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Mining for Quantum Dots

Photonics.com
May 2010
HOUSTON, May 26, 2010 — Graphene, the one-atom-thick, honeycomblike form of carbon, is the material of choice for physicists on the cutting edge of materials science.

Researchers mentored by Boris Yakobson, a Rice professor of mechanical engineering and materials science and of chemistry, have discovered the strategic extraction of hydrogen atoms from a two-dimensional sheet of graphane naturally opens up spaces of pure graphene that look — and act — like quantum dots.

That opens up a new world of possibilities for an ever-shrinking class of nanoelectronics that depend on the highly controllable semiconducting properties of quantum dots, particularly in the realm of advanced optics.


Rice researchers created these fanciful images of electron densities from their graphane-embedded quantum dot calculations. The isosurfaces depict electrons in the valance band that, in reality, would be confined within the quantum dot and demonstrate that very little charge would leak from the hydrogen-defined boundaries of such a dot. (Image: Evgeni Penev/Abhishek Singh)

The theoretical work by Abhishek Singh and Evgeni Penev, both postdoctoral researchers in co-author Yakobson's group, was published online last week in the journal ACS Nano.

Graphene is touted as a solution to stepping beyond the limits of Moore's Law. Graphane is simply graphene modified by hydrogen atoms added to both sides of the matrix, which makes it an insulator. While it's still technically only a single atom thick, graphane offers great possibilities for the manipulation of the material's semiconducting properties.

Quantum dots are crystalline molecules from a few to many atoms in size that interact with light and magnetic fields in unique ways. The size of a dot determines its bandgap — the amount of energy needed to close the circuit — and makes it tunable to a precise degree. The frequencies of light and energy released by activated dots make them particularly useful for chemical sensors, solar cells, medical imaging and nanoscale circuitry.

Singh and Penev calculated that removing islands of hydrogen from both sides of a graphane matrix leaves a well with all the properties of quantum dots, which may also be useful in creating arrays of dots for many applications.

"We arrived at these ideas from an entirely different study of energy storage in a form of hydrogen adsorption on graphene," Yakobson said. "Abhishek and Evgeni realized that this phase transformation (from graphene to graphane), accompanied by the change from metal to insulator, offers a novel palette for nanoengineering."

Their work revealed several interesting characteristics. They found that when chunks of the hydrogen sublattice are removed, the area left behind is always hexagonal, with a sharp interface between the graphene and graphane. This is important, they said, because it means each dot is highly contained; calculations show very little leakage of charge into the graphane host material. (How, precisely, to remove hydrogen atoms from the lattice remains a question for materials scientists, who are working on it, they said.)

"You have an atomlike spectra embedded within a media, and then you can play with the bandgap by changing the size of the dot," Singh said. "You can essentially tune the optical properties."

Along with optical applications, the dots may be useful in single-molecule sensing and could lead to very tiny transistors or semiconductor lasers, he said.

Challenges remain in figuring out how to make arrays of quantum dots in a sheet of graphane, but neither Singh nor Penev sees the obstacles as insurmountable.

"We think the major conclusions in the paper are enough to excite experimentalists," said Singh, who will soon leave Rice to become an assistant professor at the Indian Institute of Science in Bangalore. "Some are already working in the directions we explored."

"Their work is actually supporting what we're suggesting, that you can do this patterning in a controlled way," Penev said.

When might their calculations bear commercial fruit? "That's a tough question," Singh said. "It won't be that far, probably — but there are challenges. I don't know that we can give it a time frame, but it could happen soon."

Funding from the Office of Naval Research supported the work. Computations were performed at the Department of Defense Supercomputing Resource Center at the Air Force Research Laboratory.

For more information, visit:  www.rice.edu 




GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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