Self-organization mechanisms in a Fe-Au film: from isolated core-shell to multicore nanoparticles

Abstract

Many nanotechnological applications necessitate a high density of nanoparticles (NPs), making NP morphology control highly challenging. In this work, the morphology of bimetallic NPs formed by magnetron sputtering deposition of a Fe(3nm)-Au(2nm) bilayer film on an amorphous silica substrate is analyzed using high-angle dark-field scanning transmission electron microscopy (HAADF-STEM). While all the NPs adopt a Fe-Au core-shell chemical order, they can be sorted into three different types. Isolated NPs, displaying either a highly symmetric centered core geometry (CC-type) or an asymmetrical off-centered core geometry (OC-type), are observed despite the rather large metallic volume. The majority of the NPs however displays a multicore geometry, with Fe cores in the 10-12 nm range, larger than the NPs observed in a pure Fe (3nm) film grown under identical conditions. The driving forces leading to the different morphologies are discussed together with the role played by the environnement.

Morphology and symmetry driven by lattice accommodation in polycrystalline bcc–fcc core–shell metallic nanoparticles

“…its interaction with the oxide substrate used in the growth process is not considered here. This is experimentally justified as the adhesion of the NP on the substrate is particularly weak and does not inhibit the shell development at the interface between the NP and the oxide substrate [30]. (ii) We can reasonably assume that the boundaries joining three adjacent <111>fcc oriented grains (not experimentally accessible) do not modify the symmetry.…”

“…Once the interface is covered by several atomic layers of shell, the variant distribution is considered as frozen, due to the very huge energy barrier to be crossed to reverse a variant. However, in our growth conditions close to equilibrium [20,22,30], the easy surface diffusion of Au and Ag atoms allows the rearrangement of the shell surface to minimize the excess energy coming from surfaces. We then do not treat here of a possible differential growth rate during the growth.…”

“…Once formed the NPs are protected by a 5 nm thick amorphous silica cover layer deposited at room temperature. This procedure is the same as the one described for Fe@Au NPs in [26,30].…”

“…2(a)) reported in Fe@Au [22,26] and in CoFe@Au NPs [24], morphologies with lower symmetries were also observed. In a recent experiment [30], the Fe@Au NPs formed with a larger core (in the range 10-20 nm) were no longer centered (so called off-centered NPs). The large cores themselves were no longer completely cube-shaped (that can be accounted for by kinetic limitations preventing the interface to reach its equilibrium shape), but the strong dissymmetry of the shells was even more dramatic (Fig.…”

“…2. Morphologies of Fe@Au NPs displaying (a) a cubic symmetry with a centered core or (b) an offcentered core and a strongly dissymmetrical shell [20,22,26,30]. The AuAg alloy is another candidate to produce core@shell NPs with Fe.…”