Dissertations / Theses 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.

The general aim of Master’s thesis is to design and execute electronics in the SMT device view for thin film gas sensors and to study principle of gas sensor functionality based on semiconducting oxides. The SMT device contains temperature controller and electronics which is able to scan the concentration of gas on the sensor surface. It is designed for controlling of four sensors and has to communicate with computer for setting of initial conditions and scanning of concentration of gas. The practical part of Master’s thesis contains design and construction of electronics and software making.

Methods of monodisperse SnO2 crystallites and SnO2-TiO2 composites synthesis were developed. Obtained materials were studied using SEM, TEM, FTIR, XRD methods. Sensors based on these materials were made, and their capability of detecting hydrogen in dry and humid air was investigated. Several different approaches to improvement of sensing devises characteristics were proposed in present study: doping of one metal oxide (SnO2) with another (TiO2) in different ways (mixing or co-precipitating); use of one-dimensional nanowire as a sensing element to detect H2S; application of reactor which converts ammonia to a compound convenient for its detection; analysis of n-dimensional experimental data array with principal components method for acetone selective detection with a single sensor. For every approach, promising results indicating potential for their application are obtained.

Gas sensitive semiconductors have been known for many years and applied in static gas alarm systems for the monitoring of hazardous gases, however, their application has been limited by a lack of selectivity. In this work a semiconducting gas sensor has been configured for use as a gas chromatographic detector thus combining the sensitivity of semiconductor sensors with the selectivity of gas chromatography. The study has been confined to tin oxide devices, more specifically the Taguchi gas sensor (TGS) . The majority of this work has concentrated on the TGS 813 although the use of other TGS is described. The development of suitable instrumentation is described and rigorous optimisation of the operating parameters e.g. heater voltage and column temperature has been performed using the variable step size simplex technique. Attention was concentrated on the response of the TGS 813 to hydrogen which was used as a test gas. A novel figure of merit, response multiplied by retention time and divided by skew factor was designed so that optimum response was obtained whilst maintaining adequate chromatographic separation. Optimum conditions were verified by univariate searches and the response was observed to be most dependant upon heater voltage. A limit of detection of 20 ppb v/v of hydrogen in a 1 ml sample was obtained at optimal conditions. Illustrative analyses of hydrogen were performed in human breath and laboratory air with results found to be in close agreement with literature values. Calibration was found to be linear over at least three orders of magnitude. The response of the TGS 813 to low molecular weight alkanes has also been investigated. It was observed that different heater voltage optima existed for each of the C1-C5 alkanes and that the sensor was relatively more sensitive to the higher molecular weight compounds. As with hydrogen linear response was obtained over at least three orders of magnitude and an illustrative analysis of natural gas showed excellent agreement with known levels. A compromise optimum heater voltage was used to study the response of the TGS 813 to alcohols, aldehydes, ketones and some Cs hydrocarbons. Capillary columns were used in this investigation and it was noted that they had potentially wider application than packed columns due to the use of an inert carrier with an air make-up flow to the detector. This replaced the air carrier gas used previously which might degrade certain stationary phases. Three different types of TGS: the 813; 822 and 831 were used in a study of the response and skew factor for the detection of halogen-containing compounds. Very high skew factors were often observed, although, for some compounds it appeared that symmetrical peaks could be obtained within narrow heater voltage ranges. Skewed response was observed to be dependant upon sensor type, heater voltage and halogen proportion and type. Analysis of the three sensor types was performed and differences in potential surface area and tin oxide additives observed. The presence of additives was observed to adversely affect sensor recovery.

This thesis presents the results of applying new strategies to understand the mechanism and explore the sensing performance of metal oxide (MOX) nanowire based gas sensors by testing individual nanowire gas sensors, running density functional theory (DFT) calculations, using new materials, applying ex-situ analysis and temperature-pulsed operation mode. These MOX nanowires include SnO2, CuO decorated SnO2, CuO and ZnO, electrically contacted either individually or in bundles. With SnO2-NH3 as a model system, DFT calculations were made to draw the pictures of surface-gas interactions, which were combined with empirical modeling of individual nanowire sensors to determine the sensing mechanism of this system. The surface reaction routine that involves non lattice oxygen was found to responsible for the sensing effect. As an interfering substance to NH3 sensing, H2O was also studied in this approach. At the new material front, CuO decorated SnO2 nanowire showed significantly increased sensitivity toward H2S while keeping other gases, e.g., CO and NH3 low, offering good selectivity to this gas. Ex-situ analysis has shown that sulfurization and desulfurization reactions happened on CuO, confirming the charge transport channel depletion model proposed for this material. The less common p-type CuO was obtained with the facile thermal oxidation method. NH3, H2S and NO2 sensing have all indicated the key role of surface adsorbed oxygen species in its gas sensing. Due to its intrinsic property, the ZnO nanowires assembled onto micro hot plate (μHP) substrates by dielectrophoretic (DEP) alignment showed relative NH3 selectivity from CO. When operated in temperature-pulsed mode, sensitivity enhancement was seen at the low temperature end. Such effect was ascribed to the fast regulation of surface oxygen, H2O and NO2 in the pulsed mode. The current dissertation is organized as follows: Chapter 1 introduces the general background of the MOX gas sensors and the basic idea of computational chemistry. Chapter 2 gives a brief introduction to density functional theory, which is the major theoretical tool in this work. Chapter 3 describes the experimental and theoretical techniques that have been applied. Chapter 4 deals with the NH3/H2O sensing of SnO2 nanowire, DFT calculations and empirical modeling. The sensing mechanism of NH3 by SnO2 and the interfering principle of H2O were unveiled. Chapter 5 reports the H2S sensing of SnO2 and CuO decorated SnO2 nanowires and the study of the corresponding mechanisms. Chapter 6 explores the NH3, H2S and NO2 sensing properties of the individual CuO nanowire. The importance of surface oxygen species in gas sensing was demonstrated. Chapter 7 is about the DEP deposition of ZnO nanowires onto the μHP sensing substrate and the NH3 sensing in isothermal and temperature-pulsed mode. Chapter 8 reviews the present work, highlighting the main achievements and proposes future directions.
Aquesta tesi se centra en l’estudi dels mecanismes de detecció de gasos amb sensors basats en nanofils d’òxids metàl•lics. Amb aquest objectiu, s’han estudiat les respostes de sensors formats per un únic nanofil, s’ha modelitzat la seva resposta mitjançant càlculs DFT (Density Functional Theory) i s’han analitzat nous materials i explorat modes de funcionament no estàndards com és el polsat de temperatura. Als tres primers capítols de la dissertació se’n fa una introducció als dispositius basats amb òxids metàl•lics, es revisen els fonaments teòrics que hi ha darrera de les simulacions DFT i es presenten els mètodes experimentals que s’han fet servir per completar aquest treball doctoral. El quart capítol se n’ocupa de les interaccions del sistema SnO2-NH3 mitjançant la combinació de càlculs teòrics amb DFT i dades experimentals. Es presenta i valida el mecanisme de detecció de l’amoníac amb l’òxid d’estany així com es discuteix les interferències d’aquest contaminant amb la humitat. Al cinquè capítol es presenta la detecció de H2S amb heteroestructures formades per nanofils de SnO2 decorats amb nanopartícules de CuO. La gran sensitivitat a aquest gas que es troba experimentalment, especialment si es compara amb les respostes típiques obtingudes amb nanofils no decorats, s’ha analitzat i modelitzat. El capítol sisè explora la utilització d’òxid de coure, un semiconductor tipus p, com a sensor de gas; i les seves respostes a diferents contaminants es comparen amb les obtingudes amb l’òxid d’estany, el semiconductor tipus n de referència. Ja al capítol setè, es presenta el dipòsit controlat de nanofils de ZnO sobre hotplates mitjançant dielectroforesi (DEP) així com la millora de la sensitivitat quan els dispositius obtinguts són operats en mode de temperatura polsada. Finalment, el capítol vuitè i últim resumeix tots els capítols anteriors destacant els resultats més significatius aconseguits, i s’exploren noves línies de treball per a futures tesis.

