Electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM-EELS) enable to investigate the local electronic states at the specific atomic column selected by an incident electron probe. Even though the same elements, crystallographically non-equivalent atoms give the different energy-loss near-edge structure (ELNES) . Although SrTiO3 has a single oxygen atom in the unit cell, the chemical bonds between Ti and O have directionality as shown in Fig. 1(a), depicting the ligand field orbital with eg symmetry formed by Ti-dx2−y2 and O-p orbital. If the oxygen K-edge ELNES is acquired with an off-axis collection aperture to select a specific direction of momentum transfer, the intensity related to the transition to the unoccupied eg state should vary with O1 and O2 sites as predicted previously . In this study, we show that the directionality of local electronic states at different oxygen sites can be detected by momentum-selected STEM-EELS.
STEM-EELS measurements were performed using Cs-corrected STEM (JEM-9980TKP1 equipped with a cold-FEG and an omega filter) operated at 200 kV. Fig. 1(b) shows the setup of experiment. An electron probe with a convergence semi-angle (α) of 23 mrad incidents along the c-axis of SrTiO3 and EEL spectra are recorded with the spectrum collection aperture (β = 10 mrad) placed at the edge of the transmitted beam disc along θy direction. In order to simulate the experimental results, the electronic structure of SrTiO3 was calculated by WIEN2k  based on the density functional theory, and the O-K edge ELNESs were calculated by TELNES3 code incorporated in WIEN2k.
Fig. 2(b) shows the experimental O-K edge ELNES obtained by scanning electrons along two different O-TiO-O lines shown in Fig. 2(a). It is found that the intensity of peak b which can be assigned to the transition to the eg state is different between the two spectra. Since these spectra are acquired with a collection aperture shifted in the θy direction, the partial density of state of O-py hybridized with Ti-dx2-y2 state should predominantly contribute to the spectrum. Actually, the spectrum acquiring from the oxygen atoms aligned in the y-direction has an enhanced peak b, which is indicated that the present method can detect the directionality of Ti-O bonding at a specific oxygen site.
Fig. 3(b) shows the O-K edge ELNES calculated by adding the contribution from the two oxygen sites shown in Fig. 3(a). The features in the calculated spectra are in good agreement with that of the experimental one, which verifies the reliability of the experimental finding.
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To cite this abstract:Atsushi Yamaguchi, Mitsutaka Haruta, Takashi Nemoto, Hiroki Kurata; Probing the directionality of local electronic states in SrTiO3 by momentum-selected STEM-EELS. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/probing-the-directionality-of-local-electronic-states-in-srtio3-by-momentum-selected-stem-eels/. Accessed: September 20, 2021
EMC Abstracts - https://emc-proceedings.com/abstract/probing-the-directionality-of-local-electronic-states-in-srtio3-by-momentum-selected-stem-eels/