acase@tulane.edu / 1 / Moses Oguntoye

Zinc oxide is a semiconducting material that has received lot of attention due to its numerous proeprties such as wide direct band gap, piezoelectricity, and numerous low cost and robust methods of synthesizing nanomaterials. Its piezoelectric properties have been harnessed for use in energy production through nanogenerators, and to tune carrier transport, birthing a field known as piezotronics. However, one weakness of ZnO is that it is notoriously difficult to dope p-type. Antimony was investigated as a p-type dopant for ZnO, and found to have a stability of up to 3 years, which is completely unprecedented in the literature. Furthermore, a variety of zinc oxide structures ranging from ultra-long nanowires to thin films were produced and their piezotronic properties were demonstrated. By making p-n homojunctions using doped and undoped ZnO, enhanced nanogenerators were produced which could see application in gesture recognition. As a proof of concept, a simple photodetector was also derived from a core-shell nanowire structure. Finally, the ability to integrate this material with other semiconductors was demonstrated by growing a heterojunction with silicon nanowires, and investigating its electrical properties. All this work together lays the foundation for a fundamentally new material that could see application in future electronics, optoelectronics, and human-machine interfacing.

Piezoelectric nanogenerators (PNGs) are a new class of energy harvesting materials that show potential as a direct energy source for low powered electronics. Recently, piezoelectric polymers have been utilized for PNG technology due to low toxicity, high flexibility, and facile solution processing which provide manufacturing opportunities such as screen printing. Throughout the last decade, countless projects have focused on how to enhance the energy harvesting capabilities of these PNGs through the incorporation of nanoparticle fillers, which have been reported to enhance the piezoelectric properties of the film either directly through their intrinsic piezoelectric properties or through acting as surfaces for the interfacial nucleation of piezoelectric polymer crystals. Herein, two systems of PNGs formed from piezoelectric copolymers poly(vinylidene fluoride-co-hexafluropropylene) or poly(vinylidene fluoride-co-trifluoroethylene) mixed with high aspect ratio zinc oxide nanowires, hydroxyl functionalized multi-walled carbon nanotubes, or carboxylic acid functionalized single walled carbon nanotubes were investigated. Variations of filler type and loading are tested to determine influences on film morphology and piezoelectric properties. Power harvesting tests are conducted to directly determine the effect of nanoparticle addition on the output power of the non-poled devices. Both copolymer systems are found to exhibit a non-linear increase in output power with the increase of nanoparticle filler loading. The crystal polymorph properties of both systems are investigated by Fourier transform infrared spectroscopy. The microstructure of the poly(vinylidene fluoride-co-trifluoroethylene) films are further examined using X-ray diffraction, differential scanning calorimetry, polarized optical microscopy, and atomic force microscopy to determine the mechanism behind the increased power harvesting capabilities. As well, explanations for perceived output power from “self-poled” films are briefly explored.

This thesis present the fabrication of 2 types of soft wearable electrical devices, utilizing the 3D printing technique. The devices are capable to detect human heart pulse waves and sound waves for health evaluation and speech recognition.

Nanotechnology and nanoscience are experiencing rapid development in the last decade. Intensive research has been carried out on nanostructures synthesis and nanodevices fabrication. Due to its small size, a nanodevice usually requires an extremely small power to operate. However, to make the novel nanodevice work, an external power source is normally needed, which can either be a battery or a power source, thus, the size of the battery is usually much larger than that of the device and its life time is limited. It is highly desired to have a nanoscale size power source that harvests its energy from the environment so that it works independently and wirelessly to provide power to the nanodevices. This dissertation provides a solid solution to this dilemma based on nanotechnology. Starting from the synthesis of well aligned ZnO nanowire arrays on different substrates, an innovative method is presented first to measure the mechanical property of the as-synthesized ZnO nanowire arrays by using AFM without destroying and manipulating the sample. This technique is then extended to converte mechanical energy into electricity by scanning the nanowire arrays using a AFM tip in contact mode. Due to the unique semiconducting and piezoelectric dual properties of ZnO, mechanical energy is converted into electricity and is effectively output. This is the invention of the piezoelectric nanogenerator. Then, by replacing AFM tips using a zigzag top electrode, the first prototype direct-cirrent nanogenerator driven by ultrasonic wave has been fabricated. Further investigations have also been carried out about the effect of ZnO carrier density on the output power, and the power generating property of oligomer functionalized ZnO nanowires. This desertation established the fundamental mechanism for the nanogenerator, and it provides a new path towards self-powered nanosystems, which has key applications in in-vivo biosensing, MEMS, environmental mornitoring, defence technology and even personal electronics.

