- Email: [email protected]
Eu3 +
Journal of Non-Crystalline Solids 457 (2017) 31–35
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Spectroscopic investigations of phosphate-borate-fluoride glass doped with Tb3 +/Eu3 + D. Valiev a,⁎, K. Belikov b a b
National Research Tomsk Polytechnic University, 30 Lenin Avenue, 634050 Tomsk, Russia Institute for Single Crystals, 60 Lenin Avenue, Kharkov 61001, Ukraine
a r t i c l e
i n f o
Article history: Received 25 August 2016 Received in revised form 15 November 2016 Accepted 16 November 2016 Available online xxxx Keywords: Glass Pulse cathodoluminescence Rare earth ions Decay kinetics
a b s t r a c t The luminescence of Li2O–B2O3–P2O5–CaF2 glass doped with Tb3+/Eu3+ under electron beam excitation have been investigated in detail. The excitation spectra, photoluminescence (PL), pulse cathodoluminescence (PCL) and luminescence decay kinetics were analyzed. The energy transfer efficiency from Tb3+ to Eu3+ ions was found to increase at increased concentration of Eu3+ ions. As the concentration of europium grew, the luminescence quenching was observed. The energy transfer from Tb3+ to Eu3+ was observed to occur through level 5D4 of terbium ions. It is shown that the luminescence decay kinetics of terbium ions at 485, 544, 622 and 700 nm depends on europium concentration. © 2016 Elsevier B.V. All rights reserved.
1. Introduction At the present time, white light emission diodes (w-LED) are widely used as a light source in different areas replacing those conventional like incandescent and luminescence sources due to tunable light characteristics and long operation life-time [1,2]. A commercial white LED is typically fabricated with a blue chip coated with yellow YAG:Ce3 + phosphor [3]. The major problems of the phosphor in this application are the lack of a red light component and poor thermal stability [4], and the latter leads to degradation and change in basic light parameters. Glass used instead of phosphor can be a more suitable approach to solve the problem [5–9]. Glass doped with rare earth elements (REE) can be an alternative for conventional phosphors due to high thermal stability of the former. In addition, this glass possesses tunable color rendering index (CRI), since the glass can be varied in its composition, and low power consumption [10,11]. Synthesis of glass materials based on oxide glass doped with rare earth ions (REI) is simple, optical elements can be manufactured of any shape and size, and the cost is relatively low. Furthermore, glass can be doped, and varied composition of the host and high optical homogeneity make them an alternative to single crystals [12,13].
⁎ Corresponding author at: National Research Tomsk Polytechnic University, Institute of High Technology Physics, Department of Lasers and Lighting Engineering, Tomsk, Russia. E-mail address: [email protected] (D. Valiev).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.11.016 0022-3093/© 2016 Elsevier B.V. All rights reserved.
Synthesis of new materials with specific optical properties requires an understanding of the mechanisms of luminescence in different systems [12–15]. These are the processes of electronic excitations and energy transfer between the luminescence centers and the glass host, and the interaction between active ions [16–21]. Investigation of these processes can improve luminescent characteristics and light yield of the systems for different applications. Glass based on phosphate/borate can be used as luminescent material. Phosphate glass has good physical properties such as low melting point, high transparency in the UV region, good thermo-optic properties and low refractive index [22–25]. Borate glass is mechanically, thermally and chemically stable [16,26]. Borophosphate glass is a good candidate for photonic materials due to its physical and optical properties such as high REI solubility and high transparency in the UV region [8,27]. Europium ions as dopants are found in many glass hosts, the luminescence bands are found to be intensive in the “orange-red” spectral region, and the light yield of these ions is high. To correct the emission chromaticity and to increase the light yield, Eu-containing glass is supposed to be doped with co-dopant ions. Terbium ions can be used as co-dopants due to an intense “green” band and several bands observed in the “blue” region. White light emission can be observed for co-doped Eu3 +/Tb3 + borophosphate glass due to the combination of the Eu3 + emission bands at 425, 593, 614 and 700 nm, with the most intensive Tb3+ emission band at 544 nm [28,29]. Few reports provide data on spectroscopic properties of REIs under pulsed electron beam excitation. This research aims to study the spectral characteristics and luminescence decay in co-doped Tb3 +/Eu3 +
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D. Valiev, K. Belikov / Journal of Non-Crystalline Solids 457 (2017) 31–35
phosphate-borate-fluoride glass with variation of europium concentration under photo- and electron excitation. 2. Experimental details 2.1. Glass preparation In the present work, the glass composition Li2O–B2O3–P2O5–CaF2 doped with Tb2O3 (5 wt.%), Eu2O3 (X wt.%), where X = 0.5; 0.7; 1 wt.% has been investigated. The composition of the samples is given in Table 1. For synthesis of glass, LiPO3, H3BO3, CaF2, Tb2O3 and Eu2O3 compounds were used. All the reagents were chemically pure or extra pure grades without further purification. The samples were synthesized at the Institute for Single Crystals, National Academy of Sciences of Ukraine (Kharkov). The major component of all the prepared glass samples was lithium metaphosphate. It was chosen due to its low melting point and high solubility to different oxides and salts. This property facilitates modification of glass composition and incorporation of a large amount of dopants. In addition, lithium has a large cross section of thermal neutron capturing, and it lacks heavy element atoms in the host. Homogeneous optically transparent glass can be prepared with the volume of a perfect working surface, which needs no mechanical treatment directly in crucibles. Lithium metaphosphate was made by melting lithium dihydrogen phosphate (LiH2PO4) powder at 900 °C in a platinum crucible for 40 min. The molten mass was