Sm3+ co-doped barium gallium borosilicate glasses

Physica B: Condensed Matter 559 (2019) 8–16

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Energy transfer (Ce3+→ Sm3+) influence on PL emission of Ce3+/Sm3+ codoped barium gallium borosilicate glasses

T

T. Sambasiva Raoa, D.V. Krishna Reddya, S.K. Taherunnisaa, K.S. Rudramambaa,b, A. Siva Sesha Reddya, N. Veeraiaha, M. Rami Reddya,∗ a b

Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, 522 510, India Department of Physics, VNR Vignana Jyothi Institute of Engineering and Technology, Hyderabad, 5000 090, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Barium gallium borosilicate glasses Ce3+→ Sm3+ energy transfer Inokuti-Hirayama model

30BaOe2Ga2O3‒(27.8−x)B2O3e40SiO2exCe2O3e0.2Sm2O3(0 ≤ x ≤ 1) glasses were synthesized by conventional melting and quenching processes. The characterization of samples was performed using XRD, EDS, FTIR and Raman studies. Later, Optical absorption (OA) and photoluminescence (PL) spectral were recorded. X-ray diffraction pattern indicated amorphous nature of the samples. Raman and FTIR spectra have confirmed the presence of various, borate and silicate structural groups in the glass matrix. Structural modifications in glass matrix were clearly observed with variation of glass composition and increasing asymmetry led to depolymerisation of the glass structure. The absorption spectra exhibited nine absorption bands of Sm3+ ions in the vis–NIR regions excited from the ground stated 6H5/2. Additionally, a feeble band corresponding to 4f (2FJ) →5d (2A1g) absorption transition of Ce3+ ions (at 415 nm) is also observed in these spectra. Optical band gap (Eo) evaluated using Tauc plots is observed to decrease with Ce3+ concentration. PL emission of Sm3+ ions (recorded at λexc = 350 nm) in the co-doped glasses is observed to increase with increase of Ce2O3 content up to 0.6 mol%. The non-exponential decay curves are well fitted to I-H (Inokuti-Hirayama) model by S = 6, suggesting that the nature of the energy transfer between Ce3+→ Sm3+ in terms of dipole-dipole interaction. The CIE coordinates for 350 nm excitation of emission of 0.6CSm glass was observed to be exist in near white light region; hence these glasses may be considered for white LED's applications.

1. Introduction Recently W-LED's are mostly used due to their long lifetime, energy saving, safe, reliable, low production price and environmental pleasant; these LED's are flexible and attractive for their position of conventional incandescent and the fluorescent lamps such as a GaN based LED's [1,2]. Among various glass hosts, borosilicate glasses admixed with heavy metal oxides viz., BaO and Ga2O3 have attracted more interest in recent years science they possess high thermal and chemical stability with inherent attributes like high band gap and covalent bond energy, low thermal expansion and larger refractive index [3,4]. The addition of modifier oxide like BaO even in low concentrations to borosilicate glass network influences the glass network strongly [5–9] and make the glass to exhibit different optoelectronic effects. The admixing of Ga2O3 to the borosilicate glass systems bring interesting changes in the physical characteristics, e.g., refractive index (n) the glass transition temperature, chemical resistance and infrared transmittance are found to be improved. In view of these, Ga2O3 mixed borosilicate glasses find ∗

potential applications for IR windows, optical isolators, ultra fast optical switches and several optically operated instruments useful for communication applications [10–12]. Above all, these glasses offer highly suitable environment to host rare earth (RE) ions (with low phonon losses) that exhibit PL emission in visible NIR spectral ranges. Among different RE ions, cerium and samarium show important role as dopants, components in a glass matrix [13,14]. Extensive research has been carried out recently on several Sm3+ doped glass matrices [15–18]. These glasses are laser active materials for an extended time and also found to be suitable for various applications in under sea communications and optical amplifiers. The PL output of Sm3+ ions especially in reddish-orange region (due to 4G5/2 → 6H7/2, 6H9/2 and 6 H11/2 emission transitions) is strongly dependent on host glass composition. Ce3+ ions ([Xe] 4f1) exhibit PL emission in a broad region due to 4f ↔ 5d electronic transitions in the blue region. Hence Ce3+ doped glasses find potential applications e.g., such as blue luminescent materials, scintillators, biosensors, compact fuel cells, laser glasses and phosphors [19,20]. Additionally, these ions can be used as sensitizers

