Abstract text (incl. figure legends and references)

Electron energy loss spectroscopy with high energy resolution is interesting for a wide range of applications ranging from analytics of organic matter at very small dimensions to determining the optical properties in two-dimensional materials. In (scanning) transmission electron microscopy ((S)TEM), the energy resolving power of the instrument was determined by the type of the electron source and limited to about 300 meV before the advent of electron monochromators for (S)TEM instruments about two decades ago. In the meantime, several different designs of monochromators have been implemented and can be bought as an option to most high-end microscopes [1‑4].

There are two meaningful positions for monochromators along the optical path: Before or after the accelerator, that is, at high potential or at ground potential, respectively. While most monochromator implementations today are at high potential, ground-potential monochromators have some notable advantages, such as less sensitivity to selection-slit interactions and magnetic stray fields, independence of acceleration voltage instabilities, no performance deterioration from Boersch or Loeffler effect, and the possibility to retrofit it to existing instruments.

One of the major general drawbacks in the use of a monochromator for imaging or spectroscopy is the loss of current in the electron beam, and thus, loss of interpretable signal. Therefore, the most interesting parameter besides the actual energy resolution is the available current at this very energy resolution at a certain primary electron energy.

Here, we present a design for a ground-potential monochromator that can be retrofit to existing microscope columns. Performance estimates are given for primary electron energies of 300 keV and 30 keV with instruments equipped with a cold field emission electron source.

References:

[1] H. Rose, Ultramicroscopy 78, 13-25 (1999).

[2] P. C. Tiemeijer, Inst. Phys. Conf. Ser. 161, 191-194 (1999).

[3] O. L. Krivanek et al., Philos. Trans. R. Soc. London. Sec. A 367, 3683-3697 (2009)

[4] M. Mukai et al., Ultramicroscopy 140, 37-43 (2014).