This charge redistribution model shows how charge flows across the phase interfaces in a 2D material of molybdenum (blue) and tellurium (yellow). The red areas are electron-deficient, while the green areas are electron-rich. Voltage from a microscope tip distorts the lattice and creates dipoles at the boundary between the atoms. Image: Ajayan Research Group/Rice University.
This charge redistribution model shows how charge flows across the phase interfaces in a 2D material of molybdenum (blue) and tellurium (yellow). The red areas are electron-deficient, while the green areas are electron-rich. Voltage from a microscope tip distorts the lattice and creates dipoles at the boundary between the atoms. Image: Ajayan Research Group/Rice University.

There’s still plenty of room at the bottom to generate piezoelectricity, and engineers at Rice University and their colleagues are showing the way.

In a new study, they describe the discovery of piezoelectricity — the phenomenon by which mechanical energy turns into electrical energy — across the phase boundaries of a two-dimensional (2D) material. This could lead to the development of ever-smaller nanoelectromechanical systems, which could be used, for example, to power tiny actuators and implantable biosensors, and ultrasensitive temperature or pressure sensors.

The work is led by Rice materials scientists Pulickel Ajayan and Hanyu Zhu and their colleagues at Rice’s George R. Brown School of Engineering, together with researchers at the University of Southern California, the University of Houston, Wright-Patterson Air Force Base Research Laboratory and Pennsylvania State University. They report their findings in a paper in Advanced Materials.

The presence of piezoelectricity in 2D materials often depends on the number of layers, but synthesizing materials with a precise number of layers has been a formidable challenge, said Rice research scientist Anand Puthirath, co-lead author of the paper.

“Our question was how to make a structure that is piezoelectric at multiple thickness levels – monolayer, bilayer, trilayer and even bulk – from even non-piezoelectric material,” Puthirath said. “The plausible answer was to make a one-dimensional, metal-semiconductor junction in a 2D heterostructure, thus introducing crystallographic as well as charge asymmetry at the junction.”

The researchers found that an atomically thin system of metallic phases surrounding semiconducting islands creates a mechanical response in the 2D material’s crystal lattice when subjected to an applied voltage.

“The lateral junction between phases is very interesting, since it provides atomically sharp boundaries in atomically thin layers, something our group pioneered almost a decade before,” Ajayan said. “This allows one to engineer materials in 2D to create device architectures that could be unique in electronic applications.”

The junction is less than 10nm thick and forms when tellurium gas is introduced as molybdenum metal is forming a film on silicon dioxide in a chemical vapor deposition furnace. This process creates islands of semiconducting molybdenum telluride phases in a sea of metallic phases.

Applying a voltage to the junction between these phases via the tip of a piezoresponse force microscope generates a mechanical response. The microscope can also carefully measure the strength of the piezoelectricity created at the junction.

“The difference between the lattice structures and electrical conductivity creates asymmetry at the phase boundary that is essentially independent of the thickness,” Puthirath explained. That simplifies the preparation of 2D crystals for applications like miniaturized actuators.

“A heterostructure interface allows much more freedom for engineering materials properties than a bulk single compound,” Zhu said. “Although the asymmetry only exists at the nanoscale, it may significantly influence macroscopic electrical or optical phenomena, which are often dominated by the interface.”

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

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