Page 159 - Carbon Nanotubes

Flexibility of graphene layers in carbon nanotubes

J.F. DESPRE~ and E. DAG-
Laboratoire Marcel Mathieu, 2, avenue du President Pierre Angot
64OOO Pau, France

K. LAFDI
Materials Technology Center, Southern Illinois University at Carbondale,
Carbondale, IL 629014303
(Received 16 September 1994; accepted in revised form 9 November 1994)
Key Words - Buckeytubes; nanotubes; graphene layers

The Kratschmer-Huffman technique [ 11 has been widely formation requires a two-atom addition to the graphitic
used to synthesize fullerenes. In this technique, graphite sheet while a pentagon formation requires only one.
rods serve as electrodes in the production of a Pentagon formation may be explained by a temporary
continuous dc electric arc discharge within an inert reduction in carbon during current fluctuations of the arc
environment. When the arc is present, carbon discharge. More complex defaults (beyond isolated
evaporates from the anode and a carbon slag is deposited pentagons and hexagons) may be possible. Macroscopic
on the cathode. In 1991, Ijima et al. [2] examined models have been constructed by Conard et al. [5] to
samples of this slag. They observed a new form of determine the angles that would be created by such
carbon which has a tubular structure. These structures, defaults.
called nanotubes, are empty tubes made of perfectly To construct a nanotube growth theory, a new
coaxial graphite sheets and generally have closed ends. approach, including some new properties of nanotubes,
The number of sheets may vary from a single sheet to as must be taken. The purpose of this work is to present
many as one hundred sheets. The tube length can also graphene layer flexibility as a new property of graphitic
vary; and the diameters can be several nanometers. The materials. In previous work, the TEM characterization
tube ends are either spherical or polyhedral. The of nanotubes consists of preparing the sample by
smallest nanotube ever observed consisted of a single dispersing the particles in alcohol (ultrasonic
graphite sheet with a 0.75 nm diameter [2]. preparation). When the particles are dispersed in this
Electron diffraction studies [3] have revealed that manner, individual nanotubes are observed in a stress-
hexagons within the sheets are helically wrapped along free state, i.e. without the stresses that would be present
the axis of the nanotubes. The interlayer spacing due to other particles in an agglomeration. If one
between sheets is 0.34 nm which is slightly larger than carefully prepares a sample without using the dispersion
that of graphite (0.3354 nm). It was also reported [2] technique, we expect that a larger variety of
that the helicity aspect may vary from one nanotube to configurations may be observed.
another. Ijima et al. [2] also reported that in addition to Several carbon shapes are presented in Figure 1
nanotubes, polyhedral particles consisting of concentric in which the sample has been prepared without using
carbon sheets were also observed. ultrasonic preparation. In this figure, there are three
An important question relating to the structure of polyhedral entities (in which the two largest entities
nanotubes is: Are nanotubes made of embedded closed belong to the same family) and a nanotube. The bending
tubes, like "Russian dolls," or are they composed of a of the tube occurs over a length of several hundred
single graphene layer which is spirally wound, like a roll nanometers and results in a 60" directional change.
of paper? Ijima et al. [2] espouse the "Russian doll" Also, the general condition of the tube walls has been
model based on TEM work which shows that the same modified by local buckling, particularly in compressed
number of sheets appear on each side of the central areas. Figure 2 is a magnification of this compressed
channel. Dravid et al. [4], however, support a "paper area A contrast intensification in the tensile area near the
roll" structural model for nanotubes. compression can be observed in this unmodified
Determination of the structure of nanotubes is photograph. The inset in Figure 2 is a drawing which
crucial for two reasons: (1) to aid understanding the illustrates the compression of a plastic tube. If the tube
nanotube growth mechanism and (2) to anticipate is initially straight, buckling occurs on the concave side
whether intercalation can occur. Of the two models, of the nanotubes as it is bent. As shown in Figure 3,
only the pper roll structure can be intercalated. this fact is related to the degree of curvature of the
The closure of the graphite sheets can be nanotube at a given location. Buckling is not observed
explained by the substitution of pentagons for hexagons in areas where the radius of curvature is large, but a large
in the nanotube sheets. Six pentagons are necessary to degree of buckling is observed in severely bent regions.
close a tube (and Euler's Rule is not violated). Hexagon These TEM photographs are interpreted as
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