Electrets – electrons trapped in defects in 2D molybdenum dioxide – confer piezoelectric properties onto the material, according to researchers at Rice University. The defects (blue) appear in the material during its formation in a furnace, and generate an electric field when under pressure. Image: Ajayan Research Group/Rice University.Researchers at Rice University have found evidence of piezoelectricity in lab-grown, two-dimensional (2D) flakes of molybdenum dioxide. Their investigation showed that these surprise electrical properties are due to electrons trapped in defects throughout the 2D material, which is less than 10nm thick. They characterize these charges as electrets, which appear in some insulating materials and generate internal and external electric fields.
Piezoelectricity is a property of materials that respond to stress by generating an electric voltage across their surfaces, or generate mechanical strain in response to an applied electric field. It has many practical and scientific uses, from the conversion of a wiggling guitar string into an electrical signal to scanning microscopes like those used to make the new finding.
The researchers at Rice University's Brown School of Engineering, led by Pulickel Ajayan, discovered their micron-scale flakes exhibit a piezoelectric response that is as strong as that observed in conventional 2D piezoelectric materials as molybdenum disulfide. They report their work in a paper in Advanced Materials.
The key appears to be defects that make molybdenum dioxide's crystal lattice imperfect. When strained, the dipoles of electrons trapped in these defects seem to align, as with other piezoelectric materials, creating an electric field leading to the observed effect.
"Super thin, 2D crystals continue to show surprises, as in our study," Ajayan said. "Defect engineering is a key to engineer properties of such materials but is often challenging and hard to control."
"Molybdenum dioxide isn't expected to show any piezoelectricity," added Rice postdoctoral researcher Anand Puthirath, a co-corresponding author of the paper. "But because we're making the material as thin as possible, confinement effects come into the picture."
According to Puthirath, the effect appears in molybdenum dioxide flakes grown by chemical vapor deposition. Stopping the growth process at various points gave the researchers some control over the defects' density, if not their distribution. Lead author and Rice alumna Amey Apte said that the researchers' single-chemical, precursor-based vapor deposition technique "helps in the reproducibility and clean nature of growing molybdenum oxide on a variety of substrates".
The researchers found that the piezoelectric effect is stable at room temperature for significant timescales, with the molybdenum dioxide flakes remaining stable at temperatures up to 100°C (212°F). But annealing them for three days at 250°C (482°F) eliminated the defects and halted the piezoelectric effect.
Puthirath said the material has many potential applications. "It can be used as an energy harvester, because if you strain this material, it will give you energy in the form of electricity," he said. "If you give it voltage, you induce mechanical expansion or compression. And if you want to mobilize something at the nanoscale, you can simply apply voltage and this will expand and move that particle the way you want."
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.