- Email: [email protected]
Surface and spectral studies of Sm3+ doped Li4Ca(BO3)2 phosphors for white light emitting diodes
Accepted Manuscript Surface and spectral studies of Sm emitting diodes
3+
doped Li4Ca(BO3)2 phosphors for white light
Neharika Wazir, V.K. Singh, J. Sharma, A.K. Bedyal, Vinay Kumar, H.C. Swart PII:
S0925-8388(17)34308-6
DOI:
10.1016/j.jallcom.2017.12.118
Reference:
JALCOM 44204
To appear in:
Journal of Alloys and Compounds
Received Date: 9 October 2017 Revised Date:
2 December 2017
Accepted Date: 12 December 2017
Please cite this article as: N. Wazir, V.K. Singh, J. Sharma, A.K. Bedyal, V. Kumar, H.C. Swart, Surface 3+ and spectral studies of Sm doped Li4Ca(BO3)2 phosphors for white light emitting diodes, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.118. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Surface and spectral studies of Sm3+ doped Li4Ca(BO3)2 phosphors for white light emitting diodes
1
School of Physics, Shri Mata Vaishno Devi University, Katra-182320 (J&K) India.
Department of Physics and Astronomical Sciences, Central University of Jammu, Rahya-Suchani, Samba-181143 J&K, India 3
Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA9300, South Africa.
SC
2
RI PT
Neharika1, V. K. Singh1, J. Sharma1, A. K. Bedyal3 , Vinay Kumar2,3,4 and H.C. Swart3
Abstract
M AN U
Orange-red emitting Sm3+ doped Li4Ca(BO3)2 phosphors synthesized by combustion method were investigated for the applications in white light-emitting diodes. The phase purity and photoluminescence performance of the Li4Ca(BO3)2:Sm3+ phosphor were studied in detail using the powder X-ray diffraction technique, UV-VIS-NIR absorption and Photoluminescence measurements. X-ray photoelectron spectroscopy was used to study the surface chemical composition and the electronic states of the elements composing the phosphor. The synthesized
TE D
phosphor showed good absorption in the near ultraviolet region and four emission peaks in the visible region at 567 nm, 603 nm, 650 nm and 714 nm due to the characteristic transition of the Sm3+ ion, resulting orange-red light as predicted by Commission Internationale de l′Eclairage coordinates. The concentration quenching mechanism, bandgap, phosphorescence lifetime and
EP
quantum efficiency of Li4Ca(BO3)2:Sm3+ phosphors have been investigated. The results indicate that the Li4Ca(BO3)2:Sm3+ is a good candidate as a bright orange-red component for near UV-
AC C
excited white light emitting diodes.
Key words: - Phosphors; X-ray photoelectron spectroscopy (XPS); Photoluminescence (PL); UV-Vis spectroscopy; UV LEDs. 4
Corresponding Author: Tel: +91 1923-249658; email: [email protected] ;
[email protected]
ACCEPTED MANUSCRIPT
1. Introduction Fourth generation illumination technology focuses on the white light emitting diodes
RI PT
(WLED) as they have high illumination efficiency, low voltage consumption, good stability and color tunablity which have replaced conventional lamps [1-3]. White light illumination formed by the combination of a near UV light emitting diode semi-conductor chip with highly efficient and chemically stable tri color blue, green and red phosphors have proved to be very promising
SC
in solid state lightening [4-6]. These UV light emitting chips give an emission maximum around 400 nm. Hence, for improving the light emission in LEDs, those phosphors that are excitable in the specified range are of greater interest [7]. Although, there have been several attempts to make
M AN U
wLED’s and some have even achieved the color coordinates closer to it, but still novel white light is not yet achieved. Nowadays, many of the scientists’ attention have been attracted towards the applications of borates in high technology utilization areas as they show transparency to a wide range of wavelengths extending from ultraviolet to infrared, low absorption, high stability, easy synthesis, high laser-damage tolerance and show a variety of structure types [8-10]. Moreover, Inorganic borate phosphors have shown better absorption in the ultraviolet region
TE D
[11]. Alkaline earth borates like K2Al2B2O7: Dy3+ [12], Sr6YSc(BO3)6: Ce3+,Tb3+ [13], LiSr4(BO3)3:Ce3+, Eu3+ and Tb3+ [14], LiBF4: Ce3+and Eu2+[15] and LiBaBO3:Sm3+ [16] have been reported to have potential application in the field of solid state lighting. In our study, we have chosen borate Li4Ca (BO3) 2 as host material. Li4Ca(BO3)2 crystallizes in the orthorhombic
EP
phase with space group of pnnm having an arrangement three coordinated boron atoms, six-fold coordinated calcium atoms, partially four-fold and partially five-fold coordinated lithium atoms
AC C
[17]. Hong et al. [18] in 2008 reported the structure, electronic properties and chemical bonding of borate Li4Ca(BO3)2 under high pressure and suggested that borate Li4Ca(BO3)2 can be used as the semi-conductor optical material. Recently, in 2015 Pekgozlu et al. have reported the photoluminescence properties of Pb2+ doped Li4Ca(BO3)2 prepared by solid state reaction and found Li4Ca(BO3)2:Pb2+ as a good candidate for the broadband UV application [19]. The optical properties of rare-earth doped borates can be tuned by varying rare earth dopant concentration [20-23]. Although not much work has been done to explore the luminescence properties of lanthanide doped Li4Ca(BO3)2 as host. Also, among lanthanides, trivalent samarium is one of the important luminescent activators as it is excitable in the
ACCEPTED MANUSCRIPT
near UV region and is an efficient red-orange emitter due to 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, 11/2) transitions which makes it useful in a near UV LED lighting system [24, 25].There are no. of synthesis method that had been reported in the literature for the preparation of phosphor powder. In present study, the solution combustion method has been used because
RI PT
of its more advantages over the other methods such as it is simple, low cost, fast process which saves energy and time to produce nanocrystalline phosphors.
In this study, Sm3+ doped Li4Ca(BO3)2 phosphors were synthesized by the combustion method and their optical properties have been studied in details. The effect of Sm3+ ion on the PL
SC
emission of Li4Ca(BO3)2 host, mechanism of concentration quenching, life time, CIE coordinates and band gap studies were carried out.
M AN U
2. Experimental details
Powder samples with the stoichiometry composition of Li4Ca1-x(BO3)2: xSm3+ (x= 0.5, 1, 1.5, 2, 2.5) were synthesized by using the combustion route. Raw materials like calcium nitrate (Ca(NO3)2), lithium nitrate (LiNO3), urea (NH2CONH2), boric acid (H3BO3), samarium nitrate (Sm(NO3)3), polyethylene glycol (PEG) purchased from HiMedia, Analytical Grade were used as starting materials. These raw materials were weighted according to the stoichiometry, amounts
TE D
of urea was added as reducing agent/fuel, then an appropriate amount of PEG (2.5% of the product) was added as dispersant.The balance reaction can be given as follows: Ca(NO3)2 + 4LiNO3+ 5 NH2CONH2 +2H3BO3 + xSm(NO3)3 →
EP
Li4(1-x)Ca(1-x)Smx(BO3)2 + 13H2O + 5CO2 + 8N2 -(1)
These starting materials were then mixed in an agate mortar. The well grounded mixture when kept in a muffle furnace for auto ignited at 560°C which resulted into a white foamy
AC C
product. The product was subsequently annealed at 630°C for 3 hrs and then cooled to room temperature. Finally the samples were ground into a powder for characterization. The assynthesized phosphors was investigated by using the X-ray powder diffraction (XRD) with the X-ray Diffractometer using a Rikagu-Miniflex X-ray diffractometer (30kV, 15mA) with Cu-K radiation with Ni filter and NaI (Tl) scintillation detector. The particle morphology of these phosphors was analyzed by using a JEOL JSM 7100F scanning electron microscopy (SEM). Xray photoelectron spectroscopy (XPS) analysis was carried out using a PHI5000 Versaprobe spectrometer (Analyser Resolution 0.5 eV) using monochromatic Al Kα radiation. The photoluminescence (PL) spectra were recorded by Cary-Eclipse Florescence Spectrophotometer
ACCEPTED MANUSCRIPT
using Xenon lamp as excitation source. Reflectance spectra were recorded from a UV-VIS-2600 double beam spectrophotometer coupled with an ISR (Integrating Sphere Assembly). All the characterization was carried at room temperature.
RI PT
3. Results and discussion 3.1 XRD analysis
The XRD pattern of Li4Ca(BO3)2:Sm3+ doped phosphor is shown in fig. 1. The diffraction pattern of the prepared phosphor well matches with the standard COD Card No: 96-151-0851.
SC
Li4Ca(BO3)2: Sm3+ was having an orthorhombic crystal structure with space group pnnm or pnn2. The lattice parameters of the unit cell were calculated by using UnitCell software and found = 9.23Å, = 9.23Å, = 3.48Å and volume = 259.85 Å . No extra peak was
M AN U
observed due to Sm ion confirmed that the Sm3+ ions successfully replace the Ca2+ ions in the matrix. The exchange of divalent ions by the trivalent ions results in charge imbalance condition in the host matrix. This charge imbalance could prompt point defects/vacancies in the structure of the material. In this study, divalent calcium ions (Ca2+) of Li4Ca(BO3)2 host matrix was replaced by the trivalent samarium ions (Sm3+), which leads to charge
TE D
imbalance condition in the material. As per charge balance rule, on addition of Sm3+ ions into the Li4Ca(BO3)2 host matrix, strontium ion vacancy should formed as given by the equation below:
3Ca×Cr+2Sm×Su→2Sm×Ca+ V″Ca
EP
The average crystallite size of the particles was calculated using Debye-Scherrer’s formula [26].
(2).
AC C
The average crystallite size was found to be 43 nm when calculated by using Scherrer’s equation =
.
-(2)
where, d is the average diameter of the particles, is the wavelength of Cu (1.5406Å) radiation, (in radians) is full width half maxima (FWHM) and
!
is the Bragg angle.
ACCEPTED MANUSCRIPT
RI PT
Intensity (a.u.)
Li4Ca(BO3)2: Sm3+
50
60
332
531 611
SC
40
70
M AN U
30
002 141 112 241 511 600 521 312
230 301 311 031 330 321 231 411
211
130 121 320
310
011
20
220
110
111
COD Card No: 96-151-0851
2-Theta (degrees)
Fig. 1:XRD pattern of (a) Li4Ca(BO3)2: Sm3+ phosphor and (b) the standard COD Card No: 96-151-0851 of Li4Ca(BO3)2. 3.2 SEM Analysis
TE D
The surface morphology of the as-prepared combustion synthesized 1.5 mol% Li4Ca(BO3)2:Sm3+phosphor
was
examined
by
SEM
micrographs
at
various
magnifications as shown in fig. 2. Before SEM examination the sample was prevented from charging by coating it with an ultrathin layer of gold (Au). The SEM image shows
EP
agglomerates rather than single particle. The expected fluffy and porous morphology of the particles normally obtained from the combustion synthesis can be explained by the
AC C
fact that combustion synthesis comprises of simultaneous oxidation and reduction process along with release of large amount of gas due to gaseous decomposition of fuel. So, when the reaction mixture undergoes a violent combustion, a driving force coupled with heat and mass transfer, departs the system from its equilibrium state resulting in modulation of the surface free energy of final crystals. The variations in crystal growth form depend upon the magnitude of driving force as the crystal growth is considered primarily to occur at this phase. Thus, we can observe that the morphology shown in fig. 2(a) is an outcome of escape of large amount of gasses during combustion process. A porous network can be clearly seen in fig. 2(b) due to creation of voids when gas is
ACCEPTED MANUSCRIPT
evolved resulting in production of product of high surface area. Hence, a well expected dense flake like surface morphology overgrown on pore spaces that resembles a corn flake is observed for as-prepared 1.5 mol% Li4Ca(BO3)2:Sm3+phosphor when examined
b)
M AN U
a)
SC
RI PT
by SEM micrographs at various magnifications.
