Quantum dots are improving screens worldwide, but their cadmium content worries some
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. The firms—Nanosys, QD Vision, and Dow Chemical (Nanoco) — are racing to capture a share of the emerging market, but there may not be a place for everyone at the finish line.
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
You must be logged in to post a comment.