Effects of ambient gas and laser fluence on the compositional changes in iron oxide particle aggregated films prepared by laser ablation

Abstract

Iron oxide nanoparticle aggregated films were prepared using the excimer laser ablation technique by adopting an off-axis configuration and a gas condensation process. Sintered iron oxide targets (αFe₂O₃) were ablated in Ar, O₂, He, N₂, Ne and Xe by an ArF excimer (193 nm) under various pressures. It was found that both the ambient gas and its pressure strongly affected the composition of the deposition product. Depending on the ablation parameters, the product of ablation was comprised of Fe₂O₃ or a mixture of Fe₂O₃ and FeO. Ablation in argon at the lowest fluence of 4.9 J/cm² led to deposition of two oxides, FeO and Fe₂O₃ in the entire pressure range tested. The most pronounced change in the relative amounts of these phases was observed in the 13.3–350 Pa pressure range. Changing the Ar pressure from the lower to the upper limit of this range varied the product chemistry from being close to a single (Fe₂O₃) phase to being two phase (Fe₂O₃+FeO). Ablation experiments in all the background gases used showed that when the irradiation fluence was sufficiently high the deposition became stoichiometric (Fe/O ratio the same as that of the target) within a pressure range specific to each gas. With further increase in fluence, the pressure ranges within which the deposition was stoichiometric became broadened. Changes in the product chemistry were analyzed in terms of the energetics of the ablated species. For monoatomic gases, the maximum pressure (P_S (X)) at which the product was still a single Fe₂O₃ phase decreased monotonically with increasing atomic mass of the gas at lower fluence, and at higher fluence this dependence became non-monotonic. Diatomic gases, such as N₂ and O₂, were more efficient in cooling down the evaporated species, leading to non-stoichiometric deposition even at lower pressures.