Graphene: high mobility, electromagnets, ballistic movement.
Researchers at the Georgia Institute of Technology and Tianjin University created a functional semiconductor made from graphene that is compatible with conventional microelectronics processing methods.
“We now have an extremely robust graphene semiconductor with 10 times the mobility of silicon, and which also has unique properties not available in silicon,” said Walter de Heer, professor of physics at Georgia Tech, in a release. “We were motivated by the hope of introducing three special properties of graphene into electronics. It’s an extremely robust material, one that can handle very large currents, and can do so without heating up and falling apart.”
The team figured out how to grow epitaxial graphene on silicon carbide wafers. When made properly, the epitaxial graphene chemically bonded to the silicon carbide and started to show semiconducting properties.
“A long-standing problem in graphene electronics is that graphene didn’t have the right band gap and couldn’t switch on and off at the correct ratio,” said Lei Ma, director at the Tianjin International Center for Nanoparticles and Nanosystems at Tianjin University, in a statement. “Over the years, many have tried to address this with a variety of methods. Our technology achieves the band gap, and is a crucial step in realizing graphene-based electronics.” [1]
Researchers from Helmholtz-Zentrum Dresden-Rossendorf, University of Duisburg-Essen, Institute of High Pressure Physics, Indian Institute of Technology, University of Maryland, and the U.S. Naval Research Laboratory found that firing short terahertz pulses at micrometer-sized discs of graphene turned them into surprisingly strong electromagnets.
“We were able to generate magnetic fields in the range of 0.5 Tesla, which is roughly ten thousand times the Earth’s magnetic field,” said HZDR physicist Dr. Stephan Winnerl, in a release. These were short magnetic pulses, only about ten picoseconds or one-hundredth of a billionth of a second long.
Winnerl noted a key advantage: “With our method, the magnetic field does not reverse polarity, as is the case with many other methods. It, therefore, remains unipolar.” The team sees potential for the discovery in magnetic storage technology and spintronics. [2]
Researchers from the University of Kansas investigated the ballistic movement of electrons in graphene in real time. “Generally, electron movement is interrupted by collisions with other particles in solids,” said Ryan Scott, a doctoral student in KU’s Department of Physics & Astronomy, in a statement. “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.’”
“Light can provide energy to an electron to liberate it so that it can move freely,” Hui Zhao, professor of physics & astronomy at KU, in a release. “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 researchers designed and fabricated a four-layer artificial structure with two graphene layers separated by two other single-layer materials, molybdenum disulphide and molybdenum diselenide.
“With this strategy, we were able to guide the electrons to one graphene layer while keeping their ‘seats’ in the other graphene layer,” Scott noted. “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.”
They found the electrons on average move ballistically for about 20-trillionths of a second with a speed of 22 kilometers per second before running into something that terminates their ballistic motion. [3]
[1] Zhao, J., Ji, P., Li, Y. et al. Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide. Nature 625, 60–65 (2024). https://doi.org/10.1038/s41586-023-06811-0
[2] Han, J.W., Sai, P., But, D.B. et al. Strong transient magnetic fields induced by THz-driven plasmons in graphene disks. Nat Commun 14, 7493 (2023). https://doi.org/10.1038/s41467-023-43412-x
[3] Ryan J. Scott, Pavel Valencia-Acuna, and Hui Zhao, Spatiotemporal Observation of Quasi-Ballistic Transport of Electrons in Graphene, ACS Nano 2023 17 (24), 25368-25376 https://dx.doi.org/10.1021/acsnano.3c08816
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