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Photoluminescence properties of fully concentrated Terbium oxalate: A novel efficient green emitting phosphor
Author’s Accepted Manuscript Photoluminescence properties of fully concentrated Terbium oxalate: A novel efficient green emitting phosphor Dinu Alexander, Kukku Thomas, S Sisira, G Vimal, Kamal P Mani, P R Biju, N V Unnikrishnan, M A Ittyachen, Cyriac Joseph www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31873-0 http://dx.doi.org/10.1016/j.matlet.2016.12.002 MLBLUE21814
To appear in: Materials Letters Received date: 18 September 2016 Revised date: 24 November 2016 Accepted date: 2 December 2016 Cite this article as: Dinu Alexander, Kukku Thomas, S Sisira, G Vimal, Kamal P Mani, P R Biju, N V Unnikrishnan, M A Ittyachen and Cyriac Joseph, Photoluminescence properties of fully concentrated Terbium oxalate: A novel efficient green emitting phosphor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.12.002 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 galley proof before it is published in its final citable 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.
Photoluminescence properties of fully concentrated Terbium oxalate: A novel efficient green emitting phosphor Dinu Alexander, Kukku Thomas, Sisira S, Vimal G, Kamal P Mani, P R Biju, N V Unnikrishnan, M A Ittyachen and Cyriac Joseph* School of Pure and Applied Physics, Mahatma Gandhi University, Kerala, India -686560
Abstract A novel green emitting phosphor, Terbium oxalate decahydrate, was synthesized in crystalline form employing the hydro silica gel method. The structure of the sample was confirmed by x-ray diffraction analysis. Photoluminescence excitation spectrum of the sample shows several peaks in the deep UV to Visible region, matching well with the commercially available LED sources. Emission spectrum, recorded with representative excitation wavelengths revealed efficient luminescence, particularly the green emission at 543nm. The decay time of the sample was measured to be 0.81ms. It exhibits a colour purity of 68% with chromaticity coordinates (0.31, 0.57) which is very close to that of the European Broadcasting Union illuminant green. Graphical Abstract
Keywords: Phosphors, Crystal growth, Optical materials and properties, Luminescence
1
1. Introduction Rare earth elements have been the focus of research towards technological inventions on account of their wide spread applications with utmost precision in properties. Rare earth based optical technology has thus become an emerging field of research due to its applications in light emitting diodes, electro luminescent displays and bio-imaging [1,2]. Excellent luminescence emission is inevitable for such applications but “concentration quenching” limits the same. Low doping of RE3+ in a host yields only weak emission while heavy doping reduces the intensity by concentration quenching. Even though there are numerous reports on rare earth doped phosphor materials, fully concentrated rare earth based phosphor materials have been less investigated despite its probable higher luminescence intensity which is a pre-requisite for an efficient phosphor material [3]. In order to explore the above feature, we have synthesised Terbium oxalate crystals and studied its luminescence characteristics. Terbium is one of the most promising rare earth elements which emit intense green, weak blue, yellow and red radiations, hence suitable for applications in fluorescent lamps, solar cells, plasma display panels, etc. This paper presents a first time report on the synthesis and photoluminescence studies of Terbium oxalate decahydrate crystals, a novel green emitting fully concentrated phosphor. 2. Experimental Terbium oxalate crystals were grown by single diffusion gel technique in which the hydro silica gel acts as a three dimensional frame work for the growth of perfect crystals. Well filtered aqueous solution of sodium meta silicate (Na2SiO3.9H2O, CDH) of specific gravity 1.03gm/cc was mixed with aqueous solution of 1M oxalic acid (H2C2O4.2H2O, Merck) to obtain gel with pH 7. The top solution, prepared by mixing equal volumes of 0.5M aqueous solution of terbium nitrate pentahydrate (Tb2 (NO3)3.5H2O, Sigma Aldrich) and concentrated nitric acid, was dripped gently through the walls of the tube. The outer reactant slowly
2
diffuses through the fine pores of the gel medium and the Terbium ions combines with oxalate ions in a controlled manner to form Terbium oxalate crystals which are shown in figure 1(a). 3. Results and Discussion 3.1 X-ray powder diffraction analysis The x-ray powder diffractogram of the grown Terbium oxalate crystals was recorded using PANalytical X’pert Pro X-ray diffractometer and the pattern displayed in figure 1(b) matches well with the standard data provided in the ICDD Card no.22-8487 of Terbium oxalate decahydrate. Accordingly the diffraction pattern was indexed to monoclinic structure with space group P21/c and the lattice parameters were evaluated using Unit Cell Win software and are a=10.99Å, b=9.611Å, c=10.02Å and =114.11. The calculated lattice parameters are in good agreement with that reported in the ICDD data and hence the formation of Tb2(C2O4)3.10H2O is confirmed. Single crystal structure of Terbium oxalate decahydrate given in figure 1(c) indicates that it crystallizes in monoclinic environment with three chelating oxalate groups bonded to the metal center along with three water molecules as coordinating ligands and the remaining seven are lattice water. The EDS spectrum depicted in figure 1(d) confirms the presence of Terbium, Oxygen and Carbon and no impurity peaks are present.