Corresponding author. E-mail address: [email protected] (M.R. Reddy).

https://doi.org/10.1016/j.physb.2019.01.053 Received 3 December 2018; Received in revised form 17 January 2019; Accepted 31 January 2019 Available online 31 January 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.

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3. Results and discussions

Table 1 The details of the chemical compositions of the glasses (all in mol %). Glass

BaO

Ga2O3

B2O3

SiO2

Ce2O3

Sm2O3

Pure 0.2Ce 0.2 Sm 0.2CSm 0.4CSm 0.6CSm 0.8CSm 1.0CSm

30 30 30 30 30 30 30 30

2 2 2 2 2 2 2 2

28.0 27.8 27.8 27.6 27.4 27.2 27.0 26.8

40 40 40 40 40 40 40 40

– 0.2 – 0.2 0.4 0.6 0.8 1.0

– – 0.2 0.2 0.2 0.2 0.2 0.2

The prepared barium gallium borosilicate (BaGaBSi) glasses mixed with Ce3+/Sm3+ ions are visibly free from voids, internal cracks etc. With the average molecular weight density d and refractive index n, various physical parameters (useful for the analysis of emission spectra) are evaluated using standard formulae [23,24] and are furnished in Table 2. In Fig. 2 density d and molar volume (M.V.) are plotted as functions of Ce3+ content; d exhibited an increasing trend, whereas M.V. shows a decreasing trend with Ce2O3 content. This is an indication of increasing concentration of internal cross-linkages between different borate, silicate structural units in the glass network. XRD patterns of Ce3+/Sm3+ doped barium gallium borosilicate glasses are shown in Fig. 3. The diffractograms exhibited a broad hump with the meta-centre at about 28° (with no sharp diffraction peaks). This observation confirms amorphous nature of the samples prepared. Energy dispersive spectral studies have clearly confirmed that all the elements that are present in the original glass batch are quite intact (Fig. 4). FTIR spectra of BaOeGa2O3eB2O3eSiO2 glasses recorded in the spectral region 400-1600 cm−1 (Fig. 5) exhibited vibrational bands due to different borate (BO3, BO4 and BeOeB bending vibration), silicate (asymmetric and symmetric stretching vibrations and also rocking vibrations of SiO4 groups. Additionally, GaO4 vibrational band is also detected in these spectra. Summary of various vibrational band positions observed in these spectra is presented in Table 3 [25–31]. As Ce2O3 content is raised up to 0.6 mol%, intensity of BO3 vibrational band and asymmetric vibrational bands of silicate groups is observed to increase while that of BO4 and GaO4 structural units showed a decreasing tendency. As Ce2O3 content, is increased further, the intensity of these bands seemed to exhibit an opposite trend. Raman spectra of BaO‒Ga2O3eB2O3eSiO2 glasses recorded in the wavenumber range 200–1600 cm−1 are presented in Fig. 6, while the summary of positions of different vibrational bands is furnished in Table 4. The spectra of exhibited five intense vibrational bands related to the various groups of gallium oxide (GaO4 units), borate (BO3, BO4 and BeOeB) and SiO4 (asymmetric, symmetric stretching vibrations and SieOe Si bending vibrations) [32–34]. Similar to IR spectra, we have observed a gradual hike in the intensity of vibrational bands of BO3 units and also asymmetric vibrations of silicate groups with increase of Ce2O3 content up to 0.6 mol% at the expense of BO4 and GaO4 structural units. For further raise of Ce2O3 quantity, the intensity of these bands exhibited an opposite trend. Thus, both IR and Raman spectroscopy studies have clearly revealed an increasing degree of disconnectivity or rupturing of bonds between various structural groups with the raise of Ce2O3 dopant up to 0.6 mol% in the glass network; structural defects facilitate the decrease of quenching losses of PL output. OA spectra of Sm3+, Ce3+ ions and Ce3+/Sm3+ ions co-doped BaO‒Ga2O3eB2O3eSiO2 glass samples are recorded in the UV–Vis–NIR regions and presented in Fig. 7(a) and (b). It may be noted here that the spectrum of pure glass (rare earth free glass) has not exhibited any absorption bands. With the addition of 0.2 mol % of Sm2O3, the spectrum exhibited as many as nine distinct absorption bands due to the following electronic transitions of Sm3+ ions: 6H5/2 → 4D5/2, 6P7/2, 4F7/ 6 6 6 6 6 6 2, F9/2, F7/2, F5/2, F3/2, H15/2 and F1/2 at 365, 372, 402, 1072, 1220, 1368, 1461, 1527, and 1583 nm, respectively [35]. The spectrum of the glass doped with 0.2 mol% of Ce2O3 (0.2Ce glass) exhibited a feeble absorption band at 415 nm due to 4f (2FJ) →5d (2A1g) transition of Ce3+ ions [36,37]. When Sm3+ ions are co-doped with Ce3+ions, the absorption edge exhibited spectrally red shift. Probable reason for the red shift of the edge may be because of the probable exchange of electrons between two oxidation states of Ce ions (Ce3+↔ Ce4+) [38]. As a result, among three OA bands of Sm3+ ions located at 365, 372 and 402 nm, the first two peaks could not be visualized in the spectra of the

