Å in the VN), and further weakens the bond between host-1 and host-2. More precisely, the BB bond length increases to 1.90 Å from 1.82 Å (in the VN)11 for the VN-CN complex, while the NN bond length remains unchanged at 1.48 Å for the VB-CB complex relative to the VB (also see the right panel of Figure 2a).11 In this case, most of the deformation energy introduced by the substitution is released through the vacancy, thereby ensuring the stability of defective BNNTs. While for the VB-CN (or VN-CB) complex where two constituents, B and N, are removed from the tube and substituted by the C impurity together (see the initial structure in Figure 2b), the complex chemical bonding is formed among the C (e.g., substituting for host-2 at site B0 ) and residual two NN atoms of the vacancy (see the final structure in Figure 2b). In this case, the interaction between electronic states of different constituents 12783

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Table 1. Formation Energies (in eV) of Complex Defects and Single Ddefects in B- and N-Rich Conditions defect

B-rich

N-rich

VB

7.92

5.37

VN CB

3.99 4.07

6.54 1.52

CN

1.53

4.08

VB-CB

9.25

4.15

VN-CN

3.76

8.86

(electronic effect), superior to the deformation energy from their size difference (geometrical effect), is a crucial factor determining the configuration of complex defect. The most direct evidence is that the 51DB configuration comes from the break of one CB (or CN) bond and the formation of two CN (or CB) bonds, as shown in Figure 2b. Obviously, the formation mechanism is distinctly different from that of VB-CB (or VN-CN) complex. Since the weak NN (or BB) bond is replaced by the strong CN (or CB) bond, the VB-CN (or VN-CB) complex defect shows a very different local electronic structure as compared to the VB-CB (or VN-CN), even though they have the same 51DB configuration. This also means that they have different effects on the properties of defective BNNTs. In principle, the complexity of the transformation, involving the break and the formation of chemical bonds (electronic effects are dominant), is tantamount to reducing the possibility of the generated structure. So we can conclude that the VB-CN (or VN-CB) complex defect is not easily encountered in BNNTs, as compared to the VB-CB (or VN-CN) one. As such, this deduction is borne out by the formation energies of various complex defects mentioned below. The formation energy, Eform, of complex defect is given as Eform ¼ E½NTdef   E½NT þ mμN þ nμB  μC

ð1Þ

where E[NTdef] and E[NT] are total energies of defective and perfect BNNTs, respectively. μB, μC, and μN are the chemical potentials of the B, C, and N species, respectively, which are generally obtained from the bulk phase of boron (R-B), graphite, and the N2 gas. The variation of chemical potentials constrained by the thermodynamic equilibrium condition follows the relation9 μB þ μN ¼ μBN

ð2Þ

where μBN is the chemical potential of a BN pair in hexagonal BN. The upper limit of μB (μN) (i.e., the energy per atom of R-B and the N2 gas) corresponds to a B-rich (N-rich) growth condition. With the above equations and assumption, we can ensure the most favorable environment where the vacancy and C-doping could appear together. Herein, m and n are determined by the below correlations, which are compatible with defect complexes considered 8 > < m ¼ 2, n ¼ 0, V N  CN m ¼ 0, n ¼ 2, V B  CB ð3Þ > : m ¼ 1, n ¼ 1, V B  CN or V N  CB The calculated formation energies of C impurity-vacancy defects, as well as single defects, are listed in Table 1. Note that for single defects, the chemical potentials of the B, C, and N species in eq 1 are taken into account according to the cases

