The magnetic and transport properties of template-synthesized carbon-based and related nanomaterials

Abstract

The porous alumina template-assisted method of nanoscale materials preparation provides a simple, relatively inexpensive, yet highly controllable and repeatable process for nanomaterial synthesis. Various nanostructures can then be made utilizing the porous structure as a scaffold. In this dissertation we study the porous alumina anodization process, the synthesis of porous alumina-assisted materials, and the basic physical properties of these materials, primarily concentrating on the magnetic and transport properties. First, we study the porous alumina formation process as a function of anodization voltage, acid type, and acid concentration. We find that while acid type strongly affects the growth characteristics of porous alumina, pH does not. We also study the stability of pore formation. We characterize the two- and three-dimensional stability of the growth process. We find that in three dimensions, an unstable formation region as a function of pH and voltage will cause the formation of dendrite structures. Next, we study the synthesis of materials in the porous alumina templates. Through chemical self-assembly, electrodeposition is able to make a wide variety of nanowires and nanotubes and we seek to optimize this process. Third, we study the optical properties Au and Ag nanowire arrays embedded in porous alumina. We find that such materials have use as negative index metamaterials owing to the existence of both transverse and longitudinal surface plasmon resonances. Next, we study the basic magnetic properties of new PAni-ferromagnet composite nanostructures and compare these properties to the magnetic properties of the nanotubes and the nanowires alone. We find the high dielectric properties of the PAni to strongly shield the ferromagnetic nanowires from magnetostatic interactions. Fifth, we make devices out of carbon nanotubes synthesized by CVD in the alumina templates. We investigate the transport properties of these carbon nanotubes. Further, we find that that the contact resistances, which are normally on the order of mega-ohms for these tubes, can be lowered to the order of kilo-ohms by annealing in H2/Ar atmosphere. We find that the disorder in these carbon nanotubes allows for the uptake of H during the annealing process. These H-complex hydrogen impurities, along with C and H adatoms, induce ferromagnetism in the carbon nanotubes. We carry out a magnetic study on the annealed carbon nanotubes. Moreover, the ferromagnetism of the carbon nanotubes results in hysteric magnetoresistance. We study this effect, attributing it to strong magneto-viscosity effects and anisotropic magnetoresistance. We also study the transport and magnetotransport properties of the annealed carbon nanotubes as a function of temperature and inner diameter. We find that there is an order-disorder transition that occurs at lower temperatures that resembles behavior predicted in disordered carbon fibers by the Bright model. We also find that the nanotubes behave as one-dimensional Luttinger liquids. Finally, as a means of comparison, we fabricate and study the properties of monolayer graphene devices.