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High-Resolution Printing of Quantum Dots For Vibrant, Inexpensive Displays

Using a technique much like inkjet printing, engineers have created high-resolution patterns of quantum dots. Quantum dots (QDs) are light-emitting semiconductor nanocrystals that, used in light-emitting diodes (LEDs), hold the promise of brighter, faster displays. But there is no reliable and efficient way to pattern them at a high resolution to create multicolor pixels for displays.

John Rogers, a materials science and engineering professor at the University of Illinois in Urbana-Champaign, and his colleagues are repurposing a printing method they devised for other applications. When used with “QD ink,” it can create lines and spots that are just 0.25 micrometers wide. They made arrays and complex patterns of QDs in multiple colors, and could even print QDs on top of others of a different color. They sandwiched these patterns between electrodes to make bright QD LEDs. Details about the results were published in the journal Nano Letters.

Quantum dot TVs were big at the 2015 Consumer Electronics Show (CES) in Las Vegas. Companies such as Sony, Samsung and LG all have their own version. But the TVs demoed at CES use QDs along with blue inorganic LEDs to create a white backlight. The white light is beamed through color filters at each pixel to generate any color. The quantum dots are simply filled in a tube or painted on the entire backpanel; they don’t need to be patterned.

But companies such as QD Vision and Samsung seek a true QD display, wherein red, green and blue QD LEDs glow at each pixel. QD LED displays would be rich and vibrant thanks to the pure, bright glow of QDs. They would also be efficient and affordable because QDs require very little energy to light up, and they can easily be made into inks and printed.

Seems simple, doesn’t it? It was—except for the fact that no one had been able to print with these inks. The best that researchers had been able to do, Rogers told IEEE Spectrum, was print QDs on a surface using a patterned rubber stamp. But that wouldn’t cut it for mass production.

So Rogers and his team turned to a technique called electrohydrodynamic jet, or e-jet printing, which they developed and used to print patterns of DNA and carbon nanotubes.

The print tool in this case is a metal-coated glass capillary tube with a 5-micrometer nozzle at the end. A pneumatic system pushes the QD ink in the tube just enough that it bulges out of the tip. A voltage bias applied between the capillary and a metallic surface forces free ions in the ink to accumulate in that bulging ink drop.

“As those deposit, they deform the shape of the meniscus from circular to conical,” Rogers explains. “That terminates in the formation of droplets at the tip of the cone.”

The droplets, which are printed on the surface, are much smaller than the nozzle tip because they’re generated from the fluid cone rather than the nozzle itself.

Compared to stamping, he says, “We can do more diverse geometries more easily. There’s an inherent versatility in patterning. And we also eliminate any possible chemical contamination due to contact with the stamp surface.”

The researchers can print QDs on pretty much any surface as long as they lay it on a metal backplane. That means they could print on flexible surfaces, which is an exciting prospect for displays as well as lighting.

The ability to print QDs on top of other QDs, meanwhile, offers a new way to do color mixing. “In a display having separate RGB pixels at each location is one way,” Rogers says. “But you can also imagine mixing red and green by printing on top of one another. You could generate LEDs with any emission color at all by mixing and matching multiple quantum dots.”

Rogers says his team is now working on creating arrays of multiple nozzles, which would be important for manufacturing. Inkjet printers usually have a few hundred nozzles. The difficulty with the e-jet printing method is that the electric field at one nozzle affects the fields of neighboring nozzles. So the engineers are trying to figure out how to isolate nozzles in order to eliminate that crosstalk.

Privately-funded Solar Sail Prepared For Launch

In a parallel universe, a NASA-sponsored experimental solar sail—the largest ever built—would be launching into space next month along with the solar wind-monitoring DSCOVR spacecraft. That won’t happen in this universe: NASA cancelled the Sunjammer launch last year due to scheduling concerns, and it’s not clear when, or even if, the sail will get another opportunity to fly.

But there is a bright spot on the near horizon for solar sail tests. The space advocacy organization The Planetary Society today announced that a date has been set for the first flight of its LightSail spacecraft. The 30-centimeter-long CubeSatan increasingly popular class of small spacecraftis set to take off from Cape Canaveral Air Force Station in May as a secondary payload on an Atlas V rocket.

Solar sails are propelled by the momentum that photons impart when they bounce off a reflective surface. This method of propulsion can provide slow but steady acceleration to high speeds, which could cut down on the cost and mass of interplanetary missions. With the assistance of a powerful laser beam, the same technique might be used to propel spacecraft to interstellar mission speeds.

Solar sails “will ultimately eventually take a lot of missions a long, long way,” Planetary Society CEO Bill Nye told the New York Times. He said that this spacecraft—and a second one set to launch next year—together cost less than US $4 million to build. 

This first LightSail, integrated and tested by the Pasadena, Calif.–based Ecliptic Enterprises, will spend a month or so in orbit before it unfurls Mylar sheets along 4-meter-long booms to create a 32-square-meter sail. The first flight will be a shakedown mission to test sail deployment and other aspects of the spacecraft. But the sailing will have to wait; the spacecraft will be carried to a fairly low orbit where it will experience significant drag from the atmosphere. The second spacecraft, slated to launch in 2016 on a SpaceX Falcon Heavy rocket, will reach a higher orbit where solar pressure can be used to maneuver.

