Intense upconversion luminescen Er3-Yb3 oxyfluoride-borosilicate-glass ceramics Ba2GdF7 nanocrystals

JOURNAL OF RARE EARTHS, Vol. 28, No. 6, Dec. 2010, p. 903Foundation item: Project supported by China Postdoctoral Science Foundation funded project (20080430216), National Natural Science Foundation of China(60508014), Program for New Century Excellent Talents in University (NCET-07-0786) and Zhejiang Provincial Natural Science Foundation of China (Z4100030) Corresponding author: ZHAO Shilong (E-mail: shilong_zhao@; Tel.: +86-571-86835781) DOI: 10.1016/S1002-0721(09)60220-9Intense upconversion luminescence of Er 3+/Yb 3+ codoped oxyfluoride borosilicate glass ceramics containing Ba 2GdF 7 nanocrystalsZHAO Shilong (赵士龙)1,2, XU Shiqing (徐时清)1, DENG Degang (邓德刚)1, FAN Xianping (樊先平)2(1. College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China; 2.Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China)Received 31 August 2010; revised 3 November 2010Abstract: Er 3+/Yb 3+-codoped transparent oxyfluoride borosilicate glass ceramics containing Ba 2GdF 7 nanocrystals were prepared and spec-troscopic properties of rare earth ions were investigated. Fluoride nanocrystals Ba 2GdF 7 were successfully precipitated in glass matrix, which was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) results. In comparison with the as-made precursor, significant enhancement of upconversion luminescence was observed in the Er 3+/Yb 3+ codoped oxyfluoride glass ceramics, which may be due to the variation of coordination environment around Er 3+ and Yb 3+ ions after crystallization. The transition mechanisms of the green and red upconversion luminescence were ascribed to a two-photon process, and that of the blue upconversion luminescence was a three-photon process. Keywords: oxyfluoride glass ceramics; Er 3+/Yb 3+ codoped; spectroscopic properties; rare earthsNowadays, Er 3+ doped optical materials have attracted great interest due to their possible application as media for the upconversion laser, waveguide laser and Er 3+-doped fiber amplifier (EDFA), which is one of key elements used in the wavelength division multiplexing (WDM) network system for optical communication [1–4]. Since oxyfluoride glass ce-ramics were firstly reported by Wang and Ohwaki [5], oxy-fluoride glass ceramics doped with rare earth ions have been researched widely in the past few decades, because they possess not only higher chemical and mechanical stability than fluoride glass but also lower phonon energy than oxide glass [6,7]. By thermal treatment close to the crystallization temperature for the oxyfluoride glasses, transparent oxy-fluoride glass ceramics can be obtained, in which fluoride crystallites are dispersed in the oxide glass matrix. If the rare earth ions can be successfully incorporated into the fluoride crystals which can provide a low phonon environment, rare earth ion doped glass ceramics will exhibit excellent lumi-nescence properties.Currently, oxyfluoride glass ceramics mainly concentrate on aluminosilicate glass ceramics [8,9], oxyfluoride borosili-cate glass ceramics is seldom reported. In fact, borosilicate glasses possess lower melting temperature and easy forma-tion of fibers compared to aluminosilicate glass. Recently, Środa have successfully prepared oxyfluoride borosilicate glass ceramics containing LaOF and LaF 3 nanocrystals by heat treatment of SiO 2-B 2O 3-Na 2O-LaF 3 glass [10]. In this pa-per, a new Er 3+/Yb 3+ co-doped oxyfluoride borosilicate glass ceramics containing Ba 2GdF 7 nanocrystals was successfullyprepared and the change of upconversion luminescence were also discussed.1 Experimental1.1 Preparation of Er 3+/Yb 3+-codoped oxyfluoride boro-silicate glass ceramics The oxyfluoride borosilicate glass was prepared with the composition in mol.