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Eu triple-doped lithium aluminoborate glass for white light generation
Journal of Luminescence 192 (2017) 71–76
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Tm/Tb/Eu triple-doped lithium aluminoborate glass for white light generation
MARK
⁎
Mreedula Mungraa, Franziska Steudelb, , Bernd Ahrensa,b, Stefan Schweizera,b a
Department of Electrical Engineering, South Westphalia University of Applied Sciences, Lübecker Ring 2, 59494 Soest, Germany Fraunhofer Application Center for Inorganic Phosphors, Branch Lab of Fraunhofer Institute for Microstructure of Materials and Systems IMWS, Lübecker Ring 2, 59494 Soest, Germany
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Thulium Luminescence quenching Triple-doping Energy transfer White light
The aim was to design a multi-chromatic phosphor based on the colour mixing principle, whereby white light can be produced from an appropriate mixture of blue, green, and red emissions by a single phosphor. In this perspective, Tm2O3, Tb4O7, and Eu2O3 were simultaneously doped in single host glass matrix. A series of Tm2O3 single- as well as Tm/Tb/Eu triple-doped glasses were synthesised. The photoluminescence behaviour and luminescence quenching were investigated for Tm2O3 single-doped glasses. Keeping Tm2O3 constant at the saturation concentration in the Tm/Tb/Eu triple-doped glasses and varying the doping ratio of the two other lanthanides (Tb4O7 and Eu2O3) in the host glass allow to some extent the control of the spectral composition and consequently the generation of tunable colour impressions. The latter can also be adjusted by keeping the concentration ratio of the three lanthanides constant and varying the excitation wavelength. Upon 358-nm excitation of the Tm/Tb/Eu triple-doped glasses, white light emission is achieved and the corresponding correlated colour temperatures are also determined. The different energy transfers occurring in between the three lanthanides are investigated and possible energy transfer routes are proposed.
1. Introduction With increasing round the clock human activities, the demand for natural white light (daylight-like light) or applications necessitating white light have fuelled a significant research effort into developing efficient, more economic and sustainable white light sources. Owing to their lower power consumption, lower voltage, longer operational lifetime, smaller size, and more aesthetic appearance, white-lightemitting diodes (W-LEDs) distinguish themselves as superior to incandescent and fluorescent lamps. They are the promising components for the next-generation illumination and solid state lighting applications. W-LEDs are also associated with energy saving and hence subsequent reduction of production of greenhouse gases [1–6]. The four most common approaches to generate white light are: (1) an electronic device that mixes the light emitted from red, green, and blue LEDs, (2) blue LED chip with a yellow phosphor, (3) an ultraviolet (UV) LED chip with a combination of two (blue and yellow) or three (red, blue, and green) phosphors, and (4) an UV LED chip combined to a single multi-chromatic phosphor [2,3,7]. The current limitations of the dichromatic (blue and yellow) phosphor based W-LEDs are poor colourrendering index (CRI) caused by the absence of red light and the
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Corresponding author. E-mail address: [email protected] (F. Steudel).
http://dx.doi.org/10.1016/j.jlumin.2017.06.028 Received 6 April 2017; Received in revised form 19 May 2017; Accepted 14 June 2017 Available online 15 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
thermal degradation of the polymer-based encapsulant of the LED, which also contains the phosphor material. This degradation often entails efficiency reduction and colour properties deviation [5,8]. In order to circumvent the aforementioned issues, trichromatic (simultaneously emitting blue, green, and red lights) luminescent materials, made from glasses doped with lanthanides (Ln), are proposed as alternative encapsulation for W-LEDs. For optical activation of the Ln3+ ions, a suitable host glass is essential. The features of interest of the host glass are high optical transmittance, good Ln3+ ion solubility, thermal stability, along with mechanical resistance. Borate glasses generally satisfy these requirements and due to their low melting point, these glasses can be rather easily produced and moulded in any desired dimensions and shape [9,10]. Another important property characterising the suitability of a host for phosphors is not only the thermal stability but also the so-called quenching temperature, i.e. the temperature at which the luminescence intensity of a lanthanide ion in this host amounts to 50% of its intensity at room temperature. In the borate glass host, this is approximately 300 °C for Eu3+ and even significantly higher for Tb3+ [11]. For comparison, the quenching temperature of the conventional LED phosphor YAG:Ce3+ is at 270 °C [12].
Journal of Luminescence 192 (2017) 71–76
M. Mungra et al.
A combination of three Ln3+ ions consisting of trivalent thulium (Tm3+), terbium (Tb3+), and europium (Eu3+) ions were selected, based on their respective potential of emitting blue, green, and red luminescence reported in other host glass materials [4,5,13–16]. The photoluminescence (PL) properties of Eu3+ and Tb3+ doped borate glasses as well as the energy transfer from Tb3+ to Eu3+ have been extensively investigated and established in [6,13,14,17,18]. Within the framework of this research, the transmittance, PL, quantum efficiency, and luminescence quenching of Tm2O3 singledoped and Tm/Tb/Eu triple-doped lithium aluminoborate (LiAlB) glasses are investigated. The effect of varying the excitation wavelength and tuning the doping ratio of the three Ln3+ ions in the triple-doped glasses on the PL behaviour as well as the chromaticity of triple-doped glasses is also evaluated.
2. Experimental details For the synthesis of the LiAlB glasses, boron oxide (B2O3) as network former and lithium oxide (Li2O) as network modifier are used in a ratio of 2 to 1. To prevent devitrification of the glass, 10% of the B2O3 is substituted by aluminum oxide (Al2O3); the B2O3 to Al2O3 ratio amounts to 9 to 1. The additional Al2O3 doping also increases the strength of the glass [19]. Single-doped glasses are prepared by individually doping them with thulium oxide (Tm2O3), terbium oxide (Tb4O7), and europium oxide (Eu2O3). Additionally, glasses tripledoped with Tm2O3, Tb4O7, and Eu2O3 are also prepared. The synthesis of all the glasses are carried out using the melt quenching technique. The starting chemicals are weighed in a Pt/Au (95/5) crucible. The exact composition of all the investigated glass samples used is as listed in Table 1. The chemicals are transferred to a porcelain mortar-pestle mixer and are thoroughly grinded to achieve a homogeneous mixture. The mixture is melted at 1000 °C in the crucible over a period of approximately 3 h in a furnace. The melt is then poured on a 400 °C pre-heated brass plate, which is below the glass transition temperature (Tg = 459 °C) of undoped LiAlB glass [20]. The temperature of the brass plate is maintained constant for 3 h, before allowing the glass sample to further cool slowly to room temperature to relieve any residual mechanical and thermal stresses. The glass samples are shaped and polished to optical quality. Fig. 1 shows the appearance of the Tm2O3, Tb4O7, and Eu2O3 single-doped glasses as well as two tripledoped glasses having same Tm2O3 concentration but different levels of Tb4O7 and Eu2O3, under UV excitation.
Fig. 1. From left to right: Tm2O3, Tb4O7, and Eu2O3 single-doped glasses (top photo) and 0.3Tm/0.4Tb/0.4Eu and 0.3Tm/0.3Tb/0.3Eu triple-doped glasses (bottom photo) under 365-nm excitation (UV lamp). All values are given in mol%.
