Biomedical Engineering Reference
In-Depth Information
exposure to gaseous molecules NO
2
or NH
3
, the electrical resistance of the
semiconducting SWNT dramatically changed. The nanotube sensors exhibited
a fast response and a substantially higher sensitivity than that of other
existing solid-state sensors at room temperature. At a gate voltage of +4 V, the
conductance of the SWNT sample increased sharply by about three orders of
magnitude after 200 ppm of NO
2
was introduced. The response times of the
resistance changed by one order of magnitude for 200 ppm of NO
2
in the range
of 2-10 seconds. The conductance increase was reversible because of the NO
2
molecules' desorption from the nanotube sample. Moreover, after exposing
the recovered SWNT sample to 1% NH
3
molecules, the current versus voltage
curve showed a 100-fold conductance depletion, with a response time of
about 1-2 minutes. Therefore, the electric conductance provided information
about the target molecules. On the other hand, the back gate voltages, able to
induce a high conductance for SWNTs exposed to NO
2
or NH
3
molecules, were
about +4 V and 0 V, respectively, suggesting that the selectivity of different
molecules can be achieved by adjusting the electrical gate to set the SWNT
sample in an initial conducting or insulating state.
Interaction between CNTs and Br led to substantially change the
conductance.
22
As a prototypical electron acceptor, doped Br decreased the
resistivity of CNT at 300 K and enlarged the region where the temperature
coeficient of resistance is positive (the signature of metallic behaviour).
In addition, intercalation of charged polyiodide chains into the interstitial
channels in an SWNT rope lattice resulted in a new carbon material with a
signiicantly low resistance.
23
The mechanism of chemical sensors lies on two main aspects: (1) the
chemical nature of the molecules and (2) the CNT samples, which appear to
be hole-doped (p-type). Oxidising molecules, including NO
2
, Br, oxygen and
air, have unpaired electrons, and there would be charge transfer from SWNTs
to these molecules. Therefore, the conductivity of the p-type CNTs would
increase. Removal of these molecules from CNT samples diminishes the
charge transfer and causes a reverse effect. On the other hand, charge would
be transferred to CNTs when Lewis base molecules, such as NH
3
, are targeting
compounds. The effect of the charge on the hole would result in increased
resistance. These studies have indicated that utilising CNT-based transistors
or conductors as chemical sensors is not limited to speciic chemicals. The
nanotube electronic transport may be sensitive to other substances as long as
they can affect the amount of charge transfer between CNTs. In this way it is
possible to design many CNT transistor-based nano-sensors to detect various
types of molecules.
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