- Email: [email protected]
Rare-earth doped YF3 nanocrystals embedded in sol–gel silica glass matrix for white light generation
Journal of Luminescence 130 (2010) 2508–2511
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Rare-earth doped YF3 nanocrystals embedded in sol–gel silica glass matrix for white light generation J. Me´ndez-Ramos a, A. Santana-Alonso b, A.C. Yanes b,n, J. del-Castillo b, V.D. Rodrı´guez a a b
´nica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Departamento Fı´sica Fundamental y Experimental, Electro ´sica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Departamento Fı´sica Ba
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 January 2010 Received in revised form 19 July 2010 Accepted 23 August 2010 Available online 26 August 2010
YF3 nanocrystals triply-doped with Yb3 + , Ho3 + and Tm3 + ions embedded in amorphous silica matrix have been successfully obtained by heat treatment of precursor sol–gel glasses for the first time to our knowledge and confirmed by X-ray diffraction and luminescence measurements. Simultaneous UV and visible efficient up-conversion emissions, with well-resolved Stark structure, under 980 nm infrared pump are observed, indicating the effective partition of rare-earth ions into a crystalline-like environment of the YF3 nanocrystals. Corresponding energy transfer mechanisms have been analyzed and overall colour emission has been quantified in terms of standard chromaticity diagram. By an adequate doping level and heat treatment temperature of precursor sol–gel glasses, a bright white colour has been accomplished, close to the standard equal energy white light illumination point, with potential applications in photo-electronic devices and information processing. & 2010 Elsevier B.V. All rights reserved.
Keywords: Nano-glass-ceramics Up-conversion White light generation Rare-earths
1. Introduction The search of efficient white light generation has devoted a lot of interest due to potential applications in a variety of fields, such as displays, photo-electronic devices, solid state lasers, etc. [1–3]. An efficient white light source can be obtained by means of a cheap excitation, e.g. a commercial laser diode, an efficient absorption, species with stable excited energy levels and a controllable luminescence intensity of the three primary colours, i.e. red, green and blue (RGB). In this respect, frequency upconversion emissions of selected rare-earth (RE) optically active ions have proven to be an easy way to generate white light via multiphoton processes [4,5], by converting near infrared laser photons into visible. RE doped transparent materials show up as promising candidates for development in optical devices research such as colour displays, optical communication, biosensors, data storage, fibre amplifiers, radiation detection and solid state lasers [6–10]. These applications rely on the high efficiency, long-lived intermediate levels and narrow spectral lines of RE ions and on the physical and optical properties of host materials [11]. Thus, nanoglass-ceramics comprising RE doped fluoride nanocrystals embedded in an oxide glassy matrix emerge as high quantum efficient luminescence materials. An appropriate thermal treatment of precursor glasses allows a controllable growth of these nanocrystals, without loss of transparency, where RE ions are
n
Corresponding author. Tel.: + 34 922 318 304; fax: + 34 922 318 228. E-mail address: [email protected] (A.C. Yanes).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.08.027
selectively incorporated, while oxide glassy matrix provides chemical and mechanical stability [12]. Yttrium trifluoride (YF3) is well-known as an efficient host lattice for RE ions due to its wide band gap and the possibility of replacement of Y3 + sites by trivalent RE ions avoiding the need for charge compensation [13]. In this sense, RE doped YF3 nanocrystals present high quantum efficiency, due to its low phonon energy environment, becoming a good candidate in the field of optical nano-structured materials [14,15]. Room temperature sol–gel procedure present preparation advantages, alternative to melting quenching methods [16], to synthesize transparent nano-glass-ceramics under adequate heat treatment. Thus, the embedded nanocrystals are obtained with a controlled shape and size [17,18], which determine their physical and optical properties. In this respect, we have successfully developed sol–gel derived highly transparent RE doped nanoglass-ceramics [19–21]. In this work, we present nano-glass-ceramics comprising YF3 nanocrystals doped with Yb3 + , Ho3 + and Tm3 + prepared by heat treatment of precursor sol–gel glasses for the first time to our knowledge. Simultaneous red, green and blue up-conversion emissions result in an efficient white light with potential applications in general lighting appliances and integrated photonic devices.