Other spacecraft have used radiation pressure to adjust their orbits, and solar sails have already had a few successful missions. In 2010, the Japan’s space agency JAXA launched a solar sail called IKAROS (short for Interplanetary Kite-craft Accelerated by Radiation Of the Sun) on a mission past Venus. And in 2011, NASA successfully unfurled NanoSail-D, also a compact CubeSat, in low-Earth orbit.

The new Planetary Society mission comes some 10 years after its first solar sail spacecraft, Cosmos 1, fell to Earth after a rocket failure. 

The Planetary Society's Jason Davis writes that the team has been sharing data with NASA teams that are working on additional solar-sail-bearing CubeSat missions. The space agency aims to launch at least two such missions (not all payloads have been selected yet) on the maiden voyage of the new heavy-lift Space Launch System, which could happen as early as 2018. One of the spacecraft, Lunar Flashlight, will use its solar sail to maneuver and to reflect light onto the moon's south pole.

Entanglement On a Chip

Quantum entanglement is a phenomenon where particles act in sync, even if they are at separate ends of the universe. Links of this type are so delicate that, if anyone tried to eavesdrop on a message sent using a stream of entangled particles, the disturbance would immediately be obvious. This has led to extraordinarily secure quantum cryptography that can instantly detect any spying.

But if this phenomenon is to be used to keep cellphone conversations private or make sure that no one can sniff out your banking password, scientists have to first generate entangled photons using electronics that can fit onto a microchip. However, until now, entangled photon emitters could only be scaled down to millimeters in size, too large for on-chip applications by several orders of magnitude. In addition, such emitters required much more power than is practical for putting them on, say, a cellular handset.

Now researchers at the University of Pavia in Italy say they have developed a device that can generate a continuous supply of entangled photons and is small enough to fit on a microchip. The scientists detailed their findings on 26 January in the online edition of the journal Optica.

The key component of the device is a "micro-ring resonator," a 20-micrometer-diameter ring etched into a silicon wafer. The ring is 500 nanometers wide and 220 nanometers high. When a laser beam is directed along an optical fiber and into the device, the photons race around the ring and can become entangled. “The key to this result is the ability to confine light and matter in the same microscopic place for as long as possible to force their interaction,” Daniele Bajoni, a physicist at the University of Pavia in Italy who is a member of the research team, told IEEE Spectrum.

The device can generate 10 million entangled pairs of photons per second, and requires less than a milliwatt of power—thousands of times less than was needed by previous entangled photon emitters. The researchers employ lasers with a wavelength of 1,550 nanometers, which is often used in telecommunications. They suggest their device could be readily incorporated into existing silicon chip technologies.

Bajoni and his colleagues now aim to integrate this device onto microchips. “I would like to caution the lay reader that we are not going to see a quantum version of the Internet, in which you can send quantum-encrypted e-mails, anytime soon,” Bajoni said. “It is probable that the first applications will be point-to-point exchanges of information. For instance, one can think of quantum ATM machines where bank clients can exchange quantum cryptography keys to be used for home banking.”

MIT's Planning Algorithms are Like Siri, Except Creative and Helpful

People have trouble with realistic planning. By “people,” I mean humans in general, particularly those of us who have jobs and families and hobbies and all that other stuff that makes life variable and complicated. We can’t do it all, but we try anyway, and it frequently involves failures of varying levels of catastrophe. While there are plenty of interactive tools to assist us with scheduling, they mostly just do what we say, whether or not it makes sense. MIT engineers are trying to inject some sense into personal planning. They are trying to make a better version of Apple’s Siri virtual assistant by factoring in risks and probabilities of success, and offering alternatives, even if those alternatives bend the rules a little bit.

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The Germanium-Tin Laser: Answer to the On-Chip Data Bottleneck?

Photonics engineers dream about using light to zap data between processor cores on multicore CPU chips. By replacing copper wires, such optical interconnects could make chips much faster and more power efficient. The holy-grail for optical on-chip communication is a laser made of silicon.

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2014 U.S. Venture Funding Was Highest Since Dot-Com Boom

2014 was the biggest year for venture capital since the peak of the dot-coom boom in 2000. Venture capitalists poured US $48.3 billion across 4,356 deals with U.S. startups last year, according to a new report by PricewaterhouseCoopers and the National Venture Capital Association. That’s 61 percent higher than the $30 billion startups got from investors in 2013 and twice the $20.4 billion invested in 2009.

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Lost Beagle 2 Robot Found Intact on Mars After a Decade

A decade-long search for a lost Martian robot has finally discovered the UK-led mission’s fate. The Beagle 2 Mars Lander apparently made it down to the red planet in one piece but failed to fully deploy properly in order to make contact with mission controllers on Earth.

The discovery of the lost robot brings some measure of closure to UK and European Space Agency team members after Beagle 2’s silence following its Christmas Day descent to the red planet in 2003. But the partial deployment of  just two or three of Beagle 2’s four solar panels likely blocked the robot’s RF antenna and made it impossible for mission controllers to send a signal to revive the mission remotely.

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Obesity-Fighting Implant Approved by FDA

A medical device company in Minnesota has had a big win. It can now sell its weight-loss implant in the heaviest country on the planet. In the Lancet’s list of nations with the most obese people, the United States came out on top with 13 percent of the world’s 671 million. Now Enteromedics, the Minneapolis company, can offer those considering gastric-bypass surger—or similar drastic measures—a less radical alternative.

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