Abstract: The development and large-scale implementation of low cost, environmentally friendly gas sensors could lead to drastic improvements in environmental health and safety. Resistive-type metal oxide nano-scale gas sensors are a promising candidate to fulfill these requirements due to their relatively non-toxic materials, simple operating principles, and high sensitivity. The operating principles of resistive-type gas sensors utilize charge transfer between gas molecules and solid semiconducting metal oxides. The most widely studied metal oxide semiconductors for resistive-type sensors are SnO2, ZnO, and TiO2 due to their stability and good electrical properties . In air, oxygen adsorbs onto the sensor surface and captures electrons. For n-type materials the adsorbed oxygen will deplete the material of electrons thereby increasing the electrical resistance. When these materials are exposed to other gasses, the adsorbed oxygen may react or be displaced. When reducing gasses, such as ethanol, methane, carbon monoxide, etc., react with adsorbed oxygen, the oxygen is removed from the surface thereby releasing electrons back into the material causing a decrease in resistance for an n-type material. A large surface to volume ratio is beneficial for these processes because maximization of the resistance modulation can only occur if the adsorbed surface species capture a proportionally large number of charge carriers. For this reason, nano-scale structures are highly beneficial for gas sensing applications. To enhance gas sensor response further, heterostructures have been incorporated into sensor designs. Heterostructure materials with a lower Fermi energy will effectively drain electrons from materials with a higher Fermi energy which reduces the number of charge carriers thereby maximizing resistance modulation.
Authors: J. Walker, S. Akbar, and P. Morris
Keywords: Sensor, Heterostructure, Oxide