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Photoluminescence and energy transfer studies in Ce3+ and Sm3+ activated P2O5+K2O+Al2O3+BaF2+NaF2 glasses for solid state lighting
Optical Materials 99 (2020) 109576
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Photoluminescence and energy transfer studies in Ce3þ and Sm3þ activated P2O5þK2OþAl2O3þBaF2þNaF2 glasses for solid state lighting V. Rajeswara Rao a, Ramachari Doddoji b, Wisanu Pecharapa c, J. Kaewkhao d, Shobha Rani Depuru e, C.K. Jayasankar a, * a
Department of Physics, Sri Venkateswara University, Tirupati-517 502, India Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Bangkok 10520, Thailand d Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand e Institute of Aeronautical Engineering, Hyderabad-500 043, India b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorophosphate glasses (Ce3þ,Sm3þ):glasses Energy transfer W-LEDs
A color of blue-red emission by Ce3þ and Sm3þ activated fluorophosphate glasses (PKABfNf:P2O5þK2O þ Al2O3þBaF2þNaF2) were successfully fabricated by using usual melt and sudden quenching method. The FTIR, SEM, EDS, luminescence spectra, the decay rates, and energy transfer (ET) mechanism in the co-doped (Ce3þ, Sm3þ):PKABfNf glasses were systematically investigated in detail. The luminescence spectra of co-doped glasses show broadband blue de-excitation that belongs to Ce3þ ions and it obviously intensify the red de-excitation of Sm3þ through an effective ET process. The decrease in lifetime could support the existence of ET between Ce3þ and Sm3þ in PKABfNf glasses. This ET from Ce3þ to Sm3þ can be proposed to be the interaction due to quadrupole-quadrupole, based on the Dexter’s ET formula and Reisfeld’s approximation. Furthermore, CIE chromaticity coordinates, color correlated temperature, color purity, ET luminous efficiency (ηET), critical energy transfer distance (Rc), and ET parameter (Q) were calculated. The findings show that (Ce3þ,Sm3þ):PKABfNf glass might be a promising candidate in lighting field applications.
1. Introduction The next generation of solid-state lighting business has drawn tremendous attention for inorganic luminescent materials in virtue of their characteristic advantages consisting of considerable power con servation, high luminous efficiency, longer lifetime, enhanced bright ness and friendly to environmental conditions [1–3]. Most of the commercialized w-LEDs were normally fabricated as InGaN based blue chip combining the yellow emitting YAG:Ce3þ phosphor [4,5]. How ever, they still have the drawbacks of red emission which can be used in w-LEDs, excited through near-UV light. Thus, it is essential to search for alternative sources for blue, green or red color light. However, glasses exhibit the advantage of high homogeneity, transparency and thermal stability with low fabrication cost and also can be shaped in required size that can be sandwiched easily on LED chip [6–13]. Rare-earth activated luminescence complexes are frequently found in sensing, lighting and displaying due to their outstanding properties that include longer lifetimes and sharp emission peaks. Among number of
lanthanide ions, Sm3þ ion found to be an appropriate activator due to its de-excitation energy of 4G5/2 state with good quantum efficiency and many radiative states. Also Sm3þ ion have intense absorption in the nearUV region and it de-excites intense orange-red emission which has extensive applications for the design of visible optical devices such as glass lasers, LEDs, optical amplifiers operating in the NIR spectral region [14, 15]. Also, the Ce3þ ions show a strong and broad f-d excitation and de-excitation transitions located in UV-VIS region due to electric-dipole transition of 4f-5d [6,16–18]. As such, Ce3þ ions can act as an excellent sensitizer and transfer their energy to Sm3þ ions, thus increases the red emission that corresponds to Sm3þ ions [18–23]. Energy transfer (ET) of this nature has been reported in many systems like (Ce3þ,Sm3þ):Na2O þY2O3þSiO2, (Ce3þ,Tb3þ):nano glass-ceramics and (Sm3þ,Ce3þ):barium fluoroborate, under UV–Visible photon conversion for white LEDs and solar cell applications [23–26]. Also studied (Ce3þ,Sm3þ):Lu2SiO5þGd2 SiO5 solid solution crystals [27], (Ce3þ,Sm3þ):Y2SiO5 [28] and (Ce,Sm): CaS nano phosphors for applications in w-LEDs [29]. White light source quality has been estimated by various
* Corresponding author. E-mail address: [email protected] (C.K. Jayasankar). https://doi.org/10.1016/j.optmat.2019.109576 Received 23 October 2019; Received in revised form 24 November 2019; Accepted 25 November 2019 Available online 14 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Table 1 Selected glass compositions along with glass lables. S. No.
Composition
Label
1 2 3 4 7 8 9 10
45P2O5þ15K2Oþ10Al2O3þ19.99BaF2þ10NaF2þ0.01Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.95BaF2þ10NaF2þ0.05Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.9BaF2þ10NaF2þ0.1Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.5BaF2þ10NaF2þ0.5Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.89BaF2þ10NaF2þ0.1CeO2þ0.01Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.85BaF2þ10NaF2þ0.1CeO2þ0.05Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.8BaF2þ10NaF2þ0.1CeO2þ0.1Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.4BaF2þ10NaF2þ0.1CeO2þ0.5Sm2O3
PKABfNfSm0.01 PKABfNfSm0.05 PKABfNfSm0.1 PKABfNfSm0.5 PKABfNfCe0.1Sm0.01 PKABfNfCe0.1Sm0.05 PKABfNfCe0.1Sm0.1 PKABfNfCe0.1Sm0.5
performance parameters such as tristimulus coordinates (x, y), color rendering index (CRI), luminous efficacy of optical radiation (LER), correlated color temperature (CCT), color purity (CP) besides color component (fluorescent level) of peak emission wavelength, full-widthat-half-maximum, radiative lifetime and relative amplitude [30–33]. In general, for high quality white light generation, the most impor tant properties are CRI values that varies from 100 (high color rendition) to 100 (poorest color rendition) whereas LER should be greater than 380 lm/W at a warm CCT (i.e., CCT<4000 K). As such the luminescence of organic [34] and inorganic [14–40] materials have been investigated to tune, tailor and optimize the generation of white light. However, the fundamental relationship among CRI, LER and CCT are still not completely understood. In the case of rare earth doped glass-phosphorous, addition of fluo rides into phosphate glasses, the resultant oxyfluoride glasses exhibit the combined advantages of oxide and fluoride glasses such as physical and chemical durability, thermal stability, high transparency, low phonon energy, low melting point and mechanical durability. With these enhanced optical properties, oxyfluoride glasses are suitable for wide range of applications in optical technology [41,42]. Recently, the importance of Ce3þ with Dy3þ ions in fluorophosphate glasses for white light emission has been reported [14,43]. In this direction, the white light source parameters such as CIE co-ordinates, CCT and CP may be useful to study new glass composition of Ce3þ singly and (Ce3þ,Sm3þ) co-doped fluorophosphates (PKABfNf) glasses. Also it is quite interesting to investigate the ET process from Ce3þ to Sm3þ ions (ET:Ce3þ → Sm3þ) in PKABfNf glasses wherein Dexter’s ET formula and Reisfeld’s approximation [44–47] can be adopted to analyse the electric multi polar interactions. Therefore, the present work focuses on the fluorophosphate (PKABfNf) glasses co-doping of different (0.01, 0.05, 0.1, and 0.5 mol%) Sm3þ ions with a fixed concentration of Ce3þ (0.1 mol%) and were developed by the conventional melt and sudden quenching method. The structural and elemental mapping were determined from the FTIR, SEM and EDS analysis. The photoluminescent and in turn color coordinates, CCT and CP as well as ET process has also been studied [44–49]. The (Ce3þ,Sm3þ):PKABfNf glass emit blue (Ce3þ) to orange-reddish (Sm3þ) color and found to be useful for white light-emitting diodes.
