Bimetallic nanoparticles offer an additional degree of freedom for modifying their physical properties leading to a wide range of novel applications. We detail here our study on the Cu-Ag system, characterized by a large lattice mismatch (~12%) and non miscibility.  Nanoparticles are prepared under ultra-high vacuum (UHV) conditions, either by pulsed laser deposition (PLD) or thermal evaporation (TE). Cu nanoparticles are first synthesized by condensing Cu vapor onto a thin amorphous carbon film, supported by a TEM grid. Ag is then ablated/evaporated and deposited on the pre-existing Cu nanoparticles. For each experiment, two TEM grid fixed on the same heating support are prepared simultaneously, one with Cu and Ag and the other one with only Cu.

Samples have been prepared by PLD with nominal thicknesses of 2 nm and 1 nm for Cu and Ag respectively, at a substrate temperature of 350°C. Core-shell nanoparticles are obtained with a Cu core and Ag shell. It appears that coalescence of many Cu nanoparticles occurred during the Ag deposition, leading to large Cu core inside a bimetallic core-shell nanoparticle. However, with the same thermal history, this coalescence did not occur on the sample with Cu only, and neither on another test sample prepared with the same total nominal thickness (3 nm Cu total nominal thickness).

We demonstrate that this phenomenon depends on thermodynamics rather than kinetics, by considering the results of UHV annealing of 3 specifically designed samples: 1/ using TE instead of PLD, 2/ using samples with Ag deposited at a lower substrate temperature and 3/ using samples covered by a 3 nm thick amorphous Al₂O₃. For the two first samples, identical core-shell configurations are found, very similar to the initial sample described before. For the last sample with Al₂O₃, almost no microstructural changes are observed.

We finally explain how these results support the thermodynamic hypothesis for the rearrangement of Cu nanoparticles under Ag deposition, even if the equilibrium state is not reached.

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