Tunable emission from green to red in the GdSr2AlO5:Tb3+,Eu3+ phosphor via efficient energy transfer

Herein, a series of GdSr2AlO5:Tb3+,Eu3+ phosphors were successfully synthesized through a high temperature solid-state reaction, and their crystal structures as well as photoluminescence properties were investigated in detail. Compared to the intense emission of 5D0 → 7F1 or 5D0 → 7F2 transition of Eu3+, another strong emission corresponding to 5D0 → 7F4 was observed. Concentration quenching is not obvious in Tb3+ or Eu3+-doped GdSr2AlO5 because structure isolation and energy transfer (ET) of Gd3+ → Eu3+ and Gd3+ → Tb3+ were found. Moreover, the energy transfer process from Tb3+ to Eu3+ was verified by the overlap of luminescence spectra and the variation of lifetime. Energy transfer mechanism was determined to be a dipole–dipole interaction, and ET efficiency as well as quantum efficiency were also obtained. Moreover, the emission color of GdSr2AlO5:Tb3+,Eu3+ can be tuned from green to red by altering the ratio of Tb3+/Eu3+. These results indicate that the GdSr2AlO5:Tb3+,Eu3+ phosphor is a promising single-component white light-emitting phosphor.


Introduction
White light-emitting diodes (w-LEDs) with superior advantages, such as high energy efficiency, good physical and chemical stability, long lifetime, and environment friendliness, have been drawing attention and are regarded as the next generation of illumination sources. [1][2][3][4] The most available material for light devices involves Ce 3+ -doped yttrium aluminum garnet (YAG:Ce 3+ ) with blue-LED chips, which has been commercialized. 5,6 Because of the scarcity of red component, it encounters a low color rendering index (R a < 80) and a high color temperature (CCT > 7000 K). 7,8 To overcome these drawbacks, the combination of a UV LED chip with red (Ca 4 (PO 4 )O:Eu 2+ ), 9 green (Na 2 Y 2 B 2 O 7 :Ce 3+ ,Tb 3+ ), 10 and blue (Na x Ca 1Àx Al 2Àx Si 2+x O 8 :Eu 2+ ) 11 to achieve white-light with high R a and appropriate CCT is another approach. However, the mixture of different phosphors brings about an inevitable problem of uorescence reabsorption, resulting in the loss of luminous efficiency. 12,13 Therefore, it is obligatory to develop a single-phase white lightemitting phosphor.
Rare earth ion-doped aluminate phosphors, such as LaSr 2 -AlO 5 :Ce 3+ /Eu 2+ , 14,15 (La,Gd)Sr 2 AlO 5 :Ce, 16 and (La,Gd)Sr 2 (Al,B) O 5 :Ce, 17 have been studied extensively due to their cheap raw materials and good chemical and physical stability. The structural properties of this aluminate-based phosphor and the effect on the luminescence properties have been investigated. However, the luminescence of Ce 3+ and Eu 2+ belong to the fd transition. 18 Upon reviewing the literature, it is observed that it is difficult to nd other rare-earth ions with f-f transition for doping into the GdSr 2 AlO 5 phosphor. As is well known, codoping of different rare ions as a sensitizer and activator plays a signicant role in obtaining emission via energy transfer processes. 19 Tb 3+ and Eu 3+ with characteristic green 20,21 and red 22,23 emissions are widely used in luminescent materials, respectively. Compared to most Eu 3+ -doped phosphors with the dominant emission corresponding to 5 D 0 / 7 F 1 or 5 D 0 / 7 F 2 , the Eu 3+ -doped GdSr 2 AlO 5 phosphor shows an intensive emission related to 5

