However, the misfit strain relaxation between water-soluble sacrificial layer and oxide membrane triggers the formation of high-density cracks during the water-assisted exfoliation of freestanding oxide membrane, compromising the crystallinity and integrity of current freestanding oxide membrane. How to suppress the crack formation and obtain large-area, highly crystalline freestanding oxide membrane remains a challenge.

The team delved into the pulsed laser deposition growth window of Sr-Al-O (SAO) based water-soluble sacrificial layer and discovered a previously unknown Sr₄Al₂O₇ film which exhibits extraordinary properties. The biaxial-strained Sr₄Al₂O₇ film has a tetragonal structural symmetry, enabling coherent growth of high-quality ABO₃/SAO_T epitaxial heterostructures, which suppresses the crack formation during water-assisted release and thus enhancing the crystallinity and integrity of freestanding oxide membrane.

The team also verified the exfoliation effect of perovskite oxide films with a broad lattice constant range, and revealed that the crack-free areas of the freestanding oxide membrane released from the Sr₄Al₂O₇ sacrificial layer can span up to a few millimeters in scale, which is 1-3 orders of magnitude larger than previous freestanding oxide membrane. The crystallinity and functionalities of freestanding oxide membrane are comparable to the epitaxial membranes grown on single crystalline substrates.

Furthermore, the team discovered that the unique structure of Sr₄Al₂O₇ results in high water solubility, significantly reducing the time of water-assisted releasing processes and enhancing the efficiency of freestanding oxide membrane.

This study represents a breakthrough in improving the integrity and crystallinity of freestanding oxide membrane, which can enhance its application potential in low-dimensional flexible electronic devices.