phys. stat. sol. (a) 204, No. 6, 1762 – 1768 (2007) / DOI 10.1002/pssa.200675349
Optical properties of Ho3+–Yb3+ co-doped nanostructured SiO2 –LaF3 glass-ceramics prepared by sol–gel method J. J. Velázquez1, A. C. Yanes2, J. del Castillo2, J. Méndez-Ramos1, and V. D. Rodríguez*, 1 1
2
Departamento de Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Departamento de Física Básica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
Received 16 October 2006, revised 19 October 2006, accepted 5 January 2007 Published online 23 May 2007 PACS 78.45.+h, 78.55.Hx, 78.67.Bf, 81.05.Pj, 81.07.Bc, 81.20.Fw Transparent glass-ceramics with composition of 95SiO2 – 5LaF3 doped with 0.1 mol% of Ho3+ or co-doped with 0.1 mol% of Ho3+ and 0.3 mol% of Yb3+ were synthesized by thermal treatment of precursor sol – gel glasses. Segregation of LaF3 nanocrystals in the matrix was confirmed from a structural analysis by means of X-ray diffraction. Blue, green and red efficient up-conversion emissions were observed under 980 nm excitation at room temperature. These results could be attributed to the partition of a fraction of Ho3+ ions into the precipitated LaF3 nanocrystals. Moreover, near infrared down-conversion at 1.2 µm is also observed. The mechanisms involved in the up-conversion emissions could be ascribed to a two- and threephoton process. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Recently, there has been a great interest of doped nanoparticles to photonics systems due to luminescent properties induced by the small size [1–3]. In particular, rare earth (RE) doped materials are attractive in the area of photonic aplications [4–10]. With the appropriate choice of host matrix and RE doping, the optical properties could be considerably improved. Furthermore, RE doped oxyfluoride glass-ceramics (GCs) have generated a great attention as host materials for active optical devices, such as lasers and optical amplifiers by using optical f–f transitions of these ions. These materials can remain transparent after segregation of nano-scaled fluoride crystals. When the RE ions are incorporated into the crystalline phase, the oxyfluoride glass-ceramics combine the fabrication advantages and the high chemical, mechanical and thermal stabilities of oxide based glasses with the low phonon energy of fluoride crystals (300–400 cm–1), increasing the number and the probability of radiative transitions (reducing the non-radiative loss due to multiphonon relaxation [11–13]). At present the Ho3+ ion is one of the most widely investigated RE ions due to its applications in optical amplifiers and for up-conversion laser. Silica fibers activated with Ho3+, or co-activated with Yb3+ and Ho3+ can be used for communication purposes and for medicine applications [14]. In addition, upconversion emissions in Ho3+–Yb3+ co-doped glasses obtained by melt-quenching method have also been studied [15].
*
Corresponding author: e-mail:
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On the other hand, the sol–gel synthesis is a room temperature method that permits a homogenous doping with high concentration of dopants in a silica matrix without difficulties of conventional meltquenching techniques [16], with the possibility of developing glass-ceramics composites by adequate heat treatments. Moreover, glass-ceramics containing LaF3 nanocrystals have been studied owing to the superior solid solubility for lanthanide ions, as a consequence of the comparable ionic radius and equal valence to La3+ ion. In this sense, Ho3+–Yb3+ co-doped aluminosilicate GCs based on LaF3 have been obtained by sol–gel method [14]. In previous works we successfully obtained by sol–gel technique silica based transparent glassceramics with SnO2 [9, 10] and LaF3 nanocrystals [17] both doped with Eu3+ ions. In this work, we report the visible (VIS) and near infrared (NIR) emissions and up-conversion luminescence of Ho3+–Yb3+ codoped transparent SiO2–LaF3 glass-ceramics prepared by the sol–gel process.
