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.