Formation of aldehyde- and carboxy-terminated self-assembled monolayers on SiO2 surfaces

Aldehyde (CHO)- and carboxy (COOH)-terminated self-assembled monolayers (SAMs) were prepared on silicon substrates covered with native oxide (SiO2/Si) based on the chemisorption of triethoxysilylundecanal (TESUD) molecules and subsequent photooxidation of the terminal groups using 172-nm vacuum UV light in wavelength, respectively. In this study, we have particularly focused on the effects of atmospheric pressure during vacuum UV irradiation on the degree of the SAM photooxidation. When irradiation was conducted under a residual pressure of 10(3) Pa, the hydrophobic TESUD-SAM surface became hydrophilic and its water-contact angle fell from approximately 80 degrees to less than 5 degrees within 1 min. Film thickness also decreased from about 1.2 nm to about 0.3 nm, indicating the complete photodecomposition of the organic parts of the TESUD-SAM. On the other hand, in the case of 10(5) Pa, by selecting an appropriate time for vacuum UV irradiation, we were able to control the degree of photooxidation of the TESUD-SAM. Using a vacuum UV irradiation of 25 min at 10(5) Pa, only the topmost region of the TESUD-SAM was successfully photooxidized into COOH groups without any marked change in both film thickness and surface morphology, as evidenced by water-contact angle data, ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Copyright (C) 2008 John Wiley & Sons, Ltd.