Heterostructured B6Nx/BN Nanocable and Nanofeather Nanojunctions Limin Cao,*,† He Tian,‡ Ze Zhang,‡ Min Feng,† Zaiji Zhan,† Wenkui Wang,† and Xiangyi Zhang*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4350–4354
State Key Laboratory of Metastable Materials Science and Technology, Yanshan UniVersity, Qinhuangdao 066004, China, and Institute of Microstructure and Properties of AdVanced Materials, Beijing UniVersity of Technology, Beijing 100022, China ReceiVed August 16, 2008; ReVised Manuscript ReceiVed October 3, 2008
ABSTRACT: One of the arduous challenges in nanotechnology is to mass-fabricate useful and desired nanostructures and devices using cost-effective processes. Here we report the first creation of B6Nx/BN coaxial nanowires and branched nanofeather nanojunctions with radial heterostructures using simple nitriding processing of pure boron nanostructured precursors at 1200 °C. The produced nanostructures consist of a core nanowire with rhombohedral structure and stoichiometry of B6Nx, a metastable high pressure phase, and a hexagonal BN sheath. We suggest that the BN shell layers formed first act as the high-pressure nanocell to provide the foundation for the formation of the B6Nx core in the nitriding process. This simple process might enable the studies of high-pressure-induced phase transformation and reaction in a nanosystem at ambient pressure, and be extended to bulk fabrication of a wealth of nano-heterostructures and nanocomposites in a B-C-N-O system. Progress in nanotechnology is offering great opportunities for technology innovations in several key scientific and technological areas. However, significant economic and technical challenges, in terms of materials and processes, need to be overcome for the developments of nanotechnology to continuously satisfy the relentless consumer demand for ever-cheaper but more functional materials and devices. One of the substantial challenges is that new nanostructures need to be created and explored to serve as the functional units in the miniaturized nanosystems where high performance and reliability are required. Another practical challenge is that simple processes for reducing cost but improving efficiency are necessitated to mass-fabricate functional nanostructures at a commercial scale. Among various nanostructures, coaxial nanocable heterostructures are of particular importance as promising building blocks in developing nanoscale devices and nanocomposites.1-22 Boron and boron-rich borides constitute a fascinating class of materials which have attracted considerable attention owing to their unique structures and properties.23-32 They provide an ideal platform to explore the fundamental natures in cluster physics and structural chemistry, thanks to the unusual three-center electrondeficient bond and most varied polymorphism existing in the boron family. They possess many unrivaled properties ranging from superconducting metals to wide-band semiconductors, and are widely used in numerous technological applications, particularly in extreme environments where a refractory, light, and hard material is required.23-32 For example, B6O possesses a hardness comparable with that of cubic BN, the second hardest material;29-31 B6N, the subnitride analogue to B6O, might be metallic.32 However, the syntheses of these useful boron-based materials normally require extreme high-pressure, high-temperature conditions.29-32 Strategies to obtain these materials in useful shape-designed forms at ambient pressure are of practical importance for their technological applications. To our knowledge, the synthesis of B6N based onedimensional nanostructures has not so far been reported. Here we present the first creation of B6Nx/BN radial heterostructured nanocables and multiply connected T-and/or Y-type junction nanofeathers in bulk quantities using a simple approach of annealing the corresponding pure boron nanostructured precursors in N2 at 1200 °C. The synthesized nanostructures consist of a core B6Nx * To whom correspondence should be addressed. E-mail:
[email protected] (L.M.C.);
[email protected] (X.Y.Z.). † Yanshan University. ‡ Beijing University of Technology.
