Parametric Optimization of Visible Wavelength Gold Lattice Geometries for Improved Plasmon-Enhanced Fluorescence Spectroscopy

The exploitation of spectro-plasmonics will allow for innovations in optical instrumentation development and the realization of more efficient optical biodetection components. Biosensors have been shown to improve the overall quality of life through real-time detection of various antibody-antigen reactions, biomarkers, infectious diseases, pathogens, toxins, viruses, etc. has led to increased interest in the research and development of these devices. Further advancements in modern biosensor development will be realized through novel electrochemical, electromechanical, bioelectrical, and/or optical transduction methods aimed at reducing the size, cost, and limit of detection (LOD) of these sensor systems. One such method of optical transduction involves the exploitation of the plasmonic resonance of noble metal nanostructures. This thesis presents the optimization of the electric (E) field enhancement granted from localized surface plasmon resonance (LSPR) via parametric variation of periodic gold lattice geometries using finite difference time domain (FDTD) software. Comprehensive analyses of cylindrical, square, star, and triangular lattice feature geometries were performed to determine the largest surface E-field enhancement resulting from LSPR for reducing the LOD of plasmon-enhanced fluorescence (PEF). The design of an optical transducer engineered to yield peak E-field enhancement and, therefore, peak excitation enhancement of fluorescent labels would enable for improved emission enhancement of these labels. The methodology presented in this thesis details the optimization of plasmonic lattice geometries for improving current visible wavelength fluorescence spectroscopy

Plasma Treatment of Zinc Oxide Thin Film and Temperature Sensing Using the Zinc Oxide Thin Film

Zinc oxide is a direct and wide bandgap, II-VI semiconductor. It has large exciton binding energy, large piezoelectric constant, strong luminescence, and high thermal conductivity. These properties make zinc oxide as a suitable material for various optoelectronic applications. Vacuum based processes of fabrication of zinc oxide thin film dominate the market for their better electrical and optical properties. In this work, zinc oxide thin films were prepared by easy and low cost solution method with oriented crystal growth along (002) plane. To improve electrical and optical property of the fabricated zinc oxide thin films, films were treated with oxygen, hydrogen, and nitrogen plasmas. Oxygen plasma treatment improved the crystallinity of zinc oxide thin film. Hydrogen plasma treatments were found very effective in improving the electrical conductivity of the film sacrificing film’s transmittance. Nitrogen plasma treatment following hydrogen plasma treatment could restore the transmittance maintaining the improved electrical property. Sequential oxygen, hydrogen, and nitrogen plasma treatment decreased the resistivity of zinc oxide thin film by more than two order maintaining transmittance close to the as deposited film. This work also reports a temperature sensor based on the temperature-dependent bandgap of zinc oxide semiconductors. Transmittance measurement of the ZnO films at different temperatures showed sharp absorption edge at around 380 nm and red shift characteristics. An optical temperature sensor was established using the zinc oxide coated glass as sensing element, ultra-violet light emitting diode as light source, and a ultra-violet photodiode as light detector. Short circuit current of the photodiode was measured over a range of the zinc oxide film’s temperature. The short circuit current decreased linearly with the increase of the temperature and the sensitivity was ~0.1 μA/°C

Superfocusing, Biosensing and Modulation in Plasmonics

Plasmonics could bridge the gap between photonics and electronics at the nanoscale, by allowing the realization of surface-plasmon-based circuits and plasmonic chips in the future. To build up such devices, elementary components are required, such as a passive plasmonic lens to focus free-space light to nanometre area and an active plasmonic modulator or switch to control an optical response with an external signal (optical, thermal or electrical). This thesis partially focuses on designing novel passive and active plasmonic devices, with a specific emphasis on the understanding of the physical principles lying behind these nanoscale optical phenomena. Three passive plasmonic devices, designed by conformal transformation optics, are numerically studied, including nanocrescents, kissing and overlapping nanowire dimers. Contrary to conventional metal nanoparticles with just a few resonances, these devices with structural singularities are able to harvest light over a broadband spectrum and focus it into well-defined positions, with potential applications in high efficiency solar cells and nanowire-based photodetectors and nanolasers. Moreover, thermo-optical and electrooptical modulation of plasmon resonances are realized in metallic nanostructures integrated with either a temperature-controlled phase transition material (vanadium dioxide, VO2), or ferroelectric thin films. Taking advantage of the high sensitivity of particle plasmon resonances to the change of its surrounding environment, we develop a plasmon resonance nanospectroscopy technique to study the effects of sizes and defects in the metal-insulator phase transition of VO2 at the single-particle level, and even single-domain level. Finally, we propose and examine the use of two-dimensional metallic nanohole arrays as a refractive index sensing platform for future label-free biosensors with good surface sensitivity and high-throughput detection ability. The designed plasmonic devices have great potential implications for constructing nextgeneration optical computers and chip-scale biosensors. The developed plasmon resonance nanospectroscopy has the potential to probe the interfacial or domain boundary scattering in polycrystalline and epitaxial thin films

