Themis g2

HAADF‐STEM images were acquired on an FEI Titan Themis G2 double‐ aberration‐corrected TEM operating at 60 kV. The collection angle of HAADF detector was around 52–200 mrad, and the convergence semi angle was 25 mrad. The beam current was set about 30 pA for the annular dark‐field imaging to reduce the electron beam induced damage.

HAADF-STEM images were acquired using an aberration-corrected FEI Titan Themis G2 with an acceleration voltage of 300 kV. The convergence semi-angle is 30 mrad, and the collection semi-angle is 39–200 mrad. TEM samples were first thinned by mechanical polishing, and then milled by using a precision ion polishing system with an argon ion source. The acceleration voltage of 4 kV was used until a hole appears. Then accelerating voltage of 0.2 kV was used to remove the amorphous layer.

The lamella thickness was estimated using the STEM-EELS method. STEM-EELS was performed using an aberration-corrected FEI Titan Themis G2 at 300 kV. The typical energy resolution (half-width of the full zero-loss peak, ZLP) was 0.8 eV. For the different regions of superlattice lamella, STEM-EELS images in the low loss region were acquired and converted to a thickness map of inelastic mean free paths (t/λ) in Gatan’s Digital Micrograph software using the absolute log-ratio method for the zero-loss and the first plasmon peaks³⁷ .

The sample for STEM was obtained by focused ion beam milling via the Precision Ion Polishing System (Model 691, Gatan Inc.). High-angle annular dark-field (HAADF) images were collected at 300 kV by using an aberration-corrected FEI Titan Themis G2 with spatial resolutions up to 60 pm.

X-ray diffraction was performed on a Rigaku SmartLab automated multipurpose R-ray diffractometer using Cu K_α. STEM imaging and EDS analysis were performed on an FEI double C_s corrected Titan Themis G2 operated at 300 kV and equipped with an X-FEG electron gun. The convergence semi-angle of the probe was 17.8 mrad, the inner and outer collection angles of the STEM images were 48 and 200 mrad, respectively. The screen current used for ADF-STEM imaging and EDS analysis was 50 pA and 100 pA, respectively. During in situ experiments, the sample was heated up to a certain temperature at a heating rate of 1 °C/s. STEM image simulation was performed using multi-slice algorithm implemented in QSTEM package⁵⁰ using the same experimental parameters. The thermal diffuse scattering (TDS) was considered using the frozen phonon method with 10 configurations. The Debye Waller factors of Ge, Bi and Te used for image simulation are 2.21, 2.03 and 1.06 Ų, respectively⁵¹ . GPA strain mapping was calculated using (0006) and (01 $\overline{1}\overline{7}$ ) diffraction spots implemented in Digital Micrograph plugin GPA-v2.0.gt1⁵² .

STEM images and EDX mappings were obtained in a double aberration-corrected transmission electron microscope (FEI Titan Themis G2) operated at 300 kV with a HAADF detector (collection angle range of 48–200 mrad) and Super-X EDX detector. Transmission electron microscopy (TEM) images were obtained with an FEI-Tecnai F20 (200 kV) transmission electron microscope. The XAS experiments at the Ni K-edge were performed at the Shanghai Synchrotron Radiation Facility (SSRF,11B), and the intensities of the two S_Fe-Ni samples were multiplied by five for the EXAFS spectra. The metal content of different samples was measured by an Inductively coupled plasma optical emission spectrometer (ICP-OES) (i CAP Pro X, Thermofisher) or an Inductively coupled plasma source mass spectrometer (ICP-MS) (Aurora M90, Jenoptik). The temperature-programmed reduction (TPR) results were recorded with an infrared spectrometer (ThermoFisher Nicolet iS 50) equipped with a Deuterated Triglycine Sulfate (DTGS) detector.