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

Abstract

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.

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1. 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^1–6. In most commercial WLEDs, a combination of a blue LED chip and cerium doped yttrium aluminum garnet (YAG:Ce³⁺) 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 yellow-emitting and a narrow-band red-emitting phosphor or a mixture of green-emitting and red-emitting phosphor.^8–12 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.^13–15 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 Eu²⁺, Ce³⁺, and Mn²⁺).¹⁶ Two such hosts that can be considered for exploration are BaGa₂SiS₆ reported by Yin et al. in 2012 (ref. 17) and Ba₂Ga₈SiS₁₆ reported by Liu and group in the year 2014 (ref. 18) for the first time. Both the materials were studied for high power IR – nonlinear optical applications. Since, sulphide-containing lattices when activated by Eu²⁺ 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 (Ga³⁺) and silicon (Si⁴⁺) ions can create a protective environment to successfully enhance the stability of the sulfide phosphors and hence their luminescence properties.

When doped with Eu²⁺, 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 shift value, the phosphors could be excited from UV to blue region. The covalency and the crystal field splitting between the host allows Eu²⁺ ions to exhibit the parity-allowed 5d¹4fⁿ⁻¹ → 4fⁿ emission from the UV region to the visible spectral region.^22–26 In this research, a green-emitting Eu²⁺-activated BaGa₂SiS₆ and a tunable cyan-to-green-emitting Ba₂Ga₈SiS₁₆:Eu²⁺ 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 BaGa₂SiS₆:Eu²⁺ phosphor with a 405 nm LED chip was fabricated to demonstrate its applicability as a colour-conversion phosphor in the fabrication of WLEDs.

2. Experimental

2.1 Materials and synthesis

The Eu²⁺ -doped BaGa₂SiS₆ and Ba₂Ga₈SiS₁₆ phosphors were synthesized using BaS (Alfa Aesar, 99.7%), Ga₂S₃ (Alfa Aesar, 99.99%), Si powder (Alfa Aesar, 99.999%), and S powder (Acros, 99.999%) and EuF₂ (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 after evacuated to 10⁻⁴ torr. The ampoules were heated in a furnace to 900 °C for 8 h at 5 °C min⁻¹. Finally, the powdered phosphors were obtained after the furnace naturally cooled down to room temperature. In each case, the reactions are summarized in the following equations.

(1 − x)BaS + Ga₂S₃ + xEuF₂ + Si + 2S → (Ba_1−xEu_x)Ga₂SiS₆ (1)

(2 − 2x)BaS + 4Ga₂S₃ + 2xEuF₂ + 2Si + 4S → (Ba_1−xEu_x)₂Ga₈SiS₁₆ + SiS₂↑ (2)

2.2 Characterizations

Synchrotron X-ray Diffraction (SXRD) using the BL01C2 beamline with an X-ray wavelength of 0.774908 Å 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 refinement was carried to investigate the structure of the phosphor using the General Structure Analysis System (GSAS) software. 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 spectrofluorometer (Horiba Jobin-Yvon Fluorolog 3-2-2) measured the quantum efficiency (QE). The thermal luminescence performance was analyzed using a heating apparatus (THMS-600) fitted 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.

3. Results and discussion

3.1 Structural characterizations and crystallographic parameters of the (Ba_0.95Eu_0.05)Ga₂SiS₆ and (Ba_0.90Eu_0.10)₂Ga₈SiS₁₆ phosphors

To investigate the phases of the obtained phosphors and a knowledge of their crystal structure, the Rietveld analysis²² was performed using the single crystal structure data of BaGa₂SiS₆ (ICSD file no. 184747) and Ba₂Ga₈SiS₁₆ (ICSD file no. 194875), respectively, as references to approach a dependable approximation of the actual crystal structure. The experimental, calculated, and structural results of the SXRD refinement of (Ba_0.95Eu_0.05)Ga₂SiS₆ and (Ba_0.90Eu_0.10)₂Ga₈SiS₁₆ are shown in Fig. 1a and b, respectively.

Fig. 1 The SXRD profiles for Rietveld refinement results of (a) (Ba_0.95Eu_0.05)Ga₂SiS₆ and (b) (Ba_0.90Eu_0.10)₂Ga₈SiS₁₆. Observed intensities (cross), calculated patterns (red line), Bragg positions (tick mark), and difference plot (blue line) are presented.

The final refinement converged with weighted-profiles of R_p = 3.04% and R_wp = 4.28% of (Ba_0.95Eu_0.05)Ga₂SiS₆; R_p = 1.59% and R_wp = 2.72% of (Ba_0.90Eu_0.10)₂Ga₈SiS₁₆, thus revealing the good quality of the refinement. The crystallographic data are summarized and the selected bond lengths are available in Table 1 and 2, respectively.

