New blue quantum dot technology could lead to more energy-efficient displays

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 normally required to display full-color images—red, green, and blue—the latter proved difficult to produce.

A new method based on self-assembled chemical structures offers a solution, and a state-of-the-art imaging technique for visualizing these new blue quantum dots proved essential for their creation and analysis.

Look closely at your device’s screen and you may be able to see the individual picture elements, pixels, that make up the image. Pixels can appear in almost any color, but they’re not actually the smallest element on your screen, as they’re usually made up of subpixels that are red, green, and blue. The variable intensity of these subpixels gives individual pixels the appearance of a single color from a palette of billions.

The underlying technology behind subpixels has evolved since the days of early color TV and there are now many possible options. But the next big leap is likely to be so-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 subpixels inside them. Of the three primary colors, blue subpixels are the most important. Through a process called down-conversion, blue light is used to create green and red light. Because of this, 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 drew on our team’s knowledge of self-organizing chemistry to precisely control molecules until they form the structures we want. Think of it like building a house from bricks rather than carving one made of stone. It’s much easier to be precise, design the way you want, and it’s also more efficient and economical.”

But it’s not just how Nakamura’s team produced their blue quantum dot that’s special. when exposed to UV light, it produces almost 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 such as lead perovskite, malic acid and oleylamine. And only through self-organization can these be converted into the required form, which is a cube with 64 lead atoms, four to a side.

“Oddly enough, one of our biggest challenges was figuring out that malic acid was a key piece of our chemical puzzle. It took over a year of methodically trying different things to find it,” Nakamura said.

“Perhaps less surprisingly, the other major challenge was to determine the structure of our blue quantum dot. At 2.4 nanometers, 190 times smaller than the wavelength of the blue light we sought to create with it, the structure of a quantum dot is not can be imaged by conventional means. So we turned to an imaging tool pioneered by some of our team known as SMART-EM, or ‘cinematic chemistry,’ as we like to call it.”

Film chemistry is a development of electron microscope imaging that is more like video capture than still image capture. To capture details of the structure of the blue quantum dot, this is necessary, as the nanocrystal is actually quite dynamic, so any individual image of it will only tell a small part of its story. Unfortunately, the blue quantum dot is also quite short-lived, although this was to be expected, and the team now aims to improve its stability with the help of industrial collaboration.