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J. Phys. Chem. C 2008, 112, 5778-5783
Theoretical Study of Ni Adsorption on Single-Walled Boron Nitride Nanotubes with Intrinsic Defects Jing-xiang Zhao†,‡ and Yi-hong Ding*,† State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin UniVersity, Changchun 130023, People’s Republic of China, and Department of Chemistry, Harbin Normal UniVersity, Harbin 150080, People’s Republic of China ReceiVed: December 27, 2007; In Final Form: January 31, 2008
As is known, boron nitride nanotubes (BNNTs) are usually not defect free, and various types of defects can inevitably be formed in the process of tube growth and preparations. While previous studies only considered the chemical functionalization of “perfect” BNNTs by transition metals, there was no report on the interaction between the “defective” BNNTs and transition metal atoms. In this article, we report the first study of the adsorption of a Ni atom on three kinds of defects in an (8,0) BNNT using density functional theory (DFT). Particular attention is paid to searching for the stable configurations and the corresponding adsorption energies. We found that the existence of the three defects can drastically enhance the chemical reactivity of the (8,0) BNNT toward Ni-adsorption and have a significant effect on the charge-transfer, band gap, and density of states (DOSs) of Ni-adsorption on the BNNT. The present results are helpful not only for understanding the adsorption nature of the Ni atom on defective BNNTs but also for further designing of BNNT-based materials.
Introduction Boron nitride nanotubes (BNNTs), which represent an important class of inorganic tubular materials, have attracted significant attention since their theoretical prediction in 1994.1 Though structurally similar to carbon nanotubes (CNTs), BNNTs are quite different from their carbon counterparts. The BNNTs are stable wide-band gap semiconductors, independent of tube diameter and chirality,2 whereas the electrical performance of CNTs is a complex function of many structural parameters, which are difficult to control.3 Similar to the CNTs, BNNTs might also act as promising materials for various potential applications such as hydrogen storage media, sensor materials, catalysts, and nanoelectronic devices.4-9 Of particular interest, BNNTs have excellent mechanical properties, thermal conductivity, and resistance to oxidation at high temperatures, which makes them most valuable in nanodevices working in hazardous and high-temperature environments.4,5,8 Metal-functionalization has been found to be a very useful scheme to improve or induce some unique properties of nanotubes.10 Recently, both experimental11-13 and theoretical14,15 studies were reported on the interaction of transition metal atoms with “perfect” BNNTs (BNNTs without intrinsic defects). Yet, we are aware that pristine BNNTs are usually synthesized using the arc discharge method,16a and experimental studies have shown that BNNT is not defect free.16b Due to the limitation of the present synthetic methods, a variety of defects on BNNTs can inevitably be found, such as B or N vacancies, C and other impurity-doping, Stone-Wales (SW) defects, and rehybridization defects.17-22 At this point, “defective” BNNTs (BNNTs with intrinsic defects) are more common than “perfect” BNNTs. Therefore, theoretical studies of “defective” BNNTs should be useful not only to mimic the more “realistic” BNNTs, but also † ‡
Institute of Theoretical Chemistry. Harbin Normal University.
to understand and evaluate the effects of defects on the properties of BNNTs. Considering the potential applications of Ni nanoparticles in hydrogen storage,13b catalysts, and gas sensors, in this paper, we computationally report the first attempt on the interaction of a Ni atom on three intrinsic defects (single vacancies, StoneWales, and antisite defects) in an (8,0) BNNT through density functional theory (DFT) calculations. In particular, the difference in reactivity between the perfect tube and near the intrinsic defects on the sidewall of BNNTs toward Ni atom adsorption was focused on. The results might be useful for understanding the initial steps of metal-fabrication of BNNTs. Theoretical and Computational Details We first built a fragment of the (8,0) BNNT of 64 boron and 64 nitride atoms with 16 end-capping hydrogen atoms, B64N64H16. The hydrogen atoms were used to avoid the effects of dangling bonds at the two ends. Three kinds of defective BNNTs were chosen to interact with the single Ni atom and compared with the adsorption on perfect tubes, and the three defective BNNTs were the single B vacancy (VB: removing one boron atom from middle of the sidewall of B64N64H16), N vacancy (VN: removing one nitride atom from middle of the sidewall of B64N64H16), Stone-Wales defect (SW: rotating a B-N bond in the hexagon network by 90°), and the antisite defect (AS: rotating a B-N bond in the hexagon network by 180°), respectively. The adsorption or binding energy was defined as Eb ) E(Ni + BNNT) - E(BNNT) - E(Ni), where E(Ni + BNNT) was the total energy of the Ni adsorbed on a perfect or defective BNNT, E(BNNT) was the total of the perfect or defective BNNT, and E(Ni) was the total energy of the isolated Ni atom. A negative Eb denoted exothermic adsorption. All-electron density-functional theory calculations were performed, employing the Becke’s three-parameter hybrid exchange functional combined with the Lee, Yang, and Parr correlation
10.1021/jp7121196 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
Ni Adsorption on Single-Walled Boron Nitride Nanotubes
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Figure 1. Optimized structure of a perfect (8,0) single-walled boron nitride nanotube (BNNT).
