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Yb3+ ions
Journal of Alloys and Compounds 536 (2012) 198–203
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Upconversion emissions in YAG glass ceramics doped with Tm3+/Yb3+ ions Yuhua Li a,b, Baojiu Chen b, Xin Zhao a, Zhiqiang Wang a, Hai Lin a,b,⇑ a b
School of Textile and Material Engineering, Dalian Polytechnic University, Dalian 116034, PR China Department of Physics, Dalian Maritime University, Dalian 116026, PR China
a r t i c l e
i n f o
Article history: Received 24 March 2012 Received in revised form 1 May 2012 Accepted 2 May 2012 Available online 12 May 2012 Keywords: Tm3+/Yb3+ doped YAG glass ceramics Stark splitting Multi-photon upconversion fluorescence
a b s t r a c t Tm3+/Yb3+ doped glass ceramics, containing single phase of YAG micro-crystals with several preferred orientations and a dominant one (4 4 4) in the matrix, which present the primary particle size of 46 nm and the secondary particle size of 8 lm, have been prepared by heat-treating the Li2O– Y2O3–Al2O3–SiO2 (LYAS) precursor glasses. Judd-Ofelt parameters Xt (t = 2, 4, 6) are derived to be 5.14 1020, 1.11 1020, and 1.19 1020 cm2, respectively, indicating high inversion asymmetry and strong covalent environment in the precursor glasses. Efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ are captured in Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics. Obvious Stark splitting in the glass ceramics manifests that rare earth ions have been incorporated into YAG lattices. The comparison of spectral behaviors of the glass and glass ceramic samples will be helpful for further exploring the novel phosphors applied to illumination and display. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Yttrium aluminium garnet (Y3Al5O12, YAG in brief) materials are promising candidates for optical and lighting applications, such as high power lasers, color displays, optical sensors, and LED phosphors. This is primarily ascribed to its small thermal expansion, outstanding chemical–mechanical stability, low acoustic losses, and excellent optical properties [1–7]. Among YAG systems, glass ceramics containing YAG phase are one of the most seductive optoelectronic materials, attributing to its larger size, higher rare earth (RE) ions doping concentration, and easier fabrication compared with the single YAG crystals [8–11]. Up to now, many researchers have devoted themselves with great enthusiasms to the studies of RE doped YAG systems, and great achievements have been reported on spectral characteristics and crystal growth processes [12–21]. For example, the luminescence characteristics of Ce3+:YAG glass–ceramic phosphor for white LED [12,13], the optical properties and laser performances of Ho3+:YAG transparent ceramics [15], and the effects of nano-YAG crystallization on the structure and photoluminescence properties of Nd3+-doped glasses [18], and so on, have been investigated. Among RE ions, Tm3+ is a promising optical activator that opens the possibility for simultaneous blue and ultraviolet (UV) emissions of laser action and various applications [22–27]. Moreover,
⇑ Corresponding author at: School of Textile and Material Engineering, Dalian Polytechnic University, Dalian 116034, PR China. Tel.: +86 411 86323097; fax: +86 411 86322228. E-mail address: [email protected] (H. Lin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.009
the upconversion efficiency of Tm3+ can be substantially improved by codoping with Yb3+ under the 980 nm wavelength laser pumping, which is only completely resonant with 2F7/2 ? 2F5/2 transition absorption of Yb3+ but not resonant at all with the ground-state absorption of Tm3+. Hence, Tm3+/Yb3+ co-doped situations were widely adopted in investigating and utilizing the light radiation from Tm3+ [28–36]. In the present work, Tm3+/Yb3+ doped multi-component silicate glasses have been designed and fabricated, and then the glass ceramics containing single phase of YAG micro-crystals are derived by heat-treating the precursor glasses. The XRD and SEM results indicate that the YAG crystals grow with several preferred orientations and a dominant one (4 4 4) in the matrix, and efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ with obvious Stark splitting are exhibited in YAG glass ceramics.
2. Experiment Tm3+/Yb3+ doped glasses were prepared from high-purity Li2CO3, Y2O3, Al2O3, SiO2, Tm2O3 and Yb2O3 powders, according to the molar composition of 5Li2O–18Y2O3–29Al2O3–48SiO2 (LYAS) with additional doping of 1 wt% Tm2O3 and 2 wt% Yb2O3. The relative molar ratio among Y2O3, Al2O3 and SiO2 is 19.0:30.5:50.5, which located in the central of the glass formation region presented in Fig. 1 [37]. The well-mixed raw materials were first melted in a platinum crucible at 1620 °C for 3 h using an electric furnace, then the molten glasses were cast onto an iron plate and subsequently annealed at 500 °C for 3 h, after that cooled down slowly to room temperature. The choice of the annealed temperature at 500 °C is based on the glass transition temperature (T g ) 531 °C derived by differential thermal analysis (DTA) curve as presented in the inset of Fig. 2(a), which was carried out by a WCR-2D differential thermal analyzer. The LYAS glasses exhibit excellent
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a Philips XL40 scanning electron microscopy (SEM) and a JSM-6460LV SEM, respectively. Absorption spectrum of the glass sample was recorded using a Perkin-Elmer UV–VIS-NIR Lambda 19 double-beam spectrophotometer. Upconversion spectra were measured using a Jobin Yvon Fluorolog-3 spectrophotometer with an R928 photomultiplier tube (PMT) detector, and a commercial CW Xe-lamp was used as pump source. The dependences of luminescence intensity on pump power were determined using a Perkin-Elmer LS 55 luminescence spectrometer with an R928 PMT detector, and a 974 nm wavelength fiber-pigtailed laser diode was adopted as pump source. All the measurements were carried out at room temperature.
3. Results and discussion
Fig. 1. Glass formation region in Y2O3-Al2O3-SiO2 ternary phase diagram. Inserted photo: LYAS glasses.
The powder XRD patterns of Tm3+/Yb3+ doped LYAS glasses and the derived glass ceramics are shown in Fig. 2(a) and (b), respectively. In Fig. 2(a), the amorphous state of LYAS glasses is well confirmed. In Fig. 2(b), only a YAG phase is identified and no peak is assigned to other crystalline phase. The lattice constant for Tm3+/ Yb3+ doped YAG glass ceramics is calculated to be 1.201 nm, which is consistent with the value of pure YAG crystal phase, and the Tm3+ and Yb3+ ions can be considered to take the position of Y3+ in the lattice structure, because the radii of Tm3+ (0.094 nm) and Yb3+ (0.093 nm) are extremely similar to that of Y3+ (0.096 nm) [38,39]. The primary particle size of the sample is estimated using the Scherrer equation [40],
D¼
Fig. 2. XRD patterns of powder Tm3+/Yb3+ doped LYAS glasses (a) and derived YAG glass ceramics (b), and XRD pattern of bulk YAG glass ceramics (c). Inserted photo: YAG glass ceramics. Inset of (a): DTA curve of Tm3+/Yb3+ doped LYAS glasses.
