29 Jan 2015
Long the object of ivory tower fascination, quantum dots are entering the commercial realm. Factories that manufacture the nanomaterials are opening, and popular consumer products that use them are hitting the market.
Behind the gee-whiz technology are three companies with three different approaches to producing and delivering quantum dots.
Developed at Bell Labs in the 1980s, quantum dots are semiconducting inorganic particles small enough to force the quantum confinement of electrons. Ranging in size from 2 to 6 nm, the dots emit light after electrons are excited and return to the ground state. Larger ones emit red light, medium-sized ones emit green, and smaller ones emit blue.
Quantum dots have been proposed for all sorts of applications, including lighting and medical diagnostics, but the market that is taking off now is enhancing liquid-crystal displays (LCDs).
According to Yoosung Chung, an analyst who follows the quantum dot business for the consulting firm NPD DisplaySearch, last year saw the introduction of the first commercial display products to incorporate quantum dots: Bravia brand televisions from Sony and the Kindle Fire HDX tablet from Amazon. This year, the Chinese company TCL introduced a quantum-dot-containing TV and Taiwan’s Asus shipped a quantum dot laptop.
What quantum dots bring to displays is more vibrant colors generated with less energy. The liquid crystals in conventional LCD screens create colors by selectively filtering white light emitted by a light-emitting diode (LED) backlight, which typically runs along one edge of the screen. But that white light is broad spectrum and not optimal for producing the highly saturated reds, greens, and blues needed for lifelike images.
Jeff Yurek, a marketing manager at Nanosys, says the color performance of LCDs is only 70% of what is provided by more expensive organic light-emitting diode (OLED) displays.
Quantum-dot-enabled displays incorporate a backlight that gives off blue light, some of which the dots convert into pure red and green. The three colors combine into an improved white light that the LCDs draw on to create pictures that are almost as vivid as those achieved with OLEDs.
Moreover, because no light is wasted, energy costs are lowered. That’s important, according to Yurek, because the display accounts for half of the power consumed in a mobile device. By incorporating Nanosys’s quantum dots in its new HDX tablet, Amazon was able to cut display power consumption by 20%, he claims.
“Going from the HD to the HDX, they made a thinner, lighter, higher resolution, more colorful display with longer battery life,” Yurek says.
On the strength of demand from companies such as Amazon, Nanosys has been investing in its quantum dot plant in Milpitas, Calif. According to Yurek, the company is now completing an expansion that will more than double its output. Soon, he says, the firm will have the capacity to supply dots for 250 million 10-inch tablet devices a year.
Also expanding is QD Vision, a Lexington, Mass.-based firm founded on chemistry developed at Massachusetts Institute of Technology. Its dots can be found in Sony’s Bravia line and are set to appear in TVs made by TCL, which is the third-largest TV maker after Samsung and LG.
Seth Coe-Sullivan, QD Vision’s chief technology officer and cofounder, explains that his firm and Nanosys use the same basic manufacturing technique: They decompose organocadmium and other compounds at high heat in the presence of surfactants and solvents. The resulting monomers nucleate and form nanocrystals. Size can be controlled stoichiometrically or by thermally quenching the growing crystals.
Where the two firms differ is the way in which they embed quantum dots in a consumer product. Nanosys works with companies such as 3M to create quantum-dot-containing films that are placed between the LED backlight and the LCDs in tablets and other displays. For example, the Asus quantum-dot-containing laptop, known as the NX500 Notebook PC, incorporates the 3M/Nanosys film.
QD Vision, in contrast, encapsulates its quantum dots in a polymer matrix inside a glass tube that is placed directly against the LED backlight. It’s a hot environment but one that the dots can withstand, Coe-Sullivan says, because of how they are synthesized and packaged.
QD Vision manufactures its dots in Lexington and ships them to a contractor in Asia to be packaged in the tubes. The contractor is in the process of quadrupling capacity to 4 million tubes per month, which is enough, Coe-Sullivan says, to supply a quarter of the world’s TV industry.
He argues that his firm’s tube approach is suited to TVs and other large displays, whereas a film works better with smaller tablets and laptops. So far, marketplace adoption bears this contention out. “I honestly don’t feel our products compete with each other,” Coe-Sullivan says.
Dow, however, is throwing down the gauntlet against both approaches. Using technology licensed from the British firm Nanoco, Dow is developing cadmium-free quantum dots. It is betting that the display industry is uneasy with the cadmium content of dots from Nanosys and QD Vision and that it will flock to a cadmium-free alternative.
In September, Dow announced that it will use the Nanoco technology to build the world’s first large-scale, cadmium-free quantum dot plant at its site in Cheonan, South Korea. When the plant opens in the first half of 2015, Dow says, it will enable the manufacture of millions of quantum dot TVs and other display devices.
Dow and Nanoco haven’t disclosed the active material in their quantum dots and declined an interview with C&EN. They acknowledge that the dots contain indium but insist that they aren’t indium phosphide, as their competitors claim.
The use of one heavy metal versus another might not seem to make a big difference environmentally. But in the European Union, cadmium is one of six substances regulated by the Restriction of Hazardous Substances, or RoHS, directive. Cadmium cannot be present in electronics at levels above 100 ppm without an exemption.
Larger amounts of cadmium are allowed in LED-containing displays under an exemption that expired on July 1. Late last year, in a consultation process moderated by Oeko-Institut (Institute for Applied Ecology), a German nonprofit, the major quantum dot players made their cases for why the expiring exemption should or shouldn’t be extended.
Nanosys, QD Vision, 3M, and others lobbied for extension to at least 2019, arguing that the benefits of cadmium-based quantum dots outweigh any potential harm. One big reason is that they lower energy consumption by devices, meaning less use of coal in power plants and fewer of the cadmium emissions that can come from burning coal.
In April, Oeko recommended to the EU that the exemption be extended—but only to July 1, 2017, in light of emerging technology that could reduce or eliminate the need for cadmium quantum dots. Industry executives expect the EU to adopt the recommendation by the end of the year.
In their submissions to the consultation process, Dow and Nanoco argued that no extension is necessary because cadmium-free dots are already here. In fact, the Korea Times recently reported that LG and Samsung plan to launch cadmium-free TVs in 2015 with quantum dots from Dow.
