In modern electronics, various semiconductor materials are essential for many different functional applications, such as transistors, resonators, and sensors. Recently, performance requirements for each function have been dramatically increasing, and these semiconductor materials exhibit their best performance when they have a single-crystalline nature. However, single-crystalline materials are difficult to grow and face numerous challenges in device process compatibility, which limits their further applications. Freestanding single-crystalline membranes present exciting opportunities from both material and device perspectives. The functional single-crystalline layers are exfoliated from the bottom substrate and the material properties of the released layers can be significantly improved as they are no longer attached to a substrate. Additionally, the challenging growth conditions typically required for single-crystalline materials can be avoided by exfoliating the membrane after the growth, enabling new device opportunities. This talk will cover the fabrication processes of freestanding single-crystalline membranes and explore their potential, particularly in gas sensor applications.