Luminescence properties and its red shift of blue-emitting phosphor Na₃YSi₃O₉:Ce³⁺ for UV LED

Abstract

A series of Na₃YSi₃O₉:Ce³⁺ phosphors have been synthesized via a conventional high temperature solid-state reaction. Each crystal structure was characterized by X-ray diffraction (XRD) and refined by the Rietveld method. Luminescence properties of Na₃YSi₃O₉:Ce³⁺ phosphors such as emission red shifts, fluorescence decay curves, thermal stability, and CIE values were systematically investigated. Upon 300 nm excitation, the emission peaks of Na₃YSi₃O₉:Ce³⁺ phosphors red shift from 385 nm to 415 nm with Ce³⁺ concentration increasing from 0.002 to 0.11. Moreover, this red-shift phenomenon also occurs with an excitation wavelength from 270 nm to 340 nm as the Ce³⁺ concentration determined, which has been explained using the centroid shift and crystal field splitting. The quenching concentration of Ce³⁺ in the host Na₃YSi₃O₉ is determined to be about 3 mol% and the critical distance is calculated to be about 16.623 Å. The energy dispersion mechanism between Ce³⁺ ions was verified to be a dipole–dipole interaction. Temperature-dependent luminescence of Na₃YSi₃O₉:0.03Ce³⁺ from 25 °C to 250 °C was evaluated, and the corresponding activation energy ΔE is 0.277 eV. Not only crystal field splitting but also centroid shift plays an important role in the red-shift of the Na₃YSi₃O₉:Ce³⁺ phosphors, which may contribute to future research in designing novel solid phosphors by modifying composition of the host lattice to affect crystal field splitting and centroid shift, and then adjusting emission wavelengths to match the purposed application.

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1. Introduction

As a new generation of light sources, phosphors converting white-light-emitting diodes (WLEDs) have attracted increasing attention in academic and industrial fields due to advantages such as long lifetime, saving energy, high efficiency, and environmentally friendly character.^1,2 Commercially, the most convenient way of creating WLEDs is to combine the yellow-emitting phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) with blue LED chips.³ However, owing to the lack of a red spectral component, WLEDs often have highly correlated color temperature (CCT > 4500 K) and a low color rendering index (CRI, R_a < 75).^4–7 An alternative strategy is the combination of near-UV LED chips with red-, green-, and blue-emitting phosphors as a trichromatic approach, which will produce excellent CRI values.⁸ Therefore, it is very important to develop new phosphors to realize better optical requirements.

Due to its excellent thermal and chemical stability, low cost and excellent water-resistance, silicate materials have been widely studied.⁹ Na₃YSi₃O₉ crystallized in an orthorhombic system with space group of P2₁2₁2₁(19). These compounds possesses a mixed octahedral–tetrahedral framework, where the YO₆ octahedrals are isolated from each other by SiO₄ tetrahedrals.¹⁰ Several research reports on Na₃YSi₃O₉ phosphors have been published, such as Na₃YSi₃O₉:Bi³⁺,Eu³⁺ phosphor,¹¹ Na₃(Y_1−aLn_a)Si₃O₉ (Ln = Eu, Tb, Tm) phosphors,¹⁰ and Na₃YSi₃O₉:Tm³⁺,Dy³⁺ phosphors.¹² On the other hand, there is little research information on the Ce³⁺ doped Na₃YSi₃O₉ phosphor. A Ce³⁺-activated phosphor often shows high efficiency in many hosts because of a 4f–5d parity allowed electric dipole transition. Moreover, due to the outer shell 5d orbits of Ce³⁺ ions, the emission band peak is sensitive to the host lattice,^13,14 which can be shifted from UV to the visible range; for example, the yellow-orange-emitting CaAlSiN₃:Ce³⁺,¹³ blue-emitting Ca₈La₂(PO₄)₆O₂:Ce³⁺,¹⁵ green-emitting CaY₂Si₂S₈:Ce³⁺,¹⁶ and yellow-emitting LaSr₂AlO₅:Ce³⁺.^17,18 Herein, a series of Ce³⁺-doped Na₃YSi₃O₉ phosphors were synthesized via a conventional high temperature solid-state reaction. Crystal structures, luminescence properties, decay curves, and thermal stability were systematically investigated. The Na₃YSi₃O₉:Ce³⁺ phosphor shows blue emission and the emission band peak shifts red with increasing excitation wavelengths and Ce³⁺ content. Because electronegative vacancies affect charge distribution around the Ce³⁺ ions, the 5d energy levels shift down; thus, spectra with determined Ce³⁺ shift red and broaden under different excitation. As Ce³⁺ concentration increases, the emission shifts red and is ascribed to enhancement of the crystal field strength surrounding Ce³⁺ ions and the centroid shift of Ce³⁺ ions. Considering these factors in emission spectra, an interesting experiment may be to design solid solution phosphors via controlling crystal field splitting together with a centroid shift when seeking future novel phosphors.

