Researchers from Harvard University and Raytheon BBN Technology have observed for the first time electrons in graphene behaving like a fluid. Image: Peter Allen/Harvard SEAS.
Researchers from Harvard University and Raytheon BBN Technology have observed for the first time electrons in graphene behaving like a fluid. Image: Peter Allen/Harvard SEAS.

Since its discovery a decade ago, scientists and tech gurus have hailed graphene as a wonder material that could replace silicon in electronics, increase the efficiency of batteries, enhance the durability and conductivity of touch screens, and pave the way for cheap thermal electric energy. Graphene is one atom thick, stronger than steel, harder than diamond and one of the most conductive materials on Earth.

But several challenges must be overcome before graphene products are brought to market. Scientists are still trying to understand the basic physics of this unique material. In addition, it's very challenging to make, and even harder to make without impurities.

In a new paper published in Science, researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences (SEAS) and Raytheon BBN Technology have advanced our understanding of graphene's basic properties. For the first time, they have observed electrons in graphene behaving like a fluid.

In order to make this observation, the team had to develop improved methods for creating ultra-clean graphene and a new way measure its thermal conductivity. This research could lead to novel thermoelectric devices, as well as provide a model system for exploring exotic phenomena like black holes and high-energy plasmas. This research was led by Philip Kim, professor of physics and applied physics at SEAS.

In ordinary, three-dimensional metals, electrons hardly interact with each other. But graphene's two-dimensional, honeycomb structure acts like an electron superhighway in which all the particles have to travel in the same lane. The electrons in graphene act like massless relativistic objects, some with positive charge and some with negative charge. They move at incredible speed – 1/300 of the speed of light – and have been predicted to collide with each other ten trillion times a second at room temperature. These intense interactions between charged particles have never been observed in an ordinary metal before.

The team fabricated an ultra-clean sample of graphene by sandwiching a single sheet between tens of layers of an electrically-insulating transparent crystal with a similar atomic structure to graphene. "If you have a material that's one atom thick, it's going to be really affected by its environment," explained Jesse Crossno, a graduate student in the Kim Lab and first author of the paper. "If the graphene is on top of something that's rough and disordered, it's going to interfere with how the electrons move. It's really important to create graphene with no interference from its environment."

The fabrication technique they used was developed by Kim and his collaborators at Columbia University before he moved to Harvard University in 2014, and has now been perfected in his lab at SEAS.

Next, the team set up a kind of thermal soup of positively-charged and negatively-charged particles on the surface of the graphene, and observed how these particles flowed as thermal and electric currents. This revealed that when the strongly interacting particles in graphene were driven by an electric field, they behaved not like individual particles but like a fluid that could be described by hydrodynamics. "Instead of watching how a single particle was affected by an electric or thermal force, we could see the conserved energy as it flowed across many particles, like a wave through water," said Crossno.

This means that a small chip of graphene could be used to model the fluid-like behavior of other high-energy systems like supernovas and black holes. "Physics we discovered by studying black holes and string theory, we're seeing in graphene," said Andrew Lucas, co-author and graduate student with Subir Sachdev, a professor of physics at Harvard University. "This is the first model system of relativistic hydrodynamics in a metal."

In order to observe this hydrodynamic system, the team first needed to develop a precise way to measure how well electrons in the system carry heat. This is very difficult to do, said Kin Chung Fong, a scientist with Raytheon BBN Technology.

Materials conduct heat in two ways: through vibrations in their atomic structure or lattice; and carried by the electrons themselves. "We needed to find a clever way to ignore the heat transfer from the lattice and focus only on how much heat is carried by the electrons," Fong said.

To do so, the team turned to noise. At finite temperatures, the electrons move about randomly: the higher the temperature, the noisier the electrons. By measuring the temperature of the electrons to three decimal points, the team was able to precisely measure the thermal conductivity of the electrons.

"Converting thermal energy into electric currents and vice versa is notoriously hard with ordinary materials," said Lucas. "But in principle, with a clean sample of graphene there may be no limit to how good a device you could make."

This story is adapted from material from the John A. Paulson School of Engineering and Applied Sciences, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.