nanowire with a rhombohedral structure, a metastable high pressure phase, and a shell of hexagonal BN layers. We propose a highpressure nanocell-assisted growth mechanism; that is, the BN sheath formed in the initial stage of the nitriding reaction serves as the nanoscopic high pressure cell, for the formation of the B6Nx core nanostructure. This simple strategy might be used to rationally create other novel boron-based nano-heterostructures and nanocomposites in B-C-N-O system, and in principle, can provide nanoscale platforms to study high-pressure-induced phase transformations and reactions at ambient pressure. Our results show promising technological potential of fabricating new and hybrid nanostructures using simple processes for practical applications. The starting boron nanowire and nanofeather precursors were synthesized using the radio frequency magnetron sputtering method.33-37 In the present study, we used a mixture of smooth boron nanowires (Figure 1a) and branched nanofeathers (Figure 1b) as the starting materials. The mixture was enclosed in a Ta capsule, and then placed in a homemade high-temperature annealing furnace for nitriding treatment. The furnace chamber was first pumped down to 10-5 Torr. High-purity nitrogen gas was then introduced into the chamber to a pressure of 150 Torr. After nitriding at 1200 °C for 4 h, the furnace was naturally cooled to room temperature, and the resultant materials were characterized using scanning electron microscopy (SEM, XL30S-FEG) and transmission electron microscopy (TEM, Phillips CM200/FEG, equipped with a Gatan imaging filter). Figure 2 shows TEM studies of the as-prepared products after nitriding treatment. The obtained nanostructures possess either the smooth one-dimensional wire-like morphology (Figure 2a) or the branched multiple junction structure (Figure 2b) as those of the starting pure boron nanostructures. The smooth nanowires have diameters of 15-100 nm and are uniform throughout their entire length. In the branched nanowires, the branches locate on the same sidewall of the stem to form a series of multiple terminal T- and/or Y-nanojunctions, manifesting the unilateral feather-like morphology (Figure 2b). The abrupt interface in the diffraction contrast of the TEM image (see Figure 2a) suggests that the body and skin of the synthesized nanowires should be of different phases. Selected area electron diffraction (SAED) technique was applied to identify the phase of nanostructures. SAED patterns shown in Figure 2c-e could be well indexed with the lattice parameters of rhombohedral B6N (Joint Committee on Powder Diffraction Standards (JCPDS) card: 50-1504;
10.1021/cg800900j CCC: $40.75 2008 American Chemical Society Published on Web 10/24/2008
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Figure 2. TEM and SAED analyses of the as-produced nanostructures. (a) TEM image of a smooth nanowire showing the coaxial core-shell structure. (b) TEM image of a nanofeather revealing the branched Y- and/or T-type multiple nanojunction structure. (c-e) SAED patterns acquired on a typical smooth nanowire. These patterns correspond to the electron diffractions from [100], [122], and [4j 41] zone axes of B6N rhombohedral unit cell, respectively. The weak half-rings (corresponding to a separation of 0.33 nm) in the SAED patterns are from the electron diffraction of hexagonal BN (0002) atomic layers in the nanowire shells.
Figure 1. Scanning electron images of the starting pure boron nanostructure precursors grown on silicon substrates. (a) SEM image of the smooth boron nanowire arrays. (b) Top-view SEM image of the boron nanofeather arrays. These pure boron nanostructures were stripped off the substrates, and used as the starting materials in this study.
space group R3j m; lattice parameters: a ) 5.457 Å, c ) 12.241 Å), in agreement with the chemical analysis (see below). Analysis and diffraction simulation showed that the SAED patterns (Figure 2c-e) fit precisely to the electron diffractions from [100], [122], and [4j 41] zone axes of rhombohedral B6N lattice, respectively. The nanostructures were further characterized using high resolution transmission electron microscopy (HRTEM). HRTEM images, shown in Figure 3, highlight that both the smooth nanowires and the branched nanofeathers consist of a single-crystalline core sheathed with a few graphitic walls with an interlayer spacing of ∼0.33 nm. Referring to chemical analysis (see below) and our previous experimental results (see ref 37), we designated the shell layers to hexagonal BN (0002) planes. The lattice images from the crystalline nanowire cores match the structure of rhombohedral B6N, considering the interplanar spacings and angles between the planes. This is consistent with SAED analysis. Figure 3c depicts the junction area of a nanofeather, showing the single-crystalline nature in the junction region between the stem and branch. The same crystal planes running through the stem into the branch suggest that the atomic clean and uniform interconnection is formed (Figure 3c). Extensive electron microscopy investigations revealed that the majority of the nanostructures synthesized possess a single crystalline core, or at least consist of long single crystal sections. In addition, we should mention here that the core-shell B6Nx/BN nanofeathers produced in this study possess the B6Nx/BN heterostructure in the radial direction, and the multiterminal B6Nx/BN nanojunction structure in the lateral direction. This complex nanoarchitecture in two independent dimensions can offer significant
Figure 3. High-resolution TEM images of representative B6Nx/BN nanostructures. (a,b) HRTEM images of two typical smooth nanowires showing the (012) and (11j2) planes (a), (012) and (014j) planes (b) of rhombohedral B6N unit cell, respectively. The longitudinal axes of the two nanowires orientate roughly along the [241] direction. (c) HRTEM image of the junction region of a typical nanofeather. All the nanostructures are capped with hexagonal BN shells.
