Concentration Quenching and Luminescence Decay in Yb3+ doped Cerium Tri-metaphosphate

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ScienceDirect Materials Today: Proceedings 15 (2019) 626–632

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ICMAM-2018

Concentration Quenching and Luminescence Decay in Yb3+ doped Cerium Tri-metaphosphate S.U. Bhonsulea*, Deepali Marghadeb, S.P.Wankhedec a*

Department of Applied Physics, Priyadarshini College of Engineering, Nagpur 440019, India

b

Department of Applied Chemistry, Priyadarshini Institute of Engineering and Technology, Nagpur 440019, India. C

Department of Applied Physics, K.D.K. College of Engineering, Nagpur 440009, India.

Abstract The compounds Yb2O3 doped Ce(PO3)3 were synthesized using solid state reaction method. New results on photoluminescence in Ce(PO3)3 are reported. Energy transfer was observed from Ce3+ to Yb3+ ions. Concentration quenching was observed above 5% Yb3+ doping in Ce(PO3)3. Luminescence decay measurements established that energy transfer took place from Ce3+ to Yb3+. Lifetime measurements showed considerable reduction in lifetime of doped compounds. Non radiative energy transfer takes place from Ce3+ to Yb3+ ions. The effective mechanism of concentration quenching seems to be the multipolar interaction. Such materials have desired applications in production of C-Si solar cells. © 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON MULTIFUNCTIONAL ADVANCED MATERIALS (ICMAM-2018).

Keywords: Cerium metaphosphate; Photoluminescence; Solid state synthesis; Energy transfer; Luminescence decay

*Email address: [email protected]

2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of INTERNATIONAL CONFERENCE ON MULTIFUNCTIONAL ADVANCED MATERIALS (ICMAM-2018).

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1. Introduction The characteristic features of rare earth meta phosphates, RE(PO3)3, make them suitable candidates for usage in plasma display panels, in scintillators, as ionising radiation detectors and in mercury-free lamps[1]. These materials have simple methods of synthesis[2]. Zhang et al[3] have discussed that the host absorption efficiency and the energy transfer efficiency are equally important factors to be taken into account while using VUV-excited phosphors in mercury-free lamps. Aspects of Ce3+ luminescence in Ce(PO3)3 [4] and in Lu(PO3)3compounds[5] have already been reported. There are reports on extensive research on scintillator properties of Pr(PO3) [6] and Nd3+ luminescence in La(PO3)3[7]. Extensive details of investigations of luminescence in Y(PO3)3 doped Eu3+ has been done by Wang et al [8]. Ternane et al[9] studied La(PO3)3 and Y(PO3)3 codoped with trivalent Eu3+ and Tb3+ ions as promising luminescent phosphors. Ln(PO3)3(Ln=La to Gd) has monoclinic structure and belongs to C222 space group where Ln ion has a large radius while Ln(PO3)3(Ln=Gd to Lu and Y) has monoclinic structure with smaller radius of Ln ion and belongs to P2/c space group[10]. In both these structures, the eight fold coordination of Ln3+ ion has a profound effect on their luminescence properties. Large number of cerium compounds like CePO4, CeF3 and CeBO3 show strong luminescence. Strong luminescence has also been observed in choroaluminates [11]. In comparison with other trivalent lanthanides, Cerium is a strong contender for energy transfer studies. Its parity allowed 4f- 5d electronic transitions fall in visible region. The two wide bands generally obtained in the emission spectrum of cerium are due to 5d-4f transitions from the lowest level of 5d configuration to the 2F5/2, 2F7/2 states of the 4f1 configuration. The lifetime of 5d-4f electric dipole transitions is of the order of few nanoseconds with fast decay time. The Ce3+→Ce3+distance(R) is of the order of 15 Å to 20 Å. The transfer of energy from sensitizer to activator ions depends on this distance [12]. Concentration quenching as observed with increase in dopant concentration above a critical value is found to occur when there is strong interactions between two close active ions and there is transfer of excitation energy from one to other many a times and this energy is ultimately lost at dislocations or impurities. With reference to above discussion, photoluminescence properties of Ce(PO3)3 doped Yb3+ has been studied in this paper. Details about the energy transfer process between Ce3+ and Yb3+ are discussed and relevant mechanism is proposed. 2. Experimental The synthesis of the compounds was done by solid state reaction method. Ce(PO3)3:x%Yb(x = 0,0.01,0.03,0.05,0.06,0.08 and 0.1%) was prepared by sintering the stoichiometric mixture of cerium oxide CeO2 , NH4H2PO4 and Yb2O3. The mixture was sintered at 250, 500 and 900°C for 2, 5 and 15 hours respectively in a programmable furnace [13]. Philips PANalytical X-ray diffractometer X’pert Pro was used for X-ray diffraction measurements of the prepared phosphors to verify the formation of phase pure compounds. The photoluminescence measurements were done using Hitachi F-7000 fluorescence spectrophotometer. For Near infra-red (NIR) range, QM-51 NIR Spectrophotometer was used. Fluorescence lifetime measurements in nanoseconds range were done with Time-Correlated Single Photon Counting (TCSPC) technique using HORIBA make equipment. 3. Result and Discussion 3.1 Structural characterization Fig.1 shows the X-Ray diffraction (XRD) patterns of prepared Ce(PO3)3 : xYb3+phosphors with various concentrations of Yb3+ ions. The patterns were found to be in good agreement with ICDD file no.33-0335 for pure phase Ce(PO3)3.The XRD results show no diffraction peak shift.

