White luminescence of bismuth and samarium codoped Y2O3 phosphors

White luminescence of bismuth and samarium codoped Y2O3 phosphors

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CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

White luminescence of bismuth and samarium codoped Y2O3 phosphors D.Y. Medina Velazqueza,n, L.A. Hernández Sotoa, Á. de J. Morales Ramirezb, S. Carmona-Téllezc, E. Garfias-Garciaa, C. Falconyd, A. García Murillob a

Departamento de Materiales, Ciencias Básicas e Ingeniería, Universidad Autónoma Metropolitana-Azcapotzalco, Av. San Pablo No 180, Col. Reynosa-Tamaulipas, C.P. 02200 Mexico D.F., México b Instituto Politécnico Nacional, CIITEC IPN, Cerrada de Cecati S/N. Col. Santa Catarina, Azcapotzalco, México D.F. C.P. 02250, México c Instituto de Física de la Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán México D.F. C.P. 04510, México d Departamento de Física, Centro de Investigación y Estudios Avanzados, Av. Instituto Politécnico Nacional 2508, Col. San Pedro Zacatenco, México D.F., México Received 29 November 2014; received in revised form 19 February 2015; accepted 11 March 2015

Abstract In this work Sm3 þ (0–2.0 at%) and Bi3 þ (0–2.0 at%) doped Y2O3 luminescent powders were prepared by a sol–gel method from yttrium acetylacetonate, samarium and bismuth nitrates as metal sources. The as prepared powders (chemical composition is close to stoichiometric Y2O3) present the cubic structure from 700 1C, and at 900 1C are characterized by the presence of rounded particles with heterogeneous size of 42.9 nm. Luminescent effect of ions of Sm3 þ and Bi3 þ into Y2O3 host as was studied on heat treated powders from 800 to 1100 1C. The combination of the red luminescence from the Sm3 þ ions and the bluish from Bi3 þ , makes the synthesized phosphors candidates to be used in fabrication of phosphor-converted light-emitting diodes (LEDs). & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: D. Y2O3; Sol–gel; Phosphors; Sm–Bi; White light

1. Introduction Yttrium oxide has been extensively studied as a host matrix for a large variety of luminescent centers because its wide band gap and thermal and chemical stability [1,2]. This is the case of rare earth ions, which have been found to render excellent luminescence performances when they are incorporated in this host matrix, presenting radiative emissions associated with recombination processes within their own electronic energy levels [1–4]. Among these, Sm3 þ is one of the most reliable sources of red light emission due to its dominant emission at 608 nm associated with a radiative transition from the 4G5/2 to 6 H7/2 electronic energy levels [5]. The generation of white light emission, a major trend of luminescence displays light can be accomplished with a huge number of possible spectra. The creation of white light out of monochromatic visible-spectrum n

Corresponding author. Tel.: þ52 553189086; fax: þ 52 5553823998. E-mail address: [email protected] (D.Y. Medina Velazquez).

emitters can be based on dichromatic, trichromatic, or tetrachromatic approaches. One way of generating white light is the use of two narrow emission bands, called complementary wavelengths or complementary colors. Two complementary colors, at a certain power ratio, result in three values that are perceived as white light. Therefore, in order to obtain two complementary emissions, it has been reports the use of 2 or more dopants at the same time [6]. The present work examines the use of Bi3 þ and Sm3 þ as dopants. Bi3 þ is a P block metal, that has been employed in some oxides, due to its interesting luminescent properties, mainly its 6s2-6s6p transition (allowed electric-dipole transition), which produces mainly around 500 nm in Y2O3 [7], related with a wide blue–green emission band from 400 up to 550 nm. On the other hand, Sm3 þ it is a well-known reddish phosphor which the typical 4G3/2-6Hn transition [8]. Finally, the outcome of lighting devices, among other applications, has triggered the need of performant luminescence phosphors. In order to achieve this goal, an appropriate synthesis technique is required, for synthesis of luminescent material powders with good

http://dx.doi.org/10.1016/j.ceramint.2015.03.054 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: D.Y. Medina Velazquez, et al., White luminescence of bismuth and samarium codoped Y2O3 phosphors, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.054

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100°C 200°C 300°C 400°C 500°C 600°C 700°C 800°C 900°C

0.95 M (CH3COCH2COOCH3, Aldrich, 99%) was added as an organic chelating compound. The final sol was vigorously mixed for 2 h and then dried at 100 1C for 24 h with the aim to yield xerogel. The obtained xerogels were heat treated different temperatures (200–1100 1C) for 1 h to obtain the yttria-based luminescent powders. 2.2. Apparatus