opportunity for actualizing the functionality and complexity of building units in nanodevices and nanocomposites. The chemical composition of the product was characterized using energy dispersive X-ray (EDS) analysis and electron energy-loss
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Figure 4. EELS spectra of B6Nx nanowire (a), bulk pure R-boron (b), and BN nanotube (c). The B K edge features from B6Nx nanowire are similar to that from bulk pure boron, but are obviously different from that of BN. The insets show the magnified features of the B and N K-edges from B6Nx nanostructure (a), pure boron (b), and BN (c).
spectroscopy (EELS). The EDS spectrum revealed that the nanostructures contain boron and nitrogen; the N/B atomic ratio was quantified to be about 1:4.8. Figure 4a shows a representative EELS spectrum taken from the central part of a typical nanowire. Because the shell layers are very thin (2-4 nm) (see Figure 3), the EELS signals are mainly from the core phase. EELS spectrum presents two sharp threshold peaks beginning at ∼185 eV and ∼396 eV, corresponding to the characteristic K-shell ionization edges for B and N, respectively. EELS quantification of the spectra gives the N/B atomic ratio of about 0.21 ( 0.034, which is in good agreement with the EDS analysis. We also acquired EELS spectra from pure R-rhombohedral boron and hexagonal BN nanotube for a comparison (see Figure 4b,c). The fine structures of boron K-edges from the product nanowires show a similarity to those from bulk pure boron (B K-edges typical of icosahedral rhombohedral boron-rich phases), but a distinguishing difference from those of hexagonal BN. Our results are coincident with previous EELS studies of the B6N and other icosahedral boron-rich phases in B-C-N-O system.32 Energy-filtering imaging offers an effective method to directly map the distribution of particular elements of interest in a relatively large area with high spatial resolution. We acquired the elemental mapping of B and N in the produced BNx nanostructures. Figure 5a-c shows the bright field TEM image and the corresponding energy-filtered images of two typical BNx nanostructures. The B elemental map, shown clearly in Figure 5b, has uniformly bright contrast in the core throughout the nanowires. Careful observations reveal a reduced intensity at the peripheries of the nanowires in the B map. This suggests that the nanowire shells are boron-deficient compared to the core region where the distribution of B is homogeneous. In contrast, the N elemental map (Figure 5c) shows a bright contrast sheath and a dark core with much lower brightness, indicating that the coating is nitrogen-rich. Linescan analyses of the elemental distributions across the nanowire in the B and N maps show a hill-shaped B concentration profile and valley-like N profile (Figure 5d,e), confirming that our BNx nanostructures have a nitrogen-rich shell, and is in good agreement with the conclusion that the nanowire sheath is hexagonal BN. The chemical profiles of B and N, as shown in Figure 5d,e, also highlight two interesting features. First, two shoulder peaks (indicated by the arrows in Figure 5d) exist on both sides of the B concentration profile. Second, the
Figure 5. Chemical analyses of typical B6Nx/BN nanostructures. (a) Bright-field TEM image. (b) B elemental map. (c) N elemental map. (d, e) Elemental profiles for B and N, respectively, across a typical nanowire. (f, g) Theoretical simulations of the B and N profiles, respectively, on a B6N/BN core-shell nanocable (40-nm-diameter B6N core and 5-nm-thick BN shell). (h, i) Elemental mapping cross-sections of B and N, respectively, modeled from a concentric nanowire with B core (40 nm diameter) and BN shell (5 nm thickness).
