Effects of Phonon Confinement on Anomalous Thermalization, Energy Transfer, and Upconversion in Ln³⁺‐Doped Gd₂O₃ Nanotubes

Abstract There is a growing interest in understanding how size‐dependent quantum confinement affects the photoluminescence efficiency, excited‐state dynamics, energy‐transfer and thermalization phenomena in nanophosphors. For lanthanide (Ln³⁺)‐doped nanocrystals, despite the localized 4f states, confinement effects are induced mostly via electron–phonon interactions. In particular, the anomalous thermalization reported so far for a handful of Ln³⁺‐doped nanocrystals has been rationalized by the absence of low‐frequency phonon modes. This nanoconfinement may further impact on the Ln³⁺ luminescence dynamics, such as phonon‐assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd₂O₃:Eu³⁺ and Gd₂O₃:Yb³⁺,Er³⁺ nanotubes, exhibiting up to one order of magnitude larger than previously reported for similar materials, is reported. This anomalous thermalization induces unexpected energy transfer from Eu³⁺ C₂ to S₆ crystallographic sites, at 11 K, and ²H_11/2 → ⁴I_15/2 Er³⁺ upconversion emission; it is interpreted on the basis of the discretization of the phonon density of states, easily tuned by varying the annealing temperature (923–1123 K) in the synthesis procedure, and/or the Ln³⁺ concentration (0.16–6.60%).