Highly efficient and thermally stable luminescence of Ca₃Gd₂Si₆O₁₈:Ce³⁺,Tb³⁺ phosphors based on efficient energy transfer

Abstract

As phosphor-converted white light emitting diodes have emerged as new-generation light sources, developing phosphors with both high quantum efficiency (QE) and thermal stability is urgently required. One of the effective strategies is exploiting the nonradiative energy transfer to simultaneously improve the absorption of the activator and tune the emission of the sensitizer. In this work, we report a series of color-tunable phosphors by constructing an energy transfer system by co-doping Ce³⁺ and Tb³⁺ into Ca₃Gd₂Si₆O₁₈ (CGSO). We show that CGSO:Ce³⁺,Tb³⁺ not only exhibits color-tunable emission from violet (395 nm) to green (543 nm), but also possesses high internal QE (90.1%). Furthermore, we demonstrate that thermal stability can be significantly improved via efficient energy transfer from Ce³⁺ to Tb³⁺, which is mainly attributed to the fast energy transfer within the nearest Ce³⁺–Tb³⁺ pairs. Our results indicate that energy transfer from Ce³⁺ to Tb³⁺ provides a promising way to develop new green phosphors with high QE and superior thermal stability.

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1. Introduction

Solid-state lighting (SSL) has been developed rapidly because of the excellent features such as long service time, energy saving and eco-friendliness.^1–8 The phosphor converted white light emitting diode (w-LED), fabricated by combining LED chips with phosphors that can be efficiently excited by the chip light source, is the most widely used SSL to generate white light. Therefore, the luminescence properties of the phosphors play an important role in optimizing the final performance of w-LED devices. However, developing phosphors with high quantum efficiency (QE), excellent thermal stability and suitable emission is still challenging.^9–12

Trivalent lanthanide ions (Ln³⁺) have been utilized in discovering luminescent materials because of their intra-configurational 4f → 4f transitions, which give them pure emission colors. Unfortunately, the 4f → 4f parity-forbidden transitions also endow them with quite weak absorption of the excitation light, leading to a relatively low emission efficiency.^13–15 An alternative way of solving this problem is exploiting energy transfer (ET) from sensitizers with a large absorption cross section to Ln³⁺ by enhancing the absorptivity of excitation light in the UV or near ultraviolet (n-UV) range. Among Ln³⁺ ions, Tb³⁺ has been used as a green emission activator due to the ⁵D₄ → ⁷F₅ transition in some host materials for applications in lasers and lightings.^16–18 However, the application of Tb³⁺ is limited by not only the narrow absorption lines caused by the forbidden 4f → 4f transitions but also the broad 4f⁸–4f⁷5d¹ parity-allowed excitation band located in the deep UV region (<300 nm).^19–21 Another ion Ce³⁺ has strong 4f–5d emission bands that can not only match well with the absorption lines of Ln³⁺ but also the transition metal ions such as Mn²⁺ and Cr³⁺, and thus highly efficient ET from Ce³⁺ to acceptors can be realized.^9,22–27 Since Ce³⁺ has similar ionic radii as well as the same valence as Tb³⁺, it can act as an effective sensitizer for Tb³⁺ to obtain a high QE (>80%) in some materials, such as in Ca₂TbHf₂Al₃O₁₂:Ce³⁺,⁹ La₈Ba₂(Si₄P₂O₂₂N₂)O₂:Ce³⁺,Tb³⁺,²¹ and Y₅Si₃O₁₂N:Ce³⁺,Tb³⁺.²⁸

