Direct electron detection for atomic-resolution spectroscopic mapping under cryogenic and signal-limited conditions.

The expansion of spectroscopic mapping by STEM-EELS to cryogenic temperatures opens the door to new experiments across many fields including materials physics, biological systems, and solid-liquid interfaces. Such experiments, however, often face signal limitations due to sample sensitivity or acquisition time. Compared to traditional indirect detection systems such as charge coupled devices (CCDs), direct electron detectors (DEDs) offer improved detective quantum efficiency, narrower point spread function, and superior signal to noise ratio. Here, the performance of a Gatan K2 Summit DED is compared to a Gatan UltraScan 1000 CCD for use in signal-limited EELS experiments. Due to improved point spread function of the DED, energy resolution remains comparable to the CCD at 5 times lower dispersion, providing simultaneous access to a much broader total energy range. Furthermore, the benefits of DED enable a variety of low-signal experiments, including rapid atomic-resolution mapping of minor and high energy edges. For low per-pixel dwell time, elemental maps acquired by direct detection show increased atomic column contrast compared to those acquired with the CCD with lower acquisition dead time. Taking advantage of these performance improvements and the rapid per frame readout time, we demonstrate EELS atomic-resolution elemental mapping at cryogenic temperatures.