Nanotechnology-Enabled Sensors

4.3 Formation of Thin Films 143
There are numerous thin film deposition techniques available which are
categorized as either vapour phase deposition or liquid phase deposition.
The deposition technique determines whether the film is conformal or
non-conformal, with respect to the underlying substrate’s surface morphology.
A conformal coating covers all surfaces to a uniform depth, following
(conforming to) the substrate’s shape. A non-conformal coating is
non-uniform and does not conform to the substrate’s surface (Fig. 4.5).
conformal non-conformal
Fig. 4.5 Conformal and non-conformal coatings.
This section is an introduction of how nano-structured thin films can be
formed. In the following sections, we will describe different vapor and liquid
phase deposition techniques that are commonly utilized in both the
nanotechnology and sensor technology fields.
4.3.2 Growth of One-Dimensional Nano-structured Thin Films
Thin films made from one-dimensional nanostructures are useful for the
fabrication of both gas and liquid phase sensors. Their surface-to-volume
ratio is large and they can be deposited using either standard or low cost
microfabrication techniques. Nanobelts, nanorods, nanotubes and nanofibers
are all examples of one-dimensional nanostructures. They can be
deposited spontaneously without the help of any templates using the following
techniques: 28 (a) evaporation-condensation and vapour-liquid-solid
(VLS) growth in the gas phase (b) dissolution-condensation and solutionliquid-solid
(SLS) growth in the liquid phase. Spontaneous growth on a
surface can be realized through reduction of the Gibbs free energy associated
with the surface. Phase transformation, stress changes, and chemical
reactions can all be used to control the Gibbs free energy.