Chemisorption-Induced Polarization of Boron Nitride Nanotube - The

Jun 12, 2008 - Institute of High Performance Computing, 1 Science Park Road,# 01−01 The Capricorn Singapore Science Park II, Singapore 117528, and ...
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J. Phys. Chem. C 2008, 112, 10279–10286

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Chemisorption-Induced Polarization of Boron Nitride Nanotube Jia Zhang,*,† Kian Ping Loh,*,‡ Ping Wu,† Michael B. Sullivan,† and Jianwei Zheng† Institute of High Performance Computing, 1 Science Park Road, # 01-01 The Capricorn Singapore Science Park II, Singapore 117528, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed: February 22, 2008; ReVised Manuscript ReceiVed: April 3, 2008

We performed density functional theory (DFT) calculations to investigate the adsorption of atomic hydrogen (H) on the wrapping axis of nonpolar arm-chair (5,5) and chiral (8,4) boron nitride nanotubes (BNNTs) with a view toward understanding the chemisorption-induced polarization field in BNNTs. The adsorption of H along the zigzag B-N bonds that lie on the wrapping axis of the BNNT enhances the macroscopic polarization field. Depending on whether the B or N site near the edge of the nanotube is adsorbed with H, the direction of the polarization field, as well as the work function of the tube ends, can be changed significantly. The relationship between the chemical effect as well as the geometric distortion of the tube caused by chemisorption and the induced polarization were investigated, respectively. The results have implications for the application of BNNTs as electron emitters. 1. Introduction BN nanotubes (BNNTs) can exhibit pyro- and piezoelectric properties1 because of the heteropolar B-N bond. Mele and Kra´l showed that the wrapping of the nonpolar BN sheet to form a BN nanotube will lead to an electric polarization along the nanotube axis, which is controlled by the quantum mechanical boundary conditions on its electronic states around the circumference of the tube.2 The periodic dependence of the electric polarization on the wrapping vector arises from the phase matching of the π-derived Bloch states around the tube circumference.1,3 It is interesting to consider the effects of chemical adsorption on the nanotube in this case because functionalizing the walls of the tubes affects the π bonds and modifies the “hybridization” of the molecular orbitals, consequently perturbing the electronic Hamiltonian and the matrix elements for hopping between atomic sites. Most of the previous studies on the chemical functionalization of carbon nanotubes focused on changes produced in the electronic properties4–9 and ignore piezoelectric properties because carbon nanotubes do not have intrinsic polarization fields. However, the inherently polar BNNT provides an interesting subject for studying the relationship between the adsorption patterns of adsorbates on the walls and the effects on the polarization field, apart from studies on electronic structures of chemically functionalized BNNTs as well as on hydrogen-storage mechanisms.10–15 The direction and strength of the polarization field has implications on the work function of the tube at the ends and on the threshold of electron emission when used as a field emitter. In a previous study on zigzag (8,0) BNNTs, we found that the adsorption of H in an armchair fashion along the BN tube axis enhances the intrinsic polarization field and modifies the work function at the tube end significantly.16 This interesting result provides the motivation to further consider some fundamental issues that are still open: (i) Is it true that the walladsorption pattern on the wrapping axis can cause spontaneous * To whom correspondence should be addressed. E-mail: zhangj@ ihpc.a-star.edu.sg; [email protected]. † Institute of High Performance Computing. ‡ National University of Singapore.