valley in the intensity profile of the N distribution has a protruding central section (indicated by the arrow in Figure 5e). Considering that the total N/B atomic ratio in the products is about 1:5, and the BN shell has a higher N/B ratio of 1:1, we can conclude that the core of the synthesized nanostructure has a nominal stoichiometry of B6Nx (x < 1.2). To confirm this assignment, we performed theoretical simulation of the cross-sectional elemental mapping of B and N in the synthetic concentric cylinders of B6N/BN and B/BN. Figure 5f,g shows the calculated elemental profiles in the B6N/BN core-shell nanowire with a 40-nm-diameter core and 5-nm-thick shell. The fine features of the calculated B and N mapping crosssections are in good agreement with the experimental data. In contrast, the theoretical cross-section N profile in the B/BN nanostructure (a 40-nm-diameter B core with a 5-nm-thick BN shell) shows a central dip in the N valley (Figure 5i), which differs distinctly from the experimental results (Figure 5e). Taken together, these structure and composition data show that the produced nanostructures consist of single-crystalline B6Nx cores surrounded by the hexagonal BN walls. After a careful comparison of our experimental data with those from the known BxNy phases (JCPDS card), we found that the crystal structure and stoichiometry of our B6Nx nanowires fit well to that of bulk B6N crystal, a metastable high-pressure phase. Condon et al. first showed the existence of B6N in the direct boron-nitrogen reaction.38 By annealing boron powder in N2, they obtained a new boron subnitride metastable phase with stoichiometry of B6N and structure resembling to that of B6O. The
Communications microcrystalline B6N particles were present as the core particles surrounded by hexagonal BN coats. Hubert et al. demonstrated the first bulk synthesis of B6N1-x crystals with sizes up to 1 µm by reacting boron and hexagonal BN at a high pressure and high temperature of 7.5 GPa and 1700 °C.31 These observations give us a clue to consider the physical origin of the formation mechanism of B6Nx crystalline core nanowires in our experiments. In the initial stage of the nitriding process, the nitrogen on the nanowire surface reacts with boron atoms to form fragments of BN atomic layers according to the reaction 2B(solid) + N2(gas) f 2BN(solid). As the reaction proceeds, the BN fragments on the outer surface wrap around the host nanowire to form a nanocapsule structure. At high temperature, the B and N atoms in the BN layers can rearrange and the BN fragments interconnect to form perfect concentric cylinders, which makes the structure more energetically stable. Thus, the diameter of the BN capsule tends to shrink, and a pressure will build up inside the BN vessel. The BN sheath also separates the residual nanowire from the N2 atmosphere, and prevents the inward diffusion of N atoms into the nanowire. The BN nanocapsule formed thus acts as a high pressure cell to promote the reaction 12B+N2 f B6N. Under these conditions, the residual core nanowires inside the BN shells reacts with N atoms to form B6Nx nanostructures with the assistance of high-pressure buildup in the nanocells. As a result, the heterostructured B6Nx/BN nanocables are formed. In this model, the isolated high-pressure BN cell and lower inward migration of nitrogen atoms are responsible for the high-pressure-induced formation of nanocrystalline B6Nx phase. The temperature also takes a critical role in our experiments. At a higher nitriding temperature of 1500 °C, all the pure boron precursors were transformed into BN nanotubes.37 The nitriding temperature of 1200 °C made for the formation of B6Nx/BN core-shell nanostructure, suggesting that B6Nx phase inside the BN nanocell is thermodynamically stable at this temperature. Banhart and Ajayan revealed that carbon onions can act as nanoscopic pressure cells for diamond