Effect of flux on luminescence studies of Sr 2 SiO 4 : Eu nanophosphor.

Transcription

1 Effect of flux on luminescence studies of Sr 2 SiO 4 : Eu nanophosphor. B.S. Prathibha #1, M.S. Chandrashekara #1, H. Nagabhushana #2, B.M. Naghabhushana #3 #1 Department of Studies in Physics, University of Mysore, Manasagangothri, Mysore , India, , #2 Prof C.N.R Rao Centre for Advanced Materials, Tumkur University, Tumkur , India, , #3 Department of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore , India, Abstract Sr 2 SiO 4 : Eu 0.01 phosphors were prepared by solution combustion method at a low temperature of C by using fluxes like sodium flouride, boric acid and combination of both. The effect of flux on the luminescent properties of Sr 2 SiO 4 : Eu was investigated in detail. The crystalline size and structural properties are studied by X-ray diffraction. The peaks of Sr 2 SiO 4 are assigned by FTIR studies. Flux materials are selected to have a control over the morphology and size distribution of phosphors was examined by SEM. The Photoluminescence (PL) intensity of Sr 2 SiO 4 : Eu was more with the flux than without a flux. Thermoluminescence (TL) of Sr 2 SiO 4 : Eu was investigated using -irradiation in the dose range of 1-6 Kgy at a warming rate of 5 o C/s. The kinetic parameters are calculated from the deconvoluted TL glow curve. NaF is more superior compared to other combinations. Key words: combustion method, luminescence, flux Corresponding Author : Prathibha.B.S 1. INTRODUCTION The important application of inorganic Sr 2 SiO 4 luminescence material in energy, display, electronic field etc have been created special interest in research because of its various distinguish and special characteristic properties like high temperature, long life time, reliability, stability under high irradiation, durability in packing resins [1-3]. Further the incorporation of rare earth ions into this will still enhances the luminescence properties. The luminescence efficiency of phosphor greatly depends on the synthesis process and activator concentration. In recent years interest in TL has continued to increase not only in dosimetric application but also as a tool in solid state physics, archaeology and the earth sciences [4]. As a result so many research and advancements have been taken in TL studies. Recently Haoyi etal have synthesized RSPUBLICATION, rspublicationhouse@gmail.com Page 146

2 melilite co-doped with Eu and Dy and studied the luminescent properties. Thermoluminescent and photoluminescent properties of CdSiO 3 doped with Dy was repoeted by C. Munjunath and e tal [5]. When irradiation is incident on insulating crystals some of the deposited energy is stored in lattice at defect sites, color centers etc. upon heating the crystals this energy is released and fraction of it may be emitted as visible light prior to the onset of black body radiation this is thermoluminescence [6-7]. This method is used to identify defects in host material and most commonly used in radiation dosimetry. In addition to TL studies another important luminescence studies is PL studies. Among the next generation of solid state lightening white LED s have attracted the significant attention. White light emitting LED s can be produced by two different methods, one is to use a phosphor which alone emits all the three RGB colors and another way is to use a phosphor material to convert monochromatic light from a UV-LED or blue to a broad white spectrum [8]. Many white light emitting phosphors have been reported Hai Guo e tal have synthesized Sr 2 SiO 4 : Eu by solid state reaction using different combination of fluxes and they have studied luminescent properties which shows that β-sr 2 SiO 4 exhibit good luminescent properties [8]. The correlation between phase transition and luminescent properties of Sr 2 SiO 4 : Eu prepared by ball milling method using NH 4 Cl as flux was studied by Jee Hee Lee e tal [9]. White light emitting long lasting Pr 3+ doped Sr 2 SiO 4 phosphor was synthesized by a solid state reaction by Li Zhang e tal [10]. In phosphors the emission efficiency depends on morphology and size distribution but it is difficult to control because phosphor materials will grow of their own natural during synthesis and they have intrinsic crystal structure [11]. This problem can be solved by using flux. Flux accelerates crystal growth, increases the reaction rate, helps in the formation of well crystallized particle of appropriate size and reduces the process temperature which can lead to reduce the production cost [12]. These fluxes increase the reactivity of the constituents by dissolving at least one of the reactants and providing a medium to incubate the crystallization of the phosphor [11]. The addition of flux has great influence on the particle size distribution, crystallization process, ion diffusion, crystal growth. Further the addition of flux decreases activation energy of the reaction by forming a thin film of liquid around particles which contributes to the betterment of morphology [13] Here we synthesized Sr 2 SiO 4 : Eu powder by solution combustion method using different fluxes. The influence of different fluxes on the structural and luminescence properties of Sr 2 SiO 4 : Eu was studied in detail and the results show that the NaF flux is superior to other combination in TL studies and the combination of NaF and Boric acid is superior in PL studies which were well supported by XRD studies. 2. EXPERIMENTAL 2.1 Synthesis In the synthesis all the chemicals used were analar grade. The starting materials were Sr(NO 3 ) 2, H 2 O, fumed silica, citric acid (C 6 H 8 O 7 ), Eu 2 O 3, 0.14 gm of NaF, H 3 BO 3 and (0.07 gm of NaF + RSPUBLICATION, rspublicationhouse@gmail.com Page 147

