SELF-ASSEMBLY OF MAGNESIUM ALUMINATE DUE TO DEWETTING OF OVERLAID GOLD THIN FILM

Abstract

The self-assembly of magnesium aluminate spinel as a result of dewetting an overlaid thin film of (chiefly) gold was investigated. Thin films of gold were deposited on single-crystalline spinel substrates and were heat-treated to dewet gold film which led to self-assembly of intricate structures consisting of faceted spherical particles atop of frustums. The current work was conducted in continuation of previous studies which reported formation of such intricate structures. The most recent studies had evidently overruled a pure gold self-assembly scenario as was pointed out in preliminary investigations. It was in fact proven that these structures consist of three distinct parts: (i) a single or polycrystalline gold faceted sphere, (ii) quasi-phase interfacial bilayer, and (iii) a crystalline MgAl2O4 necking structure spontaneously risen from spinel substrate. In the current work, samples were produced through different film deposition methods of sputter, thermal evaporation, and e-beam evaporation coating which underwent thermal annealing to induce dewetting of gold film and subsequent self-assembly of intricate structures. Several characterization methods such as electron microscopy, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy, and atom probe tomography were utilized to survey the different features of the intricate structures with focus on chemical analysis of the gold-spinel interface. The results rejected the previous findings about formation of interface complexion at the boundary of gold-spinel. It was found out that gold-spinel interface is in fact an ordinary metal-oxide boundary with sharp atomic distinction and no inter-diffusion or formation of interfacial complexion. It was further discovered that dewetting pure gold thin films does not result in formation of spinel self-assembly and existence of elemental impurities of copper (Cu) in the initial film is vital in development of such structures. Finally, it was concluded that chemical composition of metallic overlayer and the heat treatment parameters most fundamentally influence formation and physical characteristics of those self-assembled structures.