La mesure de la qualité de l'air intérieur est un besoin relativement récent. Les êtres humains passent plus de 90 % de leurs temps dans un environnement fermé (pièce intérieure) qui contient plusieurs polluants gazeux. L'existence de tels contaminants gazeux dans l'air intérieur d'une pièce fermée ainsi que l'exposition à court ou à long terme à ces polluants peuvent provoquer des problèmes respiratoires et plusieurs maladies chroniques. Des études montrent que la qualité de l'air intérieur a un impact direct sur le bien-être et la productivité d'une part et sur la santé à plus long terme, d'autre part. Les COVs (composés organiques volatils) sont une classe importante de ces polluants, comme l'acétaldéhyde et le formaldéhyde provenant de matériaux utilisés dans l'aménagement intérieur (équipements informatiques, mobilier, peintures, tissu! s, sols...). Nous trouvons aussi des contaminants comme le CO2 provenant de l'utilisation intensive et d'une mauvaise aération des locaux, ainsi que le CO, et le NO2 issus de la pollution urbaine. Les bureaux, les salles de réunions, les salles de classes et les salles de travaux pratiques dans les milieux universitaires ou/et scolaires sont donc potentiellement pollués. Dans une pièce densément occupée et mal aérée la mesure du taux de COV/CO2 peut dépasser les seuils règlementaires. Ces polluants gazeux dans l'air à des concentrations importantes, faute d'une ventilation suffisante et d'un contrôle de la qualité de l'air, peut provoquer des somnolences et diminution de la productivité. La mesure et la surveillance de la qualité de l'air intérieur est donc indispensable pour assurer une meilleure qualité de vie dans les espaces de travail. Cette thèse est réalisée dans le cadre du GIS (groupement d'intérêt scientifique) neOCampus, porté par l'université Paul Sabatier et dédié au développement d'un campus innovant, connecté et durable pour une meilleure qualité de vie des usagers. Nous nous sommes intéressés au développement de micro-capteurs de gaz MOS (capteurs à oxydes métalliques) et à leur pilotage pour la surveillance de la qualité de l'air intérieur dans les bureaux, les salles de classes et les salles de réunions. L'objectif de cette étude est de suivre ces niveaux de pollution pour les corriger par des mesures d'aération des locaux. La prise de décision concernant l'action de correction de qualité de l'air est une étape essentielle du processus. Citons par exemple : la régulation de la ventilation dans une pièce en cas de dépassement du seuil autorisé pour les polluants identifiés. Dans le cadre de ces travaux, nous avons réalisé des prototypes de multi-capteurs de gaz miniaturisés et intégrés avec leur carte électronique dans une pièce témoin et capables de détecter des niveaux de pollution de l'air intérieur. [...]
Measuring indoor air quality is a relatively recent need. Humans spend more than 90% of their time in a closed environment that contains several gaseous pollutants. The existence of such gaseous contaminants in the indoor air as well as short or long term exposure to these pollutants can causes many respiratory problems and several chronic diseases. Studies show that the indoor air quality has an impact on well-being and productivity. VOCs (volatile organic compounds) such as acetaldehyde and formaldehyde are strongly presented in indoor air. This type of pollutants come from materials used in interior design (computer equipment, furniture, paints, fabrics, floors, etc.). We can also found in close envirements many others contaminants such as CO2, CO, and NO2 which come from urban pollution, intensive use of location and poor ventilation. Offices, meeting rooms, classrooms and practical work rooms in universities and / or schools are therefore potentially polluted. In a densely occupied and poorly ventilated room, the measurement of the VOC/CO2 rate may exceed the regulatory thresholds. These gaseous pollutants in the air in high concentrations, due to lack of sufficient ventilation and air quality control, can cause drowsiness and decreased productivity. Measuring and monitoring indoor air quality is therefore essential to ensure a better quality life in workspaces. This thesis is being carried out within the framework of the neOCampus GIS (scientific interest group), led by Paul Sabatier University and dedicated to the development of an innovative, connected and sustainable campus for a better quality life for users. We are interested by the development of micro-gas sensors MOS (metal oxide sensors) and the indoor air quality monitoring in offices, classrooms and meeting rooms. The objective of this study is to control these pollution levels in order to correct them through measures to ventilate the premises. Making a decision about how to correct air quality is an essential step in the process. For example: regulating ventilation in a room if the authorized threshold is exceeded for the identified pollutants. As part of this work, we produced prototypes of miniaturized multi-gas sensors integrated with their electronic card in a witness room and capable of detecting levels of indoor air pollution. These prototypes include a multi-sensor cell (with 4 independent cells), proximity electronics allowing the control and recovery of data from these cells, an IOT (internet of things) type communication module based on the LoRA protocol allowing send to the "Cloud NeoCampus", remotely and wirelessly, an indoor air quality status signal. This multi-sensor is based on semiconductor sensors based on nanostructured metal oxides synthesized at the LCC (coordination chemistry laboratory). [...]

Nanomaterials have received increasing attention during the last decades in the solid state field since they play a major role as catalyst and catalyst supports for many applications including fuel cells or gas sensors. The interest is mainly due to their high specific surface area, which leads to an increase of performance and a cost-effective solution for expensive or rare materials. However, many studies have reported the collapse of nanostructures at high temperature as one of the main drawbacks for their implementation in real devices and therefore, routes to thermally stabilize these materials must be explored. In this thesis, the unique features of ordered mesoporous materials fabricated by nanocasting are exploited to create quasi-universal thermal stabilization methodologies, allowing implementing and evaluating them in high temperature energy applications e.g. solid oxide fuel cells. The work developed is divided into seven parts. The first chapter introduces the basics of mesoporous materials, solid oxide fuel cells, catalysis and gas sensors. The second chapter focuses on the experimental procedures and the characterization tools employed. In the third chapter, a novel route to thermally stabilize 3-D open mesoporous structures is presented. The next three chapters, show the fabrication and evaluation of thermal stable mesoporous materials as electrodes for solid oxide fuel cells. Finally, chapter seven presents the suitability of mesoporous ceramic oxides as functional materials in humidity sensors.
Los nano-materiales han recibido especial atención durante estas últimas décadas en el campo del estado sólido dado el importante papel que desempeñan como catalizadores y/o soportes catalíticos en diversas aplicaciones, tales como las pilas de combustible o los sensores de gas. Este interés se debe principalmente a su elevada área específica, que da lugar a una mejora del rendimiento y es una solución efectiva para aquellas aplicaciones que requieran materiales de elevado coste. Sin embargo tal y como señalan muchos estudios, el colapso de estas nano-estructuras a elevadas temperaturas es uno de los mayores inconvenientes para su implementación en dispositivos reales, siendo por tanto necesario explorar nuevas rutas que consigan estabilizar estos materiales térmicamente. El objetivo de la presente tesis es desarrollar metodologías cuasi-universales de estabilización térmica, mediante la explotación de las características exclusivas que poseen los materiales mesoporosos ordenados fabricados a partir de un template. Lo cual nos permite implementarlos y evaluarlos en aplicaciones energéticas que operan a elevada temperatura p.ej. pilas de combustible de óxido sólido. El trabajo desarrollado se divide en siete partes. El primer capítulo introduce los fundamentos de los materiales mesoporosos, las pilas de combustible de óxido sólido, la catálisis y los sensores de gas. En el segundo capítulo se detallan los procedimientos experimentales y las técnicas de caracterización empleados. El tercer capítulo presenta una nueva metodología para estabilizar térmicamente los materiales mesoporosos de estructura 3-D abierta. Los siguientes tres capítulos, muestran la fabricación y el comportamiento electroquímico de materiales mesoporosos térmicamente estables trabajando como electrodos de pilas de combustible de óxido sólido. Por último, en el capítulo siete se demuestra la viabilidad de los óxidos cerámicos mesoporosos como materiales funcionales en sensores de humedad.

Clandestine laboratories are locations where chemistry is carried out in secret, often with the intent to produce illegal drugs or other controlled substances. These laboratories are unregulated and not maintained to a good laboratory standard, presenting a risk to first responders, bystanders and the environment. Electronic noses based on metal oxide semiconducting (MOS) gas sensors present a potential technology to create devices for the detection of clandestine activity. A range of sensors based on zinc oxide, chromium titanate and vanadium pentoxide have been manufactured and modified using zeolite material and metal ion doping. Sensor fabrication took place using a commercially available screen printer, a 3 x 3 mm alumina substrate containing interdigitated electrodes and a platinum heater track. Allmaterials were modified with the protonated forms of zeolite beta, Y, mordenite and ZSM5, by incorporating these materials into the metal oxide to make up 30 % of the total ink. Zinc oxide was also modified by indium doping; doping levels were set at 0.2, 0.5, 1 and 3-mol % indium. These materials were synthesised using a co-precipitation method. Sensors were exposed to a range of gases at operating temperatures between 250 and 500°C and concentrations between 50 ppb and 80 ppm. All tests were conducted on an in house testing rig, consisting of a 12-port sensing chamber, four mass flow controllers, six solenoid vales and supplies of compressed air and analyte gas. Modification of sensors was found to improve their responsiveness, compared to the control sensors, in almost all cases. This is due to a combination of surface area enhancements, increased adsorption of material and a more accessible microstructure. Machine learning techniques were applied to the sensor data to correctly classify the class of gas observed and to assess the overall sensor performance of each material. A high level of accuracy was achieved in determining the class of gas observed.

Investigations into a number of hetero-junction and nanoceramic materials systems for metal oxide semiconductor (MOS) gas sensing for potential environmental and bio-sensing applications are presented. The hetero-junction study encompasses investigations into various composite n-n hetero-contact systems such as WO3-ZnO and SnO2-ZnO and a p-n hetero-contact system, specifically CTO (Chromium Titanium Oxide) - ZnO. The facile fabrication of various arrays of hetero-junction MOS gas sensor devices has been demonstrated. A simple change in the compositional contribution of an individual metal oxide within a composite, exhibits the ability to tune the composite’s responsivity and selectivity. The hetero-junction systems were characterized by various techniques including Scanning Electron Microscopy (SEM), Raman spectroscopy, X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) and the influence of the physical and chemical properties of these materials towards the associated gas sensing properties, deduced. Further, the influence of fundamental properties of junctions such as contact potential and packing structure, towards the sensing properties, are also discussed. The nanomaterials study encompasses investigation into ZnO semiconducting oxides fabricated by various emerging fabrication technologies including Continuous Hydrothermal Flow Synthesis (CHFS) and other relatively high temperature routes. The chemical and physical properties of the nanoceramics have been investigated by various techniques including Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Brunauer Emmett Teller (BET) surface area measurements. The investigation demonstrates emerging techniques for the production of nanomaterials, which can be successfully used in MOS gas sensing for the desired applications. Further, the study shows that the behaviour of the nanomaterials is complex and material surface area is not the only deterministic factor of enhanced responsivities, but microstructural factors such as morphology and particle size, as well as heat-treatment conditions are all influential over the overall sensing properties. This thesis presents an overview of emerging material systems for MOS gas sensing applications.

