Size Effect in Novel Red Efficient Garnet Nanophosphor - The Journal

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Size Effect in Novel Red Efficient Garnet Nanophosphor Pawel Gluchowski, Ma#gorzata Alicja Ma#ecka, Wieslaw Strek, Witold Ryba-Romanowski, and Piotr Solarz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07890 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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The Journal of Physical Chemistry

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

ABSTRACT: Nanocrystalline powders of Eu3+ doped Gd3Ga3Al2O12 (GGAG) were prepared using modified Pechini method. The nanocrystals with the size from tens to hundreds nanometres were obtained. For growth of single crystal GGAG:Eu the Czochralski method was utilized. The crystal structure, grains morphology and optical properties of GAGG 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 huge impact on the luminescence decay time.

Introduction Rare earth (RE) doped materials are under 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 used RE ion 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 5D0 state, which has been used in lighting and displays 2. Eu3+ has been also often 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 garnet family is Y3Al5O12 (YAG) that have excellent chemical stability, high mechanical resistance and particularly the ability to accept substantial trivalent Ln3+ for diverse optical functionalities 10,11. Others garnets like: Y3Ga5O12 (YGG) 12, Gd3Ga5O12 (GGG) 13,14, (Y,Gd)3(Ga,Al)5O12 (YG)(GA)G 8 or Gd3Ga3Al2O12 (GGAG) 15,16 were also intensively studied due they 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 their excellent combination of physicochemical and optical properties there is only few reports where they were taken into account as a potential phosphors 17,18 or laser materials 19,20. The matrix GGAG belongs to so-called ‘solid solution’ host which are 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 imagine as a solution of

Gd3Ga5O12 (GGG) with Gd3Al5O12 (GAG) in molar ratio 3:2. These materials characterize bordering 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. Intention of the study is to determine impact of the powder grains 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 grains size allow us to prepare transparent nanoceramics, and because of high quantum efficiency can be used as a red phosphor for LED. Experimental The GGAG nanocrystalline powders doped with Eu3+ ions were prepared using modified Pechini method 22. Stoichiometric amounts of Eu2O3 (99.99% Alfa Aesar), Gd2O3 (99.999% Changsha Easchem Co., Limited), Ga2O3 (99.999% Changsha Easchem Co., Limited), 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 aluminium nitrate was added into the precursor 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 dryer at 80 ℃ 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 ℃ for 8 hour. 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 cubic shape with a size of 3×3×1 mm (for emission) and 12×8×4 mm (for absorption).

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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 copper Kα1,2 radiation. The Raman measurements were performed in back-scattering geometry using a Renishaw InVia Raman microscope equipped with a confocal DM 2500 Leica optical microscope and a CCD detector. The unpolarised Raman spectra were recorded under excitation with Ar laser emitting light at a 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 analysed with DigitalMicrograph program. Emission and excitation spectra were measured using an FLS980 fluorescence spectrometer from Edinburgh Instrument equipped with a 450W xenon lamp as an excitation source and a Hamamatsu 928 PMT as a 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 150 W xenon pulse lamp was used. Results and discussion Structure analysis The experimental results of XRD measured for the GGAG:Eu3+ nanocrystals calcined at different temperatures (Fig. 1) and Eu3+ doping level (supplementary: Fig. 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 Ia-3d (Z = 8) 23. Gd3+ ions enter dodecahedral 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 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 already at 800 ℃ material have right structure and introducing of Eu3+ into the host lattice does not have impact on the host crystal structure. It's a result of nearly the same ionic radii of the Eu3+ and Gd3+, 94.7 and 93.8 pm,

Fig.1. XRD patterns of GGAG:Eu3+ nanocrystalline powders calcined at different temperatures. respectively 24 and even at high doping level the structure of GGAG is not distorted. It can be observed that with increase of the calcining temperature peaks become narrower indicat-

