Metal oxide semiconductor (MOS) gas sensors have proven to be useful in many applications, ranging from detection of hazardous gases to monitoring of air quality. The demand for power efficient and high performance gas sensors has seen an increase in situations facing contemporary society. Currently it is common for sensors to employ an energy inefficient heater to provide for the optimal working temperature of the sensor. Light activation has been proposed as an alternative that could possibly improve modern gas sensors by decreasing energy utilization as well as increasing sensitivity and selectivity. The purpose of the following project is to explore the mechanisms and characteristics of light activated gas sensing using cuprous oxide (Cu2O), such that the findings may contribute to the development of power efficient gas sensors able to distinguish between gases at low concentrations. Several Cu2O-sensors with thicknesses of 300, 500 and 700 nm were examined, many of which also were doped with materials such as silver, graphene and titanium. Multiple types of measurements were performed where the sensors were exposed to nitrogen and carbon dioxide gas under illumination from one of three distinct light sources. The results show that conditions such as low light intensities, doping the sensors and air as the operating environment (compared to nitrogen gas) are beneficial for the carbon dioxide response under light activation. However, these findings are only indications and would need confirmation by additional measurements, both in terms of variation and repetition, under improved conditions.

Silicon phthalocyanines (R₂-SiPcs) are an emerging class of high-performance n-type or ambipolar organic semiconductors which have found application in organic electronic devices, including organic thin-film transistors (OTFTs), organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). Owing to their tetravalent silicon metal centre, R₂-SiPcs can be substituted with a range of axial ligands including phenols, carboxylic acids, and silanes to tune their intermolecular interactions, optical properties, electronic properties and solubility. While early reports of R₂-SiPcs have demonstrated promising results, the relationship between their structure and performance in OTFTs is poorly understood. Additionally, many OTFTs with R₂-SiPcs as semiconductor only demonstrate n-type behaviour under inert atmospheres due to their shallow lowest unoccupied orbital level below -4.1 eV making them susceptible to electron trapping by moisture and oxygen. This thesis presents developments in both the understanding of how R₂-SiPc structure influences performance, device engineering and exploration of these materials in ammonia sensors. First, I develop of structure-property relationships for a catalogue of fifteen R₂-SiPcs integrated into OTFTs including eleven materials used in OTFTs for the first time. I then explore the influence of dielectric surface chemistry on the texture of R₂-SiPc films and their resulting performance in OTFTs using silane self-assembled monolayers and para-sexiphenyl to understand the weak epitaxial growth behaviour of this class of materials. Next, I report eight novel peripherally fluorinated and axially substituted silicon phthalocyanines (R₂-FₓSiPcs) to investigate the influence of peripheral and axial fluorination on air-stable electron transport and determine the threshold for achieving air-stable n-type OTFTs. Finally, I integrate R₂-FₓSiPcs into organic heterojunction ammonia gas sensors to understand the influence of peripheral fluorination on the majority charge carrier in this device architecture.

