Size Effect in Novel Red Efficient Garnet Nanophosphor

Eu3+ is a preferable choice as an activator ion with red ... used as a red phosphor for LED. .... single crystal were measured at room temperature (Fi...
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Cite This: J. Phys. Chem. C 2017, 121, 25561-25567

Size Effect in Novel Red Efficient Garnet Nanophosphor Paweł Głuchowski,* Małgorzata Małecka, Wiesław Stręk, Witold Ryba-Romanowski, and Piotr Solarz Institute of Low Temperature and Structure and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wroclaw, Poland

J. Phys. Chem. C 2017.121:25561-25567. Downloaded from pubs.acs.org by DURHAM UNIV on 12/31/18. For personal use only.

S Supporting Information *

ABSTRACT: Nanocrystalline powders of Eu3+ doped Gd3Ga3Al2O12 (GGAG) were prepared using a modified Pechini method. The nanocrystals with the size from tens to hundreds of nanometers were obtained. For growth of single crystal GGAG:Eu the Czochralski method was utilized. The crystal structure, grain morphology, and optical properties of GGAG doped with different concentration of Eu3+ and calcined at different temperatures were investigated and discussed. The impact of grain size and Eu3+ concentration on the spectroscopic properties of Gd3Ga3Al2O12:Eu3+ was checked and analyzed. The spectroscopic properties of the nanopowders were compared to the Gd3Ga3Al2O12:Eu3+ single crystal. The influence of effective index of refraction was taken into account in the analysis of transition rates of metastable multiplet and it was found that grain size (surface area) of the nanocrystals has a huge impact on the luminescence decay time.

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INTRODUCTION Rare earth (RE) doped materials are being given great attention in the field of lighting, lasers, displays, and structural analysis because of their unique intraconfigurational f−f transitions, which occur as sharp and intense emission lines.1 One of most commonly RE ions used for doping of different matrices is Eu3+, due to its simple electronic energy level scheme among the RE ions. Eu3+ is a preferable choice as an activator ion with red emission from the 5D0 state, which has been used in lighting and displays.2 Eu3+ has also often been used as a local structure probe in determining the chemical surroundings and microscopic symmetries of different sites available in host lattices.3,4 The choice of the matrix is also very important due to strict requirements in different application areas. Garnets, having a general formula of Ln3Al5O12 (LnAG, Ln: lanthanide and Y) belong to the most known and widely studied matrices.5−8 The garnet structure can be viewed as a framework built up via corner sharing of the Al−O polyhedra, with the Ln residing in dodecahedral interstices.9 The best-known compound from the garnet family is Y3Al5O12 (YAG) that has excellent chemical stability, high mechanical resistance, and in particular the ability to accept substantial trivalent Ln3+ for diverse optical functionalities.10,11 Other garnets such as Y3Ga5O12 (YGG),12 Gd3Ga5O12 (GGG),13,14 (Y,Gd)3(Ga,Al)5O12 (YG)(GA)G,8 or Gd3Ga3Al2O12 (GGAG)15,16 were also studied in depth due their interesting optical properties. The last type of garnets were investigated in view of the very good scintillation properties after doping with Ce3+ ions. It is interesting that in spite of the excellent combination of physicochemical and optical properties, there are only a few reports where they were taken into account as a potential phosphor17,18 or laser material.19,20 The matrix GGAG belongs to a so-called “solid © 2017 American Chemical Society

solution” host which is defined as a mixture of two crystalline solids that coexist as a new crystalline solid, or crystal lattice without changing the structure,21 and can be imagined as a solution of Gd3Ga5O12 (GGG) with Gd3Al5O12 (GAG) in molar ratio 3:2. These materials characterize broadening of the absorption and emission lines in comparison to “start-up” hosts.17 In this work we present results of spectroscopic investigation of Eu3+ doped GGAG nanocrystals obtained by a modified Pechini method which was successfully applied for preparing the nanocrystalline powder. The intention of the study is to determine the impact of the powder grain size and Eu3+ concentration on the structure of the GGAG garnet and its optical properties, in comparison to single crystal. It is expected that the decrease of the grain size allows us to prepare transparent nanoceramics, and because of high quantum efficiency can be used as a red phosphor for LED.