Fig. 2. SEM images of combustion synthesized 1.5 mol% Li4Ca(BO3)2:Sm3+phosphor at a) 100nm and b) 1µm magnifications
TE D
3.3 Surface study
The XPS was used to investigate the chemical elemental composition and oxidation states of the cations in the materials. The XPS analysis of Li4Ca(BO3)2:Sm3+ phosphor was carried out by keeping the C1s peak of carbon arising due to hydrocarbon contamination fixed at the
EP
binding energy of 284.6 eV. Fig. 3 shows the full survey scan spectrum of the Li4Ca(BO3)2: Sm3+ phosphor which confirms the presence of all its constituent elements, i.e. Li, Ca, B, O
AC C
and Sm from their corresponding binding energies.
ACCEPTED MANUSCRIPT
14000 12000
Li4Ca(BO3)2: Sm3+
-O 1s
10000
0 1400 1200 1000
600
400
200
-Li 1s
RI PT
800
SC
2000
-B 1s
-Ca2p -C 1s
4000
-O KLL
6000
-Sm 3d5
C/s
8000
0
M AN U
Binding Energy
Fig. 3: XPS survey scan spectrum of Li4Ca(BO3)2: Sm3+ phosphor. Fig. 4 shows the deconvoluted XPS high resolution scan of the Li 1s. The peaks at binding energy of 57.5 eV and 55.6 eV corresponds to Li2O2 and Li2O type of bonding in the Li4Ca(BO3)2 lattice [27, 28]. The peak at binding energy of 54.9 eV arose due to the presence
TE D
of the LiOH bond and is the result of common atmospheric contamination [28]. The H2O or CO2 in the air can easily react with Li to form LiOH or Li2CO3 on the surface. Fig. 5 shows the deconvoluted XPS high resolution scan of the Ca 2p peak. The calcium
EP
2p spectrum consists of a spin orbital doublet, Ca 2p3/2 at binding energy of 346.9 eV and Ca 2p1/2 at binding energy of 350.4 eV. The three deconvoluted peaks at binding energy of 345.9, 346.8 and 347.7 eV lying in the Ca 2p3/2 spectra corresponds to metallic Ca, Ca-O and
AC C
CaCO3 type of bonding respectively in the Li4Ca(BO3)2 lattice [29,30]. Also the other three deconvoluted peaks at binding energy of 349.8, 350.9 and 351.2 eV lying in the Ca 2p1/2 spectra corresponds to metallic Ca, Ca-O and CaCO3 type of bonding, respectively in the Li4Ca(BO3)2 lattice [31,32]. The Ca-O bond arose due to the ionic characteristic of the bond and the presence of carbon on the surface of the sample gives rise to CaCO3 type of bonding in the Li4Ca(BO3)2 lattice. Fig. 6 shows the deconvoluted XPS high resolution scan of the B 1s peak. The spectrum consists of two deconvoluted peaks at binding energy of 191.2 and 192.4 eV which can be
ACCEPTED MANUSCRIPT
assigned to B-C-O and B-O type of bonding respectively in Li4Ca(BO3)2 lattice [33,34]. The presence of C atoms can be due to the expected adsorption of adventitious C, primary hydrocarbons, C–O–C, C–OH species. Fig. 7 shows the XPS high resolution scan spectra of O1s. The spectrum consists of four deconvoluted peaks which represents different lattice sites of
RI PT
oxygen in Li4Ca(BO3)2 lattice. The components with the binding energy of 531.5 eV correspond to the LiOH bond located at 54.9 eV in the Li1s core level [28]. The deconvoluted peak at the binding energy of 531.84 eV and 532.4 eV of the O1s core level confirmed the presence of the Ca-O and B-O bond in the Li4Ca(BO3)2 lattice, respectively [32,34]. Also, the peak at binding
SC
energy of 530.3 eV arosed due to the Sm2O3 bond in the Li4Ca(BO3)2 lattice [35].
35
M AN U
Li 1s
30
Li-O in Li4Ca(BO3)6
Li2O2
20 15
58
EP
10 60
LiOH/Li2CO3
TE D
C/s
25
56
54
52
50
Binding Energy (eV)
AC C
Fig. 4: The deconvoluted XPS high resolution scan of the Li 1s.
ACCEPTED MANUSCRIPT
120
2
P3/2
Ca 2p
CaO
100
CaCO3
2
P1/2
Ca in Li4Ca(BO3)2
RI PT
C/s
80 60
SC
40
354
352
M AN U
20 350
348
346
344
342
Binding Energy (eV)
Fig. 5: The deconvoluted XPS high resolution scan of the Ca 2p. 50
30
B-O in Li4Ca(BO3)2
B-C-O
EP
C/s
40
TE D
B 1s
AC C
20
195
194
193
192
191
190
189
Binding Energy (eV)
Fig. 6: The deconvoluted XPS high resolution scan of the B 1s.
ACCEPTED MANUSCRIPT
700
O1s
600
Li-OH
500 Ca-O
Sm2O3
RI PT
C/s
400 B-O
300 200
SC
100
M AN U
0 536 535 534 533 532 531 530 529 528
Binding Energy (eV) Fig. 7: The deconvoluted XPS high resolution scan of the O1s. 3.4 Spectroscopic properties
TE D
The photoluminescence excitation and emission spectra of the Li4Ca(BO3)2:Sm3+ phosphor measured at room temperature is shown in the fig. 8. The excitation spectra is measured in the range of 190 nm to 500 nm by monitoring the 4G5/2→6H7/2 transition at 603 nm. It can be seen that the excitation spectrum is composed of a charge transfer band centered at 205
EP
nm which arises due the O2-→Sm3+ charge transfer along with a series of sharp peaks from 300 nm to 500 nm, which was attributed to the 4f-4f transition of the Sm3+ at 6H5/2→3H9/2 (345nm), 4
D5/2,6P5/2 (361 nm), 4D1/2 (375 nm), 4F7/2 (403 nm), 4M19/2 (419 nm), 4G9/2 (439 nm), 4I13/2 (470
AC C
nm) [36]. The transition 6H5/2→4F7/2 centered at 403 nm has been chosen for recording the emission spectra of the Li4Ca(BO3)2:Sm3+ phosphor as it is the most prominent among all other 4f-4f transition peaks which falls in the range of the near UV-region (385-420 nm). The emission spectrum under 403 nm excitation is composed of four emission peaks which were ascribed to 4
G5/2→6H5/2, 4G5/2→6H7/2, 4G5/2→6H9/2 and 4G5/2→6H11/2 transitions of the Sm3+ and are located at
567 nm (yellow), 603 nm (orange), 650 nm (orange reddish) and 714 nm (red), respectively [37]. Among all the transitions the 4G5/2→6H7/2 located at 603 nm with an orange-red emission was dominant. This whole process is well explained by using the energy level diagram as shown in
ACCEPTED MANUSCRIPT
fig. 9. Due to the interaction of Li4Ca(BO3)2:Sm3+ phosphor with different excited wavelengths 345 nm, 361 nm, 375 nm, 403 nm, 419 nm, 439 nm and 470 nm, the Sm3+ ions excites from 6H5/2 ground level to the upper energy levels 3H9/2 (345 nm), 4D5/2,6P5/2 (361 nm), 4D1/2 (375 nm), 4F7/2 (403 nm), 4M19/2 (419 nm), 4G9/2 (439 nm), 4I13/2 (470 nm) [38]. After which these excited Sm3+
RI PT
ions relax non-radiatively to the 4G5/2 metastable state as shown in fig. 9. The non radiative transition is then followed by radiative transition from 4G5/2 energy level to 6Hj (where j= 5/2,7/2,9/2,11/2) energy levels and the energy gap is sufficient enough to emit the orange-red color. The above discussion clearly indicates that the Li4Ca(BO3)2: Sm3+ phosphor is suitable for
M AN U
a) Excitation at 603nm b) Emission at 403nm
300 CTB
7/2
Li4Ca(BO3)2 : Sm3+
200
300
EP
200
500
600
6H 11/2
4G 5/2
4G 5/2
I13/2
4
4
4
M G9/2 19/2
400
4G 5/2
6H 9/2
4 G 5/2
6H 5/2
D1/2 F7/2 4
4
P5/2
6
H9/2
3
0
b)
4f-4f
6 H5/2
100
6 H
a)
TE D
Intensity (a.u.)
400
SC
near-ultraviolet white light emitting diodes.
700
Wavelength (nm)
AC C
Fig. 8: PL excitation and emission spectra of Li4Ca(BO3)2: Sm3+ phosphor.
Moreover, the dominant transition 4G5/2→6H7/2 located at 603 nm with an orange emission is partially magnetic dipole and partially electric dipole allowed with ∆J = ± 1 as the selection rule. 4
G5/2→6H5/2 transition located at 567 nm is a magnetic dipole transition (MD) with ∆J = 0 i.e., J
≠ 0 ↔ 0 values, 4G5/2→6H9/2 located at 650 nm is an electric dipole transition (ED) with ∆J = ±2
and 4G5/2→6H11/2 located at 709 nm is a forbidden transition (∆J = ±3) [39]. The intensity ratio between ED and MD transitions elucidates the symmetrical nature of the Sm3+ in Li4Ca(BO3)2:Sm3+ phosphor. It can be noted from the fig. 8 (CQ) that the intensity of the
ACCEPTED MANUSCRIPT
4
G5/2→6H5/2(MD) transition is slightly higher than the intensity of the 4G5/2→6H9/2 (ED)
transition. A higher intensity of the MD transition with respect to the ED transition shows a symmetric nature of Sm3+ ions in the Li4Ca(BO3)2 lattice. Non radiative transitions
RI PT
3
30
H9/2 P5/2
6
4
D1/2 F7/2 4 G9/2
4
4
M19/2
4
I13/2
20
4
6
H11/2 6
H7/2
0
G5/2
714 nm
650 nm
567 nm
SC
Excitation
603 nm
470 nm
419 nm
439 nm
375 nm
M AN U
5
403 nm
10
361 nm
15 345 nm
3
-1
Energy × 10 (cm )
25
6
H9/2
6
Emission
H5/2
Sm3+ radiative transitions
TE D
Fig. 9: Energy level diagram of Li4Ca(BO3)2: Sm3+ phosphor. 3.5 Concentration quenching curve
In order to obtain the optimum doping concentration of Sm3+ion in Sm3+ doped in
EP
Li4Ca(BO3)2 phosphor, a series of Li4Ca1-x(BO3)2:xSm3+ where 0.5 ≤ x ≤ 2.5 phosphors were prepared. Fig. 10 shows the emission spectra of the samples. Under 403 nm excitation, the
AC C
emission intensity firstly increases with the increasing Sm3+ concentration up to 1.5 mol% thereafter it starts decreasing. So, the maximum emission intensity was observed at 1.5 mol% of Sm3+ doped in Li4Ca(BO3)2 phosphor and is taken as the critical concentration. This behavior might be attributed to the results of the nonradioactive relaxation present in the host matrix or Sm3+→ Sm3+internal concentration quenching [40]. The internal concentration quenching among the Sm3+ions is mainly caused by a cross relaxation process which may occur due to exchange interaction, radiation re-absorption, or a multipole-multipole interaction [41].
ACCEPTED MANUSCRIPT
116
112 110 108 106 104
100 90 80 70 60
102
60
601.5 602.0 602.5 603.0 603.5 604.0 604.5 605.0
c)
110
0.5
Wavelength (nm)
1.0 1.5 2.0 2.5 Concentration mol (%)
0.5 mol% 1.0 mol% 1.5 mol% 2.0 mol% 2.5 mol%
40
SC
a)
20 0 500
550
RI PT
80
P L In ten s ity (a .u .)
100
120
b)
114
In te n sity (a .u .)
PL Intensity (a.u.)
120
600
650
700
750
M AN U
Wavelength (nm)
Fig. 10: (a) PL spectra of Li4Ca(BO3)2: Sm3+ phosphor for different Sm3+ concentrations. Inset shows (b) a small portion of the highest peak, magnified and (c) the concentration quenching curve.