Coe-Sullivan says he’ll believe it when he sees it. “The idea that the product is just around the corner has been around for a long time,” he observes. Cadmium-free displays from LG and Samsung were expected to appear at the recent IFA electronics trade show in Berlin, he says, but ended up being a no-show.
The reason, according to cadmium dot proponents, is that indium-based dots have about half the energy efficiency and a narrower color range. “Cad-free today does not have the same performance as cadmium-containing quantum dots,” Coe-Sullivan says. QD Vision and Nanosys also contend that indium-containing quantum dots aren’t environmentally superior, pointing to indium phosphide’s presence on a list of substances being considered for inclusion in RoHS.
Meanwhile, Coe-Sullivan notes, QD Vision has moved away from the metal-alkyl precursors and phosphorus-containing solvents that can make quantum dot manufacturing hazardous. It now uses metal-carboxylate precursors and more benign alkane solvents. Last month, the shift won it one of the Environmental Protection Agency’s Presidential Green Chemistry Challenge Awards.
Chung, the DisplaySearch analyst, is watching the jousting between the cadmium and cadmium-free camps with interest, although he isn’t ready to predict a winner yet. Display makers are concerned about cadmium, he notes, yet they also have qualms about the lower efficiency of cadmium-free quantum dots.
Chung may not know which technology will prevail, but he is sure about one thing. “Now is the time for quantum dots to penetrate the market,” he says.
- Chemical & Engineering News
- ISSN 0009-2347
- Copyright © 2015 American Chemical Society
29 Jan 2015
Researchers report a high-resolution method for printing quantum dots to make light-emitting diodes (Nano Lett. 2015, DOI: 10.1021/nl503779e). With further development, the technique could be used to print pixels for richly colored, low-power displays in cell phones and other electronic devices.
Quantum dots are appealing materials for displays because engineers can finely tune the light the semiconducting nanocrystals emit by controlling their dimensions.
Electronics makers already use quantum dots in some backlit displays on the market, in which red and green quantum dots convert blue light from a light-emitting diode (LED) into white light. Quantum dots also emit light in response to voltage changes, so researchers are looking into using them in red, green, and blue pixels in displays that wouldn’t need a backlight.
Quantum dot LED displays should provide richer colors and use less power than the liquid-crystal displays (LCDs) used in many flat screens, which require filters and polarizers that reduce efficiency and limit color quality. But it’s not yet clear how quantum dot LED displays would be made commercially, says John A. Rogers, a materials scientist at the University of Illinois, Urbana-Champaign.
In 2011, researchers at Samsung made the first full-color quantum dot LED display by using a rubber stamp to pick up and transfer quantum dot inks (Nat. Photonics, DOI: 10.1038/nphoton.2011.12). As a manufacturing strategy, printing from ink nozzles would offer more flexibility to change designs on the fly, without the need for making new transfer stamps. Jet printing also would require less material, Rogers says.
Unfortunately, the resolution of conventional ink-jet printers, which use a heating element to force vapor droplets out of a nozzle, is limited. “It’s hard to get droplets smaller than about 25 µm,” Rogers says, because the smaller the nozzle diameter, the more pressure required to get the droplet out.
So for the past seven years, Rogers has been developing another method called electrohydrodynamic jet printing. This kind of printer works by pulling ink droplets out of the nozzle rather than pushing them, allowing for smaller droplets. An electric field at the nozzle opening causes ions to form on the meniscus of the ink droplet. The electric field pulls the ions forward, deforming the droplet into a conical shape. Then a tiny droplet shears off and lands on the printing surface. A computer program controls the printer by directing the movement of the substrate and varying the voltage at the nozzle to print a given pattern.
The Illinois researchers used this new method, including specialized quantum dot inks, to print lines on average about 500 nm wide. This allowed them to fabricate red and green quantum dot LEDs. They also showed they could carefully control the thickness of the printed film, which is difficult to do with stamp transfer and ink-jet printing methods.
The ultimate resolution possible with these kinds of printers is very high, says David J. Norris, a materials engineer at the Swiss Federal Institute of Technology (ETH), Zurich. Last year, Norris used a similar printing method to print spots containing as few as 10 quantum dots (Nano Lett. 2014, DOI: 10.1021/nl5026997). He says it’s even possible to place single quantum dots using electrohydrodynamic nozzles, albeit with less control and repeatability. Single-particle printing isn’t needed for making pixels for displays, but it is useful for studying other kinds of optical effects in quantum dots, he says.
- Chemical & Engineering News
- ISSN 0009-2347
- Copyright © 2015 American Chemical Society
An ultra-thin nanomaterial is at the heart of a major breakthrough by Univ. of Waterloo scientists who are in a global race to invent a cheaper, lighter and more powerful rechargeable battery for electric vehicles.
Chemistry Prof. Linda Nazar and her research team in the Faculty of Science at the Univ. of Waterloo have announced a breakthrough in lithium-sulphur battery technology in Nature Communications.
Their discovery of a material that maintains a rechargable sulphur cathode helps to overcome a primary hurdle to building a lithium-sulphur (Li-S) battery. Such a battery can theoretically power an electric car three times further than current lithium-ion batteries for the same weight—at much lower cost.
“This is a major step forward and brings the lithim-sulphur battery one step closer to reality,” said Nazar, who also holds the Canada Research Chair in Solid State Energy Materials and was named a Highly Cited Researcher by Thomson Reuters.
Nazar’s group is best known for their 2009 Nature Materials paper demonstrating the feasibility of a Li-S battery using nanomaterials. In theory, sulphur can provide a competitive cathode material to lithium cobalt oxide in current lithium-ion cells.
Sulphur as a battery material is extremely abundant, relatively light and very cheap. Unfortunately, the sulphur cathode exhausts itself after only a few cycles because the sulphur dissolves into the electrolyte solution as it’s reduced by incoming electrons to form polysulphides.
Nazar’s group originally thought that porous carbons or graphenes could stabilize the polysulphides by physically trapping them. But in an unexpected twist, they discovered metal oxides could be the key. Their initial work on a metallic titanium oxide was published earlier in August in Nature Communications.
