Luminescence and energy transfer in a color tunable CaY₄(SiO₄)₃O:Ce³⁺, Mn²⁺, Tb³⁺ phosphor for application in white LEDs

Abstract

A Ce³⁺/Mn²⁺/Tb³⁺ co-activated CaY₄(SiO₄)₃O phosphor was prepared through high temperature solid state reaction process. By means of dielectric theory of chemical bonding for complex crystals, covalence of chemical bonds in CaY₄(SiO₄)₃O was calculated. Blue emission from Ce³⁺ in different crystallographic cation sites was discussed quantitatively based on the calculation results. Dual energy transfer of Ce³⁺ → Mn²⁺ and Ce³⁺ → Tb³⁺ occurs and color tunable emission can be realized by modulation of their relative PL intensity. Energy transfer efficiency has been investigated and the process has been demonstrated to be a resonant type via a dipole–quadrupole mechanism. Critical distance R_c calculated through quenching concentration method and spectral overlap route is 7.23 and 7.55 Å, respectively. As-obtained samples show color tunable emission and the corresponding CIE chromaticity coordinates were given. White light with color coordinates (0.3339, 0.3055) and (0.3553, 0.3338), close to those of the ideal, was realized under lower energy UV light. All results show CaY₄(SiO₄)₃O:Ce³⁺, Mn²⁺, Tb³⁺ phosphor could be a promising candidate for UV chip pumped light emitting diodes.

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1. Introduction

Over the past few years, there has been increasing interest for use of light-emitting diodes (LEDs) in display and illumination applications because of their long lifetime, low energy consumption, high luminous efficiency and environment-friendly characteristics.^1,2 Present commercial method to generate white light is combining blue LEDs with yellow phosphor Y₃Al₅O₁₂:Ce³⁺.³ However, deficiency of a red component leads to low color-rendering index R_a and high correlated color temperature. To enrich red emission, additional red-emitting phosphors are usually blended.^4,5 Regrettably, the complement of red emission is usually not enough for utilization. Another alternative approach to white light is using near ultraviolet (UV) or UV LEDs to pump red, green and blue phosphors.⁶ However, in the system, blue emission efficiency is poor because of strong re-absorption of blue light by green or red phosphors. Therefore, many efforts have been devoted to develop single component white-emitting phosphors based on energy transfer between activators.

Recently, single-phased white phosphors realized by energy transfer from Ce³⁺, Eu²⁺ to Tb³⁺, Mn²⁺, Eu³⁺ in some hosts has attracted many attention, such as Ca₃Sc₂Si₃O₁₂:Ce³⁺, Mn²⁺, Ca₂PO₄Cl:Eu²⁺, Mn²⁺, MgY₄Si₃O₁₃:Ce³⁺, Mn²⁺, NaCaBO₃:Ce³⁺, Tb³⁺, Mn²⁺, Ba₂(Gd,Tb)₂Si₄O₁₃:Eu²⁺, Eu³⁺, BaY₂Si₃O₁₀:Ce³⁺, Tb³⁺, Eu³⁺ and so on.^7–12 Tb³⁺ is known to be a common activator in green emitting phosphors due to its predominant ⁵D₄ → ⁷F₅ transition at about 545 nm. Mn²⁺ activated luminescent materials have wide range emission from 500 to 700 nm depending on crystal field of host.^13,14 However, for Tb³⁺ single doped phosphor, emission is usually very weak due to strictly forbidden 4f–4f transitions of Tb³⁺.¹⁵ A similar situation happened in luminescent materials doped with Mn²⁺, because its parity and spin forbidden d–d transitions are difficult to pump. Therefore, a sensitizer like Ce³⁺ has been applied for improving emission intensity of Tb³⁺ or Mn²⁺. Ce³⁺ sensitized Tb³⁺ or Mn²⁺ activated phosphors have been widely reported and energy transfer mechanism from Ce³⁺ to Tb³⁺ or Mn²⁺ has been investigated.^16–18

