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Register a new userCharacterizing oxygen atoms in perovskite and pyrochlore oxides using ADF-STEM at a resolution of a few tens of picometers
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We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of the perovskite (LaFeO3) and pyrochlore (Yb2Ti2O7 and Pr2Zr2O7) oxides and demonstrate that both the shape and contrast of visible atomic columns in annular dark-field (ADF) images are sensitive to the presence of nearby atoms of low atomic number (e.g. oxygen). We show that point defects (e.g. oxygen vacancies), which are invisible - or difficult to observe due to limited sensitivity - in X-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore oxides. In addition, we present, for the first time, a method by which light atoms can be resolved in quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in perovskite and pyrochlore oxides.
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Nanotechnology research requires the routine use of characterization methods with high spatial resolution. These experiments are rather costly, not only from the point of view of the expensive microscopes, but also considering the need of a rather specialized equipment operator. Here, we describe the construction of an inexpensive and simple device that allows the analysis of nanoparticle in a FEG-SEM; images can be generated at high magnifications (ex. x500.000) and with nanometric resolution. It is based on the acquisition of transmitted electrons annular dark field (TE-ADF) signal; the systems can carry up to 16 TEM samples and, it is compatible with SEM sample exchange air-lock. Performance test have shown the measured ADF signal showed the atomic number and thickness dependence for transition metal nanoparticle about 10 nm in diameter. Also, the signal quality is high enough that the determination of the histogram of size distribution can be performed using a conventional image processing software, for gold particles in the range of 2-10 nm in diameter. The developed ADF device allows a much faster and cheaper high spatial resolution imaging of nanoparticle samples for routine morphological characterization and, provides an invaluable high throughput tool for an efficient sample screening.
Competitive mechanisms contribute to image contrast from dislocations in annular dark field scanning transmission electron microscopy ADF STEM. A clear theoretical understanding of the mechanisms underlying the ADF STEM contrast is therefore essential for correct interpretation of dislocation images. This paper reports on a systematic study of the ADF STEM contrast from dislocations in a GaN specimen, both experimentally and computationally. Systematic experimental ADF STEM images of the edge character dislocations revealed a number of characteristic contrast features that are shown to depend on both the angular detection range and specific position of the dislocation in the sample. A theoretical model based on electron channelling and Bloch wave scattering theories, supported by multislice simulations using Grillo s strain channelling equation, is proposed to elucidate the physical origin of such complex contrast phenomena.
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