Toxic pollutants must be detected while expending the minimum amount of energy in order to achieve the lowest possible power consumption. New approaches in gas sensor technology are needed to solve the detection problems associated with mobile sources of CO, NOx and aromatic hydrocarbons. In order to be useful as an air-monitoring system, a sensor should be able to detect toxic gases at environmentally relevant concentrations. According to the Air Quality Planning and Standards we should be able to achieve a detection limit of 9 ppm for CO, 0.053 ppm for NO2 and 0.03 ppm for sulfur dioxide. Most of the sensor devices developed so far cannot reach these limits under the necessary practical conditions. Researchers have been trying to achieve the ppb (parts per billion) level of gas sensitivity under practical conditions using different sensor fabrication methods [1], [2], [3] and [4]. It is therefore important to discuss whether sensors made of metal oxides can sense gases at room temperature and meet the demand of low power consumption. Over the last two decades, enormous efforts have been made to develop these room temperature gas sensors by doping metal oxides with a pure metal catalyst [5], [6], [7] and [8]. These efforts were successful in lowering the sensor's operating temperature to a value close to 100 °C, for some gases. Gas sensing in this temperature range is supported by the physics and chemistry of gas adsorption which is understood in terms of the mechanism of chemical and electronic sensitization [3], [9] and [10].
Recently, Peng et al.[12] reported room temperature gas sensing using silicon nanowires. This is a significant breakthrough in gas sensor technology. However, no new sensing mechanism has been proposed to account for this outstanding performance. We intend to explain here how sensing via silicon-silicon oxide (SiO) nanowires can be possible at room temperature.
Room temperature gas sensing is not possible in metal oxide nanoparticle and film based sensors according to gas adsorption chemistry and the electron transfer mechanism. In general, the metal oxide based sensors can only be operated above 200 °C. It is a well known fact that silicon oxide is a highly resistive material with very few electrons available for communicating with oxidizing and reducing gases at room temperature. Therefore, room temperature sensing using Si needs to be explained. The question arises, how is the conduction in Si-SiO activated by adsorbing so few gas molecules at room temperature? In principle SiO does not have free conduction electrons under normal conditions. The gas sensing phenomenon depends on the transfer of electrons between the sensing material and target gases, with formation or deformation of the depletion layer depending on the type of material. The oxidizing gases accept electrons while reducing gases donate electrons. The basic gas adsorption process allows reactions on a semiconductor's surface if the semiconductor is activated or if it possesses free electrons or holes. If this is the case then the charge carriers can communicate with the gas and form a built-in potential difference with a sufficient amount of resistance, which is actually known as the gas sensor signal. This observation agrees with the basic adsorption chemistry of gases above a certain temperature. Reported room temperature sensing [11] indicates that a new type of physiochemical phenomenon might be responsible for this extraordinary gas sensing behavior. Such a type of gas sensing is possible via the passivation of dangling bonds [12] or incomplete covalent bonds, or the available fast surface states caused by the large surface:volume ratio in nanowire systems. The target gases are likely to form temporary bonds with nanowire surface vacancies created due to the arbitrary oxygen content. These bonds act as bridges for electron transfer between the gas and nanowire surface. This is expected due to the instability of oxygen content in the oxide materials (SiOx, x = 1.0 − 2.0), although it is worth noting that it is difficult to control the oxygen content in any oxide material. If the compound is non-stoichiometric then it is possible for the compound to react with foreign elements, and it is said that these non-stoichiometric compounds are better receptors than the stoichiometric equivalents.
Due to the very high reactivity of the incomplete bonds or the fast surface states, the gas molecules can be broken down into their atomic components under certain conditions. The proposed passivation process of the bonds caused via the unstable oxygen results in the process of chemisorption. Due to the length difference it is easier to control the oxygen content throughout a nanoparticle than along a nanowire. Thus the composition difference (away from stoichiometry) Δx is directly related to the nanowire length or nanoparticle diameter. Therefore the bond passivation is more likely to occur in nanowire structures than in nanoparticles. Moreover, due to the wire type morphological nature of the sensing system the conduction mechanism is no longer of a hopping type, such as the single range hopping in polycrystalline materials or the variable range hopping in nanocrystalline materials. If the sensing materials are in nanowire form the charge carriers responsible for gas sensing travel along a linear path. This results in reduced carrier scattering during conduction and improves the gas sensitivity of the material.
The sensor mechanism proposed here is novel and may be useful for researchers dealing with environmental issues, as well for students studying the basic physics and chemistry of gas adsorption. It utilizes the physics and chemistry of gas adsorption in two dimensional structures such as nanowires and nanotubes.
Further Reading
[1] A. Tricoli, S.E. Pratsinis, Nature Nanotechnol, 5 (2010), p. 54
[2] R.K. Joshi, F.E. Kruis, Appl Phys Lett, 89 (2006), p. 153116
[3] G. Korotcenkov, Mater Sci Eng R, 61 (2008), p. 1
[4] N. Yamazoe, Sens Actuat B: Chem, 5 (1991), p. 7
[5] A. Diéguez et al. Sens Actuat B: Chem, 68 (2000), p. 94
[6] R.K. Joshi et al. J Nanopart Res, 8 (2006), p. 797
[7] S. Shukla et al. Sens Actuat B: Chem, 97 (2004), p. 256
[8] Y. Yamazoe et al. Sens Actuat, 4 (1983), p. 283
[9] H. Ogawa et al. J Appl Phys, 53 (1982), p. 4448
[10] M. Tiemann, Chem Euro J, 13 (2007), p. 8376
[11] K.-Q. Peng et al. Appl Phys Lett, 95 (2009), p. 243112
[12] N.K. Ali et al. Solid State Electron, 52 (2008), p. 1071