poured into a glass carbon crucible and cooled to room temperature. The obtained glass was then crushed and used as a precursor for batch preparation. The melting temperature of REE oxides was within the range of 1690–2400 °C. Lithium metaphosphate was well mixed with REE oxides in the batch, wherein the melting point of the mixture decreased. The initial batch was melted and heat-treated in platinum and glass carbon crucibles in a muffle furnace SNOL 7.2/1300. The temperature and melting time were optimized. Complete dissolution of the components occurred at temperatures above 900 °C. The melt was stirred at regular intervals to make it homogeneous. The initial temperature of the glass carbon crucible, in which glass casting was performed, was varied to control the cooling rate of the melt. The technique for glass casting was developed. The batch was first preheated at 800 °C in the muffle furnace, and then the temperature was raised up to 900… 1100 °C (depending on REE concentration). The melting time was 90 min. The homogeneous melt was poured into a heated up to 200 °C glass carbon crucible and cooled at room temperature. The resulting glass was found homogeneous, transparent and moisture resistant. The prepared samples were colorless glass plates with a thickness of 3 mm. 2.2. Measurements The PL excitation and luminescence spectra were measured with two crossed monochromators (MDR 204), PMT Hamamatsu R928, and a Xenon lamp was used as an excitation source (400 W). The PCL was studied by means of time-resolved luminescence spectrometry. The samples were excited by electron beams with pulse duration of 10– 15 ns FWHM, the energy density of the excitation was ~ 70 mJ·cm−2 and the average energy of electrons was ~ 250 keV. The source of Table 1 Composition of Li2O–B2O3–P2O5–CaF2 glass. Sample
LBPC:Tb5Eu0.5 LBPC:Tb5Eu0.7 LBPC:Tb5Eu1
Composition, wt.% Eu2O3
Tb2O3
CaF2
H3BO3
LiPO3
0.5 0.7 1
5 5 5
10 10 10
10 10 10
to 100 to 100 to 100
electrons was a compact high-current accelerator of GIN-600 type with a vacuum diode. The luminescence decay kinetics was recorded with a photomultiplier tube PMT-106 using a monochromator MDR-3 and a digital oscilloscope LeCROY6030A (350 MHz) with a resolution of 1.3 nm. The time spectra evolution was observed by recording the amplitude of the signal at a given time t relative to the end of the excitation pulse. The emission spectrum was reconstructed by the emission intensity I(t) from the luminescence decay kinetics for the given wavelength with a 10 ns resolution. The integral luminescence spectra of PCL (“spectrum per pulse”) were recorded after electron beam excitation in the range of 300– 900 nm with fiber optic spectrometer AvaSpec-2048 (2.1 nm spectral resolution). The integration time varied from 1 to 10 ms. To ensure accuracy data, the error of intensity is estimated by measuring the same sample under the same condition for several times and calculating the maximum deviation percent between each other. The error is estimated to be less than 7%. All measurements were carried out at room temperature. 3. Results and discussion The excitation spectrum for the main emission band of Tb3+ ions at 544 nm for LBPC:Tb5Eu0.5 wt.% is presented in Fig. 1a. The excitation spectrum consists of a relatively high intensity peak at around 377 nm, which corresponds to transition from 7F6 ground state to 5G6, 5 D3 excited states of Tb3+ ions. Other excitation peaks with relatively low intensity are observed at smaller wavelengths at 352, 358 and 370 nm. These peaks can be attributed to transitions from 7F6 ground state to higher populated 5G3, 5D2, 5G4 and 5G2 excited states. The excitation peaks are found to be relatively wide compared to common narrow peaks of REI, which can be attributed to overlapping of higher states of Tb3+ ions. The sets of emission peaks appearing in the “blue” (490 nm) and “green” (544 nm) regions are due to transitions 5D4 → 7F6 and 5 D4 → 7F5, respectively. In addition, the emission spectrum showed peaks due to transitions 5D4 → 7F4 (583 nm) and 5D4 → 7F3 (620 nm). It is well known that the “blue” (490 nm)/“green” (544 nm) emission ratio in trivalent terbium ions is highly dependent on the dopant ion concentration and the crystal field around the rare earth ion. For lower Tb3+ concentration, a predominant “blue” emission is often reported that goes on decreasing at increased dopant ion concentration. This phenomena was attributed to the cross relaxation of Tb3+ ions in a non radiative manner [30]. Furthermore, the “green” emission is classified as a magnetic dipole (MD) transition, an electric dipole (ED) transition is referred to as “blue” emission. The ED transition, being very sensitive to the metal surrounding, can provide information regarding the crystal filed. On the other hand, emission intensity of the MD transition hardly depends on the environment. Thus, the ratio of the ED/MD emission intensities can be related to the site symmetry around the Tb3+ ion [31]. The excitation spectrum monitored at 614 nm shows several sharp excitation peaks in the spectral range of 300–550 nm responsible for f–f transitions of Eu3+ ions (Fig. 1b). The bands at 322, 380, 395, 417, 465, 487, 526 and 532 nm correspond to transitions from lower energy states 7F0 and 7F1 to various excited states. Out of these, the most prominent excitation peak is due to 7F0 → 5L6 transition observed at 395 nm. When this transition is excited, the emission spectrum shows peaks at 591, 614, 654 and 700 nm. Relatively weak peak observed at 535 is attributed to the transition from 5D1 level to 7F manifold, whereas the peak at 591 nm is due to the transition from 5D0 level to 7F1 state. Similarly, the band at 614 nm is due to 5D0 → 7F2 transition [31,32]. The cathodoluminescence spectra measured under pulsed electron beam excitation using the “spectra per pulse” technique with time resolution are presented in Fig. 2a, b. The impact of the energetic electrons results in excitation of Eu3+ ions to 5D0 excited state. The characteristic
D. Valiev, K. Belikov / Journal of Non-Crystalline Solids 457 (2017) 31–35
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Fig. 1. Excitation spectra of LBPC:Tb5Eu0.5 wt.% for luminescence bands at 544, 614 nm and PL spectra at λex = 377 nm and 395 nm.