To build novel electronic noses for mimicking biological olfactory systems that consist of olfactory receptor arrays with large surface area and massively-diversified chemical reactivity, three dimensional (3D) vertical aligned ZnO nanowire arrays were employed as active materials for gas detection. ZnO nanowire arrays share 3D structures similar to mammalian olfactory receptor arrays, with thousands of vertical nanowires providing a high reception area which can significantly enhance the sensors’ sensitivity. Meanwhile, with different material decorations (such as SnO2, In2O3, WO3 and polymers), each array of nanowires can produce a distinguishable response for each separate analyte, which would provide a promising way to improve the selectivity. Both patterned grown well-aligned and wafer size random-distributed 3D nanowire array sensing devices are investigated. Several different types of gas sensors have been investigated in this dissertation. Metal oxide semiconductor gas sensors based on 3D metal oxides/ZnO vertical nanowire arrays have detected NO2 and H2S down to ppb level, and five gases of NO2, H2S, H2, NH3, and CO have been discriminated. Active self-powered gas sensors based on 3D metal oxides/ZnO vertical nanowire arrays have been successfully fabricated and worked well for H2S and NO2 detection. With the decoration by mixture of PEDOT polymer with metal oxide nanoparticles, ZnO vertical nanowire array gas sensors have fast response and recovery time as well as good sensitivity to volatile organic gases of acetone, methanol and ethanol. A novel ionization sensor also has been built by ZnO vertical nannowire arrays, and this device could be able to ionize air under safety operation voltage.

Abstract The aim of this thesis is to study whether nanostructured particles of WO3 could be competitive counterparts of traditional, more bulky materials in resistive gas sensor applications. Pristine and various surface decorated derivatives of three different types of WO3 nanoparticles applied on the surface of lithographically defined Si chips were used in the work to analyse the electrical behaviour of thin films when exposed to different gas atmospheres. Nanosized particles of WO3, obtained by capillary force-induced collapse of porous anodic tungsten oxide in water, were demonstrated as a sensing medium for the detection of H2 and NO analytes. Commercially available nanoparticles of WO3 were also studied. After decorating their surface with metal/metal oxide nanoparticles (Ag, PdOx and PtOx), stable aqueous dispersions were made and used for the inkjet printing of conductive patterns on test chips. Surface decoration was found to affect substantially the gas response behaviour of the materials with the largest differences in response to H2 and NO. The third type of tungsten oxide applied consisted of hydrothermally synthesized nanowires that were also surface decorated with PdO as well as with PtOx. The nanowires were suspended in water and drop cast on test chips for gas sensing measurements. The nanowire based devices allowed ultrasensitive detection of H2 even at room temperature. The results summarized in this thesis indicate that resistive gas sensors based on nanostructured tungsten oxides are excellent alternatives to existing devices utilizing porous thick films or bulky thin films. Their high sensitivity, low operating temperature and low electrical power consumption may enable the construction of portable sensors, for example by inkjet printing, thus having great potential for fast prototyping but also for large scale production at low cost
Tiivistelmä Väitöstyön tavoitteena on tutkia nanorakenteisten WO3 hiukkasten kilpailukykyä suhteessa perinteisiin suuremman kidekoon materiaaleihin resistiivisissä kaasusensorisovelluksissa. Työssä tutkittiin kolmella eri tekniikalla valmistettujen WO3 nanopartikkeleiden alkuperäisistä ja pintakäsitellyistä versioista muodostettujen ohutkalvojen sähköisiä ominaisuuksia erilaisten kaasukehien funktiona. Veden kapillaarivoimien aikaan saaman huokoisen anodisen volframioksidirakenteen romahduksen kautta saatujen WO3 nanopartikkeleiden osoitettiin toimivan havaintoväliaineena H2 ja NO kaasuille. Myös kaupallisia WO3 nanopartikkeleita tutkittiin. Partikkelien pinta päällystettiin metalli- ja metallioksidinanopartikkeleilla (Ag, PdOx and PtOx), jonka jälkeen niistä muodostettiin vakaita vesipohjaisia seoksia johtavien kuvioiden mustesuihkutulostukseen testisubstraateille. Pintakäsittelyn havaittiin vaikuttavan merkittävästi materiaalien kaasuvasteisiin erityisesti H2:n ja NO:n tapauksessa. Kolmannen tyyppinen väitöskirjassa tutkittu volframioksidimateriaali koostuu hydrotermisesti syntetisoiduista nanojohdoista, jotka ovat pintakäsitelty PdO tai PtOx nanopartikkeleilla. Nanojohdot sekoitettiin veteen ja pipetoitiin testisubstraateille kaasumittauksia varten. Tämän tyyppiset kaasusensorit olivat erityisen herkkiä H2 kaasulle jopa huoneenlämmössä. Väistökirjan tulosten mukaan nanorakenteiset volframioksidimateriaalit ovat erinomainen vaihtoehto perinteisille huokoisille paksukalvoille ja suhteellisen paksuille ohutkalvoille kaasusensorisovelluksissa. Niiden suuri herkkyys, alhainen toimintalämpötila ja matala sähkönkulutus voivat mahdollistaa kannettavien kaasusensorien valmistuksen, esimerkiksi mustesuihkuteknologilla, nopeaan testaukseen ja suuren mittakaavan tuotantoon alhaisin kustannuksin

>Magister Scientiae - MSc
The industrial safety requirements and environmental pollution have created a high demand to develop gas sensors to monitor combustible and toxic gases. As per specifications of World Health Organization (WHO) and Occupational Safety and Health Administration (OSHA), lengthy exposure to these gases lead to death which can be avoided with early detection. Semiconductor metal oxide (SMO) has been utilized as sensor for several decades. In recent years, there have been extensive investigations of nanoscale semiconductor gas sensor.

Abstract. This thesis concentrated on the analysis of the gas response properties of several metal oxide based gas sensors. A thin layer of chosen metal oxide was deposited on SGX Sensortech S.A. sensor platforms using pulsed laser deposition (PLD). Metal oxides used in the studies included tungsten trioxide (WO₃3), tin oxide zinc oxide (SnO₂-ZnO) and vanadium pentoxide (V₂O₅). The films were deposited at room temperature and various oxygen partial pressures, and were then post-annealed at 400 °C. Gas response measurements were done in two different temperatures and using several gases including nitrogen oxides (NOx), carbon monoxide (CO), hydrogen (H₂), and ammonia (NH₃). The concentration of the gases were varied during each measurement to probe the sensitivity of the sensors. Gas sensing performance of the sensors were evaluated based on material, selectivity toward different gases, and the effect of surface structure. Oxygen partial pressure during PLD had a clear impact on the structure of the oxide film. Higher pressure resulted in larger agglomerates of particles, which in general leads to lower gas sensitivity due to factors such as grain size and surface area-to-volume ratio. The measurements showed high responses to NOx for WO₃ and SnO₂-ZnO samples, as expected. Also, flipping of the response from low concentration to high concentration was observed for WO₃ and SnO₂-ZnO while V₂O₅ showed a mostly stable response.Metallioksidinanopartikkeleihin perustuvien kaasuantureiden analysointi MEMS-rakenteissa. Tiivistelmä. Tässä työssä analysoitiin useiden metallioksideihin perustuvien antureiden kaasuvasteita. Kaasuantureiden substraattina käytettiin SGX Sensortech S.A. valmistamia mikrolämmittimeen pohjautuvia MEMS-rakenteita. Substraatin päälle kasvatettiin ohut kerros valittuja metallioksideja, kuten volframioksidi (WO₃), tinaoksidin ja sinkkioksidin yhdiste (SnO₂-ZnO), ja vanadiumoksidi (V₂O₅). Kasvatusmenetelmänä käytettiin pulssilaserkasvatusta. Kasvatus tapahtui huoneenlämmössä ja useissa eri hapen osapaineissa. Kasvatuksen jälkeen anturit jälkihehkutettiin 400 °C lämpötilassa. Kaasuvastemittaukset suoritettiin kahdessa eri lämpötilassa usealle eri kaasulle, kuten typpioksideille (NOx), hiilimonoksidille (CO), vetykaasulle (H₂) ja ammoniakille (NH₃). Kaasun konsentraatiota vaihdeltiin mittausten aikana antureiden herkkyyden määrittämiseksi. Kaasuantureiden toimintakykyä arvioitiin materiaalin, selektiivisyyden ja oksidin pintarakenteen perusteella. Hapen osapaineella pulssilaserkasvatuksen aikana oli merkittävä vaikutus oksidikerroksen rakenteeseen. Suuremmassa paineessa kasvatetut kerrokset muodostivat suurempia partikkeleiden agglomeraatteja, mikä yleisesti ottaen johti heikompaan kaasuvasteeseen johtuen suuremmasta partikkelikoosta ja pienemmästä pinta-alan ja tilavuuden suhteesta. Mittauksissa nähtiin voimakkaita reaktioita typpioksidikaasuihin erityisesti SnO₂-ZnO ja WO₃ näytteiden osalta, kuten oli odotettavissa. SnO₂-ZnO ja WO₃ näytteillä oli myös havaittavissa kaasuvasteen suunnan muutos redusoivasta oksidoivaan kaasukonsentraation kasvaessa, kun taas V₂O₅5-näytteet käyttäytyivät enimmäkseen vakaasti.