(or activator) because of their strong blue emission and are highly useful to improve the luminescence emission of co-dopants like Sm3+ [21,22]. In this work we have reported the influence of concentration of Ce3+ ions on luminescence efficiency of Sm3+ ions in the visible region in barium gallium borosilicate glasses and analysed the results in the light of probable energy exchange among these rare-earth ions (Ce3+& Sm3+) and to assess their possible suitability for W-LED applications.

2. Experimental The details of the chemical composition and their nomenclature are furnished in Table 1. All the AR grade chemical compounds Ga2O3, BaCO3, H3BO3, SiO2, Ce2O3 and Sm2O3 in appropriate amounts (of 99.99% purity all in mol %) are admixed in an agate mortar and melted in a silica crucible at about 1450 °C for 20 min. The resultant melt was poured in a brass mold and subsequently annealed at about 450 °C. Then, the glasses are polished up to final dimensions of 1.0 cm × 1.0 cm x 0.1 cm for the spectroscopic studies. The photographs of the obtained samples (pre-polished) are shown in Fig. 1. The colour of the samples containing higher content of Ce2O3 appeared to be slightly yellowish; this observation indicates a possible presence of a small fraction cerium ions in Ce4+ state in such glasses. XRD studies are carried out on a XRD-6100 SHIMADZU Diffractometer in the scanning range of 10–90° (2θ) operated at 40 kV, 30 mA, using CuKα radiation. The refractive index of the glasses is measured using Abbe's refractometer at 589.3 nm wavelength. The density of these samples was measured according to Archimedes' principle with o-xylene as immersion liquid using VIBRA HT model kit to an accuracy of ± 0.001 g/cm3. The energy dispersive spectroscopy measurements were carried out on a Thermo Instruments Model Noran System 6 attached to electron scanning microscope. Fourier transform infrared spectra were recorded on SHIMADZU-IRAffinity-1SFT-IR spectrophotometer with a precision of 0.1 cm−1 in the spectral range 400–2000 cm−1 using KBr pellets. The optical absorption spectra of the samples were recorded at ambient temperature using JASCO, V-570 Spectrophotometer in the wave length range 300–2000 nm with are solution of 0.1 nm emission, excitation, and decay measurements were performed using FLS-980 Fluorescence spectrometer at room temperature with a xenon flash lamp as an excitation source at different excitation wavelengths.

