FABRICATION OF VERTICALLY ALIGNED CARBON NANOTUBES AND HORIZONTAL NANO-STRUCTURES

Fabrication of ordered anodic alumina nanopore arrays and anodization parameters including electrolyte, concentration, voltage, temperature and time have been investigated. Cobalt nanoparticles were electrodeposited at the bottom of the pores. Vertically aligned, open-tipped multi-walled carbon nanotube arrays of high density and uniformity were synthesized via a flame method on silicon substrates using a nanoporous template of anodized aluminum oxide. The diameter and length of the nanotubes are controlled by the geometry of the aluminum oxide template. It is the cobalt catalyst particles, not the porous aluminum templates, help the growth of carbon nanotubes through graphitization and bonding of carbon nanotubes to the silicon substrates. Fabrication of nano-structures has been demonstrated. Nano-trenches of 20 nm have been achieved using single-walled nanotube bundles as shadow masks, which were aligned across electrodes under high frequency AC voltage

INTEGRATION AND CHARACTERIZATION OF TOBACCO MOSAIC VIRUS BASED NANOSTRUCTURED MATERIALS IN THREE-DIMENSIONAL MICROBATTERY ARCHITECTURES

The realization of next-generation portable electronics, medical implants and miniaturized, autonomous microsystems is directly linked with the development of compact and efficient power sources and energy storage devices with high energy and power density. As the components of these devices are continuously scaled down in size, there is a growing demand for decreasing the size of their power supply as well, while maintaining performance comparable to larger assemblies. This dissertation presents a novel approach for the development of microbattery electrodes that is based on integrating both micro and nano structured components for the formation of hierarchical electrodes. These electrodes combine both high energy density (enabled by the high surface area and mass loading) with high power density (due to the small thickness of the active battery materials). The key building block technologies in this work are the bottom-up self-assembly and metallization of a biological template and the top-down microfabrication processes enabled by Microelectromechanical Systems (MEMS) technology. The biotemplate used is the Tobacco mosaic virus (TMV), a rod-like particle that can be genetically modified to express functional groups with enhanced metal binding properties. In this project, this feature is combined with standard microfabrication techniques for the synthesis of nanostructured energy-related materials as well as their hierarchical patterning in device architectures. Specifically, synthesis of anode (TiO2) and cathode (V2O5) materials for Li-ion batteries in a core/shell configuration is presented, where the TMV biomineralization is combined with atomic layer deposition of the active material. These nanostructured electrodes demonstrate high energy storage capacities, high rate capabilities and superior performance to electrodes with planar geometries. In addition, a toolbox of biofabrication processes for the defined patterning of virus-templated structures has been developed. Finally, the nanocomposite electrodes are integrated with three-dimensional micropillars to form hierarchical electrodes that maintain the high rate performance capabilities of nanomaterials while exhibiting an increase in energy density compared to nanostructures alone. This is in accordance with the increase in surface area added by the microstructures. Investigation of capacity scaling for varying active material thickness reveals underlying limitations in nanostructured electrodes and highlights the importance of this method in controlling both energy and power density with structural hierarchy. These results present a paradigm-shifting technology for the fabrication of next-generation microbatteries for MEMS and microsystems applications

Silicon nano wires solar cell

Improving the optical absorption capability of solar cells\u27 materials is a crucial factor in increasing their power conversion efficiency. To this end, the absorption can be enhanced by minimizing the reflection and the transmission out from the absorbing layer. While the reflection can be minimized using an antireflection coating, the transmission can be minimized by exploiting a light- trapping mechanism. In this thesis, the Si nanowires have been utilized to enhance the absorption and photocurrent without the need for antireflection coating, and provide high field localization, which in turn enhances the overall efficiency of the solar cell. Vertically orientated single crystalline silicon nanowire (SiNW) arrays with controlled diameters have been fabricated via a metal-assisted chemical etching (MACE) method. The diameter of the fabricated nanowires is controlled by simply varying the etching time in HF/H2O2 solution. The fabricated SiNWs have diameters ranging from 117 to 650 nm and length from 8 to 18 μm. The optical measurements show a significant difference in the reflectance/absorption of the SiNWs with different diameters, where the reflectance increases with increasing the diameter of the SiNWs. The optical absorption also has been measured at different incident light angle to determine the best angle for absorption. The best absorption angle for different diameters was 10o.The SiNWs showed significant photoluminescence (PL) emission spectra with peaks lying between 380 and 670 nm. The PL intensity increases as the diameter increases and shows red shift for peaks at ~ 670 nm. The increase or decrease of reflectivity is coincident with PL intensity at wavelength ~ 660 nm. The x-ray diffraction (XRD) patterns and high-resolution transmission electron microscope (HR-TEM) confirm the high crystallinity of the fabricated SiNWs. In addition, the Raman spectra showed a shift in the first order transverse (1TO) band toward lower frequencies compared to that usually seen for c-Si. The current-voltage characteristics have also been investigated using photoelectrochemical cell. The measurements have been done in two electrolytes; 10% HF 10% and hydrobromic acid (40%) and bromine (3%). The measurements have been done for the fabricated Si nanowires with different diameters under dark and illumination conditions. The resulted photocurrent decreases with increasing the diameter of SiNWs, which has been explained based on the Debye length of SiNWs. Full wave electromagnetic analysis has been performed using finite difference time domain simulations (FDTD) to confirm the effect of change of diameter on the optical properties of the nanowires. The simulation results show good agreement with the experimental findings for the SiNWs of different diameters. Also, the simulation has been done for different incident light angles to investigate the best incident angle that results in the highest absorption and minimum reflection