radiative transitions of Eu3+ ions from 5D0 to lower 7F1, 7F2 and 7F4 energy states result in emission peaks at 585, 614 and 700 nm, respectively [32,33]. The emission peaks (Fig. 2a) centered at 490 and 544 nm are due radiative transitions from 5D4 excited state of Tb3+ ions to 7F6 and 7 F5 states, respectively, with possible overlapping of the emission of Tb3+ ions and that of Eu3+ ions in the region of 585 and 614 nm. The emission peak with maxima at 700 nm is observed only in LBPC:Tb5Eu0.5 glass. As the concentration of co-dopants grows, the luminescence intensity is observed to decrease in the bands responsible for radiative transitions of Eu3+ ions. This may be attributed to luminescence concentration quenching. After the end of the excitation pulse, (Fig. 2b, I max) a structureless short-lived emission spectrum is recorded. The continuous spectrum luminescence is associated with the emission of the glass host. The luminescence bands of REI in the background of the nanosecond continuous spectrum are not observed (Fig. 2b). In the spectra measured at 100 μs and 1 ms after the end of the excitation pulse, the emission of the “green” spectral region at λmax = 544 nm is dominant. In addition, a number of bands associated with the radiative transitions at 414, 436, 458, 490, 544, 585 and 614 nm are observed. It is found that the emission of terbium ions in the bands at 414, 436 and 458 nm corresponds to the transitions of Tb3+ ions from 5D3 level to the ground state with microsecond duration. The luminescence in the bands at 490, 544, 585 and 622 nm corresponds to the transitions from 5D4 level with millisecond duration. By the time of 1 ms after the end of the excitation pulse, the bands in the 400–460 nm spectral region are not recorded. In [32], it was shown that in oxyfluoride
borogermanate glass doped with Eu3+/Tb3+ the effective energy transfer takes place from Tb3 + to Eu3+ ions, and it increases at increased Eu3 + ion concentration. The intensity of the luminescence of Tb3 + ions decreases dramatically at Eu3 + ion concentration grown from 1 to 4 mol.%. The authors of the study pointed out the possibility of reverse energy transfer from Eu3+ to Tb3+ ions depending on Eu3+ concentration. In our case, the energy transfer occurs from Tb3+ to Eu3+ ions. The introduction of europium ions in the glass changes the luminescence decay kinetics of terbium emission bands. Fig. 3 shows the luminescence decay kinetics for the main emission bands of terbium ions and for the band at 700 nm related to the emission transition of Eu3+ ions (5D0 → 7F4). It is found that the luminescence decay curves of Tb3+ ions deviate from the single exponential equation for all bands and can be fitted by two exponential functions, I(t) = Σ Aiexp(−t/τi), where I(t) is the luminescence intensity, Ai stands for the amplitude, t indicates the time, and τi is the decay constant of the i-th exponential component. The luminescence decay kinetics is found to have “fast” and “slow” decay components with microsecond and millisecond time intervals, respectively. The luminescence decay curves of Tb5Eu1 and Tb5Eu0.7 samples in the band at 700 nm include only a short-time component (Fig. 3). The luminescence decay in the band at 700 nm occurs in the millisecond interval for Tb5Eu0.5 glass sample. The decay times of the glass doped with Tb3 +/Eu3 + are given in Table 2. The emission bands at 436 and 458 nm originate from 5D3 to 7 F4 and 7F3, respectively, and have similar decay kinetics in all the samples with τfast ~ 5 μs and τslow ~ 55 μs. The decay kinetics of the emission
Fig. 2. PCL spectra of LBPC:Tb5EuX (X = 0.5; 0.7; 1 wt.%) measured by “spectrum per pulse” technique (a); PCL spectra of LBPC:Tb5Eu0.5 measured with time-resolution: at the initial time of Imax, 100 μs and 1 ms after the end of the excitation pulse action. The inset shows the luminescence decay kinetics for LBPC host at 470 nm.
