Synthesis of Gold Nanoparticles in Liquid Environment by Laser Ablation with Geometrically Confined Configurations: Insights to Improve Size Control and Productivity

Laser ablation of solid targets in liquid environment allows the generation of nanoparticles (NPs) with several useful properties such as high purity, easily functionalizable surface, metastable composition, or complex structure, including doped nanocrystals, core shells, hollow microspheres, nanotruffles, or nanocrescents. However, the mechanisms of NPs formation is still not well understood, and challenges remain in size control and productivity. Here, we investigate how the asymmetry intrinsic of laser-matter interaction can influence the structure and yield of gold NPs produced with nanosecond pulses. In particular, we confined the geometry of the laser ablation configuration in three ways: by reducing the thickness of the solid target from bulk size to a few tens of nanometers, by reducing the size of the laser spot on the solid target, and finally, by reducing the lateral size of the bulk target. The interpretation of results was supported with numerical simulations of heat distribution inside the metal target in the three configurations. Surprisingly, we found that only the average size of NPs is affected by target thickness, whereas NPs polydispersity is reduced by confining the ablation geometry in transversal direction to the light propagation axis, that is, by decreasing transversal target size or laser spot size. In addition, we observed a strong dependence of yield versus target thickness, suggesting that targets below similar to 0.1 mm should be avoided for optimal ablation rate. Taken together, these findings indicate that NPs formation mechanism changes with the depth of the ablated layer inside the bulk target and with the spatiotemporal temperature gradient in the material. By adding another piece to the puzzle of laser ablation synthesis in liquid solution, this study provides useful indications to improve the size distribution and productivity of laser-generated NPs.