Nanotechnology-Enabled Sensors

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Chapter 6: Inorganic Nanotechnology Enabled Sensors
Potential energy (eV)
AB+S
Fig. 6.13 The Lennard-Jones potentials.
A+B+S
Distance from the surface
In the weakest form of adsorption (physisorption) no true chemical bond
between the surface and the adsorbate is established. This bonding can be
due to the induced dipole moment of a nonpolar adsorbate interacting with
its own image charges on the polarized surface. This bonding is rather
weak in the order of 0.1 eV. However, it is observed in a large range of
applications. For instance many enzymatic interactions (Chap. 7) consist of
a large number of physical adsorptions. Physisorptive forces such as van
der Waals are purely attractive. However, many physical forces can be repulsive.
The orbitals of the approaching atom have to be orthogonal to the
substrate wave function which increases their kinetic energy. This generates
a strong repulsive force. As a result, there is a balance between the
short range Pauli repulsion and the long range attractions.
Adsorption can also be the result of a chemical reaction. Chemisorption
corresponds to the creation of chemical bonds between adsorbate and substrate,
which causes the resulting electronic structure perturbation. In gas
sensors, the target gas may be chemisorbed or physisorbed on the surface.
The interaction depends on the type of PES, gas molecules and environmental
parameters. When they adsorb on surface, molecules can be dissociated
on or diffused in the sensitive layer. 29