PL and quantum efficiency measurements are performed using a commercial quantum yields measurement system (Hamamatsu C9920–02G) coupled to a 3.3-in. integrating sphere with a xenon lamp (150 W) for excitation and a photonic multichannel analyzer (PMA 12) for detection. The quantum efficiency describes the ratio between the number of emitted photons and the number of absorbed photons. The number of absorbed photons was determined from absorption spectra, while the number of emitted photons was determined from emission spectra in the spectral range from 400 to 850 nm. Transmittance measurements were carried out using a Cary 5000 UV–Vis–NIR spectrophotometer from Agilent Techonologies. 3. Results and discussion 3.1. Transmission, quantum efficiency, and PL properties of Tm2O3 singledoped LiAlB glass Fig. 2 (left) exhibits the comparison of the transmission spectra of the Tm2O3 single-doped glasses. The glasses have high transmittance,
Table 1 Nominal composition of the investigated LiAlB glass samples and quantum efficiency (QE) values for excitation at 358 nm (Tm3+ and triple-doped glasses), 378 nm (Tb3+), and 394 nm (Eu3+). Dopant
Tm
3+
Tb3+ Eu3+ Tm3+, Tb3+, Eu3+
B2O3
Li2O
Al2O3
Tm2O3
Tb4O7
Eu2O3
Ln3+content / at%
59.99 59.94 59.88 59.82 59.70
33.33 33.30 33.27 33.23 33.17
6.67 6.66 6.65 6.65 6.63
0.01 0.10 0.20 0.30 0.50
– – – – –
– – – – –
0.00/ – / – 0.05/ – / – 0.09/ – / – 0.14/ – / – 0.23/ – / –
1 3 4 7 3
59.70 59.40 59.34
33.17 33.00 32.97
6.63 6.60 6.59
– – 0.50
0.50 – 0.50
– 1.00 0.10
– /0.49/ – – / – /0.46 0.23/ 0.46/0.05
32 83 8
59.04
32.80
6.56
1.00
0.50
0.10
3
59.34
32.97
6.59
0.30
0.40
0.40
59.46
33.03
6.61
0.30
0.30
0.30
0.46/ 0.46/0.05 0.14/ 0.37/0.18 0.14/ 0.28/0.14
Composition/mol%
QE /%
8
Fig. 2. (left) Transmission spectra of the Tm2O3 single-doped glasses. (right) Variation of absorption coefficient calculated at 358 nm wavelength (black crosses) over the range of Tm2O3 concentration in mol%; the solid line represents the linear regression of this relationship.
6
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Fig. 3. Quantum efficiency spectra of Tm2O3 single-doped LiAlB glasses. Fig. 4. PL emission spectra of Tm2O3 single-doped LiAlB glasses under 358-nm excitation (scaled up by a factor of 5 in the spectral range of 550–900 nm). The inset shows the variation of the integrated PL intensity with respect to Tm2O3 concentration.
approximately 90% in the visible spectral range, except at localised positions where transmittance decreases due to the absorption caused by Tm3+ ions in the glass matrix. The absorption lines correspond to the Tm3+-related 4f-4f transitions from the ground state, 3H6 to 1D2 (358 nm), 1G4 (468 nm), 3F2 (660 nm), 3F3 (686 nm), and 3H4 (792 nm). Absorption increases along with the concentration of Tm2O3. As shown in Fig. 2 (right), the absorption coefficient due to Tm3+, determined at 358 nm and corrected for the absorption due the base glass, increases linearly with respect to the concentration of Tm2O3 in the glasses. Fig. 3 shows the quantum efficiency spectra of the different Tm2O3 single-doped glasses. Note that the recorded quantum efficiency spectra correspond to photoluminescence excitation spectra for an integrated emission in the spectral range from 400 to 850 nm. Five LiAlB glasses, doped with 0.01, 0.1, 0.2, 0.3, and 0.5 mol% Tm2O3, are excited at 358, 468, 660, 686, and 792 nm. It is a noteworthy feature of Tm3+ luminescent behaviour that only under 358-nm excitation (3H6 → 1D2), the single-doped glasses emit bright blue luminescence while under all the other excitation wavelengths in the UV and blue spectral ranges, hardly any luminescence is recorded. The low signal-to-noise ratio for wavelengths larger than 400 nm results from the low absorption (A < 2%) of the glass samples in this spectral range. The quantum efficiency values of the Tm3+ single-doped samples for 358-nm excitation are listed in Table 1. The quantum efficiency is less than 10% due to the small energy gap between the energy levels of maximal ΔE Tm3 + = 0.87 eV and thus the high multi-phonon relaxation rate [21]. For concentrations of 0.01–0.3 mol% Tm203, the quantum efficiency increases to up to 7% and beyond that, the decrease in quantum efficiency indicates luminescence quenching for higher concentrations. For single Tb3+ and Eu3+ doping, the quantum efficiency is significantly higher and amounts to 32% and 83%, respectively, due to a larger energy gap of ΔE Tb3 + = 1.80 eV and ΔE Eu3 + = 1.51 eV , respectively (values obtained from absorption and emission spectra). The triple-doped glasses show an increase in quantum efficiency as compared to the single-doped glasses with the same thulium concentration, which is a clear hint for energy transfer from thulium to at least one of the other lanthanide ions. The energy transfer is further discussed and elaborated in Section 3.3. As presented in the emission spectra in Fig. 4, the very sharp emission line at 453 nm, responsible for the blue colour impression, predominates the emission spectra while four other bands of weaker intensities are also observed. As illustrated in Fig. 5, 358-nm excitation leads to the population of the 1D2 level, from which radiative emission in the blue spectral range occurs through the transition to the 3F4 (453 nm) level. Through multiphonon relaxation 1G4 level is populated and from thereon, through 3F2,3 levels, to the eventual population of the 3 H4 level. The radiative relaxation of the excited 1G4 and 3H4 levels
Fig. 5. Energy level diagram depicting the 358-nm excitation route and the subsequent emission routes for the Tm2O3 single-doped glasses. The values next to the arrows indicate the wavelength in nm of the emission bands.
results in four emission lines, which are ascribed to the transitions, namely 1G4→3H6 (478 nm), 1G4→3F4 (652 nm), 1G4→3H5 (755 nm), and 3 H4→3H6 (785 nm). In anology to the quantum efficiency measurements, the PL intensity increases monotonously in function with the Tm2O3 concentration in the range from 0.01 up to 0.3 mol% and from beyond there, the intensity decreases. To have a better insight on this PL variation, the dependence of the integrated PL intensities (integration performed for the range of 430–800 nm) on Tm2O3 concentration was plotted as shown in the inset of Fig. 4. The highest emission intensity occurs at 0.3 mol% (0.14 at%) concentration. The concentration-dependent quenching is evidenced by the decrease in PL intensity in between 0.3 and 0.5 mol% (0.14 and 0.23 at%) of Tm2O3 due to crossrelaxation. Nine possible cross-relaxation channels are reported in literature [22–29], five channels include excitation from the ground state: (1G4, 3H6) → (3F2, 3F4), (1G4, 3H6) → (3H4, 3H5), (1G4, 3H6) → (3H5, 3H4), (1G4, 3H6) → (3F4, 3F2), and (3H4, 3H6) → (3F4, 3F4) [22–26]. Similarly, 73
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Fig. 6. CIE 1931 (x, y) chromaticity diagram showing the position of the colour coordinates of 0.3 mol% Tm2O3, 0.5 mol% Tb4O7, and 1.0 mol% Eu2O3 single-doped glasses under 358, 377, and 393-nm excitation, respectively. The solid line represents the Planckian locus (black-body radiation) and E denotes the equal-energy point at (0.333, 0.333), used as the while-light reference.
