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Photoluminescence of Sm3+, Dy3+, and Tm3+-doped transparent glass ceramics containing CaF2 nanocrystals
Journal of Non-Crystalline Solids 355 (2009) 2668–2673
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Photoluminescence of Sm3+, Dy3+, and Tm3+-doped transparent glass ceramics containing CaF2 nanocrystals G. Lakshminarayana a,*, Rong Yang a, Mengfei Mao a, Jianrong Qiu a, I.V. Kityk b a b
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Electrical Engineering Department, Czestochowa Technological University, Al. Armii Krajowej 17/19, Czestochowa, Poland
a r t i c l e
i n f o
Article history: Received 17 November 2008 Received in revised form 5 July 2009 Available online 23 September 2009 PACS: 78.55 Qr 78.20.e 78.55.m 78.30.j Keywords: Glass ceramics Glass transition Fluorides Optical spectroscopy Nanocrystals Luminescence
a b s t r a c t Photoluminescence properties of Sm3+, Dy3+, and Tm3+-doped transparent oxyfluoride silicate glass ceramics containing CaF2 nanocrystals were reported. Emission bands of 4G5/2 ? 6H5/2 (562 nm), 4 G5/2 ? 6H7/2 (598 nm), 4G5/2 ? 6H9/2 (645 nm) and 4G5/2 ? 6H11/2 (706 nm) for the Sm3+: glass and glass ceramic, with an excitation at 6H5/2 ? 4F7/2 (402 nm) have been recorded. Of them, 4G5/2 ? 6H7/2 (598 nm) has shown a bright orange emission. With regard to the Dy3+: glass, a bright fluorescent yellow emission at 575 nm (4F9/2 ? 6H13/2) and blue emission at 481 nm (4F9/2 ? 6H15/2) have been observed, apart from 662 nm (4F9/2 ? 6H11/2) emission transition with an excitation at 386 nm (6H15/2 ? 4 I13/2 + 4F7/2) wavelength. Emission bands of 1G4 ? 3F4 (650 nm) and 1G4 ? 3H5 (795 nm) transitions for the Tm3+: glass and glass ceramic, with an excitation at 3H6 ? 1G4 (467 nm) have been observed. Of them, 1G4 ? 3F4 (650 nm) has shown bright red emission. Decay lifetime measurements were also carried out for all the observed Sm3+, Dy3+, and Tm3+-doped glass and glass ceramic emission bands. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Glasses doped with rare-earth ions are attracting great interest for fiber amplifiers, upconversion lasers, and the optical devices for three-dimensional displays. Since many fluorescent transitions of rare-earth ions of practical importance are initiated from an excited level with a small energy gap, materials with lower phonon energy are often required as a luminescent host to suppress the non-radiative loss and to obtain higher quantum efficiency of the desired fluorescence. However, most oxide glasses have large phonon energy (1100 cm1) due to the stretching vibration of network-forming oxides. Fluoride glasses have an advantage due to their low phonon energy (300–400 cm1) and higher quantum efficiency of many active transitions, but the stability and fiberizability as a practical material still remain problems. The invention of rare-earth ions-doped transparent oxyfluoride glass ceramics [1] have attracted great interest due to their excellent optical properties like fluoride crystals and good mechanical, chemical properties like oxide glasses [2,3]. The advantages of these materials are that * Corresponding author. Tel./fax: +86 571 88925079. E-mail address: [email protected] (G. Lakshminarayana). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.08.029
the rare-earth ions are incorporated selectively in the fluoride crystal phase with lower phonon energy after heat-treatment and the material remains transparent due to much smaller size of precipitated crystals than the wavelength of visible light. Among rareearth ions, Sm3+ (4f5) ion is one of the most interesting Ln ions to analyze the fluorescence properties as its emitting 4G5/2 level exhibits relatively high quantum efficiency and also shows different quenching emission channels. In recent times, glasses containing Sm3+ ions have stimulated extensive interest due to their potential application for high-density optical storage, under sea communication and colour displays [4]. Optical properties of Dy3+ ions in various glasses have attracted much practical interest because its 1.3 lm emission can be utilized for the optical amplification and its visible upconversion emission can be used as a solid state laser [5]. Among the trivalent lanthanide ions, Tm3+ ion has stable excited levels suitable for emitting blue and ultraviolet upconversion fluorescence. Due to peculiarities of the electronic structure of the Tm3+ ion, doped fused silica is not a practical amplifier material, as it is for Er. In the case of Tm3+, due to the presence of the intervening 3H5 level, the desired 3H4 ? 3F4 luminescence suffers appreciable multiphonon de-excitation when Tm3+ is doped into a host glass with high maximum phonon energy
G. Lakshminarayana et al. / Journal of Non-Crystalline Solids 355 (2009) 2668–2673
