CHAPEL HILL - Chemists at the University of North Carolina at Chapel Hill have answered an important decade-old scientific riddle that has been the subject of hundreds of research papers about hydrogen - the simplest element - and properties of silicon, which are at the heart of the microelectronics revolution.
The question has been why an energy barrier hinders hydrogen from sticking to the surface of silicon when it's well known that when hydrogen molecules come off that surface, they come off slowly. Understanding the origin of energy barriers is important because they prevent reactions from occurring except at the highest temperatures, scientists say.
"The hydrogen-silicon case was a big puzzle because if the hydrogen molecules had to huff and puff to climb onto the surface, surely they would move rapidly down this same 'hill' as they leave," said Dr. John J. Boland, professor of chemistry at UNC-CH. "What we've shown, among other things, is that surfaces like that of silicon should not be treated solely as passive platforms on which reactions take place among molecules that happen to stick to them." A report on the findings appears in the current issue (Oct. 20) of the journal Science. Graduate student Emily J. Buehler is co-author.
The reaction the two study occurs at what are called dimer sites, which are pairs of silicon atoms that rapidly tilt up and down like a see-saw, Boland said. A first clue as to what was happening came from a European research group theory that reacting dimers must be untilted and flat when hydrogen reaches the top of the energy hill, otherwise known as the "transition state." One difficulty was testing the theory since no such dimers occur in nature.
"By performing chemistry on neighboring dimers, what we did was to show that it's possible to pin a dimer so that it's untilted," Boland said. "We showed using low-temperature experiments and, more recently, density functional theory calculations that the dimers are indeed untilted and horizontal.
"We then measured the reactivity of those dimers to hydrogen molecules and found that they were more than a billion times more reactive than the tilting dimers found on the normal surface," he said. "Even more important, the reaction showed no temperature dependence, suggesting that the untilted dimer orientation is optimal and just what you want. It was amazing that neither very high nor very cold temperatures made any difference."
If the reacting hydrogen molecule and silicon dimer are really at the top of the energy hill, then no additional energy, such as heat, is needed to help out, Boland said. In that sense, the untilted dimer orientation mimics the structure of the actual transition state.
"Preparing transition states is one of the holy grails of modern chemistry," he said. "We've now learned that it's not the hydrogen molecules that are huffing and puffing up the hill but rather the dimers, or paired silicon molecules moving on the surface, that are doing it."
On a broader level, the research is important for several reasons, the scientist said. "While others have shown that chemistry can induce a modification of a surface that has novel reactivity, what we've done is much more subtle," Boland said. "We have shown for the first time that it's possible to use chemistry to prepare a surface configuration that approximates the transition state structure. By untilting the dimer see-saw, we have pushed the dimer up the activation energy hill so that it teeters at the precipice of reaction."
From now on, chemists will know that surface atoms are intimately involved in the dynamics of reactions and that to overlook their contribution would be a serious mistake, he said. The technique he and Buehler used should be valuable in studying other silicon reactions with practical applications in industry. "Chemistry of this kind is important," Boland said. "It's a bread-and-butter issue for the semiconductor industry."
Note: John J. Boland can be reached at 919-962-5098 or BOLAND@UNC.EDU
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Science