There is still enough space at the bottom to generate piezoelectricity. Rice University engineers and their colleagues are leading the way.
A new study describes the discovery of piezoelectricity—the phenomenon by which mechanical energy is converted into electrical energy—across the phase boundaries of two-dimensional materials.
The work, led by Rice materials scientists Pulikel Ajayan and Hanyu Zhu and their colleagues at Rice’s George R. Brown School of Engineering, the University of Southern California, the University of Houston, Wright-Patterson Air Force Base Research Laboratory and Pennsylvania State University, appears in Advanced materials.
The discovery could help develop ever-smaller nanoelectromechanical systems, devices that can be used, for example, to power tiny actuators and implantable biosensors, as well as ultrasensitive sensors for temperature or pressure.
The researchers show that the atomically thin system of metal regions surrounding the semiconductor islands creates a mechanical response in the material’s crystal lattice when subjected to an applied voltage.
The presence of piezoelectricity in 2D materials often depends on the number of layers, but synthesizing the materials with the correct number of layers is a huge challenge, said Rice researcher Anand Putirath, a co-author of the paper.
“Our question was how to make a structure that is piezoelectric at multiple levels of thickness — monolayer, bilayer, trilayer, and even bulk — even from a non-piezoelectric material,” Putirath said. “The plausible answer was to make a one-dimensional metal-semiconductor junction into a 2D heterostructure, thereby introducing crystallographic as well as charge asymmetry at the junction.”
“The lateral junction between the phases is very interesting because it provides atomically sharp boundaries in atomically thin layers, something our group pioneered almost a decade ago,” Ajayan said. “This allows materials to be designed in 2D to create device architectures that can be unique in electronic applications.”
The transition is less than 10 nanometers thick and is formed when tellurium gas is introduced while metallic molybdenum forms a film on silicon dioxide in a chemical vapor deposition furnace. This process creates islands of semiconductor molybdenum telluride phases in a sea of metallic phases.
Applying voltage to the junction through the tip of a piezoeffect microscope generates a mechanical response. This also carefully measures the strength of the piezoelectricity created at the junction.
“The difference between the lattice structures and the electrical conductivity creates an asymmetry at the phase boundary that is essentially independent of the thickness,” Putirath said. This simplifies the preparation of 2D crystals for applications such as miniaturized actuators.
“The heterostructure interface allows much more freedom for the properties of engineering materials than a bulk single compound,” Zhu said. “Although asymmetry only exists at the nanoscale, it can significantly affect macroscopic electrical or optical phenomena that are often dominated by the interface.”
Researcher Xiang Zhang and graduate student Rui Xu of Rice and postdoctoral researcher Aravind Krishnamoorthy of the University of Southern California co-authored the paper. Co-authors are graduate student Jiawei Lai, research professor Robert Vajtai, and Rice faculty member Venkataraman Swaminathan; Priya Vashishta, professor of chemical engineering and materials science, biomedical engineering, computer science, and physics and astronomy at the University of Southern California; graduate students Farnaz Safi Samghabadi and Dmitry Litvinov, the John and Rebecca Moores Professor at the University of Houston; David Moore and Nicholas Glavin of the Air Force Research Laboratory, Wright-Patterson Air Force Base; and Rice alumni Tiani Zhang and Fu Zhang, graduate student David Sanchez, and Mauricio Terrones, the Verne M. Willaman Professor of Physics at the University of Pennsylvania.
Zhu is an assistant professor of materials science and nanoengineering. Ajayan is the Benjamin M. and Mary Greenwood Anderson Professor of Engineering and Professor of Materials Science and Nanoengineering, Chemistry, and Chemical and Biomolecular Engineering. He also chairs Rice’s Department of Materials Science and Nanoengineering.
The Air Force Office of Scientific Research (FA9550-18-1-0072, FA9550-19RYCOR050) and the National Science Foundation (2005096) supported the research.
Materials provided by Rice University. Originally written by Mike Williams. Note: Content may be edited for style and length.