Adsorption changes of cyclohexyl isothiocyanate on gold surfaces

The adsorption and structure of cyclohexyl isothiocyanate (CHIT) on gold surfaces has been investigated by surface-enhanced Raman scattering (SERS) and scanning tunneling microscopy (STM). Depending on the concentration, the spectral changes of the NCS stretching vibration on gold nanoparticles appeared to be more conspicuous than those of cyclohexyl ring modes. Both equatorial and axial chair conformers of CHIT were found to exist at low bulk concentrations near the monolayer coverage limit, whereas the equatorial chair conformer appeared to be dominant at high bulk concentrations. It was also observed that the ring conversion of equatorial to axial conformers can easily occur at higher temperatures, and the mole fraction of the axial form is assumed to increase with increasing temperature from 30 to 60 degrees C. Alternatively, STM imaging revealed that the adsorption of CHIT molecules on Au(1 1 1) leads to the formation of disordered SAMs with a few vacancy islands, as opposed to the formation of well-ordered self-assembled monolayers (SAMs) by cyclohexanethiols.