Nanocrystalline indium oxide (In2O3)-doped tin oxide (SnO2) thin film sensor has been sol-gel dip-coated on a microelectromechanical systems (MEMS) device. The micro-sensor device is successfully utilized to sense ppm level H2 at room temperature with high sensitivity. The chamber pressure has no pronounce effect on the room temperature H2 sensitivity.

In this work we employed lithographic techniques, combined with sputtering depositions, to fabricate semiconductor metal-oxide (MOX) gas sensors with controlled grain dimensions. The basic idea is to replace the continuous sensing film of standard MOX sensors with a pattern of wires in the sub-micron scale, thus controlling the lateral size of the grains. Regarding the fabrication process, we followed two different approaches: a plain lift-off technique and a substrate patterning process. We present a comparison between the results of both the approaches. Furthermore, we tested the electrical responses to several gases and compared them with those of continuous film sensors. The experimental data highlight an improvement for the patterned sensors.

A tremendous variation in electrical resistance, from the semiconductor to metallic range, has been observed in titania nanotube arrays at room temperature, ≈ 25°C, in the presence of low ppm hydrogen gas concentrations (≤ 1000 ppm). The nanotube arrays are fabricated by anodizing titanium foil in an aqueous fluoride containing electrolyte solution. Subsequently, the arrays are annealed in an oxygen ambient, then coated with a 10 nm layer of palladium by evaporation. Electrical contacts are made by sputtering a small (e.g. 1 mm diameter) platinum disk atop the Pd coated nanotube-array. These sensors exhibit a resistance variation of the order of over 107 (1,000,000,000%) in the presence of 1000 ppm hydrogen at 23°C. To the best of our knowledge this dynamic change in electrical resistance the largest known response of any material, to any gas, at any temperature. The sensors demonstrate complete reversibility, repeatability, high selectivity, no drift and wide dynamic range. The nanoscale geometry of the nanotubes, in particular the points of tube-to-tube contact, is believed to be responsible for the outstanding hydrogen gas sensitivities.

A high sensitive and reliable In2O3 nanowire mat devices are fabricated. Detection of NO2 gas down to ppb levels was achieved. This represents orders-of-magnitude improvement over previously reported metal oxide film or nanowire/nanobelt sensors. Furthermore, this simple device shows certain selectivity to NO2 with other common chemicals such as NH3, O2, CO and H2 around.

The properties of the most important semiconductor oxides for gas sensing, such as SnO2, TiO2, WO3 and their modifications due to ion-addition or to catalyzers, have been reviewed. The oxide powders, synthesized via wet chemical routes, have been characterized both as dispersed powders and as sintered films. Deposition of thick film gas sensors was carried out through screen-printing technology. They have been studied comparing the electrical behavior under identical environmental conditions. Potential barrier heights have been measured via stimulated temperature measurements and density of ionized donors obtained via Hall Effect. A theoretical model has been developed to justify the size dependent behavior of nanocrystalline oxides.

In the present work, we report a detailed study based on transmission electron microscopy of the microstructure and composition of the Fe2O3: SnO2 nanometrical binary system obtained by sol-gel. We studied a set of samples based in a pure Fe2O3 material where we added SnO 2 from 0 to 50% Sn-Fe nominal content. The structure of our composites will change from pure a-Fe2O3 to the well-known cassiterite SnO 2 structure for high Sn content. Distortions on the structural parameters will be attributed to the presence Sn4+ ions on the α-Fe2O3 structure and Fe3+ ions on the SnO2 cassiterite structure.

Novel sensor design for high sensitivity gas sensors has been proposed for the detection of dilute NO2 using WO3 film. First, concerning nano-design of oxide particles, the disk-shaped WO3 particles (300 nm in diameter, 20 nm in thickness) were deposited on Au comb-type microelectrode (line width: 5 μm, distance between lines: 5 μm) to be WO3 thick film sensor. This sensor showed the excellent sensing properties to dilute NO2 at optimized thickness. Second, the nano-gap electrode with various gap-sizes (110–1500 nm) was fabricated by means of MEMS techniques in order to investigate the effect of microelectrode. When the gap size was decreased less than 800 nm, the sensitivity to dilute NO2 increased with decreasing gap size. This was understood from the facts that the contribution of interface resistance between particle and electrode to total sensor resistance was increased and that the sensitivity at electrode-grain interface was much larger than that at grain boundary. It was found that the designs of not only nano-particles but also nano-electrode were important for the fabrication of high sensitivity gas sensor.

