Tricolor- and White Light–Emitting Ce³⁺/Tb³⁺/Mn²⁺-Coactivated Li₂Ca₄Si₄O₁₃ Phosphor via Energy Transfer

Abstract

Single-component tunable Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺,Mn²⁺ phosphors were successfully synthesized at 950 °C. Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺ exhibits two luminescence peaking at 430 and 550 nm, which originated from the allowed 5d → 4f transition of the Ce³⁺ ion and the ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transition of the Tb³⁺ ion, respectively. Moreover, by codoping Ce³⁺ ions in the Li₂Ca₄Si₄O₁₃:Mn²⁺ system, yellow-red emission from the forbidden transition of Mn²⁺ could be enhanced. Under UV excitation, dual energy transfers (ETs), namely, Ce³⁺ → Mn²⁺ and Ce³⁺ → Tb³⁺, are present in the Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺,Mn²⁺ system. The ET process was confirmed by the overlap of the excitation spectra, variations in the emission spectra, ET efficiency, and decay times of phosphors. In addition, quantum yields and CIE chromatic coordinates are presented. The emission color of these phosphors can be tuned precisely from blue to green via ET of Ce³⁺ → Tb³⁺ and from blue to yellow via ET of Ce³⁺ → Mn²⁺. White light can also be achieved upon excitation of UV light by properly tuning the relative composition of Tb³⁺/Mn²⁺. This result indicates that the developed phosphor may be regarded as a good tunable emitting phosphor for UV light-emitting diodes.

1. Introduction

White light–emitting diode (w-LED), because of its highly efficient luminance, long service life, improved performance, and environment-friendly properties, has gained considerable attention in order to address global energy shortage.¹⁻⁵ In the generation of white light with LED chips, basically two methods are used. The first method is by mixing red, green, and blue (RGB) lights emitted by different LED chips, which, because of the required complex electro-optical design to control different colors, is seldom used. The second method is by using UV or blue LED chips and down-converting phosphors.⁶⁻⁸ To circumvent the disadvantages of these approaches, researchers have focused on investigating single-phased phosphors with RGB components, which could emit white light during energy transfer (ET) between activators in an efficient and durable way and with excellent color-rendering indices. Thus, in order to avoid high cost, device complication, and intrinsic color balance in the use of multiple-emitting components, it is crucial to find new single-component and UV chip pumped phosphors for w-LED.

Different emission colors can be obtained with single-phased phosphors by codoping ion pairs through ET, which was investigated extensively for the pairs of Ce³⁺ and Mn²⁺ or Tb³⁺ among single-component lattices. Li et al. reported on a color-tuning luminescence of Mg₂Y₈(SiO₄)₆O₂:Ce³⁺/Mn²⁺/Tb³⁺ via ET,⁹ whereas Huang and Chen described a single-composition Ca₃Y(GaO)₃(BO₃)₄:Ce³⁺,Mn²⁺,Tb³⁺ phosphor for UV-LEDs.¹⁰ Then, again, Zhang et al. investigated the ET from Ce³⁺ to Tb³⁺ and Ce³⁺ to Mn²⁺ in Ca₃Gd₇(PO₄)(SiO₄)₅O₂ host,¹¹ and Lü et al. reported some results on the spectral properties and ET of a color-tunable CaScAlSiO₆:Ce³⁺,Tb³⁺,Mn²⁺ phosphor.¹² Single-composition trichromatic Sr_3.5Y_6.5O₂(PO₄)(_1.5)(SiO₄)_4.5:Ce³⁺/Tb³⁺/Mn²⁺ and NaCaBO₃:Ce³⁺,Tb³⁺,Mn²⁺ phosphors via ET are also reported by Liu and Zhang et al.^13,14 Recently, other researchers have also suggested the ET mechanism in proper single-host lattices, such as Ca₁₉Mg₂(PO₄)₁₄:Ce³⁺,Tb³⁺,Mn²⁺ and Ca₉ZnK(PO₄)₇:Ce³⁺,Tb³⁺,Mn²⁺, indicating that the obtained samples may be a promising candidate for UV-convertible devices.¹⁵⁻¹⁸ In these systems, for either Mn²⁺ or Tb³⁺ ions, because of their prohibited ⁴T₁ → ⁶A₁ transition and 4f–4f weak absorption, respectively, both have weak emissions.^15,16 However, in codoping of Ce³⁺ with the allowed transition as sensitizer, ET will occur from Ce³⁺ to Mn²⁺ or Tb³⁺, which allows for efficient emission of Mn²⁺ and Tb³⁺ ions and tunability of the emission colors from blue to orange/red and green, respectively. This feature was used in the development of new tridoped phosphors.¹⁹⁻³⁰

