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The aim of this research work is to design, fabricate, characterize, and test nanogap-electrode based device for biochemical detection and DNA immobilization and hybridization detection. However, the focus of this research is to investigate electrically and chemically the effect of different materials and gap sizes on the nanogap electrodes design.
The aim of this research is to design, fabricate and characterize an array of planar nanogap capacitive sensor based on Poly-SiO2 structure by using size expansion technique.
Silicon's chemical stability, high natural abundance (as the second most common element in the earth's crust), mechanical stiffness, and semiconducting behavior have made it the subject of extensive scientific investigation and the material of choice for both the microelectronics and microelectromechanical device industries. The success of Moore's Law that demands continual size reduction has directed it to a central place in emerging nanoscience and nanotechnology as well. Crystalline nanowires (NWs) are one nanostructured form that silicon may take that has sparked significant interest as they can exhibit considerable confinement effects and high surface-to-volume ratios, but may be interfaced simply along one direction for the determination of material properties and implementation into new technologies. The expense and difficulty involved in the creation of semiconductor nanowires using the "top down" fabrication techniques of the microelectronics industry has promoted an explosion of chemical synthetic "bottom up" techniques to produce high quality crystalline nanowires in large quanitities. Nevertheless, bottom up synthesized Si NWs retain a new set of challenges for their successful integration into reliable, high-performance devices, which is hindered by an incomplete understanding of the factors controlling their material properties. The first chapter of this dissertation introduces the motivation for studying semiconductor NWs and the benefits of limiting the scope to silicon alone. A brief survey of the current understanding of thermal conductivity in silicon nanowires provides prime examples of how confinement effects and surface morphology may dramatically alter nanowire properties from their bulk crystal counterparts. The particular challenges to bottom up silicon nanowire device integration and characterization are noted, especially related to Si nanowires that are grown epitaxially on crystal silicon substrates, and Raman spectroscopy is introduced as a promising optical characterization and metrology tool for semiconductor nanowire based devices. Chapter two describes the vapor-liquid-solid (VLS) mechanism for the synthesis of very high quality, single-crystal silicon nanowires using Au and Pt catalyst nanoparticles. A new technique is presented for the simplified synthesis of branched silicon nanowires based on the migration of Au catalyst during an hydrogen anneal intermediate between growth stages, and the faceting behavior at synthetic stages is revealed by the analysis of electron microscope images. Synthesis of solid and porous Si nanowires based on Ag mediated electrochemical silicon etching is described as well. The third chapter specifies new processing techniques developed with future device integration of epitaxially VLS-grown Si nanowires in mind. Epitaxially bridging nanowires are shown to provide an excellent platform for single-wire electrical and mechanical property measurements. Galvanic displacement through block copolymer micelle/homopolymer surface templates is demonstrated as a means to deposit catalyst nanoparticles with controlled sizes and areal densities in a variety of geometries and with registration to photolithographic patterns. Ex situ boron doping by the direct hydrogen reduction of boron tribromide is shown to achieve active concentrations exceeding 1019 cm-3 with high axial uniformity, while avoiding the adverse impact on nanowire morphology that is often observed with in situ boron doping of silicon nanowires. Chapter four describes the characteristics of Raman spectroscopy that are relevant to studying individual semiconductor nanowires. Careful spectral measurements show that the anharmonic dependence of Raman spectra on temperature for individual Si nanowires remains unchanged from the bulk crystal for diameters down to 30 nm, regardless of surface morphology. Using this result, a new technique for measuring the thermal conductivity of individual semiconductor nanowires is then outlined based on Raman thermal mapping of individual cantilevered nanowires. Finally, the dissertation is concluded with suggestions for possible future experiments. One avenue is to probe more deeply the morphology of faceted silicon nanowires and nanotrees and its impact on their transport physics. Another possible route for further study would be to explore new characterization and metrological applications of Raman spectrocopy for semiconductor nanowires.
