Effect of Dipolar Molecule Structure on the Mechanism of Graphene-Enhanced Raman Scattering

Graphene-enhanced Raman scattering (GERS) is a promising characterization technique which uses a single layer of graphene. As the electronic coupling of adsorbates with graphene leads to enhancement in the Raman signal, it is of immense interest to explore the factors that affect the coupling of the adsorbates with graphene. To probe this effect we have designed and synthesized a series of dipolar molecules with the general structure, N-ethyl-N-(2-ethyl(1-pyrenebutyrate)-4-(4-R-phenylazo)aniline) where the R-groups are varied from methoxy (-OCH3), methyl (-CH3), hydrogen (-H), nitrile (-CN), nitro (-NO2) to tricyanofuran (TCF) groups. This systematically changes the dipole moments and electronic/optical band gap of the molecules. By noncovalently interfacing these molecules on graphene, the Raman signal is enhanced by a factor of 40-90 at the excitation wavelength of 532 nm. Measurements of the Raman enhancement factor and Raman cross section are complemented with DFT calculations to correlate the dipole moment and the energy level of the hybrid to the Raman scattering efficiency. These studies highlight the relevance of the dipolar nature of chromophores, which determines their dipole moment and the band gap, and the resulting electronic coupling to graphene which simultaneously alters the energy level of the orbitals in the molecule and the Fermi level in graphene, resulting in efficient Raman excitations and GERS.