Fig. 1. The photographs of the obtained samples BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glass system.

(pre-polished)

of

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Table 2 Various physical parameters of BaOeGa2O3eB2O3eSiO2: Ce2O3/Sm2O3 glasses. Physical properties 3

Density, d (g/cm ) ± 0.001 Molar volume (cm3/mol) ± 0.001 Ce3+ ion concentration (x1021 ions/cm3) Inter ionic distances, ri(Å) ± 0.001 Polaron radius, rp (Å) ± 0.001 Field strength, F (1015cm−2) Refractive Index ± 0.001 Reflection losses(R) Dielectric constant (∈) Boron–Boron separation (nm) ± 0.0001 Metallization factor (M) Electronic polarizability, αm (Å)

Pure

0.2CSm

0.4CSm

0.6CSm

0.8CSm

1.0CSm

3.325 28.046 – – – – 1.650 0.462 2.722 0.3194 0.5372 –

3.382 27.890 4.319 6.140 2.474 4.900 1.652 0.463 2.729 0.3174 0.5363 2.563

3.403 27.872 8.643 4.872 1.963 7.783 1.652 0.463 2.732 0.3170 0.5363 1.281

3.425 27.847 12.991 4.263 1.717 10.167 1.653 0.464 2.732 0.3166 0.5359 0.858

3.431 27.803 17.239 3.871 1.559 12.332 1.653 0.464 2.732 0.3163 0.5357 0.853

3.469 27.791 21.670 3.586 1.445 14.363 1.654 0.464 2.735 0.3158 0.5353 0.512

Fig. 4. EDS Spectrum of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

Fig. 2. Variations of density and molar volume with the Ce2O3/Sm2O3 concentration of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glass system.

Fig. 5. FT–IR spectra of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

the equation.

Fig. 3. XRD Spectrum of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

α (ν) hν = C(hν-Eo)n glasses containing higher contents of Ce2O3. However, the spectra of all the glasses seemed to have retained the third absorption band related to 6 H5/2 → 4D5/2 transition of Sm3+ ions and its intensity is gradually increased with Ce2O3 content. The intensity of absorption band of Ce3+ ions (observed at 415 nm) also exhibited increasing trend with Ce2O3 content. The NIR absorption bands of Sm3+ ions in the co-doped glasses are also observed to grow gradually with co-dopant (Ce2O3) concentration. From OA edges, the optical band gap (Eo) is estimated for the studied glass samples by drawing Tauc plots between (αhν)n vs hν as per

(1)

In Eq. (1), n takes the values 1/2 and 2 and represent indirect and direct band gaps, respectively [39], C is a constant, hv is the energy of the photon, α(ν)is absorption coefficient. The obtained plots are presented in Fig. 8(a) and (b). From the extrapolation of linear portions of the curves, the values of Eo are determined for the titled glasses and furnished in Table 5. Eo (both direct and indirect band gaps) exhibited a decreasing trend with Ce2O3 concentration up to 0.6 mol %; for further increase of Ce2O3 content an increase of Eo is observed. We have also evaluated Urbach energy ΔE by plotting ln α(ν) vs hν (Fig. 9) as per the relation; 10

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Table 3 Summary of FT–IR band positions of BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses. Pure

CSm0.2

CSm0.4

CSm0.6

CSm0.8

CSm1.0

Band assignments

456 632 820 990 1140 1390

459 626 817 986 1148 1400

458 623 813 984 1152 1406

454 619 809 979 1155 1411

460 617 804 976 1158 1418

463 615 800 970 1162 1420

Bending and rocking motion of SieOeSi GaO4 structural units Symmetric stretching vibrations of SieOeSi units Stretching vibrations of BeO bonds in BO4 units SieOeSi asymmetric vibrations BeO structural vibrations of BO3 units and other borate groups

