Absorption and luminescence studies of Sm3+ ions activated in distinct phosphate glasses for reddish orange light applications

Optical Materials 88 (2019) 7–14

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Absorption and luminescence studies of Sm3+ ions activated in distinct phosphate glasses for reddish orange light applications

T

B. Nagaraja Naicka, V. Reddy Prasada, S. Damodaraiaha, A.V. Reddyb, Y.C. Ratnakarama,∗ a b

Department of Physics, Sri Venkateswara University, Tirupati, 517502, India Reader in Physics, J B College, Kavali, Nellore, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Phosphate glass Solid state 31P NMR FTIR Absorption Photoluminescence Emission cross-section

In the present work, we report the optical properties of Sm3+ ions doped glasses having chemical compositions (70-x) NH4H2PO4+15Na2O+15M2O + xSm2O3 (where x = 0.5 mol% and M = Li, Mg, Ca, Sr, Ba). Structural details are obtained from XRD, SEM, FTIR, Raman and solid-state 31P NMR spectra. Absorption (UV-NIR), emission and lifetime measurements have been performed and discussed using Judd-Ofelt theory. Various radiative parameters are calculated and discussed. Visible emissions from 4G5/2 excited state have been characterized and the experimental lifetimes of these levels have been compared with those obtained theoretically by using J-O approach. Quantum efficiencies are reported. The CIE chromaticity co-ordinates of the present glass matrices confirmed reddish-orange light emission. Hence the prepared samples can be treated as candidates for reddish orange light applications.

1. Introduction Phosphate glasses have been technological interest in the field of materials research due to unique properties. The interesting spectroscopic properties of trivalent lanthanide (Ln3+) doped glasses making them as potential candidates among different types of glasses for their applications in laser materials [1–5]. It is an important factor to increase the laser efficiency of these materials. Hence this phenomenon has a significant application in the research and development of new laser materials and it is possible to reduce the threshold energy of laser oscillation in these materials. Reddish light region is very much useful in penetrating through human tissues, 50–200% deeper than blue or green lights. An effectual luminescence center for Sm3+ (4f5) ions in reddish orange (600–650 nm) wavelength region, possess sufficient energy to stimulate photodynamic reaction. Rare earth ions-doped phosphate glasses have been known for a long time as the efficient photoluminescent materials. Glass composition consisting of network formers and modifiers has a main effect on the luminescence and absorption of Ln3+ ions in glasses. Network modifiers of the glass matrix are alkaline earths like MgO, CaO, SrO, BaO, Li2O, Na2O, K2O, MgF2 etc. The alkali ions are thermally activated and the movement of alkali ions enables the occupancy of other ions of same valency near the surface of glass [6,7]. Though P2O5 being one of the four classic Zacarhiason glass forming

∗

oxides (along with SiO2, GeO2 and B2O3), the research and development on the nature of glass, and its applications have been limited due to its hygroscopic nature. The phosphate glasses are usually formed at low temperatures when compared to silicate glasses. Phosphate glasses have received the great importance due to their considerable applications in optical data transmission, detection and laser technology [8]. The other promising properties of phosphate glasses are high thermal expansion co-efficient, high gain density and low refractive index which are desirable for potential applications in the fields such as fabrication of optical fibers, sensors and solid state lasers [9,10]. Also the phosphate glasses include which treatment and for some radiation detection systems [9]. Glasses doped with rare earth ions (RE3+) with high luminescence efficiencies are very much useful for the fabrication of visible and infrared optical devices [11–13]. The properties of RE-ions are closely associated with their 4f-4f transitions and these properties can be tuned by the physical and chemical interactions. Among various RE3+ ions, Sm3+ ion is one emitting strong reddish orange color which has increased demand in fluorescent devices such as color displays and visible solid-state laser [14]. Sm3+ ions containing phosphate glasses are known to have an unusual elastically behavior due to valency instability and it has promising characteristics for spectral whole burning studies [15]. Recently, Ramteke et al. [16] studied optical absorption spectra of Sm3+-doped

Corresponding author. E-mail address: [email protected] (Y.C. Ratnakaram).

https://doi.org/10.1016/j.optmat.2018.11.009 Received 5 October 2018; Received in revised form 1 November 2018; Accepted 12 November 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