polarization fields for nonpolar armchair BNNTs and chiral BNNTs? (ii) If so, then what is the origin of the macroscopic polarization caused by wall-adsorption along the wrapping axis? (iii) How do the chemical effect and geometrical distortion caused by the local transformation from sp2- to sp3-type bonding during chemical functionalization influence the polarization field? Hence, in this study, a systematic investigation of various adsorption patterns of adsorbates on nonpolar (5,5) and chiral (8,4) BNNTs was formed aiming to answer these questions. 2. Methods of Calculation Density functional theory (DFT) calculations on the clean and adsorbed BNNT were performed using the Vienna Ab initio Simulation (VASP) package.17 The electron-ion interaction was modeled using the projector-augmented wave (PAW) method.18,19 The generalized gradient approximation (GGA) functional of Perdew and Wang (PW91) was used for the exchange and correlation.20 The cutoff energy was set to 400 eV. In this work, armchair (5,5) and chiral (8,4) BNNTs were considered. To simulate an open-ended BNNT, a finite structure was built with a vacuum region of 30 Å on both ends along the z axis, and a separation of at least 10 Å along the x and y axes. A 2 × 2 × 1 k-point mesh was used for the structure. The finite structure consists of 8 BN layers (containing 40 B atoms and 40 N atoms) for (5,5) BNNT, as well as 56 B and 56 N atoms for (8,4) BNNT, respectively. The dangling bonds at both ends of the tube were saturated by hydrogen atoms, and the different patterns of H adsorption on the tube walls were considered. The work function (Φ) is defined as the difference between the Fermi level (EF) and the vacuum level (φ). The vacuum level is obtained by averaging the total electrostatic potential of the system in the x-y plane as a function of z axis from the tube end, and determining the point where the electrostatic potential levels off (denoted by φ in Figures 2–5d). Here, the dipole correction was used to ensure flat vacuum potential at the two tube ends. 21,22 The difference in the vacuum levels at the two tube ends (as shown in Figures 2–5d) arises from the macroscopic polarization field in the BNNT. The polarization field here describes the direction and magnitude of the change of

10.1021/jp8015722 CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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Figure 1. Schematic drawing of three highly symmetric H atom adsorption patterns on infinite (5,5) BNNTs and corresponding top view of the optimized configuration: (a) zigzag pattern, (b) armchair pattern, (c) dimer pattern.

electrostatic potential per unit distance. In order to quantify the polarization field, the one-dimensional macroscopic average of the electrostatic potential is calculated. In the inset of Figure 2d, the tube bulk portion displays strong atomic oscillations, which can be subtracted by calculating the one-dimensional macroscopic average.23 Integrating the tube bulk portion according to the following equation,23 the macroscopic average can be obtained (the dotted line in the inset of Figure 2d).

fc(z) ) (1 ⁄ a)

z+a ⁄ 2 f(s) ds ∫z-a ⁄2

(1)

Here, jf (s) corresponds to the planar average electrostatic potential function. The slope of the macroscopic average potential cf (z) is defined as the polarization field.21 To compare the stability of adsorption patterns on (5,5) BNNTs, periodic boundary conditions were employed along the tube axis and a supercell containing 20 B and 20 N atoms was chosen to model the infinite (5,5) BNNTs. In this case, the vacuum region is 10 Å between the nanotubes. On the basis of the Monkhorst-Pack scheme,24 1 × 1 × 4 k-mesh was used along the nanotube axis. 3. Results and Discussion 3.1. Nonpolar to Polar Transition in Armchair (5,5) BNNTs. In this work, various high-symmetry adsorption patterns of hydrogen on infinite (5, 5) BNNTs with an overall surface coverage of 50% were considered. When viewed from the side, it can be seen in Figure 1 that rows of H-adsorbed B-N chains alternate with B-N chains where no H is adsorbed. Figure 1a-c illustrates the three adsorption patterns, namely, (a) zigzag pattern, where H atoms are adsorbed in a zigzag fashion along tube axis;