TABLE 1: Results for the Ni-Adsorption on the Perfect (8,0) BNNT site
Eb (eV)
bondN-Ni (Å)
bondB-Ni (Å)
BA BZ H
-1.31 -1.27 -0.80
1.87 1.88 1.97, 2.51, 2.52
2.09 2.13 2.22, 2.40, 2.41
functional, denoted as the B3LYP23 method with a Gaussian 0324 program. The basis sets used in the calculation were 6-31G25 for B and N, and 6-311++G*26 for Ni to describe atomic orbitals more accurately. In addition, the STO-3G27 basis set was used for H to avoid dangling bonds at two ends.
Figure 2. The most stable structure of (a) VN and (b) VB defects.
Results and Discussions Ni Adsorption on the Surface of a Perfect (8,0) BNNT. First, we considered the adsorption of a Ni atom on the perfect (8,0) BNNT. Several various adsorption positions were selected for the Ni atom on the perfect BNNTs as presented in Figure 1: (1) directly above a boron atom (B), (2) above a nitrogen atom (N), (3) over an axial BN bond (BA), (4) over a zigzag BN bond (BZ), and (5) above a B3N3 hexagon (H). The results for the Ni adsorption on the surface of perfect tubes were listed in Table 1. Three stable configurations are obtained at the BA, BZ, and H sites with the corresponding binding energies of -1.31, -1.27, and -0.8 eV, whereas other adsorption sites are unstable. Surely, the H-adsorption site is the least stable. The binding energy at the BA-site is slightly larger than that at the BZ-site by 0.04 eV, which is in excellent agreement with the value (0.03 eV) recently reported by Zeng et al.14 This seems to contradict with the normal thinking that the BZ-site with larger curvature is more reactive than the BA site and should have a larger binding energy, as in the case of O- and CCl2-adsorption on BNNTs.20 One possible explanation for the above discrepancy might be that for the first-row elements, the strain energy induced by bond curvature plays a significant role, whereas for higher row elements, such an effect is almost negligible due to the long bond lengths. As a result, the energy difference between the BA and BZ sites becomes very small for many transition metal adsorptions.14 Thus, considering the predictive capability of the present quantum chemical theories, the rather small energy difference indicates that the competition between the BA and BZ sties should not be considered as conclusive. At the BA site, the B-Ni and N-Ni bond lengths are 2.09 and 1.87 Å, respectively. At the BZ site, the two bond lengths are 2.13 and 1.88 Å. At the H-site, the Ni atom is located right above the center of the B3N3 hexagonal ring at the height of ∼1.6 Å and the three B-Ni bonds lengths are 2.22, 2.40, and 2.41 Å, and the three N-Ni bonds lengths are 1.97, 2.51, and 2.52 Å, respectively. Ni Adsorption on the Single Vacancies in the (8,0) BNNT. There were some theoretical studies of single vacancies (SVs) in BN tubes, showing that the formation of SVs was a useful
Figure 3. Fully optimized configurations of the Ni atom adsorbed on the (a) VN and (b) VB defect.
