Encapsulation of metal oxide nanoparticles by oxide supports during epitaxial growth

The traditional view of complex oxide heteroepitaxy is that the substrate is merely a passive bystander. Here we show that this is not always the case, in particular for a metal oxide/metal oxide interface during the synthesis of epitaxial nickel oxide (NiO) nanoislands on (001) strontium titanate (SrTiO3-STO) single crystal substrates. We find that the substrate atoms are driven to encapsulate the metal oxide nanocrystal islands - a phenomenon more commonly reported for metal/metal oxide heterogeneous catalysis where the migration of atoms from the support onto the functional nanoparticle system is driven by strong metal-support interactions (SMSI). High-resolution transmission electron microscopy (HRTEM), high angle angular dark field (HAADF) scanning transmission electron microscopy (STEM), and electron tomography (ET) are used to reveal a clear physical displacement of the interface; the atoms from the STO substrate climb and cover the bottom facets of NiO nanoparticles during the synthesis process during which a unique caldera-shaped structure is formed. It is postulated that this phenomenon occurs to lower the work function and surface energy of the system. These results indicate that the SMSI could be important in the metal oxide/metal oxide systems. In this way changes to the substrate-film interface could be exploited by controlling and modifying the properties of nanoscaled functional oxides.