Photorefractive Hybrid Materials Based on Inorganic Nano-Crystals Embedded in an Amorphous Organic Host
Final Report Abstract
The goal of the project is to manufacture inorganic nanoparticies and to add them to functional optical polymers as a dopant. Such hybrid materials promise to combine the positive properties of both materials - inorganic and organic. Aiming for reversible photorefractive materials, we selected from both material classes the most prominent candidates: Inorganic lithium niobate crystals (LiNbOs) and organic PVK-TNF polymers. The project has been conducted by two teams in close collaboration. The research group of Karsten Buse at the University of Bonn, active in the field of LiNbOs crystals, and the research group of Klaus Meerhotz at the University of Cologne, active in the field of photorefractive polymers. It turned out that the experiments as well as their understanding were more challenging than originally expected. Thus it took us also longer than we thought until first publications came out that report about hybrid materials consisting of LiNbOs as a dopant of PVK-TNF polymers. However, the results are now very clear and have relevance beyond the original scope of the applications. Three publications in internationally known journals are direct outcomes of this collaborative research effort. Several others are affected and influenced by this project. The main insights can be summarized as follows: Optical spectroscopy shows that 20-nm-diameter LiNbOs nanocrystals have still the same dielectric properties as bulk crystals, i.e., a wide band-gap with an absorption that is strongly influenced by transition-metal dopants. Putting these crystals into PVK-TNF has - even at very low nanoparticle concentrations - a very strong influence onto the photoconductivity; non-steady-state photocurrent investigations showed a drastic increase of the hole conductivity induced by the nanoparticle doping. Photorefractive studies showed also a strong increase of the photorefractive performance of the polymers - small amounts of the LiNbOs nanocrystals can replace a pretty large amount of the photosensitizer TNF. - All results are excellently explained considering the energy levels (in particular of the conduction band) of the LiNbOs nanocrystals, which allow them to trap electrons from the LUMO ("Lowest Unoccupied Molecular Orbital") levels of the photosensitizer. By this means the density and the mean-free-path length of holes is increased, being beneficial for the hole conductivity.
Publications
- "Fabrication and optical characterization of stable suspensions of iron- or copper-doped lithium niobate nanocrystals in heptane" Applied Physics B 89, 15-17 (2007)
J. R. Schwesyg, H. A. Eggert, K. Buse, E. Sliwinska, S. Khalil, M. Kaiser, K. Meerholz
- "Effect of co-sensitization in new hybrid photo-refractive materials based on PVK polymer matrix and inorganic LiNbO3 nanocrystals" Applied Physics B 95, 519-524 (2009)
E. Sliwinska, S. Mansurova; U. Hartwig, K. Buse, K. Meerholz
- "Enhancement of charge carrier transport by doping PVK-based photoconductive polymers with LiNbO3 nanocrystals" Phys. Rev. B 79, 174208 - 1-7 (2009)
S. Mansurova, K. Meerholz, E. Sliwinska, U. Hartwig, K. Buse