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ing increase of crystals size and decrease of strains in the structure. An average size of crystalline grains, cell parameter and strains were determined by Rietveld method. Analysis of the XRD line positions and width suggests that at changing the Eu3+ doping level does not change significantly the size of the grains. Also cell parameter and strains stay on the same level for all Eu3+ concentration which indicates a very good fit of the Eu3+ and Gd3+ ionic radius (Table 1). Table 1. Structural parameters of the GGAG:Eu3+ nanocrystalline powders doped with different Eu3+ concentration. Concentration Size / nm a Strains / % (x mol%) 0.2

43

12.2752

0.028

0.4

44

12.2751

0.026

0.8

46

12.2734

0.026

1.6 3.2

44 46

12.2783 12.2713

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 (D−1) of the particle is proportional to the surface to volume ratio (S/V) the increase of the lattice parameter can be related the higher surface to volume ratio in the smaller particles resulting in a higher contribution from the surface layer 25. Lower contribution of the surface in the S/V ratio changes the defects, what leads to the lattice parameter changes. Table 2. Structural parameters of the GGAG:Eu3+ nanocrystalline powders calcined at different temperatures. Calcining Temperature / Size / o C nm a Strains / % 800

27

12.3164

0.043

900

31

12.2861

0.039

1000

44

12.2783

0.027

1100

59

12.2742

0.022

1200

102

12.2683

0.014

The Raman spectra of GGAG:Eu3+ nanopowders and GGAG single crystal were measured at room temperature (Fig. 2). It was ascertained that spectra recorded for GGAG:Eu3+ nanopowders are not affected particle size distribution and decreases number of the lattice by the change of Eu3+ concentration (supplementary: Fig. S2). However, spectra recorded for nanopowders calcined at different temperaturesshow significant dissimilarities. A preliminary analysis of the Raman spectra reveals that all peaks belong to the internal vibration modes of (GaO4)5 26 and (AlO4)5 27 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 a comparison, in a single crystal sample it is located at 361.2 cm-1, and has a FWHM of 14.3 cm-1. Observed shift of the band results from the shortening of Ga–O bond in the powders what is in good


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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.

Excitation and luminescence spectra of GGAG:Eu3+ In order to obtain information about the impact of the Eu3+ concentration and nanocrystals size on the excitation spectrum of Eu3+ in GGAG matrix, appropriate measurement was made at room temperature (Figs. 4, 5). All spectra were monitored at the 609.4 nm and normalized to 7F0 → 5L6 transition (395 nm). In the spectra sharp peaks centred at 321, 364, 384, 395, 417, and 467 nm are observed, which are assigned to the 7F0,1 → 5 H4,5,6, 7F0,1 → 5D4, 7F0,1 → 5G2 + 5L7, 7F0,1 → 5L6, 7F0,1 → 5D3, and 7F0,1 → 5D2 transitions of Eu3+, respectively 28. Another three peaks observed at 274, 308, 314 can be attributed to the f–f transition of Gd3+ from the ground state 8S7/2 to 6IJ, 6P7/2 and 6 P5/2 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 2931 . The excitation energy from 6DJ, 6IJ, or 6PJ state of Gd3+ migrates

Fig.2. Room temperature Raman scattering spectra of the GGAG single crystal and GGAG:Eu3+ powders with different grain sizes. TEM images of the material calcined at 800 ℃ show grains with a diameter ranging from several to several dozens of nanometres (Fig. 3A). A sample calcined at 1000 ℃ (Fig. 3C) contained fractions of smaller and bigger grains and much more aggregates. The powder obtained at the highest temperature (Fig. 3E) shows well defined large single crystallites with the size of hundreds of nanometres. HRTEM made for all powders (Figs. 3B,D,F) show that powders are well crystallized. SAED patterns registered for the samples (supplementary: Fig. S3) show that in sample calcined at lowest temperature an amorphous phase (diffuse rings) as well as a polynanocrystalline phase exist (small spots making up a rings,each spot arising from Bragg reflection from an individual crystallite). For the sample obtained at 1000 ℃ SAED patterns indicate presence of polynanocrystalline phase as well as crystallized grains (big spots). The patterns registered for the sample calcined at highest temperature show clear, bright reflexes what indicate large well defined crystallites.