Fig. 1. FTIR Spectra of Ce3þ/Sm3þ co-doped PKABfNf glasses.
FTIR spectra were recorded on Perkin-Elmer (Frontier) spectrometer from 400 to 2500 cm 1 with spectral resolution of 10 μm. Edinburgh (FLS 980) fluorescence spectrometer have been used to measure the excitation and de-excitation spectra as well decay rates. JSM-IT500 JEOL scanning electron microscope have been used to study the morphology and composition details. The phonon energy of the host (PKABfNf) glass was estimated from the Raman spectrum as 1132 cm 1 and the same was already reported in our earlier works [14,43]. 3. Results and discussions 3.1. FTIR studies Fig. 1 presents the FTIR spectra measured from 400 to 2500 cm 1 for PKABfNfCeSm co-doped fluorophosphate glasses to analyse the struc tural details. The observed low frequency transition within the range of 460–500 cm 1 is ascribed to harmonics or bending mode of vibration of PO2 units and O–P¼O linkages. The weak transition at 473 cm 1 is due to cerium cations mode of vibrations at their network sites [50]. The transition at 642 cm 1 belongs to Al2O mode of vibrations (Al–O–Al). The band at 781 cm 1 is assigned to the symmetric stretching mode of vibration of the P–O–P groups. The transition at 970 cm 1 is labeled to symmetric mode of vibrations of the PO4 tetrahedra (P–O- ionic group). The transition at 1151 cm 1 is related to asymmetric mode of vibrational groups of phosphates. The weak band at 1321 cm 1 could be ascribed to PO2 asymmetric stretching vibrations of non-bridging oxygens (NBO). The band at 2360 cm 1 is ascribed to hydrogen bond [51–53].
2. Experimental A series of (Ce3þ,Sm3þ) co-doped and Sm3þ singly doped PKABfNf glasses of 45P2O5þ15K2Oþ10Al2O3þ(19.9-x)BaF2þ10NaF2þ0.1CeO2 þxSm2O3 (x ¼ 0.01, 0.05, 0.1, and 0.5 mol %), were fabricated by usual conventional melt and sudden quenching technique. The chemical composition with glass labels are presented in Table 1. High purity (>99.9%) compounds of Al(PO3)3, KPO3, NaF2, BaF2, Sm2O3 and CeO2 were taken. Each batch (30 g), as shown in Table 1, was well grinded and made homogenized mixture which was transferred to alumina crucible and melted at 1200 � C for 2 h. Bubble free with homogenized melt was quenched rapidly at RT on a preheated stainless steel mold and subse quently annealed at 420 � C for 16 h (to remove stress and strain) and then allowed to cool slowly to RT. The developed glasses were cut for desired dimension and polished which are characterized. 2
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gives information about the elements which are present in the PKABfNfCe0.1Sm0.5 glass. The spectrum of EDS exhibits number of peaks belong to the O, F, Na, Al, P, K, Ba, Ce and Sm elements. The percentage of elements found to be O (49.65 wt%), F (0.01 wt%), Na (4.67 wt%), Al (4.90 wt%), P (18.61 wt%), K (5.83 wt%), Ba (11.96 wt%), Ce (1.42 wt %) and Sm (1.50 wt%) in PKABfNfCe0.1Sm0.5 glass. The inset shows the SEM image which clearly suggest the absence of cracks and un-melted portions in PKABfNfCe0.1Sm0.5 glass, justifies that the glass is of amor phous nature. 3.3. Absorption studies The absorption spectral profile of PKABfNfSm0.5 singly doped glasses is depicted in Fig. 3(a) (UV–Visible) and 3(b) (NIR). The addition of Sm2O3 (0.5 mol%) to the glass, the spectrum exhibited several absorp tion bands resulting from the ground state (6H5/2) to the several excited states of 6F1/2, 6H15/2, 6F3/2,5/2,7/2,9/2,11/2, 4G5/2, 4F3/2, 4G7/2, 4I11/2,13/ 4 4 6 4 4 2, G9/2, M19/2, P3/2, L15/2,17/2, and D3/2,7/2 and the corresponding wavelengths are 1588, 1527, 1484, 1379, 1231, 1079, 945, 560, 534, 500, 473, 462, 439, 416, 401, 391, 375, 361 and 344 nm, respectively. When Ce2O3 (0.1 mol%) and Sm2O3 (0.5 mol%) is co-doped in flu orophosphates glass (PKABfNfCe0.1Sm0.5), the bands at 344, 361 and 375 nm could not be visualized well in the spectra (Fig. 3(c)), however,
Fig. 2. SEM and EDS spectra for PKABfNfCe0.1Sm0.5 glass.
3.2. SEM and EDS analyses Fig. 2 depicts the SEM (inset shows 10 μm range scan) and EDS micro graphs of PKABfNfCe0.1Sm0.5 glass, measured by using JSM-IT500 InTouchScope™ Scanning Electron Microscope. The EDS spectrum
Fig. 3. Absorption spectrum of PKABfNfSm0.5, and PKABfNfCe0.1Sm0.5 glasses in UV–Visible, and NIR region. The inset figure shows the absorption spectrum of Ce3þ singly doped PKABfNfCe0.1 glass. 3
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Fig. 4. (a) Excitation Spectrum of Sm3þ doped PKABfNfSm0.5 glass and (b) emission spectra of PKABfNf glasses for different concentrations of Sm3þ ions.
The strongest intensity band of 6H5/2 → 6P3/2 (at 402 nm) is a hyper sensitive transition which can be normally used for fluorescence exci tation [54–56]. 3.4. Photoluminescence and decay curve studies Fig. 4 presents the excitation and de-excitation spectra of PKABfNf: Sm3þ glasses. From Fig. 4(a), the excitation spectrum (λemi ¼ 597 nm) exhibit number of transitions and are assigned as 6H5/2 → 4P3/2 (318 nm), 4G9/2 (332 nm), 4D7/2 (344 nm), 4D3/2 (361 nm), 6P7/2 (374 nm), 4 L15/2 (391 nm), 6P3/2þ4F7/2þ4L13/2 (402 nm), (6P, 4P)5/2 (416 nm), 4 I15/2 (439 nm), 4I13/2 (461 nm), 4I11/2 (472 nm), 4G7/2 (500 nm) and 4 F3/2 (527 nm). Among these, intense band observed at 402 nm has been excited to measure emission spectra for various concentrations of Sm3þ:PKABfNf glasses and are presented in Fig. 4(b). The observed emission spectra in orange-red region are peaked at 561, 598, 644 and 705 nm corresponds to 4G5/2 → 6H5/2,7/2,9/2,11/2 levels, respectively [55–57]. Luminescence decay rates of 4G5/2 state for different concentration of Sm3þ:PKABfNf have also been investigated and shown in Fig. 5. For higher concentrations of Sm3þ ion, the decay rates exhibit nonexponential nature. Therefore, effective lifetime (τeff) are determined by taking the following equation. R tIðtÞdt (1) τeff ¼ R IðtÞdt
Fig. 5. Decay curves of PKABfNf glass for different concentrations of Sm3þ ions.
observed a weak band at 310 nm of Ce3þ shown in the inset of Fig. 3(c). The intensity of all these observed transitions decreases with the pres ence of cerium which means that cerium can act as a UV blocker below 400 nm and gives the information of transferring energy to Sm2O3 ions. The low energy (NIR) bands of Sm3þ is also observed in the presence of Ce2O3, as presented in Fig. 3 (d). All absorption levels existing in the region of UV–Visible are relatively weak than those observed in NIR region, since the UV–Visible transitions observed are spin forbidden.
in equation (1), I(t) refers the intensity of emission at time ‘t’. The values of τeff tabulated in Table 2 are found to be 2.35, 2.31, 2.29 and 2.02 ms for 0.01, 0.05, 0.1, and 0.5 mol% (Sm3þ), respectively. The shortening
Table 2 Lifetimes (τeff) of single Sm3þ, and Ce3þ/Sm3þ co-doped glasses, and energy transfer efficiency (ηET, %) of Ce3þ/Sm3þ co-doped fluorophosphate glasses for different concentrations (C). S. No.