Although the reported phosphors possess high efficiency, most of them suffer from poor thermal stability (see Table S1 for a detailed comparison in the ESI†). Since the heat generated during the operation results in an unavoidable emission loss, it will limit their practical applications.^29,30 In the Ce³⁺–Tb³⁺ system, most of the Ce³⁺ ions show limited thermal stability because the 4f–5d transitions of Ce³⁺ are sensitive to the crystal environment, which make the emissions of Ce³⁺ strongly depend on the structural properties of the host crystals. Considering that the excitation energy of Tb³⁺ is obtained by the ET from Ce³⁺, it will show a similar or even worse thermal stability than that of Ce³⁺ in some hosts (see Table S1 in the ESI†). In contrast, as reported by our group,^26,27 the high thermal stability can be obtained when the ET rate from Ce³⁺ to Tb³⁺ is large enough to compete with the thermal quenching rate of Ce³⁺ 5d emission, at which the excitation energy of the Ce³⁺ 5d level will be captured by the thermally stable Tb^{3+ 5}D₄ level before thermal quenching happens. Therefore, the realization of efficient ET from Ce³⁺ to Tb³⁺ in proper hosts is of importance in developing phosphors with highly efficient luminescence and superior thermal stability.

In this paper, a series of novel phosphors, Ce³⁺ singly doped and Ce³⁺, Tb³⁺ co-doped Ca₃Gd₂Si₆O₁₈ (CGSO) were prepared via the traditional high-temperature solid state reaction routes. The crystal structure and optical properties of the samples were characterized by X-ray diffraction, high resolution transmission electron microscopy (HRTEM) and photoluminescence spectra. Moreover, the internal and external QE, thermal stability, ET process from Ce³⁺ to Tb³⁺ in CGSO:Ce³⁺,Tb³⁺ as well as the application performance of phosphors under 365 nm chip drive were also investigated in detail.

2. Experimental

2.1 Sample preparation

The powder samples of Ce³⁺ singly doped and Ce³⁺, Tb³⁺ co-doped CGSO were synthesized via the high temperature solid-state reaction.³¹ The stoichiometric amounts of CaCO₃ (99.99%), Gd₂O₃ (99.99%), SiO₂ (99.99%), CeO₂ (99.99%), and Tb₄O₇ (99.99%) were used as raw materials. They were weighed and thoroughly ground in an agate mortar for 30 min. The resulting mixtures were heated at 1380 °C for 6 h under a 5% H₂/95% N₂ reducing atmosphere. Finally, the samples were cooled to room temperature in the furnace and ground again for the following characterization.

2.2 Characterizations

The crystal structure of samples was identified by X-ray diffraction (XRD) (Bruker D8 Focus diffractometer, in the 2θ range from 15° to 70° with Cu Kα radiation (λ = 1.54056 Å) operating at 40 kV and 30 mA). Structure refinement was carried out using the Rietveld method involving the FullProf program. HRTEM images and elemental distributions were obtained on a transmission electron microscope (JEM-2100F, JEOL) at an operating voltage of 200 kV. The emission and excitation spectra at room temperature and 77 K were recorded using the HITACHI F-7000 spectrometer. Fluorescence microscopy photoluminescence images (BX53M Instruments; OLYMPUS, Japan) of the powder particles were recorded in a dark-field mode using a 350 nm UV lamp excitation. The internal and external QE (IQE and EQE) were also measured using a spectrophotometer (QE-2100, Otsuka Photal Electronics). The IQE and EQE values were calculated by the quantum yield measurement software. The IQE defined as the ratio of the number of photons emitted (I_em) to the number of photons absorbed (I_abs) is expressed aswhere L_S is the emission spectrum of the sample, E_S and E_R are the spectra of excitation light with and without the sample in the integrating sphere. I_abs is calculated by the following equation:

The EQE defined as the ratio of the number of photons emitted (I_em) to the number of photons excited (I_ex) is expressed as

The fluorescence decays of Ce³⁺ were measured by an FLS920 fluorimeter (Edinburgh Instruments, Livingston, UK) using a hydrogen flash lamp (nF900; Edinburgh Instruments). The temperature-dependent photoluminescence spectra were also recorded on an F-7000 spectrometer with an external heater. A process controller (OMEGA CN76000) equipped with a thermocouple was used to measure the temperature and control the heating rate.

