Research article Received: 2 February 2013,
Revised: 8 August 2013,
Accepted: 7 October 2013
Published online in Wiley Online Library: 21 November 2013
(wileyonlinelibrary.com) DOI 10.1002/bio.2605
Synthesis and characterization of Y(1-x)Eu(x) (TTA)3(Phen) organic luminescent thin films N. Thejo Kalyani,a R. G. Atramb and S. J. Dhobleb* ABSTRACT: Yttrium is stoichiometrically doped into europium by mole percentage, during the synthesis of Y(1-x)Eu(x)(TTA)3 (Phen), using solution techniques (where x = 0.2, 0.4, 0.5, 0.6 and 0.8, TTA = thenoyltrifluoroacetone and Phen = 1,10phenanthroline).These complexes were characterized using different techniques such as X-ray diffraction, thermogravimetric/ differential thermal analysis, optical absorption and emission spectra. Thin films of the doped Eu–Y complexes were prepared on a glass substrate under a high vacuum of 10-6 Torr. The photoluminescence spectra of these thin films were recorded by exciting the sample at a wavelength of 360 nm. The emission peak for all the synthesized complexes centered at 611 nm; maximum emission intensity was obtained from Y0.6Eu0.4 (TTA)3(Phen). The results proved that these doped complexes are more economical than pure Eu(TTA)3(Phen) and are best suited as red emissive material for energy-efficient and eco-friendly organic light-emitting diodes and displays. Copyright © 2013 John Wiley & Sons, Ltd. Keywords: synthesis; OLED; displays; photoluminescence; lamp phosphor
Introduction Despite developments in flat panel displays, a great challenge in the field of organic light-emitting diodes (OLEDs) is the realization of an efficient pure red LED with a narrow emission line. Use of an organic Eu3+ complex may solve the problem to some extent (1–6). However, issues regarding the red emitter remain, such as color purity, stability and efficiency (7,8). Rare earth (RE) complexes such as europium (Eu) and samarium are attractive because of their large Stokes shift values, narrow emission bandwidths and long emission lifetimes, which make them suitable as candidates for diodes and displays (9). Eu complexes have been used as emitters to obtain a pure red emitting device. Eu β-diketonates can also be applied for microcavity remitters (10), in laser materials and cathodo-luminescent display phosphor screens because of their high photoluminescence (PL) quantum efficiencies and very sharp spectral lines (11). Hence, it is proposed to synthesize and characterize red-emitting organic complexes based on Eu. Yttrium (Y) is stoichiometrically doped into Eu by mole percentage, during the synthesis of Y(1-x)Eu(x)(TTA)3(Phen) using solution techniques. The X-ray diffraction (XRD) patterns of Y(1-x)Eu(x)(TTA)3 (Phen) powders were recorded on a Philips PW 1700 powder diffractometer Simultaneous thermogravimetric/differential thermal analysis (TGA/DTA) measurements were recorded on a Setaram TG 92 Mottlor Toller Star System in a nitrogen atmosphere. Absorption and PL measurements for a thin film of Y(1-x)Eu(x)(TTA)3(Phen) complexes were carried out on a SPECORD 50 spectrophotometer and spectrofluorometer respectively. These complexes have good solvability in basic and acidic solvents (12,13). Red OLEDs using these complexes as the emissive layer have been reported previously (14).
Experimental Synthesis of Y(1-x)Eu(x)(TTA)3(Phen) organic red phosphors
674
Y is stoichiometrically doped into Eu by mole percentage, during the synthesis of Y(1-x)Eu(x)(TTA)3(Phen), using solution techniques. The synthesis of Eu(TTA)3(Phen) is shown in Fig. 1.
Luminescence 2014; 29: 674–678
The chemical constituents for the synthesis of Y(1-x)Eu(x)(TTA)3 (Phen) complexes are given in Table 1. The chemical structure of Y(1-x)Eu(x)(TTA)3(Phen) is shown in Fig. 2. Preparation of thin film A thin film of the complex was prepared on a glass substrate using the vacuum evaporation technique. Glass substrates, 14 × 14 mm, were cleaned in methanol or alternatively in ether. The vacuum chamber was cleaned very carefully so that no particles remained inside the chamber. The Y(1-x)Eu(x)(TTA)3(Phen) powder was kept in a boat made of molybdenum metal, which has a high melting point (1400°C). The boat containing the powder sample was placed on the coil, which is connected to the electrode. After attaining a vacuum of 10-6 Torr, the current was increased very slowly until the counting began. The quartz crystal and glass substrate were placed at the same height from the boat. After a fixed count had been measured by the frequency counter, the current was decreased slowly to zero. Later, the vacuum chamber cock was closed and the films on the glass substrate were removed and kept in a desiccator. The thickness of the thin film was found to be 1000 Å.
