Tip-Enhanced Raman Spectroscopy of Self-Assembled Thiolated Monolayers on Flat Gold Nanoplates Using Gaussian-Transverse and Radially Polarized Excitations

Abstract

Tip-enhanced Raman spectroscopy (TERS) is a highly sensitive spectroscopic technique that combines the spatial resolution of scanning near-field techniques with the chemical specificity of vibrational spectroscopy. TERS is based on the excitation of the localized surface plasmon resonance at the apex of an AFM metallized tip, producing a confined and enhanced electromagnetic field. Due to the inherent local nature of TERS and its confinement in the optical near-field of the object, TERS measurements can also be used to probe monolayers adsorbed onto surfaces providing better surface specificity in addition to higher spatial resolution. We implement here gap-mode TERS using gold nanoplates functionalized with thiolated reference molecules such as alkoxy substituted azobenzene thiol and 4-nitrothiophenol. The monolayer is probed with a silver coated AFM tip in order to obtain the largest electromagnetic field enhancement from the surface plasmon localized between the silver tip and the functionalized gold surface. More specifically, we have measured the TERS spectra of the self-assembled monolayer on gold using 532 nm excitation that is linearly (Gaussian–transverse TEM00) and radially polarized. We report the nature of the collected TERS spectra for the thiolated molecules (azobenzene thiol and nitrothiophenol) that appear to be dependent on the polarization of the excitation light at the tip/substrate interface.