In this paper, we report our recent results on the photoluminescence (PL) properties and the ET mechanism of Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺,Mn²⁺ phosphor. To the best of our knowledge, no study has been conducted on Ce³⁺/Tb³⁺/Mn²⁺ codoped in the Li₂Ca₄Si₄O₁₃ host. We generated green/yellow/white lights with high color stabilities using Tb³⁺ and Mn²⁺ at different relative dopant concentrations. Additionally, we systematically investigated the mechanisms of ET from Ce³⁺ to Tb³⁺ or Mn²⁺ and their decay times.

2. Results and Discussion

2.1. X-ray Diffraction

We collected and used X-ray diffraction (XRD) patterns to study the samples in order to identify their compositions and phase purities. The representative XRD patterns for LCSO, LCSO:Ce³⁺,Tb³⁺, LCSO:Ce³⁺,Mn²⁺, and LCSO:xCe³⁺,yTb³⁺,zMn²⁺ samples are showed in Figure 1. The selected samples have pure phases, and their XRD patterns exactly originate from the pure tetragonal phase of Li₂Ca₄Si₄O₁₃ according to JCPDS file no. 31-0714. No other phase was detected, which indicates that all of the Mn²⁺, Tb³⁺, and Ce³⁺ ions are dissolved in Li₂Ca₄Si₄O₁₃.

Figure 1.

Powder XRD patterns of LCSO, LCSO:Ce³⁺,Tb³⁺, LCSO:Ce³⁺,zMn²⁺, and LCSO:xCe³⁺,yTb³⁺,zMn²⁺ samples.

2.2. Luminescence Properties of LCSO:Ce³⁺ and ET of LCSO:Ce³⁺,Tb³⁺

Both PL excitation (PLE) and PL spectra for as-prepared LCSO:xCe³⁺ samples are shown in Figure 2a. Three distinctive bands peaked at 259, 296, and 353 nm (the strongest) were found spanning a broad absorption from 200 to 400 nm for the PLE spectrum monitored at 420 nm. These peaks are caused by transitions to ²F_5/2 and ²F_7/2 state from the lowest 5d state because of spin–orbit coupling. The PL spectrum extends from 400 to 550 nm and peaks at 450 nm, caused by the 5d → 4f transitions from Ce³⁺. Figure 2b shows the concentration dependence of the emitting intensities of LCSO:xCe³⁺ with various Ce³⁺ concentrations. After excitation at 330 nm, the emitting intensities increase with Ce³⁺ ion contents until the highest point x = 0.04, after which the intensities decline because of concentration quench. Between several excitation bands of Tb³⁺ and the broad Ce³⁺ emission band, a significant spectral overlap is observed in Figure 3. Because of transitions from ⁵D₄ → ⁷F₅, a main emission line and well-known green fluorescence occur in LCSO:0.1Tb³⁺ at 543 nm. Moreover, several weak lines appear at 485, 580, and 620 nm, corresponding to ⁵D₄ to ⁷F_J (J = 3, 4, 5, 6) multiple transitions, respectively. Because of the prohibited Tb³⁺ f–f absorbing transitions in the UV region, LCSO:0.1Tb³⁺ demonstrates very weak emissions excited by UV light compared to the PL spectrum of LCSO:0.04Ce³⁺. An apparent overlap between the Tb³⁺ PLE and Ce³⁺ PL spectra indicates a possible Ce³⁺–Tb³⁺ ET in LCSO. The PLE and PL spectra in LCSO:0.04Ce³⁺,0.1Tb³⁺ are illustrated in Figure 3c. A profile with similarities to the Ce³⁺ single-doped samples appears with that of the Ce³⁺,Tb³⁺ codoped samples. The Tb³⁺ emitting intensity is enormously increased relative to Ce³⁺ emissions, which validates a Ce³⁺–Tb³⁺ ET.

Figure 2.

(a) Excitation and emission spectra of LCSO:xCe³⁺. (b) PL intensity of LCSO:xCe³⁺ samples with different Ce³⁺ contents.

Figure 3.

