Photon-to-electron conversion
Throughout decades of research on solar cells, one formula has been considered an absolute limit to the efficiency of such devices in converting sunlight into electricity: Called the Shockley-Queisser efficiency limit, it posits that the ultimate conversion efficiency can never exceed 34% for a single optimized semiconductor junction.  But now, researchers at MIT have shown there is a way to blow past that limit.

The principle behind the barrier-busting technique has been known theoretically since the 1960s, the researchers aid, but it was a somewhat obscure idea that nobody had succeeded in putting into practice. The MIT team was able, for the first time, to perform a successful “proof of principle” of the idea, which is known as singlet exciton fission. An exciton is the excited state of a molecule after absorbing energy from a photon.

In a standard photovoltaic (PV) cell, each photon knocks loose exactly one electron inside the PV material. That loose electron then can be harnessed through wires to provide an electrical current. But in the new technique, each photon can instead knock two electrons loose. This makes the process much more efficient: In a standard cell, any excess energy carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.

While others have previously “split” a photon’s energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth’s surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels. This was accomplished using an organic compound called pentacene in an organic solar cell. While that material’s ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.

Wireless “smart skin” sensors
Major bridge failures in recent years have focused attention on the need to monitor America’s highway bridges and other infrastructure. As thousands of bridges, parking garages and other structures age, improved methods for detecting deterioration could save lives and prevent economic disruption.

Researchers at the Georgia Institute of Technology are developing a novel technology that would facilitate close monitoring of structures for strain, stress and early formation of cracks. Their approach uses wireless sensors that are low cost, require no power, can be implemented on tough yet flexible polymer substrates, and can identify structural problems at a very early stage. The only electronic component in the sensor is an inexpensive radio-frequency identification (RFID) chip.

These sensor designs can be inkjet-printed on various substrates, using methods that optimize them for operation at radio frequency. The result would be low-cost, weather-resistant devices that could be affixed by the thousands to various kinds of structures.

For many engineering structures, one of the most dangerous problems is the initiation of stress concentration and cracking, which is caused by overloading or inadequate design and can lead to collapse – as in the case of the I-35W bridge failure in Minneapolis in 2007, the researchers said.  Placing a ‘smart skin’ of sensors on structural members, especially on certain high-stress hot spots that have been pinpointed by structural analysis, could provide early notification of potential trouble.

The Georgia Tech research team is focusing on wireless sensor designs that are passive, which means they need no power source. Instead, these devices respond to radio-frequency signals sent from a central reader or hub. One such reader can interrogate multiple sensors, querying them on their status at frequent intervals. The researchers’ approach utilizes a small antenna mounted on a substrate and tuned to a specific radio frequency. This technique enables the antenna itself to function as a stress sensor.

As long as the structural member to which the antenna/sensor is affixed remains entirely stable, its frequency stays the same. But even a slight deformation in the structure also deforms the antenna and alters its frequency response. The reader can detect that change at once, initiating a warning months or years before an actual collapse.

Gentler robot hands
What use is a hand without nerves, that can’t tell what it’s holding? A hand that lifts a can of soda to your lips, but inadvertently tips or crushes it in the process? Researchers at the Harvard School of Engineering and Applied Sciences (SEAS) have developed a very inexpensive tactile sensor for robotic hands that is sensitive enough to turn a brute machine into a dextrous manipulator.

Designed by researchers in the Harvard Biorobotics Laboratory at SEAS, the sensor, called TakkTile, is intended to put what would normally be a high-end technology within the grasp of commercial inventors, teachers, and robotics enthusiasts.

Despite decades of research, tactile sensing hasn’t moved into general use because it’s been expensive and fragile, normally costing about $16,000, give or take, to put tactile sensing on a research robot hand, which is really limited where people can use it. The traditional technology also uses very specialized construction techniques, which can slow down your work. Now, Takktile changes that because it’s based on much simpler and cheaper fabrication methods, the researchers said.

TakkTile takes an existing device—a tiny barometer, which senses air pressure—and adds a layer of vacuum-sealed rubber to it, protecting it from as much as 25 pounds of direct pressure. The creators said that the chips can even survive a strike from a hammer or a baseball bat. At the same time, Takktile is sensitive enough to detect a very slight touch.

An 8x5 tactile array provides gram-level sensitivity in hardware created from MEMS barometers and standard manufacturing processes. (Source: Harvard)

The result, when added to a mechanical hand, is a robot that knows what it’s touching. It can pick up a balloon without popping it. It can pick up a key and use it to unlock a door.

~Ann Steffora Mutschler

TVs with the thickness and weight of a sheet of paper will be possible someday as printed electronics technology advances. This technology is already used in organic solar cells and organic light-emitting diodes (OLEDs) that form the displays of cellphones by allowing manufacturers to literally print or roll materials onto surfaces to produce an electronically functional device.

However, one current challenge is in manufacturing at low cost in ambient conditions. In order to create light or energy by injecting or collecting electrons, printed electronics require conductors, usually calcium, magnesium or lithium, with a low-work function. These metals are chemically very reactive; they oxidize and stop working if exposed to oxygen and moisture. This is why electronics in solar cells and TVs, for example, must be covered with a rigid, thick barrier such as glass or expensive encapsulation layers.

