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

Abstract

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/r 6) and longer range polar (∝1/r to 1/r 4) interactions play in controlling the observed odd–even wetting behavior of SAMs on metal substrates as a function of surface roughness.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications

“…Getautis et al found that no vibrational bands could be identified in the VSFG spectrum of a 10% V1036 124 /90% C4 54 mixed SAMs, indicating a monolayer prepared from a mixed solution results in a more disordered structure compared to a monolayer from a pure V1036 solution, since the measured VSFG signal was proportional to the molecular ordering of the probed molecules . Moreover, VSFG has also been applied for studying some important aspects, including SAM growth mechanism exploration, conformational studies, − and functional head design . However, the high price, few commercial resources, complex data processing, and high sample preparation requirements make VSFG not a mainstream SAM-characterization tool at present.…”

Sum frequency generation spectroscopy of fluorinated organic material-based interfaces: a tutorial review

“…This change in terminal group orientation has been shown to originate “odd–even” effect in the wettability of the surfaces and can be observed by advancing contact angle measurements. 52–59…”

“…This change in terminal group orientation has been shown to originate "oddeven" effect in the wettability of the surfaces and can be observed by advancing contact angle measurements. [52][53][54][55][56][57][58][59] Changing the terminal functional group of a monolayer to a group other than the n-alkyl moiety has been shown to have a direct effect on the resulting film's physical properties. 52,[60][61][62][63][64][65][66][67][68] It has been demonstrated that using terminal fluorinated tailgroups has a significant effect on the interfacial properties of the thin films in unexpected ways.…”

Interfacial Water-Dictated Oil Adhesion Based on Ion Modulation

“…SAMs are 2D materials that are widely used in tuning the surface chemistry of materials. The structure and properties of SAMs have a subtle and investigable influence on macroscopic wetting properties. − Herein, we investigate the oil adhesion process on a carboxyl-terminated self-assembled monolayer surface (COOH-SAM) in ionic solutions and observe the interfacial water structure via SERS in this oil–water–solid three-phase system. The results show that interfacial water plays a dominant role in the adhesion of oil droplets.…”