Figure 2635a shows the EELS zero loss extraction using a power-law background model, giving a raw valence band spectrum and residual spectrum following subtraction of a power-law background model. The green arrow indicates a typical, artificially induced false feature, located in the energy region of 2-3 eV. This artifact can easily occur and is most likely induced by the incorrectly focused ZLP as during EELS alignment process the ZLP is normally optimized by only focusing the peak to maximize the intensity and less attention is paid to the low intensity tails.
Figure 2635a. EELS zero loss extraction using a power-law background model.
Note that slit can block the electron beam if the GIF camera is used to align the electron beam (with Ronchigram) for EELS acquisition in STEM mode. Therefore, for EELS acquisition in STEM mode, slit should be retracted.
To enhance EELS signal and to avoid energy offset, two alignments are needed:
i) The electron beam should be aligned by placing the Ronchigram (with a STEM-objective aperture) in the center of the GIF camera with Projector Lens Alignment (PLA) knob. This step is normally achieved by “VIEW” in DigitalMicrograph interface (see Figure 2635b).
Figure 2635b. (a) and (b) Ronchigram is not in the center of the GIF camera; (c) Ronchigram is in the center of the GIF camera.
ii) The zero-loss peak is aligned with the Zero-Loss-Peak-Alignment button (the first icon in Figure 2635c) in the DigitalMicrograph interface.
Figure 2635c. Zero-Loss-Peak-Alignment button indicated by the first icon.