Quantum dots are nanoscale crystals capable of emitting light of different colors. Display devices based on quantum dots promise greater power efficiency, brightness and color purity than previous generations of displays. Of the three colors typically required to display color images – red, green and blue – the last has proven difficult to produce. A new method based on self-organizing chemical structures offers a solution, and a cutting-edge imaging technique to visualize these new blue quantum dots has proven essential to their creation and analysis.
Look closely at your device’s screen and you may be able to see the individual picture elements, the pixels, that make up the image. Pixels can appear in almost any color, but they aren’t actually the smallest element on your screen as they’re usually made up of red, green, and blue sub-pixels. The varying intensity of these sub-pixels gives individual pixels the appearance of a single color from a palette of billions. The underlying sub-pixel technology has evolved since the days of the first color TVs, and there are now a number of possible options. But the next big leap will likely be something called quantum dot light-emitting diodes, or QD-LEDs.
QD-LED-based displays already exist, but the technology is still maturing, and current options have some drawbacks, especially when it comes to the blue sub-pixels they contain. Of the three primary colors, the blue sub-pixels are the most important. Through a process called down-conversion, blue light is used to generate green and red light. For this reason, blue quantum dots require more tightly controlled physical parameters. This often means that blue quantum dots are very complex and expensive to produce, and their quality is a critical factor in any display. But now a team of researchers led by Professor Eiichi Nakamura from the University of Tokyo’s Department of Chemistry has a solution.
“Previous design strategies for blue quantum dots were very top-down, taking relatively large chemicals and putting them through a series of processes to refine them into something that worked,” Nakamura said. “Our strategy is bottom-up. We relied on our team’s knowledge of self-organizing chemistry to precisely control molecules until they form the structures we want. Think of it as building a house out of bricks rather than carving one out of stone. It’s much easier to be precise, design the way you want, and it’s also more efficient and cost effective.”
But it’s not just how Nakamura’s team produced their blue quantum dot that’s special; when exposed to ultraviolet light, it produces nearly perfect blue light, according to the international standard for measuring color accuracy, known as BT.2020. This is due to the unique chemical composition of their dot, a hybrid mixture of organic and inorganic compounds including lead perovskite, malic acid and oleylamine. And it is only by self-organization that these can be coaxed into the required shape, which is a cube of 64 lead atoms, four to the side.
“Surprisingly, one of our biggest challenges was discovering that malic acid was a key part of our chemical puzzle. It took over a year of methodically trying different things to find it,” Nakamura said. “Perhaps less surprising is that our other main challenge was to determine the structure of our blue quantum dot. At 2.4 nanometers, 190 times smaller than the wavelength of blue light we sought to create with it, the structure of a quantum dot cannot be imaged by conventional means. So we turned to an imaging tool developed by some of our team members, known as SMART-EM, or “cinematic chemistry” as we like to call it. »
Cinematic chemistry is an evolution of electron microscope imaging that is more akin to taking a video than taking a still image. To capture the structural details of the blue quantum dot, this is essential, as the nanocrystal is actually quite dynamic, so any image of it would only tell a small part of its story. Unfortunately, the blue quantum dot also has a fairly short lifespan, although this was expected, and the team is now aiming to improve its stability using industrial collaboration.
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Material provided by University of Tokyo. Note: Content may be edited for style and length.