527. Functional films for gas sensing applications: A review

Fig. 1 The sensing mechanism scheme of semiconductor-based films.

Fig. 2 Schematic model of space charge layer and grain size of films.

Ankur Gupta, et al, Sensors for Automotive and Aerospace Applications, Chapt. 2, Springer, 2019
https://doi.org/10.1007/978-981-13-3290-6_2

(1) A gas sensor should have some characteristics, viz., higher sensitivity, selectivity for target gas, least response and recovery time, higher reproducibility, and stability.
(2) When gas sensors interact with a gas of different concentrations, the behavior of the sensing films changes due to electron or ion interaction between the desired gas and sensing films used in the sensor. The sensing films are most important part of the gas sensor. The films are prepared by chemical processing for a particular gas that it will detect this particular gas even if at low concentration.
(3) For making gas sensor consumer friendly, sensor should be wireless, room temperature based, low power consumption, and small size. In short, performance of any gas sensor depends on 3S (sensitivity, selectivity, and stability) and 3R (response, reversibility, and reliability).
(4) There are broadly three kinds of sensing films, viz., metallic, semiconductor-based, and polymer-based films.
(5) Fabrication of sensing films with aqueous chemical synthesis approach is the simplest and cost-effective way. It just requires some equipment such as arrangement to assist in the chemical reaction for essential bond formulations, some equipment to mix several chemical agents, some energy source to bring the overall molecular size down to sub-micron/nanorange, and heating arrangement to provide thermal treatment. Additional tool may be required for thin deposition to select substrates such as silicon, ITO substrate, etc.
(6) Conducting polymer could be doped by redox reaction or protonation, and oxidation reaction.
(7) After washing with dry nitrogen or air, that polymer film recovers its resistance totally or partially.
(8) VOCs gases (benzene, toluene, and others) are not reactive to conductive polymer films at room temperature.
(9) The conductivity will increase if electrical permittivity of polymer is less than analyte and decreases if electrical permittivity of polymer is greater than analyte.
(10) It is the general place to modify the selectivity by using dopants, grain size controllers, catalysts, operating temperature, etc.