The need to observe the structures and spatial distribution of nanosized catalytic particles to aid in the development of highly selective catalysts is a driving force behind the construction of a new ORNL laboratory for advanced microscopy, tentatively called the Advanced Materials Characterization Lab (AMCL). It will house three of the world’s first aberration-corrected electron microscopesinstruments that will require the most carefully controlled environment to achieve the limits of their design resolutions.
“With the new microscopes, atoms will look more like points instead of blurry blobs,” says Ted Nolan, a consultant in ORNL’s Metals and Ceramics (M&C) Division. “And we will be able to determine the arrangement of atoms in a crystal with much greater accuracy.”
ABERRATION-CORRECTED ELECTRON MICROSCOPES
One of the new microscopes, the Aberration-Corrected Electron Microscope (called the ACEM), is being built for ORNL by the Japan Electron Optics Laboratory (JEOL). It is a 200-kilovolt (kV) combination scanning transmission and conventional transmission electron microscope (STEM-TEM). With optics corrected primarily for spherical aberration, it will be capable of imaging features of matter as small as 0.7 angstrom (Å), compared with its standard resolution of 1.4 Å. An angstrom is one ten-millionth of a centimeter, or 0.1 nanometer (nm), the size of an average atom. A human hair is about 500,000 Å (50,000 nm) thick.
For researchers, this is a great leap in resolution,” says M&C Division’s Larry Allard, the ORNL scientist in charge of the ACEM project. “It’s equivalent to an athlete suddenly boosting his high-jump record from 8 feet to 16 feet.” The new laboratory will also house two additional, powerful aberration-corrected microscopes, operated under the direction of Steve Pennycook in ORNL’s Solid State Division (SSD): the 100-kV and 300-kV Z-contrast STEMs, built by VG Microscopes.
Optical aberrations are familiar to anyone who wears eyeglasses, which are simply lenses designed to compensate for the faulty shape of the lens of the eye. For 40 some years it had been proposed that magnetic fields be shaped to compensate for distortions in images “seen” through the electromagnetic lenses of electron microscopes. Thanks to a patented algorithm and other technologies developed recently by Nion Company in Kirkland, Washington, and Corrected Electron Optical Systems (CEOS) in Heidelberg, Germany, several systems for spherical aberration correction now exist. The SSD’s STEMs incorporate Nion correctors; the High Temperature Materials Laboratory (HTML) ACEM will be equipped with a CEOS corrector.
The 300-kV instrument brought attention to ORNL when it was used to show columns of silicon atoms only 0.78Å apart, a world record in 1999 for measuring distances between atoms using an electron microscope. The corrector was installed in 2001 on the 100-kV machine, bringing its resolution down from 2.2Å to 1.3Å. The ultimate resolution for imaging with the 300-kV instrument when the corrector is operational in the fall of 2002 is expected to approach 0.5Å.
The SSD microscopes will achieve their highest resolution using dark field imaging techniques, in which the higher atomic number atoms show brighter contrast (the so-called “Z-contrast” mode). “With the aberrations corrected, we are able to see single atoms more clearly and have a sharper view of different rows of atoms,” says Pennycook.
“The new microscopes will pave the way for new discoveries that have yet to be imagined,” says Allard. “They will be like the Hubbell space telescope after its faulty optics were corrected in 1993. No one was able to predict then the range of discoveries that have been made using that instrument.”
The JEOL microscope will be operated remotely from a desktop computer that will control which resolution is used. “The advantage of operating the microscope at standard resolution over aberration-corrected resolution is that we can use 10 times as much signalthe number of electrons per unit areaas we have on the Hitachi STEM,” Allard says. “As a result, we will have a 4-second exposure versus a 40-second exposure, allowing less time for sample movement, improving the contrast, and providing a better picture.”