Table 1 Structural parameters of (Ba_0.95Eu_0.05)Ga₂SiS₆ and (Ba_0.90Eu_0.10)₂Ga₈SiS₁₆ of Rietveld Refinement from SXRD data at room temperature^a

Formula	(Ba0.95Eu0.05)Ga2SiS6	(Ba0.90Eu0.10)2Ga8SiS16
a Lattice parameters: a and c in Å, V in Å^3; T = 298 K; BL01C2 beamline of NSRRC, λ = 0.774908 Å; Site Occupancy Fraction (S.O.F.); Uiso in Å^2.
Symmetry	Trigonal	Hexagonal
Space group	R3 (no. 146)	P63mc (no. 186)
Lattice parameters	a = b = 9.55032(28)	a = b = 10.83380(10)
	c = 8.63634(27)	c = 11.86626(18)
	V = 682.18(4)	V = 1206.164(25)
Rwp	4.28%	4.68%
Rp	3.04%	2.75%
χ^2	3.805	2.873

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(Ba0.95Eu0.05)Ga2SiS6
Atom	x/a	y/b	z/c	S.O.F	Uiso*100
Ba	0.00000	0.000000	0.4988(26)	0.9500	6.59
Ga	0.0537(13)	0.8276(10)	0.9498(29)	0.6700	0.30
Si	0.049351	0.7379(22)	0.8921(74)	0.3300	1.34
S(1)	0.8527(13)	0.5886(12)	0.5146(28)	1.0000	3.12
S(2)	0.8136(15)	0.7627(13)	0.8284(27)	1.0000	1.63
Eu	0.000000	0.000000	0.4988(26)	0.0500	6.59

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(Ba0.90Eu0.10)2Ga8SiS16
Atom	x/a	y/b	z/c	S.O.F	Uiso*100
Ba(1)	0.333300	0.666700	0.0253(8)	0.9500	4.53
Ba(2)	0.333300	0.666700	0.5156(8)	0.9500	2.59
Ga(1)	0.0048(17)	0.3384(11)	0.3262(65)	0.8600	1.42
Ga(2)	0.8846(5)	0.1154(5)	0.0746(13)	1.0000	2.16
Si	0.0048(17)	0.3384(11)	0.3262(65)	0.1400	1.42
S(1)	0.6625(14)	0.0040(14)	0.0164(7)	1.0000	1.72
S(2)	0.2255(11)	0.7745(11)	0.2637(18)	1.0000	1.06
S(3)	0.5593(9)	0.4407(9)	0.2791(20)	1.0000	1.61
S(4)	0.8977(11)	0.1023(11)	0.2664(18)	1.0000	0.53
S(5)	1.000000	0.000000	−0.0162(22)	1.0000	2.70
Eu(1)	0.333300	0.666700	0.0253(8)	0.0500	4.53
Eu(2)	0.333300	0.666700	0.5156(8)	0.0500	2.59

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Table 2 Selected interatomic bond distances^a of (Ba_0.95Eu_0.05)Ga₂SiS₆ and (Ba_0.90Eu_0.10)₂Ga₈SiS₁₆ at room temperature

(Ba0.95Eu0.05)Ga2SiS6		(Ba0.90Eu0.10)2Ga8SiS16
a Bond distance in Å.
(Ba/Eu)-S1	3.45104(10) (3×)	(Ba1/Eu1)-S1	3.61231(3) (6×)
(Ba/Eu)-S1^i	3.50580(8) (3×)	(Ba1/Eu1)-S2	3.47936(4) (3×)
(Ba/Eu)-S2	3.51719(8) (3×)	(Ba1/Eu1)-S3	3.54776(4) (3×)
(Ba/Eu)-S2^i	3.60984(11) (3×)	(Ba2/Eu2)-S1	3.54578(3) (6×)
Ga-S1	2.27107(7)	(Ba2/Eu1)-S2	3.60961(4) (3×)
Ga-S1^i	2.40444(7)	(Ba2/Eu1)-S3	3.71969(4) (6×)
Ga-S2	2.20099(6)	(Ga1/Si)-S1	2.25675(3)
Ga-S2^i	2.12290(5)	(Ga1/Si)-S2	2.19921(2)
		(Ga1/Si)-S3	2.19370(2)
		(Ga1/Si)-S4	2.32944(2)
		Ga2-S1	2.19539(2) (2×)
		Ga2-S4	2.28965(3)
		Ga2-S5	2.41845(2)

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The as-synthesized BaGa₂SiS₆ phosphor was found to crystallize in the space group R3 of the trigonal system, whereas the Ba₂Ga₈SiS₁₆ in the space group P6₃mc of the hexagonal system. Fig. 2a presents the exact crystal structure of BaGa₂SiS₆ viewed along the c-axis and a single Ba atomic site (BaS₁₂). Fig. 2b indicates the crystal structure of Ba₂Ga₈SiS₁₆, which shows two crystallographically-independent Ba atomic sites (Ba1 and Ba2) along with the Ga/SiS₄ tetrahedral coordination.

