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Abstract
A kind of Tm3+/Dy3+ co-doped fluorphosphate glass was prepared by the melt quenching method for continuous tunable multi-chromatic phosphors. The emission and excitation spectra of the glass showed that the color of luminescence can be tuned from blue/cold white to warm white/yellow by controlling the wavelength of exciting light. White light emission can be achieved under 353 nm and 362 nm UV light excitation. In addition, energy transfer mechanisms between Tm3+ and Dy3+ were also analyzed. The relationship between the color coordinate and the excited wavelength was also established. The Tm3+/Dy3+ co-doped fluorphosphate glass may be potential candidates for tunable multi-chromatic phosphor application.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
White light emitting diodes (W-LEDs) have been considered as a promising light source, owing to their much higher power efficiency and longer lifetime than that of conventional incandescent and fluorescent lamps [1–3]. The dominating method for LEDs to obtain white light is the combination of blue or UV chips and one or more phosphors [4, 5]. Unfortunately, such multi-component phosphors have some drawbacks, such as reabsorption and poor color balance caused by thermal quenching [6]. To overcome these deficiency, a single-component material with tunable emission is most desirable for using as multi-chromatic phosphor [7]. At present, most studies on tunable emission single-component phosphors focused on changing the species or concentration of doped ions, such as YAG: Ce (yellow) and YAG: Mn (red) [8–11]. However, it is still difficult to achieve the color continuous tunable, because a series of phosphors need to be prepared by precisely controlling the proportion of doped ions [12]. In contrast, a single material with continuous tunable emission under varying light excitation will be a great potential multi-chromatic emission phosphor.
Dy3+ is one of the important rare-earth ions which plays a major role in the production of different types of phosphors, since Dy3+ 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 [13–15]. More importantly, 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 [16–18]. When the hypersensitive transition is dominant in a Dy3+ doped material, it would appear strong yellow emission. On the other hand, Tm3+ ion has received much attention because its blue emission (4f to 5d transitions) centers at 450-455 nm. Also, such emissions are achieved by co-excitations of Tm3+/Dy3+ and controlling the energy transfer processes between Tm3+ and Dy3+ in suitable matrix, which allows to modulate the emission colors [14, 18–21].
Compared with common phosphor powder combined packaging materials, luminescent glasses are considered to be as an alternative due to their better thermal stability, homogeneous light emitting, high transparency, operational lifetime and simpler manufacture procedure. Many efforts have been made to develop rare-earth doped luminescent glasses for multi-chromatic phosphors [22–26]. Fluorophosphate glass possesses some notable properties, including low phonon energy, wide transmission range from near UV to mid-IR, low nonlinear refractive index, good stability and tailorable properties by varying the ratio of the fluoride and phosphate, which are more suitable to be the matrix of Tm3+/Dy3+ co-doped phosphors [27, 28].
In the present work, a class of Tm3+/Dy3+ co-doped fluorophosphate (FP) glass was designed and successfully prepared by the conventional melt-quenching method and its continuous tunable multi-chromatic emissions were investigated by varying UV light excitation. Furthermore, the energy transfer between Dy3+ and Tm3+ was also discussed. Significantly, the relationship between the color coordinate and the excited wavelength was also established, which was a good way to achieve color continuous tunable.
2. Experimental
The fluorphosphate glass with basic composition of 40NaPO3- 22(Al(PO3)3 + AlF3)- 18NaF- 20BaF2-1wt%Dy2O3-1wt%Tm2O3 (FP) were prepared with high-purity (~99.9%) raw materials according to a conventional melt-quenching method that employed alumina crucibles. All glass precursors were mixed in stoichiometric proportions, then melted by an electrical furnace at 980 °C for 30 min at ambient atmosphere. The bubble free and transparent glass melts were quenched on a preheated brass, annealed at a muffle furnace near the glass transition temperature (Tg) for 2 h, and then cooled down to room temperature at a rate of 1°C/min to remove the internal thermal strain. The obtained glass was cut and polished into the size of 10 mm × 15 mm × 1.5 mm for measurements. The photoluminescent (PL) spectra and the fluorescence decay lifetimes were recorded by a steady/transient state fluorescence spectrometer (FLS920P, Edinburgh, Edinburgh, UK). The UV-visible (VIS) transmission spectra were collected by using a UV/VIS/near infrared (NIR) spectrophotometer (Lambda 950, PerkinElmer, Waltham, USA) in the range of 200-800 nm. The chromaticity coordinates (x, y) were calculated based on the photoluminescent spectra by using a software. The photographs were recorded with a digital camera (D90, Nikon, Tokyo, Japan). All measurements were performed at room temperature.