2. Experimental section

2.1 Materials and synthesis

The Na₃YSi₃O₉:Ce³⁺ phosphors were prepared via a conventional high temperature solid-state reaction. All starting materials, i.e., Na₂CO₃ (A.R.), SiO₂ (A.R.), Y₂O₃ (A.R.) and CeO₂ (4 N), were weighed according to stoichiometric ratios (Sinopharm Chemical). All raw materials were mixed and ground thoroughly in an agate mortar for 1 h, and then the homogenous mixture was put in an aluminum crucible with a cover. Next, the crucible was put in a muffle furnace with continuous sintering at 1100 °C in a reducing atmosphere for 6 h. The product was cooled down to room temperature naturally, and subsequently pulverized for further measurements.

2.2 Measurement and characterization

The crystal structure of the as-prepared phosphor was determined by X-ray diffraction (XRD) analysis using a Bruker-D8 powder diffractometer (XRD) with Cu K_α (λ = 1.54078 Å). All data were collected over a 2θ range from 10° to 80°. Room temperature excitation and emission spectra were detected using a Hitachi F-4600 spectrophotometer with a 150 W Xe lamp. Temperature-dependent luminescence properties were measure using the same spectrophotometer with a self-made heating attachment and a computer-controlled electric furnace. Decay curves were recorded on a FLS980 fluorescence spectrophotometer with a nanosecond flash nF920 as the excitation resource.

3. Results and discussion

3.1 Phase formation and crystal structure

Fig. 1 shows the XRD pattern of the as-synthesized Na₃YSi₃O₉:0.03Ce³⁺ phosphor and the standard pattern (ICSD #20774) of Na₃YSi₃O₉. By comparing the diffraction peaks of XRD patterns, the XRD pattern of Na₃YSi₃O₉:0.03Ce³⁺ agrees well with the standard pattern and no other phase is observed, demonstrating that a single phase of Na₃YSi₃O₉:0.03Ce³⁺ phosphor has been obtained without any notable impurities. In Na₃YSi₃O₉, the effective ionic radii of Na⁺ (CN = 6), six-coordinated Y³⁺ and four-coordinated Si⁴⁺ are 1.02 Å, 0.88 Å, and 0.26 Å, while the ionic radii of Ce³⁺ is 1.01 Å. Considering the ionic radii differences between the doped ions and cations, there may a be potential for Ce³⁺ ions to occupy both Na⁺ sites and Y³⁺ sites in Na₃YSi₃O₉. In fact, according to Bragg's equation 2d sin θ = nλ, the shrinkage of cell volume will lead to an increase of the 2-theta value. When a larger cation is replaced by smaller doping ions, the 2-theta value shifts right.¹⁹ The main diffraction peaks shift to a higher angle in the enlarged view from 20° to 35° (Fig. 1), which indicates that Ce³⁺ ions substituted the position of Na⁺ sites in the Na₃YSi₃O₉ host lattice.

Fig. 1 Powder XRD pattern of the Na₃YSi₃O₉:0.03Ce³⁺ phosphor with the standard pattern (ICSD #20774).

