Understanding the enhanced piezoelectric response in inclined ZnO nanorods

In this work, the finite element method simulation is used to investigate the origin of enhanced piezoelectric response in the inclined ZnO nanorods (NRs) compared with the vertically aligned NRs. The inherent crystal structure of ZnO and spontaneous polarization along the c-axis give rise to a larger d₃₃ piezoelectric coefficient, hence most prior studies focus on the development and growth of vertical NRs. In contrast, our simulation results show the highest piezoelectric potential response of ∼875.5 mV in the case of the 60° inclined NR, which is ∼ 35 times higher than the vertical NR, which is ∼25 mV, where the NR length, diameter and pressure are fixed at 5 μm, 0.1 μm, 50 kPa, respectively. In addition to the enhanced piezoelectric potential, the 60° inclined NRs show a nonlinear L² behaviour with the length (L), which has not been previously predicted in simulation studies. This suggests a much higher piezoelectric potential response for longer NRs. The vertical NRs show predictable linear behaviour with length due to the axial pressure, which causes only the d₃₃ to effectively contribute to the piezoelectric potential. A novel selective coefficient suppression strategy in the FEM framework is used to study the contributions of each piezoelectric coefficient in the NR. The study shows that the generated piezoelectric potential in vertical NRs is primarily due to axial pressure, where only the d₃₃ contributed effectively. Whereas the inclined NRs show the combined effect of axial and bending stress, where the major contributions come from orders of magnitude higher bending stress levels, which causes a much higher strain and significant contributions from d₃₁ and d₁₅ in addition to d₃₃. These findings can help design efficient tactile sensors and energy harvesting devices by suggesting the required morphology. The vertical NRs can be well-suited for tactile sensing due to their higher vertical stability under pressure, whereas inclined NRs are more promising for both tactile sensing and energy harvesting, due to their enhanced piezoelectric potential response under the same loading conditions.