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

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    INTERFACIAL INTERACTIONS OF NANOTUBES: AN IN-SITU STUDY OF STRUCTURE AND REACTIONS WITH THEIR ENVIRONMENTS
    (2021) Chao, Hsin-Yun; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nanotubes have the potential to be a revolutionary material for many different applications. Though often touted as versatile and tunable materials, the difficulty of their reliable large-scale production for any specified property is a hurdle in their wide-scale implementation. Interactions at nanotube interfaces dictate overall performance of their growth, radiation resistance, and nanofluidics properties. In this dissertation, I present in-situ experiments using an environmental transmission electron microscope (ETEM). Numerous aspects of interfacial mechanisms of nanotubes are examined at the atomic scale and models considered for the observed behavior. First, I study the interface between nanotubes and catalyst particles during single-walled carbon nanotube (SWCNT) growth. The structure and phase transformation of cobalt catalysts are elucidated for inactive, active, and deactivated nanoparticles by ETEM imaging. Through in-depth studies of multiple distinct cobalt nanoparticles, I establish the dominant nanoparticle phase for SWCNT growth. I also identify the preferred lattice planes and a threshold for work of adhesion for the anchoring and liftoff of SWCNTs. Second, the nanotubes are tested for their radiation resistance properties. I study the resistance of nanotube degradation in an ionizing environment with oxygen pressure, where the damage initiates at the interface with the gas phase. Observations show boron nitride nanotubes (BNNTs) have a higher resistance to damage than carbon nanotubes (CNTs). By computing knock-on threshold energies for the atoms impacted by incoming electrons, a model can be formulated for the oxygen-assisted radiation damage pathway. I provide further validation to the model with heating experiments that demonstrate a surprising increase in damage resistance. Lastly, interfaces between nanotubes and water are studied. The goal is to capture the ordering dynamics of water at the BNNT interface using in-situ characterization at cryogenic temperatures. Water is hyper-quenched to liquid nitrogen temperatures for the formation of low density amorphous (LDA) ice. High resolution images are then acquired, preserving the original water structure. Crystallization of LDA ice is induced by both environmental heating and electron beam irradiation. I present a comparison of the structural evolutions of LDA ice with and without the presence of BNNT, which indicates the presence of nascent ordering at the interface.
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    The effect of boric acid on the growth mechanism of electrodeposited metal nanostructures
    (2014) Graham, Lauren M.; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Electrochemical deposition (ECD) has been a primary method of metal plating for a variety of applications for nearly 100 years. Its popularity is due to its straightforward design, low cost, uniform results, and successful application to a wide range of metals and substrates. Many factors have been shown to influence the composition, texture, and chemical properties of the resultant deposit, such as the current density, the nature and concentration of metal ions, the solution temperature and composition, the applied current waveform, the substrate surface, and agitation. In particular, additives play a complex role in metal deposition due to their ability to greatly alter the growth mechanism and resultant deposit structure. There exists a vast body of work related to the role of additives in various plating solutions, however the majority of investigations on additive effects are focused on planar deposition. For instance, boric acid is a common additive found in nearly all aqueous transition metal plating solutions, yet its influence on metal nanostructure deposition has not been well studied. In this work, we focus on the impact of additives on the growth of metal nanostructures. Specifically, we investigate the role of boric acid in the ECD of nickel nanotubes (NTs) and nanowires (NWs). First, we demonstrate the difference in the growth mechanism and nanostructure morphology in the presence and absence of boric acid with electron microscopy and electrochemical analyses. The ECD conditions are modified to further probe the role of boric acid in the 1D growth of nickel nanostructures. The results confirm the function of boric acid in the surface-directed growth of nickel nanostructures. Second, we employ the boric acid-controlled growth mechanism in the synthesis of advanced nickel nanostructures. The potential for the role of boric acid to be applied to the deposition of additional metals is realized through the synthesis of nickel alloy NTs and NWs. Additionally, the advantage of the boric acid-controlled surface-directed growth mechanism is demonstrated through the straightforward synthesis of segmented nanostructures and a 3D interconnected nanotube network.
