Image Modes in TEM - Lattice Images (HRTEM)
TEM | Image Formation | BF Images | DF Images | CTF | Mathematics |
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| To obtain lattice images, a large objective aperture has to be selected that allows many beams including the direct beam to pass. The image is formed by the interference of the diffracted beams with the direct beam (phase contrast). If the point resolution of the microscope is sufficiently high and a suitable crystalline sample oriented along a zone axis, then high-resolution TEM (HRTEM) images are obtained. In many cases, the atomic structure of a specimen can directly be investigated by HRTEM. | |
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The incident parallel electron beam, ideally a plane wave, interacts elastically while passing through the specimen, and the resulting modulations of its phase and amplitude are present in the electron wave leaving the specimen. The wave here, the object exit wave o(r), thus contains the information about the object structure. Unfortunately, the objective lens is not an ideal but has aberrations (astigmatism, spherical Cs and chromatic Cc aberration) that reduce image quality. The intensity distribution of the exit wave function is described by the contrast transfer function (CTF). Much HRTEM work has been done on complex oxides with a short and two long axes. If these are observed along the short axis, a structural model can often directly been derived by evaluation of the HRTEM image (examples: block structures, Nb-W oxides). This is demonstrated on the example of a new bronze-type oxide with the structure determination performed on HRTEM and ED data. Of course, the interpretation of HRTEM images has to be confirmed by image simulation (e.g., EMS program of P. Stadelmann at the CIME (Centre Interdisciplinaire de Microscopie Electronique, EPFL Lausanne). |
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modified: 29 June, 2023 by F. Krumeich | © ETH Zürich and the authors