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Structural and Electronic Properties of Fluorinated Boron Nitride Nanotubes Lin Lai,† Wei Song,† Jing Lu,*,†,‡ Zhengxiang Gao,*,† Shigeru Nagase,*,§ Ming Ni,† W. N. Mei,‡ Jianjun Liu,‡ Dapeng Yu,† and Hengqiang Ye† Mesoscopic Physics Laboratory, Department of Physics, Peking UniVersity, Beijing 100871, P. R. China, Department of Physics, UniVersity of Nebraska at Omaha, Omaha, Nebraska 68182-0266, and Department of Theoretical Molecular Science, Institute for Molecular Science, Okazaki 444-8585, Japan ReceiVed: February 24, 2006; In Final Form: May 25, 2006
The effects of F doping on the structural and electronic properties of the (5, 5) single-walled boron nitride nanotube (BNNT) are investigated by using the density functional theory method. The chemiadsorption of F maintains the hexagonal BN network, increases the lattice constant, and introduces acceptor impurity states. On the other hand, substitutional doping of F destroys the hexagonal BN network, decreases the lattice constant, but does not alter the insulating feature of the BNNT. The observed insulator-to-semiconducting transition, a lattice contraction, and a highly disordered atom arrangement in the sidewall of BNNTs upon F doping appear to be most reasonably attributed to a codoping of dominating substitutional F over chemiabsorbed F, which can induce deep donor impurity states, a lattice contraction, and a destruction of the hexagonal BN network simultaneously.
Introduction Due to their low dimension and high surface-to-volume ratio, nanotubes (NTs) have attracted much attention. Single-walled carbon nanotubes (SWCNTs) can be either metallic or semiconducting depending on their chirality and diameter.1 On the other hand, boron nitride nanotubes2-5 (BNNTs) are always semiconductors with a wide band gap of about 4-5 eV, almost independent of the tube diameters, helicity, and of whether a nanotube is single- or multiwalled.6,7 Manipulation of electronic properties of SWNTs is the requisite step for using them to realize a functional device. Functionalization is an effective method to change the electronic properties of SWCNTs.8-12 Apparently the electronic properties of functionalized SWCNTs depend on the geometrical parameters of original SWCNTs, while control of diameter and helicity of original SWCNTs is extremely difficult. The advantage of BNNT-based nanoelectronic devices lies in that there is no need to control the diameter and helicity of original BNNTs. Nevertheless, high chemical inertness of BNNT prevents them from functionalization. Recently, functionalization of BNNTs by stannic oxide coating13 and by F doping14 has been successfully realized via new synthesis method. Especially, an insulator-to-semiconductor transition, accompanied by a lattice contraction and a highly disordered atom arrangement of the sidewall, has been observed in fluorinated BNNTs, suggestive of possible application in tunable nanoscale electronic devices.14 Obviously, the atomic and electronic structures of fluorinated BNNTs is desirable. Methods In this article, we have examined systematically the effects of F doping on the atomic structure and the electronic energy * Corresponding authors e-mail:
[email protected] (J.L.), zxgao@ pku.edu.cn (Z.G.), and
[email protected] (S.G.). † Peking University. ‡ University of Nebraska at Omaha. § Institute for Molecular Science.
bands of BNNTs by using the density functional theory (DFT). We chose the (5, 5) single-walled BNNT as a representative of BNNTs, which has an indirect band gap of about 4.6 eV and a diameter of 6.8 Å. We built a supercell with nanotubes separated by 17 Å to eliminate the interaction between them. Each supercell contains two unit cells of the (5,5) BNNT. Full geometry optimization was performed for both the atomic positions and lattice lengths by using the ultrasoft pseudopotential15 plane-wave program, CASTEP,16 with four k points. The plane-wave cutoff energy of geometry optimization is 310 eV, and the convergence tolerance of force on each atom is 0.05 eV/Å. Static total energies of the relaxed structures are calculated with a larger 370 eV cutoff energy. The double numerical atomic orbital basis set17 is employed to calculate the electronic energy bands. This numerical atomic orbital basis set method can generate the band structure quite similar to that generated by ultrasoft plane-wave basis set method with much less computation time. The generalized gradient approximation by Perdew, Burke, and Ernzerhof (PBE)18 is employed for the exchange-correlation functional. Results and Discussion Four F doping ways are taken into consideration: (1) exohedral chemiadsorption, (2) endohedral chemiadsorption, (3) substitutional doping, and (4) a combined doping of exohedral chemiadsorption and substitution. In either case of chemiadsorption, the F atom prefers to be attached to B atoms of the BNNT. For a given coverage of F, several different isomers are taken into account. The coverage of chemiadsorbed F on the BNNT is defined as the ratio of the number of F atoms to the total number of B and N atoms. The lowest energy F exohedrally and endohedrally doped isomers at the F coverages of 2.5%, 5%, 7.5%, 10%, and 12.5% are displayed in Figure 1(a-j), respectively. The BNNT sidewall is somewhat distorted by F chemiadsorption. For exohedral and endohedral chemiadsorptions, the B atoms attached to by the F atoms somewhat pop outward and inward, respectively. The B-N bonds near
10.1021/jp061203y CCC: $33.50 © 2006 American Chemical Society Published on Web 07/06/2006
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Figure 1. Optimized favorable chemiadsorption configurations of F exohedrally (a-e) and endohedrally (f-j) doped (5, 5) BNNTs at the coverage of 2.5% (a,f), 5% (b,g), 7.5% (c,h), 10% (d,i), and 12.5% (e,j). Red ball: F; blue ball: N; brown ball: B.
