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Near UV–blue emission from Ce doped Y2SiO5 phosphor
Materials Science in Semiconductor Processing 31 (2015) 715–719
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Near UV–blue emission from Ce doped Y2SiO5 phosphor Yogita Parganiha a, Jagjeet Kaur a, Vikas Dubey b,n, K.V.R. Murthy c a
Department of Physics, Govt. Vishwanath Yadav Tamaskar Post Graduate Autonomous College, Durg 491001, C.G., India Department of Physics, Bhilai Institute of Technology, Kendri, Raipur, C.G., India c Display Materials Laboratory, Applied Physics Department, Faculty of Technology and Engineering, The M.S. University of Baroda, Vadodara-390001, India b
a r t i c l e i n f o
abstract
Available online 22 January 2015
Cerium doped yttrium oxy-orthosilicate (Y2SiO5:Ce3 þ ) phosphor was synthesized by modified solid state reaction method, which is low cost and useful for large scale production. A sample was characterized by X-ray diffraction (XRD) technique as well as field emission gun scanning electron microscopy. The crystallite size was calculated by Scherer's formula. The effect of Ce3 þ concentration on photoluminescence studies was interpreted. The sample shows good PL spectra in the violet and blue region. The excitation spectra monitored at 400 nm and emission spectra monitored at 360 nm. The spectrophotometric determination was calculated by Commission Internationale de I’Eclairage technique. Using this phosphor, the desired CIE values including emissions throughout the violet (360 nm) and blue (427 nm) of the spectra were achieved. Efficient blue light emitting diodes were fabricated using Ce3 þ doped phosphor based on near ultraviolet (NUV) excited LED lights. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Photoluminescence (PL) XRD FEGSEM CIE
1. Introduction Yttrium silicate [Yttrium oxy-orthosilicate (YSO) or Y2SiO5] crystal is well-known host material by accommodating appreciable amounts of rare earth ions. Ce:Y2SiO5 have been investigated in numerous papers for scintillation applications [1,2]. Y2SiO5:Ce is a blue emitting phosphor and has been studied by many physicists in order to understand the luminescent behaviour for application in the lighting industry (such as field emission displays, FEDs). The crystal structure of Y2SiO5 belongs to the rare earth oxyorthosilicates RE2SiO5 [31]. This kind of silicates has multiphase structure. From the literature it is found that Y2SiO5:Ce has two different monoclinic crystal structures. Low temperatures (synthesized at temperatures less than 1190 1C) X1-phase (much weaker luminescent intensity [3], with n
Corresponding author. Tel.: þ91 9826937919. E-mail addresses: [email protected] (Y. Parganiha), [email protected] (V. Dubey). http://dx.doi.org/10.1016/j.mssp.2014.12.070 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
space group P21/c) and a high temperature (synthesized at temperatures above 1190 1C with a melting temperature at 1980 1C) X2-phase (space group B2/c). In each of these two phases there are two possible Y3 þ sites in theY2SiO5 matrix [3,4]. These two sites are attributed to the difference in coordination numbers (CN). During the preparation method of Y2SiO5:Ce the activator Ce3 þ (radius of 0.106 nm) can easily substitute for Y3 þ (radius of 0.093 nm) thus also resulting in the two different crystallographic sites. The notations A1 and A2 are given to the two sites in the X1-phase with CN of 9 and 7. B1 and B2 are denoted for the X2-phase with CN of 6 and 7. A1 with the CN of 9 means that there are 8 oxygen bonded to yttrium and silicon and only 1 that is bonded to only yttrium [4–8]. Luminescence in Y2SiO5:Ce occurs due to characteristic transitions (in the Ce3 þ ion itself). From a standpoint of material chemistry and physics, Y2SiO5 represent interesting systems to test and develop fundamental ideas about synthesis and properties of doped-insulators. However, the research on the synthesis and morphology control of
