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

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 C-13, beyond which SAMs become "solid-like".