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    Critical behavior of superconductors and electrical transport properties of carbon nanotube thin films
    (2007-08-28) Xu, Hua; Anlage, Steven M; Lobb, Christopher J; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    With AC microwave measurements from 10 MHz up to 50 GHz and DC nano-volt level measurements we have investigated the superconducting phase transition of YBa 2 Cu 3 O 7-δ films in zero magnetic field and electrical transport properties of single walled carbon nanotube networks. We studied the microwave conductivity of YBa 2 Cu 3 O 7-δ thin films around Tc for different incident microwave power and observed that the microwave fluctuation conductivity deviates from scaling theory at low frequency around Tc. We systematically investigated the length scales involved in AC measurements and found the probed length scale depends on both frequency and current. At low current density J but high frequency ω, we observed critical behavior without hindrance from finite-size effects. However, at low current density J and low frequency ω, the experimentally probed length scale LAC may approach the thickness d of the sample, and then the critical behavior will be destroyed by finite-size effects. In this regime, we can not observe the phase transition. With very small applied microwave power, specifically -46dBm, and high frequency data, we have investigated the critical fluctuations of YBa2 Cu 3 O 7-δ thin films around Tc. It is shown that the determination of Tc is crucial for obtaining critical exponents. Improved temperature stability and conductivity calibration allow us to take high quality data at small temperature intervals (50mK). This improves the conventional data analysis method and allows a new method of extracting exponents to be developed. With these two methods, consistent values of Tc and the critical exponent were precisely determined. Experiments on 6 samples have been done and the results give a dynamical scaling exponent z=1.55±0.15. The scaling behavior of the fluctuation conductivity is also established. We have also investigated fluctuation effects of YBa2 Cu 3 O 7-δ by doing frequency-dependent microwave conductivity measurements and dc current-voltage characteristics on the same film. The dc measurement verified that the applied microwave power -46dBm in our ac measurement is small enough for the correct determination of Tc and critical exponents. For both dc and ac experiments the scaling behavior of the data was investigated. We found that the dc measurement could be affected by disorder. For high quality YBCO films and crystal, the critical exponent z is also around 1.5, which is consistent with ac measurement. Finally, using our broadband experimental technique and DC current-voltage characteristic measurement system, we measured the transport properties of single-walled carbon nanotube films. Based on the real and imaginary parts of the microwave conductivity, we calculated the shielding effectiveness for various film thickness. Shielding effectiveness of 43 dB at 10 MHz and 28 dB at 10 GHz is found for films with 90% optical transmittance, which suggests that single walled carbon nanotube(SWCNT) films are promising as a type of transparent microwave shielding material. We also investigated the frequency and electric field dependent conductivity of single walled carbon nanotube networks of various densities. We found the ac conductivity as a function of frequency follows the extended pair approximation model and increases with frequency above an onset frequency ω0 which varies over seven decades with a range of film thickness from sub-monolayer to 200 nm. The nonlinear electric field-dependent conductivity shows strong dependence on film thickness as well. Measurement of the electric field dependence of the resistance allows for the determination of the localization length scale L of localized states, which is found to systematically decrease with increasing film thickness. The onset frequency ω0 of enhanced ac conductivity and the localization length scale L of SWCNT networks are found to be correlated, and an empirical formula relating them has been proposed. Such studies will help the understanding of transport properties and broaden the applications of this novel material system.
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    Electronic Properties of Carbon Nanotubes studied in Field-Effect Transistor Geometries
    (2004-05-12) Dürkop, Tobias; Fuhrer, Michael S; Physics
    Due to their outstanding properties carbon nanotubes have attracted considerable research effort during the last decade. While they serve as an example of a 1-dimensional electron system allowing one to study fundamental quantum effects nanotubes-especially semiconducting nanotubes-are an interesting candidate for next-generation transistor application with the potential to replace silicon-based devices. I have fabricated nanotubes using chemical vapor deposition techniques with various catalysts and gas mixtures. The nanotubes produced with these techniques vary in length from 100 nm to several hundreds of micrometers. While data taken on shorter metallic and semiconducting devices show Coulomb blockade effects, the main part of this work is concerned with measurements that shed light on the intrinsic properties of semiconducting nanotubes. On devices with lengths of more than 300 um I have carried out measurements of the intrinsic hole mobility as well as the device-specific field-effect mobility. The mobility measured on these nanotube devices at room temperature exceeds that of any semiconductor known previously. Another important consideration in nanotube transistor applications are hysteresis effects. I present measurements on the time scales involved in some of these hysteresis effects and a possible application of the hysteresis for memory devices.