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August 5, 2002
Volume 80, Number 31
CENEAR 80 31 pp. 26-32
ISSN 0009-2347


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8031sci1
ZOOMING IN High-resolution TEM methods easily resolve 0.55-nm channels (blue) in porous materials such as GeSiO4, a hydrocarbon oxidation catalyst.
COURTESY OF PRATIBHA GAI
SUBTLE DETAILS of the structure of organic molecular crystals were revealed through TEM investigations conducted by Thomas' research group in the 1970s and 1980s. For example, using low-temperature techniques, the microscopists discovered a new phase of photoisomerizable anthracene, which could not be isolated in a pure state, and analyzed its structure and thermodynamic and vibrational properties. The group studied pyrene, tetracene, and other crystals using similar methods.

By focusing on each of the material's lattice dislocations and other deviations from perfect crystallinity--precisely the sort of thing to which TEM is ideally suited--Thomas' group built up a knowledge base needed for explaining singlet- and triplet-state lifetimes and other photophysical properties of the organic crystals.

Shining a beam of electrons on a thin sample and studying images and diffraction patterns to learn about the structure, dimensions, and symmetry of a specimen isn't the only way researchers use electron microscopes. Other signals that are generated as the high-energy electron beam impinges upon a specimen can provide element-specific information.

For example, X-rays are measured in energy-dispersive X-ray spectroscopy, and electrons scattered under certain conditions form the basis of electron energy-loss spectroscopy. Thomas stresses that one of the powerful features of electron microscopy is that all of the signals may be collected simultaneously. But it wasn't always that way, he notes. Some 30 years ago, instrument manufacturers were reluctant to work with Thomas to design microscopes with built-in spectrometers. Nowadays, such microscopes are widely available.

The assortment of tools included in modern microscopes can be used to gather a variety of information from tiny areas and just attogram (10–18 g) quantities of a material. But the same tools can be used to record signals while an electron beam is scanned across a specimen. Using scanning transmission electron microscopes, scientists can produce maps showing specimen composition at high spatial resolution.

Porous catalytic materials have been at the center of Thomas' research efforts in recent years. The group applied TEM methods, computational techniques, and other experimental procedures to solve the structure of a framework-substituted aluminophosphate compound known as MAPO-36. The material is used as a selective oxidation catalyst for hydrocarbons.

By finely dispersing bimetallic nanoparticles such as Pd6Ru6, Ru6Sn, and others in a silica support with 2- to 20-nm pores, scientists can prepare effective catalysts for hydrogenation of organic compounds in solvent-free reactions. Thomas' group has studied these types of materials using a TEM method that enhances contrast for elements with high atomic numbers when they sit in a light-element background--exactly the case for the metal nanoparticles in silica.

The essence of the technique, Thomas explains, is that electrons that are scattered to large angles while passing through a specimen can be collected by a suitable detector and used to form images. The intensity of these so-called Rutherford-scattered electrons is roughly proportional to the square of the atomic number (Z) of the element in the specimen from which they were scattered.

"It means that if heavy elements like ruthenium, tin, or platinum, for example, are sitting on light supports such as carbon or alumina, then the heavy elements stand out like a sore thumb," Thomas remarks. Using Rutherford-scattered electrons to study catalysts via Z-contrast imaging, Thomas' group showed that the metal particles, which appear as bright white spots in the micrographs, indeed are anchored to interior surfaces of silica pores [Acc. Chem. Res., 34, 583 (2001)].

Although high-resolution TEM techniques are employed successfully by scientists to determine the size and distribution of tiny metal particles on supports of alumina, silica, and other materials, the procedures are limited in that they provide information in just two dimensions. Researchers would benefit from knowing the 3-D distribution of the particles and details of particles' shapes and contours. That type of information could be used to understand catalyst activity, selectivity, and stability.

A new 3-D visualization technique that enables researchers to see those types of details was announced last year by Thomas, Midgley, chemistry professor Brian F. G. Johnson, and graduate student Matthew Weyland (C&EN, May 28, 2001, page 9). Known as Z-contrast tomography, the method involves recording a series of high-resolution Z-contrast images of a specimen as it is tilted incrementally relative to the electron beam. Computers are used to reconstruct 3-D images from the 2-D tilt series.

 
FULL SIZE - CLICK IMAGE
MULTITASK When a common TEM mode known as bright-field (BF) imaging is used, Pd6Ru6 particles (dark spots) in porous silica are hard to see. They stand out sharply in a dark-field imaging mode (ADF) that emphasizes heavy elements in a light-element background. Little difference between X-ray maps of Pd and Ru indicates that the particles have 1:1 stoichiometry.
COURTESY OF P. A. MIDGLEY

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