holmium-ytterbium codoped tantalum oxide

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2 Departamento de Química, FFCLRP - USP - Ribeirão Preto, Brasil .... sols were aged for 16h and afterward maintained in oven at 60ºC to obtain xerogels.
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Generation of wide color gamut visible light in NIR-excited thuliumholmium-ytterbium codoped tantalum oxide nanopowders Artur S. Gouveia-Neto1,*, Luciano A. Bueno1, Ernande B. Costa1, Elias A. Silva Jr1, Jefferson L. Ferrari2, Karmel O. Lima2, Rogéria R. Gonçalves2 1

Departamento de Física, Universidade Federal Rural de Pernambuco, Recife, Brasil 2 Departamento de Química, FFCLRP - USP - Ribeirão Preto, Brasil ABSTRACT

Multicolor visible light emitting NIR-excited Tm/Ho/Yb-codoped tantalum oxide nanopowders were produced using the sol-gel method. The generation of wide color gamut fluorescence in glass-ceramic with orthorhombic Ta2O5 nanocrystals dispersed into amorphous silica-based matrix is observed. The light emission spectroscopic properties of the rare-earth doped SiO2:Ta2O5 nanocomposites as a function of the tantalum content and temperature of annealing is examined. Simultaneously emitted multicolor fluorescence consisting of blue(480 nm), green(540 nm) and red(650 nm) upconversion signals in the SiO2:Ta2O5 system doped with holmium and thulium and sensitized with ytterbium, is demonstrated. It is also demonstrated that the proper choice of the rare-earth content and the NIR excitation power yielded the generation and control of the three primary colors and allows the emission of a balanced white overall luminescence from the glass-ceramic nanopowder samples Keywords: visible-light, nanopowder, rare-earth, tantalum, oxide, luminescence

1. INTRODUCTION Novel materials suitable for the development of solid-state red-green-blue(RGB) light emitters have drawn much scientific and technological interest lately, as the basis for future high brightness full-color display technology, back lighting, assay of biological compounds, remote sensors, optical data storage, optical printing, etc[1]. A significant number of methods have been employed in the past few decades, exploiting for instance dye-laser systems[2], LED/OLED technology[3,4], and frequency upconversion luminescence in lanthanide doped materials[5]. Recently, multicolor upconversion emission and white light generation was produced in novel glasses[6,7] and oxyfloride nanostructured materials[8]. The latter find potential applications in general lighting appliances, and integrated optical devices[9,10]. Therefore, it is of great interest, to study frequency upconversion processes in alternative phosphors materials and identify the major relaxation and interaction mechanisms of rare-earth ions implanted into the matrices. Considering solid-state hosts, nanoglasses have recently emerged as a viable alternative for photonics and biophotonics applications[1]. They are obtained by a suitable heat treatment of the precursor glass samples produced by standard oxide glass fusion and casting methods. Such heat treatment yields the precipitation of nanocrystals in which rare-earth ions are selectively concentrated. The crystal sizes are small enough to allow light transmission with no considerable scattering loss. The advantages of the nanocomposites reside in the fact that the rare-earth ions are confined in nanoscaled crystalline environments of low phonon energy, producing high quantum efficiencies and low optical absorption cross sections when compared to vitreous environment. In order to avoid the technical demands and difficulties of either crystals or glass preparation exploiting conventional melting methods under controlled atmosphere and high temperatures, the sol-gel process[11,12] was used. The sol-gel technique is one of the most widely used methods for production of bulk materials and thin films in integrated optics, because it is simple, low cost and performed at room temperatures, and possess the ability to control the purity and homogeneity of the final materials on a molecular level. *[email protected]; phone +55 81 33206483; fax +55 81 33206011

