Atomic-Scale Imaging and Spectroscopy

STEM imaging was performed using a Nion UltraSTEM100 operated at a 60 kV accelerating voltage in UHV (~10⁻⁹ mbar). The collection angles of the high-angle annular dark-field (HAADF) and medium-angle annular dark-field (MAADF) detectors were 80–300 mrad and 60–80 mrad, respectively. The probe convergence angle was 30 mrad. A Gatan PEELS 666 spectrometer, retrofitted with an Andor iXon 897 electron-multiplying charge-coupled device camera, was used for the EELS experiments^{55 (link)}. During measurements, the energy dispersion, the beam current and the EELS collection semi-angle were 1 eV per channel, 30 pA, and 35 mrad, respectively. The direct transfer of samples without ambient exposure was enabled using a STEM with a customized sample loading and transfer system^{41 (link),42 (link),56 (link)}.