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Zinc oxide nanostructures are synthesized in parallel microfluidic channels (held by the metal frame) by flowing reactants through the tubing. The microfluidic structure creates the device and also becomes the final packaged functional LED device.
Photo: Jaebum Joo; MITnews

Just when I thought I could stroll back into the macro world, two new papers were published that complement our recent posts on PCMMs, supercapacitors, nanoporous materials and other nanostructured materials.

First, out of MIT comes “Face-selective electrostatic control of hydrothermal zinc oxide nanowire synthesis,” by Jaebum Joo, et. al. (see Nature Materials, doi:10.1038/nmat3069). Using hydrothermal synthesis, the group grew zinc oxide nanowires with controlled morphologies and functional properties. Morphologies synthesized ranged from platelets to needles with aspect ratios that spanned three orders of magnitude (~0.1-100 are reported).

The article abstract says a classical thermodynamic model was used to explain the growth inhibition mechanism “by means of the competitive and face-selective electrostatic adsorption on non-zinc complex ions at alkaline conditions.” An online story from MITnews clarifies that “the key turns out to be the electrostatic properties of the zinc oxide material as it grows from a solution.” When ions from other compounds are added to the solution (from which the ZnO is grown hydrothermally), they attach electrostatically and preferentially to the wire at only the sides or the ends, which inhibits growth in those directions (i.e., face-selective). The hydrothermal synthesis process temperature was less than 60°C, which opens up the possibility of manufacturing devices on or in polymers and plastics.

The team fabricated a functional LED array of ZnO nanowires, but ZnO also can be used in battery, sensor and other optical applications. In the MIT story Joo says this method and the ability to use it to control morpholgy could be applied to other materials, for example, titanium dioxide, a possible solar cell material. Joo also says the successful use of hydrothermal synthesis to manipulate nanostructure has “the potential for large-scale manufacturing.” (Perhaps a candidate technology for Obama’s recently announced Advanced Manufacturing Partnership?)

The second paper, published in the same issue of Nature Materials, is from a group at Samsung Electronics in Korea. In an interesting departure from the more common PCMM chalcogenide approach, they looked at tantalum oxide-based bilayer structures for nonvolatile memory devices. (See “A fast, high-endurance and scalable non-volatile memory device made from asymmetric Ta₂O_5-x/TaO_2-x bilayer structures,” Nature Materials, doi:10.1038/nmat3070, by Myoung-Jae Lee, et. al.)

(Quick note: Based on the abstract and online images, this appears to be a material property- and performance-oriented paper, and it is not known whether the material is nanostructured.)

Like others researching nonvolatile memory materials, they are looking for “a material or device structure that satisfies high-density, switching-speed, endurance, retention and most importantly power — consumption criteria” (from the abstract). The paper describes an asymmetric passive switching device with an impressive cycling endurance of over 10¹² and switching times of 10 ns. They were able to demonstrate a significant reduction of switching current and, therefore, power consumption.

The paper’s abstract postulates that there may be another benefit: “[B]y combining two such devices, each with an intrinsic Schottky barrier, we eliminate any need for a discrete transistor or diode in solving issues of stray leakage current paths in high-density crossbar arrays.”

Samsung’s published interest in nonvolatile memory materials combined with IBM’s recent proof-of-concept chalcogenide device seems to comprise pretty strong evidence that the leap from lab to prototype is underway, and Si-based flash memory may soon be just that, a memory.

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Credit: Piccolo Namek, Wikipedia.

I really had intended to give the topic of LEDs a rest for a while. But while catching up on some reading, I came across a story suggesting that, while LEDs have performance advantages over incandescents and CFLs, one can’t assume that LEDs are free from disposal problems. In fact, the paper’s authors from the University of California (Davis and Irvine) suggest that LEDs may bring their own “environmental burdens.”

The researchers, who are associated with UCI’s School of Social Ecology/Program in Public Health and UCD’s Department of Chemical Engineering and Materials Science, distinguish between environmental burdens related to resource depletion (e.g., gold and silver) and those burdens related to toxicity (e.g., copper, nickel and lead).