By calculating the critical distance (Rc) between neighboring Sm3+ions we can know
TE D
more about the type of interaction mechanism in the Sm3+ doped Li4Ca(BO3)2 phosphor. According to the report of Blasse [42] the critical energy transfer distance (Rc) can be obtained by using the calculated critical concentration of the activator ion. Using the relation proposed by
EP
Blasse, the critical distance between the Sm3+ ions for energy transfer can be obtained: $ ≈ 2 &
'
()*+ ,
. /
-
-(3)
AC C
Where V is the volume of the unit cell, 0 is the critical concentration of the activator ion and Z is the number of formula per unit cell. The structure of the Li4Ca(BO3)2 compound has an orthorhombic phase, with cell volume (V) of 259.85 Å . The value of Z is 2, and the obtained value of $ is 25.48 Å. Since there is no overlap between the observed excitation and emission spectra of sensitizer and activator, so radiation re-absorption is not applicable. Further, since the distance between Sm3+ ions is larger than 5Å, the exchange interaction becomes ineffective and multipole interaction is effective [43,44].
ACCEPTED MANUSCRIPT
9.0
Experimental Fitted line
RI PT
8.9
8.7
8.5 8.4
SC
8.6 Li4Ca(BO3)2 : Sm3+ λ = 403nm
8.3 -4.2
-4.1
M AN U
log (I/x)
8.8
-4.0
-3.9
-3.8
-3.7
-3.6
log(x)
Fig. 11: Plot of log(I/x) as a function of log(x) in Li4Ca(BO3)2: Sm3+.
TE D
When the multipole interaction is involved in the energy transfer mechanism, there are several types of interactions, such as dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupolequadrupole (q-q) which are responsible for interaction between the same activators. According to
EP
the theory of Dexter [45], the multipole interaction can be calculated from the equation: 1
2
;
= 3[1 + 7(9) / ]=1
-(4)
AC C
where C,D are constants for the same excitation condition for the given host, x is the activator concentration which is greater than the critical concentration, Q is the constant of multipole interaction and is equal to 6, 8,10 for dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions, respectively.
To obtain the correct Q value, the
dependence of log (I/x) on log(x) was plotted which yielded a straight line with a slope of the fitted line which was approximated to be -1.25 as shown in fig. 11. Therefore, the value of Q is 3.75 which is closer to 6, this shows that dipole-dipole interaction plays an important role in concentration quenching for the Sm3+ doped Li4Ca(BO3)2 phosphor.
ACCEPTED MANUSCRIPT
Li4Ca(BO3)2 : Sm3+ 6 H5/2
4f-4f 80
23+ O - Sm
94 92 90 88 86 84 250
300
350
SC
70
400
450
500
550
Wavelength (nm)
50 200
400
M AN U
60
6 F7/2 6 6 H5/2 F5/2
96
Reflectance (%)
Reflectance (%)
90
6 F9/2 6 H5/2
RI PT
100
600
800
1000
1200
1400
Wavelength (nm)
Fig. 12: Diffuse reflectance spectra of Li4Ca(BO3)2:Sm3+ phosphor.
TE D
3.6 Diffuse reflectance study
Fig. 12 presents the diffuse reflectance spectrum of the Li4Ca(BO3)2:Sm3+ (1.5 mol%) phosphor in the wavelength range from 190 to 1400 nm. The spectrum has an intense broad absorption band centered at 205 nm which has been assigned to the O2-→Sm3+ charge transfer
EP
band (CTB). The peaks at higher wavelengths of 1073, 1228 and 1368 nm are ascribed to the transitions from the ground state of the 6H5/2 to 6F9/2, 6F7/2 and 6F5/2 for the Sm3+ ions [46]. The weak absorption peaks as shown in the inset of the fig. 12 are assigned to the 4f-4f transitions of
AC C
the Sm3+ ions.
3.6.1 Band gap
The band gap of the 1.5 mol% of Li4Ca(BO3)2:Sm3+ phosphor was determined from the
diffuse reflectance by using the Kubelka-Munk theory [47]. The diffuse reflectance spectra was transformed to an absorption spectrum of the phosphor as shown in fig. 13 by using the KubelkaMunk theory [48] which is given by equation (5)
ACCEPTED MANUSCRIPT
>($) =
(1=?)@ A?
=
B
-(5)
C
Where, $ = the diffuse reflectance, = absorption coefficient and D= scattering coefficient.
absorption coefficient G for a direct allowed transition as below: (GℎI)A = 3(ℎI − EF )
RI PT
The Tauc equation (6) gives us the relation between energy band gap EF = of material and
-(6)
Where ℎI = energy of photon=1239.7/ eV and 3 = constant of proportionality. The F(R) is
SC
directly proportional to G thus the above relation can be written as below:
-(7)
M AN U
[>($)ℎI]A = 3(ℎI − EF )
Equation (7) represents a straight line equation M = N9 + , where N is the slope. Thus, it is clear that on plotting a graph between [>($)ℎI]A and ℎI the intercept of the slope will represent the value of the band gap (EF ) of the phosphor shown in fig. 13. In the present case, the average band gap was estimated to be 5.13 O .
TE D
0.5
0.3
Li4Ca(BO3)2 : Sm3+
EP
[F(R)hν]2
0.4
0.2
AC C
0.1
0.0 1
Eg = 5.13 eV
2
3
4 5 Energy(eV)
6
7
Fig. 13: Energy band gap in the Li4Ca(BO3)2: Sm3+ phosphor.
3.7 Decay Curve The recorded decay curve for the 1.5 mol% Sm3+ doped Li4Ca(BO3)2phosphor is shown in fig. 14. The Sm3+ was directly excited by a 403 nm pulse light and then the emission signal
ACCEPTED MANUSCRIPT
was collected at 603 nm (emission of Sm3+) and the corresponding life time of the phosphor was determined. The curve was well fitted by a single exponential equation: T
P = P exp (− U)
- (8)
RI PT
WhereP is the initial intensity at V = 0, P is the intensity at V = V andτ is the decay lifetime. On the basis of equation (8) and decay curve in fig. 14, the effective life time for the 1.5 mol% Li4Ca(BO3)2:Sm3+
phosphor
was
determined
to
be
2.72
ms.
The
1.5
mol%
Li4Ca(BO3)2:Sm3+ phosphor showed a quantum efficiency of 56.31% as calculated by using
SC
the LUMPAC software [49]. The quantum efficiency of 1.5 mol% of the Sm3+ doped Li4Ca(BO3)2 phosphor was found to be greater than Sm3+ doped Ba3Sc0.95(PO4)3, SrBi2B2O7 and Ba4.93(BO3)2(B2O5) phosphors which was reported as 53.5 [50], 33.2 [51] and 16.0%
M AN U
[52], respectively. These results indicate that Li4Ca(BO3)2:Sm3+ (1.5 mol%) phosphor can act as a suitable candidate for n-UV w-LEDs.
6
Li4Ca(BO3)2 : Sm3+
3
λex = 403 nm λem = 603 nm
EP
2
Experimental Fitted curve
TE D
4
1
AC C
Intensity (a.u.)
5
τ
= 2.72 ms
0
0
10
20 30 40 Time (ms)
50
Fig. 14: Decay curve of the Li4Ca(BO3)2:Sm3+ phosphor.
3.8 CIE coordinate diagram
ACCEPTED MANUSCRIPT
Fig. 15 shows a digital photo of the chromaticity diagram for 1.5 mol% of Sm3+ doped Li4Ca(BO3)2phosphors excited at 403 nm. The Commission International de l’Eclairage (CIE) color coordinates calculated by using the CIE 1931 calculator program [53] were determined as (0.60, 0.40) for emission of the Li4Ca(BO3)2:Sm3+ phosphor which are
RI PT
located in the orange-red region. This confirms that the emission color of the Sm3+ ion is reddish-orange. Determination of color purity is essential for narrow banded emission. In the present case we get a narrow band emission in the red region. So we calculated the color purity of the phosphor by using the equation given below [54]: a(bc − bd )e + (fc − fd )e
× ijj%
SC
XYZY[\ ][\^_` =
a(bg − bd )e + (fg − fd )e
M AN U
where, (xs, ys) are the coordinates of the sample point, (xd, yd) are the CIE coordinates of the corresponding dominant wavelength at 600 nm, and (xi, yi) are the coordinates of the illuminant point. By putting the values for (xs, ys) = (0.600, 0.400), (xd, yd) = (0.634, 0.368) and (xi, yi) = (0.3101, 0.3162) (Illuminants C)), the color purity was found to be approximately 92 %. Hence the photometric results implies that the present can be act as
AC C
EP
with InGaN chips.
TE D
an acceptable candidate as the red component in WLED or as a red emitter in combination
Fig. 15: The CIE 1931 chromaticity diagram for Li4Ca(BO3)2:Sm3+ phosphor with the coordinates (x, y) as (0.60, 0.40) under 403 nm excitation.
ACCEPTED MANUSCRIPT
4. Conclusions A series of Sm3+ doped Li4Ca(BO3)2 phosphors were successfully synthesized by the combustion method.The XPS study confirmed the presence of Li, Ca, B, O and Sm on the
RI PT
surface of the phosphor. The Li4Ca(BO3)2:Sm3+ phosphors showed broad excitation bands ranging from 300 nm to 500 nm which match well with the wavelength of available UV chips. The emission peak consisted of four emission peaks resulting into an orange-red emission having (0.60,0.40) as CIE coordinates. The concentration-dependent emission spectra indicated the presence of concentration quenching at 1.5 mol% for the Sm3+ site in the Li4Ca(BO3)2 phosphor
SC
which was caused due to dipole-dipole interactions. Also, the Sm3+ ions showed the symmetric nature in the Li4Ca(BO3)2 phosphor. The average crystallite size of the 1.5 mol% of Sm3+ doped
M AN U
Li4Ca(BO3)2 phosphor was found to be 43 nm with an estimated band gap, effective lifetime and quantum efficiency of 5.13 eV, 2.72 ms and 56.31%, respectively. These results indicated that 1.5 mol% Li4Ca(BO3)2:Sm3+ phosphor is suitable candidate of producing warm white light for a light-conversion phosphor in the near-ultraviolet based w-LEDs. Acknowledgements
TE D
V K gratefully acknowledges the financial support provided by Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India under Extra Mural Research (EMR) funding scheme vide reference no. EMR/16/001718.
[1]
EP
References
Y. Shi, G. Zhu, M. Mikami, Y. Shimomura, Y. Wang, “Photoluminescence of green-
AC C
emitting Ca7(PO4)2(SiO4)2:Eu2+ phosphor for white light emitting diodes” Opt. Mater
Express 4 (2014) 280-287.
[2]
T. Jia, Z. Ci, Q. Wu, G. Zhu, C. Wang, Y. Wang, “Tunable luminescence and energy transfer investigation in Sr8La2[(PO4)4.5(SiO4)2(BO4)0.5](BO2):Ce3+/Mn2+ for White-lightemitting diodes” ECS J. Solid State Sci. Technol. 4 (2015) R78-R82.
[3]
L.P. Lai, Y.Z. Ping, W.Z. Jun, G.Q. Lin, “Luminescence characteristics of Eu3+ activated borate phosphor for white light emitting diode” Chin. Phys. Soc. 17 (2008) 1907-1910.
ACCEPTED MANUSCRIPT
[4]
X. Chen, Z. Xia, “Synthesis and Color-tunable luminescence of Ce3+, Tb3+codoped Sr6YSc(BO3)6 Phosphor” J. Solid State light.1(2014) 1-10.
[5]
X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen, Q. Su, “Highly thermally stable
diodes” ACS Appl. Mater. Interfaces 6 (2014) 2709-2717. [6]
RI PT
single-component white-emitting silicate glass for organic-resin-free white-light-emitting C.K. Chang, T.M. Chen, “Sr3B2O6:Ce3+, Eu2+:A potential single-phased white-emitting borate phosphor for ultraviolet light-emitting diodes” Appl. Phys. Lett. 91 (2007) 081902. [7]
J. McKittrick, M.E. Hannah, A. Piquette, J.K. Han, J.I. Choi, M. Anc, M. Galvez, H.
SC
Lugauer, J.B. Talbot, K.C. Mishra, “Phosphor selection considerations for Near-UV LED solid state lighting” ECS J. Solid State Sc. 2 (2013) R3119-R3131.