(MPE). In order to improve the quantum efficiency of the desired fluorescence to practical levels, it is necessary to make Tm-doped fibers from host glasses with intermediate to low MPE. As the energy gap between the upper 3H4 and the intervening 3H5 levels is about 4400 cm1, a host glass with maximum phonon energy 6880 cm1, corresponding to the exchange of P5 phonons, is required for the TDFA application [6]. 2. Experimental studies The oxyfluoride silicate glasses used in this work were prepared with the following composition in mol%: 45SiO2 – 20Al2O3 – 10CaO–(25 x) CaF2 – xSmF3/DyF3/TmF3 (x = 1.0 mol%) by employing the melt-quenching technique, by using high purity SiO2 (99.99%), Al2O3 (99.99%), CaCO3 (99.9%), CaF2 (99.99%), SmF3 (99.99%), DyF3 (99.99%) and TmF3 (99.99%) as raw materials. Each batch weighing about 20 g was mixed homogeneously and melted at 1400 °C for 30 min in a covered platinum crucible, in air. The melts were poured onto a cold brass plate and then pressed by another plate. These glasses are in circular designs having 2–3 cm in diameter with a thickness of about 0.3 cm and with a good transparency. Glass ceramics containing CaF2 nanocrystals were prepared by heat-treatment at 750 °C for 4 h as reported earlier [7]. The powder X-ray diffraction (XRD) profiles were obtained on a Rigaku D/MAX-RA diffractometer with a Ni-filter and CuKa (=1.542 Å) radiation with an applied voltage of 340 kV and 20 mA anode current, calibrated with Si at the rate of 2 °C/min. The excitation and emission spectra and the fluorescence decay curves were recorded by using a FLS920 fluorescence spectrophotometer. The light source for excitation and emission spectra was a 450W Xenon arc lamp with continuous spectral distribution from 190 to 2600 nm. The relative error in the measurement of fluorescence lifetime is estimated to be ±2%. For data fitting processes also, the error is ±2%. The temporal decay curves of the fluorescence signals were stored by using the attached storage digital oscilloscope. In order to compare the relative fluorescence intensity between the studied glasses, the conditions of excitation and detection systems were fixed, and the samples with the same shape were set at the same place in the experimental setup. On the basis of the signal to noise ratios, the relative errors in the spectral measurements are estimated to be ±2%. Besides, systematic errors have been deducted through the standard instrument corrections. Time-domain techniques use pulsed excitation and record the fluorescence decay function directly, for instance by time-correlated single-photon counting (TCSPC). TCSPC combines a superior signal to noise ratio with high time resolution and therefore provides maximum flexibility. The number of time channels can be made large enough so that standard multiexponential fluorescence decay analysis can be applied. Of course, TCSPC can also be used as a multidetector technique and, therefore, be used for simultaneous multiwavelength detection. For reasonable operation of a TCSPC device the average number of photons detected per signal period must be less than one. Now consider an array of detectors over which the same photons flux is spread. Because it is unlikely that the complete array detects several photons per period, it is also unlikely that several detectors of the array will detect a photon in one signal period. This is the basic idea behind multidetector TCSPC. Although several detectors are active simultaneously they are unlikely to deliver a photon pulse in the same signal period. The times of the photons detected in all detectors can therefore be measured in a single TAC (Time to Amplitude Converter). Technically, the photons of all detectors are combined into a common timing pulse line. Simultaneously, a detector number signal is generated that indicates in which of the detectors a particular photon was detected. The photon pulses are sent through the normal time measurement procedure of the TCSPC device. The detector numbers are used as a
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channel (or routing) signal for multidimensional TCSPC, routing the photons from the individual detectors into different waveform memory sections. All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 presents the XRD profiles of the Sm3+-doped glass and glass ceramic. From these profiles, it is observed that the as-made glass is completely amorphous with no diffraction peaks. After crystallization by thermal treatment at 750 °C for 4 h, the XRD profile show intense diffraction peaks, which could be all assigned to the well crystallized phases of CaF2 with cubic phase (JCPDS Card No. 35-0816) [7,8]. The CaF2 crystal phases containing Sm3+ or Ln3+ ions are considered to precipitate in both systems by heattreatment at 750 °C for 4 h because the CaF2 – LnF3 system has compositional range of solid solutions in the phase diagram and the lattice constant is lager than pure CaF2 [7]. The average size of the nanocrystals was estimated from the line broadening of the XRD peaks and Scherrer’s equation [9]:
d ¼ 0:89k=b cos hB ; where d is the average diameter of the nanocrystals, k is the wavelength of CuKa (1.54 Å) radiation, b (in radians) is full width at half maxima (FWHM) and hB is the Bragg angle. The lattice constant calculated from the peak in Fig. 1 is 5.473 Å, larger than that of pure CaF2 (5.462 Å). The average particle size was found to be 30 nm. 3.1. Samarium (Sm3+) Fig. 2 presents the excitation spectrum of 1 mol% Sm3+-doped glass ceramic, monitoring emission at 598 nm. Several excitation bands are identified which are assigned to the electronic transitions of 6H5/2 ? 4L17/2 at 360 nm, 6H5/2 ? 4P5/2 at 374 nm, 6 H5/2 ? 4F7/2 at 402 nm, 6H5/2 ? 6p5/2 at 415 nm, 6H5/2 ? 4p5/2 at 422 nm, 6H5/2 ? 4G9/2 at 450 nm, 6H5/2 ? 4I13/2 at both 462 and 467 nm due to Stark splitting of 4I13/2 state, 6H5/2 ? 4I11/2 at 473 nm, and 6H5/2 ? 4I9/2 at 480 nm [10,11]. Sm3+ ion will exhibits several overlapped excitation bands from 460–495 nm wavelength due to its several closely spaced energy levels located at these wavelengths. We also aware that, Xe-arc lamp will exhibit several closely spaced sharp emission lines between 450 and 500 nm. But we measured this Sm3+ excitation spectrum by monitoring its
Fig. 1. XRD profiles of the Sm3+ (as-made) and glass ceramic (heat-treated at 750 °C/4 h).