While the researchers found since then that nanosheets of manganese dioxide (MnO2) work even better than titanium oxides, their main goal in this paper was to clarify the mechanism at work.
“You have to focus on the fundamental understanding of the phenomenon before you can develop new, advanced materials,” said Nazar.
They found that the oxygenated surface of the ultrathin MnO2 nanosheet chemically recycles the sulphides in a two-step process involving a surface-bound intermediate, polythiosulfate. The result is a high-performance cathode that can recharge more than 2000 cycles.
The surface reaction is similar to the chemical process behind Wackenroder’s Solution discovered in 1845 during a golden age of German sulfur chemistry.
“Very few researchers study or even teach sulphur chemistry anymore,” said Nazar. “It’s ironic we had to look so far back in the literature to understand something that may so radically change our future.”
Source: Univ. of Waterloo
22 Jan 2015
A new tool capable of carrying out simultaneous nano-sized measurements could soon lead to more innovative nanotech-based products and help boost the EU economy. Indeed the tool, developed by scientists cooperating through the EU-funded UNIVSEM project, has the potential to revolutionise research and development in a number of sectors, ranging from electronics and energy to biomedicine and consumer products.
Nanotechnology, which involves the manipulation of matter at the atomic and molecular scale, has led to new materials – such as graphene – and microscopic devices that include new surgical tools and medicines. Up until now however, nanotech R&D has been hampered by the fact that it has not been possible to achieve simultaneous information on 3D structure, chemical composition and surface properties.
This is what makes the UNIVSEM project, due for completion in March 2015, so innovative. By integrating different sensors capable of measuring these different aspects of nano-sized materials, EU scientists have created a single instrument that enables researchers to work much more efficiently. By providing clearer visual and other sensory information, the tool will help scientists to manipulate nano-sized particles with greater ease and help cut R&D costs for industry.
The project team began in April 2012 by developing a vacuum chamber capable of accommodating the complex sensory tools required. In parallel, they significantly improved the capabilities of each individual analytical technique. This means that users now need just one instrument to achieve key capabilities such as vision and chemical analysis.
Preliminary tests demonstrated that the achieved optical resolution of 360 nanometres (nm) far exceeds the original 500 nm target set out at the start of the project. This should be of significant interest to numerous sectors where cost-efficient but incredibly precise measurements are required, such as in the manufacture of nano-sized surgical tools and nano-medicines.
Electronics is another key area. For example, the UNIVSEM project could help scientists learn more about the properties of quasiparticles such as plasmons. Since plasmons can support much higher frequencies than today’s silicon based chips, researchers believe they could be the future for optical connections on next-generation computer chips.
Plasmon research could also lead to the development of new lasers and molecular-imaging systems, and increase solar cell efficiencies due to their interaction with light. Another exciting area of nanotechnology concerns silver nanowires (AgNWs). These nanowires can form a transparent conductive network, and thus are a promising candidate for solar cell contacts or transparent layers in displays.
The next stage is the commercialisation of the instrument. The multi-modal tool is expected to spur nanotechnology development and enhanced quality control in numerous areas – such as the development of third generation solar cells – and create new opportunities in sectors that have until now not fully tapped into the potential of nanotechnology.
Explore further: Using nanoparticles to better protect industrial applications
More information: For further information, please visit: www.univsem.eu/
22 Jan 2015
Super-hydrophobic materials are desirable for a number of applications such as rust prevention, anti-icing, or even in sanitation uses. However, as Rochester’s Chunlei Guo explains, most current hydrophobic materials rely on chemical coatings.
In a paper published today in the Journal of Applied Physics, Guo and his colleague at the University’s Institute of Optics, Anatoliy Vorobyev, describe a powerful and precise laser-patterning technique that creates an intricate pattern of micro- and nanoscale structures to give the metals their new properties. This work builds on earlier research by the team in which they used a similar laser-patterning technique that turned metals black. Guo states that using this technique they can create multifunctional surfaces that are not only super-hydrophobic but also highly-absorbent optically.
University of Rochester’s Institute of Optics Professor Chunlei Guo has developed a technique that uses lasers to render materials hydrophobic, illustrated in this image of a water droplet bouncing off a treated sample. Credit: J. Adam Fenster/University of Rochester
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Guo adds that one of the big advantages of his team’s process is that “the structures created by our laser on the metals are intrinsically part of the material surface.” That means they won’t rub off. And it is these patterns that make the metals repel water.
“The material is so strongly water-repellent, the water actually gets bounced off. Then it lands on the surface again, gets bounced off again, and then it will just roll off from the surface,” said Guo, professor of optics at the University of Rochester. That whole process takes less than a second.
A femtosecond laser created detailed hierarchical structures in the metals, as shown in this SEM image of the platinum surface. Credit: The Guo Lab/University of Rochester
The materials Guo has created are much more slippery than Teflon—a common hydrophobic material that often coats nonstick frying pans. Unlike Guo’s laser-treated metals, the Teflon kitchen tools are not super-hydrophobic. The difference is that to make water to roll-off a Teflon coated material, you need to tilt the surface to nearly a 70-degree angle before the water begins to slide off. You can make water roll off Guo’s metals by tilting them less than five degrees.
As the water bounces off the super-hydrophobic surfaces, it also collects dust particles and takes them along for the ride. To test this self-cleaning property, Guo and his team took ordinary dust from a vacuum cleaner and dumped it onto the treated surface. Roughly half of the dust particles were removed with just three drops of water. It took only a dozen drops to leave the surface spotless. Better yet, it remains completely dry.
Guo is excited by potential applications of super-hydrophobic materials in developing countries. It is this potential that has piqued the interest of the Bill and Melinda Gates Foundation, which has supported the work.
“In these regions, collecting rain water is vital and using super-hydrophobic materials could increase the efficiency without the need to use large funnels with high-pitched angles to prevent water from sticking to the surface,” says Guo. “A second application could be creating latrines that are cleaner and healthier to use.”
Latrines are a challenge to keep clean in places with little water. By incorporating super-hydrophobic materials, a latrine could remain clean without the need for water flushing.
