Photoluminescence properties of Eu³⁺ ions in yttrium oxide nanoparticles: defect vs. normal sites

Abstract

The position of activator ions in the lattice has a fundamental effect on the luminescent properties of phosphors. In this paper, we describe the normal and defect sites of Eu³⁺ ions in a Y₂O₃ crystal lattice, their interaction and difference in physical properties. Analytically detectable amounts of defect sites reached in Y₂O₃:Eu³⁺ nanopowders with an average size of 40–50 nm were synthesized by a novel combined Pechini-foaming method. The luminescence properties of Eu³⁺ ions in defect sites are most pronounced in highly doped nanocrystalline powders (32 and 40 at%). To explain this phenomenon we suggest the mechanism of irreversible energy transfer from normal to defect sites of Eu³⁺ ions.

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1. Introduction

In recent years, materials doped with rare-earth ions (REI) have been widely used as high-performance luminescent devices, magnets, and catalysts due to their unique electronic, optical, and chemical characteristics.^1–5 Among the rare-earth compounds, yttrium oxides (Y₂O₃) have been widely used in a broad range of fields as one of the multifunctional materials. Perfect thermal stability, chemical durability, and low phonon energy are the main advantages of Y₂O₃ when used as a host matrix for phosphors.^6–8 The luminescence properties of the materials largely depend on their size, shape, dimensions, and structure.⁹ The development of nano- or micromaterials with controlled chemical–physical parameters provides opportunities in exploring new phenomena.¹⁰ Therefore, the design and controlled synthesis of inorganic materials with desired sizes and shapes have been attracting attention for many years.^11–16 Nowadays, various methods for the synthesis of rare-earth oxide doped phosphors have been developed, including the solution combustion,¹⁷ sol–gel,¹⁸ spray pyrolysis,¹⁹ hydrothermal synthesis,²⁰ and precipitation methods.²¹

Pechini technique²² (variation of sol–gel method) is an inexpensive and convenient way to prepare REI doped phosphors; but it results in formation of strongly agglomerated nanocrystalline powders.^23–25 Optimization of the synthesis process allowed us to modify the procedure and improve properties of the obtained powders. It was found that adding a foaming agent leads to the intense gas release during calcination procedure. This process prevents agglomeration and sintering of the derived particles.

Due to wide application of Y₂O₃:Eu³⁺ particles, there are a lot of papers devoted to the study of this material. In the last years manuscripts concerning comparison of different synthesis methods,²⁶ annealing temperature and doping concentration effect^27,28 and influence of cold compaction²⁹ were published. However, to the best of our knowledge, there is a lack of papers dealing with wide concentration series of Eu-doped yttria nanoparticles and highly doped samples.

The present paper is focused mainly on the study of structural and luminescence properties of nanocrystalline Y₂O₃:Eu³⁺ powders synthesized by the combined Pechini-foaming method which is first reported here. The detailed analysis of REI doping concentration effect on synthesized samples was presented. The most interesting point and one of the novel outcomes of this research is that Eu³⁺ ions could occupy not only two well-known symmetry sites in cubic Y₂O₃ host C₂ (noncentrosymmetric) and C_3i (centrosymmetric) but also defect sites. Photoluminescence properties of ions at various sites were studied at different excitation wavelengths.

2. Experimental

Nanocrystalline Y₂O₃ doped with Eu³⁺ ions was prepared by the combined Pechini-foaming method. Previously we synthesized complex oxides doped with rare earth ions using modified Pechini method.^30–32 This method reduces agglomeration but requires second calcination procedure in molten salt (potassium chloride).

The proposed combined technique is also an improved standard Pechini method but has the only one calcination stage. The main part of modification is implementation of foaming agent into polymer gel matrix in order to prevent strong agglomeration of synthesized samples. The mixture of aluminium nitrate and potassium chloride was chosen as a foaming agent.

