Key ingredient: Change in material boosts prospects of ultrafast single-photon detector

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Colorized micrograph of an ultrafast single-photon detector made of superconducting nanowires. NIST researchers use electron beam lithography to pattern the nanowires (vertical lines) on a thin film of tungsten-silicon alloy, which produces more reliable signals than the niobium nitride material used previously. Credit: Credit: Baek/NIST

By swapping one superconducting material for another, researchers at the National Institute of Standards and Technology (NIST) have found a practical way to boost the efficiency of the world's fastest single-photon detector, while also extending light sensitivity to longer wavelengths. The new tungsten-silicon alloy could make the ultrafast detectors more practical for use in quantum communications and computing systems, experiments testing the nature of reality, and emerging applications such as remote sensing.

The detector, made of superconducting nanowires, is one of several sensor designs developed or used at NIST to register individual photons (particles of light). The original nanowire detector, invented in Russia, uses wires made of niobium nitride and has a detection or quantum efficiency-ability to generate an electrical signal for each arriving photon-of less than 10 percent in its simplest, most compact model. NIST's tungsten-silicon alloy version has an efficiency of 19 to 40 percent over a broad wavelength range of 1280 to 1650 nanometers, including bands used in telecommunications. The limitations are due mainly to imperfect photon absorption, suggesting that, with further design improvements, detector efficiency could approach 100 percent reliably, researchers say.

Superconducting nanowire detectors have many advantages. They are very fast, able to count nearly a billion photons per second, and they operate over a large range of wavelengths, have low dark (false) counts, and produce strong signals, especially at telecom wavelengths. The detectors produce a signal when a photon breaks apart some of the electron pairs that carry current in the superconducting state, where the material has zero resistance. If the nanowires are narrow enough and the DC current across the device is very close to the transition between ordinary and super conductance, a resistive band temporarily forms across each wire, resulting in a measurable voltage pulse.

Niobium nitride is difficult to make into nanowires that are narrow, long, and sensitive enough to work well. NIST researchers selected the tungsten-silicon alloy mainly because it has higher energy sensitivity, resulting in more reliable signals. A photon breaks more electron pairs in the tungsten-silicon alloy than in niobium nitride. The tungsten alloy also has a more uniform and less granular internal structure, making the nanowires more reliably sensitive throughout. As a result of the higher energy sensitivity, tungsten-silicon nanowires can have larger dimensions (150 nanometers wide versus 100 nanometers or less for niobium nitride), which enlarges the detectors' functional areas to more easily capture all photons.

The NIST team now hopes to raise the efficiency of tungsten alloy detectors by embedding them in optical cavities, which trap light for extremely high absorption. High efficiency may enable the use of nanowire detectors in demanding applications such as linear optical quantum computing, which encodes information in single photons. An equally intriguing application may be an experiment to test quantum mechanics-the so-called "loophole-free Bell test." This test of what Einstein called "spooky action at a distance" depends critically on having a nearly 100-percent efficient photon detector. Tungsten-silicon detectors also are sensitive to longer wavelengths of light, in the mid-infrared range, which could be useful for applications such as laser-based remote sensing of trace gases.

More information: B. Baek, A.E. Lita, V. Verma and S.W. Nam. Superconducting a-WxSi1-x nanowire single-photon detector with saturated internal quantum efficiency from visible to 1850 nm. Applied Physics Letters 98, 251105. Published online June 21, 2011.

Provided by National Institute of Standards and Technology (news : web)

Packing the ions: Discovery boosts supercapacitor energy storage

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Computational modeling of carbon supercapacitors with the effects of surface curvature included. Credit: Jingsong Huang, ORNL

Flat is in the eye of the beholder. When you're talking about nanomaterials, however, that eye is pretty much useless unless it's looking through an electron microscope or at a computer visualization. Yet the pits and ridges on a seemingly flat surface -- so small they are invisible without such tools -- can give the material astonishing abilities. The trick for researchers interested in taking advantage of these abilities lies in understanding and, eventually, predicting how the microscopic topography of a surface can translate into transformative technologies.

Drexel University's Yury Gogotsi and colleagues recently needed an atom's-eye view of a promising supercapacitor material to sort out experimental results that were exciting but appeared illogical. That view was provided by a research team led by Oak Ridge National Laboratory (ORNL) computational chemists Bobby Sumpter and Jingsong Huang and computational physicist Vincent Meunier.

