Investigation of luminescence properties and the energy transfer mechanism of Li₆Lu(BO₃)₃:Ce³⁺, Tb³⁺ green-emitting phosphors

Abstract

Li₆Lu(BO₃)₃:Ce³⁺, Tb³⁺ phosphor was synthesized via a conventional solid-state method. The phase purity, photoluminescence properties, energy transfer mechanism, thermal stability and chromaticity coordinates were investigated. The absorption spectrum was comprised of broad bands in the UV region and the emission spectrum was consisted of characteristic peaks from both Ce³⁺ and Tb³⁺ under UV excitation. The amount of Ce³⁺ was fixed to 3 mol% while Tb³⁺ was varied from 20 to 80 mol% and the chromaticity coordinates were tuned from the blue to green region. The energy transfer mechanism between Ce³⁺ and Tb³⁺ was attributed to a quadrupole–quadrupole interaction and the critical distance was measured to be 8.12 Å. The overall performance of the phosphor was enhanced by the efficient energy transfer between Ce³⁺ and Tb³⁺ ions.

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Introduction

White light emitting diodes (W-LEDs) have provided remarkable advances in lighting technologies and displays. They have gained significant attention over conventional lighting sources due to their advantages such as low energy consumption, long operational lifetime, high efficiency, high stability and less environmental threat.^1–11 Commercially available W-LEDs are fabricated by combining a blue emitting InGaN-based LED chip covered by a yellow emitting phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG).^{1,2,4,6,7,10,11} However, YAG suffers from inevitable drawbacks such as low color rendering index (CRI) (Ra < 80) and high correlated color temperature (CCT) (T_c > 4500 K) due to the insufficient red emission in the visible spectrum.^{1–4,6–8,10,11} Moreover, blue LED chip and yellow phosphor have different degradation rates causing chromatic aberration and decreased efficiency over long period of time.¹ To overcome these defects, a new method has been developed by coupling red, green and blue tricolor phosphors with n-UV LED chip in order to produce high CRI, high color stability and good color uniformity.^1,2,6,7,10 Thus, developments of new phosphors with high chemical stability and strong absorption in UV or near-UV region with high conversion efficiency are highly crucial.^1–3,10

Borate compounds are widely used in nonlinear optics, piezo and scintillation techniques and phosphors for W-LEDs¹² due to its low synthesizing temperature and high physical and chemical stability.² Recent investigations were conducted on NaSrBO₃:Ce³⁺,² MSr₄(BO₃)₃:Ce³⁺ (M = Li and Na),³ Ca₃Y₂(BO₃)₄:Eu³⁺,⁴ Na₃La₂(BO₃)₃:Ce³⁺, Tb³⁺,⁵ KSr₄(BO₃)₃:Eu³⁺,⁷ LiSr₄(BO₃)₃:Ce³⁺, Eu²⁺,¹¹ LiCaBO₃,¹³ Ba₂Ca(BO₃)₂:Ce^3+ 14 and KCa₄(BO₃)₃:Ln³⁺ (Ln = Dy, Eu, Tb).¹⁵ (Li₆Ln(BO₃)₃ (Ln = Gd, Y) were investigated and showed promising results on W-LEDs and nonlinear optics application.^12,17–19 Li₆Lu(BO₃)₃ host was first synthesized and studied by Yang et al.¹⁶ and Fawad et al.²⁰ and it exhibited potential applications for neutron detection scintillator).

