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Luminescent properties and energy transfer behavior between Tm3+ and Dy3+ ions in co-doped phosphate glasses for white LEDs
Journal of Luminescence 178 (2016) 6–12
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Luminescent properties and energy transfer behavior between Tm3 þ and Dy3 þ ions in co-doped phosphate glasses for white LEDs Guo-hua Chen a,b,n, Le-qi Yao a, Hai-ji Zhong a, San-chuan Cui a a b
College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2016 Received in revised form 26 April 2016 Accepted 19 May 2016 Available online 26 May 2016
In order to obtain white light-emitting materials, the luminescent glasses composed of SrO–ZnO–TiO2– P2O5 co-doped with Tm3 þ and Dy3 þ ions were synthesized by a conventional melting-quenching method. The luminescent properties of Tm3 þ and Dy3 þ in zinc strontium phosphate glasses were investigated, where the emission spectra show possible energy transfer mechanism between Tm3 þ and Dy3 þ ions. The energy transfer carries out via resonant transfer modes, which is easily understood from Tm3 þ and Dy3 þ energy level diagrams. The decreasing in mean-duration time of Tm3 þ :1D2 and Dy3 þ :4F9/2 obtained from the decay curves makes a further evidence of energy transfer from Tm3 þ to Dy3 þ ions and Dy3 þ to Tm3 þ ions in the glasses. & 2016 Elsevier B.V. All rights reserved.
Keywords: Fluorescence glass White LEDs Energy transfer Optical properties
1. Introduction Currently, white light emitting diodes (W-LEDs) have been considered as a potential candidate for lighting source due to their merits of long lifetime, environmental friendliness and high efficiency, compared with conventional incandescent and fluorescence lamps [1–4]. Up to now, W-LEDs have been realized by two methods: mixing blue, green and red phosphors in appropriate proportion or one single phosphor that emit blue, green and red emissions, excited by a UV LED chip [5,6]. Although white LEDs combining a blue GaN LED chip with yellow phosphor YAG:Ce3 þ have been commercialized, they have some inherent drawbacks such as poor heat resistance, high correlated color temperature and low color rendering index due to their low thermal quenching temperature and lack of red emitting element [7]. Considering these problems, it is urgent to develop pollution-free, cheap, highly efficient white-light-emitting (WLE) materials. Compared with common phosphors used for W-LEDs, white light emitting glasses are considered to be as an alternative due to their homogeneous light emitting, simpler manufacture procedure, lower production cost, free of halo effect, and better thermal stability [3,8]. Since white light-emitting glass was first developed [9], many efforts have been made to develop luminescent glasses for white LEDs [10–13]. Nowadays, a growing number of attentions are focused on such glasses doped with rare-earth ions [3,14]. However, n
Corresponding author at: College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China. Tel.: þ 86 7732291957; fax: þ86 7732191903. E-mail addresses: [email protected], [email protected] (G.-h. Chen). http://dx.doi.org/10.1016/j.jlumin.2016.05.034 0022-2313/& 2016 Elsevier B.V. All rights reserved.
only a few studies are concerned on phosphate glasses [15,16]. Phosphate glasses own many merits, such as low softening and melting temperature, low product cost, high luminous ion doped concentration, large emission and absorption cross sections and high transparency for visible light and optical characteristics [17,18]. Dy3 þ is one of the important rare-earth ions which play a major role in the production of different types of light-emitting materials, since Dy3 þ ions possess intense emission in the blue and yellow regions, which are assigned to the 4F9/2-6H15/2 and 4F9/2-6H13/2 transitions respectively [19]. On the other hand, Tm3 þ ion 4f to 5d transitions have received much attention and there have been a lot of reports on Tm3 þ ion doped luminescent materials in the last several years for white light emitting diode [12]. Also, there have mutual energy transfer between Dy3 þ and Tm3 þ ions in other glasses [2]. In the present work, Dy3 þ /Tm3 þ single-doped and Dy3 þ /Tm3 þ co-doped zinc strontium phosphate glasses were successfully prepared by the conventional melt-quenching method and their luminescence and color coordinates were investigated by varying doping ion concentrations [20]. Furthermore, the energy transfer between Dy3 þ and Tm3 þ was also discussed. Our research, thus, wishes to provide new information in this glass system by restraining the shortcomings. This work aims to (i) investigate the luminescence properties of Tm3 þ and Dy3 þ co-doped phosphate glasses; and (ii) reveal the energy transfer mechanism between Tm3 þ and Dy3 þ ions.
2. Experiments and measurements Base glasses with the chemical composition 15SrO–43ZnO– 40P2O5–2TiO2 (SZPT, similarly hereinafter) are separately prepared
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
by doping Tm2O3 or Dy2O3 individually and also with Tm2O3 and Dy2O3 in dual combinations for different concentrations as follows in Table 1. The chemicals used in the preparation of glasses were reagent grade (Z 99%) NH4H2PO4, ZnO, SrCO3, TiO2, and high purity (Z 99.99%) rare earth ions Tm2O3 and Dy2O3 (Guo-Yao Co. Ltd, Shanghai, China). All these chemicals were weighed in 30 g batch each separately, thoroughly mixed and finely powdered using agate mortar and pestle. Then each batch of chemical mix was transferred into corundum crucible and melted in an electric furnace for 2 h at 1300 °C. These melts were quenched in between two smooth surfaced brass plates to obtain circular glass discs of 1–1.5 mm in diameter with 0.3 cm in thickness. Later, these prepared glasses were cut and the surfaces were polished, and taken to characterize luminescent properties. The optical absorption spectra were recorded on Shimadzu UV– vis–NIR spectrophotometer (UV-3600) in the wavelength range 200– 900 nm. The excitation and emission spectra, fluorescence lifetimes were recorded with a model FL3-P-TCSPC fluorescence spectrophotometer with a Xenon lamp as the excitation source. All measurements were performed at room temperature and with the same instrument parameters. The chromaticity coordinates (x, y) were calculated based on the photoluminescent spectra by using software.
