Nanoengineering of up- and down converting materials to enhance the quantum yield and investigating the effect of the density of photon states on the probability for Förster energy transfer

• Applicant: Dr. Stefan Fischer
• Subject Area: Experimental Condensed Matter Physics ; Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2014 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 263646147

This research project is concerned with upconversion of photons using rare-earth ions in host lattices. Upconversion denotes the process of the absorption of two or more photons and a subsequent emission of one photon with more energy than the single energy of each of the previously absorbed ones. The research project focuses on upconverter nanocrystals. These materials are useful for many applications including theranostics, biological imaging, optical storage, and harvesting of the solar energy. However, the upconversion quantum yield of upconverter nanocrystals is yet to low to make them relevant for the different applications.One main focus of the research project is the enhancement of the upconversion quantum yield of upconverter nanocrystals to enable a successful integration in the different application areas. The underlying limitations for the upconversion quantum yield of the upconverter nanocrystals will be investigated. Solutions to eliminate these limitations shall be found. Furthermore, fundamental physical properties of energy transfer processes will be examined. The proposed postdoctoral research project can be organized in three parts that are introduced in the following. 1. So-called core-shell upconverter nanocrystals are synthesized. The special core-shell upconverter nanocrystals feature a very homogeneous and uniform shell that shields the optical active core. The optimal thickness of the passivating shell for the highest enhancement of the upconversion quantum yield shall be evaluated. Furthermore, the significant processes reducing the upconversion quantum yield for very small nanocrystals as well as thin shell thickness will be investigated. 2. The upconverter nanocrystals are investigated in combination with a second luminescent material. The objective is to widen the effective absorption range of the upconverter due to the second luminescent material. Thus, upconverters can be used much more efficiently in many applications, such as harvesting of the solar energy. Furthermore, possible energy transfer processes between the upconverter nanocrystals and III-V semiconductor quantum dots shall be examined. 3. The upconverter nanocrystals are also investigated in combination with plasmonic and photonic structures. On the one hand, a possible enhancement of the upconversion quantum yield is examined. On the other hand, the effect of an altered local density of photon states on the probability of Förster energy transfer is investigated. Förster energy transfer constitutes the most efficient upconversion process in rare-earth ions. Consequently, it is the most significant upconversion process and of major importance for the upconversion quantum yield.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Dr. A. Paul Alivisatos