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

Fig.5. Excitation spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of grains size (inverted spectrum of the absorption of single crystal was inserted at the top of the figure for comparison). Fig. 3. TEM and HRTEM images of GGAG:Eu3+ powders calcined at 800 ℃ (A, B), 1000 ℃ (C, D), 1200 ℃ (E, F)

over the Gd3+ sub-lattice until trapped by Eu3+, and finally radiatively returns to 7FJ state to produce red light. It is observed that with increase of Eu3+ concentration the intensity of



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Gd3+ excitation peaks decreases. This is result of decrease of 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 detector and is impossible to describe 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.

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5 D0 → 7F0 transition. The emission spectrum in this region is shown in the inset of Fig. 7. Deconvolution of this band 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 5D0 → 7F2 luminescence band (left inset in Fig. 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.

It was observed that with increasing of the grains size 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 region. It can be seen that the maximum of the 7F0–5L6 peak measured for a single crystal sample is shifted to the UV. As known, 7F0–5L6 transition is electric dipole transitions, implying that it depends on the crystal field and local structures. Thus the shift of 7F0–5L6 peak observed for a single crystal points at changes of 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 (Fig. 6). The characteristic luminescence 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+.

Fig.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). 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

Fig.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. An inherent structural disorder resulting from substitution of gallium ions by aluminium ions in crystal lattice of GAGG 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 known, the 5D0 → 7F1 lines originate from the parity allowed magnetic dipole transition, while the 5D0 → 7 F2 lines from a forced electric dipole transition. According to the Judd–Ofelt theory 32,33, forced electric dipole transitions are allowed only on condition that the europium ion occupies a site without an inversion centre and is sensitive to local symmetry. To check the impact of the grain size on the degree of disorder, asymmetry ratio was calculated according to the formula: R =

I I

(D (D 5

0

5

0

→ 7 F2 → 7 F1

) )

(1)

The integrated intensity of the 5D0 → 7F2 and 5D0 → 7F1 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 is the site symmetry at the Eu3+ ion. However, contrary to this rule, hypersensitive transitions are extremely weak in K5EuLi2F10 28, where Eu3+


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ion occupies a site with low Cs symmetry. For bulk crystal of K5EuLi2F10 R is 0.348. Therefore, one can conclude that other factors like covalency degree of chemical bond plays the important rule, also.

calcined at 1200 oC (102 nm) and doped with 1.6 mol%. 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:16at.%Eu3+ 36 it can be assumed that our powders may be promising phosphors for LED. Luminescence decay of GGAG:Eu3+ 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 (supplementary: Fig. S4). The decay curves have been normalized and plotted in semi log scale. One can see that all the decay curves are straight lines, demonstrating that they follow a single exponential time dependence described by the formula:  t   I (t ) = I0 exp −  τ   exp 

Fig. 8. Impact of the GGAG:Eu3+ grains size on the degree of distortion (R). Analysis of the Fig. 8 shows that with increase of grains size the degree of distortion decreases implying that the polyhedron around the Eu3+ ion has higher symmetry. It is natural result of grain growth that changes surface/volume ion ratio. It is known that ions on the surface will have lower symmetry due to a large amount of the 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 possibility to use this garnet as commercial red phosphor. The QY was calculated according to the formula: em em (2) ∫ I GGAG:Eu −∫ I GGAG QY =

∫I

exc GGAG

ex − ∫ I GGAG : Eu

(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 the Fig. 4s. 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. Also for GGAG:Eu3+ nanopowders with different crystals size the decay of the luminescence intensity were measured at room temperature (Fig. 9). It is interesting that the 5D0 lifetime observed decreases with an increase of grains 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 behaviour is opposite.

