Ongoing nanotechnology research at Ames
Research Center is aimed at developing revolutionary
small-scale electronic devices and sensors to meet
the requirements of future NASA missions. Carbon
Nanotubes (CNT) are hollow cylinders of graphitic
carbon atoms; the cylinders have diameters of
~0.001 microns and lengths of up to several microns
(more than 100 times smaller than the components of
today's microprocessors). Depending on the orientation
of the hexagonal carbon rings in the tube
surface, some CNTs have metallic properties (that is,
the band gap is zero) and others are semiconductors
(that is, the band gap is finite, but less than 1 electron
volt).
Because of their varying electronic properties and
very small size, CNT components are expected to
have an important role in the development of future
electronic devices. In addition, CNTs have extraordinarily
large tensile modulus and tensile strength,
which places them among the strongest materials
known. These attributes make them promising
candidates for reinforcing fibers and for microelectronic-
mechanical sensors (MEMS). Single-CNT
transistors (shown schematically in figure 1) have
been fabricated and tested under ideal laboratory
conditions, but it is not known whether they will
function under typical operating conditions of
integrated circuits. Previously, modeling studies
were carried out to characterize idealized CNT
electronic devices. This year, research focused on
characterizing electronic devices and sensors under
realistic operating conditions where the CNTs are
bent, flattened, and twisted by their interactions with
substrates, metal contacts, and other devices.
Electronic properties of stretched, compressed,
and bent CNTs have been studied, using a full-valence
electron tight-binding model to calculate the
electronic density of states (DOS), bandgap and
conductance. Selected results for stretched and
compressed nanotubes are illustrated in figure 2.
Similar results have been found for other deformations.
We find that some metallic-like nanotubes
(labeled 5,5) remain conducting even when
subjected to large compressive or tensile strain, as
evidenced by their bandgaps remaining zero. However,
other conducting tubes (labeled 9,0) develop
sizeable band gaps under these conditions. Finally
the band gaps of semiconducting tubes (8,0 and 10,0)
are found to be very sensitive to the applied strain.
Metallic nanotubes, such as the (5,5), retain their
electronic properties even when subjected to large
deformations; as a result, they are promising candidates
for use in CNT-based electronic devices. On
the other hand, other conducting tubes, such as the
(9,0), are good candidates for MEMS devices, because
their conductivity decreases markedly with
increasing tensile strain. Optimal performance of
CNT devices and sensors will be achieved by selecting
the correct type of nanotube for the particular
application.
Point of Contact: R. Jaffe
(650) 604-6458
rjaffe@mail.arc.nasa.gov
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