Page 172 - Carbon Nanotubes

ONION-LIKE GRAPHITIC PARTICLES

D. UGARTE
Laboratorio National de Luz Sincrotron (CNPq/MCT), Cx. Postal 6192,
13081-970 Campinas SP, Brazil;
Institut de Physique Experimentale, Ecole Polytechnique FCdCrale de Lausanne,
1015 Lausanne, Switzerland

(Received 18 July 1994; accepted 10 February 1995)
Abstract-Nanometric graphitic structures (fullerenes, nanotubes, bucky-onions, etc.) form in different
harsh environments (electric arc, electron irradiation, plasma torch). In particular, the onion-like graphitic
particles may display a wide range of structures, going from polyhedral to nearly spherical. High-resolution
electron microscopy is the primary tool for studying these systems. On the basis of HREM observations,
we discuss the energetics and possible formation mechanism of these multi-shell fullerenes. The better un-
derstanding of the underlying processes would allow the development of an efficient production method.
Key Words-Graphite, fullerenes, HREM, nanostructures, electron irradiation.

1. INTRODUCTION large-scale synthesis of nanotubes[4]. The central part
of the deposit consists of a black powder containing
The high melting temperature of carbon materials a mixture of graphitic nanotubes and nanoparticles.
(=4000”K) has made difficult the preparation and Although this procedure allows the generation of these
study of carbon clusters. In recent years, the field of particles in macroscopic quantities, the purification of
nanometric carbon particles has found an unexpected the different soot components (e.g., nanotubes) has
and overwhelming development. In a first step, the so- not been easily performed. Centrifugation and filtra-
phisticated laser evaporation source revealed the ex- tion methods have been unsuccessful, and a rather in-
istence of fullerenes, but with the limitation that their efficient oxidation procedure (99% of the material is
study could only be performed in cluster-beam exper- lost) has allowed the production of purified nanotubes
iments[l]. Second, in 1990, the current and simple
, samples[l2].
electric arc-discharge[2] allowed the synthesis and The electric arc is a transient phenomenon, where
study of these molecules by a large number of labo- the region of the electrode producing the arc changes
ratories, generating a burst of revolutionary discov- permanently all over the surface of contact. The gen-
eries. A wide family of nanometric graphitic systems erated temperature gradients induce an important
may be synthesized by making slight modifications to range of conditions for the formation of graphitic
the electric arc experiment (nanotubes[3,4], nanopar- nanoparticles; this fact leads to wide size and shape
ticles[5-7], metal-filled nanoparticles[8-10], etc.). distributions (see Fig. la). The particles usually display
High-resolution transmission electron microscopy a clear polyhedral morphology, and a large inner empty
(HREM) is the technique best suited for the structural space (3-10 nm in diameter). Macroscopic quantities
characterization of nanometer-sized graphitic parti- of nanometric graphitic particles may be obtained by
cles. In-situ processing of fullerene-related structures a thermal treatment of “fullerene black”[ 13,141; this
may be performed, and it has been shown that carbo- method yields particles with similar structure to those
naceous materials transform themseIves into quasi- ones generated in the arc, but with a narrower size dis-
spherical onion-like graphitic particles under the effect tribution, in particular located in the 510 nm range.
of intense electron irradiation[ll]. The high-energy electron irradiation of carbona-
in this paper, we analyze the methods of synthesiz- ceous materials produces remarkably symmetrical and
ing multi-shell fullerene structures and try to gather spherical onion-like particles[ll] (see Fig. lb, 2). These
some information about their formation mechanism. particles are very stable under electron bombardment,
We also discuss some particularities of the energetics even when formed by a small number of shells (2-4)
of onion-like graphitic particles. The understanding of [15]. The generation of these quasi-spherical graphitic
the parameters involved would allow the development systems (nicknamed bucky-onions) is realized in situ in
of efficient production procedures. an electron microscope. However, the particles are only
formed in minute quantities on an electron microscopy
grid, and their study may only be performed by trans-
2. SYNTHESIS OF MULTI-SHELL FULLERENES
mission electron microscopy-associated techniques.
The electric arc is the easiest and most frequently Two major structural aspects differentiate them from
used experiment to produce onion-like particles. A the graphitic particles discussed previously: (a) the
dc arc-discharge is used to generate a carbon deposit shape of the concentrical arrangement of graphitic lay-
on the negative electrode following the procedure for ers is a nearly perfect sphere, (b) the innermost shell
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