V.R. Panse, N.S. Kokode, S.J. Dhoble, “Tb3+ doped Sr2(BO3)Cl green emitting phosphor
M AN U
[8]
for solid state lighting” Adv. Mat. Lett. 5 (2014) 604-610. [9]
Z.C. Wu, P.W.J. Liu, C. Li, W.H. Zhou, S.P. Kuang, “Improved photoluminescence properties of a new green SrB2O4:Tb3+ phosphor by charge compensation” Mater. Res. Bull. 47 (2012) 3414-3416.
[10]
J. Liu, Z.C. Wu, P. Wang, Y.M. Mei, M. Jiang, S.P. Kuang, “The effects of charge
TE D
compensation on photoluminescence properties of a new green-emitting ZnB2O4:Tb3+” Luminescence 29 (2014) 868-871.
[11]
R.S. Kumar, V. Ponnusamy, M.T. Jose, “Synthesis and photoluminescence properties of Sm3+ -doped YAl3(BO3)4 phosphor”Luminescence 29 (2013) 649-656. R.S. Palaspagar, R.P. Sonekar, S.K. Omanwar, “Novel inorganic borate host phosphor
EP
[12]
K2Al2B2O7:Dy3+ for LED based solid state lighting” AIP Conf. Proc.1536 (2013) 807[13]
AC C
808.
L. Chen, C.C. Lin, C.W. Yeh, R. S. Liu, “Light converting inorganic phosphors for white light-emitting diodes” Materials 3 (2010) 2172-2195.
[14]
X. Zhang, H. Lang , H.J. Seo, “On the Luminescence of Ce3+, Eu3+, and Tb3+ in Novel
Borate LiSr4(BO3)3” J Fluoresc. 21 (2010) 1111-1115.
[15]
S. P. Puppalwar, S. J. Dhoble, G. Singh, A. Kumar, “Blue emission in Ce3+ and Eu2+ activated lithium fluoroborate phosphors” J. Mod. Phys. 2 (2011) 1560-1566.
[16]
X. Li, C. Liu, L. Guan, W. Wei, Z. Yang, Q. Guo, G. Fu, “An ideal blue Sr3B2O6: Ce3+ phosphor prepared by sol-combustion method.” Mater. Lett. 87 (2012) 121-123.
ACCEPTED MANUSCRIPT
[17]
L. Wu, C. Wang, X.L. Chen, X.Z. Li, Y.P. Xu, Y.G. Cao , “Ab initio structure determination of new compound Li4CaB2O6” J. Sold state Chem. 177 (2004) 1847-1851.
[18]
Z. Hong, T. Jin, C. Xiu-Lu, “Structural, Electronic Properties and Chemical Bonding of
(2008) 552-555. [19]
I. Pekgozlu, E. Erdogmus, A.S. Basak, “Synthesis and photoluminescence properties of Pb2+ doped Li4CaB2O6”Optik 126 (2015) 1221-1223.
[20]
RI PT
Borate Li4CaB2O6 under High Pressure: an Ab Initio Investigation”Chin. Phys. Lett. 25
Z. Tao, Y. Huang, P. Cai, S.I. Kim, H.J. Seo, “Synthesis and efficient blue-emitting of
[21]
SC
Eu2+-activated borate fluoride BaAlBO3F2” Opt. Mater. 37 (2014) 287-292.
K. Kim, Y.M. Moon, S. Choi, H.K. Jung, S. Nahm, “Luminescent properties of a novel
62 (2008) 3925-3927. [22]
M AN U
green-emitting gallium borate phosphor under vacuum ultraviolet excitation” Mater. lett.
L. Wu, M. Ji, H. Wang, Y. Kong, Y. Zhang, “Site occupancy and photoluminescence of Sm3+ in KSr4(BO3)3:Sm3+ phosphors” Opt. Mater. Express 4 (2014) 1535-1544.
[23]
Neharika, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Synthesis and photoluminescence study of Dy3+ doped Sr3B2O6: Dy3+ nanophosphors” AIP Conf. Proc. 1591 (2014) 558-
[24]
TE D
560.
D. Rajesh, Y.C. Ratnakaram, M. Seshadri, A. Balakrishna, “Luminescence properties of Sm3+ impurities in strontium lithium bismuth borate glasses” AIP Conf. Proc. 1447 (2012) 581-582.
L.F. Shen, B.J. Chen, E.Y.B. Pun, H. Lin, “Sm3+ doped alkaline earth borate glasses as
EP
[25]
UV-visible photon conversion layer for solar cells” J. Lumin.160 (2015) 138-144. L. Alexander, H.P. Klug,” Determination of crystallite size with the X-Ray spectrometer”
AC C
[26]
J. Appl. Phys. 21 (1950) 137-142.
[27]
S. Kato, M. Bielmann, A. Borgschulte, V.Z. Herzog, A. Remhof, S. Orimob, A. Zuttel, “Effect of the surface oxidation of LiBH4 on the hydrogen desorption mechanism” Phys. Chem. Chem. Phys. 12 (2010) 10950–10955.
[28]
S.S. Pitale, I.M. Nagpure, V. Kumar, O.M. Ntwaeaborwa, J.J. Terblans, H.C. Swart, “Investigations on the low voltage cathodoluminescence stability and surface chemical behaviour using Auger and X-ray photoelectron spectroscopy on LiSrBO3:Sm3+ phosphor” Mater. Res. Bull. 46 (2011) 967-994.
ACCEPTED MANUSCRIPT
[29]
H.F Franzen, J. Merrick, M. Umana, A.S. Khan, D.T. Peterson, “XPS spectra and crystalline potentials in alkaline-earth chalcogenides and hydrides” J. Electron Spectrosc. RelatPhenom. 11 (1977) 439-443.
[30]
M. Nuño, G.L. Pesce, C.R. Bowen, P. Xenophontos, R.J. Ball, “Environmental
Build. Environ. 92 (2015) 734-742.
[31]
RI PT
performance of nano-structured Ca(OH)2/TiO2photocatalytic coatings for buildings” C.S. Gopinath, S.G. Hegde, A.V. Ramaswamy, S. Mahapatra, “Photoemission studies of polymorphic CaCO3 materials” Mater. Res. Bull. 37 (2002) 1323-1332.
M. Manhas, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Structrual, surface and
SC
[32]
luminescence properties of Ca3B2O6: Dy3+ phosphors”. Ceram. Int. 42 (2016) 5743[33]
M AN U
5753.
Y. Liu, L. Zhang, L. Cheng, W. Yang, Y. Xu, “Effect of deposition temperature on boron-doped carbon coatings deposited from a BCl3–C3H6–H2 mixture using low pressure chemical vapor deposition” Appl. Surf. Sci. 255 (2009) 8761-8768.
[34]
Neharika, V. Kumar, J. Sharma, O.M. Ntwaeaborwa and H.C. Swart, “Surface and spectral studies of green emitting Sr3B2O6:Tb3+ phosphor” J. Electron. Spectrosc. Relat.
[35]
TE D
Phenom. 206 (2016) 52-57.
Y. Uwamino, T. Ishizuka, “X-ray Photoelectron Spectroscopy Of Rare-Earth Compounds” J. Electron Spectrosc. RelatPhenom. 34 (1984) 67-78.
[36]
C.P. Reddy, V. Naresh, B.C. Babu, S. Buddhudu, “Photoluminescence and energy
EP
transfer process in Bi3+/Sm3+ Co-doped phosphate zinc lithium glasses” Adv. Mater. Phys. Chem. 4 (2014) 165-171.
A.A. Ali, “Optical properties of Sm3+ doped CaF2 bismuth borate glasses” J. Lumin. 129
AC C
[37]
(2009) 1314-1319.
[38]
S. Sailaja, C. A. Raju, C. A. Reddy, B.D.P. Raju, Y.D. Jho, B. S. Reddy, “Optical properties of Sm3+-doped cadmium bismuth borate glasses” J. Mol. Struct. 1038 (2013)
29-34.
[39]
P. Biswas, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Enhanced orange-red emission from KSrVO4:Sm3+nanophosphor for possible application in blue lightemitting diode based white LED” Mater. Res. Express 2 (2015) 025010.
ACCEPTED MANUSCRIPT
[40]
W.Z. Jun, L. P. Lai, Y.Z. Ping, G.Q. Lin, F.G. Sheng, “Energy transfer from Ce3+ to Eu2+ in LiSrBO3 and its potential application in UV-LED-Based white LEDs” Chin. Phys. Lett. 26 (2009) 117802.
[41]
M. Manhas, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Photoluminescence and
RI PT
thermoluminescence investigations of Ca3B2O6:Sm3+ phosphor” Mater. Res. Express 2 (2015) 075008. [42]
G. Blasse, B.C. Grabmaier, “Luminescent Materials” Edn 1st Berlin, New York: Springer-Verlag (1994).
Z. Yang, P. Liu, J. Li, Q. Yang, L. Lv, Y. Zhao, “A novel yellow luminescent material
SC
[43]
Ba3Y(PO4)3:Eu2+ ”J. Alloys Compd. 578 (2013) 118-120. [44]
J. Zhou, Z. Xia, “Synthesis, luminescence properties and energy transfer behavior of
[45]
M AN U
Na2CaMg(PO4)2:Eu2+,Mn2+ phosphors” J. Lumin., 146 (2014) 22-26.
D.L. Dexter, “A Theory of Sensitized Luminescence in Solids” J. Chem. Phys. 21 (1953) 836-850.
[46]
A. E. Morales, E.S. Mora, U. Pal, “Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures” Rev. Mex. Fis. 53 (2007) 18-22. A.K. Bedyal, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “A promising orange-red
TE D
[47]
emitting nanocrystalline NaCaBO3: Sm3+ phosphor for solid state lightining” Mater. Res. Express 1 (2014) 015006.
[48]
L.L. Noto, S.S. Pitale, M.A. Gusowki, J.J. Terblans, O.M. Ntwaeaborwa, H.C. Swart,
EP
“Afterglow enhancement with In3+codoping in CaTiO3:Pr3+ red phosphor” Powder Technol. 237 (2013) 141-146.
J.D.L. Dutra, T.D. Bispo, R.O. Freire, “LUMPAC Lanthanide Luminescence Software:
AC C
[49]
Efficient and User Friendly” J. Comput. Chem. 35 (2014) 772-775.
[50]
X. Ma, L. Mei, H. Liu, L. Liao, Y. Liu, K. Nie, Z. Li, “Structure and fluorescent properties of Ba3Sc(PO4)3: Sm3+ red-orange phosphor for n-UV w-LEDs” Chem. Phys.
Lett. 653 (2016) 212-215.
[51]
L. Wu, Y. Bai, L. Wu, H. Yi, Y. Kong, Y. Zhang, J. Xu, “Sm3+ and Eu3+ codoped SrBi2B2O7: a red-emitting phosphor with improved thermal stability” RSC Adv., 7 (2017) 1146-1153.
ACCEPTED MANUSCRIPT
[52]
J. Zheng, Q. Cheng, W. Chen, Z. Guo, Z. Guo, C. Chen, “Luminescence Properties of an Orange-Red Ba5(BO3)2(B2O5):Sm3+ Phosphor with High Color Purity” ECS J. Solid State Sci. Technol. 4 (2015) R72-R77.
[53]
http://in.mathworks.com/matlabcentral/fileexchange/29620-cie-coordinate-calculator.
S. Som, A.K. Kunti, V. Kumar, V. Kumar, S. Dutta, M. Chowdhury, S.K. Sharma, J.J. Terblans, H.C. Swart, Defect correlated fluorescent quenching and electron phonon coupling in the spectral transition of Eu3+ in CaTiO3 for red emission in display
EP
TE D
M AN U
SC
application, J. Appl. Phys. 115 (2014). doi:10.1063/1.4876316.
AC C
[54]
RI PT
(last assessed date: 1 May 2017)
ACCEPTED MANUSCRIPT
Highlights 1. First time, the orange-red emitting Sm3+ doped Li4Ca(BO3)2 phosphor was synthesized by using the combustion method.
