Accepted Manuscript Influence of Pr doping on the thermal, structural and optical properties of novel SLS-ZnO glasses for red phosphor Nurzilla Mohamed, Jumiah Hassan, Khamirul Amin Matori, Raba'ah Syahidah Azis, Zaidan Abdul Wahab, Zamratul Maisarah Mohd Ismail, Nur Fadilah Baharuddin, Siti Syuhaida Abdul Rashid PII: DOI: Reference:
S2211-3797(16)30735-5 http://dx.doi.org/10.1016/j.rinp.2017.03.018 RINP 625
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
22 December 2016 13 March 2017 15 March 2017
Please cite this article as: Mohamed, N., Hassan, J., Matori, K.A., Azis, R.S., Wahab, Z.A., Ismail, Z.M.M., Baharuddin, N.F., Rashid, S.S.A., Influence of Pr doping on the thermal, structural and optical properties of novel SLS-ZnO glasses for red phosphor, Results in Physics (2017), doi: http://dx.doi.org/10.1016/j.rinp.2017.03.018
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Influence of Pr doping on the thermal, structural and optical properties of novel SLS-ZnO glasses for red phosphor Nurzilla Mohamed1, Jumiah Hassan1, Khamirul Amin Matori1, Raba’ah Syahidah Azis1, Zaidan Abdul Wahab1 , Zamratul Maisarah Mohd Ismail1, Nur Fadilah Baharuddin 1, Siti Syuhaida Abdul Rashid1 Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Nurzilla Mohamed (Corresponding author) Email:
[email protected] Tel.: +60174153140 Jumiah Hassan Email:
[email protected] Khamirul Amin Matori Email:
[email protected] Raba’ah Syahidah Azis Email:
[email protected] Zaidan Abdul Wahab Email:
[email protected] Zamratul Maisarah Mohd Ismail Email:
[email protected] Nur Fadilah Baharuddin Email:
[email protected] Siti Syuhaida Abdul Rashid Email :
[email protected] Abstract
35
A novel environmental friendly strategy towards red phosphors in optoelectronic applications employing Pr6O11
36
doped SLS-ZnO with chemical composition x(Pr6O 11).100-x(SLS.ZnO) where x = 0, 1, 2, 3, 4 and 5 wt.% via
37
melt-quenching technique was successfully synthesized. The X-ray Diffraction (XRD) patterns of all these
38
glasses show broad and diffused humps, which confirm the amorphous structure of samples. The Differential
39
Thermal Calorimetry (DSC) indicated that the value of glass transition is higher from 625o C to 637oC with
40
increasingly of Pr6O11 content. Fourier Transform Infrared Spectra (FTIR) spectra display a decreasing trend
41
towards a smaller wavenumber with the increase of Pr content is due to the formation of non-bridging oxygen
42
(NBO) in SLS-ZnO host matrix. The absorption spectra had revealed the most intense absorption band at ~444
43
nm, which was assigned as excitation wavelength to determine the Photoluminescence (PL) emission intensity
44
of the glass. The indirect band gap values varies from ~2.44 ev to ~3.02 ev as a function of Pr6O11 concentration.
45
The PL emission bands at ~530 (blue), ~556(green), ~613 (red) and ~650 (red) nm increases from 0 wt.% to
46
4wt.% and slightly decreases as Pr6O11increases with a maximum at 5 wt.%. Therefore, the SLS-ZnO doped
47
with Pr6O11 as a good potential as red phosphors in an optoelectronic application in accordance with the highest
1
1
red emission intensity at ~613 nm and~650 nm.
2
Keywords: X-ray Diffraction, Differential Thermal Calorimetry, Fourier Transform Infrared Spectra,
3
Photoluminescence.