Experimental method
A series of phosphors with the composition of GdSr 2 AlO 5 :Tb 3+ , GdSr 2 AlO 5 :Eu 3+ , and GdSr 2 AlO 5 :Tb 3+ ,Eu 3+ were prepared by a conventional high temperature solid-state method under This journal is © The Royal Society of Chemistry 2018 RSC Advances PAPER a reductive atmosphere (5% H 2 + 95% N 2 ) at 1500 C. SrCO 3 (A. R.), Al 2 O 3 (A. R.), Gd 2 O 3 (A. R.), Tb 4 O 7 (99.99%), and Eu 2 O 3 (99.99%) were used as raw materials. They were stoichiometrically weighed and mixed by grinding in an agate mortar for 30 min. Aer this, they were transferred into ceramic crucibles and calcined in a high-temperature tubular furnace at 1500 C for 4 h at a heating rate of 3 C min À1 . Finally, the samples were cooled down to room temperature in the furnace and ground into powders for subsequent use.
Bruker D8 (voltage 40 kV and current 40 mA) over the 2q range from 10 to 70 with CuKa radiation (l ¼ 1.54178 A) was used to obtain the XRD patterns of GdSr 2 AlO 5 :Tb 3+ ,Eu 3+ samples. Hitachi F-7000 spectrophotometers with a 150 W xenon lamp light source were used to obtain the excitation and emission spectra. The morphology of the prepared phosphor was examined by a eld-emission scanning electron microscope (Hitachi S-4800). The decay curves were obtained using the Hamamatsu Quantaurus-Tau C11367 uorescence spectrophotometer. The chromaticity data were calculated by the CIE1931 soware. Photoluminescence absolute quantum efficiency (QE) was determined via Hamamatsu Quantaurus-QY C11347 using an integrated sphere. All the measurements were conducted at room temperature. Fig. 1(a) shows the XRD patterns of the as-prepared GdSr 2 AlO 5 host and Tb 3+ , Eu 3+ -doped phosphor as well as the standard patterns of EuSr 2 AlO 5 (JCPDS 70-2197). The patterns agreed with the crystallographic structure of the JCPDS card 70-2197, and impurity peaks were not found. This result illustrates that the obtained samples are in single phase, and doping of Tb 3+ and/or Eu 3+ ions at low concentrations does not change the crystal structure. In addition, it can be observed from Fig. 1(b) that the diffraction peak positions of the samples are shied slightly to a higher angle as compared to those of the standard patterns of EuSr 2 AlO 5 (JCPDS 70-2197); this can be due to the replacement of large-size Eu 3+ sites by the smaller-size trivalent ions. Hence, RE 3+ ions were effectively doped into the lattice; this could be conrmed by the slight change in the diffraction peaks. Fig. 1(c) displays a partial structure of GdSr 2 AlO 5 . As reported previously, the crystal structure of GdSr 2 AlO 5 is isostructural with the standard le of EuSr 2 AlO 5 (JCPDS no. 70-2197, tetragonal, space group I4/mcm). 28 Gd atoms will occupy the half 8h sites, and the other half is occupied by the Sr atoms. The cell also contains 4a sites, which are fully occupied by the Sr atoms, and the Al atoms will occupy the 4b sites. The 4c sites and 16l sites are occupied by O, denoted as O 1 and O 2 , respectively. The GdO 8 polyhedron consists of two O 1 atoms and six O 2 atoms, where the latter are shared by four AlO 4 tetrahedral on the adjacent layers, forming an isolated structure. Fortunately, isolation in space weakens the interaction between the paired ions. To some extent, the concentrating quenching effect would be avoided. 29 3.2 SEM with energy dispersive X-ray analysis sample. The surface image shows irregular particles with a size range of 5-10 mm. Moreover, as shown in Fig. 2(b), EDS is used to analyze the chemical composition of the GdSr 2 AlO 5 :Tb 3+ ,Eu 3+ phosphor, and the result conrms the presence of Gd, Sr, Al, O, Tb, and Eu elements in the phosphor. Compared with the abovementioned XRD results, the EDS results demonstrate that Tb/Eu ions have been successfully incorporated into the GdSr 2 AlO 5 lattice.

