Ultrafast lasers map electrons ‘going ballistic’ in graphene, with implications for next-generation electronic devices

Research published in ACS Nano reveals the ballistic movement of electrons in graphene in real time.

Observations at the University of Kansas Ultrafast Laser Laboratory could lead to breakthroughs in the management of electrons in semiconductors, fundamental components of most information and energy technologies.

“Typically, the movement of electrons is interrupted by collisions with other particles in solids,” said lead author Ryan Scott, a doctoral student in KU’s Department of Physics and Astronomy.

“It’s like someone running in a ballroom full of dancers. These collisions are quite frequent, around 10 to 100 billion times per second. They slow down the electrons, cause a loss of energy and generate energy. unwanted heat. Without collisions, an electron moves continuously in a solid, similar to cars on a highway or ballistic missiles in the air. We call this “ballistic transport.”

Scott carried out the laboratory experiments under the mentorship of Hui Zhao, professor of physics and astronomy at KU. They were joined in the work by Pavel Valencia-Acuna, a former KU doctoral student who is now a postdoctoral researcher at the Pacific Northwest National Laboratory.

Zhao said electronic devices using ballistic transport could potentially be faster, more powerful and more energy efficient.

“Current electronic devices, such as computers and telephones, use silicon-based field-effect transistors,” Zhao said. “In such devices, electrons can only drift at a speed on the order of a few centimeters per second due to the frequent collisions they encounter. Ballistic transport of electrons in graphene can be used in devices to fast speed and low power consumption.”

KU researchers observed the ballistic movement of graphene, a promising material for next-generation electronic devices. First discovered in 2004 and awarded the Nobel Prize in Physics in 2010, graphene is made up of a single layer of carbon atoms forming a hexagonal lattice structure, much like a football net.

“Graphene’s electrons move as if their ‘effective’ mass were zero, making them more likely to avoid collisions and move ballistically,” Scott said. “Previous electrical experiments, studying electric currents produced by voltages under various conditions, have revealed signs of ballistic transport. However, these techniques are not fast enough to track electrons as they move.”

According to the researchers, the electrons in graphene (or any other semiconductor) are like students sitting in a full classroom, where students cannot move freely because the desks are full. Laser light can free electrons to temporarily leave a desk, or “hole,” as physicists call them.

“Light can provide energy to an electron to release it so it can move freely,” Zhao explained. “This is similar to allowing a student to stand up and move away from their seat. However, unlike a neutrally charged student, an electron is negatively charged. Once the electron leaves its ‘seat,’ the seat becomes positively charged and quickly pulls the electron backwards, causing the moving electrons to disappear, as if the student were sitting down again.

Because of this effect, graphene’s ultralight electrons can only remain mobile for about a trillionth of a second before falling back into place. This short time frame presents a significant challenge for observing the movement of electrons. To address this problem, KU researchers designed and fabricated an artificial four-layer structure with two layers of graphene separated by two other single-layer materials, molybdenum disulfide and molybdenum diselenide.

“With this strategy, we were able to guide electrons to one layer of graphene while keeping their ‘seats’ in the other layer of graphene,” Scott said. “Separating them by two layers of molecules, with a total thickness of just 1.5 nanometers, requires the electrons to remain mobile for about 50 trillionths of a second, long enough for the researchers, equipped with lasers as fast as 0.1 trillionths of second., to study how they move.

Researchers use a tightly focused laser spot to release certain electrons from their sample. They trace these electrons by mapping the sample’s “reflectance,” or the percentage of light they reflect.

“We see most objects because they reflect light back to our eyes,” Scott said.

“Brighter objects have greater reflectance. On the other hand, dark objects absorb light, which is why dark clothes become hot in summer. When a moving electron moves to a certain location in the sample, it makes that spot slightly brighter by changing the way it moves.” “The electrons there interact with light. The effect is very small: even with everything optimized, one electron only changes the reflectance by 0.1 parts per million.”

To detect such a small change, the researchers released 20,000 electrons at a time, using a laser probe to reflect off the sample and measure that reflectance, repeating the process 80 million times for each data point. They found that electrons move ballistically on average for about 20 trillionths of a second at a speed of 22 kilometers per second before hitting something that ends their ballistic motion.