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Fig. 2. Excitation spectrum of 1 mol% Sm3+-doped glass ceramic.
bright visible orange emission at 598 nm .We know that RE ions will exhibit sharp emission/excitation bands in optical materials .But, the sharpness of these bands also depends on the host matrices phonon energies. Sometimes, Stark splitting may cause for broadening of these observed bands. Only the prominent excitation at 402 nm has been selected for the measurement of emission spectrum of 1 mol% Sm3+-doped glass and glass ceramic. Fig. 3 shows the emission spectra of 1 mol% Sm3+-doped glass and glass ceramic. When the 4F7/2 level (402 nm) of Sm3+ is excited, the initial population relaxes finally to the 4G5/2 level. Between 4F7/2 and 4 G5/2 levels, there are several levels with smaller energy differences, which encourage their efficient non-radiative relaxation leading to the population of the 4G5/2 state. This state is separated from the next lower-lying 6F11/2 by about 7000 cm1, which makes the multiphonon relaxation negligible. Thus, it could be stated that radiative transitions and relaxation by non-radiative energy transfer are the two main processes, which could finally depopulate the 4 G5/2 state. The emission spectra have exhibited four emission transitions, which are assigned to 4G5/2 ? 6H5/2 (562 nm), 4 G5/2 ? 6H7/2 (598 nm) and 4G5/2 ? 6H9/2 (645 nm), and 4 G5/2 ? 6H11/2 (706 nm) transitions. Sm3+-doped glass ceramic has shown stronger emissions compared to Sm3+-doped glass due to CaF2 crystalline phase. Among these four emission bands, the transition 4G5/2 ? 6H7/2 (598 nm) has shown a strong orange emission. The Sm3+: glass and glass ceramic shows a bright
Fig. 3. Emission spectra of 1 mol% Sm3+-doped glass and glass ceramic with 402 nm excitation wavelength. Inset shows the energy level scheme for observed emission transitions.
orange-reddish emission under an UV source also. The transition 4 G5/2 ? 6H7/2 with DJ = ±1 is a magnetic dipole (MD) allowed one but it is also electric dipole (ED) dominated, the other transition 4 G5/2 ? 6H9/2 is purely an ED one [12]. Generally, the intensity ratio of ED to MD transitions has been used to measure the symmetry of the local environment of the trivalent 4f ions. The greater the intensity of the ED transition, the more the asymmetry nature [13]. In the present work, 4G5/2 ? 6H9/2 (ED) transition of Sm3+ ions is slightly intense than 4G5/2 ? 6H5/2 (MD) specifying the asymmetric nature of the glass host. Fig. 3 inset shows the energy level scheme for all the observed emission transitions of Sm3+ ions. We have measured the lifetimes of observed emission transitions with the excitation wavelength (402 nm), and Fig. 4(a) and (b) presents the decay curves of these four emission transitions of Sm3+ glass and glass ceramic, respectively. From these decay curves, we observed that compared to glass, the respective glass ceramic has shown longer lifetimes for the observed emission transitions. When the interaction between luminescent ions is not important, the decay of the luminescence can be fitted to a single-exponential. After heat-treatment, the CaF2 nanocrystals will be formed and distributed in the glass matrices. The crystal field strength around each CaF2 nanocrystal is site dependent, and the macroscopic decay rate is treated by ascribing the average rates to the system. The fluorescence following a selected excitation wavelength for a particular emission band will have a single-exponential time dependent.
Fig. 4. Decay lifetime curves of 1 mol% Sm3+-doped (a) glass and (b) glass ceramic for all the observed emission transitions. (Y-axis on semi-log scale).
G. Lakshminarayana et al. / Journal of Non-Crystalline Solids 355 (2009) 2668–2673
Fig. 5. Excitation spectrum of 1 mol% Dy3+-doped glass ceramic.
3.2. Dysprosium (Dy3+) Fig. 5 shows the excitation spectrum of 1 mol% Dy3+-doped glass ceramic, monitoring emission at 575 nm. We have observed several excitation bands from this excitation spectrum, and these are assigned to the electronic transitions with the ground level 6 H15/2 to higher energy levels of Dy3+, i.e. 6H15/2 ? 4L19/2 6 6 (323 nm), H15/2 ? 4M15/2 + 6P7/2 (348 nm), H15/2 ? 4I11/2 (363 nm), 6H15/2 ? 4I13/2 + 4F7/2 (386 nm), and 6H15/2 ? 4G11/2 (425 nm) based on the energy levels reported earlier [14]. From these excitation transitions, only a prominent transition (386 nm) has been selected for the measurement of emission spectra of Dy3+: glass and glass ceramic. Fig. 6 presents the emission spectra of 1 mol% Dy3+-doped glass and glass ceramic with 386 nm excitation wavelength. When the 4I13/2 + 4F7/2 level of Dy3+ is excited with 386 nm wavelength, though this level is within the thermal excitation energy at room temperature, we do not get any fluorescence from this level. The Dy3+ ions will depopulate non-radiatively from 4I13/2 + 4F7/2 level to stable eigen state of Dy3+, 4F9/2 whose energy from ground state is 20 660 cm1. This state is separated from the next lower-lying level (6F1/2) by about 6000 cm1, what makes the multiphonon relaxation negligible inspite of high phonon energies of the host (900 cm1). It appears that only radiative transitions and relaxation by non-radiative energy transfer processes could be depopulating the 4F9/2 state. From these emission
Fig. 6. Emission spectra of 1 mol% Dy3+-doped glass and glass ceramic with 386 nm excitation wavelength. Inset shows the energy level scheme for observed emission transitions.