To further understand phase purity and occupancy of Ce³⁺ ions, a Rietveld structure refinement of the as-prepared sample was performed using the general structure analysis system (GSAS) program as shown in Fig. 2a.²⁰ The standard Na₃YSi₃O₉ was used as an initial structure model, and the refined parameters, residual factors of un-doped Na₃YSi₃O₉ and Na₃YSi₃O₉:0.03Ce³⁺ are summarized in Table 1. Results of the Rietveld refinement further demonstrate that the doping of Ce³⁺ ions did not generate any impurity in Na₃YSi₃O₉. Un-doped Na₃YSi₃O₉ and Na₃YSi₃O₉:0.03Ce³⁺ crystallized in an orthorhombic system with a space group of P2₁2₁2₁(19), and the lattice parameters for Na₃YSi₃O₉:0.03Ce³⁺ were fitted to be a = 15.0363(4) Å, b = 15.1427(5) Å, c = 15.2095(3) Å, α = β = γ = 90°, V = 3463.08(5) ų, and Z = 16, and the reliability factors are χ² = 7.048, R_wp = 17.37% and R_p = 12.81%, while the parameters for un-doped Na₃YSi₃O₉ are a = 15.0384(1) Å, b = 15.1478(0) Å, c = 15.2135(9) Å, α = β = γ = 90°, V = 3465.64(0) ų, Z = 16, χ² = 6.560, R_wp = 17.01%, R_p = 12.95%. Comparing the refined parameters of Na₃YSi₃O₉:0.03Ce³⁺ with the parameters of un-doped Na₃YSi₃O₉, we observed that values of the refined parameters were smaller after doping Ce³⁺ ions, which is caused by the fact that the larger host cations were replaced by smaller doping ions. The atomic occupancy of Ce and Na is provided in Table S1 in the ESI† which further confirmed that Ce³⁺ ions occupied the six-coordinated Na⁺ sites. Fig. 2b presents the coordination spheres of six-coordinated Na⁺ ions, forming distorted octahedrons with Si⁴⁺ ions coordinated by four oxygen atoms in a regular tetrahedron and the Y³⁺ ions in regular [YO₆]⁹⁻ octahedrons along the a-direction. The distorted [NaO₆]¹¹⁻ octahedron connected with the [SiO₄]⁴⁻ tetrahedron and [YO₆]⁹⁻ octahedron by sharing edges or corners to form a three-dimensional framework. Moreover, the distorted [NaO₆]¹¹⁻ octahedron connected with another six-coordinated Na⁺ ion by the O²⁻ point.

Fig. 2 (a) Powder XRD pattern of Na₃YSi₃O₉:0.03Ce³⁺ with its corresponding Rietveld refinement (red solid line) and residuals (blue line in the bottom); (b) the coordination sphere of six-coordinated Na⁺ in the Na₃YSi₃O₉ host matrix.

Table 1 Rietveld refinement and crystal date for Na₃YSi₃O₉ and Na₃YSi₃O₉:0.03Ce³⁺

Compound	Standard Na3YSi3O9	Un-doped Na3YSi3O9	Na3YSi3O9:0.03Ce^3+
Symmetry	Orthorhombic	Orthorhombic	Orthorhombic
Space group	P212121	P212121	P212121
a (Å)	15.0330	15.0384(1)	15.0363(4)
b (Å)	15.1420	15.1478(0)	15.1427(5)
c (Å)	15.2130	15.2135(9)	15.2095(3)
α (deg)	90	90	90
β (deg)	90	90	90
γ (deg)	90	90	90
V (Å^3)	3462.93	3465.64(0)	3463.08(5)
Z	16	16	16
Rwp%	—	17.01	17.37
Rp%	—	12.95	12.81
χ^2	—	6.560	7.048

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3.2 Photoluminescence properties and the red-shift emissions of Na₃YSi₃O₉:Ce³⁺ phosphors

Photoluminescence emission (PL, λ_ex = 300 nm) and excitation (PLE, λ_em = 402 nm) spectra of Na₃YSi₃O₉:0.03Ce³⁺ are depicted in Fig. 3a. The PLE spectrum represents a broad hump from 240 nm to 375 nm, assigned to electron transition from the 4f energy level to different 5d sub-levels of Ce³⁺ ions. Upon 300 nm excitation, the Na₃YSi₃O₉:0.03Ce³⁺ phosphor shows a broad emission band from 340 nm to 570 nm peaking at 402 nm, which is attributed to the spin-allowed 5d–4f transitions of Ce³⁺ ions. For the sake of clarity, the PL asymmetric band de-convoluted by Gaussian functions fit with good approximation with two Gaussian curves peaking at 23 490 cm⁻¹ (∼425 nm) and 25 544 cm⁻¹ (∼391 nm), as shown in Fig. 3b. The energy difference was calculated to be 2054 cm⁻¹, which is close to that between ²F_5/2 and ²F_7/2 (generally ≈2000 cm⁻¹).²¹ Herein, the two Gaussian peaks are ascribed for the transitions from the lowest 5d excited state to the ²F_5/2 and ²F_7/2 ground states of Ce³⁺ ions due to spin–orbit coupling. That is to say, there exists only one type of emission center in the Na₃YSi₃O₉ host lattice, which is consistent with Ce³⁺ ion substituting the six-coordinated Na⁺ site in Na₃YSi₃O₉.