transparency as shown in the inset of Fig. 1. The glass density was determined by the Archimedes method using distilled water and was derived to be 3.462 g cm3. Utilizing a Metricon 2010 prism coupler, the refractive indices were measured to be 1.678 and 1.649 at 632.8 and 1536 nm, respectively. For obtaining the YAG glass ceramics, LYAS glasses were heat-treated at 1350 °C for 6 h, and then annealed at 400 °C for 2 h, finally the white samples were slowly cooled down to room temperature. For optical measurements, the annealed glass and glass ceramic samples were sliced and polished with two parallel sides. X-ray diffraction (XRD) measurements for powder and bulk samples were carried out by a D/Max-3B X-ray diffractometer (40 kV, 20 mA) and a SIEMENS D500 X-ray diffractometer (40 kV, 30 mA), respectively. The surface appearance and cross-section microstructure of the glass ceramic sample were observed with
kk ; b cos h
ð1Þ
where K is a constant (0.94), k is the wavelength of X-ray (0.154 nm for CuKa), b is the corrected full width at half maximum of the strongest diffraction peak in radians, and h is the diffraction angle. The crystallite size calculated from Eq. (1) is 46 nm. The XRD pattern of bulk YAG glass ceramic sample and its appearance are presented in Fig. 2(c). Compared with the diffraction spectrum of the powder sample, the peak number of the bulk sample diminishes obviously. The strongest peak corresponding to crystal face also changes from (4 2 0) to (4 4 4), in the meanwhile, the intensity ratios of peaks on several crystal faces, such as (2 1 1), (4 2 2) and (6 4 2), to that on (4 2 0) crystal face increase significantly, demonstrating that the crystalline grains grow with several preferred orientations and a dominant one (4 4 4) in the bulk YAG glass ceramics. The SEM micrographs of surface and cross-section of the glass– ceramic sample are shown in Fig. 3(a) and (b), respectively. The YAG micro particles are irregular in shape, but they distribute along certain preferred orientations and a highly predominant one in the matrix, which is in good agreement with the bulk XRD pattern, and this pilotaxitic texture could increase yield strength and tensile strength of YAG glass ceramics effectively. Their average size is 8 lm, much larger than 46 nm calculated by Scherrer equation. The main reason is that the primary nanocrystals with high energy would assemble with interface matching coherently to minimize the surface energy and sinter by the heat treatment process [41,42]. Combined with the XRD and SEM results, it is obvious that after heat treatment, the glass ceramics containing YAG crystal phase were formed in the Tm3+/Yb3+ doped LYAS mother glasses. The absorption spectrum of Tm3+/Yb3+ doped LYAS glasses shows the typical transitions from Tm3+ and Yb3+ ions, as illustrated in Fig. 4. The spectrum consists of six well-isolated absorption bands associated with transitions from the ground 3H6 level to the excited 1D2, 1G4, 3F2,3, 3H4, 3H5 and 3F4 levels of Tm3+, and a single intense absorption band attributed to the 2F7/2 ? 2F5/2 transition of Yb3+ at 976 nm. The Judd–Ofelt intensity parameters Xt (t = 2, 4, 6) of Tm3+ are derived to be 5.14 1020, 1.11 1020, and 1.19 1020 cm2, respectively. The value of X2 is larger than 3.31 1020 and 4.29 1020 cm2 in strontium silicate and calcium fluoro phosphorous silicate glasses [43,44], revealing a strong
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Fig. 4. Absorption spectrum of 1 wt% Tm2O3 and 2 wt% Yb2O3 doped LYAS glasses. Inset: Absorption cross-section (rabs, curve 1) and calculated emission crosssection (rem, curve 2) of Yb3+ in Tm3+/Yb3+ doped LYAS glasses.
and YAG glass ceramics, respectively, thus the two samples exhibit intense blue fluorescence as shown in the inserted photos of Fig. 5, due to the absence of contribution from NIR emission to the nakedeye vision. On account of the excitation laser from output end of the SIEOR 62.5/125 lm optical fiber irradiating directly on the closely laid sample, the diameter of the laser spot is estimated to be 60–80 lm, much larger than the average size 8 lm of the YAG micro particles. Therefore, not only are the Tm3+ ions in YAG micro-crystals excited efficiently, but also the stimulated Tm3+ ions in the rest area of glass ceramics partially contribute to the upconversion fluorescences. In YAG glass ceramics, multi-peaks splitting occurs at the blue and NIR emission bands, and the strongest split sub-peaks are observed to be around 484 and 820 nm, respectively, showing 8 and 18 nm red-shifts compared with those strongest peaks of LYAS glasses. Meanwhile, peaks of the upconversion emission spectrum become sharper, and the intensity ratio of blue-toNIR emission is lower compared with that of LYAS glasses, which Fig. 3. SEM micrographs of surface (a) and cross-section (b) of Tm3+/Yb3+ doped YAG glass ceramics.
inversion asymmetry and covalent environment around Tm3+ ions [45,46]. Using the Xt values, spontaneous transition probabilities, branching ratios, and lifetimes for the optical transitions of Tm3+ in LYAS glasses have been determined and listed in Table 1. The predicated spontaneous emission probabilities for 3H4 ? 3H6, 1 G4 ? 3H6 and 1D2 ? 3H6 transitions are 1315, 1155 and 7583 s1, respectively, indicating that efficient multi-photon upconversion luminescence is reasonable to be expected under suitable excitation condition. The inset of Fig. 4 depicts the absorption cross-section and the stimulated emission cross-section derived by the reciprocity method for Yb3+. The peak values of absorption and emission cross-sections are 1.941 1020 and 1.852 1020 cm2 at 976 and 977 nm wavelengths, respectively. And the large cross-section profiles of Yb3+ are beneficial in absorbing sufficient pump energy and transferring energy to Tm3+ ions in co-doped LYAS glass system [47]. Under the excitation of the 974 nm infrared laser, the upconversion emission spectra of Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics were recorded and are presented in Fig. 5(a) and (b), respectively. The NIR, red, blue and UV upconversion emission bands peaked at around 802, 650, 476 and 365 nm are assigned to the 3H4 ? 3H6, 1G4 ? 3F4, 1G4 ? 3H6 and 1D2 ? 3H6 transitions of Tm3+, respectively. The integrated intensity ratios of blue-to-red fluorescence are derived to be 19.0:1.0 and 21.0:1.0 in LYAS glasses
Fig. 5. Emission spectra of 1 wt% Tm2O3 and 2 wt% Yb2O3 doped LYAS glasses (a) and YAG glass ceramics (b). Inserted photo in (a): Fluorescence from LYAS glasses under 570 mW, 974 nm laser light excitation. Inserted photo in (b): Fluorescence from YAG glass ceramics under 879 mW, 974 nm laser light excitation (excitation laser spot size: 60–80 lm).
