APPLIED PHYSICS LETTERS 86, 133110 共2005兲
Structure and chirality distribution of multiwalled boron nitride nanotubes A. Celik-Aktas Department of Nuclear, Plasma, and Radiological Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
J. M. Zuoa兲 Department of Materials Science and Engineering, and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
J. F. Stubbins Department of Nuclear, Plasma, and Radiological Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
C. Tang and Y. Bando National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305-0044, Japan
共Received 6 December 2004; accepted 7 February 2005; published online 24 March 2005兲 We report on a high-resolution electron diffraction study of the structure of individual multiwalled boron nitride nanotubes 共MW-BNNTs兲. The tube chirality was determined by electron diffraction. Diffraction patterns were recorded from small sections of the nanotubes, ⬃125 nm long, using the nanoarea electron diffraction technique. Accurate measurements of the MW-BNNT chiral angles and their distribution were made from diffraction patterns. Generally, the tube chiralities within each MW-BNNT are strongly correlated; clustering around a single chirality with a dispersion of a few degrees. Multihelix nanotubes were rarely observed. Statistics based on 67 nanotubes revealed a dispersion of the chiral angles 共␣兲 with some preference of tubes in the ranges of 10° 艋 ␣ 艋 15° and 25° 艋 ␣ 艋 30°. Since various properties of nanotubes depend on the tube structure 共diameter and chirality兲, the results presented here have general significances to nanotube growth and applications. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1885177兴 Multiwalled nanotubes have attracted considerable attention for their unique structure and possible applications in nanotechnology.1 An ideal multiwalled nanotube is made of nested coaxial cylindrical tubes, where the interlayer interaction is much weaker than the intralayer interaction. Predictions of boron nitride nanotube 共BNNT兲 formation were made in 1994.2 Since the first successful synthesis3 of BNNTs, they have been studied as an alternative to carbon nanotubes 共CNTs兲. Individual CNTs are either metallic or semiconducting depending on the tube diameter and chirality. On the other hand, BNNTs are expected to have a constant band gap 共⬃5.5 eV兲 regardless of the tube chirality and diameter.4 In addition, BNNTs have high oxidation resistance 共onset temperature for oxidation is ⬃800 ° C兲 compared to 400 ° C for CNTs.5 Tubular hexagonal BN also exhibits piezoelectricity.6–8 A fundamental question about nanotubes is their structure and growth mechanism. In spite of intensive research, so far there is no clear understanding about how does a tube grow and under which conditions. One of the reasons for slow progress is the lack of detailed knowledge about the tube atomic structure. Since the nanotubes are very small, traditional structure characterization tools, e.g., x-ray diffraction, do not apply. Other complexities are different synthesis conditions and possibly different growth mechanisms. The tube structure is critical to its properties. A recent ab initio study revealed that piezoelectricity of BNNTs depends on the chiral angle, the diameter, and the direction of loading.8 a兲
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For multiwalled boron nitride nanotubes 共MW-BNNTs兲, an early study has indicated a preference for zig–zag and near zig–zag tubes.9 However the study was performed using a partially focused electron probe. Consequently the diffraction pattern resolution was limited by the convergence angle. In this letter we report on a study of the structure and statistical distribution of chiral angles of individual multiwalled boron nitride nanotubes. Using the newly developed nanoarea electron diffraction technique, we have significantly improved the quality and sensitivity of electron diffraction, which allows a quantitative determination of individual nanotube structure.10 In this technique a small probe of parallel electron beam illuminates the sample. Our results showed a more even distribution of chiral angles from zig– zag to armchair configurations. There is a strong correlation between the layers of boron nitride nanotubes in most of the tubes studied here. This is consistent with the lip–lip interaction between successive layers in bulk hexagonal boron nitride where a boron atom in one layer is aligned with a nitrogen atom in the neighboring layer. The boron nitride nanotubes studied here were grown in a carbon-free chemical vapor deposition process at 1100 ° C.11 The average outer diameter of the tubes was ⬃40 nm. Transmission electron microscopy 共TEM兲 study was carried out in a JEOL 2010F TEM at a high voltage of 200 keV. High-resolution electron diffraction patterns were digitally recorded on Fuji imaging plates with a camera length of 80– 100 cm. The tube inner and outer diameters are measured directly from high-resolution electron images. The tube outer diameters range from 12 to 80 nm and the inner diameters range from 3 to 18 nm. Smaller tubes tends to have armchair ori-
0003-6951/2005/86共13兲/133110/3/$22.50 86, 133110-1 © 2005 American Institute of Physics Downloaded 04 Aug 2006 to 130.126.101.121. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 1. 共a兲 Typical electron diffraction pattern from a multiwalled BN nanotube 共␣ = 9 ° 兲. The diffraction pattern was recorded from a tube of 33 walls. 共b兲 An example of multihelix diffraction pattern. This particular diffraction pattern shows the highest degree of randomness observed in BN nanotubes so far. The tube had as many as 37 walls. 共c兲 Schematic for chiral angle measurement. Ratios of d1, d2, and d3 are used to calculate the average chiral angle.
