Fabrication of Au-Fe Janus-like particles employing solid-state dewetting: Microstructure and extraparticle precipitation of Fe

In this study, we fabricated thermodynamically stable nanoparticles containing face centered cubic Au-based, and body centered cubic Fe-based phases employing a solid-state dewetting of thin Fe/Au bilayers deposited on sapphire substrate. We found that the size distribution and morphology of the two-phase nanoparticles can be varied by changing the overall composition, the deposition order of individual sub-layers, and the parameters of dewetting and subsequent heat treatments. The presence of Fe precipitates in the Au\Fe nanoparticles results in formation of Janus-like structure, as well as in other complex microstructures with unusual shapes and topographies. It was found that coherent twin boundaries are formed in the Au(Fe) phase, and that Au(Fe) and Fe phases share a coherent interface with orientation relationship of Bain type: [001]_Fe||[011]_Au and (010)_Fe ||∼4° to (100)_Au. Additionally, Au segregation on Fe precipitate facets was detected; it was found that the Au-rich segregation layer on the (110) facet of Fe contains 16 ± 4 at.% Au. A thermodynamic possibility of extraparticle precipitation of the Fe-rich phase during particles annealing in the two-phase area of the Fe-Au phase diagram was demonstrated by considering the balance of surface energy and driving force for transformation. Also, a simple geometrical model was employed to demonstrate how the Janus-like nanoparticles can approach their equilibrium shape by formation and migration of coherent twin boundaries.