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ATOM PROBE MICROSCOPE SIGNALS MAJOR ADVANCE FOR IOWA STATE ENGINEERING
 

Andrew C. Hillier

Balaji Narasimhan

CBE Associate Professors Andrew Hillier and Balaji Narasimhan are half of a four-man team responsible for one of the most significant equipment acquisitions in the history of the College of Engineering. Together with colleagues Krishna Rajan of materials science and engineering and Sriram Sundararajan of mechanical engineering, the two will take delivery in the near future of a $1.6-million atom probe microscope (APM), one of only three such instruments in American academic institutions.

“The capabilities of the atom probe microscope will lead to advancements in areas central to energy, healthcare, transportation, and food safety and security, as well as anti-terrorism,” says Mark J. Kushner, dean of the College of Engineering. “With the help of this instrument, we look forward to Iowa State becoming a national resource in informatics-based materials research.”

Acquired through a grant from the W. M. Keck Foundation of Los Angeles, California, the APM will be the centerpiece of the new W. M. Keck Laboratory for High Throughput Atom-Scale Analysis, to be housed on the third floor of Sweeney Hall. The lab, which will be the primary research center of Iowa State’s Institute for Combinatorial Discovery (ICD), is being supported with an additional $3.12 million from the university for faculty, staff, and student support, as well as additional instrumentation and maintenance.

Atom probe microscopy is the direct descendant of the field ion microscope introduced in the 1930s, the first microscope to provide direct atomic level images. But while the earlier instrument was limited to defining the position of atoms in space on a single axis, an APM is capable not only of rendering the atomic structure of samples in three dimensions, but also of fully characterizing all of the elements comprising the sample—atom by atom.

The applications for such knowledge, Hillier observes, are practically limitless.

“If you’re into understanding things,” he says, “this is the information you need. If you’re into building things, this lets you determine what you’ve made—you can get an idea whether it’s going to perform. Or, if you’re into understanding how things fail, this could let you determine definitively how or why they failed.”

According to Hillier, the atom probe microscope applies a large electric field to a material sample that has been whittled down into a sharp tip and inserted into a cryogenically cooled analysis chamber to freeze out atomic motion. The electric field evaporates materials at the tip, peeling off atoms one layer at a time. The atoms are collected in a position-sensitive, time-of-flight mass spectrometer that characterizes each atom and fixes its precise location within the larger sample.

The net result is a picture of the sample that is stunning in its detail and richness of data.

“The microscope can produce an image with ten million atoms in a computer-generated, three-dimensional visualization that allows you to see into a particular compound,” Hillier says. “Say your sample is a high-performance aerospace alloy containing several different components. As you look through the material, you can identify where those atoms are—where aluminum is, where copper is, where any other set of materials might be in three dimensions at atomic resolution.”

In effect, Hillier continues, you’ll have “a pseudo-three-dimensional, ‘rotatable’ image you can turn around and upside down, look at the outside of, and take slices of and look inside of.” In short, he adds, “You would have the atom’s identity and the coordinate—X-Y-Z—where it is. And you would have that data set for each of those ten million atoms.”

That’s a lot of data to digest, which makes high-speed computation the necessary adjunct to the APM. Fortunately, Iowa State is home not only to Krishna Rajan, one of the world’s foremost authorities in materials informatics, but also to one of the world’s most powerful computers, an IBM BlueGene/L acquired earlier this year largely through the efforts of Srinivas Aluru, a professor in the Department of Electrical and Computer Engineering who had previously collaborated with Rajan on techniques for processing atom probe data.

Indeed, says ICD director Narasimhan, it was just this combination of people and facilities that convinced the W. M. Keck Foundation that Iowa State would be an ideal site for an APM. “Not only do we have the experimental capability,” Narasimhan observes, “but we have the data-mining capability essential for an instrument of this kind to be effective. The fact that we had the supercomputer on campus was the last piece of the puzzle—it convinced Keck that we had a unique combination of capabilities that would be beneficial for the instrument.”

Ironically, another key factor working on Iowa State’s behalf was a limitation of the APM. Typically, an APM requires that samples be highly conductive for the microscope’s electric field, making it generally incapable of imaging organic compounds. The Iowa State team, however, has developed methodologies to modify the instrument’s tips to be used with organic compounds with no loss of resolution and at the same high level of throughput, a feat Narasimhan calls “the next big leap in materials science.”

“This is the grand challenge,” Narasimhan says. “When you look at organic materials, you’re typically looking not at atoms but at molecules. So now we can do molecule-by-molecule imaging of organics, soft materials, and biomolecules, with the atom probe eliminating the requirement for conductivity that previous methods required.”

The bottom line for science and engineering across a host of disciplines at Iowa State, the researchers say, is an advantage and an opportunity that comes along infrequently.

“We hit at the right time with the right people,” Hillier says. “With the support of the W. M. Keck Foundation and the Iowa State administration, we now have a tool the vast majority of our peer institutions do not have. In terms of so many areas, access to the right tools really gives you a leg up.”

“And,” he adds, “we’re going to have access to one of the really unique tools.”