Claus-Dieter Kohl

CO-Sensor for domestic use based on high temperature stable Ga2O3 thin films

Abstract Gas sensors based on high temperature operated metal oxides, like Ga2O3 thin films show promising properties in terms of reproducibility, long-term stability against interfering gases and low cross sensitivity to humidity. In this paper a surface modification of Ga2O3 is presented which allows the set up of a sensor suitable for indoor CO monitoring. The modification based on Au-clusters on the Ga2O3 surface yields high sensitivity to CO and a distinct reduction of the cross sensitivity towards organic solvents. With the specimens, a resistance change of approx. factor 4 to 100 ppm CO in wet air is attained. By employing catalytic filters of ceramic material, cross sensitivities to organic solvents are virtually completely eliminated.

Photoreduction of mesoporous In2O3: mechanistic model and utility in gas sensing.

A model is proposed for the drop in electronic resistance of n-type semiconducting indium oxide (In(2)O(3)) upon illumination with light (350 nm, 3.5 eV) as well as for the (light-enhanced) sensitivity of In(2)O(3) to oxidizing gases. Essential features of the model are photoreduction and a rate-limiting oxygen-diffusion step. Ordered, mesoporous In(2)O(3) with a high specific surface area serves as a versatile system for experimental studies. Analytical techniques comprise conductivity measurements under a controlled atmosphere (synthetic air, pure N(2)) and temperature-resolved in-situ Fourier transform infrared (FTIR) spectroscopy. IR measurements reveal that oxygen vacancies form a donor level 0.18 eV below the conduction band.

A High Temperature Capacitive Humidity Sensor Based on Mesoporous Silica

Capacitive sensors are the most commonly used devices for the detection of humidity because they are inexpensive and the detection mechanism is very specific for humidity. However, especially for industrial processes, there is a lack of dielectrics that are stable at high temperature (>200 °C) and under harsh conditions. We present a capacitive sensor based on mesoporous silica as the dielectric in a simple sensor design based on pressed silica pellets. Investigation of the structural stability of the porous silica under simulated operating conditions as well as the influence of the pellet production will be shown. Impedance measurements demonstrate the utility of the sensor at both low (90 °C) and high (up to 210 °C) operating temperatures.

A selective H2 sensor implemented using Ga2O3 thin-films which are covered with a gas-filtering SiO2 layer

N-type semiconducting Ga 2 O 3 thin-films which are stable at high temperatures are used as a new basic material for gas sensors. This study investigates the extent to which a gas-filtering layer of compact, amorphous SiO 2 is capable of modifying gas sensitivity. Using a sputtering technique and a Si target, the SiO 2 layer, which, typically, has thicknesses of 30 nm and 300 nm, is deposited onto the Ga 2 O 3 gas sensor. It was found that sensors with this surface layer structure had an extremely high specificity for H 2 when they were operated at 700°C. Other gases that were tested included CO, CO 2 , CH 4 , isobutene, ethanol, acetone, NO, NH 3 ; variations of humidity and oxygen content were also investigated. There was a marked increase in H 2 -sensitivity between the modified and unmodified sensors.

A study of surface modification at semiconducting Ga2O3 thin film sensors for enhancement of the sensitivity and selectivity

Abstract N-type semiconducting Ga2O3 thin films which are stable at high temperatures are being used as a new basic material for gas sensors. This study is an attempt to determine to what extent coating the surface of Ga2O3 thin films with another metal oxide will produce new gas sensitivities. The process involves sputtering a modification layer which is typically 30–300 nm thick onto a readily prepared planar Ga2O3 sensor with a film thickness of 2 μm Ta2O5, WO3, NiO and AlVO4 were used to form the modification layer. It was found that in some cases there was a radical change in gas sensitivity characteristics especially with WO3, NiO and AlVO4. The observed effects depend strongly on annealing and operating temperature. These results are the basis for sensors which react sensitively to NO and NH3, for selective O2 sensors, for Ga2O3 sensors which react to reducing gases with the same sign of conducting change as p-type semiconductors and a gas-sensitive reference element.

