Dramatic Impact\nof Materials Combinations on the Chemical\nOrganization of Core–Shell Nanocrystals: Boosting the Tm<sup>3+</sup> Emission above 1600 nm

This article represents the first foray into investigating\nthe\nconsequences of various material combinations on the short-wave infrared\n(SWIR, 1000–2000 nm) performance of Tm-based core–shell\nnanocrystals (NCs) above 1600 nm. In total, six different material\ncombinations involving two different types of SWIR-emitting core NCs\n(α-NaTmF<sub>4</sub> and LiTmF<sub>4</sub>) combined with three\ndifferent protecting shell materials (α-NaYF<sub>4</sub>, CaF<sub>2</sub>, and LiYF<sub>4</sub>) have been synthesized. All corresponding\nhomo- and heterostructured NCs have been meticulously characterized\nby powder X-ray diffraction and electron microscopy techniques. The\nlatter revealed that out of the six investigated combinations, only\none led to the formation of a true core–shell structure with\nwell-segregated core and shell domains. The direct correlation between\nthe downshifting performance and the spatial localization of Tm<sup>3+</sup> ions within the final homo- and heterostructured NCs is\nestablished. Interestingly, to achieve the best SWIR performance,\nthe formation of an abrupt interface is not a prerequisite, while\nthe existence of a pure (even thin) protective shell is vital. Remarkably,\nalthough all homo- and heterostructured NCs have been synthesized\nunder the exact same experimental conditions, Tm<sup>3+</sup> SWIR\nemission is either fully quenched or highly efficient depending on\nthe type of material combination. The most efficient combination (LiTmF<sub>4</sub>/LiYF<sub>4</sub>) achieved a high photoluminescence quantum\nyield of 39% for SWIR emission above 1600 nm (excitation power density\nin the range 0.5–3 W/cm<sup>2</sup>) despite significant intermixing.\nFrom now on, highly efficient SWIR-emitting probes with an emission\nabove 1600 nm are within reach to unlock the full potential of <i>in vivo</i> SWIR imaging.