J.P. Blanc

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.

Gaseous oxidation and compensating reduction of lutetium bis-phthalocyanine and lutetium phthalo-naphthalocyanine films

Abstract Thin films are deposited from lutetium bisphthalocyanine (LuPc 2 ) and lutetium phthalo-naphthalocyanine (LuPNc) that are intrinsic, easily oxidizable, molecular semiconductors. Oxidation of the films by NO 2 or Br 2 vapours is studied by visible absorption spectroscopy on “neutral” and oxidized films and by indashsitu measurements of conductivity and sorbed dopant mass uptake. Exposures of the oxidized LuPc 2 and LuPNc films to NH 3 lead to a conductivity increase or decrease depending on the oxidation degree of the films. The whole set of results is in agreement with a model that describes the existence of a conductivity maximum in LuPc 2 and LuPNc during oxidation as the consequence of both a trapping process of the charge carriers and a reduction of their mobility when the concentration of oxidized phthalocyanine molecules becomes too high.

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.

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.

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.

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.

Gas sensitivity of InP epitaxial thin layers

Abstract The resistance of n-InP epitaxial layers is shown to increase or decrease in the presence of oxidizing (O2, NO2) or reducing (NH3) gases, respectively. The magnitude of resistance variations depends on gas concentration, on InP layer thickness and on temperature. Interpretation is based on field effect mechanisms resulting from ionization of surface-chemisorbed gas molecules. Gas sensing devices are considered.

Interaction of NO2 with lutetium bisphthalocyanine thin films

Abstract Pc 2 Lu has been described as the first molecular semiconductor. This phthalocyanine is more easily oxidized than conventional metallo-phthalocyanines. This paper deals with the effects of NO 2 gas at room temperature on the conductivity of Pc 2 Lu layers. It shows that the responses of the layers to different concentrations of NO 2 are very dependent on the substrate temperature during vacuum deposition and on the thickness of the Pc 2 Lu layer. It underlines the contributions of the crystalline and amorphous parts of the layer to the conductivity changes on doping.

NO2 detection by a resistive device based on n-InP epitaxial layers

This paper deals with a new type of NO2 sensor using n-type InP epitaxial layers as sensing material. The gas action makes the conductance of the device, measured parallel to the surface between ohmic contacts, decrease. It is shown how the thickness and the doping level of these layers affect the sensor sensitivity. A simple model is proposed to describe the gas action, based on ionization of chemisorbed NO2 molecules inducing field-effect mechanism. Calculations lead to thickness, doping level and gas concentration dependence in agreement with experimental results. The ability for environmental applications is also discussed and it is concluded that even if such devices can detect low NO2 concentrations, they suffer from limitations due to a parasitic effect of moisture.