3. Results and discussion
Figure 1(a) shows the optical absorption spectrum of the Tm3+/Dy3+ co-doped FP glass in the visible region. It is observed that the Tm3+/Dy3+ co-doped FP glass exhibits many absorption bands, which were assigned to the appropriate electronic transitions of Dy3+: 6H15/2→6P7/2, 4I11/2, 4I13/2, and Tm3+: 3H6→1D2. Figure 1(b) shows the photoluminescence excitation spectrum of the Tm3+/Dy3+ co-doped FP glass, which is monitored at the emission wavelengths of 575 (blue line) and 451 nm (red line). It can be seen that in the range from 340 to 370 nm there are three excitation bands attributed to Dy3+ (350, 364 nm) and Tm3+(358 nm), respectively. Furthermore, the three excitation bands have a overlapping region from 350 to 367 nm. Meanwhile, both absorption and excitation bands have an identical overlap, which means that the tunable excitation can be realized by changing the wavelength of UV light from 350 to 367 nm.
The emission spectra of the Tm3+/Dy3+ co-doped FP glass at 350-364 nm light excitation are shown in Fig. 2. It exhibits intense blue emission band centered at 451 nm derived from the Tm3+:1D2→3F4 transition, and greenish blue emission band of 1G4→3H6 for Tm3+ and 4F9/2→6H15/2 for Dy3+ at 477-480 nm, intense yellow emission band centered at 574 nm and a weak red emission band around 664 nm corresponding to the 4F9/2→6H(13/2, 11/2) transitions of Dy3+, respectively. As shown in Fig. 2(a), with the increase of excitation wavelength from 350 to 357 nm, it is clearly found that the intensity of the Tm3+ blue emission is increasing. But at the same time, the intensity of the Dy3+ yellow and red emissions are decreasing obviously. On the contrary, further increase excitation wavelength from 358 to 364 nm not only supplements the Dy3+ yellow emission intensity but also decreases the Tm3+ blue emission intensity, which indicates that excitation wavelength has an important effect on the luminescence properties.
Fig. 2 The emission spectra of the Tm3+/Dy3+ co-doped FP glass under the (a) 350-357 nm and (b) 358-364 nm excitation.
The Commission Internationale de L'Éclairage (CIE) 1931 chromaticity coordinates of the Tm3+/Dy3+ co-doped FP glass, which were calculated based on the corresponding emission spectra excited at 350-364 nm, are shown in Fig. 3(a, b). It is clearly found that emission color changes from yellow to blue by changing the excitation wavelength from 348 to 357 nm. With the further increase of excitation wavelength from 358 to 364 nm, it is clearly found that emission color changes from blue to yellow, the photographs of the glass under 358 nm, 362 nm and 365nm excitation are shown in Fig. 3(c). In other words, the relative intensity of the yellow emission versus blue emission changes with varying excitation wavelength.
Fig. 3 CIE(x, y) coordinate diagram that show the chromaticity points of the emissions under the (a) 348-357 nm excitation, (b) 358-365 nm excitation and the photographs of the glass under (c) 358 nm, 362 nm and 365 nm excitation.
Table 1 displays all the CIE chromaticity coordinate (x, y) values of Tm3+/Dy3+ co-doped FP glass under varying UV light excitation. As shown in Fig. 4(a), all coordinate points are depicted and fitted according to the following approximate equation:
where x and y are the values of CIE chromaticity coordinate (x, y), respectively. In order to achieve precise color control, the relationship between color coordinates and excitation wavelength was studied. As shown in Fig. 4(b), there are four intersections between the curve of color coordinates and the line of standard equal energy white light illumination (0.333), which are 353.25, 353.45, 362 and 362.2 nm, respectively. It means that by regulating the excitation wavelength, the Tm3+/Dy3+ co-doped FP glass can be used as a white light phosphor, which is close to the color coordinates of the standard equal energy white light illumination (0.333, 0.333). Besides, combining Fig. 4(a) with Fig. 4(b), the corresponding excitation wavelength can be found through the selected color coordinate. The luminescent quantum efficiency (QE) of the glass is measured using an integrating sphere at room temperature. The values of QE are listed in Table 2.
Table 1. The CIE coordinates of Tm3+/Dy3+ co-doped FP glass under varying UV light excitation
Fig. 4 (a) The relationship between x and y of color coordinates, (b) the relationship between excitation wavelength and CIE coordinates.