Singly doped Sm3þ @ λexc ¼ 402 nm C (Sm
1. 2. 3. 4.
0.01 0.05 0.1 0.5
3þ
)
Co-doped Ce3þ/Sm3þ@ λexc ¼ 290 nm
τeff (ms)@ λemi ¼ 597 nm
C (Ce3þ/Sm3þ)
τeff (ms)@ λemi ¼ 597 nm
τeff (μs)@ λemi ¼ 365 nm
ηET
2.35 2.31 2.29 2.02
0.1/0.01 0.1/0.05 0.1/0.1 0.1/0.5
2.37 2.34 2.32 2.10
7.72 7.53 7.24 7.52
5 6 12 26
4
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Fig. 6. Excitation (a and b) and emission (c and d) spectra of Ce3þ doped PKABfNf glass with different concentrations of Sm3þ ions. The inset figure shows the emission spectrum of Ce3þ singly doped PKABfNfCe0.1 glass.
nature of τeff with increase in Sm3þ concentration confirms ET between Sm3þ ions via cross-relaxation (CR).
energy of 402 nm correspond to the relaxed lowest 6P level to the 6H ground level of Sm3þ, but Ce3þ could not be visualized at this wave length as seen in Fig. 6(d). Therefore, the de-excitation spectra presented in Fig. 6(c) upon 290 nm can be tuned for the intensity of luminescence by changing the Sm3þ ions concentration. While increase in concen tration of Sm3þ, the orange-red emission (~597 nm) of Sm3þ intensities increases rapidly as shown in Fig. 7 and demonstrate the effective ET: Ce3þ → Sm3þ in PKABfNf glasses. Moreover, the ET efficiency between Ce3þ to Sm3þ can be expressed by,
3.5. Energy transfer between Ce3þ and Sm3þ For the purpose of studying the ET:Ce3þ → Sm3þ, the excitation and de-excitation spectra of PKABfNf glasses by co-doping Ce3þ (0.1 mol%) with different Sm3þ concentrations were measured and presented in Fig. 6. By monitoring the emission at 597 nm of PKABfNfCe0.1Sm0.5 glass as presented in Fig. 6(a) and (b), exhibit not only 4f-5d of Ce3þ but also 4f-4f levels of Sm3þ. This spectra also reveals the enhanced absorption intensity from 250 to 350 nm, which indicates the presence of ET:Ce3þ → Sm3þ. The luminescence intensity differs with the excitation spectra which further reveals the existence of ET:Ce3þ → Sm3þ in PKABfNf glasses. Inset of Fig. 6(b) shows the excitation spectrum of PKABfNfCe0.1 glass. The de-excitation spectra presented in Fig. 6(c) and (d) covers from 300 to 750 nm, exciting at 290 and 402 nm, respectively. The excitation
ηET ¼ 1
Ice Ice þ ISm
(2)
here ICe and ISm refers the integrated intensities of 365 (Ce3þ) and 597 nm (Sm3þ) corresponding to 5 d → 4f and 4G5/2 → 6H7/2 transitions collected from Fig. 6(c), respectively. According to eq. (2), the value of ηET is found to be 5, 6, 12, and 26% for PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 glasses, respectively. 5
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emission at 365 and 597 nm, respectively, and are presented in Fig. 9. As seen in Fig. 9(a), (b) and (c), the decay rates are not perfect exponential and therefore τeff values (Table 2) were calculated using eq. (1) for Ce3þ/Sm3þ co-doped PKABfNf glasses. It is noticed that the decay times of co-doped glasses decrease significantly with increase in concentration of Sm3þ due to ET:Ce3þ → Sm3þ. This is because of the presence of more number of Sm3þ ions around Ce3þ, when Ce3þ concentration fixed with different Sm3þ concentration. Thus, the shortening of the decay rates of Ce3þ with the increase of Sm3þ concentration could confirm the mech anism of non-radiative ET:Ce3þ → Sm3þ ion as depicted in energy level scheme of Fig. 10. Normally, the non-radiative ET surrounded by rare-earths is responsible for shortening the decay time through the exchange, electric multipolar and radiation reabsorption interactions. For exchange interaction, the Rc (distance for critical energy transfer) can be deter mined by taking the Blasse’s relation [58],
Fig. 7. Relative emission intensities of Ce3þ and Sm3þ ions in PKABfNf glasses.
3Vm 4πXc Z
�13 =
� Rc ¼ 2
(3)
here Vm represents the molar volume of the co-doped glass, Xc represents the sum of the critical concentrations of Sm3þ and Ce3þ, where the luminescence intensity decreases with Sm3þ concentration, Z represents cation’s oxidation number that occupies the Ce3þ ions in the unit vol ume. In this regard, Vm ¼ 40.88 cm3/mol, Xc ¼ 0.6, Z ¼ 4, and the value of Rc is estimated to be 17.58 Å for PKABfNfCe0.1Sm0.5 glass. Similarly, the value of Rc for PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, and PKABfNfCe0.1Sm0.1 glasses have also been investigated and found to be ~31.46, 28.24, and 25.76 Å, respectively. In this work, the value of Rc is in the decreasing order for Ce3þ-doped PKABfNf glasses with increasing of Sm3þ concentration and is significantly greater than 5 Å. Usually, exchange interaction process is not possible in ET through wide gap of more than 5 Å, and the reabsorption process is only successful when emission of Ce3þ and excitation/absorption of Sm3þ are strongly over lapping, which is also not the case in the present work. Thus, the multipolar interactions will be considered in the present work for ET. To find out the exact interaction involved in the ET process, the emission intensity ratio of Ce3þ versus Sm3þ ions can be expressed on the basis of Dexter’s ET formula and Reisfeld’s approximation by using the following relation [44–47],
Fig. 8. The spectral overlap between the emission and excitation spectrum of Ce3þ/Sm3þ co-doped PKABfNf glasses along with the emission spectrum of single Ce3þ doped PKABfNfCe0.1 glass.
I0 s3 ∝C I =
Besides, the intensity of emission of Ce3þ is to be lowered if the ET between Ce3þ and Sm3þ occurs efficiently in Ce3þ/Sm3þ co-doped sys tem. Interestingly, in the co-doped systems presented in Fig. 7, the transition intensities of Ce3þ exhibited at 365 nm (5d→4f) increases up to 0.1 mol% and becomes less at 0.5 mol% of Sm3þ due to the quenching centers generated in Ce3þ ions by varying Sm3þ concentration. Such ET from Ce3þ to Sm3þ is related to the emission of Ce3þ band (350 nm) which overlaps to the Sm3þ excitation bands of 6H5/2 → 4G9/2 (332 nm), 6 H5/2 → 4D7/2 (344 nm), 6H5/2 → 4D3/2 (361 nm), 6H5/2 → 6P7/2 (374 nm), 6H5/2 → 4L15/2þ (391 nm), 6H5/2 → 6P3/2þ4F7/2þ4L13/2 (402 nm), 6 H5/2→ (6P, 4P)5/2 (416 nm). This observation clearly indicates over lapping of broad region between the emission and excitation as pre sented in Fig. 8. Another note is that the ET can be resonate between Sm3þ and Ce3þ, the maximum emission level of Ce3þ (365 nm) and the excitation band (402 nm) of Sm3þ must be at the same wavelength in order to be in resonance. Thus, resonance could not explain the mech anism of ET in the present work, but it is exactly because of nonradiative nature explained by Dexter mechanism [44] in the following section 3.6.
