Measuring Local Electric Fields and Local Charge Densities at Electrode Surfaces using Graphene-enhanced Raman Spectroscopy (GERS)-based Stark-shifts*

We report spectroscopic measurements of the local electric fields and charge densities at electrode surfaces using graphene-enhanced Raman spectroscopy (GERS) based on the Stark-shifts of surface-bound molecules (CuPc) and the G band frequency shift in graphene. The shifts in vibrational frequencies of the graphene G band and CuPc are obtained simultaneously and correlated. The 1531 cm^-1 peak exhibits Stark-shifts and can, thus, be used to report the local electric field strength at the electrode surface under electrochemical working conditions. Computational simulations using density functional theory (DFT) predict similar Stark shifts and provide a detailed atomistic picture of the electric field-induced perturbations to the surface-bound CuPc molecules.