Includes bibliographical references (p. 80-83).

Thesis (Ph.D)--Industrial and Systems Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Wu, C. F. Jeff; Committee Co-Chair: Yuan, Ming; Committee Member: Huo, Xiaoming; Committee Member: Vengazhiyil, Roshan Joseph; Committee Member: Wang, Zhonglin. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The group of III-V semiconductors is emerging as highly attractive materials for a wide range of applications, particularly the gallium nitride family of alloys. Undoubtedly, the development of nitride-based light-emitting diodes (LEDs) and laser diodes (LDs) represented a quantum leap in the advancement of optoelectronics. The timely arrival of InGaN blue LEDs enable full-color mixing with existing red and green LEDs based on AlInGaP and GaP alloys respectively, promoting the progress of solid-state lighting and displays, while the blue-violet LDs has revolutionized high-density optical data storage in the form of blu-ray. Extensive research efforts have been conducted on producing energy-efficient and highly reliable LEDs and LDs in the past decade. Amongst potential strategies, nanotechnology promises to offer significant boosts to device performances. Nano-structure on a scale of wavelength of light exhibits prominent effects on the propagation behavior of photons. However, the formation of well-defined nanostructure relies heavily on processing techniques. Although e-beam lithography enables precise direct-writing of nano-patterns, high equipment cost and time-consuming processes make mass production impractical. On the other hand, the technique of nanosphere lithography (NSL) as adopted in the work reported in this thesis is a practical alternative approach. Uniform spheres acting as etch masks are capable of self-assembling into hexagonal close-packed arrays. The resultant nanopillar array serves as the photonic crystals (PhCs) extracting guided light while the individual pillars may act as circular cavities supporting the whispering gallery (WG) mode. Due to total internal reflections at the GaN-ambient interface with high refractive index contrast, low extraction efficiency is one of the major bottlenecks for LEDs. To assist with light extraction, the LED surfaces are textured via NSL to form PhC structures. The feature dimensions of the resultant patterns are scalable according to the diameter of nanospheres used. Such ordered closedpacked arrays are capable of promoting light extraction via the dispersion and diffraction properties of PhCs. To extend the functionality of sphere-patterned arrays, a dimension-adjusting procedure is developed to realize photonic bandgap (PBG) structures. Finite spacing between individual spheres is introduced, resulting in air-spaced nano-pillar PhCs structure with a wavelength-tunable PBG. Distinguished from typical PhCs in the form of air-holes or pillars, a clovershaped structure with a wide PBG is fabricated by dual-step NSL. The PBG structures have been exploited for suppressing lateral wave-guiding and possibly redirecting a significant proportion of trapped photons for extraction. Among various lasing mechanisms in nitride-based material, the whispering gallery mode based on cylindrical resonators is a promising candidate with attractive properties of intrinsically high Q factor, low lasing threshold and simple fabrication process. Since multiple-modes lasing from larger micro-resonators patterned by conventional photolithographic method has limited applications, smaller disk-shaped cavities patterned by NSL open up opportunities to realize short-wavelength single-mode resonators. By employing a modified NSL process, photo-pumped blue lasing modes has been demonstrated from nanoring arrays, with low threshold of ~10 mJ/cm2 and a high Q factor of ~5000. Single-mode UV lasing at 373 nm has also been observed from metal-clad pillar structures. The lasing mechanisms are all verified through finite-difference time domain simulations. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Self-ordered anodic porous alumina with a nano-honeycomb structure has recently been extensively used as templates for the synthesis of various nanomaterials for diverse applications. However, due to the insufficient knowledge on the combined electro-chemo-mechanical processes, the formation mechanism of self-ordering has been under debate for decades without clear conclusions. Also, fast fabrication of highly self-ordered and mechanically stable anodic porous alumina is still a challenge. Furthermore, the actuation behavior of anodic porous alumina upon external mechanical and electrical triggering in an electrochemical cell has not been exploited. In this work, firstly, we investigated the self-ordering mechanism by establishing a kinetics model involving the Laplacian electric potential distribution and a continuity equation for current density within the oxide body. Current densities governed by the Cabrera-Mott equation are formed by ion migration within the oxide as well as across the interfaces. The pore channel growth, due to electric-field-assisted reactions, is governed by Faraday’s law. Real-time evolution of pre-patterned pore channel growth was simulated in two-dimensional cases by finite element method. The simulations revealed a parameter domain within which pre-patterned pore channels will continue to grow in a stable manner during the subsequent anodization if the pre-patterns are commensurate with the self-ordered configurations, or these are driven into stable if the pre-patterns do not initially match