Biomedical Engineering Reference
In-Depth Information
9.6
Comparative Analysis of CNT Gas Sensors
Among the nanostructures for chemical sensors, the CNTs are
challenging in nanosciences and very attractive for solving
technological hurdles in applied research. The big interest in these
carbon nanomaterials has been sparked by the excellent electrical,
optical, mechanical, thermal, and chemical properties associated to
their unique quasi-1D structure, atomically monolayered surface,
extendedly curved configuration, combined with the ease of
theoretical investigations facilitating
calculations.
The CNTs may be considered as graphene nanocylinders; while
the graphene is a single planar sheet of sp
ab initio
-bonded carbon atoms.
On the contrary, graphite consists entirely of individual graphene
layers, which are stacked on each other. The CNTs may be composed
by a nanotubule at single wall or, in the most cases, consisting of
a concentric arrangement of many nanocylinders at double wall
or multiple walls. Such MWCNTs can reach diameters of up to
100-200 nm. An individual SWCNT can be either semiconducting
or metallic, depending on its diameter and chirality. Within a CNT
film showing no preferential specific chirality, one-third of the
nanotubes are expected to be metallic, and the remaining two-thirds
to be semiconducting. The structure of a carbon nanotube can be
completely specified through its chiral vector, which is denoted by
the chiral indices (
2
). The classification of the nanotubes depends
on geometric arrangement of the carbon atoms at the seam of
cylinders: armchair (
n, m
).
These one-dimensional carbon allotropes are of high surface
area, high reactivity, high mechanical strength, ultra-light weight,
rich electronic properties, and excellent chemical and thermal
stability. Even since the discovery of CNTs by Iijima in 1991, many
researchers have been exploring their potential in chemical detection
and biosensing applications.
CNT gas sensors are largely investigated for fabricating
miniaturized devices with different geometry of transducers and
high performance to address advanced functionalities such as
high sensitivity, high resolution, selectivity, low limit of detection,
fast response, fast recovery, rapid analysis, linearity, real-time
monitoring, in-situ operation, low power consumption, wearability,
disposable low-cost devices, integrated multiple sensors array,
wireless functionalities, and mass production.
n
=
m
), zigzag (
m
= 0), or chiral (
n
≠
m
