Using low temperatures to probe 2D materials for device applications

My computer always seems to run slow. No matter how suped-up it is, I still seem to waste time waiting for applications and documents to open.

Imagine if these devices ran 100 times faster?

Our computers and other electronic devices are, at its essence, made of microcircuits, highly miniaturized integrated circuits. These devices cram hundreds, thousands, millions, or even billions of electronic components onto tiny chips of silicon no bigger than a fingernail. At its core, it is a vehicle for electrical conduction and transport.

So the speed electricity travels through these microcircuits partially determines the speeds that your electronic devices function.

The fields of research look into fundamental properties of better materials for these devices and environments in which these microcircuits operate. That’s the focus of Rui He’s work.

He is a Ph.D. in applied physics and an Associate Professor in Electrical and Computer Engineering at Texas Tech University in Lubbock, Texas. The dry, flat, sandstorm prone city is home to research that takes a different approach to exploring physical properties of the best materials to use in devices.

“People strive to make devices faster and more energy efficient,” she said. “If we can study the fundamental properties and see if proposed materials would be suitable for these types of applications, it can help people design or make devices.”

He studies the vibrational, electronic, and magnetic properties of various materials at low temperatures: about 10 degrees Kelvin, which is equal to -263.15 Celsius or -441.67 Fahrenheit. That’s close to absolute zero.

Why test the materials at such low temperatures?

“Physical properties of materials may change significantly as a function of temperature,” she said. “For instance, lattice structures may change due to a structural phase transition, and some materials exhibit magnetic properties at low temperatures even under a zero external magnetic field. One of the examples is chromium triiodide which becomes magnetic at temperatures below about 45 Kelvin. In order to probe these materials’ physical properties, we need to cool the samples down to a low temperature.”

But why study the materials at such low temperatures, when most of the actual devices will have to function at room temperature?

“Right now, we are still at the beginning,” she said. “These properties may only exist at low temperatures, but with the rapid advance of fabrication techniques, characterization, and fundamental studies of materials, we hope to see materials that can exhibit desired electronic and magnetic properties at higher temperatures. “

He works with 2D materials. Those are layered materials whose atoms within the layers are strongly bonded, whereas the interactions between each layer are much weaker. By controlling the number of atomic layers, you could tune the optical and electronic properties. She studies traditional 2D materials like graphene and molybdenum disulfide. Recently she expands her studies to novel 2D magnetic materials like chromium triiodide. It has magnetic properties below a critical temperature.

He, in collaboration with her colleagues, Dr. Liuyan Zhao of University of Michigan and Dr. Adam Tsen of University of Waterloo, Canada, recently had their research findings published in Nature Communications, a multidisciplinary journal.

“The research we published is to report our recent study on magnetic excitations in a ferromagnetic – a substance with high magnetic permeability – 2D material chromium triiodide," He said. "We discovered terahertz frequency spin waves, which are much higher in frequency than those in conventional ferromagnetic materials.”

This, He said, will open up new opportunities for making ultrafast spintronic devices – devices that use electrons' spins for realizing spin currents that are much faster than gigahertz spintronic devices and much more energy efficient than electronics currently on the market.

"Computers run with gigahertz processors, so if we can use the technology based on the new 2D ferromagnetic materials, then we may bring the processing up to run 100 times faster or even more," He said. "Spintronics could also help reduce the heat from these devices and lower the power consumption."

He uses Raman spectroscopy to study the physical properties of her samples. She has a HORIBA LabRAM HR Evolution; a confocal Raman microscope ideally suited for both micro and macro measurements. It also offers advanced confocal imaging capabilities in 2D and 3D.

“Raman allows us to study a broad range of material properties and it's a nondestructive technique,” He said. “That makes it very important for characterization and fundamental studies of materials. With the ultra-low frequency features that we have, we can probe ultra-low frequency excitations in the material and study diverse properties of materials from interlay interactions to electronic and magnetic excitations.”

It might be years before the principles He is uncovering will go into practical use. She is working at a very basic science level, doing fundamental research.

“And then there are engineers who use these properties and try to build better devices,” she said.

Follow Us