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3. 69.5 NH4H2PO4e15Na2CO3e15Ca2CO3–0.5Sm2O3 4. 69.5 NH4H2PO4e15Na2CO3e15Sr2CO3–0.5Sm2O3 5. 69.5 NH4H2PO4e15Na2CO3e15Ba2CO3–0.5Sm2O3

phosphate glasses with Li2O as modifier and observed higher spectral intensity for the hypersensitive transition and also observed for fiber optic amplifier applications. Some of the researchers have studied various spectral investigations of Sm3+ doped phosphate and sulfophosphate glasses [17–19]. They have reported from the analysis of the spectra that oxygen atoms are converted from bridging oxygens to nonbridging oxygens in these glass matrices. Luewarasirikul and Kaewkhao et al. [20] studied Judd-Ofelt analysis in Sm3+ ions doped sodium borate glasses. Identification of rare earth elements in phosphate glasses was reported by Devangad et al. [21]. Damodaraiah et al. [22] reported optical properties of Sm3+-doped bismuth phosphate glass and observed that these glass matrices are efficient lasing materials and suitable for visible orange-red photonic applications. Various phosphate glasses consisting different alkali and alkaline earth compounds doped with Sm3+ ions were prepared and studied in the present work. It is well known that the incorporation of alkali-earth oxides (viz., LiO, MgO, CaO, SrO and BaO) in the glass structure leads to a disruption of the glass network and promotes the formation of bridging and non-bridging oxygens groups. The glass acquires the entire demanding requirement from poor chemical stability and devitrification tendency. But the presence of oxides favours low phonon energy and reduces OH absorption strongly and prevents the glass leaching. Low phonon energy glass yields low non-radiative decay and increase radiative properties leading to higher quantum efficiencies. Hence, oxide phosphate glasses with different network modifiers improve glass stability because the PeO-M (M = metal cation) bond is highly stable against atmospheric hydrolysis. Also, to the see the effect of alkali earths on various spectroscopic properties of rare earth ion, the above alkali earths have chosen and studied in the present work. In addition to that Sm3+ doped phosphate glasses have very much desired to find new red emitting materials which showed good thermal stability, Suitable for excitation by near-UV region for warm light white LEDs. Particularly, the phosphate glass consists LiO, SrO and BaO has important applications. Hence the above modifiers are used in present glass systems. In the present work, absorption and luminescence spectra of Sm3+ ions in different phosphate glasses are studied and reported. In addition to that different vibrational bands are observed from the FTIR spectra. For all these glass matrices, Raman spectra have taken and studied. The chemical shifts are reported for all these glass matrices using 31P NMR spectroscopy. From the measured spectral intensities of absorption bands, intensity parameters, Ωλ are calculated and from these, various radiative properties are studied. Stimulated emission cross-sections are also obtained for the observed emission transitions and are reported. Fluorescence decay lifetimes are measured for certain excited states in these phosphate glasses and discussed.

The measurement of density and refractive indices of the prepared glasses are explained in Ref. [22]. In the present work, the densities are in the range 2.350–2.530 g m/cm3 with an error ± 0.002 g m/cm3; the refractive indices are in the range 1.650–1.652 with an error ± 0.001 for different phosphate glasses. XRD profiles were obtained in the range of 10-800 for all the glass matrices using RIGAKU X-ray diffractometer. The SEM images were recorded using Carl Zeiss EVO MA15. The FTIR spectra were recorded using BRUKER FTIR spectrometer with 4 cm−1 spectral resolution in 500–3600 cm−1 range. Raman spectra of Sm3+ doped distinct phosphate glasses were analyzed between 50 and 1500 cm−1 using a BRUKER: RFS 27 spectrometer. 31P NMR spectra of all these samples were also obtained using a JOEL ECX400 DELTA2 NMR spectrometer. The acquisition time, pulse width and relaxation delays were 18 m s, 2.9 μs and 5s respectively. The absorption and emission measurements were made at room temperature (RT) using JASCO V–570 spectrophotometer and FLS 980, Edinburg respectively.