A la present tesis doctoral, s'han produït diferents tipus de nanoestructures basades en òxids metàl·lics, com per exemple nanofils de ZnO i octaedros d'In2O3, utilitzant el mètode de Deposició Química de Vapor (CVD) a altes temperatures. Per a la detecció d'òxid de nitrogen, s'ha descobert que la resposta dels nanofils d'ZnO està directament relacionada amb la quantitat de defectes presents en el material. Com més gran es el nombre de defectes, major és la resposta al diòxid de nitrogen. Tanmateix, per a la detecció d'etanol, la mostra que contenia un nombre mig de defectes, va ser la que dóna millors resultats. Pel que fa als octaedres de In2O3, podem dir que els octaedres d'In2O3 pur són excel·lents candidats per a la detecció de NO2, ja que tenen una excel·lent sensibilitat (0.43 ppb-1) a baixes temperatures (130ºC), mentres que la resposta a altres gasos com H2 és dos ordres de magnituds inferior en les mateixes condicions. A més a més, en presencia d'humitat, s'incrementa la sensibilitat a NO2 i, a la vegada, es redueix la sensibilitat a H2, pel que la selectivitat a NO2 també es veu incrementada. Finalment, utilitzant l'espectroscopia DRIFT, s'ha analitzat l'In2O3 expost a 1 ppm de NO2 a diferents temperatures i s'ha descobert que el mecanisme proposat per a descriure el procés de sensat es bastant més complicat del que s'ha reportat fins ara. Com a resultat de tots aquests experiments, s'ha donat un pas edanvant en l'explicació dels mecanismes de sensat dels nanofils d'ZnO i els octaedres de In2O3 a diferents temperatures.
En la presente tesis doctoral, se han producido diferentes tipos de nanoestructuras basadas en oxidos metalicos, como por ejemplo nanohilos de ZnO y ocaedros de In2O3, utilizando el método de Deposición Química de Vapor (CVD) a altas temperaturas. Para la detección de dióxido de nitrógeno, se ha descubierto que la respuesta de los nanohilos de ZnO está directamente correlacionada con la cantidad total de defectos presentes en el material. Cuanto mayor es el número de defectos, mayor es la respuesta al dióxido de nitrógeno. Sin embargo, para la detección de etanol, la muestra que contenía un número medio de defectos fue la que dio mejores resultados. Por lo que respecta a los octaedros de In2O3, podemos decir que los octaedros de In2O3 puro son excelentes candidatos para la detección de NO2, ya que poseen una excelente sensibilidad (0.43 ppb-1) a bajas temperaturas (130ºC), mientras que la respuesta a otros gases como H2 es dos órdenes de magnitud inferior en las mismas condiciones. Además, en presencia de humedad, se incrementa la sensibilidad a NO2 y, a la vez, se reduce la sensibilidad a H2, por lo que la selectividad hacia NO2 tambien se ve incrementada. Finalmente, utilizando la espectroscopia DRIFT, se ha analizado el In2O3 expuesto a 1 ppm de NO2 a diferentes temperaturas y se ha descubierto que el mecanismo propuesto para describir el proceso de senado es bastante más complicado que lo que se ha publicado hasta ahora. Como resultado de todos estos experimentos, se ha arrojado luz nueva sobre los mecanismos de sensado de los nanohilos de ZnO y los octahedros de In2O3 a diferentes temperaturas.
In the present doctoral thesis, several metal oxide nanostructures such as ZnO nanowires and In2O3 octahedra via a chemical vapour deposition (CVD) method at high temperatures. For the detection of nitrogen dioxide, it was found that the response of ZnO nanowires was directly correlated to the overall amount of defects of the material. The higher the number of defects is, the higher the response to nitrogen dioxide is. On the other hand, for the detection of ethanol, ZnO nanowires with an intermediate number of defects in which surface defects were dominant led to the best results. Additionally, regarding the In2O3 octahedra, we can say that pure In2O3 octahedra are excellent for detecting NO2 gas with an outstanding sensitivity (0.43 ppb-1) at low temperatures (130ºC), while the response to H2 remains two orders of magnitude lower under the same conditions. In addition, the presence of humidity increases the sensitivity to NO2 and, at the same time, reduces the response to H2, which results in an increased selectivity. Finally, by making use of the DRIFT spectroscopy, we have analyzed In2O3 material towards 1 ppm of NO2 at different temperatures and, it has been found that the mechanism proposed for the gas sensing is far more complicated than previously reported. As a result of these experiments, new light on the sensing mechanism of ZnO and In2O3 material towards NO2 gas at different temperatures has been shed.

Mimicking the biological olfactory systems that consist of olfactory receptor arrays with large surface area and massively-diversified chemical reactivity, three dimensional (3D) metal oxide nanowire arrays were used as the active materials for gas detection. Metal oxide nanowire arrays share similar 3D structures as the array of mammal's olfactory receptors and the chemical reactivity of nanowire array can be modified by surface coatings. In this dissertation, two standalone gas sensors based on metal oxide nanowire arrays prepared by microfabrication and in-situ micromanipulation, respectively, have been demonstrated. The sensors based on WO3 nanowire arrays can detect 50 ppb NO2 with a fast response; well-aligned CuO nanowire array present a new detection mechanism, which can identify H2S at a concentration of 500 ppb. To expand the material library of 3D metal oxide nanowire arrays for gas sensing, a general route to polycrystalline metal oxide nanowire array has been introduced by using ZnO nanowire arrays as structural templates. The effectiveness of this method for high performance gas sensing was first investigated by single-nanowire devices. The polycrystalline metal oxide coatings showed high performance for gas detection and their sensitivity can be further enhanced by catalytic noble metal decorations. To form electronic nose systems, different metal oxide coatings and catalytic decorations were employed to diversify the chemical reactivity of the sensors. The systems can detect low concentrated H2S and NO2 at room temperature down to part-per-billion level. The system with different catalytic metal coatings is also capable of discriminiating five different gases (H2S, NO2, NH3, H2 and CO).

In order to create more sensitive and accurate gas sensors, we have studied the interactions of gas mixtures on metal oxide nanoparticle decorated porous silicon interfaces. The nanoparticles control the magnitude and direction of electron transduction from the interaction of analyte gases to an extrinsic porous silicon semiconductor. These interactions can be predicted by the Inverse Hard Soft Acid Base (IHSAB) principle. Moreover, the metal oxide nanoparticles can be functionalized with nitrogen and sulfur, modifying the oxide’s band structure. These modifications are demonstrated to change analyte interactions in line with the IHSAB concept and allow for light enhanced sensors. Further we looked at how the analyte gases interact with other analyte gases on the surface of these sensors. Studying these systems does two things, first the research will lead to cheaper, more accurate gas sensors, and second it helps explore the role of nanoparticles in modifying the interactions between bulk materials (porous silicon) and molecules (analyte gases).

This thesis investigates the modification of three metal oxide semiconductor gas sensors with zeolite materials for the purposes of detecting trace concentrations of gases that have an effect on health, security, safety and the environment. SnO2, Cr2O3 and Fe2O3 were chosen as the base materials of interest. Zeolites HZSM- 5, Na-A and H-Y were incorporated into the sensing system either as admixtures with the base material or as coatings on top of it. The aim of introducing zeolites into the sensing system was to improve the performance of the otherwise unmodified sensors. Twenty-two novel zeolite-modified sensor systems are presented for the detection of a range of hydrocarbons and inorganic gases. Whilst sensors based on SnO2 systems were more responsive to gases, some sensors were also found to provide a greater degree of variability among repeat tests, particularly at lower operating temperatures i.e. 300 °C. Cr2O3 sensors modified by admixture with zeolite H-ZSM- 5 were seen to be poorly sensitive to most analytes. Cr2O3 sensors modified by admixture with zeolite Na-A and by overlayer of zeolite H-Y provided very promising sensitive and selective results towards toluene gas. Sensors based on the zeolite modification of Fe2O3 were not found to be promising candidates as gas sensors at this stage. Sensors were purposely exposed to gases that had similar molecular structures or kinetic diameters to assess the true capability of the sensors to discriminate among analytes. An array of four sensors based on n-type and p-type systems was subsequently chosen to see whether machine learning classifiers could be used to accurately discriminate among nine analytes. Using an SVM SMO classifier with a polykernel function, the model was 94.1% accurate in correctly classifying nine analytes of interest just after five seconds into the gas injection. Using an RBF kernel function, the model was 90.2% accurate in correctly classifying the data into gas type. These are very encouraging results, which highlight the importance of furthering research in this field; a sensing array based on zeolite-modified metal oxide semiconductor sensors may benefit a number of research domains by providing accurate results in a very fast and inexpensive manner.

The design, microfabrication, and CMOS integration of micro-electro-mechanical systems (MEMS) capacitive humidity sensors are presented in this work. Theoretical analysis and simulations were done to understand how sensor performance can be optimized. While CoventorWare was used for steady-state simulations, a MATLAB simulation model, based on the mathematics of moisture adsorption and diffusion, was developed for dynamic simulations. The sensors were fabricated using a process flow that has a low thermal budget (≤ 300 ○C), as well as material and chemical compatibility with IC fabrication, allowing it to support monolithic integration with CMOS circuitry for system-on-chip (SoC) designs. The fabricated sensors were tested using both deliquescent calibration salts and a humidity / temperature chamber, providing results that were used to compare the performance of various sensor designs. These experimental results, along with the simulation results, were used to devise and justify a design methodology for MEMS capacitive relative humidity sensors. The sensors showed high sensitivity over a large dynamic range, response times as fast as 1.5 seconds, and excellent long term drift as low as 0.1 %RH/year. The humidity sensors were fabricated on top of CMOS dies (TIA - transimpedance amplifier) obtained from Texas Instruments to demonstrate the capability of full monolithic integration of the MEMS sensors and IC. A very convenient and versatile methodology was reported and used for integrating the MEMS sensors above IC dies of any size. Test results show that the performance of the TIA is unaffected by the integration, while the MEMS sensors grown on top of the TIA are fully functional, thereby validating the integration procedure used and the IC-compatibility of the MEMS humidity sensor process flow.
La conception, le microfabrication, et l'intégration de CMOS des sondes capacitives micro-électro-mécaniques d'humidité des systèmes (MEMS) sont présentés dans ce travail. L'analyse et les simulations théoriques ont été faites pour comprendre comment l'exécution de sonde peut être optimisée. Tandis que CoventorWare était employé pour des simulations équilibrées, un modèle de simulation de MATLAB, basé sur les mathématiques de l'adsorption et de la diffusion d'humidité, a été développé pour des simulations dynamiques. Les sondes ont été fabriquées en utilisant un écoulement de processus qui a un bas budget thermique (○C de ≤ 300), comme la compatibilité de matériel et de produit chimique avec la fabrication d'IC, lui permettant de soutenir l'intégration monolithique avec des circuits de CMOS pour des conceptions du système-sur-puce (SoC). Les sondes fabriquées ont été examinées en utilisant les deux sels déliquescents de calibrage et une chambre d'humidité/température, fournissant les résultats qui ont été employés pour comparer l'exécution de la diverse sonde conçoit. Ces résultats expérimentaux, avec les résultats de simulation, ont été employés pour concevoir et justifier une méthodologie de conception pour les sondes capacitives d'humidité relative de MEMS. Les sondes montrées la sensibilité élevée au-dessus d'une gamme dynamique étendue, des temps de réponse plus rapidement que 1.5 seconde, et d'une excellente dérive à long terme aussi basse que 0.1 % RH/year. Les sondes d'humidité ont été fabriquées sur les matrices de CMOS (TIA - amplificateur de transimpedance) obtenues à partir de Texas Instruments pour démontrer les possibilités de la pleine intégration monolithique des sondes et de l'IC de MEMS. Une méthodologie très commode et souple a été rapportée et employée pour intégrer les sondes de MEMS au-dessus des matrices d'IC de n'importe quelle taille. Les résultats d'essai prouvent que l'exécution du TIA est inchangée par l'intégration, alors que les sondes de MEMS développées sur le TIA sont entièrement fonctionnelles, validant de ce fait le procédé d'intégration utilisé et l'IC-compatibilité de l'écoulement de processus de sonde d'humidité de MEMS.