3. Results and discussion Fig. 1(a) shows the room temperature excitation spectra of the Tm3 þ (λem ¼452 nm) and Dy3 þ (λem ¼574 nm) single-doped phosphate glass samples. For the Tm3 þ doped sample, the excitation band at 350–375 nm for the 452 nm blue light emission is discovered. The main excitation band peaking at 356 nm, which is assigned to the Tm3 þ :3H6-1D2 transition monitored at 452 nm. For the Dy3 þ doped sample, monitoring the Dy emission at 574 nm, corresponding to the 4F9/2-6H13/2 transition, the excitation maximum is located around 347 nm corresponding to 6H15/ 6 2- P7/2 transition. The other secondary excitation peaks, detected around 321 nm, 363 nm and 385 nm comply with the transitions 6 H15/2-6P3/2, 6H15/2-6P5/2 and 6H15/2-4F7/2, respectively [21–23]. Fig. 1(b) shows optical absorption spectra of the Tm3 þ and Dy3 þ single-doped SZPT glass samples recorded at room temperature in the visible region. It is observed that rare earth ion single-doped glass samples exhibit different absorption levels, and the prominent levels observed were assigned to the appropriate electronic transitions, as follows: Dy3 þ : 6 H15=2 -6 P7=2 ; 4 I11=2 ; 4 I13=2 ; 4 I15=2 ; 6 F3=2 ; 6 F5=2 Tm3 þ :3 H6 -1 D2 ; 1 G4 ; 3 F3 ; 3 H4
It can be noticed that both absorption and excitation bands of Dy3 þ and Tm3 þ single-doped SZPT glass samples have identical overlap from 350 to 370 nm. It indicates that the Tm3 þ /Dy3 þ codoped phosphate glasses can be efficiently excited by UV light. Therefore, the 353 nm was chosen as the exciting wavelength in the following experiments. The emission spectra of the two samples at 353 nm light excitation are shown in Fig. 1(c). The Tm3 þ doped sample displays one intense blue emission band centered at 452 nm derived from the Tm3 þ :1D2-3F4 transition, and small greenish blue emission bland of 1G4-3H6 for Tm3 þ at 480 nm. The Dy3 þ doped sample exhibits two intense bands centered around 482 nm (greenish blue), 574 nm (yellow) and a weak band around 663 nm (red), corresponding to the 4F9/2-6HJ (J¼ 11/2, 13/2,15/2) transitions, respectively, in which the intensity of the 4F9/2-6H13/2 yellow emission is a hypersensitive (forced electric-dipole) transition, and it exhibits a strong dependence on the ligand environment, whereas the 4F9/2-6H15/2 blue emission is insensitive to the host [24]. Thus, the yellow-to-blue (Y/B) emission intensity ratio can
7
Table 1 Chemical composition of Re3 þ (Re3 þ ¼ Tm3 þ , Dy3 þ ) ion doped phosphate glasses (mol.%). Glass no.
SrO
ZnO
P2O5
TiO2
Tm2O3
Dy2O3
A B C D E F G
15 15 15 15 15 15 15
43 43 43 43 43 43 43
40 40 40 40 40 40 40
2 2 2 2 2 2 2
0.6 0 0.6 0.6 0.6 0.4 0.8
0 0.8 0.2 0.4 0.8 0.8 0.8
differ significantly in different hosts. As shown in the inset of Fig. 1(c), we took the photos of the two samples under the 353 nm excitation. The Tm3 þ and Dy3 þ single-doped phosphate glasses display the blue and yellow colors, respectively. Fig. 2 shows the emission spectrum and CIE chromaticity diagram of glasses E excited by 352, 353, 357 and 360 nm. With the increase of excitation wavelength, it is clearly found that emission color changes from yellow to blue, as shown in Fig. 2(b), which indicates that excitation wavelength has an important effect on the luminescence properties. Fig. 3(a) shows the emission spectra of glasses C, D and E excited at 353 nm. The positions of emission peaks are nearly the same for the three samples, but the existence of small band of 1G4 -3H6 for Tm3 þ at 480 nm is uncertain, because it may be overlapped by the strong emission band of 4F9/2-6H15/2 of Dy3 þ ions at 482 nm. Blue, yellow and red emission bands are observed in the emission spectra of these glasses. Spectral comparison in Fig. 3(a) indicates that the content of Dy3 þ has an important effect on the luminescence properties of glasses with a certain content of Tm3 þ . It is observed that the intensity of blue (452 nm) coming from the 1D2-3F4 transition of Tm3 þ decreases gradually with the increase of Dy3 þ concentration, which demonstrates the existence of energy transfer from Tm3 þ ions to Dy3 þ ions. Evidently, the relative intensity of the Blue emission (482 nm), yellow emission (574 nm), red emission (663 nm) derived from the 4F9/2-6HJ (J¼15/2, 13/2, 11/2) transition of Dy3 þ ions always enhances with increasing Dy3 þ ion concentration due to more direct absorption form pump absorption by Dy3 þ ions and the potential energy transfer from Tm3 þ ions to Dy3 þ ions. The maximum intensity of the 574 nm emissions is observed at 0.8 mol% Dy3 þ ions. In other words, the relative intensity of the yellow emission versus blue emission changes with an increase in the Dy3 þ ion concentration [22]. The Commission Internationale de L'Éclairage (CIE) 1931 chromaticity coordinates of the glasses A–E, which were calculated based on the corresponding emission spectra excited at 353 nm, are shown in Fig. 3(b). It reveals that Tm3 þ and Dy3 þ single-doped SZPT glasses can emit blue and yellow light, respectively. Increasing of Dy3 þ content not only supplements the yellow emission intensity but also decreases the blue emission intensity, and it is clearly found that emission color changes from blue to white. This is due to the increasing intensity of hypersensitive transition of Dy3 þ ion induced by the increment of Dy2O3 content and the decreasing of the blue emission intensity of Tm3 þ ion. The inset of Fig. 3(b) displays that the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples, and the chromaticity coordinates of the glass E are (0.332, 0.320), which are the closest to the standard white light point (0.333, 0.333). The emission color of the glass E is white for the naked eye, and the white light is bright and dazzling, and the corresponding color temperature is 5513 K. Hence, a wide application as solid state color display materials could be provided. Fig. 4 shows the decay curves of the 452 and 574 nm emissions under excitation wavelength of 353 nm. As shown in Fig. 4(a) and (b), when the interaction between luminescent ions is not
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
347
Intensity(a.u)
H
Dy :λem=574 nm
353nm
356
385
D P
3+
Tm :glass
363 321
H
Τm :λem=452 nm
Intensity(a.u)
8
I 353nm H G I
F3
3+
Dy :glass
320
340
360
380
300
400
400
500
F9/2
Intensity (a.u.)
600
700
800
Wavelength(nm) 4
D2
H4 F3/2 F5/2
Wavelength(nm)
1
I
6
H13/2 574
3
452
F4 Dy Tm 6 F9/2 H11/2
4
482
1
480 450
G4
4
F9/2
3
H6
500
6
H15/2
663
550
600
650
700
Wavelength (nm) Fig. 1. (a) Excitation and (b) absorption spectra of the Tm3 þ (λem ¼ 452 nm) and Dy3 þ (λem ¼574 nm) single-doped phosphate glasses. (c) Emission spectra of the Tm3 þ and Dy3 þ single-doped phosphate glasses at 353 nm light excitation, and the inset is the photos of them.
