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

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.