Understanding graphene’s electrical properties at the atomic level

"If you cut it one way, it might behave more like a metal, and, if you cut it another way, it could be more like a semiconductor."
July 28, 2014

An illustration of a graphene nanoribbon shaped by the beam of a transmission electron microscope (credit: Robert Johnson)

University of Pennsylvania researchers have used a cutting-edge microscope to study the relationship between the atomic geometry of a ribbon of graphene and its electrical properties.

A deeper understanding of this relationship will be necessary for the design of graphene-based integrated circuits, computer chips, and other electronic devices.

The study was published in the journal Nano Letters.

The researchers used Brookhaven National Laboratory‘s aberration-corrected transmission electron microscope (AC-TEM). By focusing the microscope’s electron beam, the researchers were able to controllably cut sheets of graphene into ribbons with widths as small as 10 nanometers, while keeping them connected to an electrical source outside the microscope.

They then could use the AC-TEM’s nanoscopic resolution to distinguish between individual carbon atoms within those ribbons. This level of precision was necessary to determine how the carbon atoms on the edges of the nanoribbons were oriented.

Edge shape defines conductor or semiconductor properties

“We’re relating the structure of the graphene — its atomic arrangement — to its electrical transport properties,” said Drndić. “In particular, we were looking at the edges, which we were able to identify the geometry of.”

“Graphene looks like chicken wire, and you can cut up this hexagonal lattice of carbon atoms in different ways, producing different shapes on the edge,” she said. “But if you cut it one way, it might behave more like a metal, and, if you cut it another way, it could be more like a semiconductor.”

For any piece of graphene, either the pointy or flat sides of its carbon hexagons might be at the piece’s edge. Where the pointy sides face outward, the edge has a “zig-zag” pattern. Flat sides produce “armchair” pattern when they are on an edge. Any given edge might also display a mix of the two, depending on how the piece of graphene was initially cut and how that edge degrades under stress.

Because the graphene nanoribbons were connected to an electrical source while they were inside the AC-TEM, the researchers were able to simultaneously trace the outline of the ribbons and measure their conductivity. This allowed the two figures to be correlated.

“If you want to use graphene nanoribbons in computer chips, for example, you absolutely need to have this information,” Johnson said. “People have looked at these ribbons under the microscope, and people have measured their electrical properties without looking at them but never both at the same time.”

After studying the nanoribbons with relatively low levels of electron flow, the researchers turned up the intensity. The combination of the electron bombardment from the microscope and the large amount of electrons flowing through the nanoribbons caused their structures to gradually degrade. As carbon bonds within the nanoribbons broke, they became thinner and the shape of their edges changed, providing additional data points.

“By doing everything within the microscope,” Rodríguez-Manzo said, “we can just follow this transformation to the end, measuring currents for the nanoribbons even when they get smaller than 1 nanometer across. That’s five atoms wide.”

This kind of stress testing is critical to the future design of graphene electronics.

“We have to see how much current we can transport before these nanoribbons fall apart. Our data shows that this figure is high compared to copper,” Rodríguez-Manzo said.

Graphene loops: ideal interconnects

The harsh conditions also caused some of the ribbons to fold up on themselves, producing nanoscopic graphene loops. Serendipitously, the team found that these loops had desirable properties.

“When the edges wrap around and form the loops we see,” Johnson said, “it helps hold the structure together, and it makes the current density a thousand higher than what is currently state of the art. That structure would be useful in making interconnects [which are the conducting paths that connect transistors together in integrated circuits].”

Future research in this field will involve directly comparing the electrical properties of graphene nanoribbons with different widths and edge shapes.

“Once we can cut these nanoribbons atom by atom,” Drndić said, “there will be a lot more we can achieve.”

The Penn team collaborated with researchers at the Université Catholique de Louvain in Belgium and Seoul National University in South Korea. The research was supported by the National Science Foundation, the National Institutes of Health, the U.S. Department of Energy, Belgium’s Fonds de la Recherche Scientifique, South Korea’s Ministry of Education, Science and Technology and National Research Foundation and the European Union’s Graphene Flagship Consortium.

University of Pennsylvania | Draw a line with a pencil and it’s likely that somewhere along that black smudge is a material that earned two scientists the 2010 Nobel Prize in Physics. The graphite of that pencil tip is simply multiple layers of carbon atoms; where those layers are only one atom thick, it is known as graphene.The properties of a material change at the nanoscopic scale, making graphene the strongest and most conductive substance known. Instead of marking mini-golf scores on paper, this form of carbon is suited for making faster and smaller electronic circuitry, flexible touchscreens, chemical sensors, diagnostic devices, and applications yet to be imagined. Graphene is not yet as ubiquitous as plastic or silicon, however, and producing the material in bulk remains a challenge. Because graphene’s properties rely on it being only one atom thick, until recently, it was only possible to make it in small patches or flakes.Physicists at Penn have discovered a way around these limitations, and have spun out their research into a company called Graphene Frontiers.

Abstract of Nano Letters paper

Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components; this fact motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (<10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. Few-layer GNRs very frequently formed bonded-bilayers and were remarkably robust, sustaining currents in excess of 1.5 μA per carbon bond across a 5 atom-wide ribbon. We found that the intrinsic conductance of a sub-10 nm bonded bilayer GNR scaled with width as GBL(w) ≈ 3/4(e2/h)w, where w is the width in nanometers, while a monolayer GNR was roughly five times less conductive. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry. Finally, we report on simulations of quantum transport in GNRs that are in qualitative agreement with the observations.