Adsorption properties of decyl thiocyanate and decanethiol on platinum substrates studied by sum-frequency generation spectroscopy

Recently, thiocyanate groups were successfully used as precursors for thiolate assemblies. Indeed, they provide a useful alternative to thiol groups for self assembly on metallic substrates. In order to check the adsorption properties and the quality of the thiocyanate-based self-assembled monolayers (SAMs), we use Sum-Frequency Generation spectroscopy (SFG) and apply it to characterize the ad-layers of two similar molecules: decanethiol (DT) and decyl thiocyanate (DTCN) adsorbed on platinum surfaces. By comparing the SFG signals of the methyl and methylene vibration modes, we show that DTCN SAMs are less ordered than DT ones. These effects are related to the SAMs quality which depends on the molecular packing as highlighted by Scanning Tunnelling Microscopy (STM) and X-Ray Photoelectron Spectroscopy (XPS).