Friday, October 28, 2011

New Nano-Ribbon Superior To Graphene Say Researchers

Graphene, which consists of a single layer of carbon atoms, has been heralded as tomorrow's super material for fast electronics and earned a Nobel Prize for its discoverer. Now two teams of researchers at LiU have each posed a solution that addresses how to progress and develop similar materials that retain even better properties.

Speed is the major benefit when using graphene as a semiconductor in electronic components. In the two-dimensional structure, the electrons travel 100 times faster than in silicon, today’s dominant semiconductor.

“But one major problem is that it consists of a large area that must be patterned, as is the case with silicon, says Sven Stafström, professor of computational physics at Linköping University.

His alternative is graphene in the form of well defined, seven carbon atoms wide nano-ribbons, produced by hydrocarbon anthracene. In conjunction with researchers Jonas Björk at LiU and Felix Hanke at the University of Liverpool, he has published a theoretical study that shows how graphene nanoribbons form from anthracene polymers on a gold substrate. 

The results of their study are published in a recent issue of the Journal of the American Chemical Society. The principle has also been demonstrated experimentally by a Swiss research team.

With the help of a powerful supercomputer at The National Supercomputer Centre in Sweden (NSC) at Linköping University, Jonas Björk has simulated the synthesis step by step. First, two bromine atoms are removed from the molecular building blocks, and this occurs at a temperature of 200 degrees Celsius. Benzene rings of six carbon atoms in each bind loosely to one another in a long free polymer that lacks conductivity. When the temperature is raised to 400 degrees Celsius, hydrogen atoms are attracted to the gold surface and this leaves room for a carbon-carbon bond.

The reaction continues in a kind of domino effect that gradually requires less and less additional energy. The result is a linear chain of carbon atoms in the honeycomb pattern.
Credit: Linkoeping Universitet

“The final product retains very good conductivity with fast-moving electrons. We get the benefits from graphene, but none of the patterning, says Sven Stafström.

The aim is to be able to customize nanoribbons for different applications by controlling the size and shape of the molecular building blocks.

Another variant on the graphene is the theme proposed by, among others Michel Barsoum and Lars Hultman, visiting professor and professor of thin film physics at Linköping University. Their premise is that the very simple chemistry of the graphene limits the material’s area of use. A more complex structure would provide more flexible features.

In his laboratory at Drexel University, Barsoum began experimenting with the MAX-phases, of which he was one of the discoverers. These are materials consisting of carbon or nitrogen and two other elements in a stored structure.
Credit: Linkoeping Universitet

The experiment was based on flakes of titanium-aluminium-carbide (Ti3AlC2). When the researchers removed aluminium a new two-dimensional material remained; similar to graphene but with two types of atoms, carbon and titanium. Through advanced electron microscopy at Linköping University, researchers have shown how exfoliation of the MAX phase to carbide flakes works.

Scientists predict that a large number of chemical variants can be produced. The material which has been named "MXene" can be used in energy and electronics components, such as electrodes in environmentally friendly lithium-ion battery electrodes.

Contacts and sources:
Graphene Ribbons: Jonas Björk, Dept. for Computational Physics
MXene: Lars Hultman, Dept. for thin film physics
Linkoeping Universitet

Jonas Bjork, Felix Hanke and Sven Stafström. Zipping Up: Cooperative Drives the Synthesis of Graphene Ribbons The Journal of the American Chemical Society, September 2011.

Michael Naguib, Murat Kurtoglu, Volker Presser, Jun Lu, Junjie Niu, Min Heon, Lars Hultman, Yury Gogotsi, and Michel W. Barsoum: Two-Dimensional Nanocrystals Produced by exfoliation of Ti3AlC2. Advanced Materials online 22 August 2011.

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