Results and discussion XRD measurement The polycrystalline nature of the synthesized complexes was analysed using a XRD technique. XRD is a versatile, non-destructive
* Correspondence to: SJ Dhoble, Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India. E-mail: [email protected] a
Department of Applied Physics, Laxminarayan Institute of Technology, Nagpur, India
b
Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India
Copyright © 2013 John Wiley & Sons, Ltd.
Y(1-x)Eu(x)(TTA)3(Phen) organic luminescent thin films
Figure 1. Synthesis of Y(1-x)Eu(x)(TTA)3(Phen).
Table 1. The composition of the chemical constituent for the synthesis of Y(1-x) Eu(x)(TTA)3(Phen)complexes Y(1-x) Eu(x)(TTA)3(Phen)complexes
TTA (g)
Phen (g)
EuCl3 (g)
YCl3 (g)
x = 0.8 x = 0.6 x = 0.5 x = 0.4 x = 0.2
1.4725 1.4725 1.4725 1.4725 1.4725
0.3979 0.3979 0.3979 0.3979 0.3979
0.4566 0.3424 0.2854 0.2283 0.1141
0.0863 0.1726 0.2157 0.2589 0.3452
Figure 2. Chemical structure of Y(1-x)Eu(x)(TTA)3(Phen), Y and Eu are used in different mole proportions (x = 0.8, 0.6, 0.5, 0.4, 0.2).
analytical technique for the qualitative identification and quantitative estimation of various crystalline forms present in powder solid samples. It is the right tool for investigating the fine structure of matter. XRD measurements for doped complexes at different stoichiometric ratios of Eu and Y were recorded on a Philips PW 1700 and are shown in Fig. 3. The XRD results gave d-values for 100% relative intensity. Because all the diffractograms show well-resolved multiple peaks, the synthesized complexes are polycrystalline in nature. From the XRD results, d-values were also observed for 100% relative intensity. XRD measurements for Y(1-x)Eu(x)(TTA)3(Phen) are given in Table 2. From Table 2, it can be seen that the maximum d-value for 100% relative intensity is observed in complex Y0.8Eu0.2(TTA)3 (Phen). Different d-values indicate the formation of doped complexes. Variations in the d-value at different concentrations of Y3+ are shown in Fig. 4. TGA and DTA measurement
Luminescence 2014; 29: 674–678
Optical absorption spectra The absorption spectra of the Y-doped Eu(TTA)3(Phen) thin film was compared with a pure Eu(TTA)3(Phen) film of the same thickness (1000 Å), as shown in Fig. 6. The maximum optical density was found for Y0.6Eu0.4(TTA)3(Phen). The optical density of Y3+-doped complexes is shown to be maximal compared with a pure complex of Eu(TTA)3(Phen), except for Y0.8Eu0.2(TTA)3 (Phen) for which the optical density is minimal. The optical density is greater for Y0.6Eu0.4(TTA)3(Phen) than for other samples, which indicates that more molecules are excited at this particular doping. A prominent absorption band was observed at 349 nm. Absorption spectra are only characteristic of the aromatic groups of the β-diketones. In all Y-doped complexes the β-diketone is TTA. Hence, there was no appreciable shift in the wavelength maxima, only the change in the optical density which depends on the concentration of the Eu3+ ion. Comparative absorption spectra for thin films of Y(1-x)Eu(x)(TTA)3(Phen) and Eu(TTA)3(Phen) are shown in Fig. 6.
Copyright © 2013 John Wiley & Sons, Ltd.
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TGA/DTA results for Y(1-x)Eu(x)(TTA)3(Phen) are shown in Fig. 5. The curved position indicates a weight loss. From this, mportant
information can be obtained about how much weight is lost on heating a sample at a given temperature. The complex is thermally stable up to the decomposition temperature. The thermal stability of the complexes was determined by TGA measurements, whereas the formation and melting point of the complexes were determined by DTA measurements. Results for the melting temperature and thermal stability are given in Table 3. Thermal stability and melting point increased with increasing concentrations of Y doped in Eu. Among all the synthesized complexes, Eu0.2Y0.8(TTA)3(Phen) showed the highest thermal stability and melting point. Because of its very short ionic radius, Y3+ can form very strong bonds, hence as concentration of Y is increased, stronger and stronger bonds with Eu, TTA and Phen are formed, leading to an increase in the thermal stability and melting point.