3
Figure 1. (a) Grown crystals (b) Powder x-ray diffractogram (c) Single crystal structure (d) EDS spectrum of Terbium oxalate crystals 3.2 Photoluminescence studies Photoluminescence excitation The photoluminescence excitation spectrum of the grown Terbium oxalate decahydrate crystals was recorded on a HORIBA Jobin-Yvon Flouromax-4 spectrofluorometer by monitoring the characteristic emission of terbium at 543nm corresponding to (5D47F5) transition and is shown in figure 2(a) which exhibits a relatively broad excitation band and a series of sharp peaks. The broad band in the region 230-300nm is attributed to the combined effect of 4f84f75d1 transitions of Tb3+ and Tb-O charge transfer [4]. The sharp peaks at 303nm, 317nm, 325nm, 340nm, 352nm, 359nm, 377nm and 488nm originates from ground state (7F6) and are assigned to the 7F65H6, 5H7, 5D1, 5L7, 5G4, 5G6, 5G8 and 5D4 transitions of the terbium ion respectively. The most intense peak located at 369nm corresponds to the 7
F65L10 transition [5]. The excitation spectrum clearly demonstrates that this new phosphor
material can be effectively excited by the commercially available LED sources at wave lengths 277nm, 375nm and 475nm. The extinction coefficient calculated from the transmittance spectrum is found to be 5.042x10-6 at λ=369nm [6].
Figure 2. (a) Excitation spectrum (b) Emission spectra of Terbium oxalate crystals 4
Photoluminescence emission Emission spectra of the Terbium oxalate crystals were recorded using a HORIBA JobinYvon Flouromax-4 spectrofluorometer at excitation wavelengths 283, 369 and 488nm representing the DUV, NUV and visible regions and are given in figure 2(b). The emission peaks are due to the radiative transitions of electrons in the 5D4 level to lower lying 7FJ multiplets and no emission from 5D3 level is observed. The peaks at 488nm, 543nm, 583nm and 618 nm correspond to weak blue, strong green, weak yellow and red regions of the visible spectrum and are assigned to the 5D47F6, 7F5, 7F4 and 7F3 transitions respectively and the transition
5
D47F5 is the most prominent one [5]. The predominance of the electric
dipole transition (5D47F5) in the emission spectra clearly implies that terbium ions are located at non-centro symmetric sites in the host lattice. The quantum efficiency of Terbium oxalate crystal is estimated to be 77% and is high compared to other Tb3+ doped systems, which confirms the suitability of this crystal for optoelectronic applications [7]. The photoluminescence analysis reveals the significance of Terbium oxalate decahydrate crystals as a potential candidate for LED applications due to the sharp and intense green emission and high value of quantum efficiency. Generally, higher concentration of rare earth ions and water molecules decreases the luminescence efficiency, but in the present case these luminescence limiting factors affect the green emission in a rewarding manner. The 5D35D4 transition of Tb3+ ion is in resonance with the 7F67F0 transition; hence the energy from the 5D35D4 transition is absorbed by a nearby Tb3+ ion via cross relaxation [8]. Higher concentration of rare earth ions enhances nonradiative cross relaxation thereby quenching the luminescence emission, which limits the practical application of rare earth doped systems. In spite of this here the higher rate of cross relaxation induced by the full concentration of terbium ions influences the emission in an 5
advantageous manner by completely emptying the 5D3 level by nonradiative decay to 5D4 level thereby promoting the population of 5D4 which results in a high intense green emission from this level.
