P. Allongue

Semiquantitative Study of the EDC/NHS Activation of Acid Terminal Groups at Modified Porous Silicon Surfaces

Infrared spectroscopy is used to investigate the transformation of carboxyl-terminated alkyl chains immobilized on a surface into succinimidyl ester-terminated chains by reaction with an aqueous solution of N-ethyl-N-(3-(dimethylamino)propyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The acid chains are covalently grafted at the surface of hydrogenated porous silicon whose large specific surface area allows for assessing the activation yield in a semiquantitative way by infrared (IR) spectroscopy and detecting trace amounts of surface products and/or reaction products of small IR cross section. In this way, we rationalize the different reaction paths and optimize the reaction conditions to obtain as pure as possible succinimidyl ester-terminated surfaces. A diagram mapping the surface composition after activation was constructed by systematically varying the solution composition. Results are accounted for by NHS surface adsorption and a kinetic competition between the various EDC-induced surface reactions.

The preparation of flat H–Si(111) surfaces in 40% NH4F revisited

Abstract The reasons why ideally flat H–Si(111) surface can be prepared by NH 4 F etching are investigated from correlation between AFM observations and experimental conditions used for etching. It is shown that pitting may be completely suppressed if a one side polished wafer is immersed in an oxygen free solution. An analytical electrochemical study of the (111) and rough face of the same n-Si wafer is presented to yield insight into observations.

Structural characterization of organic monolayers on Si〈111〉 from capacitance measurements

Abstract This work presents new data on modifications to Si〈111〉 surfaces by reduction of diazonium salts. It is shown that a monolayer thickness requires a careful control of the amount of radicals generated. Capacitance measurements performed under accumulation at n-type Si〈111〉 modified surfaces indicate a small density of electronic states at the Sixa0∣xa0molecule interface. It is further demonstrated that analyzing capacitance data in terms of an effective dielectric constant may be used to characterize the layer structure. The influence of the molecule structure on the density of layers and the re-oxidation at modified surfaces are thus investigated. The mechanism of grafting is also discussed.

Evidence for hydrogen incorporation during porous silicon formation

The incorporation of hydrogen into Si, under different electrochemical conditions including anodization in fluoride solutions where porous silicon is formed, is studied by NRA and in situ capacitance measurements. Results suggest a large near surface concentration of H whilst simulation show that the maximum penetration depth is governed by volume diffusion of H and material removal. Diffusion coefficients are found to be dependent on electrochemical conditions and ranged between 10−13 and 10−11 cm2u2009s−1. The interplay of H permeation with porous silicon layer formation is discussed.

The titration of carboxyl-terminated monolayers revisited: in situ calibrated fourier transform infrared study of well-defined monolayers on silicon.

The acid-base equilibrium at the surface of well-defined mixed carboxyl-terminated/methyl-terminated monolayers grafted on silicon (111) has been investigated using in situ calibrated infrared spectroscopy (attenuated total reflectance (ATR)) in the range of 900-4000 cm (-1). Spectra of surfaces in contact with electrolytes of various pH provide a direct observation of the COOH <--> COO (-) conversion process. Quantitative analysis of the spectra shows that ionization of the carboxyl groups starts around pH 6 and extends over more than 6 pH units: approximately 85% ionization is measured at pH 11 (at higher pH, the layers become damaged). Observations are consistently accounted for by a single acid-base equilibrium and discussed in terms of change in ion solvation at the surface and electrostatic interactions between surface charges. The latter effect, which appears to be the main limitation, is qualitatively accounted for by a simple model taking into account the change in the Helmholtz potential associated with the surface charge. Furthermore, comparison of calculated curves with experimental titration curves of mixed monolayers suggests that acid and alkyl chains are segregated in the monolayer.

Electrochemistry of Tin in Borate Buffer Solutions: An in-situ Raman study.

The electrochemical behavior of chemically polished polycrystalline tin is investigated in borate buffer solutions at p H=7.5 using in-situ Raman measurements. Experiments reveal the appearance of “luminescence” under continuum red laser light only for a narrow range of sample potentials within the anodic plateau. The phenomenon is discussed in view of the tin oxides formed on the surface, and correlated with the assignation of the oxido-reduction processes observed in the voltammograms.