(b) armchair pattern, where H atoms are adsorbed in an armchair fashion around the circumference of the tube; and (c) dimer pattern, where H atoms are aligned along tube axis. The bond distance of B-N adsorbed with H in the zigzag pattern is 1.54 Å, which is shorter than the B-N bond distance when H adsorption follows the armchair or dimer pattern. The adsorption energy of H atoms on infinite (5,5) BNNTs is defined as Ea ) [E(BNNT+H) - E(BNNT) - nEH]/n(number of H atoms), where E(BNNT+H) is the total energy of the (5,5) BNNT +H adsorption system, E(BNNT) and EH are energies of clean (5,5) BNNTs and the isolated H atom, respectively. The exothermicity of the adsorption energies for various adsorption patterns arranged in decreasing order is zigzag (-2.22 eV/H) > armchair (-2.08 eV/H) > dimer (-1.97 eV/H); therefore, in this work we focus mainly on adsorption of H atoms on B and N along the zigzag BN chains for (5,5) BNNTs. Various scenarios concerning the tube ends can be generated depending on the terminating points of H adsorption (shown in Figures 2–5a). In order to verify the stability of all considered structures, we calculate the adsorption energy of adsorbates on the wall of open-ended BNNT first. Here, the adsorption energy is defined as Ea ) [E(H-BNNT+A) - E(H-BNNT) - nEA]/ n(number of adsorbates), where E(H-BNNT-A) stands for the energy of H-terminated (5,5) or (8,4) BNNTs with adsorbates A (A ) H, OH) on the tube wall, E(H-BNNT) corresponds to the energy of clean H-terminated (5,5) or (8,4) BNNTs, and EA is the energy of the isolated adsorbate. By definition, a negative value of Ea corresponds to exothermic adsorption. By judging from the adsorption energies listed in Table 1, we find that in all cases H adsorption on the tube wall is exothermic. The adsorption energies fall between -1.80 and -1.96 eV/H. Results show

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Figure 2. H adsorption on the zigzag chains of the (5,5) BN nanotube walls at 50% surface coverage. The adsorption did not extend to the edge atoms B1 and N2 so that dangling bonds are generated at the edge sites. (a) Schematic drawing for H adsorption pattern, (b) top view of the optimized configuration, (c) LDOS plot showing the DOS of the p orbitals corresponding to B1, N1, B2, and N2 atoms (labeled in a). H adsorption occurs on N1 and B2 induced increased LDOS density of p orbitals at the Fermi level for the adjacent N2 and B1. (d) Average planar electrostatic potential distribution along tube axis. The inset in part d shows the macroscopic average of the potential (dotted line) along the tube axis.

that the H atom has a similar adsorption stability on the wall of (5,5) BNNTs (-1.80 ∼ -1.96 eV/H) and on the wall of (8,4) BNNTs (-1.85 ∼ -1.92 eV/H). Also, the adsorption energy of the -5.38 eV/(H,OH) pair is obtained for the adsorption of H and OH on N and B sites of (5,5) BNNTs in a zigzag pattern. It can be seen that the adsorption of the (H, OH) pair is energetically more favorable compared to that of pure H atoms because of the larger electronegativity of the OH group. In the following work, we start to investigate the dependence of polarization field and work function on the terminating point of the H adsorption chain. In Figures 2–5a, for ease of reference, the terminal sites at Tip 1 or Tip 2 are denoted by B1 or N2, whereas sites adjacent to terminal B1 or terminal N2 are represented by N1 or B2. The polarization and work function at the edge can be controlled depending on whether the H adsorption at the edge terminates at N or B sites on either ends. For the scenario depicted in Figure 2a, H adsorption occurs on the zigzag chains of the tube walls, the terminal B1 or N2 atoms are bonded to hydrogen, but remain threefold coordinate. In another situation depicted in Figure 3a,

H adsorption extends throughout the zigzag chain and terminates at the last atoms in the zigzag chain. Finally in the case depicted in Figure 4a, H adsorption occurs on the terminal N2 site on one terminal, but adsorbed on the N1 site adjacent to the terminal B1 at the other terminal. The unique feature here is that unlike the previous cases H adsorption now occurs on N sites at both terminals. The corresponding work function at the two ends of the BN nanotube and the polarization field of the optimized adsorption systems is listed in Table 1. The clean (5,5) BNNT is a nonpolar tube; the work function at both H-terminated ends of the tube is similar and equal to 4.82 eV. Adsorption of H in either dimer or armchair pattern does not result in a polarization field along the tube axis. However, when H adsorbed on the zigzag B-N bonds on the tube wall in the manner shown in Figure 2a, a polarization field of 0.46 eV/Å is created along the tube axis going from Tip 1 to Tip 2, as shown in Figure 2d. The Φ at Tip 1 was reduced to 1.98 eV, and that at Tip 2 was increased to 5.34 eV. However, as shown in Figure 3a, if H adsorption extends throughout the zigzag chain to the terminal B and N