λex=352nm
E: Tm0.6Dy0.8
λex=353nm λex=357nm
Intensity(a.u)
λex=360nm
450
500
550
600
650
700
Wavelength(nm) Fig. 2. (a) Emission spectrum and (b) CIE chromaticity diagram of glasses E excited by 352, 353, 357 and 360 nm. The inset shows the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
important, the decay of the luminescence can be fitted to a single exponential. However, when the ions concentration is large enough, energy transfer appears and the decay curves become non-exponential. From all these decay curves, it is clearly observed that with increasing Dy3 þ ion concentration in all the co-doped glasses the lifetime of blue (452 nm) emission band of Tm3 þ is decreased, while the lifetime of yellow (575 nm) emission band of Dy3 þ is increased, as shown in Fig. 4(b), which is in accord with the report by Yu et al. [25]. The luminescence lifetime (τ) can be calculated by the following Eq. (1): Z τ ¼ I ðt Þdt=I0 ð1Þ
where I(t) is the luminescence intensity as a function of time t and I0 is the maximum I(t) that occurs at the initial time t0 [26]. Fitted by the Eq. (1), the lifetime values of 452 nm and 574 nm emissions are listed in Table 2. With an increase in the Dy3 þ ion concentration, the lifetime values of 452 nm emission decrease from 18.66 to 13.81 μs. This indicates that the potential energy transfer from Tm3 þ to Dy3 þ is stronger and stronger as the Dy3 þ ion concentration increases because of closing the interionic interaction distance between Tm3 þ and Dy3 þ . The lifetime values of the 574 nm emission assigned to the 4F9/2 - 6H13/2 transition of Dy3 þ ions increase from 0.48 to 0.82 ms as the Dy3 þ ion concentration increases from 0.2 to 0.8 mol%. Such results are in agreement with
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
9
C:Tm0.6Dy0.2
λex=353nm
D:Tm0.6Dy0.4
Intensity(a.u)
E:Tm0.6Dy0.8
450
500
550
600
650
700
Wavelength(nm) Fig. 3. (a) Emission spectrum of glasses C, D and E excited by 353 nm, and (b) CIE chromaticity diagram for glasses A–E under 353 nm excitation. The inset shows the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples and the photo of glass E under 353 nm light excitation, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Decay curves of the 452 nm and 575 nm emissions in the glass samples with different Dy3 þ concentrations under the 353 nm excitation.
Table 2 Lifetimes and Tm–Dy energy transfer efficiency (ηET1, ηET2) of the 452 nm and 575 nm emissions in the glass samples with different Dy3 þ and Tm3 þ concentrations excited at 353 nm. Glass no.
τ452 (us)
τ574 (ms)
ηET1 (%)
ηET2 (%)
Tm0.6 Tm0.6Dy0.2 Tm0.6Dy0.4 Tm0.6Dy0.8 Dy0.8 Tm0.4Dy0.8 Tm0.8Dy0.8
20.74 18.66 15.45 13.81 ‒ 11.09 16.79
‒ 0.48 0.66 0.82 1.10 0.95 0.63
‒ 10.03 25.51 33.41 ‒ ‒ ‒
‒ ‒ ‒ 25.45 ‒ 13.64 42.73
the results of the spectrum measurement, as shown in Fig. 3(a). From these decay lifetime values, the Tm–Dy energy transfer efficiency (ET1) between RE3 þ ions can be calculated by the following equation (2) [27]:
ηET1 ¼ 1 ðτTm=Dy =τTm Þ
ð2Þ
where τTm and τTm/Dy are defined as the lifetimes for the Tm3 þ single-doped and Tm3 þ /Dy3 þ co-doped samples, respectively. From the lifetime values listed in Table 2, the values of the Tm–Dy energy transfer efficiency are calculated. Noticeably, with an increase of Dy3 þ content, the ET1 increases monotonously, as evidenced in Table 2. Such energy transfer is favored by the overlap between the Tm:1D2-3F4 emission and Dy:6H15/2-4I15/2 absorption [2,28,29], as it can be appreciated from the spectra shown in Fig. 5(a). Since
the emission band of Tm3 þ ions at 452 nm overlaps with the broad absorption band, the 452 nm emission light may be absorbed by the transition 6H15/2 -4I15/2 for Dy3 þ ions. Thus, a potential energy transfer process from Tm3 þ to Dy3 þ ions is indicated in Tm–Dy co-doped glass. And Fig. 5(b) shows the energy level scheme for the luminescence mechanisms of Tm3 þ and Dy3 þ ions. The Tm3 þ :1D2 and Dy3 þ :6P7/2 levels are first populated by the 353 nm excitation. The population processes of 6P7/2 and 1D2 can be described as follows by: 6H15/2 þhv - 6P7/2 and 3H6 þ hv - 1D2. The Dy3 þ and Tm3 þ ions in the ground state are excited to 6P7/2 and 1D2 state, respectively. Then, Tm3 þ ions deexcite nonradiatively to 1G4 level and Dy3 þ ions to 4F9/2 level. The final relaxations of Tm3 þ :1G4, and Dy3 þ :4F9/2 to their respective ground states generate blue (452 nm), green (482 nm), yellow (574 nm) and red (664 nm) emissions. With increasing Dy3 þ ion content, the interionic interaction distance between dopants becomes more and more close [3], a dominant energy transfer from Tm3 þ to Dy3 þ can easily occur (see energy level diagram in Fig. 5(b)): ET: Tm3 þ :1D2, Dy3 þ :6H15/2 - Tm3 þ :3F4, Dy3 þ :4I15/2. This phenomenon has been also confirmed by Liu et al. [3] in Tm–Dy ions codoped aluminoborosilicate glasses for white LEDs under UV light excitation. Then, Dy3 þ ions deexcite nonradiatively to 4F9/2 level, so the 4 F9/2 level has more energy to ground states to generate yellow (574 nm) emissions. From the emission spectra displayed in Fig. 3(a), it can be noticed that an enhancement of the Dy:4F9/ 6 2- H15/2 emission at the expense of a weakening of the Tm:1D2-3F4 emission occurs by increasing the Dy3 þ content from 0.2 to 0.8 mol%. These energy transfer processes are mainly based
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
20 16 12 8 4
425
450
475
0
500
Wavelength(nm)
1
G4
3 3 F2 F 3 3 H4 3
H5 3 F4 3
H6
452nm
24
4 3+ 6 Dy : H15/2 I15/2
400
6
D2
P7/2 4 G 4 11/2 I 4 15/2 F9/2
ET1 6 6
6
Tm
3+
F1/2
664nm
F4
1
482nm
3
Energy(103cm-1)
Intensity(a.u)
3+ 1 Tm : D2
28
574nm
Dy Tm
480nm
10
H5/2 7/2 9/2 11/2 13/2
H15/2
Dy3+
Fig. 5. (a) Overlap region between dysprosium 6H15/2 - 4I15/2 absorption (solid curve) and thulium 1D2 - 3F4 emission (red dotted curve), and (b) The energy level diagrams of Dy3 þ and Tm3 þ upon 353 nm excitation, and ET1 routes between them. The solid arrows stand for the absorption and emission transitions of rare earth ions, the dashed arrows stand for the non-radiative relaxations, and the curved arrows stand for the energy transfer between Dy3 þ and Tm3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
λex=353nm
F:Tm0.4Dy0.8 E:Tm0.6Dy0.8
Intensity(a.u)
G:Tm0.8Dy0.8
450
500
550
600
650
700
Wavelength(nm) Fig. 6. (a) Emission spectrum of glasses F, E and G excited by 353 nm, and (b) CIE chromaticity diagram for glasses A, B, F, E, G under 353 nm excitation. The inset shows the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