4 5
Introduction:
6
In recent last decades, many studies have been devoted to the rare earth doped into various crystals and glasses
7
because of their interesting optical and luminescence properties are observed in the field of lasers, sensors,
8
phosphors, optoelectronic devices and photonics [1,2]. Extensive studies on optical properties of various glasses
9
have attracted more attention due to their lower cost and easy to prepare when compared ceramics and crystals
10
[3,2].Oxide glasses such as silicates, phosphate, borates and halides behaves as good as a host material and
11
network modifier as well [4]. Among these oxides glasses, silicates have previously attracted some interest for
12
optical application due to their high chemical durability, thermal stability, good rare earth ions solubility, wide
13
energy band gap and multi-color phosphorescence [5,6]. Rare earth elements act as network modifier that is
14
beneficial for producing non-bridging oxygen atoms [7]. According to Talwatkar et al. (2015) [8], doping is the
15
process of introducing a small amount of foreign element into the host matrix. Numerous reports are available
16
on the rare earth doped glass shows extraordinary photoluminescence (PL) efficiency because of 4f-4f and 4f-5d
17
electronic transitions [9,10]. Praseodymium oxide (Pr6O11) is one of rare earth material that has been used as a
18
red activator in host matrix via red emission lines at~613 nm (3Po → 3H6) and ~650nm (3Po → 3F2) [11].
19
Mahmuda et al. (2013) [12] have reported on Pr3+ doped zinc alumino bismuth borate glasses showed the
20
enhancement in the red emission spectra. The optical properties are mainly determined by the presence of the Pr
21
dopant in the host matrix material [13]. Glassy material based on ZnO have to offer immense technological
22
promises for optical application such as optoelectronic devices, due to its wide band gap (3.37 ev) and large
23
exciting bounding energy (60 mev) [4]. From the previously reported work, ZnO act as a network former that is
24
connected to the neighboring SiO4 to form Si-O-Zn bond by bridging oxygen [14]. Combination of SiO2 -ZnO
25
glass has been a promising host material for the fabrication of silicate phosphor due to their excellent properties
26
in luminescence and chemical stability [15]. The technological limitation in the production of SiO2-ZnO arises
27
from a highly cost of SiO2 since it is quite expensive. Utilization of SLS waste to synthesize SiO2 would be an
28
effective effort in reducing disposal cost and preserving environment [16]. SLS glasses consists of SiO2 (~74
29
wt.%), Na2O (~14 wt.%), CaO( ~6.4 wt.%) and MgO (~4.5 wt.%) [17] are chemically durable and relatively
30
easy to form the products at high speeds inexpensively [18].
2
1
Thus, the combination of both SLS-ZnO can be considered as good candidates for host matrix due to their
2
chemical stability, low melting point and good stability for rare earth ions [19]. There has not so far been any
3
information on optical properties of Pr-doped SLS-ZnO glass system. Therefore, this work demonstrates the
4
correlation between thermal, structural and optical properties of SLS-ZnO glass system with varying
5
concentration of Pr6O11 for the fabrication of low-cost red phosphors.
6 7
Methodology
8
The glasses with the following chemical composition x(Pr6O11).100-x(SLS.ZnO) where x = 0, 1, 2, 3, 4 and 5
9
wt.% were prepared by a conventional melt-quenching technique. The starting materials consist of ZnO (Sigma
10
Aldrich), SLS (Region Food Industries Sdn Bhd, Malaysia) and Pr6O11 (Alfa Aesar) were mixed together by a
11
dry mill in bottles for 24 hours in order to obtain the homogeneous mixtures powder. Then, a series of different
12
batches powder were melted at 1400 oC for 2 hours in alumina crucibles by an electric furnace. In water
13
quenching, the molten glass was poured directly into water to produce glass frits. Afterward, the glass frits were
14
ground and sieved to be in powder form with the size of 63 µm to get a homogeneous particle size. Kashif et al.
