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 attracted intense attention due to their potential application in surface-enhanced Raman spectroscopy (SERS). Herein, we report how to modulate the SERS response of 2D materials. First, we demonstrate that SERS based on graphene materials is inversely proportional to the functionalization degree. The covalent functionalization interrupts the conjugation of the graphene π-system, inhibiting the charge transfer between graphene and the probe molecule (Rhodamine 6G), thus reducing Raman enhancement. For graphene/MoS2 vdW heterostructures, the SERS enhancement is dominated by the vdW interaction between graphene and MoS2. A shorter interlayer distance, with stronger vdW interactions, improves the dipole–dipole interaction and the charge transfer, increasing the Raman enhancement. Moreover, the SERS intensity of graphene/MoS2 vdW heterostructures varies rapidly when the interlayer distances are less than 0.6 nm, while it varies less at interlayer distances longer than 0.6 nm. This study not only demonstrates the Raman enhancement dependence on the functionalization degree of graphene materials and the interlayer distance in graphene/MoS2 vdW heterostructures but also opens the door for controlling and predicting the SERS intensity based on 2D materials.