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Current gas sensor technology, although meeting the minimum requirements in many instances, suffers for a number of limitations. Hence, there is currently a considerable volume of research being undertaken at many laboratories of different countries. In the past, all chemical sensors and catalyst were optimized empirically by a trial and error method. Today, however, systematic research and development is becoming increasingly important in order to improve sensors and to find new sensing principles. Obtaining a long term stable gas sensor with improved sensitivity, selectivity, and low cost for mass production passes through fundamental research and material characterization to build new chemically sensitive devices or to improve existing ones. The bottom line in the design and manufacture of modern gas sensors is the transfer from ceramic(of Figaro type) to thin film gas sensors(TFGs). This transfer provides new opportunities for further microminiaturization, power consumption and cost reduction of gas sensors. Therefore, at the present time, thin film gas sensors are the basis for the design of the modern gas sensitive multi-parameter microsensor systems. Applications of these systems include environment, security, home systems, smart buildings, transportation, discrete manufacturing, process industries and so on. Microelectromechanical systems(MEMS) based integrated gas sensors present several advantages for these applications such as ease of array fabrication, small size, and unique thermal manipulation capabilities. MEMS based gas sensors; which are usually produced using a standard CMOS(Complimentary Metal Oxide Semiconductor) process, have the additional advantages of being readily realized by commercial foundries and amenable to the inclusion of on-chip electronics. In order to speed up the design and optimization of such integrated sensors, microheater designs for gas sensor applications have been presented as first part of the present thesis. As heater design is the key part for a gas sensor operation. So 3D simulations have been used to optimize micro-heater geometry. The application of MEMS Design Tool(COVENTORWARE) has been presented to the design and analysis of micro-hotplate (MHP) structures. Coupled Electro-thermal analysis provided an estimation of thermal losses and temperature distribution on the hotplate for realistic geometrical and material parameters pertinent to fabrication technology. Five microheater designs have been proposed in terms of different sizes and shapes in order to optimize the microhotplate structure to be used for gas sensor operation for the specified range of temperature and power consumption. To produce a gas sensor, which is able to detect LPG leak, thin films of tin oxide have been developed. FR sputtering has been used to deposit gas sensitive tin oxide thin filmls under various deposition conditions. Four different values of pressure in the range from high pressure(5 X 10-2 mbar) to lower pressure (2 X 10-3 mbar), three RF power values 50, 75, 100 W and varied oxygen percentage in sputtering atmosphere (0-18%) have been used to optimize the material properties of tin oxide thin films to study the sensitivity towards LPG. All the samples have been analyzed using various macro and microscopic characterization techniques. Extensive studies have been done on the sensor response for the samples deposited under different conditions. Finally the sample film deposited at 5 x 10-3 mbar, with applied power of 75 W in the presence of 8% oxygen, showed maximum sensitivity towards LPG.

The atmosphere we live in contains various kinds of chemical species, natural and artificial, some of which are vital to our life, while many others are more or less harmful. The vital gases like oxygen, humidity have to be kept at adequate levels in the living atmosphere, whereas the hazardous and toxic gases like hydrocarbons, H2, volatile organic compounds, CO2, CO, NOx, SO2, NH3, O3 etc should be controlled to be under the designated levels. The measurement technology necessary for monitoring these gases has emerged, particularly as organic fuels and other chemicals have become essential in domestic and industrial life. In addition to other applications, environmental pollution monitoring and control has become a fundamental need in the recent years. Therefore, there has been an extensive effort to develop high-performance chemical sensors of small size, rugged construction, light weight, true portability, and with better sensing characteristics such as high sensitivity, fast response and recovery times, low drift, and high degree of specificity. Among the various types of gas sensors studied, solid state gas sensors based on semiconducting metal oxides are well established, due to their advantages over the other types, and hence cover a wide range of applications. However, the widespread application of these sensors has been hindered by limited sensitivity and selectivity. Various strategies have been employed in order to improved the performance parameters of these sensors. This thesis work has two major investigations, which form two parts of the thesis. The first part of this thesis describes the efforts to improve the sensing behaviour of one of the extensively studied metal oxide gas sensors, namely, ZnO, through a novel, ultrasonic-nebulised spray pyrolyis synthesis method, employing an aqueous combustion mixture (NSPACM). The second part of the thesis deals with the ideal of gas detection by optical means through the reversible phase transformation between V2O5 and V6O13 deposited by metalorganic chemical vapor deposition(MOCVD). The introductory chapter I deals with basics of chemical sensors and the characteristic sensing parameters. Different types of gas sensors based on the phenomena employed for sensing are discussed, with an emphasis on semiconducting metal oxide gas sensors. The importance of material selection for solid state gas sensors, depending on the purpose, location, and conditions of