The UCL enhancement due to the reduction in vibrational relaxation is corroborated by the lengthening of luminescence decay lifetimes upon Sn2S6 4-exchange. When directly excited with 980 nm pulses, Yb 3+ /Tm 3+ -doped UCNPs (Figure ) exhibited a significant increase in the lifetime of the Yb 3+ 1010 nm emission, from 34 µs to 105 µs, which is also longer than the emission lifetime of oleate-capped UCNPs (16 µs). A significant but smaller increase in the lifetime is also observed for the Tm 3+ 800 nm emission (980 nm excitation) upon Sn2S6 4-exchange (154 µs) compared with that of ligand-stripped UCNPs (110 µs) and oleate-capped UCNPs (111 µs). This increase in lifetime is also observed for the 540 nm emission from Sn2S6 4--capped Yb 3+ /Er 3+ UCNPs (Figure ), which had an intensity-weighted lifetime of 197 µs, compared to a lifetime of 25 µs for ligand-stripped UCNPs and 44 µs for oleate-capped UCNPs (see Supplementary Information for details on lifetime measurements and averaging).