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CHARLES M.LIEBER, ERIC W.WONG, HONGJIE DAI, BENJAMIN W. MAYNOR AND LUKE D.BURNS Department of Chemistry and Division of Applied Sciences Harvard University, Cambridge, MA 02138 ABSTRACT

Recent research on the growth and structure of carbide nanorods is reviewed. Carbide nanorods have been prepared by reacting carbon nanotubes with volatile transition metal and main group oxides and halides. Using this approach it has been possible to obtain solid carbide nanorods of TiC, SiC, NbC, Fe3C, and BCx having diameters between 2 and 30 nm and lengths up to 20 rim. Structural studies of single crystal TiC nanorods obtained through reactions of TiO with carbon nanotubes show that the nanorods grow along both [110] and [111] directions, and that the rods can exhibit either smooth or saw-tooth morphologies. Crystalline SiC nanorods have been produced from reactions of carbon nanotubes with SiO and Si-iodine reactants. The preferred growth direction of these nanorods is [111], although at low reaction temperatures rods with [100] growth axes are also observed. The growth mechanisms leading to these novel nanomaterials have also been addressed. Temperature dependent growth studies of TiC nanorods produced using a Ti-iodine reactant have provided definitive proof for a template or topotactic growth mechanism, and furthermore, have yielded new TiC nanotube materials. Investigations of the growth of SiC nanorods show that in some cases a catalytic mechanism may also be operable. Future research directions and applications of these new carbide nanorod materials are discussed.

INTRODUCTION

There has been significant interest and speculation about the properties and potential applications of carbon nanotubes filled with other materials [1-4], for example, to form quantum wires and nanocomposites. Approaches used to prepare filled nanotubes have included in-situ arc growth using metal/carbon composites [2], and the loading of nanotubes using liquid reagents [3,4]. These methods have not yet, however, led to the preparation of significant quantities of pure, filled nanotubes, and thus direct tests of the possibly unique properties of these highly anisotropic, nanometer scale materials have not been possible. 103

Mat. Res. Soc. Symp. Proc. Vol. 410 01996 Materials Research Society

Recently, we have reported an alternative approach to the preparation of anisotropic nanoscale materials with the growth of carbide nanorods of the transition metal and main group elements [5]. This work demonstrated that chemically and physically distinct carbide nanoscale rods could be obtained via the reaction of carbon nanotubes with volatile transition metal and main group oxide or halide species (Fig. 1).

Carbn anot ube

+MO

+MXn

Metal Carbide Nanorod + CO

Metal Carbide Nanorod + n/2X 2

Figure 1. Schematic diagram of the reactions used to prepare carbide nanorods. MO corresponds to a volatile metal or main group oxide and MX. corresponds to a volatile metal or main group halide complex. By using pure carbon nanotube reactants we have been able