Figure 2. Optimized configurations of (5, 5) BNNTs substituted by (a) one (BN0.95F0.05), (b) two (BN0.9F0.1), (c) three (BN0.85F0.15), and (d) four (BN0.7F0.2) F atoms. Red ball: F; blue ball: N; brown ball: B.
the adsorbed F atoms are elongated from 1.44 Å to 1.53-1.54 Å in the exohedral adsorption case and 1.50-1.51 Å in the endohedral adsorption case. As a result, the lattice constant (4.98 Å) of the (5,5) BNNT is basically increased with increasing F coverage. At the exohedral(endohedral) F coverages of 2.5%, 5%, 7.5%, 10%, and 12.5%, the lattice constants are 4.98(4.98),
5.00(4.99), 5.00(5.00), 5.02(5.00), and 5.02(5.01) Å, respectively. Similarly, the C-C bonds near the adsorbed F atoms are also elongated in the case of fluorinated CNTs.10,12 However, the hexagonal BN network survives chemiadsorption. The lengths of endohedral F-B bonds are 1.45-1.51 Å, larger than those of exohedral F-B bonds (1.40-1.41 Å). Both of them
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Figure 3. Optimized configurations of (5, 5) BNNTs codoped by (a) one adsorbed and one substitutional F (BN0.95F0.1), (b) two adsorbed and two substitutional F (BN0.9F0.2), (c) one adsorbed and two substitutional F (BN0.9F0.15), (d) one adsorbed and three substitutional F (BN0.85F0.2). Red ball: F; blue ball: N; brown ball: B.
Figure 4. Adsorption energies versus the coverage of exohedral (triangle) and endohedral (square) F in the (5, 5) BNNTs.
are significantly greater than the F-B bond length of 1.32 Å in the BF3 molecule. The favorable substitutional site on the BNNT sidewall is the N atom.19 When the N atoms of the sidewall are substituted by F atom, the six-membered ring structure is destroyed and the F atoms pop outward, as shown in Figure 2. The cause lies in that F only can form one covalent bond with B atom. The F-B bond lengths are 1.33-1.35 Å, apparently smaller than the B-N bond length (1.44 Å). Consequently, the lattice constant of the BNNT is generally reduced with the increasing number of F substitution. It is 4.98, 4.95, 4.96, and 4.92 Å, respectively, corresponding to 1, 2, 3, and 4 F substitutions. The most stable configuration of one adsorbed F and one substitutional F is shown in Figure 3(a), where the F atom is adsorbed on the B atom that bonds only to two N atoms. Surprisingly the substitutional F atom is almost in the nanotube
Figure 5. Electronic structures of (a) the pure (5, 5) BNNT, (b-d) exohedrally and (e-g) endohedrally F doped (5, 5) BNNTs at the coverage of 2.5% (b,e), 7.5% (c,f), and 12.5% (d,g), respectively, and (h) F substituted BNNT. The Fermi level or the valence top is taken as zero-energy point in (b-g), (a), and (h), respectively.
sidewall rather than pops outward. We conjecture that the repulsion between the adjacent substitutional and adsorbed F
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J. Phys. Chem. B, Vol. 110, No. 29, 2006 14095 We defined the adsorption energy of F atom on the BNNT as
Ead ) E(BNNT + F) - E(BNNT) - n × E(F)
Figure 6. Electronic structures of BNNTs codoped by (a) (a) 1 adsorbed and 1 substitutional F (BN0.95F0.1), (b) 2 adsorbed and 2 substitutional F (BN0.9F0.2), (c) 1 adsorbed and 2 substitutional F (BN0.95F0.15), and (d) 1 adsorbed and 3 substitutional F (BN0.95F0.2). The Fermi level is taken to zero.