studied. The positive formation energy means that the formation of these defects, either single-vacancy and C-impurity or complex defect, in BNNTs is an endothermic reaction. As expected, single VB and CB are more frequently present in N-rich conditions, while single VN and CN are favored in B-rich conditions, because of lower formation energies. Our calculations also support the fact that as compared to the vacancy formation, substitutional (C) doping is much easier under the same favorable environment conditions. In practice, the vacancy can be created by the high-energy electron irradiation,22 while the substitution of C for B or N can occur in the growth of nanotubes.6 More importantly, as shown in Table 1, the formation energy of complex defect (e.g., VB-CB or VN-CN) is between those of single defects (e.g., VB and CB or VN and CN) under favorable environment conditions. This not only means that, like the constituent defects,23 the VB-CB and VN-CN complexes are preferable in N-rich conditions and B-rich conditions, respectively, but also implies that the interaction between the two types of defects does exist and is beneficial to the formation and stability of complex defects. The coherent energy, reflecting this interaction, is calculated as the difference between the formation energy of the complex defect and the sum of the formation energies of isolated defects (i.e., vacancy and C impurity). They are 2.74 and 1.76 eV, respectively, for the VB-CB and VN-CN defects. In addition, to know about the effects of interaction between defects, we further consider a larger tetragonal supercell with 5 unit cells (i.e., 1  1  5), and obtain that all the formation energies of defects and the above conclusions remain unchanged, showing the localized nature of defect states. The above results and analysis suggest the formation process of the complex defect in BNNTs as follows: under favorable environment conditions, excess C atoms first enter into the lattice of growing BNNTs, performing the substitution. In the meantime the stress and new localized states are induced around the C atoms. Because of geometrical and electronic effects, the vacancy is formed near the C atom in the subsequent growth. The sequence that C substitution occurs first and the vacancy formation follows is attributed to their formation energies (see Table 1). The impurity-assisted effect is more pronounced for the VB-CB as compared to the VN-CN. In a word, we suggest that, in the study of substitutional doping, one should also consider the presence and synergistic effects of the vacancy, at least for nanostructures and nanomaterials (such as nanotubes, nanowires, and nanofilms). So far, there are many works on the properties of simple defects in the BNNTs. It was predicted that a moment of ∼1 μB might be induced by one vacancy because of the presence of one dangling bond10,11 or by one C impurity because of the removal or injection of one electron from or into the system.7,9 Strong localization restricts, however, the magnetic coupling between the local moments of simple defects to be short-range. As a result, such defective BNNTs are nonferromagnetic,11 although a spinsplitting of the energy bands arises. Interestingly, when the system contains the same number of C impurity and vacancy like as complex defects focused on here, the total number of the electrons in it becomes even. In this case, the spin splitting of the energy bands may arise or not, which is highly dependent on the interaction between localized states of the two types of defects in essence. Our systematic study indicates that this interaction is related to the nature of the vacancy, and the relative location of C impurity and dangling bond. More precisely, for the VB-CB combination (see Figure 1a), the spin-splitting of the energy bands as well as the moment of 2 μB is observed in some 12784

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Figure 3. Total and local density of states (DOS) of (a) VB-CB and (b) VN-CN complexes. I in (a) and I0 and II0 in (b) correspond to localized defect states. The Fermi level is set to zero energy, depicted by the dashed line. (c) Isosurface plots of the squared wave functions of defect states in the gap of VB-CB and VN-CN complexes. The isosurface value is 0.05 e/Å3, depicted in yellow. Pink, blue, and gray balls represent boron, nitrogen, and carbon atoms, respectively.

