Ultrafast laser moves graphene electrons at 22 km a second

Ballistic movements of electrons in graphene have been mapped at the University of Kansas Ultrafast Laser Lab. The research could have implications for next-generation technologies through controlling electrons in semiconductors.

“Generally, electron movement is interrupted by collisions with other particles in solids,” says doctoral student Ryan Scott, lead author of a paper on the research published in the journal ACS Nano.

“This is similar to someone running in a ballroom full of dancers. These collisions are rather frequent – about 10 to 100 billion times per second. They slow down the electrons, cause energy loss, and generate unwanted heat. Without collisions, an electron would move uninterrupted within a solid, similar to cars on a freeway or ballistic missiles through the air. We refer to this as ‘ballistic transport.’”

Using ballistic transport in electronic devices could make them faster, more powerful and more energy efficient.

“Current electronic devices, such as computers and phones, utilize silicon-based field-effect transistors,” says physics professor Hui Zhao. “In such devices, electrons can only drift with a speed on the order of centimetres per second due to the frequent collisions they encounter.”

Graphene is a single layer of carbon atoms arranged in an hexagonal lattice. It was first produced in 2004 and its discoverers were awarded the Nobel Prize in Physics in 2010.

“Electrons in graphene move as if their ‘effective’ mass is zero, making them more likely to avoid collisions and move ballistically,” Scott explains. “Previous electrical experiments, by studying electrical currents produced by voltages under various conditions, have revealed signs of ballistic transport. However, these techniques aren’t fast enough to trace the electrons as they move.”

The physicists induced the electrons to move in the graphene by giving them extra energy with a laser. But the electron is only mobile for about one trillionth of a second, making it extremely difficult to observe the effects. When the electron is “liberated” from its slot in the graphene layer, the positively-charged “hole” left behind drags the negatively-charged electron back down to its original place.

To overcome this problem, the physicists created a four-layer structure with two graphene layers separated by two other single-layer materials – molybdenum disulphide and molybdenum diselenide.

Electrons were freed in one graphene layer, while electrons in the other remained stationery.

“Separating them with two layers of molecules, with a total thickness of just 1.5 nanometres, forces the electrons to stay mobile for about 50-trillionths of a second, long enough for the researchers, equipped with lasers as fast as 0.1 trillionths of a second, to study how they move,” Scott says.

On average, the electrons moved ballistically for about 20 trillionths of a second. Their top speed was 22 kilometres per second before running into something, ending their ballistic run.