Published: January 11, 2011 r2011 American Chemical Society 786 dx.doi.org/10.1021/nl104004d | Nano Lett. 2011, 11, 786–790 LETTER pubs.acs.org/NanoLett Giant Piezoelectric Size Effects in Zinc Oxide and Gallium Nitride Nanowires. A First Principles Investigation Ravi Agrawal and Horacio D. Espinosa* Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3111, United States ABSTRACT: Nanowires made of materials with noncentro- symmetric crystal structure are under investigation for their piezoelectric properties and suitability as building blocks for next-generation self-powered nanodevices. In this work, we investigate the size dependence of piezoelectric coefficients in nanowires of two such materials - zinc oxide and gallium nitride. Nanowires, oriented along their polar axis, ranging from 0.6 to 2.4 nm in diameter were modeled quantum mechanically. A giant piezoelectric size effect is identified for both GaN and ZnO nanowires. However, GaN exhibits a larger and more extended size dependence than ZnO. The observed size effect is discussed in the context of charge redistribution near the free surfaces leading to changes in local polarization. The study reveals that local changes in polarization and reduction of unit cell volume with respect to bulk values lead to the observed size effect. These results have strong implication in the field of energy harvesting, as piezoelectric voltage output scales with the piezoelectric coefficient. KEYWORDS: Gallium nitride, zinc oxide, nanowires, density functional theory, piezoelectric properties P iezoelectricity is a phenomenon in which an electric field is generated inside a material subjected to a mechanical strain or vice versa. It is caused by the nonsymmetric crystal structure of certain materials, which results in an effective change in polariza- tion in response to an applied mechanical strain. The strained material behaves like a charged capacitor with an electrostatic potential across it, which can be utilized in sensing, actuation, and energy harvesting applications. Therefore, piezoelectricity pro- vides a direct means of conversion between mechanical and electrical energy. However, at the macroscale, the electrical energy output is relatively low in comparison to mechanical energy required to strain the material. Thus at large scales, applications of piezoelectric devices are typically limited to sensors (e.g., pressure sensors 1 ) and actuators (e.g., in atomic force microscopy 2 ) where efficiency is not critical. By contrast, nanoscale offers an advantage, that is, the forces required to deform nanostructures made of piezoelectric materials are small enough to be extracted from natural sources of mechanical energy (e.g., ambient noise, wind energy, body movements, and flowing water). Therefore, thin films and nanowires are considered suitable building blocks for next-generation energy- harvesting devices. 3,4 In addition, as the size of these structures is reduced to the nanoscale (thin films and nanowires (NWs)), the conversion efficiency can be improved dramatically for the following reasons: (i) nanomaterials tolerate relatively large deformations prior to failure, which is critical as the electro- static potential generated (V) is proportional to the applied strain (ε) and piezoelectric coefficient (d 33 ), namely V µ d 33 ε; (ii) material properties are often enhanced at the nanoscale relative to the bulk due to surface effects and high surface-to- volume ratios. Recently, Wang et al. showed that ZnO NWs can act as piezoelectric nanogenerators both by deflecting individual NWs 5 and by integrating them in hybrid microfiber assemblies. 6 NW nanogenerators were also used to convert biomechanical energy (movement of a human finger and body motion of a hamster) to electrical energy. 7 Various experimental studies have probed either the electrical 8-10 or mechanical 11-16 behavior of nanowires separately; however, electromechanical coupling and its potential size-effects have not been adequately addressed. The challenges associated with such experiments include (i) difficulties in sample manipulation at the nanoscale, (ii) making appropriate electrical measurements accounting for contact resistances, (iii) measuring currents and voltages with sufficiently high resolution, and so forth. The difficulties in conducting such experiments seem to be the primary reason for a large scatter observed in the experimentally reported piezoelectric coefficients for ZnO nanostructures. For example, piezoresponse force microscopy (PFM) has revealed piezoelectric coefficient ranging from 4.41 to 7.5 pm/V for ZnO nanorods with diameters in the 150-500 nm range, 17,18 as well as values ranging from 14.3 to 26.7 pm/V for ZnO nanobelts tens of nm in thickness. 19 On the contrary, a resonance shift method has revealed a value as high as 12 000 pm/V for a 230 nm ZnO nanowire, 20 as compared to a bulk value of 12.4 pm/V. 21,22 In this work, we investigate piezoelectric size effects from a computational standpoint. First principles-based density func- tional theory (DFT) calculations were performed to model Received: November 15, 2010 Revised: December 19, 2010