configurations, such as C impurity at sites B and E, but vanish in other configurations; while the spin-splitting of the energy bands does not arise for ever in the VN-CN combination. This discrepancy is attributed to different localizations of the VB and VN states, arising from different local potentials of vacancies,10 and different stress modes (tensile or compressive) induced by the C substitution for B or N. In the special configurations of VB-CB combination (i.e., C impurity at sites B and E), the weak NN bond is not formed, and the VB exhibits an open configuration because of the combined effects of the deformation arising from C impurity and the tube curvature (geometrical effect), and the repulsion between the lone pz electron pairs (electronic effect).11 The three nitrogen atoms of the vacancy and the C impurity contribute to the moment and the spin-splitting of the energy bands. Comparatively speaking, the situation is more complicated in the most energetically preferable structures of VB-CB and VN-CN combinations, that is, the impurity at the C site (see the left panel of Figure 2a). Figure 3 shows their total and local DOS, and the squared wave functions of defect states in the gap. The bonding analysis with the LDOS shows that the cloud of the unpaired electron (i.e., dangling bond) of the host-3 overlaps the pz cloud of neighboring C impurity, forming a new bond. There are two advantageous factors for the overlapping from geometrical and electronic effects. One is the relative movements, driven by the stress, of host-3 and C impurity along the radial and circumference directions, which makes them much closer to each other. (Note: our calculations indicate that in addition to the CBhost-3 or CNhost-3 bond length, the distances outward from the tube surface for host-3 and C impurity are 0.20 and 0.0 Å in the VB-CB complex, or those are 0.24 and 0.38 Å in the VN-CN complex, affecting the angle and the length of the new bond.) The other is the charge transfer between host-3 and C impurity because of the difference of their electronegativities, which increases their ionicity. The spin configuration of the two unpaired electrons in the new bond is antiparallel, following the Pauli exclusion principle, so the respective local moments of the constituent defects vanish and the system is nonmagnetic in nature. On the other hand, the essential difference in the DOS (see parts a and b of Figure 3) indicates that the VB-CB and VN-CN defects have distinctly different electronic structures and properties, even though they have the same 51DB configuration and double

bond between C and host-3. In what follows, we will clarify that the activities or ionicities of the VB and VN with respect to the C impurity, and the localizations of the vacancy states are responsible for the complexity of the interaction between the two types of defect states, leading to different DOS, intrinsic properties, and possible applications. It is known that one VB in BNNTs leaves one unoccupied state of minority-spin in the gap.11 When the pz orbital of C impurity hybrids with the sp2 orbital (dangling bond) of Nhost3, the latter accepts the electron from the C because of its higher electronegativity than the C, and thus, the unoccupied state of minorityspin of the VB11 is occupied. The NN bond states of the VB are more localized relative to the dangling bond states of Nhost-311 and thus are not affected in substance in the bond formation and charge transfer process. For example, the NN bond length remains unchanged at 1.48 Å for the VB-CB combination relative to the VB. As a result, the peak at 0.31 eV below the Fermi level (I) is composed of new CdNhost-3 bond states and the N—N bond states (see parts a and b of Figure 3). Most of the CdNhost-3 bond states are contributed by the Nhost-3-2p states relative to the C-2p states, indicating the charge transfer in the bond formation. In addition, the C impurity and the Nhost-3, respectively, contribute to the bottom of the conduction bands and the top of the valence bands, as shown in Figure 3a. The reason is that the pz orbital of C impurity, after donating the electron, is unoccupied, and strongly hybridizes the conduction bands because of the compatibility between them. On the contrary, for the VN-CN defect, in the formation of CdBhost-3 (double) bond, the electron is transferred from the occupied gap states of majority-spin of the VN (i.e., sp2 orbital of Bhost-3)11 to the C-pz orbital, because of its lower electronegativity than the C. As a result, there are two peaks in the gap: one is at 0.24 eV below the Fermi level (I0 ), and the other is at 2.23 eV above the Fermi level (II0 ). The BB bond states of the VN is to a certain extent mixed with the dangling bond states,11 so the bond formation and charge transfer process has effect on it in nature. Typically, the unoccupied gap states are contributed by the BB bond states and a few of Bhost-3-2p states (Figure 3c). The occupied gap states are predominantly the CdBhost-3 bond states, simultaneously showing the charge-transfer between them (Figure 3c). In addition, the two gap states do not hybridize with the valence bands and the conduction bands of 12785

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Figure 4. (a) TDOS of VB-CB and VN-CN complexes using the Gaussian 03 program with the hybrid B3LYP density functional and the 6-31G(d) basis sets. I in the VB-CB complex, I0 and II0 in the VN-CN complex correspond to localized defect states. The energy zero is set in the middle of the gap. (b) Isosurfaces of defect states in the gap of VB-CB and VN-CN complexes. The values of 0.05 and 0.05 au are depicted in yellow and cyan, respectively.