This thesis addresses the problem of detecting changes in the activity of a distant gas source from the response of an array of metal oxide (MOX) gas sensors deployed in an Open Sampling System (OSS). Changes can occur due to gas source activity such as a sudden alteration in concentration or due to exposure to a different compound. Applications such as gas-leak detection in mines or large-scale pollution monitoring can benefit from reliable change detection algorithms, especially where it is impractical to continuously store or transfer sensor readings, or where reliable calibration is difficult to achieve. Here, it is desirable to detect a change point indicating a significant event, e.g. presence of gas or a sudden change in concentration. The main challenges are turbulent dispersion of gas and the slow response and recovery times of MOX sensors. Due to these challenges, the gas sensor response exhibits fluctuations that interfere with the changes of interest. The contributions of this thesis are centred on developing change detection methods using MOX sensor responses. First, we apply the Generalized Likelihood Ratio algorithm (GLR), a commonly used method that does not make any a priori assumption about change events. Next, we propose TREFEX, a novel change point detection algorithm, which models the response of MOX sensors as a piecewise exponential signal and considers the junctions between consecutive exponentials as change points. We also propose the rTREFEX algorithm as an extension of TREFEX. The core idea behind rTREFEX is an attempt to improve the fitted exponentials of TREFEX by minimizing the number of exponentials even further. GLR, TREFEX and rTREFEX are evaluated for various MOX sensors and gas emission profiles. A sensor selection algorithm is then introduced and the change detection algorithms are evaluated with the selected sensor subsets. A comparison between the three proposed algorithms shows clearly superior performance of rTREFEX both in detection performance and in estimating the change time. Further, rTREFEX is evaluated in real-world experiments where data is gathered by a mobile robot. Finally, a gas dispersion simulation was developed which integrates OpenFOAM flow simulation and a filament-based gas propagation model to simulate gas dispersion for compressible flows with a realistic turbulence model.

En aquesta tesi es detalla la metodologia per obtindre sensors de gasos basats en òxid de tungstè nanoestructurat sobre suports micromecanitzats de silici. Aquesta nanoestructuració s’ha fet mitjançant una capa d’alúmina porosa como a motlle, pel que s’ha desenvolupat una metodologia per a compatibilitzar l'anodització de l’alumini, i altres metalls com el tungstè, amb els processos estàndards del silici. S’han desenvolupat dos tipus de capes nanoestructurades, nanotubs i nanopunts de WO3. Els nanotubs s’han obtingut depositant mitjançant polvorització catòdica reactiva la capa sensible sobre alúmina porosa recobrint les parets dels pors. Els nanopunts s’han obtingut anoditzant una bicapa d’alumini i tungstè, on la primera anodització crea la alúmina porosa i la segona fa créixer els nanopunts d’òxid de tungstè en la base dels pors. S’ha analitzat la composició, morfologia i funcionament com a sensors de gasos d’ambdós materials nanoestructurats i s’han comparat els resultats amb sensors basats en materials sense nanoestructuració.
En esta tesis se detalla la metodologia para obtener sensores de gases basados en óxido de tungsteno nanoestructurado sobre soportes micromecanizados de silicio. Dicha nanoestructuración se ha obtenido empleando una capa de alúmina porosa como molde, por lo que se desarrolla una metodología para compatibilizar la anodización del aluminio, y otros metales como el tungsteno, con los procesos estándares del silicio. Se han desarrollado dos tipos de capas nanoestructuradas, nanotubos y nanopuntos de WO3. Los nanotubos se han obtenido depositando por pulverización catódica reactiva la capa sensible sobre alúmina porosa recubriendo las paredes de sus poros. Los nanopuntos se han obtenido anodizando una bicapa de aluminio y tungsteno, donde la primera anodización crea la alúmina porosa y la segunda hace crecer los nanopuntos de óxido de tungsteno en la base de los poros. Se ha analizado la composición, morfología y funcionamiento como sensores de gases en ambos casos y se han comparado los resultados con los de sensores sin nanoestructuración.
This thesis shows the methodology to obtain nanostructured tungsten oxide layer as sensing material on silicon micromachined gas sensor devices. A porous anodised alumina layer was used as pattern to obtain it, so a technique has been developed to make compatible the anodising of aluminium and other metals like tungsten with the standard silicon processes. Two different nanostructuring approaches were developed, nanotube and nanodot based tungsten oxide layers. The WO3 nanotube layer has been obtained by the tungsten oxide deposition using reactive sputtering on the porous alumina layer. As a result a continuous sensing layer coats the pores without clogging them. WO3 nanodot layers were obtained by the anodising of an aluminium and tungsten bilayer, where the first anodising process grows the porous alumina layer and the second one generates the tungsten oxide nanodots in the end of the pores. Compositional and morphological studies and the study of their behaviour as gas sensors where conducted for the two nanomaterials. The results have been compared with the flat tungsten oxide layers on micromachined gas sensors.

This PhD thesis sets out to investigate novel Silicon Carbide (SiC) based field effect devices (Schottky and transistor structures), with gas sensitive layers for monitoring hydrogen and propene gases at high temperatures. The devices developed by the author were shown to exhibit sensitivities at least 1~2 orders of magnitude (voltage shift, ƒ¢V) higher than those reported in literature. Not only did the author seek to investigate the gas sensing potential of such devices, but also he set out to study, analyse and establish the gas interaction mechanism of these novel sensors. High temperature tolerant hydrogen and hydrocarbon sensors are required in numerous applications such as: aerospace, nuclear power plant, space exploration and exhaust monitoring in automobiles. Monitoring these gases in a reliable and efficient manner is of great value in these applications, not only from a safety point of view but also for economical reasons. Hence there is an absolute necessity for simple, efficient and high performance sensors not only for monitoring and leak detection but also to function as part of a safety device to prevent accidents. The proposed sensor structure of combining SiC with gas sensitive oxide layers allow them to be operated at high temperatures, making them extremely appealing for direct or in-situ monitoring applications. The microstructural analysis performed using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Rutherford Backscattering Spectroscopy (RBS) provides no evidence of inter-diffusion between different layers, in spite of the sensors being annealing at 650‹ in O2, H2 and C3H6 atmospheres for approximately 50hrs. Samples in different conditions (as deposited, annealed and tested) were compared. The electrical properties of the MROSiC (current-voltage, I-V and capacitance-voltage, C-V characteristics) and MESFET (drain current-source drain voltage (ID-VSD) and transfer, (ãID-H2 concentration) characteristics) devices were measured in the presence and absence of H2 and C3H6. Several parameters such as barrier height, saturation currents, pinch-off voltages and channel conductance were determined from the electrical characteristics, and their influence on the device performance was studied. The authorfs proposed gas interaction model based on energy band diagram is well supported by the experimental data obtained.

Philosophiae Doctor - PhD
Transition metal oxides magneli phases present crystallographic shear structure which is of great interest in multiple applications because of their wide range of valence, which is exhibited by the transition metals. The latter affect chemical and physical properties of the oxides. Amongst them we have nanostructures VO2 system of V and O components which are studied including chemical and physical reactions based on non-equilibrium thermodynamics. Due to their structural classes of corundum, rocksalt, wurtzite, spinel, perovskite, rutile, and layer structure, these oxides are generally used as catalytic materials which are prepared by common methods under mild conditions presenting distortion or defects in the case of VO2. Existence of an intermediate phase is proved using an x-ray thermodiffraction experiment providing structural information as the nanoparticles are heated. Potential application as gas sensing device has been the first time obtained due to the high surface to volume ratio, and good crystallinity, purity of the material and presence of suitable nucleating defects sites due to its n-type semiconductor behavior. In addition, annealing effect on nanostructures VO2 nanobelts shows a preferential gas reductant of Ar comparing to the N2 gas. Also, the hysteresis loop shows that there is strong size dependence to annealing treatment on our samples. This is of great interest in the need of obtaining high stable and durable material for Mott insulator transistor and Gas sensor device at room temperature.

