Electronic ferroelectricity in monolayer graphene for multifunctional neuromorphic electronics

Ferroelectricity is intriguing for its spontaneous electric polarization, which is switchable by an external electric field. Expanding ferroelectric materials to two-dimensional limit will provide versatile applications for the development of next-generation nonvolatile devices. Conventional ferroelectricity requires the materials consisting of at least two constituent elements associated with polar crystalline structures. Monolayer graphene as an elementary two-dimensional material unlikely exhibits ferroelectric order due to its highly centrosymmetric hexagonal lattices. Nevertheless, two-dimensional moire superlattices offer a powerful way to engineer diverse electronic orders in non-polar materials. Here, we report the observations of electronic ferroelectricity in monolayer graphene by introducing asymmetric moire superlattice at the graphene/h-BN interface. Utilizing Hall measurements, the electric polarization is identified to stem from electron-hole dipoles, suggesting the electronic dynamics of the observed ferroelectricity. Standard polarization-electric field hysteresis loops, as well as unconventional multiple switchable polarization states, have been achieved. By in-situ comparing with control devices, we found that the electronic ferroelectricity in graphene moire systems is independent of layer number of graphene and the corresponding fine band structures. Furthermore, we demonstrate the applications of this ferroelectric moire structures in multi-state non-volatile data storage and the emulation of versatile synaptic behaviors, including short-term plasticity, long-term potentiation and long-term depression. This work not only enriches the fundamental understanding of ferroelectricity, but also demonstrates the promising applications of graphene in multi-state memories and neuromorphic computing.