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Figure 3. Adsorption of H along the zigzag chain to the edge atoms B1 and N2 on the (5,5) BN nanotube. B2 and N1 in this case lie on the antiparallel zigzag chain adjacent to the zigzag chain containing B1 and N2. (a) Schematic drawing for H adsorption pattern, (b) top view of the optimized configuration, (c) LDOS plot for the terminal B(N) atoms showing an increase in LDOS density at the Fermi level for adjacent N1 and B2 p orbitals after H adsorption on B1 and N2 (labeled in a), and (d) average planar electrostatic potential distribution along the tube axis.

atoms, then the direction of the polarization field is now reversed (-0.43 eV/Å), as shown in Figure 3d. The adsorption of H on a B or N site transforms a threefold coordinated site to a fourfold coordinated site, and this increased the local density of states on the adjacent sites. The effects produced on the electronic states of the adjacent sites when H bonds with a particular B or N site can be observed from the local density of states (LDOS) plot. For example Figure 3c shows that after the adsorption of H atoms on the B1 or N2 sites the LDOS of the adjacent N1 or B2 sites increased considerably. Another interesting scenario is shown in Figure 4a, where H atoms now adsorb on the N sites at the two terminals of the tube. It is noteworthy that the LDOS for the adjacent B atoms, B1 and B2, near the Fermi level, has now increased considerably. The increase in LDOS at the B end of the tube translates to a lower electrostatic potential compared to increased LDOS at the N end of the tube; therefore, in this case, the Φ at both ends are reduced significantly to 1.84 eV at Tip 1 and 1.49 eV at Tip 2, respectively. The polarization field is weaker because both ends of the tubes have the same electrostatic potential (same B atom), but in this case the work functions at both ends are lowered. Compared to a bare BN nanotube, the bonding of H to anionic N will generate a dangling bond on the adjacent

cationic B and shifts the Fermi level upward with respect to the vacuum level. Alternatively, the bonding of H to cationic B will generate a dangling bond on anionic N adjacent to the B site and shift the Fermi level downward. Therefore, the terminal with dangling bond on B will have lower electrostatic potential compared to the terminal with dangling bond on N. Controlling the hydrogenation at the terminals of the BN nanotube, to be at the N or N + 1 site along the zigzag chain can result in dipoles oriented differently because dangling bonds can be generated on either B or N sites. The adsorption of hydrogen on chiral (8,4) BNNT provides another example to illustrate the effects of different edge site adsorption. Before adsorption, the intrinsic polarization field for the H,H-end terminated (8,4) BNNT is -0.013 eV/Å, which results in Φ values of 4.57 and 5.19 eV at Tip 2 and Tip 1, respectively. The adsorption of H in a helical fashion along the chiral axis of the tube, as shown in Figure 5a, where adsorption occurs on the tube wall sites up to site N1 and B2 near the two edges, increased the polarization field to 0.35 eV/Å; the Φ at Tip 1 was reduced to 2.23 eV, whereas that at Tip 2 was increased to 5.23 eV. For the boron site, which is adjacent to N1, indicated as B1, and the nitrogen site indicated as N2, adjacent to B2, these sites are threefold coordinated and localized

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Figure 4. Adsorption of H along the zigzag chain to atoms N1 and N2 on the (5,5) nanotube. (a) Schematic drawing for H adsorption pattern, (b) top view of the optimized configuration, (c) LDOS plot for the B1, N1, B2, N2 atoms (labeled in a) showing an increase in LDOS at the Fermi level for B1 and B2 p orbitals after adsorption of H on N2 and N1, (d) average planar electrostatic potential distribution along the tube axis.