3 Intensity (au) 0.07 gm of H 3 BO 3 ) combination of both are used as fluxes keeping F/O=1 the mixture. The mixture was dissolved into a certain amount of deionized water and then stirred for few minutes. The aqueous solution was introduced into a muffle furnace maintained at a temperature of 500 o C. Initially the solution boiled and underwent dehydration followed by decomposition with escape of large amount of gaseous product. Then spontaneous ignition occurred and underwent combustion. The whole process is over within less than 5 min. After the combustion products were calcined at 1100 o C for 3 hrs. The samples were kept at this temperature for 3 hr and then cooled to room temperature in furnace. 2.2 Instruments used The crystallanity and phase purity of Sr 2 SiO 4 nanophosphor was examined by using Philips and Siemen s D5005 X-ray diffractometer using Cu Kα (λ= Å) radiation with a nickel filter. FTIR spectra were recorded with a Perkin Elmer FTIR spectrometer. UV-Vis absorption of the sample was recorded on a SL 159 ELICO UV-Vis spectrometer. TL measurements are carried out for 5 mg Sr 2 SiO 4 nanophosphor powder at room temperature using Nucleonix TL reader. 60 Co gamma source was used as ionizing radiation in the dose range 1-6 KGy. PL studies of Sr 2 SiO 4 : Eu phosphor was carried out using Horiba, (model fluorolog-3) spectrofluorometer using 450 W Xenon as excitation source. 3. RESULTS AND DISCUSSION 3.1 Powder X-ray Diffraction (XRD) The fluxes play an important role in the crystalline size distribution and luminescent efficiency. Fig 1 displays the XRD of Sr 2 SiO 4 : Eu phosphor powders with different kinds of fluxes (H 3 BO 3, NaF, and H 3 BO 3 +NaF) and without flux. A single α- phase (JCPDS ) was observed in Sr 2 SiO 4 without flux and with H 3 BO 3 flux. With NaF flux and combination of fluxes phosphor exhibits mixed phase i.e both α and β- phase (JCPDS ) co exists. The particle size was calculated in all the cases by using de-bye scherrer formula [14]. There was no much difference in the particle size, the size lies in the range of nm. Noticable changes was not observed in the flux diffraction peaks which could be found in the XRD patterns, indicating that purity of the resultant phosphor was not affected by the addition of flux. H 3 BO 3 +NaF NaF Boric acid without flux Figure 1. PXRD of Sr 2 SiO 4 : Eu phosphor without and with flux RSPUBLICATION, rspublicationhouse@gmail.com Page 148