Fig. 7. (left) Emission spectra and (right) CIE 1931 (x, y) chromaticity diagram with the corresponding chromaticity coordinates of 0.3Tm/0.4Tb/0.4Eu triple-doped glass (values given in mol%) under the excitation wavelengths of 358, 368, 377, 393, and 484 nm. The position of the emission bands due to the respective lanthanides are indicated by dashed lines.
luminescence quenching has been reported to occur at relatively low concentrations of Tm2O3 different in single-doped host matrices; at around 0.6 at% (calculated from the saturation concentration of 1.0 mol% Tm2O3 single-doped glass with the following nominal chemical composition: 55ZnO-(45-x)B2O3-x Tm2O3, x = 1) in single-doped zinc borate glasses and in between 0.5 at% and 6 at% in single-doped YVO4 crystals [30,31]. As shown in Fig. 1 under UV light illumination, the Tm2O3, Tb4O7, and Eu2O3 single-doped glasses emit blue, green, and orange-red light, respectively. Their corresponding chromaticity coordinates are depicted in Fig. 6 and the chromaticity coordinates of Tb/Eu doubledoped glasses with a fixed concentration of 0.5 mol% Tb4O7 and an increasing concentration of Eu2O3 from 0 to 1.0 mol% has been shown to lie in a straight line between the chromaticity coordinates corresponding to the Tb4O7 and Eu2O3 single-doped glasses [17,18]. Based on this principle, triple-doping with Tm2O3, Tb4O7, and Eu2O3 in a single host glass matrix and under the highest excitation energy, i.e. 358 nm, should lead to colour impressions having chromaticity coordinates lying within the area bounded by the dashed triangle, as illustrated in Fig. 6.
Fig. 8. (left) Emission spectra and (right) CIE 1976 (u′, v′) uniform diagram with corresponding chromaticity coordinates of the 0.3Tm/0.3Tb/0.3Eu (grey) and 0.3Tm/0.4Tb/ 0.4Eu (orange) triple-doped glasses (values in mol%), under 358-nm excitation. The 5step MacAdam ellipses (dashed) are indicated for light sources of nominal CCT of 5000 K and 6500 K (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. PL and colour properties of Tm/Tb/Eu triple-doped LiAlB glasses Two triple-doped glasses, 0.3Tm/0.4Tb/0.4Eu and 0.3Tm/0.3Tb/ 0.3Eu (values in mol%) are synthesised, to investigate the possibility of white light emission under UV light excitation. Fig. 7 presents emission spectra and corresponding (x, y) chromaticity coordinates of the 0.3Tm/0.4Tb/0.4 Eu (values given in mol%) triple-doped LiAlB glass under the following five different excitation wavelengths: 358, 368, 377, 393, and 484 nm. The glass emits yellowish green light (points b, c & e) under 368, 377, and 484 nm excitation wavelengths, denoting typical emissions caused by Tm2O3 in the triple-doped glass. When excited at 393 nm, the very characteristic orange-red colour impression of Eu2O3 emission is achieved as depicted by point d on the chromaticity diagram in Fig. 7 (right). Under 358-nm excitation, the chromaticity coordinates (point a; x = 0.343, y = 0.362) of the triple-doped glass lie in the white light emitting region of the CIE diagram. As such, triple-doping with Tm2O3, Tb4O7, and Eu2O3 in the LiAlB glass matrix represents a promising means to achieve white light emission. Therefore, it is inferred that theoretically any desired colour of PL emission, translated by the position of the chromaticity coordinates within the area of the triangle in Fig. 6, can be achieved by varying the excitation wavelength under which the triple-doped glass is exposed. Further, in an attempt to enhance the white-light emitting
capability of the Tm/Tb/Eu triple-doped glass, the effect of varying the doping ratio of the lanthanides in the host glass at the fixed excitation wavelength of 358 nm is investigated. Fig. 8 shows the emission spectra of two triple-doped glasses with different concentrations of Eu2O3 and Tb4O7 while keeping Tm2O3 concentration fixed at 0.3 mol% under 358-nm excitation. Emission bands are registered in the blue, green as well as red spectral regions. The blue emission is attributed to: the Tmrelated 1D2→3F4 transition at 453 nm and the Tb-related 5D4→7F6 transition at 489 nm. The green emission is ascribed to the Tb-related 5 D4→7F5 transition at 543 nm. The red emissions show one band at 588 nm, which can be attributed to the overlap of the Eu-related 5 D0→7F1 and Tb-related 5D4→7F4 transitions. The second emission band in the red spectral region is at 617 nm and originates from the overlap of the emission bands of Eu-related 5D0→7F2 and Tb-related 5D4→7F3 transitions. The last two bands of weak intensity belong to Eu-related transitions, namely 5D0→7F3,4 at 653 nm and 702 nm, respectively. As shown in Fig. 8 (right) in the CIE 1976 (u′, v′) uniform chromaticity diagram, the position of the triple-doped glass having the composition of 0.3Tm/0.4Tb/0.4Eu is nearer to the position of the equal-energy point, E, as compared to that of the 0.3Tm/0.3Tb/0.3Eu (values in mol 74
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%). In addition, the colour coordinates of the glass with lower Tb3+ and Eu3+ doping are further in the blue spectral range, resulting in a “cold white” colour impression as compared to the glass with higher Tb3+ and Eu3+ doping. This is also evident from Fig. 1. To further characterise these two white light sources, their correlated colour temperatures (CCT) are determined, using the five-step MacAdam ellipses which are approximately defined by the following equation of circle, having centre (u′c , v′c ) and radius r = 0.0055 [32]:
(u′ − u′c )2 + (v′ − v′c )2 = r 2
(1)
where (u′c , v′c ) is (0.2092, 0.4884) and (0.1951, 0.4726) for light sources of nominal CCT of 5000 K and 6500 K, respectively. The ellipses, covering the area of chromaticity coordinates matching light sources having CCT values of 5000 K and 6500 K are as shown in Fig. 8 (right). The chromaticity coordinates of the 0.3Tm/0.3Tb/0.3Eu (values in mol%) is found to be within the boundaries of the ellipse corresponding to CCT value of 6500 K. As such, this triple-doped LiAlB glass is a promising daylight-like source since the CCT value of direct sunlight varies between 5700 and 6500 K [32,7].
