Morphology- and size-dependent spectroscopic properties of Eu3+-doped Gd2O3 colloidal nanocrystals
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The synthesis, morphological characterization, and optical properties of colloidal, Eu(III) doped Gd2O3 nanoparticles with different sizes and shapes are presented. Utilizing wet chemical techniques and various synthesis routes, we were able to obtain spherical, nanodisk, nanotripod, and nanotriangle-like morphology of Gd2O3:Eu3+ nanoparticles. Various concentrations of Eu3+ ions in the crystal matrix of the nanoparticles were tested in order to establish the levels at which the concentration quenching effect is negligible. Based on the luminescence spectra, luminescence lifetimes and optical parameters, which were calculated using the simplified Judd–Ofelt theory, correlations between the Gd2O3 nanoparticles morphology and Eu3+ ions luminescence were established, and allowed to predict the theoretical maximum quantum efficiency to reach from 61 to 98 %. We have also discussed the impact of the crystal structure of Gd2O3 nanoparticles, as well as coordinating environment of luminescent ions located at the surface, on the emission spectra. With the use of a tunable femtosecond laser system and the Z-scan measurement technique, the values of the effective two-photon absorption cross-section in the wavelength range from 550 to 1,200 nm were also calculated. The nonlinear optical measurements revealed maximum multi-photon absorption in the wavelength range from 600 to 750 nm.
KeywordsOxide nanoparticles Lanthanide luminescence Judd–Ofelt theory Z-scan technique Nonlinear optics Colloidal stability
The precise control of both morphology and spectroscopic properties of lanthanide-doped nanoparticles (NPs) is of great importance for their use in sensing and labeling applications. Among various factors pertinent to optimizing the properties, it is essential to consider the influence of NPs size and shape on spectroscopic properties of optically active ions embedded in the fabricated materials (Yan and Yan 2008; Zhang et al. 2009). With well-optimized synthesis protocols, it is possible to obtain NPs with narrow size and shape distributions, and further investigate their optical properties. Several reports showed the possibility of obtaining lanthanide-doped NPs with various morphologies such as spheres, plates, rods, nanotubes, nanowires, and even 3D flowers (Cao 2004; Gai et al. 2011; Paik et al. 2013; Paik and Murray 2013; Yang et al. 2008; Zheng et al. 2010). Additionally, the combination of unique optical features of lanthanide ions (Bünzli and Eliseeva 2011), i.e., long luminescence lifetimes, sharp excitation, and emission lines, with chosen crystal matrices and the nanometric size provides effective performance of the synthesized NPs. In particular, Eu3+ ions-doped oxide NPs have shown unique luminescent properties, owing to different possible narrowband luminescence excitation pathways. The Eu3+ ions have been excited by either direct 4f-4f electronic transitions in Eu3+ ions, through the host matrix absorption, Eu–O charge transfer, indirect Gd3+ or Ce3+ ions absorption, or even with the sensitization through the organic ligands attached to the NPs surface (Chen et al. 2013; Du et al. 2013; Grzyb et al. 2013; Kumar et al. 2013; Liu et al. 2006). As a result, the Eu3+ ions-doped oxide NPs showed intense, narrowband luminescence with excited state lifetimes up to 1 ms, suitable for applications as red emitting phosphors (Blasse 1994). Another interesting feature of Eu3+-doped oxide NPs is the possibility to excite the emission through multi-photon absorption processes. Although extensive literature regarding nonlinear optical (NLO) properties of Eu3+ organic complexes is available (Andraud and Maury 2009), the reports regarding NLO properties of inorganic lanthanide-doped NPs are scarce (Nyk et al. 2011; Wawrzynczyk et al. 2013b). The measured and calculated NLO parameters are usually limited to a single wavelength, thus the dispersions of these parameters are lacking, while wide-wavelength range measurements are essential for finding optimum wavelengths for multi-photon excitation of lanthanide luminescence. Additionally, since the 4f-4f photon transitions in lanthanides are two-photon allowed (Lakowicz et al. 2001; Sztucki and Strek 1986), the NLO spectroscopy can provide additional information regarding the electronic structure of the investigated materials.
