No need to hit the iron while it’s... small!

14/01/2025 Scientists from CNRS have studied the shape evolution of gold nano-tetrapods and revealed a two-step kinetics process: the four arms—initially sharp—quickly blunt, followed by a slow retraction towards an almost spherical final shape. Deforming macroscopic metallic objects requires considerable force. At the nanoscale, however, things are quite different: surface forces alone are often sufficient to reshape nanoparticles, guiding them towards the most energetically favorable form. Understanding this "nano-metallurgy" is crucial not only for advancing fundamental knowledge but also for driving progress in nanotechnology. Yet, the processes involved are challenging to monitor in real-time. A team of researchers from several French and European laboratories [1] recently published a detailed study in Nano Letters on the transformation pathway of metastable gold particles, initially featuring four sharp arms, into nearly spherical particles. Thanks to their well-defined geometry, these "nano-tetrapods" are easier to study than "nanostars," which have similar characteristics but more varied morphologies. By combining electron microscopy, X-ray scattering, optical spectroscopy techniques, and advanced data processing, the researchers uncovered a complex kinetic process. This begins with a rapid flattening of the tips towards a {110} facet of gold’s crystalline lattice, followed by a slower reshaping phase into an isometric form. Beyond the conceptual understanding of their evolution, this study will enable the use of these nano-objects in nonlinear optical applications [2], thanks to their unique shape, which lacks a center of inversion. [1] Damien Alloyeau at Laboratoire Matériaux et Phénomènes Quantiques, (Université Paris Cité/CNRS), le Laboratoire de physique des solides (CNRS/Université de Paris-Saclay), Doru Constantin at l'Institut Charles Sadron (CNRS/Université de Strasbourg) and the European synchrotron ESRF (Grenoble). [2] Jieli Lyu et al., Shape-Controlled Second-Harmonic Scattering from Gold Nanotetrapods, J. Phys. Chem. C 126, 9831-9835 (2022). Reference : Jieli Lyu et al., Two-step reshaping of acicular gold nanoparticles, Nano Letters (2025). DOI: 10.1021/acs.nanolett.4c05601