This is evident from the emission spectra that the radiative emission
corresponding to the 5D37F6 transition of Tb3+ ion is completely absent. The energy level diagram of Terbium oxalate decahydrate explaining the cross relaxation process is presented schematically in figure 3(a). But the increased population of 5D4 level by this process alone is not sufficient to account for the observed high intense green emission considering the increased quenching possibility of this level due to the high Tb3+ concentration and the presence of water molecules. The water assisted energy transfer from oxalate ligand to the Tb3+ ions may be a possible mechanism for the enhanced luminescence of Terbium oxalate decahydrate [9, 10]. In this process the energy absorbed by the oxalate ligands, in a singletsinglet transition, populates the triplet state through an inter system crossing and then nonradiatively transfers energy to Tb3+ ions, leading to intense luminescence. This will happen only if the lowest triplet state energy level of the ligand is nearly equal to or above the resonance energy level of the rare earth ion. The lowest triplet energy level of oxalate ligand (23700cm-1) is about 3200 cm-1 higher than the 5D4 level of Tb3+ (20500cm-1), hence favours energy transfer from the oxalate ligand to Tb3+ ions [11]. 3.3 Decay analysis The decay curve recorded for the green emission (5D47F5) in Terbium oxalate crystals under 369nm excitation using a Edinburgh UV-VIS-NIR (FLS-980) spectrometer is depicted in figure 3(b). The experimental decay curve is single exponential and the decay time is observed to be 0.81ms which is suitable for LED applications. This single exponential behaviour in decay measurements clearly indicate that for Tb3+ ions there exists only one site of symmetry in oxalate lattice. The lower value of decay time confirms that the terbium ion
6
occupies a lower symmetric site in oxalate lattice and is consistent with the results obtained in emission spectrum analysis [12].
Figure 3.(a) Energy level diagram (b) Decay curve (c) CIE chromaticity diagram of Terbium oxalate crystals 3.4 CIE chromaticity diagram The colour performance of the phosphor was analysed using the CIE chromaticity diagram. The calculated chromaticity co-ordinates for Terbium oxalate was (0.31, 0.57) and the dominant wavelength was found to be 550nm corresponding to green phosphor and the corresponding CIE diagram is given in figure 3(c). The colour purity of our sample was calculated to be 68%. It is interesting to note that the chromaticity co-ordinates and colour purity of fully concentrated Terbium oxalate are very close to that for European Broadcasting union illuminant green [13]. 4. Conclusion Fully concentrated Terbium oxalate decahydrate crystals were synthesized and the structure was confirmed by powder x-ray studies. Photoluminescence investigations revealed that the sample has a high intense green emission at 543nm on exciting at 283nm, 369nm and 488nm
7
in which the 369nm yields maximum emission with a shorter decay time. The emission from 5
D3 level is absent but the nonradiative decay populates the 5D4 level by higher cross
relaxation rate. The energy transfer from oxalate ligand to the Tb3+ ions also assists the green luminescence of Terbium oxalate decahydrate. Thus higher green luminescence is achieved in fully concentrated rare earth matrix, despite the expected luminescence quenching. The sample also exhibits good colour purity, matching the standard values. The intense green emission, better colour purity and high quantum efficiency of Terbium oxalate decahydrate crystal suggest it as an efficient green phosphor for W-LED applications. It is worthwhile to further investigate in detail, the energy transfer from oxalate ligand to Tb3+ and the role of coordinated water molecule on the luminescence performance of this material, on the basis of crystal structure analysis.