Gogotsi's team discovered you can increase the energy stored in a carbon supercapacitor dramatically by shrinking pores in the material to a seemingly impossible size?seemingly impossible because the pores were smaller than the solvent-covered electric charge-carriers that were supposed to fit within them. The team published its findings in the journal Science.

The mystery was not simply academic. Capacitors are an important technology that provides energy by holding an electrical charge. They have several advantages over traditional batteries?charging and discharging nearly instantaneously and recharging over and over again, almost indefinitely, without wearing out?but they also have drawbacks?most importantly, they hold far less energy.

An electric double-layer capacitor, or supercapacitor, represents an advance on the technology that allows for far greater energy density. While in traditional capacitors two metallic plates are separated by a nonconducting material known as a dielectric, in a supercapacitor an electrolyte is able to form an electric double layer with electrode materials that have very high surface areas.

As such, supercapacitors are able to achieve the same effect within a single material, as properties of the material divide it into separate layers with a very thin, nonconducting boundary. Because they can both forgo a bulky dielectric layer and make use of the carbon's nanoscale pores, supercapacitors are able to store far more energy than their traditional counterparts in a given volume. This technology could help increase the value of energy sources that are clean, but sporadic, meting out stored energy during downtimes such as night for a solar cell or calm days for a wind turbine.

So Gogotsi's discovery was potentially ground breaking. The energy was stored in the form of ions within an electrolyte, with the ions surrounded by shells of solvent molecules and packed on the surfaces of nanoporous carbons. The researchers were able to control the size of pores in the carbon material, making them 0.7 to 2.7 nanometers. What they found was that the energy stored in the material shot up dramatically as the pores became smaller than a nanometer, even though the ions in their solvation shells could not fit into spaces that small.

"It was a mystery," Sumpter said. "Many people questioned the result at the time. Yet the experimental data was showing an incredible increase in capacitance."

Fortunately, it was a mystery that the ORNL team could unravel.

"We thought this was a perfect case for computational modeling because we could certainly simulate nanometer-sized pores," Sumpter said. "We had electronic-structure capabilities that could treat it well, so it was a very good problem for us to explore."

Using ORNL's Jaguar and Eugene supercomputers, Sumpter and his team were able to take a nanoscale look at the interaction between ion and carbon surface. A computational technique known as density functional theory allowed them to show that the phenomenon observed by Gogotsi was far from impossible. In fact, they found that the ion fairly easily pops out of its solvation shell and fits into the nanoscale pore.

"It goes in such a way that it desolvates in the bulk to get inside because there's electrostatic potential and van der Waals forces that pull it in," Sumpter explained. "There are a whole lot of different forces involved, but in fact it's very easy for it to get in."

The ORNL team and colleagues at Clemson University, Drexel University, and Georgia Tech detailed their findings in a series of publications, including Angewandte Chemie, Chemistry-A European Journal, ACS Nano, Journal of Chemical Physics C, Physical Chemistry Chemical Physics, Journal of Materials Research, and Nano Letters.

"In addition," Sumpter noted, "the microscopic bumps and divots on a carbon plate make a dramatic difference in the amount of energy that can be stored on or in it.

"When you get to the nanoscale, the surface area is huge, and the curvature, both concave and convex, can be very large. This makes a large difference in the capacitance. We derived a model that explained all the experimental data. You can back out the pieces of the model from the electronic structure calculations, and from that model you can predict capacitance for different types of curved shapes and pore sizes."

For example, he said, the calculations showed that the charge-carrying ions are stored not only by slipping into pores but also attaching to mounds in the material.

"It's a positive curvature instead of a negative curvature," Sumpter said, "and they can store and release energy even faster. So you can store ions inside a hole or you can store ions outside."

Using these and other insights gained through supercomputer simulation, the ORNL team partnered with colleagues at Rice University to develop a working supercapacitor that uses atom-thick sheets of carbon materials.

"It uses graphene on a substrate and a polymer-gel electrolyte," Sumpter explained, "so that you produce a device that is fully transparent and flexible. You can wrap it around your finger, but it's still an energy storage device. So we've gone all the way from modeling electrons to making a functional device that you can hold in your hand."

Web of international collaboration boosts worldwide nanotechnology research

Web of international collaboration boosts worldwide nanotechnology research

Despite their initial focus on national economic competitiveness, the nanotechnology research initiatives now funded by more than 60 countries have become increasingly collaborative, with nearly a quarter of all papers co-authored by researchers across borders.

Researchers from the two leading producers of nanotechnology papers – China and the United States – have become each nation's most frequent international co-authors. Though Chinese and U.S. researchers now publish roughly the same number of nanotechnology papers, the U.S. retains a lead in the quality of publications – as measured by the number of early citations.