Energy transfer plays a crucial role in enhancing the luminescent properties of rare earth ions such as Tb³⁺ and Eu³⁺ which display only narrow excitation peaks near UV region due to the forbidden f–f transition and only yield sharp and weak emission peaks.¹ Ce³⁺ is not only used as an activator but also as a sensitizer. It can generate strong excitation peaks near UV region and can provide efficient conversion to longer wavelengths.²² Several Ce³⁺ and Tb³⁺ co-doped phosphors were synthesized such as Ba₃Gd(PO₄)₃:Ce³⁺, Tb³⁺,¹ Na₃La₂(BO₃)₃:Ce³⁺, Tb³⁺,⁵ SrMgSi₂O₆:Ce, Tb,⁶ Sr₂B₅O₉Cl:Ce³⁺, Tb³⁺,⁸ BaAl₂B₂O₇:Ce³⁺, Tb³⁺,⁹ Sr₃MgSi₂O₈:Ce³⁺, Tb³⁺,¹⁰ SrAl₂B₂O₇:Ce³⁺, Tb³⁺,²¹ Ca₃Y₂Si₃O₁₂:Ce³⁺, Tb^3+ 23 and so on.

To the best of our knowledge, the luminescence properties of Li₆Lu(BO₃)₃:Ce³⁺, Tb³⁺ have not been investigated yet. Borate host phosphors have lower synthesizing temperature than that of other host phosphors, such as silicon-based, aluminum-based and nitride-based system. These borate-based phosphors, such as Sr₂B₂O₅:Ce³⁺, Tb³⁺ (1073 K), NaCaBO₃ (1123 K), KCa₄(BO₃)₃ (1073 K) have been reported.^2,15,26,27 In this study, pure phased-Li₆Lu(BO₃)₃ was obtained at a much lower temperature of 973 K with high purity, providing an energy-saving and cost effective advantages over other phosphors. In this study, the crystal structure, photoluminescence (PL) properties, color chromaticity, energy transfer mechanism between the sensitizer and activator, and thermal quenching were investigated. The results indicate that Li₆Lu(BO₃)₃:Ce³⁺, Tb³⁺ is a potential green emitting phosphor for UV-LED applications.

Experimental section

Materials and synthesis

A series of rare earth-doped Li₆Lu(BO₃)₃:xCe³⁺, yTb³⁺ (x = 0.005, 0.01, 0.03, 0.05, 0.07, 0.10 mol and y = 0.20, 0.40, 0.60, 0.70 and 0.80 mol) phosphors were synthesized via solid state reactions. The reactants used were Li₂CO₃ (99.99%, Aldrich), H₃BO₃ (99.99%, Aldrich), Lu₂O₃ (99.99, Aldrich), CeO₂ (99.99%, Aldrich) and Tb₄O₇ (99.99%, Aldrich). The stoichiometric proportions of the precursors were weighed and thoroughly ground in an agate mortar. Subsequently, the powder was heated at 973 K for 8 hours under a reducing atmosphere (15% H₂/85% N₂). The products were then cooled down to ambient temperature and ground for further analyses.

Materials characterization

The crystallinity of the as-synthesized samples were characterized using X-ray diffractometer with Cu Kα (λ = 1.5418 Å) generated at 45 kV and 30 mA. Data were gathered in the 2θ range of 10° to 80° with a scan speed of 5° min⁻¹. The luminescence properties of the samples were determined using PL/PLE at room temperature and were recorded by a Spex-Fluorolog-3 spectrophotometer equipped with 450 W Xenon light source and measured with a scan rate of 150 nm min⁻¹. Commission International de I'Eclairage (CIE) chromaticity coordinates of the samples were measured using a Laiko DT-101 color analyzer equipped with a CCD detector (Laiko Co., Tokyo, Japan).