TABLE 1: Work function (Φ), Wall-Adsorption Energy (Ea), and Polarization Field of Various Finite (5,5) and (8,4) BNNTs with H Adsorption on Ends and Wallsa Φ (eV) model

Tip 1 (B tip)

Tip 2 (N tip)

polarization field (eV/Å)

H-(5,5)BNNT H-(5,5) BNNT- zigzag p H-(5,5) BNNT- zigzag pN H-(5,5) BNNT- zigzag *H-(5,5) BNNT- zigzag H-(8,4) BNNT H-(8,4) BNNT-helix p H-(8,4) BNNT-helix H-(8,4) BNNT-helix-tube

4.82 1.98 5.53 1.84 2.05 5.19 2.23 5.31 5.11

5.34 1.49 1.49 6.10 4.57 5.23 1.56 3.48

0 0.46 -0.43 -0.01 0.64 -0.013 0.35 -0.26 -0.12

Ea (eV) -1.87/H -1.96/H -1.80/H -5.38/H,OH -1.85/H -1.92/H

a (1) H-(5,5) BNNT: Only the ends of (5,5) BNNT are terminated by H to maintain threefold coordination. (2) H-(5,5) BNNT- zigzag: as in Fig 2a, H adsorption follows zigzag pattern along the tube axis.(3) pH-(5,5) BNNT - zigzag: as in Figure 3a, H adsorption extends throughout the zigzag chain. (4) pNH-(5,5) BNNT- zigzag: as in Fig. 4a, H zigzag adsorption chain stop at the N sites on both terminals. (5) *Co-adsorption of H and OH pairs on the N and B sites on the wall, following the same pattern as (2). (6) H-(8,4) BNNT-helix: as in Fig. 5a, in which H adsorption follows a helical zigzag pattern along the tube axis, but edge atoms are not adsorbed. (7) pH-(8,4) BNNT-helix: H adsorption extends throughout the helically zigzag chain to the edge atoms. (8) H-(8,4) BNNT-helix -tube: same as (7) but without H atoms on the tube wall.

DOS states appeared in the gap when H adsorbed on the N1 and B2 sites. If H adsorption extends to the terminal B1 and N2 sites on the respective terminals, then the polarization field will now be reversed, and becomes -0.26 eV/Å.

3.2. Influence of Geometric Effect on the Polarization Field and Work Function. The macroscopic polarization along the tube axis in the zigzag BNNT can be related to the z-direction strain.25,26 The armchair BN chains occur along the

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Figure 5. Adsorption of H in a helical fashion along the chiral axis of the (8,4) BN nanotube at 50% surface coverage. (a) Schematic drawing for H adsorption pattern, (b) top view of the optimized configuration, (c) LDOS plot for the terminal B(N) and adjacent N (B) atoms showing increased LDOS at the Fermi level for B1 and N2 after H adsorption on N1 and B2 (labeled in a), (d) average planar electrostatic potential distribution along the tube axis. B1 is the terminal edge atom on one end, and N2 is the terminal edge atom on the other. The zigzag H adsorption chain, as shown in part a, terminates in this case at N1 and B2.

