Quantum technology: semiconductor “tilted” towards the insulator above room temperature

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This electron microscopy image shows the atoms in individual two-dimensional layers of tantalum sulfide before and after the heat treatment process. Prior to heat treatment, all layers are bonded with an octahedral geometry. After heat treatment, most layers are bonded with prismatic geometry. The remaining octahedral layers exhibit ordered charged density waves and have transitioned from conductor to insulator. The white scale bar represents two nanometers. Photo credit: Suk Hyun Sun

A semiconductor material that performed a quantum “switch” from conductor to insulator above room temperature was developed at the University of Michigan. It potentially brings the world closer to a new generation of quantum devices and ultra-efficient electronics.

Observed in two-dimensional layers of tantalum sulfide only one atom thick, the exotic electronic structure that underpins this quantum flip was previously only stable at ultra-cold temperatures of -100 degrees Fahrenheit. The new material remains stable up to 170 F.

“We have opened up a new playground for the future of electronic and quantum materials,” said Robert Hovden, assistant professor of materials science and engineering at UM and corresponding author of the study in Nature Communications. “This represents a whole new way to access exotic states.”

Hovden says exotic quantum properties, like the ability to switch from a conductor to an insulator, could hold the key to the next generation of computing, providing more ways to store information and faster switching between states. This could lead to much more powerful and more energy-efficient devices.

Today’s electronics use tiny electronic switches to store data; “on” is one and “off” is zero, and the data disappears when the power is turned off. Future devices could use other states, like “conductive” or “insulator” to store digital data, requiring only a brief burst of energy to switch between states rather than a constant flow of electricity.

In the past, however, such exotic behavior has only been observed in materials at super cold temperatures. The ultimate goal is to develop materials capable of rapidly “switching” from one state to another on demand and at room temperature. Hovden says this research could be an important step in that direction.

“Previous research in ultra-cold temperatures has shown that it’s possible to make these kinds of on-demand reversals happen over and over again,” he said. “That wasn’t the goal of this project, but the fact that we were able to maintain a stable flip at room temperature opens up a lot of exciting possibilities.”

The switching from conductor to insulator is supported by a phenomenon called a charge density wave – an ordered, crystal-like pattern of positive and negative electrical charges that occurs spontaneously under certain conditions.

“Charge density waves have been observed in bulk samples of tantalum sulfide before, but the material had to be at ultracold temperatures,” Hovden said. “By nesting several two-dimensional layers together, we were able to make it much more stable.”

The team started by fabricating a sample of several single-atom-thick layers of tantalum sulfide sandwiched together. Each layer was a semiconductor in what is called an octahedral state, which refers to a specific arrangement of tantalum and sulfur atoms. And while some charge density waves were present, they were too unstable and disordered to give rise to exotic behavior like a conductor-insulator flip.

But Suk Hyun Sung, a graduate researcher at Hovden’s lab and first author of the study, altered the properties of the sample by heating it in an oxygen-free environment while observing the process under an electron microscope. As the sample heated, the layers began to change, one by one, to a prismatic state – a different arrangement of the same atoms.

When most, but not all, of the layers had transitioned to the prismatic state, Sung cooled the sample to room temperature. He found that layers that remained in the octahedral state exhibited charge density waves that were orderly and stable, and thus remained at temperatures up to 170 F. Additionally, these layers had transitioned from semiconductors to insulators.

“Most 2D materials are subject to all the flaws of whatever they’re on, whatever’s in the air, which makes them very unstable,” Sung said. “But we found that when octahedral layers are nested between multiple prismatic layers, they are much more stable.”

The team is examining the phenomenon further, adjusting more process variables and testing mechanisms to control exotic behaviors stimulated by charge density waves. So far, the new discovery has given them important insight into how quantum states and two-dimensional materials work.

The work was done in part at the UM Lurie Nanofabrication Facility, and it includes contributions from researchers at Cornell University, the US Naval Research Laboratory and the Chinese Academy of Sciences.

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