operation are discussed, supporting the assertion that semiconducting metal oxides are better suited to fulfill all the requirements of modern gas sensors. Some of the effective methods to improve performance parameters including the influence of grain size, microstructure, and surface doping are described., followed by the motivation of the present thesis. The part I of the thesis is based on the resistive semiconducting metal oxide, where the system investigated was ZnO. Part one comprises Chapters 2, 3 and 4. In Chapter 2, a brief introduction to the material properties of ZnO, followed by various synthesis techniques are discussed. An overview of spray pyrolysis and combustion synthesis is followed by the details of the method employed in the present study, namely NSPACM, which is based on the above two methods, for the formation of ZnO films. A detailed description of the film deposition system built in house is presented, followed by the deposition procedure and the parameters used. Thermal study of the combustion mixture and non-combustion precursor shows the importance of the fuel, along with oxidizer, in forming the film. The films formed using combustion mixture are found to be polycrystalline, whereas films formed without combustion were found to have preferred crystallographic orientation even on an amorphous substrate, which is explained on the basis of minimization of surface energy. The observed unique microstructure with fine crystallite size and porous morphology is attributed to the combustion method employed, which is interesting from the point of view of gas sensing. Chapter 3 concerns the gas sensing study of these ZnO films. The design of the home made gas sensing system is explained in detail. The study of electrode characteristics is followed by the important steps in gas sensing measurements. ZnO gas sensors were mainly studied for their selectivity between aliphatic and aromatic hydrocarbons. The results show two regions of temperature where the sensitivity peaks for aliphatic hydrocarbons, whereas aromatic hydrocarbons show a single sensitive region. This observation can pave the way for imparting selectivity. Possible reasons for the observed behavior are mentioned. Chapter 4 describes the chemical and physical modifications done to ZnO thin films by doping with catalysts, and through the use of x-y translational stage for large-area deposition.. Homogenous distribution of catalysts achieved by the NSPACM synthesis procedure, determined by the x-ray elemental mapping, is discussed. The addition of catalysts improved the sensing both because of catalytic effects and by promoting preferred crystallographic orientation, with Ni addition showing the better effects. The use of the x-y stage in producing the films with high orientation, which improved the gas sensing behavior, is explained. Part II of the thesis comprises Chapters 5,6 and 7, and describes a detailed study of V2O5 and V6O13 thin films deposited by MOCVD for optical sensing of chemical species. In Chapter 5, a brief introduction to chemical vapor deposition is given, followed by the importance of the characteristics of CVD precursors – in particular, the importance of their thermal behavior in film formation. This is followed by the importance of vapor pressure and partial pressure studies in the MOCVD of oxides of a multivalent metal such as vanadium. Various techniques of measuring vapor pressure are listed, followed by the details of the method used in the present study employing rising temperature thermogravimetry, based on the Langmuir equation. Thermogravimetric analysis performed, both at atmospheric as well as at low pressure, using commercial and home made apparatus, respectively is discussed. A detailed description of the home made setup is also presented. Chapter 6 describes the application of the vapor pressure and partial pressure studies to the deposition of films using MOCVD. Here, a detailed description of the vanadium oxide phase diagram and the stability of various phases is presented, which points the importance of precise parameter control during the deposition to obtain pure phases. The details of the CVD setup, followed by the procedure and parameters of deposition, are presented. The films deposited at various deposition temperatures, analyzed using XRD and SEM, are discussed. The effect of temperature on the growth is explained. The effect of vapor pressure is studied by varying the precursor vaporizer temperature, with a growth temperature maintained invariant. The influence of the amount of precursor on film growth, with a particular crystalline orientation and phase content, is explained followed by the description of the deposition of pure phases of V2O5 and V6O13 through the optimization of CVD parameters. Chapter 7 deals with the optical study of the films deposited by the above method. Here, the importance of two phases of vanadium oxide, V2O5 and V6O13, to the proposed gas sensing action, is presented. Their structural similarity in terms of polyhedral arrangement in the ab plane can be the basis of a reversible phase change. The difference in the optical transmittance in two phases forms the basis for the optical method for chemical sensing. The details of the laser-based optical sensing setup, its, design and the detection method, are explained. Studies on hydrocarbon sensing with vanadium, pentoxide films are also presented. The novelty in using reversible chemical transformation of a material system for detection of reducing and oxidizing gases in the ambient gases is discussed. Chapter 8 provides a summary of the present thesis, together with the main conclusions. The work reported in this thesis has been carried out by the candidate as part of the Ph.d training programme. She hopes that this would constitute a worthwhile contribution towards the understanding and subsequent application of ZnO and oxides of vanadium(V2O5 and V6O13) as novel gas sensors which will be useful for environmental protection, as well as for safety in industrial an domestic sectors.

