Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item EXPLORATION OF NOVEL METHODS FOR THE FABRICATION AND CHARACTERIZATION OF ORGANIC FIELD-EFFECT TRANSISTORS AND EXAMINATION OF FACTORS INFLUENCING OFET PERFORMANCE(2009) Southard, Adrian Edward; Fuhrer, Michael S.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis explores novel methods for fabricating organic field effect transistors (OFETs) and characterizing OFET devices. Transfer printing is a promising process for fabricating organic thin-film devices. In this work, a transfer-printing process is developed for the polymer organic semiconductor P3HT. Pre-patterned P3HT is printed onto different dielectrics such as PMMA, polystyrene and polycarbonate. The P3HT layer is spun on a smooth silicon interface made hydrophobic by treatment with octyltrichlorosilane, which functions as a release layer. This method has distinct advantages over standard OFET fabrication methods in that 1) the active layer can be pre-patterned, 2) the solvent for the P3HT need not be compatible with the target substrate, and 3) the electrical contact formed mimics the properties of top contacts but with the spatial resolution of bottom contacts. Transparent, conducting films of carbon nanotubes (CNTs) are prepared by airbrushing, and characterized optically and electronically. OFETs with CNT films as source and drain electrodes are fabricated using various patterning techniques, and the organic/CNT contact resistance is characterized. CNT films make transparent, flexible electrodes with contact resistance comparable to that found for Au bottom-contacted P3HT transistors and comparable to CNT-film bottom-contacted pentacene transistors with CNTs deposited by other less flexible methods. A transparent OFET is demonstrated using transfer printing for the assembly of an organic semiconductor (pentacene), CNT film source, drain, and gate electrodes, and polymer gate dielectric and substrate. The dependence of the conductance and mobility in pentacene OFETs on temperature, gate voltage, and source-drain electric field is studied. The data are analyzed by extending a multiple trapping and release model to account for lowering of the energy required to excite carriers into the valence band (Poole-Frenkel effect). The temperature-dependent conductivity shows activated behavior, and the activation energy is lowered roughly linearly with the square-root of electric field, as expected for the Poole-Frenkel effect. The gate voltage dependence of the activation energy is used to extract the trap density of states, in good agreement with other measurements in the literature.Item DEVELOPMENT OF ION-MOBILITY AND MASS SPECTROMETRY FOR PROBING THE REACTIVITY OF NANOPARTICLES AND NANOCOMPOSITES(2009) Zhou, Lei; Zachariah, Michael; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosols of diameter smaller than 100 nm, usually are referred as nanoparticles or ultrafines, have received considerable interests lately as a source of building blocks to novel materials. However, our capabilities for charactering these materials are greatly limited by lack of appropriate diagnostic tools. The objective of this work is to develop new aerosol-based techniques for the characterization of nanoparticles and nanocomposites. The scope of this dissertation can be categorized in two ways. First, to provide knowledge of just how reactive a material is, we develop particle ion-mobility spectrometry and Single Particle Mass Spectrometry methods to probe the intrinsic size-dependent reactivity of individual metal particles. And second, the development of a new Time-of-Flight mass spectrometer (TOFMS) combined with a temperature jump (T-Jump) technique to study particle-particle reaction, and probe the reactivity of nanocomposite materials under combustion-like condition.Item TEMPLATE SYNTHESIZED NANOTUBES, NANOWIRES AND HETEROGENEOUS COAXIAL NANOWIRES FOR ELECTROCHEMICAL ENERGY STORAGE(2009) Liu, Ran; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Template synthesized nanomaterials have been successfully applied in electrochemical energy storage systems such as supercapacitors and lithium ion batteries. The first part of present study will list examples of applying various nanomaterials such as nanowires, nanotubes and heterostructured nanowires in different electrochemical energy storage systems for enhancing their charge/discharge rates, energy densities and power densities, etc. The following of the thesis will describe the template synthesis of nanomaterials in details. The experimental part of this thesis will concentrate on the fabrication of alumina template and the detailed experimental setups for aluminum anodization and template synthesis of nanomaterials. The rest of the thesis analyzes four cases of using template synthesized nanomaterials in electrochemical energy storage, which include my major work during my PhD studies. The first one is utilizing poly(3,4- ethylenedioxythiophene) (PEDOT) nanotubes as electrode materials for highpowered supercapacitor. The thin-walled nanotubes allow fast charge/discharge of the PEDOT to achieve high power. The second one is related to synthesis and characterization of RuO2/PEDOT composite nanotubes for supercapacitors. Loading appropriate amount of RuO2 can effectively enhance the specific capacitance of PEDOT nanotube. The third case illustrates the synthesis of MnO2/PEDOT coaxial nanowires by one step coelectrodeposition for electrochemical energy storage. The combined properties of MnO2 and PEDOT enable the coaxial nanowires to have very high specific capacitances at high current densities. Their formation mechanism will be explored and their nanostructures are tuned for optimized electrochemical properties. The final case reports the MnO2-Nanoparticles enriched PEDOT nanowires for enhanced electrochemical