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Y. Li et al. / Journal of Alloys and Compounds 536 (2012) 198–203 Table 1 Predicted emission probabilities, fluorescence branching ratios, and radiative lifetimes of Tm3+ in LYAS glasses. Transition
Energy (cm1)
[U(2)]2
[U(4)]2
[U(6)]2
Aed (s1)
Amd (s1)
b (%)
srad (ls)
3
4401 6572 12658 6392 6681 8664 13064 15236 21322 7007 13399 13687 15670 20071 22242 28329
0.0152 0.1215 0.2187 0.0050 0.0100 0.1511 0.0704 0.0042 0.0452 0.1926 0.0639 0.1637 0.1147 0.0000 0.5792 0.0000
0.4669 0.1329 0.0944 0.0695 0.0698 0.0046 0.0055 0.0186 0.0694 0.1666 0.3093 0.0714 0.0138 0.0017 0.0968 0.3144
0.0153 0.2258 0.5758 0.0413 0.2915 0.3750 0.5176 0.0642 0.0122 0.0006 0.0000 0.0000 0.2307 0.0164 0.0194 0.0916
17.14 96.95 1314.87 13.04 46.49 264.14 744.16 144.81 1154.83 239.58 993.72 1452.42 2111.94 112.48 22872.86 7583.46
7.52 14.23 0.00 0.00 2.57 25.35 105.29 9.09 0.00 0.00 51.87 79.57 0.00 0.00 0.00 0.00
1.70 7.66 90.64 0.52 1.95 11.53 33.86 6.13 46.01 0.67 2.95 4.32 5.95 0.32 64.43 21.36
689.3
3
H4 ? H5 H4 ? 3F4 H4 ? 3H6 1 G4 ? 3F2 1 G4 ? 3F3 1 G4 ? 3H4 1 G4 ? 3H5 1 G4 ? 3F4 1 G4 ? 3H6 1 D2 ? 1G4 1 D2 ? 3F2 1 D2 ? 3F3 1 D2 ? 3H4 1 D2 ? 3H5 1 D2 ? 3F4 1 D2 ? 3H6 3 3
reveal the surroundings around Tm3+ ions in the YAG glass ceramic matrix have been changed to some extent. In the LYAS precursor glasses, the RE ions were randomly distributed in the disordered structures. However, in the YAG glass ceramics, YAG micro-crystals have a cubic garnet structure (space group Ia3d) which consist of interconnected and slightly distorted octahedrons, tetrahedrons, and dodecahedrons with shared O atoms at the corner, as shown in Fig. 6. In the garnet lattice, the Y3+ is surrounded dodecahedrally by eight oxygen atoms and the Al3+ occupy both tetrahedral (fourfold) and octahedral (sixfold) coordination sites in a ratio of 3:2. As the closeness in radii of Tm3+, Yb3+ and Y3+, Tm3+ and Yb3+ ions will substitute the Y3+ sites in the dodecahedron [48]. Furthermore, the distinct differences between LYAS precursor glass and YAG glass ceramic fluorescence spectra shown in Fig. 5(a) and (b) confirm that the Tm3+ and Yb3+ ions have entered into YAG crystals formed in the glass ceramics. Influenced by crystal field, Tm3+ and Yb3+ yield large
398.4
28.2
Stark splitting, giving rise to the increase in number of sub-energy level. As a result, the probability of energy gap of Tm3+ (3F4 ? 3F3 and 3H4 ? 1G4) being close to that of Yb3+ (2F5/2 ? 2F7/2) increases greatly, which will lead to more efficient energy transfer (ET) from Yb3+ to Tm3+ under the action of multipole moments [49]. It is well known that for an unsaturated upconversion process, the transition emission intensity IUP is proportional to the mth power of the IR excitation intensity IIR [50],
IUP / Im IR ;
ð2Þ
where m represents the number of IR photons absorbed per upconversion photon emitted. A plot of log IUP versus log IIR yields a straight line with slope m. As depicted in Fig. 7, the fitted slopes for the 805.0, 650.5, 476.0 and 366.5 nm emissions of Tm3+ in LYAS glass sample are 2.06, 3.11, 3.22 and 3.85, respectively, indicating that the emissions originating from 3H4, 1G4 and 1D2 levels are as
Fig. 6. Arrangement of atoms in YAG crystal structure.
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Fig. 7. Dependence of upconversion emission intensity on excitation power of the LYAS glasses. Fig. 9. Dependence of upconversion emission intensity on excitation power of the derived YAG glass ceramics.
a result of being pumped by two-, three- and four-photon excitation processes, respectively. When pumped by 980 nm wavelength laser diodes, Yb3+ ions will absorb the NIR radiation efficiently and transfer the excitation energy to Tm3+ ions via four possible excitation processes as shown schematically in Fig. 8 [51,52]. Firstly, Tm3+ ions are excited to the 3H5 level by ET from Yb3+ and partly relax to the 3F4 level. Secondly, some of them arrive at the 3F2,3 level by excited state absorption (ESA) and ET and then relax to the lower metastable state 3H4. Radiative transition from the 3H4 level to the ground state 3H6 produces the two-photon excited NIR emission. Thirdly, a few Tm3+ ions at the 3H4 level are further excited to the 1G4 level by ESA and ET. The excited ions at the 1G4 level then relax radiatively to the ground state 3H6 and the 3F4 level, respectively, emitting the blue and the red fluorescence. Finally, a small number of Tm3+ ions at the 1G4 level are further excited to the 1 D2 level by ET, and then produce the UV radiation owing to the radiative transition of 1D2 ? 3H6. As depicted in Fig. 9, the fitted slopes for the 806.0, 652.5, 477.0 and 366.5 nm emissions of Tm3+ are 1.40, 2.36, 2.29 and 3.32 respectively, indicating the possible upconversion mechanisms of Tm3+/Yb3+ doped YAG glass ceramics are more complicated. The population distribution on 3H4 level is also believed to originate from the two-photon excitation process as illustrated above, even though the fitted slope derived from dependence of upconversion emission intensity on excitation power is only 1.40. The deviation of the slope value can be attributed to scattering and refraction of
laser light among the YAG micro particles and the residual amorphous material. The radiative transitions corresponding to the 477.0 nm blue and 652.5 nm red upconversion emissions originate from the 1G4 state, which is populated through the successive ET and ESA process, and the cooperative upconversion energy transfer (CUET) process from the Yb3+ pairs which can occur synchronously [53–55]. For the Tm3+/Yb3+ doped YAG glass ceramics, the well known three-photon excitation process including ESA and ET similar with that in Tm3+/Yb3+ doped LYAS glasses is considered to be dominated. For the two-photon CUET process, two adjacent Yb3+ in 2 F5/2 state form a pair of Yb3+, which can simultaneously transfer the two-photon energy to Tm3+ and excite it from 3H6 ground state to 1G4 state. Based on Eq. (2), the additional photons absorbed for the excitation processes of 366.5 nm UV than 477.0 nm blue upconversion emission is estimated by the following relation,
I366:5nm / Iqp IR ; I477:0nm
ð3Þ
here the value of q p is derived to be 1.05, which reveals that the 366.5 nm upconversion emission contains one more photon excitation process compared with that of 477.0 nm transition emission. Therefore, the rare UV radiation ascribed to the radiative transition of 1D2 ? 3H6 produces when a small number of Tm3+ ions at the 1G4 state are excited to the 1D2 state by ET.
4. Conclusions
Fig. 8. Energy level diagram of Tm3+ and Yb3+ with possible upconversion excitation routes in LYAS glass and YAG glass ceramic systems.
Tm3+/Yb3+ doped YAG glass ceramics have been obtained by heat-treating the LYAS precursor glasses. XRD and SEM measurements reveal that the crystal grains grow with several preferred orientations and a highly predominant one (4 4 4) in the internal network, and the primary and secondary particle sizes of YAG phase are 46 nm and 8 lm, respectively. Efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ are exhibited in Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics. Obvious Stark splitting of the Tm3+ upconversion luminescence peaks in the glass ceramics indicates that Tm3+ and Yb3+ have been incorporated into YAG lattices. The analyses of structural and spectral differences of Tm3+/Yb3+ doped LYAS precursor glasses and YAG glass ceramics provide valid information for the development of new materials applied in illumination and display.
Y. Li et al. / Journal of Alloys and Compounds 536 (2012) 198–203
Acknowledgment This research work is supported by Scientific Research Foundation for Universities from Education Bureau of Liaoning Province, China (2009A080).
