Modification of Nanostructures via Laser Processing

Modification of nanostructures via laser processing is of great interest for a wide range of applications such as aerospace and the storage of nuclear waste. The primary goal of this dissertation is to improve the understanding of nanostructures through two primary routes: the modification of aerogels and pulsed laser ablation in ethanol. A new class of materials, patterned aerogels, was fabricated by photopolymerizing selected regions of homogeneous aerogel monoliths using visible light. The characterization and fabrication of functionally graded, cellular and compositionally anisotropic aerogels and ceramics is discussed. Visible light was utilized due to it’s minimal absorption and scattering by organic molecules and oxide nanoparticles within wet gels. This allowed for the fabrication of deeply penetrating, well resolved patterns. Similarly, nanoporous monoliths with a typical aerogel core and a mechanically robust exterior ceramic layer were synthesized from silica aerogels cross-linked with polyacrylonitrile. Simple variations of the exposure geometry allowed fabrication of a wide variety of anisotropic materials without requiring layering or bonding. Nanoparticle solutions were prepared by laser ablation of metal foils (Fe and Mo) in ethanol. Ablation of Fe generated Fe3O4 and Fe3C nanoparticles which were superparamagnetic with a saturation magnetization Ms = 124 emu/g. Zero field cooled (ZFC) measurements collected at an applied field of 50 Oe displayed a maximum magnetic susceptibility at 120 K with a broad distribution. Field cooled (FC) measurements showed a thermal hysteresis indicative of temperature dependent magnetic viscosity. Pulsed laser ablation of a Mo foil in ethanol generated inhomogeneous nanoparticles where Mo and MoC coexisted within the same aggregate. Formation of these unique nanoparticles is likely due to phase separation that occurs when a high temperature carbide phase cools after the laser pulse terminates. Similarly, magnetic nanoparticle suspensions were generated by pulsed laser ablation of Fe and Mo in ethanol. The formation of several carbide phases with no discernable alloy formation was seen. A decrease in magnetization with a decrease in Fe concentration was seen which was reconciled with the decreased Fe content in the system. However, at Fe concentrations below ~ 40%, an increase in Ms and Hc was observed which was reconciled with the disappearance of the ε–Fe3C. TEM analysis showed the formation of core-shell nanoparticles and Energy Filtered TEM showed the distribution of Fe-based nanoparticles in the suspensions