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In this review paper, recent progress in the fabrication, transfer, and fundamental physical properties of freestanding oxide perovskite thin films is discussed. First, the main strategies for the synthesis and transfer of freestanding perovskite thin films are analyzed. In this initial section, particular attention is devoted to the use of water-soluble (Ca,Sr,Ba)3Al2O6 thin films as sacrificial layers, one of the most promising techniques for the fabrication of perovskite membranes. The main functionalities that have been observed in freestanding perovskite thin films are then reviewed. In doing so, the authors begin by describing the emergence of new phenomena in ultrathin perovskite membranes when released from the substrate. They then move on to a summary of the functional properties that are observed in freestanding perovskite membranes under the application of strain. Indeed, freestanding thin films offer the unique possibility to actively control the strain state far beyond what can be observed with traditional methods, allowing the investigation of the profound interplay between structural and electronic properties in oxides. Overall, this review highlights the potential of oxide-based freestanding thin films to become the preferred platform for the study of novel functionalities in perovskite oxide materials.

The integration of dissimilar materials in heterostructures has long been a cornerstone of modern materials science - seminal examples are the two-dimensional (2D) materials and van der Waals heterostructures. Recently, new methods have been developed, which enable the realization of ultra-thin freestanding oxide films approaching the 2D limit. Oxides offer new degrees of freedom, due to the strong electronic interactions, especially the 3d orbitals, which give rise to rich exotic phases. Inspired by this progress, we have developed a new platform for assembling ultra-thin freestanding oxide thin films with different materials and orientations into artificial stacks with heterointerfaces. We show that the oxide stacks can be tailored by controlling the stacking sequences as well as the twist angle between the constituent layers with atomically sharp interfaces, leading to distinct moiré patterns in the transmission electron micrographs of the full stacks. Stacking and twisting has been recognized as a key degree of structural freedom in 2D materials but until now has never been realized for oxide materials. Our approach opens unexplored avenues for fabricating artificial 3D oxide stacking structure with freestanding membranes across a broad range of complex oxide crystal structures with functionalities not available in conventional 2D materials. This article is protected by copyright. All rights reserved.