As is known, general chemical formula for apatite structure is A₁₀(XO₄)₆Z₂ (A = Ca²⁺, Ba²⁺, Ce³⁺, La³⁺, Y³⁺; X = P⁵⁺, Si⁴⁺ and Z = O²⁻, F⁻, Cl⁻, OH⁻).^19,20 CaY₄(SiO₄)₃O (CYSO), which is isostructural to apatite compound, such as La_4.67(SiO₄)₃O, La₅Si₂BO₁₃, crystallizes in a hexagonal unit cell with space group P6₃/m (no. 176) and cell parameters of a = b = 9.36 Å, c = 6.78 Å, V = 514.4 ų and Z = 2.^21,22 The lattice contains different types of cationic sites, nine-fold coordinated 4f sites with C₃ point symmetry and seven-fold coordinated 6h sites with C_s point symmetry. Y atoms occupy both of two sites and Ca atoms are located at 4f site. In view of identical valence, Ce³⁺/Tb³⁺ and Mn²⁺ are expected to substitute for Y³⁺ and Ca²⁺ in CYSO lattice, respectively.

In this paper, Ce³⁺/Mn²⁺/Tb³⁺ co-activated CYSO phosphors synthesized by conventional high temperature solid state reaction were reported. Occupation of Ce³⁺ ions in crystallographic cation sites was analyzed. Energy transfer process from Ce³⁺ to Mn²⁺ was investigated and critical distance R_c between them was calculated using concentration quenching or spectral overlap route. Color tunable emission including a blue band from Ce³⁺, several green lines of Tb³⁺ and orange band originating from Mn²⁺ was achieved. White light was generated in CYSO:0.04Ce³⁺, 0.10Mn²⁺, 0.05Tb³⁺ and CYSO:0.04Ce³⁺, 0.10Mn²⁺, 0.08Tb³⁺ under 320 nm excitation.

2. Experimental section

2.1 Preparation

A series of Ca_1−yY_4−x−z(SiO₄)₃O:xCe³⁺, yMn²⁺, zTb³⁺ were prepared by conventional high temperature solid state reaction. Doping concentration of Ce³⁺, Mn²⁺ and Tb³⁺ is x = 0.004–0.08, y = 0.005–0.50 and z = 0.02–0.10, respectively. Chemicals CaCO₃ (A.R.), Y₂O₃ (99.9%), SiO₂ (99.99%), CeO₂ (99.9%), MnCO₃ (99.99%) and Tb₄O₇ (99.9%) were used as raw materials without further purification. Starting materials were first weighed in appropriate molar ratio and mixing was carried out by wet milling in ethanol for 20 min. After being mixed well, precursors were transferred to corundum crucible, heated in a tube furnace under a reducing atmosphere of 5% H₂ and 95% N₂. Temperature of furnace was increased to 1400 °C followed by holding the temperature for 6 h.

2.2 Characterization

Powder X-ray diffraction (XRD) patterns of synthesized phosphor were recorded on a Philips XPert/MPD diffraction system with Cu Kα (λ = 1.5405 Å) radiation. Photoluminescence excitation (PLE) and emission (PL) spectra were collected using a Photon Technology International (PTI) spectrofluorimeter with a 60 W Xe arc lamp, and lifetimes were measured using a phosphorimeter attached to the main system with a Xe flash lamp (25 W).

3. Results and discussion

3.1 Phase identification and crystal structure

Fig. 1 is comparison of representative XRD patterns of Ce³⁺, Mn²⁺ or Tb³⁺ doped CYSO samples with standard pattern of Y_4.67(SiO₄)₃O (YSO, JCPDS #30-1457). Both of structure prototype of CYSO and YSO is La_4.67(SiO₄)₃O and their detailed crystal structures have been obtained from MatNavi NIMS Materials and Springer Materials Database. Their crystal structures are almost same except for cell parameters. Peak positions in XRD patterns of CYSO would have some shifts compared with that of YSO because they are determined by distance between parallel planes of atoms in crystals. It can be found from Fig. 1 that all diffraction peaks are in good agreement with those of YSO except for a slight shift resulting from different cell parameters between them. It also can be concluded that as-prepared samples are single phase and incorporations of Ce³⁺, Mn²⁺ and Tb³⁺ do not bring any significant changes in host lattice structure.