WITH AN EYE TOWARD THE FUTURE: THE NTEAM CONCEPT
ORNL scientists are also discussing future developments in aberration-corrected electron microscopy. The National Transmission Electron Achromatic Microscope (NTEAM) project is focused on the correction of both spherical and chromatic aberrations. Basically, spherical aberration occurs when a lens focuses electrons of the same energy traveling along different paths to different points. Chromatic aberration occurs when electrons of different energies (like colors of light) traveling the same path are focused to different points.
“By correcting both types of aberrations, we allow the possibility of increasing the space around the specimen during atomic-scale imaging and analysis, which provides greater flexibility for conducting in-situ measurements,” says M&C’s Ian Anderson, who received a Presidential Early Career Award for Scientists and Engineers for his development of electron beam microcharacterization techniques. “For example, we can start to think about probing the chemical state of active sites in catalysts during in-situ reactions.”
Anderson heads the ORNL Shared Research Equipment Collaborative Research Center, one of four microcharacterization centers sponsored by DOE’s Office of Basic Energy Sciences. The NTEAM project is a collaboration among the four centers, which hosted a workshop at DOE’s Lawrence Berkeley National Laboratory in July 2002 to develop the concept.
IMPROVED ATOM PROBE
Atomic-resolution imaging at ORNL is not limited to electron microscopes. Another atomic-resolution microscope that may be installed in 2003 in the AMCL is the local electrode atom probe (LEAP). The three-dimensional atom probe (3DAP), pioneered by M&C Division’s Mike Miller, achieves atomic-resolution imaging and analysis in alloys by using a time-of-flight mass spectrometer and a position-sensitive detector to reconstruct the atomic positions and elemental identities of the atoms making up a small volume of material. Miller and co-workers have already used current-generation 3DAPs to quantify the atomic-scale structure and composition of ultra-stable nanoscale clusters that are virtually invisible to any other technique.
“With the extraordinary materials properties that result when features shrink to the nanoscale,” says Miller, “the atom probe allows us to learn more and more from less and less.” The LEAP improves upon the current state of the art through miniaturization of the specimen-stage area, allowing greatly improved data collection rates and analysis of lower conductivity materials, such as thin films on semiconductor substrates.
AMCL’S ADVANCED MICROSCOPES TO HAVE BROAD SCIENTIFIC IMPACT
“We expect that the JEOL microscope will be used largely for studies in nanoscience,” Allard adds. “For example, it will be used to image metal-catalyst nanoparticles that facilitate chemical reactions to get a desired product.”
A major objective for DOE is the development of cleaner, more efficient automotive engines. One way to help achieve this goal is to better understand how catalysts work. The new microscopes will enable researchers to obtain information that may help them determine ways to make catalyst particles that are highly selective, extremely stable, and uniformly dispersed on a ceramic support, to increase the exposure of the catalyst atoms to a target gas. Selective catalysts will enable reactions in automotive engines to occur faster and at lower temperatures, without generating undesirable by-products.
High-resolution microscopes can help scientists better understand how catalysts work in emission-control systems. Using a Hitachi high-resolution TEM at HTML, Allard and Professor Bruce Gates of the University of California at Davis discovered that, after use of a particular materials-mixing process, a large fraction of atomic clusters consisting of five osmium atoms apiece were uniformly dispersed on a magnesium oxide substrate. The findings were published recently in a paper in the new journal Nano Letters.
Pennycook and his colleagues have used the 100-kV, aberration-corrected Z-contrast STEM to dispute a well-publicized claim that single crystals of single-walled carbon nanotubeshollow tubes made of carbon atomshad been self-assembled. This “finding” was published by IBM Zurich and Cambridge University researchers in a much-ballyhooed paper published in May 2001 in Science magazine.
“We found that the carbon nanotube crystals they claimed to see were really not nanotubes,” Pennycook says. “The crystals contained molybdenum, calcium, and oxygen atoms in different ratios.”
According to Pennycook, one dream of scientists is to coax the growth of nanotubes as single crystals lined up in one direction. The ORNL studies indicate that this dream has not yet been realized, but the ORNL microscopes may someday produce images that can suggest how best to reach this goal.
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