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Y. Parganiha et al. / Materials Science in Semiconductor Processing 31 (2015) 715–719
Y2SiO5 is far from sufficient as compared with other materials [9], such as metals and semiconductors [10]. Rare-earth (RE) doped yttrium oxyorthosilicate (Y2SiO5) is a material that has long coherent time associated to the small magnetic moment and nuclear spin of its constituents. These properties allowed the investigation of this material for applications such as storage of information and high-resolution nonlinear imaging processing [11,12]. More recently the slowing and storage of light was demonstrated aiming to application in quantum information and all-optical networks [13]. RE-doped Y2SiO5 has also been investigated as a solid-state near-infrared laser medium [14–16]. Yttrium silicate doped with various rare earth ions has attracted much attention. For example, Ce3 þ ions-doped Y2SiO5 is considered as a possible candidate for the replacement of ZnS:Ag as a blue phosphor in field emission displays [17]. Tb3 þ ions-doped Y2SiO5 is one of the best green emitting cathodoluminescent phosphor [18]. Eu3 þ ionsdoped Y2SiO5 is a promising candidate for coherent timedomain optical memory applications [14]. Up to now, a number of synthetic approaches such as solid-state reaction [19], sol–gel [20], flame spray pyrolysis [17], combustion [21], melting salt assisted sol–gel [22], and metallorganic decomposition [24] have been employed to prepare Y2SiO5 doped with different rare earth ions. Single phase Y2SiO5 is hard to prepare by solid-state reaction below 1700 1C because it is always accompanied by the non-reacted Y2O3 and SiO2 and other phases such as polymorphs of Y2Si2O7 [23]. The present investigation reports the synthesis, characterization and effect of variable concentration on luminescence study of Y2SiO5 phosphor. 2. Experimental To prepare Y2SiO5 with cerium (0.1 to 2.5 mol%) consists of heating stoichiometric amounts of reactant mixture is taken in alumina crucible and is fired in air at 1000 1C for 1 h for calcination after that the sintering temperature 1400 1C for 4 h in a muffle furnace. Every heating is followed by intermediate grinding using agate mortar and pestle. The Ce3 þ activated Y2SiO5 phosphor was prepared via high temperature solid state method. The starting materials were as follows: Y2O3, SiO2, CeO2 and H3BO3 (as a flux) in molar ratio were used to prepare the phosphor. The XRD measurement was carried out using Bruker D8 Advance X-ray diffractometer. The X-rays were produced using a sealed tube and the wavelength of X-ray was 0.154 nm (Cu K-alpha). The X-rays were detected using a fast counting detector based on Silicon strip technology. Observation of particle morphology was investigated by FEGSEM (field emission gun scanning electron microscope) JEOL JSM-6360. The photoluminescence (PL) emission and excitation spectra were recorded at room temperature by use of a Shimadzu RF-5301 PC spectrofluorophotometer. The excitation source was a Xenon lamp [25–30]. 3. Results and discussion Fig. 1 shows the X-ray powder diffraction (XRD) pattern of Y2SiO5:Ce3 þ (2.0 mol% of Ce) phosphor. From the XRD
Fig. 1. XRD pattern of Ce3 þ doped Y2SiO5 phosphor.
Table 1 XRD calculation of prepared phosphor (Y2SiO5:Ce3 þ 2 mol%). S. No.