Study of parameters dominating electromechanical and sensing response in ionic electroactive polymer (IEAP) transducers

Ionic electroactive polymer (IEAP) transducers are a class of smart structures based on polymers that can be designed as soft actuators or sensors. IEAP actuators exhibit a high mechanical response to an external electrical stimulus. Conversely, they produce electrical signals when subjected to mechanical force. IEAP transducers are mainly composed of four different components: The ionomeric membrane (usually Nafion) is an ion permeable polymer that acts as the backbone of the transducer. Two conductive network composite (CNC) layer on both sides of the ionomeric membrane that enhance the surface conductivity and serve as an extra reservoir to the electrolytes. The electrolytes, (usually ionic liquids (IL)), which provides the mobile ions. And two outer electrodes on both sides of the transducer to either provide a distributed applied potential across the actuators (usually gold leaves) or to collect the generated signals from the sensors (usually copper electrodes). Any variation in any of these components or the operating conditions will directly affect the performance of the IEAP transduces. In this dissertation, we studied some of the parameters dominating the performance of the IEAP transducers by varying some of the transducers components or the transducers operating conditions in order to enhance their performance. The first study was conducted to understand the influence of ionic liquid concentration on the electromechanical performance of IEAP actuators. The IL weight percentage (wt%) was varied from 10% to 30% and both the electromechanical (induced strain) and the electrochemical (the current flow across the actuators) were studied. The results from this study showed an enhanced electrochemical performance (current flow is higher for higher IL wt%) and a maximum electromechanical strain of approximately 1.4% at 22 wt% IL content. A lower induced strain was noticed for IL wt% lower or higher than 22%. The second study was to investigate the effect of changing the morphology of the CNC on the sensing performance of IEAP stress sensors. In this study, small salt molecules were added to the CNC layers. Salt molecules directly affected the morphology of the CNC layers resulting in a thicker, more porous, and high conductive CNCs. As a result, the ionic conductivity increased through the CNC layers and sensing performance was enhanced significantly. In the third study, a non-linear angular deformation (limb-like motion) was achieved by varying the CNC layers of the IEAP actuators by adding some conjugated polymers (CP) patterns during the fabrication of the actuators. It was found that the segments with the CP layers will only expand and never contract during the actuation process. Depending on the direction of motion and the location of the CP layers, different actuation shapes such as square or triangular shapes were achieved rather than the typical circular bending. In the fourth study, the influence of temperature on the electromechanical properties of the IEAP actuators was examined. In this study, both electromechanical and electrochemical studies were conducted for actuators that were operated at temperatures ranging from 25 ðC to 90 ðC. The electromechanical results showed a lower cationic curvature with increasing temperature up to 70 ðC. On the other hand, a maximum anionic curvature was achieved at 50 ðC with a sudden decrease after 50 ðC. Actuators started to lose functionality and showed unpredictable performance at temperatures higher than 70 ðC. Electrochemically, an enhancement of the ionic conductivity was resulted from increasing temperature up to 80 ðC. A sudden increase in current flow was recorded at 90 ðC indicating a shorted circuit and actuator failure. Finally, in the fifth study, protons in Nafion membranes were exchanged with other counterions of different Van der Waals volumes. The ionic conductivity was measured for IEAP membranes with different counterions at different temperatures. The results showed higher ionic conductivities across membranes with larger Van der Waals volume counterions and higher temperatures. A different ionic conductivity behavior was also noticed for temperatures ranging from 30 úC to 55 úC than temperatures between 55 úC and 70 úC after fitting the data with the Arrhenius conductivity equation

