Ferdinand Hofer

Imaging of nanometer-sized precipitates in solids by electron spectroscopic imaging

Abstract Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) can be efficiently used to detect precipitates in solids. In this work we used a GATAN imaging filter which has been attached to a 200 kV TEM to record elemental maps using inner-shell ionization edges. We have investigated a niobium alloy with nanometer-sized titanium-oxide precipitates and steels with vanadium-carbide and chromium-carbide precipitates. These precipitates could be visualized using inner-shell ionization edges (Ti L 23 , Nb M 45 , Cr L 23 , V L 23 , V M 23 and Fe L 23 ). We have compared different ESI techniques to check their validity for precipitate imaging. First, energy-filtered images can yield an enhanced contrast compared to the conventional TEM bright field, but are very sensitive to diffraction contrast in crystalline specimens and to sample thickness variation. Second, elemental maps have been recorded by using the three-window method (two pre-edge images and one post-edge image). Third, ratio images have been acquired by using the two-window method (one pre-edge window and one post-edge window). These ratio images show elemental contrast with lower noise than the elemental maps and are nearly free of the diffraction artifacts. We have successfully used ratio images to detect very small precipitates of diameters ranging from 2 to 10 nm in the materials mentioned above. However, ratio images have to be used carefully, because they are susceptible to artifacts.

Quantitative analysis of EFTEM elemental distribution images

Energy-filtering TEM (EFTEM) can be used to record elemental distribution images at nanometer resolution and with short acquisition times. In this paper we show how elemental maps can be converted into concentration maps. In order to demonstrate the application of the quantification procedures, we have chosen a sample consisting of CVD grown titanium carbonitride layers on a hard metal. Two approaches have been tested: Absolute quantification which is successfully applied to biological (amorphous) specimens yields a concentration map in terms of atoms per unit area. However, it turned out that this method is not suitable for crystalline materials due to diffraction and/or thickness variation effects. In the second method, atomic ratio maps are calculated from two elemental maps by ratioing the elemental maps and dividing them by the partial ionization cross-sections (or k-factors). This method yields concentration maps in terms of atomic ratios offering the advantage that diffraction and/or thickness variation effects are eliminated. Therefore, this method is well suited for the quantification of crystalline materials science specimens. In the second part of the paper we describe how related sets of elemental maps can be examined and combined in one chemical phase map. This can be provided by scatter diagram analysis (2-dimensional) and automatic classification procedures (n-dimensional) that show how intensities of corresponding pixels are correlated. These techniques have been applied to a typical material science specimen (Si-nitride ceramics with SiC and carbon inclusions) so that the reader may get a feeling for the advantages and limitations of these techniques in EFTEM-investigations. Finally, it is shown that the scatter diagram technique can be also applied to atomic ratio maps thus providing fully quantitative chemical phase maps.

Electron energy-loss near-edge structures of 3d transition metal oxides recorded at high-energy resolution.

Near-edge fine structures of the metal L(2,3) and O K-edges in transition metal-oxides have been studied with a transmission electron microscope equipped with a monochromator and a high-resolution imaging filter. This system enables the recording of EELS spectra with an energy resolution of 0.1eV thus providing new near-edge fine structure details which could not be observed previously by EELS in conventional TEM instruments. EELS-spectra from well-defined oxides like titanium oxide (TiO(2)), vanadium oxide (V(2)O(5)), chromium oxide (Cr(2)O(3)), iron oxide (Fe(2)O(3)), cobalt oxide (CoO) and nickel oxide (NiO) have been measured with the new system. These spectra are compared with EELS data obtained from a conventional microscope and the main spectral features are interpreted. Additionally, the use of monochromised TEMs is discussed in view of the natural line widths of K and L(2,3) edges.

Dark plasmonic breathing modes in silver nanodisks.

We map the complete plasmonic spectrum of silver nanodisks by electron energy loss spectroscopy and show that the mode which couples strongest to the electron beam has radial symmetry with no net dipole moment. Therefore, this mode does not couple to light and has escaped from observation in optical experiments. This radial breathing mode has the character of an extended two-dimensional surface plasmon with a wavenumber determined by the circular disk confinement. Its strong near fields can impact the hybridization in coupled plasmonic nanoparticles as well as couplings with nearby quantum emitters.

Electron microscopy of nanoemulsions: an essential tool for characterisation and stability assessment.

The characterisation of pharmaceutical formulations by microscopic techniques is essential to obtain reliable data about the actual morphology of the system. Since the size range of colloidal drug delivery systems has long ago reached the lower end of the nanometer scale, classical light microscopy has been replaced by electron microscopy techniques which provide sufficient resolution for the visualisation of nano-sized structures. Indeed, the superior resolution and methodological versatility of electron microscopy has rendered this technique an indispensable tool for the analysis of nanoemulsions. Microscopic analysis of these lipid-based drug delivery systems with particle sizes in the lower submicron range provides critical information about the size, shape and internal structure of the emulsion droplets. Moreover, surfactant aggregates such as liposomes or multilamellar structures which remain unnoticed during particle size measurements can be detected in this fashion. This review provides a brief overview about both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques which have been employed to characterise nanoemulsions. Of special interest are sophisticated cryo techniques of sample preparation for both TEM and SEM which deliver high-quality images of nanoemulsions in their natural state. An overview about the instrumentation and sample preparation for all presented methods is given. Important practical aspects, sources of error and common artefacts as well as recent methodological advances are discussed. Selected examples of electron microscopic studies of nanoemulsions are presented to illustrate the potential of this technique to reveal detailed and specific information.