Table 2. The luminescent quantum efficiency of the FP glass
Figures 5(a) and (b) display the Tm3+ emission decay profiles at 451 nm in the Tm3+/Dy3+ co-doped FP glass excited at 350-364 nm. It can be noticed that the varying excitation wavelength (350-357 nm) strongly affects the decay evolution, which suggests that Tm3+ to Dy3+ non-radiative energy transfer takes place. On the contrary, there is weak non-radiative energy transfer when the excited wavelength changes from 358 nm to 364 nm. The Dy3+ emission decay profiles at 575 nm excited from 350 nm to 364 nm are shown in Figs. 5(c) and 5(d). The results show that with the increase of the excitation wavelength, the fluorescence lifetime becomes shorter firstly and then gradually gets longer after the lowest point excited at 357 nm, which suggests that Dy3+ to Tm3+ non-radiative energy transfer takes place. Table 3 displays all the decay lifetime values of Tm3+/Dy3+ co-doped FP glass excited at 350-364 nm. The Tm3+ and Dy3+ average lifetimes are calculated by changing the excitation wavelengths, resulting in 25.06 and 845.28 μs for the Tm3+ and Dy3+ single-doped FP glasses, respectively.
Fig. 5 The decay curves of the 451 nm emission under the (a) 350-357 nm excitation, (b) 358-364 nm excitation and the decay curves of the 575 nm emission under the (c) 350-357 nm excitation, (d) 358-364 nm excitation for the Tm3+/Dy3+ co-doped FP glass.
Table 3. The decay lifetimes and ET efficiency of Tm3+/Dy3+ co-doped FP glass under varying UV light excitation
The efficiency of Dy3+/ Tm3+ energy transfer (ET) are evaluated according to the following expressions:
where τ0 and τ are the donor lifetimes in absence and presence of acceptors, respectively. Noticeably, with varying the wavelength from 350 to 359 nm, the ET of Dy3+→Tm3+ increases monotonously and then gradually decreases, but the ET of Tm3+→Dy3+ has the opposite phenomenon, as evidenced in Table 3. It is observed that the energy transfer of Dy3+→Tm3+ and Tm3+→Dy3+ exists simultaneously, when the excitation wavelength changes from 350 to 356 and 363 to 364 nm. In contrast, there is only energy transfer of Dy3+→Tm3+ in the Tm3+/Dy3+ co-doped FP glass excited at 357-362 nm.Figure 6 shows the energy level scheme for the energy transfer mechanisms of Tm3+ and Dy3+ ions. The Dy3+ and Tm3+ ions in the ground state are excited to 6P7/2 and 1D2 states, respectively. Then, Tm3+ ions de-excite non-radiative transit to 1G4 level and Dy3+ ions to 4F9/2 level. The final relaxations of Tm3+:1G4, and Dy3+:4F9/2 to their ground states generate blue (451 nm), green (483 nm), yellow (574 nm) and red (664 nm) emissions, respectively. The energy transfer can be described as ET1, ET2 and ET3:
The above results of the efficiency indicate the energy transfer between Tm3+ ions and Dy3+ ions (ET1, ET2 and ET3) are associated with the excitation wavelengths. It is observed that there are governing ET2 and ET3 from Dy3+ to Tm3+ when the excitation wavelength is selected at 357-362 nm. In contrast, the ET1, ET2 and ET3 exist simultaneously, when the excitation wavelength change from 350 to 356 and 363 to 364 nm. Thus there is a possibility of tuning the chromaticity parameters of the single phosphor by varying the excitation wavelength.
4. Conclusions
In summary, the Tm3+/Dy3+ co-doped FP glass with the continuous tunable multi-chromatic emission, was prepared by the high-temperature melting method. The excitation and emission properties as well as the energy transfer between Tm3+ and Dy3+ ions were investigated. The relationship between excitation wavelengths and CIE coordinates was studied. The color of FP glass can be tuned from blue/cold white to warm white/yellow by precise controlling the wavelength of exciting light. Furthermore, the white light emission can be achieved under 353 nm and 362 nm UV light excitation, which are very close to the standard equal energy white light illumination (x = 0.333, y = 0.333). All of these results indicate that the Tm3+/Dy3+ co-doped FP glass would be one of the best potential candidates for tunable multi-chromatic single component phosphor.
Funding
National Key Research and Development Program of China (Grant No. 2016YFF0100901); the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYZZ16_0252); the Research Center of Optical Communications Engineering & Technology (Grant No. ZXF20170103).
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