(4)
here I0 and I represents the emission intensity of Ce3þ singly doped (PKABfNfCe0.1) and co-doped with Sm3þ (PKABfNfCe0.1Sm0.01/0.05/0.1/ 0.5) glasses, respectively. The ‘S’ value may be taken as 10, 8 and 6 related to the interactions of quadrupole-quadrupole, dipole-quadru pole, and dipole-dipole, respectively. The dependence of (I0/I) on C10/3, C8/3, and C6/3 interactions is plotted in Fig. 11. The commendable linear fitting can be noticed only when S ¼ 10 by comparing with the highest value of R2 ¼ 0.98534 as shown in Fig. 11(c). This indicates that the ET: Ce3þ → Sm3þ in the PKABfNf glasses should mainly through the quadrupole-quadrupole interaction, a finding which is consistent with that of earlier works on Ce3þ/Tb3þ co-doped SNYPF phosphor [59]. To know the ET parameter (Q) among active ions, the Q is defined as � � 4π 3 Q¼ Γ 1 Ni R3c (5) S 3 this equation shows the possibility of calculating the Q parameter of codoped (Ce3þ,Sm3þ):PKABfNf glasses on the basis of it’s interaction be tween Rc (critical energy transfer distance) and quadrupole-quadrupole (S ¼ 10). Ni indicates the acceptor (Sm3þ) concentration, presented in Table 3 along with density (d), average molecular weight (MW) and molar volume (Vm) which are necessary for this condition. According to eq. (5), the Q values are found to be 0.49, 1.71, 3.16 and 4.09 for
3.6. Luminescence decay curve analysis To analyse the ET:Ce3þ → Sm3þ, the luminescence decay rates were measured under the excitation of 290 and 402 nm by observing the 6
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Fig. 9. Decay curves for the luminescence of Sm3þ and Ce3þ ions in PKABfNf glasses excited at λex ¼ 402 nm (a) and λex ¼ 290 nm (b and c) by monitoring the emission at 597 and 365 nm, respectively.
PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 glasses, respectively. As increasing the concentra tion of Sm3þ at a fixed Ce3þ, the Q, MW and Ni parameters continuously increases whereas Vm and Rc decreases and paves the way for efficient ET:Ce3þ → Sm3þ ions.
where n¼ (x-xe)/(y-ye) and the chromaticity epicentre is at xe ¼ 0.332 and ye ¼ 0.186. The CCT values of PKABfNfCeSm co-doped glasses are found to be 3395, 5143, 1916, and 1703 K. The lamp with <3200 K CCT values are mainly useful as warm light sources, while those with >4000 K CCT values are generally treated as a cold light source [30]. In the present work, the CCT value of optimized PKABfNfCe0.1Sm0.5 glass is 1703 K which is < 3200 K and may give the CRI>90 and LER>380 1 m/W values. Because these glasses with this CCT (<3000 K) value could generate the good quality white light. Thus the PKABfNfCe0.1Sm0.5 glass is useful for warm light sources for solid state lighting applications. The Color Purity (CP) is the weighted average of the (x, y) color coordinates relative to the coordinates of the illuminated and dominant wavelength. The CP for present glasses can be calculated as follows, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 CP ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% (7) ðxd xi Þ2 þ ðyd yi Þ2
3.7. Glass-phosphor:White light emission The emitting colors of the co-doped system with the 290 nm exci tation found to be varied from blue to orange-red in CIE chromaticity color co-ordinates as presented in Fig. 12. The calculated coordinates (x, y) for the present PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 co-doped glasses are found to be (0.375, 0.298), (0.336, 0.260), (0.431, 0.297), and (0.478, 0.323), respectively. The CIE color coordinates are derived for the intense band of 4G5/2 → 6H7/2 transition (orange-reddish) under 290 nm excitation. The Color Correlated Temperature (CCT) values are evaluated by the McCamy empirical formula [30,31] by using the following equation. CCT ¼
449n3þ3525n2-6823nþ5520.33
where (x,y) indicates the coordinates of sample point, (xd, yd) refers the coordinates of illuminate wavelength and (xi, yi) are the coordinates of white light in CIE diagram, the calculated CP values of
(6) 7
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Fig. 10. Energy transfer mechanism from Ce3þ to Sm3þ ions in PKABfNf glasses.
Fig. 11. Dependence of (Io/I) of Ce3þ on (a) C6/3, (b) C8/3, and (c) C10/3. The straight line indicate the fitting behavior in Ce3þ/Sm3þ co-doped PKABfNf glasses.
PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 glasses are 19, 17, 34, and 47%, respectively. The maximum CP of optimum (PKABfNfCe0.1Sm0.5) glass is 47%. Also we are very much interested and useful to optimize and improve further the performance of the quality of white light generation in our future studies.
Table 3 Molecular weight (MW, g/mol), density (ρ, g/cm3), molar volume (Vm ¼ MW/ρ, cm3/mol), ionic concentration per unit volume (Ni, � 1021 cm 3), critical energy transfer distance (Rc, Å), and energy transfer parameter (Q) for the present glasses. Glass
MW
ρ
Vm
Ni
Rc
Q
PKABfNfCe0.1Sm0.01 PKABfNfCe0.1Sm0.05 PKABfNfCe0.1Sm0.1 PKABfNfCe0.1Sm0.5
127.67 127.74 127.83 128.52
2.95 2.99 2.98 3.14
43.28 42.72 42.90 40.93
0.29 1.39 3.40 13.85
31.46 28.24 25.76 17.58
0.49 1.71 3.16 4.09
4. Conclusions Series of Ce3þ (0.1 mol %) and (Ce3þ (0.1)/Sm3þ (0.01–0.5 mol%) doped fluorophosphate (PKABfNf) glasses were synthesized and ana lysed systematically. The structural, morphology and compositional el ements of the glass host were determined from the FTIR, SEM and EDS measurements. The photoluminescence results show that the prepared 8
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Fig. 12. CIE chromaticity diagram of Ce3þ/Sm3þ co-doped PKABfNf glasses.
glasses have luminescence in blue as well red spectral region for singlydoped Ce3þ or Sm3þ ions. However, for co-doped (Ce3þ,Sm3þ):PKABfNf glasses, the intensity of red emission enhances, which indicates the efficient ET:Ce3þ → Sm3þ. Upon the excitation of 290 nm, the optimal PKABfNfCe0.1Sm0.5 glass exhibit high ET efficiency (ηET) of 26%. White light emission quality of PKABfNf glasses have been characterized through CIE chromaticity color co-ordinates, correlated color tempera ture (CCT) and color purity (CP). The CIE color coordinates determined from the luminescence spectra are located in blue to orange-reddish region. The mechanism of ET:Ce3þ → Sm3þ is attributed to the inter action of quadrupole-quadrupole. Decay rates of the 4G5/2 state of Sm3þ found to be non-exponential nature and their effective lifetimes are found to be shortened both in singly doped and co-doped PKABfNf glasses. The energy transfer parameter (Q) is found to be 0.49, 1.71, 3.16 and 4.09 and it increases monotonically with increasing the Sm3þ con centration, indicating that efficient ET:Ce3þ → Sm3þ in (Ce3þ,Sm3þ): PKABfNf glasses. The study reveals that the (Ce3þ,Sm3þ):PKABfNf glasses can be a promising material of blue-red spectral component in wLEDs. Authors contribution All authors listed have made a significant contribution to the research reported in the present manuscript and have Writing - original draft, read, review, editing and approved the submitted manuscript. Furthermore, all those who made substantive contributions to this work have been included in the author list. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Prof.C.K. Jayasankar is thankful to DAE-BRNS, Mumbai (No.2009/ 34/36/BRNS/3174) and UGC-BSR Faculty Fellow, New Delhi (No. F.18–1/2011 (BSR) dated 24-11-2017) for financial support. The author (CKJ) is also would like to thank King Mongkut’s Institute of Technology Ladkrabang (KMITL), Thailand for offering as Visiting Professor under Academic Melting Pot Program.
