Raman enhancement on graphene: adsorbed and intercalated molecular species.

Strong Raman scattering is observed from iodine anions adsorbed at ca. 3% coverage on single layer graphene. In addition, the Raman signal from just one bromine intercalation layer inside three and four layer thick graphenes is observed. We analyze and model the intramolecular electronic, charge-transfer, and multiple reflection electromagnetic mechanisms responsible for this unusual sensitivity. Graphene is an excellent Raman substrate for adsorbed species showing intramolecular electronic resonance, because graphene efficiently quenches interfering excited-state luminescence. The Raman sensitivity for adsorbed and intercalated molecular species is highest for single layer graphene and decreases with increasing thickness. These phenomena are compared with surface enhanced Raman spectroscopy field enhancement and "chemical" Raman processes in aggregated Ag particles and on flat, highly reflective metal surfaces. The Raman spectra of adsorbed bromine layers are not observed, despite significant charge transfer to graphene. Charge transfer from adsorbed bromine is about one-half of charge transfer from intercalated bromine. We attribute the large Raman signal for both adsorbed iodine and intercalated bromine species to intramolecular electronic resonance enhancement. The signal evolution with varying graphene thickness is explained by multiple reflection electromagnetic calculations.