2 Experimental Silica glasses with compositions of 94.9SiO2–5LaF3 doped with 0.1 mol% of Ho3+ and 94.6SiO2–5LaF3 co-doped with 0.1 mol% of Ho3+ and 0.3 mol% of Yb3+ were synthesized by a sol–gel procedure in a similar way to that of Fujihara et al. [15]. First, tetraethoxysilane (TEOS) was hydrolyzed for 1 h at room temperature with a mixed solution of ethanol and H2O, using acetic acid as a catalyst. The molar ratio of TEOS:ethanol:H2O:CH3COOH was 1:4:10:0.5. The required quantity of La(CH3COO)3 ·nH2O, HoNO3 ·5H2O and Yb(CH3COO)3 ·4H2O were dissolved in CF3COOH and H2O solution, which was slowly mixed with the initial solution. The molar ratio of RE3+ (La3+, Ho3+ and Yb3+) to CF3COOH was 1:4. The resultant homogeneous solution was stirred vigorously for 1 h at room temperature. A wet-gel was obtained by leaving the solution in a sealed container at 35 °C for 1–2 weeks. After this step, evaporation for several weeks at 35 °C was required to obtain dried samples, known as xerogels. Finally, these xerogels were heat treated in air at 800 °C in order to precipitate nanocrystallites giving rise to a transparent glass-ceramic. X-ray analysis (XRD) was carried out by using a Cu anode (Cu Kα1,2)in the 10–70 2Theta range. The UV-VIS fluorescence signals were obtained by exciting the samples with light from a 300 W Xe arc lamp passed through a 0.25 m double-grating monochromator and detecting with a 0.25 m monochromator and a photomultiplier or a liquid nitrogen cooled Ge detector. The upconverted luminescence was obtained by using a 980 nm laser diode with power up to 100 mW, focused with a 50 mm focal length lens. The spectra were corrected by the instrumental response. All spectra were collected at room temperature.
3 Results and discussion X-ray diffraction patterns of the co-doped Ho3+–Yb3+ and doped Ho3+ glass-ceramics are presented in Fig. 1. Peaks corresponding to hexagonal LaF3 nanocrystals (JPCD file 32-0483) can be clearly observed, with no second crystalline phase. From the width of the XRD peaks, the nanocrystals sizes in the obtained glass-ceramics were calculated by using Scherrer’s equation: D = Kλ/β cos θ ,
(1)
where D is the crystal size, λ the X-ray wavelength, θ the diffraction angle, β the full-width at halfmaximum (FWHM) of the diffraction peak and K is a constant depending on particle shape (currently K = 0.9). The calculated nanocrystal sizes were about 11.5 nm for Ho3+ doped glass-ceramics and 16.0 nm for Ho3+–Yb3+ co-doped glass-ceramics. These results indicate that the presence of the Yb3+ lanthanide ions favours the formation of nanocrystals during the heat treatment. Biswas et al. obtained comparable average nanocrystal sizes of 10–20 nm after heat-treatment to 1000 °C of sol–gel doped SiO2–LaF3 glasses [18]. Besides, Tanabe et al. [19] obtained quite larger crystals, over 200 nm, with loss of transparency for treatment to 750 °C. www.pss-a.com
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40 50 2 θ (degrees)
' 3
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3+
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Intensity (arb.units)
5
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G 5, H5, H6, G2
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G6
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Fig. 1 XRD patterns of 95SiO2– 5LaF3 doped with 0.1Ho3+ and co-doped with 0.1Ho3+– 0.3Yb3+ (mol%) glass-ceramics.
350
400 450 Wavelength (nm)
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Fig. 2 Excitation spectra of 95SiO2– 5LaF3 doped with 0.1Ho3+ (dashed line) and co-doped with 0.1Ho3+– 0.3Yb3+ (solid line) in mol% glass-ceramics monitoring the emission at 540 nm. Transitions go from the ground level 5I8 to the levels indicated in the figure. Spectra have been normalised to intensity of the long wavelength peaks.
Excitation spectra of the 540 nm emission, transition 5S2, 5F4 → 5I8, in Ho3+ doped and Ho3+–Yb3+ codoped glass-ceramics, are shown in Fig. 2. The relative intensity of the excitation peaks corresponding to upper lying levels decreases in the co-doped glass-ceramic respect to the single doped one. This indicates that the non-radiative decays to the 5S2, 5F4 emitting level are more efficient in the single doped sample, where the nanocrystals are smaller according to the XRD measurements. The energy level diagram of Ho3+ and Yb3+ ions is presented in Fig. 3. Excitation mechanisms along with transitions from luminescent levels are indicated. After direct pumping into the 5S2, 5F4 levels, at 534 nm, excited Ho3+ ions can decay radiatively to the ground state yielding the green emissions 5
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G3 3 3 5 G5, H5, H6, G2 5 3 G , K7 5 4 G 5 5 G 5 6 5 3 F3 , F2, K8 5 5 S2, F4
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417 nm
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0 3+
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.5
I7
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I8
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Ho
Fig. 3 Energy level diagram of Ho3+ and Yb3+ ions. Up-conversion mechanisms by excited state absorption (ESA) and energy transfer (ET) are indicated by dashed arrows. Pumping wavelengths are also indicated by up-headed solid arrows. Main emissions are indicated by solid down-headed arrows.
