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Dy3+ ions
Accepted Manuscript From structure to luminescence investigation of oxyfluoride transparent glasses and glass-ceramics doped with Eu3+/Dy3+ ions Michalina Walas, Marta Lisowska, Tomasz Lewandowski, Ana I. Becerro, Marcin Łapiński, Anna Synak, Wojciech Sadowski, Barbara Kościelska PII:
S0925-8388(19)32503-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.07.017
Reference:
JALCOM 51304
To appear in:
Journal of Alloys and Compounds
Received Date: 15 April 2019 Revised Date:
30 June 2019
Accepted Date: 2 July 2019
Please cite this article as: M. Walas, M. Lisowska, T. Lewandowski, A.I. Becerro, M. Łapiński, A. Synak, W. Sadowski, B. Kościelska, From structure to luminescence investigation of oxyfluoride transparent glasses and glass-ceramics doped with Eu3+/Dy3+ ions, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.07.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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From structure to luminescence investigation of oxyfluoride transparent glasses and glass-ceramics doped with Eu3+/Dy3+ ions
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*Michalina Walas1, Marta Lisowska1, Tomasz Lewandowski1, Ana I. Becerro2, Marcin Łapiński1, Anna Synak3, Wojciech Sadowski1 and Barbara Kościelska1 1
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Faculty of Applied Physics and Mathematics, Department of Solid State Physics, Gdańsk University of Technology, ul. Gabriela Narutowicza 11/12, 80-233 Gdańsk, Poland 2 Instituto de Ciencia de Materiales de Sevilla (CSIC-US), c/Américo Vespucio, 49, 41092 Sevilla, Spain 3 Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdańsk, ul. Wita Stwosza 57/246, 80-952 Gdańsk, Poland *corresponding author: [email protected]; [email protected]
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Keywords: white light; Eu3+; Dy3+; glasses; glass-ceramics;
Abstract
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Glasses and glass-ceramics with nominal composition 73 TeO2- 4BaO- 3Bi2O3-18SrF2-2RE2O3 (where RE=Eu, Dy) have been synthesized by conventional melt-quenching technique and subsequent heat treatment at 370˚C for 24 hours in air atmosphere. Various Eu3+ to Dy3+ molar ratio have been applied to investigate luminescence properties in both glass and glass-ceramic matrices. Especially, white light emission through simultaneous excitation of Eu3+ and Dy3+ has been studied in detail. Influence of crystalline SrF2 phase on luminescence kinetics has been determined by luminescence decay time measurements. Presence of crystalline SrF2 phase has been confirmed by X-ray diffraction technique XRD and transmission electron microscopy TEM. X-ray photoelectron spectroscopy XPS and Fourier-transform infrared spectroscopy FTIR have been applied to get further insight into structural properties of glass and glass-ceramic materials. Color tunable white light emission has been obtained using UV excitation. Influence of the SrF2 crystallization on luminescence properties of prepared materials have been described in detail. Moreover, luminescence properties and especially emission color dependence on the Eu3+ to Dy3+ molar ratio have been exhaustively studied. Color-tunable white light emission has been observed as a result of simultaneous radiative transition of both, Eu3+ and Dy3+ ions when applying UV excitation. Judd – Ofelt and other optical parameters have been calculated based on luminescence emission spectra. Achieved results confirm that tellurite glass-ceramics containing SrF2 nanocrystals are good hosts for RE3+ ions and they can be considered as new phosphors for white light emitting diodes WLEDs.
Introduction
Studies on light emitting sources have attracted considerable attention during last years because of growing requirements in terms of reliability, efficiency and environment friendliness of materials [1]. Currently the most popular light sources seem to be white light emitting diodes WLED which replaced traditional fluorescent lamps or bulks. Typical WLEDs combine YAG: Ce yellow phosphor with employment of blue chip. However, the main disadvantage of such systems is the lack of red emitter resulting in low color rendering index CRI, high correlated temperature CCT [2] and also poor thermal stability of organic epoxy resin or silicon component [3]. Thus, inorganic materials i.e. glasses or glassceramics containing optically active dopants such as RE3+ ions gained a considerable attention in last few 1
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years as promising materials for WLED phosphors and other optical applications [4]. Comparing to traditional glasses, glass-ceramics have number of advantages when consider those as matrices for RE3+ ions. For example, presence of low-phonon crystalline phase [5][6][7][8] [9][10]in glass matrix promotes RE3+ radiative transitions, good optical quality etc. keeping all advantages of glass matrix at the same time [11]. Incorporation of RE3+ into crystal lattice prevents from luminescence concentration quenching due to clusters formation [10][12][13]. Moreover, this also promotes RE3+ energy transfers between ions and from crystals to ions. White light emission originating from RE3+ ions have been obtained i.e. in Tm3+/Tb3+/Eu3+ triply-doped phosphate glass-ceramics [14], for Eu3+ doped fluorozirconate glass-ceramics containing BaCl2 nanocrystals [15] or Dy3+/Sm3+/Tb3+ doped oxyfluoride glass-ceramics [16].
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Among various inorganic phosphors, tellurite based glass-ceramics containing fluoride nanocrystals seem to be especially interesting family of host materials for optically active RE3+ ions because of unique combination of tellurite glass advantages such as good mechanical and chemical resistance, high RE3+ solubility, transparency in visible and IR region and simple synthesis method [17] [18] and favorable properties of fluoride crystallites, e.g. low phonon energy and large energy transfer coefficient between RE3+ ions in the crystal lattice [19] [20]. In case of any oxyfluoride glass-ceramics employed as hosts for optically active RE3+ ions the main requirement is optical transparency. It can be achieved if crystallites diameter does not exceed 30 nm according to [21]. In addition to much smaller size of fluoride nanocrystallites comparing to length of exciting wavelength similar refractive indices of both tellurite glass and fluoride crystal phases result in high transparency in visible and infrared region. Size of nanocrystals can be controlled during preparation process by changing heat treatment conditions, i.e. temperature or duration time [22] [23] but it also depends on glass structure [24]. Among various fluoride nanocrystals strontium fluoride SrF2 is especially attractive in terms of materials for optical applications. It is due to fact that SrF2 exhibits wide bandgap, low phonon energy, low refraction index along with fine mechanical resistivity and relatively low hygroscopic properties [25].
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There is a number of publications related to white light emission generated in tellurite glasses doped with RE3+, for example in [26][27][28][22][29]. However, tellurite glass-ceramics systems containing RE3+ for light emission applications have not been described sufficiently in literature yet. More importantly materials with fluoride nanocrystals embedded into tellurite glass matrix require significant attention as promising hosts for RE3+ applications. Miguel et al. [24] reported tellurite glass-ceramics containing ZnF2:Er3+ nanocrystals with average size of 45+/-10 nm for up-conversion applications. Similarly, up-conversion processes in tellurite glasses and glass-ceramics have been also investigated by Hou et al. in [30].
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In our previous work [31] we describe the influence of SrF2 nanocrystals on Eu3+ luminescence behavior, i.e. luminescence decay times elongation and changes of red to orange R/O ratio. Present studies are focused on further analysis of structural and luminescence properties of tellurite based glasses and glass-ceramics co-doped with Eu3+ and Dy3+ in various molar ratio. Moreover, white light emission by simultaneous UV excitation of Eu3+ and Dy3+ ions in tellurite glasses and glass-ceramics hosts has been studied in detail in order to review these materials as potential candidates for WLED phosphors. Experimental
Conventional melt quenching technique in air atmosphere has been applied in order to achieve precursor glasses (PG) and after subsequent heat treatment glass-ceramics (GC) with nominal composition (in mol%) 73TeO2-4BaO-3Bi2O3-18SrF2-x Eu2O3 /(2-x)Dy2O3 (denoted as TBBS: x Eu/ (2-x) Dy) have been synthesized (where x= 0.0 - 2.0). At first well mixed high purity raw materials: TeO2 (99.7 %), BaCO3 (99.0 %), Bi5OH(OH)9(NO3)4 (78-82 %), SrF2 (99.0 %), Eu(NO3)3 (99.9 %) and Dy(NO3)3 (99.9 %) were decomposed at approximately 600 °C for 1 h and then melted in porcelain crucibles at 800 °C for 0.5 h with subsequent temperature reduction to 700 °C for another 0.5 h. In the next step, melts were poured onto preheated steel plate and pressed by another steel plate immediately. Glass samples were then cooled 2
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to the room temperature. In order to achieve glass-ceramics glasses were heat treated in 370 °C for 24 h in air atmosphere. Parameters of heat treatment process have been established based on our previous study [31].
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Structural and luminescence properties of obtained glasses and glass-ceramics were examined by employment of various techniques. X-ray diffraction (XRD) patterns have been collected on powder samples on Philips X’PERT PLUS diffractometer with Cu-Kα radiation (λ=0.154 nm). Based on XRD data the average size of crystallites has been calculated. Fourier transform infrared spectroscopy (FTIR) measurements in the mid – infrared range were carried out on Perkin-Elmer Frontier MIR/FIR spectrometer with TGS detector. The measurements were performed on pellet samples from well shredded specimen mixed with potassium bromide KBr in weight ratio (Sample: KBr) 1:100. X-ray Photoelectron Spectroscopy analysis (XPS) was performed using X-ray photoelectron spectrometer (Omnicron NanoTechnology) with 128-channel collector. Investigated samples were pre-cleaned by Ar ion beam. XPS measurements were undertaken in ultra-high vacuum conditions, below 1.1 x 10-8 mBar. Photoelectrons were excited by an Mg-Kα X-ray source with X-ray anode operated at 15 keV and 300 W. TEM measurements have been conducted using a Philips 200CM transmission electron microscope. The powder samples were suspended in water and a drop of the suspension was then deposited on a copper grid.
