Feb 21 2010
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Hexagonal boron nitride (h-BN) is a layered material with a graphite-type structure in which planar networks of BN hexagons are regularly stacked. As a structural analogue of carbon nanotubes (CNTs) a BN nanotube (BNNT) was firstly predicted in 19941 and synthesized the next year.2 Since then, it has become one of the most intriguing non-carbon nanostructures.3
Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a bandgap of ca. 5.8 eV, basically independent of tube geometry. BNNTs possess superb chemical and thermal stability, excellent mechanical properties, and high thermal conductivity. Such unique features make these tubes promising in a variety of fields such as nanodevices, functional composites, hydrogen accumulators, etc.
In addition, recent breakthroughs in studying a new C form - graphene - a single layer of graphite, have brought to the research forefront the questions of existence and stability of its BN analog - a BN nanosheet, Figure 1.
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Figure 1. Structural models of a BN monoatomic sheet and a single-shelled BN nanotube. Alternating B and N atoms are shown in blue and red.
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Recent Advances
Significant progress has been achieved within our group at NIMS regarding the synthesis, structural analysis and property measurements of multi-walled BNNTs and nanosheets. The nanotubes are currently synthesized in gram level quantities in a single run, Figure 2.4 The BN nanosheets were also prepared at a high yield under hBN ultrasonication.5
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Figure 2. (a) Synthesis of multi-walled BN nanotubes using MgO, B and FeO precursors in a vertical induction furnace at ~1500°C;4 (b) Photo images of a BNNT product; (c, d) SEM and TEM images; (e) Histograms of the external tube diameter distribution.
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Electrical and mechanical properties of the tubes/sheets were studied in a transmission electron microscope (TEM) and an atomic force microscope (AFM). The Young's modulus of nanotubes was measured as high as ~0.8 TPa, Figure 3,6 whereas the bending modulus of BN nanosheets reached ~30 GPa, Figure 4.7
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Figure 3.(a,b) Images of a multi-walled BN nanotube under its bending and reloading in TEM. The tube fully restores its original shape due to its superb elasticity and flexibility. The insets in (a,b) show the same tube at a lower-magnification; c) A sketch showing a designed deformation experiment inside TEM using a Si AFM cantilever. The force-displacement curve is recorded in-tandem with TEM imaging.6
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Figure 4. (a) AFM topography image of a Ti/Au contacts-clamped BN nanosheet placed under the trench of a Si/SiO2 substrate; and (b) The measured bending modulus of BN nanosheets as a function of their dimensions.7
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The performance of BNNTs in a high electrical bias regime was also analyzed.8,9 Due to the ioinicity of a B-N bond, the BN tube decomposition temperature was found to be proportional to an applied electrical field, Figure 5.
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Figure 5. a) An individual multi-walled BN nanotube placed between two metal contacts in STM-TEM and biased to ~140 V.8 The appearance of amorphous B-balls (arrowed) under tube chemical decomposition is apparent. B and N elemental maps of a decomposed tube are shown in the insets; b) Color-coded map of kinetic energy of atoms in a BN nanotube placed in an electrical field at certain temperature.9 Experimental data (circles) spread around an equipotential line of 0.165 eV. The line hits 1910 K (normal temperature of BN thermal decomposition) at a zero electrical field.
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In summary, it should be emphasized that the rich application potentials of BN nanotubes and nanosheets, that have long been in the shadow of their popular C counterparts, have largely been underestimated and deserve more detailed elucidations.
References
1. A. Rubio, J.L. Corkill, M.L. Cohen, "Theory of graphitic boron nitride nanotube", Phys. Rev. B 49, 5081 (1994).
2. N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, "Boron nitride nanotubes", Science 269, 966 (1995).
3. D. Golberg , Y. Bando, C.C. Tang, C.Y. Zhi, "Boron nitride nanotubes", Adv. Mater. 19, 2413 (2007).
4. C.Y. Zhi, Y. Bando, C.C. Tang, D. Golberg, "Effective precursor for high yield synthesis of pure BN nanotubes", Sol. St Comm. 135, 67 (2005).
5. C.Y. Zhi, Y. Bando, C.C. Tang, H. Kuwahara, D. Golberg, "Large-scale fabrication of few-atomic-layer boron nitride nanosheets and their utilization in polymeric composites with improved thermal and mechanical properties", Adv. Mater. 21, 2889 (2009).
6. D. Golberg D., P.M.F.J Costa, O. Lourie, M. Mitome, C.C. Tang, C.Y. Zhi, K. Kurashima, Y. Bando, "Direct force measurements and kinking under elastic deformation of individual multiwalled boron nitride nanotubes", Nano Lett.7, 2146 (2007).
7. C. Li, Y. Bando, C.Y. Zhi, Y. Huang, D. Golberg, "Thickness-dependent bending modulus of hexagonal boron nitride nanosheets", Nanotechnology 20, 385707 (2009).
8. Z. Xu, D. Golberg, Y. Bando, "In-situ TEM-STM recorded kinetics of boron nitride nanotube failure under current flow", Nano Lett. 9, 2251 (2009).
9. Z. Xu, D. Golberg, Y. Bando, "Electrical field-assisted thermal decomposition of boron nitride nanotube: experiments and first principle calculations", Chem. Phys. Lett. 480, 110 (2009).
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