Advances in gas ionization sensors based on nanostructured materials: a review

In this review article collection of summary of research works related to gas ionization sensors based on nanostructured materials is attempted. Among physical gas sensors, gas ionization sensor is used to identify gases based on breakdown of ionized gases as each gas has a unique breakdown voltage. The sensor is generally, made of two parallel electrodes separated by a narrow gap. Nanostructures, usually metallic or semiconducting, in from of nanowires/nanorods are places between the two plates. These structures act as electric field amplifiers to enhance the electric field intensity to reduce the breakdown voltages of the gases. Numerous investigations revealed that type of materials and their physical properties play important roles in the sensor characteristics and their operations. Identifying materials to optimize the strength of electric field and generate large field enhancement is a crucial step towards this application. Carbon nanotubes were among the first candidates to utilize in these structures and in fact, they were used to prove the principle of the device. The very first developed sensor showed that using carbon nanotubes between the two parallel plates of the sensor reduces the required applied voltages to ionize gases up to one order of magnitude. Since 2003 many studies are done to improve the performance and physical properties like sensitivity, selectivity, durability and stability of the device by incorporating various materials in form of nanowires/nanorods inside the device structure. In this work, first the fundamental structure of gas ionization sensor and the theory of their operations like field emission, field ionization and quantum tunneling phenomena are presented. Then, related research works reported in the literature are summarized. This review article is partitioned based on the materials used in the structure of the device as they play the most effective role in their performances. It is started with studies on devices based on carbon nanotubes, and then those used metallic nanowires followed by works done using semiconductor nanostructure to develop the device. The review started from the oldest articles in each category carried on to the most recent published works. All essential parameters effecting the structures and operations of the reported devices including their pros and cons are discussed.