The basic necessities to sustain life are – air, water and food. Although the harmful effects due to contaminated food or water are dangerous to life, these can be reduced/avoided by controlling the intake. Whereas, in case of air, the same amount of control cannot be exercised as there is very little, one can do in case of inhalation. Maximum damage to life is due to air contamination which can be detected and prevented by using gas sensors. The proper use of these sensors not only save lives, but also minimizes social and financial loss. The objective of this thesis work is to study and explore the use of p-type semiconducting material such as CuO, as a promising gas sensing material for organic compounds (VOCs), compatible with existing silicon fabrication technology. The Thesis consist of 7 chapters: Chapter 1 covers the general introduction about gas sensors, sensor parameters, criteria for the selection of sensing material, suitability of CuO as sensing material and a brief literature survey. The second chapter includes the selection of substrate, cleaning procedures and suitable deposition method. The deposition method used in the present thesis work is DC/RF magnetron sputtering. The reactive magnetron sputtering is employed during the deposition of CuO sensing films. It also includes basic introduction about some of the common material characterization techniques. This is followed by Chapter 3 which includes the optimization of sputtering process parameters such as applied power, working pressure, Ar-O2 ratio and substrate temperature for CuO sensing film and the effect of these on surface morphology. Information on the optimized sputtering parameters for electrode film (silver and gold) deposition has also been included in this chapter. In order to study the sensing behavior of the sensor, suitable testing set-up is necessary. This leads us to Chapter 4 that discusses the development of an in-house built sensor testing setup and its automization using MATLAB. The automated testing set-up facilitates off-time data plotting as well as real-time data plotting during the sensing process. To demonstrate the working of the set-up, some initial results obtained are also included in this chapter. After ascertaining the functioning of the automated gas sensor testing set-up, detailed study on the sensing behavior of nanostructured CuO films was performed. This information along with the necessary details is included in Chapter 5. The sensing response of nanostructured CuO films has been studied for different VOCs such as alcohol, toluene and benzene. The study carried out on the effect of different surface additives like multi-walled carbon nanotubes (MWNTs), gold or platinum on ethanol sensing has also been included in this chapter. During the use of MWNTs as surface additives, different concentrations of MWNTs – 0.01 mg, 0.05 mg and 0.1 mg have been dispersed on the CuO sensing film. The sample with lowest concentration of MWNTs exhibited highest sensitivity and lower response time. It is due to the fact that, higher concentrations of MWNTs do not result into uniform dispersion over the CuO films and cover the sensing film almost completely. Operating temperature is the most important factor affecting the performance of a gas sensor. In order to maintain the operating temperature for the portable sensor, the sensor is usually integrated with a heater. The chapter 6 deals with heater optimization including design, simulation and fabrication. In this chapter, microheater as well as macro-heaters were simulated and fabricated. The fabricated macro-heater is bonded with the sensor by eutectic bonding. One of the bonded samples was studied for its sensing response. The final chapter of the thesis deals with the conclusion of present research work and the possible further work on CuO gas sensor.

