UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
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 given thesis/dissertation in DRUM.
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Item Metal hydrides as a platform for reconfigurable photonic and plasmonic elements(2021) Palm, Kevin James; Munday, Jeremy N; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metal hydrides often display dramatic changes in optical properties upon hydrogenation. These shifts make them prime candidates for many tunable optical devices from optical hydrogen sensors and switchable mirrors to physical encryption schemes. In order to design and fabricate optimized devices for any of these applications, we need to determine the optical and structural properties of these materials. In this dissertation, we design and implement an apparatus that dynamically measures the gravimetric, stress, calorimetric, and optical properties of metal hydrides as they are exposed to H2. We use this apparatus to measure the properties of 5 different pure metal hydrides (Pd, Mg, Ti, V, and Zr) and then use these properties to design tunable color filters and switchable perfect absorbers, among other devices. To widen our parameter space and to combine desirable characteristics of different metal systems, we use the same apparatus to investigate the properties of different metal alloy hydride systems including Pd-Au, Mg-Ni, Mg-Ti, and Mg-Al. We demonstrate many improved nanophotonic designs with these materials, including a thin-film physical encryption scheme with Pd-Au and a switchable solar absorber with Mg-Ti. Many of these photonic devices can be further enhanced by tailoring the substrate of the device along with the metal hydride. In this dissertation, we also investigate combining the switchable optical properties of metal hydrides with near-zero-index substrates to further enhance the optical device changes. Near-zero-index materials are ones where the refractive index is below 1 and can lead to a variety of interesting optical effects, including high absorption in surrounding materials and enhanced non-linear effects. By combining an ITO substrate with a near-zero-index resonance at ~1250 nm with a thin Pd capped Mg film, we demonstrate a switchable absorption device with >76% absorption change at 1335 nm illumination. To further explore the possibility of large-scale fabrication of these devices, we survey the properties of commercially available near-zero-index materials and report the range of attainable optical properties, showing its feasibility.Item MORPHOLOGY OF CELLULOSE AND CELLULOSE BLEND THIN FILMS(2017) Lu, Rui; Briber, Robert M.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cellulose is the most abundant, renewable, biocompatible and biodegradable natural polymer. Cellulose exhibits excellent chemical and mechanical stability, which makes it useful for applications such as construction, filtration, bio-scaffolding and packaging. It is useful to study amorphous cellulose as most reactions happen in the non-crystalline regions first and at the edge of crystalline regions. In this study, amorphous thin films of cotton linter cellulose with various thicknesses were spincoated on silicon wafers from cellulose solutions in dimethyl sulfoxide / ionic liquid mixtures. Optical microscopy and atomic force microscopy indicated that the morphology of as-cast films was sensitive to the film preparation conditions. A sample preparation protocol with low humidity system was developed to achieve featureless smooth films over multiple length scales from nanometers to tens of microns. X-ray reflectivity, X-ray diffraction, Fourier transform infrared spectroscopy and high resolution sum-frequency generation vibrational spectroscopy were utilized to confirm that there were no crystalline regions in the films. One- and three- layer models were used to analyze the X-ray reflectivity data to obtain information about roughness, density and interfacial roughness as a function of film thickness from 10-100nm. Stability tests of the thin films were conducted under harsh conditions including hot water, acid and alkali solutions. The stability of thin films of cellulose blended with the synthetic polymer, polyacrylonitrile, was also investigated. The blend thin films improved the etching resistance to alkali solutions and retained the stability in hot water and acid solutions compared to the pure cellulose films.Item Engineering thin films of magnetic alloys and semiconductor oxides at the nanoscale(2016) Xie, Ting; Gomez, R. D.; Murphy, Thomas E.