Nanotechnology-Enabled Sensors

312
Chapter 6: Inorganic Nanotechnology Enabled Sensors
When the sensitive layer consists only of a compact continuous material
and the thickness is larger than the Debye length, it can only be partly depleted
when exposed to the target gas, z g > z0.(Fig. 6.19). In this case, the
interaction does not influence the entire bulk of the material. Two levels of
resistance are established in parallel, which explains the limited sensitivity
as only the underneath layer is in contact with the electrodes. As a result,
the better choice is a thinner layer which can be fully depleted.
Fig. 6.19 The schematic representation of a porous (a) and a compact (b) sensing
layer and their energy bands. The representations shows the influence of electrode-sensing
layer contacts. RC is resistance of the electrode–metal oxide contact,
Rl1 is the resistance of the depleted region of the compact layer, R1 is the equivalent
of series resistance of Rl1 and RC, and the equivalent series resistance of ΣRgi
and RC, in the porous and compact situations, respectively. Rgi is the average inter-grain
resistance in the case of porous layer, Eb minimum of the conduction
band in the bulk, qVS band bending associated with surface phenomena on the
layer, and qVC also contains the band bending induced at the electrode–metal oxide
contact. Reprinted with permission from the Elsevier publications. 33
Contacts in gas sensors can have both electrical and electro/chemical
roles. A useful tool for determining the electrical contribution of contact
resistance is the use of transmission line measurements. 35 In this method,
the dependence of the sensor resistance is plotted as a function of the spacing
between electrodes at different ambient conditions. A fit of the experimental
data can be made using ohms law. For thin compact films, contact
resistance plays an important role as a dominant factor in resistance. 36 The
contribution of contact resistance is also extremely important for the case
in which individual nanorods, nanowires or nanobelts are to be used as
sensing layers. 37 In addition to direct current measurement methods for
measuring the resistance, ac impedance spectroscopy can also be very
useful in identifying contact-related elements, and the presence of surface