P. Clechet

Photoelectrochemical behaviour of TiO2 and formation of hydrogen peroxide

Abstract Electrochemical and photoelectrochemical behaviour of anodized titanium sheets has been investigated. It has been shown that, on a TiO 2 electrode, reduction of oxygen into hydrogen peroxide can take place. Under illumination at rest potential, we have simultaneously the photo-induced anodic oxidation of water into oxygen and the reduction of oxygen into hydrogen peroxide. This H 2 O 2 formation is a real photosynthesis reaction on a semi-conductor electrode.

Autocatalytic Deposition of Gold and Palladium onto n ‐ GaAs in Acidic Media

Deposition of gold, palladium, and gold‐palladium alloys is obtained with the help of acidic electroless baths of low metal content. The baths are prepared by suitably mixing separate solutions containing the reductant (hydroxylamine hydrochloride) or one of the desired cations respectively, with suitable additives. The stability of the baths with time is good and their replenishment easy to realize. Provided that a suitable choice of mixture of the three solutions is made, they allow the fabrication of multilayered heterostructures which could be fruitfully used for ohmic contact preparation onto without any initial activation step. The deposition mechanism of gold and palladium and the effect of adding palladium to gold is discussed.

The preparation of CHEMFET selective gates by thin silica layer grafting and their behaviour

Abstract In this work, a new method of obtaining ISFET gates is propounded. Thin silica layers on a silicon substrate are bonded by monofunctional silanes. These EOS structures are tested by an a.c. capacitance method. When γ-cyanopropyldimethyl(dimethylamino)silane is used for bonding, a selective response for silver ions is observed (35 mV per decade), while there is only a small response for hydrated silica (2 mV per decade). The bonding of docosyldimethyl(dimethylamino)silane is shown to mask part of the pH response of the hydrated silica; thus it would be possible to prepare differential reference structures in this way. Grafting reactions can be used not only for ion sensing but also for the detection of many chemical species as molecules of biological interest.

Lubrication of silicon micromechanisms by chemical grafting of long-chain molecules

Abstract A method involving the grafting of long-chain molecules on hydroxylated ceramic surfaces like silica is proposed to improve lubrication in microactuators. Preliminary experiments show that one monolayer of octadecyl and docosyl silane molecules does improve the sliding properties of silica. The friction tests are performed on a reciprocating tribometer, in a sphere and plane geometry with a contact pressure of 340 MPa. The friction-reduction effect is clearly evidenced, the value of the friction coefficient decreasing from 0.6–0.7 to nearly 0.1 in the presence of the grafted molecule layer. The lifetime of the effect is estimated as one hundred cycles. In a prospective view, these results are extended to contacts in micromechanisms.

Optimization of lubricants for silica micromotors

In this paper a method for MEMS (micro-electromechanical systems) lubrication consisting in chemical grafting of long alkyl chains onto silica is proposed. In a first step, we have compared chemical grafting with other methods of layer fabrication such as Langmuir-Blodgett films and self-assembled monolayers. In a second step, various monomolecular layers were grafted on the silica surface, in order to study the influence of the chain length and chemical structure on lubrication. Friction tests were performed on a reciprocating tribometer in a sphere on plane geometry. The plane specimen was the surface of treated silica wafers. The coefficient of friction is lowered from 0.6 for bare silica to a minimum of 0.07, depending on the lubricant monolayer.

Membranes for chemical sensors

Abstract Recent trends in the development of membranes for silicon-based chemical sensors are reviewed. The sensitization of the gate dielectric of pH ISFETs has been chosen as an illustrative working example and recent results are analysed in the three main identified directions of research: deposition and grafting of sensitive membranes; ion implantation; and direct grafting of ionophores on a dielectric surface. This paper shows that the formation of highly stable covalent bonds between the materials used in the fabrication of sensors is more and more widely used as it becomes the most promising means for fabricating efficient sensors compatible with IC technology.

Covalent immobilization of glucose oxidase onto graphitic electrodes

Abstract Graphitic substrates (plates and carbon fibre 7 μm in diameter) have been used and characterized in order to obtain glucose sensitive electrodes. Preparation of such electrodes before enzymatic sensitization and after cleaning involves a specific electrochemical pretreatment of oxidation at +2.3 V/ sce. Further covalent immobilization of glucose oxidase proceeds through a grafting between surface carboxyl group and a carbodiimide reagent. Performances of enzymatic electrodes such as their stability up to two months make them well suited for further application in the biomedical field.

Sensitization of dielectric surfaces by chemical grafting: application to pH ISFETs and REFETs

Abstract The aim of this work is to prepare a pH sensing device working in a differential arrangement between an ISFET and a REFET. Two types of insulator surfaces have been tested: the pH sensitivity of silica is 36 mV pH −1 and that of the PECVD silicon oxynitride is nernstian. The grafting of these surfaces with long-alkyl-chain silanes has taken place. The surface preparation must be well controlled in order to obtain dense, grafted, long-alkyl-chain monolayers on both insulators. The pH sensitivity of grafted silica depends on the chain length; it is 10 mV pH −1 for C22 grafted silica. For a silicon oxynitride with a surface density of SiNH/NH 2 groups of 2.6 nm −2 , the pH sensitivity of the C18 grafted surface is 15 mV pH −1 . PECVD silicon oxynitride seems to be well adapted as a new insulator for pH ISFETs.

Quantitative study of the hydroxylation and of the chemical grafting of oxidized porous silicon

Abstract A porous silica layer was obtained by anodic dissolution of silicon and thermal oxidation. The initial porous structure of silicon is preserved after well defined oxidation treatments. The silica layer was hydrated and chemically modified by grafting bromohexyl silane. The adaptation of different analytical methods allows the determination of the silanol and the graft densities. The experimental conditions for obtaining the highest densities were defined. These results will be applied to the preparation of ion sensitive membranes for ISFET.

Study of the silicon nitride/aqueous electrolyte interface on colloidal aqueous suspensions and on electrolyte/insulator/semiconductor structures

Abstract The silicon nitride/aqueous electrolyte interface has been studied on colloidal aqueous suspensions and on electrolyte/insulator/semiconductor (EIS) structures. Electrophoretic mobilities of the colloidal suspensions and capacitances of the EIS structures were measured. Before any previous exposure to water or after hydrogen fluoride etching, the silicon nitride surface shows a linear and Nernstian response for pH and its isoelectric point is 4. This behavior has been explained by the presence of a small fraction of amine groups at the silicon nitride surface. After exposure to water and to alkaline media, the silicon nitride surface behaves just like a silica surface for both systems. This is due to disappearance of the amine groups through a hydrolysis reaction.