Covalent grafting of ion-exchanging groups on porous silicon for microsystem applications

We report the chemical functionalization of porous silicon (PS) by trimethylammoniumpropyl bromide (–(CH 2) 3N(CH 3) 3 +Br −) and alkylsulfonic acid (–C x H 2 x SO 3H) groups for microsystem components. PS was prepared by electrochemical etching of a p-type silicon wafer. Samples of PS were first thermally oxidized at 300 °C and densified at 500 or 700 °C under an inert atmosphere. Mercaptopropyl and trimethylammoniumpropyl bromide groups were grafted on the PS surface via a silanization procedure. The oxidation of mercapto groups was used for the synthesis of –C x H 2 x SO 3H groups. The samples were studied by Fourier transform infrared spectroscopy (FT-IR) and temperature-programmed desorption mass spectrometry (TPD-MS). The grafting of –C x H 2 x SO 3H groups was found to be dependent on the densification treatment of the oxide layer formed at the surface of PS crystallites by low temperature oxidation. For PS samples thermally oxidized at 300 °C, –C x H 2 x SO 3H were not grafted, whereas for PS samples oxidized and densified at 500 or 700 °C, they were successfully grafted. In opposite, the densification treatment of PS samples has no significant influence on the grafting of –(CH 2) 3N(CH 3) 3 +Br − groups.