Fig. 2 (a) Schematic crystal structure of BaGa₂SiS₆ (trigonal, R3) (a) and Ba₂Ga₈SiS₁₆ (hexagonal, P6₃mc) (b) viewed down the c-axis. Orange, brown, green, light green and yellow sphere balls describe Ba1, Ba2, Ga1/Si, Ga2, S and Si atoms. The insets show the coordination environment around BaS₁₂ (left-down) of BaGa₂SiS₆, Ba1S₁₂ (right-up) and Ba2S₁₂ (right-down) of Ba₂Ga₈SiS₁₆.

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 verified by EDS measurement, as shown in Fig. S1.†

3.2 Spectroscopic study of the BaGa₂SiS₆:Eu²⁺ and Ba₂Ga₈SiS₁₆:Eu²⁺ phosphor

The PLE and PL spectra of BaGa₂SiS₆:0.01Eu²⁺ and Ba₂Ga₈SiS₁₆:0.01Eu²⁺ is represented in Fig. 3. The BaGa₂SiS₆:Eu²⁺ phosphor is excitable around 350 to 450 nm and generates a green emission peaking at 510 nm, and the Ba₂Ga₈SiS₁₆:Eu²⁺ phosphor exhibits a broadband cyan emission upon excitation at 365 nm. The two emission bands of Ba₂Ga₈SiS₁₆:Eu²⁺ are attributed to the two sites of barium atom (Ba1 and Ba2) in the Ba₂Ga₈SiS₁₆ lattice.

Fig. 3 (a) PLE/PL spectra of BaGa₂SiS₆:0.01Eu²⁺. (b) PLE/PL spectra and the PL deconvolution (dashed line) of Ba₂Ga₈SiS₁₆:0.01Eu²⁺. The insets show photos of the phosphor taken under normal light (left) and 365 nm UV light (right).

As indicted in Fig. 4, the Eu²⁺ emission peak changes from 462 and 521 nm (1% Eu²⁺) to 462 and 537 nm (15% Eu²⁺) with the increasing Eu²⁺ concentration. In addition, the intensity of the short emission decreases while that of the long emission enhances as Eu²⁺ concentration increases. This may result from the fact that Eu²⁺ occupy in different sites, viz, Ba1 and Ba2 sites in the Ba₂Ga₈SiS₁₆ lattice, and the energy transfer between Eu²⁺ ions in the two different sites occur with increasing Eu²⁺ concentration.

Fig. 4 PL spectra (a) and variation in CIE chromaticity coordinates (b) of (Ba_1−xEu_x)₂Ga₈SiS₁₆ (0.01 ≤ x ≤ 0.15) phosphor as a function of Eu²⁺ content.

Moreover, BaGa₂SiS₆:Eu²⁺ exhibits a higher external quantum efficiency (EQE) values than that of Ba₂Ga₈SiS₁₆:Eu²⁺ (Fig. 5). The comparatively high asymmetry in BaGa₂SiS₆:Eu²⁺ crystal may be the major reason for the higher EQE. With an increase in absorption, the EQE value for (Ba_1−xEu_x)Ga₂SiS₆ (0.01 ≤ x ≤ 0.10) increases and reaches maximum values of about 45.80% at x = 0.10, whereas (Ba_1−xEu_x)₂Ga₈SiS₁₆ (0.01 ≤ x ≤ 0.15) reaches maximum values of about 34.75% at x = 0.10, as shown in Fig. 5.

Fig. 5 Absorption (A) and external quantum efficiency (EQE) as a function of x in (Ba_1−xEu_x)Ga₂SiS₆ (0.01 ≤ x ≤ 0.10) and (Ba_1−xEu_x)₂Ga₈SiS₁₆ (0.01 ≤ x ≤ 0.15) under excitation at 405 and 365 nm, respectively.

3.3 Time resolved measurement and thermal luminescence properties of the BaGa₂SiS₆:Eu²⁺ and Ba₂Ga₈SiS₁₆:Eu²⁺ phosphor

The decay curve of BaGa₂SiS₆:Eu²⁺ phosphor excited at 360 nm and monitored at 506 nm is presented in Fig. 6. The measured lifetime is related to the first-order exponential equation given by²⁷

Fig. 6 Decay curve for BaGa₂SiS₆:Eu²⁺ phosphor (black line) and curve-fitting (red line) under 360 nm excitation and monitored at 506 nm.