Y(NO₃)₃ and Eu(NO₃)₃ prepared by dissolving of Y₂O₃ and Eu₂O₃ in nitric acid were used as initial salts for the synthesis process. Then one of foaming agents – aluminium nitrate – was added (molar ratio Al(NO₃)₃ : Y(NO₃)₃ = 5 : 3). To the abovementioned solution were added at first saturated solution of citric acid in distilled water (volume ratio 1 : 1), and then the potassium chloride powder; the resultant mixture was heated up to 150–200 °C. The reaction between citric acid and metal nitrates leads to the chelate complexes formation. The acid excess reacts with the salt, which leads to the potassium dihydrogen citrate formation; oxidized, it forms potassium carbonate. The aforementioned chemical reactions are shown in eqn (1).

The solution is transformed into polymer gel after the etherification reaction (adding the ethylene glycol in the solution with the ethylene glycol to citric acid solution ratio of 1 : 4). Aluminium, yttrium and europium ions are uniformly distributed in polymer network while polymer cells contain potassium carbonate. The gel was calcinated at 1000 °C for 2 hours to remove organic components. Uniformly distributed aluminium oxide reacts with potassium carbonate, which results in formation of KAlO₂ accompanied by intense gas release (eqn (2)).

Al₂O₃ + K₂CO₃ → 2KAlO₂ + CO₂↑ (T > 600 °C) (2)

As a result, Y₂O₃ nanoparticles with homogeneously distributed Eu³⁺ ions were obtained, reaction byproducts were removed by washing in distilled water. The synthesized particles were collected by centrifugation at 2800 rpm for 5 min.; repeated 3 times. The resulting washed precipitate was dried naturally or in an oven at 110 °C.

Y³⁺ ions was gradually substituted by Eu³⁺, to give wide range of Eu doping concentration varying from 2 to 40 at%. The synthesized powder samples (5 mg) and potassium bromide (300 mg) were pressed into pellets (diameter 13 mm) for luminescence studies.

X-ray diffraction patterns were registered with the powder diffractometer UltimaIV (Rigaku) in Bragg–Brentano geometry with CuKα1 radiation (λ = 1.54059 Å) in the 2θ range from 10° to 80°. Phase identification was carried out using a powder diffraction database PowderDiffractionFile (PDF-2, 2011). The unit cell parameters were estimated using TOPAS software by full-profile analysis. Electron micrograph images were obtained with SUPRA 40VP WDS scanning electron microscope (resolution 4 nm). Raman spectra were measured with Bruker SENTERRA Raman Microscope with semiconductor laser 488 nm as an excitation source. Luminescent spectra were recorded with a fluorescence spectrometer Fluorolog-3 equipped with a Xe-arc lamp (450 W power). For the lifetime measurements the europium luminescence was excited with a Xe-flash lamp (150 W power, 3 μs pulse width). All the measurements were performed at the room temperature.

3. Results and discussion

3.1 Structural analysis

Fig. 1a shows the XRD patterns of synthesized concentration series of Y₂O₃:Eu³⁺ nanophosphors (doping concentration C(Eu³⁺) = 2–40 at%). Almost all the peaks in diffraction patterns matched well to cubic phase of Y₂O₃ (JCPDS 41-1105). Low intensity peak at 2θ ≈ 19° is assigned to the formation of bayerite phase (Al(OH)₃) which was not fully removed after synthesis procedure. The broad XRD peaks suggest the formation of nano-sized particles.

Fig. 1 (a) XRD patterns of Y₂O₃:Eu³⁺ concentration series and the standard card Y₂O₃ and Al(OH)₃; (b) the dependence of the unit cell volume on the Eu³⁺-doping.

The unit cell parameters were calculated with TOPAS software using full-profile analysis. It was found that both, unit cell parameter and single-crystal cell volume, systematically increase with growth of dopant concentration. This fact can be explained by substitution of yttrium ions (r = 0.89 Å) with larger europium ions (r = 0.95 Å).³³ It should be noted that the cell volume has a linear relationship with the content of Eu³⁺, which is consistent with Vegard's law (Fig. 1b). This result demonstrates that the europium ion has been efficiently and homogeneously incorporated into the host matrix due to the similar ionic radius and chemical reactivity of Eu³⁺ and Y³⁺.^34–36 So, the obtained results testify that the most of doping Eu³⁺ ions occupy yttrium sites in crystal lattice.