nucleation and growth in the cores of carbon onions under high temperature and electron irradiation conditions.39 Golberg and coworkers demonstrated the formation of solid nitrogen inside the BN nanocages, a structural analogue of carbon onions, due to the existence of superhigh-pressure inside them.40 They also observed the high-pressure-induced phase transformation of hexagonal BN to cubic BN inside the BN high-pressure nanocells. Multiwalled carbon nanotubes as high-pressure cylinders and nanoextruders were also reported to deform, extrude, and break hard nanomaterials that are encapsulated inside the core.41 All of these experimental observations support our conclusion on the nanocell-high-pressureassisted formation mechanism of B6Nx/BN nanostructures in the nitriding reaction. We suggest that the BN nanocells in a similar process can be used as a template to study the pressure-induced phase transformation at the nanoscale as well as to synthesize other unique boron-based nanostructures that cannot be formed under normal conditions. In summary, we have demonstrated the large-scale synthesis of heterostructured B6Nx/BN nanocable and nanofeather nanojunctions using a simple nitriding approach. Though the properties of B6N are little reported, it is reasonable to expect that the B6N should share some of the very interesting physical and chemical properties with other boron-rich solids, for example, low density, high hardness, chemical inertness, unique mechanical, thermal, and electronic properties, etc. EELS studies on B6N indicated that it might be metallic.32 Hexagonal BN is a wide band gap semiconductor with excellent mechanical strength, good thermal conductivity, and strong corrosion resistance properties. It is known that the composites formed from two or more distinct materials normally possess the superb and desirable properties that are not found in the individual components. Therefore, the heterostructured B6Nx/ BN nanoscables and multiple nanojunctions described here may present the significant potentials for practical applications as functional elements in nanoelectronic and nanomechanical devices and as reinforcing elements in nanocomposites, especially working
Crystal Growth & Design, Vol. 8, No. 12, 2008 4353 at high temperature and environmentally corrosive conditions. Furthermore, our simple synthetic strategy may be used as a general approach for bulk-fabrication of a variety of intriguing boron-based nanocomposites and nanoheterostructures in B-C-N-O system, through rationally optimizing the synthetic conditions and utilizing the nanocell high-pressure effect. In this process, the precursor materials serve as the template to define and control the morphology, shape, size, chemical composition, and structural details of the products. It is feasible to prepare the patterned or predesigned arrays of the boron-based nanoarchitectures following the same approach if the well-aligned and patterned boron nanostructures are used as the starting precursors. We can expect the prospect of technological and commercial exploitations by further enhancing the power of this simple and effective strategy to introduce a large number of new functional boron-based nanostructures and nanoarchitectures.
Acknowledgment. We thank M. He and F. X. Zhang for helpful discussions. This research is supported by the Natural Science Foundation of China (Nos. 10374078 and 50525102).