4 Transmittance 3.2 Fourier Transform Infrared spectroscopy (FTIR) FTIR was employed as an auxiliary characterization alternative. The composition, quality and molecular structure of the product were analyzed by this spectroscopy. Fig 2 displays the spectra of Sr 2 SiO 4 : Eu without and with flux. Changes in the shape of peaks were observed. In case of flux the peaks are overlapped which are resolved using origin 8.1 software. The additional peak appears at 2351, 2326 (Cm -1 ) which is due to CO 2. The existence of characteristic Sr-O stretching and Si-O stretching confirmed the structure. An extended list of common vibrational modes expected in Sr 2 SiO 4 can be given as below. The observed results are in good agreement with literature [15]. 498 F 2 bending ( 4 ) 503 symmetric stretching vibration of Si-O-Si asymmetric Si-O-Si bond, Si-O F 2 ( 3 ) streching (Cm -1 ) Sr-O stretching vibrations With NaF flux Without flux Wavenumber (cm -1 ) Figure 2. FTIR spectra of Sr 2 SiO 4 : Eu phosphor without and with flux 3.3 UV-Vis spectroscopy The fig 3 shows the UV-Vis absorbance spectra of Sr 2 SiO 4 : Eu with and without flux. A sharp and prominent band with a maximum of ~206, ~225, ~275 nm along with broad absorption bands at ~424 nm are observed. A red shift was observed in the case of Sr 2 SiO 4 : Eu prepared using a combination of fluxes. The maximum absorption can arise due to the transition between valence and conduction band where as the weak absorption in the UV-Vis region is expected to arise from transition involving extrinsic states such as surface traps or defect states or impurities [16]. The smaller size particles are found to have high surface to volume ratio. This results in increase in surface to volume ratio. This results in increase in defects distribution on the surface of RSPUBLICATION, rspublicationhouse@gmail.com Page 149

5 ( h ) (ev cm ) absorbance (au) nanomaterials. Thus the lower is the particle size nanomaterials exhibit strong and broad absorption bands [17]. d c b a wavelength (cm -1 ) Figure 3a UV-Vis spectra of Sr 2 SiO 4 : Eu phosphor (a) without flux (b) Boric acid flux (c) NaF flux (d) boric acid and NaF fluxes Optical band gap energy (Eg) was estimated using the wood and Tauc relation [18] given by α (hγ Eg) 1 2 hγ Where α-optical absorption coefficient, h is the photon energy, Eg is the band gap energy for direct transitions. The plot of (αh ) 2 v/s photon energy of Sr 2 SiO 4 : Eu with flux and without flux is as shown in the figure 3b. The E g vales have been evaluated by extrapolating the linear portion of the curve in the UV-Vis absorption spectra. The optical band gap values for the samples with flux (H 3 BO 3, NaF, H 3 BO 3 +NaF) and without flux are found to be 4.34, 4.29, 4.37, 4.4 ev respectively. Due to presence of defects, distortions and strain in the lattice there is variation in the optical band gap [19]. 8.00E E+010 Without flux BA NaF BA+NaF 4.00E E E h (ev) Figure 3b UV-Vis spectra of Sr 2 SiO 4 : Eu phosphor without and with flux RSPUBLICATION, rspublicationhouse@gmail.com Page 150

