Effect of Sm3+ ions concentration on borosilicate glasses for reddish orange luminescent device applications

Journal of Non-Crystalline Solids 513 (2019) 152–158

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Effect of Sm3+ ions concentration on borosilicate glasses for reddish orange luminescent device applications

T

Nisha Deopaa, , Babloo Kumarb, Mukesh K. Sahub, P. Rekha Ranic, A.S. Raob ⁎

a

Department of Physics, Chaudhary Ranbir Singh University, Jind 126 102, Haryana, India Department of Applied Physics, Delhi Technological University, Bawana Road, New Delhi 110 042, India c Department of Physics, Koneru Lakshmaiah Educational Foundation, Vaddeswaram 522 502, Guntur (Dt.), Andhra Pradesh, India b

ARTICLE INFO

ABSTRACT

Keywords: Glasses Samarium Photoluminescence Concentration quenching Dexter theory

Zinc Bismuth Strontium Borosilicate (ZnBiSrBSi) glasses doped with varying concentrations of samarium (Sm3+) ions have been synthesized via melt quenching technique and characterized by using XRD, SEM, optical absorption, excitation, Photoluminescence (PL) and decay spectral measurements. The amorphous nature of the as prepared glass has been confirmed by XRD and SEM measurements. The PL spectra recorded for the as-prepared glasses under 403 nm excitation show four emission bands from 4G5/2 level to 6HJ (J = 5/2, 7/2, 9/2 and 11/2). Among the four emission transitions, 4G5/2 → 6H7/2 transition at 600 nm is having highest intensity. The intensity of PL spectra in the titled glasses increases with increase in Sm3+ ions concentration up to 0.5 mol% and beyond decreases due to concentration quenching. Dexter theory applied to the emission spectral features reveals the energy transfer mechanism between Sm3+- Sm3+ ions as dipole-dipole in nature. The experimental lifetimes measured from the PL decay profiles for the intense 4G5/2 → 6H7/2 transition decreases with increase in Sm3+ ions concentration due to energy transfer between Sm3+-Sm3+ ions. All the aforementioned studies finally reveal that 0.5 mol% of Sm3+ ions in ZnBiSrBSi glasses is optimum in fabricating the reddish orange luminescent devices.

1. Introduction Over the past several years, the characterization of Rare Earth (RE) ion doped glasses and crystals has been emerged as a fascinating field of research that encompasses a wide variety of scientific applications in the design and development of new optical devices such as solid state lasers, white light emitting diodes (w-LEDs), fluorescent display devices, optical detectors, wave guides and fibre amplifiers etc. [1–4]. Further RE ions doped glasses are much superior than single crystals and ceramic materials because of their transparency and high luminescence efficiency [5]. The emissions initiated by 4f-4f and 4f-5d electronic transitions in RE ions doped materials cover wide range of spectral regions starting from ultraviolet (UV) to infrared (IR). Furthermore, it is observed that the phonon energy, ligand field environment and local field symmetry around the RE ions in a host material can influence the 4f electronic transitions. Hence, it is very much essential to assess the spectroscopic properties of the RE ion in a specific host for research and development of optoelectronic devices. The oxide glasses like phosphates, borates, silicates, tellurites and borosilicates are stable hosts for getting enhanced luminescence with

⁎

RE ions and are used in many photonic applications [6–9]. Among the oxide glasses, borosilicate glasses are most promising once because of their high transparency, high doping capacity for RE ions, low melting point and high thermal stability [10–12]. However, borosilicate glasses possess high phonon energies which enhance the non-radiative emissions and thereby reduces radiative emissions [13,14]. Such redundant phonon energies of borosilicate glasses can be drastically reduced by adding heavy metal oxides such as Bi2O3 [15]. A Heavy metal oxides such as Bi2O3, because of its high polarizability and small field strength cannot act as a classical glass former; however, in presence of other oxides such as B2O3 and SiO2, it may build a network network pyramids [16,17]. Glasses based on Bi2O3 have wide range of applications in the field of glass ceramics, layers for optical and electronic devices, mechanical and thermal sensors, transmitting windows in the IR region [18,19]. The alkaline earth oxides such as strontium, magnesium, barium and calcium can change the optical basicity of the host glass and alters the energy states. Strontium oxide added to a borosilicate glass can affect physical properties of the host glass and prevents devitrification by acting as a network modifier [20]. Addition of SrO to a glass can enhance its chemical durability and decreases the molar heat

