samples were prepared
by placing 10 μL of the sample solution on 200-mesh carbon-coated
Cu grids (Electron Microscopy Science, USA). Imaging in scanning transmission
microscopy (STEM) mode and energy dispersive X-ray spectroscopy (EDS)
analysis were performed on a JEOL JEM-2100F TEM operating at 200 kV,
equipped with Gatan Orius SC 1000 (2 k × 4 k), Gatan high-angle
annular dark-field (HAADF), Gatan annular bright field (BF), and EDS
detectors (Oxford Instruments INCA EDS 80 mm X-Max detector system
with STEM capability). The EDS spectra from the whole area of the
individual particles were taken. Measurements over five particles
per sample were used for elemental analysis. The obtained EDS spectra
were quantitatively analyzed with a standardless approach and k-factors provided by the Inca software (Oxford Instruments).