For applications of MoS2 in batteries, supercapacitors, electrocatalysts, solar cells and water quality sensors, a substantially increased conductivity is required in order to achieve reasonable currents. Popularly, the metallic 1T-MoS2 phase is used, which can be prepared via a lithium intercalation process, requiring inert atmosphere processing and safety procedures. In this thesis, I demonstrate a safer and more efficient process to yield conductive MoS2 (c-MoS2). This simple and effective way to prepare few layer c-MoS2 utilizes ambient conditions and 0.06 vol% aqueous hydrogen peroxide. Part of the research effort has been to enhance the conductivity of MoS2 using the idea of green solvents (like pure water). The bulk conductivities of both peroxide and water exfoliated MoS2 are up to seven orders of magnitude higher than that of the semiconducting 2H-MoS2 phase. The samples were characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Trace amounts of hydrogen molybdenum bronze (HxMoO3-y) and sub stoichiometric MoO3-y were shown to help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. c-MoS2 was further functionalized with thiols to determine the number of residual reactive sites. I also studied the mechanism of surface functionalization of MoS2 with diazonium molecules (both direct and in-situ approach) to understand the surface properties of our material and tune the chemical and mechanical properties of conductive MoS2. An important goal of my work is to control the conductivity of the MoS2 thin films in safe and facile ways that enable their application in low-cost chemiresistive sensors for liquid environments. I fabricated chemiresistive sensors with centimeter channel lengths while maintaining low measurement voltages for pH sensing. I further measured the catalytic activity of c-MoS2 films in 0.5 M H2SO4 electrolyte solution using linear sweep voltammetry (LSV) which showed a lower Tafel value at 10 mA/cm2 current density. The lower Tafel value demonstrated that c-MoS2 has potential to use as catalyst for hydrogen evolution reaction. My study furthers the understanding of conductive forms of MoS2 and opens up a new pathway for next generation electronic and energy conversion devices. / Thesis / Doctor of Philosophy (PhD) / For applications of MoS2 in batteries, supercapacitors, electrocatalysts, solar cells and water quality sensors, a substantially increased conductivity is required in order to achieve reasonable currents. Popularly, the metallic 1T-MoS2 phase is used, which can be prepared via a lithium intercalation process, requiring inert atmosphere processing and safety procedures. In this thesis, I demonstrate a safer and more efficient process to yield conductive MoS2 (c-MoS2). This simple and effective way to prepare few layer c-MoS2 utilizes ambient conditions and 0.06 vol% aqueous hydrogen peroxide. Part of the research effort has been to enhance the conductivity of MoS2 using the idea of green solvents (like pure water). The bulk conductivities of both peroxide and water exfoliated MoS2 are up to seven orders of magnitude higher than that of the semiconducting 2H-MoS2 phase. The samples were characterized with Hall measurements, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Trace amounts of hydrogen molybdenum bronze (HxMoO3-y) and sub stoichiometric MoO3-y were shown to help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. c-MoS2 was further functionalized with thiols to determine the number of residual reactive sites. I also studied the mechanism of surface functionalization of MoS2 with diazonium molecules (both direct and in-situ approach) to understand the surface properties of our material and tune the chemical and mechanical properties of conductive MoS2. An important goal of my work is to control the conductivity of the MoS2 thin films in safe and facile ways that enable their application in low-cost chemiresistive sensors for liquid environments. I fabricated chemiresistive sensors with centimeter channel lengths while maintaining low measurement voltages for pH sensing. I further measured the catalytic activity of c-MoS2 films in 0.5 M H2SO4 electrolyte solution using linear sweep voltammetry (LSV) which showed a lower Tafel value at 10 mA/cm2 current density. The lower Tafel value demonstrated that c-MoS2 has potential to use as catalyst for hydrogen evolution reaction. My study furthers the understanding of conductive forms of MoS2 and opens up a new pathway for next generation electronic and energy conversion devices.

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In this thesis¡M As-grown and H-treated polycrystalline diamond film Metal-Insulator-Semiconductor Field-Effect-Transistor on the p-type surface semiconductive layers of undoped hydrogen-terminated CVD diamond films were successfully fabricated using a new fabrication process[1-2]. This new fabrication process of the polycrystalline diamond film MISFET exploits selected area deposition (SAD)[3] with Shadow Mask and indeed possesses low cost and less process¡Mso it was a best alternation for diamond device fabrication recently. Next, a modified equivalent circuit of the polycrystalline diamond film MISFET on the p-type surface semiconductive layers of undoped hydrogen-terminated CVD diamond films is also proposed, which consists of FET and the parasitic current conduction paths. The FET current path represents the currents flowed through the P-type conductive layer, and the parasitic current conductance paths represents the currents flowed through the barrier of metal/diamond and through both of the grain boundary and bulk diamond, respectively. In addition, the I-V characteristics of As-grown and H-treated polycrystalline diamond film MISFET has been successfully simulated by using this modified equivalent circuit. The simulation results show good agreements with the measurement. In addition, the I-V characteristics of As-grown polycrystalline diamond film MISFET, from measurement and simulation results, were great affected by the grain boundaries and bulk diamond crystallites, and their shape is similar to that of Schottky diode. Whereas, the effect of the bulk diamond crystallites and diamond grain boundaries was great decreased after hydrogen plasma treatment and the shape of the I-V characteristics is similar to that of Si-MISFET. This result is believed to have important impacts for the application of diamond device in the nearest future.

