Ideally, carbon nanotubes are unique, low-dimensional conductors
with either metallic or semiconducting properties. In practice, however,
these hollow cylinders have a strong tendency to agglomerate as they form,
resulting in either large, multi-walled nanotubes (MWNTs) with many concentric
carbon shells or else bundles, or "ropes," of aligned single-walled nanotubes
(SWNTs). Both aggregates are complex composite conductors incorporating
many weakly-coupled nanotubes, each having a different electronic structure.
This complexity remains the primary difficulty in both understanding and
developing nanotube-based electronic devices.
We have demonstrated a simple and reliable method for tailoring the
properties of these bundled nanotubes (1). The method uses selective oxidation
to remove single carbon shells from either MWNTs or SWNT bundles (2), and
it allows us, for the first time, to take advantage of the full complexity
of these conductors. For instance, we can step through and individually
characterize the concentric shells of a MWNT. By choosing from among the
different shells, we convert adjacent segments of a MWNT into either metallic
or semiconducting conductors by design. By comparing the transport behavior
of a MWNT before and after the removal of single carbon shells, we can
also quantitatively address each shell's contribution and the issue of
With SWNT ropes, similar selectivity allows us separate semiconducting
and metallic carbon shells. Using this technique, we have produced entire
arrays of high performance, nanoscale field-effect transistors (FETs) based
solely on the fraction of semiconducting SWNTs. These procedures sidestep
the need for selective nanotube synthesis and allow unprecedented flexibility
in the creation of nanoscale electronic devices, both for their study and
for practical applications.
P.G. Collins, M. Arnold, and Ph. Avouris, Science292 (2001) p. 706.
P.G. Collins, R. Martel, and Ph. Avouris, Physical Review Letters, 86 (2001) p. 3128.