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EXPERIMENTAL SECTION The GGAG nanocrystalline powders doped with Eu3+ ions were prepared using a modified Pechini method.22 Stoichiometric amounts of Eu2 O3 (99.99% Alfa Aesar), Gd2 O3 (99.999% Changsha Easchem Co., Limited), Ga2O3 (99.999% Changsha Easchem Co., Limited), and Al(NO3)3·9H2O (99.999% Alfa Aesar) were taken to give the appropriate molar ratio of all reactants. To obtain europium, gadolinium, and gallium nitrates, oxides were dissolved in nitric acid in a microwave reactor and then solutions were evaporated. Subsequently aluminum nitrate was added into the precursor Received: August 8, 2017 Revised: October 12, 2017 Published: October 18, 2017 25561

DOI: 10.1021/acs.jpcc.7b07890 J. Phys. Chem. C 2017, 121, 25561−25567

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positions, surrounded by 8 oxygens (positions 24c in Wyckoff notation) while Ga3+ and Al3+ occupies tetrahedral (24d) and octahedral (16a) positions surrounded by 4 and 6 oxygen ions, respectively. However, it has been presented in ref. 17 that the active dopant can be situated in other sites in this solid solution host. Sharp peaks indicate fine crystallization of the material and no obvious peaks of impurity phases have been observed, indicating that at 800 °C material already has correct structure and introducing Eu3+ into the host lattice does not have any impact on the host crystal structure. It is a result of almost the same ionic radii of the Eu3+ and Gd3+, 94.7 and 93.8 pm, respectively,24 and even at high doping level the structure of GGAG is not distorted. It can be observed that with an increase of the calcining temperature peaks become narrower, indicating an increase of crystal size and decrease of strain in the structure. Average size of crystalline grains, cell parameters, and strain were determined by the Rietveld method. Analysis of the XRD line positions and width suggests that changing the Eu3+ doping level does not significantly change the size of the grains. In addition, cell parameters and strains stay on the same level for all Eu3+ concentrations, which indicates a very good fit of the Eu3+ and Gd3+ ionic radii (Table 1).

solution. Nitrate solutions were added to 5 mol of citric acid and 5 mol of glycol per 1 mol of metal. After that the mixture was ultrasonically stirred for 2 h and then placed into a dryer at 80 °C for 7 days to evaporate water and to carry out the polymerization process. The resulting brown resin was then placed in a quartz crucible and heated under air atmosphere from 800 to 1200 °C for 8 h. The Gd3Ga3Al2O12 single crystal doped with 1 at. % of Eu3+ was grown in Institute of Physics, PAS, Warsaw, utilizing the Czochralski method. Single crystal samples of good optical quality possessed a cubic shape with a size of 3 × 3 × 1 mm3 (for emission) and 12 × 8 × 4 mm3 (for absorption). Structure of the samples was evaluated by X-ray powder diffraction. X-ray powder diffraction patterns were collected by a X’PERT PRO PANalytical diffractometer using Cu Kα1,2 radiation. The Raman measurements were performed in backscattering geometry using a Renishaw InVia Raman microscope equipped with a confocal DM 2500 Leica optical microscope and a CCD detector. The unpolarized Raman spectra were recorded under excitation with Ar laser emitting light at 488 nm in a single scan with a 10 s exposure time. Morphology and microstructure were investigated by TEM (Philips CM-20 SuperTwin operating at 160 kV). HRTEM images and SAED patterns were analyzed with DigitalMicrograph program. Emission and excitation spectra were measured using FLS980 fluorescence spectrometer from Edinburgh Instrument equipped with a 450W xenon lamp as an excitation source and a Hamamatsu 928 PMT detector. Both the excitation and emission monochromators were in the Czerny Turner configuration (1800 lines per mm holographic grating blazed at 300 nm, 0.2 nm resolution). All spectra were corrected for the sensitivity and wavelength of the experimental setup. The luminescence kinetics measurements were performed using Edinburgh Instrument FLS980 where for excitation a 150 W xenon pulse lamp was used.

