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) 3 Al 2 O 6 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 2D materials and van der Waals heterostructures. Recently, new methods have been developed that enable the realization of ultrathin freestanding oxide films approaching the 2D limit. Oxides offer new degrees of freedom, due to the strong electronic interactions, especially the 3d orbital electrons, which give rise to rich exotic phases. Inspired by this progress, a new platform for assembling freestanding oxide thin films with different materials and orientations into artificial stacks with heterointerfaces is developed. It is shown 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 microscopy images of the full stacks. Stacking and twisting is recognized as a key degree of structural freedom in 2D materials but, until now, has never been realized for oxide materials. This approach opens unexplored avenues for fabricating artificial 3D oxide stacking heterostructures with freestanding membranes across a broad range of complex oxide crystal structures with functionalities not available in conventional 2D materials.

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The discovery of 2D conductivity at the LaAlO 3 /SrTiO 3 interface has been linking, for over a decade, two of the major current research fields in materials science: correlated transition-metal-oxide systems and lowdimensional systems. Notably, despite the 2D nature of the interfacial electron gas, the samples are 3D objects with thickness in the mm range. This prevented researchers so far from adopting strategies that are only viable for fully 2D materials, or from effectively exploiting degrees of freedom related to strain, strain gradient and curvature. Here a method based on pure strain engineering for obtaining freestanding LaAlO 3 /SrTiO 3 membranes with micrometer lateral dimensions is demonstrated. Detailed transmission electron microscopy investigations show that the membranes are fully epitaxial and that their curvature results in a huge strain gradient, each layer showing a mixed compressive/tensile strain state. Electronic devices are fabricated by realizing ad hoc circuits for individual micromembranes transferred on silicon chips. The samples exhibit metallic conductivity and electrostatic field effect like 2D-electron systems in bulk heterostructures. The results open a new path for adding oxide functionalities into semiconductor electronics, potentially allowing for ultra-low voltage gating of a superconducting transistors, micromechanical control of the 2D electron gas mediated by ferroelectricity and flexoelectricity, and on-chip straintronics.