N. T. Kalyani et al.
Figure 3. Diffractograms of Y(1-x)Eu(x)(TTA)3(Phen), where x = 0.2, 0.4, 0.5, 0.6 and 0.8.
Table 2. XRD measurement of Y(1-x) Eu(x)(TTA)3(Phen) Complex Y0.2 Y0.4 Y0.5 Y0.6 Y0.8
Eu0.8 Eu0.6 Eu0.5 Eu0.4 Eu0.2
Angle (2θ)
d
Peak intensity (counts)
21.415 21.355 21.330 21.390 21.405
4.1459 4.1574 4.1622 4.1506 4.1478
289 388 376 396 506
(TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen)
4.1643
d-values
4.1603 4.1563 Figure 5. (a) TGA of Y(1-x)Eu(x)(TTA)3(Phen), (b) DTA of Y(1-x)Eu(x)(TTA)3(Phen).
4.1523 4.1483 4.1443
Excitation and emission measurement 0
0.2
0.4
0.6
0.8
Y mole %
676
Figure 4. Variation in d-values with Y mole % in Y(1-x)Eu(x)(TTA)3(Phen).
wileyonlinelibrary.com/journal/luminescence
Excitation and emission measurements for thin film of the complexes were studied using a conventional fluorescence spectroflurometer. Figure 7 shows the excitation spectrum for a thin film of the complexes Y(1-x)Eu(x)(TTA)3(Phen) and Eu(TTA)
Copyright © 2013 John Wiley & Sons, Ltd.
Luminescence 2014; 29: 674–678
Y(1-x)Eu(x)(TTA)3(Phen) organic luminescent thin films Table 3. Thermal stability and melting point of Eu(x)Y(1-x) (TTA)3(Phen) Complex Eu0.8Y0.2 Eu0.6Y0.4 Eu0.5Y0.5 Eu0.4Y0.6 Eu0.2Y0.8
(TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen)
Thermal stability (°C)
Melting point (°C)
335.62 335.26 336.54 341.20 344.00
245.07 245.53 247.45 246.74 250.52
Figure 8. Emission spectra for a Y(1-x)Eu(x)(TTA)3(Phen) thin film.
Figure 6. Absorption spectrum for a thin film of pure and Y-doped Eu(TTA)3 (Phen).
Figure 9. Photophysical processes in lanthanide β-diketonate complexes (antenna effect). A, absorption; F, fluorescence; P, phosphorescence; L, lanthanidecentered luminescence; ISC, intersystem crossing; ET, energy transfer (18). Figure 7. Excitation spectrum for a Y(1-x)Eu(x)(TTA)3(Phen) thin film.
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3(Phen). These complexes are excited by light in the range 250–500 nm, monitored at 611 nm. The excitation peaks for all these complexes were observed at 360 nm. Maximum PL intensity is observed for Y0.6Eu0.4(TTA)3(Phen) in the excitation spectrum. Figure 8 shows the emission spectra for thin films of Eu(x)Y(1-x)(TTA)3(Phen) and Eu(TTA)3(Phen). Emission peaks at 591, 612, 618 and 627 nm, due to the transition 5D0 → 7Fj (j = 1, 2, 3, 4), were observed in all these complexes. Maximum peak intensity was observed at 612 nm due to the 5D0 → 7F2 transition. In Y(1-x)Eu(x)(TTA)3(Phen) series, maximum PL intensity was observed for Eu0.4Y0.6(TTA)3(Phen). It can be seen that there is a enhancement of the PL intensity (emission intensity) in Y(1-x)Eu(x)(TTA)3(Phen) due to efficient energy transfer from Y3+ to Eu3+; the exception is Y0.8Eu0.2(TTA)3 (Phen). When the PL intensity of Y(1-x)Eu(x)(TTA)3(Phen) is compared with that of Eu(TTA)3(Phen) the maximum emission
output was found in Y0.6Eu0.4(TTA)3(Phen). It seems that there is a nonradiative energy transfer from the enhancing ion Y3+ to Eu3+ in the doped complexes. It is well known that the Y3+ does not possess the electronic structure of the 4f shell. The lowest excited energy level of Y3+ is more than 32 × 10-3 cm-1. These energy levels are much higher than the lowest excited triplet level T (~ 20.3 × 10-3 cm-1) of the β-diketone (TTA) (15,16). The nature of the Eu3+ transition 5D0 → 7F2 is responsible for the sharp spectral line at 611 nm. The excited state 5D0 of Eu3+ has long lifetime (ms) and because of this, there is efficient energy transfer from excited Y3+ to Eu3+, which results in an enhancement of PL when compared with the reference sample. Y(III), La(III) and Tb(III) play a key role in enhancing the luminescence in a micelle solution and proved that efficient and brighter luminescence and economical complexes can be obtained by selecting an appropriate ligand and the introduction of another metal ion into the complexes (17–20). The photophysical process in lanthanide β-diketonate complexes leading to lanthanide-centered luminescence is shown in Fig. 9.
