Raman Enhancement of a Dipolar Molecule on Graphene

We show a large enhancement in the Raman signal from a highly polarizable molecule attached to single layer graphene. Through spatial mapping of the Raman signal and wavelength-dependent Raman measurements from a dipolar chromophore latched to a graphene/SiO2 substrate and to a bare SiO2 substrate, we show that strong electronic coupling in the hybrid structure contributes to the enhancement. The dipolar molecule is a pyrene tethered Disperse Red 1 (DRIP) that noncovalently binds to graphene. Upon comparison of the Raman signal of DRIP on single layer graphene with that on a bare SiO2/Si substrate, we found that the enhancement factor is in the range 29-69 at 532 nm excitation. As the surface coverage of DRIP on graphene increases, Raman intensity also increases and saturates at a certain concentration. The saturation of the Raman signal intensity at higher DRIP concentrations were accompanied by shifts in the G band and the 2D band of graphene due to p-doping. We further show that the Raman enhancement that occurs on single layer is larger than on few layer graphene. Quantitative analysis on the Raman scattering. cross section of DRIP on graphene shows a higher Raman scattering cross section compared to that in solution confirming a strong electronic coupling. A series of all-electron ab initio calculations using density functional theory (DFT) modeled the noncovalent binding of DRIP on a large graphene fragment where the pyrene tether is interacting with the graphene fragment via pi-pi stacking interactions. The DRIP molecule has occupied energy levels that are close to the Fermi level of graphene, and these interact strongly with the semimetallic nature of graphene. As a consequence, in complete contrast to the isolated DRIP molecule, our time-dependent DFT calculations show that the orbital energies and densities for DRIP are significantly modified by the graphene substrate.