“Seeking” Artificial Photosynthesis

tiny_electronics_plants_insects_jpg_662x0_q100_crop-scaleThe excessive atmospheric carbon dioxide that is driving global climate change could be harnessed into a renewable energy technology that would be a win for both the environment and the economy. That is the lure of artificial photosynthesis in which the electrochemical reduction of carbon dioxide is used to produce clean, green and sustainable fuels.

However, finding a catalyst for reducing carbon dioxide that is highly selective and efficient has proven to be a huge scientific challenge. Meeting this challenge in the future should be easier thanks to new research results from Berkeley Lab. Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division, led a study in which bimetallic nanoparticles of gold and copper were used as the catalyst for the carbon dioxide reduction. The results experimentally revealed for the first time the critical influence of the electronic and geometric effects in the reduction reaction (“Synergistic geometric and electronic effects for electrochemical reduction of carbon dioxide using gold-copper bimetallic nanoparticles”).

gold–copper bimetallic nanoparticles

This TEM shows gold–copper bimetallic nanoparticles used as catalysts for the reduction of carbon dioxide, a key reaction for artificial photosynthesis. “Acting synergistically, the electronic and geometric effects dictate the binding strength for reaction intermediates and consequently the catalytic selectivity and efficiency in the electrochemical reduction of carbon dioxide,” Yang says. “In the future, the design of carbon dioxide reduction catalysts with good activity and selectivity will require the careful balancing of these two effects as revealed in our study.”

Yang, who also holds appointments with the University of California (UC) Berkeley and the Kavli Energy NanoSciences Institute at Berkeley, is a leading authority on nanoparticle phenomena. His most recent research has focused on nanocatalysts fashioned from metal alloys rather than a single metal such as gold, tin or copper. “By alloying, we believe we can tune the binding strength of intermediates on a catalyst surface to enhance the reaction kinetics for the carbon dioxide reduction,” he says.

“Nanoparticles provide an ideal platform for studying this effect because, through appropriate synthetic processes, we can access a wide range of compositions, sizes and shapes, allowing for a deeper understanding of catalyst performance through precise control of active sites.” In addition, Yang says, nanoparticle as catalysts have high surface-to-volume and surface-to-mass ratios that are advantageous for achieving high catalytic activity. For this new study, uniform gold–copper bimetallic nanoparticles with different compositions were assembled into ordered monolayers then observed during carbon dioxide reduction.

“The ordered monolayers served as a well-defined platform that enabled us to better understand their fundamental catalytic activity in carbon dioxide reduction,” Yang says. “Based on our observations, the activity of the gold-copper bimetallic nanoparticles can be explained in terms of the electronic effect, in which the binding of intermediates can be tuned using different surface compositions, and the geometric effect, in which the local atomic arrangement at the active site allows the catalyst to deviate from the scaling relation.” The effects Yang and his colleagues observed for gold-copper bimetallic nanoparticles should hold true for other carbon dioxide reduction catalysts as well. “We expect the effects we observed to be universal for a wide range of catalysts, as evidenced in other areas of catalysis such as the hydrogen evolution and oxygen reduction reactions,” says Dohyung Kim, a member of Yang’s research group and a collaborator in this study.

“The factors we have identified are based on the solid concept of electrocatalysis.” Knowing the influence of the electronic and geometric effects makes it possible to deduce how intermediate products in the reduction of carbon dioxide, such as carboxylic acid and carbon monoxide, will interact with the surface of a newly proposed catalyst and thereby provide the means for predicting the catalyst’s performance. Coupled with the exceptional structuring of active catalytic sites made possible by the use of nanoparticles, the path is paved, Yang and his colleagues believe, for unprecedented improvements in electrochemical carbon dioxide reduction.

“My group is now using the insights gained from this study in the design of next generation carbon dioxide reduction catalysts,” Yang says.
A paper describing this research has been published in Nature Communications entitled “Synergistic geometric and electronic effects for electrochemical reduction of carbon dioxide using gold–copper bimetallic nanoparticles.” Yang is the corresponding author and Kim is the lead author. The other co-authors are Joaquin Resasco, Yi Yu and Abdullah Mohamed Asiri.
Source: Lawrence Berkeley National Laboratory

“Green” Concrete – “Just the Hard Facts Ma’am

Green Cement 0929_CEMENT-2-rnx250Concrete can be better and more environmentally friendly by paying attention to its atomic structure, according to researchers at Rice Univ., the Massachusetts Institute of Technology (MIT) and Marseille Univ.