Zinc oxide (ZnO) is perhaps one of the most widely studied material in the last two decades. It has received so much of attention because of its incredible potential for wide ranging applications. ZnO is a wide band gap semiconductor (Eg = 3.37 eV at 300 K) with a rather large excitonic binding energy (~60 meV). This combination of properties makes it an ideal choice for several optoelectronic devices that can easily work at room temperature. ZnO is a truly multifunctional material possessing several desirable electrical, optical, optoelectronic, and piezoelectric properties. In addition, it is highly amenable to production of various kinds of nanostructures such as nanorods, nanotubes, nanoribbons, nanoneedles, etc., which makes it even more desirable for nanoscale devices. Examples of ZnO based nanodevices could include photodiodes, photodetectors, nano-lasers, field-emission devices and memristors. In order to make such devices, one could need device quality nanostructures that must be reproducible and cost effective. Naturally, one has to look for a synthesis process that has great controls and is relatively inexpensive. The study provided here shows that among the various methods available for ZnO synthesis, the microwave-assisted chemical synthesis offers outstanding advantages in terms of rapid growth of nanostructures, economical use of energy and excellent controls of process parameters. In order to produce device quality ZnO nanostructures using microwave-assisted synthesis, one has to study the effect of various process parameters and optimise them for the desired growth. Therefore, in the current study, first, a systematic study was undertaken to synthesize ZnO nanostructures both in a aqueous and non-aqueous medium and their characterization was carried out in order to understand the effect of microwave power, time of irradiation, pressure, solvent and salt concentration, etc. The goal was to develop synthesis protocols for various kinds of nanostructures that could guarantee reproducibility, good yield, and device quality structures. This study has led to successful growth of ZnO nanostructures on various substrates, vertically aligned ZnO nanorods and templated arrays of desired structures, all with outstanding properties of the structures as confirmed by XRD, MicroRaman, photoluminescence, cathodoluminescence, FESEM, TEM, PFM studies and pole figure analysis. Piezoelectric force microscopy (PFM) and physical property measurement system (PPMS, Quantum Design), have been used to study the multifunctional properties of ZnO nanostructures. The PFM is a powerful technique to measure the local piezoelectric coefficient of nanostructures and nanoscale thin films. PFM works on the converse piezoelectric effect in which electric potential is applied and mechanical strain is measured using a cantilever deflection. The PFM (Brucker’s AFM dimension Scan Assist) was used to characterize individual ZnO nanorods. Extensive studies were carried out with PFM measurements and it was observed that the nanorods consistently showed high piezoelectric coupling coefficients (d33~50-154 pm/V). It was also found that the variation in d33 depended on morphology and size of nanostructure. The multifunctional properties were observed in small ZnO nanocrystals (NCs). Such high values of piezoelectric coupling coefficients open the door for novel ZnO based nanoscale sensors and actuators. The synthesized ZnO nanostructures were further optimized and characterized keeping in view three device applications namely Field emission, Memristors and Gas Sensors. The fabrication and characterization of these three devices with ZnO nanostructure was carried out using electron beam lithography and direct laser writing micromachining. Device fabrication using lithography involved several steps such as substrate cleaning, photoresist spin coating, pre-baking, post-baking, pattern writing, developing, sputtering/deposition of material for lift-off, ZnO growth, and overlay lithography. For field emission devices, high quality, well aligned, c-axis oriented ZnO nano-needles were grown on sputter coated Ti/Pt (20nm/100nm) on SiO2/Si substrate by rapid microwave-assisted method in aqueous medium. The diameter of the tip