Thursday, June 23, 2011

Researchers break light-matter coupling strength limit in nanoscale semiconductors

New engineering research at the University of Pennsylvania demonstrates that polaritons have increased coupling strength when confined to nanoscale semiconductors. This represents a promising advance in the field of photonics: smaller and faster circuits that use light rather than electricity.

The research was conducted by assistant professor Ritesh Agarwal, postdoctoral fellow Lambert van Vugt and graduate student Brian Piccione of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science. Chang-Hee Cho and Pavan Nukala, also of the Materials Science department, contributed to the study.

Their work was published in the journal Proceedings of the National Academy of Sciences.

Polaritons are quasiparticles, combinations of physical particles and the energy they contribute to a system that can be measured and tracked as a single unit. Polaritons are combinations of photons and another quasiparticle, excitons. Together, they have qualities of both light and electric charge, without being fully either.

"An exciton is a combination of a an electron, which has negative charge and an electron hole, which has a positive charge. Light is an oscillating electro-magnetic field, so it can couple with the excitons," Agarwal said. "When their frequencies match, they can talk to one another; both of their oscillations become more pronounced."

High light-matter coupling strength is a key factor in designing photonic devices, which would use light instead of electricity and thus be faster and use less power than comparable electronic devices. However, the coupling strength exhibited within bulk semiconductors had always been thought of as a fixed property of the material they were made of.

Agarwal's team proved that, with the proper fabrication and finishing techniques, this limit can be broken.

"When you go from bulk sizes to one micron, the light-matter coupling strength is pretty constant," Agarwal said. "But, if you try to go below 500 nanometers or so, what we have shown is that this coupling strength increases dramatically."

The difference is a function of one of nanotechnology's principle phenomena: the traits of a bulk material are different than structures of the same material on the nanoscale.

"When you're working at bigger sizes, the surface is not as important. The surface to volume ratio -- the number of atoms on the surface divided by the number of atoms in the whole material -- is a very small number," Agarwal said. "But when you make a very small structure, say 100 nanometers, this number is dramatically increased. Then what is happening on the surface critically determines the device's properties."

Other researchers have tried to make polariton cavities on this small a scale, but the chemical etching method used to fabricate the devices damages the semiconductor surface. The defects on the surface trap the excitons and render them useless.

"Our cadmium sulfide nanowires are self-assembled; we don't etch them. But the surface quality was still a limiting factor, so we developed techniques of surface passivation. We grew a silicon oxide shell on the surface of the wires and greatly improved their optical properties," Agarwal said.

The oxide shell fills the electrical gaps in the nanowire surface, preventing the excitons from getting trapped.

"We also developed tools and techniques for measuring this light-matter coupling strength," Piccione said."We've quantified the light-matter coupling strength, so we can show that it's enhanced in the smaller structures,"

Being able to quantify this increased coupling strength opens the door for designing nanophotonic circuit elements and devices.

"The stronger you can make light-matter coupling, the better you can make photonic switches," Agarwal said. "Electrical transistors work because electrons care what other electrons are doing, but, on their own, photons do not interact with each other. You need to combine optical properties with material properties to make it work"

This research was supported by the Netherlands Organization for Scientific Research Rubicon Programme, the U.S. Army Research Office, the National Science Foundation, Penn's Nano/Bio Interface Center and the National Institutes of Health.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of Pennsylvania.

Journal Reference:

L. K. van Vugt, B. Piccione, C.-H. Cho, P. Nukala, R. Agarwal. One-dimensional polaritons with size-tunable and enhanced coupling strengths in semiconductor nanowires. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1102212108

When size matters: Nanotechnology for energy efficiency

Researchers at the University of Leicester are using nanotechnology to create new energy efficient materials.

With the increasing worldwide demand for energy there is a pressure to use the finite energy resources wisely whilst reducing one of the major areas of energy consumption, transportation, which accounts for more than 20% of the world's total primary energy and produces much of the world's pollution.

Alternative fuels, such as bio-fuels, hydrogen fuels, fuel cells and electric batteries, being developed by the automotive industry need further development and a considerable time for their full adaptation into transportation, including passenger cars, trucks, aircrafts and trains.

