Nanotechnology-Enabled Sensors

6.2 Density and Number of States 299
nanorods, nanowires and nanobelts have been extensively studied and employed
for their applications in the fabrication of electronic devices. Not
only 1D structures can be individually used for the fabrication of transducers
and sensors, but the incorporation of a large number of such structures
together can also be utilized to form nanostructured thin films for the fabrication
of transducers and sensing elements.
Field effect transistors based on 1D structures are now being widely investigated
in the electronic industry as an alternative for current silicon
transistor technologies. 1D structures can be directly used in the fabrication
of the mechanical transducing platforms such as resonators. They are
also employed as efficient photo sensitive elements for the fabrication of
photodiodes and sensors. These 1D nanostructures can exhibit well engineered
chemical compositions and crystallographic formation. 8,9 Metal oxides
can interact with different gas species as sensitive elements at elevated
temperatures. Metals such as Pt and Au can function as catalytic elements
for gas species. Carbon nanotubes can interact with gases such as hydrogen
and store them in their cages at room temperature. In addition, these
materials can be functionalized as 1D surfaces for chemical and bio sensing
applications.
Field-effect transistors and conductometric sensors can be fabricated using
individual 1D materials such as nanobelts and nanorods. 10,11 There are
many methods that can be used for this task. A few examples are as follows:
(a) Long bundles of metals oxides nanobelts can be dispersed in a liquid
which does not react with them. 8 Ultrasonication is used until most of the
individual nanobelts are isolated. The liquid is then dispersed onto substrates
with prefabricated metallic pads. Non-contact-mode AFM (AFM tip
hovers electrostatically over the surface) is then used for imaging and to
locate the metallic pads and the bundles. Field-effect transistors can then
be formed using the AFM, electron or ion beam lithographic methods ensuring
a selected number of bundles connect to the metal pads. Heat treatment
is generally needed to make sure that the connections between the
metal pads and the one-dimensional nanostructures are correctly formed. It
is also possible to apply the metallic contacts after the dispersion of bundles.
In this case, electron beam lithography has to be applied. An AFM
image of the FET and the schematic diagram are shown in Fig. 6.9. 8 After
forming the electrical contacts a FET is produced. 12
A typical ZnO FET is depicted in Fig. 6.9 with a gate threshold voltage
of -15 V, a switching ratio of nearly 100, and a peak conductivity of
1.25 × 10 -3 (Ω-cm) -1 . Similar characteristics have been observed in using
carbon nanotubes instead of metal oxide nanobelts. 13