Effects of Phonon Confinement on Anomalous Thermalization, Energy Transfer, and Upconversion in Ln 3R -Doped Gd 2 O 3 Nanotubes By Andreia G. Macedo, Rute A. S. Ferreira, Duarte Ananias, Ma ´rio S. Reis, Vitor S. Amaral, Luı ´s D. Carlos,* and Joa ˜o Rocha 1. Introduction Lanthanide-containing compounds have been extensively utilized as high-performance phosphors in various devices based on the electronic, optical, and chemical properties arising from 4f electrons. [1–4] Ln 3þ -hydroxide and oxide (Ln ¼ Ce, Pr, Nd, Sm, Dy, Tb, Gd, Yb, Er) nanocrystals, Y 2 O 3 :Eu 3þ /Y 2 O 3 core–shell composites, Y 2 O 3 :Eu 3þ and Gd 2 O 3 :Eu 3þ nanotubes, and Gd 2 O 3 :Yb 3þ ,Er 3þ nanorods have received considerable attention [5–14] because they exhibit shape-specific and quantum-size effects. [14–30] Size-dependent quantum confinement has important effects on the radiative and nonradiative electronic transitions in nano- particles, and tuning the particles structure at the nanometer scale may result in novel optical properties for applications in 3D displays, [31–32] light-emitting devices, [33] and bioassays. [34,35] Due to the confinement effect in Ln 3þ -nanocrystals, the phonon density of states (PDOS) becomes discrete, with a cut-off on the low-energy side. [27–30] Because the low-frequency phonons con- tribute most effectively to the nonradiative relaxation between closely spaced Stark levels, the lack of these low-frequency modes considerably modifies the Ln 3þ emission dynamics, relative to that of bulk materials. Up to now, anomalous thermalization due to the absence of low-frequency phonon modes (nanoconfinement effects) has been reported only for a few Ln 3þ -nanocrystals. [14,23,26–30] In 1998 Tissue et al. noticed the presence of broad 7 F 1 ! 5 D 1 hot bands in the excitation spectra of 4–6 nm Eu 2 O 3 particles at 12 K. A large increase in the intensity of these hot bands, relative to the normal excitation lines, was measured for particle sizes from 12 nm down to 4 nm. [23] Later, Liu et al. reported hot bands of the 4 I 15/2 ! 4 F 7/2 transition (with energies up to 200 cm 1 higher than the energy of the zero-phonon line) in the excitation spectra of Y 2 O 2 S:Er 3þ nanocrystals (20– 40 nm) below 7 K. [27,28] The direct phonon relaxation between the Stark levels of the 4 I 15/2 ground term is restricted due to the lack of low-energy modes, and consequently, the intensity of the hot bands originating from the upper 4 I 15/2 crystal field levels increases suddenly as temperature decreases below 7 K. [27–29] This inter- pretation was extended to the above-mentioned anomalous hot bands in Eu 2 O 3 particles. [30] Recently, Mercier et al. [26] and Liu et al. [14] provided experimental evidence of anomalous thermaliza- tion in Gd 2 O 3 :Eu 3þ nanoparticles and Gd 2 O 3 :Eu 3þ nanotubes, respectively. Hot bands originating from the lowest 7 F 1 Stark level, with an energy of 217 cm 1 , were observed in the excitation spectra below 50 K. Both groups interpreted this anomalous FULL PAPER www.afm-journal.de www.MaterialsViews.com [*] Prof. L. D. Carlos, A. G. Macedo, Dr. R. A. S. Ferreira, Dr. M. S. Reis, Prof. V. S. Amaral Department of Physics and CICECO Universidade de Aveiro 3810-193 Aveiro (Portugal) E-mail: lcarlos@ua.pt Dr. D. Ananias, Prof. J. Rocha Department of Chemistry and CICECO Universidade de Aveiro 3810-193 Aveiro (Portugal) DOI: 10.1002/adfm.200901772 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 3R )-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 3R -doped nanocrystals has been rationalized by the absence of low- frequency phonon modes. This nanoconfinement may further impact on the Ln 3R luminescence dynamics, such as phonon-assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd 2 O 3 :Eu 3R and Gd 2 O 3 :Yb 3R ,Er 3R 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 3R C 2 to S 6 crystallographic sites, at 11 K, and 2 H 11/2 ! 4 I 15/2 Er 3R 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 3R concentration (0.16–6.60%). 624 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2010, 20, 624–634