Chemical termination and interfacial redox behavior of freestanding SrTiO₃

Tailoring oxide heterointerfaces has sparked the search for electronic and ionic phenomena in low-dimensional, confined systems. The fabrication of freestanding oxide membranes has further expanded the possible fields of application. Based on the structural vulnerability and physical confinement of such membranes, it remains a great challenge to achieve atomically defined and single-terminated surfaces by the typical chemical treatments and to induce interfacial redox-reactions in these nanoscopic transition metal oxides. To address this, we use the sacrificial layer exfoliation route, involving an all-perovskite epitaxial layer structure to fabricate freestanding SrTiO₃ membranes with high crystallinity and defined surface morphology. To study the interfacial redox-behavior of the singly TiO₂-terminated, annealed membrane, we employ the formation of oxygen vacancies in SrTiO₃, triggered by the low-pressure deposition of a thin LaAlO₃ layer epitaxially grown on the transferred SrTio₃ layer. A mixed Ti^3+/4+ valence state is indicative of the induced transfer of oxygen ions from the confined SrTiO₃ membrane into the LaAlO₃ overlayer, resulting in an oxygen vacancy concentration of around 10²¹ in the confined SrTiO3 membrane. Our results highlight that interfacial redox-reactions can be induced in SrTiO₃ membranes, which enables the ionic engineering of confined oxide heterointerfaces based on the freestanding oxide approach.