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phys. stat. sol. (a) 204, No. 6 (2007)
I8 5
5
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S 2, F 4
I8
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Fig. 4 Visible emission spectra of 95SiO2– 5LaF3 doped with 0.1Ho3+ (dashed lines) and co-doped with 0.1Ho3+– 0.3Yb3+ (solid lines) mol% glass-ceramics under excitation at 414 nm and 534 nm. All spectra have been normalised to the maximum intensity of the 640 nm emission band.
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540 nm. Furthermore, after pumping into the 5G5 level at 414 nm and by fast multiphonon relaxation process, green emitting 5S2, 5F4 levels are also populated and therefore corresponding emission is also observed. The room temperature visible emission spectra of the Ho3+ doped and Ho3+–Yb3+ co-doped glassceramics under excitation at 414 nm (5I8 → 5G5) and 534 nm (5I8 → 5S2, 5F4) are shown in Fig. 4. Under 414 nm pump, a green emission with the maximum at about 540 nm, corresponding to the 5S2, 5F4 → 5I8 transition, is observed together with less intense blue emission at 482 nm and red emissions at 640 and 750 nm. These red emissions are also observed under 534 nm pump, see dash lines in Fig. 4. The red emission at about 640 nm can have contribution of the transitions 5F3, 5F2, 3K8 → 5I7 and 5 F5 → 5I8, see Fig. 3. For this reason, the relative intensity of this emission is lower after excitation at 534 than at 414 nm. In particular, the shorter wavelength side of the red band is enhanced indicating that the contribution of upper lying levels 5F3, 5F2, 3K8 becomes more important when pumping at 414 nm, whilst these levels are not populated when excitation is shifted to 534 nm, see energy level diagram in Fig 3. On the other hand, the 750 nm emission corresponds to the 5S2, 5F4 → 5I7 transition, as indicated by solid down-headed arrows in Fig. 3. The corresponding emitting levels can be populated by multiphonon decay from higher energy levels after excitation at 414 nm. It can be seen that the ratio between green and red emissions at 540 and 640 nm is larger in the co-doped sample [20]. These features are indicative of a higher degree of crystallinity in the Ho3+–Yb3+ co-
Fig. 5 Up-conversion emission spectra of 95SiO2– 5LaF3: 0.1Ho3+– 0.3Yb3+ (mol%) glass-ceramics obtained under 980 nm excitation at room temperature (inset: magnified spectrum in the wavelength range of 350 – 550 nm). www.pss-a.com
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J. J. Velázquez et al.: Optical properties of Ho3+– Yb3+ co-doped glass-ceramics
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Ho -Yb
Intensity (u.a.)
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F5 I8
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5
5
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I7
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3
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Fig. 6 Dependence of up-conversion emission intensity at 482 (䊏), 540 (䊉), 640 (䉱) and 750 (䉳) nm on excitation power at 980 nm in 95SiO2– 5LaF3 : 0.1 Ho3+– 0.3Yb3+ (mol%) glassceramics at room temperature. Solid lines correspond to linear fits with the slopes indicated by numbers in the figure.