References [1] F. Cornacchia, M. Alshourbagy, A. Toncelli, M. Tonelli, H. Ogino, A. Yoshikawa, T. Fukuda, J. Cryst. Growth 275 (2005) 534. [2] W. Ryba-Romanowski, P. Solarz, D. Kasprowicz, T. Runka, A. Szysiak, R. Ste˛pien´, J. Alloys Compd. 509 (2011) 6280. [3] B. Zhou, Z. Wei, Y. Zou, Y. Zhang, X. Zhong, G.L. Bourdet, J. Wang, Opt. Lett. 35 (2010) 288. [4] A. Louchet, Y.L. Du, F. Bretenaker, T. Chanelière, F. Goldfarb, I. Lorgeré, J.L. Gouët, O.G. Noël, Ph. Goldner, J. Am. Ceram. Soc. 26 (2010) 1118. [5] P. Huang, D. Chen, Y. Wang, J. Alloys Compd. 509 (2011) 3375. [6] P. Goldner, O.G. Noël, A. Louchet, F. Sèze, V. Crozatier, I. Lorgeré, F. Bretenaker, J.L. Gouët, Opt. Commun. 28 (2006) 649. [7] M. Liu, S. Wang, J. Zhang, L. An, L. Chen, Opt. Mater. 30 (2007) 370. [8] J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 89 (2006) 091114. [9] M. Dubinskii, L.D. Merkle, G.A. Newburgh, J.R. Goff, J. Am. Ceram. Soc. 34 (2005) 2116. [10] G.A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, D. Jaque, Appl. Phys. Lett. 92 (2008) 111103. [11] R. Kawai, Y. Miyasaka, K. Otsuka, T. Ohtomo, T. Narita, Opt. Express 12 (2004) 2293. [12] S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, S. Tanabe, Proc. SPIE 5941 (2005) 594111. [13] S. Tanabe, S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, Proc. SPIE 5941 (2005) 594112. [14] G. Özen, O. Forte, B.D. Bartolo, J.M. Collins, J. Appl. Phys. 102 (2007) 023110. [15] N. Ter-Gabrielyan, L.D. Merkle, E.R. Kupp, G.L. Messing, M. Dubinskii, Opt. Lett. 35 (2010) 922. [16] W. Zhang, J. Zhou, W. Liu, J. Li, L. Wang, B. Jiang, Y. Pan, X. Cheng, J. Xu, J. Alloys Compd. 506 (2010) 745. [17] S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, S. Tanabe, Proc. SPIE 5941 (2005) 594111. [18] A. Tarafder, A.R. Molla, B. Karmakar, Solid State Sci. 12 (2010) 1756. [19] M.C. Paul, S. Bysakh, S. Das, S.K. Bhadra, M. Pal, S. Yoo, M.P. Kalita, A.J. Boyland, J.K. Sahu, Mater. Sci. Eng. B 175 (2010) 108. [20] P. Haro-González, I.R. Martín, S. González-Pérez, M. Liu, S.W. Wang, N. Capuj, F. Lahoz, J. Lumin. 128 (2008) 924. [21] A. Pirri, G. Toci, D. Alderighi, M. Vannini, Opt. Express 18 (2010) 17262. [22] V.K. Tikhomirov, J. Méndez-Ramos, V.D. Rodríguez, D. Furniss, A.B. Seddon, J. Alloys Compd. 436 (2007) 216.
203
[23] X. Chen, Y. Wang, Z. Song, N. Sawanobori, M. Ohtsuka, Opt. Commun. 277 (2007) 335. [24] J. Su, F. Song, H. Tan, L. Han, F. Zhou, J. Tian, G. Zhang, Z. Cheng, H. Chen, J. Phys. D: Appl. Phys. 39 (2006) 2094. [25] S. Xu, Z. Yang, G. Wang, J. Zhang, S. Dai, L. Hu, Z. Jiang, Spectrochim. Acta, Part A 60 (2004) 3025. [26] Y. Wang, N. Can, P.D. Townsend, J. Lumin. 131 (2011) 1864. [27] S. Yu, Z. Yang, S. Xu, Opt. Mater. 31 (2009) 1723. [28] S.F. León-Luis, J. Abreu-Afonso, J. Peña-Martínez, J. Méndez-Ramos, A.C. Yanes, J. Castillo, V.D. Rodríguez, J. Alloys Compd. 479 (2009) 557. [29] F. Cornacchia, E. Favilla, A. Toncelli, M. Tonelli, J. Doerschel, S. Ganschow, Appl. Phys. B: Lasers Opt. 75 (2002) 641. [30] C. Cao, W. Qin, J. Zhang, Y. Wang, G. Wang, G. Wei, P. Zhu, L. Wang, L. Jin, Opt. Commun. 281 (2008) 1716. [31] D. Kasprowicz, A. Majchrowski, E. Michalski, J. Alloys Compd. 509 (2011) 6354. [32] S. Ye, B. Zhu, J. Luo, J. Chen, G. Lakshminarayana, J. Qiu, Opt. Express 16 (2008) 8989. [33] L. Huang, S. Shen, A. Jha, J. Non-Cryst. Solids 345&346 (2004) 349. [34] G. Qin, W. Qin, C. Wu, S. Huang, D. Zhao, J. Zhang, S. Lu, Opt. Commun. 242 (2004) 215. [35] A. Kermaoui, J.P. Denis, G. Özen, P. Goldner, F. Pellé, B. Blanzat, Opt. Commun. 110 (1994) 581. [36] D. Shi, Q. Zhang, G. Yang, C. Zhao, Z. Yang, Z. Jiang, J. Alloys Compd. 466 (2008) 373. [37] A. Makehima, Y. Tamura, T. Sakaino, J. Am. Chem. Soc. 61 (1997) 247. [38] A. Stevenson, X. Li, M. Martinez, J. Anderson, D. Suchy, E. Kupp, E. Dickey, K. Mueller, G. Messing, J. Am. Ceram. Soc. 94 (2011) 1380. [39] I. Etchart, I. Hernández, A. Huignard, M. Bérard, M. Laroche, W. Gillin, R. Curry, A. Cheetham, J. Appl. Phys. 109 (2011) 063104. [40] D. Chen, E. Jordan, M. Gell, J. Am. Ceram. Soc. 91 (2008) 2759. [41] Y. Letichevsky, L. Sominski, J. Moren, A. Gedanken, New J. Chem. 29 (2005) 1445. [42] L. Su, A. Tok, F. Boey, CrystEngComm 11 (2009) 1880. [43] B. Peng, T. Izumitani, Opt. Mater. 4 (1995) 797. [44] Y. Gandhi, M.V.R. Rao, C.S. Rao, I.V. Kityk, N. Veeraiah, J. Lumin. 131 (2011) 1443. [45] Y. Yang, B. Yao, B. Chen, C. Wang, G. Ren, X. Wang, Opt. Mater. 29 (2007) 1159. [46] M. Naftaly, S. Shen, A. Jha, Appl. Opt. 39 (2000) 4979. [47] K. Li, Q. Zhang, G. Bai, S. Fan, J. Zhang, L. Hu, J. Alloys Compd. 504 (2010) 573. [48] A. Tarafder, A. Molla, B. Karmakar, J. Am. Ceram. Soc. 93 (2010) 3244. [49] L. Wang, C. Cao, X. Xue, D. Zhao, D. Zhang, K. Zheng, N. Liu, F. Shi, C. He, W. Qin, J. Fluorine Chem. 130 (2009) 1059. [50] R. Balakrishnaiah, D. Kim, S. Yi, K. Jang, H. Lee, J. Jeong, Opt. Mater. 31 (2009) 959. [51] T. Passuello, F. Piccinelli, M. Pedroni, M. Bettinelli, F. Mangiarini, R. Naccache, F. Vetrone, J.A. Capobianco, A. Speghini, Opt. Mater. 33 (2011) 643. [52] Y. Zhang, L. Yang, H. Han, J. Zhong, Opt. Commun. 282 (2009) 2857. [53] R. Wang, L. Liu, J. Sun, Y. Qian, Y. Zhang, Y. Xu, Opt. Commun. 285 (2012) 957. [54] Q. Zhang, T. Li, Z. Jiang, Appl. Phys. Lett. 87 (2005) 171911. [55] S. Xiao, X. Yang, J. Ding, X. Yan, J. Phys. Chem. C 111 (2007) 8161.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Upconversion emissions in YAG glass ceramics doped with Tm3+/Yb3+ ions Yuhua Li a,b, Baojiu Chen b, Xin Zhao a, Zhiqiang Wang a, Hai Lin a,b,⇑ a b
School of Textile and Material Engineering, Dalian Polytechnic University, Dalian 116034, PR China Department of Physics, Dalian Maritime University, Dalian 116026, PR China
a r t i c l e
i n f o
Article history: Received 24 March 2012 Received in revised form 1 May 2012 Accepted 2 May 2012 Available online 12 May 2012 Keywords: Tm3+/Yb3+ doped YAG glass ceramics Stark splitting Multi-photon upconversion fluorescence