The international team of scientists has created computational models to help concrete manufacturers fine-tune mixes for general applications.

Rice materials scientist Rouzbeh Shahsavari said the team created what it considers a game-changing strategy for an industry that often operates under the radar but is still the third-largest source of carbon dioxide released to the atmosphere.

Nature Communications published the open-access study online.

Green Cement 0929_CEMENT-2-rnx250

The annual worldwide production of more than 20 billion tons of concrete contributes 5 to 10% of carbon dioxide, according to the researchers; only transportation and energy surpass it as producers of the greenhouse gas.

There are benefits to be gained for the environment and for construction by optimizing the process, said Shahsavari, an assistant professor of civil and environmental engineering at Rice. “The heart of concrete is C-S-H—that’s calcium, silicate and hydrate (water). There are impurities, but C-S-H is the key binder that holds everything together, so that’s what we focused on.

“In a nutshell, we tried to decode the phases of C-S-H across different chemistries, thereby improving the mechanical properties of concrete in a material way.”

The years-long study involved analysis of “defect attributes” for concrete, Shahsavari said. One was in the ratio of calcium to silicon, the basic elements of concrete. Another looked at the topology of atomic-level structures, particularly the location of defects and the bonds between “medium-range” calcium and oxygen or silicon and oxygen atoms—that is, atoms that aren’t directly connected but still influence each other. The combination of these defects gives concrete its properties, he said.

Shahsavari noted a previous work by the team defined average chemistries of cement hydrates. (Cement is the component in concrete that contains calcium and silicon.)

“C-S-H is one of the most complex structured gels in nature, and the topology changes with different chemistries, from highly ordered layers to something like glass, which is highly disordered. This time, we came up with a comprehensive framework to decode it, a kind of genome for cement,” he said.

The team looked at defects in about 150 mixtures of C-S-H to see how the molecules lined up and how their regimentation or randomness affected the product’s strength and ductility.

The ratio of calcium to silicon is critical, Shahsavari said. “For strength, a lower calcium content is ideal,” he said. “You get the same strength with less material, and because calcium is associated with the energy-intensive components of concrete, you use less rebar and you save energy in transporting the raw material. Also, it’s more environmentally friendly because you put less carbon dioxide into the atmosphere.”

Alternately, a higher ratio of calcium (indeed there is a sweet spot) provides more fracture toughness, which may be better for buildings and bridges that need to give a little due to wind and other natural forces like earthquakes or well cement subjected to downhole pressure or temperature variation.

“This is the first time we’ve been able to see new degrees of freedom in the formation of concrete based on the molecular topology,” Shahsavari said. “We learned that at any given calcium/silicon ratio, there may be 10 to 20 different molecular shapes, and each has a distinct mechanical property.

“This will open up enormous opportunities for researchers to optimize concrete from the molecular level up for certain applications,” he said. “There has been a lot of work in metals and semiconductors, but understanding how defects work in cement was far from obvious, and there was pretty much no basic work done at this level.

“So I would say this is perhaps one of the most important discoveries in cement science this century.”

Source: Rice Univ.

New research points to graphene as a flexible, low-cost touchscreen solution

Nano Wires 2147496528_220x220New research published today in the journal Advanced Functional Materials suggests that could soon replace current touchscreen technology, significantly reducing production costs and allowing for more affordable, flexible displays.

The majority of today’s touchscreen devices, such as tablets and smartphones are made using (ITO) which is both expensive and inflexible. Researchers from the University of Surrey and AMBER, the materials science centre based at Trinity College Dublin have now demonstrated how graphene-treated nanowires can be used to produce flexible touchscreens at a fraction of the current cost.

Using a simple, scalable and inexpensive method the researchers produced hybrid electrodes, the building blocks of touchscreen technology, from and graphene.

Nano Wires 2147496528_220x220

Dr Alan Dalton from the University of Surrey said, “The growing market in devices such as wearable technology and bendable smart displays poses a challenge to manufacturers. They want to offer consumers flexible, touchscreen technology but at an affordable and realistic price. At the moment, this market is severely limited in the materials to hand, which are both very expensive to make and designed for rigid, flat devices.”