Fig. 1 XRD patterns of Ce³⁺, Mn²⁺, Tb³⁺ doped or co-doped CaY₄(SiO₄)₃O and Y_4.67(SiO₄)₃O standard card.

After phase identification was done, in order to know site occupation of Ce³⁺, Mn²⁺, Tb³⁺ in CYSO host lattice, detailed crystal structure is presented in Fig. 2, from which it can be observed that there are four kinds of cationic sites. Y³⁺ ions occupy two kinds of cationic sites, in which 6h sites (labeled by Y1) with seven oxygen coordinated and 4f sites (labeled by Y2) with nine. It is interesting that 4f sites are occupied by Ca²⁺ and Y³⁺ ions with equal probability. Si⁴⁺ (r = 0.26 Å, CN = 4) are too small to be replaced, and Ce³⁺ and Tb³⁺ ions are expected to substitute Ca²⁺ or Y³⁺ sites based on their ionic radii [r(Ce³⁺) = 1.196 Å, r(Tb³⁺) = 1.095 Å, r(Ca²⁺) = 1.18 Å, and r(Y³⁺) = 1.075 Å for coordination number CN = 9; r(Ce³⁺) = 1.07 Å, r(Tb³⁺) = 0.98 Å, and r(Y³⁺) = 0.96 Å for CN = 7]. Mn²⁺ ions intend to enter Ca²⁺ sites because of their same valence and similar ionic radii. In the following, substitution by Ce³⁺ will be discussed from quantitative point of view.

Fig. 2 Unit cell representation of CaY₄(SiO₄)₃O.

3.2 Ce³⁺ site preference analysis

PL excitation and emission spectra of CYSO:0.04Ce³⁺ are presented in Fig. 3. As-prepared samples exhibit a broad emission band in wavelength range of 300–600 nm with peak at about 400 nm under 320 nm UV light excitation. Excitation spectrum consists of a strong broad band at 320 nm and a weak shoulder band peaking at about 360 nm. The asymmetric excitation band can be fitted by a sum of three Gaussian peaks at 300, 326 and 361 nm. As illustrated in Fig. 3, PL spectrum is also asymmetric and can be deconvoluted into four Gaussian components, with peaks at 373 nm (26 810 cm⁻¹), 396 nm (25 253 cm⁻¹), 426 nm (23 474 cm⁻¹) and 472 nm (21 186 cm⁻¹), respectively. Energy difference for the first two is 1557 cm⁻¹ and that for the other two is 2288 cm⁻¹. It is known that energy difference of splitting of ²F_7/2 and ²F_5/2 ground states of Ce³⁺ is about 2000 cm⁻¹. Energy difference between 426 nm (23 474 cm⁻¹) and 472 nm (21 186 cm⁻¹) is in agreement with that between ²F_7/2 and ²F_5/2 energy levels of Ce³⁺. Combined with three excitation bands in Fig. 3, it can be concluded that broad emission band is originated from three different sites occupied by Ce³⁺ ions. Hexagonal CYSO provides three different cationic sites, 4f site filled up with equivalent amount of Ca and Y, 6h site occupied only by Y atoms. Among them, 6h site has a free oxygen ion (not linked to any SiO₄ group) to coordinate and therefore is expected to be of higher covalency than 4f site. In order to compare intuitively, we calculated covalency f_C of every chemical bond and give them in Table 1. As shown in Table 1, covalency is 0.1855 for Y1–O4, and the value for other Y1–O bonds are also higher than that of Y2–O and Ca–O chemical bonds. Therefore, three excitation bands at 300, 326 and 361 nm can be assigned to 4f → 5d transitions of Ce³⁺ ions in Y2, Ca and Y1 site, respectively.

