Towards Fully Quantitative Electron Energy-Loss Spectroscopy with High Spatial Resolution

Electron energy-loss spectroscopy (EELS) is an analytical microscopy technique which measures the energy lost by electrons that have been inelastically scattered as they interact with a thin sample. It yields information on composition and electronic structure but quantification of this information is challenging due to the large dynamic range of the output spectra. Electrons which have lost zero (or very small amounts) of energy saturate the EELS detector whilst inelastically scattered electrons are often hidden in the background spectral noise. An electrostatic fast beam switch (FBS) uses rapid beam blanking to avoid saturation of the detector and allows sequential acquisition of the two signals with an adequate signal to noise ratio. More importantly, the FBS uses the same optical conditions for both types of signal which is essential for quantification studies. Scott et al. [1] showed that full quantification of EELS is possible on a 200kV transmission electron microscope (TEM) using a FBS. In the current research project a modified FBS system has been implemented on a 300kV Titan scanning transmission electron microscope (STEM). The aim of this research is to use recognised quantification techniques to obtain the absolute concentration of atoms in the sample and to investigate the achievable spatial resolution for fully quantitative EELS. The FBS was tested on two systems: an Al / Al2O3 sample and a BaTiO3 / CoFe2O4 interface. Good agreement was achieved between the measured atomic concentrations and their literature values for Al, Ba and Ti. The other elements experienced problems relating to the mean free path and cross-section calculations. The highest spatial resolution achieved in these experiments was 0.6nm. This can be increased further by reducing the electron probe size but other factors, such as electron beam broadening in the sample and delocalisation of the inelastic signal cause the spatial resolution to decrease