N. T. Kalyani et al. Absorption of electrons takes place in singlet states, i.e. from S0 to S1 of the ligand, and reaches T1 via intersystem crossing (ISC). A part of the energy absorbed by the electrons comes to the ground state S0 of the ligand. While crossing, the remaining energy is transferred to the lanthanide ion, leading to luminescence, also known as the antenna effect. So, in order to check the enhancement in the emission intensity of Y(1-x) Eu(x)(TTA)3Phen in the solid form, Y is stoichiometrically doped into Eu.
Conclusions From above studies, it can be concluded that all the doped Y(1-x) Eu(x)(TTA)3(Phen) complexes were crystalline in nature. TGA shows that thermal stability of Y(1-x)Eu(x)(TTA)3(Phen) phosphors ranged between 335.26 and 344°C. Broad absorption peaks were observed at 349 nm for all thin films of Y-doped Eu complexes. No shift in absorption peak maximum was observed, indicating that the absorption peak did not depend on the metal ion, but on aromatic group of the β-diketone (TTA). However, variations in optical density were observed, which may due to the different concentrations of metal ions. A prominent sharp red emission line was observed at 611 nm, when excited at 360 nm. This shows that the emission intensity increases for the Y(1-x)Eu(x) series of complexes, when compared with pure Eu(TTA)3(Phen). the maximum emission intensity was observed for Y0.6Eu0.4 (TTA)3(Phen). Hence, these complexes can be employed in ecofriendly and energy-saving OLEDs and also in other optical devices, where intense red emission light is required.
References 1. McGehee MD, Bergstedt T, Zhang C, Saab AP, O’Regan MB, Bazan GC, et al. Narrow bandwidth luminescence from blends with energy transfer from semiconducting conjugated polymers to europium complexes. Adv Mater 1999;11:1349–54. 2. Zhao D, Hong Z, Liang C, Zhao D, Liu X, Li W, et al. Enhanced electroluminescence of europium(III) complex by terbium(III) substitution in organic light emitting diodes. Thin Solid Films 2000;363:208–10.
3. Liang CJ, Hong ZR, Lin XY, Zhao DX, Li WL, Peng JB, et al. Organic electroluminescent devices using europium. Thin Solid Films 2000;359:14–16. 4. Pyo SW, Lee SP, Lee HS, Kwon OK, Hoe HS, Lee SH, et al. Synthesis and light-emitting properties of C60-containing poly(1-phenyl-1butyne)s. Thin Solid Films 2000;363:232–5. 5. Zhu W, Jiang Q, Lu Z, Wei X, Xie M, Zou D, et al. Red electroluminescence from a novel europium β-diketone complex with acylpyrazolone ligand, with acylpyrazolone ligand. Synthetic Met 2000;111:445–7. 6. Zhao D, Li W, Hong Z, Liu X, Liang C, Zhao D. White light emitting organic electroluminescent devices using lanthanide binuclear complexes. J Lumin 1999;82:2105–9. 7. Burrows PE, Forest SR, Sibely SP, Thompson ME. Color-tunable organic light-emitting devices. Appl Phys Lett 1996;69:2959–61. 8. Yoshino K, Fujii A, Nakayama H, Lee S, Naka A, Ishikawa M. Optical properties of poly(disilanyleneoligothienylene)s and their doping characteristics. J Appl Phys 1999;85:414–18. 9. Kuriki K, Koike Y, Okamoto Y. Plastic optical fiber lasers and amplifiers containing lanthanide complexes. Chem Rev 2002;102:2347–56. 10. Kido J, Hayase H, Hongawa K, Nagai K, Okuana K. Bright red lightemitting organic electroluminescent devices having a europium complex as an emitter. Appl Phys Lett 1994;65:2124–8. 11. Blasse G Luminescence of rare earth ions at the end of the century. J Alloy Compd 1993:193;17–21. 12. Kalyani NT, Dhoble SJ, Pode RB. Enhancement of photoluminescence in various EuxRe(1-x)TTA3Phen(Re = Y, Tb) complexes molecularly doped in PMMA. Ind J Phys 2012;86:613–18. 13. Kalyani NT, Dhoble SJ, Ahn J-S, Pode RB. Optical properties of EuxRe (1 x)(TTA)3Phen organic complexes in different solvents. J Korean Phys Soc 2010; 57:746–51. 14. Kalyani NT, Dhoble SJ. Organic light emitting diodes: energy saving lighting technology—a review. Renew Sust Energ Rev 2012;16:2696–723. 15. Metlay M. The fluorescence of the europium and terbium dibenzoylmethides. J Electrochem Soc 1964;111:1253–5. 