Page 122 - Carbon Nanotubes

112 X. K. WANG et al.
was subtracted from the data. The absolute accuracy
of the mass susceptibility relative to the “standard”
value of the graphite crystal was about 1070.

2.4 Transport property measurements
The transport properties were measured using stan-
dard dc (for the Hall effect) and ac (for MR) four-
terminal techniques. The instrument used in this study
was the same one we employed for the measurements
of magnetic properties. The contact configuration is
shown schematically by the inset in Fig. 6 (a). The I-V
characteristic was measured to ensure Ohmic behav-
ior so that no hot electron effects were present. The
magnetic field was applied perpendicular to the tube
axis. The measured MR and apparent Hall coefficient
showed essentially the same temperature and field de-
pendence, regardless of the samples used and the dis-
tance between the potential contacts, implying that the
samples were homogeneous. For example, the resid-
ual resistivity ratio, R(300 K)/R(5 K), measured on
different single buckybundles agreed with each other
within 1 %. In what follows we present the data taken
on a single buckybundle having a diameter of 60 pm,
the distance between the two potential contacts being
350 pm.
Fig. 1. (a) A cross-sectional TEM image of a bundle of
buckytubes; (b) an HREM image of a single bundle of bucky-
tubes with their axes parallel to the bundle axis.
3. RESULTS AND DISCUSSIONS
3.1 Structural properties
Buckytubes were observed for the first time by has been suggested[l7] that the growth pattern, as well
HREM[ 1,2] and their structural properties were sub- as many properties of buckytubes, are intimately re-
sequently characterized. In this section, we will briefly lated to their helicacy. Here, we present the visible ob-
describe observations of the structure of a bundle of servations of frozen growth stage of buckytubes and
buckytubes, evidence for a helical growth of bucky- derivatives suggesting a helical growth mechanism.
tubes and their derivatives, and the single-shell Figure 2 shows two HREM images of buckytubes
structures. seen end-on (i.e., the axis of the tube being parallel
3.1.1 The structure of buckybundles. Both to the electron beam). The hollow center region is ap-
cross-sectional and high-resolution electron micros- parent, indicating the obvious tubular nature of the
copy images of a single bundle are shown in Fig. 1 (a) tubes. Strain contrast was evident in all these images,
(end-on-view) and 1 (b) (side view of a single bundle). which is reminiscent of disclination type defects[l8, 191.
The end-on view shows that the tubes are composed If we follow the individual inner-shell graphitic sheets
of concentric graphitic sheets. The spacing between the around, shown in Fig. 2 (a), we observe that the ter-
adjacent graphitic sheets is about 0.34 nm. The thin- mination is incomplete; that is, one extra graphitic
nest tube in this specimen, consisting of 8 carbon- sheet is associated with one portion of the inner shell
hexagon sheets, has an outside diameter of 8 nm. The compared with its opposite side. Figure 2 (b) shows
largest one, consisting of 48 sheets, has an outside di- that six graphitic sheets are seen to wrap around a
ameter of about 30 nm. It is worth noting that al- thicker buckytube. In other words, the tubes are more
though the tubes have a wide range of diameters, they of a rolled carpet geometry rather than the Russian
tend to be packed closely together. The side view of Doll-type structure in our sample.
the sing16 bundle directly reveals that the bundle con- We also observed that the rounded particulates in
sists of closely packed buckytubes running parallel to the transition region between the “black ring” and the
one another, these images clearly demonstrate that the outer shell of the deposited rod are a collection of
bundle is actually a bundle of buckytubes. Since the completely closed graphitic sheets with a helical pat-
valence requirements of all atoms in a buckytube (with tern of inner shells. Figure 3 shows a larger buckyfoot-
two sealed ends) are satisfied, the interaction among ball containing smaller inner footballs that seem to
buckytubes should be Van der Waals in nature. There- grow inside the larger one. The inner footballs clearly
fore, it is energetically favorable for buckytubes packed display extra unterminated graphitic sheets, indicative
closely together to form a “buckybundle.” of helical growth. These observations strongly suggest
3.1.2 The helicacy of buckytubes. The heli- that Fig. 3 represents buckyfootballs that form through
cacy of buckytubes is an interesting phenomenon. It a helical growth of the sheets analogous to that of