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2. EXPERIMENTAL

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The Ln3+ doped SiO2:Ta2O5 nanocomposites were prepared by the sol-gel method[11,12]. Solutions, with final (Si+Ta) concentration of 0.445 mol.L-1 were obtained with 70SiO2:30Ta2O5 molar ratio. As precursors we have used the tantalum ethoxide (99.98%) and tetraethoxysilane (98%) as received and ethanolic lanthanide chloride solution. The ethanolic ytterbium, thulium and holmium chloride solution was prepared from the respective oxide by dissolution in 0.1 mol.l-1 hydrochloric acid aqueous solution, followed by careful drying and dilution in ethanol to prepare the stock ethanolic solution.A mixture of TEOS, anhydrous ethanol and hydrochloric acid was first prepared, with a TEOS:HCl volume ratio of 1:50. Concomitantly, it was mixed tantalum ethoxide and 2-ethoxyetanol, with a 2ethoxietanol:tantalum ethoxide volume ratio of 10:1; and the lanthanide was added as ethanolic solution of LnCl3 with an Ln:(Si+Ta) molar concentration of 1.0 mol% Yb3+, 0.2 mol% Ho3+ and x mol% Tm3+( with x=0.1, 0.2, 0.3, 0.4, 0.5). The solutions containing the TEOS and tantalum ethoxide were mixed and left at room temperature, under stirring for 30 minutes. Subsequently, it was filtered with a 0.2 μm Millipore filter. Later on, at room temperature a 0.27 mol.L-1 hydrochloric acid aqueous solution was added to the final solution, with a TEOS:HClmolar ratio of 1:0.07. The resulting sols were aged for 16h and afterward maintained in oven at 60ºC to obtain xerogels. Following, the samples were annealed in an electrical furnace at 900ºC, 1000 ºC and 1100 ºC for 8h to obtain nanocomposites. Crystalline structure were evaluated by X-ray diffraction (XRD) using a Siemens-Bruker D5005 diffractometer, with Kα Cu radiation, λ = 1.5418 Å, graphite monochromator, 0.03º, 2s per step, and 15 to 70º 2θ range. High Resolution Transmission Electron Microscopy (HRTEM) images for the nanocomposites were acquired using a Philips CM 200 microscope. In order to perform spectroscopic measuremnts, the powders were placed into transparent acrylic 0.5 mm thick cubic cells with 1.0x1.0x3.0 cm3. The luminescence signal was collected and directed to a fiber integrated UV-VIS spectrograph(Ocean Optics USB 2000) with resolution of ~1.0 nm. The NIR excitation source was a cw diode laser operated at 975 nm with maximum output power of 70 mW.

Ta2O5 (Comercial)

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Figure 1- X-ray diffractogramms of the nanocomposites annealed at 900ºC, 1000 ºC and 1100 ºC.

3. RESULTS AND DISCUSSION The diffractograms for the xerogels are shown in Figure 1. A structural change is clearly observed as a result of the heat treatment. The diffractogramms do not show any indication of silica crystallization, however a crystallization of tantalum oxide occurs and it is dependent upon the annealing temperature. In Figure 2, one observes an amorphous structure for the nanocomposite annealed at 900ºC, while, in the case of the samples annealed at 1000ºC and 1100ºC, diffraction patterns clearly appear and the diffractograms display a characteristic profile of nanocrystalline systems. The diffraction patterns clearly portrayed a significant peak narrowing when the temperature was increased from 1000ºC to

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1100ºC. The diffraction patterns were attributed to the orthorhombic L-Ta2O5 structure with a = 6.1980 ± Å, b = 40.2900 ± Å, c = 3.8880 ± Å, α = β = δ = 90º, belonging to space group P21212 according to JCPDS Card number 250922 [13]. In agreement with Stephenson and Roth [14], the L-Ta2O5 orthorhombic structure can be represented by a chain of 8 edge-sharing pentagons, where the unit cell contains 22 thantalum atoms and 55 oxigen atoms, resulting in twelve positions for the Ta atoms, with a small difference between them. In previous work, it was observed that the Ln3+ replace the different Ta5+ sites and consequently an inhomogeneous broadening of the emission band could be detected. Figure 2 displays HRTEM images for the samples with Si:Ta molar ratio of 70:30, annealed at 900, 1000, and 1100 ºC. It can be noted the presence of Ta2O5 spherical shaped nanoparticles dispersed in the SiO2- based amorphous matrix, even for the compound annealed at 900 ºC. The Ta2O5 nanoparticles appear well dispersed in the SiO2 amorphous environment. The average size increases when the temperature increased reaching a maximum value of 12 nm for the highest temperature of annealing.

