Enhanced Metal-Insulator Transition in Freestanding VO₂ Down to 5 nm Thickness

Ultrathin freestanding membranes with a pronounced metal-insulator transition (MIT) have huge potential for future flexible electronic applications as well as provide a unique aspect for the study of lattice-electron interplay. However, the reduction of the thickness to an ultrathin region (a few nm) is typically detrimental to the MIT in epitaxial films, and even catastrophic for their freestanding form. Here, we report an enhanced MIT in VO₂-based freestanding membranes, with a lateral size up to millimeters and the VO₂ thickness down to 5 nm. The VO₂ membranes were detached by dissolving a Sr₃Al₂O₆ sacrificial layer between the VO₂ thin film and the c-Al₂O₃(0001) substrate, allowing the transfer onto arbitrary surfaces. Furthermore, the MIT in the VO₂ membrane was greatly enhanced by inserting an intermediate Al₂O₃ buffer layer. In comparison with the best available ultrathin VO₂ membranes, the enhancement of MIT is over 400% at a 5 nm VO₂ thickness and more than 1 order of magnitude for VO₂ above 10 nm. Our study widens the spectrum of functionality in ultrathin and large-scale membranes and enables the potential integration of MIT into flexible electronics and photonics.