Apart from being efficient luminescence centers, Eu3+ ions are also very good luminescent probes of local crystalline environment, which enable correlation between spectral and structural properties of lanthanide-doped hosts. It has been proved that the ratio of the intensities of 5D0 → 7F2 and 5D0 → 7F1 transitions in Eu3+ ions (asymmetry ratio, R) together with Judd–Ofelt intensity parameters (Ω2 and Ω4) can be considered indicative of the distortion in the symmetry of the local environment of the Eu3+ emitters (Boyer et al. 2004; Judd 1962; Kumar et al. 2013; Ofelt 1962). The intensity parameters Ω2 and Ω4 were successfully related with NaGdF4 and Gd2O3 NPs sizes (Bednarkiewicz et al. 2005; Liu et al. 2006), while Eu3+ ions luminescence was successfully used to probe the surface effects, originating from high surface to volume ratio in nanosized materials (Banski et al. 2013; Chang et al. 2007; Jia et al. 2010). Furthermore, Wiglusz et al. (Wiglusz et al. 2010) showed elegant correlation between Eu3+ luminescence decays and relative amount of Eu3+ ions located at different crystallographic sites of hydroxyapatites.
Here we report the synthesis and spectroscopic investigations of Gd2O3 NPs doped with Eu3+ ions. The obtained nanosized Gd2O3:Eu3+ NPs presented spherical, nanodisk, nanotripod, and nanotriangle-like morphologies. The various sizes and shapes of those NPs resulted from changes introduced in the thermal decomposition reaction, through modifying lanthanide precursors to capping ligands ratio, or by addition of Li+ ions. In order to perform detailed spectroscopic investigation, we have synthesized those NPs with various concentrations (0.5, 1, 2, 3, 4, and 5 mol%) of Eu3+ ions. The analysis of luminescence spectra, luminescence lifetimes, and Judd–Ofelt parameters revealed different concentration quenching for Eu3+ ions depending on the size and shape of Gd2O3 nanocrystals. We have also performed wide-wavelength range Z-scan measurements in order to calculate the values of effective two-photon absorption cross-section (σ2,eff) for representative Gd2O3 NPs of different shapes. The Z-scan experiments revealed multi-photon absorption in the wavelength range from 600 to 750 nm. Furthermore, the values of σ2,eff and molecular mass scaled parameters (σ2,eff/M) obtained for Gd2O3:Eu3+ NPs were compared with the values obtained for other types of inorganic NPs.
Materials and methods
Transmission Electron Microscopy (TEM) images and Selected Area Diffraction (SEAD) patterns were obtained by a FEI Tecnai G2 20 X-TWIN microscope. X-ray powder diffraction (XRD) measurements were recorded on a STOE diffractometer with Ge-filtered CuKα1 radiation. The excitation spectra were recorded with a Hitachi F-4500 spectrofluorimeter. The emission spectra and luminescence decay curves were obtained under excitation from the third harmonic (266 nm) of a Ti:Sapphire laser pumped by a Nd:YAG LOTIS TII laser (Belarus). The luminescence signals were detected by a Synapse CCD camera (HORIBA JOBIN–YVON). Fluorescence decay curves were recorded with a photomultiplier (HAMAMATSU R928) whose output was fed into a LeCroy Wave Surfer 425 digital oscilloscope. The NLO measurements were performed using a femtosecond laser system consisting of a Quantronix Integra-C Ti:Sapphire regenerative amplifier which produces ~130 fs, 800 nm pulses with 1 kHz repetition rate, and 1 mJ energy per pulse, pumping a Quantronix Palitra-FS optical parametric amplifier (OPA). Details of the experiment were described in our previous papers (Nyk et al. 2011, 2012). Samples for Z-scan measurements were prepared in 1-mm-thick sealed glass cuvettes. Scans of the investigated Gd2O3:Eu3+ NPs were preceded by scans of a 4.6 mm-thick silica glass plate used as a reference, and a cuvette with pure solvent, the latter being used in order to eliminate the contributions of the cuvette walls and the solvent itself to the signals (Samoc et al. 2003). The open-aperture (OA) Z-scan traces were fitted using expressions derived by Sheik-Bahae (Sheikbahae et al. 1990). This allowed us to calculate the values of effective two-photon absorption cross-section σ2,eff referring to individual Gd2O3:Eu3+ NPs.