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Table 1. Structural Parameters of the GGAG:Eu3+ Nanocrystalline Powders Doped with Different Eu3+ Concentration concentration (mol %)

size/nm

a

strain/%

0.2 0.4 0.8 1.6 3.2

43 44 46 44 46

12.2752 12.2751 12.2734 12.2783 12.2713

0.028 0.026 0.026 0.027 0.027

When the powders were calcined at higher temperatures, size of the grains start to increase, and at the same time strains decrease (Table 2). Because the reciprocal of the diameter

RESULTS AND DISCUSSION

Structure Analysis. The experimental results of XRD measured for the GGAG:Eu3+ nanocrystals calcined at different temperatures (Figure 1) and Eu3+ doping level (Figure S1) match well with the cubic structure of the Gd3Ga3Al2O12 (ICSD 192182). The density of this compound is 6.63 g/ cm3. The GGAG crystallize in the cubic crystal structure with space group Ia3̅d (Z = 8).23 Gd3+ ions enter dodecahedral

Table 2. Structural Parameters of the GGAG:Eu3+ Nanocrystalline Powders Calcined at Different Temperatures calcining temperature/°C

size/nm

a

strain/%

800 900 1000 1100 1200

27 31 44 59 102

12.3164 12.2861 12.2783 12.2742 12.2683

0.043 0.039 0.027 0.022 0.014

(D−1) of the particle is proportional to the surface to volume ratio (S/V), the increase of the lattice parameter can be related to the higher surface to volume ratio in the smaller particles resulting in a higher contribution from the surface layer.25 A lower contribution of the surface in the S/V ratio changes the defects, which leads to the lattice parameter changes. The Raman spectra of GGAG:Eu3+ nanopowders and GGAG single crystal were measured at room temperature (Figure 2). It was ascertained that spectra recorded for GGAG:Eu3+ nanopowders are not affected by the particle size distribution and the number of the lattice is decreased by the change of Eu3+ concentration (Figure S2). However, spectra recorded for nanopowders calcined at different temperatures show significant dissimilarities. A preliminary analysis of the Raman spectra

Figure 1. XRD patterns of GGAG:Eu3+ nanocrystalline powders calcined at different temperatures. 25562

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arising from Bragg reflection from an individual crystallite). For the sample obtained at 1000 °C, SAED patterns indicate the presence of a polynanocrystalline phase as well as crystallized grains (big spots). The patterns registered for the sample calcined at highest temperature show clear, bright reflexes which indicate large well-defined crystallites. Excitation and Luminescence Spectra of GGAG:Eu3+. In order to obtain information about the impact of the Eu3+ concentration and nanocrystal size on the excitation spectrum of Eu3+ in GGAG matrix, an appropriate measurement was made at room temperature (Figures 4, 5). All spectra were

Figure 2. Room temperature Raman scattering spectra of the GGAG single crystal and GGAG:Eu3+ powders with different grain sizes.

reveals that all peaks belong to the internal vibration modes of (GaO4)526 and (AlO4)527 tetrahedra. In particular, a line at the Raman frequency ωR ∼ 362 cm−1, related to the F2g(υ3) symmetry vibration modes in the Gd3Ga3Al2O12, shifts and its width changes. For the smallest grains (27 nm) it is located at 363.7 cm−1 and has a fwhm of 27.2 cm−1. For higher grain sizes the line position and fwhm are as follows: 31 nm: 363.4, 23.3 cm−1; 44 nm: 362.8, 21.6 cm−1; 59 nm: 361.8, 21.1 cm−1; 102 nm: 361.5, 20.4 cm−1. For comparison, in a single crystal sample it is located at 361.2 cm−1 and has a fwhm of 14.3 cm−1. The observed shift of the band results from the shortening of the Ga−O bond in the powders which is in good agreement with the XRD spectra. A detailed analysis of the influence of impurities and grain size on all internal and external vibrations observed in the spectrum will be the subject of a separate study. TEM images of the material calcined at 800 °C show grains with a diameter ranging from several to several dozens of nanometers (Figure 3A). A sample calcined at 1000 °C (Figure

Figure 4. Excitation spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of Eu3+ concentration.