Fig. 10. Energy level diagrams depicting the 358-nm excitation route and the routes for radiative emissions (blue, green, and red arrows) and energy transfers (dashed arrows) occurring from Tm3+→ Tb3+ (ET 1), Tm3+→ Eu3+ (ET 2), and Tb3+→ Eu3+ (ET 3) in the triple-doped glasses (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Energy transfers 3+
ions through energy transfer. As such, the possible routes for the following energy transfers at 300 K are investigated and described as follows:
3+
While the energy transfer from Tb to Eu has been thoroughly investigated and confirmed in Tb/Eu double-doped borate glasses [17,14], it is interesting to observe what happens when Tm2O3 is also doped along with Tb4O7 and Eu2O3 in a single host glass matrix. Fig. 9 shows the emission spectra of xTm/0.5Tb/0.1Eu with x = 0.5 and 1.0 (values in mol%) triple-doped glasses under 358-nm excitation. Changing the concentration of Tm2O3 from 1.0 to 0.5 mol% in the tripledoped glasses results in an improvement of the characteristic Tm2O3 emission at 454 nm, due to the inverse relationship between PL intensity and Tm3+ concentration beyond the saturation concentration (refer to Section 3.1). Another spectral observation of interest is the enhancement of PL intensity of both the Tb-related and Eu-related emissions, despite that the doping concentrations of Tb4O7 and Eu2O3 are constant in both triple-doped glasses. This can be due to eventual energy transfers from the excited Tm3+ to both Tb3+ and Eu3+ along with the well-known energy transfer from Tb3+ to Eu3+. For energy transfer to be possible, the states of the acceptor must be identical or differ insignificantly from the emitting states of the donor. Satisfaction of this criterion ensures that the excitation energy through resonance energy transfer from donor is effectively reabsorbed by the acceptor, resulting in the population of its emitting level, with possibility of subsequent PL emission [33–36]. Under 358-nm excitation, Tm3+ emits blue light at 453 nm, as illustrated in Fig. 10. Given that the excited energy states of Eu3+ and Tb3+ lies below that of Tm3+, there exists the possibility that Tm3+ sensitizes both Tb3+ and Eu3+
1. Tm3 + → Tb3 + The transition bearing the least energy difference (37 meV) and thus most probable channel of this energy transfer is through the nonradiative relaxation corresponding to the Tm3+-related 1G4→3H6 transition and Tb3+ excitation from 7F6 to 5D4, as illustrated in Fig. 10. 2. Tm3 + → Eu3 + and Tb3 + → Eu3 + The non-radiative coupling of the Tm3+-related 1G4→3H6 transition with the Eu3+-related 7F6 to 5D4 transition satisfies the resonant energy transfer condition, with the least energy difference of 7 meV (Table 2). The non-radiative energy transferred from the Tm3 + 1G4 energy level triggers the excitation of the Eu3 + 7F1 electrons (35% population at room temperature [6]) to the 5D2 level, as illustrated in the partial energy diagram in Fig. 10. From the 5D2 level, nonradiative multi-phonon relaxation, followed by radiative emission through Eu3+-related 5D0→7F1 and 5D0→7F2 transitions, occur. In our previous works [6,17,14,18], the most probable route (Fig. 10) of energy transfer from Tb3+ to Eu3+ is via Tb3+-related 5 D4→7F4 transition to Eu3+-related 7F1→5D0 transition, with energy difference between the transitions of donor and acceptor amounting to 14 meV, while the second one is via Tb3+-related 5D4→7F5 transition to Eu3+-related 7F0→5D0 transition, with an energy difference of 138 meV [6,14,18]. 4. Conclusion The PL spectra of the Tm2O3 single-doped glasses clearly Table 2 Energy difference (meV) between Tm3+, Tb3+, and Eu3+ transitions. Ln3+
transitions
energy difference / meV Tm3+ transitions D2 → 3F4
Tb3+ Eu3+
Fig. 9. Emission spectra of the xTm/0.5Tb/0.1Eu ( x = 0.5, 1.0 ) triple-doped glasses under 358-nm excitation. All values are given in mol%.
75
7
5
7
5
F6 → F0 → 7 F0 → 7 F0 → 7 F1 → 7 F1 → 7 F1 →
D4 D0 5 D1 5 D2 5 D0 5 D1 5 D2
1
1
G4 → 3H6
175 593 378 76 661 446 144
37 456 241 – 524 309 7
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demonstrated their potential as blue luminescent material. Quantum efficiency and spectral PL intensity of 0.3 mol% Tm2O3 single-doped glass was found to be higher than that of 0.5 mol%, confirming that luminescence quenching occurs beyond the saturation concentration of 0.3 mol%. Consequently, 0.3 mol% Tm2O3 was used in the synthesis of the Tm/Tb/Eu triple-doped glasses to optimise the blue spectral component in the general PL spectrum. Further, triple-doping with Tm2O3, Tb4O7, and Eu2O3 in a single host glass matrix and variation of the concentration ratio of these dopants widen the possibility of producing various colour impressions under a particular excitation wavelength. A white colour impression was achieved through 358-nm excitation of the 0.3Tm/0.3Tb/0.3Eu (values in mol%) triple-doped LiAlB glass with a CCT of 6500 K, making these glasses promising candidates as thermally stable encapsulants for white LEDs. Likewise, PL characteristics and consequently colour impression can be tailored to requirement of a particular application by selection of appropriate concentration ratio of dopants and varying the excitation wavelength, as shown in the CIE chromaticity diagram in Fig. 7. Triple-doping is favourable in that a broad range of excitation energies can lead to different colour impressions using a single glass. The different energy transfers namely, Tm3 + → Tb3 +, Tm3 + → Eu3 +, and Tb3 + → Eu3 +, as well as their possible routes were established. Acknowledgement The authors wish to thank the “Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen” for its financial support to the Fraunhofer Application Center for Inorganic Phosphors in Soest. In addition, the authors would like to thank the German Federal Ministry for Education and Research (BMBF) for its support to the South Westphalia University of Applied Sciences within the FHprofUnt 2014 project “LED-Glas” (project no. 03FH056PX4) and the German Science Foundation (DFG) for their support within the project SCHW 721/8-1. References [1] R. Mueller-Mach, G. Mueller, M.R. Krames, H.A. Höppe, F. Stadler, W. Schnick, T. Juestel, P. Schmidt, Highly efficient all-nitride phosphor-converted white light emitting diode, Phys. Status Solidi (a) 202 (9) (2005) 1727–1732. [2] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Prospects for LED lighting, Nat. Photonics 3 (4) (2009) 180–182. [3] C.H. Huang, T.M. Chen, W.R. Liu, Y.C. Chiu, Y.T. Yeh, S.M. Jang, A single-phased emission-tunable phosphor Ca9Y(PO4)7: Eu2+, Mn2+ with efficient energy transfer for white-light-emitting diodes, ACS Appl. Mater. Interfaces 2 (1) (2010) 259–264. [4] C.J. Zhao, J.L. Cai, R.Y. Li, S.L. Tie, X. Wan, J.Y. Shen, White light emission from Eu3+/Tb3+/Tm3+ triply-doped aluminoborate glass excited by UV light, J. NonCryst. Solids 358 (3) (2012) 604–608. [5] Y. Jin, W. Lu, J. Zhang, Z. Hao, X. Zhang, Luminescence properties of Tb3+, Eu3+, Tm3+ Co-doped Na5La(MoO4)4 for white light-emitting diode, J. Nanosci. Nanotechnol. 14 (5) (2014) 3683–3686. [6] F. Steudel, S. Loos, B. Ahrens, S. Schweizer, Quantum efficiency and energy transfer processes in rare-earth doped borate glass for solid-state lighting, J. Lumin. 170 (2016) 770–777. [7] E.F. Schubert, Light-Emitting Diodes, Cambridge University Press, 2006. [8] S. Yi, W.J. Chung, J. Heo, Phosphor-in-glasses composites containing light diffusers for high color uniformity of white-light emitting diodes, J. Solid State Lighting 2 (1) (2015) 1–6. [9] H. Lin, E.Y.B. Pun, X. Wang, X. Liu, Intense visible fluorescence and energy transfer in Dy3+, Tb3+, Sm3+ and Eu3+ doped rare-earth borate glasses, J. Alloys Compd.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Tm/Tb/Eu triple-doped lithium aluminoborate glass for white light generation
MARK
⁎
Mreedula Mungraa, Franziska Steudelb, , Bernd Ahrensa,b, Stefan Schweizera,b a
Department of Electrical Engineering, South Westphalia University of Applied Sciences, Lübecker Ring 2, 59494 Soest, Germany Fraunhofer Application Center for Inorganic Phosphors, Branch Lab of Fraunhofer Institute for Microstructure of Materials and Systems IMWS, Lübecker Ring 2, 59494 Soest, Germany
b
A R T I C L E I N F O
A B S T R A C T
Keywords: Thulium Luminescence quenching Triple-doping Energy transfer White light
The aim was to design a multi-chromatic phosphor based on the colour mixing principle, whereby white light can be produced from an appropriate mixture of blue, green, and red emissions by a single phosphor. In this perspective, Tm2O3, Tb4O7, and Eu2O3 were simultaneously doped in single host glass matrix. A series of Tm2O3 single- as well as Tm/Tb/Eu triple-doped glasses were synthesised. The photoluminescence behaviour and luminescence quenching were investigated for Tm2O3 single-doped glasses. Keeping Tm2O3 constant at the saturation concentration in the Tm/Tb/Eu triple-doped glasses and varying the doping ratio of the two other lanthanides (Tb4O7 and Eu2O3) in the host glass allow to some extent the control of the spectral composition and consequently the generation of tunable colour impressions. The latter can also be adjusted by keeping the concentration ratio of the three lanthanides constant and varying the excitation wavelength. Upon 358-nm excitation of the Tm/Tb/Eu triple-doped glasses, white light emission is achieved and the corresponding correlated colour temperatures are also determined. The different energy transfers occurring in between the three lanthanides are investigated and possible energy transfer routes are proposed.
