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Core-shell nanoparticles with evolving forms

The mystery of the golden pyramids is solved

par Guy Molénat - publié le

Core-shell iron-golf nanoparticles synthesized by UHV growth show an astonishing evolution of their morphology as the amount of gold increases, changing from a polyhedral shape to an iron cube coated by truncated gold pyramids. The origin of this evolution has been elucidated thanks to an experimental analysis, an analytical model and numerical simulations at the atomic scale.

Nano-objects associating a noble metal such as gold (Au) or silver (Ag) and a magnetic metal such as iron (Fe) combine the optical and magnetic properties of both metals and have enhanced magneto-plasmonic properties. Thus, Fe-Au nanoparticles have potential applications in plasmonics, catalysis and in the biomedical field.

We have observed an astonishing evolution of the morphology of Fe-Au core-shell nanoparticles elaborated by a UHV technique. The core (iron) and the shell (gold) have the same polyhedral shape when the thickness of the shell is of the order of 2 to 3 monolayers, then the nanoparticles adopt a regular but more complex shape based on gold pyramids epitaxied on an iron cube when the volume of gold is greater than that of iron [1].


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Evolution de la morphologie en fonction du rapport de volume coquille(Au)/cœur(Fe), observée expérimentalement (haut), prédite par un modèle analytique (milieu) et modélisée par des simulations Monte-Carlo (bas).

These morphologies cannot be predicted intuitively, as iron crystallizes in the bcc phase while the noble metal is fcc. Through an original approach combining experimental analysis, analytical modeling and numerical simulations, we have been able to elucidate this evolution as a function of the volume ratio between the two metals.

We have developed an analytical model using DFT calculations of surface and interface energies as data. This model, adapted to epitaxy on a nano-substrate of variable shape and experimental size (2 nm and more), investigates the competition between three driving forces : wetting, gold surface energy minimization, and interface energy minimization. These driving forces compete or cooperate according to the shell/core volume ratio and to the core size, leading to the different observed equilibrium morphologies [2].

On the other hand, we have developed Fe-Au interaction potentials whose parameters reproduce these driving forces computed by DFT, which allowed us to simulate the shell growth on iron cores of predefined shapes (polyhedron or cube). Although the sizes that can be modelled (of the order of 1 nm) are smaller than the experimental ones, the obtained shell morphologies are very similar to the experimental ones. Furthermore, this approach allows us to predict that for the Fe-Ag couple, other equilibrium shapes will be obtained [3].


References :

[1] Role of the shell thickness in the core transformation of magnetic core(Fe)-shell(Au) nanoparticles, P. Benzo, S. Combettes, B. Pecassou, N. Combe, M. Benoit, M. Respaud, and M. J. Casanove, Phys. Rev. Mat. 3, 096001 (2019) (doi.org/ 10.1103/PhysRevMaterials.3.096001)

[2] Equilibrium shape of core(Fe)-shell(Au) nanoparticles as a function of the metals volume ratio, A. Ponchet, S. Combettes, P. Benzo, N. Tarrat, M. J. Casanove and M. Benoit, J. Appl. Phys. 128, 055307 (2020) (doi.org/10.1063/5.0014906)

[3] How interface properties control the equilibrium shape of core–shell Fe–Au and Fe–Ag nanoparticles, S. Combettes, J. Lam, P. Benzo, A. Ponchet, M. J. Casanove, F. Calvo and M. Benoit, Nanoscale 12, 18079 (2020) (doi.org/10.1039/D0NR04425C)


Magali Benoit, Patrizio Benzo