VIBRATIONAL SUM-FREQUENCY GENERATION (VSFG) SPECTRA OF N-ALKYLTRICHLOROSILANES CHEMISORBED ON QUARTZ PLATE

Organic thin films prepared by chemical adsorption of n-alkyltrichlorosilanes (CH3(CH2)mSiCl3; m=6,7,13,17,18) on quartz plates were investigated by vibrational sum-frequency generation (VSFG) spectroscopy. The vibrational resonances by CH stretch bands of methyl and methylene groups were examined taking advantage of the fact that the bands associated with methylene groups in the trans conformation do not contribute to the VSFG signal. The density of adsorbed molecules and that of all-trans conformational sequences of alkyl chains decreased when the substrates were pretreated by oxygen plasma indicating the depletion of surface hydroxyls, which are the adsorption sites. The g-t conformation sequences on the plasma-treated substrates were found to increase by the length of the alkyl chain and the total number of gauche conformations exceeded that of adsorbed molecules when the number of CH2 groups in an alkyl chain is around 8.