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Preparation and characterization of Yttrium based luminescence phosphors
Optical Materials xxx (2017) 1e9
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Preparation and characterization of Yttrium based luminescence phosphors L.E. Muresan a, M. Ayvacikli b, J. Garcia Guinea c, A. Canimoglu d, Y. Karabulut b, N. Can b, e, * a
Babes- Bolyai University, Raluca Ripan Institute for Research in Chemistry, Fantanele 30, 400294 Cluj-Napoca, Romania Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey c Museo Nacional Ciencias Naturales, Jose Gutierrez Abascal 2, Madrid 28006, Spain d Omer Halisdemir University, Faculty of Arts and Sciences, Physics Department, Nigde, Turkey e Physics Department, Jazan University, P.O. Box 114, 45142 Jazan, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t s
Article history: Received 9 January 2017 Received in revised form 24 January 2017 Accepted 25 January 2017 Available online xxx
Ce doped Yttrium aluminate modified by replacing different molar part of aluminium or gallium (Y3Al5xGaxO12) and Yttrium silicate phosphors activated with Ce and Tb (Y2SiO5:Ce3þ,Tb3þ) were synthesized by solid state reaction and a gel combustion method, respectively. X-ray diffraction and Scanning electron microscope (SEM) techniques are used to identify their structures and morphologies. Luminescence characteristics are measured and spectroscopic data confirm that Y2SiO5:Ce3þ, Tb3þ phosphors can be effectively excited upon UV excitation light and X-ray irradiation, resulting in intense blue and green emissions, respectively. This energy transfer takes place by means of a non-radiative process inside Ce3þ-Tb3þ clusters formed in the host matrix. Tb3þ doped Y2SiO5 yields both blue emission 5D3 / 7Fj (j ¼ 3,4,5,6) and green emission 5D4 / 7FJ (J ¼ 3,4,5,6) of Tb3þ. Y3Al5xGaxO12:Ce3þ phosphors exhibit a broad blue emission band originating from allowed 5d-4f transition of the Ce3þ ions under different excitation sources but the broad emission band shifts with increasing of Ga3þ content. This work presents a quantitative understanding of host material's on dopant's luminescence properties and thereby provides an optimization guideline, which is extremely demanding for the development of new luminescent materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Y3Al5O12 garnet Y2SiO5 Rare earth doping XRD Luminescence
1. Introduction Rare-earth ion doped phosphor materials have gathered growing interests in modern life as technologically important components which are extensively applied to lighting, and plasma display panels over the past several years. Therefore, Y2SiO5 has attracted much attention in recent decades due to their excellent chemical and thermal stability. When Y2SiO5 having a band gap of 7.4 eV (insulator) are doped with an activator such as Ce an energy level structure occurs inside the wide band gap where the 5d to 4f transition takes place. As the shapes and positions of the emission bands are different for different activators in the host matrix the luminescence properties allow them to be an attractive luminescent material for potential applications (i.e. plasma displays [1], laser materials [2] and high energy phosphors [3]). As is well
* Corresponding author. Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey. E-mail address: [email protected] (N. Can).
known, Ce3þ with parity allowed 4f-5d and 5d-4f transitions is not only one of the most popular activators for phosphors but also good sensitizer in co-doped materials. Ce3þ shows an intense excitation and a broad yellow emission band [4e6]. Other activators such as Tb3þ [7,8], Sm3þ [9], Eu3þ [10,11] have been widely used in the literature for this purpose. Dy3þ/Tm3þ [12], Eu3þ/Ce3þ [13], Pr3þ/ Ce3þ [13] co-doped systems were also investigated for achieving pure white light emission. Besides, efficient energy transfer mechanisms between Tb3þ and Dy3þ [14], Tm3þ and Dy3þ [15] were reported. Yttrium aluminium garnet (YAG), when substituted with several mole or atomic percent of the activator ion Ce3þ to replace Y3þ, is a luminescent material that has nearly ideal PL properties for excitation by a blue solid-state light source [16,17]. Traditional YAG:Ce phosphor is produced by conventional solid phase method at a high temperature but recently new and economical wet chemical methods such as sol-gel [18], combustion method [19] and coprecipitation [20] have been developed for synthesis of the YAG:Ce phosphor. However, the traditional solid-phase method is still dominant in all of them. A necessary condition to see the
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Fig. 1. XRD patterns of (a) Y2SiO5:Ce3þ, Tb3þ. Reference patterns from Powder Diffraction File (PDF) are also included for comparison.
factors that govern how the YAG:Ce luminescence depends on codoping and, ultimately to be able to control the color of the YAG:Ce phosphor using codoping, is to acquire a detailed knowledge of the local structures of the double or multiple substitutional defects on the atomistic level as well as of their electronic structures [16,21,22]. From this starting point, co-dopants not only play an important role as co-activators but also as wavelength shifters [23]. For instance, substituting Y3þ with larger ions (e.g. Gd3þ and La3þ) shifts the broad yellow luminescence band of Ce3þ to long wavelength side (red shift), which can improve color rendering properties while substitution of Al3þ with Ga3þ shifts luminescence towards a shorter wavelength (blue shift) [24,25]. This work presents studies on (i) some new understanding regarding structural and the luminescence properties of Y2SiO5:Ce, Tb powders (ii) structural analysis and the luminescence characteristics of the Ce doped Y3Al5xGaxO12 compounds excited using different excitation sources in which Al3þ ions were replaced with Ga3þ in the garnet host lattice. 2. Experimental 2.1. Materials 2.1.1. Yttrium silicate (Y2SiO5: YSO) Yttrium silicate phosphors single and multiple doped with
Ce,Tb, were prepared by gel combustion. Raw materials used in this study for phosphor synthesis were commercially available yttrium oxide Y2O3 (99.9%, Aldrich), cerium nitrate Ce(NO3)3$6H2O (extra pure, Merck), terbium nitrate Tb(NO3)3$5H2O (extrapure, Merck), Laspartic acid (99.0%, Alfa Aesar), and tetraethoxysilane (TEOS- 98% Alfa Aesar). 2.1.2. Yttrium aluminate garnet (Y3Al5xGaxO12:YAG) The preparation of cerium activated yttrium aluminate based phosphors was achieved by solid state reaction route from homogeneous mixtures that contain raw oxide precursors such as: Y2O3 (Alfa Aesar 99.9%), Al2O3 (Merk), Ga2O3 (99.99% - Jansen); Ce(NO3)3$5H2O (extra pure Merck). 2.2. Synthesis of the phosphors 2.2.1. YSO:Ce3þ,Tb3þ The synthesis starts with the preparation of the gels from a mixture solution containing metallic nitrates, TEOS and aspartic acid. The solution mixture containing stoichiometric amounts of Y3þ, RE3þ and Si4þ are prepared in order to produce phosphors with general formula Y2xy CexTbySiO5. The terbium doping concentration in terms of mole fraction is varied from 0 to 0.12. The second stage of the synthesis consists in the combustion of the gels that take place in a heating mantle at 250 C. The thermal synthesis of as
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Fig. 2. Particle morphology of selected Y2SiO5 phosphors doped at different concentrations obtained by the gel-combustion method: (a) Ce %0.75, Tb 0% (b) Ce %0.75, Tb 0.5% (c) Ce %0.75, Tb 0.75% (d) Ce %0.75, Tb 1.5% (e) Ce %0.75, Tb 3% (f) Ce %0.75, Tb 6%.
