Impurity diffusion in magic-size icosahedral clusters.

Diana Nelli,Fabio Pietrucci,Riccardo Ferrando

doi:10.1063/5.0060236

Abstract

Atomic diffusion is at the basis of chemical ordering transformations in nanoalloys. Understanding the diffusion mechanisms at the atomic level is therefore a key issue in the study of the thermodynamic behavior of these systems and, in particular, of their evolution from out-of-equilibrium chemical ordering types often obtained in the experiments. Here, the diffusion is studied in the case of a single-atom impurity of Ag or Au moving within otherwise pure magic-size icosahedral clusters of Cu or Co by means of two different computational techniques, i.e., molecular dynamics and metadynamics. Our simulations reveal unexpected diffusion pathways, in which the displacement of the impurity is coupled with the creation of vacancies in the central part of the cluster. We show that the observed mechanism is quite different from the vacancy-mediated diffusion processes identified so far, and we demonstrate that it can be related to the presence of non-homogeneous compressive stress in the inner part of the icosahedral structure.

Highlights

The physical and chemical properties of nanoalloys strongly depend on their chemical ordering, i.e., the pattern in which the two elements are arranged within the nanoparticle
Diffusion processes are enhanced at the nanoscale, we find that the diffusion of the impurity is not activated at room temperature in a time affordable to our simulations
The reconstruction of free energy profiles has not been attempted because our MetaD simulations were unable to reach the diffusive regime. This is due to two reasons: (i) the initial configurations used in the following are largely energetically unfavorable so that returning back is highly unlikely and (ii) there are no constraints forbidding arbitrary deformations of the clusters so that the metadynamics bias, which forces the system to go away from the already visited regions of the collective variables (CVs) space, leads to unphysical cluster shapes

Summary

INTRODUCTION

Bimetallic nanoparticles, i.e., nanoalloys, have recently received considerable attention due to their possible applications in several different fields, such as optics, catalysis, energy storage, biosensing, and nanomedicine. The physical and chemical properties of nanoalloys strongly depend on their chemical ordering, i.e., the pattern in which the two elements are arranged within the nanoparticle. We focus on the very fundamental process of the diffusion in nanoalloys, i.e., the displacement of one atom of species A within a volume occupied by atoms of species B To this end, we use molecular dynamics techniques to simulate the diffusion of a single-atom impurity of either Ag or Au within otherwise pure nanoparticles of Cu and Co, respectively. Our simulations show that the displacement of the impurity within the icosahedral matrix takes place through some unexpected diffusion mechanims, in which vacancies are created in a completely different way compared to what is described in the literature We demonstrate that this new type of diffusion process is due to the peculiar properties of the icosahedral structure

MODEL AND METHODS

RESULTS AND DISCUSSION

Impurity diffusion from the central site of the icosahedral matrix

Impurity diffusion from the second and third shell of the icosahedral matrix

Impurity diffusion from the center to the surface

CONCLUSIONS