Biomedical Engineering Reference
In-Depth Information
(5-1000 ppm) gas sensing applications, at operating temperature of
200°C. Nanoclusters of different MWCNT-growth catalysts (Fe and
Co) were predeposited onto substrates to promote the growth of
carbon nanotube films with different structural properties. It was
demonstrated that the gas sensitivity of the MWCNT films depends
on catalyst used for their growth with highest gas sensitivity achieved
for Co-grown MWCNT films. SEM images (see Fig. 9.13) showed that
the average diameter of the Fe-grown MWCNTs was higher than that
of Co-grown MWCNTs offering a different specific gas adsorption area
and density of growth. The chemiresistor gas response was attributed
to electrical
-type conductivity in semiconducting MWCNTs and
the electrical charge transfer was found to be the major sensing
mechanism. The results obtained, shown in Fig. 9.13, demonstrated
that the MWCNT sensors exhibited high gas sensitivity, fast response
and reversibility, good repeatability and reproducibility, and sub-
ppm range detection limit with the gas sensing properties of the
MWCNT films tailored by catalyst employed in the PECVD-growth
process.
p
Figure 9.14
(a) Sensitivity of the electrical resistance
R
of SWCNT films
to gas exposure at a temperature
of 290 K. The nanotube
resistance switched by 10-15% as the chamber surrounding
the sample was alternately flooded with air or evacuated.
Identical results were obtained if pure dry oxygen was
used rather than air, indicating that oxygen was the source
of the effect. (b) Sensitivity to environmental conditions of
thermoelectric power
T
= 350 K. Both magnitude and
the sign can be altered by oxygen exposure. In vacuum,
S
at
T
S
is
n
-type, with
a larger magnitude. Many previous transport measurements
have found that as-prepared nanotubes are strongly
-type, whereas in an oxygen environment,
S
is
p
-type,
with no proper theoretical explanation given. Apparently
such nanotubes were inadvertantly “air-doped.” This figure
is reprinted with permission from
p
Science
[1].