Figure 2- HRTEM images of nanocomposites annealed at 900ºC, 1000 ºC and 1100 ºC

Figure 3 presents typical upconversion emission spectra of radiation emanating from Tm/Ho/Yb multiple-doped samples excited with a fixed laser power of 55 mW at 975 nm.

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Wavelength(nm) Figure 3-Upconversion emission spectra of the triply-doped (0.5Tm/0.2Ho/1.0Yb) at different annealing temperatures

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The emission signals exhibited the distinct red-green-blue(RGB) signals around 480, 540, and 650 nm, and were identified as due to the 1G4→3H6(Tm), 5S2(5F4)→5I8(Ho), 5F5→5I8, 1G4→3H4(Tm)+5F5→5I8(Ho) transitions, respectively. The excitation power dependence of the emission signals presented a cubic power law for the green, blue, and red emissons indicating the participation of three(slope ∼ 3) pump photons in the upconversion excitation mechanism, as shown in graphs of Figure 4. 5

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Figure 4- Upconversion intensity as a function of excitation power for 0.5Tm/0.2Ho/1.0Yb sample

Based upon the emission wavelengths and excitation power dependence, one would conclude that the green emission is sustained by population of the 5S2(5F4) excited-state, resulting from successive transfers from Yb3+ ions to Ho and crossrelaxation connecting 3H4(Tm) to 5I7 (Ho)[14]. This cross-relaxation mechanism becomes evident when one see the sequence of X-rays spectra for the temperatures of 900 oC, 1000 oC, and 1100 oC and the sequence of optical spectra of Figure 3. As can be seen, when the nanocrystals start to evolve, the thulium emission signals(blue and NIR) substantially diminish when compared to holmium emissions in the glassy environment. In that samples, the rare-earths ions are embeded in the nanocrystals and the distances between them favors the cross-relaxation between Tm and Ho ions. It can also be noted a significant reduction in the emission bandwidth when the annealing temperature is raised, which corroborate the low phonon-energy environment of the nanocrystals. The red signal is a result of the fluorescence contribution from Tm and Ho ions. The excitation mechanism for green, and red emissions associated to holmium ions was accomplished through two successive transfers from Yb3+ ions. In the first transfer, a holmium ion is excited from its ground-state 5I8 level to the 5I6 level and the exceeding energy (~1600 cm-1) is transferred to the host matrix. Finally, a second transfer from a nearby ytterbium ion excites the same holmium ion from the 5I6 excited-state to the 4S2(5F4) thermalized emitting levels and the exceeding energy is again transferred to the host matrix as optical phonons. The excited Ho3+ ion at the 4S2(5F4) then is demoted radiatively to the ground-state and 5I7 state producing green around 540 nm signal. There exist two possible energy upconversion mechanisms accounting for the resulting red emission band recorded in our measurements. The red emission can be obtained by populating the 5F5 level by nonradiative phononassisted relaxation from the 4S2(5F4) excited-state. The holmium ion is excited from its ground-state 5I8 level to the 5I6 level and the exceeding energy (~1600 cm-1) is transferred to the host matrix. From the 5I6 level, it relaxes nonradiatively to the 5I7 level and another pump photon is absorbed populating the 5F5 excited-state emitting level producing the recorded red emission signal. The thulium ions were excited via sequential energy-transfer from ytterbium, as illustrated in the simplified energy-level diagram of Figure 5.

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Figure 5- Simplified energy-level diagram for the thulim-holmium-ytterbium system

In the first step, an incident pump photon at 975 nm is absorbed by an Yb3+ ion promoting it to the 2F5/2 excited state level. The excited Yb3+ relaxes and nonresonantly transfers its energy to a nearby Tm3+ ion, exciting it to the 3H5 excited-state level. The thulium ion in the 3H5 excited-state relaxes nonradiatively to the 3H4 metastable level. From that level, a second energy-transfer process takes place from the same or another neighbor excited Yb3+, and promotes it to the 3F2,3 level. The 3F2,3 state also relaxes by a multiphonon-assisted process to the 3F4 state. The signature of this step of the upconversion process is the 800 nm emission owing to the 3F4→3H6 transition, which was obiously detected in our measurements. Finally, a third energy-transfer process is effectuated exciting the Tm3+ ion in the 3F4 to the 1G4 upper excited-state. From the 1G4 level the Tm3+ ions radiatively relaxe to the 3H6 ground-state generating the intense upconversion fluorescence signal around 480 nm. The emission band around 650 nm is assigned to both the 1G4→3H4 and 3F2,3→3H6 transitions. It is important to point out the excitation power tunability of the CIE 1931 chromaticity diagram coordinates of the overall visible emission light as depicted in Figure 6.

high power

low power

Figure 6- CIE 1931 chromaticity diagram coordinates dependence upon excitation power for 900 oC anealing temperature.