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Photoluminescence and energy transfer studies in Ce3þ and Sm3þ activated P2O5þK2OþAl2O3þBaF2þNaF2 glasses for solid state lighting V. Rajeswara Rao a, Ramachari Doddoji b, Wisanu Pecharapa c, J. Kaewkhao d, Shobha Rani Depuru e, C.K. Jayasankar a, * a
Department of Physics, Sri Venkateswara University, Tirupati-517 502, India Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Bangkok 10520, Thailand d Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand e Institute of Aeronautical Engineering, Hyderabad-500 043, India b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Fluorophosphate glasses (Ce3þ,Sm3þ):glasses Energy transfer W-LEDs
A color of blue-red emission by Ce3þ and Sm3þ activated fluorophosphate glasses (PKABfNf:P2O5þK2O þ Al2O3þBaF2þNaF2) were successfully fabricated by using usual melt and sudden quenching method. The FTIR, SEM, EDS, luminescence spectra, the decay rates, and energy transfer (ET) mechanism in the co-doped (Ce3þ, Sm3þ):PKABfNf glasses were systematically investigated in detail. The luminescence spectra of co-doped glasses show broadband blue de-excitation that belongs to Ce3þ ions and it obviously intensify the red de-excitation of Sm3þ through an effective ET process. The decrease in lifetime could support the existence of ET between Ce3þ and Sm3þ in PKABfNf glasses. This ET from Ce3þ to Sm3þ can be proposed to be the interaction due to quadrupole-quadrupole, based on the Dexter’s ET formula and Reisfeld’s approximation. Furthermore, CIE chromaticity coordinates, color correlated temperature, color purity, ET luminous efficiency (ηET), critical energy transfer distance (Rc), and ET parameter (Q) were calculated. The findings show that (Ce3þ,Sm3þ):PKABfNf glass might be a promising candidate in lighting field applications.
1. Introduction The next generation of solid-state lighting business has drawn tremendous attention for inorganic luminescent materials in virtue of their characteristic advantages consisting of considerable power con servation, high luminous efficiency, longer lifetime, enhanced bright ness and friendly to environmental conditions [1–3]. Most of the commercialized w-LEDs were normally fabricated as InGaN based blue chip combining the yellow emitting YAG:Ce3þ phosphor [4,5]. How ever, they still have the drawbacks of red emission which can be used in w-LEDs, excited through near-UV light. Thus, it is essential to search for alternative sources for blue, green or red color light. However, glasses exhibit the advantage of high homogeneity, transparency and thermal stability with low fabrication cost and also can be shaped in required size that can be sandwiched easily on LED chip [6–13]. Rare-earth activated luminescence complexes are frequently found in sensing, lighting and displaying due to their outstanding properties that include longer lifetimes and sharp emission peaks. Among number of
lanthanide ions, Sm3þ ion found to be an appropriate activator due to its de-excitation energy of 4G5/2 state with good quantum efficiency and many radiative states. Also Sm3þ ion have intense absorption in the nearUV region and it de-excites intense orange-red emission which has extensive applications for the design of visible optical devices such as glass lasers, LEDs, optical amplifiers operating in the NIR spectral region [14, 15]. Also, the Ce3þ ions show a strong and broad f-d excitation and de-excitation transitions located in UV-VIS region due to electric-dipole transition of 4f-5d [6,16–18]. As such, Ce3þ ions can act as an excellent sensitizer and transfer their energy to Sm3þ ions, thus increases the red emission that corresponds to Sm3þ ions [18–23]. Energy transfer (ET) of this nature has been reported in many systems like (Ce3þ,Sm3þ):Na2O þY2O3þSiO2, (Ce3þ,Tb3þ):nano glass-ceramics and (Sm3þ,Ce3þ):barium fluoroborate, under UV–Visible photon conversion for white LEDs and solar cell applications [23–26]. Also studied (Ce3þ,Sm3þ):Lu2SiO5þGd2 SiO5 solid solution crystals [27], (Ce3þ,Sm3þ):Y2SiO5 [28] and (Ce,Sm): CaS nano phosphors for applications in w-LEDs [29]. White light source quality has been estimated by various
* Corresponding author. E-mail address: [email protected] (C.K. Jayasankar). https://doi.org/10.1016/j.optmat.2019.109576 Received 23 October 2019; Received in revised form 24 November 2019; Accepted 25 November 2019 Available online 14 December 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
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Table 1 Selected glass compositions along with glass lables. S. No.
Composition
Label
1 2 3 4 7 8 9 10
45P2O5þ15K2Oþ10Al2O3þ19.99BaF2þ10NaF2þ0.01Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.95BaF2þ10NaF2þ0.05Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.9BaF2þ10NaF2þ0.1Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.5BaF2þ10NaF2þ0.5Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.89BaF2þ10NaF2þ0.1CeO2þ0.01Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.85BaF2þ10NaF2þ0.1CeO2þ0.05Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.8BaF2þ10NaF2þ0.1CeO2þ0.1Sm2O3 45P2O5þ15K2Oþ10Al2O3þ19.4BaF2þ10NaF2þ0.1CeO2þ0.5Sm2O3
PKABfNfSm0.01 PKABfNfSm0.05 PKABfNfSm0.1 PKABfNfSm0.5 PKABfNfCe0.1Sm0.01 PKABfNfCe0.1Sm0.05 PKABfNfCe0.1Sm0.1 PKABfNfCe0.1Sm0.5
performance parameters such as tristimulus coordinates (x, y), color rendering index (CRI), luminous efficacy of optical radiation (LER), correlated color temperature (CCT), color purity (CP) besides color component (fluorescent level) of peak emission wavelength, full-widthat-half-maximum, radiative lifetime and relative amplitude [30–33]. In general, for high quality white light generation, the most impor tant properties are CRI values that varies from 100 (high color rendition) to 100 (poorest color rendition) whereas LER should be greater than 380 lm/W at a warm CCT (i.e., CCT<4000 K). As such the luminescence of organic [34] and inorganic [14–40] materials have been investigated to tune, tailor and optimize the generation of white light. However, the fundamental relationship among CRI, LER and CCT are still not completely understood. In the case of rare earth doped glass-phosphorous, addition of fluo rides into phosphate glasses, the resultant oxyfluoride glasses exhibit the combined advantages of oxide and fluoride glasses such as physical and chemical durability, thermal stability, high transparency, low phonon energy, low melting point and mechanical durability. With these enhanced optical properties, oxyfluoride glasses are suitable for wide range of applications in optical technology [41,42]. Recently, the importance of Ce3þ with Dy3þ ions in fluorophosphate glasses for white light emission has been reported [14,43]. In this direction, the white light source parameters such as CIE co-ordinates, CCT and CP may be useful to study new glass composition of Ce3þ singly and (Ce3þ,Sm3þ) co-doped fluorophosphates (PKABfNf) glasses. Also it is quite interesting to investigate the ET process from Ce3þ to Sm3þ ions (ET:Ce3þ → Sm3þ) in PKABfNf glasses wherein Dexter’s ET formula and Reisfeld’s approximation [44–47] can be adopted to analyse the electric multi polar interactions. Therefore, the present work focuses on the fluorophosphate (PKABfNf) glasses co-doping of different (0.01, 0.05, 0.1, and 0.5 mol%) Sm3þ ions with a fixed concentration of Ce3þ (0.1 mol%) and were developed by the conventional melt and sudden quenching method. The structural and elemental mapping were determined from the FTIR, SEM and EDS analysis. The photoluminescent and in turn color coordinates, CCT and CP as well as ET process has also been studied [44–49]. The (Ce3þ,Sm3þ):PKABfNf glass emit blue (Ce3þ) to orange-reddish (Sm3þ) color and found to be useful for white light-emitting diodes.
Fig. 1. FTIR Spectra of Ce3þ/Sm3þ co-doped PKABfNf glasses.
