Tuning Upconversion through Ho3+/Nd3+ Co-sensitized Cascade Energy Transfer

Lanthanide-doped upconversion nanocrystals (UCNCs) have attracted significant interest, driven by their emerging applications in photonics and anticounterfeiting. However, conventional UCNCs are excited by 808, 980, or 1540 nm, and there remains a gap within the 808-980 nm region. Here, we present a conceptual design for an energy-migratory Nd sublattice in Ho-sensitized multilayer core-shell UCNCs, enabling UC luminescence from a broad range of lanthanide ions under 897 nm excitation. This excitation wavelength is enabled by resonant transitions within Ho3+ and Nd3+, serving as an intermediate "buffer" layer to bridge the energy mismatch between Ho3+5F5 and Yb3+2F5/2 levels. Transient absorption measurements provide direct evidence for the energy transfer pathways at the Ho3+/Nd3+ interface. Moreover, optimizing the interlayer thickness enhances cascaded energy transfer and UC emission under 897 nm excitation. This work expands the available excitation wavelength range of UCNCs and opens new opportunities for information security.