PL and PLE spectra in LCSO:0.04Ce³⁺ (a), LCSO:0.1Tb³⁺ (b), and LCSO:0.04Ce³⁺,0.1Tb³⁺ (c).

To understand the effect of ET on the luminescent properties (Figure 4), we studied PL spectra dependence on Tb³⁺ dopant contents for LCSO:0.04Ce³⁺,yTb³⁺ phosphors excited by UV (λ_ex = 330 nm) (y = 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12). The PL intensities of Ce³⁺ decrease monotonically with increasing Tb³⁺ concentrations because of the effective Ce³⁺–Tb³⁺ ET. Meanwhile, those of Tb³⁺ at 550 nm increase until the y value is above 0.06 when the emission intensities of Tb³⁺ are saturated. Moreover, the Tb³⁺ emitting intensities start to decrease when the Tb³⁺ dopant content (y) is over 0.06 because of the internal quench of concentrations of Tb³⁺–Tb³⁺.

Figure 4.

PL spectra of LCSO:0.04Ce³⁺,yTb³⁺ (y = 0–0.12) under 330 nm UV excitation.

The Ce³⁺ decay curves were measured for the investigation of the ET processes in LCSO:Ce³⁺,Tb³⁺. The Ce³⁺ decay curves in LCSO:0.04Ce³⁺,yTb³⁺ phosphor (y = 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) samples were also collected (Figure 5). The lifetimes can be defined in equation τ_D = ∫₀^∞I_D(t)dt, where I_D(t) is the decay function of donors in the absence of acceptors, which have normalized to its initial intensity. The Ce³⁺–Tb³⁺ ET features are reflected by the fluorescence intensities, which decrease with increasing Tb³⁺ concentrations.³¹⁻³³ The efficiency of Ce³⁺–Tb³⁺ ET is calculated with the formula

1

where τ₀ and τ are the decay lifetimes of the sensitizer Ce³⁺ in the absence and presence of activator Tb³⁺, respectively. The calculated value is illustrated in Figure 5, which will increase monotonously with the y values until the highest 45% corresponding to a y value of 0.12.

Figure 5.

Fluorescence decay curves of Ce³⁺ as a function of the Tb³⁺ contents. Inset: The calculated ET efficiency with different doping Tb³⁺ contents.

Ce³⁺ to Tb³⁺ ET occurs during electric–multipole interaction and could be determined through spectral overlap method, as follows, in line with Dexter’s ET expression for multipolar interactions³⁴⁻³⁶

2

where I and I₀ respectively represent the luminescence intensities of the samples in the present and absent state of Tb³⁺, and C represents total Ce³⁺ and Tb³⁺ concentration. The correlation of (I₀/I) with C^n/3 with n = 6, 8, and 10 corresponds to a dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interaction. The linear fittings of the relationship between C^n/3 and (I₀/I) are demonstrated in Figure 6, and a good linear correlation is obtained at n = 8, indicating a dipole–quadrupole for Ce³⁺–Tb³⁺ ET. According to spectral overlap theory, R_C can be calculated by the formula

3

where f_q represents oscillator strength in quadrupole transitions, ∫F_S(E)F_A(E)/E⁴ dE represents spectral overlapping among the normalized shape of the Ce³⁺ emission F_S(E) and the Tb³⁺ excited F_A(E), E (eV) is the energy in the transfer, and R_C (Å) represents the sensitizer–acceptor average distance. Correspondingly, during the sensitizer’s emissions, it is approximately 0.0051 eV^–5. λ_S (Å) of the wavelength position. So far, it has not been obtained for the oscillator strength in Tb³⁺ quadrupole transitions (f_q). However, Verstegen et al. suggested that 10^–3 to 10^–2 is approximately the f_q/f_d ratio, where f_d = 10^–6 is the Tb³⁺ electric dipole transition strength.³⁶ According to the parameters mentioned above, R_C^dq of dipole–quadrupole interactions is 11.53–15.38 Å.

Figure 6.

Dependence of I/I₀ vs C^n/3 of Ce³⁺ and Tb³⁺ on C^6/3, C^8/3, and C^10/3 in LCSO: Ce³⁺,Tb³⁺.