2. Experimental Silica glasses with composition 90SiO2–10YF3 co-doped with 0.3Yb3 + –0.1Ho3 + , 0.3Yb3 + –0.1Tm3 + and triply-doped with
J. Me´ndez-Ramos et al. / Journal of Luminescence 130 (2010) 2508–2511
0.3 Yb3 + –xHo3 + –0.1 Tm3 + with x ¼0.1, 0.05 and 0.025 (mol%) were obtained by sol–gel method as described in Ref. [20]. Tetraethoxysilane (TEOS) Si(OCH2CH3)4, used as a source of SiO2, 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. As a source of Y, Y(CH3COO)3 xH2O was used. The required quantities of Y(CH3COO)3 xH2O, Yb(CH3COO)3 xH2O, Ho(CH3COO)3 xH2O and Tm(CH3COO)3 xH2O were dissolved in CF3COOH and H2O solution, which was slowly mixed with the initial solution. The molar ratio of metal ions to CF3COOH was 1:4. In order to obtain a homogeneous solution, the resultant one was stirred vigorously for 1 h at room temperature. A highly transparent gel was obtained by leaving the resultant homogeneous solution in a sealed container at 35 1C for several days. Then, the gels were dried by slow evaporation of residual water and solvent. Finally, these sol–gel glasses were heat-treated in air at 675 1C, optimum temperature in order to achieve controlled precipitation of nanocrystallites, giving rise to transparent nano-glass-ceramics. X-ray powder diffraction (XRD) patterns of the samples were recorded with a Philips X’Pert Pro diffractometer equipped with a primary monochromator, Cu Ka1,2 radiation, and an X’Celerator detector. The XRD patterns were collected with a step of 0.0161 in the 2y angular range 10–901 and acquisition time of 2 h. Furthermore, the patterns were corrected by using LaB6. Luminescence measurements were obtained by exciting the samples with a laser diode at 980 nm with pump power up to 200 mW, focused with a 4 microscope objective, and detected through a 0.25 m monochromator equipped with a photomultiplier. All spectra were collected at room temperature and corrected by the instrumental response.
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Fig. 1. XRD patterns of 90SiO2–10YF3 RE doped nano-glass-ceramics heat-treated at 675 1C along with corresponding xero-gel sample. Standard peaks of YF3 (JCPDS 74-0911) are included for comparison purposes.
3. Results and discussion 3.1. Structural analysis Fig. 1 shows XRD patterns of 90SiO2–10YF3 xero-gel and a glass-ceramic corresponding to a sample heat-treated at 675 1C, optimum temperature for nano-crystalline precipitation without lost of transparency. A broad diffraction curve of xero-gel indicates a predominant glassy environment, while reflection peaks of the glass ceramics shows the precipitation of YF3 nanocrystals embedded into the glassy host corresponding to a crystallization in the orthorhombic system (JCPDS File no. 74-0911) [22,23]. The average nanocrystals radii were calculated from the XRD patterns by using the Scherrer’s equation, obtaining values around 7 nm. 3.2. Up-conversion luminescence spectra Infrared to visible up-conversion emissions under 980 nm pump are obtained in nano-glass-ceramics samples co-doped with Yb3 + –Tm3 + and Yb3 + –Ho3 + , see spectra presented in Fig. 2. Up-conversion emission bands have been labelled according to electronic transitions of involved RE ions. The proposed energy transfer mechanisms and main emission transitions are depicted in the schematic energy level diagrams of Yb3 + , Ho3 + and Tm3 + ions shown in Fig. 3. The Yb3 + ions act as sensitizer ions for Ho3 + acceptor ions due to a higher doping level and larger absorption cross-section. In this way, under 980 nm pumping, and by successive energy transfer processes from excited Yb3 + :2F5/2 ions, the 5I6, 5F5, (5S2,5F4) and 5G3 levels of Ho3 + ions are populated. Therefore, UV–vis emissions are obtained at 482, 540 and 650 nm, assigned
Fig. 2. Up-conversion emission spectra in 90SiO2–10YF3 co-doped with 0.3Yb3 + – 0.1Ho3 + and 0.3Yb3 + –0.1Tm3 + (mol%) nano-glass-ceramics heat-treated at 675 1C under 980 nm excitation at 200 mW pump power. Spectra have been normalized to their corresponding maximum intensities.
to (5F3,5F2,3K8)-5I8, (5S2,5F4)-5I8, (5F3,5F2,3K8)-5I7 and 5F5-5I8 transitions respectively, see energy level diagram of Fig. 3. It should be noticed that sharp structure and well-resolved Stark components of emissions bands suggest the effective partition of Ho3 + ions into a crystalline-like environment of the YF3 nanocrystals. It is worth mentioning that the observed RE luminescence would came mainly from those ions residing inside the nanocrystals and therefore shielded from remaining OH groups located in the glassy matrix or near to the nanocrystals. Additionally, Fig. 2 also shows up-conversion emission spectrum of the Yb3 + –Tm3 + co-doped nano-glass-ceramics. The Yb3 + ions also act as sensitizer ions for Tm3 + acceptor ions and therefore,
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Fig. 3. Energy level diagrams of Yb3 + , Ho3 + and Tm3 + ions. Main up-conversion emissions are indicated by solid arrows with corresponding mechanisms by dash arrows. Dot arrows labelled by (1) and (2) indicate cross-relaxation processes among Tm3 + and Ho3 + ions.