BNNTs because of the incompatibility between them.24 The nature and characteristics of the valence bands and the bottom of the conduction bands of BNNTs are completely preserved in such a defective system (Figure 3b). In addition, it is recognized that the hybrid functionals are usually more accurate to describe localized states than the PBE. So here we use the Gaussian 03 program25 with the hybrid B3LYP density functional26 and the 6-31G(d) basis sets to check localized states of complex defects mentioned as above. A cylindrical BN cage, with the same length of a 1  1  3 supercell under the periodic boundary condition, is chosen to represent the defective (10,0) BNNT, and H-terminal is used to avoid the boundary effect.27 The defect states in the gap (i.e., I in the VB-CB complex, I0 and II0 in the VN-CN complex) are, as shown in Figure 4, consistent with the results provided by the PBE (Figure 3). They correspond to the occupied CdNhost-3 bond states and the NN bond states in the VB-CB complex and the unoccupied BB bond states and the occupied CdBhost-3 bond states in the VN-CN complex, respectively. This indicates that our DFT calculations, with the PBE exchange-correlation functional in the GGA, could provide a reasonably reliable description for localized states of defects in BNNTs. It has been well recognized that due to the existence of defect states,8,28 the defects significantly can affect molecular interactions in both natural and artificial systems, such as some selfassemblies of nanostructures or nanomaterials following the “bottom-up” strategy. The VB-CB and VN-CN defects in BNNTs certainly will have very different chemical properties because the density and distribution of their localized gap states are different. Here, as a representative example, we concentrate on the adsorption selectivity of BNNTs with complex defects. According to the nature and characteristics of electronic states of complex defects, we select the H and F atoms as adsorbates, which not only are frequently present in the experiments of BNNTs1 but also have opposite oxidationreduction properties. For example, they will selectively adsorb on the VB and VN of BNNTs, respectively.11 Here the adsorption energy is defined as Eads ¼ Et ðNTÞ þ Et ðAÞ  Et ðNT þ AÞ

ð4Þ

where Et(A), Et(NT), and Et(NTþA), respectively, denote the total energies of isolated H (or F) atom, defective NTs without

and with H (or F). Hereafter the adsorption energy values of different situations are shown in the brackets. For a vacancy, the host-3 is generally the preferred reaction site because of the dangling bond.8,11 In the C impurity-vacancy defect, however, this dangling bond is passivated by the neighboring C impurity because the double bond is formed between them. As a result, the activities of the vacancy and the C impurity are drastically reduced, which can be demonstrated by the below adsorption energy values. As a reference, the adsorption energy of the F (H) atom on the VN (VB) is 6.76 (4.51) eV in the separated state, where the C impurity is far away from the vacancy. Our systematic studies show that although both the F and H atoms can steadily adsorb on the two complex defects, which seems to be different from the cases in the isolated vacancy,11 the C impurity acts as a new adsorption site for the F atom in the VB-CB defect, and for the H atom in the VN-CN defect. That is, the adsorption selectivity of complex defect is reflected in adsorbates on the vacancy or the C impurity. As listed in Table 2, in the VB-CB defect, the H atom would preferably adsorb on the VB (1.83 eV) rather than on the C impurity (1.18 eV), which is attributed to the different electronegativities of C and N. The F atom, excluded by the VB, would be trapped by the C impurity, but the adsorption energy (2.66 eV) is smaller than that in the separated state (4.56 eV). While, in the VN-CN defect, the F atom would preferably adsorb on the Bhost-3 atom (5.41 eV) as compared to on the C impurity (2.86 eV). And the H atom could adsorb on the C impurity (2.67 eV) and the Bhost-3 (2.50 eV), only the adsorption energies are smaller than the corresponding values (4.06 and 3.84 eV) in the separated state, respectively. Here the effect of dangling bond passivation by the neighboring substitutional atom is similar to that of the H-termination, but differently, the size and morphology of the vacancy of the former remain basically unchanged. At this rate, a stable and “inert” bore is preserved on the surface of the BNNT. The passivated vacancy can be seen as a door for foreign atom or molecule inserting into the BNNTs for encapsulation.29 The number of electrons of defective system becomes odd due to the introduce of a H/F atom . Figures 5 and 6 show the DOS and spin densities of five stable H/F-adsorbed BNNTs with the complex defects. It can be seen that the spin-splitting of the energy bands arises, and a local moment of 1 μB arises from the 12786

