New 2D material spins at the edges

An international team of researchers has fabricated an atomically thin material and shown that its exotic and durable properties make it a promising candidate for a budding branch of electronics known as ‘spintronics’. The team included researchers from the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), the University of California, Berkeley and Stanford University.

The material – known as 1T'-WTe₂ – bridges two flourishing fields of research. One is two-dimensional (2D) materials, which include monolayer materials such as graphene that behave in different ways than their thicker forms. The other is topological materials, in which electrons can zip around in predictable ways with next to no resistance and no regard for defects that would ordinarily impede their movement. At the edges of 1T'-WTe₂, the spin of electrons – a particle property that functions a bit like a compass needle pointing either north or south – and their momentum are closely tied and predictable.

This latest experimental evidence shows the potential for using 1T'-WTe₂ as a test subject in next-gen applications, such as a new breed of electronic devices that manipulate its spin property to carry and store data more efficiently than present-day devices. These traits are fundamental to the field known as spintronics.

"This material should be very useful for spintronics studies," said Sung-Kwan Mo, a physicist and staff scientist at Berkeley Lab's Advanced Light Source (ALS) who co-led the study, which is reported in a paper in Nature Physics.

The material is called a topological insulator because its interior surface does not conduct electricity, restricting its electrical conductivity (the flow of electrons) to the edges.

"The flow of electrons is completely linked with the direction of their spins, and is limited only to the edges of the material," Mo explained. "The electrons will travel in one direction, and with one type of spin, which is a useful quality for spintronics devices." Such devices could conceivably carry data more efficiently, with lesser power demands and heat build-up than is typical for present-day electronic devices.

"We're excited about the fact that we have found another family of materials where we can both explore the physics of 2D topological insulators and do experiments that may lead to future applications," said Zhi-Xun Shen, a professor in physical sciences at Stanford University and advisor for science and technology at the SLAC National Accelerator Laboratory, who also co-led the research effort. "This general class of materials is known to be robust and to hold up well under various experimental conditions, and these qualities should allow the field to develop faster."

The material was fabricated and studied at the ALS, an X-ray research facility known as a synchrotron. Shujie Tang, a visiting postdoctoral researcher at Berkeley Lab and Stanford University, and a co-lead author of the study, was instrumental in growing three-atom-thick crystalline samples of the material in a highly purified, vacuum-sealed compartment at the ALS, using a process known as molecular beam epitaxy. The high-purity samples were then studied at the ALS using a technique known as angle-resolved photoemission spectroscopy (ARPES), which provides a powerful probe of a materials' electron properties.

"After we refined the growth recipe, we measured it with ARPES. We immediately recognized the characteristic electronic structure of a 2D topological insulator," Tang said, based on theory and predictions. "We were the first ones to perform this type of measurement on this material."

But because the conducting part of this material, at its outermost edge, measured only a few nanometers – thousands of times thinner than the X-ray beam's focus – it was difficult to positively identify all of the material's electronic properties. So collaborators at UC Berkeley performed additional measurements at the atomic scale using a technique known as scanning tunneling microscopy (STM). "STM measured its edge state directly, so that was a really key contribution," Tang said.

This research effort, which began in 2015, involved more than two dozen researchers in a variety of disciplines. The research team also benefited from computational work at Berkeley Lab's National Energy Research Scientific Computing Center (NERSC).

Two-dimensional materials have unique electronic properties that are considered key to adapting them for spintronics applications. Research groups around the world are now focused on tailoring these materials for specific uses by selectively stacking different 2D materials.

"Researchers are trying to sandwich them on top of each other to tweak the material as they wish – like Lego blocks," Mo said. "Now that we have experimental proof of this material's properties, we want to stack it up with other materials to see how these properties change."

A common problem in creating such designer materials from atomically thin layers is that materials typically have nanoscale defects that can be difficult to eliminate and that can affect their performance. But because 1T'-WTe2 is a topological insulator, its electronic properties are by nature resilient.

"At the nanoscale it may not be a perfect crystal," Mo said, "but the beauty of topological materials is that even when you have less than perfect crystals, the edge states survive. The imperfections don't break the key properties."

Going forward, the researchers aim to develop larger samples of the material and to discover how to selectively tune and accentuate specific properties. In addition, they are studying ‘sister materials’ of 1T'-WTe₂, which have similar properties but are also known to be light-sensitive. These materials could thus possess useful properties for solar cells and for optoelectronics, which control light for use in electronic devices.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.