prepared precursors was performed at 1400 C, in air, for 4 h and lead to the formation of the phosphor samples. The final stage of the synthesis consists on the washing, drying and sieving of the product. A detailed description of the synthesis of Y2SiO5:Ce3þ,Tb3þ phosphor samples has been presented in our previous works [26,27]. 2.2.2. YAG:Ce3þ The synthesis consists in thorough homogenization with water for 15 min of a mixture that contains stoichiometric amounts of metallic oxides (Y2O3, Ga2O3, Gd2O3, Al2O3) as generator for host lattice and cerium nitrate as activator source. The molar ratio between Al2O3 and (Y-Ce) is 1.66. The cerium content is 1.0% Ce/ (Y þ Ce). After drying at 115 C, the powder mixtures were fired initially at 1200 C for 4 h in open air atmosphere and followed by slow cooling at the room temperature. Finally, the phosphors samples washed with distilled water were subsequently dried and sieved. Different synthesis mixtures were prepared containing variable amounts of gallium to obtain phosphors having the general formula: Y2.97Ce0.03GaxAl5xO12, where x is in the range 0e5. 2.3. Characterization of phosphor samples The X-ray diffraction (XRD) patterns of the prepared samples were evaluated by a Philips PW-1710/00 diffractometer using CuKa (1.5418 Å) radiation. The XRD patterns were recorded by step scanning from 2 to 80 2q and then compared with the XRD PDF2 card files of the Joint Committee on Powder Diffraction Standards using Xpowder diffraction software. The cathodoluminescence (CL) spectra were recorded using a MONOCL3 Gatan probe with a blue sensitive photomultiplier tube (PMT) equipped with ESEM XL30 microscope. The microscope has a chemical EDS probe. PMT used for UV and visible detection covers the entire wavelength range of 250e850 nm and it is most sensitive in the blue parts of the spectrum. The RL measurements were obtained exciting the powder samples with an X-ray unit with a Machlett OEG-50A tube
operating with a current of 15 mA and a voltage of 30 kV delivering a dose rate of 30 Gy/min. Light collection is performed using a Jobin Yvon spectrometer, equipped with a liquid nitrogen cooled CCD detector. We have performed photoluminescence (PL) measurements using a deuterium lamp LDD-400 as a source of UV light. Various excitation wavelengths in the present work were selected with a grating monochromator MDR-2 in the 200e400 nm range. Emissions were collected by a Shamrock monochromator SR-303i-B. Photoluminescence excitation spectroscopy (PLE) was performed using a prism monochromator SPM-2 and detected with a Hamamatsu photomultiplier tube [28,29]. 3. Results and discussions 3.1. Structural properties 3.1.1. YSO:Ce3þ,Tb3þ XRD patterns for the Y2SiO5:Ce3þ,Tb3þ phosphors samples with varying Ce3þ and Tb3þ concentrations are shown in Fig. 1. All of the diffraction peaks are in accordance with those of Y2SiO5 (PDF 36e1476) which indicates that Tb3þ and Ce3þ dopant ions do not give rise to any significant changes to the crystal structure of the host material. When taking into consideration the effect of ionic radius of cations with different values of coordination number Ce3þ (1.01 Å) and Tb3þ (0.923 Å) dopants can presumably occupy Y3þ (0.9 Å) sites. As can be seen from Fig. 1 we observed the strong and sharp diffractions peaks. This indicate that the phosphors obtained have a high degree of crystallization which is advantageous for luminescence [30]. A SEM study was carried out to investigate the surface morphology and crystallite sizes of the phosphors produced by a gel combustion method. The SEM images of Y2SiO5:Ce3þ,Tb3þ samples prepared with addition of Tb3þ co-dopant ion are recorded and displayed in Fig. 2(aef). As seen from the Fig. 2 the powders synthesized by this method consist of agglomerates with spongy
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Fig. 3. XRD patterns of Ce3þ doped Y3Al5xGaxO12 garnets with Ga content in the x ¼ 0e5 range. Reference patterns from Powder Diffraction File (PDF) are also included for comparison.
aspect and round spherical shape crystalline with particle size within the range of 95e120 nm. The agglomerates are tightly bound each other to form relatively large aggregated structures. 3.1.2. YAG:Ce3þ The XRD patterns of Y3Al5xGaxO12:Ce3þ phosphors for various Ga concentration were shown in Fig. 3. The XRD patterns clearly show the diffraction peaks are shifted towards lower angles with increasing the Ga3þ content. This type of shifts of XRD peaks
indicate a lattice expansion due to Ga substitution for Al in the compound which seems plausible as the ionic radius Ga3þ (0.47 nm) is bigger that of Al3þ (0.30 nm). The SEM observation of Y3Al5xGaxO12:Ce3þ in Fig. 4a revealed that phosphor particles such as shape and size without Ga dopant accommodated in aluminate host lattice is relatively homogenous. The particles have not well defined morphology since they melted together. The homogeneity of the sample decreases as Ga is incorporated in the aluminate lattice as seen Fig. 4(bed).
