Stephan Kujawa

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.

Application of Electron Tomography for Semiconductor Device Analysis

The validity of electron tomography for imaging of high Z and low Z materials is evaluated using high aspect ratio VIAs and a DRAM as an example. The influence of the tilt‐range on the 3D reconstruction is tested and it is shown that reliable 3D analysis is possible for up to 300 nm thick samples with a resolution of up to 1–2 nm.

Focused Ion Beam Preparation of Specimens for Micro-Electro-Mechanical System-based Transmission Electron Microscopy Heating Experiments

Micro-electro-mechanical systems (MEMS)-based heating holders offer exceptional control of temperature and heating/cooling rates for transmission electron microscopy experiments. The use of such devices is relatively straightforward for nano-particulate samples, but the preparation of specimens from bulk samples by focused ion beam (FIB) milling presents significant challenges. These include: poor mechanical integrity and site selectivity of the specimen, ion beam damage to the specimen and/or MEMS device during thinning, and difficulties in transferring the specimen onto the MEMS device. Here, we describe a novel FIB protocol for the preparation and transfer of specimens from bulk samples, which involves a specimen geometry that provides mechanical support to the electron-transparent region, while maximizing the area of that region and the contact area with the heater plate on the MEMS chip. The method utilizes an inclined stage block that minimizes exposure of the chip to the ion beam during milling. This block also allows for accurate and gentle placement of the FIB-cut specimen onto the chip by using simultaneous electron and ion beam imaging during transfer. Preliminary data from Si and Ag on Si samples are presented to demonstrate the quality of the specimens that can be obtained and their stability during in situ heating experiments.