On the relationship between oxygen K edge fine structure and cobalt/nickel content in magnetite-based core-shell nanoparticles

Iron oxide nanoparticles (NPs) are used in a broad range of applications and offer the possibility of fine tuning their properties by forming hybrid core-shell structures. Scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) is frequently used to study such NPs at the local scale. Specifically, the energy loss near edge structure (ELNES) provides rich information on the local chemistry. Here, the dependence between the ELNES signal of the oxygen K edge and the incorporation of cobalt and nickel into the shell of magnetite NPs is investigated quantitatively using an intensity ratio between specific energy windows of the O-K edge. The analysis shows that both the local and global cobalt and nickel content of a NP influence this relationship. In addition, the overlap between the Fe-L₁ and Ni-L_2,3 edges is studied, the impact of a supporting film on the ELNES analysis is discussed, and the importance of a clean sample preparation is demonstrated.