Figure 5. Excitation spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of grain size (inverted spectrum of the absorption of single crystal was inserted at the top of the figure for comparison).

monitored at the 609.4 nm and normalized to 7F0 → 5L6 transition (395 nm). In the spectra, sharp peaks centered at 321, 364, 384, 395, 417, and 467 nm are observed, which are assigned to the 7F0,1 → 5H4,5,6, 7F0,1 → 5D4, 7F0,1 → 5G2 + 5L7, 7 F0,1 → 5L6, 7F0,1 → 5D3, and 7F0,1 → 5D2 transitions of Eu3+, respectively.28 Another three peaks observed at 274, 308, and 314 nm can be attributed to the f−f transition of Gd3+ from the ground state 8S7/2 to 6IJ, 6P7/2, and 6P5/2 levels. Contribution of these lines to the excitation spectrum of Eu3+ luminescence indicates that an energy transfer from the host and Gd3+ to Eu3+ ions occurs. The energy transfer of Gd3+ to Eu3+ has been widely reported in many hosts.29−31 The excitation energy from 6 DJ, 6IJ, or 6PJ state of Gd3+ migrates over the Gd3+ sublattice until it is trapped by Eu3+, and finally radiatively returns to the 7 FJ state to produce red light. It is observed that with increase of Eu3+ concentration the intensity of Gd3+ excitation peaks

Figure 3. TEM and HRTEM images of GGAG:Eu3+ powders calcined at 800 °C (A, B), 1000 °C (C, D), 1200 °C (E, F).

3C) contained fractions of smaller and larger grains and many more aggregates. The powder obtained at the highest temperature (Figure 3E) shows well-defined large single crystallites with the size of hundreds of nanometers. HRTEM for all powders (Figures 3B,D,F) shows that powders are well crystallized. SAED patterns registered for the samples (Figure S3) show that in the samples calcined at lowest temperature an amorphous phase (diffuse rings) as well as a polynanocrystalline phase exist (small spots making up a rings; each spot 25563

DOI: 10.1021/acs.jpcc.7b07890 J. Phys. Chem. C 2017, 121, 25561−25567

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The Journal of Physical Chemistry C decreases. This is a result of decreasing concentration of Gd3+ in the matrix and weakening of the energy transfer between both ions. In the excitation spectrum we also observed broad charge-transfer (CT) bands at 240−290 nm (primarily for single crystal), which are caused by the electron transfer from the 2p6 orbital of O2− ion to the empty 4f6 orbital of Eu3+.30 Since Eu3+ has a 4f6 configuration, it needs to gain one more electron to achieve the half-filled 4f7 orbital, which is relatively stable compared to partially filled configurations. When Eu3+ is linked to the O2− ligand, there is a chance of electron transfer from O2− to Eu3+ to form Eu2+−O−. In the powders this band saturated the detector, and it is impossible to describe the tendency according to the Eu3+ concentration or grain size of GGAG:Eu3+. It is worth mentioning that an increase of the dopant concentration entails the increase of the absorption band intensities in the UV−blue region. It was observed that with increasing grain size the intensity of the peaks assigned to the Eu3+ and Gd3+ f−f transitions decreases in the UV range and increases in the blue and green regions. It can be seen that the maximum of the 7F0−5L6 peak measured for a single crystal sample is shifted to the UV. As is known, the 7F0−5L6 transition is an electric dipole transition, implying that it depends on the crystal field and local structures. Thus, the shift of the 7F0−5L6 peak observed for a single crystal points to changes of the local environments surrounding Eu3+ ions. The emission spectra for GGAG:Eu3+ single crystal and nanopowders as a function of Eu3+ concentration were measured at room temperature using the 395 nm Xe lamp excitation line (Figure 6). The characteristic luminescence