Fig. 3 (a) The PL (λ_ex = 300 nm) and PLE (λ_em = 402 nm) spectra of Na₃YSi₃O₉:0.03Ce³⁺; (b) the PL spectrum of Na₃YSi₃O₉:0.03Ce³⁺ and its Gaussian components.

Fig. 4a and b present the PL spectra of Na₃YSi₃O₉:0.03Ce³⁺ and Na₃YSi₃O₉:0.09Ce³⁺ excited with various excitation wavelengths, respectively. The emission of Na₃YSi₃O₉:xCe³⁺ samples excited under different wavelengths (270–340 nm) are shown in Fig. S1 and a summary is listed in Table S2.† When the excitation wavelength gradually increased from 270 nm to 340 nm, the emission of Na₃YSi₃O₉:0.03Ce³⁺ shifted to longer wavelengths from 398 nm to 418 nm, and the emissions of Na₃YSi₃O₉:0.09Ce³⁺ shifted from 411 nm to 431 nm. Here, due to the fixed Ce³⁺ concentration, the explanation of the red shift by crystal field splitting is excluded. The ∼20 nm red shift is supposed to be ascribed to a 5d centroid shift of Ce³⁺. Because of different valence states and the charge mismatch originating from doping ions Ce³⁺ on the Na⁺ site in Na₃YSi₃O₉, two vacancies with one Ce³⁺ ion would exist according to the formal equation: .²² Vacancies with a charge would appear because of the different valence states and the possible charge compensation behavior originating from Ce³⁺ ions substituting for the Na⁺ sites. Meanwhile, the electronegative vacancies will affect charge distribution with its chemical environment around the Ce³⁺ ion, and is reflected by the centroid shift with 5d levels of Ce³⁺.²³ Since the bond length and bond angle of six-coordinated Na⁺ are different, as shown in Fig. 2b, the effect of vacancies on Ce³⁺ could not be always identical to all doping Ce³⁺ ions in Na₃YSi₃O₉. Therefore, the effect of these vacancies not only shifts down the centroid of 5d levels leading to the emission red shift, but also broadens the spectra (Fig. 4).

Fig. 4 (a) The normalized PL spectra of Na₃YSi₃O₉:0.03Ce³⁺ excited with different excitation wavelengths; (b) the normalized PL spectra of Na₃YSi₃O₉:0.09Ce³⁺ excited with different excitation wavelengths.

Fig. 5 illustrates the PL spectra of Na₃YSi₃O₉ with different Ce³⁺ ion concentrations under 300 nm excitation. Except for intensity, the PL spectra of Na₃YSi₃O₉:xCe³⁺ (x = 0.002, 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11) have similar spectral profiles with a continuous 30 nm red-shift of Ce³⁺ emission from 385 nm to 415 nm as the concentration of Ce³⁺ increases gradually, exhibiting a larger shift than that of Fig. 4a and b.

Fig. 5 (a) The PL spectra of Na₃YSi₃O₉:xCe³⁺ (x = 0.002, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11) phosphors under 300 nm excitation; (b) the normalized PL spectra of Na₃YSi₃O₉:xCe³⁺ excited under 300 nm.

Comparing the 20 nm red shift of the 5d centroid shift in Fig. 4 with the 30 nm red-shift of the Ce³⁺ emission as the concentration of Ce³⁺ increased in Fig. 5, the difference of that red shift should be noted. Mostly, the emission shift phenomenon results from a crystal field splitting effect as referenced in other works.^24–26 Therefore in this paper, not only crystal field splitting should be considered, but the centroid shift should be also taken into account with the red shift.

It is established that the Dieke diagram of 4f energy levels is almost invariant while the energies of 5d states are influenced 50 times stronger by the host compound than those of 4f states.²⁷ When lanthanide ions are present in a compound, the lowest level of the 5d configuration would shift down as the emission shifted red. The down shift D(3+,A) of the first 4f–5d transition of Ce³⁺ is caused from two contributions, namely, centroid shift (ε_c) and crystal field splitting (ε_cfs),^15,28 as illustrated in Fig. 6. The centroid shift is the shift of 5d centroid energy relative to free Ce³⁺ ions, which is 6.35 eV (51 230 cm⁻¹), while crystal field splitting is the energy difference between the lowest and highest 5d-level.^15,28 The 4f–5d transition energy of Ce³⁺ ions doped in Na₃YSi₃O₉ can be written as:²⁷

E_fd(3+,A) = E_fd(3+,free) − D(3+,A) (1)

where E_fd(3+,free) is the energy for the first 4f–5d transition of free Ce³⁺ ions. The relationship between D(3+,A) and ε_c(3+,A) and ε_cfs(3+,A) can be related as:²⁹where 1/r(3+,A) is the fraction of ε_cfs that adds to the red shift, and which depends on the type of polyhedron.