1. Introduction With increasing round the clock human activities, the demand for natural white light (daylight-like light) or applications necessitating white light have fuelled a significant research effort into developing efficient, more economic and sustainable white light sources. Owing to their lower power consumption, lower voltage, longer operational lifetime, smaller size, and more aesthetic appearance, white-lightemitting diodes (W-LEDs) distinguish themselves as superior to incandescent and fluorescent lamps. They are the promising components for the next-generation illumination and solid state lighting applications. W-LEDs are also associated with energy saving and hence subsequent reduction of production of greenhouse gases [1–6]. The four most common approaches to generate white light are: (1) an electronic device that mixes the light emitted from red, green, and blue LEDs, (2) blue LED chip with a yellow phosphor, (3) an ultraviolet (UV) LED chip with a combination of two (blue and yellow) or three (red, blue, and green) phosphors, and (4) an UV LED chip combined to a single multi-chromatic phosphor [2,3,7]. The current limitations of the dichromatic (blue and yellow) phosphor based W-LEDs are poor colourrendering index (CRI) caused by the absence of red light and the
⁎
Corresponding author. E-mail address: [email protected] (F. Steudel).
http://dx.doi.org/10.1016/j.jlumin.2017.06.028 Received 6 April 2017; Received in revised form 19 May 2017; Accepted 14 June 2017 Available online 15 June 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
thermal degradation of the polymer-based encapsulant of the LED, which also contains the phosphor material. This degradation often entails efficiency reduction and colour properties deviation [5,8]. In order to circumvent the aforementioned issues, trichromatic (simultaneously emitting blue, green, and red lights) luminescent materials, made from glasses doped with lanthanides (Ln), are proposed as alternative encapsulation for W-LEDs. For optical activation of the Ln3+ ions, a suitable host glass is essential. The features of interest of the host glass are high optical transmittance, good Ln3+ ion solubility, thermal stability, along with mechanical resistance. Borate glasses generally satisfy these requirements and due to their low melting point, these glasses can be rather easily produced and moulded in any desired dimensions and shape [9,10]. Another important property characterising the suitability of a host for phosphors is not only the thermal stability but also the so-called quenching temperature, i.e. the temperature at which the luminescence intensity of a lanthanide ion in this host amounts to 50% of its intensity at room temperature. In the borate glass host, this is approximately 300 °C for Eu3+ and even significantly higher for Tb3+ [11]. For comparison, the quenching temperature of the conventional LED phosphor YAG:Ce3+ is at 270 °C [12].
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A combination of three Ln3+ ions consisting of trivalent thulium (Tm3+), terbium (Tb3+), and europium (Eu3+) ions were selected, based on their respective potential of emitting blue, green, and red luminescence reported in other host glass materials [4,5,13–16]. The photoluminescence (PL) properties of Eu3+ and Tb3+ doped borate glasses as well as the energy transfer from Tb3+ to Eu3+ have been extensively investigated and established in [6,13,14,17,18]. Within the framework of this research, the transmittance, PL, quantum efficiency, and luminescence quenching of Tm2O3 singledoped and Tm/Tb/Eu triple-doped lithium aluminoborate (LiAlB) glasses are investigated. The effect of varying the excitation wavelength and tuning the doping ratio of the three Ln3+ ions in the triple-doped glasses on the PL behaviour as well as the chromaticity of triple-doped glasses is also evaluated.
2. Experimental details For the synthesis of the LiAlB glasses, boron oxide (B2O3) as network former and lithium oxide (Li2O) as network modifier are used in a ratio of 2 to 1. To prevent devitrification of the glass, 10% of the B2O3 is substituted by aluminum oxide (Al2O3); the B2O3 to Al2O3 ratio amounts to 9 to 1. The additional Al2O3 doping also increases the strength of the glass [19]. Single-doped glasses are prepared by individually doping them with thulium oxide (Tm2O3), terbium oxide (Tb4O7), and europium oxide (Eu2O3). Additionally, glasses tripledoped with Tm2O3, Tb4O7, and Eu2O3 are also prepared. The synthesis of all the glasses are carried out using the melt quenching technique. The starting chemicals are weighed in a Pt/Au (95/5) crucible. The exact composition of all the investigated glass samples used is as listed in Table 1. The chemicals are transferred to a porcelain mortar-pestle mixer and are thoroughly grinded to achieve a homogeneous mixture. The mixture is melted at 1000 °C in the crucible over a period of approximately 3 h in a furnace. The melt is then poured on a 400 °C pre-heated brass plate, which is below the glass transition temperature (Tg = 459 °C) of undoped LiAlB glass [20]. The temperature of the brass plate is maintained constant for 3 h, before allowing the glass sample to further cool slowly to room temperature to relieve any residual mechanical and thermal stresses. The glass samples are shaped and polished to optical quality. Fig. 1 shows the appearance of the Tm2O3, Tb4O7, and Eu2O3 single-doped glasses as well as two tripledoped glasses having same Tm2O3 concentration but different levels of Tb4O7 and Eu2O3, under UV excitation.
Fig. 1. From left to right: Tm2O3, Tb4O7, and Eu2O3 single-doped glasses (top photo) and 0.3Tm/0.4Tb/0.4Eu and 0.3Tm/0.3Tb/0.3Eu triple-doped glasses (bottom photo) under 365-nm excitation (UV lamp). All values are given in mol%.
