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
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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