Fig. 3 Excitation and emission spectra of CaY_3.96(SiO₄)₃O:0.04Ce³⁺.

Table 1 Bond length d and calculated covalency of chemical bond for Y1 (6h site), Y2 and Ca (4f site)

Bond	d (Å)	fC	Bond	d (Å)	fC	Bond	d (Å)	fC
Y1–O1	2.3607	0.0941	Y2–O1	2.7328	0.0537	Ca–O1	2.7328	0.082
Y1–O1	2.4232	0.0935	Y2–O2	2.4148	0.0556	Ca–O2	2.4148	0.0864
Y1–O2	2.4265	0.0934	Y2–O3	2.3998	0.0557	Ca–O3	2.3998	0.0867
Y1–O3	2.6973	0.0917
Y1–O4	2.2442	0.1855

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3.3 Photoluminescence properties and energy transfer

To optimize doping concentration of Ce³⁺ and evaluate its influence on emission intensity, Fig. 4(a) and (c) give PL spectra of CYSO:xCe³⁺ (x = 0.004, 0.012, 0.02, 0.03, 0.04, 0.06) under 290 and 320 nm excitations, respectively. Emission intensities firstly increase with increasing Ce³⁺ concentration, and reach the maximum value at x = 0.04, then decrease as a result of concentration quenching. Therefore, doping concentration of Ce³⁺ of samples was fixed at x = 0.04.

Fig. 4 (a) and (c) PL spectra of CaY_4−x(SiO₄)₃O:xCe³⁺; (b) and (d) PL intensity of CaY_4−x(SiO₄)₃O:xCe³⁺ as a function of Ce³⁺ concentration.

After Ce³⁺ was incorporated, Mn²⁺ was introduced into CYSO host lattice. Due to forbidden transitions of Mn²⁺, there must exists energy transfer from Ce³⁺ to Mn²⁺ to get bright red luminescence. Therefore PL spectra of CYSO:0.04Ce³⁺ and CYSO:0.2Mn²⁺, and PL excitation spectrum of CYSO:0.2Mn²⁺ are presented in Fig. 5 in a comparable way. Mn²⁺ single doped sample exhibits an orange emission band centered at about 588 nm and its excitation bands are in near UV and visible regions. The excitation spectrum consists of several bands centering at 322 (342), 364, 406 and 470 nm, corresponding to transitions of Mn²⁺ ion from ground state ⁶A₁(⁶S) to ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G), ⁴E(⁴G)], and ⁴T₁(⁴G), respectively.²³ Broad emission band from 500 to 750 nm peaking at 588 nm is ascribed to spin-forbidden ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn²⁺. As shown in Fig. 5, there is a significant overlap between emission spectra of Ce³⁺ and excitation spectrum of Mn²⁺, indicating possibility of effective resonant energy transfer from Ce³⁺ to Mn²⁺. Therefore, a series of samples with fixed Ce³⁺ concentration and variable Mn²⁺ content was prepared to study luminescent properties and energy transfer process.

Fig. 5 PL excitation spectra of Ca_0.8Y₄(SiO₄)₃O:0.2Mn²⁺ and PL spectra of CaY_3.96(SiO₄)₃O:0.04Ce³⁺ and Ca_0.8Y₄(SiO₄)₃O:0.2Mn²⁺.