Corresponding author. E-mail address: [email protected] (N. Deopa).

https://doi.org/10.1016/j.jnoncrysol.2019.03.025 Received 13 January 2019; Received in revised form 15 March 2019; Accepted 16 March 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

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of dissolution [21]. Jagan Mohini et al. studied the influence of strontium on structure, bioactivity and corrosion behavior of B2O3-SiO2Na2O-CaO glasses and found that, addition of SrO optimally improved the bioactivity of glass [22]. Glasses containing SrO are playing a vital role in medical fields especially in bone generation [23]. Annapoorani et al. studied the luminescence behavior and lasing potentiality of Er3+ ions in strontium telluroborate glasses [24]. ZnO with its large excitation binding energy, low toxicity, non-hygroscopic nature and intrinsic emitting property can act as a network former as well as modifier when added to a host glass. This in turn helps the host glass in enhancing its radiative emissive capacity [25,26]. All the aforementioned scientific patronages offered by B2O3, SiO2, SrO, Bi2O3 and ZnO have motivated us to prepare a germane glassy system namely Zinc Bismuth Strontium Borosilicate (ZnBiSrBSi) glass doped with RE ions to understand their potentiality in photonic applications. Among the RE ions, Sm3+ ions are most auspicious once because of its potential applications in high density optical storage, solid state lightening, color displays and undersea communication [27,28]. Sm3+ ions doped glassy matrices possess excellent reddish orange luminescence due to its three prominent transitions (4G5/2 → 6H5/2, 4G5/2 → 6H7/2 and 4G5/2 → 6H9/2). Further, Sm3+ ions doped glasses are having significant importance for the pre-clinical radiation treatment of cancer in Micro beam Radiation Therapy (MRT). These glasses are used as high power reddish orange laser sources in MRT [29]. Furthermore, Sm3+ ions doped glasses are also used as amber LEDs employed in day time running lights (vehicles, traffic signals) [30]. Pawar et al. studied the spectroscopic investigations on Sm3+ ions doped lead alumino borate (LAB) glasses to understand their suitability for orange LED applications. They found that 0.5 mol% of Sm3+ ions in LAB glass is aptly suitable to emit intense orange light at 599 nm and most suitable to fabricate devices for orange LED applications [31]. Yuliantini et al. investigated the Sm3+ ions doped zind alumino barium borate glasses and found that under 403 nm excitation the glasses under investigation are emitting intense orange emission at 598 nm, potential for laser applications in orange region [32]. The effect of glass composition on luminescent properties of Sm3+ ions in alumino silicate glasses were investigated by Herrmann et al. and found that the peralkaline sodium, potassium, strontium and barium alumino silicate glasses show notably higher luminescence lifetimes than the other glass compositions of similar atomic weight [33]. Udaya Kumar et al. studied the effect of Bi2O3 modifier on the spectroscopic properties of Sm3+ ions in binary boro‑bismuth glasses and found that 0.5 mol% of Sm2O3 doped in B30Bi70 glass shows highest emission cross-section and is most suitable in developing visible lasers at around 647 nm [34]. All the aforementioned studies motivated us to prepare a glassy system with relatively less phonon energies and most suitable for reddish orange luminescent device applications. In this paper, the photoluminescence properties of Sm3+ ions doped borosilicate glasses have been studied by using various spectroscopic techniques to understand the usage of borosilicate glasses in reddish-orange luminescent device applications.

Fig. 1. XRD spectrum of an un-doped glass.

another electric furnace at 350 °C. Finally, Sm3+ doped ZnBiSrBSi glass samples were obtained to study their luminescent properties. The XRD pattern for the un-doped glass sample was monitored by using PAN analytical X'pert PRO in the 2θ range 10 to 40°. The SEM image was recorded by Hitachi VP-SEM S-3700 N at an accelerating voltage of 15 KV and an emission current of 116 μA. The optical absorption spectra were recorded by JASCO MODEL V-670 UV–vis-NIR spectrophotometer with a spectral resolution of 0.1 nm. The PL, PL Excitation (PLE) and PL decay spectral measurements were recorded by using Hitachi-F7000 fluorescence spectrophotometer with 150 W Xenon lamp as an excitation source. All the characterizations were performed at room temperature. 3. Results & discussion 3.1. XRD & SEM spectral analysis Fig. 1 shows the XRD spectrum of an un-doped ZnBiSrBSi glass. A broad hump in XRD spectrum clearly confirms the amorphous nature of the as-prepared glass. The surface image of an un-doped sample was recorded using SEM as shown in Fig. 2. The homogenous phase appearing in Fig. 2 confirms the glassy nature of the ZnBiSrBSi host glass. This result is also in consistent with the XRD pattern shown in Fig. 1.

