C. Maleysson

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.

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.

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.

Electrical properties of Au/doped PPy/In structures

Abstract Polypyrrole (PPy) thin films have been electrochemically polymerized and doped in various electrolytes in order to prepare Au/PPy-anion/In sandwich structures where the anion incorporated from the electrolyte is BF 4 − ,ClO 4 − ,PF 6 − , or Fe(CN) 6 3−/4− . The current-voltage characteristics of these devices have been studied as a function of the dopant anion and the ambient atmosphere. Symmetric non ohmic or rectifying I–V were obtained for PPy doped with BF 4 − ,ClO 4 − , PF 6 − respectively in dry and humid atmosphere, whereas Au/PPy-Fe(CN) 6 /In structures exhibited ohmic I–V with as-grown PPy and Schottky type I–V with partially electrochemically dedoped PPy.

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.

Conductivity and gas species content of (AlPcF)n thin films exposed to O2 or NO2

Abstract (AlPcF) n thin films were exposed to O 2 or NO 2 diluted in N 2 . Variations of electrical conductivity and of O 2 or NO 2 species mass in the film were simultaneously recorded at room temperature all along exposures to these gases. Experimental results are explained by such phenomena as sorption of O 2 or NO 2 in the phthalocyanine film, exchange of sorbed species, and chemisorption (charge transfer).