Supplementary Material
The sample prepared for the scanning transmission electron microscopy (STEM) and electron energy loss
spectroscopy (EELS) was prepared from a 8nm Fe3O4 film on ZnO by cutting a cross section using a
focused ion beam facility. The average thickness of the cross section sample is estimated at approximately
30nm by the log ratio method from a low electron energy loss spectrum.
The STEM-EELS experiments were performed on the Qu-Ant-EM microscope at the University of
Antwerp. The instrument is a FEI Titan 80-300 “cubed” microscope fitted with a double aberrationcorrector for both probe-forming and imaging lenses, and a GIF Quantum energy filter. The microscope
was operated at 300kV acceleration voltage. The convergence angle was 21mrad in the EELS linescan and
a collection angle of 100mrad was chosen. The beam current was approximately 30pA, and dwell time of
EELS was 2s per pixel. For the high-angle annular dark field (HAADF) STEM image, the inner angle of
HAADF detector was 62.4mrad. Figure 1 shows a low magnification overview HAADF STEM image of
the sample.
Figure 1 Low magnification HAADF STEM image of the sample. The morphology of the Fe 3O4 film is visible
as well as the good crystallographic quality of the ZnO substrate. The polycrystalline area on the left is an
additional Pt capping which was deposited during the preparation of the TEM cross section sample and was
not part of the original sample.
EELS spectra were measured for 100 points along a line from well inside the ZnO substrate to the surface
of the Fe3O4 film. The Dual EELS mode of the Quantum spectrometer is selected to record both the Low
Loss and high loss spectra sequentially. Taking the low loss as a reference we have obtain the Fe L edge
with absolute energy calibration and we avoid the common problem in EELS that exact energy positions
of peaks are not known. This is essential to get the valence information from the spectra as valence is
mostly affecting the peak position. The resulting EELS intensity over energy loss and z position is shown
as Figure 3 & 5 in the main manuscript.
A simulation and fitting approach of the spectral shape of the EELS is used to derive the spectral weight
contributions of Fe2+ and Fe3+, respectively. The original EELS spectrum image data was first background
subtracted by fitting a commonly-used power law model. Using two reference spectra for Fe3+ and Fe2+ as
individual components in a linear combination, all the spectra in the SI were fitted and the profile of the
spectral weights was generated. The least squares fitting to the Fe L2,3 edges was performed using
EELSMODEL.
Figure 2 Experimental EELS of a single pixel in the EELS linescan fitted to a linear combination of Fe2+ and
Fe3+ component from reference compounds. The chemical shift of the Fe L 3 edge between the Fe2+ and Fe3+
compound is approximately 1.6eV.