Molybdenum trioxide sol-gel matrices were tested for their ability to host biomaterials in their structures without adversely affecting the activity of the encapsulated biomolecules. The sol-gel composites were prepared at room temperature; the urease was added during the hydrolysis step to ensure proper encapsulation in the pore structure of the sol. Phosphate buffer solution was added during this stage to maintain the pH value of the sol in the bio-compatible range. The activity of incorporated urease was tested by exposing composite sol-gel material to standard urea test solutions and measuring the amount of gaseous ammonia (released by the reaction of urea with water in the presence of urease) using an ammonia electrode. The concentration of urea solutions ranged from 1mM to 10mM. An electronic olfactory system (EOS 835) was also employed to detect the presence of gaseous ammonia. Gas sensing tests were done to verify the sensitivity of the matrix to ammonia. Microstructural analysis was carried out by means of scanning and transmission electron microscopy. It was found that urease retained its activity inside molybdenum trioxide sol-gels that are sensitive to ammonia. These hybrid nanoporous composites are useful in the field of biosensors and fuel cells.

Factors limiting potential use of langasite as a gas sensor platform at elevated temperatures include excessive conductive and viscous damping, deviations from stoichiometry and chemical instability. This paper focuses on viscous damping which can be described by an effective viscosity of the resonator material. Based on a one-dimensional description of the resonator device, the material constants of langasite are determined as a function of temperature. Theeffective viscosity of the resonator material and the bulk conductivity are found to exhibit nearly the same activation energy at temperatures from 350 to 600 °C. Based on that fact, it is most likely that the predominant conduction mechanism, i.e. oxygen ion movement, controls the viscous damping in that temperature range. Therefore, intentional doping must suppress the oxygen conductivity to minimize losses. The effect of such dopants is presently under investigation. Further, pre-annealing specimens for several hours above 1050 °C is necessary, for the given sample dimensions, to establish reproducible materials properties. Based on oxygen diffusion data, it can be concluded that the oxygen stoichiometry of langasite specimens becomes equilibrated during the pre-annealing runs.

Results of investigation aimed at the improvement of gas sensitive characteristics of tin dioxide thin film's based gas sensors through their treatment in high frequent (HF) oxygen plasma and sensibilization by means of doping are presented.

Tin dioxide thin films were deposited from alcohol solutions of SnCl45H2O by means of chemical spray pyrolysis method on alumina substrate at the temperatures in the range from 400 to 500°C. The thickness of films was varied in the range 20–100 nm.

Surface treatment of obtained tin dioxide layers was carried out in pure oxygen plasma at room temperature and 3-4 times increase of the resistance and gas sensitivity was obtained at that. These parameters' changing is discussed from the point of view of thin film's stoichiometry alteration, for control of which there were used IR and SIMS measurements. Obtained results are compared with results of the film's high temperature (∼600oC) thermal annealing on air.

To increase thin film's gas sensitivity and selectivity the doping with Pd and Cu was applied. Doping was carried out also through chemical spray pyrolysis method from the solutions of corresponding compounds of Pd and Cu. Such modification of SnO2 surface hasallowed to increasing sensitivity to H2 up to 5.103 rel. units (high selectivity) at the simultaneous shift of sensitivity maximum from 350 to 150°C. Doping with Cu has allowed to increasing 4-5 times sensitivity and selectivity toward CH4 and C3H8 gases in air and reducing operating temperatures from 500–520°C to 350–370°C.

It is concluded that combination of HF oxygen plasma tin dioxide film' treatment and its surface sensibilization is an effective way to considerably improvement of gas sensitive and selective characteristics of tin dioxide based thin film gas sensors (TFGS).