References [1] C.Guantong, Z.Weidong, L.Ronghui, L.Yuanhonh, H.Yunsheng, H.Huaqiang, J. Rare Earths 31(2013) 944-949. [2] S.Setua, D.Menon, A.Ashok, S.Nair, M.Koyakutty, Biomaterials 31(2010)714-729. [3] F.Hong, L.Zhou, L. Li, Q Xia, M.Ye, Mater. Res. Bull. 60(2014)252-257. [4] L.P. Singh, N.P. Singh, K.Srivastava, Dalton Tran. 44(2015)6457-6465. [5] W.T.Carnall, P.R.Fields, K.Rajnak, J. Chem. Phys. 49(1968)4447-4449. [6] B.K.Periyasamy, R.S.Jebas, N.Gopalakrishnan, T.Balasubramanian, Mat. Let. 61(2007)4246-4249. [7] G.Wakefield, P.J.Dobson, J.L.Hutchison, J. Phys. Chem. Solids 60(1999)503–508. [8] F.Steudel, S.Loos, B.Ahrens, S.Schweizer, J. Lumin. 170(2016)770-777. [9] A.J.Calahorro, D.F.Jimenez, A.S.Castill, M.L.Viseras, A.R.Dieguez, Polyhedron 52(2013)315-320.
8
[10] A.I.Voloshin, N.M.Shavaleev, V.P. Kazakov, J. Photochem. Photobiol. A Chem. 134(2000)111-117. [11] P.Wang, R.Fan, X. Liu, L.Wang,Y.Yang,W. Cao, B.Yang, Q.W. Hasi, Q. Su, Y. Mu, CrystEngComm 15(2013)1931-1949. [12] D.F.Parra, A.Mucciolo, H.F.Britto, J. Appl. Polym. Sci. 94(2004)865-870. [13] J.J.Battalla, A.N.Meza-Rocha, G.Munoz, I.Camarillo, U.Caldino, Opt. Mater. 58(2016)406-411.
Highlights A novel fully concentrated intense green emitting phosphor is identified. First time report on the photoluminescence of Terbium oxalate decahydrate crystal. Efficient green luminescence is due to the ligand to Tb3+ energy transfer. The CIE chromaticity coordinates suggest enhanced color purity for green emission. Short decay time indicate the suitability for LED applications.
9
PII: DOI: Reference:
S0167-577X(16)31873-0 http://dx.doi.org/10.1016/j.matlet.2016.12.002 MLBLUE21814
To appear in: Materials Letters Received date: 18 September 2016 Revised date: 24 November 2016 Accepted date: 2 December 2016 Cite this article as: Dinu Alexander, Kukku Thomas, S Sisira, G Vimal, Kamal P Mani, P R Biju, N V Unnikrishnan, M A Ittyachen and Cyriac Joseph, Photoluminescence properties of fully concentrated Terbium oxalate: A novel efficient green emitting phosphor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.12.002 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 galley proof before it is published in its final citable 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.