Results
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Optical absorption measurements were performed using Perkin-Elmer Lambda 35 UV-Vis spectrophotometer. Photoluminescence emission (PL) and excitation (PLE) spectra were collected in a Horiba Yvon spectrofluorometer Fluorolog 3. Both spectra were corrected by the spectral response of the equipment and the intensity of the excitation source. Luminescence decay curves were measured in the same equipment using a pulsed lamp at the characteristic emission wavelength of Eu3+ (611 nm) and Dy3+ (573 nm) with 393 and 390 nm excitation wavelengths, respectively.
X-ray diffraction measurements
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X-ray diffraction measurements were carried out in order to investigate structural properties of precursor glasses and glass-ceramics. Figure 1 shows XRD patterns of TBBS: xEu / (2-x)Dy (PG) and TBBS: xEu / (2-x)Dy (GC) series. In case of PG series only broad amorphous halos are visible suggesting lack of long range order in the samples structure. However, in GC series several sharp diffraction peaks arisen on the amorphous halos may be observed suggesting successful growth of the crystalline phase after heat treatment process in applied conditions. All of the diffraction peaks may be assigned to the International Centre for Diffraction Data JCPDS No. 86 – 2418 SrF2 cubic structure with the space group Fm3m (225). Based on the XRD data of GC series, the average diameter (D) size of SrF2 crystallites in each sample have been calculated from the most intense diffraction peaks (111) in each pattern according to Scherrer’s formula: D = (K λ) / (β cos Θ),
where K = 0.9, λ – wavelength of the Cu Kα radiation, β – structural broadening of peak profiles (in radians) and Θ – diffraction angle. The average diameter sizes of SrF2 crystallites in GC series have been presented in Table X.
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Sample x = 2.0 x = 1.5 x = 1.0 x = 0.5 x = 0.0
Diameter size ± 1 (nm) 23 26 31 30 29
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Table 1. Calculated crystallites size in glass-ceramics (GC) samples heat treated in 370˚C for 24 h.
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It may be concluded that chosen heat treatment procedure parameters lead to formation of SrF2 nanocrystals with average diameter size below 30 nm which is particularly important in terms of optical transparency of glass-ceramics [21].
FTIR measurements
FTIR studies were carried out to examine the PG and GC network’s structure. Obtained results are shown in Figure 2. It is well known that tellurite glasses network is mainly constructed from TeO4 subunits in trigonal bipyramids (tbp). However, addition of structure modifiers leads to formation of TeO3+1 and TeO3 units as a result of creation of non-bridging oxygen [32].
XPS study
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In our study, we found that the main broad band at approximately 510 – 860 cm-1 spectral region is characteristic to tellurite glasses [33] and it may be deconvoluted into three features. The first one at approx. 588 cm-1 is related to Te-O-Te vibrations in TeO4 tbp, whereas the next signal at approx. 684 cm-1 is caused by symmetric stretching vibrations of Te-O bonds in TeO4 tbp unit or Te-O-Te linkages. The latter feature at approx. 775 cm-1 may be the result of Te-O stretching vibrations in TeO3 trigonal pyramids (tp) or TeO3+1 polyhedral group. Such results are consistent with our previous reports [31][22] and other scientist’s work, e.g. [34] [35] on tellurite - based glasses and glass-ceramics.
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XPS measurements were undertaken in order to examine PG and GC material’s structure, especially to analyze the influence of thermal treatment on valence states of elements and chemical bonds in PG and GC samples. In our previous work, we established [31] that heat treatment procedure does not affect other elements such as Eu or Te. Therefore, in this study we focused only on Sr doublet characterization as its character may significantly change after crystallization process. Figure 3 shows exemplary XPS spectra of Sr in TBBS: 2.0 Dy PG and GC samples. Both spectra consist of the Sr 3d spinorbit doublet. Deconvolution of these bands into four separated Gaussian-Lorentzian peaks suggests presence of more than one chemical states of Sr in both PG and GC matrices. Based on literature [25] we assigned red (or 1 and 3) peaks at 133.0 and 135.0 eV to the Sr 3d5/2 and Sr 3d3/2 in Sr-O. On the other hand blue (or 2 and 4) peak’s doublet with energies equaled approximately 133.5 and 135.5 eV can be attributed to the Sr 3d5/2 and Sr 3d3/2 in Sr-F. The intensity ratio of Sr-O doublet decreases from 79 % to 40 % after crystallization whereas intensity ratio of Sr-F doublet increases from 21 % to 60 % at the same time in GC sample. It can be concluded that heat treatment procedure leads to formation of Sr-F bonds at the expense of Sr-O in GC sample which is a result of SrF2 crystallization in glass matrix. Taking into account molar composition of prepared glass-ceramics with 18 mol% of SrF2 together with XPS data it 4
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may be found that approximately 10 mol% of SrF2 turned into crystallites after heat-treatment process. Moreover, results achieved through XPS method are in a good agreement with XRD data presented above and with our previous report.
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TEM study
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TEM measurements have been conducted in order to confirm formation of crystallites embedded in glass matrix as an effect of heat treatment procedure. Figure 4 shows the TEM micrograph corresponding to the TBBS: 2.0 Dy (GC) sample. A significant number of crystallites with average size of approximately 20 – 30 nm have been found homogenously distributed in examined glass – ceramics. TEM results along with XRD patterns and XPS studies confirm successful formation of SrF2 nanocrystals in heat treatment process.
UV-Vis study
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Ultraviolet-visible spectroscopy UV-Vis spectra were undertaken to provide absorption characteristic GC materials. Figure 5 shows UV-Vis spectra of TBBS: xEu/ (2-x)Dy (GC series) in 430 – 550 nm spectral region. Bands at 452 nm may be attributed to the 4f – 4f transitions of Dy3+ (6H15/2→4I15/2) whereas the next peaks at 464, 525 and 532 nm are due to the 4f – 4f transitions of Eu3+ (7F0→5D2,1 and 7 F0,1→5D1, respectively).
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In our previous study we found out that optical transparency of GC samples in TBBS: RE3+ is comparable to the transparency of PG. Therefore formation of SrF2 nanocrystals into glass matrix has no negative effect on optical transparency. According to [21] the maximum size of nanocrystals should be much smaller than light wavelength and does not exceed 30 nm to avoid light scattering including Rayleigh scattering affecting an optical transparency of materials. Moreover crystalline phase should have high site symmetry. In our study the average size of nanocrystalline SrF2 phase does not exceed 30 nm and since SrF2 have large site symmetry it may be concluded that obtained glass-ceramics exhibit good optical transparency as expected.
