Optical upconversion from lanthanide-doped nanoparticles is promising for a variety of applications ranging from bioimaging, optogenetics, nanothermometry, super-resolution nanoscopy and volumetric displays to solar cells. Despite remarkable progress made in enhancing upconversion to fuel these applications, achieving luminescence of upconversion nanoparticles (UCNPs) that is comparable to or higher than the bulk counterparts has been challenging due to nanoscale-induced quenching effects. Here we demonstrate a size-dependent lanthanide energy transfer effect in a conceptual design of hexagonal sodium yttrium fluoride (NaYF₄) core–shell–shell NaYF₄@NaYF₄:Yb/Tm@NaYF₄ UCNPs with depleted surface quenching. We show that precise control over the domain size (or the thickness of the middle shell doped with ytterbium (Yb) and thulium (Tm) from 1.2 to 13 nm) increases the lanthanide energy transfer efficiency (from 30.2 to 50.4%) and amplifies the upconversion quantum yield to a high value of 13.0 ± 1.3% in sub-50 nm UCNPs (excitation: 980 nm, 100 W cm⁻²), which is around fourfold higher than the micrometre-scale hexagonal NaYF₄:Yb/Tm bulk counterparts. Spectroscopic studies and theoretical microscopic modelling reveal that long-range lanthanide energy transfer (>9.5 nm) takes place and underlies the observed size-dependent phenomena. Demonstration of size-dependent lanthanide energy transfer and upconversion quantum yields at the nanoscale transforms our long-existing conceptual understanding of lanthanide energy transfer (size independence), thereby having important implications for applications of lanthanide nanophotonics and biophotonics.

镧系元素掺杂纳米粒子的光学上转换在生物成像、光遗传学、纳米测温、超分辨率纳米显微镜和体积显示器到太阳能电池等多种应用中具有广阔的前景。尽管在增强上转换以推动这些应用方面取得了显着进展，但由于纳米级诱导的猝灭效应，实现与本体对应物相当或更高的上转换纳米粒子（UCNP）的发光一直具有挑战性。在这里，我们在具有耗尽表面淬火的六方氟化钇钠（NaYF ₄）核-壳-壳NaYF ₄ @NaYF ₄ :Yb/Tm@NaYF ₄ UCNPs的概念设计中展示了尺寸依赖的镧系元素能量转移效应。我们表明，精确控制域尺寸（或掺杂镱（Yb）和铥（Tm）的中间壳的厚度从1.2到13 nm）可以提高镧系元素能量转移效率（从30.2％到50.4％）并放大在低于 50 nm 的 UCNP（激发：980 nm，100 W cm ^-2 ）中，上转换量子产率高达 13.0 ± 1.3% ，比微米级六方 NaYF ₄ :Yb/Tm 块体对应物高出约四倍。光谱研究和理论微观模型揭示了长程镧系元素能量转移（> 9.5 nm）的发生，并且是观察到的尺寸相关现象的基础。在纳米尺度上证明尺寸依赖性的镧系元素能量转移和上转换量子产率改变了我们长期以来对镧系元素能量转移（尺寸独立性）的概念理解，从而对镧系元素纳米光子学和生物光子学的应用具有重要意义。