the self-ordered configurations. This was verified in experimentally observed pore channel growth under the guidance of pre-patterns made by focused-ion-beam milling. Furthermore, the simulations revealed that ionization reaction on (001) oriented Al grain is relatively easier than that on (101) grain, which results in stable and unstable pore channel growth on (001) and (101) Al grains, respectively, both of which were observed from the simulations and experiments. Secondly, a scheme on quantitative evaluation of self-ordering qualities in anodic porous alumina has been developed, based on which we systematically searched the optimum self-ordering conditions, by varying the key anodization factors, including substrate grain orientation, electrolyte concentration, temperature, voltage, and time. A high acid concentration and high temperature anodization method was found. Compared with conventional methods, the present method can realize fast formation of highly self-ordered, and mechanically stable anodic porous alumina under a continuous range of anodization voltage with tunable interpore distances. Thirdly, reversible bending was found in anodic porous alumina-Al composites upon cyclic electric actuation, as directly observed by an optical microscope and detected by in situ nanoindentation. The bending is thought to be the result of charge-induced surface stresses in the nanoporous alumina. The results suggest a new type of composite materials for applications as micro-scale actuators to transform electrical energy into mechanical energy. Furthermore, the composite exhibits significant softening during in situ nanoindentation when the estimated maximum stress underneath the indenter is exerted on the metal/oxide interface. Softening was further verified by in situ microindentation. Electron microscopy examination indicated that the softening is due to a combination of high compression stress and electric field acting near the interface, which enhance ionization reaction and cause the interface to move faster into the substrate. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

Zero valent iron (ZVI) has demonstrated its reactivity and effectiveness for in-situ groundwater and soil remediation. The potential of the high reducing activity of nanoscale ZVI (nZVI) for environmental decontamination has attracted more attentions in recent years, as nZVI may be injected with water to the pollution sites for in-situ remediation. However, rapid oxidation and instant agglomeration of nZVI make it difficult for large-scale engineering application. Effort has been made to improve the stability and mobility of nZVI for effective in-situ remediation. In the present study, a novel Ca-based surface coating method has been developed for protection of nZVI and enhancement of its transport in environmental applications. A simple thermal deposition method was employed to coat a Ca-based layer on the surface of micro- or nano- ZVI particles in water or methanol environment. According to microscopic observations, Ca(OH)2 nano-layer was formed on the ZVI surface. A clear core-shell structure was observed for the coated nZVI/Ca(OH)2 particles based on the TEM observations. The Ca(OH)2 coating layer had a thickness about one fifth of the nZVI diameter and the Ca to Fe ratio was below 0.2. With the Ca(OH)2 shell, nZVI particles can be effectively protected against corrosion according to the standard natural spray corrosion tests. Thus, the Ca(OH)2 coating layer is able to greatly improve the stability of nZVI during storage, transportation and application. In addition, based on the result of the dissolution tests, the Ca(OH)2 shell could be readily dissolved in water with a low Ca content or a low ionic strength. After dissolution of the Ca(OH)2 shell, the reactivity of nZVI was found to be at the similar level as bare nZVI, which could remove Cr(VI) from water by more than 90% in about 20 min. The pseudo-first order rate constants for Cr(VI) reduction by bare nZVI and nZVI/ Ca(OH)2 after shell dissolution were 0.064 and 0.072 min-1, respectively. Moreover, the Ca(OH)2 coating shell would not only function as a protection layer but also improve the mobility of nZVI particles in in-situ applications. The aggregation and sedimentation of nZVI/Ca(OH)2 particles became considerably slower compared to bare nZVI without the coating. Clean-bed water filtration tests were conducted with sand and glass columns to evaluate the mobility and transport of nZVI in porous media. The results show that bare nZVI in the particle suspension deposited mostly at the top of the filters with little penetration. In comparison, the nZVI/Ca(OH)2 particles were able to penetrate through the filter media during the filtration process, and the dark iron particles could fill up the entire filter columns. The penetration rate increased from nearly 0 m/hr for bare nZVI to 0.43 m/hr for nZVI/Ca(OH)2 through the filter media. The Ca-based coating materials are known as of low cost and environmentally friendly. Thus, the new coating method developed in this study provides a cost-effective means for both the protection of nZVI and improvement of its transport and delivery in porous media for environmental decontamination. