Non-miscible bimetallic systems have been widely studied, in the bulk state and more recently in thin film configurations. Cu-Ag is one of these face-centered cubic systems, allowing probing the effects of lattice mismatch and surface and interfacial energies. It is however new to consider the same kind of system in the nanosize range. We can wonder whether the same growth mecanisms apply when the substrate is now a cluster rather than an infinite surface. We detail here our study on the Cu-Ag system, characterized by a large lattice mismatch (~12%) and a non-alloying tendency in the bulk state, through the use of Transmission Electron Microscopy (TEM) techniques corrected from lens aberrations.

Nanoparticles are prepared under ultra-high vacuum conditions by pulsed laser deposition. Cu nanoparticles are first synthesized by condensing Cu vapor onto a thin amorphous carbon film supported by an electron microscopy grid. Ag is then ablated/evaporated and deposited on the pre-existing Cu nanoparticles.

It is worth noting that for all the nanoparticles characterized, Cu and Ag are in a perfect epitaxial relationship with all crystallographic directions corresponding. Moreover, we observe interfacial dislocations accommodating the lattice misfit (a_Cu=0.3615 and a_Ag = 0.4086). The spacing between the dislocations corresponds nicely to the theoretical one. The other striking result is the observation of Ag nanoislands on the surface of the particles. Since the STEM technique can be thickness sensitive, we believe that in the present case the contrast corresponds to a “low resolution” topographic map somewhat identical to what an STM image would produce. A triangular superstructure is clearly distinguishable on the surface of the particle, that corresponds very likely to the 9x9 Ag on Cu₍₁₁₁₎ superstructure. 

We conclude by underlying the similarities observed for the Cu-Ag system between thin films and nanostructures configurations.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/18458 must be provided.

1. Growth of CuAg core-shell nanoparticles: silver-enhanced cluster diffusion on amorphous carbon
2. Inside the Moiré Patterns: a high resolution TEM study