"Despite ten years of emphasis by governments on national nanotechnology initiatives, we find that patterns of nanotechnology research collaboration and funding transcend country boundaries," said Phillip Shapira, study co-author and a professor in the School of Public Policy at the Georgia Institute of Technology. "For example, we found that U.S. and Chinese researchers have developed a relatively high level of collaboration in nanotechnology research. Each country is the other's leading collaborator in nanotechnology R&D."

The findings were part of a new study of nanotechnology publishing reported Dec. 2 in the online edition of the journal Nature. The research was sponsored by the National Science Foundation-supported Center for Nanotechnology in Society at Arizona State University (CNS-ASU).

Sparked by programs such as the National Nanotechnology Initiative (NNI) in the United States, leading industrial nations have launched nanotechnology research programs that invested more than $8 billion in public funds in 2008 alone. China, Germany, Japan and Korea are among the many countries that have launched major governmental programs to develop their national nanotechnology capabilities as part of efforts to boost future economic growth.

"There is widespread anticipation that nanotechnology will be a critical component in addressing global challenges in such areas as energy, environment, health care, security and sustainability," explained Shapira, who is also a professor of innovation at the University of Manchester. "At the same time, nanotechnology may be a key driver in the next wave of technology-led economic growth and investment. Governments around the world are hoping that their often massive investments in nanotechnology R&D will lead not only to economic, but also to significant societal returns."

Though the revolutionary advances that nanotechnology promises are still off into the future, Shapira noted that the investments made so far have led to "a noticeable shift toward innovation in the past few years as companies are beginning to market a wide range of products and devices whose performance has been enhanced by nanoscale science and engineering."

The study was conducted by Shapira and collaborator Jue Wang, an assistant professor at Florida International University. It used data mining techniques to study funding acknowledgements that have been available since 2008 in the Web of Science – one of the leading international databases of scientific publications. Shapira and Wang analyzed more than 91,000 papers published worldwide between August 2008 and July 2009.

They found that although researchers from 152 nations were represented in the survey, just 15 countries represented 90 percent of the papers. The top four countries by author affiliation were the United States (23 percent), China (22 percent), Germany (8 percent) and Japan (8 percent). Papers authored by researchers from more than one nation – which constituted 23 percent of those examined – were assigned to more than one country.

Though the United States and China now produce approximately the same number of papers, the U.S. maintains significant advantages.

"Compared with Chinese counterparts, papers authored by U.S. researchers still have a substantial lead in terms of citation quality and U.S. corporate activity in nanotechnology innovation remains rather larger," Shapira said. "However, Chinese quality is improving and an increasing number of Chinese companies are becoming engaged in developing and commercializing nano-enabled products."

The study analyzed the funding sources cited in a sub-set of 61,300 papers that were supported by grants. The National Natural Science Foundation of China was the top funder, with more than 10,200 publications representing 16.7 percent of all sponsored papers. Second was the U.S. National Science Foundation with 6,700 publications. Rounding out the top five were the Ministry of Science and Technology of China, the European Union's R&D programs, and the U.S. Department of Health and Human Services – which includes the National Institutes of Health.

Eight sponsors saw at least 10 percent of the papers they funded garner five or more citations within a year of publication – the study's definition of an "early-citation" paper. This group is led by four U.S. agencies: the National Institutes of Health, the National Science Foundation, the Department of Energy, and the Department of Defense.

About three percent of U.S. papers reported co-funding from the Chinese National Natural Science Foundation, while a similar proportion of Chinese papers report co-funding from the U.S. National Science Foundation.

"Although these numbers are still low relative to purely nationally-funded papers, they signal a significant trend as China has taken over from European countries as America's leading international collaborator by volume in nanotechnology research," Shapira explained. "China's scientific relationships do, of course, extend beyond the United States, and China has emerged as the hub for nanotechnology research collaboration in Asia."

The study also found that sponsors concentrating their funding in fewer institutions had lower research impact as measured by early citation counts.

"Our starting hypothesis is that when groups from multiple institutions vie for funding, there is increased competition, review processes are less partial, and there are more opportunities to select the most improving projects," Shapira explained.

With increasing budget pressures, growth in nanotechnology funding appears unlikely. How should countries invest their limited funding for greatest benefit?

"One way would be to foster more high-quality international collaborations, perhaps by opening funding competitions to international researchers and by offering travel and mobility awards for domestic researchers to increase alliances with colleagues in other countries," the researchers suggested in their paper.