Results and discussion

XRD and crystal structure investigation

The X-ray powder diffraction (XRD) patterns of Li₆Lu(BO₃)₃ doped with 0.03Ce³⁺, 0.80Tb³⁺ and 0.03Ce³⁺, 0.65Tb³⁺ are presented in Fig. 1. The data indicate that the peaks of the samples were consistent with the standard JCPDS #83-0843 and no impurities were observed even with heavy doping of Tb³⁺ ions. Therefore, pure-phased samples were obtained. Lu³⁺ ions were successfully substituted by Ce³⁺ ions and Tb³⁺ ions due to their comparable ionic radii (Lu³⁺ = 0.977 Å, Ce³⁺ = 1.14 Å, Tb³⁺ = 1.04 Å). Li₆Lu(BO₃)₃ host phosphor belongs to the monoclinic system of Li₆RE(BO₃)₃ (RE = Gd, Y, Yb, Ho) and space group of P2₁/c.^12,16,17 The lattice parameters were calculated to be a = 0.7236 nm, b = 1.6584 nm, c = 0.6693 nm and β = 105.42°. The crystal structure of Li₆Lu(BO₃)₃ is shown in Fig. 2(a). The structural unit of Li₆Ln(BO₃)₃ (Ln = Gd, Y, Lu) consists of isolated boron with a triangular coordination with oxygen atoms, Li⁺ ions surrounded by four or five oxygen atoms forming trigonal bipyramids or tetrahedron prisms, and distorted eight-fold Lu³⁺ with C₁ site symmetry. LuO₈ polyhedra are connected with each other by common edges along the direction oblique to the c axis.^9,12,18

Fig. 1 XRD patterns of different samples synthesized at 973 K for 8 hours under 15% H₂/85% N₂ reducing atmosphere.

Fig. 2 (a) Crystal structure of Li₆Lu(BO₃)₃ and (b) coordination environment of Lu³⁺ ion.

Photoluminescence properties of Li₆Lu(BO₃)₃:xCe³⁺

Fig. 3 illustrates the PL/PLE of Li₆Lu(BO₃)₃:xCe³⁺. When the sample is excited at 350 nm it displays an asymmetric broad band from 360–480 nm. This phenomenon was due to the dual emission of Ce³⁺ attributed to the parity allowed transition from the lowest 5d level to the ²F_5/2 and ²F_7/2 ascribed to the spin–orbit coupling of the 4f ground state of Ce³⁺.^6,9,10,23 The emission band can be decomposed into two Gaussian profiles with maximum peaks at approximately 382 nm (26 178 cm⁻¹) and 415 nm (24 096 cm⁻¹) depicted in the inset with a difference of approximately 2082 cm⁻¹. This result is comparable to the theoretical value of ≈2000 cm⁻¹.^2,10,11 The excitation spectrum monitored at 400 nm consists of two broad bands with maximum peak at 350 nm which was due to the 4f → 5d transition of Ce³⁺.^1,2,6,8,9,21

Fig. 3 PL/PLE of Li₆Lu_0.97(BO₃)₃:0.03Ce³⁺ excited at 350 nm and monitored at 400 nm. Inset shows the Gaussian deconvolution of Ce³⁺ emission.

A series of 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10 mol concentration of Ce³⁺ were prepared and is presented in Fig. 4. It is evident that the emission intensity of samples initially increased as Ce³⁺ concentration increased until it reached maximum intensity at 3 mol%. Further increase of Ce³⁺ concentration resulted in decreased intensity due to the concentration quenching of the Ce³⁺. Also, it is noticeable that the emission spectra are shifted to a longer wavelength at higher Ce³⁺ concentration and the peak emission is red shifted from 388 nm to 398 nm which is attributed to the increase of crystal field effect.⁸

Fig. 4 PL of Li₆Lu_1−x(BO₃)₃:xCe³⁺ (x = 0.5, 1, 3, 5, 7 and 10 mol%) excited at 350 nm. Inset: PL intensity of Li₆Lu(BO₃)₃:Ce³⁺ as a function of Ce³⁺ concentration.