a b s t r a c t Tm3+/Yb3+ doped glass ceramics, containing single phase of YAG micro-crystals with several preferred orientations and a dominant one (4 4 4) in the matrix, which present the primary particle size of 46 nm and the secondary particle size of 8 lm, have been prepared by heat-treating the Li2O– Y2O3–Al2O3–SiO2 (LYAS) precursor glasses. Judd-Ofelt parameters Xt (t = 2, 4, 6) are derived to be 5.14 1020, 1.11 1020, and 1.19 1020 cm2, respectively, indicating high inversion asymmetry and strong covalent environment in the precursor glasses. Efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ are captured in Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics. Obvious Stark splitting in the glass ceramics manifests that rare earth ions have been incorporated into YAG lattices. The comparison of spectral behaviors of the glass and glass ceramic samples will be helpful for further exploring the novel phosphors applied to illumination and display. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Yttrium aluminium garnet (Y3Al5O12, YAG in brief) materials are promising candidates for optical and lighting applications, such as high power lasers, color displays, optical sensors, and LED phosphors. This is primarily ascribed to its small thermal expansion, outstanding chemical–mechanical stability, low acoustic losses, and excellent optical properties [1–7]. Among YAG systems, glass ceramics containing YAG phase are one of the most seductive optoelectronic materials, attributing to its larger size, higher rare earth (RE) ions doping concentration, and easier fabrication compared with the single YAG crystals [8–11]. Up to now, many researchers have devoted themselves with great enthusiasms to the studies of RE doped YAG systems, and great achievements have been reported on spectral characteristics and crystal growth processes [12–21]. For example, the luminescence characteristics of Ce3+:YAG glass–ceramic phosphor for white LED [12,13], the optical properties and laser performances of Ho3+:YAG transparent ceramics [15], and the effects of nano-YAG crystallization on the structure and photoluminescence properties of Nd3+-doped glasses [18], and so on, have been investigated. Among RE ions, Tm3+ is a promising optical activator that opens the possibility for simultaneous blue and ultraviolet (UV) emissions of laser action and various applications [22–27]. Moreover,
⇑ Corresponding author at: School of Textile and Material Engineering, Dalian Polytechnic University, Dalian 116034, PR China. Tel.: +86 411 86323097; fax: +86 411 86322228. E-mail address: [email protected] (H. Lin). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.009
the upconversion efficiency of Tm3+ can be substantially improved by codoping with Yb3+ under the 980 nm wavelength laser pumping, which is only completely resonant with 2F7/2 ? 2F5/2 transition absorption of Yb3+ but not resonant at all with the ground-state absorption of Tm3+. Hence, Tm3+/Yb3+ co-doped situations were widely adopted in investigating and utilizing the light radiation from Tm3+ [28–36]. In the present work, Tm3+/Yb3+ doped multi-component silicate glasses have been designed and fabricated, and then the glass ceramics containing single phase of YAG micro-crystals are derived by heat-treating the precursor glasses. The XRD and SEM results indicate that the YAG crystals grow with several preferred orientations and a dominant one (4 4 4) in the matrix, and efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ with obvious Stark splitting are exhibited in YAG glass ceramics.
2. Experiment Tm3+/Yb3+ doped glasses were prepared from high-purity Li2CO3, Y2O3, Al2O3, SiO2, Tm2O3 and Yb2O3 powders, according to the molar composition of 5Li2O–18Y2O3–29Al2O3–48SiO2 (LYAS) with additional doping of 1 wt% Tm2O3 and 2 wt% Yb2O3. The relative molar ratio among Y2O3, Al2O3 and SiO2 is 19.0:30.5:50.5, which located in the central of the glass formation region presented in Fig. 1 [37]. The well-mixed raw materials were first melted in a platinum crucible at 1620 °C for 3 h using an electric furnace, then the molten glasses were cast onto an iron plate and subsequently annealed at 500 °C for 3 h, after that cooled down slowly to room temperature. The choice of the annealed temperature at 500 °C is based on the glass transition temperature (T g ) 531 °C derived by differential thermal analysis (DTA) curve as presented in the inset of Fig. 2(a), which was carried out by a WCR-2D differential thermal analyzer. The LYAS glasses exhibit excellent
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a Philips XL40 scanning electron microscopy (SEM) and a JSM-6460LV SEM, respectively. Absorption spectrum of the glass sample was recorded using a Perkin-Elmer UV–VIS-NIR Lambda 19 double-beam spectrophotometer. Upconversion spectra were measured using a Jobin Yvon Fluorolog-3 spectrophotometer with an R928 photomultiplier tube (PMT) detector, and a commercial CW Xe-lamp was used as pump source. The dependences of luminescence intensity on pump power were determined using a Perkin-Elmer LS 55 luminescence spectrometer with an R928 PMT detector, and a 974 nm wavelength fiber-pigtailed laser diode was adopted as pump source. All the measurements were carried out at room temperature.
3. Results and discussion
Fig. 1. Glass formation region in Y2O3-Al2O3-SiO2 ternary phase diagram. Inserted photo: LYAS glasses.
The powder XRD patterns of Tm3+/Yb3+ doped LYAS glasses and the derived glass ceramics are shown in Fig. 2(a) and (b), respectively. In Fig. 2(a), the amorphous state of LYAS glasses is well confirmed. In Fig. 2(b), only a YAG phase is identified and no peak is assigned to other crystalline phase. The lattice constant for Tm3+/ Yb3+ doped YAG glass ceramics is calculated to be 1.201 nm, which is consistent with the value of pure YAG crystal phase, and the Tm3+ and Yb3+ ions can be considered to take the position of Y3+ in the lattice structure, because the radii of Tm3+ (0.094 nm) and Yb3+ (0.093 nm) are extremely similar to that of Y3+ (0.096 nm) [38,39]. The primary particle size of the sample is estimated using the Scherrer equation [40],
D¼
Fig. 2. XRD patterns of powder Tm3+/Yb3+ doped LYAS glasses (a) and derived YAG glass ceramics (b), and XRD pattern of bulk YAG glass ceramics (c). Inserted photo: YAG glass ceramics. Inset of (a): DTA curve of Tm3+/Yb3+ doped LYAS glasses.
