Upconversion nanocrystals have been extensively investigated for optical imaging and biomedical applications1,2. However, their photoluminescence is strongly attenuated by surface quenching as the nanocrystal size diminishes3. Despite considerable efforts4,5, the quenching mechanism remains poorly understood. Here we report that surface coordination of bidentate picolinic acid molecules to NaGdF4:Yb/Tm nanoparticles enhances four-photon upconversion by 11,000-fold. Mechanistic studies indicate that surface ligand coordination reconstructs orbital hybridization and crystal-field splitting, minimizing the energy difference between the 4f orbitals of surface and inner lanthanide sensitizers. The 4f-orbital energy resonance facilitates energy migration within the ytterbium sublattice, impeding energy diffusion to surface defects and ultimately enhancing energy transfer to the emitters. Moreover, ligand coordination can exert energy-level reconstruction with a ligand–sensitizer separation of over 2 nm. These findings offer insights into the development of highly emissive nanohybrids and provide a platform for constructing optical interrogation systems at single-particle levels.
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All relevant data that support the findings of this work are available from the corresponding author on reasonable request.
First-principles calculations were performed using the commercially available Vienna Ab initio Simulation Package. The codes used to calculate the distance between lanthanide ions and ligands are provided in the Supplementary Information.
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X.L. acknowledges support from the NUS NANONASH Programme (NUHSRO/2020/002/NanoNash/LOA; R143000B43114), the Singapore Ministry of Education (MOE2017-T2-2-110) and the Agency for Science, Technology and Research (A*STAR) (grant no. A1883c0011). H.X. acknowledges support from the National Natural Science Foundation of China (NSFC) (grant no. 92061205) and Young Innovative Team Supporting Projects of Heilongjiang Province.
The authors declare no competing interests.
Peer review information Nature Photonics thanks Marco Bettinelli, Dayong Jin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Xu, H., Han, S., Deng, R. et al. Anomalous upconversion amplification induced by surface reconstruction in lanthanide sublattices. Nat. Photon. 15, 732–737 (2021). https://doi.org/10.1038/s41566-021-00862-3