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Alkyl monolayers formed by hydrosilylation on H-Si(111) surfaces

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 students and post-docs : T.L. Nguyen Le (2014), D. Aureau (2008), A. Faucheux (2005)

Grafting of alkyl monolayers through formation of covalent Si-C bonds is obtained through the hydrosilylation reaction between alkenes H2C=CH-(CH2)n-2X (X = CH3, COOH, COOEt) and a hydrogenated silicon surface. The reaction can be activated either thermally, or by UV irradiation, or by Lewis-acid catalysis. Grafting can be achieved in neat liquids or using precursors diluted in a solvent.

We have studied the structure of alkyl monolayers (chain density, orinetation, etc) on model H-Si(111) surfaces (see page). The functionalized monolayers (-COOH or -COOEt) can be used as starting layers for anchoring molecular moieties, in connection with our projects on photo-switchable surfaces (see page) and metal-organic film growth (see page). The grafting processes have been transposed and validated for grafting monolayers on porous silicon and amorphous (methylated) silicon, in connection with our activities in fluorescent and LSPR biosensors (see page) and Li-ion batteries (see page).

• Non-functional monolayers : Grafting processes have been optimized in order to obtain layers of high compacity and prevent oxide formation at the interface. AFM observations show that the layers are homogeneous and that grafting does not affect the stepped structure of atomic terraces of the H-Si(111) surface (see page) (Fig. 1a). XPS measurements confirm the formation of Si-C bonds (see C1s spectrum, Fig. 1b) and the absence of interfacial oxide (see Si2p spectrum, Fig. 1cb). In agreement with the ideal structure of these interfaces, electric characterizations (Fig. 1d) indicate a very low density of electronic surface states. The chain density within the monolayers is 3 1014 /cm2, according to our quantitative XPS and calibrated infrared measurements (see page).

Fig. 1 : Set of measurements characterizing alkyl monolayers grafted on a H-Si(111) surface : (a) AFM image ; (b-c) XPS C1s et Si2p spectra evidencing the formation of a Si6C bond and the absence of interfacial oxide ; (d) Capacity – potential curves measured for chain lengths from C8 to C16. These curves are characteristic of a quasi- ideal behavior of the interface and the presence of a very low electronic-state density.

• COOH–terminated monolayers and mixed –CH3/–COOH monolayers : Structural characteristics of 100% acid monolayers are the same as those described for non-functional layers. However, a special rinse (hot acetic acid) is needed for removing residual physisorbed acid chains (dimer formation, Fig. 2b). IR spectra in Figure 2a show that the νCO band intensity measures at a 100% acid surface is decreased by a factor of two after such a rinse (spectrum (b)). In perfect correlation, contact-mode AFM images show that the surface topography is perfectly homogeneous and that no unwanted adsorbed species is displaced by the tip during imaging (Figs. 2c-d).

Mixed monolayers containing –CH3 and –COOH terminal groups are easily obtained by grafting in a solution obtained from the mixing of the two alkene precursors in variable proportions. IR measurements show that the mixed monolayer composition (determined from the IR calibration of the νCO band) is systematically enriched in acid (Fig. 2e). The spatial repartition of acid chains within the monolayers is currently under study.

Fig. 2  : Set measurements characterizing alkyl monolayers functionalized with carboxylic–COOH head groups, grafted on H-Si(111) surfaces : (a) IR spectra of a 100% acid surface, before and after the specific rinse ; (b) Scheme of the dimer formation at the surface ; (c-d) AFM images of a 100% acid surface before and after the specific rinse. (e) Chemical composition chimique of mixed monolayers as a function of the grafting solution composition.

Publications :

P. Gorostiza, C. Henry de Villeneuve, Q. Y. Sun, F. Sanz, X. Wallart, R. Boukherroub, and P. Allongue, "Water Exclusion at the Nanometer Scale Provides Long-Term Passivation of Silicon (111) Grafted with Alkyl Monolayers," The Journal of Physical Chemistry B 110, 5576-5585 (2006).

X. Wallart, C. Henry de Villeneuve, and P. Allongue, “Truly quantitative XPS characterization of organic monolayers on silicon : Study of alkyl and alkoxy monolayers on H-Si(111)”, Journal of the American Chamical Society 127, 7871-7878 (2005).

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).