Photoluminescence properties of fully concentrated Terbium oxalate: A novel efficient green emitting phosphor Dinu Alexander, Kukku Thomas, Sisira S, Vimal G, Kamal P Mani, P R Biju, N V Unnikrishnan, M A Ittyachen and Cyriac Joseph* School of Pure and Applied Physics, Mahatma Gandhi University, Kerala, India -686560
Abstract A novel green emitting phosphor, Terbium oxalate decahydrate, was synthesized in crystalline form employing the hydro silica gel method. The structure of the sample was confirmed by x-ray diffraction analysis. Photoluminescence excitation spectrum of the sample shows several peaks in the deep UV to Visible region, matching well with the commercially available LED sources. Emission spectrum, recorded with representative excitation wavelengths revealed efficient luminescence, particularly the green emission at 543nm. The decay time of the sample was measured to be 0.81ms. It exhibits a colour purity of 68% with chromaticity coordinates (0.31, 0.57) which is very close to that of the European Broadcasting Union illuminant green. Graphical Abstract
Keywords: Phosphors, Crystal growth, Optical materials and properties, Luminescence
1
1. Introduction Rare earth elements have been the focus of research towards technological inventions on account of their wide spread applications with utmost precision in properties. Rare earth based optical technology has thus become an emerging field of research due to its applications in light emitting diodes, electro luminescent displays and bio-imaging [1,2]. Excellent luminescence emission is inevitable for such applications but “concentration quenching” limits the same. Low doping of RE3+ in a host yields only weak emission while heavy doping reduces the intensity by concentration quenching. Even though there are numerous reports on rare earth doped phosphor materials, fully concentrated rare earth based phosphor materials have been less investigated despite its probable higher luminescence intensity which is a pre-requisite for an efficient phosphor material [3]. In order to explore the above feature, we have synthesised Terbium oxalate crystals and studied its luminescence characteristics. Terbium is one of the most promising rare earth elements which emit intense green, weak blue, yellow and red radiations, hence suitable for applications in fluorescent lamps, solar cells, plasma display panels, etc. This paper presents a first time report on the synthesis and photoluminescence studies of Terbium oxalate decahydrate crystals, a novel green emitting fully concentrated phosphor. 2. Experimental Terbium oxalate crystals were grown by single diffusion gel technique in which the hydro silica gel acts as a three dimensional frame work for the growth of perfect crystals. Well filtered aqueous solution of sodium meta silicate (Na2SiO3.9H2O, CDH) of specific gravity 1.03gm/cc was mixed with aqueous solution of 1M oxalic acid (H2C2O4.2H2O, Merck) to obtain gel with pH 7. The top solution, prepared by mixing equal volumes of 0.5M aqueous solution of terbium nitrate pentahydrate (Tb2 (NO3)3.5H2O, Sigma Aldrich) and concentrated nitric acid, was dripped gently through the walls of the tube. The outer reactant slowly
2
diffuses through the fine pores of the gel medium and the Terbium ions combines with oxalate ions in a controlled manner to form Terbium oxalate crystals which are shown in figure 1(a). 3. Results and Discussion 3.1 X-ray powder diffraction analysis The x-ray powder diffractogram of the grown Terbium oxalate crystals was recorded using PANalytical X’pert Pro X-ray diffractometer and the pattern displayed in figure 1(b) matches well with the standard data provided in the ICDD Card no.22-8487 of Terbium oxalate decahydrate. Accordingly the diffraction pattern was indexed to monoclinic structure with space group P21/c and the lattice parameters were evaluated using Unit Cell Win software and are a=10.99Å, b=9.611Å, c=10.02Å and =114.11. The calculated lattice parameters are in good agreement with that reported in the ICDD data and hence the formation of Tb2(C2O4)3.10H2O is confirmed. Single crystal structure of Terbium oxalate decahydrate given in figure 1(c) indicates that it crystallizes in monoclinic environment with three chelating oxalate groups bonded to the metal center along with three water molecules as coordinating ligands and the remaining seven are lattice water. The EDS spectrum depicted in figure 1(d) confirms the presence of Terbium, Oxygen and Carbon and no impurity peaks are present.
3
Figure 1. (a) Grown crystals (b) Powder x-ray diffractogram (c) Single crystal structure (d) EDS spectrum of Terbium oxalate crystals 3.2 Photoluminescence studies Photoluminescence excitation The photoluminescence excitation spectrum of the grown Terbium oxalate decahydrate crystals was recorded on a HORIBA Jobin-Yvon Flouromax-4 spectrofluorometer by monitoring the characteristic emission of terbium at 543nm corresponding to (5D47F5) transition and is shown in figure 2(a) which exhibits a relatively broad excitation band and a series of sharp peaks. The broad band in the region 230-300nm is attributed to the combined effect of 4f84f75d1 transitions of Tb3+ and Tb-O charge transfer [4]. The sharp peaks at 303nm, 317nm, 325nm, 340nm, 352nm, 359nm, 377nm and 488nm originates from ground state (7F6) and are assigned to the 7F65H6, 5H7, 5D1, 5L7, 5G4, 5G6, 5G8 and 5D4 transitions of the terbium ion respectively. The most intense peak located at 369nm corresponds to the 7
F65L10 transition [5]. The excitation spectrum clearly demonstrates that this new phosphor
material can be effectively excited by the commercially available LED sources at wave lengths 277nm, 375nm and 475nm. The extinction coefficient calculated from the transmittance spectrum is found to be 5.042x10-6 at λ=369nm [6].
