Quantum dot technology breakthrough allows researchers to create near-perfect superstructures out of these tiny crystals.
Quantum dots are nano-sized semiconductor particles whose emission color can be tuned by changing their dimensions. They feature near-unity emission quantum yields and narrow emission bands, which result in excellent color purity. Their properties have singled them out as the next big thing in various fields of technology, particularly illuminating mediums.
The dots are incredibly tiny -- each of these crystals is only made up of around 5,000 atoms. Because of their physical properties, including their ability to emit or absorb light of different wavelengths depending on how they're manipulated, there has been a lot of interest in applying them in various fields of science and technology. But we've never been able to successfully tie the dots together without using another substance. These structures' properties were degraded by the second fraction, severely hampering our use of quantum dots.
Now, a team from Cornell University’s School of Chemical and Biomolecular Engineering has found a way to overcome this obstacle, arranging quantum dots together in an almost perfect structure.
Image credits Kevin Whitham, Cornell University.
Previous efforts have found that when placed on a fluid surface the crystals could be fused together, as they would float similarly to oil on water. However this negatively impacted the quantum dots' properties, hampering the effectiveness of the structure as a whole.
“Previously, they were just thrown together, and you hoped for the best,” says lead researcher Tobias Hanrath in a telephone interview with The Christian Science Monitor.
“It was like throwing a couple thousand batteries into a bathtub and hoping you get charge flowing from one end to the other.”
Dr. Hanrath and his team's breakthrough will finally allow us to connect the dots without another substance that would impact their purity and structure. This finding represents the culmination of several years’ work for the team, which the professor described as “playing lego but with atomic-sized building blocks.”
“If you take several quantum dots, all perfectly the same size, and you throw them together, they’ll automatically align into a bigger crystal,” Hanrath says.
“It’s the same idea as a bucket of tennis balls automatically assuming an ordered pattern, or stacking cannonballs on top of each other.”
The team started from some of their previously published research, including a 2013 paper published in Nano Lettersin which they detailed a method of tying the dots through controlled displacement of connector molecules, called ligand. That paper referred to "connecting the dots" - i.e. electronically tying each quantum dot - as being one of the most persistent hurdles to be overcome.
Now, the team has found a way to make the crystals not only arrange themselves in an orderly fashion, but also stick to one another. This enables the creation of crystal superlattices that are defect-free.
But there is still a way to go before quantum dots can leave the lab for a screen near you. The structure of the superlattice, while superior to ligand-connected nanocrystal solids, is still limited in its electron wave function. In essence, the lattice isn't perfectly uniform in structure because the crystals aren't all identical in size.
“Take silicon,” says Hanrath. “Every silicon atom is the same size. In our case, the building blocks are almost the same size, but there is 5 percent variability in diameter, so you can’t make a perfect crystal superstructure, but as far as you can, we’ve pushed it to the point of perfection.”
The full paper, titled "Charge transport and localization in atomically coherent quantum dot solids" has been published online in the journal Nature Materials and is available here.