Photoluminescence study
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Photoluminescence measurements have been conducted to gain a detailed information about emission and excitation characteristic of Eu3+ and Dy3+, specifically to study the possibility of white light emission by subsequent excitation of both Eu3+ and Dy3+ ions. Moreover influence of SrF2 nanocrystals formation in glass matrix on luminescence properties of materials has been examined in detail. Normalized excitation (PLE) and emission (PL) spectra of TBBS: 2.0 Eu and TBBS: 2.0 Dy (PG) and (GC) are presented in Figure 6. Observation of TBBS: 2.0 Eu (PG and GC) at λem= 611 nm resulted in several excitation bands at: 363, 366, 376, 382, 393, 401 and 415 nm that were visible in PLE spectra. These bands may be assigned to 4f - 4f transitions of Eu3+: 7F0 ⟶ 5D4, 5G4,2, 7L6, 5D3, respectively. In PLE spectra of TBBS: 2.0 Dy (PG and GC) at λem= 573 nm observation one can distinguish several bands at 351, 365, 387 and 425 nm that correspond to 4f – 4f transitions of Dy3+: 6H15/2 → 6P7/2, 4P7/2, 4I13/2 and 4 G11/2. It is clearly noticeable that after heat treatment intensity of excitation bands in UV spectral region increased significantly in comparison to PG curves in TBBS: 2.0 Eu and TBBS: 2.0 Dy (GC) samples. Such behavior is more likely caused by change of RE3+ surroundings after SrF2 nanocrystals formation and thus more effective excitation of RE3+ may be achieved in GC samples. Although tellurite glasses 5
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have relatively low phonon energy (around 700 cm-1) among glass matrices presence of strontium fluoride nanocrystals with considerably smaller phonon energy (approximately 300 cm-1 [36]) leads to further decrease of phonon energy in RE3+ surroundings and thus supports effective excitation and emission of these ions [37]. Similar phenomenon has been observed and described before [38]. Emission spectra of TBBS: 2.0 Eu and TBBS: 2.0 Dy (PG and GC) have been recorded for λexc = 393 and 387 nm, respectively. Emission spectra of TBBS: 2.0 Eu (PG and GC) consist of several bands originating from Eu3+ radiative transitions at 511 nm (5D2⟶7F3), 534 (5D1⟶7F1), 554 nm (5D1⟶7F2), 578 nm (5D0⟶7F0), 590 nm (5D0⟶7F1), 611 nm (5D0⟶7F2), 653 nm (5D0⟶7F3) and 700 nm (5D0⟶7F4). It is worth noticing that presence of transitions from higher excited states (e.g. 5D1,2) suggests low phonon environment of Eu3+ ions in studied materials. When applying λexc = 387 nm excitation, bands at 480, 573 and 662 nm due to Dy3+: 4F9/2⟶6HJ (J = 15/2, 13/2, 11/2) radiative transitions can be distinguished in emission spectra of TBBS: 2.0 Dy (PG and GC). It is well known that luminescence of RE3+ is strongly dependent on the surroundings. Comparing TBBS: 2.0 Eu and TBBS: 2.0 Dy (PG) and (GC) emission spectra it can be seen that red-to-orange (R/O) intensity ratio of Eu3+: 5D0→7F2 electric dipole (R) to 5D0→7F1 magnetic dipole 4 (O) transitions as well as yellow-to-blue (Y/B) ratio of Dy3+ F9/2⟶6H13/2 electric dipole (Y) to 4 6 F9/2⟶ H15/2 magnetic dipole (B) transitions change after crystallization process. The 5D0→7F2 and 4 F9/2⟶6H13/2 electric dipole transitions of Eu3+ and Dy3+ are known to be hypersensitive to the local symmetry of Eu3+ [39] [35] [40]. Decrease of R/O and Y/B ratio after crystallization of SrF2 in glass matrix suggests increasing symmetry of the ligand field of RE3+ ions and thus incorporation of Eu3+ and Dy3+ into crystal lattice of SrF2. In our study we found that R/O ratio changes from 2.81 in precursor glass sample to 2.30 in glass-ceramics whereas Y/B decreases from 1.50 in PG to 1.20 in GC. Similar effect have been described in e.g. [15] and [41].
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Excitation spectra of TBBS: x Eu/ (2-x) Dy (x= 1.5; 1.0 and 0.5) obtained with λem= 611 nm and λem= 573 nm are presented in Figure 7 (a) and 7 (b), respectively. Employment of λem= 573 nm results in appearance of several bands due to Dy3+ excitation similar to these presented in Figure 6 whereas after observation with λem= 611 nm Eu3+ transitions can be observed in examined spectra. Based on the results presented in Figure 7 (a) and (b) two excitation wavelengths 390 and 393 nm have been chosen in order to investigate simultaneous color – tunable white light emission of Eu3+ and Dy3+. Emission spectra of TBBS: x Eu/ (2-x) Dy PG and GC series for λexc=390 and 393 nm are shown in Figure 8 (a) and (b), respectively. After excitation by λexc=390 nm wavelength emission bands originating predominantly from Dy3+ ions are observable at 489, 573 and 662 nm. However, with increasing x value from 0.5 to 1.5 bands due to Eu3+ radiative transitions at 534, 590, 611, 653 and 700 nm grow significantly. On the other hand when comparing spectra of PG and GC materials intensity of main 4F9/2⟶6H15/2 (Dy3+) and 5D0→7F1,2 (Eu3+) transitions increase after heat treatment suggesting that most likely Eu3+ and Dy3+ incorporate into SrF2 crystal lattice with lower phonon energies than glass matrix. Therefore radiative transitions of Eu3+ and Dy3+ become more effective in GC comparing to PG. Emission spectra achieved with λexc=393 nm wavelength consist of both Eu3+ at and Dy3+ bands. For higher x values emission originating from Eu3+ ions is more prominent however with decreasing Eu3+ concentration with subsequent increasing of Dy3+ amount emission from Dy3+ becomes more significant. In this case however intensity of Dy3+: 4 F9/2⟶6H15/2, 13/2 transitions decreases slightly after heat treatment process which may be caused by e.g. energy transfer from Dy3+ to Eu3+ ions. Similar results have been obtained in for example [27].
CIE chromaticity study In order to evaluate the emission color of the Eu3+ and Dy3+ co-doped tellurite glasses and glassceramics containing SrF2 nanocrystals the International Commission on Illumination CIE chromaticity coordinates (x and y) and correlated color temperature CCT have been calculated (Table 2) and presented in a form of CIE chromaticity diagrams (Figure 9 (a) and (b)). The emission color may be changed by using different excitation wavelength or by varying the Eu3+ to Dy3+ molar ratio. In present study under 6
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390 nm excitation (Fig. 9 (a)) all samples containing Eu3+ and Dy3+ ions exhibit warm to neutral white luminescence with x value changes from 1.5 (warm white [39]) to 0.0 (neutral white [42]). After employment of 393 nm excitation (Fig. 9 (b)) emission color of samples (x = 2.0; 1.5 and 1.0) was found to be red, orange and orange – pink, respectively but after for x value equal to 0.5 it turned out that examined system emits warm white light. Therefore, addition of 0.5 mol% of Eu3+ to 1.5 mol% of Dy3+ results in change of the overall light color from neutral to warm white. In present study we additionally examined influence of crystallization of SrF2 on overall emission color of TBBS: x Eu/ (2-X) Dy systems. SrF2 nanocrystallites presence in GC samples leads to quite significant change in CCTs when applying both 390 and 393 nm excitation wavelengths. Decreasing of CCTs in these cases is connected to change of red-to-orange R/O and yellow-to-blue Y/B ratio of Eu3+ and Dy3+ emission bands, respectively as a result of SrF2 nanocrystals formation and thus, symmetry change of RE3+ local environment.
Table 2. Calculated x, y and CCT for λexc = 390 and 393 nm excitation.
PG GC x = 1.5 PG GC x = 1.0 PG GC x = 0.5 PG GC x = 0.0* PG (λexc = 387 nm) GC
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x = 2.0
λexc = 390 nm CCT (K) λexc = 393 nm CCT (K) x y x y 0.61 0.37 1289 0.62 0.37 1223 0.41 0.39 3354 0.57 0.38 1537 0.42 0.38 3027 0.57 0.37 1490 0.37 0.39 4272 0.48 0.38 2203 0.39 0.39 3894 0.52 0.38 1833 0.36 0.40 4658 0.44 0.39 2858 0.36 0.39 4572 0.46 0.38 2484 0.36 0.41 4842 0.35 0.39 5097 -
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In order to get better insight into the luminescence kinetics the luminescence decay curves associated with Dy3+: 4F9/2→6H13/2 (λexc= 387 nm) and Eu3+: 5D2→7F0 (λexc= 393 nm) transitions have been measured and presented in Figure 10 (a) and (b), respectively. Decay times of RE3+ particular transitions are known to depend on several factors such as host matrix or RE3+ concentration. It was found that luminescence decays from both 5D0 and 4F9/2 states of Eu3+ and Dy3+ respectively can be described as twoexponential decays according to the following equation: I(t) = A1 exp (-t/τ1) + A2 exp (-t/τ2)
where A1 and A2 are amplitudes of respective decay components, τ1 and τ2 are short and long luminescence lifetime components contributing to the average lifetime 〈 τavg 〉 [43]. τ1 is related with RE3+ in low symmetry surroundings whereas τ2 is due to RE3+ in higher symmetry of the crystal field . The average lifetimes can be calculated using following equation: 〈τavg〉= [
t I t dt]/[
I t dt] = (τ12 I1 + τ22 I2)/( τ1 I1 + τ2 I2)
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Table 3. Fitting parameters of the luminescence decays for TBBS: x Eu / (2-x) Dy (GC) systems. A1 (%) A2 (%) 29.5 70.5 50.5 49.5 56.0 44.0 62.3 37.7 -
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x=2.0 x=1.5 x=1.0 x=0.5 x=0.0
Eu3+: 5D2→7F0 Dy3+: 4F9/2→6H13/2 τ1 (ms) τ2 (ms) 〈 τavg 〉 (ms) A1 (%) A2 (%) τ1 (µs) τ2 (µs) 〈 τavg 〉(µs) 0.431 1.039 0.950 0.375 0.890 0.735 63.3 36.7 0.249 0.066 0.225 0.320 0.802 0.640 63.6 36.4 0.210 0.053 0.190 0.332 0.866 0.659 62.9 37.0 0.195 0.044 0.177 63.6 36.4 0.162 0.035 0.148
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The average lifetimes of Eu3+: 5D2→7F0 and Dy3+: 4F9/2→6H13/2 transitions have been found to be strongly related to Eu3+ to Dy3+ concentration ratio. As shown in Table 3. The average lifetime of 5D2→7F0 transition decreases with decreasing Eu3+ content. In this case, slow decay component reached 1.039 ms for x = 2.0 with fast decay component equaled 0.431 ms at the same time. Values of τ1 and τ2 decrease for the lower values of x however the influence of fast decay component increase systematically with smaller Eu3+ content. Thus, the average lifetime of Eu3+: 5D2→7F0 〈 τavg 〉 changes from 0.950 to 0.659 ms with decreasing of x. Such results are with a good agreement with many scientific researches, e.g. [40] [44]. Additionally, the average luminescence lifetime of Dy3+: 4F9/2→6H13/2 transition increases with decreasing Dy3+ content from 0.148 to 0.225 ms. This fact may be also explained as follows: according to much research, i.e. [45] decay lifetimes of Dy3+ ions strongly depend on Dy3+ concentration.