Photoluminescence properties of Li₆Lu_1−y(BO₃)₃:yTb³⁺

The ground state for Tb³⁺ ions with 4f⁸ configuration is ⁷F_J. When Tb³⁺ is excited it is promoted to the 5d shell which generates two 4f⁷5d¹ excitation states, a high spin state with ⁹D_J configurations or a low spin state with ⁷D_J configurations. According to Hund's rule, ⁹D_J levels have lower energy; as a result the transitions from ⁷F_J → ⁹D_J are spin–forbidden while ⁷F_J → ⁷D_J transitions are spin-allowed.²³ Fig. 5 shows the PL/PLE of Li₆Lu_1−y(BO₃)₃:yTb³⁺. The absorption spectrum monitored at 543 nm is comprised of narrow peaks from 200–275 nm attributed to the spin allowed 4f⁸ → 4f⁷5d¹ transition and sharp peaks from 300–400 nm located at 281, 303, 319, 342, 354, 360 and 377 nm due to the forbidden 4f–4f transitions of Tb³⁺ ions from ⁷F₆, to ⁵I_J, ⁵H_J, ⁵D_0,1, ⁵G_2,3,4, ⁵D₂, ⁵L₁₀, and ⁵D₃ levels, respectively.⁸ The highest peak is centered at 377 nm indicating that the phosphor can be efficiently excited by n-UV chips. Tb³⁺ can be an activator for blue and green phosphors. At lower concentration, it emits blue color on the other hand, at higher concentrations, it exhibits green emission. When the sample is excited at 377 nm, the emission spectrum exhibits four sharp peaks located at 489 nm, 543 nm, 591 nm and 618 nm which are assigned to the ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3) ascribed to the characteristic transition of Tb³⁺, respectively. The phosphor exhibits green emitting hue due to the maximum peak at 543 nm corresponding to ⁵D₄ → ⁷F₅ transition that is due to magnetic dipole transition.

Fig. 5 PL/PLE of Li₆Lu_0.40(BO₃)₃:0.60Tb³⁺ excited at 377 nm and monitored at 543 nm.

Different concentrations of Li₆Lu_1−y(BO₃)₃:yTb³⁺ (y = 0.20–0.80 mol) were synthesized and its corresponding emission intensities are illustrated in Fig. 6. The luminescence intensity of samples increased gradually as the concentration of Tb³⁺ ions increased. It reached maximum intensity at 60 mol% and began to decrease. This phenomenon is a result of the concentration quenching of the Tb³⁺ ions. ⁵D₃ → ⁵D₄ is resonant with ⁷F₆ → ⁷F₀ transition therefore, the emission due to ⁵D₃ → ⁷F_J transitions are often quenched at high Tb³⁺ content because of the cross relaxation ⁵D₃ + ⁷F₆ → ⁵D₄ + ⁷F₀.^1,9 The concentration quenching is higher than most Tb³⁺ doped phosphors. A possible reason is linked to the structure of Li₆Lu(BO₃)₃ which the zigzag structure of the activator ion restrict the energy migration to one dimension. Therefore, the probability of the migrating excitation to encounter the randomly distributed killer site is lessened.^10,12,17

Fig. 6 PL of Li₆Lu_1−y(BO₃)₃:yTb³⁺ (y = 20, 40, 50, 60, 70, and 80 mol%) excited at 377 nm.

Photoluminescence properties of Li₆Lu_0.97−y(BO₃)₃:0.03Ce³⁺, yTb³⁺

The 5d–4f transition of Ce³⁺ is electric dipole allowed and is evidently stronger compared to the 4f–4f intra-configurational transition of Tb³⁺. Fig. 7(a)–(c) depict the PL/PLE comparison of singly doped Ce³⁺, Tb³⁺ and Ce³⁺–Tb³⁺ co-doped Li₆Lu(BO₃)₃ phosphor. It is evident that there is a spectral overlap between the emission spectrum of Ce³⁺ in Fig. 7(a) and excitation spectrum of Tb³⁺ in Fig. 7(b) which implies that energy transfer between Ce³⁺ and Tb³⁺ is expected in Li₆Lu(BO₃)₃ host. In Fig. 7(c), there is an obvious overlap between the emission and excitation spectra of Ce³⁺–Tb³⁺ co-doping that verifies that the energy absorbed by Ce³⁺ is efficiently transferred to Tb³⁺ thus, Ce³⁺ is an efficient sensitizer for Tb³⁺. Furthermore, there is a broad absorption band at UV region attributed to Ce³⁺ which makes it suitable to be excited by n-UV LED chip.