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Fig. 4. SEM images of Ce3þ doped Y3Al5xGaxO12 samples (x ¼ 0,1,3,4).
Fig. 5. (a) Excitation and emission spectra of Y1.985Ce0.015SiO5 (b) Excitation and emission spectra of Y1.985Tb0.015SiO5 (c) Concentration dependence of PL intensity of Ce3þ broad emission band located at 400 nm with a shoulder at 420 nm (d) Concentration dependence of PL intensity of Tb3þ sharp emission lines located at 416 nm 488 nm and 544 nm.
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Fig. 6. RL emission spectra of (a) YSO:Ce with different Ce content and (b) YSO:0.015Ce, Tb with different Tb content.
3.2. Luminescence properties 3.2.1. YSO:Ce3þ,Tb3þ This study was performed to determine the influence of Tb3þ content on the luminescence properties of Y2SiO5:Ce3þ. Photoluminescence (PL) excitation and emission spectra of Y2SiO5:Ce3þ
phosphor samples ranging from UV to green regions are displayed in Fig. 5a. It is clear to see that the excitation spectrum consists of three major bands appeared at 265 nm, 300 nm and 356 nm which are attributed to the transitions from the 4f ground state (2F5/2) to the different crystal field split levels of 5d orbits. Under the excitation with 355 nm the emission spectrum consists of a doublet
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peak appeared at 400 nm which is attributed to the 5d / 4f radiative electron transitions [31,32]. As far as Tb3þ doped Y2SiO5, the excitation spectrum of this phosphor consists of one strong band peaked at 242 nm. The emission spectrum of this phosphor exhibits several emission bands over the spectral the range of 350e650 nm which are assigned to the transition 5D3 / 7Fj (j ¼ 6,5,4,3) (See Fig. 5b). The main green emission band of Tb centers, for which the maximum is located at 544 nm, belongs to the magnetic dipole 5D4 / 7F5 inside Tb3þ ion with 4f7 configurations. As can be seen from Fig. 5c emission intensities at 400 and 420 nm decrease with increasing of Ce3þ doping concentration. We suggest that the most probable mechanism causing this decreasing pattern is concentration quenching effect. Fig. 5d shows behaviour of emission lines peaked at 416 nm, 488 nm and 544 nm. It is easy to see that the emission lines centred at 544 nm and 488 nm increase sharply as the Tb3þ doping concentration is increased up to 6%. After that the intensity decreases significantly. We suggest that this is due to concentration quenching. Nevertheless, the emission line peaked at 416 nm exhibits different pattern compared to the other emission lines. Fig. 6a depicts the RL spectra measured of the YSO:Ce3þ phosphors with different concentration under continuous X-ray irradiation at the room temperature. As seen from the Fig. 6a, RL spectra do not reveal major change in peak shape or position with increasing Ce doping concentration. Since Ce3þ ion is expected to occupying two distinct lattice sites in the host material due to the different coordination numbers, there could be several emission peaks hidden under the broad Ce3þ emission band. This emission band can be decomposed using Gaussian peak fitting methods. Our aim in this work does not involve it but we will work
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on this fitting process in the future. Fig. 6b shows the emission spectra of the samples based on different Tb3þ dopant concentration. A clear feature from the spectra can be easily observed. Broad emission band centred at 420 nm in the wavelength range of 350e600 nm decreases with increasing of the Tb3þ doping concentration, whereas that of Tb3þ was simultaneously found to increase monotonically. The most plausible explanation according to our RL spectra is that the Ce3þ ions act as effective sensitizer for emission lines of Tb3þ ions and this gives rise to an efficient energy transfer between Ce3þ and Tb3þ due to parity allowed electric dipole of 4f-5d transition of Ce3þ. In other words, Ce3þ can strongly absorb radiation and efficiently transfer a part of its energy to Tb3þ. The energy level diagram in Fig. 7 depicts the energy transfer from Ce3þ and Tb3þ in YSO host material under the excitation with X-ray radiation. This is in good agreement with earlier results [33,34]. 3.2.2. YAG:Ce3þ RL measurements were performed at the room temperature under continuous X-ray irradiation with a resolution of 0.05 nm, in order to obtain some idea of the influence of Dy ions on the host material. As given in Fig. 8, the RL intensity of the Y3Al5xGaxO12:Ce3þ phosphors increases gradually with increasing of the doping concentration of the Ga ions. RL emission intensity did not reach a maximum value even though the x value is 5. The
Fig. 8. RL Luminescence spectra of Ce3þ doped Y3Al5xGaxO12 garnets with Ga content in the x ¼ 0e5 range.
Fig. 7. The energy level model for the energy transfer processes between Ce3þ and Tb3þ ions.
Fig. 9. Comparison of PL spectrum Y2.22Gd0.75Ce0.03Ga2Al2O12 at room temperature.
of
Y2.97Ce0.03Ga2Al3O12
and
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Fig. 10. Isometric views of low temperature PL from (a) Y2.22Gd0.75Ce0.03Ga2Al2O12 and (b) Y2.97Ce0.03Ga2Al3O12. The excitation was made using 325 nm line of a He-Cd laser.
most plausible explanation is that Ga3þ ions preferentially replace octahedral Al3þ lattice sites. Note that RL data of Y3Ga5O12:Ce3þ in Fig. 8 does not show significant RL emission spectrum at room temperature. Fig. 9 shows the PL emission spectra for Y2.22Gd0.75Ce0.03Ga2Al2O12 and Y2.97Ce0.03Ga2Al3O12 taken at room temperature. As seen from Fig. 9, there is a clear difference between phosphor materials with co-doped Gd and without Gd in terms of PL intensity and full width of maximum (FWHM). The PL intensity of broad band due to the Ce3þ 5d1/4f transitions results in the decrease of the intensity when Gd3þ is incorporated into the Y2.97Ce0.03Ga2Al3O12. Fig. 10 illustrates contour maps of the PL of Y2.22Gd0.75Ce0.03Ga2Al2O12 and Y2.97Ce0.03Ga2Al3O12. As seen in Fig. 10, Ce3þ related broad band luminescence located at 516 nm decreases with increasing temperature. We have observed different luminescence pattern when Gd3þ was accommodated in the Y2.97Ce0.03Ga2Al3O12. The luminescence intensity of broad band located at 516 nm is low but the peak intensity clearly increases with decreasing the temperature. At low temperature PL measurements, one would expect to see significantly better resolution and more intense emissions since the phonon emissions are reduced. It is worth noting that the broad band feature does show no spectral shift from 10 to 300K as expected because there is shielding of the transitions in the configuration of rare earth ions. Interestingly, decreasing luminescence intensity value obtained for YGd0.75AlGa2O12:Ce at room temperature can be attributed to further decrease in the band gap and in the crystal field splitting, which reduces the energy difference between the 5d1 excited state and the bottom of the conduction band. It leads to luminescence quenching due to thermal ionization of 5d1 excited state of Ce3þ emission center [35,36].
