Significantly Boosted Upconversion Emission in Cryogenic Er@Yb@Y Core–Shell–Shell Nanostructures

Abstract Recent advances reveal that due to the cross‐relaxation restriction, impressive upconversion (UC) enhancement (≈100‐folds) can be achieved in cryogenic Er³⁺‐rich core‐inert shell nanostructures (e.g., NaErF₄@NaYF₄), which opens up exciting opportunities in diverse frontier applications. However, further promotion of UC intensity is still highly desired, in which the rational design of nanostructures can play a key role. Herein, it is demonstrated that adopting an active shell design will constantly benefit the UC within a wide temperature range (40–300 K). Specifically, through constructing the luminescent core@active shell@inert shell sandwich nanostructure (e.g., NaErF₄@NaYbF₄@NaYF₄), 8.3–73‐folds UC enhancement will be achieved (taking the corresponding core@inert shell structures as competitors). Moreover, from spectral‐domain and time‐domain spectroscopic experiments, the relevant UC enhancement is convincingly attributed to a temperature‐dependent energy injection process (from the active shell to the luminescent core). More interestingly, the unique property of the material makes a temperature‐induced high‐level encryption application possible, which is obtained by employing the nanomaterials on a quick response (QR) code. These results not only deepen the UC mechanism in multi‐layer nanostructures, but also introduce an expanded dimension (via low temperatures) in information security.