energy storage capacity. Large amount of the MnO2 nanoparticles can be loaded into PEDOT nanowires after they are soaked in KMnO4 solution. Thus loaded MnO2 nanoparticles effective enhance the energy densities of PEDOT nanowires without causing too much volume expansion to them.Item MULTIPHOTON ABSORPTION: FABRICATION, FUNCTIONALIZATION AND APPLICATIONS(2009) Li, Linjie; Fourkas, John T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite the remarkable progress in micro/nano-scale fabrication that has occurred over the last decades, feature sizes are still restricted by the diffraction limit. The resolution in conventional photolithography is generally constrained to approximately one quarter of a wavelength (lamda) of the light used. Multiphoton absorption polymerization (MAP) offers another option for high-resolution fabrication. Using nonlinear optical and chemical effects, MAP can generate features with a transverse dimension as small as 80 nm using 800-nm laser excitation. MAP has the additional capability of fabricating arbitrary 3D structures, which is essential in many applications. Details of MAP fabrication setup and process are described in this thesis. Novel optical devices have been fabricated with MAP. One drawback of MAP is that the resolution in axial direction remains about three to five times poorer because of the shape of the laser focal point. A novel technique called Resolution Augmentation through Photo-Induced Deactivation (RAPID) lithography has been developed to overcome this issue. With RAPID, resolution of 40 nm in axial direction has been achieved. The aspect ratio of the volume element of MAP has been reduced from about 3 to 0.5. Selective functionalization of polymeric microstructure has been performed in two ways. In the first approach, microstructures are fabricated with hybrid resists that permits the chemical functionality only applies to one material. The second method is able to pattern both binary and gray-scale functionalities onto polymer surface. The density of the surface functional groups is determined by the intensity of the exposed light. The nonlinear novelty of multiphoton absorption has not only been realized in MAP, it also shows promise for multiphoton absorption based microscopy. Photoluminescence from noble metal nanostructures has been used for two-photon imaging of living cells. Multiphoton Absorption Induced Luminescence (MAIL) has been used to monitor the targeting and endocytosis of goldnanoparticles to human umbilical vein endothelial cells. Field-enhanced phenomena have been studied with MAIL and MAP.Item Core/Shell and Alloy Nanoparticles of Transition Metals for Heterogeneous Catalysis: Bridging the Gap between Experiment and Theory(2008) Alayoglu, Selim; Eichhorn, Bryan W; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis describes the structural and catalytic properties of the architecturally-controlled bimetallic nanoparticles (NPs) of transition metals. In this study, bimetallic nanoparticles with well-defined architectures were synthesized, characterized and evaluated toward various heterogeneous reactions. Random alloy nanoparticles were compared to the core/shell nanoparticles (M@M' NPs where M is the core metal and M' is the shell metal), which is the synthetic counterpart of the theoretically well-studied Near Surface Alloys (NSAs). Thus, the long existing experimental gap with the theory can be bridged via the systematic evaluation of such architecturally-controlled bimetallic NPs. The M@Pt (M=Ru, Rh, Ir, Pd and Au) and Ru@M' (M'=Rh and Pd) core/shell NPs of tunable core sizes and shell thicknesses, and the PtRu alloy and PtRh alloy NPs of various compositions were prepared via poly-ol reduction reactions by using sequential deposition techniques. Seed NPs for the core/shell systems were synthesized via either poly-ol or NaBH4 reduction reactions. The wet-chemical co-deposition technique was employed to synthesize the alloy NPs. The core/shell and alloy NPs were characterized by using a combination of TEM, STEM-EDS, XRD, and FT-IR and Micro Raman -CO probe experiments. Full structural analysis employing techniques such as Extended X-Ray Absorption Fine Structure (EXAFS) and atomic Pair Distribution Function (PDF) was also performed for the 4.1 nm Ru@Pt NPs comprising of 3.0 nm cores and 1-2 MLs thick shells and the 4.4 nm Pt50Ru50 alloy NPs. Through collaborations, the nanoparticle structures were also modeled through EXAFS analyses, PDF fits, Rietveld Refinements and Debye Function simulations. The well-characterized core/shell and alloy NPs were evaluated for preferential oxidation of CO in H2 feeds (PROX). Catalytically, the core/shell NPs were superior to their alloy counterparts with similar particle sizes and identical compositions. The PROX reactivities of the M@Pt (M=Ru, Rh, Ir, Pd and Au) core/shell NPs increased in the order of Au@Pt < Pd@Pt < Ir@Pt < Rh@Pt < Ru@Pt, which is predicted by the NSA theory. Density Functional Theory (DFT) calculations performed by Prof. Mavrikakis at the University of Wisconsin helped elucidate the thermo-chemistry beyond the enhanced PROX activities and the observed surface reactivity trends for the core/shell architectures. The decreased equilibrium surface coverage of CO as well as the new H2-assisted O2 dissociation pathway on the electronically-altered Pt shells were suggested to bring on the room temperature CO oxidation and the subsequent H2 activation with enhanced PROX selectivity. The surface reactivities toward PROX and benzene hydrogenation reactions of the composition series of the PtRu alloy NPs exhibited the `Volcano' behavior, which invoked the Hammer-Norskov theory. The preliminary benzene hydrogenation results on the Ru@Pt NPs system presented in this study also showed a structure dependent correlation in surface activity.