Influence of synthesis parameters on characteristics of silver nanoparticles obtained by laser ablation of metal in liquid

The ablation of silver in water by high-power laser pulses of nanosecond duration is investigated. It is shown that the resulting silver nanoparticles (NPs) have a multimodal size distribution. A detailed analysis of the surface plasmon resonance (SPR) spectra of the obtained NPs makes it possible to isolate 3 fractions with an average size of 5, 50, and more than 100 nm. It is assumed that particles with a diameter of more than 100 nm are formed as a result of the removal of the liquid-droplet phase directly from the target surface due to the hydrodynamic mechanism. Particles with sizes of 5 and 50 nm are formed due to condensation at different stages of cooling of the vapor-plasma formation. The proportion of these fractions in the silver sol is determined by the effective laser power density on the target surface. It has been established that, at the laser power density of more than 3.5×10⁸ W/cm², the ablation process is significantly affected by the screening of laser pulse by a vapor-plasma formation near the target surface. With an increase of the synthesis duration the processes of interaction of laser radiation with a liquid medium begin to affect due to an increase in the optical density of the resulting silver NPs sol. As a result, a decrease of the laser pulses energy occurs, leading to a decrease in the ablation efficiency. On the other hand, under the action of laser radiation, fragmentation of large NPs occurs, and the optical transparency of the sol in the near-IR range increases. The competitive course of these two processes leads to a stepwise dependence of the ablation rate on the duration of synthesis. It is shown that laser pulses irradiation of silver NPs sol after the end of the synthesis process can significantly narrow the particle size distribution and increase the stability of the resulting suspension. (authors)