2. Experimental The Sm3+ doped ZnBiSrBSi glasses were synthesized by using the melt quench technique with the chemical composition (30-x) B2O3–45 SiO2-10Bi2O3-5ZnO-10SrCO3–(x) Sm2O3 (where, x = 0.1, 0.5, 1.0, 1.5 and 2.0 mol% abbreviated as glass A, glass B, glass C, glass D and glass E respectively). The RE ions doped glasses were prepared by taking B2O3, SiO2, Bi2O3, ZnO, SrCO3, and Sm2O3 as starting materials. All the chemicals in above proportion were mixed and crushed in an agate mortar to get homogeneous mixture. The smooth mixture was then placed in an alumina crucible and heated at 1100 °C in an electric furnace for about 5 h and the desired melts were obtained. The red hot melt was then quickly quenched between two pre-heated brass plates to form glass samples of uniform thickness. Further, to remove air bubbles and thermal strains, the glass samples were annealed for about 4 h in

Fig. 2. SEM image of an un-doped glass. 153

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ZnBiSrBSi glasses, ϑa is the wavenumber of the same transition of an aqua ion. The values of parameters for glass A to E are 1.026, 1.024, 1.023, 1.021 and 1.020 respectively. The values of bonding parameters (δ) for as-prepared glasses can be evaluated by using the following expression [38,39]

=

1

× 100

(2)

where, is the average value of β. Further, the bonding parameter (δ) highly dependents on the ligand field environment near the RE ions. The δ parameter may be negative for ionic bonding and positive for covalent type bonding. The calculated values of δ from glass A to E are −0.025, −0.023, −0.022, −0.020 and −0.01 respectively. This indicates that the as-prepared glasses are showing ionic bonding between Samarium and Oxygen and this nature gradually decreases with increase in the concentration of Sm3+ ions the titled glasses. 3.2.2. Band gap and Urbach's analysis The optical band gap energy (Eopt) of the as-prepared glasses were evaluated by using Davis and Mott's expression [40] Fig. 3. Absorption spectrum of 0.5 mol% of Sm3+ ions in ZnBiSrBSi glass (glass B).

h = B (h

Eopt )n

(3)

where, α is absorption coefficient, B is bonding trailing parameter, h is Plank's constant, ϑ is frequency and Eopt is optical band gap energy. The parameter “n” can have 1/2, 2, 1/3 and 3 values corresponding to direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions respectively [41]. The Eopt values can be evaluated from Tauc's plot by extrapolating the liner region of the plot drawn between (αhυ)1/2and hυ as shown in Fig. 4. The evaluated optical band gap for glass B is 3.24 eV.

Therefore, the result obtained from Figs. 1 & 2 clearly confirms the amorphous nature in the as-prepared host glass. 3.2. Absorption spectral analysis Fig. 3 depicts the optical absorption spectrum of 0.5 mol% of Sm3+ ions doped ZnBiSrBSi glass (glass B) in 350–1600 nm region. The spectrum consists of 4 peaks [4D3/2 + (4D, 6P)5/2, 6P3/2, (6P, 4P)5/2 and 4 I11/2 at 360, 400, 438 & 475 nm respectively] in ultraviolet (UV) to visible (Vis) region; 5 peaks [6F11/2, 6F9/2, 6F7/2, 6F5/2 and 6F3/2 at 948, 1078, 1225, 1369 & 1470 nm respectively] in near infrared (NIR) region. The absorption spectra of all the as-prepared glasses under investigation are similar to glass B in band position except some variation in their intensities. The band assignments are done as per the report given by Carnal et al. [35]. Due to strong absorption of the host glass in the UV–Vis region, some other absorption bands could not appear in that region. For Sm3+ ions doped ZnBiSrBSi glasses, the transitions that follows the selection rules |∆J| ≤ 6 are electric dipole induced in nature and the transitions that follows the selection rules |∆J| = 0, ± 1 are magnetic dipole in nature [27,28]. It is reported by Jorgensen and Judd [36] that, the intensity of some of the transitions is very much sensitive towards the environment. These electric dipole transition are known as hypersensitive transitions and obeys the selection rules of quadrupole transitions |∆S| = 0, |∆L| ≤ 2, |∆J| ≤ 2 [36,37]. It can be seen from Fig. 3 that the transition corresponding to 6H5/2 → 6P3/2 and 6H5/2 → 6 F7/2 are hypersensitive in nature and have highest intensity among all the bands of the Sm3+ ions doped ZnBiSrBSi glasses.

