The sensing of CO2 at room temperature enables the prospects towards low power and low cost CO2 gas sensors and has a high demand in both industrial and domestic applications. In this work we report a detailed work function read out (Kelvin probe) based analysis on copper oxide nanoparticles (CuO-NPs) as a new CO2 gas sensitive material. The reversible interactions of CO2 with the thick CuO-NPs layer result in a work function change of about 42 mV for dry air and approximately 97 mV for humid conditions (r.h. = 20%), at a CO2 concentration step of 400 to 4000 ppm. The CO2 gas sensing mechanism at room temperature is studied by Fourier transform infrared (FTIR) spectroscopy and explained by thermodynamical calculations. The correlation found between the FTIR spectrum and thermodynamical studies suggest the reversible formation of hydroxocarbonates (malachite, azurite) which is responsible for the gas sensing effect. Moreover, the results concerning long term signal stability over time present suitability towards indoor gas sensing applications. The results indicated in this paper give a new direction to metal oxide based nanoparticles as sufficiently fast and sensitive ambient CO2 detecting materials.

Even small traces of carbon monoxide in the fuel gas lead to a degradation of performance in low temperature polymer electrolyte membrane (PEM) fuel cell systems. Therefore a sensor which is capable of detecting CO in a ppm range in a hydrogen rich atmosphere is a great advantage. In this talk a new gas sensitive layer to be used as suspended gate in an existing CCFET based sensor system is presented. The new gate material is based on a platinum/gold alloy and covered with a thin polymethylmethacrylat (PMMA) layer. A simple experiment was designed to qualify the sensitive layer measuring the work function change with a Kelvin probe system. Here measurements will be presented in an atmosphere of 95% argon, 4% hydrogen and 1% carbon dioxide. The carbon monoxide concentration was up to 50ppm.

The reliable detection of CO2 is a key for demand controlled ventilation (DCV) in buildings, which allows energy savings up to 30%. Presently, optical detection methods are only in use for a small percentage of building ventilation systems due to the relatively high cost of these sensing systems. A reliable low cost CO2 sensor would allow for widespread use of DCV. In the past, various transducer principles have been used for CO2 detection and a number of sensitive materials have been investigated including organically modified silicates [1], also called siloxanes. Due to their ability to form an acid base reaction with the molecule CO2, materials using primary amino groups are particularly suitable for the detection of CO2. This has been verified using heteropolysiloxanes in mass sensitive [1] and capacitive sensors [2]. Another recent approach makes use of the workfunction readout of these materials, where CO2 detection at room temperature was demonstrated using a labscale Kelvin Probe setup [3]. The underlying detection mechanism has been carefully examined by Stegmeier et al [4].

Floating gate FET (FGFET) gas sensors based on work function readout allow using a wide range of materials to be included as sensing materials. Longterm stability of the FGFET signal is influenced by unintended surface conductivity. A novel active operation mode presented in this work uses voltage pulses at the suspended gate to increase baseline stability and selectivity. This transient readout strategy allows differentiation between capacitive, volume-based effects and surface-located response, yielding two physically independent readout signals. Using this readout, the baseline stability as well as the selectivity for e.g. a FGFETbased humidity sensor can be increased significantly for polymeric and porous isolating sensing layers.

A new type of hydrogen sensor based on a floating gate field effect transistor (FG-FET) with a n-octadecyltrichlorsilane (ODTS) modified platinum layer of 20 nm in thickness as a gas sensitive layer is presented. The modifications stabilize the sensor signal even at temperatures up to 125degC. The surface of polycrystalline platinum is well known to act as sensitive layer for hydrogen detection in a FG -FET at room temperature. In the presence of hydrogen containing air the work function of a platinum surface is reduced up to 0.5 eV by the adsorption of atomic hydrogen. Unfortunately at temperatures above 60degC a high coverage of atomic oxygen occurs at the platinum surface. This raises the work function again, immediately after the hydrogen exposure so that the hydrogen concentration is no longer exactly detectable. With the deposition of a thin layer of ODTS, we were able to modify the platinum surface preventing it from being covered by oxygen during hydrogen exposure. A detailed model of the reactions at the platinum surface, which leads to the shift in work function, is given. Changes of the reaction paths by the modification are explained.

