Study of organic grafting of the silicon surface from 4-nitrobenzene diazonium tetrafluoroborate

The hydrogenated silicon surface has outstanding electronic properties. However, its resistance to oxidation is insufficient. An alternative is the substitution of the SiH bonds with Si-organic groups. This modification of the silicon surface by grafting of organic molecules was carried out by electrochemical reduction of 4-nitrobenzene diazonium tetrafluoroborate in an aqueous medium containing HF and H ₂SO ₄. The choice fell on this electrochemical reaction because it allows for fast grafting. The reduction of nitrobenzene diazonium is confirmed by the presence of a voltammetric peak around -0.1 V/SCE. The grafting was also characterized by in situ infrared spectroscopy (FTIR) which, via the detection of vibrations characteristic of chemical bonds, allows one to identify the chemical functions present. In addition, electrochemical impedance measurements allowed us to approach the interfacial mechanisms. It appears that the cathodic grafting leads to the formation of a polymeric layer, but the same grafting also occurs spontaneously within a few tens of seconds at open circuit potential, an expected phenomenon indeed in view of the reduction potential of 4-nitrobenzene diazonium.