Tip-enhanced Raman spectroscopy of 4,4′-bipyridine and 4,4′-bipyridine N,N'-dioxide adsorbed on gold thin films

A tip-enhanced Raman spectroscopy (TERS) set-up added on a scanning tunneling microscope (STM) was constructed and was used to study the molecular arrangement of 4,4′-bipyridine (4,4′-BiPy) and 4,4′-bipyridine N,N'-dioxide (4,4′-BiPyO₂) adsorbed on Au(111). Over a monolayer of 4,4′-BiPy, in the ambient atmosphere, intense TERS signals were recorded, involving AuN stretching vibration at 185 cm^{- 1}. At the early stage of 4,4′-BiPy adsorption, the AuN stretching signal was not detected. By the aid of the theoretical calculations of the Raman scattering intensities for each vibrational normal mode, a standing-up, tilted orientation at monolayer coverage and a vertical orientation with the longitudinal molecular axis parallel to the surface at low surface coverage was concluded. Adsorption of 4,4′-BiPyO₂ on Au(111) resulted in formation of a (6 × 9) adlattice. Moderately intense TERS signals were observed, containing the NO stretching modes and the in-plane ring vibrational modes. Neither AuO nor AuN stretching modes were observed. 4,4′-BiPyO₂ was laid with the longitudinal axis parallel to the surface, and the molecular plane vertical to the surface. These results demonstrate that STM-TERS is an effective method for simultaneous imaging and vibrational analysis, facilitating identification of the adsorbed layers with the nanometer-scale spatial resolution and monolayer sensitivity.