Olivier Segut

Elaboration of ammonia gas sensors based on electrodeposited polypyrrole—Cobalt phthalocyanine hybrid films

The electrochemical incorporation of a sulfonated cobalt phthalocyanine (sCoPc) in conducting polypyrrole (PPy) was done, in the presence or absence of LiClO4, in order to use the resulting hybrid material for the sensing of ammonia. After electrochemical deposition, the morphological features and structural properties of polypyrrole/phthalocyanine hybrid films were investigated and compared to those of polypyrrole films. A gas sensor consisting in platinum microelectrodes arrays was fabricated using silicon microtechnologies, and the polypyrrole and polypyrrole/phthalocyanine films were electrochemically deposited on the platinum microelectrodes arrays of this gas sensor. When exposed to ammonia, polymer-based gas sensors exhibited a decrease in conductance due to the electron exchange between ammonia and sensitive polymer-based layer. The characteristics of the gas sensors (response time, response amplitude, reversibility) were studied for ammonia concentrations varying from 1 ppm to 100 ppm. Polypyrrole/phthalocyanine films exhibited a high sensitivity and low detection limit to ammonia as well as a fast and reproducible response at room temperature. The response to ammonia exposition of polypyrrole films was found to be strongly enhanced thanks to the incorporation of the phthalocyanine in the polypyrrole matrix.

Towards organic film passivation of germanium wafers using diazonium salts: Mechanism and ambient stability

Germanium is well-known for its good electronic properties, but also for the poor passivation quality of its natural or thermally-grown oxide layer. The robust passivation of Ge surfaces is thus a crucial step on the way to its integration in electronics at nanoscale. Before passivation, the natural oxide layer must be removed from the surface. Different methods were investigated and compared. The surfaces were analysed by X-ray photoelectron spectroscopy and by atomic force microscopy to determine the impact of the etching on the roughness. Oxide-free Cl-terminated surfaces were chosen and functionalized with various arenediazonium salts. The grafting is a fast reaction (less than 30 min) and proceeds in soft conditions (room temperature in acetonitrile solutions). The functionalization was found to proceed even at low temperature (−18 °C). The modified surfaces were mainly studied by XPS and AFM. The morphology of the organic layer was influenced by the temperature and duration of the grafting as well as by the presence of substituents on the aromatic ring. We demonstrated that smooth oxide-free Ge surfaces could be prepared by a cyclic oxidation- rinsing- etching procedure and could be functionalized with various arenediazonium salts. The stability of the organic layer was compared to the stability of a self-assembled monolayer of alkanethiolates. The absolute coverage was also determined. This constitutes a first step towards Ge crystal passivation by organic thin films.

A bridged low band gap A–D–A quaterthiophene as efficient donor for organic solar cells

α,ω-Bis(dicyanovinyl)quaterthiophene 1 with a median 4,4-diethyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene has been synthesized. UV-Vis absorption data show that the covalent bridging of the inner 2,2′-bithiophene leads to a significant reduction of the HOMO–LUMO gap essentially due to an increase of the HOMO level as confirmed by electrochemical and theoretical results. X-ray diffraction analysis of a single crystal of 1 shows that except for the out-of-plane ethyl groups, the conjugated system displays a quasi-planar geometry while the molecular packing exhibits strong π-stacking interactions and multiple short intermolecular contacts. Quaterthiophene 1 has been used as active donor material in organic solar cells of various architectures including bi-layer planar hetero-junctions and hybrid co-evaporated bulk hetero-junctions with C60 as electron acceptor material. A maximum conversion efficiency of 4.30% is obtained with a hybrid co-evaporated device. These results are discussed in terms of structure–properties relationships with reference to the open-chain parent α,ω-bis(dicyanovinyl)quaterthiophene 2.