Crystalplex Corp., a very small 12-year-old company with headquarters in Pittsburgh, was recently awarded a patent for an important piece of IP for making “air-stable” quantum dots. And last Friday, I had an exclusive telephone interview with Ken Acer, Crystalplex’s Director of Business Development.
As I’ve mentioned in this space in recent weeks, the next generational change in quantum-dot (QD) technology will be air-stable QDs. “Air-stable” is probably not the best term for the technology, but it refers to surrounding each QD with a passivating layer that protects the dot from water and oxygen, which are killers of QD lifetime. All current commercial QDs lack such a layer, and must be protected by placing them inside a protective film, as 3M does in its Quantum Dot Enhancement Film (QDEF) with the QDs supplied by Nanosys; or by placing them inside a glass tube, as QD Vision has done with its Color IQ optical element.
Behind closed doors at SID Display Week 2016, Nanosys showed a very early technology demo of air-stable QDs. Nanosys was not revealing information on the passivating shell then, and in a recent email exchange Nanosys’s Jeff Yurek said the company is still not ready to release that information. However, said Yurek, “We plan to show a great new demo of what we would call ‘air-processable’ quantum dots at CES. After CES, we are planning a paper on this topic for SID which will go into more detail.”
Crystalplex Adds a Shell
Crystalplex, on the other hand, is ready and willing to discuss its technology in detail now. The company creates a a sapphire (aluminum oxide) shell, which is the subject of the recently allowed patent. The company coats its QD core with a 5-atom thickness of aluminum, cools it to 100 C, and then admits air to oxidize the aluminum to sapphire. The sapphire blocks air and moisture, and, being transparent, allows photons to enter and exit the QD, which is an obvious requirement.
As important as this is, the way Crystalplex makes the core of its QDs is even more interesting. But first a little — very little — quantum mechanics. The thing QDs do that interests display people is that they down-convert incoming photons. That is, an incoming blue photon excites the dot, which then emits a photon of a longer wavelength, such as green or red. This process occurs in an atomic-scale potential well in the QD, and it is the size of the well — which is determined by the exterior size of the QD — that determines the emitted wavelength of the photon. A larger QD emits a photon closer to the red side of the visible spectrum. The dots are grown from a liquid soup at an elevated temperature. If you think of a glass beaker on a hot plate, that is just how QDs are grown in laboratory quantities by all the suppliers except one (QMC). The longer the soup cooks, the bigger the QDs get, so it is critical that the dots grow at the same rate, that the growth can be terminated at just the right point so the dots have the correct diameter, and that there is a high degree of consistency from dot to dot. From a process point of view, this is doable but not easy.
Acer pointed out that there is another issue. Because the QDs that produce different colors have different sizes and, therefore, different surface/volume ratios, they inherently degrade at different rates when exposed to high photon flux — that is, to really intense light. And here is where Crystalplex’s novel approach comes in.
All of Crystalplex’s dots are the same exterior size, and the output color is tuned by interior composition. The company’s “soup” contains cadmium oxide dissolved in octadecene and oleic acid at 300 C, to which selenium and sulphur are added at the same time. In the soup, cadmium selenide is formed first; cadmium sulfide is formed last. The result is a CdSe core and a CdS layer, with a region in between where the CdSe and CdS are mixed, with the concentration of CdSe diminishing as you move further away from the center. Crystalplex calls this an “alloy gradient core.” This gradient forms a “soft wall” to the quantum well, with the wall’s location and the quantum well’s size determined by the relative concentrations of selenium and sulphur.
Acer emphasized that the processing time is not critical. Once the process proceeds to maturity, you can continue to cook the soup and the characteristics of the QDs do not change. This, said Acer, is a much more controllable and more precise process than tuning by dot size.
Another benefit, said Acer, is that the company’s process uses simple metals, which are inexpensive, and solvents that are ‘almost free’. Combined with the simpler process, when the process is scaled up, Crystalplex dots will be slightly less expensive (with their sapphire shells) than are competing dots without a passivating shell, Acer said.
And then there’s an additional cost-saving at the component level because the dots don’t have to be passivated. Acer: “Nanosys has done such a good job of decreasing the cost of their quantum dots that the dots in a layer of QDEF cost less than the [5-layer film sandwich] 3M puts them in.”
Crystalplex Lifetime Unmatchable
Acer emphasized that Crystalplex believes the main differentiator of its Sapphire-passivated alloy-gradient technology is its stability under accelerated aging. “The predicted lifetime under accelerated conditions is 30,000 hours,” said Acer. “No QD supplier to our knowledge can match this performance.”
Our conversation was long and detailed, and Acer was unusually open, so there is much more to say than I have room for here, But you can learn more at SID Display Week in Los Angeles this coming May, where Crystalplex will be exhibiting for the first time.
Ken Werner is Principal of Nutmeg Consultants, specializing in the display industry, manufacturing, technology, and applications, including mobile devices and television. He consults for attorneys, investment analysts, and companies re-positioning themselves within the display industry or using displays in their products. You can reach him at [email protected].
