Untersuchungen an funktionalisierten ZnO-Nanodrahtstrukturen: Struktureller Aufbau, elektronische Kopplung und Ladungsträgerdynamik

Abstract

The knowledge of the electronic and optical coupling as well as electron transfer across the internal interfaces of hybrid nanostructures opens possibilities to specifically tailor their microscopic transport and luminescence processes by combining specific properties of different organic and inorganic material systems. In this thesis, two different hybrid systems are studied, ZnO nanowire/CdSe quantum dot structures and nanostructures coated with p-type polymers. First, CdSe quantum dots (QDs) with different organic linker molecules are attached to ZnO nanowires (NWs) to study luminescence dynamics and electron tunneling from the QDs to the nanowires in time-resolved photoluminescence (PL) and photoconductivity measurements. After linking the QDs to the ZnO NW surface, photo-induced electron tunneling from an excited state of the QD into the conduction band of the nanowire becomes visible by a clear decrease of the PL decay time of the QDs. By comparing the PL transients of QDs in solution with those of QDs linked to ZnO NWs, the photo-induced electron transfer (PET) process between excited states of the QD and the nanowire is demonstrated and discussed in the frame of a rate equation model. Efficient electron tunneling is confirmed by a strong enhancement of the photocurrent through the functionalized nanowires. The tunneling rate can be controlled by using different organic linker molecules. Surface functionalization of ZnO nanostructures by QD systems with different QD sizes and surface modifications will lead to hybrid solar cells with high absorption over a wide spectral range, and a high energy conversion efficiency. Secondly, the coating of ZnO nanowires and GaN microrods with p-conductive polymers (polypyrrole, poly(3,4-ethylenedioxythiophene)) is analyzed by scanning electron microscopy, energy dispersive X-ray spectroscopy and photoluminescence spectroscopy. For the fabrication of hybrid ZnO/polymer and GaN/polymer core-shell nanostructures, oxidative chemical vapor deposition (oCVD) is used. oCVD, compared to wet-chemical processes, is a completely solventless, dry process where both, the oxidizing agent and the monomer are provided in the gaseous phase. The thickness and homogeneity of the polymer coating depend on the amount of the oxidizing agent (here FeCl3), the substrate and the substrate temperature. With oCVD deposition a 15nm thin and homogeneous polypyrrole layer on the ZnO nanowire surface is demonstrated, whereas, the primary optical properties of ZnO are not affected. A controlled deposition of the polymer shell with a thickness control in the nanometer range is required to tailor the electronic and optical properties. This offers a huge potential for the realization of efficient light-emitting devices