Optimization of luminescence properties of Tb3+-doped a-Sr2P2O7 phosphor synthesized by combustion method
Luminescence Material Laboratory,Department of Physics,Faculty of Science,The M.S. University of Baroda
Display Materials Laboratory, Applied Physics Department,Faculty of Technology and Engineering, The M.S. University of Baroda
作者简介:*Nimesh Prafulbhai Patel e-mail:nimesh.0112@gmail.com;
收稿日期:6 July 2015
Optimization of luminescence properties of Tb3+-doped a-Sr2P2O7 phosphor synthesized by combustion method
Nimesh Prafulbhai Patel Mangalampalli Srinivas Dhaval Modi Vishwnath Verma Kota Venkata Ramana Murthy
Luminescence Material Laboratory,Department of Physics,Faculty of Science,The M.S. University of Baroda
Display Materials Laboratory, Applied Physics Department,Faculty of Technology and Engineering, The M.S. University of Baroda
Abstract:
In this paper, thermoluminescence(TL)properties of rare earth Tb3+-doped α-Sr2 P2 O7 were examined after β-irradiation and photoluminescence(PL)properties of samples were examined for proper excitation.All the samples were synthesized by high-temperature combustion method. The X-ray diffraction(XRD)and Fourier transform infrared(FTIR) spectroscopy characterization confirms the formation of pure α-phase with crystallized in orthorhombic structure of samples.The PL emission spectra of all samples exhibit characteristic green emission peaks of Tb3+ where the peak at545 nm has the highest emission intensity for Tb3+ concentration of 5.0 mol%. The TL glow curves of β-irradiated Tb3+-doped α-Sr2 P2 O7 phosphors were recorded at different heating rates of 2, 4, and 6 K·s-1. TL curves of all sample exhibit combination of two peaks: peak at420 K shifts toward higher temperature, while peak at525 K remains unaffected with the increase in Tb3+concentration as well as fading effect. The activation energy and kinetic parameters of the samples were evaluated using thermoluminescence peak shape method.
Keyword:
Rare earth doping; Strontium pyrophosphate; β-Irradiation; Photoluminescence; Thermoluminescence;
Received: 6 July 2015
1 Introduction
Pyrophosphate-based materials are scientifically important because of their luminescent,dielectric,semiconducting,catalytic,magnetic,fluorescent,and ion-exchange properties.Because of remarkable properties,the synthesis of inorganic pyrophosphate materials is a dynamic field of research,especially in luminescence aspects
The TL properties of pure and doped strontium pyrophosphates were widely examined,because the rare earth activated inorganic phosphors are extensively used in a variety of applications in lamp phosphors,color displays,radiation dosimetry,and X-ray imaging
In this paper,PL property of phosphor was examined for various Tb3+concentrations and the TL property of Tb3+-activatedα-Sr2P2O7 was explained on the basis of fading effect.The activation energy,frequency factors,and order of kinetics of the glow curves were calculated by peak shape method with curve fitting information.PL studies suggest the phosphor as excellent green phosphor and TL of phosphor is also quite good important.
2 Experimental
Tb3+-doped Sr2P2O7 phosphors were synthesized by hightemperature combustion method.The samples with various Tb3+concentrations (0.5 mol%,1.0 mol%,1.5 mol%,2.0mol%,2.5 mol%and 5.0 mol%) were synthesized.The starting reactants SrCO3,(NH4)2HPO4 and Tb4O7(99.99%)(all in analytical reagent (AR)) were taken as per stoichiometric proportion and grinded in an agate mortar with urea (AR;15 wt%of the mixture) to make homogeneous mixture.Urea was taken as a flux in this method to raise the combustion process.The grinded mixture was placed in an alumina crucible and heated at 1200℃for 3 h in a muffle furnace in air and then naturally cooled down to room temperature.The sample in the form of pure white fine powder was obtained after grinding the product
The prepared samples were first characterized by X-ray diffractometer (XRD,Bruker D8;Cu Kαradiation,λ=0.15406 nm,40 kV and 40 mA) to examine crystal phase formation and structural parameters of phosphors.The XRD measurements were carried out for glancing angle incidence detector at an angle of 2°for 2θvalues of 10°-60°in steps of 0.02°.Fourier transform infrared (FTIR) spectra of the samples were recorded by FTIR-4100 type A IR spectrometer in transmittance mode in wavenumber range of400-4000 cm-1.The PL properties of the phosphor were examined using a Shimadzu spectrofluorometer (1503R-PC).PL excitation and emission spectra were recorded by a xenon lamp through a monochromator.The TL glow curves ofβ-irradiated samples were taken using a NUCLEONIX TL analyzer type TL1009 at different heating rates (β) of 2,4,and 6 K·s-1.