Energy harvesting using piezoelectric nanomaterials provides an opportunity for advancement towards self-powered systems. Self-powered systems are a new emerging technology, which allows the use of a system or a device that perform a function without the need for external power source like for example, a battery or any other type of source. This technology can for example use harvested energy from sources around us such as ambient mechanical vibrations, noise, and human movement, etc. and convert it to electric energy using the piezoelectric effect. For nanoscale devices, the size of traditional batteries is not suitable and will lead to loss of the concept of “nano”. This is due to the large size and the relatively large magnitude of the delivered power from traditional sources. The development of a nanogenerator (NG) to convert energy from the environment into electric energy would facilitate the development of some self-powered systems relying on nano- devices. The main objective of this thesis is to fabricate a piezoelectric Zinc Oxide (ZnO) NGs for low frequency (˂ 100 Hz) energy harvesting applications. For that, different types of NGs based on ZnO nanostructures have been carefully developed, and studied for testing under different kinds of low frequency mechanical deformations. Well aligned ZnO nanowires (NWs) possessing high piezoelectric coefficient were synthesized on flexible substrates using the low temperature hydrothermal route. These ZnO NWs were then used in different configurations to demonstrate different low frequency energy harvesting devices. Using piezoelectric ZnO NWs, we started with the fabrication of sandwiched NG for hand writing enabled energy harvesting device based on a thin silver layer coated paper substrate. Such device configurations can be used for the development of electronic programmable smart paper. Further, we developed this NG to work as a triggered sensor for wireless system using foot-step pressure. These studies demonstrate the feasibility of using ZnO NWs piezoelectric NG as a low-frequency self-powered sensor, with potential applications in wireless sensor networks. After that, we investigated and fabricated a sensor on PEDOT: PSS plastic substrate either by one side growth technique or by using double sided growth. For the first growth technique, the fabricated NG has been used as a sensor for acceleration system; while the fabricated NG by the second technique has worked as anisotropic directional sensor. This fabricated configurations showed stability for sensing and can be used in surveillance, security, and auto-mobil applications. In addition to that, we investigated the fabrication of a sandwiched NG on plastic substrates. Finally, we demonstrated that doping ZnO NWs with extrinsic element (such as Ag) will lead to the reduction of the piezoelectric effect due to the loss of crystal symmetry. A brief summary into future opportunities and challenges are also presented in the last chapter of this thesis.

The advent of additive manufacturing has opened up new frontiers in developing metal structures that can have complex geometries, composite structures made of dissimilar metals, and metal structures with embedded sensing and actuation capabilities. These types of structures are possible with ultrasonic additive manufacturing (UAM); a novel manufacturing technology that combines additive manufacturing through the ultrasonic welding of thin metal foils with computer numerical control (CNC) milling. However, the process suffers from a critical limitation, i.e., a range of build heights within which bonding between a foil and the substrate cannot be originated. <br>This work has two research objectives, the first is a fundamental understanding of the complex dynamic interaction between the substrate and ultrasonic horn, or sonotrode. Specifically, it focuses on the effects that specific modes of vibration have on the dynamic response of the substrate. The second objective is to utilize the UAM process to create metal structures with an embedded sensor that can detect contact or impact. In addressing the first objective, a semi-analytical model was developed to determine the response to three forcing descriptions that approximate the interfacial friction between the foil and substrate induced by sonotrode compression and excitation. Several observations can be seen in the results: as the height increases the dominant modes of vibration change, the modes of vibration excited also change during a single weld cycle as the sonotrode travels across the length of the substrate, and finally the three forcing models do not have a significant impact on the substrate response trends with height and during the weld cycle. <br>In addressing the second objective, three prototypes were created by embedding a triboelectric nanogenerator (TENG) sensor within an AL3003 metal-matrix. TENGs utilize contact electrification between surfaces of dissimilar materials, typically polymers, combined with electrostatic induction to generate electrical energy from a mechanical excitation. The sensors demonstrate a discernible response over a 1-5 Hz frequency range. In addition, the sensors have a linear relationship between output voltage and a mechanically applied load, and have the ability to sense contact through both touch and due to an impacting object.

