Optical properties of organic and semiconductor nanostructures

Abstract

Nanostructures have at least one of their dimensions in the range 1-100 nm. Fabricating new, well-defined nanoscale objects and studying their physical properties is of importance, because it can lead to the development of potentially useful materials, novel device applications, and the discovery of new physical phenomena. This thesis contains a comprehensive optical study of one-dimensional self-assembled ring- and wire-shaped nanostructures and two-dimensional semiconductor quantum well (QW) structures. The self-assembled nanorings and nanowires are fabricated by dropcasting a solution of porphyrin dodecamer molecules and monofunctional oligo(p-phenylene vinylene) (MOPV4) molecules, respectively, on a surface and allowing the solvent to evaporate. Using atomic force microscopy we show that we achieve a high level of control over the production process. Moreover, polarized fluorescence microscopy images on individual nanorings and nanowires show a pronounced emission and absorption. Using a general model that calculates the polarized response of a stack of molecules, we have determined the orientation of the molecules within the structures and we show that the internal moleculair order is very high. The optical properties of an 8 nm CdTe/CdMgTe QW are dominated by excitons (an electron bound to a hole) and trions (two electrons bound to a hole) at cryogenic temperatures. The formation of trions out of excitons and electrons is found to be determined by a dynamical equilibrium. This dynamical equilibrium consists of a chemical equilibrium, relating the trion, exciton, and electron populations, modified by finite formation and recombination times of (charged) excitons, as is evidenced by a magnetic-field dependent photoluminescence and far infrared study. Incorporation of the trion formation scheme into a rate equation model gives a proper description of the experimental data, leading to values of the formation, recombination and spin-flip times of trions and excitons that are in good agreement with results of time-resolved experiments in the literature.