16. Crosby GA, Whan RE, Alire RM. Intramolecular energy transfer in rare earth chelates. Role of the triplet state. J Chem Phys 1961;34:743–51. 17. Binnemans K. Rare-earth beta-diketonates. In: Gschneidner KA, Bunzli J-C, Pecharsky VK, editors. Handbook on the physics and chemistry of rare earths, Vol. 35. Amsterdam: Elsevier, 1976:107–272. 18. Kido J, Nagai K, Okamoto Y. Organic electroluminescent devices using lanthanide complexes. J Alloy Compd 1993;192:30–3. 19. Kido J, Nagai K, Okamoto Y. Electroluminescence from polysilane film doped with europium complex. Chem Lett 1991;235:1267–70. 20. Zhang X, Sun R, Zheng Q, Kobayashi T, Li W. Temperature-dependent electroluminescence from (Eu,Gd) coordination complexes. Appl Phys Lett 1997:71;2596.
678 wileyonlinelibrary.com/journal/luminescence
Copyright © 2013 John Wiley & Sons, Ltd.
Luminescence 2014; 29: 674–678
Revised: 8 August 2013,
Accepted: 7 October 2013
Published online in Wiley Online Library: 21 November 2013
(wileyonlinelibrary.com) DOI 10.1002/bio.2605
Synthesis and characterization of Y(1-x)Eu(x) (TTA)3(Phen) organic luminescent thin films N. Thejo Kalyani,a R. G. Atramb and S. J. Dhobleb* ABSTRACT: Yttrium is stoichiometrically doped into europium by mole percentage, during the synthesis of Y(1-x)Eu(x)(TTA)3 (Phen), using solution techniques (where x = 0.2, 0.4, 0.5, 0.6 and 0.8, TTA = thenoyltrifluoroacetone and Phen = 1,10phenanthroline).These complexes were characterized using different techniques such as X-ray diffraction, thermogravimetric/ differential thermal analysis, optical absorption and emission spectra. Thin films of the doped Eu–Y complexes were prepared on a glass substrate under a high vacuum of 10-6 Torr. The photoluminescence spectra of these thin films were recorded by exciting the sample at a wavelength of 360 nm. The emission peak for all the synthesized complexes centered at 611 nm; maximum emission intensity was obtained from Y0.6Eu0.4 (TTA)3(Phen). The results proved that these doped complexes are more economical than pure Eu(TTA)3(Phen) and are best suited as red emissive material for energy-efficient and eco-friendly organic light-emitting diodes and displays. Copyright © 2013 John Wiley & Sons, Ltd. Keywords: synthesis; OLED; displays; photoluminescence; lamp phosphor
Introduction Despite developments in flat panel displays, a great challenge in the field of organic light-emitting diodes (OLEDs) is the realization of an efficient pure red LED with a narrow emission line. Use of an organic Eu3+ complex may solve the problem to some extent (1–6). However, issues regarding the red emitter remain, such as color purity, stability and efficiency (7,8). Rare earth (RE) complexes such as europium (Eu) and samarium are attractive because of their large Stokes shift values, narrow emission bandwidths and long emission lifetimes, which make them suitable as candidates for diodes and displays (9). Eu complexes have been used as emitters to obtain a pure red emitting device. Eu β-diketonates can also be applied for microcavity remitters (10), in laser materials and cathodo-luminescent display phosphor screens because of their high photoluminescence (PL) quantum efficiencies and very sharp spectral lines (11). Hence, it is proposed to synthesize and characterize red-emitting organic complexes based on Eu. Yttrium (Y) is stoichiometrically doped into Eu by mole percentage, during the synthesis of Y(1-x)Eu(x)(TTA)3(Phen) using solution techniques. The X-ray diffraction (XRD) patterns of Y(1-x)Eu(x)(TTA)3 (Phen) powders were recorded on a Philips PW 1700 powder diffractometer Simultaneous thermogravimetric/differential thermal analysis (TGA/DTA) measurements were recorded on a Setaram TG 92 Mottlor Toller Star System in a nitrogen atmosphere. Absorption and PL measurements for a thin film of Y(1-x)Eu(x)(TTA)3(Phen) complexes were carried out on a SPECORD 50 spectrophotometer and spectrofluorometer respectively. These complexes have good solvability in basic and acidic solvents (12,13). Red OLEDs using these complexes as the emissive layer have been reported previously (14).