% of 56SiO 2-12B 2O 3-18Na 2O-8BaF 2- 4.5GdF 3-0.1ErF 3-0.5YbF 3. The mixed materials were melted in a covered alumina crucible at 1350 ºC for about 40 min. Then the glass was quenched onto a preheated brass mold. The quenched sample was annealed at 450 ºC for 2 h and cooled slowly down to room temperature. The glass sample was cut and polished into 10 mm×5 mm×2 mm size. The polished samples were then heat treated at 650, 670 and 690 ºC, chosen from the different thermal analysis (DTA) measurements, for 2 h to form transparent glass ceramics (denoted as GC-650, GC-670 and GC-690, respectively) through crystallization.1.2 Characterization of Er 3+/Yb 3+-codoped oxyfluorideborosilicate glass ceramics The DTA experiment of the as-made precursor sample was carried out on Netzsch DTA 404PC in N 2 atmosphere at a heating rate of 10 K/min. X-ray diffraction (XRD) measure-ments were performed on a powder diffractometer (Rigaku D/max2550V/PC) with Cu K α radiation (λ=0.154 nm). The904 JOURNAL OF RARE EARTHS, Vol. 28, No. 6, Dec. 2010microstructure of GC-670 was analyzed by a transmission electron microscope (TEM, Philips-FEI-Tecnai G2 F30) equipped with the energy dispersive X-ray spectroscopy (EDX). Upconversion luminescence was taken with a Jobin- Yvon Frolog3 fluorescence spectrophotometer using 980 nm light from a laser diode. All measurements were carried out at room temperature.2 Results and discussionThe DTA curve of the as-made precursor is presented in Fig. 1. The glass transition temperature is about 490 ºC, and two exothermic peaks are observed at around 670 and 730 °C respectively. XRD analysis demonstrates that the former peak is ascribed to the precipitation of Ba 2GdF 7 phase [11] and the latter corresponded to the bulk crystallization of the glass matrix. Therefore, the as-made precursor was heat-treated at 650, 670 and 690 ºC for 2 h to form transparent glass ceramics. Due to its distortion for the sample GC-690 after heat treat-ment, the sample GC-670 was selected as the object of study. The XRD patterns of the as-made precursor and glass ce-ramics heat-treated at different temperatures are shown in Fig. 2. For the as-made precursor, besides the amorphous diffuse humps, several diffraction peaks corresponding to Ba 2GdF 7 phase also appear, indicating the precipitation of Ba 2GdF 7 during the glass-melt quenching. With the increaseFig. 1 DTA curve of the as-made precursor recorded at a heatingrate of 10 K/minFig. 2 XRD patterns of the as-made precursor and glass ceramicsin heat-treatment temperature, the Ba 2GdF 7 diffraction peaks tend to be sharper, which indicates the gradual formation of Ba 2GdF 7 nanocrystals. Based on the peak widths, the mean crystallite size is evaluated by the Scherrer equation: D hkl =cos K λβθ(1)where D hkl is the crystal size at the vertical direction of (hkl ), λ is the wavelength of X-ray, θ is the angle of (hkl) diffrac-tion, β is the full width at half-maximum of the peak (FWHM) of the diffraction peak and the constant K =0.90. The determined mean crystallite sizes are 22.1, 27.9 and 32.3 nm for GC-650, GC-670 and GC-690 samples, respectively. Fig. 3(a) shows the TEM image of Er 3+/Yb 3+ codoped GC-670, which demonstrates that the precipitated spherical nanoparticles sized 25–35 nm are distributed homogenously among the glass matrix. The EDX spectra taken from the glass matrix and a selected nanoparticle are shown in Fig. 3 (b) and (c), respectively. The spectrum from the glassy matrix shows strong signals from Si and O elements and weak signals from Ba, Gd and F elements (the presence of Cu is due to the use of copper grid supporting TEM specimen). In contrast, the spectrum from the selected nanoparticle gives strong signal Ba, Gd, F, Si and O elements and weak signals from Er and Yb elements, which indicates that Er 3+ and Yb 3+ ions concentratedmainly inside or on the surfaces of the crystallites.Fig. 3 TEM image of Er 3+/Yb 3+codoped GC-670 (a), EDX curve taken from glass matrix (b) and EDX curve taken from a selectednanoparticle (c)ZHAO Shilong et al., Intense upconversion luminescence of Er3+/Yb3+ codoped oxyfluoride borosilicate glass ceramics (905)Fig. 4 presents the upconversion spectra of 0.1 mol.% Er3+ and 0.5 mol.