3. Results and discussions 3.1. X-ray diffraction, SEM analysis The X-ray diffraction patterns of all the prepared glass samples are shown in Fig. 1. The observed broad and diffused peaks in the diffraction pattern confirm the amorphous nature of the present glass matrices. The SEM images for Li, Mg, Ca, Sr and Ba phosphate glasses were not shown, as they are similar in nature. These SEM images have not shown in any grains which confirming the glassy nature.

3.2. FTIR and Raman analysis FTIR spectra of Sm3+ ion doped different phosphate glasses are shown in Fig. 2. Figure shows seven bands nearly at ∼535, ∼765, ∼890, ∼1265, ∼1640, ∼2360, and ∼2923 cm−1. Assignments of various bands and their positions in different phosphate glasses observed in the present work are given below.

2. Experimental procedure In the present work, the glass samples were synthesized using the chemical compositions, (70-x) NH4H2PO4+15Na2O +15M2O + xSm2O3 (where x = 0.5 mol% and M = Li, Mg, Ca, Sr, Ba). These glass samples were prepared by the standard melt quenching method using high purity (99.99%) chemicals ammonium dihydrogen orthophosphate (NH4H2PO4), sodium carbonate (Na2CO3), lithium carbonate (Li2CO3), magnesium carbonate (Mg2CO3), calcium carbonate (Ca2CO3), strontium carbonate (Sr2CO3), barium carbonate (Ba2CO3) and samarium oxide (Sm2O3). The raw materials of about 10 g m were weighed as per the above molar compositions. Pure and rare earth doped glass samples were prepared in electric muffle furnace at 1100 °C in a porcelain crucible as described in our earlier papers [22]. The final compositions of the prepared glass matrix are given below: 1. 69.5 NH4H2PO4e15Na2CO3e15Li2CO3–0.5Sm2O3 2. 69.5 NH4H2PO4e15Na2CO3e15Mg2CO3–0.5Sm2O3

Fig. 1. XRD profiles of 0.5 mol% Sm3+-doped different phosphate glasses. 8

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Fig. 3. Raman spectra of 0.5 mol% Sm3+ doped different phosphate glasses.

Fig. 2. FTIR spectra of 0.5 mol% of Sm3+ doped different phosphate glasses.

Energy (cm−1)

Band assignment

Reference

528 (Li), 531 (Mg), 538 (Ca), 531 (Sr) and 533 (Ba) 765 (Li), 768 (Mg), 779 (Ca), 779 (Sr) and 775 (Ba) 896 (Li), 888 (Mg), 892 (Ca), 887 (Sr) and 892 (Ba) 1256 (Li), 1279 (Mg), 1279 (Ca), 1271 (Sr) and 1265 (Ba) 1646 (Li), 1639 (Mg), 1638 (Ca), 1627 (Sr) and 1646 (Ba) 2383 (Li), 2404 (Mg), 2362 (Ca), 2369 (Sr) and 2361 (Ba) 2924 (Li), 2926 (Mg), 2921 (Ca), 2929 (Sr)and 2925 (Ba)

Bending vibrations of PeO bonds

[25]

Symmetric stretching vibrations of PeOeP rings Asymmetric stretching vibrations of PeOeP rings Asymmetric stretching vibrations mode of doubly bonded oxygen (P]O) group

[23,24] [25–27] [27]

Bending vibrations of OeH groups

[24]

Hydrogen bonding

[28]

Bending vibrations of H2O groups

[28]

Wave number (cm−1)

Raman peak assignment

Reference

341 (Li), 297 (Mg), 328(Ca), 347(Sr) and 330(Ba) 515 (Li), 538 (Mg), 521 (Ca), 509 (Sr) and 521(Ba) 630 (Li), 620 (Mg), 631 (Ca), 627(Sr) and 632 (Ba) 700 (Li), 691 (Mg), 694 (Ca), 694 (Sr) and 687 (Ba) 1082 (Li), 1084 (Mg), 1096 (Ca), 1096 (Sr) and 1086 (Ba) 1168 (Li), 1164 (Mg), 1171 (Ca), 1163 (Sr) and 1161 (Ba) 1262 (Li), 1292 (Mg), 1260 (Ca), 1254 (Sr) and 1256 (Ba)

PO4 tetrahedra bending vibrations of OePeO units bending vibrations of OePeO rings

[29]

symmetric stretching vibrations of PeOeP rings symmetric stretching vibrations mode PeOeP group of bridging oxygen asymmetric stretching vibrations of PeO- units

[30]

[34]

[30] [31]

symmetric stretching vibrations mode OePeO group of non-bridging oxygen

[32–34]

asymmetric stretching vibrations mode OePeO group of non-bridging oxygen

[32–34]

In the Raman spectrum, the band observed between 1160 and 1170 cm−1 plays major role on the luminescence behavior. Among various phosphate glasses, lithium phosphate glass shows high peak intensity.

