NaYF4:Yb,Er-upconversion nanoparticles: Systematic enhancement of the luminescence efficiency by unraveling the processes of energy loss

晶体化合物NaYF4代表了目前已知的Yb3+/Er3+和Yb3+/Tm3+掺杂上转换材料的最佳主体晶格。上转换材料在近红外光谱区（NIR区）激发后发射可见光。虽然上转换发光是一个非线性光学过程，因为在可见光范围内发射一个光子需要吸收至少两个近红外光子，但相对较低的光强度足以有效地激发发射。原因是近红外光子可以被顺序吸收而不是同时吸收，因为上转换机制只涉及掺杂离子的亚稳能级。在过去的几年中，已经开发了合成方法，可以制备Yb3+/Er3+和Yb3+/Tm3+掺杂NaYF4上转换材料的胶体纳米晶体，具有明确的晶体形态和非常窄的粒径分布。与半导体纳米晶体（量子点）类似，也开发了将未掺杂的NaYF4薄壳沉积到上转换粒子表面的方法。由于能量转移是通过相邻的掺杂离子进行的，因此这种未掺杂的壳层显著减少了激发能从掺杂粒子核向核-壳粒子表面的转移。这种核壳粒子比无壳粒子表现出更强的上转换发射，因为壳显著减少了粒子表面的能量损失过程。然而，与其他掺杂纳米粒子或量子点相比，核壳粒子的发光仍然弱于相应的块状材料。虽然上转换核壳粒子的电子显微镜图像证实可以制备厚度非常均匀的壳，但上转换发射效率仍然比本体上转换荧光粉的效率低近一个数量级。因此，我们提出了一个合作研究项目，其中一个研究小组专门研究上转换粒子的合成，另一个研究小组在确定绝对发光量子产率和时间相关发光测量领域具有强大的背景。在这个研究项目中，我们的目标是通过研究几个不同的参数对粒子合成的影响来揭示上转换纳米晶体效率低的原因。基于这些结果，我们将尝试制备改进的上转换颗粒，这些上转换颗粒缺乏降低当今纳米晶体上转换效率的发光猝灭机制。

The crystalline compound NaYF4 represents the best host lattice known today for Yb3+/Er3+ and Yb3+/Tm3+ doped upconversion materials. Upconversion materials emit visible light upon excitation in the near-infrared spectral region (NIR region). Although upconversion luminescence is a non-linear optical process as the emission of one photon in the visible range requires the absorption of at least two NIR photons, comparatively low light intensities are sufficient to efficiently excite the emission. The reason is that the NIR photons can be absorbed sequentially rather than simultaneously because the upconversion mechanism involves only metastable energy levels of the dopant ions. During the last years synthesis procedures have been developed allowing to prepare colloidal nanocrystals of Yb3+/Er3+ and Yb3+/Tm3+ doped NaYF4 upconversion materials with well defined crystal morphologies and very narrow particle size distributions. Similar to semiconductor nanocrystals (quantum dots), also methods have been developed to deposit a thin shell of undoped NaYF4 onto the surface of upconversion particles. This undoped shell significantly reduces the transfer of excitation energy from the doped particle core to the surface of the core-shell particles since the energy transfer takes place via adjacent dopant ions. Such core-shell particles display much stronger upconversion emission than particles without shell, since the shell significantly reduces energy loss processes on the particle surface. In contrast to other doped nanoparticles or quantum dots, however, the luminescence of the core-shell particles is still weaker than that of the corresponding bulk material. Although electron microscopy images of upconversion core-shell particles verify that shells with very uniform thickness can be prepared, the efficiency of the upconversion emission is still almost an order of magnitude lower than the efficiency of bulk upconversion phosphors. We therefore propose a collaborative research project involving one research group being specialized on the synthesis of upconversion particles and a second research group having a strong background in the determination of absolute luminescence quantum yields and the field of time-dependent luminescence measurements. Within this research project, we are aiming to unravel the reason for the low efficiency of the upconversion nanocrystals by investigating the influence of several different parameters in the particle synthesis. Based on these results, we will try to prepare improved upconversion particles which lack the luminescence quenching mechanisms that reduce the upconversion efficiency of todays nanocrystals.