This is the second in a series of three papers examining nanomaterial formation in laser ablation in liquids (LAL). Here we study the effect of the laser wavelength and fluence on the mass yield and size distribution of nanoparticles prepared by laser ablation of a platinum target immersed in water. For all wavelengths tested, laser fluences in the range of 10–70J∕cm2 resulted in spheroidal, nonagglomerated platinum nanoparticles with sizes ranging from 1to30nm. Nanoparticle size distributions are found to be composed of two modes that are attributed to thermal vaporization and explosive boiling mechanisms. The peak of the smaller size mode remains nearly constant at 3nm for all laser conditions, which is suggested to be due to the strong confinement of the vapor plume by the liquid. The larger size mode peaks in the range of 5–15nm with a population that is strongly dependent on the laser parameters. It is concluded that changes in the mean size reported in many earlier studies on LAL of metal targets are a result of the relative quantity of nanoparticles from each mechanism rather than direct control over the ablation process. Additionally, it was observed that the yield of platinum nanoparticles was significantly larger for 1064nm wavelength at fluences greater than 10J∕cm2. The maximum ablation rate was approximately 4.4mg∕h, with an estimated ablation and collection efficiency of 0.9μg∕J. Dependence of the mass yield on wavelength and fluence is seen to be dependent primarily on the extent of the explosive mechanism.
This is the first in a series of three papers aimed at better understanding the processes that lead to nanomaterial formation during laser ablation of solid targets in liquids. Here we study the variation of the target surface morphology versus laser fluence and wavelength in order to suggest an ablation mechanism. A key finding is that an explosive ablation mechanism is prominent for a wide range of laser fluences for all wavelengths tested. Interestingly, however, ultraviolet (355nm) and infrared (1064nm) wavelengths show characteristically different explosive behaviors. In the infrared case, numerous large craters with diameters around 20μm form at localized points within the laser irradiated area. In contrast, ultraviolet ablation results in a striking transition to nanoscale surface roughness across the entire irradiated area. This texture is attributed to spinodal decomposition at the molten target surface. We propose that the wavelength and fluence dependence of the ablation craters can be explained by the amount of energy absorbed in the target. The consequences of the ablation mechanism for nanomaterial synthesis are discussed.
This is the third paper in our series studying the laser-target-liquid interactions occurring in laser ablation in liquids (LAL). Here, laser ablation of a platinum target in pure water at 355nm wavelength is studied as a function of laser energy. We describe three distinct reaction regimes between the ablated target species and water at different laser focusing conditions. At low laser fluence (<10J∕cm2), material removal is caused by laser heating of the platinum surface and the primary products are small clusters with a large percentage of platinum atoms in a nonzero oxidation state. At intermediate fluences (10–70J∕cm2), platinum nanoparticles are the primary products. Our previous studies demonstrated that in this fluence regime ablation occurs through both thermal vaporization and explosive ejection of molten droplets. In both cases reactivity is small due to the low reactivity of platinum with water. At high fluences (>70J∕cm2), we find large, faceted particles that are attributed to the drying of PtOx gels formed by reactive plasma etching of the target. Taken together these results demonstrate that significant tunability in the target-liquid interaction is possible during nanomaterial synthesis by LAL.
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