Report presents results of research aimed the investigation of possibility to use such chalcogenide materials as vitreous As2S3, As2Se3 and their solid solutions for gas sensors applications. For a long time these materials were well-known as materials mostly used for optical information registration. Developed approach allows to widening the sphere of application of given materials and create new class of gas sensitive devices.

Thin film gas sensitive elements on the base of solid solutions of (As2S3)x-(As2Se3)1−x (where x=0; 0.3; 0.5; 0.7 and 1.0) were obtained by means of the thermal evaporation under vacuum conditions 10−5 Torr. Thin films can be deposited on both hard and flexible substrate. Formed films had specific resistance on the level 1010–1014 Ohm.cm (on dependence on composition) at the thickness of 1–2 μm.

It was established that CGS thin films possess gas sensitivity S, determined as S=(Ggas-Gait)/Gair, on the level 2–3 relative units for CO and 7–8 for hydrogen at the 100 ppm concentration levels of both gases in atmosphere already at the room temperature that allows to excluding the heater from the gas sensor construction decreasing through that both manufacturing and operational cost.

It is concluded that chalcogenide semiconductor materials can be considered as perspective materials for gas sensor application, including space exploration application.

Due to tightening restrictions on combustion exhaust emissions, low-cost sensors are desired for monitoring NOx production in high-temperature exhaust streams. This paper reports the characterization of Silicon Carbide MOSFET NO sensors for use in combustion exhaust monitoring. SiC depletion-mode MOSFETs were fabricated using a thermally-grown silicon dioxide gate dielectric and a Pt catalytic metal gate electrode. SiC MOSFET gas sensors were characterized at temperatures as high as 525°C in an ambient of synthetic air and NO (50–200 ppm) for 30 hours with no degradation.

Formation processes of porous silicon on insulating substrate were studied. It was demonstrated that both electrochemical and chemical formation methods allow to transform heavily doped p-type polycrystalline silicon into homogeneous porous silicon. Porous silicon was successfully used as sacrificial layer in the fabrication process of microbridge structures.

Thin films in the SnO2-In2O3 system, with relative concentrations of the two oxides ranging from 2% to 98% (molar percentage of the oxide), were deposited by sol-gel and solution processes. The films for the physical characterizations were deposited onto oxidized silicon substrates, while the films for the gas-sensing tests were deposited onto alumina. The starting solutions were characterized by FTIR spectroscopy, while the films on silicon, heated at various temperatures, were characterized by X-ray diffraction and SEM observations. The interaction between the two systems is particularly evident in the case of the system described by a 50% In2O3-50% SnO2 nominal composition. The crystallization on In2O3 during the film heat-treatment hinders the crystallization of SnO2, thus Sn is dispersed as an n-dopant in the In2O3 lattice, indeed showing a current signal, in the gas-sensing test, two orders of magnitude higher than the pure film. The response of the mixed-oxide based device to NO2 is better than pure In2O3. On the other hand, the response (relative resistance change) of pure SnO2 to low (from 0.1 to 1 ppm) NO2 concentrations ranges from 150 to 300, a result that can be correlated with the nanostructure of the film, which, from SEM and XRD results, seems constituted by very small grains.

In2O3 nanowire and carbon nanotube transistors were used to study the chemical gating effects in response to LDL particles. Low density lipoprotein (LDL) cholesterol in blood constitutes a risk factor for coronary artery disease (heart attack). The interactions of LDL particles with these two different surfaces were investigated. The degree of LDL particles binding to carbon nanotubes was ten-fold higher than to In2O3 nanowires possibly owing to the hydrophobic/hydrophilic interactions. The conductance of field effect transistors (FET) based on nanowires and nanotubes showed complementary responses after exposure to LDL particles. While In2O3 nanowire transistors exhibited higher conductance accompanied by a negative shift of the threshold voltage, nanotube transistors displayed a lower conductance. This phenomenon was attributed to the complementary doping between the n-type In2O3 nanowires and p-type carbon nanotubes.