under a 980 nm pumping, energy transfer processes from Yb3 + to Tm3 + ions populate 3H5, 3F3,2 and 1G4 levels of Tm3 + [24]. The 1D2 level of Tm3 + cannot be populated directly by energy transfer processes from 2F5/2 level of Yb3 + to promote Tm3 + ions from 1G4 to 1D2 due to the large energy mismatch. Instead, the crossrelaxation process 3F3,2 + 3H4-3H6 + 1D2 of Tm3 + ions may play an important role in the population mechanism [25]. By other side, 1 I6 level can be fed by means of energy transfer from Yb3 + ions rising Tm3 + ions from 1D2 to 1I6 level. Subsequently, UV, blue and red up-conversion emissions can be observed in Fig. 2. The remarkably high intensities of UV and blue emissions of Tm3 + ions are favoured by the low phonon energy environment of YF3 nanocrystals host. These emissions are located at 350–360 nm assigned to 1 I6-3F4, 1D2-3H6 and at 450–480 nm assigned to 1D2-3F4 and 1 G4-3H6 transitions, respectively. Therefore, this supports the assumption of the partition of RE ions into a crystalline-like environment of YF3 nanoparticles, precipitated during heat treatment, as it was observed in XRD measurements of Fig. 1. Next, Fig. 4 shows up-conversion emissions of the Yb3 + –Ho3 + – Tm3 + doped nano-glass-ceramics under 980 nm pumping, with varying Ho3 + doping level. Once more, their well-resolved Stark components can be clearly observed, indicating a crystalline-like environment for RE ions in these triply-doped samples. By comparison with above presented co-doped samples shown in Fig. 2, it is observed that when Tm3 + ions are added, red emissions are enhanced more than the expected combination of Yb3 + –Tm3 + and Yb3 + –Ho3 + samples. This fact can be explained in terms of energy transfer processes from Tm3 + to Ho3 + ions, indicated by dot lines (1) and (2) in the energy level diagrams of Fig. 3, i.e cross-relaxation channels given by (3F4:Tm3 + + 5I8:Ho3 + -3H6: Tm3 + + 5I7:Ho3 + ) and (3H4:Tm3 + + 5I6:Ho3 + -3H6:Tm3 + + 5F3,2 3K8: Ho3 + ). In addition, the diminishment of UV and blue emissions of Tm3 + when increase in the concentration of Ho3 + can be also attributed to the depopulation of 3F4 level of Tm3 + ions due the above commented energy transfer processes among Tm3 + and Ho3 + ions [26,27]. Finally, in order to generate white light, a balanced contribution of the three primary colours, i.e. red, green and blue (RGB), must be achieved. In this sense, up-conversion emissions of Yb3 + –Ho3+ –Tm3 + triply-doped sampled heat-treated at 675 1C shown above in Fig. 4, provide simultaneous RGB up-conversion emissions. In particular, the most balanced contribution corresponds to the 0.025 mol% Ho3 + -doped sample. Moreover, the overall emitted colour can be quantified by a coordinate point in the CIE standard chromaticity diagram [28], where any colour can be plotted in
Fig. 4. Up-conversion emission spectra in 90SiO2–10YF3 triply-doped with 0.3Yb3 + , xHo3 + and 0.1Tm3 + (mol%) nano-glass-ceramics heat-treated at 675 1C, with x ¼0.1, 0.05 and 0.025, under 980 nm excitation at 200 mW pump power. Spectra have been normalized to the maximum of the 660 nm emission band.
terms of (x,y) colour coordinates, representing the actual chromaticity seen by the naked eye after a correction to sensitive receptors of the eye. Thus, Fig. 5 shows colour coordinates of Yb3 + –Tm3 + and Yb3 + –Ho3 + co-doped samples, along with data corresponding to the Yb3 + –Ho3 + –Tm3 + triply-doped samples, under 200 mW pump power and heat-treated at 675 1C. Intense blue and red colour is obtained for the Yb3 + –Tm3 + and Yb3 + – Ho3 + co-doped samples, with colour coordinates (0.19, 0.12) and (0.50, 0.42), respectively, see Fig. 5. On the other side, for the Yb3 + –Ho3 + –Tm3 + triply-doped samples, a bright white colour is obtained for the 0.025 mol% Ho3 + -doped sample with colour coordinates (0.34, 0.33) near to the standard equal energy white colour. The increase in the concentration of Ho3 + ions to 0.05 and 0.1 (mol%) provides colour tuneability towards the reddish zone with colour coordinates (0.42, 0.32) and (0.47, 0.33), respectively. This fact can be partially due to the enhancement of crossrelaxation energy transfer occurring among Tm3 + and Ho3 + ions as discussed above.