2θ
FWHM
hkl
D crystallite size (nm)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
22.91 24.99 29.11 30.66 33.56 35.08 39.37 41.17 48.64 50.16 52.78 57.34
0.18 0.20 0.25 0.32 0.12 0.23 0.15 0.31 0.40 0.34 0.34 0.32
21 1 11 2 01 3 40 2 31 2 41 1 42 0 03 1 31 4 43 1 62 2 04 2
42 40 34 23 50 38 44.5 24.6 20 22 22 23
pattern, the peak indexed revealed the pure monoclinic phase of Y2SiO5. They are in good accordance with JCPDS card no. 36-1476. It is indicated that there is no impurity phase among the phosphor sample. From these experimental results we can conclude that Ce3 þ ions have been introduced in to the Y2SiO5 lattice, and do not cause any change in the monoclinic structure. It was revealed that the introduction of Ce3 þ ions did not influence the crystal structure of the phosphor matrix. Any effect of flux (H3BO3) was not found in XRD pattern. The crystallite size calculated by Scherer formula presents in Table 1. It confirms the formation of nano crystallites in phosphors Y2SiO5:Ce3 þ (2 mol%). The phosphor particles should have a flower like structure and high luminescence efficiency for successful applications. Indeed, phosphor particles with a flower shape minimize light scattering on their surfaces and therefore, improve the efficiency of light, emission and the brightness of such phosphor. To show the evolution particle morphology and size of synthesized Y2SiO5: Ce3 þ powders, in Fig. 2(a and b), we display the FEGSEM photographs at two different magnifications ( 5000 & 20,000). It shows good connectivity with different grain size and particle morphology. Fig. 3 shows the excitation spectra of the as synthesized YSO phosphor doped with 2 mol% of Ce3 þ at room temperature. The two distinct peaks in Fig. 3 might therefore be due to the excitation to two different energy levels in either the host’s band gap (other luminescent centres) or to
Y. Parganiha et al. / Materials Science in Semiconductor Processing 31 (2015) 715–719
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Fig. 2. (a and b) FEGSEM micrographs of prepared phosphors (Y2SiO5:Ce3 þ 2 mol%).
Fig. 3. Excitation spectra of Y2SiO5:Ce3 þ (2 mol%) for 400 nm emission.
energy levels in the 5d1 configuration in the Ce3 þ ion inside the band gap or both. The excitation peak (360 nm) in this case may most probable be due to the CT of electrons from the 2p orbital of O2 to the 4f orbital of Ce3 þ and the other peak is due to the 5d field splitting. Emission spectra for 360 nm excitation band indicated the familiar broad band emission of Y2SiO5:Ce3 þ as can be seen in Fig. 4(a). Under excitation wavelength of 360 nm, the emission spectrum of synthesized YSO:Ce with variable concentration of Ce3 þ phosphor shows a broad emission band extending in UV–Visible region from 370 to 550 nm (Fig. 4a) with the maximum intensity at 427 nm, which is attributed to the electron transition from the 5d lowest energy level of Ce3 þ to the 2F5/2 to 2F7/2 manifolds split by spin–orbit coupling [32,33]. The broadness of the emission peak is ascribed to emission from more than one energy level. In the PL spectra which shows linear response with concentration up to 1.5 mol% of Ce3 þ after that concentration quenching occurs (Fig. 4b) and the luminescence intensity decreases with increasing the Ce3 þ concentration [39,40]. The emission peak centred at 427 nm (2.99 eV, near UV– violet–blue region) was attributed to radiative recombination
Wavelength (nm)
Fig. 4. (a) PL Emission spectra of Y2SiO5:Ce3 þ phosphor. (b) Concentration vs intensity plot for PL emission spectra.
of photo-generated hole with an electron occupying surface defects namely the oxygen vacancies, F-centres (oxygen ion vacancy occupied by two electrons)/F-centres (oxygen ion vacancy occupied by single electrons)/surface states.
