Z Saghi

Reconstruction of 3D morphology of polyhedral nanoparticles

The three-dimensional (3D) faceting morphology of ceria nanoparticles is analysed using transmission electron microscopy (TEM)-based computed tomography on the nanometre scale. A novel tomography mode of electron energy loss spectroscopic imaging using a single energy window for inelastically scattered electrons is introduced and found to be reliable and fast for freestanding nanoparticles. To compare the new tomographic method with other methods, we provide the first comprehensive application of three complementary TEM-based imaging techniques, including bright field TEM and annular dark field specific TEM (STEM). Traditional bright-field TEM tomography is found to be applicable, in spite of obvious artefacts, for crystalline particles of constant composition. However, the safest interpretation is achieved by a combined recording of bright field and spectroscopic images.

Three-dimensional chemical analysis of tungsten probes by energy dispersive x-ray nanotomography

The chemical distribution of oxide layers around functional tungsten nanotips is studied using electron tomography. Three-dimensional element distribution functions are derived for such tips, giving insight into the subsurface chemistry. Energy dispersive x-ray (EDX) spectroscopy is coupled to computed tomography to reconstruct slices across the tip. It is finally shown how the surface reconstruction by geometric tomography from annular dark field scanning transmission electron microscopy images can be combined with EDX tomography reconstructions to reduce backprojection artefacts and improve the sharpness of the surface contours.

Electron tomography of regularly shaped nanostructures under non‐linear image acquisition

Electron tomography allows the 3D quantitative characterization of nanostructures, provided a monotonic relationship is fulfilled between the projected signal and the atomic number and thickness of the specimen. This requirement is not satisfied if the micrographs are affected by (i) diffraction contrast, (ii) detector saturation or (iii) contrast inversion due to absorption (high‐angle scattering) at high thickness. Artefacts related to the non‐monotonic tomography acquisition are examined using computer simulations and experimental tilt series of tungsten tips and CeO2 nanoparticles. Conditions are derived under which in spite of the non‐linear artefacts the information is sufficient for reconstructing the 3D morphology of convex objects by geometric tomography.

Model based atomic resolution tomography

Electron tomography is a valuable three-dimensional characterization method, but the technique has so far been limited to a nanometer scale resolution, and therefore complementary two-dimensional structural analysis is generally performed using other techniques. In this paper, computer simulations of the latest transmission electron microscope technologies are performed on a CeO2 supercell to investigate the extension of electron tomography to the atomic scale resolution, from a tilt series of high resolution electron microscopy (HREM) images. We showed that high-voltage and aberration-corrected microscopes were both capable of retrieving adjacent oxygen and Ce atoms for a thickness up to 2.5 nm. In the case where only a few tilt angle projections are in zone-axis orientations, we introduced a hybrid tomography method, whereby a lattice-resolved tomogram obtained from a few zone-axis projections is combined with the external shape reconstruction of the crystal from binary projections. Finally, we discusse...

Tomographic nanofabrication of ultrasharp three-dimensional nanostructures

We present the extension of electron tomography from a pure characterization technique into a three-dimensional nanofabrication technique using focused electron beams in the scanning transmission electron microscope. Two-dimensional surface patterning techniques are insufficient to achieve full three-dimensional nanosculpting, instead, multiple sample rotations under the beam are required. We demonstrate fabrication of ultrafine tips and hollow structures with applications in scanning probe microscopy, atom probe tomography, or in magnetoconductance of nanobridges. A sharpened tip radius of <3 nm is achieved, an order of magnitude finer than with standard methods.

3D Reconstruction of SPM Probes by Electron Tomography

Three-dimensional morphological and compositional structures of tungsten tips consisting of layered amorphous oxide shell and crystalline W core are reconstructed by electron tomography using both coherent and incoherent imaging modes. The fidelity of the reconstruction is dependent on three criteria, suppression of unwanted crystal orientation contrast in the crystalline core, nonlinear intensity-thickness relations above a certain thickness limit, and artefacts due to missing angular ranges when acquiring a tilt series of images. Annular dark field (ADF), and EDX chemical mapping are discussed as alternatives to standard bright field (BF) TEM imaging.

Atomic motion on various surfaces of ceria nanoparticles in comparison

Aberration corrected HRTEM has been successfully applied to image the atomic motion at the edges and surfaces of nanoparticles of ceria of various types under electron irradiation. Here we identify movements not only on {100} facets, but also on {110} and even {111} facets, previously considered stable. However, the degree of movement varies strongly and HRTEM is evidently the preferred technique to measure relative stability at high spatial resolution as it does not require extended surfaces as in scanning probe microscopy (SPM) or chemical methods. The advantage of aberration correction shows in suppression of contrast from the carbon support films and the absence of delocalisation fringes at particle edges, apart from improving point resolution.

Nanoscale sculpting of ferromagnetic structures by electron beam ablation

We report an electron beam (e-beam) fabrication method to produce sub-5 nm structures, e.g. nanohole-arrays, nanojunctions and nanotips inside a TEM. The method is demonstrated using ferromagnetic nickel cross-sectional TEM specimens as well as electrochemically etched nickel tips. Different e-beam shapes and electron guns are compared, including point versus line focus and field-emission versus LaB6 guns. As an extension of this 2-D patterning of nanostructures, a 3-D nanofabrication technique has been introduced using a high tilt tomographic holder. An electrochemically etched nickel tip of initially 18 nm in radius is sculpted down to sub-5 nm diameter. In some cases, mostly in 3-D nanofabrication, a protrusion during hole drilling was observed and confirmed by EELS as pure nickel.

Hybrid tomography for structural and chemical 3D imaging on the nanoscale

Electron tomograms can be reconstructed from a variety of projection imaging techniques in TEM, some of which are sensitive to the surface or the bulk, to the structure or chemistry, or operate under different resolution regimes. We introduce a concept of combining two acquisition series using two different complementary modes to form a single hybrid tomogram. As the main application, composite nanoobjects are presented for which a low-resolution 3D chemical mapping mode for the object interior can be combined with a high-resolution surface sensitive mode to precisely define the object exterior shape.

MRT letter: full-tilt electron tomography with a piezo-actuated rotary drive.

Piezoelectric nanoactuation, which is rapidly becoming established as state‐of‐the‐art positioning control in transmission electron microscopy (TEM), is extended here to include a rotational degree of freedom. A piezoelectric goniometer with both translational and rotary drive action has been designed with high level of miniaturization to fit into a standard TEM specimen holder shaft without compromising any of the performance of the default TEM goniometer and without any modifications to the TEM. Enhanced functionality of such a goniometer‐in‐goniometer is outlined and experimental results for electron tomography of nanostructures over a full tilt range of views, without any missing angles, are demonstrated. Microsc. Res. Tech., 2008.