Sensor chemiresistor-type

Electrical conduction is also very important for many devices that exploit the huge area of surface or interface per unit volume in zero-dimensional nanostructured materials such as nanoporous materials, granular materials, nanocomposites, and nanoparticle assemblies. Examples of such devices are chemiresistor-type sensors, solar cells, light-emitting diodes, and energy-storage cells. From the point of view of electron... 

The resistance variation is measured by Keithley unit as shown. The chemiresistor-type sensor is mounted on hot plate (filament heater) which is coupled with ceramic base stand. The two leads of thin copper wires are attached to interdigited electrodes by silver paste for electrical connections. The electrical connections for gas sensing measurements, thermocouple and temperature variation are made using instrumentation feed through. The response of sensor is monitored in terms of the normalized resistance calculated by response = Ro/Rg and the sensitivity factor is monitored in terms of % sensitivity = dRJRo. Where dR is the variation in resistance of sensing materials after exposure to gas, Rg is the resistance of the sensor in presence of gas and Rq is the initial resistance with gas. 

A chemical microsensor can be defined as an extremely small device that detects components in gases or Hquids (52—55). Ideally, such a sensor generates a response which either varies with the nature or concentration of the material or is reversible for repeated cycles of exposure. Of the many types of microsensors that have been described (56), three are the most prominent the chemiresistor, the bulk-wave piezoelectric quartz crystal sensor, and the surface acoustic wave (saw) device (57). 

Chemical sensors can be of the type that sense the species directly, such as ion-selective electrodes, or the type that sense the species indirectly through the change in another physical property produced by the species. An example of this latter type is the so-called "chemiresistor", in which the presence of the species being sensed modifies the electrical resistance of a transducer... 

Chemiresistors represent the most common and simple type of sensor that incorporates organic materials. Generally speaking, the chemiresistor consists of a pair of electrodes that form contacts with the chemosensing material, which is deposited onto an insulating mechanical support (Fig. 5.2). 

In this section, the performances of electrical gas sensors based on SWCNTs are reviewed. Three types of gas sensors are introduced field-effect transistor based on individual SWCNT and chemiresistor or field-effect transistor based on SWCNT network. 

Conductivity is undoubtedly the most closely investigated physical parameter of EPs. It is heavily dependent on the level of the primary doping, i.e. on the concentration of the counterion. Addition of another type of electroactive molecule to the polymer can be viewed as a secondary doping and, as expected, it also affects the conductivity of the EP, albeit usually less than the primary doping. Nevertheless, this incremental change of conductivity is the basis for operation of conductimetric chemical sensors, better known as chemiresistors. 

CNT-based gas sensors have shown good electrical response as well. It was established that chemiresistors and chemical field-effect transistors are probably the most promising types of gas sensors based on CNTs (Kong et al. 2000 Bondavalli et al. 2009 Wang and Yeow 2009 Zhang and Zhang 2009 Hu, et al. 2010). Typical configuration of such sensors is shown in Fig. 1.10. It has to be pointed out that for this kind of sensor the research has essentially focused on SWCNTs, because MWCNTs are only metallic and therefore unsuitable to fabricate chemiresistors and transistors. 

As can be seen, II-VI semiconductor compounds can be used as sensing materials in all types of gas sensors, including chemiresistors, SAW, heterojunction based, and optical. They can be applied to surface functionalizing and composites forming as well. The application of II-VI saniconductor compounds in quantum dots-based gas sensors will be discussed in Chap. 5 (Vol. 2). Operating characteristics of several Il-VI-based gas sensors are shown in Figs. 5.18 and 5.19. 

Low concentrations of ammonia in urban environments were measured, at room temperature, using chemiresistor gas sensors modified by SWCNT concentrations of 20 ppb were measured and the detection limit was estimated as 3 ppb." Trace amines in the vapour phase at sub-ppb levels were measured by a robust p-type organic nanoflbril composite material obtained by surface doping." ... 

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