In-depth profile analysis of oxide films by radiofrequency glow discharge optical emission spectrometry (rf-GD-OES): possibilities of depth-resolved solid-state speciation

Abstract

Radiofrequency glow discharge (rf-GD) coupled to optical emission spectrometry (OES) provides depth-resolved elemental composition information on a wide variety of sample types. However, quantitative studies in protective coating materials (e.g. oxides, nitrides, etc.) are severely limited by the lack of certified standards containing known amounts of oxygen and nitrogen. The potential of rf-GD-OES for quantification of oxide films has been investigated in this work using an alternative quantification methodology with correction for dc bias voltage. Oxygen calibration curves were obtained by combining two types of standards; bulk certified reference materials and well-characterised coated calibration samples. Three oxide films, including anodic alumina and iron oxide films were studied at 700 Pa of pressure and 30 W of forward power. The qualitative and quantitative in-depth profiles proved satisfactory results for depths and concentrations, in agreement with nominal values. In addition, in order to evaluate the possibilities of solid-state speciation with rf-GD-OES, the metal to oxygen ratios (Al/O for the anodic alumina film and Fe/O for the iron oxide films) in the quantitative depth profiles were investigated. Preliminary results for the oxidation states of aluminium and iron were properly compared with X-ray diffraction (XRD) measurements, demonstrating the excellent capabilities of the GD-OES technique. Further possible improvements are discussed and it is believed that the proposed speciation strategy holds a great deal of promise in the solid-state speciation area.