The groups goals was to test whether LEDs could be considered “hazardous wastes” as defined by United States and California standards, look at how the threat might vary across different LED types and look at the overall life-cycle impact of LEDs. The latter was done, in part, to help designers and manufacturers make safer products and to help waste disposers and recyclers know how to handle LEDs that are already making their way to landfills.

Their findings, published in Environmental Science & Technology, were that some LEDs did pose a threat of leaching toxic materials if disposed of improperly, but the threat was largely related to LED color and intensity. In fact, with one exception, all LEDs exceeded Cali’s silver, nickel, lead and copper standards. The one exception is low-intensity yellow LEDs. One type of LED — low-intensity reds — exceeded federal lead standards.

The groups methods were pretty straight forward: Grind up LEDs and expose the resultant flecks, nuggets and specks to the equivalent of multiyear bath in acid rain, and then test for toxic materials in the runoff.

This isn’t the first time these researchers have used this type of approach. For example, UCI’s Oladele A. Ogunseitan has been grinding up and testing cell phones and other commercial electronics for some time. Ogunseitan has been the principal investigator in NSF-sponsored study on strategies for addressing e-wastes. Another group member, UCD’s Julie M. Shoenung, runs the school’s Lead Campus activities that are part of the Research and Education in Green Materials program.

In an online story, Gizmag writer Darren Quick, reports:

Ogunseitan blames the situation on a lack of proper product testing before LEDs were presented as a more efficient replacement for incandescent bulbs - which are now being phased out around the world. Although a law requiring more stringent testing for such products was scheduled to begin on January 1st in California, it was opposed by industry groups, and Governor Arnold Schwarzenegger put it on hold before leaving office.

“Every day we don’t have a law that says you cannot replace an unsafe product with another unsafe product, we’re putting people’s lives at risk,” said Ogunseitan. “And it’s a preventable risk.”

One point of this group’s work is that the time to act is now. LEDs were already entering the waste stream from auto industry applications (front and rear lights) and hitting the mass market in the form of cheap and ubiquitous holiday lights.

On a practical level, the group suggests that anyone having to clean up broken LEDs should treat the situation as if approaching broken CFLs. Wear gloves, mask and use special brooms and other equipment to gather the debris. They also go so far as to suggest special precautions for emergency responders to highway accidents.

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90 kg sapphire crystal. Credit: Thermal Technology.

I’ve touched on LEDs a couple of times in the last few days, and in doing that research I got the distinct impression that because of strong demand in the LED markets, companies that have a hand in making the sapphire substrates for LEDs are doing quite well, both financially and technically.

For example, Illinois-based Rubicon Technology announced a few weeks ago that it has begun making 12-inch wafers for LEDs. Echoing the history of how silicon-wafer size growth has been playing a role over the last two decades in driving down semiconductor chip prices, polished sapphire wafers have grown from the two-inch standard size, to then four-inch and eight-inch. Thus, the 12-inch wafer marks a substantial achievement.

Part of the trick of making larger wafers is forming the large boules of pure sapphire. Currently, the LED industry seems to be centered on boules in the 80–90 kg range, but Rubicon has demonstrated that it can produce a 200-kg crystal.

In a press release, company president and CEO Raja Parvez, Rubicon President and CEO brags, “Rubicon’s ability to affordably produce larger wafers, free of defects, is key to helping industries that make and use LEDs scale to the volumes necessary to support the growth needed in the general lighting and consumer electronics.[…] our customers can depend on us for uniform, particulate-free sapphire wafers as well as flat, stress-free wafers. High quality sapphire wafers help our customers produce high quality LED wafers at volumes supporting the LED supply chain.”

In 2010, Rubicon said it had inked a $71 million six-inch wafer deal with an unnamed “major LED chip manufacturer” that extends through 2011. No mention is made in the new announcement about who might be interested in buying the 12-inch wafers, but the company goes out of its way to mention that Philips Lumildes and Lextar Electronics are using the six-inch variety.

