Atomic-resolution elemental mapping at cryogenic temperatures enabled by direct electron detection

Abstract

Spectroscopic mapping by scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) is a powerful technique for determining the structure and chemistry of a wide range of materials and interfaces. The extension of this technique to cryogenic temperatures opens the door to new experiments across many fields including materials physics, energy storage and conversion, and biology. Such experiments, however, often face signal limitations due to sample sensitivity or the need for rapid data acquisition under less stable cryogenic conditions. Compared to traditional indirect detection systems such as charge coupled devices (CCDs), direct electron detectors (DEDs) offer improved detective quantum efficiencies, narrower point spread functions, and superior signal-to-noise ratios. Here, we compare the performance of a Gatan K2 Summit DED to an UltraScan 1000 CCD for use in signal-limited atomic-resolution STEM-EELS experiments. Due to its improved point spread function, the DED's energy resolution remains comparable to that of the CCD at a 5 times lower dispersion, providing simultaneous access to a much broader total energy range without sacrificing spectral resolution. More importantly, the benefits of direct detection enable a variety of low-signal experiments, including atomic-resolution mapping of minor and high energy edges such as the La-M$_{2,3}$ edge at 1123 eV and the Bi-M$_{4,5}$ edge at 2580 eV. For rapid acquisitions at 400 spectra per second, elemental maps recorded with the DED show an up to 40 percent increase in atomic lattice fringe contrast compared to those acquired with the CCD. Taking advantage of these performance improvements and the fast readout of the K2 DED, we use direct detection STEM-EELS to demonstrate atomic-resolution elemental mapping at cryogenic temperatures.