Luigi Mele

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.

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.

MEMS-based Heating Element for in-situ Dynamical Experiments on FIB/SEM Systems

In-situ observation of microstructural evolution of solids such as recrystallization, grain growth and phase changes in SEM is important for various fields of material science and industry research. This technique requires reliable discrimination of differently oriented crystal phases combined with useful spatial and temporal resolution and with fast and precise control of specimen temperature. While the requirements on spatial and temporal resolution are satisfied by current SEMs with resolution below 1 nm and 100 Hz frame rate, existing heating holders for bulk samples only allow for heating rates up to 300oC per minute (5oC/s). Long ramping time, which is required during heating experiments done using these devices, may cause unwanted sample changes (e.g. oxidation or recrystallization) before the temperature range of interest is reached. Thermal radiation of massive heating holders decreases quality of material contrast imaging as the commonly used detectors of backscattered electrons become saturated by thermally emitted photons. MEMS-based heating holder [1], [2] in combination with in-situ site specific sample preparation using a FIB/SEM system brings significant improvement in instrumentation for in-situ heating experiments inside the SEM chamber.

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

Particle Imaging and Flow Visualization of In-situ TEM Nanoreactors

Loading the nano particles into a nanoreactor can be problematic due to surface charge, uniformity of particle size distribution and the design of the nanoreactor. In order to study the behavior of particle motion in the nanoreactor, a qualitative flow visualization of colloidal polystyrene suspension in the nanoreactor was analyzed. Three different particle sizes were tested: 0.2, 0.5 and 1 µm. A short wave length light source (i.e., mercury lamp) was used to illuminate a suspension of fluorescent PS particles. The particle motion was visualized and recorded using an inverted Zeiss Axiovert 200 fluorescence microscope, a CCD camera and processed using DAVIS imaging software. The loading method was to put one drop of suspension in the inlet, then let the capillary forces suck the suspension into the nanoreactor channel. In this study we used two types of nanoreactor. The first nanoreactor was wafer bonded nanoreactor [2]. The nanoreactor was fabricated based on silicon fusion bonding and thin film encapsulation for sealed lateral electrical feedthroughs. The schematic cross section of the device is shown in Figure 1. The device had a narrow channel of 2 µm height which enabled the particles up to 1 µm particle size to enter. During the loading some bubbles formed on the heating area. The disappeared with time as the flow of the suspension continues. The dried nanoreactor showed that many particles were present on the heating area. The second nanoreactor was fabricated by surface micromachining [3]. The 3D sketch of the device is shown in Figure 2. The device had a shallow channel of 0.5 µm. To increase the stiffness and prevent bulging, the top and bottom parts of the device are held by a line of pillars with the spacing of 20 µm along the channel. During the fabrication, some holes were formed on the SiNx layer for sacrificial etching, to form the channel, and to do internal coating of the channel. At the end of the fabrication, the holes were plugged by PECVD SiNx. The design of these plugs caused some particles to be trapped inside during loading. The trapped particles and crowded pillars along the channel caused a heavy traffic jam in the inlet. Only low concentration suspensions could flow into the channel. When the solvent evaporated, it also swept away some particles then flowed into the outlet/inlet holes, leaving only a small amount of particles on the heating area.