FTIR spectra were recorded on Perkin-Elmer (Frontier) spectrometer from 400 to 2500 cm 1 with spectral resolution of 10 μm. Edinburgh (FLS 980) fluorescence spectrometer have been used to measure the excitation and de-excitation spectra as well decay rates. JSM-IT500 JEOL scanning electron microscope have been used to study the morphology and composition details. The phonon energy of the host (PKABfNf) glass was estimated from the Raman spectrum as 1132 cm 1 and the same was already reported in our earlier works [14,43]. 3. Results and discussions 3.1. FTIR studies Fig. 1 presents the FTIR spectra measured from 400 to 2500 cm 1 for PKABfNfCeSm co-doped fluorophosphate glasses to analyse the struc tural details. The observed low frequency transition within the range of 460–500 cm 1 is ascribed to harmonics or bending mode of vibration of PO2 units and O–P¼O linkages. The weak transition at 473 cm 1 is due to cerium cations mode of vibrations at their network sites [50]. The transition at 642 cm 1 belongs to Al2O mode of vibrations (Al–O–Al). The band at 781 cm 1 is assigned to the symmetric stretching mode of vibration of the P–O–P groups. The transition at 970 cm 1 is labeled to symmetric mode of vibrations of the PO4 tetrahedra (P–O- ionic group). The transition at 1151 cm 1 is related to asymmetric mode of vibrational groups of phosphates. The weak band at 1321 cm 1 could be ascribed to PO2 asymmetric stretching vibrations of non-bridging oxygens (NBO). The band at 2360 cm 1 is ascribed to hydrogen bond [51–53].
2. Experimental A series of (Ce3þ,Sm3þ) co-doped and Sm3þ singly doped PKABfNf glasses of 45P2O5þ15K2Oþ10Al2O3þ(19.9-x)BaF2þ10NaF2þ0.1CeO2 þxSm2O3 (x ¼ 0.01, 0.05, 0.1, and 0.5 mol %), were fabricated by usual conventional melt and sudden quenching technique. The chemical composition with glass labels are presented in Table 1. High purity (>99.9%) compounds of Al(PO3)3, KPO3, NaF2, BaF2, Sm2O3 and CeO2 were taken. Each batch (30 g), as shown in Table 1, was well grinded and made homogenized mixture which was transferred to alumina crucible and melted at 1200 � C for 2 h. Bubble free with homogenized melt was quenched rapidly at RT on a preheated stainless steel mold and subse quently annealed at 420 � C for 16 h (to remove stress and strain) and then allowed to cool slowly to RT. The developed glasses were cut for desired dimension and polished which are characterized. 2
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gives information about the elements which are present in the PKABfNfCe0.1Sm0.5 glass. The spectrum of EDS exhibits number of peaks belong to the O, F, Na, Al, P, K, Ba, Ce and Sm elements. The percentage of elements found to be O (49.65 wt%), F (0.01 wt%), Na (4.67 wt%), Al (4.90 wt%), P (18.61 wt%), K (5.83 wt%), Ba (11.96 wt%), Ce (1.42 wt %) and Sm (1.50 wt%) in PKABfNfCe0.1Sm0.5 glass. The inset shows the SEM image which clearly suggest the absence of cracks and un-melted portions in PKABfNfCe0.1Sm0.5 glass, justifies that the glass is of amor phous nature. 3.3. Absorption studies The absorption spectral profile of PKABfNfSm0.5 singly doped glasses is depicted in Fig. 3(a) (UV–Visible) and 3(b) (NIR). The addition of Sm2O3 (0.5 mol%) to the glass, the spectrum exhibited several absorp tion bands resulting from the ground state (6H5/2) to the several excited states of 6F1/2, 6H15/2, 6F3/2,5/2,7/2,9/2,11/2, 4G5/2, 4F3/2, 4G7/2, 4I11/2,13/ 4 4 6 4 4 2, G9/2, M19/2, P3/2, L15/2,17/2, and D3/2,7/2 and the corresponding wavelengths are 1588, 1527, 1484, 1379, 1231, 1079, 945, 560, 534, 500, 473, 462, 439, 416, 401, 391, 375, 361 and 344 nm, respectively. When Ce2O3 (0.1 mol%) and Sm2O3 (0.5 mol%) is co-doped in flu orophosphates glass (PKABfNfCe0.1Sm0.5), the bands at 344, 361 and 375 nm could not be visualized well in the spectra (Fig. 3(c)), however,
Fig. 2. SEM and EDS spectra for PKABfNfCe0.1Sm0.5 glass.
3.2. SEM and EDS analyses Fig. 2 depicts the SEM (inset shows 10 μm range scan) and EDS micro graphs of PKABfNfCe0.1Sm0.5 glass, measured by using JSM-IT500 InTouchScope™ Scanning Electron Microscope. The EDS spectrum
Fig. 3. Absorption spectrum of PKABfNfSm0.5, and PKABfNfCe0.1Sm0.5 glasses in UV–Visible, and NIR region. The inset figure shows the absorption spectrum of Ce3þ singly doped PKABfNfCe0.1 glass. 3
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Fig. 4. (a) Excitation Spectrum of Sm3þ doped PKABfNfSm0.5 glass and (b) emission spectra of PKABfNf glasses for different concentrations of Sm3þ ions.
The strongest intensity band of 6H5/2 → 6P3/2 (at 402 nm) is a hyper sensitive transition which can be normally used for fluorescence exci tation [54–56]. 3.4. Photoluminescence and decay curve studies Fig. 4 presents the excitation and de-excitation spectra of PKABfNf: Sm3þ glasses. From Fig. 4(a), the excitation spectrum (λemi ¼ 597 nm) exhibit number of transitions and are assigned as 6H5/2 → 4P3/2 (318 nm), 4G9/2 (332 nm), 4D7/2 (344 nm), 4D3/2 (361 nm), 6P7/2 (374 nm), 4 L15/2 (391 nm), 6P3/2þ4F7/2þ4L13/2 (402 nm), (6P, 4P)5/2 (416 nm), 4 I15/2 (439 nm), 4I13/2 (461 nm), 4I11/2 (472 nm), 4G7/2 (500 nm) and 4 F3/2 (527 nm). Among these, intense band observed at 402 nm has been excited to measure emission spectra for various concentrations of Sm3þ:PKABfNf glasses and are presented in Fig. 4(b). The observed emission spectra in orange-red region are peaked at 561, 598, 644 and 705 nm corresponds to 4G5/2 → 6H5/2,7/2,9/2,11/2 levels, respectively [55–57]. Luminescence decay rates of 4G5/2 state for different concentration of Sm3þ:PKABfNf have also been investigated and shown in Fig. 5. For higher concentrations of Sm3þ ion, the decay rates exhibit nonexponential nature. Therefore, effective lifetime (τeff) are determined by taking the following equation. R tIðtÞdt (1) τeff ¼ R IðtÞdt
Fig. 5. Decay curves of PKABfNf glass for different concentrations of Sm3þ ions.
observed a weak band at 310 nm of Ce3þ shown in the inset of Fig. 3(c). The intensity of all these observed transitions decreases with the pres ence of cerium which means that cerium can act as a UV blocker below 400 nm and gives the information of transferring energy to Sm2O3 ions. The low energy (NIR) bands of Sm3þ is also observed in the presence of Ce2O3, as presented in Fig. 3 (d). All absorption levels existing in the region of UV–Visible are relatively weak than those observed in NIR region, since the UV–Visible transitions observed are spin forbidden.
in equation (1), I(t) refers the intensity of emission at time ‘t’. The values of τeff tabulated in Table 2 are found to be 2.35, 2.31, 2.29 and 2.02 ms for 0.01, 0.05, 0.1, and 0.5 mol% (Sm3þ), respectively. The shortening
Table 2 Lifetimes (τeff) of single Sm3þ, and Ce3þ/Sm3þ co-doped glasses, and energy transfer efficiency (ηET, %) of Ce3þ/Sm3þ co-doped fluorophosphate glasses for different concentrations (C). S. No.