Experimental Synthesis of Y(1-x)Eu(x)(TTA)3(Phen) organic red phosphors
674
Y is stoichiometrically doped into Eu by mole percentage, during the synthesis of Y(1-x)Eu(x)(TTA)3(Phen), using solution techniques. The synthesis of Eu(TTA)3(Phen) is shown in Fig. 1.
Luminescence 2014; 29: 674–678
The chemical constituents for the synthesis of Y(1-x)Eu(x)(TTA)3 (Phen) complexes are given in Table 1. The chemical structure of Y(1-x)Eu(x)(TTA)3(Phen) is shown in Fig. 2. Preparation of thin film A thin film of the complex was prepared on a glass substrate using the vacuum evaporation technique. Glass substrates, 14 × 14 mm, were cleaned in methanol or alternatively in ether. The vacuum chamber was cleaned very carefully so that no particles remained inside the chamber. The Y(1-x)Eu(x)(TTA)3(Phen) powder was kept in a boat made of molybdenum metal, which has a high melting point (1400°C). The boat containing the powder sample was placed on the coil, which is connected to the electrode. After attaining a vacuum of 10-6 Torr, the current was increased very slowly until the counting began. The quartz crystal and glass substrate were placed at the same height from the boat. After a fixed count had been measured by the frequency counter, the current was decreased slowly to zero. Later, the vacuum chamber cock was closed and the films on the glass substrate were removed and kept in a desiccator. The thickness of the thin film was found to be 1000 Å.
Results and discussion XRD measurement The polycrystalline nature of the synthesized complexes was analysed using a XRD technique. XRD is a versatile, non-destructive
* Correspondence to: SJ Dhoble, Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India. E-mail: [email protected] a
Department of Applied Physics, Laxminarayan Institute of Technology, Nagpur, India
b
Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India
Copyright © 2013 John Wiley & Sons, Ltd.
Y(1-x)Eu(x)(TTA)3(Phen) organic luminescent thin films
Figure 1. Synthesis of Y(1-x)Eu(x)(TTA)3(Phen).
Table 1. The composition of the chemical constituent for the synthesis of Y(1-x) Eu(x)(TTA)3(Phen)complexes Y(1-x) Eu(x)(TTA)3(Phen)complexes
TTA (g)
Phen (g)
EuCl3 (g)
YCl3 (g)
x = 0.8 x = 0.6 x = 0.5 x = 0.4 x = 0.2
1.4725 1.4725 1.4725 1.4725 1.4725
0.3979 0.3979 0.3979 0.3979 0.3979
0.4566 0.3424 0.2854 0.2283 0.1141
0.0863 0.1726 0.2157 0.2589 0.3452
Figure 2. Chemical structure of Y(1-x)Eu(x)(TTA)3(Phen), Y and Eu are used in different mole proportions (x = 0.8, 0.6, 0.5, 0.4, 0.2).