RI PT
2. XPS studies confirm the presence of all the possible elements in Li4Ca(BO3)2:Sm3+ phosphor.
AC C
EP
TE D
M AN U
SC
3. A new amber emitting phosphor for near UV-excited light emitting diodes is projected.
3+
doped Li4Ca(BO3)2 phosphors for white light
Neharika Wazir, V.K. Singh, J. Sharma, A.K. Bedyal, Vinay Kumar, H.C. Swart PII:
S0925-8388(17)34308-6
DOI:
10.1016/j.jallcom.2017.12.118
Reference:
JALCOM 44204
To appear in:
Journal of Alloys and Compounds
Received Date: 9 October 2017 Revised Date:
2 December 2017
Accepted Date: 12 December 2017
Please cite this article as: N. Wazir, V.K. Singh, J. Sharma, A.K. Bedyal, V. Kumar, H.C. Swart, Surface 3+ and spectral studies of Sm doped Li4Ca(BO3)2 phosphors for white light emitting diodes, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.12.118. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
Surface and spectral studies of Sm3+ doped Li4Ca(BO3)2 phosphors for white light emitting diodes
1
School of Physics, Shri Mata Vaishno Devi University, Katra-182320 (J&K) India.
Department of Physics and Astronomical Sciences, Central University of Jammu, Rahya-Suchani, Samba-181143 J&K, India 3
Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA9300, South Africa.
SC
2
RI PT
Neharika1, V. K. Singh1, J. Sharma1, A. K. Bedyal3 , Vinay Kumar2,3,4 and H.C. Swart3
Abstract
M AN U
Orange-red emitting Sm3+ doped Li4Ca(BO3)2 phosphors synthesized by combustion method were investigated for the applications in white light-emitting diodes. The phase purity and photoluminescence performance of the Li4Ca(BO3)2:Sm3+ phosphor were studied in detail using the powder X-ray diffraction technique, UV-VIS-NIR absorption and Photoluminescence measurements. X-ray photoelectron spectroscopy was used to study the surface chemical composition and the electronic states of the elements composing the phosphor. The synthesized
TE D
phosphor showed good absorption in the near ultraviolet region and four emission peaks in the visible region at 567 nm, 603 nm, 650 nm and 714 nm due to the characteristic transition of the Sm3+ ion, resulting orange-red light as predicted by Commission Internationale de l′Eclairage coordinates. The concentration quenching mechanism, bandgap, phosphorescence lifetime and
EP
quantum efficiency of Li4Ca(BO3)2:Sm3+ phosphors have been investigated. The results indicate that the Li4Ca(BO3)2:Sm3+ is a good candidate as a bright orange-red component for near UV-
AC C
excited white light emitting diodes.
Key words: - Phosphors; X-ray photoelectron spectroscopy (XPS); Photoluminescence (PL); UV-Vis spectroscopy; UV LEDs. 4
Corresponding Author: Tel: +91 1923-249658; email: [email protected] ;
[email protected]
ACCEPTED MANUSCRIPT
1. Introduction Fourth generation illumination technology focuses on the white light emitting diodes
RI PT
(WLED) as they have high illumination efficiency, low voltage consumption, good stability and color tunablity which have replaced conventional lamps [1-3]. White light illumination formed by the combination of a near UV light emitting diode semi-conductor chip with highly efficient and chemically stable tri color blue, green and red phosphors have proved to be very promising
SC
in solid state lightening [4-6]. These UV light emitting chips give an emission maximum around 400 nm. Hence, for improving the light emission in LEDs, those phosphors that are excitable in the specified range are of greater interest [7]. Although, there have been several attempts to make
M AN U
wLED’s and some have even achieved the color coordinates closer to it, but still novel white light is not yet achieved. Nowadays, many of the scientists’ attention have been attracted towards the applications of borates in high technology utilization areas as they show transparency to a wide range of wavelengths extending from ultraviolet to infrared, low absorption, high stability, easy synthesis, high laser-damage tolerance and show a variety of structure types [8-10]. Moreover, Inorganic borate phosphors have shown better absorption in the ultraviolet region
TE D
[11]. Alkaline earth borates like K2Al2B2O7: Dy3+ [12], Sr6YSc(BO3)6: Ce3+,Tb3+ [13], LiSr4(BO3)3:Ce3+, Eu3+ and Tb3+ [14], LiBF4: Ce3+and Eu2+[15] and LiBaBO3:Sm3+ [16] have been reported to have potential application in the field of solid state lighting. In our study, we have chosen borate Li4Ca (BO3) 2 as host material. Li4Ca(BO3)2 crystallizes in the orthorhombic
EP
phase with space group of pnnm having an arrangement three coordinated boron atoms, six-fold coordinated calcium atoms, partially four-fold and partially five-fold coordinated lithium atoms
AC C
[17]. Hong et al. [18] in 2008 reported the structure, electronic properties and chemical bonding of borate Li4Ca(BO3)2 under high pressure and suggested that borate Li4Ca(BO3)2 can be used as the semi-conductor optical material. Recently, in 2015 Pekgozlu et al. have reported the photoluminescence properties of Pb2+ doped Li4Ca(BO3)2 prepared by solid state reaction and found Li4Ca(BO3)2:Pb2+ as a good candidate for the broadband UV application [19]. The optical properties of rare-earth doped borates can be tuned by varying rare earth dopant concentration [20-23]. Although not much work has been done to explore the luminescence properties of lanthanide doped Li4Ca(BO3)2 as host. Also, among lanthanides, trivalent samarium is one of the important luminescent activators as it is excitable in the
ACCEPTED MANUSCRIPT
near UV region and is an efficient red-orange emitter due to 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, 11/2) transitions which makes it useful in a near UV LED lighting system [24, 25].There are no. of synthesis method that had been reported in the literature for the preparation of phosphor powder. In present study, the solution combustion method has been used because
RI PT
of its more advantages over the other methods such as it is simple, low cost, fast process which saves energy and time to produce nanocrystalline phosphors.
In this study, Sm3+ doped Li4Ca(BO3)2 phosphors were synthesized by the combustion method and their optical properties have been studied in details. The effect of Sm3+ ion on the PL
SC
emission of Li4Ca(BO3)2 host, mechanism of concentration quenching, life time, CIE coordinates and band gap studies were carried out.
M AN U
2. Experimental details
Powder samples with the stoichiometry composition of Li4Ca1-x(BO3)2: xSm3+ (x= 0.5, 1, 1.5, 2, 2.5) were synthesized by using the combustion route. Raw materials like calcium nitrate (Ca(NO3)2), lithium nitrate (LiNO3), urea (NH2CONH2), boric acid (H3BO3), samarium nitrate (Sm(NO3)3), polyethylene glycol (PEG) purchased from HiMedia, Analytical Grade were used as starting materials. These raw materials were weighted according to the stoichiometry, amounts
TE D
of urea was added as reducing agent/fuel, then an appropriate amount of PEG (2.5% of the product) was added as dispersant.The balance reaction can be given as follows: Ca(NO3)2 + 4LiNO3+ 5 NH2CONH2 +2H3BO3 + xSm(NO3)3 →
EP
Li4(1-x)Ca(1-x)Smx(BO3)2 + 13H2O + 5CO2 + 8N2 -(1)
These starting materials were then mixed in an agate mortar. The well grounded mixture when kept in a muffle furnace for auto ignited at 560°C which resulted into a white foamy
AC C
product. The product was subsequently annealed at 630°C for 3 hrs and then cooled to room temperature. Finally the samples were ground into a powder for characterization. The assynthesized phosphors was investigated by using the X-ray powder diffraction (XRD) with the X-ray Diffractometer using a Rikagu-Miniflex X-ray diffractometer (30kV, 15mA) with Cu-K radiation with Ni filter and NaI (Tl) scintillation detector. The particle morphology of these phosphors was analyzed by using a JEOL JSM 7100F scanning electron microscopy (SEM). Xray photoelectron spectroscopy (XPS) analysis was carried out using a PHI5000 Versaprobe spectrometer (Analyser Resolution 0.5 eV) using monochromatic Al Kα radiation. The photoluminescence (PL) spectra were recorded by Cary-Eclipse Florescence Spectrophotometer
ACCEPTED MANUSCRIPT
using Xenon lamp as excitation source. Reflectance spectra were recorded from a UV-VIS-2600 double beam spectrophotometer coupled with an ISR (Integrating Sphere Assembly). All the characterization was carried at room temperature.
RI PT
3. Results and discussion 3.1 XRD analysis
The XRD pattern of Li4Ca(BO3)2:Sm3+ doped phosphor is shown in fig. 1. The diffraction pattern of the prepared phosphor well matches with the standard COD Card No: 96-151-0851.
SC
Li4Ca(BO3)2: Sm3+ was having an orthorhombic crystal structure with space group pnnm or pnn2. The lattice parameters of the unit cell were calculated by using UnitCell software and found = 9.23Å, = 9.23Å, = 3.48Å and volume = 259.85 Å . No extra peak was
M AN U
observed due to Sm ion confirmed that the Sm3+ ions successfully replace the Ca2+ ions in the matrix. The exchange of divalent ions by the trivalent ions results in charge imbalance condition in the host matrix. This charge imbalance could prompt point defects/vacancies in the structure of the material. In this study, divalent calcium ions (Ca2+) of Li4Ca(BO3)2 host matrix was replaced by the trivalent samarium ions (Sm3+), which leads to charge
TE D
imbalance condition in the material. As per charge balance rule, on addition of Sm3+ ions into the Li4Ca(BO3)2 host matrix, strontium ion vacancy should formed as given by the equation below:
3Ca×Cr+2Sm×Su→2Sm×Ca+ V″Ca
EP
The average crystallite size of the particles was calculated using Debye-Scherrer’s formula [26].
(2).
AC C
The average crystallite size was found to be 43 nm when calculated by using Scherrer’s equation =
.
-(2)
where, d is the average diameter of the particles, is the wavelength of Cu (1.5406Å) radiation, (in radians) is full width half maxima (FWHM) and
!
is the Bragg angle.
ACCEPTED MANUSCRIPT
RI PT
Intensity (a.u.)
Li4Ca(BO3)2: Sm3+
50
60
332
531 611
SC
40
70
M AN U
30
002 141 112 241 511 600 521 312
230 301 311 031 330 321 231 411
211
130 121 320
310
011
20
220
110
111
COD Card No: 96-151-0851
2-Theta (degrees)
Fig. 1:XRD pattern of (a) Li4Ca(BO3)2: Sm3+ phosphor and (b) the standard COD Card No: 96-151-0851 of Li4Ca(BO3)2. 3.2 SEM Analysis
TE D
The surface morphology of the as-prepared combustion synthesized 1.5 mol% Li4Ca(BO3)2:Sm3+phosphor
was
examined
by
SEM
micrographs
at
various
magnifications as shown in fig. 2. Before SEM examination the sample was prevented from charging by coating it with an ultrathin layer of gold (Au). The SEM image shows
EP
agglomerates rather than single particle. The expected fluffy and porous morphology of the particles normally obtained from the combustion synthesis can be explained by the
AC C
fact that combustion synthesis comprises of simultaneous oxidation and reduction process along with release of large amount of gas due to gaseous decomposition of fuel. So, when the reaction mixture undergoes a violent combustion, a driving force coupled with heat and mass transfer, departs the system from its equilibrium state resulting in modulation of the surface free energy of final crystals. The variations in crystal growth form depend upon the magnitude of driving force as the crystal growth is considered primarily to occur at this phase. Thus, we can observe that the morphology shown in fig. 2(a) is an outcome of escape of large amount of gasses during combustion process. A porous network can be clearly seen in fig. 2(b) due to creation of voids when gas is
ACCEPTED MANUSCRIPT
evolved resulting in production of product of high surface area. Hence, a well expected dense flake like surface morphology overgrown on pore spaces that resembles a corn flake is observed for as-prepared 1.5 mol% Li4Ca(BO3)2:Sm3+phosphor when examined
b)
M AN U
a)
SC
RI PT
by SEM micrographs at various magnifications.
