Biomedical Engineering Reference
In-Depth Information
Fig. 2.40
The CNT gas
CNT
sensor
SiO
2
layer
Substrate (Si of high resistivity or
doped Si)
CNTs
electrode
substrate
S1
S2
S3
Fig. 2.41
CNT gas sensing array
that donate electrons, such as NH
3
, interact with native
p
-type CNT, they change
the charge carrier density in the CNT and, accordingly, the conductance of the
CNT. This mechanism is applicable whether we are dealing with pristine CNTs
or functionalized CNTs. A typical CNT gas sensor is displayed in Fig.
2.40
.
If the substrate is high-resistivity Si, the above devices are termed as chemire-
sistors. On the contrary, if the substrate is doped Si, it acts as a gate, and the entire
device works as a FET transistor; therefore, it is called chemFET.
The sensitive material could be a single CNT between contacts, a bundle, but in
the majority of cases, it is a CNT thin film, termed also as CNT network. In 2000, the
first gas sensor containing a single semiconducting CNT was demonstrated (
Kong
et al. 2000
) at room temperature. The chemFET was of p-type and the conductance
changed several orders of magnitude in time intervals of few seconds after exposure
to NH
3
and NO
2
. Similar works have started using various gases and using either
single CNTs, CNT networks, single sensors, or arrays of sensors, as in Fig.
2.41
(
Qi
et al. 2003
). In this figure, S1, S2, S3 denote three sensors. Alcohol vapors, dimethyl
methylphosphonate, and other gases were detected in sub-ppm concentration using
these CNT gas sensors.
A much simple solution was adopted by drop-casting single-walled CNTs diluted
in dimethylformamide (DMF) onto interdigitated gold electrodes. This type of
sensor had a linear response from ppb up to ppm levels, with detection limits of
262 ppb for nitrotoluene and 44 ppb for NO
2
(
Li et al. 2003
). The modern versions
of such CNT gas sensors use CNT inks and print directly the sensor on a flexible
substrate. Other types of CNT gas sensors are possible, such as chemicapacitors,
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