The triboelectric nanogenerator (TENG) has attracted global attention in the fields of power sources and self-powered sensors. By coupling the omnipresent triboelectrification effect and the electrical induction effect, the TENGs can transduce ambient mechanical energy into electrical energy. Such energy could be consumed instantaneously or stored for later use. In this way, they could be deployed distributedly to be compatible power sources in the era of the internet of things (IoTs), completing the powering structure that is currently relying on power plants. Also, the electrical signals can reflect the environment changes around the TENGs. Thus, the TENGs can serve as self-powered sensors in the IoTs. In this work, we adopted two approaches for TENG fabrication: the thermal drawing method (TDP) and 3D printing. With TDP, we have fabricated scalable fiber-based triboelectric nanogenerators (FTENG), which have been woven into textiles by an industrial loom for wearable use. This fabrication process can supply FTENG on a large scale and fast speed, bridging the gap between the TENG and weaving industry. With 3D printing, we have fabricated TENGs that are compatible with the shape of arbitrary substrates. They have been used as biocompatible sensors: human-skin-compatible TENG has been used to recognize silent speech in real-time by sensing the chin movement; the porcine-kidney-shaped fiber mesh has been used to monitor the perfusion rate of the organ. These works have extended the territory of TENGs and can be critical components in the IoTs. / Ph.D. / Portable electronic devices have become important components in our daily lives, and we are entering the era of the Internet of Things (IoTs), where everyday objects can be interconnected by the internet. While electricity is essential to all of these devices, the traditional power sources are commonly heavy and bulky and need to be recharged or directly connected to the immobile power plants. Researchers have been working to address this mismatch between the device and power systems. The triboelectric nanogenerators (TENG) are good candidates because they can harvest energy in the ambient environment. The users can use them to generate electricity by merely making the rubbing motion. In this work, we report two fabrication methods of the fiber-based triboelectric nanogenerators (FTENG). With the thermal drawing process, we have fabricated sub-kilometer-long FTENG and wove it with the regular cotton yarn into textiles. The wearable power source is human friendly as it does not induce any extra weight load for the user. Besides, we have demonstrated that such long fibers can work as self-powered distributed sensors, such as a Morse code generator. With 3D printing, we have fabricated FTENG-based devices that conform to the working substrates, which can be any shape. We have employed them as biofriendly sensors to translate the chin movement during speaking to language and to monitor the perfusion rate of a pig kidney. The FTENGs have offered excellent comfortability to the users and can play a vital role in reframing the power structure to be compatible with IoTs.