A postgraduate researcher with the Department of Engineering, Sinan Kandemir is fabricating light and strong resistant materials with nano-additives to create lighter components for automotive and aerospace industries that will help improve energy efficiency, minimise CO2 emissions and preserve the environment.

By using a novel processing technique, ultrasonic method, to disperse aluminium-based nano-particles homogenously through the liquid, his research promises quicker results while the industry is making advances with alternative fuels.

Kandemir explained: "The Kyoto agreement and the European Commission suggest that the automotive manufacturers should reduce their vehicle weight to minimise CO2 emissions and conserve finite oil (fossil fuel) reserves.

"Although light materials, including aluminium and magnesium, have been proposed to replace denser materials, such as steel in the automotive industry, they exhibit low strength. Nano-sized ceramic particles can be incorporated into light metals to modify the physical properties of established materials in a huge variety of automotive components.

"These nano-composite materials save weight and offer greater performance whilst contributing to the fuel efficiency and reducing green house gases released into the atmosphere."

Kandemir is supervised by an internationally renowned engineer, Head of the Mechanics of Materials Group in the Department of Engineering, Professor Helen Atkinson FREng, who commented: "Nanocomposites are fascinating materials with potentially excellent properties. I am very much looking forward to the overall results of the project."

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of Leicester, via AlphaGalileo.

Under pressure, sodium, hydrogen could undergo a metamorphosis, emerging as superconductor

In the search for superconductors, finding ways to compress hydrogen into a metal has been a point of focus ever since scientists predicted many years ago that electricity would flow, uninhibited, through such a material.

Liquid metallic hydrogen is thought to exist in the high-gravity interiors of Jupiter and Saturn. But so far, on Earth, researchers have been unable to use static compression techniques to squeeze hydrogen under high enough pressures to convert it into a metal. Shock-wave methods have been successful, but as experiments with diamond anvil cells have shown, hydrogen remains an insulator even under pressures equivalent to those found in Earth's core.

To circumvent the problem, a pair of University at Buffalo chemists has proposed an alternative solution for metallizing hydrogen: Add sodium to hydrogen, they say, and it just might be possible to convert the compound into a superconducting metal under significantly lower pressures.

The research, published June 10 in Physical Review Letters, details the findings of UB Assistant Professor Eva Zurek and UB postdoctoral associate Pio Baettig.

Using an open-source computer program that UB PhD student David Lonie designed, Zurek and Baettig looked for sodium polyhydrides that, under pressure, would be viable superconductor candidates. The program, XtalOpt <>, is an evolutionary algorithm that incorporates quantum mechanical calculations to determine the most stable geometries or crystal structures of solids.

In analyzing the results, Baettig and Zurek found that NaH9, which contains one sodium atom for every nine hydrogen atoms, is predicted to become metallic at an experimentally achievable pressure of about 250 gigapascals -- about 2.5 million times Earth's standard atmospheric pressure, but less than the pressure at Earth's core (about 3.5 million atmospheres).

"It is very basic research," says Zurek, a theoretical chemist. "But if one could potentially metallize hydrogen using the addition of sodium, it could ultimately help us better understand superconductors and lead to new approaches to designing a room-temperature superconductor."

By permitting electricity to travel freely, without resistance, such a superconductor could dramatically improve the efficiency of power transmission technologies.

Zurek, who joined UB in 2009, conducted research at Cornell University as a postdoctoral associate under Roald Hoffmann, a Nobel Prize-winning theoretical chemist whose research interests include the behavior of matter under high pressure.

In October 2009, Zurek co-authored a paper with Hoffman and other colleagues in the Proceedings of the National Academy of Sciences predicting that LiH6 -- a compound containing one lithium atom for every six hydrogen atoms -- could form as a stable metal at a pressure of around 1 million atmospheres.

Neither LiH6 and NaH9 exists naturally as stable compounds on Earth, but under high pressures, their structure is predicted to be stable.