But Georgia Tech researchers have introduced what appears to be a universal technique to reduce the work function of a conductor. They spread a very thin layer of a polymer, approximately one to 10 nanometers thick, on the conductor’s surface to create a strong surface dipole. The interaction turns air-stable conductors into efficient, low-work function electrodes. The commercially available polymers can be easily processed from dilute solutions in solvents such as water and methoxyethanol, are inexpensive, environmentally friendly and compatible with existent roll-to-roll mass production techniques.

After introducing what appears to be a universal technique to reduce the work function of a conductor in printable electronics, a team led by Georgia Tech's Bernard Kippelen has developed the first completely plastic solar cell. (Source: Georgia Tech)

Bernard Kippelen, director of Georgia Tech’s Center for Organic Photonics and Electronics said, “Replacing the reactive metals with stable conductors, including conducting polymers, completely changes the requirements of how electronics are manufactured and protected. Their use can pave the way for lower cost and more flexible devices.” To illustrate the new method, Kippelen and his peers evaluated the polymers’ performance in organic thin-film transistors and OLEDs and have built a prototype of the first-ever, completely plastic solar cell.

–Ann Steffora Mutschler

Nanoeconomics
Nano and green technology frequently go hand in hand, but so far the focus has been on the “gee whiz” factor rather than how much this stuff really costs. An ongoing study by Georgia Tech is about to change all of that.

Researchers Philip Shapira and Jan Youtie have launched a study into the impact of nanotechnologies on energy, the environment and safe drinking water, balancing efficiency gains and performance against the net energy, environmental, carbon and other costs associated with production.

This multidisciplinary research is a first, and it could pave the way for stripping away some of the hype about new technology and replacing it with cold hard facts. Among the areas under study are nano-enabled solar cells, nanogenerators using piezoelectric materials, energy storage, nano insulation and nano additives for fuel. This one should be interesting.

Brown Fields
Next time someone talks about greenhouse gases and the need for a better energy policy, blame fertilizer. UC Berkeley researchers analyzed the nitrous oxide in the atmosphere and discovered that fertilizer is a major contributing factor.

Nitrous oxide, aka laughing gas, is the same stuff that was used by dentists to make people forget about having cavities filled in their teeth. But in the atmosphere, nitrous oxide is the third-most potent greenhouse gas—after carbon dioxide and methane. Cows, auto exhaust, and coal-fired industrial facilities are still a problem, but so are the chemicals used to make crops and lawns grow faster and bigger.

The Cape Grim Baseline Air Pollution Station in Tasmania. Source: UC Berkeley.

The scientists came to their conclusions after analyzing air samples taken from Antarctic ice, called “fim” air, dating back to 1940 and air taken from an atmospheric monitoring station at Cape Grim in Tasmania, which has archived air since 1978. Nitrous oxide is clearly on the rise in the stratosphere, which no doubt will lead to some interesting changes in the application and content of fertilizer.

–Ed Sperling

Piezoelectric Patterns
A piezoelectric effect is the relationship between mechanical stress and voltage in solid materials. The concept has been around since the late 1800s, when it was discovered that crushing crystals of things like salt and sugar produced small electrical charges.

The common applications are quartz clocks, airbags and cigarette lighters, and they all rely on standard crystalline materials. But researchers at Georgia Tech are now working with ferroelectric nanotubes, which have a large piezoelectric effect, and turning them into different shapes. The result: The ability to manipulate the shape and the piezoelectric response—and to get a much higher piezoelectric response.

Ferroelectric nanotubes. Source: Georgia Tech.

This is particularly important for energy harvesting and nanoelectromechanical systems, or nanoMEMS.

Naazanin Bassiri-Gharb, assistant professor of mechanical engineering, said the school is using a new nano-manufacturing method to create 3D nanostructures. Being able to control the manufacturing is particularly important to the success of these materials commercially—particularly in the semiconductor world where proven manufacturing control is a requirement.

Faster Cooling
The process of distillation is as old as hard liquor, and the same process—condensation—has been used since the introduction of cooling. But actually figuring out how to get more out of condensation isn’t something that received a lot of thought.

MIT researchers have taken a deeper look at how droplets form, and how to change the structure of the collecting surfaces so that the droplets for more quickly. The goal, according to the researchers, is more efficient power plants and desalination plants. But it also could improve heat dissipation in computer chips using more advanced heat sinks.

Rethinking condensation. Source: MIT.

Key to this whole process is being extremely precise about the spacing and the width to height ratios and roughness of nanoscale pillars. The condensation drops form on top of the pillars, rather than on the surface, which makes them far more effective in releasing those drops.

The result? MIT claims a heat transfer of up to 71% compared to flat surfaces used in existing condensers.

–Ed Sperling

By Brian Fuller
Silicon CMOS is a tough act to follow. The workhorse building block for the world’s electronics has been delivering for system designers for a half century. Despite hand-wringing over its apparent scalability limits, it shows only vague signs of slowing down.