Judd - Ofelt analysis
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To fully describe luminescence behavior of prepared glasses and glass- ceramics Judd – Ofelt analysis have been perform. Especially, the effect of crystallization process onto emission has been investigated and for this purpose TBBS: 2Eu (PG) and (GC) were chosen as those possess unique properties among all rare earth ions. Judd - Ofelt parameters Ωλ were calculated based on luminescence emission spectra using JOES application software. Detailed information regarding software and calculations may be found in [46] .Other radiative properties as radiative transition probabilities A, total radiative transition probability AT, the fluorescence branching ratio β, the stimulated emission cross section σ and asymmetry ratio R/0 were also calculated based on emission spectra and JO parameters.
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Because of 5D0→7F1 pure magnetic dipole transition of Eu3+ is independent on the host matrix it can be calculated exactly and used as a reference for other electric dipole transitions of Eu3+ originating from 5D0 excited level. Value of 5D0→7F1 magnetic dipole transition strength DMD equals 9.6x10-42 esu2cm2. Reduced matrix elements │J = 0││Uλ││J’│2 simplifies further calculations of JO parameters due to values different than 0 only for J’ = 2, 4, 6, where λ = 2, 4, 6, respectively [46]. For those λ: U2= 0.0031, U4= 0.0023, U6= 0.0002. Dipole strengths for electric dipole transitions are given by: DλED = e2ΩλUλ
Whereas the integrated 5D0→7Fκ transition intensity is given by: Jκ = ∫Iκ (ν͂) dν͂ Radiative transition probabilities A may be calculated using JO parameters: Aλ = [(64π4ν͂3λ)/3h]*[(nλ(nλ2+2)2/9]*DλED, A1 = (64π4ν͂31)/3h)*n31DMD
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where ν͂κ – the average wavenumber in cm-1, nλ is a refractive index dependent on the wavelength and h – Plack constant, h= 6.63x10-27 erg s. JO parameters may be calculated from the ratio of the integrated intensities of transitions 5D0→7Fλ and MD transition 5D0→7F1 using formula:
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Ωλ= [(DMD ν͂31 )/(e2 ν͂3λUλ)] [(9n31)/(nλ(n2λ+2)2] [Jλ/J1]
Results of JO parameters calculations are given in Table 4.
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Table 4. Calculated Judd – Ofelt parameters, radiative transition probabilities, branching ratios, emission cross sections, total radiative transitions probabilities and asymmetry ratios of TBBS: 2Eu precursor glass (PG) and glass-ceramic (GC) samples. TBBS: 2Eu (PG) 3.95 1.32
TBBS: 2Eu (GC) 3.21 1.37
172 504 80
171 407 82
0.23 (0.23) 0.65 (0.67) 0.09 (0.11)
0.26 (0.26) 0.60 (0.62) 0.11 (0.12)
4.42 18.40 5.00
4.55 14.04 5.00
756 2.78
660 2.27
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JO parameter (x 10-20 cm2) Ω2 Ω4 Radiative transition probabilities (s-1) A (5D0→7F1) A (5D0→7F2) A (5D0→7F4) Branching ratio (-) Experimental (Theoretical) β (5D0→7F1) β (5D0→7F2) β (5D0→7F4) Emission cross section (x 10-22 cm2) σ (5D0→7F1) σ (5D0→7F2) σ (5D0→7F4) Total Radiative transition probability AT (s-1) Asymmetry ratio R/O (-)
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High value of Ω2 compare to Ω4 for both PG and GC sample suggests Eu3+ ions occupy mostly low symmetry sites. Ω2 parameter is known to be structure sensitive and it depends on covalence of RE3+ bonds. Since Eu – O bond in glass matrix is highly covalent calculated Ω2 is also relatively high and such results are comparable with literature, i.e. [47]. On the other hand Ω4 and Ω6 are related to rigidity of glasses [48] and Ω4 seems to depend also on long range effects related with crystal lattice [13]. Thus, crystallization process results in increase of Ω4 (from 1.32 in PG to 1.37 x 10-20 cm2 for GC) and simultaneous decrease of Ω2 (from 3.95 to 3.21 x 10-20 cm2 for PG and GC, respectively). This is most probably related with grown of SrF2 nanocystals and since Eu3+ may replace Sr2+ ions in crystal lattice with higher symmetry sites comparing to glass matrix value of Ω4 parameter increases slightly after heat treatment. Observed Ω2 to Ω4 trend is typical for glass and glass-ceramics materials. Because 5D0→7F6 transition band is located in NIR range it was not measureable by the spectrofluorometer thus Ω6 value has not been calculated.
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Radiative transition probabilities A( 5D0→7F1) obtained for PG and GC remained almost unchanged whereas value of A(5D0→7F2) decreased significantly from 504 s-1 to 407 s-1 after crystallization which is in the good agreement with PL results as 5D0→7F2 transition itself remains predominant in the emission spectra of PG and GC materials. AR values calculated for PG (766 s-1) and GC (660 s-1) confirm faster depopulation of 5D0 excited state and shorter Eu3+ lifetimes in PG than in GC which has been presented in previous section. The β value for specific transition represents occurrence of particular electron transition as a result of number of photons emitted from a specific energy level. The highest β values have been obtained for 5D0→7F2 transition: 0.65 and 0.60 for PG and GC, respectively. Experimental results are in a good agreement with theoretical values which confirms accuracy of analysis. Values of σ were found the highest for 5D0→7F2 transition of Eu3+ in both studied materials: 18.40 (PG) and 14.04 (x 10-22cm2) (GC). Moreover, asymmetry ratio R/O has been found to decrease from 2.78 to 2.27 after crystallization process confirming results presented in previous section.
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Acknowledgements
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Eu3+/Dy3+ doped oxyfluoride glasses and glass-ceramics containing SrF2 nanocrystals have been successfully synthesized using simple melt-quenching and subsequent heat treatment procedure. XRD and TEM results confirmed formation of SrF2 nanocrystalline phase in the glass matrix whereas based on XPS study increase of signals due to Sr-F bonds has been found. Based on luminescence study it can be concluded that Eu3+ and Dy3+ ions incorporate into SrF2 crystal lattice due to decrease of R/O and Y/B intensity ratio which is related to change the surroundings of RE3+ into one with higher symmetry site in SrF2. Moreover, due to significantly lower phonon energy of strontium fluoride phase than tellurite glass matrix, luminescence excitation and emission intensity increase have been observed. Optical properties along with calculated JO and optical parameters confirm that oxyfluoride glasses and glass-ceramics are excellent host for RE3+ ions. Simultaneous emission originating from both, Eu3+ and Dy3+ ions have been achieved by using 390 and 393 nm excitation wavelengths and as a result, color tunable white light emission have been obtained. The overall emission color changes from neutral white for TBBS: 2.0 Dy (PG and GC) to warm white with adding of Eu3+ ions and reaches red when only Eu3+ ions are present in PG and GC.
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This research has been supported by the grant 2015/17/B/ST5/03143 financed by National Science Centre (A.S.). This work was supported by CSIC projects PIC2016FR1 and PIE 201560E056.
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Figure Captions
Figure 1. XRD patterns of TBBS: xEu / (2-x) Dy (precursor glass: PG – bottom) and (glass-ceramics: GC heat treated in 370˚C for 24 h). Figure 2. FTIR spectra of TBBS: 2.0 Dy (PG and GC) (left) and deconvolution of GC spectrum into three signals (right).
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Figure 3. XPS spectra of Sr in TBBS: 2.0 Dy (PG and GC, top and bottom respectively). Figure 4.TEM image of TBBS: 2.0 Dy (GC) glass-ceramic sample heat treated in 370˚C for 24 h. Figure 5. UV-Vis absorbance spectra of TBBS: xEu/ (2-x) Dy (GC) serie.
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Figure 6. Excitation (PLE) and emission (PL) spectra of TBBS: 2Eu (PG and GC) and TBBS: 2Dy (PG and GC). Figure 7 (a) and (b). Excitation (PLE) spectra of TBBS: x Eu/ (2-x) Dy (x= 1.5; 1.0 and 0.5) PG and GC.
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Figure 8 (a) and (b). Emission (PL) spectra of TBBS: xEu/ (2-x) Dy (x=0.5-1.5) PG and GC for λexc=390 and 393 nm. Figure 9 (a) and (b). CIE chromaticity diagrams of TBBS: xEu/ (2-x)Dy (x=0.5-1.5) for λexc=390 nm (left) and λexc=393 nm (right) excitation wavelength. Figure 10 (a) and (b). Luminescence decay curves of TBBS: 2.0 Dy (left) and TBBS: 2.0 Eu (right) achieved with λexc = 387 and 393 nm excitation, respectively.