Fig. 6 Schematic diagram of the centroid shift ε_c(3+,A), crystal field splitting ε_cfs(3+,A), red shift D(3+,A) and emission of Ce³⁺ in a certain compound.

Origin of the centroid shift is very complicated and determined by chemical (covalence) and physical (polarizability) properties of the anions coordinating Ce³⁺.^27,28 Based on the model that describes the correlated motion between 5d electron and ligand electrons, the 5d centroid shift for Ce³⁺ can be given as:^15,30

where R_i is the distance between Ce³⁺ and anion i in the undistorted lattice. The summation is over all N anions that coordinate Ce³⁺ and 0.6ΔR is a correction for lattice relaxation around Ce³⁺. Note α_spⁱ (10⁻³⁰ m⁻³) is the spectroscopic polarizability of anion i, and it is an important parameter that includes the effects of correlated motion and covalence between Ce³⁺ and also the anions and other possible contributions to the centroid shift. For the oxides:^15,30,31

α^o_sp = 0.33 + 4.8/χ_av² (4)

where χ_av is the weighted average of the electronegativity of the cations in the oxide compounds. Due to the electronegative defect around Ce³⁺ ions, the value of χ_av for Ce³⁺ in the Na₃YSi₃O₉ would be smaller and the 5d centroid shift ε_c of Ce³⁺ is more obvious according to eqn (3) and (4). Naturally, the 4f–5d transition energy of Ce³⁺ ions with the vacancies would be lower.

Crystal field splitting is related to the shape and size of the first anion coordination polyhedron around Ce³⁺.^27,28 There is an empirical relationship between ε_cfs and the average distance (R) from the central ion to its ligand anions:^15,28

ε_cfs = β_ployR⁻² (5)

The β_ploy values are in the ratio of 1, 0.89, 0.79, 0.42, and 0.42 for octahedral, cubic, dodecahedral, tricapped trigonal prism, and cuboctahedral, respectively;²⁸ anyhow, it is a constant for Na₃YSi₃O₉. The ε_cfs would decrease with the average distance (R) increasing, as obtained from eqn (5). The smaller Ce³⁺ ions substituted in the position of six-coordinated Na⁺ sites in Na₃YSi₃O₉ leads to the shrinkage of cell volume, which then would decrease the average distance (R) and increase the strength of the crystal field. With the doping of Ce³⁺ ions, more electronegative vacancies will affect charge distribution of its chemical environment around the luminescence center Ce³⁺. Therefore, the centroid shift of Ce³⁺ is larger and the energy of the 5d-levels of Ce³⁺ is lower. Moreover, the shrinkage effect will be more effective and the strength of the crystal field increases with Ce³⁺ ions' concentration, leading to larger energy difference between the lowest and highest 5d-level. Finally, the lowest 5d-level of Ce³⁺ ions shift down, resulting in the emissions of the Na₃YSi₃O₉:xCe³⁺ red shift, too. Thus, both centroid shift and crystal field splitting play important roles in the red shift of Na₃YSi₃O₉:xCe³⁺ phosphors.²⁷

Emission intensities increase with Ce³⁺ contents until a maximum intensity is achieved, and then the intensity begins to decrease with x beyond the critical concentration due to energy dispersion between Ce³⁺ ions, namely, a concentration quenching effect.³² The concentration-dependent luminescence spectra are shown in Fig. 7a, and the critical concentration of Ce³⁺ in Na₃YSi₃O₉ is 0.03 mol. The critical distance (R_c) for energy transfer among Ce³⁺ ions is necessary to further understand the interaction mechanisms, which was often calculated by using a concentration quenching method and the relationship given by Blasse:³³

