Role of the shell thickness in the core transformation of magnetic core(Fe)-shell(Au) nanoparticles

Abstract

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 probecorrected 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.

Equilibrium shape of core(Fe)–shell(Au) nanoparticles as a function of the metals volume ratio

“…These two morphologies are highly reproducible and several NPs of various nanometric sizes similar to the ones displayed in Fig. 1(a) and 1(b) can be found in [7] (and supplemental material) and in [9]. The two morphologies are not size distributed, but NPs with well-formed pyramids as in Fig.…”

“…This contrasts with Fe@Au NPs, a study case where the Fe core and the Au shell crystallize respectively in the body-centered cubic lattice (bcc) lattice and facecentered cubic (fcc) lattice. Whereas remaining in a core-shell geometry, both core and shell morphologies of Fe@Au NPs strongly evolve as a function of the respective volumes of the two metals [7]. In this article, we aim at uncovering the mechanisms responsible for this important morphological evolution.…”

“…In this article, we aim at uncovering the mechanisms responsible for this important morphological evolution. As for some other nanostructures [8], the continuous approach adopted here provides a theoretical frame to explore the energy landscape as a function of the core and shell morphologies for NPs of any size, particularly in the typical experimental range (from 1 to 50 nm [7,[9][10][11]), and as such is a complementary approach to atomistic simulations.…”

“…In previous studies [7,9] nanometric AuFe based NPs were formed at high temperature (600°C -800°C) in an ultra-high vacuum growth process by the sequential deposition of Fe then Au. They exhibit a regular core@shell shape where Fe forms a single crystal with a polyhedral shape, entirely covered by a polycrystalline Au shell.…”

“…The {001}Au/{001}Fe interface [7] presents a perfect epitaxial relationship thanks to a 45° rotation of {001}Au with respect to {001}Fe (Fig. 1(c)).…”