Directional ionic transport across the oxide interface enables low-temperature epitaxy of rutile TiO2.

Heterogeneous interfaces exhibit the unique phenomena by the redistribution of charged species to equilibrate the chemical potentials. Despite recent studies on the electronic charge accumulation across chemically inert interfaces, the systematic research to investigate massive reconfiguration of charged ions has been limited in heterostructures with chemically reacting interfaces so far. Here, we demonstrate that a chemical potential mismatch controls oxygen ionic transport across TiO2/VO2 interfaces, and that this directional transport unprecedentedly stabilizes high-quality rutile TiO2 epitaxial films at the lowest temperature (≤ 150 °C) ever reported, at which rutile phase is difficult to be crystallized. Comprehensive characterizations reveal that this unconventional low-temperature epitaxy of rutile TiO2 phase is achieved by lowering the activation barrier by increasing the "effective" oxygen pressure through a facile ionic pathway from VO2-δ sacrificial templates. This discovery shows a robust control of defect-induced properties at oxide interfaces by the mismatch of thermodynamic driving force, and also suggests a strategy to overcome a kinetic barrier to phase stabilization at exceptionally low temperature.