Pergamon Carbon, Vol. 33, No. 7, pp. 903-914, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.6223/95 $9.50 + .OO 0008-6223(95)00019-4 CARBON NANOTUBES WITH SINGLE-LAYER WALLS zyxwvutsrqponmlkj CHING-HWA KIANG,‘,~ WILLIAM A. GODDARD III,2 ROBERT BEYERS,’ and DONALD S. BETHUNE ’ ‘IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099, U.S.A. *Materials and Molecular Simulation Center, Beckman Institute, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, U.S.A. (Received 1 November 1994; accepted 10 February 1995) Abstract-Macroscopic quantities of single-layer carbon nanotubes have recently been synthesized by co- condensing atomic carbon and iron group or lanthanide metal vapors in an inert gas atmosphere. The nano- tubes consist solely of carbon, sp*-bonded as in graphene strips rolled to form closed cylinders. The structure of the nanotubes has been studied using high-resolution transmission electron microscopy. Iron group catalysts, such as Co, Fe, and Ni, produce single-layer nanotubes with diameters typically between 1 and 2 nm and lengths on the order of micrometers. Groups of shorter nanotubes with similar diameters can grow radially from the surfaces of lanthanide carbide nanoparticles that condense from the gas phase. If the elements S, Bi, or Pb (which by themselves do not catalyze nanotube production) are used together with Co, the yield of nanotubes is greatly increased and tubules with diameters as large as 6 nm are pro- duced. Single-layer nanotubes are anticipated to have novel mechanical and electrical properties, includ- ing very high tensile strength and one-dimensional conductivity. Theoretical calculations indicate that the properties of single-layer tubes will depend sensitively on their detailed structure. Other novel structures, including metallic crystallites encapsulated in graphitic polyhedra, are produced under the conditions that lead to nanotube growth. Key Words-Carbon, nanotubes, fiber, cobalt, catalysis, fullerenes, TEM. 1. INTRODUCTION The discovery of carbon nanotubes by Iijima in 1991 [l] created much excitement and stimulated extensive re- search into the properties of nanometer-scale cylindri- cal carbon networks. These multilayered nanotubes were found in the cathode tip deposits that form when a DC arc is sustained between the graphite electrodes of a fullerene generator. They are typically composed of 2 to 50 concentric cylindrical shells, with outer di- ameters typically a few tens of nm and lengths on the order of pm. Each shell has the structure of a rolled up graphene sheet-with the sp* carbons forming a hexagonal lattice. Theoretical studies of nanotubes have predicted that they will have unusual mechani- cal, electrical, and magnetic properties of fundamen- tal scientific interest and possibly of technological importance. Potential applications for them as one- dimensional conductors, reinforcing fibers in super- strong carbon composite materials, and sorption material for gases such as hydrogen have been sug- gested. Much of the theoretical work has focussed on single-layer carbon tubules as model systems. Meth- ods to experimentally synthesize single-layer nanotubes were first discovered in 1993, when two groups inde- pendently found ways to produce them in macroscopic quantities[2,3]. These methods both involved co- vaporizing carbon and a transition metal catalyst and produced single-layer nanotubes approximately 1 nm in diameter and up to several microns long. In one case, Iijima and Ichihashi produced single-layer nanotubes by vaporizing graphite and Fe in an Ar/CH4 atmo- sphere. The tubes were found in the deposited soot[2]. Bethune et al., on the other hand, vaporized Co and graphite under helium buffer gas, and found single- layer nanotubes in both the soot and in web-like ma- terial attached to the chamber walls[3]. 2. SYNTHESIS OF SINGLE-LAYER CARBON NANOTUBES In a typical experiment to produce single-layer nanotubes, an electric arc is used to vaporize a hollow graphite anode packed with a mixture of metal or metal compound and graphite powder. Two families of metals have been tried most extensively to date: transition metals such as Fe, Co, Ni, and Cu, and lan- thanides, notably Cd, Nd, La, and Y. While these two metal groups both catalyze the formation of single- layer nanotubes, the results differ in significant ways. The iron group metals have been found to produce high yields of single-layer nanotubes in the gas phase, with length-to-diameter ratios as high as several thou- sand[2-1 I]. To date, no association between the nano- tubes and metal-containing particles has been clearly demonstrated. In contrast, the tubes formed with lan- thanide catalysts, such as Gd, Nd, and Y, are shorter and grow radially from the surface of lo-100 nm di- ameter particles of metal carbide[8,10,12-151, giving rise to what have been dubbed “sea urchin” parti- cles[ 121. These particles are generally found in the soot deposited on the chamber walls. Some other results fall in between or outside these main groups. In the case of nickel, in addition to long, straight nanotubes in the soot, shorter single-layer 903