High temperature Pd-SiC Schottky diode gas sensors are known to thermally degrade due to interdiffusion and reaction at the metal-semiconductor interface. To understand and possibly eliminate this problem, detailed surface studies of thermally induced Pd-SiC surface interactions have been performed. These experiments compare standard 6H-SiC (0001) surfaces typical of those used in device fabrication with periodically stepped surfaces prepared by high temperature hydrogen etching. The Pd films range in thickness from the monolayer level (∼0.4 nm) to actual device dimensions (∼46.5 nm) and are deposited under ultrahigh vacuum conditions at ∼50 °C. These films are characterized in-situ using Auger electron spectroscopy both before and after annealing at 670 °C. The Auger lineshapes provide quantitative information on the chemistry of the reaction products. Ex-situ atomic force microscopy is used to characterize changes in surface morphology.

In the study, low power micro gas sensors operated at room temperature for the detection of NH3 and NOx gases are proposed. As candidate material of gas sensor for NH3 gas detection at room temperature, polyaniline(PANi) synthesized by chemical polymerization was selected. And Te(Tellurium) thin film was used for NOx gas detection at room temperature. By using these sensing materials, micro gas sensors for room temperature operation were prepared and measured the response characteristics for NH3 and NOx.

In case of PANi sensor, the structure was inverted staggered FET type having advantage of useful one for Lab-On-a-Chip. The operating principle of the sensor is based on the change in work function of PANi film caused by adsorption of gas molecules in air on the film surface. The change in work function was measured indirectly from that in gate voltage of the FET device. The responses to various gases (NH3, CH4, Methanol and CH3CN) were obtained in gate voltage step mode in R.H. 30%. And in case of Te sensor, the sensing material was thermally evaporated on glass substrate. The thickness and annealing temperature were 500 Å −2000 Å and 100 °C −300 °C, respectively. The Te-based micro gas sensor exhibited high sensitivity to NOx and good selectivity against CO and hydro-carbon gases. And by adding Ti to Te film, the sensor has a good selectivity to CO gas.

A gas sensor micro-device using both thermoelectric film and catalyst film has been developed on the platform of micro-hotplate. Thermoelectric thin film of B-doped SiGe was deposited on the Si3N4/SiO2/Si substrate by helicon sputtering method and thermal annealing was carried out to crystallize the as-deposited amorphous-like film. With increasing the annealing temperature and time, the crystallization of the SiGe thin film progressed, resulting in high carrier mobility and large absolute value of Seebeck coefficient. The hydrogen sensitivity of the micro-thermoelectric gas sensors was investigated.

Polycrystalline samples of the perovskites: SrPb1−xBixO3−δ (x = 0, 0.1) were prepared from the dissolution of stoichiometric amounts of Sr(NO3), Pb(NO3)2 and Bi(NO3)3 in aqueous media. The solutions were heat-dried at 75°C obtaining powder precursors. To determine the temperature of formation of these phases, thermal analyses were made on them. Calcination at 650°C for 6 h produced pure samples, which were characterized by X-ray powder diffraction. The surface microstructure of powders was analyzed by scanning electron microscopy, and showed that this preparation procedure yield micron-sized particles, which formed agglomerations. To evaluate their gas sensing behavior, the resulting powders were finely ground and mixed with acetone to form thick-films. The electrical conductivity vs. temperature of films was measured in dry air, O2 and CO2, and resulted that SrPb0.9Bi0.1O3−δ has a better sensitivity to O2 compared to SrPbO3−δ, caused by the partial substitution of lead by bismuth.

Printing is a promising technique to fabricate commercial organic electronic devices such as OLED, TFT, solar cells and sensors. In this investigation, we report the application of high volume flexographic printing, which makes low cost and batch production possible. Commercial RFID tags have been printed using metallic inks with organic compounds in the Printing Applications Lab (PAL) at Rochester Institute of Technology (RIT). Polyaniline ink was prepared and printed by flexography in the form of test targets and Interdigitated electrodes (IDE). The conductivity can be controlled by different levels of doping. Furthermore, multiple impression printing was used to print overlapping functional layers to obtain all printed organic electronic devices like chemical vapor and humidity sensors. Optical profilometry and SEM were used to analyze the surface and interface structure. Sensitivity was measured and compared with commercial devices.