The formation of core–shell (CS) nanoparticles (NPS) often requires complex procedures. Due to minimization of interface energy, we show that colloidal Fe–Au CS NPs are obtained in one step, by laser ablation of bimetallic thin films in liquid.

The application of an Au-Fe nanoalloy is determined by its internal phase structure. Our experimental and theoretical findings explain how the prevalence of either a core-shell or a disordered solid solution structure is ruled by the target composition and the particle diameter. Furthermore, we found metastable phases not predefined by the bulk phase diagram.

Three-dimensional morphology changes of bimetallic nanoparticles (NPs) with nominal composition Au50Fe50 and Au20Fe80, generated by pulsed laser ablation in liquid, are monitored in situ and ex situ via scanning transmission electron microscopy and electron tomography. The samples are made up of a chemically segregated core–shell (CS) NPs structure, with an Au-rich shell and Fe-rich core, and solid solution (SS) NPs in the pristine state. Further, the examinations reveal information about a sequence of characteristic changes from the pristine metastable and intermediate ultrastructures up to thermodynamically stable products. In the case of the Au20Fe80 sample, a metastable spherical CS morphology is transformed at equilibrium conditions into a cube-shaped Fe-rich core faceted by truncated Au-rich pyramids. For the Au50Fe50 sample, the Au-rich shell is solved into the Fe-rich core, and chemically homogeneous (SS) NPs are formed. Interestingly, this transformation was proven to occur via an intermediate ultrastructure with lamellar segregation, not previously reported as a transient state during in situ heating. On the basis of these observations, a correlation between the composition and the morphology at equilibrium is suggested, in accordance with the bulk phase diagram of Au–Fe. At the same time, our examinations directly prove that laser ablation synthesis creates nonequilibrium NP morphologies, frozen in metastable, spherical core–shell particles.

Bimetallic core−shell nanoparticles (CSNPs), where a ferromagnetic core (e.g., Co) is surrounded by a noblemetal thin plasmonic shell (e.g., Au), are highly interesting for applications in biomedicine and catalysis. Chemical synthesis of such structures, however, requires multistep procedures and often suffers from impaired oxidation resistance of the core. Here, we utilized a one-step environmentally friendly laser ablation in liquid technique to fabricate colloidal Co−Au CSNPs with core−shell yields up to 78% in mass. An in-depth analysis of the CSNPs down to single-particle levels revealed the presence of a unique nested core−shell structure with a very thin gold-rich shell, a nanocrystalline ε-cobalt sublayer, and a nested gold-rich core. The generated Co−Au CSNPs feature soft magnetic properties, while all gold-rich phases (thin shells and nested cores) exhibit a face-centered cubic solid solution with substantial cobalt substitution. The experimental findings are backed by refined thermodynamic surface energy calculations, which more accurately predict the predominance of solid solution and core−shell phase structures in correlation with particle size and nominal composition. Based on the Co−Au bulk phase diagram and in conjunction with previously reported results on the Fe−Au core−shell system as well as Co− Pt controls, we deduce four general rules for core−shell formation in non-or partially miscible laser-generated bimetallic nanosystems.