lnα(ν) = hν/ΔE + constant

(recorded at λexc = 350 nm) of BaO‒Ga2O3eB2O3eSiO2 glass mixed with 0.6 mol% of Ce2O3 (0.6Ce) are shown in Fig. 10. Excitation spectrum exhibited a broad band at about 350 nm and same was used for recording PL spectra. The PL spectrum 0.6Ce glass exhibited an emission band at 400 nm. These bands (both emission and excitation) are identified as being due to 4f ↔ 5d electronic transitions of Ce3+ ions. Fig. 11 shows the excitation (monitored at λemi = 600 nm) and emission (λexc = 402 nm) spectra of BaOeGa2O3eB2O3eSiO2 glass containing 0.2 mol% of Sm2O3. The spectra exhibited several excitation bands assigned to 6H5/2 → 4G9/2 (315 nm), 4K17/2 (344 nm), 4D3/2 (361 nm), 6P7/2 (374 nm),6P3/2 (402 nm) 6P5/2 (416 nm),4M17/2 (437 nm),4I13/2 (460 nm), 4M15/2 (468 nm) and 4I11/2 (473 nm) [40]. Among these, the 6H5/2 → 6P3/2 excitation band observed at 402 nm is found to be more intense and same was used for recording PL spectra of

(2)

Urbach energy,ΔE, represents the width of the tail of localised states in the band gap. From the slopes of these plots, the values of ΔE are evaluated and are tabulated in Table 5. The value of ΔE is found to be the maximal (0.358 eV) for the glass 0.6CSm and found to be the minimal (0.334eV) for the glass 1.0 CSm. The lowest value of Eo and the highest value of ΔE observed 0.6CSm suggests the higher degree of disorder or the higher concentration of imperfections viz., non-bridging oxygen's, broken bonds in the samples. Such higher degree of internal depolymerisation minimizes the quenching losses and facilitates to increase luminescence output. The excitation (monitored at λemi = 400 nm)and emission spectra

Symmetric streching vibrations of SiO4

BO3 units

Asymmetric vibrations of Si-O-Si

GaO4 units Bending vibrations of Si-O-Si

1.0CSm

Intensity(arb.units)

0.8CSm

0.6CSm

0.4CSm

0.2CSm

Pure

200

400

600

800

1000

1200

1400

-1

Raman shift (cm ) Fig. 6. Raman spectra of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses. 11

1600

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Table 4 Summary of Raman band positions of BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses. Pure

CSm0.2

CSm0.4

CSm0.6

CSm0.8

CSm1.0

Band assignments

460 620 812 1102 1380

459 626 810 1106 1384

458 623 808 1110 1389

454 619 805 1113 1392

460 617 802 1116 1394

463 615 800 1122 1398

Bending vibrations of SieOeSi GaO4 units Symmetric stretching vibrations of SiO4 Asymmetric vibrations of SieOeSi BO3 units

electronic transitions of Sm3+ ions. Interestingly, as mentioned above, PL spectra of Ce3+ doped glass exhibited a broad band the spectral region of 370–470 nm when excited at 350 nm. In this region several excitation bands of Sm3+ ions are observed in the excitation spectra of Sm3+ doped glasses. This observation indicates there is broad overlapping region in these two spectra as shown in the Fig. 12. In the same figure, the emission spectra (recorded at λex = 350 nm) of glasses doped with different concentrations of Ce2O3 are presented. The spectra indicated the gradual increase of PL intensity of Ce3+ ions with shifting of the metacentre towards slightly higher wavelength. A strong overlapping of the emission band with excitation spectra of Sm2O3 doped glasses is visualized in the spectrum of 0.6Ce glass. Such overlapping leads to pumping of energy from cerium ions to samarium ions when they are co-doped as per Dexter theory [1]. Such PL spectra of Ce3+ doped samples were recorded at three excitation wavelengths viz., 330, 350 and 402 nm and are superposed with the excitation spectra of Sm3+ ions doped glasses. The comparison of the spectra indicated maximum overlapping region of the two spectra (Sm3+ excitation spectra) and Ce3+ emission spectra recorded at

a)

Pure

6H 5/2

1.3

0.2Csm

0.2Sm 0.2Csm 0.4Csm

Absorption coefficient (cm-1)

4F 7/2

0.6CSm 0.8CSm 1.0CSm 0.8

4D 5/

4f(2FJ) 6P

5d(2A1g)

7/2

0.3 300

350

400 Wavelength (nm)

500

450

Fig. 7. a Optical absorption spectra of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses in UV–VIS region. b Optical absorption spectra of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses in NIR region.

