Biomedical Engineering Reference
In-Depth Information
The semiconducting CNTs have the chiral angle
ϕ
fall between 0° and
30° and
n
-
m
≠ 3
l
. The semiconductivity of CNTs was reported in some
experiments.
3,4
When analysed by scanning tunneling spectroscopy, the
chiral CNTs showed low conductance at low bias voltage but signiicantly
reduced resistance at high voltage. Chiral CNTs showed several kinks at high
bias voltage, while the metal CNTs showed a constant conductance.
3
The
energy gap was measured of the order of ~0.5 eV. Furthermore, the Odom
group's research clearly showed the conductance of the CNTs with indices of
(12, 3) and (14, 3), following the model of metal and semiconducting CNTs,
respectively.
4
The metallic CNTs with ballistical electron transport are very good
conductors. Because of the speciic wrapping of CNTs, the electronic structure
of CNTs is nearly one-dimensional, making the electronic transport in CNT
ballistical.
5
This transport renders CNTs highly conductive with essentially no
heating.
6,7
Previous studies showed that the room-temperature resistances
of most metallic CNTs were below 100 kΩ and that some were around 15
k
Ω
, while the lowest resistance values were observed at 7 kΩ. These indings
suggested metallic CNTs as excellent materials for producing electronic
devices.
In addition to the transportation of electrons, propagation of phonons
along the CNTs was also observed ballistically.
8
The phonon mean free path of
the CNTs was estimated around hundreds of nanometres, comparable to the
length of some CNTs, which may range from a few micrometres to hundreds
of nanometres. This indicates that the phonons have only a few scattering
events between the thermal reservoirs and that the phonon transport is
“nearly” ballistic. Therefore CNTs could be good conductors for thermal
transport. The theoretical prediction was supported by the data that the
measured room temperature thermal conductivity for an individual multi-
wall carbon nanotube (MWNT) was 3000 W/m K, greater than that of natural
diamond and the basal plane of graphite (both 2000 W/m K).
With nanoscale dimension and special structure, CNTs often show
quantum characteristics. The prediction that the conductance is quantised
when electrons low ballistically through CNTs
9
was proven by experiment
designed by Frank,
7
who measured the conductance of CNTs dipped into
liquid metal surface. The conductance of CNTs remained zero when the
depth of the dipping was below 2 μm but jumped up and remained at a
constant value when the insertion length was from 2 to 2.5 μm. The result
was inconsistent with classical conductors and indicated that the CNTs were
ballistic conductors and quantum resistors. Besides the electric conductance,
the thermal conductance was also observed as quantised
10
and correlated with
theoretical predictions for phonon transport in a ballistic, one-dimensional
channel.
11
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