SEM image of Y₂O₃:Eu³⁺ 2 at% phosphor is presented in Fig. 2. As one can see, nanocrystalline powder consists of rather small spherical nanoparticles. Average size of these nanoparticles is about 40–50 nm with standard deviation of σ ≈ 6 nm.

Fig. 2 SEM image of Y₂O₃:Eu³⁺ 2 at% nanopowder.

Fig. 3 shows Raman spectra of Y₂O₃:Eu³⁺ powders with various doping concentration normalized to the most intense band in each of them. These spectra were measured in the spectral region of 80–700 cm⁻¹.

Fig. 3 Raman spectra of Y₂O₃:Eu³⁺ concentration series.

According to the factor group theory investigation,³⁷ twenty two Raman modes have been predicted for the cubic Y₂O₃ lattice belonging to the Ia3 space group with C type structure:

Γ^RS = 4A_g + 4E_g + 14F_g (3)

The major peak assigned as A_g + F_g mode is observed at about 370 cm⁻¹.³⁸ The high intensity of this band compared to the others indicates a large polarizability variation during the vibration.³⁹ The low-intensity line at about 590 cm⁻¹ could be assigned to F_g mode. Lines centered at the spectral region 450–550 cm⁻¹ are quite rarely observed in the Raman spectrum of Eu³⁺-doped Y₂O₃ samples. One of these lines has been previously reported in Y₂O₃:Eu³⁺ nanoparticles⁴⁰ and was attributed to the Eu³⁺ doping effect without any further explanation. It should be noted that these bands do not correspond to any of the first-order Raman-active modes of cubic or monoclinic Y₂O₃ and Eu₂O₃ clusters.^41–44 In order to determine the origin of these bands Raman spectra were measured using solid state laser 532 nm as an excitation source. It turned out that no line was observed in the spectral range 450–550 cm⁻¹. Therefore, these bands could not be Raman lines and most likely correspond to the luminescence of Eu³⁺ ions located at positions with C_3i symmetry sites. Energy levels of Eu³⁺ ions that occupy C_3i site are higher than that for C₂ site. So, the energy transfer to the C₂ site occurs, and the transfer efficiency increases with increasing of Eu³⁺ concentration.^45,46 Thus, the proposed mechanism also explains the observed intensity decrease with increasing of doping concentration.

It should be noted that the spectra do not show the expected number of bands. Some bands are quite broad, and some of them are likely overlapped, at least partially. Probably, nanoscale dimension of samples lead to the inhomogeneous strains and to the band broadening.⁴⁷ Furthermore, some Raman bands can overlap with luminescence lines. As can be seen from Fig. 3, doping concentration also affects the position of Raman lines. Red shift of peaks was observed along with the increase of Eu³⁺ ions level (inset of Fig. 3). This shift can be explained by changes in the effective mass and effective potential.^48,49 Substitution to an element with larger mass and larger electrons number leads to shift toward a lower wavenumber. It was found that 40% substitution of yttrium (89 Da (dalton), 36 electrons) ions for europium (152 Da, 60 electrons) ions results in 13 cm⁻¹ shift of the most intense vibration wavenumber.

3.2 Luminescent properties

Photoluminescence spectrum of Y₂O₃:Eu³⁺ 12 at% nanopowder is presented in Fig. 4a. The measurement was performed in the spectral range 500–750 nm upon 265 nm. Observed spectrum consists of characteristic narrow lines corresponding to the intra-configurational f–f transitions. It is well known that the luminescence intensity of the transitions between different j-levels depends on the site symmetry of Eu³⁺. Eu³⁺ ions could be located at two symmetry sites in cubic Y₂O₃ host: C₂ (noncentrosymmetric) and C_3i (centrosymmetric). In general, electric dipole transitions are forbidden for centrosymmetric site because the energy levels have the same parity. In this case, only weak magnetic dipole transitions occur. Therefore, for the Eu³⁺ located at C_3i site, only the magnetic dipole transitions can be observed. For noncentrosymmetric sites, the crystal-field introduces an opposite-parity part to the potential energy of the crystal levels, which makes the electric dipole transitions possible.^50,51 So, for the case of Eu³⁺ ions at C₂ site, the electric dipole transitions are also allowed.⁴⁶

Fig. 4 (a) Emission spectrum of Y₂O₃:Eu³⁺ 12 at% nanophosphor; (b) concentration dependence of ⁵D₀–⁷F₂ intensity.

