Nanostructured materials and devices for sensing and energy harvesting applications

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Bilkent University
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A closer look into the fundamental challenges of the modern world reveals that the increasing demand for energy threatens the evolution of science and technology. Energy-efficiency is thus a fundamental issue in engineering nano-devices. An important path to achieve high efficiency is to convert the mechanical energy into electrical energy using piezoelectric and triboelectric energy harvesting circuitries, hence enabling self-powered systems at nanoscale. The utilization of novel piezoelectric and triboelectric energy harvesting materials introduces the opportunity of manufacturing flexible, wearable and stretchable self-powered devices. In this thesis, we introduced a new fabrication technique, new strategies and practical approaches for developing high performance triboelectric and piezoelectric materials and devices for flexible electronics, artificial skin and energy harvesting applications. The first part of the thesis focuses on the development of piezoelectric nanoribbons. Poly (vinylidene fluoride) and its copolymer Poly (vinylidene fluoride)-co-tri (fluoroethylene) were used to fabricate spontaneously high piezoelectric nanoribbons. We measured the record-high piezoelectric charge coefficient from our ribbons, because the high stress and high temperature used in the fabrication can enhance their properties. In addition, proof of principle devices for energy harvesting and sensing were fabricated using nanoribbons. The achievements in this part of the thesis can be listed as: i) We obtained extraordinary high aspect ratio, globally oriented, polymer encapsulated, and high piezoelectric microribbon and nanoribbon arrays. ii) Due to process conditions (shear stress and temperature) used in thermal fiber drawing, as-produced micro and nanoribbons contain high amount of polar phase without requiring any electrical poling. iii) We developed a new technique for characterizing and analyzing multiferroic characteristics of nano-objects, which consist of parallel evaluation of instrumental, numerical and analytical data. iv) To our knowledge, we achieved the highest piezoelectric charge coefficient from our ribbons in the literature. v) We enhanced stability of the piezoelectric ribbons by increasing the Curie temperature above its melting point due to processing conditions. vi) We observed and explained a new phase transformation mechanism in polymer piezoelectric ribbons. vii) The state-of-the-art ab initio calculations, which explain the phase transformation mechanism of molecules during the fiber drawing with the effect of shear, tensile forces and temperature, were included in detail. The second part is about developing high energy output triboelectric generators. A high performance multi-layered triboelectric generator was developed using chalcogenide nanostructures. This part of the thesis details the following achievements: i) We demonstrated that not only polymer, but also semiconductor chalcogenide materials can be used in triboelectric applications, for the first time. ii) For the first time, we proposed and demonstrated that the fluorination of nanostructured surfaces increases triboelectric performance significantly. iii) We introduced a multi-layered triboelectric generator which is very promising for real applications such as acoustic wave and vibration detection, and energy harvesting with very high power output (0.51 Watt) in comparison with the literature. iv) We used a 3D printing technique to produce our device, which is low-cost and appropriate for rapid prototyping and mass production. v) We explained the device theory for the triboelectric nanogenerator, which aligned well with our experimental results.

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Iterative size reduction technique, Energy harvesting, Triboelectricity, Piezoelectricity, Poly (vinylidene fluoride), Flexible electronics, Nanoelectronics
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