But challenges still remain to be addressed before these applications can become a reality, Guo states. It currently takes an hour to pattern a 1 inch by 1 inch metal sample, and scaling up this process would be necessary before it can be deployed in developing countries. The researchers are also looking into ways of applying the technique to other, non-metal materials.
Guo and Vorobyev use extremely powerful, but ultra-short, laser pulses to change the surface of the metals. A femtosecond laser pulse lasts on the order of a quadrillionth of a second but reaches a peak power equivalent to that of the entire power grid of North America during its short burst.
Guo is keen to stress that this same technique can give rise to multifunctional metals. Metals are naturally excellent reflectors of light. That’s why they appear to have a shiny luster. Turning them black can therefore make them very efficient at absorbing light. The combination of light-absorbing properties with making metals water repellent could lead to more efficient solar absorbers – solar absorbers that don’t rust and do not need much cleaning.
Guo’s team had previously blasted materials with the lasers and turned them hydrophilic, meaning they attract water. In fact, the materials were so hydrophilic that putting them in contact with a drop of water made water run “uphill”.
Guo’s team is now planning on focusing on increasing the speed of patterning the surfaces with the laser, as well as studying how to expand this technique to other materials such as semiconductors or dielectrics, opening up the possibility of water repellent electronics.
22 Jan 2015
First report of Comprehensive Initiative on Technology Evaluation offers new framework for assessment.
It’s a challenge development agencies, nongovernmental organizations, and consumers themselves face every day: With so many products on the market, how do you choose the right one?
Now MIT researchers have released a report that could help answer that question through a new framework for technology evaluation. Their report — titled “Experimentation in Product Evaluation: The Case of Solar Lanterns in Uganda, Africa” — details the first experimental evaluations designed and implemented by the Comprehensive Initiative on Technology Evaluation (CITE), a U.S. Agency for International Development (USAID)-supported program led by a multidisciplinary team of faculty, staff, and students.
Building an evaluation framework
CITE’s framework is based on the idea that evaluating a product from a technical perspective alone is not enough, according to CITE Director Bishwapriya Sanyal, the Ford International Professor in MIT’s Department of Urban Studies and Planning.
“There are many products designed to improve the lives of poor people, but there are few in-depth evaluations of which ones work, and why,” Sanyal says. “CITE not only looks at suitability — how well does a product work? — but also at scalability — how well does it scale? — and sustainability — does a product have sticking power, given social, economic, and environmental context?”
CITE seeks to integrate each of these criteria — suitability, scalability, and sustainability — to develop a deep understanding of what makes products successful in emerging economies. The program’s evaluations and framework are intended to better inform the development community’s purchasing decisions.
“CITE’s work is incredibly energizing for the development community,” said Ticora V. Jones, director of the USAID Higher Education Solutions Network. “These evaluations won’t live on a shelf. The results are actionable. It’s an approach that could fundamentally transform the way we choose, source, and even design technologies for development work.”
Evaluating solar lanterns in Uganda
In summer 2013, a team of MIT faculty and students set off for western Uganda to conduct CITE’s evaluation of solar lanterns. Researchers conducted hundreds of surveys with consumers, suppliers, manufacturers, and nonprofits to evaluate 11 locally available solar lantern models.
To assess each product’s suitability, researchers computed a ratings score from 0 to 100 based on how the product’s attributes and features fared. “Attributes” included characteristics inherent to solar lanterns, such as brightness, run time, and time to charge. “Features” included less-central characteristics, such as a lantern’s ability to charge a cellphone.
The importance of cellphone charging was a surprising and noteworthy finding, Sanyal says.
“One of the things that stuck with me was that [consumers] were most concerned with whether or not the solar lantern charged their cellphone. It was a feature we never expected would be so important,” Sanyal says. “For some, having connections may be more valuable than having light.”
Learning from partnerships
CITE worked with USAID to select solar lanterns as the product family for its first evaluation. Sanyal says evaluating solar lanterns allowed CITE to learn from USAID’s existing partnership with Solar Sister, a social enterprise that distributes solar lanterns in Uganda, a country where few people have access to light after dark.
CITE researchers also worked closely with Jeffrey Asher, a former technical director at Consumer Reports, to learn from an existing product-evaluation model.
Evaluating products in a laboratory at MIT or Consumer Reports is much different than evaluating them in rural Uganda, but both are important, says Asher, who is a co-author of the CITE report.
“Consumer Reports’ greatest challenge has been evaluating products that are currently in the U.S. marketplace,” Asher says. “CITE has found that, in developing countries, we have to be even more nimble to keep up with an ever-changing market.”
Putting CITE’s results to work
Over the next two years, CITE will hone its approach, using experimental evaluations of technologies like water filters, post-harvest storage solutions, and malaria rapid-diagnostic tests to design a replicable approach that development professionals can use in their day-to-day work, Sanyal says.
“We’re aiming to make our evaluation process leaner, less expensive, and more nimble, while maintaining rigor. That’s our challenge, looking forward,” Sanyal says.
David Nicholson, director of the environment, energy, and climate change technical support unit at the international development organization Mercy Corps, says evaluation tools like CITE’s can be invaluable in making procurement decisions, especially when organizations are working with finite resources.
“Development agencies like Mercy Corps are increasingly looking to the commercial sector for solutions to long-term development challenges,” says Nicholson, who did not participate in the CITE research. “Evaluations like this can help program managers make informed decisions on which commercial products are most suitable for the program goals and the target communities.”
CITE’s research is funded by the USAID U.S. Global Development Lab. CITE is led by MIT’s Department of Urban Studies and Planning and supported by MIT’s D-Lab, Public Service Center, Sociotechnical Systems Research Center, and Center for Transportation and Logistics.
In addition to Sanyal and Asher, co-authors on the CITE report include Daniel Frey, Derek Brine, Jennifer Green, Jonars Spielberg, Stephen Graves, and Olivier de Weck.
See Also: “MIT a Linchpin of Major New USAID Program”
Institute researchers aim to spur development and evaluation of useful technologies to help the world’s poor.
22 Jan 2015
Princeton University researchers have built a rice grain-sized laser powered by single electrons tunneling through artificial atoms known as quantum dots. The tiny microwave laser, or “maser,” is a demonstration of the fundamental interactions between light and moving electrons.