atoms keeps the substitutional F from popping outward. The most stable configurations of two adsorbed and two substitutional F, one adsorbed and two substitutional F, and one adsorbed and three substitutional F are shown in Figure 3(bd), respectively. The substitutional F atoms remain nearly in the nanotube sidewall. The exohedral F-B bonds have a length of 1.34-1.36 Å, while the F-B bonds in the sidewall have a length of 1.39-1.41 Å. According to previous calculations, the adsorbed and substitutional F atoms give rise to adverse effect on the lattice constant of the BNNT (expansion for adsorbed F while contraction for substitutional F). The lattice constants of the codoped BNNT, therefore, depend on the ratio of adsorbed to substitutional F. The lattice constant of the (5,5) BNNT is slightly elongated from 4.98 Å to 4.99 Å and 5.00 Å in BN0.95F0.05 (1 adsorbed and 1 substitutional F) and BN0.9F0.1 (2 adsorbed and 2 substitutional F), respectively. On the contrary, it is contracted to 4.94 Å (0.8%) in BN0.95F0.15 (1 adsorbed and 2 substitutional F) and 4.90 Å (1.6%) in BN0.95F0.2 (1 adsorbed and 3 substitutional F).
The calculated Ead as a function of F coverage is shown in Figure 4. For exohedral F adsorption, Ead increases nearly linearly with the F coverage as the F coverage is smaller than 0.25. The average adsorption energy is about -3.0 eV per F atom, suggestive of higher thermal stability. For endohedral F adsorption, the value of Ead increases in a nonlinear way with the F coverage but does not show saturation sign till the F coverage reaches 0.125. The average adsorption energy of endohedral F doping is -1.8 to -2.2 eV per F atom, suggestive of lower stability of endohedral F doping than the exohedral one. The reported doping concentration of F was about 5%.14 Hence, further functionalization by F appears to be possible. The electronic structures of F exohedrally, endohedrally, and substitutionally doped BNNTs are shown in Figure 5(b-d),(e-g), and (h), respectively. Those of codoped BNNTs are shown in Figure 6. Exohedrally and endohedrally F doped BNNTs are typical p-type semiconductors with shallow acceptor states. The number of acceptor states increases with increasing F coverage. These shallow acceptor states are localized in the proximity of the impurity atoms, as can be seen from their orbital wave function shown in Figure 7. On the other hand, F substitutionally doped BNNT remains an insulator (or wide energy gap semiconductor) with an energy gap of 3.8 eV. Similar p-type character is also found in F exohedrally doped (10,0) BNNT in a very recent work of Yang et al.19 The codoped (5,5) BNNTs are n-type semiconductors with deep donor impurity states. These deep donor impurity states are localized in the proximity of the impurity atoms too, as shown in Figure 8. They are 1.5-1.8 eV below the conduction band and 3.1 eV above the valance states. We notice that in codoped (10,0) BNNT with one adsorbed and one substantial F atom, the impurity state is about 1.4 eV above the valence top and 2.0 eV below the conduction band bottom and appears more like a deep acceptor state. In addition to the significantly reduced electrical resistivity, two facts are crucial to identify the experimental F doping
Figure 7. Isosurfaces of square wave functions of the acceptor states at Γ point for (a-b) exohedrally and (c-d) endohedrally F doped BNNTs at the coverage of 2.5% (a,c) and 12.5% (b,d), respectively. The iso-value is taken as 0.05 au.
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Figure 8. Isosurfaces of square wave functions of the donor states at Γ point for codoped BNNTs: (a) 1 adsorbed and 1 substitutional F (BN0.95F0.1), (b) 2 adsorbed and 2 substitutional F (BN0.9F0.2), (c) 1 adsorbed and 2 substitutional F (BN0.95F0.15), and (d) 1 adsorbed and 3 substitutional F (BN0.95F0.2). The iso-value is taken as 0.05 au.