Fig. 6 depicts PL spectra of CYSO:0.04Ce³⁺, yMn²⁺ (y = 0, 0.05, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5) under excitation of 320 nm. As content of Mn²⁺ increases from 0 to 0.5, PL intensity of Mn²⁺ increases sharply firstly, then gets to the maximum, and at last quenches. In addition, one can also find that emission band of Mn²⁺ shifts to longer wavelength with its increasing concentration. The peak is located at about 580 nm at y = 0.05 and 588 nm at y = 0.5. This phenomenon can be explained by enhancement of crystal field strength surrounding Mn²⁺ ions. Crystal field strength around Mn²⁺ has been proposed to be:

where D_q is measurement of crystal field strength, z is charge of anion, r is radius of d wave function, e is charge of an electron, and R is distance between central ion and its ligands. For as-prepared samples, red-shift occurred obviously when Mn²⁺ concentration is larger than 30%. This is because when Mn²⁺ ions were introduced into CYSO host lattice, cell constants of the host would decrease since ionic radius of Mn²⁺ (r = 0.96 Å, CN = 8) is smaller than that of Ca²⁺ ion (r = 1.18 Å, CN = 9), resulting in increase of crystal field strength surrounding Mn²⁺.

Fig. 6 Dependence of PL spectra of Ca_1−yY_3.96(SiO₄)₃O:0.04Ce³⁺, yMn²⁺ on variable y.

Different from Mn²⁺, PL intensity of Ce³⁺ decreases monotonically as increasing Mn²⁺ content, which means there exists resonant energy transfer from Ce³⁺ to Mn²⁺ in CYSO:0.04Ce³⁺, yMn²⁺ phosphors. It is well known that critical distance for energy transfer from sensitizer to activator by exchange interaction should be shorter than 4 Å.²⁴ Critical distance R_c among activators can be expressed by:²⁵

where X_c is total concentration, N is the number of lattice sites in unit cell and V is volume of the unit cell. For CYSO, N = 10 and V = 514.4 ų and critical concentration X_c, at which luminescence intensity of Ce³⁺ is half of that in sample without Mn²⁺, is 0.26. Critical distance R_c is determined to be 7.23 Å. This value is longer than 4 Å, indicating little possibility of energy transfer via exchange interaction mechanism. Therefore, energy transfer from Ce³⁺ to Mn²⁺ mainly takes place via electric multi-polar interaction.

On basis of Dexter's energy transfer formula of exchange and multipolar interactions and Reisfeld's approximation, following relation can be satisfied:^26,27

where η₀ and η are the luminescence quantum efficiency of Ce³⁺ in absence and presence of Mn²⁺, C is sum of concentration of Ce³⁺ and Mn²⁺. Among them, corresponds to exchange interaction, and with n = 6, 8 and 10, are related to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. Value of η₀/η can be approximately calculated by the ratio of relative luminescence intensities:where I_S0 is intrinsic luminescence intensity of Ce³⁺, and I_S that of Ce³⁺ in presence of Mn²⁺.

We plot ln(IS0/IS) versus CCe3++Mn2+, as well as IS0/IS against CCe3++Mn2+n/3, and illustrate them in Fig. 7. It is obvious that a better linear behavior can be found when n = 6 and 8, which implying that energy transfer mechanism from Ce3+ to Mn2+ is not a quadrupole–quadrupole interaction type.

Fig. 7 Dependence of ln(IS0/IS) of Ce3+ on CCe3++Mn2+ (a), IS0/IS on CCe3++Mn2+n/3, n = 6, 8, 10 for (b)–(d).

In order to further confirm the type of energy transfer, decay curves of CYSO:0.04Ce³⁺, yMn²⁺ (y = 0, 0.05, 0.20, 0.25, 0.30) phosphors excited at 320 nm and monitored at 400 nm are shown in Fig. 8. The curves were well fitted by a single exponential equation:

I = A₁ exp(−t/τ₁) (5)

where I is emission intensity, A is a constant and τ is lifetime. Value of τ for Ce³⁺ emission in CYSO:0.04Ce³⁺, yMn²⁺ (y = 0, 0.05, 0.20, 0.25, 0.30) are calculated to be 65.5, 52.8, 43.4, 37.7 and 32.2 ns, respectively, and decreased with increasing Mn²⁺ doping concentration, supporting energy transfer from Ce³⁺ to Mn²⁺. The energy transfer efficiency η_T can be calculated by following formula:²⁸where τ_S0 and τ_S are lifetimes of Ce³⁺ in absence and presence of Mn²⁺, respectively. Value of η_T was calculated and plotted as a function of Mn²⁺ concentration y, and is shown in inset of Fig. 8. It was found η_T increases with increasing Mn²⁺ content.