The commercial blue phosphor BAM:Eu²⁺, the as-synthesized CGSO:4%Ce³⁺,50%Tb³⁺, commercial red phosphor K₂SiF₆:Mn⁴⁺ and a UV-LED chip (365 nm) were used to fabricate a w-LED. The electroluminescence spectra of the fabricated w-LEDs were recorded using a HAAS 2000 photoelectric measuring system (350–1100, EVERFINE, China). The LED was operated at a bias current of 20 mA and a voltage of 3.4 V. All the measurements were conducted at room temperature unless mentioned specially.

3. Results and discussion

3.1 Structure analysis

All the synthesized samples were characterized by powder XRD to verify the purity. Fig. 1a shows the representative XRD patterns of CGSO:4%Ce³⁺, CGSO:4%Ce³⁺,10%Tb³⁺ as well as CGSO:4%Ce³⁺,50%Tb³⁺, and the calculated XRD pattern of CGSO from the Rietveld structural refinement is also shown for comparison. Clearly, all the obtained XRD patterns are in good agreement with that of calculated CGSO and no impurity phase is detected. The results indicate that the introduction of Ce³⁺ and/or Tb³⁺ into the CGSO host retains the phase purity and the doping ions are homogeneously doped in the host lattice at the current doping level. The diffraction peaks are slightly shifted to the high diffraction degree with the increase of Tb³⁺ concentration, which can be theoretically explained by the lattice shrinkage due to the substitution of smaller Tb³⁺ (r = 1.04 Å when CN = 8; r = 0.98 Å when CN = 7; r = 0.92 Å when CN = 6) for larger Gd³⁺ (r = 1.05 Å when CN = 8; r = 1.00 Å when CN = 7; r = 0.94 Å when CN = 6).³² In order to further investigate the structure of the as-synthesized samples, Rietveld structural refinement for the composition of CGSO:4%Ce³⁺ was performed by using the previously reported crystallographic data of Ca₃Y₂(Si₃O₉)₂ as a starting model.³³ The observed, calculated and the different patterns of the XRD refinement of CGSO:4%Ce³⁺ are shown in Fig. 1b, and the crystallographic data and refinement parameters of CGSO:4%Ce³⁺ are summarized in Table S2 in the ESI.† All the final weighted R factors (R_wp) are acceptable, which confirmed the phase purity of the sample. As illustrated in Fig. 1c and d, the obtained CGSO crystallizes in the monoclinic space group C2/c (15) according to the refinement results and there are three cation sites to occupy for Ca²⁺ and Gd³⁺. The first site is coordinated with eight oxygen atoms of which six O atoms are connected with two Si₃O₉ rings by sharing edges, and the remaining two are linked with the other two Si₃O₉ rings by sharing apices. The second site is coordinated with seven O atoms of which three O atoms comprise two edges of a Si₃O₉ ring, and the other four O atoms are connected with Si₃O₉ rings by sharing apices. The third site is coordinated with six O atoms from six Si₃O₉ rings.^33,34 Considering the relatively large ionic radii of Ce³⁺, it has the possibility to occupy the three different sites when introduced into the CGSO crystal.

Fig. 1 (a) XRD patterns of CGSO:4%Ce³⁺, CGSO:4%Ce³⁺,10%Tb³⁺ and CGSO:4%Ce³⁺,50%Tb³⁺; (b) Rietveld refinements of the XRD pattern of CGSO:4%Ce³⁺; (c) crystal structure of CGSO; (d) the coordination environment of Ca/Gd units in the lattice. (e) HRTEM images of CGSO; (f) morphology and elemental distributions of CGSO:4%Ce³⁺,50%Tb³⁺ (the scale bar is 1 μm).