3.3. Luminescence spectral studies In order to study the luminescent properties of Sm3+ ions doped ZnBiSrBSi glasses, it is essential to identify the excitation wavelength. For this purpose, excitation spectrum was recorded in the range of 300 to 500 nm by monitoring the emission at 600 nm wavelength. The excitation spectrum of glass B is shown in the blue spectra in Fig. 5 (PLE), which confirms the several excitation peaks located in the n-UV as well as in blue region. The recorded excitation peaks were centered at 344, 361, 374, 403, 416, 437 and 476 nm owing to the sharp line transition from 6H5/2 ground state to various excited energy state of Sm3+ ion. The peaks at 344, 361, 374, 403, 416, 437 and 476 nm were assigned6H5/2 →

3.2.1. Nephelauxetic effect and bonding parameters The nephelauxetic effect is used to estimate the covalency between RE ion and oxygen anion bond in the host matrix and this effect arises due to partially filled f-shell. It is studied that due to the impact of nephelauxetic effect, the 4f electronic orbitals of RE ions get distorted. Which may further leads to the contraction of energy level structure of RE ion and in turn responsible for wavelength shift. To know about the nature of bonding between Sm3+ ions and oxygen ligands in ZnBiSrBSi glasses, nephelauxetic ratio (β) was estimated by using the following formula [26,27,37]

=

c

(1)

a

where, ϑc is the wavenumber of a particular transition of Sm

3+

Fig. 4. Tauc plot for 0.5 mol% of Sm3+ ions in ZnBiSrBSi glass (glass B).

ion 154

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orange region. The intensity of forced ED transition is hypersensitive in nature, which can be affected by the crystal field of the ligand atoms while MD transition is insensitive to the crystal field environment [47]. In the current Sm3+ activated glass system, the MD transitions are dominated over the ED transition. 3.3.1. Effect of activator ion (Sm3+) concentration on emission Emission properties of RE ions doped glasses mostly depend on the concentration of RE ions in host glass. In order to optimize the activator concentration, glasses are doped with different amounts of Sm3+ ions (0.1, 0.5, 1.0, 1.5 and 2.0 mol%) concentration. Fig. 6 demonstrated the emission intensity variation as a function of activator ion concentration in ZnBiSrBSi glasses under 403 nm excitation wavelength. The emission intensity changes with varying concentration of Sm3+ ions, while the shape of all emission peak does not change. Emission intensity increases up to 0.5 mol% of Sm3+ ion and beyond that intensity rapidly decreases due to concentration quenching as presented in inset image of Fig. 6 (a). Concentration quenching arises due to the energy transfer among the activator ions which leads the non-radiative energy transfer through the cross-relaxation process. The critical distance between activator ions decrease with increasing activator concentration, which raises the possibility of the non-radiative energy transfer. These non-radiative energy transfer can be dipole-dipole, dipole-quadrupole, and quadrupole-quadrupolein nature, which can be identified by employing Dexter theory. According to Dexter's theory, the emission intensity I and the activator ion concentration x are related as follows [48,49]:

Fig. 5. (PLE) Excitation spectrum of glass B recorded at 600 nm emission and PL emission spectrum recorded under the excitation wavelength at 403 nm. 4

