F heterostructures via ionic gating*

We demonstrate reversible voltage control between metal/insulator and insulator/insulator lateral patterns of epitaxial perovskite oxides and oxyfluorides via electrolytic gating, leading to electric field modulation of anisotropic transport and optical responses. Lateral stripes of SrFeO_2.5/SrFeO₂F were patterned on conducting SrTiO₃:Nb substrates through lithographically-defined topochemical fluorination reactions. Using a back-contact geometry, biasing across an ionic gel results in a conversion of the SrFeO_2.5 brownmillerite regions to SrFeO₃ perovskite, while SrFeO₂F remains unchanged after biasing. The application of an oxidizing bias (V_G = -3 V) converts the all-insulating SrFeO_2.5/SrFeO₂F pattern into SrFeO₃/SrFeO₂F metal/insulator lateral structures with highly anisotropic in-plane transport and strong optical contrast between stripes. The application of a positive bias (V_G = 1 V) results in reduction of the SrFeO₃ phase, while the SrFeO₂F regions again remain unchanged. The brownmillerite oxide regions can be reversibly reverted to the SrFeO₃ perovskite phase with a negative gate bias. [Adv. Funct. Mater. 2208434 (2022)]