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Table 2. Adsorption Sites and Rnergies Eads (in eV) of H and F Atoms on Single Defects and Complex Defects of (10,0) BNNTa H

a

F

Nhost-3 (separated VB)

4.51

Nhost-3 (separated VB)

2.40

Bhost-3 (separated VN)

3.84

Bhost-3 (separated VN)

6.76

C (separated impurity)

4.06

C (separated impurity)

4.56

Nhost-3 (VB-CB)

1.83

Nhost-3 (VB-CB)

-

C (VB-CB) Bhost-3 (VN-CN)

1.18 2.50

C (VB-CB) Bhost-3 (VN-CN)

2.66 5.41

C (VN-CN)

2.67

C (VN-CN)

2.86

A positive value corresponds to an exothermal reaction. Figure 6. Spin densities of the VB-CB defect with one adsorbed (a) H on the VB and (b) F on the C impurity and the VN-CN defect with one adsorbed (c) F on the VN and H on (d) the C impurity and (e) the VN. The spin density isosurface at a value of 0.08 au is depicted in yellow.

On the other hand, a crucial issue for the applications of doped systems in spintronic devices is whether local moments induced by the defect states can lead to a collective magnetism by the longrange coupling. Further calculations show that the antiferromagnetic and ferromagnetic states of F/H-adsorbed BNNTs with the complex defects are nearly degenerate, indicating a short-range coupling, which is attributed to very localized defect states (see Figure 6). Here two complex defects with the F/H atom were arranged along the axis in the 1  1  4 supercell. For possible spintronic applications, we will further explore the dependencies of the adatom-induced moment on the concentrations and distributions of adatoms and complex defects, and the long-range or short-range coupling between the adatom-induced moments.

Figure 5. Spin-polarized TDOS of the VB-CB defect with one adsorbed (a) H on the VB and (b) F on the C, and the VN-CN defect with one adsorbed (c) F on the VN, and H on (d) the C and (e) the VN. The Fermi level is set to zero.

host-3 or C impurity that does not interact with the H/F atom. For example, for H adsorption on VB-CB, the electrons are injected into the host-N from the H atom, so the CdNhost3 (double) bond is broken, leading to the appearance of unpaired electron of the C impurity and the local moment (see Figure 6a). This is similar to the case of the CB.9 As such, for F adsorbed on VB-CB, the electrons are transferred from the C impurity to the F atom, so the CdNhost-3 (double) bond is broken and the resulting unpaired electron (or dangling bond) of the Nhost-3 induces the local moment (see Figure 6b). This is similar to the case of the VB.11 Obviously, for F (H) adsorption on the Bhost-3 of the VN-CN, the C impurity is responsible for the local moment of 1 μm (see parts c and e of Figure 6) as in the case of the CN.9 The interesting phenomenon that the H/F adsorption induces the local moment or unpaired electron of the C impurity or the vacancy in the complex defect may be useful in some certain situations, such as magnetic detection and manipulation.