Un dels majors problemes experimentats pels sistemes de detecció de gasos basats en sensors d'òxids metàl·lics és la seva manca de reproduibilitat, estabilitat i selectivitat. A fi i a efecte d'intentar resoldre aquest problemes, diferents estratègies han estat desenvolupades en paral·lel. Algunes es relacionen a la millora dels materials i d'altres impliquen el condicionament o el pre-tractament de les mostres. Les més emprades han consistit en aprofitar que els sensors presenten sensibilitats solapades per construir matrius de sensors i emprar tècniques de processament del senyal o bé utilitzar característiques de la resposta dinàmica dels sensors. En els darrers anys, modular la temperatura de treball del sensors d'òxids metàl·lics s'ha convertit en un dels mètodes més utilitzats per incrementar-ne la selectivitat. Això s'esdevé així donat que la resposta del sensor varia amb la seva temperatura de treball. Per això, en determinats casos, mesurant la resposta d'un sensor a n temperatures de treball diferents pot ser equivalent a tenir una matriu de n sensors diferents. Això permet obtenir informació multivariant de cada sensor individualment i ajuda a mantenir baixa la dimensionalitat del sistema de mesura per resoldre una determinada aplicació. Malgrat que molts i bons resultats han estat publicats dins aquest àmbit, la tria de les freqüències emprades en la modulació de la temperatura de treball dels sensor ha consistit fins ara en un procés empíric que no garanteix la obtenció dels millors resultats per una determinada aplicació. En aquest context, el principal objectiu d'aquesta tesi doctoral ha consistit en desenvolupar un mètode sistemàtic que permeti determinar quines són les freqüències de modulació òptimes que caldria emprar per resoldre un determinat problema d'anàlisi de gasos. Aquest mètode, extret del camp d'identificació de sistemes, ha esta desenvolupat i implementat per primer cop dins l'àmbit dels sensors de gasos. Aquest consisteix en estudiar la resposta dels sensors en presència de gasos mentre la temperatura de treball dels sensors és modulada per un senyal pseudo-aleatori de longitud màxima. Aquest senyals comparteixen algunes propietats amb el soroll blanc, i per tant poden ajudar a estimar la resposta lineal d'un sistema amb no-linealitats (per exemple, la resposta impulsional d'un sistema sensor-gas). El procés d'optimització es duu a terme mitjançant la selecció entre els components espectrals de les estimacions de la resposta impulsional, d'aquells que millor ajuden bé a discriminar o a quantificar els gasos objectiu dins una aplicació d'anàlisi de gasos donada. Tenint en compte que els components espectrals estan directament relacionats amb les xvii Improving the performance of micro-machined metal oxide gas sensors: Optimization of the temperature modulation mode via pseudo-random sequences. freqüències de modulació, la tria d'uns pocs components espectrals resulta en la determinació de les freqüències òptimes de modulació. En els primer experiments, senyals binaris pseudo-aleatoris van ser emprats per modular la temperatura de treball de sensors de gasos basats en òxids metàl·lics micro-mecanitzats dins d'un rang comprès entre 0 i 112,5 Hz. La freqüència superior és lleugerament superior a la frequència de tall de les membranes dels sensors. El resultat principal derivat d'aques estudi va ser que les freqüències de modulació interessants es trobaven en un rang comprès entre 0 i 1 Hz. Això és comprensible donat que la cinètica de les reaccions i dels processos d'adsorció que es produeixen en la superfície dels sensors són lentes i si aquestes s'han de veure modificades per la modulació térmica, llavors caldran senyals de modulació de baixa freqüència. Això explica perquè s'han vingut emprant senyals moduladores de temperatura en el rang dels mHz, malgrat que les membranes d'un dispositiu micromecanitzat presenten respostes tèrmiques molts més ràpides (típicament de l'ordre de 100 Hz). En els experiments que continuaren els primers, un mètode evolucionat per determinar les freqüències de modulació tèrmica òptimes va ser implementat. Aquest es basa en l'ús de seqüències pseudo-aleatòries multi-nivell de longitud màxima. Els senyals de tipus multi-nivell van ser considerats en substitució dels senyals binaris ja que els primers permeten obtenir una millor estimació que els segons de la dinàmica lineal d'un sistema amb no linealitats. I és ben conegut que els sensors de gasos basats en òxids metàl·lics presenten no linealitat en la seva resposta. Aquests estudis sistemàtics van ser completament validats mitjançant la síntesi de senyals multi-sinusoïdals amb les freqüències prèviament identificades emprant sequències pseudo-aleatòries. Quan la temperatura de treball dels sensors va ser modulada amb un senyal, el contingut freqüencial del qual era l'òptim, els gasos i les mescles de gasos considerades van poder ser discriminades perfectament i es va mostrar la possibilitat d'obtenir models de calibració acurats per predir la concentració dels gasos. En alguns casos, aquest procés de validació es va portar a terme emprant sensors que no havien estat utilitzats durant el procés d'optimització (per exemple, una agrupació de sensors diferent però del mateix lot de fabricació). En resum, el nou mètode desenvolupat en aquesta tesi per seleccionar les freqüències de modulació òptimes s'ha mostrat consistent i efectiu. El mètode és d'aplicació general i podria ser emprat en qualsevol problema d'anàlisi de gasos o bé estès a altres tipus de sensors (per exemple sensors polimèrics). Les contribucions científiques d'aquesta tesi s'han recollit en quatre articles en revistes internacionals i 13 llibres d'actes de conferències.
Uno de los mayores problemas experimentados en los sistemas de detección de gases basados en dispositivos de óxidos metálicos es su falta de reproducibilidad, estabilidad y selectividad. Con el fin de intentar resolver estos problemas, diferentes estrategias han sido desarrolladas en paralelo. Algunas de ellas se relacionan con la mejora de los materiales y otras implican acondicionamiento o pre-tratamiento de las muestras. Otras estrategias ampliamente empleadas consisten en aprovechar que los sensores presentan sensibilidades solapadas para construir matrices de sensores y emplear técnicas de procesamiento de señal o bien utilizar características de la respuesta dinámica de los sensores.En los últimos años, modular la temperatura de trabajo de los sensores de óxidos metálicos se ha convertido en uno de los métodos más utilizados para incrementar su selectividad. Esto se debe a, dado que la respuesta del sensor varía con su propia temperatura de trabajo, entonces, en determinados casos, midiendo la respuesta de un sensor a n temperaturas de trabajo diferentes, es equivalente a tener una matriz de n sensores diferentes. Esto permite obtener información multivariante de cada sensor individualmente y ayuda a mantener baja la dimensionalidad del sistema de medida para resolver una determinada aplicación. A pesar de los buenos resultados que han sido publicados dentro de este ámbito, la selección de las frecuencias empleadas en la modulación de la temperatura de trabajo de los sensores ha consistido, hasta el momento, en un proceso empírico lo que no garantiza la obtención de los mejores resultados para una determinada aplicación.En este contexto, el principal objetivo de esta tesis doctoral ha consistido en desarrollar un método sistemático que permita determinar cuales son las frecuencias de modulación óptimas que podrían emplearse para resolver un determinado problema de análisis de gases. Este método, extraído del campo de identificación de sistemas, ha sido desarrollado e implementado por primera vez dentro del ámbito de los sensores de gases. Éste consiste en estudiar la respuesta de los sensores en presencia de gases mientras la temperatura de trabajo de los sensores es modulada mediante una señal pseudo-aleatoria de longitud máxima. Estas señales comparten algunas propiedades con el ruido blanco, y por tanto pueden ayudar a estimar la respuesta lineal de un sistema con no-linealidades (por ejemplo, la respuesta impulsional de un sistema sensor-gas).El proceso de optimización es llevado a cabo mediante la selección entre las componentes espectrales de las estimaciones de la respuesta impulsional, de aquellas que más ayudan ya sea a discriminar o a cuantificar los gases objetivo dentro de una aplicación de análisis de gases dada. Teniendo en cuenta que las componentes espectrales están directamente relacionadas con las frecuencias de modulación, la selección de unas pocas componentes espectrales resulta en la determinación de las frecuencias optimas de modulación.En los primeres experimentos, señales binarias pseudo-aleatorias fueron utilizadas para modular la temperatura de trabajo de los sensores de gases basados en óxidos metálicos micro-mecanizados en un rango comprendido entre 0 a 112.5 Hz. La frecuencia superior es ligeramente mayor a la frecuencia de corte de las membranas de los sensores. El resultado principal derivado de estos estudios fue que las frecuencias de modulación interesantes se encuentran en un rango comprendido entre 0 y 1 Hz. Esto es comprensible dado que la cinética de las reacciones y de los procesos de adsorción que se producen en la superficie del sensor son lentos y si estos se han de alterar mediante la modulación térmica, se habrá de elaborar señales de modulación a bajas frecuencias. Esto explica por que se han venido empleado señales moduladoras de temperatura en el rango de los mHz, a pesar que las membranas de un dispositivo micro-mecanizado presentan respuestas mucho más rápidas (típicamente en el orden de los 100 Hz).En los experimentos posteriores a los primeros, un método evolucionado para determinar las frecuencias de modulación óptimas de los sensores micro-mecanizados fue implementado, el cual se basa en el uso de secuencias pseudo-aleatorias multi-nivel de longitud máxima (MLPRS). Las señales de tipo multi-nivel fueron consideradas en lugar de las binarias ya que las primeras permiten obtener una mejor estimación que las segundas de la dinámica lineal de un sistema con no linealidades. Y es bien conocido que los sensores de gases basados en óxidos metálicos presentan no-linealidades en su respuesta.Estos estudios sistemáticos fueron completamente validados mediante la síntesis de señales multi-senoidales con las frecuencias previamente identificadas utilizando secuencias pseudo-aleatorias. Cuando la temperatura de trabajo de los sensores fue modulada por una señal, el contenido frecuencial de la cual es el óptimo, los gases y mezclas de gases considerados pudieron ser discriminados perfectamente y se verificó la posibilidad de obtener modelos de calibración precisos para predecir la concentración de los gases. En algunos casos, estos procesos de validación se llevaron a cabo con sensores que no habían sido utilizados durante el proceso de optimización (por ejemplo, una agrupación de sensores diferentes pero del mismo lote de fabricación).En resumen, El nuevo método desarrollado in esta tesis para seleccionar las frecuencias de modulación optimas se a mostrado consistente y efectivo. El método es de aplicación general y podría ser utilizado en cualquier problema de análisis de gases o bien extendido a otro tipo de sensores (por ejemplo sensores poliméricos).Las contribuciones científicas de esta tesis se han recogido en 4 artículos en revistas internacionales y trece actas de conferencias.
One of the major problems in gas sensing systems that use metal oxide devices is the lack of reproducibility, stability and selectivity. In order to tackle these troubles experienced with metal oxide gas sensors, different strategies have been developed in parallel. Some of these are related to the improvement of materials, or the use of sample conditioning and pre-treating methods. Other widely used techniques include taking benefit of the unavoidable partially overlapping sensitivities by using sensor arrays and pattern recognition techniques or the use of dynamic features from the gas sensor response.In the last years, modulating the working temperature of metal oxide gas sensors has been one of the most used methods to enhance sensor selectivity. This occurs because, since, the sensor response is different at different working temperatures, and therefore, measuring the sensor response at n different temperatures is, in some cases, similar to the use of an array comprising n different sensors. This allows for measuring multivariate information from every single sensor and helps in keeping low the dimensionality of the measurement system needed to solve a specific application. Although the good results reported, until now, the selection of the frequencies used to modulate the working temperature remained an empirical process and that is not an accurate method to ensure that the best results are reached for a given application.In view of this context, the principal objective of this doctoral thesis was to develop a systematic method to determine which are the optimal temperature modulation frequencies to solve a given gas analysis problem. This method, which is borrowed from the field of system identification, has been developed and introduced for the first time in the area of gas sensors. It consists of studying the sensor response to gases when the operating temperature is modulated via maximum-length pseudo-random sequences. Such signals share some properties with white noise and, therefore, can be of help to estimate the linear response of a system with non-linearity (e.g., the impulse response of a sensor-gas system).The optimization process is conducted by selecting among the spectral components of the impulse response estimates, the few that better help either discriminating or quantifying the target gases of a given gas analysis application. Since spectral components are directly related to modulating frequencies, the selection of spectral components results in the determination of the optimal temperature modulating frequencies.In the first experiments, pseudo-random binary signals (PRBS) were employed to modulate the working temperature of micro-machined metal oxide gas sensors in a frequency range from 0 up to 112.5 Hz. The upper frequency is slightly higher than the cutoff frequency of the sensor membranes. The outcome of this initial study was that the important modulating frequencies were in the range between 0 and 1 Hz. This is understandable, since the kinetics of reaction and adsorption processes taking place at the sensor surface (i.e., physisorption/chemisorption/ionosorption) are slow and if these are to be altered by the thermal modulation, low frequency modulating signals need to be devised. This explains why low-frequency temperature-modulating signals (i.e. in the mHz range) have been used with micro-hotplate gas sensors, even though the thermal response of their membranes is much faster (typically, near 100 Hz).In the experiments that followed the first ones, an evolved method to determine the optimal temperature modulating frequencies for micro-hotplate gas sensors was introduced, which was based on the use of maximum length multilevel pseudo-random sequences (MLPRS). Multilevel signals were considered instead of the binary ones because the former can provide a better estimate than the latter of the linear dynamics of a process with non-linearity. And it is well known that temperature-modulated metal oxide gas sensors present non-linearity in their response.These systematic studies were fully validated by synthesizing multi-sinusoidal signals at the optimal frequencies previously identified using pseudo-random sequences. When the sensors had their operating temperatures modulated by a signal with a frequency content that corresponded to the optimal, the gases and gas mixtures considered could be perfectly discriminated and the building of accurate calibration models to predict gas concentration was found to be possible. In some cases, the validation process was conducted on sensors that had not been used for optimization purposes (e.g. a different sensor array from the same fabrication batch).Summarizing, the new method developed in this thesis for selecting the optimal modulating frequencies is shown to be consistent and effective. The method applies generally and could be used in any gas analysis problem or extended to other type of sensors (e.g. conducting polymer sensors).The scientific contributions of this thesis are collected in four journal papers and thirteen conference proceedings.

