Two-dimensional (2D) materialsâas thin as a single layer of atomsâhave intrigued scientists with their flexibility, elasticity, and unique electronic properties since first being discovered in materials such as graphene in 2004. Some of these materials can be especially susceptible to changes in their material properties as they are stretched and pulled. Under applied strain, they have been predicted to undergo phase transitions as disparate as superconducting in one moment to nonconducting the next, or optically opaque in one moment to transparent in the next.
Now, University of Rochester researchers have combined 2D materials with oxide materials in a new way, using a transistor-scale device platform, to fully explore the capabilities of these changeable 2D materials to transform electronics, optics, computing, and a host of other technologies.
âWeâre opening up a new direction of study,â says , assistant professor of electrical and computer engineering and physics. âThereâs a huge number of 2D materials with different propertiesâand if you stretch them, they will do all sorts of things.â
The platform developed in Wuâs lab, configured much like traditional transistors, allows a small flake of a 2D material to be deposited onto a ferroelectric material. Voltage applied to the ferroelectricâwhich acts like a transistorâs third terminal, or gateâstrains the 2D material by the piezoelectric effect, causing it to stretch. That, in turn, triggers a phase change that can completely alter the way the material behaves. When the voltage is turned off, the material retains its phase until an opposite polarity voltage is applied, causing the material to revert to its original phase.
âThe ultimate goal of two-dimensional straintronics is to take all of the things that you couldnât control before, like the topological, superconducting, magnetic, and optical properties of these materials, and now be able to control them, just by stretching the material on a chip,â Wu says.
âIf you do this with topological materials you could impact quantum computers, or if you do it with superconducting materials you can impact superconducting electronics.â
Maxing out Mooreâs Law
In a paper in , Wu and his students describe using a thin film of two-dimensional molybdenum ditelluride (MoTe2) in the device platform. When stretched and unstretched, the MoTe2 changes from a low conductivity semiconductor material to a highly conductive semimetallic material and back again.
âIt operates just like a field effect transistor. You just have to put a voltage on that third terminal, and the MoTe2 will stretch a little bit in one direction and become something thatâs conducting. Then you stretch it back in another direction, and all of a sudden you have something that has low conductivity,â Wu says.
The process works at room temperature, he adds, and, remarkably, ârequires only a small amount of strainâweâre stretching the MoTe2 by only 0.4 percent to see these changes.â
Mooreâs Law famously predicts that the number of transistors in a dense, integrated circuit will double about every two years.
Yet technology is nearing the limits at which traditional transistors can be scaled down in size. So, as we reach the limits of Mooreâs Law, the technology developed in Wuâs lab could have far-reaching implications in moving past these limitations in the quest for ever faster, more enhanced computing power.
Wuâs platform has the potential to perform the same functions as a transistor with far less power consumption since power is not needed to retain the conductivity state. Moreover, it minimizes the leakage of electrical current due to the steep slope at which the device changes conductivity with applied gate voltage. Both of these issuesâhigh power consumption and leakage of electrical currentâhave constrained the performance of traditional transistors at the nanoscale.
âThis is the first demonstration,â Wu adds. âNow itâs up to researchers to figure out how far it goes.â
No strain, no gain
One advantage of Wuâs platform is that it is configured much like a traditional transistor, making it easier to eventually adapt into current electronics. However, more work is needed before the platform reaches that stage. Currently, the device can operate only 70 to 100 times in the lab before device failure. While the endurance of other non-volatile memories, like flash, are much higher, they also operate much slower than the ultimate potential of the strain-based devices being developed in Wuâs lab.
âDo I think itâs a challenge that can be overcome? Absolutely,â says Wu, who will be working on the problem with Hesam Askari, an assistant professor of mechanical engineering at ÌÇĐÄlogo also a coauthor on the paper. âItâs a materials engineering problem that we can solve as we move forward in our understanding how this concept works.â
They will also explore how much strain can be applied to various two-dimensional materials without causing them to break. Determining the ultimate limit of the concept will help guide researchers to other phase-change materials as the technology moves forward.
Wu, who completed his PhD in physics at the University of California, Berkeley, was a postdoctoral scholar in the Materials Science Division at Argonne National Laboratory before he joined the University of Rochester as an assistant professor in the and the in 2017.
He started with a single undergraduate student in his labâArfan Sewaket â19, who was spending the summer as a Xerox ÌÇĐÄlogo Fellow. She helped Wu set up a temporary lab, then was the first to try out the device concept and the first to demonstrate its feasibility. Since then, four graduate students in Wuâs labâlead author Wenhui Hou, Ahmad Azizimanesh, Tara Peña, and Carla Watsonââhave done so much workâ to document the deviceâs properties and refine it, creating about 200 different versions to this point, Wu says. All are listed with Sewaket as coauthors, along with Askari and Ming Liu of Xiâan Jiaotong University in China.