Fig. 8 Decay curves of Ce³⁺ emission in Ca_1−yY_3.96(SiO₄)₃O:0.04Ce³⁺, yMn²⁺ excited at 320 nm and monitored at 400 nm. The inset shows energy transfer efficiency η_T on Mn²⁺ concentration.

From eqn (3), (4) and (6), following equation can be presented:

Plots of τS0/τS and CCe3++Mn2+n/3 (n = 6, 8) based on above equation are shown in Fig. 9. Better linear behavior was observed when n = 8, implying energy transfer from Ce3+ to Mn2+ occurred via dipole–quadrupole mechanism.

Fig. 9 Dependence of τS0/τS of Ce3+ on CCe3++Mn2+n/3, n = 6, 8 for (a) and (b).

In view of this kind of interaction, critical distance from sensitizer to activator can be calculated through spectral overlap route and its formula is displayed as follows:^26,29

where λ_S (in Å) is wavelength position of sensitizer's emission, f_q is oscillator strength of involved absorption transition of acceptor, E is energy involved in transfer (in eV), and represents spectral overlap between normalized shapes of sensitizer Ce³⁺ emission f_S(E) and activator Mn²⁺ excitation f_A(E). In this case, the value is determined to be 0.21766 eV⁻⁵. As a result, critical distance R_c is calculated to be 7.55 Å, which is in good agreement with that obtained using concentration quenching method. This result can further testify electric dipole–quadrupole interaction is responsible for energy transfer process from Ce³⁺ to Mn²⁺ in CYSO samples.

3.4 Luminescence and chromaticity

Until here, energy transfer mechanism from Ce³⁺ to Mn²⁺ has been confirmed, in order to obtain white light, Tb³⁺ was introduced as a co-dopant. Fig. 10 gives PL spectra of CYSO:0.04Ce³⁺, 0.10Mn²⁺, zTb³⁺ phosphors with different Tb³⁺ concentration of z = 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, and 0.10. The spectra under 320 nm excitation consist of several emission bands in visible region. Among them one peaking at 400 nm is from transition of Ce³⁺, and sharp bands at 485, 543 (549), 584 (590) and 620 nm are due to transitions of Tb³⁺ from ⁵D₄ to ⁷F₆, ⁷F₅, ⁷F₄, ⁷F₃ energy levels, respectively. Broad orange emission band of Mn²⁺ from 500 to 700 nm overlaps with that of Tb³⁺. From curves in Fig. 10, it can be seen that with increasing Tb³⁺ concentration, PL intensity of Tb³⁺ increased. In order to compare chromaticity of samples with and without Tb³⁺ dopants, x and y values of Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates for CYSO:0.04Ce³⁺, yMn²⁺ (y = 0, 0.05, 0.1, 0.2, 0.25, 0.3, 0.4) and CYSO:0.04Ce³⁺, 0.10Mn²⁺, zTb³⁺ (z = 0.05, 0.08) samples under 320 nm excitation were calculated and presented in Table 2. Corresponding chromaticity diagram was shown in Fig. 11.

Fig. 10 PL spectra of Ca_0.90Y_3.96–z(SiO₄)₃O:0.04Ce³⁺, 0.10Mn²⁺, zTb³⁺.

