Nanotechnology-Enabled Sensors

252 Chapter 5: Characterization Techniques for Nanomaterials
In addition to secondary electrons there are also high-energy electrons,
originating in the electron beam (producing X-rays), that are backscattered
from the specimen interaction volume. These electrons may be
used to detect contrast between areas with different chemical compositions.
SEM can monitor the formation and growth of thin films and nanostructures.
In nanotechnology enabled sensing applications, the SEM plays a
vital role in helping researchers understand the interaction between the
sensing layer/media and the analyte. This is because sensitivity is strongly
dependant on surface morphology and topography. An SEM image of ZnO
nanorods that have been fabricated via a liquid phase deposition technique
is shown in Fig. 5.31. The figure shows highly cylindrical nanorods protruding
from the surface, whose diameter can be estimated at ~100 nm.
Since the advent of high resolution electron microscopes, features as small
as 1 nm can be resolved. This is illustrated in Fig. 5.32 which shows the
distribution of FePt nanoparticles that have been stabilized on Si substrates
with amino-silanes. 87
For many thin film based gas sensors, the surface to volume ratio of a
sensing layer film can influence its sensitivity. High resolution SEM images
of carbon nanotubes, with large surface to volume ratio, employed for
nitrous oxide gas sensing are shown in Fig. 5.33. 88 Multilayered materials
and nanostructures can also be studied by examining their cross sections.
The cross sectional SEM images of different organic multi-layers used in
biosensing applications are shown in Fig. 5.34, 89 From these SEM images
it is possible to observe their conformation to the substrate as well as the
thickness of each layers. All information (topological and morphological)
obtained using SEM can be correlated to the response of the sensors, and
enable researchers to optimize sensor performance.