Raman Enhancement of Molecules Sandwiched between van der Waals Materials.

Raman enhancement has been widely observed for molecules adsorbed on van der Waals (vdW) materials including graphene, MoS2, hexagonal boron nitride, and transition-metal dichalcogenides. This enhancement is often dominated by a chemical mechanism. However, how the Raman response evolves when a molecule is sandwiched between two vdW layers remains unclear. Here, using copper phthalocyanine (CuPc) as a probe, we compare three sandwich structures: graphene/CuPc/TiS2, graphene/CuPc/TiSe2, and graphene/CuPc/graphene, spanning asymmetric to near-symmetric dual-interface geometries. We find that adding a second interface in contact with the molecule can suppress, moderate, or enhance the Raman response of CuPc depending on the contacting material pair and vibrational symmetry. The graphene/CuPc/graphene sandwich shows stronger enhancement than single-interface CuPc/graphene, with an excitation wavelength dependence. These results show that the dual-interface Raman response is influenced by interface-dependent charge-transfer effects and increased nonradiative damping. This work provides an experimental framework for understanding Raman enhancement in molecule-sandwiched structures.