3 - 9 Year in Review

According to Sweet, the plan was for thestudents to run 24 experiments using six ofthe NSLS macromolecular crystallographybeamlines for 60 hours straight. At about 9p.m. on Tuesday, however, after just onegroup had begun to collect data, the synchro-tron went down. It took until about 1:30 a.m.to get it up and running.

But the students took the delay in stride,working in groups of eight with their scientistadvisors to plan how best to use their time.When last seen, they were huddled aroundbeam-line computer terminals, clustered inconversation, and getting on with their work.

“It’s really nice to be able to concentrateon this aspect of collecting data and solvingstructures, which is the last part of experi-mentation in x-ray crystallography,” saidSrikripa Devarakonda, a Ph.D. student fromthe University of Virginia. This was her firsttrip to a synchrotron. And with a backgroundin physics, she seemed genuinely excited tosee how it worked, like a kid in a candy store.

[Editor’s note: Reprinted with permis-sion from the BNL Bulletin - May 4, 2001.]

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�Dancing Triangles� Intrigue Chemists, PhysicistsKaren McNulty Walsh

The pattern of triangles and hexagons shown heretakes on an even more fascinating beauty when yourealize it is made up of individual atoms. The imagewas made by Jan Hrbek, Acting Chair of the ChemistryDepartment, and collaborators from Sandia NationalLaboratory in Livermore, California, using scanning tun-neling microscopy (STM). It shows the arrangement ofsulfur atoms (bright spots) on a layer of copper over aruthenium substrate.

“We are trying to understand how one metal be-haves on top of another, and how sulfur affects thatinteraction,” Hrbek says.

Layered pairs of metals are commonly used in cata-lysts-such as those that clean pollutants from automo-bile exhaust in catalytic converters. Copper on ruthe-nium, Hrbek says, would make a particularly goodmodel catalyst.

But sulfur, a common air pollutant, can “poison” thecatalyst, destroying its effectiveness. “There is a lot ofchemistry and physics involved in understanding this,”he says.

Shining a light on the problemWork being

done at the Na-tional SynchrotronLight Source(NSLS) by HubertZajonz of thePhysics Depart-ment and DoonGibbs, leader ofthe Solid StatePhysics group inPhysics, mayshine some lighton the problem, lit-erally. The physicists are using x-rays to determine howmetallic atoms like copper arrange themselves on ru-thenium. “We want to know what the structures are asyou put down more and more copper, how they change,and how that depends on temperature,” Gibbs says.

At BNL’s National Synchrotron Light Source, looking at equipment thatis being used for the recent week-long course are: (clockwise, from left)Emma Jakobsson from Uppsala University, Sweden; Bob Sweet of theBiology Department, who designed and helped run the program; OrenRosenberg from Rockefeller University & Yale University; and SrikripaDevarakonda from the University of Virginia.

“Dancing Triangles”

3 - 10NSLS Activity Report 2001

Zajonz adds, “We think that this knowledge is cru-cial for understanding, controlling, and designing bime-tallic catalysts.”

By beaming x-rays at samples and studying howthe beams scatter, the physicists can pinpoint how theatoms line up, and look for changes as copper atomsare added.

Using analytical techniques developed by Zajonz,the physicists have found that a single layer of copperatoms follows the layout of the ruthenium atoms in thesubstrate.

“But when you put down a second layer, even justone atom more than a single layer, there is a sizeableshift of positions,” Gibbs says. The result is a stripedpattern. Additional copper results in the formation ofbulk copper “islands” about 100 atoms across.

The next step will be to look at the effect of addingsulfur.

Complementary approachesWith Hrbek’s STM technique, which essentially

scans the surface atom by atom using a needle-likeprobe, the sulfur atoms are seen to organize themselvesinto triangles and closely packed hexagons, while thecopper stripes bend to form trigons. All this happensbefore the sulfur reacts chemically with the copper andalters the catalytic activity.

Hrbek describes the arrangement as dancing tri-angles: “If you want a pair to tango properly, you haveto get them together. This is exactly what you see, sul-fur moving around and trying to find the right place forgetting together with copper.”

The x-ray technique, by contrast, scans larger ar-eas and multiple layers, including the atoms in the ru-thenium substrate. “It’s completely complementary toSTM,” Gibbs says, “so it pays for us to work together.”

Hrbek agrees, adding, “We can learn more by do-ing experiments in collaboration.” Using both techniqueswill help establish whether the patterns Hrbek’s groupis observing occur over large areas.

In addition to helping scientists understand the cata-lytic reactions, the atomic arrangements of copper, ru-thenium, and other metals might turn out to be usefulfor other purposes.For example, Hrbek says, one couldimagine using the tiny grids as templates for buildingnanoscale structures, or using the techniques to makeclusters of metallic particles with precisely controlledsize distribution and chemistry.

“I have no idea what to use it for,” says Gibbs, de-scribing one of the intricate arrangements. But he wantsto learn more, saying, “I think this is just beautiful.”

[Editor’s note: Reprinted with permission from theBNL Bulletin - May 25, 2001.]

At the National Synchrotron Light Source are: (from left) Hubert Zajonz, PhysicsDepartment, Jan Hrbek, Chemistry Department, and Doon Gibbs, Physics.