Singly doped Sm3þ @ λexc ¼ 402 nm C (Sm
1. 2. 3. 4.
0.01 0.05 0.1 0.5
3þ
)
Co-doped Ce3þ/Sm3þ@ λexc ¼ 290 nm
τeff (ms)@ λemi ¼ 597 nm
C (Ce3þ/Sm3þ)
τeff (ms)@ λemi ¼ 597 nm
τeff (μs)@ λemi ¼ 365 nm
ηET
2.35 2.31 2.29 2.02
0.1/0.01 0.1/0.05 0.1/0.1 0.1/0.5
2.37 2.34 2.32 2.10
7.72 7.53 7.24 7.52
5 6 12 26
4
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Fig. 6. Excitation (a and b) and emission (c and d) spectra of Ce3þ doped PKABfNf glass with different concentrations of Sm3þ ions. The inset figure shows the emission spectrum of Ce3þ singly doped PKABfNfCe0.1 glass.
nature of τeff with increase in Sm3þ concentration confirms ET between Sm3þ ions via cross-relaxation (CR).
energy of 402 nm correspond to the relaxed lowest 6P level to the 6H ground level of Sm3þ, but Ce3þ could not be visualized at this wave length as seen in Fig. 6(d). Therefore, the de-excitation spectra presented in Fig. 6(c) upon 290 nm can be tuned for the intensity of luminescence by changing the Sm3þ ions concentration. While increase in concen tration of Sm3þ, the orange-red emission (~597 nm) of Sm3þ intensities increases rapidly as shown in Fig. 7 and demonstrate the effective ET: Ce3þ → Sm3þ in PKABfNf glasses. Moreover, the ET efficiency between Ce3þ to Sm3þ can be expressed by,
3.5. Energy transfer between Ce3þ and Sm3þ For the purpose of studying the ET:Ce3þ → Sm3þ, the excitation and de-excitation spectra of PKABfNf glasses by co-doping Ce3þ (0.1 mol%) with different Sm3þ concentrations were measured and presented in Fig. 6. By monitoring the emission at 597 nm of PKABfNfCe0.1Sm0.5 glass as presented in Fig. 6(a) and (b), exhibit not only 4f-5d of Ce3þ but also 4f-4f levels of Sm3þ. This spectra also reveals the enhanced absorption intensity from 250 to 350 nm, which indicates the presence of ET:Ce3þ → Sm3þ. The luminescence intensity differs with the excitation spectra which further reveals the existence of ET:Ce3þ → Sm3þ in PKABfNf glasses. Inset of Fig. 6(b) shows the excitation spectrum of PKABfNfCe0.1 glass. The de-excitation spectra presented in Fig. 6(c) and (d) covers from 300 to 750 nm, exciting at 290 and 402 nm, respectively. The excitation
ηET ¼ 1
Ice Ice þ ISm
(2)
here ICe and ISm refers the integrated intensities of 365 (Ce3þ) and 597 nm (Sm3þ) corresponding to 5 d → 4f and 4G5/2 → 6H7/2 transitions collected from Fig. 6(c), respectively. According to eq. (2), the value of ηET is found to be 5, 6, 12, and 26% for PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 glasses, respectively. 5
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emission at 365 and 597 nm, respectively, and are presented in Fig. 9. As seen in Fig. 9(a), (b) and (c), the decay rates are not perfect exponential and therefore τeff values (Table 2) were calculated using eq. (1) for Ce3þ/Sm3þ co-doped PKABfNf glasses. It is noticed that the decay times of co-doped glasses decrease significantly with increase in concentration of Sm3þ due to ET:Ce3þ → Sm3þ. This is because of the presence of more number of Sm3þ ions around Ce3þ, when Ce3þ concentration fixed with different Sm3þ concentration. Thus, the shortening of the decay rates of Ce3þ with the increase of Sm3þ concentration could confirm the mech anism of non-radiative ET:Ce3þ → Sm3þ ion as depicted in energy level scheme of Fig. 10. Normally, the non-radiative ET surrounded by rare-earths is responsible for shortening the decay time through the exchange, electric multipolar and radiation reabsorption interactions. For exchange interaction, the Rc (distance for critical energy transfer) can be deter mined by taking the Blasse’s relation [58],
Fig. 7. Relative emission intensities of Ce3þ and Sm3þ ions in PKABfNf glasses.
3Vm 4πXc Z
�13 =
� Rc ¼ 2
(3)
here Vm represents the molar volume of the co-doped glass, Xc represents the sum of the critical concentrations of Sm3þ and Ce3þ, where the luminescence intensity decreases with Sm3þ concentration, Z represents cation’s oxidation number that occupies the Ce3þ ions in the unit vol ume. In this regard, Vm ¼ 40.88 cm3/mol, Xc ¼ 0.6, Z ¼ 4, and the value of Rc is estimated to be 17.58 Å for PKABfNfCe0.1Sm0.5 glass. Similarly, the value of Rc for PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, and PKABfNfCe0.1Sm0.1 glasses have also been investigated and found to be ~31.46, 28.24, and 25.76 Å, respectively. In this work, the value of Rc is in the decreasing order for Ce3þ-doped PKABfNf glasses with increasing of Sm3þ concentration and is significantly greater than 5 Å. Usually, exchange interaction process is not possible in ET through wide gap of more than 5 Å, and the reabsorption process is only successful when emission of Ce3þ and excitation/absorption of Sm3þ are strongly over lapping, which is also not the case in the present work. Thus, the multipolar interactions will be considered in the present work for ET. To find out the exact interaction involved in the ET process, the emission intensity ratio of Ce3þ versus Sm3þ ions can be expressed on the basis of Dexter’s ET formula and Reisfeld’s approximation by using the following relation [44–47],
Fig. 8. The spectral overlap between the emission and excitation spectrum of Ce3þ/Sm3þ co-doped PKABfNf glasses along with the emission spectrum of single Ce3þ doped PKABfNfCe0.1 glass.
I0 s3 ∝C I =
Besides, the intensity of emission of Ce3þ is to be lowered if the ET between Ce3þ and Sm3þ occurs efficiently in Ce3þ/Sm3þ co-doped sys tem. Interestingly, in the co-doped systems presented in Fig. 7, the transition intensities of Ce3þ exhibited at 365 nm (5d→4f) increases up to 0.1 mol% and becomes less at 0.5 mol% of Sm3þ due to the quenching centers generated in Ce3þ ions by varying Sm3þ concentration. Such ET from Ce3þ to Sm3þ is related to the emission of Ce3þ band (350 nm) which overlaps to the Sm3þ excitation bands of 6H5/2 → 4G9/2 (332 nm), 6 H5/2 → 4D7/2 (344 nm), 6H5/2 → 4D3/2 (361 nm), 6H5/2 → 6P7/2 (374 nm), 6H5/2 → 4L15/2þ (391 nm), 6H5/2 → 6P3/2þ4F7/2þ4L13/2 (402 nm), 6 H5/2→ (6P, 4P)5/2 (416 nm). This observation clearly indicates over lapping of broad region between the emission and excitation as pre sented in Fig. 8. Another note is that the ET can be resonate between Sm3þ and Ce3þ, the maximum emission level of Ce3þ (365 nm) and the excitation band (402 nm) of Sm3þ must be at the same wavelength in order to be in resonance. Thus, resonance could not explain the mech anism of ET in the present work, but it is exactly because of nonradiative nature explained by Dexter mechanism [44] in the following section 3.6.