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D. Valiev, K. Belikov / Journal of Non-Crystalline Solids 457 (2017) 31–35
Fig. 3. PCL decay kinetic curves for LBPC:Tb3+/Eu3+ glass with different concentrations of Eu3+ ions measured for the wavelengths: 490 nm, 544 nm, 622 nm and 700 nm.
bands originated from 5D4 excited states shows the “fast” component. As the Eu3+ ion concentration grows, the luminescence decay time decreases. The decay time of the “slow” component in 5D4 → 7F5 (544 nm) transition decreases from ~2.6 ms to ~1.9 ms at Eu3+ ion concentration increased from 0.5 to 1 wt.%. A similar behavior of the emission decay time can be observed for all the bands originated from 5D4 excited state of the Tb3+ ion at increased Eu3+ content. Two different types of excitation have a different effect on luminescent characteristics of the investigated glass. Under steady state mode of excitation (λ = 377 nm), the energy can be absorbed by the 5G6 level or higher levels of Tb3+ ions. Then fast multistep relaxations occur and the electrons finally reach the 5D4 levels of Tb3+ ions. The excited electrons populate at 5D4 and 5 D3 levels that give rise to intensive emissions via 5D4 → 7FJ (J = 5, 4, and 3) and 5D3 → 7FJ (J = 2, 1, and 0) transitions. In this case, emission
in the 400–460 nm spectral range (transitions from 5D4 to 7FJ (2, 1 and 0)) cannot be observed. Meanwhile, Tb3+ ions in the 5DJ emitting state may also transfer the absorbed energy to the excited energy levels of Eu3+ ions through the dominant electric quadrupole–quadrupole interaction [32] (Fig. 4). The energy-transfer efficiency from Tb3+ to Eu3+ ions is related to the concentration of Eu3+ ions. The higher concentration of Eu3+ ions leads to higher energy-transfer efficiency. When the 395 nm light excites the Tb3+/Eu3+ ions co-doped phosphate-boratefluoride glass, electrons are supposed to first populate the 5D4 level of Eu3+. Next, there are multistep relaxations, and the electrons in different 5DJ (J = 4, 3, and 1) levels finally reach the 5D0 level of Eu3+ ions. Subsequently, the populated 5D0 level of Eu3+ ions results in emissions via 5D0 → 7FJ (J = 1, 2, 3 and 4). A different process can be observed under nanosecond electron excitation. When the high energy electron pulse transmits to the glass
Table 2 The luminescence decay time of glass samples doped with Tb3+/Eu3+. λem, [nm]
Transition
436 458 490 544 585 622 700
5
D3 5 D3 5 D4 5 D4 5 D4 5 D4 5 D0
→ → → → → → →
7
F4 7 F3 7 F6 7 F5 7 F4 7 F3 7 F4 (Eu3+)
Tb5Eu0.5
Tb5Eu0.7
Tb5Eu1
τfast
τslow
τfast
τslow
τfast
τslow
~5 ± 0.5 μs ~5 ± 0.5 μs ~0.74 ± 0.07 ~0.73 ± 0.07 ~0.78 ± 0.07 ~0.74 ± 0.07 ~0.74 ± 0.07
~55 ± 5 μs ~37 ± 0.3 μs ~3.14 ± 0.3 ms ~2.69 ± 0.26 ms ~3.03 ± 0.3 ms ~2.21 ± 0.02 ms ~2.20 ± 0.02 ms
~5 ± 0.5 μs ~5 ± 0.5 μs ~0.61 ± 0.06 ms ~0.65 ± 0.06 ms ~0.7 ± 0.07 ms ~0.74 ± 0.07 ms –
~55 ± 5 μs ~36 ± 3 μs ~1.95 ± 0.1 ms ~2.23 ± 0.23 ms ~2.18 ± 0.2 ms ~2.2 ± 0.2 ms –
~5 ± 0.5 μs ~100 ± 10 ns ~0.6 ± 0.06 ms ~0.62 ± 0.06 ms ~0.5 ± 0.05 ms ~0.55 ± 0.05 ms –
~55 ± 5 μs ~37 ± 3 μs ~1.9 ± 0.1 ms ~1.98 ± 0.1 ms ~1.76 ± 0.1 ms ~1.66 ± 0.1 ms –
ms ms ms ms ms
D. Valiev, K. Belikov / Journal of Non-Crystalline Solids 457 (2017) 31–35
Fig. 4. Energy level diagram, emission characteristics of Tb3+ and Eu3+ ions in phosphateborate-fluoride glass.