Low temperature plasma processing of platinum porphyrins for the development of metal nanostructured layers

This article establishes the bases for a vacuum and plasma supported methodology for the fabrication at mild temperatures of nanostructured platinum in the form of porous layers and nanocolumns using platinum octaethylporphyrin as precursor. In addition, the application of these materials as tunable optical filters and nano-counterelectrodes is proved. On one hand, the transparency in the ultraviolet-visible-near infrared range can be adjusted precisely between 70% and 1% by tuning the deposition and processing conditions, obtaining a high spectral planarity. Deviations of the spectra from an ideal flat filter are below 4%, paving the way to the fabrication of neutral density filters. The transparency limit values yield a sheet resistivity of ¿1350 and 120 ¿ ¿-1, respectively. On the other hand, the catalytic properties of the nanostructures are further demonstrated by their implementation as counterelectrodes of excitonic solar cells surpassing the performance of commercial platinum as counterelectrode in a 20% of the overall cell efficiency due to simultaneous enhancement of short-circuit photocurrent and open-circuit photovoltage. One of the most interesting features of the developed methodology is its straightforward application to other metal porphyrins and phthalocyanines readily sublimable under mild vacuum and temperature conditions.Junta de AndaluciaTEP8067 FQM-6900 FQM 1851 P12-FQM-2265España MinecoMAT2013-40852-R MAT2013-42900-P MAT2013-47192-C3-3-RMAT2016-79866-RMINECO-CSIC 201560E055

Doctor of Philosophy

dissertationTiO2 is an extensively studied material due to its nontoxic, environmental friendly, corrosion-resistant nature and wide band gap (~3 eV). TiO2 nanotubes (T-NTs) synthesized via electrochemical anodization have been studied extensively, with particular focus on their electrical and optical properties. The advantage of T-NT is the large surface area to volume ratio. T-NT has been used to demonstrate many applications such as sensors and energy harvesting. These applications have traditionally been demonstrated via T-NT synthesized on Ti foil. However, there is currently no commercially available T-NT- based device, which may be due to a lack of fabrication techniques, to make such devices on a large scale. One of the requirements for fabricating compact T-NT- based devices is the need for a stable and planar substrate. The titanium foils commonly used for T-NT synthesis are mechanically flexible, making them more prone to bending, limiting the integration of T-NT with microfabrication techniques. Here, we present the synthesis of T-NT on Si wafer at room temperature from direct current (D.C.) sputtered as well as e-beam evaporated thin Ti film. Hundred nm SiO2 was used to electrically isolate the T-NT from the substrate. We demonstrate integration of the synthesis of T-NT with photolithography, which is one of the most important requirements for scaling up a T-NT-based device. The T-NT was stable up to 500oC, which is required for improved charge transport. The T-NT was 1.4 times longer than the thickness of the Ti film, showing selective electric field-assisted etching of Ti by the electrolyte. We also report site-specific and patterned growth of the T-NT. The effect of properties of thin films such as grain size, residual stress and density on the morphology of T-NT was studied to improve the stability and quality of the T-NT. We demonstrate the synthesis of TiO2-WO3 composite nanotubes for photoelectrochemical cells with up to a 40% increase in photocurrent in comparison to plain T-NT. The T-NT was extensively studied and characterized using SEM, AFM, UV-Vis spectroscopy, XRD, and XPS

Porous Silicon

Porous silicon is a sponge-like structure of monocrystalline silicon which although accidentally discovered, soon became one of the most well-researched silicon structures. Its properties and applications have been the main subject of several books and more than a dozen review articles. However, a survey of porous silicon fabrication methods has not been published even though more than 20 different routes have been developed to synthesize this material. This chapter briefly discusses the properties of porous silicon, describes its fabrication methods, and introduces its applications