Effect of molecular spacers on surface-enhanced attenuated total reflection infrared spectra of bulk liquids

The absorbance of bands in the infrared spectrum of species located within 5 nm of the surface of metal nanoparticles deposited on dielectric substrates is known to be increased by between one and three orders of magnitude through the phenomenon known as surface-enhanced infrared absorption (SEIRA). The mechanism of SEIRA is still not fully understood. For example, under certain circumstances, the band shapes in SEIRA spectra become asymmetric. One cause of this asymmetry that has been postulated is a chemical interaction between the molecule and the metal surface. In a previous paper, we showed that adsorbate bands first become asymmetric when the metal nanoparticles start to percolate (coalesce) and that a chemical interaction is not needed to induce this effect. In this paper, we report the results of experiments in which a horizontal internal reflection element is coated with silver nanoparticles to a nominal thickness of between 5 and 10 nm. At the onset of percolation, the absorption bands of molecules in contact with the surface of the silver nanoparticles become asymmetric, whether these molecules are chemisorbed thiols or bulk solvents such as n-heptane that have little interaction with the silver surface. A self-assembled monolayer (SAM) of p-fluorothiophenol (PFTP) acts as a molecular spacer between the metal and any solvent with which it is in contact. The asymmetry of the PFTP bands is the same in the absence and presence of the solvent, whereas the asymmetry of the solvent bands was reduced when the silver surface is coated with a SAM of PFTP. From these results, we deduce that the band asymmetry is caused by an electrical interaction that occurs very close to the surface of the metal nanoparticles.