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

本研究项目涉及在主晶格中使用稀土离子的光子上转换。上转换是指吸收两个或两个以上的光子，随后发射的一个光子的能量比先前吸收的每个光子的单个能量多的过程。该研究项目的重点是上变频纳米晶体。这些材料在包括治疗学、生物成像、光存储和太阳能收集在内的许多应用中都很有用。然而，上转换纳米晶体的上转换量子产率还很低，不足以使其适用于不同的应用。研究项目的一个主要重点是提高上转换纳米晶体的上转换量子产率，使其能够成功地集成在不同的应用领域。上转换纳米晶体上转换量子产率的潜在限制将被研究。必须找到消除这些限制的办法。此外，能量传递过程的基本物理性质将被检查。拟建博士后研究项目可分为以下三部分进行介绍。1. 合成了所谓的核壳上变换器纳米晶体。特殊的核壳上变频纳米晶体具有非常均匀和均匀的外壳，可以屏蔽光学有源核心。应评估钝化壳的最佳厚度，以最大程度地提高上转换量子产率。此外，还将研究降低非常小的纳米晶体和薄壳厚度的上转换量子产率的重要过程。2. 研究了上变换器纳米晶体与第二种发光材料的结合。目的是为了扩大上变频器的有效吸收范围，由于第二发光材料。因此，上变频器可以在许多应用中更有效地使用，例如太阳能的收集。此外，上转换器纳米晶体和III-V半导体量子点之间可能的能量传递过程将被检查。3. 并结合等离子体和光子结构对上变换器纳米晶体进行了研究。一方面，研究了提高上转换量子产率的可能性。另一方面，研究了局域光子态密度变化对Förster能量转移概率的影响。Förster能量转移是稀土离子中最有效的上转换过程。因此，它是最重要的上转换过程，对上转换量子产率具有重要意义。

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.