J.P. Germain

Influence of peripheral electron-withdrawing substituents on the conductivity of zinc phthalocyanine in the presence of gases. Part 2: oxidizing gases

Thin layers of zinc hexadecafluorophthalocyanine ZnF16Pc and of zinc phthalocyanine ZnPc were prepared by vapor deposition. Their conductivity was measured during exposures to the reducing gases NH3 and H2 diluted in N2. The conductivity of ZnF16Pc increases in the presence of NH3 whereas ZnPc exhibits no sensitivity to this gas. Correlatively, the sensitivity to H2 is higher for ZnF16Pc than for ZnPc. This behavior is related to redox potentials of the two phthalocyanines. Gas sensing applications are considered.

Doping mechanisms of phthalocyanines by oxidizing gases: Application to gas sensors

The present paper proposes a model for the action of the oxidizing gases O2 and NO2 on the electrical conductivity of polyfluoro-aluminum-phthalocyanine thin films. The proposed doping mechanism makes it possible to separate the physical process (physisorption, diffusion, displacement of previously adsorbed species) and the chemical process (chemisorption or charge transfer). Because of their different kinetics, these two processes are clearly seen in the transient response of the film conductivity which follows a sudden change in the temperature. Several experimental results obtained in our laboratory, or quoted in the literature, can be explained by our model, especially the power-law dependence of the conductivity on the gas concentration. Conclusions are drawn about the use of such films as sensitive elements for gas sensors.

Interaction of NO2 with copper phthalocyanine thin films I: Characterization of the copper phthalocyanine films

Abstract Thin films of copper phthalocyanine (CuPc) have been deposited by vacuum evaporation on glass or alumina substrates maintained at different temperatures. The morphology and the crystallinity of as-deposited films and of heat-treated and/or NO 2 -treated films have been studied by electron microscopy and X-ray diffraction. The crystallinity of the films increases with substrate temperature during deposition: films deposited at room temperature or below are rather amorphous, whereas films deposited at 80°C or 200°C exhibit a highly uniaxial order of their crystallites; this order is altered by heat treatment or NO 2 doping. Scanning electron microscopy shows that the morphology of the film surface also depends on deposition temperature and that heat treatments induce the growth of whiskers from the surface. The structural changes are also observed through the temperatures of maximum conductivity of the CuPc films doped with NO 2 . The sensitivity of CuPc to NO 2 could be applied to gas sensing.

Gas sensitivity of phthalocyanine thin films

Abstract Thin films of copper phthalocyanine (CuPc) and poly(phthalocyanine of aluminium and fluor) (AlPcF)n were exposed to concentration steps of the diluted oxidizing gases O2, NO2 and Cl2, and their conductivity variations were recorded at the same time. These are in good agreement with the model we recently proposed to describe gas-phthalocyanine interactions, leading to the law: (conductivity)α(gas concentration)s, which is often experimentally observed. The sensitivities s of the two phthalocyanines to O2 and NO2 are calculated at various temperatures.

Investigation, for NH3 gas sensing applications, of the Nb2O5 semiconducting oxide in the presence of interferent species such as oxygen and humidity

Abstract Semiconducting oxides, like Nb2O5, can be used as sensitive materials in the manufacturing of gas sensors based on conductivity variation measurements. Since these materials are sensitive to many gases, precautions must be taken when they are operated. In the present paper, the sensitivity of Nb2O5 to NH3 is studied. NH3 behaves like an electron donor and induces an increase of the N-type semiconductor Nb2O5 conductivity. In presence of oxygen, the conductivity decreases and the sensitivity of the Nb2O5 oxide to the NH3 gas is much lower than in absence of oxygen. Humidity injects electronic carriers in the Nb2O5 material and acts like an electron donor. In presence of humidity, the conductivity of the Nb2O5 layer to NH3 is improved. Those results are interpreted by using a model where the sensor resistance is dominated by the grain boundary resistance.