on Tm–Dy energy transition via resonant transfer modes, which enhances the emission intensity of Dy3 þ ions, and decreases those of Tm3 þ ions. Fig. 6(a) depicts the emission spectra of glasses F, E and G excited at 353 nm. The positions of emission peaks are nearly the same for the three samples. Spectral comparison in Fig. 6(a) indicates that the content of Tm3 þ has an important effect on the luminescence properties of glasses with a certain content of Dy3 þ . It is observed that the intensity of blue emission (482 nm), yellow emission (574 nm), red emission (663 nm) derived from the 4F9/2 - 6HJ (J¼15/2, 13/2, 11/2) transition of Dy3 þ ions decreases gradually with the increase of Tm3 þ concentration, which demonstrates the existence of energy transfer from Dy3 þ ions to Tm3 þ ions. Evidently, the relative intensity of the blue (452 nm) coming from the 1 D2 - 3F4 transition of Tm3 þ always enhances with increasing Tm3 þ ion concentration due to more direct absorption form pump absorption by Tm3 þ ions and the potential energy transfer from Dy3 þ to Tm3 þ ions. The maximum intensity of the 452 nm emissions is observed at 0.8 mol% Tm3 þ ions. In other words, the relative intensity of the blue emission versus yellow emission changes with an increase in the Tm3 þ ion concentration. The CIE 1931 chromaticity coordinates of the glasses A, B, F, E and G, which were calculated based on the corresponding emission spectra excited at 353 nm, are shown in Fig. 6(b). It reveals
that Tm3 þ and Dy3 þ single-doped SZPT glasses can emit blue and yellow light, respectively. Increasing of Tm3 þ content not only supplements the blue emission intensity but also decreases the yellow emission intensity, and it is clearly found that emission color changes from yellow to white. This is due to the increasing intensity of hypersensitive transition of Tm3 þ ion induced by the increment of Tm2O3 content and the decreasing of the yellow emission intensity of Dy3 þ ion. The inset of Fig. 6(b) displays that the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples, and the chromaticity coordinate of the glass E is (0.332, 0.320), which is the closest to the standard white light point (0.333, 0.333). The emission color of the glass E is white for the naked eye, and the white light is bright and dazzling, and the corresponding color temperature is 5513 K, as shown in Fig. 3(b). Fig. 7 shows the decay curves of the 452 and 574 nm emissions excited at 353 nm. Similarly, from all these decay curves, it is clearly observed that with increasing Tm3 þ ion concentration in all the co-doped glasses the lifetime of blue (452 nm) emission band of Tm3 þ is increased, while the lifetime of yellow (575 nm) emission band of Dy3 þ is decreased, as shown in Fig. 7(b). Fitted by the Eq. (1), the lifetime values of 452 nm and 574 nm emissions are also listed in Table 2. With an increase in the Tm3 þ ion concentration, the lifetime values of 574 nm emission decrease from 1.10 ms to 0.63 ms. This indicates that the potential energy transfer
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
11
Tm Dy
28
H15/2 G4
20 16 12 8 4
425
450
475
500
525
550
0
1 G4 3 3 F2 F 3 3 H4 3 H5 3 F4 3 H6
Wavelength(nm)
452nm
1
6 P7/2 4 G 4 11/2 I 4 15/2 F9/2
ET2 6 F1/2 6 H5/2 7/2 9/2
480nm
3+ 3 Tm : H6
400
1 D2
24
Intensity(a.u)
Dy : F9/2
Energy(103cm-1)
3+ 4
6
482nm 574nm 664nm
Fig. 7. Decay curves of the 452 nm and 575 nm emissions in the glass samples with different Tm3 þ concentrations under the 353 nm excitation.
11/2 13/2
3+
Tm
6 H15/2
Dy3+
Fig. 8. (a) Overlap region between dysprosium 3H6 - 1G4 absorption (solid curve) and thulium 4F9/2 - 6H15/2 emission (red dotted curve), and (b) The energy level diagrams of Dy3 þ and Tm3 þ upon 353 nm excitation, and ET2 routes between them. The solid arrows stand for the absorption and emission transitions of rare earth ions, the dashed arrows stand for the non-radiative relaxations, and the curved arrows stand for the energy transfer between Dy3 þ and Tm3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
from Dy3 þ to Tm3 þ is stronger and stronger as the Tm3 þ ion concentration increases because of closing the interionic interaction distance between Tm3 þ and Dy3 þ ions. The lifetime values of the 452 nm emission assigned to the 1D2 - 3F4 transition of Tm3 þ ions increase from 11.09 μs to 16.79 μs as the Tm3 þ ion concentration increases from 0.4 to 0.8 mol%. Such results are in agreement with the results of the spectrum measurement, as shown in Fig. 6(a). Based on these decay lifetime values, the Dy– Tm energy transfer efficiency (ET2) between RE3 þ ions can be calculated by the following Eq. (3):
ηET2 ¼ 1 ðτDy=Tm =τDy Þ
ð3Þ
where τDy and τDy/Tm are defined as the lifetimes for the Dy3 þ single-doped and Dy3 þ /Tm3 þ co-doped samples, respectively. From the lifetime values listed in Table 2, the values of the ET2 are calculated. Evidently, the ET2 increases monotonously with an increase of Tm3 þ content, as evidenced in Table 2. Likewise, such energy transfer is favored by the overlap between the Dy:4F9/2 - 6H15/2 emission and Tm:3H6 - 1G4 absorption, as it can be appreciated from the spectra shown in Fig. 8(a). Since the emission band of Dy3 þ ions at 482 nm overlaps with the broad absorption band, the 482 nm emission light may be absorbed by the transition 3H6 - 1G4 for Tm3 þ ions. Thus, a potential energy transfer process from Dy3 þ to Tm3 þ ions is indicated in Dy–Tm co-doped glasses. The energy level scheme for the luminescence mechanisms of Tm3 þ and Dy3 þ ions is shown in Fig. 8(b). A governing energy transfer from Dy3 þ to Tm3 þ can
easily occur: ET2:Dy3 þ :4F9/2, Tm3 þ :3H6 - Dy3 þ :6H15/2, Tm3 þ :1G4. These energy transfer processes are mainly based on Dy–Tm energy transition via resonant transfer modes, which enhances the emission intensity of Tm3 þ ions, and decreases that of Dy3 þ ions.
4. Conclusion Tm3 þ /Dy3 þ co-doped SZPT glass samples are prepared via melting-quenching method. The luminescence properties and energy transfer mechanism between Tm3 þ and Dy3 þ ions have been investigated. The obtained conclusions are as follows: 1. Both excitation wavelength and ions doping concentration have an important effect on the luminescence properties in the asprepared co-doped glass samples. 2. Mutual energy transfer between Dy3 þ and Tm3 þ ions exists, and the potential energy transfer becomes stronger as the Tm3 þ or Dy3 þ ion concentration increases. 3. Tm3 þ and Dy3 þ single-doped samples excited by 353 nm can emit blue and yellow light, respectively. The tunable luminescent color can be realized with increasing Dy3 þ or Tm3 þ contents. And white light emission can be achieved in the Tm3 þ , Dy3 þ co-doped glass samples. This finding provides an easy approach to fabricate luminescent glass with tunable optical parameters and finds their broad applications as potential candidate for white light emitting diodes.
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC No.51362005), Natural Science Foundation of Guangxi Province, China (2013GXNSFDA019026), and Program for Postgraduate Joint Training Base of GUET-GLESI (No. 20141103-12-Z).