Figure 2. (a) Excitation spectrum (b) Emission spectra of Terbium oxalate crystals 4
Photoluminescence emission Emission spectra of the Terbium oxalate crystals were recorded using a HORIBA JobinYvon Flouromax-4 spectrofluorometer at excitation wavelengths 283, 369 and 488nm representing the DUV, NUV and visible regions and are given in figure 2(b). The emission peaks are due to the radiative transitions of electrons in the 5D4 level to lower lying 7FJ multiplets and no emission from 5D3 level is observed. The peaks at 488nm, 543nm, 583nm and 618 nm correspond to weak blue, strong green, weak yellow and red regions of the visible spectrum and are assigned to the 5D47F6, 7F5, 7F4 and 7F3 transitions respectively and the transition
5
D47F5 is the most prominent one [5]. The predominance of the electric
dipole transition (5D47F5) in the emission spectra clearly implies that terbium ions are located at non-centro symmetric sites in the host lattice. The quantum efficiency of Terbium oxalate crystal is estimated to be 77% and is high compared to other Tb3+ doped systems, which confirms the suitability of this crystal for optoelectronic applications [7]. The photoluminescence analysis reveals the significance of Terbium oxalate decahydrate crystals as a potential candidate for LED applications due to the sharp and intense green emission and high value of quantum efficiency. Generally, higher concentration of rare earth ions and water molecules decreases the luminescence efficiency, but in the present case these luminescence limiting factors affect the green emission in a rewarding manner. The 5D35D4 transition of Tb3+ ion is in resonance with the 7F67F0 transition; hence the energy from the 5D35D4 transition is absorbed by a nearby Tb3+ ion via cross relaxation [8]. Higher concentration of rare earth ions enhances nonradiative cross relaxation thereby quenching the luminescence emission, which limits the practical application of rare earth doped systems. In spite of this here the higher rate of cross relaxation induced by the full concentration of terbium ions influences the emission in an 5
advantageous manner by completely emptying the 5D3 level by nonradiative decay to 5D4 level thereby promoting the population of 5D4 which results in a high intense green emission from this level.
This is evident from the emission spectra that the radiative emission
corresponding to the 5D37F6 transition of Tb3+ ion is completely absent. The energy level diagram of Terbium oxalate decahydrate explaining the cross relaxation process is presented schematically in figure 3(a). But the increased population of 5D4 level by this process alone is not sufficient to account for the observed high intense green emission considering the increased quenching possibility of this level due to the high Tb3+ concentration and the presence of water molecules. The water assisted energy transfer from oxalate ligand to the Tb3+ ions may be a possible mechanism for the enhanced luminescence of Terbium oxalate decahydrate [9, 10]. In this process the energy absorbed by the oxalate ligands, in a singletsinglet transition, populates the triplet state through an inter system crossing and then nonradiatively transfers energy to Tb3+ ions, leading to intense luminescence. This will happen only if the lowest triplet state energy level of the ligand is nearly equal to or above the resonance energy level of the rare earth ion. The lowest triplet energy level of oxalate ligand (23700cm-1) is about 3200 cm-1 higher than the 5D4 level of Tb3+ (20500cm-1), hence favours energy transfer from the oxalate ligand to Tb3+ ions [11]. 3.3 Decay analysis The decay curve recorded for the green emission (5D47F5) in Terbium oxalate crystals under 369nm excitation using a Edinburgh UV-VIS-NIR (FLS-980) spectrometer is depicted in figure 3(b). The experimental decay curve is single exponential and the decay time is observed to be 0.81ms which is suitable for LED applications. This single exponential behaviour in decay measurements clearly indicate that for Tb3+ ions there exists only one site of symmetry in oxalate lattice. The lower value of decay time confirms that the terbium ion
6
occupies a lower symmetric site in oxalate lattice and is consistent with the results obtained in emission spectrum analysis [12].
