Quantum plasmons demonstrated in atomic-scale nanoparticles

 Addressing a half-decade-old debate, engineers at Stanford have positively identified the presence of plasmons, the collective oscillations of electrons, in individual metal particles as small as one nanometer in diameter. The discovery could impact nanotechnology.

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 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.

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. 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.

Longstanding debate

The science of tiny metal particles has perplexed physicists and engineers for decades. As metallic particles near about 10 nanometers 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.

By 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.

"Particles 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."

Noble metals

In 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 field.

Additionally, 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.

Depending 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 plasmon resonances.

Standing to benefit

Nanotechnology 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 into cells.

There 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 artificial fuels.

Aiding and abetting

The 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.

E-STEM 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.

"With this new microscope, we can resolve individual atoms within the nanoparticle," said Dionne, "and we can directly observe these particles' quantum plasmon resonances."

Ai 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

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.

"This 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."

Story Source:

The above story is reprinted from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Jonathan A. Scholl, Ai Leen Koh, Jennifer A. Dionne. Quantum plasmon resonances of individual metallic nanoparticles. Nature, 2012; 483 (7390): 421 DOI: 10.1038/nature10904

Subatomic quantum memory in diamond demonstrated

Physicists working at the University of California, Santa Barbara and the University of Konstanz in Germany have developed a breakthrough in the use of diamond in quantum physics, marking an important step toward quantum computing. The results are reported in this week's online edition of Nature Physics.

The physicists were able to coax the fragile quantum information contained within a single electron in diamond to move into an adjacent single nitrogen nucleus, and then back again using on-chip wiring.

"This ability is potentially useful to create an atomic-scale memory element in a quantum computer based on diamond, since the subatomic nuclear states are more isolated from destructive interactions with the outside world," said David Awschalom, senior author. Awschalom is director of UCSB's Center for Spintronics & Quantum Computation, professor of physics, electrical and computer engineering, and the Peter J. Clarke director of the California NanoSystems Institute.

Awschalom said the discovery shows the high-fidelity operation of a quantum mechanical gate at the atomic level, enabling the transfer of full quantum information to and from one electron spin and a single nuclear spin at room temperature. The process is scalable, and opens the door to new solid-state quantum device development.

Scientists have recently shown that it is possible to synthesize thousands of these single electron states with beams of nitrogen atoms, intentionally creating defects to trap the single electrons. "What makes this demonstration particularly exciting is that a nitrogen atom is a part of the defect itself, meaning that these sub-atomic memory elements automatically scale with the number of logical bits in the quantum computer," said lead author Greg Fuchs, a postdoctoral fellow at UCSB.

Rather than using logical elements like transistors to manipulate digital states like "0" or "1," a quantum computer needs logical elements capable of manipulating quantum states that may be "0" and "1" at the same time. Even at ambient temperature, these defects in diamond can do exactly that, and have recently become a leading candidate to form a quantum version of a transistor.

However, there are still major challenges to building a diamond-based quantum computer. One of these is finding a method to store quantum information in a scalable way. Unlike a conventional computer, where the memory and the processor are in two different physical locations, in this case they are integrated together, bit-for-bit.

"We knew that the nitrogen nuclear spin would be a good choice for a scalable quantum memory -- it was already there," said Fuchs. "The hard part was to transfer the state quickly, before it is lost to decoherence."

Awschalom explained: "A key breakthrough was to use a unique property of quantum physics -- that two quantum objects can, under special conditions, become mixed to form a new composite object." By mixing the quantum spin state of the electrons in the defect with the spin state of the nitrogen nucleus for a brief time -- less than 100 billionths of a second -- information that was originally encoded in the electrons is passed to the nucleus.

"The result is an extremely fast transfer of the quantum information to the long-lived nuclear spin, which could further enhance our capabilities to correct for errors during a quantum computation," said co-author Guido Burkard, a theoretical physicist at the University of Konstanz, who developed a model to understand the storage process.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of California - Santa Barbara, via EurekAlert!, a service of AAAS.

Journal Reference:

G. D. Fuchs, G. Burkard, P. V. Klimov, D. D. Awschalom. A quantum memory intrinsic to single nitrogen–vacancy centres in diamond. Nature Physics, 2011; DOI: 10.1038/nphys2026