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The precise measurement of the crystal plane spacing and angle of the coating layer through the FFT of TEM images, and the comparison with the crystallographic database is an effective method for identifying the phase of the coated material. However, it must be recognized that this method has limitations in thin-layer analysis, and it is necessary to combine it closely with elemental analysis (EDS/EELS) to obtain convincing conclusions.
FFT is a powerful tool for determining the crystal structure of the coating layer, but it usually cannot be used as the "only" identification method. To more reliably determine the coated material, it is strongly recommended that you adopt the "multimodal combination" strategy:
TEM-FFT + SAED: In addition to local FFT, you can try to use selected area electron diffraction (SAED) to separately diffract the coating layer area (if large enough) to obtain a "cleaner" diffraction pattern.
TEM-FFT + STEM-EDS/EELS line scan/surface scan: In STEM mode, perform a line scan on the cross-sectional sample to precisely confirm the distribution of each element (especially the elements specific to the coating layer) at the interface, and clearly identify the chemical boundary of the coating layer. Perform a surface scan in the coating layer area to determine its elemental composition and uniformity of distribution. Correspond the structural information obtained from FFT with the chemical information obtained from EDS/EELS one by one, which is the most powerful evidence.
Combining phase characterization: Although XRD does not show the peaks of the coating layer (possibly because it is too thin or amorphous), techniques that are sensitive to the surface, such as Raman spectroscopy, XPS, etc., can provide information on the chemical bonds and phase of the coating layer, and can be used to confirm the results of TEM analysis.