IV. CONCLUSION The structure, stability, and formation mechanism of complex defect (i.e., C impurity-vacancy) in BNNTs were systematically studied by ab initio calculations. The impurity and the vacancy tend to bind together to form an “inert” pentagonnonagon pair because of geometrical and electronic effects. The VB-CB and VN-CN defects are, respectively, preferable in N-rich and B-rich conditions. A new bond is formed between host-3 of the vacancy and C impurity, which leads to the pairing of unpaired electrons in them or the saturation of the dangling bond of the vacancy. The local moments of the C impurity and the vacancy are quenched, and their activities are reduced. The localized gap states of VB-CB and VN-CN defects show different density and distribution, which correspond to different oxidationreduction properties, such as adsorption selectivity. It is suggested that active dangling bonds can be saturated by excess electrons of dopants, even for larger and more complex vacancies. Such complex defect systems might have more potential in chemical applications, such as ion sieve and filter. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] phys.tsinghua.edu.cn (W.D.).

(G.Z.);

dwh@

’ ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (Grant Nos. 2011CB606405 and 12787

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The Journal of Physical Chemistry C 2009CB929400), and the National Natural Science Foundation of China.

’ REFERENCES (1) (a) Golberg, D.; Bando, Y; Tang, C. C.; Zhi, C. Y. Adv. Mater. 2007, 19, 2413. (b) Hao, S. G.; Zhou, G.; Duan, W. H.; Wu, J.; Gu, B.-L. J. Am. Chem. Soc. 2008, 130, 5257. (2) (a) Rubio, A.; Corkill, J. L.; Cohen, M. L. Phys. Rev. B 1994, 49, 5081. (b) Blase, X.; Rubio, A.; Louie, S. G.; Cohen, M. L. Europhys. Lett. 1994, 28, 335. (c) Chen, Y.; Zou, J.; Campbell, S. J.; Caer, G. L. Appl. Phys. Lett. 2004, 84, 2430. (3) Blase, X.; Rubio, A.; Louie, S. G.; Cohen, M. L. Europhys. Lett. 1994, 28, 335. (4) (a) Zhi, C. Y.; Guo, J. D.; Bai, X. D.; Wang, E. G. J. Appl. Phys. 2002, 91, 5325. (b) Golberg, D.; Dorozhkin, P. S.; Bando, Y.; Dong, Z.-C.; Tang, C. C.; Uemura, Y.; Grobert, N.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Appl. Phys. A: Mater. Sci. Process. 2003, 76, 499. (c) Golberg, D.; Bando, Y.; Dorozhkin, P.; Dong, Z.-C. MRS Bull. 2004, 29, 38. (5) (a) Tang, C. C.; Bando, Y.; Huang, Y.; Yue, S. L.; Gu, C. Z.; Xu, F. F.; Golberg, D. J. Am. Chem. Soc. 2005, 127, 6552. (b) Liu, H. T.; Zhou, G.; Yan, Q. M.; Wu, J.; Gu, B.-L.; Duan, W. H. Phys. Rev. B 2007, 75, 125410. (6) (a) Zhou, G.; Duan, W. H. J. Nanosci. Nanotechnol. 2005, 5, 1421. (b) Zhou, G.; Duan, W. H. Doped Nanomaterials and Nanodevices; Chen, W., Ed.; American Scientific Publishers: Los Angeles, 2008. (7) (a) Wu, R. Q.; Liu, L.; Peng, G. W.; Feng, Y. P. Appl. Phys. Lett. 2005, 86, 122510. (b) Si, M. S.; Xue, D. S. Europhys. Lett. 2006, 76, 664. (8) Kang, H. S. J. Phys. Chem. B 2006, 110, 4621. (9) Schmidt, T. M.; Baierle, R. J.; Piquini, P.; Fazzio, A. Phys. Rev. B 2003, 67, 113407. (10) Hao, S. G.; Zhou, G.; Duan, W. H.; Wu, J.; Gu, B.