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Theoretical edge
ORNL team's nanoscale theory and simulation underpin collaborator's discoveries
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Nanoscale electrical circuitry is the goal of research into heterojunctions between metallic atoms imbedded in carbon nanotubes. |
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Vincent Meunier and Bobby Sumpter, both of the Computer Science & Mathematics Division and Center for Nanophase Materials Sciences, are co-authors on two papers that represent important advances in carbon-based materials research.
The researchers’ theory-based computational simulations have enabled collaborators at several institutions to understand why their experimental materials exhibit useful properties.
“Our collaborators are the best in the world at synthesis and characterization of these materials, but at ORNL we have the theory and simulation capabilities to confirm and develop an atomistic understanding of what they are seeing,” says Bobby.
An article in the March 26 issue of Science describes work on graphene, which is a single sheet of graphite that, because of its essentially two-dimensional structure, has unique electrical and magnetic characteristics. Nanoribbons made from graphene have the potential for nanoscale electronics applications.
However, the material is difficult to work with. Theoretical and experimental studies show that the edges of the material—such as a ribbon’s—strongly influence its properties. Lithographic or chemical etching of the material leaves rough edges, which disrupts the charge-carrier properties.
“Graphene has all the great properties of carbon nanotubes but is a far more simple material. However, you have to have smooth edges or it can compromise the electrical properties,” Bobby says.
“Like a coastline, the edges of the graphene are rocky, not straight, “Vincent says. “The experimenters devised a way to clean the edges, not just removing the ‘rocks,’ but restructuring them by injecting electrons. The effect of the current is like knitting the edges one after another. You can almost see it in real time under the microscope.”
Researchers at the Massachusetts Institute of Technology and the Laboratory for Nanoscience and Nanotechnology Research (LINAN) in Mexico used a method called Joule heating, in which a sample of graphene ribbon is suspended between a holder and a scanning-tunneling microscope tip. A current is then sent through the ribbon.
The resulting material displays sharp zig-zag and armchair-shaped edges as a result of the annealing process. Meunier and Sumpter’s computational simulations explained why the Joule heating had that effect on the materials—the carbon edges vaporize and then reconstruct at the higher, voltage-induced temperature, forming cleaner edges.
“There was no real explanation why it was working. Our simulations, with some quantum-based calculations , explained why it was happening that way,” Vincent says.
“Graphene is one of the hottest topics in the materials science literature. With this process, significant improvements can be obtained which open the door to graphene material for electronics and composite materials,” Bobby says.
The previous week, Proceedings of the National Academy of Sciences published another article with Sumpter and Meunier as co-authors. Their theoretical and simulation work, combined with experiments performed at the University of Strasbourg and LINAN, has contributed to a better understanding of how carbon nanotubes and metallic crystals can be used to build robust nanoscale contacts.
Researchers are interested in forming such contacts with carbon nanotubes and metal nanocrystals, since this is fundamental to the development of a multitude of electronic applications. Previously, in multiwalled carbon nanotube materials, the electrical connections between metallic crystals in the nanotube structures occurred only at the outermost wall. This rather feeble connectivity presented the technical barrier to fabricating a nanotube-metal composite material with suitable electrical properties.
“If you want to use nanowires in a device, you have to use it in the real, macroscopic world. At a nano level, these connections are pretty poor. This work shows an approach to connecting the nanoscale world to the ‘real,’ or macroscopic, world,” Vincent says.
Again, theoretical calculations and computational simulations by Vincent and Bobby revealed why a process of controlled electron irradiation results in heterojunctions with much stronger metal-to-metal interfaces.
“Our modeling enabled us to develop a complete understanding of the bonding at the interfaces seen by the experiments,” Bobby says.
The resulting “ultimate nanocontacts” appear very promising for making functional, nanotech-based electronic and ferromagnetic devices.
Vincent and Bobby’s collaborators on the graphene project (Science) were Xiaojing Jia, Mario Hoffman, Hyungbin Son, Ya-Ping Hsieh, Alfonso Reina, Jing Kong and Mildred Dresselhaus of MIT; and Jessica Campos-Delgado, Jose Manuel Romo-Herrera and Mauricio Terrones of Mexico’s LINAN.
The far-flung collaborators on the ultimate nanocontacts research (PNAS) are Julio Rodriguez-Manzo and Florian Banhart, Institut de Physique et Chimie des Materiaux in France; Mauricio Terrones and Humberto Terrones at LINAN, Mexico; Nicole Grobert at Oxford University, UK; Pulickel M. Ajayan at Rice University; Mingsheng Wang, Yoshio Bando and Dmitri Golberg at Japan’s National Institute for Materials Science.
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