Figure 7. Normalized luminescence spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of grains size. On top is presented photo of GGAG:Eu3+ powders under UV lamp excitation (254 nm) for different powder grains size.

reveals three weak emission bands at 576.4, 579, and 579.7 nm pointing at the presence of at least three different sites for the Eu3+ ions in agreement with GGAG:Sm.17 It is interesting to notice that their intensity decreases with increasing grain size and eventually becomes negligible for a single crystal sample although the integrated intensity observed for the rest of transitions increases. Examination of the 5 D 0 → 7 F 2 luminescence band (left inset in Figure 7) reveals that its width decreases with increasing grain size. This finding indicates that the change of grain size affects local environment around the Eu3+ ions. It is well-known that in contrast to ordered crystals spectral lines of lanthanide impurities in disordered crystalline hosts are inhomogeneously broadened and their widths depend markedly on the degree of a structural disorder of the host. An inherent structural disorder resulting from substitution of gallium ions by aluminum ions in crystal lattice of GGAG is enhanced in GGAG:Eu3+ nanopowders due to important contribution of strongly defected surface layer. A high degree of disorder for the Eu3+ sites in the GGAG host is observed for smallest grains and with increase of grains size the disorder start to decrease. As is known, the 5D0 → 7F1 lines originate from the parity allowed magnetic dipole transition, while the 5D0 → 7F2 lines from a forced electric dipole transition. According to the Judd−Ofelt theory,32,33 forced electric dipole transitions are allowed only on the condition that the europium ion occupies a site without an inversion center and is sensitive to local symmetry. To check the impact of the grain size on the degree of disorder, the asymmetry ratio was calculated according to the formula

Figure 6. Luminescence spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of Eu3+ concentration. Photographs shown on the top depict the effect of Eu3+ concentration on luminescence of GGAG:Eu3+ powders under UV lamp excitation (254 nm).

bands located at around 580, 590, 610, 650, and 700 nm are associated with transitions from the excited 5D0 state to multiplets of the ground term, 7FJ (J = 0, 1, 2, 3, 4), respectively. The most intense emission was recorded for the sample with 1.6 mol % of Eu3+. It is well-known that the Eu3+ ion is a powerful structural probe to show the presence of different sites accommodating the lanthanide ions in the lattice. Particularly meaningful is the Eu3+ emission band observed around 580 nm, related to the 5 D0 → 7F0 transition. The emission spectrum in this region is shown in the inset of Figure 7. Deconvolution of this band

R=

I(5D0 → (7F2) I(5D0 → (7F1)

(1)

The integrated intensity of the D0 → F2 and D0 → F1 transitions can be considered as indicative of the asymmetry of the coordination polyhedron around the Eu3+ ion.34,35 In particular, the lower the R value is, the higher the site symmetry at the Eu3+ ion. However, contrary to this rule, hypersensitive transitions are extremely weak in K5EuLi2F10,28 where Eu3+ ion occupies a site with low Cs symmetry. For bulk crystal of K5EuLi2F10, R is 0.348. Therefore, one can conclude that other 5

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5

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DOI: 10.1021/acs.jpcc.7b07890 J. Phys. Chem. C 2017, 121, 25561−25567

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see that all the decay curves are straight lines, demonstrating that they follow a single exponential time dependence described by the formula

factors like covalency degree of chemical bond plays the important rule, also. Analysis of Figure 8 shows that with the increase of grain size the degree of distortion decreases implying that the polyhedron