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Luminescent properties and energy transfer behavior between Tm3 þ and Dy3 þ ions in co-doped phosphate glasses for white LEDs Guo-hua Chen a,b,n, Le-qi Yao a, Hai-ji Zhong a, San-chuan Cui a a b
College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 January 2016 Received in revised form 26 April 2016 Accepted 19 May 2016 Available online 26 May 2016
In order to obtain white light-emitting materials, the luminescent glasses composed of SrO–ZnO–TiO2– P2O5 co-doped with Tm3 þ and Dy3 þ ions were synthesized by a conventional melting-quenching method. The luminescent properties of Tm3 þ and Dy3 þ in zinc strontium phosphate glasses were investigated, where the emission spectra show possible energy transfer mechanism between Tm3 þ and Dy3 þ ions. The energy transfer carries out via resonant transfer modes, which is easily understood from Tm3 þ and Dy3 þ energy level diagrams. The decreasing in mean-duration time of Tm3 þ :1D2 and Dy3 þ :4F9/2 obtained from the decay curves makes a further evidence of energy transfer from Tm3 þ to Dy3 þ ions and Dy3 þ to Tm3 þ ions in the glasses. & 2016 Elsevier B.V. All rights reserved.
Keywords: Fluorescence glass White LEDs Energy transfer Optical properties
1. Introduction Currently, white light emitting diodes (W-LEDs) have been considered as a potential candidate for lighting source due to their merits of long lifetime, environmental friendliness and high efficiency, compared with conventional incandescent and fluorescence lamps [1–4]. Up to now, W-LEDs have been realized by two methods: mixing blue, green and red phosphors in appropriate proportion or one single phosphor that emit blue, green and red emissions, excited by a UV LED chip [5,6]. Although white LEDs combining a blue GaN LED chip with yellow phosphor YAG:Ce3 þ have been commercialized, they have some inherent drawbacks such as poor heat resistance, high correlated color temperature and low color rendering index due to their low thermal quenching temperature and lack of red emitting element [7]. Considering these problems, it is urgent to develop pollution-free, cheap, highly efficient white-light-emitting (WLE) materials. Compared with common phosphors used for W-LEDs, white light emitting glasses are considered to be as an alternative due to their homogeneous light emitting, simpler manufacture procedure, lower production cost, free of halo effect, and better thermal stability [3,8]. Since white light-emitting glass was first developed [9], many efforts have been made to develop luminescent glasses for white LEDs [10–13]. Nowadays, a growing number of attentions are focused on such glasses doped with rare-earth ions [3,14]. However, n
Corresponding author at: College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China. Tel.: þ 86 7732291957; fax: þ86 7732191903. E-mail addresses: [email protected], [email protected] (G.-h. Chen). http://dx.doi.org/10.1016/j.jlumin.2016.05.034 0022-2313/& 2016 Elsevier B.V. All rights reserved.
only a few studies are concerned on phosphate glasses [15,16]. Phosphate glasses own many merits, such as low softening and melting temperature, low product cost, high luminous ion doped concentration, large emission and absorption cross sections and high transparency for visible light and optical characteristics [17,18]. Dy3 þ is one of the important rare-earth ions which play a major role in the production of different types of light-emitting materials, since Dy3 þ ions possess intense emission in the blue and yellow regions, which are assigned to the 4F9/2-6H15/2 and 4F9/2-6H13/2 transitions respectively [19]. On the other hand, Tm3 þ ion 4f to 5d transitions have received much attention and there have been a lot of reports on Tm3 þ ion doped luminescent materials in the last several years for white light emitting diode [12]. Also, there have mutual energy transfer between Dy3 þ and Tm3 þ ions in other glasses [2]. In the present work, Dy3 þ /Tm3 þ single-doped and Dy3 þ /Tm3 þ co-doped zinc strontium phosphate glasses were successfully prepared by the conventional melt-quenching method and their luminescence and color coordinates were investigated by varying doping ion concentrations [20]. Furthermore, the energy transfer between Dy3 þ and Tm3 þ was also discussed. Our research, thus, wishes to provide new information in this glass system by restraining the shortcomings. This work aims to (i) investigate the luminescence properties of Tm3 þ and Dy3 þ co-doped phosphate glasses; and (ii) reveal the energy transfer mechanism between Tm3 þ and Dy3 þ ions.
2. Experiments and measurements Base glasses with the chemical composition 15SrO–43ZnO– 40P2O5–2TiO2 (SZPT, similarly hereinafter) are separately prepared
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
by doping Tm2O3 or Dy2O3 individually and also with Tm2O3 and Dy2O3 in dual combinations for different concentrations as follows in Table 1. The chemicals used in the preparation of glasses were reagent grade (Z 99%) NH4H2PO4, ZnO, SrCO3, TiO2, and high purity (Z 99.99%) rare earth ions Tm2O3 and Dy2O3 (Guo-Yao Co. Ltd, Shanghai, China). All these chemicals were weighed in 30 g batch each separately, thoroughly mixed and finely powdered using agate mortar and pestle. Then each batch of chemical mix was transferred into corundum crucible and melted in an electric furnace for 2 h at 1300 °C. These melts were quenched in between two smooth surfaced brass plates to obtain circular glass discs of 1–1.5 mm in diameter with 0.3 cm in thickness. Later, these prepared glasses were cut and the surfaces were polished, and taken to characterize luminescent properties. The optical absorption spectra were recorded on Shimadzu UV– vis–NIR spectrophotometer (UV-3600) in the wavelength range 200– 900 nm. The excitation and emission spectra, fluorescence lifetimes were recorded with a model FL3-P-TCSPC fluorescence spectrophotometer with a Xenon lamp as the excitation source. All measurements were performed at room temperature and with the same instrument parameters. The chromaticity coordinates (x, y) were calculated based on the photoluminescent spectra by using software.
