Electron Beam Imaging and Analysis

EELS 2D spectrum images were acquired on a Titan Themis (FEI Company) using a K2 Summit direct electron detector in counting mode (Gatan Inc, Pleasanton, CA). This direct electron detector with high quantum efficiency (DQE up to 80%) allowed simultaneous acquisition of all relevant inner shell ionization (core loss) edges at high energy resolution and low background levels despite the low dose required to minimize beam damage.^{[43 (link)]} The entrance aperture was 5 mm, the energy dispersion 0.5 eV/channel. The beam current was 4.0-8.5 pA, the dwell time 2.5 μs/pixel. STEM-ADF images were recorded in parallel. Concentration maps were extracted by fitting and subtracting the pre-edge background with a linear combination of power laws and integrating the intensity under the EELS edge of interest. MCR analysis of the Mg-L₂,₃ edge region (Fig. S6) was performed as described previously.^{[44 (link)]}