way of modifying the electronic, magnetic, mechanical, and adsorption properties.17,19,28,29 A combination of electron microscopy and theoretical calculations revealed that in both cases local bond reconstruction occurs changing the dimension around the missing atom where a (5,9) pair appears.15,17,19 In Figure 2, we showed the most stable optimization structures of N-vacancy (VN), B-vacancy (VB), and the formation of the (5,9) pair. We noted that the B2-B3 (VN) and the N2-N3 (VB) bonds subside in the tube, and their bond lengths are 1.79 and 1.52 Å. The two-coordinated atoms in both defects protrude from the circumference of the tube. Furthermore, the tube with vacancy defects has a magnetic moment of 1 µB and the spin density located on the under-coordinated boron (B1 in Figure 2a) and nitrogen atom (N1 in Figure 2b) is 0.65 and 0.95 e, respectively, according to the Mulliken population analysis. To investigate the Ni adsorption on the VN and VB defects in (8,0) BNNT, we initially placed a single Ni atom on the under-coordinated boron or nitrogen atom. Figure 3 shows the optimized structures of the Ni atom adsorbed on VN and VB. We found that the structures of VN and VB by the formation of a (5,9) pair are destroyed by the Ni adsorption, resulting in the
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Zhao and Ding
TABLE 2: Results for the Ni-Adsorption on the Vacancies Defects in the (8,0) BNNT
B1-N1 B1-N2 B2-B3 Ni-B1 Ni-B2 Ni-B3
bond length (Å)
bond length (Å)
VN defect
Ni-VN
VB defect
Ni-VB
1.44 1.44 1.79
1.49 1.49 2.24 1.88 1.95 1.95
1.45 1.45 1.52
1.47 1.47 2.98 1.80 1.85 1.85
N1-B1 N1-B2 N2-N3 Ni-N1 Ni-N2 Ni-N3
Figure 4. The most stable structure of SW defects.
TABLE 3: Binding Energy (Eb in eV) for Ni Atom Adsorbed on Various Defects and the Spin Density of Ni Atom Ni-VN Ni-VB Ni-SW1 Ni-SW2 Ni-SW3 Ni-AS1 Ni-AS2
Eb
spin density
-4.69 -7.25 -1.70 -2.95 -2.07 -2.27 -2.22
0.54 1.02 0.00 0.00 0.00 0.00 0.00
breaking of the B2-B3 (VN) and N2-N3 (VB) bonds in the pentagon ring and thus the forming the Ni-B (VN) or Ni-N (VB) bonds. The corresponding bond lengths are given in Table 2. The binding of atomic Ni to the VN and VB defects is strongly exothermic with Eb values of -4.69 and -7.25 eV, respectively, as shown in Table 3, which is much larger than that of Niadsorption on perfect BN tubes with the adsorption energy of -1.31 eV. Moreover, through the further examination of Table 3, we noted that the spin density of the Ni atom of Ni-VN and Ni-VB systems is 0.54 and 1.02 e, respectively, while the spin density of other atoms is negligible, indicating that the spin density has been transferred to the Ni atom from the uncoordinated boron (VN) or nitrogen atom (VB). From the above discussions, it can be obviously seen that the presence of vacancy defects indeed significantly enhances the reactivity of BNNTs toward the Ni atom. Ni Adsorption on the Stone-Wales Defects in the (8,0) BNNT. The Stone-Wales (SW) defect was generated by rotating a B-N bond in the hexagon network by 90°, resulting intwopentagon-heptagonpairs(5-7-7-5).Itwasreported20,30-32 that there are two possible SW defects depending on the orientation of the rotated B-N bond. Previous theoretical calculations of isolated intrinsic defects clearly indicated that the reactivity near the SW defect is generally higher than that at the perfect BNNTs due to the formation of frustrated B-B and N-N bonds.20,30-32 The most stable structure of SW defect in (8,0) BNNT and the calculated bond lengths are displayed in Figure 4, which is quite consistent with previous results.20,32 Upon Ni adsorption on SW defect in (8,0) BNNT, three stable configurations on top of the frustrated B1-B2, N1-N2, and B1-N1 bonds are obtained, labeled Ni-SW1, Ni-SW2, and Ni-SW3, respectively (Figure 5a,b,c), whereas the adsorption at the other sites is unstable and the Ni atom will be relocated to take one of the three stable configurations after optimization. Compared with the perfect (8,0) BNNT, the three configurations of Ni-SW1, Ni-SW2, and Ni-SW3 are all strongly exothermic with Eb of -1.70, -2.95, and -2.07 eV as shown in Table 3. Interestingly, the Ni adsorbed on the N1-N2 bond (NiSW2 configuration) is the most stable configuration. This is different from the Ni-adsorption on SW defects in single-walled carbon nanotubes whose most stable configurations of Niadsorption are on the central bonds formed by the 7-7 ring fusion of SW defects in carbon nanotubes.33 Furthermore, when
Figure 5. Fully optimized structures of the Ni atom adsorbed on the (a) B1-N1, (b) N1-N2, and (c) B1-B2 bond of SW defect in (8,0) BNNT.