Synthesis of Eu3+ doped Gd2O3 nanocrystals
All chemicals for synthesis of Gd2O3:Eu3+ NPs were purchased from Sigma Aldrich or POCH S.A. (Poland), and used without further purification. In order to study the dependence of NPs sizes and shapes on Eu3+ optical properties, we synthesized Gd2O3 NPs with spherical, nanodisk, nanotripod, and nanotriangle-like NPs morphology. For nanosphere- and nanodisk-shaped Gd2O3 NPs, we utilized the synthesis protocol reported by Cao (Cao 2004). In a typical synthesis procedure, gadolinium acetate hydrate (1 or 2 mM) and europium acetate hydrate (1 or 2 mM) were dissolved in a mixture of 1.7 ml oleylamine, 1 ml oleic acid, and 2.7 ml octadecene at ~100 °C. The solution was degassed and heated to temperature around 300 °C, and then cooled to room temperature after 1 h. The obtained Gd2O3 NPs were precipitated with a mixture of methanol and acetone (2:1 v/v), and washed several times with methanol. Finally, the NPs were dissolved in 4 ml of CHCl3. For nanotripod- and nanotriangle-shaped Gd2O3 NPs, we followed the report by Paik et al. (Paik et al. 2013). In a typical synthesis procedure, lithium hydroxide (6 mM) was dissolved and degassed in a mixture of 12 ml of oleic acid, 18 ml of oleylamine, and 30 ml of 1-octadecene at ~100 °C. Next, gadolinium acetate (3 mM) and europium acetate (3 mM) were added into the solution, followed by the evaporation at ~100 °C. The yellowish, transparent solution was then heated to ~300 °C under N2 flow. Identical amounts of the reaction solution were picked up with a syringe after 45 min, 1.5, and 3.5 h, cooled to room temperature, precipitated with a mixture of methanol and acetone (2:1 v/v), and washed several times with methanol. Finally, the NPs were dissolved in 4 ml of CHCl3. The intentional Eu3+ ions content in the crystal matrix in the prepared samples was as follows: Gd2O3:x %Eu3+ (x = 0.5, 1, 2, 3, 4, and 5).
Results and discussion
Morphology and crystal structure
Introducing Li+ ions into the reaction results in the formation of thin nanotripod- and nanotriangle-shaped Gd2O3:Eu3+ NPs. As it was previously shown by Paik et al. (2013), by prolonging the reaction time the NPs shape is transformed from nanotripods (Fig. 2a) to nanotriangles (Fig. 2c) throughout the transitional form (Fig. 2b). The obtained Gd2O3:Eu3+ NPs possessed much narrower size and shape distribution and, in comparison with previously obtained nanospheres and nanodiscs, were much bigger. The average diameter of Gd2O3:Eu3+ NPs was ~40 nm. Based on the SAED (insets in Fig. 2a–c) and XRD patterns (Fig. 2d), we could again assign the obtained NPs to the Gd2O3 cubic form, as expected for the case of bigger NPs, and the diffraction pattern rings were sharp and more intense. The broadening of the peaks at the XRD patterns results from the small thickness of obtained Gd2O3:Eu3+ NP, which did not exceed 2 nm. The influence of the Li+ ions on the size and shape of the obtained Gd2O3 NPs may be related to the direct doping of Li+ ions into the crystal matrix, causing lattice expansion (Paik et al. 2013). Extensive work concerning the influence of alkali ions (Li+, Na+, and K+) on the formation of fluoride NPs has been performed by (Xue et al. 2013) showing that, depending on the ionic radius of the added alkali ions, formation of various fluoride NPs morphologies may occur.
