Substrate Roughness and Tilt Angle Dependence of Sum-Frequency Generation Odd–Even Effects in Self-Assembled Monolayers

Role of substrate type, gold or silver, and surface roughness on the parity odd–even effect in n-alkanethiolate (n = 10–16) self-assembled monolayers (SAMs), materials of potential importance to molecular scale electronics, is studied using vibrational sum-frequency generation (SFG) spectroscopy. An inverted odd–even effect is observed for SAMs on Ag substrates relative to SAMs on Au. The metal-specific SFG spectra in the methyl and methylene stretching regions provide a sensitive probe of the in situ cant angle and molecular twist dependence of the interfacial group orientation. Within the precision of these measurements, disorder due to gauche defects is not evident in this spectral analysis. SFG methyl vibrational frequencies and line widths show parity and substrate dependence. Metal substrate roughness, an under-reported experimental parameter, affects the extent of odd–even methyl orientation anisotropy. Parity-dependent methyl orientation effects are seen on Ag beyond the roughness limit established by hydrophobicity data on Au. The SFG analysis predicts an ∼2.8 nm rms roughness limit for odd–even effects on Ag substrates. These SFG results further confirm the role competing pairwise short-range dispersive (∝1/r⁶) and longer range polar (∝1/r to 1/r⁴) interactions play in controlling the observed odd–even wetting behavior of SAMs on metal substrates as a function of surface roughness.