Since Raman first discovered the phenomena a breakthrough that earned him Nobel Prize engineers have dreamed of using it in everyday devices to identify the molecular composition and structures of substances, but for many materials the strength of the extra colors of reflected light was too weak to be seen even with the most sophisticated laboratory equipment.
Researchers discovered in the 1970s that the Raman signals were much stronger if the substance to be identified is placed on a rough metal surface or tiny particles of gold or silver. The technique, known as surface enhanced Raman scattering (SERS), showed great promise, but even after four decades of research has proven difficult to put to practical use. The strong signals appeared only at a few random points on the sensor surface, making it difficult to predict where to measure the signal and resulting in a weak overall signal for such a sensor.
Abandoning the previous methods for designing and manufacturing the sensors, Chou and his colleagues developed a completely new SERS architecture: a chip studded with uniform rows of tiny pillars made of metals and insulators.
One secret of the Chou team's design is that their pillar arrays are fundamentally different from those explored by other researchers. Their structure has two key components: a cavity formed by metal on the top and at the base of each pillar; and metal particles of about 20 nanometers in diameter, known as plasmonic nanodots, on the pillar wall, with small gaps of about 2 nanometers between the metal components.
The small particles and gaps significantly boost the Raman signal. The cavities serve as antennae, trapping light from the laser so it passes the plasmonic nanodots multiple times to generate the Raman signal rather than only once. The cavities also
|Contact: Chris Emery|
Princeton University, Engineering School