Browsing by Subject "nanowires"
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Item Curvature-directed Crystallization of Polymer Dielectrics in Nanopores(2013-11-26) Reid, DariyaThe formation of isotactic poly(propylene) (iPP) and polycarbonate (PC) nanowires of tunable diameter was demonstrated by melt-wetting the polymer into nanoporous anodic alumina. The crystallization process was analyzed using differential scanning calorimetry. A transition from hetero- to homogeneous crystallization was observed for iPP as the pore diameter decreased. Also, the Avrami exponent decreased as the pore diameter decreased, indicating preferential 1-D crystallization for diameters of 40 nm or less. X-ray diffraction studies showed that the polymer chains crystallize into the ?-phase and that they preferentially orient along the a- and b-axis, perpendicular to the pore wall. Directed crystallization of iPP may enhance the materials electrical and mechanical properties.Item Femtosecond time-resolved spectroscopy of coherent oscillations in nanomaterials(2009-05-15) Jerebtsov, Serguei NikolaevichThe interaction of laser radiation with a material can excite coherent lattice vibration. The observation of such periodic motion of the atoms in the lattice provides information on the properties of the material. In the present work a femtosecond pump-probe technique was applied for studies of acoustic vibrations in nanoparticles and nanowires, and coherent optical phonons in thin films. The elastic properties of spherical Ag nanoparticles and Ag and Bi nanowires were studied in a dual-color femtosecond pump-probe experiment. The results of the period determinations of the acoustic vibrations, obtained from the time-domain measurements with low intensity pump pulses, together with the information about the size of the particles, were used to determine the elastic constants of the materials. Also changes in the measured acoustical response of the Ag nanowires under high intensity laser excitation were studied. In addition the coherent optical phonon excitation in a Bi film was studied in a femtosecond pump-probe experiment. A red-shift of the phonon frequency at the high photoexcitation density was observed. To separate the effect of the lattice softening and the lattice anharmonicity the excitation with two pump pulses was employed. Numerical simulations, which took into account the evolution of the spatial inhomogeneity of the excitation density, were carried out and compared to the experimental results.Item Template-based Ferromagnetic Nanowires and Nanotubes: Fabrication and Characterization(2013-05-01) Wei, ZhiyuanThis dissertation describes experimental studies of the structures and properties, and their correlations in ferromagnetic nanowires and nanotubes fabricated using porous templates. Ferromagnetic Ni and Fe nanowires with diameters 30 ~ 250 nm were electroplated into the pores of anodic aluminum oxide membranes. The effects of nanowire diameter on structural and magnetic properties were investigated. The microstructures of these nanowires were studied using X-ray diffraction and selected-area electron diffraction measurements. The magnetic properties of the nanowires were investigated using magnetic hysteresis measurements and magnetic force microscopy. Additionally, ferromagnetic Ni-P nanotubes were fabricated using an electroless chemical deposition method. Structure and composition analyses were conducted using X-ray diffraction and energy-dispersive spectroscopy. The magnetic properties of the nanotube arrays and the electronic properties of individual nanotubes were studied. Hysteresis measurements revealed that the 250-nm diameter Ni nanowires had a poor squareness in their hysteresis loops, indicating the existence of multi-domain states. In comparison, the squareness in the hysteresis loops of 60-nm and 30-nm Ni nanowires was much improved, suggesting the existence of single domain states in these smaller diameter nanowires. Magnetic force microscopy measurements confirmed the magnetic domain structures suggested by magnetic hysteresis measurements. Similar investigations of Fe nanowires with diameters of 250 nm and 60 nm found that they all have multidomain magnetic structures. This is expected based on their material properties and polycrystalline structures. Furthermore, magnetic structures of Y-branches and multi-wire clusters were also studied using magnetic force microscopy. The as-prepared Ni-P nanotubes had an amorphous structure. Following a heat treatment, however, a structural phase transformation from the amorphous phase to a crystalline phase was observed using X-ray diffraction measurements. The tetragonal crystalline phase of Ni3P and the face-centered-cubic phase of Ni were confirmed via simulations by the GSAS software. The high Ni3P content accounts for the semiconducting behavior and a low magnetic anisotropy observed in the Ni-P nanotubes.Item Vapor transport techniques for growing macroscopically uniform zinc oxide nanowires(2009-08) Baker, Chad Allan; Hall, Matthew John; Shi, Li, Ph. D.ZnO nanowires were grown using carbothermal reduction and convective vapor phase transport in a tube furnace. Si <100> substrates that were 20 mm x 76.2 mm were sputter coated with 2 nm to 50 nm gold which formed nanoparticles on the order of 50 nm in diameter through a process of Ostwald ripening upon being heated. Growth temperatures were varied from 800ºC to 1000ºC, flow rates were varied from 24 sccm to 3300 sccm, and growth durations were varied from 8 minutes to 5 hours. Vapor phase Zn, CO, and CO2, produced by carbothermal reduction and suspended in an Ar atmosphere, were flowed over the Si substrates. The Au nanoparticles formed an eutectic alloy with Zn, causing them to become liquid nanodroplets which catalyzed vapor-liquid-solid nanowire growth. The nanowires were also synthesized by self-catalyzing vapor-solid growth in some cases. Using the tube furnace never resulted in more than 50% of the substrate being covered by nanowires. It was found that a bench-top furnace could achieve nearly 100% nanowire coverage by placing the 20 mm x 76.2 mm sample face down in a quartz boat less than 2 mm above the source powder. This was because minimizing the distance between the sample and the source powder was critical to achieve macroscopically uniform growth consistently.