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Determining monolayer composition by calibrated IR spectroscopy

par Rosso Michel - publié le , mis à jour le

Participants : P. Allongue, J.-N. Chazalviel, A.C. Gouget-Laemmel, C. Henry de Villeneuve, A. Moraillon, F. Ozanam
PhD student : A. Faucheux (2005)

The quantitative determination of the functional groups present at a functionalized surface is a crucial piece of information for correlating the surface physico-chemical properties to their composition. This need is present at each step of the building of a surface molecular architecture. We have developed a method for calibrating the intensity of IR bands characteristic of organic monolayers grafted at a silicon surface. This way, the surface concentration in molecular chains and functional groups present at the surface is determined (see page, page and page).

The method is based on IR measurements (in ATR geometry) performed using a silicon prism in contact with a solution containing molecular species bearing the vibrational group(s) whose absorption should be quantitatively determined. In the conditions sketched in Fig. 1a, the IR evanescent wave is absorbed by the solution chemical species on a probing depth δ. IR spectra are recorded in a and p polarization for increasing concentrations of the absorbing molecules (Fig. 1b), which allows for correlating absorbances abs0s and abs0p to the concentration C of the vibrators contained in solution (Fig. 1c). A linear variation should be observed above a given concentration C.
The amount of vibrators present in the grafted monolayer is dependent on their absorbance in p and s polarization. It can be decomposed according to the projections of the associated dynamic dipole of the vibrations on directions parallel and perpendicular to the surface (N = N// + Nperp).

Figure 1 (a) Principle of the calibration of the absorbance of a specific vibrator. For instance, solutions of decanoic acid in decane are used for calibrating the IR band of the carbonyl function of carboxylic acids ; (b) Set of IR spectra recorded for increasing concentration of decanoic acid ; (c) Variation of the intensity (in s and p polarization) of the C=O band as a function of the concentration of decanoic acid

Surface and solution measurements have now to be put together through the following equations. These equations take into account the difference in the interfacial electric field intensity with respect to the electric field orientation. They include the electric intensities (Ix, Iy, Iz) at the Si/air interface (measurements on the grafted monolayer) and (I0x, I0y, I0z) at the Si/solution interface (measurements in solution).

The electric field components Ix, Iy, Izand I0x, I0y, I0z and the probing depth of the evanescent wave are computed for the wavelength λ corresponding to the vibrator of interest and the incidence angle φ with respect to the normal to the prism surface.

Publication :
A. Faucheux, A. C. Gouget-Laemmel, C. Henry de Villeneuve, R. Boukherroub, F. Ozanam, P. Allongue, and J. –N. Chazalviel, "Well-Defined Carboxyl-Terminated Alkyl Monolayers Grafted onto H-Si(111) : Packing Density from a Combined AFM and Quantitative IR Study," Langmuir 22 (1), 153-162 (2005).