pattern: (1) a 1% lattice contraction is observed upon F doping.14 (2) The extremely broad peak of the in-plane B-N transverse vibration in Fourier transformation infrared spectrum and scanning electron microscopy and transmission electron microscopy observation all point to a highly disordered atom arrangement in the BN sidewall.14 In other words, the BN hexagonal ring structure is destroyed. The codoping with dominating substitutional F over adsorbed F, represented by the two structures shown in Figure 3(c),(d), not only induces a metal-to-semiconductor transition but also gives rise to a lattice contraction of about 1% and a break of the hexagonal BN network and therefore accounts for the experiments most reasonably. If the doping is performed after the growth stage of BNNTs, the substitutional doping is difficult and chemiadsorption is supposed to dominate over substitutional doping. However, the actual doping is carried out at the growth stage of BNNTs rather than after the growth stage.14 In such a case, the doping pattern is not dominated by the final state energy but by the dynamic process; therefore, a codoping by dominant substitutional over adsorbed F is fully possible. In view of the coexistence of a variety of chirality in the BNNT sample, both electrons and holes should contribute to the electrical conductivity. The conductivity type of the experimental F doped BNNTs can be either n-type if the dominant BNNTs in the sample are armchair type or p-type if the dominant BNNTs in the sample are zigzag type. Experimentally F doping concentration is up to about 5 atom %,14 and it is probably responsible for the measured decrease of 3 orders of magnitude in electrical resistivity upon F doping. Further theoretical calculation will help understand the remarkably reduced electrical resistivity. Conclusions In summary, the effects of F doping on the structural and electronic properties of BNNTs are investigated by the DFT method. Chemisorption of F gives rise to a lattice expansion and shallow acceptor impurity states, meanwhile keeping the hexagonal ring network structure of the sidewall. On the other hand, the codoping with dominating substitutional F over adsorbed F can lead to a lattice contraction, a highly disordered
atom arrangement of the BN sidewall and deep donor or acceptor impurity states depending on the chirality. All the available characterizations of the synthesized fluorinated BNNTs appear to point to a codoping of dominating substitutional F over adsorbed F. It is expected that the present results for the fluorinated BNNTs will activate further works on their electronic applications. Acknowledgment. We thank Professor C. C. Tang for useful discussions. This work was supported by the NSFC (Grant No. 10474123, 10434010, and 20131040), National 973 project (No. 2002CB613505, MOST of China), 211, 985, and Creative Team Projects of MOE of China, and Nebraska Research Initiative (No. 4132050400, U.S.A.). Our calculations were partially carried out in the HP Cluster of the Calculation Center of Science and Engineering of Peking University. References and Notes (1) Hamada, N.; Sawada, S. L.; Oshiyama, A. Phys. ReV. Lett. 1992, 68, 1579-1581. (2) Bengu, E.; Marks, L. D. Phys. ReV. Lett. 2001, 86, 2385-2387. (3) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966-967. (4) Golberg, D.; Bando, Y.; Han, W.; Kurashima, K.; Sato, T. Chem. Phys. Lett. 1999, 308, 337-342. (5) Loiseau, A.; Willaime, F.; Demoncy, N.; Hug, G.; Pascard, H. Phys. ReV. Lett. 1996, 76, 4737-4740. (6) Blase, X.; Rubio, A.; Louie, S. G.; Cohen, M. L. Europhys. Lett. 1994, 28, 335. (7) Rubio, A.; Corkill, J. L.; Cohen, M. L. Phy. ReV. B 1994, 49, 50815084. (8) Hu, H.; Zhao, B.; Hamon, M. A.; Kamaras, K.; Itkis, M. E.; Haddon, R. C. J. Am. Chem. Soc. 2003, 125, 14893-14900. (9) Kamaras, K.; Itkis, M. E.; Hu, H.; Zhao, B.; Haddon, R. C. Science 2003, 301, 1501-1501. (10) Park, K. A.; Choi, Y. S.; Lee, Y. H.; Kim, C. Phys. ReV. B 2003, 68, 045429. (11) Zhao, J. J.; Park, H.; Han, J.; Lu, J. P. J. Phys. Chem. B 2004, 108, 4227-4230. (12) Kudin, K. N.; Bettinger, H. F.; Scuseria, G. E. Phys. ReV. B 2001, 63, 045413. (13) Han, W. Q.; Zettl, A. J. Am. Chem. Soc. 2003, 125, 2062-2063. (14) Tang, C. C.; Bando, Y.; Huang, Y.; Yue, S. L.; Gu, C. Z.; Xu, F. F.; Golberg, D. J. Am. Chem. Soc. 2005, 127, 6552-6553. (15) Vanderbilt, D. Phys. ReV. B 1990, 41, R7892.
Fluorinated Boron Nitride Nanotubes (16) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmatskaya, E. V.; Nobes, R. H. Int. J. Quantum Chem. 2000, 77, 895-910. (17) Delley, B. J. Chem. Phys. 2000, 113, 7756-7764.
J. Phys. Chem. B, Vol. 110, No. 29, 2006 14097 (18) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (19) Xiang, H. J.; Yang, J. L.; Hou, J. G.; Zhu, Q. S. Appl. Phys. Lett. 2005, 87, 243113.
