This article represents the first foray into investigating
the
consequences of various material combinations on the short-wave infrared
(SWIR, 1000–2000 nm) performance of Tm-based core–shell
nanocrystals (NCs) above 1600 nm. In total, six different material
combinations involving two different types of SWIR-emitting core NCs
(α-NaTmF4 and LiTmF4) combined with three
different protecting shell materials (α-NaYF4, CaF2, and LiYF4) have been synthesized. All corresponding
homo- and heterostructured NCs have been meticulously characterized
by powder X-ray diffraction and electron microscopy techniques. The
latter revealed that out of the six investigated combinations, only
one led to the formation of a true core–shell structure with
well-segregated core and shell domains. The direct correlation between
the downshifting performance and the spatial localization of Tm3+ ions within the final homo- and heterostructured NCs is
established. Interestingly, to achieve the best SWIR performance,
the formation of an abrupt interface is not a prerequisite, while
the existence of a pure (even thin) protective shell is vital. Remarkably,
although all homo- and heterostructured NCs have been synthesized
under the exact same experimental conditions, Tm3+ SWIR
emission is either fully quenched or highly efficient depending on
the type of material combination. The most efficient combination (LiTmF4/LiYF4) achieved a high photoluminescence quantum
yield of 39% for SWIR emission above 1600 nm (excitation power density
in the range 0.5–3 W/cm2) despite significant intermixing.
From now on, highly efficient SWIR-emitting probes with an emission
above 1600 nm are within reach to unlock the full potential of in vivo SWIR imaging.