Biomedical Engineering Reference
In-Depth Information
ultra-low power consumption, or alternatively as a channel in high-
performance field-effect transistors. The mechanical properties of
the MWCNTs with intermediate diameter (5-20 nm) are of particular
interest for fabricating NEMS or MEMS. The metallic single-walled
carbon nanotubes (SWCNTs) can be important as nanoscale
interconnections or nanoelectrodes.
The discovery of CNTs in 1991 by Iijima [23] in the format of
multiwalled nanotubules, and then in 1993 [24] his design and
fabrication of CNTs in the format of single-walled nanotubes with a
diameter of 1 nm, has generated a great interest among worldwide
researchers to explore their unique mix of electrical, optical, thermal,
mechanical, physical, and chemical properties to develop high-
performance sensing devices.
CNTs may be considered as graphene cylinders [25]. Graphene is
a single planar sheet of sp
2
-bonded carbon atoms. Graphite consists
entirely of individual graphene layers, which are stacked on each
other. The properties of graphite already give a first hint toward the
remarkable properties of CNTs, such as high mobility of the order of
10
4
2
-1
-1
at room temperature, in-plane electrical resistivity as
low as ranging 1-50
cm
V
s
µΩ.
cm, thermal conductivity of graphite as high
-1
-1
as 500 Wm
.
The basic structure of CNTs in the geometry of SWCNTs can
be thought of as a rolled-up graphene sheet in which the edges of
the sheet are joined together to form a seamless tube. By changing
the direction in the roll-up, different chiralities can be created.
Furthermore, several tubes of different diameter can be fitted into
each other to make MWCNTs. These consist of concentric cylindrical
shells of graphene sheets coaxially arranged around a central hollow
area. The diameter of the single nanotubules is at very narrow size
distribution of 1-5 nm. The individual CNTs can be either metallic
or semiconducting depending on their chirality and diameter, thus
they electrically behave as a metal or semiconductor. For sensing
applications, the semiconducting CNTs are useful for gas detection.
In fact, exposure to target chemical species generally shifts the Fermi
level in the individual semiconducting CNTs and determines also an
intertube change modulation based on the hopping mechanism in
the CNTs networked films with subsequent changes in the electrical
conductance of the individual nanotubes and nanotube-based layers
[26-28].
K