FTIR spectra exhibit prominent bands due to structural units at 540550 cm−1 is attributed to the bending vibrations of PeO bonds [25]. The band at 760-780 cm−1 is attributed to the symmetric stretching vibrations of PeOeP rings [23,24]. The band at 880-890 cm−1 is owing to asymmetric stretching vibrations of PeOeP groups linked with linear metaphosphate chain [25–27]. The strong band nearly at 12551280 cm−1 is attributed to the asymmetric stretching vibration mode of the doubly bonded oxygen (P]O) group [27]. The weak bands for magnesium, calcium and strontium phosphate glasses at 1630 cm−1 are assigned to OeH bending vibrations. The bands at 2360-2400 cm−1 is due to hydrogen bonding [28]. For strontium phosphate glass the band at 2923 cm−1 is attributed to bending vibrations of H2O. Finally the band at 2920-2930 cm−1 is attributed to PeOeH hydroxyl groups. There is no much change in the position of peaks for different glass matrices. Fig. 3 shows the Raman spectra of different Sm3+ doped phosphate glasses. In the present work, it exhibits seven bands clearly at ∼340, ∼515, ∼630, ∼695, ∼1090, ∼1164 and ∼1260 cm−1 in all the glass matrices. Two intense peaks centered at 700 cm−1 and 1168 cm−1 are attributed to the symmetric stretching mode of PeOeP bridging mode and asymmetric stretching mode of non-bridging oxygens groups respectively [29]. Assignments of various Raman bands and their energies in different phosphate glasses observed are given below.

3.3.

31

P NMR spectroscopy

31 P solid state NMR spectra obtained for Sm3+ doped different phosphate glasses are shown in Fig. 4. It is noticed from figure that, Li, Mg, Ca, Sr and Ba phosphate glasses exhibited the chemical shifts at −22.32, −24.43, −23.83, −23.17 and −21.80 ppm respectively. The peaks observed for the prepared phosphate glass matrices are due to Q2 (metaphosphate) species, resulting to ionic bonds between cations and non-bridging oxygens [35]. The signal have single symmetric Q2 peak indicating the non existence of Q0, Q1 and Q3 structural units.

3.4. Absorption spectra and spectral intensities Absorption spectra of Sm3+ doped different phosphate glasses in the regions (a) UV-VIS and (b) NIR are shown in Fig. 5. Spectra show clearly the absorption bands corresponding to transitions 6H5/2 → 4D3/2, 6 P7/2, 6P3/2, 6P5/2, 4G9/2, 4I11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6F15/2 and 6F1/2 in all the five glass matrices. Table 1 shows the observed energies of different absorption bands in Li, Mg, Ca, Sr and Ba phosphate glasses. In the present work, these absorption bands were identified according to Dieke given in Ref. [36]. The spectral intensities of various absorption 9

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Fig. 4.

31

P NMR spectra of different phosphate glasses.

presented in Table 3 for all the glass matrices. It is observed that these J-O parameters are in order Ω4 > Ω6 > Ω2 for all glass matrices. Among the three J-O parameters, Ω2 parameter indicates the covalency and change in symmetry of the ligand around the RE ion [40]. In the present work, covalency was found decreasing in the order Ca→Mg→Sr→Ba→

spectra were obtained both from theoretical and experimental and are presented in Table 2. From the table, it is observed that Li glass showed higher and Ba glass showed lower spectral intensities for most of the transitions. Using Judd-Ofelt (J-O) theory [37–39], J-O parameters are obtained from the measured spectral intensities and these values are

Fig. 5. (a) UV–Visible (b) NIR absorption spectra of Sm3+ doped different phosphate glasses. 10

