Gerald Kothleitner

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.

Tuning the Magnetic Properties of Metal Oxide Nanocrystal Heterostructures by Cation Exchange

For three types of colloidal magnetic nanocrystals, we demonstrate that postsynthetic cation exchange enables tuning of the nanocrystal’s magnetic properties and achieving characteristics not obtainable by conventional synthetic routes. While the cation exchange procedure, performed in solution phase approach, was restricted so far to chalcogenide based semiconductor nanocrystals, here ferrite-based nanocrystals were subjected to a Fe2+ to Co2+ cation exchange procedure. This allows tracing of the compositional modifications by systematic and detailed magnetic characterization. In homogeneous magnetite nanocrystals and in gold/magnetite core shell nanocrystals the cation exchange increases the coercivity field, the remanence magnetization, as well as the superparamagnetic blocking temperature. For core/shell nanoheterostructures a selective doping of either the shell or predominantly of the core with Co2+ is demonstrated. By applying the cation exchange to FeO/CoFe2O4 core/shell nanocrystals the Neél temperature of the core material is increased and exchange-bias effects are enhanced so that vertical shifts of the hysteresis loops are obtained which are superior to those in any other system.

Optimization of the Signal to Noise Ratio in EFTEM Elemental Maps with Regard to Different Ionization Edge Types

Abstract An energy filtering transmission microscope can provide information about the spatial distribution of an element by selecting an energy loss corresponding to a characteristic inner-shell excitation. One parameter affecting the obtainable resolution, as well as the detection limit of such core-loss images (elemental maps), is the signal to noise ratio. Statistical noise due to low inelastic cross-sections makes weak-contrast features invisible, leading to a loss of resolution and a deterioration of the detection limits. The information content therefore is largely determined by the noise level in the recorded maps. As a consequence, improving the detection limits means first optimizing the signal to noise ratio (SNR). Since electron energy-loss spectroscopy (EELS) is the basis for energy filtering, EEL spectra can be used as a basis to calculate optimal values of the SNR for different ionization edges and the results shall be directly applicable to core-loss imaging. In this paper we describe the procedures which have been employed for these calculations and discuss the results, giving useful information about some setup parameters for elemental mapping with different edges. The calculated imaging parameters can then be interpreted in terms of the attainable spatial resolution in the elemental maps.

High resolution EELS using monochromator and high performance spectrometer: comparison of V2O5 ELNES with NEXAFS and band structure calculations.

Using single crystal V2O5 as a sample, we tested the performance of the new aberration corrected GATAN spectrometer on a monochromatised 200 kV FEG FEI (S)TEM. The obtained V L and O K ELNES were compared with that obtained in a common GATAN GIF and that in the new spectrometer, without monochromatised beam. The performance of the new instrumentation is impressive: recorded with an energy-resolution of 0.22 eV, the V L(3) edge reveals all the features due to the bulk electronic structure, that are also revealed in near-edge X-ray absorption fine structure (NEXAFS) with a much higher energy-resolution (0.08 eV). All features of the ELNES and NEXAFS are in line with a theoretical spectrum derived from band-structure calculations.

The influence of beam defocus on volume growth rates for electron beam induced platinum deposition.

Electron beam induced deposition (EBID) is a versatile method for the controlled fabrication of conducting, semi-conducting and non-conducting structures down to the nanometer scale. In contrast to ion beam induced deposition, EBID processes are free of sputter effects, ion implantation and massive heat generation; however, they have much lower deposition rates. To push the deposition efficiency further towards its intrinsic limits, the individual influences of the process parameters have to be explored. In this work a platinum pre-cursor is used for the deposition of conducting nanorods on highly oriented pyrolytic graphite. The study shows the influence of a beam defocus during deposition on the volume growth rates. The temporal evolution of volume growth rates reveals a distinct maximum which is dependent on the defocus introduced, leading to an increase of deposited volumes by a factor 2.5 after the same deposition times. The observed maximum is explained by an increasing and saturating electron yield contributing to the final deposition process and constantly decreasing diffusion abilities of the pre-cursor molecules toward the tip of the nanorods, which is further supported by dwell time experiments.

On the quantitativeness of EDS STEM.

Chemical mapping using energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) has recently shown to be a powerful technique in analyzing the elemental identity and location of atomic columns in materials at atomic resolution. However, most applications of EDS STEM have been used only to qualitatively map whether elements are present at specific sites. Obtaining calibrated EDS STEM maps so that they are on an absolute scale is a difficult task and even if one achieves this, extracting quantitative information about the specimen - such as the number or density of atoms under the probe - adds yet another layer of complexity to the analysis due to the multiple elastic and inelastic scattering of the electron probe. Quantitative information may be obtained by comparing calibrated EDS STEM with theoretical simulations, but in this case a model of the structure must be assumed a priori. Here we first theoretically explore how exactly elastic and thermal scattering of the probe confounds the quantitative information one is able to extract about the specimen from an EDS STEM map. We then show using simulation how tilting the specimen (or incident probe) can reduce the effects of scattering and how it can provide quantitative information about the specimen. We then discuss drawbacks of this method - such as the loss of atomic resolution along the tilt direction - but follow this with a possible remedy: precession averaged EDS STEM mapping.

Formation of bimetallic clusters in superfluid helium nanodroplets analysed by atomic resolution electron tomography

Structure, shape and composition are the basic parameters responsible for properties of nanoscale materials, distinguishing them from their bulk counterparts. To reveal these in three dimensions at the nanoscale, electron tomography is a powerful tool. Advancing electron tomography to atomic resolution in an aberration-corrected transmission electron microscope remains challenging and has been demonstrated only a few times using strong constraints or extensive filtering. Here we demonstrate atomic resolution electron tomography on silver/gold core/shell nanoclusters grown in superfluid helium nanodroplets. We reveal morphology and composition of a cluster identifying gold- and silver-rich regions in three dimensions and we estimate atomic positions without using any prior information and with minimal filtering. The ability to get full three-dimensional information down to the atomic scale allows understanding the growth and deposition process of the nanoclusters and demonstrates an approach that may be generally applicable to all types of nanoscale materials.

Optimization of postgrowth electron-beam curing for focused electron-beam-induced Pt deposits

The authors use focused electron-beam-induced Pt deposition from a gaseous (CH3)3CH3C5H4Pt precursor for the fabrication of electrically conductive structures consisting of Pt nanocrystals embedded in a carbon containing matrix. Recently it has been demonstrated that the electrical resistivity of such deposits can be strongly improved via postgrowth electron irradiation. This study shows very strong evidence that incompletely and nondissociated precursor molecules incorporated within the deposits during deposition are the key elements for efficient e-beam curing. During the early stages of e-beam curing these fragments are further dissociated, which leads to slight growth of the Pt nanocrystals. This is further supported by variable growth regime experiments during deposition which can be used to enhance the incorporation of incompletely and nondissociated precursor molecules, resulting in higher curing efficiencies and lower electrical resistivities. The absence of a predominant graphitization of the sur...

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.