Real-time visualization of the ballistic movement of electrons in graphene is provided by research published in ACS Nano.
Ultrafast Laser Lab. Image Credit: KU Marketing Communications
The findings, obtained at the Ultrafast Laser Lab at the University of Kansas, could open the way for significant advancements in the control of semiconductor electrons, which are essential elements of most information and energy technologies.
Generally, electron movement is interrupted by collisions with other particles in solids. 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 air. We refer to this as ‘ballistic transport.
Ryan Scott, Study Lead Author and Doctoral Student, Department of Physics & Astronomy, The University of Kansas
Scott worked with Hui Zhao, a KU physics and astronomy professor, as his mentor while conducting the lab experiments. Pavel Valencia-Acuna, a former Ph.D. student at KU who is currently a postdoctoral researcher at the Northwest Pacific National Laboratory, joined them in their study.
Ballistic transfer, according to Zhao, could enable quicker, more potent, and more energy-efficient electronic gadgets.
Current electronic devices, such as computers and phones, utilize silicon-based field-effect transistors. In such devices, electrons can only drift with a speed on the order of centimeters per second due to the frequent collisions they encounter. The ballistic transport of electrons in graphene can be utilized in devices with fast speed and low energy consumption.
Hui Zhao, Professor, Physics and Astronomy, The University of Kansas
Graphene is composed of a single layer of carbon atoms, creating a hexagonal lattice structure. It was first discovered in 2004 and was given the Nobel Prize in Physics in 2010. Graphene exhibits ballistic mobility, as demonstrated by KU researchers.
Scott added, “Electrons in graphene move as if their ‘effective’ mass is zero, making them more likely to avoid collisions and move ballistically. 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 researchers compared electrons in graphene (or any other semiconductor) to students seated in a packed classroom with desks occupied, preventing students from moving about freely. Physicists refer to these desks as “holes,” and the laser light has the ability to temporarily release electrons from them.
Zhao added, “Light can provide energy to an electron to liberate it so that it can move freely. This is similar to allowing a student to stand up and walk away from their seat. However, unlike a charge-neutral student, an electron is negatively charged. Once the electron has left its ‘seat,’ the seat becomes positively charged and quickly drags the electron back, resulting in no more mobile electrons — like the student sitting back down.”
The super-light electrons in graphene can only remain mobile for a minuscule fraction of a second due to this effect before returning to their original position. This little period of time makes it extremely difficult to see the electrons’ motion. To solve this issue, the KU scientists created a four-layer artificial structure comprising two graphene layers divided by two additional single-layer materials: molybdenum diselenide and molybdenum disulfide.
“With this strategy, we were able to guide the electrons to one graphene layer while keeping their ‘seats’ in the other graphene layer. Separating them with two layers of molecules, with a total thickness of just 1.5 nanometers, 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 trillionth of a second, to study how they move,” Scott noted.
To release part of the electrons in their sample, the researchers utilize a laser point that is precisely concentrated. By charting the sample’s “reflectance,” or the fraction of light it reflects, they are able to trace these electrons.
Scott further stated, “We see most objects because they reflect light to our eyes. Brighter objects have larger reflectance. On the other hand, dark objects absorb light, which is why dark clothes become hot in the summer. When a mobile electron moves to a certain location of the sample, it makes that location slightly brighter by changing how electrons in that location interact with light. The effect is very small — even with everything optimized, one electron only changes the reflectance by 0.1 part per million.”
The researchers released 20,000 electrons at once to detect such a minute change. They then measured the sample’s reflectance by reflecting a probe laser off it, repeating the procedure 80 million times for every data point. They discovered that before encountering an object that stops its ballistic motion, electrons travel ballistically for an average of 22 kilometers per second, or 20 trillionths of a second.
A grant from the Department of Energy’s Physical Behavior of Materials program provided funding for the study.
According to Zhao, his group is presently attempting to improve the material design to direct electrons more effectively to the appropriate graphene layer and is also looking for ways to increase the ballistic distance that electrons can travel.
Journal Reference:
Scott, R. J., et. al. (2023) Spatiotemporal Observation of Quasi-Ballistic Transport of Electrons in Graphene. ACS Nano. doi:10.1021/acsnano.3c08816.