4. Conclusions YF3 nanocrystals co-doped with Yb3 + –Tm3 + and Yb3 + –Ho3 + and triply-doped with Yb3 + –Ho3 + –Tm3 + partitioned into silica matrix have been successfully developed by adequate heat treatment of sol–gel precursor glasses for the first time to our
J. Me´ndez-Ramos et al. / Journal of Luminescence 130 (2010) 2508–2511
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Desarrollo Regional FEDER (SolSubC200801000286) and Ministerio de Ciencia y Tecnologı´a of Spain Government (FIS 200602980) for financial support.
References
Fig. 5. CIE standard chromaticity diagram including colour coordinates of 90SiO2– 10YF3 nano-glass-ceramics heat-treated at 675 1C co-doped with 0.3Yb3 + –0.1Ho3 + , 0.3Yb3 + –0.1Tm3 + and triply-doped with 0.3Yb3 + –xHo3 + –0.1Tm3 + (mol%) with x¼0.1, 0.05, 0.025, respectively, under 980 nm excitation at 200 mW pump power.
knowledge. XRD measurements confirmed the precipitation of YF3 nanocrystals and luminescence features, such as wellresolved Stark structure and high energy up-conversion emissions, point out the incorporation of RE ions into the nanocrystals. Corresponding up-conversion mechanisms and cross-relaxation energy transfer processes, among Tm3 + and Ho3 + ions, have been analyzed. Bright white colour matching the standard equal energy point in the CIE diagram was achieved in the 0.3Yb3 + , 0.025Ho3 + and 0.1Tm3 + (mol%) triply-doped sample heat-treated at 675 1C. Moreover, easy colour tuneability by adjusting Ho3 + concentration is achieved in the triply-doped samples, showing up potential applications in photonic integrated devices and colour displays.
Acknowledgments The authors would like to thank Agencia Canaria de Investigacio´n, Innovacio´n y Sociedad de la Informacio´n y Fondo Europeo de
¨ [1] T. Trupke, A. Shalav, B.S. Richards, P. Wurfel, M.A. Green, Sol. Energy Mater Sol. Cells 90 (2006) 3327. ¨ E. Osiac, Opt. Mater. 26 (2004) 365. [2] H. Scheife, G. Huber, E. Heumann, S. Bar, [3] Y. Ying, A.P. Alivisatos, Nature 437 (2005) 664. [4] D. Matsuura, Appl. Phys. Lett. 81 (2002) 4526. [5] S. Sivakumar, F.C.J.M. van Veggel, M. Raudsepp, J. Am. Chem. Soc. 127 (2005) 12464. [6] L.H. Huang, X.R. Liu, W. Xu, B.J. Chen, J.L. Lin, J. Appl. Phys. 90 (2001) 5550. [7] S.Q. Man, E.Y.B. Pun, P.S. Chung, Appl. Phys. Lett. 77 (2000) 483. [8] S. Xu, Z. Yang, J. Zhang, G. Wang, S. Dai, L. Hu, Z. Jiang, Chem. Phys. Lett. 385 (2004) 263. [9] D.A. Parthenopoulos, P.M. Rentzepis, Science 245 (1989) 843. [10] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996) 1185. [11] A. Polman, Physica B 300 (2001) 78. [12] G.H. Beall, L.R. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. [13] V. Pankratov, M. Kirm, H. von Seggern, J. Lumin. 113 (2005) 143. [14] M. Zhang, H. Fan, B. Xi, X. Wang, C. Dong, Y. Qian, J. Phys. Chem. C 111 (2007) 6652. [15] R.X. Yan, Y.D. Li, Adv. Funct. Mater. 15 (2005) 763. [16] C.J. Brinker, G.W. Scherer, in: Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, 1990. [17] S.R. Shannigrahi, R.N.P. Choudhary, H.N. Acharya, T.P. Sinha, J. Appl. Phys. 85 (1713) 1999. [18] A.C. Yanes, J. del-Castillo, M.E. Torres, J. Peraza, V.D. Rodrı´guez, J. Me´ndezRamos, Appl. Phys. Lett. 85 (2004) 2343. [19] V.D. Rodrı´guez, J. del-Castillo, A.C. Yanes, J. Me´ndez-Ramos, M.E. Torres, J. Peraza, Opt. Mater. 29 (2007) 1557. [20] A.C. Yanes, A. Santana-Alonso, J. Me´ndez-Ramos, J. del-Castillo, V.D. Rodrı´guez, J. Alloys Compd. 480 (2009) 706. [21] J. Me´ndez-Ramos, A.C. Yanes, A. Santana-Alonso, J. del-Castillo and V.D. Rodrı´guez, J. Nanosci. Nanotechnol. 10.1166/jnn.2010.1871 (2010). [22] F. Weng, D. Chen, Y. Wang, Y. Yu, P. Huang, H. Lin, Ceram. Int. 35 (2009) 2619. [23] W. Luo, Y. Wang, Y. Cheng, F. Bao, L. Zhou, Mater. Sci. Eng. B 127 (2006) 218. [24] J. Me´ndez-Ramos, J.J. Vela´zquez, A.C. Yanes, J. del-Castillo, V.D. Rodrı´guez, Phys. Status Solidi (a) 205 (2008) 330. [25] W. Guofeng, Q. Weiping, W. Lili, W. Guodong, Z. Peifen, Z. Daisheng, D. Fuheng, J. Rare Earth 27 (2009) 330. [26] L. Ki-Soo, P. Babu, L. Sun-Kyun, P. Van-Thai, D.S. Hamilton, J. Lumin. 