Despite considerable research, the location of an aliovalent dopant into SnO₂ nanoparticles is far to be clarified. The aim of the present study on trivalent lanthanide doped SnO₂ is to differentiate between substitutional versus interstitial and surface versus bulk doping, delineate the bulk and surface defects induced by doping and establish an intrinsic dopant distribution. We evidence for the first time a complex distribution of intrinsic nature composed of substitutional isolated, substitutional associates with defects as well as surface centers. Such multi-modal distribution is revealed for Eu and Sm, while Pr, Tb and Dy appear to be distributed mostly on the SnO₂ surface. Like the previously reported case of Eu, Sm displays a long-lived luminescence decaying in the hundreds of ms scale which is likely related to a selective interaction between the traps and the substitutional isolated center. Analyzing the time-gated luminescence, we conclude that the local lattice environment of the lattice Sn is not affected by the particle size, being remarkably similar in the ~2 and 20 nm particles. The photocatalytic measurements employed as a probe tool confirm the conclusions from the luminescence measurements concerning the nature of defects and the temperature induced migration of lanthanide dopants.

Pure and europium (Eu³⁺) doped cerium dioxide (CeO₂) nanocrystals have been synthesized by a novel oil-in-water microemulsion reaction method under soft conditions. In-situX-ray diffraction and RAMAN spectroscopy, high-resolution transmission electron microscopy, UV/Vis diffuse-reflectance and Fourier transform infrared spectroscopy as well as time-resolved photoluminescence spectroscopy were used to characterize the nanaocrystals. The as-synthesized powders are nanocrystalline and have a narrow size distribution centered on 3 nm and high surface area of ∼250 m² g⁻¹. Only a small fraction of the europium ions substitutes for the bulk, cubic Ce⁴⁺ sites in the europium-doped ceria nanocrystals. Upon calcination up to 1000 °C, a remarkable high surface area of ∼120 m² g⁻¹ is preserved whereas an enrichment of the surface Ce⁴⁺ relative to Ce³⁺ ions and relative strong europium emission with a lifetime of ∼1.8 ms and FWHM as narrow as 10 cm⁻¹ are measured. Under excitation into the UV and visible spectral range, the europium doped ceria nanocrystals display a variable emission spanning the orange-red wavelengths. The tunable emission is explained by the heterogeneous distribution of the europium dopants within the ceria nanocrystals coupled with the progressive diffusion of the europium ions from the surface to the inner ceria sites and the selective participation of the ceria host to the emission sensitization. Effects of the bulk-doping and impregnation with europium on the ceria host structure and optical properties are also discussed.

Pure and europium (Eu³⁺) doped ZrO₂ synthesized by an oil-in-water microemulsion reaction method were investigated by in situ and ex situ X-ray diffraction (XRD), ex situ Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), steady state and time-resolved photoluminescence (PL) spectroscopies. Based on the Raman spectra excited at three different wavelengths i.e. 488, 514 and 633 nm and measured in the spectral range of 150–4000 cm⁻¹ the correlation between the phonon spectra of ZrO₂ and luminescence of europium is clearly evidenced. The PL investigations span a variety of steady-state and time resolved measurements recorded either after direct excitation of the Eu³⁺ f–f transitions or indirect excitation into UV charge-transfer bands. After annealing at 500 °C, the overall Eu³⁺ emission is dominated by Eu³⁺ located in tetragonal symmetry lattice sites with a crystal-field splitting of the ⁵D₀–⁷F₁ emission of 20 cm⁻¹. Annealing of ZrO₂ at 1000 °C leads to a superposition of Eu³⁺ emissions from tetragonal and monoclinic lattice sites with monoclinic crystal-field splitting of 200 cm⁻¹ for the ⁵D₀–⁷F₁ transition. At all temperatures, a non-negligible amorphous/disordered content is also measured and determined to be of monoclinic nature. It was found that the evolutions with calcination temperature of the average PL lifetimes corresponding to europium emission in the tetragonal and monoclinic sites and the monoclinic phase content of the Eu³⁺ doped ZrO₂ samples follow a similar trend. By use of specific excitation conditions, the distribution of europium on the amorphous/disordered surface or ordered/crystalline sites can be identified and related to the phase content of zirconia. The role of zirconia host as a sensitizer for the europium PL is also discussed in both tetragonal and monoclinic phases.

In the recent years, there is an extensive effort concentrated towards the development of nanoparticles with near-infrared emission within the so called second or third biological windows induced by excitation outside 800–1000 nm range corresponding to the traditional Nd (800 nm) and Yb (980 nm) sensitizers. Here, we present a first report on the near-infrared (900–1700 nm) emission of significant member of cubic sesquioxides, Er-Lu ₂ O ₃ nanoparticles, measured under both near-infrared up-conversion and low energy X-ray excitations. The nanoparticle compositions are optimized by varying Er concentration and Li addition. It is found that, under ca. 1500 nm up-conversion excitation, the emission is almost monochromatic (>93%) and centered at 980 nm while over 80% of the X-ray induced emission is concentrated around 1500 nm. The mechanisms responsible for the up-conversion emission of Er - Lu ₂ O ₃ are identified by help of the up-conversion emission and excitation spectra as well as emission decays considering multiple excitation/emission transitions across visible to near-infrared ranges. Comparison between the emission properties of Er-Lu ₂ O ₃ and Er-Y ₂ O ₃ induced by optical and X-ray excitation is also presented. Our results suggest that the further optimized Er-doped cubic sesquioxides represent promising candidates for bioimaging and photovoltaic applications.

Although the emission of lanthanide (Ln) ions doped cubic sesquioxides has been subject of extensive studies, there are fundamental issues still to be elucidated. Specifically, compared to the well-understood emission of Ln at C₂ sites, the emission at the inversion S₆ (C_3i) sites, representing ¼ of the total sites, has been identified only for Pr, Nd, Eu, Sm and Yb. Here, we present a first report and improved identification of the emission, excitation and decay properties of Tb, Dy and Pr at S₆ sites in Y₂O₃ by use of time-gated luminescence spectroscopy. The green emission of Tb at S₆ sites is characterized by an intensity ratio relative to the cyan band at 490 nm of 10 compared to 2 measured for Tb at C₂ sites. Dy at S₆ sites displays a relatively intense, near-infrared emission at 765 nm which is red-shifted by ca. 200 nm relative to yellow peaked emission of Dy at C₂ sites. The emission lifetimes of 9.4 and 4.8 ms, associated with Tb and Dy at S₆ sites, exceed by a factor of 5 and 10 those of C₂ counterparts. It is also found that Dy may be regarded as a sensitive probe for the inversion symmetry, comparable with the more recognized Eu. The participation of Ln at S₆ sites to the up-conversion emission is revealed for the first time in Sm doped Y₂O₃ and Eu /Tb, Yb co-doped Y₂O₃ and explained in terms of successive ground state and excited state absorptions and cooperative energy transfer, respectively.