Simulated SERS spectra of 4,4′-bipyridine/gold single molecule junction in different conductance states: ab initio molecular dynamics approach

The SERS spectra of 4,4′-bipyridine/gold single molecule junction in different conductance states at room temperature are calculated by performing ab initio molecular dynamics simulations, in connection with a Fourier transform of the polarizability autocorrelation function, to illustrate that the Raman peaks of the ON state are enhanced by an additional one or two orders of magnitude in comparison with the OFF state. Considering the relative intensities of SERS spectra, the largest enhanced peak for the OFF state is the ring breathing mode due to the simultaneous contribution from structural change, vibrational coupling, and charge transfer. For the ON state, the C–H bending mode has the largest enhancement due to structural change and charge transfer effects. Finally, the SERS spectra of 4,4′-bipyridine junction in the the ON state with perpendicular and tilted orientations are examined to understand the reason why the tilted junction has lower SERS spectrum intensity with doublet feature of the C–C stretching mode.