102 (2003) 737. [27] D. Wang, Y. Guo, G. Sun, J. Li, L. Zhao, G. Xu, J. Alloys Compd. 451 (2008) 122. [28] CIE, International Commission on Illumination, Colorimetry: Official Recommendations of the International Commission on Illumination, Publication CIE no. 15 (E-1.3.1), Bureau Central de la CIE, Paris, 1991.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Rare-earth doped YF3 nanocrystals embedded in sol–gel silica glass matrix for white light generation J. Me´ndez-Ramos a, A. Santana-Alonso b, A.C. Yanes b,n, J. del-Castillo b, V.D. Rodrı´guez a a b
´nica y Sistemas, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Departamento Fı´sica Fundamental y Experimental, Electro ´sica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain Departamento Fı´sica Ba
a r t i c l e in f o
a b s t r a c t
Article history: Received 13 January 2010 Received in revised form 19 July 2010 Accepted 23 August 2010 Available online 26 August 2010
YF3 nanocrystals triply-doped with Yb3 + , Ho3 + and Tm3 + ions embedded in amorphous silica matrix have been successfully obtained by heat treatment of precursor sol–gel glasses for the first time to our knowledge and confirmed by X-ray diffraction and luminescence measurements. Simultaneous UV and visible efficient up-conversion emissions, with well-resolved Stark structure, under 980 nm infrared pump are observed, indicating the effective partition of rare-earth ions into a crystalline-like environment of the YF3 nanocrystals. Corresponding energy transfer mechanisms have been analyzed and overall colour emission has been quantified in terms of standard chromaticity diagram. By an adequate doping level and heat treatment temperature of precursor sol–gel glasses, a bright white colour has been accomplished, close to the standard equal energy white light illumination point, with potential applications in photo-electronic devices and information processing. & 2010 Elsevier B.V. All rights reserved.
Keywords: Nano-glass-ceramics Up-conversion White light generation Rare-earths
1. Introduction The search of efficient white light generation has devoted a lot of interest due to potential applications in a variety of fields, such as displays, photo-electronic devices, solid state lasers, etc. [1–3]. An efficient white light source can be obtained by means of a cheap excitation, e.g. a commercial laser diode, an efficient absorption, species with stable excited energy levels and a controllable luminescence intensity of the three primary colours, i.e. red, green and blue (RGB). In this respect, frequency upconversion emissions of selected rare-earth (RE) optically active ions have proven to be an easy way to generate white light via multiphoton processes [4,5], by converting near infrared laser photons into visible. RE doped transparent materials show up as promising candidates for development in optical devices research such as colour displays, optical communication, biosensors, data storage, fibre amplifiers, radiation detection and solid state lasers [6–10]. These applications rely on the high efficiency, long-lived intermediate levels and narrow spectral lines of RE ions and on the physical and optical properties of host materials [11]. Thus, nanoglass-ceramics comprising RE doped fluoride nanocrystals embedded in an oxide glassy matrix emerge as high quantum efficient luminescence materials. An appropriate thermal treatment of precursor glasses allows a controllable growth of these nanocrystals, without loss of transparency, where RE ions are
n
Corresponding author. Tel.: + 34 922 318 304; fax: + 34 922 318 228. E-mail address: [email protected] (A.C. Yanes).
