Werner Grogger

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.

Fabrication of n‐ and p‐Type Organic Thin Film Transistors with Minimized Gate Overlaps by Self‐Aligned Nanoimprinting

[∗] U. Palfi nger , C. Auner , H. Gold , A. Haase , J. Kraxner , G. Jakopic , J. R. Krenn , B. Stadlober Institute of Nanostructured Materials and Photonics Joanneum Research GmbH Franz-Pichlerstrasse 30, A-8160 Weiz (Austria) Fax: 0043-316-876-2710 Telephone: 0043-316-876-2721 E-mail: barbara.stadlober@joanneum.at T. Haber , M. Sezen , W. Grogger Institute for Electron Microscopy Graz University of Technology Steyrergasse 17, A-8010 Graz (Austria) G. Domann Fraunhofer-Institut für Silicatforschung ISC Neunerplatz 2, D-97082 Würzburg (Germany)

High resolution electron microscopy of Ag-clusters in crystalline and non-crystalline morphologies grown inside superfluid helium nanodroplets

We present a first investigation of structural properties of Ag clusters with a diameter of up to 5.5 nm grown inside superfluid helium nanodroplets (He(N)) and deposited on an amorphous C surface. With high resolution transmission electron microscope images we are able to show that in addition to the crystalline face centered cubic (fcc) structure, noncrystalline icosahedral (Ih), and decahedral (Dh) morphologies are grown. Relative abundances (56% fcc, 31% Dh, and 13% Ih) as well as the size distribution of each morphology (mean diameters d(fcc)=2.62(5) nm, d(Dh)=3.34(7) nm, and d(Ih)=3.93(2) nm) do not reflect the situation expected from pure energetic considerations, where small Ihs should be followed by medium sized Dhs and large fccs. Instead, kinetic factors seem to play an important role in the formation of these structures, as it appears to be the case for clusters formed by inert gas aggregation. Considering the low temperatures (0.37 K) and extremely high cooling rates, we discuss basic ideas that might lead to a qualitative picture of the cluster formation process inside He(N).

Energy-filtering TEM at high magnification: spatial resolution and detection limits.

Energy-filtering TEM (EFTEM) has turned out to be a very efficient and rapid tool for the chemical characterization of a specimen on a nanometer and even subnanometer length scale. Especially, the detection and measurement of very thin layers has become a great application of this technique in many materials science fields, e.g. semiconductors and hard disk technology. There, the reliability of compositional profiles is an important issue. However, the experimentally obtainable spatial resolution strongly influences the appearance of a thin layer in an EFTEM image, when dimensions reach subnanometer levels, which mainly leads to a broadening of the layer in the image. This fact has to be taken into account, when measuring the thickness of such a thin layer. Additionally, the convolution decreases contrast which makes the layer less visible in the image and finally determines the detection limit. In this work we present a systematic study on specifically designed Mn/PdMn multilayer test specimens to explore the practical aspects of spatial resolution and detection limits in EFTEM. Although specific to the ionization edges used, we will present general conclusions about the practical limitations in terms of EFTEM spatial resolution. Additionally, work will be shown about low energy-loss imaging of thin oxide layers, where delocalization is the main factor responsible for broadening.

Silicon: The key element in early stages of biocalcification.

Biocalcification is a widespread process of forming hard tissues like bone and teeth in vertebrates. It is also a topic connecting life sciences and earth sciences: calcified skeletons and shells deposited as sediments represent the earths fossil record and are of paramount interest for biogeochemists trying to get an insight into the past of our planet. This study reports on the role of silicon in the early biocalcification steps, where silicon and calcium were detected on the surface of cyanobacteria (initial stage of lacustrine calcite precipitation) and in crustacean cuticles. By using innovative methodological approaches of correlative microscopy (AFM in combination with analytical TEM: EFTEM, EELS) the chemical form of silicon in biocalcifying matrices and organic-inorganic particles is determined. Previously, silicon was reported to be localized in active growth areas in the young bone of vertebrates. We have found evidence that biocalcification in evolutionarily distant organisms involves very similar initial phases with silicon as a key element at the organic-inorganic interface.

Monochromated, spatially resolved electron energy-loss spectroscopic measurements of gold nanoparticles in the plasmon range.

Gold nanoparticles show optical properties different from bulk material due to resonance phenomena which depend on local structure and geometry. Electron energy-loss spectrometry (EELS) in scanning transmission electron microscopy (STEM) allows the spatially resolved measurement of these properties at a resolution of few nanometers. In this work, the first monochromated measurements of gold nanoparticles (spheres, rods and triangles) are presented. Due to the improved energy resolution of about 0.2 eV, surface plasmon excitations at energies below 1 eV could be accurately measured from raw experimental data.

Microstructure and ionic conductivity of strontium-substituted lanthanum cobaltites

Abstract La 0.4 Sr 0.6 CoO 3− δ powders were synthesized by the glycine nitrate process. X-ray powder diffraction and selected area electron diffraction (SAED) in the transmission electron microscope (TEM) were used to determine the basic crystal structure of the perovskite samples. Additionally, energy-filtering transmission electron microscopy (EFTEM) was used to evaluate the homogeneity of the samples at the nanometer level. The ionic conductivity σ i of La 0.4 Sr 0.6 CoO 3− δ was obtained from galvanostatic polarization experiments as a function of oxygen non-stoichiometry for 3− δ values between 2.70 and 2.81 (corresponding to p O 2 values between 10 −4 and 10 −1 bar) at 775 and 825 °C. It could be observed that σ i shows a maximum which shifts towards larger values of the oxygen non-stoichiometry with increasing temperature. The maximum value was observed at 3− δ =2.79 and 2.77 for 775 and 825 °C, respectively. This behavior has been reported to be indicative of the formation of vacancy-ordered structures which are expected to lower the mobility of oxygen vacancies. The TEM investigations revealed a superstructure within microdomains which were crystallographically oriented perpendicular to each other and were found to be around 100 nm in size.

Analysis of native structures of soft materials by cryo scanning probe tomography

We propose a universal tool for structural analysis of soft/frozen-hydrated materials at a macromolecular level over a wide range of temperatures (−120 to +50 °C). The cryogenic scanning probe microscope presented here allows detailed investigation and serial-section 3D tomography of soft matter under native-like conditions.