Academic literature on the topic 'Semiconductor metal oxide gas sensors'

Composite zeolite-semiconducting metal oxide gas sensors have been produced using standard screen printing techniques. Zeolites A, ferrierite, ZSM-5, mordenite, ! and Y, in their acid form, have been incorporated as overlayers or admixtures (ZSM-5, mordenite and !) to tungsten trioxide (WO3) and titanium doped chromium oxide (CTO) thick films screen printed on interdigitated electrode substrates. These composite sensors, in addition to unmodified control sensors, were evaluated for selectivity to specific concentrations of CO, NH3, NO2 and C2 – C4 alkanes, alkenes and alcohols. A new gas sensing rig (AA Rig) capable of housing 8 sensors in a gas-tight enclosure with capability for sequential delivery of test gases over the sensors, control of sensor operating temperature via integrated heater track including DC resistance sensor conductivity measurements was designed and built. Arrays of sensors in batches of 8 were operated at a stable temperature of 400 °C and their responses to low concentration of test gases was monitored and recorded with view to the assembly of array of sensors possessing biased specificity for simple or complex gas mixtures. The dynamic responses of 16 sensors comprising unmodified WO3 and CTO controls (2 different thickness layers per oxide), zeolites H-ZSM-5, H-Mordenite and H-! overlaid and admixed sensors was shown to exhibit degrees of variance and gas specific patterns in their gas responses. The pattern of response of the 8 WO3 sensor array was found to exhibit repeatable and reproducible ‘fingerprints’ for 30 ppm CO, 25 ppm NH3 and 0.5 ppm NO2. Additionally, this WO3 array exhibited distinctive selectivity to 50 ppm alkenes and alcohols for good gas specific fingerprints with primary and secondary alcohol discrimination. Array comprising WO3 and CTO control, H-ZSM-5 and H-Mordenite admixed sensors exhibited significant NO2 selectivity in the binary mixture of CO and NO2, thus demonstrating capability for environmental monitoring.

A sensor is a technological device or biological organ that detects, or senses, a signal or physical condition and chemical compounds. Technological developments in the recent decades have brought along with it several environmental problems and human safety issues to the fore. In today's world, therefore, sensors, which detect toxic and inflammable chemicals quickly, are necessary. Gas sensors which form a subclass of chemical sensors have found extensive applications in process control industries and environmental monitoring. The present thesis reports the attempt made in development of Zinc oxide thin film based gas sensors. ZnO is sensitive to many gases of interest like hydrocarbons, hydrogen, volatile organic compounds etc. They exhibit high sensitivity, satisfactory stability and rapid response. In the present work the developed sensors have been tested for their sensitivity for a typical volatile organic compound, acetone. An objective analysis of the various substrates namely borosilicate glass, sintered alumina and hard anodized alumina, has been performed as a part of this work. The substrates were evaluated for their electrical insulation and thermal diffusivity. The microstructure of the gas sensitive film on the above mentioned substrates was studied by SEM technique. The gas sensitive Zinc oxide film is deposited by D.C reactive magnetron sputtering technique with substrate bias arrangement. The characterization of the as-deposited film was performed by XRD, SEM and EDAX techniques to determine the variation of microstructure, crystallite size, orientation and chemical composition with substrate bias voltage. The thesis also describes the development of the gas sensor test setup, which has been used to measure the sensing characteristics of the sensor. It was observed that the ZnO sensors developed with higher bias voltages exhibited improved sensitivity to test gas of interest. Gas sensors essentially measure the concentration of gas in its vicinity. In order to determine the distribution of gas concentration in a region, it is necessary to network sensors at remote locations to a host. The host acts as a gateway to the end user to determine the distribution of gas concentration in a region. However, wireless gas sensor networks have not found widespread use because of two inherent limitations: Metal oxide gas sensors suffer from output drift over time; frequent recalibration of a number of sensors is a laborious task. The gas sensors have to be maintained at a high temperature to perform the task of gas sensing. This is power intensive operation and is not well suited for wireless sensor network. This thesis reports an exploratory study carried out on the applicability of gas sensors in wireless gas sensor network. A simple prototype sensing node has been developed using discrete electronic components. A methodology to overcome the problem of frequent calibration of the sensing nodes, to tackle the sensor drift with ageing, is presented. Finally, a preliminary attempt to develop a strategy for using gas sensor network to localize the point of gas leak is given.