Intensity / a.u

The IR spectra of samples were recorded in the range 1900– 400 cm  1 using Fourier transform infrared spectroscopy (FTIR 2000, Perkin-Elmer) and KBr pelleting technique. Xray diffraction patterns were obtained at room temperature on a powder diffractometer (Bruker D8Advance) using Cu Kα radiation (1.5418 Å). In order to inspect the morphology and sizes of the samples, conventional Scanning Electron Microscopy was carried out on a JEM-2200FS operating at 80 keV. The photoluminescent properties were recorded employing a Hitachi F700 spectrophotometer. 3. Results and discussion

4000

3500

3000

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2000

wavenumber / cm

1500

1000

500

-1

Fig. 1. FT-IR spectra of Y2O3:1.0 at% Sm3 þ , 1.0 at% Bi3 þ powders as function of the annealing temperature.

luminescence characteristics and at a low cost. Among the synthesis procedures employed with the elaboration of yttrium oxide powders are: polyol, spray pyrolysis and sol–gel [9–11]. Sol–gel is a low cost and useful technique to produce advanced phosphors, a big advantage for a sol–gel technique above others, is that the incorporation of dopants can be readily achieved by a simple mix of the proper reactive source with that used for the synthesis of powders. In particular, the incorporation of rare earth ions into yttrium oxide, synthesized by the sol–gel technique has been previously reported [11–14]. In the present work, photoluminescence emission behavior of Bi3 þ and Sm3 þ co doped yttrium oxide powders synthesized by the sol–gel technique and structurally are reported. 2. Experimental

3.1. Infrared analysis In order to study the infrared vibrations and establish the chemical evolution of the sol during the process, the effect during the annealing temperature (100–900 1C) was monitored using FTIR, as shown in Fig. 1. A characteristic band centered at 3450 cm  1 related with the O–H stretching (ν) deformation vibrations of H2O is observed at 100 1C and up to 700 1C. However, this vibration band is practically absent at 800 1C. Two bands located around 1560 and 1400 cm  1 are ascribed to the C– O and CQO stretching vibrations associated with acetylacetonate, as well as the bands located at 580–680 cm  1, related with acetylacetonate group [15]. Additional bands appear in 1080– 940 cm  1, and are attributed to the COO  asymmetric and symmetric vibrations, arising from acetic acid. Two bands centered occur at 570 cm  1 and 470 cm  1 are present from 500 1C up to 700 1C, a corresponding to Y–O vibration, associated with C–Y2O3, both bands increases with higher annealing temperatures. Those features suggest a better stoichiometry as well as the Y2O3 powders crystallization [16], as was confirmed by XRD observations.

2.1. Preparation of Bi3 þ and Sm3 þ codoped Y2O3 luminescent powders

3.2. Structural and morphological properties

Sm–Bi co-doped Y2O3 powders were prepared using the conventional sol–gel process. A yellow and transparent sol was prepared by dissolving yttrium (III)-2,4-pentadionate, Y (CH3COCHCO–CH3)3 (Alfa Aesar, 99.9%) precursor in ethanol under stirring at 65 1C for 1 h (0.19 M). Samarium and bismuth nitrates, Sm(NO3)3, Bi(NO3)3 (99% and 99.1% respectively, Alfa Aesar), were added in order to obtain desired composition (0–2.0 at% Bi3 þ with an Sm3 þ concentration fixed at 1.0 at%, and 0– 2.0 at% Sm3 þ with an Bi3 þ concentration fixed at 1.0 at%). Thereafter, the pH was adjusted by the incorporation of acetic acid, 0.15 M (CH3COOH, Fermont 98%) and 2–4-pentanedione,

Fig. 2 shows the XRD patterns of Y2O3: Sm3 þ , Bi3 þ powders annealed at different temperatures. The xerogel heat treated at 500 1C (not showed) presents an amorphous character. The Y2O3 cubic structure (JCPDS 251200) spatial group Iā3 (a¼ 10.604 Å) is clearly observed from 700 1C. In this case no evidence of the monoclinic structure was observed, which is a possible subproduct in the sol–gel process [17]. Additionally, the diffraction peaks become sharper as the temperature increases, as an effect of crystallinity enhancement. Fig. 3 shows the calculated crystal sizes according to Scherrer's equation, taking into account the line broadening of the diffracted

Please cite this article as: D.Y. Medina Velazquez, et al., White luminescence of bismuth and samarium codoped Y2O3 phosphors, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.054

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3

45

222 211

440

400 411

431

622 611

Intensity / a.u

40

Crystallite size / nm

35

30

40

50

60

25

70

2θ / ° Fig. 2. XRD patterns of Y2O3:1.0 at% Sm function of the annealing temperature.

