Nanotechnology-Enabled Sensors

6.3 Gas Sensing with Nanostructured Thin Films 323
Transition layer
Substrate
Textured layer
Fig. 6.25 Simplified cross section of a metal oxide film containing a transition
layer.
A large number of substrates can be employed for the deposition of gas
sensitive metal oxide films. These include: various dielectric polycrystalline
Al2O3-based substrates; Al2O3 single crystals with specific crystallographic
orientation; SiO2 deposited on Si substrate; MgO substrate; SiC
substrate; glass ceramics; etc.
The structure of nanocrystals can greatly depend on the substrate’s material
and lattice structure. An example of ZnO thin films deposited on four
different substrates and consequently taking different forms is shown in
Chap. 2 in the sputtering deposition method section.
The substrate can also have a significant influence on the mechanical
structure of the deposited thin films. Metal oxide thin films can be mechanically
strained by the substrate. 39 By selecting substrates with a favourable
lattice parameters and thermal expansion coefficients, it is also
possible to minimize the mechanical strain. 28
6.3.6 Metal Oxides Modification by Additives
Mixing metal oxides with metals that function as catalysts, the use of
binary compounds and complex multi-component materials, as well as
doping, are among the most common methods utilized to enhance or tailor
the gas sensing performance of metal oxide thin films. 28,44-52 These changes
can be introduced to the base oxide matrix during the deposition process or