The researchers built the device — which uses about one-billionth the electric current needed to power a hair dryer — while exploring how to use quantum dots, which are bits of semiconductor material that act like single atoms, as components for quantum computers.
“It is basically as small as you can go with these single-electron devices,” said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.
Princeton University researchers have built a rice grain-sized microwave laser, or “maser,” powered by single electrons that demonstrates the fundamental interactions between light and moving electrons, and is a major step toward building quantum-computing systems out of semiconductor materials. A battery forces electrons to tunnel one by one through two double quantum dots located at each end of a cavity (above), moving from a higher energy level to a lower energy level and in the process giving off microwaves that build into a coherent beam of light. (Photo courtesy of Jason Petta, Department of Physics)
The device demonstrates a major step forward for efforts to build quantum-computing systems out of semiconductor materials, according to co-author and collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology. “I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices,” Taylor said.
The original aim of the project was not to build a maser, but to explore how to use double quantum dots — which are two quantum dots joined together — as quantum bits, or qubits, the basic units of information in quantum computers.
Yinyu Liu, first author of the study and a graduate student in Princeton’s Department of Physics, holds a prototype of the device. (Photo by Catherine Zandonella, Office of the Dean for Research)
“The goal was to get the double quantum dots to communicate with each other,” said Yinyu Liu, a physics graduate student in Petta’s lab. The team also included graduate student Jiri Stehlik and associate research scholar Christopher Eichler in Princeton’s Department of Physics, as well as postdoctoral researcher Michael Gullans of the Joint Quantum Institute.
Because quantum dots can communicate through the entanglement of light particles, or photons, the researchers designed dots that emit photons when single electrons leap from a higher energy level to a lower energy level to cross the double dot.
Each double quantum dot can only transfer one electron at a time, Petta explained. “It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person,” he said. “They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned — they are trapped in all three spatial dimensions.”
When the power (P) is turned on, single electrons (small arrows) begin to flow through the two double quantum dots (Left DQD and Right DQD) from the drain (D) to the source (S). As the electrons move from the higher energy level to the lower energy level, they give off particles of light in the microwave region of the spectrum. These microwaves bounce off mirrors on either side of the cavity (k-in and k-out) to produce the maser’s beam. (Photo courtesy of Science/AAAS)
The researchers fabricated the double quantum dots from extremely thin nanowires (about 50 nanometers, or a billionth of a meter, in diameter) made of a semiconductor material called indium arsenide. They patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, which control the energy levels in the dots.
To construct the maser, they placed the two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero, around minus 459 degrees Fahrenheit. “This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance,” Taylor said.
When the device was switched on, electrons flowed single-file through each double quantum dot, causing them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity to build into a coherent beam of microwave light.
One advantage of the new maser is that the energy levels inside the dots can be fine-tuned to produce light at other frequencies, which cannot be done with other semiconductor lasers in which the frequency is fixed during manufacturing, Petta said. The larger the energy difference between the two levels, the higher the frequency of light emitted.
A double quantum dot as imaged by a scanning electron microscope. Current flows one electron at a time through two quantum dots (red circles) that are formed in an indium arsenide nanowire. (Photo courtesy of Science/AAAS)
Claire Gmachl, who was not involved in the research and is Princeton’s Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers, said that because lasers, masers and other forms of coherent light sources are used in communications, sensing, medicine and many other aspects of modern life, the study is an important one.
“In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron,” Gmachl said. “The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources.”
The paper, “Semiconductor double quantum dot micromaser,” was published in the journal Science on Jan. 16, 2015. The research was supported by the David and Lucile Packard Foundation, the National Science Foundation (DMR-1409556 and DMR-1420541), the Defense Advanced Research Projects Agency QuEST (HR0011-09-1-0007), and the Army Research Office (W911NF-08-1-0189).
22 Jan 2015
Reducing the amount of sunlight that bounces off the surface of solar cells helps maximize the conversion of the sun’s rays to electricity, so manufacturers use coatings to cut down on reflections. Now scientists at the U.S. Department of Energy’s Brookhaven National Laboratory show that etching a nanoscale texture onto the silicon material itself creates an antireflective surface that works as well as state-of-the-art thin-film multilayer coatings.
Their method, described in the journal Nature Communications and submitted for patent protection, has potential for streamlining silicon solar cell production and reducing manufacturing costs. The approach may find additional applications in reducing glare from windows, providing radar camouflage for military equipment, and increasing the brightness of light-emitting diodes.
“For antireflection applications, the idea is to prevent light or radio waves from bouncing at interfaces between materials,” said physicist Charles Black, who led the research at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility.
Preventing reflections requires controlling an abrupt change in “refractive index,” a property that affects how waves such as light propagate through a material. This occurs at the interface where two materials with very different refractive indices meet, for example at the interface between air and silicon. Adding a coating with an intermediate refractive index at the interface eases the transition between materials and reduces the reflection, Black explained.
“The issue with using such coatings for solar cells,” he said, “is that we’d prefer to fully capture every color of the light spectrum within the device, and we’d like to capture the light irrespective of the direction it comes from. But each color of light couples best with a different antireflection coating, and each coating is optimized for light coming from a particular direction. So you deal with these issues by using multiple antireflection layers. We were interested in looking for a better way.”
For inspiration, the scientists turned to a well-known example of an antireflective surface in nature, the eyes of common moths. The surfaces of their compound eyes have textured patterns made of many tiny “posts,” each smaller than the wavelengths of light. This textured surface improves moths’ nighttime vision, and also prevents the “deer in the headlights” reflecting glow that might allow predators to detect them.
“We set out to recreate moth eye patterns in silicon at even smaller sizes using methods of nanotechnology,” said Atikur Rahman, a postdoctoral fellow working with Black at the CFN and first author of the study.
The scientists started by coating the top surface of a silicon solar cell with a polymer material called a “block copolymer,” which can be made to self-organize into an ordered surface pattern with dimensions measuring only tens of nanometers. The self-assembled pattern served as a template for forming posts in the solar cell like those in the moth eye using a plasma of reactive gases-a technique commonly used in the manufacture of semiconductor electronic circuits.
