Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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    Materials for large-area electronics: characterization of pentacene and graphene thin films by ac transport, Raman spectroscopy, and optics
    (2010) Lenski, Daniel Roy; Fuhrer, Michael S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation explores techniques for fabricating and characterizing two classes of novel materials which may be useful for large-area electronics applications: organic semiconductors and graphene. Organic semiconductors show promise for large-area electronics because of their low cost, compatibility with a variety of substrates, and relative ease of fabricating and patterning thin-film transistors (TFTs). Nearly all published work has focused on the dc electronic transport properties of these materials, rather than their ac behavior, which could be affected by their polycrystalline, granular structure. To address this, I have constructed a model of organic TFTs based on lossy transmission lines, and determined the relationship between the film conductivity and the overall device behavior for a bottom-contacted TFT. I apply this transmission-line framework to interpret my experiments on pentacene TFTs designed in a special long-channel geometry to hasten the onset of high-frequency effects. The experiments reveal an intrinsic frequency-dependent conductivity of polycrystalline pentacene, which can be understood within the context of the universal dielectric response model of ac conduction in disordered solids. The results are important for establishing practical limits on pentacene's ac performance. Graphene is a two-dimensional crystalline form of carbon, with a remarkably simple structure. It is a gapless semiconductor with an extremely high mobility and very high optical transparency, attracting great interest both for its possible uses as a replacement for silicon and as a transparent conducting material. I have synthesized large-area films of graphene via atmospheric-pressure chemical vapor deposition (CVD) on copper substrates, adapting a low-pressure CVD method previously reported to produce exclusively monolayer graphene films. I have transferred the graphene films to insulating SiO2, and characterized them using optical transparency, Raman spectroscopy, and atomic-force microscopy, observing significant differences from the measured properties of widely studied mechanically-exfoliated graphene. I analyze the strengths and weaknesses of these three techniques for distinguishing films of different layer number, and relate them to uncertainties in the known properties of one- and few-layer graphene. I conclude that atmospheric-pressure CVD of graphene on copper produces significant areas of multilayer, rotationally-misoriented graphene, in a significant departure from results on low-pressure CVD of graphene on copper.
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    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.
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    INTERFACE EFFECTS ON NANOELECTRONICS
    (2009) Conrad, Brad Richard; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nanoelectronics consist of devices with active electronic components on the nanometer length scale. At such dimensions most, if not all, atoms or molecules composing the active device region must be on or near a surface. Also, materials effectively confined to two dimensions, or when subject to abrupt boundary conditions, generally do not behave the same as materials inside three dimensional, continuous structures. This thesis is a quantitative determination of how surfaces and interfaces in organic nanoelectronic devices affect properties such as charge transport, electronic structure, and material fluctuations. Si/SiO2 is a model gate/gate dielectric for organic thin film transistors, therefore proper characterization and measurement of the effects of the SiO2/organic interface on device structures is extremely important. I fabricated pentacene thin film transistors on Si/SiO2 and varied the conduction channel thickness from effectively bulk (~40nm) to 2 continuous conducting layers to examine the effect of substrate on noise generation. The electronic spectral noise was measured and the generator of the noise was determined to be due to the random spatial dependence of grain boundaries, independent of proximity to the gate oxide. This result led me to investigate the mechanisms of pentacene grain formation, including the role of small quantities of impurities, on silicon dioxide substrates. Through a series of nucleation, growth and morphology studies, I determined that impurities assist in nucleation on SiO2, decreasing the stable nucleus size by a third and increasing the overall number of grains. The pentacene growth and morphology studies prompted further exploration of pentacene crystal growth on SiO2. I developed a method of making atomically clean ultra-thin oxide films, with surface chemistry and growth properties similar to the standard thick oxides. These ultra-thin oxides were measured to be as smooth as cleaned silicon and then used as substrates for scanning tunneling microscopy of pentacene films. The increased spatial resolution of this technique allowed for the first molecular resolution characterization of the standing-up pentacene crystal structure near the gate dielectric, with molecules oriented perpendicular to the SiO2 surface. Further studies probed how growth of C60 films on SiO2 and pentacene surfaces affected C60 morphology and electronic structure to better understand solar cell heterojunctions.