Luminescence properties
The excitation spectrum of GdSr 2 AlO 5 :2%Tb 3+ is shown in Fig. 3(a). The excitation spectrum monitored at 548 nm exhibits a broad band and several weak peaks. The broad band from  This journal is © The Royal Society of Chemistry 2018 200 nm to 250 nm centered at 220 nm is ascribed to the fd transitions of the Tb 3+ ions, whereas the excitation peaks at 275 and 310 nm originate from the transitions of 8 S 7/2 / 6 I 7/2 and 8 S 7/2 / 6 P 7/2 of the Gd 3+ ions, respectively. 30,31 The appearance of strong excitation sharp bands of Gd 3+ ions conrms that there is an efficient energy transfer from Gd 3+ to Tb 3+ . The emission spectra of the phosphor under 220, 275, and 310 nm excitation in the wavelength range from 400-650 nm are also depicted in Fig. 3(a). There are two groups of lines in the emission spectrum: one group in the region from 400 to 480 nm is derived from the 5 D 3 / 7 F J (J ¼ 5, 4, and 3) transitions of Tb 3+ . The second groups of lines in the wavelength range from 480 to 650 nm originate from the 5 D 4 / 7 F J (J ¼ 6, 5, 4, and 3) transitions. Among these emissions, the 5 D 4 / 7 F 5 transition with a maximum at about 548 nm exhibits the strongest emission intensity. The emission intensity increases with the increasing concentration of Tb 3+ , as presented in Fig. 3(b). Concentration quenching does not occur in GdSr 2 AlO 5 :Tb 3+ . Fig. 3(c) and (d) present the excitation and emission spectra as well as concentration-dependent luminescence intensity of GdSr 2 AlO 5 :yEu 3+ . The excitation spectrum of the Eu 3+ -activated sample monitoring the emission wavelength at 705 nm contains a broad band in the wavelength range from 220 to 350 nm, corresponding to the charge transfer transition of Eu 3+ (O 2À / Eu 3+ ) and several characteristic narrow peaks at 383 nm and 393 nm. They are attributed to 7 F 0 / 5 G 2 and 7 F 0 / 5 L 6 electronic transitions of the Eu 3+ ions. In this band, there should be absorption of transitions from 8 S 7/2 to 6 I 7/2 level of Gd 3+ , which is too weak and is overlapped with the strong CTB of the O 2À / Eu 3+ . 32,33 The emission spectrum consists of multiple band emissions at 587, 607, 622, 657, and 705 nm, which are assigned to the 5 D 0 / 7 F J (J ¼ 0, 1, 2, 3, and 4) transitions of Eu 3+ ions, 34 respectively. Among these emission peaks, the two most intense lines are the emissions located at 622 and 705 nm. Obviously, the magnetic dipole transition of 5 D 0 / 7 F 1 is stronger than the 5 D 0 / 7 F 2 electronic transition; this indicates that Eu 3+ ions in the GdSr 2 AlO 5 host occupy a site with a non-inversion symmetry; this is consistent with its structure. Compared to other Eu 3+ phosphors, GdSr 2 AlO 5 :Eu 3+ provides more red components to improve R a . Similarly, there is no evident concentration quenching with the change in the Eu 3+ concentration.
The foundation of energy transfer is the spectral overlapping Tb 3+ emission and the Eu 3+ excitation. As is clearly shown in Fig. 4, there are some overlaps between the emission of 5 D 3 / 7 F 6 (Tb 3+ ) and the excitation of 7 F 0 / 5 G 2,3,4,5 , 5 L 8 (Eu 3+ ).
Moreover, the excitation of 7 F 0 / 5 D 3 , 5 L 6 (Eu 3+ ) overlaps with the emission of 5 D 3 / 7 F 5 (Tb 3+ ). 35 To further investigate the energy transfer process, a series of samples of GdSr 2 AlO 5 :2%Tb 3+ ,x%Eu 3+ (x ¼ 0, 0.5, 1, 2, 3, and 5) were prepared whose emission spectra under the excitation at 275 nm are presented in Fig. 5(a). The characteristic sharp emission peaks of Tb 3+ and Eu 3+ are observed. The tendency in the variation of Tb 3+ and Eu 3+ differs signicantly. The emission intensity of Tb 3+ at 548 nm decreases remarkably with an increase in the concentration of Eu 3+ , whereas the emission intensity of Eu 3+ increases monotonously. Fig. 5(b) depicts the contribution of different ions in the corresponding emission spectra. The G/R (green to red) ratio continuously decreases notably. Based on the prerequisite of the concentration of Tb 3+ being a constant value, explanation for this phenomenon should be the evidence of energy transfer from Tb 3+ to Eu 3+ .
In spite of steady-state spectra, kinetic spectra are necessary to better analyze the energy transfer mechanism of Tb 3+ / Eu 3+ in GdSr 2 AlO 5 . Fig. 6(a) shows the uorescence decay curves of Tb 3+ emissions, which are obtained under 275 nm excitation and detected at 548 nm. The luminescence curves can be tted well with the double-exponential expression: 36,37 where I and I 0 represent the luminescence intensity at time t and 0, A 1 and A 2 are equal to constants, t is the time, and s 1 and s 2 are the rapid and slow lifetimes for exponential components, respectively. As a function of these parameters, the average luminescence lifetime (s) was determined by the following equation:  Fig. 4 Spectral overlapping of the PL spectrum of GdSr 2 AlO 5 :Tb 3+ and the PLE spectrum of GdSr 2 AlO 5 :Eu 3+ .
The values of s are calculated to decrease when the Eu 3+ concentration increases; this indicates that energy transfer occurs from Tb 3+ to Eu 3+ , as expected. Furthermore, the energy transfer efficiency is calculated by the expression as follows: 38 where h T represents the energy transfer efficiency, and I s and I so represent the luminescence intensity of Tb 3+ in the presence and absence of Eu 3+ , respectively. The results of h T are shown in Fig. 6(b). It can be found that the efficiencies from Tb 3+ to Eu 3+ increase with the increasing Eu 3+ concentration, which can reach the maximum value of 83%. Therefore, the energy transfer from Tb 3+ to Eu 3+ ions is efficient to improve Eu 3+ luminescence.