Emission spectrum is dominated by the forced electric dipole transition ⁵D₀–⁷F₂ with maximum at 610.2 nm. Maximum of magnetic dipole transition ⁵D₀–⁷F₁ slightly differs for different symmetry sites. Very weak spectral line centered at 581.2 nm was observed at low-doped samples. This line can be assigned to the Eu³⁺ ions located at C_3i site. Other three lines centered at 586.2, 592 and 598.6 nm correspond to the Eu³⁺ ions located at C₂ site. Transitions ⁵D₀–⁷F₀ (579.2 nm), ⁵D₀–⁷F₃ (649.6, 656.6, 661.4 nm) and ⁵D₀–⁷F₄ (686.4, 692.4, 706, 707.6, 711.2 nm) are also observed in emission spectrum. Moreover, the spectrum contains a low-intensity transition from a higher excited level ⁵D₁: ⁵D₁–⁷F₁ (532.2 and 537 nm).

Concentration dependence of the integrated intensity of the most strong transition ⁵D₀–⁷F₂ (610.2 nm) is presented in Fig. 4b. The optimal doping concentration is determined by two competitive effects: on the one hand, an increase of the doping concentration means an increase of luminescence centers number and thus radiative recombination. On the other hand, there is also an increase of the cross-relaxation probability which enhances the efficiency of the nonradiative processes.²⁹ In the Fig. 4b we observe that increase of Eu³⁺ concentration leads to the increase of the luminescence intensity only up to 12 at%. Further increase of doping concentration results in the intensity reduction. This fact indicates the concentration of quenching effects. It has been reported in earlier studies that there are possibilities of pairing and aggregation of dopant ions at higher doping concentrations where fraction of activators can act as quenchers in yttria lattice.⁵² It was also shown that optimum doping concentration depends on crystallite size, surface area, synthesis method etc. Dependence of quenching concentration on particle size was studied by Zhang et al.⁵³ It was found that quenching concentration is 6 mol%, 13 mol% and 18 mol% for 3000 nm, 40 nm and 5 nm size particles respectively. In our research optimum concentration was determined to be 12 at% which coincide with optimum concentration for nanophosphors with similar particles dimensions prepared by colloidal precipitation method.⁵⁴

Excitation spectrum of nanocrystalline phosphor Y₂O₃:Eu³⁺ 12 at% for electric–dipole transition ⁵D₀–⁷F₂ monitored at λ_em = 610.2 nm is shown in Fig. 5. The spectrum consists of intensive broad band and several sharp weak lines in the longer-wavelength region. The observed broad band can be assigned to the charge transfer (CT) between Eu³⁺ and O²⁻, an electron transfers from O²⁻ (2p⁶) orbital to the empty orbital of 4f⁶ for Eu³⁺.^55,56 Other weak lines are attributed to the typical intra-configurational f–f transitions of the Eu³⁺ ion: ⁷F₀–⁵L₈ (322 nm), ⁷F₀–⁵D₄ (362.5 nm), ⁷F₀–⁵L₇ (381.5 nm), ⁷F₀–⁵L₆ (393.5 nm), ⁷F₀–⁵D₃ (415 nm), ⁷F₀–⁵D₂ (465 nm), ⁷F₀–⁵D₄ (362.5 nm), ⁷F₀–⁵L₇ (381.5 nm), ⁷F₀–⁵L₆ (393.5 nm), ⁷F₀–⁵D₃ (415 nm), ⁷F₀–⁵D₂ (465 nm), ⁷F₀–⁵D₁ (532.5 nm) and ⁷F₁–⁵D₀ (587 nm).

Fig. 5 Excitation spectrum of Y₂O₃:Eu³⁺ 12 at% nanophosphor (λ_em = 610.2 nm).