6 Pl Intensity (a.u) 3.4 Photoluminescence (PL) Figure 4a and 4b gives the excitation and emission spectra of Sr 2 SiO 4 : Eu with and without flux. The excitation spectra are performed under 613 nm emission. Broad excitation spectra are observed in the range of nm in a phosphor without flux, with NaF flux and with H 3 BO 3 flux. Broad bands are due to charge transfer band (O 2 Eu 3+ ) [20]. Eu 3+ and O 2- two near by ions forms Eu 3+ _O 2- hence there will be transfer of electron from filled 2p orbital of O 2- to partially filled 4f orbit of Eu 3+. As a result oxidation of O 2- and reduction of Eu 3+ takes place. But in Sr 2 SiO 4 : Eu phosphor prepared using combination of fluxes sharp excitation peaks are observed around 318 nm ( 7 F 0 5 H 6 ), 361 nm ( 7 F 0 5 D 4 ), 376 nm ( 7 F 0 5 G 2 ), 382 nm ( 7 F 0 5 G 5 ), 393 nm ( 7 F 0 5 L 6 ). These bands are related to the intra configurational 4f-4f transition of Eu 3+ in the host lattices. It was observed that 4f- 4f transitions and charge transfer band are two types of excitation of the trivalent lanthanide ions in crystals [3]. The 4f orbital is shielded from the surroundings by the filled 5 S 2 and 5 P 6 orbital. Therefore the influence of the host lattice on the optical transitions within the 4fn configuration is small and 4f- 4f transition is sharp lines c without flux NaF BA+NaF BA b a d wavelength (nm) Figure 4a: Excitation spectra of Sr 2 SiO 4 : Eu (a) without flux (b) NaF (c) H 3 BO 3 +NaF (d) H 3 BO 3 They exhibit an emission bands from 580 nm 705 nm under 393 excitation. According to literature survey these bands are ascribed to the 5 D 0 7 F j transitions of Eu 3+ [21]. The emission intensity increases with the addition of NaF flux and it increases little further with the combination of fluxes. The increases in the PL intensity in the combination of fluxes is attributed to improvement in the crystallanity which reduces the defect in the lattice and on the surface of phosphor [LL ] in addition to this existence of β- Sr 2 SiO 4 phase. This variation in PL intensity in Sr 2 SiO 4 :Eu treated with different fluxes were mainly due to method of preparation, RSPUBLICATION, rspublicationhouse@gmail.com Page 151

7 PL intensity (a.u) particle size effect, presence of concentration of defects etc. this clearly confirms that the crystalline morphology plays an important role in luminescence enhancement [22] without flux BA NaF BA+NaF b a Wavelength (nm) Figure 4b: Emission spectra of Sr 2 SiO 4 : Eu (a) without flux (b) NaF (c) H 3 BO 3 +NaF (d) H 3 BO 3 However boric acid did not have a positive effect on the PL properties of phosphor. The PL intensity decreases with the addition of boric acid. This is may be due to single α- phase, and unclean surface [11]. Figure 4a and 4b represents the excitation and emission spectra of the samples without and with flux which exhibit an emission bands centered around 579 nm ( 5 D 0 7 F 0 ), 588, 592 nm ( 5 D 0 7 F 1 ), 613, 618 nm ( 5 D 0 7 F 2 ), 651, 656 nm ( 5 D 0 7 F 3 ), 702 nm ( 5 D 0 7 F 4 ) [3]. In addition to these peaks Sr 2 SiO 4 : Eu phosphor prepared by using combination of NaF and H 3 BO 3 flux shows extra emission bands around 431 nm which lies in the blue region. Thus this phosphor exhibits an emission in all the three (Red, Green, Blue) regions. 3.5 Thermoluminescence (TL) When ionizing radiation such as -rays, X-rays, β-rays, α-particles, energetic ions are incident on an insulator or semiconductor there will be emission of light from these materials following the previous absorption of energy from the ionizing radiation. This emission is known as thermally stimulated emission [23-24]. The emitted light intensity by the phosphor on being heated reflects the irradiation dose given to it [14]. The plot of emitted intensity verses temperature gives rise to TL glow curve. The TL glow curve of 1KGy - irradiated Sr 2 SiO 4 : Eu sample with flux and without flux are as shown in the Fig 5a. All the samples are warmed at a heating rate of 5 o C/S. A broad well resolved TL glow peak at C was observed for the samples without flux. However for the phosphor with flux (NaF) the peak arises at C. RSPUBLICATION, rspublicationhouse@gmail.com Page 152