For nearly as many years, it seems, the next great material or alternative to silicon CMOS has popped into the industry’s consciousness promising to be the next big thing—next year. Gallium arsenide, for example, has been next year’s hit technology for four decades.

The latest “it” material, however, could actually deliver on its early hype, and in the process enable the industry not only to continue scaling but to drive deep into previously unexpected depths of low-power design. Graphene—the two-dimensional crystalline form of carbon—first emerged as a term in the late 1980s and gained traction in 2004 when researchers at the University of Manchester extracted graphene layers from graphite—basically using Scotch tape—and then placing on silicon dioxide on a silicon wafer.

This time it’s different (maybe)
The material exhibited fantastic characteristics, including high electron mobility compared to silicon, twice the storage capacity of ultracapacitors, and it was rugged to boot. What’s more, its characteristics apparently remain stable down to the molecular level, unlike other materials used in semiconductor design. The graphene promise is such that in just the past three years, research papers are being written on graphene at the rate of one a day.

“There are two features that make graphene exceptional,” Kirill Bolotin, assistant professor in the Vanderbilt Department of Physics and Astronomy, said in a recent interview. “First, its molecular structure is so resistant to defects that researchers have had to hand-make them to study what effects they have. Second, the electrons that carry electrical charge travel much faster and generally behave as if they have far less mass than they do in ordinary metals or superconductors.”

Where some see glowing walls made of graphene circuitry and other exotic applications, people like James Meindl see an answer to scaling. Keynoting at the recent ISSCC (International Solid State Circuits Conference) in San Francisco, Meindl, director of the Joseph M. Pettit Microelectronics Research Center at the Georgia Institute of Technology in Atlanta, said: ”We will continue to scale vigorously for the next 15 years. Beyond silicon microchip technology, revolutionary developments in nanoelectronics, perhaps centering on graphene, may evolve.”

That’s music to the ears of many who, despite CMOS’s dogged determination, seeing scaling hitting a wall in the next decade.

“Look at Intel’s roadmap. They’re looking at 4nm in 2022,” said Michael Keating, a Synopsys Fellow. “As long as they’re charging down that road, graphene’s going to be a second-class citizen. But my guess is 2022 is not realistic for 4nm. Silicon will be seriously in trouble in that decade.”

“The reason graphene’s interesting is so much progress has been made in such a short time frame,” he added.

What’s all the fuss?
Graphene—a one-atom-thick planar sheet of sp2-bonded carbon atoms that resembles chicken wire—has a lot going for it.

Meindl, speaking at ISSCC, gave a half-dozen reasons graphene is going to win in the marketplace, including:

• No other known material has a higher mechanical strength-to-weight ratio.
• Carrier mobility exceeds 200,000-cm2/Vs.
• The capacity to conduct current densities as large as one thousand times greater than copper without electromigration.
• Graphene can serve as a source, channel drain regions of a field effect transistor (FET) and as an interconnect.

In addition to all the big talk, there’s been action.
• Fujitsu Laboratories Ltd. has developed a method to form graphene transistors directly on the entire surface of large-scale insulating substrates at low temperatures while employing popular chemical-vapor deposition (CVD) techniques.
• IBM in 2007 fabbed graphene field-effect transistors (FETs) using a single layer of carbon atoms atop a silicon wafer at its T.J. Watson Research Center at Yorktown Heights, N.Y.
• In February, IBM built, on 2-inch wafers, RF graphene transistors running at 100-GHz and operating at room temperature.
• At around the same time, Penn State researchers announced they have developed a way to fabricate graphene sheets on 4-inch wafers.
• Last year, Bolotin, working with colleagues at Columbia University, managed to get graphene to exhibit the fractional quantum Hall effect, where the electrons create new particles with electrical charges that are a fraction that of individual electrons, according to work published in the journal Nature.
• A venture-backed Austin, Texas, company, Graphene Energy, is working to commercialize graphine for energy storage.
• Javad Rafiee, a doctoral student at Rensselaer Polytechnic Institute developed a method of ultra-efficient hydrogen storage based on graphene. His approach stores hydrogen with 14 percent efficiency, better than any other material attempted to date.

What’s the catch?
There’s always a catch. While graphene is easier to manufacture than its cousin, the carbon nanotube, it’s no slam dunk yet. To date, there hasn’t been a simple way to create the p- and n-type devices required for CMOS transistors. But Georgia Tech recently reported it has used an electron beam doping process that simplifies the transistor manufacture.

In addition, graphene has no band gap so there’s no way to turn them “off.” But even that hurdle is being brought down. Researchers at Lawrence Berkeley National Lab last year engineered a controllable band gap in bilayer graphene—at room temperature.

When will we know when graphene gets the “next-year’s technology” monkey off its back for good?

Maybe relatively soon, suggested Synopsys’ Keating.
“CMOS has had an incredible run. It’s foolish to bet against CMOS. (But) graphene every year makes significant progress. It’s absolutely the promising thing right now. We’re a decade away.”