Physical Sciences and Mathematics Commons^™

Neutron Scattering Studies Of Phosphate Proton Conductors, Amal Bajes Al-Wahish

Doctoral Dissertations

Proton ceramic fuel cells operating in the intermediate temperature range of 300-500 °C offer potentially revolutionary advantages over existing fuel cells because expensive noble metal catalysts would not be needed, and in situ reforming of liquid bio-fuels such as ethanol or methanol would be possible.The chief obstacle facing intermediate fuel cells is the lack of a suitable electrolyte in the operating temperature range. A good electrolyte is thermally and chemically stable, inexpensive, environmentally friendly, and has a proton conductivity on the order of 10^-2 S cm^-1 [Siemens per centimeter] at 400 °C. Acceptor-doped lanthanum orthophosphate is an …

Studies On The Wrinkling Of Thin Polymer Films Floating On Liquid, Kamil B. Toga

Doctoral Dissertations

This dissertation aims to broaden our understanding on wrinkling instabilities occurring on floating polymeric sheets, and tries to establish innovative methods that exploit these patterns in studies on material behavior and interfacial phenomena. We will address three major topics in this thesis including, i) characterization of the conditions required to buckle an annular disc, ii) characterization of wrinkles occurring around a droplet/bubble placed on a membrane that is kept taut at the liquid-air interface, and iii) using wrinkling patterns as a probe to understand the interfacial behavior and dynamics of ultrathin films. The first project in this thesis is about …

Morphology Characterization Of Low Band Gap Polymer-Based Organic Photovoltaics, Feng Liu

Doctoral Dissertations

In bulk heterojunction (BHJ) thin film organic photovoltaics (OPV), morphology control is critical to obtain good device efficiency. Nanoscale phase separation that creates bicontinuous interpenetrating structure on a size scale commensurate with exciton diffusion length (~10 nm) is thought to be the ideal morphology. Results obtained from this work indicate that morphology can be affected by chemical structure of the polymer, processing conditions, blending ratio and post treatments. Physical properties of the material, such as crystallinity, crystal orientation, material interactions and miscibility, surface energy and particle aggregations are critical for determining the morphology and thus the device performance. Previous investigations …

Optical And Scanning Probe Studies Of Isolated Poly (3-Hexylthiophene) Nanofibers, Mina Baghgarbostanabad

Doctoral Dissertations

Plastic electronics have an essential role in the future technologies owing to their compelling characteristics such as light weight, biocompatibity, low cost fabrication, and tunable optoelectronic properties. However, the performance of polymer-based devices strongly depends on the efficiency of exciton formation and dynamics that are themselves strongly sensitive to polymer molecular packing and structural order. Therefore, the current challenge in achieving high efficiency is establishing a correlation between molecular packing and exciton coupling. P3HT nanofibers represent an attractive platform for studying optical and electronic properties of exciton coupling because their nominal (highly crystalline) internal chain packing structure is known. A …

Growth, Structure, Electronic And Transport Properties Of Yttrium Disilicide Nanowires, Saban Mustafa Hus

Doctoral Dissertations

The electronic properties of low-dimensional materials deviate significantly from their bulk counterparts. Especially in quasi one-dimensional (1D) materials, a small number of structural defects can lead to strong electron localization. Electrons may also display unusual collective behavior in 1D. As integrated circuits continue to shrink in size, there is an increasing need for understanding and possibly manipulating electronic transport in quasi 1D materials. Here, we focus on electrical transport in self-assembled YSi₂ [yttrium disilicide] nanowires on Si(001). Being just a few atoms wide, these nanowires are one of the closest experimental realizations of a 1D conductor. YSi₂ nanowires …

Study Of Ferroelectric Oxides And Field Effect In Complex Oxides Heterostructures, Lu Jiang

Doctoral Dissertations

With the rapid development of technology, the need for novel materials and state-of-theart devices is growing fast. Complex oxides which have strongly correlated electrons are favorable candidates for materials industry, due to their rich phase diagrams and multiple functions. Especially, ferroelectric oxides is very promising materials in the industry for storage, due to their bistable polarization states triggered by external electrical field. This thesis is centered on ferroelectric oxides, analyzing their lattice structures and investigating the interface of ferroelectrics and other complex oxides to examine the potential of the heteostructures in the application in electronic devices.