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Table 1 Observed band positions (cm−1) and their assignments of different excited levels of Sm3+ doped different phosphate glasses. Transition 6 H5/2→

Li

Mg

Ca

Sr

Ba

4

27855.2 26809.6 25000.0 24154.5 22831.2 21052.6 9293.6 8143.3 7262.1 6738.5 6468.3 6301.1

27777.7 26809.6 25000.0 24213.1 22831.1 21052.6 9276.4 8149.9 7278.0 6738.5 6459.9 6321.1

27855.2 26881.7 25000.0 24213.0 22831.2 21097.1 9319.6 8149.9 7267.4 6729.4 6485.0 6317.1

27932.9 26809.6 25000.0 24213.1 22831.0 21052.6 9310.9 8149.9 7256.8 6738.5 6480.8 6313.1

27855.2 26881.7 25000.0 24154.5 22831.3 21097.0 9302.3 8156.6 7272.7 6738.5 6497.7 6317.1

D3/2 6 P7/2 6 P3/2 6 P5/2 4 G9/2 4 I11/2 6 F9/2 6 F7/2 6 F5/2 6 F3/2 6 F15/2 6 F1/2

Table 3 Judd -Ofelt parameters (Ωλ ×10−20) and their trends of Sm3+ doped different phosphate glass matrices. Glass

Ω2

Ω4

Ω6

Trend

Reference

Li Mg Ca Sr Ba NBZPSm05 LBTAF Calibo KPbFb

0.117 0.692 1.231 0.489 0.302 0.11 0.27 0.98 2.32

6.475 5.359 5.986 3.460 4.088 4.44 2.52 5.04 3.04

5.042 3.537 5.217 3.024 3.680 4.10 2.47 4.73 2.83

Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2 Ω4 > Ω6 > Ω2

present present present present present [41] [42] [43] [44]

work work work work work

H5/2 → 4I11/2 with the wavelengths 359, 373, 401, 412, 438 and 475 nm respectively. From the spectrum, it is found that, the emission peak at 401 nm (6P3/2) is dominant among all the other peaks. The emission spectra of Sm3+ doped different phosphate glass matrices under the excitation wavelength 401 nm are shown in Fig. 7. It is consists four emission transitions, 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/ 6 4 6 2 → H9/2 and G5/2 → H11/2 at 565, 597, 645, and 715 nm respectively. Among these transitions, the peak due to the transition 4G5/2 → 6 H7/2 appeared at 597 nm is found to be most intense than the other transitions which corresponds to characteristic of reddish-orange region. The intensity ratio between electric dipole (4G5/2 → 6H7/2) and magnetic dipole (4G5/2 → 6H9/2) transitions measures the symmetry of the trivalent 4f ions. Among all the glass matrices, the intensity of 4G5/ 6 4 6 2 → H7/2 (MD) transition is greater than G5/2 → H9/2 (ED) transition indicating the presence of symmetric nature in all glass matrices. The intensity of transition 4G5/2 → 6H7/2 is decreasing in order Ca > Li > Sr > Mg > Ba in different phosphate glasses. Table 5 gives the emission band positions (λP), effective bandwidths (Δυeff), radiative transition probabilities (AR), peak stimulated emission cross-sections (σP) and branching ratios (βexp and βcal) of 4G5/2 state of Sm3+ doped different phosphate glasses. βexp values are obtained from relatives areas of the emission peaks. βcal values are obtained from the J-O theory. It is observed that effective bandwidths are higher in Ca and lower in Mg glass matrix. The stimulated emission cross-section (σP) is an important parameter which signifies the potential lasing transitions in that glass matrix. Generally large stimulated emission cross-sections for any transitions indicate good laser transitions. In the present work, σP value is higher in Li and lower in Sr phosphate glasses for 4G5/2 → 6 H7/2 transition. Hence among the five glass matrices Li phosphate glass is more stable for reddish orange laser light application. Branching ratio values are also higher in lithium glass matrix for this transition indicating lasing transition. 6