4. Conclusions A series of novel Y2SiO5:Ce3þ, Tb3þ and (Y3Al5xGaxO12): Ce3þ, Gd3þ were prepared by a gel combustion method and solid state reaction, respectively. The dependence of Ce3þ and Tb3þ ions concentration on the structure, morphology and photoluminescence and radioluminescence of the phosphor samples has been investigated in detail. By co-doping with Tb3þ ions, the Ce3þ doped Y2SiO5 is observed to show the following features in terms of structural and luminescence properties:
(i) Observation of phase (X2) at high reaction temperature with round-spherical morphology (ii) Mean size of 100 nm in diameter (iii) Main blue and green emission peaks located at 487 nm and 544 nm corresponding to electric and magnetic dipole allowed transition of Tb3þ ion, respectively (iv) Tunable emission of the Tb3þ ion from blue to green region after Ce3þ excitation (v) A decrease of the Ce3þ overall emission (vi) An energy transfer process between Ce3þ and Tb3þ ions As regards (Y3Al5xGaxO12):Ce3þ, Gd3þ phosphor materials we have yielded the following main conclusions. (i) Shifting of the diffraction peaks due to lattice expansion (ii) Observation of Ce3þ related luminescence under partial substitution of Ga3þ with Al3þ (iii) A sharp decrease of the Ce3þ emission intensity in the case of Gd doped (Y3Al5xGaxO12):Ce3þ and observation different luminescence emission pattern in the range of 10e300 K On the basis of the promising findings illustrated here, further research to investigate luminescence properties of the samples based on parameters such as excitation wavelength, laser power, excitation source is still continuing and will be presented in our future papers. References [1] Z.H. Zhang, Y.H. Wang, Y. Hao, W.J. Liu, Synthesis and VUV photoluminescence of green-emitting X2-Y2SiO5:Tb3þ phosphor for PDP application, J. Alloy. Comp. 433 (2007) L12eL14. [2] F. Thibault, D. Pelenc, B. Chambaz, M. Couchaud, J. Petit, B. Viana, Growth and characterization of Yb3þ doped Y2SiO5 layers on Y2SiO5 substrate for laser applications, Opt. Mater. 30 (2008) 1289e1296. [3] F. Thibault, D. Pelenc, B. Chambaz, M. Couchaud, J. Petit, B. Viana, Growth and characterization of Yb3þ doped Y2SiO5 layers on Y2SiO5 substrate for laser applications, Opt. Mater. 30 (2008) 1289e1296. [4] L. Chen, C.-C. Lin, C.-W. Yeh, R.-S. Liu, Light converting inorganics phosphors for white light emitting diodes, Materials 3 (2010) 2172e2195. [5] L.E. Muresan, B.F. Oprea, A.I. Cadis, I. Perhaita, O. Ponta, Studies on Y2SiO5:Ce phosphors prepared by gel combustion using new fuels, J. Alloys Comp. 615 (2014) 795e803. [6] G. Ramakrishna, H. Nagabhushana, R.B. Basavaraj, S.C. Prashantha, S.C. Sharma, Ramachandra Naik, K.S. Anantharaju, Green synthesis structural characterization and photoluminescence properties of Sm3þ co-doped Y2SiO5:
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[21] Y.X. Pan, W. Wang, G.K. Liu, S. Skanthakumar, R.A. Rosenberg, X.Z. Guo, K.K. Li, Correlation between structure variation and luminescence red shift in YAG: Ce, J. Alloys Compd. 488 (2009) 638e642. [22] M. Kottaisamy, P. Thiyagarajan, J. Michra, M.S. Ramachandra Rao, Color tuning of Y3Al5O12:Ce phosphor and their blend for white LEDs, Mat. Res. Bull. 43 (2008) 1657e1663. [23] H.S. Jang, W.B. Im, D.C. Lee, D.Y. Jeon, S.S. Kim, Enhancement of red spectral emission intensity of Y3Al5O12:Ce3þ phosphor via Pr co-doping and Tb substitution for the application to white LEDs, J. Lumin. 126 (2007) 371e377. [24] W.W. Holloway, M. Kestigian, Optical properties of cerium-activated garnet crystals, J. Opt. Soc. Am. 59 (1969) 60e63. [25] T.Y. Tien, E.F. Gibbons, R.G. Delosh, P.J. Zacmanidis, D.E. Smith, H.L. Stadler, Optical properties of cerium-activated garnet crystals, J. Electrochem. Soc. 120 (1973) 278e281. [26] L.E. Muresan, A.I. Cadis, I. Perhaita, O. Ponta, O. Pana, L. Trinkler, B. Berzina, V. Korsaks, Influence of Vinyltriethoxysilane Concentration on Structural and Luminescent Characteristics of Cerium Doped Yttrium Based Silicate Phosphors, vol. 41, 2015, pp. 13179e13188. [27] L.E. Muresan, A.I. Cadis, I. Perhaita, E. Indrea, Morpho-structural and luminescence investigations on yttrium silicate based phosphors prepared with different precipitating agents, Cent. Eur. J. Chem. 12 (2014) 1023e1031. [28] A. Canimoglu, Y. Karabulut, M. Ayvacikli, L.E. Muresan, I. Perhaita, L. BarbuTudoran, J. Garcia Guinea, T. Karali, N. Can, Optical spectroscopy of the Cedoped multicomponent garnets, Appl. Radiat. Isotopes 114 (2016) 114e120. [29] L.K.S. de Herval, Y. Tuncer Arslanlar, M. Ayvacikli, F. Iikawa, J.A. Nobrega, ~o Gobato, N. Can, M. Henini, M.P.F. de Godoy, Enhancement P.S. Pizani, Y. Galva of the luminescence intensity by co-doping Mn2þ into Er3þ -doped SrAl2O4, J. Lumin. 163 (2015) 17e20. [30] H. Luo, J. Liu, X. Zheng, L. Han, K. Ren, X. Yu, Enhanced photoluminescence of Sr3SiO5:Ce3þ and tunable yellow emission of Sr3SiO5:Ce3þ, Eu2þ by Al3þ charge compensation for W-LEDs, J. Mater. Chem. 22 (2012) 15887e15893. [31] H. Jiao, N. Zhang, X. Jing, D. Jiao, Influence of rare earth elements (Sc, La, Gd and Lu) on the luminescent properties of green phosphor Y2SiO5:Ce,Tb, Opt. Mater. 29 (2007) 1023e1028. [32] L.E. Muresan, B.F. Oprea, A.I. Cadis, I. Perhaita, O. Ponta, Studies on Y2SiO5:Ce phosphors prepared by gel combustion using new fuels, J. Alloys Compd. 615 (2014) 795e803. [33] A. Kar, A. Patra, Impacts of coreeshell structures on properties of lanthanidebased nanocrystals: crystal phase, lattice strain, downconversion, upconversion and energy transfer, Nanoscale 4 (2012) 3608e3619. [34] J.A. Gonzales-Ortega, E.M. Tejeda, N. Perea, G.A. Hirate, E.J. Bosze, J. McKittrick, White light emission from rare earth activated yttrium silicate nanocrystalline powders and thin films, Opt. Mater. 27 (2005) 1221e1227. [35] Y. Wu, G. Ren, Energy transfer and radiative recombination processes in (Gd, Lu)3Ga3Al2O12:Pr3þ scintillators, Opt. Mater. 35 (2013) 2146e2154. [36] J. Ueda, K. Aishima, S. Tanabe, Temperature and Compositional Dependence of Optical and Optoelectronic Properties in Ce3þ-doped Y3Sc2Al3xGaxO12(x ¼ 0, 1, 2, 3), vol. 35, 2013, pp. 1952e1957.