The resulting surface nanotexture served to gradually change the refractive index to drastically cut down on reflection of many wavelengths of light simultaneously, regardless of the direction of light impinging on the solar cell.
“Adding these nanotextures turned the normally shiny silicon surface absolutely black,” Rahman said.
Solar cells textured in this way outperform those coated with a single antireflective film by about 20 percent, and bring light into the device as well as the best multi-layer-coatings used in the industry.
“We are working to understand whether there are economic advantages to assembling silicon solar cells using our method, compared to other, established processes in the industry,” Black said.
Hidden layer explains better-than-expected performance
One intriguing aspect of the study was that the scientists achieved the antireflective performance by creating nanoposts only half as tall as the required height predicted by a mathematical model describing the effect. So they called upon the expertise of colleagues at the CFN and other Brookhaven scientists to help sort out the mystery.
“This is a powerful advantage of doing research at the CFN-both for us and for academic and industrial researchers coming to use our facilities,” Black said. “We have all these experts around who can help you solve your problems.”
Using a combination of computational modeling, electron microscopy, and surface science, the team deduced that a thin layer of silicon oxide similar to what typically forms when silicon is exposed to air seemed to be having an outsized effect.
“On a flat surface, this layer is so thin that its effect is minimal,” explained Matt Eisaman of Brookhaven’s Sustainable Energy Technologies Department and a professor at Stony Brook University. “But on the nanopatterned surface, with the thin oxide layer surrounding all sides of the nanotexture, the oxide can have a larger effect because it makes up a significant portion of the nanotextured material.”
Said Black, “This ‘hidden’ layer was the key to the extra boost in performance.”
The scientists are now interested in developing their self-assembly based method of nanotexture patterning for other materials, including glass and plastic, for antiglare windows and coatings for solar panels.
This research was supported by the DOE Office of Science.
Scientists at the US Department of Energy’s Oak Ridge National Laboratory are learning how the properties of water molecules on the surface of metal oxides can be used to better control these minerals and use them to make products such as more efficient semiconductors for organic light emitting diodes and solar cells, safer vehicle glass in fog and frost, and more environmentally friendly chemical sensors for industrial applications.
The behavior of water at the surface of a mineral is determined largely by the ordered array of atoms in that area, called the interfacial region. However, when the particles of the mineral or of any crystalline solid are nanometer-sized, interfacial water can alter the crystalline structure of the particles, control interactions between particles that cause them to aggregate, or strongly encapsulate the particles, which allows them to persist for long periods in the environment. As water is an abundant component of our atmosphere, it is usually present on nanoparticle surfaces exposed to air.
A great scientific challenge is to develop ways to look closely at the interfacial region and understand how it determines the properties of nanoparticles. The ORNL researchers are taking advantage of two of the lab’s signature strengths—neutron and computational sciences—to reveal the influence of just a few monolayers of water on the behavior of materials.
In a set of papers published in the Journal of the American Chemical Society and the Journal of Physical Chemistry C, the team of researchers studied cassiterite (SnO2, a tin oxide), representative of a large class of isostructural oxides, including rutile (TiO2). These minerals are common in nature, and water wets their surfaces. The behavior of water confined on the surface of metal oxides readily relates to applications in such diverse areas as heterogeneous catalysis, protein folding, environmental remediation, mineral growth and dissolution, and light-energy conversion in solar cells, to name just a few.
When metal oxide nanoparticles are produced, they spontaneously adsorb water from the atmosphere, bonding it to their surface, explained Hsiu-Wen Wang, a research scientist currently at the ORNL–University of Tennessee Joint Institute for Neutron Sciences who performed this research while conducting a postdoctoral fellowship in the Chemical Sciences Division (CSD) at ORNL.
This water can interfere with the function of SnO2-containing products in surprising ways that are hard to predict. Wang’s team used neutron scattering at ORNL’s Spallation Neutron Source (SNS) to help understand the role that bound water plays in the stability of SnO2 nanoparticles and to learn more about the bound water’s structure and dynamics. Wang said neutrons are perfect for studying light elements such as the hydrogen and oxygen that make up water, and molecular dynamics simulations are an ideal tool to reinforce the observations. In fact, hydrogen is essentially invisible to X-ray and electron beams but scatters neutrons strongly, making neutron diffraction and inelastic scattering the ideal tools for probing the properties of water and other hydrogen-bearing species.
“When we drive all the water off the surface of the nanoparticles, this destabilizes the structure of the nanoparticles, and they grow larger,” said David J. Wesolowski, a co-author and Wang’s supervisor when she worked in CSD.
“The lifetime of engineered nanoparticles in the environment is an important environmental safety and health issue,” Wesolowski said. “We show that water sorbed on the nanoparticles, which naturally happens when they are exposed to normal humid air, prolongs their lifetimes as nanomaterials, thus prolonging their potential environmental impacts. In addition, the high surface area of nanoparticles is desirable. If the particles grow, which happens as they are heated and dehumidified, their surface area drops rapidly.”
To remove sorbed water, the nanoparticles are heated under vacuum. Water dissipation begins at around 250°C (nearly 500°F, or about as hot as you can set your kitchen’s oven). Much energy is required to drive off the water completely from the nanoparticles, which stay stable to these relatively high temperatures precisely because of the presence of the bound water. Once the water begins to dissipate, destabilization begins. Before completing this study, researchers did not know to what degree the removal of water would cause destabilization.
“It may be that the surfaces without water have different and useful chemical properties, but because water is everywhere in the environment, it is very important to know that the surfaces of oxide nanoparticles are likely to be already covered with a few molecular layers of water,” Wesolowski said.
Researchers used SNS’s Nanoscale-Ordered Materials Diffractometer (NOMAD) instrument to determine the structure of water on cassiterite nanoparticle surfaces, as well as the structure of the particles themselves. NOMAD is dedicated to local structure studies of various materials from liquids to nanoparticles, using the neutron scattering pattern produced during experiments, said Mikhail Feygenson, NOMAD instrument scientist.
“The combination of the high neutron flux of SNS and the wide detector coverage of NOMAD enables rapid data collection on very small samples, like our nanoparticles,” Feygenson said. “NOMAD is much faster than similar instruments around the world. In fact, the measurements of our samples that took about 24 hours of NOMAD time could have required as much as a full week on a similar instrument at another lab.”
