Just as the single-crystal silicon¹ wafer forever changed the nature of communication 60 years ago, a group of Cornell researchers is hoping its work with quantum dot solids - crystals made out of crystals - can help usher² in a new era in electronics. The 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 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 coherence³ 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 semiconductor⁴ nanocrystals, with anticipated applications in energy absorption and light emission⁵.

 

"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.

 

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

 

This latest work has grown out of previous published research by the Hanrath group, including a 2013 paper published in Nano Letters that reported a new approach to connecting quantum dots through controlled displacement⁶ of a connector molecule⁷, called a ligand. That paper referred to "connecting the dots" - i.e. electronically coupling each quantum dot - as being one of the most persistent⁸ hurdles⁹ to be overcome.

 

That barrier seems to have been cleared with this new research. The strong coupling of the nanocrystals leads to formation of energy bands that can be manipulated based on the crystals' makeup¹⁰, and could be the first step toward discovering and developing other artificial materials with controllable electronic structure.

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