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Preparation and characterization of Yttrium based luminescence phosphors L.E. Muresan a, M. Ayvacikli b, J. Garcia Guinea c, A. Canimoglu d, Y. Karabulut b, N. Can b, e, * a
Babes- Bolyai University, Raluca Ripan Institute for Research in Chemistry, Fantanele 30, 400294 Cluj-Napoca, Romania Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey c Museo Nacional Ciencias Naturales, Jose Gutierrez Abascal 2, Madrid 28006, Spain d Omer Halisdemir University, Faculty of Arts and Sciences, Physics Department, Nigde, Turkey e Physics Department, Jazan University, P.O. Box 114, 45142 Jazan, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t s
Article history: Received 9 January 2017 Received in revised form 24 January 2017 Accepted 25 January 2017 Available online xxx
Ce doped Yttrium aluminate modified by replacing different molar part of aluminium or gallium (Y3Al5xGaxO12) and Yttrium silicate phosphors activated with Ce and Tb (Y2SiO5:Ce3þ,Tb3þ) were synthesized by solid state reaction and a gel combustion method, respectively. X-ray diffraction and Scanning electron microscope (SEM) techniques are used to identify their structures and morphologies. Luminescence characteristics are measured and spectroscopic data confirm that Y2SiO5:Ce3þ, Tb3þ phosphors can be effectively excited upon UV excitation light and X-ray irradiation, resulting in intense blue and green emissions, respectively. This energy transfer takes place by means of a non-radiative process inside Ce3þ-Tb3þ clusters formed in the host matrix. Tb3þ doped Y2SiO5 yields both blue emission 5D3 / 7Fj (j ¼ 3,4,5,6) and green emission 5D4 / 7FJ (J ¼ 3,4,5,6) of Tb3þ. Y3Al5xGaxO12:Ce3þ phosphors exhibit a broad blue emission band originating from allowed 5d-4f transition of the Ce3þ ions under different excitation sources but the broad emission band shifts with increasing of Ga3þ content. This work presents a quantitative understanding of host material's on dopant's luminescence properties and thereby provides an optimization guideline, which is extremely demanding for the development of new luminescent materials. © 2017 Elsevier B.V. All rights reserved.
Keywords: Y3Al5O12 garnet Y2SiO5 Rare earth doping XRD Luminescence
1. Introduction Rare-earth ion doped phosphor materials have gathered growing interests in modern life as technologically important components which are extensively applied to lighting, and plasma display panels over the past several years. Therefore, Y2SiO5 has attracted much attention in recent decades due to their excellent chemical and thermal stability. When Y2SiO5 having a band gap of 7.4 eV (insulator) are doped with an activator such as Ce an energy level structure occurs inside the wide band gap where the 5d to 4f transition takes place. As the shapes and positions of the emission bands are different for different activators in the host matrix the luminescence properties allow them to be an attractive luminescent material for potential applications (i.e. plasma displays [1], laser materials [2] and high energy phosphors [3]). As is well
* Corresponding author. Manisa Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Muradiye-Manisa, Turkey. E-mail address: [email protected] (N. Can).
known, Ce3þ with parity allowed 4f-5d and 5d-4f transitions is not only one of the most popular activators for phosphors but also good sensitizer in co-doped materials. Ce3þ shows an intense excitation and a broad yellow emission band [4e6]. Other activators such as Tb3þ [7,8], Sm3þ [9], Eu3þ [10,11] have been widely used in the literature for this purpose. Dy3þ/Tm3þ [12], Eu3þ/Ce3þ [13], Pr3þ/ Ce3þ [13] co-doped systems were also investigated for achieving pure white light emission. Besides, efficient energy transfer mechanisms between Tb3þ and Dy3þ [14], Tm3þ and Dy3þ [15] were reported. Yttrium aluminium garnet (YAG), when substituted with several mole or atomic percent of the activator ion Ce3þ to replace Y3þ, is a luminescent material that has nearly ideal PL properties for excitation by a blue solid-state light source [16,17]. Traditional YAG:Ce phosphor is produced by conventional solid phase method at a high temperature but recently new and economical wet chemical methods such as sol-gel [18], combustion method [19] and coprecipitation [20] have been developed for synthesis of the YAG:Ce phosphor. However, the traditional solid-phase method is still dominant in all of them. A necessary condition to see the
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Fig. 1. XRD patterns of (a) Y2SiO5:Ce3þ, Tb3þ. Reference patterns from Powder Diffraction File (PDF) are also included for comparison.
