Ally Winning, European Editor, PSD
The piezoelectric effect is quite an unusual phenomenon that could become very important to the electronics industry in the next few years. The effect is achieved in applicable materials by applying mechanical stress to provide an electrical charge. The electric field is created because the mechanical stress shifts the positive and negative charge centres in the material. The amount of current that can be taken from the material is not usually too large, but if it can power a sensor, a microcontroller, and a wireless node for a short period of time, usually a few milliseconds, until a reading is taken and transmitted, it would be an ideal power source for sensor nodes in IoT applications. Using piezoelectricity would enable those nodes to operate without a battery, and therefore make them much smaller. They’d also be able to operate in a wider variety of environments, and require less maintenance for battery changes. And even better for these types of application, scientists have just discovered that the piezoelectric effect happens on an even smaller scale than previously thought.
Researchers have just released the results of a new study that has found that piezoelectricity works across the phase boundaries of two-dimensional materials. The work was undertaken by Pulickel Ajayan, 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. The results of the study have been published in Advanced Materials.
The researcher’s work could enable the development of smaller nanoelectromechanical systems that can power devices such as minute actuators, implantable biosensors, or ultrasensitive temperature and pressure sensors. The research showed the atomically thin system of a metallic domain surrounding semiconducting 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 a precise number of layers has been a difficult 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 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 lateral junction between phases provides atomically sharp boundaries in atomically thin layers, allowing the one engineering of materials in 2D to create device architectures that could be unique in electronic applications.”
The junction is under 10 nanometers thick and is formed when tellurium gas is introduced while molybdenum metal forms a film on silicon dioxide in a chemical vapor deposition furnace. This process creates islands of semiconducting molybdenum telluride phases in the sea of metallic phases. Applying voltage to the junction via the tip of a piezoresponse force microscope generates a mechanical response. That also carefully measures the strength of 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 clarified. Simplifying the preparation of 2D crystals for applications like miniaturized actuators.”
“A heterostructure interface allows 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.”
Zhu is an assistant professor of materials science and nanoengineering. Ajayan is the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of materials science and nanoengineering, chemistry, and chemical and biomolecular engineering. He is also chair of Rice’s Department of Materials Science and NanoEngineering.
The Air Force Office of Scientific Research and the National Science Foundation supported the research.