4 years ago

Advanced Methods in Transmission Electron Microscopy

Advanced Methods in Transmission Electron Microscopy

Knut Müller,

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 Introduction As this talk is supposed to be a tutorial, we will introduce to transmission electron microscopy (TEM) with an apparently simple question: What is microscopy? The greek origin of this word suggests that it has to do with „looking at small things“, that is: We have an object, optics, and we want to get a magnified image. Of course this is not the full story: An image originates especially from the interaction of the irradiation with our specimen, thus it will depend on the source. Electrons, which we will of course focus on in this talk, interact with the crystal Coulomb potential. We might furthermore want to control the shape of the so-called probe, which is the primary wave at the specimen entrance surface. A condenser system will do this job. The experimental part of microscopy still goes much further, as indicated coloured here: Resolving the energy spectrum of both the transmitted electrons and the emitted X-rays can give powerful insights to chemical and solid state properties. However, this picture of microscopy is still not complete. Any of these aspects shown on the experimental side must also be treated theoretically. For example, the source is strongly connected with the theory of coherence. The optical systems present in the condenser and imaging stage require detailed understanding of imaging theory. The most complicated part is the theoretical treatment of the electron-specimen interaction, since quantum theory including quantum statistics is involved. Finally, digital image processing is usually needed to work quantitatively. This picture is still not complete: The complexness of the theoretical aspects requires efficient numerical models and usually still capable computers to yield a quantitative result. The messsage of this introduction is now as follows: Microscopy means much more than magnifying. It stands for working at experimental, theoretical and numerical frontiers to cope with up-to-date physical problems. It should therefore be distinguished strictly from photography. 2/13

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 Outline Here you see the Titan 80/300 which has been installed in Bremen in 2007. Electrons are emitted from a field emission gun and accelerated to 300keV, then they pass the 3-condenser lens system and are directed at the specimen, situated nearly in the center of the microscope. We are very proud to have an image aberration corrector and a biprism (to do holography) below the objective lens. Just above the screen you see the high-angle annular dark field (HAADF) detector for the STEM signal. The energy filter below the microscope cannot be seen on this image. Finally, we see the energy dispersive X-ray detector top right above the specimen. Our presentation will pick out the most interesting aspects shown in the introduction with a focus on experimental results. It is divided into two main parts: Conventional and Scanning TEM. The conventional part starts with a modicum of imaging theory and it will demonstrate the power of aberration correction. We will present exit wave reconstruction results, as well as traditional techniques for compositional analysis at atomic scale, namely strain-state analysis and the CELFA technique. In the Scanning TEM (STEM) part, we will firstly outline the state-of-the-art of quantitative chemical compositional analysis from dark field images. Then, elemental maps from X-ray analysis and tomography results will follow for nanotube-like structures. We will -as a consequence of limited time- not talk about the topics STEM-EELS, TEM-tomography, TEM-holography, quantitative diffraction work, theory and simulation methods. Conventional TEM In conventional transmission electron microscopy, lenses play a central role. To get an impression of the quality of electron lenses, this picture shows the logo of Bremen University viewed through a wine glass filled with water. It becomes clear that we have to understand the image distortions introduced by a lens in detail before we can do quantitative work, that is, 3/13

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