TABLE 4: Results for the Ni-Adsorption on the SW Defects in the (8,0) BNNT bond length (Å)
B1-N1 B1-B2 N1-N2 Ni-B1 Ni-B2 Ni-N1 Ni-N2
SW defect
Ni-SW1a
Ni-SW2b
Ni-SW3c
1.43 1.71 1.49
1.53 1.71 1.51 1.95 2.05
1.43 1.70 2.55
1.47 2.09 1.52 1.88 1.90
1.85
1.75 1.77
a The Ni adsorbed on the top of the B1-N1 bond. b The Ni adsorbed on the top of the N1-N2 bond. c The Ni adsorbed on the top of the B1-B2 bond of Figure 4.
bonded with the Ni atom, the distances between B1 and N1 (Figure 5a), two N (N1-N2) (Figure 5b), and two B (B1-B2) atoms (Figure 5c) is increased to 1.53, 2.55, and 2.09 Å from their equilibrium bond lengths of 1.43, 1.49, and 1.71 Å, respectively (Table 4). Therefore, the interaction between the
Ni Adsorption on Single-Walled Boron Nitride Nanotubes
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5781 TABLE 5: Results for the Ni-Adsorption on the AS Defects in the (8,0) BNNT bond length (Å)
Figure 6. The most stable structure of AS defects.
B1-N1 B1-B2 B1-B3 N1-N2 N1-N3 Ni-B1 Ni-B2 Ni-N1
AS defect
Ni-AS1a
Ni-AS2b
1.40 1.67 1.67 1.45 1.45 1.83
1.52 1.66 1.66 1.50 1.50 2.02
1.43 1.97 1.70 1.44 1.45 1.91 1.91
1.83
a
The Ni adsorbed on the top of the B1-N1 bond. b The Ni adsorbed on the top of the B1-B2 bond of Figure 5.
TABLE 6: Charge Transfer and the HOMO-LUMO Gap (Band Gap) of BNNT charge transfer (e) perfect tube VN defect VB defect SW defect AS defect Ni-perfect tube Ni-VN defect Ni-VB defect
Figure 7. Fully optimized structures of the Ni atom adsorbed on the (a) B1-N1 and (b) B1-B2 bond of AS defect in (8,0) BNNT.
Ni atom and BNNTs with SW-defects is significantly stronger than that of perfect BNNTs. Ni Adsorption on the Antisite Defects in the (8,0) BNNT. Different from carbon nanotubes (CNTs), there are two kinds of atoms in BNNTs. Thus a new defect can be formed by rotating the B-N bond in the hexagon network of BNNTs by 180°, and we label it the “antisite (AS) defect”. Compared with other intrinsic defects, no report has been given to the AS defect, especially the electronic and adsorption properties. There also exist the B-B and N-N bonds in the most stable AS defect and the hexagon network of BNNTs still remains as Figure 6. We noted that the B1 atom relaxed outward somewhat from the tube surface and the bond lengths of B1-B2, B1-N1, and N1-N2 are 1.67, 1.40, and 1.45 Å, respectively. To investigate the Ni adsorption on AS defect in an (8,0) BNNT, the sites near the AS defect were taken into account as reaction centers. Two stable structures on top of B1-N1 and B1-B2 (labeled Ni-AS1 and Ni-AS2, respectively) were obtained after full relaxation (Figure 7a,b) and the corresponding adsorption energies are given in Table 3. Similar to other defects, the adsorption energies of the Ni atom at B1-N1 and B1-B2 sites of the AS defect are -2.27 (Ni-AS1 configuration) and -2.22 eV (Ni--S2 configuration), which is more exothermic than that of perfect BNNTs. Furthermore, at the B1-N1 site, the B1-Ni and N1-Ni bond lengths are 2.02 and 1.83 Å, respectively, and the B1-N1 bond is lengthen to 1.52 Å. At the B1-B2 site, the B1-Ni and B2-Ni bond distances are 1.91 Å. However, the B1-B2 bond is elongated to 1.97 Å from 1.67 Å (Table 5). Obviously, the existence of AS defects in BNNTs facilitates the adsorption of the Ni atom. Charge Transfer, Band Gap, and Density of States (DOSs) When the transition metal atom is adsorbed to the surface of
Ni-SW defect1 Ni-SW defect2 Ni-SW defect3 Ni-AS defect1 Ni-AS defect2
0.12 -0.03 0.62 0.07 -0.26 0.31 0.15 0.35
HOMO--LUMO (eV) 4.97 3.50b 4.00c 4.54b 2.67c 4.89 4.16 2.90 2.71a 4.01b 1.87b 3.00c 3.03 3.73 3.02 2.97 2.49
a HOMO-LUMO gap of the R orbital. b HOMO-LUMO gap of the β orbital. c BNNT containing defects and Ni adsorbed tubes are the most stable configurations, as described in the text. The positive values of the charge transfers suggest a charge loss of Ni, whereas negative values indicate a charge gain of Ni.