entation. Otherwise, there is not a strong correlation between the tube diameter and the chirality. In the nanoarea electron diffraction mode, the electron beam is focused at the front focal plane of the objective lens by using an additional magnetic lens 共mini lens兲 共see Ref. 10兲. The sample is illuminated with a small probe of parallel electron beam. In this study, we have used a 27 m condenser aperture which produces a parallel beam of ⬃125 nm in diameter. Figure 1共a兲 shows a representative diffraction pattern from an individual multiwalled boron nitride nanotube. To understand the diffraction pattern, first, there will be two sets of hexagonal diffraction spots from each wall of the nanotube depending on the chirality of the nanotube. In the special cases of armchair and zigzag tubes, these two sets of spots will overlap and a single hexagonal set of diffraction spots will be observed. If the chiral angle of each wall is different and randomly distributed, the diffraction pattern in general will have twice as many hexagons as the number of walls distributed anywhere from 0° to 30°. In contrast, the diffraction pattern of the MW-BNNT of Fig. 1共a兲 shows a narrow range of chiral angle distribution. The overall diffraction pattern mimics the single wall CNT diffraction pattern rather than that of multiwalled carbon nanotubes.12–14 For the vast majority of BNNTs studied here chirality of all, or most of the walls, lie within a few degrees of the average chirality. Rarely, multihelix cases are observed 关see Fig. 1共b兲兴. The diffraction pattern of MW-BNNTs can be characterized by an average chiral angle and a width of distributions, Fig. 1共c兲. The chiral angle ␣ is defined as
冉冑
␣ = arctan
冊
1 2d2 − d3 . 3 d3
共1兲
Here d is the distance from the middle of the band to the equatorial line, Fig. 1共c兲. In this method, the determination of ␣ relies on the ratios of distances between diffraction spots and equatorial line. The ratios are not effected by the tilt of
the nanotube with respect to electron beam.10 Therefore this method is more accurate than a direct measurement of chiral angles. This approach was first proposed by Gao et al.10 Figure 2共a兲 shows the chiral angle distribution of MWBNNTs. The initial qualitative observations showed that the BNNTs have zig–zag or near zig–zag chirality.9,11,15–17 The quantitative measurements here revealed a more even distribution of the chiral angles from zig–zag 共0°兲 to armchair 共30°兲 configurations. The chirality of MW-BNNTs tends to peak around 10°–15° and 25°–30°. The difference between our results and previous work on BNNTs is a significant improvement in electron diffraction data quality and a better chiral angle measurement method, which has an accuracy of less than 0.2°.10 The differences in the sample growth conditions is also a possibility, which can not be ruled out at this stage. Figure 2共b兲 shows the relation between the dispersion of chiral angles within individual BNNTs 关spread of d2, Fig. 1共c兲兴 and the average chiral angle of the corresponding nanotubes. A correlation between the chiral angle and the width of ¯ 0兲 peak is observed: The chiral angle versus width plot 共011 resembles a bell-shaped curve which suggest wider distribution in chiral angles around 12° 关see Fig. 2共b兲兴. The width changes from 0.85° to 5.83° for chiral angles 1.4° and 11.67°, respectively. The observations here suggest that the layers in BNNTs are strongly correlated. As in the case of bulk hBN, boron atoms in one layer must be aligned with nitrogen atoms in the neighboring layer. Because of the cylindrical nature of the nanotubes this structure cannot be perfect. Therefore edge dislocations must be introduced to relieve the stress in the tube, Fig. 3. In addition, truncated layers at the innermost surface of the BNNTs are sometimes observed. Movement of edge dislocations in small BNNTs under electron beam radiation was observed in situ by Golberg et al.18 Thus, the truncation can be explained based on the edge dislocation motion during synthesis.
