Stanford engineers find elusive plasmons in tiny metal particles, a boost to nanotechnology

Colored artwork using Surface Plasmon Resonances

Artist Kate Nichols creates structurally colored artwork using Surface Plasmon Resonances, the same phenomenon described by Scholl and Dionne. (Photo: Donald Felton/Almac Camera)



March 28, 2012

By Andrew Myers

After 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.

Stanford scientists have shown that a phenomenon known as plasmon resonance occurs at very small scales, offering a new understanding of quantum phyics that could lead to improved solar catalysis and targeted cancer treatments.

The new discovery by Stanford engineers was reported recently in the cover story of the journal Nature.

When light hits a metal, electrons on the surface collectively oscillate in waves, called plasmons, that travel out like ripples on a pond. The new research shows that plasmons exist in smaller particles than had been shown before. The research reveals the presence and clear quantum-influenced nature of plasmons in individual metal particles as small as 1 nanometer in diameter, about 100 atoms in total.

"Particles of this size are valuable in engineering. They are more sensitive and more reactive than bulk materials and could prove very useful in nanotechnology," said Jennifer Dionne, an assistant professor of materials science and engineering at Stanford and the study's senior author.

Plasmons are an area of intense research focus and a key driver of engineering at the nanoscale. However, as metals become smaller, obtaining experimental data about the nature of plasmons becomes extremely challenging. For over five decades, scientists have debated the nature of plasmons at these smallest of scales.

"Until now, however, we hadn't been able to take full advantage of the optical and electronic properties of these tiny particles because we didn't have a complete picture of the science," said Jonathan Scholl, a doctoral candidate in Dionne's lab and first author of the paper. "This paper provides the foundation for nanoengineering a new class of metal particles made up of between 100 and 10,000 atoms."

Plasmon resonances in relatively small metal particles are not new. They are visible in the vibrant hues of the great stained-glass windows of the world. More recently, engineers have used them to develop new, light-activated cancer treatments and to enhance light absorption in photovoltaics and photocatalysis.

Stained-glass windows
"The windows of Notre Dame Cathedral and Stanford Memorial Church derive their color from metal nanoparticles embedded in the glass. When the windows are illuminated, the nanoparticles scatter specific colors of light. The color depends on the size and geometry of the metal particles," said Dionne.

"While scientists have found a number of applications for larger nanoparticles, quantum-sized metal particles have remained largely underutilized," said Scholl.

Science has a solid understanding of plasmons in larger metal particles based mostly on classical physics. Below a threshold of about 10 nanometers in diameter, however, at what is described as the quantum scale, the classical physics breaks down and quantum mechanics takes over.

At this scale, the particles begin to demonstrate unique physical and chemical properties that larger counterparts of the very same materials do not. Additional and important physical properties can occur when plasmons are constrained in extremely small spaces, at the scale of the nanoparticles Dionne and Scholl studied.

A nanoparticle of silver measuring just a few atoms across, for instance, will respond to photons and electrons in ways profoundly different from a larger particle or slab of silver. By clearly illustrating the details of this classical-to-quantum transition, Scholl and Dionne have pushed the study of plasmons, a field known as plasmonics, into a new realm.

"Our study allows researchers, for the first time, to directly correlate a quantum-sized particle's geometry – its shape and size – with its plasmon resonances," said Dionne.

Interesting applications
Exploring the size-dependent nature of plasmons at the extreme nanoscale could open up some interesting applications.

"We might discover novel electronic or photonic devices based on excitation and detection of plasmons. Or, there could be opportunities in quantum optics, bio-imaging and therapeutics," said Dionne.

Medical science, for instance, has devised a way to use nanoparticles excited by light to burn away cancer cells, a process known as photothermal ablation. Metal nanoparticles are affixed with molecular appendages that attach exclusively to cancerous cells in the body. When irradiated with infrared light the plasmons in the metal begin to vibrate and the nanoparticles heat up, burning away the cancer while leaving the surrounding healthy tissue unaffected.

The metal particles used in these applications today, however, are relatively large. The use of smaller particles like those described in this research could prove more easily integrated into cells and might therefore improve the accuracy and the effectiveness of these technologies.

In a similar vein, the greater surface-area-to-volume ratios offered by atomic-scale nanoparticles could improve rates and efficiencies in catalytic processes like water-splitting and artificial photosynthesis, yielding clean and renewable sources of energy from artificial fuels.

Elegant and versatile
The researchers concluded by explaining the physics of their discovery through an elegant and versatile analytical model based on well-known quantum mechanical principles.

"Technically speaking, we've created a relatively simple, computationally light model that describes plasmonic systems where classical theories have failed," said Scholl.

The researchers' ability to observe plasmons in particles of such small size was abetted by the powerful, multimillion-dollar environmental scanning transmission electron microscope (E-STEM) installed recently at Stanford's Center for Nanoscale Science and Engineering, one of just a handful of such microscopes in the world.

E-STEM imaging was used in conjunction with electron energy-loss spectroscopy (EELS) – a research technique that measures the change of an electron's energy as it passes through a material – to determine the shape and behavior of individual nanoparticles. Combined, E-STEM and EELS allowed the team to address many of the ambiguities of previous investigations.

Ai Leen Koh, a research scientist at the Stanford Nanocharacterization Laboratory, contributed to this study. The work was funded by the National Science Foundation Graduate Research Fellowship Program, the Stanford Terman Fellowship and the Robert N. Noyce Family Faculty Fellowship.

Andrew Myers is associate director of communications for the Stanford University School of Engineering.