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Transparent oxyfluoride glass-ceramics with SrF2 nanocrystals have been prepared Color-tunable white light emission has been obtained by adjusting Eu3+/ Dy3+ molar ratio SrF2 crystallization influence onto luminescence of Eu3+ and Dy3+ has been studied Judd-Ofelt parameters have been calculated using JOES application software
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• • • •
S0925-8388(19)32503-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.07.017
Reference:
JALCOM 51304
To appear in:
Journal of Alloys and Compounds
Received Date: 15 April 2019 Revised Date:
30 June 2019
Accepted Date: 2 July 2019
Please cite this article as: M. Walas, M. Lisowska, T. Lewandowski, A.I. Becerro, M. Łapiński, A. Synak, W. Sadowski, B. Kościelska, From structure to luminescence investigation of oxyfluoride transparent glasses and glass-ceramics doped with Eu3+/Dy3+ ions, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.07.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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From structure to luminescence investigation of oxyfluoride transparent glasses and glass-ceramics doped with Eu3+/Dy3+ ions
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*Michalina Walas1, Marta Lisowska1, Tomasz Lewandowski1, Ana I. Becerro2, Marcin Łapiński1, Anna Synak3, Wojciech Sadowski1 and Barbara Kościelska1 1
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Faculty of Applied Physics and Mathematics, Department of Solid State Physics, Gdańsk University of Technology, ul. Gabriela Narutowicza 11/12, 80-233 Gdańsk, Poland 2 Instituto de Ciencia de Materiales de Sevilla (CSIC-US), c/Américo Vespucio, 49, 41092 Sevilla, Spain 3 Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdańsk, ul. Wita Stwosza 57/246, 80-952 Gdańsk, Poland *corresponding author: [email protected]; [email protected]
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Keywords: white light; Eu3+; Dy3+; glasses; glass-ceramics;
Abstract
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Glasses and glass-ceramics with nominal composition 73 TeO2- 4BaO- 3Bi2O3-18SrF2-2RE2O3 (where RE=Eu, Dy) have been synthesized by conventional melt-quenching technique and subsequent heat treatment at 370˚C for 24 hours in air atmosphere. Various Eu3+ to Dy3+ molar ratio have been applied to investigate luminescence properties in both glass and glass-ceramic matrices. Especially, white light emission through simultaneous excitation of Eu3+ and Dy3+ has been studied in detail. Influence of crystalline SrF2 phase on luminescence kinetics has been determined by luminescence decay time measurements. Presence of crystalline SrF2 phase has been confirmed by X-ray diffraction technique XRD and transmission electron microscopy TEM. X-ray photoelectron spectroscopy XPS and Fourier-transform infrared spectroscopy FTIR have been applied to get further insight into structural properties of glass and glass-ceramic materials. Color tunable white light emission has been obtained using UV excitation. Influence of the SrF2 crystallization on luminescence properties of prepared materials have been described in detail. Moreover, luminescence properties and especially emission color dependence on the Eu3+ to Dy3+ molar ratio have been exhaustively studied. Color-tunable white light emission has been observed as a result of simultaneous radiative transition of both, Eu3+ and Dy3+ ions when applying UV excitation. Judd – Ofelt and other optical parameters have been calculated based on luminescence emission spectra. Achieved results confirm that tellurite glass-ceramics containing SrF2 nanocrystals are good hosts for RE3+ ions and they can be considered as new phosphors for white light emitting diodes WLEDs.
Introduction
Studies on light emitting sources have attracted considerable attention during last years because of growing requirements in terms of reliability, efficiency and environment friendliness of materials [1]. Currently the most popular light sources seem to be white light emitting diodes WLED which replaced traditional fluorescent lamps or bulks. Typical WLEDs combine YAG: Ce yellow phosphor with employment of blue chip. However, the main disadvantage of such systems is the lack of red emitter resulting in low color rendering index CRI, high correlated temperature CCT [2] and also poor thermal stability of organic epoxy resin or silicon component [3]. Thus, inorganic materials i.e. glasses or glassceramics containing optically active dopants such as RE3+ ions gained a considerable attention in last few 1
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years as promising materials for WLED phosphors and other optical applications [4]. Comparing to traditional glasses, glass-ceramics have number of advantages when consider those as matrices for RE3+ ions. For example, presence of low-phonon crystalline phase [5][6][7][8] [9][10]in glass matrix promotes RE3+ radiative transitions, good optical quality etc. keeping all advantages of glass matrix at the same time [11]. Incorporation of RE3+ into crystal lattice prevents from luminescence concentration quenching due to clusters formation [10][12][13]. Moreover, this also promotes RE3+ energy transfers between ions and from crystals to ions. White light emission originating from RE3+ ions have been obtained i.e. in Tm3+/Tb3+/Eu3+ triply-doped phosphate glass-ceramics [14], for Eu3+ doped fluorozirconate glass-ceramics containing BaCl2 nanocrystals [15] or Dy3+/Sm3+/Tb3+ doped oxyfluoride glass-ceramics [16].
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Among various inorganic phosphors, tellurite based glass-ceramics containing fluoride nanocrystals seem to be especially interesting family of host materials for optically active RE3+ ions because of unique combination of tellurite glass advantages such as good mechanical and chemical resistance, high RE3+ solubility, transparency in visible and IR region and simple synthesis method [17] [18] and favorable properties of fluoride crystallites, e.g. low phonon energy and large energy transfer coefficient between RE3+ ions in the crystal lattice [19] [20]. In case of any oxyfluoride glass-ceramics employed as hosts for optically active RE3+ ions the main requirement is optical transparency. It can be achieved if crystallites diameter does not exceed 30 nm according to [21]. In addition to much smaller size of fluoride nanocrystallites comparing to length of exciting wavelength similar refractive indices of both tellurite glass and fluoride crystal phases result in high transparency in visible and infrared region. Size of nanocrystals can be controlled during preparation process by changing heat treatment conditions, i.e. temperature or duration time [22] [23] but it also depends on glass structure [24]. Among various fluoride nanocrystals strontium fluoride SrF2 is especially attractive in terms of materials for optical applications. It is due to fact that SrF2 exhibits wide bandgap, low phonon energy, low refraction index along with fine mechanical resistivity and relatively low hygroscopic properties [25].
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There is a number of publications related to white light emission generated in tellurite glasses doped with RE3+, for example in [26][27][28][22][29]. However, tellurite glass-ceramics systems containing RE3+ for light emission applications have not been described sufficiently in literature yet. More importantly materials with fluoride nanocrystals embedded into tellurite glass matrix require significant attention as promising hosts for RE3+ applications. Miguel et al. [24] reported tellurite glass-ceramics containing ZnF2:Er3+ nanocrystals with average size of 45+/-10 nm for up-conversion applications. Similarly, up-conversion processes in tellurite glasses and glass-ceramics have been also investigated by Hou et al. in [30].
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In our previous work [31] we describe the influence of SrF2 nanocrystals on Eu3+ luminescence behavior, i.e. luminescence decay times elongation and changes of red to orange R/O ratio. Present studies are focused on further analysis of structural and luminescence properties of tellurite based glasses and glass-ceramics co-doped with Eu3+ and Dy3+ in various molar ratio. Moreover, white light emission by simultaneous UV excitation of Eu3+ and Dy3+ ions in tellurite glasses and glass-ceramics hosts has been studied in detail in order to review these materials as potential candidates for WLED phosphors. Experimental
Conventional melt quenching technique in air atmosphere has been applied in order to achieve precursor glasses (PG) and after subsequent heat treatment glass-ceramics (GC) with nominal composition (in mol%) 73TeO2-4BaO-3Bi2O3-18SrF2-x Eu2O3 /(2-x)Dy2O3 (denoted as TBBS: x Eu/ (2-x) Dy) have been synthesized (where x= 0.0 - 2.0). At first well mixed high purity raw materials: TeO2 (99.7 %), BaCO3 (99.0 %), Bi5OH(OH)9(NO3)4 (78-82 %), SrF2 (99.0 %), Eu(NO3)3 (99.9 %) and Dy(NO3)3 (99.9 %) were decomposed at approximately 600 °C for 1 h and then melted in porcelain crucibles at 800 °C for 0.5 h with subsequent temperature reduction to 700 °C for another 0.5 h. In the next step, melts were poured onto preheated steel plate and pressed by another steel plate immediately. Glass samples were then cooled 2
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to the room temperature. In order to achieve glass-ceramics glasses were heat treated in 370 °C for 24 h in air atmosphere. Parameters of heat treatment process have been established based on our previous study [31].
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Structural and luminescence properties of obtained glasses and glass-ceramics were examined by employment of various techniques. X-ray diffraction (XRD) patterns have been collected on powder samples on Philips X’PERT PLUS diffractometer with Cu-Kα radiation (λ=0.154 nm). Based on XRD data the average size of crystallites has been calculated. Fourier transform infrared spectroscopy (FTIR) measurements in the mid – infrared range were carried out on Perkin-Elmer Frontier MIR/FIR spectrometer with TGS detector. The measurements were performed on pellet samples from well shredded specimen mixed with potassium bromide KBr in weight ratio (Sample: KBr) 1:100. X-ray Photoelectron Spectroscopy analysis (XPS) was performed using X-ray photoelectron spectrometer (Omnicron NanoTechnology) with 128-channel collector. Investigated samples were pre-cleaned by Ar ion beam. XPS measurements were undertaken in ultra-high vacuum conditions, below 1.1 x 10-8 mBar. Photoelectrons were excited by an Mg-Kα X-ray source with X-ray anode operated at 15 keV and 300 W. TEM measurements have been conducted using a Philips 200CM transmission electron microscope. The powder samples were suspended in water and a drop of the suspension was then deposited on a copper grid.