wrapping axis of the zigzag tube, so adsorption of H on these were found to produce the largest z-direction strain. Sai et al.25 investigated the piezoelectric properties of BNNT and found that the armchair BNNT has a longitudinal response for torsion, but not for uniaxial extension or compression. As shown in Figure 2b, the 50%-H coverage zigzag adsorption pattern deformed the tube to a pentagonal prism, whereas other adsorption patterns like dimer or armchair do not induce significant distortion at the tube ends. In order to study the influence of the wall geometry on the polarization field, we removed the adsorbed hydrogen after geometry optimization, while “freezing” the distorted shape of the tube. In this case, it was found that increasing the coverage of adsorbed hydrogen causes a proportionately higher degree of wall distortion, so the effects on the polarization field can be systematically studied by varying the wall coverage of H at 0, 0.1, 0.2, 0.3, 0.4, and 0.5 (see Figure 6). The Φ at two ends of the wall-distorted tube was calculated, and the corresponding results are listed in Table 2. Table 2 shows that the degree of tube-wall distortion is directly proportional to the induced electric polarization. The 50% coverage has the largest polarization field of -0.137 eV/Å and the largest ∆Φ (ΦTip2 - ΦTip1) of 0.93 eV.

3.3. Influence of Chemical Effect on the Polarization Field and Work Function. The adsorption of H atoms on the wall of the (5,5) BNNT in a zigzag pattern creates an opposite and increased polarization field (0.46 eV/Å) compared to the field induced by the distortion of tube wall (-0.137 eV/Å). Therefore, the net polarization field results from the stronger influence of the chemical effects; these have also been found to be true for the chiral (8,4) and zigzag (8,0) BNNTs. Our calculations show that if the H atoms adsorbed on the tube walls are removed then the magnitude of the polarization field decreases from 0.35 to -0.12 eV/Å for the (8,4) BNNT (see Table 1). These results indicate clearly that the chemical effects due to H wall adsorption are more prominent than geometric effects. A further demonstration of the chemical effect is to substitute the more electronegative OH for H, thereby studying the pairwise adsorption of H and OH on the N and B sites of the (5,5) BNNT in a zigzag fashion. Such an adsorption may result from the hydroxylation of BNNT by water at elevated temperatures. In this study, 50% surface coverage by H and OH is considered. As shown in Table 1, the adsorption of H and OH on N and B creates a polarization field of 0.64 eV/Å, which is larger than that of the H adsorption case (0.46 eV/Å).

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Figure 6. Optimized structures (top view) of the (5,5) BN nanotube with (H,H) pairs adsorbed on the wall in a zigzag pattern at surface coverages of (a) 0.5, (b) 0.4, (c) 0.3, (d) 0.2, (e) 0.1, and (f) 0. For 50% surface coverage, the (H,H) pairs were adsorbed on five zigzag chains.

TABLE 2: Work Function (Φ), Fermi Level (EF), and Polarization Field of (5,5) BNNT after the Removal of (H,H) Pairs Adsorbed on the Wall at Different Surface Coverage, While Maintaining the Structure of the Tube in Its Distorted Form Φ (eV) H-(5,5)BNNT-0 H-(5,5)BNNT-tube-0.1 H-(5,5)BNNT-tube-0.2 H-(5,5)BNNT-tube-0.3 H-(5,5)BNNT-tube-0.4 H-(5,5)BNNT-tube-0.5

Tip1

Tip 2

EF (eV)

polarization field (eV/Å)

4.82 4.52 4.63 4.73 4.81 4.87

4.28 4.17 4.07 4.00 3.94

-4.34 -3.92 -3.92 -3.92 -3.93 -3.93

0 -0.027 -0.063 -0.092 -0.116 -0.137

BN nanotubes have been considered as candidates for fieldemission materials; they can potentially offer more robust performance in poor vacuums compared to carbon nanotubes because of their resistance to oxidation. Cumings and Zettl27 have measured stable field-emission currents from capped BN nanotubes; they reported that the nanotubes are insulating at low electrical bias, but showed stable, reversible breakdown current at high bias. Because of the insulating nature of the BN nanotube, current transport at low bias is not possible. One possible strategy to modify the electron transport properties is the chemical functionalization of the tube. It has been reported by Zhu and Bando that functionalizing the walls of the BN nanotube can impart metallicity on the tube, judging from