volume, it may excite the host and generate charge carriers. After relaxation, the electron returns to the ground state and emits the energy in the visible spectral range. This means that higher electron doses generate a greater number of electron-hole pairs, which consequently exhibits greater lifetimes for full recombination. As a result, we can observe population of higher excitation states and processes of radiative recombination from 5D4 to 7FJ (J = 6, 5, 4, and 3) and from 5D3 to 7FJ (J = 2, 1, and 0) transitions which were not observed in steady state luminescence spectra. Finally, the produced secondary electrons directly or indirectly excite Eu3+ ions and the integral luminescence spectra of both Tb3+ and Eu3+ ions as can be recorded. 4. Conclusion Lithium–phosphate–borate–fluoride glass samples doped with Tb3+/Eu3+ with different europium concentrations have been prepared and their luminescence properties were investigated. Time-resolved luminescence of Eu3+ and Tb3+ ions excited in the glass host by nanosecond electron pulses was studied. A structureless short-lived emission spectrum was recorded after the end of the excitation pulse. The nature of this luminescence with a continuous spectrum is related to the emission of the glass host. Europium concentration increased from 0.5 to 1 wt.% caused quenching of the main emission bands of Eu3+ ions and increase in the luminescence intensity of Tb3 + ions. The emission peak with the maxima at 700 nm observed in the LBPC:Tb5Eu0.5 sample only. The study revealed energy transfer from terbium ions to europium ions. It was found that the characteristic decay time of the main luminescence bands of terbium ions at 490, 544 and 622 nm depends on the concentration of europium ions. References [1] S. Nakamura, T. Mukai, M. Senoh, Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes, Appl. Phys. Lett. 64 (1994) 1687.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Spectroscopic investigations of phosphate-borate-fluoride glass doped with Tb3 +/Eu3 + D. Valiev a,⁎, K. Belikov b a b
National Research Tomsk Polytechnic University, 30 Lenin Avenue, 634050 Tomsk, Russia Institute for Single Crystals, 60 Lenin Avenue, Kharkov 61001, Ukraine
a r t i c l e
i n f o
Article history: Received 25 August 2016 Received in revised form 15 November 2016 Accepted 16 November 2016 Available online xxxx Keywords: Glass Pulse cathodoluminescence Rare earth ions Decay kinetics
a b s t r a c t The luminescence of Li2O–B2O3–P2O5–CaF2 glass doped with Tb3+/Eu3+ under electron beam excitation have been investigated in detail. The excitation spectra, photoluminescence (PL), pulse cathodoluminescence (PCL) and luminescence decay kinetics were analyzed. The energy transfer efficiency from Tb3+ to Eu3+ ions was found to increase at increased concentration of Eu3+ ions. As the concentration of europium grew, the luminescence quenching was observed. The energy transfer from Tb3+ to Eu3+ was observed to occur through level 5D4 of terbium ions. It is shown that the luminescence decay kinetics of terbium ions at 485, 544, 622 and 700 nm depends on europium concentration. © 2016 Elsevier B.V. All rights reserved.
1. Introduction At the present time, white light emission diodes (w-LED) are widely used as a light source in different areas replacing those conventional like incandescent and luminescence sources due to tunable light characteristics and long operation life-time [1,2]. A commercial white LED is typically fabricated with a blue chip coated with yellow YAG:Ce3 + phosphor [3]. The major problems of the phosphor in this application are the lack of a red light component and poor thermal stability [4], and the latter leads to degradation and change in basic light parameters. Glass used instead of phosphor can be a more suitable approach to solve the problem [5–9]. Glass doped with rare earth elements (REE) can be an alternative for conventional phosphors due to high thermal stability of the former. In addition, this glass possesses tunable color rendering index (CRI), since the glass can be varied in its composition, and low power consumption [10,11]. Synthesis of glass materials based on oxide glass doped with rare earth ions (REI) is simple, optical elements can be manufactured of any shape and size, and the cost is relatively low. Furthermore, glass can be doped, and varied composition of the host and high optical homogeneity make them an alternative to single crystals [12,13].
⁎ Corresponding author at: National Research Tomsk Polytechnic University, Institute of High Technology Physics, Department of Lasers and Lighting Engineering, Tomsk, Russia. E-mail address: [email protected] (D. Valiev).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.11.016 0022-3093/© 2016 Elsevier B.V. All rights reserved.
Synthesis of new materials with specific optical properties requires an understanding of the mechanisms of luminescence in different systems [12–15]. These are the processes of electronic excitations and energy transfer between the luminescence centers and the glass host, and the interaction between active ions [16–21]. Investigation of these processes can improve luminescent characteristics and light yield of the systems for different applications. Glass based on phosphate/borate can be used as luminescent material. Phosphate glass has good physical properties such as low melting point, high transparency in the UV region, good thermo-optic properties and low refractive index [22–25]. Borate glass is mechanically, thermally and chemically stable [16,26]. Borophosphate glass is a good candidate for photonic materials due to its physical and optical properties such as high REI solubility and high transparency in the UV region [8,27]. Europium ions as dopants are found in many glass hosts, the luminescence bands are found to be intensive in the “orange-red” spectral region, and the light yield of these ions is high. To correct the emission chromaticity and to increase the light yield, Eu-containing glass is supposed to be doped with co-dopant ions. Terbium ions can be used as co-dopants due to an intense “green” band and several bands observed in the “blue” region. White light emission can be observed for co-doped Eu3 +/Tb3 + borophosphate glass due to the combination of the Eu3 + emission bands at 425, 593, 614 and 700 nm, with the most intensive Tb3+ emission band at 544 nm [28,29]. Few reports provide data on spectroscopic properties of REIs under pulsed electron beam excitation. This research aims to study the spectral characteristics and luminescence decay in co-doped Tb3 +/Eu3 +