PL and quantum efficiency measurements are performed using a commercial quantum yields measurement system (Hamamatsu C9920–02G) coupled to a 3.3-in. integrating sphere with a xenon lamp (150 W) for excitation and a photonic multichannel analyzer (PMA 12) for detection. The quantum efficiency describes the ratio between the number of emitted photons and the number of absorbed photons. The number of absorbed photons was determined from absorption spectra, while the number of emitted photons was determined from emission spectra in the spectral range from 400 to 850 nm. Transmittance measurements were carried out using a Cary 5000 UV–Vis–NIR spectrophotometer from Agilent Techonologies. 3. Results and discussion 3.1. Transmission, quantum efficiency, and PL properties of Tm2O3 singledoped LiAlB glass Fig. 2 (left) exhibits the comparison of the transmission spectra of the Tm2O3 single-doped glasses. The glasses have high transmittance,
Table 1 Nominal composition of the investigated LiAlB glass samples and quantum efficiency (QE) values for excitation at 358 nm (Tm3+ and triple-doped glasses), 378 nm (Tb3+), and 394 nm (Eu3+). Dopant
Tm
3+
Tb3+ Eu3+ Tm3+, Tb3+, Eu3+
B2O3
Li2O
Al2O3
Tm2O3
Tb4O7
Eu2O3
Ln3+content / at%
59.99 59.94 59.88 59.82 59.70
33.33 33.30 33.27 33.23 33.17
6.67 6.66 6.65 6.65 6.63
0.01 0.10 0.20 0.30 0.50
– – – – –
– – – – –
0.00/ – / – 0.05/ – / – 0.09/ – / – 0.14/ – / – 0.23/ – / –
1 3 4 7 3
59.70 59.40 59.34
33.17 33.00 32.97
6.63 6.60 6.59
– – 0.50
0.50 – 0.50
– 1.00 0.10
– /0.49/ – – / – /0.46 0.23/ 0.46/0.05
32 83 8
59.04
32.80
6.56
1.00
0.50
0.10
3
59.34
32.97
6.59
0.30
0.40
0.40
59.46
33.03
6.61
0.30
0.30
0.30
0.46/ 0.46/0.05 0.14/ 0.37/0.18 0.14/ 0.28/0.14
Composition/mol%
QE /%
8
Fig. 2. (left) Transmission spectra of the Tm2O3 single-doped glasses. (right) Variation of absorption coefficient calculated at 358 nm wavelength (black crosses) over the range of Tm2O3 concentration in mol%; the solid line represents the linear regression of this relationship.
6
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Fig. 3. Quantum efficiency spectra of Tm2O3 single-doped LiAlB glasses. Fig. 4. PL emission spectra of Tm2O3 single-doped LiAlB glasses under 358-nm excitation (scaled up by a factor of 5 in the spectral range of 550–900 nm). The inset shows the variation of the integrated PL intensity with respect to Tm2O3 concentration.
approximately 90% in the visible spectral range, except at localised positions where transmittance decreases due to the absorption caused by Tm3+ ions in the glass matrix. The absorption lines correspond to the Tm3+-related 4f-4f transitions from the ground state, 3H6 to 1D2 (358 nm), 1G4 (468 nm), 3F2 (660 nm), 3F3 (686 nm), and 3H4 (792 nm). Absorption increases along with the concentration of Tm2O3. As shown in Fig. 2 (right), the absorption coefficient due to Tm3+, determined at 358 nm and corrected for the absorption due the base glass, increases linearly with respect to the concentration of Tm2O3 in the glasses. Fig. 3 shows the quantum efficiency spectra of the different Tm2O3 single-doped glasses. Note that the recorded quantum efficiency spectra correspond to photoluminescence excitation spectra for an integrated emission in the spectral range from 400 to 850 nm. Five LiAlB glasses, doped with 0.01, 0.1, 0.2, 0.3, and 0.5 mol% Tm2O3, are excited at 358, 468, 660, 686, and 792 nm. It is a noteworthy feature of Tm3+ luminescent behaviour that only under 358-nm excitation (3H6 → 1D2), the single-doped glasses emit bright blue luminescence while under all the other excitation wavelengths in the UV and blue spectral ranges, hardly any luminescence is recorded. The low signal-to-noise ratio for wavelengths larger than 400 nm results from the low absorption (A < 2%) of the glass samples in this spectral range. The quantum efficiency values of the Tm3+ single-doped samples for 358-nm excitation are listed in Table 1. The quantum efficiency is less than 10% due to the small energy gap between the energy levels of maximal ΔE Tm3 + = 0.87 eV and thus the high multi-phonon relaxation rate [21]. For concentrations of 0.01–0.3 mol% Tm203, the quantum efficiency increases to up to 7% and beyond that, the decrease in quantum efficiency indicates luminescence quenching for higher concentrations. For single Tb3+ and Eu3+ doping, the quantum efficiency is significantly higher and amounts to 32% and 83%, respectively, due to a larger energy gap of ΔE Tb3 + = 1.80 eV and ΔE Eu3 + = 1.51 eV , respectively (values obtained from absorption and emission spectra). The triple-doped glasses show an increase in quantum efficiency as compared to the single-doped glasses with the same thulium concentration, which is a clear hint for energy transfer from thulium to at least one of the other lanthanide ions. The energy transfer is further discussed and elaborated in Section 3.3. As presented in the emission spectra in Fig. 4, the very sharp emission line at 453 nm, responsible for the blue colour impression, predominates the emission spectra while four other bands of weaker intensities are also observed. As illustrated in Fig. 5, 358-nm excitation leads to the population of the 1D2 level, from which radiative emission in the blue spectral range occurs through the transition to the 3F4 (453 nm) level. Through multiphonon relaxation 1G4 level is populated and from thereon, through 3F2,3 levels, to the eventual population of the 3 H4 level. The radiative relaxation of the excited 1G4 and 3H4 levels
Fig. 5. Energy level diagram depicting the 358-nm excitation route and the subsequent emission routes for the Tm2O3 single-doped glasses. The values next to the arrows indicate the wavelength in nm of the emission bands.
results in four emission lines, which are ascribed to the transitions, namely 1G4→3H6 (478 nm), 1G4→3F4 (652 nm), 1G4→3H5 (755 nm), and 3 H4→3H6 (785 nm). In anology to the quantum efficiency measurements, the PL intensity increases monotonously in function with the Tm2O3 concentration in the range from 0.01 up to 0.3 mol% and from beyond there, the intensity decreases. To have a better insight on this PL variation, the dependence of the integrated PL intensities (integration performed for the range of 430–800 nm) on Tm2O3 concentration was plotted as shown in the inset of Fig. 4. The highest emission intensity occurs at 0.3 mol% (0.14 at%) concentration. The concentration-dependent quenching is evidenced by the decrease in PL intensity in between 0.3 and 0.5 mol% (0.14 and 0.23 at%) of Tm2O3 due to crossrelaxation. Nine possible cross-relaxation channels are reported in literature [22–29], five channels include excitation from the ground state: (1G4, 3H6) → (3F2, 3F4), (1G4, 3H6) → (3H4, 3H5), (1G4, 3H6) → (3H5, 3H4), (1G4, 3H6) → (3F4, 3F2), and (3H4, 3H6) → (3F4, 3F4) [22–26]. Similarly, 73
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Fig. 6. CIE 1931 (x, y) chromaticity diagram showing the position of the colour coordinates of 0.3 mol% Tm2O3, 0.5 mol% Tb4O7, and 1.0 mol% Eu2O3 single-doped glasses under 358, 377, and 393-nm excitation, respectively. The solid line represents the Planckian locus (black-body radiation) and E denotes the equal-energy point at (0.333, 0.333), used as the while-light reference.