15
(2012) studied the effect of different conventional melt quenching technique and observed that the samples
16
poured in water were amorphous [20]. They also reported that, the samples poured at air and small droplets on
17
the cold surface were detected by the presence of crystalline structure in the glass matrix. Thus, water quenching
18
is suitable choice to produce transparent glasses compared than those of following conventional melt quenching
19
technique. The structural analysis of glass samples were determined by X-ray Diffraction (XRD) (Philips X’Pert
20
Diffractometer model 7602 EA Almelo with CuKα radiation), Differential Thermal Calorimetry (DSC)
21
(Diamond Pyris TG/DTA (Perkin Elmer)) and Fourier Transform Infrared Spectra (FTIR) (Perkin Elmer
22
Spectrum 100).The absorption and optical band gap were recorded in the 400-500 nm intervals using the
23
Ultraviolet-Visible (UV-Vis) Spectrophotometer (Lambda 35, Perkin Elmer). The Photoluminescence Excitation
24
(PLE) measurement were performed on a spectrometer model of Perkin Elmer LS 55 Fluorescence instrument
25
with an excitation wavelength of 444 nm.
26 27 28
Results and Discussions
29
X-ray diffraction (XRD)
30
Fig.1 shows XRD patterns of x(Pr6O11).100-x(SLS.ZnO)] where x = 0,1,2,3,4 and 5 wt.% respectively. All
3
1
presented glasses exhibit a broad diffraction characteristics and the absence of any crystalline sharp peak that
2
confirms the amorphous structure of the glasses.
3 4
Thermal properties
5
The glass transition (Tg) and crystallization temperatures (Tc) were investigated by DSC thermogram are shown
6
in Fig.2. It is seen that the Tg shifted to higher temperature with increasingly of Pr6O11 content from 625oC to
7
637oC. In view of this, a significant increase of Tg proves network bonding between Pr3+ ions and the host glass
8
[21,22]. Tg is attributed to reversible transition in amorphous materials from a brittle state into a rubber- like a
9
state [23]. It is found that, the crystallization peak (Tc) increases from 714 to 748 oC with increasing amount of
10
Pr6O11. The thermal stability ( ∆T ) for this glass is determined by the difference between the Tc and Tg, as
11
shown in below equation:
12
∆T = (Tc- Tg)
(1)
13 14
According to the literatures [24,25], the thermal stability of glass higher than 100oC can be considered as good
15
thermal stability. Characteristics temperature such as glass transition temperature (Tg), crystallization
16
temperature (Tc) and thermal stability (∆T) are listed in table 1. From these results, samples with high Pr6O11
17
such as 5 wt.% Pr have a more stable glass forming ability. It means that, Pr6O11 plays an important role as a
18
network modifier in the improvement of thermal stability [26,22]. In order to achieve a good thermal stability,
19
the introduction of Pr6O11 into the glass structure seems to be a good idea to increase the thermal stability of the
20
glass.
21 22 23
Fourier Transform Infrared Spectra (FTIR)
24
Fig.3 shows the FTIR spectra of all the samples with different Pr3+ doping. As shown in Fig.3, the bands ranging
25
from 400 to about 525 cm-1 which due to Si–O–Si bending vibration [7]. The broad band centered in the 683-
26
703 cm-1 range can be assigned to asymmetric stretching of the ZnO4 groups. These broad bands are also
27
associated to the presence of amorphous character [27]. The band positions agree well with those of the XRD
28
results reported there is no presence of any crystalline peak that confirms the glass structure. The bands in the
29
range of 892-922 cm-1 are originated from the asymmetric stretching of SiO4 [28]. The shoulder bands locate at
4
1
1117 to 1214 cm-1 attributed to the stretching vibration of Si-O-Si [27]. It is observed that the whole spectrum of
2
Pr- doped SLS-ZnO glasses seems same spectral pattern is obtained from the 0 wt.% Pr (undoped SLS-ZnO).
