Optical properties of (single) nanostructures in high magnetic fields

Contains fulltext : 65653_optiprofs.pdf (publisher's version ) (Open Access)In this thesis we discuss the optical properties of nanostructures, which are generally determined by the internal structure of the nanostructure. As such, nanostructures have a high potential for use in future applications. The relation between the internal structure and those optical properties is not completely understood. We employ high magnetic fields and use fluorescence microscopy and spectroscopy to investigate this relation. We present the results of a study on the internal order of single molecular fibres. Self-assembled stacks of tetra(p-phenylene\-vinylene)-based molecules (OPV) form micrometre long fibres on graphite by end-to-end alignment of the stacks upon transfer to the solid support. We employ polarised fluorescence microscopy to determine the level of internal order of single OPV fibres. A dipole model is used to explain the observed polarisation ratio ($R \sim 2$). We find that the dielectric properties of the solid support have a strong depolarising influence on the fluorescence of the fibres. This effect, combined with the depolarising effects of the experimental setup, accounts for the rather low polarisation ratio. Therefore, our results suggest that the OPV fibres have a high degree of internal order. Low temperature photoluminescence spectroscopy is used to investigate the exciton levels in cadmium-selenide/cadmium-sulfide core/shell nanocrystals. We employ both resonant and non-resonant photoluminescence spectroscopy on nanocrystal quantum dots and nanocrystal quantum rods to study the optical properties of the different exciton levels. The existing model was extended to include the effects of a magnetic field on the excitonic levels. We find that the photoluminescence properties of the lowest energy exciton level in both type of nanocrystals is determined by an intricate interplay of phonon-assisted and zero-phonon transitions. This competition can be influenced by a magnetic field or by the composition of the nanocrystal shell. This provides a basis for a new model.Radboud Universiteit Nijmegen, 17 februari 2009Promotores : Maan, J.C., Christianen, P.C.M.143 p