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

晶体化合物NaYF 4代表了目前已知的用于Yb 3 +/Er 3+和Yb 3 +/Tm 3+掺杂的上转换材料的最佳基质晶格。上转换材料在近红外光谱区域（NIR区域）中激发时发射可见光。虽然上转换发光是非线性光学过程，因为在可见光范围内发射一个光子需要吸收至少两个NIR光子，但是相对低的光强度足以有效地激发发射。原因是NIR光子可以被顺序地而不是同时地吸收，因为上转换机制仅涉及掺杂剂离子的亚稳态能级。在过去的几年中，已经开发了允许制备具有良好定义的晶体形态和非常窄的粒度分布的Yb 3 +/Er 3+和Yb 3 +/Tm 3+掺杂的NaYF 4上转换材料的胶体纳米晶体的合成程序。类似于半导体纳米晶体（量子点），也已经开发了将未掺杂的NaYF 4的薄壳存款到上转换颗粒的表面上的方法。该未掺杂的壳显著减少了激发能量从掺杂的颗粒核到核-壳颗粒的表面的转移，因为能量转移经由相邻的掺杂剂离子发生。这种核-壳颗粒显示出比没有壳的颗粒强得多的上转换发射，因为壳显著减少了颗粒表面上的能量损失过程。然而，与其他掺杂的纳米颗粒或量子点相比，核-壳颗粒的发光仍然弱于相应的块体材料的发光。尽管上转换核-壳颗粒的电子显微镜图像证实可以制备具有非常均匀厚度的壳，但是上转换发射的效率仍然比本体上转换磷光体的效率低几乎一个数量级。因此，我们提出了一个合作研究项目，涉及一个研究小组正在专门合成的上转换粒子和第二个研究小组有很强的背景，在确定绝对发光量子产率和时间依赖性发光测量领域。在这个研究项目中，我们的目标是通过研究粒子合成中几个不同参数的影响来解开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，其缺乏降低当今纳米晶体的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.