In Situ X-Ray Studies of Remote Epitaxy of Perovskite Oxides on Graphene

Remote epitaxy is a novel synthesis technique that allows for the fabrication of thin, freestanding single crystals and nanomembranes.It relies on a sacrificial graphene layer 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. Use of a more universal interlayer can facilitate the development of “stacktronics”, where a variety of single crystalline materials can be laid atop each other at low to moderate temperatures, leading to the synthesis of complex van der Waals heterostructures. 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. Thus, advances in stacktronics require improved understanding of remote epitaxy and insight into the impact of interlayer thickness and deposition conditions on the nucleation and growth of thin films. 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/>In this talk, we will demonstrate a set of <i>in situ </i>studies of perovskite oxide thin film synthesis on polycrystalline graphene few layers, using synchrotron X-ray scattering to investigate the deposition of SrTiO₃ (STO) and LaNiO₃ (LNO) by molecular beam epitaxy (MBE) onto graphene-coated STO (001) substrates. X-ray phase retrieval methods are 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. We observe the effect of oxygen partial pressure on graphene-coated STO, finding that most of the graphene is removed at <i>p</i>O₂ = 2 × 10⁻⁴ Torr at 760°C. Lower oxygen partial pressures can be used for the growth of SrTiO₃ on graphene, which was confirmed by the deposition of 12UC STO on 1ML G. The crystallinity of the film, however, depends on both its thickness and the thickness of the interlayer: for example, 12UC STO is epitaxial when grown on 1ML G but polycrystalline when grown on 2ML G; 3UC STO is epitaxial on 4ML G but becomes more polycrystalline with increasing film thickness. The crystal quality appears to depend on the additional electron density lying above the topmost graphene layer (&gt; 9 Å), as nuclei formed on these regions may have poor interaction with the underlying crystal substrate. We also find the etching of graphene in an ozone environment for LNO remote epitaxy even with the presence of an STO buffer layer on graphene interlayers. So, thicker oxide buffer layers and the use of single crystalline graphene may be necessary for the synthesis of nickelate heterostructures on graphene.