Abstract
The synthesis of chemically clean and environmentally friendly nanoparticles through pulsed laser ablation in liquids has shown a number of advantages over conventional chemical synthesis methods and has evolved into a thriving research field attracting various applications. While eclipsed by the rapid progress of nanoparticle generation, controlled modification of surface morphology and microstructure through laser processing in liquids has also been demonstrated to be a viable technique. The fundamental understanding of processes leading to the nanoparticle generation and surface structuring, however, still remains elusive. A hybrid computational model combining coarse-grained representation of liquid and a fully atomistic description of laser interactions with metal targets is developed to investigate the microscopic mechanisms of nanoparticle generation and surface structuring in pulsed laser ablation in liquids.
The simulations reveal the critical role of dynamic interactions between ablation plume and liquid environment resulting in multiple nanoparticle generation mechanisms corresponding to distinctively different sizes of nanoparticles. The presence of liquid can effectively decelerate the ejected ablation plume, and the liquid in contact with the plume is heated up to supercritical state leading to a formation of a low-density vapor region (precursor of a cavitation bubble observed in experiments). One of nanoparticle generation mechanisms is nucleation and growth of small nanoparticles (mostly < 10 nanometers) from vapor-phase metal atoms in the low-density vapor region. This nanoparticle generation mechanism is observed consistently regardless target geometry, material type and pulse duration. For short laser pulses (< 10 picoseconds), simulations reveal the formation of a dense molten metal layer at plume-liquid interface due to the conditions of strong stress confinement. The surface of the molten metal layer rapidly develops complex morphological features attributed to the Rayleigh-Taylor instability. A subsequent evolution of hydrodynamic instabilities at the plume-liquid interface can result in the layer disintegration and/or ejection of large (10s of nanometers) nanoparticles into the liquid environment. The computational predictions are supported by a series of stroboscopic videography and double pulse experiments performed by collaborators from the University of Duisburg-Essen, Germany. Two distinctive nanoparticle generations mechanisms responsible for two characteristic nanoparticle sizes can be related to the bimodal nanoparticle size distributions commonly observed in short pulse laser ablation in liquids. The simulations also predict that fast quenching induced by the interaction with liquid environment results in crystallization of nanoparticles on nanosecond timescale. Extending the simulations to longer, sub-nanosecond and nanosecond, laser pulses reveals that “gentler” ablation conditions do not produce the accumulation of a thick molten metal layer at the plume-liquid interface, and the ablation plume remains as a mixture of metal vapor and small droplets confined by liquid. In addition to the aforementioned generation of nanoparticles through the nucleation and growth from metal vapor in the expanding low-density region, the interaction of the ablation plume with liquid environment results in a condensation of a thin transient metal layer at the plume-liquid interface, which ruptures into large nanoparticles (10s of nanometers). The supercritical liquid streaming through ruptured layer deeper into the ablation plume induces rapid cooling and massive nucleation of nanoparticles with sizes mostly less than 10 nm throughout the ablation plume.
Series of simulations are also performed to investigate the potential role of liquid environment in laser-assisted surface engineering. The presence of liquid is found to suppress nucleation of sub-surface voids characteristic for laser processing in vacuum and provide an additional pathway for surface cooling, thus facilitating the formation of nanocrystalline surface layer. In the irradiation regime of photomechanical spallation, the liquid environment is shown to prevent the complete separation of the spalled layer from the target, leading to the formation of large subsurface voids stabilized by rapid cooling and solidification. The subsequent irradiation of the laser-modified surface is found to result in a more efficient ablation and nanoparticle generation, thus suggesting the possibility of the incubation effect in multipulse laser ablation in liquids. Finally, the effect of liquid environment on the recently invented surface nanostructuring technique, single-pulse ablative generation of laser induced periodic surface structures, is also investigated. The presence of liquid induces complex hydrodynamic flow that not only affects the characteristics of the periodic surface structures, but also suggests the possibility of surface hyperdoping through liquid-assisted laser processing.
The initial applications of the hybrid computational model demonstrate its unique capability to reveal microscopic mechanisms responsible for nanoparticle generation and surface structuring. The model can be further applied to facilitate the design of innovative experimental techniques aimed fine-tuning nanoparticle distributions, engineering desirable alloy nanoparticles, and producing surface layers with unusual non-equilibrium structures and phase compositions.