Photoluminescence properties and Judd–Ofelt parameters of Gd2O3:Eu3+ nanocrystals
In order to find Eu3+ ions doping level at which the concentration quenching is the weakest in the Gd2O3 matrix, as well as to investigate the influence of nanocrystals’ shape and crystal structure on Eu3+ emission, we have measured luminescence spectra and 5D0 excited state lifetimes for Gd2O3:Eu3+ NPs of different shapes, as a function of rising Eu3+ concentrations. In the case of nanospheres, the observed luminescence lines were considerably broadened in comparison with the larger Gd2O3:Eu3+ NPs (Fig. 3a). Those Gd2O3:Eu3+ NPs had the highest surface to volume ratio, and the largest number of Eu3+ ions was exposed to the interaction with organic ligands as well as solvent molecules (Liu et al. 2006; Wawrzynczyk et al. 2013a). The slight increase in Gd2O3:Eu3+ NPs’ size resulted in evident narrowing of the emission lines for the nanodiscs, nanotripods, and nanotriangles (Fig. 3b and Fig. 4). We have also observed roughly even intensity of the two peaks characteristic for the 5D0 → 7F2 electronic transition in Eu3+ ions. The Gd2O3 NPs are obtained either in the monoclinic or cubic crystal structure of nanopowders, with emission spectra maximum at longer wavelengths (~620 nm) for the monoclinic form, and blue-shifted (~610 nm) for the cubic one (Seo et al. 2009). The increased ratio of Eu3+ emission intensities at 620 nm to the peak at 610 nm for the investigated cubic Gd2O3 NPs may be rationalized by high surface to volume ratio. In such a case, a significant number of Eu3+ ions are located at NPs’ surface, and thus, Eu3+ display a lower coordination number and emission spectra similar to monoclinic polycrystalline powders, in which Eu3+ ions occupy lower symmetry. The same relationships were observed by (Si et al. 2005) for Eu2O3 NPs. Gd2O3:Eu3+ colloidal NPs emission spectra, with peaks at 610 and 620 nm having roughly even intensities, have been also reported by (Paik et al. 2013). The highest luminescence intensity was observed at 4 % Eu3+ doping level for nanospheres and nanodiscs (Fig. 3), at 5 % for nanotripods and at 4 % for nanotriangles (Fig. 4). However, these values were found somewhat susceptible to the concentration of the Gd2O3:Eu3+ NPs in the chloroform colloidal dispersion, sample experimental setup alignment, and fluctuation in the excitation light intensity.
The luminescence lifetime measurements provide more reliable information regarding optimum Eu3+ concentration in the host matrix. In general, longer 5D0 excited state luminescence lifetimes indicate lower quenching arising from surface defects, vibrational modes of ligands molecules, and impurities inside the crystal matrix. Apart from these external quenching mechanisms, additional concentration quenching may occur in heavily Eu3+ doped hosts, which originates from the distance dependent non-radiative cross relaxation between neighbor Eu3+ ions (Blasse 1994; Singh et al. 2008). For nanospheres and nanodiscs, the observed luminescence decays were double exponential, with the short lifetime component in the range from 271 ± 4 μs up to 761 ± 5 μs, and from 535 ± 6 μs up to 926 ± 2 μs in the case of nanospheres and nanodiscs, respectively (Fig. 6). The long luminescence lifetime components varied in the range from 1.18 ± 0.01 μs up to 1.60 ± 0.02 μs, and from 1.34 ± 0.01 μs up to 1.59 ± 0.02 μs in the case of nanospheres and nanodiscs, respectively (Fig. 6). The longest decay times, for both τ1 and τ2, were measured for 3 and 4 % of Eu3+ doping level concentration for nanospheres and nanodiscs, respectively. The relative maximum changes of the measured long lifetime components in relation to the Eu3+ level concentration in the matrix were found to be ~30 and ~20 % for nanospheres and nanodiscs, respectively.
The observed lower Eu3+ concentration quenching in nanotripods and nanotriangles could be attributed to the size of the Gd2O3 NPs. In smaller nanosized particles, the non-radiative quenching centers distribute randomly with a considerable fluctuation within the particle, and at the same time, the increased surface quenching effect in smaller particles effectively competes with cross-relaxation between Eu3+ ions (Li and Hong 2007). Additionally, in smaller NPs, the quenching traps distribute randomly, and resonance energy transfer between neighbor Eu3+ ions can occur only within one particle due to the hindrance by the particle surface. When increasing the concentration of luminescence centers, quenching occurs first in particles containing many traps, while those particles containing fewer traps quench only at high concentration (Jiang et al. 2003). Therefore, quenching occurs preferentially at higher Eu3+ concentration in smaller NPs. For gaining better insight into the spectroscopic properties of synthesized Gd2O3:Eu3+ NPs, we have also calculated and analyzed Judd–Ofelt intensity parameters (Judd 1962; Ofelt 1962).