Fig. 7. (left) Emission spectra and (right) CIE 1931 (x, y) chromaticity diagram with the corresponding chromaticity coordinates of 0.3Tm/0.4Tb/0.4Eu triple-doped glass (values given in mol%) under the excitation wavelengths of 358, 368, 377, 393, and 484 nm. The position of the emission bands due to the respective lanthanides are indicated by dashed lines.
luminescence quenching has been reported to occur at relatively low concentrations of Tm2O3 different in single-doped host matrices; at around 0.6 at% (calculated from the saturation concentration of 1.0 mol% Tm2O3 single-doped glass with the following nominal chemical composition: 55ZnO-(45-x)B2O3-x Tm2O3, x = 1) in single-doped zinc borate glasses and in between 0.5 at% and 6 at% in single-doped YVO4 crystals [30,31]. As shown in Fig. 1 under UV light illumination, the Tm2O3, Tb4O7, and Eu2O3 single-doped glasses emit blue, green, and orange-red light, respectively. Their corresponding chromaticity coordinates are depicted in Fig. 6 and the chromaticity coordinates of Tb/Eu doubledoped glasses with a fixed concentration of 0.5 mol% Tb4O7 and an increasing concentration of Eu2O3 from 0 to 1.0 mol% has been shown to lie in a straight line between the chromaticity coordinates corresponding to the Tb4O7 and Eu2O3 single-doped glasses [17,18]. Based on this principle, triple-doping with Tm2O3, Tb4O7, and Eu2O3 in a single host glass matrix and under the highest excitation energy, i.e. 358 nm, should lead to colour impressions having chromaticity coordinates lying within the area bounded by the dashed triangle, as illustrated in Fig. 6.
Fig. 8. (left) Emission spectra and (right) CIE 1976 (u′, v′) uniform diagram with corresponding chromaticity coordinates of the 0.3Tm/0.3Tb/0.3Eu (grey) and 0.3Tm/0.4Tb/ 0.4Eu (orange) triple-doped glasses (values in mol%), under 358-nm excitation. The 5step MacAdam ellipses (dashed) are indicated for light sources of nominal CCT of 5000 K and 6500 K (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. PL and colour properties of Tm/Tb/Eu triple-doped LiAlB glasses Two triple-doped glasses, 0.3Tm/0.4Tb/0.4Eu and 0.3Tm/0.3Tb/ 0.3Eu (values in mol%) are synthesised, to investigate the possibility of white light emission under UV light excitation. Fig. 7 presents emission spectra and corresponding (x, y) chromaticity coordinates of the 0.3Tm/0.4Tb/0.4 Eu (values given in mol%) triple-doped LiAlB glass under the following five different excitation wavelengths: 358, 368, 377, 393, and 484 nm. The glass emits yellowish green light (points b, c & e) under 368, 377, and 484 nm excitation wavelengths, denoting typical emissions caused by Tm2O3 in the triple-doped glass. When excited at 393 nm, the very characteristic orange-red colour impression of Eu2O3 emission is achieved as depicted by point d on the chromaticity diagram in Fig. 7 (right). Under 358-nm excitation, the chromaticity coordinates (point a; x = 0.343, y = 0.362) of the triple-doped glass lie in the white light emitting region of the CIE diagram. As such, triple-doping with Tm2O3, Tb4O7, and Eu2O3 in the LiAlB glass matrix represents a promising means to achieve white light emission. Therefore, it is inferred that theoretically any desired colour of PL emission, translated by the position of the chromaticity coordinates within the area of the triangle in Fig. 6, can be achieved by varying the excitation wavelength under which the triple-doped glass is exposed. Further, in an attempt to enhance the white-light emitting
capability of the Tm/Tb/Eu triple-doped glass, the effect of varying the doping ratio of the lanthanides in the host glass at the fixed excitation wavelength of 358 nm is investigated. Fig. 8 shows the emission spectra of two triple-doped glasses with different concentrations of Eu2O3 and Tb4O7 while keeping Tm2O3 concentration fixed at 0.3 mol% under 358-nm excitation. Emission bands are registered in the blue, green as well as red spectral regions. The blue emission is attributed to: the Tmrelated 1D2→3F4 transition at 453 nm and the Tb-related 5D4→7F6 transition at 489 nm. The green emission is ascribed to the Tb-related 5 D4→7F5 transition at 543 nm. The red emissions show one band at 588 nm, which can be attributed to the overlap of the Eu-related 5 D0→7F1 and Tb-related 5D4→7F4 transitions. The second emission band in the red spectral region is at 617 nm and originates from the overlap of the emission bands of Eu-related 5D0→7F2 and Tb-related 5D4→7F3 transitions. The last two bands of weak intensity belong to Eu-related transitions, namely 5D0→7F3,4 at 653 nm and 702 nm, respectively. As shown in Fig. 8 (right) in the CIE 1976 (u′, v′) uniform chromaticity diagram, the position of the triple-doped glass having the composition of 0.3Tm/0.4Tb/0.4Eu is nearer to the position of the equal-energy point, E, as compared to that of the 0.3Tm/0.3Tb/0.3Eu (values in mol 74
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%). In addition, the colour coordinates of the glass with lower Tb3+ and Eu3+ doping are further in the blue spectral range, resulting in a “cold white” colour impression as compared to the glass with higher Tb3+ and Eu3+ doping. This is also evident from Fig. 1. To further characterise these two white light sources, their correlated colour temperatures (CCT) are determined, using the five-step MacAdam ellipses which are approximately defined by the following equation of circle, having centre (u′c , v′c ) and radius r = 0.0055 [32]:
(u′ − u′c )2 + (v′ − v′c )2 = r 2
(1)
where (u′c , v′c ) is (0.2092, 0.4884) and (0.1951, 0.4726) for light sources of nominal CCT of 5000 K and 6500 K, respectively. The ellipses, covering the area of chromaticity coordinates matching light sources having CCT values of 5000 K and 6500 K are as shown in Fig. 8 (right). The chromaticity coordinates of the 0.3Tm/0.3Tb/0.3Eu (values in mol%) is found to be within the boundaries of the ellipse corresponding to CCT value of 6500 K. As such, this triple-doped LiAlB glass is a promising daylight-like source since the CCT value of direct sunlight varies between 5700 and 6500 K [32,7].