Fig. 7 (a). PL/PLE of Li₆Lu(BO₃)₃:0.01Ce³⁺ excited at 350 nm and monitored at 400 nm; (b) PL/PLE of Li₆Lu(BO₃)₃:0.60Tb³⁺ excited at 377 nm and monitored at 543 nm; (c) PL/PLE of Li₆Lu(BO₃)₃:0.03Ce³⁺,0.65Tb³⁺ excited at 350 nm and monitored at 543 nm.

Energy transfer between sensitizer and activator enhances the emission intensity. Fig. 8 illustrates the investigation of series of phosphors co-doped with 3 mol% of Ce³⁺ and varying concentrations of Tb³⁺ (y = 20–80 mol%) excited at 350 nm. The broad band situated from 360–450 nm is ascribed to the 5d–4f transition of Ce³⁺ ions while the narrow peaks 475–650 nm are assigned to the ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3) transitions of Tb³⁺ with maximum peak at 543 nm emitting a green color. The PL intensity initially increased as the concentration of Tb³⁺ increased until it reached its optimum concentration at x = 0.03, y = 0.65. As the Tb³⁺ concentration further increased, the emission intensity decreased as a result of concentration quenching. Conversely, the intensity of Ce³⁺ is inversely proportional to the concentration of Tb³⁺. These results denote that an effective energy transfer occurred. Moreover, comparing the PL of Fig. 8 with Fig. 6, it is highly evident that the intensity of Ce³⁺–Tb³⁺ co-doping is higher than singly doped Tb³⁺ proving that Ce³⁺ is an efficient sensitizer.

Fig. 8 PL of Li₆Lu(BO₃)₃:0.03Ce³⁺, yTb³⁺ (y = 20, 40, 60, 65, 70 and 80 mol%) excited at 350 nm and monitored at 543 nm.

Schematic diagram of energy transfer

The energy levels and energy transfer mechanism are illustrated in the schematic diagram in Fig. 9. When excited at 350 nm, Ce³⁺ ions are excited to the 5d configuration then the ions relax non-radiatively to the lowest of 5d state. The excited ions return to its ground state at ²F_5/2 and ²F_7/2 levels with a consequent broad band emission. As a consequence of the matched energy levels of Ce³⁺ and Tb³⁺, the sensitizer (Ce³⁺) can transfer its energy to the activator (Tb³⁺) thence causing Tb³⁺ ions to excite from its ground state. Tb³⁺ ions relax at level ⁵D₃ with subsequent ⁵D₃ → ⁷F_J transitions. Cross-relaxation might occur between ⁵D₃ and ⁵D₄ simultaneously with increasing amount of Tb³⁺ generating stronger emissions and higher intensities at ⁵D₄ → ⁷F_J transitions while leading to a decrease in ⁵D₃ emission. Finally, the excited Tb³⁺ ions return to its ground state with a subsequent emission of green luminescence.^1,6

Fig. 9 The schematic energy levels of Ce³⁺ and Tb³⁺ with energy transfer mechanism.

Ce³⁺ → Tb³⁺ energy transfer

In order to further analyze the energy transfer between Ce³⁺ and Tb³⁺, Fig. 10 presents the energy transfer efficiency of Ce³⁺ and Tb³⁺, 4f–5d transition of Ce³⁺ and ⁵D₄ → ⁷F₅ transition of Tb³⁺ all as a function of Tb³⁺ concentration in Li₆Lu(BO₃)₃ phosphor excited at 350 nm. The energy transfer efficiency (η_ET) can be calculated using following equation:^1,6,8,9where I_S and I_S0 are the luminescence intensities of Ce³⁺ in the presence and absence of Tb³⁺, respectively. The value of η_ET increased gradually with an increase of Tb³⁺ concentration. When Tb³⁺ concentration increased to 65 mol%, the η_ET increased to 99.60% indicating efficient energy transfer between the sensitizer and activator. The intensities of 4f–5d transition of Ce³⁺ decreased as the amount of Tb³⁺ increased thus the energy of Ce³⁺ ions is successfully transferred to its neighboring Tb³⁺ ions. On the other hand, Tb³⁺ initially increased until it decreased an optimum concentration at 65 mol% and eventually decreased with further addition of Tb³⁺ into the host.