Fig. 2. SEM images of combustion synthesized 1.5 mol% Li4Ca(BO3)2:Sm3+phosphor at a) 100nm and b) 1µm magnifications
TE D
3.3 Surface study
The XPS was used to investigate the chemical elemental composition and oxidation states of the cations in the materials. The XPS analysis of Li4Ca(BO3)2:Sm3+ phosphor was carried out by keeping the C1s peak of carbon arising due to hydrocarbon contamination fixed at the
EP
binding energy of 284.6 eV. Fig. 3 shows the full survey scan spectrum of the Li4Ca(BO3)2: Sm3+ phosphor which confirms the presence of all its constituent elements, i.e. Li, Ca, B, O
AC C
and Sm from their corresponding binding energies.
ACCEPTED MANUSCRIPT
14000 12000
Li4Ca(BO3)2: Sm3+
-O 1s
10000
0 1400 1200 1000
600
400
200
-Li 1s
RI PT
800
SC
2000
-B 1s
-Ca2p -C 1s
4000
-O KLL
6000
-Sm 3d5
C/s
8000
0
M AN U
Binding Energy
Fig. 3: XPS survey scan spectrum of Li4Ca(BO3)2: Sm3+ phosphor. Fig. 4 shows the deconvoluted XPS high resolution scan of the Li 1s. The peaks at binding energy of 57.5 eV and 55.6 eV corresponds to Li2O2 and Li2O type of bonding in the Li4Ca(BO3)2 lattice [27, 28]. The peak at binding energy of 54.9 eV arose due to the presence
TE D
of the LiOH bond and is the result of common atmospheric contamination [28]. The H2O or CO2 in the air can easily react with Li to form LiOH or Li2CO3 on the surface. Fig. 5 shows the deconvoluted XPS high resolution scan of the Ca 2p peak. The calcium
EP
2p spectrum consists of a spin orbital doublet, Ca 2p3/2 at binding energy of 346.9 eV and Ca 2p1/2 at binding energy of 350.4 eV. The three deconvoluted peaks at binding energy of 345.9, 346.8 and 347.7 eV lying in the Ca 2p3/2 spectra corresponds to metallic Ca, Ca-O and
AC C
CaCO3 type of bonding respectively in the Li4Ca(BO3)2 lattice [29,30]. Also the other three deconvoluted peaks at binding energy of 349.8, 350.9 and 351.2 eV lying in the Ca 2p1/2 spectra corresponds to metallic Ca, Ca-O and CaCO3 type of bonding, respectively in the Li4Ca(BO3)2 lattice [31,32]. The Ca-O bond arose due to the ionic characteristic of the bond and the presence of carbon on the surface of the sample gives rise to CaCO3 type of bonding in the Li4Ca(BO3)2 lattice. Fig. 6 shows the deconvoluted XPS high resolution scan of the B 1s peak. The spectrum consists of two deconvoluted peaks at binding energy of 191.2 and 192.4 eV which can be
ACCEPTED MANUSCRIPT
assigned to B-C-O and B-O type of bonding respectively in Li4Ca(BO3)2 lattice [33,34]. The presence of C atoms can be due to the expected adsorption of adventitious C, primary hydrocarbons, C–O–C, C–OH species. Fig. 7 shows the XPS high resolution scan spectra of O1s. The spectrum consists of four deconvoluted peaks which represents different lattice sites of
RI PT
oxygen in Li4Ca(BO3)2 lattice. The components with the binding energy of 531.5 eV correspond to the LiOH bond located at 54.9 eV in the Li1s core level [28]. The deconvoluted peak at the binding energy of 531.84 eV and 532.4 eV of the O1s core level confirmed the presence of the Ca-O and B-O bond in the Li4Ca(BO3)2 lattice, respectively [32,34]. Also, the peak at binding
SC
energy of 530.3 eV arosed due to the Sm2O3 bond in the Li4Ca(BO3)2 lattice [35].
35
M AN U
Li 1s
30
Li-O in Li4Ca(BO3)6
Li2O2
20 15
58
EP
10 60
LiOH/Li2CO3
TE D
C/s
25
56
54
52
50
Binding Energy (eV)
AC C
Fig. 4: The deconvoluted XPS high resolution scan of the Li 1s.
ACCEPTED MANUSCRIPT
120
2
P3/2
Ca 2p
CaO
100
CaCO3
2
P1/2
Ca in Li4Ca(BO3)2
RI PT
C/s
80 60
SC
40
354
352
M AN U
20 350
348
346
344
342
Binding Energy (eV)
Fig. 5: The deconvoluted XPS high resolution scan of the Ca 2p. 50
30
B-O in Li4Ca(BO3)2
B-C-O
EP
C/s
40
TE D
B 1s
AC C
20
195
194
193
192
191
190
189
Binding Energy (eV)
Fig. 6: The deconvoluted XPS high resolution scan of the B 1s.
ACCEPTED MANUSCRIPT
700
O1s
600
Li-OH
500 Ca-O
Sm2O3
RI PT
C/s
400 B-O
300 200
SC
100
M AN U
0 536 535 534 533 532 531 530 529 528
Binding Energy (eV) Fig. 7: The deconvoluted XPS high resolution scan of the O1s. 3.4 Spectroscopic properties
TE D
The photoluminescence excitation and emission spectra of the Li4Ca(BO3)2:Sm3+ phosphor measured at room temperature is shown in the fig. 8. The excitation spectra is measured in the range of 190 nm to 500 nm by monitoring the 4G5/2→6H7/2 transition at 603 nm. It can be seen that the excitation spectrum is composed of a charge transfer band centered at 205
EP
nm which arises due the O2-→Sm3+ charge transfer along with a series of sharp peaks from 300 nm to 500 nm, which was attributed to the 4f-4f transition of the Sm3+ at 6H5/2→3H9/2 (345nm), 4
D5/2,6P5/2 (361 nm), 4D1/2 (375 nm), 4F7/2 (403 nm), 4M19/2 (419 nm), 4G9/2 (439 nm), 4I13/2 (470
AC C
nm) [36]. The transition 6H5/2→4F7/2 centered at 403 nm has been chosen for recording the emission spectra of the Li4Ca(BO3)2:Sm3+ phosphor as it is the most prominent among all other 4f-4f transition peaks which falls in the range of the near UV-region (385-420 nm). The emission spectrum under 403 nm excitation is composed of four emission peaks which were ascribed to 4
G5/2→6H5/2, 4G5/2→6H7/2, 4G5/2→6H9/2 and 4G5/2→6H11/2 transitions of the Sm3+ and are located at
567 nm (yellow), 603 nm (orange), 650 nm (orange reddish) and 714 nm (red), respectively [37]. Among all the transitions the 4G5/2→6H7/2 located at 603 nm with an orange-red emission was dominant. This whole process is well explained by using the energy level diagram as shown in
ACCEPTED MANUSCRIPT
fig. 9. Due to the interaction of Li4Ca(BO3)2:Sm3+ phosphor with different excited wavelengths 345 nm, 361 nm, 375 nm, 403 nm, 419 nm, 439 nm and 470 nm, the Sm3+ ions excites from 6H5/2 ground level to the upper energy levels 3H9/2 (345 nm), 4D5/2,6P5/2 (361 nm), 4D1/2 (375 nm), 4F7/2 (403 nm), 4M19/2 (419 nm), 4G9/2 (439 nm), 4I13/2 (470 nm) [38]. After which these excited Sm3+
RI PT
ions relax non-radiatively to the 4G5/2 metastable state as shown in fig. 9. The non radiative transition is then followed by radiative transition from 4G5/2 energy level to 6Hj (where j= 5/2,7/2,9/2,11/2) energy levels and the energy gap is sufficient enough to emit the orange-red color. The above discussion clearly indicates that the Li4Ca(BO3)2: Sm3+ phosphor is suitable for
M AN U
a) Excitation at 603nm b) Emission at 403nm
300 CTB
7/2
Li4Ca(BO3)2 : Sm3+
200
300
EP
200
500
600
6H 11/2
4G 5/2
4G 5/2
I13/2
4
4
4
M G9/2 19/2
400
4G 5/2
6H 9/2
4 G 5/2
6H 5/2
D1/2 F7/2 4
4
P5/2
6
H9/2
3
0
b)
4f-4f
6 H5/2
100
6 H
a)
TE D
Intensity (a.u.)
400
SC
near-ultraviolet white light emitting diodes.
700
Wavelength (nm)
AC C
Fig. 8: PL excitation and emission spectra of Li4Ca(BO3)2: Sm3+ phosphor.
Moreover, the dominant transition 4G5/2→6H7/2 located at 603 nm with an orange emission is partially magnetic dipole and partially electric dipole allowed with ∆J = ± 1 as the selection rule. 4
G5/2→6H5/2 transition located at 567 nm is a magnetic dipole transition (MD) with ∆J = 0 i.e., J
≠ 0 ↔ 0 values, 4G5/2→6H9/2 located at 650 nm is an electric dipole transition (ED) with ∆J = ±2
and 4G5/2→6H11/2 located at 709 nm is a forbidden transition (∆J = ±3) [39]. The intensity ratio between ED and MD transitions elucidates the symmetrical nature of the Sm3+ in Li4Ca(BO3)2:Sm3+ phosphor. It can be noted from the fig. 8 (CQ) that the intensity of the
ACCEPTED MANUSCRIPT
4
G5/2→6H5/2(MD) transition is slightly higher than the intensity of the 4G5/2→6H9/2 (ED)
transition. A higher intensity of the MD transition with respect to the ED transition shows a symmetric nature of Sm3+ ions in the Li4Ca(BO3)2 lattice. Non radiative transitions
RI PT
3
30
H9/2 P5/2
6
4
D1/2 F7/2 4 G9/2
4
4
M19/2
4
I13/2
20
4
6
H11/2 6
H7/2
0
G5/2
714 nm
650 nm
567 nm
SC
Excitation
603 nm
470 nm
419 nm
439 nm
375 nm
M AN U
5
403 nm
10
361 nm
15 345 nm
3
-1
Energy × 10 (cm )
25
6
H9/2
6
Emission
H5/2
Sm3+ radiative transitions
TE D
Fig. 9: Energy level diagram of Li4Ca(BO3)2: Sm3+ phosphor. 3.5 Concentration quenching curve
In order to obtain the optimum doping concentration of Sm3+ion in Sm3+ doped in
EP
Li4Ca(BO3)2 phosphor, a series of Li4Ca1-x(BO3)2:xSm3+ where 0.5 ≤ x ≤ 2.5 phosphors were prepared. Fig. 10 shows the emission spectra of the samples. Under 403 nm excitation, the
AC C
emission intensity firstly increases with the increasing Sm3+ concentration up to 1.5 mol% thereafter it starts decreasing. So, the maximum emission intensity was observed at 1.5 mol% of Sm3+ doped in Li4Ca(BO3)2 phosphor and is taken as the critical concentration. This behavior might be attributed to the results of the nonradioactive relaxation present in the host matrix or Sm3+→ Sm3+internal concentration quenching [40]. The internal concentration quenching among the Sm3+ions is mainly caused by a cross relaxation process which may occur due to exchange interaction, radiation re-absorption, or a multipole-multipole interaction [41].
ACCEPTED MANUSCRIPT
116
112 110 108 106 104
100 90 80 70 60
102
60
601.5 602.0 602.5 603.0 603.5 604.0 604.5 605.0
c)
110
0.5
Wavelength (nm)
1.0 1.5 2.0 2.5 Concentration mol (%)
0.5 mol% 1.0 mol% 1.5 mol% 2.0 mol% 2.5 mol%
40
SC
a)
20 0 500
550
RI PT
80
P L In ten s ity (a .u .)
100
120
b)
114
In te n sity (a .u .)
PL Intensity (a.u.)
120
600
650
700
750
M AN U
Wavelength (nm)
Fig. 10: (a) PL spectra of Li4Ca(BO3)2: Sm3+ phosphor for different Sm3+ concentrations. Inset shows (b) a small portion of the highest peak, magnified and (c) the concentration quenching curve.