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I8
Excitation power (mW)
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doped sample, according to our above conclusion based on XRD measurements presented in the previous section. Low phonon energy typical of like-crystalline environment favours the emission from this higher 5S2, 5F4 green emitting level. Moreover, it is expected to have a fraction of Ho3+ ions partitioned into the precipitated LaF3 nanocrystals, accordingly to previous result obtained by the authors in similar samples doped with other rare earth ions as Eu3+ and Er3+ [17]. By exciting the Yb3+ transition (2F7/2 → 2F5/2) at 980 nm, and after non-resonant energy transfer to Ho3+ ions (see Fig. 3), visible up-conversion are presented in Fig. 5; i.e. three main emissions at 540, 640 and 750 nm in the green-red part of the spectrum, and additionally weak blue emissions at 389, 417 and 482 nm corresponding to the transitions 5G4, 3K7 → 5I8, 5G5 → 5I8 and 5F3, 5F2, 3K8 → 5I8 respectively. The up-conversion luminescence was bright enough to be perceptible by naked eye. The green at 540 nm and the blue at 482 nm upconversion emissions present features, i.e. sharper structure and better-resolved Stark components, similar to the corresponding emission spectrum obtained by direct excitation at 414 shown in Fig. 4. This result points out that this up-conversion emission comes from the same Ho3+ ions, which are partitioned into the LaF3 nano-crystalline environment as discussed previously. Moreover, the presence of high-energy upconversion emissions coming from upper-lying levels of Ho3+ indicates low phonon energy of the nano-crystalline environment. The dependence of the intensity of the blue, green and red up-conversion emissions on pumping power was analyzed. The results are presented in log–log plots in Fig. 6. Green and red emissions exhibit a close to quadratic dependence on the excitation power. This result reveals upconversion mechanisms consist in two-photon processes. However, a slope of 2.2 for the blue emission at 482 nm indicates that three-photon process is involved. The up-conversion mechanisms are depicted in Fig. 3. There are two possible energy up-conversion mechanisms: stepwise phonon-assisted two/three excited state absorption (ESA) or energy transfer (ET) [21], followed by multiphonon relaxation to populate emitting level. After infrared excitation of Yb3+ ions, non-resonant ESA/ET processes excite Ho3+ ions to the 5F4, 5S2, excited states levels, de-exciting radiatively to the ground-state 5I8 giving rise to the intense green emission at 540 nm and to the first excited level 5I7 generating the red emission at 750 nm. The luminescence from the 5F5 level, populated by multiphonon relaxation from higher energy levels or by direct population ESA/ET of infrared photon from 5I7 level of Ho3+ ions (see Fig. 3), mainly originates the red emission at 640 nm. Additionally, emission corresponding to the 5F3, 5F2, 3K8 → 5I7 transition of Ho3+ ions also contributes to the red emission located at 640 nm. Moreover, it should be noticed that the ratio between green and red emissions at 540 and 640 nm differs substantially from direct excitation measurements that were shown in Fig. 2. This fact indicates that the additional upconversion mechanism of ESA/ET of infrared photon from 5I7 level of Ho3+ ions results in the increment observed for the red emission band at 640 nm. At last, blue emissions at 389, 417, and 482 nm are obtained after three-photon processes and following non-radiative relaxation to the corresponding emitting levels, i.e the 5G4, 3K7, 5G5 and the 5 F3, 5F2, 3K8 and from there, to the ground-state 5I8 by radiative transitions, as indicated in Fig. 3. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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phys. stat. sol. (a) 204, No. 6 (2007) 3+
Fig. 7 Room temperature near-infrared down-conversion emission spectrum of 95SiO2– 5LaF3 : 0.1Ho3+– 0.3Yb3+ (mol%) glass-ceramic under 980 nm excitation.
3+ 5
I8
5
I6
Intensity (u.a.)
Ho -Yb
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1160
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Wavelength (nm)
Finally, Fig. 7 shows the room temperature near infrared down-conversion emission of the Ho3+ ions at about 1.2 µm under 980 nm pumping (minimum loss telecommunication window) in the Ho3+–Yb3+ co-doped glass-ceramic sample. The broad emission band of the Ho3+ ions corresponds to the 5I6 → 5I8 transition. The FWHM of this emission band is approximately 60 nm, similar to the values observed in Ho3+ doped GCs [22]. This also points out that the Ho3+ ions are in fluoride environments in the GCs studied and indicates the interest of these materials for wavelength multiplexing applications.
4 Conclusion Sol–gel synthesized 95SiO2–5LaF3 nano-glass-ceramics doped with Ho3+ and co-doped with Ho3+–Yb3+ have been characterized by means of X-ray diffraction, showing the precipitation of LaF3 hexagonal nanocrystals during thermal treatment. In spite of the low LaF3 concentration, most Ho3+ ions are partitioned into the LaF3 nanocrystals. Ho3+–Yb3+ co-doped glass-ceramics present higher crystallinity than the Ho3+-doped one. The emissions coming from upper-lying levels are favoured by low phonon energy typical of like-crystalline environment for the Ho3+ ions. As a consequence, efficient blue, green and red upconversion emissions were observed under NIR excitation at 980 nm and following non-resonant energy transfer from Yb3+ to Ho3+ ions. Moreover 1.2 µm down-conversion emission was observed under NIR excitation at 980 nm in Ho3+–Yb3+ co-doped glass-ceramic sample, interesting from the viewpoint of potential applications in the telecommunication window. Acknowledgements The authors would like to thank Gobierno Autónomo de Canarias (PI042005/039), Ministerial de Ciencia y Tecnología (FIS 2006-02980) and Universidad de La Laguna (Beca SEGAI) for financial support.
