Valence state map of iron oxide thin film obtained from electron spectroscopy imaging series

This paper demonstrates the applicability of electron-spectroscopic imaging (ESI) for valence-state mapping of the iron oxide system. We have previously developed a set of signal-processing methods for an ESI series, to allow mapping of sp²/sp³ ratio, dielectric function and energy bandgap. In this study, these methods are applied to generate a valence-state map of an iron oxide thin film (Fe/α-Fe₂O₃). Two problems, data undersampling and a convolution effect associated with extraction of the image-spectrum from the core loss image series, were overcome by using cubic-polynomial interpolation and maximum-entropy deconvolution. As a result, the reconstructed image-spectrum obtained from the ESI series images has a quality as good as that of conventional electron energy-loss spectra. The L₃/L₂ ratio of the reconstructed ESI spectrum is determined to be 3.30 ± 0.30 and 5.0 ± 0.30 for Fe and α-Fe₂O₃, respectively. Our L₃/L₂ ratio mapping shows an accurate correspondence across the Cu/Fe/α-Fe₂O₃ region. The effect of delocalization and chromatic aberration on the ESI resolution is discussed and estimated to be about 2 nm for the case of L₃/L₂ ratio mapping.