Researchers "expand" the ability of 2D materials to change technology



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Researchers extend the ability of 2D materials to change technology

Artistic representation of a 2D material in phase change with the help of a platform-scale transistor developed in the laboratory of Stephen Wu, professor Assistant of Electrical and Computer Engineering and Physics at the University of Rochester. Credit: Illustration of the University of Rochester / Michael Osadciw

Two-dimensional (2D) materials, as thin as a single layer of atoms, intrigued scientists with their unique flexibility, elasticity and electronic properties, discovered in materials such as graphene in 2004. Some of these materials can be : particularly sensitive to changes in their material properties as they are stretched and drawn. Under an applied constraint, it is expected that they undergo phase transitions as disparate as superconducting at one time or non-conductive, or optically opaque at one time or transparent at another time.

Researchers at the University of Rochester are now combining 2D materials with oxides in a new way, using a transistor-scale platform, to fully exploit the capabilities of these variable 2D materials to transform electronics, optics, computing and a host. other technologies.

"We are opening a new direction of study," said Stephen Wu, an assistant professor of electrical and computer engineering and physics. "There are a lot of two-dimensional materials with different properties – and if you stretch them, they will do all kinds of things."

The platform developed in Wu's laboratory, configured in the same way as traditional transistors, allows to deposit a small chip of a 2D material on a ferroelectric material. The voltage applied to the ferroelectric, which acts as the third terminal or gate of a transistor, constrains the 2D material by the piezoelectric effect, which causes it to stretch. This, in turn, triggers a phase change that can completely change the behavior of the material. When the voltage is cut, the material retains its phase until a voltage of opposite polarity is applied, which brings the material back to its original phase.

"The ultimate goal of the two-dimensional constraint is to take everything you could not control before, like the topological, superconducting, magnetic and optical properties of these materials, and now be able to control them by stretching the material on a chip Wu said.

"If you do that with topological materials, you could have an impact on quantum computers, or if you do it with superconducting materials, you can have an impact on the superconducting electronic components."

In a paper in Nature NanotechnologyWu and his students describe the use of a two-dimensional molybdenum ditelluride (MoTe2) thin film in the device platform. Stretched and unstretched, the MoTe2 goes from a low-conductivity semiconductor material to a highly conductive semi-metallic material, and vice versa.

"It works like a field effect transistor, just put a voltage on that third terminal, and the MoTe2 stretches a bit in one direction to become something of a driver. other direction, and all of a sudden, you have something that has low conductivity, "Wu says.

The process runs at room temperature, he adds, and remarkably, "only requires a small amount of stress – we only lengthen the MoTe2 by 0.4% to see these changes."

Moore's law predicts that the number of transistors in a dense integrated circuit doubles every two years or so.

However, as technology approaches the limits to which the size of traditional transistors can be reduced, as we reach the end of Moore's law, the technology developed in Wu's laboratory could have a profound impact on the surpassing these limits the powerful and faster computing continues.

Wu's platform has the potential to perform the same functions as a transistor with a much lower power consumption, since no power supply is needed to maintain the conductivity state. In addition, it minimizes electrical current leakage due to the steep slope on which the device changes conductivity with the applied gate voltage. These two problems – high power consumption and electrical current leakage – have limited the performance of traditional nanoscale transistors.

"It's the first demonstration," Wu adds. "It's now up to researchers to determine where this is going."

One of the advantages of the Wu platform is that it is configured pretty much like a traditional transistor, which facilitates its subsequent adaptation to the current electronics. However, there is still a long way to go before the platform reaches this stage Currently, the device can only run 70 to 100 times in the lab before a device failure. Although the endurance of other nonvolatile memories, such as flash memory, is much higher, they also work much more slowly than the ultimate potential of the strain-based devices developed in Wu's laboratory.

"Do I think it's a challenge that can be overcome? Absolutely," said Wu, who will work on the problem with Hesam Askari, assistant professor of mechanical engineering in Rochester, also co-author of the journal . "It's a problem of materials engineering that we can solve as we better understand how this concept works."

They will also explore the amount of voltage that can be applied to various two-dimensional materials without causing them to break. Determining the ultimate limit of the concept will help researchers turn to other phase-change materials as technology evolves.

Wu, who completed his Ph.D. in physics at the University of California at Berkeley, was a postdoctoral fellow at the Materials Science Division at the Argonne National Laboratory before joining the University of Rochester as an Assistant Professor in the Department of Materials Science. Electrical and Computer Engineering and Physics Department in 2017.

He started with just one undergraduate student in his lab – Arfan Sewaket 19, who spent the summer as a Xerox researcher. She helped Wu set up a temporary laboratory, then was the first to experiment with the concept of the device and the first to demonstrate its feasibility.

Since then, four graduate students from Wu's laboratory – the main author, Wenhui Hou, Ahmad Azizimanesh, Tara Pen? A and Carla Watson "worked a great deal" to document the properties of the device and tweak it, creating about 200 different versions. Point, says Wu. All are listed with Sewaket as coauthors, with Askari and Ming Liu of Xi'an University Jiaotong in China.


The constraint allows new applications of 2D materials


More information:
Non-volatile ferroelectric phase change transistor MoTe2 at ambient temperature based on constraints, Nature Nanotechnology, DOI: 10.1038 / s41565-019-0466-2, https://www.nature.com/articles/s41565-019-0466-2

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