Ferroelectric-assisted charge-transfer plasmons in graphene/CuInP2S6.

Two-dimensional materials and their van der Waals heterostructures provide a versatile platform for novel optoelectronic functionalities. Interfacial charge transfer in such stacks enables high-quality graphene plasmons, but the charge-doping is fixed by work-function differences between constituent layers. Ferroelectric substrates have been explored for non-volatile, switchable doping, yet oxide ferroelectrics yield only modest doping due to imperfect interfaces. Here, we report efficient charge transfer between graphene and the van der Waals ferroelectric CuInP2S6, where the atomically-sharp interface yields unexpectedly large doping and high-quality plasmons, despite orders-of-magnitude smaller polarizations compared to oxide ferroelectrics. Infrared nano-imaging directly visualizes graphene plasmons whose doping depends on the adjacent ferroelectric layer. Combined nano-infrared and piezoresponse force microscopies reveal that graphene doping and CuInP2S6 polarization increase with thickness and saturate in tandem, linking charge-transfer doping and out-of-plane ferroelectricity. Our results establish ferroelectric-assisted charge-transfer plasmons in graphene/CuInP2S6 as a promising route to probe and control nanoscale ferroelectric textures.