The transition to larger diameter wafers in LED production has started. Earlier in 2010, Rubicon announced that the company entered into a $71 million agreement with a major LED chip manufacturer for which Rubicon will provide six-inch polished substrates. Companies such as Philips Lumiled and Lextar Electronics have announced six-inch production of LED wafers built on sapphire.

The company describes itself as “a vertically-integrated manufacturer with capabilities in crystal growth, high precision core drilling, wafer slicing, surface lapping, large-diameter polishing and wafer cleaning processes. Demand seems strong (according to Bloomberg.com, Parvez last week said that wafer prices are up 30 percent compared to the previous quarter) and investors seem to like what they see in Rubicon: When the company released is quarterly report Feb. 18, Rubicon’s stock price jumped 20 percent.

Another example of the industry see doing well is Thermal Technology. Unlike the vertically integrated Rubicon, Thermal Technology focuses on making and selling “crystal growth equipment and high temperature furnace systems.” In other words, it sells the equipment so other companies can make the boules and then slice and prepare the sapphire wafers.

In early December 2010, Thermal Technology — a privately held company — announced it would begin marketing equipment to make 90 kg boules using a Kyropoulos growth method. The announcement must have struck a chord because is released another announcement Feb. 10, 2011, that trumpets that the company has already received 59 orders for the new crystal grower “from customers in Taiwan, Korea and China. In total, these growers will produce 5.2 million TIE (two-in-equivalents) per year.”

About 10 seconds into this recent video, you can see Sino-America Silicon Products, one of the company’s LED-producing customers in Taiwan, celebrate  the creation of one of the 90 kg crystal grown with the Thermal Technology system

Matt Mede, Thermal Technology’s president and CEO, seems pleased. He says in a press release, “Previously, the Russian growers were the industry standard. The superiority of our design, crystal size and tool capability are quickly making Thermal Technology the industry leader in this market sector,” Mede continues.

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R50 vacuum insulation panel, 30×48x1 inch. Credit: ThermalVisions.

Nano R&D is opening up new energy-conservation vistas, but a new report from Lux Research claims that the adoption of existing nano-enabled products, such as aerogels, low-friction coatings and quantum dot LEDs, could yield a huge energy-consumption reduction in three countries representative of nations with developed economies: Japan, Germany and the United States.

How huge? Lux, which provides strategic advice and intelligence on emerging technologies, says that these nano products could cut energy consumption by 12%. No one is advocating this (settle down Ohio and West Virginia), but Lux says that is the equivalent of shutting down all of the coal-powered generation plants in the U.S.

Lux researcher and lead author of the report David Hwang says researchers examined energy usage in residential, commercial, industrial and transportation sectors, and then interviewed nanotech product developers, manufacturers and end users. Generally speaking, they discovered that the U.S. would benefit most from nano in the automobile sector, Germany from heating application for residential and commercial use and Japan from lighting applications. Here is how Lux summarized its observations:

• “The ubiquitous American automobile drives most energy consumption in the U.S. Hence, low-friction tribological coatings and lightweight nanocomposites offer the biggest opportunities — 1.8% and 3.6%, respectively — for reducing energy consumption. With 3.9% total energy spent on lighting in the commercial, industrial, and residential sectors, QD-enabled LED lighting could cut another 2.5% from the U.S.’s energy consumption in 2020.”

• “Germany’s energy consumption is dominated by space heating. In 2009, 28.9% of the country’s total consumption went to heating living and work spaces in the residential and commercial sectors. Consequently, nano-enabled insulation could help lower countrywide consumption by 6.8%, representing almost half of Germany’s total potential savings. Adoption of tribological coatings and nanocomposites in German automobiles could cut another 3.8% from the country’s total consumption by 2020.

• “In 2009, 4.6% of Japan’s energy was used for general space illumination. With the large efficiency increases that QD-enabled LEDs deliver, the country could shave off 3.3% of total consumption by 2020.”

