Stanford engineering researchers Jonathan Scholl, Ai Leen Koh and assistant professor Jennifer Dionne at the controls of the powerful Titan environmental scanning transmission electron microscope (E-STEM) installed recently at Stanford’s Center for Nanoscale Science and Engineering. Photo: Andrea Baldi/Jennifer Dionne | Stanford University
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
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 particle’s 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.
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.
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.”
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.
science of tiny metal particles has perplexed physicists and engineers
for decades. As metallic particles near about 10 nm in diameter,
classical physics breaks down. The particles begin to demonstrate unique
physical and chemical properties that bulk counterparts of the very
same materials do not. A nanoparticle of silver measuring a few atoms
across, for instance, will respond to photons and electrons in ways
profoundly different from a larger particle or slab of silver.
clearly illustrating the details of this classical-to-quantum
transition, Scholl and Dionne have pushed the field of plasmonics into a
new realm that could have lasting consequences for catalytic processes
such as artificial photosynthesis, for cancer research and treatment,
and even quantum computing.
at this scale are more sensitive and more reactive than bulk
materials,” said Dionne. “But we haven’t been able to take full
advantage of their optical and electronic properties without a complete
picture of the science. This paper provides the foundation for new
avenues of nanotechnology entering the 100-to-10,000 atom regime.”
recent years, engineers have paid particular attention to nanoparticles
of the noble metals: silver, gold, palladium, platinum and so forth.
These metals are well known to support localized surface plasmon
resonances in larger particles. Plasmons are the collective oscillation
of electrons at the metal surface in response to light or an electric
other important physical properties can be driven when plasmons are
constrained in extremely small spaces, like the nanoparticles Dionne and
Scholl studied, a phenomenon known as quantum confinement.
on the shape and size of the particle, therefore, quantum confinement
can dominate a particle’s electronic and optical response. This research
allows scientists, for the first time, to directly correlate a
quantum-sized plasmonic particle’s geometry—its shape and size—with its
Standing to benefit
stands to benefit from this new understanding. 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 called ligands that
attach exclusively to chemical receptors on cancerous cells. When
irradiated with infrared light, the plasmons begin to resonate and the
metal nanoparticles heat up, burning away the cancerous cells while
leaving the surrounding healthy tissue unaffected. The use of smaller
nanoparticles in these therapies might improve their accuracy and the
effectiveness, particularly since they can be more easily integrated
is great promise for such small nanoparticles in catalysis, as well.
The greater surface-area-to-volume ratios offered by atomic-scale
nanoparticles could could significantly improve catalyic rates and
efficiencies and provide advances in water-splitting and artificial
photosynthesis, yielding clean and renewable energy sources from
Aiding and abetting
researchers’ ability to observe plasmons in particles of such small
size was abetted by the powerful, multi-million 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.
imaging was used in conjunction with electron energy-loss spectroscopy
(EELS)—a research technique that measures the change in an electron’s
energy as it passes through a material—to determine the shape and
behavior of individual nanoparticles. Combined, STEM and EELS allowed
the team to address many of the ambiguities of previous investigations.
this new microscope, we can resolve individual atoms within the
nanoparticle,” said Dionne, “and we can directly observe these
particles’ quantum plasmon resonances.”
Leen Koh, a research scientist at the Stanford Nanocharacterization
Laboratory, and co-author of the paper, noted: “Even though plasmons can
be probed using both light and electrons, electron excitation is
advantageous in that it allows us to image the nanoparticle down to the
atomic level and study its plasmon resonances at the same time.”
Scholl added, “Someday, we might use this microscope to watch reactions in progress to better understand and optimize them.”
Elegant and versatile
researchers concluded by explaining the physics of their discovery
through an elegant and versatile analytical model based on well-known
quantum mechanical principles.
speaking, we’ve created a relatively simple, computationally light
model that describes plasmonic systems where classical theories have
failed,” said Scholl.
paper represents fundamental research. We have clarified what was an
ambiguous scientific understanding and, for the first time, directly
correlated a particle’s geometry with its plasmonic resonance for
quantum-sized particles,” summarized Dionne. “And this could have some
very interesting, and very promising, implications and applications.”
research was made possible by the National Science Foundation Graduate
Research Fellowship Program, the Stanford Terman Fellowship and the
Robert N. Noyce Family Faculty Fellowship.