Fig. 10. Energy level diagrams depicting the 358-nm excitation route and the routes for radiative emissions (blue, green, and red arrows) and energy transfers (dashed arrows) occurring from Tm3+→ Tb3+ (ET 1), Tm3+→ Eu3+ (ET 2), and Tb3+→ Eu3+ (ET 3) in the triple-doped glasses (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Energy transfers 3+
ions through energy transfer. As such, the possible routes for the following energy transfers at 300 K are investigated and described as follows:
3+
While the energy transfer from Tb to Eu has been thoroughly investigated and confirmed in Tb/Eu double-doped borate glasses [17,14], it is interesting to observe what happens when Tm2O3 is also doped along with Tb4O7 and Eu2O3 in a single host glass matrix. Fig. 9 shows the emission spectra of xTm/0.5Tb/0.1Eu with x = 0.5 and 1.0 (values in mol%) triple-doped glasses under 358-nm excitation. Changing the concentration of Tm2O3 from 1.0 to 0.5 mol% in the tripledoped glasses results in an improvement of the characteristic Tm2O3 emission at 454 nm, due to the inverse relationship between PL intensity and Tm3+ concentration beyond the saturation concentration (refer to Section 3.1). Another spectral observation of interest is the enhancement of PL intensity of both the Tb-related and Eu-related emissions, despite that the doping concentrations of Tb4O7 and Eu2O3 are constant in both triple-doped glasses. This can be due to eventual energy transfers from the excited Tm3+ to both Tb3+ and Eu3+ along with the well-known energy transfer from Tb3+ to Eu3+. For energy transfer to be possible, the states of the acceptor must be identical or differ insignificantly from the emitting states of the donor. Satisfaction of this criterion ensures that the excitation energy through resonance energy transfer from donor is effectively reabsorbed by the acceptor, resulting in the population of its emitting level, with possibility of subsequent PL emission [33–36]. Under 358-nm excitation, Tm3+ emits blue light at 453 nm, as illustrated in Fig. 10. Given that the excited energy states of Eu3+ and Tb3+ lies below that of Tm3+, there exists the possibility that Tm3+ sensitizes both Tb3+ and Eu3+
1. Tm3 + → Tb3 + The transition bearing the least energy difference (37 meV) and thus most probable channel of this energy transfer is through the nonradiative relaxation corresponding to the Tm3+-related 1G4→3H6 transition and Tb3+ excitation from 7F6 to 5D4, as illustrated in Fig. 10. 2. Tm3 + → Eu3 + and Tb3 + → Eu3 + The non-radiative coupling of the Tm3+-related 1G4→3H6 transition with the Eu3+-related 7F6 to 5D4 transition satisfies the resonant energy transfer condition, with the least energy difference of 7 meV (Table 2). The non-radiative energy transferred from the Tm3 + 1G4 energy level triggers the excitation of the Eu3 + 7F1 electrons (35% population at room temperature [6]) to the 5D2 level, as illustrated in the partial energy diagram in Fig. 10. From the 5D2 level, nonradiative multi-phonon relaxation, followed by radiative emission through Eu3+-related 5D0→7F1 and 5D0→7F2 transitions, occur. In our previous works [6,17,14,18], the most probable route (Fig. 10) of energy transfer from Tb3+ to Eu3+ is via Tb3+-related 5 D4→7F4 transition to Eu3+-related 7F1→5D0 transition, with energy difference between the transitions of donor and acceptor amounting to 14 meV, while the second one is via Tb3+-related 5D4→7F5 transition to Eu3+-related 7F0→5D0 transition, with an energy difference of 138 meV [6,14,18]. 4. Conclusion The PL spectra of the Tm2O3 single-doped glasses clearly Table 2 Energy difference (meV) between Tm3+, Tb3+, and Eu3+ transitions. Ln3+
transitions
energy difference / meV Tm3+ transitions D2 → 3F4
Tb3+ Eu3+
Fig. 9. Emission spectra of the xTm/0.5Tb/0.1Eu ( x = 0.5, 1.0 ) triple-doped glasses under 358-nm excitation. All values are given in mol%.
75
7
5
7
5
F6 → F0 → 7 F0 → 7 F0 → 7 F1 → 7 F1 → 7 F1 →
D4 D0 5 D1 5 D2 5 D0 5 D1 5 D2
1
1
G4 → 3H6
175 593 378 76 661 446 144
37 456 241 – 524 309 7
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demonstrated their potential as blue luminescent material. Quantum efficiency and spectral PL intensity of 0.3 mol% Tm2O3 single-doped glass was found to be higher than that of 0.5 mol%, confirming that luminescence quenching occurs beyond the saturation concentration of 0.3 mol%. Consequently, 0.3 mol% Tm2O3 was used in the synthesis of the Tm/Tb/Eu triple-doped glasses to optimise the blue spectral component in the general PL spectrum. Further, triple-doping with Tm2O3, Tb4O7, and Eu2O3 in a single host glass matrix and variation of the concentration ratio of these dopants widen the possibility of producing various colour impressions under a particular excitation wavelength. A white colour impression was achieved through 358-nm excitation of the 0.3Tm/0.3Tb/0.3Eu (values in mol%) triple-doped LiAlB glass with a CCT of 6500 K, making these glasses promising candidates as thermally stable encapsulants for white LEDs. Likewise, PL characteristics and consequently colour impression can be tailored to requirement of a particular application by selection of appropriate concentration ratio of dopants and varying the excitation wavelength, as shown in the CIE chromaticity diagram in Fig. 7. Triple-doping is favourable in that a broad range of excitation energies can lead to different colour impressions using a single glass. The different energy transfers namely, Tm3 + → Tb3 +, Tm3 + → Eu3 +, and Tb3 + → Eu3 +, as well as their possible routes were established. Acknowledgement The authors wish to thank the “Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen” for its financial support to the Fraunhofer Application Center for Inorganic Phosphors in Soest. In addition, the authors would like to thank the German Federal Ministry for Education and Research (BMBF) for its support to the South Westphalia University of Applied Sciences within the FHprofUnt 2014 project “LED-Glas” (project no. 03FH056PX4) and the German Science Foundation (DFG) for their support within the project SCHW 721/8-1. References [1] R. Mueller-Mach, G. Mueller, M.R. Krames, H.A. Höppe, F. Stadler, W. Schnick, T. Juestel, P. Schmidt, Highly efficient all-nitride phosphor-converted white light emitting diode, Phys. Status Solidi (a) 202 (9) (2005) 1727–1732. [2] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Prospects for LED lighting, Nat. Photonics 3 (4) (2009) 180–182. [3] C.H. Huang, T.M. Chen, W.R. Liu, Y.C. Chiu, Y.T. Yeh, S.M. Jang, A single-phased emission-tunable phosphor Ca9Y(PO4)7: Eu2+, Mn2+ with efficient energy transfer for white-light-emitting diodes, ACS Appl. Mater. Interfaces 2 (1) (2010) 259–264. [4] C.J. Zhao, J.L. Cai, R.Y. Li, S.L. Tie, X. Wan, J.Y. Shen, White light emission from Eu3+/Tb3+/Tm3+ triply-doped aluminoborate glass excited by UV light, J. NonCryst. Solids 358 (3) (2012) 604–608. [5] Y. Jin, W. Lu, J. Zhang, Z. Hao, X. Zhang, Luminescence properties of Tb3+, Eu3+, Tm3+ Co-doped Na5La(MoO4)4 for white light-emitting diode, J. Nanosci. Nanotechnol. 14 (5) (2014) 3683–3686. [6] F. Steudel, S. Loos, B. Ahrens, S. Schweizer, Quantum efficiency and energy transfer processes in rare-earth doped borate glass for solid-state lighting, J. Lumin. 170 (2016) 770–777. [7] E.F. Schubert, Light-Emitting Diodes, Cambridge University Press, 2006. [8] S. Yi, W.J. Chung, J. Heo, Phosphor-in-glasses composites containing light diffusers for high color uniformity of white-light emitting diodes, J. Solid State Lighting 2 (1) (2015) 1–6. [9] H. Lin, E.Y.B. Pun, X. Wang, X. Liu, Intense visible fluorescence and energy transfer in Dy3+, Tb3+, Sm3+ and Eu3+ doped rare-earth borate glasses, J. Alloys Compd.
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