Block Copolymer Nanolithography for Sub-50 nm Structure Applications

As high technology device patterns are continuing to move towards decreasing critical dimensions and increasing pattern density, there is a need for lithography to move in the same direction. Block copolymer (BCP) lithography is a promising technique, which has single digit nanometer resolution, has a pattern periodicity of about 7-200 nm, and easily scales up to large area at a low cost. The use of BCPs with high immiscibility of constituent blocks, so-called high-Chi material, enables smaller pattern dimensions and is therefore of special interest. However, for lithography techniques to be applicable, also integration into existing nanofabrication processes is necessary. Furthermore, development of techniques to perform sub 10 nm pattern transfer is an enabler for continued device development. This dissertation first provides an overview of the BCP lithography field, to thereafter study the selective infiltration synthesis of alumina into the maltoheptaose block in high-Chi poly(styrene)-block-maltoheptaose of 12 nm pattern periodicity. The infiltration was studied using neutron reflectometry, and a subsequent sub-10 nm pattern transfer was performed into silicon. Also, it studies the process of surface reconstruction of high-Chi poly(styrene)-block-poly(4-vinylpyridine) of 50 nm pattern periodicity, more specifically the effect of time and temperature on pore diameter. Furthermore, pattern transfer of the surface reconstructed BCP film into silicon nitride, and selective area metal-organic vapor phase epitaxy (SA-MOVPE) of indium arsenide vertical nanowires on a silicon platform, using directed self-assembly is demonstrated. By directing the self-assembly along different crystal directions of the substrate, two vertical nanowire configurations were grown. Demonstration of gate all-around stack deposition of oxide/metal to the densely packed nanowire configurations was thereafter made. The results have contributed to the knowledge on BCP lithography and pattern transfer in the sub 50 nm regime, enabling new approaches for applications such as vertical nanowire, or fin transistors

Advancing nanofabrication processes for the generation of multifunctional surfaces

Ubiquitous in the natural world, micro- and/or nano-structured surfaces can afford simultaneous control over a range of interfacial properties; providing an attractive solution for where the accumulation of fluids (fog/rain/oil) and bacteria, and the mismanaged interaction of photons, can impede the safety or efficiency of the surface. Although surfaces found in nature provide a wealth of inspiration, replicating the structures synthetically persists to be a challenge, particularly so when striving for scalability and simplicity to encourage industrial/commercial uptake. Furthermore, the fabrication challenges become amplified when aiming for sub-wavelength structures; often necessary to unlock or enhance additional functionality. In this thesis, I present novel fabrication routes based on lithography and reactive ion etching (RIE) to achieve a range of ordered structures at the nano-scale in glass and silicon, and further replicate the resultant structures into polymers. I explore scalable masking techniques including block copolymer (BCP) lithography, laser interference lithography (LIL) and nanoimprint lithography (NIL), to achieve a series of pitches from 50 – 600 nm. By coupling the masking with novel combinations of etching chemistries, and taking advantage of the etch resistivity of different materials, I fabricate high aspect ratio nanostructures through simplified processes and demonstrate their ability to target applications in wettability, photonics and anti-bacterial action. Specifically, for silicon and glass nanocones, I focus on their anti-fogging, superhydrophobic, anti-reflective and anti-bacterial properties. I also investigate the impact of the nanostructure morphology on a sub-class of water-repellent surfaces, namely, slippery liquid infused porous surfaces, and their ability to retain lubricant under dynamic conditions; continuing on the theme of smart nanostructure design and simplified fabrication to pave a route to multifunctional surfaces. It is anticipated that the surfaces and their properties will find use as car windscreens, coatings for solar panels, high-rise glass facades, and high-touch surfaces to name a few

Hybrid Plasmonic Nanoantennas: Fabrication, Characterization, and Application

As optical counterpart of microwave antennas, plasmonic nanoantennas are important nanoscale devices for converting propagating optical radiation into confined/enhanced electromagnetic fields. Presently, nanoantennas, with a typical size of 200-500 nm, have found their applications in bio-sensing, bio-imaging, energy harvesting, and disease cure and prevention. With the device feature size of next generation IC goes down to 22 nm or smaller, and biological/chemical sensing reaches the Gene’s level, the sizes of the corresponding nanoantennas have to be scaled down to sub-100nm level. In the literature, these sub-100nm nanoantennas are referred as deep subwavelength nanoantennas as size of such miniaturized nanoantennas is only a fraction of the wavelength of applied visible light range (390nm-750nm)

Electrical and Optical Properties of Upgraded Metallurgical Grade Silicon Solar Cells.

