| 1. |
- Carstensen, Hauke
(författare)
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Self-assembly of magnetic particles
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Self-assembly is the spontaneous formation of larger structures from small building blocks. This process is driven and determined by the interactions between the constituents. Examples of self assembly are found almost everywhere and, in particular, biological systems in general rely on a hierarchical formation of structures over a range of length scales. Technologically, self-assembly can be used to form mesoscopic structures and artificial crystals. In the case of particles with micrometer size suspended in a liquid phase, it is possible to use optical microscopy for the the investigation of self-assembly.In this thesis, the self-assembly of microbeads with tunable magnetic interactions is studied, based on the statistic analysis of microscope images and computer simulations. Magnetic and non-magnetic microbeads are suspended in a ferrofluid, which is a dispersion of magnetic nanoparticles in water. As a result, the magnetic properties of the microbeads in the ferrofluid are altered and can be described by effective magnetic susceptibilities and magnetic dipole moments, which can be tuned continuously. The liquid is confined between glass slides and effectively the microbeads are studied in a 2D geometry under a magnetic field, applied either in- or out-of-plane. The resulting structures are detected by image analysis algorithms, analyzed and correlated to the dipolar interaction between the beads, as well as to macroscopic quantities, like the particle density and ratio. For the in-plane field a phase transition from square to hexagonal lattice is observed. This phase transition is explained by the change in dipole interaction between the microbeads as the moments change from anti-parallel to parallel alignment. For the out-of-plane field the situation becomes diverse and more phases appear. It turns out that the phase formation in this case is strongly dependent on the bead ratio, density and interactions.We identify regions in the phase diagram, where isolated beads, percolated structures, and crystals dominate. To cover a wide parameter range the experiments are complemented by computer simulations. The tools developed in this thesis enable us to construct phase diagrams extracted from direct imaging and dependence on the extracted relevant parameters.
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| 2. |
- Melander, Emil, 1985-
(författare)
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Magnetoplasmonic nanostructures
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Surfaces that are nanopatterned, metallic, and magnetic can support surface plasmon resonances, providing an alternative and effective way to reconfigure flat optical components. Utilising a range of near- and far-field characterisation techniques, the optical and magneto-optical properties of lithographically patterned thin magnetic films are investigated.A magneto-optical diffractometer was designed, assembled, and commissioned to characterise periodic magneto-plasmonic nanostructures. For Ni and Co nanostructured antidot arrays, enhanced values of the magneto-optical Kerr rotation were recorded for energies and angles corresponding to excitations of surface plasmon polaritons. This enhancement was found to be thickness dependent. Modification of the optical properties via applied transverse magnetic fields and the excitation of surface plasmon polaritons, was demonstrated for an antidot array of pure Ni. The excitation was also shown to enhance the generation of second harmonics, as well as further activate nonlinear-optical mechanisms.In order to fully resolve and explain the source of this remarkable magneto-optical activity, near field probing techniques were used. This allows for mapping the electric near-field with a sub-wavelength resolution, thereby revealing the interplay between the light and the nanostructured lattice. The measurements show that the electric near field intensification, induced by plasmon excitation, increases the polarisation conversion, which correlates to the observed magneto-optical Kerr rotation.
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| 3. |
- Nouhi, Shirin, 1987-
(författare)
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Structure formation at solid/liquid interfaces : Understanding self-assembly and environmental challenges
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work described in the present dissertation has explored the structure of particles and molecules at solid/liquid interfaces, aiming to understand the physics of self-organizing systems and use this knowledge to address some environmental issues. Surface-sensitive neutron scattering techniques, such as reflectometry and grazing incidence small angle scattering, have been used as the primary tool to investigate structures in proximity to an interface. Some of the challenges in the interpretation of neutron scattering data are discussed, and new methods for analyzing the signal have been proposed.It was shown that charged stabilized colloidal particles can self-assemble and form large areas (20 cm2) of crystalline structures, close to a smooth solid surface extending to depths of several micrometers, while orienting themselves into smaller crystallites in the bulk of the suspension. The adsorption of proteins from the seeds of different species of Moringa trees on alumina, silica and polystyrene surfaces was studied, as a means for using proteins from different sources and with different properties, for the water clarification step in the purification process. The seed proteins also showed to enable locking the structure of colloidal particles at the solid/liquid interface, acting as a molecular glue.Perfluorinated surfactants (PFASs), widely used in industrial, pharmaceutical and food packing products, have been identified as emerging pollutants, raising a global concern for the environment and wildlife. The present study has shown how PFASs molecules of different fluorocarbon chain length and with different functional groups, create defects in model membranes by partitioning and removing phospholipids from the bilayer, making the bilayer thin and less dense.The effect of interface roughness was studied on the lamellar structure of a non-ionic surfactant. Concentrated solutions of the surfactant have been shown to form well-ordered and well-aligned structures at a smooth interface, which could be modified further by simply heating the sample. However it was found that even small roughness, of the same order as the bilayer thickness, can distort the structure to a depth of several micrometers from the interface. Heating the sample could improve the alignment but not as much as that formed at a smooth surface.
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