"One of the things that I always like to emphasize is that chemistry is very different under high pressures," Zurek says. "Our chemical intuition is based upon our experience at one atmosphere. Under pressure, elements that do not usually combine on the Earth's surface may mix, or mix in different proportions. The insulator iodine becomes a metal, and sodium becomes insulating. Our aim is to use the results of computational experiments in order to help develop a chemical intuition under pressure, and to predict new materials with unusual properties."

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University at Buffalo.

Glowing 'Cornell Dots': Potential cancer diagnostic tool set for human trials

The U.S. Food and Drug Administration (FDA) has approved the first clinical trial in humans of a new technology: Cornell Dots, brightly glowing nanoparticles that can light up cancer cells in PET-optical imaging.

A paper describing this new medical technology is published online in the Journal of Clinical Investigation (July 2011). This is a collaboration between Memorial Sloan-Kettering Cancer Center (MSKCC), Cornell University, and Hybrid Silica Technologies, a Cornell business start-up.

For the first time, scientists report a uniquely advanced and comprehensive characterization of Cornell Dots -- an ultra small, cancer-targeted, multimodal silica nanoparticle -- which has recently been approved as an "investigational new drug" (IND) by the FDA for a first-in-human clinical trial, says Michelle S. Bradbury, M.D., of the Memorial Sloan-Kettering Cancer Center and an assistant professor of radiology at Weill Cornell Medical College.

Cornell Dots are silica spheres less than 8 nanometers in diameter that enclose several dye molecules. (A nanometer is one-billionth of a meter, about the length of three atoms in a row.) The silica shell, essentially glass, is chemically inert and small enough to pass through the body and out in the urine. For clinical applications, the dots are coated with polyethylene glycol (PEG) so the body will not recognize them as foreign substances.

A guiding light within the body: To make the dots stick to tumor cells, organic molecules that bind to tumor surfaces or even specific locations within tumors can be attached to the PEG shell. When exposed to near-infrared light, the dots fluoresce much brighter than dye to serve as a beacon to identify the target cells. The technology, the researchers say, enables visualization during surgical treatment, showing invasive or metastatic spread to lymph nodes and distant organs, and can show the extent of treatment response.

Hooisweng Ow, a coauthor of the paper and once a graduate student working with Ulrich Wiesner, Cornell Professor of Materials Science and Engineering, developed first-generation Cornell dots in 2005. Together, Wiesner, Ow and Kenneth Wang, have co-founded the company Hybrid Silica Technologies (HST) to commercialize the invention. The combined team of MSKCC, Cornell and HST researchers is now in the process of forming a new commercial entity in New York City that will help transition the research into commercial products that will benefit cancer patient care.

"This is the first FDA IND approved inorganic particle platform of its class and properties that can be used for multiple clinical indications, two of which are explored: cancer targeting for diagnostics and future therapeutic diagnostics, as well as cancer disease staging and tumor burden assessment via lymph node mapping," says Bradbury.

The Cornell Dots were optimized for efficient renal clearance, allowing the body to pass them through the kidneys.

In addition, the scientists were able to perform real-time imaging of lymphatic drainage patterns and particle clearance rates, as well as sensitively detect nodal metastases. Nodal mapping is now being pursued under a new award of a BioAccelerate NYC Prize from the Partnership for New York City and the New York City Economic Development Corporation, which is expected to lead to another clinical trial in humans.

The lead authors of the paper are Miriam Benezra and Oula Penate-Medina, who are researchers at MSKCC. Bradbury and Wiesner are the senior authors.

Story Source:

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

Journal Reference:

Miriam Benezra, Oula Penate-Medina, Pat B. Zanzonico, David Schaer, Hooisweng Ow, Andrew Burns, Elisa DeStanchina, Valerie Longo, Erik Herz, Srikant Iyer, Jedd Wolchok, Steven M. Larson, Ulrich Wiesner, Michelle S. Bradbury. Multimodal silica nanoparticles are effective cancer-targeted probes in a model of human melanoma. Journal of Clinical Investigation, 2011; DOI: 10.1172/JCI45600