Cette étude est dédiée à l’optimisation de capteurs chimiques de gaz de type résistifs à base de dioxyde d’étain (SnO2) pour l’application en ligne d’échappement automobile. Les capteurs sont élaborés par la technique de sérigraphie qui permet la production en masse de capteurs robustes sur substrat alumine. En vue de l’application automobile visée, les capteurs ont été optimisés par l’ajout d’une couche protectrice poreuse déposée sur l’élément sensible de SnO2. Le comportement de ces capteurs a été étudié en fonction de différents paramètres, comme la température et la vitesse des gaz, représentatifs de conditions d’échappement automobile. De plus, une modélisation « simple » de la réponse électrique des capteurs en fonction de la concentration d’un ou plusieurs gaz polluants cibles a été proposée. / This study is dedicated to the optimization of chemical gas sensors based on resistive type tin dioxide (SnO2) for automotive exhaust application. The sensors were produced by screen-printing technique which allows mass production of robust sensors on alumina substrate. In regards of the automotive application, the sensors were optimized by adding a porous protective layer deposited on the sensing element SnO2. The behaviour of this type of gas sensors was studied depending on gas parameters such as gas temperature and velocity, representative of real operation conditions. In addition, a “simple” modelisation of the electrical response of sensors depending on the concentration of one or more gaseous targets is proposed.

A novel MOSFET gas sensor for the investigation has been developed. Its configuration resembles a"normally on"n-type thin-film transistor (TFT) with a gas sensitive metal oxide as a channel. The device used in the experiments only differs from common TFTs in the gate configuration. In order to allow gas reactions with the SnO2-surface, the gate is buried under the semiconducting layer. Without any gate voltage, the device works as a conventional metal oxide gas sensor. Applied gate voltages affect the channel carrier concentration and surface potential of the metal oxide, thus causing a change in sensitivity. The results of the gas measurements are in accordance with the electric adsorption effect, which was postulated by Fedor Wolkenstein 1957, and arises the possibility to operate a semiconductor gas sensor at relatively low temperatures and, thereby, be able to integrate CMOS electronics for processing of measurements at the same chip.

In this study, zinc oxide (ZnO) nanotip and nanorod were grown on glass substrate by aqueous solution deposition (ASD). Both characteristics of the two nanostructures were investigated. For fabrication of ZnO nanostructure UV photodetector, In-Zn inter-digitated metal electrode was evaporated on the top of the grown ZnO nanostructure to form the contact via. Compared with the common value (375 nm), both the peaks from the PL spectra of ZnO nanotip and nanorod are red-shifted (409 nm) due to the massive defects in nanotip and nanorod. In order to improve the photosensiblity, heterojunction of ZnO nanostructure/TiO2 film was prepared and were made into UV photodetector. Photoresponses of both nanotip and nanorod were improved after N2O annealing at 300oC. With the heterojunction of ZnO 1D nanostructure on TiO2 film, the photoresponses of both ZnO nanotip/TiO2 film can reach to 22.85, and the rise time and decay time are 40 and 82 seconds, respectively. On the other side, the photoresponses of both ZnO nanorod/TiO2 film can reach to 27.44, and the rise time and decay time are 22 and 133 seconds, respectively.

Nanopore covered microporous silicon interfaces have been formed via an electrochemical etch for gas sensor applications. Rapid reversible and sensitive gas sensors have been fabricated. Both top-down and bottom-up approaches are utilized in the process. A nano-pore coated micro-porous silicon surface is modified selectively for sub-ppm detection of NH3, PH3, NO, H2S, SO2. The selective depositions include electrolessly generated SnO2, CuxO, AuxO, NiO, and nanoparticles such as TiO2, MgO doped TiO2, Al2O3, and ZrO2. Flow dynamics are analyzed via numerical simulations and response data. A general coating selection method for chemical sensors is established via an extrapolation on the inverse of the Hard-Soft Acid-Base concept.

<p>A novel MOSFET gas sensor for the investigation has been developed. Its configuration resembles a"normally on"n-type thin-film transistor (TFT) with a gas sensitive metal oxide as a channel. The device used in the experiments only differs from common TFTs in the gate configuration. In order to allow gas reactions with the SnO2-surface, the gate is buried under the semiconducting layer. Without any gate voltage, the device works as a conventional metal oxide gas sensor. Applied gate voltages affect the channel carrier concentration and surface potential of the metal oxide, thus causing a change in sensitivity. The results of the gas measurements are in accordance with the electric adsorption effect, which was postulated by Fedor Wolkenstein 1957, and arises the possibility to operate a semiconductor gas sensor at relatively low temperatures and, thereby, be able to integrate CMOS electronics for processing of measurements at the same chip.</p>

A mass manufacturable impedance based, palladium doped porous silicon sensor, was fabricated for hydrogen detection. The sensor was built using electrochemical etching to produce mesoporous silicon. Four nanometers of palladium was defused directly into the porous silicon and another four nanometers of Pd was deposited on the defused surface to enhance sensing. The sensor was tested in a sealed chamber in which the impedance was measured while hydrogen in nitrogen was ranged from 0-2 percent. Unlike conventional hydrogen sensors this sensor responded at room temperature to changes in hydrogen concentration. The electrical impedance response due to adsorption and desorption of hydrogen reacted relatively quickly due to the nanoparticle nature of palladium diffusion in and Pd evaporation on porous silicon.