0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.08.027
selectively incorporated, while oxide glassy matrix provides chemical and mechanical stability [12]. Yttrium trifluoride (YF3) is well-known as an efficient host lattice for RE ions due to its wide band gap and the possibility of replacement of Y3 + sites by trivalent RE ions avoiding the need for charge compensation [13]. In this sense, RE doped YF3 nanocrystals present high quantum efficiency, due to its low phonon energy environment, becoming a good candidate in the field of optical nano-structured materials [14,15]. Room temperature sol–gel procedure present preparation advantages, alternative to melting quenching methods [16], to synthesize transparent nano-glass-ceramics under adequate heat treatment. Thus, the embedded nanocrystals are obtained with a controlled shape and size [17,18], which determine their physical and optical properties. In this respect, we have successfully developed sol–gel derived highly transparent RE doped nanoglass-ceramics [19–21]. In this work, we present nano-glass-ceramics comprising YF3 nanocrystals doped with Yb3 + , Ho3 + and Tm3 + prepared by heat treatment of precursor sol–gel glasses for the first time to our knowledge. Simultaneous red, green and blue up-conversion emissions result in an efficient white light with potential applications in general lighting appliances and integrated photonic devices.
2. Experimental Silica glasses with composition 90SiO2–10YF3 co-doped with 0.3Yb3 + –0.1Ho3 + , 0.3Yb3 + –0.1Tm3 + and triply-doped with
J. Me´ndez-Ramos et al. / Journal of Luminescence 130 (2010) 2508–2511
0.3 Yb3 + –xHo3 + –0.1 Tm3 + with x ¼0.1, 0.05 and 0.025 (mol%) were obtained by sol–gel method as described in Ref. [20]. Tetraethoxysilane (TEOS) Si(OCH2CH3)4, used as a source of SiO2, 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. As a source of Y, Y(CH3COO)3 xH2O was used. The required quantities of Y(CH3COO)3 xH2O, Yb(CH3COO)3 xH2O, Ho(CH3COO)3 xH2O and Tm(CH3COO)3 xH2O were dissolved in CF3COOH and H2O solution, which was slowly mixed with the initial solution. The molar ratio of metal ions to CF3COOH was 1:4. In order to obtain a homogeneous solution, the resultant one was stirred vigorously for 1 h at room temperature. A highly transparent gel was obtained by leaving the resultant homogeneous solution in a sealed container at 35 1C for several days. Then, the gels were dried by slow evaporation of residual water and solvent. Finally, these sol–gel glasses were heat-treated in air at 675 1C, optimum temperature in order to achieve controlled precipitation of nanocrystallites, giving rise to transparent nano-glass-ceramics. X-ray powder diffraction (XRD) patterns of the samples were recorded with a Philips X’Pert Pro diffractometer equipped with a primary monochromator, Cu Ka1,2 radiation, and an X’Celerator detector. The XRD patterns were collected with a step of 0.0161 in the 2y angular range 10–901 and acquisition time of 2 h. Furthermore, the patterns were corrected by using LaB6. Luminescence measurements were obtained by exciting the samples with a laser diode at 980 nm with pump power up to 200 mW, focused with a 4 microscope objective, and detected through a 0.25 m monochromator equipped with a photomultiplier. All spectra were collected at room temperature and corrected by the instrumental response.
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Fig. 1. XRD patterns of 90SiO2–10YF3 RE doped nano-glass-ceramics heat-treated at 675 1C along with corresponding xero-gel sample. Standard peaks of YF3 (JCPDS 74-0911) are included for comparison purposes.
