A research team led by Prof. WU Wenbing and Prof. WANG Linfei from the University of Science and Technology of China (USTC), in collaboration with Prof. SI Liang’ s team from Northwest University, developed a new water-soluble sacrificial layer, “super-tectragonal” Sr₄Al₂O₇ (SAO_T), with broad tunability in lattice constants, which can be used to prepare high-quality freestanding oxide membrane. Their work was published in Science on January 26^th.

Freestanding oxide membrane is a type of low-dimensional quantum material that maintains single-crystal properties even after removing the substrate, possessing both the coupled properties of multi-degree-of-freedom in correlated electron systems and the structural flexibility of two-dimensional materials. Its unique properties like superelasticity offer the potential to induce novel quantum-derived phenomena and develop ultrathin flexible electronic devices.

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.

Schematic illustration of water-assisted exfoliation of freestanding oxide membranes from SAO_C and SAO_T sacrificial layers (Image by Prof. WU Wenbing’s team).

To address the problem, 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 exhibit 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 (3.85 to 4.04 Å), 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. In addition, the crystallinity and functionalities of the team’s 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.

Crystal structures of water-soluble Sr₄Al₂O₇ sacrificial layer (Image by Prof. WU Wenbing’s team).

This discovery is a breakthrough in improving the integrity and crystallinity of freestanding oxide membrane, which can enhance its application potential in low-dimensional flexible electronic devices. Reviewers highly commend this work, saying that “(this work) has the potential to have broad impact on oxide electronics from multiple perspectives”.

Paper link: https://doi.org/10.1126/science.adi6620

(Written by HUANG Rui, edited by ZHANG Yihang, USTC News Center)