0.5

b

6H 5/2 6F 9/2

6F 5/2 6F 7/2

0.45

6F 3/2

6H 15/2

6F 1/2

Absorption coefficient (cm)-1

1.0CSm 0.4

0.8CSm 0.6CSm

0.35 0.4CSm 0.3

0.2CSm

0.25

0.2 1000

0.2Sm Pure 1100

1200

1300

1400

1500

1600

Wavelength (nm) Fig. 7. (continued)

λexc = 350 nm. Hence, we have chosen λexc = 350 nm to record PL spectra of co-doped glasses. Fig. 13 shows PL spectra recorded at λexc = 350 nm of the co-doped

Sm2O3 doped samples. PL spectra recorded in the visible region exhibited four significant luminescence bands assigned to 4G5/2 → 6H5/2 (564 nm), 6H7/2 (601 nm), 6H9/2 (648 nm) and 6H11/2 (711 nm)

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Fig. 9. Urbach plots for BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

overlapping region of these two spectra when the content of Ce2O3 is raised beyond 0.6 mol % may account for the decrease in the PL intensity of Ce2O3/Sm2O3 doped BaO‒Ga2O3eB2O3eSiO2 glass samples [41,42]. The colour of any light source can be evaluated by colour matching functions x (λ), y (λ) and z(λ). The chromaticity colour coordinates were computed from PL spectra using the Commission International de l’Eclairage (CIE) system [43]using:

Table 5 Cut-off wavelength, direct, indirect band gap and Urbach energy values of BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses. Cut-off wavelength (nm) ± 1

Direct band gap (eV) ± 0.01

Indirect band gap (eV) ± 0.01

Urbach energy (eV) ± 0.001

Pure 0.2CSm 0.4CSm 0.6CSm 0.8CSm 1.0CSm

306 378 382 394 376 360

3.48 2.97 2.94 2.89 3.07 3.12

3.47 2.96 2.91 2.86 3.05 3.11

0.294 0.348 0.354 0.358 0.338 0.334

X X+Y+Z

(3)

y=

Y X+Y+Z

(4)

In Eq. (4), X, Y, Z represent tristimulus parameters that give the information on the stimulation of red (R), green (G) and blue (B) primary colours that are required to match P(λ) (known as colour wavelength). In Table 6, the estimated (x, y) coordinates for studied samples are furnished. The values of chromaticity coordinates for these samples are found to be (0.168, 0.152), (0.588, 0.401), (0.198, 0.189), (0.214, 0.195), (0.287, 0.259), (0.264, 0.218) and (0.243, 0.201) for 0.2Ce, 0.2Sm, 0.2CSm, 0.4CSm, 0.6CSm, 0.8CSm and 1.0CSm glasses, respectively, for the luminescence spectra recorded at excitation wavelength of 350 nm. The colour coordinates of samarium and cerium mixed glasses obtained for 0.6CSm (0.287, 0.259) are found to be nearer to the standard values of ideal white light (Fig. 14). These evaluated colour coordinates suggest the titled glasses are useful for white LEDs. Fig. 15 represents the decay profile of PL emission of co-doped glasses recorded at λexc = 350 nm and λemi = 400 nm. These curves are well fitted with bi-exponential function and the resultant PL intensity I (t) that contains fast (τfd) and slow (τsd)decay components, is represented, by

