Electron Tomography of SPM Probes, Nanoparticles and Precipitates
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Electron Tomography of SPM Probes, Nanoparticles and Precipitates Xiaojing Xu, Zineb Saghi, Guang Yang, Yong Peng, Beverley Inkson, Ralph Gay, and Günter Möbus Dept Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, United Kingdom
ABSTRACT Nanoscale tomographic reconstructions from objects with diameters of 100nm or smaller can only be achieved non-destructively with transmission electron tomography. The application of this technique to W tips, which are common probes for scanning tunneling microscopy and nanoindentation, is demonstrated with emphasis on visualizing oxide layers and functionally attached nanoparticles. For the reconstruction of facetted free-standing catalyst nanoparticles, such as CeO2 octahedra, we propose a combination of energy-filtered (EF) and bright field (BF) TEM tomography to achieve high fidelity of the projection relationship via EFTEM, due to its incoherent imaging mode, and high resolution definition of the particle circumference from the BF tomogram. Finally, electron tomography applications to CeO2 nanoprecipitates embedded in a multicomponent oxide glass matrix are shown, which comprises the first tomographic 3D reconstruction of a nanoscale dendrite. INTRODUCTION Electron tomography [1] is the only generally applicable 3D imaging technique for nondestructive nanotechnology-based materials problems in order to reconstruct volumes of submicron size with nanometer resolution, and has recently been successfully transferred from the soft materials [2-3] to the hard materials area [4-6]. The prerequisites for a successful tomography experiment normally include: (i) a high enough tilt range within the transmission electron microscopy (TEM) goniometer to avoid the missing-wedge artefacts, (ii) a small enough tilt increment to avoid star artefacts in the reconstruction, (iii) a projection contrast mechanism which suppresses non-linearities and crystal-orientation dependencies due to Bragg-scattering, (iv) enough contrast and detail in the images to enable alignment of the tilt-series. The importance of incoherent imaging modes, such as STEM mode with annular dark field detector (ADF-STEM) and energy-filtered TEM (EFTEM) to suppress the crystalline diffraction contrast has been stressed in the past [5-6], and a first comparison of bright field (BF-TEM) vs dark field published [7]. In this paper, we report on the recent progress in the field of electron tomography of nanomaterials in an ultra-narrow-gap high-resolution TEM (HRTEM) goniometer using novel specimen mounting schemes. We use three categories of nanoobjects: (i) Conical metal probes with a few nm radius of tip-curvature such as used for nanoindentation or scanning probe microscopy (SPM) imaging; (ii) Ceria nanoparticles free-standing on carbon-film as examples of crystallographically faceted nanoobjects; and (iii) Nanoscale precipitates embedded in a glass matrix with a non-facetted roundish or dendritic structure. Each of these three examples represent classes of applications of electro
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