3 Theory
The activation energy (Ea),frequency factor (s),and order of kinetics (b) of glow curves were calculated using Chen's peak shape method
The general equation for evaluating activation energy(Ea) is given as:
where k is the Boltzmann's constant,γ=τ,δ,ω,and cγand by are constants which can be calculated using following formula:
Peak shape method is also very useful for estimating the order of kinetics of the TL peaks.As per this method,the order of kinetics (b) was determined by the geometric shape factor (μg).In general,suggested values of geometric factor areμg=0.42 for the first-order kinetics andμg=0.52 for the second-order kinetics
4 Results and discussion
4.1 XRD analysis
Figure 1 shows XRD patterns of Sr2P2O7 phosphors doped with Tb3+concentrations of 0.5 mol%,2.5 mol%,and5.0 mol%.The XRD patterns illustrate that the structural formation of all samples are very similar to that of the ofα-Sr2P2O7 (JCPDS No.24-1011),in which Sr2P2O7 has anα-phase orthorhombic crystal structure with Pnam space group(JCPDS No.24-1011).The doping of Tb3+does not show any measurable influence on the host structure
Fig.1 XRD patterns of Tb-doped Sr2P2O7 phosphors
4.2 FTIR results
FTIR transmittance spectra of Tb3+-doped Sr2P2O7 were recorded using KBr pellets.KBr pellet of all samples were prepared in a 99:1 weight ratio of KBr to Sr2P2O7 for total weight at a thickness of 1 mm.Figure 2 shows FTIR spectra of Tb3+-doped Sr2P2O7 phosphors in the wavenumber range of 400-2000 cm-1.The FTIR spectra show characteristic absorption bands of pyrophosphate group (P2O7)-4 at 746 and 1190 cm-1,which explain the formation of pyrophosphate groups
Fig.2 FTIR spectra of Tb3+--doped Sr2P2O7 phosphors
4.3 PL properties
Figure 3 a shows excitation spectrum of Tb3+-dopedα-Sr2P2O7 phosphor monitored at 545-nm emission wavelength.The excitation spectra exhibits a single band peaked at 232 nm due to the 4f8-44f75d1 (f-d) transition of Tb3+.An excitation band illustrates that the phosphor can be excited through the UV ligth of about 232 nm.Figure 3b illustrates the PL emission spectra,recorded by exciting the samples at their optimized excitation wavelength (i.e.,232 nm) to determine the PL emission from samples.It is important to find out the detailed information of the nature of the Tb3+luminescence center formed inside host lattice.The emission spectra do not give all information about luminescence center,due to the high degeneracy of the Tb3+levels involved in several transitions.
The PL emission spectra of the Tb3+-dopedα-Sr2P2O7show peaks at 415,436,469,491,545 and 584 nm as a consequence of the characteristics radiative transitions of Tb3+.All defined transitions occurring correspond to the5D3-7F5,5D3-7F4,5D3-7F3,5D4-7F6,5D4-7F5,and5D4-7F4,respectively.The observed high intense peak at545 nm can be attributed to green emission which is a characteristic emission of Tb3+.Tb3+replaces the host Sr2+from lattice site without any structural deformation,which is mainly responsible for contraction of unit cell of host Sr2P2O7.The doping ions occupy the host lattice site and create metastable states inside the energy gap of host,which is the reason for the occurrence of PL emissions.It is inferred that the energy transfer process occurs between host (Sr2P2O7) and activator Tb3+,and a very strong green emission occurs at 545 nm because of 5D4-7F5 electronic transition.
Fig.3 Excitation spectra of Tb3+-dopedα-Sr2P2O7 recorded at emission wavelength of 545 nm a,PL excitation spectra of Tb3+-dopedα-Sr2P2O7recorded at excitation wavelength of 232 nm b,International Commission on Illumination CIE (Commission internationale de l'eclairage) co-ordinates of Tb3+-dopedα-Sr2P2O7 phosphors c,and effect of doping Tb3+concentration on emission intensity of hostα-Sr2P2O7 d
Tb3+doping ion is effectively observed through the characteristic emission lines,where the emission intensity is lower for small Tb3+concentrations,indicating that the dopant can be clustered or incorporated onto the lattice sites of hostα-Sr2P2O7.The chromaticity coordinates of PL emission spectra recorded at 232-nm excitation are shown in Fig.3c.The chromaticity coordinates of Tb3+-dopedα-Sr2P2O7 phosphor are X=0.316,Y=0.673,which are falling into the green region in the chromaticity diagram
and remain the same for all Tb3+concentrations.Figure 3d exhibits the variation of intensity for two dominant transitions 5D4-7F6 and 5D4-7F5 with various Tb3+doping concentrations.The intensity of emission occurred due to5D4-7F5 transition is 2-3 times higher that of 5D4-7F6transition.Concentration quenching effect of doping was not studied,but the green emission could be optimized for5 mol%Tb3+.