factors that govern how the YAG:Ce luminescence depends on codoping and, ultimately to be able to control the color of the YAG:Ce phosphor using codoping, is to acquire a detailed knowledge of the local structures of the double or multiple substitutional defects on the atomistic level as well as of their electronic structures [16,21,22]. From this starting point, co-dopants not only play an important role as co-activators but also as wavelength shifters [23]. For instance, substituting Y3þ with larger ions (e.g. Gd3þ and La3þ) shifts the broad yellow luminescence band of Ce3þ to long wavelength side (red shift), which can improve color rendering properties while substitution of Al3þ with Ga3þ shifts luminescence towards a shorter wavelength (blue shift) [24,25]. This work presents studies on (i) some new understanding regarding structural and the luminescence properties of Y2SiO5:Ce, Tb powders (ii) structural analysis and the luminescence characteristics of the Ce doped Y3Al5xGaxO12 compounds excited using different excitation sources in which Al3þ ions were replaced with Ga3þ in the garnet host lattice. 2. Experimental 2.1. Materials 2.1.1. Yttrium silicate (Y2SiO5: YSO) Yttrium silicate phosphors single and multiple doped with
Ce,Tb, were prepared by gel combustion. Raw materials used in this study for phosphor synthesis were commercially available yttrium oxide Y2O3 (99.9%, Aldrich), cerium nitrate Ce(NO3)3$6H2O (extra pure, Merck), terbium nitrate Tb(NO3)3$5H2O (extrapure, Merck), Laspartic acid (99.0%, Alfa Aesar), and tetraethoxysilane (TEOS- 98% Alfa Aesar). 2.1.2. Yttrium aluminate garnet (Y3Al5xGaxO12:YAG) The preparation of cerium activated yttrium aluminate based phosphors was achieved by solid state reaction route from homogeneous mixtures that contain raw oxide precursors such as: Y2O3 (Alfa Aesar 99.9%), Al2O3 (Merk), Ga2O3 (99.99% - Jansen); Ce(NO3)3$5H2O (extra pure Merck). 2.2. Synthesis of the phosphors 2.2.1. YSO:Ce3þ,Tb3þ The synthesis starts with the preparation of the gels from a mixture solution containing metallic nitrates, TEOS and aspartic acid. The solution mixture containing stoichiometric amounts of Y3þ, RE3þ and Si4þ are prepared in order to produce phosphors with general formula Y2xy CexTbySiO5. The terbium doping concentration in terms of mole fraction is varied from 0 to 0.12. The second stage of the synthesis consists in the combustion of the gels that take place in a heating mantle at 250 C. The thermal synthesis of as
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Fig. 2. Particle morphology of selected Y2SiO5 phosphors doped at different concentrations obtained by the gel-combustion method: (a) Ce %0.75, Tb 0% (b) Ce %0.75, Tb 0.5% (c) Ce %0.75, Tb 0.75% (d) Ce %0.75, Tb 1.5% (e) Ce %0.75, Tb 3% (f) Ce %0.75, Tb 6%.
prepared precursors was performed at 1400 C, in air, for 4 h and lead to the formation of the phosphor samples. The final stage of the synthesis consists on the washing, drying and sieving of the product. A detailed description of the synthesis of Y2SiO5:Ce3þ,Tb3þ phosphor samples has been presented in our previous works [26,27]. 2.2.2. YAG:Ce3þ The synthesis consists in thorough homogenization with water for 15 min of a mixture that contains stoichiometric amounts of metallic oxides (Y2O3, Ga2O3, Gd2O3, Al2O3) as generator for host lattice and cerium nitrate as activator source. The molar ratio between Al2O3 and (Y-Ce) is 1.66. The cerium content is 1.0% Ce/ (Y þ Ce). After drying at 115 C, the powder mixtures were fired initially at 1200 C for 4 h in open air atmosphere and followed by slow cooling at the room temperature. Finally, the phosphors samples washed with distilled water were subsequently dried and sieved. Different synthesis mixtures were prepared containing variable amounts of gallium to obtain phosphors having the general formula: Y2.97Ce0.03GaxAl5xO12, where x is in the range 0e5. 2.3. Characterization of phosphor samples The X-ray diffraction (XRD) patterns of the prepared samples were evaluated by a Philips PW-1710/00 diffractometer using CuKa (1.5418 Å) radiation. The XRD patterns were recorded by step scanning from 2 to 80 2q and then compared with the XRD PDF2 card files of the Joint Committee on Powder Diffraction Standards using Xpowder diffraction software. The cathodoluminescence (CL) spectra were recorded using a MONOCL3 Gatan probe with a blue sensitive photomultiplier tube (PMT) equipped with ESEM XL30 microscope. The microscope has a chemical EDS probe. PMT used for UV and visible detection covers the entire wavelength range of 250e850 nm and it is most sensitive in the blue parts of the spectrum. The RL measurements were obtained exciting the powder samples with an X-ray unit with a Machlett OEG-50A tube
operating with a current of 15 mA and a voltage of 30 kV delivering a dose rate of 30 Gy/min. Light collection is performed using a Jobin Yvon spectrometer, equipped with a liquid nitrogen cooled CCD detector. We have performed photoluminescence (PL) measurements using a deuterium lamp LDD-400 as a source of UV light. Various excitation wavelengths in the present work were selected with a grating monochromator MDR-2 in the 200e400 nm range. Emissions were collected by a Shamrock monochromator SR-303i-B. Photoluminescence excitation spectroscopy (PLE) was performed using a prism monochromator SPM-2 and detected with a Hamamatsu photomultiplier tube [28,29]. 3. Results and discussions 3.1. Structural properties 3.1.1. YSO:Ce3þ,Tb3þ XRD patterns for the Y2SiO5:Ce3þ,Tb3þ phosphors samples with varying Ce3þ and Tb3þ concentrations are shown in Fig. 1. All of the diffraction peaks are in accordance with those of Y2SiO5 (PDF 36e1476) which indicates that Tb3þ and Ce3þ dopant ions do not give rise to any significant changes to the crystal structure of the host material. When taking into consideration the effect of ionic radius of cations with different values of coordination number Ce3þ (1.01 Å) and Tb3þ (0.923 Å) dopants can presumably occupy Y3þ (0.9 Å) sites. As can be seen from Fig. 1 we observed the strong and sharp diffractions peaks. This indicate that the phosphors obtained have a high degree of crystallization which is advantageous for luminescence [30]. A SEM study was carried out to investigate the surface morphology and crystallite sizes of the phosphors produced by a gel combustion method. The SEM images of Y2SiO5:Ce3þ,Tb3þ samples prepared with addition of Tb3þ co-dopant ion are recorded and displayed in Fig. 2(aef). As seen from the Fig. 2 the powders synthesized by this method consist of agglomerates with spongy
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Fig. 3. XRD patterns of Ce3þ doped Y3Al5xGaxO12 garnets with Ga content in the x ¼ 0e5 range. Reference patterns from Powder Diffraction File (PDF) are also included for comparison.