Li indicating higher and lower covalencies of SmeO bond in Ca and Li phosphate glasses respectively. From Table 3, it is observed that Ca has higher covalency than that in ZnOeBaF2eNaF2 [41], LBTAF [42], Calibo [43] glasses and lower than that in KPbFb [44] glass. Ω4 and Ω6 parameters indicate the viscous nature and dielectric properties of the glass, which are influenced by the vibronic transitions of RE ions bound to the ligand atoms [45,46]. In the present work, the obtained value of Ω4 parameter is higher in Li phosphate glass and lower in Sr phosphate glass indicating higher and lower viscosities of the glass matrix. 3.5. Radiative parameters The J-O intensity parameters, Ωλ (λ = 2, 4, 6) have been used to calculate radiative parameters for certain excited states of Sm3+ doped different phosphate glasses using the expressions given in Ref. [47] and are listed in Table 4 for lithium phosphate glass matrix. From the table, it is noticed that total radiative transition probability (AT) is higher for 6 F11/2 state and lower for 6F1/2 state among different excited states. From the magnitude of branching ratios, it is found that the transitions 6 F7/2 → 6H5/2 and 6F1/2 → 6H7/2 are having higher values (> 0.5) when compared to other transitions in these glass matrices. It is also noticed that the transition, 6F7/2 → 6H5/2 gives higher absorption cross-section (Σ) than other transitions in all glass matrices. 3.6. Visible luminescence spectra The visible excitation spectrum of Sm3+ ions doped lithium phosphate glass was recorded at room temperature for a 0.5 mol% doped sample. The spectrum is shown in Fig. 6 and consists of six excitation peaks due to transitions, 6H5/2 → 4D3/2, 6P7/2, 6P3/2, 6P5/2, 4G9/2 and

Table 2 Experimental (fexp ×10−6) and calculated (fcal ×10−6) spectral intensities of different absorption bands of Sm3+ doped different phosphate glass matrices. Transition 6 H5/2→

4

D3/2 P7/2 6 P3/2 6 P5/2 4 G9/2 4 I11/2 6 F9/2 6 F7/2 6 F5/2 6 F3/2 6 F15/2 6 F1/2 RMS deviation 6

Li

Mg

Ca

Sr

Ba

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

fexp

fcal

1.41 2.15 6.76 0.54 0.53 2.17 4.75 6.84 4.01 1.07 0.38 0.10 ± 0.459

1.22 2.51 7.18 1.03 0.12 1.18 4.69 6.85 3.64 1.65 0.03 0.03

1.64 1.10 6.32 0.48 0.65 1.78 3.09 5.27 2.97 1.41 0.32 0.06 ± 0.480

1.01 1.76 5.93 0.85 0.08 0.85 3.32 5.07 3.06 1.52 0.02 0.23

1.00 1.37 5.69 0.39 0.39 1.53 5.48 6.48 4.01 1.33 0.69 0.12 ± 0.641

1.13 2.59 6.63 0.96 0.12 1.22 4.82 6.87 3.44 1.82 0.04 0.41

0.81 0.82 4.11 0.28 0.24 1.59 2.93 3.92 2.11 0.65 0.38 0.09 ± 0.398

0.65 1.49 3.83 0.55 0.07 0.70 2.79 3.98 1.97 0.99 0.02 0.16

0.46 0.69 3.30 0.24 0.28 0.96 3.31 4.94 2.69 0.69 0.33 0.04 ± 0.565

0.77 1.29 1.48 0.65 0.08 0.85 3.38 4.80 2.31 1.10 0.02 0.10

11

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Table 4 Energies of transitions (ʋ,cm−1), radiative transition probabilities (AR, s−1), radiative branching ratios (βR) and integrated absorption cross sections (Σ X1018) for different excited states of Sm3+ doped lithium phosphate glass matrix. Transitions

Υ

AR

βR

Σ

G5/2 → F11/2 F9/2 6 F7/2 6 F5/2 6 F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2

7188 8518 9669 10550 11074 11344 11511 12740 14145 15471 16677 17711

0 0.8 4.6 5.7 0.1 0 0 7.3 53.6 92.1 202.9 126.9

0 0.002 0.009 0.012 0 0 0 0.015 0.11 0.189 0.417 0.261

0 0.01 0.02 0.02 0 0 0 0.02 0.13 0.19 0.35 0.2

AT = 494.2 τR = 2024

F11/2 → 6F9/2 F7/2 6 F5/2 6 F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2

