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 2H11/2 → 4I15/2 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%).

Photoluminescent properties of ZrO2: Tm3+, Tb3+, Eu3+ powders—A combined experimental and theoretical study

“…The Energy transfer (ET2) between Tb 3+ and Eu 3+ has been extensively studied, in order to understand the photoluminescent behavior [78]. In fact, the luminescence intensities of various rare-earth ions can be enhanced or quenched by the energy transfer from other co-doped rare-earth ions [79][80][81]. ET2 between Tb 3+ and Eu 3+ may occur in hosts, such as tungstates, zeolite-Y, yttria, porous silicon, borate, hydrate, and molybdates [82][83][84][85].…”

Triple excitation with dual emission in paramagnetic ZnO:Er3+ nanocrystals

“…ZnO is a unique multifunctional semiconductor due to its direct band gap (3.37 eV) that can host 4f levels of RE ions [7][8][9] to exhibit excellent optical properties, 5,6 anisotropic structure and spontaneous polarization that render ZnO properties such as lasing, piezoelectric and ferroelectric behaviour 10,11 and also dilute magnetism with appropriate doping. 12,13 Among all RE ions, Er 3+ ion is most suitable for upconversion of IR photons with wavelength 1 mm and longer due to its level spacing of ground 4 I 15/2 level and lowest excitable 4 I 13/2 and 4 I 11/2 levels.…”

Lanthanide upconversion luminescence at the nanoscale: fundamentals and optical properties

“…This decrease has a significant impact on all processes where the probability of interaction with a phonon is involved, such as multiphonon recombination and phonon-assisted energy transfer whose probabilities decrease with nanoparticle size. As a result of the reduced DOS with nanoparticle size, the relevant rates should decrease as well [79][80][81] . Moreover, in nanocrystals, the phonons being confined no longer have well-defined wave vectors, leading to the relaxation of the selection rules.…”