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The results also indicate that one can color tune the overall light emitted by the samples, proply ajusting the rare-earth concentrations, particularly the Tm conentration, as demonstrated by bthe CIE 1931 chromaticity diagram presented in Figure 7.

0.2Tm 0.1Tm 0.3Tm 0.5Tm 0.4Tm 1.0Tm

Figure 7- CIE 1931 chromaticity diagram coordinates dependence upon Tm concentration for fixed 1.0Yb/0.2Ho/xTm

4. CONCLUSION Multicolor visible light emitting NIR-excited Tm/Ho/Yb-codoped tantalum oxide nanopowders were produced and tested as new phosphor materials. The generation of wide color gamut fluorescence in glass-ceramic with orthorhombic Ta2O5 nanocrystals dispersed into amorphous silica-based matrix was observed. Multicolor simultaneously emitted fluorescence consisting of blue(480 nm), green(540 nm) and red(650 nm) upconversion signals in the SiO2:Ta2O5 system doped with holmium and thulium and sensitized with ytterbium, was observed. The proper choice of the rareearth content and the NIR excitation power yielded the generation and control of the three primary colors and allowed the emission of a balanced white or tunable overall luminescence from the glass-ceramic nanopowders

ACKNOWLEDGEMENTS The financial support for this research by CNPq(472328/2008-05) and FACEPE(Fundação de Amparo a Ciência e Tecnologia do Estado de Pernambuco) APQ 0504-1.05/08, Instituto Nacional de Ciência e Tecnologia(INFOS) is gratefully acknowledged. Elias A. da Silva Jr. is supported by a graduate studentship from FACEPE.

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REFERENCES [1] P. N. Prasad, Nanophotonics(Wiley, New York, 2004) [2] Y. Saito , M. Kato, A. Nomura, and T. Kano, Appl. Phys. Lett. 56 (1990) 811 [3] A. Niko, S. Tasch, F. Meghdadi, C. Brandstatter, and G. Leising, J. Appl. Phys. 82 (1997) 4177 [4] Y. Yang, and S-C. Shang, Appl. Phys. Lett. 77 (2000) 936 [5] E. Downing, L. Hesselink, J. Ralston, and R. Macfarlane, Science 273 (1996) 1185 [6] A. S. Gouveia-Neto, L. A. Bueno, R. F. do Nascimento, E. A. da Silva, E. B. da Costa, and V. B. do Nascimento, Appl. Phys. Lett. 91 (2007) 091114 [7] D. Chen, Y. Wang, K. Zheng, T. Guo, Y. Yu, and P. Huang, Appl. Phys. Lett. 91 (2007) 251903 [8] D. Chen, Y. Wang, Y. Yu, P. Huang, and F. Weng, J. Solid-state Chem. 181 (2008) 2763 [9] C. Feldmann, T. Justel, C. R. Ronda, and P. J. Schmidt, Adv. Funct. Mater. 13 (2003) 511 [10] D. Matsuura, Appl. Phys. Lett. 81 (2002) 4526 [11] S. Fujihara, C. Mochizuki, T. Kimura, J. Non-Cryst Solids 244, 267 (1999) [12] X. Y. Liu, P. D. Sawant, Appl. Phys. Lett. 79, 3518 (2001) [13] Joint Committee on Powder Diffraction Standards, Diffraction Data File, No. 025-0922 - JCPDS International Center for Diffraction Data, Pennsylvania, 1991. [14] N. C. Stephenson and R. S. Roth, Acta Cryst. B27 1037 (1971) [15] X. Zou, and H. Toratani, J. Non-Cryst. Solids 195, 113(1996)

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