Kosmas Galatsis

p- and n-type Fe-doped SnO2 gas sensors fabricated by the mechanochemical processing technique

Fe-doped SnO2 sensors were fabricated using micromechanical synthesis technique. The Fe-doped sensor was compared to pure SnO2. Fe-doped SnO2 responded as a p-type semiconductor to oxygen concentrations of up to 10% at 300 °C. As the temperature increased to 400°C, the material responded as an n-type semiconductor. Furthermore, a higher surface area and smaller grains size diameters were achieved when doping SnO2 with Fe. This translated into improved dynamic gas sensing properties and also improved responses to gases such as ethanol.

Investigation of the oxygen gas sensing performance of Ga2O3 thin films with different dopants

Abstract The oxygen gas sensing performance of Ga2O3 semiconducting thin films doped with Ce, Sb, W and Zn have been investigated. These thin films have been prepared by the sol–gel process and were deposited on sapphire transducers with inter-digital electrodes and a platinum heater integrated. The sensors were exposed to various concentrations of oxygen gas in an ambient of nitrogen and the gas sensing performance has been examined. The responses of sensors doped with Ce, Sb, W and Zn were stable and reproducible at their respective operating temperatures. It was observed that Ga2O3 films doped with Ce, Zn and W are promising for oxygen gas sensing applications.

Novel Love mode surface acoustic wave based immunosensors

Love mode surface acoustic wave (SAW) immunosensors were developed. The Love mode guiding layers in these sensors are ZnO and SiO2 thin films. It is shown that mass sensitivity of the devices with ZnO layer are larger than that of with SiO2 guiding layers. A system comprising dual delay line, based on ZnO/90° rotated ST-cut quartz crystal devices, was set up for conducting the immunosensing experiments. In these experiments adsorption of rat immunoglobulin G onto the active area of the sensor was monitored. Upon exposure to solutions containing IgG, the operational frequency of the system changed due to the interaction of IgG and a gold surface on the active area of the sensor. Mass sensitivities were measured and frequency responses to various IgG concentrations are presented.

Semiconductor MoO3–TiO2 thin film gas sensors

Abstract The O 2 , CO and NO 2 gas sensing properties of MoO 3 –TiO 2 thin films have been studied. The sol–gel process was employed to fabricate MoO 3 –TiO 2 thin films onto sapphire and alumina transducers for gas sensing measurements. It was found the MoO 3 dominated sensors have a lower optimal operating temperature of 370°C than the TiO 2 dominated sensors. The response of the sensors was stable and reproducible at operating temperatures below 400°C. However, once the films were exposed to temperatures higher than 400°C, repeatable gas sensing results could not be achieved. The low evaporation temperature of MoO 3 component of the mixed system is believed to be the cause of the sensors instability and irreversibility at high operating temperatures.

A Pt/Ga/sub 2/O/sub 3/-ZnO/SiC Schottky diode-based hydrocarbon gas sensor

In this paper, a novel metal-reactive insulator-silicon carbide device with a catalytic layer for hydrocarbon gas-sensing is presented. This structure, employed as a Schottky diode, utilizes sol-gel prepared Ga/sub 2/O/sub 3/-ZnO layer as the reactive insulator. The sensor has been exposed to propene gas, which lowers the barrier height of the diode. The responses were stable and repeatable at operating temperatures between 300 and 600/spl deg/C. The response to propene in different ambients was examined. The effect of diode bias has been investigated by analyzing the sensors response to various propene concentrations when held at constant currents of 2 and 8 mA.

Comparison of layered based SAW sensors

A comprehensive investigation of layered surface acoustic wave (SAW) devices is presented. These types of SAW devices have a remarkable performance for gas and liquid sensing applications as confinement of energy in the layers generally increases their sensitivity. Different modes of wave propagations and the effect of the guiding layer thickness and material on the layered devices are discussed in this paper.

Study of novel Love mode surface acoustic wave filters

Novel Love mode filters based on ZnO and SiO/sub 2//90/spl deg/ rotated ST-cut quartz crystal structure were fabricated. A comprehensive study was carried out to show the capabilities of such filters. The periodicity of the fingers is 50 /spl mu/m and the thickness of the SiO/sub 2/ and ZnO layers ranges from 0.2 to 7.2 /spl mu/m. Electromechanical coupling coefficient, capacitance per unit wavelengths of finger pairs and temperature coefficient of frequency are studied in terms of thickness of the wave-guiding layers.

Microcharacterisation and gas sensing properties of mechanochemically processed nanosized iron-doped SnO/sub 2/

SnO/sub 2/ nanoparticles, undoped and doped with 1, 2 and 5 mol% Fe/sub 2/O/sub 3/, were synthesised by mechanochemical processing. XRD analysis, for heat treatment at 600/spl deg/C, revealed the average crystallite size to decrease progressively from 9.5 nm for undoped SnO/sub 2/, to 5.3 nm for SnO/sub 2/ doped with 5% Fe/sub 2/O/sub 3/. HRTEM imaging revealed the presence of a number of crystal defects. Thin films for electrical characterisation were prepared by spin coating and subsequent annealing, with the response to O/sub 2/ measured. The resistance increased for increasing dopant concentration. The response to 1000 ppm, 1% and 10% O/sub 2/ was measured, with the highest response found for the 2%-doped sample. The 5%-doped sample exhibited a weak p-type response at 400/spl deg/C and below, with a transition to n-type at 450/spl deg/C. The thin films prepared from the Fe/sub 2/O/sub 3/-doped SnO/sub 2/ nanopowders have potential for the development of gas sensors.

Microstructure characterization of sol-gel prepared MoO3–TiO2 thin films for oxygen gas sensors

Binary metal–oxide MoO3–TiO2 films have been prepared using the sol-gel technique. The thin films were annealed at several temperatures including 400, 450, 500, 550, and 600 °C for 1 h. The morphology, crystalline structure, and chemical composition of the films have been analyzed using scanning electron microscopy (SEM) and atomic force microscopy, x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), and x-ray photoelectron spectroscopy (XPS) techniques. The SEM analysis showed that there are two different sizes of grains in the films annealed at temperatures of 400, 450, 500, and 550 °C. One grain type is small with 20–100 nm; the other is a large grain type several micrometers in length. The XRD analysis revealed that the films annealed at 400 °C were a mixture of orthorhombic and hexagonal MoO3 phases. The films annealed at 450 °C showed an increase in the hexagonal phase. A preferential orientation growth along the (100) plane of the hexagonal phase and the (010) plane of the orthor...

Low-temperature InOx thin films for O3 and NO2 gas sensing

The desirable electrical properties of InOx thin films and their response towards oxidizing gases has promoted InOx to be recognized as a promising material for gas sensors. In this study, InOx films in the thickness range of 10-1000 nm were deposited onto Corning 7059 glass substrates by dc magnetron sputtering. Their structural, electrical, and O3 and NO2 sensing properties were analyzed. Structural investigations carried out by XRD and AFM showed a strong correlation between crystallinity, surface topology and gas sensitivity. Moreover, the electrical conductivity exhibited a change of over six orders of magnitude during the processes of photoreduction and oxidation. The films deposited on alumina transducers were calibrated towards O3 and NO2 at temperatures from 50-300 °C. The sensors show promising characteristics as they exhibited reproducible and stable responses. The 50 nm thin film had a response of over 10 towards 50 ppb of ozone operating at 50°C, while the 20 nm film had a response of over 22 towards 0.1 ppm of NO2 at 100°C.