Characterising the surface and interior chemistry of core–shell nanoparticles using scanning transmission electron microscopy

A method for extracting core and shell spectra from core–shell particles with varying core to shell volume fractions is described. The method extracts the information from a single EELS spectrum image of the particle. The distribution of O and N was correctly reproduced for a nanoparticle with a TiN core and Ti-oxide shell. In addition, the O distribution from a nanoparticle with a Cu core and a Cu-oxide shell was obtained, and the extracted Cu L2,3-core and shell spectra showed the required change in EELS near edge fine structure. The extracted spectra can be used for multiple linear least squares fitting to the raw data in the spectrum image. The effect of certain approximations on numerical accuracy, such as treating the nanoparticle as a perfect sphere, as well as the intrinsic detection limits of the technique have also been explored. The technique is most suitable for qualitative, rather than quantitative, work.

Research Highlights► A new method for extracting core and shell EELS spectra from a core–shell nanoparticle is developed. ► The technique utilises spectrum imaging in a scanning transmission electron microscope (STEM). ► The composition and bonding environment at the core and shell regions can be analysed.► The method is applied to TiN/Ti-oxide and Cu/Cu-oxide core–shell nanoparticles.► Numerical accuracy and intrinsic limits of the technique are also discussed.