Energy transfer mechanism
The type of energy transfer should depend on either the exchange interaction or the multipolar interaction. It relies on the critical distance between the Tb 3+ (sensitizer) and Eu 3+ (activator), and the critical distance is expressed by the following formula: 39 where V represents the cell volume, N is equal to the number of cations in the unit cell, and x c is the total concentration of Tb 3+ and Eu 3+ . In this case, N ¼ 4 and V ¼ 498.64 A 3 . The corresponding distance is calculated to be more than 5 A, indicating that Tb 3+ / Eu 3+ energy transfer would tend to be a multipolar interaction. On account of energy transfer mechanism for multipolar interaction, the following expression can be given: 40,41 I so /I s f C n/3 (5) where I so and I s are the luminescence intensity of the sensitizer without and with activator. C represents the doping concentration of Tb 3+ and Eu 3+ ion. The values for n ¼ 6, 8, and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions. Results are illustrated in Fig. 7. The best linear t was obtained when n ¼ 6, which conrmed energy transfer through dipole-dipole mechanism. The detailed Gd 3+ / Tb 3+ , Gd 3+ / Eu 3+ , and Tb 3+ / Eu 3+ energy transfer processes are shown in Fig. 8. The electrons were excited from the 8 S 7/2 ground state to the 6 I J excited state of Gd 3+ under 275 nm excitation. Because the energy levels 5 D 3 of Tb 3+ and 5 D 1 state of Eu 3+ are lower than the 6 I J state of Gd 3+ in the diagram, the energy transfer processes from Gd 3+ / Tb 3+ and Gd 3+ / Eu 3+ occur simultaneously. In addition, the electrons absorbing energy were excited from the 7 F 6 ground state to the 5 D 3 excited state of Tb 3+ . The energy transfer process occurs between Tb 3+ and Eu 3+ activators since the 5 D 3 state of Tb 3+ lies higher than the 5 D 1 state of Eu 3+ . Then, the electrons from the excited state of 5 D 3 (Tb 3+ ) and 5 D 1 (Eu 3+ ) relaxed to 5 D 4 and 5 D 0 , respectively. Finally, green to red emission is yielded by electrons returning to their respective ground states.

CIE chromaticity coordinate and QE
The Commission Internationale de L'Eclairage (CIE 1931) chromaticity coordinates of GdSr 2 AlO 5 :2%Tb 3+ ,x%Eu 3+ phosphors under the excitation at 275 nm are presented in Fig. 9. The CIE coordinates shied from (0.320, 0.466) to (0.574, 0.403) for the samples x ¼ 0 to x ¼ 5% correspond to green, yellow-green, and red. It is clear that the coordinate Y changes linearly with the X, the expression of which can be shown as below: The equation proves that the range of chromaticity coordinates can be obtained by adjusting the ratio of Tb 3+ and Eu 3+ mathematically. The as-prepared phosphor emits bright tunable visible light, implying its potential application in multicolor displays, and meets the requirement for application in n-UV LEDs.
For photo-luminescence application, the importance of quantum efficiency (QE) should be considered. According to the method described by De Mello et al., QE can be calculated by the following equation: 42 where L S is the emission of the sample, E S equals to the spectrum of the sample excited by the light, and E R represents the spectrum of the excitation light without the sample. The results are listed in Fig. 10. Under 275 nm sexcitation, the calculated QE of GdSr 2 AlO 5 :2%Tb 3+ ,2%Eu 3+ is 78.5%, which demonstrates a high QE.

Conclusions
The GdSr 2 AlO 5 :Tb 3+ ,Eu 3+ phosphors were prepared by a high temperature solid-state reaction method, and their luminescence properties were investigated in detail. Due to REO 8 structure isolation, damping of the concentration quenching would occur. Energy transfer from Gd 3+ / Eu 3+ and Gd 3+ / Tb 3+ was found in the excitation spectrum. Moreover, the luminescence properties and the transient spectra conrmed the efficiency energy transfer from Tb 3+ to Eu 3+ . The mechanism it demonstrated is dipole-dipole interaction. GdSr 2 AlO 5 :-Tb 3+ ,Eu 3+ phosphors were able to provide multi-color emission from green, yellow, orange, and nally to deep red by the different ratios of Tb 3+ /Eu 3+ . Additionally, the QE can reach about 78.5%. All the results indicate that the series of GdSr 2 -AlO 5 :Tb 3+ ,Eu 3+ phosphors can be a candidate for color tunable luminescence materials used for n-UV LEDs.

Conflicts of interest
There are no conicts to declare.