The second step of the study took place at SNS on the Fine-Resolution Fermi Chopper Spectrometer (SEQUOIA), which allows for forefront research on dynamical processes in materials. “This part of the study focuses on the role of surface hydrogen bonds and the surface water vibrational properties,” said Alexander Kolesnikov, SEQUOIA instrument scientist.
The NOMAD and SEQUOIA studies enabled the research team to validate computational models they created to fully capture the structural ordering of the surface-bound water on the SnO2 nanocrystals. Integrating neutron scattering experiments with classical and first principles molecular dynamics simulations provided evidence that strong hydrogen bonds—as strong as in water under ultrahigh pressure of >500,000 atm—drive water molecules to dissociate at the interfaces and result in a weak interaction of the hydrated SnO2 surface with additional water layers.
“The results are significant in demonstrating many new features of surface-confined water that can provide general guidance into tuning of surface hydrophilic interactions at the molecular level,” said Jorge Sofo, professor of physics at Pennsylvania State University.
More information: H.-W. Wang, M. DelloStritto, N. Kumar, A. I. Kolesnikov, P. R. C. Kent, J. D. Kubicki, D. J. Wesolowski, and J. O. Sofo, “Vibrational density of states of strongly H-bonded interfacial water: Insights from inelastic neutron scattering and theory.” The Journal of Physical Chemistry C, 118, 10805–1083 (2014); DOI: dx.doi.org/10.1021/jp500954v
Quantum dots glow a specific color when they are hit with any kind of light. Here, a vial of green quantum dots are activated by a blue LED backlight system.
If you look at the CES 2015 word cloud—a neon blob of buzz radiating from the Nevada desert, visible from space—much of it is a retweet of last year’s list. Wearables. 4K. The Internet of Things, still unbowed by its stupid name. Connected cars. HDR. Curved everything. It’s the same-old, same-old, huddled together for their annual #usie at the butt-end of a selfie stick.
But there at the margin, ready to photobomb the shot, is the new kid: quantum dot. It goes by other names, too, which is confusing, and we’ll get to that in a minute. Regardless of what you call it, QD was all over CES this year, rubbing shoulders with the 4K crowd. You may have heard people say it’s all hype. Those people can go pound sand. Quantum dot is gonna be the next big thing in TVs, bringing better image quality to cheaper sets.
A Quantum-Dot TV Is an LCD TV
The first thing to know is quantum-dot televisions are a new type of LED-backlit LCD TV. The image is created just like it is on an LCD screen, but quantum-dot technology enhances the color.
On an LCD TV, you have a backlight system, which is a bank of LEDs mounted at the edge of the screen or immediately behind it. That light is diffused, directed by a light-guide plate and beamed through a polarized filter. The photons then hit a layer of liquid crystals that either block the light or allow it to pass through a second polarized filter.
Before it gets to that second polarizer, light passes through a layer of red, blue, and green (and sometimes yellow) color filters. These are the subpixels. Electrical charges applied to the subpixels moderate the blend of colored light visible on the other side. This light cocktail creates the color value of each pixel on the screen.
With a quantum-dot set, there are no major changes to that process. The same pros and cons cited for LCD TVs also apply. You can have full-array backlit quantum-dot sets with local-dimming technology (Translation: good for image uniformity and deeper blacks). There can be edge-lit quantum-dot sets with no local dimming (Translation: thinner, but you may see light banding and grayer blacks). You can have 1080p quantum-dot sets, but you’re more likely to see only 4K quantum-dot sets because of the industry’s big push toward UltraHD/4K resolution.
But a Quantum-Dot TV Is Different
In a quantum-dot set, the changes start with the color of the backlight. The LEDs in most LCD TVs emit white light, but those in quantum-dot televisions emit blue light. Both types actually use blue LEDs, but they’re coated with yellow phosphor in normal LCD televisions and therefore emit white light.
Here’s where the quantum dots come in. The blue LED light drives the blue hues of the picture, but red and green light is created by the quantum dots. The quantum dots are either arranged in a tube—a “quantum rail”—adjacent to the LEDs or in a sheet of film atop the light-guide plate.
Quantum dots have one job, and that is to emit one color. They excel at this. When a quantum dot is struck by light, it glows with a very specific color that can be finely tuned. When those blue LEDs shine on the quantum dots, the dots glow with the intensity of angry fireflies.
“Blue is an important part of the spectrum, and it’s the highest-energy portion—greater than red or green,” explains John Volkmann, chief marketing officer at QD Vision, which makes quantum dots for several TVs and monitors. “You start with high energy light and refract it to a lower energy state to create red or green… Starting with red or green would be pushing a rock uphill.”
Quantum dots are tiny, and their size determines their color. There are two sizes of dots in these TVs. The “big” ones glow red, and they have a diameter of about 50 atoms. The smaller ones, which glow green, have a diameter of about 30 atoms. There are billions of them in a quantum-dot TV.
If you observed quantum-dot light with a spectrometer, you would see a very sharp and narrow emission peak. Translation: Pure red and pure green light, which travels with the blue light through the polarizers, liquid crystals, and color filters.
Because that colored light is the good stuff, quantum dots have an advantage over traditional LCD TVs when it comes to vivid hues and color gamut. In a normal LCD, white light produced by the LEDs has a wider spectrum. It’s kind of dirty, with a lot of light falling in a color range unusable by the set’s color filters.
“A filter is a very lossy thing,” says Nanosys President and CEO Jason Hartlove. Nanosys makes film-based quantum-dot systems for several products. “When you purify the color using a color filter, then you will get practically no transmission through the filter. The purer the color you start with, the more relaxed the filter function can be. That translates directly to efficiency.”
So with a quantum-dot set, there is very little wasted light. You can get brighter, more-saturated, and more-accurate colors. The sets I saw in person at CES 2015 certainly looked punchier than your average LCD.
That Sounds Expensive
There’s no doubt that quantum-dot TVs will cost more than normal LCDs—especially because they’re likely to be 4K sets. But quantum-dot is getting a lot of buzz because its cheaper than OLED.