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spectra, we have observed three emission bands centered at 481, 575, and 622 nm which could be assigned to 4F9/2 ? 6H15/2, 4 F9/2 ? 6H13/2, and 4F9/2 ? 6H11/2 transitions from Dy3+-doped glass and glass ceramic. Among these, the transition 6F9/2 ? 6H13/2 has shown bright yellow emission intensity i.e. a major part of the emission intensity is contained in the 4F9/2 ? 6H13/2 transition. Dy3+-doped glass ceramic has shown stronger emissions compared to Dy3+-doped glass due to the CaF2 crystalline phase present in it. The Dy3+-doped glass ceramic showed a bright yellow emission under an UV source also. Fig. 6 inset presents the energy level scheme for all the observed emission transitions of Dy3+-doped glass and glass ceramic. We have measured the lifetimes of blue (481 nm) yellow (575 nm) and red (662 nm) emission transitions with excitation wavelength at 386 nm for Dy3+-doped glass and glass ceramic, and presented in Fig.7(a) and (b). From these decay curves, we observed that compared to glass, glass ceramic has shown longer lifetimes for the observed emission transitions. The non-exponentially in the decay curves of lanthanide doped materials usually arises from ion – ion interactions as the concentration of dopant is increased, usually due to cross-relaxation processes. But, in our present study, the longer lifetime of glass ceramic compared with respective glass confirms a fact that long-range crystalline ordering may be crucial for the effects observed because the site coordination of the Dy3+ ions is so strongly defined. Previously, for CaMoO4:Dy3+ single crystals, the decay curves measured in the 10–170 K temperature range are not exponential and obey the Inokuti – Hirayama model for energy transfer for an electric quad-
Fig. 7. Decay lifetime curves of 1 mol% Dy3+-doped (a) glass and (b) glass ceramic for all the observed emission transitions. (Y-axis on semi-log scale).
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rupole – quadrupole interaction in the absence of diffusion among the donors [15]. Recently, we reported spectral results of various 1 mol% RE ions-doped oxyfluoride glasses/glass ceramics [16–21], in which the room temperature decay curves were well fitted to the single-exponential as in the present case. Recently, it is reported that in Er3+ and Ho3+-doped nanocrystalline yttria, emission spectra obtained were with less intensity compared with bulk materials. Also, the lifetimes of the excited states were found to be significantly faster in the nanocrystal samples than in the bulk samples [22,23]. 3.3. Thulium (Tm3+) Fig. 8 shows the emission spectra of 1 mol% Tm3+-doped glass and glass ceramic with 467 nm excitation. From these spectra, emission bands centered at 650 nm (1G4 ? 3F4) and 795 nm (1G4 ? 3H5) are observed from both glass and glass ceramic. Of them, 1G4 ? 3F4 (650 nm) has shown bright red emission. Tm3+doped glass ceramic has shown stronger emissions compared to Tm3+-doped glass due to the CaF2 crystalline phase present in it. Fig. 8 inset (a) shows the excitation spectrum of 1 mol% Tm3+doped glass ceramic, monitoring emission at 650 nm from which a broad excitation band centered at 467 nm (3H6 ? 1G4) is observed. And Fig. 8 inset (b) presents the energy level scheme of all the observed emission transitions of Tm3+-doped glass and glass ceramic. We have measured the lifetimes of the observed emission transitions with the excitation wavelength 467 nm, and Fig. 9 presents the decay curves of these two emission transitions of Tm3+ glass and glass ceramic, respectively. From these decay curves, we observed that compared to glass, glass ceramic has shown longer lifetimes for the observed emission transitions. For all the studied rare-earth ions, compared with glasses, glass ceramics have shown stronger emissions due to the crystalline phase present in it. This fact unambiguously indicates that for the crystalline states, the doped ions should be incorporated into the fixed crystalline site positions. Structurally, glass is a continuous random network lacking both symmetry and periodicity. In the case of glasses the coordination of the rare-earth ions becomes more delocalized. As a consequence, the oscillator strengths are also more dispersed. Additionally, the contribution of the phonon sub-system gives more contribution to electron – phonon an harmonicity [24]. Moreover, all the lifetimes show a strong increase after heat-treatment. Generally, it is known that the phonon energy of Si – O is 1000 cm1 and that of Ca – F is 400 cm1. The increase in lumines-
Fig. 9. Decay lifetime curves of 1 mol% Tm3+-doped glass and glass ceramic for all the observed emission transitions. (Y-axis on semi-log scale).
