Novel Eu2+-activated thiogallate phosphors for white LED applications: structural and spectroscopic analysis

Novel Eu2+-activated BaGa2SiS6 and Ba2Ga8SiS16 thiogallate phosphors were prepared by solid-state reaction route. The BaGa2SiS6:Eu2+ phosphor generated a green emission upon excitation at 405 nm, whereas the Ba2Ga8SiS16:xEu2+ phosphor could be tuned from cyan to green range with increasing Eu2+ concentration upon excitation at 365 nm. Additionally, the thermal luminescence properties of the thiogallate phosphors were investigated in the temperature range of 25 to 250 °C. A warm-white LED is fabricated using the combination of a 405 nm blue InGaN-based LED chip with the green-emitting BaGa2SiS6:0.01Eu2+ phosphor, and red-emitting Sr2Si5N8:Eu2+ commercial phosphor with the CRI value of ∼88 and the CCT of 4213 K.


Introduction
Phosphor-converted white light emitting diodes (PC-WLEDs) have emerged as one of the most promising and eco-friendly white-light sources for general illumination, consuming less energy than conventional incandescent light sources [1][2][3][4][5][6] . In most commercial WLEDs, a combination of a blue LED chip and cerium doped yttrium aluminum garnet (YAG:Ce 3+ ) phosphor is used for the generation of white light. 7 But this combination has the shortcomings of poor colour rendering index (CRI) and a high correlated colour temperature (CCT) due to the absence of emission in the red spectral region which restricts its use in commercial lighting. 8,9 There are several approaches considered for achieving high CRI and cool color correlated temperature such as; a combination of a blue LED chip with a yellowemitting and a narrow-band red-emitting phosphor or a mixture of green-emitting and red-emitting phosphor. [8][9][10][11][12] The use of a near-ultraviolet (n-UV) LED chip or an ultraviolet (UV) LED chip when combined with a mixture of red, green and blue phosphors can also improve the CRI value and the CCT values of the white light generated. [13][14][15] Hence there is a need to develop new phosphors for obtaining the optimal requirements for a high-quality white light for general commercial lighting applications.
One of the main approaches involved in the designing of new phosphors includes the exploration of host compounds from existing structural model and then the proper selection of suitable activators (such as broadband emitting Eu 2+ , Ce 3+ , and Mn 2+ ). 16 Two such hosts that can be considered for exploration are BaGa 2 SiS 6 reported by Yin et al. in 2012 (ref. 17) and Ba 2 -Ga 8 SiS 16 reported by Liu and group in the year 2014 (ref. 18) for the rst time. Both the materials were studied for high power IR nonlinear optical applications. Since, sulphide-containing lattices when activated by Eu 2+ provide a longer emission wavelength due to their higher nephelauxetic effect compared to nitrides and oxides, 19-21 the structural design of these two materials opens up the possibility to study them for LED applications. The gallium (Ga 3+ ) and silicon (Si 4+ ) ions can create a protective environment to successfully enhance the stability of the sulde phosphors and hence their luminescence properties.
When doped with Eu 2+ , the emission spectrum is in general characterized by a single broad band emission which can be deconvoluted into two or more number of emission bands depending on the number of cationic sites replaced by Eu which can also help in achieving higher CRI values for WLED applications. Also, due to the small Stokes shi value, the phosphors could be excited from UV to blue region. The covalency and the crystal eld splitting between the host allows Eu 2+ ions to exhibit the parity-allowed 5d 1 4f nÀ1 / 4f n emission from the UV region to the visible spectral region. [22][23][24][25][26] In this research, a green-emitting Eu 2+ -activated BaGa 2 SiS 6 and a tunable cyanto-green-emitting Ba 2 Ga 8 SiS 16 :Eu 2+ phosphors were reported which show broad excitation range. The structural and luminescent properties of the phosphors were investigated in detail. In addition, a LED device using the BaGa 2 SiS 6 :Eu 2+ phosphor with a 405 nm LED chip was fabricated to demonstrate its applicability as a colour-conversion phosphor in the fabrication of WLEDs.