changes in UV absorption spectra.28 In this work, we consider an open-ended BN nanotube, which can be generated by laser shearing of a closed nanotube, or by channel-confined growth in anodic alumina template.29,30 Recent work had also shown the possibility of precisely cutting nanotubes with a low-energy electron beam to create size-selected nanotube segments.31 For open-ended tubes, the tube ends are typically more reactive than the tube wall and can be selectively functionalized under controlled conditions. For example, the tube ends can be exposed to atomic hydrogen from a radical beam source in vacuum. The wrapping axis of the tube typically terminates in either B or N along the zigzag bond directions, and functionalization with H, either at these edge atoms or at sites adjacent to these edge atoms, can polarize the ends significantly. The terminating point of H adsorption on the zigzag chain can affect whether the dangling bond is generated on the B or N site at the terminal, and consequently affects the direction of the dipole. We found that for polar tubes such as (8,0) or (8,4) functionalizing the edge atoms with H results in increased polarization of the ends. In the case of nonpolar tubes such as the (5,5), functionalizing the tube walls along the zigzag bond direction can result in dangling bonds on cationic B at one end, and anionic N at the other, thus polarizing the tube. Because of the induced macroscopic polarization field along the tube, the open-ended BNNT, which has been functionalized at the edges, may show a strong response to an electrical field. In the presence of electrical field, the growth direction of such polarized BN nanotubes can be aligned with the electrical field more readily than carbon nanotubes.32 An electrical field can be applied during the growth of the vertical array of a BN nanotube to direct the terminal with the positive dipole to face outward, such that a low work function surface can be obtained. Another implication is the condition of negative electron affinity (NEA) on the H-adsorbed tube ends, which arises from the creation of a strong dipole at these ends due to N-H or B-H bonds. Our results show that when the hydrogen adsorbed on N sites is adjacent to a terminal B site the work function at the open tube end can be lower than 2 eV, which is smaller than the band gap of the BN nanotube, indicating the presence of NEA. The NEA condition is favorable for electron emission because electrons that have quasi-thermalized to the conduction band minimum at the tip can spontaneously escape, since the vacuum level at the tip is now situated below the conduction band minimum.33 In addition, the adsorption of H on the N site near the terminal creates a donor state near the conduction band, which can be a source for electron emission. Komatsu and co-workers34 reported intense electron emission from cone-shaped boron nitride emitters; they suggested the electron emission to be favored by a stable surface dipole layer formed by N-H bonds. The results of this work suggest that creating hydrogenated states near the edges of the BN nanotube and changing the local bonding to sp3 can increase the polarization field along the tube and enhances electron emission from the edges. 4. Conclusions Adsorption of hydrogen on the walls of the nonpolar (5,5) or polar (8,4) BN nanotubes, along the direction of the zigzag bond, can result in a macroscopic polarization field along the tube axis that is larger than the intrinsic polarization field. The work function at one terminal of the BN nanotube can be lowered significantly with respect to the other terminal when the zigzag H adsorption pattern terminates on a N site at one edge, and on a B site at the other edge. If the terminating point of H adsorption on the zigzag chain ends at N sites on both

