Hybrid Tomography of Nanostructures in the Electron Microscope
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Hybrid Tomography of Nanostructures in the Electron Microscope Z Saghi, T Gnanavel, X J Xu, and G Möbus Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK ABSTRACT A variety of tomographic experiments and modes for electron tomography of nanostructures are introduced, derived from the general concepts of quantitative computed tomography, binarised geometric tomography, including shape-from-silhouette, and spectroscopic chemical mapping. Our emphasis is on working out concepts of combining at least two of these tomography modes in order to share their respective advantages and improve the overall reconstruction quality. In this work, the following three hybrid modes are presented: (i) ADF-STEM tomography and EDX tomography into high-resolution 3D chemical mapping, (ii) geometric tomography and lattice-resolved backprojection into HREM-tomography for convex bodies, and (iii) geometric tomography and e-beam nanosculpting into “tomographic nanofabrication”. INTRODUCTION Electron tomography in the transmission electron microscope (TEM) [1] is the oldest form of “nanotomography” reaching nanometer-scale resolution in 3D from its beginnings in 1968. After three decades of tomography based on bright field conventional TEM imaging, the last ten years have seen a large number of new signals being introduced for electron tomography. We classify all the old and new modes into major groups (Fig 1), separating the cases of: (i) Quantitative density mapping (equivalent to x-ray computed tomography, CT), where the signal is proportional to a materials property, e.g. density, integrated along the projected thickness. The absorption index is the most common example and images can be easily inverted or linearised by applying a logarithm function (see Fig 1b, top line). (ii) Z-contrast mapping is a special case of (i) where the signal is proportional to atomic number times density. However, different from case (iii), multiple elements are integrated/mixed rather than isolated in compound materials. Examples of this category are high angle annular dark field scanning TEM (HAADF-STEM) [2] or weak phase object phase contrast imaging of thin specimens (see Fig 1b, bottom line). (iii) Spectroscopic imaging, where the signal is the integrated density of one element only, even if part of a compound crystal. This has been realized by energy filtered TEM (EFTEM) [3], electron energy loss spectroscopy (EELS) spectrum imaging, and energy dispersive X-ray (EDX) mapping [4]. Fig 1d,e show the two projections of a core-shell particle with the spectrometer tuned to each element. (iv) Silhouette or shadow mapping is a case of “Geometric Tomography” [5] and operates on binarised signals (0 or 1) derived from any of the above modes (see fig 1c). The backprojection of binarised projection images leads to an approximate solution for the 3D structure with the following features: - Any internal density fluctuations are ignored. Only a surface is reconstructed. The method therefore is particularly powerful for c
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