In order to study effect of the excitation wavelength upon the luminescence properties, emission spectra of Y₂O₃:Eu³⁺ concentration series were measured. 4 various pumping wavelengths were used: (1) λ_ex = 265 nm (CT); (2) λ_ex = 322 nm (Eu³⁺, ⁷F₀–⁵L₈); (3) λ_ex = 393.5 nm (Eu³⁺, ⁷F₀–⁵L₆); (4) λ_ex = 465 nm (Eu³⁺, ⁷F₀–⁵D₂). Spectra normalized to the intensity of 610.2 nm peak are presented in Fig. 6. Normalized emission spectra do not depend on Eu³⁺ content upon 265 and 465 nm. However, the increase of Eu³⁺ doping concentration leads to the significant changes when excited at 322 and 393.5 nm. Shoulder of the forced electric dipole transition ⁵D₀–⁷F₂ is appeared in the longer-wavelength region. Also, intensity of the very weak line centered at 699 nm rise dramatically. The observed changes could be assigned to the luminescence of Eu³⁺ ions in a third site completely different to the well-known C₂ or C_3i sites.

Fig. 6 Emission spectra of Y₂O₃:Eu³⁺ concentration series upon (a) λ_ex = 265 nm, (b) λ_ex = 322 nm, (c) λ_ex = 465 nm and (d) λ_ex = 393.5 nm.

Excitation spectra of Y₂O₃:Eu³⁺ 40 at% nanophosphors for lines centered at 711 and 699 nm (forced electric dipole transition ⁵D₀–⁷F₄) are presented in Fig. 7. Monitored emission bands are assigned to C₂ and unknown site of Eu³⁺ ions, respectively. Shown excitation spectra were normalized in respect to the intensity of 393.5 nm line. As it can be seen, the positions of the observed excitation bands do not depend on luminescence center site. However, the excitation efficiency of luminescence is different. So, we can draw a conclusion that Eu³⁺ ions situated at unknown sites are mostly excited through Eu³⁺ ions located at C₂ and C_3i sites.

Fig. 7 Excitation spectra of Eu³⁺ ions situated at different sites in Y₂O₃:Eu³⁺ 40 at% nanophosphor (λ_em1 = 699 nm; λ_em2 = 711 nm).

3.3 Kinetics of luminescence

In addition to the steady-state measurements, important information about luminescence properties of synthesized nanophosphors could be obtained from fluorescence kinetics. Fig. 8 shows emission spectra of Y₂O₃:Eu³⁺ 2, 12 and 40 at% nanopowders and wavelength of spectral lines where lifetimes were measured. All these lines are assigned to the transitions from metastable excited level ⁵D₀ to the lower levels.

Fig. 8 Emission spectra and position of spectral lines for lifetime measurements of Eu³⁺-doped Y₂O₃ phosphors (λ_ex = 393.5 nm).

Lifetimes monitored at different spectral lines are listed in the Table 1. Analysing measured lifetimes in Y₂O₃:Eu³⁺ 2 at% and Y₂O₃:Eu³⁺ 12 at% we can see that there are two very different observed lifetimes of ⁵D₀ level for the same sample. It is well known that the occupation of different symmetry sites leads to the distinction of lifetimes. So, the presence of two different lifetimes could be explained by various positions of Eu³⁺ ions in crystal lattice. Measured lifetimes are similar for all emission lines except 699 nm. This suggests that the luminescence bands are attributed to Eu³⁺ ions occupying C₂ sites in crystal lattice. 699 nm emission peak has different lifetime and hence it can be assigned to the Eu³⁺ ions situated at defect sites.