aspect and round spherical shape crystalline with particle size within the range of 95e120 nm. The agglomerates are tightly bound each other to form relatively large aggregated structures. 3.1.2. YAG:Ce3þ The XRD patterns of Y3Al5xGaxO12:Ce3þ phosphors for various Ga concentration were shown in Fig. 3. The XRD patterns clearly show the diffraction peaks are shifted towards lower angles with increasing the Ga3þ content. This type of shifts of XRD peaks
indicate a lattice expansion due to Ga substitution for Al in the compound which seems plausible as the ionic radius Ga3þ (0.47 nm) is bigger that of Al3þ (0.30 nm). The SEM observation of Y3Al5xGaxO12:Ce3þ in Fig. 4a revealed that phosphor particles such as shape and size without Ga dopant accommodated in aluminate host lattice is relatively homogenous. The particles have not well defined morphology since they melted together. The homogeneity of the sample decreases as Ga is incorporated in the aluminate lattice as seen Fig. 4(bed).
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Fig. 4. SEM images of Ce3þ doped Y3Al5xGaxO12 samples (x ¼ 0,1,3,4).
Fig. 5. (a) Excitation and emission spectra of Y1.985Ce0.015SiO5 (b) Excitation and emission spectra of Y1.985Tb0.015SiO5 (c) Concentration dependence of PL intensity of Ce3þ broad emission band located at 400 nm with a shoulder at 420 nm (d) Concentration dependence of PL intensity of Tb3þ sharp emission lines located at 416 nm 488 nm and 544 nm.
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Fig. 6. RL emission spectra of (a) YSO:Ce with different Ce content and (b) YSO:0.015Ce, Tb with different Tb content.
3.2. Luminescence properties 3.2.1. YSO:Ce3þ,Tb3þ This study was performed to determine the influence of Tb3þ content on the luminescence properties of Y2SiO5:Ce3þ. Photoluminescence (PL) excitation and emission spectra of Y2SiO5:Ce3þ
phosphor samples ranging from UV to green regions are displayed in Fig. 5a. It is clear to see that the excitation spectrum consists of three major bands appeared at 265 nm, 300 nm and 356 nm which are attributed to the transitions from the 4f ground state (2F5/2) to the different crystal field split levels of 5d orbits. Under the excitation with 355 nm the emission spectrum consists of a doublet
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peak appeared at 400 nm which is attributed to the 5d / 4f radiative electron transitions [31,32]. As far as Tb3þ doped Y2SiO5, the excitation spectrum of this phosphor consists of one strong band peaked at 242 nm. The emission spectrum of this phosphor exhibits several emission bands over the spectral the range of 350e650 nm which are assigned to the transition 5D3 / 7Fj (j ¼ 6,5,4,3) (See Fig. 5b). The main green emission band of Tb centers, for which the maximum is located at 544 nm, belongs to the magnetic dipole 5D4 / 7F5 inside Tb3þ ion with 4f7 configurations. As can be seen from Fig. 5c emission intensities at 400 and 420 nm decrease with increasing of Ce3þ doping concentration. We suggest that the most probable mechanism causing this decreasing pattern is concentration quenching effect. Fig. 5d shows behaviour of emission lines peaked at 416 nm, 488 nm and 544 nm. It is easy to see that the emission lines centred at 544 nm and 488 nm increase sharply as the Tb3þ doping concentration is increased up to 6%. After that the intensity decreases significantly. We suggest that this is due to concentration quenching. Nevertheless, the emission line peaked at 416 nm exhibits different pattern compared to the other emission lines. Fig. 6a depicts the RL spectra measured of the YSO:Ce3þ phosphors with different concentration under continuous X-ray irradiation at the room temperature. As seen from the Fig. 6a, RL spectra do not reveal major change in peak shape or position with increasing Ce doping concentration. Since Ce3þ ion is expected to occupying two distinct lattice sites in the host material due to the different coordination numbers, there could be several emission peaks hidden under the broad Ce3þ emission band. This emission band can be decomposed using Gaussian peak fitting methods. Our aim in this work does not involve it but we will work
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on this fitting process in the future. Fig. 6b shows the emission spectra of the samples based on different Tb3þ dopant concentration. A clear feature from the spectra can be easily observed. Broad emission band centred at 420 nm in the wavelength range of 350e600 nm decreases with increasing of the Tb3þ doping concentration, whereas that of Tb3þ was simultaneously found to increase monotonically. The most plausible explanation according to our RL spectra is that the Ce3þ ions act as effective sensitizer for emission lines of Tb3þ ions and this gives rise to an efficient energy transfer between Ce3þ and Tb3þ due to parity allowed electric dipole of 4f-5d transition of Ce3þ. In other words, Ce3þ can strongly absorb radiation and efficiently transfer a part of its energy to Tb3þ. The energy level diagram in Fig. 7 depicts the energy transfer from Ce3þ and Tb3þ in YSO host material under the excitation with X-ray radiation. This is in good agreement with earlier results [33,34]. 3.2.2. YAG:Ce3þ RL measurements were performed at the room temperature under continuous X-ray irradiation with a resolution of 0.05 nm, in order to obtain some idea of the influence of Dy ions on the host material. As given in Fig. 8, the RL intensity of the Y3Al5xGaxO12:Ce3þ phosphors increases gradually with increasing of the doping concentration of the Ga ions. RL emission intensity did not reach a maximum value even though the x value is 5. The
Fig. 8. RL Luminescence spectra of Ce3þ doped Y3Al5xGaxO12 garnets with Ga content in the x ¼ 0e5 range.
