Page 123 - Carbon Nanotubes

Properties of buckytubes and derivatives 113
multilayer buckytubes. Because the insertion of pent-
agons and heptagons produced positive and negative
curvature of the hexagonal network, respectively, the
complicated morphologies of single-shell buckytubes
were probably a result of the insertion of pentagons,
heptagons, or various combinations into the single-
shell buckytube. We believe that it is much easier to in-
corporate pentagonal and heptagonal defects in these
single-shell structures than in multilayer buckytubes.

3.2 Magnetic properties
In the first paper reporting the observation of c60,
Kroto et al. suggested that the molecule was aromatic-
like with its inner and outer surfaces covered with a
“sea” of a-electrons[ 181. Accordingly, Cm should ex-
hibit a large diamagnetic susceptibility associated with
a a-electron ring current. This suggestion appeared to
be supported by NMR chemical shift measurements
[ 19,231. Theoretical calculations of the magnetic sus-
ceptibility of c60 performed by various groups were
not consistent; some authors predicted a vanishingly
small diamagnetism[24-261 and others predicted strong
diamagnetism (as in an aromatic system)[27,28]. Sus-
ceptibility measurements on C60 and C,o showed that
the value for C,,, was about twice that of c60, but
that both were very sma11[25,29]. Another theoretical
calculation by Ajiki & Ando[30] showed that the cal-
culated magnetic susceptibility for the magnetic field
perpendicular to the carbon nanotube axis was about
three orders of magnitude larger than the parallel
value. It is, then, of interest to extend the measure-
ments to buckytubes.
With the availability of bulk samples of carbon
nanotubes (bundles of buckytubes), a systematic study
of the magnetic properties became possible. A cylin-
drical bulk deposit, with a diameter of 10 mm, con-
sisting of an inner core and an outer cladding, was
Fig. 2. Two HREM images of frozen growth of buckytubes formed on the graphite cathode during the arc process.
seen end-on.
The outer cladding, a gray shell, was composed mainly
of amorphous carbon and buckydoughnuts[3 11. The
inner core, with a diameter of 8 mm, consisted of an
single-crystal growth through a screw dislocation array of rather evenly spaced, parallel, and closely
mechanism[2]. packed bundles approximately 50 micrometers in di-
3.1.3 The structure of single-shell bucky- ameter. HREM revealed that the buckybundles in our
tubes. Single-shell buckytubes were first synthesized best sample were comprised of buckytubes running
by Iijima and Ichihashi[20] and Bethune et al. [21]. parallel to one another[32]. A buckytube in each bun-
Unlike their reports, where the single-shell buckytubes dle could be pictured as a rolled-up graphitic sheet
were found on the chamber wall, we found a large with a diameter ranging from 8 A to 300 A and a
number of monolayer buckytubes in a deposited rod length of a few microns. The tube was capped by sur-
which built up on the cathode of the dc arc[22]. The faces involving 6 pentagons (per layer) on each
synthesis of the single-shell buckytubes used in this end[l,2]. The purity of the bulk sample was examined
study was similar to that described in the experimen- by energy dispersive X-ray analysis. No elements
tal section. However, instead of using a monolithic heavier than carbon were observed. The C60 powder
graphite anode, we utilized a composite anode consist- was extracted from the soot formed in the same cham-
ing of copper rods inserted inside a graphite anode in ber employed for production of the buckytubes. The
a variety of ways. The single-shell buckytubes assumed purity of the c60 powder used in this study was bet-
a large variety of shapes (shown in Fig. 4), such as ter than 99% as examined by high-performance liq-
buckytents, buckydomes, and giant fullerenes, and uid chromatography.
much shorter tubes than in previous reports. Protru- The measured mass susceptibility values for bucky-
sions, indentations, and bending were quite common, bundle (both x: and xi), C6,,, the gray-shell mate-
unlike the faceted or smooth-faceted morphologies of rial, the polycrystalline graphite anode, and the