As shown in Fig. 1e, the fine structure observation of CGSO:4%Ce³⁺,50%Tb³⁺ was performed by HRTEM to corroborate the lattice contraction of the solid solution. The clear lattice fringes in the HRTEM image indicate high crystallinity of the sample. The distance between adjacent fringes is estimated to be 6.144 Å, corresponding well with the d(1̄1̄1) spacing (6.096 Å) of the refined structure of CGSO:4%Ce³⁺. Fig. 1f depicts the morphology and elemental distributions of CGSO:4%Ce³⁺,50%Tb³⁺. It can be seen from the morphology that the sample displays slightly agglomerate spheres due to the high reaction temperature. The elemental distribution images give direct evidence that Ca, Gd, Si, Ce, Tb, and O homogeneously distribute in CGSO:4%Ce³⁺,50%Tb³⁺.

3.2 Photoluminescence properties

The excitation and emission spectra of CGSO:4%Ce³⁺ at room temperature are described in Fig. 2a. The excitation spectrum monitored at 395 nm exhibits a broad excitation band ranging from 230 to 380 nm, which is assigned to the Ce³⁺ parity-allowed 4f–5d transitions. Upon 325 nm excitation, the Ce³⁺ doped CGSO shows an intense violet emission and the corresponding emission spectrum consists of an asymmetric emission band peaking at 395 nm, originating from the lowest 5d level to the doublet ground levels (²F_5/2 and ²F_7/2) of the 4f configurations of Ce³⁺. To confirm the existence of more than one site that is occupied by Ce³⁺ ions, the low-temperature (77 K) emission spectrum of CGSO:0.5%Ce³⁺ was also measured. As shown in Fig. 2b, an additional shoulder located at 360 nm appears in the short-wavelength side of the emission spectrum. The emission spectrum can be well decomposed into four Gaussian bands on the energy scale, peaking at 355 nm (28 159 cm⁻¹), 374 nm (26 709 cm⁻¹), 385 nm (25 944 cm⁻¹) and 414 nm (24 140 cm⁻¹), respectively (see Fig. S1 for details, ESI†), which suggests that Ce³⁺ ions occupy at least two different sites, considering that the energy separation between the ²F_7/2 and ²F_5/2 ground levels of Ce³⁺ is about 1800 cm⁻¹. Besides, Ce³⁺ highly prefers sites with large coordination number because of its relatively large ionic radius (1.01 Å), and it has been reported that the emission of six-coordinated Ce³⁺ in oxides is usually in the long-wavelength range (>550 nm), owing to the strong crystal field splitting.^35–37 Therefore, Ce³⁺ only occupies eight- and seven-coordinated sites in the CGSO crystal.

Fig. 2 (a) The excitation and emission spectra of CGSO:4%Ce³⁺ at room temperature; (b) the emission spectrum of CGSO:0.5%Ce³⁺ at 77 K, the emission spectrum is fitted to display the Gaussian curves (dashed lines) characteristic of Ce³⁺; (c) the normalized emission spectra of sample series CGSO:x%Ce³⁺ (x = 0.5–20) under 325 nm excitation; (d) the emission intensities of Ce³⁺ as a function of Ce³⁺ concentration under 325 nm excitation.

In order to further investigate the luminescence properties, a series of Ce³⁺ with different doping content, CGSO:x%Ce³⁺ (0.5 ≤ x ≤ 20), were synthesized and the normalized emission spectra are presented in Fig. 2c. Previously, with the increase of Ce³⁺ concentration, the peak position of the Ce³⁺ emission band shifts to longer wavelength under 325 nm excitation from 381 nm to 397 nm. The above red-shift phenomenon is usually ascribed to the increase of the probability of ET from Ce³⁺ ions at higher 5d energy levels to those at the lower energy levels in a Ce³⁺ heavily doped host, leading to the energy decrease of the short-wavelength emission.^38,39 Consequently, the peak positions of the emission spectra change with the doping process. The dependence of the Ce³⁺ emission intensity on its doping concentration is also displayed in Fig. 2d. The emission intensity of Ce³⁺ is found to increase gradually with the increase of Ce³⁺ contents, and it reaches the maximum at x = 4%, after which (x > 4%) the emission intensity decreases due to concentration quenching.⁴⁰