Fig. 10 Dependence of Ce³⁺ emission (4f → 5d), Tb³⁺ emission (⁵D₄ → ⁷F₅) and energy transfer efficiency as a function of Tb³⁺.

Besides from spectral overlap, efficient energy transfer involves strong interactions. It may transpire under radiative transfer through photons which is nearly distance independent and non-radiative transfer which is associated with the resonance between the donor and acceptor. It can be either exchange interaction or multipolar interaction.^24,25 To calculate the critical distance (R_c) between Ce³⁺ ions and Tb³⁺ ions, the following equation is used:^9,10,24

where V is the volume of the unit cell (V = 762.43 ų), χ_c is the total concentration of Ce³⁺ and Tb³⁺ at maximum intensity (χ_c = 0.68) and N is the number of host cations in the unit cell (N = 4). Using eqn (2) the calculated critical distance is approximately 8.12 Å. R_c should be less than 5 Å in an exchange interaction. Thus, the principle governing the energy transfer is multipolar interaction.^5,24 According to Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the following relationship is obtained:^1,6,8,9,24where η and η₀ are the luminescence quantum efficiencies of Ce³⁺ with and without Tb³⁺ ions present, respectively. The ratio η/η₀ can be estimated by the ratio of relative emission intensities, I_S0/I_S in the absence and presence of activator. C_Ce+Tb is the total concentration of Ce³⁺ and Tb³⁺ and n = 6, 8 and 10 corresponds to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole mechanisms, respectively.^1,6,8,9,24

In order to further comprehend the energy transfer mechanism between Ce³⁺ and Tb³⁺ ions, Fig. 11(a)–(c) illustrate the relationships of I_S0/I_S and C^n/3. Among these three, Fig. 10(c) demonstrates the most commendable linear relationship. Therefore, it signifies that the energy transfer mechanism between Ce³⁺ and Tb³⁺ in Li₆Lu(BO₃)₃ host is governed by quadrupole–quadrupole interaction.

Fig. 11 The dependence of I_S0/I_S of Ce³⁺ on (a) C_Ce–Tb^6/3, (b) C_Ce–Tb^8/3and (c) C_Ce–Tb^10/3.

Aside from high luminescence, another crucial property of LED phosphors is thermal stability predominantly for high power LEDs in which the operating temperature can reach up to 450 K.¹² Fig. 12(a) depicts the temperature dependence of the luminescence intensity of Li₆Lu(BO₃)₃:0.03Ce³⁺, 0.65Tb³⁺ excited at 350 nm in the temperature range of 298 K to 573 K. The maximum peak is constantly located at 543 nm even at higher temperature. The emission intensity progressively decreased as the temperature increased due to thermal quenching. Fig. 12(b) presents the normalized intensity of the thermal luminescence. At 373 K, the thermal luminescence is reduced by almost 12%, at 473 K the intensity is decreased to 50% and at 573 K, the thermal luminescence is reduced by approximately 76%.

Fig. 12 (a) Thermal luminescence of Li₆Lu_0.32(BO₃)₃:0.03Ce³⁺, 0.65Tb³⁺ excited at 350 nm. The insets shows the (b) normalized intensity as a function of temperature and (c) the graph of ln[(I₀/I) − 1] versus 1/κT.

To promote better understanding of the relationship between the temperature and luminescence intensity and to determine the activation energy for thermal quenching, the gathered data were fitted to the Arrhenius equation:^12,14

where I(T) is the intensity at given temperature T, I₀ is the initial intensity, A is a constant, κ is the Boltzmann constant (8.617 × 10 ⁻⁵ eV K⁻¹) and E is the activation energy for thermal quenching. Fig. 12(c) plots the ln[(I₀/I) − 1] versus 1/κT and the activation energy is calculated to be 0.320 eV. The high activation energy indicates good thermal stability.

