Impact of the Interlayer Distance between Graphene and MoS2 on Raman Enhancement

Two-dimensional (2D) materials and their van der Waals(vdW) heterostructures,particularly graphene and graphene/MoS2, have attractedintense attention due to their potential application in surface-enhancedRaman spectroscopy (SERS). Herein, we report how to modulate the SERSresponse of 2D materials. First, we demonstrate that SERS based ongraphene materials is inversely proportional to the functionalizationdegree. The covalent functionalization interrupts the conjugationof the graphene & pi;-system, inhibiting the charge transfer betweengraphene and the probe molecule (Rhodamine 6G), thus reducing Ramanenhancement. For graphene/MoS2 vdW heterostructures, theSERS enhancement is dominated by the vdW interaction between grapheneand MoS2. A shorter interlayer distance, with strongervdW interactions, improves the dipole-dipole interaction andthe charge transfer, increasing the Raman enhancement. Moreover, theSERS intensity of graphene/MoS2 vdW heterostructures variesrapidly when the interlayer distances are less than 0.6 nm, whileit varies less at interlayer distances longer than 0.6 nm. This studynot only demonstrates the Raman enhancement dependence on the functionalizationdegree of graphene materials and the interlayer distance in graphene/MoS2 vdW heterostructures but also opens the door for controllingand predicting the SERS intensity based on 2D materials.