perfect BNNTs, a certain amount of charge was transferred from the metal atoms to the BNNT, leaving the metal atom in a cationic form.13 How did the existence of defects influence the charge transfer from the Ni atom to the BNNT in the present work? To shed light on this problem, we analyzed the charge transfer with the Mulliken population analysis as summarized in Table 6. Interestingly, we found that the charge transfer behavior depends on the Ni-adsorption sites, for example, the Ni atom gains 0.26 e when adsorbed on the N1-N2 site in the SW defect, whereas the Ni atom bound to the B1-N1 bond in the SW defect loses 0.07 e. Similar charge transfer behavior can also be observed for Ni-adsorption on other kinds of defects in BNNT as shown in Table 6. A number of investigations14,15,22,29,34,35 have indicated that the introduction of defects would disrupt the delocalized π bonds and considerably increase the conductivity of BNNTs. To explore the effect of Ni atom adsorption on the electronic properties of BNNTs, we plot the density of states (DOSs) of perfect and defective (8,0) BNNT before and after Ni atom adsorption as shown in Figure 8. It can be seen that for the SW (Figure 8c) and AS defects (Figure 8e) in (8,0) BNNT, only slight changes of the DOSs of BNNTs are found, compared to the DOSs of perfect BNNTs (Figure 8a). The HOMO-LUMO gaps are decreased from 4.97 eV (perfect BNNTs) to 4.89 eV (SW defects) and 4.16 eV (AS defects), respectively. This can be understood from the fact that SW and AS defects well preserve the sp2 bonding character of the original network and
5782 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Zhao and Ding the moderate change of electronic states is mainly contributable to the rotated bond. Unlike the case of the SW and AS defects, the DOS of BNNTs are drastically changed due to the forming vacancy defects and the introduction of an unpaired electron. As discussed above, the vacancy defect in BNNTs has an unpaired valence electron, resulting in spin-splitting.33b Thus two split one-electron levels are introduced within the band gap (see arrows in Figure 8g,i), which reduces the band gap effectively. In the VB system, the occupied (spin-up) state is about 0.43 eV higher than the valence band, which mainly comes from the unpaired N atom with spin density of 0.65 e. In the case of the VN defect, there are also two spin-splitting defective states within the gap region, coming from the contribution of the unpaired B atom with a spin density of 0.95 e. Figure 8 further shows that upon adsorption of the Ni atom on perfect and defective BNNTs, the DOSs near the Fermi level undergo many significant changes. Compared with the perfect tube (Figure 8a), the adsorption of the Ni atom not only causes the originally degenerate states to be split due to the breaking of nanotube symmetry, but also clearly introduces a new level in the original energy gap of perfect or defective tubes, reducing the band gap (Table 6). Furthermore, we find that the band gap of perfect, VN, VB, SW, and AS defect upon the Ni atom decreases by 2.07, 0.79, 2.67, 1.86, and 1.19 eV, respectively. Interestingly, this variation of the band gap seems to correlate with the charge transfer from the Ni atom to the tubes. It is worth noting that upon a single Ni atom, the two spin-splitting levels in the band gap still remain (Figure 8h,j) in vacancy defects, but a slight shift of spin-up level toward the Fermi level is found, which originates from the contribution of Ni atom whose spin density is 0.54 and 1.02 e in Ni-VB and Ni-VN systems. We note that in a recent theoretical work by Duan and co-workers,34 the spin-splitting was also introduced in open BNNTs. The difference lies in the way of creating spin-splitting. The spin-polarization originates from all unpaired B or N atoms at open ends of open BNNTs, whereas it comes from an unpaired B or N atom within the defective BNNTs. In short, our DFT calculations demonstrated that the reactivity of the defective BNNTs toward Ni atom is higher than that of the perfect BNNTs toward Ni. The intrinsic defects play a decisive role. Moreover, the Ni-functionalization of defective BNNTs can induce attractive features, for example, rendering Ni as the active center in chemical reactions, and can bind with other molecules. More importantly, we can tailor the electronic properties of BNNTs by metal-functionalization for application in chemical sensor and nanoelectrical devices under some special conditions. Therefore the present investigations shed light on the nature of the interaction of the Ni atom with more “realistic” BNNTs, namely “defective” BNNTs, which are useful to further applications in sensors and catalysts, among others. Conclusion
Figure 8. Density of states (DOSs) of (8,0) BNNT (a) perfect, (b) Ni-adsorbed on perfect BNNT, (c) SW defect, (d) Ni-adsorbed on SW defect, (e) AS defect, (f) Ni-adsorbed on AS defect, (g) VN defect, (h) Ni-adsorbed on VN defect, (i) VB defect, and (j) Ni-adsorbed on VB defect. Ef denotes Fermi levels.