Table 1 Lifetime of ⁵D₀ level measured at different transitions for Eu³⁺-doped Y₂O₃ phosphors

Wavelength (nm)	Lifetime (ms)
	2 at%	12 at%	40 at%
592	1.29 ± 0.03	0.90 ± 0.02	0.22 ± 0.01
610	1.34 ± 0.04	0.91 ± 0.02	0.15 ± 0.01
630	1.26 ± 0.03	0.99 ± 0.02	0.14 ± 0.01
699	0.36 ± 0.01	0.32 ± 0.01	0.23 ± 0.01
708	1.22 ± 0.03	0.90 ± 0.02	—

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Photoluminescence decays of Y₂O₃:Eu³⁺ 2, 12 and 40 at% nanopowders are presented in Fig. 9a. Doping concentration effect on the ⁵D₀ level lifetime of Eu³⁺ ions occupying both normal and defect positions in crystal lattice was studied to understand further the luminescence properties (Fig. 9b). Emission was excited using 393.5 nm pumping in both cases. Fluorescence decays of Eu³⁺ ions at C₂ sites were measured for the electric dipole transition ⁵D₀–⁷F₂ (610.2 nm). Experimental curves of Y₂O₃ samples doped up to 16 at% were fitted by single exponential function. However, higher doped samples show non mono-exponential decay. Correct fits required a double-exponential model to be used. The average lifetime was calculated by a double exponential fit.⁵⁷

where A₁ and A₂ are pre-exponential factors; τ₁ and τ₂ are lifetimes. The change of fitting model can be explained by the following reason. An increasing concentration of the dopant forces allocation of Eu³⁺ ions in surface sites. So the non-exponential decay can occur due to the different decays of the Eu³⁺ ions at the surface and Eu³⁺ ions in the volume of the nanoparticles.^58–60

Fig. 9 (a) Experimental data and single exponential fit of Eu³⁺-doped Y₂O₃ phosphors (λ_ex = 393.5 nm); (b) ⁵D₀ level lifetime of Eu³⁺ ions occupied different positions in crystal lattice as a function of Eu³⁺ doping level.

As it can be seen from Fig. 9b, concentration dependence consists of two parts. Eu³⁺ concentration increase up to 24 at% leads to a significant decrease of ⁵D₀ lifetime. So, we observed the drop from 1.31 ms to 0.23 ms. Further increase of Eu³⁺ concentration results in much more smooth lifetime decrease.

Compared to the lifetime of Eu³⁺ at C₂ site, there are only small changes for the lifetime of Eu³⁺ at defect sites. When doping level reaches 24 at%, ⁵D₀ lifetime of Eu³⁺ ions at both positions becomes almost the same.

To understand this fact, relative positions of europium ions should be considered. At low doping concentrations the distance between the Eu³⁺ is very large. Therefore, there is almost no interaction between the Eu³⁺ ions occupied different (normal and defect) sites. However, the distances between the ions become shorter with the increase of doping level and the interaction between them becomes stronger.⁶¹ So, the probability of energy transfer between different sites increases.

3.4 Energy transfer processes of Eu³⁺ ions

Excitation spectra of luminescence lines centered at 610.2 nm, 699 nm and 711 nm were measured in order to study the interaction between Eu³⁺ ions situated at normal and defect positions. For all the above-mentioned luminescence lines the ratio of integrated intensities of ⁷F₀–⁵L₆ (393.5 nm) and ⁷F₀–⁵D₂ (465 nm) excitation transitions was calculated as a function of Eu³⁺ doping concentration (Fig. 10a). It was found that in case of 610.2 nm and 711 nm emission this ratio remains constant for all concentration range, whereas in case of 699 nm emission there are two different parts of the studied dependence. In the first part, when doping concentration does not exceed 16 at% the intensity ratio also remains constant. It should be noted that calculated intensity ratio is about 4 times higher than for other emission wavelengths. In the second part, gradual increase of intensity ratio with increase of Eu³⁺ content was observed. It can be seen that there is linear dependence between the studied ratio and the doping level for high Eu³⁺ concentration. Growth of the intensity ratio can be explained as follows. As mentioned above, the increase of doping concentration leads to the decrease of distances between Eu³⁺ ions. Hence, the probability of energy transfer between Eu³⁺ ions which occupied normal and defect sites in crystal lattice is enhanced. This fact is also confirmed by kinetics measurements (for Y₂O₃:Eu 24 at% sample lifetimes of Eu³⁺ ions, which situated at different positions in crystal lattice, become almost the same). The Eu³⁺ energy levels depend on occupied site due to the crystal field variation. Scheme of energy levels is presented in Fig. 10b. Based on emission spectra (Fig. 6) we can conclude that energy levels of Eu³⁺ ions occupied defects sites shift towards a longer-wavelength region compared to the Eu³⁺ ions occupied normal sites. It is well known that resonance energy transfer is more probable than the phonon-assisted one. Due to the fast thermalization between sublevels and level positions, energy transfer from normal to defect sites can be resonant whereas back transfer is always phonon-assisted. Thus, the probability of energy transfer from normal to defect sites is much higher than the probability of back transfer. The Eu^{3+ 5}L₈ and ⁵L₆ energy levels reached by 322 and 393.5 nm excitation, respectively, are quite broad and probably these levels overlaps or are close to the defect Eu³⁺ excited state levels and hence the energy transfer can occur. The Eu^{3+ 5}D₂ energy level reached by 465 nm excitation is narrow and therefore there could be no overlap with the Eu³⁺ defect ⁵D₂ levels. Energy transfer requires a multiphonon process and is unlikely. Increase of the Eu³⁺ concentration will increase energy transfer from Eu³⁺ in the C_3i and C₂ sites to the Eu³⁺ defect site, and the studied intensity ratio in Fig. 10a is expected to increase.