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: This study developed and optimised zinc oxide (ZnO) nanowire-based nanogenerator. The nanogenerator works on the piezoelectric effect that is, a mechanical force is converted to an electrical voltage. The ZnO nanowires are piezoelectric and when any force is applied to the nanowires an output voltage is generated. This ZnO nanowire-based nanogenerator can be used to power small electronic devices, such as pacemakers. The nanogenerator can also be incorporated into clothes and shoes to generate electricity to charge a cell phone for example. The problem experienced currently is that the nanogenerator does not generate enough electricity to be of practical use and needs to be further optimised. Simulations and mathematical models were used to identify areas where the nanogenerator could be optimised in order to increase the output voltage. It is shown that the morphology of the nanowires can have a considerable effect on the output voltage. For this reason the growth of the nanowires was investigated first. Different methods were used to propagate the nanowires in order to select the method that, on average, has the highest output voltage. Accordingly, one parameter at a time and design of experiments were used to optimise the nanowire growth. Consequently, these two methods were used to optimise the growth parameters with the respect to the output voltage. The aqueous solution method was found to yield nanowires that give the highest generated output voltage. After growing over 600 nanowire samples, optimal growth parameters for this method were found. These optimal growth parameters were subsequently used to grow nanowires that were used to manufacture the nanogenerator. The nanowires were grown on a solid substrate and hence the nanogenerator was also manufactured on the solid substrate. Through various optimisations of the manufacturing process the maximum output voltage achieved was about 500 mV. However, this output voltage is too low to be of practical use, even though the output has been raised considerably. The main problem was found to be the fact that the contact between the nanowires and the electrode was weak due to contamination. A new method was therefore required where the electrode and the nanowires would be in proper contact to ensure that higher output voltages were achieved. Subsequently, a flexible nanogenerator was manufactured in order to solve this problem. Accordingly, the nanowires were grown on the flexible polyimide film and a buffer layer was then spun onto the flexible substrate, leaving only the nanowire tips exposed. The electrode was then sputtered on top of this buffer layer, covering the nanowire tips. This ensured proper contact between the nanowires and the electrode. The nanogenerator, which was manufactured with non-optimal growth parameters, gives a maximum voltage output of 1 V, double the maximum achieved with the solid nanogenerator. When the optimal growth parameters were used the output voltage was raised to 2 V. Various optimisation techniques were performed on the nanogenerator, including plasma treatment and annealing and the use of various materials in the buffer layer. Combining these optimisation methods subsequently led to an optimised nanogenerator that can generate an output voltage of over 5 V. This was achieved after over 1200 nanogenerators had been manufactured. However, the output voltage was not in a usable form. Circuitry was therefore developed to transform the voltage generated by the nanogenerator to a useable form. The best circuit, the LTC3588, was used to power an LED for 10 seconds. The completed device was found to achieve a power output of 0.3 mW, enough for small electronic devices. / AFRIKAANSE OPSOMMING: ‘n Sink-oksied (ZnO) nanodraad gebaseerde nanogenerator is ontwikkeld en geöptimeer. Die nanogenerator werk met behulp van die piezoelektriese effek - meganiese krag work omgesit in ‘n elektriese spanning. Die ZnO nanodrade is piezoelektries en wanneer ‘n krag op die drade aangewend word, word ‘n uittree spanning gegenereer. Die nanogenerator kan gebruik word om klein elektroniese toestelle, soos ‘n pasaangeër, van krag te voorsien. Die nanogenerator kan in klere en skoene geïnkorporeer word om elektrisiteit op te wek vir die laai van ‘n selfoon. Die probleem is egter dat die nanogenerator tans nie genoeg krag opwek om prakties van nut te wees nie en verdere optimasie word benodig. Simulasies en wikundige modelle work gebruik om areas te identifiseer waar die nanogenerator geöptimeer kan word, met die doel om die uittreespanning te verhoog. Dit word bewys dat die morfologie van die nanodrade ‘n groot effek het op die uittreespanning. Dus word die groei van die nanodrade eerste ondersoek. Verskillende metodes word gebruik om die nanodrade te groei en die beste metode, wat die hoogste uittreespanning op gemiddeld verskaf, word gekies. Een parameter op ‘n slag en ontwerp van eksperimente word gebruik om die nanodraad groei te optimeer. Die groei parameters word geöptimeer deur van die twee metodes gebruik te maak, en die optimeering word gedoen in terme van die uittreespanning. Die oplossing groei metode lei tot nanodrade wat die hoogste uittreespanning verskaf. Na oor die 600 nanodraad monsters gegroei is, is die optimale parameters gevind. Hierdie optimale parameters word uitsluitlik gebruik om die nanogenerator te vervaardig. Die nanodrade word op ‘n soliede substraat gegroei en dus word die nanogenerator op dieselfde soliede substraat vervaardig. Verskeie metodes is gebruik om die vervaardiging te optimeer en die hoogste uittreespanning wat bereik is, is 500 mV. Die uittreespanning is te laag om van praktiese nut te wees alhoewel dit heelwat verhoog is. Die grootste probleem is die swak kontak tussen die nanodrade en die elektrode, wat veroorsaak word deur kontaminasie. ‘n Nuwe metode word verlang wat beter kontak tussen die nanodrade en elektrode sal verseker. ‘n Buigbare nanogenerator is vervaardig om die probleem op te los. Die nanodrade word nou op ‘n buigbare film gegroei. ‘n Bufferlaag word tussen die nanodrade in gedraai, tot net die punte van die nanodrade nog sigbaar is. Die elektrode word bo-op die bufferlaag gedeponeer, wat behoorlike kontak tussen die nanodrade en elektrode verseker. Die nanogenerator wat met nie-optimale groei parameters vervaardig is, bereik ‘n uittreespanning van 1 V, dubbel die soliede nanogenerator. Met optimale groei parameters word die uittreespanning tot 2 V verhoog. Verskeie optimasie tegnieke word op die nanogenerator toegepas. Die metodes sluit in suurstof plasma behandeling, verhitting en die inkorporasie van verskillende materiale in die bufferlaag. ‘n Kombinasie van die metodes geïnkorporeer in een nanogenerator lei tot ‘n uittreespanning van 5 V. Die uittreespanning is bereik na oor die 1200 nanogenerators vervaardig is. The uittreespanning is nog nie in ‘n bruikbare vorm nie. Spesiale stroombane is ontwikkel wat die nanogenerator spanning omskakel na ‘n bruikbare vorm. Die beste stroombaan, die LTC3588, kan ‘n LED aanskakel vir 10 sekondes. The toestel kan ook 0.3mWuittreekrag voorsien, genoeg vir klein elektroniese toestelle om te werk.

