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Electrochemical Grafting of Organic Monolayers on Oxide Free Silicon Surfaces

par Rosso Michel - publié le , mis à jour le

Participants : P. Allongue, J.-N Chazalviel, C. Henry de Villeneuve, F. Ozanam

In this work we showed that silicon surfaces can be functionalized by promoting an electrochemical reaction leading to the formation of organic radicals in the liquid phase. We start with hydrogenated Si surfaces. The different methods described below lead to the formation of covalent Si-C bonds. Depending on the systems and/or the electrochemical conditions, layers of variable thickness are obtained from monolayers to polymeric films. When the radicals formed in solution react with the groups grafted on the surface, the layer thickness can be controlled by controlling the electrochemical charge.

Oxidation of Grignard reagents R-MgX
The oxidation of saturated Grignards (R-MgCnH2n+1) leads to alkyl monolayers Si-CnH2n+1. Only the oxidation of CH3MgBr allows for a full Si-H → Si-CH3 conversion (100% coverage). In the case of unsaturated Grignards a thick polymer film may be obtained.
The reaction mechanism is :

 RMgX + h+ → R+ MgX+
 ≡SiH + R → ≡Si+ RH
 ≡Si+ RMgX + h+ → ≡SiR + MgX+

Reduction of diazonium salts R-C6H4-N2+ :
This versatile method allows grafting a wide variety of functional end-groups (R=Br, COOH, C≡N…) in an aqueous or an organic electrolyte, according to the following reaction mechanism :

 R-C6H4-N2+ + e- → R-C6H4 + N2
 ≡SiH + R-C6H4 → ≡Si + R-C6H4-H
 ≡Si + R-C6H4-N2+ + e- → ≡Si-C6H4-R + N2

An accurate tuning of the electrical charge is necessary to form a dense and ordered molecular monolayer (Fig. 1b).

Fig. 1 : In situ STM images showing the (1x1) atomic structure of a H-Si(111) surface prior to (a) and after the grafting of a bromophenyl monolayer (b).

Publications :
[1] S. Fellah, F. Ozanam, J.-N. Chazalviel, J. Vigneron, A. Etcheberry and M. Stchakovsky, "Grafting and polymer formation on silicon from unsaturated Grignards : I- Aromatic precursors", J. Phys. Chem. B 110 (2006) 1665-1672.

[2] S. Fellah, A. Teyssot, F. Ozanam, J.-N. Chazalviel, J. Vigneron and A. Etcheberry, "Kinetics of Electrochemical Derivatization of the Silicon Surface by Grignards", Langmuir, 18 (2002) 5851-5860

[3] A. Teyssot, A. Fidélis, S. Fellah, F. Ozanam and J.-N. Chazalviel, "Anodic grafting of organic groups on the silicon surface", Electrochimica Acta, 47 (2002) 2565-2571

[4] A. Fidélis, F. Ozanam and J.-N. Chazalviel, "Fully methylated, atomically flat (111) silicon surface", Surf. Sci. 444 (2000) L7-L10

[5] C. H. de Villeneuve, J. Pinson, M. C. Bernard, and P. Allongue, "Electrochemical Formation of Close-Packed Phenyl Layers on Si(111)," J. Phys. Chem. B 101 (14), 2415-2420 (1997).

[6] P. Allongue, C. Henry de Villeneuve, J. Pinson, F. Ozanam, J. -N. Chazalviel, and X. Wallart, "Organic monolayers on Si(111) by electrochemical method," Electrochim. Acta 43 (19-20), 2791-2798 (1998).

[7] P. Allongue, C. Henry de Villeneuve, G. Cherouvrier, R. Cortes, "Phenyl layers by electrochemical reduction of aryl diazonium salts : Monolayer versus multiplayer formation and growth process", J. Electroanal. Chem. 550-551 (2003) 161-175