3
However, these spectrum bands show a decreasing trend towards smaller wavenumber with the increase Pr
4
content. This is a signature of the formation of non-bridging oxygen (NBOs) due to presence of Pr in SLS-ZnO
5
leads to distinct shift of the band position [7,29,30]
6 7 8
Absorption
9
The absorption spectra of Pr3+ doped SLS-ZnO glasses at different concentration in the wavelength range of
10
250-750 nm are represented in Fig.4. Optical absorption provides information regarding structural properties,
11
band gap energy and induces transition of electronics [31]. It can be seen that there is no sharp absorption band
12
of 0 wt.% Pr (undoped SLS-ZnO). Meanwhile, absorption spectra of Pr- doped SLS-ZnO are comparatively
13
sharp as the concentration of Pr6O11 increases. From these spectra, the absorption bands at ~444, ~467 and ~483
14
nm are which were assigned to the transition of 3H4→3P2, 3H4→3P1 and 3H4→ 3P0 respectively [11,32]. These
15
transitions have been labeled as hypersensitive transitions and strongly related on the neighbouring ligands in
16
the Praseodymium [33,34]. It is known that, Pr3+ ions has the normal outer electronic configuration 4s2,4p6,4d10,
17
4f2, 5s2,5p6 and a ground state 3H4 with angular momentum ,J = 4 [35]. According to Russel Sanders coupling
18
scheme, the splitting of the J levels into a stark component by ligand field in the Pr causes larger probability of
19
interaction between praseodymium 4f wave function and the ligand wave function [36]. This leads to stronger Pr
20
bonds with the surrounding ligands. For all Pr- doped glasses, the sharpness band was found at 444 nm and it
21
was used as excitation wavelength to study the photoluminescence emission properties.
22 23
Indirect band gap
24
Glassy materials are strongly related on a band tailing of density of states [37]. According to Chimalong et al.
25
(2010) the transition occurring in glass are indirect type at higher photon energy [5]. A relationship between
26
absorption coefficient () and phonon energy (ℎѵ) in order to determine the optical band gap was proposed by
27
Davids and Mot [38] and is given by the following equation:
28 29 30
hѵ = hѵ −
5
(2)
1 2 3
= the optical absorption coefficient
4
ℎѵ = energy of the incident photons
5
B= constant called the band tailing parameter
6
=optical band gap
7
= the index which has different values (2, 3, 1/2, and 1/3) corresponding to indirect allowed, indirect forbidden,
8
direct allowed and direct forbidden transitions respectively.
9 10
Therefore, the optical band gap ( ) for allowed indirect transition (n=2) can be calculated by extrapolating the
11
linear region of the typical plot (hѵ)/ versus ℎѵ as shown in fig.5. It is observed, indirect band gap ( )
12
values increased from ~2.44 to ~3.02 ev with increasing Pr6O11 concentration from 0 to 5 wt.%. It is known that,
13
the high of carrier concentration can influence on band gap variation [39]. In this case, Pr adding may enhance
14
the concentration of free carriers and impurity ions by perturbation the energy states of the carriers and thereby
15
increase of band gap. This behavior can be explained in term of Burstein- Moss effect (BME) which is usually
16
accompanied by band gap renormalization in degenerate semiconductor [40].
17 18
Photoluminescence (PL)
19
The emission spectrum of SLS-ZnO glasses doped Pr 3+ at different concentration in the wavelength range 500-
20
700 nm obtained by monitoring the excitation at 444 nm is shown in Fig.6. The reasons for choosing 444 nm as
21
excitation wavelength that corresponds to the absorption of Pr in SLS-ZnO host lattice. The excitation of
22
commercial blue LED is supposed in the range of ~430-500 nm [41].Therefore, the excitation in the blue region
23
(~444 nm) leads to emission in the red region [42].
24
It is believed, the wide green emission band at ~575 nm with low intensity for undoped SLS-ZnO is associated
25
with the recombination between a single ionized oxygen vacancy in ZnO and a photogenerated hole [43, 44].
