Measurements

The EELS measurements and STEM imaging are performed with a FEI Titan TEM equipped with a monochromator and probe corrector. The microscope is operated in monochromated scanning TEM (STEM) mode at an acceleration voltage of 300 kV, providing a spot size of approximately 0.5 nm and an energy resolution of 0.1 eV (measured as the full-width at half-maximum of the zero-loss peak). The microscope is equipped with a GIF Tridiem electron energy-loss spectrometer and the Gatan DigiScan acquisition system. It can record an entire EELS intensity map in 10 to 20 min, depending on the number of pixels. We use a C3 aperture size of 30 μm, a camera length of 38 mm, an entrance aperture of 2.5 mm and a spectral dispersion of 0.01 eV per pixel in the EELS measurements. In addition, we utilize the automatic drift and dark current correction function included in the acquisition system. The individual EELS spectrum of the EELS intensity maps (with pixel sizes typically of 2–2.5 nm) are recorded with acquisition times of approximately 1 ms.