cence is connected to the reduction in multiphonon relaxation for Sm3+, Dy3+ and Tm3+ ions embedded in crystals which suffer from less competitive non-radiative relaxation with respect to the RE ions in the glassy environment. Crystallization plays an important role in the observed enhanced emission properties; in particular, in the IR oscillators, their wideness and spectral shift are very important. This reflects the fixed position of the rare-earth ions for the crystalline states with respect to the glass-like states. 4. Conclusions In summary, transparent glass ceramics containing CaF2 nanocrystals with Sm3+, Dy3+, and Tm3+ as dopants were prepared for their optical characterization. Emission bands of 562, 598, 645 and 706 nm for the Sm3+: glass and glass ceramic have been recorded in which 598 nm emission bands have shown a bright orange emission. With regard to the Dy3+: glass, a bright fluorescent yellow emission at 575 nm and blue emission at 481 nm have been observed, apart from 662 nm emission transition. Emission bands of 650 and 795 nm transitions for the Tm3+: glass and glass ceramic, with an excitation at 467 nm have been observed in which 650 nm has shown bright red emission. These glasses or glass ceramics with bright visible luminescence in orange (Sm3+), yellow (Dy3+), and red (Tm3+) regions should have potential technological applications in optoelectronic materials and displays. Acknowledgments This work was financially supported by National Natural Science Foundation of China (Grant Nos. 50672087 and 60778039), National Basic Research Program of China (2006CB806007) and National High Technology Program of China (2006AA03Z304). This work was also supported by the program for Changjiang Scholars and Innovative Research Team in University (IRT0651). References
Fig. 8. Emission spectra of 1 mol% Tm3+-doped glass and glass ceramic with 467 nm excitation wavelength. Inset shows (a) excitation spectrum of Tm3+-doped glass ceramic (b) energy level scheme for all the observed emission transitions.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Photoluminescence of Sm3+, Dy3+, and Tm3+-doped transparent glass ceramics containing CaF2 nanocrystals G. Lakshminarayana a,*, Rong Yang a, Mengfei Mao a, Jianrong Qiu a, I.V. Kityk b a b
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Electrical Engineering Department, Czestochowa Technological University, Al. Armii Krajowej 17/19, Czestochowa, Poland
a r t i c l e
i n f o
Article history: Received 17 November 2008 Received in revised form 5 July 2009 Available online 23 September 2009 PACS: 78.55 Qr 78.20.e 78.55.m 78.30.j Keywords: Glass ceramics Glass transition Fluorides Optical spectroscopy Nanocrystals Luminescence
a b s t r a c t Photoluminescence properties of Sm3+, Dy3+, and Tm3+-doped transparent oxyfluoride silicate glass ceramics containing CaF2 nanocrystals were reported. Emission bands of 4G5/2 ? 6H5/2 (562 nm), 4 G5/2 ? 6H7/2 (598 nm), 4G5/2 ? 6H9/2 (645 nm) and 4G5/2 ? 6H11/2 (706 nm) for the Sm3+: glass and glass ceramic, with an excitation at 6H5/2 ? 4F7/2 (402 nm) have been recorded. Of them, 4G5/2 ? 6H7/2 (598 nm) has shown a bright orange emission. With regard to the Dy3+: glass, a bright fluorescent yellow emission at 575 nm (4F9/2 ? 6H13/2) and blue emission at 481 nm (4F9/2 ? 6H15/2) have been observed, apart from 662 nm (4F9/2 ? 6H11/2) emission transition with an excitation at 386 nm (6H15/2 ? 4 I13/2 + 4F7/2) wavelength. Emission bands of 1G4 ? 3F4 (650 nm) and 1G4 ? 3H5 (795 nm) transitions for the Tm3+: glass and glass ceramic, with an excitation at 3H6 ? 1G4 (467 nm) have been observed. Of them, 1G4 ? 3F4 (650 nm) has shown bright red emission. Decay lifetime measurements were also carried out for all the observed Sm3+, Dy3+, and Tm3+-doped glass and glass ceramic emission bands. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Glasses doped with rare-earth ions are attracting great interest for fiber amplifiers, upconversion lasers, and the optical devices for three-dimensional displays. Since many fluorescent transitions of rare-earth ions of practical importance are initiated from an excited level with a small energy gap, materials with lower phonon energy are often required as a luminescent host to suppress the non-radiative loss and to obtain higher quantum efficiency of the desired fluorescence. However, most oxide glasses have large phonon energy (1100 cm1) due to the stretching vibration of network-forming oxides. Fluoride glasses have an advantage due to their low phonon energy (300–400 cm1) and higher quantum efficiency of many active transitions, but the stability and fiberizability as a practical material still remain problems. The invention of rare-earth ions-doped transparent oxyfluoride glass ceramics [1] have attracted great interest due to their excellent optical properties like fluoride crystals and good mechanical, chemical properties like oxide glasses [2,3]. The advantages of these materials are that * Corresponding author. Tel./fax: +86 571 88925079. E-mail address: [email protected] (G. Lakshminarayana). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.08.029
the rare-earth ions are incorporated selectively in the fluoride crystal phase with lower phonon energy after heat-treatment and the material remains transparent due to much smaller size of precipitated crystals than the wavelength of visible light. Among rareearth ions, Sm3+ (4f5) ion is one of the most interesting Ln ions to analyze the fluorescence properties as its emitting 4G5/2 level exhibits relatively high quantum efficiency and also shows different quenching emission channels. In recent times, glasses containing Sm3+ ions have stimulated extensive interest due to their potential application for high-density optical storage, under sea communication and colour displays [4]. Optical properties of Dy3+ ions in various glasses have attracted much practical interest because its 1.3 lm emission can be utilized for the optical amplification and its visible upconversion emission can be used as a solid state laser [5]. Among the trivalent lanthanide ions, Tm3+ ion has stable excited levels suitable for emitting blue and ultraviolet upconversion fluorescence. Due to peculiarities of the electronic structure of the Tm3+ ion, doped fused silica is not a practical amplifier material, as it is for Er. In the case of Tm3+, due to the presence of the intervening 3H5 level, the desired 3H4 ? 3F4 luminescence suffers appreciable multiphonon de-excitation when Tm3+ is doped into a host glass with high maximum phonon energy
