An example of the quantum dot solids developed by researchers from Cornell University. Image: Kevin Whitham, Cornell University.
An example of the quantum dot solids developed by researchers from Cornell University. Image: Kevin Whitham, Cornell University.

Just as the single-crystal silicon wafer forever changed the nature of communication 60 years ago, a group of researchers from Cornell University is hoping its work with the nanocrystals known as quantum dots can help usher in a new era in electronics.

The team, led by Tobias Hanrath, associate professor in Cornell’s School of Chemical and Biomolecular Engineering, and graduate student Kevin Whitham, has fashioned two-dimensional superstructures out of these single-crystal building blocks. Using a pair of chemical processes, the lead-selenium nanocrystals can be synthesized into larger crystals, which are then fused together to form atomically-coherent square super-lattices.

The difference between these and previous crystalline structures is the atomic coherence of each crystal, which is just 5nm in size. They're not connected by a substance between each crystal – they're connected to each other. This means the electrical properties of these superstructures are potentially superior to existing semiconductor nanocrystals, with anticipated applications in energy absorption and light emission.

"As far as level of perfection, in terms of making the building blocks and connecting them into these superstructures, that is probably as far as you can push it," Hanrath said, referring to the atomic-scale precision of the process. A paper on this research is published in Nature Materials.

This latest work grew out of previously published research by the Hanrath group, including a 2013 paper in Nano Letters that reported a new approach to connecting quantum dots through controlled displacement of a connector molecule, called a ligand. That paper described ‘connecting the dots’ – i.e. electronically coupling each quantum dot – as one of the most persistent hurdles still to be overcome.

That barrier now seems to have been cleared with this new research. The strong coupling of the nanocrystals leads to the formation of energy bands that can be manipulated via the crystals' makeup, and could be the first step towards discovering and developing other artificial materials with controllable electronic structures.

Still, Whitham said, more work needs to be done to bring the group's work out of the laboratory and into the real world. The structure of the Hanrath group's super-lattice, while superior to ligand-connected nanocrystal solids, still has multiple sources of disorder due to the fact the nanocrystals are not all identical. This creates defects, which limit electron wave function.

"I see this paper as sort of a challenge for other researchers to take this to another level," Whitham said. "This is as far as we know how to push it now, but if someone were to come up with some technology, some chemistry, to provide another leap forward, this is sort of challenging other people to say, 'How can we do this better?'"

Hanrath said the discovery can be viewed in one of two ways, depending on whether you see the glass as half empty or half full. "It's the equivalent of saying, 'Now we've made a really large single-crystal wafer of silicon, and you can do good things with it,'" he said, referencing the game-changing communications discovery of the 1950s. "That's the good part, but the potentially bad part of it is we now have a better understanding that if you wanted to improve on our results, those challenges are going to be really, really difficult."

This story is adapted from material from Cornell University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.