Bert Freitag

Detection of Single Atoms and Buried Defects in Three Dimensions by Aberration-Corrected Electron Microscope with 0.5-Å Information Limit

The ability of electron microscopes to analyze all the atoms in individual nanostructures is limited by lens aberrations. However, recent advances in aberration-correcting electron optics have led to greatly enhanced instrument performance and new techniques of electron microscopy. The development of an ultrastable electron microscope with aberration-correcting optics and a monochromated high-brightness source has significantly improved instrument resolution and contrast. In the present work, we report information transfer beyond 50 pm and show images of single gold atoms with a signal-to-noise ratio as large as 10. The instruments new capabilities were exploited to detect a buried Sigma3 {112} grain boundary and observe the dynamic arrangements of single atoms and atom pairs with sub-angstrom resolution. These results mark an important step toward meeting the challenge of determining the three-dimensional atomic-scale structure of nanomaterials.

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.

Enhanced Detection Sensitivity with a New Windowless XEDS System for AEM Based on Silicon Drift Detector Technology

For many years now, the combination of the modern S/TEM system (scanning/transmission electron microscope) with the X-ray energy dispersive spectrometer (XEDS) has resulted in Analytical Electron Microscopes (AEMs) able to deliver both high-resolution imaging and elemental composition maps in the same instrument. This ability to correlate local elemental composition with microstructure has greatly broadened the applications realm of the S/TEM instrument. The boundaries of performance for many of these applications are now determined by limits in XEDS system detection sensitivity. In this article, we describe an AEM with greatly enhanced detection sensitivity due to a number of innovations in the system architecture, including: a high-brightness Schottky FEG source, four detectors integrated deeply into the objective lens, windowless silicon drift detector technology with shutters, and high-speed electronics readout. This new system architecture provides many performance benefits, such as improved light element detection, better sample tilt response, faster mapping, and especially enhanced system detection sensitivity.

Surface Chemistry of Ag Particles: Identification of Oxide Species by Aberration-Corrected TEM and by DFT Calculations

In the 1990s, by means of spectroscopic methods, Ertl et al. found surface and subsurface oxygen atoms on and in Ag catalysts. Three species of atomic oxygen with distinct structural and energetic properties were identified. According to their TDS behavior (TDS: thermal desorption spectroscopy), a surface atomic species was termed the a form, and a bulk-dissolved species of lower interaction energy the b form. Finally, g-oxygen was identified as strongly interacting atomic oxygen with high electron density, incorporated into the top atomic surface layer of Ag. As silver catalysts are used in many reactions, for example, hydrogenation of unsaturated aldehydes, partial oxidation of methanol to formaldehyde, and oxidative coupling of methane to ethane and ethylene, the discovery of surface and subsurface oxygen atoms in Ag is of great significance for understanding the catalytic reaction steps and mechanisms of silver catalysts. However, the location of surface and subsurface oxygen atoms remains an unanswered question in Ag catalysis and, perhaps more importantly, it is unclear whether nonmodel industrial catalysts exhibit the same surface chemistry. High-resolution transmission electron microscopy (HRTEM) has been widely used to study the morphology and structure of catalysts. It provides detailed information on the microand nanostructure of catalysts. By aligning the normal of a given surface perpendicular to the incident electron beam, the surface structure and its relationship to the underlying bulk structure can be investigated. However, due to the artefacts caused by spherical aberration of magnetic imaging lenses, conventional TEM is not optimally suited to obtaining readily interpretable images of catalyst surfaces. One major artefact is the delocalization of image details, which appears as an extension of the perimeter of a sample beyond the actual surface. In this study, we investigated an Ag/SiO2 catalyst using a TEM with a sphericalaberration corrector that can compensate for these problems and thus provide detailed information about the structure of the surfaces of a silver-based catalyst. With the support of DFT calculations, the positions of aand g-oxygen on the surfaces of Ag particles have been determined for the first time. Furthermore, the presence of local surface oxygen atoms or oxide was verified. To investigate Ag particles by a direct imaging technique, spatial frequencies in the band between 4.24 and 4.89 nm 1 representing Ag(111) and Ag(200) lattice-plane distances of 0.236 and 0.204 nm, respectively, must be transferred with the same contrast. Under the experimental conditions shown in Figure 1 the acquired high-resolution electron micrograph makes the surface terminations of the Ag particle clearly observable (Figure S1 and Figure 2). The internal crystalline structure of the Ag particles extends to the surface, where it is terminated abruptly in different ways. Steps consisting of one or two atom rows on the (111) facet are observed.

Electron Tomography on Micrometer-Thick Specimens with Nanometer Resolution

Transmission electron microscopy (TEM) is a well-established technique to explore matter down to the atomic scale. TEM tomography methods have been developed to obtain volume information at the mesoscopic dimensions of devices or complex mixtures of multiphase objects with nanometer resolution, but these methods are in general only applicable to relatively thin specimens with a few hundred nanometer thickness at most. Here we introduce an approach based on scanning TEM (STEM) tomography that pushes the resolution in three dimensions down to a few nanometers for several micrometer ultrathick specimens using a conventional TEM with 300 kV accelerating voltage, and we demonstrate its versatility for materials research and nanotechnology.

XEDS STEM tomography for 3D chemical characterization of nanoscale particles.