,where : and : are integrated intensities of emission and excitation bands of the doped sample and

Table 3. QY paremeters calculated for GGAG:Eu powders with different dopant concentration and grain sizes. Grain Size / nm QY / % 27

Eu3+ Concentration / mol%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


31

44

0.2

2.9

0.4

4.7

0.8

7.7

1.6 3.2

5.8

6.3

19.0

59

102

20.1

21.7 Fig.9. The luminescence kinetics of GGAG:Eu3+ single crystal and nanopowders in function of grain sizes.

8.2

and are intensities measured for reference powder. The results of the calculation show are summarized in Table 3 and show that highest QY (21.7 %) was measured for the powder

Decrease of decay time with increase of grains 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 rate of electronic



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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: τm ~

(4)

λ0 2

(

)

2

1  2 f ( ED)  neff + 2  neff 3 

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 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 grains 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 (Fig. 10).

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Table 4. Influence of medium refractive index nmed on: measured lifetime, transitions rate, effective index of refraction, and filling factor in the frame of Meltzer et al. model 39. Indexes of refraction nmed after Ref 41, except bulk crystal after Ref. 17. Medium

nmed

Lifetime

Transition rate

ms

W [s-1]

neff

Filling factor x

Air

1.00

5.44

183.82

1.65

0.69

Water

1.33

4.57

218.82

1.74

0.66

Trichloromethane (chloroform)

1.45

4.20

238.10

1.79

0.67

Methylbenzene (toluene)

1.50

4.10

243.90

1.80

0.66

Bulk crystal

1.95

3.07

325.73

1.95

1.00

Conclusions The effect of Eu3+ doping level and annealing temperature of the powders on their crystal structure was analysed. Based on the XRD patterns, SEM/TEM images and Raman spectra 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 grains 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 lifetime of the 5 D0 decreases with the grains size. It was also observed that increase the grain size leads to decrease of luminescence decay time of Eu3+. This phenomenon has been explained in relation to the grains surface area and filling factor related to the effective refractive index of nanocrystals and medium surrounding the powder. It was observed significant difference between luminescence properties of nanoparticles and single crystals (supplementary: Table S1). The small grain size of the powders calcined at 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 colour in white LED systems.

Fig.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 a comparison.

AUTHOR INFORMATION

The same amount 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: 1.95 17), and it is shown that match very well.

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.

Corresponding Author * [email protected]

Author Contributions

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

Supporting Information


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This information is available free of charge via the Internet at http://pubs.acs.org XRD patterns of GGAG:Eu3+ nanocrystalline powders with different Eu3+ doping level, Room temperature Raman scattering spectra of the GGAG:Eu3+ powders in 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.