30

20 3þ

, 1.0 at% Bi



powders as

15

peak due to the effect of crystal size 0:9λ β cos θ

ð1Þ

where D is the crystal size of the powder, λ is the wavelength of the Cu Kα radiation (0.15406 nm), β is the full-width at halfmaximum (FWHM) of the peak, and θ is the Bragg angle of the X-ray diffraction peak. As the annealing temperature increase, the crystal size increases from 14 to 38 nm from 700 to 1100 1C, respectively. As shown in Fig. 4, as the concentration of Bi3 þ ion increases from 0 to 1.5 at%, for a fixed Sm3 þ 1 at% on Y2O3 nanophosphors, the lattice parameter arises from 10.631 to 10.647 Å. This effect may be due to the larger ionic radius of Bi3 þ 1.03 Å than that of Sm3 þ ion 0.96 Å and Y3 þ ion 0.89 Å. A similar effect has been observed [18] for Y2O3: Eu3 þ , Bi3 þ nanophosphors, and it is stated that this behavior could be consequence of the substitution of Bi3 þ ion into the Y3 þ sites. SEM micrographs of Y2O3:Sm3 þ 1.0 at%, Bi3 þ 1.0 at% phosphors annealed at 900 1C are shown in Fig. 5a. As can be observed, the phosphors are agglomerated, and composed of individual particles with a preferential rounded morphology. The average particle size from these observations is E 42.9 nm (Fig. 5b). As expected, as the annealing temperature increases to 1100 1C, the particles size enhances (Fig. 5c and d) to 85.8 nm.

10 700

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900

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1100

Temperature / °C Fig. 3. Crystallite size evolution of Y2O3:1.0 at% Sm3 þ , 1.0 at% Bi3 þ powders.

10.648

3+

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Y2O3: 1 at. % Sm , X at. % Bi

10.646 10.644

o

Lattice Parameter / A



10.642 10.640 10.638 10.636 10.634 10.632 10.630 0.0

0.2

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Bi / at. %

Fig. 4. Lattice parameter as function of the Bi3 þ concentration.

3.3. Photoluminescent studies The excitation spectrum of Y2O3 powder with 0.5% of Sm, 0.25% of Bi synthesized at 1100 1C is presented in the Fig. 6, showing that the principal excitation arises from Bi3 þ with it main emission band peaking at 335 nm. The Bi3 þ ion has this ground state in 1S0, and the 6s6p excited states give rise to the triplet

levels 3P0, 3P1, 3P2 and to the 1P1 singlet state, so then excitation peak at 335 correspond to the 1S0-3P0 and 1S0-3P1 transitions [19,20]. Additionally, the Sm3 þ ion is also excited and a second and third peak appear at 378 nm and 405 nm, corresponding to 6 H5/2-6P5/2 and 6H5/2 4F7/2 transitions [21,22].

Please cite this article as: D.Y. Medina Velazquez, et al., White luminescence of bismuth and samarium codoped Y2O3 phosphors, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.054

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D.Y. Medina Velazquez et al. / Ceramics International ] (]]]]) ]]]–]]]

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42.9 nm

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14 12 10 8 6 4 2 0 20 30 40 50 60 70 80 90 Particle size / nm

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85.8 nm

Frecuency / a.u

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40 60 80 100 120 140 160 180 Particle size / nm

Fig. 5. (a) SEM micrograph of Y2O3:1.0 at% Sm3 þ , 1.0 at% Bi3 þ powders annealed at 900 1C, (b) particle size distribution, (c) SEM micrograph of Y2O3:1.0 at% Sm3 þ , 1.0 at% Bi3 þ powders annealed at 1100 1C, and (d) particle size distribution.

Fig. 6. Photoluminescence excitation of a Y2O3 powder with 0.5% of Sm, 0.25% of Bi sythesized at 1100 1C.

Fig. 7. Photoluminescence emission of a Y2O3 powder with 0.5% of Sm, 0.25% of Bi synthesized at 1100 1C.

