Two-Dimensional Heterostructure as a Platform for Surface-Enhanced Raman Scattering

Raman enhancement on a flat nonmetallic surface has attracted increasing attention, ever since the discovery of graphene enhanced Raman scattering. Recently, diverse two-dimensional layered materials have been applied as a flat surface for the Raman enhancement, attributed to different mechanisms. Looking beyond these isolated materials, atomic layers can be reassembled to design a heterostructure stacked layer by layer with an arbitrary chosen sequence, which allows the flow of charge carriers between neighboring layers and offers novel functionalities. Here, we demonstrate the heterostructure as a novel Raman enhancement platform. The WSe₂ (W) monolayer and graphene (G) were stacked together to form a heterostructure with an area of 10 mm × 10 mm. Heterostructures with different stacked structuress are used as platforms for the enhanced Raman scattering, including G/W, W/G, G/W/G/W, and W/G/G/W. On the surface of the heterostructure, the intensity of the Raman scattering is much stronger compared with isolated layers, using the copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman enhancement effect on heterostructures depends on stacked methods. Phonon modes of CuPc have the strongest enhancement on G/W. W/G and W/G/G/W have a stronger enhancement than that on the isolated WSe₂ monolayer, while lower than the graphene monolayer. The G/W/G/W/substrate demonstrated a comparable Raman enhancement effect than the G/W/substrate. These differences are due to the different interlayer couplings in heterostructures related to electron transition probability rates, which are further proved by first-principle calculations and probe-pump measurements.