3. Results and discussion Fig. 1(a) shows the room temperature excitation spectra of the Tm3 þ (λem ¼452 nm) and Dy3 þ (λem ¼574 nm) single-doped phosphate glass samples. For the Tm3 þ doped sample, the excitation band at 350–375 nm for the 452 nm blue light emission is discovered. The main excitation band peaking at 356 nm, which is assigned to the Tm3 þ :3H6-1D2 transition monitored at 452 nm. For the Dy3 þ doped sample, monitoring the Dy emission at 574 nm, corresponding to the 4F9/2-6H13/2 transition, the excitation maximum is located around 347 nm corresponding to 6H15/ 6 2- P7/2 transition. The other secondary excitation peaks, detected around 321 nm, 363 nm and 385 nm comply with the transitions 6 H15/2-6P3/2, 6H15/2-6P5/2 and 6H15/2-4F7/2, respectively [21–23]. Fig. 1(b) shows optical absorption spectra of the Tm3 þ and Dy3 þ single-doped SZPT glass samples recorded at room temperature in the visible region. It is observed that rare earth ion single-doped glass samples exhibit different absorption levels, and the prominent levels observed were assigned to the appropriate electronic transitions, as follows: Dy3 þ : 6 H15=2 -6 P7=2 ; 4 I11=2 ; 4 I13=2 ; 4 I15=2 ; 6 F3=2 ; 6 F5=2 Tm3 þ :3 H6 -1 D2 ; 1 G4 ; 3 F3 ; 3 H4
It can be noticed that both absorption and excitation bands of Dy3 þ and Tm3 þ single-doped SZPT glass samples have identical overlap from 350 to 370 nm. It indicates that the Tm3 þ /Dy3 þ codoped phosphate glasses can be efficiently excited by UV light. Therefore, the 353 nm was chosen as the exciting wavelength in the following experiments. The emission spectra of the two samples at 353 nm light excitation are shown in Fig. 1(c). The Tm3 þ doped sample displays one intense blue emission band centered at 452 nm derived from the Tm3 þ :1D2-3F4 transition, and small greenish blue emission bland of 1G4-3H6 for Tm3 þ at 480 nm. The Dy3 þ doped sample exhibits two intense bands centered around 482 nm (greenish blue), 574 nm (yellow) and a weak band around 663 nm (red), corresponding to the 4F9/2-6HJ (J¼ 11/2, 13/2,15/2) transitions, respectively, in which the intensity of the 4F9/2-6H13/2 yellow emission is a hypersensitive (forced electric-dipole) transition, and it exhibits a strong dependence on the ligand environment, whereas the 4F9/2-6H15/2 blue emission is insensitive to the host [24]. Thus, the yellow-to-blue (Y/B) emission intensity ratio can
7
Table 1 Chemical composition of Re3 þ (Re3 þ ¼ Tm3 þ , Dy3 þ ) ion doped phosphate glasses (mol.%). Glass no.
SrO
ZnO
P2O5
TiO2
Tm2O3
Dy2O3
A B C D E F G
15 15 15 15 15 15 15
43 43 43 43 43 43 43
40 40 40 40 40 40 40
2 2 2 2 2 2 2
0.6 0 0.6 0.6 0.6 0.4 0.8
0 0.8 0.2 0.4 0.8 0.8 0.8
differ significantly in different hosts. As shown in the inset of Fig. 1(c), we took the photos of the two samples under the 353 nm excitation. The Tm3 þ and Dy3 þ single-doped phosphate glasses display the blue and yellow colors, respectively. Fig. 2 shows the emission spectrum and CIE chromaticity diagram of glasses E excited by 352, 353, 357 and 360 nm. With the increase of excitation wavelength, it is clearly found that emission color changes from yellow to blue, as shown in Fig. 2(b), which indicates that excitation wavelength has an important effect on the luminescence properties. Fig. 3(a) shows the emission spectra of glasses C, D and E excited at 353 nm. The positions of emission peaks are nearly the same for the three samples, but the existence of small band of 1G4 -3H6 for Tm3 þ at 480 nm is uncertain, because it may be overlapped by the strong emission band of 4F9/2-6H15/2 of Dy3 þ ions at 482 nm. Blue, yellow and red emission bands are observed in the emission spectra of these glasses. Spectral comparison in Fig. 3(a) indicates that the content of Dy3 þ has an important effect on the luminescence properties of glasses with a certain content of Tm3 þ . It is observed that the intensity of blue (452 nm) coming from the 1D2-3F4 transition of Tm3 þ decreases gradually with the increase of Dy3 þ concentration, which demonstrates the existence of energy transfer from Tm3 þ ions to Dy3 þ ions. Evidently, the relative intensity of the Blue emission (482 nm), yellow emission (574 nm), red emission (663 nm) derived from the 4F9/2-6HJ (J¼15/2, 13/2, 11/2) transition of Dy3 þ ions always enhances with increasing Dy3 þ ion concentration due to more direct absorption form pump absorption by Dy3 þ ions and the potential energy transfer from Tm3 þ ions to Dy3 þ ions. The maximum intensity of the 574 nm emissions is observed at 0.8 mol% Dy3 þ ions. In other words, the relative intensity of the yellow emission versus blue emission changes with an increase in the Dy3 þ ion concentration [22]. The Commission Internationale de L'Éclairage (CIE) 1931 chromaticity coordinates of the glasses A–E, which were calculated based on the corresponding emission spectra excited at 353 nm, are shown in Fig. 3(b). It reveals that Tm3 þ and Dy3 þ single-doped SZPT glasses can emit blue and yellow light, respectively. Increasing of Dy3 þ content not only supplements the yellow emission intensity but also decreases the blue emission intensity, and it is clearly found that emission color changes from blue to white. This is due to the increasing intensity of hypersensitive transition of Dy3 þ ion induced by the increment of Dy2O3 content and the decreasing of the blue emission intensity of Tm3 þ ion. The inset of Fig. 3(b) displays that the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples, and the chromaticity coordinates of the glass E are (0.332, 0.320), which are the closest to the standard white light point (0.333, 0.333). The emission color of the glass E is white for the naked eye, and the white light is bright and dazzling, and the corresponding color temperature is 5513 K. Hence, a wide application as solid state color display materials could be provided. Fig. 4 shows the decay curves of the 452 and 574 nm emissions under excitation wavelength of 353 nm. As shown in Fig. 4(a) and (b), when the interaction between luminescent ions is not
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
347
Intensity(a.u)
H
Dy :λem=574 nm
353nm
356
385
D P
3+
Tm :glass
363 321
H
Τm :λem=452 nm
Intensity(a.u)
8
I 353nm H G I
F3
3+
Dy :glass
320
340
360
380
300
400
400
500
F9/2
Intensity (a.u.)
600
700
800
Wavelength(nm) 4
D2
H4 F3/2 F5/2
Wavelength(nm)
1
I
6
H13/2 574
3
452
F4 Dy Tm 6 F9/2 H11/2
4
482
1
480 450
G4
4
F9/2
3
H6
500
6
H15/2
663
550
600
650
700
Wavelength (nm) Fig. 1. (a) Excitation and (b) absorption spectra of the Tm3 þ (λem ¼ 452 nm) and Dy3 þ (λem ¼574 nm) single-doped phosphate glasses. (c) Emission spectra of the Tm3 þ and Dy3 þ single-doped phosphate glasses at 353 nm light excitation, and the inset is the photos of them.