This thesis describes an investigation into the suitability of complementary metal oxide semiconductor (CMOS) active pixel sensor (APS) devices for scientific imaging applications. CMOS APS offer a number of advantages over the established charge-coupled device (CCD) technology, primarily in the areas of low power consumption, high-speed parallel readout and random (X-Y) addressing, increased system integration and improved radiation hardness. The investigation used a range of newly designed Test Structures in conjunction with a range of custom developed test equipment to characterise device performance. Initial experimental work highlighted the significant non-linearity in the charge conversion gain (responsivity) and found the read noise to be limited by the kTC component due to resetting of the pixel capacitance. The major experimental study investigated the contribution to dark signal due to hot-carrier injection effects from the in-pixel transistors during read-out and highlighted the importance of the contribution at low signal levels. The quantum efficiency (QE) and cross-talk were also investigated and found to be limited by the pixel fill factor and shallow depletion depth of the photodiode. The work has highlighted the need to design devices to explore the effects of individual components rather than stand-alone imaging devices and indicated further developments are required for APS technology to compete with the CCD for high-end scientific imaging applications. The main areas requiring development are in achieving backside illuminated, deep depletion devices with low dark signal and low noise sampling techniques.

Four different trivalent and tetravalent metal phthalocyanine systems (chlorogallium, chloroindium, vanadyl, and titanyl phthalocyanines) were used singly to prepare thin films (0.05-2.0 micron thickness) on gold, optically transparent substrates. The photoelectronic properties of these electrodes could be modified either by altering the growth conditions (i.e. rate of sublimation, cleanliness of substrate) or by dosing the thin films with either hydrogen or oxygen at elevated temperatures (150°C). The properties of these thin films were monitored by electron microscopy, UV-visible spectrophotometry, X-ray and Ultra-violet surface spectroscopies, and a variety of electrochemical and photoelectrochemical techniques. All four systems behaved in a manner similar to a p-type semiconductor when prepared at rapid rates (10-20 A/min) on gold substrates. In the dark, for contacting redox couples with Eᵒ’ values negative of +0.6V, the phthalocyanine electrodes showed negligible dark currents. Upon illumination, the photoelectrodes only produced positive photopotentials. Chlorogallium phthalocyanine thin films could be made to produce both positive and negative photopotentials when grown at slow rates (1-5 A/min) on clean, gold substrates. These chlorogallium phthalocyanine electrodes regained the properties of a p-type semiconductor after being dosed with oxygen for 48 hours at 150°C. X-ray Photoelectron Spectroscopy confirmed the presence of a high concentration of oxygen at the surface of all of the p-type phthalocyanine electrodes. The oxygen may accept electron density from the phthalocyanine macrocycle to cause the Fermi level to move down in energy toward its valence band edge. Dosing the film with hydrogen caused the electrode to exhibit its original intrinsic characteristics. This variability in electrical properties as a function of gas dopant may lead to the development of a sensitive gas sensing device. Ultra-violet Photoelectron Spectroscopy, as well as molecular orbital calculations, were applied to the chlorogallium phthalocyanine system to determine the molecular orbital contributions to its valence and conduction bands. Photoelectrochemical cells made from electrodes of chlorogallium and vanadyl phthalocyanines exhibited power conversion efficiencies in excess of 0.1%. The vanadyl and titanyl phthalocyanine electrodes were also effective catalysts for the photoreduction of H⁺ to H₂.

Effects of urban air pollution on health and environment have lead researchers to find economic air quality monitoring regulations. Since tin dioxide (SnO2) was demonstrated as a gas sensing device in 1962, tin oxide based thin film sensors have been widely studied due to their high sensitivity and fast response. The main advantages of using tin oxide sensors are their low cost, small size and low power consumption for mobile system applications. But, in order SnO2 based sensors to meet low concentration of gases they should be highly upgraded in sensitivity, selectivity and stability. This study was focused on the capacity of dopants in the SnO2 layer to increase the sensitivity of the sensor in detecting carbon monoxide. 1 wt. % Pd promoted and 0.1 wt. % Na-1 % Pd promoted SnO2 multilayer thin films were produced by sol-gel technique followed by spin coating route on soda-lime glass substrates. The EDX and SEM studies showed the surface composition and the surface structure is homogeneous throughout the films. The film thickness was determined app. 450 nm from the SEM image of the cross-section, after coating 8 layers. The experiments conducted at several temperatures namely 150, 175 and 200oC, in oxygen free and 1% oxygen containing atmospheres showed that the responses at higher temperatures in the presence of oxygen were much sharper with respect to others. Besides, Na promoted test sensors showed larger responses with shorter response time in oxygen free atmospheres at relatively lower temperatures. The results showed that the sensor signal is not directly correlated with the carbon dioxide production in oxygen free atmospheres.