where V is the volume of the unit cell, x_c is the critical concentration, and N is the number of cations in the unit cell. Herein, the values are V = 3463.08 ų, x_c = 0.03, and N = 48. Thus, the R_c of Ce³⁺ was calculated to be 16.623 Å. There are three mechanisms for the nonradiative energy transfer: exchange interaction, radiation reabsorption, and electric multipolar interaction.³⁴ Owing to the typical critical distance of the exchange interaction being about 5 Å, the exchange interaction only fits the energy transfer of forbidden transitions.^35,36 Therefore, the electric multipolar interactions are dominant in the energy transfer process. According to Dexter^'s theory, the mechanism of the interaction between Ce³⁺ ions can be expressed by the following equation:^35,37in which x is the activator concentration, not less than the critical concentration, I/x is the emission intensity (I) per activator concentration (x), and k and β are constants for the same excitation condition for a given host lattice. θ is a function of electric multipolar character and θ = 6, 8, 10 corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. As represented in Fig. 7b, the correlation between lg(I/x) and lg(x) can be fitted linearly with a slope of −1.639 equaling to . The θ value is determined to be 4.917, which is close to 6. Therefore, the concentration quenching mechanism of Na₃YSi₃O₉:xCe³⁺ is mainly accounted for by a dipole–dipole interaction.

Fig. 7 (a) The dependence of Ce³⁺ emission intensity as a function of the Ce³⁺ content; (b) linear fitting of the relationship of lg(I/x) vs. lg(x) in Na₃YSi₃O₉:xCe³⁺ (x = 0.002–0.11) phosphors beyond the quenching concentration.

To further explore the energy transfer process, the decay curves of Ce³⁺ ions in Na₃YSi₃O₉:xCe³⁺ phosphors by monitoring 402 nm excited by 320 nm were also measured (Fig. 8). As discussed above, the electronegative vacancies will affect the interaction of the charge distribution with its chemical environment around Ce³⁺ ions, which would modify fluorescence dynamics of the Ce³⁺ ions. Results revealed that the fluorescence decays deviated from the single-exponential rule, and this deviation is more obvious with doping Ce³⁺ content. The decay curves were fitted with a second-order exponential decay model as:²⁴

I = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (8)

where I is the luminescence intensity, A₁ and A₂ are constants, and τ₁ and τ₂ are the short and long lifetimes for exponential components. The average decay time τ can be calculated as:

τ = (A₁τ₁² + A₁τ₂²)/(A₁τ₁ + A₁τ₂) (9)

Fig. 8 Decay curves and lifetimes of Ce³⁺ emission in Na₃YSi₃O₉:xCe³⁺ (x = 0.002–0.09) phosphors (excited at 320 nm, monitored at 402 nm).

The average decay times τ are calculated to be 36.734, 33.862, 31.921, 27.442, 25.806, and 23.233 ns for Na₃YSi₃O₉:xCe³⁺ (x = 0.002, 0.01, 0.03, 0.05, 0.07, and 0.09) phosphors. All the results show that the lifetime τ of the 5d¹–4f¹ transitions of Ce³⁺ ions in Na₃YSi₃O₉ decreases with Ce³⁺ concentration, indicating energy dispersion between Ce³⁺ ions. The measured lifetime is also related to the total relaxation rate expressed as:¹¹

1/τ = 1/τ₀ + A_nr + P_t (10)

where τ₀ is the radiative lifetime, A_nr is the nonradiative rate due to multi-phonon relaxation, and P_t is the energy transfer rate between Ce³⁺ ions. With increasing Ce³⁺ concentration, the distance between Ce³⁺ ions decreases. Then, both the Ce³⁺–Ce³⁺ energy transfer rate (corresponding to P_t) and the probability of energy transfer to quenching sites (corresponding to A_nr) increase. As a result, the lifetimes are shortened with increasing concentrations of Ce³⁺.

