Correlation of Point Defects with Piezoelectric Voltage in Strained ZnO Microwires
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Date
2022-05
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The Ohio State University
Abstract
ZnO microwires are biocompatible semiconductors that are piezoelectric and can turn mechanical strain into an electric potential. This is due to the lack of central symmetry in the ZnO hexagonal wurtzite crystal structure. These microstructures have been used to create bio-nanogenerators that can harvest the body's natural movement into power for implanted biomedical devices. Nanogenerators can replace lithium-ion batteries in implanted devices which are relatively large, have a limited lifetime requiring multiple replacement surgeries, and can be dangerous for the patient should the battery fail. However, ZnO microwire based devices have an electric potential output that is not always efficient enough to replace lithium-ion batteries. Electrically - active native point defects can have a significant role in macroscale electrical properties. These defects can be electrically charged and have electric fields that can interact with the piezo- electric fields, causing them to redistribute, but this effect has yet to be investigated as a possible solution to improving the device efficiency. This study aims to correlate defects with changes in piezoelectric potential, enabling researchers fabricating ZnO microwire-based nanogenerators to understand the impact of defects on device output and suggest growth and processing treatments to control these defects.
ZnO microwires were grown using a vapor-solid method via carbothermal reduction in a quartz tube furnace. Wires were then cast on a Si substrate with a native oxide layer and then grounded to the substrate using conductive silver paste. Results presented here involve Microwire A with a 2.36 µm diameter and ~700 µm length and Microwire B with a 2.26 µm diameter and ~800 µm length. Cathodoluminescence spectroscopy (CLS) and Kelvin Probe Force Microscopy (KPFM) were used to characterize the defects and surface potential difference of the wire, respectively, for the strained and unstrained wire case. Both microwires were able to show a decrease in microwire tip voltage and decrease in electrically active defects as a result of strain, a result not yet reported. Microwire A had a tip voltage of 1.19 V drop to 593 mV and defect concentration ratio relative to the bandgap of 2.2 drop 1.1 after strain. Microwire B showed a tip voltage decrease from 700 mV to 600 mV and defect concentration ratio decrease from 3.0 to 1.7 after strain. The KPFM potential maps also showed an increased surface potential on the tensile end of the microwire after strain. This aligns with previous reports of a piezoelectric field formed radially along the wire. Due to the results reported here, this radial electric field is thought to cause the electrically active defects in ZnO microwires to redistribute longitudinally along the microwire causing the increased voltage at the tip. These results suggest that controlled nano- and microwire defect addition can significantly increase bio-nanogenerator device efficiency due to the effect electrically active defects have on the generated piezoelectric fields.
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Keywords
Piezoelectricity, ZnO Microwires, Point Defects in Materials, Energy Harvesting, Functional Oxide