transparency as shown in the inset of Fig. 1. The glass density was determined by the Archimedes method using distilled water and was derived to be 3.462 g cm3. Utilizing a Metricon 2010 prism coupler, the refractive indices were measured to be 1.678 and 1.649 at 632.8 and 1536 nm, respectively. For obtaining the YAG glass ceramics, LYAS glasses were heat-treated at 1350 °C for 6 h, and then annealed at 400 °C for 2 h, finally the white samples were slowly cooled down to room temperature. For optical measurements, the annealed glass and glass ceramic samples were sliced and polished with two parallel sides. X-ray diffraction (XRD) measurements for powder and bulk samples were carried out by a D/Max-3B X-ray diffractometer (40 kV, 20 mA) and a SIEMENS D500 X-ray diffractometer (40 kV, 30 mA), respectively. The surface appearance and cross-section microstructure of the glass ceramic sample were observed with
kk ; b cos h
ð1Þ
where K is a constant (0.94), k is the wavelength of X-ray (0.154 nm for CuKa), b is the corrected full width at half maximum of the strongest diffraction peak in radians, and h is the diffraction angle. The crystallite size calculated from Eq. (1) is 46 nm. The XRD pattern of bulk YAG glass ceramic sample and its appearance are presented in Fig. 2(c). Compared with the diffraction spectrum of the powder sample, the peak number of the bulk sample diminishes obviously. The strongest peak corresponding to crystal face also changes from (4 2 0) to (4 4 4), in the meanwhile, the intensity ratios of peaks on several crystal faces, such as (2 1 1), (4 2 2) and (6 4 2), to that on (4 2 0) crystal face increase significantly, demonstrating that the crystalline grains grow with several preferred orientations and a dominant one (4 4 4) in the bulk YAG glass ceramics. The SEM micrographs of surface and cross-section of the glass– ceramic sample are shown in Fig. 3(a) and (b), respectively. The YAG micro particles are irregular in shape, but they distribute along certain preferred orientations and a highly predominant one in the matrix, which is in good agreement with the bulk XRD pattern, and this pilotaxitic texture could increase yield strength and tensile strength of YAG glass ceramics effectively. Their average size is 8 lm, much larger than 46 nm calculated by Scherrer equation. The main reason is that the primary nanocrystals with high energy would assemble with interface matching coherently to minimize the surface energy and sinter by the heat treatment process [41,42]. Combined with the XRD and SEM results, it is obvious that after heat treatment, the glass ceramics containing YAG crystal phase were formed in the Tm3+/Yb3+ doped LYAS mother glasses. The absorption spectrum of Tm3+/Yb3+ doped LYAS glasses shows the typical transitions from Tm3+ and Yb3+ ions, as illustrated in Fig. 4. The spectrum consists of six well-isolated absorption bands associated with transitions from the ground 3H6 level to the excited 1D2, 1G4, 3F2,3, 3H4, 3H5 and 3F4 levels of Tm3+, and a single intense absorption band attributed to the 2F7/2 ? 2F5/2 transition of Yb3+ at 976 nm. The Judd–Ofelt intensity parameters Xt (t = 2, 4, 6) of Tm3+ are derived to be 5.14 1020, 1.11 1020, and 1.19 1020 cm2, respectively. The value of X2 is larger than 3.31 1020 and 4.29 1020 cm2 in strontium silicate and calcium fluoro phosphorous silicate glasses [43,44], revealing a strong
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Fig. 4. Absorption spectrum of 1 wt% Tm2O3 and 2 wt% Yb2O3 doped LYAS glasses. Inset: Absorption cross-section (rabs, curve 1) and calculated emission crosssection (rem, curve 2) of Yb3+ in Tm3+/Yb3+ doped LYAS glasses.
and YAG glass ceramics, respectively, thus the two samples exhibit intense blue fluorescence as shown in the inserted photos of Fig. 5, due to the absence of contribution from NIR emission to the nakedeye vision. On account of the excitation laser from output end of the SIEOR 62.5/125 lm optical fiber irradiating directly on the closely laid sample, the diameter of the laser spot is estimated to be 60–80 lm, much larger than the average size 8 lm of the YAG micro particles. Therefore, not only are the Tm3+ ions in YAG micro-crystals excited efficiently, but also the stimulated Tm3+ ions in the rest area of glass ceramics partially contribute to the upconversion fluorescences. In YAG glass ceramics, multi-peaks splitting occurs at the blue and NIR emission bands, and the strongest split sub-peaks are observed to be around 484 and 820 nm, respectively, showing 8 and 18 nm red-shifts compared with those strongest peaks of LYAS glasses. Meanwhile, peaks of the upconversion emission spectrum become sharper, and the intensity ratio of blue-toNIR emission is lower compared with that of LYAS glasses, which Fig. 3. SEM micrographs of surface (a) and cross-section (b) of Tm3+/Yb3+ doped YAG glass ceramics.
inversion asymmetry and covalent environment around Tm3+ ions [45,46]. Using the Xt values, spontaneous transition probabilities, branching ratios, and lifetimes for the optical transitions of Tm3+ in LYAS glasses have been determined and listed in Table 1. The predicated spontaneous emission probabilities for 3H4 ? 3H6, 1 G4 ? 3H6 and 1D2 ? 3H6 transitions are 1315, 1155 and 7583 s1, respectively, indicating that efficient multi-photon upconversion luminescence is reasonable to be expected under suitable excitation condition. The inset of Fig. 4 depicts the absorption cross-section and the stimulated emission cross-section derived by the reciprocity method for Yb3+. The peak values of absorption and emission cross-sections are 1.941 1020 and 1.852 1020 cm2 at 976 and 977 nm wavelengths, respectively. And the large cross-section profiles of Yb3+ are beneficial in absorbing sufficient pump energy and transferring energy to Tm3+ ions in co-doped LYAS glass system [47]. Under the excitation of the 974 nm infrared laser, the upconversion emission spectra of Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics were recorded and are presented in Fig. 5(a) and (b), respectively. The NIR, red, blue and UV upconversion emission bands peaked at around 802, 650, 476 and 365 nm are assigned to the 3H4 ? 3H6, 1G4 ? 3F4, 1G4 ? 3H6 and 1D2 ? 3H6 transitions of Tm3+, respectively. The integrated intensity ratios of blue-to-red fluorescence are derived to be 19.0:1.0 and 21.0:1.0 in LYAS glasses
Fig. 5. Emission spectra of 1 wt% Tm2O3 and 2 wt% Yb2O3 doped LYAS glasses (a) and YAG glass ceramics (b). Inserted photo in (a): Fluorescence from LYAS glasses under 570 mW, 974 nm laser light excitation. Inserted photo in (b): Fluorescence from YAG glass ceramics under 879 mW, 974 nm laser light excitation (excitation laser spot size: 60–80 lm).
