A color-tunable NaCaY(PO4)2:Eu2+, Mn2+ was synthesized by a solid state reaction. NaCaY(PO4)2 crystallizes in the hexagonal structure system with space group of P6222 and Z = 1. The NaCaY(PO4)2:Eu2+ exhibits blue-greenish emission and broad excitation bands corresponding to the allowed f→d electronic transition of Eu2+. In addition, via the design of efficient energy transfer from Eu2+ to Mn2+, a high quality of white-emitting light could be generated in the optimized composition of NaCaY(PO4)2:1%Eu2+, 0.5%Mn2+ with CIE coordinates of (0.3389,0.3531) and CRI of 82, which is superior than that of blue chip and YAG phosphors. The results indicate that as-synthesized NaCaY(PO4)2:Eu2+, Mn2+ phosphors exhibits the potential to be an n-UV convertible phosphor.
© 2014 Optical Society of America
Nowadays, white light-emitting diodes (LEDs) are the most promising technique for applications of general lighting and display due to the merits of environmental friendly, energy saving, high brightness, and long life time [1,2]. The conventional approach for making white light are by using blue-emitting chip and yellow-emitting phosphor (Y3Al5O12:Ce3+). The drawbacks of these combinations, however, are low color-rendering index (CRI) and high correlated color temperature (CCT) due to red emission deficiency in the visible spectrum . From the points of view, the white LEDs fabricated using near ultraviolet (n-UV) LED (380~420 nm) coupled with red, green, and blue-emitting phosphors have attracted much attention due to the advantages of color purity and excellent color rendering. Thus, the development of new phosphors for UV LED applications is highly desirable. Among these systems, phosphate-based phosphors have drawn much attention, such as Ca5(PO4)2SiO4:Eu2+ , Mg3Ca3(PO4)4:Eu2+ , Na2Ca4(PO4)3F:Eu2+,Tb3+ , Rb2CaP2O7:Eu2+ , Ca15(PO4)2(SiO4)6:Eu2+ , Ca2YF4PO4:Eu2+, Mn2+ , Ca3Mg3(PO4)4 , Ca8La2(PO4)6O2:Ce3+/Eu2+ , and Sr4La2Ca4(PO4)6O2:Ce3+ . Apart from phosphate phosphors, other potential phosphors, such as Ba2Zn3Si3O11:Eu2+, Ba3Si6O9N4:Eu2+, Sr3Si6O3N8:Eu2+ and (Ba1-xSrx)9Sc2Si6O24:Ce3+, Li+ have also been studied and reported in 2013 [13–16]. To the best of our knowledge, the luminescence properties of NaCaY(PO4)2:Eu2+, NaCaY(PO4)2:Mn2+, and NaCaY(PO4)2:Eu2+,Mn2+ (NCYP:Eu,Mn) have not been reported in the literature. In this study, we report a novel emission-tunable white-emitting phosphor, NCYP:Eu,Mn, in which energy transfer mechanism between Eu2+ and Mn2+ was investigated.
In this paper, we first report on the crystal structure, luminescence properties as well as thermal stability and energy transfer mechanism between Eu2+ ion and Mn2+ ion. The emission hue of as-synthesized phosphors could be tuned by changing the ratio of Eu/Mn via the energy transfer between sensitizer of blue-emission Eu2+ and activator of red-emission Mn2+, respectively. The results demonstrate that NCYP:Eu2+, Mn2+ are potential phosphors for n-UV applications.
In this study, a series of NaCaY(PO4)2:Eu2+, Mn2+ phosphors were prepared by a solid-state reaction in where the constituent raw materials NaCO3 (99,99%), CaCO3 (99.99%), Y2O3, (NH3)2HPO4 (99.99%) Eu2O3 (99,99%) and MnO (99,99%) (all from Aldrich Chemicals, Milwaukee, WI, U.S.A) were weighed in stoichiometric proportions. The powder mixtures were sintered under a reducing atmosphere (15%H2/85%N2) at 1350°C for 8 h. The products were then cooled to room temperature in the furnace, ground, and pulverized for further measurements. The phase purity of the as-prepared samples were identified by powder X-ray diffraction (XRD) using a Bruker AXS D8 advanced automatic diffractometer with Cu-Kα radiation (λ = 1.5418 Å) operating at 40 kV and 30 mA. The XRD profiles were collected in the range of 10° < 2θ < 80°. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured at room temperature by a Spex Fluorolog-3 spectrofluorometer (Instruments S.A., N.J., U.S.A) equipped with a 450W Xe light source and double excitation monochromators. The powder samples were compacted and excited and emitted fluorescence was detected by a Hamamatsu Photonics R928 type photomultiplier perpendicular to the excitation beam. The spectral response of the measurement system is calibrated automatically on start up. The Commission International de I’Eclairage (CIE) chromaticity coordinates for all samples were determined by using a Laiko DT-100 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan).
