High-pressure raman spectroscopy of molecular monolayers adsorbed on a metal surface

The effects of high pressure up to 10 GPa on organic molecules in the form of a solid or a self-assembled monolayer (SAM) on a metal surface are investigated by Raman spectroscopy. The SAMs studied are benzenethiol (BT) and benzenemethylthiol (BMT) on polycrystalline Ag(111). Raman spectra of the SAMs are obtained using a periodically nanostructured Ag surface that provides surface-enhanced Raman scattering (SERS), All the vibrational modes studied show a pressure-dependent blue-shift. These blue-shifts are remarkably similar for the SAMs and solids. There are some minor differences between the SAM and solid-state spectra in the v _18asymmetric in-plane CH-bend and in the v _Cs CS-stretch. The v ₁₈ mode in BT blue-shifts more in the SAM than in the solid, but in BMT it blue-shifts less in the SAM than in the solid. The greater blue-shift for the BT SAM is attributed to the much lower surface coverage of BT, causing greater compressibility. In the SAM, v _CS blue-shifts slower in BT than in the solid, but in BMT the blue-shifts are the same in the SAM and in the solid. It is noted that strain is more easily transmitted from phenyl to the C-S linkage in BT than in BMT.