Dlott, Fang and Seong demonstrated their technique by measuring the distribution of local enhancements for benzenethiolate molecules on a substrate of silver-coated nanospheres 330 nanometers in diameter.
To characterize the surface, the researchers first measured the initial Raman intensity. Then they put in a weak killer pulse, which destroyed the molecules in the hottest spots. After measuring the new Raman intensity, they put in a bigger pulse and destroyed the molecules in slightly colder spots. The researchers continued with bigger and bigger pulses until all the benzenethiolate molecules were destroyed.
"We found the hottest spots comprised just 63 molecules per million, but contributed 24 percent of the overall Raman intensity," Dlott said. "We also found the coldest spots contained 61 percent of the molecules, but contributed only 4 percent of the overall intensity."
Measurements like these, of the distribution of local site enhancements, will help researchers design better scattering surfaces for sensor applications.
Prior to this work, no one knew if the Raman intensity was dominated by a small number of hot molecules or a large number of cold ones. Dlott, Fang and Seong have answered that important scientific question; not just with a yes or no, but with a full determination of exactly how many molecules there are in each level of hot or cold.
"Now, when evaluating a new surface-enhanced Raman material, instead of knowing just the average intensity, we know the highest, the lowest, and everything in between," Dlott said.
|Contact: James E. Kloeppel|
University of Illinois at Urbana-Champaign