Page 124 - Carbon Nanotubes

I14 X. K. WANC et al.

_- .

I.
..
. .
5'n m

.. , .

Fig. 3. Three small buckyfootballs appear to grow inside a large buckyfootball.

graphite crystal are shown in Table 1. The measured emu/g, which is near the literature value (implying the
mass susceptibility of -0.35 x emu/g for c60 is behaviors of the remtiining materials do not involve
consistent with the literature value[29], and is 30 times impurities arising from the source material). The mea-
smaller than that of buckytubes. The c60 powder sured susceptibility of the gray-shell material, which
shows the strongest magnetic field dependence of the consists of amorphous carbon mixed with fragments
susceptibility and does not exhibit diamagnetism un- of buckytubes and buckydoughnuts, is close to (but
til H is greater than 0.5 T; saturation is observed for larger than) that of the source rod.
fields greater than 3 T. The c60 results, involving a The measured magnetic susceptibility of multilayer
very small diamagnetic susceptibility and a strong buckytubes for xkis approximately half x; of graph-
magnetic field dependence, appear to support the ite. This can be interpreted as follows. We recall that
Elser-Haddon result where a cancellation occurs be- crystalline graphite is a semimetal with a small band
tween the diamagnetic and paramagnetic contribu- overlap and a low density of carriers (lO-l8/cm3)[33,
tions[26]. The small measured susceptibility of C, sug- 341; the in-plane effective mass is small (m * - & mo).
gests that if it (or possibly other fullerenes) is present The bending of the graphite planes necessary to form
as a contaminant in the buckybundle matrix (which is a buckytube changes the band parameters. The rele-
likely at some level) its contribution will be small. vant dimensionless parameter is the ratio a/R, where
From Table 1, we see that the measured suscepti- a (=3.4 A) is the lattice constant and R is the bucky-
bility of the polycrystalline graphite anode (used to tube radius. For R = 20 A, the shift is expected to
produce the fullerenes measured here) is 6.50 x alter the nature of the conductivity[l3-161. In our
buckybundle samples, most of material involves
buckytubes with R > 100 A confirmed by statistical
analysis of TEM data, and we assume that the elec-

Table 1. Measured room-temperature susceptibilities

SusceDtibilitv
Material Symbol x emu/g
Buckybundle: axis parallel
to H XB I -10.75
Buckybundle: axis
perpendicular to H -9.60

Gray-shell material - -7.60
Graphite: c-axis parallel to H xk -21.10
Graphite: c-axis perpendicular
to H Xc' -0.50
Fig. 4. Various morphologies of single-shell buckytubes.