Energy harvesting and energy storage are two most important technologies in today's green and renewable energy science. As for energy harvesting, the fundamental science and practically applicable technologies are not only essential in realizing the self-powered electronic devices and systems, but also tremendously helpful in meeting the rapid-growing world-wide energy consumptions. Mechanical energy is one of the most universally-existing, diversely-presenting, but usually-wasted energies in the natural environment. Owing to the limitations of the traditional technologies for mechanical energy harvesting, it is highly desirable to develop new technology that can efficiently convert different types of mechanical energy into electricity. On the other hand, the electricity generated from environmental energy often needs to be stored before used to drive electronic devices. For the energy storage units such as Li-ion batteries as the power sources, the limited lifetime is the prominent problem. Hybridizing energy harvesting devices with energy storage units could not only provide new solution for this, but also lead to the realization of sustainable power sources. In this dissertation, the research efforts have led to several critical advances in a new technology for mechanical energy harvesting—triboelectric nanogenerators (TENGs). Previous to the research of this dissertation, the TENG only has one basic mode—the contact mode. Through rational structural design, we largely improved the output performance of the contact-mode TENG and systematically studied their characteristics as a power source. Beyond this, we have also established the second basic mode for TENG—the lateral sliding mode, and demonstrated sliding-based disk TENGs for harvesting rotational energy and wind-cup-based TENGs for harvesting wind energy. In order to expand the application and versatility of TENG by avoid the connection of the electrode on the moving part, we further developed another basic mode—freestanding-layer mode, which is capable of working with supreme stability in non-contact mode and harvesting energy from any free-moving object. Both the grating structured and disk-structured TENGs based on this mode also display much improved long-term stability and very high energy conversion efficiency. For the further improvement of the TENG’s output performance from the material aspect, we introduced the ion-injection method to study the maximum surface charge density of the TENG, and for the first time unraveled its dependence on the structural parameter—the thickness of the dielectric film. The above researches have largely propelled the development of TENGs for mechanical energy harvesting and brought a big potential of impacting people’s everyday life. Targeted at developing sustainable and independent power sources for electronic devices, efforts have been made in this dissertation to develop new fundamental science and new devices that hybridize the nanogenerator-based mechanical energy harvesting and the Li-ion-battery-based energy storage process into a single-step process or in a single device. Through hybridizing a piezoelectric nanogenerator with a Li-ion battery, a self-charging power cell has been demonstrated based on a fundamentally-new mechanical-to-electrochemcial process. The triboelectric nanogenerator as a powerful technology for mechanical energy harvesting has also been hybridized with a Li-ion battery into a self-charging power unit. This new concept of device can sustainably provide a constant voltage for the non-stop operation of electronic devices.