-L. J. Am. Chem. Soc. 2006, 128, 8453. (11) Liu, R.; Li, J.; Zhou, G. J. Phys. Chem. C 2010, 114, 4357. (12) Li, J.; Zhou, G.; Chen, Y.; Gu, B.-L.; Duan, W. H. J. Am. Chem. Soc. 2009, 131, 1796. (13) (a) Carbon-Based Magnetism: An Overview of Metal Free CarbonBased Compounds and Materials; Makarova, T., Palacio, F., Eds.; Elsevier: Amsterdam, 2005. (b) Talapatra, S.; Ganesan, P. G.; Kim, T.; Vajtai, R.; Huang, M.; Shima, M.; Ramanath, G.; Srivastava, D.; Deevi, S. C.; Ajayan, P. M. Phys. Rev. Lett. 2005, 95, 097201. (c) Krasheninnikov, A. V.; Banhart, F. Nature Phys. 2007, 6, 723. (14) (a) Shibayama, Y.; Sato, H.; Enoki, T.; Endo, M. Phys. Rev. Lett. 2000, 84, 1744. (b) Ohldag, H.; Tyliszczak, T.; H€ohne, R.; Spemann, D.; Esquinazi, P.; Ungureanu, M.; Butz, T. Phys. Rev. Lett. 2007, 98, 187204. (15) (a) Tenne, R. Nature 2004, 431, 640. (b) Krusin-Elbaum, L.; Newns, D. M.; Zeng, H.; Derycke, V.; Sun, J. Z.; Standstorm, R. Nature 2004, 431, 672. (16) Dev, P.; Xue, Y.; Zhang, P. H. Phys. Rev. Lett. 2008, 100, 117204. (17) (a) Golberg, D.; Han, W.; Bando, Y.; Bourgeois, L.; Kurashima, K.; Sato, T. J. Appl. Phys. 1999, 86, 2364. (b) Golberg, D.; Bando, Y.; Kurashima, K.; Sato, T. Chem. Phys. Lett. 2000, 323, 185. (18) (a) Kresse, G.; Furthm€uller, J. Comput. Mater. Sci. 1996, 6, 15. (b) Kresse, G.; Furthm€uller, J. Phys. Rev. B 1996, 54, 011169. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (20) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (21) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (22) (a) Zobelli, A.; Ewels, C. P.; Gloter, A.; Seifert, G.; Stephan, O.; Csillag, S.; Colliex, C. Nano Lett. 2006, 6, 1955. (b) Zobelli, A.; Gloter, A.; Ewels, C. P.; Colliex, C. Phys. Rev. B 2008, 77, 045410. (23) (a) Weng-Sieh, Z.; Cherrey, K.; Chopra, N. G.; Blase, X.; Miyamoto, Y.; Rubio, A.; Cohen, M. L.; Louie, S. G.; Zettl, A.; Gronsky, R. Phys. Rev. B 1995, 51, 11229. (b) Han, W.; Cumings, J.; Huang, X.; Bradley, K.; Zettl, A. Chem. Phys. Lett. 2001, 346, 368. (24) Li, J.; Zhou, G.; Liu, H. T.; Duan, W. H. Chem. Phys. Lett. 2006, 426, 148.

ARTICLE

(25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.02; Gaussian, Inc.: Wallingford, CT, 2004. (26) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (27) Zhou, G.; Duan, W. H.; Gu, B.-L. Chem. Phys. Lett. 2001, 333, 344. (28) (a) Wang, C. C.; Zhou, G.; Liu, H. T.; Wu, J.; Qiu, Y.; Gu, B.-L.; Duan, W. H. J. Phys. Chem. B 2006, 110, 10266. (b) Wang, C. C.; Zhou, G.; Wu, J.; Gu, B.-L.; Duan, W. H. Appl. Phys. Lett. 2006, 89, 173130. (c) Zhang, J.; Zou, H. L.; Qing, Q.; Yang, Y. L.; Li, Q. W.; Liu, Z. F.; Guo, X. Y.; Du, Z. L. J. Phys. Chem. B 2003, 107, 3712. (29) (a) Mickelson, W.; Aloni, S.; Han, W. Q.; Cumings, J.; Zettl, A. Science 2003, 300, 467. (b) Han, W. Q.; Chang, C. W.; Zettl, A. Nano Lett. 2004, 4, 1355.

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