The luminescence decay times τ was calculated to be 290.59 ns for BaGa₂SiS₆:Eu²⁺, the result is acceptable for the parity-allowed 4f⁶5d¹ → 4f⁷ transitions of Eu²⁺ and rapid enough for LED lighting applications. Moreover, the well-fitting results by an exponential decay with a single component illustrate that Eu²⁺ ions occupy only one site in the BaGa₂SiS₆ host.

The decay curves of Ba₂Ga₈SiS16:Eu²⁺ phosphor monitored at 462 nm (τ = 300.00 ns) and monitored at 521 nm (τ = 410.90 ns) under 360 nm excitation are illustrated in Fig. 7a and b, respectively. The results indicated that the Eu²⁺ ions occupied the two different Ba²⁺ ions coordination environment in the Ba₂Ga₈SiS₁₆ host.

Fig. 7 Decay curve for Ba₂Ga₈SiS₁₆:Eu²⁺ phosphor (black line) and curve-fitting (red line) under 360 nm excitation and monitored at (a) 462 nm and (b) 521 nm.

Both thiosilicate phosphors were found to remain intact when left 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 BaGa₂SiS₆:Eu²⁺, Ba₂Ga₈SiS₁₆:Eu²⁺, and BaGa₂S₄:Eu²⁺ over the range 25 to 250 °C. The thermal stability of the as-prepared thiogallate phosphors is stronger than that of the ternary sulfide BaGa₂S₄:Eu²⁺, which perhaps belongs to the stiff (SiS₄) tetrahedral network that makes the host more stable. The lower left inset of Fig. 8 presents the calculated thermal activation energy (E_a) expressed by the following equation:²⁸

where I₀ and I(T) represent the PL integrated intensity at room temperature and testing temperature (25–250 °C), respectively, and k the Boltzmann constant. The values of E_a for BaGa₂SiS₆:Eu²⁺, Ba₂Ga₈SiS₁₆:Eu²⁺, and BaGa₂S₄:Eu²⁺ were estimated to be 0.268, 0.233 and 0.218 eV, respectively.

Fig. 8 Temperature dependence of relative PL integrated intensity for BaGa₂SiS₆:Eu²⁺, Ba₂Ga₈SiS₁₆:Eu²⁺, and BaGa₂S₄:Eu²⁺ over the range 25 to 250 °C. The inset shows the fitted PL integrated intensity and the calculated thermal activation energy (E_a) as a function of temperature.

3.4 CIE chromaticity coordinates and performance of LED devices based on BaGa₂SiS₆:Eu²⁺ phosphor

To reveal the prospective use of BaGa₂SiS₆:Eu²⁺ for PC-WLED application, the BaGa₂SiS₆:0.01Eu²⁺ phosphor was utilized to fabricate a WLED device driven by 100 to 300 mA current with red-emitting Sr₂Si₅N₈:Eu²⁺ and a 405 nm LED chip shown in Fig. 9.

Fig. 9 (a) EL spectra of the device using 405 nm LED chip with green-emitting BaGa₂SiS₆:Eu²⁺ phosphor, and red-emitting Sr₂Si₅N₈:Eu²⁺ phosphor and (b) variation in CIE chromaticity coordinates of the WLED operated under different currents (100 to 300 mA). (c) CIE chromaticity coordinates of the used phosphors and the fabricated LED are presented. The insets show the used phosphors and the LED device photos recorded under 365 nm excitation.

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 shifted slightly. The results demonstrated the excellent colour stability of the BaGa₂SiS₆:0.01Eu²⁺ 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.

4. Conclusions

In summary, we have investigated two new Eu²⁺-doped BaGa₂SiS₆ and Ba₂Ga₈SiS₁₆ thiogallate phosphors. The crystal structure and luminescence performance of both the phosphors were studies in detail. The results reveal that the green-emitting Eu²⁺-doped BaGa₂SiS₆ and the colour-tunable Eu²⁺-doped Ba₂Ga₈SiS₁₆ could be excited over a broad range of wavelength thus generating a broadband emission. The green-emitting Eu²⁺-doped BaGa₂SiS₆ phosphor was integrated together with a red-emitting phosphor and a blue chip to obtain a warm-white-light 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 conflicts of interest to declare.

Acknowledgements

This research was financially supported by Ministry of Science and Technology of Taiwan (R.O.C.) under Contract No. MOST104-2113-M-009-018-MY3.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra01113c

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