Lead author, Dr Izabela Jurewicz from the University of Surrey commented, “Our work has cut the amount of expensive nanowires required to build such touchscreens by more than fifty times as well as simplifying the production process. We achieved this using graphene, a material that can conduct electricity and interpret touch commands whilst still being transparent.”

Co-author, Professor Jonathan Coleman, AMBER, added, “This is a real alternative to ITO displays and could replace existing touchscreen technologies in electronic devices. Even though this material is cheaper and easier to produce, it does not compromise on performance.”

“We are currently working with industrial partners to implement this research into future devices and it is clear that the benefits will soon be felt by manufacturers and consumers alike.”

Explore further: Conductive nanofiber networks for flexible, unbreakable, and transparent electrodes

New “gold nanocluster” Technology Revolutionizes Solar Power

QDOT images 6Scientists at Western University have discovered that a small molecule created with just 144 atoms of gold can increase solar cell performance by more than 10 per cent.




These findings, published recently by the high-impact journal Nanoscale, represent a game-changing innovation that holds the potential to take solar power mainstream and dramatically decrease the world’s dependence on traditional, resource-based sources of energy, says Giovanni Fanchini from Western’s Faculty of Science.

Fanchini, the Canada Research Chair in Carbon-based Nanomaterials and Nano-optoelectronics, says the new technology could easily be fast-tracked and integrated into prototypes of solar panels in one to two years and solar-powered phones in as little as five years.

“Every time you recharge your cell phone, you have to plug it in,” says Fanchini, an assistant professor in Western’s Department of Physics and Astronomy. “What if you could charge mobile devices like phones, tablets or laptops on the go? Not only would it be convenient, but the potential energy savings would be significant.”

The Western researchers have already started working with manufacturers of solar components to integrate their findings into existing and are excited about the potential.

“The Canadian business industry already has tremendous know-how in solar manufacturing,” says Fanchini. “Our invention is modular, an add-on to the existing production process, so we anticipate a working prototype very quickly.”

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Making nanoplasmonic enhancements, Fanchini and his team use “gold nanoclusters” as building blocks to create a flexible network of antennae on more traditional to attract an increase of light. While nanotechnology is the science of creating functional systems at the molecular level, nanoplasmonics investigates the interaction of light with and within these systems.

“Picture an extremely delicate fishnet of gold,” explains Fanchini explains, noting that the antennae are so miniscule they are unseen even with a conventional optical microscope. “The fishnet catches the light emitted by the sun and draws it into the active region of the solar cell.”


According to Fanchini, the spectrum of light reflected by gold is centered on the yellow colour and matches the light spectrum of the sun making it superior for such antennae as it greatly amplifies the amount of sunlight going directly into the device.

“Gold is very robust, resilient to oxidization and not easily damaged, making it the perfect material for long-term use,” says Fanchini. “And gold can also be recycled.”

It has been known for some time that larger gold nanoparticles enhance solar cell performance, but the Western team is getting results with “a ridiculously small amount” – approximately 10,000 times less than previous studies, which is 10,000 times less expensive too.

Explore further: Using solar energy to turn raw materials into ingredients for everyday life

Provided by University of Western Ontario

‘Greener,’ low-cost transistor heralds advance in flexible electronics


As tech company LG demonstrated this summer with the unveiling of its 18-inch flexible screen, the next generation of roll-up displays is tantalizingly close. Researchers are now reporting in the journal ACS Nano a new, inexpensive and simple way to make transparent, flexible transistors — the building blocks of electronics — that could help bring roll-up smartphones with see-through displays and other bendable gadgets to consumers in just a few years.

Washington, DC | Posted on September 24th, 2014

Yang Yang and colleagues note that transistors are traditionally made in a multi-step photolithography process, which uses light to print a pattern onto a glass or wafer. Not only is this approach costly, it also involves a number of toxic substances. Finding a greener, less-expensive alternative has been a challenge. Recently, new processing techniques using metal oxide semiconductors have attracted attention, but the resulting devices are lacking in flexibility or other essential traits. Yang’s team wanted to address these challenges.

LG Logo 111712_1657_LGLogoRedes6

The researchers developed inks that create patterns on ultrathin, transparent devices when exposed to light. This light sensitivity precludes the need for harsh substances or high temperatures. “The main application of our transistors is for next-generation displays, like OLED or LCD displays,” said Yang. “Our transistors are designed for simple manufacturing. We believe this is an important step toward making flexible electronics widely accessible.”

