The data on film type nanosensors reviewed in this article are mainly published in 2008 and 2009. In Section 2, gas sensor configurations and measurements, performance parameters, as well as theoretical fundamentals of gas sensors based on 1D nanostructures will be introduced, while the material system, cited are focused on undoped metal oxide nanowires or nanobelts. Section 3 features modified 1D metal oxide nanostructures, as well as heterostuctures. In Section 4, novel gas sensors based on novel operation principles such as the ��electronic nose��, the self-heated gas sensor and the optical gas sensor will be elaborated on. Finally, Section 5 summarizes the whole article and indicates possible future developments in one-dimensional metal oxide nanosensors.
2.
?Fundamentals of Gas Sensors Based on Metal Oxide 1D Nanostructures2.1. Fabrication and Characterization of Gas SensorsUp to now, one-dimensional metal oxide nanostructures sensors have been characterized in three ways: conductometric, field effect transistor (FET) and impedometric ones. Conductometric sensors are based on resistance changes caused by exposure of the sensor surface to a target species. So far, two types of conductometric nanowire gas sensors have been mainly fabricated: one is the film type, in which a film composed of nanowires is contacted by pairs of metal electrodes on a substrate (Figure 1a) or a ceramic tube (Figure 1b); the other is the single nanowire type in which a single nanowire bridges two metal electrodes on a heavily doped silicon substrate covered with SiO2 acting as insulating layer between the nanowire/electrode combinations and the conducting silicon (Figure 2).
In the fabrication of film type nanosensors, nanowires products are usually pulverized to a pulp state and either directly painted or screen-painted [16] onto the substrates or tubes. But other approaches are reported. Batimastat Sometimes nanowire growth is integrated into device fabrication [17-20]: SiO2/Si substrates with patterned metal coatings were used to catalyze the growth of the metal oxide nanowires and the coating also acts as electrodes contacting Cilengitide the sensing material.
This type of sensor has lower contact resistance compared to the previous one because the nanowire growth process is integrated into device fabrication. Well-aligned nanowire arrays have been fabricated into nanosensors to explore benefits brought about by orderliness.Figure 1.(a) MEMS structures with interdigitated electrode [7]. (b) Schematics of nanowire gas sensors on ceramic tube [26].Figure 2.The schematic of the single nanowire field effect transistor.