References (1) Dong, Y. J.; Yu, G. H.; McAlpine, M. C.; Lu, W.; Lieber, C. M. Nano Lett. 2008, 8, 386. (2) Tian, B. Z.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Huang, J. L.; Lieber, C. M. Nature 2007, 449, 885. (3) Hu, Y.; Churchill, H. O. H.; Reilly, D. J.; Xiang, J.; Lieber, C. M.; Marcus, C. M. Nat. Nanotechnol. 2007, 2, 622. (4) Xiang, J.; Vidan, A.; Tinkham, M.; Westervelt, R. M.; Lieber, C. M. Nat. Nanotechnol. 2006, 1, 208. (5) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Nature 2006, 441, 489. (6) Lauhon, L. J.; Gudiksen, M. S.; Wang, D. L.; Lieber, C. M. Nature 2002, 420, 57. (7) He, J. H.; Zhang, Y. Y.; Liu, J.; Moore, D.; Bao, G.; Wang, Z. L. J. Phys. Chem. C 2007, 111, 12152. (8) Chueh, Y. L.; Hsieh, C. H.; Chang, M. T.; Chou, L. J.; Lao, C. S.; Song, J. H.; Gan, J. Y.; Wang, Z. L. AdV. Mater. 2007, 19, 143. (9) Chueh, Y. L.; Chou, L. J.; Wang, Z. L. Angew. Chem., Int. Ed. 2006, 45, 7773. (10) Wang, X. D.; Gao, P. X.; Li, J.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2002, 14, 1732. (11) Liu, C. B.; Li, R. W.; Belik, A.; Golberg, D.; Bando, Y.; Cheng, H. M. Appl. Phys. Lett. 2006, 88, 043105. (12) Li, Y. B.; Dorozhkin, P.; Bando, Y.; Golberg, D. AdV. Mater. 2005, 17, 545. (13) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Golberg, D. AdV. Mater. 2005, 17, 1964. (14) Tang, C. C.; Bando, Y.; Golberg, D.; Mitome, M.; Ding, X. X.; Qi, S. Appl. Phys. Lett. 2004, 85, 106. (15) Ma, R. Z.; Bando, Y. Chem. Phys. Lett. 2003, 367, 219. (16) Greene, L. E.; Law, M.; Yuhas, B. D.; Yang, P. D. J. Phys. Chem. C 2007, 111, 18451. (17) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. D. J. Phys. Chem. B 2006, 110, 22652. (18) Dong, A; Tang, R; Buhro, W. E. J. Am. Chem. Soc. 2007, 129, 12254. (19) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P. Nano Lett. 2007, 7, 2951. (20) McCann, J. T.; Li, D.; Xia, Y. J. Mater. Chem. 2005, 15, 735. (21) Shi, W. S.; Peng, H. Y.; Xu, L.; Wang, N.; Tang, Y. H.; Lee, S. T. AdV. Mater. 2000, 12, 1927. (22) Zhang, Y.; Suenaga, K.; Colliex, C.; Iijima, S. Science 1998, 281, 973. (23) Emin, D. Phys. Today 1987, 40, 55. (24) Greenwood, N. N. In ComprehensiVe Inorganic Chemistry; Bailar, J. C., Emele´us, H. J, Nyholm, R.; Trotman-Dickenson, A. F., Eds.; Pergamon: Oxford, 1973; Vol. 1, p 665. (25) Boron and Refractory Borides; Matkovich V. I., Ed.; Springer-Verlag: New York, 1977. (26) Nelmes, R. J.; Loveday, J. S.; Wilson, R. M.; Marshall, W. G.; Besson, J. M.; Klotz, S.; Hamel, G.; Aselage, T. L.; Hull, S. Phys. ReV. Lett. 1995, 74, 2268. (27) Eremets, M. I.; Struzhkin, V. V.; Mao, H. K.; Hemley, R. J. Science 2001, 293, 272.
4354 Crystal Growth & Design, Vol. 8, No. 12, 2008 (28) Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.; Akimitsu, J. Nature 2001, 410, 63. (29) Hubert, H.; Devouard, B.; Garvie, L. A. J.; O’Keeffe, M.; Buseck, P. R.; Petuskey, W. T.; McMillan, P. F. Nature 1998, 391, 376. (30) He, D. W.; Zhao, Y.; Daemen, L.; Qian, J.; Shen, T. D.; Zerda, T. W. Appl. Phys. Lett. 2002, 81, 643. (31) Hubert, H.; Garvie, L. A. J.; Buseck, P. R.; Petuskey, W. T.; McMillan, P. F. J. Solid State Chem. 1997, 133, 356. (32) Garvie, L. A. J.; Hubert, H.; Petuskey, W. T.; McMillan, P. F.; Buseck, P. R. J. Solid State Chem. 1997, 133, 365. (33) Cao, L. M.; Zhang, Z.; Sun, L. L.; Gao, C. X.; He, M.; Wang, Y. Q.; Li, Y. C.; Zhang, X. Y.; Li, G.; Zhang, J.; Wang, W. K. AdV. Mater. 2001, 13, 1701. (34) Cao, L. M.; Hahn, K.; Scheu, C.; Ruhle, M.; Wang, Y. Q.; Zhang, Z.; Gao, C. X.; Li, Y. C.; Zhang, X. Y.; He, M.; Sun, L. L.; Wang, W. K. Appl. Phys. Lett. 2002, 80, 4226.
Communications (35) Cao, L. M.; Hahn, K.; Wang, Y. Q.; Scheu, C.; Zhang, Z.; Gao, C. X.; Li, Y. C.; Zhang, X. Y.; Wang, W. K.; Ruhle, M. AdV. Mater. 2002, 14, 1294. (36) Cao, L. M.; Tian, H.; Zhang, Z.; Zhang, X. Y.; Gao, C. X.; Wang, W. K. Nanotechnology 2004, 15, 139. (37) Cao, L. M.; Zhang, X. Y.; Tian, H.; Zhang, Z.; Wang, W. K. Nanotechnology 2007, 18, 155605. (38) Condon, J. B.; Holcombe, C. E.; Johnson, D. H.; Steckel, L. M. Inorg. Chem. 1976, 15, 2173. (39) Banhart, F.; Ajayan, P. M. Natrure 1996, 382, 433. (40) Golberg, D.; Bando, Y.; Sato, T.; Grobert, N.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. J. Chem. Phys. 2002, 116, 8523. (41) Sun, L.; Banhart, F.; Krasheninnikov, A. V.; Rodrg´uez-Manzo, J. A.; Terrones, M.; Ajayan, P. M. Science 2006, 312, 1199.
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