The emission spectrum of Y2O3 powder with 0.5 at% Sm and 0.25 at% Bi synthesized at 1100 1C is shown in the Fig. 7. The maximum emission peaks were found at 500 nm and 615 nm, which can be used for obtain white color emission [22–24], after 385 nm excitation. The emission spectrum shows a broad between 350 to 500 nm with a maximum at

500 nm corresponding 3P1-1S0 transition of Bi3 þ , and the corresponding 4G3/2-6H5/2(564 nm), 4G3/2-6H7/2(600 nm), 4 G3/2-6H9/2 (645 nm) transitions of the Sm3 þ . The energy transfer of Bi3 þ to Sm3 þ ions where observed for all the codoped materials, this transference presents a better efficiency for a determined concentrations that was studied in the studied

Please cite this article as: D.Y. Medina Velazquez, et al., White luminescence of bismuth and samarium codoped Y2O3 phosphors, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.054

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0% 0.25% 0.5% 0.75% 1% 1.25% 1.5% 2%

5

Intensit y co u n

2000000

ts

2500000

1500000 1000000 500000

400 450

Wa 500 vel 550 en gth 600 (n m 650 )

0 2% 1.5% 1.25% 1% 0.75% 0.5% 0.25% 0%

Fig. 8. Photoluminescence emission of a Y2O3 700 1C annealed powders (0–2) at% of Sm, 1 at% Bi.

of Bi and Sm systems as a function of a set of Sm–Bi concentrations Fig. 8 shows the samarium (600 nm) and bismuth (500 nm) intensity powders thermally treated at 700 1C as a function of bismuth concentration (0–2) at% and a fixed samarium concentration of 1.0 at%. In this figure is shown that the addition of bismuth an enhanced emission intensity of Sm3 þ samarium was observed, this enhancement has a maximum at 0.25 at% Bi3 þ . The process involved correspond to the energy transfer from Bi3 þ to Sm3 þ , and the increasing distortion and crystal asymmetry occurring with the addition of Bi ions [24]. The increasing of bismuth concentration produces the formation of bismuth aggregations playing a role of trapping centers which decrease the emission of both ions. Fig. 9 presents the emission intensity of the both ions as a function of Sm concentration (0–2) at% when the concentration of Bi is fixed at 1 at%. The maximum emission is performed at 0.5 at% Sm. The variation of the emission intensity with the increasing temperature is shown in Fig. 10. In this figure is observed an enhancement of intensity after 1000 1C heat treatment but the white color is observed at 1100 1C. Fig. 11 shows on a semi-logarithmic scale, at 505 and 608 nm emission peaks when excited by 335 nm light for Y2O3:Bi, Y2O3:Sm and Y2O3:Bi–Sm samples. All the data for every sample is best fitted by a double exponential decay; it is observable that the large time exponential is dominant. This excitation (335 nm) is associated with a charge transfer from the host matrix (Y2O3) to Bi and Sm impurities; the times decay are showed at Table 1. The total average time decay (τmean), was calculated using the following equation, and the values are also presented at Table 1. τmean ¼

Aτ21 þ Bτ22 Aτ1 þ Bτ2

ð10 Þ

where A and B are the corresponding amplitudes associated with each time decay.

Fig. 9. Photoluminescence emission of Y2O3 700 1C annealed powders (0–2) at% Bi, 1 at% Sm.

Fig. 10. Photoluminescence emission of Y2O3 (800–1100) 1C annealed powders 0.5 at Sm, 0.25 at% Bi.

According to this results, there are not enough evidence that suggest an energy transfer from bismuth to samarium; due that time decay of Bi into Y2O3:Bi and Y2O3:Bi–Sm samples are essentially the same. However, times decay have a notorious difference between Y2O3:Sm and Y2O3:Sm–Bi samples; this effect could be attributed to a charge transfer, and more future studies are necessary to establish the origin of this process. Finally, the observed emission spectra for the co-doped Sm– Bi powder match the requirements of complementary light for white-emitting devices. As stated by Xiaoyun Guo [25], the corresponding complementary wavelengths for the 485– 486 nm would be 611.1–629.3 nm in order to achieve white light, which is the synthetized phosphors. Fig. 12 shows the correlation between Bi and Sm emissions and its CIE coordinates is presented for samples synthetized at 800, 900,

Please cite this article as: D.Y. Medina Velazquez, et al., White luminescence of bismuth and samarium codoped Y2O3 phosphors, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.054

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Fig. 11. Time decay of the luminescence intensity at the 505 nm emission peak when excited by 207 nm light for Y2O3:Bi3 þ Sm3 þ .