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phosphate-borate-fluoride glass with variation of europium concentration under photo- and electron excitation. 2. Experimental details 2.1. Glass preparation In the present work, the glass composition Li2O–B2O3–P2O5–CaF2 doped with Tb2O3 (5 wt.%), Eu2O3 (X wt.%), where X = 0.5; 0.7; 1 wt.% has been investigated. The composition of the samples is given in Table 1. For synthesis of glass, LiPO3, H3BO3, CaF2, Tb2O3 and Eu2O3 compounds were used. All the reagents were chemically pure or extra pure grades without further purification. The samples were synthesized at the Institute for Single Crystals, National Academy of Sciences of Ukraine (Kharkov). The major component of all the prepared glass samples was lithium metaphosphate. It was chosen due to its low melting point and high solubility to different oxides and salts. This property facilitates modification of glass composition and incorporation of a large amount of dopants. In addition, lithium has a large cross section of thermal neutron capturing, and it lacks heavy element atoms in the host. Homogeneous optically transparent glass can be prepared with the volume of a perfect working surface, which needs no mechanical treatment directly in crucibles. Lithium metaphosphate was made by melting lithium dihydrogen phosphate (LiH2PO4) powder at 900 °C in a platinum crucible for 40 min. The molten mass was poured into a glass carbon crucible and cooled to room temperature. The obtained glass was then crushed and used as a precursor for batch preparation. The melting temperature of REE oxides was within the range of 1690–2400 °C. Lithium metaphosphate was well mixed with REE oxides in the batch, wherein the melting point of the mixture decreased. The initial batch was melted and heat-treated in platinum and glass carbon crucibles in a muffle furnace SNOL 7.2/1300. The temperature and melting time were optimized. Complete dissolution of the components occurred at temperatures above 900 °C. The melt was stirred at regular intervals to make it homogeneous. The initial temperature of the glass carbon crucible, in which glass casting was performed, was varied to control the cooling rate of the melt. The technique for glass casting was developed. The batch was first preheated at 800 °C in the muffle furnace, and then the temperature was raised up to 900… 1100 °C (depending on REE concentration). The melting time was 90 min. The homogeneous melt was poured into a heated up to 200 °C glass carbon crucible and cooled at room temperature. The resulting glass was found homogeneous, transparent and moisture resistant. The prepared samples were colorless glass plates with a thickness of 3 mm. 2.2. Measurements The PL excitation and luminescence spectra were measured with two crossed monochromators (MDR 204), PMT Hamamatsu R928, and a Xenon lamp was used as an excitation source (400 W). The PCL was studied by means of time-resolved luminescence spectrometry. The samples were excited by electron beams with pulse duration of 10– 15 ns FWHM, the energy density of the excitation was ~ 70 mJ·cm−2 and the average energy of electrons was ~ 250 keV. The source of Table 1 Composition of Li2O–B2O3–P2O5–CaF2 glass. Sample
LBPC:Tb5Eu0.5 LBPC:Tb5Eu0.7 LBPC:Tb5Eu1
Composition, wt.% Eu2O3
Tb2O3
CaF2
H3BO3
LiPO3
0.5 0.7 1
5 5 5
10 10 10
10 10 10
to 100 to 100 to 100
electrons was a compact high-current accelerator of GIN-600 type with a vacuum diode. The luminescence decay kinetics was recorded with a photomultiplier tube PMT-106 using a monochromator MDR-3 and a digital oscilloscope LeCROY6030A (350 MHz) with a resolution of 1.3 nm. The time spectra evolution was observed by recording the amplitude of the signal at a given time t relative to the end of the excitation pulse. The emission spectrum was reconstructed by the emission intensity I(t) from the luminescence decay kinetics for the given wavelength with a 10 ns resolution. The integral luminescence spectra of PCL (“spectrum per pulse”) were recorded after electron beam excitation in the range of 300– 900 nm with fiber optic spectrometer AvaSpec-2048 (2.1 nm spectral resolution). The integration time varied from 1 to 10 ms. To ensure accuracy data, the error of intensity is estimated by measuring the same sample under the same condition for several times and calculating the maximum deviation percent between each other. The error is estimated to be less than 7%. All measurements were carried out at room temperature. 3. Results and discussion The excitation spectrum for the main emission band of Tb3+ ions at 544 nm for LBPC:Tb5Eu0.5 wt.% is presented in Fig. 1a. The excitation spectrum consists of a relatively high intensity peak at around 377 nm, which corresponds to transition from 7F6 ground state to 5G6, 5 D3 excited states of Tb3+ ions. Other excitation peaks with relatively low intensity are observed at smaller wavelengths at 352, 358 and 370 nm. These peaks can be attributed to transitions from 7F6 ground state to higher populated 5G3, 5D2, 5G4 and 5G2 excited states. The excitation peaks are found to be relatively wide compared to common narrow peaks of REI, which can be attributed to overlapping of higher states of Tb3+ ions. The sets of emission peaks appearing in the “blue” (490 nm) and “green” (544 nm) regions are due to transitions 5D4 → 7F6 and 5 D4 → 7F5, respectively. In addition, the emission spectrum showed peaks due to transitions 5D4 → 7F4 (583 nm) and 5D4 → 7F3 (620 nm). It is well known that the “blue” (490 nm)/“green” (544 nm) emission ratio in trivalent terbium ions is highly dependent on the dopant ion concentration and the crystal field around the rare earth ion. For lower Tb3+ concentration, a predominant “blue” emission is often reported that goes on decreasing at increased dopant ion concentration. This phenomena was attributed to the cross relaxation of Tb3+ ions in a non radiative manner [30]. Furthermore, the “green” emission is classified as a magnetic dipole (MD) transition, an electric dipole (ED) transition is referred to as “blue” emission. The ED transition, being very sensitive to the metal surrounding, can provide information regarding the crystal filed. On the other hand, emission intensity of the MD transition hardly depends on the environment. Thus, the ratio of the ED/MD emission intensities can be related to the site symmetry around the Tb3+ ion [31]. The excitation spectrum monitored at 614 nm shows several sharp excitation peaks in the spectral range of 300–550 nm responsible for f–f transitions of Eu3+ ions (Fig. 1b). The bands at 322, 380, 395, 417, 465, 487, 526 and 532 nm correspond to transitions from lower energy states 7F0 and 7F1 to various excited states. Out of these, the most prominent excitation peak is due to 7F0 → 5L6 transition observed at 395 nm. When this transition is excited, the emission spectrum shows peaks at 591, 614, 654 and 700 nm. Relatively weak peak observed at 535 is attributed to the transition from 5D1 level to 7F manifold, whereas the peak at 591 nm is due to the transition from 5D0 level to 7F1 state. Similarly, the band at 614 nm is due to 5D0 → 7F2 transition [31,32]. The cathodoluminescence spectra measured under pulsed electron beam excitation using the “spectra per pulse” technique with time resolution are presented in Fig. 2a, b. The impact of the energetic electrons results in excitation of Eu3+ ions to 5D0 excited state. The characteristic
D. Valiev, K. Belikov / Journal of Non-Crystalline Solids 457 (2017) 31–35
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Fig. 1. Excitation spectra of LBPC:Tb5Eu0.5 wt.% for luminescence bands at 544, 614 nm and PL spectra at λex = 377 nm and 395 nm.