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Fig.1. X-ray diffraction patterns of pure and Yb3+ doped Ce(PO3)3.

3.2 Crystal Structure of Ce(PO3)3 The structure of Ce(PO3)3 is similar to the structures of La, Pr, Nd and Gd C-type polyphosphates[14]. It has orthorhombic structure with C2221 space group. The structure consists of helical polyphosphate chains of six tetrahedra running along the c-axis.Fig.2 shows the Ce3+ having eightfold coordination with oxygen O atoms of the polyphosphate chains. Ce with one P and one O atom are placed on twofold rotation axes. The Ce-O bond length varies from 2.390 to 2.406 Å. The nearest Ce-Ce distance is 4.23 Å whereas the distance between the chains is 7Å [15].

Fig.2. 8 Coordinated Ce3+ ions in Ce(PO3)3

3.3 Photo-Luminescence (PL) spectra for Ce(PO3)3 The PL emission and excitation spectra of pure Ce(PO3)3 are shown in Fig.3 curves (a) & (b). The emission spectra(curve a)shows an prominent peak at 324 nm (30864 cm-1, 3.83eV) upon 254 nm excitation and an unresolved peak at 306 nm (32679 cm-1, 4.05eV). The excitation spectra (curve b) shows a broad band in the range 221 to 316 nm with 3 peaks at 294.4 (33967cm-1, 4.13eV), 270.4 (36982cm-1, 4.59eV) and 237nm (42194cm-1, 5.23eV) along with a shoulder at 258 nm (38759cm-1, 4.81eV). These bands are due to the f–d transitions between 2F5/2 (4f1) and crystal field components of 2D (5d1) excited state. This resulted in Stokes shift calculations of about 3103 cm-1.

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Fig.3- PL Spectra of Ce(PO3)3 (a) Emission for 254 nm excitation (b) Excitation for 324 nm emission

3.4 PL spectra for Ce(PO3)3:xYb3+ PL emission and excitation spectra of Ce(PO3)3:xYb3+(1%,3%,5%,6%,8%,10%) are shown in Fig.4(curves a-n). Curve (a) shows the excitation spectrum for 979 nm emission of 1mol% doped Yb3+sample. Yb3+ emission spectra for 297 nm excitation are shown in curves (b) to (g). As the concentration of Yb3+ ions increases, the emission intensity of Ce3+ remarkably decreases and the NIR emission intensity of Yb3+ ions initially increases and then decreases above 5% due to concentration quenching.

Fig.4. Ce3+ →Yb3+ energy transfer in Ce(PO3)3: xYb3+ (a) Excitation spectra for 1% Yb3+ (for 979 nm emission) Emission spectra for various concentrations of Yb3+ (for 297 nm excitation), b) 1% (c) 3% (d) 5% (e) 6% (f) 8% (g) 10% (h)-(m) Ce3+ emission for 298 nm excitation (n) Excitation for 330 nm (Ce3+) emission The inset shows the dependence of Ce3+ (330 nm) and Nd3+ (979 nm) emission intensities on the Yb3+ concentration in Ce(PO3)3: xYb3+

Fig.5 gives a broader view of emission spectra of Ce(PO3)3:xYb3+ (x=0.01, x=0.03, x=0.05, x=0.06, x=0.08, x=0.1) for 297 nm excitation. The inset in this figure shows the initial increase in NIR emission intensity of Yb3+ ions. The emission intensity of Yb3+ ions then decreases as the concentration of Yb3+ ions increases due to concentration quenching [16]. The energy transfer mechanism seems to be the Cooperative Energy transfer (CET mechanism) taking place from Ce3+ to Yb3+ ions. CET mechanism is explained in Fig 6. Ce3+ absorbs a high energy photon and then transfers the energy to two Yb3+ ions leading to emission of two NIR photons [17]. The Table 1 summarizes the PL results.