4.4 TL properties
Figure 4 shows the TL glow curves fitted by deconvolution method forα-Sr2P2O7:0.5 mol%Tb,to formulate the trap depth created inside the band structure ofα-Sr2P2O7phosphors by doping Tb3+.All the samples were irradiated for dose of 10 J·kg-1 using Sr90β-source.The fading effect afterβ-irradiation on glow curves of all Tb3+-dopedα-Sr2P2O7 samples recorded at heating rate of 6 K·s-1 are depicted in Fig.5.The glow curves for various Tb3+concentrations demonstrate that the intensity of TL glow curve decreases from Day 1 to Day 20,in which the peak temperature of the first peak at 420 K shifts toward higher temperature,while that of the second peak at 525 K remains unaffected.It is also observed that at highest Tb3+concentration (5.0 mol%),the second peak at 525 K of glow curve almost vanishes due to energy transfer process;therefore,the intensity of the first peak increases dominantly.TL glow curves analysis were done by Chen's peak shape method by evaluating order of kinetics (b) and activation energy (Ea) recorded at a different heating rates of 2,4,and 6 K·s-,and the glow curves were fitted by deconvolution method.From the curve fitting method,it is found that the glow curve consists of two peaks,the first peak is centered at around 420-435 K,and the second peak is centered at around 525 K.
Fig.4 Glow curves fitting ofα-Sr2P2O7:0.5 mol%Tb3+recorded at heating rate of 6 K·s-1
Fig.5 Fading effect on glow curves of Tb3+-dopedα-Sr2P2O7 recorded at heating rate of 6 K·s-1:a Sr2P2O7:0.5 mol%Tb,b Sr2P2O7:1.0 mol%Tb,c Sr2P2O7:1.5 mol%Tb,d Sr2P2O7:2.0 mol%Tb,e Sr2P2O7:2.5 mol%Tb,and f Sr2P2O7:5.0 mol%Tb
All estimated parameters like energy of trap depths (activation energy)(Ea),order of kinetics (b),and frequency factor (s) at different heating rates are mentioned in Table 1.The energy of trap depth for the first fitted curve peaked at420 K is around 0.8-0.9 eV and that of the second fitted curve peaked at 525 K is around 0.5-0.6 eV.The calculated values of frequency factor (s) are around 1×106-1×1011 s-1,which is very small compared to that of the expected lattice vibration of 1×1012-1×1014 s-1
Figure 6 shows the variation in activation energy (Ea)with different Tb3+doping concentrations.The activation energies of TL glow curve calculated for both peaks for the different doping concentrations are significantly uniform.The results indicate that the same TL mechanism occurs in all samples,revealing that the phosphor could be good for thermoluminescence dosimeter (TLD).
5 Conclusion
Tb3+-dopedα-Sr2P2O7 phosphors were synthesized by high-temperature combustion method using urea as a flux.XRD and FTIR studies confirm that the host has orthorhombic structure which remains unchanged with various Tb3+concentrations.The results of PL and TL studies reveal that the luminescence properties increase with doping Tb3+concentration.PL studies reveal thatα-Sr2P2O7 doped with 5.0 mol%Tb3+shows excellent green emission for a given excitation,and this material has potential to be used as a green luminescent material for display systems in solid state lighting applications.TL analyses of glow curve reveal that the TL glow curve has two peaks analogous to traps created within band structure of host because of Tb3+doping.The calculated parameters of the glow curve imply that it is of second-order kinetic.The activation energy (Ea) and frequency factor (s) are about 0.8-0.9 eV and 1×109-1×1011 s-1 for the curve with maximum temperature of around 420 K,respectively,and 0.5-0.6 eV and 1×106-1×107 s-1 for the curve with maximum temperature of 525 K.TL parameters suggest phosphor as a good TL material.
Table 1 Activation energy,frequency factor,and order of kinetic calculated using peak shape method
Fig.6 Discrepancy of activation energy of Tb3+-dopedα-Sr2P2O7 with different doping concentrations at heating rate of 4 K·s-1:a E(τ),b E(δ),and c E(ω)
参考文献
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