Mechanism of Improved Au Nanoparticle Size Distributions Using Simultaneous Spatial and Temporal Focusing for Femtosecond Laser Irradiation of Aqueous KAuCl4

The production of gold nanoparticles (AuNPs) by irradiation of aqueous [AuCl4]- with femtosecond laser pulses is investigated using simultaneous spatial and temporal focusing (SSTF) and compared to the results of conventional geometric focusing (GF). The effects of capping agent, laser power, reaction conditions in the cuvette, and laser chirp are studied, and we find that SSTF produces smaller particles with fewer irregular structures and fewer outlying large particles than GF in all cases except for one, in which the particle size distributions are the same. The difference is primarily ascribed to the intrinsic plasma properties of the two geometries: SSTF produces a plasma that is more homogeneous and spatially symmetric than that of GF, promoting efficient intrinsic mixing of the solution.