2.3. Luminescence Properties and ET of LCSO:Ce³⁺,Mn²⁺

As a transition metal ion with 3d⁵ configuration, its position of the broad emitting bands is largely determined by its host. In LCSO host, the intensity luminescence of Mn²⁺ emission is too weak to be detected owing to prohibited Mn²⁺ d–d transitions. Thus, the PLE and PL spectra of a series of Mn²⁺ and Ce³⁺ codoped LCSO phosphors are illustrated in Figure 7. The PLE spectrum of Mn²⁺ emissions is similar to that of Ce³⁺. Thus, Mn²⁺ ions can be excited by UV or near-UV light, which strongly verify an effective Ce³⁺–Mn²⁺ ET. The effective resonance-type Ce³⁺–Mn²⁺ ET is relatively common and was observed in several Ce³⁺ and Mn²⁺ codoped phosphors, including Ca₃Sc₂Si₃O₁₂:Ce³⁺,Mn²⁺ and Ca₉Y(PO₄)₇:Ce³⁺,Mn²⁺.^37,38 The PL spectrum demonstrates multicolor emissions consisting of an orange band excited at 365 nm caused by Mn²⁺⁴T₁–⁶A₁ transitions and a blue band caused by Ce³⁺ f–d transitions, respectively. Therefore, the red emission in LCSO:Ce³⁺,Mn²⁺ phosphors originates from ET from Ce³⁺ to Mn²⁺. The detailed PL spectra of the series of LCSO:0.04Ce³⁺,zMn²⁺ phosphors are shown in Figure 8, which are excited by a wavelength of 330 nm, with the z value from 0 to 0.24. With increasing Mn²⁺ concentration, two broad bands with emission peaks at 450 and 580 nm are detected. Because of Ce³⁺–Mn²⁺ ET, the emission intensities of Mn²⁺ ions increase systematically. Additionally, concentration quench may induce the decrease in Mn²⁺ emission intensity.

Figure 7.

PL and PLE spectra of LCSO:0.04Ce³⁺,0.2Mn²⁺.

Figure 8.

Variation of PL spectra of LCSO:0.04Ce³⁺,zMn²⁺ (z = 0–0.24) phosphors excited with 330 nm UV light.

In LCSO:0.04Ce³⁺,zMn²⁺ (z = 0, 0.04, 0.08, 0.12, 0.16, 0.2, 0.24), the decay curves of Ce³⁺ fluorescence were measured (Figure 9a), which reveals that the Ce³⁺ lifetime decreases with increased Mn²⁺ concentrations and verifies its Ce³⁺–Mn²⁺ ET efficiency (calculated with eq 1; refer to Figure 9b). The efficiency of ET significantly increases, and the maximum value is approximately 30.4% of Mn²⁺ concentration at z = 0.24, which clarifies that Ce³⁺ contributes to the yellow emission energy of Mn²⁺. The relationship of (I₀/I) to C^n/3 is shown in Figure 10. Similar to the investigated and observed results of other research groups, the ET of Ce³⁺–Mn²⁺ arises from the dipole–quadrupole mechanism.^39,40

Figure 9.

(a) Decay curves of Ce³⁺ in LCSO:0.02Ce³⁺,zMn²⁺ (z = 0, 0.12, and 0.24) under 330 nm excitation and monitored at 430 nm. (b) Lifetimes of the sensitizer Ce³⁺ and the ET efficiency (η_Ce–Tb) with different doping Mn²⁺ contents.

Figure 10.

Experimental data plots of I/I₀ vs C^n/3.

