Probing the Effect of Dye-2D Material Configuration on their Surface Enhanced Raman Spectra

Graphene-enhanced Raman Spectroscopy (GERS) is a powerful technique enabling reproducible and sensitive detection of various biochemical species enabled by graphene’s atomically flat surface, charge transfer characteristics and high quenching ability. GERS works on the highly charge and surface-sensitive mechanism of chemical enhancement and thus requires an in-depth understanding of the analyte-graphene interaction to tune the overall enhancement. Here we choose planar and non-planar dye molecules from the Rhodamine family as analytes and study their GERS enhancement in two configurations: dye on top or bottom of graphene. For dyes on top of graphene, we observe a very high enhancement for stretching vibrational modes and a greater degree of fluorescence quenching, indicating more significant dye molecule adsorption than the dye-on-bottom configuration. Non-planar dye molecules also show an abnormal enhancement of vibrational modes associated with out-of-plane chemical species implying their flattening to increase conformity with the graphene surface, which is not observed in the case of planar dye molecules or on top of MoS₂ thus showing the importance of structural matching in GERS. First principle calculations of the resonance Raman spectra assignments of the dye molecules support the experimentally observed molecular flattening. Our work gives a deeper understanding of the effect of analyte-graphene interaction on GERS signals and paves the way for a robust graphene-based platform for biochemical sensing.