Ultrafast laser ablation of gold in liquids: Effect of laser pulse overlap-induced surface porosity on size distribution of formed nanoparticles

Pulsed Laser Ablation in Liquids (PLAL) manifested itself as a powerful tool for the synthesis of nanoparticles (NPs) from a variety of materials of high demand in biomedicine. The mechanisms and regimes of nanostructures formation, however, still require clarification in order to better control the final NPs characteristics. Here, we present a numerical study of femtosecond laser-ablative production of metal (gold) NPs in water ambient using an advanced atomistic continuum approach, combining the Molecular Dynamics (MD) and Two Temperature Model into the frames of a single MD-TTM computational method. The model describes non-equilibrium laserinduced phase transitions at atomic level and accounts for the effect of free carriers in continuum. With the MDTTM model we investigated the effect of slight porosity arising due to a partial overlap of laser craters during the scanning of laser beam over the target surface under a high repetition rate of laser pulses. For that purpose, we perform a simulation of 270 fs laser pulse interaction with solid and porous gold targets at the incident fluence of 2.5 J/cm2. The obtained results revealed the manifestation of different regimes of ablation and different yield of the obtained NPs. The simulation results are compared with the corresponding experimental data. We found that depending on the scanning speed the ablation can follow thermal and spallation mechanisms of the material ejection, which are responsible for the appearance of fine and course populations of NPs correspondingly. In the case of clean target's surface ablation, when at high scanning speed of 3840 mm/s each pulse hits a fresh area, the material ejection is governed by both mechanisms, which leads to the appearance of bimodal population of NPs. Alternatively, a moderate scanning speed of 840 mm/s results in a partial overlap of the sequential laser spots on the surface and generated slight porosity removes the accumulation of laser-induced stresses. The spallation mechanism of the material ejection is therefore suppressed, which results in generation of a monomodal fine population of NPs with a 20 times larger yield. The obtained data are of importance to predict and control size characteristics of laser-synthesized nanomaterials.