Pattern formation and magnetism in pulsed laser-induced self-organized nanostructures from single and bilayer metallic films
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
Self-organization via nanosecond pulsed laser melting of thin metal films is attractive as a reliable and low cost method to create surface metallic nanostructures. The potential applications of these resulting structures are related to their magnetic, optical, plasmonic, and magneto-optical properties, such as in high density magnetic storage, energy harvesting, and sensing. In this dissertation experimental and theoretical mechanisms of nanoscale patterning by dewetting self-organization was explored in nanoscopic thickness single and bilayer metal films. Magnetism in the resulting nanostructures was also explored. Experimental techniques included thin film deposition, laser irradiation, scanning and transmission electron microscopy, atomic force and magnetic force microscopy. A thermodynamic model of dewetting based on comparing the rate of change of free energy to frictional loss via viscous dissipation was developed. From this model the length scale characteristics of the self-organization in single and bilayer metal systems was determined. The model was successfully applied to experimental behavior of pulsed-laser induced dewetting of Ag, as well as for Co, where intrinsic thermocapillary effects influenced dewetting. Another finding was that the metal films show a thickness dependent dewetting morphology. This was investigated in detail for Ag on SiO2 substrates and it was found that for films less then 9.5 nm, the early stages of dewetting consisted of bicontinuous structures, while above, it consisted of regular sized holes. This observation was consistent with a previously published theoretical argument that morphology was based on the film-thickness dependent location of the minimum in the free energy curvature. In this model, bicontinuous patterns occur for thickness below the curvature minimum, while holes occur above it. For bilayer self-organization, the immiscible metals Ag and Co were investigated on SiO2 substrates. The thermodynamic theory predicted substantially different length scales for the two bilayer configurations, Ag/Co/SiO2 and Co/Ag/SiO2. This behavior was corroborated by experimental measurements based on pulsed laser dewetting. The difference arises due to change in the sign and magnitude of intermolecular forces for the two configurations. The nanoparticles resulting from bilayer self-organization consisted of granular nanostructure with each nanoparticle containing phase separatedl grains of Ag and Co, thus representing nanoscale nanocomposites. Extensive magnetic investigations were performed on hemispherical shaped nanoparticles of elements: Co, Fe, Ni), alloys: FeCo) and composites: CuCo), created by the laser dewetting process. By utilizing magnetic force microscopy technique, the magnetic anisotropy of individual nanoparticles was determined. The most important discovery was a size-dependent single-domain magnetic anisotropy due to which, smaller particles had an in-plane magnetic orientation while larger particles were out-of-plane. The reason for the unusual out-of-plane orientation for the hemispherical particles was a residual strain in the nanoparticles following the large cooling rates under laser melting. The resulting magnetoelastic energy was sufficient to overcome shape and magnetocrystalline anisotropy energies. These investigations present a fundamental picture of nanostructure synthesis via pulsed laser-induced dewetting and self-organization in single and bilayer films and the magnetic behavior of the processed materials.
Krishna, Hare, "Pattern formation and magnetism in pulsed laser-induced self-organized nanostructures from single and bilayer metallic films" (2010). All Theses and Dissertations (ETDs). 188.
Permanent URL: http://dx.doi.org/10.7936/K7RF5S4R