Plasmons resonate in atomic-scale metal particles

Addressing five decades of debate, Stanford engineers determine how collective electron oscillations, called plasmons, behave in individual metal particles as small as just a few nanometers in diameter. This knowledge may open up new avenues in nanotechnology ranging from solar catalysis to biomedical therapeutics.

San Francisco artist Kate Nichols creates structurally colored artwork using Surface Plasmon Resonances, the same phenomenon described by Scholl and Dionne. This image is a selected view from Nichols's two-part installation at The Leonardo Museum. Nature chose the same image to grace the cover of its issue featuring the Scholl/Koh/Dionne research. Credit: Kate Nichols. Through the Looking Glass 1. Silver nanoparticles on glass. 2011. In situ at The Leonardo Museum, Salt Lake City. Photo: Donald Felton/Almac Camera. www.katenicholsstudio.com

The physical phenomenon of plasmon resonances in small metal particles has been apparent for centuries. They are visible in the vibrant hues of the great stained-glass windows of the world. More recently, plasmon resonances have been used by engineers to develop new, light-activated cancer treatments and to enhance light absorption in photovoltaics and photocatalysis.

"The stained-glass windows of Notre Dame Cathedral and Stanford Chapel derive their color from metal nanoparticles embedded in the glass. When the windows are illuminated, the nanoparticles scatter specific colors depending on the particles' size and geometry " said Jennifer Dionne, an assistant professor of materials science and engineering at Stanford and the senior author of a new paper on plasmon resonances to be published in the journal Nature.

In the study, the team of engineers report the direct observation of plasmon resonances in individual metal particles measuring down to one nanometer in diameter, just a few atoms across.

"Plasmon resonances at these scales are poorly understood," said Jonathan Scholl, a doctoral candidate in Dionne's lab and first author of the paper. "So, this class of quantum-sized metal nanoparticles has gone largely under-utilized in engineering. Exploring their size-dependent nature could open up some interesting applications at the nanoscale."

The research could lead to novel electronic or photonic devices based on excitation and detection of plasmons in these extremely small particles, the engineers said.

"Alternatively, there could be opportunities in catalysis, quantum optics, and bio-imaging and therapeutics," added Dionne.

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