Page 56 - Carbon Nanotubes

CARBON NANOTUBES WITH SINGLE-LAYER WALLS

CHING-HWA KIANG,^^' WILLIAM A. GODDARD 111: 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 91 125, 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 roup or lanthanide metal vapors in an inert gas atmosphere. The nano-
K
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 Bethune et al., on the other hand, vaporized Co and
graphite under helium buffer gas, and found single-
The discovery of carbon nanotubes by Iijima in 1991 [I]
created much excitement and stimulated extensive re- layer nanotubes in both the soot and in web-like ma-
search into the properties of nanometer-scale cylindri- terial attached to the chamber walls[3].
cal carbon networks. These multilayered nanotubes
were found in the cathode tip deposits that form when 2. SYNTHESIS OF SINGLELAYER
a DC arc is sustained between the graphite electrodes CARBON NANOTUBES
of a fullerene generator. They are typically composed
of 2 to 50 concentric cylindrical shells, with outer di- In a typical experiment to produce single-layer
ameters typically a few tens of nm and lengths on the nanotubes, an electric arc is used to vaporize a hollow
order of pm. Each shell has the structure of a rolled graphite anode packed with a mixture of metal or
up graphene sheet-with the sp2 carbons forming a metal compound and graphite powder. Two families
hexagonal lattice. Theoretical studies of nanotubes of metals have been tried most extensively to date:
have predicted that they will have unusual mechani- transition metals such as Fe, Co, Ni, and Cu, and lan-
cal, electrical, and magnetic properties of fundamen- thanides, notably Gd, Nd, La, and Y. While these two
tal scientific interest and possibly of technological metal groups both catalyze the formation of single-
importance. Potential applications for them as one- layer nanotubes, the results differ in significant ways.
dimensional conductors, reinforcing fibers in super- The iron group metals have been found to produce
strong carbon composite materials, and sorption high yields of single-layer nanotubes in the gas phase,
material for gases such as hydrogen have been sug- with length-to-diameter ratios as high as several thou-
gested. Much of the theoretical work has focussed on sand[2-11]. To date, no association between the nano-
single-layer carbon tubules as model systems. Meth- tubes and metal-containing particles has been clearly
ods to experimentally synthesize single-layer nanotubes demonstrated. In contrast, the tubes formed with lan-
were first discovered in 1993, when two groups inde- thanide catalysts, such as Gd, Nd, and Y, are shorter
pendently found ways to produce them in macroscopic and grow radially from the surface of 10-100 nm di-
quantities[2,3]. These methods both involved co- ameter particles of metal carbide[8,10,12-15], giving
vaporizing carbon and a transition metal catalyst and rise to what have been dubbed “sea urchin” parti-
produced single-layer nanotubes approximately 1 nm cles[ 121. These particles are generally found in the soot
in diameter and up to several microns long. In one case, deposited on the chamber walls.
Iijima and Ichihashi produced single-layer nanotubes Some other results fall in between or outside these
by vaporizing graphite and Fe in an Ar/CH4 atmo- main groups. In the case of nickel, in addition to long,
sphere. The tubes were found in the deposited soot[2]. straight nanotubes in the soot, shorter single-layer
47