This dissertation concerns the development, fabrication and integration in a gas sensing microdevice of a novel 3-dimensional (3D) nanostructured metal-oxide semiconducting film that effectively merges the benefits of inorganic nanomaterials with the simplicity offered by non-lithographic electrochemistry-based preparation techniques. The film is synthesized via the porous-anodic-alumina-assisted anodizing of an Al/Nb metal bilayer sputter-deposited on a SiO2/Si substrate and is basically composed of a 200 nm thick NbO2 layer holding an array of upright-standing spatially separated Nb2O5 nanocolumns, being 50 nm wide, up to 900 nm long and of 8109 cm2 population density. The nanocolumns work as semiconducting nano-channels, whose resistivity is greatly impacted by the surface and interface reactions. Either Pt or Au patterned electrodes are prepared on the top of the nanocolumn array using an innovative sensor design realized by means of microfabrication technology or via a direct original point electrodeposition technique, followed by selective dissolution of the alumina overlayer. For gas-sensing tests the film is mounted on a standard TO-8 package using the wire-bonding technique. Electrical characterization of the 3D niobium-oxide nanofilm reveals asymmetric electron transport properties due to a Schottky barrier that forms at the Au/Nb2O5 or Pt/Nb2O5 interface. Effects of the active film morphology, structure and composition on the electrical and gas-sensing performance focusing on sensitivity, selectivity, detection limits and response/recovery rates are explored in experimental detection of hydrogen gas and ammonia. The fast and intensive response to H2 confirms the potential of the 3D niobium-oxide nanofilm as highly appropriate active layer for sensing application. A computer-aided microfluidics simulation of gas diffusion in the 3D nanofilm predicts a possibility to substantially improve the gas-sensing performance through the formation of a perforated top electrode, optimizing the film morphology, altering the crystal structure and by introducing certain innovations in the electrode design. Preliminary experiments show that a 3D nanofilm synthesized from an alternative Al/W metal bilayer is another promising candidate for advanced sensor applications. The techniques and materials employed in this work are advantageous for developing technically simple, cost-effective and environmentally friendly solutions for practical micro- and nanodevices, where the well-defined nano-channels for charge carriers and surface reactions may bring unprecedented benefits.

Modification of the properties of solid state structures, used for gas sensing is important task in making detection and measurement systems of volatile chemical compounds. These properties depend on material, inner structure and interaction with gas atmosphere. In hybrid materials (solid-biomolecular) biochemical recognition plays important role in gas sensing mechanism. In this work the methodologies of the SPM was applied for characterization of the local point and local area properties in the gas sensitive MO films with the nanoscaled thickness that can be used for nanosystems and hybrid materials in novel types of chemical detectors. In this dissertation morphology and physical properties of metal oxide films with thickness from a few to about 50 nm was investigated and described a relationship between the gas response and film thickness. It was experimentally shown that effects of external influence on the properties of the surface nanostructures can be described by the specific characteristics of the scanning probe spectroscopy displaying the dependences of the probe contact electric current on both the probe potential and the probe pressing force. An original method based on the SPM probe controlled electrical current was proposed for the formation of nanosystems with various electrical properties on the surfaces of thin MO films.
Kryptingas kietojo kūno darinių, naudojamų išorinio dujų poveikio detekcijai, savybių keitimas yra vienas iš aktualiausių uždavinių, sprendžiamų kuriant lakiųjų cheminių junginių poveikio atpažinimo ir matavimo sistemas. Šias savybes lemia darinių medžiaga, jų struktūra bei sąveikos su dujine aplinka ypatumai, kurie hibridiniuose dariniuose iš kietojo kūno ir biomolekulių gali būti lemiami dar ir biocheminiu atpažinimu. Šiame darbe tiriami dujoms jautrūs hibridiniai dariniai ir nanosistemos, integruotos metalo oksido plėvelėse, Skenuojančio zondo mikroskopijos (SZM) metodais. Disertacijoje susieti itin plonų (<30-50 nm) SnOx sluoksnių varžos atsako į dujas bei elektrinių savybių ypatumai su sluoksnių morfologija, priklausančia nuo auginimo sąlygų ir trukmės. Eksperimentiškai įrodyta, jog SZM lokalinių srovių tyrimai, priklausomai nuo matavimo parametrų, leidžia atskirai aprašyti technologiškai keičiamas dujoms jautrių darinių charakteristikas ir tik nanosistemose vykstančius procesus, kurie, kai kuriais atvejais, gali būti stebimi ir tipiškuose dujoms jautrių darinių taikymuose. Sukurtas originalus metodas, tinkantis nanostruktūroms metalo oksidų paviršiuje formuoti bei tų struktūrų elektrinėms savybėms keisti. Skirtingai nuo literatūroje žinomo paviršiaus nanooksidinimo, pritaikyto formuoti cheminiam poveikiui atsparias dangas, mūsų metodas leidžia formuoti įvairaus elektrinio laidumo nanostruktūras metalo oksidų paviršiuje.

Les équipes micro-capteurs (IM2NP) et capteurs de gaz (LMMA) développent des capteurs à base de couches minces de CuxO et étudient leurs réponses électriques sous O3. Les travaux de cette thèse ont pour but de mieux comprendre l’interaction solide-gaz à l’échelle atomique en simulant l’adsorption de l’O3 sur les surfaces (111) du CuO et du Cu2O. Pour cela nous avons utilisé la DF T (Density Functional Theory) dans le cadre de deux approximations de la fonctionnelle : la LDA (Local Density Approximation) et la GGA (Generalized Gradient Approximation).Pour le CuO, la correction de Hubbard (DF T + U) a été également prise en compte pour reproduire correctement les comportements semiconducteuret antiferromagnétique du matériau. Tous les calculs ont été menés avec le code SIESTA et montrent que pour les deux matériaux, l'ozone s’adsorbe sur la surface sans défauts, sans se dissocier, induisant un dopage p du matériau. Ceci est en accord avec la diminution de la résistance électrique mesurée expérimentalement sous ozone. Ensuite, l’ozone se dissocie en formant une molécule de O2 et un atome d’oxygène qui restent adsorbés. Cette étape ne semble pas modifier le dopage. Par contre lorsque le capteur n’est plus en présence d'O3, la molécule d’O2 désorbe et le dopage est annihilé. Dans ce mécanisme les énergies mises en jeu sont du même ordre de grandeur pour CuO ou pour Cu2O (allant de −3 eV à −1 eV). Dans l’objectif de développer un capteur de gaz, le CuO, plus facile à obtenir par les techniques de dépôt courantes en microélectronique, semble donc être plus pertinent que le Cu2O, qui a une réponse similaire (voire moindre) mais dont il est difficile d’obtenir une phase pure
Micro-sensors (IM2NP) and gas sensors (LMMA) team develop sensors based on CuO and Cu2O thin layers and study their electrical responses to O3. The aim of this thesis is a better understanding of the solid-gas interactions at the atomic scale by simulating the adsorption of O3 molecule on the (111) surfaces of CuO and Cu2O. Simulations were performed using the DF T (Density Functional Theory) within two functional approximations : the LDA (Local Density Appriximation) and GGA (Generalized Gradient Approximation). In the case of CuO, the Hubbard correction (DF T + U) was taken into account to properly reproduce the semiconductor and antiferromagnetic behaviors of the material. All calculations were performed with the SIESTA code and show that for the CuO as for Cu2O, O3 is adsorbed on the defect-free surface, without dissociating inducing a p-doping of the material. This observation is consistent with the decrease in electrical resistance measured experimentally under ozone. In a second stage ozone dissociates into a molecule of O2 and an oxygen atom which remains adsorbed. This step does not appear to change the doping. However, when the sensor is no longer in the presence of ozone, O2 molecule is desorbed and doping disappears. In this mechanism, the energies involved during the adsorption or the dissociation of ozone are of the same order of magnitude for CuO or Cu2O (ranging from −1 eV to −3 eV). Aiming to develop a gas sensor, and since the CuO material is easier to obtain by standard deposition techniques (RF sputtering), it seems to be more appropriate than the Cu2O, which has a similar response (even lower) but is more difficult to synthesize in a pure phase

The field of infrared (IR) detector technology has shown vast improvements in terms of speed and performance over the years. Specifically the dynamic range (DR) and sensitivity of detectors showed significant improvements. The most commonly used technique of implementing these IR detectors is the use of charge-coupled devices (CCD). Recent developments show that the newly investigated bipolar complementary metal-oxide semiconductor (BiCMOS) devices in the field of detector technology are capable of producing similar quality detectors at a fraction of the cost. Prototyping is usually performed on low-cost silicon wafers. The band gap energy of silicon is 1.17 eV, which is too large for an electron to be released when radiation is received in the IR band. This means that silicon is not a viable material for detection in the IR band. Germanium exhibits a band gap energy of 0.66 eV, which makes it a better material for IR detection. This research is aimed at improving DR and sensitivity in IR detectors. CCD technology has shown that it exhibits good DR and sensitivity in the IR band. CMOS technology exhibits a reduction in prototyping cost which, together with electronic design automation software, makes this an avenue for IR detector prototyping. The focus of this research is firstly on understanding the theory behind the functionality and performance of IR detectors. Secondly, associated with this, is determining whether the performance of IR detectors can be improved by using silicon germanium (SiGe) BiCMOS technology instead of the CCD technology most commonly used. The Simulation Program with Integrated Circuit Emphasis (SPICE) was used to realise the IR detector in software. Four detectors were designed and prototyped using the 0.35 µm SiGe BiCMOS technology from ams AG as part of the experimental verification of the formulated hypothesis. Two different pixel structures were used in the four detectors, which is the silicon-only p-i-n diodes commonly found in literature and diode-connected SiGe heterojunction bipolar transistors (HBTs). These two categories can be subdivided into two more categories, which are the single-pixel-single-amplifier detectors and the multiple-pixel-single-amplifier detector. These were needed to assess the noise performance of different topologies. Noise influences both the DR and sensitivity of the detector. The results show a unique shift of the detecting band typically seen for silicon detectors to the IR band, accomplished by using the doping feature of HBTs using germanium. The shift in detecting band is from a peak of 250 nm to 665 nm. The detector still accumulates radiation in the visible band, but a significant portion of the near-IR band is also detected. This can be attributed to the reduced band gap energy that silicon with doped germanium exhibits. This, however, is not the optimum structure for IR detection. Future work that can be done based on this work is that the pixel structure can be optimised to move the detecting band even more into the IR region, and not just partially.
Dissertation (MEng)--University of Pretoria, 2013.
Electrical, Electronic and Computer Engineering
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