Development of HRTEM

In order to improve the point resolution, a number of important custom or home-built instruments at higher voltages ( 500-600 keV) were developed during the 1970s (including at the Universities of Cambridge (UK) and Kyoto (Japan)). Some of these impressive and highly specialized instruments were, 

In HRTEM studies of complex catalyst structures, complementary multislice image simulations using the dynamical theory of electron diffraction (Cowley 1981) may be necessary for the nanostructural analysis and to match experimental images with theory. 

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The majority of STEM instruments are simply conventional TEMs with the addition of scanning coils. As a result, these non-dedicated STEMs are capable of TEM/ STEM, as well as SEM imaging for thicker samples. The development of HRTEMs and dedicated STEMs with lens aberration correction have now pushed the... 

The breakthrough in wide applications of HRTEM came with the development of the first state-of-the-art medium-voltage (200 kV) HRTEM by... 

Boyes, Gai and coworkers at the University of Oxford (in association with JEOL Ltd) (33) and by Thomas and coworkers at the University of Cambridge (also in association with JEOL Ltd) (34,35). The key points of these developments were that the instrument had a resolution similar to that of the best home-built HRTEM instruments ( 2 A) at a small fraction of the cost, and it came in a user-friendly package, achieving the full theoretical performance routinely while fitting in a standard laboratory and requiring no special buildings. Incremental improvements in resolution (—1.3-1.6 A) were achieved later with the development of a 400-kV HRTEM (36). 

In the development of Gai and Boyes (87,88,90), the ECELL, atomic-resolution (HRTEM), STEM, hot stage and PEELS/Gatan imaging filter (GIF) functionalities were combined in a single instalment. The combination is required to aid simultaneous dynamic structure and composition of the reactor contents. 

Several other approaches have been followed towards quantitative HRTEM imaging. One approach is the development of new hardware to correct for or alleviate some of the aberrations in the image, e.g. spherical aberration corrector (Rose 1990, Haider et al. 1995) and three-fold astigmatism corrector (Overwijk et al. 1997). An alternative approach is the development of new methods to retrieve the exit wave function, e.g. off-axis holography (Eichte 1986, Lichte and Rau 1994) and focal-series reconstruction (FSR) (Coene et al. 1992, 1996, Thust et al. 1996a). While each approach has its distinct advantages, we are only going to discuss focal-series reconstruction in this paper. 

With rapid development of zeotypic materials and mesoporous solids and their application in heterogeneous catalysis, HRTEM shows its advantages in distinguishing the ultrastructural features [40, 41], Carbon materials are used as support in catalytic reactions due to some of their specific characteristics and many publications report the TEM investigations on various forms of carbon related materials [42-48],... 

Important ongoing developments in HRTEM that are expected to be valuable in catalysis research include the correction of spherical aberrations in electron microscope lenses and monochromatization of the electron beam for improvement of the spatial and spectral resolution. Recently, scanning-TEM (STEM) of atomically dispersed lanthanum atoms on alumina (63) has provided e.x situ aberration-corrected images, but it is noteworthy that there is no technical limitation in applying the correction devices to instruments used for making measurements of samples in reactive environments. 

The authors gratefully acknowledge J. B. Wagner, T. W. Hansen, C. Lopez-Cartes, J. Sehested, S. Dahl, C. H. J. Jacobsen, A. M. Molenbroek, H. Topsoe, B. S. Clausen, J. R. Rostrup-Nielsen, F. Abild-Pedersen, and J. K. Norskov for contributions to the research included in the present article. We thank the China Technical Consultants Foundation, Taiwan, for financial support and the FEI Company for a fruitful collaboration on the development of the in situ HRTEM facility at Haldor Topsoe A/S. 

Recognition of the differences between single-crystal model catalysts and supported nanoparticles, mentioned above, stimulated the development of nanoparticle model catalysts (14,34,37,62,63,70-83,99). The most straightforward approach to their preparation is to grow metal nanoparticles on a single crystal of the support material. Figure 3c shows a HRTEM image of a Au/MgO model catalyst prepared... 

Although HRTEM has atomic resolution, most images of oxides show only metal atoms, as oxygen s contribution to the image contrast is not significant. As a result of the development of Cs-corrected TEM, it has become possible to see directly oxygen atoms [13]. Even then, determination of the positions and occupation factors of oxygen in oxides still has to be rely on diffraction methods. 

Nanoparticle characterization by high-resolution transmission electron microscopy (HRTEM) and diffraction (TED) requires model catalysts with thicknesses <100 mn. This led to the development of thin film model catalysts which could be... 

It will become possible to follow in situ the evolution of the internal structure of the catalysts during a reaction by the newly developed environmental HRTEM... 

There is a major drawback, however, to the preparation of carbon onions by electron bombardment the amounts obtained are extremely small and thus render the examination of bulk properties virtually impossible. Only high-energy electron sources outside an HRTEM would enable the production of macroscopic amounts. S till a further development of this method is of considerable interest as the carbon onions made from diamond are very uniform in quality. 

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