Friday, February 26, 2016

Quantum Dot Solids Could Revolutionize Silicon Wafers

Quantum Dots
Quantum dot solids could become the next-generation silicon wafers. Credit: Kevin Whitham, Cornell University

Scientists may be nearing a breakthrough which could revolutionize the semiconductor industry. Just as the single-crystal silicon wafer forever changed the nature of communication 60 years ago, a group of Cornell University researchers is hoping its work with quantum dot solids - crystals made out of crystals - can help usher in a new era in electronics.


The chemical engineers have fashioned two-dimensional superstructures out of single-crystal building blocks. Through a pair of chemical processes, the lead-selenium nanocrystals 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 to each other. The electrical properties of these superstructures potentially are superior to existing semiconductor nanocrystals, with anticipated applications in energy absorption and light emission.



The strong coupling of the nanocrystals 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 artificial materials with controllable electronic structure.

"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," said research lead Tobias Hanrath, associate professor in the Robert Frederick Smith School of Chemical and Biomolecular Engineering.


The structure of the experimental superlattice still has multiple sources of disorder due to the fact that all nanocrystals are not identical. This creates defects, which limit electron wave function.



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 1950's. "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."



Hanrath group's paper, "Charge transport and localization in atomically coherent quantum dot solids," is published in this month's issue of Nature Materials.

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