REFERENCES (1) Rao, G. M.; Seeta Rama Raju, G.; Hussain, S. K.; Pavitra, E.; Rao, P. S. V. S.; Yu, J. S., Tunable Emissions via the White Region from Sr2Gd8(SiO4)6O2:RE3+ (RE3+: Dy3+, Tm3+, Eu3+) Phosphors, New J. Chem. 2016, 40 (7), 6214–6227. (2) Riwotzki, K.; Haase M., Colloidal YVO4:Eu and YP0.95V0.05O4:Eu Nanoparticles: Luminescence and Energy Transfer Processes, J. Phys. Chem. B, 2001, 105 (51), 12709–12713. (3) Qi, S.; Huang, Y.; Li, Y.; Cai, P.; Kim, S. Il; Seo, H. J. J., Probe Spectrum Measurements of Eu3+ Ions as a Relevant Tool for Monitoring in Vitro Hydroxyapatite Formation in a New Borate Biomaterial, Mater. Chem. B 2014, 2 (37), 6387–6396. (4) Jia, G.; Tanner, P. A.; Duan, C.-K.; Dexpert-Ghys, J., Eu3+ Spectroscopy: A Structural Probe for Yttrium Orthoborate Phosphors, J. Phys. Chem. C 2010, 114 (6), 2769–2775. (5) Rathaiah, M.; Haritha, P.; Lozano-Gorrín, A. D.; Babu, P.; Jayasankar, C. K.; Rodríguez-Mendoza, U. R.; Lavín, V.; Venkatramu, V., Stokes and Anti-Stokes luminescence in Tm3+/Yb3+-Doped Lu3Ga5O12 Nano-Garnets: a Study of Multipolar Interactions and Energy Transfer Dynamics, Phys. Chem. Chem. Phys. 2016, 18 (21), 14720–14729. (6) Li, R.; Xu, X.; Su, L.; Sai, Q.; Xia, C.; Yang, Q.; Xu, J.; Strzęp, A.; Półkoszek, A., Crystal Characterization and Optical Spectroscopy of Eu3+-Doped CaGdAlO4 Single Crystal Fabricated by the Floating Zone Method, Chinese Opt. Lett. 2016, 14 (2), 021602–021606. (7) Lemański, K.; Dereń, P. J.; Gągor, A.; Stręk, W., Spectroscopic Investigations of Gd3Sc2Ga3O12 Garnet Doped with Cr3+ and Nd3+ Ions, J. Rare Earths 2009, 27 (4), 560–563. (8) Kamada, K.; Yanagida, T.; Pejchal, J.; Nikl, M.; Endo, T.; Tsutumi, K.; Fujimoto, Y.; Fukabori, A.; Yoshikawa, A., ScintillatorOriented Combinatorial Search in Ce-doped (Y,Gd)3(Ga,Al)5O12 Multicomponent Garnet Compounds, J. Phys. D. Appl. Phys. 2011, 44 (50), 505104. (9) Xu, Y.-N.; Ching, W. Y., Electronic Structure of Yttrium Aluminum Garnet (Y3Al5O12), Phys. Rev. B 1999, 59 (16), 10530– 10535. (10) Binnemans, K.; Görller-Walrand, C., Magnetic Circular Dichroism and Optical Absorption Spectra of Eu3+ in Y3Al5O12 (YAG), J. Chem. Soc., Faraday Trans. 1996, 92 (13), 2487–2493. (11) Marciniak, L.; Bednarkiewicz, A.; Drabik, J.; Trejgis, K.; Strek, W., Optimization of Highly Sensitive YAG:Cr3+, Nd3+ Nanocrystal-Based Luminescent Thermometer Operating in an Optical Window of Biological Tissues, Phys. Chem. Chem. Phys. 2017, 19 (10), 7343–7351. (12) Venkatramu, V.; León-Luis, S. F.; Rodríguez-Mendoza, U. R.; Monteseguro, V.; Manjón, F. J.; Lozano-Gorrín, A. D.; Valiente, R.; Navarro-Urrios, D.; Jayasankar, C. K.; Muñoz, A.; et al, Synthesis, Structure and Luminescence of Er3+-Doped Y3Ga5O12 Nano-Garnets, J. Mater. Chem. 2012, 22 (27), 13788–13799. (13) Guyot, Y.; Canibano, H.; Goutaudier, C.; Novoselov, A.; Yoshikawa, A.; Fukuda, T.; Boulon, G., Yb3+-doped Gd3Ga5O12 Garnet Single Crystals Grown by the Micro-Pulling Down Technique for Laser Application. Part 2: Concentration Quenching Analysis and Laser Optimization, Opt. Mater. 2006, 28 (1–2), 1–8. (14) Lisiecki, R.; Ryba-Romanowski, W.; Solarz, P.; Strzęp, A.; Berkowski, M., Effect of Temperature on Optical Spectra and

