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 Micelle and Aggregate Formation in Amphiphilic Block Copolymer Solutions(2010) Clover, Bryna Christine; Greer, Sandra C; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The amphiphilic nature of many block copolymers causes self-aggregation and micelle formation in solvents that are miscible with only one of the block polymers (selective solvents). Micelle and aggregate formation of amphiphilic block copolymers in selective solvents is a function of temperature and concentration. Such self-aggregation has been examined here in a variety of block copolymer systems. In dilute solutions of Pluronic P85 (PEO26PPO40PEO26) (where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)) in D2O, transitions between clustered unimers, spherical micelles, cylindrical micelles, and finally lamellar micelles were observed with increasing temperature. The effect of pressure on this system was examined through small angle neutron scattering (SANS) techniques. At temperatures above 95 oC, a new phase of “demixed lamellae” was observed. Pressure effects on the transition temperatures between the phases of this system were investigated. The self-aggregation of Reverse Pluronic 17R4 (PPO14PEO24PPO14) in D2O has also been examined. The phase diagram of this system was determined through visual cloud-point techniques. Three distinct regions have been observed in solutions of this system, as a function of temperature and concentration: a cloudy, one-phase region; a clear, one-phase system; and a region of phase separation. Copolymer structures were examined in the clear and cloudy one-phase regions through SANS and dynamic light scattering (DLS) techniques. A network, or clustering, of unimers was observed in the cloudy phase. Aggregates in the clear, one-phase region could not be identified definitively as micelles. Finally, micellization of PEO132-PB89 (where PB is polybutadiene) has been studied in solutions of deuterated methanol and deuterated cyclohexane. Spherical micelles were observed in solutions of deuterated methanol. These micelles change little in size or shape over a 50 oC temperature span. The difference in aggregates in protonated and deuterated solvents was also examined. In deuterated cyclohexane, the copolymer formed flexible, cylindrical micelles below 40 oC. These micelles became spherical in shape at higher temperatures.Item Development of self-assembled ZnO nanostructures in diblock copolymers on large area Si wafers and gas sensor applications(2008-05-27) Ali, Hasina Afroz; Iliadis, Agis A.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)ZnO nanoparticles with improved optical properties and increased surface areas have the potential for advanced optoelectronic, gas sensor and biosensor applications. In order to exploit these unique properties of ZnO nanoparticles for the realization of nanoscale devices, we developed novel techniques for the self-assembly and functionalization of ZnO nanoparticles through diblock copolymers on large area (100)Si surfaces. These novel techniques allowed us to subsequently develop the first ZnO nanoparticle based device. Thus, a novel ZnO-nanocomposite/Si n-p heterojunction diode and a high performance hydrogen gas nanosensor have been developed, for the first time. The thesis presents the novel technique developed for the self-assembly of ZnO nanostructures with spherical morphology through diblock copolymers on large area Si substrates. Correlation between the physical parameters of the nanoparticles and the copolymers was evaluated from AFM studies. Control of the nanoparticle size and density was achieved by varying copolymer block lengths. The largest nanoparticles had average sizes of 250 nm and densities of 1x107cm-2 while the smallest nanoparticles had average sizes of 20nm and densities of 1x101010cm-2. XRD studies showed that the wurtzite crystal structured nanoparticles assumed the same orientation (100) as the Si substrate, indicating a pseudo-epitaxial nanostructure. Room temperature photoluminescence studies showed quantum confinement effects with a blue shift from 372 nm (large particles) to 363 nm (small particles). A broad defect related green-yellow luminescence centered at 555 nm indicative of n-type conductivity of the nanoparticles was also observed. The n-type nanoparticles on p-type Si resulted in the development of a ZnO-nanocomposite/pSi n-p heterojunction diode for the first time. The nanodiode showed good rectification and low leakage currents. LogI-V characteristics gave built-in voltages of 0.69 and 0.7 eV, saturation currents of 2 and 2.34 x10-8A, and ideality factors of 5.9 and 5.7 for the small and large particles, respectively. The transport mechanisms of the nano-diodes were studied. C-V characteristics showed abrupt p-n junctions, suggesting an intimate junction interface consistent with the pseudo-epitaxial nature of the structure. A novel hydrogen gas nanosensor based on the ZnO-nanocomposite/Si heterojunction diode was developed for high sensitivity, rapid, room temperature sensing. Response and recovery times were reduced by a factor of 100 and smaller and denser nanoparticles were found to be faster and more sensitive.