analytical technique for the qualitative identification and quantitative estimation of various crystalline forms present in powder solid samples. It is the right tool for investigating the fine structure of matter. XRD measurements for doped complexes at different stoichiometric ratios of Eu and Y were recorded on a Philips PW 1700 and are shown in Fig. 3. The XRD results gave d-values for 100% relative intensity. Because all the diffractograms show well-resolved multiple peaks, the synthesized complexes are polycrystalline in nature. From the XRD results, d-values were also observed for 100% relative intensity. XRD measurements for Y(1-x)Eu(x)(TTA)3(Phen) are given in Table 2. From Table 2, it can be seen that the maximum d-value for 100% relative intensity is observed in complex Y0.8Eu0.2(TTA)3 (Phen). Different d-values indicate the formation of doped complexes. Variations in the d-value at different concentrations of Y3+ are shown in Fig. 4. TGA and DTA measurement
Luminescence 2014; 29: 674–678
Optical absorption spectra The absorption spectra of the Y-doped Eu(TTA)3(Phen) thin film was compared with a pure Eu(TTA)3(Phen) film of the same thickness (1000 Å), as shown in Fig. 6. The maximum optical density was found for Y0.6Eu0.4(TTA)3(Phen). The optical density of Y3+-doped complexes is shown to be maximal compared with a pure complex of Eu(TTA)3(Phen), except for Y0.8Eu0.2(TTA)3 (Phen) for which the optical density is minimal. The optical density is greater for Y0.6Eu0.4(TTA)3(Phen) than for other samples, which indicates that more molecules are excited at this particular doping. A prominent absorption band was observed at 349 nm. Absorption spectra are only characteristic of the aromatic groups of the β-diketones. In all Y-doped complexes the β-diketone is TTA. Hence, there was no appreciable shift in the wavelength maxima, only the change in the optical density which depends on the concentration of the Eu3+ ion. Comparative absorption spectra for thin films of Y(1-x)Eu(x)(TTA)3(Phen) and Eu(TTA)3(Phen) are shown in Fig. 6.
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TGA/DTA results for Y(1-x)Eu(x)(TTA)3(Phen) are shown in Fig. 5. The curved position indicates a weight loss. From this, mportant
information can be obtained about how much weight is lost on heating a sample at a given temperature. The complex is thermally stable up to the decomposition temperature. The thermal stability of the complexes was determined by TGA measurements, whereas the formation and melting point of the complexes were determined by DTA measurements. Results for the melting temperature and thermal stability are given in Table 3. Thermal stability and melting point increased with increasing concentrations of Y doped in Eu. Among all the synthesized complexes, Eu0.2Y0.8(TTA)3(Phen) showed the highest thermal stability and melting point. Because of its very short ionic radius, Y3+ can form very strong bonds, hence as concentration of Y is increased, stronger and stronger bonds with Eu, TTA and Phen are formed, leading to an increase in the thermal stability and melting point.
N. T. Kalyani et al.
Figure 3. Diffractograms of Y(1-x)Eu(x)(TTA)3(Phen), where x = 0.2, 0.4, 0.5, 0.6 and 0.8.
Table 2. XRD measurement of Y(1-x) Eu(x)(TTA)3(Phen) Complex Y0.2 Y0.4 Y0.5 Y0.6 Y0.8
Eu0.8 Eu0.6 Eu0.5 Eu0.4 Eu0.2
Angle (2θ)
d
Peak intensity (counts)
21.415 21.355 21.330 21.390 21.405
4.1459 4.1574 4.1622 4.1506 4.1478
289 388 376 396 506
(TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen)
4.1643
d-values
4.1603 4.1563 Figure 5. (a) TGA of Y(1-x)Eu(x)(TTA)3(Phen), (b) DTA of Y(1-x)Eu(x)(TTA)3(Phen).
4.1523 4.1483 4.1443
Excitation and emission measurement 0
0.2
0.4
0.6
0.8
Y mole %
676
Figure 4. Variation in d-values with Y mole % in Y(1-x)Eu(x)(TTA)3(Phen).
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Excitation and emission measurements for thin film of the complexes were studied using a conventional fluorescence spectroflurometer. Figure 7 shows the excitation spectrum for a thin film of the complexes Y(1-x)Eu(x)(TTA)3(Phen) and Eu(TTA)
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Luminescence 2014; 29: 674–678
Y(1-x)Eu(x)(TTA)3(Phen) organic luminescent thin films Table 3. Thermal stability and melting point of Eu(x)Y(1-x) (TTA)3(Phen) Complex Eu0.8Y0.2 Eu0.6Y0.4 Eu0.5Y0.5 Eu0.4Y0.6 Eu0.2Y0.8
(TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen) (TTA)3(Phen)
Thermal stability (°C)
Melting point (°C)
335.62 335.26 336.54 341.20 344.00
245.07 245.53 247.45 246.74 250.52
Figure 8. Emission spectra for a Y(1-x)Eu(x)(TTA)3(Phen) thin film.