3. Results and discussion
3.1. XRD analysis of as-synthesized NaCaY(PO4)2:1%Eu2, 10%Mn2+ phosphors
Figure 1 shows the XRD patterns of a series of NaCaY(PO4)2:1% Eu2+, 10% Mn2+ phosphors and standard KCaY(PO4)2 with JCPDS 51-1632. The XRD patterns of NaCaY(PO4)2 were in consistent with that of KCaY(PO4)2. The result indicates that NaCaY(PO4)2 and KCaY(PO4)2 were iso-structure. And via the doping of Eu2+ and Mn2+ into the host, no any impurity phase was detected by XRD measurements. According to the report of JCPDS 51-1632, NaCaY(PO4)2 crystallizes in the hexagonal structure with space group of P6222. The lattice constants of reported KCaY(PO4)2 are: a = 6.903 Å, c = 6.331 Å, V = 261.26 Å3 and Z = 1. The crystal structure of NaCaY(PO4)2, shown in the inset of Fig. 1, consists of chains of edge sharing (Ca,Y)O8 polyhedra interconnected by corner sharing. The crystal structure composes of PO4, CaO8, YO8 and NaO8 polyhedra in the lattice. The ionic radii for eight-coordinated Ca2+, Na+ and Y3+ atoms are 1.15, 1.18 and 1.02 Å, respectively. The ionic radius of Eu2+ for eight-coordinated is 1.25 Å, while that for eight-coordinated Mn2+ is 0.96 Å. Thus, for the consideration of ionic radii matching, these doping ions of Eu2+ and Mn2+ should occupy the Ca2+ ions sites in the NaCaY(PO4)2 host.
3.2. Luminescence properties of NaCaY(PO4)2:Eu2+ phosphor
Figure 2 shows the PL and PLE spectra of NaCaY(PO4)2:1%Eu2+. The excitation spectrum displayed a band from 300 nm to 450 nm, which is corresponded to the typically transition from the ground state of Eu2+ to its field-splitting levels at the 5d1 state. The broad band nature demonstrates that the as-synthesized phosphor could be excited from UV, nUV to blue LED applications. The PL spectrum shows that NaCaY(PO4)2:1%Eu2+ exhibited a blue-greenish emitting band peaking at 493 nm under optimal excitation at 376 nm. The Stoke shift is the difference between positions of the band maxima of the excitation and emission spectra, and it was estimated to be 6311 cm−1 in NaCaY(PO4)2:1%Eu2+. A symmetric emission band demonstrated that Eu2+ ions occupy 8-coordinated Ca2+ site due to the matching of ionic radii and charges. The inset of Fig. 2 displays the PL intensity of as a function of doped Eu2+ content. The optimal doping concentration was observed at 0.5 mol%. The PL intensity of NaCaY(PO4)2:Eu2+ was found to decline dramatically as the content of Eu2+ exceeded 0.5 mol% due to concentration quenching. The phenomenon of concentration quenching is mainly caused by energy transfer among Eu2+ ions and the possibility increases with the Eu2+ concentration. The internal quantum efficiency of NaCaY(PO4)2:0.5%Eu2+ and BaMgAl10O17:Eu2+ phosphor (Kasei Optonix Ltd.) were found to be 30.3% and 92.0% at the excitation wavelength of 365 nm. It is believed that the quantum efficiency of NaCaY(PO4)2:Eu2+ phosphors could be further enhanced by tuning the synthetic conditions.