By calculating the critical distance (Rc) between neighboring Sm3+ions we can know
TE D
more about the type of interaction mechanism in the Sm3+ doped Li4Ca(BO3)2 phosphor. According to the report of Blasse [42] the critical energy transfer distance (Rc) can be obtained by using the calculated critical concentration of the activator ion. Using the relation proposed by
EP
Blasse, the critical distance between the Sm3+ ions for energy transfer can be obtained: $ ≈ 2 &
'
()*+ ,
. /
-
-(3)
AC C
Where V is the volume of the unit cell, 0 is the critical concentration of the activator ion and Z is the number of formula per unit cell. The structure of the Li4Ca(BO3)2 compound has an orthorhombic phase, with cell volume (V) of 259.85 Å . The value of Z is 2, and the obtained value of $ is 25.48 Å. Since there is no overlap between the observed excitation and emission spectra of sensitizer and activator, so radiation re-absorption is not applicable. Further, since the distance between Sm3+ ions is larger than 5Å, the exchange interaction becomes ineffective and multipole interaction is effective [43,44].
ACCEPTED MANUSCRIPT
9.0
Experimental Fitted line
RI PT
8.9
8.7
8.5 8.4
SC
8.6 Li4Ca(BO3)2 : Sm3+ λ = 403nm
8.3 -4.2
-4.1
M AN U
log (I/x)
8.8
-4.0
-3.9
-3.8
-3.7
-3.6
log(x)
Fig. 11: Plot of log(I/x) as a function of log(x) in Li4Ca(BO3)2: Sm3+.
TE D
When the multipole interaction is involved in the energy transfer mechanism, there are several types of interactions, such as dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupolequadrupole (q-q) which are responsible for interaction between the same activators. According to
EP
the theory of Dexter [45], the multipole interaction can be calculated from the equation: 1
2
;
= 3[1 + 7(9) / ]=1
-(4)
AC C
where C,D are constants for the same excitation condition for the given host, x is the activator concentration which is greater than the critical concentration, Q is the constant of multipole interaction and is equal to 6, 8,10 for dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions, respectively.
To obtain the correct Q value, the
dependence of log (I/x) on log(x) was plotted which yielded a straight line with a slope of the fitted line which was approximated to be -1.25 as shown in fig. 11. Therefore, the value of Q is 3.75 which is closer to 6, this shows that dipole-dipole interaction plays an important role in concentration quenching for the Sm3+ doped Li4Ca(BO3)2 phosphor.
ACCEPTED MANUSCRIPT
Li4Ca(BO3)2 : Sm3+ 6 H5/2
4f-4f 80
23+ O - Sm
94 92 90 88 86 84 250
300
350
SC
70
400
450
500
550
Wavelength (nm)
50 200
400
M AN U
60
6 F7/2 6 6 H5/2 F5/2
96
Reflectance (%)
Reflectance (%)
90
6 F9/2 6 H5/2
RI PT
100
600
800
1000
1200
1400
Wavelength (nm)
Fig. 12: Diffuse reflectance spectra of Li4Ca(BO3)2:Sm3+ phosphor.
TE D
3.6 Diffuse reflectance study
Fig. 12 presents the diffuse reflectance spectrum of the Li4Ca(BO3)2:Sm3+ (1.5 mol%) phosphor in the wavelength range from 190 to 1400 nm. The spectrum has an intense broad absorption band centered at 205 nm which has been assigned to the O2-→Sm3+ charge transfer
EP
band (CTB). The peaks at higher wavelengths of 1073, 1228 and 1368 nm are ascribed to the transitions from the ground state of the 6H5/2 to 6F9/2, 6F7/2 and 6F5/2 for the Sm3+ ions [46]. The weak absorption peaks as shown in the inset of the fig. 12 are assigned to the 4f-4f transitions of
AC C
the Sm3+ ions.
3.6.1 Band gap
The band gap of the 1.5 mol% of Li4Ca(BO3)2:Sm3+ phosphor was determined from the
diffuse reflectance by using the Kubelka-Munk theory [47]. The diffuse reflectance spectra was transformed to an absorption spectrum of the phosphor as shown in fig. 13 by using the KubelkaMunk theory [48] which is given by equation (5)
ACCEPTED MANUSCRIPT
>($) =
(1=?)@ A?
=
B
-(5)
C
Where, $ = the diffuse reflectance, = absorption coefficient and D= scattering coefficient.
absorption coefficient G for a direct allowed transition as below: (GℎI)A = 3(ℎI − EF )
RI PT
The Tauc equation (6) gives us the relation between energy band gap EF = of material and
-(6)
Where ℎI = energy of photon=1239.7/ eV and 3 = constant of proportionality. The F(R) is
SC
directly proportional to G thus the above relation can be written as below:
-(7)
M AN U
[>($)ℎI]A = 3(ℎI − EF )
Equation (7) represents a straight line equation M = N9 + , where N is the slope. Thus, it is clear that on plotting a graph between [>($)ℎI]A and ℎI the intercept of the slope will represent the value of the band gap (EF ) of the phosphor shown in fig. 13. In the present case, the average band gap was estimated to be 5.13 O .
TE D
0.5
0.3
Li4Ca(BO3)2 : Sm3+
EP
[F(R)hν]2
0.4
0.2
AC C
0.1
0.0 1
Eg = 5.13 eV
2
3
4 5 Energy(eV)
6
7
Fig. 13: Energy band gap in the Li4Ca(BO3)2: Sm3+ phosphor.
3.7 Decay Curve The recorded decay curve for the 1.5 mol% Sm3+ doped Li4Ca(BO3)2phosphor is shown in fig. 14. The Sm3+ was directly excited by a 403 nm pulse light and then the emission signal
ACCEPTED MANUSCRIPT
was collected at 603 nm (emission of Sm3+) and the corresponding life time of the phosphor was determined. The curve was well fitted by a single exponential equation: T
P = P exp (− U)
- (8)
RI PT
WhereP is the initial intensity at V = 0, P is the intensity at V = V andτ is the decay lifetime. On the basis of equation (8) and decay curve in fig. 14, the effective life time for the 1.5 mol% Li4Ca(BO3)2:Sm3+
phosphor
was
determined
to
be
2.72
ms.
The
1.5
mol%
Li4Ca(BO3)2:Sm3+ phosphor showed a quantum efficiency of 56.31% as calculated by using
SC
the LUMPAC software [49]. The quantum efficiency of 1.5 mol% of the Sm3+ doped Li4Ca(BO3)2 phosphor was found to be greater than Sm3+ doped Ba3Sc0.95(PO4)3, SrBi2B2O7 and Ba4.93(BO3)2(B2O5) phosphors which was reported as 53.5 [50], 33.2 [51] and 16.0%
M AN U
[52], respectively. These results indicate that Li4Ca(BO3)2:Sm3+ (1.5 mol%) phosphor can act as a suitable candidate for n-UV w-LEDs.
6
Li4Ca(BO3)2 : Sm3+
3
λex = 403 nm λem = 603 nm
EP
2
Experimental Fitted curve
TE D
4
1
AC C
Intensity (a.u.)
5
τ
= 2.72 ms
0
0
10
20 30 40 Time (ms)
50
Fig. 14: Decay curve of the Li4Ca(BO3)2:Sm3+ phosphor.
3.8 CIE coordinate diagram
ACCEPTED MANUSCRIPT
Fig. 15 shows a digital photo of the chromaticity diagram for 1.5 mol% of Sm3+ doped Li4Ca(BO3)2phosphors excited at 403 nm. The Commission International de l’Eclairage (CIE) color coordinates calculated by using the CIE 1931 calculator program [53] were determined as (0.60, 0.40) for emission of the Li4Ca(BO3)2:Sm3+ phosphor which are
RI PT
located in the orange-red region. This confirms that the emission color of the Sm3+ ion is reddish-orange. Determination of color purity is essential for narrow banded emission. In the present case we get a narrow band emission in the red region. So we calculated the color purity of the phosphor by using the equation given below [54]: a(bc − bd )e + (fc − fd )e
× ijj%
SC
XYZY[\ ][\^_` =
a(bg − bd )e + (fg − fd )e
M AN U
where, (xs, ys) are the coordinates of the sample point, (xd, yd) are the CIE coordinates of the corresponding dominant wavelength at 600 nm, and (xi, yi) are the coordinates of the illuminant point. By putting the values for (xs, ys) = (0.600, 0.400), (xd, yd) = (0.634, 0.368) and (xi, yi) = (0.3101, 0.3162) (Illuminants C)), the color purity was found to be approximately 92 %. Hence the photometric results implies that the present can be act as
AC C
EP
with InGaN chips.
TE D
an acceptable candidate as the red component in WLED or as a red emitter in combination
Fig. 15: The CIE 1931 chromaticity diagram for Li4Ca(BO3)2:Sm3+ phosphor with the coordinates (x, y) as (0.60, 0.40) under 403 nm excitation.
ACCEPTED MANUSCRIPT
4. Conclusions A series of Sm3+ doped Li4Ca(BO3)2 phosphors were successfully synthesized by the combustion method.The XPS study confirmed the presence of Li, Ca, B, O and Sm on the
RI PT
surface of the phosphor. The Li4Ca(BO3)2:Sm3+ phosphors showed broad excitation bands ranging from 300 nm to 500 nm which match well with the wavelength of available UV chips. The emission peak consisted of four emission peaks resulting into an orange-red emission having (0.60,0.40) as CIE coordinates. The concentration-dependent emission spectra indicated the presence of concentration quenching at 1.5 mol% for the Sm3+ site in the Li4Ca(BO3)2 phosphor
SC
which was caused due to dipole-dipole interactions. Also, the Sm3+ ions showed the symmetric nature in the Li4Ca(BO3)2 phosphor. The average crystallite size of the 1.5 mol% of Sm3+ doped
M AN U
Li4Ca(BO3)2 phosphor was found to be 43 nm with an estimated band gap, effective lifetime and quantum efficiency of 5.13 eV, 2.72 ms and 56.31%, respectively. These results indicated that 1.5 mol% Li4Ca(BO3)2:Sm3+ phosphor is suitable candidate of producing warm white light for a light-conversion phosphor in the near-ultraviolet based w-LEDs. Acknowledgements
TE D
V K gratefully acknowledges the financial support provided by Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India under Extra Mural Research (EMR) funding scheme vide reference no. EMR/16/001718.
[1]
EP
References
Y. Shi, G. Zhu, M. Mikami, Y. Shimomura, Y. Wang, “Photoluminescence of green-
AC C
emitting Ca7(PO4)2(SiO4)2:Eu2+ phosphor for white light emitting diodes” Opt. Mater
Express 4 (2014) 280-287.
[2]
T. Jia, Z. Ci, Q. Wu, G. Zhu, C. Wang, Y. Wang, “Tunable luminescence and energy transfer investigation in Sr8La2[(PO4)4.5(SiO4)2(BO4)0.5](BO2):Ce3+/Mn2+ for White-lightemitting diodes” ECS J. Solid State Sci. Technol. 4 (2015) R78-R82.
[3]
L.P. Lai, Y.Z. Ping, W.Z. Jun, G.Q. Lin, “Luminescence characteristics of Eu3+ activated borate phosphor for white light emitting diode” Chin. Phys. Soc. 17 (2008) 1907-1910.
ACCEPTED MANUSCRIPT
[4]
X. Chen, Z. Xia, “Synthesis and Color-tunable luminescence of Ce3+, Tb3+codoped Sr6YSc(BO3)6 Phosphor” J. Solid State light.1(2014) 1-10.
[5]
X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen, Q. Su, “Highly thermally stable
diodes” ACS Appl. Mater. Interfaces 6 (2014) 2709-2717. [6]
RI PT
single-component white-emitting silicate glass for organic-resin-free white-light-emitting C.K. Chang, T.M. Chen, “Sr3B2O6:Ce3+, Eu2+:A potential single-phased white-emitting borate phosphor for ultraviolet light-emitting diodes” Appl. Phys. Lett. 91 (2007) 081902. [7]
J. McKittrick, M.E. Hannah, A. Piquette, J.K. Han, J.I. Choi, M. Anc, M. Galvez, H.
SC
Lugauer, J.B. Talbot, K.C. Mishra, “Phosphor selection considerations for Near-UV LED solid state lighting” ECS J. Solid State Sc. 2 (2013) R3119-R3131.
