Piezoelectric Nanostructures - Synthesis, Alignment, and Electrical Response to Strain

Abstract

Piezoelectric nanostructures can convert mechanical deformation into electrical signal, and have applications in mechanical energy harvesting and strain sensing. While nanostructures have unique advantages compared with bulk materials, their synthesis and assembly are more challenging, and their response to mechanical deformation is complex. The objective of this research is to have a better control and a deeper understanding of piezoelectric nanostructures. The synthesis and assembly processes will be improved, and the electrical response to strain will be studied in details. First, a new method to synthesize piezoelectric nanostructures is developed. Growth of nanostructures is similar to that of grasses - from the bottom up. Controlling the growth orientation is important for the optimization of device performance, and requires a highly engineered substrate in general. In this research, a textured polycrystalline film is used as an inexpensive substrate to fulfill that requirement. The textured film is coated conformally on various surface topographies and allows the epitaxial growth of nanostructures with vertical, tilted, or lateral orientations. Second, a new method to organize piezoelectric nanostructures is developed. Alignment and transfer of a large quantity of nanostructures at the same time is a critical step in the fabrication of energy harvesters, and has been achieved in this work through a Spinning-Langmuir-Film technique. In this method, a surfactant-enhanced shear flow aligns inorganic and organic nanostructures, which could be easily transferred to other substrates and ready for device fabrication. The areal density of the align nanostructures can be controlled in a wide range. Various factors that may affect the alignment process are studied systematically. Third, a special type of piezoelectric nanostructures with semiconductivity is investigated in depth for the application in strain sensing. Mechanical strain induces multiple changes in the electrical property of those materials simultaneously - a piezoresistive effect from the semiconductor band structure change, and a piezotronic effect from the coupling between the piezoelectric polarization and the semiconductor interface. A general method to separate the piezotronic and piezoresistive effects is reported in this thesis based on modified four-point measurements. The piezotronic effect is found to be more sensitive to strain than the piezoresistive effect.