One may notice from Fig. 2c that most of the emission of Ce³⁺ singly doped CGSO is shorter than 450 nm in wavelength, which limits its application in w-LEDs as a blue phosphor. However, the Ce³⁺ ion can be used as an effective sensitizer due to the 4f–5d transitions, transferring its energy to Tb³⁺ ions. As shown in Fig. 3a, for Tb³⁺ singly doped CGSO, when monitoring the well-known green emission at 543 nm due to the ⁵D₄ → ⁷F₅ characteristic transitions of Tb³⁺, the excitation spectrum contains not only the typical strong excitation band in the range of <300 nm that is attributed to the first spin-allowed 4f–5d transitions of Tb³⁺, but also some weak excitation lines in the range of 350–380 nm and at 484 nm due to the 4f–4f forbidden transition lines of Tb³⁺. Upon UV light (243 nm) excitation, the emission spectrum of Tb³⁺ is dominated by the green emission at 543 nm due to its large values of the branching ratio. A series of strong emission lines at 490, 585 and 622 nm that originate from the ⁵D₄ → ⁷F_J (J = 6, 4, and 3) characteristic transitions of Tb³⁺ are also shown in the spectrum. Obviously, Tb³⁺ singly doped CGSO is useless for w-LEDs because it cannot be effectively excited by the n-UV light. However, the obvious overlap between the 4f–5d emission band of Ce³⁺ (350–510 nm) and the 4f–4f excitation lines of Tb³⁺ around 484 nm indicates the possibility of ET from Ce³⁺ to Tb³⁺, making it possible to tune the emission color from violet to green in the Ce³⁺ and Tb³⁺ co-doped CGSO. In Fig. 3a, for Ce³⁺ and Tb³⁺ co-doped CGSO, the excitation spectrum for monitoring Tb³⁺ emission at 543 nm shows the remarkable 5d bands of Ce³⁺ in the range of 300–400 nm, which is similar to that of Ce³⁺ emission at 395 nm. Moreover, upon excitation at 325 nm, not only the emission band of Ce³⁺ but also the strong emission lines peaking at 543 nm of Tb³⁺ appear in the emission spectrum. The above results indicate the occurrence of ET from Ce³⁺ to Tb³⁺ in Ce³⁺ and Tb³⁺ co-doped CGSO (see Fig. S2, ESI†). Therefore, Ce³⁺ ions can serve as a sensitizer for the Tb³⁺ activated phosphor to enhance the absorptivity of excitation light in the n-UV region, and then achieving an efficient green emission for w-LEDs based on n-UV LEDs.

Fig. 3 (a) The excitation and emission spectra of CGSO:60%Tb³⁺ and CGSO:4%Ce³⁺,60%Tb³⁺; (b) the emission spectra of sample series CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 10, 50) under 325 nm excitation and the insets show the fluorescence microscopy photoluminescence images of CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 10, 50) under 350 nm excitation; (c) the emission intensities of Ce³⁺ and Tb³⁺, the ET efficiency η_ET′ as a function of Tb³⁺ concentration under 325 nm excitation; (d) fluorescence decay curves of Ce³⁺ in CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0–96) phosphors (excited at 325 nm, monitored at 395 nm).

