Quantum dots are nanoscale crystals that can give off different colours of light. Displays that use quantum dots promise to use less power, be brighter, and have more accurate colours than older displays. Red, green, and blue are the three colours that are usually needed to show full-colour images, but blue has been challenging to fabricate.
Researchers at the University of Tokyo have developed a solution based on chemical structures that can arrange themselves. A cutting-edge imaging technique for visualizing these blue quantum dots was also critical to their creation and analysis.
Quantum Dot Light Emitting Diodes
Electron microscope images of experimental samples using different chemical combinations. Credit: ©2022 Nakamura et al.
If you look closely at your device’s screen, you might be able to see the individual picture elements, or pixels, that make up the image.
Pixels can be almost any colour, but they are not the smallest element on your screen because they are typically made up of red, green, and blue subpixels. The variable intensity of these subpixels gives the appearance of a single colour from a palette of billions to individual pixels.
Since the early days of colour television, the underlying technology behind subpixels has evolved, and there are now several options. The next big step, however, is likely to be quantum dot light-emitting diodes (QD-LEDs).
There are already displays based on QD-LEDs, but the technology is still maturing, and the available options have some drawbacks, particularly in regard to the blue subpixels within them. Blue subpixels are the most significant of the three primary colours.
Self-organizing And Bottom Up
By a process known as down-conversion, blue light is converted into green and red light. Blue quantum dots require more stringently controlled physical parameters as a result.
Because of this, it is often hard and expensive to make blue quantum dots, and their quality is important for any display. Now, though, a group of scientists led by Professor Eiichi Nakamura of the University of Tokyo’s Department of Chemistry has come up with a solution.
Previous blue quantum dot design strategies were very top-down, taking relatively large chemical substances and refining them through a series of processes into something that worked.
“Our strategy is bottom up. We built 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 from stone. It’s much easier to be precise, design the way you want, and is more efficient and cost effective too,”
said Nakamura.
Cinematic Chemistry
But it’s not just how Nakamura’s team created their blue quantum dot; when exposed to ultraviolet light, it generates nearly perfect blue light, according to the international colour accuracy standard known as BT.2020.
This is because their dot has a special chemical composition that combines both organic and inorganic substances, such as lead perovskite, malic acid, and oleylamine. And they can only be forced into the necessary shape, a cube of 64 lead atoms, four to a side, through self-organization.
“Surprisingly, one of our biggest challenges was in finding that malic acid was a key piece of our chemical puzzle. It took over a year methodically trying different things to find it,”
said Nakamura.
Stills from the video captured using “cinematic chemistry” of the blue quantum dot, including an illustration showing the atomic arrangement of the sample. Credit: ©2022 Nakamura et al.
The team’s other main challenge was determining the structure of their blue quantum dot, which was perhaps less surprising. The structure of a quantum dot cannot be imaged conventionally because it is 2.4 nanometers in size, 190 times smaller than the wavelength of the blue light they sought to create with it.
As a result, they turned to SMART-EM, an imaging tool pioneered by some of their team, or ‘cinematic chemistry,’ as they like to call it.
Cinematic chemistry is an evolution of electron microscope imaging that is more akin to filmmaking than still photography. This is critical for capturing details of the structure of the blue quantum dot, as the nanocrystal is actually quite dynamic, and any single image of it would only tell a small part of its story.
Unfortunately, the blue quantum dot is also quite short-lived, which was expected, and the team is now working to improve its stability through industrial collaboration.
Reference:
Olivier J. G. L. Chevalier, Takayuki Nakamuro, et al. Precision synthesis and atomistic analysis of deep blue cubic quantum dots made via self-organization. Journal of the American Chemical Society (2022). DOI: 10.1021/jacs.2c08227