Table 2 CIE chromaticity coordinates for CYSO:0.04Ce³⁺, yMn²⁺ (y = 0, 0.05, 0.10, 0.20, 0.25, 0.30, 0.40) and CYSO:0.04Ce³⁺, 0.10Mn²⁺, zTb³⁺ (z = 0.05, 0.08) samples

No. of points	Sample composition	CIE (x, y)
1	CYSO:0.04Ce^3+	(0.1649, 0.1040)
2	CYSO:0.04Ce^3+, 0.05Mn^2+	(0.2808, 0.2195)
3	CYSO:0.04Ce^3+, 0.10Mn^2+	(0.3379, 0.2745)
4	CYSO:0.04Ce^3+, 0.20Mn^2+	(0.4278, 0.3564)
5	CYSO:0.04Ce^3+, 0.25Mn^2+	(0.4569, 0.3798)
6	CYSO:0.04Ce^3+, 0.30Mn^2+	(0.4792, 0.3974)
7	CYSO:0.04Ce^3+, 0.40Mn^2+	(0.4968, 0.4035)
8	CYSO:0.04Ce^3+, 0.10Mn^2+, 0.05Tb^3+	(0.3339, 0.3055)
9	CYSO:0.04Ce^3+, 0.10Mn^2+, 0.08Tb^3+	(0.3553, 0.3338)

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Fig. 11 CIE chromaticity diagram for Ca_1−yY_3.96(SiO₄)₃O:0.04Ce³⁺, yMn²⁺ (y = 0, 0.05, 0.10, 0.20, 0.25, 0.30, 0.40) and Ca_0.90Y_3.96−z(SiO₄)₃O:0.04Ce³⁺, 0.10Mn²⁺, zTb³⁺ (z = 0.05, 0.08) samples.

From chromaticity diagram in Fig. 11, it can be observed that light emitted by Ce³⁺ single doped CYSO phosphor is blue. After incorporation of Mn²⁺, chromaticity coordinates for CYSO:0.04Ce³⁺, yMn²⁺ can be tuned from blue (0.1649, 0.1040) to light purple red and finally to orange (0.4968, 0.4035), and that for CYSO:0.04Ce³⁺, 0.10Mn²⁺ (point 3, (0.3379, 0.2745)) is very close to ideal white light. Therefore, concentration of Mn²⁺ was fixed to be y = 0.10 and green component from Tb³⁺ is increased with its increasing content from z = 0.02 to 0.10. A near white light was achieved in CYSO:0.04Ce³⁺, 0.10Mn²⁺, 0.05Tb³⁺ and CYSO:0.04Ce³⁺, 0.10Mn²⁺, 0.08Tb³⁺, as given in Table 2.

4. Conclusion

In conclusion, Ce³⁺/Mn²⁺/Tb³⁺ co-activated CaY₄(SiO₄)₃O phosphors were synthesized and their photoluminescence properties were investigated. Ce³⁺ ions were confirmed to substitute three kinds of cationic sites and generated three excitation bands. Spectral overlap between emission of Ce³⁺ and excitation of Mn²⁺ indicates energy transfer occurred among them. Mechanism of the energy transfer is a resonant type through a dipole–quadrupole interaction. Critical distance calculated using concentration quenching method and spectral overlap route was to be 7.23 and 7.55 Å, respectively. Color tunable emission from blue to orange was realized after introduction of Mn²⁺. White light with CIE chromaticity coordinates (0.3339, 0.3055) and (0.3553, 0.3338) was achieved in sample CaY₄(SiO₄)₃O:0.04Ce³⁺, 0.10Mn²⁺, 0.05Tb³⁺ and CaY₄(SiO₄)₃O:0.04Ce³⁺, 0.10Mn²⁺, 0.08Tb³⁺, respectively. Preliminary studies have implied that the Ce³⁺/Mn²⁺/Tb³⁺ co-activated CaY₄(SiO₄)₃O phosphors may serve as a potential phosphor for UV chip pumped white light emitting diodes.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015060315). This work also supported by the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning. The Ce³⁺/Mn²⁺/Tb³⁺ activated CaY₄(SiO₄)₃O phosphor was supplied by the Display and Lighting Phosphor Bank at Pukyong National University.

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