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Y. Li et al. / Journal of Alloys and Compounds 536 (2012) 198–203 Table 1 Predicted emission probabilities, fluorescence branching ratios, and radiative lifetimes of Tm3+ in LYAS glasses. Transition
Energy (cm1)
[U(2)]2
[U(4)]2
[U(6)]2
Aed (s1)
Amd (s1)
b (%)
srad (ls)
3
4401 6572 12658 6392 6681 8664 13064 15236 21322 7007 13399 13687 15670 20071 22242 28329
0.0152 0.1215 0.2187 0.0050 0.0100 0.1511 0.0704 0.0042 0.0452 0.1926 0.0639 0.1637 0.1147 0.0000 0.5792 0.0000
0.4669 0.1329 0.0944 0.0695 0.0698 0.0046 0.0055 0.0186 0.0694 0.1666 0.3093 0.0714 0.0138 0.0017 0.0968 0.3144
0.0153 0.2258 0.5758 0.0413 0.2915 0.3750 0.5176 0.0642 0.0122 0.0006 0.0000 0.0000 0.2307 0.0164 0.0194 0.0916
17.14 96.95 1314.87 13.04 46.49 264.14 744.16 144.81 1154.83 239.58 993.72 1452.42 2111.94 112.48 22872.86 7583.46
7.52 14.23 0.00 0.00 2.57 25.35 105.29 9.09 0.00 0.00 51.87 79.57 0.00 0.00 0.00 0.00
1.70 7.66 90.64 0.52 1.95 11.53 33.86 6.13 46.01 0.67 2.95 4.32 5.95 0.32 64.43 21.36
689.3
3
H4 ? H5 H4 ? 3F4 H4 ? 3H6 1 G4 ? 3F2 1 G4 ? 3F3 1 G4 ? 3H4 1 G4 ? 3H5 1 G4 ? 3F4 1 G4 ? 3H6 1 D2 ? 1G4 1 D2 ? 3F2 1 D2 ? 3F3 1 D2 ? 3H4 1 D2 ? 3H5 1 D2 ? 3F4 1 D2 ? 3H6 3 3
reveal the surroundings around Tm3+ ions in the YAG glass ceramic matrix have been changed to some extent. In the LYAS precursor glasses, the RE ions were randomly distributed in the disordered structures. However, in the YAG glass ceramics, YAG micro-crystals have a cubic garnet structure (space group Ia3d) which consist of interconnected and slightly distorted octahedrons, tetrahedrons, and dodecahedrons with shared O atoms at the corner, as shown in Fig. 6. In the garnet lattice, the Y3+ is surrounded dodecahedrally by eight oxygen atoms and the Al3+ occupy both tetrahedral (fourfold) and octahedral (sixfold) coordination sites in a ratio of 3:2. As the closeness in radii of Tm3+, Yb3+ and Y3+, Tm3+ and Yb3+ ions will substitute the Y3+ sites in the dodecahedron [48]. Furthermore, the distinct differences between LYAS precursor glass and YAG glass ceramic fluorescence spectra shown in Fig. 5(a) and (b) confirm that the Tm3+ and Yb3+ ions have entered into YAG crystals formed in the glass ceramics. Influenced by crystal field, Tm3+ and Yb3+ yield large
398.4
28.2
Stark splitting, giving rise to the increase in number of sub-energy level. As a result, the probability of energy gap of Tm3+ (3F4 ? 3F3 and 3H4 ? 1G4) being close to that of Yb3+ (2F5/2 ? 2F7/2) increases greatly, which will lead to more efficient energy transfer (ET) from Yb3+ to Tm3+ under the action of multipole moments [49]. It is well known that for an unsaturated upconversion process, the transition emission intensity IUP is proportional to the mth power of the IR excitation intensity IIR [50],
IUP / Im IR ;
ð2Þ
where m represents the number of IR photons absorbed per upconversion photon emitted. A plot of log IUP versus log IIR yields a straight line with slope m. As depicted in Fig. 7, the fitted slopes for the 805.0, 650.5, 476.0 and 366.5 nm emissions of Tm3+ in LYAS glass sample are 2.06, 3.11, 3.22 and 3.85, respectively, indicating that the emissions originating from 3H4, 1G4 and 1D2 levels are as
Fig. 6. Arrangement of atoms in YAG crystal structure.
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Fig. 7. Dependence of upconversion emission intensity on excitation power of the LYAS glasses. Fig. 9. Dependence of upconversion emission intensity on excitation power of the derived YAG glass ceramics.
a result of being pumped by two-, three- and four-photon excitation processes, respectively. When pumped by 980 nm wavelength laser diodes, Yb3+ ions will absorb the NIR radiation efficiently and transfer the excitation energy to Tm3+ ions via four possible excitation processes as shown schematically in Fig. 8 [51,52]. Firstly, Tm3+ ions are excited to the 3H5 level by ET from Yb3+ and partly relax to the 3F4 level. Secondly, some of them arrive at the 3F2,3 level by excited state absorption (ESA) and ET and then relax to the lower metastable state 3H4. Radiative transition from the 3H4 level to the ground state 3H6 produces the two-photon excited NIR emission. Thirdly, a few Tm3+ ions at the 3H4 level are further excited to the 1G4 level by ESA and ET. The excited ions at the 1G4 level then relax radiatively to the ground state 3H6 and the 3F4 level, respectively, emitting the blue and the red fluorescence. Finally, a small number of Tm3+ ions at the 1G4 level are further excited to the 1 D2 level by ET, and then produce the UV radiation owing to the radiative transition of 1D2 ? 3H6. As depicted in Fig. 9, the fitted slopes for the 806.0, 652.5, 477.0 and 366.5 nm emissions of Tm3+ are 1.40, 2.36, 2.29 and 3.32 respectively, indicating the possible upconversion mechanisms of Tm3+/Yb3+ doped YAG glass ceramics are more complicated. The population distribution on 3H4 level is also believed to originate from the two-photon excitation process as illustrated above, even though the fitted slope derived from dependence of upconversion emission intensity on excitation power is only 1.40. The deviation of the slope value can be attributed to scattering and refraction of
laser light among the YAG micro particles and the residual amorphous material. The radiative transitions corresponding to the 477.0 nm blue and 652.5 nm red upconversion emissions originate from the 1G4 state, which is populated through the successive ET and ESA process, and the cooperative upconversion energy transfer (CUET) process from the Yb3+ pairs which can occur synchronously [53–55]. For the Tm3+/Yb3+ doped YAG glass ceramics, the well known three-photon excitation process including ESA and ET similar with that in Tm3+/Yb3+ doped LYAS glasses is considered to be dominated. For the two-photon CUET process, two adjacent Yb3+ in 2 F5/2 state form a pair of Yb3+, which can simultaneously transfer the two-photon energy to Tm3+ and excite it from 3H6 ground state to 1G4 state. Based on Eq. (2), the additional photons absorbed for the excitation processes of 366.5 nm UV than 477.0 nm blue upconversion emission is estimated by the following relation,
I366:5nm / Iqp IR ; I477:0nm
ð3Þ
here the value of q p is derived to be 1.05, which reveals that the 366.5 nm upconversion emission contains one more photon excitation process compared with that of 477.0 nm transition emission. Therefore, the rare UV radiation ascribed to the radiative transition of 1D2 ? 3H6 produces when a small number of Tm3+ ions at the 1G4 state are excited to the 1D2 state by ET.
4. Conclusions
Fig. 8. Energy level diagram of Tm3+ and Yb3+ with possible upconversion excitation routes in LYAS glass and YAG glass ceramic systems.
Tm3+/Yb3+ doped YAG glass ceramics have been obtained by heat-treating the LYAS precursor glasses. XRD and SEM measurements reveal that the crystal grains grow with several preferred orientations and a highly predominant one (4 4 4) in the internal network, and the primary and secondary particle sizes of YAG phase are 46 nm and 8 lm, respectively. Efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ are exhibited in Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics. Obvious Stark splitting of the Tm3+ upconversion luminescence peaks in the glass ceramics indicates that Tm3+ and Yb3+ have been incorporated into YAG lattices. The analyses of structural and spectral differences of Tm3+/Yb3+ doped LYAS precursor glasses and YAG glass ceramics provide valid information for the development of new materials applied in illumination and display.
Y. Li et al. / Journal of Alloys and Compounds 536 (2012) 198–203
Acknowledgment This research work is supported by Scientific Research Foundation for Universities from Education Bureau of Liaoning Province, China (2009A080).