26
For Pr doped SLS-ZnO, four absorption bands are observed at ~530 nm (blue), ~556 nm (green), ~613 nm (red)
27
and ~650 nm (red) which are ascribed to the electric dipole transitions of 3Po → 3H4 ,3Po → 3H5, 3Po → 3H6 and
28
3
29
SLS-ZnO due to absence of Pr3+ in this host matrix. According to Li et al. (2014), the enhancement of the
30
emission bands intensities after Pr3+ ion doping is accordance with the successful substitution of Pr ions into Zn
Po→ 3F2 respectively[33,45,46,47]. There is no emission band in the range of ~600 to 650 nm for undoped
6
1
sites in the SLS-ZnO host lattice [48]. The red emission bands located at ~613 nm and ~650 nm are attributable
2
to the 3Po → 3H6 and 3Po → 3F2 transition were more intense as compared to other observed transitions. These red
3
emission bands should be of value in improving the low Color Rendering Index (CRI) in White Light Emitting
4
Diodes (WLEDs) as reported by Yang et al. (2011) [49]. It can be seen that intensity of the 3Po → 3H6 transitions
5
increases when passing from 1 wt.% Pr to sample 4 wt.% Pr and then slightly decreases for sample 5 wt.% of
6
Pr6O11 content. This increase in intensity may due to the difference between inner states of Pr3+ ions in SLS-ZnO
7
host matrix. This behavior can be explained by the effect of Schottky barrier that produces the levels occupied
8
by an excess electron in the grain boundary and those levels are assigned as a recombination center [50,51,52].
9
In this case, the energy transfer might occur between levels of Pr3+ ions in SLS-ZnO and induced defects that
10
lead to the formation of non-bridging oxygen (NBO). Thus, the increment of nonbridging oxygens (NBOs)
11
which are related to the radiative transition in the vicinity of the Pr3+ ions indicates a shift toward the visible
12
region with larger emission wavelengths [53]. This assumption was supported by FTIR results that addition of
13
Pr6O11 oxide into host lattice provides NBOs for network structure. However, it is observed that by increasing
14
the Pr3+ concentration from 4 wt.% to 5 wt.%, the emission intensity slightly decreased suggesting luminescence
15
quenching [11]. According to Remteke et al. (2014), the increase in rigidity of the glasses leads to decrease in
16
intermolecular Pr-O distance and will provide Pr3+ ions close together in the host matrix [10]. This is assigned to
17
the decrease of emission intensity that indicates, there is no efficient energy transfer process occurred between
18
Pr3+ ions in SLS-ZnO host matrix
19 20
Conclusions
21
Novel red phosphors Pr-doped SLS-ZnO glasses with chemical composition x(Pr6O11).100-x(SLS.ZnO) where x
22
= 0, 1, 2 ,3 ,4 and 5 wt.% have been fabricated successfully by melt-quenching method. The amorphous phase of
23
glass phosphor was confirmed by XRD analysis. A significant increment of Tg values from 625 oC for the host
24
glass to 637oC for the glass with maximum Pr6O11 content is observed, indicating network bonding between Pr3+
25
ions into the SLS-ZnO host matrix. FTIR spectrum has clearly indicated shifted to lower wavelength as
26
concentration Pr6O11 increased due to the formation of non-bridging oxygen (NBOs). Absorption spectra of Pr-
27
doped SLS-ZnO exhibit the sharpness bands at ~444 nm (3H4→3P2) with the increasing of Pr6O11 concentration
28
are due to the ligand field in the praseodymium. The addition of Pr6O11 may lead to an increase concentration of
29
free carriers and impurity ions by perturbation the energy states of the carriers, which increase in optical energy
30
band gap. The emission intensity enhances with an increase of Pr6O11 concentration up to 4 wt.% and reduces
7
1
with a further increase of 5 wt.% due to concentration quenching phenomena. It can be found that, the sharp and
2
well-defined red emission bands mainly originated to 3Po → 3H6 (~613) and 3Po → 3F2 (~650 nm) transition with
3
excitation wavelength at 444 nm. These results indicate that the investigated of SLS-ZnO glass doped with
4
Pr6O11 concentration without incorporating expensive material can be a promising alternative to the
5
conventional red phosphor with a satisfactory chemical stability and high efficiency in optoelectronic
6
applications
7 8
Acknowledgments
9
The authors gratefully acknowledge the financial support for this study from the Malaysian Ministry of
10
Education (MOE), Ministry of Science, Technology and Innovation (MOSTI) and Universiti Putra Malaysia
11
through Research University Grant Scheme (RUGS 05-02-12-2180RU), Science Fund, and Putra Grant.
