Lanthanide-doped upconversion (UC) phosphors absorb low-energy infrared light and convert it into higher-energy visible light. Despite over 10 years of development, it has not been possible to synthesize nanocrystals (NCs) with UC efficiencies on a par with what can be achieved in bulk materials. To guide the design and realization of more efficient UC NCs, a better understanding is necessary of the loss pathways competing with UC. Here we study the excited-state dynamics of the workhorse UC material β-NaYF4 co-doped with Yb3+ and Er3+. For each of the energy levels involved in infrared-to-visible UC, we measure and model the competition between spontaneous emission, energy transfer between lanthanide ions, and other decay processes. An important quenching pathway is energy transfer to high-energy vibrations of solvent and/or ligand molecules surrounding the NCs, as evidenced by the effect of energy resonances between electronic transitions of the lanthanide ions and vibrations of the solvent molecules. We present a microscopic quantitative model for the quenching dynamics in UC NCs. It takes into account cross-relaxation at high lanthanide-doping concentration as well as Förster resonance energy transfer from lanthanide excited states to vibrational modes of molecules surrounding the UC NCs. Our model thereby provides insight in the inert-shell thickness required to prevent solvent quenching in NCs. Overall, the strongest contribution to reduced UC efficiencies in core-shell NCs comes from quenching of the near-infrared energy levels (Er3+: 4I11/2 and Yb3+: 2F5/2), which is likely due to vibrational coupling to OH- defects incorporated in the NCs during synthesis.

中文翻译：

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NaYF4：Er3 +，Yb3 +上转换纳米晶体的猝灭途径。

掺杂镧系元素的上转换（UC）磷光体吸收低能红外光并将其转换为高能可见光。尽管有十多年的发展，但尚无法合成具有UC效率的纳米晶体（NC），而纳米晶体（NCs）的合成能力却可以与散装材料中实现的纳米晶体媲美。为了指导更高效的UC NC的设计和实现，必须更好地了解与UC竞争的损失途径。在这里，我们研究了掺有Yb3 +和Er3 +的主力UC材料β-NaYF4的激发态动力学。对于从红外到可见光的UC涉及的每个能级，我们测量并建模自发发射，镧系离子之间的能量转移以及其他衰减过程之间的竞争。一个重要的淬灭途径是能量转移到NC周围的溶剂和/或配体分子的高能振动，这通过镧系元素离子的电子跃迁与溶剂分子的振动之间的能量共振效应得以证明。我们提出了在UC NCs淬火动力学的微观定量模型。它考虑到了在高镧系元素掺杂浓度下的交叉弛豫以及从镧系元素激发态到UC NCs周围分子振动模式的Förster共振能量转移。因此，我们的模型提供了防止NC中溶剂淬灭所需的惰性壳厚度的见解。总体而言，对核-壳型NC降低UC效率的最大贡献来自对近红外能级（Er3 +：4I11 / 2和Yb3 +：2F5 / 2）的淬灭，