1330 2481 3362 3886 4156 4323 5552 6957 8283 9489 10523

0.3 1.9 3.8 0 122.4 2.9 316 458.2 432.3 257.6 76.2

0 0.001 0.002 0 0.073 0.002 0.19 0.275 0.26 0.155 0.046

0.08 0.15 0.16 0 3.44 0.08 4.98 4.6 3.06 1.39 0.33

AT = 1671.6 τR = 598.2

F9/2 → 6F7/2 F5/2 6 F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2

1251 2132 2656 2926 3093 4322 5727 7053 8259 9293

0.2 1.1 1.4 55.7 0.4 59.3 131.8 321 528.5 442.4

0 0.001 0.001 0.036 0 0.039 0.086 0.209 0.345 0.289

0.06 0.12 0.1 3.16 0.02 1.54 1.95 3.13 3.76 2.49

AT = 1541.8 τR = 648.6

F7/2 → 6F5/2 F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2

881 1405 1675 1842 3071 4476 5802 7008 8042

0 0.1 7.7 0.5 30.5 63.5 160.2 245.5 597.9

0 0 0.007 0 0.028 0.058 0.146 0.224 0.546

0 0.02 1.33 0.07 1.57 1.54 2.31 2.43 4.49

AT = 1105.9 τR = 904.2

F5/2 → 6F3/2 6 H15/2 6 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2

524 794 961 2190 3595 4921 6127 7161

0 0.4 0 14.6 58.6 55.2 281 335.7

0 0.001 0 0.02 0.08 0.075 0.383 0.457

0 0.31 0 1.48 2.2 1.11 3.64 3.18

AT = 745.5 τR = 1341.3

F3/2 → 6H15/2 F1/2 6 H13/2 6 H11/2 6 H9/2 6 H7/2 6 H5/2

271 438 1667 3072 4398 5604 6638

0 0 7 32.7 169.3 103.1 195.7

0 0 0.014 0.066 0.341 0.208 0.394

0 0 1.22 1.68 4.25 1.59 2.16

AT = 507.8 τR = 1969.2

F1/2 → 6H13/2 H11/2 6 H9/2 6 H7/2 6 H5/2

1229 2634 3960 5166 6200

1.4 46.1 90.9 183.9 8

0.004 0.145 0.285 0.576 0.025

0.45 3.23 2.82 3.35 0.1

AT = 330.3 τR = 3027.5

4

6

6

6 6

6 6

6 6

6

6 6

6 6

Fig. 6. Excitation spectrum of Sm3+ doped lithium phosphate glass.

Fig. 7. Visible emission spectra of Sm3+ doped different phosphate glasses.

presented in Fig. 8. From figure, it is found that the decay curves exemplify the single-exponential behavior. Table 6 gives measured and calculated lifetimes of 4G5/2 state in different phosphate glasses and the quantum efficiencies (η). It is observed that, the lifetime decay constants of 4G5/2 excited state are 1.96, 1.60, 1.95, 1.94 and 1.94 ms for Li, Mg, Ca, Sr and Ba glass matrices respectively. The differences in τexp and τcal values shows the presence of higher vibrational frequencies answerable for non-radiative decay processes which is observed due to the presence OH− ions in the five glass matrices [29]. The quantum efficiency value, η is higher in Li glass when compared with the five glass matrices studied.

3.8. CIE color coordinates and generation of reddish orange light Commission International deI'eclairage (CIE) color coordinates of Sm3+ doped different phosphate glasses are computed from luminescence spectra using CIE 1931 chromaticity as shown in Fig. 9. In the figure, the measured values of color coordinates for the different phosphate glass samples falls within the reddish orange region. Emitted light is calculated with the help of the color coordinates such as x, y of Sm3+ different phosphate glasses using the equations given in Ref.