Relaxation Dynamics of Sm3+ in Gd3Ga5O12 Single Crystals, J. Alloys Compd. 2014, 582, 208–212. (15) Tyagi, M.; Meng, F.; Koschan, M.; Donnald, S. B.; Rothfuss, H.; Melcher, C. L., Effect of Codoping on Scintillation and Optical Properties of a Ce-Doped Gd3Ga3Al2O12 Scintillator, J. Phys. D. Appl. Phys. 2013, 46 (47), 475302. (16) Kamada, K.; Shoji, Y.; Kochurikhin, V. V.; Nagura, A.; Okumura, S.; Yamamoto, S.; Yeom, J. Y.; Kurosawa, S.; Pejchal, J.; Yokota, Y.; et al, Large Size Czochralski Growth and Scintillation Properties of Mg2+ Co-doped Ce:Gd3Ga3Al2O12, IEEE Trans. Nucl. Sci. 2016, 63 (2), 443–447. (17) Solarz, P.; Głowacki, M.; Berkowski, M.; RybaRomanowski, W., Growth and Spectroscopy of Gd3Ga3Al2O12 (GGAG) and Evidence of Multisite Positions of Sm3+ Ions in Solid Solution Matrix, J. Alloys Compd. 2016, 689, 359–365. (18) Niedźwiedzki, T.; Ryba-Romanowski, W.; Komar, J.; Głowacki, M.; Berkowski, M., Excited State Relaxation Dynamics and Up-Conversion Phenomena in Gd3(Al,Ga)5O12 Single Crystals Co-Doped with Erbium and Ytterbium, J. Lumin. 2016, 177, 219–227. (19) Kuwano, Y.; Saito, S.; Hase, U., Crystal Growth and Optical Properties of Nd:GGAG, J. Cryst. Growth 1988, 92 (1–2), 17–22. (20) Lisiecki, R.; Solarz, P.; Niedźwiedzki, T.; RybaRomanowski, W.; Głowacki, M., Gd3Ga3Al2O12 Single Crystal Doped with Dysprosium: Spectroscopic Properties and Luminescence Characteristics, J. Alloys Compd. 2016, 689, 733– 739. (21) Perkowitz, S., Encyclopedia Britannica, Inc., 2009; p https://www.britannica.com/science/solid-solution. (22) Pechini, M. P. Method of Preparing Lead and Alkaline Earth Titanates and Niobates and Coating Method Using the Same to Form a Capacitor. US Patent No 3,330,697, 1967. (23) Sackville Hamilton, A. C.; Lampronti, G. I.; Rowley, S. E.; Dutton, S. E., Enhancement of the Magnetocaloric Effect Driven by Changes in the Crystal Structure of Al-Doped GGG, Gd3Ga5xAlxO12 (0≤x≤5), J. Phys. Condens. Matter 2014, 26 (11), 116001. (24) Shannon, R. D., Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides Acta Crystallogr. Sect. A 1976, 32 (5), 751–767. (25) Vaqueiro, P.; López-quintela, M. A., Synthesis of Yttrium Aluminium Garnet by the Citrate Gel Process, J. Mater. Chem. 1998, 8 (1), 161–163. (26) Venkatramu, V.; Giarola, M.; Mariotto, G.; Enzo, S.; Polizzi, S.; Jayasankar, C. K.; Piccinelli, F.; Bettinelli, M.; Speghini, A., Nanocrystalline Lanthanide-Doped Lu3Ga15O3 Garnets: Interesting Materials for Light-Emitting Devices, Nanotechnology 2010, 21 (17), 175703. (27) Kostić, S.; Lazarević, Z. Ž.; Radojević, V.; Milutinović, A.; Romčević, M.; Romčević, N. Ž.; Valčić, A., Study of Structural and Optical Properties of YAG and Nd:YAG Single Crystals, Mater. Res. Bull. 2015, 63, 80–87. (28) Solarz, P.; Gajek, Z., Optical Transitions and Enhanced Angular Overlap Model for the Low Symmetry Europium(III) System, J. Phys. Chem. C 2010, 114 (24), 10937–10946. (29) Singh, B. P.; Parchur, A. K.; Ningthoujam, R. S.; Ansari, A. A.; Singh, P.; Rai, S. B., Enhanced Photoluminescence in CaMoO4:Eu3+ by Gd3+ Co-Doping, Dalt. Trans. 2014, 43 (12), 4779–4789. (30) Raju, G. S. R.; Pavitra, E.; Yu, J. S., Pechini Synthesis of Lanthanides (Eu3+/Tb3+ or Dy3+) Ions Activated BaGd2O4 Nanostructured Phosphors: an Approach for Tunable Emissions, Phys. Chem. Chem. Phys. 2014, 16 (34), 18124–18140. (31) Meijerink, A.; Nuyten, J.; Blasse, G., Luminescence and Energy Migration in (Sr,Eu)B4O7, a System with a 4f7-4f65d Crossover in the Excited State, J. Lumin. 1989, 44 (1), 19–31. (32) Judd, B. R., Optical Absorption Intensities of RareEarth Ions, Phys. Rev. 1962, 127 (3), 750–761. (33) Ofelt, G. S., Intensities of Crystal Spectra of Rare-Earth Ions, J. Chem. Phys. 1962, 37 (3), 511–520.



