Effect of charged line defects on conductivity in graphene: Numerical Kubo and analytical Boltzmann approaches

Charge carrier transport in single-layer graphene with one-dimensional charged defects is studied theoretically. Extended charged defects, considered an important factor for mobility degradation in chemically-vapor-deposited graphene, are described by a self-consistent Thomas-Fermi potential. A numerical study of electronic transport is performed by means of a time-dependent real-space Kubo approach in honeycomb lattices containing millions of carbon atoms, capturing the linear response of realistic size systems in the highly disordered regime. Our numerical calculations are complemented with a kinetic transport theory describing charge transport in the weak scattering limit. The semiclassical transport lifetimes are obtained by computing scattered amplitudes within the second Born approximation. The transport electron-hole asymmetry found in the semiclassical approach is consistent with the Kubo calculations. In the strong scattering regime, the conductivity is found to be a sublinear function of electronic density and weakly dependent on the Thomas-Fermi screening wavelength. We attribute this atypical behavior to the extended nature of one-dimensional charged defects. Our results are consistent with recent experimental reports.Comment: 15 pages, 9 figure

Advanced Nanoelectromechanical Systems for Next Generation Energy Harvesting

The ever-increasing desire to produce portable, mobile and self-powered wireless micro-/nano systems (MNSs) with extended lifetimes has lead to the significant advancement in the area of mechanical energy harvesting over the last few years and it has been possible not only because has nanotechnology evolved as a powerful tool for the manipulation of matter on an atomic, molecular, and supramolecular scale, but also different micro-/nano fabrication techniques have enabled researchers and scientists to create, visualize, analyse and manipulate nano-structures, as well as to probe their nano-chemistry, nano-mechanics and other properties within the systems. The dissertation first discusses briefly about energy harvesting technologies for self-powered MNSs, for example a wireless aircraft structural health monitoring (SHM) system, with a particular focus on piezoelectric nanogenerators (PENG) and triboelectric nanogenerators (TENG) as they are the most promising approaches for converting ambient tiny mechanical energy into electrical energy efficiently and effectively and then it analyzes the theoretical and experimental methodologies for efficient energy harvesting using PENG, TENG and hybrid devices. The piezoelectric property intertwined with the semiconducting behaviour of different ZnO nanostructures has made them ideal candidate for piezoelectric energy harvesting, also intensive and state-of-the-art research has been going on to enhance the performance of the PENG devices based on 1D and 2D ZnO nanostructures. In this work, a high performance and consolidated PENG device based on the integration of ZnO nanowires and nanoplates on the same substrate has been demonstrated, that produces an output electrical power of 8.4 µW/cm2 at the matched load of 10MΩ that manifests their ability for powering up different MNSs. Since hybrid nanogenerators (HNG) integrate different types of harvesters in a single unit, where several energy sources can be leveraged either simultaneously or individually, in the next part of this work, a HNG device integrating PENG and TENG components has been designed, fabricated and characterized where PENG and TENG parts mutually enhance the performance of each other resulting an instantaneous peak power density of 1.864mW/cm2 and subsequently the device has been used to charge several commercial capacitors to corroborate their potential for aircraft SHM applications. Moreover, the hybrid device exhibits strong potential for wearable electronics as it can harvest energy from human walking and normal hand movements. However, successful implementation of self-powered electronics, such as a wireless aircraft SHM depends not only on the performance of individual parts but also on components integration within the system, where each device/system node within the network consists of a low-power microcontroller unit, high-performance data-processing/storage units, a wireless signal transceiver, ultrasensitive sensors based on a micro-/nano electro-mechanical system, and most importantly the embedded powering units. This dissertation aims to deepen the understanding of the different energy harvesting methods utilizing the knowledge of nanoscale phenomena and nanofabrication tools along with the associated prospects and challenges and thus, this research in the field of energy harvesting using advanced nano electro-mechanical systems could have a substantial impact on many areas, ranging from the fundamental study of new nanomaterial properties and different effects in nanostructures to diverse applications

Influence of Mechanical Properties on the Piezoelectric Response of UV-Cured Composite Films Containing Different ZnO Morphologies

ZnO flower-like (ZFL) and needle (ZLN) structures were synthesized and embedded into UV-curable acrylic resin (EB), with the aim to study the effect of filler loading on the piezoelectric properties of the resulting composite films. The composites showed uniform dispersion of fillers within the polymer matrix. However, by increasing the filler amount, the number of aggregates increased, and ZnO fillers appeared not to be perfectly embedded in polymer film, indicating poor interaction with acrylic resin. The filler content increase caused an increase in glass transition temperature (Tg) and a decrease in storage modulus in the glassy state. In particular, compared with pure UV-cured EB (Tg = 50 °C), 10 wt.% ZFL and ZLN presented Tg values of 68 and 77 °C, respectively. The piezoelectric response generated by the polymer composites was good when measured at 19 Hz as a function of the acceleration; the RMS output voltages achieved at 5 g were 4.94 and 1.85 mV for the composite films containing ZFL and ZLN, respectively, at their maximum loading levels (i.e., 20 wt.%). Further, the RMS output voltage increase was not proportional to the filler loading; this finding was attributable to the decrease in the storage modulus of the composites at high ZnO loading rather than the dispersion of filler or the number of particles on the surface