8 TL intensity (au) TL intensity (a.u) TL glow peak was observed in H 3 BO 3 flux and combination of fluxes very weak signals are observed in TL this clearly shows that these fluxes do not play a significant role in creating trapping levels in host material. It was observed that the TL intensity is found to be higher in Sr 2 SiO 4 : Eu prepared with NaF flux. The increases in intensity is due to an increase in surface to volume ratio, creation of more depths by the addition of fluxes, smaller crystalline size of the phosphor compared to other phosphor [19] without flux BA NaF BA+NaF Temperature ( o c) Figure 5a: TL glow curve of 1KGy - irradiated Sr 2 SiO 4 : Eu sample with and without flux Sr 2 SiO 4 : Eu phosphor prepared with NaF flux was taken, exposed to different doses of - irradiation (1-6 KGy) and TL glow curves are recorded at the same heating rate which was as shown in the Fig 5b. It was found that the TL intensity increases with the increase in the dose reaches maximum for 5KGy and then decreases. This is because the increase in surface to volume ratio results in higher surface barrier energy for the nanoparticles. At lower dose the energy derived is not sufficient to overcome the barrier energy hence the defect creation is more thus the number of defects created in the particle keeps on increasing with increase in the dose and hence the TL intensity increases KGy 2KGy 3KGy 4KGy 5KGy 6KGy Temperature ( o C) Figure 5b: TL glow curve of 1-6 KGy - irradiated Sr 2 SiO 4 : Eu with NaF flux RSPUBLICATION, rspublicationhouse@gmail.com Page 153

9 Intensity (a.u) Intensity (a.u) The TL glow curve for 5KGy -irradiated was deconvoluted using the origin 8.0 software. Figure 6 shows the deconvoluted glow curve, which was used to estimate the kinetic parameters such as activation energy (E) i.e the measure of the energy required to eject an electron from the defect center to the conduction band, order of kinetics (b) i.e measure of the probability that a free electron gets ret rapped and frequency factor (s) rate of electron ejection using chen s set of equation [25]. The results are tabulated in the table below Experimemntal Peak1 Peak2 PeakSum Temperature ( 0 C) Figure 6: the de convoluted glow curve Figure 7 shows the plot of TL intensity verses dose. It was observed that the intensity increases with increase in the dose i.e linear nature. A material meant to serve as a dosimeter for high dose must have its response to be directly proportional. Response was sublinear at 3KGy after that it shows a linear nature I V/S dose Dose (KGy) Figure 6: Plot of intensity versus dose RSPUBLICATION, rspublicationhouse@gmail.com Page 154

10 Table 1: The kinetic parameters for Sr 2 SiO 4 phosphors estimated from Chen s glow peak shape method. Average T m ( o C) µ g E τ E ω E δ E (ev) S (s -1 ) Peak X Peak CONCLUSIONS Sr 2 SiO 4 : Eu was synthesized by solution combustion method using fluxes. The existence of pure α- phase and mixed phase (α and β) was confirmed from PXRD. FTIR spectrum revealed that addition of fluxes caused some change in vibrational modes of silicates and band structure within the material. The band gap value was obtained from the UV-Vis spectrum. A white light emitting phosphor was successfully prepared and the combination of fluxes (NaF + H 3 BO 3 ) gives rise to β- monoclinic phase along with α-orthorhombic phase which enhanced the luminescent performance of the phosphor. The TL results obtained shows that the synthesized material responds to TL. More over the Sr 2 SiO 4 : Eu phosphor prepared using NaF flux shows meaning full results compared to those synthesized by using boric acid and combination of fluxes. REFERENCES 1. Jee Hee Lee, Young Jin Kim, Material science and engineering B, 146,99-102, (2008). 2. Shanshan yao, Donghua Chan, Optics and laser technology 40, , (2008). 3. QIAO Yanmin, ZHANG Xinbo, YE Xiao, CHEN Yan, GUO Hai, Journal of rare earths 27, 323, (2009). 4. R.K Bull Nuclear traks radiation measurements 11, , (1986). 5. C.Manjunath, D.V. Sunitha, H.Nagabhushana, B.M.Nagabhushana, S.C. Sharma, R.P.S. Chakradar, spectra chemical act (2012). 6. E. Pekpak, A. Yilmaz, G. Ozbayoglu, Journal of alloys and compounds , (2011). 7. K.Madhukumar, H.K. Varma, Manoj Komath, T.S.Elias, V. Padmanabhan and C.M.K. Nair Bulletin material science 30, , (2007). RSPUBLICATION, rspublicationhouse@gmail.com Page 155

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