V.R. Panse, N.S. Kokode, S.J. Dhoble, “Tb3+ doped Sr2(BO3)Cl green emitting phosphor
M AN U
[8]
for solid state lighting” Adv. Mat. Lett. 5 (2014) 604-610. [9]
Z.C. Wu, P.W.J. Liu, C. Li, W.H. Zhou, S.P. Kuang, “Improved photoluminescence properties of a new green SrB2O4:Tb3+ phosphor by charge compensation” Mater. Res. Bull. 47 (2012) 3414-3416.
[10]
J. Liu, Z.C. Wu, P. Wang, Y.M. Mei, M. Jiang, S.P. Kuang, “The effects of charge
TE D
compensation on photoluminescence properties of a new green-emitting ZnB2O4:Tb3+” Luminescence 29 (2014) 868-871.
[11]
R.S. Kumar, V. Ponnusamy, M.T. Jose, “Synthesis and photoluminescence properties of Sm3+ -doped YAl3(BO3)4 phosphor”Luminescence 29 (2013) 649-656. R.S. Palaspagar, R.P. Sonekar, S.K. Omanwar, “Novel inorganic borate host phosphor
EP
[12]
K2Al2B2O7:Dy3+ for LED based solid state lighting” AIP Conf. Proc.1536 (2013) 807[13]
AC C
808.
L. Chen, C.C. Lin, C.W. Yeh, R. S. Liu, “Light converting inorganic phosphors for white light-emitting diodes” Materials 3 (2010) 2172-2195.
[14]
X. Zhang, H. Lang , H.J. Seo, “On the Luminescence of Ce3+, Eu3+, and Tb3+ in Novel
Borate LiSr4(BO3)3” J Fluoresc. 21 (2010) 1111-1115.
[15]
S. P. Puppalwar, S. J. Dhoble, G. Singh, A. Kumar, “Blue emission in Ce3+ and Eu2+ activated lithium fluoroborate phosphors” J. Mod. Phys. 2 (2011) 1560-1566.
[16]
X. Li, C. Liu, L. Guan, W. Wei, Z. Yang, Q. Guo, G. Fu, “An ideal blue Sr3B2O6: Ce3+ phosphor prepared by sol-combustion method.” Mater. Lett. 87 (2012) 121-123.
ACCEPTED MANUSCRIPT
[17]
L. Wu, C. Wang, X.L. Chen, X.Z. Li, Y.P. Xu, Y.G. Cao , “Ab initio structure determination of new compound Li4CaB2O6” J. Sold state Chem. 177 (2004) 1847-1851.
[18]
Z. Hong, T. Jin, C. Xiu-Lu, “Structural, Electronic Properties and Chemical Bonding of
(2008) 552-555. [19]
I. Pekgozlu, E. Erdogmus, A.S. Basak, “Synthesis and photoluminescence properties of Pb2+ doped Li4CaB2O6”Optik 126 (2015) 1221-1223.
[20]
RI PT
Borate Li4CaB2O6 under High Pressure: an Ab Initio Investigation”Chin. Phys. Lett. 25
Z. Tao, Y. Huang, P. Cai, S.I. Kim, H.J. Seo, “Synthesis and efficient blue-emitting of
[21]
SC
Eu2+-activated borate fluoride BaAlBO3F2” Opt. Mater. 37 (2014) 287-292.
K. Kim, Y.M. Moon, S. Choi, H.K. Jung, S. Nahm, “Luminescent properties of a novel
62 (2008) 3925-3927. [22]
M AN U
green-emitting gallium borate phosphor under vacuum ultraviolet excitation” Mater. lett.
L. Wu, M. Ji, H. Wang, Y. Kong, Y. Zhang, “Site occupancy and photoluminescence of Sm3+ in KSr4(BO3)3:Sm3+ phosphors” Opt. Mater. Express 4 (2014) 1535-1544.
[23]
Neharika, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Synthesis and photoluminescence study of Dy3+ doped Sr3B2O6: Dy3+ nanophosphors” AIP Conf. Proc. 1591 (2014) 558-
[24]
TE D
560.
D. Rajesh, Y.C. Ratnakaram, M. Seshadri, A. Balakrishna, “Luminescence properties of Sm3+ impurities in strontium lithium bismuth borate glasses” AIP Conf. Proc. 1447 (2012) 581-582.
L.F. Shen, B.J. Chen, E.Y.B. Pun, H. Lin, “Sm3+ doped alkaline earth borate glasses as
EP
[25]
UV-visible photon conversion layer for solar cells” J. Lumin.160 (2015) 138-144. L. Alexander, H.P. Klug,” Determination of crystallite size with the X-Ray spectrometer”
AC C
[26]
J. Appl. Phys. 21 (1950) 137-142.
[27]
S. Kato, M. Bielmann, A. Borgschulte, V.Z. Herzog, A. Remhof, S. Orimob, A. Zuttel, “Effect of the surface oxidation of LiBH4 on the hydrogen desorption mechanism” Phys. Chem. Chem. Phys. 12 (2010) 10950–10955.
[28]
S.S. Pitale, I.M. Nagpure, V. Kumar, O.M. Ntwaeaborwa, J.J. Terblans, H.C. Swart, “Investigations on the low voltage cathodoluminescence stability and surface chemical behaviour using Auger and X-ray photoelectron spectroscopy on LiSrBO3:Sm3+ phosphor” Mater. Res. Bull. 46 (2011) 967-994.
ACCEPTED MANUSCRIPT
[29]
H.F Franzen, J. Merrick, M. Umana, A.S. Khan, D.T. Peterson, “XPS spectra and crystalline potentials in alkaline-earth chalcogenides and hydrides” J. Electron Spectrosc. RelatPhenom. 11 (1977) 439-443.
[30]
M. Nuño, G.L. Pesce, C.R. Bowen, P. Xenophontos, R.J. Ball, “Environmental
Build. Environ. 92 (2015) 734-742.
[31]
RI PT
performance of nano-structured Ca(OH)2/TiO2photocatalytic coatings for buildings” C.S. Gopinath, S.G. Hegde, A.V. Ramaswamy, S. Mahapatra, “Photoemission studies of polymorphic CaCO3 materials” Mater. Res. Bull. 37 (2002) 1323-1332.
M. Manhas, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Structrual, surface and
SC
[32]
luminescence properties of Ca3B2O6: Dy3+ phosphors”. Ceram. Int. 42 (2016) 5743[33]
M AN U
5753.
Y. Liu, L. Zhang, L. Cheng, W. Yang, Y. Xu, “Effect of deposition temperature on boron-doped carbon coatings deposited from a BCl3–C3H6–H2 mixture using low pressure chemical vapor deposition” Appl. Surf. Sci. 255 (2009) 8761-8768.
[34]
Neharika, V. Kumar, J. Sharma, O.M. Ntwaeaborwa and H.C. Swart, “Surface and spectral studies of green emitting Sr3B2O6:Tb3+ phosphor” J. Electron. Spectrosc. Relat.
[35]
TE D
Phenom. 206 (2016) 52-57.
Y. Uwamino, T. Ishizuka, “X-ray Photoelectron Spectroscopy Of Rare-Earth Compounds” J. Electron Spectrosc. RelatPhenom. 34 (1984) 67-78.
[36]
C.P. Reddy, V. Naresh, B.C. Babu, S. Buddhudu, “Photoluminescence and energy
EP
transfer process in Bi3+/Sm3+ Co-doped phosphate zinc lithium glasses” Adv. Mater. Phys. Chem. 4 (2014) 165-171.
A.A. Ali, “Optical properties of Sm3+ doped CaF2 bismuth borate glasses” J. Lumin. 129
AC C
[37]
(2009) 1314-1319.
[38]
S. Sailaja, C. A. Raju, C. A. Reddy, B.D.P. Raju, Y.D. Jho, B. S. Reddy, “Optical properties of Sm3+-doped cadmium bismuth borate glasses” J. Mol. Struct. 1038 (2013)
29-34.
[39]
P. Biswas, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Enhanced orange-red emission from KSrVO4:Sm3+nanophosphor for possible application in blue lightemitting diode based white LED” Mater. Res. Express 2 (2015) 025010.
ACCEPTED MANUSCRIPT
[40]
W.Z. Jun, L. P. Lai, Y.Z. Ping, G.Q. Lin, F.G. Sheng, “Energy transfer from Ce3+ to Eu2+ in LiSrBO3 and its potential application in UV-LED-Based white LEDs” Chin. Phys. Lett. 26 (2009) 117802.
[41]
M. Manhas, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “Photoluminescence and
RI PT
thermoluminescence investigations of Ca3B2O6:Sm3+ phosphor” Mater. Res. Express 2 (2015) 075008. [42]
G. Blasse, B.C. Grabmaier, “Luminescent Materials” Edn 1st Berlin, New York: Springer-Verlag (1994).
Z. Yang, P. Liu, J. Li, Q. Yang, L. Lv, Y. Zhao, “A novel yellow luminescent material
SC
[43]
Ba3Y(PO4)3:Eu2+ ”J. Alloys Compd. 578 (2013) 118-120. [44]
J. Zhou, Z. Xia, “Synthesis, luminescence properties and energy transfer behavior of
[45]
M AN U
Na2CaMg(PO4)2:Eu2+,Mn2+ phosphors” J. Lumin., 146 (2014) 22-26.
D.L. Dexter, “A Theory of Sensitized Luminescence in Solids” J. Chem. Phys. 21 (1953) 836-850.
[46]
A. E. Morales, E.S. Mora, U. Pal, “Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures” Rev. Mex. Fis. 53 (2007) 18-22. A.K. Bedyal, V. Kumar, O.M. Ntwaeaborwa, H.C. Swart, “A promising orange-red
TE D
[47]
emitting nanocrystalline NaCaBO3: Sm3+ phosphor for solid state lightining” Mater. Res. Express 1 (2014) 015006.
[48]
L.L. Noto, S.S. Pitale, M.A. Gusowki, J.J. Terblans, O.M. Ntwaeaborwa, H.C. Swart,
EP
“Afterglow enhancement with In3+codoping in CaTiO3:Pr3+ red phosphor” Powder Technol. 237 (2013) 141-146.
J.D.L. Dutra, T.D. Bispo, R.O. Freire, “LUMPAC Lanthanide Luminescence Software:
AC C
[49]
Efficient and User Friendly” J. Comput. Chem. 35 (2014) 772-775.
[50]
X. Ma, L. Mei, H. Liu, L. Liao, Y. Liu, K. Nie, Z. Li, “Structure and fluorescent properties of Ba3Sc(PO4)3: Sm3+ red-orange phosphor for n-UV w-LEDs” Chem. Phys.
Lett. 653 (2016) 212-215.
[51]
L. Wu, Y. Bai, L. Wu, H. Yi, Y. Kong, Y. Zhang, J. Xu, “Sm3+ and Eu3+ codoped SrBi2B2O7: a red-emitting phosphor with improved thermal stability” RSC Adv., 7 (2017) 1146-1153.
ACCEPTED MANUSCRIPT
[52]
J. Zheng, Q. Cheng, W. Chen, Z. Guo, Z. Guo, C. Chen, “Luminescence Properties of an Orange-Red Ba5(BO3)2(B2O5):Sm3+ Phosphor with High Color Purity” ECS J. Solid State Sci. Technol. 4 (2015) R72-R77.
[53]
http://in.mathworks.com/matlabcentral/fileexchange/29620-cie-coordinate-calculator.
S. Som, A.K. Kunti, V. Kumar, V. Kumar, S. Dutta, M. Chowdhury, S.K. Sharma, J.J. Terblans, H.C. Swart, Defect correlated fluorescent quenching and electron phonon coupling in the spectral transition of Eu3+ in CaTiO3 for red emission in display
EP
TE D
M AN U
SC
application, J. Appl. Phys. 115 (2014). doi:10.1063/1.4876316.
AC C
[54]
RI PT
(last assessed date: 1 May 2017)
ACCEPTED MANUSCRIPT
Highlights 1. First time, the orange-red emitting Sm3+ doped Li4Ca(BO3)2 phosphor was synthesized by using the combustion method.
RI PT
2. XPS studies confirm the presence of all the possible elements in Li4Ca(BO3)2:Sm3+ phosphor.
AC C
EP
TE D
M AN U
SC
3. A new amber emitting phosphor for near UV-excited light emitting diodes is projected.