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The thesis aims to exploit properties of thin films for applications such as spintronics, UV detection and gas sensing. Nanoscale thin films devices have myriad advantages and compatibility with Si-based integrated circuits processes. Two distinct classes of material systems are investigated, namely ferromagnetic thin films and semiconductor oxides. To aid the designing of devices, the surface properties of the thin films were investigated by using electron and photon characterization techniques including Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), and energy-dispersive X-ray spectroscopy (EDS). These are complemented by nanometer resolved local proximal probes such as atomic force microscopy (AFM), magnetic force microscopy (MFM), electric force microscopy (EFM), and scanning tunneling microscopy to elucidate the interplay between stoichiometry, morphology, chemical states, crystallization, magnetism, optical transparency, and electronic properties. Specifically, I studied the effect of annealing on the surface stoichiometry of the CoFeB/Cu system by in-situ AES and discovered that magnetic nanoparticles with controllable areal density can be produced. This is a good alternative for producing nanoparticles using a maskless process. Additionally, I studied the behavior of magnetic domain walls of the low coercivity alloy CoFeB patterned nanowires. MFM measurement with the in-plane magnetic field showed that, compared to their permalloy counterparts, CoFeB nanowires require a much smaller magnetization switching field , making them promising for low-power-consumption domain wall motion based devices. With oxides, I studied CuO nanoparticles on SnO2 based UV photodetectors (PDs), and discovered that they promote the responsivity by facilitating charge transfer with the formed nanoheterojunctions. I also demonstrated UV PDs with spectrally tunable photoresponse with the bandgap engineered ZnMgO. The bandgap of the alloyed ZnMgO thin films was tailored by varying the Mg contents and AES was demonstrated as a surface scientific approach to assess the alloying of ZnMgO. With gas sensors, I discovered the rf-sputtered anatase-TiO2 thin films for a selective and sensitive NO2 detection at room temperature, under UV illumination. The implementation of UV enhances the responsivity, response and recovery rate of the TiO2 sensor towards NO2 significantly. Evident from the high resolution XPS and AFM studies, the surface contamination and morphology of the thin films degrade the gas sensing response. I also demonstrated that surface additive metal nanoparticles on thin films can improve the response and the selectivity of oxide based sensors. I employed nanometer-scale scanning probe microscopy to study a novel gas senor scheme consisting of gallium nitride (GaN) nanowires with functionalizing oxides layer. The results suggested that AFM together with EFM is capable of discriminating low-conductive materials at the nanoscale, providing a nondestructive method to quantitatively relate sensing response to the surface morphology.Item Synthesis and characterization of multiferroic thin films(2008-07-02) Lim, Sung Hwan; Salamanca-Riba, Lourdes; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiferroic materials and multiferroic materials systems which simultaneously exhibit ferroelectricity and magnetism have attracted great attention because of their exotic physical properties and their potential applications which utilize coupling of magnetism and ferroelectricity. The goal of this thesis was to study multiferroic materials systems in thin film and multilayer forms in order to explore the possibility of fabricating room temperature thin film devices. In particular, we have focused on two types of multiferroic materials systems: 1) intrinsic multiferroic/magnetoelectric thin film materials and 2) magnetostrictive/ piezoelectric bilayer systems for investigation of the strain-mediated magnetoelectric (ME) effect. BiFeO3 is an intrinsic multiferroic which displays ferroelectricity and antiferromagnetism at room temperature, and thus of strong interest for ambient device applications. In this thesis, we have extensively investigated the role of microstructure on the properties of BiFeO3 thin films. We studied multiphase formation in Bi-Fe-O thin films, and found that formation of secondary phases such as α-Fe2O3, γ-Fe2O3, and Fe3O4 increased overall saturation magnetization and released the misfit strain of the BiFeO3 grains in the films. We have studied several aspects of the ME effect which are directly relevant to possible novel device applications. Electric field tunable spintronic devices using the ME effect have been proposed. In one such device configuration, the desired effect is electric field tuning of giant magnetoresistance or tunnel magnetoresistance through control of exchange bias via the ME effect. We have investigated the feasibility of such a device using exchange-biased Co/Pt multilayers on Cr2O3 thin films. The strain-mediated ME effect at the interface of magnetostrictive/ piezoelectric bilayers has been widely used to demonstrate magnetic field detection with extremely high sensitivity. Although the overall mechanism of such an effect is known, the details of the bilayer interfaces and how they affect the coupling is not understood. In order to directly observe the strain-mediated ME coupling effect, we fabricated bilayer thin film structures and performed in-situ dynamic observation of magnetic domains while an electric-field was being applied using Lorentz transmission electron microscopy. Electric-field induced motion of magnetic domain boundaries in the magnetostrictive layer was observed for the first time.