Communications of the ACM
Quantum Dot Solids: This Generation's Silicon Wafer?
The electrical properties of these superstructures developed at Cornell are potentially superior to existing semiconductor quantum dots
Credit: Kevin Whitham
Just as the single-crystal silicon wafer forever changed the nature of electronics 60 years ago, a group of Cornell researchers is hoping its work with quantum dot solids — crystals made out of crystals — can help usher in a new era in electronics.
The multidisciplinary team, led by Tobias Hanrath, associate professor in the Robert Frederick Smith School of Chemical and Biomolecular Engineering, and graduate student Kevin Whitham, has fashioned two-dimensional superstructures out of single-crystal building blocks. Through directed assembly and attachment processes, the lead selenide quantum dots are synthesized into larger crystals, then fused together to form atomically coherent square superlattices.
The difference between these and previous crystalline structures is the atomic coherence of each 5-nanometer crystal (a nanometer is one-billionth of a meter). They're not connected by a substance between each crystal — they're connected directly to each other. The electrical properties of these superstructures potentially are superior to existing semiconductor quantum dots, with anticipated applications in solar cells and other electronic devices.
"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 says, referring to the atomic-scale precision of the process.
The research is described in "Charge Transport and Localization in Atomically Coherent Quantum Dot Solids," published in the journal Nature Materials.
Whitham, a doctoral candidate in the field of materials science and engineering, did most of the experimentation and is lead author of the work. Also contributing were: Jun Yang, postdoctoral researcher; Benjamin H. Savitzky, graduate student in the field of physics; Lena Kourkoutis, assistant professor and Morgan Sesquicentennial Faculty Fellow in applied and engineering physics; and Frank Wise, the Samuel B. Eckert Professor of Engineering.
This latest work has grown out of previous published research by the Hanrath group, including "Confined-but-Connected Quantum Solids via Controlled Ligand Displacement," a 2013 paper published in Nano Letters, that reported a new approach to connecting quantum dots through controlled displacement of a connector molecule, called a ligand. That paper referred to "connecting the dots" — i.e. electronically coupling each quantum dot — as being one of the most persistent hurdles to be overcome.
That barrier seems to have been cleared with this new research. The strong coupling of the quantum dots leads to formation of energy bands that can be manipulated based on the crystals' makeup, and could be the first step toward discovering and developing other novel materials with programmable electronic structure.
Still, Whitham says, more work must be done to bring the group's findings from the lab to technological fruition. The structure of the superlattice, while superior to ligand-connected quantum dot solids, still has multiple sources of disorder due to the fact that, in contrast to atoms in an ordinary solid, no two quantum dots are truly identical. This creates defects, which limits how far the electron wave function can spread.
"I see this paper as a challenge for other researchers to take this to another level," Whitham says. "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 challenging other people to say, 'How can we do this better?'"
Hanrath says 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 access to high-quality samples yields new understanding and technological advances,'" Hanrath says, referencing the game-changing electronics 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 work made use of the Cornell Center for Materials Research, which is supported by the U.S. National Science Foundation through its Materials Research Science and Engineering Center program. X-ray scattering was conducted at the Cornell High Energy Synchrotron Source, which is supported by the NSF and the National Institutes of Health.
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