Fig. 7. The energy level model for the energy transfer processes between Ce3þ and Tb3þ ions.
Fig. 9. Comparison of PL spectrum Y2.22Gd0.75Ce0.03Ga2Al2O12 at room temperature.
of
Y2.97Ce0.03Ga2Al3O12
and
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Fig. 10. Isometric views of low temperature PL from (a) Y2.22Gd0.75Ce0.03Ga2Al2O12 and (b) Y2.97Ce0.03Ga2Al3O12. The excitation was made using 325 nm line of a He-Cd laser.
most plausible explanation is that Ga3þ ions preferentially replace octahedral Al3þ lattice sites. Note that RL data of Y3Ga5O12:Ce3þ in Fig. 8 does not show significant RL emission spectrum at room temperature. Fig. 9 shows the PL emission spectra for Y2.22Gd0.75Ce0.03Ga2Al2O12 and Y2.97Ce0.03Ga2Al3O12 taken at room temperature. As seen from Fig. 9, there is a clear difference between phosphor materials with co-doped Gd and without Gd in terms of PL intensity and full width of maximum (FWHM). The PL intensity of broad band due to the Ce3þ 5d1/4f transitions results in the decrease of the intensity when Gd3þ is incorporated into the Y2.97Ce0.03Ga2Al3O12. Fig. 10 illustrates contour maps of the PL of Y2.22Gd0.75Ce0.03Ga2Al2O12 and Y2.97Ce0.03Ga2Al3O12. As seen in Fig. 10, Ce3þ related broad band luminescence located at 516 nm decreases with increasing temperature. We have observed different luminescence pattern when Gd3þ was accommodated in the Y2.97Ce0.03Ga2Al3O12. The luminescence intensity of broad band located at 516 nm is low but the peak intensity clearly increases with decreasing the temperature. At low temperature PL measurements, one would expect to see significantly better resolution and more intense emissions since the phonon emissions are reduced. It is worth noting that the broad band feature does show no spectral shift from 10 to 300K as expected because there is shielding of the transitions in the configuration of rare earth ions. Interestingly, decreasing luminescence intensity value obtained for YGd0.75AlGa2O12:Ce at room temperature can be attributed to further decrease in the band gap and in the crystal field splitting, which reduces the energy difference between the 5d1 excited state and the bottom of the conduction band. It leads to luminescence quenching due to thermal ionization of 5d1 excited state of Ce3þ emission center [35,36].
4. Conclusions A series of novel Y2SiO5:Ce3þ, Tb3þ and (Y3Al5xGaxO12): Ce3þ, Gd3þ were prepared by a gel combustion method and solid state reaction, respectively. The dependence of Ce3þ and Tb3þ ions concentration on the structure, morphology and photoluminescence and radioluminescence of the phosphor samples has been investigated in detail. By co-doping with Tb3þ ions, the Ce3þ doped Y2SiO5 is observed to show the following features in terms of structural and luminescence properties:
(i) Observation of phase (X2) at high reaction temperature with round-spherical morphology (ii) Mean size of 100 nm in diameter (iii) Main blue and green emission peaks located at 487 nm and 544 nm corresponding to electric and magnetic dipole allowed transition of Tb3þ ion, respectively (iv) Tunable emission of the Tb3þ ion from blue to green region after Ce3þ excitation (v) A decrease of the Ce3þ overall emission (vi) An energy transfer process between Ce3þ and Tb3þ ions As regards (Y3Al5xGaxO12):Ce3þ, Gd3þ phosphor materials we have yielded the following main conclusions. (i) Shifting of the diffraction peaks due to lattice expansion (ii) Observation of Ce3þ related luminescence under partial substitution of Ga3þ with Al3þ (iii) A sharp decrease of the Ce3þ emission intensity in the case of Gd doped (Y3Al5xGaxO12):Ce3þ and observation different luminescence emission pattern in the range of 10e300 K On the basis of the promising findings illustrated here, further research to investigate luminescence properties of the samples based on parameters such as excitation wavelength, laser power, excitation source is still continuing and will be presented in our future papers. References [1] Z.H. Zhang, Y.H. Wang, Y. Hao, W.J. Liu, Synthesis and VUV photoluminescence of green-emitting X2-Y2SiO5:Tb3þ phosphor for PDP application, J. Alloy. Comp. 433 (2007) L12eL14. [2] F. Thibault, D. Pelenc, B. Chambaz, M. Couchaud, J. Petit, B. Viana, Growth and characterization of Yb3þ doped Y2SiO5 layers on Y2SiO5 substrate for laser applications, Opt. Mater. 30 (2008) 1289e1296. [3] F. Thibault, D. Pelenc, B. Chambaz, M. Couchaud, J. Petit, B. Viana, Growth and characterization of Yb3þ doped Y2SiO5 layers on Y2SiO5 substrate for laser applications, Opt. Mater. 30 (2008) 1289e1296. [4] L. Chen, C.-C. Lin, C.-W. Yeh, R.-S. Liu, Light converting inorganics phosphors for white light emitting diodes, Materials 3 (2010) 2172e2195. [5] L.E. Muresan, B.F. Oprea, A.I. Cadis, I. Perhaita, O. Ponta, Studies on Y2SiO5:Ce phosphors prepared by gel combustion using new fuels, J. Alloys Comp. 615 (2014) 795e803. [6] G. Ramakrishna, H. Nagabhushana, R.B. Basavaraj, S.C. Prashantha, S.C. Sharma, Ramachandra Naik, K.S. Anantharaju, Green synthesis structural characterization and photoluminescence properties of Sm3þ co-doped Y2SiO5:
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Please cite this article in press as: L.E. Muresan, et al., Preparation and characterization of Yttrium based luminescence phosphors, Optical Materials (2017), http://dx.doi.org/10.1016/j.optmat.2017.01.044