Fig. 8. (a)&8(b) Tauc plots to evaluate (a) direct band gap, (b) in-direct band gap of BaoeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

Sample

x=

I(t) = A1 exp[− glasses (containing fixed concentration of Sm2O3) and varying concentration of Ce2O3 (from 0.2 to 1.0 mol %). The spectra exhibited the emission bands due to Ce3+ ions and also those of Sm3+ ions that are observed in the spectra of individually doped glasses. Moreover, the intensity of emission bands of Sm3+ ions exhibited an increasing trend (at simultaneous decay of Ce3+ emission band observed at 400 nm) with the increase of Ce3+ ion concentration and the maximal intensity is observed at 0.6 mol% of Ce3+ ions. This observation suggests the maximum energy transfer from Ce3+ to Sm3+ ions in this glass. This is obviously because of larger overlapping in the emission spectra of Ce3+ ions and the excitation spectrum of Sm3+ ions. The decreasing

t t ] + A2 exp[− ] τfd τsd

(5)

The average lifetime (τavg) is represented by,

τavg =

2 2 A1 τ fd + A2 τsd

A1 τfd + A2 τsd

(6)

The values of τavg estimated using Eq. (6) are found to be 38.0, 29.2, 27.5, 22.0, 23.8 and 25.2ns for 0.2Ce, 0.2CSm, 0.4CSm, 0.6CSm, 0.8CSm and 1.0CSm samples, respectively. The decreasing trend of τavg with increase of Ce2O3 content clearly suggests the energy transfer from Ce3+→Sm3+. From I–H theory, the emission intensity is expressed as 13

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4f emi =

5d

5d

4f

400 nm

ex

= 350 nm

Intensity (a.u.)

0.6Ce excitation 0.6Ce emission

250

300

350

400

450

500

550

Wavelength (nm) Fig. 10. The excitation (monitored at λemi = 400 nm) and emission spectra (λex = 350 nm) of BaOeGa2O3eB2O3eSiO2 glass doped with 0.6 mol% of Ce2O3.

5d

4f

ex=350nm

Intensity(arb.units)

6 4

H7/2

G5/2

6 6

0.2Ce 0.2CSm 0.4CSm 0.6CSm 0.8CSm 1.0CSm

H9/2

H5/2

6

370

420

470

520

570

620

H11/2

670

720

Wavelength(nm) Fig. 13. Luminescence spectra of BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 doped glasses excited at 350 nm.

Table 6 The CIE 1931 chromaticity colour coordinates BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

Fig. 11. The excitation (monitored at λemi = 600 nm) and emission (λex = 402 nm). Spectra of BaOeGa2O3eB2O3eSiO2 glass doped with 0.2 mol% of Sm2O3.

S.No

1 2 3 4 5 6 7

Intensity (a.u.)

1.0Ce 0.8Ce 0.6Ce 0.4Ce 0.2Ce 0.2Sm

I(t) = I0 exp{ −

300

350

400

450

500

Glass

0.2Ce 0.2 Sm 0.2CSm 0.4CSm 0.6CSm 0.8CSm 1.0CSm

t t 3 − Q ( ) S} τo τo

(x,

y)

of

The chromaticity coordinates x

y

0.168 0.588 0.198 0.214 0.287 0.264 0.243

0.152 0.401 0.189 0.195 0.259 0.218 0.201

(7)

In Eq.(12), S takes the values 6, 8 & 10 that represent the dipole–dipole, dipole– quadrupole and quadrupole–quadrupole interactions between Ce2O3 and Sm2O3 respectively, at t sec. after the excitation, τ0 is the inherent decay time of the donors in the absence of acceptors, Q is the energy transfer parameter and it is defined as,

550

Wavelength (nm)

Fig. 12. Overlap region between Ce2O3 (emission) and Sm2O3 (excitation) BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3doped glasses.