The authors acknowledge funding from the National Science Foundation.


About American Chemical Society
The American Chemical Society is a nonprofit organization chartered by the U.S. Congress. With more than 161,000 members, ACS is the world’s largest scientific society and a global leader in providing access to chemistry-related research through its multiple databases, peer-reviewed journals and scientific conferences. Its main offices are in Washington, D.C., and Columbus, Ohio.

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Nanoco has very bright future

Nanoco has very bright future Nanoco’s (NANO) cadmium-free quantum dots are in demand. They provide much better brightness, colour and power efficiency than existing LED technology, making them a must-have for the big electronics manufacturers. Finally, partner Dow Chemical (DOW) is building a plant to make them, which means big profits for Nanoco. Even after surging by 20%, prospects appear undervalued.

A global licensing deal with Dow was signed early in 2013, but the factory in South Korea has taken longer than expected to get past the planning stage, held up by Dow’s attempts to secure volume commitments from Korea’s major television manufacturers. That backing has clearly been secured and other big names are likely.

That’s why this is such big news and why investors have rushed into the shares. Nanoco traded close to 200p when the Dow deal emerged and was above 180p a year ago, but shareholders grew impatient and bailed out, sending the price as low as 85p.

But even at 147p, the shares could be cheap. Kicking off construction at the plant triggers a milestone payment, estimated at about $2 million (£1.22 million). Commercial production of Nanoco quantum dots – to be marketed by Dow under the Trevista brand – is expected to begin in the first half of 2015, followed by substantial royalty revenue for Nanoco, likely from the fourth quarter.

Broker Liberum is excited. It reckons the quantum dots will be sold at $80,000 per kilogram (kg), with each of the 21 million 55-inch TV sets sold annually using about 1gm. Even if only half use cadium-free dots demand could top 10,000kg. Tablets, notebook PCs and smartphones will use them, too.

We are therefore modelling Dow to expand capacity to 2,400kg by 2016 and further to 4,800kg by 2017,” says Liberum, which forecasts an increase in revenue from £5 million in the year to July 2015 to £51 million in 2017, generating earnings per share (EPS) of 17.1p.

On Liberum’s target multiple of 15 times earnings, Nanoco would be valued at almost 260p. “We therefore strongly recommend buying the stock here, which we see as an exciting long term technology growth story available at an attractive price,” it says.

Expect more colour when full-year results are published on 14 October.

This article is for information and discussion purposes only and does not form a recommendation to invest or otherwise. The value of an investment may fall. The investments referred to in this article may not be suitable for all investors, and if in doubt, an investor should seek advice from a qualified investment adviser.

Hybrid graphene–giant nanocrystal quantum dot assemblies

September 24, 2014

photonic-nanostructure-coupling-300x257Coupling two distinct nanostructures often leads to a new class of hybrid material with unique properties and functionalities. On the other hand, it could produce worthless hybrids, as one nanostructure could neutralize the useful properties of the other.

Graphene, an atomically thin sheet of carbon, exhibits unique electrical and optical properties that are highly beneficial for a wide range of electronic, optoelectronic, and photovoltaic applications. Semiconductor nanocrystals (NCs) that exhibit exceptional optical properties derived from 3D quantum confinement of their charge carriers (i.e., electrons and holes), have also inspired many applications spanning from biomedical imaging to solid state lighting and quantum communication. When NCs are coupled to graphene, excitons (i.e., electron–hole pairs) created in the NCs by the absorption of light can recombine nonradiatively, subsequently transferring their energy to graphene via near-field interactions (i.e, Förster energy transfer).

As a result, the NC’s photoluminescence (PL) is quenched strongly. While such an effect may be useful for energy harvesting and sensing applications, it neutralizes the exceptional light emission properties of the NCs and therefore severely inhibits the use of NC–graphene hybrid systems in applications such as light-emitting diodes, lasers, and as single photon sources.


