Quantum dots are nanoscale crystals that can emit light of different colors. Display devices based on quantum dots promise higher energy efficiency, brightness and color purity than previous display generations. Of the three colors typically required to display full color images – red, green and blue – the last has proven difficult to produce. A new method based on self-assembling chemical structures offers a solution, and a state-of-the-art imaging technique to visualize these novel blue quantum dots proved essential for their fabrication and analysis.
Look closely at your device’s screen and you may be able to make out the individual picture elements, pixels, that make up the image. Pixels can come in almost any color, but they’re actually not the smallest element on your screen, as they’re typically made up of red, green, and blue sub-pixels. The variable intensity of these sub-pixels makes each pixel appear like a single color out of a palette of billions. The underlying technology behind subpixels has evolved from the days of early color television, and there are now a number of possible options. However, the next big leap is likely to be so-called quantum dot light-emitting diodes or QD-LEDs.
Displays based on QD-LEDs already exist, but the technology is still maturing, and current options have some drawbacks, particularly around the blue sub-pixels they contain. Of the three primary colors, blue sub-pixels 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 manufacture and their quality is a critical factor in any display. But now a team of researchers led by Professor Eiichi Nakamura of the University of Tokyo’s Faculty of Chemistry has found a solution.
“Previous design strategies for blue quantum dots were very top-down, taking relatively large chemical entities and running them through a series of processes to refine them into something that worked,” Nakamura said. “Our strategy is bottom-up. We have built on our team’s knowledge of self-assembling chemistry to precisely control molecules until they form the desired structures. Think of it like building a house out of bricks instead of carving one out of stone. It’s a lot easier to be precise, get the design the way you want it, and it’s more efficient and less expensive.”
But it’s not just the way Nakamura’s team produced their blue quantum dot that’s special; When exposed to ultraviolet light, it produces near-perfect blue light, according to the international standard for measuring color accuracy known as BT.2020. This is due to the unique chemistry of their dot, a hybrid blend of organic and inorganic compounds including lead perovskite, malic acid and oleylamine. And only through self-organization can these be brought into the desired shape, namely a cube of 64 lead atoms, four on a side.
“Surprisingly, one of our biggest challenges was figuring out that malic acid is 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, 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 the blue light we tried to produce with it, the structure of a quantum dot cannot be imaged by conventional means. So we turned to an imaging tool that was developed by some of our team members and is known as SMART-EM, or “cinematic chemistry” as we like to call it.”
Film chemistry is an evolution of electron microscopic imaging that is more akin to capturing a video than capturing a still image. This is essential for capturing details of the structure of the blue quantum dot, as the nanocrystal is actually quite dynamic, so any single image of it would only tell a small part of its story. Unfortunately, the blue quantum dot is also quite short-lived, as expected, and the team now aim to improve its stability through an industrial collaboration.
Materials provided by University of Tokyo. Note: Content can be edited for style and length.