Flexible electronics : materials and sensor fabrication

This dissertation demonstrates how to fabricate piezoelectric/pyroelectric thin films by using different printing techniques. These techniques could replace vacuum techniques for manufacturing piezoelectric/pyroelectric sensors. Ink-jet, screen and stencil printing techniques were developed to print these devices. This work outlines attempts to develop a solution processable conductive ink for ink-jet printing. It then details the printing of commercial conductive ink on flexible substrates employing the three printing methods. Raman spectroscopy and Fourier transform infrared spectroscopy, are both used to investigate the structure of the P(VDF-TrFE) films. Optical microscopy is used to investigate the thickness and uniformity of the deposited films. The formulation of P(VDF-TrFE) for printing is also described for the three printing methods. Piezoelectric accelerometers have been developed and demonstrated. The sensors are axial compression piezoelectric accelerometers which measure impacts in the direction perpendicular to the sensors themselves. When the sensors are moved downward the top electrode tends to move upward, inducing charge via the piezoelectric effect. The sensors were mounted on an electrodynamic shaker and tested with an input vibration up to 1.5 g s at 100 Hz. The test data show that the accelerometers track the frequency of the input vibration; the output increases with increasing input acceleration. A comparison of the three printing methods to fabricate sensors on flexible substrates with commercial conductive inks and formulated P(VDF-TrFE) ink specific to the print method with similar geometries produces the following conclusions: Excellent adhesion of the commercial silver ink for screen and stencil printing has been achieved. The stencil printed silver films are smoother and more uniform than the screen printed films. Adhesion of the commercial PEDOT/PSS ink-jettable was successful. However, smoothness and uniformity were issues that need to be resolved. Also, when the ink-jetted PDOT/PSS films were exposed to high temperatures the films tended to crack and adhesion was lost. Functional devices were fabricated with screen and stencil printing quickly. In a one day period, multiple sheets of functional devices were obtained with both printing methods. Ink-jet printing, on the other hand, required greater then twenty four hours to fabricate one sheet of sensors even when the sensor size was reduced. The cost of masks/cartridges was $0.75,$1.68 and $59 per layer for stencil, screen and ink-jet printing respectively. The ink-jet print system cartridges were manufactured for one time use, whereas the masks were reusable for both screen and stencil printing. The best stencil and screen printed accelerometers demonstrated a voltage sensitivity of 145 mV/g. It is believed that the performance of these sensors can be enhanced with an automated printing system that is equipped with optical vision and automated alignment systems. The successful development of printed devices demonstrates that these print methods will be beneficial to the future of flexible electronics

Design and fabrication of a D 33 -mode piezoelectric micro-accelerometer

Abstract(#br)In this paper, a D 33 -mode piezoelectric micro-accelerometer with Pb 1.1 (Zr 0.52 Ti 0.48 )O 3 (PZT) thin film is designed, fabricated and tested. Both the polarization and deformation directions of the piezoelectric thin film are horizontal in this structure. With the high sensitivity and natural frequency, the D 33 -mode piezoelectric micro-accelerometer possesses improved practicality. The influence of filling factor ( Γ ) and interdigital electrode width ( b ) on the output voltage is analyzed in this work. The micro-electro-mechanical systems technology is then used to fabricate the piezoelectric accelerometer device, which is based on the Sol–Gel PZT piezoelectric thin film. Performance of the piezoelectric accelerometer micro-devices with the different Γ and b is..

Hybrid Structural Composites with Energy Harvesting Capabilities

Hybrid materials have received significant interest due to the potential enhancements they provide over traditional materials such as sensing, actuating, energy scavenging, thermal management, and vibration damping. While traditional materials can be utilized for either one of these functions or loadbearing, the hybrid materials are superior as they allow combination of a wide array of functionalities whilst being suitable for load-bearing purposes. The goal of this thesis is to elucidate the synergistic effects of hybridization of two piezoelectric materials; zinc oxide nanowires (ZnO NWs) and thin film of lead zirconium titanate (PZT) on the mechanical and energy harvesting of beams made from plain-woven carbon fiber reinforced epoxy composites (CFRPs). ZnO NWs have, by contrast, displayed great promises. While not only being a very strong piezoelectric material, it enhanced the mechanical and dynamic properties of the composite due to the increased surface area and mechanical interlocking. However, the aspect of energy scavenging is somewhat limited due to the weak piezoelectrical effects of ZnO nanowires. In this thesis, the prospects of ZnO NWs are exploited further to improve both their production processes and piezoelectric performance. Combining ZnO NWs grown on carbon fibers combined with other piezoelectrical materials has not yet been implemented but appears to be encouraging. This is the focus of this thesis. Despite that the composite comprising the combination of the two piezoelectric materials showed a minor drop in tensile strength and damping characteristics, the substantial gain in both stiffness (25.8 % increase compared to plain composite) and the electrical power gain (733.94 % more than that for ZnO NWs) is very promising for future application of the hybrid material into real engineering problems. A comprehensive study utilizing available commercial finite element software to simulate and foresee the behavior of hybrid materials was also carried out. The simulations agreed qualitatively with the experimental observations and explanations of the discrepancies between the model and experiment setup were discussed. Despite the preliminary promising results, more work is necessary to exploit the full potential of these material by optimizing the design of the energy harvesting devices and establishing more feasible models that treat the electromechanical coupling of these multifunctional hybrid composites more realistically