CIE coordinates of Li₆Lu_0.97(BO₃)₃:0.03Ce³⁺ and Li₆Lu_0.97−y(BO₃)₃:0.03Ce³⁺, yTb³⁺ phosphors

Commission International de I'Eclairage (CIE) chromaticity coordinates of the samples are presented in Fig. 13 and summarized in Table 1. The CIE coordinates illustrate that with increasing Tb³⁺ concentration, the chromaticity is tuned from blue to green as a result of the cross-relaxation between ⁵D₃ level and ⁵D₄ level. Table 1 also shows the relative intensity of ⁵D₄ → ⁷F₅ transition and it indicates that the influence of energy transfer between Ce³⁺ and Tb³⁺ has significantly improved the luminescence properties of the phosphor. At 350 nm, the relative intensity of Li₆Lu(BO₃)₃:0.03Ce³⁺, 0.65Tb³⁺ was enhanced by 178%. It is also noticeable that Li₆Lu_0.97(BO₃)₃:0.03Ce³⁺ has high color purity.

Fig. 13 CIE coordinates of Li₆Lu_0.97(BO₃)₃:0.03Ce³⁺ and Li₆Lu_0.97−y(BO₃)₃:0.03Ce³⁺, yTb³⁺ (y = 0.20 to 0.75) phosphors under 350 nm excitation.

Table 1 Chromaticity coordinates of Li₆Lu_0.97(BO₃)₃:0.03Ce³⁺ and Li₆Lu_0.97−y(BO₃)₃:0.03Ce³⁺, yTb³⁺ phosphors

Phosphors	Excitation wavelength (nm)	CIE chromaticity coordinates		^5D4 → ^7F5 Relative intensity
		x	y
1. Li6Lu0.97(BO3)3:0.03Ce^3+	350	0.162	0.015	0
2. Li6Lu0.77(BO3)3:0.03Ce^3+,0.20Tb^3+	350	0.317	0.519	1.00
3. Li6Lu0.57(BO3)3:0.03Ce^3+,0.40Tb^3+	350	0.334	0.570	1.43
4. Li6Lu0.37(BO3)3:0.03Ce^3+,0.60Tb^3+	350	0.340	0.586	1.50
5. Li6Lu0.32(BO3)3:0.03Ce^3+,0.65Tb^3+	350	0.340	0.588	1.78
6. Li6Lu0.27(BO3)3:0.03Ce^3+,0.70Tb^3+	350	0.342	0.589	1.64
7. Li6Lu0.22(BO3)3:0.03Ce^3+,0.75Tb^3+	350	0.342	0.593	1.53

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Conclusions

A series of tunable blue to green emitting Li₆Lu_0.97−y(BO₃)₃:0.03Ce³⁺, yTb³⁺ phosphors were successfully synthesized via conventional solid state reaction. Pure phased samples were successfully obtained even at high Tb³⁺ doping concentration. The energy absorbed by Ce³⁺ ions were efficiently transferred to Tb³⁺ since the 5d energy level of Ce³⁺ ions is close to the ⁵D₃ level of Tb³⁺ ions resulting in improved photoluminescence properties of the phosphors. The gathered results indicate that Ce³⁺ is an efficient sensitizer for Tb³⁺. The energy transfer is governed by a resonant type quadrupole–quadrupole interaction and the energy transfer critical distance is calculated to be 8.12 Å. The high activation energy denotes high thermal stability of the phosphor. The color of Li₆Lu(BO₃)₃:Ce³⁺, Tb³⁺ phosphor can be modified from blue to green under UV radiation and shows a great potential for W-LED applications.

Acknowledgements

This research is supported by the National Science Council of Taiwan under contract numbers: 102-2221-E-033-050-MY2 and 102-3011-P-033-003.

Notes and references

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