In most peoples’ eyes, OLED TVs are the best tech available. But they’re expensive to build and expensive to buy—you’re looking at $3,500 to as much as $20,000—and the manufacturing process differs in several key ways. That’s a big reason LG is the only company putting big money into building them.
Conversely, quantum-dot sets don’t require overhauling the LCD fabrication process, and they produce a much wider color gamut than traditional LCDs. They’re closer to OLED in color performance, and they also can get brighter. That’s important for HDR video.
“The attraction to the OEM is that this is a pure drop-in solution,” says Nanoco CEO Michael Edelman, whose company makes quantum-dot film in a licensing deal with Dow Chemical. “They remove a diffuser sheet in front of the light-guide plate and replace it with quantum-dot film. Nothing in the supply chain gets changed, nothing in the factory gets changed. They get, in some cases, better than OLED-type color at a fraction of the cost.”
As you’d expect, companies making film-based and tube-based solutions are touting each approach as superior. QD Vision claims its tube-based approach is easier and cheaper to implement, and it can boost the color performance of cheaper edge-lit LCD sets. According to QD Vision, the oxygen-barrier film needed for film-based dots is costly, which explains why Nanoco and Nanosys are partnering with Dow and 3M for that film.
Film-based suppliers say their method has the upper hand due to “light coupling,” or the ability to feed all that quantum-dot light directly into a light-guide plate. The film layer also purportedly works better with full-array backlight systems, which will be used in a lot of UHD and HDR TVs.
Super! So This Is OLED for Less Money?
Not entirely. Color gamut is important, but it’s only one aspect of picture quality. Because these are LCD sets, they won’t have the blackest blacks, super-wide viewing angles, and amazing contrast of OLED. And while the extra brightness and saturation makes onscreen colors really pop, all that luminance may create light bleeding.
Some quantum dots also contain cadmium, which is toxic at high levels—think “factory emission” levels rather than “sealed tube or film in your TV” levels. Still, there are health and environmental concerns, especially if a bunch of quantum-dot TVs end up in landfills. The European Union restricts the use of cadmium in household appliances. Some quantum-dot producers are marketing their product as cadmium-free. QD Vision, which supplies quantum dots for TCL’s new flagship 4K TV, Sony’s well-reviewed 2013 Triluminos sets, and Philips and AOC monitors, still uses cadmium.
“There are only a couple of materials that deliver on the promise of quantum dots,” says QD Vision’s Volkmann. “The other is based on indium. Cadmium is superior with respect to delivering higher-quality color, meaning a broader color gamut. But also much more energy-efficient at converting blue light to other forms of light that allow you to fill out that spectrum. The folks making indium-based solutions like to paint cadmium as the bad guy… Cadmium is under observation by different regulatory agencies around the world, but it turns out indium is too.”
Nanosys, which produces both cadmium and cadmium-free quantum dots, agrees that cadmium-based dots are more efficient.
“Cadmium-based materials have a narrower spectral width,” says Nanosys’s Hartlove. “More pure color. And what that means is the other things the system has to do in order to keep that color pure, the burden on the rest of the system is reduced.”
Hartlove also says that cadmium may be a greener solution. The cad selenide crystal used in quantum dots isn’t as toxic as pure metallic cadmium, and the efficiency of their color-producing ways has benefits.
“The type of power we generate in the US from coal-based power plants throws cadmium into the atmosphere,” says Hartlove. “That’s one of the byproducts of burning coal. And you look at the net cadmium content over this whole lifecycle, and it turns out that cadmium sequestration is actually net better for the environment.”
Why Isn’t Everybody Calling It “Quantum Dot”?
Each manufacturer with a quantum-dot TV set seemingly has a different name for the technology. Samsung likes “nano-crystal semiconductors.” Sony has new Triluminos TVs that “incorporate the same benefits as quantum dots.” LG, TCL, Hisense, and Changhong are actually calling it quantum dot, which is nice.
“The term quantum dot is generic,” says Hartlove. “Each company kind of wants to grab this for their own and brand it their own way. That will probably lead to some consumer confusion… but I think most of the industry will converge on a way to describe this technology.”
There are slight differences between the technologies everyone’s using, but they’re variations on a theme. The differences center on whether the TVs are edge-lit or back-lit with quantum dots, and whether the systems use cadmium- or indium-based quantum dots.
Who Is Making Quantum Dots?
At this stage, three companies are the big players in the quantum-dot TV landscape.
QD Vision specializes in glass-tube “edge-lit” components, and its systems will be found in TCL TVs and monitors from Philips and AOC. It supplied the quantum-dot component for Sony’s 2013 Triluminos sets, but Sony recently ditched the company in favor of another.
Nanoco focuses on cadmium-free, film-based quantum dot systems. They have a licensing deal with Dow Chemical, and Dow is currently building a factory in South Korea to ramp up production of quantum-dot film. Nanoco’s cadmium-free technology will be found in LG’s quantum-dot TVs in 2015.
Nanosys is another film-based producer that has partnered with 3M on the film-sheet tech. It makes both cadmium-based and cadmium-free quantum dots. They are the company behind Amazon’s HDX 7 display and the Asus Zenbook NX500, and Samsung licenses the cadmium-free quantum-dot tech in its new SUHD 4K sets from Nanosys. Nanosys is also working with Panasonic, Hisense, TCL, Changhong, and Skyworth on future TVs.
When Can I Get One, and What Will It Cost?
The new TVs showcased at CES each year usually start hitting stores in the spring, but some higher-end models don’t arrive until the fall. That’s a little bit of a wait, but it’s probably for the best—there are UltraHD content-delivery complications to work out, anyway.
The TV we know the most about in terms of pricing is TCL’s 55-inch H9700, and we still don’t know much. It’s already available in China for around $2,000 U.S., and TCL representatives at CES hinted that it will be close to that mark when it hits the U.S.
Expect that to be at the low end of the quantum-dot price bracket; LG, Samsung, and Sony generally have pricy TVs, and similar 4K LCDs from last year—minus the quantum dots—went in the $2,000 to $3,000 range for a 55-incher. For this initial wave of quantum-dot TVs, most MSRPs will probably fall between $2,500 to $4,000 for a 55-inch 4K set.