For safety reasons, while handling fuel cells, hydrogen concentrations of 0.1 -3% and above need to be detected. Low power hydrogen sensors, based on a Field Effect Transistor (FET), have been in use for about 25 years. In the past platinum and palladium were often used as gas sensitive layers. Unfortunately in the required concentration range, the Pt based sensors have a poor selectivity at room temperature and were not stable at operating temperatures above 60degC. To solve this problem Pt with a porous tin oxide (SnO2) top layer is used as a chemically sensitive electrode in a Floating Gate Field Effect Transistor (FG -FET). The results show that the SnO2 film on Pt stabilizes the sensor signal response between room temperature and 135degC. Also the sensor response time with t50 < 10s is quite fast and the cross sensitivity to other gases compared to pure Pt is reduced.

Gas sensors based on hybrid suspended gate field effect devices are used for the detection of specific gases occurring in test fires. This sensor type can be operated with a power consumption<1mW and is therefore best suited for the use in fire detectors. Different floating gate FET (FGFET) sensors designed for the detection of CO, CO2 and NO2 were tested in standard test fires regarding EN54. Significant sensor signals were obtained for all test fires enabling a reliable and fast detection of fires. In addition, a separation of fire types is possible by signal evaluation combining signals of different sensors.

NO2 sensitive suspended gate (SG) field effect transistors (FET) based on copper phthalocyanine (CuPc) thin layers have been produced and investigated. The sensor structure is a hybrid one. The SG includes the CuPc sensing layer deposited in an independent process by thermal evaporation on the gate electrode (Au/alumina wafer). The transducer (FET) platform realised in standard complementary-metal-oxide-semiconductor (C-MOS) technology and containing several measuring and reference channels is mounted using the flip-chip technology over the gate structure. The sensing layers and the sensors are showing low detection limit (<50ppb) and good sensitivity (20–70mV/concentration decade), selectivity and reproducibility.

Ammonia sensitive field effect transistors (FET) realised in a hybrid flip-chip technology have been produced and investigated. The ammonia sensitive element, the suspended gate (SG) in the transistor structure, is a thin polyacrylic acid (PAA) layer prepared by spray deposition on a gold plated alumina substrate. The transistor platform was designed for and fabricated by the standard complementary-metal-oxide-semiconductor (CMOS) technology, which offers good industrialisation prospects. The intrinsic transconductance of the suspended gate setup is about 16μA/V. The access of the analysed gaseous sample to the air gap of the device occurs, spontaneously, by diffusion. The electrical output of the device to ammonia exposure is in the range of 50mV/decade of concentration variation. The sensor can be operated at low temperatures (25–60°C) and presents good performance.

A Lundstrom-FET and a SGFET with Pt suspended gate have been integrated together on a single chip in CMOS technology, and operated as a hydrogen sensor. Sensor sensitivities have been determined and the reactions to target gases have been measured. While the Lundstrom-FET is more sensitive at low H2 concentrations, the SGFET's measuring range extends to higher concentrations. So the paired setup enlarges the sensitivity range of the sensor. At the same time the signal reliability, e.g. in security applications, is increased due to different cross sensitivities of the two devices. A modified CMOS process is used which allows the integration of operation and evaluation electronics.

For the first time, a supertip on a [111] W base tip has been self-imaged by field-ion microscopy with atomic resolution. It is a tiny crystalline protrusion of a few nm in diameter.

Optical near-field microscopy provides a variety of contrast mechanisms with possible nanometre resolution. For the first time we used optical absorption contrast in order to analyze ion-irradiated silicon films. A finely focused 17 keV Ne+ beam produces local amorphous areas with an enhanced optical extinction coefficient in a crystalline Si matrix. The photon scanning tunnelling microscope, a variant of the optical near-field microscopes, is very suitable to image the damaged areas with sufficient spatial resolution. The mechanism of the contrast origin is discussed.

The formation of amorphous zones in thin heteroepitaxial layers of silicon has been investigated by optical transmission spectroscopy with ion species, dose, intensity and irradiation temperature as the main parameters. The dependence of the recrystallization of the irradiated layers on annealing temperature and time has been studied. The optical contrast between crystalline and amorphous domains is sufficient to allow applications for masking purposes and information storage.