References [1] F. Cornacchia, M. Alshourbagy, A. Toncelli, M. Tonelli, H. Ogino, A. Yoshikawa, T. Fukuda, J. Cryst. Growth 275 (2005) 534. [2] W. Ryba-Romanowski, P. Solarz, D. Kasprowicz, T. Runka, A. Szysiak, R. Ste˛pien´, J. Alloys Compd. 509 (2011) 6280. [3] B. Zhou, Z. Wei, Y. Zou, Y. Zhang, X. Zhong, G.L. Bourdet, J. Wang, Opt. Lett. 35 (2010) 288. [4] A. Louchet, Y.L. Du, F. Bretenaker, T. Chanelière, F. Goldfarb, I. Lorgeré, J.L. Gouët, O.G. Noël, Ph. Goldner, J. Am. Ceram. Soc. 26 (2010) 1118. [5] P. Huang, D. Chen, Y. Wang, J. Alloys Compd. 509 (2011) 3375. [6] P. Goldner, O.G. Noël, A. Louchet, F. Sèze, V. Crozatier, I. Lorgeré, F. Bretenaker, J.L. Gouët, Opt. Commun. 28 (2006) 649. [7] M. Liu, S. Wang, J. Zhang, L. An, L. Chen, Opt. Mater. 30 (2007) 370. [8] J. Dong, A. Shirakawa, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, Appl. Phys. Lett. 89 (2006) 091114. [9] M. Dubinskii, L.D. Merkle, G.A. Newburgh, J.R. Goff, J. Am. Ceram. Soc. 34 (2005) 2116. [10] G.A. Torchia, A. Rodenas, A. Benayas, E. Cantelar, L. Roso, D. Jaque, Appl. Phys. Lett. 92 (2008) 111103. [11] R. Kawai, Y. Miyasaka, K. Otsuka, T. Ohtomo, T. Narita, Opt. Express 12 (2004) 2293. [12] S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, S. Tanabe, Proc. SPIE 5941 (2005) 594111. [13] S. Tanabe, S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, Proc. SPIE 5941 (2005) 594112. [14] G. Özen, O. Forte, B.D. Bartolo, J.M. Collins, J. Appl. Phys. 102 (2007) 023110. [15] N. Ter-Gabrielyan, L.D. Merkle, E.R. Kupp, G.L. Messing, M. Dubinskii, Opt. Lett. 35 (2010) 922. [16] W. Zhang, J. Zhou, W. Liu, J. Li, L. Wang, B. Jiang, Y. Pan, X. Cheng, J. Xu, J. Alloys Compd. 506 (2010) 745. [17] S. Fujita, S. Yoshihara, A. Sakamoto, S. Yamamoto, S. Tanabe, Proc. SPIE 5941 (2005) 594111. [18] A. Tarafder, A.R. Molla, B. Karmakar, Solid State Sci. 12 (2010) 1756. [19] M.C. Paul, S. Bysakh, S. Das, S.K. Bhadra, M. Pal, S. Yoo, M.P. Kalita, A.J. Boyland, J.K. Sahu, Mater. Sci. Eng. B 175 (2010) 108. [20] P. Haro-González, I.R. Martín, S. González-Pérez, M. Liu, S.W. Wang, N. Capuj, F. Lahoz, J. Lumin. 128 (2008) 924. [21] A. Pirri, G. Toci, D. Alderighi, M. Vannini, Opt. Express 18 (2010) 17262. [22] V.K. Tikhomirov, J. Méndez-Ramos, V.D. Rodríguez, D. Furniss, A.B. Seddon, J. Alloys Compd. 436 (2007) 216.
203
[23] X. Chen, Y. Wang, Z. Song, N. Sawanobori, M. Ohtsuka, Opt. Commun. 277 (2007) 335. [24] J. Su, F. Song, H. Tan, L. Han, F. Zhou, J. Tian, G. Zhang, Z. Cheng, H. Chen, J. Phys. D: Appl. Phys. 39 (2006) 2094. [25] S. Xu, Z. Yang, G. Wang, J. Zhang, S. Dai, L. Hu, Z. Jiang, Spectrochim. Acta, Part A 60 (2004) 3025. [26] Y. Wang, N. Can, P.D. Townsend, J. Lumin. 131 (2011) 1864. [27] S. Yu, Z. Yang, S. Xu, Opt. Mater. 31 (2009) 1723. [28] S.F. León-Luis, J. Abreu-Afonso, J. Peña-Martínez, J. Méndez-Ramos, A.C. Yanes, J. Castillo, V.D. Rodríguez, J. Alloys Compd. 479 (2009) 557. [29] F. Cornacchia, E. Favilla, A. Toncelli, M. Tonelli, J. Doerschel, S. Ganschow, Appl. Phys. B: Lasers Opt. 75 (2002) 641. [30] C. Cao, W. Qin, J. Zhang, Y. Wang, G. Wang, G. Wei, P. Zhu, L. Wang, L. Jin, Opt. Commun. 281 (2008) 1716. [31] D. Kasprowicz, A. Majchrowski, E. Michalski, J. Alloys Compd. 509 (2011) 6354. [32] S. Ye, B. Zhu, J. Luo, J. Chen, G. Lakshminarayana, J. Qiu, Opt. Express 16 (2008) 8989. [33] L. Huang, S. Shen, A. Jha, J. Non-Cryst. Solids 345&346 (2004) 349. [34] G. Qin, W. Qin, C. Wu, S. Huang, D. Zhao, J. Zhang, S. Lu, Opt. Commun. 242 (2004) 215. [35] A. Kermaoui, J.P. Denis, G. Özen, P. Goldner, F. Pellé, B. Blanzat, Opt. Commun. 110 (1994) 581. [36] D. Shi, Q. Zhang, G. Yang, C. Zhao, Z. Yang, Z. Jiang, J. Alloys Compd. 466 (2008) 373. [37] A. Makehima, Y. Tamura, T. Sakaino, J. Am. Chem. Soc. 61 (1997) 247. [38] A. Stevenson, X. Li, M. Martinez, J. Anderson, D. Suchy, E. Kupp, E. Dickey, K. Mueller, G. Messing, J. Am. Ceram. Soc. 94 (2011) 1380. [39] I. Etchart, I. Hernández, A. Huignard, M. Bérard, M. Laroche, W. Gillin, R. Curry, A. Cheetham, J. Appl. Phys. 109 (2011) 063104. [40] D. Chen, E. Jordan, M. Gell, J. Am. Ceram. Soc. 91 (2008) 2759. [41] Y. Letichevsky, L. Sominski, J. Moren, A. Gedanken, New J. Chem. 29 (2005) 1445. [42] L. Su, A. Tok, F. Boey, CrystEngComm 11 (2009) 1880. [43] B. Peng, T. Izumitani, Opt. Mater. 4 (1995) 797. [44] Y. Gandhi, M.V.R. Rao, C.S. Rao, I.V. Kityk, N. Veeraiah, J. Lumin. 131 (2011) 1443. [45] Y. Yang, B. Yao, B. Chen, C. Wang, G. Ren, X. Wang, Opt. Mater. 29 (2007) 1159. [46] M. Naftaly, S. Shen, A. Jha, Appl. Opt. 39 (2000) 4979. [47] K. Li, Q. Zhang, G. Bai, S. Fan, J. Zhang, L. Hu, J. Alloys Compd. 504 (2010) 573. [48] A. Tarafder, A. Molla, B. Karmakar, J. Am. Ceram. Soc. 93 (2010) 3244. [49] L. Wang, C. Cao, X. Xue, D. Zhao, D. Zhang, K. Zheng, N. Liu, F. Shi, C. He, W. Qin, J. Fluorine Chem. 130 (2009) 1059. [50] R. Balakrishnaiah, D. Kim, S. Yi, K. Jang, H. Lee, J. Jeong, Opt. Mater. 31 (2009) 959. [51] T. Passuello, F. Piccinelli, M. Pedroni, M. Bettinelli, F. Mangiarini, R. Naccache, F. Vetrone, J.A. Capobianco, A. Speghini, Opt. Mater. 33 (2011) 643. [52] Y. Zhang, L. Yang, H. Han, J. Zhong, Opt. Commun. 282 (2009) 2857. [53] R. Wang, L. Liu, J. Sun, Y. Qian, Y. Zhang, Y. Xu, Opt. Commun. 285 (2012) 957. [54] Q. Zhang, T. Li, Z. Jiang, Appl. Phys. Lett. 87 (2005) 171911. [55] S. Xiao, X. Yang, J. Ding, X. Yan, J. Phys. Chem. C 111 (2007) 8161.