3.3 Thermal stability of Na₃YSi₃O₉:Ce³⁺ phosphor

Thermal stability is important for practical applications due to its significant influence on light output and CRI. Temperature-dependent PL spectra of the Na₃YSi₃O₉:0.03Ce³⁺ phosphor excited by 300 nm are depicted in Fig. 9. When the temperature increased from 25 °C to 250 °C, the luminescence intensity decreased gradually to 56.97% at 100 °C. Thermal stability of Na₃YSi₃O₉:0.03Ce³⁺ and commercial BAM:Eu²⁺ is shown in the inset of Fig. 9. Thermal quenching can be explained by a configuration coordinate diagram. At high temperature, the electron–phonon interaction is intensive, and the electrons in the 5d excited state could be thermally activated to the 4f ground state through the crossing point between the excited state and the ground state. The nonradiative thermal relaxation process results in decreased luminescence intensity.³⁸ The full width at the half maximum (FWHM) of PL spectrum increases with the temperature, which can be described by using the Boltzmann distribution:³⁹where W₀ is the FWHM at 0 K, hv represents the vibrational phonon energy which interacts with the electronic transitions, S means the Huang–Rhys parameter, and k is the Boltzmann constant. It is assumed that hv is the same for both 4f ground state and 5d excited state of Ce³⁺ ions. When the temperature increases, the electron–phonon interaction becomes dominate and the excited electrons spread to higher vibration levels, and then, the increasing transition from different levels result in the increase of FWHM.³⁶

Fig. 9 Temperature-dependent PL spectra of Na₃YSi₃O₉:0.03Ce³⁺ phosphor; the inset shows the emission of Na₃YSi₃O₉:0.03Ce³⁺ and the commercial BAM as a function of temperature when excited at 300 nm.

To further investigate the relationship of emission intensity with temperature, the activation energy (ΔE) for thermal quenching can be estimated using the Arrhenius equation:⁴⁰

where I₀ is the initial intensity, I_T is the intensity at different temperatures, c is a constant, ΔE is the activation energy for thermal quenching, and k is the Boltzmann constant (8.62 × 10⁻⁵ eV). According to the equation, the activation energy ΔE is calculated to be 0.277 eV by the relationship of ln[(I₀/I_T) − 1] against 1/kT, as shown in Fig. 10.

Fig. 10 The linear fitting of ln[(I₀/I_t) − 1] vs. 1/kT for thermal quenching of Na₃YSi₃O₉:0.03Ce³⁺; the inset shows the configuration coordinate diagram for an explanation of thermal quenching.

3.4 CIE color coordinates

The variations of Commission International de L' Eclairage (CIE) chromaticity coordinates of the Na₃YSi₃O₉:xCe³⁺ (x = 0.002, 0.01, 0.03, 0.05, 0.07, 0.09, and 0.11) phosphors are determined based on the PL spectra excited at 300 nm (Fig. 11). The CIE chromaticity diagram is tuned from (0.149, 0.057) to (0.164, 0.172) with increasing concentrations of Ce³⁺ ions from 0.002 to 0.11. The result indicates that the chromaticity coordinates of Na₃YSi₃O₉:xCe³⁺ phosphors can be modulated over a wide range by appropriately changing the concentrations.

Fig. 11 CIE chromaticity diagram of Na₃YSi₃O₉:xCe³⁺ (x = 0.002–0.11) phosphors excited at 300 nm.

4. Conclusion

In summary, a serious of blue-emitting Na₃YSi₃O₉:Ce³⁺ phosphors were synthesized via a traditional solid-state reaction. Results of X-ray diffraction (XRD) and Rietveld refinement analysis indicate that the doping of Ce³⁺ ions did not cause any impurities in Na₃YSi₃O₉. Concentration quenching of Na₃YSi₃O₉:Ce³⁺ occurs and the critical quenching concentration was determined to be about 3 mol%. The energy transfer critical distance was calculated to be about 16.626 Å. The corresponding quenching mechanism was verified to be a dipole–dipole interaction. The activation energy ΔE of Ce³⁺ ion was calculated to be 0.277 eV. The CIE chromaticity coordinates of Na₃YSi₃O₉:xCe³⁺ phosphors can be tuned in a wide range from (0.149, 0.057) to (0.164, 0.172) with Ce³⁺ ions increasing from 0.002 to 0.11. Upon 300 nm excitation, the emission peaks of Na₃YSi₃O₉:0.03Ce³⁺ phosphors shifted to a longer wavelength of 20 nm as the Ce³⁺ concentration was determined and to 30 nm when Ce³⁺ concentration increased from 0.002 to 0.11. The spectral red-shift phenomenon is explained by a centroid shift and crystal field splitting. Therefore, some novel phosphors, especially in solid solution, may be prepared by combining centroid shift and crystal field splitting to adjust an emission for application.

Acknowledgements

This work is financially supported by the National Nature Science Foundation of China (11104366), the Natural Science Foundation Project of Chong Qing (cstc2014jcyjA50018), Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1500913, KJ1500937) and the Science and Technology Innovation Project of CQUT.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra03813e

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