Article in HTML

Author(s): Dipti Sahu, D.P. Bisen, Nameeta Brahme, Kanchan Tiwari, Aastha Sahu

Email(s): dipti8332@gmail.com

Address: School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur (C.G.) 492010, India.
School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur (C.G.) 492010, India.
School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur (C.G.) 492010, India.
Govt. Nagarjuna P.G. College of Science, Raipur (C.G.).
Guru Ghasidas Vishwavidyalaya, Bilaspur (C.G.), India.

Corresponding Author: dipti8332@gmail.com

Published In:   Volume - 36,      Issue - 1,     Year - 2023


Cite this article:
Sahu, Bisen, Brahme, Tiwari and Sahu (2023). Structural, Compositional and Photoluminescence Studies of Li4SrCa(SiO4)2: Eu3+ Red Phosphor Synthesized by Solid State Reaction Method. Journal of Ravishankar University (Part-B: Science), 36(1), pp. 104-112.



Structural, Compositional and Photoluminescence Studies of Li4SrCa(SiO4)2: Eu3+ Red Phosphor Synthesized by Solid State Reaction Method

Dipti Sahua, D.P. Bisena, Nameeta Brahmea, Kanchan Tiwarib, Aastha Sahuc

aSchool of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur (C.G.) 492010, India

bGovt. Nagarjuna P.G. College of Science, Raipur (C.G.)

cGuru Ghasidas Vishwavidyalaya, Bilaspur (C.G.), India

Corresponding Author: dipti8332@gmail.com

Abstract

In this paper, Eu3+ doped phosphor Li4SrCa(SiO4)2 is successfully synthesized via a solid-state reaction method at high temperatures.  Structure characterization is determined using X-ray diffraction (XRD). Surface morphology was analyzed using Field emission scanning electron microscopy(FESEM). The EDX spectra confirm the elements present in Li4SrCa(SiO4)2:Eu3+ phosphor. Photoluminescence(PL) spectra of Li4SrCa(SiO4)2:Eu3+ phosphor was efficiently excited by UV range 220-450nm and under the excitation 395nm phosphors shows good intensity with an orange-red intense peak at 591nm and 619 nm was due to 4D0-7F1 and 4D0-7F2 transition respectively.  Commission International del’ Eclairage (CIE) color coordinate was calculated, which confirms the Eu3+ doped phosphor with a CIE value of (x=0.6259, y=0.3737). This phosphor is considered to be a new promising orange-red emitting phosphor for WLEDs application. This phosphor may be used for solid-state lighting.

Keywords: Solid state reaction, X-ray diffraction, FESEM, EDX, Photoluminescence.

 

Introduction

Luminescence is the most attractive phenomenon of light which is shown by the phosphor when the charge carriers are excited by some higher energy[1]. These phosphors can be named luminescent materials and rare-earth-doped luminescent materials are studied in many fields such as; display devices, LEDs, FEDs and fluorescence labels, solar cells etc[1–3]. In present days, increasing demand for developing white light emitting diodes (WLEDs) due to their long lifetime, high quantum yield, better optical properties, saving energy, reliability, safety and environmental friendly characteristics[3–5].

There are two different approaches that can be used for fabricated phosphor-converted white-light-emitting diodes(pc-WLEDs), which are fabricated by using  blue LED chip and commercial yellow phosphor Y3Al5O12: Ce3+ (YAG: Ce3+) phosphor. However, due to the deficiency of the  red component, the phosphor emits cold white light with high correlated color temperature (CCT >4500 K) and poor color rendering index (CRI) which is generally unappealing for home use[6,7]. In this way, to overcome these problem pc-WLED devices can also be fabricated by another method where combines the tri-color RGB (red, green, and blue) phosphors with near-ultraviolet(350-410nm) LED chips[6–8]. Therefore, the supplement of the red component plays a very important role for generation of warm white light. According to the composition of phosphor and their properties, a suitable host with effective rare earth activators are fundamentally necessary for white LEDs. Rare earth ions Eu3+ and Sm3+ are popular used red emission ions. Eu3+ doped phosphors produced narrow red emission in the visible region because the emission spectra of  Eu3+ ions are due to transition from excited level 5D0-7Fj(j=0,1,2,3,4) levels[9]. We choose the composition Li4SrCa(SiO4)2 as a host because of their high physical and chemical stability also host should provide metal ions that match well with the Rare earth ions as well as europium ion so that they can easily be substituted with the host.

