Role of the Shell Thickness in the Core Transformation of Magnetic Core(fe)-Shell(au) Nanoparticles

Fe-Au core-shell nanoparticles embedded in an amorphous alumina matrix are synthesized at high temperature using magnetron sputtering. The nanoparticles display a single-crystal Fe core covered by a crystalline Au shell epitaxially grown on the different core facets. The morphology of the grown nanoparticles is analyzed by transmission electron microscopy, while their structural details are resolved at the atomic scale using probe-corrected high-angle annular dark-field scanning transmission microscopy. Two different epitaxial relationships at the Au/Fe interface are observed at low Au coverage, whereas only one type of interface orientation remains when the shell thickness exceeds 3 monolayers (MLs). This leads to a drastic Fe core transformation from a Wulff shaped crystal towards a nanocube. This core transformation drives a surface reconstruction resulting in a combination of open and close-packed strain free facets, offering different opportunities for molecule binding. In addition, our experiments show that the magnetic properties of the Fe core are preserved by the Au shell and that the 10 nm size of the grown nanoparticles favors a superparamagnetic behavior, suitable for biomedical applications.