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Organic solar cells (OSCs) have recently attracted considerable research interest. However, there is a mismatch between their optical absorption length and charge transport scale. Attempts to optimize both the optical and electrical properties of the photoactive layer of OSCs have inevitably resulted in demands for rationally designed device architecture. Plasmonic nanostructures have recently been introduced into solar cells to achieve highly efficient light harvesting. The remaining challenge is to improve OSC performance using plasmonic nanotechnology, a challenge taken up by the research reported in this thesis. I systematically investigated two types of plasmonic effect: localized plasmonic resonances (LPRs) and surface plasmonic resonances (SPRs). Broadband plasmonic absorption is obviously highly desirable when the LPR effect is adopted in OSCs. Unfortunately, typical nanomaterials possess only a single resonant absorption peak, which inevitably limits the power conversion efficiency (PCE) enhancement to a narrow spectral range. To address this issue, I combined Ag nanomaterials of different shapes, including nanoparticles and nanoprisms. The incorporation of these mixed nanomaterials into the active layer resulted in wide band absorption improvement. My results suggest a new approach to achieving greater overall enhancement through an improvement in broadband absorption. I also explored the SPR effect induced by a metal patterned electrode with two parts. Most reports to date on back reflector realization involve complicated and costly techniques. In this research, however, I adopted a polydimethylsiloxane (PDMS)-nanoimprinted method to produce patterned back electrodes in OSCs directly, which is a very simple and efficient technique for realizing high-performance OSCs in industrial processes. Besides, a remaining challenge is that plasmonic effects are strongly sensitive to light polarization, which limits plasmonic applications in practice. To address this issue, I designed three-dimensional patterns as the back electrode of inverted OSCs, which simultaneously achieved highly efficient and polarization-independent plasmonic OSCs. In addition to investigating the two types of plasmonic effect individually, I also investigated their integrated function by introducing both LPRs and SPRs in one device structure. With the aim of achieving high-performance OSCs, I first demonstrated experimentally a dual metal nanostructure composed of Au nanoparticles (i.e. LPRs) embedded in the active layer and an Ag nanograting electrode (i.e. SPRs) as the back reflectors in inverted OSCs, which can generate a very strong electric field, in a single junction to improve the light absorption of solar cells. As a result, the PCE of the OSC reached 9.1%, making it one of the best-performing OSCs reported to date. In addition, as an important extension, I subsequently achieved tremendous near-field enhancement owing to multiple couplings, including nanoparticle-nanoparticle (LPR-LPR) couplings and nanoparticle-film (LPR-SPR) couplings, by designing a novel nanoparticle-film coupling system through the introduction of ultrathin monolayer graphene as a well-defined sub-nanogap between the Ag nanoparticles and Ag film. The graphene sub-nanogap is the thinnest nanogap (in atomic scale terms) to date, and thus constitutes a promising light-trapping strategy for improving future OSC performance. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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With the progress of microfabrication and nanofabrication technologies, there has been a reawakened interest in the possibility of controlling the propagation of light in various materials periodically structured at a scale comparable to, or slightly smaller than the wavelength. We can now engineer materials with periodic structures to implement a great variety of optical phenomena. These include well known effects, such as dispersing a variety of wavelength to form a spectrum and diffracting light and controlling its propagation directions, to new ones such as prohibiting the propagation of light in certain directions at certain wavelengths and localizing light with defects in some artificially synthesized dielectric materials. Advances in this field have had tremendous impact on modern optical and photonic technologies. This doctoral research was aimed at investigating some of the physics and applications of periodic structures for building blocks of the optical communication and interconnection system. Particular research emphasis was placed on the exploitation of innovative periodic structure-based optical and photonic devices featuring better functionality, higher performance, more compact size, and easier fabrication. Research topics extended from one-dimensional periodic-structure-based wavelength-division-multiplexing (WDM) optical interconnects (beam wavelength selection devices), and liquid crystal beam steerers (beam steering devices), to two-dimensional periodic-structure-based silicon photonic-crystal thermo-optic and electro-optic modulators (beam switching devices). This research was specifically targeted to seek novel and effective solutions to some long-standing technical problems, such as the limited wavelength coverage of coarse WDM devices, small bandwidth of highly dispersed dense WDM devices, low deflection efficiency of high-resolution liquid crystal beam steerers, slow switching speed, large device size, and high power consumption of silicon optical modulators, among others. For each subtopic, research challenges were presented and followed by the proposed solutions with extensive theoretical analysis. The proposals were then verified by experimental implementations. Experimental results were carefully interpreted and the future improvements were also discussed.