Multimodal X-Ray Studies of Remote Epitaxy of Functional Oxides and Enhanced Functionalities on Graphene

Future technologies are likely to exploit flexible heterostructures exhibiting multifunctional properties constructed from multiple materials. One technique for the synthesis of such systems relies on remote epitaxy, which is a novel synthesis technique that allows for the fabrication of thin, freestanding single crystals and nanomembranes. It relies on a sacrificial layer (e.g. graphene) between a thin film and a single-crystalline substrate: during film deposition, the electronic interactions across the graphene are strong enough to enable epitaxial growth but weak enough to allow mechanical release of the film. Others have demonstrated methods for the fabrication of freestanding structures, but the procedures are often materials-specific in terms of the interlayer permitting epitaxial growth. The remote epitaxy technique can be used to create single crystal heterostructures comprised of stacked epitaxial films, their properties optimized by minimizing incompatibilities between the different materials. Details regarding nucleation and growth via remote epitaxy remain unknown, however, due to the many difficulties in studying synthesis in the growth environment with atomic-scale resolution. This necessitates <i>in situ</i> studies sensitive to the atomic-level structure conducted in the growth environment. <i>In situ</i> measurements are particularly important for the synthesis of complex oxides such as perovskite oxides, where small changes to the degree of oxygen incorporation can impact the properties of the film/interface as well as degrade the graphene interlayer.<br/><br/>Here, in this talk, we will firstly demonstrate an <i>in situ</i> synchrotron X-ray investigation of perovskite oxide (e.g. SrTiO₃and LaNiO₃) thin film growth by molecular beam epitaxy onto graphene few layer coated SrTiO₃ (001) substrates. X-ray phase retrieval methods were used to reconstruct the electron density profiles from X-ray crystal truncation rods measured under different growth conditions. Our <i>in situ</i> observations combined with post-growth spectroscopy provide a number of key insights regarding graphene in the synthesis environment and the resulting effects on the complex oxide/graphene heterostructure. Furthermore, we show that the graphene buffer layer could also enchance the physical propeties of a functional oxide thin film grown by the remote epitaxy. We will present the study conducted on epitaxial VO₂ thin films to assess to the effect of remote epitaxy on the metal–insulator transition (MIT). The epitaxial VO₂ heterostructures were synthesized on both bare Al₂O₃ (0001) substrates and Al₂O₃ substrates coated with a bilayer graphene. While both systems exhibit the MIT, the film grown by remote epitaxy on graphene demonstrates improved transport properties. Electrical transport measurements show that the on/off ratio is enhanced by a factor of ~7.5 and the MIT switching temperature window is narrower for VO₂ thin films grown on graphene. By characterizing the heterostructures with a suite of X-ray structural, chemical, and spectroscopic tools, we find that the graphene interlayer inhibits oxygen vacancy diffusion from Al₂O₃ (0001) during the VO₂ growth, resulting in improved electrical behaviors at the MIT.