Fabrication of FeCoNi medium-entropy alloy nanoparticles by high-repetition-rate UV picosecond laser ablation in water

Abstract

Laser ablation in liquid (LAL) is a rapid heating and cooling process that enables the formation of a variety of nanomaterials. Extensive LAL studies for metals, semiconductors, insulators, and carbons have been reported. Further, alloys are worthwhile target to investigate the applicability of LAL. We report the LAL of equiatomic FeCoNi medium-entropy alloy (MEA) in water using a high-repetition-rate UV picosecond laser with a galvanometric scanner. The extinction spectra of the colloidal solution, morphology, elemental composition, and size distribution of MEA nanoparticles (NPs) were investigated as a function of laser-irradiation time, laser power, and scanning speed. Electron microscopy revealed that chain-like superstructures longer than 10 µm consisting of Ni-rich FeCoNi MEA-core Fe-shell spherical NPs of approximately 50-nm diameter were formed. These superstructures were not originated from the NP agglomeration during sample preparation for microscopy or the successive laser irradiation of colloids, but from the overstriking ablation on the target surface. Generally, the LAL target was irradiated by spatially and temporally separated near-infrared or visible laser pulses to efficiently produce NPs. Overstriking of UV ultrashort laser pulses was shown to be a valuable strategy for modifying the elemental composition of MEA NPs and for forming their superstructures.