Oxidation Mapping by Postpeak Technique

Abstract

Department of Physics, University of Alberta, Edmonton T6G 2G7, CanadaSpatially resolved energy -loss near-edge structure (ELNES) reveals geometric and electronic structure of materials at high spatial resolution [1]. In scanning transmission ele ctron microscope (STEM), ELNES allows to study the valence state of atoms at interfaces of a nanocomposite system. The obtainable signal-to-noise ratio (SNR), set by specimen drift and radiation damage, is an obstacle in obtaining interpretable ELNES data at high spatial resolution. The white line ratio (WLR) is often used as a “fingerprinting” technique in identifying the oxidation of transition metals (TMs) [2], but hard to extract when the SNR is low. We recently found that a broad postpeak following the L-edge, observed in thin specimens where plural scattering is negligible, is characteristic of TMs in an oxidized state [3]. The postpeak was applied to identify surface oxidation of nanoparticles (NP) by examining average oxidation state of many NPs [4]. Here we show that the postpeak can be a convenient and sensitive indicator of oxidation of single iron NP by forming point analysis using a STEM with a small (0.5 nm) probe. The measurements were performed on isolated Fe NPs embedded in silica, shown in the high-angle annular dark-field (HAADF) image (Fig. 1a). The data were obtained using a JEOL 2200 FS (a 200 kV Schottky field emission instrument equipped with an W filter). Fig. 1b shows the ELNES of Fe L-edges recorded at different sites of a selected particle, as labelled in HAADF image in Fig. 1a. The signal comes mostly from the surface of the NP when the beam is focused at its edge, and mainly from the interior when the beam is focused at the center. A broad postpeak, about 40 eV above the onset of Fe L