Silicon(Si) accounts for more than ~ 90 % of solar cell market due to its advantages of earth abundance, good reliability, performance, and a wealth of Si materials processing knowledge. However, as the photovoltaic industry matures, there have been more demands on lowering the cost of solar cells, which is mainly dominated by the cost of starting materials. Currently two major approaches are pursued to reduce the cost of Si- based solar cells per watt: the adoption of low-cost silicon such as metallurgical-grade (MG) Si or upgraded metallurgical-grade (UMG) Si, and reducing the usage of Si by producing ultrathin solar modules. UMG-Si is generally obtained by special heat treatment of MG- Si and is a much cost–efficient material compared to the solar-grade Si. However, UMG-Si contains high level of various metal impurities and defects which leads to diminished diffusion length and poor performance. Therefore, in order to achieve efficient photo-generated charge collection from a p-n junction made from low quality Si, the thickness of the solar cell should be within the diffusion length, particularly less than ~ 20 µm for the application of UMG-Si. Si thickness in this range does not allow sufficient light absorption and thus, designing of the structure of ultrathin solar cells to have optically thick active layer, so that the light absorbance can be improved, becomes very important. Strategies to enhance optical absorbance in the solar cells include dielectric-anti reflection coating, surface texturing and exploitation of surface plasmon resonance. Among them, the surface plasmon resonance, which is the collective oscillation of conduction electrons stimulated by incident light at the interface between a metallic (Ag, Au, Pt) nanostructure and a dielectric, has been an emerging method for achieving the light trapping in ultrathin Si solar cells. This thesis presents ultrathin Si solar cells generated from UMG-Si wafers incorporating combinations of nanostructures that enable use of surface plasmon resonance, light scattering feature, and anti-reflection layers. Detailed studies of electrical and optical properties of the resulting solar cells provide useful design considerations for future MG-Si based and any classes of solar cell systems.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/99886/1/jykwon_1.pd

Synthesis and gas sensing properties of inorganic semiconducting, p-n heterojunction nanomaterials

En aquesta tesis utilitzant principalment Aerosol Assited Chemical Vapor Deposition, AACVD, com a metodologia de síntesis d'òxid de tungstè nanoestructurat s'han fabricat diferents sensors de gasos. Per tal d'estudiar la millora en la selectivitat i la sensibilitat dels sensors de gasos basats en òxid de tungstè aquest s'han decorat, via AACVD, amb nanopartícules d'altres òxids metàl·lics per a crear heterojuncions per tal d'obtenir un increment en la sensibilitat electrònica, les propietats químiques del material o bé ambdues. En particular, s'han treballat en diferents sensors de nanofils d'òxid de tungstè decorats amb nanopartícules d'òxid de níquel, òxid de cobalt i òxid d'iridi resultant en sensors amb un gran increment de resposta i selectivitat cap al sulfur d'hidrogen, per a l'amoníac i per a l'òxid de nitrogen respectivament a concentracions traça. A més a més, s'han estudiat els mecanismes de reacció que tenen lloc entre les espècies d'oxigen adsorbides a la superfície del sensor quan interactua amb un gas. I també s'ha treballat en intentar controlar el potencial de superfície de les capes nanoestructurades per tal de controlar la deriva en la senyal al llarg del temps, quan el sensor està operant, a través d'un control de temperatura.En esta tesis utilizando principalmente Aerosol Assited Chemical Vapor Deposition, AACVD, como metodología de síntesis de óxido de tungsteno nanoestructurado se han fabricado diferentes sensores de gases. Para estudiar la mejora en la selectividad y la sensibilidad de los sensores de gases basados en óxido de tungsteno estos se han decorado, vía AACVD, con nanopartículas de otros óxidos metálicos para crear heterouniones para obtener un incremento en la sensibilidad electrónica, las propiedades químicas del material o bien ambas. En particular, se han trabajado en diferentes sensores de nanohilos de óxido de tungsteno decorados con nanopartículas de óxido de níquel, óxido de cobalto y óxido de iridio resultante en sensores con un gran incremento de respuesta y selectividad hacia el sulfuro de hidrógeno, para el amoníaco y para el óxido de nitrógeno respectivamente a concentraciones traza. Además, se han estudiado los mecanismos de reacción que tienen lugar entre las especies de oxígeno adsorbidas en la superficie del sensor cuando interactúa con un gas. Y también se ha trabajado en intentar controlar el potencial de superficie de las capas nanoestructuradas para controlar la deriva en la señal a lo largo del tiempo, cuando el sensor está trabajando, a través de un control de temperatura.In this thesis, using mainly Aerosol Assited Chemical Vapor Deposition, AACVD, as a synthesis methodology for nanostructured tungsten oxide, different gas sensors have been manufactured. To study the improvement in the selectivity and sensitivity of gas sensors based on tungsten oxide, they have been decorated, via AACVD, with nanoparticles of other metal oxides to create heterojunctions to obtain an increase in electronic sensitivity, in the chemical properties of the material or at the same time in both. Particularly, we have worked on different tungsten oxide nanowire sensors decorated with nanoparticles of nickel oxide, cobalt oxide and iridium oxide resulting in sensors with a large increase in response and selectivity towards hydrogen sulfide, for ammonia. and for nitrogen oxide respectively at trace concentrations. In addition, the reaction mechanisms that take place between oxygen species adsorbed on the sensor surface when it interacts with a gas have been also studied. Furthermore, efforts have been put on trying to control the surface potential of the nanostructured layers to control the drift in the signal over time, when operating the sensors, through temperature control