Materials and synthesis
The Eu 2+ -doped BaGa 2 SiS 6 and Ba 2 Ga 8 SiS 16 phosphors were synthesized using BaS (Alfa Aesar, 99.7%), Ga 2 S 3 (Alfa Aesar, 99.99%), Si powder (Alfa Aesar, 99.999%), and S powder (Acros, 99.999%) and EuF 2 (Alfa Aesar, 99.9%) as raw ingredients. The ingredients were homogeneously mixed and ground in a glove box under nitrogen atmosphere and loaded into the quartz glass ampoules, which were then sealed off aer evacuated to 10 À4 torr. The ampoules were heated in a furnace to 900 C for 8 h at 5 C min À1 . Finally, the powdered phosphors were obtained aer the furnace naturally cooled down to room temperature. In each case, the reactions are summarized in the following equations.

Characterizations
Synchrotron X-ray Diffraction (SXRD) using the BL01C2 beamline with an X-ray wavelength of 0.774908Å was used to analyse the phase purity of the synthesized products by at the National Synchrotron Radiation Research Centre (NSRRC) in Hsinchu, Taiwan. The X-ray Rietveld renement was carried to investigate the structure of the phosphor using the General Structure Analysis System (GSAS) soware. JEOL JSM-7401F operated at voltage of 5 kV was used to perform the scanning electron microscopy (SEM) morphological analysis and energy dispersive X-ray spectroscopy (EDS) analysis. The photoluminescence spectra and the time resolved measurement of the phosphors were obtained using a FS5 Fluorescence Spectrometer (Edinburgh Instruments) with a 450 W xenon lamp and a TCSPC (Time Correlated Single Photon Counting) module in combination with EPLED-360 picosecond pulsed light emitting diode laser system as the excitation source respectively. An integrating sphere whose inner face was coated with Spectralon equipped with a spectrouorometer (Horiba Jobin-Yvon Fluorolog 3-2-2) measured the quantum efficiency (QE). The thermal luminescence performance was analyzed using a heating apparatus (THMS-600) tted with PL equipment. The electroluminescence (EL) spectra were performed by Sphere-Optics integrating sphere with LED measurement starter packages (Onset, Inc.) recording at different current in the range of 100-300 mA.    Fig. 1a and b, respectively. The nal renement converged with weighted-proles of R p ¼ 3.04% and R wp ¼ 4.28% of (Ba 0.95 Eu 0.05 )Ga 2 SiS 6 ; R p ¼ 1.59% and R wp ¼ 2.72% of (Ba 0.90 Eu 0.10 ) 2 Ga 8 SiS 16 , thus revealing the good quality of the renement. The crystallographic data are summarized and the selected bond lengths are available in Table 1 and 2, respectively.
The as-synthesized BaGa 2 SiS 6 phosphor was found to crystallize in the space group R3 of the trigonal system, whereas the Ba 2 Ga 8 SiS 16 in the space group P6 3 mc of the hexagonal system. Fig. 2a presents the exact crystal structure of BaGa 2 SiS 6 viewed along the c-axis and a single Ba atomic site (BaS 12 ). Fig. 2b indicates the crystal structure of Ba 2 Ga 8 SiS 16 , which shows two crystallographically-independent Ba atomic sites (Ba1 and Ba2) along with the Ga/SiS 4 tetrahedral coordination.
The grain size and morphology of the two phosphor particles characterized by SEM show that the as-synthesized phosphor was composed of irregular granular micro crystals. The nominal stoichiometry was also veried by EDS measurement, as shown in Fig. S1. †      Fig. 4, the Eu 2+ emission peak changes from 462 and 521 nm (1% Eu 2+ ) to 462 and 537 nm (15% Eu 2+ ) with the increasing Eu 2+ concentration. In addition, the intensity of the short emission decreases while that of the long emission enhances as Eu 2+ concentration increases. This may result from the fact that Eu 2+ occupy in different sites, viz, Ba1 and Ba2 sites in the Ba 2 Ga 8 SiS 16 lattice, and the energy transfer between Eu 2+ ions in the two different sites occur with increasing Eu 2+ concentration.