Percolation and gas sensitivity in nanocrystalline metal oxide films

We use Monte Carlo simulations to study the influence of gas adsorption on the conductance of nanocrystalline metal oxide films. The films are modeled by a network of intergranular contacts with conductances that depend on the amount of adsorbed gas molecules, and take into account a broad distribution of grain sizes and the possibility that ultrasmall nanograins can be insulating. Using percolation theory, we show that below a critical gas concentration (detection limit), the film can be insulating due to the absence of a percolating cluster of conducting grains. Above this detection limit, the conductance of the film increases rapidly. The detection limit can be tuned by the grain size and the mean coordination number in the film.

HIGHLY SENSITIVE NO2 SENSOR DEVICE FEATURING A JFET-LIKE TRANSDUCER MECHANISM

Abstract The conductance of a thin SnO 2 film in contact with platinum is used to detect NO 2 down to the 10 ppb range with a cross sensitivity to CO of the order of only 10 −4 . The SnO 2 layer thickness of 60 nm is chosen of the order of the depletion layer caused by the action of adsorbed oxygen ions and by the Schottky contact at the platinum interface. By a strong sintering pretreatment potential barriers between the grains are avoided and a diffusion of the ambient gas into the layer is suppressed. The width of the conductive channel in the middle of the SnO 2 layer is reduced in the presence of NO 2 adsorbed on the surface. This strongly electrophilic molecule forms acceptor surface states energetically below the acceptor states of the adsorbed oxygen thus enhancing the electron depletion.

Gas sensing fundamentals

Part I: Concepts High-Temperature Gas Sensors by Denny Richter and Holger Fritze Insect Olfaction as a Natural Blue-Print of Gas-Sensors? by Bernhard Weissbecker and Stefan Schutz Sensor Arrays, Virtual Multisensors, Data Fusion and Gas Sensor Data Evaluation by Peter Reimann and Andreas Schutze Part II: Material and Structure Carbon Nanotube Gas Sensors by Michele Penza, Phil Martin and John Yeow New Sensing Model of (mesoporous) In2O3 by Thorsten Wagner, Nicola Donato and Michael Tiemann SAW and Functional Polymers by Adnan Mujahida and Franz L. Dickert Part III: Transducer Functions Percolation effects in metal oxide gas sensors and related systems by Tilman Sauerwald and Stefanie Russ Calorimetric Gas Sensors for Hydrogen Peroxide Monitoring in Aseptic Food Processes by Patrick Kirchner, Steffen Reisert and Michael J. Schoning Group III-nitride chemical nanosensors with optical readout by Jorg Teubert, Sumit Paul, Andreas Helwig, Gerhard Muller and Martin Eickhoff

Percolation model of a nanocrystalline gas sensitive layer

A model for the nanocrystalline layer of a semiconducting gas sensor is developed that takes into account the possibility of having ultrasmall compensated grains that are insulating. The probability that an ultrasmall grain is insulating depends on the diameter D and the number of next neighbors (the coordination number k). The occurrence of insulating grains leads to specific percolation type effects on the conductivity of the layer, by which its sensitivity can be improved.

New Mesoporous Metal Oxides as Gas Sensors

Abstract Nanoporous semiconducting metal oxide materials (In 2 O 3 , WO 3 ) with uniform pore systems, large specific surface areas (80 m 2 g −1 ), and pore-wall crystallinity were prepared by structure replication, using KIT-6 silica as a template. The products show improved properties as gas sensors (for methane or butanone) as compared to non-porous samples. In addition, porous WO 3 samples with comparable porosity (prepared by conventional ‘soft templating’, using pluronic P123), which are amorphous on the atomic length scale show poorer gas sensitivity, indicating that crystallinity is a crucial factor in the sensing process.