Results
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Optical absorption measurements were performed using Perkin-Elmer Lambda 35 UV-Vis spectrophotometer. Photoluminescence emission (PL) and excitation (PLE) spectra were collected in a Horiba Yvon spectrofluorometer Fluorolog 3. Both spectra were corrected by the spectral response of the equipment and the intensity of the excitation source. Luminescence decay curves were measured in the same equipment using a pulsed lamp at the characteristic emission wavelength of Eu3+ (611 nm) and Dy3+ (573 nm) with 393 and 390 nm excitation wavelengths, respectively.
X-ray diffraction measurements
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X-ray diffraction measurements were carried out in order to investigate structural properties of precursor glasses and glass-ceramics. Figure 1 shows XRD patterns of TBBS: xEu / (2-x)Dy (PG) and TBBS: xEu / (2-x)Dy (GC) series. In case of PG series only broad amorphous halos are visible suggesting lack of long range order in the samples structure. However, in GC series several sharp diffraction peaks arisen on the amorphous halos may be observed suggesting successful growth of the crystalline phase after heat treatment process in applied conditions. All of the diffraction peaks may be assigned to the International Centre for Diffraction Data JCPDS No. 86 – 2418 SrF2 cubic structure with the space group Fm3m (225). Based on the XRD data of GC series, the average diameter (D) size of SrF2 crystallites in each sample have been calculated from the most intense diffraction peaks (111) in each pattern according to Scherrer’s formula: D = (K λ) / (β cos Θ),
where K = 0.9, λ – wavelength of the Cu Kα radiation, β – structural broadening of peak profiles (in radians) and Θ – diffraction angle. The average diameter sizes of SrF2 crystallites in GC series have been presented in Table X.
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Sample x = 2.0 x = 1.5 x = 1.0 x = 0.5 x = 0.0
Diameter size ± 1 (nm) 23 26 31 30 29
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Table 1. Calculated crystallites size in glass-ceramics (GC) samples heat treated in 370˚C for 24 h.
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It may be concluded that chosen heat treatment procedure parameters lead to formation of SrF2 nanocrystals with average diameter size below 30 nm which is particularly important in terms of optical transparency of glass-ceramics [21].
FTIR measurements
FTIR studies were carried out to examine the PG and GC network’s structure. Obtained results are shown in Figure 2. It is well known that tellurite glasses network is mainly constructed from TeO4 subunits in trigonal bipyramids (tbp). However, addition of structure modifiers leads to formation of TeO3+1 and TeO3 units as a result of creation of non-bridging oxygen [32].
XPS study
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In our study, we found that the main broad band at approximately 510 – 860 cm-1 spectral region is characteristic to tellurite glasses [33] and it may be deconvoluted into three features. The first one at approx. 588 cm-1 is related to Te-O-Te vibrations in TeO4 tbp, whereas the next signal at approx. 684 cm-1 is caused by symmetric stretching vibrations of Te-O bonds in TeO4 tbp unit or Te-O-Te linkages. The latter feature at approx. 775 cm-1 may be the result of Te-O stretching vibrations in TeO3 trigonal pyramids (tp) or TeO3+1 polyhedral group. Such results are consistent with our previous reports [31][22] and other scientist’s work, e.g. [34] [35] on tellurite - based glasses and glass-ceramics.
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XPS measurements were undertaken in order to examine PG and GC material’s structure, especially to analyze the influence of thermal treatment on valence states of elements and chemical bonds in PG and GC samples. In our previous work, we established [31] that heat treatment procedure does not affect other elements such as Eu or Te. Therefore, in this study we focused only on Sr doublet characterization as its character may significantly change after crystallization process. Figure 3 shows exemplary XPS spectra of Sr in TBBS: 2.0 Dy PG and GC samples. Both spectra consist of the Sr 3d spinorbit doublet. Deconvolution of these bands into four separated Gaussian-Lorentzian peaks suggests presence of more than one chemical states of Sr in both PG and GC matrices. Based on literature [25] we assigned red (or 1 and 3) peaks at 133.0 and 135.0 eV to the Sr 3d5/2 and Sr 3d3/2 in Sr-O. On the other hand blue (or 2 and 4) peak’s doublet with energies equaled approximately 133.5 and 135.5 eV can be attributed to the Sr 3d5/2 and Sr 3d3/2 in Sr-F. The intensity ratio of Sr-O doublet decreases from 79 % to 40 % after crystallization whereas intensity ratio of Sr-F doublet increases from 21 % to 60 % at the same time in GC sample. It can be concluded that heat treatment procedure leads to formation of Sr-F bonds at the expense of Sr-O in GC sample which is a result of SrF2 crystallization in glass matrix. Taking into account molar composition of prepared glass-ceramics with 18 mol% of SrF2 together with XPS data it 4
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may be found that approximately 10 mol% of SrF2 turned into crystallites after heat-treatment process. Moreover, results achieved through XPS method are in a good agreement with XRD data presented above and with our previous report.
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TEM study
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TEM measurements have been conducted in order to confirm formation of crystallites embedded in glass matrix as an effect of heat treatment procedure. Figure 4 shows the TEM micrograph corresponding to the TBBS: 2.0 Dy (GC) sample. A significant number of crystallites with average size of approximately 20 – 30 nm have been found homogenously distributed in examined glass – ceramics. TEM results along with XRD patterns and XPS studies confirm successful formation of SrF2 nanocrystals in heat treatment process.
UV-Vis study
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Ultraviolet-visible spectroscopy UV-Vis spectra were undertaken to provide absorption characteristic GC materials. Figure 5 shows UV-Vis spectra of TBBS: xEu/ (2-x)Dy (GC series) in 430 – 550 nm spectral region. Bands at 452 nm may be attributed to the 4f – 4f transitions of Dy3+ (6H15/2→4I15/2) whereas the next peaks at 464, 525 and 532 nm are due to the 4f – 4f transitions of Eu3+ (7F0→5D2,1 and 7 F0,1→5D1, respectively).
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In our previous study we found out that optical transparency of GC samples in TBBS: RE3+ is comparable to the transparency of PG. Therefore formation of SrF2 nanocrystals into glass matrix has no negative effect on optical transparency. According to [21] the maximum size of nanocrystals should be much smaller than light wavelength and does not exceed 30 nm to avoid light scattering including Rayleigh scattering affecting an optical transparency of materials. Moreover crystalline phase should have high site symmetry. In our study the average size of nanocrystalline SrF2 phase does not exceed 30 nm and since SrF2 have large site symmetry it may be concluded that obtained glass-ceramics exhibit good optical transparency as expected.
Photoluminescence study
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Photoluminescence measurements have been conducted to gain a detailed information about emission and excitation characteristic of Eu3+ and Dy3+, specifically to study the possibility of white light emission by subsequent excitation of both Eu3+ and Dy3+ ions. Moreover influence of SrF2 nanocrystals formation in glass matrix on luminescence properties of materials has been examined in detail. Normalized excitation (PLE) and emission (PL) spectra of TBBS: 2.0 Eu and TBBS: 2.0 Dy (PG) and (GC) are presented in Figure 6. Observation of TBBS: 2.0 Eu (PG and GC) at λem= 611 nm resulted in several excitation bands at: 363, 366, 376, 382, 393, 401 and 415 nm that were visible in PLE spectra. These bands may be assigned to 4f - 4f transitions of Eu3+: 7F0 ⟶ 5D4, 5G4,2, 7L6, 5D3, respectively. In PLE spectra of TBBS: 2.0 Dy (PG and GC) at λem= 573 nm observation one can distinguish several bands at 351, 365, 387 and 425 nm that correspond to 4f – 4f transitions of Dy3+: 6H15/2 → 6P7/2, 4P7/2, 4I13/2 and 4 G11/2. It is clearly noticeable that after heat treatment intensity of excitation bands in UV spectral region increased significantly in comparison to PG curves in TBBS: 2.0 Eu and TBBS: 2.0 Dy (GC) samples. Such behavior is more likely caused by change of RE3+ surroundings after SrF2 nanocrystals formation and thus more effective excitation of RE3+ may be achieved in GC samples. Although tellurite glasses 5
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have relatively low phonon energy (around 700 cm-1) among glass matrices presence of strontium fluoride nanocrystals with considerably smaller phonon energy (approximately 300 cm-1 [36]) leads to further decrease of phonon energy in RE3+ surroundings and thus supports effective excitation and emission of these ions [37]. Similar phenomenon has been observed and described before [38]. Emission spectra of TBBS: 2.0 Eu and TBBS: 2.0 Dy (PG and GC) have been recorded for λexc = 393 and 387 nm, respectively. Emission spectra of TBBS: 2.0 Eu (PG and GC) consist of several bands originating from Eu3+ radiative transitions at 511 nm (5D2⟶7F3), 534 (5D1⟶7F1), 554 nm (5D1⟶7F2), 578 nm (5D0⟶7F0), 590 nm (5D0⟶7F1), 611 nm (5D0⟶7F2), 653 nm (5D0⟶7F3) and 700 nm (5D0⟶7F4). It is worth noticing that presence of transitions from higher excited states (e.g. 5D1,2) suggests low phonon environment of Eu3+ ions in studied materials. When applying λexc = 387 nm excitation, bands at 480, 573 and 662 nm due to Dy3+: 4F9/2⟶6HJ (J = 15/2, 13/2, 11/2) radiative transitions can be distinguished in emission spectra of TBBS: 2.0 Dy (PG and GC). It is well known that luminescence of RE3+ is strongly dependent on the surroundings. Comparing TBBS: 2.0 Eu and TBBS: 2.0 Dy (PG) and (GC) emission spectra it can be seen that red-to-orange (R/O) intensity ratio of Eu3+: 5D0→7F2 electric dipole (R) to 5D0→7F1 magnetic dipole 4 (O) transitions as well as yellow-to-blue (Y/B) ratio of Dy3+ F9/2⟶6H13/2 electric dipole (Y) to 4 6 F9/2⟶ H15/2 magnetic dipole (B) transitions change after crystallization process. The 5D0→7F2 and 4 F9/2⟶6H13/2 electric dipole transitions of Eu3+ and Dy3+ are known to be hypersensitive to the local symmetry of Eu3+ [39] [35] [40]. Decrease of R/O and Y/B ratio after crystallization of SrF2 in glass matrix suggests increasing symmetry of the ligand field of RE3+ ions and thus incorporation of Eu3+ and Dy3+ into crystal lattice of SrF2. In our study we found that R/O ratio changes from 2.81 in precursor glass sample to 2.30 in glass-ceramics whereas Y/B decreases from 1.50 in PG to 1.20 in GC. Similar effect have been described in e.g. [15] and [41].