In order to investigate the ET process from Ce³⁺ to Tb³⁺ in the CGSO host further, the emission intensities for a fixed Ce³⁺ concentration at 4 mol% and various Tb³⁺ concentrations y% were measured under the excitation at 325 nm of Ce³⁺. As shown in Fig. 3b, with the increases of y from 0 to 50, the green emission intensities of Tb³⁺ peaking at 543 nm due to ⁵D₄ → ⁷F₅ transitions of Tb³⁺ increase accompanied by the decline of the violet emission band of Ce³⁺. The green emission line at 543 nm dominates the emission spectrum gradually (see Fig. 3b), resulting from the enhanced ET from Ce³⁺ to Tb³⁺ as well as the tremendous absorption coefficient of the 4f–5d allowed transition of Ce³⁺, which is visually reflected by the inset fluorescence microscopy photoluminescence images shown in Fig. 3b of the selected CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 10, 50) phosphors under 350 nm excitation. As shown in Fig. 3c, the emission intensity of Tb³⁺ reaches a maximum value at y = 50, beyond which it starts to decrease because of the Tb³⁺–Tb³⁺ internal concentration quenching. The decay curves of Tb^{3+ 5}D₄ → ⁷F₅ emission (543 nm) in CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 30, 96) under 325 nm excitation were also measured and are shown in Fig. S3 (ESI†). One can find that the decay curves of Tb³⁺ decrease gradually with increasing Tb³⁺ concentration due to the energy diffusion among Tb³⁺ ions, which finally results in the above concentration quenching.^26,41

To judge the possibility of the as-synthesized phosphors for the practical application in w-LEDs, the key parameters, IQE, I_abs and EQE of the Ce³⁺ singly doped and Ce³⁺, Tb³⁺ co-doped CGSO, were measured, as shown in Table 1 (the corresponding QE measurements are also shown in Fig. S4, ESI†). It is obvious that the IQE and EQE increase with the introduction of Tb³⁺ when compared with that of the Ce³⁺ singly doped sample (see Table 1), indicating the absence of QE loss during the ET process from Ce³⁺ to Tb³⁺. Additionally, the IQE of CGSO:4%Ce³⁺,50%Tb³⁺ was determined to be as high as 90.1% while its EQE was reached up to 72% under 325 nm excitation. While Some reported Ce³⁺/Eu²⁺ and Tb³⁺ co-doped phosphors, including their chemical composition, the effective excitation wavelength and QE are summarized in Table S1 (ESI†) for detailed comparison. One can find that the QE of CGSO:4%Ce³⁺,50%Tb³⁺ is higher than that of most reported phosphors, implying its potential application for w-LEDs as a green phosphor. It is noteworthy that the QE in Ce³⁺–Tb³⁺ is apparently higher than that in Eu²⁺–Tb³⁺ because of the existence of the unreduced Eu³⁺ in the as-synthesized samples.

Table 1 The IQE, I_abs and EQE of CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 10, 30, 50)

Samples	IQE (%)	I abs (%)	EQE (%)
CGSO:4%Ce^3+	81.1	85.1	69.0
CGSO:4%Ce^3+,10%Tb^3+	95.1	84.6	80.5
CGSO:4%Ce^3+,30%Tb^3+	94.3	81.8	77.1
CGSO:4%Ce^3+,50%Tb^3+	90.1	80.4	72.4

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In order to understand the ET mechanism in CGSO:Ce³⁺,Tb³⁺, the critical distance for the ET from Ce³⁺ to Tb³⁺, defined as the distance at which the probability of ET is equal to that of the radiative transition of Ce³⁺, is estimated using the following equation, as proposed by Blasse:⁴²

where V is the volume of the unit cell, X_C is the critical concentration, and N is the number of total sites to be occupied by the doped ions in the CGSO unit cell. For the CGSO:Ce³⁺,Tb³⁺ lattice, V = 1552.05 ų, N = 8, and X_C = 0.48, which is the sum of the concentration of Ce³⁺ and Tb³⁺. Therefore, R_c is calculated to be 9.17 Å, which means that the average distance between Ce³⁺ and its nearest Tb³⁺ is larger than 5 Å, indicating that an exchange interaction may play a negligible role in this situation. In CGSO, the shortest Tb³⁺–Tb³⁺ distance is 3.7 Å, but a large amount of Tb³⁺ (>60 mol%) is needed to completely quench the Ce³⁺ emission, and thus we believe that the ET from Ce³⁺ to Tb³⁺ mainly takes places via an electric dipole–quadrupole interaction,^26,27 which can also be verified by the linear fitting results according to Dexter's ET expressions of multipolar interactions and Reisfeld's approximation as shown in Fig. S5 (ESI†). Moreover, the next-nearest neighbor Tb³⁺ of Ce³⁺ (6.6 Å) in CGSO is about two orders of magnitude smaller than that for the nearest neighbor Tb³⁺ of Ce³⁺ (3.7 Å), indicating that ET from Ce³⁺ to Tb³⁺ mainly occurs within the nearest Ce³⁺–Tb³⁺ pairs requiring high concentration of Tb³⁺.

