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Bonding and structure of a reconstructed (001) surface of SrTiO3 from TEM


The determination of the atomic structure and the retrieval of information about reconstruction and bonding of metal oxide surfaces is challenging owing to the highly defective structure and insulating properties of these surfaces. Transmission electron microscopy (TEM) offers extremely high spatial resolution (less than one ångström) and the ability to provide systematic information from both real and reciprocal space. However, very few TEM studies1,2,3 have been carried out on surfaces because the information from the bulk dominates the very weak signals originating from surfaces. Here we report an experimental approach to extract surface information effectively from a thickness series of electron energy-loss spectra containing different weights of surface signals, using a wedge-shaped sample. Using the (001) surface of the technologically important compound strontium titanate, SrTiO3 (refs 4, 5, 6), as a model system for validation, our method shows that surface spectra are sensitive to the atomic reconstruction and indicate bonding and crystal-field changes surrounding the surface Ti cations. Very good agreement can be achieved between the experimental surface spectra and crystal-field multiplet calculations based on the proposed atomic surface structure optimized by density functional calculations3. The distorted TiO6−x units indicated by the proposed model can be viewed directly in our high-resolution scanning TEM images. We suggest that this approach be used as a general method to extract valuable spectroscopic information from surface atoms in parallel with high-resolution images in TEM.

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Figure 1: STEM-HAADF image and diffraction pattern of a [001] oriented SrTiO 3 sample.
Figure 2: Experimental spectra.
Figure 3: High-resolution images.
Figure 4: Crystal-field multiplet calculations of Ti L 2,3 -edge EEL spectra.

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We are grateful to NSERC for Discovery and Strategic Grants supporting this work. The microscopy was carried out at the Canadian Centre for Electron Microscopy, a National Facility supported by NSERC and McMaster University. G.-z.Z. thanks J. LeBeau for help and advice on the simulations of the PACBEDs.

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G.-z.Z. and G.A.B. conceived and are responsible for the experiments, performed the simulations and jointly wrote the paper. G.R. contributed to the analysis of crystal-field multiplet simulations and the final version of this manuscript.

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Correspondence to Gianluigi A. Botton.

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The authors declare no competing financial interests.

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Zhu, Gz., Radtke, G. & Botton, G. Bonding and structure of a reconstructed (001) surface of SrTiO3 from TEM. Nature 490, 384–387 (2012).

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