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Excitation spectra of TBBS: x Eu/ (2-x) Dy (x= 1.5; 1.0 and 0.5) obtained with λem= 611 nm and λem= 573 nm are presented in Figure 7 (a) and 7 (b), respectively. Employment of λem= 573 nm results in appearance of several bands due to Dy3+ excitation similar to these presented in Figure 6 whereas after observation with λem= 611 nm Eu3+ transitions can be observed in examined spectra. Based on the results presented in Figure 7 (a) and (b) two excitation wavelengths 390 and 393 nm have been chosen in order to investigate simultaneous color – tunable white light emission of Eu3+ and Dy3+. Emission spectra of TBBS: x Eu/ (2-x) Dy PG and GC series for λexc=390 and 393 nm are shown in Figure 8 (a) and (b), respectively. After excitation by λexc=390 nm wavelength emission bands originating predominantly from Dy3+ ions are observable at 489, 573 and 662 nm. However, with increasing x value from 0.5 to 1.5 bands due to Eu3+ radiative transitions at 534, 590, 611, 653 and 700 nm grow significantly. On the other hand when comparing spectra of PG and GC materials intensity of main 4F9/2⟶6H15/2 (Dy3+) and 5D0→7F1,2 (Eu3+) transitions increase after heat treatment suggesting that most likely Eu3+ and Dy3+ incorporate into SrF2 crystal lattice with lower phonon energies than glass matrix. Therefore radiative transitions of Eu3+ and Dy3+ become more effective in GC comparing to PG. Emission spectra achieved with λexc=393 nm wavelength consist of both Eu3+ at and Dy3+ bands. For higher x values emission originating from Eu3+ ions is more prominent however with decreasing Eu3+ concentration with subsequent increasing of Dy3+ amount emission from Dy3+ becomes more significant. In this case however intensity of Dy3+: 4 F9/2⟶6H15/2, 13/2 transitions decreases slightly after heat treatment process which may be caused by e.g. energy transfer from Dy3+ to Eu3+ ions. Similar results have been obtained in for example [27].
CIE chromaticity study In order to evaluate the emission color of the Eu3+ and Dy3+ co-doped tellurite glasses and glassceramics containing SrF2 nanocrystals the International Commission on Illumination CIE chromaticity coordinates (x and y) and correlated color temperature CCT have been calculated (Table 2) and presented in a form of CIE chromaticity diagrams (Figure 9 (a) and (b)). The emission color may be changed by using different excitation wavelength or by varying the Eu3+ to Dy3+ molar ratio. In present study under 6
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390 nm excitation (Fig. 9 (a)) all samples containing Eu3+ and Dy3+ ions exhibit warm to neutral white luminescence with x value changes from 1.5 (warm white [39]) to 0.0 (neutral white [42]). After employment of 393 nm excitation (Fig. 9 (b)) emission color of samples (x = 2.0; 1.5 and 1.0) was found to be red, orange and orange – pink, respectively but after for x value equal to 0.5 it turned out that examined system emits warm white light. Therefore, addition of 0.5 mol% of Eu3+ to 1.5 mol% of Dy3+ results in change of the overall light color from neutral to warm white. In present study we additionally examined influence of crystallization of SrF2 on overall emission color of TBBS: x Eu/ (2-X) Dy systems. SrF2 nanocrystallites presence in GC samples leads to quite significant change in CCTs when applying both 390 and 393 nm excitation wavelengths. Decreasing of CCTs in these cases is connected to change of red-to-orange R/O and yellow-to-blue Y/B ratio of Eu3+ and Dy3+ emission bands, respectively as a result of SrF2 nanocrystals formation and thus, symmetry change of RE3+ local environment.
Table 2. Calculated x, y and CCT for λexc = 390 and 393 nm excitation.
PG GC x = 1.5 PG GC x = 1.0 PG GC x = 0.5 PG GC x = 0.0* PG (λexc = 387 nm) GC
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x = 2.0
λexc = 390 nm CCT (K) λexc = 393 nm CCT (K) x y x y 0.61 0.37 1289 0.62 0.37 1223 0.41 0.39 3354 0.57 0.38 1537 0.42 0.38 3027 0.57 0.37 1490 0.37 0.39 4272 0.48 0.38 2203 0.39 0.39 3894 0.52 0.38 1833 0.36 0.40 4658 0.44 0.39 2858 0.36 0.39 4572 0.46 0.38 2484 0.36 0.41 4842 0.35 0.39 5097 -
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In order to get better insight into the luminescence kinetics the luminescence decay curves associated with Dy3+: 4F9/2→6H13/2 (λexc= 387 nm) and Eu3+: 5D2→7F0 (λexc= 393 nm) transitions have been measured and presented in Figure 10 (a) and (b), respectively. Decay times of RE3+ particular transitions are known to depend on several factors such as host matrix or RE3+ concentration. It was found that luminescence decays from both 5D0 and 4F9/2 states of Eu3+ and Dy3+ respectively can be described as twoexponential decays according to the following equation: I(t) = A1 exp (-t/τ1) + A2 exp (-t/τ2)
where A1 and A2 are amplitudes of respective decay components, τ1 and τ2 are short and long luminescence lifetime components contributing to the average lifetime 〈 τavg 〉 [43]. τ1 is related with RE3+ in low symmetry surroundings whereas τ2 is due to RE3+ in higher symmetry of the crystal field . The average lifetimes can be calculated using following equation: 〈τavg〉= [
t I t dt]/[
I t dt] = (τ12 I1 + τ22 I2)/( τ1 I1 + τ2 I2)
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Table 3. Fitting parameters of the luminescence decays for TBBS: x Eu / (2-x) Dy (GC) systems. A1 (%) A2 (%) 29.5 70.5 50.5 49.5 56.0 44.0 62.3 37.7 -
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Eu3+: 5D2→7F0 Dy3+: 4F9/2→6H13/2 τ1 (ms) τ2 (ms) 〈 τavg 〉 (ms) A1 (%) A2 (%) τ1 (µs) τ2 (µs) 〈 τavg 〉(µs) 0.431 1.039 0.950 0.375 0.890 0.735 63.3 36.7 0.249 0.066 0.225 0.320 0.802 0.640 63.6 36.4 0.210 0.053 0.190 0.332 0.866 0.659 62.9 37.0 0.195 0.044 0.177 63.6 36.4 0.162 0.035 0.148
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The average lifetimes of Eu3+: 5D2→7F0 and Dy3+: 4F9/2→6H13/2 transitions have been found to be strongly related to Eu3+ to Dy3+ concentration ratio. As shown in Table 3. The average lifetime of 5D2→7F0 transition decreases with decreasing Eu3+ content. In this case, slow decay component reached 1.039 ms for x = 2.0 with fast decay component equaled 0.431 ms at the same time. Values of τ1 and τ2 decrease for the lower values of x however the influence of fast decay component increase systematically with smaller Eu3+ content. Thus, the average lifetime of Eu3+: 5D2→7F0 〈 τavg 〉 changes from 0.950 to 0.659 ms with decreasing of x. Such results are with a good agreement with many scientific researches, e.g. [40] [44]. Additionally, the average luminescence lifetime of Dy3+: 4F9/2→6H13/2 transition increases with decreasing Dy3+ content from 0.148 to 0.225 ms. This fact may be also explained as follows: according to much research, i.e. [45] decay lifetimes of Dy3+ ions strongly depend on Dy3+ concentration.