To further confirm the ET interaction between Ce³⁺ and Tb³⁺, the decay curves of Ce³⁺ 5d–4f emission (395 nm) on the concentration of Tb³⁺ in CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0–96) under 325 nm excitation were obtained. One can find from Fig. 3d that the decay curve of Ce³⁺ deviates from the single exponential profile with increasing Tb³⁺ doping content, which indicates the occurrence of nonradiative ET from Ce³⁺ to Tb³⁺. The effective lifetime of Ce³⁺ is defined as:

where I(t) is the intensity at time t. As shown in Fig. 3d, the effective lifetime decreases monotonically with Tb³⁺ concentration, strongly demonstrating the ET from Ce³⁺ to Tb³⁺. The ET efficiencies η_ET from Ce³⁺ to Tb³⁺ were also calculated based on the effective lifetime as given in Table S3 (ESI†), from which one can find that η_ET increases with y and reaches 63% at y = 96, indicating an enhanced ET with the increase of Tb³⁺ concentration. However, the ET efficiency from Ce³⁺ to Tb³⁺, labeled as η_ET′, can also be calculated based on the Tb³⁺ concentration dependence of the emission intensity of Ce³⁺, as listed in Table S3 (ESI†), and the value of η_ET′ is as high as 96% for y = 96 (see Fig. 3c). As mentioned in our previous works,^26,27 the greater difference between the values of η_ET and η_ET′ further proves the possibility of the undetected fast decay component in the decay curve of Ce³⁺. Therefore, the ET from Ce³⁺ to Tb³⁺ is a fast process that mainly occurs from some “quick quenching” Ce³⁺ ions to their nearest neighbor Tb³⁺.

3.3 Photoluminescence thermal stability

Another key characteristic with which phosphors should be judged is the luminescence thermal stability, since the operating temperature is usually high in the LED/laser diode device. Fig. 4a shows the temperature dependences of Ce³⁺, Tb³⁺ and the total emission intensities in CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 5, 50). Under the excitation of 325 nm, the emission intensities of Ce³⁺ with various y values show similar thermal quenching behavior with the increasing temperature. The relative intensity drops to nearly 75% when the temperature is raised up from 30 °C to 150 °C, demonstrating a relatively poor thermal stability of Ce³⁺ in CGSO due to the larger thermal ionization of the excited 5d electron caused by the smaller bandgap of the CGSO host. Furthermore, Tb³⁺ emissions decline very slowly compared with that of Ce³⁺ with increasing temperature and it still maintains 92% even at 210 °C for y = 50, which indicates a superior thermal stability of Tb³⁺ in CGSO. Thus, the introduction of Tb³⁺ into Ce³⁺ doped CGSO considerably improves the thermal stability of the total emission intensity with increasing y. The maintained total emission intensity has been raised from 75% for y = 0 to 92% for y = 50 at 150 °C, indicating a comparable or superior thermal stability than most of the reported phosphors (Table S1, ESI†). To better understand the relationship between the total emission intensities and temperature, the activation energy ΔE_a of CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 5, 50) is given in Fig. S6 (ESI†). Obviously, ΔE_a exhibits a slight increment with the increase of y, which further indicates the improved thermal stability of the total emission intensity with the introduction of Tb³⁺. As mentioned in our previous works,^26,27 the Tb³⁺ concentration dependent thermal stability should be attributed to the fast ET from Ce³⁺ to Tb³⁺ within the nearest Ce³⁺–Tb³⁺ pairs as well as the energy diffusion among Tb³⁺ ions, which is supported by the large different decline rates between effective lifetime and emission intensity of Ce³⁺. Moreover, as shown in Fig. 4b–d, the locations of the emission peaks show nearly no change with increasing temperature due to the weak electron–phonon coupling between the 4f electrons and their surroundings, indicating the excellent fluorescence thermal stability of CGSO:4%Ce³⁺,50%Tb³⁺.