Layer-by-layer (LbL) assembly was used to produce highly conductive thin films with carbon black (CB) and polyelectrolytes. The effects of sonication and pHadjustment of the deposition mixtures on the conductivity and transparency of deposited films were studied. Drying temperature was also evaluated with regard to thin film resistance. Sonication and oven drying at 70oC produced films with the lowest sheet resistance (~ 1500 Ω/sq), which corresponds to a bulk resistivity of 0.2 Ω⋠cm for a 14- bilayer film that is 1.3 μm thick. Increasing the pH of the PAA-stabilized mixture and decreasing the pH of the PEI-stabilized mixture resulted in films with 70% transparency due to thinner deposition from increased polymer charge density. Varying the number of bilayers allows both sheet resistance and optical transparency to be tailored over a broad range. Variation of deposition mixture composition led to further reduction of sheet resistance per bilayer. A 14 bilayer film, made from mixtures of 0.25wt% carbon black in 0.05wt% PAA and plain 0.1wt% PEI, was found to have a sheet resistance of approximately 325 Ω/sq. Bulk resistivity was not improved due to the film being 8 μm thick, but this combination of small thickness and low resistance is an order of magnitude better than carbon black filled composites made via traditional melt or solution processing. Applications for this technology lie in the areas of flexible electronics, electrostatic charge dissipation, and electromagnetic interference shielding.

In this thesis, three types of conductive membranes were fabricated and characterized for potential energy applications such as fuel cells and solar photovoltaics. First, a single layer conductive polypyrrole (PPy) membrane was synthesized and activated. Through image analysis, surface pore geometry changes were analyzed. The single layer PPy membrane was proposed as a possible additional layer or coating in polymer electrolyte membrane fuel cells. Next, a novel adaptive trilayer PPy membrane was fabricated. The membranes were activated, and characterized through changes in surface wrinkle, roughness and contact angle. A dynamic range of surface properties were observed. Lastly, conductive fibrous membranes were fabricated with electrospinning. Two methods were utilized to spin conductive fibers including the incorporation of multi-walled carbon nanotubes (MWCNT) in polystyrene (PS) and the utilization of vapor phase polymerization (VPP) to chemically synthesize PPy on electrospun FeCl3/PS oxidant fibers. Properties including fiber morphology, thermal stability and conductivity were characterized.

In this thesis, three types of conductive membranes were fabricated and characterized for potential energy applications such as fuel cells and solar photovoltaics. First, a single layer conductive polypyrrole (PPy) membrane was synthesized and activated. Through image analysis, surface pore geometry changes were analyzed. The single layer PPy membrane was proposed as a possible additional layer or coating in polymer electrolyte membrane fuel cells. Next, a novel adaptive trilayer PPy membrane was fabricated. The membranes were activated, and characterized through changes in surface wrinkle, roughness and contact angle. A dynamic range of surface properties were observed. Lastly, conductive fibrous membranes were fabricated with electrospinning. Two methods were utilized to spin conductive fibers including the incorporation of multi-walled carbon nanotubes (MWCNT) in polystyrene (PS) and the utilization of vapor phase polymerization (VPP) to chemically synthesize PPy on electrospun FeCl3/PS oxidant fibers. Properties including fiber morphology, thermal stability and conductivity were characterized.

Philosophiae Doctor - PhD / This project is aimed at fabricating a smart material. Zinc oxide and vanadium dioxide have received a great deal of attention in recent years because they are used in various applications. ZnO semiconductor in particular has a potential application in optoelectronic devices such as light emitting diodes (LED), sensors and in photovoltaic cell industry as a transparent electrode. VO2 also has found application in smart windows, solar technology and infrared smart devices. Hence the need to synthesis or fabricate a new smart material using VO2 and an active ZnO based nano-composites family in which ZnO matrix will be hosting thermally active VO2 nano-crystals is the basis of this study. Since VO2 behave as an MIT Mott’s type oxides and exhibits a thermally driven semiconductor-metal phase transition at about 68 oC and as a direct result ZnO:VO2 nano-composites would exhibit a reversible and modulated optical transmission in the infra-red (IR) while maintaining a constant optical transmission in the UV-Vis range. The synthesis is possible by pulsed laser deposition and ion implantation. Synthesis by pulsed laser deposition will involve thin films multilayer fabrication. ZnO buffer layer thin film will be deposited on the glass and ZnO single crystals and subsequent layer of VO2 and ZnO will be deposited on the substrate. X-ray diffraction (XRD) reveals that the series of ZnO thin films deposited by Pulsed Laser Deposition (PLD) on glass substrates has the hexagonal wurtzite structure with a c-axis preferential orientation. In addition the XRD results registered for VO2 samples indicate that all thin films exhibits a monoclinic VO2 (M) phase. UV-Vis NIR measurements of multilayered structures showed the optical tunability at the near-IR region and an enhanced transparency (>30 %) at the visible range.