Figure 6. Absorption spectrum for a thin film of pure and Y-doped Eu(TTA)3 (Phen).
Figure 9. Photophysical processes in lanthanide β-diketonate complexes (antenna effect). A, absorption; F, fluorescence; P, phosphorescence; L, lanthanidecentered luminescence; ISC, intersystem crossing; ET, energy transfer (18). Figure 7. Excitation spectrum for a Y(1-x)Eu(x)(TTA)3(Phen) thin film.
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3(Phen). These complexes are excited by light in the range 250–500 nm, monitored at 611 nm. The excitation peaks for all these complexes were observed at 360 nm. Maximum PL intensity is observed for Y0.6Eu0.4(TTA)3(Phen) in the excitation spectrum. Figure 8 shows the emission spectra for thin films of Eu(x)Y(1-x)(TTA)3(Phen) and Eu(TTA)3(Phen). Emission peaks at 591, 612, 618 and 627 nm, due to the transition 5D0 → 7Fj (j = 1, 2, 3, 4), were observed in all these complexes. Maximum peak intensity was observed at 612 nm due to the 5D0 → 7F2 transition. In Y(1-x)Eu(x)(TTA)3(Phen) series, maximum PL intensity was observed for Eu0.4Y0.6(TTA)3(Phen). It can be seen that there is a enhancement of the PL intensity (emission intensity) in Y(1-x)Eu(x)(TTA)3(Phen) due to efficient energy transfer from Y3+ to Eu3+; the exception is Y0.8Eu0.2(TTA)3 (Phen). When the PL intensity of Y(1-x)Eu(x)(TTA)3(Phen) is compared with that of Eu(TTA)3(Phen) the maximum emission
output was found in Y0.6Eu0.4(TTA)3(Phen). It seems that there is a nonradiative energy transfer from the enhancing ion Y3+ to Eu3+ in the doped complexes. It is well known that the Y3+ does not possess the electronic structure of the 4f shell. The lowest excited energy level of Y3+ is more than 32 × 10-3 cm-1. These energy levels are much higher than the lowest excited triplet level T (~ 20.3 × 10-3 cm-1) of the β-diketone (TTA) (15,16). The nature of the Eu3+ transition 5D0 → 7F2 is responsible for the sharp spectral line at 611 nm. The excited state 5D0 of Eu3+ has long lifetime (ms) and because of this, there is efficient energy transfer from excited Y3+ to Eu3+, which results in an enhancement of PL when compared with the reference sample. Y(III), La(III) and Tb(III) play a key role in enhancing the luminescence in a micelle solution and proved that efficient and brighter luminescence and economical complexes can be obtained by selecting an appropriate ligand and the introduction of another metal ion into the complexes (17–20). The photophysical process in lanthanide β-diketonate complexes leading to lanthanide-centered luminescence is shown in Fig. 9.
N. T. Kalyani et al. Absorption of electrons takes place in singlet states, i.e. from S0 to S1 of the ligand, and reaches T1 via intersystem crossing (ISC). A part of the energy absorbed by the electrons comes to the ground state S0 of the ligand. While crossing, the remaining energy is transferred to the lanthanide ion, leading to luminescence, also known as the antenna effect. So, in order to check the enhancement in the emission intensity of Y(1-x) Eu(x)(TTA)3Phen in the solid form, Y is stoichiometrically doped into Eu.
Conclusions From above studies, it can be concluded that all the doped Y(1-x) Eu(x)(TTA)3(Phen) complexes were crystalline in nature. TGA shows that thermal stability of Y(1-x)Eu(x)(TTA)3(Phen) phosphors ranged between 335.26 and 344°C. Broad absorption peaks were observed at 349 nm for all thin films of Y-doped Eu complexes. No shift in absorption peak maximum was observed, indicating that the absorption peak did not depend on the metal ion, but on aromatic group of the β-diketone (TTA). However, variations in optical density were observed, which may due to the different concentrations of metal ions. A prominent sharp red emission line was observed at 611 nm, when excited at 360 nm. This shows that the emission intensity increases for the Y(1-x)Eu(x) series of complexes, when compared with pure Eu(TTA)3(Phen). the maximum emission intensity was observed for Y0.6Eu0.4 (TTA)3(Phen). Hence, these complexes can be employed in ecofriendly and energy-saving OLEDs and also in other optical devices, where intense red emission light is required.
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