In contrast to the above scenario, scientists from the Center for Integrated Nanotechnologies, Los Alamos National Laboratory, show that when a new class of nanocrystals called giant-NQDs (g-NQDs) are coupled to graphene, bi-excitons (BX), i.e., pairs of excitons, optically excited in g-NQDs, experience efficient recombination and subsequent simultaneous emission of photon pairs. A joint effort with scientists from the Theoretical Division, (Los Alamos National Laboratory) resulted in a model providing an insight into this interesting phenomenon. The model predicts the formation of excess charge density, named a ‘charge puddle’, within the graphene sheet right underneath a photocharged g-NQD. In this case, the g-NQD acts as a gate electrode, controlling the amount of charge in the puddle. Collective oscillations of the puddle’s charges results in a new localized plasmon mode interacting with the excitons and bi-excitons of the g-NQD. This interaction gives rise to the observed enhancement in photon emission.

These findings reveal a tremendous potential for graphene-g-NQD hybrids in applications requiring highly efficient single-photon and photon-pair emission, including lasing and entangled photon sources. The discovery of a new plasmon mode also has several important implications in the fields of plasmonics, photonics, and quantum optics. Specifically, extending the tunability of graphene plasmon modes to the visible spectral range should dramatically expand the optical functionality of graphene plasmonics. Furthermore, the formation of this new plasmon mode directly underneath the g-NQD provides a perfect solution to the general problem of quantum emitter–plasmonic cavity alignment hindering a realization of strong plasmon–exciton coupling. Access to such a coupling regime may ultimately open a new route towards quantum plasmonics.

Immune system is key ally in cyberwar against cancer: Rice University study yields new two-step strategy for weakening cancer

Rice Cancer 50167Abstract:
Research by Rice University scientists who are fighting a cyberwar against cancer finds that the immune system may be a clinician’s most powerful ally.

Houston, TX | Posted on September 23rd, 2014

“Recent research has found that cancer is already adept at using cyberwarfare against the immune system, and we studied the interplay between cancer and the immune system to see how we might turn the tables on cancer,” said Rice University’s Eshel Ben-Jacob, co-author of a new study this week in the Early Edition of the Proceedings of the National Academy of Sciences.

Ben-Jacob and colleagues at Rice’s Center for Theoretical Biological Physics (CTBP) and the University of Texas MD Anderson Cancer Center, developed a computer program that modeled a specific channel of cell-to-cell communication involving exosomes. Exosomes are tiny packets of proteins, messenger RNA and other information-coding segments that both cancer and immune cells make and use to send information to other cells.

“Basically, exosomes are small cassettes of information that are packed and sealed inside small nanoscale vesicles,” Ben-Jacob said. “These nanocarriers are addressed with special markers so they can be delivered to specific types of cells, and they contain a good deal of specific information in the form of signaling proteins, snippets of RNA, microRNAs and other data. Once taken by the target cells, these nanocarriers can order cells to change what they are doing and in some cases even change their identity.”

Rice Cancer 50167A cancer cell under attack by lymphocytes.

CREDIT: thinkstockphotos.com/Rice University

Ben-Jacob said recent research showed that dendritic cells use exosomal communications to carry out their specialized role as moderators of and mediators between the innate and adaptive immune systems. The innate and adaptive immune systems use different strategies to protect the body from disease. The innate system guards against all threats at all times and is the first to act even against unrecognized invaders. In contrast, the adaptive immune system acts more efficiently, and in a specific way, against recognized, established threats. Dendritic cells, which are part of both the innate and adaptive systems, share information and help “coach” the adaptive system’s hunter-killer cells about which cells to target and how best to destroy them.

“We were inspired to do this research by two papers — one that showed how the dendritic cells use the exosome to fight cancer and another that showed how cancer cells co-opt the exosomal system both to prevent the bone marrow from making dendritic cells and disable dendritic cells’ coaching abilities,” Ben-Jacob said. “This is cyberwarfare, pure and simple. Cancer uses the immune systems’ own communications network to attack not the soldiers but the generals that are coordinating the body’s defense.”

To examine the role of exosome-mediated cell-to-cell communication in the battle between cancer in the immune system, Ben-Jacob and postdoctoral fellow Mingyang Lu, the study’s first author, worked with CTBP colleagues to create a computer model that captured the special aspects of the exosomal exchange between cancer cells, dendritic cells and the other cells in the immune system.

“You should imagine there is a tug-of-war between the cancer and the immune system,” said study co-author and CTBP co-director José Onuchic, Rice’s Harry C. and Olga K. Wiess Professor of Physics and Astronomy. “Sometimes one side wins and sometimes the other. The question is whether we can understand this battle enough to use radiotherapy or chemotherapy in such a way as to change the balance of the tug-of-war in favor of the immune system.”