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The energy levels of these defect centres exist in the forbidden energy gap of YSO [34–37]. The deep blue UV-band may attribute to recombination of a delocalized electron close to the conduction band with a single charged state of surface oxygen vacancy, according to Wang’s proposal [37]. Normalized fluorescence spectra of YSO:Ce with different Ce concentration the same. Commission Internationale de I’Eclairage (CIE) chromaticity coordinate for Ce3þ doped YSO phosphor were calculated using the blue LED with the excitation at 360 nm as shown in Fig. 5 and the values are found in violet–blue region. Their corresponding location has been marked in Fig. 5 with cross in violet–blue region. This clearly shows YSO sample can be used for blue light emitting applications and its chromaticity coordinate is x¼0.175, y¼0.061.The results indicate that Y2SiO5:Ce3 þ (1.5%) phosphors can be selected as a potential candidate for LED (Light Emitting Diode) application. However, the relative intensity of the emission bands which provide the fundamental colours balance for violet—blue-light emission was achieved with the 1.5 mol% sample with the spectrum (Fig. 4a) providing the CIE 1931 chromaticity. If one increases the activator concentration even further, the emission intensity commences to decrease owing to concentration quenching. This concentration quenching is due to the
increase in the ion–ion interaction provoked by the shorter distance between interacting activators as the concentration increases. The fluorescence light spectral profile as a function of the activator concentration was examined and the results indicated that the chromaticity coordinates of the overall emission light changed resulting in different colours of the overall emission light [38,41–43], for different concentrations as can be observed. The emission band shows intense violet– blue emission of prepared phosphor. Fig. 6 shows the energy level diagram of possible emission of the prepared phosphor.
4. Conclusions Y2SiO5:Ce3 þ doped phosphor successfully synthesized by solid state reaction method. XRD pattern confirms that synthesized sample shows monoclinic structure. The crystallites size was found to be 22–50 nm. XRD studies confirm the phosphors are in single phase and nano crystallites. FEGSEM images show the formation of nano flower with irregular shape and sizes. It can be seen that the particles have a compact distribution and flower like shape. The PL emission was observed in near UV–violet– blue range with emission wavelength 370–550 nm when excited at 360 nm. The present phosphor can act as single host for violet–blue light emission in display devices. The results indicate that Y2SiO5:Ce3 þ (1.5%) phosphors can be selected as a potential candidate for LED (Light Emitting Diode) with excitation at 360 nm. References
Fig. 5. CIE 1931 coordinate for Ce3 þ doped phosphor Ce3 þ :YSO.
Fig. 6. Energy level diagram for possible transition of Y2SiO5:Ce phosphor.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Near UV–blue emission from Ce doped Y2SiO5 phosphor Yogita Parganiha a, Jagjeet Kaur a, Vikas Dubey b,n, K.V.R. Murthy c a
Department of Physics, Govt. Vishwanath Yadav Tamaskar Post Graduate Autonomous College, Durg 491001, C.G., India Department of Physics, Bhilai Institute of Technology, Kendri, Raipur, C.G., India c Display Materials Laboratory, Applied Physics Department, Faculty of Technology and Engineering, The M.S. University of Baroda, Vadodara-390001, India b
a r t i c l e i n f o
abstract
Available online 22 January 2015
Cerium doped yttrium oxy-orthosilicate (Y2SiO5:Ce3 þ ) phosphor was synthesized by modified solid state reaction method, which is low cost and useful for large scale production. A sample was characterized by X-ray diffraction (XRD) technique as well as field emission gun scanning electron microscopy. The crystallite size was calculated by Scherer's formula. The effect of Ce3 þ concentration on photoluminescence studies was interpreted. The sample shows good PL spectra in the violet and blue region. The excitation spectra monitored at 400 nm and emission spectra monitored at 360 nm. The spectrophotometric determination was calculated by Commission Internationale de I’Eclairage technique. Using this phosphor, the desired CIE values including emissions throughout the violet (360 nm) and blue (427 nm) of the spectra were achieved. Efficient blue light emitting diodes were fabricated using Ce3 þ doped phosphor based on near ultraviolet (NUV) excited LED lights. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Photoluminescence (PL) XRD FEGSEM CIE
1. Introduction Yttrium silicate [Yttrium oxy-orthosilicate (YSO) or Y2SiO5] crystal is well-known host material by accommodating appreciable amounts of rare earth ions. Ce:Y2SiO5 have been investigated in numerous papers for scintillation applications [1,2]. Y2SiO5:Ce is a blue emitting phosphor and has been studied by many physicists in order to understand the luminescent behaviour for application in the lighting industry (such as field emission displays, FEDs). The crystal structure of Y2SiO5 belongs to the rare earth oxyorthosilicates RE2SiO5 [31]. This kind of silicates has multiphase structure. From the literature it is found that Y2SiO5:Ce has two different monoclinic crystal structures. Low temperatures (synthesized at temperatures less than 1190 1C) X1-phase (much weaker luminescent intensity [3], with n
Corresponding author. Tel.: þ91 9826937919. E-mail addresses: [email protected] (Y. Parganiha), [email protected] (V. Dubey). http://dx.doi.org/10.1016/j.mssp.2014.12.070 1369-8001/& 2015 Elsevier Ltd. All rights reserved.