2.4. Quantum Yields and Chromaticity of LCSO:Ce³⁺,Tb³⁺,Mn²⁺

The absolute internal quantum yield (QY) of the obtained phosphors was measured to explore the potential of the as-synthesized LCSO:Ce³⁺,Tb³⁺,Mn²⁺ phosphors. Upon excitation at 365 nm UV radiation, the absolute internal QY of LCSO:0.04Ce³⁺,yTb³⁺ (y = 0–12%) phosphors are 60.3, 62.1, 64.5, 67.3, 61.7, 55.9, and 47.2%. For LCSO:0.04Ce³⁺,zMn²⁺ (z = 0–24%) samples, the absolute internal QY is 65.3, 48.8, 31.6, 27.2, 23.4, 20.9, and 13.1%. The internal QY values decrease obviously with the increase in Mn²⁺ concentration, which may be in relation to two factors: First, the energy should be lost in the Ce³⁺ → Mn²⁺ ET process. Second, the internal quantum efficiency can be expressed as η_in = W_R/(W_R + W_NR), where W_R and W_NR represent the radiative transition rate and the nonradiative decay rate, respectively. This suggests that the nonradiative decay rate is much higher than the radiative decay rate, leading to low internal quantum efficiency of the Mn²⁺ ions. The maximum internal QY in the as-prepared samples can reach 67.3%. The high QYs of our samples indicate that LCSO functions well to dope rare-earth ions as a matrix. For LCSO:Ce³⁺,yTb³⁺,zMn²⁺ at varied contents (y and z), their CIE chromaticity diagrams is shown in Figure 11. The color point evolves from blue (0.155, 0.112) to white (0.249, 0.267) by changing the Mn²⁺ concentrations and from blue (0.155, 0.112) to green (0.298, 0.524) by changing the content of Tb³⁺. The tunable color point results from the ET processes of Ce³⁺ → Tb³⁺ and Ce³⁺ → Mn²⁺, which is deduced from the energy-level model in Figure 12. Upon excitation at 365 nm, Ce³⁺ ions can be effectively excited. Then some Ce³⁺ ions go back to the ground state of ²F_7/2 or ²F_5/2 spontaneously by a radiative process, and other excited Ce³⁺ ions transfer its energy to the ⁵D₂/⁵D₃ state of Tb³⁺ and ⁴A₁ (4G) state of Mn²⁺. Finally, Tb³⁺ and Mn²⁺ ions will emit a green and red emission band, respectively. In particular, the inset in Figure 11 shows a series of digital photos of the selected LCSO:0.04Ce³⁺,yTb³⁺,zMn²⁺ phosphors upon UV lamp excitation at 365 nm. The material exhibits tunable color emission. LCSO:0.04Ce³⁺,0.24Mn²⁺ can emit white light and thus can be used as a single-component multicolor-emitting phosphor for UV LEDs.

Figure 11.

CIE chromaticity diagram of the as-prepared samples under 365 nm UV lamp excitation and the selected digital photos.

Figure 12.

Energy-level diagram of ET from Ce³⁺ ions to Tb³⁺/Mn²⁺.

3. Conclusions

In summary, we have synthesized a series of emission-tunable Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺,Mn²⁺ phosphors by solid-state reaction. The ETs from Ce³⁺ to Tb³⁺ and from Ce³⁺ to Mn²⁺ in Li₂Ca₄Si₄O₁₃ were observed. The ET from Ce³⁺ to Tb³⁺ in Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺ phosphors is a resonant type via the dipole–quadrupole mechanism, and the calculated critical distance (R_C) is 11.53–15.38 Å. The ET from Ce³⁺ to Mn²⁺ was investigated based on PL and PLE spectra, decay curves, and effect of Ce³⁺/Mn²⁺ concentration. Color emission and luminescence internal QY (13–67%) can be tuned by controlling the contents of Ce³⁺, Tb³⁺, and Mn²⁺. Hence, Li₂Ca₄Si₄O₁₃:Ce³⁺,Tb³⁺,Mn²⁺ could be a white light-emitting phosphor for UV-excited w-LEDs.

4. Experimental Section

4.1. Sample Preparation

Li₂Ca_{4–x–y–z}Si₄O₁₃:xCe³⁺,yTb³⁺,zMn²⁺ (LCSO:xCe³⁺,yTb³⁺,zMn²⁺) were synthesized by a high-temperature solid-state reaction. Stoichiometric amounts of starting materials, including CaCO₃ (A. R.), Li₂CO₃ (A. R.), SiO₂ (A. R.), CeO₂ (99.99%), Tb₄O₇ (99.99%), and MnCO₃ (A. R.), were thoroughly mixed in an agate mortar for 0.5 h. The mixed powders were calcined at 950 °C under a reducing atmosphere of H₂ (5%) and N₂ (95%) for 4 h. The samples were cooled to room temperature to obtain the final products.

4.2. Measurements and Characterization

The structure and crystallinity of the sample was identified by powder XRD analysis (Bruker AXS D8, Cu Kα radiation, λ = 1.5405 Å, 40 kV, 40 mA). PL and PLE spectra were collected on a Hitachi F7000 spectrometer (700 V, 150 W xenon lamp, EX/EM slit of 2.5 nm, scanning rate of 1200 nm/min). Luminescence decay profiles were obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) by using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). PL internal QYs were measured by using the absolute PL QY measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). All of the measurements were performed at room temperature.

The authors declare no competing financial interest.