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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Item Heat dissipation in current carrying multiwalled carbon nanotubes(2014) Voskanian, Norvik; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding thermal transport is of great interest in combatting the excess heat generated in current electronic circuits. In this dissertation we provide insight and progress in thermal transport in current carrying MWCNT. Chapter 1 gives an overview of the work presented in this dissertation, quickly discusses the motivation for studying heat dissipation in current carrying carbon nanotubes, and outlines the key findings. The chapter outlines the unique remote heating phenomena observed in Joule heated MWCNTs, as well as, the process in which the research led to the discovery of a detection method for near-field heat transfer. The physical properties of carbon nanotubes are discussed in Chapter 2 and the relevant heat transfer mechanisms are introduced. Chapter 3 outlines some of the previous experimental work in studying thermal properties of nanotubes. The results presented in this dissertation rely on previously measured thermal conductivity and thermal contact resistance for nanotubes and thus a discussion of these results is critical. The fabrication process for the measured devices is presenter in Chapter 4. In addition, chapter 4 provides a detailed discussion of the measurement technique employed to probe the thermal properties of the devices presented in Chapter 5 and 6. Chapter 5 discusses the findings in regard to heat dissipation for a current carrying MWCNT supported on a SiN substrate. The results provide definitive proof of substrate heating via hot electrons; a process which can not be explained using traditional Joule heating model and requires the presence of an additional remote heating mechanism. Analysis of the results indicate a reduction in remote Joule heating which led to a series of controlled experiments presented in Chapter 6 in an effort to study substrate thermal conductivity, kSiN, variations as a function of voltage. In this chapter we outline the experimental and simulated results which indicate the remarkable ability of our technique to detect near-field thermal radiation. The enhanced thermal transport via near-field radiation is of great interest for scientific and engineering purposes but its detection has proven difficult. This thesis provides evidence of the sensitivity of the electron thermal microscopy technique to measure near-field radiation.Item SPECTROSCOPIC ENHANCEMENT FROM NOBLE METALLIC NANOPARTICLES(2011) Tsai, Shu-Ju; Phaneuf, Raymond J.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Resonant coupling of localized surface plasmon resonances (LSPRs) in noble metallic nanostructures to incident radiation and the related subject of localized behavior of electromagnetic waves are currently of great interest due to their potential application to sensors, biochemical assays, optical transmission, and photovoltaic devices. My thesis research is made up of two related parts. In part one I examined enhanced fluorescence in dye molecules in proximity to Ag nanostructures. In part two I studied the effect of Au nanostructure arrays on the performance of poly(3-hexylthiophene-2,5-diyl) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) organic solar cells (OSCs). Nanostructures were fabricated by two different methods: e-beam lithography (top down) and spray pyrolysis (bottom up). Using e-beam lithography, we produced arrays of nanostructures with well defined shapes, sizes, and spacings. By systematically varying these topographical parameters, we measured their effect on nanometer-sized metallic structure-enhanced fluorescence (nMEF) and on absorption and external quantum efficiency (EQE) in OSC devices as a function of optical wavelength. In analyzing experimental results, we carried out numerical simulations of the local electric field under incident light, across plasmonic resonances. The comparison between the calculated local field squared and measured fluorescence/EQE provides physical insight on the configuration- dependence of these two processes. Our results indicate that local field enhancement near nanostructures is dominant in nMEF, and that the local field is strongly affected by the substrate and device architectures. For the OSCs, both measurements and calculations show that absorbance within the active layer is enhanced only in a narrow band of wavelengths (~640-720 nm) where the active layer is not very absorbing for our prototype nanopillar-patterned devices. The peak enhancement for 180 nm wide Au nanopillars was approximately 60% at 675nm. The corresponding resonance involves both localized surface plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. Finally, we explore the role of the size of the nanostructures in such a device on the optical absorption in the OSC active layer. We find that small Au nanopillars produce strong internal absorption resulting in Joule heating, and suppressing the desired enhancement in EQE in OSC devices.