space group P21/c) and a high temperature (synthesized at temperatures above 1190 1C with a melting temperature at 1980 1C) X2-phase (space group B2/c). In each of these two phases there are two possible Y3 þ sites in theY2SiO5 matrix [3,4]. These two sites are attributed to the difference in coordination numbers (CN). During the preparation method of Y2SiO5:Ce the activator Ce3 þ (radius of 0.106 nm) can easily substitute for Y3 þ (radius of 0.093 nm) thus also resulting in the two different crystallographic sites. The notations A1 and A2 are given to the two sites in the X1-phase with CN of 9 and 7. B1 and B2 are denoted for the X2-phase with CN of 6 and 7. A1 with the CN of 9 means that there are 8 oxygen bonded to yttrium and silicon and only 1 that is bonded to only yttrium [4–8]. Luminescence in Y2SiO5:Ce occurs due to characteristic transitions (in the Ce3 þ ion itself). From a standpoint of material chemistry and physics, Y2SiO5 represent interesting systems to test and develop fundamental ideas about synthesis and properties of doped-insulators. However, the research on the synthesis and morphology control of
716
Y. Parganiha et al. / Materials Science in Semiconductor Processing 31 (2015) 715–719
Y2SiO5 is far from sufficient as compared with other materials [9], such as metals and semiconductors [10]. Rare-earth (RE) doped yttrium oxyorthosilicate (Y2SiO5) is a material that has long coherent time associated to the small magnetic moment and nuclear spin of its constituents. These properties allowed the investigation of this material for applications such as storage of information and high-resolution nonlinear imaging processing [11,12]. More recently the slowing and storage of light was demonstrated aiming to application in quantum information and all-optical networks [13]. RE-doped Y2SiO5 has also been investigated as a solid-state near-infrared laser medium [14–16]. Yttrium silicate doped with various rare earth ions has attracted much attention. For example, Ce3 þ ions-doped Y2SiO5 is considered as a possible candidate for the replacement of ZnS:Ag as a blue phosphor in field emission displays [17]. Tb3 þ ions-doped Y2SiO5 is one of the best green emitting cathodoluminescent phosphor [18]. Eu3 þ ionsdoped Y2SiO5 is a promising candidate for coherent timedomain optical memory applications [14]. Up to now, a number of synthetic approaches such as solid-state reaction [19], sol–gel [20], flame spray pyrolysis [17], combustion [21], melting salt assisted sol–gel [22], and metallorganic decomposition [24] have been employed to prepare Y2SiO5 doped with different rare earth ions. Single phase Y2SiO5 is hard to prepare by solid-state reaction below 1700 1C because it is always accompanied by the non-reacted Y2O3 and SiO2 and other phases such as polymorphs of Y2Si2O7 [23]. The present investigation reports the synthesis, characterization and effect of variable concentration on luminescence study of Y2SiO5 phosphor. 2. Experimental To prepare Y2SiO5 with cerium (0.1 to 2.5 mol%) consists of heating stoichiometric amounts of reactant mixture is taken in alumina crucible and is fired in air at 1000 1C for 1 h for calcination after that the sintering temperature 1400 1C for 4 h in a muffle furnace. Every heating is followed by intermediate grinding using agate mortar and pestle. The Ce3 þ activated Y2SiO5 phosphor was prepared via high temperature solid state method. The starting materials were as follows: Y2O3, SiO2, CeO2 and H3BO3 (as a flux) in molar ratio were used to prepare the phosphor. The XRD measurement was carried out using Bruker D8 Advance X-ray diffractometer. The X-rays were produced using a sealed tube and the wavelength of X-ray was 0.154 nm (Cu K-alpha). The X-rays were detected using a fast counting detector based on Silicon strip technology. Observation of particle morphology was investigated by FEGSEM (field emission gun scanning electron microscope) JEOL JSM-6360. The photoluminescence (PL) emission and excitation spectra were recorded at room temperature by use of a Shimadzu RF-5301 PC spectrofluorophotometer. The excitation source was a Xenon lamp [25–30]. 3. Results and discussion Fig. 1 shows the X-ray powder diffraction (XRD) pattern of Y2SiO5:Ce3 þ (2.0 mol% of Ce) phosphor. From the XRD
Fig. 1. XRD pattern of Ce3 þ doped Y2SiO5 phosphor.