Hwang says the reference to nano-enabled insulation includes things such as aerogels, vacuum insulated panels with aerogel and other foam cores, polyurethane foams and coated glasses, although he says that coated glass, thermochromic glass, etc., will play a bigger role in locales where cooling is more important than heating. He mentions that Cabot and Aspen are finding significant sales success in the European insulation market.

In addition, he notes predicts that ceramic materials can play a big role in energy savings in the automotive composites field.

Regarding QD-enabled LEDs makers, Hwang says that joint efforts in the field, for example between Nexxus Lighting and QD Vision, NN Labs and Renaissance, and Nanosys and Samsung.

Nexxus/QD Vision introduced the first commercial LED lamp line that uses QDs in May 2009, and actually started to ship the bulbs in March 2010. The allure of QDs is that it improves the color quality and warms LED light, thereby avoiding some of the pitfalls that have limited the acceptance of CFL bulbs. The Nexxus/QD Vision approach is to apply a thin film of QD to the external face of LEDs. The companies say their “Array Quantum LED” bulbs are six times more efficient than incandescent ones. They aren’t cheap, with some online suppliers selling them for $100+ per bulb.

NN Lab’s angle is that it’s QDs are made from indium phosphide. It describes this as an environment-friendly alternative to the typical cadmium selenide-based QDs, developed in collaboration with the University of Arkansas. NNCrystal, a wholly owned subsidiary of Hangzhou Najing Technology Co., has licensed NN Lab’s Qshift Lucid and Coral technologies and is working with Renaissance Lighting to introduce a line of Solia-branded lighting products. This is also a coating-on-LED approach.

Hwang says that the 12% cut in energy use is an extremely optimistic number based on a 10-year model. He cautions that 1.6% is a more realistic energy decrease, but says several external factors, such as government policy and the price of oil, can significantly influence the adoption of these technologies. He also says the five-year cycle for automotive development will introduce a lag between adoption of a technology and a product hitting that marketplace.

Hwang also tells me that they chose the three countries because they were good proxies for developed economies in the Americas, Europe in Asia.

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Graphic illustrates a single row of nanowires (cylinders with red tops) with fin-shaped
nanowalls extending outward. Credit: NIST.

Speaking of ZnO nanowires, chemists at NIST who were perfecting new methods of creating these nanowire report that they have stumbled upon a way to create light-emitting nanowires that operate like very tiny LEDs.

TEM of four rows of the nanowires and nanowalls. Credit: NIST.

The chemists, Babak Nikoobakht and Andrew Herzing, had already found a way to grow ZnO nanowires horizontally across a substrate that was seeded with gold nanoparticles. These nanoparticles serve as growth sites and medium for the crystallization of zinc oxide molecules: As the zinc oxide nanocrystal grows, it pushes the gold nanoparticle along the surface of the substrate (“surface-directed” fabrication).

While working with a gallium nitride substrate, Nikoobakht and Herzing, unexpectedly found that when they increased the thickness of the gold nanoparticle from less than 8 nm to approximately 20 nm, the nanowires sprouted a secondary structure. Visually, the structure is said to look like a “dorsal fin.” Technically, it is a nanowall that forms where the ZnO portion is electron-rich and the gallium nitride portion is electron-poor.

At this location, the nanowall becomes a p-n heterojunction where electrons can flow across it when voltage is applied to the nanowire-nanowall combination. This flow of electrons produces light, which led the researchers to dub it a “nano LED.”

According to NIST

“Unlike previous techniques for producing heterojunctions, the NIST “surface-directed” fabrication method makes it easy to locate individual heterojunctions on the surface. This feature is especially useful when a large number of heterojunctions must be grouped in an array so that they can be electrically charged as a light-emitting unit.”

The duo says the simple design of the nanowires means that is scalable to literally any platform size. For example, the nano LEDs, with further improvements, could be used as light sources and detectors in photonic devices or lab-on-a-chip platforms.

TEM of nano LEDs emitting light. Credit: NIST.

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