  Ultrathin freestanding membranes with a pronounced metal-insulator transition
(MIT) provides huge potential in future flexible electronic applications as
well as a unique aspect of 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 millimetres and VO2 thickness down to 5
nm. The VO2-membranes were detached by dissolving a Sr3Al2O6 sacrificial layer
between the VO2 thin film and c-Al2O3(0001) substrate, allowing a transfer onto
arbitrary surfaces. Furthermore, the MIT in the VO2-membrane was greatly
enhanced by inserting an intermediate Al2O3 buffer layer. In comparison to the
best available ultrathin VO2-membranes, the enhancement of MIT is over 400% at
5 nm VO2 thickness and more than one 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.