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Fig. 15. (continued) Table 7 The experimental (τexp, ns) life time, energy transfer parameter (Q), critical transfer distance (R0, nm), donor-acceptor coupling constant (CDA X 10−46 cm6/s), energy transfer efficiency (ηET,%), probability of energy transfer (PET,s−1) of BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses. Fig. 14. The color space chromaticity diagram BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3doped glasses excited at 350 nm.

Glass

τexp

Q

R0

CDA

ηET

PET

0.2Ce 0.2CSm 0.4CSm 0.6CSm 0.8CSm 1.0CSm

38.0 29.2 27.5 22.0 23.8 25.2

– 0.36 0.49 0.60 0.58 0.55

– 4.84 4.23 3.42 3.59 3.85

– 604.2 271.1 75.80 100.6 155.0

– 23.15 27.63 42.01 37.36 33.68

– 0.007 0.010 0.019 0.015 0.013

of

furnished in Table 7. Energy transfer efficiency from Ce2O3 to Sm2O3 was evaluated using [45].

ηET = 1 −

τc τco

(10)

where ETηET is the energy transfer efficiency, τc is average luminescence intensity decay time for both sensitizer (Ce3+) and activator (Sm3+) and τco represents average decay time of donor ions. The probability energy transfer (PET) in terms of lifetimes is given by

PET =

4π 3 Γ (1 − )No Ro3 3 S

(8)

4. Conclusions

In Eq. (8), as per Ref. [44], the parameter Γ (1– 3 ) takes the values S 1.77(if S = 6, it represents dipole-dipole interactions), 1.43(if S = 8, it represent dipole–quadrupole interactions) and 1.30 (if S = 10, it represents quadrupole–quadrupole interactions). The symbol R0 represents critical transfer distance while N0 denotes acceptor concentration of trivalent rare earth (Sm3+ ions). The donator-acceptor interaction parameter can be presented as

CDA =

Ro(S ) τO

(11)

From the decay curve, ηET value is observed to vary from 23.15 to 42.01% and PET to change from 0.007 to 0.019. These results indicated that the glasses co-doped with 0.6 mol% of Ce2O3 exhibited the maximal energy transfer efficiency and energy transfer probability. The probable energy transfer mechanism from Ce3+ to Sm3+ ions is shown in energy level diagram (Fig. 16).

Fig. 15. Decay Curves (monitored at λex = 350 nm, λemi = 400 nm) a) Non exponential curve fit b) I–H fit of BaOeGa2O3eB2O3eSiO2:Ce2O3/Sm2O3 glasses.

Q=

1 1 − o τc τc

Barium gallium borosilicate glasses of the composition 30BaO‒2Ga2O3‒(27.8‒x) B2O3e40SiO2‒0. 2Sm2O3:xCe2O3 (with x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized by melt-quenching method. The detailed influence of concentration of Ce3+ co-dopant on luminescence features of Sm3+ ions is investigated. The PL emission of the co-doped samples (recorded at λexc = 350 nm) exhibited maximum intensity when the concentration of co-dopant (Ce2O3) is about 0.6 mol %. Such result indicated maximum energy pumping from Ce3+→ Sm3+ ions; this could be possible because of larger overlapping between the PL and excitation spectra of Ce3+ and Sm3+ ions, respectively. The results of auxiliary structural studies viz., FTIR and Raman were also used in analysing the emission spectral results of co-doped glasses. We

(9)

The non-exponential nature of IeH model obtained (fitted with S = 6 Fig. 15 b) thus suggested dipole–dipole interaction are prevalent between Ce3+ and Sm3+ ions. Calculated values of CDA, R0 and Q are 15

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T. Sambasiva Rao et al.

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]

Fig. 16. Partial Energy level diagram of BaOeGa2O3eB2O3eSiO2:Ce2O3/ Sm2O3 glasses.

[31] [32]

have also estimated CIE colour coordinates for the emission spectra recorded at λexc = 350 nm. Obtained values of the coordinates are found to be nearer to those of white light emission, and hence it is concluded that BaO‒Ga2O3eB2O3eSiO2: Ce3+/Sm3+ glasses may be useful for white LED's.

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