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Conductivity of gold nanoparticle thin films and magnetoresistance of metallic thin films embedded with periodic arrays of cobalt nanoparticles
Thin films of 7 nm gold nanoparticles were fabricated via a direct write electron beam exposure. The electrons caused the nanoparticles to stick to the substrate and the unexposed areas were rinsed off. After the lift off, the films were annealed. This caused Ostwald ripening. Electrical and structural properties were studied. It was found that the initial film thickness is the crucial factor of the post annealing properties. In case of films thinner than 50 nm, it was found that the particles ripen to form insulating islands hundreds of nanometers in size and seperated from each other. If the film thickness is increased beyond 50 nm, the ripening causes the islands to grow so much, that at least one percolating pathway for charge transport is formed. Metallic conductivity was observed in a wide temperature range (2–350 K). It is possible to create films that display hopping conductivity, if the initial film thickness is at approximately 50 nm. In a second experiment, polystyrene—block-poly(4-vinylpyridine) (PS-P4VP) diblock copolymer was used to create a hexagonal lattice of cobalt nanoparticles. The size scale of the particles was about 12–15 nm with a 25–30 nm separation between them. This artificially arranged array of magnetic "impurities" was covered with a thin metallic film. Copper or palladium were used and the thickness was varied between 13–40 nm. It was found that these nanoengineered films show thermal hysteresis in resistance versus temperature measurements, the shape of which can be manipulated by applying a magnetic field. The effect was more pronounced in the cobalt-copper samples. Further, magnetoresistance measurements showed oscillations as a function of applied field and temperature. Again, the effects shown in the cobalt-copper samples were significantly more pronounced.
Dickert, Stefan, "Conductivity of gold nanoparticle thin films and magnetoresistance of metallic thin films embedded with periodic arrays of cobalt nanoparticles" (2013). Doctoral Dissertations Available from Proquest. AAI3556242.