Biomedical Engineering Reference
In-Depth Information
the tube walls, and to whether the gas molecules were physisorbed
or chemisorbed. Kunadian
. [82] constructed a thermoelectric
device based on MWCNTs with a Seebeck coefficient measured as
about 20
et al
µ
V/K at the temperature of 300 K. Doping of the MWCNTs
resulted in an increase in the Seebeck coefficient. Small
. [83]
measured the thermoelectric power of the individual CNTs with a
Seebeck coefficient of about 40
et al
V/K at the temperature of 300 K.
These thermoelectric properties of the CNTs are useful to design a
temperature nanosensor and thin-film thermocouples.
The optical properties of the SWCNTs have been deeply studied.
Various extensive reports are shared in the nanoscience community
[54, 84]. Because of the van Hove singularities in the electronic
density of states in the SWCNTs, the optical Raman spectroscopy
can provide powerful diameter-selective observations of the CNTs
from a sample containing nanotubes with different diameters. Since
the optical properties of the SWCNTs depend on their diameter
and chirality, a spectroscopic characterization tool to investigate
the geometry and structure of the nanotubes is very important to
distinguish optically metallic and semiconducting CNTs, which can
be separated in the observed resonant Raman signal. Thus, they are
identified with a proper optical band gap. SWCNTs with different
diameter distribution have been synthesized and optical absorption
spectra have been measured. Kataura
µ
. [85] investigated their
optical properties and demonstrated that three large absorption
bands due to the optical transitions between spike-like density of
states were observed from infrared to visible region. Comparing
with the calculated energy band, it has been concluded that the
first and the second lowest absorption bands are due to the optical
transitions between spikes in semiconductor phases and the third
one is due to that in metallic phase. Furthermore, they found that
the absorption peaks sensitively shifted to higher energy side with
decreasing tube diameters, as the band calculation predicted. As
an example, for a (12,0) zigzag nanotube with a diameter of about
1 nm, a band gap of 0.05 eV was measured; while for a (15,0) zigzag
nanotube with a diameter of about 1.2 nm, a band gap of 0.04 eV was
measured as well [54].
High-quality metallic nanotubes at low biases (<200 meV) are
ballistic electrical conductors [86]. This means that low energy
electrons can travel several micrometers without collision at room
temperature; although for higher biases the electrons are scattered
by optical phonons. Semiconducting nanotubes can also achieve
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