12 13
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16
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Z. Khalkhali, Z. Hamnabard,S.Sadat, A. Qazvini, S.Baghshahi. Opt Mater. 34, 850 (2012)
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
10
1 2 3 4
Table 1 : Glass transition temperature, Tg, temperature of crystallization, Tc and thermal stability range ∆T of x(Pr6O11).100-x(SLS.ZnO)
5 6
Sample
Tg (oC)
Tc(oC)
7
0% Pr 1% Pr 2% Pr
625 627 629
714 737 729
∆T(oC)= (Tc - Tg) 89 110 100
3% Pr
632
738
106
4% Pr 5% Pr
635 637
740 748
105 111
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
11
5% Pr
Intensity (a.u)
4% Pr
3% Pr 2% Pr 1% Pr
0% Pr 10
20
30
40
50
2 Theta (20
1 2
Fig.1: XRD pattern of x(Pr6O11).100-x(SLS.ZnO)
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 12
60
70
80
90
0% Pr 1% Pr 2% Pr 3% Pr 4% Pr 5% Pr
714
endo ← ∆T → exo
Tg= 625 737
Tg= 627 Tg= 629
729 738
Tg= 632 Tg= 635
740
Tg= 637
400
450
500
550
600
748
650 o
Temperature( C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Fig.2: DSC patterns x(Pr6O11).100-x(SLS.ZnO)
13
700
750
1177
5% Pr
1183
4% Pr
1190
3% Pr
1201
2% Pr
1204
1% Pr
683 892
487
686 895
494
Transmittance (a.u)
689 902
504
696 909
511
700 912
0% Pr
1214
518
703 922 525
400
600
800
1000
1200
1400 -1
wavenumber (cm )
1 2
Fig.3 : FTIR patterns x(Pr6O11).100-x(SLS.ZnO)
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
14
1600
1800
2000
Absorbance (a.u)
0% 1% 2% 3% 4% 5%
444
468 481
400
450
500
Wavelength (nm)
1 2
Pr Pr Pr Pr Pr Pr
Fig.4 : Absorption spectra of x(Pr6O11).100-x(SLS.ZnO)
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
15
550
600
3.1
0% 1% 2% 3% 4% 5%
3.0 2.9
1/2
Egindirect (ev)
2.8 2.7
Pr Pr Pr Pr Pr Pr
2.6 2.5
(cm eν)
2.4 1
-1
0
2
3
4
5
(αhν)
1/2
Pr6011 (%)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
h ν (eν)
1 2
Fig.5 : Indirect band gap of x(Pr6O11).100-x(SLS.ZnO). Inset: Relationship between indirect band gap and
3
Pr6O11 concentration
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 16
1 2
3
Po
3
3
3
500
H6
Po
3
Po
3
F2
H5
550
600
Wavelength (nm)
3 4
3
H4
Intensity (a.u)
Po
3
0% Pr 1% Pr 2% Pr 3% Pr 4% Pr 5% Pr
Fig.6: Emission peak of x(Pr6O11).100-x(SLS.ZnO)
5 6 7 8 9 10 11 12
17
650
700
