Classification Based on Sensing Materials

Sensors based on semiconductor metal oxide (SMO) thin films are the most promising among solid state gas sensors, due to their small dimensions, low cost, on-line operation, and high compatibility with microelectronic processing [60]. They have been used extensively for gas sensing based on film conductivity changes [61-63]. Intense research and development have been conducted to design highly sensitive, selective and stable gas sensors since Seiyama first observed gas sensing effects in Zinc Oxide (ZnO) [64]. Later, the range of sensitive materials was extended to SnO2, TiO2, WO3, In2O3 and other oxides. Semiconductor

metal oxide based gas sensors are used for environmental and emission monitoring, automotive, domestic, industrial and medical applications.

The gas sensing mechanism in these materials is governed by the reactions which occur at the sensor surface between the thin film sensitive layer and the target gas molecules. It involves chemisorption of oxygen on the oxide surface followed by charge transfer during the reactions of oxygen with target gas molecules [65]. The adsorbed gas atoms inject electrons into or extract electrons from the semiconducting material, depending on whether they are reducing or oxidizing agents, respectively [66]. This mechanism results in a change of the film conductivity, which corresponds to the gas concentration. Although semiconductor metal oxide gas sensors are promising, low selectivity, high power consumption and lack of long term stability have prevented their use in more demanding applications [67]. In the literature, there are several approaches to reduce these limitations, such as use of catalysts and promoters, multi-sensor array systems, optimization of sensors’ operating temperature and using materials in nanostructured forms.

The performance of MOS gas sensors improves with a reduction in the size of the oxide particles [10], as the entire thickness of the sensitive layer can be affected by the redox

reaction during the interaction process. As a result, the performance of a gas sensor is directly related to granularity, porosity and ratio of exposed surface area to volume. Recent advances in the synthesis, structural characterization and investigation of physical properties of nanostructured metal oxides provide the opportunity to greatly improve the response of these materials based sensors for gas sensing. A detailed explanation of the gas sensing mechanisms and current status of nanostructured semiconducting metal oxide gas sensors is given in Chapter 2.

Solid-State Semiconductor Gas Sensors

Sensors that fall under this category encompass field effect transistors (FET’s), Schottky diodes, and capacitors. Gas sensitive devices based on field effect transistors are generally called GASFETs or suspended gate SGFETs. The specific name of the sensor in the literature denotes the gate material and/or the set-up. These types of sensors consist of a thin catalytic metal layer (platinum or palladium) deposited on top of an insulating oxide layer such as SiO2 and in some cases silicon (Si) or silicon carbide (SiC) substrates. The manufacturing

process and packaging technology are similar to standard integrated circuits. The sensors are small, measuring less than 1x1 mm2_.

The working principle of these type sensors is based on the reactions between the target gas molecules and the catalytic surface. The reaction products and intermediary products may polarise and adsorb at the metal surface or spill over to the uncovered parts of the insulator surface. For example, hydrogen atoms, formed by reactions of hydrogen or hydrogen- containing species, diffuse through the catalytic metal and form dipoles at the metal- insulator interface [68]. The polarised species at the insulator surface and polarised hydrogen atoms at the metal-insulator interface are in equilibrium with the concentration in the gas phase. They form a dipole layer, which adds to the electric field between the metal and semiconductor, altering the voltage between the gate and the source electrode. This causes a shift in the current-voltage characteristics of the device. In practice, the gas response is measured as the change in the externally applied gate voltage which is required to keep the current through the device constant.

Optimisation of sensor performance to different gases can be achieved by: changing the operating temperature, as the dehydrogenation and the catalytic process is temperature dependent; the metal on the catalytic surface; and the thickness and morphology of the gate metal [69]. These sensors can thus be made sensitive to a broad range of hydrogen- containing or polar compounds. They are stable and exhibit a relatively low sensitivity to humidity.

Conducting Polymer (CP) Based Sensors

Polymer gas sensors based on measuring resistance changes in thin film structures have been extensively studied. Among the family of conducting polymers, polyaniline and polypyrrole are possibly the most studied polymers for sensing applications due to their simple synthesis, environmental stability and straightforward non-redox acid doping/base dedoping process to control conductivity [14]. For sensing applications, the suitable polymers have to have a conjugated π–electron system along the polymer backbone. However, the choice of the polymer is limited to intrinsically conducting polymers (ICP) or those which can be made conducting by doping the polymer with counter ions using reducing (n-doping) or oxidising (p-doping) processes. Conducting polymer layers can be incorporated in many different types of transducers, including conductometric, SAW and optical transducers [70].

Usually a thin polymer film is directly deposited onto the sensor substrate by electrochemical or chemical polymerisation. Drop casting, dip and spin coating, screen printing, layer by layer self assembly and Langmuir-Blodgett techniques can also be used to deposit thin films onto the substrates [71].

Conducting polymer sensors operate at room temperature as opposed to the MOS sensors described previously. Their advantages also include high sensitivity, small size, low production costs and ease of deposition on a wide variety of substrates [72]. Disadvantages include the reproducibility of fabrication (poor batch-to-batch reproducibility), strong humidity interference, and base line drift over time due to oxidation processes or changes in the conformation by exposure to inappropriate compounds.

The sensitivity of these type sensors can be altered over a wide range by incorporating functional side groups to the polymer backbone, the selection of doping ions, the variation of the polymer chain length, condition of the polymerisation and the use of nanostructured forms [72]. Among these options, the nanostructured forms of polymer appear to be the most promising to develop highly sensitive and stable gas sensors. A detailed explanation of the gas sensing mechanisms and current status of nanostructured polymer based gas sensors is given in Chapter 2.