The precise determination of the type and concentration of gases is of increasing importance in numerous applications. Despite the diverse operating principles of today´s gas sensors, technological trends can be summarized with the keyword miniaturization, because of the resulting benefits such as integrability and energy efficiency. This work deals with the development and fabrication of novel nanowire based gas sensors, which in comparison to conventional devices have an advantageous combination of high sensitivity and selectivity with low power consumption and small size. On the basis of grown silicon nanowires, sensors based on the functional principle of classical Schottky barrier field effect transistors with abrupt metal-semiconductor contacts are fabricated. The sensing performance of the devices is investigated with respect to the detection of ammonia. Ammonia concentrations down to 170 ppb are measured with a sensor response of more than 160 % and a theoretical limit of detection of 20 ppb is determined. Selectivity investigations show that no cross sensitivity to most common solvents occurring in living spaces exists. Moisture influences on the device are studied and reveal that the sensor responds within seconds, making it potentially suitable as humidity sensor. Moreover, it is shown that a higher relative humidity and higher temperatures decrease the sensor sensitivity. In terms of possible applications, it is a great advantage that the maximum sensitivity is achieved at 25 °C. With respect to sensitivity and selectivity an enhancement is demonstrated compared to most nanosensors known from the literature. Hence, the technology offers the potential to complement conventional measurement systems in future sensor technology especially in portable applications. / Die präzise Bestimmung der Art und Konzentration von Gasen erlangt in zahlreichen Anwendungsgebieten zunehmend an Bedeutung. Trotz der vielfältigen Wirkprinzipien heutiger Gassensoren lassen sich die technologischen Trends mit dem Schlagwort Miniaturisierung zusammenfassen, da sich daraus entscheidende Vorteile wie Integrierbarkeit und Energieeffizienz ergeben. Diese Arbeit beschäftigt sich mit der Entwicklung und Herstellung neuartiger nanodrahtbasierter Gassensoren, welche im Vergleich zu klassischen Sensoren eine vorteilhafte Kombination von hoher Sensitivität und Selektivität bei geringem Stromverbrauch und geringer Größe aufweisen. Auf der Grundlage gewachsener Silizium-Nanodrähte werden Sensoren mit abrupten Metall-Halbleiter-Kontakten hergestellt, welche auf dem Funktionsprinzip klassischer Schottkybarrieren-Feldeffekttransistoren beruhen. Die Eignung der Sensoren wird in Bezug auf die Detektion von Ammoniak untersucht. Dabei kann eine minimale Ammoniakkonzentration von 170 ppb mit einer Signaländerung von mehr als 160 % gemessen werden, wobei die theoretische Nachweisgrenze mit 20 ppb ermittelt wird. Selektivitätsuntersuchungen zeigen, dass keine Querempfindlichkeit gegenüber den am häufigsten in Wohnräumen vorkommenden Lösungsmitteln besteht. Feuchtigkeitseinflüsse auf den Sensor werden untersucht und es wird nachgewiesen, dass der Sensor Ansprechzeiten im Sekundenbereich besitzt, was ihn zu einem potenziell geeigneten Feuchtigkeitssensor macht. Darüber hinaus wird gezeigt, dass eine höhere relative Luftfeuchtigkeit und höhere Umgebungstemperaturen die Sensorsensitivität verringern. In Bezug auf mögliche Einsatzgebiete stellt die maximale Empfindlichkeit bei 25 °C einen großen Vorteil da. Bezogen auf Sensitivität und Selektivität wird somit eine Verbesserung im Vergleich zu den meisten aus der Literatur bekannten Nanosensoren demonstriert. Damit bietet die Technologie das Potential, konventionelle Messsysteme in zukünftiger Sensorik vor allem in portablen Anwendungen zu ergänzen.