Table 1 XRD calculation of prepared phosphor (Y2SiO5:Ce3 þ 2 mol%). S. No.
2θ
FWHM
hkl
D crystallite size (nm)
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
22.91 24.99 29.11 30.66 33.56 35.08 39.37 41.17 48.64 50.16 52.78 57.34
0.18 0.20 0.25 0.32 0.12 0.23 0.15 0.31 0.40 0.34 0.34 0.32
21 1 11 2 01 3 40 2 31 2 41 1 42 0 03 1 31 4 43 1 62 2 04 2
42 40 34 23 50 38 44.5 24.6 20 22 22 23
pattern, the peak indexed revealed the pure monoclinic phase of Y2SiO5. They are in good accordance with JCPDS card no. 36-1476. It is indicated that there is no impurity phase among the phosphor sample. From these experimental results we can conclude that Ce3 þ ions have been introduced in to the Y2SiO5 lattice, and do not cause any change in the monoclinic structure. It was revealed that the introduction of Ce3 þ ions did not influence the crystal structure of the phosphor matrix. Any effect of flux (H3BO3) was not found in XRD pattern. The crystallite size calculated by Scherer formula presents in Table 1. It confirms the formation of nano crystallites in phosphors Y2SiO5:Ce3 þ (2 mol%). The phosphor particles should have a flower like structure and high luminescence efficiency for successful applications. Indeed, phosphor particles with a flower shape minimize light scattering on their surfaces and therefore, improve the efficiency of light, emission and the brightness of such phosphor. To show the evolution particle morphology and size of synthesized Y2SiO5: Ce3 þ powders, in Fig. 2(a and b), we display the FEGSEM photographs at two different magnifications ( 5000 & 20,000). It shows good connectivity with different grain size and particle morphology. Fig. 3 shows the excitation spectra of the as synthesized YSO phosphor doped with 2 mol% of Ce3 þ at room temperature. The two distinct peaks in Fig. 3 might therefore be due to the excitation to two different energy levels in either the host’s band gap (other luminescent centres) or to
Y. Parganiha et al. / Materials Science in Semiconductor Processing 31 (2015) 715–719
717
Fig. 2. (a and b) FEGSEM micrographs of prepared phosphors (Y2SiO5:Ce3 þ 2 mol%).
