Nanoparticles associating a noble metal and a ferromagnetic metal are appealing from a magneto-plasmonics point of view, in addition to the problematics of magnetic anisotropy tailoring (interface anisotropy, phase transformation) and of nanoalloy original geometries. Because Co and Au are immiscible in the bulk phase, and since fcc cobalt and gold have highly different cell parameters, chemically separated structures (core-shell type) are expected for nanoparticles.

We have studied CoAu cluster assemblies, with a diameter between 3 and 10 nm, prepared by low energy cluster beam deposition (LECBD) where nanoparticles are formed in out-of-equilibrium conditions by laser vaporization, then deposited on a substrate under ultrahigh vacuum conditions and protected by a capping layer (amorphous carbon, to avoid oxidation). The nanoparticles’ structure and chemical arrangement (see figures) have been investigated by HRTEM, STEM-HAADF and STEM-EELS before and after annealing (2h around 500°C). A mapping of the low energy electronic excitations has also been performed by STEM-EELS, which is a challenge on such small nano-objects.

As prepared particles are found to be inhomogeneous (as deduced from EELS measurements), with interatomic distances always corresponding to pure gold, and they appear to be surrounded by a shell of lighter HAADF intensity which rapidly transforms upon electron beam exposure in STEM. After annealing, a phase separation is observed and CoAu nanoparticles adopt a core-shell structure where, as observed by HRTEM (with a clearly visible difference of inter-plane distances between Co and Au regions), STEM-HAADF and STEM-EELS, an off-centered cobalt core is surrounded by a gold shell (see figure). For both as-prepared and annealed nanoparticles, the Co is not oxidized thanks to the efficient protection of the thin carbon layer. Using low-loss STEM-EELS mapping, we are able to observe some surface contributions, which may reflect collective electronic resonances (surface plasmons), with significant differences between as-prepared and annealed CoAu nanoparticles (in particular, an intense peak is detected on the gold-rich regions, around 6 eV).

These results shed light on the atomic-scale behavior of the Co-Au nanoalloy, here in an amorphous carbon matrix, which will be compared to other dielectric matrices, and will help us to understand the original magnetic properties of these hybrid nanoparticles. 

Figures:

HRTEM image of an as-prepared CoAu nanoparticle (a) and of an annealed particle (b), where two distinct zones are visible. Elemental mapping of Co and Au from STEM-EELS measurements (Co L and Au M intensity maps, corresponding to the blue and yellow colors), for an as-prepared CoAu nanoparticle (c) and an annealed particle (d). EELS spectrum for annealed CoAu nanoparticles embedded in carbon, measured at different locations (indicated on the HAADF image on the right), and maps corresponding to three different windows of energy, where a surface and volume contribution can be distinguished.

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EMC Abstracts - https://emc-proceedings.com/abstract/chemical-arrangement-and-surface-effects-in-coau-nanoparticles/