Piezoelectricity across Two‐dimensional Phase Boundaries

Significance 

A new study describes the discovery of piezoelectricity, the phenomenon by which mechanical energy turns into electrical energy across phase boundaries of two-dimensional materials. The work published in the research journal Advanced Materials and led by Rice materials scientist Professor Pulickel Ajayan. The discovery could aid in the development of ever-smaller nanoelectromechanical systems, devices that could be used, for example, to power tiny actuators and implantable biosensors, and ultrasensitive temperature or pressure sensors. The researchers show 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 formidable challenge. The authors objective was to make a structure that is piezoelectric at multiple thickness levels monolayer, bilayer, trilayer and even bulk from even non-piezoelectric material. 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 lateral junction between phases is very interesting, since it provides atomically sharp boundaries in atomically thin layers, something the research team pioneered almost a decade before. This allowed the team to engineer materials in 2D to create device architectures that could be unique in electronic applications. The junction is less than 10 nanometers thick and forms 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. 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. Although the asymmetry only exists at the nanoscale, it may significantly influence macroscopic electrical or optical phenomena, which are often dominated by the interface. Furthermore, the new study suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials.

Piezoelectricity across Two‐dimensional Phase Boundaries - Advances in Engineering

About the author

Professor Ajayan He joined the mechanical engineering and materials science department of Rice University, as the Benjamin M. and Mary Greenwood Anderson Professor in Engineering from July 2007. From 2014, he is the founding chair of the new department of Materials Science and NanoEngineering at Rice University.

Professor Ajayan’s research interests include synthesis and structure-property relations of nanostructures and nanocomposites, materials science and applications of nanomaterials, energy storage, and phase stability in nanoscale systems. He is one of the pioneers in the field of carbon nanotubes and was involved in the early work on the topic along with the NEC group. He has published one book and 1000 journal papers with nearly 95,000 citations and an h-index of 150, based on ISI database. He has given more than 375 invited talks including several named lectures, keynote and plenary lectures in several countries and at several international conferences.

Reference

Anand B. Puthirath et al, Piezoelectricity across Twodimensional Phase Boundaries, Advanced Materials (2022). DOI: 10.1002/adma.202206425

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