FIG. 2. 共a兲 Distribution of chiral angle in multiwalled BN nanotubes. 共b兲 Distribution of d2 spread calculated in terms of chiral angle using Eq. 共1兲. Average width of d2 is 3.2°. Continuous curve is a polynomial fit. Downloaded 04 Aug 2006 to 130.126.101.121. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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which suggests a strong interlayer correlation. Detailed analysis of diffraction patterns revealed a dispersion of chiral angles 共␣兲 with some preference of tubes with 10° 艌 ␣ 艌 15° and 25° 艌 ␣ 艌 30°. Work on electron microscopy characterization was supported by DOE DEFG02-01ER45923 and DEFG0291ER4539 and uses the TEM facility of Center for Microanalysis of Materials at FS-MRL. V. Pokropivnyi, Powder Metall. Met. Ceram. 41, 123 共2002兲. A. Rubio, J. Corkill, and M. Cohen, Phys. Rev. B 49, 5081 共1994兲. 3 N. Chopra, R. Luyken, K. Cherrey, C. Crespi, M. Cohen, S. Luuie, and A. Zettl, Science 269, 966 共1995兲. 4 X. Blase, A. Rubio, S. Louie, and M. Cohen, Europhys. Lett. 28, 335 共1994兲. 5 Y. Chen, J. Zou, S. Campbell, and G. Caer, Appl. Phys. Lett. 84, 2430 共2004兲. 6 E. Mele and P. Kral, Phys. Rev. Lett. 88, 056803 共2002兲. 7 S. Nakhmanson, A. Calzolari, V. Meunier, J. Bernholc, and M. Nardelli, Phys. Rev. B 67, 235406 共2003兲. 8 N. Sai and E. Mele, Phys. Rev. B 68, 241405 共2003兲. 9 D. Goldberg, Y. Bando, K. Kurashima, and T. Sato, Solid State Commun. 116, 1 共2000兲. 10 M. Gao, J. Zuo, R. Twesten, and I. Petrov, Appl. Phys. Lett. 82, 2703 共2003兲. 11 C. Tang, Y. Bando, T. Sato, and K. Kurashima, Chem. Commun. 共Cambridge兲 2002, 1290. 12 A. Lucas, F. Moreau, and P. Lambin, Rev. Mod. Phys. 74, 1 共2002兲. 13 M. Kociak, K. Hirahara, K. Suenaga, and S. Iijima, Eur. Phys. J. B 32, 457 共2003兲. 14 S. Amelinckx, A. Lucas, and P. Lambin, Rep. Prog. Phys. 62, 1471 共1999兲. 15 R. Ma, Y. Bando, T. Sato, and K. Kurashima, Chem. Mater. 13, 2965 共2001兲. 16 L. Bourgeois, Y. Bando, and T. Sato, J. Phys. D 33, 1902 共2000兲. 17 D. Goldberg, W. Han, Y. Bando, L. Bourgeois, K. Kurashima, and T. Sato, J. Appl. Phys. 86, 2364 共1999兲. 18 D. Goldberg, Y. Bando, M. Eremets, K. Takemura, K. Kurashima, K. Tamiya, and H. Yusa, Commun. Pure Appl. Math. 279, 191 共1997兲. 19 J. Zuo, I. Vartanyants, M. Gao, R. Zhang, and L. Nagahara, Science 300, 1419 共2003兲. 1 2
FIG. 3. Typical HRTEM image of a BN nanotube showing the resolved layers and the atomic structure tube. The inset shows an edge dislocation among the walls and truncated layers at the innermost surface of the nanotube. Dislocations and defects are common in BN nanotubes because of the strong interlayer interaction and the strained tubular structure.
The structure of MW-BNNTs revealed here is significantly different from that of multiwalled CNTs. In CNTs chirality of each layer is independent of the other layers as observed in 2 and 3 walled carbon nanotubes.12,13,19 As the number of walls increased up to 20 or so some grouping among the chiralities of the different walls was reported.12,14 In multiwalled boron nitride nanotubes the structure is dominated by a single chirality for the whole tube. In conclusion, electron diffraction from individual multiwalled nanotubes mimics single wall nanotube structure,
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