Biomedical Engineering Reference
In-Depth Information
A model of gas adsorption into CNTs networks has been proposed
by Meyyappan
[28] to distinguish two different effects in
the electrical charge transfer:
et al.
intratube modulation
and
intertube
modulation
. In the first case, a direct charge transfer between gas
molecules and an individual nanotube can lead to the modulation of
the Fermi level in the semiconducting tubes causing a conductivity
change. In the second case, gas adsorption occurs in the interstitial
space between nanotubes to form a CNT-molecule junction leading
to a hopping mechanism for intertube charge transfer between
individual nanotubes in the bundle with a consequent change in the
conductivity. These effects of charge modulation can be enhanced
depending on adsorbed target gas, tube nanostructure, and defective
status in the nanomaterials. It also demonstrated that the recovery
time for NO
desorption is very long, on the order of various hours,
because of the high bonding energy between SWCNTs and adsorbed
NO
2
molecules. A strategy proposed to reduce the recovery time was
the UV light illumination upon recovery cycle in simultaneous with
recovery nitrogen gas. The UV exposure decreased the desorption
energy barrier to promote the NO
2
desorption up to a few minutes.
However, this UV light method was already employed by Dai
et al.
2
[259] to accelerate molecular photodesorption from SWCNTs
by using an ultraviolet light source with a wavelenght of 254 nm,
intensity of 2 mW/cm
2
×
15
2
, photon flux of 2.5
10
/cm
s.
. [147, 262] demonstrated that PECVD-grown CNT
sensors were fabricated onto Si
Cantalini
et al
/Si substrates equipped with Pt
electrodes for gas sensing applications. The CNTs diameter ranged
30-40 nm and length 100-200 nm. CNTs showed cross-sensitivities
toward NH
N
3
4
, H
O, and C
H
OH and exhibited high sensitivity toward
3
2
2
5
NO
gas in the gas concentrations range of 10-100 ppb. The highest
2
NO
gas sensitivity was measured at 165°C working temperature.
No response was found toward CO and CH
2
gases in the operating
temperature range of 25-250°C. Additionally, NO
4
gas sensitivity
resulted to be improved by annealing the as-grown films at
temperatures higher than 330°C. This was attributed to structural
changes induced by temperature exceeding 250°C providing a
support for a phase-transition causing an electrical change from
a metallic to a semiconducting response of the CNT film. This
temperature-dependence of the electrical resistance in the CNT film
has been deeply studied by the same group, as reported by Valentini
et al
2
. [27]. Similar results on electrical properties of the SWCNT layers