Table 1 Decay times of Bi into Y2O3:Bi and Y2O3:Bi–Sm and Sm in Y2O3:Sm and Y2O3:Bi–Sm samples. Type of sample

τ1 (μs)

τ2 (μs)

τmean

Y2O3:Bi 18.4070.10 696.707 46.41 36.6677.17 18.6770.11 776.357 27.47 46.7878.47 Y2O3:Bi–Sm, observing Bi Y2O3:Sm 67.8071.76 651.217 6.44 297.6577.04 Y2O3:Sm–Bi, observing Sm 358.8674.29 1371.507 9.28 948.4677.31

1000 and 1100 1C and doped 1 at% Bi and Sm. It is observable that for a perfect ratio Bi/Sm, the emission intensities values of 1 are expected. Thus values close to 1 are acceptable as white light, all items synthetized between 800 and 1000 1C present Bi/Sm values higher than 1, this is due to Bi emission is bigger than the Sm ones, and the light emission of these samples are blue dominantly. Just samples synthetized at 1100 1C present Bi/Sm values very close to 1 (1.024) and its light emission is white, in other hand the CIE coordinates of the samples shown a good approximation to the white color, with values of (0.223, 0.356), (0.228, 0.355), (0.218, 0.347) and (0.238, 0.352) for the 800, 900, 1000, and 1100 1C annealing temperature respectively; at temperature of 1100 1C, C–Y2O3 matrix is completely formed and cubic structure is achieved, it is well known that a crystalline field increase luminescence in rare earths ions as Sm3 þ [8,26,27]. 4. Conclusions The photoluminescent properties of Y2O3:Sm3 þ , Bi3 þ phosphors, prepared from the sol–gel method, have been investigated. The synthetized powders present the cubic structure from 700 1C, and have a broad excitation band with a maximum at 335 nm, corresponding to the 1S0-3P0 and 1 S0-3P1 transitions. The emission spectrum exhibited a strong band at 500 nm, corresponding to 3P1-1S0 transition of Bi3 þ ,

Fig. 12. Correlation between Bi and Sm intensity and CIE coordinates as function of the annealing temperature.

along with the typical 4G3/2-6Hn emissions from Sm3 þ (564– 645 nm). Finally, it has been stated that both emissions from Bi3 þ and Sm3 þ could be considered as complementary, and therefore, the system is candidate for white-emitting devices. Acknowledgments The authors wish to thank IPN-México SIP20140033, Departamento de materiales, UAM-México No. 2260244 and DGAPA-UNAM México for financial support. Also, wish to acknowledge the technical assistance of Z. Rivera, from physics department of Cinvestav-IPN, to Eng. Oscar Francisco Rivera Dominguez and Eng. Maribel Pacheco Ramos for their help. References [1] X. Bai, H. Song, G. Pan, Z. Liu, S. Lu, W. Di, X. Ren, Y. Lei, Q. Dai, L. Fan, Luminescent enhancement in europium-doped yttria nanotubes coated with yttria, Appl. Phys. Lett. 88 (2006). [2] L. Robindro, R.S. Ningthoujam, V. Sudarsan, I. Srivastava, S. Dorendrajit, G.K. Dey, S.K. Kulshreshtha, Luminescence study on Eu3 þ doped Y2O3 nanoparticles: particle size, concentration and core– shell formation effects, Nanotechnology 19 (2008) 055201. [3] N.C. Chang, Fluorescence and stimulated emission from trivalent europium in yttrium oxide, J. Appl. Phys. 34 (1963) 3500–3504. [4] S. Mukherjee, V. Sudarsan, R.K. Vatsa, S.V. Godbole, R.M. Kadam, U.M. Bhatta, A.K. Tyagi, Effect of structure, particle size and relative concentration of Eu3 þ and Tb3 þ ions on the luminescence properties of Eu3 þ co-doped Y2O3:Tb nanoparticles, Nanotechnology 19 (2008) 325704. [5] M. Yu, J. Lin, Z. Wang, J. Fu, S. Wang, H.J. Zhang, Y.C. Han, Fabrication, patterning, and optical properties of nanocrystalline YVO4:A (A¼ Eu3 þ , Dy3 þ , Sm3 þ , Er3 þ ) phosphor films via sol–gel soft lithography, Chem. Mater. 14 (2002) 2224–2231. [6] S. Carmona-Tellez, C. Falcony, M. Aguilar-Frutis, G. Alarcon-Flores, M. Garcia-Hipólito, R. Martinez-Martinez, White light emitting transparent double layer stack of Al2O3:Eu3 þ , Tb3 þ , and Ce3 þ films deposited by spray pyrolysis, ECS J. Solid State Sci. Technol. 2 (2013).

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Please cite this article as: D.Y. Medina Velazquez, et al., White luminescence of bismuth and samarium codoped Y2O3 phosphors, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.03.054