R6G on Graphene: High Raman Detection Sensitivity, Yet Decreased Raman Cross-Section

Several recent studies have demonstrated the use of single and few-layer graphene as a substrate for the enhancement of Ranian scattering by adsorbed molecules in a method termed graphene-enhanced Raman spectroscopy (GERS). Here we determine the resonance Raman scattering cross-section for the dye molecule rhodamine 6G (R6G) adsorbed on bilayer graphene. For the 1650 cm(-1) R6G mode, we obtain a cross-section of 5.1 x 10(-24) cm(2).molecule(-1), a greater than 3-fold reduction from the previously reported solution value. We show that the absorption spectrum of adsorbed R6G can be measured using micro-optical contrast spectroscopy, and we find that detuning of the molecular resonance explains the decreased Raman scattering cross-section. We find no evidence for a graphene Raman enhancement process. We also study the graphene thickness dependence of the adsorbed R6G Raman signal and show that a model incorporating electromagnetic interference effects can qualitatively explain the decrease in signal with increasing graphene thickness.