was found to be 1~2 nm and the length of the rod was approximately 3~5μm. For a particular batch the tip size, morphology, and lengths were found to be the same and highly repeatable. Pole figure analysis revealed that nanorods were highly oriented towards <002> direction. Field-emission measurements using the ZnO nanoneedles arrays as cathode showed very low turn-on electric field of 0.9 V/μm and a very high field enhancement factor ~ 20200. Such a high emission current density, low turn-on electric field, and high field enhancement factor are attributed to the high aspect ratio, narrow tip size, high quality and single crystallinity of the nanoneedles. The high emission current density, high stability, low threshold electric field (0.95 V/μm) and low turn-on field make the ZnO nanoneedle arrays one of the ideal candidates for field-emission displays and field emission sensors. In the suitability of ZnO nanostructures for memristor application it was found that the single crystalline ZnO nanorods were not suitable as they did not show memristive behaviour but the ZnO nanorods with native defects exhibited considerable memristive behaviour. Therefore the microwave-assisted grown ZnO nanorods with defects were used to fabricate memristive devices. Single and multiple ZnO nanorods based memristors were fabricated using electron beam lithography. These devices were characterized electrically by measuring the hysteresis in the I/V characteristics. A high degree of repeatability has been established in terms of growth, device fabrication, and measurements. The switching in single nanorod based devices was found to have “ON-to- OFF” resistance ratio of approximately 104 and current switching ratio (ION/IOFF) of 106. Gas sensing based on electrical resistance change depends on absorption and desorption rate of gases on the analyte which is governed by surface properties, morphologies and activation energy. Therefore, various morphologies of nanostructure were grown for gas sensing application. Through experimentation, the emphasis shifted to c-axis oriented ZnO nanostructures on SiO2 substrate for gas sensing. The c-axis orientation of ZnO nanostructures was preferred mainly due to its huge surface area. The measurements showed that the c-axis oriented ZnO nanorods were excellent hydrogen sensors, able to detect H2 as low concentration as 2 ppm, even when the sensing temperature is as low as 200 ˚C. However, oxygen sensing was achieved at a higher temperature (300 ˚C). Thus, the study undertaken in this thesis presents a microwave based rapid and economical method for synthesizing high quality, device grade ZnO nanostructures, their extensive characterization that shows the multifunctional properties of these structures, and there examples of varied device applications of the synthesized nanostructures as field emitters, memristors, and gas sensors.

Potentiometrisch betriebene Festelektrolytsensoren auf der Basis von Yttriumoxid-stabilisiertem Zirconium(IV)-oxid als festem Oxidionenleiter weisen einen für elektrochemische Sensoren ungewöhnlich breiten Messbereich von über 30 Zehnerpotenzen sowie eine vergleichsweise hohe chemische, thermische und mechanische Stabilität auf. Dadurch konnten sich diese Sensoren in einem sehr weiten Applikationsbereich etablieren, der hauptsächlich die Abgaskontrolle von Verbrennungsprozessen betrifft. In der vorliegenden Arbeit wird der Frage nachgegangen, ob mit Festelektrolytsensoren (FES) bei coulometrischer oder potentiodynamischer Betriebsweise, die gegenüber dem potentiometrischen Prinzip Vorteile im Hinblick auf Sensitivität beziehungsweise Selektivität bieten, weitere Applikationsfelder erschlossen werden können. Dazu durchgeführte Untersuchungen, Weiterentwicklungen und Optimierungen an coulometrisch betriebenen Festelektrolytsensoren sowie deren Einbindung in ein chromatographisches Messsystem zielten auf die Applikation zur Bestimmung von Spurenbestandteilen in Gasen und Flüssigkeiten ab. Mit den Ergebnissen wird beispielhaft ein neues Anwendungsfeld für FES bei der kontinuierlichen Überwachung von Biogasprozessen eröffnet. Zur Erreichung der Ziele wird zunächst gezeigt, wie der Messbereich potentiostatisch betriebener coulometrischer FES hin zu Spurenkonzentrationen im Vol.