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Advanced Methods in Transmission Electron Microscopy

Advanced Methods in Transmission Electron Microscopy

Knut Müller,

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 from the saphire nanotube and a carbon signal from the carbon coated film. Furthermore, potassium could be present in the sample. To create an elemental map, an EDX spectrum is recorded for each scan point in STEM mode. Then, energy windows can be selected around certain spectral peaks, so that the respective peak intensity can be mapped against the position of the STEM probe. Here we can see the elemental maps for aluminium, oxygen, carbon and potassium in a region around one end of a sapphire nanotube. On the left, the HAADF signal is shown again. In the oxygen and the aluminium maps the signal in the area of the nanotube is -as expected- high. The carbon map shows principally the carbon coated film. However, this signal should be distributed homogeneously throughout the depicted region since the film is also present under the nanotube. Thus we see that the EDX signal can be difficult to interprete quantitavely, and the application/development of valid models to correct e.g. for thickness effects and flourescence is maybe a future task, as current models often give only rough quantitative estimates. As a last topic we want to talk about electron tomography in STEM mode. Electron tomography The sketch in the upper left corner depicts that projections of an object do not give any information about the real three-dimensional structure. The principle of tomography is to tilt the specimen -in our case between +70° and -70°- to get different projections in subsequent images. The graphic in the lower half of the slide shows that one can even not distinguish if two separate or one large particle is present from one projection alone, whereas projections for different specimen tilts reveal that two separate particles is the true answer. To get an idea of the 3D structure of the object the recorded projections have to be backprojected. The more tilt angles are used in this process the better the results, especially for the resolution of the reconstruction in 3D-space. 12/13

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 In this series some snapshots from a STEM tilt series of the sapphire nanotube are shown. About one hundred of these images were then used to reconstruct a part of the nanotube and to produce a 3D model of the nanotube surface. The reconstructed nanotube is shown from 3 different perspectives here. The cross-section does not show rotational symmetry, as one could expect from the projections shown on the previous slide. As the elongation occurs in beam direction, this artefact is caused by the so-called missing wedge: As the tilt range of our specimen holder is limited to ±70°, some projections are missing to reconstruct the full 3D information correctly. Let us finally introduce our group here, and use it to demonstrate the missing wedge phenomenon in 2D heuristically. If we look at the Fourier transform of our group on the right, we can cut out the information of some spatial frequencies in triangularly shaped regions to get in impression of what a missing wedge results in. Then we transform back, and it can clearly be seen that features stay sharp in horizontal direction, whereas they smear out in the direction where we applied the missing wedge. For example our heads are elongated vertically. Thank you for your attention. Please contact us for further details or to get this script as PDF. ● AG Rosenauer Institut für Festkörperphysik Bereich Elektronenmikroskopie Universität Bremen www.ifp.uni-bremen.de ● Knut Müller mueller@ifp.uni-bremen.de ● Katharina Gries kgries@ifp.uni-bremen.de 13/13

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