NOVEL SELF-ASSEMBLY OF CRYSTALLINE MgAl2O4 NANOSTRUCTURES PROMOTED BY ANNEALING A GOLD OVERLAYER ON A (111) MgAl2O4 SUBSTRATE

Abstract

<p>The solid state dewetting characteristics of thin gold films sputtered onto (111) MgAl₂O₄ substrates were investigated. Prior research done on this system reported discovering the self-assembly of intricately shaped nanostructures, consisting of a faceted sphere lying above a truncated triangular pyramid, formed by applying a specific two stage heating profile. The current work was done to provide deeper insight on the odd self-assembly observed in this system. The results indicate that the intricate structures are not purely gold self-assemblies, but in fact consist of three distinct materials: a single crystal or polycrystalline gold faceted sphere, separated by an interfacial boundary layer, from above a crystalline MgAl₂O₄ necking structure that spontaneously developed from the initially flat substrate. The boundary separating these two assemblies is confined within a thin, sharp region of a third material consisting of Au and O elements. The composition and crystalline nature of the individual nanostructures were studied using high angle annular dark-field imaging, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy employed by a high resolution transmission electron microscope. Two-dimensional X-ray diffraction texture analysis revealed that the gold nanoparticles are crystalline, with majority of the maximum intensity signal corresponding to the epitaxial alignment of the gold nanoparticles with the substrate. The MgAl₂O₄ necking structures were found to be sensitive to both the annealing profile, and the thickness of the gold film, which influenced the distinguishable presence, size, and footprint of the MgAl₂O₄ nanostructures. Atomic force microscopy and scanning electron microscopy results were consistent with the gold overlayer playing an essential role in the self-assembly of MgAl₂O₄ nanostructures. While the fundamental mechanisms that govern this phenomenon are not entirely clear, the presented results do provide insight into the role of interfaces in heteroepitaxial systems, especially the self-assembly of crystalline nanostructures from a previously stable substrate.</p>