This work presents a new low-temperature fabrication process of metal oxide nanostructures that allows high-aspect ratio zinc oxide (ZnO) and titanium dioxide (TiO2) nanowires and nanotubes to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 µm) aluminum film deposited on a silicon and lithium niobate substrate (LiNbO3). This technique is in sharp contrast to traditional free-standing thick film methods and the use of an integrated thin aluminum film greatly enhances the utility of such methods. We have demonstrated the method by integrating ZnO nanowires, TiO2 nanowires, and multiwall TiO2 nanotubes onto the metal gate of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), and the delay line of a surface acoustic wave (SAW) device to form an integrated ChemFET (Chemical Field-Effect Transistor) and a orthogonal frequency coded (OFC) SAW gas sensor. The resulting metal oxide nanostructures of 1-1.7 µm in height and 40-100 nm in diameter offer an increase of up to 220X the surface area over a standard flat metal oxide film for sensing applications. The metal oxide nanostructures were characterized by SEM, EDX, TEM and Hall measurements to verify stoichiometry, crystal structure and electrical properties. Additionally, the electrical response of ChemFETs and OFC SAW gas sensors with ZnO nanowires, TiO2 nanowires, and multiwall TiO2 nanotubes were measured using 5-200 ppm ammonia as a target gas at room temperature (24ºC) showing high sensitivity and reproducible testing results.

The problem of rapid detection of bacteria for 'in-field' applications is addressed by way of a portable 'electronic nose' comprised of five metal oxide semiconductor (MOS) gas sensors in an array for the discrimination of volatile organic compounds (VOCs) associated with bacteria species such as Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). A prototype portable sensor array unit (PSA unit) is presented capable of heating and taking measurements from five MOS gas sensors using a 7-volt power source. An array of five sensors based on zinc oxide (ZnO) is produced suitable for the operational requirements for portable applications. This was achieved be means of zeolite modification where a selection of these microporous aluminosilicalite structures (H-ZSM-5, H-ZSM-22 and H-Y) were incorporated into the ZnO sensor as admixtures, overlayers, admixtures as an overlayer and admixtures with overlayers using commercial screen-printing methods. Unique signal patterns towards ethanol and acetone, two key markers identified for the model bacteria selected for this thesis, were achieved at low ppm concentrations (a detection limit of 2 ppm is reported) using just one MOS material with various zeolite modification strategies.

Low dimensional semiconductors are promising materials with diverse range of applications in a variety of fields. Specifically, in recent times low dimensional oxide and sulfide based semiconductors are regarded as materials that can have potential applications in chemical gas sensor, optoelectronic devices and memristor. How ever, in some cases it is envisioned that appropriate doping as well as phase stabilization is important in enhancing their material properties. This work presents the synthesis, characterization and application of various (pristine and doped) quasi-one dimensional metal oxides (TiO2, VO2) and two-dimensional materials (CuO thin film, MoS2). Some practical protocols for stabilization of specific phases at ambient conditions via a new method of doping in VO2 nanostructures with aluminum, is demonstrated. Similarly, a temperature-doping level phase diagram for the free-standing nanostructures in the temperature range close to the ambient conditions was presented. TiO2 nanowire was doped during growth and electrical measurements on individual TiO2 single crystal nanowires indicate that light in visible range can induce electron-hole pair formation. Furthermore, gas sensing (CO, H2) measurements taken under visible light irradiation imply that photo-activated chemical oxidization on the surface of TiO2 nanowires occurs, which is responsible for the observed measurements. Further, the effect of self heating in some nanostructures was also explored. Since self-heating is a prospective power-efficient energy delivery channel to the conductometric chemical sensors that require elevated temperatures for their operation, the unprecedentedly low power consumption can be achieved via minimizing the heat dissipation in the optimized device architecture. By investigating the heat dissipation in these devices we show that the thermal, electrical and chemical properties of the self-heated semiconducting nanowires appear to be strongly coupled with each other at nanoscale. This opens up unique opportunity to fabricate low power nanoscopic sensing leading to an ultra-small and power efficient single nanostructure gas recognition system. The CuO film based lateral devices were fabricated and studied for its resistive switching behavior. A good, stable and reproducible threshold RS performance of CuO film was obtained by electrical measurement. Finally, the micro-flake MoS2 based FET photoelectronic device was fabricated (using mechanically exfoliated MoS2) and its electronic and photoelectronic properties were investigated. We show that though the FET mobility values of MoS2 microflake is in the average range, but the photo-responsivity is much higher compared to most of others similar sulfide based 2D layered materials.