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(MPE). In order to improve the quantum efficiency of the desired fluorescence to practical levels, it is necessary to make Tm-doped fibers from host glasses with intermediate to low MPE. As the energy gap between the upper 3H4 and the intervening 3H5 levels is about 4400 cm1, a host glass with maximum phonon energy 6880 cm1, corresponding to the exchange of P5 phonons, is required for the TDFA application [6]. 2. Experimental studies The oxyfluoride silicate glasses used in this work were prepared with the following composition in mol%: 45SiO2 – 20Al2O3 – 10CaO–(25 x) CaF2 – xSmF3/DyF3/TmF3 (x = 1.0 mol%) by employing the melt-quenching technique, by using high purity SiO2 (99.99%), Al2O3 (99.99%), CaCO3 (99.9%), CaF2 (99.99%), SmF3 (99.99%), DyF3 (99.99%) and TmF3 (99.99%) as raw materials. Each batch weighing about 20 g was mixed homogeneously and melted at 1400 °C for 30 min in a covered platinum crucible, in air. The melts were poured onto a cold brass plate and then pressed by another plate. These glasses are in circular designs having 2–3 cm in diameter with a thickness of about 0.3 cm and with a good transparency. Glass ceramics containing CaF2 nanocrystals were prepared by heat-treatment at 750 °C for 4 h as reported earlier [7]. The powder X-ray diffraction (XRD) profiles were obtained on a Rigaku D/MAX-RA diffractometer with a Ni-filter and CuKa (=1.542 Å) radiation with an applied voltage of 340 kV and 20 mA anode current, calibrated with Si at the rate of 2 °C/min. The excitation and emission spectra and the fluorescence decay curves were recorded by using a FLS920 fluorescence spectrophotometer. The light source for excitation and emission spectra was a 450W Xenon arc lamp with continuous spectral distribution from 190 to 2600 nm. The relative error in the measurement of fluorescence lifetime is estimated to be ±2%. For data fitting processes also, the error is ±2%. The temporal decay curves of the fluorescence signals were stored by using the attached storage digital oscilloscope. In order to compare the relative fluorescence intensity between the studied glasses, the conditions of excitation and detection systems were fixed, and the samples with the same shape were set at the same place in the experimental setup. On the basis of the signal to noise ratios, the relative errors in the spectral measurements are estimated to be ±2%. Besides, systematic errors have been deducted through the standard instrument corrections. Time-domain techniques use pulsed excitation and record the fluorescence decay function directly, for instance by time-correlated single-photon counting (TCSPC). TCSPC combines a superior signal to noise ratio with high time resolution and therefore provides maximum flexibility. The number of time channels can be made large enough so that standard multiexponential fluorescence decay analysis can be applied. Of course, TCSPC can also be used as a multidetector technique and, therefore, be used for simultaneous multiwavelength detection. For reasonable operation of a TCSPC device the average number of photons detected per signal period must be less than one. Now consider an array of detectors over which the same photons flux is spread. Because it is unlikely that the complete array detects several photons per period, it is also unlikely that several detectors of the array will detect a photon in one signal period. This is the basic idea behind multidetector TCSPC. Although several detectors are active simultaneously they are unlikely to deliver a photon pulse in the same signal period. The times of the photons detected in all detectors can therefore be measured in a single TAC (Time to Amplitude Converter). Technically, the photons of all detectors are combined into a common timing pulse line. Simultaneously, a detector number signal is generated that indicates in which of the detectors a particular photon was detected. The photon pulses are sent through the normal time measurement procedure of the TCSPC device. The detector numbers are used as a
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channel (or routing) signal for multidimensional TCSPC, routing the photons from the individual detectors into different waveform memory sections. All the measurements were carried out at room temperature. 3. Results and discussion Fig. 1 presents the XRD profiles of the Sm3+-doped glass and glass ceramic. From these profiles, it is observed that the as-made glass is completely amorphous with no diffraction peaks. After crystallization by thermal treatment at 750 °C for 4 h, the XRD profile show intense diffraction peaks, which could be all assigned to the well crystallized phases of CaF2 with cubic phase (JCPDS Card No. 35-0816) [7,8]. The CaF2 crystal phases containing Sm3+ or Ln3+ ions are considered to precipitate in both systems by heattreatment at 750 °C for 4 h because the CaF2 – LnF3 system has compositional range of solid solutions in the phase diagram and the lattice constant is lager than pure CaF2 [7]. The average size of the nanocrystals was estimated from the line broadening of the XRD peaks and Scherrer’s equation [9]:
d ¼ 0:89k=b cos hB ; where d is the average diameter of the nanocrystals, k is the wavelength of CuKa (1.54 Å) radiation, b (in radians) is full width at half maxima (FWHM) and hB is the Bragg angle. The lattice constant calculated from the peak in Fig. 1 is 5.473 Å, larger than that of pure CaF2 (5.462 Å). The average particle size was found to be 30 nm. 3.1. Samarium (Sm3+) Fig. 2 presents the excitation spectrum of 1 mol% Sm3+-doped glass ceramic, monitoring emission at 598 nm. Several excitation bands are identified which are assigned to the electronic transitions of 6H5/2 ? 4L17/2 at 360 nm, 6H5/2 ? 4P5/2 at 374 nm, 6 H5/2 ? 4F7/2 at 402 nm, 6H5/2 ? 6p5/2 at 415 nm, 6H5/2 ? 4p5/2 at 422 nm, 6H5/2 ? 4G9/2 at 450 nm, 6H5/2 ? 4I13/2 at both 462 and 467 nm due to Stark splitting of 4I13/2 state, 6H5/2 ? 4I11/2 at 473 nm, and 6H5/2 ? 4I9/2 at 480 nm [10,11]. Sm3+ ion will exhibits several overlapped excitation bands from 460–495 nm wavelength due to its several closely spaced energy levels located at these wavelengths. We also aware that, Xe-arc lamp will exhibit several closely spaced sharp emission lines between 450 and 500 nm. But we measured this Sm3+ excitation spectrum by monitoring its
Fig. 1. XRD profiles of the Sm3+ (as-made) and glass ceramic (heat-treated at 750 °C/4 h).
