Spectroscopic evidence for the origin of odd–even effects in self-assembled monolayers and effects of substrate roughness

Abstract

This paper reports the effects of substrate roughness on the odd–even effect in n-alkanethiolate self-assembled monolayers (SAMs) probed by vibrational sum frequency generation (SFG) spectroscopy. By fabricating SAMs on surfaces across the so-called odd–even limit, we demonstrate that differentiation of the vibrational frequencies of CH₃ from SAMs derived from alkyl thiols with either odd (SAM^O) or even (SAM^E) numbers of carbons depends on the roughness of the substrate on which they are formed. Odd–even oscillation in SFG susceptibility amplitudes was observed for spectra derived from SAM^E and SAM^O fabricated on flat surfaces (RMS roughness = 0.4 nm) but not on rougher surfaces (RMS roughness = 2.38 nm). In addition, we discovered that local chemical environments for the terminal CH₃ group have a chain-length dependence. There seems to be a transition at around C₁₃, beyond which SAMs become “solid-like”.