Rapid Communications Strong piezoelectricity in individual GaN nanowires

GaN nanowires are promising building blocks for future nanoelectronics, optoelectronic devices, and nanogenerators. Here, we report on strong piezoelectricity in individual single-crystal GaN nanowires revealed by direct measurement of the piezoelectric constant using piezoresponse force microscopy. Our experimental results show that individual c-axis GaN nanowires, with a characteristic dimension as small as 65 nm, show a shear piezoelectric constant of d15∼ 10 pm/V, which is several times that measured in bulk. The revealed strong piezoelectricity could open promising opportunities for application of GaN nanowires in nanowire-based sensors and generators for self-powered nanodevices. Semiconductor nanowires have been proposed as building blocks for nanoscale electronic, optoelectronic devices, and nanogenerators for self-powered nanodevices. Recent demonstrations of energy harvesting from individual piezoelectric nanowires added another important dimension to the possible application of these nanostructures in future devices. The proposed nanogenerators for energy harvesting, in either nanowire or nanoribbon form, rely on the piezoelectric and semiconducting properties of these nanostructures. Piezoelectric materials convert mechanical deformation directly into electrical charges, which can be harvested and used for driving small-power nanodevices. Furthermore, nanostructures such as nanowires, due to their excellent mechanical properties arising from a low defect density, can undergo large deformation, producing larger electric charges, rendering them attractive for energy applications. Here, GaN nanowires are studied because of their technological potential, including their wide band gap, high thermal conductivity, temperature stability, and piezoelectricity. Nanodevices based on GaN such as light emitting diodes (LEDs), blue emitting lasers, nanowire-based field-effect transistors, and many more have been developed. Piezoelectricity in GaN nanowires has been demonstrated through voltage (charge) generation under applied mechanical deformation, and it is shown that the output voltage is higher than in ZnO nanowires. Also using an improved atomic force microscopy (AFM) cross-sectional method, voltage generation as high as 150 mV for GaN nanowires was recently reported. However, there has been no report on direct quantification of the piezoelectric constants in individual GaN nanowires. Given that application of these nanowires in future energy devices would require a knowledge of their piezoelectric constants, it is highly relevant to quantify them. Herein, we report on the first quantification of the shear piezoelectricity in individual c-axis single-crystal GaN nanowires, using piezoresponse force microscopy (PFM). PFM is a method based on AFM for probing converse piezoelectric effects at the nanoscale. It has been extensively used for probing piezoelectric and ferroelectric properties with nanometer spatial resolution. Specifically, PFM has been used to investigate piezoelectricity in ZnO nanobelts, PZT nanowires, BaTiO3 nanowires, [16,17] individual collagen fibrils, and thin films of III-nitride materials. In addition to experimental studies, recent quantum mechanical calculations based on density functional theory have revealed a giant piezoelectric size effect for ZnO and GaN nanowires ranging from 0.6 to 2.4 nm in diameter, with GaN showing a more enhanced size-dependence. Surface reconstruction and associated volume change, together with change in local polarization, was proposed as the mechanism of the observed size effect. These previous results underscore the need to perform experimental studies on individual nanowires in order to quantify their piezoelectric response.