H9/2, 6H5/2→ 4D3/2, 6H5/2 → 6P7/2, 6H5/2 → 4F7/2, 6H5/2 → 6M19/2, H5/2 → 4G9/2 and 6H5/2 → 4I13/2+ 4I11/2 + 4M15/2 due to the forbidden f-f transitions of Sm3+ ion, respectively [42–44]. From the excitation spectra, it can be seen that among all the bands, the transition 6H5/2 → 4 F7/2 (403 nm) is having highest intensity. The excited spectra also indicate that the as prepared glass can be efficiently excited by n-UV bare chip. The Orange spectrum in the range of 500 to 750 nm in the Fig. 5 (PL) illustrates the emission spectrum of the glass B (0.5 mol% of Sm3+ ions doped ZnBiSrBSi glass) under the n-UV excitation (λex = 403 nm). The emission spectrum consists of four peaks centered at 563, 600, 646 and 708 nm owing to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2 and 4 G5/2 → 6H11/2 transitions of Sm3+ ions, respectively. As per the spectroscopic selection rules, the nature of 4G5/2 → 6H5/2 and 4G5/2 → 6H7/2 transitions are magnetic dipole (MD), while 4G5/2 → 6H9/2 and 4G5/2 → 6H11/2 transitions are purely force electric dipole (ED) in nature. The transition at 600 nm assigned to 4G5/2 → 6H7/2 transition is MD in nature but here forced electric dipole transition dominates. Hence the 4G5/2 → 6H7/2 transition is partially MD and partially ED in nature [45,46]. Among these emission peaks, the most intense peak centered at 600 nm is situated in reddish 6

log

I = logf c

slog (c ) d

(4)

where, I represents the emission intensity, c is the activator ion concentration, d (= 3) is the dimension of compound, f is a constant and independent of activator ion concentration. The value of s was estimated by using slope of the linear fitted plot between log (I/c) and log (c) as shown in inset image of Fig. 6 (b). The s parameter can have the values 6, 8, and 10, which indicates dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interaction respectively [50,51]. The value of s is found to be 6 for the present Sm3+ activated ZnBiSrBSi glasses. Hence, it is concluded that dipole-dipole type non-radiative energy transfer among the Sm3+ ions is responsible for the concentration quenching in Sm3+ ions doped ZnBiSrBSi glasses.

Fig. 6. Emission spectra of different glasses with varying Sm3+ ion concentration (x = 0.1, 0.5, 1.0, 1.5, 2.0 and 2.5 mol%) under excitations at λex = 403 nm. The inset image represents (a) the intensity vs Sm3+ ion concentration plot and (b) Relationship of log (I/c) with log (c) plot. 155

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Fig. 8. CIE diagram of glass B under λex = 380 nm. Fig. 7. Partial energy level diagram of Sm3+ activated ZnBiSrBSi glass and possible cross relaxation channels.

chromaticity coordinates have been calculated on the basis of emission spectra under 403 nm excitation, which are for glass A (0.595, 0.401), glass B (0.596, 0.402), glass C (0.594, 0.404), glass D (0.591, 0.406) and for glass E (0.599, 0.398). The calculated values of the CIE chromaticity coordinates are situated in CIE reddish orange region (in Fig. 8) for glass B. Hence, it is expected that the glass B show favourable properties for n-UV excited LED driven reddish orange component for warm white light display devices.

Fig. 7 illustrates the partial energy level diagram of Sm3+ ions activated glass. Solid line represents the radiative transition and dotted line signifying the non-radiative transition. All the excitation levels above 4G5/2 swiftly undertake non-radiative relaxation to this state due to small energy variance. The radiative visible transition takes place from the level 4G5/2 to various lower energy levels 6H5/2, 6H7/2, 6H9/2 and 6H11/2 because of enough energy difference between them [52,53] The non-radiative energy transfer takes place due to the small energy gape between nearest energy levels. As per the energy matching rules there are four possible cross relaxation (CR) channels among Sm3+ions, which are as follows: 10,793 cm−1) ≈ (6H5/2 → 6F11/2; CR1: (4G5/2 → 6F5/2; −1 10,471 cm ) CR2: (4G5/2 → 6F7/2; 9947 cm−1) ≈ (6H5/2 → 6F9/2; 9090 cm−1) CR3: (4G5/2 → 6F9/2; 7784 cm−1) ≈ (6H5/2 → 6F7/2; 7931 cm−1) CR4: (4G5/2 → 6F11/2; 7407 cm−1) ≈ (6H5/2 → 6F5/2; 7085 cm−1)