radiative transitions of Eu3+ ions from 5D0 to lower 7F1, 7F2 and 7F4 energy states result in emission peaks at 585, 614 and 700 nm, respectively [32,33]. The emission peaks (Fig. 2a) centered at 490 and 544 nm are due radiative transitions from 5D4 excited state of Tb3+ ions to 7F6 and 7 F5 states, respectively, with possible overlapping of the emission of Tb3+ ions and that of Eu3+ ions in the region of 585 and 614 nm. The emission peak with maxima at 700 nm is observed only in LBPC:Tb5Eu0.5 glass. As the concentration of co-dopants grows, the luminescence intensity is observed to decrease in the bands responsible for radiative transitions of Eu3+ ions. This may be attributed to luminescence concentration quenching. After the end of the excitation pulse, (Fig. 2b, I max) a structureless short-lived emission spectrum is recorded. The continuous spectrum luminescence is associated with the emission of the glass host. The luminescence bands of REI in the background of the nanosecond continuous spectrum are not observed (Fig. 2b). In the spectra measured at 100 μs and 1 ms after the end of the excitation pulse, the emission of the “green” spectral region at λmax = 544 nm is dominant. In addition, a number of bands associated with the radiative transitions at 414, 436, 458, 490, 544, 585 and 614 nm are observed. It is found that the emission of terbium ions in the bands at 414, 436 and 458 nm corresponds to the transitions of Tb3+ ions from 5D3 level to the ground state with microsecond duration. The luminescence in the bands at 490, 544, 585 and 622 nm corresponds to the transitions from 5D4 level with millisecond duration. By the time of 1 ms after the end of the excitation pulse, the bands in the 400–460 nm spectral region are not recorded. In [32], it was shown that in oxyfluoride
borogermanate glass doped with Eu3+/Tb3+ the effective energy transfer takes place from Tb3 + to Eu3+ ions, and it increases at increased Eu3 + ion concentration. The intensity of the luminescence of Tb3 + ions decreases dramatically at Eu3 + ion concentration grown from 1 to 4 mol.%. The authors of the study pointed out the possibility of reverse energy transfer from Eu3+ to Tb3+ ions depending on Eu3+ concentration. In our case, the energy transfer occurs from Tb3+ to Eu3+ ions. The introduction of europium ions in the glass changes the luminescence decay kinetics of terbium emission bands. Fig. 3 shows the luminescence decay kinetics for the main emission bands of terbium ions and for the band at 700 nm related to the emission transition of Eu3+ ions (5D0 → 7F4). It is found that the luminescence decay curves of Tb3+ ions deviate from the single exponential equation for all bands and can be fitted by two exponential functions, I(t) = Σ Aiexp(−t/τi), where I(t) is the luminescence intensity, Ai stands for the amplitude, t indicates the time, and τi is the decay constant of the i-th exponential component. The luminescence decay kinetics is found to have “fast” and “slow” decay components with microsecond and millisecond time intervals, respectively. The luminescence decay curves of Tb5Eu1 and Tb5Eu0.7 samples in the band at 700 nm include only a short-time component (Fig. 3). The luminescence decay in the band at 700 nm occurs in the millisecond interval for Tb5Eu0.5 glass sample. The decay times of the glass doped with Tb3 +/Eu3 + are given in Table 2. The emission bands at 436 and 458 nm originate from 5D3 to 7 F4 and 7F3, respectively, and have similar decay kinetics in all the samples with τfast ~ 5 μs and τslow ~ 55 μs. The decay kinetics of the emission
Fig. 2. PCL spectra of LBPC:Tb5EuX (X = 0.5; 0.7; 1 wt.%) measured by “spectrum per pulse” technique (a); PCL spectra of LBPC:Tb5Eu0.5 measured with time-resolution: at the initial time of Imax, 100 μs and 1 ms after the end of the excitation pulse action. The inset shows the luminescence decay kinetics for LBPC host at 470 nm.