Judd–Ofelt analysis of Eu3+ doped Gd2O3 NPs with different shapes: intensity parameters (Ω2 and Ω4), emission quantum efficiency (η), lifetimes (τ), non-radiative (Anrad), radiative (Arad), and total (Atotal) rates for observed transitions
Samples Gd2O3:x %Eu3+
Ω2 (10−20 cm2)
Ω4 (10−20 cm2)
0.5 % Eu3+
1 % Eu3+
2 % Eu3+
3 % Eu3+
4 % Eu3+
5 % Eu3+
0.5 % Eu3+
1 % Eu3+
2 % Eu3+
3 % Eu3+
4 % Eu3+
5 % Eu3+
0.5 % Eu3+
1 % Eu3+
2 % Eu3+
3 % Eu3+
4 % Eu3+
5 % Eu3+
Intermediate (Nanotripods → Nanotriangles)
0.5 % Eu3+
1 % Eu3+
2 % Eu3+
3 % Eu3+
4 % Eu3+
5 % Eu3+
0.5 % Eu3+
1 % Eu3+
2 % Eu3+
3 % Eu3+
4 % Eu3+
5 % Eu3+
The Ω2 intensity parameter reflects the hypersensitive behavior of the 5D0 → 7F2 transitions, and the observed relatively high Ω2 values, when compared to Eu3+ doped NaYF4 NPs (Bednarkiewicz et al. 2005) result from the presence of highly electronegative O2- ions in the Gd2O3 crystal structure. We have also observed slightly lower Ω2 values for nanosphere and nanodisc-like shaped Gd2O3:Eu3+ NPs in comparison with Gd2O3:Eu3+ NPs of other shapes. It may be due to the fact, that in smaller NPs, the lanthanide ion tends to increase its local symmetry (Malyukin et al. 2003). Additionally, in the synthesis of nanotripods, intermediate form, and nanotriangles, Li+ ions have been used, and since the ionic radius of this alkali metal is different from Gd3+ ions, the crystal lattice is distorted, and therefore, the symmetry around the Eu3+ ions can be disturbed. The observed higher Ω2 values for bigger Gd2O3:Eu3+ NPs can be thus indicative of efficient incorporation of Li+ ions inside the crystal structure of matrix. Additionally, the colloidal character of studied Gd2O3:Eu3+ NPs and influence of organic ligands attached to the NPs surface can influence the values of Ω2. Kumar et al. (Kumar et al. 2013) reported lower Ω2 values for nanosized Gd2O3:Eu3+ NPs; however, those nanophosphors did not have any organic molecules attached to the surface. In the case of Ω4 intensity parameter, no size or shape dependency was observed. Only for nanodiscs the values of Ω4 were slightly higher.
It is also very important to study the influence of both Gd2O3:Eu3+ NPs size and shape on the radiative contribution for the depopulation of the emitting level, and consequently for the emission quantum efficiency. Arad was higher for nanotripods and nanotriangles, when compared to the nanospheres, with simultaneous decrease of non-radiative contribution. This is why the relatively high η values, ranging from ~60 % for nanospheres up to ~90 % for nanotripods, were observed suggesting high efficiencies of energy transfer and efficient Eu3+ luminescence excitation in Gd2O3 matrix of those shapes.
Nonlinear optical properties of Gd2O3:Eu3+ nanocrystals
Comparison of NLO parameter σ2,eff and merit factor σ2,eff/M for different types of inorganic nonlinear absorbers
σ2,eff/M (GM mol/g)
Nano-disk-shaped Gd2O3:Eu3+ NPs
Nanotripod-shaped Gd2O3:Eu3+ NPs
Nanotriangle-shaped Gd2O3:Eu3+ NPs
NaYF4:2 %Er3+, 20 %Yb3+ NPs
Nyk et al. (2011)
Wawrzynczyk et al. (2013b)
CdSe quantum dots (4.2 nm)
Nyk et al. (2012)
CdS quantum dots (5 nm)
Szeremeta et al. (2013)
10 × 35 nm gold nanorods
Olesiak-Banska et al. (2012)
For the case of the studied in this work Gd2O3:Eu3+ NPs of different shapes the most significant values of σ2,eff were ~4,700, 7,600, ~26,700 GM for nanodiscs, nanotripods, and nanotriangles at 600, 700, and 700 nm, respectively. For the sake of the comparisons of multiple photon absorption merit, one needs to scale the values of σ2,eff with the Gd2O3:Eu3+ NPs size. The calculated merit factors σ2,eff/M (M being the molecular weight of a particle) were ~0.01, 0.03, 0.01 for nanodiscs, nanotripods, and nanotriangles, respectively. These values are therefore an order of magnitude lower than the corresponding ones for YVO4 (Wawrzynczyk et al. 2013b) and NaYF4 (Nyk et al. 2011) lanthanide-doped NPs, and two orders of magnitude lower than those for some two-photon absorbing chromophores (Schwich et al. 2011). However, the 5 nm large CdS quantum dots exhibited similar value of σ2,eff/M merit factor. In comparison with lanthanide-doped NPs, previously investigated in our group (Nyk et al. 2011; Wawrzynczyk et al. 2013b), as well as quantum dots and metallic NPs (Nyk et al. 2012; Olesiak-Banska et al. 2012; Szeremeta et al. 2013), the Gd2O3:Eu3+ NPs seem to be only moderately efficient multi-photon absorbing luminophores.