% YbF3 codoped glass and GC-670 excited at 980 nm with the power of 200 mW. The emission bands can be assigned to 2H9/2→4I15/2 (404 nm), 2H11/2→4I15/2 (520 nm), 4S3/2→4I15/2 (538 nm), and 4F9/2→4I15/2 (644 nm) transitions, respectively. Only weak upconversion luminescence of Er3+ is observed in the as-made precursor. However, intense green and red upconversion luminescence of Er3+ was obvi-ously observed in GC-670 by naked eyes. Especially, in the ultraviolet and visible region, besides the emission band at 404 nm, weak emission bands centered at 379 nm and 490 nm are recorded, which correspond to 4G11/2→4I15/2 and2H11/2→4I15/2 transitions of Er3+ ions, respectively. As far as we know, the 379 nm upconversion band was only reported in a low phonon energy crystal Cs3Er2Br9[12] and Er3+-doped tellurium-germanate oxyfluoride glass ceramics[13]. It is well known that the intensity of upconversion luminescence of rare earth ions is very sensitive to the multiphonon relaxation rate, which strongly depends on the phonon energy of the host matrix. Usually, the lower the phonon energy of host matrix is, the smaller the multiphonon relaxation probability is. The approximate frequencies of the highest-energy pho-nons in the borate and silicate glasses are 1400 and 1100 cm–1, respectively. However, the phonon frequency of BaF2 crystal is 346 cm–1 [14] and that of GdF3 crystal is 400 cm–1 [15]. Due to much smaller phonon energy of Ba2GdF7 nanocrystals than the oxide glass, the upconversion luminescence of Er3+ in the Ba2GdF7 nanocrystals would be stronger than that in the as-made precursor. Furthermore, the upconversion lumi-nescence intensity of the glass ceramics increased signifi-cantly with increasing crystallization temperature[8].The fluorescence decay curves and fitted curves of Er3+ for 540 nm emission corresponding to Er3+:4S3/2→4I15/2 transition in the as-made precursor and GC-670 are shown in Fig. 5, which provides important information to evaluate the Er3+ ions surroundings. When the excitation is produced in an excited state of higher energy than the emitting level, the fluorescence decay would not be purely exponential. The fluorescence decay curve could be described by a mean de-cay lifetime (τm ) given by(2)Fig. 4 Upconversion luminescence of Er3+ ions in the as-made precursor and GC-670where I(t) is the dependence of photoluminescence intensity on the time t, and I max is the maximum of I(t) and I(t0)= I max[16]. On the basis of Eq. (2), the mean lifetime of Er3+ in the as-made precursor and GC-670 are calculated to be about 175.02 μs and 265.66 μs, respectively. The results clearly show that fluoride nanocrystals are precipitated in the glass host which leads to much longer 4S3/2 lifetime[17].In the upconversion processes, there is the following rela-tion between the emission intensity I em and the infrared (IR) excitation intensity I ex:I em∝(I ex)n(3) where n is the number of IR photons absorbed per visible photon emitted. Therefore, a plot of lg(I em) versus lg(I ex) should yield a straight line with the slope n[18], shown in Fig. 6. The slopes of 404, 520, 538 and 644 nm emissions are 2.65, 1.98, 1.90 and 1.88, respectively. These results indicate that green and red emission mechanisms can be described as a two-photon mechanism. The blue emission at 404 nm can beattributed to a three-photon mechanism.Fig. 5 Fluorescence decay curves of 4S3/2 level of the Er3+ in the as-made precursor and GC-670Fig. 6 Power dependence of Ba2GdF7: Er3+, Yb3+ upconversion in-tensity for GC-6703 ConclusionsEr3+/Yb3+-doped transparent oxyfluoride glass ceramics containing Ba2GdF7 nanocrystals were prepared by heat treatment of 56SiO2-12B2O3-18Na2O-8BaF2-4.5GdF3-0.1ErF3- 0.5YbF3 glasses. The results indicated that Ba2GdF7906 JOURNAL OF RARE EARTHS, Vol. 28, No. 6, Dec. 2010nanocrystals were formed in the glass matrix and a part of Er3+ and Yb3+ ions were incorporated into the precipitated fluoride nanocrystals after heat treatment. 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