A fresh look at the structure of aromatic thiols on Au surfaces from theory and experiment

A detailed study of the adsorption structure of self-assembled monolayers of 4-nitrothiophenol on the Au(111) surface was performed from a theoretical perspective via first-principles density functional theory calculations and experimentally by Raman and vibrational sum frequency spectroscopy (vSFS) with an emphasis on the molecular orientation. Simulations-including an explicit van der Waals (vdW) description-for different adsorbate structures, namely, for (root 3 x root 3), (2 x 2), and (3 x 3) surface unit cells, reveal a significant tilting of the molecules toward the surface with decreasing coverage from 75 degrees down to 32 degrees tilt angle. vSFS suggests a tilt angle of 50 degrees, which agrees well with the one calculated for a structure with a coverage of 0.25. Furthermore, calculated vibrational eigenvectors and spectra allowed us to identify characteristic in-plane (NO2 scissoring) and out-of-plane (C-H wagging) modes and to predict their strength in the spectrum in dependence of the adsorption geometry. We additionally performed calculations for biphenylthiol and terphenylthiol to assess the impact of multiple aromatic rings and found that vdW interactions are significantly increasing with this number, as evidenced by the absorption energy and the molecule adopting a more upright-standing geometry. Published under an exclusive license by AIP Publishing.

Automated summary

A study on the adsorption structure of 4-nitrothiophenol self-assembled monolayers on the Au(111) surface was conducted using both theoretical calculations and experimental spectroscopy, revealing a decreasing tilt angle of molecules towards the surface with decreasing coverage. Additionally, calculations for biphenylthiol and terphenylthiol showed an increase in van der Waals interactions with multiple aromatic rings, leading to a more upright-standing molecular geometry.