Based on their findings, Ben-Jacob and Onuchic say the answer is likely yes. In particular, the CTBP model found that the presence of exosomes creates a situation where three possible cancer states can exist, and one of the states — an intermediate state in which cancer is neither strong nor weak but the immune system is on high alert — could be the key for a new therapeutic approach and with reduced side effects.

“When exosomes are not included, there are only two possible states — one where cancer is strong and the immune system is weak and the other where cancer is weak and the immune system is strong,” Ben-Jacob said.

Although the state where cancer is weakened is preferable, there is a growing body of clinical evidence that suggests it is very difficult to force cancer directly from the strong to the weak position, in part because radiation and chemotherapeutic treatments also weaken the immune system as they weaken cancer.

“It is fairly common that a cancer recedes following treatment only to return stronger than ever in just a few months or weeks,” said study co-author Sam Hanash, professor of clinical cancer prevention and director of the Red and Charlene McCombs Institute for the Early Detection and Treatment of Cancer at MD Anderson. “The new model captures this dynamic and suggests alternative scenarios whereby the immune system does its job fighting the cancer.”

Ben-Jacob said the team showed that it was possible to force cancer from the strong to the moderate state by alternating cycles of radiation or chemotherapy with immune-boosting treatments.

“Our model shows that just a few of these treatment-boosting cycles can alter the cancer-immune balance to help the immune system bring the cancer to the moderate state,” Ben-Jacob said. “Once in the intermediate state, cancer can be brought further down to the weak state by a few short pulses of immune boosting.

“It is much more effective to use a two-step process and drive cancer from the strong to the intermediate state and then from the intermediate to the weak state,” he said. “Without the exosome — the cancer-immune cyberwar nanocarriers — and the third state, this two-step approach wouldn’t be possible.”

Ben-Jacob is a senior investigator at CTBP, adjunct professor of biochemistry and cell biology at Rice and the Maguy-Glass Chair in Physics of Complex Systems and professor of physics and astronomy at Tel Aviv University.

In addition to Ben-Jacob, Onuchic and Lu, study co-authors include Rice graduate student Bin Huang and Sam Hanash, director of the Red and Charline McCombs Institute for the Early Detection and Treatment of Cancer at the University of Texas MD Anderson Cancer Center. The research was supported by the Cancer Prevention and Research Institute of Texas, the National Science Foundation and the Tauber Family Funds.


About Rice University
Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation’s top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is home to the Baker Institute for Public Policy. With 3,920 undergraduates and 2,567 graduate students, Rice’s undergraduate student-to-faculty ratio is just over 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice is highly ranked for best quality of life by the Princeton Review and for best value among private universities by Kiplinger’s Personal Finance.

Follow Rice News and Media Relations on Twitter @RiceUNews.

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Nanotechnology leads to better, cheaper LEDs for phones and lighting

130807133432Princeton, NJ | Posted on September 24th, 2014

Using a new nanoscale structure, the researchers, led by electrical engineering professor Stephen Chou, increased the brightness and efficiency of LEDs made of organic materials (flexible carbon-based sheets) by 57 percent. The researchers also report their method should yield similar improvements in LEDs made in inorganic (silicon-based) materials used most commonly today.

The method also improves the picture clarity of LED displays by 400 percent, compared with conventional approaches. In an article published online August 19 in the journal Advanced Functional Materials, the researchers describe how they accomplished this by inventing a technique that manipulates light on a scale smaller than a single wavelength.

“New nanotechnology can change the rules of the ways we manipulate light,” said Chou, who has been working in the field for 30 years. “We can use this to make devices with unprecedented performance.”


A LED, or light emitting diode, is an electronic device that emits light when electrical current moves through two terminals. LEDs offer several advantages over incandescent or fluorescent lights: they are far more efficient, compact and have a longer lifetime, all of which are important in portable displays.

Current LEDs have design challenges; foremost among them is to reduce the amount of light that gets trapped inside the LED’s structure. Although they are known for their efficiency, only a very small amount of light generated inside an LED actually escapes.

“It is exactly the same reason that lighting installed inside a swimming pool seems dim from outside – because the water traps the light,” said Chou, the Joseph C. Elgin Professor of Engineering. “The solid structure of a LED traps far more light than the pool’s water.”