λex=352nm
E: Tm0.6Dy0.8
λex=353nm λex=357nm
Intensity(a.u)
λex=360nm
450
500
550
600
650
700
Wavelength(nm) Fig. 2. (a) Emission spectrum and (b) CIE chromaticity diagram of glasses E excited by 352, 353, 357 and 360 nm. The inset shows the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
important, the decay of the luminescence can be fitted to a single exponential. However, when the ions concentration is large enough, energy transfer appears and the decay curves become non-exponential. From all these decay curves, it is clearly observed that with increasing Dy3 þ ion concentration in all the co-doped glasses the lifetime of blue (452 nm) emission band of Tm3 þ is decreased, while the lifetime of yellow (575 nm) emission band of Dy3 þ is increased, as shown in Fig. 4(b), which is in accord with the report by Yu et al. [25]. The luminescence lifetime (τ) can be calculated by the following Eq. (1): Z τ ¼ I ðt Þdt=I0 ð1Þ
where I(t) is the luminescence intensity as a function of time t and I0 is the maximum I(t) that occurs at the initial time t0 [26]. Fitted by the Eq. (1), the lifetime values of 452 nm and 574 nm emissions are listed in Table 2. With an increase in the Dy3 þ ion concentration, the lifetime values of 452 nm emission decrease from 18.66 to 13.81 μs. This indicates that the potential energy transfer from Tm3 þ to Dy3 þ is stronger and stronger as the Dy3 þ ion concentration increases because of closing the interionic interaction distance between Tm3 þ and Dy3 þ . The lifetime values of the 574 nm emission assigned to the 4F9/2 - 6H13/2 transition of Dy3 þ ions increase from 0.48 to 0.82 ms as the Dy3 þ ion concentration increases from 0.2 to 0.8 mol%. Such results are in agreement with
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
9
C:Tm0.6Dy0.2
λex=353nm
D:Tm0.6Dy0.4
Intensity(a.u)
E:Tm0.6Dy0.8
450
500
550
600
650
700
Wavelength(nm) Fig. 3. (a) Emission spectrum of glasses C, D and E excited by 353 nm, and (b) CIE chromaticity diagram for glasses A–E under 353 nm excitation. The inset shows the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples and the photo of glass E under 353 nm light excitation, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Decay curves of the 452 nm and 575 nm emissions in the glass samples with different Dy3 þ concentrations under the 353 nm excitation.
Table 2 Lifetimes and Tm–Dy energy transfer efficiency (ηET1, ηET2) of the 452 nm and 575 nm emissions in the glass samples with different Dy3 þ and Tm3 þ concentrations excited at 353 nm. Glass no.
τ452 (us)
τ574 (ms)
ηET1 (%)
ηET2 (%)
Tm0.6 Tm0.6Dy0.2 Tm0.6Dy0.4 Tm0.6Dy0.8 Dy0.8 Tm0.4Dy0.8 Tm0.8Dy0.8
20.74 18.66 15.45 13.81 ‒ 11.09 16.79
‒ 0.48 0.66 0.82 1.10 0.95 0.63
‒ 10.03 25.51 33.41 ‒ ‒ ‒
‒ ‒ ‒ 25.45 ‒ 13.64 42.73
the results of the spectrum measurement, as shown in Fig. 3(a). From these decay lifetime values, the Tm–Dy energy transfer efficiency (ET1) between RE3 þ ions can be calculated by the following equation (2) [27]:
ηET1 ¼ 1 ðτTm=Dy =τTm Þ
ð2Þ
where τTm and τTm/Dy are defined as the lifetimes for the Tm3 þ single-doped and Tm3 þ /Dy3 þ co-doped samples, respectively. From the lifetime values listed in Table 2, the values of the Tm–Dy energy transfer efficiency are calculated. Noticeably, with an increase of Dy3 þ content, the ET1 increases monotonously, as evidenced in Table 2. Such energy transfer is favored by the overlap between the Tm:1D2-3F4 emission and Dy:6H15/2-4I15/2 absorption [2,28,29], as it can be appreciated from the spectra shown in Fig. 5(a). Since
the emission band of Tm3 þ ions at 452 nm overlaps with the broad absorption band, the 452 nm emission light may be absorbed by the transition 6H15/2 -4I15/2 for Dy3 þ ions. Thus, a potential energy transfer process from Tm3 þ to Dy3 þ ions is indicated in Tm–Dy co-doped glass. And Fig. 5(b) shows the energy level scheme for the luminescence mechanisms of Tm3 þ and Dy3 þ ions. The Tm3 þ :1D2 and Dy3 þ :6P7/2 levels are first populated by the 353 nm excitation. The population processes of 6P7/2 and 1D2 can be described as follows by: 6H15/2 þhv - 6P7/2 and 3H6 þ hv - 1D2. The Dy3 þ and Tm3 þ ions in the ground state are excited to 6P7/2 and 1D2 state, respectively. Then, Tm3 þ ions deexcite nonradiatively to 1G4 level and Dy3 þ ions to 4F9/2 level. The final relaxations of Tm3 þ :1G4, and Dy3 þ :4F9/2 to their respective ground states generate blue (452 nm), green (482 nm), yellow (574 nm) and red (664 nm) emissions. With increasing Dy3 þ ion content, the interionic interaction distance between dopants becomes more and more close [3], a dominant energy transfer from Tm3 þ to Dy3 þ can easily occur (see energy level diagram in Fig. 5(b)): ET: Tm3 þ :1D2, Dy3 þ :6H15/2 - Tm3 þ :3F4, Dy3 þ :4I15/2. This phenomenon has been also confirmed by Liu et al. [3] in Tm–Dy ions codoped aluminoborosilicate glasses for white LEDs under UV light excitation. Then, Dy3 þ ions deexcite nonradiatively to 4F9/2 level, so the 4 F9/2 level has more energy to ground states to generate yellow (574 nm) emissions. From the emission spectra displayed in Fig. 3(a), it can be noticed that an enhancement of the Dy:4F9/ 6 2- H15/2 emission at the expense of a weakening of the Tm:1D2-3F4 emission occurs by increasing the Dy3 þ content from 0.2 to 0.8 mol%. These energy transfer processes are mainly based
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
20 16 12 8 4
425
450
475
0
500
Wavelength(nm)
1
G4
3 3 F2 F 3 3 H4 3
H5 3 F4 3
H6
452nm
24
4 3+ 6 Dy : H15/2 I15/2
400
6
D2
P7/2 4 G 4 11/2 I 4 15/2 F9/2
ET1 6 6
6
Tm
3+
F1/2
664nm
F4
1
482nm
3
Energy(103cm-1)
Intensity(a.u)
3+ 1 Tm : D2
28
574nm
Dy Tm
480nm
10
H5/2 7/2 9/2 11/2 13/2
H15/2
Dy3+
Fig. 5. (a) Overlap region between dysprosium 6H15/2 - 4I15/2 absorption (solid curve) and thulium 1D2 - 3F4 emission (red dotted curve), and (b) The energy level diagrams of Dy3 þ and Tm3 þ upon 353 nm excitation, and ET1 routes between them. The solid arrows stand for the absorption and emission transitions of rare earth ions, the dashed arrows stand for the non-radiative relaxations, and the curved arrows stand for the energy transfer between Dy3 þ and Tm3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
λex=353nm
F:Tm0.4Dy0.8 E:Tm0.6Dy0.8
Intensity(a.u)
G:Tm0.8Dy0.8
450
500
550
600
650
700
Wavelength(nm) Fig. 6. (a) Emission spectrum of glasses F, E and G excited by 353 nm, and (b) CIE chromaticity diagram for glasses A, B, F, E, G under 353 nm excitation. The inset shows the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
