The influence of fluid nature on femtosecond laser ablation properties of SiO2/Si target and synthesis of ultrafine-grained Si nanoparticles
Nanocrystalline silicon nanoparticles with a median crystallite size of 3-4 nm, several crystalline phases and defects (e.g. twin boundary) were produced by femtosecond laser processing of SiO2/Si target in various organic fluids. Furthermore, nanoscaled amorphous oxide layer and a few atomic layers of graphite shell were detected in ethanol and 2-butanol correspondingly. The ultrafast laser pulses may manipulate nanostructures at the atomic level and generate a high density of defects; this may be correlated to significant thermal stresses on nanoparticles and rapid condensation of primary nanoparticles with high cooling rates. Size distribution width and polydispersity index slightly increased with increasing laser fluence in ethanol. In 2-butanol, the maximum ablation volume was observed. The specific ablation rates in 2-butanol and ethanol were approximately three times higher than n-hexane. The lowest ablation efficiency in n-hexane can be associated with femtosecond laser-induced photolysis and pyrolysis of solvent molecules, as total energy deposition on the material may be reduced due to the formation of carbonaceous products. The roughened zones (average roughness of ~400 nm) in circumferences of the ablated craters in 2-butanol may be related to a correlation between the erosive power of the vapour bubble collapse and higher pressure at the bubble wall in relatively high dynamic viscosity fluids. Furthermore, sputtering of a pristine surface by releasing nanoparticles from the collective collapse of up-flow vapour bubbles can also contribute to the generation of roughened regions.