Judd - Ofelt analysis
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To fully describe luminescence behavior of prepared glasses and glass- ceramics Judd – Ofelt analysis have been perform. Especially, the effect of crystallization process onto emission has been investigated and for this purpose TBBS: 2Eu (PG) and (GC) were chosen as those possess unique properties among all rare earth ions. Judd - Ofelt parameters Ωλ were calculated based on luminescence emission spectra using JOES application software. Detailed information regarding software and calculations may be found in [46] .Other radiative properties as radiative transition probabilities A, total radiative transition probability AT, the fluorescence branching ratio β, the stimulated emission cross section σ and asymmetry ratio R/0 were also calculated based on emission spectra and JO parameters.
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Because of 5D0→7F1 pure magnetic dipole transition of Eu3+ is independent on the host matrix it can be calculated exactly and used as a reference for other electric dipole transitions of Eu3+ originating from 5D0 excited level. Value of 5D0→7F1 magnetic dipole transition strength DMD equals 9.6x10-42 esu2cm2. Reduced matrix elements │J = 0││Uλ││J’│2 simplifies further calculations of JO parameters due to values different than 0 only for J’ = 2, 4, 6, where λ = 2, 4, 6, respectively [46]. For those λ: U2= 0.0031, U4= 0.0023, U6= 0.0002. Dipole strengths for electric dipole transitions are given by: DλED = e2ΩλUλ
Whereas the integrated 5D0→7Fκ transition intensity is given by: Jκ = ∫Iκ (ν͂) dν͂ Radiative transition probabilities A may be calculated using JO parameters: Aλ = [(64π4ν͂3λ)/3h]*[(nλ(nλ2+2)2/9]*DλED, A1 = (64π4ν͂31)/3h)*n31DMD
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where ν͂κ – the average wavenumber in cm-1, nλ is a refractive index dependent on the wavelength and h – Plack constant, h= 6.63x10-27 erg s. JO parameters may be calculated from the ratio of the integrated intensities of transitions 5D0→7Fλ and MD transition 5D0→7F1 using formula:
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Ωλ= [(DMD ν͂31 )/(e2 ν͂3λUλ)] [(9n31)/(nλ(n2λ+2)2] [Jλ/J1]
Results of JO parameters calculations are given in Table 4.
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Table 4. Calculated Judd – Ofelt parameters, radiative transition probabilities, branching ratios, emission cross sections, total radiative transitions probabilities and asymmetry ratios of TBBS: 2Eu precursor glass (PG) and glass-ceramic (GC) samples. TBBS: 2Eu (PG) 3.95 1.32
TBBS: 2Eu (GC) 3.21 1.37
172 504 80
171 407 82
0.23 (0.23) 0.65 (0.67) 0.09 (0.11)
0.26 (0.26) 0.60 (0.62) 0.11 (0.12)
4.42 18.40 5.00
4.55 14.04 5.00
756 2.78
660 2.27
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JO parameter (x 10-20 cm2) Ω2 Ω4 Radiative transition probabilities (s-1) A (5D0→7F1) A (5D0→7F2) A (5D0→7F4) Branching ratio (-) Experimental (Theoretical) β (5D0→7F1) β (5D0→7F2) β (5D0→7F4) Emission cross section (x 10-22 cm2) σ (5D0→7F1) σ (5D0→7F2) σ (5D0→7F4) Total Radiative transition probability AT (s-1) Asymmetry ratio R/O (-)
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High value of Ω2 compare to Ω4 for both PG and GC sample suggests Eu3+ ions occupy mostly low symmetry sites. Ω2 parameter is known to be structure sensitive and it depends on covalence of RE3+ bonds. Since Eu – O bond in glass matrix is highly covalent calculated Ω2 is also relatively high and such results are comparable with literature, i.e. [47]. On the other hand Ω4 and Ω6 are related to rigidity of glasses [48] and Ω4 seems to depend also on long range effects related with crystal lattice [13]. Thus, crystallization process results in increase of Ω4 (from 1.32 in PG to 1.37 x 10-20 cm2 for GC) and simultaneous decrease of Ω2 (from 3.95 to 3.21 x 10-20 cm2 for PG and GC, respectively). This is most probably related with grown of SrF2 nanocystals and since Eu3+ may replace Sr2+ ions in crystal lattice with higher symmetry sites comparing to glass matrix value of Ω4 parameter increases slightly after heat treatment. Observed Ω2 to Ω4 trend is typical for glass and glass-ceramics materials. Because 5D0→7F6 transition band is located in NIR range it was not measureable by the spectrofluorometer thus Ω6 value has not been calculated.
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Radiative transition probabilities A( 5D0→7F1) obtained for PG and GC remained almost unchanged whereas value of A(5D0→7F2) decreased significantly from 504 s-1 to 407 s-1 after crystallization which is in the good agreement with PL results as 5D0→7F2 transition itself remains predominant in the emission spectra of PG and GC materials. AR values calculated for PG (766 s-1) and GC (660 s-1) confirm faster depopulation of 5D0 excited state and shorter Eu3+ lifetimes in PG than in GC which has been presented in previous section. The β value for specific transition represents occurrence of particular electron transition as a result of number of photons emitted from a specific energy level. The highest β values have been obtained for 5D0→7F2 transition: 0.65 and 0.60 for PG and GC, respectively. Experimental results are in a good agreement with theoretical values which confirms accuracy of analysis. Values of σ were found the highest for 5D0→7F2 transition of Eu3+ in both studied materials: 18.40 (PG) and 14.04 (x 10-22cm2) (GC). Moreover, asymmetry ratio R/O has been found to decrease from 2.78 to 2.27 after crystallization process confirming results presented in previous section.
Conclusions
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Eu3+/Dy3+ doped oxyfluoride glasses and glass-ceramics containing SrF2 nanocrystals have been successfully synthesized using simple melt-quenching and subsequent heat treatment procedure. XRD and TEM results confirmed formation of SrF2 nanocrystalline phase in the glass matrix whereas based on XPS study increase of signals due to Sr-F bonds has been found. Based on luminescence study it can be concluded that Eu3+ and Dy3+ ions incorporate into SrF2 crystal lattice due to decrease of R/O and Y/B intensity ratio which is related to change the surroundings of RE3+ into one with higher symmetry site in SrF2. Moreover, due to significantly lower phonon energy of strontium fluoride phase than tellurite glass matrix, luminescence excitation and emission intensity increase have been observed. Optical properties along with calculated JO and optical parameters confirm that oxyfluoride glasses and glass-ceramics are excellent host for RE3+ ions. Simultaneous emission originating from both, Eu3+ and Dy3+ ions have been achieved by using 390 and 393 nm excitation wavelengths and as a result, color tunable white light emission have been obtained. The overall emission color changes from neutral white for TBBS: 2.0 Dy (PG and GC) to warm white with adding of Eu3+ ions and reaches red when only Eu3+ ions are present in PG and GC.
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This research has been supported by the grant 2015/17/B/ST5/03143 financed by National Science Centre (A.S.). This work was supported by CSIC projects PIC2016FR1 and PIE 201560E056.
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Figure Captions
Figure 1. XRD patterns of TBBS: xEu / (2-x) Dy (precursor glass: PG – bottom) and (glass-ceramics: GC heat treated in 370˚C for 24 h). Figure 2. FTIR spectra of TBBS: 2.0 Dy (PG and GC) (left) and deconvolution of GC spectrum into three signals (right).
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Figure 3. XPS spectra of Sr in TBBS: 2.0 Dy (PG and GC, top and bottom respectively). Figure 4.TEM image of TBBS: 2.0 Dy (GC) glass-ceramic sample heat treated in 370˚C for 24 h. Figure 5. UV-Vis absorbance spectra of TBBS: xEu/ (2-x) Dy (GC) serie.
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Figure 6. Excitation (PLE) and emission (PL) spectra of TBBS: 2Eu (PG and GC) and TBBS: 2Dy (PG and GC). Figure 7 (a) and (b). Excitation (PLE) spectra of TBBS: x Eu/ (2-x) Dy (x= 1.5; 1.0 and 0.5) PG and GC.
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Figure 8 (a) and (b). Emission (PL) spectra of TBBS: xEu/ (2-x) Dy (x=0.5-1.5) PG and GC for λexc=390 and 393 nm. Figure 9 (a) and (b). CIE chromaticity diagrams of TBBS: xEu/ (2-x)Dy (x=0.5-1.5) for λexc=390 nm (left) and λexc=393 nm (right) excitation wavelength. Figure 10 (a) and (b). Luminescence decay curves of TBBS: 2.0 Dy (left) and TBBS: 2.0 Eu (right) achieved with λexc = 387 and 393 nm excitation, respectively.
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Highlights:
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Transparent oxyfluoride glass-ceramics with SrF2 nanocrystals have been prepared Color-tunable white light emission has been obtained by adjusting Eu3+/ Dy3+ molar ratio SrF2 crystallization influence onto luminescence of Eu3+ and Dy3+ has been studied Judd-Ofelt parameters have been calculated using JOES application software
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