David Kirkpatrick

May 26, 2010

Graphene as quantum dots

Nanoelectronics is a major — and important — field right now, and graphene and its cousin graphane are very important materials research components. Both of the nanomaterials are getting a lot of  hype, particularly graphene, but there’s far too much smoke for there not to be at least a little fire. It’s exciting to keep watch on the news to see the breakthroughs as they happen, and eventually cover real-world, market-ready uses for graphene and graphane.

The release:

Graphane yields new potential

Rice physicists dig theoretical wells to mine quantum dots

Graphane is the material of choice for physicists on the cutting edge of materials science, and Rice University researchers are right there with the pack – and perhaps a little ahead.

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.

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 and will be on the cover of the print version in June. Rice was recently named the world’s No. 1 institution for materials science research by a United Kingdom publication.

Graphene has become the Flat Stanley of materials. The one-atom-thick, honeycomb-like form of carbon may be two-dimensional, but it seems to be everywhere, 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 band gap – 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 atom-like spectra embedded within a media, and then you can play with the band gap 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.”

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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.

July 31, 2009

Introducing graphane

Filed under: Science — Tags: , , , , , — David Kirkpatrick @ 3:58 pm

I’ve done plenty of blogging on the nanomaterial graphene, now here’s an introduction to graphane, its insulating offshoot. Just like with graphene, there’s high hopes for graphane applications.

The release:

From graphene to graphane, now the possibilities are endless

Ever since graphene was discovered in 2004, this one-atom thick, super strong, carbon-based electrical conductor has been billed as a “wonder material” that some physicists think could one day replace silicon in computer chips.

But graphene, which consists of carbon atoms arranged in a honeycomb lattice, has a major drawback when it comes to applications in electronics – it conducts electricity almost too well, making it hard to create graphene-based transistors that are suitable for integrated circuits.

In August’s Physics World, Kostya Novoselov – a condensed-matter physicist from the Manchester University group that discovered graphene — explains how their discovery of graphane, an insulating equivalent of graphene, may prove more versatile still.

Graphane has the same honeycomb structure as graphene, except that it is “spray-painted” with hydrogen atoms that attach themselves to the carbon. The resulting bonds between the hydrogen and carbon atoms effectively tie down the electrons that make graphene so conducting. Yet graphane retains the thinness, super-strength, flexibility and density of its older chemical cousin.

One advantage of graphane is that it could actually become easier to make the tiny strips of graphene needed for electronic circuits. Such structures are currently made rather crudely by taking a sheet of the material and effectively burning away everything except the bit you need. But now such strips could be made by simply coating the whole of a graphene sheet – except for the strip itself – with hydrogen. The narrow bit left free of hydrogen is your conducting graphene strip, surrounded by a much bigger graphane area that electrons cannot go down.

As if this is not enough, the physicists in Manchester have found that by gradually binding hydrogen to graphene they are able to drive the process of transforming a conducting material into an insulating one and watch what happens in between.

Perhaps most importantly of all, the discovery of graphane opens the flood gates to further chemical modifications of graphene. With metallic graphene at one end and insulating graphane at the other, can we fill in the divide between them with, say, graphene-based semiconductors or by, say, substituting hydrogen for fluorine?

As Professor Novoselov writes, “Being able to control the resistivity, optical transmittance and a material’s work function would all be important for photonic devices like solar cells and liquid-crystal displays, for example, and altering mechanical properties and surface potential is at the heart of designing composite materials. Chemical modification of graphene – with graphane as its first example – uncovers a whole new dimension of research. The capabilities are practically endless.”

 

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