3. Results and discussion 3.1. Structural analysis Fig. 1 shows XRD patterns of 90SiO2–10YF3 xero-gel and a glass-ceramic corresponding to a sample heat-treated at 675 1C, optimum temperature for nano-crystalline precipitation without lost of transparency. A broad diffraction curve of xero-gel indicates a predominant glassy environment, while reflection peaks of the glass ceramics shows the precipitation of YF3 nanocrystals embedded into the glassy host corresponding to a crystallization in the orthorhombic system (JCPDS File no. 74-0911) [22,23]. The average nanocrystals radii were calculated from the XRD patterns by using the Scherrer’s equation, obtaining values around 7 nm. 3.2. Up-conversion luminescence spectra Infrared to visible up-conversion emissions under 980 nm pump are obtained in nano-glass-ceramics samples co-doped with Yb3 + –Tm3 + and Yb3 + –Ho3 + , see spectra presented in Fig. 2. Up-conversion emission bands have been labelled according to electronic transitions of involved RE ions. The proposed energy transfer mechanisms and main emission transitions are depicted in the schematic energy level diagrams of Yb3 + , Ho3 + and Tm3 + ions shown in Fig. 3. The Yb3 + ions act as sensitizer ions for Ho3 + acceptor ions due to a higher doping level and larger absorption cross-section. In this way, under 980 nm pumping, and by successive energy transfer processes from excited Yb3 + :2F5/2 ions, the 5I6, 5F5, (5S2,5F4) and 5G3 levels of Ho3 + ions are populated. Therefore, UV–vis emissions are obtained at 482, 540 and 650 nm, assigned
Fig. 2. Up-conversion emission spectra in 90SiO2–10YF3 co-doped with 0.3Yb3 + – 0.1Ho3 + and 0.3Yb3 + –0.1Tm3 + (mol%) nano-glass-ceramics heat-treated at 675 1C under 980 nm excitation at 200 mW pump power. Spectra have been normalized to their corresponding maximum intensities.
to (5F3,5F2,3K8)-5I8, (5S2,5F4)-5I8, (5F3,5F2,3K8)-5I7 and 5F5-5I8 transitions respectively, see energy level diagram of Fig. 3. It should be noticed that sharp structure and well-resolved Stark components of emissions bands suggest the effective partition of Ho3 + ions into a crystalline-like environment of the YF3 nanocrystals. It is worth mentioning that the observed RE luminescence would came mainly from those ions residing inside the nanocrystals and therefore shielded from remaining OH groups located in the glassy matrix or near to the nanocrystals. Additionally, Fig. 2 also shows up-conversion emission spectrum of the Yb3 + –Tm3 + co-doped nano-glass-ceramics. The Yb3 + ions also act as sensitizer ions for Tm3 + acceptor ions and therefore,
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J. Me´ndez-Ramos et al. / Journal of Luminescence 130 (2010) 2508–2511
Fig. 3. Energy level diagrams of Yb3 + , Ho3 + and Tm3 + ions. Main up-conversion emissions are indicated by solid arrows with corresponding mechanisms by dash arrows. Dot arrows labelled by (1) and (2) indicate cross-relaxation processes among Tm3 + and Ho3 + ions.
under a 980 nm pumping, energy transfer processes from Yb3 + to Tm3 + ions populate 3H5, 3F3,2 and 1G4 levels of Tm3 + [24]. The 1D2 level of Tm3 + cannot be populated directly by energy transfer processes from 2F5/2 level of Yb3 + to promote Tm3 + ions from 1G4 to 1D2 due to the large energy mismatch. Instead, the crossrelaxation process 3F3,2 + 3H4-3H6 + 1D2 of Tm3 + ions may play an important role in the population mechanism [25]. By other side, 1 I6 level can be fed by means of energy transfer from Yb3 + ions rising Tm3 + ions from 1D2 to 1I6 level. Subsequently, UV, blue and red up-conversion emissions can be observed in Fig. 2. The remarkably high intensities of UV and blue emissions of Tm3 + ions are favoured by the low phonon energy environment of YF3 nanocrystals host. These emissions are located at 350–360 nm assigned to 1 I6-3F4, 1D2-3H6 and at 450–480 nm assigned to 1D2-3F4 and 1 G4-3H6 transitions, respectively. Therefore, this supports the assumption of the partition of RE ions into a crystalline-like environment of YF3 nanoparticles, precipitated during heat treatment, as it was observed in XRD measurements of Fig. 1. Next, Fig. 4 shows up-conversion emissions of the Yb3 + –Ho3 + – Tm3 + doped nano-glass-ceramics under 980 nm pumping, with varying Ho3 + doping level. Once more, their well-resolved Stark components can be clearly observed, indicating a crystalline-like environment for RE ions in these triply-doped samples. By comparison with above presented co-doped samples shown in Fig. 2, it is