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

本研究计画系关于在基质晶格中使用稀土离子进行光子上转换。上转换表示吸收两个或更多个光子并且随后发射一个光子的过程，该光子具有比先前吸收的每个光子的单个能量更多的能量。该研究项目的重点是上转换器纳米晶体。这些材料可用于许多应用，包括治疗诊断学、生物成像、光存储和太阳能的收集。然而，上转换器纳米晶体的上转换量子产率还很低，使得它们与不同的应用相关。研究项目的一个主要焦点是提高上转换器纳米晶体的上转换量子产率，以使其能够成功地集成在不同的应用领域中。将研究上转换纳米晶体的上转换量子产率的潜在限制。此外，还将研究能量传递过程的基本物理性质。该博士后研究项目可以分为三个部分，在下面介绍。1.合成所谓的核-壳上转换器纳米晶体。特殊的核-壳上转换器纳米晶体具有非常均匀和均匀的壳，其屏蔽光学活性核。应评估用于上转换量子产率的最高增强的钝化壳的最佳厚度。此外，显着的过程中减少非常小的纳米晶体以及薄壳层厚度的上转换量子产率将进行调查。2.结合第二发光材料研究上转换器纳米晶体。目的是由于第二发光材料而加宽上转换器的有效吸收范围。因此，上变频器可以在许多应用中更有效地使用，例如收集太阳能。此外，将检查上转换器纳米晶体和III-V族半导体量子点之间可能的能量转移过程。3.上转换器纳米晶体也研究了与等离子体和光子结构相结合。一方面，上转换量子产率的可能的增强检查。另一方面，改变局部光子态密度的影响的概率的福斯特能量转移的影响进行了研究。Förster能量转移构成了稀土离子中最有效的上转换过程。因此，它是最显著的上转换过程，并且对于上转换量子产率具有重大意义。

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.