References [1] [2] [3] [4] [5] [6] [7]
G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994). R. N. Bhargava, J. Lumin. 70, 85 (1996). M. Nogami, T. Enomoto, and T. Hayakawa, J. Lumin. 97, 147 (2002). A. Patra, C. S. Friend, R. Kapoor, and P. N. Prasad, Appl. Phys. Lett. 83, 284 (2003). H. X. Zhang, C. H. Kam, Y. Zhou, X. Q. Han, S. Buddhudu, and Y. L. Lam, Opt. Mater. 15, 47 (2000). N. Chiodini, A. Paleari, D. DiMartino, and G. Spinolo, Appl. Phys. Lett. 81, 1702 (2002). E. De la Rosa Cruz, L. A. Díaz Torres, R. A. Rodríguez Rojas, M. A. Meneses Nava, and O. Barbosa García, Appl. Phys. Lett. 83, 4903 (2003). [8] D. Pacifici, G. Franzò, F. Iacona, S. Boninelli, A. Irrera, M. Miritello, and F. Priolo, Mater. Sci. Eng. B 105, 197 (2003). [9] A. C. Yanes, J. Del Castillo, M. E. Torres, J. Peraza, V. D. Rodríguez, and J. Méndez-Ramos, Appl. Phys. Lett. 85, 2343 (2004).
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[10] J. del Castillo, V. D. Rodríguez, A. C. Yanes, J. Méndez-Ramos, and M. E. Torres, Nanotechnol. 16, 303 (2005). [11] J. Méndez-Ramos, V. Lavín, I. R. Martín, U. R. Rodríguez-Mendoza, J. A. González-Almeida, V. D. Rodríguez, A. D. Lozano-Gorrín, and P. Nuñez, J. Alloys Compd. 323/324, 753 (2001). [12] S. Xu, G. Wang, J. Zhang, S. Dai, L. Hu, and Z. Jiang, J. Non-Cryst. Solids 336, 230 (2004). [13] Y. Zhanci, H. Shihua, L. Shaozhe, and C. Baojiu, J. Non-Cryst. Solids 343, 154 (2004). [14] A. V. Kir’yanov, V. P. Minkovich, Yu. O. Barmenkov, M. A. Martinez Gamez, and A. Martinez-Rios, J. Lumin. 111, 1 (2005). [15] I. R. Martin, V. D. Rodriguez, V. Lavin, and U. R. Rodriguez-Mendoza, J. Alloys Compd. 275–277, 345 – 348 (1998). [16] S. Fujihara, C. Mochizuki, and T. Kimura, J. Non-Cryst. Solids 244, 267 (1999). [17] A. C. Yanes, J. del Castillo, J. Méndez-Ramos, V. D. Rodríguez, M. E. Torres, and J. Arbiol, Opt. Mater. 29, 999 (2007). V. D. Rodríguez, J. del Castillo, A. C. Yanes, J. Méndez-Ramos, M. E. Torres, and J. Peraza, Opt. Mater., in press (2007). [18] A. Biswas, G. S. Maciel, C. S. Friend, and P. N. Prasad, J. Non-Cryst. Solids 316, 393 (2003). [19] S. Tanabe, H. Hayashi, T. Hanada, and N. Onodera, Opt. Mater. 19, 343 (2002). [20] J. Qiu and A. Makishima, Adv. Mater. 5, 313 – 317 (2004). [21] A. S. Gouveia-Neto, E. B. da Costa, L. A. Bueno, and S. J. L. Ribeiro, J. Lumin. 110, 79 (2004). [22] K. Driesen, V. K. Tikhomirov, C. Görller-Warland, V. D. Rodríguez, and A. B. Seldon, Appl. Phys. Lett. 88, 73111 (2006).
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