⎛ t ⎞ ⎟⎟ I(t ) = I0 exp⎜⎜ − ⎝ τexp ⎠

(3)

where I(t) is a luminescence intensity, I0 is the initial intensity, t is the time after the excitation pulse, and τexp the experimental decay time. It can be seen that for all concentrations of europium ions the 5D0 decay rate is very similar (average value gives 5.26 ms), implying that no concentration quenching occurs. The calculated results of the decay times are shown in Figure 4. The lifetime (τ) of the 5D0 level in GGAG:Eu3+ single crystal is found to be 3.07 ms, a value markedly smaller than 4.00 ms observed for Eu3+ ions in Y3Al5O12 garnet.37 In addition, for GGAG:Eu3+ nanopowders with different crystal size the decay of the luminescence intensity were measured at room temperature (Figure 9). It is interesting that the 5D0

Figure 8. Impact of the GGAG:Eu3+ grain size on the degree of distortion (R).

around the Eu3+ ion has higher symmetry. It is a natural result of grain growth that changes the surface/volume ion ratio. It is known that ions on the surface will have lower symmetry due to a large amount of structure defects. Eu3+ ions in the volume become organized and the degree of disorder decreases. For all samples we measured quantum yield (QY) to check the possibility of using this garnet as commercial red phosphor. The QY was calculated according to the formula QY =

em em − ∫ IGGAG ∫ IGGAG:Eu exc ex − ∫ IGGAG:Eu ∫ IGGAG

em IGGAG:Eu

(2)

exc IGGAG:Eu

Figure 9. Luminescence kinetics of GGAG:Eu3+ single crystal and nanopowders as a function of grain sizes.

where and are integrated intensities of emission and excitation bands of the doped sample and Iem GGAG and Iexc GGAG are intensities measured for reference powder. The results of the calculation show are summarized in Table 3 and

lifetime observed decreases with an increase of grain size. For a single crystal sample the decay time is almost two times shorter than for smallest grains (27 nm). Usually when crystal structure is better organized and lanthanide ions are fully coordinated the decay times become longer. In the case of GGAG:Eu3+ this behavior is opposite. Decrease of decay time with increase of grain size (smaller surface area) is observed due to lowered radiative transition rate arising from the change of effective refractive index.38 In other words, the influence of grain size on the rate of electronic relaxation processes is associated with the local electric field in dielectric medium. This effect has been described by Melzer et al.39 as a function of the index of refraction of the medium. They assumed that the dependence of measured lifetime (τm) on the index of refraction (n) arises from the change in the density of states for photons in the medium of reduced light velocity and the modification of the polarizability of the surrounding medium. In this case, the radiative lifetime can be related to the refractive index using the equation

Table 3. QY Parameters Calculated for GGAG:Eu Powders with Different Dopant Concentration and Grain Sizes QY/% Eu3+ Concentration/mol % 0.2 0.4 0.8 1.6 3.2

grain size/nm 27

5.8

31

6.3

44 2.9 4.7 7.7 19.0 8.2

59

102

20.1

21.7

show that highest QY (21.7%) was measured for the powder calcined at 1200 °C (102 nm) and doped with 1.6 mol %. By comparing to the photoluminescence quantum yield measured for YAG powders doped with various concentration of Eu3+ where PLQY was about 15.5% for the YAG:16 atom %Eu3+36, it can be assumed that our powders may be promising phosphors for LED. Luminescence Decay of GGAG:Eu 3+ . To further investigate the luminescence dynamics, the luminescence decay curves of the Eu3+ in GGAG:Eu3+ single crystal and nanopowders with different concentration of Eu3+ were measured at room temperature (Figure S4). The decay curves have been normalized and plotted on semi log scale. One can

τm ∼

λ02 2 1 f (ED)⎡⎣ 3 (neff 2 + 2)⎤⎦ neff

(4)

where f(ED) is the oscillator strength for the electric dipole transition, λ0 is the wavelength in vacuum, and neff is the effective index of refraction for the medium. The effective index 25565