Considering the potential applications of metal-nanotube materials and the difficulty in reality to synthesize perfect BNNTs without defects as well as the higher reactivity of “defective” BNNTs, we investigated the adsorption of the Ni atom on the more “realistic”, namely “defective”, BNNTs by employing DFT calculations. To explore possible effects of the existence of intrinsic defects on the structures, adsorptions, and electronic properties, main focuses are laid on stable configurations of Ni-adsorption on different sites of intrinsic defects, the corresponding binding energies, and the charge-transfer and band gaps. The results show that the forming of defects in
Ni Adsorption on Single-Walled Boron Nitride Nanotubes BNNTs facilitates the adsorption of the Ni atom and the nature of the interaction is explained according to the difference of Ni-adsorption between perfect and defective BNNTs. Our work indicates that the intrinsic defects have important promoting effects on functionalizing modification of the electronic properties of BNNTs induced by metal atoms, so they would have unique application potential in chemical sensors and BNNTsbased devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20103003, 20573046), Excellent Young Teacher Foundation of Ministry of Education of China, Excellent Young People Foundation of Jilin Province, and Program for New Century Excellent Talents in University (NCET). References and Notes (1) Rubio, A.; Corkill, J. L.; Cohen, M. L. Phys. ReV. B 1994, 49, 5081. (2) Blase, X.; Rubio, A.; Louie, S. G.; Cohen, M. L. Eur. Phys. Lett. 1994, 28, 335. (3) Mele, E. J.; KrGl, P. Phys. ReV. Lett. 2002, 88, 056803. (4) Hernandez, E.; Goze, C. P.; Bernier, A.; Rubio, A. Phys. ReV. Lett. 1998, 80, 4502. (5) Suryavanshi, A. P.; Yu, M.; Wen, J.; Tang, C.; Bando, Y. Appl. Phys. Lett. 2004, 84, 2527. (6) Xiao, Y.; Yan, X. H.; Cao, J. X.; Ding, J. W.; Mao, Y. L.; Xiang, J. Phys. ReV. B 2004, 69, 205415. (7) Chang, C. W.; Han, W. Q.; Zettl, A. Appl. Phys. Lett. 2005, 86, 173102. (8) Han, W. Q.; Bando, Y.; Kurashima, K.; Sato, T. Appl. Phys. Lett. 1998, 73, 3085. (9) Golberg, D.; Bando, Y.; Tang, C.; Zhi, C. Y. AdV. Mater. 2007, 19, 2413. (10) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Small 2006, 2, 182. (11) Han, W. Q.; Zettl. A. J. Am. Chem. Soc. 2003, 125, 2062. (12) Zhi, C. Y.; Bando, Y.; Tang, C.; Golberg, D. J. Phys. Chem. B 2006, 110, 1525. (13) (a) Sainsbury, T.; Ikuno, T.; Okawa, D.; Pacile, D.; Frechet, J. M. J.; Zettl, A. J. Phys. Chem. C 2007, 111, 12992. (b) Shin, W. H.; Yang, S. H.; Goddard, W. A.; Kang, J. K. Appl. Phys. Lett. 2006, 88, 053111. (14) Wu, X. J.; Zeng, X. C. J. Chem. Phys. 2006, 125, 044711.
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