Blasse  pointed out that the critical transfer distance (Rc) is approximately equal to twice the radius of a sphere with the volume of the unit cell:
The temperature dependence of the PL spectra of NaCaY(PO4)2:1%Eu2+ under excitation at 365 nm is shown in Fig. 3(a).The activation energy (Ea) can be expressed by
where Io and I are luminescence intensity of NCYP:1%Eu2+ at room temperature and the testing temperature, respectively; A is a constant and k is Boltzmann’s constant (8.617 × 10−5 eV K−1). By linear fitting, Ea was found to be 0.201 eV. The inset in Fig. 3(a) compare the thermal stability of as-synthesis NCYP:Eu and commercial phosphor – Ba2SiO4:Eu2+. The PL intensity of NCYP:1%Eu2+ at 100°C and 150°C was 80% and 65% of that measured at room temperature, respectively. The thermal stability is superior to that of commodity for higher temperature of 100°C. Figure 3(b) and inset show the CIE coordinates with different temperature. The deviation of x and y from 25°C to 150°C were 0.0048 and 0.0018, respectively, which demonstrate that its good stability of chromaticity against temperature.
Figure 4 shows the emission spectra of NCYP:1%Eu2+,x%Mn2+ phosphors (x = 0, 0.5, 1, 2, 4, 5, 7 and 10) under 376 nm excitation. The inset in Fig. 4 displays the energy transfer efficiency with different Eu2+/Mn2+ ratio in the NCYP host. By co-doping Eu2+ and Mn2+ in the host, the phosphors generated blue-greenish and red emission bands centering at 493 nm (4f65d1 → 4f7 transition of Eu2+) and 630 nm (4T1(4G) → 6A1(6S) transition of Mn2+). The intensity of Eu2+ blue emission at 493 nm decreased as the Mn2+ content increased to x. The intensity of red emission at 630 nm increased as the Mn2+ content increased, reached a maximum at x = 3 mol%, and then decreased when x exceeded 3 mol%. The apparent decrease in the PL intensity for Mn2+ with x > 3 mol% is primarily due to the concentration quenching effect. The energy transfer efficiency of Eu2+ and Mn2+ as a function of Mn2+ concentration showed energy transfer efficiency of 92%, which is much higher than other phosphate phosphors in our group [18,19]. Thus, NCYP:Eu,Mn could be a very potential phosphor for UVLED applications.
According to Dexter’s energy transfer formula of multi-polar interaction, the following relation can be obtained :Figures 5(a) and 5(b) illustrate the relationships between (IS0/IS) versus Cα/3, revealing a linear behavior only when α = 8. In most case, the energy transfer of Eu-Mn is diope-quadrupole mechanism. This implies that the energy transfer from sensitizer Eu2+ to activator Mn2+ follows a non-radiative dipole-quadrupole mechanism, which is similar to the results of previous reports [18,19]. Figures 6(a) and 6(b) demonstrate the corresponding CIE coordinates and photos of phosphors excited at 365 nm of NCYP:x%Eu2+ (x = 0.5, 1, 3, 5, 7 and 10) excited at 365 nm; (b) NCYP:1%Eu2+, y%Mn2+ (y = 0, 0.5, 1, 3, 5, 7 and 10), respectively. The CIE coordinates are (0.2940,0.3095), (0.3389,0.3531), (0.3902,0.3826), (0.4480,0.4076), (0.5000,0.4153), (0.5257,0.4134), and (0.5843,0.3939), respectively. The results indicate that the emission color of the novel NCYP:Eu2+,Mn2+ phosphors could be easily controlled by tuning the ratio of Eu2+/Mn2+ to obtain different colors.
In summary, a color-tunable and white-emitting NaCaY(PO4)2:Eu2+, Mn2+ phosphors are firstly reported. The energy transfer between Eu2+ ions in NCYP occurred via electric multipolar interaction, and the critical transfer distance was estimated to be 46.38 Å. The emission color could be widely controlled from blue (0.2940,0.3095), white (0.3389,0.3531) to red (0.5843,0.3939) via tuning the ratio of Eu/Mn in the NaCaY(PO4)2. The results indicates that NCYP:Eu2+, Mn2+ can be used for n-UV convertible white-emitting phosphors.
The work is financially supported from NSC under contracts no. of 102-2221-E-033-050-MY2 and 102-3011-P-033-003.
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