Moreover, BaGa 2 SiS 6 :Eu 2+ exhibits a higher external quantum efficiency (EQE) values than that of Ba 2 Ga 8 SiS 16 :Eu 2+ (Fig. 5). The comparatively high asymmetry in BaGa 2 SiS 6 :Eu 2+ crystal may be the major reason for the higher EQE. With an increase in absorption, the EQE value for (Ba 1Àx Eu x )Ga 2 SiS 6 (0.01 # x # 0. The decay curve of BaGa 2 SiS 6 :Eu 2+ phosphor excited at 360 nm and monitored at 506 nm is presented in Fig. 6. The measured lifetime is related to the rst-order exponential equation given by 27    The luminescence decay times s was calculated to be 290.59 ns for BaGa 2 SiS 6 :Eu 2+ , the result is acceptable for the parityallowed 4f 6 5d 1 / 4f 7 transitions of Eu 2+ and rapid enough for LED lighting applications. Moreover, the well-tting results by an exponential decay with a single component illustrate that Eu 2+ ions occupy only one site in the BaGa 2 SiS 6 host. The decay curves of Ba 2 Ga 8 SiS16:Eu 2+ phosphor monitored at 462 nm (s ¼ 300.00 ns) and monitored at 521 nm (s ¼ 410.90 ns) under 360 nm excitation are illustrated in Fig. 7a and b, respectively. The results indicated that the Eu 2+ ions occupied the two different Ba 2+ ions coordination environment in the Ba 2 Ga 8 SiS 16 host.
Both thiosilicate phosphors were found to remain intact when le in the air at ambient temperature for 10-14 days with 60-70% relative humidity. Thermal luminescence quenching property of a phosphor plays an important role for LED applications. Fig. 8 shows temperature dependence of relative PL integrated intensity for BaGa 2 SiS 6 :Eu 2+ , Ba 2 Ga 8 SiS 16 :Eu 2+ , and BaGa 2 S 4 :Eu 2+ over the range 25 to 250 C. The thermal stability of the as-prepared thiogallate phosphors is stronger than that of the ternary sulde BaGa 2 S 4 :Eu 2+ , which perhaps belongs to the stiff (SiS 4 ) tetrahedral network that makes the host more stable. The lower le inset of Fig. 8 presents the calculated thermal activation energy (E a ) expressed by the following equation: 28 where I 0 and I(T) represent the PL integrated intensity at room temperature and testing temperature (25- To reveal the prospective use of BaGa 2 SiS 6 :Eu 2+ for PC-WLED application, the BaGa 2 SiS 6 :0.01Eu 2+ phosphor was utilized to fabricate a WLED device driven by 100 to 300 mA current with red-emitting Sr 2 Si 5 N 8 :Eu 2+ and a 405 nm LED chip shown in Fig. 9.
The EL intensity of the blue, green, and red bands of the white LED device increased while increasing the forward-biased current from 100 to 300 mA, and the saturation phenomenon was not observed even at a high forward current of 300 mA, as illustrated in Fig. 9a. As shown in Fig. 9b, with an increase in the driving current, the CIE chromaticity coordinates shied slightly. The results demonstrated the excellent colour stability of the BaGa 2 SiS 6 :0.01Eu 2+ phosphor. In Fig. 9c the studies indicate that this novel green phosphor is a potential candidate for white LED, especially for the generation of warm white light with an optimum CRI of 88 and a CCT value of 4213 K.

Conclusions
In summary, we have investigated two new Eu 2+ -doped BaGa 2 -SiS 6 and Ba 2 Ga 8 SiS 16 thiogallate phosphors. The crystal structure and luminescence performance of both the phosphors were studies in detail. The results reveal that the green-emitting Eu 2+ -doped BaGa 2 SiS 6 and the colour-tunable Eu 2+ -doped Ba 2 -Ga 8 SiS 16 could be excited over a broad range of wavelength thus generating a broadband emission. The green-emitting Eu 2+doped BaGa 2 SiS 6 phosphor was integrated together with a redemitting phosphor and a blue chip to obtain a warm-whitelight LED device an optimum CRI value of 88 and CCT value of 4213 K. Our investigation indicates the potential use of this phosphor in phosphor-converted LED for the lighting application.

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