Analysis of the Atomic Scale Defect Chemistry in Oxygen Deficient Materials by STEM
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practical applications, we therefore need to understand the defect chemistry on the fundamental atomic scale. The route to characterizing oxide materials on this level is afforded by the combination of Zcontrast imaging [21 and electron energy loss spectroscopy (EELS) 13] in the scanning transmission electron microscope (STEM). These correlated techniques [41 allow direct images of crystal and defect structures to be obtained, the compositions to be quantified and the effect of the structures on the local electronic properties (i.e. oxygen coordination and cation valence) to be assessed [51. In this paper we discuss the use of these techniques in the JEOL 2010F STEM 16] to analyze the defect chemistry in SrCoO3_. This material is chosen to demonstrate the use of the techniques as it is known to exist in a variety of phases with different crystal structures, compositions and valence states of cobalt and can be highly oxygen deficient [7}. EXPERIMENTAL TECHNIQUES The basis of the STEM techniques is the ability to form a probe of atomic dimensions on the surface of the specimen (Figure 1). In the case of the JEOL 2010F STEM, the size of the probe that can be achieved is 0.14nm 161, which corresponds to the resolution of the Z-contrast image and the energy loss spectrum for a specimen in a zone-axis orientation 141. 69 Mat. Res. Soc. Symp. Proc. Vol. 589 0 2001 Materials Research Society
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Figure 1: Schematic of the detector arrangement in the JEOL 2010F STEM. Z-Contrast Imaging Z-contrast imaging in the STEM uses the high angle scattering from a sample collected on an annular detector (Figure 1). This integrated intensity is then synchronously displayed on a TV screen while the electron probe is scanned across the specimen. The integration over the annular detector averages over the interference effects from adjacent atoms and thus each atom can be considered as an independent scatterer [2,8]. Therefore, the image intensity may be described as an object function, which is sharply peaked at the atomic locations, convolved with the probe collection angles the cross-section for this scattering is intensity profile. For high inner approximately proportional to Z 2 , where Z is the atomic number [8]. As a result of this simple image formation mechanism, many of the problems associated with phase contrast microscopy are removed and a simple, direct interpretation of the Z-contrast image is possible [2]. Since the width of the object function is typically of the order of 0.02nm, the spatial resolution of the image is determined by the microscope probe size (0.14nm for the JEOL 2010F). This means that for a specimen aligned with a crystallographic pole along the beam direction, providing the atomic spacing of interest is larger than the probe diameter, atomic columns may be illuminated individually and an atomic resolution image may be obtained. Electron Energy Loss Spectroscopy As can be seen from Figure 1, the annular detector used for Z-contrast imaging do
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