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(34) Ebendorff-Heidepriem, H.; Ehrt, D. J., Spectroscopic Properties of Eu3+ and Tb3+ Ions for Local Structure Investigations of Fluoride Phosphate and Phosphate Glasses, Non. Cryst. Solids 1996, 208 (3), 205–216. (35) Binnemans, K., Interpretation of Europium(III) Spectra, Coord. Chem. Rev. 2015, 295, 1–45. (36) Kolesnikov, I. E.; Tolstikova, D. V.; Kurochkin, A. V.; Manshina, A. A.; Mikhailov, M. D., Eu3+ Concentration Effect on Luminescence Properties of YAG:Eu3+ Nanoparticles, Opt. Mater. 2014, 37, 306–310. (37) Masaharu Mitsunaga; Naoshi Uesugi., Linear and Nonlinear Spectroscopy of Eu3+ in Crystals, J. Lumin. 1991, 48–49, 459–462. (38) Christensen, H. P.; Gabbe, D. R.; Jenssen, H. P., Fluorescence Lifetimes for Neodymium-Doped Yttrium

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Aluminum Garret and Yttrium Oxide Powders, Phys. Rev. B 1982, 25 (3), 1467–1473. (39) Meltzer, R. S.; Feofilov, S. P.; Tissue, B.; Yuan, H. B., Dependence of Fluorescence Lifetimes of Y2O3:Eu3+ Nanoparticles on the Surrounding Medium, Phys. Rev. B 1999, 60 (20), R14012–R14015. (40) Li, J.; Li, J.-G.; Zhang, Z.; Wu, X.; Liu, S.; Li, X.; Sun, X.; Sakka, Y., Effective Lattice Stabilization of Gadolinium Aluminate Garnet (GdAG) via Lu3+ Doping and Development of Highly Efficient (Gd,Lu)AG:Eu3+ Red Phosphors, Sci. Technol. Adv. Mater. 2012, 13 (3), 35007. (41) Haynes, W. M. CRC Handbook of Chemistry and Physics, 90th Edition (CD-ROM Version 2010); Lide, D. R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2010


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XRD patterns of GGAG:Eu3+ nanocrystalline powders calcined at different temperatures 201x140mm (300 x 300 DPI)



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Room temperature Raman scattering spectra of the GGAG single crystal and GGAG:Eu3+ powders with different grain sizes 201x140mm (300 x 300 DPI)


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TEM and HRTEM images of GGAG:Eu3+ powders calcined at 800 ℃ (A, B), 1000 ℃ (C, D), 1200 ℃ (E, F) 119x79mm (300 x 300 DPI)



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Excitation spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of Eu3+ concentration. 201x140mm (300 x 300 DPI)


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Excitation spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of grains size (inverted spectrum of the absorption of single crystal was inserted at the top of the figure for comparison) 201x140mm (300 x 300 DPI)



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Luminescence spectra of the GGAG:Eu3+ single crystal and nanopowders as a function of Eu3+ concentration. Photo-graphs shown on the top depict the effect of Eu3+ concentration on luminescence of GGAG:Eu3+ powders under UV lamp excitation (254 nm). 201x140mm (300 x 300 DPI)


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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. 201x140mm (300 x 300 DPI)



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Impact of the GGAG:Eu3+ grains size on the degree of distortion (R) 201x140mm (300 x 300 DPI)


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The luminescence kinetics of GGAG:Eu3+ single crystal and nanopowders in function of grain sizes 201x140mm (300 x 300 DPI)



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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 a comparison 201x140mm (300 x 300 DPI)


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Graphic abstract 635x242mm (72 x 72 DPI)


Size Effect in Novel Red Efficient Garnet Nanophosphor - The Journal

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