Pleun Dona

Visualization of oscillatory behaviour of Pt nanoparticles catalysing CO oxidation

Many catalytic reactions under fixed conditions exhibit oscillatory behaviour. The oscillations are often attributed to dynamic changes in the catalyst surface. So far, however, such relationships were difficult to determine for catalysts consisting of supported nanoparticles. Here, we employ a nanoreactor to study the oscillatory CO oxidation catalysed by Pt nanoparticles using time-resolved high-resolution transmission electron microscopy, mass spectrometry and calorimetry. The observations reveal that periodic changes in the CO oxidation are synchronous with a periodic refacetting of the Pt nanoparticles. The oscillatory reaction is modelled using density functional theory and mass transport calculations, considering the CO adsorption energy and the oxidation rate as site-dependent. We find that to successfully explain the oscillations, the model must contain the phenomenon of refacetting. The nanoreactor approach can thus provide atomic-scale information that is specific to surface sites. This will improve the understanding of dynamic properties in catalysis and related fields.

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

A MEMS-based heating holder for the direct imaging of simultaneous in-situ heating and biasing experiments in scanning/transmission electron microscopes

The introduction of scanning/transmission electron microscopes (S/TEM) with sub‐Angstrom resolution as well as fast and sensitive detection solutions support direct observation of dynamic phenomena in‐situ at the atomic scale. Thereby, in‐situ specimen holders play a crucial role: accurate control of the applied in‐situ stimulus on the nanostructure combined with the overall system stability to assure atomic resolution are paramount for a successful in‐situ S/TEM experiment. For those reasons, MEMS‐based TEM sample holders are becoming one of the preferred choices, also enabling a high precision in measurements of the in‐situ parameter for more reproducible data.

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.

Live Imaging of Reversible Domain Evolution in BaTiO3 on the Nanometer Scale Using In Situ STEM and TEM

1. Department of Physics and Astronomy, School of Mathematics and Physics, Queens University Belfast, UK, BT7 1NN 2. FEI Company, Europe NanoPort, Achtseweg Noord 5, 5651 GG Eindhoven, The Netherlands There is an increasing interest in novel ferroic materials, especially in device applications such as transistors, memory devices, tunneling barriers, etc.. The functionality of such materials is enabled by the reversible switching between equivalent states (or domains) that form to minimize the system’s free energy. This switching behavior depends strongly on the domain structure pattern and their mobility under external stimuli (electrical, mechanical, temperature, etc.). There is a strong need to study this switching in detail. Nanoscale domain structures and their specific switching behavior strongly influence the material responses and properties such as dielectric permittivity, piezoelectric coefficients and remnant polarization. Fortunately in-situ (scanning) transmission electron microscopy (S/TEM) represents a powerful technique for studying ferroic materials and their switching behavior with resolutions down to the atomic scale. Here, the domain pattern evolution in BaTiO

Electron Microscopy Advances for Studies of Catalysis at Atomic-Resolution and at Ambient Pressure Levels

1. Haldor Topsoe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark 2. CINF, Technical University of Denmark, Fysikvej building 307, 2800 Kgs. Lyngby, Denmark 3. ChemE, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands 4. FEI Company, Acthtseweg Noord 5, 5651 GG Eindhoven, The Netherlands 5. DIMES-ECTM, Delft University of Technology, P.O. Box 5053, 2600 GB Delft, The Netherlands 6. Leiden Probe Microscopy BV, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands 7. Albemarle Catalyst Company BV, P.O. Box 37650, 1030 BE Amsterdam, The Netherlands