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Fig. 2. Excitation spectrum of 1 mol% Sm3+-doped glass ceramic.
bright visible orange emission at 598 nm .We know that RE ions will exhibit sharp emission/excitation bands in optical materials .But, the sharpness of these bands also depends on the host matrices phonon energies. Sometimes, Stark splitting may cause for broadening of these observed bands. Only the prominent excitation at 402 nm has been selected for the measurement of emission spectrum of 1 mol% Sm3+-doped glass and glass ceramic. Fig. 3 shows the emission spectra of 1 mol% Sm3+-doped glass and glass ceramic. When the 4F7/2 level (402 nm) of Sm3+ is excited, the initial population relaxes finally to the 4G5/2 level. Between 4F7/2 and 4 G5/2 levels, there are several levels with smaller energy differences, which encourage their efficient non-radiative relaxation leading to the population of the 4G5/2 state. This state is separated from the next lower-lying 6F11/2 by about 7000 cm1, which makes the multiphonon relaxation negligible. Thus, it could be stated that radiative transitions and relaxation by non-radiative energy transfer are the two main processes, which could finally depopulate the 4 G5/2 state. The emission spectra have exhibited four emission transitions, which are assigned to 4G5/2 ? 6H5/2 (562 nm), 4 G5/2 ? 6H7/2 (598 nm) and 4G5/2 ? 6H9/2 (645 nm), and 4 G5/2 ? 6H11/2 (706 nm) transitions. Sm3+-doped glass ceramic has shown stronger emissions compared to Sm3+-doped glass due to CaF2 crystalline phase. Among these four emission bands, the transition 4G5/2 ? 6H7/2 (598 nm) has shown a strong orange emission. The Sm3+: glass and glass ceramic shows a bright
Fig. 3. Emission spectra of 1 mol% Sm3+-doped glass and glass ceramic with 402 nm excitation wavelength. Inset shows the energy level scheme for observed emission transitions.
orange-reddish emission under an UV source also. The transition 4 G5/2 ? 6H7/2 with DJ = ±1 is a magnetic dipole (MD) allowed one but it is also electric dipole (ED) dominated, the other transition 4 G5/2 ? 6H9/2 is purely an ED one [12]. Generally, the intensity ratio of ED to MD transitions has been used to measure the symmetry of the local environment of the trivalent 4f ions. The greater the intensity of the ED transition, the more the asymmetry nature [13]. In the present work, 4G5/2 ? 6H9/2 (ED) transition of Sm3+ ions is slightly intense than 4G5/2 ? 6H5/2 (MD) specifying the asymmetric nature of the glass host. Fig. 3 inset shows the energy level scheme for all the observed emission transitions of Sm3+ ions. We have measured the lifetimes of observed emission transitions with the excitation wavelength (402 nm), and Fig. 4(a) and (b) presents the decay curves of these four emission transitions of Sm3+ glass and glass ceramic, respectively. From these decay curves, we observed that compared to glass, the respective glass ceramic has shown longer lifetimes for the observed emission transitions. When the interaction between luminescent ions is not important, the decay of the luminescence can be fitted to a single-exponential. After heat-treatment, the CaF2 nanocrystals will be formed and distributed in the glass matrices. The crystal field strength around each CaF2 nanocrystal is site dependent, and the macroscopic decay rate is treated by ascribing the average rates to the system. The fluorescence following a selected excitation wavelength for a particular emission band will have a single-exponential time dependent.
Fig. 4. Decay lifetime curves of 1 mol% Sm3+-doped (a) glass and (b) glass ceramic for all the observed emission transitions. (Y-axis on semi-log scale).
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Fig. 5. Excitation spectrum of 1 mol% Dy3+-doped glass ceramic.
3.2. Dysprosium (Dy3+) Fig. 5 shows the excitation spectrum of 1 mol% Dy3+-doped glass ceramic, monitoring emission at 575 nm. We have observed several excitation bands from this excitation spectrum, and these are assigned to the electronic transitions with the ground level 6 H15/2 to higher energy levels of Dy3+, i.e. 6H15/2 ? 4L19/2 6 6 (323 nm), H15/2 ? 4M15/2 + 6P7/2 (348 nm), H15/2 ? 4I11/2 (363 nm), 6H15/2 ? 4I13/2 + 4F7/2 (386 nm), and 6H15/2 ? 4G11/2 (425 nm) based on the energy levels reported earlier [14]. From these excitation transitions, only a prominent transition (386 nm) has been selected for the measurement of emission spectra of Dy3+: glass and glass ceramic. Fig. 6 presents the emission spectra of 1 mol% Dy3+-doped glass and glass ceramic with 386 nm excitation wavelength. When the 4I13/2 + 4F7/2 level of Dy3+ is excited with 386 nm wavelength, though this level is within the thermal excitation energy at room temperature, we do not get any fluorescence from this level. The Dy3+ ions will depopulate non-radiatively from 4I13/2 + 4F7/2 level to stable eigen state of Dy3+, 4F9/2 whose energy from ground state is 20 660 cm1. This state is separated from the next lower-lying level (6F1/2) by about 6000 cm1, what makes the multiphonon relaxation negligible inspite of high phonon energies of the host (900 cm1). It appears that only radiative transitions and relaxation by non-radiative energy transfer processes could be depopulating the 4F9/2 state. From these emission
Fig. 6. Emission spectra of 1 mol% Dy3+-doped glass and glass ceramic with 386 nm excitation wavelength. Inset shows the energy level scheme for observed emission transitions.