The most notable feature …

Theoretical Studies Of The Growth And Functionality Of Layered Materials, Wei Chen

Doctoral Dissertations

In this thesis, we present several projects on the growth and functionality of layered materials, using density functional theory (DFT) method and phenomenological modeling approach. Beyond the understanding of growth mechanisms and exploration of properties, we propose novel avenues to realize controllable growth processes and layered materials with desirable properties. The contents have three major parts:

(1) Graphene growth on Cu(111) and Ni(111) substrates. We first demonstrate that the inherent multi-orientational degeneracy of the graphene islands on Cu(111) in the early stages of nucleation could result in the prevalence of grain boundaries (GBs). Next, we propose a possible solution to …

Probing The Size Dependent Chemical Properties Of Metals In Reduced Dimension, Xiangshi Yin

Doctoral Dissertations

Heterogeneously catalyzed reactions typically start with adsorption and dissociation of reactant molecules on the surface of a solid catalyst. In many instances, this is followed by surface diffusion of the adsorbed species, chemical reaction, and removal of the product molecule. According to the Sabatier principle, optimal catalytic performance requires that the bonding between the adsorbate molecule and the surface should neither be too strong nor too weak. This bonding strength is directly related to the catalyst’s surface electronic structure and hence, electronic structure modification would seem a promising approach for tuning catalytic activity.

There have been many studies along this …

Quantum Tuning Of Plasmons In Ultrathin Metal Films, Ao Teng

Doctoral Dissertations

The surface plasmon is a coherent charge density oscillation localized at a metal surface. It can couple with light and the resulting plasmon-polariton hybrid mode is confined to volumes that are much smaller than the classical diffraction limit of light. Nano-plasmonics is a rapidly evolving field where light manipulation at the nanoscale may lead to novel applications. However, as the size of plasmonic devices approaches the quantum-size regime, the macroscopic picture of plasmon may no longer be valid. To elucidate the influence of the discretization of the single particle spectrum on the collective plasmon response, we performed a systematic study …

Quantum Levitation Using Metamaterials, Venkatesh K. Pappakrishnan

Doctoral Dissertations

The emergence of an attractive vacuum force (Casimir force) between two purely dielectric materials can lead to an increase in the friction and the stiction effects in nanoscale devices, resulting in degradation or decreased performance. Thus, it is of high practical importance that the conditions for the reversal of the Casimir force from attractive to repulsive are identified. Although the repulsive Casimir force has been considered for high dielectric materials as an intermediate (between the plates) medium, so far no realistic system has been proposed that can demonstrate quantum levitation with air/vacuum as a host medium. Since air is the …

Synthesis And Characterization Of Magnetic Nanowires Prepared By Chemical Vapor Deposition, Siwei Tang

Doctoral Dissertations

Various metal silicide and germanide magnetic nanowires were synthesized using a home-built CVD [chemical vapor deposition] system. The morphology, composition, and magnetic properties of the nanowires were studied and correlated with growth parameters such as temperature, pressure, time, and source-substrate distance.

One of the compositions targeted for synthesis was MnSi [manganese silicide]. In bulk, this material orders helimagnetically at T_c [curie temperature] = 30K, with a helical pitch of about 20 nm. After extensive study, we learned that the thickness of the silicon dioxide layer on the substrate is a critical parameter for the growth of MnSi nanowires. An …

Neutron Scattering Studies Of Cuprates And Iron Pnictides, Mengshu Liu

Doctoral Dissertations

Presented within are neutron scattering studies of several different high temperature superconducting materials: BaFe_1.9Ni_0.1As₂ [Barium Iron Nickel Arsenic], BaFe_1.85Ni_0.15As₂ [Barium Iron Nickel Arsenic], Ba_0.67K_0.33Fe₂As₂ [Barium Potassium Iron Arsenic], and Pr_0.88LaCe_0.12CuO_4-y [Praseodymium Lanthanum Cerium Copper Oxide]. The main focus is on the magnetic excitations within the systems.

For BaFe_1.9Ni_0.1As₂ [Barium Iron Nickel Arsenic], we measured the intensity of its magnetic excitations and compared the results with excitations in antiferromagnetic non-superconducting BaFe₂As_{2 …}