3.7. Luminescence decay analysis Luminescence decay curves of 4G5/2 level of Sm3+ in different phosphate glasses with excitation 401 nm and emission at 597 nm are 12

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Table 5 Emission band positions (λp, nm), effective bandwidths (Δʋeff, cm−1), peak stimulated emission cross-sections (σp, x 10−20 cm2) and branching ratios (βR) of certain emission transitions of Sm3+ doped different phosphate glasses. Glass

Transition

Λp

Δʋeff

σp

βexp

βcal

Li

4

G5/2 → H5/2 H7/2 H9/2 6 H11/2 4 G5/2 → 6H5/2 6 H7/2 6 H9/2 6 H11/2 4 G5/2 → 6H5/2 6 H7/2 6 H9/2 6 H11/2 4 G5/2 → 6H5/2 6 H7/2 6 H9/2 6 H11/2 4 G5/2 → 6H5/2 6 H7/2 6 H9/2 6 H11/2

561.1 597.3 644.1 705.3 560.6 596.4 643.3 705.2 561.4 597.1 643.2 705.3 560.6 597.2 643.5 707.1 560.5 596.6 643.3 706.2

261.2 311.9 330.0 328.4 273.3 304.1 314.7 324.5 279.4 320.1 324.5 315.9 252.3 310.6 347.9 308.9 245.8 320.5 333.3 335.6

7.4 11.2 5.6 3.9 5.0 7.9 4.7 2.7 5.8 9.4 0.5 3.3 3.7 5.6 2.9 2.0 4.5 6.5 3.3 2.1

0.26 0.45 0.18 0.11 0.25 0.39 0.20 0.10 0.24 0.39 0.21 0.10 0.24 0.40 0.20 0.10 0.25 0.41 0.19 0.10

0.07 0.46 0.40 0.09 0.07 0.44 0.38 0.09 0.07 0.45 0.41 0.09 0.06 0.44 0.42 0.08 0.07 0.45 0.38 0.09

6

6 6

Mg

Ca

Sr

Ba

Fig. 9. CIE chromaticity diagram showing chromaticity coordinates (x, y) for 0.5 Sm3+ doped different phosphate glasses.

light applications. 4. Conclusions Samarium doped phosphate glasses with different compositions were prepared by employing melt quenching technique. Spectroscopic examinations such as absorption, emission and decay spectral measurements were done to select the better glass composition for reddishorange photonic applications. Radiative properties have been determined from the analysis of both absorption and emission spectra of observed fluorescent levels of Sm3+ ions using Judd-Ofelt theory. The emission spectra of the prepared glasses exhibit two strong bands corresponding to transitions 4G5/2 → 6H7/2 (orange) and 4G5/2→ 6H 9/2 (red). The effective bandwidths, stimulated emission cross-sections and experimental branching ratios were calculated. From decay graphs, the experimental lifetime (τexp) values have been measured and they are in agreement with radiative lifetime (τR) values which were used to determine the quantum efficiencies (η) of the prepared glasses. Among all the prepared glasses, Li glass having comparatively higher values of emission cross-sections, branching ratios and quantum efficiencies. Hence Li glass could be a potential candidate for visible reddish-orange emission which can be further supported by the CIE coordinates. All the aforementioned studies ultimately gave picture that among all the Sm3+ doped phosphate glasses studied, Li glass is more appropriate for the lighting applications in reddish-orange region.

Fig. 8. Decay curves of 4G5/2 level of Sm3+ doped different phosphate glasses. Table 6 Experimental (τexp) (ms) and calculated (τcal) (ms) lifetimes and quantum efficiencies (ɳ) (%) of 4G5/2 level of Sm3+ doped different phosphate glasses. Glass

τexp

τcal

ɳ

Reference

Li Mg Ca Sr Ba NSMgP NSCaP NSBaP

1.96 1.60 1.95 1.94 1.94 1.22 1.39 1.74

2.05 2.80 2.27 3.99 3.41 2.30 2.34 2.40

95.60 57.14 85.90 48.62 56.89 53.0 59.4 72.5

present present present present present [29] [29] [29]

work work work work work

Acknowledgement The authors acknowledge MoU-DAE-BRNS Project (No. 2009/34/ 36/BRNS/3174), Department of Physics, S.V. University, Tirupati, India for extending the experimental facility.

[44]. Among five phosphate glasses, CIE coordinate (x, y) values are almost same and were found to be (0.601, 0.384); (0.601, 0.384); (0.601, 0.381); (0.6, 0.380); (0.61, 0.384). Hence the prepared Sm3+ doped all five phosphate glasses might be useful for reddish-orange

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