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spectra, we have observed three emission bands centered at 481, 575, and 622 nm which could be assigned to 4F9/2 ? 6H15/2, 4 F9/2 ? 6H13/2, and 4F9/2 ? 6H11/2 transitions from Dy3+-doped glass and glass ceramic. Among these, the transition 6F9/2 ? 6H13/2 has shown bright yellow emission intensity i.e. a major part of the emission intensity is contained in the 4F9/2 ? 6H13/2 transition. Dy3+-doped glass ceramic has shown stronger emissions compared to Dy3+-doped glass due to the CaF2 crystalline phase present in it. The Dy3+-doped glass ceramic showed a bright yellow emission under an UV source also. Fig. 6 inset presents the energy level scheme for all the observed emission transitions of Dy3+-doped glass and glass ceramic. We have measured the lifetimes of blue (481 nm) yellow (575 nm) and red (662 nm) emission transitions with excitation wavelength at 386 nm for Dy3+-doped glass and glass ceramic, and presented in Fig.7(a) and (b). From these decay curves, we observed that compared to glass, glass ceramic has shown longer lifetimes for the observed emission transitions. The non-exponentially in the decay curves of lanthanide doped materials usually arises from ion – ion interactions as the concentration of dopant is increased, usually due to cross-relaxation processes. But, in our present study, the longer lifetime of glass ceramic compared with respective glass confirms a fact that long-range crystalline ordering may be crucial for the effects observed because the site coordination of the Dy3+ ions is so strongly defined. Previously, for CaMoO4:Dy3+ single crystals, the decay curves measured in the 10–170 K temperature range are not exponential and obey the Inokuti – Hirayama model for energy transfer for an electric quad-
Fig. 7. Decay lifetime curves of 1 mol% Dy3+-doped (a) glass and (b) glass ceramic for all the observed emission transitions. (Y-axis on semi-log scale).
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rupole – quadrupole interaction in the absence of diffusion among the donors [15]. Recently, we reported spectral results of various 1 mol% RE ions-doped oxyfluoride glasses/glass ceramics [16–21], in which the room temperature decay curves were well fitted to the single-exponential as in the present case. Recently, it is reported that in Er3+ and Ho3+-doped nanocrystalline yttria, emission spectra obtained were with less intensity compared with bulk materials. Also, the lifetimes of the excited states were found to be significantly faster in the nanocrystal samples than in the bulk samples [22,23]. 3.3. Thulium (Tm3+) Fig. 8 shows the emission spectra of 1 mol% Tm3+-doped glass and glass ceramic with 467 nm excitation. From these spectra, emission bands centered at 650 nm (1G4 ? 3F4) and 795 nm (1G4 ? 3H5) are observed from both glass and glass ceramic. Of them, 1G4 ? 3F4 (650 nm) has shown bright red emission. Tm3+doped glass ceramic has shown stronger emissions compared to Tm3+-doped glass due to the CaF2 crystalline phase present in it. Fig. 8 inset (a) shows the excitation spectrum of 1 mol% Tm3+doped glass ceramic, monitoring emission at 650 nm from which a broad excitation band centered at 467 nm (3H6 ? 1G4) is observed. And Fig. 8 inset (b) presents the energy level scheme of all the observed emission transitions of Tm3+-doped glass and glass ceramic. We have measured the lifetimes of the observed emission transitions with the excitation wavelength 467 nm, and Fig. 9 presents the decay curves of these two emission transitions of Tm3+ glass and glass ceramic, respectively. From these decay curves, we observed that compared to glass, glass ceramic has shown longer lifetimes for the observed emission transitions. For all the studied rare-earth ions, compared with glasses, glass ceramics have shown stronger emissions due to the crystalline phase present in it. This fact unambiguously indicates that for the crystalline states, the doped ions should be incorporated into the fixed crystalline site positions. Structurally, glass is a continuous random network lacking both symmetry and periodicity. In the case of glasses the coordination of the rare-earth ions becomes more delocalized. As a consequence, the oscillator strengths are also more dispersed. Additionally, the contribution of the phonon sub-system gives more contribution to electron – phonon an harmonicity [24]. Moreover, all the lifetimes show a strong increase after heat-treatment. Generally, it is known that the phonon energy of Si – O is 1000 cm1 and that of Ca – F is 400 cm1. The increase in lumines-
Fig. 9. Decay lifetime curves of 1 mol% Tm3+-doped glass and glass ceramic for all the observed emission transitions. (Y-axis on semi-log scale).
cence is connected to the reduction in multiphonon relaxation for Sm3+, Dy3+ and Tm3+ ions embedded in crystals which suffer from less competitive non-radiative relaxation with respect to the RE ions in the glassy environment. Crystallization plays an important role in the observed enhanced emission properties; in particular, in the IR oscillators, their wideness and spectral shift are very important. This reflects the fixed position of the rare-earth ions for the crystalline states with respect to the glass-like states. 4. Conclusions In summary, transparent glass ceramics containing CaF2 nanocrystals with Sm3+, Dy3+, and Tm3+ as dopants were prepared for their optical characterization. Emission bands of 562, 598, 645 and 706 nm for the Sm3+: glass and glass ceramic have been recorded in which 598 nm emission bands have shown a bright orange emission. With regard to the Dy3+: glass, a bright fluorescent yellow emission at 575 nm and blue emission at 481 nm have been observed, apart from 662 nm emission transition. Emission bands of 650 and 795 nm transitions for the Tm3+: glass and glass ceramic, with an excitation at 467 nm have been observed in which 650 nm has shown bright red emission. These glasses or glass ceramics with bright visible luminescence in orange (Sm3+), yellow (Dy3+), and red (Tm3+) regions should have potential technological applications in optoelectronic materials and displays. Acknowledgments This work was financially supported by National Natural Science Foundation of China (Grant Nos. 50672087 and 60778039), National Basic Research Program of China (2006CB806007) and National High Technology Program of China (2006AA03Z304). This work was also supported by the program for Changjiang Scholars and Innovative Research Team in University (IRT0651). References
Fig. 8. Emission spectra of 1 mol% Tm3+-doped glass and glass ceramic with 467 nm excitation wavelength. Inset shows (a) excitation spectrum of Tm3+-doped glass ceramic (b) energy level scheme for all the observed emission transitions.
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