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Fig. 3. PCL decay kinetic curves for LBPC:Tb3+/Eu3+ glass with different concentrations of Eu3+ ions measured for the wavelengths: 490 nm, 544 nm, 622 nm and 700 nm.
bands originated from 5D4 excited states shows the “fast” component. As the Eu3+ ion concentration grows, the luminescence decay time decreases. The decay time of the “slow” component in 5D4 → 7F5 (544 nm) transition decreases from ~2.6 ms to ~1.9 ms at Eu3+ ion concentration increased from 0.5 to 1 wt.%. A similar behavior of the emission decay time can be observed for all the bands originated from 5D4 excited state of the Tb3+ ion at increased Eu3+ content. Two different types of excitation have a different effect on luminescent characteristics of the investigated glass. Under steady state mode of excitation (λ = 377 nm), the energy can be absorbed by the 5G6 level or higher levels of Tb3+ ions. Then fast multistep relaxations occur and the electrons finally reach the 5D4 levels of Tb3+ ions. The excited electrons populate at 5D4 and 5 D3 levels that give rise to intensive emissions via 5D4 → 7FJ (J = 5, 4, and 3) and 5D3 → 7FJ (J = 2, 1, and 0) transitions. In this case, emission
in the 400–460 nm spectral range (transitions from 5D4 to 7FJ (2, 1 and 0)) cannot be observed. Meanwhile, Tb3+ ions in the 5DJ emitting state may also transfer the absorbed energy to the excited energy levels of Eu3+ ions through the dominant electric quadrupole–quadrupole interaction [32] (Fig. 4). The energy-transfer efficiency from Tb3+ to Eu3+ ions is related to the concentration of Eu3+ ions. The higher concentration of Eu3+ ions leads to higher energy-transfer efficiency. When the 395 nm light excites the Tb3+/Eu3+ ions co-doped phosphate-boratefluoride glass, electrons are supposed to first populate the 5D4 level of Eu3+. Next, there are multistep relaxations, and the electrons in different 5DJ (J = 4, 3, and 1) levels finally reach the 5D0 level of Eu3+ ions. Subsequently, the populated 5D0 level of Eu3+ ions results in emissions via 5D0 → 7FJ (J = 1, 2, 3 and 4). A different process can be observed under nanosecond electron excitation. When the high energy electron pulse transmits to the glass
Table 2 The luminescence decay time of glass samples doped with Tb3+/Eu3+. λem, [nm]
Transition
436 458 490 544 585 622 700
5
D3 5 D3 5 D4 5 D4 5 D4 5 D4 5 D0
→ → → → → → →
7
F4 7 F3 7 F6 7 F5 7 F4 7 F3 7 F4 (Eu3+)
Tb5Eu0.5
Tb5Eu0.7
Tb5Eu1
τfast
τslow
τfast
τslow
τfast
τslow
~5 ± 0.5 μs ~5 ± 0.5 μs ~0.74 ± 0.07 ~0.73 ± 0.07 ~0.78 ± 0.07 ~0.74 ± 0.07 ~0.74 ± 0.07
~55 ± 5 μs ~37 ± 0.3 μs ~3.14 ± 0.3 ms ~2.69 ± 0.26 ms ~3.03 ± 0.3 ms ~2.21 ± 0.02 ms ~2.20 ± 0.02 ms
~5 ± 0.5 μs ~5 ± 0.5 μs ~0.61 ± 0.06 ms ~0.65 ± 0.06 ms ~0.7 ± 0.07 ms ~0.74 ± 0.07 ms –
~55 ± 5 μs ~36 ± 3 μs ~1.95 ± 0.1 ms ~2.23 ± 0.23 ms ~2.18 ± 0.2 ms ~2.2 ± 0.2 ms –
~5 ± 0.5 μs ~100 ± 10 ns ~0.6 ± 0.06 ms ~0.62 ± 0.06 ms ~0.5 ± 0.05 ms ~0.55 ± 0.05 ms –
~55 ± 5 μs ~37 ± 3 μs ~1.9 ± 0.1 ms ~1.98 ± 0.1 ms ~1.76 ± 0.1 ms ~1.66 ± 0.1 ms –
ms ms ms ms ms
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Fig. 4. Energy level diagram, emission characteristics of Tb3+ and Eu3+ ions in phosphateborate-fluoride glass.
volume, it may excite the host and generate charge carriers. After relaxation, the electron returns to the ground state and emits the energy in the visible spectral range. This means that higher electron doses generate a greater number of electron-hole pairs, which consequently exhibits greater lifetimes for full recombination. As a result, we can observe population of higher excitation states and processes of radiative recombination from 5D4 to 7FJ (J = 6, 5, 4, and 3) and from 5D3 to 7FJ (J = 2, 1, and 0) transitions which were not observed in steady state luminescence spectra. Finally, the produced secondary electrons directly or indirectly excite Eu3+ ions and the integral luminescence spectra of both Tb3+ and Eu3+ ions as can be recorded. 4. Conclusion Lithium–phosphate–borate–fluoride glass samples doped with Tb3+/Eu3+ with different europium concentrations have been prepared and their luminescence properties were investigated. Time-resolved luminescence of Eu3+ and Tb3+ ions excited in the glass host by nanosecond electron pulses was studied. A structureless short-lived emission spectrum was recorded after the end of the excitation pulse. The nature of this luminescence with a continuous spectrum is related to the emission of the glass host. Europium concentration increased from 0.5 to 1 wt.% caused quenching of the main emission bands of Eu3+ ions and increase in the luminescence intensity of Tb3 + ions. The emission peak with the maxima at 700 nm observed in the LBPC:Tb5Eu0.5 sample only. The study revealed energy transfer from terbium ions to europium ions. It was found that the characteristic decay time of the main luminescence bands of terbium ions at 490, 544 and 622 nm depends on the concentration of europium ions. References [1] S. Nakamura, T. Mukai, M. Senoh, Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes, Appl. Phys. Lett. 64 (1994) 1687.
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