Face-Dependent Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy of 2,2′-Bipyridine on Au(100) and Au(111)

Abstract

We have investigated the potential-dependent assembly of 2,2′-bipyridine molecules on both Au(100) and Au(111) surfaces using a newly developed Shell-Isolated Nanoparticle Enhanced-Raman Spectroscopy (SHINERS) technique. We present potential-dependent SHINERS spectra of 2,2′-bipyridine adsorbed on both surfaces collected under anodic as well as cathodic polarization. A series of processes were characterized by the analysis of the data set with Perturbation Correlation Moving Window Two-Dimensional Spectroscopy (PCMW2D) and Two-Dimensional Correlation Spectroscopy (2DCOS). Exquisite spectral detail was achieved and allowed for the characterization of the complicated ring breathing mode and C–C inter-ring stretching modes that are diagnostic of molecular orientation on the surfaces. Detection of several occluded vibration peaks was also made possible with SHINERS. Analysis reveals that in very negative potentials 2,2′-bipyridine adsorbs in a disordered, mixed state with both π-flat cis and several different vertically N-bound cis orientations, in contrast to previously published reports. Our findings provide insight into 2,2′-bipyridine adsorption on Au single crystals and also powerfully combine SHINERS with two-dimensional correlation analysis to yield a more detailed view of spectral transitions.