We present a tomography technique which couples scanning transmission electron microscopy (STEM) and X-ray energy dispersive spectrometry (XEDS) to resolve 3D distribution of elements in nanoscale materials. STEM imaging when combined with XEDS mapping using a symmetrically arranged XEDS detector design around the specimen overcomes many of the obstacles in 3D chemical imaging of nanoscale materials and successfully elucidates the 3D chemical information in a large field of view of the transmission electron microscopy (TEM) sample. We employed this technique to investigate 3D distribution of Nickel (Ni), Manganese (Mn) and Oxygen (O) in a Li1.2Ni0.2Mn0.6O2 (LNMO) nanoparticle used as a cathode material in Lithium (Li) ion batteries. For this purpose, 2D elemental maps were acquired for a range of tilt angles and reconstructed to obtain 3D elemental distribution in an isolated LNMO nanoparticle. The results highlight the strength of this technique in 3D chemical analysis of nanoscale materials by successfully resolving Ni, Mn and O elemental distributions in 3D and discovering the new phenomenon of Ni surface segregation in this material. Furthermore, the comparison of simultaneously acquired high angle annular dark field (HAADF) STEM and XEDS STEM tomography results shows that XEDS STEM tomography provides additional 3D chemical information of the material especially when there is low atomic number (Z) contrast in the material of interest.

An integrated Silicon Drift Detector System for FEI Schottky Field Emission Transmission Electron Microscopes

Silicon Drift Detectors (SDD) [1] are rapidly replacing Si(Li) detectors for EDX microanalysis in SEM, but have yet to have an impact in the S/TEM world. Main reason for this difference is the low count rate created by thin S/TEM samples compared to the bulk samples in SEM . These low count rates make EDX mapping a very slow process in S/TEM. However, the recent introduction of higher brightness electron sources [2] and probe Cs-correctors has led to significantly increased beam currents in small electron probes and, potentially, to higher EDX count rates. Since a key advantage of the SDD is the high count rate capability, the throughput improvement compared to the Si(Li) detectors will be considerable in these new instruments. Compared to SEM, the smaller excited volumes obtained with the atomic-scale probes in the new S/TEM instruments can lead to radiation damage of beam-sensitive materials before the analysis is completed. Therefore S/TEM microanalysis needs not only the higher count rate capability, but also higher collection efficiency of the X-rays generated, in order to reduce the dose on the sample. In this paper we present a new prototype EDX detector system for an FEI 200kV TEM/STEM, in which FEI has integrated a detector system consisting of multiple SDDs, placed symmetrically around the electron beam axis in the objective lens chamber without affecting the S/TEM resolution. The SDDs with a total active area of 120 mm were designed by PN Sensor to fit into the FEI design to achieve a quantum leap in solid angle of collection compared to previous designs in S/TEMs. The SDDs are cooled to achieve the optimum energy resolution, typically below 130 eV. The windowless design allows for better sensitivity for light-element detection than conventional thin-window detectors. The specially designed front-end electronics and ultra fast multi-channel pulse processor are provided by Bruker AXS MA in collaboration with FEI. The processor is capable of fast mapping with pixel dwell times down to a few microseconds and >100 kcps count rates per channel. Compared to currently available Si(Li) detectors the anticipated count rates will be an order of magnitude higher with the new detector. Additionally the new high brightness gun of FEI (X-FEG) [2] increases the brightness of the electron source compared to conventional Schottky sources, leading to a further increase in count rate, and an equivalent significant decrease in mapping time at the same spatial resolution. This improvement is illustrated in Fig. 1 where the relative minimum detectable mass MDM ~ (t.P.P/B) (t=analysis time, P=elemental peak counts, P/B = peak-to-background ratio) [3] is shown for conventional and new EDX detector count rates at the same spatial resolution. Fig.1 also compares the MDM with EELS and, for the specific case of strontium titanate, shows that the new EDX detector is expected to be more sensitive than EELS. Further results will be reported at the conference. Microsc Microanal 15(Suppl 2), 2009 Copyright 2009 Microscopy Society of America doi: 10.1017/S1431927609094288 208

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.

Benefits of microscopy with super resolution

Transmission Electron Microscopy developed from an imagingtool into a quantitative electron beam characterization tool that locallyaccesses structure, chemistry, and bonding in materials with sub Angstromresolution. Experiments utilize coherently and incoherently scatteredelectrons. In this contribution, the interface between gallium nitrideand sapphire as well as thin silicon gate oxides are studied tounderstand underlying physical processes and the strength of thedifferent microscopy techniques. An investigation of the GaN/sapphireinterface benefits largely from the application of phase contrastmicroscopy that makes it possible to visualize dislocation corestructures and single columns of oxygen and nitrogen at a closest spacingof 85 pm. In contrast, it is adequate to investigate Si/SiOxNy/poly-Siinterfaces with incoherently scattered electrons and electronspectroscopy because amorphous and poly crystalline materials areinvolved. Here, it is demonstrated that the SiOxNy/poly-Si interface isrougher than the Si/SiOx interface, that desirable nitrogen diffusiongradients can be introduced into the gate oxide, and that a nitridationcoupled with annealing increases its physical width while reducing theequivalent electrical oxide thickness to values approaching 1.2 nm.Therefore, an amorphous SiNxOy gate dielectric seems to be a suitablesubstitute for traditional gate oxides to further increase device speedby reducing dimensions in Si technology.

An integrated multiple silicon drift detector system for transmission electron microscopes

A new EDX system, consisting of multiple SDDs has been developed for an FEI 200kV TEM/STEM in which the SDDs, designed by PN Sensor with a total collection angle approaching 1 srad has been obtained by placing 4 SDDs symmetrically around the electron beam axis in the objective lens chamber. The massive increase in solid angle of collection compared to previous designs in S/TEMs leads to a huge reduction in the time for EDX mapping. First results from the detector are reported.