Ripples warp graphene’s conductivity

U. BUFFALO (US) — Folds or bends in graphene act like construction zones in a superhighway—making it difficult for electric charges to travel smoothly through.

Graphene is the thinnest and strongest material known to man, consisting of a single layer of carbon atoms linked in a honeycomb-like arrangement.

The material’s structure makes it incredibly conductive: Under ideal circumstances, when graphene is completely flat, electric charges speed through it without encountering many obstacles.

But conditions are not always optimal.

Chemists have used synchrotron light sources to show how when graphene is bent, the electron cloud lining its surface becomes folded as well, harming its conductivity.

“When we imaged the electron cloud, you can imagine this big fluffy pillow, and we saw that the pillow is bent here and there,” says Sarbajit Banerjee, assistant professor of chemistry at the University at Buffalo.

To create the images and understand the factors perturbing the electron cloud, Banerjee and colleagues employed two techniques that required use of a synchrotron: scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (NEXAFS), a type of absorption spectroscopy.

The experiments were further supported by computer simulations performed on computing clusters at the Lawrence Berkeley National Laboratory.

“Using simulations, we can better understand the measurements our colleagues made using X-rays, and better predict how subtle changes in the structure of graphene affect its electronic properties,” says David Prendergast, a staff scientist at Berkeley Lab.

“We saw that regions of graphene were sloped at different angles, like looking down onto the slanted roofs of many houses packed close together.”

Besides documenting how folds in graphene distort its electron cloud, researchers discovered that contaminants that cling to graphene during processing linger in valleys where the material is uneven, uniquely distorting the electron cloud by changing the strength with which its bound to the underlying atoms.

Because graphene’s electrical conductivity matches that of copper, and because its thermal conductivity is the best of any known material, the material has generated excitement in industries including computing, energy, and defense.

But the new research, published in the journal Nature Communications, suggests that companies hoping to incorporate graphene into products such as conductive inks, ultrafast transistors, and solar panels could benefit from more basic research on the nanomaterial. Improved processes for transferring flat sheets of graphene onto commercial products could greatly increase those products’ efficiency.

“A lot of people know how to grow graphene, but it’s not well understood how to transfer it onto something without it folding onto itself,” Banerjee says. “It’s very hard to keep straight and flat, and our work is really bringing home the point of why that’s so important.”

Dotted lines show distinctive regions of graphene that are sloped at different angles. (Credit: Brian J. Schultz)

“Graphene is going to be very important in electronics,” says PhD candidate Brian Schultz, one of three graduate students who were lead authors on the study.

“It’s going to be one of the most conductive materials ever found, and it has the capability to be used as an ultrahigh-frequency transistor or as a possible replacement for silicon chips, the backbone of current commercial electronics.

“When graphene was discovered, people were just so excited that it was such a good material that people really wanted to go with it and run as fast as possible.

“But what we’re showing is that you really have to do some fundamental research before you understand how to process it and how to get it into electronics.”

The research was supported in part by the Department of Energy Office of Science and a New York State Energy Research and Development Authority grant.

More news from University at Buffalo: