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

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 (Ln 3+ )‐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 Ln 3+ ‐doped nanocrystals has been rationalized by the absence of low‐frequency phonon modes. This nanoconfinement may further impact on the Ln 3+ luminescence dynamics, such as phonon‐assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd 2 O 3 :Eu 3+ and Gd 2 O 3 :Yb 3+ ,Er 3+ 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 Eu 3+ C 2 to S 6 crystallographic sites, at 11 K, and 2 H 11/2 → 4 I 15/2 Er 3+ 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 Ln 3+ concentration (0.16–6.60%).