Lattice‐Engineered Lanthanide Nanocrystals with Tunable Near‐Infrared‐IIb Lifetime

Lattice engineering in lanthanide nanocrystals (LnNCs) is fundamental for tailoring their versatile optical properties, yet it remains underexplored in manipulating the downconversion characteristics. Here, we engineer the lattices from distorted to strain‐graded in LnNCs by CaF 2 deposition on vacancy‐controlled β‐Na x GdF 3+x :Yb,A (A = Tm/Er, x = 0.5, 0.75, 1) NCs. We demonstrate that strain‐graded lattices with high phonon energy enhance the multi‐phonon orbit‐lattice relaxation (MPR) in the downconversion process more effectively than distorted lattices, which helps to extend the near‐infrared‐IIb (NIR‐IIb, 1500–1700 nm) lifetime of Er 3+ /Tm 3+ by 2∼4 times. Subsequent epitaxial CaF 2 growth further attenuates surface phonon coupling, enabling exponential tunability of NIR‐IIb lifetime (Tm 3+ : 0.74∼6.28 ms; Er 3+ : 0.055∼9.2 ms). Finally, we showcase the potential of these LnNCs in time‐resolved multiplexed bioimaging.