Fig. 10 (a) Ratio of ⁷F₀–⁵L₆ and ⁷F₀–⁵D₂ intensities as a function of Eu³⁺ doping level; (b) scheme of possible energy transfer.

To gain some insight of the luminescence properties of Eu³⁺ ions in Y₂O₃ nanophosphors, 4f–4f intensity theory was applied to determinate the radiative and nonradiative transition rates and quantum efficiencies. These parameters were obtained from emission spectra using the technique first reported by Kodaira et al.⁶² Calculations have been performed according to the procedure described in articles.^63,64 Briefly, the Einstein coefficients A_0−λ for spontaneous emission can be obtained from the relationship:

where ν_0−λ and I_0−λ are, respectively, the frequency and intensity of the corresponding transition ⁵D₀–⁷F_λ in the emission spectrum. Due to the magnetic character of the ⁵D₀–⁷F₁ transition and its weak dependence on crystal field effect, the value of the A₀₋₁ coefficient is taken as a constant and is equal to 50 s⁻¹. The radiative transition rate (A_rad) is the sum of all the A_0−λ values. The observed lifetime of excited state, radiative and nonradiative (A_nrad) transition rates are related through the following equation:

The quantum efficiency of the ⁵D₀ level may be calculated from:

We studied effect of excitation wavelength on the calculated values. The obtained results are listed in the Table 2. It should be noted that uncertainties in calculated values do not exceed 10%.

Table 2 Radiative (A_rad), nonradiative (A_nrad) and total (A_total) transition rates of the ⁵D₀ level, and quantum efficiencies (η) obtained upon different excitations as a function of Eu³⁺ doping concentration in Y₂O₃:Eu³⁺

C(Eu^3+) (at%)	Atotal (s^−1)	λex (nm)
		Arad (s^−1)				Anrad (s^−1)				η (%)
		265	322	393.5	465	265	322	393.5	465	265	322	393.5	465
2	760	380	350	370	400	380	410	400	360	50	46	48	52
4	760	340	350	330	390	420	410	430	370	45	46	43	51
8	960	420	410	390	420	540	550	570	550	44	42	41	43
12	1100	420	410	380	420	680	690	720	680	38	37	35	38
16	1500	420	410	380	420	1100	1100	1100	1100	28	28	25	28
24	4400	420	390	360	410	3900	4000	4000	3900	10	9	8	9
32	5000	400	340	320	390	4600	4700	4700	4600	8	7	6	8
40	6700	410	320	310	370	6300	6300	6400	6300	6	5	5	6

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Analyzing the calculated radiative transition rate we can see that it slightly changed with the increase of doping level. Slight variations of this probability could be explained by the crystal field changes. In spite of similar values of the ionic radii, substitution of Y³⁺ ions (r = 0.89 Å) to Eu³⁺ ions (r = 0.95 Å)¹⁸ affects the position of Eu³⁺ energy levels and the oscillator strength. It should be noted that the calculated radiative probabilities were almost the same for all excitation wavelengths. Special attention should be paid to the highly-doped samples (32 and 40 at%). Their radiative transition rates are smaller when excited at 322 and 393.5 nm, compared with 265 and 465 nm. This can be explained by significant emission of Eu³⁺ ions at defect sites upon 322 and 393.5 nm (Fig. 6). Due to the partial overlap of emission lines attributed to the electric dipole transitions of normal and defect ions, we obtained averaged radiative transition rate of Eu³⁺ ions occupied both normal and defect sites in crystal lattice.