In fact, a rudimentary LED emits only about 2 to 4 percent of the light it generates. The trapped light not only makes the LEDs dim and energy inefficient, it also makes them short-lived because the trapped light heats the LED, which greatly reduces its lifespan.

“A holy grail in today’s LED manufacturing is light extraction,” Chou said.

Engineers have been working on this problem. By adding metal reflectors, lenses or other structures, they can increase the light extraction of LEDs. For conventional high-end, organic LEDs, these techniques can increase light extraction to about 38 percent. But these light-extraction techniques cause the display to reflect ambient light, which reduces contrast and makes the image seem hazy.

To combat the reflection of ambient light, engineers now add light-absorbing materials to the display. But Chou said such materials also absorb the light from the LED, reducing its brightness and efficiency by as much as half.

The solution presented by Chou’s team is the invention of a nanotechnology structure called PlaCSH (plasmonic cavity with subwavelength hole-array). The researchers reported that PlaCSH increased the efficiency of light extraction to 60 percent, which is 57 percent higher than conventional high-end organic LEDs. At the same time, the researchers reported that PlaCSH increased the contrast (clarity in ambient light) by 400 percent. The higher brightness also relieves the heating problem caused by the light trapped in standard LEDs.

Chou said that PlaCSH is able to achieve these results because its nanometer-scale, metallic structures are able to manipulate light in a way that bulk material or non-metallic nanostructures cannot.

Chou first used the PlaCSH structure on solar cells, which convert light to electricity. In a 2012 paper, he described how the application of PlaCSH resulted in the absorption of as much as 96 percent of the light striking solar cells’ surface and increased the cells’ efficiency by 175 percent. Chou realized that a device that was good at absorbing light from the outside could also be good at radiating light generated inside the device – offering an efficient solution for both light extraction and the reduction of light reflection.

“From a view point of physics, a good light absorber, which we had for the solar cells, should also be a good light radiator,” he said. “We wanted to experimentally demonstrate this is true in visible light range, and then use it to solve the key challenges in LEDs and displays.”

The physics behind PlaCSH are complex, but the structure is relatively simple. PlaCSH has a layer of light-emitting material about 100 nanometers thick that is placed inside a cavity with one surface made of a thin metal film. The other cavity surface is made of a metal mesh with incredibly small dimensions: it is 15 nanometers thick; and each wire is about 20 nanometers in width and 200 nanometers apart from center to center. (A nanometer is one hundred-thousandth the width of a human hair.)

Because PlaCSH works by guiding the light out of the LED, it is able to focus more of the light toward the viewer. The system also replaces the conventional brittle transparent electrode, making it far more flexible than most current displays.

“It is so flexible and ductile that it can be weaved into a cloth,” Chou said.

Another benefit for manufacturers is cost. The PlaCSH organic LEDs were made by nanoimprint, a technology Chou invented in 1995, which creates nanostructures in a fashion similar to a printing press producing newspapers.

“It is cheap and extremely simple,” Chou said.

Princeton has filed patent applications for both organic and inorganic LEDs using PlaCSH. Chou and his team are now conducting experiments to demonstrate PLaCSH in red and blue organic LEDs, in addition the green LEDs used in the current experiments. They also are demonstrating the system in inorganic LEDs.

Besides Chou, the paper’s authors are Wei Ding, Yuxuan Wang and Hao Chen, graduate students in electrical engineering at Princeton. Support for the research was provided in part by the Defense Advanced Research Projects Agency and the Office of Naval Research. Chou recently was awarded a major grant from the U.S. Department of Energy to further advance the use of PlaCSH as a solution for energy-efficient lighting.


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Quantum Dots and other world-changing stuff

Vicki Saunders

Yesterday I had the pleasure of attending the Perimeter Institute for Theoretical Physics for a special meeting of the Emmy Noether Council, Board of Directors and the Leadership Council. Every time I go there I get so excited by what they are doing and can literally feel myself changing as I learn more about their work. Waterloo is known as Quantum Valley and has the largest concentration of people building quantum computers anywhere in the world.
A friend of mine once said, Canada is just one energy innovation away from a collapsed economy (meaning if someone finds an energy alternative to oil, immediately, we are pooched). I see PI as our bet against that happening.
Mike Lazaridis said that he thinks we’ll have one world changing discovery in the next 10 years and then the other discoveries coming out of PI are going to dramatically transform our world.

Here are…

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