In past decades, many studies have been already done in this field using  Eu3+ doped phosphor. Z. Xu et al. has reported adjustable double center emission of Eu3+ and Eu2+ codoped Ca2Al2SiO7 phosphor[10]. Sadaf Gauhar M. et al. have reported color tunability in Sm3+/Eu3+ activated/ co-activated Sr6Ca4(PO4)6F2 phosphor[11]. The photoluminescence studies of novel red-emitting phosphor Eu3+ activated KMg4(PO4)3 phosphor have been reported by Chaitali M Mehare et al.[12]. Chandrahasya M. Nandanwar et al. have reported BiPO4:Eu3+ phosphors[13]. Yatish R. Parauha et al. have reported Dy3+, Eu3+ co-doped La(PO4) phosphor[14].

In the present paper, we are being used Li4SrCa(SiO4)2   as host materials. Even earlier in this work, some authors have obtained predictable results by substituting rare earth and transition metals in alkaline earth silicates containing phosphor[15,17–23].

 

1.     Material and Methods

1.1.Synthesis of phosphor

Series of Li4SrCa(SiO4)2:xEu3+(x=1-3mol%) phosphor were prepared by solid-state reaction method at high temperature, this is the most commonly employed technique for the synthesis of silicates. High-temperature solid state reaction method produces phosphors with desired phase in their structurally pure form and charming luminescence characteristics[24]. The raw material used were Li2CO3 (A.R.), SrCO3(A.R.), CaCO3(A.R.), SiO2(A.R.), and Eu2O3(A.R.), the stoichiometric amount of the all raw materials were mixed thoroughly by grinding for 2-3h with the help of agate mortar and pastel. The mixer was pre-calcination at 6000C for 5h and fired at 9000C for 12h using an alumina crucible in an air atmosphere, subsequent final product was obtained after cooling in furnace at room temperature and then crushed into fine powder. The process was repeated for all concentrations (1-3 mol%) of dopant Eu3+ separately.

Crystal structure and phase purity of all synthesized powder samples was done by X-ray Diffraction [D2 phase model:08 discover, Bruker, Germany, Cu Kα radiation λ=1.54Å]. Surface morphological studies were done by FESEM(CARL ZEISS UHR FESEM MODEL GEMINI SEM 50 KMAT). Photoluminescence studies of the prepared sample were examined by spectrofluoro-photometer RF-5301PC (Shimadzu) provided with an inbuilt 150W xenon lamp as an excitation source.

 

Result and Discussion

1.2.        XRD Pattern

Figure 1 XRD pattern of pure and Li4SrCa(SiO4)2:Eu3+phosphor


Fig.2(a) represent the XRD pattern of pure Li4SrCa(SiO4)2 and Li4SrCa(SiO4)2:xEu3+(x=1,2.3mol%) phosphor. XRD data were examined by fixed incidence wavelength X-ray (1.54 Å for Cu Kα) and diffraction peaks measured ranging from 200 to 60°. XRD pattern of the prepared sample, it could be seen that all diffraction peaks are well matched with the standard pattern JCPDS 83-0763, suggesting that the Li4SrCa(SiO4)2 single phase is obtained.  Li4SrCa(SiO4)2 crystal structure has orthorhombic space group Pbcm with a = 0.4983(2) nm, b = 0.9930(2) nm, c = 1.4057(2) nm and v = 0.6955 nm3. The Average crystallite size of Li4SrCa(SiO4)2:Eu3+ phosphors was calculated using Debey Scherrer relation D = kλ/βcosθ, where k is the shape factor and that value is 0.9, λ is the wavelength of X-ray (1.54 Å), β is the FWHM and θ is the peak position[3,26]. The calculated average crystallite size of Li4SrCa(SiO4)2:Eu3+ phosphor were 37nm.

2.2  Field Emission Scanning electron microscopy (FESEM)

Figure 2 displayed the FESEM images. Surface morphological properties of synthesized phosphor Li4SrCa(SiO4)2:Eu3+ was examined by FESEM. The FESEM images show that particles are highly agglomerated and particles are non-uniform. The prepared sample had a porous  structure , which might be due to the emanation of gas showing a porous nature and particle size in some micrometers, shown in Fig. 2(A-D).