Nonradiative transition rate is monotonically raised together with doping level. However, the dramatic increase (∼3000 s⁻¹) was observed for 24 at% doped sample. It should be noted that from this doping level the decay curve became non exponential. Therefore, we can draw the conclusion that decay curve shape is probably changed due to the nonradiative processes. There are different types of these processes: multiphonon relaxation, quenching on impurities remaining after synthesis (e.g. OH⁻ group), migration and cross-relaxation. Detailed description of these processes was given in our previous paper.^65,66 Taking into account low energy of phonons in Y₂O₃ host and large energy gap between the excited level ⁵D₀ and the nearest lower level it can be concluded that multiphonon relaxation is impossible. Probability of quenching on impurities cannot be neglected due to the presence of a few OH⁻ groups that can be adsorbed from the atmosphere. Cooperative processes could lead to the diffusion process between europium ions when the involved levels are identical (migration) or to self-quenching when they are different (cross-relaxation).⁶⁷ In case of Eu³⁺ ions the ⁵D₀ level is not involved in the self-quenching processes while ⁵D₁ is. However, both ⁵D₀ and ⁵D₁ levels are involved in diffusion processes.

Only cooperative processes are strongly dependent on the doping level among all considered nonradiative processes. So, increase of nonradiative transition rate is most likely connected with the growth of energy transfer efficiency to impurities.

As can be seen, quantum efficiencies do not depend on the excitation wavelength. The same values of radiative transition rate and growth of nonradiative transition rate results in decrease of quantum efficiencies with the increase of doping level. It was found that η decreased from 50% for 2 at% to 6% for 40 at%, almost nine times.

4. Conclusions

Nanocrystalline Eu³⁺-doped Y₂O₃ materials were synthesized by novel combined Pechini-foaming technique. Structural analysis indicated the formation of pure cubic phase nanocrystals without any impurity. The obtained samples consist of nanoparticles with average size about 40–50 nm. Vibrational spectrum was dominated by phonon centered at about 370 cm⁻¹. It was found that the increasing Eu³⁺ doping concentration leads to the red shift of Raman lines. Intense emission of the nanophosphors due to doping with the Eu³⁺ ions are assigned to transitions occurring between the ⁵D₀ excited state and ⁷F_J ground states of these ions. Most efficiently emission was activated by charge transfer between Eu³⁺ and O²⁻ and subsequent energy transfer to the dopant ions. Optimal doping concentration was determined to be 12 at% in terms of luminescence intensity. Steady-state and kinetic measurements confirm existence of normal and defect sites in Y₂O₃ crystal lattice for europium ions. The excitation mechanism of Eu³⁺ ions occupying defect sites was proposed. It was found that energy transfer from normal to defect sites increases with growth of doping level due to the decrease of distance between Eu³⁺ ions. Radiative and nonradiative transition rates were calculated using the model of f–f transition intensities upon different excitation wavelengths. It was found that radiative transition rate slightly depends on excitation wavelength and doping level. Averaged radiative transition rate of Eu³⁺ ions occupying both normal and defect sites in crystal lattice was obtained for high doping concentration upon 322 and 393.5 nm excitations. Concentration quenching is caused by the growth of energy transfer efficiency to impurities.

Acknowledgements

This research has been supported by The Ministry of Education and Science of the Russian Federation (# 14.604.21.0078, RFMEFI60414X0078). Experimental investigations were carried out in the “Center for Optical and Laser materials research”, “Research Centre for X-ray Diffraction Studies”, and “Interdisciplinary Resource Center for Nanotechnology” (Saint Petersburg State University).

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