(4)
here I0 and I represents the emission intensity of Ce3þ singly doped (PKABfNfCe0.1) and co-doped with Sm3þ (PKABfNfCe0.1Sm0.01/0.05/0.1/ 0.5) glasses, respectively. The ‘S’ value may be taken as 10, 8 and 6 related to the interactions of quadrupole-quadrupole, dipole-quadru pole, and dipole-dipole, respectively. The dependence of (I0/I) on C10/3, C8/3, and C6/3 interactions is plotted in Fig. 11. The commendable linear fitting can be noticed only when S ¼ 10 by comparing with the highest value of R2 ¼ 0.98534 as shown in Fig. 11(c). This indicates that the ET: Ce3þ → Sm3þ in the PKABfNf glasses should mainly through the quadrupole-quadrupole interaction, a finding which is consistent with that of earlier works on Ce3þ/Tb3þ co-doped SNYPF phosphor [59]. To know the ET parameter (Q) among active ions, the Q is defined as � � 4π 3 Q¼ Γ 1 Ni R3c (5) S 3 this equation shows the possibility of calculating the Q parameter of codoped (Ce3þ,Sm3þ):PKABfNf glasses on the basis of it’s interaction be tween Rc (critical energy transfer distance) and quadrupole-quadrupole (S ¼ 10). Ni indicates the acceptor (Sm3þ) concentration, presented in Table 3 along with density (d), average molecular weight (MW) and molar volume (Vm) which are necessary for this condition. According to eq. (5), the Q values are found to be 0.49, 1.71, 3.16 and 4.09 for
3.6. Luminescence decay curve analysis To analyse the ET:Ce3þ → Sm3þ, the luminescence decay rates were measured under the excitation of 290 and 402 nm by observing the 6
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Fig. 9. Decay curves for the luminescence of Sm3þ and Ce3þ ions in PKABfNf glasses excited at λex ¼ 402 nm (a) and λex ¼ 290 nm (b and c) by monitoring the emission at 597 and 365 nm, respectively.
PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 glasses, respectively. As increasing the concentra tion of Sm3þ at a fixed Ce3þ, the Q, MW and Ni parameters continuously increases whereas Vm and Rc decreases and paves the way for efficient ET:Ce3þ → Sm3þ ions.
where n¼ (x-xe)/(y-ye) and the chromaticity epicentre is at xe ¼ 0.332 and ye ¼ 0.186. The CCT values of PKABfNfCeSm co-doped glasses are found to be 3395, 5143, 1916, and 1703 K. The lamp with <3200 K CCT values are mainly useful as warm light sources, while those with >4000 K CCT values are generally treated as a cold light source [30]. In the present work, the CCT value of optimized PKABfNfCe0.1Sm0.5 glass is 1703 K which is < 3200 K and may give the CRI>90 and LER>380 1 m/W values. Because these glasses with this CCT (<3000 K) value could generate the good quality white light. Thus the PKABfNfCe0.1Sm0.5 glass is useful for warm light sources for solid state lighting applications. The Color Purity (CP) is the weighted average of the (x, y) color coordinates relative to the coordinates of the illuminated and dominant wavelength. The CP for present glasses can be calculated as follows, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 CP ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% (7) ðxd xi Þ2 þ ðyd yi Þ2
3.7. Glass-phosphor:White light emission The emitting colors of the co-doped system with the 290 nm exci tation found to be varied from blue to orange-red in CIE chromaticity color co-ordinates as presented in Fig. 12. The calculated coordinates (x, y) for the present PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 co-doped glasses are found to be (0.375, 0.298), (0.336, 0.260), (0.431, 0.297), and (0.478, 0.323), respectively. The CIE color coordinates are derived for the intense band of 4G5/2 → 6H7/2 transition (orange-reddish) under 290 nm excitation. The Color Correlated Temperature (CCT) values are evaluated by the McCamy empirical formula [30,31] by using the following equation. CCT ¼
449n3þ3525n2-6823nþ5520.33
where (x,y) indicates the coordinates of sample point, (xd, yd) refers the coordinates of illuminate wavelength and (xi, yi) are the coordinates of white light in CIE diagram, the calculated CP values of
(6) 7
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Fig. 10. Energy transfer mechanism from Ce3þ to Sm3þ ions in PKABfNf glasses.
Fig. 11. Dependence of (Io/I) of Ce3þ on (a) C6/3, (b) C8/3, and (c) C10/3. The straight line indicate the fitting behavior in Ce3þ/Sm3þ co-doped PKABfNf glasses.
PKABfNfCe0.1Sm0.01, PKABfNfCe0.1Sm0.05, PKABfNfCe0.1Sm0.1, and PKABfNfCe0.1Sm0.5 glasses are 19, 17, 34, and 47%, respectively. The maximum CP of optimum (PKABfNfCe0.1Sm0.5) glass is 47%. Also we are very much interested and useful to optimize and improve further the performance of the quality of white light generation in our future studies.
Table 3 Molecular weight (MW, g/mol), density (ρ, g/cm3), molar volume (Vm ¼ MW/ρ, cm3/mol), ionic concentration per unit volume (Ni, � 1021 cm 3), critical energy transfer distance (Rc, Å), and energy transfer parameter (Q) for the present glasses. Glass
MW
ρ
Vm
Ni
Rc
Q
PKABfNfCe0.1Sm0.01 PKABfNfCe0.1Sm0.05 PKABfNfCe0.1Sm0.1 PKABfNfCe0.1Sm0.5
127.67 127.74 127.83 128.52
2.95 2.99 2.98 3.14
43.28 42.72 42.90 40.93
0.29 1.39 3.40 13.85
31.46 28.24 25.76 17.58
0.49 1.71 3.16 4.09
4. Conclusions Series of Ce3þ (0.1 mol %) and (Ce3þ (0.1)/Sm3þ (0.01–0.5 mol%) doped fluorophosphate (PKABfNf) glasses were synthesized and ana lysed systematically. The structural, morphology and compositional el ements of the glass host were determined from the FTIR, SEM and EDS measurements. The photoluminescence results show that the prepared 8
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Fig. 12. CIE chromaticity diagram of Ce3þ/Sm3þ co-doped PKABfNf glasses.
glasses have luminescence in blue as well red spectral region for singlydoped Ce3þ or Sm3þ ions. However, for co-doped (Ce3þ,Sm3þ):PKABfNf glasses, the intensity of red emission enhances, which indicates the efficient ET:Ce3þ → Sm3þ. Upon the excitation of 290 nm, the optimal PKABfNfCe0.1Sm0.5 glass exhibit high ET efficiency (ηET) of 26%. White light emission quality of PKABfNf glasses have been characterized through CIE chromaticity color co-ordinates, correlated color tempera ture (CCT) and color purity (CP). The CIE color coordinates determined from the luminescence spectra are located in blue to orange-reddish region. The mechanism of ET:Ce3þ → Sm3þ is attributed to the inter action of quadrupole-quadrupole. Decay rates of the 4G5/2 state of Sm3þ found to be non-exponential nature and their effective lifetimes are found to be shortened both in singly doped and co-doped PKABfNf glasses. The energy transfer parameter (Q) is found to be 0.49, 1.71, 3.16 and 4.09 and it increases monotonically with increasing the Sm3þ con centration, indicating that efficient ET:Ce3þ → Sm3þ in (Ce3þ,Sm3þ): PKABfNf glasses. The study reveals that the (Ce3þ,Sm3þ):PKABfNf glasses can be a promising material of blue-red spectral component in wLEDs. Authors contribution All authors listed have made a significant contribution to the research reported in the present manuscript and have Writing - original draft, read, review, editing and approved the submitted manuscript. Furthermore, all those who made substantive contributions to this work have been included in the author list. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Prof.C.K. Jayasankar is thankful to DAE-BRNS, Mumbai (No.2009/ 34/36/BRNS/3174) and UGC-BSR Faculty Fellow, New Delhi (No. F.18–1/2011 (BSR) dated 24-11-2017) for financial support. The author (CKJ) is also would like to thank King Mongkut’s Institute of Technology Ladkrabang (KMITL), Thailand for offering as Visiting Professor under Academic Melting Pot Program.
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