Tunable upconversion of holmium sublattice through interfacial energy transfer for anti-counterfeiting.

Photon upconversion is a fascinating phenomenon that can convert low-energy photons to high-energy photons efficiently. However, most previous relevant research has been focused on upconversion systems with a sufficiently low lanthanide emitter concentration, such as 2 mol% for Er3+ in an Er-Yb coupled system. Realizing the upconversion from lanthanide heavily doped systems in particular, the emitter sublattice is still a challenge. Here, we report a mechanistic strategy to achieve the intense upconversion of the holmium sublattice in a core-shell-based nanostructure design through interfacial energy transfer channels. This design allowed a spatial separation of Ho3+ and sensitizers (e.g., Yb3+) into different regions and unwanted back energy transfers between them could then be minimized. By taking advantage of the dual roles of Yb3+ as both a migrator and energy trapper, a gradual color change from red to yellowish green was achievable upon 808 nm excitation, which could be further markedly enhanced by surface attaching indocyanine green dyes to facilitate the harvesting of the incident excitation energy. Moreover, emission colors could be tuned by applying non-steady state excitation. Such a fine-tunable color behavior holds great promise in anti-counterfeiting. Our results present a facile but effective conceptual model for the upconversion of the holmuim sublattice, which is helpful for the development of a new class of luminescent materials toward frontier applications.