Fig. 4 (a) The emission intensities of Ce³⁺, Tb³⁺ and the total emission intensities in CGSO:4%Ce³⁺,y%Tb³⁺ (y = 0, 5, 50) as a function of the temperature; temperature dependent emission spectra of CGSO:4%Ce³⁺ (b), CGSO:4%Ce³⁺,5%Tb³⁺ (c), and CGSO:4%Ce³⁺,50%Tb³⁺ (d) under 325 nm excitation.

3.4 Electroluminescence spectrum of the fabricated w-LED device

To demonstrate that CGSO:4%Ce³⁺,50%Tb³⁺ can serve as a potential green color component for w-LEDs, a prototype of w-LEDs was fabricated by combining a 365 nm UV LED chip with a mixture of commercial blue phosphor BAM:Eu²⁺, the as-synthesized CGSO:4%Ce³⁺,50%Tb³⁺ and commercial red phosphor KSF:Mn⁴⁺. The emission spectrum of the as-fabricated w-LEDs is shown in Fig. 5, where the weight ratio of the three phosphors is 0.7 : 50 : 6. The corresponding luminous efficiency (LE), CCT, CRI (R_a), and CIE chromaticity coordinates were determined to be 15.73 lm W⁻¹, 5028 K, 61.4 and (0.3417, 0.3197), respectively. The w-LED packaging results indicate that the as-synthesized CGSO:4%Ce³⁺,50%Tb³⁺ is a promising candidate as a green emitting phosphor based on n-UV LEDs.

Fig. 5 The emission spectrum of the fabricated w-LED based on the UV-LED chip (365 nm) and BAM:Eu²⁺, CGSO:4%Ce³⁺,50%Tb³⁺ and KSF:Mn⁴⁺ phosphors.

4. Conclusions

In summary, we obtained a high IQE (90.1%) and EQE (72.4%) of green emission in Ce³⁺ and Tb³⁺ activated CGSO. Since Tb³⁺ can be well sensitized by Ce³⁺, the Ce³⁺ and Tb³⁺ co-doped samples can be effectively excited by the n-UV excitation. The emission color of the co-doped samples can also be tuned from violet to green due to the enhanced ET from Ce³⁺ to Tb³⁺. Moreover, CGSO:4%Ce³⁺,50%Tb³⁺ shows excellent thermal stability wherein 92% of the emission intensity can be retained when the temperature is elevated to 150 °C, which is attributed to the fast ET from Ce³⁺ to Tb³⁺ as well as the energy diffusion among Tb³⁺ ions. Finally, the green phosphor CGSO:4%Ce³⁺,50%Tb³⁺ was employed to fabricate a w-LEDs lamp, indicating its potential to serve as a green phosphor. The results can provide a new insight in developing potential inorganic luminescent materials with high luminescence efficiency as well as good thermal stability.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the National Natural Science Foundation of China (Grant No. 11904186, 11864028 and 51962026), the Natural Science Foundation of Inner Mongolia (Grant No. 2019BS01003), the Research Program of Science and Technology at Universities of Inner Mongolia (Grant No. NJZZ20001) and the Program of Higher-Level Talents of Inner Mongolia University (Grant No. 5185108).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S6, Tables S1–S3. See DOI: 10.1039/d0tc04449k

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