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Researchers Simulate Detailed Behavior of Electrons in Graphene Nanoribbons

Achieving further reductions in the size of electronic components requires new materials. This would help make devices such as smartphones and laptops faster and more efficient. Minuscule nanostructures of the novel material graphene show potential in this respect.

The graphene nanoribbon (center) consists of a single layer of honeycomb carbon atoms. The ribbon is only a few carbon atoms wide and has different electrical properties depending on its shape and width. The local density of the electrons is increased at the edges, as the dark red areas in the boxes show. Image Credit: © Jan-Philip Joost, AG Bonitz.

Graphene comprises a single layer of carbon atoms and, apart from other things, has excellent electrical conductivity. But the higher level of spatial confinement in such nanostructures intensely impacts their electronic properties.

A team guided by Professor Michael Bonitz of the Institute for Theoretical Physics and Astrophysics (ITAP) at the Christian-Albrechts-Universität zu Kiel (CAU) has successfully mimicked the in-depth behavior of electrons in these special nanostructures using an advanced computational model. This knowledge is vital for the potential application of graphene nanostructures in electronic devices.

Precise Simulation of the Properties of Electrons in Nanostructures

Two research teams were independently successful last year in fabricating narrow, atomically precise graphene nanoribbons and measuring their electron energies. The width of the nanoribbons differs in a precisely regulated manner. Each section of the nanoribbons includes its own energy states with their own electronic structure.

However, the measurement results could not be completely reproduced by previous theoretical models.

Michael Bonitz, Professor, Institute for Theoretical Physics and Astrophysics, Christian-Albrechts-Universität zu Kiel

Bonitz also heads the Chair of Statistical Physics at ITAP.

Along with his PhD student Jan-Philip Joost and their Danish colleague Professor Antti-Pekka Jauho from the Technical University of Denmark (DTU), they created an enhanced model, which resulted in a very good agreement with the experiments. The physicists reported their theoretical results in the recent issue of the distinguished journal Nano Letters.

The origin of the new and more accurate computer simulations was the hypothesis that the deviations between the experiment and earlier models were caused by the details of the mutual repulsion of the electrons.

Although this Coulomb interaction, which also occurs in metals, was in fact roughly included in previous simulations, the effect is a lot greater in the small graphene nanoribbons, and requires a comprehensive analysis. The electrons are evicted from their primary energy states and have to “search” for other places.

We were able to prove that correlation effects due to the Coulomb interaction of the electrons have a dramatic influence on the local energy spectrum.

Michael Bonitz, Professor, Institute for Theoretical Physics and Astrophysics, Christian-Albrechts-Universität zu Kiel

The Shape of Nanoribbons Determines Their Electronic Properties

The way the permissible energy values of the electrons rely on the width, length, and shape of the nanostructures has been verified by the researchers by analyzing many such nanoribbons. “The energy spectrum also changes when the geometry of the nanoribbons, their width and shape, is modified,” adds Joost.

For the first time, our new data allow precise predictions to be made as to how the energy spectrum can be controlled by specifically varying the shape of the nanoribbons.

Antti-Pekka Jauho, Professor, Technical University of Denmark

The team believes that these predictions will also be verified experimentally and set in motion the development of new nanostructures. Such systems could have significant implications for the further miniaturization of electronics.


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