on Tm–Dy energy transition via resonant transfer modes, which enhances the emission intensity of Dy3 þ ions, and decreases those of Tm3 þ ions. Fig. 6(a) depicts the emission spectra of glasses F, E and G excited at 353 nm. The positions of emission peaks are nearly the same for the three samples. Spectral comparison in Fig. 6(a) indicates that the content of Tm3 þ has an important effect on the luminescence properties of glasses with a certain content of Dy3 þ . It is observed that the intensity of blue emission (482 nm), yellow emission (574 nm), red emission (663 nm) derived from the 4F9/2 - 6HJ (J¼15/2, 13/2, 11/2) transition of Dy3 þ ions decreases gradually with the increase of Tm3 þ concentration, which demonstrates the existence of energy transfer from Dy3 þ ions to Tm3 þ ions. Evidently, the relative intensity of the blue (452 nm) coming from the 1 D2 - 3F4 transition of Tm3 þ always enhances with increasing Tm3 þ ion concentration due to more direct absorption form pump absorption by Tm3 þ ions and the potential energy transfer from Dy3 þ to Tm3 þ ions. The maximum intensity of the 452 nm emissions is observed at 0.8 mol% Tm3 þ ions. In other words, the relative intensity of the blue emission versus yellow emission changes with an increase in the Tm3 þ ion concentration. The CIE 1931 chromaticity coordinates of the glasses A, B, F, E and G, which were calculated based on the corresponding emission spectra excited at 353 nm, are shown in Fig. 6(b). It reveals
that Tm3 þ and Dy3 þ single-doped SZPT glasses can emit blue and yellow light, respectively. Increasing of Tm3 þ content not only supplements the blue emission intensity but also decreases the yellow emission intensity, and it is clearly found that emission color changes from yellow to white. This is due to the increasing intensity of hypersensitive transition of Tm3 þ ion induced by the increment of Tm2O3 content and the decreasing of the yellow emission intensity of Dy3 þ ion. The inset of Fig. 6(b) displays that the CIE chromaticity coordinate (x, y) values of luminescent color for glass samples, and the chromaticity coordinate of the glass E is (0.332, 0.320), which is the closest to the standard white light point (0.333, 0.333). The emission color of the glass E is white for the naked eye, and the white light is bright and dazzling, and the corresponding color temperature is 5513 K, as shown in Fig. 3(b). Fig. 7 shows the decay curves of the 452 and 574 nm emissions excited at 353 nm. Similarly, from all these decay curves, it is clearly observed that with increasing Tm3 þ ion concentration in all the co-doped glasses the lifetime of blue (452 nm) emission band of Tm3 þ is increased, while the lifetime of yellow (575 nm) emission band of Dy3 þ is decreased, as shown in Fig. 7(b). Fitted by the Eq. (1), the lifetime values of 452 nm and 574 nm emissions are also listed in Table 2. With an increase in the Tm3 þ ion concentration, the lifetime values of 574 nm emission decrease from 1.10 ms to 0.63 ms. This indicates that the potential energy transfer
G.-h. Chen et al. / Journal of Luminescence 178 (2016) 6–12
11
Tm Dy
28
H15/2 G4
20 16 12 8 4
425
450
475
500
525
550
0
1 G4 3 3 F2 F 3 3 H4 3 H5 3 F4 3 H6
Wavelength(nm)
452nm
1
6 P7/2 4 G 4 11/2 I 4 15/2 F9/2
ET2 6 F1/2 6 H5/2 7/2 9/2
480nm
3+ 3 Tm : H6
400
1 D2
24
Intensity(a.u)
Dy : F9/2
Energy(103cm-1)
3+ 4
6
482nm 574nm 664nm
Fig. 7. Decay curves of the 452 nm and 575 nm emissions in the glass samples with different Tm3 þ concentrations under the 353 nm excitation.
11/2 13/2
3+
Tm
6 H15/2
Dy3+
Fig. 8. (a) Overlap region between dysprosium 3H6 - 1G4 absorption (solid curve) and thulium 4F9/2 - 6H15/2 emission (red dotted curve), and (b) The energy level diagrams of Dy3 þ and Tm3 þ upon 353 nm excitation, and ET2 routes between them. The solid arrows stand for the absorption and emission transitions of rare earth ions, the dashed arrows stand for the non-radiative relaxations, and the curved arrows stand for the energy transfer between Dy3 þ and Tm3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
from Dy3 þ to Tm3 þ is stronger and stronger as the Tm3 þ ion concentration increases because of closing the interionic interaction distance between Tm3 þ and Dy3 þ ions. The lifetime values of the 452 nm emission assigned to the 1D2 - 3F4 transition of Tm3 þ ions increase from 11.09 μs to 16.79 μs as the Tm3 þ ion concentration increases from 0.4 to 0.8 mol%. Such results are in agreement with the results of the spectrum measurement, as shown in Fig. 6(a). Based on these decay lifetime values, the Dy– Tm energy transfer efficiency (ET2) between RE3 þ ions can be calculated by the following Eq. (3):
ηET2 ¼ 1 ðτDy=Tm =τDy Þ
ð3Þ
where τDy and τDy/Tm are defined as the lifetimes for the Dy3 þ single-doped and Dy3 þ /Tm3 þ co-doped samples, respectively. From the lifetime values listed in Table 2, the values of the ET2 are calculated. Evidently, the ET2 increases monotonously with an increase of Tm3 þ content, as evidenced in Table 2. Likewise, such energy transfer is favored by the overlap between the Dy:4F9/2 - 6H15/2 emission and Tm:3H6 - 1G4 absorption, as it can be appreciated from the spectra shown in Fig. 8(a). Since the emission band of Dy3 þ ions at 482 nm overlaps with the broad absorption band, the 482 nm emission light may be absorbed by the transition 3H6 - 1G4 for Tm3 þ ions. Thus, a potential energy transfer process from Dy3 þ to Tm3 þ ions is indicated in Dy–Tm co-doped glasses. The energy level scheme for the luminescence mechanisms of Tm3 þ and Dy3 þ ions is shown in Fig. 8(b). A governing energy transfer from Dy3 þ to Tm3 þ can
easily occur: ET2:Dy3 þ :4F9/2, Tm3 þ :3H6 - Dy3 þ :6H15/2, Tm3 þ :1G4. These energy transfer processes are mainly based on Dy–Tm energy transition via resonant transfer modes, which enhances the emission intensity of Tm3 þ ions, and decreases that of Dy3 þ ions.
4. Conclusion Tm3 þ /Dy3 þ co-doped SZPT glass samples are prepared via melting-quenching method. The luminescence properties and energy transfer mechanism between Tm3 þ and Dy3 þ ions have been investigated. The obtained conclusions are as follows: 1. Both excitation wavelength and ions doping concentration have an important effect on the luminescence properties in the asprepared co-doped glass samples. 2. Mutual energy transfer between Dy3 þ and Tm3 þ ions exists, and the potential energy transfer becomes stronger as the Tm3 þ or Dy3 þ ion concentration increases. 3. Tm3 þ and Dy3 þ single-doped samples excited by 353 nm can emit blue and yellow light, respectively. The tunable luminescent color can be realized with increasing Dy3 þ or Tm3 þ contents. And white light emission can be achieved in the Tm3 þ , Dy3 þ co-doped glass samples. This finding provides an easy approach to fabricate luminescent glass with tunable optical parameters and finds their broad applications as potential candidate for white light emitting diodes.
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC No.51362005), Natural Science Foundation of Guangxi Province, China (2013GXNSFDA019026), and Program for Postgraduate Joint Training Base of GUET-GLESI (No. 20141103-12-Z).
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