Electron microscopical characterization of Sn/SnSb composite electrodes for lithium-ion batteries

Abstract Lithium storage alloys such as Sn/SnSb are promising new anode materials for Li-ion batteries. Due to a proper design of the active Sn/SnSb material as well as the composite electrode, capacities exceeding 500 mAh g −1 have been achieved with this system for more than 30 cycles. The observation of micro- and nano-structural changes in the composite electrode during charge/discharge cycling is of immense importance for a further improvement of the cycling performance. Electron microscopy (SEM and TEM) in combination with analytical techniques (EFTEM, EDXS and EELS) has been used for the characterization of Sn/SnSb raw powder as well as the Sn/SnSb composite electrodes. The pristine morphology and the changes of morphology during cycling of the electrode material have been studied. Furthermore, the chemical composition and particularly compositional fluctuations within the composite material have been investigated using EFTEM and EDXS. The electron microscopy results indicate that parts of the active material get finer during the initial cycles. Moreover, amorphous regions are detected in the cycled material. The experimental results are discussed with regard to the reaction mechanism of SnSb with Li.

Precipitation of NBC in a model austenitic steel

A model Fe–30 wt% Ni, 0.1 C, 1.61 Mn, 0.1 Nb microalloyed steel, that simulates conventional microalloyed C–Mn steels, but does not transform from the austenite phase on cooling, is reported. Plane strain compression testing was undertaken at 950°C at a constant true strain rate of 10 s−1. Samples were deformed in a two stage process. An initial true strain of 0.25–0.45 was followed by unloading, a hold of 1–1000 s and a final deformation to a total true strain of 0.5–0.9. A single deformation was undertaken under identical conditions, but to the total true strain of the double deformation tests. Electron spectroscopic imaging (ESI) in the TEM was used to determine precipitate size and distribution. A 1 s hold time between equal strains of ϵ=0.25 was sufficient for appreciable strain induced precipitation, although 40% static recrystallisation occurred during the hold time. Precipitation occurred entirely on dislocations, present principally as microband walls but also as a rudimentary cell structure within the microbands. No evidence was found for NbC precipitation in the matrix, which therefore remains supersaturated with Nb. NbC particle diameter was in the range 2.5–15 nm, with a density of 3.8×1021 particles/m3 for a 100 s delay period between two strains of ϵ=0.45 at 950°C. Both the size and number density are consistent with those observed in conventional microalloyed C–Mn steels. The behaviour of the model microalloyed Fe–30 Ni steel is discussed in relation to the data on conventional microalloyed steels.

Investigation of the formation of CuInS2 nanoparticles by the oleylamine route: comparison of microwave-assisted and conventional syntheses.

The formation of copper indium disulfide nanoparticles via the oleylamine route using copper iodide, indium chloride, and elemental sulfur has been investigated by applying conventional thermal heating as well as microwave irradiation. Oleylamine thereby acts as a capping ligand as well as a solvent. In an initial set of experiments, the onset of the reaction was determined to be around 115 °C by an in situ X-ray study using Synchrotron radiation. Using comparatively low synthesis temperatures of 120 °C, it is already possible to obtain nanoparticles of 2-4 nm with both heating methods but with irregular shape and size distribution. By applying higher temperatures of 220 °C, more crystalline and larger nanoparticles were obtained with slight differences in crystallite size and size distribution depending on the synthesis route. The size of the nanoparticles is in the range of 3-10 nm depending on the heating time. Using microwave irradiation, it is possible to obtain nanoparticles in only 90 s of total synthesis time. Control experiments to probe a nonthermal microwave effect were carried out ensuring an identical experimental setup, including the heating profile, the stirring rate, and the volume and concentration of the solutions. These experiments clearly demonstrate that for the preparation of CuInS(2) nanoparticles described herein no differences between conventional and microwave heating could be observed when performed at the same temperature. The nanoparticles obtained by microwave and thermal methods have the same crystal phase, primary crystallite size, shape, and size distribution. In addition, they show no significant differences concerning their optical properties.

New examples for near-edge fine structures in electron energy loss spectroscopy

Abstract The electron energy loss near-edge structures of the K and L edges of elements occurring in sulfides and oxides of copper and zinc have been investigated. Furthermore, the K edge of carbon in some carbonates and the L edges of sulfur and phosphorus in sulfates and phosphates have been measured and the near-edge fine structures of these edges have been found to be characteristic for the complex ions CO 2- 3 , SO 2- 4 . Thus, near-edge fine structures of EELS edges can be very useful as a fingerprint for rapid identification of chemical compounds such as carbonates, sulfates, phosphates and some oxides in the TEM.

Universal dispersion of surface plasmons in flat nanostructures

Dimensionality has a significant impact on the optical properties of solid-state nanostructures. For example, dimensionality-dependent carrier confinement in semiconductors leads to the formation of quantum wells, quantum wires and quantum dots. While semiconductor properties are governed by excitonic effects, the optical response of metal nanostructures is dominated by surface plasmons. Here we find that, in contrast to excitonic systems, the mode dispersions in plasmonic structures of different dimensionality are related by simple scaling rules. Employing electron energy loss spectroscopy, we show that the modes of silver nanodisks can be scaled to the surface and edge modes of extended silver thin films. We thereby introduce a general and intuitive ordering scheme for plasmonic excitations with edge and surface modes as the elementary building blocks.