Molecular conformation of n-alkyl monolayers covalently bonded to Si(1 1 1) probed by infrared–visible sum-frequency spectroscopy

Infrared–visible sum-frequency spectra of n-alkyl (CnH2n+1; n=10–13, 18) monolayers anchored on the Si(1 1 1) surface through a Si–C covalent bond were measured for the C–H stretch region. Spectral shapes exhibited azimuth-orientation dependence compatible with the C3v symmetry of the substrate, indicating that the monolayers were epitaxially constructed on the substrate. Three methyl bands dominated each spectrum, and two-weak methylene bands were identified. Two distinct intensity patterns of the methyl bands were observed depending on n. The observed spectra were interpreted with reference to reported molecular models of the monolayer that consist of alkyl chains with an all-trans head part and a twisted stem.