Novel nanotubes and encapsulated nanowires
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Applied Physics A Materials Science & Processing Springer-Verlag 1998
Novel nanotubes and encapsulated nanowires M. Terrones1 , W.K. Hsu1 , A. Schilder2 , H. Terrones3 , N. Grobert1 , J.P. Hare1 , Y.Q. Zhu1 , M. Schwoerer2 , K. Prassides1 , H.W. Kroto1 , D.R.M. Walton1 1 School of Chemistry, Physics and Environmental Science, University of Sussex Brighton, BN1 9QJ, UK 2 Lehrstuhl für Experimentalphysik II, Universität Bayreuth and Bayreuther Institut für Makromolekülforschung 3 Instituto de F´ısica, UNAM, Apartado Postal 20-364, 01000 M´ exico, D. F.
(BIMF), D-95440 Bayreuth, Germany
Received: 31 July 1997/Accepted: 6 October 1997
Abstract. Carbon nanotubes, with or without encapsulated material, generated by arc discharge and electrolytic techniques have been studied. Microcrystals of refractory carbides (i.e. NbC, TaC, MoC), contained in nanotubes and polyhedral particles, produced by arcing electrodes of graphite/metal mixtures, were analysed by high hesolution transmission electron microscopy (HRTEM) and X-ray powder diffraction. Encapsulation of MoC was found to give rise to an unusual stable form, namely face-centered-cubic MoC. SQUID measurements indicate that the encapsulated carbides exhibit superconducting transitions at about 10–12 K, thus they differ from carbon nanotubes/nanoparticles which do not superconduct. Four-probe and microwave (contactless) conductivity measurements indicate that most of the analysed samples behave as semiconductors. However, metallic transport was observed in specimens containing single conglomerated carbon nanotube bundles and boron-doped carbon nanotubes. Novel metallic β-Sn nanowires were produced by electrolysis of graphite electrodes immersed in molten LiCl/SnCl2 mixtures. Prolonged electron irradiation of these nanowires leads to axial growth and to dynamic transformations. These observations suggest ways in which materials may be modified by microencapsulation and irradiation. PACS: 61.48.+c; 73.61.Wp
Soon after carbon nanotubes were prepared in bulk by the arc discharge technique [1], the suggestion was made that it might be possible to encapsulate metals within the inner core of the nanotubes. Such a proposal followed logically from the successful preparation of fullerenes containing endohedral metals [2, 3]. Elements such as Pb, Bi, Cs, S, and Se were first introduced by heating the elements with openended nanotubes [4–6] (sometimes in the presence of oxygen). Capillarity and wetting plays an important role in the process. Generally, only low-surface-tension substances can be introduced into nanotubes [6] in this way. In subsequent experiments, a graphite anode was drilled and packed with a metal. Arcing then led to the formation at
the cathode of nanotubes and polyhedral particles, containing both encapsulated metals and carbides [7–12]. A problem arises with this technique in that the nanotubes are only partly filled, a fact which may be disadvantageous if nanoscale applications (e.g. conduction) are to be considered. Recently, Loisseau and Pascard described
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