Fig. 3. Excitation spectra of Y2SiO5:Ce3 þ (2 mol%) for 400 nm emission.
energy levels in the 5d1 configuration in the Ce3 þ ion inside the band gap or both. The excitation peak (360 nm) in this case may most probable be due to the CT of electrons from the 2p orbital of O2 to the 4f orbital of Ce3 þ and the other peak is due to the 5d field splitting. Emission spectra for 360 nm excitation band indicated the familiar broad band emission of Y2SiO5:Ce3 þ as can be seen in Fig. 4(a). Under excitation wavelength of 360 nm, the emission spectrum of synthesized YSO:Ce with variable concentration of Ce3 þ phosphor shows a broad emission band extending in UV–Visible region from 370 to 550 nm (Fig. 4a) with the maximum intensity at 427 nm, which is attributed to the electron transition from the 5d lowest energy level of Ce3 þ to the 2F5/2 to 2F7/2 manifolds split by spin–orbit coupling [32,33]. The broadness of the emission peak is ascribed to emission from more than one energy level. In the PL spectra which shows linear response with concentration up to 1.5 mol% of Ce3 þ after that concentration quenching occurs (Fig. 4b) and the luminescence intensity decreases with increasing the Ce3 þ concentration [39,40]. The emission peak centred at 427 nm (2.99 eV, near UV– violet–blue region) was attributed to radiative recombination
Wavelength (nm)
Fig. 4. (a) PL Emission spectra of Y2SiO5:Ce3 þ phosphor. (b) Concentration vs intensity plot for PL emission spectra.
of photo-generated hole with an electron occupying surface defects namely the oxygen vacancies, F-centres (oxygen ion vacancy occupied by two electrons)/F-centres (oxygen ion vacancy occupied by single electrons)/surface states.
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The energy levels of these defect centres exist in the forbidden energy gap of YSO [34–37]. The deep blue UV-band may attribute to recombination of a delocalized electron close to the conduction band with a single charged state of surface oxygen vacancy, according to Wang’s proposal [37]. Normalized fluorescence spectra of YSO:Ce with different Ce concentration the same. Commission Internationale de I’Eclairage (CIE) chromaticity coordinate for Ce3þ doped YSO phosphor were calculated using the blue LED with the excitation at 360 nm as shown in Fig. 5 and the values are found in violet–blue region. Their corresponding location has been marked in Fig. 5 with cross in violet–blue region. This clearly shows YSO sample can be used for blue light emitting applications and its chromaticity coordinate is x¼0.175, y¼0.061.The results indicate that Y2SiO5:Ce3 þ (1.5%) phosphors can be selected as a potential candidate for LED (Light Emitting Diode) application. However, the relative intensity of the emission bands which provide the fundamental colours balance for violet—blue-light emission was achieved with the 1.5 mol% sample with the spectrum (Fig. 4a) providing the CIE 1931 chromaticity. If one increases the activator concentration even further, the emission intensity commences to decrease owing to concentration quenching. This concentration quenching is due to the
increase in the ion–ion interaction provoked by the shorter distance between interacting activators as the concentration increases. The fluorescence light spectral profile as a function of the activator concentration was examined and the results indicated that the chromaticity coordinates of the overall emission light changed resulting in different colours of the overall emission light [38,41–43], for different concentrations as can be observed. The emission band shows intense violet– blue emission of prepared phosphor. Fig. 6 shows the energy level diagram of possible emission of the prepared phosphor.
4. Conclusions Y2SiO5:Ce3 þ doped phosphor successfully synthesized by solid state reaction method. XRD pattern confirms that synthesized sample shows monoclinic structure. The crystallites size was found to be 22–50 nm. XRD studies confirm the phosphors are in single phase and nano crystallites. FEGSEM images show the formation of nano flower with irregular shape and sizes. It can be seen that the particles have a compact distribution and flower like shape. The PL emission was observed in near UV–violet– blue range with emission wavelength 370–550 nm when excited at 360 nm. The present phosphor can act as single host for violet–blue light emission in display devices. The results indicate that Y2SiO5:Ce3 þ (1.5%) phosphors can be selected as a potential candidate for LED (Light Emitting Diode) with excitation at 360 nm. References
Fig. 5. CIE 1931 coordinate for Ce3 þ doped phosphor Ce3 þ :YSO.
Fig. 6. Energy level diagram for possible transition of Y2SiO5:Ce phosphor.
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