Evaluation of atmospheric pollution by two semiconductor gas sensors

Abstract After a brief introduction about: the different gaseous pollutants present in the atmosphere; the cause of their concentration rise; their consequences on human health; and the different devices used to monitor each gas, two types of semiconductor gas sensors are presented. The first of them is a simple InP-based resistive sensor. The sensitive layer is a thin n-type InP epitaxial layer grown on a semi-insulating InP substrate. The electrical resistance, increasing in the presence of oxidising gases, is measured between two ohmic contacts. The second sensor is a thin film of copper phthalocyanine deposited by sublimation on an alumina substrate. The conductivity of the phthalocyanine layer increases in the presence of oxidising gases. Influence of NO 2 and O 3 , especially at low concentration, is investigated. Influence of other parameters like operating temperature or interfering gases is also studied. In conclusion, optimisation of sensor characteristics to improve the evaluation of oxidising air pollutants concentrations is discussed.

Gas-sensing properties of metallo-phthalocyanine thin films as a function of their crystalline structure

Abstract PcM (M = Cu, Fe, Pb, Li) thin films are studied as sensitive elements of semiconducting gas sensors. After exposure to oxidizing gases (NO 2 , O 2 , Cl 2 , I 2 ), the electrical conductivities of the materials go through a maximum at temperature T m , which corresponds to the maximum sensitivity to the gas and thus to the temperature of operation as gas sensors. We describe the evolution of T m with the morphology of the film and with the heat and gaseous treatments, and its consequences with regard to the gas-sensing application.

Elaboration and tests of microelectronically designed gas sensors with phthalocyanine sensitive layers

Abstract Microsens has developed miniaturized integrated semiconductor gas sensors using standard microelectronic technologies; these sensors include a semiconducting metal oxide layer (SnO2, Nb2O5) and an integrated heater on a silicon substrate. The selectivity and sensitivity of the devices to oxidizing or reducing gases depend on the metal oxide, on its doping and on the selection of an appropriate working temperature. Recently, in collaboration with LASMEA, the same microelectronic device has been used to develop gas sensors on which the metal oxide layer is replaced by a phthalocyanine thin film. Phthalocyanines are known to be sensitive to oxidizing gases at ppm concentrations, and the specificity of their responses makes them potential gas sensors or complementary elements of SnO2 in th realization of multi-array integrated gas sensors. This papers describes the successive steps of the manufacturing of miniaturized phthalocyanine gas sensors and shows that deposition of phthalocyanine thin films by evaporation is compatible with microelectronic technologies. Responses of CuPc and AlPcFn films to NO2, HCl and CO are reported. Each gas induces conductivity variations on both phthalocyanines; these variations are qualitatively and quantitatively different depending on the gas-phthalocyanine pair.

A metallic oxide gas sensor array for a selective detection of the CO and NH3 gases

Abstract An analysis of single gases using non-selective sensor elements is presented. It contains two steps: the identification of an unknown substance and the estimation of its concentration. To prepare the identification step, a calibration procedure is done to associate a class with each definite gas. In order to do this classification, the signals of the sensor array are transformed into quantities independent of the gas concentration but characteristic for its chemical compound. Then, the analysis parameters of an unknown substance allow to identify it with one of the calibration classes. After the identification of the gas, its concentration can be estimated with the model which is specific for each class of a single gas. The accuracy of this estimation is discussed.

Air quality evaluation by monolithic InP-based resistive sensors

Abstract After a brief summary on environmental pollution and commonly used materials for NO 2 and O 3 sensing, simple InP-based resistive sensors are studied. The gas sensitive device is a thin n-type InP epitaxial layer grown on a semi-insulating InP substrate. The electrical resistance of the layer, measured between ohmic contacts, increases in the presence of oxidising gases, the most important variations occurring in NO 2 and ozone-containing atmospheres. A complete study performed in laboratory at a moderate operating temperature of 80°C on the sensor exposed to nitrogen dioxide enabled to obtain a precise NO 2 calibration curve. In an outdoor application, only NO 2 , ozone and humidity seem to act on the sensor resistance, confirming laboratory experiments. One of these devices was used to monitor air pollution in an urban atmosphere. The resistance changes were compared with the readings of a commercial analysing equipment. The results clearly show that the sensor resistance variation follow the total O 3 and NO 2 concentrations variation in air. Different operating methods are discussed for these type of sensors, depending on what of these two pollutants has to be measured.