Optically Driven AuCu Alloying in Single Nanoparticles Revealed by In Situ TEM

AuCu alloys are versatile materials whose physical and optical properties can be finely tuned through composition, making precise control over the Au/Cu ratio highly desirable. Here, we demonstrate that pulsed laser excitation provides a promising route to dynamically drive and control alloy formation in single nanoparticles. We track the structural and compositional evolution of individual Au@TiO₂ core-shell nanorods under optical excitation in situ using transmission electron microscopy combined with picosecond pulsed laser illumination. Laser irradiation mobilizes copper-containing species from the carbon support, which migrate to the nanorod surface and alloy with gold to form a homogeneous AuCu alloy. The copper fraction can be tuned with laser fluence, reaching values above 80%, before the local temperature exceeds the threshold for copper sublimation and a gold-rich structure is partially restored. Time-resolved diffraction shows that the process is reversible but kinetically asymmetric: incorporation is rapid, while the slower evaporative removal allows the copper fraction to be tuned more finely on the return path. These results demonstrate that, under pulsed laser excitation, nanoparticle composition can be actively driven, observed, and frozen on demand at the single-particle level through control of laser fluence and illumination time.