Transistors are the fundamental components of computer processors. The dimensions of transistors used in microprocessors are decreasing every year and the challenge of maintaining this trend now requires nanoscale dimensions. A potential method of achieving nanoscale dimensions is using atomic clusters as building blocks. It is therefore desirable to investigate transistor-like behaviour in cluster devices. Traditionally, transistor devices are made from semiconducting materials. It was therefore proposed that gated behaviour would be observable in devices that are fabricated from germanium clusters. A germanium cluster source was designed and built. Field effects were successfully observed in films of germanium clusters. Immediately after deposition, the gate effect of germanium cluster films was insignificant. As the films slowly oxidized in vacuum, a decrease in the overall carrier concentration was observed which lead to an increase in the gate effect, with a maximum change in resistance observed of 12%. When films of germanium clusters were exposed to air, a resistance decrease was observed, attributed to water vapour adsorbing on the surface. The phenomenon was further investigated and the proposed resistance change mechanism involves water vapour creating surface defects which act as donors and cause the electron concentration in the film to increase. Films of germanium clusters were sensitive to hydrogen concentrations above 1% in air, with up to a factor of 25 decrease in resistance observed at room temperature for 5% hydrogen concentration. Thin films were found to be most sensitive. The higher sensitivity was attributed to the larger surface-to-volume ratio. The proposed mechanism for sensing is that defects are created on the surface of the film, which in turn act as donors which cause the electron concentration in the film to increase. Bismuth is a semimetal and gate effects have previously been observed in bismuth nanowires. Parallel bismuth nanowires of 300nm diameter were successfully deposited at a distance of 200nm apart allowing one of the wires to be used as a gate. The gate effects observed in bismuth cluster structures were weak and inconclusive, with a small gate effect (change in resistance of 0.1%) observed at 11K in some devices.

A theoretical model is developed for calculating the sensitivity of resistive sensors based on aligned multiwall carbon nanotubes (MWCNT) embedded in the pores of alumina membranes. Aligned MWCNTs offer more surface area as each CNT acts as a landing site for detecting gas species. The MWCNTs behave as a p-type semiconducting layer; when the bus bar contacts are placed at either end of the top surface the resistance between the contacts responds to oxidizing (resistance decreases) and reducing gases (resistance increases). The model presented in this thesis aims to understand the device resistance dependence upon the MWCNT resistance, and the sensitivity dependence upon the device structure and design. The model was utilized for enhancing the sensitivity of MWCNT sensors for ammonia (30% sensitivity) and nitrogen dioxide (40% sensitivity) gases. Experimental results from sensitivity measurements are compared with theoretical predictions.

In this thesis, the author proposed and developed nanostructured materials based Surface Acoustic Wave (SAW) and conductometric transducers for gas sensing applications. The device fabrication, nanostructured materials synthesis and characterization, as well as their gas sensing performance have been undertaken. The investigated structures are based on two structures: lithium niobate (LiNbO3) and lithium tantalate (LiTaO3). These two substrates were chosen for their high electromechanical coupling coefficient. The conductometric structure is based on langasite (LGS) substrate. LGS was selected because it does not exhibit any phase transition up to its melting point (1470°C). Four types of nanostructured materials were investigated as gas sensing layers, they are: polyaniline, polyvinylpyrrolidone (PVP), graphene and antimony oxide (Sb2O3). The developed nanostructured materials based sensors have high surface to volume ratio, resulting in high sensitivity towards di¤erent gas species. Several synthesis methods were conducted to deposit nanostructured materials on the whole area of SAW based and conductometric transducers. Electropolymerization method was used to synthesize and deposit polyaniline nanofibers on 36° YX LiTaO3 and 64° YX LiNbO3 SAW substrates. By varying several parameters during electropolymerization, the effect on gas sensing properties were investigated. The author also extended her research to successfully develop polyaniline/inorganic nanocomposites based SAW structures for room temperature gas sensing applications. Via electrospinning method, PVP fibres and its composites were successfully deposited on 36° YX LiTaO3 SAW transducers. Again in this method, the author varied several parameters of electrospinning such as distance and concentration, and investigated the effect on gas sensing performance. Graphene-like nano-sheets were synthesized on 36° YX LiTaO3 SAW devices. This material was synthesized by spin-coating graphite oxide (GO) on the substrate and then exposin g the GO to hydrazine to reduce it to graphene. X-ray photoelectron spectroscopy (XPS) and Raman characterizations showed that the reduced GO was not an ideal graphene. This information was required to understand the properties of the deposited graphene and link its properties to the gas sensing properties. Thermal evaporation method was used to grow Sb2O3 nanostructures on LGS conductometric transducers. Using this method, different nanoscale structures such as nanorods and lobe-like shapes were found on the gold interdigitated transducers (IDTs) and LGS substrate. The gas sensing performance of the deposited nanostructured Sb2O3 based LGS conductometric sensors was investigated at elevated temperatures. The gas sensing performance of the investigated nanostructured materials/SAW and conductometric structures provide a way for further investigation to future commerciallization of these types of sensors.