Thermoelectric devices, which convert temperature gradients into electricity, have the potential to harness waste heat to improve overall energy efficiency. However, current thermoelectric devices are not cost-effective for most applications due to their low efficiencies and high material costs. To improve the overall conversion efficiency, thermoelectric materials should possess material properties that closely resemble a "phonon glass" and an "electron crystal". The desired low thermal and high electrical conductivities allow the thermoelectric device to maintain a high temperature gradient while effectively transporting current. Unfortunately, thermal transport and electrical transport are a closely coupled phenomena and it is difficult to independently engineer each specific conduction mechanism in conventional materials. One strategy to realize this is to generate nanostructured silicon (e.g. silicon nanowires (SiNWs)), which have been shown to reduce thermal conductivity ([kappa]) through enhanced phonon scattering while theoretically preserving the electronic properties; therefore, improving the overall device efficiency. The ability to suppress phonon propagation in nanostructured silicon, which has a bulk phonon mean free path ~ 300 nm at 300 K, has raised substantial interest as an ultra-low [kappa] material capable of reducing the thermal conductivity up to three orders of magnitude lower than that of bulk silicon. While the formation of porous silicon and SiNWs has individually been demonstrated as promising methods to reduce [kappa], there is a lack of research investigating the thermal conductivity in SiNWs containing porosity. We fabricated SiNW arrays using top-down etching methods (deep reactive ion etching and metal-assisted chemical etching) and by tuning the diameter with different patterning methods and tuning the internal porosity with different SiNW etching conditions. The effects of both the porosity and the SiNW dimensions at the array scale are investigated by measuring [kappa] of vertical SiNW arrays using a nanosecond time-domain thermoreflectance technique. In addition to thermoelectric devices, vertical SiNW arrays, due to their anisotropic electronic and optical properties, large surface to volume ratios, resistance to Li-ion pulverization, ability to orthogonalize light absorption and carrier transport directions, and trap light, make vertical SiNW arrays important building blocks for various applications. These may include sensors, solar cells, and Li-ion batteries. Many of these applications benefit from vertical SiNW arrays fabricated on non-silicon based substrates which endow the final devices with the properties of flexibility, transparency, and light-weight while removing any performance limitation of the silicon fabrication substrate. We then developed two vertical transfer printing methods (V-TPMs) that are used to detach SiNW arrays from their original fabrication substrates and subsequently attach them to any desired substrate while retaining their vertical alignment over a large area. The transfer of vertically aligned arrays of uniform length SiNWs is desirable to remove the electrical, thermal, optical, and structural impact from the fabrication substrate and also to enable the integration of vertical SiNWs directly into flexible and conductive substrates. Moreover, realization of a thermoelectric device requires the formation of electrical contacts on both sides of the SiNW arrays. We formed metallic contacts on both ends of the SiNW arrays with a mechanical supporting and electrical insulating polymer in between. Electrical characterization of the SiNW devices exhibited good current-voltage (I-V) characteristics independent of substrates materials and bending conditions. We believe the V-TPMs developed in this work have great potential for manufacturing practical thermoelectric devices as well as high performing, scalable SiNW array devices on flexible and conducting substrates.
Research on lab-on-a-chip devices fabricated by traditional microfabrication techniques are gaining importance in the field of analytical chemistry. In particular there is an increasing demand for miniaturized biosensors based on enzymes owing to their applications in clinical analysis. With respect to Capillary Electrophoresis (CE) microchips, enzymes can impart a greater degree of selectivity. The technique of electrochemical polymerization was used to immobilize glucose oxidase on the CE microchip. Polypyrrole was used an immobilization matrix. The second chapter describes the experimental procedure for the immobilization of glucose oxidase on the CE microchip device. A two-step procedure was employed, which involved a thin layer of polypyrrole followed by a thick layer of polypyrrole/glucose oxidase film. The third chapter deals with the results obtained using a CE microchip.
