Functional oxide membranes with a freestanding form have the potential for applications in flexible devices. However, the currently used materials have been limited to perovskite structural systems.

Prof. LU Nianpeng's group from the Institute of Physics of the Chinese Academy of Sciences synthesized a new hexagonal BaAl₂O₄ film, which is a hexagonal structure completely different from cubic or tetragonal material systems. The study was published in Nature Materials.

This BaAl₂O₄ film is a hexagonal structure greatly different from cubic or tetragonal materials, which can be widely used to exfoliate the six-fold and three-fold symmetric membranes. It can dissolve rapidly in water within a sub-minute, and remains highly stable against air, oxygen and ammonia, even at high temperatures. The combination of fast water solubility and exceptional thermal and chemical stability allows in-situ and ex-situ growth of high-quality functional films.

By using this hexagonal BaAl₂O₄ as a water-soluble sacrificial layer, researchers fabricated a broad range of freestanding membranes, including the multiferroic material YMnO₃, lithium-ion battery cathode LiCoO₂, antiferromagnetic oxide α-Fe₂O₃, field effect channel material In₂O₃, typical p-type semiconductor NiO, ultra-wide bandgap semiconductor β-Ga₂O₃, and superconducting nitride TiN.

During the film growth, researchers revealed a unique interfacial strain relaxation mechanism between the films and the substrate, which guaranteed the high-crystallinity of the grown films and the corresponding freestanding membranes.

The application of exfoliated membrane is illustrated by the wide bandgap semiconductor β-Ga₂O₃. The fabricated flexible solar-blind photodetectors from the high-quality β-Ga₂O₃ membranes have good performance and excellent mechanical flexibility, achieving a large photo-to-dark current ratio (10⁴) and a very high detection responsivity (3.79×10³ mA/W).