Evolution of the composition of nanoparticles formed by the nanosecond Nd:YAG laser irradiation of an aluminium target in N2–O2 gas mixtures

Laser ablation of metallic targets in gas atmospheres gives rise to the formation of nanoparticle (NP) powders when high laser irradiances (i.e., > 1 GW/cm² for aluminium targets) are used. The properties of the NPs so formed vary as a function of the laser irradiation parameters and the pressure and composition of the ambient gas. Here, we studied the composition and structure of NPs formed by laser ablation of Al targets with a nanosecond Nd:YAG laser emitting at 1064 nm. The laser irradiance was 2.5 GW/cm2. N2–O2 gas mixtures containing different amounts of nitrogen (100–80 vol% N2) were used as ambient gas at atmospheric pressure. The influence of the ambient gas composition on the morphology and the structure of the NPs was studied ex-situ by scanning and transmission electron microscopy, X-ray diffraction and micro-Raman spectroscopy. Their composition of light elements was determined by ion beam analysis. It was shown that together with metallic aluminium, different amounts of aluminium nitride and aluminium oxynitride were formed in the NPs as a function of the nitrogen concentration in the ambient gas. Below 95 vol% N2, the concentration of nitride phases decreases strongly. However, the formation of aluminium oxide (Al2O3) was not observed, whereas it was detected in NPs formed in air. A discussion of these results that takes into account the dynamics of the plasma plume is proposed.