3.4. Photoluminescence decay spectral analyses Photoluminescence decay lifetime value includes the radiative and non-radiative decay time of the samples. The radiative decay is mainly affected by the crystal symmetry around the RE ions, while non-radiative decay dominant owing to energy transfer and multi photon emission. The decay spectral profiles have been recorded for the asprepared glasses (A, B, C, D & E) under the excitation of 403 nm, while emission is fixed at 600 nm wavelength. These decay profile monitored the characteristics transition from the levels 4G5/2 of Sm3+ ion. Fig. 9 depicts the decay profile of Sm3+ions doped ZnBiSrBSi glasses showing single exponential nature for lower concentration and bi-exponential nature for higher concentration [55,56]. The non-exponential behavior arises due to the efficient energy transfer between two nearest Sm3+ ions. It has been observed that at high concentration of activator ion, separation between active ion centers deceases, which leads the

3.3.2. CIE chromaticity coordinates The color emitted by any RE ions doped material can be better understood from its PL spectrum by using Commission Internationale de L'Eclairage (CIE) chromaticity coordinates system [54]. The following three set of equations having specified power spectral density P (λ) define the degree of stimulation which is needed to match the color of specified P(λ)

X=

x ( )P ( )d

(5)

Y=

y ( )P ( )d

(6)

Z=

z ( )P ( )d

(7)

where, X,Y and Z are the tristimulus values which give stimulation value for RGB (red, green, blue) colors to match the color of P(λ). On the other hand x , y and z are matching functions. The CIE chromaticity coordinates were estimated by using the following formulae

x=

X X+Y+Z

(8)

y=

Y X+Y+Z

(9)

As discussed in earlier section, the emission peaks at 563, 600, 646 and 708 nm are located in yellowish green, reddish orange, red and week red regions respectively. So, the overall color of emission can be predicated with the help of color chromaticity coordinates. The color

Fig. 9. Photoluminescence decay curve of glasses (A, B, C, D & E) recorded under the excitation and emission wavelength at 403 and 600 nm respectively. 156

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strong interaction and in turn excited energy transfer takes place among them [57]. The average decay time of glasses A, B, C, D and E are 2.090, 1.619, 1.447, 1.243 and 1.124 μs respectively. It is observed that the average decay time of the glasses decreases with the increasing concentration of Sm3+ ions which is due to the energy transfer among Sm3+ ions at shorter distances.

[11] [12] [13]

4. Conclusions

[14]

In summary, spectral studies of ZnBiSrBSi glasses doped with Sm3+ ions concentration were performed by various characterization techniques. The non-crystalline nature of as-prepared glasses was confirmed using XRD and SEM analysis. The bonding parameter and optical energy band gap was evaluated by using absorption spectra of Sm3+ ions doped ZnBiSrBSi glasses. The negative values of bonding parameter have been clearly identified ionic nature of bonding between Sm3+ ions and its surrounding ligands which decreases with increase in RE ion concentration. The PL studies under 403 nm excitation exhibit reddish orange emissions with intense peak centred at 600 nm corresponding to the transition 4G5/2 → 6H7/2. The optimized concentration of Sm3+ ions is found to be 0.5 mol%. Further it has been observed that the energy transfer between Sm3+ ions via dipole-dipole interaction is responsible for concentration quenching. The CIE chromaticity coordinates of the as-prepared glasses were evaluated and found in reddish orange region of the visible spectrum. The experimental lifetime recorded for the asprepared glasses show gradual change from single exponential to nonexponential resulting decrease in experimental lifetime due to energy transfer processes. The aforementioned results confirm potentiality of these glasses in photonic device application. Particularly glass B with 0.5 mol% of Sm3+ ions in ZnBiSrBSi glass (optimized glass) is aptly suitable for visible reddish orange luminescent device applications.

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Acknowledgements

[25]

One of the authors, Dr. Nisha Deopa is very much thankful to the Hon'ble Vice-Chancellor, Prof. R.B. Solanki, Chaudhary Ranbir Singh University, Jind, Haryana for his encouragement. The author, Prof. A.S. Rao is thankful to Department of Science and Technology (DST), Govt. of India, New Delhi for the award of a major research project (EMR/ 2016/007766) to him.

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