-ppb-Bereich erweitert werden kann. Hierfür werden Fehlereinflüsse untersucht, die die Nachweisgrenze dieser Sensoren beeinflussen. Durch die Entwicklung rauscharmer elektronischer Sensoransteuerungen, durch die Optimierung von Betriebsparametern sowie durch die Bestimmung der elektronischen Leitfähigkeit wird die Nachweisgrenze von FES verglichen mit dem bisherigen Forschungsstand um vier Zehnerpotenzen verringert. Als Ergebnis dieser Weiterentwicklungen liegen die Nachweisgrenzen für den FES im Durchflussbetrieb nun bei unter 5 Vol.-ppb für die Analyte H2, O2 und CH4. Zur Steigerung der Selektivität von FES werden zwei Möglichkeiten aufgezeigt. Einerseits werden bei cyclovoltammetrischer Betriebsweise für H2-, O2- oder CO-haltige Messgase im Konzentrationsbereich unterhalb von 10^2 Vol.-ppm lineare Zusammenhänge zwischen den Konzentrationen und den Peakeigenschaften Höhe und Fläche gefunden. Auf diese Weise konnte H2 an einer katalytisch hochaktiven Pt-Elektrode in Anwesenheit eines Überschusses an Sauerstoff mit hoher Selektivität erfasst werden. Andererseits wird die Selektivität potentiostatisch betriebener coulometrischer FES drastisch gesteigert, indem diese einer gaschromatographischen Trenneinheit nachgeschaltet werden. Im Konzentrationsbereich von 10^−1 bis 10^4 Vol.-ppm zeigte sich für H2 und CH4 ein lineares Ansprechverhalten, die Nachweisgrenzen des chromatographischen Messsystems lagen für diese Gase bei 55 bzw. 40 Vol.-ppb. Mit einem neuartigen In-situ-Messsystem, das auf dem Prinzip der kontinuierlichen membranfreien Gasextraktion und anschließender intervallmäßiger chromatographischer Trennung und Detektion mit einem potentiostatisch betriebenen coulometrischen FES basiert, wurden im Gärmedium von Biogasanlagen Spuren von gelöstem H2 und O2 sowie das vielfach höher konzentrierte CH4 parallel erfasst. Es wird gezeigt, dass Instabilitäten im Biogas-Entstehungsprozess, die durch Überfütterung des Fermenters hervorgerufen werden, anhand des Verlaufs des gelösten H2 deutlich früher erkannt werden, als es durch die H2-Bestimmung im Biogas durch kommerziell verfügbare Gassensoren möglich ist. Auf diese Weise ließ sich mit dem FES ein praxistaugliches langzeitstabiles, robustes und wartungsarmes Messsystem für diese Kenngrößen entwickeln. Bei der coulometrischen Bestimmung von Essigsäure mit dem FES kommt es zur Blockierung der Platinelektroden. Infrarotspektroskopische Untersuchungen des Abgases aus dem FES belegen die thermische Zersetzung dieses Analyts bei 750 °C, die mit der Bildung eines Kohlenstofffilms auf der messgasseitigen Pt-Elektrode einhergeht. Diese Blockierung führt zur Peakverbreiterung und verhindert so die Detektion der Carbonsäuren mit zwei bis fünf Kohlenstoffatomen im Molekül. In dieser Arbeit wird gezeigt, dass dieser ungünstige thermische Zerfall durch die Einbringung einer beheizbaren Katalyseeinheit in die Gasleitung zwischen Gaschromatograph und FES verhindert werden kann. Die Säuren zerfallen dann an der Pt-Oberfläche des Katalysators bei 800 °C, so dass nur die gasförmigen Zerfallsprodukte in den FES gelangen, wo sie ohne die Bildung von Pyrolyseprodukten an den Elektroden coulometrisch umgesetzt werden. Mittels Austausch des FES durch einen Flammenionisationsdetektor konnte mit dem In-situ-Messsystem gelöste Essigsäure über einen Zeitraum von achtzehn Tagen im Gärmedium einer Biogasanlage mit hinreichender Langzeitstabilität erfasst werden. Damit werden in der vorliegenden Arbeit wesentliche Beiträge zur Weiterentwicklung von coulometrischen Festelektrolytsensoren im Hinblick auf die Erniedrigung der Nachweisgrenze, die Erhöhung der Selektivität und die Verbreiterung des Anwendungsspektrums geleistet. / Potentiometric solid electrolyte sensors made of the solid oxygen ion conductor 'yttria stabilized zirconia' exhibit a very broad measuring range of more than 30 orders of magnitude as well as comparatively high chemical, thermal and mechanical stability. Therefore, these sensors were established in a large application range which covers mainly the field of exhaust gas control of combustion processes. This work tries to answer the question if it is possible to address new fields of application with coulometrically or potentiodynamically