Neurological disorders produce serious cognitive and motor disabilities, and they account for 7% of total global disease burden, measured in disability‐adjusted life years. In addition, the life expectancy has been continuously growing during the last decades and, as a consequence, neurodegenerative disorders are becoming more prevalent representing a larger part of the healthcare efforts and expenses. Every year, treating brain conditions accounts for 35% of Europe’s disease burden with a yearly cost of €798.000 million. Despite the efforts performed in the medical and science fields, the cure for most of the neurological disorders is far from being achieved. One of the most efficient strategies to treat neurological disorders is the use of implanted electrodes to produce neural electrical stimulation. Furthermore, electrodes are one of the main neural interfaces used in diagnostics techniques, and for the study of the neural activity in basic investigation, in vitro and in vivo. Despite their enormous potential, these electrodes face nowadays limitations. Generally, they are too big, producing unspecific stimulation that can lead to secondary effects. Their size reduction is limited by the associated impedance increase, which restricts their charge‐injection to the tissue...
For applications in mobile and remote sensing platforms, microsupercapacitors are attractive energy storage devices due to their robust lifetimes and high specific power capacity. Utilization of green electrolytes in these devices reduces environmental impact and simplifies packaging by avoiding the stringent oxygen and moisture free conditions required for organic and ionic liquid based electrolytes. Porous silicon nanowire based microsupercapacitor electrode materials are promising for on chip applications using an environmentally benign aqueous electrolyte, 1 M KCl, however they are prone to oxidation. A silicon carbide coating was found to mitigate this issue. The fabrication techniques, involving low-temperature electroless etching of silicon, are compatible with current integrated circuit processing methods and may be readily integrated at the micro device level. The electrode materials are in good electrical contact with the underlying substrate and require no additional current collector. The base porous silicon nanowires are coated with a thin silicon carbide passivation layer by low pressure chemical vapor deposition. The demonstrated capacitance of the electrode materials, ~1700 [mu]F/cm2 projected area, is comparable to other carbon based microsupercapacitor electrodes, remains stable over many charge/discharge cycles, and maintains capacitive behavior over a wide range of charge/discharge rates. An improved passivation method for the porous silicon nanowires has also been developed. The selective coating procedure deposits an ultra-thin (~ 1-3 nm) carbon sheath over the nanowires and passivates them. The ultra-thin nature of the coating enables solvent access to the pore area and hence a large improvement of active specific surface over the SiC coated PSiNWs discussed above. The electrochemical performance of these coated nanowires is characterized in both an aqueous electrolyte and an ionic liquid electrolyte. Specific capacitance values reaching 325 mF cm 2 are achieved in ionic liquid, and calculations indicate that the theoretical maximum capacitance of the pristine wires is reached. TEM studies confirm the coating thickness and its conformality. Raman spectroscopy indicates that the carbon in the coating is mainly sp2 hybridized, with corresponding high conductivity. At the time of writing, these materials represent the largest specific energy microsupercapacitor electrode published. A test device is prepared and demonstrated powering an LED. The testing results of silicon carbide (SiC) nanowires (NW) as an electrode material for micro-supercapacitors is described. SiC NWs are grown on a SiC thin film coated with a thin Ni catalyst layer via chemical vapor deposition. A specific capacitance of ~240 μF cm-2 is demonstrated. Charge-discharge studies demonstrate the SiC nanowires exhibit exceptional stability, with 95% capacitance retention after 2×105 charge/discharge cycles in an environmentally benign, aqueous electrolyte. Doping of the nanowires with nitrogen through the addition of 5 at% ammonia to the precursor gas flow rate improves the conductivity of the nanowire films by over an order of magnitude leading to increased power capabilities. A method to transfer silicon and silicon carbide nanowire arrays to arbitrary substrates while maintaining electrical contact through the entire array is elucidated. The nanowires are grown on graphene sheets on SiO2 coupons. The graphene acts as both the flexible material for maintaining structural continuity and electrical contact through the array during transfer. The SiO2 acts as the sacrificial growth substrate which is etched after growth in order to release the nanowire/graphene hybrid. The nanowire/graphene hybrids are structurally characterized by XRD and electron microscopy. Good electrical contact is confirmed through testing of the SiCNW/graphene hybrids as supercapacitor electrode materials in an aqueous electrolyte. The specific capacitance, ~340 mF cm-2, is similar to SiCNW arrays grown on oxide while the electrical conductivity is improved and cycling stability tests show less than a 1% decrease in capacitance after 10,000 cycles.