observed that when Tm3 + ions are added, red emissions are enhanced more than the expected combination of Yb3 + –Tm3 + and Yb3 + –Ho3 + samples. This fact can be explained in terms of energy transfer processes from Tm3 + to Ho3 + ions, indicated by dot lines (1) and (2) in the energy level diagrams of Fig. 3, i.e cross-relaxation channels given by (3F4:Tm3 + + 5I8:Ho3 + -3H6: Tm3 + + 5I7:Ho3 + ) and (3H4:Tm3 + + 5I6:Ho3 + -3H6:Tm3 + + 5F3,2 3K8: Ho3 + ). In addition, the diminishment of UV and blue emissions of Tm3 + when increase in the concentration of Ho3 + can be also attributed to the depopulation of 3F4 level of Tm3 + ions due the above commented energy transfer processes among Tm3 + and Ho3 + ions [26,27]. Finally, in order to generate white light, a balanced contribution of the three primary colours, i.e. red, green and blue (RGB), must be achieved. In this sense, up-conversion emissions of Yb3 + –Ho3+ –Tm3 + triply-doped sampled heat-treated at 675 1C shown above in Fig. 4, provide simultaneous RGB up-conversion emissions. In particular, the most balanced contribution corresponds to the 0.025 mol% Ho3 + -doped sample. Moreover, the overall emitted colour can be quantified by a coordinate point in the CIE standard chromaticity diagram [28], where any colour can be plotted in
Fig. 4. Up-conversion emission spectra in 90SiO2–10YF3 triply-doped with 0.3Yb3 + , xHo3 + and 0.1Tm3 + (mol%) nano-glass-ceramics heat-treated at 675 1C, with x ¼0.1, 0.05 and 0.025, under 980 nm excitation at 200 mW pump power. Spectra have been normalized to the maximum of the 660 nm emission band.
terms of (x,y) colour coordinates, representing the actual chromaticity seen by the naked eye after a correction to sensitive receptors of the eye. Thus, Fig. 5 shows colour coordinates of Yb3 + –Tm3 + and Yb3 + –Ho3 + co-doped samples, along with data corresponding to the Yb3 + –Ho3 + –Tm3 + triply-doped samples, under 200 mW pump power and heat-treated at 675 1C. Intense blue and red colour is obtained for the Yb3 + –Tm3 + and Yb3 + – Ho3 + co-doped samples, with colour coordinates (0.19, 0.12) and (0.50, 0.42), respectively, see Fig. 5. On the other side, for the Yb3 + –Ho3 + –Tm3 + triply-doped samples, a bright white colour is obtained for the 0.025 mol% Ho3 + -doped sample with colour coordinates (0.34, 0.33) near to the standard equal energy white colour. The increase in the concentration of Ho3 + ions to 0.05 and 0.1 (mol%) provides colour tuneability towards the reddish zone with colour coordinates (0.42, 0.32) and (0.47, 0.33), respectively. This fact can be partially due to the enhancement of crossrelaxation energy transfer occurring among Tm3 + and Ho3 + ions as discussed above.
4. Conclusions YF3 nanocrystals co-doped with Yb3 + –Tm3 + and Yb3 + –Ho3 + and triply-doped with Yb3 + –Ho3 + –Tm3 + partitioned into silica matrix have been successfully developed by adequate heat treatment of sol–gel precursor glasses for the first time to our
J. Me´ndez-Ramos et al. / Journal of Luminescence 130 (2010) 2508–2511
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Desarrollo Regional FEDER (SolSubC200801000286) and Ministerio de Ciencia y Tecnologı´a of Spain Government (FIS 200602980) for financial support.
References
Fig. 5. CIE standard chromaticity diagram including colour coordinates of 90SiO2– 10YF3 nano-glass-ceramics heat-treated at 675 1C co-doped with 0.3Yb3 + –0.1Ho3 + , 0.3Yb3 + –0.1Tm3 + and triply-doped with 0.3Yb3 + –xHo3 + –0.1Tm3 + (mol%) with x¼0.1, 0.05, 0.025, respectively, under 980 nm excitation at 200 mW pump power.
knowledge. XRD measurements confirmed the precipitation of YF3 nanocrystals and luminescence features, such as wellresolved Stark structure and high energy up-conversion emissions, point out the incorporation of RE ions into the nanocrystals. Corresponding up-conversion mechanisms and cross-relaxation energy transfer processes, among Tm3 + and Ho3 + ions, have been analyzed. Bright white colour matching the standard equal energy point in the CIE diagram was achieved in the 0.3Yb3 + , 0.025Ho3 + and 0.1Tm3 + (mol%) triply-doped sample heat-treated at 675 1C. Moreover, easy colour tuneability by adjusting Ho3 + concentration is achieved in the triply-doped samples, showing up potential applications in photonic integrated devices and colour displays.
Acknowledgments The authors would like to thank Agencia Canaria de Investigacio´n, Innovacio´n y Sociedad de la Informacio´n y Fondo Europeo de
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