operated solid electrolyte sensors (ses) because of their generally higher sensitivity and selectivity compared to potentiometrically operated ses. Investigations, advancements and optimizations executed for this aim on coulometrically operated ses as well as the integration of these sensors into a chromatographic measuring system were directed on the detection of traces of analytes in gas mixtures and liquids. The results of this work unlock a new field of application for ses in the continuous monitoring of biogas processes. For the achievement of these goals it is firstly demonstrated how the measuring range of potentiostatically operated coulometric ses can be expanded in the direction of trace concentrations in the range of some vol.-ppb. Therefore, error sources influencing the detection limit are investigated. Compared to the current state of research, this limit is decreased by four orders of magnitute by developing low-noise sensor controllers, by optimizing operation conditions and by determining the electronic conductivity of the solid electrolyte material. As a result, the detection limits of the sensor operating in continuous flow-through mode range now below 5 vol.-ppb for the analytes H2, O2 and CH4. Furthermore, two approaches for the increasement of the selectivity of ses are presented. One of them concerns an optimized cyclovoltammetric operation of these sensors, resulting in a linear increase of peak height and peak area with increasing concentrations up to 10^2 vol.-ppm for H2, O2 or CO in nitrogen based gas mixtures. Thus, hydrogen could be detected on a Pt electrode with high catalytic activity in presence of an excess of oxygen in the measuring gas. The second approach is directed on the significant improvement of selectivity by operating coulometric ses in potentiostatic mode downstream of a gas chromatographic separation unit. For H2 and CH4 this chromatographic measuring system exhibited linear operation in the concentration range from 10^-1 - 10^4 vol.-ppm and offered detection limits of 55 and 40 vol.-ppb respectively. A novel in-situ measuring system is based on continuous membrane-free extraction, followed by periodic chromatographic separation and subsequent coulometric detection by a potentiostatically operated coulometric ses. With this measuring system, traces of H2 and O2 as well as the much larger amount of generated CH4 were determined simultaneously in the digestion medium of biogas plants. It is shown that instabilities in the microbial biogas process which are caused by fermenter overfeeding can be realized on the basis of the course of dissolved hydrogen. The novel measuring system indicates these instabilities much earlier than commercially available hydrogen sensors positioned in the biogas stream. Thus, a practicable longterm-stable, robust and low-maintenance measuring system could be developed for these parameters with the use of ses. The ses equipped with platinum electrodes shows electrode blocking during the coulometric measurement of acetic acid. Infrared spectrometric investigations of the ses exhaust gas clearly indicate thermal decomposition of this analyte at 750 °C, which is accompanied with carbon film formation on the Pt electrode surface. This blockage leads to peak broadening and therefore prevents appropriate detection of carboxylic acids containing between 2 and 5 carbon atoms. It could be demonstated in this work that this detrimental thermal decomposition on the ses electrodes could be circumvented by integrating a heated Pt catalyst between separation column and ses detector. The acids decompose then at the Pt surfaces of the catalyst at 800 °C and the decomposition products are detected by ses immediatly without formation of pyrolysis products on the electrodes. By replacing the ses in the measuring system with a flame ionization detector, acetic acid could be measured with appropriate long-term stability in the digestion medium of a biogas plant over a period of eighteen days. In summary this work presents substancial contributions to the advancement of coulometric solid electrolyte sensors by lowering their detection limits, increasing their selectivity and thus broadening their application spectrum significantly.