Spectroscopic coincidence experiments at nanoseconds time resolution in TEM

Abstract text (incl. figure legends and references)

Electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDX) are two frequently used analytical techniques in transmission electron microscopy. Both techniques rely on the inelastic scattering of the incoming electron with the core level states of an atom to determine the chemical composition of the material. The two techniques differ with respect to which particle is detected. For EELS the energy of the inelastically scattered electron is measured and in EDX the energy of an emitted X-ray, which is created during the decay of the excited atom, is detected. Hence, for every X-ray emitted there was at least one electron that gave part of its energy to excite the atom which could give the impression that they convey very similar information. However, the energy resolution for EDX is typically 100 times worse compared to EELS due to the detection mechanism of X-rays (100 eV). On the other hand, EELS has a low signal-to-background ratio, (SNR) making it hard to detect low concentration elements. Since these two techniques arise from the same physical process, it should be possible to find a correlation in time between such an inelastic electron and x-ray. By only selecting an electron when an X-ray is detected nearby in time Kruit el al. [1] were able to detect this time correlation between the electrons and X-rays.

Since individual electron and X-ray events need to be detected at nanosecond time resolution, a dedicated setup needed to be developed. In the present work, the current setup will be described where we were able to get a temporal resolution of ~250 ns without filtering events on the hardware level. Hence the conventional EELS and EDX signals are retained. Furthermore, we will show how by applying such a time correlation constraint it is possible to remove the background signal in EELS (see Fig.1) and obtain EDX spectra without having any prior knowledge on the shape of this background [2]. This is especially valuable for EELS where conventionally an empirical power-law function is used to fit the background where a pre-edge is needed to extrapolate a fitted background signal. Additionally, it will be shown how one can determine the collection efficiency [JV1] of both EELS and EDX without having the need for a reference sample and knowledge on beam current. Finally, it is discussed how using the time correlation between EELS and EDX can improve the signal-to-noise ratio for trace elements where the signal-to-background is too low for EELS and the energy resolution for EDX is insufficient to discriminate between overlapping x-ray peaks.

[1] P. Kruit et al. Detection of X-rays and electron energy loss events in time coincidence. Ultramicroscopy, 13(3):205-213,1984.

[2] D. Jannis, K. Müller-Caspary et al., Spectroscopic coincidence experiments in transmission electron microscope, Appl. Phys. Lett. 114, 143101 (2019)

[3] D. Jannis et al., Coincidence Detection of EELS and EDX Spectral Events in the Electron Microscope, Appl. Sci. 2021, 11, 9058

[4] D.J., A.B. and J.V. acknowledge funding from the Flemish Research Fund FWO under project no. G093417N and G042920N, J.V. acknowledges funding from No 823717 ESTEEM3[JV1] , K.M.-C. acknowledges funding from the FWO under contract G042920N