DOI: 10.1021/acs.jpcc.7b07890 J. Phys. Chem. C 2017, 121, 25561−25567

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results it was shown that Eu3+ concentration does not affect the structure of GGAG, whereas an increase of the calcination temperature brings about an increase of the grain size. The analysis of the excitation, emission, and decay times revealed that the energy transfer from gadolinium to europium occurs, the optimal concentration of Eu3+ is 1.6%, and the lifetime of the 5D0 decreases with the grain size. It was also observed that increase of the grain size leads to decrease of luminescence decay time of Eu3+. This phenomenon has been explained in relation to the grain surface area and filling factor related to the effective refractive index of nanocrystals and medium surrounding the powder. A significant difference was observed between luminescence properties of nanoparticles and single crystals (Table S1). The small grain size of the powders calcined at the lowest temperature and cubic structure of the GGAG:Eu3+ material suggest that it will be possible to obtain translucent or even transparent ceramic. The emission wavelength suggest that this material can be used as a red phosphor to warm the color in white LED systems.

of refraction consists of two components: refractive index of the nanoparticles n and the medium surrounded the particles nmed. According to that, the definition of neff is neff = xn + (1 − x)nmed

(5)

where x is the ″filling factor″ showing what fraction of volume is occupied by the luminescent compound, in our case the GGAG:Eu3+ garnet. Decrease of the decay time of the Eu3+ emission with increase of the grain size was also observed for the (Gd,Lu)AG:Eu3+.40 To prove the impact of the medium index of refraction on the lifetime of the 5D0 level in GGAG:1% Eu3+, the decays were measured for powder suspended in different media (Figure 10).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07890. XRD patterns of GGAG:Eu3+ nanocrystalline powders with different Eu3+ doping level, room temperature Raman scattering spectra of the GGAG:Eu3+ powders as a function of Eu3+ concentration, SAED patterns registered for the GGAG:Eu3+ powder calcined at different temperatures, luminescence decay curves for GGAG:Eu3+ nanopowders with different concentrations of Eu3+, table with comparison of the luminescence properties of GGAG:Eu3+nanoparticles and single crystal (PDF)

Figure 10. Luminescence decay curves for GGAG:Eu3+ nanopowders embedded in media differing in index of refraction (left) and a plot of corresponding lifetime values versus indexes of refraction (right). Lifetime value measured with a single crystal sample is included for comparison.

The same amounts of the nanopowder with the smallest grains (27 nm) were taken to make a suspension in air, water, trichloromethane (chloroform), and methylbenzene (toluene). The indexes of refraction were taken after ref 41 for sodium D lines in the yellow region (589 nm) and are presented in Table 4. The calculated decay times were also compared to that obtained for bulk crystal (with n: 1.9517), and is shown to match very well.

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Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.

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CONCLUSIONS The effect of Eu3+ doping level and annealing temperature of the powders on their crystal structure was analyzed. Based on the XRD patterns, SEM/TEM images, and Raman spectra

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Centre, Poland under grant number DEC-2014/15/B/ST5/05062.

Table 4. Influence of Medium Refractive Index nmed on Measured Lifetime, Transitions Rate, Effective Index of Refraction, and Filling Factor in the Frame of the Meltzer et al. model39a medium Air Water Trichloromethane (chloroform) Methylbenzene (toluene) Bulk crystal a

nmed

lifetime [ms]

transition rate W [s−1]

neff

filling factor x

1.00 1.33 1.45

5.44 4.57 4.20

183.82 218.82 238.10

1.65 1.74 1.79

0.69 0.66 0.67

1.50

4.10

243.90

1.80

0.66

1.95

3.07

325.73

1.95

1.00

AUTHOR INFORMATION

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REFERENCES

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Indexes of refraction nmed after ref 41, except bulk crystal after ref 17. 25566

DOI: 10.1021/acs.jpcc.7b07890 J. Phys. Chem. C 2017, 121, 25561−25567

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DOI: 10.1021/acs.jpcc.7b07890 J. Phys. Chem. C 2017, 121, 25561−25567