Heat-induced alloying in individual Au@Ag core-shell nanoparticles as a function of size, shape, and presence of defects

Abstract text (incl. figure legends and references)

The recent and ongoing development of advanced colloidal synthetic techniques has allowed scientists to routinely produce bimetallic nanoparticles of various shapes, sizes, composition, and elemental distribution. Novel synthetic routes enable precise control of specific optical properties based on surface plasmon resonance. Thus, nanoplasmonic-based materials are of great interest for the utilization in biosensing, photocatalysts, medicine, data storage, etc. However, operation in real conditions (e.g. at elevated temperatures) may cause particle reshaping and redistribution of metals between the core and shell of the particle, which in turn gradually alter the properties of nanoplasmonics. Hence, a deeper understanding of the influence of the size, shape, and presence of defects on the nature of such processes is essential for the further development of nanoplasmonic-based technologies.

The recently developed combination of fast tomography based on High Angle Annular Dark Field Scanning Transmission Electron Microscopy with in situ heating holders has enabled the investigation of heat-induced processes at the single nanoparticle level with high spatial resolution in 3 dimensions (3D). Using this approach, we evaluated the influence of various parameters (size, shape, defect structure) on heat-induced elemental redistribution in Au@Ag core-shell nanoparticles qualitatively and quantitatively.

In more detail, the elemental redistribution at high temperature in single-crystalline Au@Ag nanocubes with similar size and composition, but with different shapes of the core (truncated octahedra and sphere), was shown to be uniform along all directions (Figure 1c-d, upper rows). By performing 3D simulations of diffusion based on Fick"s law, similar diffusion constants were found (Figure 1a-b and Figure 1c-d, lower rows). Additionally, our investigation indicated faster alloying kinetics for pentatwinned Au@Ag nanorods in comparison to single-crystalline nanorods of comparable sizes and composition. This may be related to the presence of twinning planes, which cause the formation of distortions and vacant sites in the crystal lattice, facilitating diffusion of atoms. Finally, the influence of the core shape was demonstrated to be negligible in the case of two pentatwinned nanorods with different Au cores (rod and bipyramid).

In conclusion, using our fast tomography approach we gained a fundamental insight into the nature of elemental diffusion at high temperature and its dependence on various factors, e.g. size, shape, and presence of twinning defects. We believe, that detailed knowledge of thermal stability and/or thermal degradation of bimetallic nanoplasmonic materials will drastically improve the application of these materials in the future.

The project has received funding from European Research Council (ERC Consolidator Grant 815128, REALNANO) and European Commission (grant 731019, EUSMI).

Figure 1. (a) – (b) Comparison of the alloying progress of single-crystalline nanocubes (with octahedral and spherical core, respectively) to diffusion simulations performed for different diffusion coefficients. (c) – (d) The upper rows show slices through the experimentally determined 3D elemental distribution at different stages of alloying of single-crystalline nanocubes (with octahedral and spherical core, respectively). The lower rows display slices through the simulated 3D elemental distribution using the optimal diffusion coefficient.