Page 132 - Carbon Nanotubes

122 J.-P. ISSI et ul.

Fig. 1. Topographic STM scan of a bundle of nanotubes. STS data were collected at points (I), (2), and
(3), on tubes with diameters of 8.7, 4.0, and 1.7 nm, respectively. The diameters were determined from
image cross-sections of the variations in height in a direction perpendicular to the tubes (adapted from
Olk et u/.[ll]).

individual multilayer tubules with diameters ranging bules (trace 2 for a tubule with diameter = 4.0 nm, and
from 2 to 10 nm, prepared by the standard carbon-arc trace 3 for a tubule with diameter = 1.7 nm) show pla-
technique, were examined. STM measurements were teaus in the I/V characteristics at zero current. This
taken first, and a topographic STM scan of a bundle rectifying behavior is the signature of semiconducting
of nanotubes is shown in Fig. 1. The exponential re- tubules. The dI/dV plot in the inset crudely mimics a
lation between the tunneling current and the tip-to- 1D density of states, the peaks in the dI/dV plot be-
tubule distance was experimentally verified to confirm
that the tunneling measurements pertain to the tubule
and not to contamination on the tubule surface. From
these relations, barrier heights were measured to estab-
lish the range in which the current-voltage character-
istics can be taken for further STS studies. The image
in Fig. 1 is used to determine the diameter of the in-
dividual tubule on which the STS scans are carried
out. During brief interruptions in the STM scans, the
instrument was rapidly switched to the STS mode of
operation, and I-V plots were made on the same re-
gion of the same tubule as was characterized for its di-
ameter by the STM measurement. The I-V plots for
three typical tubules, identified (1)-(3), are shown in
Fig. 2. The regions (1)-(3) correspond to interruptions
in the STM scans at the locations identified by crosses
in the topographic scan, Fig. 1. Although acquisition
of spectroscopic data in air can be complicated by
contamination-mediated effects on the tunneling gap,
several studies on a wide variety of surfaces have been
reported[l2]. Trace (1) in Fig. 2, taken on a tube with Fig. 2. Current-voltage characteristics taken at points (l), (2),
and (3) in Fig. 3. The top insert shows the conductance versus
8.7 nm diameter, has an ohmic behavior, providing ev- voltage plot, for the data taken at point (3) (adapted from
idence for the metallic nature of that tubule. Two tu- Olk et ul.[ll]).