Fig.2(A–D) FESEM images of the synthesized Li4SrCa(SiO4)2:Eu3+ phosphor

 

2.3 Energy Dispersive X-Ray Spectroscopy (EDX)

 The chemical composition of synthesized phosphor is studied by Energy dispersive X-ray spectroscopy which is shown in Fig. 3 and Table 1 representing the appropriate weight and atomic%  which confirms the elements present in the prepared phosphor. From EDX spectra we can see no other peak identified apart from lithium (Li), strontium(Sr), Calcium(Ca), silicon (Si), oxygen(O), and Europium(Eu)

Table 1 Chemical composition of phosphor

Element

Li K

O K

Si K

Ca K

Sr L

Eu L

Total

Weight %

0.8

57.5

10.2

9.7

19.9

1.9

100

Atomic %

2.6

78.9

7.9

5.3

5.0

0.3

100

 

 

Fig.3  EDX spectrum of Li4SrCa(SiO4)2:Eu3+ phosphor

2.3  Photoluminescence Spectra

In the present paper, we examined the PL characteristic of the prepared powder sample. The most important part is their plotting and understanding of PL spectra.  PL excitation and emission spectra of Li4SrCa(SiO4)2:xEu3+(x= x=1,2.3mol%) phosphor are shown in Fig.4. The PL excitation spectra for all samples are carried out emission wavelength of 591 nm which exhibits a broad absorption from to 450 nm and a representative plot for 3 mol% Eu3+ is shown in Fig. 4(a). Here the peaks are located in this UV range 220-450nm with charge transfer band(CTB) which is due to the transition from 2p orbital of O2- ions to the 4f orbital of Eu3+ ions. In the excitation spectra, the strongest sharp peak is observed at 395 nm corresponding to the 7F0 -5L6 transition of Eu3+. Other peaks are observed  at 320(7F0 -5H6), 363(7F0 -5D4), 385(7F0 - 5L7 ) and 395(7F0 -5L6)[3,27]. The range 220 to 450nm is most significant for white LEDs because when RGB phosphor is excited by UV excitation then it is produced white light, hence this range is very useful and it may be promising excitation for white LEDs.

In fig.4(b) shows the PL emission spectra of Li4SrCa(SiO4)2:Eu3+ ranging at 450-700nm. Under the excitation of 395nm. Two strong emission peaks were observed, the first at 591nm near the orange region due to (4D0-7F1) transition and the second peak at the red region at 619nm due to the (4D0-7F2) transition. Fig.4(c) shows the emission spectra of Li4SrCa(SiO4)2:Eu3+  phosphor with concentration varying from 1 to 3 mol% and seen that same profile for each concentration i.e. the peak position does not change or shifted when increasing the concentration of dopant Eu3+. The only effect of doping concentration observed is that it enhances the emission intensity with increasing concentration of dopant does not affect the crystal structure as all the emission peaks are of the same nature[26,28].

The CIE (Commission International de I'Eclairage) 1931 chromaticity diagram of Li4SrCa(SiO4)2:0.03Eu3+  phosphor is shown in Fig. 4(d). The CIE chromaticity coordinate of prepared samples was investigated from the corresponding PL emission data under the excitation of 395 nm wavelength. The values of CIE coordinates for Li4SrCa(SiO4)2:0.03Eu3+  phosphor were x=0.6283, y=0.3713 placed in the orange-red corner[27,28].


                                  

Fig.A PL Excitation Spectra and Fig.B show Emission spectra of Li4SrCa(SiO4)2:0.03Eu3+  phosphor. Fig.C Emission spectra for different concentrations. Fig.D CIE diagram of Li4SrCa(SiO4)2:0.03Eu3+  phosphor

 

Conclusion

In this paper, Li4SrCa(SiO4)2:Eu3+ phosphor was prepared by the traditional high-temperature solid state reaction method, and for better crystallinity, we have synthesized with 6000C(pre calcination) and 9000C (firing) in air atmosphere. This synthesized phosphor has an orthorhombic crystal structure. XRD patterns confirmed that the prepared undoped phosphor and Eu3+ doped phosphors are single phase and no impurity peaks are identified. FESEM analysis shows that the synthesized phosphor and the particle sizes were in the range of the sub-micrometer. The EDX spectra confirm the present elements in Li4SrCa(SiO4)2:Eu3+ phosphor.  Photoluminescence studies reveal that, Eu3+-activated Li4SrCa(SiO4)2 exhibit bright orange-red region. PL spectra were monitored by excitation at λex. = 395nm and emission spectra have two emission peaks found at 591 and 619. CIE chromaticity diagram indicated that the prepared phosphor has the orange-red color of luminescence and excellent color stability, revealing that the Li4SrCa(SiO4)2:Eu3+ phosphor is suitable for the red component of WLEDs.

 

Acknowledgments

The author is very grateful to the PRSU scholarship, IIT Bhilai for the SEM and EDX studies and Central Instrumentation Facility.

 

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