Figure 3.(a) Energy level diagram (b) Decay curve (c) CIE chromaticity diagram of Terbium oxalate crystals 3.4 CIE chromaticity diagram The colour performance of the phosphor was analysed using the CIE chromaticity diagram. The calculated chromaticity co-ordinates for Terbium oxalate was (0.31, 0.57) and the dominant wavelength was found to be 550nm corresponding to green phosphor and the corresponding CIE diagram is given in figure 3(c). The colour purity of our sample was calculated to be 68%. It is interesting to note that the chromaticity co-ordinates and colour purity of fully concentrated Terbium oxalate are very close to that for European Broadcasting union illuminant green [13]. 4. Conclusion Fully concentrated Terbium oxalate decahydrate crystals were synthesized and the structure was confirmed by powder x-ray studies. Photoluminescence investigations revealed that the sample has a high intense green emission at 543nm on exciting at 283nm, 369nm and 488nm
7
in which the 369nm yields maximum emission with a shorter decay time. The emission from 5
D3 level is absent but the nonradiative decay populates the 5D4 level by higher cross
relaxation rate. The energy transfer from oxalate ligand to the Tb3+ ions also assists the green luminescence of Terbium oxalate decahydrate. Thus higher green luminescence is achieved in fully concentrated rare earth matrix, despite the expected luminescence quenching. The sample also exhibits good colour purity, matching the standard values. The intense green emission, better colour purity and high quantum efficiency of Terbium oxalate decahydrate crystal suggest it as an efficient green phosphor for W-LED applications. It is worthwhile to further investigate in detail, the energy transfer from oxalate ligand to Tb3+ and the role of coordinated water molecule on the luminescence performance of this material, on the basis of crystal structure analysis.
References [1] C.Guantong, Z.Weidong, L.Ronghui, L.Yuanhonh, H.Yunsheng, H.Huaqiang, J. Rare Earths 31(2013) 944-949. [2] S.Setua, D.Menon, A.Ashok, S.Nair, M.Koyakutty, Biomaterials 31(2010)714-729. [3] F.Hong, L.Zhou, L. Li, Q Xia, M.Ye, Mater. Res. Bull. 60(2014)252-257. [4] L.P. Singh, N.P. Singh, K.Srivastava, Dalton Tran. 44(2015)6457-6465. [5] W.T.Carnall, P.R.Fields, K.Rajnak, J. Chem. Phys. 49(1968)4447-4449. [6] B.K.Periyasamy, R.S.Jebas, N.Gopalakrishnan, T.Balasubramanian, Mat. Let. 61(2007)4246-4249. [7] G.Wakefield, P.J.Dobson, J.L.Hutchison, J. Phys. Chem. Solids 60(1999)503–508. [8] F.Steudel, S.Loos, B.Ahrens, S.Schweizer, J. Lumin. 170(2016)770-777. [9] A.J.Calahorro, D.F.Jimenez, A.S.Castill, M.L.Viseras, A.R.Dieguez, Polyhedron 52(2013)315-320.
8
[10] A.I.Voloshin, N.M.Shavaleev, V.P. Kazakov, J. Photochem. Photobiol. A Chem. 134(2000)111-117. [11] P.Wang, R.Fan, X. Liu, L.Wang,Y.Yang,W. Cao, B.Yang, Q.W. Hasi, Q. Su, Y. Mu, CrystEngComm 15(2013)1931-1949. [12] D.F.Parra, A.Mucciolo, H.F.Britto, J. Appl. Polym. Sci. 94(2004)865-870. [13] J.J.Battalla, A.N.Meza-Rocha, G.Munoz, I.Camarillo, U.Caldino, Opt. Mater. 58(2016)406-411.
Highlights A novel fully concentrated intense green emitting phosphor is identified. First time report on the photoluminescence of Terbium oxalate decahydrate crystal. Efficient green luminescence is due to the ligand to Tb3+ energy transfer. The CIE chromaticity coordinates suggest enhanced color purity for green emission. Short decay time indicate the suitability for LED applications.
9