We have investigated the correlation between the size and shape of colloidal Gd2O3 NPs and the Eu3+ luminescence. In the case of lanthanide-doped inorganic NPs, the size and shape effect on spectroscopic features is more ambiguous, than for QDs or metallic NPs. Surface to volume ratio, type of surface attached ligands molecules, as well as deliberately introduced defects in the crystal structure of NPs can have great influence of their optical properties. For the purpose of that work, we have obtained Gd2O3:Eu3+ NPs shaped as nanospheres, nanodiscs, nanotripods, and nanotriangles. Narrow size and shape distribution of obtained Gd2O3:Eu3+ NPs allowed us to relate the observed changes in Eu3+ luminescence with morphology of NPs. The initially observed narrowing and splitting of 5D0 → 7FJ transitions for bigger ~40 nm NPs (nanotripods and nanotriangles) could be attributed to the changing homogeneous crystal fields, when compared to the smaller ~5 nm in size Gd2O3 NPs. In such small NPs, the crystal field changes significantly from grain to grain due to the higher dispersion of lattice constants. In addition, a higher number of Eu3+ ions are located close to the surface of the Gd2O3 particle. It is expected that those surface states influence the energies and splitting of 5D0 → 7FJ transitions as well. This effect is more important for a small diameter particle (~ 5 nm) than for a bigger one (~40 nm), due to the higher probability of the Eu3+ ions being close to the surface of Gd2O3 grains, if we consider only random distribution of atoms in the Gd2O3:Eu3+ nanomatrix. Additionally, a high number of Eu3+ ions located near or at the surface of the investigated colloidal nanosized Gd2O3 resulted in emission spectra with shapes similar to those of the monoclinic form of Gd2O3 NPs. Such ions demonstrate a lower coordinating number, and behave as if they were embedded in lower symmetry sites, causing noticeable changes in the shape of emission spectra. Based on emission spectra and kinetics, we have estimated the optimum Eu3+ ions doping level concentration in the host matrix. The higher optimum (3 and 4 %) europium (III) concentrations were found for smaller nanosphere and nanodiscs shaped Gd2O3:Eu3+ NPs in comparison with 0.5 and 1 % for nanotripods and nanotriangles. Those differences were also related to the size of the studied NPs. The spectroscopic properties of the synthesized Gd2O3:Eu3+ NPs were also evaluated in the frame of Judd–Ofelt theory. The intensity parameter Ω2 showed lower hypersensitive behavior of nanospheres and nanodiscs, and also indirectly proved the incorporation of Li+ ions inside the Gd2O3 crystal matrix. The hypersensitive character of Ω2 parameter resulted also from the colloidal character of synthesized Gd2O3:Eu3+ NPs, and interaction between Eu3+ ions situated close to the NP surface and capping ligands molecules. Additionally, both radiative rates and quantum efficiencies were higher for the nanotripods and the nanotriangles. Finally, we have investigated the NLO properties of differently shaped Gd2O3:Eu3+ NPs. The smallest, nanosphere shaped, Gd2O3:Eu3+ NPs did not show any measurable NLO absorption in the whole spectral range. However, we have observed NLO absorption in the bigger Gd2O3:Eu3+ NPs in the spectral range between 600 nm and 750 nm. The strength of the NLO absorption scaled with the Gd2O3:Eu3+ NPs size, and was the highest for the nanotripod-shaped particles. By calculating the σ2,eff/M merit factor, and comparison with other lanthanide-doped NPs, quantum dots, and metallic NPs, we have concluded that the investigated Gd2O3:Eu3+ NPs are from ten to a hundred times less efficient than efficient two-photon absorbing chromophores.
DW, MS, and MN acknowledge financial support from the National Science Centre under Grant No. DEC-2012/05/N/ST5/00601, by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology, as well as from the Foundation for Polish Science under START 2014 and WELCOME programmes. A.B. acknowledges financial support from the National Science Centre under Grant No. DEC-2012/05/E/ST5/03901.
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