10286 J. Phys. Chem. C, Vol. 112, No. 27, 2008 ends, then the magntitude of the macroscopic polarization field is reduced, but in this case the work function at both ends is reduced simultaneously. Our results suggest that chemical functionalization of the BN nanotube, combined with selective cutting of the BN tube edge to expose different terminating sites, afford the tuning of the work function and local density of states of the tube ends, which can produce effects in enhanced electron emission when the BN nanotube array is applied for field emission. References and Notes (1) (a) Bernholc, J.; Nakhmanson, S. M.; Nardelli, M. B.; Meunier, V. Comput. Sci. Eng. 2004, 6, 12. (b) Mele, E. J.; Kra´l, P. Phys. ReV. Lett. 2002, 88, 056803. (2) Kra´l, P.; Mele, E. J.; Toma´nek, D. Phys. ReV. Lett. 2000, 85, 1512. (3) Kim, K. S.; Bae, D. J.; Kim, J. R.; Park, K. A.; Lim, S. C.; Kim, J.-J.; Choi, W. B.; Park, C. Y.; Lee, Y. H. AdV. Mater. 2002, 14, 1818. (4) Park, K. A.; Seo, K.; Lee, Y. H. J. Phys. Chem. B 2005, 109, 8967. (5) Gu¨lseren, O.; Yildirim, T.; Ciraci, S. Phys. ReV. B 2002, 66, 121401. (6) Park, K. A.; Choi, Y. S.; Lee, Y. H. Phys. ReV. B 2003, 68, 045429. (7) Pan, H.; Feng, Y. P.; Lin, J. Y. J. Phys.: Condens. Matter 2006, 18, 5175. (8) Yang, C -K.; Zhao, J.; Lu, J. P. Phys. ReV. B 2002, 66, 041403. (9) . (10) Han, S. S.; Lee, S. H.; Kang, J, K.; Lee, H. M. Phys. ReV. B 2005, 72, 113402. (11) Wu, X.; Yang, J.; Hou, J. G.; Zhu, Q. J. Chem. Phys. 2004, 121, 8481. (12) Lai, L.; Song, W.; Lu, J.; Gao, Z.; Nagase, S.; Ni, M.; Mei, W. N.; Liu, J.; Yu, D.; Ye, H. J. Phys. Chem. B 2006, 110, 14092. (13) Zhou, Z.; Zhao, J.; Chen, Z.; Gao, X.; Yan, T.; Wen, B.; Schleyer, P. V. R. J. Phys. Chem. B 2006, 110, 13363.

Zhang et al. (14) Zhou, Z.; Zhao, J.; Chen, Z.; Schleyer, P. V. R. J. Phys. Chem. B 2006, 110, 25678. (15) Zhang, J.; Loh, K. P.; Yang, S. W.; Wu, P. Appl. Phys. Lett. 2005, 87, 243105. (16) Zhang, J.; Loh, K. P.; Sullivan, M. B.; Zheng, J. W.; Wu, P. J. Appl. Phys. 2006, 99, 104309. (17) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 48, 13115. (b) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (18) Bloechl, P. E. Phys. ReV. B 1994, 50, 17953. (19) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758. (20) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (21) Meunier, V.; Roland, C.; Bernholc, J.; Nardelli, M. B. Appl. Phys. Lett. 2002, 81, 46. (22) Bengtsson, L. Phys. ReV. B 1999, 59, 12301. (23) Baldereschi, A.; Baroni, S.; Resta, R. Phys. ReV. Lett. 1988, 61, 734. (24) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (25) Sai, N.; Mele, E. J. Phys. ReV. B 2003, 68, 241405. (26) Michalski, P. J.; Sai, N.; Mele, E. J. Phys. ReV. Lett. 2005, 95, 116803. (27) Cumings, J.; Zettl, A. Solid State Commun. 2004, 129, 661. (28) Zhi, C.; Bando, Y.; Tang, C.; Golberg, D. Phys ReV B 2006, 74, 153413. (29) Golberg, D.; Bando, Y. Appl. Phys. Lett. 2001, 79, 415. (30) Shelimov, K. B.; Moskovits, M. Chem. Mater. 2000, 12, 250. (31) Yuzvinsky, T. D.; Fennimore, A. M.; Mickelson, W.; Esquivias, C.; Zettl, A. Appl. Phys. Lett. 2005, 86, 053109. (32) Nojeh, A.; Ural, A.; Pease, R. F.; Dai, H. J. Vac. Sci. Technol., B. 2004, 26, 3421. (33) Schoenbach, K. H.; Verhappen, R.; Tessnow, T.; Peterkin, F. E. Appl. Phys. Lett. 1996, 68, 13. (34) Komatsu, S.; Okudo, K.; Kazami, D.; Golberg, D.; Li, Y.; Moriyoshi, Y.; Shiratani, M.; Okada, K. J. Phys. Chem. B 2004, 108, 5182.

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