Letter pubs.acs.org/NanoLett
Flexural Electromechanical Coupling: A Nanoscale Emergent Property of Boron Nitride Bilayers Karel-Alexander N. Duerloo and Evan J. Reed* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ABSTRACT: The symmetry properties of atomically thin boron nitride (BN) monolayers endow them with piezoelectric properties, whereas the bulk parent crystal of stacked BN layers is not piezoelectric. This suggests potential for unusual electromechanical properties in the few layer regime. In this work, we explore this regime and discover that a bilayer consisting of two BN monolayers exhibits a strong mechanical coupling between curvature and electric fields. Using a mechanical model with parameters obtained from density functional theory, we find that these bilayers amplify in-plane piezoelectric displacements by exceedingly large factors on the order of 103−104. We find that this type of electromechanical coupling is an emergent nanoscale property that occurs only for the case of two stacked BN monolayers. KEYWORDS: Atomically thin materials, flexoelectricity, piezoelectricity, piezotronics, nanoelectromechanical systems (NEMS), two-dimensional materials
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he interplay between electrostatic and mechanical behavior exhibited by piezoelectric materials has made piezoelectric components an essential feature of modern lowpower, high-precision technology. In addition, piezoelectric materials are highly relevant to many nanoscale applications where electromechanical coupling plays a key role. In an emerging field termed piezotronics, piezoelectric nanowires have been used as prototype transistors, diodes, and mechanical sensors and generators.1−3 These devices work by virtue of the direct piezoelectric effect, where through application of some force to a noncentrosymmetric dielectric crystal, polarization, bound charges, and associated electric fields are generated commensurately. The direct piezoelectric effect is accompanied by the converse piezoelectric effect, whereby external application of an electric field causes a piezoelectric crystal to develop mechanical strains or stresses that are proportional to the applied field.4 The converse piezoelectric effect has potential to improve upon the state of the art in nanoscale technologies such as tweezers,5 motors6 and high-frequency switches7−10 in which an electrical contact is made and broken. While direct and converse piezoelectricity are very technologically relevant materials properties, there are two important practical constraints that limit their utility: (1) piezoelectricity is manifested only by noncentrosymmetric insulators and semiconductors,4 severely limiting the scope of engineering materials that can be used to build a piezoelectric device, and (2) strains and relative displacements generated by the converse effect are typically modest (
