STEM-EELS imaging of complex oxides and interfaces
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Introduction
Spectrum imaging with atomic resolution
With the advent of spherical aberration correction in the electron microscope, the last decade witnessed a major revolution in real space characterization techniques, making atomic resolution spectroscopic characterization accessible to virtually any materials scientist. The enhanced capabilities of modern aberration-corrected electron microscopes allow spatial resolutions of the order of 0.1 nm or below to be routinely reproduced in laboratories and universities around the world.1–10 This article will review some state-of-the-art applications of electron energy-loss spectroscopy (EELS) in the aberration-corrected scanning transmission electron microscope (STEM) to oxide materials. STEM-EELS is now capable of not just producing elemental maps with atomic resolution6–8 but also detecting individual atoms in materials11 and producing elemental maps of impurities in low concentrations.12,13 An example of such a possibility is summarized in the spectrum image14,15 shown in Figure 1. Individual La atoms in a CaTiO3 matrix can be detected by annular dark field or Z-contrast imaging and also by EELS (Figure 1a–b),11 and a quantification of the number of La atoms per atomic column in a Ca0.95La0.05TiO3 layer can be attempted by analyzing the La elemental maps (Figure 1c).12,13
Atomic resolution spectroscopic imaging in the electron microscope is becoming an unprecedented tool for chemical identification in real space. When it comes to applications to low dimensionality systems, such as 2D interfaces between different materials, EELS imaging permits unambiguously identifying the interface structure atomic plane by atomic plane, atomic column by atomic column. An example is shown in Figure 2, where an image of a ferromagnetic/superconducting/ferromagnetic La0.7Ca0.3MnO3/YBa2Cu3O7–x/La0.7Ca0.3MnO3 (LCMO/YBCO/ LCMO) heterostructure is depicted. 16 This system is the oxide counterpart of classic ferromagnetic/superconducting superlattices, which were obtained to study the interplay of these competing order parameters.17–22 The interface structure and atomic plane stacking sequence has a direct impact on the physical properties of these heterostructures. Atomic resolution spectroscopic images show how in this system, the YBCO interface unit cell is not complete: a CuO chain atomic plane is missing, and hence the YBCO BaO plane at the interface is bound to the first MnO2 plane of the manganite (see the sketch in Figure 2).19 This interface structure has tremendous implications: charge transfer from the ferromagnet into the superconductor20–22 gives rise to an orbital reconstruction
Maria Varela, Oak Ridge National Laboratory; [email protected] Jaume Gazquez, Institute of Materials Science of Barcelona; [email protected] Stephen J. Pennycook, Oak Ridge National Laboratory; [email protected] DOI: 10.1557/mrs.2011.330
© 2012 Materials Research Society
MRS BULLETIN • VOLUME 37 • JANUARY 2012 • www.mrs.org/bulletin
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STEM-EELS IMAGING OF COMPLEX OXIDES AND INTERFACES
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