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Fig.5. Emission spectra of Ce(PO3)3:xYb3+ (x=0.01, x=0.03, x=0.05, x=0.06, x=0.08, x=0.1)

Fig.6- Ce3+ to Yb3+energy transfer mechanism

Table 1 Intensity, Wavelength And Concentration of Yb3+ in Ce(PO3)3

Phosphors

Emission Peak Wavelength (nm ) (λ ext.= 297 nm)

Relative Intensity (a. u)

Ce(PO3)3:Yb(1%)

979,1007,1023

125,36.7,36.7

Ce(PO3)3:Yb(3%)

979,1007,1023

138,36.5,36.2

Ce(PO3)3:Yb(5%)

979,1007,1023

179,48.5,48.2

Ce(PO3)3:Yb(6%)

979,1007,1023

89.6,23.4,23.2

Ce(PO3)3:Yb(8%)

979,1007,1023

65,19.7,20

Ce(PO3)3:Yb(10%)

979,1007,1023

60,19.8,19.9

3.5 Luminescence decay measurements Lifetime measurements were done for pure and doped compounds. The resolved curves ‘a’ to‘d’ in figure 7 verified that there is indeed a transfer of energy from sensitizer Ce3+ ions to doped Yb3+ ions. As the amount of doping was increased the lifetime was found to be reducing from10 ns to 1.2 ns. The calculations of average lifetime of undoped and doped samples for various Yb3+ concentrations is shown in the inset of figure 7.

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Fig.7. Luminescence decay curves corresponding to Ce3+ emission in Ce(PO3)3: xYb3+ (a)x=0 (b) x=0.01 (c) x=0.05 and (d) x=0.1 under 295 nm excitation. Inset show lifetime for various Yb3+ concentrations

The energy transfer efficiency

ETE

%

was obtained from following equation:

ETE  1 

x 0

(1)

where τx and τo are the lifetimes of undoped and doped samples respectively[18,19]. Table 2 presents the lifetime (τ) and energy transfer efficiency data (

%) for various concentrations of Yb3+.

Table 2 Lifetime and Energy Transfer Efficiency % in Ce(PO3)3: xYb3+ S. No. 1 2 3 4

x mol% Yb3+ 0 1 5 10

τ(ns) 10 8.6 2.2 1.2

ɳ ETE% 0 14 78 88

Photoluminescence measurements for undoped and doped compounds depicts that concentration quenching occurs above a critical concentration of 5% of Yb3+. To examine what type of mechanism is responsible for such type of non-radiative transfer of energy from Ce3+ to Yb3+ ions, critical energy transfer distance Rc was calculated using equation 2. aV 1/ 3 RC  2( ) (2) 4X C N where XC is the critical concentration, N is the number of cation sites in the unit cell, and V is the volume of the unit cell. For Ce(PO3)3, V (Å3) =711.4,N=4 and Xc=0.05, the value of Rc was calculated as 18.94Å. The non-radiative energy transfer of the luminescence is either by electric multipole–multipole interaction [21,22] or exchange interaction [20]. For critical distance greater than 5 Å, multipole–multipole interaction is important. When the distance is less than 5 Å, exchange interaction is effective. Since the Rc value for Ce(PO3)3: Yb3+ was calculated as 18.94 Å, the mechanism of concentration quenching in Ce(PO3)3:Yb3+ seems to be the multipolar interaction [23]. 4. Conclusions Yb3+ doped Ce(PO3)3 was obtained successfully using solid state reaction method. Strong photoluminescence is obtained in Ce(PO3)3 which points toward weak Ce3+- Ce3+ interaction. This is an anticipated result as the shortest Ce3+–Ce3+ distance is 4.23 Å. Photoluminescence measurements have shown that concentration quenching is observed above a critical concentration of 5% Yb3+. There is a transfer of excitation energy from cerium to ytterbium ions. Lifetime decay measurements have shown a decline of lifetime from 10 ns to 1.2 ns. Thus the

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transfer of energy from Ce3+to Yb3+ ions is supported by analysis of lifetime data. Critical energy transfer distance (Rc) was obtained as 18.94 Å. Thus the energy transfer mechanism can be recognized as multipole–multipole interaction. The potential applications of these phosphors are in development of new luminescent materials for white LED’s, Lasers, energy saving fluorescent lamps and for spectral conversion in C-Si solar cells. Acknowledgements This work received financial support and assistance from I.C.M.R, New Delhi and Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India. The lifetime measurements were done by Sophisticated Analytical Instrument facility (SAIF) IIT, Chennai, India. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

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